Fuel Cells
327
PEFCs are the most widely used fuel cells in transport applications. It is
estimated that more than 90% of the FCVs on the road are equipped with
PEFCs (Cropper, Geiger, and Jolli 2004). PEFCs exhibit high-power density at
operating temperatures of about 70°C. There are still some drawbacks with
the current state-of-the-art PEFCs: one issue being the low CO tolerance.
Most PEFC stacks require CO levels well below 10 ppm to mitigate losses
in performance. A second factor is the operating temperature of the PEFCs
that may require an unacceptably large radiator area for cooling the fuel cell
stack (Mallant 2003). These two factors have led to research efforts in proton
conducting membranes at elevated temperatures (Li et al. 2003; Savogado
2004). It is projected that these developments will take a long time before
they will find a use in systems for practical applications (Bruijn 2005). This
type of high-temperature PEFCs can at best be regarded as second-generation systems. The conventional low-temperature PEFC stacks will be the first
products in practical applications requiring relatively pure hydrogen. PEFC
stacks in transport applications will be used in a hybrid configuration with
batteries/super capacitors.
For auxiliary power units (APUs) in FCVs, SOFCs are also being considered
as a possible candidate. SOFCs are characterized by their high-operatingtemperature; the state-of-the-art ytria stabilized zirconia based SOFC has an
operating temperature of about 1000°C. There are two basic configurations of
SOFCs, tubular and planar. Tubular SOFCs are regarded as suitable for large
scale stationary applications while planar SOFCs are preferred for automotive applications due to their higher power density (Singhal and Kendall
2003). SOFCs offer two major advantages over PEFCs, namely no need for a
platinum catalyst and greater fuel flexibility without any external reforming.
However, their high-operating temperature and consequent lower durability and long start-up times, pose significant hurdles for use in automotive
applications (Ormerod 2003). Advances in materials have brought in intermediate temperature (IT) SOFCs operating between 500 and 750°C (Brett
et al. 2008). The lower operating temperature for IT-SOFCs, and consequent
faster start-up time, greater durability, and lower cost materials, make them
a possible candidate for automotive applications. The IT-SOFCs offer highfuel flexibility, efficiency, and more tolerance to impurities than PEFCs and,
unlike PEFCs, do not require external fuel reforming. However, they exhibit
reduced activity toward oxygen reduction reactions at the cathode in relation to high-temperature SOFCs (Shao and Haile 2004). But, IT-SOFCs might
be promising only as APUs in heavy-duty FCVs (Steele and Heinzel 2001).
The use of SOFCs in automotives would best be limited to APUs and is not
seen viable for automotive purposes in general due to the specific requirements associated with traction, and in particular start-up behavior and
dynamic load changes (Oosterkamp et al. 2006). In light of the foregoing,
only PEFCs are considered suitable for vehicular applications by automotive
manufacturers.
327
PEFCs are the most widely used fuel cells in transport applications. It is
estimated that more than 90% of the FCVs on the road are equipped with
PEFCs (Cropper, Geiger, and Jolli 2004). PEFCs exhibit high-power density at
operating temperatures of about 70°C. There are still some drawbacks with
the current state-of-the-art PEFCs: one issue being the low CO tolerance.
Most PEFC stacks require CO levels well below 10 ppm to mitigate losses
in performance. A second factor is the operating temperature of the PEFCs
that may require an unacceptably large radiator area for cooling the fuel cell
stack (Mallant 2003). These two factors have led to research efforts in proton
conducting membranes at elevated temperatures (Li et al. 2003; Savogado
2004). It is projected that these developments will take a long time before
they will find a use in systems for practical applications (Bruijn 2005). This
type of high-temperature PEFCs can at best be regarded as second-generation systems. The conventional low-temperature PEFC stacks will be the first
products in practical applications requiring relatively pure hydrogen. PEFC
stacks in transport applications will be used in a hybrid configuration with
batteries/super capacitors.
For auxiliary power units (APUs) in FCVs, SOFCs are also being considered
as a possible candidate. SOFCs are characterized by their high-operatingtemperature; the state-of-the-art ytria stabilized zirconia based SOFC has an
operating temperature of about 1000°C. There are two basic configurations of
SOFCs, tubular and planar. Tubular SOFCs are regarded as suitable for large
scale stationary applications while planar SOFCs are preferred for automotive applications due to their higher power density (Singhal and Kendall
2003). SOFCs offer two major advantages over PEFCs, namely no need for a
platinum catalyst and greater fuel flexibility without any external reforming.
However, their high-operating temperature and consequent lower durability and long start-up times, pose significant hurdles for use in automotive
applications (Ormerod 2003). Advances in materials have brought in intermediate temperature (IT) SOFCs operating between 500 and 750°C (Brett
et al. 2008). The lower operating temperature for IT-SOFCs, and consequent
faster start-up time, greater durability, and lower cost materials, make them
a possible candidate for automotive applications. The IT-SOFCs offer highfuel flexibility, efficiency, and more tolerance to impurities than PEFCs and,
unlike PEFCs, do not require external fuel reforming. However, they exhibit
reduced activity toward oxygen reduction reactions at the cathode in relation to high-temperature SOFCs (Shao and Haile 2004). But, IT-SOFCs might
be promising only as APUs in heavy-duty FCVs (Steele and Heinzel 2001).
The use of SOFCs in automotives would best be limited to APUs and is not
seen viable for automotive purposes in general due to the specific requirements associated with traction, and in particular start-up behavior and
dynamic load changes (Oosterkamp et al. 2006). In light of the foregoing,
only PEFCs are considered suitable for vehicular applications by automotive
manufacturers.