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Concept
Solid oxide electrolyzer cell
Summary
A solid oxide electrolyzer cell (SOEC) is a solid oxide fuel cell that runs in regenerative mode to achieve the electrolysis of water (and/or carbon dioxide) by using a solid oxide, or ceramic, electrolyte to produce hydrogen gas (and/or carbon monoxide) and oxygen.
The production of pure hydrogen is compelling because it is a clean fuel that can be stored, making it a potential alternative to batteries, methane, and other energy sources (see hydrogen economy). Electrolysis is currently the most promising method of hydrogen production from water due to high efficiency of conversion and relatively low required energy input when compared to thermochemical and photocatalytic methods.
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 a solid oxide fuel cell. 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: 2 O2− → O2 + 4 e−
Cathode: H2O + 2 e− → H2 + O2−
Net Reaction: 2 H2O → 2 H2 + O2
Electrolysis of water at 298 K (25 °C) requires 285.83 kJ of energy per mole 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.
The general function of the electrolyzer cell is to split water in the form of steam into pure H2 and O2. Steam is fed into the porous cathode. When a voltage is applied, the steam moves to the cathode-electrolyte interface and is reduced to form pure H2 and oxygen ions. The hydrogen gas then diffuses back up through the cathode and is collected at its surface as hydrogen fuel, while the oxygen ions are conducted through the dense electrolyte.
Official source
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