3.2.1.2. Proton exchange membrane electrolyzer. PEM electroyzers
build upon the recent advances in PEM fuel cell technology [127].
PEM-based electrolyzers typically use Pt black, iridium, ruthenium,
and rhodium for electrode catalysts and a Nafionmembrane
which not only separates the electrodes, but acts as a gas separator
[159,166]. In PEM electrolyzers water is introduced at the anode
where it is split into protons and oxygen [127]. The protons travel
through the membrane to the cathode, where they are recombined
into hydrogen [127]. The O2 gas remains behind with the
unreacted water. There is no need for a separations unit.
Depending on the purity requirements a drier may be used to
remove residual water after a gas/liquid separations unit. PEM
electrolyzers have low ionic resistances and therefore high
currents of >1600 mA cm2 can be achieved while maintaining
high efficiencies of 55–70% [20,159]. The reactions at the anode
and cathode are: