Chemistry Reference
In-Depth Information
Fig. 20.2
Graph of the ideal potential
for a hydrogen/oxygen fuel cell as a
function of temperature.
than the ideal potential due to losses within the
cell. Three main sources contribute to these losses,
namely activation polarisation, ohmic polarisation
and concentration polarisation, as shown in Fig.
20.3.
Activation polarisation arises due to slow kinetics
of the electrochemical reaction at the electrode
surface and typically may be in the region of 50-
100 mV. Ohmic polarisation arises due to resistance
of the electrolyte to the flow of ions and resistance
of the electrodes to the flow of electrons. Concen-
tration polarisation arises due to the formation of
concentration gradients close to the electrode sur-
faces as a result of slow diffusion, solution or disso-
lution of reaction products away from the electrodes.
The overall result of these polarisation losses is to
increase the anode potential and decrease the
cathode potential, and hence to reduce the cell
voltage. The polarisation can be reduced by modify-
ing the cell design, e.g. by using better electrocata-
lysts or thinner cell components, and by changing
the temperature or pressure. A compromise often is
required between operating at a higher temperature
and/or pressure and causing unacceptable stability or
durability problems to the cell components.
As temperature increases, D
G
also increases and
hence the ideal fuel cell potential will decrease as
shown in Fig. 20.2. Because the different fuel cell
types operate under different temperature regimes,
they will have different ideal cell potentials.
In comparison, the theoretical efficiency of a con-
ventional heat engine is limited by the Carnot effi-
ciency. which is a function of the upper temperature
(
T
U
) and the lower temperature (
T
L
) of the thermal
cycle:
TT
T
UL
U
Carnot efficiency of a heat engine
=
At lower operating temperatures the fuel cell effi-
ciency is significantly higher than the heat engine.
The efficiency of a heat engine increases with oper-
ating temperature and at approximately 1200 K may
actually be higher than a fuel cell. However, the need
for motion within a heat engine at such high tem-
peratures will present serious material problems.
The relaxing of the temperature requirement enables
greater flexibility in fuel cell design to achieve higher
efficiency.
The fuel cell potential also will be altered by the
reaction kinetics of the chemical process involved,
according to the Nernst equation:
3 Fuel Cell Technology
RT
nF
[
]
05
O
EE
=-
ln
P
P
(
P
)
water
hydrogen
oxygen
Over the last 40 years each of the main types of fuel
cell have been developed and improved to give
higher electrical efficiency, longer operating lives and
lower cost of components and supporting systems.
where
R
is the gas constant,
T
is the temperature,
and
P
is the partial pressure. The voltage obtained
from the fuel cell when a current is drawn is lower
