Impedance analysis of Sr-substituted CePO 4 with mixed protonic and p-type electronic conduction

Abstract

Members of the solid-solution series Ce 1− x Sr x PO 4− 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 O 2 and 10%H 2:90%N 2. Conductivity increases with dopant concentration ( x), oxygen partial pressure ( pO 2) and water vapour partial pressure ( pH 2O) reaching ∼3.5 × 10 −3 S cm −1 at 600 °C for x = 0.02 in wet O 2. Activation energies ( E a) for the bulk conductivity of Ce 0.98Sr 0.02PO 4− δ below 650 °C are 0.44 and 0.78 eV for wet oxidising and wet reducing conditions, respectively. A moderate but positive pO 2 + n power-law dependence ( n < 1/10) of conductivity is exhibited in the pO 2 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 CO 2 atmosphere in the temperature range 25–1200 °C.