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 deg. 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 deg. 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 deg. 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 deg. 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