Water effect on the conductivity behavior of NH₄PO₃-based electrolytes for intermediate temperature fuel cells

The electrical conductivity at intermediate temperature of 150-250 °C and the activation energy for conductivity of composite proton conductors, 2NH₄PO₃-(NH₄)₂Mn(PO₃)₄ and 2NH₄PO₃-(NH₄)₂SiP₄O₁₃, were investigated as a function of water vapor pressure, P_{H2 O}. The activation energy decreased linearly with the natural logarithm of P_{H2 O}, indicating that water is chemically adsorbed to the electrolytes. The decrease in activation energy is possibly caused by formation of hydrogen bonds between the adsorbed water and the electrolytes. In addition, the pre-exponential factor of Arrhenius equation, σ₀, increased with P_{H2 O}. This suggests that the adsorbed water may generate additional mobile protons for the composite electrolyte. Therefore, the enhancement in the electrical conductivity of a NH₄PO₃-based electrolyte in a water-vapor rich atmosphere originates possibly from the decrease in activation energy as well as the increase in mobile proton concentration.