Microkinetic Modeling of the Oxygen Evolution Reaction (OER) on Hydrous Iridium Oxide during Dynamic Operation

Abstract

In an energy system which is based on intermittent renewable sources, efficient energy storage and conversion technologies are needed. Proton exchange membrane (PEM) electrolysis is a promising technology for the storage of electrical energy in the form of hydrogen. In PEM electrolyzers, the oxygen evolution reaction (OER) is seen as the bottleneck due to its high overpotential. To enhance the overall performance, highly active but stable OER catalysts are essential. Oxidized iridium shows good activity for the OER and is more stable compared to other materials such as ruthenium oxide[1]. It has been shown that electrochemically grown hydrous iridium oxide is particularly favorable[2]. A better understanding of the changes of its complex surface structure and chemistry during dynamic operations is crucial for practical applications and for obtaining deep insights in factors that affect its activity and stability.In the present work we investigate the surface states of hydrous iridium during the OER. For this purpose, we implement a dynamic physical model to simulate the formation of different types of intermediate states. A complete set of microkinetic equations is derived, solved and validated with cyclovoltammetric data (see figure 1). By solving the surface species balances dynamically, the phenomena that are observable in cyclovoltammetric, chronoamperometric and impedance spectroscopic experiments can be interpreted quantitatively[3].Applying this method onto hydrous iridium oxide we are able to study the degree of oxidation of the iridium atoms at the surface at various electrode potentials. In a wide potential region from 0.4 V vs RHE up to the OER potential range, the simulation predicts the amount of adsorbed -OH, -O, -OOH and -OO species, which determine the overall oxidation state of the iridium. The characteristic peaks and shoulders in the cyclovoltammogram are assigned to the single reaction steps. By comparing different mechanisms, a deep understanding of the predominant reaction pathway is gained. Furthermore, the quantification of microkinetic rate constants allows us to define rate determining reaction steps during the dynamic operation. With this study we confirm that the mechanism in figure 1, in which two parallel reaction pathways are present, can describe the experimental cyclovoltammogram quantitatively.[1] S. Cherevko et al., Oxygen and hydrogen evolution reactions on Ru, RuO 2 , Ir, and IrO 2 thin film electrodes in acidic and alkaline electrolytes: A comparative study on activity and stability, Catalysis Today 262 (2016) 170–180[2] S. Cherevko et al., Oxygen evolution activity and stability of iridium in acidic media. Part 2. –Electrochemically grown hydrous iridium oxide, Journal of Electroanalytical Chemistry 774 (2016) 102–110[3] U. Krewer et al., Impedance spectroscopic analysis of the electrochemical methanol oxidation kinetics, Journal of Electroanalytical Chemistry, 589 (2006) 148–159 Figure 1