High-rate and selective conversion of CO 2 from aqueous solutions to hydrocarbons.

Electrochemical carbon dioxide (CO 2 ) conversion to hydrocarbon fuels, such as methane (CH 4 ), offers a promising solution for the long-term and large-scale storage of renewable electricity. To enable this technology, CO 2 -to-CH 4 conversion must achieve high selectivity and energy efficiency at high currents. Here, we report an electrochemical conversion system that features proton-bicarbonate-CO 2 mass transport management coupled with an in-situ copper (Cu) activation strategy to achieve high CH 4 selectivity at high currents. We find that open matrix Cu electrodes sustain sufficient local CO 2 concentration by combining both dissolved CO 2 and in-situ generated CO 2 from the bicarbonate. In-situ Cu activation through alternating current operation renders and maintains the catalyst highly selective towards CH 4 . The combination of these strategies leads to CH 4 Faradaic efficiencies of over 70% in a wide current density range (100 - 750 mA cm -2 ) that is stable for at least 12 h at a current density of 500 mA cm -2 . The system also delivers a CH 4 concentration of 23.5% in the gas product stream.