Solid oxide electrolyser cell
Encyclopedia
Electrolysis of water
Electrolysis of water is the decomposition of water into oxygen and hydrogen gas due to an electric current being passed through the water.-Principle:...
and which uses a solid oxide, or ceramic
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
, electrolyte
Electrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....
to produce oxygen and hydrogen gas.
Principle
Solid oxide electrolyzer cells operate at temperatures which allow high-temperature electrolysisHigh-temperature electrolysis
High-temperature electrolysis is a method currently being investigated for the production of hydrogen from water with oxygen as a by-product.-Efficiency:...
to occur, typically between 500 and 850 °C. These operating temperatures are similar to those conditions for an SOFC. The net cell reaction yields hydrogen and oxygen gases. The reactions for one mole
Mole (unit)
The mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12 , the isotope of carbon with atomic weight 12. This corresponds to a value...
of water are shown below, with oxidation of water occurring at the anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
and reduction
Reduction
Reduction, reduced, or reduce may refer to:- Chemistry :* Reduction, part of a reduction-oxidation reaction where oxygen is being removed from a compound.** Reduced gas, a gas with a low oxidation number...
of water occurring at the cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
.
Cathode: H2O ---> 1/2O2 + 2H+ + 2e-
Anode: 2H2O + 2e- ---> H2 + 2OH-
Net Reaction: H2O ---> H2 + 1/2O2
Electrolysis of water at 298 K (25 °C) requires 285.83 kJ of energy 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
Joule heating, also known as ohmic heating and resistive heating, is the process by which the passage of an electric current through a conductor releases heat. It was first studied by James Prescott Joule in 1841. Joule immersed a length of wire in a fixed mass of water and measured the temperature...
of an electrolysis cell, which may be utilized in the water splitting
Water splitting
Water splitting is the general term for a chemical reaction in which water is separated into oxygen and hydrogen. Efficient and economical water splitting would be a key technology component of a hydrogen economy. Various techniques for water splitting have been issued in water splitting patents in...
process at high temperatures. Research is ongoing to add heat from external heat sources such as concentrating solar thermal collectors
Solar power
Solar energy, radiant light and heat from the sun, has been harnessed by humans since ancient times using a range of ever-evolving technologies. Solar radiation, along with secondary solar-powered resources such as wind and wave power, hydroelectricity and biomass, account for most of the available...
and geothermal
Geothermal power
Geothermal energy is thermal energy generated and stored in the Earth. Thermal energy is the energy that determines the temperature of matter. Earth's geothermal energy originates from the original formation of the planet and from radioactive decay of minerals...
sources.
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 temperatureOperating temperature
An operating temperature is the temperature at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the...
, 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
Thermal expansion is the tendency of matter to change in volume in response to a change in temperature.When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation. Materials which contract with increasing temperature are rare; this effect is...
mismatch and chemical stability issues such as diffusion
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
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
Fuel cell
A fuel cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent. Hydrogen is the most common fuel, but hydrocarbons such as natural gas and alcohols like methanol are sometimes used...
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
High pressure electrolysis
High-pressure electrolysis is the electrolysis of water by decomposition of water into oxygen and hydrogen gas due to the passing of an electric current through the water. The difference with a standard proton exchange membrane electrolyzer is the compressed hydrogen output around 120–200 Bar ...
, unitized regenerative fuel cell
Unitized regenerative fuel cell
A unitized regenerative fuel cell 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 electricity. Both modes are done with the same fuel cell...
s and regenerative fuel cells. However, current research is being conducted to investigate systems in which a solid oxide cell may be run in either direction efficiently.
Applications
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 syngasSyngas
Syngas is the name given to a gas mixture that contains varying amounts of carbon monoxide and hydrogen. Examples of production methods include steam reforming of natural gas or liquid hydrocarbons to produce hydrogen, the gasification of coal, biomass, and in some types of waste-to-energy...
by electrolyzing water vapor and carbon dioxide. This conversion could be useful for energy generation and energy storage applications.
See also
- Glossary of fuel cell termsGlossary of fuel cell termsThe Glossary of fuel cell terms lists the definitions of many terms used within the fuel cell industry. The terms in this fuel cell glossary may be used by fuel cell industry associations, in education material and fuel cell codes and standards to name but a few. –...
- Hydrogen technologiesHydrogen technologiesHydrogen technologies are technologies that relate to the production and use of hydrogen. Hydrogen technologies are applicable for many uses....