Cobalt-Doped MnFeCrO₄ Spinel as the Fuel Electrode for Solid Oxide Electrolysis Cells

Solid oxide electrolysis cells (SOECs) are devices that convert electrical energy into chemical energy and are widely used for hydrogen production from steam or CO generation from CO₂. Spinel-type oxides are commonly used as a coating on metal interconnects because of their high electrical conductivity (σ) in the air and matching thermal expansion with that of the electrolyte. They have been used as oxygen electrodes in solid oxide cells in recent years, but their application in fuel electrodes was less studied. In this research, MnFeCr_1-xCo_xO₄ (x = 0, 0.05, 0.2) spinel oxides with a σ around 0.1 S cm^-1 were used as the fuel electrode of an SOEC. The MnFeCr_0.95Co_0.05O₄ (MFCC5) and MnFeCr_0.8Co_0.2O₄ (MFCC20) with Co doping on the Cr site decomposed partially during the water electrolysis, producing CoFe alloy and MnO particles on the parent spinels, forming a metal/oxide interface that further enhanced the electrocatalytic performance and durability of the cell. MFCC5 was able to achieve a current density of 1550 mA cm^-2 at 1.6 V and maintained a current density of about 600 mA cm^-2 after the 150 h durability test at 1.3 V at 800 °C. The Faraday efficiency was close to unity. The in situ exsolution process driven by the cathodic current significantly changed the performance of the cell, but a high performance was still maintained. In situ exsolution of the metals from the reducible cations was a feasible way of designing the spinel-type cathode for SOECs.