The role of defect chemistry in strontium titanates utilised for high temperature steam electrolysis
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 La0.3Sr0.7TiO3+δ, A-site-deficient La0.2Sr0.7TiO3 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 La0.3Sr0.7TiO3+δ to be the best in terms of highest current density and lowest polarisation resistance, followed by La0.2Sr0.7TiO3 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 (Rs) was independent of pH2O for (La,Sr)TiO3 perovskites, while Rs 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.