Water Oxidation Catalysts Research Articles

Coordination tuning of FeNi-HMT Framework derived effective hybrid catalysts for water oxidation.

FeNi-based hybrid materials are promising oxygen evolution reaction (OER) catalysts for water electrolysis in hydrogen generation. In this work, the coordination tuning of FeNi-HMT frameworks was achieved by simply changing the Fe/Ni ratios using hexamethylenetetramine (HMT) as an organic ligand, and the derived hybrid FeNi catalysts with varied compositions were probed for OER. Incorporating varying amounts of Fe3+ by adjusting the Ni/Fe ratio results in different metal-organic framework (MOF) structures, and higher Fe feed leads to the formation of amorphous structures due to the coordination structure destruction from the weaker coordination capacity of Fe3+ compared to Ni2+ combining with the tertiary amine ligand. Among them, the FeNi-HMT (with the Fe/Ni molar ratio of 1/1) derived catalyst, consisting of Fe0.36Ni0.64 alloy/Ni0.4Fe2.6O4 spinel oxide heterostructures supported by graphitized carbon matrix, exhibits the highest OER performance. The unique structure facilitates significant electron transfer at the alloy/spinel interface due to the large work function difference between each phase. This strong electronic effect downshifts the d-band center of the catalyst and optimizes the binding energies to the crucial oxygenated intermediates, thereby promoting the OER kinetics. This work highlights the importance of the coordination tuning of FeNi-HMT frameworks for highly efficient catalyst development.

Analyzing the Temperature Dependence of Titania Photocatalysis: Kinetic Competition between Water Oxidation Catalysis and Back Electron–Hole Recombination

Analyzing the Temperature Dependence of Titania Photocatalysis: Kinetic Competition between Water Oxidation Catalysis and Back Electron–Hole Recombination

Novel Inorganic-Organic Dual-Photosensitizing Dinuclear-Metal Self-Assembly System for Efficient Artificial Photosynthesis without Sacrificial Electron Donors.

Owing to the unique synergistic effect, dinuclear-metal-molecule-catalysts (DMCs) show excellent performance in catalytic fields. However, for overall photocatalytic CO2 reaction (CO2RR), it is still a challenge to construct well-matched photosensitizer (PS) components for DMCs-based photocatalysts. Inorganic-quantum-dot PS possesses capacities of multiple exciton generation and catalyzing water oxidation but is incompatible with DMCs. In contrast, organic PS can be covalently linked with DMCs but inescapable of using sacrificial electron donors. For overall photocatalytic CO2RR, organic-inorganic dual-photosensitizing system might be a promising candidate. Herein, we employed covalent-linking and electrostatic-driven approaches to construct the self-assembly of pyrene-sensitized Co2L DMCs (Py-Co2L) and perovskite (PVK) quantum dots, i.e., PVK@[Py-Co2L]. Using H2O as an electron donor, PVK@[Py-Co2L] realized 105.24 μmol·g-1·h-1 CO yield in photocatalytic CO2RR, much higher than PVK (15.44 μmol·g-1·h-1) and PVK@Co2L (32.30 μmol·g-1·h-1), ascribing to the efficient photogenerated charge separation and transfer. The experimental results and theoretical investigations demonstrated that the pyrene linked on Co2L boosted the electron delivery from PVK to DMCs. Besides, this strategy could also be extended to the photocatalytic H2 evolution coupled with alcohol oxidation. As a proof-of-concept, our work lightens the integration of DMCs, organic and inorganic PS components, promoting the development of photocatalysis without sacrificial electron donors.

Facet Engineering of Cobalt Manganese Oxide for Highly Stable Acidic Oxygen Evolution Reaction

AbstractDeveloping cost‐effective, robust, and durable catalysts for water oxidation in acidic conditions remains a significant challenge for the practical application of proton exchange membrane electrolyzers. Cobalt‐based spinel catalysts, known for their effective oxygen evolution reaction (OER) activity in acidic environments, are promising alternatives to the costly iridium‐based catalysts. However, their application is often limited by poor stability under corrosive acidic conditions and oxidative potential. Here it is demonstrated that doping Co2MnO4 with Ni effectively regulates the structural and electronic properties of the catalysts, enabling stable operation for over 285 h at 100 mA cm−2 in 0.5 m H2SO4. This stability enhancement is primarily due to the increased exposure of the (400) facet, a result supported by theoretical studies. The proton exchange membrane water electrolysis (PEMWE) performance of Ni(5%)Co2MnO4 further demonstrates its potential, achieving 1 A cm−2 at a cell voltage of 2.3 V, with minimal degradation over 100 h at 500 mA cm−2. This study not only provides insights into the design of advanced OER catalysts through the doping of heteroatoms but also offers a pathway to enhance the sustainability and economic viability of acidic OER applications.

Mo-doped α-MnO2 for Enhanced Electrocatalytic Water Oxidation.

Manganese is a key metal involved in the catalysis of natural photosynthesis. Thus, the investigation of Mn-based electrocatalysts for water oxidation is of high importance. This work reports the doping of Mo into α-MnO2 nanorods to improve the water oxidation performance. The doping of Mo can transform the microstructure of α-MnO2 from nanorods into nanosphere superstructures. As a dopant, Mo expands the α-MnO2 lattice to result in a decrease in the average oxidation state of Mn and the generation of oxygen vacancies, which are beneficial to water oxidation catalysis. Under optimized doping, the OER overpotential of Mo/α-MnO2 is reduced by 80 mV (at 10 mA/cm2) compared with pure α-MnO2.

Voltage-Driven Molecular Photoelectrocatalysis of Water Oxidation.

Molecular photocatalysis and photoelectrocatalysis have been widely used to conduct oxidation-reduction processes ranging from fuel generation to electroorganic synthesis. We recently showed that an electrostatic potential drop across the double layer contributes to the driving force for electron transfer (ET) between a dissolved reactant and a molecular catalyst immobilized directly on the electrode surface. In this article, we report voltage-driven molecular photoelectrocatalysis with a prevalent homogeneous water oxidation catalyst, (bpy)Cu (II), which was covalently attached to the carbon surface and exhibited photocatalytic activity. The strong potential dependence of the photooxidation current suggests that the electrostatic potential drop across the double layer contributes to the driving force for ET between a water molecule and the excited state of surface-bound (bpy)Cu (II). Scanning electrochemical microscopy (SECM) was used to analyze the products and determine the faradaic efficiencies for the generation of oxygen and hydrogen peroxide. Unlike electrocatalytic water oxidation by (bpy)Cu (II) in the dark, which produces only O2, the voltage-driven photooxidation includes an additional 2e- pathway generating H2O2. DFT calculations show that the applied voltage and the presence of light can alter the activation energy for the rate-determining water nucleophilic attack steps, thereby increasing the reaction rate of photo-oxidation of water and opening the 2e- pathway. These results suggest a new route for designing next-generation hybrid molecular photo(electro)catalysts for water oxidation and other processes.

Interfacial Electron Coupling in Au Nanoparticle/CoFe Layered Double Hydroxide Nanosheet Heterostructures as Water Oxidation Catalysts

Interfacial Electron Coupling in Au Nanoparticle/CoFe Layered Double Hydroxide Nanosheet Heterostructures as Water Oxidation Catalysts

Tailoring of Zinc Oxide-based microstructures to efficiently promote piezocatalytic water oxidation and oxygen production

Two zinc oxide (ZnO) nanomaterials, exhibiting distinct morphologies, were synthesized in wurtzite phase through chemical precipitation. The elongated shape rod-like ZnO (R-ZnO) exhibited more pronounced piezoelectric characteristics compared to the more compact flower-like ZnO (F-ZnO). Detailed characterization including piezoforce microscopy, finite element simulation, revealed that the remarkable piezoelectric properties of R-ZnO are due to its strongly anisotropic structure, enabling significant mechanical deformation under ultrasound in aqueous suspension. The piezocatalytical degradation of these materials was assessed using methylene blue as a test organic dye across various ultrasound frequencies, with R-ZnO consistently outperforming F-ZnO. This highlighted the impact of structural deformability on piezocatalytic efficiency. Additionally, the capability of R-ZnO to facilitate oxygen evolution from water through ultrasound-induced stress was explored in an oxygen-free environment. Our findings demonstrate that R-ZnO can effectively catalyze water oxidation and produce oxygen directly, showcasing its potential as a standalone catalyst for environmental remediation and sustainable chemical processes.

{Co4O4} Cubanes in a conducting polymer matrix as bio-inspired molecular oxygen evolution catalysts

Exploration of efficient molecular water oxidation catalysts for long-term application remains a key challenge for the conversion of renewable energy sources into fuels. Cuboidal {Co4O4} complexes keep attracting interest as molecular water oxidation catalysts as they combine features of both heterogeneous and homogeneous catalysis with bio-inspired motifs. However, the application of many cluster-based catalysts for the oxygen evolution reaction still requires new stabilization strategies. Drawing inspiration from the stabilizing effects of natural polymers, we introduce a conductive polymer-hybrid approach to covalently immobilize {Co4O4} cubane oxo clusters as oxygen evolution catalysts. Polypyrrole is applied as an efficient p-type conducting polymer that promotes hole transfer during the oxygen evolution reaction, resulting in higher turnover frequency compared to the pristine {Co4O4} oxo cluster and heterogeneous Co-oxide benchmarks. The asymmetric coordination of {Co4O4} not only mitigates catalyst decomposition pathways, but also increases the catalytic efficiency by exposing a directed cofacial dihydroxide motif during catalysis.

Unraveling the Role of the Nitrate Ion and Solvent Water on the O─O Bond Formation Step in Fe─TAML Catalyzed Water Oxidation

AbstractDensity functional theory‐based molecular dynamics (DFT‐MD) combined with an explicit solvation model were employed to further elucidate the O─O bond formation step in Fe─TAML catalyzed water oxidation reaction. The water nucleophilic attack (WNA) and nitrate nucleophilic attack (NNA) on the oxo group of the high‐valent [TAML+•─Fe5+═O] species were calculated to have comparable active barriers (24 kcal/mol versus 22 kcal/mol). This suggests nitrate ion can behave as a cocatalyst to promote the O─O bond formation. More importantly, a crucial role of the presence and thermal motion of solvent water in the NNA process was observed. This was quantified by an increase of the activation energy barrier by 4 kcal/mol, determined by comparing the explicit solvent DFT‐MD simulation with implicit solvent static DFT calculation.

A Proton-Conductive Co(II)-Polyoxometalate Acts as a Precatalyst for Efficient Electrocatalytic Water Oxidation.

A cobalt(II)-containing polyoxometalate, [H3O]5[{Co(H2O)4}3{Na(H2O)4}W12O42]·3H2O (Co-POM), has been isolated in a one-step facile aqueous synthesis and characterized unambiguously using single-crystal X-ray crystallography along with routine spectral analysis. The paratungstate cluster anion [W12O42]12- coordinates with {CoII(H2O)4}2+ and {Na(H2O)4}+ complex cations resulting in the formation of the water-insoluble Co-POM compound having three-dimensional (3-D) extended structure. Motivated by the protonated water molecules existing as the counter cations in Co-POM, herein, we demonstrate the detailed proton conductivity studies of the Co-POM, reaching a value of 1.04 × 10-2 S cm-1 at 80 °C and 98% relative humidity (RH). The temperature- and humidity-dependent proton conductivity in Co-POM is governed by Grotthus mechanism with Ea = 0.25 eV. In addition, we examined the electrochemical behavior of Co-POM, in an alkaline borate buffer where it is found to be electrochemically unstable and acts as a precatalyst (and not a true catalyst) for oxygen evolution reaction (OER). We also discuss the "post-mortem" analysis of the postelectrolysis sample to identify the active species which turns out to be a cobalt oxide material (Co3O4) incorporating small amounts of tungsten. Thus, in the present electrocatalysis work, the Co-POM molecule transforms into an efficient water oxidation catalyst (WOC).

Construction of Ag/AgCl@MIL-53(Fe): Achieving Efficient Photocatalytic Water Oxidation via the Plasma-Phase Silver and Heterojunction Synergistic Effect

The development of highly efficient water oxidation catalysts is a bottleneck in achieving artificial photosynthesis, as water oxidation is a complex process involving multiple electron and proton transfers. To further improve the photocatalytic water oxidation performance of MIL-53(Fe), a series of Ag/AgCl@MIL-53(Fe) samples with different Fe:Ag ratios were synthesized by hydrothermal methods and used in the photocatalytic water oxidation reactions. XRD characterization showed the successful preparation of Ag/AgCl@MIL-53(Fe) heterostructure catalysts. The reaction results showed that sample AAM-2 (Fe:Ag=5:1) had the best photocatalytic water oxidation performance, with the highest TOF value of 0.14 mmol/(g·s) and quantum efficiency of 39.0 % under the conditions of catalyst mass of 1 mg and pH=9.0 for boric acid-borax buffer solution. Measurements of photocurrent indicated that the AAM-2 samples doped with Ag/AgCl had higher photocurrent densities, thus improving the photocatalytic performance. This provides new insights for constructing highly efficient porous MOFs catalysts.

Structural development on Ru and RuO2 electrodes during oxygen evolution – a soft X-ray absorption spectroscopy approach

Time resolved in-situ X-ray absorption spectroscopy (XAS) in soft X-ray region was used to characterize polarized interphase on Ru and Ru oxide based electrodes under oxygen evolution reaction (OER) conditions. XAS spectra were used to align the type and population of oxygen-containing species formed at electrodes at anodic potentials with local electronic structure of the OER catalyst. The soft XAS data do not identify a single rate limiting process for OER at potentials negative to 1.7 V vs. RHE. Individual intermediates of the oxygen evolution process coexist at the surface at potentials preceding the actual OER onset. The OER is accompanied with redistribution of the electron density resulting for a start of the catalytic cycle reflecting the transfer of the electrons from water to the external circuit during catalyzed water oxidation. The observed spectral behavior indicates a confinement of the OER to the coordination unsaturated sites (cus) at the surface.

Facile synthesis of spherical-shaped CeC2–TiO2 nanocomposite electrocatalyst with boosted alkaline OER

Developing noble metal-free multifunctional electrocatalysts to catalyze water oxidation is vital for future renewable energy systems. Herein, through several defect modifications, the CeC2–TiO2 nanocomposite on the strength of enriched oxygen vacancies was successfully fabricated by facile hydrothermal method on SS (stainless steel) substrate, attributed to the conductive ability for fast electron transportation. A well-defined phase growth, vibrational bonding of metal atoms, morphological determination, and elemental composition for synthesized heterostructured electrocatalysts were revealed by XRD, FTIR, TEM, and EDX, respectively. XPS has complied to understand the moderate binding energies of CeC2–TiO2 for OER intermediates, which signifies the strong interactions between electronic states of Ce, C, Ti, and O atoms. With the plentiful catalytic active sites, the resultant CeC2–TiO2 entails low overpotentials of only 169 mV and 108.8 mV/dec Tafel slope to afford 10 mA cm−2 for OER in 1.0 M KOH are tested through cyclic and linear sweep voltammogram measurements, thereby exhibit promising activity in alkaline water electrolysis. EIS measurements revealed a decreased polarization resistance for the composite dominated by the small semicircle diameter, corresponding to the adsorption of intermediates. Finally, observed results strongly recommended that the CeC2–TiO2 catalyst exhibited superior OER performance due to excellent electrical chemical coupling between CeC2 and TiO2.

Synthesis, characterization and application of quinoxaline-based organic dyes as anodic sensitizers in photoelectrochemical cells

Three new D-A-π-A organic dyes (1a-c) containing a 2,3-diphenylquinoxaline core as the main chromophore were designed to act as sensitizers for the TiO2-based photoanode of dye-sensitized photoelectrochemical cells (DS-PEC) for water splitting. The dyes structures featured a cyanoacrylic acid group as the terminal acceptor/anchoring moiety and three different donor groups of moderate strength, namely mono- and dialkoxy-substituted benzenes, which were introduced to modulate the energies of the respective HOMO levels and enable the electron transfer from a Ru-based molecular water oxidation catalyst (WOC). Dyes 1a-c were synthesized following a C–H activation strategy and fully characterized. In addition, the previously known catalyst Ru(bda)(PyP)2 (bda = 2,2′-bipyridine-6,6′-dicarboxylate; PyP = pyridin-4-methyl phosphonic acid) was also prepared by modification of a reported procedure. The dynamics of charge transfer processes involving the dyes adsorbed on nanocrystalline TiO2 films were investigated by means of transient absorption spectroscopy. Photo-electrochemical experiments carried out on photoanodes functionalized with 1a-c and Ru(bda)(PyP)2 in 0.1 M aq. Na2SO4 electrolyte showed the production of photocurrents up to ca. 0.15 mA cm−2 at + 0.5 V vs. NHE, and highlighted how modifications of the TiO2 staining procedure can lead to significant differences in cell performances.

Construction of Mesoporous Aluminosilicate Thin Films on ITO Electrodes for Immobilizing a Cationic Ru(II) Water Oxidation Catalyst

Aluminosilicate (Al‐SiO2) thin films with vertically aligned mesochannels were successfully synthesized on ITO electrodes and employed for the immobilization of a cationic Ru(II) water oxidation catalyst without requiring linker groups. Optimal synthesis conditions yielded uniform mesoporous Al‐SiO2 films with tunable Al content, high surface area (568 m2/g), 3.94 nm pore size, and 155 nm thickness. Electrochemical studies confirmed the presence of the immobilized Ru complex undergoing diffusion‐controlled Ru(III/II) and Ru(IV/III) electron transfer. The Ru loading reached 4.71 nmol/cm2 at Si/Al = 9.6, with higher Al content enhancing loading amounts via cation exchange. The Ru‐modified electrode exhibited high electrocatalytic water oxidation activity, achieving 75.3% Faradaic efficiency and a turnover number of 298.6 for O2 evolution for 1 hour. This work provides a new approach to construct porous environments on an electrode surface to immobilize positively charged transition‐metal complexes as catalysts, offering potential applications in the development of electrocatalytic systems for energy conversion.

Rare Earth‐Induced CoO6 Octahedral Distortion in Perovskite: An Efficient Catalytic Platform for RuO2‐Catalyzed Water Oxidation

Rare Earth‐Induced CoO₆ Octahedral Distortion in Perovskite: An Efficient Catalytic Platform for RuO₂‐Catalyzed Water Oxidation

High-Performance Low-Iridium Catalyst for Water Oxidation: Breaking Long-Ranged Order of IrO2 by Neodymium Doping.

Exploring efficacious low-Irelectrocatalysts for oxygen evolution reaction (OER) is crucial for large-scale application of proton exchange membrane water electrolysis (PEMWE). Herein, an efficient non-precious lanthanide-metal-doped IrO2 electrocatalyst is presented for OER catalysis by doping large-ionic-radius Nd into IrO2 crystal. The doped Nd breaks the long-ranged order structure by triggering the strain effect and thus inducing an atomic rearrangement of Nd─IrO2 involving the forming of Nd─O─Ir bonds along with an increased amount of oxygen vacancies (Ov), giving rise of a long-ranged disorder but a short-ranged order structure. The formed Nd─O─Ir bonds tailor the electronic structure of Ir, leading to a lowered d-band center that weakens intermediates absorption on Ir sites. Moreover, doping Nd triggers Nd─IrO2 to catalyze OER mainly through lattice oxygen mechanism (LOM) by activating lattice oxygen owing to abundant Ov. The optimal catalyst only requires a relatively low overpotential of 263 mV@10mA cm-2 with a high mass activity of 216.98 A gIr -1 (at 1.53V) (eightfold of commercial IrO2), and also shows a superior durability at 50mA cm-2 (20h) than commercial IrO2 (3h) due to the oxidation-suppressing effect induced by Nd doping. This work offers insights into designing high-performance low-Ir electrocatalysts for PEMWE application.

Bioinspired octanuclear copper cubane complex as an efficient molecular electrocatalyst for water oxidation

Water oxidation catalysts (WOCs) with excellent activity and stability are highly desired. Numerous efforts have been dedicated to develop WOCs mimicking the cubic structure and activity of Mn4CaO5 cluster in oxygen-evolving center (OEC) of photosynthesis system II (PSII). Herein, a octanuclear Cu cluster [Cu8(dpy{OH}O)8(OAc)4]4+ (1, dpy{OH}O− = the monoanion of the hydrated, gem-diol form of di-2-pyridyl ketone) that contains two Cu4O4 cubic fragments with bioinspired structure mimicking the Mn4CaO5 cluster of OEC of natural PSII is developed as efficient homogeneous catalyst for electrochemical water oxidation under near neutral condition for the first time. It exhibits outstanding water oxidation catalytic activity via accelerating oxygen evolution with onset overpotential of 730 mV and appreciable turnover frequency of 20.1 s−1. Collective experiments together confirmed the homogeneity and stability of cluster 1 under long-term water oxidation catalytic cycle. Kinetic study reveals the water nucleophilic attack (WNA) catalytic mechanism occurs on the coordination unsaturated Cu center of the cluster. This work provides the inspiration of the development of robust bioinspired catalyst for electrochemical water oxidation.

Intermolecular O-O Bond Formation between High-Valent Ru-oxo Species.

Despite extensive research on water oxidation catalysts over the past few decades, the relationship between high-valent metal-oxo intermediates and the O-O bond formation pathway has not been well clarified. Our previous study showed that the high spin density on O in RuV=O is pivotal for the interaction of two metal-oxyl radical (I2M) pathways. In this study, we found that introducing an axially coordinating ligand, which is favorable for bimolecular coupling, into the Ru-pda catalyst can rearrange its geometry. The shifts in geometric orientation altered its O-O bond formation pathway from water nucleophilic attack (WNA) to I2M, resulting in a 70-fold increase in water oxidation activity. This implies that the I2M pathway is concurrently influenced by the spin density on oxo and the geometry organization of the catalysts. The observed mechanistic switch and theoretical studies provide insights into controlling reaction pathways for homogeneous water oxidation catalysis.