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Electrochemical Nitrogen Reduction: The Energetic Distance to Lithium
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ACS Energy Letters

Cite this: ACS Energy Lett. 2024, 9, 10, 4947–4952
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https://doi.org/10.1021/acsenergylett.4c01638
Published September 19, 2024

Copyright © 2024 The Authors. Published by American Chemical Society. This publication is licensed under

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Abstract

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Energy-efficient electrochemical reduction of nitrogen to ammonia could help in mitigating climate change. Today, only Li- and recently Ca-mediated systems can perform the reaction. These materials have a large intrinsic energy loss due to the need to electroplate the metal. In this work, we present a series of calculated energetics, formation energies, and binding energies as fundamental features to calculate the energetic distance between Li and Ca and potential new electrochemical nitrogen reduction systems. The featured energetic distance increases with the standard potential. However, dimensionality reduction using principal component analysis provides an encouraging picture; Li and Ca are not exceptional in this feature space, and other materials should be able to carry out the reaction. However, it becomes more challenging the more positive the plating potential is.

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The reduction of N2 to NH3 is a critical process for the growth of plants in nature and food in crops, for the chemical industry, and as an energy carrier. Although dinitrogen (N2) is highly abundant in the atmosphere, it does not react easily and is extremely difficult to activate.

At high temperatures and pressures activating N2 is possible via the industrial Haber–Bosch process (1) with high energy efficiency thanks to a highly integrated process and economy of scale. However, it consumes H2 typically delivered from steam methane reforming (SMR), resulting in immense energy consumption and CO2 emissions. This limitation has driven researchers toward the discovery of alternatives.

At ambient conditions, different routes prevail including enzymatic, homogeneous, and electrocatalytic activation. (2) The enzymatic activation of N2 happens in nature by the nitrogenase enzyme. The active site in the enzyme appears to be iron as an essential transition metal, and it typically contains molybdenum (FeMo-nitrogenase is the most common form), (3,4) with a middle carbon atom. (5) The operation of nitrogenase at ambient conditions to allow nitrogen reduction to ammonia has been long debated, (6) and new findings for nitrogenase are still relevant today. (7,8) Homogeneous activation is inspired by the observation of the nitrogenase enzyme and is driven by well-defined molecular coordination complexes. (9) In the first instance, the complexes only fixated nitrogen. (10) Later they facilitated protonation of nitrogen, (11) and finally conversion to ammonia was achieved. (12−14)

Electrocatalytic activation and direct electrochemical N2 reduction to NH3 could provide a sustainable alternative for small-scale production. The only electrode upon which multiple groups have provided unequivocal evidence that electrochemical nitrogen reduction can take place is in situ deposited Li in an organic electrolyte. (15−19) The discovery of a working Ca system was recently provided by Fu et al., (20) while a two-step electrochemical ammonia synthesis has been shown on Mg, (21) proving Mg to form a metal-nitride and subsequently dissolve it to yield ammonia. However, no continuous production with Mg has been shown to date. Other systems such as metal electrodes in aqueous solution produce such low yields of ammonia that it is impossible to distinguish it from background contamination. (22) In optimizing the Li-based system, researchers have been successful in achieving close to 100% selectivity for nitrogen reduction to ammonia. (18) Although the Li (and Ca) based route shows a viable pathway to ammonia, both systems suffer from a low energy efficiency of ∼28% due to the −3 V plating potential. A recent analysis compares the energy efficiency of these electrochemical systems with the Haber–Bosch process, (23) suggesting a maximum cell potential of 0.38 V to reach energy parity with Haber–Bosch and ruling out alkali metals such as Li and Ca on that single metric. Thus, these systems can only compete when ammonia price is not the key metric and rather soft parameters dominate, such as handling and production on-site, and for small/limited/specialized usage.

There is a pressing need to establish the features that enable electrochemical nitrogen reduction under ambient conditions. (24) Using density functional theory (DFT), we previously searched for the rules of nitrogen fixation. (25) Several metals, including Mg, Ca, Cr, and Mo, showed a similar reactivity to Li toward N2, both in terms of adsorption energy and the propensity to form a bulk nitride. In a separate study, (20) Fu et al. discovered that Ca can also electrochemically convert N2 to NH3, giving credence to our predictions (notably, our experiments were unsuccessful). It would seem that the electrochemical route is dependent on a unique combination of properties that may not only be catalytic. Properties that have been suggested are dinitrogen binding, (7) nitrogen binding, (26) nitrogen dissociation, (27) and the transport of reactants and products through the solid–electrolyte interphase layer (SEI). (28) In particular, the functionality of the SEI layer is difficult to probe in experiments and computations. In the battery literature, one can find insights into the SEI in Al, Mg (29,30), and Ca (31−33) batteries. However, the SEI for a battery needs to be ion conductive but otherwise passive, while the SEI for a nitrogen reduction system has the requirement that reactants and proton sources can reach the material and products can leave the material through the SEI. (34) Thus, insights can be found in battery literature but are not necessarily translatable to electrochemical nitrogen reduction.

In this work, we investigate the energetic distances of materials from Li and Ca as electrodes capable of electrochemical conversion of N2 to NH3. We focus on calculating a series of material properties across the periodic table from first principles: phase formation energies (ΔHname), reaction energies between phases (ΔEMx), and binding energies on surfaces (ΔE*N), where the subscripts name, Mx, and *N refer to the name of formation energies, name of phase reaction (M being the element in the periodic table), and binding energy, respectively. These calculated phase energies and binding energies provide a solid data set to capture trends across the periodic table and allow us to estimate energetic distances to Li and Ca. We then hypothesize that the most likely materials to work for electrochemical nitrogen reduction are materials with the shortest distances to Li and Ca. All data is provided in Tables S1–S3, and examples of calculated DFT structures of Li are given in Figure S1.

Examples of reactions for formation energies (ΔHNitride), reaction energies between phases (ΔEMx), and binding energies on surfaces (ΔE*N) are

xM+12yN2→MxNy
(1)
MpNq+3q2H2→pxMx+qNH3
(2)
12N2+→*N*
(3)
From the reactions, we calculate these properties as
ΔHNitride=EMxNy−xEM−12yEN2
(4)
ΔEMx=pxEMx−EMpNq+qENH3−3q2EH2
(5)
ΔE*N=E*N−E*−12EN2
(6)
Here the asterisks, *, refers to a reaction intermediate on the electrode surface, E is the obtained DFT energy, and the rest of the equations are given in the Supporting Information. In total this allows us to investigate 16 different materials features; 8 different phase formation energies (ΔHNitride, ΔHHydride, ΔHOxide, ΔHMxOyHz, ΔHMxCyOz, ΔHMxFy, ΔHXN3, ΔHX3N), and 4 phase reaction energies (ΔEMxOyHz, ΔEMxOy, ΔEMxHy, ΔEMx), and 4 binding energies (ΔE*N, ΔE*N2, ΔE*NH2, ΔE*NH3).

For some elements of the periodic table, certain features (such as ΔHMxCyOz) would not have a corresponding defined structure or would cause difficulties in converging DFT simulations. In such cases, we carried out linear regressions to predict missing values, as shown in Figure S5, and values are colored red in Tables S1–S3.

The crystalline phases are selected as the most stable structure from the Materials Project database. (35) If the most stable nitride phase has isolated nitrogen atoms we denoted it the “cleaved phase”, and if it does not have isolated nitrogen atoms we denote it the “coupling phase”, similar to our previous work. (25) To assess the formation of the correct SEI layer we have calculated the stability of bulk ionic salt phases, specifically ΔHMxOyHz, (36,37) ΔHMxCyOz, (38,39) and ΔHMxFy (18,40) phases. We chose these compounds as descriptors toward the formation of organic or inorganic SEI species as our earlier study suggested that electrochemical nitrogen reduction is facilitated by the presence of an inorganic SEI. (36) For binding energies, we use only nitrogen reduction reaction intermediates, but several additional features could, in principle, be included in an extended analysis.

For the above-discussed materials properties, we use statistical analysis to

  • Display the Z-score for Li and Ca for each material property. The score is given as, Z = (x – μ)/σ, where x is the value of Li or Ca, μ is the mean, and σ is the standard deviation obtained from the material property. Values of Z above 3 (or ∼2) indicate that the material is an outlier or exceptional material with respect to the entire data set.

  • Calculate the energetic distance to Li and Ca. This will show which material across the periodic table has overall similar thermodynamic characteristics to Li and Ca.

  • Use principal component analysis (PCA) of scaled features to project the feature space down to two dimensions and assess which materials are closest to Li and Ca.

The binding energies of nitrogen (ΔE*N) and the calculated energetic distance to Li versus the standard reduction potential are shown for the materials across the periodic table in Figure 1. The atomic nitrogen binding energy of Li and Ca is neither weak nor strong, which could correspond to a Sabatier principle (Figure 1a). However, multiple other elements have close to similar binding (Sr, Ba, Nb, Fe, W, Mo, etc.) which suggests that on the basis of the atomic nitrogen alone these could electrochemically reduce N2 to NH3, as we previously suggested. (25) The notion that other materials should function is also supported by the statistical standard scores provided in the plot for Li and Ca, which do not show any statistical significance as the score is well below 2. The standard score ZLi = −0.34 and ZCa = −0.91 for ΔE*N means that both materials have a ΔE*N value within 0.34 and 0.91 standard deviation of the mean for this material property, respectively. Analysis of all additional features versus the standard reduction potential is shown in Figure S2. We do not, in any case, find significant values for both Li and Ca, showing that none of these materials are exceptional or outliers with respect to singular material features used in this work. The most significant values are Z = −2.13 for Li with the formation energy of Li3N phase, while Z = −2.17 for Ca with the binding energy of *N2. It should also be noted that in particular, the binding energy of nitrogen (ΔE*N) varies quite a lot for s-block elements depending on the lattice constant of the unit cell (see convergence checks in Figure S3 and S4).

Figure 1

Figure 1. (a) Calculated binding energies of nitrogen (ΔE*N) versus standard reduction potential. Horizontal lines indicate Li and Ca (working electrodes). A histogram and a probability density distribution are plotted together with the Z-score values. Neither Li nor Ca is exceptional in that regard, as a Z-score >2 gives a data point outside of 95% of the data assuming a normal distribution. (b) Calculated distance to Li, with m being the metals and f the features, such as the formation energies and binding energies, plotted as a function of the standard reduction potential. Ca is the material closest to Li, and with increasing standard potential the further away the materials energetics are. “Cleaved phase” means that the material forms a nitride phase with isolated nitrogen atoms.

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To compare all features in one dimension versus standard reduction potential, we define an effective energetic distance from the formation energies and binding energies as

DLi=∑f(Efm−EfLi)2
(7)
with m being the metals and f the features. Interestingly, this analysis shown in Figure 1b depicts that the closest energetically material to Li is Ca. The performance of Ca has only recently been demonstrated. (20) This highlights the fact that the energetic distance calculations could provide opportunities for close-lying materials. However, it also displays that materials which have close to similar nitrogen binding (ΔE*N) to Li, such as W, Mo, etc. as shown in Figure 1a, can have a distance very far from Li when including multiple material features. Unfortunately, it can even be observed that the material’s distance to Li as a function of plating potential is almost linear, depicting that material similar to Li will have similar energy efficiency. This also implies that an energy-efficient electrochemical system will use a different chemistry, hence ruling out battery chemistries in the future. To display the importance of having less plating potential, a recent energy efficiency analysis has been provided. (23,41)

While the scalar distance to Li provides an effective visualization, it may be hiding more complex patterns, and some features with high variation could carry the weight of the distance. It can be helpful to scale all features to obtain similar weights and carry out dimensionality reduction. We choose PCA to reduce the dimensions while preserving as much of the variance in the data as possible, and previously PCA has been successful in obtaining insights into the CO2 reduction reaction, which also depends on a complex multifeature space. (42) In the PCA the data is linearly transformed into a new coordinate system such that the directions (principal components) capture the largest variation in the data. This means that principal component 1 captures the most of the variation, principal component 2 the second most, etc., until the number of principal components hits the original data set dimensions and the explained variance reaches 100%. We have exemplified this in Figure S7.

A two-dimensional PCA was applied for (a) DFT calculated energies only and (b) the inclusion of predicted values where DFT values were missing (see red data in Tables S1–S3, and analysis in Figure S5), and the results are provided in Figure 2. For both plots, around ∼67% and ∼14% of the variation in the data set is captured by the first and second principal components, respectively. This gives a total of ∼81% explained variations by only these two dimensions (see also Figure S7). We have colored Li and Ca with a larger blue point to show these working materials at the perimeter in the plots but with several nearby materials showing again that neither Li nor Ca are unique. We have also co-plotted the original feature dimensions with the green length as weight in the plot to display their direction projected into the first and second principal components, as this can help to interpret the meaning of the principal component. Note that some features point in a similar direction, which means that these features are more correlated than others (e.g., ΔE*NH3 and ΔHNitride) in Figure 2a.

Figure 2

Figure 2. Principal component analysis of the nitrogen reduction feature space: formation energies (ΔH), binding energies (ΔE), and the standard reduction potential (VSHE) for (a) DFT energies and (b) including data with linear regression predicted values noted by star points. “Cleave phase” means that the material forms a nitride phase with isolated nitrogen atoms. Data are shown in Tables S1 and S2.

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Among materials that have a nitride phase with isolated nitrogen atoms (denoted “cleaved phase”), Mg and Al are close to Li (Figure 2a). We have recently tested these materials with selected salt and electrolyte components. (25) Using the predicted values in Figure 2b shows that also Ti, Be, Zr, or Hf look interesting. However, it is not given that any of these materials form an SEI layer suitable for sustained electrocatalysis, and ideally an electrolyte interphase should be formed in the aqueous electrolyte as investigated for aqueous batteries. (43)

Since reduction potential is a property we would like to constrain to positive values, we performed a similar PCA (Figure S8), excluding this parameter from our variables. This clearly shows that there is a negligible change in PCAs. One can also see in Figure 2a,b that the materials of interest lie almost on a line orthogonal to the co-plotted original VSHE component, which challenges energy-efficient improvements.

In conclusion, our analysis depicts that Mg or Al is most likely to work, given that they are battery materials (can form a stable SEI), have a close distance to both Li and Ca, and form a dissociative nitride phase. If a dissociative nitride phase is not needed, one should also experiment with Sr, Ba, and Sc. Unfortunately, all of these single-component materials would imply a marginal improvement in the energy efficiency of the system. To bring substantial improvement one could search for complex multicomponent electrodes or a completely different setup such as intermediate temperature electrocatalysis (improving kinetics), high nitrogen pressure (improving nitrogen coverage), or a controllable artificial transport layer (moderate access to protons and metal cations but facilitate access to N2).

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsenergylett.4c01638.

  • Computational details, calculations of features, convergence checks, data in table format and filling of missing data, and additional plots (PDF)

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Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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Acknowledgments

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A.B. acknowledges support from the Novo Nordisk Foundation Start Package grant (Grant number NNF23OC0084996) and the Pioneer Center for Accelerating P2X Materials Discovery (CAPeX), DNRF grant number P3. This project was also supported with funding from Samsung Electronics Ltd. (SAIT). Via our membership of the UK’s HEC Materials Chemistry Consortium, which is funded by EPSRC (EP/X035859/1), this work used the ARCHER2 UK National Supercomputing Service (http://www.archer2.ac.uk). R.T. and M.M.T acknowledge the Royal Academy of Engineering (Chair in Emerging Technologies Fellowship). R.T. and I.E.L.S. acknowledges the European Research Council (ERC) European under the Union’s Horizon 2020 research and innovation programme (Grant no: 866402)

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    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.
  16. 16
    Tsuneto, A.; Kudo, A.; Sakata, T. Lithium-mediated electrochemical reduction of high pressure N2 to NH3. J. Electroanal. Chem. 1994, 367, 183– 188,  DOI: 10.1016/0022-0728(93)03025-K
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    Lithium-mediated electrochemical reduction of high pressure N2 to NH3
    Tsuneto, Akira; Kudo, Akihiko; Sakata, Tadayoshi
    Journal of Electroanalytical Chemistry (1994), 367 (1-2), 183-8CODEN: JECHES ISSN:.
    Lithium-mediated electrochem. redn. of N2 to NH3 was achieved. NH3 was formed with significant current efficiency (up to 8%) by the electrolysis of a soln. of LiClO4 (0.2M) + ethanol (0.18M) in THF under an atm. pressure of N2. The current efficiency for the NH3 formation increased with the N2 pressure, and quite a high current efficiency (59%) was obtained by carrying out the electrolysis under 50 atm of N2. The NH3 formation efficiency depended strongly on the kind and the amt. of proton source (alc., carboxylic acid, and water) added to the electrolysis medium, as well as on the electrode material. NH3 was even formed (3.7% current efficiency) when air was used as a source of N2.
  17. 17
    Lazouski, N.; Schiffer, Z. J.; Williams, K.; Manthiram, K. Understanding Continuous Lithium-Mediated Electrochemical Nitrogen Reduction. Joule 2019, 3, 1127– 1139,  DOI: 10.1016/j.joule.2019.02.003
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    17
    Understanding Continuous Lithium-Mediated Electrochemical Nitrogen Reduction
    Lazouski, Nikifar; Schiffer, Zachary J.; Williams, Kindle; Manthiram, Karthish
    Joule (2019), 3 (4), 1127-1139CODEN: JOULBR; ISSN:2542-4351. (Cell Press)
    Ammonia is a large-scale commodity chem. that is crucial for producing nitrogen-contg. fertilizers. Electrochem. methods have been proposed as renewable and distributed alternatives to the incumbent Haber-Bosch process, which utilizes fossil fuels for ammonia prodn. Herein, we report a mechanistic study of lithium-mediated electrochem. nitrogen redn. to ammonia in a non-aq. system. The rate laws of the main reactions in the system were detd. At high current densities, nitrogen transport limitations begin to affect the nitrogen redn. process. Based on these observations, we developed a coupled kinetic-transport model of the process, which we used to optimize operating conditions for ammonia prodn. The highest Faradaic efficiency obsd. was 18.5% ± 2.9%, while the highest prodn. rate obtained was (7.9 ± 1.6) × 10-9 mol cm-2 s-1. Our understanding of the reaction network and the influence of transport provides foundational knowledge for future improvements in continuous lithium-mediated ammonia synthesis.
  18. 18
    Du, H. L.; Chatti, M.; Hodgetts, R. Y.; Cherepanov, P. V.; Nguyen, C. K.; Matuszek, K.; MacFarlane, D. R.; Simonov, A. N. Electroreduction of nitrogen with almost 100% current-to-ammonia efficiency. Nature 2022, 609, 722– 727,  DOI: 10.1038/s41586-022-05108-y
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    18
    Electroreduction of nitrogen with almost 100% current-to-ammonia efficiency
    Du, Hoang-Long; Chatti, Manjunath; Hodgetts, Rebecca Y.; Cherepanov, Pavel V.; Nguyen, Cuong K.; Matuszek, Karolina; MacFarlane, Douglas R.; Simonov, Alexandr N.
    Nature (London, United Kingdom) (2022), 609 (7928), 722-727CODEN: NATUAS; ISSN:1476-4687. (Nature Portfolio)
    In addn. to its use in the fertilizer and chem. industries1, ammonia is currently seen as a potential replacement for carbon-based fuels and as a carrier for worldwide transportation of renewable energy2. Implementation of this vision requires transformation of the existing fossil-fuel-based technol. for NH3 prodn.3 to a simpler, scale-flexible technol., such as the electrochem. lithium-mediated nitrogen-redn. reaction3,4. This provides a genuine pathway from N2 to ammonia, but it is currently hampered by limited yield rates and low efficiencies4-12. Here we investigate the role of the electrolyte in this reaction and present a high-efficiency, robust process that is enabled by compact ionic layering in the electrode-electrolyte interface region. The interface is generated by a high-concn. imide-based lithium-salt electrolyte, providing stabilized ammonia yield rates of 150 ± 20 nmol s-1 cm-2 and a current-to-ammonia efficiency that is close to 100%. The ionic assembly formed at the electrode surface suppresses the electrolyte decompn. and supports stable N2 redn. Our study highlights the interrelation between the performance of the lithium-mediated nitrogen-redn. reaction and the physicochem. properties of the electrode-electrolyte interface. We anticipate that these findings will guide the development of a robust, high-performance process for sustainable ammonia prodn.
  19. 19
    Izelaar, B.; Ripepi, D.; van Noordenne, D. D.; Jungbacker, P.; Kortlever, R.; Mulder, F. M. Identification, Quantification, and Elimination of NOx and NH3 Impurities for Aqueous and Li-Mediated Nitrogen Reduction Experiments. ACS Energy Letters 2023, 8, 3614– 3620,  DOI: 10.1021/acsenergylett.3c01130
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    Fu, X.; Niemann, V. A.; Zhou, Y.; Li, S.; Zhang, K.; Pedersen, J. B.; Saccoccio, M.; Andersen, S. Z.; Enemark-Rasmussen, K.; Benedek, P.; Xu, A.; Deissler, N. H.; Mygind, J. B. V.; Nielander, A. C.; Kibsgaard, J.; Vesborg, P. C. K.; No̷rskov, J. K.; Jaramillo, T. F.; Chorkendorff, I. Calcium-mediated nitrogen reduction for electrochemical ammonia synthesis. Nat. Mater. 2024, 23, 101– 107,  DOI: 10.1038/s41563-023-01702-1
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    20
    Calcium-mediated nitrogen reduction for electrochemical ammonia synthesis
    Fu, Xianbiao; Niemann, Valerie A.; Zhou, Yuanyuan; Li, Shaofeng; Zhang, Ke; Pedersen, Jakob B.; Saccoccio, Mattia; Andersen, Suzanne Z.; Enemark-Rasmussen, Kasper; Benedek, Peter; Xu, Aoni; Deissler, Niklas H.; Mygind, Jon Bjarke Valbaek; Nielander, Adam C.; Kibsgaard, Jakob; Vesborg, Peter C. K.; Noerskov, Jens K.; Jaramillo, Thomas F.; Chorkendorff, Ib
    Nature Materials (2024), 23 (1), 101-107CODEN: NMAACR; ISSN:1476-1122. (Nature Portfolio)
    Ammonia (NH3) is a key commodity chem. for the agricultural, textile and pharmaceutical industries, but its prodn. via the Haber-Bosch process is carbon-intensive and centralized. Alternatively, an electrochem. method could enable decentralized, ambient NH3 prodn. that can be paired with renewable energy. The first verified electrochem. method for NH3 synthesis was a process mediated by lithium (Li) in org. electrolytes. So far, however, elements other than Li remain unexplored in this process for potential benefits in efficiency, reaction rates, device design, abundance and stability. In our demonstration of a Li-free system, we found that calcium can mediate the redn. of nitrogen for NH3 synthesis. We verified the calcium-mediated process using a rigorous protocol and achieved an NH3 Faradaic efficiency of 40 ± 2% using calcium tetrakis(hexafluoroisopropyloxy)borate (Ca[B(hfip)4]2) as the electrolyte. Our results offer the possibility of using abundant materials for the electrochem. prodn. of NH3, a crit. chem. precursor and promising energy vector.
  21. 21
    Krebsz, M.; Hodgetts, R. Y.; Johnston, S.; Nguyen, C. K.; Hora, Y.; MacFarlane, D. R.; Simonov, A. N. Reduction of dinitrogen to ammonium through a magnesium-based electrochemical process at close-to-ambient temperature. Energy Environ. Sci. 2024, 17, 4481– 4487,  DOI: 10.1039/D4EE01090F
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    Choi, J.; Suryanto, B. H. R.; Wang, D.; Du, H.-L.; Hodgetts, R. Y.; Ferrero Vallana, F. M.; MacFarlane, D. R.; Simonov, A. N. Identification and elimination of false positives in electrochemical nitrogen reduction studies. Nat. Commun. 2020, 11, 5546,  DOI: 10.1038/s41467-020-19130-z
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    22
    Identification and elimination of false positives in electrochemical nitrogen reduction studies
    Choi, Jaecheol; Suryanto, Bryan H. R.; Wang, Dabin; Du, Hoang-Long; Hodgetts, Rebecca Y.; Ferrero Vallana, Federico M.; MacFarlane, Douglas R.; Simonov, Alexandr N.
    Nature Communications (2020), 11 (1), 5546CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
    Ammonia is of emerging interest as a liquefied, renewable-energy-sourced energy carrier for global use in the future. Electrochem. redn. of N2 (NRR) is widely recognized as an alternative to the traditional Haber-Bosch prodn. process for ammonia. However, though the challenges of NRR expts. have become better understood, the reported rates are often too low to be convincing that redn. of the highly unreactive N2 mol. has actually been achieved. This perspective critically reassesses a wide range of the NRR reports, describes exptl. case studies of potential origins of false-positives, and presents an updated, simplified exptl. protocol dealing with the recently emerging issues.
  23. 23
    Jin, D.; Chen, A.; Lin, B.-L. What Metals Should Be Used to Mediate Electrosynthesis of Ammonia from Nitrogen and Hydrogen from a Thermodynamic Standpoint?. J. Am. Chem. Soc. 2024, 146, 12320– 12323,  DOI: 10.1021/jacs.4c02754
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    Westhead, O.; Jervis, R.; Stephens, I. E. L. Is lithium the key for nitrogen electroreduction?. Science 2021, 372, 1149– 1150,  DOI: 10.1126/science.abi8329
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    24
    Is lithium the key for nitrogen electroreduction?
    Westhead, Olivia; Jervis, Rhodri; Stephens, Ifan E. L.
    Science (Washington, DC, United States) (2021), 372 (6547), 1149-1150CODEN: SCIEAS; ISSN:1095-9203. (American Association for the Advancement of Science)
    The Haber-Bosch process converts nitrogen (N2) and hydrogen (H2) into ammonia (NH3) over iron-based catalysts. Today, 50% of global agriculture uses Haber-Bosch NH3 in fertilizer. Efficient synthesis requires enormous energy to achieve extreme temps. and pressures, and the H2 is primarily derived from methane steam reforming. Hence, the Haber-Bosch process accounts for at least 1% of global greenhouse gas emissions (). Electrochem. N2 redn. to make NH3, powered by renewable electricity under ambient conditions, could provide a localized and greener alternative. On page 1187 of this issue, Suryanto et al. () report highly efficient and stable electrochem. N2 redn. based on a recyclable proton donor. This study builds on earlier work showing that an electrolyte contg. a lithium salt in an org. solvent with a sacrificial proton donor was unmatched in its ability to unequivocally reduce N2 (, ). In both studies, it is still unclear why lithium is so crit.
  25. 25
    Tort, R.; Bagger, A.; Westhead, O.; Kondo, Y.; Khobnya, A.; Winiwarter, A.; Davies, B. J. V.; Walsh, A.; Katayama, Y.; Yamada, Y.; Ryan, M. P.; Titirici, M.-M.; Stephens, I. E. L. Searching for the Rules of Electrochemical Nitrogen Fixation. ACS Catal. 2023, 13, 14513– 14522,  DOI: 10.1021/acscatal.3c03951
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    Singh, A. R.; Rohr, B. A.; Statt, M. J.; Schwalbe, J. A.; Cargnello, M.; No̷rskov, J. K. Strategies toward Selective Electrochemical Ammonia Synthesis. ACS Catal. 2019, 9, 8316– 8324,  DOI: 10.1021/acscatal.9b02245
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    Strategies toward Selective Electrochemical Ammonia Synthesis
    Singh, Aayush R.; Rohr, Brian A.; Statt, Michael J.; Schwalbe, Jay A.; Cargnello, Matteo; Noerskov, Jens K.
    ACS Catalysis (2019), 9 (9), 8316-8324CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    The active and selective electroredn. of atm. N (N2) to NH3 (NH3) using energy from solar or wind sources at the point of use would enable a sustainable alternative to the Haber-Bosch process for fertilizer prodn. While the process is thermodynamically possible, exptl. attempts thus far have required large overpotentials and produced primarily H (H2). In this Perspective, insights from electronic structure calcns. of the energetics of the process, combined with mean-field microkinetic modeling, can be used to (1) understand the activity and selectivity challenges in electrochem. NH3 synthesis and (2) propose alternative strategies toward an economically viable process. In particular, the authors develop the theor. understanding for two promising actionable avenues that are gaining interest in the exptl. literature, (1) circumventing the scaling relations between adsorbed surface intermediates and (2) using nonaq. electrolytes to suppress the competing H evolution reaction.
  27. 27
    McEnaney, J. M.; Singh, A. R.; Schwalbe, J. A.; Kibsgaard, J.; Lin, J. C.; Cargnello, M.; Jaramillo, T. F.; No̷rskov, J. K. Ammonia synthesis from N2 and H2O using a lithium cycling electrification strategy at atmospheric pressure. Energy Environ. Sci. 2017, 10, 1621– 1630,  DOI: 10.1039/C7EE01126A
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    27
    Ammonia synthesis from N2 and H2O using a lithium cycling electrification strategy at atmospheric pressure
    McEnaney, Joshua M.; Singh, Aayush R.; Schwalbe, Jay A.; Kibsgaard, Jakob; Lin, John C.; Cargnello, Matteo; Jaramillo, Thomas F.; Noerskov, Jens K.
    Energy & Environmental Science (2017), 10 (7), 1621-1630CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    Ammonia prodn. is imperative to providing food for a growing world population. However, the primary method of synthetic ammonia prodn., the Haber Bosch process, is resource demanding and unsustainable. Here we report a novel ammonia prodn. strategy, exemplified in an electrochem. lithium cycling process, which provides a pathway to sustainable ammonia synthesis via the ability to directly couple to renewable sources of electricity and can facilitate localized prodn. Whereas traditional aq. electrochem. approaches are typically dominated by the hydrogen evolution reaction (HER), we are able to circumvent the HER by using a stepwise approach which separates the redn. of N2 from subsequent protonation to NH3, thus our synthesis method is predominantly selective for ammonia prodn. D. functional theory calcns. for thermodn. and diffusion energy barrier insights suggest that Li-based materials are well suited to carry out this process, though other materials may also be useful. The three steps of the demonstrated process are LiOH electrolysis, direct nitridation of Li, and the exothermic release of ammonia from Li3N, which reproduces the LiOH, completing the cycle. The process uses N2 and H2O at atm. pressure and reasonable temps., and, while approaching industrial level electrolytic current densities, we report an initial current efficiency of 88.5% toward ammonia prodn.
  28. 28
    Chang, W.; Jain, A.; Rezaie, F.; Manthiram, K. Lithium-mediated nitrogen reduction to ammonia via the catalytic solid–electrolyte interphase. Nature Catalysis 2024, 7, 231– 241,  DOI: 10.1038/s41929-024-01115-6
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    Ng, K. L.; Amrithraj, B.; Azimi, G. Nonaqueous rechargeable aluminum batteries. Joule 2022, 6, 134– 170,  DOI: 10.1016/j.joule.2021.12.003
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    Nonaqueous rechargeable aluminum batteries
    Ng, Kok Long; Amrithraj, Brohath; Azimi, Gisele
    Joule (2022), 6 (1), 134-170CODEN: JOULBR; ISSN:2542-4351. (Cell Press)
    A review. Promises for safe, affordable, environmentally sustainable, and high-performance energy storage technologies have spurred an increased interest in nonaq. rechargeable Al batteries (RABs) worldwide. However, the complex Al electrochem. involved in existing nonaq. RABs has invoked more comprehensive assessments on the implications of overall cell chemistries to the actual battery performance metrics. In this review, we present a summary of reported cathode materials and their corresponding charge storage mechanisms. We critically discuss the implications of overall cell chemistries to the actual battery performance metrics and outline the fundamental and practical limitations of existing RAB chemistries. We also highlight discrepancies in the proposed mechanisms of several RAB systems and further emphasize the importance of an accurate elucidation of the underlying charge storage mechanism involved. We discuss ion migration kinetics in existing electrodes and outline design guidelines for enhancing their performance. Lastly, we provide our perspectives to better understand existing RAB chemistries as they are critically relevant for future research directed at advancing the deployment of nonaq. RABs.
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    Leung, O. M.; Schoetz, T.; Prodromakis, T.; Ponce de Leon, C. Review─Progress in Electrolytes for Rechargeable Aluminium Batteries. J. Electrochem. Soc. 2021, 168, 056509  DOI: 10.1149/1945-7111/abfb36
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    Review-progress in electrolytes for rechargeable aluminium batteries
    Leung, Oi Man; Schoetz, Theresa; Prodromakis, Themis; Ponce de Leon, Carlos
    Journal of the Electrochemical Society (2021), 168 (5), 056509CODEN: JESOAN; ISSN:1945-7111. (IOP Publishing Ltd.)
    A review. The growing demand for safe, sustainable and energy-dense energy storage devices has spurred intensive investigations into post-lithium battery technologies. Rechargeable aluminum batteries are promising candidates for future electrochem. energy storage systems due to the high theor. volumetric capacity of aluminum and its natural abundance in the Earth's crust, but their practical application is currently hindered by the limitations of presently available electrolytes. In this review, we highlight the key considerations needed to optimize the electrolyte design in relation to the aluminum battery system and critically assess the current state of knowledge and new concepts in liq. and quasi-solid polymer electrolytes, focusing primarily on non-aq. systems. We then discuss the challenges and approaches in developing polymer electrolytes and finally provide an overview of the opportunities in quasi-solid electrolytes which could pave the way to achieving further improvements in aluminum batteries.
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    Forero-Saboya, J. D.; Tchitchekova, D. S.; Johansson, P.; Palacín, M. R.; Ponrouch, A. Interfaces and Interphases in Ca and Mg Batteries. Advanced Materials Interfaces 2022, 9, 2101578,  DOI: 10.1002/admi.202101578
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    Forero-Saboya, J.; Davoisne, C.; Dedryvère, R.; Yousef, I.; Canepa, P.; Ponrouch, A. Understanding the nature of the passivation layer enabling reversible calcium plating. Energy Environ. Sci. 2020, 13, 3423– 3431,  DOI: 10.1039/D0EE02347G
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    Understanding the nature of the passivation layer enabling reversible calcium plating
    Forero-Saboya, Juan; Davoisne, Carine; Dedryvere, Remi; Yousef, Ibraheem; Canepa, Pieremanuele; Ponrouch, Alexandre
    Energy & Environmental Science (2020), 13 (10), 3423-3431CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    As for other multivalent systems, the interface between the calcium (Ca) metal anode and the electrolyte is of paramount importance for reversible plating/stripping. Here, we combined exptl. and theor. approaches to unveil the potential solid electrolyte interphase (SEI) components enabling facile Ca plating. Borates compds., in the form of cross-linked polymers are suggested as divalent conducting component. A pre-passivation protocol with such SEI is demonstrated and allows to broaden the possibility for electrolyte formulation. We also demonstrated a 10-fold increase in Ca plating kinetics by tuning the cation solvation structure in the electrolyte limiting the degree of contact ion pair.
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    Wang, D.; Gao, X.; Chen, Y.; Jin, L.; Kuss, C.; Bruce, P. G. Plating and stripping calcium in an organic electrolyte. Nat. Mater. 2018, 17, 16– 20,  DOI: 10.1038/nmat5036
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    Plating and stripping calcium in an organic electrolyte
    Wang, Da; Gao, Xiangwen; Chen, Yuhui; Jin, Liyu; Kuss, Christian; Bruce, Peter G.
    Nature Materials (2018), 17 (1), 16-20CODEN: NMAACR; ISSN:1476-1122. (Nature Research)
    There is considerable interest in multivalent cation batteries, such as those based on magnesium, calcium or aluminum. Most attention has focused on magnesium. In all cases the metal anode represents a significant challenge. Recent work has shown that calcium can be plated and stripped, but only at elevated temps., 75 to 100°, with small capacities, typically 0.165 mAh cm-2, and accompanied by significant side reactions. Here we demonstrate that calcium can be plated and stripped at room temp. with capacities of 1 mAh cm-2 at a rate of 1 mA cm-2, with low polarization (∼100 mV) and in excess of 50 cycles. The dominant product is calcium, accompanied by a small amt. of CaH2 that forms by reaction between the deposited calcium and the electrolyte, Ca(BH4)2 in THF (THF). This occurs in preference to the reactions which take place in most electrolyte solns. forming CaCO3, Ca(OH)2 and calcium alkoxides, and normally terminate the electrochem. The CaH2 protects the calcium metal at open circuit. Although this work does not solve all the problems of calcium as an anode in calcium-ion batteries, it does demonstrate that significant quantities of calcium can be plated and stripped at room temp. with low polarization.
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    Steinberg, K.; Yuan, X.; Klein, C. K.; Lazouski, N.; Mecklenburg, M.; Manthiram, K.; Li, Y. Imaging of nitrogen fixation at lithium solid electrolyte interphases via cryo-electron microscopy. Nature Energy 2023, 8, 138– 148,  DOI: 10.1038/s41560-022-01177-5
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    Jain, A.; Ong, S. P.; Hautier, G.; Chen, W.; Richards, W. D.; Dacek, S.; Cholia, S.; Gunter, D.; Skinner, D.; Ceder, G.; Persson, K. A. Commentary: The Materials Project: A materials genome approach to accelerating materials innovation. APL Materials 2013, 1, 011002  DOI: 10.1063/1.4812323
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    Commentary: The Materials Project: A materials genome approach to accelerating materials innovation
    Jain, Anubhav; Ong, Shyue Ping; Hautier, Geoffroy; Chen, Wei; Richards, William Davidson; Dacek, Stephen; Cholia, Shreyas; Gunter, Dan; Skinner, David; Ceder, Gerbrand; Persson, Kristin A.
    APL Materials (2013), 1 (1), 011002/1-011002/11CODEN: AMPADS; ISSN:2166-532X. (American Institute of Physics)
    Accelerating the discovery of advanced materials is essential for human welfare and sustainable, clean energy. In this paper, we introduce the Materials Project (www.materialsproject.org), a core program of the Materials Genome Initiative that uses high-throughput computing to uncover the properties of all known inorg. materials. This open dataset can be accessed through multiple channels for both interactive exploration and data mining. The Materials Project also seeks to create open-source platforms for developing robust, sophisticated materials analyses. Future efforts will enable users to perform rapid-prototyping'' of new materials in silico, and provide researchers with new avenues for cost-effective, data-driven materials design. (c) 2013 American Institute of Physics.
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    Spry, M.; Westhead, O.; Tort, R.; Moss, B.; Katayama, Y.; Titirici, M.-M.; Stephens, I. E. L.; Bagger, A. Water Increases the Faradaic Selectivity of Li-Mediated Nitrogen Reduction. ACS Energy Letters 2023, 8, 1230– 1235,  DOI: 10.1021/acsenergylett.2c02792
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    Li, K.; Andersen, S. Z.; Statt, M. J.; Saccoccio, M.; Bukas, V. J.; Krempl, K.; Sažinas, R.; Pedersen, J. B.; Shadravan, V.; Zhou, Y.; Chakraborty, D.; Kibsgaard, J.; Vesborg, P. C. K.; No̷rskov, J. K.; Chorkendorff, I. Enhancement of lithium-mediated ammonia synthesis by addition of oxygen. Science 2021, 374, 1593– 1597,  DOI: 10.1126/science.abl4300
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    Enhancement of lithium-mediated ammonia synthesis by addition of oxygen
    Li, Katja; Andersen, Suzanne Z.; Statt, Michael J.; Saccoccio, Mattia; Bukas, Vanessa J.; Krempl, Kevin; Sazinas, Rokas; Pedersen, Jakob B.; Shadravan, Vahid; Zhou, Yuanyuan; Chakraborty, Debasish; Kibsgaard, Jakob; Vesborg, Peter C. K.; Noerskov, Jens K.; Chorkendorff, Ib
    Science (Washington, DC, United States) (2021), 374 (6575), 1593-1597CODEN: SCIEAS; ISSN:1095-9203. (American Association for the Advancement of Science)
    Owing to the worrying increase in carbon dioxide concns. in the atm., there is a need to electrify fossil-fuel-powered chem. processes such as the Haber-Bosch ammonia synthesis. Lithium-mediated Electrochem. nitrogen redn. has shown preliminary promise but still lacks sufficient faradaic efficiency and ammonia formation rate to be industrially relevant. Here, we show that oxygen, previously believed to hinder the reaction, actually greatly improves the faradaic efficiency and stability of the lithium-mediated nitrogen redn. when added to the reaction atm. in small amts. With this counterintuitive discovery, we reach record high faradaic efficiencies of up to 78.0 ± 1.3% at 0.6-0.8 mol% oxygen in 20 bar of nitrogen. Exptl. x-ray anal. and theor. microkinetic modeling shed light on the underlying mechanism.
  38. 38
    Westhead, O.; Spry, M.; Bagger, A.; Shen, Z.; Yadegari, H.; Favero, S.; Tort, R.; Titirici, M.; Ryan, M. P.; Jervis, R.; Katayama, Y.; Aguadero, A.; Regoutz, A.; Grimaud, A.; Stephens, I. E. L. The role of ion solvation in lithium mediated nitrogen reduction. J. Mater. Chem. A 2023, 11, 12746– 12758,  DOI: 10.1039/D2TA07686A
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    The role of ion solvation in lithium mediated nitrogen reduction
    Westhead, O.; Spry, M.; Bagger, A.; Shen, Z.; Yadegari, H.; Favero, S.; Tort, R.; Titirici, M.; Ryan, M. P.; Jervis, R.; Katayama, Y.; Aguadero, A.; Regoutz, A.; Grimaud, A.; Stephens, I. E. L.
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2023), 11 (24), 12746-12758CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    Since its verification in 2019, there have been numerous high-profile papers reporting improved efficiency of lithium-mediated electrochem. nitrogen redn. to make ammonia. However, the literature lacks any coherent investigation systematically linking bulk electrolyte properties to electrochem. performance and Solid Electrolyte Interphase (SEI) properties. In this study, we discover that the salt concn. has a remarkable effect on electrolyte stability: at concns. of 0.6 M LiClO4 and above the electrode potential is stable for at least 12 h at an applied c.d. of -2 mA cm-2 at ambient temp. and pressure. Conversely, at the lower concns. explored in prior studies, the potential required to maintain a given N2 redn. current increased by 8 V within a period of 1 h under the same conditions. The behavior is linked more coordination of the salt anion and cation with increasing salt concn. in the electrolyte obsd. via Raman spectroscopy. Time of flight secondary ion mass spectrometry and XPS reveal a more inorg., and therefore more stable, SEI layer is formed with increasing salt concn. A drop in faradaic efficiency for nitrogen redn. is seen at concns. higher than 0.6 M LiClO4, which is attributed to a combination of a decrease in nitrogen soly. and diffusivity as well as increased SEI cond. as measured by electrochem. impedance spectroscopy.
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    McShane, E. J.; Niemann, V. A.; Benedek, P.; Fu, X.; Nielander, A. C.; Chorkendorff, I.; Jaramillo, T. F.; Cargnello, M. Quantifying Influence of the Solid-Electrolyte Interphase in Ammonia Electrosynthesis. ACS Energy Letters 2023, 8, 4024– 4032,  DOI: 10.1021/acsenergylett.3c01534
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    Li, S.; Zhou, Y.; Li, K.; Saccoccio, M.; Sažinas, R.; Andersen, S. Z.; Pedersen, J. B.; Fu, X.; Shadravan, V.; Chakraborty, D.; Kibsgaard, J.; Vesborg, P. C.; No̷rskov, J. K.; Chorkendorff, I. Electrosynthesis of ammonia with high selectivity and high rates via engineering of the solid-electrolyte interphase. Joule 2022, 6, 2083– 2101,  DOI: 10.1016/j.joule.2022.07.009
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    Electrosynthesis of ammonia with high selectivity and high rates via engineering of the solid-electrolyte interphase
    Li, Shaofeng; Zhou, Yuanyuan; Li, Katja; Saccoccio, Mattia; Sazinas, Rokas; Andersen, Suzanne Z.; Pedersen, Jakob B.; Fu, Xianbiao; Shadravan, Vahid; Chakraborty, Debasish; Kibsgaard, Jakob; Vesborg, Peter C. K.; Noerskov, Jens K.; Chorkendorff, Ib
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    Ammonia is a large-scale commodity essential to fertilizer prodn., but the Haber-Bosch process leads to massive emissions of carbon dioxide. Electrochem. ammonia synthesis is an attractive alternative pathway, but the process is still limited by low ammonia prodn. rate and faradaic efficiency. Herein, guided by our theor. model, we present a highly efficient lithium-mediated process enabled by using different lithium salts, leading to the formation of a uniform solid-electrolyte interphase (SEI) layer on a porous copper electrode. The uniform lithium-fluoride-enriched SEI layer provides an ammonia prodn. rate of 2.5 ± 0.1μmol s-1 cm-2geo at a c.d. of -1 A cm-2geo with 71% ± 3% faradaic efficiency under 20 bar nitrogen. Exptl. X-ray anal. reveals that the lithium tetrafluoroborate electrolyte induces the formation of a compact and uniform SEI layer, which facilitates homogeneous lithium plating, suppresses the undesired hydrogen evolution as well as electrolyte decompn., and enhances the nitrogen redn.
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    Electrolyte Interphases in Aqueous Batteries
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    A review. The narrow electrochem. stability window of water poses a challenge to the development of aq. electrolytes. In contrast to non-aq. electrolytes, the products of water electrolysis do not contribute to the formation of a passivation layer on electrodes. As a result, aq. electrolytes require the reactions of addnl. components, such as additives and co-solvents, to facilitate the formation of the desired solid electrolyte interphase (SEI) on the anode and cathode electrolyte interphase (CEI) on the cathode. This review highlights the fundamental principles and recent advancements in generating electrolyte interphases in aq. batteries.

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  1. Luis Miguel Azofra, José Miguel Doña-Rodríguez, Douglas R. MacFarlane, Alexandr N. Simonov. Mechanism of the N2 Cleavage Promoted by Lithium vs Other Alkali and Alkaline-Earth Metals. The Journal of Physical Chemistry C 2025, 129 (2) , 1198-1205. https://doi.org/10.1021/acs.jpcc.4c07438
  2. Yuanyuan Zhou, Xianbiao Fu, Ib Chorkendorff, Jens K. Nørskov. Electrochemical Ammonia Synthesis: The Energy Efficiency Challenge. ACS Energy Letters 2025, 10 (1) , 128-132. https://doi.org/10.1021/acsenergylett.4c02954
  3. Pawel J. Kulesza, Iwona A. Rutkowska, Anna Chmielnicka, Beata Rytelewska, Olena Siamuk, Adam Gorczynski, Violetta Patroniak. Development of catalytic systems for reduction of electrochemically inert inorganic molecules: Carbon dioxide and nitrogen. Current Opinion in Electrochemistry 2025, 51 , 101657. https://doi.org/10.1016/j.coelec.2025.101657
  4. Diwakar Singh, Samad Razzaq, Shohreh Faridi, Kai S. Exner. Selective nitrogen reduction reaction on single-atom centers of molybdenum-based MXenes by pulsing the electrochemical potential. Materials Today 2025, 1 https://doi.org/10.1016/j.mattod.2025.03.016
  5. Boaz Izelaar, Mahinder Ramdin, Alexander Vlierboom, Mar Pérez-Fortes, Deanne van der Slikke, Asvin Sajeev Kumar, Wiebren de Jong, Fokko M. Mulder, Ruud Kortlever. Techno-economic assessment of different small-scale electrochemical NH 3 production plants. Energy & Environmental Science 2024, 17 (21) , 7983-7998. https://doi.org/10.1039/D4EE03299C
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  • Abstract

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    Figure 1

    Figure 1. (a) Calculated binding energies of nitrogen (ΔE*N) versus standard reduction potential. Horizontal lines indicate Li and Ca (working electrodes). A histogram and a probability density distribution are plotted together with the Z-score values. Neither Li nor Ca is exceptional in that regard, as a Z-score >2 gives a data point outside of 95% of the data assuming a normal distribution. (b) Calculated distance to Li, with m being the metals and f the features, such as the formation energies and binding energies, plotted as a function of the standard reduction potential. Ca is the material closest to Li, and with increasing standard potential the further away the materials energetics are. “Cleaved phase” means that the material forms a nitride phase with isolated nitrogen atoms.

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    Figure 2

    Figure 2. Principal component analysis of the nitrogen reduction feature space: formation energies (ΔH), binding energies (ΔE), and the standard reduction potential (VSHE) for (a) DFT energies and (b) including data with linear regression predicted values noted by star points. “Cleave phase” means that the material forms a nitride phase with isolated nitrogen atoms. Data are shown in Tables S1 and S2.

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  • References

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      The Haber-Bosch process conventionally reduces N2 to NH3 at 200 bar and 500°. Under ambient conditions, i.e., room temp. and ambient pressure, N2 can be converted into NH3 by the nitrogenase mol. and Li-contg. solid electrodes in nonaq. media. The authors explore the catalyst space for the N2 redn. reaction under ambient conditions. The authors describe N2 redn. from the *N2 binding energy vs. the *H binding energy; under std. conditions, no catalyst can bind and reduce *N2 without producing H2. The authors show why a selective catalyst for N2 redn. will also likely be selective for CO2 redn., but N2 redn. is intrinsically more challenging than CO2 redn. Only by modulating the reaction pathway, like nitrogenase, or by tuning chem. potentials, like the Haber-Bosch and the Li-mediated process, N2 can be reduced.
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      Catalytic Reduction of Dinitrogen to Ammonia at a Single Molybdenum Center
      Yandulov, Dmitry V.; Schrock, Richard R.
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      Dinitrogen (N2) was reduced to ammonia at room temp. and 1 atm with molybdenum catalysts that contain tetradentate [HIPTN3N]3- triamidoamine ligands {such as [HIPTN3N]Mo(N2), where [HIPTN3N]3- is [{3,5-(2,4,6-i-Pr3C6H2)2C6H3NCH2CH2}3N]3-} in heptane. Slow addn. of the proton source [{2,6-lutidinium}{BAr'4}, where Ar' is 3,5-(CF3)2C6H3] and reductant (decamethyl chromocene) was crit. for achieving high efficiency (∼66% in four turnovers). Numerous previous x-ray studies, along with previous isolation and characterization of six proposed intermediates in the catalytic reaction under noncatalytic conditions, suggest that N2 was reduced at a sterically protected, single molybdenum center that cycled from Mo(III) through Mo(VI) states.
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      Arashiba, K.; Miyake, Y.; Nishibayashi, Y. A molybdenum complex bearing PNP-type pincer ligands leads to the catalytic reduction of dinitrogen into ammonia. Nat. Chem. 2011, 3, 120– 125,  DOI: 10.1038/nchem.906
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      Arashiba, Kazuya; Miyake, Yoshihiro; Nishibayashi, Yoshiaki
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      The synthesis of transition metal-dinitrogen complexes and the stoichiometric transformation of their coordinated dinitrogen into ammonia and hydrazine have been the subject of considerable research, with a view to achieving nitrogen fixation under ambient conditions. Since a single example in 2003, no examples have been reported of the catalytic conversion of dinitrogen into ammonia under ambient conditions. The dimolybdenum-dinitrogen complex bearing PNP pincer ligands was found to work as an effective catalyst for the formation of ammonia from dinitrogen, with 23 equiv. of ammonia being produced with the catalyst (12 equiv. of ammonia are produced based on the molybdenum atom of the catalyst). This is another successful example of the catalytic and direct conversion of dinitrogen into ammonia under ambient reaction conditions. We believe that the results described in this Article provide valuable information with which to develop a more effective nitrogen-fixation system under mild reaction conditions.
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      Anderson, J. S.; Rittle, J.; Peters, J. C. Catalytic conversion of nitrogen to ammonia by an iron model complex. Nature 2013, 501, 84– 87,  DOI: 10.1038/nature12435
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      Catalytic conversion of nitrogen to ammonia by an iron model complex
      Anderson, John S.; Rittle, Jonathan; Peters, Jonas C.
      Nature (London, United Kingdom) (2013), 501 (7465), 84-87CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
      The redn. of nitrogen (N2) to ammonia (NH3) is a requisite transformation for life. Although it is widely appreciated that the Fe-rich cofactors of nitrogenase enzymes facilitate this transformation, how they do so remains poorly understood. A central element of debate was the exact site or sites of N2 coordination and redn. In synthetic inorg. chem., an early emphasis was placed on Mo because it probably is an essential element of nitrogenases and because it had been established that well-defined Mo model complexes could mediate the stoichiometric conversion of N2 to NH3. This chem. transformation can be performed in a catalytic fashion by two well-defined mol. systems that feature Mo centers. However, it is now thought that Fe is the only transition metal essential to all nitrogenases, and recent biochem. and spectroscopic data have implicated Fe instead of Mo as the site of N2 binding in the FeMo-cofactor. Here, the authors describe a tris(phosphine)borane-supported Fe complex that catalyzes the redn. of N2 to NH3 under mild conditions, and in which >40 per cent of the proton and reducing equiv are delivered to N2. The authors' results indicate that a single Fe site may be capable of stabilizing the various NxHy intermediates generated during catalytic NH3 formation. Geometric tunability at Fe imparted by a flexible Fe-B interaction in the authors' model system seems to be important for efficient catalysis. Probably the interstitial C atom recently assigned in the nitrogenase cofactor may have a similar role, perhaps by enabling a single Fe site to mediate the enzymic catalysis through a flexible Fe-C interaction.
    15. 15
      Andersen, S. Z.; Čolić, 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.; No̷rskov, J. K.; Chorkendorff, I. A rigorous electrochemical ammonia synthesis protocol with quantitative isotope measurements. Nature 2019, 570, 504– 508,  DOI: 10.1038/s41586-019-1260-x
      15
      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.
    16. 16
      Tsuneto, A.; Kudo, A.; Sakata, T. Lithium-mediated electrochemical reduction of high pressure N2 to NH3. J. Electroanal. Chem. 1994, 367, 183– 188,  DOI: 10.1016/0022-0728(93)03025-K
      16
      Lithium-mediated electrochemical reduction of high pressure N2 to NH3
      Tsuneto, Akira; Kudo, Akihiko; Sakata, Tadayoshi
      Journal of Electroanalytical Chemistry (1994), 367 (1-2), 183-8CODEN: JECHES ISSN:.
      Lithium-mediated electrochem. redn. of N2 to NH3 was achieved. NH3 was formed with significant current efficiency (up to 8%) by the electrolysis of a soln. of LiClO4 (0.2M) + ethanol (0.18M) in THF under an atm. pressure of N2. The current efficiency for the NH3 formation increased with the N2 pressure, and quite a high current efficiency (59%) was obtained by carrying out the electrolysis under 50 atm of N2. The NH3 formation efficiency depended strongly on the kind and the amt. of proton source (alc., carboxylic acid, and water) added to the electrolysis medium, as well as on the electrode material. NH3 was even formed (3.7% current efficiency) when air was used as a source of N2.
    17. 17
      Lazouski, N.; Schiffer, Z. J.; Williams, K.; Manthiram, K. Understanding Continuous Lithium-Mediated Electrochemical Nitrogen Reduction. Joule 2019, 3, 1127– 1139,  DOI: 10.1016/j.joule.2019.02.003
      17
      Understanding Continuous Lithium-Mediated Electrochemical Nitrogen Reduction
      Lazouski, Nikifar; Schiffer, Zachary J.; Williams, Kindle; Manthiram, Karthish
      Joule (2019), 3 (4), 1127-1139CODEN: JOULBR; ISSN:2542-4351. (Cell Press)
      Ammonia is a large-scale commodity chem. that is crucial for producing nitrogen-contg. fertilizers. Electrochem. methods have been proposed as renewable and distributed alternatives to the incumbent Haber-Bosch process, which utilizes fossil fuels for ammonia prodn. Herein, we report a mechanistic study of lithium-mediated electrochem. nitrogen redn. to ammonia in a non-aq. system. The rate laws of the main reactions in the system were detd. At high current densities, nitrogen transport limitations begin to affect the nitrogen redn. process. Based on these observations, we developed a coupled kinetic-transport model of the process, which we used to optimize operating conditions for ammonia prodn. The highest Faradaic efficiency obsd. was 18.5% ± 2.9%, while the highest prodn. rate obtained was (7.9 ± 1.6) × 10-9 mol cm-2 s-1. Our understanding of the reaction network and the influence of transport provides foundational knowledge for future improvements in continuous lithium-mediated ammonia synthesis.
    18. 18
      Du, H. L.; Chatti, M.; Hodgetts, R. Y.; Cherepanov, P. V.; Nguyen, C. K.; Matuszek, K.; MacFarlane, D. R.; Simonov, A. N. Electroreduction of nitrogen with almost 100% current-to-ammonia efficiency. Nature 2022, 609, 722– 727,  DOI: 10.1038/s41586-022-05108-y
      18
      Electroreduction of nitrogen with almost 100% current-to-ammonia efficiency
      Du, Hoang-Long; Chatti, Manjunath; Hodgetts, Rebecca Y.; Cherepanov, Pavel V.; Nguyen, Cuong K.; Matuszek, Karolina; MacFarlane, Douglas R.; Simonov, Alexandr N.
      Nature (London, United Kingdom) (2022), 609 (7928), 722-727CODEN: NATUAS; ISSN:1476-4687. (Nature Portfolio)
      In addn. to its use in the fertilizer and chem. industries1, ammonia is currently seen as a potential replacement for carbon-based fuels and as a carrier for worldwide transportation of renewable energy2. Implementation of this vision requires transformation of the existing fossil-fuel-based technol. for NH3 prodn.3 to a simpler, scale-flexible technol., such as the electrochem. lithium-mediated nitrogen-redn. reaction3,4. This provides a genuine pathway from N2 to ammonia, but it is currently hampered by limited yield rates and low efficiencies4-12. Here we investigate the role of the electrolyte in this reaction and present a high-efficiency, robust process that is enabled by compact ionic layering in the electrode-electrolyte interface region. The interface is generated by a high-concn. imide-based lithium-salt electrolyte, providing stabilized ammonia yield rates of 150 ± 20 nmol s-1 cm-2 and a current-to-ammonia efficiency that is close to 100%. The ionic assembly formed at the electrode surface suppresses the electrolyte decompn. and supports stable N2 redn. Our study highlights the interrelation between the performance of the lithium-mediated nitrogen-redn. reaction and the physicochem. properties of the electrode-electrolyte interface. We anticipate that these findings will guide the development of a robust, high-performance process for sustainable ammonia prodn.
    19. 19
      Izelaar, B.; Ripepi, D.; van Noordenne, D. D.; Jungbacker, P.; Kortlever, R.; Mulder, F. M. Identification, Quantification, and Elimination of NOx and NH3 Impurities for Aqueous and Li-Mediated Nitrogen Reduction Experiments. ACS Energy Letters 2023, 8, 3614– 3620,  DOI: 10.1021/acsenergylett.3c01130
      There is no corresponding record for this reference.
    20. 20
      Fu, X.; Niemann, V. A.; Zhou, Y.; Li, S.; Zhang, K.; Pedersen, J. B.; Saccoccio, M.; Andersen, S. Z.; Enemark-Rasmussen, K.; Benedek, P.; Xu, A.; Deissler, N. H.; Mygind, J. B. V.; Nielander, A. C.; Kibsgaard, J.; Vesborg, P. C. K.; No̷rskov, J. K.; Jaramillo, T. F.; Chorkendorff, I. Calcium-mediated nitrogen reduction for electrochemical ammonia synthesis. Nat. Mater. 2024, 23, 101– 107,  DOI: 10.1038/s41563-023-01702-1
      20
      Calcium-mediated nitrogen reduction for electrochemical ammonia synthesis
      Fu, Xianbiao; Niemann, Valerie A.; Zhou, Yuanyuan; Li, Shaofeng; Zhang, Ke; Pedersen, Jakob B.; Saccoccio, Mattia; Andersen, Suzanne Z.; Enemark-Rasmussen, Kasper; Benedek, Peter; Xu, Aoni; Deissler, Niklas H.; Mygind, Jon Bjarke Valbaek; Nielander, Adam C.; Kibsgaard, Jakob; Vesborg, Peter C. K.; Noerskov, Jens K.; Jaramillo, Thomas F.; Chorkendorff, Ib
      Nature Materials (2024), 23 (1), 101-107CODEN: NMAACR; ISSN:1476-1122. (Nature Portfolio)
      Ammonia (NH3) is a key commodity chem. for the agricultural, textile and pharmaceutical industries, but its prodn. via the Haber-Bosch process is carbon-intensive and centralized. Alternatively, an electrochem. method could enable decentralized, ambient NH3 prodn. that can be paired with renewable energy. The first verified electrochem. method for NH3 synthesis was a process mediated by lithium (Li) in org. electrolytes. So far, however, elements other than Li remain unexplored in this process for potential benefits in efficiency, reaction rates, device design, abundance and stability. In our demonstration of a Li-free system, we found that calcium can mediate the redn. of nitrogen for NH3 synthesis. We verified the calcium-mediated process using a rigorous protocol and achieved an NH3 Faradaic efficiency of 40 ± 2% using calcium tetrakis(hexafluoroisopropyloxy)borate (Ca[B(hfip)4]2) as the electrolyte. Our results offer the possibility of using abundant materials for the electrochem. prodn. of NH3, a crit. chem. precursor and promising energy vector.
    21. 21
      Krebsz, M.; Hodgetts, R. Y.; Johnston, S.; Nguyen, C. K.; Hora, Y.; MacFarlane, D. R.; Simonov, A. N. Reduction of dinitrogen to ammonium through a magnesium-based electrochemical process at close-to-ambient temperature. Energy Environ. Sci. 2024, 17, 4481– 4487,  DOI: 10.1039/D4EE01090F
      There is no corresponding record for this reference.
    22. 22
      Choi, J.; Suryanto, B. H. R.; Wang, D.; Du, H.-L.; Hodgetts, R. Y.; Ferrero Vallana, F. M.; MacFarlane, D. R.; Simonov, A. N. Identification and elimination of false positives in electrochemical nitrogen reduction studies. Nat. Commun. 2020, 11, 5546,  DOI: 10.1038/s41467-020-19130-z
      22
      Identification and elimination of false positives in electrochemical nitrogen reduction studies
      Choi, Jaecheol; Suryanto, Bryan H. R.; Wang, Dabin; Du, Hoang-Long; Hodgetts, Rebecca Y.; Ferrero Vallana, Federico M.; MacFarlane, Douglas R.; Simonov, Alexandr N.
      Nature Communications (2020), 11 (1), 5546CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
      Ammonia is of emerging interest as a liquefied, renewable-energy-sourced energy carrier for global use in the future. Electrochem. redn. of N2 (NRR) is widely recognized as an alternative to the traditional Haber-Bosch prodn. process for ammonia. However, though the challenges of NRR expts. have become better understood, the reported rates are often too low to be convincing that redn. of the highly unreactive N2 mol. has actually been achieved. This perspective critically reassesses a wide range of the NRR reports, describes exptl. case studies of potential origins of false-positives, and presents an updated, simplified exptl. protocol dealing with the recently emerging issues.
    23. 23
      Jin, D.; Chen, A.; Lin, B.-L. What Metals Should Be Used to Mediate Electrosynthesis of Ammonia from Nitrogen and Hydrogen from a Thermodynamic Standpoint?. J. Am. Chem. Soc. 2024, 146, 12320– 12323,  DOI: 10.1021/jacs.4c02754
      There is no corresponding record for this reference.
    24. 24
      Westhead, O.; Jervis, R.; Stephens, I. E. L. Is lithium the key for nitrogen electroreduction?. Science 2021, 372, 1149– 1150,  DOI: 10.1126/science.abi8329
      24
      Is lithium the key for nitrogen electroreduction?
      Westhead, Olivia; Jervis, Rhodri; Stephens, Ifan E. L.
      Science (Washington, DC, United States) (2021), 372 (6547), 1149-1150CODEN: SCIEAS; ISSN:1095-9203. (American Association for the Advancement of Science)
      The Haber-Bosch process converts nitrogen (N2) and hydrogen (H2) into ammonia (NH3) over iron-based catalysts. Today, 50% of global agriculture uses Haber-Bosch NH3 in fertilizer. Efficient synthesis requires enormous energy to achieve extreme temps. and pressures, and the H2 is primarily derived from methane steam reforming. Hence, the Haber-Bosch process accounts for at least 1% of global greenhouse gas emissions (). Electrochem. N2 redn. to make NH3, powered by renewable electricity under ambient conditions, could provide a localized and greener alternative. On page 1187 of this issue, Suryanto et al. () report highly efficient and stable electrochem. N2 redn. based on a recyclable proton donor. This study builds on earlier work showing that an electrolyte contg. a lithium salt in an org. solvent with a sacrificial proton donor was unmatched in its ability to unequivocally reduce N2 (, ). In both studies, it is still unclear why lithium is so crit.
    25. 25
      Tort, R.; Bagger, A.; Westhead, O.; Kondo, Y.; Khobnya, A.; Winiwarter, A.; Davies, B. J. V.; Walsh, A.; Katayama, Y.; Yamada, Y.; Ryan, M. P.; Titirici, M.-M.; Stephens, I. E. L. Searching for the Rules of Electrochemical Nitrogen Fixation. ACS Catal. 2023, 13, 14513– 14522,  DOI: 10.1021/acscatal.3c03951
      There is no corresponding record for this reference.
    26. 26
      Singh, A. R.; Rohr, B. A.; Statt, M. J.; Schwalbe, J. A.; Cargnello, M.; No̷rskov, J. K. Strategies toward Selective Electrochemical Ammonia Synthesis. ACS Catal. 2019, 9, 8316– 8324,  DOI: 10.1021/acscatal.9b02245
      26
      Strategies toward Selective Electrochemical Ammonia Synthesis
      Singh, Aayush R.; Rohr, Brian A.; Statt, Michael J.; Schwalbe, Jay A.; Cargnello, Matteo; Noerskov, Jens K.
      ACS Catalysis (2019), 9 (9), 8316-8324CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      The active and selective electroredn. of atm. N (N2) to NH3 (NH3) using energy from solar or wind sources at the point of use would enable a sustainable alternative to the Haber-Bosch process for fertilizer prodn. While the process is thermodynamically possible, exptl. attempts thus far have required large overpotentials and produced primarily H (H2). In this Perspective, insights from electronic structure calcns. of the energetics of the process, combined with mean-field microkinetic modeling, can be used to (1) understand the activity and selectivity challenges in electrochem. NH3 synthesis and (2) propose alternative strategies toward an economically viable process. In particular, the authors develop the theor. understanding for two promising actionable avenues that are gaining interest in the exptl. literature, (1) circumventing the scaling relations between adsorbed surface intermediates and (2) using nonaq. electrolytes to suppress the competing H evolution reaction.
    27. 27
      McEnaney, J. M.; Singh, A. R.; Schwalbe, J. A.; Kibsgaard, J.; Lin, J. C.; Cargnello, M.; Jaramillo, T. F.; No̷rskov, J. K. Ammonia synthesis from N2 and H2O using a lithium cycling electrification strategy at atmospheric pressure. Energy Environ. Sci. 2017, 10, 1621– 1630,  DOI: 10.1039/C7EE01126A
      27
      Ammonia synthesis from N2 and H2O using a lithium cycling electrification strategy at atmospheric pressure
      McEnaney, Joshua M.; Singh, Aayush R.; Schwalbe, Jay A.; Kibsgaard, Jakob; Lin, John C.; Cargnello, Matteo; Jaramillo, Thomas F.; Noerskov, Jens K.
      Energy & Environmental Science (2017), 10 (7), 1621-1630CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      Ammonia prodn. is imperative to providing food for a growing world population. However, the primary method of synthetic ammonia prodn., the Haber Bosch process, is resource demanding and unsustainable. Here we report a novel ammonia prodn. strategy, exemplified in an electrochem. lithium cycling process, which provides a pathway to sustainable ammonia synthesis via the ability to directly couple to renewable sources of electricity and can facilitate localized prodn. Whereas traditional aq. electrochem. approaches are typically dominated by the hydrogen evolution reaction (HER), we are able to circumvent the HER by using a stepwise approach which separates the redn. of N2 from subsequent protonation to NH3, thus our synthesis method is predominantly selective for ammonia prodn. D. functional theory calcns. for thermodn. and diffusion energy barrier insights suggest that Li-based materials are well suited to carry out this process, though other materials may also be useful. The three steps of the demonstrated process are LiOH electrolysis, direct nitridation of Li, and the exothermic release of ammonia from Li3N, which reproduces the LiOH, completing the cycle. The process uses N2 and H2O at atm. pressure and reasonable temps., and, while approaching industrial level electrolytic current densities, we report an initial current efficiency of 88.5% toward ammonia prodn.
    28. 28
      Chang, W.; Jain, A.; Rezaie, F.; Manthiram, K. Lithium-mediated nitrogen reduction to ammonia via the catalytic solid–electrolyte interphase. Nature Catalysis 2024, 7, 231– 241,  DOI: 10.1038/s41929-024-01115-6
      There is no corresponding record for this reference.
    29. 29
      Ng, K. L.; Amrithraj, B.; Azimi, G. Nonaqueous rechargeable aluminum batteries. Joule 2022, 6, 134– 170,  DOI: 10.1016/j.joule.2021.12.003
      29
      Nonaqueous rechargeable aluminum batteries
      Ng, Kok Long; Amrithraj, Brohath; Azimi, Gisele
      Joule (2022), 6 (1), 134-170CODEN: JOULBR; ISSN:2542-4351. (Cell Press)
      A review. Promises for safe, affordable, environmentally sustainable, and high-performance energy storage technologies have spurred an increased interest in nonaq. rechargeable Al batteries (RABs) worldwide. However, the complex Al electrochem. involved in existing nonaq. RABs has invoked more comprehensive assessments on the implications of overall cell chemistries to the actual battery performance metrics. In this review, we present a summary of reported cathode materials and their corresponding charge storage mechanisms. We critically discuss the implications of overall cell chemistries to the actual battery performance metrics and outline the fundamental and practical limitations of existing RAB chemistries. We also highlight discrepancies in the proposed mechanisms of several RAB systems and further emphasize the importance of an accurate elucidation of the underlying charge storage mechanism involved. We discuss ion migration kinetics in existing electrodes and outline design guidelines for enhancing their performance. Lastly, we provide our perspectives to better understand existing RAB chemistries as they are critically relevant for future research directed at advancing the deployment of nonaq. RABs.
    30. 30
      Leung, O. M.; Schoetz, T.; Prodromakis, T.; Ponce de Leon, C. Review─Progress in Electrolytes for Rechargeable Aluminium Batteries. J. Electrochem. Soc. 2021, 168, 056509  DOI: 10.1149/1945-7111/abfb36
      30
      Review-progress in electrolytes for rechargeable aluminium batteries
      Leung, Oi Man; Schoetz, Theresa; Prodromakis, Themis; Ponce de Leon, Carlos
      Journal of the Electrochemical Society (2021), 168 (5), 056509CODEN: JESOAN; ISSN:1945-7111. (IOP Publishing Ltd.)
      A review. The growing demand for safe, sustainable and energy-dense energy storage devices has spurred intensive investigations into post-lithium battery technologies. Rechargeable aluminum batteries are promising candidates for future electrochem. energy storage systems due to the high theor. volumetric capacity of aluminum and its natural abundance in the Earth's crust, but their practical application is currently hindered by the limitations of presently available electrolytes. In this review, we highlight the key considerations needed to optimize the electrolyte design in relation to the aluminum battery system and critically assess the current state of knowledge and new concepts in liq. and quasi-solid polymer electrolytes, focusing primarily on non-aq. systems. We then discuss the challenges and approaches in developing polymer electrolytes and finally provide an overview of the opportunities in quasi-solid electrolytes which could pave the way to achieving further improvements in aluminum batteries.
    31. 31
      Forero-Saboya, J. D.; Tchitchekova, D. S.; Johansson, P.; Palacín, M. R.; Ponrouch, A. Interfaces and Interphases in Ca and Mg Batteries. Advanced Materials Interfaces 2022, 9, 2101578,  DOI: 10.1002/admi.202101578
      There is no corresponding record for this reference.
    32. 32
      Forero-Saboya, J.; Davoisne, C.; Dedryvère, R.; Yousef, I.; Canepa, P.; Ponrouch, A. Understanding the nature of the passivation layer enabling reversible calcium plating. Energy Environ. Sci. 2020, 13, 3423– 3431,  DOI: 10.1039/D0EE02347G
      32
      Understanding the nature of the passivation layer enabling reversible calcium plating
      Forero-Saboya, Juan; Davoisne, Carine; Dedryvere, Remi; Yousef, Ibraheem; Canepa, Pieremanuele; Ponrouch, Alexandre
      Energy & Environmental Science (2020), 13 (10), 3423-3431CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      As for other multivalent systems, the interface between the calcium (Ca) metal anode and the electrolyte is of paramount importance for reversible plating/stripping. Here, we combined exptl. and theor. approaches to unveil the potential solid electrolyte interphase (SEI) components enabling facile Ca plating. Borates compds., in the form of cross-linked polymers are suggested as divalent conducting component. A pre-passivation protocol with such SEI is demonstrated and allows to broaden the possibility for electrolyte formulation. We also demonstrated a 10-fold increase in Ca plating kinetics by tuning the cation solvation structure in the electrolyte limiting the degree of contact ion pair.
    33. 33
      Wang, D.; Gao, X.; Chen, Y.; Jin, L.; Kuss, C.; Bruce, P. G. Plating and stripping calcium in an organic electrolyte. Nat. Mater. 2018, 17, 16– 20,  DOI: 10.1038/nmat5036
      33
      Plating and stripping calcium in an organic electrolyte
      Wang, Da; Gao, Xiangwen; Chen, Yuhui; Jin, Liyu; Kuss, Christian; Bruce, Peter G.
      Nature Materials (2018), 17 (1), 16-20CODEN: NMAACR; ISSN:1476-1122. (Nature Research)
      There is considerable interest in multivalent cation batteries, such as those based on magnesium, calcium or aluminum. Most attention has focused on magnesium. In all cases the metal anode represents a significant challenge. Recent work has shown that calcium can be plated and stripped, but only at elevated temps., 75 to 100°, with small capacities, typically 0.165 mAh cm-2, and accompanied by significant side reactions. Here we demonstrate that calcium can be plated and stripped at room temp. with capacities of 1 mAh cm-2 at a rate of 1 mA cm-2, with low polarization (∼100 mV) and in excess of 50 cycles. The dominant product is calcium, accompanied by a small amt. of CaH2 that forms by reaction between the deposited calcium and the electrolyte, Ca(BH4)2 in THF (THF). This occurs in preference to the reactions which take place in most electrolyte solns. forming CaCO3, Ca(OH)2 and calcium alkoxides, and normally terminate the electrochem. The CaH2 protects the calcium metal at open circuit. Although this work does not solve all the problems of calcium as an anode in calcium-ion batteries, it does demonstrate that significant quantities of calcium can be plated and stripped at room temp. with low polarization.
    34. 34
      Steinberg, K.; Yuan, X.; Klein, C. K.; Lazouski, N.; Mecklenburg, M.; Manthiram, K.; Li, Y. Imaging of nitrogen fixation at lithium solid electrolyte interphases via cryo-electron microscopy. Nature Energy 2023, 8, 138– 148,  DOI: 10.1038/s41560-022-01177-5
      There is no corresponding record for this reference.
    35. 35
      Jain, A.; Ong, S. P.; Hautier, G.; Chen, W.; Richards, W. D.; Dacek, S.; Cholia, S.; Gunter, D.; Skinner, D.; Ceder, G.; Persson, K. A. Commentary: The Materials Project: A materials genome approach to accelerating materials innovation. APL Materials 2013, 1, 011002  DOI: 10.1063/1.4812323
      35
      Commentary: The Materials Project: A materials genome approach to accelerating materials innovation
      Jain, Anubhav; Ong, Shyue Ping; Hautier, Geoffroy; Chen, Wei; Richards, William Davidson; Dacek, Stephen; Cholia, Shreyas; Gunter, Dan; Skinner, David; Ceder, Gerbrand; Persson, Kristin A.
      APL Materials (2013), 1 (1), 011002/1-011002/11CODEN: AMPADS; ISSN:2166-532X. (American Institute of Physics)
      Accelerating the discovery of advanced materials is essential for human welfare and sustainable, clean energy. In this paper, we introduce the Materials Project (www.materialsproject.org), a core program of the Materials Genome Initiative that uses high-throughput computing to uncover the properties of all known inorg. materials. This open dataset can be accessed through multiple channels for both interactive exploration and data mining. The Materials Project also seeks to create open-source platforms for developing robust, sophisticated materials analyses. Future efforts will enable users to perform rapid-prototyping'' of new materials in silico, and provide researchers with new avenues for cost-effective, data-driven materials design. (c) 2013 American Institute of Physics.
    36. 36
      Spry, M.; Westhead, O.; Tort, R.; Moss, B.; Katayama, Y.; Titirici, M.-M.; Stephens, I. E. L.; Bagger, A. Water Increases the Faradaic Selectivity of Li-Mediated Nitrogen Reduction. ACS Energy Letters 2023, 8, 1230– 1235,  DOI: 10.1021/acsenergylett.2c02792
      There is no corresponding record for this reference.
    37. 37
      Li, K.; Andersen, S. Z.; Statt, M. J.; Saccoccio, M.; Bukas, V. J.; Krempl, K.; Sažinas, R.; Pedersen, J. B.; Shadravan, V.; Zhou, Y.; Chakraborty, D.; Kibsgaard, J.; Vesborg, P. C. K.; No̷rskov, J. K.; Chorkendorff, I. Enhancement of lithium-mediated ammonia synthesis by addition of oxygen. Science 2021, 374, 1593– 1597,  DOI: 10.1126/science.abl4300
      37
      Enhancement of lithium-mediated ammonia synthesis by addition of oxygen
      Li, Katja; Andersen, Suzanne Z.; Statt, Michael J.; Saccoccio, Mattia; Bukas, Vanessa J.; Krempl, Kevin; Sazinas, Rokas; Pedersen, Jakob B.; Shadravan, Vahid; Zhou, Yuanyuan; Chakraborty, Debasish; Kibsgaard, Jakob; Vesborg, Peter C. K.; Noerskov, Jens K.; Chorkendorff, Ib
      Science (Washington, DC, United States) (2021), 374 (6575), 1593-1597CODEN: SCIEAS; ISSN:1095-9203. (American Association for the Advancement of Science)
      Owing to the worrying increase in carbon dioxide concns. in the atm., there is a need to electrify fossil-fuel-powered chem. processes such as the Haber-Bosch ammonia synthesis. Lithium-mediated Electrochem. nitrogen redn. has shown preliminary promise but still lacks sufficient faradaic efficiency and ammonia formation rate to be industrially relevant. Here, we show that oxygen, previously believed to hinder the reaction, actually greatly improves the faradaic efficiency and stability of the lithium-mediated nitrogen redn. when added to the reaction atm. in small amts. With this counterintuitive discovery, we reach record high faradaic efficiencies of up to 78.0 ± 1.3% at 0.6-0.8 mol% oxygen in 20 bar of nitrogen. Exptl. x-ray anal. and theor. microkinetic modeling shed light on the underlying mechanism.
    38. 38
      Westhead, O.; Spry, M.; Bagger, A.; Shen, Z.; Yadegari, H.; Favero, S.; Tort, R.; Titirici, M.; Ryan, M. P.; Jervis, R.; Katayama, Y.; Aguadero, A.; Regoutz, A.; Grimaud, A.; Stephens, I. E. L. The role of ion solvation in lithium mediated nitrogen reduction. J. Mater. Chem. A 2023, 11, 12746– 12758,  DOI: 10.1039/D2TA07686A
      38
      The role of ion solvation in lithium mediated nitrogen reduction
      Westhead, O.; Spry, M.; Bagger, A.; Shen, Z.; Yadegari, H.; Favero, S.; Tort, R.; Titirici, M.; Ryan, M. P.; Jervis, R.; Katayama, Y.; Aguadero, A.; Regoutz, A.; Grimaud, A.; Stephens, I. E. L.
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2023), 11 (24), 12746-12758CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      Since its verification in 2019, there have been numerous high-profile papers reporting improved efficiency of lithium-mediated electrochem. nitrogen redn. to make ammonia. However, the literature lacks any coherent investigation systematically linking bulk electrolyte properties to electrochem. performance and Solid Electrolyte Interphase (SEI) properties. In this study, we discover that the salt concn. has a remarkable effect on electrolyte stability: at concns. of 0.6 M LiClO4 and above the electrode potential is stable for at least 12 h at an applied c.d. of -2 mA cm-2 at ambient temp. and pressure. Conversely, at the lower concns. explored in prior studies, the potential required to maintain a given N2 redn. current increased by 8 V within a period of 1 h under the same conditions. The behavior is linked more coordination of the salt anion and cation with increasing salt concn. in the electrolyte obsd. via Raman spectroscopy. Time of flight secondary ion mass spectrometry and XPS reveal a more inorg., and therefore more stable, SEI layer is formed with increasing salt concn. A drop in faradaic efficiency for nitrogen redn. is seen at concns. higher than 0.6 M LiClO4, which is attributed to a combination of a decrease in nitrogen soly. and diffusivity as well as increased SEI cond. as measured by electrochem. impedance spectroscopy.
    39. 39
      McShane, E. J.; Niemann, V. A.; Benedek, P.; Fu, X.; Nielander, A. C.; Chorkendorff, I.; Jaramillo, T. F.; Cargnello, M. Quantifying Influence of the Solid-Electrolyte Interphase in Ammonia Electrosynthesis. ACS Energy Letters 2023, 8, 4024– 4032,  DOI: 10.1021/acsenergylett.3c01534
      There is no corresponding record for this reference.
    40. 40
      Li, S.; Zhou, Y.; Li, K.; Saccoccio, M.; Sažinas, R.; Andersen, S. Z.; Pedersen, J. B.; Fu, X.; Shadravan, V.; Chakraborty, D.; Kibsgaard, J.; Vesborg, P. C.; No̷rskov, J. K.; Chorkendorff, I. Electrosynthesis of ammonia with high selectivity and high rates via engineering of the solid-electrolyte interphase. Joule 2022, 6, 2083– 2101,  DOI: 10.1016/j.joule.2022.07.009
      40
      Electrosynthesis of ammonia with high selectivity and high rates via engineering of the solid-electrolyte interphase
      Li, Shaofeng; Zhou, Yuanyuan; Li, Katja; Saccoccio, Mattia; Sazinas, Rokas; Andersen, Suzanne Z.; Pedersen, Jakob B.; Fu, Xianbiao; Shadravan, Vahid; Chakraborty, Debasish; Kibsgaard, Jakob; Vesborg, Peter C. K.; Noerskov, Jens K.; Chorkendorff, Ib
      Joule (2022), 6 (9), 2083-2101CODEN: JOULBR; ISSN:2542-4351. (Cell Press)
      Ammonia is a large-scale commodity essential to fertilizer prodn., but the Haber-Bosch process leads to massive emissions of carbon dioxide. Electrochem. ammonia synthesis is an attractive alternative pathway, but the process is still limited by low ammonia prodn. rate and faradaic efficiency. Herein, guided by our theor. model, we present a highly efficient lithium-mediated process enabled by using different lithium salts, leading to the formation of a uniform solid-electrolyte interphase (SEI) layer on a porous copper electrode. The uniform lithium-fluoride-enriched SEI layer provides an ammonia prodn. rate of 2.5 ± 0.1μmol s-1 cm-2geo at a c.d. of -1 A cm-2geo with 71% ± 3% faradaic efficiency under 20 bar nitrogen. Exptl. X-ray anal. reveals that the lithium tetrafluoroborate electrolyte induces the formation of a compact and uniform SEI layer, which facilitates homogeneous lithium plating, suppresses the undesired hydrogen evolution as well as electrolyte decompn., and enhances the nitrogen redn.
    41. 41
      Kani, N. C.; Goyal, I.; Gauthier, J. A.; Shields, W.; Shields, M.; Singh, M. R. Pathway toward Scalable Energy-Efficient Li-Mediated Ammonia Synthesis. ACS Appl. Mater. Interfaces 2024, 16, 16203– 16212,  DOI: 10.1021/acsami.3c19499
      There is no corresponding record for this reference.
    42. 42
      Christensen, O.; Bagger, A.; Rossmeisl, J. The Missing Link for Electrochemical CO2 Reduction: Classification of CO vs HCOOH Selectivity via PCA, Reaction Pathways, and Coverage Analysis. ACS Catal. 2024, 14, 2151– 2161,  DOI: 10.1021/acscatal.3c04851
      There is no corresponding record for this reference.
    43. 43
      Sui, Y.; Ji, X. Electrolyte Interphases in Aqueous Batteries. Angew. Chem., Int. Ed. 2024, 63, e202312585  DOI: 10.1002/anie.202312585
      43
      Electrolyte Interphases in Aqueous Batteries
      Sui, Yiming; Ji, Xiulei
      Angewandte Chemie, International Edition (2024), 63 (2), e202312585CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. The narrow electrochem. stability window of water poses a challenge to the development of aq. electrolytes. In contrast to non-aq. electrolytes, the products of water electrolysis do not contribute to the formation of a passivation layer on electrodes. As a result, aq. electrolytes require the reactions of addnl. components, such as additives and co-solvents, to facilitate the formation of the desired solid electrolyte interphase (SEI) on the anode and cathode electrolyte interphase (CEI) on the cathode. This review highlights the fundamental principles and recent advancements in generating electrolyte interphases in aq. batteries.

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