Characterization and testing of strontium titanium ferrite-based solid oxide cells for reversible and co-electrolysis operation

This study investigates the electrocatalytic performance and the preliminary durability of SrTi_0.3Fe_0.7O_3-δ (STF) and Sr_0.95(Ti_0.3Fe_0.63Ni_0.07)O_3-δ (STF-Ni) as fuel electrodes for solid oxide cells operated at 750 °C in reversible mode with H₂/H₂O and CO/CO₂ mixtures, and in H₂O/CO₂ co-electrolysis mode. STF-Ni achieved a peak power density of 415 mW/cm² under 3 % humidified H₂, and maintained 96 % of its performance over 96 h of H₂/H₂O reversible operation. In co-electrolysis, STF-Ni delivered a maximum current density of 540 mA/cm² at 1.4 V, with 15 % performance drop at 1.2 V after 162 h. Reversible operation with CO/CO₂ mixtures proved most challenging, as surface SrCO₃ formation caused significant degradation, and stable performance was reached only on STF-Ni. STF-Ni consistently demonstrated superior stability compared to STF across all the operative modes. A 0D model was utilized to analyze the I/V curves of STF-Ni: a power-law rate for the fuel electrode's kinetics with H₂ and H₂O mixtures was extracted, and the kinetic insensitivity to the CO₂ amount in co-electrolysis was proved. Complementary characterization using XRD, TPR, SEM, and TEM techniques highlighted the role of exsolution. After exposure to CO₂, compositional changes in exsolved Ni-Fe nanoparticles were observed, with selective reincorporation of Fe into the perovskite structure.