Electrochemical acetate production from high-pressure gaseous and liquid CO₂

Electroreduction of CO₂ (CO₂RR) to high-value chemicals is important to environmental and energy landscapes, with one of the challenges being direct and efficient acetate production. Here we demonstrate a Cu(OH)₂-derived Cu/CuO_x catalyst that achieves 87% Faradaic efficiency for the CO₂RR to acetate in KOH electrolytes with borate additives under 58 atm CO₂(g). Dynamic electrolytic speciation reveals high and low concentrations of dissolved CO₂(aq) and proton donor HCO₃⁻, respectively, as the key to acetate. In situ Raman spectroscopy suggests that the oxygen-bound bidentate intermediate *OC^•O* formed as the acetate precursor on Cu(I) under high pressures. Introducing CO₂(l) to the interface between CO₂(g) and the electrolyte by melting CO₂(s) further increases [CO₂(aq)] and boosts the acetate partial current density to 86 mA cm^–2 with 71% Faradaic efficiency. A Cu²⁺ cross-linked alginate coating layer on the cathode surface drastically enhances the durability of Cu/CuO_x catalyst over a 20 h reaction, affording a potassium acetate yield of 30 mg h⁻¹ cm⁻² directly from the CO₂RR. [Figure not available: see fulltext.]