Environmental Engineering Reference
In-Depth Information
paper or cloth, able to conduct the electrons exiting from the anode and entering
the cathode. The GDL does not participate in the electrochemical reaction, but
carries on many important functions, in particular: electron transfer between cat-
alyst layer and bipolar plates, diffusion of gaseous reactants from bipolar plates to
catalyst layers, water transfer between catalyst layers and bipolar plates, heat
transfer from reaction sites to bipolar plates, mechanical separation between
membrane and bipolar plates. Typical values of thickness in commercial GDL are
comprised between 100 and 400 lm, with densities ranging from 0.2 to 0.7 g/cm
3
and porosity between 70 and 80% [
34
].
While both through-plane and in-plane electronic conductivities play a key role
in controlling the capability of the GDL to transfer electrons from reaction sites to
bipolar plates, the porous nature of the GDL material guarantees an effective
diffusion of all reactant molecules from the high concentration region outside of
the GDL to the inner side at lower concentration, next to the catalyst layer, where
the molecules are consumed by the electrochemical reaction. An effect of spread
out of the gases thorough the porous structure ensures the uniform contact of gas
molecules with the entire surface of the membrane. As the mean free path of gas
molecules is many orders of magnitude lower than pore diameters of a GDL,
the diffusion of reactants is not regulated by a Knudsen mechanism, but mainly by
the convective flow resistance [
34
]. The GDL porosity is also fundamental for
water management inside the cell, as it offers a pathway for the transfer outside the
MEA of liquid water produced by the cathodic reaction, preventing flooding
phenomena at 100% RH (relative humidity), but also permits water coming from
external humidification systems to reach the membrane and avoid electrolyte
drying out at higher operative temperature and low RH (see
Sect. 4.5
).
The importance of this aspect has determined the tendency to treat the surface of
the GDL with different substances. While the hydrophilicity of GDL could help the
membrane correct humidification at low RH, they are in general treated with
hydrophobic polymeric microporous sheets (mostly PTFE, or fluorinated ethylene
propylene), privileging flooding issues [
35
,
36
]. The effect of hydrophobic polymer
content on liquid water and oxygen gas transport has been investigated, with results
showing how the trade-off between hydrophobicity and permeability can be solved
by an optimal PTFE loading in the GDL (20%) that reduces the mass transport
limitations and increases the oxygen diffusion kinetics [
37
]. On the other hand other
studies have been recently presented about the possibility to use additional coatings,
different from hydrophobic polymers, with the aim of finding an optimal balance
between hydrophilic and hydrophobic properties and of extracting the maximum
power from the stack. In particular, thin films of nano-scaled inorganic oxides
(Al
2
O
3
, TiO
2
) have been deposited on hydrophobic commercial GDL with for-
mation of three-dimensional surface structures able to improve both transport of
gases and water retention in low-humidity conditions [
38
]. Chemical vapor depo-
sition techniques have been used to deposit microporous carbon nanotubes struc-
tures on commercial macroporous carbon papers, without addition of hydrophobic
coatings. This type of GDL has shown significant advantages in terms of
mechanical robustness; thanks to the reinforcing presence of multi-walled carbon
