The move towards a sustainable net-zero economy will require hydrogen as part of an integrated solution since its high specific energy is amenable to sectors which are currently prohibitive to batteries, including high-temperature heat and certain transport applications. Hydrogen production through electrolysis of water enabled by renewable energies (green hydrogen) is key to this integration. Ceramic electrolyser cells which operate at high temperature are advantageous for their thermodynamic and electrochemical efficiency, requiring a lower electrical energy demand than the low temperature alkaline and polymer analogues.
In this work, two-layer Ruddlesden-Popper-type (RP) compositions based on Sr₃Fe₂O_7-d with potential high mixed oxide-ionic- electronic conductivity and thermal compatibility with oxide-ion or proton-conducting electrolytes (CGO, BZCY) are investigated as cobalt-free air electrodes for solid oxide electrolysis cells. The compositions Sr₃Fe₂O_7-d, Sr₃FeNiO_7-d, Sr_2.8Pr_0.2FeNiO_7-d and Sr_2.8La_0.2FeNiO_7-d were synthesised by the Pechini method and sintering conditions were explored for achieving phase purity in the temperature range 1000 – 1250 °C with and without O₂ flow. Crystal structure was investigated by high-resolution transmission electron microscopy and Rietveld refinement employing X-ray powder diffraction data. Microstructure was studied by scanning electron microscopy, and thermal evolution and oxygen loss by thermogravimetry. Electrical conductivity of sintered bars was measured by the 4-probe direct-current method in air, O₂ and N₂ atmospheres, indicating that Ni doping resulted in electrical conductivities superior to 200 S·cm-1 at 600 °C in oxygen. Electrode polarisation measurements performed on Ce_0.9Gd_0.1O_2-d electrolyte with anodic bias indicate the promise of cobalt-free RP-type phases as air electrodes in ceramic electrolyser cells.