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ArticleApril 1, 2024

Electrocatalytic Nitrate and Nitrite Reduction toward Ammonia Using Cu₂O Nanocubes: Active Species and Reaction Mechanisms
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Lichen Bai
Lichen Bai
Department of Interface Science, Fritz-Haber-Institute of Max-Planck-Society, Faradayweg 4-6, 14195 Berlin, Germany
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https://orcid.org/0000-0003-1452-6129
Federico Franco
Federico Franco
Department of Interface Science, Fritz-Haber-Institute of Max-Planck-Society, Faradayweg 4-6, 14195 Berlin, Germany
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https://orcid.org/0000-0002-1166-1256
Janis Timoshenko
Janis Timoshenko
Department of Interface Science, Fritz-Haber-Institute of Max-Planck-Society, Faradayweg 4-6, 14195 Berlin, Germany
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Clara Rettenmaier
Clara Rettenmaier
Department of Interface Science, Fritz-Haber-Institute of Max-Planck-Society, Faradayweg 4-6, 14195 Berlin, Germany
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Fabian Scholten
Fabian Scholten
Department of Interface Science, Fritz-Haber-Institute of Max-Planck-Society, Faradayweg 4-6, 14195 Berlin, Germany
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Hyo Sang Jeon
Hyo Sang Jeon
Department of Interface Science, Fritz-Haber-Institute of Max-Planck-Society, Faradayweg 4-6, 14195 Berlin, Germany
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Aram Yoon
Aram Yoon
Department of Interface Science, Fritz-Haber-Institute of Max-Planck-Society, Faradayweg 4-6, 14195 Berlin, Germany
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Martina Rüscher
Martina Rüscher
Department of Interface Science, Fritz-Haber-Institute of Max-Planck-Society, Faradayweg 4-6, 14195 Berlin, Germany
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Antonia Herzog
Antonia Herzog
Department of Interface Science, Fritz-Haber-Institute of Max-Planck-Society, Faradayweg 4-6, 14195 Berlin, Germany
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https://orcid.org/0000-0002-4402-3007
Felix T. Haase
Felix T. Haase
Department of Interface Science, Fritz-Haber-Institute of Max-Planck-Society, Faradayweg 4-6, 14195 Berlin, Germany
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https://orcid.org/0000-0003-1646-4312
Stefanie Kühl
Stefanie Kühl
Department of Interface Science, Fritz-Haber-Institute of Max-Planck-Society, Faradayweg 4-6, 14195 Berlin, Germany
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See Wee Chee
See Wee Chee
Department of Interface Science, Fritz-Haber-Institute of Max-Planck-Society, Faradayweg 4-6, 14195 Berlin, Germany
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Arno Bergmann
Arno Bergmann
Department of Interface Science, Fritz-Haber-Institute of Max-Planck-Society, Faradayweg 4-6, 14195 Berlin, Germany
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https://orcid.org/0000-0001-5071-6806
Roldan Cuenya Beatriz*
Roldan Cuenya Beatriz
Department of Interface Science, Fritz-Haber-Institute of Max-Planck-Society, Faradayweg 4-6, 14195 Berlin, Germany
*Email: [email protected]
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https://orcid.org/0000-0002-8025-307X

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Journal of the American Chemical Society

Cite this: J. Am. Chem. Soc. 2024, 146, 14, 9665–9678

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https://doi.org/10.1021/jacs.3c13288

Published April 1, 2024

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Abstract

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The electrochemical reduction of nitrate (NO₃^–) and nitrite (NO₂^–) enables sustainable, carbon-neutral, and decentralized routes to produce ammonia (NH₃). Copper-based materials are promising electrocatalysts for NO_x^– conversion to NH₃. However, the underlying reaction mechanisms and the role of different Cu species during the catalytic process are still poorly understood. Herein, by combining quasi in situ X-ray photoelectron spectroscopy (XPS) and operando X-ray absorption spectroscopy (XAS), we unveiled that Cu is mostly in metallic form during the highly selective reduction of NO₃^–/NO₂^– to NH₃. On the contrary, Cu(I) species are predominant in a potential region where the two-electron reduction of NO₃^– to NO₂^– is the major reaction. Electrokinetic analysis and in situ Raman spectroscopy was also used to propose possible steps and intermediates leading to NO₂^– and NH₃, respectively. This work establishes a correlation between the catalytic performance and the dynamic changes of the chemical state of Cu, and provides crucial mechanistic insights into the pathways for NO₃^–/NO₂^– electrocatalytic reduction.

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Introduction

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Ammonia (NH₃) is an essential feedstock for the industrial production of various chemicals, including fertilizers, synthetic fibers, and pharmaceuticals. (1−3) It also serves as a potential carbon-free energy carrier with high energy density and excellent hydrogen storage capacity. (4,5) Nowadays, large-scale NH₃ production is supplied by the Haber–Bosch process, which is largely dependent on fossil fuels (used for the generation of H₂ from CH₄) and contributes significantly to the emissions of carbon dioxide. (1−3) Moreover, the increasing rate of anthropogenic nitrogen fixation has led to a persistent imbalance in the nitrogen cycle, (6,7) ultimately resulting in an accumulation of harmful nitrate (NO₃^–) and nitrite (NO₂^–) in groundwater, which could have serious negative impact on both the environment (3,8−10) and the human health. (3,11,12) The electrochemical reduction of nitrate (NO₃RR) and nitrite (NO₂RR), powered by renewable energy sources, has recently attracted growing interest as a sustainable and carbon-neutral route to produce NH₃ directly from the industrial and agricultural wastewater. (3,6,13−15) This approach would favor a decentralized NH₃ production in local communities. (13−15)

Due to the extreme complexity of the overall NO₃RR and NO₂RR processes, which involve multiple proton/electron-transfer steps and intermediates, suitable catalysts are essential to control the selectivity toward a specific target product and to drive an efficient NH₃ production for practical and technological applications. (3,15−17) Copper-based materials are promising catalysts for electrochemical NO₃RR and NO₂RR due to the properties of low cost, high selectivity for NH₃ production and relatively low overpotentials. (3,16,17) Several strategies including alloying, (18−20) doping, (21−23) facet control (24−27) and defect engineering (24,28−30) were employed to further improve the catalytic performance of Cu catalysts. However, the underlying reaction mechanisms and the identity of active sites responsible for NH₃ formation of Cu-based catalysts remain under debate. (21,31−33) For instance, CuO nanowire arrays were reported to undergo an in situ electrochemical reconstruction to generate a mixed Cu/Cu₂O active phase, which serves as a catalyst for NH₃ production. (31) Several highly active Cu-based bimetallic catalysts were reported to be in metallic state under NO₃RR conditions. (18,21) Elsewhere, highly active defective CuO nanostructures were converted to mixed CuO/Cu₂O under catalytic NO_xRR conditions, with no evidence reported for metallic Cu. (28) Some recent studies have also considered cuprous(I) oxide (Cu₂O) to be the real catalyst for ammonia formation, which is supposed to reduce the energy barrier for NH₃ formation and suppress the competing hydrogen evolution reaction (HER). (32,33)

Here, we use Cu₂O nanocubes (NCs) as model precatalysts for both NO₃RR and NO₂RR due their well-defined structures and initial composition, combined with the absence of surface-adsorbed ligands, and track the chemical state of Cu as a function of the applied potential to establish a correlation with the NH₃/NO₂^– selectivity. A combination of grazing incidence X-ray diffraction (GI-XRD), transmission electron microscopy (TEM), quasi in situ X-ray photoelectron spectroscopy (XPS), and operando X-ray absorption spectroscopy (XAS) measurements were used to follow the dynamic changes of Cu₂O NCs occurring both in the bulk (beneath the surface) and on the surface under electrocatalytic NO₃RR and NO₂RR conditions. We reveal that Cu(I) species are predominant during the initial two-electron NO₃^–-to-NO₂^– reduction step at low overpotentials, whereas metallic copper decisively contributes to multielectron stepwise reduction of NO₃^–/NO₂^– to NH₃ at more cathodic potentials. Possible reaction pathways are further proposed based on in situ Raman spectroscopy and electrokinetic analysis.

Results and Discussion

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Characterization and Electrocatalytic NO₃RR and NO₂RR Performances of Cu₂O NCs

The Cu₂O NCs were prepared via a wet-chemistry method, where copper sulfate was reduced by l-ascorbic acid in alkaline condition (see the Experimental section). (34) The cubic morphology of the particles is confirmed by TEM (Figures 1a and S1a,b) and high angular annular dark field scanning transmission electron microscopy (HAADF-STEM, Figure S1c). The size of the Cu₂O NCs is in the range of 30–40 nm (Figures 1a and S1a,c) and energy dispersive X-ray (EDX) elemental mapping images show uniform distribution of copper and oxygen (Figure S1c–e). The powder X-ray diffraction (PXRD) pattern (Figure S1f) reveals a crystallographic profile matching with only that of the cubic Cu₂O phase (JCPDS no. 00-005-0667), which is also in accordance with the corresponding fast Fourier-transfer (FFT) pattern (Figure S1b) of the high-resolution (HR) TEM image (Figure S1a).

The NO₃RR and NO₂RR performances of the Cu₂O NCs were initially evaluated in an H-type electrochemical cell, using 0.1 M Na₂SO₄ electrolyte with added NaNO₃ or NaNO₂ (8 mM) (Figures S2–S8 and Supporting Information Note 1). Despite the initial close-to-neutral pH of the electrolyte, ammonia formation and proton consumption occurring during NO₃RR and NO₂RR inevitably lead to a pH increase of the unbuffered bulk solution. (3,14) In order to minimize the pH variation during the catalytic tests, the electrochemical behavior of Cu₂O NCs was also investigated at alkaline pH (Supporting Information Note 1). An analogous electrochemical behavior was observed for all the explored pH conditions.

The cyclic voltammograms (CVs) of Cu₂O NCs show the presence of quasi-reversible R1/O1 peaks assigned to Cu(I)/Cu(0) transition, which are observed regardless of the addition of NO₃^–/NO₂^– (Figure 1b). R1 is followed by two irreversible reductive features (R2 and R3) occurring at more cathodic potentials (Figure 1b, red). The R2 wave exhibits an onset just after the R1 peak and is absent when NO₃^– is replaced by an equimolar amount of NO₂^– in the electrolyte, suggesting the reduction of NO₃^– to NO₂^–. (35,36) The R3 wave occurring at more negative potentials with a maximum current at ca. −0.14 V_RHE, accounts for the further reduction of NO₃^–/NO₂^– to products with lower oxidation state of nitrogen, as revealed by the similarity of the curves recorded in NO₃^–- and NO₂^–-containing electrolytes, respectively (Figure 1b, red and blue). The quasi-reversible R4/O4 peak probably originates from the desorption/adsorption of *OH from metallic Cu, (37) which are formed during the catalytic NO₃^– and NO₂^– reduction (see the Electrokinetic Analysis section for a detailed discussion). Moreover, a direct comparison with the polarization curve performed in the absence of NO₃^–/NO₂^– (Figure 1b, black) indicates that the NO₃^–/NO₂^– activation on Cu₂O NCs occurs at considerably milder potentials than HER.

Electrolysis experiments were carried out at varying applied potentials and the spent electrolyte solution was analyzed by UV–vis spectroscopy to quantify the product distribution (Figures 1c and S9, S10). The accuracy of the UV–vis spectroscopy method is corroborated by ¹H NMR analysis and the related ¹⁵N isotopic labeling experiments confirm that NO₃^–/NO₂^– are the sources of the produced ammonia (Figures S11–S13, Supporting Information Note 1). At mild cathodic potentials, NO₃RR catalyzed by Cu₂O NCs results in a selective NO₃^– conversion into NO₂^–, with a maximum faradaic efficiency (FE) of 75 ± 2% at 0.10 V_RHE. However, NH₃ formation progressively increases with overpotential and becomes the predominant product at the expense of NO₂^– with a FE(NH₃) > 80% between −0.20 and −0.40 V_RHE. In particular, a maximum FE of 88 ± 5% and a molar yield rate of 45 ± 3 μmol h^–1 cm^–2 (or 7570 ± 473 μg_product h^–1 mg_catalyst^–1), respectively, are obtained for ammonia formation at −0.30 V_RHE, whereas the FE for NO₂^– production is below 10% (Figures 1c and S14a,b).

The electrocatalytic performance of Cu₂O NCs was also explored in the presence of an equimolar amount of NO₂^–. The catalyst exhibits close to or exceeds 90% FE for NH₃ formation in a wide range of applied potentials, from 0.10 V to −0.40 V_RHE (Figure 1d). The higher FE(NH₃) of NO₂RR compared to those of NO₃RR suggests faster kinetics for NO₂^– activation and reduction by Cu₂O NCs.

A comparison of the NO₃RR performances of Cu₂O NCs at different pH values shows analogous trends of activity and selectivity, with higher applied overpotential leading to higher FE(NH₃) (Figures 1c, S4a, S5a,b, S14a,b and S15). A slight improvement of NH₃ selectivity is observed at 0.0 and −0.1 V_RHE when the pH is increased to 12.9 (Figures 1c and S4a, S15a, Supporting Information Note 1). The Cu₂O NCs also exhibit excellent NO₂RR performances in both pH 12.9 and initially neutral electrolyte, leading to FE(NH₃) > 90% in a wide potential range (Figures S4b,S5c,d and S16).

It is noteworthy that the Cu₂O NCs exhibit better FE(NH₃) compared to other Cu-based materials such as electropolished Cu foil (38) and electrodeposited Cu catalyst (39) under identical conditions (Figure S17). The excellent NO₃RR performance and well-defined initial structures/compositions make our Cu₂O NCs a good candidate for mechanistic insight.

Correlation of the Oxidation State to Catalytic Performance

In order to elucidate the evolution of structure and chemical state of Cu₂O NCs during the electrocatalytic NO₃RR, and to correlate these changes to the catalytic properties, we investigated the catalyst using complementary ex situ and in situ/operando techniques. Quasi in situ X-ray photoelectron spectroscopy (XPS) was employed to analyze the evolution of surface chemical state of the Cu₂O NCs upon NO₃RR in pH 12 electrolyte. Two potentials with distinct corresponding main NO₃RR products: 0.10 V_RHE for NO₂^– or −0.30 V_RHE for NH₃ were chosen for the XPS measurements, respectively. According to the fitting results of Cu LMM X-ray Auger Electron spectra (XAES, Figure 2a, Table S1), Cu(I) still constitutes the majority of the species present in the catalyst near-surface region (70% content) after NO₃RR at 0.10 V_RHE (Figure 2a, Table S1). The rest of the near-surface region is formed by metallic copper. On the other hand, at more negative potentials (−0.30 V_RHE), where ammonia is predominantly formed, an almost complete reduction of Cu(I) to Cu(0) is observed on the catalyst surface, as indicated by the presence of 92% metallic copper (Figure 2a, Table S1). These findings indicate the presence of a mixture of Cu/Cu₂O in the near-surface of the catalyst layer in a low overpotential regime, where NO₂^– is the main product, while metallic Cu is predominant in the NH₃ forming regime.

Figure 2. Quasi in situ and postreaction ex situ characterization of Cu₂O NCs after NO₃RR. (a) Quasi in situ XPS Cu LMM AES of Cu₂O NCs after NO₃RR (1 h) at different potentials. (b,c) Ex situ GI-PXRD of Cu₂O NCs before and after NO₃RR at 0.10 V_RHE (b) and −0.30 V_RHE (c). TEM of Cu₂O NCs after NO₃RR at 0.10 V_RHE (d) and −0.30 V_RHE (e). Electrolyte: 0.1 M NaSO₄ + 8 mM NaNO₃, pH 12.

Consistent results were also obtained by grazing incidence X-ray diffraction (GI-XRD) measurements of Cu₂O NCs after NO₃RR. After 0.5 h of reaction at 0.10 V_RHE, the catalyst is mostly in the form of Cu₂O, whereas only a small peak at ∼43° corresponding to the metallic Cu(111) Bragg peak is observed. Longer reaction times (2 h) tend to generate more metallic and crystalline Cu species, as indicated by the growth of the Cu(200) Bragg peak at ∼50°, while simultaneously the intensity of the Cu₂O-related Bragg peaks decrease (Figure 2b). After NO₃RR at a more negative applied potential (−0.30 V_RHE), the enhanced reduction of Cu₂O ultimately leads to a significant formation of metallic Cu(0) already after short reaction times (0.5 h, Figure 2c), whereas the characteristic Bragg peaks corresponding to the Cu₂O phase can be barely seen after 2 h (Figure 2c).

The transition from Cu₂O NCs to metallic copper is paralleled by the change of morphology and crystallinity. After NO₃RR at 0.1 V_RHE, most of the Cu₂O NCs are preserved (Figures 2d, and S18a–c). Some of the cubes are distorted, while the FFT patterns corresponding to both Cu₂O and metallic copper are observed (Figure S19a–c). At −0.3 V_RHE, the cubes are completely fractioned and changed to particulate aggregates (Figures 2e, S18d,e, S19d,e). The FFT patterns show the features of mixed polycrystalline Cu₂O and Cu (Figures S18f and S19f).

To identify the Cu local atomic structure and chemical state during NO₃RR, the dynamic structural changes of Cu₂O NCs were investigated by means of operando XAS. Cu K-edge X-ray absorption near edge structure (XANES) data were analyzed using linear combination fitting (Figure 3a). Spectra for bulk CuO, Cu₂O and Cu foil were used as references. In the NO₃^– containing electrolyte (pH 12), the XANES data reveal that the as-prepared sample contains almost exclusively Cu(I) species. Nonetheless, we observed a gradual conversion of Cu(I) to Cu(II) species at open circuit potential (OCP), as also observed in the literature. (40) This can be explained at least partially by the interactions with the X-ray beam and the presence of the electrolyte. In the regime of two-electron conversion of NO₃^– to NO₂^– at 0.10 V_RHE (Figure 1c), the Cu(II) species are reduced. However, the reduction of Cu(I) to Cu(0) is sluggish and in the bulk nearly 70% of Cu(I) remains after more than 1 h electrolysis (Figure 3b). When the potential is shifted to optimal conditions for NH₃ production (−0.30 V_RHE, Figure 1c), the remaining Cu(I) is rapidly reduced to Cu(0), resulting in more than 70% of copper being in the Cu(0) state after 15 min (Figure 3b). The fraction of the Cu(0) reaches ca. 85% after 1 h (Figure 3b), suggesting that our NCs remain partially oxidized. A direct comparison of the XANES results with the NO₃RR selectivity data displays a strong correlation between the FE(NH₃) and the formation of metallic Cu (Figure 3b). The extended X-ray absorption fine structure (EXAFS) analysis confirms the findings from XANES. Raw Cu K-edge EXAFS spectra are shown in Figure S20a. Figure 3c shows the Fourier transformed EXAFS spectra. EXAFS fitting results are summarized in Figures 3d and S20b. At OCP, the Fourier-transformed EXAFS spectra for our catalyst show two distinct peaks with a peak at 1.5 Å (phase uncorrected) corresponding to the Cu–O bond, while the peak at 2.9 Å we attribute to Cu–Cu distances in a Cu₂O-like structure. Upon longer reaction times at 0.10 V_RHE, the intensity of both peaks slowly decreases, while a new peak emerges at 2.1 Å, suggesting the formation of Cu–Cu distances in the metallic Cu phase. The Cu–Cu coordination number (CN) at 0.10 V_RHE is around 4, that is, approximately three times lower than the expected Cu–Cu CNs in bulk Cu metal. This suggests that only ca. one-third of the Cu species is in a metallic state, which agrees well with the XANES analysis results. At −0.30 V_RHE, the Cu–Cu contribution increases rapidly, with a Cu–Cu CN reaching around 10 after 30 min. At the same time, the EXAFS features related to Cu₂O are diminished. Hence, the EXAFS analysis results confirm that the Cu(I) species dominates in a potential region where NO₂^– is the main product of NO₃RR, while Cu(0) is the main species at more negative potential where the FE(NH₃) is maximized.

Figure 3. Operando XAS data for Cu₂O NCs during NO₃RR. (a) Normalized Cu K-edge XANES spectra collected at OCP, 0.10 V_RHE and −0.30 V_RHE. (b) Linear combination analysis (LCA) results of the XANES spectra, showing the variation of the Cu oxidation state. The full squares indicate the time-dependent FE(NO₂^–) (red) and FE(NH₃) (blue) obtained by NO₃RR electrolysis experiments. The first data point is measured at 0.10 V_RHE for 2 h, while the other data points are measured at −0.30 V_RHE with the corresponding reaction times. The error bars are calculated based on three independent experiments. (c) Fourier-transformed (FT) Cu K-edge EXAFS spectra of Cu₂O NCs at OCP, 0.10 V_RHE, and −0.30 V_RHE. (d) EXAFS fitting results showing the evolution of Cu–Cu and Cu–O coordination numbers. The error bars indicate the fitting uncertainty. Electrolyte: pH 12, 0.1 M Na₂SO₄ + 8 mM NaNO₃.

The potential-dependent evolution of Cu₂O NCs under NO₃RR was further investigated using operando XAS data collected under a stepwise-applied potential in the range from 0.3 V to −0.5 V_RHE, Figure S21). At 0.3 V_RHE, where the R1 peak emerged in the CV (Figures 1b and S21), the oxidation state of the catalyst still remains apparently unchanged, suggesting that the reduction of the catalyst bulk phase lags behind the reduction of the catalyst surface. The conversion of Cu(I) to Cu(0) starts between +0.1 V_RHE and −0.1 V_RHE and the reduction becomes faster at more negative applied potentials, where the increase of FE(NH₃) was observed (Figures 1c and S21). Notably, the Cu₂O NCs cannot be fully reduced even at more negative applied bias (−0.5 V_RHE, Figure S21). An analogous behavior has been recently reported by us for similar Cu₂O nanocubes during the electrocatalytic CO₂ reduction. (41) Notably, even the catalyst surface that is directly in contact to the electrolyte still contains a small amount of oxidized Cu (∼8%) when subjected to −0.3 V_RHE for 1 h according to our quasi in situ XPS data (Figure 2a, Table S1). Therefore, the observed changes in oxidation state of Cu and its concentration distribution during reaction predominantly reflects the intrinsic kinetic properties of the catalyst, although we cannot completely rule out that a minor fraction of the Cu₂O are partially electrochemically inaccessible in the operando XAS experiments due to the higher catalyst loading on the porous gas diffusion electrode.

The Cu₂O NCs display similar potential-dependent dynamic behavior at different electrolyte pH values (Figures S21–S24). Nonetheless, while the onset potential for Cu(I) reduction is almost pH-independent, a faster transformation of Cu(I) to Cu(0) is observed in alkaline electrolytes, consistent with the improvement of the apparent FE(NH₃) at 0.0 and −0.1 V_RHE when the pH is increased (Figures 1c and S4a, S15a, S23).

Similar trends are obtained also from operando XAS measurements and quasi in situ XPS carried out under NO₂RR conditions (Figures S25 and S26, Table S1), as well as TEM images after NO₂RR (Figures S27 and S28). While the FE(NH₃) of NO₂RR was found to be less sensitive to copper oxidation state change, being >90% in a wide range of potentials (Figure 1d), the ammonia production rate rapidly increases as more metallic copper species are generated (Figure S14c,d).

Electrokinetic Analysis

To get deeper information on the underlying reaction mechanism of NO₂RR/NO₃RR, we performed a comprehensive electrokinetic study. Therefore, we recorded the polarization curves (Figure S29) of Cu₂O NCs in the presence of 8 mM NaNO₃ at pH values from 11.5 to 12.9, to determine the pH-dependent reaction kinetics. The quasi-reversible peaks before catalytic onset shift negatively with the electrolyte pH by ca. −80 mV/pH, implying that more than one proton is involved per Cu site during the Cu(I)/Cu(0) transition (Figure S29c,d). The Tafel slopes are in the range of 102–180 mV/dec (Figures 4a and S30), indicating that the first electron transfer for the activation of nitrate is the rate-determining step (RDS) in the investigated pH range at low overpotentials, according to the quasi-equilibrium Langmuir adsorption model assumption (Supporting Information Note 2). Moreover, from −0.75 to −0.84 V_Ag/AgCl, the logarithm of the current density exhibits −0.70 ± 0.12 order on the pH values (Figure S31), indicating the RDS mostly consumes one proton, while water may partly be the hydrogen source (Supporting Information Note 4). Due to the nitrate adsorption equilibrium (Supporting Information Note 4), the reaction order of nitrate is less than one (0.64 ± 0.02), in the similar potential range (Figure S32). These results suggest that the RDS of NO₃RR at low overpotential is a proton-coupled electron transfer (PCET) step.

To get mechanistic insight into the redox events occurring at higher overpotentials, scan rate dependence studies were performed in the presence of nitrate (pH 12) to complement the Tafel slope analysis (Figure 4b). The peak current of the quasi-reversible R1 feature shows a linear dependence on the square root of the scan rate, both in a static solution and under stirring, while the currents are almost unchanged in the same scan rate (Figures 4b and S33, S34). This implies that the Cu(I)/Cu(0) redox transition is not restricted to the catalyst’s surface (42) and is limited by the charge transfer in the bulk material. (43,44) Unlike R1, both the R2 and R3 waves are irreversible and the scan rate dependence can be eliminated under stirring (Figures 4b and S33), manifesting that the peak current at high overpotentials is limited by mass transport of the reactant to the electrode–electrolyte interface. (42) In a solution without forced convection, the shift of the peak potential (E_p) linearly varies with the logarithm of the scan rate (log ν) based on the following equation (Supporting Information Note 3) (42,45)

| \frac{{d E}_{p}}{d l o g ν} | = \frac{2.303 R T}{2 {F n}_{r} α_{r}} = \frac{29.6 m V}{n_{r} α_{r}}

(1)

where n_r is the number of transferred electrons in RDS (typically 1) and α_r is the transfer coefficient of RDS. For R2, a peak shift of 47.7 mV/dec (Figure S35a) leads to an α_r equal to 0.62, which is consistent with a Tafel slope value of ∼100 mV/dec regarding the first electron activation of nitrate as the RDS. By using the Nicholson–Shain equation (Supporting Information Note 3), (42,45) the number of electrons of the R2 wave is determined to be ∼1.7 (Figure S35b), suggesting rather a two-electron reduction of NO₃^– to NO₂^–. A similar kinetic analysis for R3 leads to an estimated number of transferred electrons equal to 5.5, which suggests a six-electron reduction of NO₃^– to hydroxylamine (NH₂OH), commonly reported as a key intermediate involved in the NO₃RR route to ammonia on copper (Figure S36). (26,27,46) Although the uncertainty in the electrode area determination due to the electrode porosity is expected to induce prominent errors in the n value, the ratio of n(R3)/n(R2) is always around 3 (2.9 ± 0.2). The reaction kinetics of NO₃RR were also investigated using rotating disk electrode (RDE) and by analyzing the Koutecký–Levich plots (42) (Figures 4c and S37, Supporting Information Note 3), the number of transferred electrons at 0.05 V and −0.30 V_RHE are calculated as 2.3 and 6.1, respectively, in accordance with the scan rate dependence analysis. Unlike R1–R3, the quasi-reversible R4/O4 peak is observed only in the presence of NO₃^–/NO₂^– (Figure 1b), and the peak current of R4 shows linear dependence on the scan rate (Figure S38), corresponding to surface adsorbed species. (42) We assigned R4/O4 as desorption/adsorption of *OH on metallic Cu, (37) which is generated during NO₃^–/NO₂^– reduction.

In agreement with previous reports, (32,47) NH₂OH is not detected as a reaction product after NO₃RR on Cu₂O NCs in alkaline electrolytes, suggesting that it rapidly converts to ammonia under electrochemical reductive conditions. This is confirmed by the nearly quantitative ammonia formation [FE(NH₃) > 90%] upon electrolysis carried out in the presence of 8 mM NH₂OH (pH 12) at potentials from 0.1 to −0.3 V_RHE (Figure S39a). These results validate that NH₂OH is a key intermediate species involved in the pathway for the multielectron conversion of NO₃^–/NO₂^– to ammonia. Interestingly, the presence of NH₂OH in solution induces a strong loss of reversibility of R1 with an increased current density compared to the analogous voltammetric profiles in the presence of NO₃^–, which indicates a facile catalytic reduction of NH₂OH on metallic Cu(0) (Figures S39b,c). However, the very large Tafel slope value (435 mV/dec, Figure S39d) suggests a chemical reduction of NH₂OH on Cu(0) without involving interfacial charge transfer. According to the observed *OH desorption/adsorption R4/O4 peaks (Figures 1b, 4b, and S38), we propose that the RDS of NH₂OH reduction is the dissociation of NH₂OH to adsorbed *NH₂ and *OH, consistent with the observed apparent 6-electron reduction of NO₃^–, despite the final product being NH₃ (Figures S36 and S37b).

The pH-dependence of NO₂RR (from pH 11.5 to 12.9) was found to be similar to that of NO₃RR (Figures 4d, and S40–S48). However, upon increasing the pH value, the Tafel slope gradually decreases from ∼100 to ∼60 mV/dec (Figures 4d and S41), suggesting a change of the mechanism from PCET activation of nitrite to a single-electron transfer pre-equilibrium step (PES) preceding a pure chemical RDS. (48) This change is probably due to more available adsorbed hydrogen (*H) generated from the Volmer step in alkaline conditions (assumed as PES), with the chemical hydrogenation of nitrite becoming rate-determining (Supporting Information Note 2 and 4). (49) Similar as NO₃RR, one proton is involved in RDS or PES in total (Figures S31 and S42). Furthermore, both, the scan rate dependence and the RDE studies suggest the involvement of four electrons during NO₂RR, in agreement with the generation of NH₂OH as an intermediate species in the route to ammonia (Figures S46–S48, Supporting Information Note 4).

Reaction Intermediates

To verify the proposed formation of reaction intermediates during NO₃RR and NO₂RR, we performed in situ Raman spectroscopy of the Cu₂O NCs (Figures 5 and S49–S51). The primary goal was to prove the formation of the NH₂OH intermediate and therefore, we additionally performed control in situ Raman spectroscopy experiments using NH₂OH as reactant in the absence of nitrate/nitrite (Figure S52) and also under HER conditions (Figure S53). For all cases (Figures S49b, S50b, S52b, and S53), only Cu₂O characteristic peaks (415, 530, and 625 cm^–1) (34) and peaks related to the G-band of the carbon substrate (1580 cm^–1) (34) can be observed under OCP and at potentials more positive than 0.3 V_RHE. Cu₂O characteristic peaks started to disappear at a cathodic potential of 0.3 V_RHE, which agrees well with the Cu(I)/Cu(0) redox peaks in the CVs (Figure 1b). At more negative potentials than 0.1 V_RHE, the oxide-related spectroscopic region (300–800 cm^–1) significantly changed, showing broad features that suggest the presence of adsorbed hydroxyl ions on metallic Cu and remnant disordered Cu₂O. (50,51) The oxide features can still be observed when the potential is decreased to −0.3 V_RHE, which agrees well with our findings based on operando XAS (Figure 3b). Note that upon the reduction of the Cu₂O nanocubes, the intensity of the Raman signals is significantly enhanced due to surface plasmonic effects of nanostructured metallic Cu. (34)

Figure 5. In situ Raman spectroscopy data of Cu₂O NCs. (a) Enlarged in situ Raman spectra during NO₃RR in the range of 950–1650 cm^–1 Raman shift, with the applied potentials ranging from the 0.1 to −0.3 V_RHE. Full spectra are shown in Figure S49a, Supporting Information. Electrolyte: pH 12, 0.1 M Na₂SO₄ + 8 mM NaNO₃. The NO₃^–, NO₂^–, NH₂OH and NH₃ related peaks are labeled with red, violet, orange, and blue dotted lines, respectively. The asterisk denoted the atoms adsorbed on the catalyst surface. Peaks at 983 and 1065 cm^–1 correspond to SO₄^2– and CO₃^2– in solution (labeled with a black dotted line), (34,36) respectively, as they always appeared in the electrolyte containing Na₂SO₄ (pH 12). (b) In situ Raman spectra in the range of 1100–1250 cm^–1 during NO₃RR (left, pH 12, 0.1 M Na₂SO₄ + 8 mM NaNO₃), NO₂RR (middle, pH 12, 0.1 M Na₂SO₄ + 8 mM NaNO₂), NH₂OH reduction (right, pH 12, 0.1 M Na₂SO₄ + 8 mM NH₂OH). The NH₂OH related peaks are labeled with orange dashed lines. The intensity of NO₃RR Raman spectra is manually increased five times for clarity.

To determine the Raman bands induced by the NH₂OH intermediate and its reduction to NH₃, we performed in situ Raman spectroscopy in a NH₂OH-containing electrolyte (Figures 5b and S52) and identified the peaks at 870, 1190, and 1526 cm^–1 corresponding to NH₂ rocking, symmetric wagging vibration of NH₂ and antisymmetric N–H bending in NH₂OH (26,52) respectively, in the potential regime of 0.3 to −0.1 V_RHE. Notably, we also identified several peaks related to NH₃, including NH₃ rocking (785 cm^–1), (36) symmetric bending vibrations of the HNH (1320 and 1371 cm^–1), (53) ν(T₄) antisymmetric NH₂ deformation of NH₄⁺ (1455 cm^–1), (26,54) and the antisymmetric bending vibration of the HNH of NH₃ (1550 and 1590 cm^–1). (36,53) A further decrease in the potential results in attenuated peak intensity, probably due to much faster conversion rate of NH₂OH and production rate of NH₃. The in situ Raman results are consistent with our electrochemical kinetic data, indicating the reduction of NH₂OH to NH₃ on metallic Cu.

In situ Raman spectra acquired during NO₃RR between 950 and 1650 cm^–1 served to monitor the most important adsorbates under electrocatalytically relevant potentials between 0.1 and −0.3 V_RHE (Figure 5a). Adsorption of nitrate is evidenced between 0.1 and −0.3 V_RHE from Raman peaks at 1005 cm^–1, corresponding to the NO stretching vibration from the unidentate nitrate (*ONO₂^–). (53) The peak at 1098 cm^–1 that resembles those observed for copper-nitrate complexes also implies interaction between copper and nitrate. (55) Broad peaks at 1400 cm^–1 are assigned to antisymmetric vibrations of the NO₂ group in nitrate. (36)

Furthermore, we have identified adsorbed NO₂^– species between 0.1 and −0.3 V_RHE following the Raman bands at 1128 and 1266 cm^–1. (36,53) These bands are typically assigned to the symmetric and antisymmetric stretching vibration of the adsorbed NO₂ in a nitro configuration (*NO₂^–). Additionally, the Raman band at 1298 cm^–1 is typically assigned to the chelating nitrito configuration (*ONO*^–), while the peak at 1440 cm^–1 is related to bridging nitro configuration on the Cu surface (*N*OO^–). (36,53) Concurrently with the NO₂^–-related bands, broad features between 1500 and 1630 cm^–1 appeared cathodically at 0.1 V_RHE. These broad peaks might consist of several peaks related to intermediates or products of NO₂^– reduction, such as ν(N═O) of HNO* (1530 cm^–1), (36) antisymmetric bending vibration of the HNH of NH₃ (1550, 1590 cm^–1), (36,53) and O–H bending of water (1610 cm^–1). (56) The broad peaks from 2000 to 2100 cm^–1 indicate that *NO species might be involved in NO₃RR (Figure S49c). (57)

Most importantly, we identified weak and broad Raman bands assigned to the N–H bending (1526 cm^–1) and the NH₂ symmetric wagging vibration of NH₂OH (1190 cm^–1) (26,54) during NH₃ formation below 0.0 V_RHE, proving the presence of NH₂OH intermediate during NO₃RR to NH₃ (Figure 5a,b). Qualitatively, we found that the relative spectral weight of the nitrite-related bands decreases below −0.1 V_RHE, while the intensity of the NH₂OH- and NH₃-related bands increases below 0.0 V_RHE, consistent with NO₂^– as the main product in the low overpotential region, while NH₃ is observed as the main product at high overpotential.

In terms of NO₂RR (Figures S50–S51), nitrite-related peaks at 1128 cm^–1 were observed. (53) Small broad peaks from 1280 to 1410 cm^–1 might consist of several peaks related to adsorbed *ONO*^– (1298 cm^–1, chelating nitrito) (36) and ammonia (1320 and 1371 cm^–1, symmetric bending vibrations of the HNH). (36,53) The overlapping peaks at 1440 and 1458 cm^–1 are related to bridging nitro (*N*OO^–) and NH₄⁺, (26,36,53,54) respectively, according to Raman spectroscopy of NO₃RR and NH₂OH reduction. ν(N═O) of HNO* (1530 cm^–1) (36) and antisymmetric bending vibration of the HNH of NH₃ (1550, 1590 cm^–1) (36,53) appeared as well. These findings suggest that NO₂RR and NO₃RR share common intermediates and reaction steps. Notably, the peaks related to NH₂OH at 1190 cm^–1 emerge at more positive potential and they are more obvious under NO₂RR condition (Figure 5b), comparing to those of NO₃RR, manifesting more facile reaction kinetics for NO₂RR.

Active Species and Reaction Mechanisms

Based on the aforementioned experimental results, the evolution of the catalyst and the active species can be schematically depicted in Figure 6a. The overall eight-electron conversion of NO₃^– to NH₃ takes place via two main potential-dependent steps. The initial two-electron activation and reduction of NO₃^– to NO₂^– occurs in a potential region (ca. 0.1 V_RHE) where the original cuprous(I) oxide layer in the as-prepared Cu₂O NCs is partially and slowly reduced to metallic Cu(0), while remaining predominantly Cu(I) in both the near-surface and in the bulk of the sample. Hence, the catalyst surface features a mixed Cu/Cu₂O phase composition at mild applied voltages, where both, Cu(I) and Cu(0) species can act as active sites for NO₃^– adsorption and activation toward NO₂^– formation.

Figure 6. Active species and possible reaction mechanisms. (a) Schematics of the structural and oxidation state change of the catalyst under NO₃RR/NO₂RR conditions. (b) Proposed mechanisms for NO₃RR/NO₂RR to ammonia over Cu₂O NCs. The species that been determined based on experimental evidence (either detected by in situ Raman or deduced from electrokinetic analysis) are in bold. The left side (yellow background) correspond to reaction conditions where Cu(I)/Cu(0) species are present, while on the right-hand side (red background) the catalyst is mostly metallic.

At larger overpotentials (ca. −0.3 V_RHE), NH₃ is produced with high efficiency and selectivity by further NO₂^– reduction (Figure 6a). In this applied potential range, the catalyst is mainly metallic, whose formation rate from the initial Cu₂O precatalyst strongly correlates with the FE(NH₃) formation (Figure 3b). While we cannot rule out that the small amount of residual Cu(I) species may play some roles in the process, these data indicate that metallic Cu(0) represents the main catalytic sites for the NO₃^–/NO₂^– conversion to NH₃. We hypothesize that in addition to chemical reasons, the enhanced electrical conductivity of metallic copper is beneficial for the multielectron reduction of NO₃^–/NO₂^–, thus favoring the selectivity toward NH₃. In this regard, the observation that increasing overpotentials and electrolysis times lead to an improved NH₃-to-NO₂^– selectivity is consistent with a sluggish reduction of the initial semiconducting cuprous(I) oxide centers to the active Cu(0). Notably, a rapid increase of the Cu–Cu coordination number observed during operando XAS (Figure 3d) is indicative of severe surface restructuring and particle aggregation occurring during NO₃RR to NH₃. An analogous behavior has been recently reported for single-atom Cu precatalysts undergoing a dynamic reconstruction driven by the applied potential during NO₃RR to form the real Cu(0) NP catalyst. (58)

We note that previous studies led to contrasting conclusions about the oxidation states and active phases of copper-based catalysts for NO₃RR. (21,31−33) Such discrepancies may not only originate from the different nature of the catalysts (or precatalysts), but are also related to the interpretation of the data based in many prior examples on an ex situ postreaction characterization of the catalysts, usually influenced by the rapid oxidation of surface Cu(0) back to Cu(I) upon even short time exposures to ambient conditions. A single characterization technique, even under working conditions, also does not provide a complete view of the chemical and structural evolution of complex electrocatalysts. Thus, our data and conclusions highlight the importance of combining various in situ/operando techniques to reveal the evolution of electrocatalysts and formation the catalytically active species upon exposure to different chemical environments, different applied potentials and during various reaction times. Although some reports previously showed the existence of a mixed Cu/Cu₂O under NO₃RR conditions, (31) we unambiguously show here that the emergence of metallic Cu correlates with NH₃ selectivity and activity, while Cu(I) species can only catalyze the two-electron reduction of nitrate.

Based on the results of the electrokinetic analysis and spectroscopic data, we propose that the reduction of NO₃^– to NH₃ proceeds via the main pathways depicted in Figure 6b, and Supporting Information Note 4. The RDS of the NO₃RR is first PCET activation of adsorbed NO₃^–, while the second PCET further generates *NO₂^–. These two steps can be promoted by either Cu(I) or Cu(0) species while further reduction of *NO₂^– requires Cu(0). A recent work (59) demonstrated that the conversion of NO₃^– to NO₂^– can be easily realized by the spontaneous redox reaction between metallic Co and NO₃^–. To check whether the generated metallic Cu can be oxidized by nitrate in our case, we performed similar in situ Raman experiments (Figure S54). We first applied −0.3 V_RHE for 15 min in 0.1 M Na₂SO₄ electrolyte at pH 12 to reduce the Cu₂O to metallic Cu. After this prereduction, we quickly added 0.5 M NaNO₃ to the 0.1 M Na₂SO₄ electrolyte, resulting in 8 mM NaNO₃ in the electrolyte and monitored the evolution of the Raman bands under OCP. We observed the characteristic Raman peaks related to Cu₂O quickly emerge and stabilize after 1 min in both, the nitrate-containing (Figure S54a) and nitrate-free electrolyte (Figure S54b). We also did not detect any NO₂^– by UV–vis spectrometry for 4 min after adding NO₃^–. Therefore, we conclude that the spontaneous chemical reaction between Cu and nitrate does not happen in our catalyst.

The RDS of NO₂RR can either be a PCET reduction of *NO₂^– or the hydrogenation of *NO₂^– via *H, which is probably generated from water dissociation PES, depending on the pH values. The exposure of metallic Cu(0) sites on the surface may favor the *H adsorption, thus enabling the hydrogenation step. (49,60) According to the number of transferred protons and electrons estimated by the electrokinetic analysis, *NO₂OH^– and *NOOH^– can be proposed as probable intermediate adsorbed species formed after activation of nitrate and nitrite, respectively. These intermediates were also suggested by several computational studies. (61,62)

Finally, in agreement with the determination of six electrons for NO₃RR and four electrons for NO₂RR by scan-rate dependence and RDE studies, we propose the formation of NH₂OH as a possible intermediate involved in the route to NH₃ in alkaline electrolyte, (22,26,31,63) which is also confirmed by in situ Raman spectroscopy (Figure 5). The reduction of NH₂OH to NH₃ likely occurs on metallic Cu via a chemical step which involves the dissociation of *NH₂OH to form the adsorbed *NH₂ and *OH. The *NH₂ intermediate can further react with water to produce ammonia and generate another *OH, while the *OH is rapidly reduced to release OH^–. The reduction of NO₂^– to NH₂OH possibly involves *NO, *HNO as the intermediates.

It should be however noted that the mechanistic insights obtained here are only applicable to the NO₃RR in an unbuffered alkaline electrolyte, and that alternative mechanism might be possible when working under acidic media (Figure S55, and Supporting Information Note 4).

Conclusions

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In summary, we have presented a detailed picture for a fundamental understanding of the ammonia electrosynthesis reaction from NO_x^– sources on copper, taking well-defined Cu₂O nanocubes as model precatalysts. Combining electrochemistry and in situ/operando spectroscopy, we found a direct correlation between the potential- and time-dependent changes of the chemical state of the catalyst during NO₃RR/NO₂RR and the catalytic selectivity, providing insights into the main active species involved in the process. The sluggish reduction of Cu(I) to metallic Cu at low overpotentials results in low NH₃ production rates and efficiencies, suggesting that the Cu(I) species can only catalyze the two-electron NO₃^–-to-NO₂^– reduction. On the other hand, the increased formation rate of Cu(0) at high overpotentials strongly correlates with improved NH₃ Faradaic efficiency and/or yield rate, thus reflecting the crucial role of metallic Cu toward stepwise multielectron reduction of NO₃^– and NO₂^– to NH₃. We proposed an electrokinetic model consistent with the experimental data, highlighting the main pathways possibly leading to NO₂^– and NH₃, respectively. Our data suggest that the RDS of NO₃RR involves a PCET activation of nitrate, while the rate of NO₂RR is limited by either a PCET activation or a direct hydrogenation of nitrite, depending on the electrolyte’s pH. We also proposed that the final step leading to NH₃ is limited by a chemical reaction involving NH₂OH.

This study highlights the importance of using in situ/operando spectroscopic tools to unravel the chemical evolution process and active species of the catalyst at different potentials and reaction times. The mechanistic understanding provided here for the ammonia electrocatalytic synthesis offer several clues for future advanced catalyst design. In particular, we have demonstrated that it is crucial to stabilize copper in the metallic state under lower overpotential as well as to facilitate the generation and stabilization of adsorbed *H.

Methods and Experimental Details

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Synthesis of Cu₂O NCs

The Cu₂O NCs were synthesized by using a slight modification of a previously reported surfactant-free procedure. (34) Briefly, 1 mL of a 0.1 M CuSO₄ solution (prepared from CuSO₄·4H₂O, Sigma-Aldrich, 99.995% trace metals basis) was added to 91.5 mL of Milli-Q water (18.2 MΩ) and the mixture was stirred for 10 min. Then, 1 M sodium hydroxide (3.5 mL, Alfa Aesar, >97%) was poured to the mixture under stirring, followed after 10 s by the addition of a freshly prepared 0.25 M l-ascorbic acid solution (4 mL, Sigma-Aldrich, reagent grade). The mixture was further stirred at room temperature for 15 min. The particles were retrieved by centrifugation and washed with an ethanol/water mixture (1:1) three times and ethanol one time. The solid sample was finally dispersed in 6 mL ethanol and stored in the fridge. Inductively coupled plasma mass spectrometry (ICP-MS, iCAP RQ, Thermo Fisher Scientific) data indicated that the mass concentration of catalyst suspension is 1.2 ± 0.1 mg/mL.

X-Ray Diffraction

X-ray diffraction (XRD) was performed on a Bruker D8 Advance diffractometer equipped with a Cu source, a primary Goebel mirror and a secondary equatorial Soller slit as well as an energy-dispersive detector operated in 0D mode. To investigate the electrode samples of this study, grazing incident mode was applied with an incident angle of 1° and a measurement range of 10–85° with a step size of 0.04°. All the manipulations were conducted with minimal but inevitable air exposure, which may cause partial reoxidation of the surface but not of the bulk. The sample was placed on a special sampler holder having an airtight dome to minimize air contact during the measurement. Furthermore, a low-background silicon plate was used as sample support in the holder. Immediately after the experiments, the catalyst containing carbon paper electrode (geometric active area of 0.5 × 0.5 cm) was removed from the reaction vessel and rinsed thoroughly with Milli-Q water. After carefully drying with an Ar flow, the electrode was fixed to the sample holder with the help of some stripes of Kapton tape. To minimize the contact to air, the holder was transferred to a N₂-filled glovebox as fast as possible. With the closed dome the sample was measured immediately afterward. Between the end of the electrolysis experiment and the transfer of the sample into the glovebox, ca. 5–10 min passed.

Transmission Electron Microscopy

The size and morphology of the Cu₂O NCs were determined by transmission electron microscopy (TEM) using an FEI Talos F200X microscope. TEM samples were prepared by placing 10 μL of the as-synthesized Cu₂O NCs suspensions (with a five-time dilution with pure ethanol) on a nickel grid (400 mesh with lacey carbon film, PLANO GmbH) and allowing it to dry in air overnight. For investigating the catalysts after reaction, the carbon paper electrodes that are loaded with the catalyst were initially subjected to 2 h electrolysis at a certain potential. Then the electrodes are quickly washed in ultrapure water and dried under nitrogen flow. After that, the dry electrodes are put in ethanol and sonicated for 1 h. Finally, 10 μL suspensions are drop-casted on nickel grids for the TEM measurements.

Quasi In Situ X-Ray Photoelectron Spectroscopy

Quasi in situ XPS measurements were carried out using a sample to analyzer angle of 90°. The system employed was a commercial hemispherical analyzer (SPECS GmbH, Phoibos 150 equipped with a MCD-9 Detector) using a pass Energy of 15 eV and a monochromatic XRM X-ray source (SPECS GmbH) utilizing an Al anode (E = 1486.7 eV). A source power of P = 300 W was used. The electrochemical cell employed was directly attached to the ultrahigh vacuum (UHV) system, allowing for a sample transfer from electrochemical conditions to the UHV in a protective Ar atmosphere. (64)

Operando X-Ray Absorption Spectroscopy

Operando XAS measurements were performed at the Cu K-edge (8979 eV) in fluorescence mode at the SOLEIL SAMBA beamline. Supporting measurements at different applied potentials were also done at ALBA CLAESS beamline.

For experiments at SOLEIL, a Si(220) monochromator was used for energy selection. Intensity of incident X-ray radiation was measured with an ionization chamber filled with a 50/50 mixture of N₂ and He. Fluorescence from the sample was detected using an energy-selective 35-channel Ge detector. For experiments at ALBA, a Si(111) monochromator was used, and 6-channel energy-selective silicon drift detector was used to collect fluorescence data.

For operando XAS measurements, the samples were deposited on carbon paper, which acted both as a working electrode and X-ray window in our in-house built single compartment electrochemical cell. (65) Pt mesh was used as a counter electrode, and a leak-free Ag/AgCl electrode was used as a reference electrode. The applied potential was controlled by a BioLogic potentiostat. During the experiment, continuous circulation of the electrolyte was ensured by a peristaltic pump, and Ar was bubbled through the cell.

XAS data extraction, alignment, and normalization were done using the Athena software. (66) Linear combination fitting of XANES spectra was done by using an in-house developed set of Wolfram Mathematica scripts. EXAFS data fitting was done using the FEFFIT code. (66) Fitting was carried out in the R-space in the range between 1.0 and 2.8 Å (between 1.0 and 2.0 Å for the as-prepared fully oxidized samples). Fourier transform for EXAFS data fitting was carried out in the k-range between 3.0 and 10.0 Å^–1. The EXAFS signal was modeled as sum of Cu–O and Cu–Cu contributions, where the corresponding bond lengths, coordination numbers, and disorder factors were treated as fitting parameters. For the as-prepared fully oxidized samples the Cu–Cu bond contribution was not included. An additional fitting parameter was the correction to the photoelectron reference energy value (ΔE₀) value. The amplitude reduction factor (S₀² factor) was obtained by fitting the EXAFS spectra for Cu₂O and Cu foil references (the obtained values are 0.7 and 0.92, respectively). The R-factors, characterizing the EXAFS fit quality, were lower than 0.5% for the all nearly phase-pure spectra (i.e., spectra, corresponding to the fully oxidized as-prepared samples, and to nearly fully reduced samples after prolonged exposure to NO₃RR). For samples in the intermediate state the R-factors were higher, but still below 3%. The increased R-factor in this case is due to the presence of a remaining Cu₂O-like phase, where the Cu–Cu bond from the second coordination shell (not included in the fit) partially overlaps with the contribution of the first shell Cu–Cu bond in metallic Cu phase.

In Situ Raman Spectroscopy

The in situ Raman spectra were obtained by using a Renishaw (InVia Reflex) Raman instrument with confocal Raman microscope. To perform the in situ experiments in an electrolyte, a water immersion objective with a long working distance (Leica microsystems, 63×, numerical aperture of 0.9) was chosen. During the experiments, the objective was protected from the electrolyte by a Teflon film (DuPont, 0.013 mm thickness). Fifteen μL of water was used between the film and the objective to match the refractive index, which ensures efficient collection of the Raman signal. The objective with long working distance is necessary to avoid diffusion hindrance problems during the Raman measurements. A 633 nm laser was employed and the laser power was 5%. The extended mode was chosen and the acquisition time was 10 s. A Si wafer (520.6 cm^–1) was used to calibrate the Raman frequency before the experiment. The electrochemical measurements were performed in a home-built spectroelectrochemical cell made of Teflon and controlled by a Biologic SP-240 potentiostat. The cell was equipped with a reference electrode (leak-free Ag/AgCl, Alvatek), a counter electrode (Pt ring), and a working electrode with the catalyst drop-casted on carbon paper. Typically, a 12.5 mL electrolyte was filled in the cell, with continuous Ar bubbling during the experiments, to remove oxygen and alleviate mass transport issues. For each applied potential, the spectra were collected after 30 s and the final data were obtained using the averaged results of 5–8 tests to improve the quality.

Electrochemical Experiments

All the electrochemical experiments were carried out by using an Autolab potentiostat (PGSTAT 302N) and a custom-made H-type electrochemical cell, whereby the cathodic and anodic compartments were separated by an anion exchange membrane (Selemion AMV, AGC Inc.). A platinum gauze (MaTecK, 3600 mesh cm^–2) and a leak-free Ag/AgCl electrode (LF-1, Alvatek) were used as the counter and the reference electrodes, respectively. In a typical experiment, each compartment was filled with 18 mL 0.1 M Na₂SO₄ electrolyte (anhydrous, 99.99%, Suprapur). For nitrate electroreduction tests, the cathodic electrolyte solution contained 8 mM NaNO₃ (112 ppm of NO₃–N, Sigma-Aldrich, ReagentPlus, ≥99.0%). For ¹⁵N labeling nitrate electroreduction experiments, equimolar concentrations of Na¹⁴NO₃ and Na¹⁵NO₃ (Sigma-Aldrich, ≥98 atom % ¹⁵N, ≥99%) were employed, respectively. Before each experiment, both, the anodic and cathodic solutions were deaerated by continuously bubbling Ar (grade 6.0, 99.9999%) with a 20 mL min^–1 flow rate (Bronkhorst). During the chronoamperometric measurements, a constant Ar flow (20 mL min^–1) was kept to maintain the inert atmosphere. The potential values were reported vs Ag/AgCl electrode (V_Ag/AgCl) or vs the reversible hydrogen electrode (V_RHE) for experiments carried out in unbuffered near-neutral or alkaline electrolytes, respectively. For all the electrolytes, the cyclic voltammograms (CV) were recorded at a scan rate of 50 mV s^–1 and five consecutive cycles between −0.20 V and −1.40 V_Ag/AgCl were performed to achieve steady-state polarization curves and to achieve a stable and reproducible state of the electrocatalyst prior to the catalytic testing. For the electrolysis experiments, a linear sweep voltammogram (5 mV s^–1) was performed from −0.30 V_Ag/AgCl to the cathodic potential followed by chronoamperometry at the latter potential for 2 h. The LSV and electrolysis experiments are applied with 800 rpm stirring using a stirring bar.

For NO₂RR experiments, all the steps are similar as those of NO₃RR, except 8 mM NaNO₂ (99%, for analysis, Sigma-Aldrich) is the reactant instead of NaNO₃. For ¹⁵N isotope labeling experiments, 8 mM Na¹⁵NO₂ (98 atom % ¹⁵N, 95% chemical pure, Sigma-Aldrich) is used.

Tafel plots were recorded using staircase voltammetry. The electrode was preactivated in pH 12, 0.1 M Na₂SO₄ + 8 mM NaNO₃ for 1 h, at −1.20 V_Ag/AgCl (−0.30 V_RHE). Then the activated electrode was measured in a fresh electrolyte with pH values ranging from 11.5 to 12.9, containing 8 mM NaNO₃/NaNO₂/NH₂OH. The electrolyte pH was tuned by adding 4 M NaOH, and the values were monitored by a VWR pH1100L pH meter. The potential was ranging from −0.60 to −1.40 V_Ag/AgCl, with a ramping step of 50 mV. For each data point, the potential was held for 60 s, and then the current was recorded. The electrolyte is stirred using a stirring bar at 800 rpm.

For acquisition of scan rate dependence current–potential data of both NO₃RR and NO₂RR (pH 12), the electrode was preactivated in pH 12, 0.1 M Na₂SO₄ + 8 mM NaNO₃ for 1 h, at −1.20 V_Ag/AgCl (−0.30 V_RHE). The activated electrode was then put in a fresh pH 12, 0.1 M Na₂SO₄ + 8 mM NaNO₃/NaNO₂ electrolyte. The CV curves were recorded between −0.20 and −1.40 V_Ag/AgCl, starting from −0.20 V_Ag/AgCl. The scan rate was sequentially set as 5, 10, 20, 30, 50, 75, and 100 mV/s. For the CV curves recorded without stirring, after each measurement, a 1000 rpm stirring was applied for 1 min to remove the concentration gradient of the reactants/products near the electrode caused by the reaction. In this way, the initial state of the solution can be restored. After collecting the CV curves recorded without stirring, the electrode was put in a fresh electrolyte, following the same procedure to acquire the CV curves recorded with stirring (800 rpm).

For RDE experiments, the electrode was also preactivated in pH 12, 0.1 M Na₂SO₄ + 8 mM NaNO₃ for 1 h, at −1.20 V_Ag/AgCl (−0.30 V_RHE), with a rotating speed of 900 rpm. The activated electrode was then put in a fresh pH 12, 0.1 M Na₂SO₄ + 8 mM NaNO₃/NaNO₂ electrolyte. The LSV curves were recorded between −0.40 to −1.40 V_Ag/AgCl, starting from −0.40 V_Ag/AgCl. The rotation rate was sequentially set as 50, 100, 200, 300, 400, 600, 900, 1200, and 1600 rpm. After each measurement, a 900 rpm rotation was applied for 1 min to restore the initial state of solution.

Note that for all the measurements of the electrokinetic studies, the values of the solution resistance were measured by the current-interruption method, and then the CV curves, LSV curves, or steady state J–V curves were manually iR corrected.

For the electrode preparation, 0.98 mL catalyst suspension in ethanol (presonicated for 1 h) and 20 μL of Nafion solution (Sigma-Aldrich, ∼5 wt % in a mixture of alcohols and water) (0.1% v/v Nafion) were mixed together. After sonicating the suspension for 20 min, 50 μL of the catalyst mixture were drop-casted on both sides of a 0.5 × 0.5 cm Toray carbon paper (Alfa Aesar, TGP-H-60) to provide a geometric catalyst mass loading of 0.1 mg cm^–2 (A = 0.5 cm²). The electrode was dried under ambient condition for least 3 h before the electrochemical measurements. For the RDE experiments, 0.49 mL catalyst suspension in ethanol (presonicated for 1 h) was mixed together with 0.49 mL ultrapure water and 20 μL of Nafion solution (Sigma-Aldrich, ∼5 wt % in a mixture of alcohols and water) (0.1% v/v Nafion). After sonicating the suspension for 20 min, 20 μL of the catalyst mixture were drop-casted on a 0.196 cm² RDE. The electrode was dried under ambient condition for at least 1 h before electrochemical measurements.

Spectrophotometric Product Analysis and Detection

UV–vis spectroscopy (Agilent Cary 60) was used to detect and quantify ammonia and nitrite as possible products of NO₃RR and NO₂RR, following the experimental procedures recently reported by previous literature. (49,67) The original samples were properly diluted in order to match the suitable detection range for spectrophotometric analysis of each analyte and the sample absorbance was measured in the range of 400–800 nm.

Determination of Ammonia

The indophenol-blue method was used for the spectrophotometric determination of ammonia. Briefly, 500 μL of phenol nitroprusside solution (P6994, Sigma-Aldrich) and 500 μL of alkaline hypochlorite solution (A1727, Sigma-Aldrich) were added to 2 mL of a diluted electrolytic sample. The solution was incubated for 30 min at room temperature in the dark before recording the UV–vis spectrum. Standard solutions with known NH₄Cl (99.998%, Sigma-Aldrich) concentration were used to construct a calibration curve. For electrocatalytic NO₃RR experiments, the Faradaic efficiency [FE(NH₃)] and the yield rate (r_NH₃, based on mmol h^–1 cm^–2 or μg_product h^–1 mg_catalyst^–1) for ammonia formation were calculated according to eq 2, eqs 3, and 4), (68−70) respectively

F E_{N H_{3}} = \frac{(8 F \times C_{N H_{3}} \times V)}{(M M_{N H_{3}} \times Q)} \times 100 %

(2)

r_{N H_{3}} = \frac{(C_{N H_{3}} \times V)}{(M M_{N H_{3}} \times A \times t)}

(3)

r_{N H_{3}} = \frac{(C_{N H_{3}} \times V)}{(m_{C a t} \times t)}

(4)

where C_NH₃ is the mass concentration of NH₃(aq) (mg/L), V is the volume of electrolyte in the cathode compartment (18 mL), F is the Faradaic constant (96,485 C mol^–1), MM_NH₃ is the molar mass of NH₃, Q is the charge passed during electrolysis, m_Cat is the total mass on the electrode (for 0.5 cm² electrode is 0.05 mg), A is the geometric surface of the electrode (0.5 cm²), and t is the electrolysis time.

Determination of Nitrite

For nitrite (NO₂^–) quantification, 3 mL of the diluted sample were added to a glass vial containing 35 mg of white powder from a commercial nitrite test kit (photometric 0.002–1.00 mg L^–1 NO₂–N, 0.007–3.28 mg L^–1 NO₂^–, Spectroquant, Merck). The mixture was then incubated for 10 min at room temperature in the dark before UV–vis analysis. Analogously to ammonia, a calibration curve was constructed by using standard NaNO₂ (99%, for analysis, Sigma-Aldrich) concentrations. For electrocatalytic NO₃RR experiments, the Faradaic efficiency [FE(NO₂^–)] and the yield rate (r_NO₂^–, based on mmol h^–1 cm^–2 or μg_product h^–1 mg_catalyst^–1) for nitrite formation were calculated according to (eq 5) and (eqs 6 and 7), respectively

F E_{N O_{2}^{-}} = \frac{(2 F \times c_{N O_{2}^{-}} \times V)}{(M M_{N O_{2}^{-}} \times Q)} \times 100 %

(5)

r_{N O_{2}^{-}} = \frac{(c_{N O_{2}^{-}} \times V)}{(M M_{N O_{2}^{-}} \times A \times t)}

(6)

r_{N O_{2}^{-}} = \frac{(c_{N O_{2}^{-}} \times V)}{(m_{C a t} \times t)}

(7)

where c_NO₂^–is the mass concentration of NO₂^–(aq) (mg/L), MM_NO₂^– is the molar mass of NO₂^–, the other parameters are similar like those for calculating ammonia FE and yield rate.

Nuclear Magnetic Resonance Spectroscopy

¹H NMR spectra were provided by using a 600-MHz instrument (JEOL) available at the Freie Universität of Berlin. In a typical measurement for ammonia quantification, 10 mL of the used electrolyte solution were added to a clean glass vial containing 5.0 mg of maleic acid used as an internal standard (ReagentPlus, ≥99% (HPLC), Sigma-Aldrich), followed by the addition of 2.5 mL H₂SO₄ 4 M to acidify the mixture. After ensuring complete miscibility, the nuclear magnetic resonance tube was filled by mixing 500 μL of the mixture and 50 μL of deuterium oxide (D₂O, 99.9 atom %D, Sigma-Aldrich). The concentration of ¹⁴NH₄⁺/¹⁵NH₄⁺ in the samples was derived from the integral ratio of the triplet/doublet ammonium signal and the singlet of the maleic acid standard. Standard solutions with known ¹⁴NH₄Cl (99.998%, Sigma-Aldrich) and ¹⁵NH₄Cl (Sigma-Aldrich, ≥98 atom % ¹⁵N, ≥99%) concentration were used to construct calibration curves for ¹⁴NH₄⁺ and ¹⁵NH₄⁺ determination, respectively.

Supporting Information

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Additional TEM images, quasi in situ XPS, in situ Raman, operando XAS results, and electrochemical data are supplied as Supporting Information (PDF)

ja3c13288_si_001.pdf (4.94 MB)

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Author Information

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Corresponding Author
- Roldan Cuenya Beatriz - Department of Interface Science, Fritz-Haber-Institute of Max-Planck-Society, Faradayweg 4-6, 14195 Berlin, Germany; https://orcid.org/0000-0002-8025-307X; Email: [email protected]
Authors
- Lichen Bai - Department of Interface Science, Fritz-Haber-Institute of Max-Planck-Society, Faradayweg 4-6, 14195 Berlin, Germany; https://orcid.org/0000-0003-1452-6129
- Federico Franco - Department of Interface Science, Fritz-Haber-Institute of Max-Planck-Society, Faradayweg 4-6, 14195 Berlin, Germany; Present Address: Department of Chemical and Pharmaceutical Sciences, University of Trieste, 34127 Trieste, Italy; https://orcid.org/0000-0002-1166-1256
- Janis Timoshenko - Department of Interface Science, Fritz-Haber-Institute of Max-Planck-Society, Faradayweg 4-6, 14195 Berlin, Germany
- Clara Rettenmaier - Department of Interface Science, Fritz-Haber-Institute of Max-Planck-Society, Faradayweg 4-6, 14195 Berlin, Germany
- Fabian Scholten - Department of Interface Science, Fritz-Haber-Institute of Max-Planck-Society, Faradayweg 4-6, 14195 Berlin, Germany
- Hyo Sang Jeon - Department of Interface Science, Fritz-Haber-Institute of Max-Planck-Society, Faradayweg 4-6, 14195 Berlin, Germany; Present Address: Technological Convergence Center, Korea Institute of Science and Technology (KIST), 02792 Seoul, Republic of Korea
- Aram Yoon - Department of Interface Science, Fritz-Haber-Institute of Max-Planck-Society, Faradayweg 4-6, 14195 Berlin, Germany
- Martina Rüscher - Department of Interface Science, Fritz-Haber-Institute of Max-Planck-Society, Faradayweg 4-6, 14195 Berlin, Germany
- Antonia Herzog - Department of Interface Science, Fritz-Haber-Institute of Max-Planck-Society, Faradayweg 4-6, 14195 Berlin, Germany; https://orcid.org/0000-0002-4402-3007
- Felix T. Haase - Department of Interface Science, Fritz-Haber-Institute of Max-Planck-Society, Faradayweg 4-6, 14195 Berlin, Germany; https://orcid.org/0000-0003-1646-4312
- Stefanie Kühl - Department of Interface Science, Fritz-Haber-Institute of Max-Planck-Society, Faradayweg 4-6, 14195 Berlin, Germany
- See Wee Chee - Department of Interface Science, Fritz-Haber-Institute of Max-Planck-Society, Faradayweg 4-6, 14195 Berlin, Germany
- Arno Bergmann - Department of Interface Science, Fritz-Haber-Institute of Max-Planck-Society, Faradayweg 4-6, 14195 Berlin, Germany; https://orcid.org/0000-0001-5071-6806
Author Contributions
L. Bai and F. Franco contributed equally to this work.
Funding
Open access funded by Max Planck Society.
Notes
The authors declare no competing financial interest.

Acknowledgments

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The authors thank Walter Wachsmann (FHI-MPG) and Dr. Andreas Schäfer (Freie Universität Berlin) for the ICP-MS and ¹H NMR measurements, respectively. We acknowledge the funding from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)-project no. 406944504-SPP 2080 and German Excellence Strategy-EXC 2008-390540038-UniSysCat. Additional funding was obtained from the Bundesministerium für Bildung und Forschung (BMBF) collaborative Project (Catlab, 03EW0015A), and the European Research Council (ERC) under grant ERC-OPERANDOCAT (ERC-725915). L.B. acknowledges the support from the Early Postdoc Mobility Fellowship (P2ELP2_199800) of the Swiss National Science Foundation. A.Y. thanks the Alexander von Humboldt Foundation (AvH) for supporting her with an AvH postdoctoral research grant. C.R., A.H. and F.T.H. acknowledge support by the IMPRS (International Max Planck Research Schools) Elementary Processes in Physical Chemistry. We acknowledge Dr. Andrea Zitolo at SOLEIL synchrotron (France) and Dr. Carlo Marini at ALBA synchrotron (Spain) for the assistance in using beamlines.

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Manassaram, D. M.; Backer, L. C.; Moll, D. M. A review of nitrates in drinking water: maternal exposure and adverse reproductive and developmental outcomes. Environ. Health Perspect. 2006, 114 (3), 320– 327, DOI: 10.1289/ehp.8407
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Mensinga, T. T.; Speijers, G. J.; Meulenbelt, J. Health implications of exposure to environmental nitrogenous compounds. Toxicol. Rev. 2003, 22 (1), 41– 51, DOI: 10.2165/00139709-200322010-00005
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There is no corresponding record for this reference.
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Lim, J.; Fernández, C. A.; Lee, S. W.; Hatzell, M. C. Ammonia and Nitric Acid Demands for Fertilizer Use in 2050. ACS Energy Lett. 2021, 6 (10), 3676– 3685, DOI: 10.1021/acsenergylett.1c01614
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Ammonia and Nitric Acid Demands for Fertilizer Use in 2050
Lim, Jeonghoon; Fernandez, Carlos A.; Lee, Seung Woo; Hatzell, Marta C.
ACS Energy Letters (2021), 6 (10), 3676-3685CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
A review. Access to nitrogen-based fertilizers is crit. to maximize agricultural yield, as nitrogen is the most common rate-limiting nutrient. Nearly all nitrogen-based fertilizers rely on ammonia and nitric acid as feedstocks, and thus the demand for these chems. is heavily dependent on the global population and food demand. Over the next three decades, the global population will continue to dictate the market size and value of ammonia and nitric acid, which consequently will have a significant impact on our energy infrastructure. Here, we discuss the potential for carbon-free electrocatalytic nitrogen redn., nitrogen oxidn., and nitrate redn. to meet fertilizer manufg. demands. We also explore various growth scenarios to predict the 2050 market size and value for ammonia and nitric acid. We highlight that if the current approaches for manufg. ammonia and nitric acid remain const., carbon emissions from the prodn. of fixed fertilizer feedstocks could exceed 1300 MtCO2eq/yr, prompting a strong need for green alternatives.
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Katsounaros, I. On the assessment of electrocatalysts for nitrate reduction. Curr. Opin. Electrochem. 2021, 28, 100721, DOI: 10.1016/j.coelec.2021.100721
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On the assessment of electrocatalysts for nitrate reduction
Katsounaros, Ioannis
Current Opinion in Electrochemistry (2021), 28 (), 100721CODEN: COEUCY; ISSN:2451-9111. (Elsevier B.V.)
A review. The electrochem. redn. of nitrate is attracting attention in the context of producing ammonia, besides the traditional removal to harmless N2. To make progress in this complex reaction and facilitate the search for active and selective catalysts, we need to establish generalized testing protocols which will enable to compare and complement data from different labs. The purpose of this article is to raise awareness on the importance of (i) soln. processes that involve products of the electrode reaction, (ii) detn. of products with appropriate, product-specific quant. methods, (iii) the strong sensitivity of the reaction on exptl. parameters, (iv) the cell design for the sepn. of anode from cathode processes, and (v) the increase in the interfacial and soln. pH that occurs during the electrolysis at high current densities.
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van Langevelde, P. H.; Katsounaros, I.; Koper, M. T. M. Electrocatalytic Nitrate Reduction for Sustainable Ammonia Production. Joule 2021, 5 (2), 290– 294, DOI: 10.1016/j.joule.2020.12.025
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Zhang, X.; Wang, Y.; Liu, C.; Yu, Y.; Lu, S.; Zhang, B. Recent advances in non-noble metal electrocatalysts for nitrate reduction. Chem. Eng. J. 2021, 403, 126269, DOI: 10.1016/j.cej.2020.126269
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Recent advances in non-noble metal electrocatalysts for nitrate reduction
Zhang, Xi; Wang, Yuting; Liu, Cuibo; Yu, Yifu; Lu, Siyu; Zhang, Bin
Chemical Engineering Journal (Amsterdam, Netherlands) (2021), 403 (), 126269CODEN: CMEJAJ; ISSN:1385-8947. (Elsevier B.V.)
A review. Nitrate pollution has become a serious global problem, threatening human health and ecosystems. The electrochem. redn. has emerged as an energy-efficient and environmental-friendly technol. to remove nitrate from H2O. Recently, nonnoble metal electrocatalysts have attracted increasing attention in nitrate redn. due to their great advantages in terms of low cost, high activity, and large-scale application potential. This review highlights the latest research progress in the area of nonnoble metal materials for electrochem. nitrate redn. The mechanistic insight into the electrochem. redn. of nitrate is briefly discussed. Meanwhile, numerous examples in this field are collected and analyzed. Some strategies employed to improve the performance of nitrate electroredn. are also presented. Finally, the challenges and prospects in this field are discussed. This review hopes to guide the design and development of efficient nonnoble metal electrocatalysts for nitrate redn. on a large scale.
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Lu, X.; Song, H.; Cai, J.; Lu, S. Recent development of electrochemical nitrate reduction to ammonia: A mini review. Electrochem. Commun. 2021, 129, 107094, DOI: 10.1016/j.elecom.2021.107094
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Recent development of electrochemical nitrate reduction to ammonia: A mini review
Lu, Xingmei; Song, Haoqiang; Cai, Jinmeng; Lu, Siyu
Electrochemistry Communications (2021), 129 (), 107094CODEN: ECCMF9; ISSN:1388-2481. (Elsevier B.V.)
A review. Nitrate (NO3-) pollution has become increasingly prominent due to industry and agriculture. Electrochem. redn. can convert NO-3 into high value-added ammonia (NH3) and remove NO3- pollution. This review focuses on the latest research progress in the field of electrochem. nitrate redn. reaction (NO3-RR) to NH3 . The mechanism of NO3-RR is briefly discussed. Catalysts, as well as qual. and quant. methods for the detection of NH3 are also summarized. Finally, the challenges and prospects in this field are discussed. We hope this mini review aids researchers in the design and development of advanced research strategies for electrochem. NO3--to-NH3 processes.
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Wang, Y.; Xu, A.; Wang, Z.; Huang, L.; Li, J.; Li, F.; Wicks, J.; Luo, M.; Nam, D. H.; Tan, C. S.; Ding, Y.; Wu, J.; Lum, Y.; Dinh, C. T.; Sinton, D.; Zheng, G.; Sargent, E. H. Enhanced Nitrate-to-Ammonia Activity on Copper-Nickel Alloys via Tuning of Intermediate Adsorption. J. Am. Chem. Soc. 2020, 142 (12), 5702– 5708, DOI: 10.1021/jacs.9b13347
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Enhanced Nitrate-to-Ammonia Activity on Copper-Nickel Alloys via Tuning of Intermediate Adsorption
Wang, Yuhang; Xu, Aoni; Wang, Ziyun; Huang, Linsong; Li, Jun; Li, Fengwang; Wicks, Joshua; Luo, Mingchuan; Nam, Dae-Hyun; Tan, Chih-Shan; Ding, Yu; Wu, Jiawen; Lum, Yanwei; Dinh, Cao-Thang; Sinton, David; Zheng, Gengfeng; Sargent, Edward H.
Journal of the American Chemical Society (2020), 142 (12), 5702-5708CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Electrochem. conversion of nitrate (NO3-) into NH3 (NH3) recycles N and offers a route to the prodn. of NH3, which is more valuable than dinitrogen gas. However, today's development of NO3- electroredn. remains hindered by the lack of a mechanistic picture of how catalyst structure may be tuned to enhance catalytic activity. Here the authors demonstrate enhanced NO3- redn. reaction (NO3-RR) performance on Cu50Ni50 alloy catalysts, including a 0.12 V upshift in the half-wave potential and a 6-fold increase in activity compared to those obtained with pure Cu at 0 V vs. reversible H electrode (RHE). Ni alloying enables tuning of the Cu d-band center and modulates the adsorption energies of intermediates such as *NO3-, *NO2, and *NH2. Using d. functional theory calcns., the authors identify a NO3-RR-to-NH3 pathway and offer an adsorption energy-activity relation for the CuNi alloy system. This correlation between catalyst electronic structure and NO3-RR activity offers a design platform for further development of NO3-RR catalysts.
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Mattarozzi, L.; Cattarin, S.; Comisso, N.; Gerbasi, R.; Guerriero, P.; Musiani, M.; Vázquez-Gómez, L.; Verlato, E. Electrodeposition of Compact and Porous Cu-Zn Alloy Electrodes and Their Use in the Cathodic Reduction of Nitrate. J. Electrochem. Soc. 2015, 162 (6), D236– D241, DOI: 10.1149/2.1041506jes
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Electrodeposition of Compact and Porous Cu-Zn Alloy Electrodes and Their Use in the Cathodic Reduction of Nitrate
Mattarozzi, Luca; Cattarin, Sandro; Comisso, Nicola; Gerbasi, Rosalba; Guerriero, Paolo; Musiani, Marco; Vazquez-Gomez, Lourdes; Verlato, Enrico
Journal of the Electrochemical Society (2015), 162 (6), D236-D241CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
Compact and porous Cu-Zn alloys were electrodeposited from citrate baths. The latter deposits, consisting of a spongy material with a macroscopic interconnected porosity (pore diam. of tens of microns), were obtained at large current densities (-3 A cm-2) causing strong hydrogen evolution. Porous deposits with compns. between Cu62Zn38 and Cu91Zn9 were obtained by varying the ions concns. in the deposition baths, with optimal morphol. for the Cu70Zn30 compn. The lattice parameter of the samples, estd. from XRD data, showed in the explored range a linear dependence on compn., consistent with formation of a solid soln. of Zn in Cu. The alloy materials were tested in nitrate redn. in alkali by voltammetry, chronoamperometry and const. potential electrolysis. Their performances were compared with those of similar (compact and porous) Cu electrodes. The nitrate redn. current decayed rapidly at compact Cu electrodes and was stable at the other electrodes, being the highest at porous Cu70Zn30. In all cases, the main product was ammonia.
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Yin, H.; Chen, Z.; Xiong, S.; Chen, J.; Wang, C.; Wang, R.; Kuwahara, Y.; Luo, J.; Yamashita, H.; Peng, Y.; Li, J. Alloying effect-induced electron polarization drives nitrate electroreduction to ammonia. Chem Catal. 2021, 1 (5), 1088– 1103, DOI: 10.1016/j.checat.2021.08.014
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Alloying effect-induced electron polarization drives nitrate electroreduction to ammonia
Yin, Haibo; Chen, Zhen; Xiong, Shangchao; Chen, Jianjun; Wang, Chizhong; Wang, Rong; Kuwahara, Yasutaka; Luo, Jingshan; Yamashita, Hiromi; Peng, Yue; Li, Junhua
Chem Catalysis (2021), 1 (5), 1088-1103CODEN: CCHAE9; ISSN:2667-1093. (Elsevier Inc.)
Electrocatalytic conversion of nitrate (NO-3) to ammonia (NH3) holds significant potential in the control of nitrogen oxide (NOx) from stationary sources. However, previous studies on reaction intermediates remain unclear. Here we report that PdCu/Cu2O hybrids with mesoporous hollow sphere structure show high selectivity (96.70%) and Faradaic efficiency (94.32%) for NH3 synthesis from NO-3. Detailed characterizations demonstrate that (1) Pd enables electron transfer (Pd 3d → Cu 3d) and causes the polarization of Cu 3d orbitals by forming partial PdCu alloys, which makes Pd electron deficient but offers empty orbits to adsorb NO-3, and (2) electron-rich Cu is more conducive to the occurrence of NO-3 redn. The mutual confirmation of online differential electrochem. mass spectrometry and d. functional theory calcns. demonstrates that PdCu alloys block the generation of *NOH intermediate and facilitate the formation of *N, providing a new mechanism for NH3 synthesis from NO-3 redn. reactions.
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Chen, F. Y.; Wu, Z. Y.; Gupta, S.; Rivera, D. J.; Lambeets, S. V.; Pecaut, S.; Kim, J. Y. T.; Zhu, P.; Finfrock, Y. Z.; Meira, D. M.; King, G.; Gao, G.; Xu, W.; Cullen, D. A.; Zhou, H.; Han, Y.; Perea, D. E.; Muhich, C. L.; Wang, H. Efficient conversion of low-concentration nitrate sources into ammonia on a Ru-dispersed Cu nanowire electrocatalyst. Nat. Nanotechnol. 2022, 17 (7), 759– 767, DOI: 10.1038/s41565-022-01121-4
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Efficient conversion of low-concentration nitrate sources into ammonia on a Ru-dispersed Cu nanowire electrocatalyst
Chen, Feng-Yang; Wu, Zhen-Yu; Gupta, Srishti; Rivera, Daniel J.; Lambeets, Sten V.; Pecaut, Stephanie; Kim, Jung Yoon Timothy; Zhu, Peng; Finfrock, Y. Zou; Meira, Debora Motta; King, Graham; Gao, Guanhui; Xu, Wenqian; Cullen, David A.; Zhou, Hua; Han, Yimo; Perea, Daniel E.; Muhich, Christopher L.; Wang, Haotian
Nature Nanotechnology (2022), 17 (7), 759-767CODEN: NNAABX; ISSN:1748-3387. (Nature Portfolio)
Electrochem. converting nitrate ions, a widely distributed nitrogen source in industrial wastewater and polluted groundwater, into ammonia represents a sustainable route for both wastewater treatment and ammonia generation. However, it is currently hindered by low catalytic activities, esp. under low nitrate concns. Here we report a high-performance Ru-dispersed Cu nanowire catalyst that delivers an industrial-relevant nitrate redn. current of 1 A cm-2 while maintaining a high NH3 Faradaic efficiency of 93%. More importantly, this high nitrate-redn. catalytic activity enables over a 99% nitrate conversion into ammonia, from an industrial wastewater level of 2,000 ppm to a drinkable water level <50 ppm, while still maintaining an over 90% Faradaic efficiency. Coupling the nitrate redn. effluent stream with an air stripping process, we successfully obtained high purity solid NH4Cl and liq. NH3 soln. products, which suggests a practical approach to convert wastewater nitrate into valuable ammonia products. D. functional theory calcns. reveal that the highly dispersed Ru atoms provide active nitrate redn. sites and the surrounding Cu sites can suppress the main side reaction, the hydrogen evolution reaction.
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Liu, H.; Lang, X.; Zhu, C.; Timoshenko, J.; Ruscher, M.; Bai, L.; Guijarro, N.; Yin, H.; Peng, Y.; Li, J.; Liu, Z.; Wang, W.; Cuenya, B. R.; Luo, J. Efficient Electrochemical Nitrate Reduction to Ammonia with Copper-Supported Rhodium Cluster and Single-Atom Catalysts. Angew. Chem., Int. Ed. 2022, 61 (23), e202202556 DOI: 10.1002/anie.202202556
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Efficient Electrochemical Nitrate Reduction to Ammonia with Copper-Supported Rhodium Cluster and Single-Atom Catalysts
Liu, Huimin; Lang, Xiuyao; Zhu, Chao; Timoshenko, Janis; Ruescher, Martina; Bai, Lichen; Guijarro, Nestor; Yin, Haibo; Peng, Yue; Li, Junhua; Liu, Zheng; Wang, Weichao; Cuenya, Beatriz Roldan; Luo, Jingshan
Angewandte Chemie, International Edition (2022), 61 (23), e202202556CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
The electrochem. nitrate redn. reaction (NITRR) provides a promising soln. for restoring the imbalance in the global nitrogen cycle while enabling a sustainable and decentralized route to source ammonia. Here, we demonstrate a novel electrocatalyst for NITRR consisting of Rh clusters and single-atoms dispersed onto Cu nanowires (NWs), which delivers a partial c.d. of 162 mA cm-2 for NH3 prodn. and a Faradaic efficiency (FE) of 93% at -0.2 V vs. RHE. The highest ammonia yield rate reached a record value of 1.27 mmol h-1 cm-2. Detailed investigations by ESR, in situ IR spectroscopy, differential electrochem. mass spectrometry and d. functional theory modeling suggest that the high activity originates from the synergistic catalytic cooperation between Rh and Cu sites, whereby adsorbed hydrogen on Rh site transfers to vicinal *NO intermediate species adsorbed on Cu promoting the hydrogenation and ammonia formation.
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Hao, R.; Tian, L.; Wang, C.; Wang, L.; Liu, Y.; Wang, G.; Li, W.; Ozin, G. A. Pollution to solution: A universal electrocatalyst for reduction of all NO_x-based species to NH₃. Chem Catal. 2022, 2 (3), 622– 638, DOI: 10.1016/j.checat.2022.01.022
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Pollution to solution: A universal electrocatalyst for reduction of all NOx-based species to NH3
Hao, Ran; Tian, Lu; Wang, Cai; Wang, Lu; Liu, Yuping; Wang, Guichang; Li, Wei; Ozin, Geoffery A.
Chem Catalysis (2022), 2 (3), 622-638CODEN: CCHAE9; ISSN:2667-1093. (Elsevier Inc.)
NOx-based species, including all NO, NO2, NO-2, and NO-3 nitrogen oxides, are considered major industrial pollutants responsible for numerous environmental issues. Here, a bimetallic electrocatalyst based on copper and iron is developed that enables efficient electrochem. conversion of all NOx-based species to NH3. The key to success is the tunability of the d-band energy through introduction of iron to copper, which improves the adsorption energies of reaction intermediates. Details of the reaction pathway are provided by in situ Fourier transform IR spectroscopy complemented by d. functional theory. Electrochem. prodn. of green NH3 from nitrogen oxide pollutants using renewable electricity is a sustainable soln. for a serious environmental problem.
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Hu, Q.; Qin, Y.; Wang, X.; Wang, Z.; Huang, X.; Zheng, H.; Gao, K.; Yang, H.; Zhang, P.; Shao, M.; He, C. Reaction intermediate-mediated electrocatalyst synthesis favors specified facet and defect exposure for efficient nitrate-ammonia conversion. Energy Environ. Sci. 2021, 14 (9), 4989– 4997, DOI: 10.1039/D1EE01731D
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Reaction intermediate-mediated electrocatalyst synthesis favors specified facet and defect exposure for efficient nitrate-ammonia conversion
Hu, Qi; Qin, Yongjie; Wang, Xiaodeng; Wang, Ziyu; Huang, Xiaowan; Zheng, Hongju; Gao, Keru; Yang, Hengpan; Zhang, Peixin; Shao, Minhua; He, Chuanxin
Energy & Environmental Science (2021), 14 (9), 4989-4997CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
The electrochem. nitrate (NO3-) redn. reaction (NO3-RR), with much faster kinetics than the nitrogen (N2) redn., provides new opportunities to harvest ammonia (NH3) under ambient conditions. However, the NH3 prodn. rate of NO3-RR is still much inferior to that of the industrial Haber-Bosch route due to the lack of robust electrocatalysts for suppressing the hydrogen evolution reaction (HER) at large current densities. Herein, we demonstrate an electrocatalyst synthesis strategy based on the in situ electrochem. redn. of ultrathin copper-oxide nanobelts under NO3-RR conditions, which favorably exposes Cu(100) facets and abundant surface defects, thereby markedly facilitating the NO3-RR yet hindering the HER. We discover that the intermediates of NO3-RR (i.e., N*) can serve as capping agents for controlling the exposed facets during the redn. Impressively, in alk. media, the NO3-RR catalyzed by defective Cu(100) facets gives a NH3 yield rate which is 2.3-fold higher than that of the Haber-Bosch process. The synergy of Cu(100) facets and defects, which upshifts the d band center of Cu, is the key to excellent performance. The reaction intermediate-mediated strategy demonstrated in this study offers a fresh concept and robust methodol. for directional electrocatalyst synthesis to achieve markedly enhanced performance.
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Patil, S. B.; Liu, T. R.; Chou, H. L.; Huang, Y. B.; Chang, C. C.; Chen, Y. C.; Lin, Y. S.; Li, H.; Lee, Y. C.; Chang, Y. J.; Lai, Y. H.; Wen, C. Y.; Wang, D. Y. Electrocatalytic Reduction of NO₃^– to Ultrapure Ammonia on {200} Facet Dominant Cu Nanodendrites with High Conversion Faradaic Efficiency. J. Phys. Chem. Lett. 2021, 12 (33), 8121– 8128, DOI: 10.1021/acs.jpclett.1c02236
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Electrocatalytic Reduction of NO3- to Ultrapure Ammonia on {200} Facet Dominant Cu Nanodendrites with High Conversion Faradaic Efficiency
Patil, Shivaraj B.; Liu, Ting-Ran; Chou, Hung-Lung; Huang, Yu-Bin; Chang, Chia-Che; Chen, Yi-Chia; Lin, Ying-Sheng; Li, Hsin; Lee, Yi-Cheng; Chang, Yuan Jay; Lai, Ying-Huang; Wen, Cheng-Yen; Wang, Di-Yan
Journal of Physical Chemistry Letters (2021), 12 (33), 8121-8128CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
Nitrate (NO3-) redn. reaction (NtRR) is considered as a green alternative method for the conventional method of NH3 synthesis (Haber-Bosch process), which is known as a high energy consuming and large CO2 emitting process. Herein, the copper nanodendrites (Cu NDs) grown along with the {200} facet as an efficient NtRR catalyst have been successfully fabricated and investigated. It exhibited high Faradaic efficiency of 97% at low potential (-0.3 V vs RHE). Furthermore, the 15NO3- isotope labeling method was utilized to confirm the formation of NH3. Both exptl. and theor. studies showed that NtRR on the Cu metal nanostructure is a facet dependent process. Dissocn. of NO bonding is supposed to be the rate-detg. step as NtRR is a spontaneously reductive and protonation process for all the different facets of Cu. D. functional theory (DFT) calcns. revealed that Cu{200} and Cu{220} offer lower activation energy for dissocn. of NO compared to that of Cu{111}.
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Pérez-Gallent, E.; Figueiredo, M. C.; Katsounaros, I.; Koper, M. T. M. Electrocatalytic reduction of Nitrate on Copper single crystals in acidic and alkaline solutions. Electrochim. Acta 2017, 227, 77– 84, DOI: 10.1016/j.electacta.2016.12.147
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Electrocatalytic reduction of Nitrate on Copper single crystals in acidic and alkaline solutions.
Perez-Gallent, Elena; Figueiredo, Marta C.; Katsounaros, Ioannis; Koper, Marc T. M.
Electrochimica Acta (2017), 227 (), 77-84CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
Nitrate redn. on Cu (100) and Cu (111) surfaces in alk. and acidic solns. was studied by electrochem. methods (cyclic voltammetry, rotating disk electrode) coupled with online and in situ characterization techniques (mass spectrometry, ion chromatog. and Fourier transformed IR spectroscopy) to evaluate the reaction mechanism and products on the different surfaces. Electrochem. results show that redn. of nitrate in alk. media on Cu is structure sensitive. The onset potential on Cu (100) is +0.1 V vs. RHE, ∼50 mV earlier than on Cu (111). The onset potentials for nitrate redn. on Cu (100) and Cu (111) in acidic media are rather similar. Anal. techniques show a diverse product distribution for both surfaces and for both electrolytes. Whereas in acidic media both Cu electrodes show the formation of NO and NH3, in alk. media Cu reduces nitrate to nitrite and further to hydroxylamine. In alk. media, Cu (100) is a more active surface for the formation of hydroxylamine than Cu (111).
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Bae, S.-E.; Stewart, K. L.; Gewirth, A. A. Nitrate adsorption and reduction on Cu (100) in acidic solution. J. Am. Chem. Soc. 2007, 129 (33), 10171– 10180, DOI: 10.1021/ja071330n
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Nitrate Adsorption and Reduction on Cu(100) in Acidic Solution
Bae, Sang-Eun; Stewart, Karen L.; Gewirth, Andrew A.
Journal of the American Chemical Society (2007), 129 (33), 10171-10180CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Nitrate adsorption and redn. on Cu(100) in acidic soln. was studied by electrochem. methods, in situ electrochem. scanning tunneling microscopy (EC-STM), surface enhanced Raman spectroscopy (SERS), and d. functional theory (DFT) calcns. Electrochem. results show that redn. of nitrate starts at -0.3 V vs. Ag/AgCl and reaches max. value at -0.58 V. Over the entire potential region interrogated adlayers composed of nitrate, nitrite, or other intermediates are obsd. by using in situ STM. From the open-circuit potential (OCP) to -0.22 V vs. Ag|AgCl, the nitrate ion is dominant and forms a (2 × 2) adlattice on the Cu(100) surface while nitrate forms a dominantly c(2 × 2) structure from -0.25 to -0.36 V. The interconversion between the nitrate and nitrite adlattices is obsd. DFT calcns. indicate that both nitrate and nitrite are 2-fold coordinated to the Cu(100) surface.
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Daiyan, R.; Tran-Phu, T.; Kumar, P.; Iputera, K.; Tong, Z.; Leverett, J.; Khan, M. H. A.; Asghar Esmailpour, A.; Jalili, A.; Lim, M.; Tricoli, A.; Liu, R.-S.; Lu, X.; Lovell, E.; Amal, R. Nitrate reduction to ammonium: from CuO defect engineering to waste NO_x-to-NH₃ economic feasibility. Energy Environ. Sci. 2021, 14 (6), 3588– 3598, DOI: 10.1039/D1EE00594D
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Nitrate reduction to ammonium: from CuO defect engineering to waste NOx-to-NH3 economic feasibility
Daiyan, Rahman; Tran-Phu, Thanh; Kumar, Priyank; Iputera, Kevin; Tong, Zizheng; Leverett, Joshua; Khan, Muhammad Haider Ali; Asghar Esmailpour, Ali; Jalili, Ali; Lim, Maggie; Tricoli, Antonio; Liu, Ru-Shi; Lu, Xunyu; Lovell, Emma; Amal, Rose
Energy & Environmental Science (2021), 14 (6), 3588-3598CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
Crit. to the feasibility of electrochem. redn. of waste NOx (NOxRR), as a sustainable pathway and to close the NOx cycle for the emerging NH3 economy, is the requirement of inexpensive, scalable and selective catalysts that can generate NH4+ with high yield, as indicated by our economic modeling. To this end, we carry out d. functional theory (DFT) calcns. to investigate the possible contribution of oxygen vacancy (OV) defects in NOxRR catalysis, discovering that an increase in defect d. within CuO is leading to a decrease in adsorption energy for NO3- reactants. Using these findings as design guidelines, we develop defective CuO nanomaterials using flame spray pyrolysis (FSP) and mild plasma treatment, that can attain a NH4+ yield of 520μmol cm-2 h-1 at a cell voltage of 2.2 V within a flow electrolyzer with good stability over 10 h of operation. Through our mechanistic investigation, we establish the beneficial role of oxygen vacancy defects (with one free electron) in CuO for NOxRR and we reveal a direct correlation of oxygen vacancy d. with the NH4+ yield, arising from improved NO3- adsorption, as evidenced from our theor. calcns. Our findings on defect engineering to improve NH4+ yield and its economic feasibility display the potential of NOxRR as an alternative pathway to generate green NH3, which can also serve as an energy vector for the emerging hydrogen economy and close the NOx cycle.
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Xu, Y.; Wang, M.; Ren, K.; Ren, T.; Liu, M.; Wang, Z.; Li, X.; Wang, L.; Wang, H. Atomic defects in pothole-rich two-dimensional copper nanoplates triggering enhanced electrocatalytic selective nitrate-to-ammonia transformation. J. Mater. Chem. A 2021, 9 (30), 16411– 16417, DOI: 10.1039/D1TA04743D
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Atomic defects in pothole-rich two-dimensional copper nanoplates triggering enhanced electrocatalytic selective nitrate-to-ammonia transformation
Xu, You; Wang, Mingzhen; Ren, Kaili; Ren, Tianlun; Liu, Mengying; Wang, Ziqiang; Li, Xiaonian; Wang, Liang; Wang, Hongjing
Journal of Materials Chemistry A: Materials for Energy and Sustainability (2021), 9 (30), 16411-16417CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
The development of efficient catalysts for electrocatalytic selective conversion of nitrate pollutants into valuable ammonia is a project of far-reaching importance. This work demonstrated the in situ electroredn. of pre-synthesized CuO nanoplates into defect-rich metallic Cu nanoplates and evaluated their electrocatalytic nitrate-to-ammonia activity. Concd. at. defects in the as-converted Cu nanoplates could favor the adsorption, enrichment, and confinement of nitrate ions and pivotal reaction intermediates, selectively promoting eight-electron redn. (NH3 formation). Consequently, the resultant defect-rich Cu nanoplates exhibit a significant ammonia prodn. rate of 781.25μg h-1 mg-1, together with excellent nitrate conversion (93.26%), high ammonia selectivity (81.99%), and good electrocatalytic stability, superior to the defect-free Cu nanoplate counterpart. Isotope labeling expts. demonstrated that the source of ammonia was from nitrate. Both 1H NMR and colorimetric methods were used to quantify the ammonia yield.
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Xu, Y.; Ren, K.; Ren, T.; Wang, M.; Wang, Z.; Li, X.; Wang, L.; Wang, H. Ultralow-content Pd in-situ incorporation mediated hierarchical defects in corner-etched Cu₂O octahedra for enhanced electrocatalytic nitrate reduction to ammonia. Appl. Catal., B 2022, 306 (5), 121094, DOI: 10.1016/j.apcatb.2022.121094
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Wang, Y.; Zhou, W.; Jia, R.; Yu, Y.; Zhang, B. Unveiling the Activity Origin of a Copper-based Electrocatalyst for Selective Nitrate Reduction to Ammonia. Angew. Chem., Int. Ed. 2020, 59 (13), 5350– 5354, DOI: 10.1002/anie.201915992
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Unveiling the Activity Origin of a Copper-based Electrocatalyst for Selective Nitrate Reduction to Ammonia
Wang, Yuting; Zhou, Wei; Jia, Ranran; Yu, Yifu; Zhang, Bin
Angewandte Chemie, International Edition (2020), 59 (13), 5350-5354CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
Unveiling the active phase of catalytic materials under reaction conditions is important for the construction of efficient electrocatalysts for selective nitrate redn. to NH3. The origin of the prominent activity enhancement for CuO (faradaic efficiency: 95.8%, Selectivity: 81.2%) toward selective nitrate electroredn. to NH3 was probed. 15N isotope labeling expts. showed that NH3 originated from nitrate redn. 1H NMR spectroscopy and colorimetric methods were performed to quantify NH3. In situ Raman and ex situ expts. revealed that CuO was electrochem. converted into Cu/Cu2O, which serves as an active phase. The combined results of online differential electrochem. mass spectrometry (DEMS) and DFT calcns. demonstrated that the electron transfer from Cu2O to Cu at the interface could facilitate the formation of *NOH intermediate and suppress the H evolution reaction, leading to high selectivity and faradaic efficiency.
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Yuan, J.; Xing, Z.; Tang, Y.; Liu, C. Tuning the Oxidation State of Cu Electrodes for Selective Electrosynthesis of Ammonia from Nitrate. ACS Appl. Mater. Interfaces 2021, 13 (44), 52469– 52478, DOI: 10.1021/acsami.1c10946
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Gong, Z.; Zhong, W.; He, Z.; Liu, Q.; Chen, H.; Zhou, D.; Zhang, N.; Kang, X.; Chen, Y. Regulating surface oxygen species on copper (I) oxides via plasma treatment for effective reduction of nitrate to ammonia. Appl. Catal., B 2022, 305 (5), 121021, DOI: 10.1016/j.apcatb.2021.121021
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Zhan, C.; Dattila, F.; Rettenmaier, C.; Bergmann, A.; Kuhl, S.; Garcia-Muelas, R.; Lopez, N.; Cuenya, B. R. Revealing the CO Coverage-Driven C-C Coupling Mechanism for Electrochemical CO₂ Reduction on Cu₂O Nanocubes via Operando Raman Spectroscopy. ACS Catal. 2021, 11 (13), 7694– 7701, DOI: 10.1021/acscatal.1c01478
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Revealing the CO Coverage-Driven C-C Coupling Mechanism for Electrochemical CO2 Reduction on Cu2O Nanocubes via Operando Raman Spectroscopy
Zhan, Chao; Dattila, Federico; Rettenmaier, Clara; Bergmann, Arno; Kuehl, Stefanie; Garcia-Muelas, Rodrigo; Lopez, Nuria; Cuenya, Beatriz Roldan
ACS Catalysis (2021), 11 (13), 7694-7701CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
Electrochem. redn. of carbon dioxide (CO2RR) is an attractive route to close the carbon cycle and potentially turn CO2 into valuable chems. and fuels. However, the highly selective generation of multicarbon products remains a challenge, suffering from poor mechanistic understanding. Herein, we used operando Raman spectroscopy to track the potential-dependent redn. of Cu2O nanocubes and the surface coverage of reaction intermediates. In particular, we discovered that the potential-dependent intensity ratio of the Cu-CO stretching band to the CO rotation band follows a volcano trend similar to the CO2RR Faradaic efficiency for multicarbon products. By combining operando spectroscopic insights with D. Functional Theory, we proved that this ratio is detd. by the CO coverage and that a direct correlation exists between the potential-dependent CO coverage, the preferred C-C coupling configuration, and the selectivity to C2+ products. Thus, operando Raman spectroscopy can serve as an effective method to quantify the coverage of surface intermediates during an electrocatalytic reaction.
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Dima, G.; De Vooys, A.; Koper, M. Electrocatalytic reduction of nitrate at low concentration on coinage and transition-metal electrodes in acid solutions. J. Electroanal. Chem. 2003, 554–555, 15– 23, DOI: 10.1016/S0022-0728(02)01443-2
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Electrocatalytic reduction of nitrate at low concentration on coinage and transition-metal electrodes in acid solutions
Dima, G. E.; de Vooys, A. C. A.; Koper, M. T. M.
Journal of Electroanalytical Chemistry (2003), 554-555 (), 15-23CODEN: JECHES ISSN:. (Elsevier Science B.V.)
A comparative study was performed to det. the reactivity of nitrate ions at 0.1M on 8 different polycryst. electrodes (Pt, Pd, Rh, Ru, Ir, Cu, Ag and Au) in acidic soln. using cyclic voltammetry (CV), chronoamperometry and differential electrochem. mass spectroscopy (DEMS). Cyclic voltammetry shows that the current densities for nitrate redn. depend strongly on the nature of the electrode. The activities decrease in the order Rh>Ru>Ir>Pd and Pt for the transition-metal electrodes and in the order Cu>Ag>Au for the coinage metals. The rate-detg. step on Ru, Rh, Ir, Pt, Cu, and Ag is the redn. of nitrate to nitrite, as is evident from the Tafel slope, the kinetic reaction order in nitrate, and the anion effect. Transfer expts. with Pt suggest that chemisorbed nitric oxide is the key surface intermediate in the nitrate redn. Since online mass spectrometry (DEMS) measurements on Pt and Rh show no formation of gaseous products such as nitric oxide (NO), nitrous oxide (N2O) or N2, probably NH3 and hydroxylamine are the main products on transition-metal electrodes. This is in agreement with the known mechanism for NO redn., which forms N2O or N2 only if NO is in soln. On Cu, DEMS measurements show the prodn. of gaseous NO, which is explained by the weaker binding of NO to Cu as compared to the transition metals.
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Butcher, D. P.; Gewirth, A. A. Nitrate reduction pathways on Cu single crystal surfaces: Effect of oxide and Cl^–. Nano Energy 2016, 29, 457– 465, DOI: 10.1016/j.nanoen.2016.06.024
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Nitrate reduction pathways on Cu single crystal surfaces: Effect of oxide and Cl-
Butcher, Dennis P. Jr.; Gewirth, Andrew A.
Nano Energy (2016), 29 (), 457-465CODEN: NEANCA; ISSN:2211-2855. (Elsevier Ltd.)
The origin of different nitrate redn. activity between the (100), (111), and (110) faces of Cu is examd. using vibrational spectroscopy and calcns. Shell isolated nanoparticle enhanced Raman spectroscopy (SHINERS) reveals a suite of intermediates from the nitrate redn. process on Cu(100), Cu(111), and Cu(110) including NO-2 and HNO. All three faces show similar intermediates, suggesting the same mechanism is operative on all of them. Crit. to the redn. pathway on the bare Cu surfaces is the redn. of nitrate to nitrite concomitant with partial oxidn. of the Cu surface. This priming action facilitates nitrate redn. and reduces overpotentials, particularly on the Cu(111) and Cu(110) faces, which are more susceptible to oxidn. Decoration of the surfaces with Cl- suppresses nitrate redn., resulting in higher overpotentials and lower c.d. NH3 is obsd. by SHINERS as a direct nitrate redn. product in the presence of Cl-, rather than NOx species obsd. on the bare Cu surfaces, indicating a reaction pathway unique from the bare, undecorated surface.
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Schouten, K. J.; Gallent, E. P.; Koper, M. T. M. The electrochemical characterization of copper single-crystal electrodes in alkaline media. J. Electroanal. Chem. 2013, 699, 6– 9, DOI: 10.1016/j.jelechem.2013.03.018
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The electrochemical characterization of copper single-crystal electrodes in alkaline media
Schouten, Klaas Jan P.; Gallent, Elena Perez; Koper, Marc T. M.
Journal of Electroanalytical Chemistry (2013), 699 (), 6-9CODEN: JECHES; ISSN:1873-2569. (Elsevier B.V.)
The use of single-crystals in electrochem. requires careful characterization of the surface structure. This paper addresses the characterization of Cu single-crystals using blank cyclic voltammetry in alk. media. The adsorption and desorption of OH species in the underpotential region of Cu2O formation in alk. media occur at different potentials on Cu(111) and Cu(100), whereas OH adsorption on Cu(110) is not obsd. in this potential region. This allows for a direct distinction of the Cu(hkl) basal planes. The adsorption of OH on Cu(111) induces a reconstructed adlayer on the surface. On Cu(322), a stepped surface with 5 atom wide (111) terraces, OH adsorption is obsd. in the same potential range as on Cu(111), but on Cu(3 2 2) reconstruction does not seem to take place. This is explained by the fact that the unit cell of the reconstructed layer is much larger than the (111) terrace width of Cu(322) and, therefore, reconstruction cannot take place. Cu(911), having 5 atom wide (100) terraces, exhibits the same voltammetric features as Cu(100), but with a lower intensity. This is explained by the lower amt. of (100) terraces present on this surface.
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Arán-Ais, R. M.; Scholten, F.; Kunze, S.; Rizo, R.; Roldan Cuenya, B. The role of in situ generated morphological motifs and Cu(I) species in C₂₊ product selectivity during CO₂ pulsed electroreduction. Nat. Energy 2020, 5 (4), 317– 325, DOI: 10.1038/s41560-020-0594-9
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The role of in situ generated morphological motifs and Cu(I) species in C2+ product selectivity during CO2 pulsed electroreduction
Aran-Ais, Rosa M.; Scholten, Fabian; Kunze, Sebastian; Rizo, Ruben; Roldan Cuenya, Beatriz
Nature Energy (2020), 5 (4), 317-325CODEN: NEANFD; ISSN:2058-7546. (Nature Research)
The efficient electrochem. conversion of CO2 provides a route to fuels and feedstocks. Copper catalysts are well-known to be selective to multicarbon products, although the role played by the surface architecture and the presence of oxides is not fully understood. Here we report improved efficiency towards ethanol by tuning the morphol. and oxidn. state of the copper catalysts through pulsed CO2 electrolysis. We establish a correlation between the enhanced prodn. of C2+ products (76% ethylene, ethanol and n-propanol at -1.0 V vs. the reversible hydrogen electrode) and the presence of (100) terraces, Cu2O and defects on Cu(100). We monitored the evolution of the catalyst morphol. by anal. of cyclic voltammetry curves and ex situ at. force microscopy data, whereas the chem. state of the surface was examd. via quasi in situ XPS. We show that the continuous regeneration of defects and Cu(I) species synergistically favors C-C coupling pathways.
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Reyter, D.; Belanger, D.; Roue, L. Elaboration of Cu-Pd films by coelectrodeposition: application to nitrate electroreduction. J. Phys. Chem. C 2009, 113 (1), 290– 297, DOI: 10.1021/jp805484t
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Elaboration of Cu-Pd Films by Coelectrodeposition: Application to Nitrate Electroreduction
Reyter, David; Belanger, Daniel; Roue, Lionel
Journal of Physical Chemistry C (2009), 113 (1), 290-297CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
Nanocryst. Cu-Pd films were synthesized over a wide range of compns. by coelectrodeposition of Pd and Cu in a 1 M NaCl soln. contg. both CuCl2 and PdCl2 in various proportions. The deposition potential was fixed at -0.5 V vs. a SCE. These coatings were characterized by SEM coupled to energy dispersive x-ray anal. (SEM-EDX), XRD, and XPS. These analyses revealed a fine and homogeneous distribution of Pd and Cu within and over the whole surface of the film. Depending upon the Cu(II)/Pd(II) ratio in soln., monophased Pd-rich films (Pd95Cu5 or Pd88Cu12 alloys) or biphased films (contg. Pd80Cu20 and Cu phases in different proportions) were obtained. Theses materials were tested as electrocatalysts for nitrate redn. in alk. media. Electrochem. measurements showed that biphasic (Pd80Cu20 + Cu) materials displayed the best electrocatalytic activity toward nitrate redn. Results of prolonged electrolysis also proved that the selectivity of the modified electrodes clearly depends not only on the applied potential but also on their structure and chem. compn. At -1.3 V vs. Hg/HgO, all the electrodes (except pure Pd, which is inactive for nitrate redn.) mainly produced NH3. However, at -0.93 V vs. Hg/HgO, biphasic Cu-Pd electrode composed of 77% Pd80Cu20 + 23% Cu successfully reduced nitrate to nitrogen with a current efficiency approaching 76%.
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Weatherup, R. S.; Wu, C. H.; Escudero, C.; Perez-Dieste, V.; Salmeron, M. B. Environment-Dependent Radiation Damage in Atmospheric Pressure X-ray Spectroscopy. J. Phys. Chem. B 2018, 122 (2), 737– 744, DOI: 10.1021/acs.jpcb.7b06397
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Environment-dependent radiation damage in atmospheric pressure x-ray spectroscopy
Weatherup, Robert S.; Wu, Cheng Hao; Escudero, Carlos; Perez-Dieste, Virginia; Salmeron, Miquel B.
Journal of Physical Chemistry B (2018), 122 (2), 737-744CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)
Atm. pressure x-ray spectroscopy techniques based on soft x-ray excitation can provide powerful interface-sensitive chem. information about a solid surface immersed in a gas or liq. environment. However, x-ray illumination of such dense phases can lead to the generation of considerable quantities of radical species by radiolysis. Soft x-ray absorption measurements of Cu films in both air and aq. alkali halide solns. reveal that this can cause significant evolution of the Cu oxidn. state. In air and NaOH (0.1 M) solns., the Cu is oxidized toward CuO, while the addn. of small amts. of CH3OH to the soln. leads to redn. toward Cu2O. For Ni films in NaHCO3 solns., the oxidn. state of the surface is found to remain stable under x-ray illumination and can be electrochem. cycled between a reduced and oxidized state. We provide a consistent explanation for this behavior based on the products of x-ray-induced radiolysis in these different environments and highlight a no. of general approaches that can mitigate radiolysis effects when performing operando x-ray measurements.
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Möller, T.; Scholten, F.; Thanh, T. N.; Sinev, I.; Timoshenko, J.; Wang, X.; Jovanov, Z.; Gliech, M.; Roldan Cuenya, B.; Varela, A. S. Electrocatalytic CO₂ reduction on CuO_x nanocubes: tracking the evolution of chemical state, geometric structure, and catalytic selectivity using operando spectroscopy. Angew. Chem., Int. Ed. 2020, 59 (41), 17974– 17983, DOI: 10.1002/anie.202007136
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Electrocatalytic CO2 Reduction on CuOx Nanocubes: Tracking the Evolution of Chemical State, Geometric Structure, and Catalytic Selectivity using Operando Spectroscopy
Moller Tim; Thanh Trung Ngo; Wang Xingli; Jovanov Zarko; Gliech Manuel; Strasser Peter; Scholten Fabian; Timoshenko Janis; Roldan Cuenya Beatriz; Sinev Ilya; Varela Ana Sofia
Angewandte Chemie (International ed. in English) (2020), 59 (41), 17974-17983 ISSN:.
The direct electrochemical conversion of carbon dioxide (CO2 ) into multi-carbon (C2+ ) products still faces fundamental and technological challenges. While facet-controlled and oxide-derived Cu materials have been touted as promising catalysts, their stability has remained problematic and poorly understood. Herein we uncover changes in the chemical and morphological state of supported and unsupported Cu2 O nanocubes during operation in low-current H-Cells and in high-current gas diffusion electrodes (GDEs) using neutral pH buffer conditions. While unsupported nanocubes achieved a sustained C2+ Faradaic efficiency of around 60 % for 40 h, the dispersion on a carbon support sharply shifted the selectivity pattern towards C1 products. Operando XAS and time-resolved electron microscopy revealed the degradation of the cubic shape and, in the presence of a carbon support, the formation of small Cu-seeds during the surprisingly slow reduction of bulk Cu2 O. The initially (100)-rich facet structure has presumably no controlling role on the catalytic selectivity, whereas the oxide-derived generation of under-coordinated lattice defects, can support the high C2+ product yields.
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Bard, A. J.; Faulkner, L. R.; Leddy, J.; Zoski, C. G. Electrochemical Methods: Fundamentals and Applications; Wiley: New York, 1980; Vol. 2.
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Danaee, I.; Jafarian, M.; Mirzapoor, A.; Gobal, F.; Mahjani, M. G. Electrooxidation of methanol on NiMn alloy modified graphite electrode. Electrochim. Acta 2010, 55 (6), 2093– 2100, DOI: 10.1016/j.electacta.2009.11.039
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Electrooxidation of methanol on NiMn alloy modified graphite electrode
Danaee, I.; Jafarian, M.; Mirzapoor, A.; Gobal, F.; Mahjani, M. G.
Electrochimica Acta (2010), 55 (6), 2093-2100CODEN: ELCAAV; ISSN:0013-4686. (Elsevier B.V.)
Ni and Ni-Mn alloy modified graphite electrodes (G/Ni and G/NiMn) prepd. by galvanostatic deposition were examd. for their redox process and electrocatalytic activities towards the oxidn. of MeOH in alk. solns. The methods of cyclic voltammetry (CV), chronoamperometry (CA) and impedance spectroscopy (EIS) were employed. In CV studies, in the presence of MeOH NiMn alloy modified electrode shows a significantly higher response for MeOH oxidn. The peak current of the oxidn. of Ni hydroxide increase is followed by a decrease in the corresponding cathodic current in presence of MeOH. The anodic peak currents show linear dependency upon the square root of scan rate. This behavior is the characteristic of a diffusion controlled process. Under the CA regime the reaction followed a Cottrell-Ian behavior and the diffusion coeff. of MeOH is 4 × 10-6 cm2 s-1. A mechanism based on the electro-chem. generation of Ni3+ active sites and their subsequent consumptions by MeOH were discussed and the corresponding rate law under the control of charge transfer was developed and kinetic parameters were derived. The charge transfer resistance accessible both theor. and through the EIS were used as criteria for derivation of the rate const.
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Manjunatha, H.; Venkatesha, T. V.; Suresh, G. S. Electrochemical studies of LiMnPO₄ as aqueous rechargeable lithium-ion battery electrode. J. Solid State Electrochem. 2012, 16 (5), 1941– 1952, DOI: 10.1007/s10008-011-1593-3
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Nicholson, R. S.; Shain, I. Theory of stationary electrode polarography. Single scan and cyclic methods applied to reversible, irreversible, and kinetic systems. Anal. Chem. 1964, 36 (4), 706– 723, DOI: 10.1021/ac60210a007
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Theory of stationary electrode polarography. Single scan and cyclic methods applied to reversible, irreversible, and kinetic systems
Nicholson, Richard S.; Shain, Irving
(1964), 36 (4), 706-23CODEN: ANCHAM; ISSN:0003-2700.
A numerical method is developed for solving the integral equations obtained from the boundary value problems, and extensive data were calcd. which permit construction of stationary electrode polarograms from theory. Correlations of kinetic and exptl. parameters make it possible to develop diagnostic criteria so that unknown systems can be characterized by studying the variation of peak current, half-peak potential, or ratio of anodic to cathodic peak currents as a function of rate of voltage scan. 51 references.
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Simpson, B. K.; Johnson, D. C. Electrocatalysis of Nitrate Reduction at Copper-Nickel Alloy Electrodes in Acidic Media. Electroanalysis 2004, 16 (7), 532– 538, DOI: 10.1002/elan.200302790
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Electrocatalysis of nitrate reduction at copper-nickel alloy electrodes in acidic media
Simpson, Brett K.; Johnson, Dennis C.
Electroanalysis (2004), 16 (7), 532-538CODEN: ELANEU; ISSN:1040-0397. (Wiley-VCH Verlag GmbH & Co. KGaA)
The cathodic redn. of NO3- in 1.0 M HClO4 was studied by voltammetry at pure Ni and Cu electrodes, and three Cu-Ni alloy electrodes of varying compn., all configured as rotated disks. Voltammetric data obtained using these hydrodynamic electrodes demonstrate significantly improved activity for NO3- redn. at Cu-Ni alloy electrodes as compared to the pure Ni and Cu electrodes. This observation is explained from the synergistic benefit of different surface sites for adsorption of H-atoms, generated by cathodic discharge of H+ at Ni-sites, and adsorption of NO3- at Cu-sites on these binary alloy electrodes. Koutecky-Levich plots indicate that the cathodic response for NO3- at a Cu75Ni25 electrode corresponds to an 8-electron redn., which is consistent with prodn. of NH3. In comparison, the cathodic response at Cu50Ni50 and Cu25Ni75 electrodes corresponds to a 6-electron redn., which is consistent with prodn. of NH2OH. Flow injection data obtained using Cu50Ni50 and Cu25Ni75 electrodes with 100-μL injections exhibit detection limits for NO3- of ∼0.95 μM (∼95 pmol) and 0.60 μM (∼60 pmol), resp.
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Liu, Q.; Liu, Q.; Xie, L.; Ji, Y.; Li, T.; Zhang, B.; Li, N.; Tang, B.; Liu, Y.; Gao, S.; Luo, Y.; Yu, L.; Kong, Q.; Sun, X. High-Performance Electrochemical Nitrate Reduction to Ammonia under Ambient Conditions Using a FeOOH Nanorod Catalyst. ACS Appl. Mater. Interfaces 2022, 14 (15), 17312– 17318, DOI: 10.1021/acsami.2c00436
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High-Performance Electrochemical Nitrate Reduction to Ammonia under Ambient Conditions Using a FeOOH Nanorod Catalyst
Liu, Qin; Liu, Qian; Xie, Lisi; Ji, Yuyao; Li, Tingshuai; Zhang, Bing; Li, Na; Tang, Bo; Liu, Yang; Gao, Shuyan; Luo, Yonglan; Yu, Lingmin; Kong, Qingquan; Sun, Xuping
ACS Applied Materials & Interfaces (2022), 14 (15), 17312-17318CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
Electrocatalytic nitrate redn. is promising as an environmentally friendly process to produce high value-added ammonia with simultaneous removal of nitrate, a widespread nitrogen pollutant, for water treatment; however, efficient electrocatalysts with high selectivity are required for ammonia formation. In this work, FeOOH nanorod with intrinsic oxygen vacancy supported on carbon paper (FeOOH/CP) is proposed as a high-performance electrocatalyst for converting nitrate to ammonia at room temp. When operated in a 0.1 M phosphate-buffered saline (PBS) soln. with 0.1 M NaNO3, FeOOH/CP is able to obtain a large NH3 yield of 2419μg h-1 cm-2 and a surprisingly high Faradic efficiency of 92% with excellent stability. D. functional theory calcn. demonstrates that the potential-detg. step for nitrate redn. over FeOOH (200) is *NO2H + H+ + e- → *NO + H2O.
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Bai, L.; Hsu, C.-S.; Alexander, D. T. L.; Chen, H. M.; Hu, X. Double-atom catalysts as a molecular platform for heterogeneous oxygen evolution electrocatalysis. Nat. Energy 2021, 6 (11), 1054– 1066, DOI: 10.1038/s41560-021-00925-3
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Double-atom catalysts as a molecular platform for heterogeneous oxygen evolution electrocatalysis
Bai, Lichen; Hsu, Chia-Shuo; Alexander, Duncan T. L.; Chen, Hao Ming; Hu, Xile
Nature Energy (2021), 6 (11), 1054-1066CODEN: NEANFD; ISSN:2058-7546. (Nature Portfolio)
The oxygen evolution reaction (OER) is an essential anode reaction for the generation of fuels through water splitting or CO2 electroredn. Mixed metal oxides contg. Co, Fe or Ni have proved to be the most promising OER electrocatalysts in alk. media. However, the active sites and reaction mechanisms of these catalysts are difficult to study due to their heterogeneous nature. Here we describe a general synthesis of Co-, Fe- and Ni-contg. double-atom catalysts from their single-atom precursors via in situ electrochem. transformation. Characterization reveals mol.-like bimetallic active sites for these supported catalysts. For each catalyst, we propose a catalytic cycle; all exhibit bimetallic cooperation and follow a similar O-O bond-forming step. However, the mechanisms diverge in the site and source of OH- for O-O bond formation, as well as the order of proton and electron transfer. Our work demonstrates double-atom catalysts as an attractive platform for fundamental studies of heterogeneous OER electrocatalysts.
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Wang, Y.; Wang, C.; Li, M.; Yu, Y.; Zhang, B. Nitrate electroreduction: mechanism insight, in situ characterization, performance evaluation, and challenges. Chem. Soc. Rev. 2021, 50 (12), 6720– 6733, DOI: 10.1039/D1CS00116G
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Nitrate electroreduction: mechanism insight, in situ characterization, performance evaluation, and challenges
Wang, Yuting; Wang, Changhong; Li, Mengyang; Yu, Yifu; Zhang, Bin
Chemical Society Reviews (2021), 50 (12), 6720-6733CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
A review. Excessive nitrate ions in the environment break the natural nitrogen cycle and become a significant threat to human health. So far, many phys., chem., and biol. techniques have been developed for nitrate remediation, but most of them require high post-processing costs and rigorous treatment conditions. In contrast, nitrate electroredn. is promising because it utilizes green electrons as reductants under ambient conditions. The recognition and mastering of the nitrate reaction mechanism is the premise for the design and synthesis of efficient electrocatalysts for the selective redn. of nitrate. In this regard, this aims to provide an insight into the electrocatalytic mechanism of nitrate redn., esp. combined with in situ electrochem. characterization and theor. calcns. over different kinds of materials. Moreover, the performance evaluation parameters and std. test methods for nitrate electroredn. are summarized to screen efficient materials. Finally, an outlook on the current challenges and promising opportunities in this research area is discussed. This provides a guide for development of electrocatalysts for selective nitrate redn. with a fascinating performance and accelerates the development of sustainable nitrogen chem. and engineering.
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Bodappa, N.; Su, M.; Zhao, Y.; Le, J. B.; Yang, W. M.; Radjenovic, P.; Dong, J. C.; Cheng, J.; Tian, Z. Q.; Li, J. F. Early Stages of Electrochemical Oxidation of Cu(111) and Polycrystalline Cu Surfaces Revealed by in Situ Raman Spectroscopy. J. Am. Chem. Soc. 2019, 141 (31), 12192– 12196, DOI: 10.1021/jacs.9b04638
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Early Stages of Electrochemical Oxidation of Cu(111) and Polycrystalline Cu Surfaces Revealed by in Situ Raman Spectroscopy
Bodappa, Nataraju; Su, Min; Zhao, Yu; Le, Jia-Bo; Yang, Wei-Min; Radjenovic, Petar; Dong, Jin-Chao; Cheng, Jun; Tian, Zhong-Qun; Li, Jian-Feng
Journal of the American Chemical Society (2019), 141 (31), 12192-12196CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Studying the chem. nature of the adsorbed intermediate species on well-defined Cu single crystal substrates is crucial in understanding many electrocatalytic reactions. Herein, the authors systematically study the early stages of electrochem. oxidn. of Cu(111) and polycryst. Cu surfaces in different pH electrolytes using in situ shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). On Cu(111), for the 1st time, the authors identified surface OH species which convert to chemisorbed O before forming Cu2O in alk. (0.01M KOH) and neutral (0.1M Na2SO4) electrolytes; while at the Cu(poly) surface, the authors only detected the presence of surface hydroxide. Whereas, in a strongly acidic soln. (0.1M H2SO4), sulfate replaces the hydroxyl/oxy species. This results improves the understanding of the reaction mechanisms of various electrocatalytic reactions.
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Niaura, G. Surface-enhanced Raman spectroscopic observation of two kinds of adsorbed OH^– ions at copper electrode. Electrochim. Acta 2000, 45 (21), 3507– 3519, DOI: 10.1016/S0013-4686(00)00434-5
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Surface-enhanced Raman spectroscopic observation of two kinds of adsorbed OH- ions at copper electrode
Niaura, G.
Electrochimica Acta (2000), 45 (21), 3507-3519CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Science Ltd.)
The surface of a polycryst. roughened Cu electrode in 1 M NaOH soln., was studied in situ using surface-enhanced Raman spectroscopy (SERS). Cu2O, adsorbed OH- ions, and water mols. were detected as the electrode potential was varied from open circuit value to -1.20 V vs. SHE. The vibrational spectrum of Cu2O consisted of three main peaks located at 150, 528, and 623 cm-1. The intense and narrow feature at 150 cm-1 is highly characteristic, and could be used for SER monitoring of Cu2O. Two different states of adsorbed OH- ions, giving Cu-OH vibrations around 450-470 cm-1 and 540-580 cm-1, were detected. The distinct nature of the bands was shown by opposite isotopic frequency shifts changing the solvent from H2O to D2O. The frequency of the 1st band decreased by ∼12 cm-1, while the frequency of the 2nd band increased by ∼35 cm-1 in D2O solns. These differences were explained in terms of distinct surface ligation and the formation of strong hydrogen bonds between water mols. and the 2nd type of adsorbed OH- ion. Water mols. were obsd. at the interface at an applied potential -1.20 V.
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Giguère, P. A.; Liu, I. Infrared spectrum, molecular structure, and thermodynamic functions of hydroxylamine. Can. J. Chem. 1952, 30 (12), 948– 962, DOI: 10.1139/v52-115
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Fang, J. Y.; Zheng, Q. Z.; Lou, Y. Y.; Zhao, K. M.; Hu, S. N.; Li, G.; Akdim, O.; Huang, X. Y.; Sun, S. G. Ampere-level current density ammonia electrochemical synthesis using CuCo nanosheets simulating nitrite reductase bifunctional nature. Nat. Commun. 2022, 13 (1), 7899, DOI: 10.1038/s41467-022-35533-6
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Ampere-level current density ammonia electrochemical synthesis using CuCo nanosheets simulating nitrite reductase bifunctional nature
Fang, Jia-Yi; Zheng, Qi-Zheng; Lou, Yao-Yin; Zhao, Kuang-Min; Hu, Sheng-Nan; Li, Guang; Akdim, Ouardia; Huang, Xiao-Yang; Sun, Shi-Gang
Nature Communications (2022), 13 (1), 7899CODEN: NCAOBW; ISSN:2041-1723. (Nature Portfolio)
The development of electrocatalysts capable of efficient redn. of nitrate (NO3-) to ammonia (NH3) is drawing increasing interest for the sake of low carbon emission and environmental protection. Herein, we present a CuCo bimetallic catalyst able to imitate the bifunctional nature of copper-type nitrite reductase, which could easily remove NO2- via the collaboration of two active centers. Indeed, Co acts as an electron/proton donating center, while Cu facilitates NOx- adsorption/assocn. The bio-inspired CuCo nanosheet electrocatalyst delivers a 100 ± 1% Faradaic efficiency at an ampere-level c.d. of 1035 mA cm-2 at -0.2 V vs. Reversible Hydrogen Electrode. The NH3 prodn. rate reaches a high activity of 4.8 mmol cm-2 h-1 (960 mmol gcat-1 h-1). A mechanistic study, using electrochem. in situ Fourier transform IR spectroscopy and shell-isolated nanoparticle enhanced Raman spectroscopy, reveals a strong synergy between Cu and Co, with Co sites promoting the hydrogenation of NO3- to NH3 via adsorbed *H species. The well-modulated coverage of adsorbed *H and *NO3 led simultaneously to high NH3 selectivity and yield.
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Figueiredo, M. C.; Souza-Garcia, J.; Climent, V.; Feliu, J. M. Nitrate reduction on Pt (111) surfaces modified by Bi adatoms. Electrochem. Commun. 2009, 11 (9), 1760– 1763, DOI: 10.1016/j.elecom.2009.07.010
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Castro, P. M.; Jagodzinski, P. W. FTIR and Raman spectra and structure of Cu(NO₃)⁺ in aqueous solution and acetone. Spectrochim. Acta, Part A 1991, 47 (12), 1707– 1720, DOI: 10.1016/0584-8539(91)80008-7
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Wang, Y. H.; Zheng, S.; Yang, W. M.; Zhou, R. Y.; He, Q. F.; Radjenovic, P.; Dong, J. C.; Li, S.; Zheng, J.; Yang, Z. L.; Attard, G.; Pan, F.; Tian, Z. Q.; Li, J. F. In situ Raman spectroscopy reveals the structure and dissociation of interfacial water. Nature 2021, 600 (7887), 81– 85, DOI: 10.1038/s41586-021-04068-z
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In situ Raman spectroscopy reveals the structure and dissociation of interfacial water
Wang, Yao-Hui; Zheng, Shisheng; Yang, Wei-Min; Zhou, Ru-Yu; He, Quan-Feng; Radjenovic, Petar; Dong, Jin-Chao; Li, Shunning; Zheng, Jiaxin; Yang, Zhi-Lin; Attard, Gary; Pan, Feng; Tian, Zhong-Qun; Li, Jian-Feng
Nature (London, United Kingdom) (2021), 600 (7887), 81-85CODEN: NATUAS; ISSN:1476-4687. (Nature Portfolio)
Understanding the structure and dynamic process of water at the solid-liq. interface is an extremely important topic in surface science, energy science and catalysis1-3. As model catalysts, atomically flat single-crystal electrodes exhibit well-defined surface and elec. field properties, and therefore may be used to elucidate the relationship between structure and electrocatalytic activity at the at. level4,5. Hence, studying interfacial water behavior on single-crystal surfaces provides a framework for understanding electrocatalysis6,7. However, interfacial water is notoriously difficult to probe owing to interference from bulk water and the complexity of interfacial environments8. Here, we use electrochem., in situ Raman spectroscopic and computational techniques to investigate the interfacial water on atomically flat Pd single-crystal surfaces. Direct spectral evidence reveals that interfacial water consists of hydrogen-bonded and hydrated Na+ ion water. At hydrogen evolution reaction (HER) potentials, dynamic changes in the structure of interfacial water were obsd. from a random distribution to an ordered structure due to bias potential and Na+ ion cooperation. Structurally ordered interfacial water facilitated high-efficiency electron transfer across the interface, resulting in higher HER rates. The electrolytes and electrode surface effects on interfacial water were also probed and found to affect water structure. Therefore, through local cation tuning strategies, we anticipate that these results may be generalized to enable ordered interfacial water to improve electrocatalytic reaction rates.
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Li, Y.; Cheng, C.; Han, S.; Huang, Y.; Du, X.; Zhang, B.; Yu, Y. Electrocatalytic Reduction of Low-Concentration Nitric Oxide into Ammonia over Ru Nanosheets. ACS Energy Lett. 2022, 7 (3), 1187– 1194, DOI: 10.1021/acsenergylett.2c00207
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Yang, J.; Qi, H.; Li, A.; Liu, X.; Yang, X.; Zhang, S.; Zhao, Q.; Jiang, Q.; Su, Y.; Zhang, L.; Li, J. F.; Tian, Z. Q.; Liu, W.; Wang, A.; Zhang, T. Potential-Driven Restructuring of Cu Single Atoms to Nanoparticles for Boosting the Electrochemical Reduction of Nitrate to Ammonia. J. Am. Chem. Soc. 2022, 144 (27), 12062– 12071, DOI: 10.1021/jacs.2c02262
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Potential-Driven Restructuring of Cu Single Atoms to Nanoparticles for Boosting the Electrochemical Reduction of Nitrate to Ammonia
Yang, Ji; Qi, Haifeng; Li, Anqi; Liu, Xiaoyan; Yang, Xiaofeng; Zhang, Shengxin; Zhao, Qiao; Jiang, Qike; Su, Yang; Zhang, Leilei; Li, Jian-Feng; Tian, Zhong-Qun; Liu, Wei; Wang, Aiqin; Zhang, Tao
Journal of the American Chemical Society (2022), 144 (27), 12062-12071CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Restructuring is ubiquitous in thermocatalysis and of pivotal importance to identify the real active site, yet it is less explored in electrocatalysis. Herein, by using operando x-ray absorption spectroscopy in conjunction with advanced electron microscopy, the authors reveal the restructuring of the as-synthesized Cu-N4 single-atom site to the nanoparticles of ~ 5 nm during the electrochem. redn. of nitrate to NH3, a green NH3 prodn. route upon combined with the plasma-assisted oxidn. of N. The redn. of Cu2+ to Cu+ and Cu0 and the subsequent aggregation of Cu0 single atoms occurs concurrently with the enhancement of the NH3 prodn. rate, both of them are driven by the applied potential switching from 0.00 to -1.00 V vs. RHE. The max. prodn. rate of NH3 reaches 4.5 mg cm-2 h-1 (12.5 molNH3 gCu-1 h-1) with a faradaic efficiency of 84.7% at -1.00 V vs. RHE, outperforming most of the other Cu catalysts reported previously. After electrolysis, the aggregated Cu nanoparticles are reversibly disintegrated into single atoms and then restored to the Cu-N4 structure upon being exposed to an ambient atm., which masks the potential-induced restructuring during the reaction. The synchronous changes of the Cu0 percentage and the NH3 faradaic efficiency with the applied potential suggests that the Cu nanoparticles are the genuine active sites for nitrate redn. to NH3, which is corroborated with both the post-deposited Cu NP catalyst and d. functional theory calcns.
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Han, S.; Li, H.; Li, T.; Chen, F.; Yang, R.; Yu, Y.; Zhang, B. Ultralow overpotential nitrate reduction to ammonia via a three-step relay mechanism. Nat. Catal. 2023, 6 (5), 402– 414, DOI: 10.1038/s41929-023-00951-2
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Ultralow overpotential nitrate reduction to ammonia via a three-step relay mechanism
Han, Shuhe; Li, Hongjiao; Li, Tieliang; Chen, Fanpeng; Yang, Rong; Yu, Yifu; Zhang, Bin
Nature Catalysis (2023), 6 (5), 402-414CODEN: NCAACP; ISSN:2520-1158. (Nature Portfolio)
Ammonia plays a substantial role in agriculture and the next generation of carbon-free energy supply. Electrocatalytic nitrate redn. to NH3 is attractive for nitrate removal and NH3 prodn. under ambient conditions. However, the energy efficiency is limited by the high reaction overpotential. Here we propose a three-step relay mechanism composed of a spontaneous redox reaction, electrochem. redn. and electrocatalytic redn. to overcome this issue. RuxCoy alloys were designed and adopted as model catalysts. Ru15Co85 exhibits an onset potential of +0.4 V vs. reversible hydrogen electrode, and an energy efficiency of 42 ± 2%. The high performance results in a low prodn. cost of US$0.49 ± 0.02 per kg of ammonia. The high nitrate redn. performances on Ru15Fe85 and Ru15Ni85 also highlight the promising potential of the relay mechanism.
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Wang, H.; Guo, Y.; Li, C.; Yu, H.; Deng, K.; Wang, Z.; Li, X.; Xu, Y.; Wang, L. Cu/CuO_x In-Plane Heterostructured Nanosheet Arrays with Rich Oxygen Vacancies Enhance Nitrate Electroreduction to Ammonia. ACS Appl. Mater. Interfaces 2022, 14 (30), 34761– 34769, DOI: 10.1021/acsami.2c08534
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Cu/CuOx In-Plane Heterostructured Nanosheet Arrays with Rich Oxygen Vacancies Enhance Nitrate Electroreduction to Ammonia
Wang, Hongjing; Guo, Yanan; Li, Chunjie; Yu, Hongjie; Deng, Kai; Wang, Ziqiang; Li, Xiaonian; Xu, You; Wang, Liang
ACS Applied Materials & Interfaces (2022), 14 (30), 34761-34769CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
The artificial ammonia synthesis via electrochem. nitrate redn. has met increasing research interest, but it is still necessary to develop advanced catalysts with high nitrate-to-ammonia capability. Herein, we propose and demonstrate a one-step method to construct binder-free Cu foam-supported oxygen vacancy-rich Cu/CuOx in-plane heterostructured nanosheet arrays (Cu/CuOx/CF). In addn. to exposing ample active sites, the two-dimensional nanosheet morphol. greatly facilitates the mass/charge-transfer process during electrocatalysis. Besides, the in-plane heterojunctions and rich oxygen vacancies induced synergistic effect can modulate the electronic structure of active sites and thus tune the adsorption properties of the reactant intermediates and inhibit the formation of undesirable byproducts, which is conducive to the further improvement of nitrate redn. activity. As a result, these advantages endow the Cu/CuOx/CF with superior performance for ammonia synthesis via nitrate electroredn., achieving high ammonia selectivity (95.00%) and Faradaic efficiency (93.58%).
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Wang, Y.; Shao, M. Theoretical Screening of Transition Metal-N₄-Doped Graphene for Electroreduction of Nitrate. ACS Catal. 2022, 12 (9), 5407– 5415, DOI: 10.1021/acscatal.2c00307
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Theoretical Screening of Transition Metal-N4-Doped Graphene for Electroreduction of Nitrate
Wang, Yian; Shao, Minhua
ACS Catalysis (2022), 12 (9), 5407-5415CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
Electrochem. nitrate redn. reaction (NO3RR) is an advantageous conversion technol. for nitrate removal and NH3 synthesis. Single-atom catalysts, owing to their utmost metal atom use efficiency, are promising electrocatalysts for NO3RR but are rarely studied in systematic ways. A theor. screening was performed on transition metal-N4-doped graphene (TM-N4/C) as active and selective electrocatalysts for NO3RR, where detailed reaction mechanisms and activity origins are explored. Volcano plots of activity trends show that Cu- and Pt-N4/C are highly active for NO3RR following the NH3 and N2 formation pathways, resp., whose activities can be attributed to the optimal NO and N adsorptions. A contour plot of selectivity trend shows that Re- and Pt-N4/C are highly selective toward NH3 and N2 formations, resp. This work provides theor. insights into the rational design of TM-N4/C catalysts for NO3RR and opportunities for efficient nitrate removal and NH3 synthesis strategies.
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Niu, H.; Zhang, Z.; Wang, X.; Wan, X.; Shao, C.; Guo, Y. Theoretical Insights into the Mechanism of Selective Nitrate-to-Ammonia Electroreduction on Single-Atom Catalysts. Adv. Funct. Mater. 2020, 31 (11), 2008533, DOI: 10.1002/adfm.202008533
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Reyter, D.; Bélanger, D.; Roué, L. Study of the electroreduction of nitrate on copper in alkaline solution. Electrochim. Acta 2008, 53 (20), 5977– 5984, DOI: 10.1016/j.electacta.2008.03.048
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Study of the electroreduction of nitrate on copper in alkaline solution
Reyter, David; Belanger, Daniel; Roue, Lionel
Electrochimica Acta (2008), 53 (20), 5977-5984CODEN: ELCAAV; ISSN:0013-4686. (Elsevier B.V.)
The electrocatalytic activity of a Cu electrode for the electroredn. of nitrate in alk. medium was studied by linear sweep voltammetry at stationary and rotating disk electrodes. Nitrate-redn. products generated upon prolonged electrolyzes at different potentials were quantified. Adsorption phenomena assocd. with the nitrate electroredn. process were characterized by electrochem. quartz crystal microbalance (EQCM) expts. This data revealed that nitrate electroredn. process strongly depends on the applied potential. Firstly, at ∼-0.9 V vs. Hg/HgO, the electroredn. of adsorbed nitrate anions to nitrite anions was identified as the rate-detg. step of the nitrate electroredn. process. Between -0.9 and -1.1 V, nitrite is reduced to hydroxylamine. However, during long-term electrolyzes, hydroxylamine is not detected and presumably because it is rapidly reduced to NH3. At potential more neg. than -1.1 V, nitrite is reduced to NH3. At ∼-1.45 V, i.e. just before the hydrogen evolution reaction, the abrupt decrease of the cathodic current is due to the electrode poisoning by adsorbed hydrogen. During the 1st minutes of nitrate electrolysis, a decrease of the Cu electrode activity was obsd. at the 3 studied potentials (-0.9, -1.1 and -1.4 V). From polarization and EQCM measurements, this deactivation is attributed to the adsorption of nitrate-redn. products, blocking the electrode surface and slowing down the nitrate electroredn. rate. However, the Cu electrode can be reactivated by the periodic application of a square wave potential pulse at -0.5 V, which causes the desorption of poisoning species.
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Scholten, F.; Sinev, I.; Bernal, M.; Roldan Cuenya, B. Plasma-Modified Dendritic Cu Catalyst for CO₂ Electroreduction. ACS Catal. 2019, 9 (6), 5496– 5502, DOI: 10.1021/acscatal.9b00483
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Plasma-Modified Dendritic Cu Catalyst for CO2 Electroreduction
Scholten, Fabian; Sinev, Ilya; Bernal, Miguel; Roldan Cuenya, Beatriz
ACS Catalysis (2019), 9 (6), 5496-5502CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
Efficient and active catalysts with high selectivity for hydrocarbons and other valuable chems. during stable operation are crucial. We have taken advantage of low-pressure oxygen plasmas to modify dendritic Cu catalysts and were able to achieve enhanced selectivity toward C2 and C3 products. Utilizing operando spectroscopic methods such as X-ray absorption fine-structure spectroscopy (XAFS) and quasi in situ XPS, we obsd. that the initial presence of oxides in these catalysts before the reaction plays an inferior role in detg. their catalytic performance as compared to the overall catalyst morphol. This is assigned to the poor stability of the CuxO species in these materials under the conditions of electrocatalytic conversion of CO2 (CO2RR). Our findings shed light into the strong structure/chem. state-selectivity correlation in CO2RR and open venues for the rational design of more effective catalysts through plasma pretreatments.
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Timoshenko, J.; Roldan Cuenya, B. In Situ/Operando Electrocatalyst Characterization by X-ray Absorption Spectroscopy. Chem. Rev. 2021, 121 (2), 882– 961, DOI: 10.1021/acs.chemrev.0c00396
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In Situ/Operando Electrocatalyst Characterization by X-ray Absorption Spectroscopy
Timoshenko, Janis; Roldan Cuenya, Beatriz
Chemical Reviews (Washington, DC, United States) (2021), 121 (2), 882-961CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
A review. During the last decades, x-ray absorption spectroscopy (XAS) has become an indispensable method for probing the structure and compn. of heterogeneous catalysts, revealing the nature of the active sites and establishing links between structural motifs in a catalyst, local electronic structure, and catalytic properties. Here the fundamental principles of the XAS method and describe the progress in the instrumentation and data anal. approaches undertaken for deciphering x-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra are discussed. Recent usages of XAS in the field of heterogeneous catalysis, with emphasis on examples concerning electrocatalysis, will be presented. The latter is a rapidly developing field with immense industrial applications but also unique challenges in terms of the exptl. characterization restrictions and advanced modeling approaches required. This review will highlight the new insight that can be gained with XAS on complex real-world electrocatalysts including their working mechanisms and the dynamic processes taking place in a chem. reaction. More specifically, the authors will discuss applications of in situ and operando XAS to probe the catalyst's interactions with the environment (support, electrolyte, ligands, adsorbates, reaction products, and intermediates) and its structural, chem., and electronic transformations as it adapts to the reaction conditions.
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Ravel, B.; Newville, M. ATHENA, ARTEMIS, HEPHAESTUS: data analysis for X-ray absorption spectroscopy using IFEFFIT. J. Synchrotron Radiat. 2005, 12 (4), 537– 541, DOI: 10.1107/s0909049505012719
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ATHENA, ARTEMIS, HEPHAESTUS: data analysis for x-ray absorption spectroscopy using IFEFFIT
Ravel, B.; Newville, M.
Journal of Synchrotron Radiation (2005), 12 (4), 537-541CODEN: JSYRES; ISSN:0909-0495. (Blackwell Publishing Ltd.)
A software package for the anal. of x-ray absorption spectroscopy (XAS) data is presented. This package is based on the IFEFFIT library of numerical and XAS algorithms and is written in the Perl programming language using the Perl/Tk graphics toolkit. The programs described here are: (i) ATHENA, a program for XAS data processing, (ii) ARTEMIS, a program for EXAFS data anal. using theor. stds. from FEFF and (iii) HEPHAESTUS, a collection of beamline utilities based on tables of at. absorption data. These programs enable high-quality data anal. that is accessible to novices while still powerful enough to meet the demands of an expert practitioner. The programs run on all major computer platforms and are freely available under the terms of a free software license.
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Andersen, S. Z.; Colic, V.; Yang, S.; Schwalbe, J. A.; Nielander, A. C.; McEnaney, J. M.; Enemark-Rasmussen, K.; Baker, J. G.; Singh, A. R.; Rohr, B. A.; Statt, M. J.; Blair, S. J.; Mezzavilla, S.; Kibsgaard, J.; Vesborg, P. C. K.; Cargnello, M.; Bent, S. F.; Jaramillo, T. F.; Stephens, I. E. L.; Norskov, J. K.; Chorkendorff, I. A rigorous electrochemical ammonia synthesis protocol with quantitative isotope measurements. Nature 2019, 570 (7762), 504– 508, DOI: 10.1038/s41586-019-1260-x
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A rigorous electrochemical ammonia synthesis protocol with quantitative isotope measurements
Andersen, Suzanne Z.; Colic, Viktor; Yang, Sungeun; Schwalbe, Jay A.; Nielander, Adam C.; McEnaney, Joshua M.; Enemark-Rasmussen, Kasper; Baker, Jon G.; Singh, Aayush R.; Rohr, Brian A.; Statt, Michael J.; Blair, Sarah J.; Mezzavilla, Stefano; Kibsgaard, Jakob; Vesborg, Peter C. K.; Cargnello, Matteo; Bent, Stacey F.; Jaramillo, Thomas F.; Stephens, Ifan E. L.; Noerskov, Jens K.; Chorkendorff, Ib
Nature (London, United Kingdom) (2019), 570 (7762), 504-508CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
The electrochem. synthesis of ammonia from nitrogen under mild conditions using renewable electricity is an attractive alternative1-4 to the energy-intensive Haber-Bosch process, which dominates industrial ammonia prodn. However, there are considerable scientific and tech. challenges5,6 facing the electrochem. alternative, and most exptl. studies reported so far have achieved only low selectivities and conversions. The amt. of ammonia produced is usually so small that it cannot be firmly attributed to electrochem. nitrogen fixation7-9 rather than contamination from ammonia that is either present in air, human breath or ion-conducting membranes9, or generated from labile nitrogen-contg. compds. (for example, nitrates, amines, nitrites and nitrogen oxides) that are typically present in the nitrogen gas stream10, in the atm. or even in the catalyst itself. Although these sources of exptl. artifacts are beginning to be recognized and managed11,12, concerted efforts to develop effective electrochem. nitrogen redn. processes would benefit from benchmarking protocols for the reaction and from a standardized set of control expts. designed to identify and then eliminate or quantify the sources of contamination. Here we propose a rigorous procedure using 15N2 that enables us to reliably detect and quantify the electrochem. redn. of nitrogen to ammonia. We demonstrate exptl. the importance of various sources of contamination, and show how to remove labile nitrogen-contg. compds. from the nitrogen gas as well as how to perform quant. isotope measurements with cycling of 15N2 gas to reduce both contamination and the cost of isotope measurements. Following this protocol, we find that no ammonia is produced when using the most promising pure-metal catalysts for this reaction in aq. media, and we successfully confirm and quantify ammonia synthesis using lithium electrodeposition in tetrahydrofuran13. The use of this rigorous protocol should help to prevent false positives from appearing in the literature, thus enabling the field to focus on viable pathways towards the practical electrochem. redn. of nitrogen to ammonia.
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Wu, Z. Y.; Karamad, M.; Yong, X.; Huang, Q.; Cullen, D. A.; Zhu, P.; Xia, C.; Xiao, Q.; Shakouri, M.; Chen, F. Y.; Kim, J. Y. T.; Xia, Y.; Heck, K.; Hu, Y.; Wong, M. S.; Li, Q.; Gates, I.; Siahrostami, S.; Wang, H. Electrochemical ammonia synthesis via nitrate reduction on Fe single atom catalyst. Nat. Commun. 2021, 12 (1), 2870, DOI: 10.1038/s41467-021-23115-x
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Electrochemical ammonia synthesis via nitrate reduction on Fe single atom catalyst
Wu, Zhen-Yu; Karamad, Mohammadreza; Yong, Xue; Huang, Qizheng; Cullen, David A.; Zhu, Peng; Xia, Chuan; Xiao, Qunfeng; Shakouri, Mohsen; Chen, Feng-Yang; Kim, Jung Yoon; Xia, Yang; Heck, Kimberly; Hu, Yongfeng; Wong, Michael S.; Li, Qilin; Gates, Ian; Siahrostami, Samira; Wang, Haotian
Nature Communications (2021), 12 (1), 2870CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
Electrochem. converting nitrate, a widespread water pollutant, back to valuable ammonia is a green and delocalized route for ammonia synthesis, and can be an appealing and supplementary alternative to the Haber-Bosch process. However, as there are other nitrate redn. pathways present, selectively guiding the reaction pathway towards ammonia is currently challenged by the lack of efficient catalysts. Here we report a selective and active nitrate redn. to ammonia on Fe single atom catalyst, with a maximal ammonia Faradaic efficiency of ∼ 75% and a yield rate of up to ∼ 20,000μg h-1 mgcat.-1 (0.46 mmol h-1 cm-2). Our Fe single atom catalyst can effectively prevent the N-N coupling step required for N2 due to the lack of neighboring metal sites, promoting ammonia product selectivity. D. functional theory calcns. reveal the reaction mechanisms and the potential limiting steps for nitrate redn. on atomically dispersed Fe sites.
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Wang, W.; Chen, J.; Tse, E. C. M. Synergy between Cu and Co in a Layered Double Hydroxide Enables Close to 100% Nitrate-to-Ammonia Selectivity. J. Am. Chem. Soc. 2023, 145 (49), 26678– 26687, DOI: 10.1021/jacs.3c08084
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70
Wang, Y.; Sun, M.; Zhou, J.; Xiong, Y.; Zhang, Q.; Ye, C.; Wang, X.; Lu, P.; Feng, T.; Hao, F.; Liu, F.; Wang, J.; Ma, Y.; Yin, J.; Chu, S.; Gu, L.; Huang, B.; Fan, Z. Atomic coordination environment engineering of bimetallic alloy nanostructures for efficient ammonia electrosynthesis from nitrate. Proc. Natl. Acad. Sci. U.S.A. 2023, 120 (32), e2306461120 DOI: 10.1073/pnas.2306461120
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Abstract
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Figure 1
Figure 1. Characterization and electrocatalytic NO₃RR and NO₂RR performances of Cu₂O NCs. (a) TEM image of the as prepared Cu₂O NCs. (b) Linear scan voltammograms (LSV) of Cu₂O NCs without adding NO₃^–/NO₂^– sources (black) and in the presence of NaNO₃ (red) and NaNO₂ (blue), respectively, using a scan rate of 50 mV/s. (c,d) Potential-dependent FEs for NH₃/NO₂^– production during 2 h chronoamperometric NO₃RR (c) and NO₂RR (d). The error bars represent the standard deviation from three independent experiments. Experimental conditions: 0.1 M Na₂SO₄ electrolyte (pH 12) + 8 mM NaNO₃ or NaNO₂.
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Figure 2
Figure 2. Quasi in situ and postreaction ex situ characterization of Cu₂O NCs after NO₃RR. (a) Quasi in situ XPS Cu LMM AES of Cu₂O NCs after NO₃RR (1 h) at different potentials. (b,c) Ex situ GI-PXRD of Cu₂O NCs before and after NO₃RR at 0.10 V_RHE (b) and −0.30 V_RHE (c). TEM of Cu₂O NCs after NO₃RR at 0.10 V_RHE (d) and −0.30 V_RHE (e). Electrolyte: 0.1 M NaSO₄ + 8 mM NaNO₃, pH 12.
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Figure 3
Figure 3. Operando XAS data for Cu₂O NCs during NO₃RR. (a) Normalized Cu K-edge XANES spectra collected at OCP, 0.10 V_RHE and −0.30 V_RHE. (b) Linear combination analysis (LCA) results of the XANES spectra, showing the variation of the Cu oxidation state. The full squares indicate the time-dependent FE(NO₂^–) (red) and FE(NH₃) (blue) obtained by NO₃RR electrolysis experiments. The first data point is measured at 0.10 V_RHE for 2 h, while the other data points are measured at −0.30 V_RHE with the corresponding reaction times. The error bars are calculated based on three independent experiments. (c) Fourier-transformed (FT) Cu K-edge EXAFS spectra of Cu₂O NCs at OCP, 0.10 V_RHE, and −0.30 V_RHE. (d) EXAFS fitting results showing the evolution of Cu–Cu and Cu–O coordination numbers. The error bars indicate the fitting uncertainty. Electrolyte: pH 12, 0.1 M Na₂SO₄ + 8 mM NaNO₃.
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Figure 4
Figure 4. Electrokinetic analysis of NO₃RR and NO₂RR in alkaline conditions. (a) Tafel plots of Cu₂O NCs at different pH (11.5–12.9) derived from staircase voltammetry. Electrolyte: 0.1 M Na₂SO₄ with 8 mM NaNO₃ at different pH values. (b) Scan rate-dependent CVs of Cu₂O NCs at pH 12.0. Typical reductive peaks are labeled. (c) Rotating rate dependent LSVs of Cu₂O NCs on rotating disk electrodes at pH 12.0. (d) Tafel plots of Cu₂O NCs at different pH (11.5–12.9) derived from staircase voltammetry. Electrolyte: 0.1 M Na₂SO₄ with 8 mM NaNO₂ with different pH values.
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Figure 5
Figure 5. In situ Raman spectroscopy data of Cu₂O NCs. (a) Enlarged in situ Raman spectra during NO₃RR in the range of 950–1650 cm^–1 Raman shift, with the applied potentials ranging from the 0.1 to −0.3 V_RHE. Full spectra are shown in Figure S49a, Supporting Information. Electrolyte: pH 12, 0.1 M Na₂SO₄ + 8 mM NaNO₃. The NO₃^–, NO₂^–, NH₂OH and NH₃ related peaks are labeled with red, violet, orange, and blue dotted lines, respectively. The asterisk denoted the atoms adsorbed on the catalyst surface. Peaks at 983 and 1065 cm^–1 correspond to SO₄^2– and CO₃^2– in solution (labeled with a black dotted line), (34,36) respectively, as they always appeared in the electrolyte containing Na₂SO₄ (pH 12). (b) In situ Raman spectra in the range of 1100–1250 cm^–1 during NO₃RR (left, pH 12, 0.1 M Na₂SO₄ + 8 mM NaNO₃), NO₂RR (middle, pH 12, 0.1 M Na₂SO₄ + 8 mM NaNO₂), NH₂OH reduction (right, pH 12, 0.1 M Na₂SO₄ + 8 mM NH₂OH). The NH₂OH related peaks are labeled with orange dashed lines. The intensity of NO₃RR Raman spectra is manually increased five times for clarity.
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Figure 6
Figure 6. Active species and possible reaction mechanisms. (a) Schematics of the structural and oxidation state change of the catalyst under NO₃RR/NO₂RR conditions. (b) Proposed mechanisms for NO₃RR/NO₂RR to ammonia over Cu₂O NCs. The species that been determined based on experimental evidence (either detected by in situ Raman or deduced from electrokinetic analysis) are in bold. The left side (yellow background) correspond to reaction conditions where Cu(I)/Cu(0) species are present, while on the right-hand side (red background) the catalyst is mostly metallic.
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References
This article references 70 other publications.
1. 1
  Qing, G.; Ghazfar, R.; Jackowski, S. T.; Habibzadeh, F.; Ashtiani, M. M.; Chen, C. P.; Smith, M. R.; Hamann, T. W. Recent Advances and Challenges of Electrocatalytic N₂ Reduction to Ammonia. Chem. Rev. 2020, 120 (12), 5437– 5516, DOI: 10.1021/acs.chemrev.9b00659
  1
  Recent Advances and Challenges of Electrocatalytic N2 Reduction to Ammonia
  Qing, Geletu; Ghazfar, Reza; Jackowski, Shane T.; Habibzadeh, Faezeh; Ashtiani, Mona Maleka; Chen, Chuan-Pin; Smith, Milton R.; Hamann, Thomas W.
  Chemical Reviews (Washington, DC, United States) (2020), 120 (12), 5437-5516CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
  A review. Global NH3 prodn. reached 175 million metric tons in 2016, 90% of which is produced from high purity N2 and H2 gases at high temps. and pressures via the Haber-Bosch process. Reliance on natural gas for H2 prodn. results in large energy consumption and CO2 emissions. Concerns of human-induced climate change are spurring an international scientific effort to explore new approaches to NH3 prodn. and reduce its C footprint. Electrocatalytic N2 redn. to NH3 is an attractive alternative that can potentially enable NH3 synthesis under milder conditions in small-scale, distributed, and on-site electrolysis cells powered by renewable electricity generated from solar or wind sources. This review provides a comprehensive account of theor. and exptl. studies on electrochem. N fixation with a focus on the low selectivity for redn. of N2 to NH3 vs. protons to H2. A detailed introduction to NH3 detection methods and the execution of control expts. is given as they are crucial to the accurate reporting of exptl. findings. The main part of this review focuses on theor. and exptl. progress that was achieved under a range of conditions. Finally, comments on current challenges and potential opportunities in this field are provided.
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2. 2
  Wang, M.; Khan, M. A.; Mohsin, I.; Wicks, J.; Ip, A. H.; Sumon, K. Z.; Dinh, C.-T.; Sargent, E. H.; Gates, I. D.; Kibria, M. G. Can sustainable ammonia synthesis pathways compete with fossil-fuel based Haber-Bosch processes?. Energy Environ. Sci. 2021, 14 (5), 2535– 2548, DOI: 10.1039/D0EE03808C
  2
  Can sustainable ammonia synthesis pathways compete with fossil-fuel based Haber-Bosch processes?
  Wang, Miao; Khan, Mohd A.; Mohsin, Imtinan; Wicks, Joshua; Ip, Alexander H.; Sumon, Kazi Z.; Dinh, Cao-Thang; Sargent, Edward H.; Gates, Ian D.; Kibria, Md Golam
  Energy & Environmental Science (2021), 14 (5), 2535-2548CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
  As renewable electricity prices continue to decline, interest grows in alternative routes for the synthesis of sustainable fuels and chems., including ammonia. Considering demand for fertilizers, as well as its future potential as a dispatchable energy vector, sustainable synthesis of ammonia is being explored as an alternative to the capital- and carbon-intensive fossil-fuel-driven Haber-Bosch process. Here we assess stages along a transition to the sustainable synthesis of ammonia, looking at economic feasibility and climate impacts compared to the incumbent Haber-Bosch without and with CO2 capture. This anal. enables us to suggest technol. thresholds for sustainable synthesis of ammonia to become economically and environmentally favorable. When driven by renewable energy sources, the water electrolyzer (near $400 per kW) coupled Haber-Bosch process will reach cost parity near 2.5 cents per kWh electricity. In the case of direct electrochem. ammonia synthesis, achieving cost-parity using the same 2.5 cents per kWh electricity will rely on achieving major advances in performance: an electrolysis full-cell energy efficiency exceeding 40% at a c.d. of 0.5 A cm-2. Once this operating performance is reached, elec.-powered ammonia synthesis will bring climate benefits when coupled with low-carbon electricity (<180 gCO2e per kWh), achievable when over half of today's U.S. electricity generation is supplied by renewable energy sources. We conclude with a forward-looking perspective on the key challenges and opportunities for sustainable ammonia synthesis routes to be competitive with the incumbent Haber-Bosch process in the future.
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3. 3
  Wang, J.; Feng, T.; Chen, J.; Ramalingam, V.; Li, Z.; Kabtamu, D. M.; He, J.-H.; Fang, X. Electrocatalytic nitrate/nitrite reduction to ammonia synthesis using metal nanocatalysts and bio-inspired metalloenzymes. Nano Energy 2021, 86, 106088, DOI: 10.1016/j.nanoen.2021.106088
  3
  Electrocatalytic nitrate/nitrite reduction to ammonia synthesis using metal nanocatalysts and bio-inspired metalloenzymes
  Wang, Jing; Feng, Tao; Chen, Jiaxin; Ramalingam, Vinoth; Li, Zhongxiao; Kabtamu, Daniel Manaye; He, Jr-Hau; Fang, Xiaosheng
  Nano Energy (2021), 86 (), 106088CODEN: NEANCA; ISSN:2211-2855. (Elsevier Ltd.)
  A review. Ammonia (NH3) is attracted as a potential carbon free energy carrier and as important feedstock for most of the fertilizers, chems., pharmaceutical related products. NH3 is industrially produced by conventional Haber-Bosch process under harsh exptl. conditions (high temp. and high pressure), and this process requires high-energy consumption and produces large amt. of CO2 emissions into the atm. Therefore, there is an urgent need to develop an alternative and sustainable route for NH3 prodn. under ambient conditions. Recently, electrocatalytic N2 redn. to NH3 prodn. has attracted as a potential approach, but achieving high NH3 yield and Faradaic efficiency, and avoiding competitive hydrogen-evolution reaction (HER) are still challenging. Nitrate/nitrite (NO-3/NO-2) is the widely reported contaminant for eutrophication and carcinogens, which can be utilized as a nitrogen resource for electrocatalytic NO-3/NO-2 redn. to NH3 (NRA) via eight/six-electron transfer process. Unfortunately, electrocatalytic NRA using metal nanomaterials are rarely investigated. In this review, we discuss the electrocatalytic NRA performance contg. reactivity, selectivity, Faradaic efficiency and cycling stability of metal nanocatalysts, bio-inspired metalloenzymes and bioelectrochem. system. After this overview, we investigate the key factors, rate-detg. step and the reaction mechanism that controlling the NRA performance. Finally, we summarize the challenges and future pathways guiding the design of effective nanomaterials and reaction systems to promote the industrial application of electrocatalytic NRA.
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4. 4
  Elishav, O.; Mosevitzky Lis, B.; Miller, E. M.; Arent, D. J.; Valera-Medina, A.; Grinberg Dana, A.; Shter, G. E.; Grader, G. S. Progress and Prospective of Nitrogen-Based Alternative Fuels. Chem. Rev. 2020, 120 (12), 5352– 5436, DOI: 10.1021/acs.chemrev.9b00538
  4
  Progress and Prospective of Nitrogen-Based Alternative Fuels
  Elishav, Oren; Mosevitzky Lis, Bar; Miller, Elisa M.; Arent, Douglas J.; Valera-Medina, Agustin; Grinberg Dana, Alon; Shter, Gennady E.; Grader, Gideon S.
  Chemical Reviews (Washington, DC, United States) (2020), 120 (12), 5352-5436CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
  A review. Alternative fuels are essential to enable the transition to a sustainable and environmentally friendly energy supply. Synthetic fuels derived from renewable energies can act as energy storage media, thus mitigating the effects of fossil fuels on environment and health. Their economic viability, environmental impact, and compatibility with current infrastructure and technologies are fuel and power source specific. Nitrogen-based fuels pose one possible synthetic fuel pathway. In this , we discuss the progress and current research on utilization of nitrogen-based fuels in power applications, covering the complete fuel cycle. We cover the prodn., distribution, and storage of nitrogen-based fuels. We assess much of the existing literature on the reactions involved in the ammonia to nitrogen atom pathway in nitrogen-based fuel combustion. Furthermore, we discuss nitrogen-based fuel applications ranging from combustion engines to gas turbines, as well as their exploitation by suggested end-uses. Thereby, we evaluate the potential opportunities and challenges of expanding the role of nitrogen-based mols. in the energy sector, outlining their use as energy carriers in relevant fields.
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5. 5
  Wu, S.; Salmon, N.; Li, M. M.-J.; Bañares-Alcántara, R.; Tsang, S. C. E. Energy Decarbonization via Green H₂ or NH₃?. ACS Energy Lett. 2022, 7 (3), 1021– 1033, DOI: 10.1021/acsenergylett.1c02816
  5
  Energy Decarbonization via Green H2 or NH3?
  Wu, Simson; Salmon, Nicholas; Li, Molly Meng-Jung; Banares-Alcantara, Rene; Tsang, Shik Chi Edman
  ACS Energy Letters (2022), 7 (3), 1021-1033CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
  A review. Due to the global need to decarbonize, it is increasingly clear that we are heading toward a new era of massive renewable energy generation and utilization to substitute fossil fuels. However, pragmatic means for renewable energy storage, transportation, and utilization at a very large scale are not yet fully developed and require careful scrutiny. In this Focus Review, we will consider the thermodn., kinetic barriers, material challenges, current science and technol. readiness, and geog. locations for green hydrogen vs. green ammonia as energy vectors for suitable sustainability of our future economy.
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6. 6
  Wang, Z.; Richards, D.; Singh, N. Recent discoveries in the reaction mechanism of heterogeneous electrocatalytic nitrate reduction. Catal. Sci. Technol. 2021, 11 (3), 705– 725, DOI: 10.1039/D0CY02025G
  6
  Recent discoveries in the reaction mechanism of heterogeneous electrocatalytic nitrate reduction
  Wang, Zixuan; Richards, Danielle; Singh, Nirala
  Catalysis Science & Technology (2021), 11 (3), 705-725CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
  A review. The importance of maintaining a balanced N cycle and rectifying the accumulation of nitrate in H2O streams creates a need for technologies that can convert nitrate species. Heterogeneous electrocatalytic nitrate redn. (NO3RR) is a promising technol. because it can use renewable electricity to convert nitrate to N or NH3 without chem. reductants, H gas, or the prodn. of biol. waste. This review discusses the fundamental mechanism of nitrate redn. by exploring the rate-detg. conversion of nitrate to nitrite and NO, and the selectivity pathways that lead to NH3, N gas, and N oxides. Addnl., it explores several important techniques for evaluating nitrate redn. electrocatalysts, including methods to quantify the electrochem. active surface area and the product selectivity. This review highlights the activity and selectivity trends for electrocatalysts which include metals, alloys, and more recent work on sulfides, oxysulfides, oxides, phosphides, and N-doped materials. Factors that influence the reactivity and selectivity trends, such as adsorption energy of the intermediates, reaction conditions, and surface contaminants are also discussed. Overall, this review shows how mechanistic studies led to a better understanding of NO3RR. Targeted and controlled NO3RR studies can lead to better synergy between exptl. and computational results, predictive mechanistic understanding, and discovery of new NO3RR electrocatalysts.
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7. 7
  Rosca, V.; Duca, M.; de Groot, M. T.; Koper, M. T. Nitrogen cycle electrocatalysis. Chem. Rev. 2009, 109 (6), 2209– 2244, DOI: 10.1021/cr8003696
  7
  Nitrogen Cycle Electrocatalysis
  Rosca, Victor; Duca, Matteo; de Groot, Matheus T.; Koper, Marc T. M.
  Chemical Reviews (Washington, DC, United States) (2009), 109 (6), 2209-2244CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
  A review. Electrocatalytic reactions of inorg. nitrogen compds. are discussed from ammonia oxidn. to nitrate redn. Emphasis was focused on mechanisms suggested for electrocatalysis by (modified) metal, often Pt, electrodes with brief comparisons by way of the same reactions catalyzed by enzymes. Hydrazine oxidn., hydroxylamine oxidn. and redn., nitrogen redn., nitrous oxide redn., nitric oxide redn. and oxidn., nitrite and nitrous acid redn. and oxidn., etc. are discussed.
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8. 8
  Duca, M.; Koper, M. T. M. Powering denitrification: the perspectives of electrocatalytic nitrate reduction. Energy Environ. Sci. 2012, 5 (12), 9726– 9742, DOI: 10.1039/c2ee23062c
  8
  Powering denitrification: the perspectives of electrocatalytic nitrate reduction
  Duca, Matteo; Koper, Marc T. M.
  Energy & Environmental Science (2012), 5 (12), 9726-9742CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
  A review is given. The electrocatalytic removal of nitrate from polluted water is a promising alternative to bacterial denitrification, provided that full selectivity to harmless N, which can be returned to the atm., is achieved. This perspective article discusses the state-of-the-art of research on electrocatalytic denitrification, critically evaluating the obstacles still hampering large-scale application of this technique. The milestones of fundamental research focusing on the cathode reaction is 1st dealt with, followed by their translation into electrochem. reactors of practical interest. A short foray into the novel field of bioelectrochem. reactors is presented. Challenges and opportunities pertaining to these 3 topics are analyzed.
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9. 9
  Diaz, R. J.; Rosenberg, R. Spreading Dead Zones and Consequences for Marine Ecosystems. Science 2008, 321 (5891), 926– 929, DOI: 10.1126/science.1156401
  9
  Spreading Dead Zones and Consequences for Marine Ecosystems
  Diaz, Robert J.; Rosenberg, Rutger
  Science (Washington, DC, United States) (2008), 321 (5891), 926-929CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
  A review concerning spreading dead zones (hypoxia; extremely low dissolved O2 concns.) in coastal seawater and serious consequences for marine ecosystems is given. Topics discussed include: global nature of eutrophication-induced hypoxia; degree of hypoxia; progression of hypoxia; ecosystem responses; missing biomass; recovery; and prospects for change.
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10. 10
  Galloway, J. N.; Aber, J. D.; Erisman, J. W.; Seitzinger, S. P.; Howarth, R. W.; Cowling, E. B.; Cosby, B. J. The nitrogen cascade. Bioscience 2003, 53 (4), 341– 356, DOI: 10.1641/0006-3568(2003)053[0341:TNC]2.0.CO;2
  There is no corresponding record for this reference.
11. 11
  Manassaram, D. M.; Backer, L. C.; Moll, D. M. A review of nitrates in drinking water: maternal exposure and adverse reproductive and developmental outcomes. Environ. Health Perspect. 2006, 114 (3), 320– 327, DOI: 10.1289/ehp.8407
  There is no corresponding record for this reference.
12. 12
  Mensinga, T. T.; Speijers, G. J.; Meulenbelt, J. Health implications of exposure to environmental nitrogenous compounds. Toxicol. Rev. 2003, 22 (1), 41– 51, DOI: 10.2165/00139709-200322010-00005
  There is no corresponding record for this reference.
13. 13
  Lim, J.; Fernández, C. A.; Lee, S. W.; Hatzell, M. C. Ammonia and Nitric Acid Demands for Fertilizer Use in 2050. ACS Energy Lett. 2021, 6 (10), 3676– 3685, DOI: 10.1021/acsenergylett.1c01614
  13
  Ammonia and Nitric Acid Demands for Fertilizer Use in 2050
  Lim, Jeonghoon; Fernandez, Carlos A.; Lee, Seung Woo; Hatzell, Marta C.
  ACS Energy Letters (2021), 6 (10), 3676-3685CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
  A review. Access to nitrogen-based fertilizers is crit. to maximize agricultural yield, as nitrogen is the most common rate-limiting nutrient. Nearly all nitrogen-based fertilizers rely on ammonia and nitric acid as feedstocks, and thus the demand for these chems. is heavily dependent on the global population and food demand. Over the next three decades, the global population will continue to dictate the market size and value of ammonia and nitric acid, which consequently will have a significant impact on our energy infrastructure. Here, we discuss the potential for carbon-free electrocatalytic nitrogen redn., nitrogen oxidn., and nitrate redn. to meet fertilizer manufg. demands. We also explore various growth scenarios to predict the 2050 market size and value for ammonia and nitric acid. We highlight that if the current approaches for manufg. ammonia and nitric acid remain const., carbon emissions from the prodn. of fixed fertilizer feedstocks could exceed 1300 MtCO2eq/yr, prompting a strong need for green alternatives.
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14. 14
  Katsounaros, I. On the assessment of electrocatalysts for nitrate reduction. Curr. Opin. Electrochem. 2021, 28, 100721, DOI: 10.1016/j.coelec.2021.100721
  14
  On the assessment of electrocatalysts for nitrate reduction
  Katsounaros, Ioannis
  Current Opinion in Electrochemistry (2021), 28 (), 100721CODEN: COEUCY; ISSN:2451-9111. (Elsevier B.V.)
  A review. The electrochem. redn. of nitrate is attracting attention in the context of producing ammonia, besides the traditional removal to harmless N2. To make progress in this complex reaction and facilitate the search for active and selective catalysts, we need to establish generalized testing protocols which will enable to compare and complement data from different labs. The purpose of this article is to raise awareness on the importance of (i) soln. processes that involve products of the electrode reaction, (ii) detn. of products with appropriate, product-specific quant. methods, (iii) the strong sensitivity of the reaction on exptl. parameters, (iv) the cell design for the sepn. of anode from cathode processes, and (v) the increase in the interfacial and soln. pH that occurs during the electrolysis at high current densities.
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15. 15
  van Langevelde, P. H.; Katsounaros, I.; Koper, M. T. M. Electrocatalytic Nitrate Reduction for Sustainable Ammonia Production. Joule 2021, 5 (2), 290– 294, DOI: 10.1016/j.joule.2020.12.025
  There is no corresponding record for this reference.
16. 16
  Zhang, X.; Wang, Y.; Liu, C.; Yu, Y.; Lu, S.; Zhang, B. Recent advances in non-noble metal electrocatalysts for nitrate reduction. Chem. Eng. J. 2021, 403, 126269, DOI: 10.1016/j.cej.2020.126269
  16
  Recent advances in non-noble metal electrocatalysts for nitrate reduction
  Zhang, Xi; Wang, Yuting; Liu, Cuibo; Yu, Yifu; Lu, Siyu; Zhang, Bin
  Chemical Engineering Journal (Amsterdam, Netherlands) (2021), 403 (), 126269CODEN: CMEJAJ; ISSN:1385-8947. (Elsevier B.V.)
  A review. Nitrate pollution has become a serious global problem, threatening human health and ecosystems. The electrochem. redn. has emerged as an energy-efficient and environmental-friendly technol. to remove nitrate from H2O. Recently, nonnoble metal electrocatalysts have attracted increasing attention in nitrate redn. due to their great advantages in terms of low cost, high activity, and large-scale application potential. This review highlights the latest research progress in the area of nonnoble metal materials for electrochem. nitrate redn. The mechanistic insight into the electrochem. redn. of nitrate is briefly discussed. Meanwhile, numerous examples in this field are collected and analyzed. Some strategies employed to improve the performance of nitrate electroredn. are also presented. Finally, the challenges and prospects in this field are discussed. This review hopes to guide the design and development of efficient nonnoble metal electrocatalysts for nitrate redn. on a large scale.
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17. 17
  Lu, X.; Song, H.; Cai, J.; Lu, S. Recent development of electrochemical nitrate reduction to ammonia: A mini review. Electrochem. Commun. 2021, 129, 107094, DOI: 10.1016/j.elecom.2021.107094
  17
  Recent development of electrochemical nitrate reduction to ammonia: A mini review
  Lu, Xingmei; Song, Haoqiang; Cai, Jinmeng; Lu, Siyu
  Electrochemistry Communications (2021), 129 (), 107094CODEN: ECCMF9; ISSN:1388-2481. (Elsevier B.V.)
  A review. Nitrate (NO3-) pollution has become increasingly prominent due to industry and agriculture. Electrochem. redn. can convert NO-3 into high value-added ammonia (NH3) and remove NO3- pollution. This review focuses on the latest research progress in the field of electrochem. nitrate redn. reaction (NO3-RR) to NH3 . The mechanism of NO3-RR is briefly discussed. Catalysts, as well as qual. and quant. methods for the detection of NH3 are also summarized. Finally, the challenges and prospects in this field are discussed. We hope this mini review aids researchers in the design and development of advanced research strategies for electrochem. NO3--to-NH3 processes.
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18. 18
  Wang, Y.; Xu, A.; Wang, Z.; Huang, L.; Li, J.; Li, F.; Wicks, J.; Luo, M.; Nam, D. H.; Tan, C. S.; Ding, Y.; Wu, J.; Lum, Y.; Dinh, C. T.; Sinton, D.; Zheng, G.; Sargent, E. H. Enhanced Nitrate-to-Ammonia Activity on Copper-Nickel Alloys via Tuning of Intermediate Adsorption. J. Am. Chem. Soc. 2020, 142 (12), 5702– 5708, DOI: 10.1021/jacs.9b13347
  18
  Enhanced Nitrate-to-Ammonia Activity on Copper-Nickel Alloys via Tuning of Intermediate Adsorption
  Wang, Yuhang; Xu, Aoni; Wang, Ziyun; Huang, Linsong; Li, Jun; Li, Fengwang; Wicks, Joshua; Luo, Mingchuan; Nam, Dae-Hyun; Tan, Chih-Shan; Ding, Yu; Wu, Jiawen; Lum, Yanwei; Dinh, Cao-Thang; Sinton, David; Zheng, Gengfeng; Sargent, Edward H.
  Journal of the American Chemical Society (2020), 142 (12), 5702-5708CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Electrochem. conversion of nitrate (NO3-) into NH3 (NH3) recycles N and offers a route to the prodn. of NH3, which is more valuable than dinitrogen gas. However, today's development of NO3- electroredn. remains hindered by the lack of a mechanistic picture of how catalyst structure may be tuned to enhance catalytic activity. Here the authors demonstrate enhanced NO3- redn. reaction (NO3-RR) performance on Cu50Ni50 alloy catalysts, including a 0.12 V upshift in the half-wave potential and a 6-fold increase in activity compared to those obtained with pure Cu at 0 V vs. reversible H electrode (RHE). Ni alloying enables tuning of the Cu d-band center and modulates the adsorption energies of intermediates such as *NO3-, *NO2, and *NH2. Using d. functional theory calcns., the authors identify a NO3-RR-to-NH3 pathway and offer an adsorption energy-activity relation for the CuNi alloy system. This correlation between catalyst electronic structure and NO3-RR activity offers a design platform for further development of NO3-RR catalysts.
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19. 19
  Mattarozzi, L.; Cattarin, S.; Comisso, N.; Gerbasi, R.; Guerriero, P.; Musiani, M.; Vázquez-Gómez, L.; Verlato, E. Electrodeposition of Compact and Porous Cu-Zn Alloy Electrodes and Their Use in the Cathodic Reduction of Nitrate. J. Electrochem. Soc. 2015, 162 (6), D236– D241, DOI: 10.1149/2.1041506jes
  19
  Electrodeposition of Compact and Porous Cu-Zn Alloy Electrodes and Their Use in the Cathodic Reduction of Nitrate
  Mattarozzi, Luca; Cattarin, Sandro; Comisso, Nicola; Gerbasi, Rosalba; Guerriero, Paolo; Musiani, Marco; Vazquez-Gomez, Lourdes; Verlato, Enrico
  Journal of the Electrochemical Society (2015), 162 (6), D236-D241CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
  Compact and porous Cu-Zn alloys were electrodeposited from citrate baths. The latter deposits, consisting of a spongy material with a macroscopic interconnected porosity (pore diam. of tens of microns), were obtained at large current densities (-3 A cm-2) causing strong hydrogen evolution. Porous deposits with compns. between Cu62Zn38 and Cu91Zn9 were obtained by varying the ions concns. in the deposition baths, with optimal morphol. for the Cu70Zn30 compn. The lattice parameter of the samples, estd. from XRD data, showed in the explored range a linear dependence on compn., consistent with formation of a solid soln. of Zn in Cu. The alloy materials were tested in nitrate redn. in alkali by voltammetry, chronoamperometry and const. potential electrolysis. Their performances were compared with those of similar (compact and porous) Cu electrodes. The nitrate redn. current decayed rapidly at compact Cu electrodes and was stable at the other electrodes, being the highest at porous Cu70Zn30. In all cases, the main product was ammonia.
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20. 20
  Yin, H.; Chen, Z.; Xiong, S.; Chen, J.; Wang, C.; Wang, R.; Kuwahara, Y.; Luo, J.; Yamashita, H.; Peng, Y.; Li, J. Alloying effect-induced electron polarization drives nitrate electroreduction to ammonia. Chem Catal. 2021, 1 (5), 1088– 1103, DOI: 10.1016/j.checat.2021.08.014
  20
  Alloying effect-induced electron polarization drives nitrate electroreduction to ammonia
  Yin, Haibo; Chen, Zhen; Xiong, Shangchao; Chen, Jianjun; Wang, Chizhong; Wang, Rong; Kuwahara, Yasutaka; Luo, Jingshan; Yamashita, Hiromi; Peng, Yue; Li, Junhua
  Chem Catalysis (2021), 1 (5), 1088-1103CODEN: CCHAE9; ISSN:2667-1093. (Elsevier Inc.)
  Electrocatalytic conversion of nitrate (NO-3) to ammonia (NH3) holds significant potential in the control of nitrogen oxide (NOx) from stationary sources. However, previous studies on reaction intermediates remain unclear. Here we report that PdCu/Cu2O hybrids with mesoporous hollow sphere structure show high selectivity (96.70%) and Faradaic efficiency (94.32%) for NH3 synthesis from NO-3. Detailed characterizations demonstrate that (1) Pd enables electron transfer (Pd 3d → Cu 3d) and causes the polarization of Cu 3d orbitals by forming partial PdCu alloys, which makes Pd electron deficient but offers empty orbits to adsorb NO-3, and (2) electron-rich Cu is more conducive to the occurrence of NO-3 redn. The mutual confirmation of online differential electrochem. mass spectrometry and d. functional theory calcns. demonstrates that PdCu alloys block the generation of *NOH intermediate and facilitate the formation of *N, providing a new mechanism for NH3 synthesis from NO-3 redn. reactions.
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21. 21
  Chen, F. Y.; Wu, Z. Y.; Gupta, S.; Rivera, D. J.; Lambeets, S. V.; Pecaut, S.; Kim, J. Y. T.; Zhu, P.; Finfrock, Y. Z.; Meira, D. M.; King, G.; Gao, G.; Xu, W.; Cullen, D. A.; Zhou, H.; Han, Y.; Perea, D. E.; Muhich, C. L.; Wang, H. Efficient conversion of low-concentration nitrate sources into ammonia on a Ru-dispersed Cu nanowire electrocatalyst. Nat. Nanotechnol. 2022, 17 (7), 759– 767, DOI: 10.1038/s41565-022-01121-4
  21
  Efficient conversion of low-concentration nitrate sources into ammonia on a Ru-dispersed Cu nanowire electrocatalyst
  Chen, Feng-Yang; Wu, Zhen-Yu; Gupta, Srishti; Rivera, Daniel J.; Lambeets, Sten V.; Pecaut, Stephanie; Kim, Jung Yoon Timothy; Zhu, Peng; Finfrock, Y. Zou; Meira, Debora Motta; King, Graham; Gao, Guanhui; Xu, Wenqian; Cullen, David A.; Zhou, Hua; Han, Yimo; Perea, Daniel E.; Muhich, Christopher L.; Wang, Haotian
  Nature Nanotechnology (2022), 17 (7), 759-767CODEN: NNAABX; ISSN:1748-3387. (Nature Portfolio)
  Electrochem. converting nitrate ions, a widely distributed nitrogen source in industrial wastewater and polluted groundwater, into ammonia represents a sustainable route for both wastewater treatment and ammonia generation. However, it is currently hindered by low catalytic activities, esp. under low nitrate concns. Here we report a high-performance Ru-dispersed Cu nanowire catalyst that delivers an industrial-relevant nitrate redn. current of 1 A cm-2 while maintaining a high NH3 Faradaic efficiency of 93%. More importantly, this high nitrate-redn. catalytic activity enables over a 99% nitrate conversion into ammonia, from an industrial wastewater level of 2,000 ppm to a drinkable water level <50 ppm, while still maintaining an over 90% Faradaic efficiency. Coupling the nitrate redn. effluent stream with an air stripping process, we successfully obtained high purity solid NH4Cl and liq. NH3 soln. products, which suggests a practical approach to convert wastewater nitrate into valuable ammonia products. D. functional theory calcns. reveal that the highly dispersed Ru atoms provide active nitrate redn. sites and the surrounding Cu sites can suppress the main side reaction, the hydrogen evolution reaction.
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22. 22
  Liu, H.; Lang, X.; Zhu, C.; Timoshenko, J.; Ruscher, M.; Bai, L.; Guijarro, N.; Yin, H.; Peng, Y.; Li, J.; Liu, Z.; Wang, W.; Cuenya, B. R.; Luo, J. Efficient Electrochemical Nitrate Reduction to Ammonia with Copper-Supported Rhodium Cluster and Single-Atom Catalysts. Angew. Chem., Int. Ed. 2022, 61 (23), e202202556 DOI: 10.1002/anie.202202556
  22
  Efficient Electrochemical Nitrate Reduction to Ammonia with Copper-Supported Rhodium Cluster and Single-Atom Catalysts
  Liu, Huimin; Lang, Xiuyao; Zhu, Chao; Timoshenko, Janis; Ruescher, Martina; Bai, Lichen; Guijarro, Nestor; Yin, Haibo; Peng, Yue; Li, Junhua; Liu, Zheng; Wang, Weichao; Cuenya, Beatriz Roldan; Luo, Jingshan
  Angewandte Chemie, International Edition (2022), 61 (23), e202202556CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
  The electrochem. nitrate redn. reaction (NITRR) provides a promising soln. for restoring the imbalance in the global nitrogen cycle while enabling a sustainable and decentralized route to source ammonia. Here, we demonstrate a novel electrocatalyst for NITRR consisting of Rh clusters and single-atoms dispersed onto Cu nanowires (NWs), which delivers a partial c.d. of 162 mA cm-2 for NH3 prodn. and a Faradaic efficiency (FE) of 93% at -0.2 V vs. RHE. The highest ammonia yield rate reached a record value of 1.27 mmol h-1 cm-2. Detailed investigations by ESR, in situ IR spectroscopy, differential electrochem. mass spectrometry and d. functional theory modeling suggest that the high activity originates from the synergistic catalytic cooperation between Rh and Cu sites, whereby adsorbed hydrogen on Rh site transfers to vicinal *NO intermediate species adsorbed on Cu promoting the hydrogenation and ammonia formation.
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23. 23
  Hao, R.; Tian, L.; Wang, C.; Wang, L.; Liu, Y.; Wang, G.; Li, W.; Ozin, G. A. Pollution to solution: A universal electrocatalyst for reduction of all NO_x-based species to NH₃. Chem Catal. 2022, 2 (3), 622– 638, DOI: 10.1016/j.checat.2022.01.022
  23
  Pollution to solution: A universal electrocatalyst for reduction of all NOx-based species to NH3
  Hao, Ran; Tian, Lu; Wang, Cai; Wang, Lu; Liu, Yuping; Wang, Guichang; Li, Wei; Ozin, Geoffery A.
  Chem Catalysis (2022), 2 (3), 622-638CODEN: CCHAE9; ISSN:2667-1093. (Elsevier Inc.)
  NOx-based species, including all NO, NO2, NO-2, and NO-3 nitrogen oxides, are considered major industrial pollutants responsible for numerous environmental issues. Here, a bimetallic electrocatalyst based on copper and iron is developed that enables efficient electrochem. conversion of all NOx-based species to NH3. The key to success is the tunability of the d-band energy through introduction of iron to copper, which improves the adsorption energies of reaction intermediates. Details of the reaction pathway are provided by in situ Fourier transform IR spectroscopy complemented by d. functional theory. Electrochem. prodn. of green NH3 from nitrogen oxide pollutants using renewable electricity is a sustainable soln. for a serious environmental problem.
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24. 24
  Hu, Q.; Qin, Y.; Wang, X.; Wang, Z.; Huang, X.; Zheng, H.; Gao, K.; Yang, H.; Zhang, P.; Shao, M.; He, C. Reaction intermediate-mediated electrocatalyst synthesis favors specified facet and defect exposure for efficient nitrate-ammonia conversion. Energy Environ. Sci. 2021, 14 (9), 4989– 4997, DOI: 10.1039/D1EE01731D
  24
  Reaction intermediate-mediated electrocatalyst synthesis favors specified facet and defect exposure for efficient nitrate-ammonia conversion
  Hu, Qi; Qin, Yongjie; Wang, Xiaodeng; Wang, Ziyu; Huang, Xiaowan; Zheng, Hongju; Gao, Keru; Yang, Hengpan; Zhang, Peixin; Shao, Minhua; He, Chuanxin
  Energy & Environmental Science (2021), 14 (9), 4989-4997CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
  The electrochem. nitrate (NO3-) redn. reaction (NO3-RR), with much faster kinetics than the nitrogen (N2) redn., provides new opportunities to harvest ammonia (NH3) under ambient conditions. However, the NH3 prodn. rate of NO3-RR is still much inferior to that of the industrial Haber-Bosch route due to the lack of robust electrocatalysts for suppressing the hydrogen evolution reaction (HER) at large current densities. Herein, we demonstrate an electrocatalyst synthesis strategy based on the in situ electrochem. redn. of ultrathin copper-oxide nanobelts under NO3-RR conditions, which favorably exposes Cu(100) facets and abundant surface defects, thereby markedly facilitating the NO3-RR yet hindering the HER. We discover that the intermediates of NO3-RR (i.e., N*) can serve as capping agents for controlling the exposed facets during the redn. Impressively, in alk. media, the NO3-RR catalyzed by defective Cu(100) facets gives a NH3 yield rate which is 2.3-fold higher than that of the Haber-Bosch process. The synergy of Cu(100) facets and defects, which upshifts the d band center of Cu, is the key to excellent performance. The reaction intermediate-mediated strategy demonstrated in this study offers a fresh concept and robust methodol. for directional electrocatalyst synthesis to achieve markedly enhanced performance.
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25. 25
  Patil, S. B.; Liu, T. R.; Chou, H. L.; Huang, Y. B.; Chang, C. C.; Chen, Y. C.; Lin, Y. S.; Li, H.; Lee, Y. C.; Chang, Y. J.; Lai, Y. H.; Wen, C. Y.; Wang, D. Y. Electrocatalytic Reduction of NO₃^– to Ultrapure Ammonia on {200} Facet Dominant Cu Nanodendrites with High Conversion Faradaic Efficiency. J. Phys. Chem. Lett. 2021, 12 (33), 8121– 8128, DOI: 10.1021/acs.jpclett.1c02236
  25
  Electrocatalytic Reduction of NO3- to Ultrapure Ammonia on {200} Facet Dominant Cu Nanodendrites with High Conversion Faradaic Efficiency
  Patil, Shivaraj B.; Liu, Ting-Ran; Chou, Hung-Lung; Huang, Yu-Bin; Chang, Chia-Che; Chen, Yi-Chia; Lin, Ying-Sheng; Li, Hsin; Lee, Yi-Cheng; Chang, Yuan Jay; Lai, Ying-Huang; Wen, Cheng-Yen; Wang, Di-Yan
  Journal of Physical Chemistry Letters (2021), 12 (33), 8121-8128CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
  Nitrate (NO3-) redn. reaction (NtRR) is considered as a green alternative method for the conventional method of NH3 synthesis (Haber-Bosch process), which is known as a high energy consuming and large CO2 emitting process. Herein, the copper nanodendrites (Cu NDs) grown along with the {200} facet as an efficient NtRR catalyst have been successfully fabricated and investigated. It exhibited high Faradaic efficiency of 97% at low potential (-0.3 V vs RHE). Furthermore, the 15NO3- isotope labeling method was utilized to confirm the formation of NH3. Both exptl. and theor. studies showed that NtRR on the Cu metal nanostructure is a facet dependent process. Dissocn. of NO bonding is supposed to be the rate-detg. step as NtRR is a spontaneously reductive and protonation process for all the different facets of Cu. D. functional theory (DFT) calcns. revealed that Cu{200} and Cu{220} offer lower activation energy for dissocn. of NO compared to that of Cu{111}.
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26. 26
  Pérez-Gallent, E.; Figueiredo, M. C.; Katsounaros, I.; Koper, M. T. M. Electrocatalytic reduction of Nitrate on Copper single crystals in acidic and alkaline solutions. Electrochim. Acta 2017, 227, 77– 84, DOI: 10.1016/j.electacta.2016.12.147
  26
  Electrocatalytic reduction of Nitrate on Copper single crystals in acidic and alkaline solutions.
  Perez-Gallent, Elena; Figueiredo, Marta C.; Katsounaros, Ioannis; Koper, Marc T. M.
  Electrochimica Acta (2017), 227 (), 77-84CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
  Nitrate redn. on Cu (100) and Cu (111) surfaces in alk. and acidic solns. was studied by electrochem. methods (cyclic voltammetry, rotating disk electrode) coupled with online and in situ characterization techniques (mass spectrometry, ion chromatog. and Fourier transformed IR spectroscopy) to evaluate the reaction mechanism and products on the different surfaces. Electrochem. results show that redn. of nitrate in alk. media on Cu is structure sensitive. The onset potential on Cu (100) is +0.1 V vs. RHE, ∼50 mV earlier than on Cu (111). The onset potentials for nitrate redn. on Cu (100) and Cu (111) in acidic media are rather similar. Anal. techniques show a diverse product distribution for both surfaces and for both electrolytes. Whereas in acidic media both Cu electrodes show the formation of NO and NH3, in alk. media Cu reduces nitrate to nitrite and further to hydroxylamine. In alk. media, Cu (100) is a more active surface for the formation of hydroxylamine than Cu (111).
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27. 27
  Bae, S.-E.; Stewart, K. L.; Gewirth, A. A. Nitrate adsorption and reduction on Cu (100) in acidic solution. J. Am. Chem. Soc. 2007, 129 (33), 10171– 10180, DOI: 10.1021/ja071330n
  27
  Nitrate Adsorption and Reduction on Cu(100) in Acidic Solution
  Bae, Sang-Eun; Stewart, Karen L.; Gewirth, Andrew A.
  Journal of the American Chemical Society (2007), 129 (33), 10171-10180CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Nitrate adsorption and redn. on Cu(100) in acidic soln. was studied by electrochem. methods, in situ electrochem. scanning tunneling microscopy (EC-STM), surface enhanced Raman spectroscopy (SERS), and d. functional theory (DFT) calcns. Electrochem. results show that redn. of nitrate starts at -0.3 V vs. Ag/AgCl and reaches max. value at -0.58 V. Over the entire potential region interrogated adlayers composed of nitrate, nitrite, or other intermediates are obsd. by using in situ STM. From the open-circuit potential (OCP) to -0.22 V vs. Ag|AgCl, the nitrate ion is dominant and forms a (2 × 2) adlattice on the Cu(100) surface while nitrate forms a dominantly c(2 × 2) structure from -0.25 to -0.36 V. The interconversion between the nitrate and nitrite adlattices is obsd. DFT calcns. indicate that both nitrate and nitrite are 2-fold coordinated to the Cu(100) surface.
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28. 28
  Daiyan, R.; Tran-Phu, T.; Kumar, P.; Iputera, K.; Tong, Z.; Leverett, J.; Khan, M. H. A.; Asghar Esmailpour, A.; Jalili, A.; Lim, M.; Tricoli, A.; Liu, R.-S.; Lu, X.; Lovell, E.; Amal, R. Nitrate reduction to ammonium: from CuO defect engineering to waste NO_x-to-NH₃ economic feasibility. Energy Environ. Sci. 2021, 14 (6), 3588– 3598, DOI: 10.1039/D1EE00594D
  28
  Nitrate reduction to ammonium: from CuO defect engineering to waste NOx-to-NH3 economic feasibility
  Daiyan, Rahman; Tran-Phu, Thanh; Kumar, Priyank; Iputera, Kevin; Tong, Zizheng; Leverett, Joshua; Khan, Muhammad Haider Ali; Asghar Esmailpour, Ali; Jalili, Ali; Lim, Maggie; Tricoli, Antonio; Liu, Ru-Shi; Lu, Xunyu; Lovell, Emma; Amal, Rose
  Energy & Environmental Science (2021), 14 (6), 3588-3598CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
  Crit. to the feasibility of electrochem. redn. of waste NOx (NOxRR), as a sustainable pathway and to close the NOx cycle for the emerging NH3 economy, is the requirement of inexpensive, scalable and selective catalysts that can generate NH4+ with high yield, as indicated by our economic modeling. To this end, we carry out d. functional theory (DFT) calcns. to investigate the possible contribution of oxygen vacancy (OV) defects in NOxRR catalysis, discovering that an increase in defect d. within CuO is leading to a decrease in adsorption energy for NO3- reactants. Using these findings as design guidelines, we develop defective CuO nanomaterials using flame spray pyrolysis (FSP) and mild plasma treatment, that can attain a NH4+ yield of 520μmol cm-2 h-1 at a cell voltage of 2.2 V within a flow electrolyzer with good stability over 10 h of operation. Through our mechanistic investigation, we establish the beneficial role of oxygen vacancy defects (with one free electron) in CuO for NOxRR and we reveal a direct correlation of oxygen vacancy d. with the NH4+ yield, arising from improved NO3- adsorption, as evidenced from our theor. calcns. Our findings on defect engineering to improve NH4+ yield and its economic feasibility display the potential of NOxRR as an alternative pathway to generate green NH3, which can also serve as an energy vector for the emerging hydrogen economy and close the NOx cycle.
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29. 29
  Xu, Y.; Wang, M.; Ren, K.; Ren, T.; Liu, M.; Wang, Z.; Li, X.; Wang, L.; Wang, H. Atomic defects in pothole-rich two-dimensional copper nanoplates triggering enhanced electrocatalytic selective nitrate-to-ammonia transformation. J. Mater. Chem. A 2021, 9 (30), 16411– 16417, DOI: 10.1039/D1TA04743D
  29
  Atomic defects in pothole-rich two-dimensional copper nanoplates triggering enhanced electrocatalytic selective nitrate-to-ammonia transformation
  Xu, You; Wang, Mingzhen; Ren, Kaili; Ren, Tianlun; Liu, Mengying; Wang, Ziqiang; Li, Xiaonian; Wang, Liang; Wang, Hongjing
  Journal of Materials Chemistry A: Materials for Energy and Sustainability (2021), 9 (30), 16411-16417CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
  The development of efficient catalysts for electrocatalytic selective conversion of nitrate pollutants into valuable ammonia is a project of far-reaching importance. This work demonstrated the in situ electroredn. of pre-synthesized CuO nanoplates into defect-rich metallic Cu nanoplates and evaluated their electrocatalytic nitrate-to-ammonia activity. Concd. at. defects in the as-converted Cu nanoplates could favor the adsorption, enrichment, and confinement of nitrate ions and pivotal reaction intermediates, selectively promoting eight-electron redn. (NH3 formation). Consequently, the resultant defect-rich Cu nanoplates exhibit a significant ammonia prodn. rate of 781.25μg h-1 mg-1, together with excellent nitrate conversion (93.26%), high ammonia selectivity (81.99%), and good electrocatalytic stability, superior to the defect-free Cu nanoplate counterpart. Isotope labeling expts. demonstrated that the source of ammonia was from nitrate. Both 1H NMR and colorimetric methods were used to quantify the ammonia yield.
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30. 30
  Xu, Y.; Ren, K.; Ren, T.; Wang, M.; Wang, Z.; Li, X.; Wang, L.; Wang, H. Ultralow-content Pd in-situ incorporation mediated hierarchical defects in corner-etched Cu₂O octahedra for enhanced electrocatalytic nitrate reduction to ammonia. Appl. Catal., B 2022, 306 (5), 121094, DOI: 10.1016/j.apcatb.2022.121094
  There is no corresponding record for this reference.
31. 31
  Wang, Y.; Zhou, W.; Jia, R.; Yu, Y.; Zhang, B. Unveiling the Activity Origin of a Copper-based Electrocatalyst for Selective Nitrate Reduction to Ammonia. Angew. Chem., Int. Ed. 2020, 59 (13), 5350– 5354, DOI: 10.1002/anie.201915992
  31
  Unveiling the Activity Origin of a Copper-based Electrocatalyst for Selective Nitrate Reduction to Ammonia
  Wang, Yuting; Zhou, Wei; Jia, Ranran; Yu, Yifu; Zhang, Bin
  Angewandte Chemie, International Edition (2020), 59 (13), 5350-5354CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
  Unveiling the active phase of catalytic materials under reaction conditions is important for the construction of efficient electrocatalysts for selective nitrate redn. to NH3. The origin of the prominent activity enhancement for CuO (faradaic efficiency: 95.8%, Selectivity: 81.2%) toward selective nitrate electroredn. to NH3 was probed. 15N isotope labeling expts. showed that NH3 originated from nitrate redn. 1H NMR spectroscopy and colorimetric methods were performed to quantify NH3. In situ Raman and ex situ expts. revealed that CuO was electrochem. converted into Cu/Cu2O, which serves as an active phase. The combined results of online differential electrochem. mass spectrometry (DEMS) and DFT calcns. demonstrated that the electron transfer from Cu2O to Cu at the interface could facilitate the formation of *NOH intermediate and suppress the H evolution reaction, leading to high selectivity and faradaic efficiency.
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32. 32
  Yuan, J.; Xing, Z.; Tang, Y.; Liu, C. Tuning the Oxidation State of Cu Electrodes for Selective Electrosynthesis of Ammonia from Nitrate. ACS Appl. Mater. Interfaces 2021, 13 (44), 52469– 52478, DOI: 10.1021/acsami.1c10946
  There is no corresponding record for this reference.
33. 33
  Gong, Z.; Zhong, W.; He, Z.; Liu, Q.; Chen, H.; Zhou, D.; Zhang, N.; Kang, X.; Chen, Y. Regulating surface oxygen species on copper (I) oxides via plasma treatment for effective reduction of nitrate to ammonia. Appl. Catal., B 2022, 305 (5), 121021, DOI: 10.1016/j.apcatb.2021.121021
  There is no corresponding record for this reference.
34. 34
  Zhan, C.; Dattila, F.; Rettenmaier, C.; Bergmann, A.; Kuhl, S.; Garcia-Muelas, R.; Lopez, N.; Cuenya, B. R. Revealing the CO Coverage-Driven C-C Coupling Mechanism for Electrochemical CO₂ Reduction on Cu₂O Nanocubes via Operando Raman Spectroscopy. ACS Catal. 2021, 11 (13), 7694– 7701, DOI: 10.1021/acscatal.1c01478
  34
  Revealing the CO Coverage-Driven C-C Coupling Mechanism for Electrochemical CO2 Reduction on Cu2O Nanocubes via Operando Raman Spectroscopy
  Zhan, Chao; Dattila, Federico; Rettenmaier, Clara; Bergmann, Arno; Kuehl, Stefanie; Garcia-Muelas, Rodrigo; Lopez, Nuria; Cuenya, Beatriz Roldan
  ACS Catalysis (2021), 11 (13), 7694-7701CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
  Electrochem. redn. of carbon dioxide (CO2RR) is an attractive route to close the carbon cycle and potentially turn CO2 into valuable chems. and fuels. However, the highly selective generation of multicarbon products remains a challenge, suffering from poor mechanistic understanding. Herein, we used operando Raman spectroscopy to track the potential-dependent redn. of Cu2O nanocubes and the surface coverage of reaction intermediates. In particular, we discovered that the potential-dependent intensity ratio of the Cu-CO stretching band to the CO rotation band follows a volcano trend similar to the CO2RR Faradaic efficiency for multicarbon products. By combining operando spectroscopic insights with D. Functional Theory, we proved that this ratio is detd. by the CO coverage and that a direct correlation exists between the potential-dependent CO coverage, the preferred C-C coupling configuration, and the selectivity to C2+ products. Thus, operando Raman spectroscopy can serve as an effective method to quantify the coverage of surface intermediates during an electrocatalytic reaction.
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35. 35
  Dima, G.; De Vooys, A.; Koper, M. Electrocatalytic reduction of nitrate at low concentration on coinage and transition-metal electrodes in acid solutions. J. Electroanal. Chem. 2003, 554–555, 15– 23, DOI: 10.1016/S0022-0728(02)01443-2
  35
  Electrocatalytic reduction of nitrate at low concentration on coinage and transition-metal electrodes in acid solutions
  Dima, G. E.; de Vooys, A. C. A.; Koper, M. T. M.
  Journal of Electroanalytical Chemistry (2003), 554-555 (), 15-23CODEN: JECHES ISSN:. (Elsevier Science B.V.)
  A comparative study was performed to det. the reactivity of nitrate ions at 0.1M on 8 different polycryst. electrodes (Pt, Pd, Rh, Ru, Ir, Cu, Ag and Au) in acidic soln. using cyclic voltammetry (CV), chronoamperometry and differential electrochem. mass spectroscopy (DEMS). Cyclic voltammetry shows that the current densities for nitrate redn. depend strongly on the nature of the electrode. The activities decrease in the order Rh>Ru>Ir>Pd and Pt for the transition-metal electrodes and in the order Cu>Ag>Au for the coinage metals. The rate-detg. step on Ru, Rh, Ir, Pt, Cu, and Ag is the redn. of nitrate to nitrite, as is evident from the Tafel slope, the kinetic reaction order in nitrate, and the anion effect. Transfer expts. with Pt suggest that chemisorbed nitric oxide is the key surface intermediate in the nitrate redn. Since online mass spectrometry (DEMS) measurements on Pt and Rh show no formation of gaseous products such as nitric oxide (NO), nitrous oxide (N2O) or N2, probably NH3 and hydroxylamine are the main products on transition-metal electrodes. This is in agreement with the known mechanism for NO redn., which forms N2O or N2 only if NO is in soln. On Cu, DEMS measurements show the prodn. of gaseous NO, which is explained by the weaker binding of NO to Cu as compared to the transition metals.
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36. 36
  Butcher, D. P.; Gewirth, A. A. Nitrate reduction pathways on Cu single crystal surfaces: Effect of oxide and Cl^–. Nano Energy 2016, 29, 457– 465, DOI: 10.1016/j.nanoen.2016.06.024
  36
  Nitrate reduction pathways on Cu single crystal surfaces: Effect of oxide and Cl-
  Butcher, Dennis P. Jr.; Gewirth, Andrew A.
  Nano Energy (2016), 29 (), 457-465CODEN: NEANCA; ISSN:2211-2855. (Elsevier Ltd.)
  The origin of different nitrate redn. activity between the (100), (111), and (110) faces of Cu is examd. using vibrational spectroscopy and calcns. Shell isolated nanoparticle enhanced Raman spectroscopy (SHINERS) reveals a suite of intermediates from the nitrate redn. process on Cu(100), Cu(111), and Cu(110) including NO-2 and HNO. All three faces show similar intermediates, suggesting the same mechanism is operative on all of them. Crit. to the redn. pathway on the bare Cu surfaces is the redn. of nitrate to nitrite concomitant with partial oxidn. of the Cu surface. This priming action facilitates nitrate redn. and reduces overpotentials, particularly on the Cu(111) and Cu(110) faces, which are more susceptible to oxidn. Decoration of the surfaces with Cl- suppresses nitrate redn., resulting in higher overpotentials and lower c.d. NH3 is obsd. by SHINERS as a direct nitrate redn. product in the presence of Cl-, rather than NOx species obsd. on the bare Cu surfaces, indicating a reaction pathway unique from the bare, undecorated surface.
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37. 37
  Schouten, K. J.; Gallent, E. P.; Koper, M. T. M. The electrochemical characterization of copper single-crystal electrodes in alkaline media. J. Electroanal. Chem. 2013, 699, 6– 9, DOI: 10.1016/j.jelechem.2013.03.018
  37
  The electrochemical characterization of copper single-crystal electrodes in alkaline media
  Schouten, Klaas Jan P.; Gallent, Elena Perez; Koper, Marc T. M.
  Journal of Electroanalytical Chemistry (2013), 699 (), 6-9CODEN: JECHES; ISSN:1873-2569. (Elsevier B.V.)
  The use of single-crystals in electrochem. requires careful characterization of the surface structure. This paper addresses the characterization of Cu single-crystals using blank cyclic voltammetry in alk. media. The adsorption and desorption of OH species in the underpotential region of Cu2O formation in alk. media occur at different potentials on Cu(111) and Cu(100), whereas OH adsorption on Cu(110) is not obsd. in this potential region. This allows for a direct distinction of the Cu(hkl) basal planes. The adsorption of OH on Cu(111) induces a reconstructed adlayer on the surface. On Cu(322), a stepped surface with 5 atom wide (111) terraces, OH adsorption is obsd. in the same potential range as on Cu(111), but on Cu(3 2 2) reconstruction does not seem to take place. This is explained by the fact that the unit cell of the reconstructed layer is much larger than the (111) terrace width of Cu(322) and, therefore, reconstruction cannot take place. Cu(911), having 5 atom wide (100) terraces, exhibits the same voltammetric features as Cu(100), but with a lower intensity. This is explained by the lower amt. of (100) terraces present on this surface.
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38. 38
  Arán-Ais, R. M.; Scholten, F.; Kunze, S.; Rizo, R.; Roldan Cuenya, B. The role of in situ generated morphological motifs and Cu(I) species in C₂₊ product selectivity during CO₂ pulsed electroreduction. Nat. Energy 2020, 5 (4), 317– 325, DOI: 10.1038/s41560-020-0594-9
  38
  The role of in situ generated morphological motifs and Cu(I) species in C2+ product selectivity during CO2 pulsed electroreduction
  Aran-Ais, Rosa M.; Scholten, Fabian; Kunze, Sebastian; Rizo, Ruben; Roldan Cuenya, Beatriz
  Nature Energy (2020), 5 (4), 317-325CODEN: NEANFD; ISSN:2058-7546. (Nature Research)
  The efficient electrochem. conversion of CO2 provides a route to fuels and feedstocks. Copper catalysts are well-known to be selective to multicarbon products, although the role played by the surface architecture and the presence of oxides is not fully understood. Here we report improved efficiency towards ethanol by tuning the morphol. and oxidn. state of the copper catalysts through pulsed CO2 electrolysis. We establish a correlation between the enhanced prodn. of C2+ products (76% ethylene, ethanol and n-propanol at -1.0 V vs. the reversible hydrogen electrode) and the presence of (100) terraces, Cu2O and defects on Cu(100). We monitored the evolution of the catalyst morphol. by anal. of cyclic voltammetry curves and ex situ at. force microscopy data, whereas the chem. state of the surface was examd. via quasi in situ XPS. We show that the continuous regeneration of defects and Cu(I) species synergistically favors C-C coupling pathways.
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39. 39
  Reyter, D.; Belanger, D.; Roue, L. Elaboration of Cu-Pd films by coelectrodeposition: application to nitrate electroreduction. J. Phys. Chem. C 2009, 113 (1), 290– 297, DOI: 10.1021/jp805484t
  39
  Elaboration of Cu-Pd Films by Coelectrodeposition: Application to Nitrate Electroreduction
  Reyter, David; Belanger, Daniel; Roue, Lionel
  Journal of Physical Chemistry C (2009), 113 (1), 290-297CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
  Nanocryst. Cu-Pd films were synthesized over a wide range of compns. by coelectrodeposition of Pd and Cu in a 1 M NaCl soln. contg. both CuCl2 and PdCl2 in various proportions. The deposition potential was fixed at -0.5 V vs. a SCE. These coatings were characterized by SEM coupled to energy dispersive x-ray anal. (SEM-EDX), XRD, and XPS. These analyses revealed a fine and homogeneous distribution of Pd and Cu within and over the whole surface of the film. Depending upon the Cu(II)/Pd(II) ratio in soln., monophased Pd-rich films (Pd95Cu5 or Pd88Cu12 alloys) or biphased films (contg. Pd80Cu20 and Cu phases in different proportions) were obtained. Theses materials were tested as electrocatalysts for nitrate redn. in alk. media. Electrochem. measurements showed that biphasic (Pd80Cu20 + Cu) materials displayed the best electrocatalytic activity toward nitrate redn. Results of prolonged electrolysis also proved that the selectivity of the modified electrodes clearly depends not only on the applied potential but also on their structure and chem. compn. At -1.3 V vs. Hg/HgO, all the electrodes (except pure Pd, which is inactive for nitrate redn.) mainly produced NH3. However, at -0.93 V vs. Hg/HgO, biphasic Cu-Pd electrode composed of 77% Pd80Cu20 + 23% Cu successfully reduced nitrate to nitrogen with a current efficiency approaching 76%.
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40. 40
  Weatherup, R. S.; Wu, C. H.; Escudero, C.; Perez-Dieste, V.; Salmeron, M. B. Environment-Dependent Radiation Damage in Atmospheric Pressure X-ray Spectroscopy. J. Phys. Chem. B 2018, 122 (2), 737– 744, DOI: 10.1021/acs.jpcb.7b06397
  40
  Environment-dependent radiation damage in atmospheric pressure x-ray spectroscopy
  Weatherup, Robert S.; Wu, Cheng Hao; Escudero, Carlos; Perez-Dieste, Virginia; Salmeron, Miquel B.
  Journal of Physical Chemistry B (2018), 122 (2), 737-744CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)
  Atm. pressure x-ray spectroscopy techniques based on soft x-ray excitation can provide powerful interface-sensitive chem. information about a solid surface immersed in a gas or liq. environment. However, x-ray illumination of such dense phases can lead to the generation of considerable quantities of radical species by radiolysis. Soft x-ray absorption measurements of Cu films in both air and aq. alkali halide solns. reveal that this can cause significant evolution of the Cu oxidn. state. In air and NaOH (0.1 M) solns., the Cu is oxidized toward CuO, while the addn. of small amts. of CH3OH to the soln. leads to redn. toward Cu2O. For Ni films in NaHCO3 solns., the oxidn. state of the surface is found to remain stable under x-ray illumination and can be electrochem. cycled between a reduced and oxidized state. We provide a consistent explanation for this behavior based on the products of x-ray-induced radiolysis in these different environments and highlight a no. of general approaches that can mitigate radiolysis effects when performing operando x-ray measurements.
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41. 41
  Möller, T.; Scholten, F.; Thanh, T. N.; Sinev, I.; Timoshenko, J.; Wang, X.; Jovanov, Z.; Gliech, M.; Roldan Cuenya, B.; Varela, A. S. Electrocatalytic CO₂ reduction on CuO_x nanocubes: tracking the evolution of chemical state, geometric structure, and catalytic selectivity using operando spectroscopy. Angew. Chem., Int. Ed. 2020, 59 (41), 17974– 17983, DOI: 10.1002/anie.202007136
  41
  Electrocatalytic CO2 Reduction on CuOx Nanocubes: Tracking the Evolution of Chemical State, Geometric Structure, and Catalytic Selectivity using Operando Spectroscopy
  Moller Tim; Thanh Trung Ngo; Wang Xingli; Jovanov Zarko; Gliech Manuel; Strasser Peter; Scholten Fabian; Timoshenko Janis; Roldan Cuenya Beatriz; Sinev Ilya; Varela Ana Sofia
  Angewandte Chemie (International ed. in English) (2020), 59 (41), 17974-17983 ISSN:.
  The direct electrochemical conversion of carbon dioxide (CO2 ) into multi-carbon (C2+ ) products still faces fundamental and technological challenges. While facet-controlled and oxide-derived Cu materials have been touted as promising catalysts, their stability has remained problematic and poorly understood. Herein we uncover changes in the chemical and morphological state of supported and unsupported Cu2 O nanocubes during operation in low-current H-Cells and in high-current gas diffusion electrodes (GDEs) using neutral pH buffer conditions. While unsupported nanocubes achieved a sustained C2+ Faradaic efficiency of around 60 % for 40 h, the dispersion on a carbon support sharply shifted the selectivity pattern towards C1 products. Operando XAS and time-resolved electron microscopy revealed the degradation of the cubic shape and, in the presence of a carbon support, the formation of small Cu-seeds during the surprisingly slow reduction of bulk Cu2 O. The initially (100)-rich facet structure has presumably no controlling role on the catalytic selectivity, whereas the oxide-derived generation of under-coordinated lattice defects, can support the high C2+ product yields.
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42. 42
  Bard, A. J.; Faulkner, L. R.; Leddy, J.; Zoski, C. G. Electrochemical Methods: Fundamentals and Applications; Wiley: New York, 1980; Vol. 2.
  There is no corresponding record for this reference.
43. 43
  Danaee, I.; Jafarian, M.; Mirzapoor, A.; Gobal, F.; Mahjani, M. G. Electrooxidation of methanol on NiMn alloy modified graphite electrode. Electrochim. Acta 2010, 55 (6), 2093– 2100, DOI: 10.1016/j.electacta.2009.11.039
  43
  Electrooxidation of methanol on NiMn alloy modified graphite electrode
  Danaee, I.; Jafarian, M.; Mirzapoor, A.; Gobal, F.; Mahjani, M. G.
  Electrochimica Acta (2010), 55 (6), 2093-2100CODEN: ELCAAV; ISSN:0013-4686. (Elsevier B.V.)
  Ni and Ni-Mn alloy modified graphite electrodes (G/Ni and G/NiMn) prepd. by galvanostatic deposition were examd. for their redox process and electrocatalytic activities towards the oxidn. of MeOH in alk. solns. The methods of cyclic voltammetry (CV), chronoamperometry (CA) and impedance spectroscopy (EIS) were employed. In CV studies, in the presence of MeOH NiMn alloy modified electrode shows a significantly higher response for MeOH oxidn. The peak current of the oxidn. of Ni hydroxide increase is followed by a decrease in the corresponding cathodic current in presence of MeOH. The anodic peak currents show linear dependency upon the square root of scan rate. This behavior is the characteristic of a diffusion controlled process. Under the CA regime the reaction followed a Cottrell-Ian behavior and the diffusion coeff. of MeOH is 4 × 10-6 cm2 s-1. A mechanism based on the electro-chem. generation of Ni3+ active sites and their subsequent consumptions by MeOH were discussed and the corresponding rate law under the control of charge transfer was developed and kinetic parameters were derived. The charge transfer resistance accessible both theor. and through the EIS were used as criteria for derivation of the rate const.
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44. 44
  Manjunatha, H.; Venkatesha, T. V.; Suresh, G. S. Electrochemical studies of LiMnPO₄ as aqueous rechargeable lithium-ion battery electrode. J. Solid State Electrochem. 2012, 16 (5), 1941– 1952, DOI: 10.1007/s10008-011-1593-3
  There is no corresponding record for this reference.
45. 45
  Nicholson, R. S.; Shain, I. Theory of stationary electrode polarography. Single scan and cyclic methods applied to reversible, irreversible, and kinetic systems. Anal. Chem. 1964, 36 (4), 706– 723, DOI: 10.1021/ac60210a007
  45
  Theory of stationary electrode polarography. Single scan and cyclic methods applied to reversible, irreversible, and kinetic systems
  Nicholson, Richard S.; Shain, Irving
  (1964), 36 (4), 706-23CODEN: ANCHAM; ISSN:0003-2700.
  A numerical method is developed for solving the integral equations obtained from the boundary value problems, and extensive data were calcd. which permit construction of stationary electrode polarograms from theory. Correlations of kinetic and exptl. parameters make it possible to develop diagnostic criteria so that unknown systems can be characterized by studying the variation of peak current, half-peak potential, or ratio of anodic to cathodic peak currents as a function of rate of voltage scan. 51 references.
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46. 46
  Simpson, B. K.; Johnson, D. C. Electrocatalysis of Nitrate Reduction at Copper-Nickel Alloy Electrodes in Acidic Media. Electroanalysis 2004, 16 (7), 532– 538, DOI: 10.1002/elan.200302790
  46
  Electrocatalysis of nitrate reduction at copper-nickel alloy electrodes in acidic media
  Simpson, Brett K.; Johnson, Dennis C.
  Electroanalysis (2004), 16 (7), 532-538CODEN: ELANEU; ISSN:1040-0397. (Wiley-VCH Verlag GmbH & Co. KGaA)
  The cathodic redn. of NO3- in 1.0 M HClO4 was studied by voltammetry at pure Ni and Cu electrodes, and three Cu-Ni alloy electrodes of varying compn., all configured as rotated disks. Voltammetric data obtained using these hydrodynamic electrodes demonstrate significantly improved activity for NO3- redn. at Cu-Ni alloy electrodes as compared to the pure Ni and Cu electrodes. This observation is explained from the synergistic benefit of different surface sites for adsorption of H-atoms, generated by cathodic discharge of H+ at Ni-sites, and adsorption of NO3- at Cu-sites on these binary alloy electrodes. Koutecky-Levich plots indicate that the cathodic response for NO3- at a Cu75Ni25 electrode corresponds to an 8-electron redn., which is consistent with prodn. of NH3. In comparison, the cathodic response at Cu50Ni50 and Cu25Ni75 electrodes corresponds to a 6-electron redn., which is consistent with prodn. of NH2OH. Flow injection data obtained using Cu50Ni50 and Cu25Ni75 electrodes with 100-μL injections exhibit detection limits for NO3- of ∼0.95 μM (∼95 pmol) and 0.60 μM (∼60 pmol), resp.
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47. 47
  Liu, Q.; Liu, Q.; Xie, L.; Ji, Y.; Li, T.; Zhang, B.; Li, N.; Tang, B.; Liu, Y.; Gao, S.; Luo, Y.; Yu, L.; Kong, Q.; Sun, X. High-Performance Electrochemical Nitrate Reduction to Ammonia under Ambient Conditions Using a FeOOH Nanorod Catalyst. ACS Appl. Mater. Interfaces 2022, 14 (15), 17312– 17318, DOI: 10.1021/acsami.2c00436
  47
  High-Performance Electrochemical Nitrate Reduction to Ammonia under Ambient Conditions Using a FeOOH Nanorod Catalyst
  Liu, Qin; Liu, Qian; Xie, Lisi; Ji, Yuyao; Li, Tingshuai; Zhang, Bing; Li, Na; Tang, Bo; Liu, Yang; Gao, Shuyan; Luo, Yonglan; Yu, Lingmin; Kong, Qingquan; Sun, Xuping
  ACS Applied Materials & Interfaces (2022), 14 (15), 17312-17318CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
  Electrocatalytic nitrate redn. is promising as an environmentally friendly process to produce high value-added ammonia with simultaneous removal of nitrate, a widespread nitrogen pollutant, for water treatment; however, efficient electrocatalysts with high selectivity are required for ammonia formation. In this work, FeOOH nanorod with intrinsic oxygen vacancy supported on carbon paper (FeOOH/CP) is proposed as a high-performance electrocatalyst for converting nitrate to ammonia at room temp. When operated in a 0.1 M phosphate-buffered saline (PBS) soln. with 0.1 M NaNO3, FeOOH/CP is able to obtain a large NH3 yield of 2419μg h-1 cm-2 and a surprisingly high Faradic efficiency of 92% with excellent stability. D. functional theory calcn. demonstrates that the potential-detg. step for nitrate redn. over FeOOH (200) is *NO2H + H+ + e- → *NO + H2O.
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48. 48
  Bai, L.; Hsu, C.-S.; Alexander, D. T. L.; Chen, H. M.; Hu, X. Double-atom catalysts as a molecular platform for heterogeneous oxygen evolution electrocatalysis. Nat. Energy 2021, 6 (11), 1054– 1066, DOI: 10.1038/s41560-021-00925-3
  48
  Double-atom catalysts as a molecular platform for heterogeneous oxygen evolution electrocatalysis
  Bai, Lichen; Hsu, Chia-Shuo; Alexander, Duncan T. L.; Chen, Hao Ming; Hu, Xile
  Nature Energy (2021), 6 (11), 1054-1066CODEN: NEANFD; ISSN:2058-7546. (Nature Portfolio)
  The oxygen evolution reaction (OER) is an essential anode reaction for the generation of fuels through water splitting or CO2 electroredn. Mixed metal oxides contg. Co, Fe or Ni have proved to be the most promising OER electrocatalysts in alk. media. However, the active sites and reaction mechanisms of these catalysts are difficult to study due to their heterogeneous nature. Here we describe a general synthesis of Co-, Fe- and Ni-contg. double-atom catalysts from their single-atom precursors via in situ electrochem. transformation. Characterization reveals mol.-like bimetallic active sites for these supported catalysts. For each catalyst, we propose a catalytic cycle; all exhibit bimetallic cooperation and follow a similar O-O bond-forming step. However, the mechanisms diverge in the site and source of OH- for O-O bond formation, as well as the order of proton and electron transfer. Our work demonstrates double-atom catalysts as an attractive platform for fundamental studies of heterogeneous OER electrocatalysts.
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49. 49
  Wang, Y.; Wang, C.; Li, M.; Yu, Y.; Zhang, B. Nitrate electroreduction: mechanism insight, in situ characterization, performance evaluation, and challenges. Chem. Soc. Rev. 2021, 50 (12), 6720– 6733, DOI: 10.1039/D1CS00116G
  49
  Nitrate electroreduction: mechanism insight, in situ characterization, performance evaluation, and challenges
  Wang, Yuting; Wang, Changhong; Li, Mengyang; Yu, Yifu; Zhang, Bin
  Chemical Society Reviews (2021), 50 (12), 6720-6733CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
  A review. Excessive nitrate ions in the environment break the natural nitrogen cycle and become a significant threat to human health. So far, many phys., chem., and biol. techniques have been developed for nitrate remediation, but most of them require high post-processing costs and rigorous treatment conditions. In contrast, nitrate electroredn. is promising because it utilizes green electrons as reductants under ambient conditions. The recognition and mastering of the nitrate reaction mechanism is the premise for the design and synthesis of efficient electrocatalysts for the selective redn. of nitrate. In this regard, this aims to provide an insight into the electrocatalytic mechanism of nitrate redn., esp. combined with in situ electrochem. characterization and theor. calcns. over different kinds of materials. Moreover, the performance evaluation parameters and std. test methods for nitrate electroredn. are summarized to screen efficient materials. Finally, an outlook on the current challenges and promising opportunities in this research area is discussed. This provides a guide for development of electrocatalysts for selective nitrate redn. with a fascinating performance and accelerates the development of sustainable nitrogen chem. and engineering.
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50. 50
  Bodappa, N.; Su, M.; Zhao, Y.; Le, J. B.; Yang, W. M.; Radjenovic, P.; Dong, J. C.; Cheng, J.; Tian, Z. Q.; Li, J. F. Early Stages of Electrochemical Oxidation of Cu(111) and Polycrystalline Cu Surfaces Revealed by in Situ Raman Spectroscopy. J. Am. Chem. Soc. 2019, 141 (31), 12192– 12196, DOI: 10.1021/jacs.9b04638
  50
  Early Stages of Electrochemical Oxidation of Cu(111) and Polycrystalline Cu Surfaces Revealed by in Situ Raman Spectroscopy
  Bodappa, Nataraju; Su, Min; Zhao, Yu; Le, Jia-Bo; Yang, Wei-Min; Radjenovic, Petar; Dong, Jin-Chao; Cheng, Jun; Tian, Zhong-Qun; Li, Jian-Feng
  Journal of the American Chemical Society (2019), 141 (31), 12192-12196CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Studying the chem. nature of the adsorbed intermediate species on well-defined Cu single crystal substrates is crucial in understanding many electrocatalytic reactions. Herein, the authors systematically study the early stages of electrochem. oxidn. of Cu(111) and polycryst. Cu surfaces in different pH electrolytes using in situ shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). On Cu(111), for the 1st time, the authors identified surface OH species which convert to chemisorbed O before forming Cu2O in alk. (0.01M KOH) and neutral (0.1M Na2SO4) electrolytes; while at the Cu(poly) surface, the authors only detected the presence of surface hydroxide. Whereas, in a strongly acidic soln. (0.1M H2SO4), sulfate replaces the hydroxyl/oxy species. This results improves the understanding of the reaction mechanisms of various electrocatalytic reactions.
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51. 51
  Niaura, G. Surface-enhanced Raman spectroscopic observation of two kinds of adsorbed OH^– ions at copper electrode. Electrochim. Acta 2000, 45 (21), 3507– 3519, DOI: 10.1016/S0013-4686(00)00434-5
  51
  Surface-enhanced Raman spectroscopic observation of two kinds of adsorbed OH- ions at copper electrode
  Niaura, G.
  Electrochimica Acta (2000), 45 (21), 3507-3519CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Science Ltd.)
  The surface of a polycryst. roughened Cu electrode in 1 M NaOH soln., was studied in situ using surface-enhanced Raman spectroscopy (SERS). Cu2O, adsorbed OH- ions, and water mols. were detected as the electrode potential was varied from open circuit value to -1.20 V vs. SHE. The vibrational spectrum of Cu2O consisted of three main peaks located at 150, 528, and 623 cm-1. The intense and narrow feature at 150 cm-1 is highly characteristic, and could be used for SER monitoring of Cu2O. Two different states of adsorbed OH- ions, giving Cu-OH vibrations around 450-470 cm-1 and 540-580 cm-1, were detected. The distinct nature of the bands was shown by opposite isotopic frequency shifts changing the solvent from H2O to D2O. The frequency of the 1st band decreased by ∼12 cm-1, while the frequency of the 2nd band increased by ∼35 cm-1 in D2O solns. These differences were explained in terms of distinct surface ligation and the formation of strong hydrogen bonds between water mols. and the 2nd type of adsorbed OH- ion. Water mols. were obsd. at the interface at an applied potential -1.20 V.
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52. 52
  Giguère, P. A.; Liu, I. Infrared spectrum, molecular structure, and thermodynamic functions of hydroxylamine. Can. J. Chem. 1952, 30 (12), 948– 962, DOI: 10.1139/v52-115
  There is no corresponding record for this reference.
53. 53
  Fang, J. Y.; Zheng, Q. Z.; Lou, Y. Y.; Zhao, K. M.; Hu, S. N.; Li, G.; Akdim, O.; Huang, X. Y.; Sun, S. G. Ampere-level current density ammonia electrochemical synthesis using CuCo nanosheets simulating nitrite reductase bifunctional nature. Nat. Commun. 2022, 13 (1), 7899, DOI: 10.1038/s41467-022-35533-6
  53
  Ampere-level current density ammonia electrochemical synthesis using CuCo nanosheets simulating nitrite reductase bifunctional nature
  Fang, Jia-Yi; Zheng, Qi-Zheng; Lou, Yao-Yin; Zhao, Kuang-Min; Hu, Sheng-Nan; Li, Guang; Akdim, Ouardia; Huang, Xiao-Yang; Sun, Shi-Gang
  Nature Communications (2022), 13 (1), 7899CODEN: NCAOBW; ISSN:2041-1723. (Nature Portfolio)
  The development of electrocatalysts capable of efficient redn. of nitrate (NO3-) to ammonia (NH3) is drawing increasing interest for the sake of low carbon emission and environmental protection. Herein, we present a CuCo bimetallic catalyst able to imitate the bifunctional nature of copper-type nitrite reductase, which could easily remove NO2- via the collaboration of two active centers. Indeed, Co acts as an electron/proton donating center, while Cu facilitates NOx- adsorption/assocn. The bio-inspired CuCo nanosheet electrocatalyst delivers a 100 ± 1% Faradaic efficiency at an ampere-level c.d. of 1035 mA cm-2 at -0.2 V vs. Reversible Hydrogen Electrode. The NH3 prodn. rate reaches a high activity of 4.8 mmol cm-2 h-1 (960 mmol gcat-1 h-1). A mechanistic study, using electrochem. in situ Fourier transform IR spectroscopy and shell-isolated nanoparticle enhanced Raman spectroscopy, reveals a strong synergy between Cu and Co, with Co sites promoting the hydrogenation of NO3- to NH3 via adsorbed *H species. The well-modulated coverage of adsorbed *H and *NO3 led simultaneously to high NH3 selectivity and yield.
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54. 54
  Figueiredo, M. C.; Souza-Garcia, J.; Climent, V.; Feliu, J. M. Nitrate reduction on Pt (111) surfaces modified by Bi adatoms. Electrochem. Commun. 2009, 11 (9), 1760– 1763, DOI: 10.1016/j.elecom.2009.07.010
  There is no corresponding record for this reference.
55. 55
  Castro, P. M.; Jagodzinski, P. W. FTIR and Raman spectra and structure of Cu(NO₃)⁺ in aqueous solution and acetone. Spectrochim. Acta, Part A 1991, 47 (12), 1707– 1720, DOI: 10.1016/0584-8539(91)80008-7
  There is no corresponding record for this reference.
56. 56
  Wang, Y. H.; Zheng, S.; Yang, W. M.; Zhou, R. Y.; He, Q. F.; Radjenovic, P.; Dong, J. C.; Li, S.; Zheng, J.; Yang, Z. L.; Attard, G.; Pan, F.; Tian, Z. Q.; Li, J. F. In situ Raman spectroscopy reveals the structure and dissociation of interfacial water. Nature 2021, 600 (7887), 81– 85, DOI: 10.1038/s41586-021-04068-z
  56
  In situ Raman spectroscopy reveals the structure and dissociation of interfacial water
  Wang, Yao-Hui; Zheng, Shisheng; Yang, Wei-Min; Zhou, Ru-Yu; He, Quan-Feng; Radjenovic, Petar; Dong, Jin-Chao; Li, Shunning; Zheng, Jiaxin; Yang, Zhi-Lin; Attard, Gary; Pan, Feng; Tian, Zhong-Qun; Li, Jian-Feng
  Nature (London, United Kingdom) (2021), 600 (7887), 81-85CODEN: NATUAS; ISSN:1476-4687. (Nature Portfolio)
  Understanding the structure and dynamic process of water at the solid-liq. interface is an extremely important topic in surface science, energy science and catalysis1-3. As model catalysts, atomically flat single-crystal electrodes exhibit well-defined surface and elec. field properties, and therefore may be used to elucidate the relationship between structure and electrocatalytic activity at the at. level4,5. Hence, studying interfacial water behavior on single-crystal surfaces provides a framework for understanding electrocatalysis6,7. However, interfacial water is notoriously difficult to probe owing to interference from bulk water and the complexity of interfacial environments8. Here, we use electrochem., in situ Raman spectroscopic and computational techniques to investigate the interfacial water on atomically flat Pd single-crystal surfaces. Direct spectral evidence reveals that interfacial water consists of hydrogen-bonded and hydrated Na+ ion water. At hydrogen evolution reaction (HER) potentials, dynamic changes in the structure of interfacial water were obsd. from a random distribution to an ordered structure due to bias potential and Na+ ion cooperation. Structurally ordered interfacial water facilitated high-efficiency electron transfer across the interface, resulting in higher HER rates. The electrolytes and electrode surface effects on interfacial water were also probed and found to affect water structure. Therefore, through local cation tuning strategies, we anticipate that these results may be generalized to enable ordered interfacial water to improve electrocatalytic reaction rates.
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57. 57
  Li, Y.; Cheng, C.; Han, S.; Huang, Y.; Du, X.; Zhang, B.; Yu, Y. Electrocatalytic Reduction of Low-Concentration Nitric Oxide into Ammonia over Ru Nanosheets. ACS Energy Lett. 2022, 7 (3), 1187– 1194, DOI: 10.1021/acsenergylett.2c00207
  There is no corresponding record for this reference.
58. 58
  Yang, J.; Qi, H.; Li, A.; Liu, X.; Yang, X.; Zhang, S.; Zhao, Q.; Jiang, Q.; Su, Y.; Zhang, L.; Li, J. F.; Tian, Z. Q.; Liu, W.; Wang, A.; Zhang, T. Potential-Driven Restructuring of Cu Single Atoms to Nanoparticles for Boosting the Electrochemical Reduction of Nitrate to Ammonia. J. Am. Chem. Soc. 2022, 144 (27), 12062– 12071, DOI: 10.1021/jacs.2c02262
  58
  Potential-Driven Restructuring of Cu Single Atoms to Nanoparticles for Boosting the Electrochemical Reduction of Nitrate to Ammonia
  Yang, Ji; Qi, Haifeng; Li, Anqi; Liu, Xiaoyan; Yang, Xiaofeng; Zhang, Shengxin; Zhao, Qiao; Jiang, Qike; Su, Yang; Zhang, Leilei; Li, Jian-Feng; Tian, Zhong-Qun; Liu, Wei; Wang, Aiqin; Zhang, Tao
  Journal of the American Chemical Society (2022), 144 (27), 12062-12071CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Restructuring is ubiquitous in thermocatalysis and of pivotal importance to identify the real active site, yet it is less explored in electrocatalysis. Herein, by using operando x-ray absorption spectroscopy in conjunction with advanced electron microscopy, the authors reveal the restructuring of the as-synthesized Cu-N4 single-atom site to the nanoparticles of ~ 5 nm during the electrochem. redn. of nitrate to NH3, a green NH3 prodn. route upon combined with the plasma-assisted oxidn. of N. The redn. of Cu2+ to Cu+ and Cu0 and the subsequent aggregation of Cu0 single atoms occurs concurrently with the enhancement of the NH3 prodn. rate, both of them are driven by the applied potential switching from 0.00 to -1.00 V vs. RHE. The max. prodn. rate of NH3 reaches 4.5 mg cm-2 h-1 (12.5 molNH3 gCu-1 h-1) with a faradaic efficiency of 84.7% at -1.00 V vs. RHE, outperforming most of the other Cu catalysts reported previously. After electrolysis, the aggregated Cu nanoparticles are reversibly disintegrated into single atoms and then restored to the Cu-N4 structure upon being exposed to an ambient atm., which masks the potential-induced restructuring during the reaction. The synchronous changes of the Cu0 percentage and the NH3 faradaic efficiency with the applied potential suggests that the Cu nanoparticles are the genuine active sites for nitrate redn. to NH3, which is corroborated with both the post-deposited Cu NP catalyst and d. functional theory calcns.
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59. 59
  Han, S.; Li, H.; Li, T.; Chen, F.; Yang, R.; Yu, Y.; Zhang, B. Ultralow overpotential nitrate reduction to ammonia via a three-step relay mechanism. Nat. Catal. 2023, 6 (5), 402– 414, DOI: 10.1038/s41929-023-00951-2
  59
  Ultralow overpotential nitrate reduction to ammonia via a three-step relay mechanism
  Han, Shuhe; Li, Hongjiao; Li, Tieliang; Chen, Fanpeng; Yang, Rong; Yu, Yifu; Zhang, Bin
  Nature Catalysis (2023), 6 (5), 402-414CODEN: NCAACP; ISSN:2520-1158. (Nature Portfolio)
  Ammonia plays a substantial role in agriculture and the next generation of carbon-free energy supply. Electrocatalytic nitrate redn. to NH3 is attractive for nitrate removal and NH3 prodn. under ambient conditions. However, the energy efficiency is limited by the high reaction overpotential. Here we propose a three-step relay mechanism composed of a spontaneous redox reaction, electrochem. redn. and electrocatalytic redn. to overcome this issue. RuxCoy alloys were designed and adopted as model catalysts. Ru15Co85 exhibits an onset potential of +0.4 V vs. reversible hydrogen electrode, and an energy efficiency of 42 ± 2%. The high performance results in a low prodn. cost of US$0.49 ± 0.02 per kg of ammonia. The high nitrate redn. performances on Ru15Fe85 and Ru15Ni85 also highlight the promising potential of the relay mechanism.
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60. 60
  Wang, H.; Guo, Y.; Li, C.; Yu, H.; Deng, K.; Wang, Z.; Li, X.; Xu, Y.; Wang, L. Cu/CuO_x In-Plane Heterostructured Nanosheet Arrays with Rich Oxygen Vacancies Enhance Nitrate Electroreduction to Ammonia. ACS Appl. Mater. Interfaces 2022, 14 (30), 34761– 34769, DOI: 10.1021/acsami.2c08534
  60
  Cu/CuOx In-Plane Heterostructured Nanosheet Arrays with Rich Oxygen Vacancies Enhance Nitrate Electroreduction to Ammonia
  Wang, Hongjing; Guo, Yanan; Li, Chunjie; Yu, Hongjie; Deng, Kai; Wang, Ziqiang; Li, Xiaonian; Xu, You; Wang, Liang
  ACS Applied Materials & Interfaces (2022), 14 (30), 34761-34769CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
  The artificial ammonia synthesis via electrochem. nitrate redn. has met increasing research interest, but it is still necessary to develop advanced catalysts with high nitrate-to-ammonia capability. Herein, we propose and demonstrate a one-step method to construct binder-free Cu foam-supported oxygen vacancy-rich Cu/CuOx in-plane heterostructured nanosheet arrays (Cu/CuOx/CF). In addn. to exposing ample active sites, the two-dimensional nanosheet morphol. greatly facilitates the mass/charge-transfer process during electrocatalysis. Besides, the in-plane heterojunctions and rich oxygen vacancies induced synergistic effect can modulate the electronic structure of active sites and thus tune the adsorption properties of the reactant intermediates and inhibit the formation of undesirable byproducts, which is conducive to the further improvement of nitrate redn. activity. As a result, these advantages endow the Cu/CuOx/CF with superior performance for ammonia synthesis via nitrate electroredn., achieving high ammonia selectivity (95.00%) and Faradaic efficiency (93.58%).
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61. 61
  Wang, Y.; Shao, M. Theoretical Screening of Transition Metal-N₄-Doped Graphene for Electroreduction of Nitrate. ACS Catal. 2022, 12 (9), 5407– 5415, DOI: 10.1021/acscatal.2c00307
  61
  Theoretical Screening of Transition Metal-N4-Doped Graphene for Electroreduction of Nitrate
  Wang, Yian; Shao, Minhua
  ACS Catalysis (2022), 12 (9), 5407-5415CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
  Electrochem. nitrate redn. reaction (NO3RR) is an advantageous conversion technol. for nitrate removal and NH3 synthesis. Single-atom catalysts, owing to their utmost metal atom use efficiency, are promising electrocatalysts for NO3RR but are rarely studied in systematic ways. A theor. screening was performed on transition metal-N4-doped graphene (TM-N4/C) as active and selective electrocatalysts for NO3RR, where detailed reaction mechanisms and activity origins are explored. Volcano plots of activity trends show that Cu- and Pt-N4/C are highly active for NO3RR following the NH3 and N2 formation pathways, resp., whose activities can be attributed to the optimal NO and N adsorptions. A contour plot of selectivity trend shows that Re- and Pt-N4/C are highly selective toward NH3 and N2 formations, resp. This work provides theor. insights into the rational design of TM-N4/C catalysts for NO3RR and opportunities for efficient nitrate removal and NH3 synthesis strategies.
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62. 62
  Niu, H.; Zhang, Z.; Wang, X.; Wan, X.; Shao, C.; Guo, Y. Theoretical Insights into the Mechanism of Selective Nitrate-to-Ammonia Electroreduction on Single-Atom Catalysts. Adv. Funct. Mater. 2020, 31 (11), 2008533, DOI: 10.1002/adfm.202008533
  There is no corresponding record for this reference.
63. 63
  Reyter, D.; Bélanger, D.; Roué, L. Study of the electroreduction of nitrate on copper in alkaline solution. Electrochim. Acta 2008, 53 (20), 5977– 5984, DOI: 10.1016/j.electacta.2008.03.048
  63
  Study of the electroreduction of nitrate on copper in alkaline solution
  Reyter, David; Belanger, Daniel; Roue, Lionel
  Electrochimica Acta (2008), 53 (20), 5977-5984CODEN: ELCAAV; ISSN:0013-4686. (Elsevier B.V.)
  The electrocatalytic activity of a Cu electrode for the electroredn. of nitrate in alk. medium was studied by linear sweep voltammetry at stationary and rotating disk electrodes. Nitrate-redn. products generated upon prolonged electrolyzes at different potentials were quantified. Adsorption phenomena assocd. with the nitrate electroredn. process were characterized by electrochem. quartz crystal microbalance (EQCM) expts. This data revealed that nitrate electroredn. process strongly depends on the applied potential. Firstly, at ∼-0.9 V vs. Hg/HgO, the electroredn. of adsorbed nitrate anions to nitrite anions was identified as the rate-detg. step of the nitrate electroredn. process. Between -0.9 and -1.1 V, nitrite is reduced to hydroxylamine. However, during long-term electrolyzes, hydroxylamine is not detected and presumably because it is rapidly reduced to NH3. At potential more neg. than -1.1 V, nitrite is reduced to NH3. At ∼-1.45 V, i.e. just before the hydrogen evolution reaction, the abrupt decrease of the cathodic current is due to the electrode poisoning by adsorbed hydrogen. During the 1st minutes of nitrate electrolysis, a decrease of the Cu electrode activity was obsd. at the 3 studied potentials (-0.9, -1.1 and -1.4 V). From polarization and EQCM measurements, this deactivation is attributed to the adsorption of nitrate-redn. products, blocking the electrode surface and slowing down the nitrate electroredn. rate. However, the Cu electrode can be reactivated by the periodic application of a square wave potential pulse at -0.5 V, which causes the desorption of poisoning species.
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64. 64
  Scholten, F.; Sinev, I.; Bernal, M.; Roldan Cuenya, B. Plasma-Modified Dendritic Cu Catalyst for CO₂ Electroreduction. ACS Catal. 2019, 9 (6), 5496– 5502, DOI: 10.1021/acscatal.9b00483
  64
  Plasma-Modified Dendritic Cu Catalyst for CO2 Electroreduction
  Scholten, Fabian; Sinev, Ilya; Bernal, Miguel; Roldan Cuenya, Beatriz
  ACS Catalysis (2019), 9 (6), 5496-5502CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
  Efficient and active catalysts with high selectivity for hydrocarbons and other valuable chems. during stable operation are crucial. We have taken advantage of low-pressure oxygen plasmas to modify dendritic Cu catalysts and were able to achieve enhanced selectivity toward C2 and C3 products. Utilizing operando spectroscopic methods such as X-ray absorption fine-structure spectroscopy (XAFS) and quasi in situ XPS, we obsd. that the initial presence of oxides in these catalysts before the reaction plays an inferior role in detg. their catalytic performance as compared to the overall catalyst morphol. This is assigned to the poor stability of the CuxO species in these materials under the conditions of electrocatalytic conversion of CO2 (CO2RR). Our findings shed light into the strong structure/chem. state-selectivity correlation in CO2RR and open venues for the rational design of more effective catalysts through plasma pretreatments.
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65. 65
  Timoshenko, J.; Roldan Cuenya, B. In Situ/Operando Electrocatalyst Characterization by X-ray Absorption Spectroscopy. Chem. Rev. 2021, 121 (2), 882– 961, DOI: 10.1021/acs.chemrev.0c00396
  65
  In Situ/Operando Electrocatalyst Characterization by X-ray Absorption Spectroscopy
  Timoshenko, Janis; Roldan Cuenya, Beatriz
  Chemical Reviews (Washington, DC, United States) (2021), 121 (2), 882-961CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
  A review. During the last decades, x-ray absorption spectroscopy (XAS) has become an indispensable method for probing the structure and compn. of heterogeneous catalysts, revealing the nature of the active sites and establishing links between structural motifs in a catalyst, local electronic structure, and catalytic properties. Here the fundamental principles of the XAS method and describe the progress in the instrumentation and data anal. approaches undertaken for deciphering x-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra are discussed. Recent usages of XAS in the field of heterogeneous catalysis, with emphasis on examples concerning electrocatalysis, will be presented. The latter is a rapidly developing field with immense industrial applications but also unique challenges in terms of the exptl. characterization restrictions and advanced modeling approaches required. This review will highlight the new insight that can be gained with XAS on complex real-world electrocatalysts including their working mechanisms and the dynamic processes taking place in a chem. reaction. More specifically, the authors will discuss applications of in situ and operando XAS to probe the catalyst's interactions with the environment (support, electrolyte, ligands, adsorbates, reaction products, and intermediates) and its structural, chem., and electronic transformations as it adapts to the reaction conditions.
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66. 66
  Ravel, B.; Newville, M. ATHENA, ARTEMIS, HEPHAESTUS: data analysis for X-ray absorption spectroscopy using IFEFFIT. J. Synchrotron Radiat. 2005, 12 (4), 537– 541, DOI: 10.1107/s0909049505012719
  66
  ATHENA, ARTEMIS, HEPHAESTUS: data analysis for x-ray absorption spectroscopy using IFEFFIT
  Ravel, B.; Newville, M.
  Journal of Synchrotron Radiation (2005), 12 (4), 537-541CODEN: JSYRES; ISSN:0909-0495. (Blackwell Publishing Ltd.)
  A software package for the anal. of x-ray absorption spectroscopy (XAS) data is presented. This package is based on the IFEFFIT library of numerical and XAS algorithms and is written in the Perl programming language using the Perl/Tk graphics toolkit. The programs described here are: (i) ATHENA, a program for XAS data processing, (ii) ARTEMIS, a program for EXAFS data anal. using theor. stds. from FEFF and (iii) HEPHAESTUS, a collection of beamline utilities based on tables of at. absorption data. These programs enable high-quality data anal. that is accessible to novices while still powerful enough to meet the demands of an expert practitioner. The programs run on all major computer platforms and are freely available under the terms of a free software license.
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67. 67
  Andersen, S. Z.; Colic, V.; Yang, S.; Schwalbe, J. A.; Nielander, A. C.; McEnaney, J. M.; Enemark-Rasmussen, K.; Baker, J. G.; Singh, A. R.; Rohr, B. A.; Statt, M. J.; Blair, S. J.; Mezzavilla, S.; Kibsgaard, J.; Vesborg, P. C. K.; Cargnello, M.; Bent, S. F.; Jaramillo, T. F.; Stephens, I. E. L.; Norskov, J. K.; Chorkendorff, I. A rigorous electrochemical ammonia synthesis protocol with quantitative isotope measurements. Nature 2019, 570 (7762), 504– 508, DOI: 10.1038/s41586-019-1260-x
  67
  A rigorous electrochemical ammonia synthesis protocol with quantitative isotope measurements
  Andersen, Suzanne Z.; Colic, Viktor; Yang, Sungeun; Schwalbe, Jay A.; Nielander, Adam C.; McEnaney, Joshua M.; Enemark-Rasmussen, Kasper; Baker, Jon G.; Singh, Aayush R.; Rohr, Brian A.; Statt, Michael J.; Blair, Sarah J.; Mezzavilla, Stefano; Kibsgaard, Jakob; Vesborg, Peter C. K.; Cargnello, Matteo; Bent, Stacey F.; Jaramillo, Thomas F.; Stephens, Ifan E. L.; Noerskov, Jens K.; Chorkendorff, Ib
  Nature (London, United Kingdom) (2019), 570 (7762), 504-508CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
  The electrochem. synthesis of ammonia from nitrogen under mild conditions using renewable electricity is an attractive alternative1-4 to the energy-intensive Haber-Bosch process, which dominates industrial ammonia prodn. However, there are considerable scientific and tech. challenges5,6 facing the electrochem. alternative, and most exptl. studies reported so far have achieved only low selectivities and conversions. The amt. of ammonia produced is usually so small that it cannot be firmly attributed to electrochem. nitrogen fixation7-9 rather than contamination from ammonia that is either present in air, human breath or ion-conducting membranes9, or generated from labile nitrogen-contg. compds. (for example, nitrates, amines, nitrites and nitrogen oxides) that are typically present in the nitrogen gas stream10, in the atm. or even in the catalyst itself. Although these sources of exptl. artifacts are beginning to be recognized and managed11,12, concerted efforts to develop effective electrochem. nitrogen redn. processes would benefit from benchmarking protocols for the reaction and from a standardized set of control expts. designed to identify and then eliminate or quantify the sources of contamination. Here we propose a rigorous procedure using 15N2 that enables us to reliably detect and quantify the electrochem. redn. of nitrogen to ammonia. We demonstrate exptl. the importance of various sources of contamination, and show how to remove labile nitrogen-contg. compds. from the nitrogen gas as well as how to perform quant. isotope measurements with cycling of 15N2 gas to reduce both contamination and the cost of isotope measurements. Following this protocol, we find that no ammonia is produced when using the most promising pure-metal catalysts for this reaction in aq. media, and we successfully confirm and quantify ammonia synthesis using lithium electrodeposition in tetrahydrofuran13. The use of this rigorous protocol should help to prevent false positives from appearing in the literature, thus enabling the field to focus on viable pathways towards the practical electrochem. redn. of nitrogen to ammonia.
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68. 68
  Wu, Z. Y.; Karamad, M.; Yong, X.; Huang, Q.; Cullen, D. A.; Zhu, P.; Xia, C.; Xiao, Q.; Shakouri, M.; Chen, F. Y.; Kim, J. Y. T.; Xia, Y.; Heck, K.; Hu, Y.; Wong, M. S.; Li, Q.; Gates, I.; Siahrostami, S.; Wang, H. Electrochemical ammonia synthesis via nitrate reduction on Fe single atom catalyst. Nat. Commun. 2021, 12 (1), 2870, DOI: 10.1038/s41467-021-23115-x
  68
  Electrochemical ammonia synthesis via nitrate reduction on Fe single atom catalyst
  Wu, Zhen-Yu; Karamad, Mohammadreza; Yong, Xue; Huang, Qizheng; Cullen, David A.; Zhu, Peng; Xia, Chuan; Xiao, Qunfeng; Shakouri, Mohsen; Chen, Feng-Yang; Kim, Jung Yoon; Xia, Yang; Heck, Kimberly; Hu, Yongfeng; Wong, Michael S.; Li, Qilin; Gates, Ian; Siahrostami, Samira; Wang, Haotian
  Nature Communications (2021), 12 (1), 2870CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
  Electrochem. converting nitrate, a widespread water pollutant, back to valuable ammonia is a green and delocalized route for ammonia synthesis, and can be an appealing and supplementary alternative to the Haber-Bosch process. However, as there are other nitrate redn. pathways present, selectively guiding the reaction pathway towards ammonia is currently challenged by the lack of efficient catalysts. Here we report a selective and active nitrate redn. to ammonia on Fe single atom catalyst, with a maximal ammonia Faradaic efficiency of ∼ 75% and a yield rate of up to ∼ 20,000μg h-1 mgcat.-1 (0.46 mmol h-1 cm-2). Our Fe single atom catalyst can effectively prevent the N-N coupling step required for N2 due to the lack of neighboring metal sites, promoting ammonia product selectivity. D. functional theory calcns. reveal the reaction mechanisms and the potential limiting steps for nitrate redn. on atomically dispersed Fe sites.
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69. 69
  Wang, W.; Chen, J.; Tse, E. C. M. Synergy between Cu and Co in a Layered Double Hydroxide Enables Close to 100% Nitrate-to-Ammonia Selectivity. J. Am. Chem. Soc. 2023, 145 (49), 26678– 26687, DOI: 10.1021/jacs.3c08084
  There is no corresponding record for this reference.
70. 70
  Wang, Y.; Sun, M.; Zhou, J.; Xiong, Y.; Zhang, Q.; Ye, C.; Wang, X.; Lu, P.; Feng, T.; Hao, F.; Liu, F.; Wang, J.; Ma, Y.; Yin, J.; Chu, S.; Gu, L.; Huang, B.; Fan, Z. Atomic coordination environment engineering of bimetallic alloy nanostructures for efficient ammonia electrosynthesis from nitrate. Proc. Natl. Acad. Sci. U.S.A. 2023, 120 (32), e2306461120 DOI: 10.1073/pnas.2306461120
  There is no corresponding record for this reference.
Supporting Information
Supporting Information
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.3c13288.
- Additional TEM images, quasi in situ XPS, in situ Raman, operando XAS results, and electrochemical data are supplied as Supporting Information (PDF)
- ja3c13288_si_001.pdf (4.94 MB)
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Electrocatalytic Nitrate and Nitrite Reduction toward Ammonia Using Cu2O Nanocubes: Active Species and Reaction MechanismsClick to copy article linkArticle link copied!

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Characterization and Electrocatalytic NO3RR and NO2RR Performances of Cu2O NCs

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Correlation of the Oxidation State to Catalytic Performance

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Electrokinetic Analysis

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Reaction Intermediates

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Active Species and Reaction Mechanisms

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Spectrophotometric Product Analysis and Detection

Determination of Ammonia

Determination of Nitrite

Nuclear Magnetic Resonance Spectroscopy

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Electrocatalytic Nitrate and Nitrite Reduction toward Ammonia Using Cu₂O Nanocubes: Active Species and Reaction Mechanisms
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Characterization and Electrocatalytic NO₃RR and NO₂RR Performances of Cu₂O NCs

Synthesis of Cu₂O NCs