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Journal articleBozal-Ginesta C, Rao RR, Mesa CA, et al., 2022,
Spectroelectrochemistry of Water Oxidation Kinetics in Molecularversus Heterogeneous Oxide Iridium Electrocatalysts
, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, Vol: 144, Pages: 8454-8459, ISSN: 0002-7863- Cite
- Citations: 35
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Journal articleScott SB, Rao RR, Moon C, et al., 2022,
The low overpotential regime of acidic water oxidation part I: the importance of O<sub>2</sub> detection
, ENERGY & ENVIRONMENTAL SCIENCE, Vol: 15, Pages: 1977-1987, ISSN: 1754-5692- Cite
- Citations: 40
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Journal articleScott SB, Sorensen JE, Rao RR, et al., 2022,
The low overpotential regime of acidic water oxidation part II: trends in metal and oxygen stability numbers
, ENERGY & ENVIRONMENTAL SCIENCE, Vol: 15, Pages: 1988-2001, ISSN: 1754-5692- Cite
- Citations: 69
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Journal articleRao RR, Corby S, Bucci A, et al., 2022,
Spectroelectrochemical analysis of the water oxidation mechanism on doped nickel oxides
, Journal of the American Chemical Society, Vol: 144, Pages: 7622-7633, ISSN: 0002-7863Metal oxides and oxyhydroxides exhibit state-of-the-art activity for the oxygen evolution reaction (OER); however, their reaction mechanism, particularly the relationship between charging of the oxide and OER kinetics, remains elusive. Here, we investigate a series of Mn-, Co-, Fe-, and Zn-doped nickel oxides using operando UV–vis spectroscopy coupled with time-resolved stepped potential spectroelectrochemistry. The Ni2+/Ni3+ redox peak potential is found to shift anodically from Mn- < Co- < Fe- < Zn-doped samples, suggesting a decrease in oxygen binding energetics from Mn- to Zn-doped samples. At OER-relevant potentials, using optical absorption spectroscopy, we quantitatively detect the subsequent oxidation of these redox centers. The OER kinetics was found to have a second-order dependence on the density of these oxidized species, suggesting a chemical rate-determining step involving coupling of two oxo species. The intrinsic turnover frequency per oxidized species exhibits a volcano trend with the binding energy of oxygen on the Ni site, having a maximum activity of ∼0.05 s–1 at 300 mV overpotential for the Fe-doped sample. Consequently, we propose that for Ni centers that bind oxygen too strongly (Mn- and Co-doped oxides), OER kinetics is limited by O–O coupling and oxygen desorption, while for Ni centers that bind oxygen too weakly (Zn-doped oxides), OER kinetics is limited by the formation of oxo groups. This study not only experimentally demonstrates the relation between electroadsorption free energy and intrinsic kinetics for OER on this class of materials but also highlights the critical role of oxidized species in facilitating OER kinetics.
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Journal articleLee TH, Rao RR, Pacalaj RA, et al., 2022,
A Dual Functional Polymer Interlayer Enables Near-Infrared Absorbing Organic Photoanodes for Solar Water Oxidation
, ADVANCED ENERGY MATERIALS, Vol: 12, ISSN: 1614-6832- Cite
- Citations: 7
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Journal articleVonrueti N, Rao R, Giordano L, et al., 2022,
Implications of Nonelectrochemical Reaction Steps on the Oxygen Evolution Reaction: Oxygen Dimer Formation on Perovskite Oxide and Oxynitride Surfaces
, ACS CATALYSIS, Vol: 12, Pages: 1433-1442, ISSN: 2155-5435 -
Journal articleKatayama Y, Kubota R, Rao RR, et al., 2021,
Direct Observation of Surface-Bound Intermediates During Methanol Oxidation on Platinum Under Alkaline Conditions
, JOURNAL OF PHYSICAL CHEMISTRY C, Vol: 125, Pages: 26321-26331, ISSN: 1932-7447- Author Web Link
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- Citations: 3
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Journal articleCorby S, Rao R, Steier L, et al., 2021,
The kinetics of metal oxide photoanodesfrom charge generation to catalysis
, Nature Reviews Materials, Vol: 6, Pages: 1136-1155, ISSN: 2058-8437Generating charge carriers with lifetimes long enough to drive catalysis is a critical aspect for both photoelectrochemical and photocatalytic systems and a key determinant of their efficiency. This review addresses the charge carrier dynamics underlying the performance of metal oxides as photoanodes and their ability to drive photoelectrochemical water oxidation, alongside wider comparison with metal oxide function in photocatalytic and electrocatalytic systems. We start by highlighting the disparity between the ps–ns lifetimes of electron and holes photoexcited in bulk metal oxides versus the ms –s timescale of water oxidation catalysis. We go onto review recent literature of the dominant kinetic processes determining photoanode performance, namely charge generation, polaron formation and charge trapping, bulk and surface recombination, charge separation and extraction, and finally the kinetics of water oxidation catalysis. With each topic, we review current understanding and note areas of remaining uncertainty or controversy. We discuss the potential for material selection and examine approaches such as doping, nanostructuring, junction formation and/or co-catalyst deposition to enhance performance. Critically, we examine how such performance enhancements can be understood from analyses of carrier dynamics and propose design guidelines for further material or device optimisation.
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Journal articleBozal-Ginesta C, Rao RR, Mesa CA, et al., 2021,
Redox-state kinetics in water-oxidation IrOx electrocatalysts measured by operando spectroelectrochemistry
, ACS Catalysis, Vol: 11, Pages: 15013-15025, ISSN: 2155-5435Hydrous iridium oxides (IrOx) are the best oxygen evolution electrocatalysts available for operation in acidic environments. In this study, we employ time-resolved operando spectroelectrochemistry to investigate the redox-state kinetics of IrOx electrocatalyst films for both water and hydrogen peroxide oxidation. Three different redox species involving Ir3+, Ir3.x+, Ir4+, and Ir4.y+ are identified spectroscopically, and their concentrations are quantified as a function of applied potential. The generation of Ir4.y+ states is found to be the potential-determining step for catalytic water oxidation, while H2O2 oxidation is observed to be driven by the generation of Ir4+ states. The reaction kinetics for water oxidation, determined from the optical signal decays at open circuit, accelerates from ∼20 to <0.5 s with increasing applied potential above 1.3 V versus reversible hydrogen electrode [i.e., turnover frequencies (TOFs) per active Ir state increasing from 0.05 to 2 s–1]. In contrast, the reaction kinetics for H2O2 is found to be almost independent of the applied potential (increasing from 0.1 to 0.3 s–1 over a wider potential window), indicative of a first-order reaction mechanism. These spectroelectrochemical data quantify the increase of both the density of active Ir4.y+ states and the TOFs of these states with applied positive potential, resulting in the observed sharp turn on of catalytic water oxidation current. We reconcile these data with the broader literature while providing a unique kinetic insight into IrOx electrocatalytic reaction mechanisms, indicating a first-order reaction mechanism for H2O2 oxidation driven by Ir4+ states and a higher-order reaction mechanism involving the cooperative interaction of multiple Ir4.y+ states for water oxidation.
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Journal articleHuang B, Rao RR, You S, et al., 2021,
Cation- and pH-Dependent Hydrogen Evolution and Oxidation Reaction Kinetics
, JACS AU, Vol: 1, Pages: 1674-1687- Author Web Link
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- Citations: 66
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