Efficient Methane Electrosynthesis Enabled by Tuning Local CO₂ Availability

The electroreduction of carbon dioxide (CO₂RR) to valuable chemicals is a promising avenue for the storage of intermittent renewable electricity. Renewable methane, obtained via CO₂RR using renewable electricity as energy input, has the potential to serve as a carbon-neutral fuel or chemical feedstock, and it is of particular interest in view of the well-established infrastructure for its storage, distribution, and utilization. However, CO₂RR to methane still suffers from low selectivity at commercially relevant current densities (>100 mA cm^-2). Density functional theory calculations herein reveal that lowering *CO₂ coverage on the Cu surface decreases the coverage of the *CO intermediate, and then this favors the protonation of *CO to *CHO, a key intermediate for methane generation, compared to the competing step, C-C coupling. We therefore pursue an experimental strategy wherein we control local CO₂ availability on a Cu catalyst by tuning the concentration of CO₂ in the gas stream and regulate the reaction rate through the current density. We achieve as a result a methane Faradaic efficiency (FE) of (48 ± 2)% with a partial current density of (108 ± 5) mA cm^-2 and a methane cathodic energy efficiency of 20% using a dilute CO₂ gas stream. We report stable methane electrosynthesis for 22 h. These findings offer routes to produce methane with high FE and high conversion rate in CO₂RR and also make direct use of dilute CO₂ feedstocks.