14 Introduction
cheap materials that can be used as catalysts. A more elaborate description of
these catalysts will be given in section 2.5. Both for the anode and cathode Ni
based materials can be used as electrodes in alkaline systems. At this point it
should be noted that while the potential necessary for driving the reaction at the
oxygen electrode is approximately the same in alkaline and acid, the hydrogen
electrode requires a larger driving force or overpotential in alkaline compared to
acid electrolyte 36.
In contrast to the alkaline based cells, the Polymer Electrolyte Membrane cells,
known as PEM cells, utilize a solid membrane, based on a proton conducting
sulfonated teon polymer. This type of membrane was originally developed by
Dupont in the 1960's and is known as Naonr. Today, many other brands
exist, oering similar membranes with great performance and stability. Using
this type of membrane allows for very thin (Membrane Electrode Assembly down
to 0.5 mm 37) but mechanically stable electrochemical cells leading to a highly
compact electrolyzer design 38. Furthermore, the superior ionic conductivity
enables high current densities and low operation temperature facilitates fast
start up times 39. In gure 1.14 a comparison between the performances of
the traditional alkaline electrolyzer and a PEM electrolyzer can be seen. They
are compared on the basis of eciency as function of the current density.
05001000150020002500
100
80
60
40
20
0
Traditionalalkaline
electrolyserPEM
electrolyser
Traditional
alkaline
operating
range
VoltageEfficiency(HHV)/%
j/mAcm-2
PEM
operating
range
Figure 1.14: Figure showing the operational ranges of two types of electrolyzers.
The voltage eciency is plotted as function of the current density. For traditional
alkaline electrolyzers an eciency between 70 and 80 % means operating between 250
and 500 mA/cm2. Conversely, the PEM electrolyzer can operate in the same eciency
range with current densities between 1500 and 2000 mA/cm2. Data for this gure has
been adapted from 39.