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Figure 35: Layout of alkaline electrolysis for AEL
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An electrolyzer consists of several stacks of electrolysis cells that are connected in series in each stack. The oxidation reaction occurs at the anode and the reduction reaction at the cathode in the cell, but the reactions differ according to the types of electrolysis. The efficiency of electrolysis decreases, or conversely, the overpotential increases with rising current density, decreasing temperature and increasing pressure.
The main types of water electrolyzers are alkaline (AEL), polymer electrolyte membrane (PEMEL) and solid oxide (SOEL) electrolyzer. Recently a new type of alkaline polymer electrolyte membrane electrolyzer, called the anionic exchancge membrane electrolyzer (AEMEL) is being developed. It combines the advantages of AEL and PEMEL.
a. Alkaline Electrolysis (AEL)
This is the most common and mature technology used in water electrolysis, which has been applied for large-scale hydrogen production in the MW-scale already at the beginning of the 20th century. Alkaline electrolysis is highly efficient, cheap, durable and long-lasting. The purity of hydrogen and oxygen is very high, 99.7% - 99.9% for hydrogen and 99%-99.8% for oxygen, which can be further increased to above 99.999% by catalytic gas purification (deoxidiser).
When a DC voltage is applied to the electrolysis cell, water is split to form hydrogen at the cathode releasing hydroxide anions that pass through the diaphragm and recombine at the anode to form oxygen. The partial reaction at the electrodes is given by:
V Cathode
20H_ → ½O2 + H2O + 2e_ Anode
