Electric Field Effects in Oxygen Reduction Kinetics: Rationalizing pH Dependence at the Pt(111), Au(111), and Au(100) Electrodes

Abstract

In this work, we develop a new microkinetic model for the oxygen reduction reaction (ORR) which incorporates field effects into the established computational hydrogen electrode model. We find that the field can significantly alter the binding energy of ORR adsorbates, particularly *OOH, *O 2 , and *H 2 O 2 . By considering these effects, we find that the model gives polarization curves and rotating ring disk electrode currents consistent with those found experimentally on the Pt(111), Au(111), and Au(100) electrodes, allowing us to predict onset potentials for both overall ORR activity and activity toward hydrogen peroxide as well as reproduce experimental Tafel slopes. In particular, we resolve the peculiar behavior observed on the Au(100) surface, where increasing pH leads to greatly enhanced 4e-ORR activity, without strongly affecting 2e-ORR activity. We then use these predictions to better understand activity across all ORR catalysts, showing that weak-binding catalysts are more influenced by pH because of the strong field effects on the *OOH adsorbate. Finally, we argue that considering field effects can expand the search for improved ORR catalysts by allowing us to deconvolute the activity dependence of catalysts on the standard hydrogen electrode and the reversible hydrogen electrode.

Probing the Effects of Acid Electrolyte Anions on Electrocatalyst Activity and Selectivity for the Oxygen Reduction Reaction

“…[4,26,27,33,34] For example, solvated ions near the surface may stabilize or destabilize certain adsorbates via direct non-covalent chemical interaction(s) of the solvated-ion structure with adsorbates and/or indirectly by inducing localized electric fields perturbing absorbates. [12,26,27,33,34] A simplified microenvironment schematic summarizing the anion interactions with the electrocatalyst and/or reaction ORR intermediates we hypothesize to be possible is shown in Figure 3.…”

“…One possible avenue towards modulating ORR activity and selectivity aside from material engineering (e. g. alloying and micro‐structuring) is to control the composition of the electrochemical double layer, for example by tuning the pH and/or the ions at or near the catalyst surface. ORR activity and selectivity have been observed to vary as a function of pH [8,12,13] . The overall pH effect, on the ORR, namely that 4 e − selectivity and overall activity tend to increase with increasing pH, has been explained in terms of the pH dependent adsorption thermodynamics of O 2 , [8] and more recently [12] by effect of the electrode electric field corresponding to the absolute applied potential ( U SHE ), which significantly affects the binding energies of *O 2 , *OOH, and H 2 O 2 * [8,12] .…”

“…[8,12,13] The overall pH effect, on the ORR, namely that 4e À selectivity and overall activity tend to increase with increasing pH, has been explained in terms of the pH dependent adsorption thermodynamics of O 2 , [8] and more recently [12] by effect of the electrode electric field corresponding to the absolute applied potential (U SHE ), which significantly affects the binding energies of *O 2 , *OOH, and H 2 O 2 *. [8,12] Moreover, the pH effect has been shown to affect weak-binding materials, for which the applied electric field becomes more negative with increasing pH, more significantly than strong-binding materials. [12,13] However, while significant that pH plays a large role in modulating activity and selectivity, the overall catalytic performance is also impacted by changes in material stability as a function of pH.…”

“…[8,12] Moreover, the pH effect has been shown to affect weak-binding materials, for which the applied electric field becomes more negative with increasing pH, more significantly than strong-binding materials. [12,13] However, while significant that pH plays a large role in modulating activity and selectivity, the overall catalytic performance is also impacted by changes in material stability as a function of pH. Changing the electrolyte species, at constant pH, on the other hand, could help control the local double layer electrochemical microenvironment [4] and modulate the activity of a material via anion effects without compromising stability.…”

Interaction of CO with Gold Under Gas Phase and Electrochemical Environments

“…Computational mechanistic investigations have recently begun to include the impact of solvation and field, which can have a critical impact on reaction energetics. [8][9][10][11] This work presents a theoretical and experimental investigation of the binding characteristics of CO on Au under both gas phase and electrochemical conditions. By analyzing previously reported TPD data, we determined the equilibrium coverage of CO adsorbed on (211) and (310) single crystals under atmospheric CO pressure.…”