| Summary: | The electrocatalytic conversion of CO<sub>2</sub> on a Cu electrode has the potential to produce valuable chemicals such as hydrocarbons and oxygenated compounds. While the influence of electrolyte cation on the activity and selectivity of the CO<sub>2</sub> reduction reaction (CO<sub>2</sub>RR) on Cu has been widely observed, the specific mechanism through which cation species affect the CO<sub>2</sub>RR remains unclear and subject to debate. In this work, the CO<sub>2</sub>RR in the carbonate electrolytes containing different alkali metals (Li<sup>+</sup>, Na<sup>+</sup>, K<sup>+</sup>, Rb<sup>+</sup>, and Cs<sup>+</sup>) was investigated at potentials from −0.1 to −1.1 V (vs. RHE) over a Cu electrode using electrochemical techniques. Charge transfer kinetics, adsorption of species, and mass transport were considered comprehensively during the analysis. It is found that several factors can play a role in the CO<sub>2</sub>RR, including hydrated cation adsorption, preferential hydrolysis, and interaction between the cation and adsorbed species, with the dominating factor determined by the external bias and cation species. Consequently, a coherent interpretation of the influence of electrolyte cations on the intrinsic kinetics of the CO<sub>2</sub>RR has been put forward. We envision that these insights will greatly contribute to the development of efficient catalytic systems and the optimization of catalytic conditions, thereby advancing progress toward commercial applications in this field.
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