Solid oxide electrolyzer cell
[4] Solid oxide electrolyzer cells operate at temperatures which allow high-temperature electrolysis[5] to occur, typically between 500 and 850 °C.
These operating temperatures are similar to those conditions for a solid oxide fuel cell.
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.
[7] The general function of the electrolyzer cell is to split water in the form of steam into pure H2 and O2.
When a voltage is applied, the steam moves to the cathode-electrolyte interface and is reduced to form pure H2 and oxygen ions.
The most common electrolyte, again similar to solid-oxide fuel cells, is a dense ionic conductor consisting of ZrO2 doped with 8 mol-% Y2O3 (also known as YSZ, ytrium-stabilized zirconia).
Yttrium(III) oxide (Y2O3) is added to mitigate the phase transition from the tetragonal to the monoclinic phase on rapid cooling, which can lead to cracks and decrease the conductive properties of the electrolyte by causing scattering.
[9] Some other common choices for SOEC are Scandia stabilized zirconia (ScSZ), ceria based electrolytes or lanthanum gallate materials.
Despite the material similarity to solid oxide fuel cells, the operating conditions are different, leading to issues such as high steam concentrations at the fuel electrode and high oxygen partial pressures at the electrolyte/oxygen electrode interface.
Recent studies have found that doping LSM with scandium to form LSMS promotes mobility of oxide ions in the cathode, increasing reduction kinetics at the interface with the electrolyte and thus leading to higher performance at low temperatures than traditional LSM cells.
However, further development of the sintering process parameters is required to prevent precipitation of scandium oxide into the LSM lattice.
In particular, the processing temperature and concentration of scandium in the LSM lattice are being researched to optimize the properties of the LSMS cathode.
[13] New materials are being researched such as lanthanum strontium manganese chromate (LSCM), which has proven to be more stable under electrolysis conditions.
Scandium-doped LCSM (LSCMS) is also being researched as a cathode material due to its high ionic conductivity.
However, the rare-earth element introduces a significant materials cost and was found to cause a slight decrease in overall mixed conductivity.
Nonetheless, LCSMS materials have demonstrated high efficiency at temperatures as low as 700 °C.
[16] In addition, impregnating LSM electrode with Gd-doped CeO2 (GDC) nanoparticles was found to increase cell lifetime by preventing delamination at the electrode/electrolyte interface.
In a 2010 study, it was found that neodymium nickelate as an anode material provided 1.7 times the current density of typical LSM anodes when integrated into a commercial SOEC and operated at 700 °C, and approximately 4 times the current density when operated at 800 °C.
The increased performance is postulated to be due to higher "overstoichimoetry" of oxygen in the neodymium nickelate, making it a successful conductor of both ions and electrons.
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[20] In principle, the process of any fuel cell could be reversed, due to the inherent reversibility of chemical reactions.
However, current research is being conducted to investigate systems in which a solid oxide cell may be run in either direction efficiently.
[22] Fuel cells operated in electrolysis mode have been observed to degrade primarily due to anode delamination from the electrolyte.
The delamination is a result of high oxygen partial pressure build up at the electrolyte-anode interface.
The maximum stress induced can be expressed in terms of the internal oxygen pressure using the following equation from fracture mechanics:[23] where c is the length of the crack or pore and
exceeds the theoretical strength of the material, the crack will propagate, macroscopically resulting in delamination.
Delamination of the anode from the electrolyte increases the resistance of the cell and necessitates higher operating voltages in order to maintain a stable current.
[26] Higher applied voltages increases the internal oxygen partial pressure, further exacerbating the degradation.
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.
[27] Mega-watt scale SOEC have been installed in Rotterdam, using industrial waste heat to reach its operating temperature of 850°C .
