Time-resolved observations of water oxidation intermediates on a cobalt oxide nanoparticle catalyst

H/D Isotope Effects Reveal Factors Controlling Catalytic Activity in Co-Based Oxides for Water Oxidation

Understanding the mechanism for electrochemical water oxidation is important for the development of more efficient catalysts for artificial photosynthesis. A basic step is the proton-coupled electron transfer, which enables accumulation of oxidizing equivalents without buildup of a charge. We find that substituting deuterium for hydrogen resulted in an 87% decrease in the catalytic activity for water oxidation on Co-based amorphous-oxide catalysts at neutral pH, while ¹⁶O-to-¹⁸O substitution lead to a 10% decrease. In situ visible and quasi-in situ X-ray absorption spectroscopy reveal that the hydrogen-to-deuterium isotopic substitution induces an equilibrium isotope effect that shifts the oxidation potentials positively by approximately 60 mV for the proton coupled Co^II/III and Co^III/IV electron transfer processes. Time-resolved spectroelectrochemical measurements indicate the absence of a kinetic isotope effect, implying that the precatalytic proton-coupled electron transfer happens through a stepwise mechanism in which electron transfer is rate-determining. An observed correlation between Co oxidation states and catalytic current for both isotopic conditions indicates that the applied potential has no direct effect on the catalytic rate, which instead depends exponentially on the average Co oxidation state. These combined results provide evidence that neither proton nor electron transfer is involved in the catalytic rate-determining step. We propose a mechanism with an active species composed by two adjacent Co^IV atoms and a rate-determining step that involves oxygen-oxygen bond formation and compare it with models proposed in the literature.

Vibrational sum-frequency generation spectroscopy of electrode surfaces: studying the mechanisms of sustainable fuel generation and utilisation

The electrocatalytic oxidation of water coupled to the reduction of carbon dioxide, to make carbon based products, or the reduction of protons to provide hydrogen, offers a sustainable route to generating useful fuels.

Synthesis from a layered double hydroxide precursor for a highly efficient oxygen evolution reaction

A hybrid of Co₃Se₄ and FeSe₂ prepared via a facile hydrothermal method achieves an efficient OER activity during water splitting.

Ultrafast probes at the interfaces of solar energy conversion materials

Transient reflection, photoreflectance and attenuated total reflection spectroscopy are developed to understand the ultrafast interfacial dynamics of solar conversion materials.

Quantitative in situ13C NMR studies of the electro-catalytic oxidation of ethanol

The newly developed pouch cells offer a sensitive method to analyse various products of electrocatalytic reactions, especially of the alcohol oxidation reaction.

Reactive Cobalt⁻Oxo Complexes of Tetrapyrrolic Macrocycles and N-based Ligand in Oxidative Transformation Reactions

High-valent cobalt–oxo complexes are reactive transient intermediates in a number of oxidative transformation processes e.g., water oxidation and oxygen atom transfer reactions. Studies of cobalt–oxo complexes are very important for understanding the mechanism of the oxygen evolution center in natural photosynthesis, and helpful to replicate enzyme catalysis in artificial systems. This review summarizes the development of identification of high-valent cobalt–oxo species of tetrapyrrolic macrocycles and N-based ligands in oxidation of organic substrates, water oxidation reaction and in the preparation of cobalt–oxo complexes.

Enhanced Photoelectrochemical Water Oxidation Performance on BiVO4 by Coupling of CoMoO4 as a Hole-Transfer and Conversion Cocatalyst

Manipulation of interfacial charge separation and transfer is one of the primary breakthroughs to improve the water oxidation activity and stability of BiVO₄ photoanode. In the present work, a CoMoO₄-coupled BiVO₄ (BiVO₄/CoMoO₄) film was designed and prepared as the photoanode for photoelectrochemical (PEC) water oxidation. Compared with the bare BiVO₄ film, obviously improved PEC water oxidation performance was observed on the BiVO₄/CoMoO₄ film. Specifically, a higher water oxidation photocurrent density of 3.04 mA/cm² at 1.23 V versus RHE was achieved on the BiVO₄/CoMoO₄ photoanode, which is of about 220% improvement over bare BiVO₄ photoanode (1.34 mA/cm² at 1.23 V vs RHE). In addition, the BiVO₄/CoMoO₄ film photoanode was of better stability and faster hole-to-oxygen kinetics for water oxidation, without significant activity attenuation for 6 h of reaction at 0.65 V versus RHE. The enhanced water oxidation performance on the BiVO₄/CoMoO₄ film photoanode can be ascribed to the synergistic effect of the following factors: (i) thermodynamically, the photogenerated holes of BiVO₄ are directionally transferred to CoMoO₄ through their physical coupling interface and valance band potential matching; and (ii) kinetically, the transferred holes induce the formation of Co³⁺-active sites on CoMoO₄ that could synergistically oxidize H₂O to molecular O₂ with stable activity.

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.

Direct oxygen isotope effect identifies the rate-determining step of electrocatalytic OER at an oxidic surface

Understanding the mechanism of water oxidation to dioxygen represents the bottleneck towards the design of efficient energy storage schemes based on water splitting. The investigation of kinetic isotope effects has long been established for mechanistic studies of various such reactions. However, so far natural isotope abundance determination of O₂ produced at solid electrode surfaces has not been applied. Here, we demonstrate that such measurements are possible. Moreover, they are experimentally simple and sufficiently accurate to observe significant effects. Our measured kinetic isotope effects depend strongly on the electrode material and on the applied electrode potential. They suggest that in the case of iron oxide as the electrode material, the oxygen evolution reaction occurs via a rate-determining O−O bond formation via nucleophilic water attack on a ferryl unit.

Understanding reaction mechanisms is crucial for catalyst design. Here, natural-abundance isotope quantifications of O₂ yield mechanistically significant reaction kinetic isotope effects for water oxidation over metal oxide electrodes, the bottleneck step of water electrolysis.

Understanding the visible-light photocatalytic activity of GaN:ZnO solid solution: the role of Rh2-y Cr y O3 cocatalyst and charge carrier lifetimes over tens of seconds

A persistent challenge for the widespread deployment of solar fuels is the development of high efficiency photocatalysts combined with a low-cost preparation and implementation route. Since its discovery in 2005, GaN:ZnO solid solution has been a benchmark overall water splitting photocatalyst. Notably, GaN:ZnO functionalised with an appropriate proton reduction cocatalyst is one of the few particulate photocatalyst systems that can generate hydrogen and oxygen directly from water using visible light. However, the reasons underlying the remarkable visible light activity of GaN:ZnO are not well understood and photophysical studies of GaN:ZnO have been limited to date. Using time-resolved optical spectroscopies, we investigated the charge carrier dynamics of GaN:ZnO and the effect of Rh2-y Cr y O3 proton reduction cocatalyst. Here we show that charge trapping and trap state filling play an important role in controlling the photophysics of GaN:ZnO. We also find that electrons transfer to Rh2-y Cr y O3 on sub-microsecond timescales, important to reduce the electron concentration within GaN:ZnO and promote hole accumulation. Operando measurements showed that the water oxidation process is the rate determining process, and that the dependence of the rate of water oxidation on the accumulated hole density is similar to common metal oxides photoanodes such as TiO2, α-Fe2O3, and BiVO4. Remarkably, we show that the recombination timescale of holes accumulated on the surface of GaN:ZnO is on the order of 30 s, distinctly longer than for metal oxides photoanodes. We conclude that the unusual visible light activity of GaN:ZnO is a result of large electron-hole spatial separation due to the preferential flow of holes to the GaN-rich surface and efficient electron extraction by the cocatalyst. Our studies demonstrate that in depth spectroscopic investigations of the charge carrier dynamics of photocatalysts yield important information to understand their behaviour, and identify key properties to deliver outstanding performance.

Catalytic Oxidation of Benzyl Alcohol Using Nanosized Cu/Ni Schiff-Base Complexes and Their Metal Oxide Nanoparticles

In this work, nanosized Cu and Ni Schiff-base complexes, namely ahpvCu, ahpnbCu, and ahpvNi, incorporating imine ligands derived from the condensation of 2-amino-3-hydroxypyridine, with either 3-methoxysalicylaldehyde (ahpv) or 4-nitrobenzaldehyde (ahpnb), were synthesized using sonochemical approach. The structure and properties of the new ligands and their complexes with Ni(II) and Cu(II) were determined via infrared (IR), nuclear magnetic resonance (NMR), electronic spectra (UV-Vis), elemental analysis (CHN), thermal gravimetric analysis (TGA), molar conductivity (Λm), and magnetic moment (μeff). The combined results revealed the formation of 1:1 (metal: ligand) complexes for ahpvCu and ahpvNi and 1:2 for ahpnbCu. Additionally, CuO and NiO nanoparticles were prepared by calcination of the respective nanosized Cu/Ni complexes at 500 °C, and characterized by powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). Significantly, the as-prepared nanosized Schiff-base Cu/Ni complexes and their oxides showed remarkable catalytic activity towards the selective oxidation of benzyl alcohol (BzOH) in aqueous H2O2/ dimethylsulfoxide (DMSO) solution. Thus, catalytic oxidation of BzOH to benzaldehyde (BzH) using both ahpvCu complex and CuO nanoparticles in H2O2/DMSO media at 70 °C for 2 h yielded 94% and 98% BzH, respectively, with 100% selectivity.

Two-site H2O2 photo-oxidation on haematite photoanodes

H2O2 is a sacrificial reductant that is often used as a hole scavenger to gain insight into photoanode properties. Here we show a distinct mechanism of H2O2 photo-oxidation on haematite (α-Fe2O3) photoanodes. We found that the photocurrent voltammograms display non-monotonous behaviour upon varying the H2O2 concentration, which is not in accord with a linear surface reaction mechanism that involves a single reaction site as in Eley-Rideal reactions. We postulate a nonlinear kinetic mechanism that involves concerted interaction between adions induced by H2O2 deprotonation in the alkaline solution with adjacent intermediate species of the water photo-oxidation reaction, thereby involving two reaction sites as in Langmuir-Hinshelwood reactions. The devised kinetic model reproduces our main observations and predicts coexistence of two surface reaction paths (bi-stability) in a certain range of potentials and H2O2 concentrations. This prediction is confirmed experimentally by observing a hysteresis loop in the photocurrent voltammogram measured in the predicted coexistence range.

Characterization of peroxo reaction intermediates in the water oxidation process on hematite surfaces

We use density functional theory-based calculations to study structural, electronic, and magnetic properties of two key reaction intermediates on a hematite, [Formula: see text]-Fe₂O₃, photoanode during the solar-driven water splitting reaction. Both intermediates contain an oxygen atom bonded to a surface iron atom. In one case, the adsorbed oxygen also forms a peroxo bond with a lattice oxygen from hematite; in the second case no such bond is formed. Both configurations are energetically equivalent and are related to the overpotential-determining step in the oxygen evolution reaction. The calculated reaction path for the breaking of the peroxo bond shows a barrier of about 0.86 eV for the transformation between the two intermediates. We explain this high barrier with the drastically different electronic and magnetic structure, which we also analyze using maximally localized Wannier functions. Photo-generated electron holes are shown to localize preferentially close to the reaction center at the surface in both configurations. In the case of the oxo species, this localization favors subsequent electron transfer steps during the oxygen evolution cycle. In the case of the peroxo configuration, this fact together with the high barrier for breaking the oxygen-oxygen bond indicates a possible loss mechanism due to hole trapping. Graphical Abstract Calculated spin density at a hematite surface with peroxo intermediate.

Novel Cobalt Germanium Hydroxide for Electrochemical Water Oxidation

Developing efficient and stable oxygen evolution catalyst (OEC) is a critical step to overcome the sluggish kinetics of water oxidation. Here, we hydrothermally synthesized a novel OEC, cobalt germanium hydroxide, CoGeO₂(OH)₂. The inherent Co-bonded hydroxyl groups facilitate the formation of active oxygen evolution reaction intermediates. Meanwhile, the facile leaching of Ge at the OEC-electrolyte interface contributes to surface reconstruction, generating Co-based (oxy)hydroxides, which would weaken its lattice constraint and suppress the excessive corrosion in the OEC bulk. As a result, CoGeO₂(OH)₂ reveals good catalytic activity and stability. This CoGe-based OEC achieves the overpotential at 10 mA cm^-2 (η_@10mA) of ∼340 mV, and the turnover frequency of ∼0.08 s^-1. And the electrolysis kept at ∼10 mA cm^-2 could be sustained for over 350 h. In addition, this p-type CoGeO₂(OH)₂ is demonstrated to be an effective electrocatalytic overlayer on n-type Ta₃N₅ photoanode, remarkably decreasing the onset for nearly 400 mV and increasing the photocurrent density at 1.23 V_RHE about 3.8 times.

Metal Organic Framework Derived Materials: Progress and Prospects for the Energy Conversion and Storage

Exploring new materials with high efficiency and durability is the major requirement in the field of sustainable energy conversion and storage systems. Numerous techniques have been developed in last three decades to enhance the efficiency of the catalyst systems, control over the composition, structure, surface area, pore size, and moreover morphology of the particles. In this respect, metal organic framework (MOF) derived catalysts are emerged as the finest materials with tunable properties and activities for the energy conversion and storage. Recently, several nano- or microstructures of metal oxides, chalcogenides, phosphides, nitrides, carbides, alloys, carbon materials, or their hybrids are explored for the electrochemical energy conversion like oxygen evolution, hydrogen evolution, oxygen reduction, or battery materials. Interest on the efficient energy storage system is also growing looking at the practical applications. Though, several reviews are available on the synthesis and application of MOF and MOF derived materials, their applications for the electrochemical energy conversion and storage is totally a new field of research and developed recently. This review focuses on the systematic design of the materials from MOF and control over their inherent properties to enhance the electrochemical performances.

Resolution of Electronic and Structural Factors Underlying Oxygen-Evolving Performance in Amorphous Cobalt Oxide Catalysts

Non-noble-metal, thin-film oxides are widely investigated as promising catalysts for oxygen evolution reactions (OER). Amorphous cobalt oxide films electrochemically formed in the presence of borate (CoBi) and phosphate (CoPi) share a common cobaltate domain building block, but differ significantly in OER performance that derives from different electron-proton charge transport properties. Here, we use a combination of L edge synchrotron X-ray absorption (XAS), resonant X-ray emission (RXES), resonant inelastic X-ray scattering (RIXS), resonant Raman (RR) scattering, and high-energy X-ray pair distribution function (PDF) analyses that identify electronic and structural factors correlated to the charge transport differences for CoPi and CoBi. The analyses show that CoBi is composed primarily of cobalt in octahedral coordination, whereas CoPi contains approximately 17% tetrahedral Co(II), with the remainder in octahedral coordination. Oxygen-mediated 4 p-3 d hybridization through Co-O-Co bonding was detected by RXES and the intersite dd excitation was observed by RIXS in CoBi, but not in CoPi. RR shows that CoBi resembles a disordered layered LiCoO₂-like structure, whereas CoPi is amorphous. Distinct domain models in the nanometer range for CoBi and CoPi have been proposed on the basis of the PDF analysis coupled to XAS data. The observed differences provide information on electronic and structural factors that enhance oxygen evolving catalysis performance.

In-situ developed carbon spheres function as promising support for enhanced activity of cobalt oxide in oxygen evolution reaction

Highly active, stable electrocatalyst for oxygen evolution reaction (OER) is sincerely required for the practical application of water splitting to get rid from the sluggish reaction kinetics and the stability issue. Here, Co₃O₄ is studied as OER catalyst and to improve the electrocatalytic activity, carbon is chosen as the conducting support. A simple and cost-effective synthetic route is developed for the synthesis of Co₃O₄ on carbon support following hydrothermal route using various hydrolyzing agents. The heterostructure 'Co₃O₄/C' perform well in OER as a non-precious metal catalyst. The best Co₃O₄/C electrocatalyst can generate 10 and 30 mA/cm² current densities upon application of 1.623 V and 1.678 V vs. RHE whereas, bare Co₃O₄ can generate current density of 10 and 30 mA/cm² upon application of 1.677 and 1.754 V vs. RHE. Carbon in the heterostructure helps to improve the conductivity and at the same time enhances the charge transfer ability which further leads to increase current density and stability to the catalyst. Co₃O₄/C can generate unaltered current density up to 1000 cycles.

Catalysis by design: development of a bifunctional water splitting catalyst through an operando measurement directed optimization cycle

A critical challenge in energy research is the development of earth abundant and cost-effective materials that catalyze the electrochemical splitting of water into hydrogen and oxygen at high rates and low overpotentials. Key to addressing this issue lies not only in the synthesis of new materials, but also in the elucidation of their active sites, their structure under operating conditions and ultimately, extraction of the structure-function relationships used to spearhead the next generation of catalyst development. In this work, we present a complete cycle of synthesis, operando characterization, and redesign of an amorphous cobalt phosphide (CoP x ) bifunctional catalyst. The research was driven by integrated electrochemical analysis, Raman spectroscopy and gravimetric measurements utilizing a novel quartz crystal microbalance spectroelectrochemical cell to uncover the catalytically active species of amorphous CoP x and subsequently modify the material to enhance the activity of the elucidated catalytic phases. Illustrating the power of our approach, the second generation cobalt-iron phosphide (CoFePx) catalyst, developed through an iteration of the operando measurement directed optimization cycle, is superior in both hydrogen and oxygen evolution reactivity over the previous material and is capable of overall water electrolysis at a current density of 10 mA cm-2 with 1.5 V applied bias in 1 M KOH electrolyte solution.

In Situ Visualization of Electrocatalytic Reaction Activity at Quantum Dots for Water Oxidation

Exploring electrocatalytic reactions on the nanomaterial surface can give crucial information for the development of robust catalysts. Here, electrocatalytic reaction activity at single quantum dots (QDs) loaded silica microparticle involved in water oxidation is visualized using electrochemiluminescence (ECL) microscopy. Under positive potential, the active redox centers at QDs induce the generation of hydroperoxide surface intermediates as coreactants to remarkably enhance ECL emission from luminol derivative molecules for imaging. For the first time, in situ visualization of the catalytic activity of water oxidation with QDs catalyst was achieved, supported by a linear relation between ECL intensity and turn over frequency. A very slight diffusion trend attributed to only the luminol species proved in situ capture of hydroperoxide surface intermediates at catalytic active sites of QDs. This work provides tremendous potential in online imaging of electrocatalytic reactions and visual evaluation of catalyst performance.

Water Splitting on Rutile TiO2 -Based Photocatalysts

Water splitting using a semiconductor photocatalyst with sunlight has long been viewed as a potential means of large-scale H₂ production from renewable resources. Different from anatase TiO₂ , rutile enables preferential water oxidation, which is useful for the construction of a Z-scheme water-splitting system. The combination of rutile TiO₂ with a suitable H₂ -evolution photocatalyst such as a Pt-loaded BaZrO₃ -BaTaO₂ N solid solution enables solar-driven water splitting into H₂ and O₂ . While rutile TiO₂ is a wide-gap semiconductor with a bandgap of 3.0 eV, co-doping of rutile TiO₂ with certain metal ions and/or nitrogen produces visible-light-driven photocatalysts, which are also useful as a component for water oxidation in visible-light-driven Z-scheme water splitting. The key to achieving highly efficient water oxidation is to maintain a charge balance of dopants in the rutile, because single doping typically produces trap states that capture photogenerated electrons and/or holes. Here we provide a concise summary of rutile TiO₂ -based photocatalysts for water-splitting systems.

Electrochemical trapping of metastable Mn3+ ions for activation of MnO2 oxygen evolution catalysts

Electrodeposited manganese oxide films are promising catalysts for promoting the oxygen evolution reaction (OER), especially in acidic solutions. The activity of these catalysts is known to be enhanced by the introduction of Mn3+ We present in situ electrochemical and X-ray absorption spectroscopic studies, which reveal that Mn3+ may be introduced into MnO2 by an electrochemically induced comproportionation reaction with Mn2+ and that Mn3+ persists in OER active films. Extended X-ray absorption fine structure (EXAFS) spectra of the Mn3+-activated films indicate a decrease in the Mn-O coordination number, and Raman microspectroscopy reveals the presence of distorted Mn-O environments. Computational studies show that Mn3+ is kinetically trapped in tetrahedral sites and in a fully oxidized structure, consistent with the reduction of coordination number observed in EXAFS. Although in a reduced state, computation shows that Mn3+ states are stabilized relative to those of oxygen and that the highest occupied molecular orbital (HOMO) is thus dominated by oxygen states. Furthermore, the Mn3+(Td) induces local strain on the oxide sublattice as observed in Raman spectra and results in a reduced gap between the HOMO and the lowest unoccupied molecular orbital (LUMO). The confluence of a reduced HOMO-LUMO gap and oxygen-based HOMO results in the facilitation of OER on the application of anodic potentials to the δ-MnO2 polymorph incorporating Mn3+ ions.

Identifying MnVII-oxo Species during Electrochemical Water Oxidation by Manganese Oxide

Identifying surface active intermediate species is essential to reveal the catalytic mechanism of water oxidation by metal-oxides-based catalysts and to develop more efficient catalysts for oxygen-oxygen bond formation. Here we report, through electrochemical methods and ex situ infrared spectroscopy, the identification of a Mn^VII = O intermediate during catalytic water oxidation by a c-disordered δ-MnO_x with an onset-potential-dependent reduction peak at 0.93 V and an infrared peak at 912 cm⁻¹. This intermediate is proved to be highly reactive and much more oxidative than permanganate ion. Therefore, we propose a new catalytic mechanism for water oxidation catalyzed by Mn oxides, with involvement of the Mn^VII = O intermediate in a resting state and the Mn^IV−O−Mn^VII = O as a real active species for oxygen-oxygen bond formation.

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A reactive Mn^VII-oxo intermediate was identified during water oxidation by a MnO_x

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The Mn^VII-oxo species was proved to be much more oxidative than permanganate ion

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The Mn^IV−O−Mn^VII = O moiety is a real highly active state for O–O bond formation

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A new mechanism for Mn oxide-catalyzed electrocatalytic water oxidation is proposed

Co/CoOx Nanoparticles Embedded on Carbon for Efficient Catalysis of Oxygen Evolution and Oxygen Reduction Reactions

The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are important electrochemical reactions to realize clean energy technologies. Herein, we prepared a hybrid electrocatalyst consisting of Co/CoO_x nanoparticles embedded in amorphous carbon through the simple pyrolysis of cobalt-based zeolitic terephthalate frameworks. The pyrolysis temperature significantly influenced the structure morphology and catalytic behavior. Good contact between Co/CoO_x and amorphous carbon resulted in a high catalytic efficiency. The hybrid obtained under pyrolysis temperature of 600 °C exhibited the highest performance for OER, offering a stable current density of 10 mA cm^-2 at 277 mV in basic media. Besides good OER behavior, it also showed good ORR performance [onset potential: ∼0.87 V vs. the reversible hydrogen electrode (RHE), diffusion-limiting current density: ∼4.9 mA cm^-2 ]. This work describes a novel and efficient catalyst, and greatly expands the scope of low-cost Co-based electrocatalysts for various electrochemical reactions without the need for N-containing ligands.

NiOOH Exfoliation-Free Nickel Octahedra as Highly Active and Durable Electrocatalysts Toward the Oxygen Evolution Reaction in an Alkaline Electrolyte

A layered β-NiOOH crystal with undercoordinated facets is an active and economically viable nonnoble catalyst for the oxygen evolution reaction (OER) in alkaline electrolytes. However, it is extremely difficult to enclose the β-NiOOH crystal with undercoordinated facets because of its inevitable crystal transformation to γ-NiOOH, resulting in the exfoliation of the catalytic surfaces. Herein, we demonstrate {111}-faceted Ni octahedra as the parent substrates whose surfaces are easily transformed to catalytically active β-NiOOH during the alkaline OER. Electron microscopic measurements demonstrate that the horizontally stacked β-NiOOH on the surfaces of Ni octahedra has resistance to further oxidation to γ-NiOOH. By contrast, significant crystal transformation and thus the exfoliation of the γ-NiOOH sheets can be observed on the surfaces of Ni cubes and rhombic dodecahedra (RDs). Electrocatalytic measurements show that the β-NiOOH formed on Ni octahedra exhibits highly enhanced OER durability compared to the Ni cubes, Ni RDs, and the state-of-the-art Ir/C catalysts.

In situ X-ray absorption spectroscopy of transition metal based water oxidation catalysts

X-ray absorption studies of the geometric and electronic structure of primarily heterogeneous Co, Ni, and Mn based water oxidation catalysts are reviewed. The X-ray absorption near edge and extended X-ray absorption fine structure studies of the metal K-edge, characterize the metal oxidation state, metal-oxygen bond distance, metal-metal distance, and degree of disorder of the catalysts. These properties guide the coordination environment of the transition metal oxide radical that localizes surface holes and is required to oxidize water. The catalysts are investigated both as-prepared, in their native state, and under reaction conditions, while transition metal oxide radicals are generated. The findings of many experiments are summarized in tables. The advantages of future X-ray experiments on water oxidation catalysts, which include the limited data available of the oxygen K-edge, metal L-edge, and resonant inelastic X-ray scattering, are discussed.

Stable iridium dinuclear heterogeneous catalysts supported on metal-oxide substrate for solar water oxidation

Atomically dispersed catalysts refer to substrate-supported heterogeneous catalysts featuring one or a few active metal atoms that are separated from one another. They represent an important class of materials ranging from single-atom catalysts (SACs) and nanoparticles (NPs). While SACs and NPs have been extensively reported, catalysts featuring a few atoms with well-defined structures are poorly studied. The difficulty in synthesizing such structures has been a critical challenge. Here we report a facile photochemical method that produces catalytic centers consisting of two Ir metal cations, bridged by O and stably bound to a support. Direct evidence unambiguously supporting the dinuclear nature of the catalysts anchored on α-Fe₂O₃ is obtained by aberration-corrected scanning transmission electron microscopy (AC-STEM). Experimental and computational results further reveal that the threefold hollow binding sites on the OH-terminated surface of α-Fe₂O₃ anchor the catalysts to provide outstanding stability against detachment or aggregation. The resulting catalysts exhibit high activities toward H₂O photooxidation.

In situ electrochemical development of copper oxide nanocatalysts within a TCNQ nanowire array: a highly conductive electrocatalyst for the oxygen evolution reaction

It is highly desired to develop efficient earth-abundant electrocatalysts for the oxygen evolution reaction (OER) in alkaline media. In this communication, we report the in situ electrochemical conversion of a nanoarray of Cu(tetracyanoquinodimethane), Cu(TCNQ), an inorganic-organic hybrid, on Cu foam into CuO nanocrystals confined in a highly conductive nanoarray via anode oxidation. As a 3D catalyst electrode, the resulting CuO-TCNQ/CF shows high OER activity and demands an overpotential of only 317 mV to drive a geometrical catalytic current density of 25 mA cm^-2. Notably, this catalyst also demonstrates strong long-term electrochemical durability. This study provides us with a universal strategy toward topotactic room-temperature preparation of conductive nanoarrays with confined transition metal nanocatalysts for practical applications.

The role of carbonate in electro-catalytic water oxidation by using Ni(1,4,8,11-tetraazacyclotetradecane)2

NiL_i²⁺ are good electro-catalysts for water oxidation in phosphate or carbonate buffers. The results point out that the active oxidizing agents are L(X)Ni^IVOH^{4-(3-n+1)/(2-n+1)}, where X = PO₄H_n^(3-n)- or CO₃H_n^(2-n)- formed from LNi^IVX₂via a mechanism involving an acid catalyzed O-P or O-C bond heterolysis. Carbonate behaves differently from phosphate as it is a non-innocent ligand and it can be oxidized.