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