A possible low-intermediate temperature proton conductor based on silicon oxide phosphate

Abstract

The main material studied in this project is silicon oxide phosphate, often referred to in the literature as Si₅O(PO₄)₆. This material has highly unusual coordination of the silicon (octahedral, as well as the more common tetrahedral). The structure is hexagonal, it has been assigned to space group R -3 and lattice parameters a ≈ 7.85 Å, c ≈ 24.14 Å. This work’s main focus is on understanding the interplay between structure and properties in order to enhance protonic conductivity for a fuel cell electrolyte. Silicon oxide phosphate was synthesised with the solid-state method, using a gel precursor made from H₃PO₄, water and SiO₂. Various compositions were made with different P/Si starting ratios, ranging between 0.57 - 1.5. There were small but significant differences in the a,b axes for the different compositions that corresponded to conductivity behaviour of hydrothermally treated P-Si compositions. This correlation was also found to appear in ³¹P NMR for the chemical shift at - 44 ppm for untreated P-Si compositions as well as in the temperatures of the DTA peaks for the hydrothermally treated compositions. This all implies that this particular P-Si system with the addition of water becomes a ternary system that enables protonic conductivity. A proposed mechanism for the protonic conductivity is given where it is suggested that protons flow along the internal channels of the structure using two waters that provide dual pathways for protons. This is possible through utilization of a proton thought to be in the structure (a P_OH bond of 1.57 Å). Protonic conductivity could further be increased in the system by incorporating 85% H₃PO₄ in the P-Si materials, thus these materials act as matrices for the phosphoric acid. Another composition, Ge₅O(PO₄)₆ with 5% extra germanium, was hydrothermally treated and found to have protonic conductivity at higher temperatures than the silicon oxide phosphate analogues.