Steps Of Water Oxidation Research Articles

Ligand effect on switching the rate-determining step of water oxidation in atomically precise metal nanoclusters

The ligand effects of atomically precise metal nanoclusters on electrocatalysis kinetics have been rarely revealed. Herein, we employ atomically precise Au25 nanoclusters with different ligands (i.e., para-mercaptobenzoic acid, 6-mercaptohexanoic acid, and homocysteine) as paradigm electrocatalysts to demonstrate oxygen evolution reaction rate-determining step switching through ligand engineering. Au25 nanoclusters capped by para-mercaptobenzoic acid exhibit a better performance with nearly 4 times higher than that of Au25 NCs capped by other two ligands. We deduce that para-mercaptobenzoic acid with a stronger electron-withdrawing ability establishes more partial positive charges on Au(I) (i.e., active sites) for facilitating feasible adsorption of OH– in alkaline media. X-ray photo-electron spectroscopy and theoretical study indicate a profound electron transfer from Au(I) to para-mercaptobenzoic acid. The Tafel slope and in situ Raman spectroscopy suggest different ligands trigger different rate-determining step for these Au25 nanoclusters. The mechanistic insights reported here can add to the acceptance of atomically precise metal nanoclusters as effective electrocatalysts.

O-hetero-Fe-N3.6 active sites in ZIF-derived carbon nanotubes for the electrocatalytic oxygen evolution reaction

In order to clarify competitive reactions between lattice oxygen oxidation mechanism (LOM) with lower energy barrier and conventional adsorbate evolution mechanism (AEM), O-hetero-Fe-N3.6 active site was built in ZIF-derived carbon nanotubes to transform traditional AEM mechanism on iron-unsaturated active site into LOM mechanism during electrocatalytic oxygen evolution reaction (OER). In-situ attenuated total reflectance infrared (ATR-FTIR) and X-ray absorption fine structure results confirmed the formation of Fe-O bonds and a Fe-N3.6 structure, indicating that O-hetero-Fe-N3.6 active sites were built. A key intermediate O* was detected on ATR-FTIR, which suggests that OER proceeded via LOM mechanism. The alkaline OER activity and stability are enhanced on the O-hetero-Fe-N3.6 active sites to accelerate rate-determining step of water oxidation. The ZIF-derived carbon nanotubes expose more O-hetero-Fe-N3.6 active sites to facilitate electron and mass transfer during electrocatalytic OER. The Fe-NC catalyst exhibits excellent OER activity with an overpotential of 235 mV at 10 mA cm−2 and Tafel slope of 98 mV dec−1 in 1 M KOH.

Rational Design of a Low-Dimensional and Metal-free Heterostructure for Efficient Water Oxidation: DFT and Experimental Studies.

A nitrogen-doped fullerene dimer is synthesized and compounded with multi-walled carbon nanotubes (MWCNTs) to construct a low-dimensional and metal-free 0D-1D heterostructure for electrocatalytic water oxidation. The (C59N)2/MWCNTs heterostructure exhibits a highly efficient performance, as verified by both first-principles density functional theory and experimental studies. The *O → *OOH process is confirmed as the rate-determining step of water oxidation. The negatively charged N-doping leads to electronic redistribution and intermolecular charge transfer and thus reduces the uphill free energies of intermediates on the (C59N)2/MWCNTs interface. Therefore, the (C59N)2/MWCNTs heterostructure has great potential to emit light and heat in metal-free-based electrocatalytic water oxidation.

Heat‐Triggered Ferri‐to‐Paramagnetic Transition Accelerates Redox Couple‐Mediated Electrocatalytic Water Oxidation

AbstractRedox couple oxidation as an initial step of water oxidation may be key for high electricity consumption in the electrochemical oxygen evolution reaction (OER). Here, a heat‐induced magnetic transition strategy is reported to speed up the oxidation kinetics of redox couples. The activation energy of Ni2+/Ni3+ redox couple oxidation is sharply decreased by heating the Ni0.67Fe0.33OxHy catalyst above a Curie temperature (Tc) of 70 °C. In such a strategy, heat instead of electricity drives the spin flipping of Ni2+/Ni3+ oxidation through heat‐sensitive ferri‐to‐paramagnetic spin state changes. As a result of magnetic transition‐assisted efficient heat‐electricity coupling, Ni0.67Fe0.33OxHy exhibits the lowest OER overpotential of 221 mV at 100 mA cm−2 at 90 °C in alkaline electrolytes, outperforming the benchmark IrO2 catalyst. The findings provide new insights into the design of efficient heat–electricity complementary OER devices.

Switching the O–O bond-formation mechanism by controlling water activity

Switching the O–O bond-formation mechanism by controlling water activity

Tuning the Proton-Coupled Electron-Transfer Rate by Ligand Modification in Catalyst-Dye Supramolecular Complexes for Photocatalytic Water Splitting.

In view of the considerably high activation energy barrier of the O−O bond formation photocatalytic step in water oxidation, it is essential to understand if and how nonadiabatic factors can accelerate the proton‐coupled electron transfer (PCET) rate in this process to find rational design strategies facilitating this step. Herein, constrained ab initio molecular dynamics simulations are performed to investigate this rate‐limiting step in a series of catalyst‐dye supramolecular complexes functionalized with different alkyl groups on the catalyst component. These structural modifications lead to tunable thermodynamic driving forces, PCET rates, and vibronic coupling with specific resonant torsional modes. These results reveal that such resonant coupling between electronic and nuclear motions contributes to crossing catalytic barriers in PCET reactions by enabling semiclassical coherent conversion of a reactant into a product. Our results provide insight on how to engineer efficient catalyst‐dye supramolecular complexes by functionalization with steric substituents for high‐performance dye‐sensitized photoelectrochemical cells.

Proton Acceptor near the Active Site Lowers Dramatically the O-O Bond Formation Energy Barrier in Photocatalytic Water Splitting.

The O–O bond formation process via water nucleophilic attack represents a thermodynamic and kinetic bottleneck in photocatalytic water oxidation because of the considerably high activation free energy barrier. It is therefore of fundamental significance and yet challenging to find strategies to facilitate this reaction. The microscopic details of the photocatalytic water oxidation step involving the O–O bond formation in a catalyst–dye supramolecular complex are here elucidated by density functional theory-based Car–Parrinello molecular dynamics simulations in the presence of an extra proton acceptor. Introducing a proton acceptor group (OH–) in the hydration shell near the catalytic active site accelerates the rate-limiting O–O bond formation by inducing a cooperative event proceeding via a concerted proton-coupled electron-transfer mechanism and thus significantly lowering the activation free energy barrier. The in-depth insight provides a strategy for facilitating the photocatalytic water oxidation and for improving the efficiency of dye-sensitized photoelectrochemical cells.

Switch of the Rate-Determining Step of Water Oxidation by Spin-Selected Electron Transfer in Spinel Oxides

The 3d transition-metal oxides are a class of promising catalysts for the oxygen evolution reaction (OER) in alkaline media. Manganese-based spinel oxides usually do not show impressive OER perform...

Insights into Proton-Transfer Pathways during Water Oxidation in Photosystem II.

Water oxidation by photosystem II (PSII) involves the release of O2, electrons, and protons at the oxygen-evolving complex (OEC). These processes are facilitated by a hydrogen-bonded network of amino acid residues and waters surrounding the OEC. It is crucial to probe the proton-transfer pathways from the OEC as proton release helps to maintain the charge balance required for efficient water oxidation. In this study, we generate point mutations in the cyanobacterium Synechocystis sp. PCC 6803 at secondary-shell amino acid residues surrounding the OEC: D2-K317, D1-S169, CP43-R357, D1-D61, and D1-N181. We employ direct experimental methods to study the O2 evolution rate under varying pH ranging from 3-8. The pH dependence follows a bell-shaped curve in both wild-type and mutated PSII from which we can derive the effective acidic pKa. The effective acidic pKa provides insights into the protonation states of the amino acid residues participating in the proton-transfer process during the rate-determining step of water oxidation. The presence of an additional effective pKa in D1-S169A PSII and D2-K317A PSII indicates the possibility of multiple proton-transfer pathways during the rate-determining step of water oxidation. We also studied the O2 evolution rate in H2O and D2O with varying pL (L = H or D) to identify the amino acid residues participating in the proton-transfer process. We find that replacing the positively charged lysine with a neutral alanine in D2-K317A PSII and aspartate with alanine in D1-D61A PSII significantly enhances the kinetic solvent isotope effect (KSIE), indicating that proton transfer becomes rate-limiting at the optimal pH in these mutated PSII. However, the KSIE remains unchanged for D1-N181A, D1-S169A, and CP43-R357K PSII. Thus, perturbing the channel defined by the D1-D61 and D2-K317 residues strongly hampers the proton-transfer mechanism, and in turn, the water oxidation reaction of PSII. Hence, our study provides a direct experimental probe to identify that the D1-D61 and D2-K317 residues participate in the proton-transfer process. These results, thereby, provide us a deeper understanding of the proton-transfer processes in the water oxidation mechanism.

Evidence of two-state reactivity in water oxidation catalyzed by polyoxometalate-based complex [Mn3(H2O)3(SbW9O33)2]12−

In this work, the key OO bond formation step in water oxidation catalyzed by a polyoxometalate-based complex [Mn3(H2O)3(SbW9O33)2]12− was theoretically investigated. The MnV-oxo species rather than MnIV-oxo species, is computed to be responsible for OO bond formation via water nucleophilic attack mechanism. During OO bond formation based on MnV-oxo species, two-state reactivity is found and the energy profile switches from the 13et state to the 15et state through a minimum energy crossing point, which significantly reduces the activation barrier. The observed spin inversion phenomenon in OO bond formation is attributed to the exchange-enhanced reactivity for 15et state, but not for 13et state.

Computational Characterization of Single-Electron Transfer Steps in Water Oxidation

The presence of single-electron transfer (SET) steps in water oxidation processes catalyzed by first-row transition metal complexes has been recently recognized, but the computational characterization of this type of process is not trivial. We report a systematic theoretical study based on density functional theory (DFT) calculations on the reactivity of a specific copper complex active in water oxidation that reacts through two consecutive single-electron transfers. Both inner-sphere (through transition state location) and outer-sphere (through Marcus theory) mechanisms are analyzed. The first electron transfer is found to operate through outer-sphere, and the second one through inner-sphere. The current work proposes a scheme for the systematic study of single-electron transfer in water oxidation catalysis and beyond.

Mechanistic Insight into Dioxygen Evolution from Diastereomeric μ-Peroxo Dinuclear Co(III) Complexes Based on Stoichiometric Electron-Transfer Oxidation.

Stoichiometric electron-transfer (ET) oxidation of two diastereomeric μ-peroxo-μ-hydroxo dinuclear Co(III) complexes with tris(2-pyridylmethyl)amine (TPA) was examined to scrutinize the reaction mechanism of O2 evolution from the peroxo complexes, as seen in the final step in water oxidation by a Co(III)-TPA complex. The two isomeric Co(III)-peroxo complexes were synthesized and selectively isolated by recrystallization under different conditions. Although cyclic voltammograms of the two isomers in aqueous solutions showed one reversible wave at 1.1 V vs NHE at pH 2.0, two oxidation waves were observed at 1.0 and 1.4 V at pH 7.0 in the aqueous solutions, the latter of which is responsible for the O2-releasing process. At pH 7, one diastereomer showed higher reactivity than the other in O2 evolution, indicating the importance of structures of the μ-peroxo complexes in the reaction. In order to clarify the O2-evolving mechanism, we performed electron paramagnetic resonance (EPR) and resonance Raman (RR) measurements for characterizing one-electron oxidized species: The observed EPR and RR signals supported the formation of μ-superoxo-μ-hydroxo dinuclear Co(III) complexes; however, no characteristic difference was observed between two isomers in the EPR parameters including g values and superhyperfine coupling constants. ET-oxidation rate constants of the isomers were determined to be much faster than the O2-evolving rate constants, indicating that the O2-releasing step is the rate-determining step in the O2 evolution through the stoichiometric ET oxidation of the dinuclear Co(III)-μ-peroxo complexes. Therefore, the difference of reactivity in the O2 evolution for the two isomers should be derived from the thermodynamic stability of two-electron oxidized species of the dinuclear Co(III)-μ-peroxo complexes, μ-dioxygen-μ-hydroxo dinuclear Co(III) intermediates.

DFT characterization on the effect of redox-inactive cation Ca2+ on water oxidation by CoII-based cuboidal catalyst

The activity of the biomimetic CoII-based cuboidal water oxidation catalyst (WOC) [CoII4(hmp)4(μ-OAc)2(μ2-OAc)2(H2O)] ([CoII4O4]) and its Ca2+-substitute [CoII3CaO4] were theoretically investigated to evaluate the effect of redox-inactive Ca2+ on water oxidation by CoII-based cuboidal catalyst. Compared with [CoII4O4], the smaller redox potential of [CoII3CaO4] results in the more efficient oxidation-equivalents accumulation for water oxidation. By contrast, the water oxidation by [CoII3CaO4] is identified to be more thermodynamically superior than that for [CoII4O4]. The energy barrier of key OO bond formation step in water oxidation by [CoII3CaO4] is 22.0 kcal/mol and lower than that found for [CoII4O4] (25.0 kcal/mol).

An Overview of Significant Achievements in Ruthenium-Based Molecular Water Oxidation Catalysis

Fossil fuels (coal, oil, natural gas) are becoming increasingly disfavored as long-term energy options due to concerns of scarcity and environmental consequences (e.g., release of anthropogenic CO2). Hydrogen gas, on the other hand, has gained popularity as a clean-burning fuel because the only byproduct from its reaction with O2 is H2O. In recent decades, hydrogen derived from water splitting has been a topic of extensive research. The bottleneck of the water splitting reaction is the difficult water oxidation step (2H2O → O2 + 4H+ + 4e−), which requires an effective and robust catalyst to overcome its high kinetic barrier. Research in water oxidation by molecular ruthenium catalysts enjoys a rich history spanning nearly 40 years. As the diversity of novel ligands continues to widen, the relationship between ligand geometry or electronics, and catalyst activity is undoubtedly becoming clearer. The present review highlights, in the authors’ opinion, some of the most impactful discoveries in the field and explores the evolution of ligand design that has led to the current state of the art.

Oxygen-Oxygen Bond Cleavage and Formation in Co(II)-Mediated Stoichiometric O2 Reduction via the Potential Intermediacy of a Co(IV) Oxyl Radical.

In reactions of significance to alternative energy schemes, metal catalysts are needed to overcome kinetically and thermodynamically difficult processes. Often, high-oxidation-state, high-energy metal oxo intermediates are proposed as mediators in elementary steps involving O-O bond cleavage and formation, but the mechanisms of these steps are difficult to study because of the fleeting nature of these species. Here we utilized a novel dianionic pentadentate ligand system that enabled a detailed mechanistic investigation of the protonation of a cobalt(III)-cobalt(III) peroxo dimer, a known intermediate in oxygen reduction catalysis to hydrogen peroxide. It was shown that double protonation occurs rapidly and leads to a low-energy O-O bond cleavage step that generates a Co(III) aquo complex and a highly reactive Co(IV) oxyl cation. The latter was probed computationally and experimentally implicated through chemical interception and isotope labeling experiments. In the absence of competing chemical reagents, it dimerizes and eliminates dioxygen in a step highly relevant to O-O bond formation in the oxygen evolution step in water oxidation. Thus, the study demonstrates both facile O-O bond cleavage and formation in the stoichiometric reduction of O2 to H2O with 2 equiv of Co(II) and suggests a new pathway for selective reduction of O2 to water via Co(III)-O-O-Co(III) peroxo intermediates.

O-O Radical Coupling: From Detailed Mechanistic Understanding to Enhanced Water Oxidation Catalysis.

A deeper mechanistic understanding of the key O-O bond formation step of water oxidation by the [Ru(bda)(L)2] (bdaH2 = 2,2'-bipyridine-6,6'-dicarboxylic acid; L is a pyridine or isoquinoline derivative) family of catalysts is reached through harmonious experimental and computational studies of two series of modified catalysts with systematic variations in the axial ligands. The introduction of halogen and electron-donating substituents in [Ru(bda)(4-X-py)2] and [Ru(bda)(6-X-isq)2] (X is H, Cl, Br, and I for the pyridine series and H, F, Cl, Br, and OMe for the isoquinoline series) enhances the noncovalent interactions between the axial ligands in the transition state for the bimolecular O-O coupling, resulting in a lower activation barrier and faster catalysis. From detailed transition state calculations in combination with experimental kinetic studies, we find that the main contributor to the free energy of activation is entropy due to the highly organized transition states, which is contrary to other reports. Previous work has considered only the electronic influence of the substituents, suggesting electron-withdrawing groups accelerate catalysis, but we show that a balance between polarizability and favorable π-π interactions is the key, leading to rationally devised improvements. Our calculations predict the catalysts with the lowest Δ G⧧ for the O-O coupling step to be [Ru(bda)(4-I-py)2] and [Ru(bda)(6,7-(OMe)2-isq)2] for the pyridine and isoquinoline families, respectively. Our experimental results corroborate these predictions: the turnover frequency for [Ru(bda)(4-I-py)2] (330 s-1) is a 10-fold enhancement with respect to that of [Ru(bda)(py)2], and the turnover frequency for [Ru(bda)(6-OMe-isq)2] reaches 1270 s-1, two times faster than [Ru(bda)(isq)2].

Revisiting Tractable Strategies to Determine the Activity/Inactivity of Electrocatalysts towards O2/•OH Production

The measurement of the electroactivity of anode materials toward the generation of hydroxyl radicals or oxygen evolution from water oxidation is overriding to develop novel catalysts with tuned electrocatalytic applications. Thus, this work experimentally revisits chemical and electrochemical characterization techniques with the aims of establishing a comparison of the electrochemical performance of typical IrO2, RuO2, SnO2-Sb, and Boron doped diamond (BDD) electrodes toward H2O oxidation under feasible experimental conditions, and highlighting those conditions where variability cannot be removed. Tafel slopes corrected for ohmic resistance and active areas are 127, 175, 42, 48 mV dec−1 for BDD, SnO2-Sb, IrO2 and RuO2, respectively; while in the same order, the onset potential for water oxidation is: 2.49, 2.29, 1.47, 1.38 V vs SHE, measured at a current density of 0.5 mA cm−2. Additionally, a new interpretation is presented for the analysis of electrochemical impedance spectroscopy (EIS), and its connection with the OER mechanism: in the potential zone where Tafel is applicable is possible to appreciate the dielectric properties of the substrate for non-active anodes and elucidating the variation in the rate-determining step of water oxidation. EIS results suggest the possibility to use the time-constant associated with this stage as a parameter of the active or non-active behavior of the above electrodes. The advantages and drawbacks of different trapping molecules used to detect the •OH generation in electrocatalysis is discussed. The photoluminescence of coumarin could be used as a fast and reliable method for the evaluation of the electrocatalyst activity toward the •OH production.

Atomically manipulated proton transfer energizes water oxidation on silicon carbide photoanodes

Preferential exposure of Si-face of SiC will mechanistically shift the rate limiting step of water oxidation from sluggish proton-coupled electron transfer on C-face to a more energy-favorable electron transfer.

Achieving solar overall water splitting with hybrid photosystems of photosystem II and artificial photocatalysts.

Solar overall water splitting is a promising sustainable approach for solar-to-chemical energy conversion, which harnesses solar irradiation to oxidize water to oxygen and reduce the protons to hydrogen. The water oxidation step is vital but difficult to achieve through inorganic photocatalysis. However, nature offers an efficient light-driven water-oxidizing enzyme, photosystem II (PSII). Here we report an overall water splitting natural-artificial hybrid system, in which the plant PSII and inorganic photocatalysts (for example, Ru/SrTiO3:Rh), coupled with an inorganic electron shuttle [Fe(CN)6(3-)/Fe(CN)6(4-)], are integrated and dispersed in aqueous solutions. The activity of this hybrid photosystem reaches to around 2,489 mol H2 (mol PSII)(-1) h(-1) under visible light irradiation, and solar overall water splitting is also achieved under solar irradiation outdoors. The optical imaging shows that the hybrid photosystems are constructed through the self-assembly of PSII adhered onto the inorganic photocatalyst surface. Our work may provide a prototype of natural-artificial hybrids for developing autonomous solar water splitting system.

Aligning Electronic and Protonic Energy Levels of Proton‐Coupled Electron Transfer in Water Oxidation on Aqueous TiO2

AbstractThe high overpotential in water oxidation on anodes is a limiting factor for the large‐scale application of photoelectrochemical cells. To overcome this limitation, it is essential to understand the four proton‐coupled electron transfer (PCET) steps in the reaction mechanism and their implications to the overpotential. Herein, a simple scheme to compute the energies of the PCET steps in water oxidation on the aqueous TiO2 surface using a hybrid density functional is described. An energy level diagram for fully decoupled electron‐ and proton‐transfer reactions in which both electronic and protonic levels are placed on the same potential scale is also described. The level diagram helps to visualize the electronic and protonic components of the overpotential, and points out what are needed to improve. For TiO2, it is found that its catalytic activity is due to aligning the protonic energy levels in the PCET steps, while improving the activity requires also aligning the electronic levels.