Gold-Stabilized Copper Enables Anodic Hydrogen Evolution for Ultralow-Voltage CO-to-Ethylene Electrolysis

Electrochemical upgrading from CO₂ and CO to ethylene has typically been coupled with the oxygen evolution reaction (OER), whose high standard reduction potential leads to full-cell voltages above 2.2 V at 200 mA/cm². Here we explored an alternative anodic reaction, where furfural is oxidized to furoic acid, a reaction having a low onset potential (E⁰ ≈ 0.05 V vs. RHE), and which reaction is accompanied by the evolution of H₂: an anodic hydrogen evolution reaction (a-HER). In early experiments, copper oxide as a-HER catalyst exhibit limited stability (< 10 min) and activity (80 mA/cm² at 0.8 V_cell). We found, using operando spectroscopy, that hydroxide forms on the surface of copper and deactivates the desired a-HER process. When we screened candidate metal dopants, we found the best to be Au, for it served to stabilize the Cu surface, enablinained a-HER: 260 mA cm^-2 at 0.8 V_cell and stable operation for 16 h. Integrated into a paired CO-to-ethylene electrolyzer, this delivered 0.92 V_fullcell at 400 mA cm^-2, required 40 GJ electricity per ton of ethylene, and co-produced 460 kg H₂ per ton ethylene. To enable comparison with CO₂-to-ethylene reports, which require an additional CO₂-to-CO step, we estimate ∼ 69 GJ/tonC₂H₄.