Electro‐Conversion of a Cu‐Based Metal–Organic Framework Gel Into a Copper‐Sulfide Electrocatalyst: Toward Selective CO ₂ Electroreduction to Formic Acid

Copper sulfide‐based materials (Cu _x S) have been shown to exhibit promising electrocatalytic activity toward the electrochemical CO ₂ reduction reaction (CO ₂ RR) into formic acid. Nevertheless, obtaining fine control over the structure and chemical composition of these catalysts is often a key obstacle that limits the ability to extract CO ₂ RR‐based structure–activity relations. Here, to this end, we show that using an electrochemical conversion of a copper‐based metal–organic framework into Cu _x S, one can achieve precise tuning of the resulting catalyst's composition as well as its copper‐to‐sulfur atomic distribution. As an outcome, it allowed us to synthesize a Cu ₂ S‐rich electrocatalyst, which substantially improves formic acid Faradaic efficiency to above 90%. Furthermore, via operando Raman spectroscopy analysis we were able to: (i) determine that the Cu _x S converts CO ₂ ‐to‐formic acid by a carbon‐bound *COOH intermediate, and (ii) follow the Cu _x S's electrochemical transformation during catalytic working conditions. Consequently, these findings provide new tools to develop highly tunable and efficient electrocatalysts for a large variety of electrochemical applications.