Water Splitting Reaction Research Articles

A decade of breakthroughs: MOF-graphene oxide catalysts for water splitting efficiency

Abstract Burning fossil fuels has significantly worsened environmental pollution, particularly due to the release of carbon dioxide emissions. The global efforts to promote renewable energy solutions, like electrocatalytic water splitting, have gained momentum. Scientists are focusing on the development of sustainable methods like water splitting to reduce dependence on conventional fuels. Developing affordable and effective electrocatalysts is crucial for multifunctional electrochemical water splitting (ECWS). In comparison to traditional electrocatalysts, metal-organic frameworks (MOFs) exhibit favorable catalytic performance for electrochemical water decomposition because of their plentiful porosity, surface area, and topologies for enhanced production of hydrogen (H2) and oxygen (O2) gas. When combined with MOF, graphene creates a synergistic hybrid nanomaterial that is more stable, adaptable, and durable. The primary goal of this review article is to conduct an in-depth investigation of the latest advancements in MOFs and MOF-GO electrocatalysts for water electrolysis. Herein, we have covered the plausible mechanism for the overall water-splitting electrocatalytic processes and several important factors influencing their electrocatalytic response. We also discussed the recent progress in the performance and stability of MOFs and MOF-GO electrocatalysts for water-splitting reactions. Finally, the article highlights the challenges and application of MOF and MOF-GO composites and the future preference for water-splitting applications.

Engineering active sites in ternary CeO2-CuO-Mn3O4 heterointerface embedded in reduced graphene oxide for boosting water splitting activity

The rational design of highly efficient and stable bifunctional catalysts for overall water splitting is vitally important. In this study, to increase the active catalytic sites of CeO2 for electrochemical water splitting, a ternary CeO2-CuO-Mn3O4 heterostructure, synthesized by coprecipitation method, is loaded on reduced graphene oxide (rGO) nanosheets in different amounts to produce CeO2-CuO-Mn3O4@rGO nanocomposites. It is found that CeO2-CuO-Mn3O4@rGO nanocomposites show higher electrocatalytic activity than unsupported samples, and the best activity is observed when the wieght ratio of CeO2-CuO-Mn3O4 is three times that of rGO. The CeO2-CuO-Mn3O4@rGO(3:1) requires low overpotentials of 130 and 270 mV for hydrogen and oxygen evolution reactions (HER and OER) at a current density of 10 mA cm−2. Furthermore, this material demonstrates a large electrochemically active surface area, low charge transfer resistance, suitable kintics, and high long-term stability for both OER and HER. Additionally, when CeO2-CuO-Mn3O4@rGO(3:1) is used as self-supported electrodes for the overall water splitting reaction, a low cell voltage of 1.68 V is obtained. This superior performance is due to: (i) active multi-metal sites that produce strong synergistic effects; (ii) the high conductivity of rGO, which faciliate favorable electron transfer; and (iii) the homogenous anchoring of CeO2-CuO-Mn3O4 on rGO, which increases the number of active sites available on the catalyst surface.

Engineering Lattice Strain in Co-Doped NiMoO4 for boosting Methanol Oxidation Reaction.

Nickel-based molybdates have attracted considerable attention owing to their distinctive isomorphous structure. In this study, pristine NiMoO4 and Co-doped Ni1-xCoxMoO4 were synthesized and investigated for their electrocatalytic activity in methanol oxidation and methanol-assisted water splitting reactions. Through a comprehensive exploration of the structure-property relationship, it was found that the optimal coexistence of α and β molybdate phases, induced by Co doping, led to lattice strain and facilitated the presence of essential catalytic descriptors such as higher oxidation states of Ni and surface oxygen vacancies within the lattice. These factors contributed to the enhanced electrocatalytic activity of Ni0.7Co0.3MoO4 in methanol oxidation and hydrogen evolution reaction. Detailed kinetic studies were conducted to further elucidate the mechanisms involved. Overall, these findings highlight the promising potential of Ni0.7Co0.3MoO4 as an effective catalyst for electrochemical methanol upgrading in conjunction with water splitting, with implications for sustainable energy conversion technologies.

Dynamic regulation of ferroelectric polarization using external stimuli for efficient water splitting and beyond.

Establishing and regulating the ferroelectric polarization in ferroelectric nano-scale catalysts has been recognized as an emerging strategy to advance water splitting reactions, with the merits of improved surface charge density, high charge transfer rate, increased electronic conductivity, the creation of real active sites, and optimizing the chemisorption energy. As a result, engineering and tailoring the ferroelectric polarization induced internal electric field provides significant opportunities to improve the surface and electronic characteristics of catalysts, thereby enhancing the water splitting reaction kinetics. In this review, an interdisciplinary and comprehensive summary of recent advancements in the construction, characterization, engineering and regulation of the polarization in ferroelectric-based catalysts for water splitting is provided, by exploiting a variety of external stimuli. This review begins with a detailed overview of the classification, benefits, and identification methodologies of the ferroelectric polarization induced internal electric field; this offers significant insights for an in-depth analysis of ferroelectric-based catalysts. Subsequently, we explore the underlying structure-activity relationships for regulating the ferroelectric polarization using a range of external stimuli which include mechanical, magnetic, and thermal fields to achieve efficient water splitting, along with a combination of two or more fields. The review then highlights emerging strategies for multi-scale design and theoretical prediction of the relevant factors to develop highly promising ferroelectric catalysts for efficient water splitting. Finally, we present the challenges and perspectives on the potential research avenues in this fascinating and new field. This review therefore delivers an in-depth examination of the strategies to engineer the ferroelectric polarization for the next-generation of water electrolysis devices, systems and beyond.

Semiconducting Overoxidized Polypyrrole Nano-Particles for Photocatalytic Water Splitting.

Capturing sunlight to fuel the water splitting reaction (WSR) into O2 and H2 is the leitmotif of the research around artificial photosynthesis. Organic semiconductors have now joined the quorum of materials currently dominated by inorganic oxides, where for both families of compounds the bandgaps and energies can be adjusted synthetically to perform the Water Splitting Reaction. However, elaborated and tedious synthetic pathways are necessary to optimize the photophysical properties of organic semiconductors. This study reports here, that when pyrrole dissolved distilled water is exposed to high energy radiation, this leads to the formation of nanostructured spherical polypyrrole (Nano-PPy) particles that are characterized as overoxidized polypyrrole. Electrochemical studies and Tauc's plot highlight the production of a semiconducting material with a bandgap of ≈1.8eV with the conduction band at ≈-0.5V and a valence band at ≈+1.3V vs NHE. When suspended in water and under irradiation at wavelengths higher than 420nm, Nano-PPy materials lead to O2 evolution, while electrons and protons can be recovered in the form of reduced quinone. Interestingly, upon intermittent visible irradiation and dark phases, a consumption of the evolved O2 from oxidation of water is observed. This concomitant O2 reduction is found to produce H2O2.

Enhanced Hydrogen Evolution Reaction of a Zn+2-Stabilized Tungstate Electrocatalyst

Due to their diverse properties and functionalities, cost-effective transition metal-based nanomaterials have been rigorously studied for electrochemical applications. Ultrathin nanosheets have been identified as the most effective electrodes for catalyzing water-splitting reactions in both acidic and alkaline environments. Here, we reported ZnWO4, a member of the tungstate family, as an effective electrocatalyst for promoting the electrochemical hydrogen evolution reaction. The Zn+2-stabilized tungstate showed a remarkable cathodic reaction during the water-splitting reaction with low overpotential (136 mV at 10 mA cm−2) and small HER kinetics (Tafel Slope = 75.3 mV dec−1) and long-term cyclic durability. The high-valence tungsten stabilized with divalent Zn+2 promotes electron transfer during the reaction, making it an advanced electrocatalyst for green hydrogen production.

Atomic Imaging of the Surface Termination and Reconstruction of Low and High Index Iridium Oxide Surfaces and Insights into Their Facet-Dependent Oxygen Evolution Activities.

Resolving the atomic surface structure, particularly surface termination or reconstruction, is essential for understanding the catalytic properties of metal oxides. Although rutile phase iridium dioxide (IrO2) is the state-of-the-art electrocatalyst for the oxygen evolution reaction (OER) in water splitting, the atomic-level surface structures of IrO2 remain largely unexplored, limiting our understanding of its facet-dependent OER activities. Herein, we perform aberration-corrected integrated differential phase contrast scanning transmission electron microscopy of the low- and high-index surface structures of spindle-shaped IrO2 nanorods and reveal distinct surface terminations and/or reconstructions on different surfaces. Notably, the (110) surface shows a predominantly top-/bridge-oxygen termination and high structural stability without obvious surface reconstruction. In contrast, the (001) and (101) surfaces, where all surface Ir atoms are coordinatively unsaturated, undergo significant reconstruction. Additionally, a high-index (321) surface composed of (110) terraces is identified and exhibits a distinct [IrO] surface termination, indicating a weaker binding energy between Ir with O. Density functional theory calculations reveals weakened oxygen-binding energies on both the reconstructed (101) surface and the high-index (321) surface, predicting substantially lower limiting OER overpotentials compared to the (110) surface. These findings provide an important structural basis for understanding the OER activities of IrO2 surfaces.

A brief review on the progress of MXene-based catalysts for electro- and photochemical water splitting for hydrogen generation.

The development and generation of affordable and highly efficient energy, particularly hydrogen, are one of the best approaches to address the challenges posed by the depletion of non-renewable energy sources. Hydrogen energy, as a green and ecosystem-friendly source with zero carbon emission, can be generated through various methods, including water splitting (HER/OER) via either photo- or electrocatalytic reactions. To implement these reactions effectively in practical applications, it is highly desirable to develop extremely efficient and cost-effective catalytic materials that are comparable to contemporary catalysts. MXenes, a family of newly discovered 2D transition metal carbides, nitrides, or carbonitrides with surface termination groups, such as -OH, -O, and -F, have emerged as promising materials and substrates for photo- and electrocatalytic applications due to their unique characteristics. These include excellent conductivity provided by the transition metals, hydrophilic nature imparted by the surface termination groups, high mechanical stability, fast electronic transmission and extremely high surface area-to-volume ratios. In this review, we provide detailed insights into the synthesis, properties, and catalytic applications of MXenes. We systematically outline the photo- and electrocatalytic water splitting reactions carried out by various MXene-based heterostructures, supported by experimental data. A thorough deliberation on the structure-activity associations of reported catalysts and a basic understanding of the electrocatalytic applications of MXenes are also included. Furthermore, we offer an insight into the upcoming tasks, challenges, prospects and new research strategies for MXenes in water splitting applications. A noteworthy recognition of the design and optimization of extremely efficient MXene-based catalysts in water splitting applications is therefore offered in this review.

Cobalt Hydroxide Modification of TiO2 Nanosheets for Visible-Light-Responsive Photocatalysts.

To make full use of sunlight for water splitting reactions for hydrogen production, a visible-light-driven photocatalyst was developed by modifying TiO2 nanosheets with Co(OH)2. By adding an aqueous Co(NO3)2·6H2O solution to a TiO2 nanosheet suspension, the TiO2 nanosheets aggregated and Co(OH)2 was formed. In the ultraviolet-visible (UV-vis) diffuse reflectance spectrum of the photocatalyst, new absorption bands attributable to Co(OH)2 and the interfacial charge transfer between Co(OH)2 and the TiO2 nanosheets appeared at around 600 and 400 nm, respectively. The photocatalytic activity of Co(OH)2/TiO2 nanosheets was evaluated in terms of the O2 evolution reaction in an aqueous AgNO3 solution, finding that the reaction proceeds under visible light. Furthermore, the investigation of the wavelength dependence of the photocatalytic activity revealed that the photocatalytic reaction on Co(OH)2/TiO2 nanosheets proceeds via Co(OH)2 photocatalysis and interfacial charge transfer between Co(OH)2 and the TiO2 nanosheets under visible light irradiation.

Co0.85Se/V3O4@NC composite catalyst for improved methanol oxidation reaction in water splitting

Co0.85Se/V3O4@NC composite catalyst for improved methanol oxidation reaction in water splitting

Regulating the electronic structure of CoMoO4via La doping for efficient and durable electrochemical water splitting reactions

Metal molybdates (M′MoO4, M = Fe, Co, and Ni) are recognized as active catalysts for water-splitting reactions.

Oxygen evolution activity of nickel-based phosphates and effects of their electronic orbitals.

Metal phosphate-type compounds have been utilized in diverse applications, and their distinctive chemical properties have recently opened avenues for their use as catalysts. Metal phosphates have previously demonstrated significant electrocatalytic activity for the anodic oxygen evolution reaction (OER) in electrochemical water splitting. However, the critical factors influencing OER electrocatalysis on Ni-based phosphates have been insufficiently explored. We herein demonstrate nickel (Ni)-based phosphates-monoclinic Ni3(PO4)2, monoclinic Ni2P2O7, and monoclinic Ni2P4O12-as exemplary materials exhibiting outstanding OER activity in alkaline media. These Ni-based phosphates exhibit superior OER overpotentials compared to conventional Ni-based oxides (NiO) and phosphides (Ni2P). Additionally, their OER-specific activity surpasses that of the rare metal-based benchmark, IrO2, and previously reported state-of-the-art crystalline electrocatalysts comprising nonprecious metals. Long-term durability tests show that Ni3(PO4)2 maintains its OER activity even after 1000 repeated potential cycles while retaining its elemental composition and Raman spectrum. To understand the excellent OER activities of Ni-based phosphates, the atomic configurations within their crystals are examined. Remarkably, a clear correlation between Ni-O bond length and OER overpotentials is observed in both Ni-based phosphates and NiO, i.e., shorter Ni-O bond lengths are highly beneficial for the OER. Density functional theory (DFT) calculations revealed that the outstanding OER activities of Ni-based phosphates are facilitated by their favorable electronic orbitals, which strengthen the Ni-O bond and improve the adsorption of OER intermediates on Ni sites. This mechanism is substantiated by DFT calculations employing surface slab models, where the adsorption of OER intermediates on the surface of Ni-based phosphates is more energetically favorable than on the surface of NiO. Hence, Ni-based phosphates are promising OER electrocatalysts, and this study provides important guidelines to further improve Ni-based electrocatalysts.

Synergistic flower-like N-doped Co/Mo2C catalyst for hydrogen evolution reaction in universal pH and high-performance alkaline water-splitting

Synergistic flower-like N-doped Co/Mo2C catalyst for hydrogen evolution reaction in universal pH and high-performance alkaline water-splitting

AuCu bimetallic nanocluster-modified titania nanotubes for photoelectrochemical water splitting: composition-dependent atomic arrangement and activity.

The photoelectrochemical (PEC) water splitting reaction of bimetallic AuxCu1-x (x = 1, 0.75, 0.5, 0.25 and 0) nanocluster-decorated TiO2 nanotube (TNT) photoanodes was investigated using a solar simulator. A strong enhancement in the anodic photocurrent relative to pristine TNTs was found with clear composition-dependent PEC activity, increasing with the Cu content and peaking at Au0.25Cu0.75. Electron microscopy and X-ray absorption fine structure spectra recorded at both Au and Cu edges identified a clear composition-dependent atomic arrangement of the spherical nanoclusters on anatase TNTs, resulting mostly from a time-dependent restructuring of the original metallic nanoalloys in the ambient environment. With time, Cu segregates from the alloy to form a surface oxide layer surrounding a pure gold metallic core in the gold-rich nanoclusters (x = 0.75 and 0.50) or a face centered tetragonal (fct)-intermetallic Au0.5Cu0.5 nanoalloy in copper-rich (x = 0.25) particles. In pure Cu nanoclusters, a metallic Cu core is stabilized by surrounding Cu2O and CuO. The enhanced PEC activity is attributed to a synergy between Au and Cu that upon segregation produces bifunctional catalytic sites consisting of a metallic Au/AuCu alloy and copper oxide at the surface of the nanoclusters. The photoactivity under solar light illumination is boosted by the plasmonic response of the metal. The ordered structure of the fct-AuCu alloy present in the most active Au0.25Cu0.75 may explain its higher stability and photocatalytic performance. Hence, this work provides insight into the relationship between the atomic-level structure of AuxCu1-x nanoalloys on TNTs and their PEC activity.

Facile synthesis of a new nanocomposite based on Ba(II)-substituted phosphomolybdate anchored on manganese oxide, PMo11Ba@MnO2, for oxygen evolution of water splitting reaction

Facile synthesis of a new nanocomposite based on Ba(II)-substituted phosphomolybdate anchored on manganese oxide, PMo11Ba@MnO2, for oxygen evolution of water splitting reaction

Mesoporous Co-MoS2 with sulfur vacancies: A bifunctional electrocatalyst for enhanced water-splitting reactions in alkaline media.

As a graphene-like layered material, molybdenum disulfide (MoS2), has attracted increasing attentions for its promising application in electrocatalysis. Whereas MoS2 still suffers from the sluggish reaction kinetics in oxygen evolution reaction (OER) due to the low density of active sites in most exposed planes. In this work, high density of active sites on MoS2 basal planes has been obtained by synthesizing mesoporous MoS2 with Co doping and sulfur vacancies (VS). The synergy of the mesoporous structure, Co doping, and sulfur vacancies resulted in optimized bifunctional electrocatalytic activity for both hydrogen evolution reaction (HER) and OER in alkaline media. The overpotential required to achieve a current density of 10mA cm-2 (denoted as η10) is 34 mV for HER and 268 mV for OER, respectively. The two-electrode electrolyzer constructed with the as-prepared cobalt-doped mesoporous MoS2 electrodes exhibited a low bias (η10=1.58V) for overall water splitting. Density functional theory (DFT) calculations confirm the significance of Co doping and the S vacancy defects, which lowers the Gibbs free energy (ΔG) for the formation of the corresponding intermediates.

MoSSe-graphene hybrid material as an advanced catalyst for hydrogen evolution reaction in water splitting

Abstract Hydrogen, with its high energy density and environmentally friendly nature, holds great promise as a future energy. The development of efficient catalysts for the hydrogen evolution reaction (HER) in water splitting is necessary. In this study, we focused on synthesizing a hybrid material MoSSe-Graphene (MoSSe-Gr) as a catalyst for HER. The MoSSe-Gr hybrid catalyst was synthesized through a solvothermal method followed by calcination under an inert atmosphere at 800 °C. Morphology and structural analysis of the catalyst was analyzed using SEM, TEM, XRD and Raman spectroscopy. The MoSSe-Gr hybrid catalyst exhibited remarkable catalytic activity for HER, showing significantly lower overpotentials of 90 mV and 200 mV at current densities of 1 and 10 mA cm−2, respectively, comparable to the benchmark Pt/C catalyst. Furthermore, the MoSSe-Gr catalyst demonstrated long-term electrochemical stability during HER over a 24 h period in an acidic medium. Furthermore, the MoSSe-Gr catalyst demonstrated long-term electrochemical stability during HER in 24 h duration in an acidic medium. The MoSSe-Gr hybrid catalyst shows great potential as an efficient and stable catalyst for HER, promoting sustainable hydrogen production.

Cation-Modulated Ni/Ni3N Compound Heterojunctions as Highly Efficient Bifunctional Electrocatalysts for Water Splitting.

The exploration and rational design of high-performance, durable, and non-precious-metal bifunctional oxygen electrocatalysts are highly desired for the large-scale application of overall water splitting. Herein, an effective and straightforward coupling approach was developed to fabricate high-performance bifunctional OER/HER electrocatalysts based on core-shell nanostructure comprising a Ni/Ni3N core and a NiFe(OH)x shell. The as-prepared Ni/Ni3N@NiFe(OH)x-4 catalyst exhibited low overpotentials of 57 and 243 mV at 10 mA cm-2 for the HER and OER in 1.0 m KOH, respectively, superior to most bifunctional oxygen electrocatalysts reported so far. Compared to the unmodified Ni/Ni3N, the Ni/Ni3N@NiFe(OH)x-4 catalyst exhibited a 43.3-fold increase in mass activity for the OER and an 8.7-fold increase for the HER, as well as a 29.5-fold increase in intrinsic activity for the OER and a 2.6-fold increase for the HER. When employed as both the cathode and the anode of the electrolyzer for the overall water splitting reaction, its voltage was reduced to 1.58 V at 10 mA cm-2. This surface reconstruction method increased the electrochemically active surface area and enhanced the catalytic activity. Furthermore, in situ Raman spectroscopy revealed that the Fe etching reduced the onset potential for the active phase NiOOH, promoted its formation, and accelerated the reaction kinetics, thereby enhancing the overall electrocatalytic performance of the catalyst.

Photoelectrochemical Investigation of Hole Scavengers for Photocatalytic Hydrogen Evolution Reaction on Perovskite‐Type Niobate Nanosheets

AbstractHole‐scavengers are a crucial part of particulate photocatalytic systems. The use of hole‐scavengers increases the photocatalytic activity by preventing electron‐hole recombination. The holes are used in photo‐oxidation of the hole‐scavenger molecules increasing the probability of electrons involved in photoreduction of water. In this study, different hole‐scavengers utilized in photocatalytic water splitting reaction were tested. The impact of different hole scavengers on photocatalytic reactions were investigated on [Ca2Nb3O10]− 2D nanosheets mechanistically. Cyclic voltammetry and chronoamperometry tests were employed to assess the impact of hole scavengers on photocatalytic efficiency. The photoelectrochemical tests agree with the results of the photocatalytic reactions. Methanol and EDTA give the largest oxidation photocurrent and the best photocatalytic activity.

New Layered Chalcogenide-Type Metal Sulfide Materials with Improved Properties for Solar Fuel Production Applications.

Novel lamellar chalcogenide materials, named as ITQ-75, are synthesized, focusing on the advancement and alteration of metal (tin and zinc) sulfide-based microstructured materials. These are achieved via hydro(solvo)thermal processes in the presence of N-heterocyclic aromatic structural directing agents. The comprehensive characterization of these materials included fine-tuning their electronic structure through a metal doping strategy and enhancing their accessibility by modifying the synthesis gel composition. This modification involved altering the gel's viscosity or incorporating mesoporogen agents such as saccharide moieties. The most promisingly modified ITQ-75-type materials demonstrated optical band gap values of ≈2.0eV, falling within the optimal range for efficient solar fuel production processes. Furthermore, the photocatalytic performance of these optimized lamellar chalcogenides is assessed using the water-splitting reaction for hydrogen generation in the gas phase and without any sacrificial reagent. These new noble metal-free materials are revealed to be among the most efficient to date (up to 7µmolH2hcm- 2). The results confirm the potential of these materials as promising photocatalysts for solar fuel production applications.