Dynamic Cation Enrichment during Pulsed CO₂ Electrolysis and the Cation-Promoted Multicarbon Formation

Recently, pulsed electrolysis has been demonstrated as an emerging electrochemical technique that significantly promotes the performance of various electrocatalysis applications. The ionic nature of aqueous electrolytes implies a likely change in ionic distribution under these alternating potential conditions. However, despite the well-known importance of cations, the impact of pulsed electrolysis on the cation distribution remains unexplored as well as its influences on the performance. Herein, we explore the cation effects on the pulsed electrochemical CO₂ reduction (p-CO₂RR) using the most widely utilized alkali metal cations, including Li⁺, Na⁺, K⁺, and Cs⁺. It is discovered that the nature of cations can significantly influence the product ratio of C₂₊ over C₁ (mostly CH₄) during p-CO₂RR in an order of Li⁺< Na⁺< K⁺< Cs⁺, much more profoundly than those of static cases. We report direct experimental evidence for the cation enrichment caused by pulsed electrolysis, depending on the radius of the hydrated ions. With further quasi-in situ analysis of the catalyst surface, the cation-promoted Cu dissolution-and-redeposition process was identified; this is found to alter the surface CuO/Cu ratio during the pulsed process. We demonstrate that both the cation enrichment and the cation-adjusted surface CuO/Cu composition impact the C₂₊/C₁ ratio through the control of the surface-adsorbed CO population. These results reveal the presence of pulse-induced cation redistribution in emerging pulsed electrolysis techniques and provide a comprehensive understanding of alkali metal cation effects for improving the selectivity of p-CO₂RR.