Behaviors of Si, B, Al, and Na during electrochemical reduction of borosilicate glass in molten CaCl₂

The electrochemical reduction of borosilicate glass in molten CaCl₂ at 1123 K was investigated. The behaviors of the constituent elements, i.e., Si, B, Al, Na, and K, were estimated using potential-pO²⁻ diagrams constructed from the thermodynamic data for the species. The diagrams suggested that the first cathodic wave in the cyclic voltammogram results from the reduction of the B₂O₃ component. The dissolution of the Na₂O and K₂O components, which was predicted from the diagrams, was confirmed by energy dispersive X-ray analysis of a borosilicate glass plate after immersion into molten CaCl₂ without electrolysis. The scanning electron microscopy/wavelength dispersive X-ray mappings and Raman spectrum for borosilicate glass reduced at 0.9 V vs. Ca²⁺/Ca indicated that SiO₂ and B₂O₃ are reduced to Si and B-Si compound. The formation of Ca-Al-O compounds owing to the increase of O²⁻ ions is suggested. The pO²⁻ range during electrolysis at 0.9 V was indicated to be 2.95-3.46