The role of defect chemistry in strontium titanates utilised for high temperature steam electrolysis

Abstract

The significant role of perovskite defect chemistry brought about by the A-site doping of strontium titanate with lanthanum on high temperature steam electrolysis properties is demonstrated. Solid oxide electrolysis cells based on oxygen-excess La_0.3Sr_0.7TiO_3+δ, A-site-deficient La_0.2Sr_0.7TiO₃ and undoped SrTiO₃ perovskite hydrogen electrodes (cathodes) were considered. Steam electrolysis performance was largely independent of the presence or absence of hydrogen in the cathode inlet, reflecting the redox stability of perovskites and representing a possible advantage over the state-of-the-art Ni/yttria-stabilised zirconia cermet cathode. Electrochemical testing in 47%H₂O/53%N₂ atmosphere at 900 °C revealed La_0.3Sr_0.7TiO_3+δ to be the best in terms of highest current density and lowest polarisation resistance, followed by La_0.2Sr_0.7TiO₃ and SrTiO₃. Surface area effects, electronic conductivity and oxide ion mobility were considered to be among the determining factors. Furthermore a dramatic order-of-magnitude difference between the characteristic relaxation frequencies of oxygen-excess and A-site-deficient titanates was observed. Steam partial pressure (pH₂O)-dependency measurements revealed an added benefit of La-doping in that series resistance (R_s) was independent of pH₂O for (La,Sr)TiO₃ perovskites, while R_s increased with pH₂O for undoped SrTiO₃. Electrochemical testing was complemented by X-ray diffraction, electronic conductivity, particle size, BET, porosity and scanning electron microscopy measurements.