Environmental Engineering Reference
In-Depth Information
Fig. 4.4 Schematic voltage-
current relationship of a fuel
cell
Theoretical voltage
Open Circuit Voltage (OCV)
Ohmic Loss
Activation Overpotential
Diffusion Overpotentail
Current density, I cell / A cm -2
is heavier than the weight, it lifts the weight in reverse. This corresponds to the
electrical process of breaking down the water in electrolysis to produce hydrogen.
4.2.2
Concept of Efficiency
With any energy conversion device using limited resources, energy conversion ef-
ficiency is an important factor. However, efficiency data as issued in performance
specifications for various energy conversion devices are often based on different
assumptions, and may be misleading when such assumptions are not appreciated.
We thus first consider the knowledge required to assess the efficiency of conversion
of chemical energy to and from electrical energy.
Thermodynamic Aspects In conversion from chemical energy, it is necessary to
know how much energy is released in the chemical reactions involved. Also to know
the driving force under which the chemical reaction proceeds. These values are
available from thermodynamic databases established by our scientific predecessors.
For example, Table 4.1 is an example of the output of a personal computer-based
thermodynamic database MALT, showing the change of energy when the H 2 O mol-
ecule is formed by the reaction of H 2 with O 2 at around 1 atmosphere (10 5 Pa).
In Table 4.1a , the column headed by Δ r H is the heat of reaction at each tem-
perature ( H is the enthalpy. Δ r shows it is the amount that changes as a result of the
reaction). The negative sign shows that the energy of the system is reduced- in other
words, heat is released. The column headed by Δ r G ( G is the Gibbs energy) is the
driving force to promote the reaction. A negative sign means that the reaction shown
here will proceed spontaneously, and the value is the maximum amount of energy
that can be converted into electrical (work) in the fuel cell.
If we look at the change in values in Table 4.1a with temperature, the tempera-
ture dependence of Δ r H is seen to be small. It can also be seen that the absolute
value of Δ r G becomes smaller as the temperature rises. In general, as the tempera-
ture is increased, the discrete state becomes more stable than forming a large mol-
ecule. So, at higher temperatures, the state of H 2 + 1/2O 2 is more stable than the
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