New Insights into the Water Oxidation Mechanism from Kinetic Isotope Effect Studies
Authors: Tse Edmund C. M. , Hoang Thao Thi Huong , Varnell Jason A. , Gewirth Andrew A. 
Published: February 2020

The ability to regulate the chemistry of oxygen represents a monumental step toward the utilization of sustainable energy from renewable energy sources. To materialize this vision, mediating the oxygen evolution reaction (OER, 4 OH– → 2 H2O + 4 e– + O2 in alkaline or 2 H2O → O2 + 4 H+ + 4 e– in acid) at low overpotential is necessary to the widespread deployment of water splitting electrolyzers. Currently, the performance of state-of-the-art electrolyzers is limited by the lethargic kinetics of the reaction involving oxygen. Oxygen chemistry has thus attracted a tremendous amount of attention over the past fifty years to prepare robust, scalable and competent OER catalysts.
The efficiency of water splitting electrolyzers is dictated predominantly by the OER overpotential at the anode and the stability of the anode material. Ir and Ru are the anodes of choice in acidic electrolyte with low OER overpotentials. However, the application of Ir/Ru-based anodes is hindered by the prohibitive high price and undesirable long-term durability of these precious metal catalysts. Earth-abundant and inexpensive non-precious metal (NPM) catalysts with low overpotential and appreciable stability in basic condition are instrumental to the development of efficient alkaline electrolyzers. However, OER overpotentials using Ni/Co-based materials range from approximately 50-150 mV relative to the thermoneutral potential of 1.48 V versus RHE. Real-world thermodynamic efficiencies for water splitting are only ∼75% with commercially available Ni-based catalysts. Despite the extensive effort devoted to these areas, catalyst design utilizing NPM materials to eliminate the high OER overpotential is still lacking because the OER mechanism is not fully understood.
The kinetic isotope effect (KIE) is a general method to study the reaction mechanism of many types of chemical transformations. Specifically, the substitution of hydrogen with deuterium has been carried out extensively due to the large differences in reaction rates arising from the reduced mass differences between the isotopes. For electrocatalysis involving protons, Conway et al. investigated KIE of the hydrogen evolution reaction catalyzed by Pt and Yeager et al. conducted a similar KIE study on oxygen reduction reaction catalyzed by Pt. However, KIE studies are not prevalent in the field of electrochemistry. Therefore, we seek to expand the use of KIE studies to further understand the OER process at the molecular level.
Here, we launched a comparative KIE study in conjunction with in situ potential-dependent Raman spectroscopy on several NPM OER catalysts to identify the rate-determining step (RDS) of the intricate OER involving multiple proton-coupled electron transfer (PCET) steps. We interrogated the difference in OER response of NPM OER catalysts in the condition at which they are stable and active. Our results corroborate that a rate-limiting bond breaking or forming event likely occurs at an occluded site (Scheme 1a,b) on the electrode surface with adjacent OH functionalities (Scheme 1c). The mechanistic insight gained from our KIE study of the OER should be broadly useful to the electrochemical and bioinorganic communities that are interested in both the fundamental aspects of these bio-relevant PCET processes and the development of active, robust, and inexpensive catalysts for practical energy conversion devices under operation conditions in the near future.

Acknowledgements. E.C.M.T. acknowledges a Croucher Foundation Scholarship. We thank the National Science Foundation (Grant CHE-1309731) for support of this research.

Scheme 1 is presented here.

Figure 1