2 H2 + 4 OH-
2 H2O + 4 e- + O2
Electrolyseur pour la production d'hydrogène et d'oxygène.
Électrolyseur, générateur d'hydrogène.
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An electrolyser or electrolyser is used to produce high-purity hydrogen. Applying electric current causes a chemical dissociation (decomposition) of water (H2O) to form hydrogen (H2) and oxygen (O2). Since only renewable raw materials are required for production, electrolysers are a green technology. Due to the high hydrogen purity of 99.999%, this technology is ideal for supplying fuel cells. A hydrogen supply on site in different sizes is possible. The resulting gas must be dried. This drying is already integrated into our systems.
In water electrolysis, a distinction can be made between alkaline and PEM electrolysis:
In alkaline electrolysis (AEL), the electrodes are made of metal and have a very high long-term stability. Usually dimensionsstable Anodes (DSA) made of iron or titanium are used. On top of this electrically conductive electrode a catalyst layer with a large surface
called ECSA (Electrocchemical active surface area) is applied. This can consist of noble metal oxide or raney nickel, for example. A permeable diaphragm is used as the membrane.
In earlier designs, the so-called cell gap design, the electrodes were attached at a certain distance from the membrane in order to remove the produced gas. The distance varied between a few centimeters and millimeters. In modern AEL electrolysers, the zero gap design is used.
Here the electrodes are placed directly on the membrane to reduce electrical resistance and allow greater current densities. With this structure, the resulting gas escapes through pores in the electrodes. A liquid electrolyte is also necessary for this structure.
Significantly higher gas flows can be realized with this method compared to, e.g. PEM electrolysis. An alkaline electrolyser is therefore particularly recommended for very large applications.
The speed of the reaction depends primarily on two factors. The higher the temperature, the faster the reaction and the less voltage is needed.
On the other hand, too high temperature is difficult to handle. That is why our systems work at a temperature of approx. 85° C.
Secondly, the rate is affected by the type of ions used in the electrolyte. Potassium shows a significantly better ion mobility than, e.g. sodium.
By adding caustic potash (KOH; potassium hydroxide solution), an electrolyte is generated that conducts significantly better than water. This achieves rapid gas separation at the electrodes.
The splitting of hydrogen therefore takes place in a basic environment and can be described by the following reaction equation:
| HER (Hydrogen evolution reaction): |
4 H2O + 4 e- 2 H2 + 4 OH-
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| OER (Oxygen evolution reaction): |
4 OH- 2 H2O + 4 e- + O2
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The electrodes are separated by a permeable membrane (diaphragm). This enables the transport of hydroxide ions (OH-), but prevents the exchange of the resulting gases oxygen and hydrogen, both in dissolved form and in the form of gas bubbles. In order to allow the diffusion of ions in aqueous solution, the membrane must have hydrophilic properties. Today, this is achieved through a combination of hydrophobic organic polymers such as PTFE or polysulfone as the carrier material and hydrophilic ceramics such as potassium titanate or zirconium oxide. Such membranes are chemically and mechanically stable and allow the pores to be filled with electrolyte. Applying a DC voltage produces oxygen at the anode and hydrogen at the cathode.
The alkaline hydrogen production technology is mature and is characterized by low production costs. With our alkaline electrolysers, it is currently possible to generate hydrogen from 5 Nm3/h-2000 Nm3/h. The working pressure of such a system is ≤16 bar. A partial load operation of 30-100% is possible. The separating membrane is not perfect. The proportion of gases that are let through depends only on the concentrations and not on the quantities of gas produced. Therefore, an undesired mixing of the gases has a greater effect in the lower partial load range than in full operation.
| Alkaline electrolyser (SinoHy) | |||
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In the Proton-Exchange-Membran electrolyser (polymer electrolyte membrane) in contrast to alkaline electrolysis, works in an acidic environment. This is why it is also referred to as an acidic electrolyser. Since the ionic mobility of hydrogen ions (H+) is higher than that of hydroxide ions (OH-), higher speeds can be achieved here than with alkaline electrolysis. The dissotiaion of water is shown ind the reaction scheme below.
| HER (Hydrogen evolution reaction): |
4 H+ + 4 e- 2 H2
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| OER (Oxygen evolution reaction): |
H2O 4 H+ + 4 e- + O2
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The anode is usually carbon coated with platinum (Pt) or molybdenum sulfide; the cathode consits of iridium or ruthenium oxide. A solid polymer is used as the electrolyte. As a result, no corrosion takes place and easy maintenance is possible. The heart of this method is the polymer membrane (proton exchange membrane). This is a semi-permeable membrane that allows the transport of protons but prevents the exchange of gases such as oxygen and hydrogen. In this process, distilled water is dissosiated into hydrogen and oxygen with the help of electricity. A voltage is applied to the electrodes and water is added on the anode side. Due to the catalytic effect of the noble metal, water is decomposed on the anode side. This creates oxygen (O2), free electrons and hydrogen ions. The H+ ions diffuse through the proton-permeable PEM membrane to the cathode and are converted to hydrogen there. The contacting of the electrodes works through current collectors. The resulting gases are collected using a channel structure and carried out of the cell.
In comparison, PEM hydrogen generation technology has the advantages of faster start-up, no corrosion, easy maintenance, fewer components and higher current densities. Higher pressures than with alkaline electrolysis can be achieved and partial load operation starting at 5% is already possible. High manufacturing costs are the main limiting factors of PEM hydrogen production technology.
| PEM electrolyser (SinoHy) | |||
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Good water quality is required for all types of electrolysers. Based on your water analysis data, we can optionally integrate water treatment into our systems.
Both after alkaline electrolysis and after PEM electrolysis, there is residual moisture in the produced hydrogen gas. The necessary drying is already integrated in our systems. Gas drying can be done by cooling the gas and by a swing adsorption dryer. (SAD; Swing Adsorption Dryer), which consists of a TSA (Ttemperature s wing adsorption) or PSA (pressure swing adsorption). Such a system consists of two columns filled with molecular sieves based on silicate or aluminum oxide, which alternately absorb moisture or are regenerated. Switching from adsorption to regeneration takes place through an induced temperature or pressure change.
Crystec will be pleased to engineer a cost effective system to satisfy your most demanding and exacting requirements.
