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.