Proton Conductivity and Solid Acidity of Mg-, In-, and Al-Doped SnP[sub 2]O[sub 7]

Mg-, In-, and Al-doped were synthesized, and their potential as electrolytes for fuel cells was evaluated in terms of their performance and durability. The overall conductivity of Mg-doped increased with increasing content and reached a maximum at 10 mol %. However, this was smaller than that obtained for doping with 10 mol % and 5 mol % , especially at temperatures below . This result was interpreted to be a consequence of lower proton mobility rather than of smaller proton concentration in Mg-doped , probably due to the electrostatic restriction by against proton transport. Moreover, to investigate the influence of the doping on the fuel-cell properties, fuel-cell tests were conducted using , , and as electrolytes at a temperature of . Compared with and cells, the cell showed a somewhat higher Ohmic resistance but exhibited a comparable polarization resistance, which is the main component of the total electrical resistance. More importantly, the doping led to an improved stability of the Pt catalyst and carbon support at high potentials. This beneficial effect was attributed to a large reduction in the acidity of resulting from the basicity of MgO.