Fuel Cells for Automotive Applications - Hydrogen Fuel Cells for Road Vehicles - page 82

would simplify the fuel cell cooling device, making possible the use of radiators
currently adopted in internal combustion engine vehicles, with a gain in weight
energy density and then in overall efficiency [ 15 ]. On the other hand, the heat
recovered by a high-temperature PEM fuel cell would be higher, making more
attractive the applications in the co-generation field. Lastly, also the water man-
agement would be greatly simplified, because in a PEM fuel cell operating above
100C water on the membrane would exist only in vapor state, while the absence of
liquid water would increase the exposed area of electrocatalysts, thus improving the
diffusion of reactants into the reaction layer [ 16 ].
High-temperature operation in PEM fuel cells using Nafion-like membranes is
presently impeded by polymer degradation above 120C and increase of mem-
brane resistance as a consequence of loss of hydration (alteration of proton
transport mechanism due to reduction of water nano-domains). In particular,
conductivity losses correlated to low humidity conditions on Nafion-like mem-
branes can reach some orders of magnitude [ 17 ], strongly increasing the Ohmic
losses (see Sect. 3.3.1 ) with lowering of voltage, power, and efficiency. This has
determined the direction of studies aimed at finding new materials able to over-
come the above limitations [ 18 ]. These materials can be subdivided into four
classes: (1) modified perfluorosulfonic acids, (2) non-fluorinated hydrocarbon
polymers, (3) inorganic-organic composites, and (4) acid-base polymers (poly-
benzimidazoles, PBI). The first class of materials is based on the incorporation of
hydrophilic inorganic additives into the perfluorinated membrane to increase the
polymer swelling and the binding energy of water. Several hygroscopic additives
have been proposed for incorporation in Nafion membranes (Zr(HPO 4 ) 2 , SiO 2 ,
TiO 2 ) with different methods of preparation, obtaining composite materials
characterized by variable water retention and electrochemical performance [ 19 ,
20 ]. The second approach is based on the utilization of aromatic polymers as
membrane backbone, which are cheaper than perfluorinated ionomers, and can
contain polar groups characterized by high water absorption in a wide temperature
range [ 21 ]. Thermal and chemical stability of these materials are the main limi-
tations hindering their practical application. The third solution is based on the
rationale of using an inert organic polymer as binding medium for a large quantity
of an inorganic proton conductor of high performance [ 22 ]. As high proton con-
ductivity materials are often crystalline, they are suspended in inert organic
polymers, such as polyvinylidene fluoride (PVDF), nevertheless the difficulty of
obtaining satisfactory film-forming properties is the main limitation of this option.
Acid-base polymers represent the state of the art in the field of high-temperature
polymeric electrolytes, and are essentially constituted by a basic polymer doped
with a non-volatile inorganic acid or blended with a polymeric acid [ 23 ].
PBI is today considered the most interesting basic polymer for preparation of
acid-base membranes, especially if doped with phosphoric acid [ 24 ]. In Fig. 3.3
the scheme of the commercial PBI known as Celazole is shown.
The aromatic nuclei of PBI are responsible of its good characteristics of
chemical stability, whereas the basic functional groups act as proton acceptors like
a normal acid-base reaction. Amphoteric acids, such as phosphoric or phosphonic