Electrolyzer
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| Electrolyzer | |
|---|---|
 Low pressure Proton Exchange Membrane water electrolyzer | |
| Inventor | Sir William Robert Grove |
| Invented | 1839 |
An electrolyzer (American English) or electrolyser (British English) is a device that uses electrolysis to separate some chemical compounds into their constituent parts. However, in most cases this word is associated with Water Electrolyzers (WE), devices that consume water and electrical energy to produce hydrogen and oxygen. The hydrogen produced by electrolyzers can be used in various processes, such as replenishing hydrogen fuel cells, producing fertilizers, manufacturing steel, and other applications.
Design
An electrolyzer typically consists of several components: a cell containing an electrolyte with a feedstock, and two electrodes (the anode and cathode). When an electric potential difference is applied between the electrodes, an electric current flows through the cell. Depending on the type of electrolyzer, an electrolyte can be represented as a membrane, that passes only certain ions or as a liquid solution increasing the electrical conductivity of the whole solution inside the cell.
Types of water electrolyzers
There are 4 main types of water electrolyzers: Alkaline Electrolyzers, Proton Exchange Membrane Electrolyzers, Anion Exchange Membrane Electrolyzers and Solid Oxide Electrolyzers.
An alkaline electrolyzer (AWE or AEL), uses an aqueous electrolyte solution – usually potassium hydroxide (KOH) or sodium hydroxide (NaOH), with a porous separator. Alkaline electrolyzers typically operate at low temperatures, about 333 K - 363 K.
Proton Exchange Membranes (PEM) or Solid Polymer Electrolyte (SPE) are thin polymer electrolyte membranes that passes only positively charged hydrogen ions. At the start of the process, liquid water is supplied to the anode side of the electrolyzer.
Anion Exchange Membrane Electrolyzers (AEM or AEMWE), also known as Zero-gap Alkaline Electrolyzers, are electrolyzers using technology that combines the low-cost benefits of alkaline electrolysis and the compact, zero-gap design of Proton Exchange Membranes, allowing only anions to pass.
Solid Oxide Electrolysers (SOEC) are electrolyzers that use high temperatures to reduce electricity requirements. They require temperatures between 773 K and 1273 K.
Applications
Power plants
In some powerful power plants hydrogen is used to transmit heat from the power generating winding enclosure to the heat exchanges known as H2 coolers, since hydrogen is 7 times more thermal conductivity than air, and dry hydrogen is non‑corrosive to winding insulation and metal parts. This practice was first used in 1930s-40s at Port Washington Power Plant, as at the one of the most innovative power plants of that time. Typically, hydrogen pressure in systems like that must be at least 207 kPa(g), where the G letter inside parentheses means that the value is measured in gauge pressure, the pressure measured relatively to atmospheric pressure and using atmospheric pressure as the zero point. There are two ways to maintain the pressure in the system: deliver hydrogen in cylinders or make it on site, but to make it n site often is more preferred, since in this way hydrogen doesn't need to be stored and can be produced when it's necessary.
Outer space
Although electrolyzers are most commonly used to produce hydrogen, they are also employed for the production of oxygen in space environments. The Oxygen Generation System on the International Space Station uses electrolysis to produce oxygen for astronauts, and generates hydrogen as byproducts. It constitutes an on-board Proton-Exchange Membrane electrolyzer.
Energy storage
Based on the common classification of hydrogen types, there is so-called Green Hydrogen. Its energy source is mainly renewable electricity. The cycle of green hydrogen consists of an electrolyzer that produces a hydrogen and oxygen and the exact opposite device – a fuel cell, which consumes hydrogen and oxygen, but produces water and electricity. Oxygen is usually taken from the surrounding air, using compressors, although it's also possible to operate without them.
Fertilizers
Hydrogen is an essential element for ammonia production, and the base element of ammonium fertilizers. Electrolyzers allows us to produce them on an industrial scale using renewable energy sources.
Aluminum extraction
- 2 Al2O3 → 4 Al + 3 O2
As the word "electrolyzer" typically refers us to water electrolyzers, they can also be used to extract aluminum from aluminium oxide. Since aluminum oxide is classified as an electrical insulator, it needs to be melted so that it can conduct electricity. The melting point of aluminum oxide is over 2,000 K, so it is dissolved in molten cryolite, essentially – an electrolyte, which lowers the melting point of the alloy to approximately 1,200 K, thus allows us to spend less energy and time to start the process of electrolysis.
Problems
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Despite the advantages of water electrolyzers, there are problems in their use. One of the main problems is the risk of hydrogen explosions. In order to avoid mixing the fuel with an oxidizer, namely oxygen, from an electolyzer itself or the surrounding air, hydrogen purity is kept high.