Progress in tracking electrochemical CO2 reduction intermediates over single-atom catalysts by operando ATR-SEIRAS

Due to the involvement of multiple proton-coupled electron transfer (PCET) steps in the electrochemical carbon dioxide reduction reaction (CO₂RR), single-atom catalysts (SACs) serve as an ideal platform for studying such complex chemical reaction processes. Their structural simplicity and homogeneity facilitate understanding the structure-performance relationship as well as reaction mechanisms in CO₂RR. To clearly identify the dynamic intermediate transformation processes in CO₂RR taking place over SACs and study the impact of local reaction environments on CO₂RR performance, operando attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) provides a valuable tool.

Recently, a research team led by Prof. Bin Liu from City University of Hong Kong, China, and Prof. Chenliang Su from Shenzhen University, China, published the latest review article in the field of electrochemical in-situ infrared spectroscopy. This review summarizes the operando ATR-SEIRAS and its key applications in CO₂RR catalyzed by SACs. It covers topics including a brief introduction to the surface enhancement mechanism of electrochemical in situ infrared spectroscopy, the formation mechanisms of C1 and C2 products, the function of operando ATR-SEIRAS in investigating the mechanisms of single/dual-atom catalysts in converting CO₂/CO to C1 and C2 products, and methods of using spectroscopic information to determine interfacial H₂O and local pH at the electrode. Finally, they also provide some perspectives on the future development of operando ATR-SEIRAS. The results were published in Chinese Journal of Catalysis (https://doi.org/10.1016/S1872-2067(24)60068-9).

This review summarizes the application of in-situ infrared spectroscopy to electrochemical CO₂RR studies. Firstly, the surface enhancement mechanism of electrochemically attenuated total reflective surface enhancement infrared spectroscopy is briefly introduced. Then, the key role of in-situ attenuated total reflective surface-enhanced infrared spectroscopy in studying the dynamic evolution of CO₂/CORR reactions catalyzed by mono/diatomic catalysts is discussed in detail. Subsequently, the relevant information of the in-situ infrared spectrogram to determine the interfacial water and the method of quantifying the local pH of the electrode are briefly described. In-situ electrochemical infrared spectroscopy has great potential to advance our understanding of the fundamental aspects of CO₂RR. By elucidating reaction intermediates in detail, it is possible to better understand the interactions of catalysts, electrolytes, and adsorbed species in guiding the selectivity and activity of the reaction. Although significant progress has been made, the current recognition of CO₂RR is still incomplete, and the opportunities and challenges are as follows:

1) In-situ spectroscopic techniques with much higher resolution is very necessary for effectively tracking the dynamic behaviour of key intermediates involved in CO₂RR, especially when the characteristic peaks of CO₂RR intermediates/products are overlapped with others. Currently, researchers are still unable to resolve key intermediate products from overlapping spectra using purely optical methods. Today, deep learning algorithms play a significant role in fields such as natural language processing and smart healthcare. Therefore, they provide a potential solution for the qualitative identification of components directly from mixed feature peaks. Applying deep learning algorithms to the analysis of overlapped in situ infrared spectra can elevate the study of CO₂RR mechanism to new heights.

2) A comprehensive understanding of CO₂RR mechanisms and the intricate structure-performance relationships requires synergizing multiple in situ spectroelectrochemical approaches with theoretical simulations. In recent years, density functional theory (DFT) has been widely applied in fundamental and applied research across fields such as chemistry, materials science, and life sciences. With continuous improvements in theoretical frameworks and advances in computational technology, DFT allows for the investigation of potential reaction intermediates in CO₂RR. By comparing the changes in free energy of protonation at each step, DFT can identify the most suitable reaction pathway. This guides experimental efforts to optimize reaction conditions and improve catalyst performance, making an indelible contribution to the CO₂RR.

3) Operando mechanistic studies of CO₂RR at industrial-scale reaction rates are rare. Membrane electrode assembly (MEA) electrolyzers, known for their "zero-gap" design, effectively address the limitations of flow-cell reactors in CO₂RR by directly interfacing the cathodic gas diffusion electrode (GDE) and anode using membrane. Despite its potential, MEA technology faces challenges in CO₂RR, including the need for stable systems, enhanced selectivity, production of valuable multi-carbon products, and improved CO₂ conversion efficiency. Therefore, there is a need to develop in-situ spectroscopic techniques that can be applied under the operating conditions of MEA electrolyzers. These techniques can be used to study the impact of membrane electrode structures on the activity and selectivity of CO₂RR and correlate real-time information on the three-phase boundary with the observed CO₂RR performance. Establishing a feedback loop between molecular-level understanding and MEA performance under operando CO₂RR conditions will help us develop more efficient electrolyzer reactors.

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About the Journal

Chinese Journal of Catalysis is co-sponsored by Dalian Institute of Chemical Physics, Chinese Academy of Sciences and Chinese Chemical Society, and it is currently published by Elsevier group. This monthly journal publishes in English timely contributions of original and rigorously reviewed manuscripts covering all areas of catalysis. The journal publishes Reviews, Accounts, Communications, Articles, Highlights, Perspectives, and Viewpoints of highly scientific values that help understanding and defining of new concepts in both fundamental issues and practical applications of catalysis. Chinese Journal of Catalysis ranks among the top one journals in Applied Chemistry with a current SCI impact factor of 15.7. The Editors-in-Chief are Profs. Can Li and Tao Zhang.

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