Electrolytic Synthesis of White Phosphorus Is Promoted in Oxide-Deficient Molten Salts

Elemental white phosphorus (P₄) is a key feedstock for the entire phosphorus-derived chemicals industry, spanning everything from herbicides to food additives. The electrochemical reduction of phosphate salts could enable the sustainable production of P₄; however, such electrosynthesis requires the cleavage of strong, inert P-O bonds. By analogy to the promotion of bond activation in aqueous electrolytes with high proton activity (Brønsted-Lowry acidity), we show that low oxide anion activity (Lux-Flood acidity) enhances P-O bond activation in molten salt electrolytes. We develop electroanalytical tools to quantify the oxide dependence of phosphate reduction, and find that Lux acidic phosphoryl anhydride linkages enable selective, high-efficiency electrosynthesis of P₄ at a yield of 95% Faradaic efficiency. These fundamental studies provide a foundation that may enable the development of low-carbon alternatives to legacy carbothermal synthesis of P₄.