Electrolysis of Molten Polyphosphate Salts Generates P₄ and O₂: Selectivity, Kinetics and Stability Behind a Promising Alternative to Carbothermal Phosphate Reduction

Abstract

Elemental white phosphorus is a vital feedstock chemical for a vast network of industries, but it is currently produced via a carbothermal reduction rife with environmental and efficiency problems. In this work, we use the molten salt electrolysis of sodium polyphosphates to investigate an alternative means of white phosphorus production. In order to collect robust and meaningful data on these traditionally challenging systems, we introduce a comprehensive toolkit of techniques and design elements, such as a sodium reference electrode and graphite pseudoreference couple, working electrode geometries that prevent gas blockage and inert container and separator materials. Via high-temperature product collection at graphite working electrodes in tandem with electrokinetic analysis, we identify white phosphorus as the major cathodic product with high selectivity and infer a rate-limiting formation of a P³⁺ intermediate involved in its formation. Simultaneously, we find that graphite oxidizes much more efficiently as an anode than the coke used in the industrial thermal process, evolving a mixture of carbon dioxide (96%) and carbon monoxide (4%) instead of exclusively carbon monoxide. When the graphite anode is replaced by a metal anode, we also identify oxygen as the major, and ostensibly only long-term, anodic product. While iridium metal anodes require a higher overpotential for oxygen evolution relative to platinum and gold, they also demonstrate vastly superior corrosion resistance, with corrosion rates as low as 0.93 mm yr⁻¹ in Lux-basic melt compositions. Platinum and gold ions released during corrosion also re-reduce in solution to form elemental metal particulates and gaseous oxygen, resulting in roughly 100% oxygen evolution efficiency in all cases and allowing for catalyst material to partially be reclaimed. Finally, we find that the Lux acidity of the melt has a variety of significant effects on the energetics of melt reactions, simultaneously promoting the reduction of phosphates to phosphorus while suppressing the oxidation of graphite or evolution of oxygen and promoting metal anode corrosion. Taken together, these findings indicate that a viable carbon-free electrogeneration of white phosphorus is feasible under appropriate solvent conditions and electrode material choices, and this electrolysis represents an appealing alternative to legacy processes.