Understanding the Hydrogen and Oxygen Evolution Reactions through Microkinetic Models
Authors: Marshall Aaron Timothy , Herritsch Alfred 
Published: February 2020

The hydrogen and oxygen evolution reactions are two common processes found in both existing and emerging industrial processes. As these reactions require the input of electrical energy, the development of electrocatalytic materials to decrease the energy requirements of these reactions is important. These electrocatalysts are used to decrease the overpotentials at a given current density, by either increasing the exchange current density or decreasing the apparent Tafel slope of the reaction. Despite the fact these two reactions are relatively simple and that there have been significant improvements in the understanding of electrocatalytic theory and advancements in computational screening methods [1], much of the development of electrocatalytic materials still relies on trial and error rather than using a catalysis by design approach [2, 3].

The performance of any electrocatalytic material can be determined by fabricating the material into electrodes and measuring the current density as a function of overpotential, ideally under standardised conditions [4, 5]. While the overpotential at a certain current density is the only parameter required to determine if the developed electrocatalytic material has any practical relevance, in most cases it is also useful to understand why the material has the observed electrocatalytic performance. This understanding can then (in theory) be used to either design better electrocatalytic materials or enhance the underlying theory of the electrode reactions of interest. Such understanding may come from electrochemical techniques like cyclic voltammetry and electrochemical impedance spectroscopy, or in-situ spectroscopic or structural analysis of the electrocatalytic material, or by fitting experimental data to microkinetic models.

Given that electrocatalytic reactions are multi-step processes [6], understanding how electrode performance is related to the surface coverage of reaction intermediates and the rate determining step is critical in designing better electrocatalysts. However as the reaction intermediates during the hydrogen and oxygen evolution reactions are similar to species within the electrolyte (e.g. adsorbed H, OH, O), directly quantifying these surface bound intermediates by spectroscopy is difficult [7]. Microkinetic models avoid this challenge by allowing the simulation of steady-state or dynamic electrode behavior for a given set of kinetic parameters. This approach was successfully used in some of the earliest work on the hydrogen and oxygen evolution reactions by showing that the reaction mechanism and rate determining steps can be differentiated by measuring the Tafel slope and reaction order with respect to pH [8, 9]. More recent work shows that these microkinetic models can also discover unexpected behaviour such as non-Tafel-like polarisation curves [10, 11].

In the present work, the simulation of steady-state polarization curves and chronoamperometry hydrogen and oxygen evolution reactions following standard reaction mechanisms is discussed. These simulations show that under some conditions, unexpected Tafel slopes can be observed and that multiple sets of kinetic parameters can result in almost identical polarisation curves. The latter clearly highlights the difficultly in determining kinetic parameters from steady-state experimental data. To address this, we show how the simulation of dynamic electrode behaviour can be used to differentiate between sets of kinetic parameters.

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