Diffusion-convection layer oxidant concentration

We assume that the concentration distribution within the diffusion—convection layer can be treated in the similar way to that described in Chapter 2, and then the concentration distribution of the oxidant near the electrode surface can be schematically expressed in Figure 5.2. Thus, the diffusion—convection current density (ioc.o) can be expressed in a similar form to those Eqns (2.57) and (2.58) ... 

In order to get the current—potential relationship on the RDE, particularly the expression of limiting current density as the function of the electrode rotating rate and the reactant concentration, Pick s second law has to be used to give the equations of reactant concentration change with time at the steady-state situation of diffusion—convection. When the surface concentration of oxidant reaches zero during the reaction at the steady-state situation, the concentration distribution within the diffusion—convection layer is not changing with time anymore, meaning that the diffusion rate is... 

Figure 4-7 Concept of electrochemical reaction increasing the diffusion layer thickness (concentration polarization) of analyte via a reduction (or oxidation) at the surface of the working electrode. As time (t) increases, the diffusion layer thickness grows quickly to a value that is determined by degree of convection in the sample solution.

Convection refers to fhe fransport of the reactant or product species by bulk fluid motion driven by natural or applied mechanical forces. The natural convection limitations are due to convective transport caused by differences in densities as a result of temperature or concentration. The species transport to the interface can also be limited by fhe fuel cell flow sfrucfures and fheir conditions. For example, in PEMFC, blockage of flow channels or pore structures in diffusion or elecfrode-cafalysf layers owing to the liquid phase can restrict the supply of fhe reactant to the interface. Accumulation of inert gases that do not participate in chemical reaction will limit the partial pressure of the reactant at the interface. This results to decreased reactions at the interface. The accumulation of chemical impurities at the reaction sites will prevent adsorption of desired reactant species. For example, in PEMFC, the presence of carbon monoxide degrades the platinum catalyst because the platinum preferentially adsorbs carbon monoxide, leaving few reaction sites for hydrogen adsorption and oxidation. This leads to high anodic overpotential. 

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