Electrode Reactions on Heterogeneous Surfaces

The two basic premises of the discussion of electrode kinetics in the preceding chapter are  

Condition a is fulfilled in a strict sense for liquid metal electrodes like 

Condition b can be met for homogeneous surfaces by the design of the cell, for instance by planar arrangement of test and counter electrode. Polarization [1—4] tends to produce a secondary current distribution that is less non-uniform than the primary distribution in the absence of polarization. If certain parameters that are characteristic [1—4] for the arrangement of the electrodes, the conductivity of the electrolyte, and the polarization are chosen properly, the current distribution can be considered uniform on a homogeneous surface. 

Condition a is not fulfilled for real surfaces of polycrystalline material used commonly in the study of electrocatalytic reactions. Each of the rate constants may cover a large spectrum of values when passing across the surface. If the spectrum is large, the current distribution need no longer be uniform even when the conditions of uniformity for a homogeneous surface are met. 

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