Chemistry Reference
In-Depth Information
the maximum efficiency of a fuel cell falls with
increase in temperature, most fuel cells operate at
temperatures significantly higher than ambient
temperature because of two principal factors:
Fig. 19.8 for a low-temperature cell. The factors
identifiable from the curve of Fig. 19.8 include:
(1)
Electrode activation losses.
These losses are due
to the inherent slow reaction kinetics (electrode
polarisation), particularly at the cathode
(oxygen reduction), in low-temperature cells.
They appear as an initial relatively rapid fall
in voltage with applied current. They can be
reduced by using more effective catalysts with
higher exchange current densities (
j
o
), operating
at higher temperatures, increasing the catalyst
roughness or surface area and increasing the
available reactant concentration.
(2)
Ohmic losses.
These losses essentially are due to
resistance to the flow of electrons in various con-
nections and materials in the cell and the flow
of ions in the electrolyte(s). They appear as an
approximately linear variation of voltage with
current density in the mid-range of Fig. 19.8.
Highly conducting electrodes and electrolytes
and thin electrolyte regions reduce the effect.
(3)
Mass transport.
The losses associated with mass-
transport limitations or concentration polarisa-
tion appear at high current densities as a typical
rapid loss in cell voltage. The electrocatalyst
becomes starved of fuel or oxidant and thus
limits the associated electrode reaction. The
effect may be due to changes in bulk composi-
tions of reactants, e.g. influence of nitrogen in
air on oxygen supply, or may be due to interfa-
(1)
Waste heat generated at higher temperatures is
much more useful than from lower temperature
cells and leads to highly efficient combined heat
and power systems
(2)
Voltage losses, i.e. cell polarisation factors, are
typically lower at higher temperatures, so higher
temperatures ultimately can lead to higher fuel
cell efficiencies (and power densities, i.e. watts
per kilogram)
Fuel cell operating voltage
The actual operating voltage of a fuel cell is affected
by many factors, including the fuel and oxidant
supply and the type and characteristics of the cell
materials and components [10]. High pressures of
hydrogen and oxygen generally are attractive for
better fuel cell performance, although of course this
has to be considered in the overall cost of supply and
performance of the system. In practice, one of the
attractions of fuel cells is the high power density
that can be achieved, certainly in comparison with
batteries. High power densities require significant
current or current density to be drained from the
cell, with the typical resultant effect on the cell
voltage—current density characteristics as shown in
Fig. 19.
8
Typical current-voltage
characteristics of a low-temperature
fuel cell.
