Butler-Volmer type rate coefficients

The adsorption (Eqs. 2.1, 3.1) and desorption (Eqs. 2.2, 3.2) of Xads (X = OH, AN) on Pt surface occur at finite rates that depend on potential E) and temperature (7). These adsorption and desorption rates are modeled using Butler-Volmer type rate coefficients as ... 

Both the frequency of the well and its depth cancel, so that the free energy of activation is determined by the height of the maximum in the potential of mean force. The height of this maximum varies with the applied overpotential (see Fig. 13). To a first approximation this dependence is linear, and a Butler-Volmer type relation should hold over a limited range of potentials. Explicit model calculation gives transfer coefficients between zero and unity there is no reason why they should be close to 1/2. For large overpotentials the barrier disappears, and the rate will then be determined by ion transport. 

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