Solid state protonic conductor NH₄PO₃-(NH₄)₂Mn(PO₃)₄ for intermediate temperature fuel cells

A new proton-conductive composite of NH₄PO₃-(NH₄)₂Mn(PO₃)₄ was synthesized and characterized as a potential electrolyte for intermediate temperature fuel cells that operated around 250 °C. Thermal gravimetric analysis and X-ray diffraction investigation showed that (NH₄)₂Mn(PO₃)₄ was stable as a supporting matrix for NH₄PO₃. The composite conductivity, measured using impedance spectroscopy, improved with increasing the molar ratio of NH₄PO₃ in both dry and wet atmospheres. A conductivity of 7 mS cm^-1 was obtained at 250 °C in wet hydrogen. Electromotive forces measured by hydrogen concentration cells showed that the composite was nearly a pure protonic conductor with hydrogen partial pressure in the range of 10²-10⁵ Pa. The proton transference number was determined to be 0.95 at 250 °C for 2NH₄PO₃-(NH₄)₂Mn(PO₃)₄ electrolyte. Fuel cells using 2NH₄PO₃-(NH₄)₂Mn(PO₃)₄ as an electrolyte and the Pt-C catalyst as an electrode were fabricated. Maximum power density of 16.8 mW/cm² was achieved at 250 °C with dry hydrogen and dry oxygen as the fuel and oxidant, respectively. However, the NH₄PO₃-(NH₄)₂Mn(PO₃)₄ electrolyte is not compatible with the Pt-C catalyst, indicating that it is critical to develop new electrode materials for the intermediate temperature fuel cells.