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

Despite numerous experimental and theoretical studies devoted to the oxygen evolution reaction (OER), the mechanism of the OER on transition metal oxides remains controversial. This is in part owing 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 using the steady-state approximation. The analysis was performed for an ideal electrocatalyst without scaling of the intermediates as well as for one on the top of a volcano relation and one on each leg of the volcano relation which exhibits scaling of the intermediates. For these four cases, the number of possible Tafel slopes strongly depends on 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.