Efficient multicarbon formation in acidic CO₂ reduction via tandem electrocatalysis

The electrochemical reduction of CO2 in acidic conditions enables high single-pass carbon efficiency. However, the competing hydrogen evolution reaction reduces selectivity in the electrochemical reduction of CO2, a reaction in which the formation of CO, and its ensuing coupling, are each essential to achieving multicarbon (C2+) product formation. These two reactions rely on distinct catalyst properties that are difficult to achieve in a single catalyst. Here we report decoupling the CO2-to-C2+ reaction into two steps, CO2-to-CO and CO-to-C2+, by deploying two distinct catalyst layers operating in tandem to achieve the desired transformation. The first catalyst, atomically dispersed cobalt phthalocyanine, reduces CO2 to CO with high selectivity. This process increases local CO availability to enhance the C-C coupling step implemented on the second catalyst layer, which is a Cu nanocatalyst with a Cu-ionomer interface. The optimized tandem electrodes achieve 61% C2H4 Faradaic efficiency and 82% C2+ Faradaic efficiency at 800 mA cm(-2) at 25 degrees C. When optimized for single-pass utilization, the system reaches a single-pass carbon efficiency of 90 +/- 3%, simultaneous with 55 +/- 3% C2H4 Faradaic efficiency and a total C2+ Faradaic efficiency of 76 +/- 2%, at 800 mA cm(-2) with a CO2 flow rate of 2 ml min(-1).