Calculation of the Tafel slope and reaction order of the oxygen evolution reaction between pH 12 and pH 14 for the adsorbate mechanism

TL;DR

This study uses microkinetic analysis to clarify how Tafel slopes and reaction orders vary with pH in the oxygen evolution reaction, revealing complexities in identifying rate-limiting steps.

Contribution

It provides a detailed analysis of how Tafel slopes and reaction orders depend on surface coverage and pH, offering guidelines for experimental identification of limiting steps.

Findings

Tafel slopes depend on preceding steps and surface coverage.

Tafel slope varies with pH due to changing intermediate coverage.

Reaction orders can help distinguish rate-limiting steps.

Abstract

Despite numerous experimental and theoretical studies devoted to the oxygen evolution reaction, the mechanism of the OER on transition metal oxides remains controversial. This is in part owed to the ambiguity of electrochemical parameters of the mechanism such as the Tafel slope and reaction orders. We took the most commonly assumed adsorbate mechanism and calculated the Tafel slopes and reaction orders with respect to pH based on microkinetic analysis. We demonstrate that number of possible Tafel slopes strongly depends on a number of preceding steps and surface coverage. Furthermore, the Tafel slope becomes pH dependent when the coverage of intermediates changes with pH. These insights complicate the identification of a rate-limiting step by a single Tafel slope at a single pH. Yet, simulations of reaction orders complementary to Tafel slopes can solve some ambiguities to distinguish between possible rate-limiting steps. The most insightful information can be obtained from the low overpotential region of the Tafel plot. The simulations in this work provide clear guidelines to experimentalists for the identification of the limiting steps in the adsorbate mechanism using the observed values of the Tafel slope and reaction order in pH-dependent studies.