Proton conductors capable of operating in the temperature range of 100–600 °C have received great interest for many application areas such as fuel cells, sensors, and electrolyzers; however, very few materials that can satisfy these demands have been reported to date. Here, we report a promising candidate for a solid electrolyte in intermediate-temperature electrochemical devices. First, MP₂O₇ –MO₂ composite ceramics (M = Sn, Si, Ti, and Zr) were prepared by reacting a porous MO₂ substrate with an 85% H₃PO₄ solution at 600 °C. Although all the tested MO₂ could react with H₃PO₄ to form the corresponding metal pyrophosphate layers on the surfaces of the exterior and interior substrate, the extent of MP₂O₇ growth in the obtained samples and their relative density were strongly dependent on the M species. These factors were the best for the SnP₂O₇ –SnO₂ composite ceramic, yielding the highest electrical conductivity in the temperature range of interest. Next, different low valence cations (cation = Mg²⁺ , Sc³⁺ , Ga³⁺ , Y³⁺ , In³⁺ , La³⁺ , Sm³⁺ , Eu³⁺ , and Gd³⁺ ) were doped into the SnP₂O₇ –SnO₂ composite ceramic. The most positive effect on the electrical conductivity was observed when Sm³⁺ was used as the dopant; the electrical conductivity of this sample was approximately one order of magnitude higher than that of the non-doped sample, especially at low temperatures below 250 °C. Fourier transform infrared (FTIR) and proton magic angle spinning (MAS) nuclear magnetic resonance (NMR) analyses revealed that the quantity of protons incorporated in the SnP₂O₇ layer and their mobility were increased by the doping of Sm into the composite ceramic. As a consequence, the electrical conductivity of the Sm-doped SnP₂O₇ –SnO₂ composite ceramic reached ∼10⁻² S cm⁻¹ in the temperature range of 100–600 °C. Proton conduction in this sample was also investigated by various electrochemical techniques. The protons function as the predominant charge carrier in the sample, and the proton transport number was almost unity in both static and dynamic conditions.

Dense SnP₂O₇ –SnO₂ composite ceramics were prepared by reacting a porous SnO₂ substrate with an 85% H₃PO₄ solution at elevated temperatures. At 300 °C and higher, SnO₂ reacted with H₃PO₄ to form an SnP₂O₇ layer on exterior and interior surfaces in the substrate, the growth rate of which increased with increasing reaction temperature. Finally, at 600 °C, the pores of this composite ceramic were perfectly closed and its electrical conductivity became several orders of magnitude higher than that of the SnO₂ substrate alone. Proton conduction was demonstrated in this composite ceramic using electrochemical measurements and various analytical techniques. Comparison of the observed electromotive force with the theoretical value in two gas concentration cells demonstrated that this composite ceramic is a pure ion conductor, wherein the predominant ion species are protons. Fourier transform infrared (FT-IR) and proton magic angle spinning (MAS) nuclear magnetic resonance (NMR) analyses revealed that the protons interacted with lattice oxide ions in the SnP₂O₇ layer to form hydrogen bonds. An H/D isotope effect suggested that proton conduction in this composite ceramic was based on a proton-hopping mechanism. The proton conductivity in this material reached ∼10⁻² S cm⁻¹ in the temperature range of 250–600 °C.