α-PbO₂-type niobate as efficient cathode materials for steam and CO₂ electrolysis

α-PbO₂-type Mn_0.5Ti_0.5NbO₄-based oxides are studied as cathode materials in solid oxide electrolysis cell (SOEC) for the direct electrolysis of steam or CO₂ at 800 °C. Comparison between Mn_0.5Ti_0.5NbO₄ and the Fe-doped ones, Fe_0.3Mn_0.35Ti_0.35NbO₄ and Fe_0.6Mn_0.2Ti_0.2NbO₄, indicates that the Fe3+ doping increases the electric conductivity in air but decreases the stability of the α-PbO₂ structure in reducing atmosphere as in Fe_0.6Mn_0.2Ti_0.2NbO₄. The electric conductivity in reducing atmosphere is found to be ascribed to Ti⁴⁺-O-Ti³⁺ and Fe³⁺-O-Fe²⁺ in Mn_0.5Ti_0.5NbO₄ and Fe doped ones, respectively. The steam or CO₂ electrolsysis indicates that 30% Fe³⁺ substitution for Mn/Ti decreases the polarization resistance at low bias, but 60% Fe doping increases the polarization resistance dramatically than Mn_0.5Ti_0.5NbO₄. The excessive Fe³⁺ doping is found to induce the phase transformation and delamination on the cathode/electrolyte interface under a cathodic current. The electrolyser with Fe_0.3Mn_0.35Ti_0.35NbO₄ cathode on zirconia-based electrolyte imparts a stable current density of 2.32 Acm⁻² at −1.6 V if Ar-88% H₂O is used as feeding gas. Analysis of the gas product indicates that H₂ is produced and the Faradaic efficiency at −1.4 V and −1.6 V is found to be 100% and 93%, respectively.