A solid oxide electrolyzer cell (SOEC) is a
solid oxide fuel cell
A solid oxide fuel cell (or SOFC) is an electrochemical conversion device that produces electricity directly from oxidizing a fuel. Fuel cells are characterized by their electrolyte material; the SOFC has a solid oxide or ceramic electrolyte.
A ...
that runs in
regenerative mode to achieve the
electrolysis of water
Electrolysis of water, also known as electrochemical water splitting, is the process of using electricity to decompose water into oxygen and hydrogen gas by electrolysis. Hydrogen gas released in this way can be used as hydrogen fuel, or remi ...
(and/or carbon dioxide) by using a solid oxide, or
ceramic
A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain ...
,
electrolyte
An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon dis ...
to produce
hydrogen gas
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, a ...
(and/or
carbon monoxide
Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...
) 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
The hydrogen economy is using hydrogen to decarbonize economic sectors which are hard to electrify, essentially, the "hard-to-abate" sectors such as cement, steel, long-haul transport etc. In order to phase out fossil fuels and limit climate ch ...
). 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.
Principle
Solid oxide electrolyzer cells operate at temperatures which allow
high-temperature electrolysis
High-temperature electrolysis (also HTE or steam electrolysis) is a technology for producing hydrogen from water at high temperatures.
Efficiency
High temperature electrolysis is more efficient economically than traditional room-temperature elect ...
to occur, typically between 500 and 850 °C. These
operating temperature
An operating temperature is the allowable temperature range of the local ambient environment at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the de ...
s are similar to those conditions for a
solid oxide fuel cell
A solid oxide fuel cell (or SOFC) is an electrochemical conversion device that produces electricity directly from oxidizing a fuel. Fuel cells are characterized by their electrolyte material; the SOFC has a solid oxide or ceramic electrolyte.
A ...
. The net cell reaction yields hydrogen and oxygen gases. The reactions for one
mole
Mole (or Molé) may refer to:
Animals
* Mole (animal) or "true mole", mammals in the family Talpidae, found in Eurasia and North America
* Golden moles, southern African mammals in the family Chrysochloridae, similar to but unrelated to Talpida ...
of water are shown below, with
oxidation
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
of water occurring at the
anode
An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemonic is ...
and
reduction of water occurring at the
cathode
A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in whi ...
.
Anode: 2 O
2− → O
2 + 4 e
−
Cathode: H
2O + 2 e
− → H
2 + O
2−
Net Reaction: 2 H
2O → 2 H
2 + O
2
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
Joule heating, also known as resistive, resistance, or Ohmic heating, is the process by which the passage of an electric current through a conductor (material), conductor produces heat.
Joule's first law (also just Joule's law), also known in c ...
of an electrolysis cell, which may be utilized in the
water splitting
Water splitting is the chemical reaction in which water is broken down into oxygen and hydrogen:
:2 H2O → 2 H2 + O2
Efficient and economical water splitting would be a technological breakthrough that could underpin a hydrogen economy, base ...
process at high temperatures. Research is ongoing to add heat from external heat sources such as
concentrating solar thermal collectors and
geothermal sources.
Operation
The general function of the electrolyzer 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 steam moves to the cathode-electrolyte interface and is reduced to form pure H
2 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. The electrolyte must be dense enough that the steam and hydrogen gas cannot diffuse through and lead to the recombination of the H
2 and O
2−. At the electrolyte-anode interface, the oxygen ions are oxidized to form pure oxygen gas, which is collected at the surface of the anode.
Materials
Solid oxide electrolyzer cells follow the same construction of 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, again similar to solid-oxide fuel cells, is a dense ionic conductor consisting of ZrO
2 doped with 8 mol-% Y
2O
3 (also known as YSZ, ytrium-stabilized zirconia).
Zirconium dioxide
Zirconium dioxide (), sometimes known as zirconia (not to be confused with zircon), is a white crystalline oxide of zirconium. Its most naturally occurring form, with a monoclinic crystalline structure, is the mineral baddeleyite. A dopant stabi ...
is used because of its high strength, high melting temperature (approximately 2700 °C) and excellent corrosion resistance.
Yttrium(III) oxide
Yttrium oxide, also known as yttria, is Y2 O3. It is an air-stable, white solid substance.
The thermal conductivity of yttrium oxide is 27 W/(m·K).
Uses Phosphors
Yttria is widely used to make Eu:YVO4 and Eu:Y2O3 phosphors that give the red ...
(Y
2O
3) 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. 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. 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.
Fuel Electrode (Cathode)
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. 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. New materials are being researched such as lanthanum strontium manganese chromate (LSCM), which has proven to be more stable under electrolysis conditions. 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.
Oxygen Electrode (Anode)
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. In addition, impregnating LSM electrode with Gd-doped CeO
2 (GDC) nanoparticles was found to increase cell lifetime by preventing delamination at the electrode/electrolyte interface. The exact mechanism by how this happen needs to be explore further. In a 2010 study, it was found that
neodymium nickelate
Neodymium nickelate is a nickelate of neodymium with a chemical formula NdNiO3. In this compound, the neodymium atom is in the +3 oxidation state.
Preparation
Neodymium nickelate can be prepared by dissolving neodymium(III) oxide and nickel(II) ...
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.
Considerations
Advantages of solid oxide-based regenerative fuel cells include high efficiencies, as they are not limited by
Carnot efficiency.
Additional advantages include long-term stability, fuel flexibility, low emissions, and low operating costs. However, the greatest disadvantage is the high
operating temperature
An operating temperature is the allowable temperature range of the local ambient environment at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the de ...
, which results in long start-up times and break-in times. The high operating temperature also leads to mechanical compatibility issues such as
thermal expansion
Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions.
Temperature is a monotonic function of the average molecular kinetic ...
mismatch and chemical stability issues such as
diffusion
Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
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.
However, a given
fuel cell
A fuel cell is an electrochemical cell that converts the chemical energy of a fuel (often hydrogen) and an oxidizing agent (often oxygen) into electricity through a pair of redox reactions. Fuel cells are different from most batteries in requ ...
is usually optimized for operating in one mode and may not be built in such a way that it can be operated in reverse. Fuel cells operated backwards may not make very efficient systems unless they are constructed to do so such as in the case of solid oxide electrolyzer cells,
high pressure electrolyzers,
unitized regenerative fuel cell A unitized regenerative fuel cell (URFC) is a fuel cell based on the proton exchange membrane which can do the electrolysis of water in regenerative mode and function in the other mode as a fuel cell recombining oxygen and hydrogen gas to produce e ...
s and
regenerative fuel cell A regenerative fuel cell or reverse fuel cell (RFC) is a fuel cell run in reverse mode, which consumes electricity and chemical B to produce chemical A. By definition, the process of any fuel cell could be reversed. However, a given device is usuall ...
s. However, current research is being conducted to investigate systems in which a solid oxide cell may be run in either direction efficiently.
Delamination
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:
:
where c is the length of the crack or pore and
is the radius of curvature of the crack or pore. If
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. The internal pressure of oxygen at the electrolyte- anode interface was modelled as:
: