Probing the Dynamics of Low-Overpotential CO₂-to-CO Activation on Copper Electrodes with Time-Resolved Raman Spectroscopy

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1. 1

   Birdja, Y. Y.; Pérez-Gallent, E.; Figueiredo, M. C.; Göttle, A. J.; Calle-Vallejo, F.; Koper, M. T. M. Advances and Challenges in Understanding the Electrocatalytic Conversion of Carbon Dioxide to Fuels. Nat. Energy 2019, 4, 732– 745,  DOI: 10.1038/s41560-019-0450-y

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   Advances and challenges in understanding the electrocatalytic conversion of carbon dioxide to fuels

   Birdja, Yuvraj Y.; Perez-Gallent, Elena; Figueiredo, Marta C.; Gottle, Adrien J.; Calle-Vallejo, Federico; Koper, Marc T. M.

   Nature Energy (2019), 4 (9), 732-745CODEN: NEANFD; ISSN:2058-7546. (Nature Research)

   A review. The electrocatalytic redn. of carbon dioxide is a promising approach for storing (excess) renewable electricity as chem. energy in fuels. Here, we review recent advances and challenges in the understanding of electrochem. CO2 redn. We discuss existing models for the initial activation of CO2 on the electrocatalyst and their importance for understanding selectivity. Carbon-carbon bond formation is also a key mechanistic step in CO2 electroredn. to high-d. and high-value fuels. We show that both the initial CO2 activation and C-C bond formation are influenced by an intricate interplay between surface structure (both on the nano- and on the mesoscale), electrolyte effects (pH, buffer strength, ion effects) and mass transport conditions. This complex interplay is currently still far from being completely understood. In addn., we discuss recent progress in in situ spectroscopic techniques and computational techniques for mechanistic work. Finally, we identify some challenges in furthering our understanding of these themes.
2. 2

   Nitopi, S.; Bertheussen, E.; Scott, S. B.; Liu, X.; Engstfeld, A. K.; Horch, S.; Seger, B.; Stephens, I. E. L.; Chan, K.; Hahn, C.; Nørskov, J. K.; Jaramillo, T. F.; Chorkendorff, I. Progress and Perspectives of Electrochemical CO₂ Reduction on Copper in Aqueous Electrolyte. Chem. Rev. 2019, 119, 7610– 7672,  DOI: 10.1021/acs.chemrev.8b00705

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   Progress and Perspectives of Electrochemical CO2 Reduction on Copper in Aqueous Electrolyte

   Nitopi, Stephanie; Bertheussen, Erlend; Scott, Soren B.; Liu, Xinyan; Engstfeld, Albert K.; Horch, Sebastian; Seger, Brian; Stephens, Ifan E. L.; Chan, Karen; Hahn, Christopher; Noerskov, Jens K.; Jaramillo, Thomas F.; Chorkendorff, Ib

   Chemical Reviews (Washington, DC, United States) (2019), 119 (12), 7610-7672CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)

   A review. To date, copper is the only heterogeneous catalyst that has shown a propensity to produce valuable hydrocarbons and alcs., such as ethylene and ethanol, from electrochem. CO2 redn. (CO2R). There are variety of factors that impact CO2R activity and selectivity, including the catalyst surface structure, morphol., compn., the choice of electrolyte ions and pH, the electrochem. cell design, etc. Many of these factors are often intertwined, which can complicate catalyst discovery and design efforts. Here we take a broad and historical view of these different aspects and their complex interplay in CO2R catalysis on Cu, with the purpose of providing new insights, crit. evaluations, and guidance to the field with regards to research directions and best practices. First, we describe the various exptl. probes and complementary theor. methods that have been used to discern the mechanisms by which products are formed, and next we present our current understanding of the complex reaction networks for CO2R on Cu. Then we analyze two key methods that have been used in attempts to alter the activity and selectivity of Cu: nanostructuring and the formation of bimetallic electrodes. Finally, we offer some perspectives on the future outlook for electrochem. CO2R.
3. 3

   Garg, S.; Li, M.; Weber, A. Z.; Ge, L.; Li, L.; Rudolph, V.; Wang, G.; Rufford, T. E. Advances and Challenges in Electrochemical CO₂ Reduction Processes: An Engineering and Design Perspective Looking beyond New Catalyst Materials. J. Mater. Chem. A 2020, 8, 1511– 1544,  DOI: 10.1039/C9TA13298H

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   Advances and challenges in electrochemical CO2 reduction processes: an engineering and design perspective looking beyond new catalyst materials

   Garg, Sahil; Li, Mengran; Weber, Adam Z.; Ge, Lei; Li, Liye; Rudolph, Victor; Wang, Guoxiong; Rufford, Thomas E.

   Journal of Materials Chemistry A: Materials for Energy and Sustainability (2020), 8 (4), 1511-1544CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)

   A review. Electrochem. CO2 redn. (CO2R) is one of several promising strategies to mitigate CO2 emissions. Electrochem. processes operate at mild conditions, can be tuned to selective products, allow modular design, and provide opportunities to integrate renewable electricity with CO2 redn. in carbon-intensive manufg. industries such as iron and steel making. In recent years, significant advances have been achieved in the development of highly efficient and selective electrocatalysts for CO2R. However, to realize fully the potential benefits of new electrocatalysts in low cost, large scale CO2R electrolyzers requires advances in design and engineering of the CO2R process. In this review, the state-of-the-art is examd. in electrochem. CO2R technologies, and highlight how the efficiency of CO2R processes can be improved through electrolyzer configuration, electrode structure, electrolyte selection, pH control, and the electrolyzer's operating pressure and temp. Although a comprehensive review of catalytic materials is beyond this review's scope, how other engineering and design decisions may also affect CO2R reaction pathways because of effects on mass transfer rates, the electrode surface chem., interactions with intermediate reaction species, and rates of charge transfer is illustrated.
4. 4

   Kim, J. J.; Summers, D. P.; Frese, K. W. Reduction of CO₂ and CO to Methane on Cu Foil Electrodes. J. Electroanal. Chem. Interfacial Electrochem. 1988, 245, 223– 244,  DOI: 10.1016/0022-0728(88)80071-8

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   Reduction of carbon dioxide and carbon monoxide to methane on copper foil electrodes

   Kim, J. J.; Summers, D. P.; Frese, K. W., Jr.

   Journal of Electroanalytical Chemistry and Interfacial Electrochemistry (1988), 245 (1-2), 223-44CODEN: JEIEBC; ISSN:0022-0728.

   The electrochem. redn. of CO2 was studied on Cu foil electrodes in 0.5M KHCO3, pH 7.6. The highest CH4 formation rates of 22 and 0° were 8 × 10-5 (17 mA cm-2) and 1.1 × 10-4 (23 mA cm-2) mol cm-2 h-1 at -2.0 and -2.3 V vs. SCE, resp. The onset potentials for both CH4 and ethylene were -1.5 to -1.6 V vs. SCE. It was shown that CH4 is formed from CO at a 50 fold lower rate than from CO2 with an onset potential of -1.5 to -1.6 V vs. SCE. The CH4 formation rate is higher if the surface is prepd. by cleaning with HCl rather than HNO3 or oxidn. in air. Tafel slopes for the CH4 partial current were 174 and 110 mV/decade for the HNO3 and HCl pretreatment, resp. Electrochem. assisted dissocn. of adsorbed CO by electron transfer is suggested as the rate-detg. step.
5. 5

   Zhu, Q.; Sun, X.; Yang, D.; Ma, J.; Kang, X.; Zheng, L.; Zhang, J.; Wu, Z.; Han, B. Carbon Dioxide Electroreduction to C₂ Products over Copper-Cuprous Oxide Derived from Electrosynthesized Copper Complex. Nat. Commun. 2019, 10, 3851,  DOI: 10.1038/s41467-019-11599-7

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   Carbon dioxide electroreduction to C2 products over copper-cuprous oxide derived from electrosynthesized copper complex

   Zhu Qinggong; Sun Xiaofu; Yang Dexin; Ma Jun; Kang Xinchen; Han Buxing; Sun Xiaofu; Yang Dexin; Kang Xinchen; Han Buxing; Zheng Lirong; Zhang Jing; Wu Zhonghua; Han Buxing

   Nature communications (2019), 10 (1), 3851 ISSN:.

   Efficient electroreduction of carbon dioxide to multicarbon products in aqueous solution is of great importance and challenging. Unfortunately, the low efficiency of the production of C2 products limits implementation at scale. Here, we report reduction of carbon dioxide to C2 products (acetic acid and ethanol) over a 3D dendritic copper-cuprous oxide composite fabricated by in situ reduction of an electrodeposited copper complex. In potassium chloride aqueous electrolyte, the applied potential was as low as -0.4 V vs reversible hydrogen electrode, the overpotential is only 0.53 V (for acetic acid) and 0.48 V (for ethanol) with high C2 Faradaic efficiency of 80% and a current density of 11.5 mA cm(-2). The outstanding performance of the electrode for producing the C2 products results mainly from near zero contacting resistance between the electrocatalysts and copper substrate, abundant exposed active sites in the 3D dendritic structure and suitable copper(I)/copper(0) ratio of the electrocatalysts.
6. 6

   Garza, A. J.; Bell, A. T.; Head-Gordon, M. Mechanism of CO₂ Reduction at Copper Surfaces: Pathways to C₂ Products. ACS Catal. 2018, 8, 1490– 1499,  DOI: 10.1021/acscatal.7b03477

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   Mechanism of CO2 Reduction at Copper Surfaces: Pathways to C2 Products

   Garza, Alejandro J.; Bell, Alexis T.; Head-Gordon, Martin

   ACS Catalysis (2018), 8 (2), 1490-1499CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)

   From constraints from reported exptl. observations and d. functional theory simulations, the authors propose a mechanism for the redn. of CO2 to C2 products on Cu electrodes. To model the effects of an applied potential bias on the reactions, calcns. are carried out with a variable, fractional no. of electrons on the unit cell, which is optimized so that the Fermi level matches the actual chem. potential of electrons (i.e., the applied bias); an implicit electrolyte model allows for compensation of the surface charge so that neutrality is maintained in the overall simulation cell. The authors' mechanism explains the presence of the seven C2 species that were detected in the reaction, as well as other notable exptl. observations. Also, the authors' results shed light on the difference in activities toward C2 products between the (100) and (111) facets of Cu. The authors compare the authors' methodologies and findings with those in other recent mechanistic studies of the Cu-catalyzed CO2 redn. reaction.
7. 7

   Wang, X.; Wang, Z.; Zhuang, T. T.; Dinh, C. T.; Li, J.; Nam, D. H.; Li, F.; Huang, C. W.; Tan, C. S.; Chen, Z.; Chi, M.; Gabardo, C. M.; Seifitokaldani, A.; Todorović, P.; Proppe, A.; Pang, Y.; Kirmani, A. R.; Wang, Y.; Ip, A. H.; Richter, L. J.; Scheffel, B.; Xu, A.; Lo, S. C.; Kelley, S. O.; Sinton, D.; Sargent, E. H. Efficient Upgrading of CO to C₃ Fuel Using Asymmetric C-C Coupling Active Sites. Nat. Commun. 2019, 10, 5186,  DOI: 10.1038/s41467-019-13190-6

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   Efficient upgrading of CO to C3 fuel using asymmetric C-C coupling active sites

   Wang Xue; Wang Ziyun; Zhuang Tao-Tao; Dinh Cao-Thang; Li Jun; Nam Dae-Hyun; Li Fengwang; Tan Chih-Shan; Seifitokaldani Ali; Todorovic Petar; Proppe Andrew; Pang Yuanjie; Wang Yuhang; Ip Alexander H; Scheffel Benjamin; Xu Aoni; Sargent Edward H; Li Jun; Gabardo Christine M; Pang Yuanjie; Sinton David; Huang Chun-Wei; Lo Shen-Chuan; Chen Zitao; Chi Miaofang; Proppe Andrew; Kelley Shana O; Kirmani Ahmad R; Richter Lee J; Kelley Shana O

   Nature communications (2019), 10 (1), 5186 ISSN:.

   The electroreduction of C1 feedgas to high-energy-density fuels provides an attractive avenue to the storage of renewable electricity. Much progress has been made to improve selectivity to C1 and C2 products, however, the selectivity to desirable high-energy-density C3 products remains relatively low. We reason that C3 electrosynthesis relies on a higher-order reaction pathway that requires the formation of multiple carbon-carbon (C-C) bonds, and thus pursue a strategy explicitly designed to couple C2 with C1 intermediates. We develop an approach wherein neighboring copper atoms having distinct electronic structures interact with two adsorbates to catalyze an asymmetric reaction. We achieve a record n-propanol Faradaic efficiency (FE) of (33 ± 1)% with a conversion rate of (4.5 ± 0.1) mA cm(-2), and a record n-propanol cathodic energy conversion efficiency (EEcathodic half-cell) of 21%. The FE and EEcathodic half-cell represent a 1.3× improvement relative to previously-published CO-to-n-propanol electroreduction reports.
8. 8

   Kim, D.; Kley, C. S.; Li, Y.; Yang, P. Copper Nanoparticle Ensembles for Selective Electroreduction of CO₂ to C–C₃ Products. Proc. Natl. Acad. Sci. U. S. A. 2017, 114, 10560– 10565,  DOI: 10.1073/pnas.1711493114

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   Copper nanoparticle ensembles for selective electroreduction of CO2 to C2-C3 products

   Kim, Dohyung; Kley, Christopher S.; Li, Yifan; Yang, Peidong

   Proceedings of the National Academy of Sciences of the United States of America (2017), 114 (40), 10560-10565CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)

   Direct conversion of carbon dioxide to multicarbon products remains as a grand challenge in electrochem. CO2 redn. Various forms of oxidized copper have been demonstrated as electrocatalysts that still require large overpotentials. Here, we show that an ensemble of Cu nanoparticles (NPs) enables selective formation of C2-C3 products at low overpotentials. Densely packed Cu NP ensembles underwent structural transformation during electrolysis into electrocatalytically active cube-like particles intermixed with smaller nanoparticles. Ethylene, ethanol, and n-propanol are the major C2-C3 products with onset potential at -0.53 V (vs. reversible hydrogen electrode, RHE) and C2-C3 faradaic efficiency (FE) reaching 50% at only -0.75 V. Thus, the catalyst exhibits selective generation of C2-C3 hydrocarbons and oxygenates at considerably lowered overpotentials in neutral pH aq. media. In addn., this approach suggests new opportunities in realizing multicarbon product formation from CO2, where the majority of efforts has been to use oxidized copper-based materials. Robust catalytic performance is demonstrated by 10 h of stable operation with C2-C3 c.d. 10 mA/cm2 (at -0.75 V), rendering it attractive for solar-to-fuel applications. Tafel anal. suggests reductive CO coupling as a rate detg. step for C2 products, while n-propanol (C3) prodn. seems to have a discrete pathway.
9. 9

   Zheng, Y.; Vasileff, A.; Zhou, X.; Jiao, Y.; Jaroniec, M.; Qiao, S. Z. Understanding the Roadmap for Electrochemical Reduction of CO₂ to Multi-Carbon Oxygenates and Hydrocarbons on Copper-Based Catalysts. J. Am. Chem. Soc. 2019, 141, 7646– 7659,  DOI: 10.1021/jacs.9b02124

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   Understanding the Roadmap for Electrochemical Reduction of CO2 to Multi-Carbon Oxygenates and Hydrocarbons on Copper-Based Catalysts

   Zheng, Yao; Vasileff, Anthony; Zhou, Xianlong; Jiao, Yan; Jaroniec, Mietek; Qiao, Shi-Zhang

   Journal of the American Chemical Society (2019), 141 (19), 7646-7659CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

   A review. Electrochem. redn. of CO2 to high-energy-d. oxygenates and hydrocarbons beyond CO is important for long-term and large-scale renewable energy storage. However, the key step of the C-C bond formation needed for the generation of C2 products induces an addnl. barrier on the reaction. This inevitably creates larger overpotentials and greater variety of products as compared to the conversion of CO2 to C1 products. Therefore, an in-depth understanding of the catalytic mechanism is required for advancing the design of efficient electrocatalysts to control the reaction pathway to the desired products. Herein, we present a crit. appraisal of redn. of CO2 to C2 products focusing on the connection between the fundamentals of reaction and efficient electrocatalysts. An in-depth discussion of the mechanistic aspects of various C2 reaction pathways on copper-based catalysts is presented together with consideration of practical factors under electrocatalytic operating conditions. By providing some typical examples illustrating the benefit of merging theor. calcns., surface characterization, and electrochem. measurements, we try to address the key issues of the ongoing debate toward better understanding electrochem. redn. of CO2 at the at. level and envisioning the roadmap for C2 products generation.
10. 10

    Gao, D.; Arán-Ais, R. M.; Jeon, H. S.; Roldan Cuenya, B. Rational Catalyst and Electrolyte Design for CO₂ Electroreduction towards Multicarbon Products. Nat. Catal. 2019, 2, 198– 210,  DOI: 10.1038/s41929-019-0235-5

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    Rational catalyst and electrolyte design for CO2 electroreduction towards multicarbon products

    Gao, Dunfeng; Aran-Ais, Rosa M.; Jeon, Hyo Sang; Roldan Cuenya, Beatriz

    Nature Catalysis (2019), 2 (3), 198-210CODEN: NCAACP; ISSN:2520-1158. (Nature Research)

    A review. The CO2 electroredn. reaction (CO2RR) to fuels and feedstocks is an attractive route to close the anthropogenic carbon cycle and store renewable energy. The generation of more reduced chems., esp. multicarbon oxygenate and hydrocarbon products (C2+) with higher energy densities, is highly desirable for industrial applications. However, selective conversion of CO2 to C2+ suffers from a high overpotential, a low reaction rate and low selectivity, and the process is extremely sensitive to the catalyst structure and electrolyte. Here we discuss strategies to achieve high C2+ selectivity through rational design of the catalyst and electrolyte. Current state-of-the-art catalysts, including Cu and Cu-bimetallic catalysts, as well as some alternative materials, are considered. The importance of taking into consideration the dynamic evolution of the catalyst structure and compn. are highlighted, focusing on findings extd. from in situ and operando characterizations. Addnl. theor. insight into the reaction mechanisms underlying the improved C2+ selectivity of specific catalyst geometries and compns. in synergy with a well-chosen electrolyte are also provided.
11. 11

    Kortlever, R.; Shen, J.; Schouten, K. J. P.; Calle-Vallejo, F.; Koper, M. T. M. Catalysts and Reaction Pathways for the Electrochemical Reduction of Carbon Dioxide. J. Phys. Chem. Lett. 2015, 6, 4073– 4082,  DOI: 10.1021/acs.jpclett.5b01559

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    Catalysts and Reaction Pathways for the Electrochemical Reduction of Carbon Dioxide

    Kortlever, Ruud; Shen, Jing; Schouten, Klaas Jan P.; Calle-Vallejo, Federico; Koper, Marc T. M.

    Journal of Physical Chemistry Letters (2015), 6 (20), 4073-4082CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)

    A review. The electrochem. redn. of CO2 has gained significant interest recently as it has the potential to trigger a sustainable solar-fuel-based economy. In this Perspective, we highlight several heterogeneous and mol. electrocatalysts for the redn. of CO2 and discuss the reaction pathways through which they form various products. Among those, copper is a unique catalyst as it yields hydrocarbon products, mostly methane, ethylene, and ethanol, with acceptable efficiencies. As a result, substantial effort has been invested to det. the special catalytic properties of copper and to elucidate the mechanism through which hydrocarbons are formed. These mechanistic insights, together with mechanistic insights of CO2 redn. on other metals and mol. complexes, can provide crucial guidelines for the design of future catalyst materials able to efficiently and selectively reduce CO2 to useful products.
12. 12

    Kuhl, K. P.; Cave, E. R.; Abram, D. N.; Jaramillo, T. F. New Insights into the Electrochemical Reduction of Carbon Dioxide on Metallic Copper Surfaces. Energy Environ. Sci. 2012, 5, 7050– 7059,  DOI: 10.1039/c2ee21234j

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    New insights into the electrochemical reduction of carbon dioxide on metallic copper surfaces

    Kuhl, Kendra P.; Cave, Etosha R.; Abram, David N.; Jaramillo, Thomas F.

    Energy & Environmental Science (2012), 5 (5), 7050-7059CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)

    We report new insights into the electrochem. redn. of CO2 on a metallic copper surface, enabled by the development of an exptl. methodol. with unprecedented sensitivity for the identification and quantification of CO2 electroredn. products. This involves a custom electrochem. cell designed to maximize product concns. coupled to gas chromatog. and NMR for the identification and quantification of gas and liq. products, resp. We studied copper across a range of potentials and obsd. a total of 16 different CO2 redn. products, five of which are reported here for the first time, thus providing the most complete view of the reaction chem. reported to date. Taking into account the chem. identities of the wide range of C1-C3 products generated and the potential-dependence of their turnover frequencies, mechanistic information is deduced. We discuss a scheme for the formation of multi-carbon products involving enol-like surface intermediates as a possible pathway, accounting for the obsd. selectivity for eleven distinct C2+ oxygenated products including aldehydes, ketones, alcs., and carboxylic acids.
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    Monteiro, M. C. O.; Dattila, F.; Hagedoorn, B.; García-Muelas, R.; López, N.; Koper, M. T. M. Absence of CO₂ Electroreduction on Copper, Gold and Silver Electrodes without Metal Cations in Solution. Nat. Catal. 2021, 4, 654– 662,  DOI: 10.1038/s41929-021-00655-5

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    Absence of CO2 electroreduction on copper, gold and silver electrodes without metal cations in solution

    Monteiro, Mariana C. O.; Dattila, Federico; Hagedoorn, Bellenod; Garcia-Muelas, Rodrigo; Lopez, Nuria; Koper, Marc T. M.

    Nature Catalysis (2021), 4 (8), 654-662CODEN: NCAACP; ISSN:2520-1158. (Nature Portfolio)

    The electrocatalytic redn. of carbon dioxide is widely studied for the sustainable prodn. of fuels and chems. Metal ions in the electrolyte influence the reaction performance, although their main role is under discussion. Here we studied CO2 redn. on gold electrodes through cyclic voltammetry and showed that, without a metal cation, the reaction does not take place in a pure 1 mM H2SO4 electrolyte. We further investigated the CO2 redn. with and without metal cations in soln. using scanning electrochem. microscopy in the surface-generation tip-collection mode with a platinum ultramicroelectrode as a CO and H2 sensor. CO is only produced on gold, silver or copper if a metal cation is added to the electrolyte. D. functional theory simulations confirmed that partially desolvated metal cations stabilize the CO2- intermediate via a short-range electrostatic interaction, which enables its redn. Overall, our results redefine the reaction mechanism and provide definitive evidence that pos. charged species from the electrolyte are key to stabilize the crucial reaction intermediate. [graphic not available: see fulltext].
14. 14

    de Gregorio, G. L.; Burdyny, T.; Loiudice, A.; Iyengar, P.; Smith, W. A.; Buonsanti, R. Facet-Dependent Selectivity of Cu Catalysts in Electrochemical CO₂ Reduction at Commercially Viable Current Densities. ACS Catal. 2020, 10, 4854– 4862,  DOI: 10.1021/acscatal.0c00297

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    Facet-Dependent Selectivity of Cu Catalysts in Electrochemical CO2 Reduction at Commercially Viable Current Densities

    De Gregorio, Gian Luca; Burdyny, Thomas; Loiudice, Anna; Iyengar, Pranit; Smith, Wilson A.; Buonsanti, Raffaella

    ACS Catalysis (2020), 10 (9), 4854-4862CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)

    Despite substantial progress in the electrochem. conversion of CO2 into value-added chems., the translation of fundamental studies into com. relevant conditions requires addnl. efforts. Here, the authors study the catalytic properties of tailored Cu nanocatalysts under com. relevant current densities in a gas-fed flow cell. Their facet-dependent selectivity is retained in this device configuration with the advantage of further suppressing H prodn. and increasing the faradaic efficiencies toward the CO2 redn. products compared to a conventional H-cell. The combined catalyst and system effects result in state-of-the art product selectivity at high current densities (in the range 100-300 mA/cm2) and at relatively low applied potential (≥-0.65 V vs. RHE). Cu cubes reach an ethylene selectivity of up to 57% with a corresponding mass activity of 700 mA/mg, and Cu octahedra reach a methane selectivity of up to 51% with a corresponding mass activity of 1.45 A/mg in 1 M KOH.
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    Baricuatro, J. H.; Kwon, S.; Kim, Y. G.; Cummins, K. D.; Naserifar, S.; Goddard, W. A. Operando Electrochemical Spectroscopy for CO on Cu(100) at pH 1 to 13: Validation of Grand Canonical Potential Predictions. ACS Catal. 2021, 11, 3173– 3181,  DOI: 10.1021/acscatal.0c05564

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    15

    Operando Electrochemical Spectroscopy for CO on Cu(100) at pH 1 to 13: Validation of Grand Canonical Potential Predictions

    Baricuatro, Jack H.; Kwon, Soonho; Kim, Youn-Geun; Cummins, Kyle D.; Naserifar, Saber; Goddard, William A., III

    ACS Catalysis (2021), 11 (5), 3173-3181CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)

    Electrochem. redn. of CO2 to value-added products is an attractive strategy to address issues of increasing atm. CO2 concn. Cu is the only pure metal catalyst known to electrochem. convert CO2 to appreciable amts. of oxygenates and hydrocarbons such as EtOH, CH4, and C2H4, but the Faraday efficiencies are too low and the onset potentials are too high. To discover electrocatalytic systems better than Cu, the authors use in silico strategies based on new grand canonical potential (GCP) methods, but the complexity of the electrode-electrolyte interface makes it difficult to validate the accuracy of GCP. Operando electrochem. polarization-modulation IR spectroscopy (PMIRS) provides a performance benchmark for theor. tools that account for the vibrational stretching frequencies of surface-bound CO, νCO, as a function of pH and applied potential U. The authors show here that GCP calcns. of the surface coverages of H*, OH*, and CO* on Cu(100) as a function of U lead to excellent predictions of the potential-dependent νCO and its shift with pH from 1 to 13. This validation justifies the use of GCP for predicting the performance of catalyst designs.
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    Liu, G.; Lee, M.; Kwon, S.; Zeng, G.; Eichhorn, J.; Buckley, A. Y.; Toste, F. D.; Goddard, W. A., III; Toma, F. M. CO₂ Reduction on Pure Cu Produces Only H₂ after Subsurface O Is Depleted: Theory and Experiment. Proc. Natl. Acad. Sci. U. S. A. 2021, 118, e2012649118  DOI: 10.1073/pnas.2012649118

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    16

    CO2 reduction on pure Cu produces only H2 after subsurface O is depleted: Theory and experiment

    Liu, Guiji; Lee, Michelle; Kwon, Soonho; Zeng, Guosong; Eichhorn, Johanna; Buckley, Aya K.; Toste, F. Dean; Goddard, William A. III; Toma, Francesca M.

    Proceedings of the National Academy of Sciences of the United States of America (2021), 118 (23), e2012649118CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)

    We elucidate the role of subsurface oxygen on the prodn. of C2 products from CO2 redn. over Cu electrocatalysts using the newly developed grand canonical potential kinetics d. functional theory method, which predicts that the rate of C2 prodn. on pure Cu with no O is ∼500 times slower than H2 evolution. In contrast, starting with Cu2O, the rate of C2 prodn. is >5,000 times faster than pure Cu(111) and comparable to H2 prodn. To validate these predictions exptl., we combined time-dependent product detection with multiple characterization techniques to show that ethylene prodn. decreases substantially with time and that a sufficiently prolonged reaction time (up to 20 h) leads only to H2 evolution with ethylene prodn. ∼1,000 times slower, in agreement with theory. This result shows that maintaining substantial subsurface oxygen is essential for long-term C2 prodn. with Cu catalysts.
17. 17

    Yang, H.; Negreiros, F. R.; Sun, Q.; Xie, M.; Sementa, L.; Stener, M.; Ye, Y.; Fortunelli, A.; Goddard, W. A.; Cheng, T. Predictions of Chemical Shifts for Reactive Intermediates in CO₂ Reduction under Operando Conditions. ACS Appl. Mater. Interfaces 2021, 13, 31554– 31560,  DOI: 10.1021/acsami.1c02909

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    17

    Predictions of Chemical Shifts for Reactive Intermediates in CO2 Reduction under Operando Conditions

    Yang, Hao; Negreiros, Fabio Ribeiro; Sun, Qintao; Xie, Miao; Sementa, Luca; Stener, Mauro; Ye, Yifan; Fortunelli, Alessandro; Goddard III, William A.; Cheng, Tao

    ACS Applied Materials & Interfaces (2021), 13 (27), 31554-31560CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)

    The electroredn. of CO2 into value-added products is a significant step toward closing the global C loop, but its performance remains far from meeting the requirement of any practical application. The insufficient understanding of the reaction mechanism is one of the major causes that impede future development. Although several possible reaction pathways are proposed, significant debates exist due to the lack of exptl. support. The authors provide opportunities for expts. to validate the reaction mechanism by providing predictions of the core-level shifts (CLS) of reactive intermediates, which can be verified by the XPS data in the expt. The authors 1st validated the authors' methods from benchmark calcns. of cases with reliable expts., from which the authors reach consistent predictions with exptl. results. Then, the authors conduct theor. calcns. under conditions close to the operando exptl. ones and predict the C 1s CLS of 20 reactive intermediates in the CO2 redn. reaction (CO2RR) to CH4 and C2H4 on a Cu(100) catalyst by carefully including solvation effects and applied voltage (U). The results presented in this work should be guidelines for future expts. to verify and interpret the reaction mechanism of CO2RR.
18. 18

    Dattila, F.; Garclá-Muelas, R.; López, N. Active and Selective Ensembles in Oxide-Derived Copper Catalysts for CO₂ Reduction. ACS Energy Lett. 2020, 5, 3176– 3184,  DOI: 10.1021/acsenergylett.0c01777

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    18

    Active and Selective Ensembles in Oxide-Derived Copper Catalysts for CO2 Reduction

    Dattila, Federico; Garcia-Muelas, Rodrigo; Lopez, Nuria

    ACS Energy Letters (2020), 5 (10), 3176-3184CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)

    Copper catalysts are unique in CO2 redn. as they allow the formation of C2+ products. Depending on the catalysts' synthesis, product distribution varies significantly: while Cu nanoparticles produce mainly methane and hydrogen, oxide-derived copper leads to ethylene and ethanol. Here, by means of ab initio mol. dynamics on oxygen-depleted models, we identified the ensembles controlling catalytic performance. Upon reconstruction and irresp. of the starting structure, recurrent patterns defined by their coordination and charges appear: metallic Cu0, polarized Cuδ+, and oxidic Cu+. These species combine to form 14 ensembles. Among them, 4-(6-)coordinated Cu adatoms and Cu3δ+O3 are responsible for tethering CO2, while metastable near-surface oxygens in fcc-(111) or (100)-like Cu domains promote C-C bond formation via glyoxylate species, thus triggering selective C2+ prodn. at low onset potentials. Our work provides guidelines for modeling complex structural rearrangements under CO2 redn. conditions and devising new synthetic protocols toward an enhanced catalytic performance.
19. 19

    Mandal, L.; Yang, K. R.; Motapothula, M. R.; Ren, D.; Lobaccaro, P.; Patra, A.; Sherburne, M.; Batista, V. S.; Yeo, B. S.; Ager, J. W.; Martin, J.; Venkatesan, T. Investigating the Role of Copper Oxide in Electrochemical CO₂ Reduction in Real Time. ACS Appl. Mater. Interfaces 2018, 10, 8574– 8584,  DOI: 10.1021/acsami.7b15418

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    19

    Investigating the Role of Copper Oxide in Electrochemical CO2 Reduction in Real Time

    Mandal, Lily; Yang, Ke R.; Motapothula, Mallikarjuna Rao; Ren, Dan; Lobaccaro, Peter; Patra, Abhijeet; Sherburne, Matthew; Batista, Victor S.; Yeo, Boon Siang; Ager, Joel W.; Martin, Jens; Venkatesan, T.

    ACS Applied Materials & Interfaces (2018), 10 (10), 8574-8584CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)

    Cu oxides were of considerable interest as electrocatalysts for CO2 redn. (CO2R) in aq. electrolytes. However, their role as an active catalyst in reducing the required overpotential and improving the selectivity of reaction compared with that of polycryst. Cu remains controversial. Here, the authors introduce the use of selected-ion flow tube mass spectrometry, in concert with chronopotentiometry, in situ Raman spectroscopy, and computational modeling, to study CO2R on Cu2O nanoneedles, Cu2O nanocrystals, and Cu2O nanoparticles. The authors show exptl. that the selective formation of gaseous C2 products (i.e., ethylene) in CO2R is preceded by the redn. of the Cu oxide (Cu2OR) surface to metallic Cu. From d. functional theory modeling, CO2R products are not formed as long as Cu2O is present at the surface because Cu2OR is kinetically and energetically more favorable than CO2R.
20. 20

    Zhan, C.; Dattila, F.; Rettenmaier, C.; Bergmann, A.; Kühl, S.; García-Muelas, R.; López, N.; Roldan Cuenya, B. Revealing the CO Coverage-Driven C–C Coupling Mechanism for Electrochemical CO₂ Reduction on Cu₂O Nanocubes via Operando Raman Spectroscopy. ACS Catal. 2021, 11, 7694– 7701,  DOI: 10.1021/acscatal.1c01478

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    20

    Revealing the CO Coverage-Driven C-C Coupling Mechanism for Electrochemical CO2 Reduction on Cu2O Nanocubes via Operando Raman Spectroscopy

    Zhan, Chao; Dattila, Federico; Rettenmaier, Clara; Bergmann, Arno; Kuehl, Stefanie; Garcia-Muelas, Rodrigo; Lopez, Nuria; Cuenya, Beatriz Roldan

    ACS Catalysis (2021), 11 (13), 7694-7701CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)

    Electrochem. redn. of carbon dioxide (CO2RR) is an attractive route to close the carbon cycle and potentially turn CO2 into valuable chems. and fuels. However, the highly selective generation of multicarbon products remains a challenge, suffering from poor mechanistic understanding. Herein, we used operando Raman spectroscopy to track the potential-dependent redn. of Cu2O nanocubes and the surface coverage of reaction intermediates. In particular, we discovered that the potential-dependent intensity ratio of the Cu-CO stretching band to the CO rotation band follows a volcano trend similar to the CO2RR Faradaic efficiency for multicarbon products. By combining operando spectroscopic insights with D. Functional Theory, we proved that this ratio is detd. by the CO coverage and that a direct correlation exists between the potential-dependent CO coverage, the preferred C-C coupling configuration, and the selectivity to C2+ products. Thus, operando Raman spectroscopy can serve as an effective method to quantify the coverage of surface intermediates during an electrocatalytic reaction.
21. 21

    Eilert, A.; Cavalca, F.; Roberts, F. S.; Osterwalder, J.; Liu, C.; Favaro, M.; Crumlin, E. J.; Ogasawara, H.; Friebel, D.; Pettersson, L. G. M.; Nilsson, A. Subsurface Oxygen in Oxide-Derived Copper Electrocatalysts for Carbon Dioxide Reduction. J. Phys. Chem. Lett. 2017, 8, 285– 290,  DOI: 10.1021/acs.jpclett.6b02273

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    21

    Subsurface Oxygen in Oxide-Derived Copper Electrocatalysts for Carbon Dioxide Reduction

    Eilert, Andre; Cavalca, Filippo; Roberts, F. Sloan; Osterwalder, Jurg; Liu, Chang; Favaro, Marco; Crumlin, Ethan J.; Ogasawara, Hirohito; Friebel, Daniel; Pettersson, Lars G. M.; Nilsson, Anders

    Journal of Physical Chemistry Letters (2017), 8 (1), 285-290CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)

    Cu electrocatalysts derived from an oxide showed extraordinary electrochem. properties for the CO2 redn. reaction (CO2RR). Using in situ ambient pressure XPS and quasi in situ EELS in a transmission electron microscope, there is a substantial amt. of residual O in nanostructured, oxide-derived Cu electrocatalysts but no residual Cu oxide. From these findings in combination with d. functional theory simulations, probably residual subsurface O changes the electronic structure of the catalyst and creates sites with higher CO binding energy. If such sites are stable under the strongly reducing conditions found in CO2RR, these findings would explain the high efficiencies of oxide-derived Cu in reducing CO2 to multicarbon compds. such as ethylene.
22. 22

    Fan, Q.; Zhang, X.; Ge, X.; Bai, L.; He, D.; Qu, Y.; Kong, C.; Bi, J.; Ding, D.; Cao, Y.; Duan, X.; Wang, J.; Yang, J.; Wu, Y. Manipulating Cu Nanoparticle Surface Oxidation States Tunes Catalytic Selectivity toward CH₄ or C₂₊ Products in CO₂ Electroreduction. Adv. Energy Mater. 2021, 11, 2101424,  DOI: 10.1002/aenm.202101424

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    22

    Manipulating Cu Nanoparticle Surface Oxidation States Tunes Catalytic Selectivity toward CH4 or C2+ Products in CO2 Electroreduction

    Fan, Qikui; Zhang, Xue; Ge, Xiaohu; Bai, Licheng; He, Dongsheng; Qu, Yunteng; Kong, Chuncai; Bi, Jinglei; Ding, Dawei; Cao, Yueqiang; Duan, Xuezhi; Wang, Jin; Yang, Jian; Wu, Yuen

    Advanced Energy Materials (2021), 11 (36), 2101424CODEN: ADEMBC; ISSN:1614-6840. (Wiley-Blackwell)

    Herein, a facile seed-assisted strategy for prepg. Cu nanoparticles (NPs) with polyvinyl pyrrolidone (PVP) capping is presented. Compared to the Cu NPs with deficient PVP protection, the Cu NPs capped with a sufficient amt. of PVP remain almost completely as Cu0 species. In contrast, the Cu NPs that are considered PVP deficient form an oxide structure in which the inner layer is face-centered cubic Cu and the outer layer is, at least in part, made up of Cu2O species. Furthermore, to eliminate CO2 mol. diffusion and simultaneously obtain significant c.d. (200 mA cm-2) for industrial applications, a flow cell configuration is used for carbon dioxide electro redn. reaction (CO2RR) testing in 0.5 M potassium hydroxide soln. The Cu NPs with zero valence deliver Faradaic efficiencies (FEs) for the CO2 redn. to CH4 of over 70%, with a c.d. exceeding 200 mA cm-2, outstripping the performances of the majority of the reported CO2 electrocatalysts. Interestingly, the distribution of products catalyzed by the Cu NPs with +1 valence includes multicarbon products (C2+) such as C2H4, C2H5OH, CH3COOH, and C3H7OH with combined FEs of >80%, with current densities of up to 300 mA cm-2. The above results unambiguously establish that surface oxidn. of Cu species plays a crucial role in the CO2RR.
23. 23

    Lim, C. F. C.; Harrington, D. A.; Marshall, A. T. Altering the Selectivity of Galvanostatic CO₂ Reduction on Cu Cathodes by Periodic Cyclic Voltammetry and Potentiostatic Steps. Electrochim. Acta 2016, 222, 133– 140,  DOI: 10.1016/j.electacta.2016.10.185

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    23

    Altering the selectivity of galvanostatic CO2 reduction on Cu cathodes by periodic cyclic voltammetry and potentiostatic steps

    Lim, C. F. C.; Harrington, D. A.; Marshall, A. T.

    Electrochimica Acta (2016), 222 (), 133-140CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)

    Anal. of the product selectivity and potential of Cu cathodes during galvanostatic CO2 redn. is reported. Initially, clean Cu cathodes are more selective for H evolution, but as the Cu is slowly poisoned by CO2 redn. products (most likely C) the cathode potential becomes more neg., which in turn drives the formation of CH4, C2H4 and CO. As the accumulation of surface poisons continues, the selectivity towards CH4 and C2H4 begins to decrease due to the loss in neighboring reaction sites that support the hydrogenation of COads by Hads. In an attempt to avoid these changes in product selectivity, periodic cyclic voltammetry and potentiostatic steps were used throughout extended periods of galvanostatic CO2 redn. Contrary to previous literature, temporarily interrupting galvanostatic CO2 redn. with short periods at potentials between -0.5 and -0.1 V vs. Ag|AgCl suppresses the formation of CH4, CO and C2H4. Probably this is due to the partial removal or oxidn. of adsorbed CO2 redn. intermediates and that this clean cathode surface is more active for the H evolution reaction. However, when brief potentiostatic steps (84 and 200 s) were conducted at more neg. potentials (-1.2 V vs. Ag|AgCl), the CO2 redn. selectivity could be switched from CH4 to CO, and maintained for at least 2 h. This change in selectivity probably is caused by an increase in the surface coverage of COads (at the expense of Hads) during the brief -1.2 V steps, which then enables the Cu cathode to switch between multiple steady-state surface coverages when the cathodic current is re-applied.
24. 24

    Bui, J. C.; Kim, C.; Weber, A. Z.; Bell, A. T. Dynamic Boundary Layer Simulation of Pulsed CO₂ Electrolysis on a Copper Catalyst. ACS Energy Lett. 2021, 6, 1181– 1188,  DOI: 10.1021/acsenergylett.1c00364

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    24

    Dynamic Boundary Layer Simulation of Pulsed CO2 Electrolysis on a Copper Catalyst

    Bui, Justin C.; Kim, Chanyeon; Weber, Adam Z.; Bell, Alexis T.

    ACS Energy Letters (2021), 6 (4), 1181-1188CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)

    Pulsed electrolysis was demonstrated to improve the faradaic efficiency (FE) to C2+ products during the electrochem. redn. of CO2 over a Cu catalyst, but the nature of this enhancement is poorly understood. Herein, the authors developed a time-dependent continuum model of pulsed CO2 electrolysis on Cu in 0.1M CsHCO3 that faithfully represents the exptl. obsd. effects of pulsed electrolysis. Pulsing results in dynamic changes in the pH and CO2 concn. near the Cu surface, which lead to an enhanced C2+ FE as a consequence of repeatedly accessing a transient state of heightened pH and CO2 concn. at high cathodic overpotential. Using these insights, a variety of pulse shapes were explored to establish operating conditions that maximize the rate of C2+ product formation and minimize the rates of H2 and C1 product formation.
25. 25

    Kim, C.; Weng, L. C.; Bell, A. T. Impact of Pulsed Electrochemical Reduction of CO₂ on the Formation of C₂₊ Products over Cu. ACS Catal. 2020, 10, 12403– 12413,  DOI: 10.1021/acscatal.0c02915

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    25

    Impact of Pulsed Electrochemical Reduction of CO2 on the Formation of C2+ Products over Cu

    Kim, Chanyeon; Weng, Lien-Chun; Bell, Alexis T.

    ACS Catalysis (2020), 10 (21), 12403-12413CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)

    The authors report the results of exptl. and theor. studies aimed at developing a detailed understanding of how pulsed electrolysis alters the prodn. of the temporal evolution of products over Cu and in particular increases the formation of C2+ products. The catalyst is a Cu film sputtered onto the surface of a PTFE membrane, through which the products of CO2 redn. are sampled for anal. by differential electrochem. mass spectroscopy (DEMS). To avoid changes in the catalyst morphol., the cathode potential is set at -0.8 V vs. RHE and -1.15 V vs. RHE. The faradaic efficiency (FE) for H evolution reaction (HER) minimizes and for the CO2 redn. reaction (CO2RR) maximizes when the durations at each potential are 10 s. Under these conditions, the FE for the HER decreases to 11%, relative to 22% for static electrolysis, at -1.15 V vs. RHE, and the FE for the CO2RR increases to 89%, relative to 78% for static electrolysis. Pulsed electrolysis also increases the FE for C2+ products from 68% for static electrolysis to 81%. Temporal anal. of the products by DEMS reveals that while the variation in product concns. near the cathode begins in synchrony at the start of pulsed electrolysis, the concn. of C2H4 increases and those of CO and H2 decrease with extended time. The authors attribute these trends to an increase in the ratio of adsorbed CO to H on the catalyst surface. Simulation of pulsed electrolysis also shows that during the period when the cathode is at -0.8 V vs. RHE, the local concn. of CO2 in the electrolyte near the cathode builds up. This inventory then allows electrolysis during the period at -1.15 V vs. RHE to occur with a higher CO2 concn. than could be achieved for static electrolysis. The net effect of alternating cathode potentials is to enhance the local concn. of CO2, which favors the progress of the CO2RR relative to the HER and in particular the formation of C2+ products.
26. 26

    Kimura, K. W.; Fritz, K. E.; Kim, J.; Suntivich, J.; Abruña, H. D.; Hanrath, T. Controlled Selectivity of CO₂ Reduction on Copper by Pulsing the Electrochemical Potential. ChemSusChem 2018, 11, 1781– 1786,  DOI: 10.1002/cssc.201800318

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    26

    Controlled Selectivity of CO2 Reduction on Copper by Pulsing the Electrochemical Potential

    Kimura, Kevin W.; Fritz, Kevin E.; Kim, Jiyoon; Suntivich, Jin; Abruna, Hector D.; Hanrath, Tobias

    ChemSusChem (2018), 11 (11), 1781-1786CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)

    We demonstrate a simple strategy to enhance the CO2 redn. reaction (CO2RR) selectivity by applying a pulsed electrochem. potential to a polycryst. copper electrode. By controlling the pulse duration, we show that the hydrogen evolution reaction (HER) is highly suppressed to a fraction of the original value (<5 % faradaic efficiency) and selectivity for the CO2RR dramatically improves (>75 % CH4 and >50 % CO faradaic efficiency). We attribute the improved CO2RR selectivity to a dynamically rearranging surface coverage of hydrogen and intermediate species during the pulsing. Our finding provides new insights into the interplay of transport and reaction processes as well as timescales of competing pathways to enable new opportunities to tune CO2RR selectivity by adjusting the pulse profile. Addnl., the pulsed potential method we describe can be easily applied to other catalysts materials to improve their CO2RR selectivity.
27. 27

    Nogami, G.; Ltagaki, H.; Shiratsuch, R. Pulsed Electroreduction of CO₂ on Copper Electrodes-II. J. Electrochem. Soc. 1994, 141, 1138– 1142,  DOI: 10.1149/1.2054886

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    27

    Pulsed electroreduction of CO2 on copper electrodes - II

    Nogami, Gyoichi; Itagaki, Hideo; Shiratsuchi, Ryuichi

    Journal of the Electrochemical Society (1994), 141 (5), 1138-42CODEN: JESOAN; ISSN:0013-4651.

    The pulsed method newly developed by the present authors was applied to electroredn. of CO2 on Cu electrodes. The optimum cathodic bias for generating CH4 and C2H4 was -2.6 V vs. SCE, and the optimum anodic bias for generation of C2H4 was slightly more anodic than that of CH4. Total faradaic efficiency ηHc for the generation of CH4 and C2H4 became a max. at 10° and reached ∼65%. Also, ηHc increased with increasing redn. time in strong contrast with conventional potentiostatic electroredn. in which ηHc decreased drastically with the redn. time due to the poisoning effect of the products. Probably some surface intermediate formed by the interaction between CO2 and the thin oxide layer on a Cu electrode is finally reduced to CH4 to C2H4, depending on the anodic reactions taking place during the anodic period.
28. 28

    Didomenico, R. C.; Hanrath, T. Pulse Symmetry Impacts the C₂ Product Selectivity in Pulsed Electrochemical CO₂ Reduction. ACS Energy Lett. 2022, 7, 292– 299,  DOI: 10.1021/acsenergylett.1c02166

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    28

    Pulse Symmetry Impacts the C2 Product Selectivity in Pulsed Electrochemical CO2 Reduction

    DiDomenico, Rileigh Casebolt; Hanrath, Tobias

    ACS Energy Letters (2022), 7 (1), 292-299CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)

    Pulsed electrochem. CO2 redn. has emerged as an attractive approach to direct product selectivity and activity. The versatility of the pulse profile creates opportunities to study fundamental processes and optimize reaction conditions. The authors examd. the effects of applied pulse potential, duration, and shape to understand the interfacial reaction environment with an eye toward optimized C2 product formation. The authors present an electrochem. anal. to show that upper pulse potentials with pos. anodic current (indicative of anion coadsorption) improve reaction stability and enhance C2 selectivity (reaching 76% FE). Whereas changing pulse duration had little to no effect on C2 selectivity, pulse symmetry significantly affected selectivity. Notably, sym. pulses most selectively produce C2 products. The relation between pulse symmetry and selectivity in the context of COads coverage and C-C coupling reaction energy landscape as a result of anion coadsorption, increased interfacial charge, and elec. field variation effects are discussed.
29. 29

    Jeon, H. S.; Timoshenko, J.; Rettenmaier, C.; Herzog, A.; Yoon, A.; Chee, S. W.; Oener, S.; Hejral, U.; Haase, F. T.; Roldan Cuenya, B. Selectivity Control of Cu Nanocrystals in a Gas-Fed Flow Cell through CO₂ Pulsed Electroreduction. J. Am. Chem. Soc. 2021, 143, 7578– 7587,  DOI: 10.1021/jacs.1c03443

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    29

    Selectivity Control of Cu Nanocrystals in a Gas-Fed Flow Cell through CO2 Pulsed Electroreduction

    Jeon, Hyo Sang; Timoshenko, Janis; Rettenmaier, Clara; Herzog, Antonia; Yoon, Aram; Chee, See Wee; Oener, Sebastian; Hejral, Uta; Haase, Felix T.; Roldan Cuenya, Beatriz

    Journal of the American Chemical Society (2021), 143 (19), 7578-7587CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    The authors have taken advantage of a pulsed CO2 electroredn. reaction (CO2RR) approach to tune the product distribution at industrially relevant current densities in a gas-fed flow cell. The authors compared the CO2RR selectivity of Cu catalysts subjected to either potentiostatic conditions (fixed applied potential of -0.7 VRHE) or pulsed electrolysis conditions (1 s pulses at oxidative potentials ranging from Ean = 0.6 to 1.5 VRHE, followed by 1 s pulses at -0.7 VRHE) and identified the main parameters responsible for the enhanced product selectivity obsd. in the latter case. Herein, two distinct regimes were obsd.: (i) for Ean = 0.9 VRHE the authors obtained 10% enhanced C2 product selectivity (FEC2H4 = 43.6% and FEC2H5OH = 19.8%) in comparison to the potentiostatic CO2RR at -0.7 VRHE (FEC2H4 = 40.9% and FEC2H5OH = 11%), (ii) while for Ean = 1.2 VRHE, high CH4 selectivity (FECH4 = 48.3% vs. 0.1% at const. -0.7 VRHE) was obsd. Operando spectroscopy (XAS, SERS) and ex situ microscopy (SEM and TEM) measurements revealed that these differences in catalyst selectivity can be ascribed to structural modifications and local pH effects. The morphol. reconstruction of the catalyst obsd. after pulsed electrolysis with Ean = 0.9 VRHE, including the presence of highly defective interfaces and grain boundaries, was found to play a key role in the enhancement of the C2 product formation. In turn, pulsed electrolysis with Ean = 1.2 VRHE caused the consumption of OH- species near the catalyst surface, leading to an OH-poor environment favorable for CH4 prodn.
30. 30

    Timoshenko, J.; Bergmann, A.; Rettenmaier, C.; Herzog, A.; Arán-Ais, R. M.; Jeon, H. S.; Haase, F. T.; Hejral, U.; Grosse, P.; Kühl, S.; Davis, E. M.; Tian, J.; Magnussen, O.; Roldan Cuenya, B. Steering the Structure and Selectivity of CO₂ Electroreduction Catalysts by Potential Pulses. Nat. Catal. 2022, 5, 259– 267,  DOI: 10.1038/s41929-022-00760-z

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    30

    Steering the structure and selectivity of CO2 electroreduction catalysts by potential pulses

    Timoshenko, Janis; Bergmann, Arno; Rettenmaier, Clara; Herzog, Antonia; Aran-Ais, Rosa M.; Jeon, Hyo Sang; Haase, Felix T.; Hejral, Uta; Grosse, Philipp; Kuehl, Stefanie; Davis, Earl M.; Tian, Jing; Magnussen, Olaf; Roldan Cuenya, Beatriz

    Nature Catalysis (2022), 5 (4), 259-267CODEN: NCAACP; ISSN:2520-1158. (Nature Portfolio)

    Convoluted selectivity trends and a missing link between reaction product distribution and catalyst properties hinder practical applications of the electrochem. CO2 redn. reaction (CO2RR) for multicarbon product generation. Here we employ operando X-ray absorption and X-ray diffraction methods with subsecond time resoln. to unveil the surprising complexity of catalysts exposed to dynamic reaction conditions. We show that by using a pulsed reaction protocol consisting of alternating working and oxidizing potential periods that dynamically perturb catalysts derived from Cu2O nanocubes, one can decouple the effect of the ensemble of coexisting copper species on the product distribution. In particular, an optimized dynamic balance between oxidized and reduced copper surface species achieved within a narrow range of cathodic and anodic pulse durations resulted in a twofold increase in ethanol prodn. compared with static CO2RR conditions. This work thus preps. the ground for steering catalyst selectivity through dynamically controlled structural and chem. transformations.
31. 31

    An, H.; Wu, L.; Mandemaker, L. D. B.; Yang, S.; de Ruiter, J.; Wijten, J. H. J.; Janssens, J. C. L.; Hartman, T.; van der Stam, W.; Weckhuysen, B. M. Sub-Second Time-Resolved Surface-Enhanced Raman Spectroscopy Reveals Dynamic CO Intermediates during Electrochemical CO₂ Reduction on Copper. Angew. Chem., Int. Ed. 2021, 60, 16576– 16584,  DOI: 10.1002/anie.202104114

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    31

    Sub-Second Time-Resolved Surface-Enhanced Raman Spectroscopy Reveals Dynamic CO Intermediates during Electrochemical CO2 Reduction on Copper

    An, Hongyu; Wu, Longfei; Mandemaker, Laurens D. B.; Yang, Shuang; de Ruiter, Jim; Wijten, Jochem H. J.; Janssens, Joris C. L.; Hartman, Thomas; van der Stam, Ward; Weckhuysen, Bert M.

    Angewandte Chemie, International Edition (2021), 60 (30), 16576-16584CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)

    The electrocatalytic CO2 redn. reaction (CO2RR) into hydrocarbons is a promising approach for greenhouse gas mitigation, but many details of this dynamic reaction remain elusive. Here, time-resolved surface-enhanced Raman spectroscopy (TR-SERS) is employed to successfully monitor the dynamics of CO2RR intermediates and Cu surfaces with sub-second time resoln. Anodic treatment at 1.55 V vs. RHE and subsequent surface oxide redn. (<-0.4 V vs. RHE) induced roughening of the Cu electrode surface, which resulted in hotspots for TR-SERS, enhanced time resoln. (down to ≈0.7 s) and 4-fold improved CO2RR efficiency toward ethylene. With TR-SERS, the initial restructuring of the Cu surface was followed (<7 s), after which a stable surface surrounded by increased local alky. was formed. The authors' measurements revealed that a highly dynamic CO intermediate, with a characteristic vibration <2060 cm-1, is related to C-C coupling and ethylene prodn. (-0.9 V vs. RHE), whereas lower cathodic bias (-0.7 V vs. RHE) resulted in gaseous CO prodn. from isolated and static CO surface species with a distinct vibration at 2092 cm-1.
32. 32

    Pan, Z.; Wang, K.; Ye, K.; Wang, Y.; Su, H. Y.; Hu, B.; Xiao, J.; Yu, T.; Wang, Y.; Song, S. Intermediate Adsorption States Switch to Selectively Catalyze Electrochemical CO₂ Reduction. ACS Catal. 2020, 10, 3871– 3880,  DOI: 10.1021/acscatal.9b05115

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    Intermediate Adsorption States Switch to Selectively Catalyze Electrochemical CO2 Reduction

    Pan, Zhangweihao; Wang, Kun; Ye, Kaihang; Wang, Ying; Su, Hai-Yan; Hu, Bihua; Xiao, Juan; Yu, Tongwen; Wang, Yi; Song, Shuqin

    ACS Catalysis (2020), 10 (6), 3871-3880CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)

    Electrochem. CO2 redn. (CO2R) powered by renewable energy to convert CO2 mols. into formate is of great interest. It is still challenging to develop an efficient CO2R catalyst with high selectivity. Herein, the authors adjust the adsorption states of CO2- intermediates to improve the selectivity of CO2 toward formate by doping S to Cu-based electrocatalysts. S doping could stabilize the reductive-state Cu as the active site for CO2R. The vibration models of CO2- intermediates within in situ Raman spectroscopy reveal that the selectivity improvement is ascribed to the change of the adsorption state from coexisting O*CO- and OC*O*- to the dominating OC*O*-. The electrocatalyst manifests high selectivity and activity toward formate (max. faradaic efficiency ≤76.5% and max. partial c.d. 21.06 mA cm-2).
33. 33

    Li, H.; Wei, P.; Gao, D.; Wang, G. In Situ Raman Spectroscopy Studies for Electrochemical CO₂ Reduction over Cu Catalysts. Current Opinion in Green and Sustainable Chemistry 2022, 34, 100589  DOI: 10.1016/j.cogsc.2022.100589

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    In situ Raman spectroscopy studies for electrochemical CO2 reduction over Cu catalysts

    Li, Hefei; Wei, Pengfei; Gao, Dunfeng; Wang, Guoxiong

    Current Opinion in Green and Sustainable Chemistry (2022), 34 (), 100589CODEN: COGSC4; ISSN:2452-2236. (Elsevier B.V.)

    An accurate understanding of reaction mechanisms is crucial for the rational design of highly efficient catalytic materials for electrochem. CO2 redn. reaction (CO2RR). In situ characterization methods are powerful to reveal structure-performance correlations of working catalysts under reaction conditions. Electrochem. in situ Raman spectroscopy is able to probe catalyst structures as well as reaction intermediates on/near catalyst surfaces in an electrochem. environment. In this short review, we briefly introduce the principle of electrochem. in situ Raman spectroscopy and highlight recent advances of its applications in tracking structure evolution of catalyst surfaces and identifying reaction intermediates during CO2RR over selected Cu catalysts. The research challenges and opportunities of investigating CO2RR mechanisms using electrochem. in situ Raman spectroscopy are also proposed.
34. 34

    Moradzaman, M.; Mul, G. In Situ Raman Study of Potential-Dependent Surface Adsorbed Carbonate, CO, OH, and C Species on Cu Electrodes During Electrochemical Reduction of CO₂. ChemElectroChem 2021, 8, 1478– 1485,  DOI: 10.1002/celc.202001598

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    In Situ Raman Study of Potential-Dependent Surface Adsorbed Carbonate, CO, OH, and C Species on Cu Electrodes During Electrochemical Reduction of CO2

    Moradzaman, Mozhgan; Mul, Guido

    ChemElectroChem (2021), 8 (8), 1478-1485CODEN: CHEMRA; ISSN:2196-0216. (Wiley-VCH Verlag GmbH & Co. KGaA)

    Using in situ surface-enhanced Raman spectroscopy (SERS), and 13C/12C and D2O/H2O isotopic labeling for assignment, we show potential dependent transients in surface compn. of Cu-catalyzed electrochem. redn. of CO2 in carbonate soln. First, redn. of Cu(I)oxide is accompanied by adsorption of predominantly monodentate carbonate at ∼1067 cm-1 starting in the potential range from [+0.2 V∼-0.2 V]. Contrary to recently advocated hypotheses, and based on the significant presence at anodic potential, a band in this potential range at ∼1540 cm-1 can be assigned to bidentate carbonate. As expected, appearance of surface CO was obsd. in the range of [-0.4 V∼-1.0 V], clearly identified by the Cu-CO vibration at 360 cm-1. Most importantly, at the more neg. end of this potential range, we identified the formation of surface OH, and for the first time a surface Cu-C species, showing Raman bands at ∼25 cm-1 (Cu-OH) and ∼500 cm-1 (Cu-C), resp. In the potential range of [-1.0 V∼-1.4 V], surface CO disappears, while the Cu-OH and Cu-C species are persistent. Interestingly pos. polarization at >0.1 V removes these species and restores the surface to Cu(I)oxide, rendering the surface processes completely reversible. Implications of this study for mechanistic understanding of electrode deactivation and practical operation are discussed.
35. 35

    Chernyshova, I. V.; Somasundaran, P.; Ponnurangam, S. On the Origin of the Elusive First Intermediate of CO₂ Electroreduction. Proc. Natl. Acad. Sci. U. S. A. 2018, 115, E9261– E9270,  DOI: 10.1073/pnas.1802256115

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    35

    On the origin of the elusive first intermediate of CO2 electroreduction

    Chernyshova, Irina V.; Somasundaran, Ponisseril; Ponnurangam, Sathish

    Proceedings of the National Academy of Sciences of the United States of America (2018), 115 (40), E9261-E9270CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)

    The authors resolve the long-standing controversy about the 1st step of the CO2 electroredn. to fuels in aq. electrolytes by providing direct spectroscopic evidence that the 1st intermediate of the CO2 conversion to formate on Cu is a carboxylate anion *CO2- coordinated to the surface through one of its C-O bonds. The authors identify this intermediate and gain insight into its formation, its chem. and electronic properties, as well as its dependence on the electrode potential by taking advantage of a cutting-edge methodol. that includes operando surface-enhanced Raman scattering (SERS) empowered by isotope exchange and electrochem. Stark effects, reaction kinetics (Tafel) anal., and d. functional theory (DFT) simulations. The SERS spectra are measured on an operating Cu surface. These results advance the mechanistic understanding of CO2 electroredn. and its selectivity to CO and formate.
36. 36

    Gunathunge, C. M.; Li, X.; Li, J.; Hicks, R. P.; Ovalle, V. J.; Waegele, M. M. Spectroscopic Observation of Reversible Surface Reconstruction of Copper Electrodes under CO₂ Reduction. J. Phys. Chem. C 2017, 121, 12337– 12344,  DOI: 10.1021/acs.jpcc.7b03910

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    36

    Spectroscopic Observation of Reversible Surface Reconstruction of Copper Electrodes under CO2 Reduction

    Gunathunge, Charuni M.; Li, Xiang; Li, Jingyi; Hicks, Robert P.; Ovalle, Vincent J.; Waegele, Matthias M.

    Journal of Physical Chemistry C (2017), 121 (22), 12337-12344CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)

    The ability of Cu to catalyze the electrochem. redn. of CO2 greatly depends on its nanoscale surface morphol. While previous studies found evidence of irreversible changes of Cu nanoparticle and thin film electrodes following electrolysis, the authors present here the 1st observation of the reversible reconstruction of electrocatalytic Cu surfaces induced by the adsorbed CO intermediate. Using attenuated total internal reflection IR and surface-enhanced Raman spectroscopies, the reversible formation of nanoscale metal clusters on the electrode is revealed by the appearance of a new C≡O absorption band characteristic of CO bound to undercoordinated Cu atoms and by the strong enhancement of the surface-enhanced Raman effect. The authors' study shows that the morphol. of the catalytic Cu surface is not static but dynamically adapts with changing reaction conditions.
37. 37

    Chang, X.; Xiong, H.; Xu, Y.; Zhao, Y.; Lu, Q.; Xu, B. Determining Intrinsic Stark Tuning Rates of Adsorbed CO on Copper Surfaces. Catal. Sci. Technol. 2021, 11, 6825– 6831,  DOI: 10.1039/D1CY01090E

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    Determining intrinsic stark tuning rates of adsorbed CO on copper surfaces

    Chang, Xiaoxia; Xiong, Haocheng; Xu, Yifei; Zhao, Yaran; Lu, Qi; Xu, Bingjun

    Catalysis Science & Technology (2021), 11 (20), 6825-6831CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)

    The abrupt change in potential between the electrode and the electrolyte, and the resulting interfacial elec. field, is the driving force in electrochem. reactions. For surface mediated electrocatalytic reactions, the interfacial elec. field is believed to have a key impact on the stability and reactivity of adsorbed intermediates. However, the exact mechanisms remain a topic of discussion. In this context, reliable measurements of the interfacial elec. field are a prerequisite in understanding how it influences the rate and product distribution in electrochem. reactions. The vibrational Stark effect of adsorbates, such as CO, offers an accessible means to assess the interfacial elec. field strength by detg. the shift of vibrational peaks of the adsorbates with potential, i.e., the Stark tuning rate. However, the vibrational Stark effect could be convoluted with the dynamical dipole coupling effect of the adsorbates on weak binding surfaces such as Cu, thus complicating the detn. of the intrinsic Stark tuning rate. In this work, we report a general and effective strategy of detg. the intrinsic Stark tuning rate by removing the impact of the dynamical coupling of adsorbed CO on the Cu surface with surface enhanced IR absorption spectroscopy. A similar intrinsic Stark tuning rate of ∼33 cm V-1 was obtained on oxide-derived Cu in different electrolyte pH of 7.2, 10.9 and 12.9, indicating the pH independence of the interfacial elec. field. Investigations on different Cu electrodes show that the intrinsic Stark tuning rates on (electro)chem. deposited films are close to 33 cm V-1, while particulate Cu catalysts show a similar value of ∼68 cm V-1. These observations indicate that aggregate morphol., rather than the size and shape of individual catalyst particles, has a more prominent impact on the interfacial elec. field.
38. 38

    Jiang, S.; D’Amario, L.; Dau, H. Copper Carbonate Hydroxide as Precursor of Interfacial CO in CO₂ Electroreduction. ChemSusChem 2022, 15, e202102506  DOI: 10.1002/cssc.202102506

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    38

    Copper Carbonate Hydroxide as Precursor of Interfacial CO in CO2 Electroreduction

    Jiang, Shan; D'Amario, Luca; Dau, Holger

    ChemSusChem (2022), 15 (8), e202102506CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)

    Copper electrodes are esp. effective in catalysis of C2 and further multi-carbon products in the CO2 redn. reaction (CO2RR) and therefore of major technol. interest. The reasons for the unparalleled Cu performance in CO2RR are insufficiently understood. Here, the electrode-electrolyte interface was highlighted as a dynamic phys.-chem. system and determinant of catalytic events. Exploiting the intrinsic surface-enhanced Raman effect of previously characterized Cu foam electrodes, operando Raman expts. were used to interrogate structures and mol. interactions at the electrode-electrolyte interface at subcatalytic and catalytic potentials. Formation of a copper carbonate hydroxide (CuCarHyd) was detected, which resembles the mineral malachite. Its carbonate ions could be directly converted to CO at low overpotential. These and further expts. suggested a basic mode of CO2/carbonate redn. at Cu electrodes interfaces that contrasted previous mechanistic models: the starting point in carbon redn. was not CO2 but carbonate ions bound to the metallic Cu electrode in form of CuCarHyd structures. It was hypothesized that Cu oxides residues could enhance CO2RR indirectly by supporting formation of CuCarHyd motifs. The presence of CuCarHyd patches at catalytic potentials might result from alkalization in conjunction with local elec. potential gradients, enabling the formation of metastable CuCarHyd motifs over a large range of potentials.
39. 39

    Eilert, A.; Roberts, F. S.; Friebel, D.; Nilsson, A. Formation of Copper Catalysts for CO₂ Reduction with High Ethylene/Methane Product Ratio Investigated with in Situ X-Ray Absorption Spectroscopy. J. Phys. Chem. Lett. 2016, 7, 1466– 1470,  DOI: 10.1021/acs.jpclett.6b00367

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    39

    Formation of Copper Catalysts for CO2 Reduction with High Ethylene/Methane Product Ratio Investigated with In Situ X-ray Absorption Spectroscopy

    Eilert, Andre; Roberts, F. Sloan; Friebel, Daniel; Nilsson, Anders

    Journal of Physical Chemistry Letters (2016), 7 (8), 1466-1470CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)

    Nanostructured copper cathodes are among the most efficient and selective catalysts to date for making multicarbon products from the electrochem. carbon dioxide redn. reaction (CO2RR). We report an in situ X-ray absorption spectroscopy investigation of the formation of a copper nanocube CO2RR catalyst with high activity that highly favors ethylene over methane prodn. The results show that the precursor for the copper nanocube formation is copper(I)-oxide, not copper(I)-chloride as previously assumed. A second route to an electrochem. similar material via a copper(II)-carbonate/hydroxide is also reported. This study highlights the importance of using oxidized copper precursors for constructing selective CO2 redn. catalysts and shows the precursor oxidn. state does not affect the electrocatalyst selectivity toward ethylene formation.
40. 40

    Spodaryk, M.; Zhao, K.; Zhang, J.; Oveisi, E.; Züttel, A. The Role of Malachite Nanorods for the Electrochemical Reduction of CO₂ to C₂ Hydrocarbons. Electrochim. Acta 2019, 297, 55– 60,  DOI: 10.1016/j.electacta.2018.11.124

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    40

    The role of malachite nanorods for the electrochemical reduction of CO2 to C2 hydrocarbons

    Spodaryk, Mariana; Zhao, Kun; Zhang, Jie; Oveisi, Emad; Zuttel, Andreas

    Electrochimica Acta (2019), 297 (), 55-60CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)

    The electrochem. redn. of CO2 to higher hydrocarbons is a very challenging process that has high potential for the storage of large amts. of renewable energy with a high gravimetric and volumetric energy d. The distribution of hydrocarbons from the electrocatalytic redn. of CO2 is primarily detd. by the interaction of the cathode material with the CO2 in the electrolyte. While the research on the electrochem. CO2 redn. focuses on the cathode metal and surface structure of the metals, recently evidence was found that the metal itself may not be the active species but rather the product formed from the metal and CO2. In this paper, we report about the synthesis, catalytic activity and selectivity of nanostructured metal carbonate, i.e. malachite, as a highly active catalyst for the electrochem. synthesis of C2 hydrocarbons. These first results obtained on Cu2(OH)2CO3 nanorod-structured "trees" show that carbonate, not the pure metal, is the active catalytic species. This new catalyst favors the prodn. of ethylene (C2H4) and ethane (C2H6) with significantly higher Faradaic efficiency than that of the pure Cu surface.
41. 41

    Henckel, D. A.; Counihan, M. J.; Holmes, H. E.; Chen, X.; Nwabara, U. O.; Verma, S.; Rodríguez-López, J.; Kenis, P. J. A.; Gewirth, A. A. Potential Dependence of the Local pH in a CO₂ Reduction Electrolyzer. ACS Catal. 2021, 11, 255– 263,  DOI: 10.1021/acscatal.0c04297

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    41

    Potential Dependence of the Local pH in a CO2 Reduction Electrolyzer

    Henckel, Danielle A.; Counihan, Michael J.; Holmes, Hannah E.; Chen, Xinyi; Nwabara, Uzoma O.; Verma, Sumit; Rodriguez-Lopez, Joaquin; Kenis, Paul J. A.; Gewirth, Andrew A.

    ACS Catalysis (2021), 11 (1), 255-263CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)

    Quantifying the local pH of a gas diffusion electrode undergoing CO2 redn. is a complicated problem owing to a multitude of competing processes, both electrochem.- and transport-related, possibly affecting the pH at the surface. Here, the authors present surface-enhanced Raman spectroscopy (SERS) and electrochem. data evaluating the local pH of Cu in an alk. flow electrolyzer for CO2 redn. The local pH is evaluated by using the ratio of the SERS signals for HCO3- and CO32-. The local pH is both substantially lower than expected from the bulk electrolyte pH and exhibits dependence on applied potential. Anal. of SERS data reveals that the decrease in pH is assocd. with the formation of malachite [Cu2(OH)2CO3, malachite] due to the presence of sol. Cu(II) species from the initially oxidized electrode surface. After this initial layer of malachite is depleted, the local pH maintains a value >11 even at currents exceeding -20 mA/cm2.
42. 42

    Tromans, D.; RuSun, R. Anodic Behavior of Copper in Weakly Alkaline Solutions. J. Electrochem. Soc. 1992, 139, 1946– 1950

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43. 43

    González, S.; Pérez, M.; Barrera, M.; González Elipe, A. R.; Souto, R. M. Mechanism of Copper Passivation in Aqueous Sodium Carbonate-Bicarbonate Solution Derived from Combined X-Ray Photoelectron Spectroscopic and Electrochemical Data. J. Phys. Chem. B 1998, 102, 5483– 5489,  DOI: 10.1021/jp981069k

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    Mechanism of Copper Passivation in Aqueous Sodium Carbonate-Bicarbonate Solution Derived from Combined X-ray Photoelectron Spectroscopic and Electrochemical Data

    Gonzalez, S.; Perez, M.; Barrera, M.; Gonzalez Elipe, A. R.; Souto, R. M.

    Journal of Physical Chemistry B (1998), 102 (28), 5483-5489CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)

    XPS was used to characterize the passivating films anodically formed on Cu in NaHCO3 and Na2CO3 aq. solns. (9-11 pH range). The influence of potential and soln. pH on the compn. and protective characteristics of the passivating layers was considered. The anal. of the exptl. data supports the composed nature of the formed films. Direct evidence regarding the addn. of carbonate species to the surface layers in potential ranges pos. to the onset of Cu(II) oxides formation was achieved from XPS data. Specific components of the XPS signals attributable to carbonate species present in the passivating films could be identified.
44. 44

    Simon, G. H.; Kley, C. S.; Roldan Cuenya, B. Potential-Dependent Morphology of Copper Catalysts During CO₂ Electroreduction Revealed by In Situ Atomic Force Microscopy. Angew. Chem., Int. Ed. 2021, 60, 2561– 2568,  DOI: 10.1002/anie.202010449

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    Potential-Dependent Morphology of Copper Catalysts During CO2 Electroreduction Revealed by In Situ Atomic Force Microscopy

    Simon, Georg H.; Kley, Christopher S.; Roldan Cuenya, Beatriz

    Angewandte Chemie, International Edition (2021), 60 (5), 2561-2568CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)

    Electrochem. AFM is a powerful tool for the real-space characterization of catalysts under realistic electrochem. CO2 redn. (CO2RR) conditions. The evolution of structural features ranging from the micrometer to the at. scale could be resolved during CO2RR. Using Cu(100) as model surface, distinct nanoscale surface morphologies and their potential-dependent transformations from granular to smoothly curved mound-pit surfaces or structures with rectangular terraces are revealed during CO2RR in 0.1 M KHCO3. The d. of undercoordinated copper sites during CO2RR is shown to increase with decreasing potential. In situ at.-scale imaging reveals specific adsorption occurring at distinct cathodic potentials impacting the obsd. catalyst structure. These results show the complex interrelation of the morphol., structure, defect d., applied potential, and electrolyte in copper CO2RR catalysts.
45. 45

    Singhal, A.; Pai, M. R.; Rao, R.; Pillai, K. T.; Lieberwirth, I.; Tyagi, A. K. Copper(I) Oxide Nanocrystals - One Step Synthesis, Characterization, Formation Mechanism, and Photocatalytic Properties. Eur. J. Inorg. Chem. 2013, 2013, 2640– 2651,  DOI: 10.1002/ejic.201201382

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    45

    Copper(I) Oxide Nanocrystals - One Step Synthesis, Characterization, Formation Mechanism, and Photocatalytic Properties

    Singhal, Anshu; Pai, Mrinal R.; Rao, Rekha; Pillai, Kodanthakurup T.; Lieberwirth, Ingo; Tyagi, Avesh K.

    European Journal of Inorganic Chemistry (2013), 2013 (14), 2640-2651CODEN: EJICFO; ISSN:1434-1948. (Wiley-VCH Verlag GmbH & Co. KGaA)

    We report here two different simple, one-pot, and low cost chem. synthetic routes for the prepn. of Cu2O nanocrystals: (a) thermal decompn. of copper-org. precursors copper(II) acetate or copper(II) acetylacetonate in long chain org. solvents oleyl alc. and trioctylamine, resp., at 170 °C and (b) a surfactant-free solvothermal approach involving the reaction of copper(II) acetylacetonate in acetone at 140 °C. The structure and morphol. of the nanocrystals have been characterized in detail by XRD, FTIR spectroscopy, Raman spectroscopy, and high-resoln. transmission electron microscopy (HRTEM). The optical properties of the nanocrystals have been explored by diffuse-reflectance spectroscopy (DRS) and a blue shift of the optical band gap of the nanocrystals is obsd. owing to size effects. Based on the FTIR, GC-MS, and 13C{1H} NMR studies of post-reaction solns., different formation mechanisms for the Cu2O nanocrystals, which depend on the synthetic approach, have been proposed. Oleyl alc. and trioctylamine play dual roles as solvents and mild reductants and reduce CuII species to CuI species during the course of the thermal decompn. reactions. The solvothermal reaction of copper(II) acetylacetonate in acetone possibly proceeds by acetylacetone-mediated redn. of Cu2+ to Cu+ in the absence of any reducing agent. The potential of Cu2O nanocrystals as photocatalytic materials for hydrogen generation from water/methanol (2:1) mixts. under UV/Vis irradn. has also been evaluated. The results show that all the nanocystalline Cu2O samples generate H2.
46. 46

    Debbichi, L.; Marco De Lucas, M. C.; Pierson, J. F.; Krüger, P. Vibrational Properties of CuO and Cu₄O₃ from First-Principles Calculations, and Raman and Infrared Spectroscopy. J. Phys. Chem. C 2012, 116, 10232– 10237,  DOI: 10.1021/jp303096m

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    Vibrational Properties of CuO and Cu4O3 from First-Principles Calculations, and Raman and Infrared Spectroscopy

    Debbichi, L.; Marco de Lucas, M. C.; Pierson, J. F.; Kruger, P.

    Journal of Physical Chemistry C (2012), 116 (18), 10232-10237CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)

    A combined exptl. and theor. study is reported on the vibrational properties of tenorite CuO and paramelaconite Cu4O3. The optically active modes were measured by Raman scattering and IR absorption spectroscopy. First-principles calcns. were carried out with the LDA+U approach to account for strong electron correlation in the Cu oxides. The vibrational properties were computed ab initio using the so-called direct method. Agreement is found between the measured Raman and IR peak positions and the calcd. phonon frequencies at the Brillouin zone center, which allows the assignment of all prominent peaks of the Cu4O3 spectra. Through a detailed anal. of the displacement eigenvectors, a close relation exists between the Raman modes of CuO and Cu4O3.
47. 47

    Jiang, S.; Klingan, K.; Pasquini, C.; Dau, H. New Aspects of Operando Raman Spectroscopy Applied to Electrochemical CO₂ Reduction on Cu Foams. J. Chem. Phys. 2019, 150, 041718  DOI: 10.1063/1.5054109

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    New aspects of operando Raman spectroscopy applied to electrochemical CO2 reduction on Cu foams

    Jiang, Shan; Klingan, Katharina; Pasquini, Chiara; Dau, Holger

    Journal of Chemical Physics (2019), 150 (4), 041718/1-041718/12CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)

    The mechanism of electrochem. CO2 redn. (CO2RR) on copper surfaces is still insufficiently understood. Operando Raman spectroscopy is ideally suited to elucidate the role of adsorbed reaction intermediates and products. For a Cu foam material which has been previously characterized regarding electrochem. properties and product spectrum, 129 operando spectra are reported, covering the spectral range from 250 to 3300 cm-1. (1) The dendritic foam structure facilitates surface-enhanced Raman spectroscopy (SERS) and thus electrochem. operando spectroscopy, without any further surface manipulations. (2) Both Raman enhancement and SERS background depend strongly on the elec. potential and the "history" of preceding potential sequences. (3) To restore the plausible intensity dependencies of Raman bands, normalization to the SERS background intensity is proposed. (4) Two distinct types of *CO adsorption modes are resolved. (5) Hysteresis in the potential-dependent *CO desorption supports previous electrochem. analyses; satg. *CO adsorption may limit CO formation rates. (6) HCO3- likely deprotonates upon adsorption so that exclusively adsorbed carbonate is detectable, but with strong dependence on the preceding potential sequences. (7) A variety of species and adsorption modes of reaction products contg. C-H bonds were detected and compared to ref. solns. of likely reaction products, but further investigations are required for assignment to specific mol. species. (8) The Raman bands of adsorbed reaction products depend weakly or strongly on the preceding potential sequences. In future investigations, suitably designed potential protocols could provide valuable insights into the potential-dependent kinetics of product formation, adsorption, and desorption. (c) 2019 American Institute of Physics.
48. 48

    Anderson, G. R. The Raman Spectra of Carbon Dioxide in Liquid H₂O and D₂O. J. Phys. Chem. 1976, 81, 273– 276

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    There is no corresponding record for this reference.
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    King, H. E.; Geisler, T. Tracing Mineral Reactions Using Confocal Raman Spectroscopy. Minerals 2018, 8, 158,  DOI: 10.3390/min8040158

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    Tracing mineral reactions using confocal Raman spectroscopy

    King, Helen E.; Geisler, Thorsten

    Minerals (Basel, Switzerland) (2018), 8 (4), 158/1-158/15CODEN: MBSIBI; ISSN:2075-163X. (MDPI AG)

    Raman spectroscopy is a powerful tool used to identify mineral phases, study aq. solns. and gas inclusions as well as providing crystallinity, crystallog. orientation and chem. of mineral phases. When united with isotopic tracers, the information gained from Raman spectroscopy can be expanded and includes kinetic information on isotope substitution and replacement mechanisms. This review will examine the research to date that utilizes Raman spectroscopy and isotopic tracers. Beginning with the Raman effect and its use in mineralogy, the review will show how the kinetics of isotope exchange between an oxyanion and isotopically enriched water can be detd. in situ. Moreover, we show how isotope tracers can help to unravel the mechanisms of mineral replacement that occur at the nanoscale and how they lead to the formation of pseudomorphs. Finally, the use of isotopic tracers as an in situ clock for mineral replacement processes will be discussed as well as where this area of research can potentially be applied in the future.
50. 50

    Monteiro, M. C. O.; Mirabal, A.; Jacobse, L.; Doblhoff-Dier, K.; Barton, S. C.; Koper, M. T. M. Time-Resolved Local pH Measurements during CO₂ Reduction Using Scanning Electrochemical Microscopy: Buffering and Tip Effects. JACS Au 2021, 1, 1915– 1924,  DOI: 10.1021/jacsau.1c00289

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    Time-Resolved Local pH Measurements during CO2 Reduction Using Scanning Electrochemical Microscopy: Buffering and Tip Effects

    Monteiro, Mariana C. O.; Mirabal, Alex; Jacobse, Leon; Doblhoff-Dier, Katharina; Barton, Scott Calabrese; Koper, Marc T. M.

    JACS Au (2021), 1 (11), 1915-1924CODEN: JAAUCR; ISSN:2691-3704. (American Chemical Society)

    The electrochem. redn. of CO2 is widely studied as a sustainable alternative for the prodn. of fuels and chems. The electrolyte's bulk pH and compn. play an important role in the reaction activity and selectivity and can affect the extent of the buildup of pH gradients between the electrode surface and the bulk of the electrolyte. Quantifying the local pH and how it is affected by the soln. species is desirable to gain a better understanding of the CO2 redn. reaction. Local pH measurements can be realized using Scanning Electrochem. Microscopy (SECM); however, finding a pH probe that is stable and selective under CO2 redn. reaction conditions is challenging. Here, we have used our recently developed voltammetric pH sensor to perform pH measurements in the diffusion layer during CO2 redn. using SECM, with high time resoln. Using a 4-hydroxylaminothiophenol (4-HATP)/4-nitrosothiophenol (4-NSTP) functionalized gold ultramicroelectrode, we compare the local pH developed above a gold substrate in an argon atm., when only hydrogen evolution is taking place, to the pH developed in a CO2 atmosphere. The pH is monitored at a fixed distance from the surface, and the sample potential is varied in time. In argon, we observe a gradual increase of pH, while a plateau region is present in CO2 atmosphere due to the formation of HCO3- buffering the reaction interface. By analyzing the diffusion layer dynamics once the sample reaction is turned "off", we gain insightful information on the time scale of the homogeneous reactions happening in soln. and on the time required for the diffusion layer to fully recover to the initial bulk concn. of species. In order to account for the effect of the presence of the SECM tip on the measured pH, we performed finite element method simulations of the fluid and reaction dynamics. The results show the significant localized diffusion hindrance caused by the tip, so that in its absence, the pH values are more acidic than when the tip is present. Nonetheless, through the simulation, we can account for this effect and est. the real local pH values across the diffusion layer.
51. 51

    Raciti, D.; Mao, M.; Park, J. H.; Wang, C. Local pH Effect in the CO₂ Reduction Reaction on High-Surface-Area Copper Electrocatalysts. J. Electrochem. Soc. 2018, 165, F799,  DOI: 10.1149/2.0521810jes

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    Local pH Effect in the CO2 Reduction Reaction on High-Surface-Area Copper Electrocatalysts

    Raciti, David; Mao, Mark; Park, Jun Ha; Wang, Chao

    Journal of the Electrochemical Society (2018), 165 (10), F799-F804CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)

    The local pH on electrode surfaces is known to play an important role in the electrochem. redn. of CO2, which could alter the chem. kinetics and mol. transport under the reaction conditions. Here we report the study of local pH effect on the catalytic performance of high-surface-area Cu electrocatalysts. The electroredn. of CO2 was systematically investigated on three types of Cu nanowires with distinct surface roughness factors and nanostructures. The measured electrocatalytic activities and selectivities were further correlated to the simulated local pH on the electrode surface. It was revealed that the high local pH induced by the prodn. of hydroxide from the reaction beneficially suppresses the evolution of hydrogen and enhances the selectivity toward multi-carbon products, but detrimentally limits the transport of CO2 mols. at large current densities. An optimal range of local pH is detd. for the electroredn. of CO2, which is insightful for improving the design of electrodes for more efficient energy conversion and chem. transformations.
52. 52

    Ryu, J.; Wuttig, A.; Surendranath, Y. Quantification of Interfacial PH Variation at Molecular Length Scales Using a Concurrent Non-Faradaic Reaction. Angewandte Chemie International Edition 2018, 130, 9444– 9448,  DOI: 10.1002/ange.201802756

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    Yang, K.; Kas, R.; Smith, W. A. In Situ Infrared Spectroscopy Reveals Persistent Alkalinity near Electrode Surfaces during CO₂ Electroreduction. J. Am. Chem. Soc. 2019, 141, 15891– 15900,  DOI: 10.1021/jacs.9b07000

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    53

    In Situ Infrared Spectroscopy Reveals Persistent Alkalinity near Electrode Surfaces during CO2 Electroreduction

    Yang, Kailun; Kas, Recep; Smith, Wilson A.

    Journal of the American Chemical Society (2019), 141 (40), 15891-15900CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    Over the past decade, electrochem. CO2 redn. has become a thriving area of research with the aim of converting electricity to renewable chems. and fuels. Recent advances through catalyst development have significantly improved selectivity and activity. However, drawing potential dependent structure-activity relations has been complicated, not only due to the ill-defined and intricate morphol. and mesoscopic structure of electrocatalyts, but also by immense concn. gradients existing between the electrode surface and bulk soln. Here, by using in-situ surface enhanced IR absorption spectroscopy (SEIRAS) and computational modeling, we explicitly show that commonly used strong phosphate buffers cannot sustain the interfacial pH during CO2 electroredn. on Cu electrodes at relatively low current densities, <10 mA/cm2. The pH near the electrode surface was obsd. to be as much as 5 pH units higher compared to bulk soln. in 0.2M phosphate buffer at potentials relevant to the formation of hydrocarbons (1 V vs. RHE), even on smooth polycryst. copper electrodes. Drastically increasing the buffer capacity did not stand out as a viable soln. for the problem as the concurrent prodn. of H increased dramatically, which resulted in a breakdown of the buffer in a narrow potential range. These unforeseen results imply that most of the studies, if not all, on electrochem. CO2 redn. to hydrocarbons in CO2 satd. aq. solns. were evaluated under mass transport limitations on Cu electrodes. We underscore that the large concn. gradients on electrodes with high local c.d. (e.g., nanostructured) have important implications on the selectivity, activity, and kinetic anal., and any attempt to draw structure-activity relationships must rule out mass transport effects.
54. 54

    Li, J.; Chang, X.; Zhang, H.; Malkani, A. S.; Cheng, M. J.; Xu, B.; Lu, Q. Electrokinetic and in Situ Spectroscopic Investigations of CO Electrochemical Reduction on Copper. Nat. Commun. 2021, 12, 3264,  DOI: 10.1038/s41467-021-23582-2

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    Electrokinetic and in situ spectroscopic investigations of CO electrochemical reduction on copper

    Li, Jing; Chang, Xiaoxia; Zhang, Haochen; Malkani, Arnav S.; Cheng, Mu-jeng; Xu, Bingjun; Lu, Qi

    Nature Communications (2021), 12 (1), 3264CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)

    Rigorous electrokinetic results are key to understanding the reaction mechanisms in the electrochem. CO redn. reaction (CORR), however, most reported results are compromised by the CO mass transport limitation. In this work, we detd. mass transport-free CORR kinetics by employing a gas-diffusion type electrode and identified dependence of catalyst surface speciation on the electrolyte pH using in-situ surface enhanced vibrational spectroscopies. Based on the measured Tafel slopes and reaction orders, we demonstrate that the formation rates of C2+ products are most likely limited by the dimerization of CO adsorbate. CH4 prodn. is limited by the CO hydrogenation step via a proton coupled electron transfer and a chem. hydrogenation step of CO by adsorbed hydrogen atom in weakly (7 < pH < 11) and strongly (pH > 11) alk. electrolytes, resp. Further, CH4 and C2+ products are likely formed on distinct types of active sites.
55. 55

    Buzgar, N.; Apopei, A. I. The Raman Study of Certain Carbonates. Geologie, Tomul 2009, 2, 97– 112

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56. 56

    Chan, H. Y. H.; Takoudis, C. G.; Weaver, M. J. Oxide Film Formation and Oxygen Adsorption on Copper in Aqueous Media as Probed by Surface-Enhanced Raman Spectroscopy. J. Phys. Chem. B 1999, 103, 357– 365,  DOI: 10.1021/jp983787c

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    Oxide Film Formation and Oxygen Adsorption on Copper in Aqueous Media As Probed by Surface-Enhanced Raman Spectroscopy

    Chan, Ho Yeung H.; Takoudis, Christos G.; Weaver, Michael J.

    Journal of Physical Chemistry B (1999), 103 (2), 357-365CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)

    The electrode potential-dependent formation of oxygen species on copper in non-complexing aq. media, encompassing oxide phase films and adsorbed oxygen/hydroxide, are explored at different pH values by means of surface-enhanced Raman spectroscopy (SERS). This technique provides a monolayer-sensitive in-situ vibrational probe, which can follow potential-dependent surface speciation on voltammetric or longer time scales. In alk. NaClO4 electrolytes (pH 13), the cyclic voltammetric peaks assocd. with copper oxide phase-film formation and removal are correlated quant. with simultaneously acquired SER spectral sequences. The latter indicate the sequential formation of Cu2O and then mixed Cu2O/Cu(OH)2 layers, diagnosed by the appearance of metal-oxygen lattice vibrations at 625/525 and 460 cm-1, resp. The potential-dependent speciation is in concordance with the Pourbaix diagram, certifying the "bulk-phase" nature of the films. The Raman band intensity-film thickness correlation (the latter deduced from the voltammetric Coulombic charges) indicate that the vibrational spectral responses are limited to the first 15-20 monolayers, consistent with earlier SERS observations and theor. predictions. Weaker bands at ca. 800 and 460 cm-1 are discernible at more neg. potentials, suggestive of hydroxide adsorption. Similar, although thinner, oxide films were deduced to form in neutral 0.1 M NaClO4. In the addnl. presence of chloride under these conditions, a potential-sensitive competition between the formation of a CuCl and a more passivating Cu2O phase film was evident from SERS. While oxide phase films are absent on copper in 0.1 M H2SO4 and 0.1 M HClO4, an adsorbed oxygen species was nonetheless detected from a broad SERS band at ca. 625 cm-1. This feature, which was deduced to involve oxygen rather than hydroxyl from an absence of a frequency shift upon H/D solvent isotopic substitution, is evident throughout most of the "polarizable potential" region on copper in acid, ca. -0.7 to -0.1 V vs SCE. The likely nature and reasons for its remarkable prevalence on copper in acidic media are discussed with ref. to the recent literature.
57. 57

    Niaura, G. Surface-Enhanced Raman Spectroscopic Observation of Two Kinds of Adsorbed OH–ions at Copper Electrode. Electrochim. Acta 2000, 45, 3507– 3519,  DOI: 10.1016/S0013-4686(00)00434-5

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    Surface-enhanced Raman spectroscopic observation of two kinds of adsorbed OH- ions at copper electrode

    Niaura, G.

    Electrochimica Acta (2000), 45 (21), 3507-3519CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Science Ltd.)

    The surface of a polycryst. roughened Cu electrode in 1 M NaOH soln., was studied in situ using surface-enhanced Raman spectroscopy (SERS). Cu2O, adsorbed OH- ions, and water mols. were detected as the electrode potential was varied from open circuit value to -1.20 V vs. SHE. The vibrational spectrum of Cu2O consisted of three main peaks located at 150, 528, and 623 cm-1. The intense and narrow feature at 150 cm-1 is highly characteristic, and could be used for SER monitoring of Cu2O. Two different states of adsorbed OH- ions, giving Cu-OH vibrations around 450-470 cm-1 and 540-580 cm-1, were detected. The distinct nature of the bands was shown by opposite isotopic frequency shifts changing the solvent from H2O to D2O. The frequency of the 1st band decreased by ∼12 cm-1, while the frequency of the 2nd band increased by ∼35 cm-1 in D2O solns. These differences were explained in terms of distinct surface ligation and the formation of strong hydrogen bonds between water mols. and the 2nd type of adsorbed OH- ion. Water mols. were obsd. at the interface at an applied potential -1.20 V.
58. 58

    Zhang, Y.; Gao, X.; Weaver, M. J. Nature of Surface Bonding on Voltammetrically Oxidized Noble Metals in Aqueous Media as Probed by Real-Time Surface-Enhanced Raman Spectroscopy. J. Phys. Chem. 1993, 97, 8656– 8663,  DOI: 10.1021/j100135a020

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    58

    Nature of surface bonding on voltammetrically oxidized noble metals in aqueous media as probed by real-time surface-enhanced Raman spectroscopy

    Zhang, Yun; Gao, Xiaoping; Weaver, Michael J.

    Journal of Physical Chemistry (1993), 97 (33), 8656-63CODEN: JPCHAX; ISSN:0022-3654.

    The nature of the metal-O bonding formed during the initial phase of voltammetric electrooxidn. of Pt, Rh, Ru, and Au surfaces in aq. media was examd. by surface-enhanced Raman spectroscopy (SERS). The 1st 3 surfaces were formed by electrodeposition as ultrathin (∼3 monolayer) films on a SERS-active Au substrate, enabling intense Raman spectra to be obtained for oxidn. of the transition-metal overlayers. Sequences of SER spectra were typically obtained in real time during cyclic potential excursions in acidic (0.1M HClO4) and basic (0.1M KOH) media, enabling the evolution of the surface vibrational properties to be correlated with the simultaneous voltammetric (current-potential) response. Several Raman bands are evident upon surface electrooxidn. within the frequency range ∼250-850 cm-1, corresponding to metal-O vibrational modes. On Au, Pt, and Rh, a broad vibrational band at 500-600 cm-1 appears close to the onset of irreversible surface oxidn. as discerned voltammetrically and is removed upon oxide redn. during the subsequent reverse voltammetric sweep. Together with this feature, ascribed to metal-O stretching within a place-exchanged oxide/hydroxide film, a narrower band at ∼300 cm-1 is obsd. on Pt and Rh in acid, assigned to a bending vibration involving terminally bound O atoms. On Au in base, but not on Pt and Rh, a sep. feature at 420-490 cm-1 is also obsd. at lower potentials, ascribed to specifically adsorbed hydroxide ions, i.e., not involving metal-oxygen place exchange. Ru electrooxidn. yielded more complex spectral behavior, featuring the potential-dependent appearance of several bands at 300-800 cm-1, ascribed to the formation of Ru oxides of differing oxidn. state. A distinction between M-O and M-OH vibrations was undertaken by D solvent isotope shifts. The tendency to form predominantly surface metal oxides (M-O) rather than hydroxides (M-OH) is described. These vibrational features are compared with related spectral observations for metal surfaces dosed with O in gas-phase (and vacuum) environments and for bulk-phase metal oxides. The similarities and differences in the O surface bonding are assessed.
59. 59

    Akemann, W.; Otto, A. Vibrational Modes of CO Adsorbed on Disordered Copper Films. J. Raman Spectrosc. 1991, 22, 797– 803,  DOI: 10.1002/jrs.1250221212

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    Vibrational modes of carbon monoxide adsorbed on disordered copper films

    Akemann, W.; Otto, A.

    Journal of Raman Spectroscopy (1991), 22 (12), 797-803CODEN: JRSPAF; ISSN:0377-0486.

    The Raman spectrum of CO adsorbed on copper films featuring at.-scale roughness reveals four vibrational modes: a CO stretching mode at 2102 cm-1, a restricted rotation at 282 cm-1 and restricted translations normal and parallel to the local surface at 355 and 24 cm-1. The spectrum originates from a minority of CO mols. adsorbed at surface defect sites via a single coordination bond. The bond strength is increased compared with adsorption at facet sites, presumably owing to increased σ-donation promoted by a local depletion of conduction electron d. A splitting of the translational mode into two sep. frequencies, expected for adsorption sites with rotational symmetry less than C3v, was not obsd. The intensities and line shapes of the SERS bands are sensitive to a thermally induced rearrangement of the mols. within the adsorbate layer. Spectra from annealed substrate films indicate dominant excitation of the rotational mode on surfaces with a lesser degree of at.-scale roughness. This is explained by the relevance of Herzberg-Teller contributions to the inelastic scattering of photoexcited electrons by the surface mols.
60. 60

    Louisia, S.; Kim, D.; Li, Y.; Gao, M.; Yu, S.; Roh, I.; Yang, P. The Presence and Role of the Intermediary CO Reservoir in Heterogeneous Electroreduction of CO₂. Proc. Natl. Acad. Sci. U. S. A. 2022, 119, e2201922119  DOI: 10.1073/pnas.2201922119

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61. 61

    Gunathunge, C. M.; Li, J.; Li, X.; Hong, J. J.; Waegele, M. M. Revealing the Predominant Surface Facets of Rough Cu Electrodes under Electrochemical Conditions. ACS Catal. 2020, 10, 6908– 6923,  DOI: 10.1021/acscatal.9b05532

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    61

    Revealing the Predominant Surface Facets of Rough Cu Electrodes under Electrochemical Conditions

    Gunathunge, Charuni M.; Li, Jingyi; Li, Xiang; Hong, Julie J.; Waegele, Matthias M.

    ACS Catalysis (2020), 10 (12), 6908-6923CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)

    Metal electrodes with rough surfaces are often found to convert CO or CO2 to hydrocarbons and oxygenates with high selectivity and at high reaction rates in comparison with their smooth counterparts. The at.-level morphol. of a rough electrode is likely one key factor responsible for its comparatively high catalytic selectivity and activity. However, few methods are capable of probing the at.-level structure of rough metal electrodes under electrocatalytic conditions. As a result, the nuances in the at.-level surface morphol. that control the catalytic characteristics of these electrodes have remained largely unexplored. Because the C≡O stretching frequency of atop-bound CO (COatop) depends on the coordination of the underlying metal atom, the IR spectrum of this reaction intermediate on the Cu electrode could, in principle, provide structural information about the catalytic surface during electrolysis. However, other effects, such as dynamic dipole coupling, easily obscure the dependence of the frequency on the surface morphol. Further, in the limit of low COatop coverage, where coupling effects are small, the C≡O stretching frequencies of COatop on Cu(111) and Cu(100) facets are virtually identical. Therefore, from the C≡O stretching frequency, it is not straightforward to distinguish between these two ubiquitous surface facets, which exhibit vastly different CO redn. activities. Herein, key features of the at.-level surface morphol. of rough Cu electrodes can be inferred from the potential dependence of the line shape of the C≡O stretching band of COatop. Specifically, the authors compared two types of rough Cu thin-film electrodes that are routinely employed in the context of surface-enhanced IR absorption spectroscopy (SEIRAS). Cu films that are electrochem. deposited on Si-supported Au films (CuAu-Si) are poor catalysts for the redn. of CO to ethylene in comparison to Cu films (Cu-Si) that are electrolessly deposited onto Si crystals. As quantified by differential electrochem. mass spectrometry (DEMS), the onset potential for ethylene is ~ 200 ± 65 mV more cathodic for CuAu-Si than that for Cu-Si. To reveal the origin of the disparate catalytic properties of Cu-Si and CuAu-Si, the authors probed the surfaces of the electrodes with cyclic voltammetry (CV) and SEIRAS. The CV characterization suggests that the (111) surface facet predominates on CuAu-Si, whereas the (100) facet is more common on Cu-Si. SEIRAS reveals that the line shape of the C≡O stretching of COatop is composed of two bands that are attributable to COatop on terrace and defect sites. The different surface structures manifest themselves as starkly different potential dependences of the line shape of the C≡O stretching mode of COatop on the two types of electrodes. With a simple Boltzmann model that considers the different adsorption energies of COatop on terrace and defect sites, and the resulting COatop populations on terrace and defect sites, the obsd. electrode-specific potential dependence of the line shape is consistent with the presence of different predominant terrace sites on the two types of films. This strategy for assessing the at.-level morphol. is not restricted to SEIRAS but could also be applied to the C≡O stretching bands recorded with surface-enhanced Raman spectroscopy (SERS), which is suitable for probing a wide range of rough Cu electrodes. Therefore, with this work, the authors establish the potential dependence of the C≡O stretching band of COatop as a probe of the at.-level surface structure of rough metal electrodes under electrochem. conditions. When it is coupled with complementary techniques, this methodol. provides essential structural information for further improvement in the reaction selectivity of rough metal electrodes.
62. 62

    Hori, Y.; Koga, O.; Watanabe, Y.; Matsuo, T. FTIR Measurements of Charge Displacement Adsorption of CO on Poly-and Single Crystal (100) of Cu Electrodes. Electrochim. Acta 1998, 44, 1389– 1395,  DOI: 10.1016/S0013-4686(98)00261-8

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    62

    FTIR measurements of charge displacement adsorption of CO on poly- and single crystal (100) of Cu electrodes

    Hori, Y.; Koga, O.; Watanabe, Y.; Matsuo, T.

    Electrochimica Acta (1998), 44 (8-9), 1389-1395CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Science Ltd.)

    CO is reversibly adsorbed on a Cu electrode with simultaneous charge transfer, as obsd. by voltammetric measurements. In-situ FTIR measurements were conducted with a Cu(100) electrode in a phosphate buffer soln. (pH 6.8) and a polycryst. Cu electrode in a carbonate buffer soln. (pH 10.3). With increase of the neg. potential, the IR absorption band of anions (phosphate anion and CO32-) diminishes at the potential of the charge transfer, whereas that of the adsorbed CO increases. Specifically adsorbed anions will remain still on the electrode surface below the potential of zero charge (pzc) to some extent. The apparent charge transfer is obsd. at a potential below the pzc, where CO is adsorbed, displacing specifically adsorbed anions.
63. 63

    Koga, O.; Teruya, S.; Matsuda, K.; Minami, M.; Hoshi, N.; Hori, Y. Infrared Spectroscopic and Voltammetric Study of Adsorbed CO on Stepped Surfaces of Copper Monocrystalline Electrodes. Electrochim. Acta 2005, 50, 2475– 2485,  DOI: 10.1016/j.electacta.2004.10.076

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    63

    Infrared spectroscopic and voltammetric study of adsorbed CO on stepped surfaces of copper monocrystalline electrodes

    Koga, O.; Teruya, S.; Matsuda, K.; Minami, M.; Hoshi, N.; Hori, Y.

    Electrochimica Acta (2005), 50 (12), 2475-2485CODEN: ELCAAV; ISSN:0013-4686. (Elsevier B.V.)

    Voltammetric and IR spectroscopic measurements were carried out to study adsorbed CO on 2 series of Cu single crystal electrodes n(111)(111) and n(111)(100) in 0.1M KH2PO4 + 0.1M K2HPO4 at 0°. Reversible voltammetric waves were obsd. <-0.55 V vs. SHE for adsorption of CO which displaces preadsorbed phosphate anions. The elec. charge of the redox waves is proportional to the step atom d. for both single crystal series. This fact indicates that phosphate anions are specifically adsorbed on the step sites <-0.55 V vs. SHE. Voltammetric measurements indicated that (111) terrace of Cu is covered with adsorbed CO <-0.5 V vs. SHE. Nevertheless, no IR absorption band of adsorbed CO is detected from (111) terrace. Presence of adsorbed CO on (111) terrace is presumed which is not visible by the p.d. spectroscopy used. IR spectroscopic measurements showed that CO is reversibly adsorbed with an on-top manner on Cu single crystal electrodes of n(111)(111) and n(111)(100) with approx. same wavenumber of C-O stretching vibration of 2070 cm-1. The IR band intensity is proportional to the step atom d. Thus CO is adsorbed on (111) or (100) steps on the single crystal surfaces. An anal. of the IR band intensity suggested that one CO mol. is adsorbed on every two or more Cu step atom of the monocryst. surface. The spectroscopic data were compared with those reported for uhv system. The C-O stretching wavenumber of adsorbed CO in the electrode-electrolyte system is 30-40 cm-1 lower than those in uhv system.
64. 64

    Vavra, J.; Shen, T. H.; Stoian, D.; Tileli, V.; Buonsanti, R. Real-Time Monitoring Reveals Dissolution/Redeposition Mechanism in Copper Nanocatalysts during the Initial Stages of the CO₂ Reduction Reaction. Angew. Chem. 2021, 60, 1347– 1354,  DOI: 10.1002/anie.202011137

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    64

    Real-time Monitoring Reveals Dissolution/Redeposition Mechanism in Copper Nanocatalysts during the Initial Stages of the CO2 Reduction Reaction

    Vavra, Jan; Shen, Tzu-Hsien; Stoian, Dragos; Tileli, Vasiliki; Buonsanti, Raffaella

    Angewandte Chemie, International Edition (2021), 60 (3), 1347-1354CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)

    Size, morphol., and surface sites of electrocatalysts have a major impact on their performance. Understanding how, when, and why these parameters change under operating conditions is of importance for designing stable, active, and selective catalysts. Herein, we study the reconstruction of a Cu-based nanocatalysts during the startup phase of the electrochem. CO2 redn. reaction by combining results from electrochem. in situ transmission electron microscopy with operando X-ray absorption spectroscopy. We reveal that dissoln. followed by redeposition, rather than coalescence, is the mechanism responsible for the size increase and morphol. change of the electrocatalyst. Furthermore, we point out the key role played by the formation of copper oxides in the process. Understanding of the underlying processes opens a pathway to rational design of Cu electro (re)deposited catalysts and to stability improvement for catalysts fabricated by other methods.
65. 65

    Zhang, G.; Zhao, Z. J.; Cheng, D.; Li, H.; Yu, J.; Wang, Q.; Gao, H.; Guo, J.; Wang, H.; Ozin, G. A.; Wang, T.; Gong, J. Efficient CO₂ Electroreduction on Facet-Selective Copper Films with High Conversion Rate. Nat. Commun. 2021, 12, 5745,  DOI: 10.1038/s41467-021-26053-w

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    Efficient CO2 electroreduction on facet-selective copper films with high conversion rate

    Zhang, Gong; Zhao, Zhi-Jian; Cheng, Dongfang; Li, Huimin; Yu, Jia; Wang, Qingzhen; Gao, Hui; Guo, Jinyu; Wang, Huaiyuan; Ozin, Geoffrey A.; Wang, Tuo; Gong, Jinlong

    Nature Communications (2021), 12 (1), 5745CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)

    Tuning the facet exposure of Cu could promote the multi-carbon (C2+) products formation in electrocatalytic CO2 redn. Here we report the design and realization of a dynamic deposition-etch-bombardment method for Cu(100) facets control without using capping agents and polymer binders. The synthesized Cu(100)-rich films lead to a high Faradaic efficiency of 86.5% and a full-cell electricity conversion efficiency of 36.5% towards C2+ products in a flow cell. By further scaling up the electrode into a 25 cm2 membrane electrode assembly system, the overall current can ramp up to 12 A while achieving a single-pass yield of 13.2% for C2+ products. An insight into the influence of Cu facets exposure on intermediates is provided by in situ spectroscopic methods supported by theor. calcns. The collected information will enable the precise design of CO2 redn. reactions to obtain desired products, a step towards future industrial CO2 refineries.
66. 66

    Lum, Y.; Yue, B.; Lobaccaro, P.; Bell, A. T.; Ager, J. W. Optimizing C-C Coupling on Oxide-Derived Copper Catalysts for Electrochemical CO₂ Reduction. J. Phys. Chem. C 2017, 121, 14191– 14203,  DOI: 10.1021/acs.jpcc.7b03673

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    66

    Optimizing C-C Coupling on Oxide-Derived Copper Catalysts for Electrochemical CO2 Reduction

    Lum, Yanwei; Yue, Binbin; Lobaccaro, Peter; Bell, Alexis T.; Ager, Joel W.

    Journal of Physical Chemistry C (2017), 121 (26), 14191-14203CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)

    Cu electrodes, prepd. by redn. of oxidized metallic Cu are reported to exhibit higher activity for the electrochem. redn. of CO2 and better selectivity towards C2 and C3 (C2+) products than metallic Cu that was not pre-oxidized. The authors report here a study of the effects of four different prepns. of oxide-derived electrocatalysts on their activity and selectivity for CO2 redn., with particular attention given to the selectivity to C2+ products. All catalysts were tested for CO2 redn. in 0.1M KHCO3 and 0.1M CsHCO3 at applied voltages at -0.7 V to -1.0 V vs. RHE. The best performing oxide-derived catalysts show up to ∼70% selectivity to C2+ products and only ∼3% selectivity to C1 products at -1.0 V vs. RHE when CsHCO3 was used as the electrolyte. In contrast, the selectivity to C2+ products decreases to ∼56% for the same catalysts tested in KHCO3. By studying all catalysts under identical conditions, the key factors affecting product selectivity could be discerned. These efforts reveal that the grain size and surface area of the oxide-derived layer are crit. parameters affecting selectivity. A high selectivity to C2+ products is attained at an overpotential of -1 V vs. RHE by operating at a c.d. sufficiently high to achieve a moderately high pH near the catalyst surface but not so high as to cause a significant redn. in the local concn. of CO2. Based on recent theor. studies, a high pH suppresses the formation of C1 relative to C2+ products. At the same time however, a high local CO2 concn. is necessary for the formation of C2+ products.
67. 67

    Klingan, K.; Kottakkat, T.; Jovanov, Z. P.; Jiang, S.; Pasquini, C.; Scholten, F.; Kubella, P.; Bergmann, A.; Roldan Cuenya, B.; Roth, C.; Dau, H. Reactivity Determinants in Electrodeposited Cu Foams for Electrochemical CO₂ Reduction. ChemSusChem 2018, 11, 3449– 3459,  DOI: 10.1002/cssc.201801582

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    Reactivity determinants in electrodeposited Cu foams for electrochemical CO2 reduction

    Klingan, Katharina; Kottakkat, Tintula; Jovanov, Zarko P.; Jiang, Shan; Pasquini, Chiara; Scholten, Fabian; Kubella, Paul; Bergmann, Arno; Roldan Cuenya, Beatriz; Roth, Christina; Dau, Holger

    ChemSusChem (2018), 11 (19), 3449-3459CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)

    CO2 redn. is of significant interest for the prodn. of nonfossil fuels. The reactivity of eight Cu foams with substantially different morphologies was comprehensively investigated by anal. of the product spectrum and in situ electrochem. spectroscopies (X-ray absorption near edge structure, extended X-ray absorption fine structure, XPS, and Raman spectroscopy). The approach provided new insight into the reactivity determinants: The morphol., stable Cu oxide phases, and *CO poisoning of the H2 formation reaction are not decisive; the electrochem. active surface area influences the reactivity trends; macroscopic diffusion limits the proton supply, resulting in pronounced alkalization at the CuCat surfaces (operando Raman spectroscopy). H2 and CH4 formation was suppressed by macroscopic buffer alkalization, whereas CO and C2H4 formation still proceeded through a largely pH-independent mechanism. C2H4 was formed from two CO precursor species, namely adsorbed *CO and dissolved CO present in the foam cavities.
68. 68

    Mattei, E.; de Vivo, G.; de Santis, A.; Gaetani, C.; Pelosi, C.; Santamaria, U. Raman Spectroscopic Analysis of Azurite Blackening. J. Raman Spectrosc. 2008, 39, 302– 306,  DOI: 10.1002/jrs.1845

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    Raman spectroscopic analysis of azurite blackening

    Mattei, E.; de Vivo, G.; De Santis, A.; Gaetani, C.; Pelosi, C.; Santamaria, U.

    Journal of Raman Spectroscopy (2008), 39 (2), 302-306CODEN: JRSPAF; ISSN:0377-0486. (John Wiley & Sons Ltd.)

    Azurite is a basic copper carbonate pigment largely employed in painting realization. The areas painted with azurite are easily alterable and are often less resistant than the other parts of artworks. The azurite alteration in a black pigment, the copper oxide (tenorite), has been studied by micro-Raman spectroscopy. The blackening can be due to thermal or chem. alterations: in the second case the alterations being due to the presence of alk. conditions. Laser-induced degrdn. of azurite has been studied as a function of the grain size. The results show that the temp. of the grains decreases as the size increases, and azurite degrades into tenorite only below the crit. value of 25 μm. To study the chem. alteration of azurite, the pigment has been applied on the plaster of terracotta samples and analyzed at different pH values by micro-Raman spectroscopy. As opposed to most part of the anal. techniques, it can detect the presence of both azurite and tenorite mols. in the same micro areas, and provides a valuable tool to det. azurite degrdn.
69. 69

    Sanchez, M. P.; Souto, R. M.; Barrera, M.; Gonzalez, S.; Salvarezza, R. C.; Arvia, A. J. A Mechanistic Approach to the Electroformation of Anodic Layers on Copper and Their Electroreduction in Aqueous Solutions Containing NaHCO₃ and Na₂CO₃. Electrochim. Acta 1993, 38, 703– 715,  DOI: 10.1016/0013-4686(93)80242-R

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    Vavra, J.; Dattila, F.; Kormányos, A.; Cherevko, S.; Lopéz, N.; Buonsanti, R. Cu⁺ Transient Species Mediate Cu Catalyst Reconstruction during CO₂ Electroreduction. April 12, 2022. ChemRxiv.  DOI: 10.26434/chemrxiv-2022-3cr9k (accessed 2022-07-21).

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