Visualization of the Diffusion Pathway of Protons in (NH₄)₂Si_0.5Ti_0.5P₄O₁₃ as an Electrolyte for Intermediate-Temperature Fuel Cells

We demonstrate that (NH₄)₂Si_0.5Ti_0.5P₄O₁₃ is an excellent proton conductor. The crystallographic information concerning the hydrogen positions is unraveled from neutron-powder-diffraction (NPD) data for the first time. This study shows that all the hydrogen atoms are connected though H bonds, establishing a two-dimensional path between the [(Si_0.5Ti_0.5)P₄O₁₃^2-]_n layers for proton diffusion across the crystal structure by breaking and reconstructing intermediate H-O=P bonds. This transient species probably reduces the potential energy of the H jump from an ammonium unit to the next neighboring NH₄⁺ unit. Both theoretical and experimental results support an interstitial-proton-conduction mechanism. The proton conductivities of (NH₄)₂Si_0.5Ti_0.5P₄O₁₃ reach 0.0061 and 0.024 S cm^-1 in humid air at 125 and 250 °C, respectively. This finding demonstrates that (NH₄)₂Si_0.5Ti_0.5P₄O₁₃ is a promising electrolyte material operating at 150-250 °C. This work opens up a new avenue for designing and fabricating high-performance inorganic electrolytes.