Download The performance of a fuel cell can be divided into three main

The performance of a fuel cell can be divided into three main regions, each controlled by
different physical and chemical phenomena. The sharp voltage drop of the first region is
associated with the activation resistance in the cell. This resistance is attributed to the
type of catalyst and the catalyst surface area that is both in contact with the electrolyte
and the electrical network in the electrode and accessible to the reacting gases. Lowering
this resistance will raise the whole polarization curve.
The gradual drop in voltage of the second region is attributed to the ohmic resistance in
the cell and the depletion of the reactive gas at the catalyst surface. The ohmic resistance
comes from ionic resistance of the membrane, electronic and contact resistance of the
electrodes, bipolar plates and current collectors. Lowering this resistance will result in a
lower slope in the E-I curve and consequently higher power densities at higher energy
efficiencies.
The sharp voltage drop in the third region is attributed to the mass transport limitation,
which occurs when the transport of the reactant to the reaction surface fails to keep up
with the reaction. This phenomenon is especially severe at the cathode of the fuel cell
where oxygen is the reactant because of the presence of liquid water within the porous
structure of the electrode and on the catalystlmembrane surface. This liquid water, which
is the product of the cathodic reaction and proton transport from the anode, acts as an
additional barrier to the transport of oxygen to the reaction sites. Minimizing this
resistance will allow the ohmic region to be extended and result in much higher power
density operation.
Voltage loss in the first region can be reduced by using catalysts with lower activation
resistance and increasing the catalyst surface available for reaction per unit volume of
electrode. Currently, platinum is the best catalyst available. Note that the voltage loss by
gas crossover can be minimized by using a thicker membrane and keeping the membrane
well hydrated. However, any reduction in voltage loss by gas crossover by using a thicker
membrane must be considered against the additional ohmic voltage loss of a thicker
membrane.
Voltage loss in the second region can be reduced by employing thinner membranes and
membranes with lower ionic and water transport resistance and humidifying the anode
gas stream (and the cathode gas if air is used). The last voltage loss region, associated
with mass transport limitation, can be reduced by using flow fields that can remove liquid
water from the cathode more effective, like the interdigitated flow fields.
As described above, the performance of a fuel cell can be analyzed by plotting its cell
potential versus the current density and analyzing its three voltage loss regions.
Performance in these three regions can be used to determine the performance of a fuel
cell at various operating conditions and to compare one fuel cell design to another.
Voltage losses in each region are indicative of how well or poorly a fuel cell performs.
Lower voltages losses lead to higher power densities and therefore better performance.
Figure 4.2 below illustrates how the cell potential and power density change as the
individual resistance is reduced.
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