Electrical conductivity of M(2+)-doped (M = Mg, Ca, Sr, Ba) cerium pyrophosphate-based composite electrolytes for low-temperature proton conducting electrolyte fuel cells

M(2+)-doped (M = Mg, Ca, Sr, Ba) cerium pyrophosphate (Ce(0.9)M(0.1)P(2)O(7))-based electrolytes were synthesized by digesting CeO(2) and dopant precursor in 85% H(3)PO(4). The phase composition and microstructure of sintered samples is studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The variation of electrical conductivity with temperature was studied by electrochemical impedance spectroscopy (EIS) in dry and humid atmosphere for the possible application as electrolyte in proton conducting ceramic electrolyte fuel cells in 90-200 degree C range. The conductivity of M(2+)-doped CeP(2)O(7) was very low ( less than or equal to 10(-8)S/cm) in unhumidified conditions at the temperatures Ca > Sr > Ba order, both in unhumidified and humid conditions. The effect of sintering temperature on the microstructure and conductivity was also studied. Ce(0.9)Mg(0.1)P(2)O(7) sample sintered at 450 degree C showed maximum conductivity at 90 degree C due to the large contribution of incorporated water species whereas those sintered at 650 and 750 degree C showed maximum conductivity at 150 degree C due to the higher contribution from the material bulk. The variation in conductivity with different dopant cations is explained on the basis of dopant solubility and P(2)O(7)(4-) sub-lattice distortion, and polarizability of the dopant cation.