Chemistry Reference
In-Depth Information
cial mass transport effects at the electrocata-
lyst/electrolyte/electrode interface. For example,
a water film may form over the electrocatalyst
surface, which may inhibit diffusion of oxygen
to the catalyst.
(4)
Open-circuit losses.
Here, particularly for low-
temperature cells, a significant loss in voltage
below the thermodynamic theoretical value at
open circuit (zero current flow) is seen, which
can be several hundred millivolts. The losses are
associated with fuel crossover and internal cur-
rents. Fuel crossover is the diffusion of hydrogen
through the electrolyte to the cathode catalyst,
where it reacts with oxygen to waste electrons.
This is essentially an internal current production
that serves to polarise the cathode and does not
appear as an external current. The effect is influ-
enced by the nature of the electrocatalyst, its
structure and the dispersion of catalyst in the
electrode matrix. The internal currents gener-
ated are typically low, probably <2mAcm
-2
, but
are sufficient to cause local cell polarisation. A
similar effect is produced also at the anode due
to oxygen crossover but, due to faster anode
kinetics, the effect on cell voltage is not as pro-
nounced as at the cathode.
(1)
Electrode and electrode structure.
Platinum is
acknowledged as the best catalyst for anode and
cathode. This catalyst typically is supported on
small (2-10 nm) particles of carbon to produce
highly active material with a high surface area.
The supported catalyst often is bound with
poly(tetrafluoroethene) (PTFE) to repel water
from the porous matrix, to facilitate evaporation
and to prevent flooding of electrode pores. The
catalyst region must be in intimate contact with
the polymer electrolyte to enable efficient ion
(H
+
) transfer from anode to cathode. To this
end, dispersed polymer electrolyte also can be
introduced into the electrode matrix. Further-
more, the catalyst particles must maintain
good electronic contact with each other through
effective contact of the carbon supports. In
addition, the whole catalyst matrix must be con-
nected electrically to adjacent cells or the exter-
nal circuit and load. This is one function of
the 'gas diffusion layer' that serves a dual role
as a gas distributor and current collector. The
gas diffusion layer is typically carbon cloth or
paper.
(2)
Polymer electrolyte and humidification.
The basic
polymer electrolyte membrane used in PEM cells
is a perfluorosulfonic acid/PFTE copolymer. A
typical 'ionomer' structure, shown in Fig. 19.10,
is referred to under the tradename Nafion, made
by DuPont. The SO
3
-
ion group is highly
hydrophilic and creates regions (clusters of side
chains) that absorb large quantities of water,
even though the polymer backbone is very
hydrophobic, in which the H
+
ions are able to
move by attraction to the fixed sulfonate groups.
Fuel cell structure
The development of any particular fuel cell system is
complex, requiring detailed science and engineering
of the cell components and stack system [11]. If, as
an example, we consider the PEM fuel cell, shown
schematically in Fig. 19.9, the factors that can be
identified include:
H
3
Fig. 19.9
Schematic diagram of a
polymer electrolyte fuel cell.
