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 °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.