Proton conduction in AIII0.5BV0.5P2O7 compounds at intermediate temperatures

Abstract

Proton conductors capable of operation between 100 and 400 °C are attractive electrochemical materials due to their high utility value in energy and environmental applications. However, such proton conductors have not yet made headway in the marketplace, due to insufficient proton conductivity. Here, we present new types of metal pyrophosphates as promising candidates for intermediate temperature proton conductors. A series of A^III_0.5B^V_0.5P₂O₇ (A^IIIB^V = InSb, SbSb, FeSb, GaNb, FeNb, YNb, GaTa, AlTa, FeTa, YTa, BiTa, and SmTa) compounds were synthesized, of which In_0.5Sb_0.5P₂O₇, Fe_0.5Nb_0.5P₂O₇, and Fe_0.5Ta_0.5P₂O₇ exhibited the highest proton conductivities in the temperature ranges of 50–100 °C (0.045 S cm⁻¹@100 °C), 100–200 °C (0.12 S cm⁻¹@150 °C), and 200–400 °C (0.18 S cm⁻¹@250 °C), respectively, in unhumidified air. The proton conductivity of these three compounds was further enhanced by the introduction of A^III or B^V deficiency into the bulk. Consequently, Fe_0.4Ta_0.5P₂O₇ exhibited the highest proton conductivity of 0.27 S cm⁻¹ at 300 °C in unhumidified air. Such high proton conductivity values were also observed under fuel cell operating conditions. The environments of protons in the bulk of these compounds were monitored using Fourier transform infrared (FT-IR) spectroscopy, temperature-programmed desorption (TPD), and proton magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. Protons were incorporated into the compounds for charge-compensation of the deficient A^III or B^V cations, which results in an increase in the quantity of protons. More importantly, the mobility of the protons was also enhanced. Various electrochemical measurements demonstrate that proton conduction is dominant in these compounds, where the protons migrate according to a hopping mechanism.