Tailoring Borate Mediator Species Enables Industrial CO Production with Improved Overall Energy Efficiency by Sustainable Molten Salt CO₂ Electrolysis

The electrochemical conversion of CO₂ into CO represents a promising strategy for mitigating excessive global greenhouse gas emissions. Nevertheless, achieving industrial-scale electrochemical CO₂-to-CO conversion with enhanced selectivity and reduced energy consumption presents significant challenges. In this study, a borate-enhanced molten salt process for CO₂ capture and electrochemical transformation is employed, achieving over 98% selectivity for CO and over 55% energy efficiency without the necessity for complex and costly electrocatalysts. Cathodic CO₂ electro-reduction (CO₂ER) with the anodic oxygen evolution reaction (OER) at an overall current density of 500 mA cm^-2 using non-nanostructured transition-metal plate electrodes at 650 °C is coupled. By regulating the electrolyte's oxo-basicity with earth-abundant borax (Na₂B₄O₇), a borate-enhanced electrolyte is established that accelerates the overall electrochemical reaction efficiently. This system involved a series of well-designed target borate species (BO₃ ^3-, BO₂ ^-, and B₄O₇ ^2-) that acted as mediators shuttling between the cathode and anode, favoring CO as the primary cathodic product. Manipulating the atmosphere above the anode facilitated a spontaneous transformation of borates, further enhancing OER performance with long-term operational stability over a cumulative period of 50 h, while also reducing overall energy consumption. This work presents a cost-effective strategy for the industrial-scale production of CO derived from CO₂, contributing to a lower carbon footprint by establishing a sustainable borate-mediated closed loop.