Electrochemical carbon dioxide (CO2) conversion to hydrocarbon fuels, such as methane (CH4), offers a promising solution for the long-term and large-scale storage of renewable electricity. To enable this technology, CO2-to-CH4conversion must achieve high selectivity and energy efficiency at high currents. Here, we report an electrochemical conversion system that features proton-bicarbonate-CO2mass transport management coupled with an in-situ copper (Cu) activation strategy to achieve high CH4selectivity at high currents. We find that open matrix Cu electrodes sustain sufficient local CO2concentration by combining both dissolved CO2and in-situ generated CO2from the bicarbonate. In-situ Cu activation through alternating current operation renders and maintains the catalyst highly selective towards CH4. The combination of these strategies leads to CH4Faradaic 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 CH4concentration of 23.5% in the gas product stream.