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-CH₄ conversion must achieve high selectivity and energy efficiency at high currents. Here, we report an electrochemical conversion system that features proton-bicarbonate-CO₂ mass transport management coupled with an in-situ copper (Cu) activation strategy to achieve high CH₄ selectivity at high currents. We find that open matrix Cu electrodes sustain sufficient local CO₂ concentration by combining both dissolved CO₂ and in-situ generated CO₂ from 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 CH₄ 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 CH₄ concentration of 23.5% in the gas product stream.