Solid oxide electrolyser cell
A solid oxide electrolyzer cell (SOEC) is a solid oxide fuel cell that runs in regenerative mode to achieve the electrolysis of water and which uses a solid oxide, or ceramic, electrolyte to produce oxygen and hydrogen gas.
Solid oxide electrolyzer cells operate at temperatures which allow high-temperature electrolysis to occur, typically between 500 and 850 °C. These operating temperatures are similar to those conditions for an SOFC. The net cell reaction yields hydrogen and oxygen gases. The reactions for one mole of water are shown below, with oxidation of water occurring at the anode and reduction of water occurring at the cathode.
Anode: H2O ---> 1/2O2 + 2H+ + 2e-
Cathode: 2H2O + 2e- ---> H2 + 2OH-
Net Reaction: H2O ---> H2 + 1/2O2
Electrolysis of water at 298 K (25 °C) requires 285.83 kJ of energy in order to occur, and the reaction is increasingly endothermic with increasing temperature. However, the energy demand may be reduced due to the Joule heating of an electrolysis cell, which may be utilized in the water splitting process at high temperatures. Research is ongoing to add heat from external heat sources such as concentrating solar thermal collectors and geothermal sources.
Advantages of solid oxide-based regenerative fuel cells include high efficiencies, as they are not limited by Carnot efficiency. Additional advantages include long-term stability, fuel flexibility, low emissions, and low operating costs. However, the greatest disadvantage is the high operating temperature, which results in long start-up times and break-in times. The high operating temperature also leads to mechanical compatibility issues such as thermal expansion mismatch and chemical stability issues such as diffusion between layers of material in the cell
In principle, the process of any fuel cell could be reversed, due to the inherent reversibility of chemical reactions. However, a given fuel cell is usually optimized for operating in one mode and may not be built in such a way that it can be operated in reverse. Fuel cells operated backwards may not make very efficient systems unless they are constructed to do so such as in the case of solid oxide electrolyzer cells, high pressure electrolyzers, unitized regenerative fuel cells and regenerative fuel cells. However, current research is being conducted to investigate systems in which a solid oxide cell may be run in either direction efficiently.
SOECs have possible application in fuel production, carbon dioxide recycling, and chemicals synthesis. In addition to the production of hydrogen and oxygen, an SOEC could be used to create syngas by electrolyzing water vapor and carbon dioxide. This conversion could be useful for energy generation and energy storage applications.
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