Page 86 - Introduction to Transfer Phenomena in PEM Fuel Cells

Charge Transfer Phenomena 75
The straight line in Figure 2.5 shows the evolution of the ohmic voltage
losses that a fuel cell can have.
The losses due to this ohmic polarization (also called resistance) can be
minimized by reducing the resistance of the electrolyte (by decreasing the
thickness, for example), or by increasing the ionic conductivity of the
electrolyte (nature of the material, temperature increase, etc.).

2.5.3. Concentration polarization

Losses of concentration are due to the decrease in concentration of the
reactants on the surface of the electrodes. They appear for high currents
where the concentration of the reagents decreases greatly on the surface of
the electrodes. In the case of air (which normally contains 0.21% oxygen),
the fraction of oxygen at the surface of the electrodes will decrease
significantly, depending on the current intensity, causing a drop in partial
pressure of oxygen (the supply of oxygen is blocked by nitrogen) and
therefore the voltage. In other words, it is the reaction rate that decreases.
The same phenomenon occurs for the anode, which is supplied with
hydrogen. The Nernst equation calculates the voltage loss if it goes from an
initial concentration (arrival of reactants) to a final concentration. The
extreme case being where the final concentration is equal to zero; in this
case, we define a limiting current density (j L) [BOU 07] and the
concentration overpotential can be written in the following form:

RT  j 
η = ⋅ ln  L  [2.76]
conc
nF  j − j 
L
The limiting current density (j L) is a measure of the maximum flow rate
of the gases; the current density (i) cannot be greater than (j L). At this
limiting current density, the concentration of the gases at the electrodes
becomes almost zero and the voltage drops quite significantly.

Using the Butler–Volmer equation (reaction kinetics) leads to a similar
result with a general equation in the form:

 j 
η conc = ⋅  L  [2.77]
cln
 j − j 
L