Insight on Tafel slopes from a microkinetic analysis of aqueous electrocatalysis for energy conversion

TLDR

Tafel slopes are often assumed to arise from extreme surface coverages, yet they are actually coverage‑dependent, and the Butler‑Volmer equation is frequently misapplied in electrocatalytic Tafel analyses. This study revisits microkinetic analyses of aqueous HER, HOR, ORR, and OER to illustrate the limitations of the Butler‑Volmer expression and to provide a more accurate kinetic description. The authors performed detailed kinetic analyses that model coverage‑dependent Tafel slopes for these reactions. Their analyses establish a benchmark linking experimentally observed Tafel slopes to specific rate‑determining steps, show that oversimplified assumptions yield inaccurate surface descriptions, and expose the shortcomings of the Butler‑Volmer equation in electrocatalysis.

Abstract

Abstract Microkinetic analyses of aqueous electrochemistry involving gaseous H 2 or O 2 , i.e., hydrogen evolution reaction (HER), hydrogen oxidation reaction (HOR), oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), are revisited. The Tafel slopes used to evaluate the rate determining steps generally assume extreme coverage of the adsorbed species (θ ≈ 0 or ≈1), although, in practice, the slopes are coverage-dependent. We conducted detailed kinetic analyses describing the coverage-dependent Tafel slopes for the aforementioned reactions. Our careful analyses provide a general benchmark for experimentally observed Tafel slopes that can be assigned to specific rate determining steps. The Tafel analysis is a powerful tool for discussing the rate determining steps involved in electrocatalysis, but our study also demonstrated that overly simplified assumptions led to an inaccurate description of the surface electrocatalysis. Additionally, in many studies, Tafel analyses have been performed in conjunction with the Butler-Volmer equation, where its applicability regarding only electron transfer kinetics is often overlooked. Based on the derived kinetic description of the HER/HOR as an example, the limitation of Butler-Volmer expression in electrocatalysis is also discussed in this report.