Structure- and Electrolyte-Sensitivity in CO Electroreduction.

The utilization of fossil fuels (i.e., coal, petroleum, and natural gas) as the main energy source gives rise to serious environmental issues, including global warming caused by the continuously increasing level of atmospheric CO. To deal with this challenge, fossil fuels are being partially replaced by renewable energy such as solar and wind. However, such energy sources are usually intermittent and currently constitute a very low portion of the overall energy consumption. Recently, the electrochemical conversion of CO to chemicals and fuels with high energy density driven by electricity derived from renewable energy has been recognized as a promising strategy toward sustainable energy. The activation and reduction of CO, which is a thermodynamically stable and kinetically inert molecule, is extremely challenging. Although the participation of protons in the CO electroreduction reaction (CORR) helps lower the energy barrier, high overpotentials are still needed to efficiently drive the process. On the other hand, the concurrent hydrogen evolution reaction (HER) under CORR conditions leads to lower selectivity toward CORR products. Electrocatalysts that are highly active and selective for multicarbon products are urgently needed to improve the energy efficiency of CORR. The reduction of CO involves multiple proton-electron transfers and has many complex intermediates. Recent reports have shown that the relative stability of the intermediates on the surface of catalysts determines final reaction pathways as well as the product selectivity. Furthermore, this reaction displays a strong structure-sensitivity. The atomic arrangement, electronic structure, chemical composition, and oxidation state of the catalysts significantly influence catalyst performance. Fundamental understanding of the dependence of the reaction mechanisms on the catalyst structure would guide the rational design of new nanostructured CORR catalysts. As a reaction proceeding in a complex environment containing gas/liquid/solid interfaces, CORR is also intensively affected by the electrolyte. The electrolyte composition in the near surface region of the electrode where the reaction takes place plays a vital role in the reactivity. However, the former might also be indirectly determined by the bulk electrolyte composition via diffusion. Adding to the complexity, the structure, chemical state and surface composition of the catalysts under reaction conditions usually undergo dynamic changes, especially when adsorbed ions are considered. Therefore, in addition to tuning the structure of the electrocatalysts, being able to also modify the electrolyte provides an alternative method to tune the activity and selectivity of CORR. In situ and operando characterization methods must be employed to gain in depth understanding on the structure- and electrolyte-sensitivity of real CORR catalysts under working conditions. This Account provides examples of recent advances in the development of nanostructured catalysts and mechanistic understanding of CORR. It discusses how the structure of a catalyst (crystal orientation, oxidation state, atomic arrangement, defects, size, surface composition, segregation, etc.) influences the activity and selectivity, and how the electrolyte also plays a determining role in the reaction activity and selectivity. Finally, the importance of in situ and operando characterization methods to understand the structure- and electrolyte-sensitivity of the CORR is discussed.