Proton conduction in non-doped and acceptor-doped metal pyrophosphate (MP2O7) composite ceramics at intermediate temperatures

Abstract

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.