Environmental Engineering Reference
In-Depth Information
trade name), is manufactured as a membrane roughly 175
m
m thick with the
appearance of clear cellophane wrapping paper. When humidified, Nafion conducts
positive ions and blocks negative ions. The negative ions must follow a shunt path
to complete their circuit around the polymer. A catalyst is necessary to speed up
the oxidation process in the PEM fuel cell. Today, the most popular catalyst is
platinum - a very expensive metal.
The bipolar plates in Figure 10.47 consist of gas diffusers and current collec-
tors. The current collectors or backing layers are typically made of a porous carbon
cloth to which flow diffusers are pressed to guide the gases across the cell. Current
collectors on both anode and cathode sides of the basic fuel cell are strapped to
succeeding cells in a stack so that the terminal voltage is equal to the number of
cells times the potential of each cell.
The current capacity of a fuel cell stack is proportional to the bipolar plate
area. Stack voltage, as already noted, is determined from the number of inter-
connected cells. Fuel cell power density has increased steadily over the past decade
from 150 W/L to nearly 1 kW/L. Current density in a pure H
2
gas fed PEM stack
today is at 1,300 A/cm
2
at peak power (
V
cell
¼
0.6 V) and the corresponding
specific power
g
P
¼
V
cell
J
cell
¼
780 W/cm
2
.
500.4
20
400.4
300.4
10
200.4
100.4
0.4
0
0
500
1,000
1,500
0
20
40
60
80
100
V
(100% H
2
)
V
(40% H
2
)
P
aux
/
P
stack
Current density (A/cm
2
)
P
stack
(%)
(a)
(b)
Figure 10.48 Stack voltage and auxiliary power demand (90 kW, 400 cell stack):
(a) stack voltage versus current density, (b) auxiliary power versus
stack output power
In Figure 10.48(a) the variation of stack voltage with loading is shown for a
400 cell stack as used in the Toyota FCHV and in (b) the approximate consumption
of gross stack output to power the supporting subsystems.
Supporting subsystems for a fuel cell stack, other than a reformer if used, is the
air compressor to force air through the labyrinth of cells and a thermal management
system to cool the stack plus the water management system to drain and store the
effluent. These ancillary subsystems can be put in perspective by noting the com-
ponents used in the Toyota FCHV [35]. Toyota's FCHV uses a 90 kW PEM stack
composed of 400 cells that develop between 0.6 and 0.7 V/cell at rated output
(240-492 V
dc
). The PEM stack membranes are ultra-thin, platinum rich polymers
with a graphite separator. The balance of the plant (BOP) consists of humidifier, air
compressor, and thermal and water management systems. The FCHV has a control
unit, fuel cell stack and propulsion motor integrated under-hood with a total system
