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Power Conversion and Control for Fuel Cell Systems in Transportation 293

The efficiency of the PEMFC is about 40%–50%, and its operating temperature is about
80 °C–100 °C. Owing to low-temperature operation, faster response to load transients, and short
start-up time (less than a minute), the PEMFC is suitable for propulsion applications. Several
industries are developing this type of fuel cells for propulsion and other applications. However, the
PEMFC requires pure hydrogen, and drastic performance degradation occurs even from a small
amount of carbon monoxide, resulting in poisoning of the electrodes. A sophisticated water manage-
ment system is also required because of the continuous water generation at the cathode and to keep
the membrane at a certain humidity level. Although significant research has been done to reduce the
level of platinum coating on the electrodes, this requirement makes the proton exchange membrane
(PEM)-based systems relatively expensive.
PEMFCs operate at low temperatures (less than 100 °C), making them temperature compatible
with many of today’s automotive systems and also allowing for a faster start-up. However, due to
a relatively small temperature gradient to the ambient atmosphere, the waste heat produced is low
grade and requires large heat exchangers. Also, for the PEM electrolyte to operate properly it must
be hydrated, resulting in the need for sophisticated water management and the humidification of
the incoming fuel and oxygen flows. This hydration also causes issues because automobiles must
operate below the freezing point of water, with typical specifications for today’s automotive compo-
nents being in the range of –30 °C to –40 °C. Finally, to achieve the high power density, the incom-
ing reactant gases are pressurized to 2–3 atmospheres to increase power output, resulting in the need
for high-pressure and high-flow air compressors. PEMFCs have been demonstrated in systems with
the size ranging from 1 W to 250 kW.

12.1.2 SOFC
A solid oxide fuel cell (SOFC) usually uses a hard ceramic material of solid zirconium oxide and a
small amount of yttria, instead of a liquid electrolyte, allowing operating temperatures to reach about
1000 °C. The solid electrolyte is coated on both sides with specialized porous electrode materials.
The hydrogen is supplied at the anode, and oxygen, usually from air, at the cathode. At these high
operating temperatures, oxygen ions (with a negative charge) migrate through the electrolyte to the
anode. Electrons generated at the anode travel through an external load to the cathode, completing
the circuit and supplying electric power along the way.
SOFC requires a simple reforming process and may not need an external reformer. SOFC operates
at extremely high temperatures in the range of 700 °C–1000 °C and hence has less compelling require-
ments for reformate quality and uses carbon monoxide as fuel. High-grade heat exhaust of SOFC
allows for smaller heat exchangers and the cogeneration option to produce extra power, thus increasing
the overall system efficiency. Because the electrolyte is in a solid state and does not require hydration,
water management is not a concern. The by-product is steam rather than liquid water, which must be
drained out in a PEM system. In addition, fuel versatility makes SOFCs suitable for large to very large
stationary power generation applications. SOFC has also been demonstrated as an auxiliary power unit
(APU) in automotive systems [4, 5]. The SOFC is very suitable for combined heat and power (CHP)
and for hybrid power generation (or cogeneration), where the exhaust of the SOFC can be used to run
a turbine to generate additional electric power. The start-up time of the SOFC system is long because
of its high-temperature operation and therefore is not suitable for propulsion applications.

12.1.3 MCFC

Molten carbonate fuel cell (MCFC) uses high-temperature compounds of salt (like a mixture of
sodium and lithium or magnesium or lithium and potassium) carbonates (chemically CO ) as the
3
electrolyte and is the only fuel cell that requires CO supply. Their nickel electrode catalysts are
2
inexpensive compared to the platinum used in PEMFC. Carbonate ions from the electrolyte are used