Efficient multicarbon formation in acidic CO 2 reduction via tandem electrocatalysis.

The electrochemical reduction of CO 2 in acidic conditions enables high single-pass carbon efficiency. However, the competing hydrogen evolution reaction reduces selectivity in the electrochemical reduction of CO 2 , a reaction in which the formation of CO, and its ensuing coupling, are each essential to achieving multicarbon (C 2+ ) product formation. These two reactions rely on distinct catalyst properties that are difficult to achieve in a single catalyst. Here we report decoupling the CO 2 -to-C 2+ reaction into two steps, CO 2 -to-CO and CO-to-C 2+ , by deploying two distinct catalyst layers operating in tandem to achieve the desired transformation. The first catalyst, atomically dispersed cobalt phthalocyanine, reduces CO 2 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% C 2 H 4 Faradaic efficiency and 82% C 2+ Faradaic efficiency at 800 mA cm -2 at 25 °C. When optimized for single-pass utilization, the system reaches a single-pass carbon efficiency of 90 ± 3%, simultaneous with 55 ± 3% C 2 H 4 Faradaic efficiency and a total C 2+ Faradaic efficiency of 76 ± 2%, at 800 mA cm -2 with a CO 2 flow rate of 2 ml min -1 .