Research progress on fuel assisted solid oxide electrolysis cells

The combustion of fossil fuels has resulted in significant environmental degradation, with the emission of vast quantities of CO₂ and other greenhouse gases contributing to a pronounced greenhouse effect. Solid Oxide Electrolysis Cell (SOEC) shows great potential for CO₂ conversion and renewable power storage due to the advantage of high electrolysis rate and energy conversion efficiency. However, during SOEC operation, the air electrode side is directly exposed to air and produces oxygen. This exposure creates an oxygen concentration gradient cross the electrode, which results in a certain open circuit voltage. Consequently, overcoming this voltage is necessary to maintain the electrolysis process. This process results in a considerable loss of electrical energy and negative economic impacts. The higher electrical energy consumption further hampers the commercial application of SOEC technology. Fuel-assisted solid oxide electrolysis cell (FA-SOEC) can greatly reduce the electrical energy consumption of the electrolysis cell by feeding fuel into the air electrode side and providing the chemical energy of the fuel to the electrolysis reaction. This review comprehensively introduced working principle, assisted fuel types, operating voltage and the recent research progress on FA-SOEC, the details are as follows: (1): The working principle of FA-SOEC and SOEC are compared. The fuel electrode of FA-SOEC operates in the same way as SOEC and consumes less electrical energy than SOEC. Through supplying the reducing substances (C, CH₄, CO, etc.) to the air electrode (the assisted fuel electrode), the oxygen evoluted from the electrolyte can be consumed, which can largely overcome the overpotential of oxygen evolution reaction (OER) process, thereby reducing the partial pressure of O₂ and reaching the equilibrium potential of electrolysis. (2): The reaction principles of the fuel-assisted electrode with various fuels have been summarized, which demonstrates that the wide range of fuel choices. Meanwhile, FA-SOEC can generate syngas, polycarbon-based olefins, and other products via the partial oxidation reaction (POM) and oxidative coupling reaction (OCM) of hydrocarbon fuels. (3): The thermodynamic and kinetics comparison between the FA-SOEC and SOEC is discussed. Compared to the SOEC, FA-SOEC enables the electrolysis at lower energy consumption and operation voltage. These results demonstrate that the electrical energy needed for electrolysis can be greatly decreased via FA-SOEC. (4): The progress research of FA-SOEC in 1D and 2D numerical models are summarized. Numerical simulation research has been focused on the different performance of with and without fuel assisted on SOEC. Meanwhile, the operation conditions, assisted fuel types, and factors for electrochemical performance are systematically evaluated by the numerical simulation research. (5): The progress research of fuel-assisted electrode materials on metal-ceramic/metal electrode materials and perovskite electrode materials are also summarized, including the basic requirement and issues. Meanwhile, the methods for improving the electrochemical activity of the electrode materials are discussed, such as elemental doping, infiltration modification, and exsolution for perovskite electrodes. Finally, the key problems and challenges, and several suggestions are proposed for the future research. In summary, FA-SOEC is a novel and promising electrochemical reaction device for energy storage applications.