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PEM Electrolyzer
Hydrogen and oxygen can be produced by the electrolysis of water. Electrolysis is an electrochemical process through which a substance (the electrolyte) is decomposed when an external DC voltage is applied to two electrodes (cathode and anode) that are in contact with the electrolyte. For electrolysis to happen the DC voltage must be equal to exceed a certain material-dependent threshold voltage known as the decomposition voltage. Different types of electrolyzers are usually distinguished by their type of electrolyte and/or electrodes.

PEM electrolyzers have a particularly simple and compact design. The central component is proton-conducting polymer membrane which is coated with a layer of catalyst material on either side. These two layers are the electrodes of the cell.

When a DC voltage greater than the decomposition voltage of water is applied to its electrodes, the PEM electrolyzer splits pure water into hydrogen and oxygen. The theoretical decomposition voltage of water is 1.23V, however, because of transition resistances, somewhat higher voltages are necessary in practice.

Higher power electrolyzers are built as stacks in which individual electrolyzers are connected in series and voltages are added.

PEM electrolyzers have efficiencies of up to ~85%.

How PEM electrolyzers work
Suppose a DC voltage is applied to the PEM electrolyzer electrodes (solar panel on the transparency). At the anode (electrode on the right) water is oxidized, leaving oxygen, protons H+- ions) and free electrons. While the oxygen gas can be collected directly at the anode, the protons (yellow +) migrate through the proton-conducting membrane to the cathode where they are reduced to hydrogen (the electrons for this are provided by the external circuit).

Cathode reaction : 4H+ + 4e- → 2H2
Anode reaction : 2H2O → 4H+ + 4e- +O2

Source: h-tec GmbH © h-tec GmbH
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