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 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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