Page 280 - Biofuels Refining and Performance

Fuel Cells 259

a plate, is pressed against the outer surface of each backing layer. The
plate serves the dual role of a flow field and current collector. The side
of the plate next to the backing layer contains channels machined into
the plate. The plates are made of a lightweight, strong, gas-impermeable,
electron-conducting material; graphite or metals are commonly used,
although composite material plates are now being developed. Electrons
produced by the oxidation of hydrogen move through the anode, through
the backing layer, and through the plate before they can exit the cell,
travel through an external circuit, and reenter the cell at the cathode
plate. In a single fuel cell, these two plates are the last of the compo-
nents making up the cell.
In a fuel cell stack, current collectors are the bipolar plates; they
make up over 90% of the volume and 80% of the mass of a fuel cell stack
[11, 15, 16].

Water and air management. Although water is a product of the fuel cell
reaction and is carried out of the cell during its operation, it is nec-
essary that both the fuel and air entering the fuel cell be humidified.
This additional water keeps the polymer electrolyte membrane
hydrated. The humidity of the gases has to be carefully controlled, as too
little water dries up the membrane and prevents it from conducting

the H ions and the cell current drops. If the air flow past the cathode
is too slow, the air cannot carry all the water produced at the cathode
out of the fuel cell, and the cathode “floods.” Cell performance deteri-
orates because not enough oxygen is able to penetrate the excess
liquid water to reach the cathode catalyst sites. Cooling is required
to maintain the temperature of a fuel cell stack at about 80 C, and the
product water produced at the cathode at this temperature is both
liquid and vapor.

Performance of the PEM fuel cell [3, 11, 16]. Energy conversion in a fuel
cell is given by the relation:

Chemical energy of the fuel electric energy heat energy
Power is the rate at which energy (E) is made available (P dE/dt, or
E P t). The power delivered by a cell is the product of the current
(I ) drawn and the terminal voltage (V ) at that current (P IV watts).
In order to compute power delivered by a fuel cell, we have to know the
cell voltage and load current. The ideal (maximum) cell voltage (E ) for
the hydrogen/air fuel cell reaction (H 1/2O → H O) at a specific tem-
2
2
2
perature and pressure is calculated from the maximum electrical energy
G
W G nFE or E
el
nF