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The gas diffusion layer must be a highly conductive material for both fuel cell and
electrolysis applications. It must have a porous structure to bring the reactants to the PEM
for fuel cell and to expel the products for the electrolysis. In the conventional fuel cells, the
gas diffusion layers are usually porous carbon matrix, such as carbon cloth or carbon paper.
However, this structure is not suitable for water electrolysis due to the oxidation of carbon
with active oxygen species, such as oxygen atom or hydroxyl free radicals, at high positive
potentials of anode (Song et al. 2006, Petersson et al. 2006).
Gas diffusion layer of an anode electrode should not be a hydrophobic material.
Thus, PTFE loading generally decreases the efficiency of the cell similar to the PTFE
loading effect on the catalyst layer (Ioroi et al. 2003). Woven metal cloths, expanded metal
sheets, perforated metal sheets or metal foams which are made up of corrosive resistive
metals, such as titanium, zirconium, hafnium, niobium and tantalum, are used as the
electrolyzer gas diffusion plates (Petersson et al. 2006).
Another approach for making electrolysis gas diffusion layer is to promote the
traditional carbon matrix used in the fuel cells with a suitable metal(s). This approach aims
to form an oxygen molecule rapidly before the atoms starts to diffuse the gas diffusion
layer (Song et al. 2006) proposed it as a new cathode for electrolysis cell which had a water
reservoir placed inside the cell contacting with the membrane, and with the Toray carbon
paper used as the gas diffusion layer. After the electrolysis operation, no corrosion of the
oxygen electrode occurred because the water did not come in direct contact with the
electrode and the active oxygen species were combined before reaching the gas diffusion
layer. However, their cell structure was complicated and the gap between anode and
cathode was wide which caused less voltage efficient electrolysis operation (Song et al
2006).
2.2.1.4. Bipolar Plates
All the fuel cells and electrolyzers (with the exception of laboratory bench scale
ones) are constructed with many cells connected in series. Similar to the serially connected
battery systems, the serially connected fuel cell systems could generate electricity at high
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