Impedance analysis of Sr-substituted CePO4 with mixed protonic and p-type electronic conduction

Members of the solid-solution series Ce1−xSrxPO4−δ (x = 0, 0.01, 0.02) with mixed protonic and electronic transport have been synthesized by a nitrate-decomposition method followed by sintering at 1450 °C. Impedance spectroscopy is employed to estimate the bulk electrical conductivity in wet (∼0.03 atm) and dry atmospheres of O2 and 10%H2:90%N2. Conductivity increases with dopant concentration (x), oxygen partial pressure (pO2) and water vapour partial pressure (pH2O) reaching ∼3.5 × 10−3 S cm−1 at 600 °C for x = 0.02 in wet O2. Activation energies (Ea) for the bulk conductivity of Ce0.98Sr0.02PO4−δ below 650 °C are 0.44 and 0.78 eV for wet oxidising and wet reducing conditions, respectively. A moderate but positive pO2+n power-law dependence (n < 1/10) of conductivity is exhibited in the pO2 range 10−2.5 to 10−1 atm, consistent with mixed ionic and p-type electronic transport. Thermogravimetric analysis indicates that the Sr-doped materials are stable in a CO2 atmosphere in the temperature range 25–1200 °C.