Hydrogen Utilization: Fuel Cells - Hydrogen Generation, Storage, and Utilization - page 157

that the PEMFC has captured attention and is the leading fuel cell candidate
as power sources for transportation, small-scale power generation, and por-
table applications [18].
The proton conducting membrane is the vital component of a PEMFC,
since that is what makes it possible to attain high power densities [7]. A
major breakthrough in the field of PEMFCs came with the use of Nafion
membranes by DuPont. These membranes possess a higher acidity and also
a higher conductivity and are far more stable than the polystyrene sulfonate
membranes. Furthermore, composite membranes have been widely explored
to improve the membrane structure and conductivity. Novel membranes have
also been prepared by new techniques, such as radiation grafting or plasma
polymerization, which have proved to be mechanically and electrochemi-
cally stable for PEMFC applications. Moreover, membranes in PEMFC are
usually filled with water to keep the conductivity high, since proton transport
through a wet membrane is similar to that of an aqueous solution. Water
management in the membrane is one of the major issues in PEMFC technol-
ogy. One way to improve the water management is to humidify the gases
coming into the fuel cell. Another form of water management can be found
in the direct hydration of the membrane by mounting porous fiber wicks.
Electrodes for PEMFCs are generally porous gas diffusion electrodes to
ensure the supply of reactant gases to the active zones where the noble metal
catalyst is in contact with the ionic and electronic conductor [20]. Pt and
Pt-based alloy are the best electrocatalysts for both hydrogen oxidation and
oxygen reduction [21]. One of the major problems with the Pt electrocataly-
sis for hydrogen electrode is its low tolerance to CO in H 2 from reformed
fuels, so the development of strategies to improve the stabilities of Pt based
catalysts and search for other novel electrocatalysts have attracted wide
researchers [22-24]. The basic designs for platinum catalysts are summa-
rized in Figure 9.3, categorized by overall geometry of the catalyst and its
support, and then further subdivided according to structural morphology and
composition [16, 25, 26]. Comparison of the results shows that kinetic activ-
ity can change by nearly an order of magnitude when the catalyst is a discrete
nanoparticle or a polycrystalline thin film, and that catalyst surface area per
unit volume can affect the maximum achievable current density [27]. Also,
the volume occupied by the nonactive support influences current density,
while aspect ratios determine the packing of the catalyst supports and hence
porosity and free-radical scavenging [28].
9.2.3 Phosphoric Acid Fuel Cell (PAFC)
PAFC is a type of fuel cell that uses liquid phosphoric acid as an electrolyte.
It was developed in the mid-1960s and field-tested in the 1970s. Since then,