Environmental Engineering Reference
In-Depth Information
3.3.1.2 Losses Due to Fuel Crossover and Internal Current
Even if the polymeric membrane in a PEM fuel cell is designed to permit only the
passage of hydrated hydrogen ion, some electric conductivity and gas permeation
cannot be avoided. Then at open-circuit, when no current can be observed through
the external circuit, two phenomena can occur at the anode side of a PEM fuel
cells, called fuel crossover and internal current. They can be described as follows:
fuel crossover: some not oxidized hydrogen can pass through the electrolyte
membrane to reach the cathode, where it can be directly oxidized by oxygen,
wasting two electrons for each hydrogen molecule. The hydrogen crossover
depends on membrane permeability and thickness, and on difference between
hydrogen partial pressure on the two side of the membrane.
internal current: some hydrogen is oxidized at the anode side with production
of two electrons per molecule. These do not flow through the external circuit
(which is open) but pass through the electrolyte membrane directly to the cathode.
Both the above phenomena produce the same effect to transfer electrons directly
from anode to cathode and to consume a small quantity of hydrogen at open-
circuit, in spite of no useful current is generated. Then this hydrogen consumption
correspond to a current density associated with electron flow through the elec-
trolyte membrane, which is a current density subtracted from that necessary for
useful work production. This loss could be considered negligible in energy terms,
but for conditions close to open-circuit the consequent reduction of potential is
significant. The occurring of internal current allows the application of Eq.
3.30
also
at open-circuit, using the internal current density in the ratio i/i
0
at the place of the
external one. For the entire range of current densities Eq.
3.30
becomes:
E
¼
E
V
act
;
in
¼
E
RT
i
þ
i
in
i
0
ð
3
:
31
Þ
aF
ln
where the total cell current density is sum of the current density circulating
through the external circuit far from the equilibrium conditions (i) and of the
current density lost through the membrane at open-circuit (i
in
). Setting i = 0in
Eq.
3.31
, it is possible to calculate the open-circuit voltage of a PEM fuel cell,
which generally results significantly lower with respect to the theoretical value. At
practical current density values, that is when an appreciable electron flow is
present in the external circuit, the difference between hydrogen concentrations on
the two sides of the membrane decreases, then the driving force for hydrogen
permeation is strongly reduced, and the crossover losses become negligible (a
typical value for i
in
is 2 mA/cm
2
[
50
]).
3.3.1.3 Losses Due to Electric Resistance
The sharp slope change of the polarization curve in Fig.
3.5
suggests that after the
first steep voltage diminution up to 2 A, attributed to activation and internal
current polarization, another type of losses become predominant.
