Gold-in-copper at low *CO coverage enables efficient electromethanation of CO2

The renewable-electricity-powered CO₂ electroreduction reaction provides a promising means to store intermittent renewable energy in the form of valuable chemicals and dispatchable fuels. Renewable methane produced using CO₂ electroreduction attracts interest due to the established global distribution network; however, present-day efficiencies and activities remain below those required for practical application. Here we exploit the fact that the suppression of *CO dimerization and hydrogen evolution promotes methane selectivity: we reason that the introduction of Au in Cu favors *CO protonation vs. C-C coupling under low *CO coverage and weakens the *H adsorption energy of the surface, leading to a reduction in hydrogen evolution. We construct experimentally a suite of Au-Cu catalysts and control *CO availability by regulating CO₂ concentration and reaction rate. This strategy leads to a 1.6× improvement in the methane:H₂ selectivity ratio compared to the best prior reports operating above 100 mA cm^-2. We as a result achieve a CO₂-to-methane Faradaic efficiency (FE) of (56 ± 2)% at a production rate of (112 ± 4) mA cm^-2.