Electrochemically Driven Cation Exchange Enables the Rational Design of Active CO 2 Reduction Electrocatalysts

Metal oxides or sulfides are considered to be one of the most promising CO 2 reduction reaction (CO 2 RR) precatalysts, owing to their electrochemical conversion in situ into highly active electrocatalytic species. However, further improvement of the performance requires new tools to gain fine control over the composition of the active species and its structural features [e.g., grain boundaries (GBs) and undercoordinated sites (USs)], directly from a predesigned template material. Herein, we describe a novel electrochemically driven cation exchange (ED‐CE) method that enables the conversion of a predesigned CoS 2 template into a CO 2 RR catalyst, Cu 2 S. By means of ED‐CE, the final Cu 2 S catalyst inherits the original 3 D morphology of CoS 2 , and preserves its high density of GBs. Additionally, the catalyst's phase structure, composition, and density of USs were precisely tuned, thus enabling rational design of active CO 2 RR sites. The obtained Cu 2 S catalyst achieved a CO 2 ‐to‐formate Faradaic efficiency of over 87 % and a record high activity (among reported Cu‐based catalysts). Hence, this study opens the way for utilization of ED‐CE reactions to design advanced electrocatalysts.