IEEE Electrification Magazine - March 2018 - 51

An MEA is a
proton-conducting
membrane coated
on both sides with
two electrodes:
an anode and
a cathode.

fuel cell are obtained from a sophisticated mixture of a catalyst, electron
conductor, and proton conductor.
Careful choices of the ingredients,
composition, and coating process
have continuously contributed to
the increase in electric power generation per unit area of meas. in
addition, the development of thin,
mechanically strong membranes
has further increased the power
density by enhancing the proton
conductivity and reducing the
thickness of the fuel-cell stack.
The combined result of these technological advances
is appreciable. Figure 3 shows a polarization curve,
which compares the characteristics of fuel-cell stacks in
2013 and 2018 and indicates a 60% increase in power
density during these five years. Unfortunately, the number of FCevs currently available in the automotive market remains insufficient for a statistical analysis, but
some typical numbers related to the driving performance of FCevs are: a 0-60 mi/h time of 12.5 s, a top
speed of 100 mi/h, and a driving range of 260 mi.
although these figures are not comparable to those of
supercars, one can readily agree that the FCevs currently available have the necessary performance levels for
commuting and traveling. moreover, the next-generation
FCevs, which incorporate all technological progress during the development period, will show better performances with the same size fuel-cell stacks as the
predecessor, longer driving range with reduced weight
and increased efficiency, or both (Figure 4).

Durability
durability, which is not easy to achieve, particularly because
of the tradeoff between performance and endurance, is
another important factor for the success of FCevs. Similar to any other industrial good, a fuel-cell system and

2018
2013
Stack Voltage

an mea is where the electrochemical
reaction occurs, and it takes the form
of a thin sheet. The bipolar plates
electrically connect the neighboring
fuel cells and form flow channels on
both sides of an mea, through which
hydrogen and oxygen are distributed
and the water produced is removed.
another key component, the gas diffusion layer (gdl), is an electron-conducting porous material inserted
between an mea and a bipolar plate
to enhance the roles of bipolar plates.
To increase the power density of a
fuel-cell stack, meas that generate more watts per unit
area and thinner bipolar plates and gdls without reducing their electron-conducting and fluid-transferring capabilities must be developed.
graphite was used as a material for bipolar plates in
most prototype fuel cells because of its machinability, conductivity, and resistance to corrosion. However, the brittleness of graphite sets a lower limit for the thickness of the
graphite bipolar plates and makes them unsuitable for
high-power-density applications. metal bipolar plates that
are made from stamped, thin metal sheets to form a flowfield pattern have been widely used for this application.
The problem of corrosion in harsh environments (heat
and acidic chemicals) within fuel cells has been handled
by a careful selection of the material and by proper coating treatments. recently, the use of porous plates instead
of flow-field patterned plates has further increased the
power density. in fact, flow-field patterned plates can also
be considered as porous layers, in a wide sense, with the
characteristic pore size within the millimeter range,
whereas porous plates have pores within the submillimeter range. This change in pore size of bipolar plates should
be in harmony with that of gdls, which have pores in the
micrometer range, and complete the cascade of porosity
to nanometer-sized pores in meas. The combined optimizations of bipolar plates and gdls have enabled thinner
but functionally better fuel cells.
meas have a different and more material-oriented
development strategy compared to that of bipolar plates
and gdls. as the name suggests, an mea is a protonconducting membrane coated on both sides with two
electrodes: an anode and a cathode. when hydrogen
and oxygen are supplied to the anode and cathode,
respectively, the hydrogen atoms are split into protons
and electrons. The membrane then selectively conducts
protons to the cathode, whereas the electrons go through
an external electric circuit to reach the cathode and
generate electricity. Finally, water is produced at the
cathode by combining protons, electrons, and oxygen.
Since this electrochemical reaction is spontaneous but
slow, catalysts, which are commonly platinum, are
mixed into the electrodes. Therefore, the electrodes of a

Stack Current Density
Figure 3. A representation of improved performance of the fuel-cell
stacks. (Image courtesy of Hyundai Motor Company.)

IEEE Electrific ation Magazine / ma r c h 201 8

Table of Contents for the Digital Edition of IEEE Electrification Magazine - March 2018