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

The renewable-electricity-driven carbon dioxide reduction reaction (CO 2 RR) is potentially a pathway to closing the anthropogenic carbon cycle; however, it is challenging to couple a fast cathodic CO 2 RR 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 CO 2 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 (Na 2 B 4 O 7 ) as an electrolyte additive, where the borates acting as O 2– (an oxo-base) shuttles that constantly mediate between cathode and anode can concurrently facilitate CO 2 RR and OER. In particular, it can respectively sustain CO 2 RR 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 CO 2 RR and simultaneously enhancing OER.