(52f) Influence of Elementary Step Reversibility on the Mechanistic Interpretation of Tafel Slope

The measurement of Tafel slope is a core tenet of electrocatalysis. Indeed, the identification of rate-determining steps (RDS) is commonly based on the observation of “cardinal” Tafel slopes—which take the form 60 mV/dec × 1/n where n reflects, in part, the number of electrons transferred before the RDS, or “pre-RDS”. This paradigmatic approach is predicated on the pre-equilibrium approximation (PEA)—that all pre-RDS steps are equilibrated. This assumption is fundamental to the mechanistic interpretation of Tafel slope and is often accepted without scrutiny. As a result, there exists a profound tacit knowledge gap within the field of electrocatalysis regarding the limitations of the PEA and the consequences of its breakdown on the mechanistic meaning of Tafel slope.

In this work, we formulate a simple, universal equation that exactly quantifies the accuracy of the PEA and the consequences of de-equilibration of pre-RDS steps on observed Tafel slope. These developments are applied to (i) the chlorine evolution reaction (CER) on Pt-Ir bimetallic catalysts and (ii) the oxygen reduction reaction (ORR) on iron phthalocyanine (FePc) catalysts. In each example, we reinterpret reported rate-potential dependencies to demonstrate that account of the approach-to-equilibrium of elementary steps is critical to clarifying the mechanistic meaning of observed Tafel slope. In the case of the CER, we demonstrate that shifts in Tafel slope from ~40 mV/dec to ~120 mV/dec at low and high overpotential respectively do not indicate a shift in the identity of the rate-determining step or the catalyst resting state. Instead, this transition in Tafel slope is a direct manifestation of the de-equilibration of pre-RDS elementary steps. These same principles are demonstrated to explain potential-dependent transitions in Tafel slope during FePc-catalyzed ORR and lead us to conclude that elementary step approach-to-equilibrium is a key descriptor of catalysis of comparable importance to rate-determining-step identification.