Region III Concentration Polarization

Restriction of the rate of transport to the electrode can occur for a variety of reasons  

Gas-Phase Diffusion Limitation The diffusion of reactant in the gas phase is limited to some value. In principle, we can imagine that there is a maximum rate that perfume can diffuse from the front of a room to the back of the room. Now consider the perfume is being consumed at the back of the room. The consumption rate would be limited to the maximum perfume diffusion rate, as the reaction rate is limited by reactant availability. At the double layer, a greater polarization will be required to attract the required adsorbed species for reaction in a limiting condition. 

Liquid-Phase Accumulation and Pore Blockage Limitation In low-temperature fuel cells such as the PEFC, liquid water accumulation and blockage in the pores of the electrolyte, diffusion media, or flow channels of the anode or cathode can reduce the transport rate of reactant to the catalyst. Voltage loss by this phenomenon is generically ismioAflooding. This topic is discussed in greater detail in Chapters 5 

Surface Blockage by Impurity Coverage In this case, some impurity becomes adsorbed on reaction sites, preventing adsorption of the desired reactant. This is commonly seen in low-temperature fuel cells when a CO impurity is present. Carbon monoxide preferentially adsorbs on platinum catalyst sites in the anode and has a very high polarization for oxidative removal. 

In Chapter 3, we learned that the Nemst equation could be used to predict the change in equilibrium voltage from concentration changes of the reactants and products. At the time, we noted that there was also a kinetic effect of the concentration. Once there is reaction at the electrode, the system is not in thermodynamic equilibrium and these effects must be considered. From a combination of the thermodynamic (Nemst) and exchange current density dependency [Eq. (4.31)], the voltage adjustment as a result of the change in oxygen concentration from the reference value (1 atm) on the cathode can be shown as 

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