Selective CO2 Electroreduction with Enhanced Oxygen Evolution Efficiency in Affordable Borate-Mediated Molten Electrolyte

The renewable-electricity-driven carbon dioxide reduction reaction (CO2RR) is potentially a pathway to closing the anthropogenic carbon cycle; however, it is challenging to couple a fast cathodic CO2RR that endows tunable selectivity with a kinetically matched anodic oxygen evolution reaction (OER) at a promising energy efficiency. Here, we report a cost-effective strategy to convert CO2 into C/CO (tunable selectivity of over 70%) with enhanced OER kinetics in water-soluble Na-K-based molten carbonate by modulating the electrolyte's oxo-acidity using earth-abundant borax (Na2B4O7) as an electrolyte additive, where the borates acting as O2– (an oxo-base) shuttles that constantly mediate between cathode and anode can concurrently facilitate CO2RR and OER. In particular, it can respectively sustain CO2RR at a stable current density of 100 mA cm–2 with a considerable OER current density of ∼300 mA cm–2. The optimal energy efficiency can reach up to over 60%, opening avenues for efficiently manipulating CO2RR and simultaneously enhancing OER.