Enhancing Proton Mobility in Silicophosphoric Acid by Replacing Si with Al

Electrochemical devices that can operate at temperatures of 200-300 °C are expected to become the next-generation energy conversion devices in fuel cells and electrosynthesis, which are important for achieving carbon neutrality. Proton conductors based on phosphate glasses are being developed as candidate materials for such devices. We recently developed a glass proton conductor by using silicophosphoric acid based on the idea of solidifying phosphoric acid with silicon as a cross-linking glass framework. However, the proton conductivity was low owing to protons being trapped by silicon. In this study, silicon was replaced with aluminum to mitigate the proton-trapping effect and increase proton conductivity. Proton conductivity was found to improve by 1 order of magnitude without thermal stability being compromised. Nuclear magnetic resonance and Raman and X-ray photoelectron spectroscopy analyses confirmed that no significant change in the glass structure occurred when silicon was replaced with aluminum. Thus, the improvement in proton conductivity was due to the difference in electrical properties between aluminum and silicon; this difference weakened the proton-trapping effect.