Composite Cathode Based on Redox-Reversible Nb₂TiO₇ for Direct High-Temperature Steam Electrolysis

Ni/YSZ fuel electrodes can only operate under strongly reducing conditions for steam electrolysis in an oxide-ion-conducting solid oxide electrolyzer (SOE). In atmosphere with a low content of H₂ or without H₂, cathodes based on redox-reversible Nb₂TiO₇ provide a promising alternative. The reversible changes between oxidized Nb₂TiO₇ and reduced Nb_1.33Ti_0.67O₄ samples are systematically investigated after redox-cycling tests. The conductivities of Nb₂TiO₇ and reduced Nb_1.33Ti_0.67O₄ are studied as a function of temperature and oxygen partial pressure and correlated with the electrochemical properties of the composite electrodes in a symmetric cell and SOE at 830 oC. Steam electrolysis is then performed using an oxide-ion-conducting SOE based on a Nb_1.33Ti_0.67O₄ composite fuel electrode at 830 oC. The current-voltage and impedance spectroscopy tests demonstrate that the reduction and activation of the fuel electrode is the main process at low voltage; however, the steam electrolysis dominates the entire process at high voltages. The Faradic efficiencies of steam electrolysis reach 98.9% when 3%H₂O/Ar/4%H₂ is introduced to the fuel electrode and 89% for that with introduction of 3%H₂O/Ar.