Solid oxide electrolyzer cell

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). Zirconium dioxide is used because of its high strength, high melting temperature (approximately 2700 °C) and excellent corrosion resistance. 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.[10] A recent study found that periodic cycling a cell between electrolyzer and fuel cell modes reduced the oxygen partial pressure build up and drastically increased the lifetime of the electrolyzer cell.[11]

The most common fuel electrode material is a Ni doped YSZ. However, high steam partial pressures and low hydrogen partial pressures at the Ni-YSZ interface causes oxidation of the nickel which results in catalyst degradation.[12] Perovskite-type lanthanum strontium manganese (LSM) is also commonly used as a cathode material. 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. These precipitate particles are problematic because they can impede electron and ion conduction. 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.[14] LSCM has high redox stability, which is crucial especially at the interface with the electrolyte. 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.[15]

Lanthanum strontium manganate (LSM) is the most common oxygen electrode material. LSM offers high performance under electrolysis conditions due to generation of oxygen vacancies under anodic polarization that aid oxygen diffusion.[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.[17] The exact mechanism by how this happen needs to be explore further. 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.[18]

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. Pores in the electrolyte-anode material act to confine high oxygen partial pressures inducing stress concentration in the surrounding material. The maximum stress induced can be expressed in terms of the internal oxygen pressure using the following equation from fracture mechanics:[23]

${\displaystyle \sigma _{max}=2P_{O2}({\frac {c}{\rho }})^{1/2}}$

where c is the length of the crack or pore and ${\displaystyle \rho }$ is the radius of curvature of the crack or pore. If ${\displaystyle \sigma _{max}}$ exceeds the theoretical strength of the material, the crack will propagate, macroscopically resulting in delamination.

Virkar et al. created a model to calculate the internal oxygen partial pressure from the oxygen partial pressure exposed to the electrodes and the electrolyte resistive properties.[24] The internal pressure of oxygen at the electrolyte- anode interface was modelled as:

${\displaystyle P_{O2}^{a}=P_{O2}^{Ox}\exp \left[-{\frac {4F}{RT}}\left\{{\frac {E_{a}r_{e}^{a}}{R_{e}}}-{\frac {(E_{a}-E_{N})r_{i}^{a}}{R_{i}}}\right\}\right]}$

${\displaystyle =P_{O2}^{Ox}\exp \left[-{\frac {4F}{RT}}\left\{(\phi ^{Ox}-\phi ^{a})-{\frac {(E_{a}-E_{N})r_{i}^{a}}{R_{i}}}\right\}\right]}$

where ${\displaystyle P_{O2}^{Ox}}$ is the oxygen partial pressure exposed to the oxygen electrode (anode), ${\displaystyle r_{e}^{a}}$ is the area specific electronic resistance at the anode interface, ${\displaystyle r_{i}^{a}}$ is the area specific ionic resistance at the anode interface, ${\displaystyle E_{a}}$ is the applied voltage, ${\displaystyle E_{N}}$ is the Nernst potential, ${\displaystyle R_{e}}$ and ${\displaystyle R_{i}}$ are the overall electronic and ionic area specific resistances respectively, and ${\displaystyle \phi ^{Ox}}$ and ${\displaystyle \phi ^{a}}$ are the electric potentials at the anode surface and the anode electrolyte interface respectively.[25]

In electrolysis mode ${\displaystyle \phi ^{Ox}}$>${\displaystyle \phi ^{a}}$ and ${\displaystyle E_{a}}$>${\displaystyle E_{N}}$. Whether ${\displaystyle P_{O2}^{a}}$ is greater than ${\displaystyle P_{O2}^{Ox}}$ is dictated by whether (${\displaystyle \phi ^{Ox}}$-${\displaystyle \phi ^{a}}$ ) or ${\displaystyle {\frac {E_{a}r_{e}^{a}}{R_{e}}}}$ is greater than ${\displaystyle {\frac {(E_{a}-E_{N})r_{i}^{a}}{R_{i}}}}$ . The internal oxygen partial pressure is minimized by increasing the electronic resistance at the anode interface and decreasing the ionic resistance at anode interface.

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.