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Why the microkinetic modeling of experimental tafel plots requires knowledge of the reaction intermediate's binding energy

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The quasi-equilibrium description approaches the steady-state model, but only if the binding strength of the reaction intermediate is sufficiently weakly or strongly (|∆G TD| > 0.2 eV).


Knowledge of the relation between electrocatalytic activity and the rate-determining step or the surface coverage of reaction intermediates is crucial to design next-generation electrocatalysts that may contribute to the sustainability of our society. Commonly, microkinetic models making use of the quasi-equilibrium or steady-state assumptions are applied to correlate potential mechanistic descriptions to experimental data, which are usually depicted in the form of a Tafel plot. Yet, there is a discrepancy in the literature on the utility of the quasi-equilibrium and steady-state conditions, both of which are approximations per se. This inconsistency is the starting point of the present work, which compares the quasi-equilibrium and steady-state approaches for the analysis of a Tafel plot by the concept of free-energy diagram for a two-electron process. A correlation between the two frameworks is deduced and important guidelines for the application of the quasi-equilibrium and steady-state assumptions are obtained. While the quasi-equilibrium approach is a suitable approximation for the analysis of a linear Tafel line without change in the Tafel slope, it may fail in the evaluation of a Tafel plot with two linear Tafel regimes. There, the binding energy of the reaction intermediate governs whether the quasi-equilibrium model is justified, or the steady-state approach is needed. This implies that density functional theory calculations are indispensably required as a supplement to analyze a Tafel plot with two different Tafel slopes, a situation that is particularly observed for highly active electrocatalysts.

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