Reaction kinetics in cobalt-based electrocatalyst for water oxidation Shima Farhoosh , Michael Haumann, Holger Dau Department of Physics, Free University Berlin, Germany Hydrogen production through water splitting, powered by renewable electricity or direct sunlight is considered as a promising method to reduce reliance on fossil fuels. Cobalt-based oxy-hydroxide electrocatalyst (CoCat) is notable for its high activity in neutral pH environment. [1-2] It is well-known that cobalt ions undergo two distinct redox reactions prior to initiating the oxygen evolution reaction (OER) and formation of O-O bond. [3] Our research aims to investigate the kinetics of redox transitions and OER by utilizing operando spectroscopic methods. We have employed X-ray and UV-vis absorption spectroscopy, coupled with electrochemical measurements, such as step potential experiments, to monitor changes in oxidation state of cobalt ions during the reaction. Additionally, electrochemical impedance spectroscopy (EIS) was used to further explore the electrochemical processes occurring at the catalyst-electrolyte interface and studying the kinetics and mechanism of oxygen evolution reaction. Through a detailed analysis of X-ray and optical time traces, simultaneously recorded current density transients, as well as simulation of impedance data, we found that two redox transitions (Co II ↔Co III ↔Co IV ) of interacting cobalt sites occur rapidly within milliseconds, while the OER proceeds significantly slower. [4] We also examined possible rate-limiting factors, including film thickness, mass transport, electrical and proton transport. [4] References 1. S. Liu et al., Adv. Energy Mater. 2022, 12 (46), 2202914. 2. S. Liu, S. Farhoosh et al., Adv. Sustainable Syst. 2023, 7 (7), 2300008.
3. C. Pasquini et al., Anal. Bioanal. Chem. 2021, 413 (21), 5395. 4. S. Farhoosh et al., Adv. Energy Mater. 2025, 15, 2403818.
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