A carbon-efficient bicarbonate electrolyzer

CO2 electroreduction (CO2ER) is a promising route to carbon-neutral production of various chemicals and fuels. Carbon efficiency is one of the most pressing problems for CO2ER today. While there have been studies on anion exchange membrane (AEM) electrolyzers with CO2(gas) and bipolar membrane (BPM) electrolyzers with HCO3–(aq) feedstock, both suffer from significant carbon efficiency loss. In AEM electrolyzers, this is due to carbonate anion crossover, whereas in BPM electrolyzers, the exsolution of CO2(gas) from the bicarbonate solution is the culprit. Here, we first elucidate the root cause of the low carbon efficiency of liquid bicarbonate electrolyzers with thermodynamic calculations, then achieve carbon-efficient CO2ER by adopting a near-neutral-pH cation exchange membrane (CEM) and CO2(gas) partial pressure management, with tin nanoparticle catalysts. We have converted highly concentrated bicarbonate solution to solid formate fuel with a yield (carbon efficiency) of > 96%. The device test was demonstrated at 100 mA cm–2 with a full-cell voltage of 3.1 V for over 100 h. This strategy enables full conversion of HCO3–(aq) feedstock to energy-dense solid formate fuel at ambient pressure and temperature with renewable electricity. Importantly, it can power direct formate fuel cells (DFFCs) which exhibit promising power density for seasonal energy storage.