Synthesis and characterization of dense SnP2O7–SnO2 composite ceramics as intermediate-temperature proton conductors

Abstract

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.