Potential Step, High Overpotential Region Chrono-amperometry

2 Potential Step, High Overpotential Region (Chrono-amperometry) 

Our second example is also a potential step experiment. Here, however, it is assumed that the kinetics of the reaction is slow, and equilibrium is not maintained at the interface. Moreover, we assume that the potential range studied is far from equilibrium, so that only the forward reaction need be considered. In the original literature this condition was referred to as the totally irreversible case, a term which we consider to be. rather misleading, because the course of the reaction can be reversed, if a potential step in the opposite direction is applied. 

The first five initial and boundary conditions of the differential equation listed above remain imchanged, only the sixth (Eq. (14.24)) is different. The new boundary condition is obtained by relating the current density to the heterogeneous rate constant of the forward reaction and to the flux of reactant at the electrode surface  

The solution of the diffusion equation yields the current density as a function of time and potential, as  

The heterogeneous rate constant kh depends on potential exponentially, following a Tafel-like relationship  

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