Solid oxide electrolyzer cell

SOEC-60 cell stack.

A solid oxide Elektrolyseurzelle ( English solid oxide electrolyzer cell , SOEC) is a solid oxide fuel cell (SOFC), which in the reverse mode , is operated by the electrolysis of water (and / or carbon dioxide to achieve). Using a solid oxide or ceramic , electrolyte produce hydrogen gas (and / or carbon monoxide ) and oxygen. Making pure hydrogen is compelling as it is a clean fuel that is easy to store. This makes it a potential alternative to batteries, which have a relatively low storage capacity and currently produce a lot of waste. Electrolysis is currently the most promising method for generating hydrogen from water, as it has a high conversion efficiency and requires relatively little energy input compared to thermochemical and photocatalytic methods.

principle

Solid oxide electrolyzer cells work at typical temperatures between 500 and 850 ° C, at which it uses high-temperature electrolysis. These operating temperatures are similar to the conditions for an SOFC . The net cell reaction results in hydrogen and oxygen gases. The reactions for one mole of water are listed below, with oxidation of water taking place at the anode and reduction of water at the cathode .

${\displaystyle {\ce {Anode: O^{2}- -> {\frac {1}{2}}O2 + 2e-}}}$

${\displaystyle {\ce {Kathode: H2O + 2e- -> H2 + O^{2}-}}}$

${\displaystyle {\ce {Nettoreaktion: H2O -> H2 + {\frac {1}{2}}O2}}}$

Electrolysis of water at 298 K (25 ° C) requires 285.83 kJ of energy per mole to occur. The reaction is increasingly endothermic with increasing temperature . However, the energy demand can increase due to the Joule heating of an electrolysis cell that can be used in water electrolysis at high temperatures. Research is currently underway to bring in heat from external sources such as concentrating solar thermal collectors and geothermal sources .

business

The general function of the electrolytic cell is to split water in the form of steam into pure H ₂ and O ₂ . Steam is fed into the porous cathode. When a voltage is applied, the vapor moves to the cathode-electrolyte interface and is reduced to form pure H ₂ and oxygen ions. The hydrogen gas then diffuses back through the cathode and is collected on its surface as hydrogen fuel gas as the oxygen ions are passed through the dense electrolyte. The electrolyte must be so dense that steam and the hydrogen gas cannot diffuse through and prevent the recombination of H ₂ and O ²⁻ . At the interface between the electrolyte and the anode, the oxygen ions are oxidized to form pure oxygen gas, which is collected on the surface of the anode.

materials

Solid oxide electrolysis cells follow the same structure as a solid oxide fuel cell, consisting of a fuel electrode (cathode), an oxygen electrode (anode) and a solid oxide electrolyte.

electrolyte

The most common electrolyte, which is also similar to solid oxide fuel cells, is a dense ion conductor made of ZrO ₂ doped with 8 mol% Y ₂ O ₃ (also known as YSZ ). Zirconium dioxide is used because of its high strength and melting temperature (approx. 2700 ° C) and excellent corrosion resistance. Y ₂ O ₃ is added in order to soften the phase transition from the tetragonal to the monoclinic phase during rapid cooling, which can lead to cracks and reduce the conductivity of the electrolyte by scattering. Some other common options for SOEC are scandium- stabilized zirconia , ceria- based electrolytes, or lanthanum - gallate materials. Despite the material similarity to solid oxide fuel cells, the operating conditions are different, which leads to problems such as high vapor concentrations at the fuel electrode and high oxygen partial pressure at the interface between the electrolyte and the oxygen electrode. A recent study found that periodically switching a cell between electrolyzer and fuel cell modes reduced the oxygen partial pressure and dramatically extended the life of the electrolyzer cell.

Fuel electrode (cathode)

The most common fuel electrode material is a Ni-doped YSZ, but high vapor partial pressures and low hydrogen partial pressures at the Ni-YSZ interface caused the nickel to oxidize and lead to irreversible degradation. Perovskite-type lanthanum strontium manganese (LSM) is also commonly used as a cathode material. Recent studies have shown that doping LSM with scandium to form LSMS promotes the mobility of oxide ions in the cathode, increases the reduction kinetics at the interface with the electrolyte, and thus leads to higher performance at low temperatures than conventional LSM cells. However, further development of the sintering process parameters is necessary in order to prevent the precipitation of scandium oxide into the LSM lattice. These precipitate particles are problematic because they can hinder the conduction of electrons and ions. In particular, the processing temperature and the concentration of scandium in the LSM are examined in order to optimize the properties of the LSMS cathode. New materials such as Lanthanum Strontium Manganese Chromate (LSCM), which has been shown to be more stable under electrolysis conditions, are currently being investigated. LSCM has a high redox stability , which is particularly important at the interface with the electrolyte. Scandium -doped LCSM (LSCMS) is also being researched as a cathode material because of its high ionic conductivity. The rare earth element, however, has a significant material cost and has been found to cause a slight decrease in the total mixed conductivity. Nonetheless, LCSMS materials have already proven to be highly efficient at temperatures of 700 ° C.

Oxygen electrode (anode)

Lanthanum strontium manganate (LSM) is the most common oxygen electrode material. LSM offers high performance under electrolysis conditions, since oxygen vacancies are created under anodic polarization, which support oxygen diffusion. In addition, impregnating the LSM electrode with GDC nanoparticles was found to extend the life of the cells by preventing delamination at the electrode-electrolyte interface. The exact mechanism by which this is done needs further study. A 2010 study 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 Operation at 800 ° C. It is believed that the increased performance is due to a higher "overstoichimetry" of the oxygen in the neodymium nickelate, making it a successful conductor for both ions and electrons.

The advantages of regenerative fuel cells based on solid oxide include high efficiencies, as they are not limited to their Carnot efficiency . Further advantages are long-term stability, fuel flexibility, low emissions and low operating costs. The main disadvantage, however, is the high operating temperature . This leads to long start-up times. The high operating temperature also leads to mechanical problems such as thermal expansion , mismatches and chemical stability problems 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. A given fuel cell is typically optimized to operate in one mode and must not be built to operate in reverse. Fuel cells operated in reverse may not be very efficient systems unless they are designed to do so, as is the case with solid oxide electrolyzer cells. High pressure electrolyzers, reversible fuel cells and regenerative fuel cells. However, research is currently being carried out to investigate systems in which a solid oxide cell can operate efficiently in both directions.

Delamination

It has been observed that fuel cells operated in the electrolysis mode deteriorate mainly due to anode detachment from the electrolyte. The detachment is the result of a high partial pressure of oxygen at the interface between the electrolyte and anode. Pores in the electrolyte anode material act to limit high oxygen partial pressures that induce stress concentration in the surrounding material. The maximum induced stress can be expressed in terms of internal oxygen pressure using the following equation from fracture mechanics:

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

where c is the length of the crack or pore and is the radius of curvature of the crack or pore. If the theoretical strength of the material is exceeded, the crack will spread, which macroscopically leads to delamination. ${\displaystyle \rho }$ ${\displaystyle \sigma _{max}}$

Virkar et al. created a model for calculating the internal oxygen partial pressure from the oxygen partial pressure exposed to the electrodes with the electrolyte-resistant properties. The internal pressure of oxygen at the electrolyte-anode interface was modeled 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 is the partial pressure of oxygen that is exposed to the oxygen electrode (anode), is the area-specific electronic resistance at the anode interface , is the area-specific ionic resistance at the anode interface, is the applied voltage, is the Nernst potential, and is the total specific resistances of the electronic and the ionic surface, and and are the electrical potentials at the anode surface and the anode-electrolyte interface, respectively. ${\displaystyle P_{O2}^{Ox}}$ ${\displaystyle r-e^{a}}$ ${\displaystyle r_{i}^{a}}$ ${\displaystyle E_{a}}$ ${\displaystyle E_{N}}$ ${\displaystyle R_{e}}$ ${\displaystyle R_{i}}$ ${\displaystyle \phi ^{Ox}}$ ${\displaystyle \phi ^{a}}$

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

The separation of the anode from the electrolyte increases the resistance of the cell and requires higher operating voltages to maintain a stable current. Higher applied voltages increase the internal partial pressure of oxygen and worsen the degradation.

Applications

SOECs could be used in fuel production, carbon dioxide recycling and chemical synthesis. In addition to producing hydrogen and oxygen, a SOEC could be used to produce synthesis gas through the electrolysis of water vapor and carbon dioxide. This conversion could be important for power generation and storage applications.

The Massachusetts Institute of Technology plans to use the method on the March 2020 Rover Mission as a means of generating oxygen for both human consumption and liquid rocket fuel.

Operating conditions

The SOEC module can be operated in three different operating modes: thermoneutral, exothermic and endothermic. In the exothermic mode, the stack temperature rises during operation due to the heat build-up and this heat is used to preheat the inlet gas. Therefore, the external heat source is not needed while the power consumption increases. In the endothermic batch operation mode, there is an increase in thermal energy consumption and a decrease in electrical energy consumption and hydrogen production as the average current density also decreases. The third mode is thermoneutral, in which the heat generated by irreversible losses equals the heat required by the reaction. Since there is a loss, an external heat source is required. This mode consumes more electricity than the endothermic operating mode.

Web links

• 2007 DOE Hydrogen Program Review
• RELHY

Individual evidence

1. ↑ Yun Zheng, Jianchen Wang, Bo Yu, Wenqiang Zhang, Jing Chen, Jinli Qiao, Jiujun Zhang: A review of high temperature co-electrolysis of HO and CO to produce sustainable fuels using solid oxide electrolysis cells (SOECs): advanced materials and technology . In: Chem. Soc. Rev. . 46, No. 5, 2017, pp. 1427–1463. doi : 10.1039 / C6CS00403B . PMID 28165079 .
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5. ^ Greg Tao: A Reversible Planar Solid Oxide Fuel-Fed Electrolysis Cell and Solid Oxide Fuel Cell for Hydrogen and Electricity Production Operating on Natural Gas / Biomass Fuels . Materials and Systems Research, Inc., Salt Lake City, Utah, 2007, doi : 10.2172 / 934689 ( energy.gov [PDF]).
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11. ↑ Xiangling Yue, Aiyu Yan, Min Zhang, Lin Liu, Yonglai Dong, Mojie Cheng: Investigation on scandium-doped manganate La0.8Sr0.2Mn1 - xScxO3 −δ cathode for intermediate temperaturesolid oxide fuel cells . In: Journal of Power Sources . tape 185 , no. 2 , December 1, 2008, p. 691-697 , doi : 10.1016 / j.jpowsour.2008.08.038 .
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25. ↑ MOXIE - An MIT oxygen-creating instrument has been selected to fly on the upcoming Mars 2020 mission
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