Mixed protonic and electronic conductors are promising materials for electrodes in hydrogen-related electrochemical devices and are essential for the realization of high-performance fuel cells, electrolysis cells, and hydrogen separation membranes. Materials exhibiting oxide ionic conduction in addition to protonic and electronic conduction termed as the triple-conducting oxides recently attract much attention as a proton-conducting mixed conductor; however, their working temperature is limited at >600 °C. Because the operating temperatures of fuel cells and electrolysis cells are expected to be lower, extending into the intermediate temperature range of <500 °C, it is necessary to explore new mixed protonic and electronic conductors that work by a different mechanism than conventional ones. Hydrogen is known to dissolve in n-type oxide semiconductors and behave as a donor according to the following reaction: H₂ → 2Hi^・＋ 2e’. The protons generated by this reaction are expected to be highly mobile because their countercharges are not immobile point defects but electrons that are delocalized throughout the crystal. Here, the dissolution of hydrogen into the Nb-doped TiO₂ crystal and its mixed protonic and electronic conductivity in H₂ were investigated.

Total electrical conductivity that corresponds to the partial electronic conductivity varied in response to the atmosphere (air or H₂) and was larger in H₂ than in air. It was over 1 Scm⁻¹ at above 200 °C in H₂. The electron carrier density in air was 1.3×10¹⁸ cm⁻³ and increased to 1.3×10¹⁹ cm⁻³ after annealing in H₂. Thermal desorption spectrometry showed the H₂ and H₂O gas emissions only from the sample annealed in H₂. The amount of H₂ and H₂O released was converted to the proton densities in the Nb-doped TiO₂ of 2.8×10¹⁹ and 3.5×10²⁰ cm⁻³, respectively. Since the increase in electron carrier density and the amount of H₂ released after H₂ annealing were almost the same, the increase in total electrical conductivity in H₂ is due to the increase in electron carrier density resulting from the dissolution of hydrogen and its ionization according to the above reaction. The reason why protons released as H₂O do not generate electron carriers is not yet clear. The partial protonic conductivity was determined by dc and ac impedance methods using a proton-conducting glass electrolyte as an electron-blocking electrode. It varied from 2×10⁻⁴ Scm⁻¹ at 200 °C to 1×10⁻³ Scm⁻¹ at 250 °C with an activation energy of 0.74 eV. Its partial protonic conductivity of 1×10⁻³ Scm⁻¹ at 250 °C is only an order of magnitude smaller than that of BaZrO₃-based electrolyte at ~600 °C, which is extremely high despite the low temperature. Assuming that all dissolved protons with a density of 3.8×10²⁰ cm⁻³ contribute to the protonic conductivity, their diffusion coefficient was calculated to be 7×10⁻⁷ cm²s⁻¹ at 250 °C, which is larger than the previously reported 1×10⁻⁸ cm²s⁻¹ for pure TiO₂ [1]. This may be because the electron carriers introduced by the Nb donor spread throughout the crystal, facilitating proton transport.

The result that Nb-doped TiO₂ exhibits high protonic and electronic conductivity at around 250 °C in H₂ strongly suggests that n-type oxide semiconductors dissolving hydrogen could be a new field to explore mixed protonic and electronic conductors. Furthermore, this finding has great potential for extending the operating temperatures of hydrogen-related electrochemical devices to lower temperatures.