Elucidating the diffusion pathway of protons in ammonium polyphosphate: a potential electrolyte for intermediate temperature fuel cells

Abstract

Ammonium polyphosphate (NH₄PO₃) is a potential electrolyte material for intermediate temperature fuel cells (150–250 °C). The crystal structure of NH₄PO₃, including the H positions, is unravelled by neutron powder diffraction (NPD) data by successive Fourier synthesis from the polyphosphate backbone. The structure consists of zig–zag chains aligned along the [001] directions of tetrahedral phosphate PO₄ units that are connected through O3 atoms with a P–O3–P angle of 126.3(5)°. The proton conductivity mechanism of NH₄PO₃ is clarified from the thermal evolution of the structure. It shows that some H atoms subtly shift at high temperatures, resulting in a weakening of certain H-bonds, thus increasing the lability of those H atoms involved in the proton conduction mechanism. Conductivity measurements in humid air and H₂ of NH₄PO₃ show high proton conductivities of 1.2 × 10⁻⁵ to 2.61 × 10⁻³ S cm⁻¹ and 2.2 × 10⁻⁵ to 2.69 × 10⁻³ S cm⁻¹, respectively, in the temperature range of 50 °C to 275 °C.