Water Oxidation Intermediates Research Articles

Direct propylene epoxidation via water activation over Pd-Pt electrocatalysts.

Direct electrochemical propylene epoxidation by means of water-oxidation intermediates presents a sustainable alternative to existing routes that involve hazardous chlorine or peroxide reagents. We report an oxidized palladium-platinum alloy catalyst (PdPtOx/C), which reaches a Faradaic efficiency of 66 ± 5% toward propylene epoxidation at 50 milliamperes per square centimeter at ambient temperature and pressure. Embedding platinum into the palladium oxide crystal structure stabilized oxidized platinum species, resulting in improved catalyst performance. The reaction kinetics suggest that epoxidation on PdPtOx/C proceeds through electrophilic attack by metal-bound peroxo intermediates. This work demonstrates an effective strategy for selective electrochemical oxygen-atom transfer from water, without mediators, for diverse oxygenation reactions.

A dinuclear nickel peroxycarbonate complex: CO2 addition promotes H2O2 release.

Nickel coordination compounds featuring Ni-O bonds are key structural motifs in both bioinorganic and synthetic chemistries. They serve as precursors for organic substrate oxidation and are commonly invoked intermediates in water oxidation and oxygen reduction schemes. Herein, we disclose a series of well-defined dinuclear nickel complexes that, upon treatment with CO2 and H2O2, afford the first nickel-bound peroxycarbonate. This unprecedented nickel-oxygen intermediate is stabilized by hydrogen bonding templated across the bimetallic core. Contrasting copper and iron analogues, the nickel peroxycarbonate reversibly dissociates H2O2, a process that is shown to be accelerated by exogenous CO2.

The mechanism of water oxidation using transition metal-based heterogeneous electrocatalysts.

The water oxidation reaction, a crucial process for solar energy conversion, has garnered significant research attention. Achieving efficient energy conversion requires the development of cost-effective and durable water oxidation catalysts. To design effective catalysts, it is essential to have a fundamental understanding of the reaction mechanisms. This review presents a comprehensive overview of recent advancements in the understanding of the mechanisms of water oxidation using transition metal-based heterogeneous electrocatalysts, including Mn, Fe, Co, Ni, and Cu-based catalysts. It highlights the catalytic mechanisms of different transition metals and emphasizes the importance of monitoring of key intermediates to explore the reaction pathway. In addition, advanced techniques for physical characterization of water oxidation intermediates are also introduced, for the purpose of providing information for establishing reliable methodologies in water oxidation research. The study of transition metal-based water oxidation electrocatalysts is instrumental in providing novel insights into understanding both natural and artificial energy conversion processes.

DFT Electronic Structure and Vibrational Signatures of Intermediates in Water Oxidation on Perovskite NaTaO3

DFT Electronic Structure and Vibrational Signatures of Intermediates in Water Oxidation on Perovskite NaTaO₃

Flash Joule heating to enhance water oxidation of hematite photoanode via mediating with an oxidized carbon overlayer

Flash Joule heating technique as an innovative and promising strategy has been extensively utilized to synthesize various functionalized nanomaterials within a very short time. This work describes a facile synthesis of an oxidized carbon overlayer-mediated hematite photoanode through a simple NaH2PO2-immersion followed by a flash Joule heating treatment. It discloses that on account of the high conductivity and abundant oxygen-functionalized carbon groups of carbon overlayer, the surface charge transfer of as-resulted photoanode is greatly facilitated and meanwhile the adsorption strength of water oxidation intermediates on Fe sites is improved. Moreover, the oxidized carbon overlayer could confine sufficient P atoms into the hematite nanostructure to realize P-doping to dramatically promote the bulk charge behaviors. Consequently, benefiting from these multiple effects, an enhanced photocurrent density of 2.07 mA cm−2 at 1.23 V vs. RHE is achieved for the as-resulted photoanode. Further, a comparable photocurrent density of 2.37 mA cm−2 at 1.23 V vs. RHE is yielded after the loading of FeNiOOH cocatalyst. This study, for the first time, demonstrates the flash Joule heating strategy for the post-treatment of hematite photoanode, which opens up a novel and facile avenue for the development of efficient photoelectrodes.

In situ observation of photoelectrochemical water oxidation intermediates for selective biomass upgrading with simultaneous hydrogen production

In a water-splitting reaction, the anodic biomass upgrading with the simultaneous cathodic hydrogen production is a promising sustainable energy development for future needs. In the electrocatalytic water oxidation process, the reactive intermediates generated in the oxygen evolution reaction (OER) can be used for the selective oxidation of organic molecules to value-added chemicals. Herein, we fabricated a multifunctional catalyst, composed of phosphate-functionalized few-layer phosphorene (FLP-P) and bismuth ferrite (BFO) nanosheets (referred to as FLP-P-BFO), for photoelectrochemical (PEC) water-splitting reactions with the advantages of an efficient charge separation and the hybrid excitations of electrons and photons. At the cathode, the PEC-assisted FLP-P-BFO-catalytic reaction exhibits excellent hydrogen evolution reaction (HER) activity of a low overpotential of 110 mV at 10 mA cm−2 and a small Tafel slope of 51 mV dec-1. At the anode, the high FLP-P-BFO-catalytic efficiency is attributed to the bifunctional mechanism, in which the functionalized phosphate groups help to stabilize the Fe-OOH intermediate, thus mitigating the energy demand in the OER process. The bifunctional mechanism was validated by both pH-dependent and isotope-labeling examinations. With the assistance of in situ Raman spectroscopy, the optimal electrochemical conditions for the maximal production of the Fe-OOH intermediate in OER were obtained for the biomass upgrading of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) with 100% conversion and selectivity, but without reducing the HER activity. The anodic biomass conversion, from HMF (a carbon–neutral feedstock) to the high value-added FDCA (a monomer of bio-plastics), with the simultaneous cathodic H2 production is a promising sustainable energy innovation. This novel strategy of employing PEC-assisted FLP-P-BFO-catalytic biomass upgrading to value-added products is extendable to combine the cathodic HER with many other anodic hydrocarbon oxidations for future energy-related applications.

Direct Detection of FeVI Water Oxidation Intermediates in an Aqueous Solution

AbstractIn situ detection of highly‐oxidized metal intermediates is the key to identifying the active center of an oxygen evolution reaction (OER) catalyst, but it remains challenging for NiFe‐based catalysts in an aqueous solution under working conditions. Here, by utilizing the dynamic stability of the FeVIO42− intermediates in a self‐healing water oxidation cycle of NiFe‐based catalyst, the highly‐oxidized FeVI intermediates leached into the electrolyte are directly detected by simple spectroelectrochemistry. Our results provide direct evidence that Fe is the active center in NiFe‐based OER catalysts. Furthermore, it is revealed that the incorporation of Co into NiFe‐based catalyst facilitates the formation of FeVI active species, thus enhancing the OER activity of NiCoFe‐based catalyst. The insights into the mechanisms for the sustainable generation of FeVI active species in these NiFe‐based catalysts lay the foundation for the design of more efficient and stable OER catalysts.

Direct Detection of FeVI Water Oxidation Intermediates in an Aqueous Solution.

In situ detection of highly-oxidized metal intermediates is the key to identifying the active center of an oxygen evolution reaction (OER) catalyst, but it remains challenging for NiFe-based catalysts in an aqueous solution under working conditions. Here, by utilizing the dynamic stability of the FeVI O4 2- intermediates in a self-healing water oxidation cycle of NiFe-based catalyst, the highly-oxidized FeVI intermediates leached into the electrolyte are directly detected by simple spectroelectrochemistry. Our results provide direct evidence that Fe is the active center in NiFe-based OER catalysts. Furthermore, it is revealed that the incorporation of Co into NiFe-based catalyst facilitates the formation of FeVI active species, thus enhancing the OER activity of NiCoFe-based catalyst. The insights into the mechanisms for the sustainable generation of FeVI active species in these NiFe-based catalysts lay the foundation for the design of more efficient and stable OER catalysts.

2D Bismuthene as a Functional Interlayer between BiVO4 and NiFeOOH for Enhanced Oxygen‐Evolution Photoanodes

AbstractBiVO4 has attracted wide attention for oxygen‐evolution photoanodes in water‐splitting photoelectrochemical devices. However, its performance is hampered by electron‐hole recombination at surface states. Herein, partially oxidized two‐dimensional (2D) bismuthene is developed as an effective, stable, functional interlayer between BiVO4 and the archetypal NiFeOOH co‐catalyst. Comprehensive (photo)electrochemical and surface photovoltage characterizations show that NiFeOOH can effectively increase the lifetime of photogenerated holes by passivating hole trap states of BiVO4; however, it is limited in influencing electron trap states related to oxygen vacancies (VO). Loading bismuthene on BiVO4 photoanodes increases the density of VO that are beneficial for the oxygen evolution reaction via the formation of oxy/hydroxyl‐based water oxidation intermediates at the surface. Moreover, bismuthene increases interfacial band bending and fills the VO‐related electron traps, leading to more efficient charge extraction. With the synergistic interaction of bismuthene and NiFeOOH on BiVO4, this composite photoanode achieves a 5.8‐fold increase in photocurrent compared to bare BiVO4 reaching a stable 3.4 (±0.2) mA cm–2 at a low bias of +0.8 VRHE or 4.7(±0.2) mA cm–2 at +1.23 VRHE. The use of 2D bismuthene as functional interlayer provides a new strategy to enhance the performance of photoanodes.

Superlative photoelectrochemical properties of 3D MgCr-LDH nanoparticles influencing towards photoinduced water splitting reactions

In the present work, we report the synthesis of single system three-dimensional (3D) open porous structure of MgCr-LDH nanoparticles in a substrate-free path by using one-step formamide assisted hydrothermal reaction followed by visible light irradiation for significant photoelectrochemical (PEC) properties that manifest towards photocatalytic H2 and O2 production. The as-prepared nanostructured materials were characterized by various physico-chemical characterization techniques. Moreover, this unique synthetic approach produces 3D open porous network structure of MgCr-LDH nanoparticles, which were formed by stacking of numerous 2D nanosheets, for effective light harvestation, easy electronic channelization and unveil superlative PEC properties, including high current density (6.9 mA/cm2), small Tafel slope of 82 mV/decade, smallest arc of the Nyquist plot (59.1 Ω cm−2) and photostability of 6000 s for boosting water splitting activity. In addition, such perfectly self-stacked 2D nanosheets in 3D MgCr-LDH possess more surface active defect sites as enriched 50% oxygen vacancy resulting a good contact surface within the structure for effective light absorption along with easy electron and hole separation, which facilitates the adsorption of protons and intermediate for water oxidation. Additionally, the Cr3+ as dopant pull up the electrons from water oxidation intermediates, thereby displaying superior photocatalytic H2 and O2 production activity of 1315 μmol/h and 579 μmol/h, respectively. Therefore, the open 3D morphological aspects of MgCr-LDH nanoparticles with porous network structure and high surface area possess more surface defect sites for electron channelization and identified as distinct novel features of this kind of materials for triggering significant PEC properties, along with robustly enhance the photocatalytic water splitting performances.

Lowering the Water Oxidation Overpotential by Spin-Crossover in Cobalt Hexacyanoferrate

The oxygen evolution reaction (OER) is limited by the inherent linear scaling relationships of its reaction intermediates. Manipulating the spin configuration of the water oxidation intermediates allows us to overcome these constraints. Cobalt hexacyanoferrate (CoFe-PB) is an efficient and robust water oxidation catalyst and further known as a magnetic switch. Its versatile electronic structure renders it a potential candidate for magnetic tuning of the OER. Herein, we used first-principles density functional theory calculations to describe the OER on two different CoFe-PB model systems and evaluated the possibility for spin-crossover (SCO) of their resting states. We show that SCO during OER can significantly lower the overpotential by 0.7 V, leading to an overpotential of around 0.3 V, which is in agreement with the experimentally measured value. Applying an external potential >1.5 V vs SHE, the SCO-assisted pathway becomes largely favored and most likely the predominant reaction pathway.

Polymer Modification of Surface Electronic Properties of Electrocatalysts.

Finding alternative ways to tailor the electronic properties of a catalyst to actively and selectively drive reactions of interest has been a growing research topic in the field of electrochemistry. In this Letter, we investigate the tuning of the surface electronic properties of electrocatalysts via polymer modification. We show that when a nickel oxide water oxidation catalyst is coated with polytetrafluoroethylene, stable Ni–CFx bonds are introduced at the nickel oxide/polymer interface, resulting in shifting of the reaction selectivity away from the oxygen evolution reaction and toward hydrogen peroxide formation. It is shown that the electron-withdrawing character of the surface fluorocarbon molecule leaves a slight positive charge on the water oxidation intermediates at the adjacent active nickel sites, making their bonds weaker. The concept of modifying the surface electronic properties of an electrocatalyst via stable polymer modification offers an additional route to tune multipathway reactions in polymer/electrocatalyst environments, like with ionomer-modified catalysts or with membrane electrode assemblies.

The Role of Surface States on Reduced TiO2@BiVO4 Photoanodes: Enhanced Water Oxidation Performance through Improved Charge Transfer

The efficient transfer of photogenerated carriers and improved stability against corrosion are essential to maximize the performance of photoanodes. Herein, a reduced catalytic layer formed on a TiO2 protected BiVO4 (R-TiO2@BiVO4) photoanode has been prepared for progress on both fronts. Specifically, R-TiO2@BiVO4 photoanodes at pH 8 displayed a high photocurrent of 2.1 mA cm–2 at 1.23 VRHE and a more negative onset potential of 234 mVRHE compared to pristine BiVO4. We here discovered two surface states on BiVO4 photoanodes through photoelectrochemical impedance studies. In contrast, only one of them, located at higher potential, appeared on oxygen-vacancy-rich R-TiO2@BiVO4 photoanodes. For BiVO4 photoanodes, the first surface state (SS1) is located near the onset potential (∼0.45 VRHE), while the second surface state (SS2) sits near the water oxidation potential (∼1.05 VRHE). However, SS1 at lower energetics, which originated from water oxidation intermediates with slow kinetics, is passivated in R-TiO2@BiVO4 photoanodes. In contrast, the hole densities in SS2 at higher energetics were greatly enhanced in R-TiO2@BiVO4 photoanodes, due to the increased accumulation of intermediates with fast water oxidation kinetics. Therefore, SS2 is proposed as a reaction center, which is related to the amount and occupancy of oxygen vacancies. Additionally, surface recombination centers in BiVO4 photoanodes are passivated by TiO2, which prevents electron trapping into the irreversible surface conversion of VO2+ to VO2+. These observations provide fundamental understanding of the role of surface states and of the function of oxygen vacancies in BiVO4 photoanodes. Our study offers detailed insight into key strategies for optimal photoelectrochemical performance through surface property tuning.

Construction of nickel- and iron-coordinated poly(5-amino-1,10-phenanthroline) film for electrocatalytic water oxidation reactions

Metalation of the electro-polymerized 5-amino-1,10-phenanthroline film (poly(5Aphen)) is carried out in methanol solutions containing nickel and iron ions, and the characterization results suggest the formation of metal complexes. The metal ion-coordinated films are able to electrochemically catalyze water oxidation reaction (WOR) in alkaline solutions. The most active electrode (Ni0.55Fe0.45-poly(5Aphen)/CP) exhibits only 264 mV overpotentials to reach 10 mA cm−2 water oxidation current densities, and is robust towards long-term galvanostatic test. Cyclic voltammetry suggests the existence of electron interactions between Fe3+ centers and the adjacent Ni2+ centers. Electrokinetic studies indicate that the presence of Fe3+ increases the adsorption energies of the water oxidation intermediates OH*, which leads to enhanced WOR catalytic activities. Ex-situ characterizations after the prolonged electrolysis suggest that the formation of amorphous metal hydroxides that are also active towards WOR. This simple preparation method can also be applied to prepare other transition metal ion-coordinated poly(5Aphen) for electrocatalysis.

Observation of a potential-dependent switch of water-oxidation mechanism on Co-oxide-based catalysts

Summary O–O bond formation is a key elementary step of the water-oxidation reaction. However, it is still unclear how the mechanism of O–O coupling depends on the applied electrode potential. Herein, using water-in-salt electrolytes, we systematically altered the water activity, which enabled us to probe the O–O bond-forming mechanism on heterogeneous Co-based catalysts as a function of applied potential. We discovered that the water-oxidation mechanism is sensitive to the applied potential: At relatively low driving force, the reaction proceeds through an intramolecular oxygen coupling mechanism, whereas the water nucleophilic attack mechanism prevails at high driving force. The observed mechanistic switch has major implications for the understanding and control of the water-oxidation reaction on heterogeneous catalysts.

Capturing Manganese Oxide Intermediates in Electrochemical Water Oxidation at Neutral pH by In Situ Raman Spectroscopy.

Electrochemical water splitting is a promising means to produce eco-friendly hydrogen fuels. Inspired by the Mn4 CaO5 cluster in nature, substantial works have been performed to develop efficient manganese (Mn)-based heterogeneous catalysts. Despite improvements in catalytic activity, the underlying mechanism of the oxygen evolution reaction (OER) is not completely elucidated owing to the lack of direct spectroscopic evidence for the active Mn-oxo moieties. We identify water oxidation intermediates on the surface of Mn3 O4 nanoparticles (NPs) in the OER at neutral pH by in situ Raman spectroscopy. A potential-dependent Raman peak was detected at 760 cm-1 and assigned to the active MnIV =O species generated during water oxidation. Isotope-labeling experiments combined with scavenger experiments confirmed the generation of surface terminal MnIV =O intermediates in the Mn-oxide NPs. This study provides an insight into the design of systems for the observation of reaction intermediates.

Capturing Manganese Oxide Intermediates in Electrochemical Water Oxidation at Neutral pH by In Situ Raman Spectroscopy

AbstractElectrochemical water splitting is a promising means to produce eco‐friendly hydrogen fuels. Inspired by the Mn4CaO5 cluster in nature, substantial works have been performed to develop efficient manganese (Mn)‐based heterogeneous catalysts. Despite improvements in catalytic activity, the underlying mechanism of the oxygen evolution reaction (OER) is not completely elucidated owing to the lack of direct spectroscopic evidence for the active Mn‐oxo moieties. We identify water oxidation intermediates on the surface of Mn3O4 nanoparticles (NPs) in the OER at neutral pH by in situ Raman spectroscopy. A potential‐dependent Raman peak was detected at 760 cm−1 and assigned to the active MnIV=O species generated during water oxidation. Isotope‐labeling experiments combined with scavenger experiments confirmed the generation of surface terminal MnIV=O intermediates in the Mn‐oxide NPs. This study provides an insight into the design of systems for the observation of reaction intermediates.

Hydrophobic/Hydrophilic Directionality Affects the Mechanism of Ru-Catalyzed Water Oxidation Reaction

From the study of supramolecular dimers of [RuVO(pda)]+ (pda = 1,10-phenanthroline-2,9-dicarboxylic acid) and [RuVO(bda)]+ (bda = 2,2′-bipyridine-6,6′-dicarboxylate) complexes, the O–O bond-forming intermediates in water oxidation, we found orientational distinction induced by the pda and bda ligands. The bda complex prefers the front-to-front geometry, while the pda complex favors the front-to-back geometry in the formation of prereactive geometry. In the bda complex, the hydrophobic oxo will point at another oxo with the bda directed toward water, which favors the I2M mechanism. In the pda complex, the hydrophobic oxo instead is directed toward a more hydrophobic phenanthroline moiety of the pda of another species, which disfavors I2M. The binding free energy of the nonproductive front-to-back of pda is 3 kcal mol–1 more stable than that of the prereactive dimer. This incorrect orientation leads to an additional rearrangement required before the O–O bond can be formed. Estimation of the rate constant shows 2 orders of magnitude lower reactivity for the I2M mechanism of the pda complex relative to the bda complex, which makes the water nucleophilic attack mechanism competitive.

Theoretical Insight into the Role of Defects and Facets in the Selectivity of Products in Water Oxidation over Bismuth Vanadate (BiVO4)

Bismuth vanadate (BiVO₄) is an efficient photocatalyst or photoanode in water oxidation. However, the product selectivity in water oxidation over the most exposed clean and defective (001) and (101) facets is not clear. In this work, we investigated the water oxidation reaction on clean and oxygen deficient (001) and (101) facets and on Mo/W-doped facets using density functional theory. The pure (001) facet, we found, promotes OH radical formation. In the presence of oxygen vacancies, the O₂ evolution is preferred on the (101) facet with an overpotential of 0.52 V. On doping with Mo/W, the H₂O₂ evolution is preferred on the (101) facet with the overpotential of about 0.40 V; while on the (001) facet, the O₂ evolution is preferred with the overpotential of about 0.5 V. We also found that, on the (101) facet, there is a strong charge transfer from Bi atoms to the intermediates of water oxidation; but on the (001) facet, there is not. Our calculations could guide the design of the photocatalyst toward specific products of the water oxidation reaction, such as an OH radical, H₂O₂, or O₂. Moreover, the results show that the preferred sorption on the (101) facet may be a reason for the photoinduced charge separation between (101) and (001) facets of BiVO₄.

Selecting between two transition states by which water oxidation intermediates decay on an oxide surface

Although catalytic mechanisms on electrode surfaces have been proposed for decades, the pathways by which the product’s chemical bonds evolve from the initial charge-trapping intermediates have not been resolved. Here, we discover a reactive intermediate population with states in the middle of a semiconductor’s bandgap that reveal the dynamics of two parallel transition state pathways for their decay. After phototriggering the water oxidation reaction from the n-SrTiO3 surface, the microsecond decay of the intermediates affirms transition state theory through two distinct time constants, the primary kinetic salt and H/D kinetic isotope effects, realistic activation barrier heights and transition state theory pre-factors. Furthermore, we show that the reaction conditions can be adjusted to allow selection between the two pathways, one characterized by a labile intermediate facing the electrolyte (the oxyl), and the other by a lattice oxygen (the bridge). In summary, we experimentally isolate an important activation barrier in multi-electron transfer water oxidation and, in doing so, identify competing mechanisms for O2 evolution at surfaces. Elucidating reaction mechanisms on electrode surfaces is of utmost importance. Now, using canonical transition state theory, Cuk and colleagues show the competing pathways by which the charge-trapping intermediates of the water oxidation reaction on n-SrTiO3 decay towards the next step in the reaction.