Another role of CO-formation catalyst in acidic tandem CO2 electroreduction: Local pH modulator

Electrochemical CO2 reduction on Cu-based catalysts is a promising technique to convert CO2 to high-value C2 and C3 feedstocks. High carbon efficiency can be achieved in acidic electrolytes, but Cu-based catalysts show suppressed activity toward C2+ formation in acidic conditions. Acid removes the oxygen-containing species on Cu, which are necessary for C–C coupling. In this work, a gas diffusion electrode (GDE)/Cu/Ni-N-C tandem configuration, in which Ni-N-C served as a CO2-to-CO catalyst, expressed a 5-time enhancement of C2+ formation activity compared with GDE/Cu. Electrochemical measurements and finite element simulations indicate the improved C2+ formation activity was due to the elevated local pH rather than the increased CO concentration in the Cu catalyst layer. The major function of the CO-formation catalyst in the tandem system working in an acidic condition is to modulate the local pH near the Cu catalyst instead of producing CO intermediate for Cu.