Solid state protonic conductor NH 4PO 3–(NH 4) 2Mn(PO 3) 4 for intermediate temperature fuel cells

Abstract

A new proton-conductive composite of NH 4PO 3–(NH 4) 2Mn(PO 3) 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 (NH 4) 2Mn(PO 3) 4 was stable as a supporting matrix for NH 4PO 3. The composite conductivity, measured using impedance spectroscopy, improved with increasing the molar ratio of NH 4PO 3 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 10 2–10 5 Pa. The proton transference number was determined to be 0.95 at 250 °C for 2NH 4PO 3–(NH 4) 2Mn(PO 3) 4 electrolyte. Fuel cells using 2NH 4PO 3–(NH 4) 2Mn(PO 3) 4 as an electrolyte and the Pt–C catalyst as an electrode were fabricated. Maximum power density of 16.8 mW/cm 2 was achieved at 250 °C with dry hydrogen and dry oxygen as the fuel and oxidant, respectively. However, the NH 4PO 3–(NH 4) 2Mn(PO 3) 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.