In Situ Generation of Pseudorutile Oxide as the Cathode for Direct Electrolysis of CO₂

The conversion of CO₂ into CO in high-temperature solid oxide electrolysis cells (SOECs) is an attractive route for the CO₂ utilization using the intermittent renewables. The low-cost and highly catalytic cathode is important for the direct electrolysis of pure CO₂. In this study, non-perovskite Fe_0.5Mg_0.25+0.5Ti_0.25-0.5Nb_1-MoO₄ oxides (denoted as Mo-x when x is equal to 0, 0.1, and 0.2) are evaluated as the cathode of an SOEC for the direct electrolysis of CO₂. Mo⁶⁺ doping converted the wolframite Mo-0 into an α-PbO₂-type with cation disordering, while further doping to Mo-0.2 showed a wolframite with cation ordering again. The SOEC with Mo-0.2 as the cathode exhibits the best electrochemical performance for the direct electrolysis of CO₂ as a large portion of the oxide converted into oxygen-deficient pseudorutile-type oxide with a nominal formula of M₅O₉ (M = cation). The pseudorutile, a crystallographic shear phase of rutile, can be obtained after 60 h of direct electrolysis in CO₂ at a 1.3 V bias rather than a reduction under 5% H₂. The SOEC with Mo-0.2 as the cathode imparted a stable current density of 0.45 A cm^-2, which could be related to the production of pseudorutile decorated with nanoparticles of MoO₂. These results show that molybdenum doping is an effective strategy for developing oxygen-deficient rutile (pseudorutile) for the electrolysis of CO₂.