Replacing Sodium Ions with Protons in Vanadophosphate Glass: Suppression of Electronic Conduction by Local Structural Change around Vanadium Ions

Charge carrier conversion in alkali oxide-incorporated transition metal oxide-containing glasses has been investigated extensively; however, the influences of protons on this phenomenon have not been systematically probed. In this study, we employed electrochemical alkali (Na+)–proton substitution (APS) in an attempt to fabricate a mixed protonic–electronic conductor and thus prepared 30HO1/2–6VO5/2–14VO2–50PO5/2 glass from 30NaO1/2–10VO5/2–10VO2–50PO5/2 precursor glass. The precursor glass was a mixed ionic–electronic conductor; however, the product APS-treated glass was a pure proton conductor, contrary to expectations. The suppression of electronic conduction in the APS-treated glass was investigated using Mott’s theoretical expression of small polaron hopping conduction in semiconducting glasses. Our findings indicated that the V ions in the APS-treated and precursor glass specimens existed mainly as VO6 and VO4 with large and small electron wave function decay constants, respectively. Therefore, in the APS-treated glass, the electron wave functions between adjacent V ions did not overlap, resulting in miniscule electronic conductivity. The proton conductivity of the APS-treated glass (1 × 10–5 S·cm–1 at 300 °C) was lower than that of conventional APS-derived proton-conducting phosphate glasses, owing to the low proton mobility stemming from the strong proton-trapping feature of the nonbridging oxygen in VO6.