Publications
49 results found
Kwon S, Stoerzinger KA, Rao R, et al., 2024, Facet-Dependent Oxygen Evolution Reaction Activity of IrO2 from Quantum Mechanics and Experiments., J Am Chem Soc
The diversity of chemical environments present on unique crystallographic facets can drive dramatic differences in catalytic activity and the reaction mechanism. By coupling experimental investigations of five different IrO2 facets and theory, we characterize the detailed elemental steps of the surface redox processes and the rate-limiting processes for the oxygen evolution reaction (OER). The predicted complex evolution of surface adsorbates and the associated charge transfer as a function of applied potential matches well with the distinct redox features observed experimentally for the five facets. Our microkinetic model from grand canonical quantum mechanics (GC-QM) calculations demonstrates mechanistic differences between nucleophilic attack and O-O coupling across facets, providing the rates as a function of applied potential. These GC-QM calculations explain the higher OER activity observed on the (100), (001), and (110) facets and the lower activity observed for the (101) and (111) facets. This combined study with theory and experiment brings new insights into the structural features that either promote or hinder the OER activity of IrO2, which are expected to provide parallels in structural effects on other oxide surfaces.
Liang C, Katayama Y, Tao Y, et al., 2024, Role of Electrolyte pH on Water Oxidation for Iridium Oxides., J Am Chem Soc, Vol: 146, Pages: 8928-8938
Understanding the effect of noncovalent interactions of intermediates at the polarized catalyst-electrolyte interface on water oxidation kinetics is key for designing more active and stable electrocatalysts. Here, we combine operando optical spectroscopy, X-ray absorption spectroscopy (XAS), and surface-enhanced infrared absorption spectroscopy (SEIRAS) to probe the effect of noncovalent interactions on the oxygen evolution reaction (OER) activity of IrOx in acidic and alkaline electrolytes. Our results suggest that the active species for the OER (Ir4.x+-*O) binds much stronger in alkaline compared with acid at low coverage, while the repulsive interactions between these species are higher in alkaline electrolytes. These differences are attributed to the larger fraction of water within the cation hydration shell at the interface in alkaline electrolytes compared to acidic electrolytes, which can stabilize oxygenated intermediates and facilitate long-range interactions between them. Quantitative analysis of the state energetics shows that although the *O intermediates bind more strongly than optimal in alkaline electrolytes, the larger repulsive interaction between them results in a significant weakening of *O binding with increasing coverage, leading to similar energetics of active states in acid and alkaline at OER-relevant potentials. By directly probing the electrochemical interface with complementary spectroscopic techniques, our work goes beyond conventional computational descriptors of the OER activity to explain the experimentally observed OER kinetics of IrOx in acidic and alkaline electrolytes.
Moss B, Svane KL, Nieto-Castro D, et al., 2024, Cooperative Effects Drive Water Oxidation Catalysis in Cobalt Electrocatalysts through the Destabilization of Intermediates., J Am Chem Soc, Vol: 146, Pages: 8915-8927
A barrier to understanding the factors driving catalysis in the oxygen evolution reaction (OER) is understanding multiple overlapping redox transitions in the OER catalysts. The complexity of these transitions obscure the relationship between the coverage of adsorbates and OER kinetics, leading to an experimental challenge in measuring activity descriptors, such as binding energies, as well as adsorbate interactions, which may destabilize intermediates and modulate their binding energies. Herein, we utilize a newly designed optical spectroelectrochemistry system to measure these phenomena in order to contrast the behavior of two electrocatalysts, cobalt oxyhydroxide (CoOOH) and cobalt-iron hexacyanoferrate (cobalt-iron Prussian blue, CoFe-PB). Three distinct optical spectra are observed in each catalyst, corresponding to three separate redox transitions, the last of which we show to be active for the OER using time-resolved spectroscopy and electrochemical mass spectroscopy. By combining predictions from density functional theory with parameters obtained from electroadsorption isotherms, we demonstrate that a destabilization of catalytic intermediates occurs with increasing coverage. In CoOOH, a strong (∼0.34 eV/monolayer) destabilization of a strongly bound catalytic intermediate is observed, leading to a potential offset between the accumulation of the intermediate and measurable O2 evolution. We contrast these data to CoFe-PB, where catalytic intermediate generation and O2 evolution onset coincide due to weaker binding and destabilization (∼0.19 eV/monolayer). By considering a correlation between activation energy and binding strength, we suggest that such adsorbate driven destabilization may account for a significant fraction of the observed OER catalytic activity in both materials. Finally, we disentangle the effects of adsorbate interactions on state coverages and kinetics to show how adsorbate interactions determine the observed Tafel slopes. Cruciall
Guo C, Benzie P, Hu S, et al., 2024, Extensive photochemical restructuring of molecule-metal surfaces under room light., Nat Commun, Vol: 15
The molecule-metal interface is of paramount importance for many devices and processes, and directly involved in photocatalysis, molecular electronics, nanophotonics, and molecular (bio-)sensing. Here the photostability of this interface is shown to be sensitive even to room light levels for specific molecules and metals. Optical spectroscopy is used to track photoinduced migration of gold atoms when functionalised with different thiolated molecules that form uniform monolayers on Au. Nucleation and growth of characteristic surface metal nanostructures is observed from the light-driven adatoms. By watching the spectral shifts of optical modes from nanoparticles used to precoat these surfaces, we identify processes involved in the photo-migration mechanism and the chemical groups that facilitate it. This photosensitivity of the molecule-metal interface highlights the significance of optically induced surface reconstruction. In some catalytic contexts this can enhance activity, especially utilising atomically dispersed gold. Conversely, in electronic device applications such reconstructions introduce problematic aging effects.
Luo H, Yukuhiro VY, Fernandez PS, et al., 2022, Role of Ni in PtNi Bimetallic Electrocatalysts for Hydrogen and Value-Added Chemicals Coproduction via Glycerol Electrooxidation, ACS CATALYSIS, Vol: 12, Pages: 14492-14506, ISSN: 2155-5435
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- Citations: 4
Peng J, Schwalbe-Koda D, Akkiraju K, et al., 2022, Human- and machine-centred designs of molecules and materials for sustainability and decarbonization, NATURE REVIEWS MATERIALS, Vol: 7, Pages: 991-1009, ISSN: 2058-8437
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- Citations: 17
Rao RR, Mesa CA, Durrant JR, 2022, Better together, NATURE CATALYSIS, Vol: 5, Pages: 844-845, ISSN: 2520-1158
Bozal-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
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- Citations: 12
Scott 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
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- Citations: 14
Scott 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
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- Citations: 22
Rao 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: 0002-7863
Metal 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.
Lee 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
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- Citations: 7
Vonrueti 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
Katayama 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
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- Citations: 3
Corby 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: 2058-8437
Generating 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.
Bozal-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: 2155-5435
Hydrous 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.
Huang 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
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- Citations: 66
Wang T, Zhang Y, Huang B, et al., 2021, Enhancing oxygen reduction electrocatalysis by tuning interfacial hydrogen bonds, NATURE CATALYSIS, Vol: 4, Pages: 753-762, ISSN: 2520-1158
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- Citations: 81
Hwang J, Rao RR, Giordano L, et al., 2021, Regulating oxygen activity of perovskites to promote NO<i><sub>x</sub></i> oxidation and reduction kinetics, NATURE CATALYSIS, Vol: 4, Pages: 663-673, ISSN: 2520-1158
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- Citations: 44
Bucci A, Garcia-Tecedor M, Corby S, et al., 2021, Self-supported ultra-active NiO-based electrocatalysts for the oxygen evolution reaction by solution combustion, JOURNAL OF MATERIALS CHEMISTRY A, Vol: 9, Pages: 12700-12710, ISSN: 2050-7488
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- Citations: 12
Rao RR, Huang B, Katayama Y, et al., 2021, pH- and Cation-Dependent Water Oxidation on Rutile RuO<sub>2</sub>(110), JOURNAL OF PHYSICAL CHEMISTRY C, Vol: 125, Pages: 8195-8207, ISSN: 1932-7447
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- Citations: 24
Huang B, Myint KH, Wang Y, et al., 2021, Cation-Dependent Interfacial Structures and Kinetics for Outer-Sphere Electron-Transfer Reactions, JOURNAL OF PHYSICAL CHEMISTRY C, Vol: 125, Pages: 4397-4411, ISSN: 1932-7447
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- Citations: 28
Rao RR, Stephens IEL, Durrant JR, 2021, Understanding What Controls the Rate of Electrochemical Oxygen Evolution, JOULE, Vol: 5, Pages: 16-18, ISSN: 2542-4351
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- Citations: 14
Rao RR, Tulodziecki M, Han B, et al., 2021, Reactivity with Water and Bulk Ruthenium Redox of Lithium Ruthenate in Basic Solutions, ADVANCED FUNCTIONAL MATERIALS, Vol: 31, ISSN: 1616-301X
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- Citations: 5
Mesa CA, Rao RR, Francas L, et al., 2020, Reply to: Questioning the rate law in the analysis of water oxidation catalysis on haematite photoanodes, NATURE CHEMISTRY, Vol: 12, Pages: 1099-+, ISSN: 1755-4330
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- Citations: 8
Kawaguchi T, Rao RR, Lunger JR, et al., 2020, Stern layers on RuO<sub>2</sub> (100) and (110) in electrolyte: Surface X-ray scattering studies, JOURNAL OF ELECTROANALYTICAL CHEMISTRY, Vol: 875, ISSN: 1572-6657
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- Citations: 4
Rao RR, Kolb MJ, Giordano L, et al., 2020, Operando identification of site-dependent water oxidation activity on ruthenium dioxide single-crystal surfaces, NATURE CATALYSIS, Vol: 3, Pages: 516-525, ISSN: 2520-1158
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- Citations: 137
Escudero-Escribano M, Biegel CM, Kamat PV, 2020, Women Scientists at the Forefront of Energy Research: A Virtual Issue, Part 2, ACS ENERGY LETTERS, Vol: 5, Pages: 623-633, ISSN: 2380-8195
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- Citations: 1
Hwang J, Feng Z, Charles N, et al., 2019, Tuning perovskite oxides by strain: Electronic structure, properties, and functions in (electro)catalysis and ferroelectricity, MATERIALS TODAY, Vol: 31, Pages: 100-118, ISSN: 1369-7021
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- Citations: 140
Mezzavilla S, Katayama Y, Rao R, et al., 2019, Activity-or Lack Thereof-of RuO2-Based Electrodes in the Electrocatalytic Reduction of CO2, JOURNAL OF PHYSICAL CHEMISTRY C, Vol: 123, Pages: 17765-17773, ISSN: 1932-7447
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