Environmental Engineering Reference
In-Depth Information
electrodes must offer an adequate support for highly dispersed catalyst particles.
These are the reaction sites which have to be reached not only by gaseous reactants,
but also by electrons and protons. Therefore, the catalyst layer must be in intimate
contact with a porous and electrically conductive structure for gas and electrons
transfer (GDL, see Sect. 3.2.3 ) and with the electrolytic membrane.
The most diffused catalyst for both semi-reactions is platinum, which is sup-
ported on carbon powders (typically 40 nm) in order to optimize the dispersion of
metal particles and the active surface area, then to increase the reaction rate. The
carbonaceous material of the support assures the conduction of the electrons
produced at the anode and received by the cathode. The ratio Pt/C has to be
optimized, in fact while a thin layer of carbon support (high Pt/C) could give
advantages on the rate of proton transfer and gas permeation into the catalyst layer,
a lower Pt/C ratio (smaller Pt particles) permits a larger surface area to be
obtained. Moreover, the impregnation of catalyst particles with a calibrated
amount of proton conductor (the same ionomer used for membrane) allows all
catalyst particles to be reached by protons and extends the three-phase boundary
contact between gas, electrolyte, and catalyst, with great benefit in terms of
reduction of platinum concentration. Optimal values of Pt/C ratios for current
realizations have been found to be comprised between 10 and 40 wt%, with a
platinum loading of 0.4 mg/cm 2 [ 28 ], whereas the optimized content of ionomer in
the catalyst layer results to be in the range 20-50 wt%, in dependence of fabri-
cation method and platinum loading [ 29 ]. At this regard novel carbon supports are
currently under development to improve PEM fuel cell performance, in particular
materials able to assure more efficient proton conduction from the catalytic sites to
the membrane are considered. Recently, the grafting of polymers onto carbon
black surfaces has been proposed, and promising results have been obtained with
polystyrene sulfonic-grafted carbon black as support of platinum-ruthenium-based
catalysts, in terms of reduced ohmic drop and increased proton transfer with
respect to commercial carbonaceous supports [ 30 ]. As carbon supports can
undergo electrochemical oxidation within the PEM cell in some severe operative
conditions, such as fast dynamic phases and repeated start up/shut down actions,
the platinum surface area can be reduced with worsening of cell performance
[ 31 , 32 ]. About this issue some studies have proposed the use of carbon nanocages
as catalyst support, evidencing the possibility to significantly reduce the electro-
chemical carbon corrosion; thanks to both the strong hydrophobic nature and
graphitic structure of these materials [ 33 ].
3.2.3 The MEA: GDL
Two GDL, whose main task is to allow uniform access of gaseous reactants to the
catalyst layer, are located on both anode and cathode side, and can be considered
as an integrant part of the MEA. They are interposed between the catalyst layer
and bipolar plates, and are constituted by a porous carbonaceous material, such as
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