Enhanced Proton Transport in Nb-Doped Rutile TiO₂: A Highly Useful Class of Proton-Conducting Mixed Ionic Electronic Conductors

Mixed protonic and electronic conductors are essential materials as electrodes for electrochemical devices that use hydrogen as a reactant or product, such as fuel cells and steam electrolyzers. As the demand for devices operating at intermediate temperatures (<500 °C) is increasing, it has become highly desirable to develop optimized mixed protonic and electronic conductors for this temperature range. In this study, we investigated hydrogen dissolution and proton transport in Nb-doped rutile TiO₂ (Ti_0.96Nb_0.04O₂), an oxide semiconductor. Ti_0.96Nb_0.04O₂ dissolved hydrogen at 450 °C in a hydrogen atmosphere, incorporating 2.1 × 10²⁰ cm^-3 of protons to the semiconductor, which is 10-100 times higher than that in undoped TiO₂. This resulted in a proton conductivity of 4 × 10^-3 S cm^-1 at 250 °C. Given its electronic conductivity of 2 S cm^-1 at 250 °C, Ti_0.96Nb_0.04O₂ has been identified as an excellent mixed protonic and electronic conductor. The proton diffusion coefficient reached ∼1 × 10^-5 cm² s^-1 at 250 °C, which is over 1000 times higher than that of undoped TiO₂ and 10-1000 times higher than those of well-known perovskite-type proton conductors represented by Ba(Zr,Y)O₃. This enhanced proton density and diffusion is attributed to Nb doping, which stabilizes and delocalizes the electrons introduced by hydrogen dissolution while shielding negative charge centers. This previously unreported mechanism for improving proton conductivity provides a novel design guideline for mixed protonic and electronic conductors, paving the way for the development of future materials in hydrogen-related electrochemical applications.