High-temperature electrolysis

High-temperature electrolysis (also HTE or steam electrolysis) is a technology for producing hydrogen from water at high temperatures. High-temperature electrolysis (also HTE or steam electrolysis) is a technology for producing hydrogen from water at high temperatures. High temperature electrolysis is more efficient economically than traditional room-temperature electrolysis because some of the energy is supplied as heat, which is cheaper than electricity, and also because the electrolysis reaction is more efficient at higher temperatures. In fact, at 2500 °C, electrical input is unnecessary because water breaks down to hydrogen and oxygen through thermolysis. Such temperatures are impractical; proposed HTE systems operate between 100 °C and 850 °C. The efficiency improvement of high-temperature electrolysis is best appreciated by assuming that the electricity used comes from a heat engine, and then considering the amount of heat energy necessary to produce one kg hydrogen (141.86 megajoules), both in the HTE process itself and also in producing the electricity used. At 100 °C, 350 megajoules of thermal energy are required (41% efficient). At 850 °C, 225 megajoules are required (64% efficient). The selection of the materials for the electrodes and electrolyte in a solid oxide electrolyser cell is essential. One option being investigated for the process used yttria-stabilized zirconia (YSZ) electrolytes, nickel-cermet steam/hydrogen electrodes, and mixed oxide of lanthanum, strontium and cobalt oxygen electrodes.