Metal organic frameworks as electrocatalysts: Hydrogen evolution reactions and overall water splitting

The development of clean energy technologies to protect the environment is an important demand of the times. Electrocatalysis is emerging as a promising method for evolution of hydrogen and overall water splitting. Nowadays, metal organic frameworks (MOFs) have emerged as electrocatalysts having uniformly distributed active sites and high electrical conductivity. This review summarizes the latest advances in heterogeneous catalysis by MOFs and their composite/derivatives for efficient hydrogen evolution reaction (HER) and water splitting. Pristine MOFs with their recent development are summarized first followed by composites of MOFs with their enhanced electrocatalytic performances. Overall water splitting by using bifunctional electrocatalysts derived from MOFs with different synthetic approaches is provided and this review gives the metal-based categorisation of precursor MOFs. Different strategies to improve chemical stability, conductivity, and overall electrocatalytic properties have been discussed. In the last, perspectives on the synthesis of efficient MOF-based electrocatalyst materials are provided.     

Recent strategies to improve the catalytic activity of pristine MOFs and their derived catalysts in electrochemical water splitting

In recent years, MOF-based materials, including pristine metal-organic frameworks (MOFs) and their derivatives, are one kind of the most popular materials, and have been widely used in biomedicine, sensing, energy storage, and catalysis due to their unique pore structures, abundantly accessible metal nodes, large specific surface areas, tailorable function, and morphology. Most of them have shown satisfactory performances in specific fields. However, there are still some urgent problems need to be solved in the application of MOF-based materials in electrocatalytic water splitting, especially the facile and controllable synthesis, and limited activity and stability. Herein, first this article mainly reviewed the applications and challenges of MOF-based materials in electrocatalytic water splitting. Then, typical development strategies (e.g., construction of multi-metallic site of MOF-based materials, structural morphology designing and controlling, engineering of doping, defect, vacancy, coupling with conductive carrier or constructing heterostructures) were highlighted in this review. Finally, perspectives on the development of MOF-based materials were provided for fabricating more efficient water splitting electrocatalysts.     

Engineering MOF-based nanocatalysts for boosting electrocatalytic water splitting

Electrochemical water splitting is now appearing as a promising strategy for the renewable production of clean hydrogen. Constructing advanced electrocatalysts for boosting these processes is intensely expected to reduce their overpotentials and facilitate the practical applications. Benefitting from their tunable compositions, excellent porosities, and ultrahigh surface areas, metal-organic frameworks (MOFs) are thus emerging as the nonprecious candidates for driving overall water splitting. Recent years have witnessed the rapid development of MOFs toward electrochemical water splitting and beyond. Herein, we have presented the most pivotal progresses in recent research on engineering advanced MOFs for boosting electrochemical water splitting. This review is started by manifesting the advantages and disadvantages of MOFs. Subsequent, some advanced strategies for the effective modifications of MOFs are also highlighted. Finally, a summary about the future directions and challenges are also presented to provide guidance for the further development of more efficient MOF-based electrocatalysts.     

Metal organic frameworks: Mastery in electroactivity for hydrogen and oxygen evolution reactions

Electrochemical water splitting is recognized as a conspicuous technique for sustainable and an alternative energy storage systems. Fabricating different catalysts for electrocatalysis is highly desirable to decrease the overpotential and ease practical applications. Metal-organic-frameworks (MOFs) have obtained significant consideration recently due to tunable porous structure, superior catalytic activity, and high surface area. Owing to the properties of MOF, these materials can be employed as catalysts for overall water splitting applications. Herein, the most recent advancement in MOFs for an efficient electrochemical water splitting are demonstrated. Primarily, the basics and reaction mechanisms of water splitting were summarized which is followed by the recent improvements in electrocatalytic properties of pristine MOFs, guest@MOFs, MOF derived different metallic compounds and carbon-based catalytic materials. The fast growing innovations in the electrocatalytic activities and their fundamental mechanisms are comprehensively summarized. Finally, a thorough discussion on the current challenges and future outlooks in water splitting is provided.     

Recent progress in photocatalysts for overall water splitting

Photocatalytic splitting of water with solar energy is considered as the most promising approach for the production of hydrogen fuel. However, its solar to hydrogen conversion efficiency is much below the industrial requirement (10%). This situation has stimulated intensive efforts to improve photocatalytic overall water splitting (namely, simultaneously providing unassisted oxidation and reduction of water), leading to the invention of novel catalysts in the recent years. The evaluation of these recent progresses constitutes this review article, with emphasis on the strategies employed for the development of catalysts. The catalysts were deeply reviewed and were classified into four types: (a) perovskite compounds, (b) metal oxides (sulfides and nitrides), (c) Bi‐ and In‐based materials, and (d) multicomponent catalysts. Furthermore, the challenges that remain with the process and catalysts and the potential advances were discussed as an outlook for future research.     

External and internal dual-controls: Tunable cavity and Ru–O–Co bond bridge synergistically accelerate the RuCoCu-MOF/CF nanorods for urea-assisted energy-saving hydrogen production

Currently, there are still many limitations in the research of conductive metal-organic frameworks (MOFs) in the field of electrocatalysis. On the one hand, most MOFs have a solid construction, seriously hindering their mass transfer process. On the other hand, innate bonding is not conducive to the optimization of electronic structures and the excitation of intrinsic active sites. Therefore, external/internal dual-control of MOFs is urgently needed to break the shackles of their activity. Herein, the hollow RuCoCu-MOF/CF nanorods with tunable cavities are directionally constructed by a self-sacrificial template method. Benefiting from the exact morphological control and the unique Ru–O–Co bond bridge, RuCoCu-MOF/CF exhibits superior performances for alkaline hydrogen evolution reaction (HER) and urea oxidation reaction (UOR). Surprisingly, a record-breaking voltage of 1.402 V drives a current density of 10 mA cm⁻² for urea-assisted overall water splitting under alkaline conditions, greatly promoting the development of energy-efficient hydrogen production technology. This work firstly constructed the MOF-based self-supporting electrode with ultra-high urea-assisted hydrogen production and urea degradation performances via the dual controls of the cavity size and chemical bond bridge. This points out the direction for the development of unique integrated electrodes for both hydrogen production and decontamination.     

Metal-organic frameworks as highly efficient electrodes for long cycling stability supercapacitors

Among a large variety of energy storage technologies, supercapacitors possess special advantages such as rapid charge/discharge, high power density, safety, and environmental friendliness to meet the requirement of specific applications. The common electrode materials of supercapacitors, including porous carbon, conductive polymers, and metal oxides/hydroxides, have their own benefits and drawbacks in energy density and stability. Owing to the big surface area and controllable porosity, the metal-organic frameworks (MOFs) have been explored as important candidates for supercapacitor applications. This mini-review focuses on the recent advances of MOF-based materials including pristine MOFs, MOFs composite materials, and MOF-derived materials in the development of long cycling life supercapacitors. The devices discussed here mean those with capacitive retention rates of more than 90% after 10,000 cycles and high energy density. In addition, we also describe the fundamental knowledge of supercapacitors, highlight the stabilization mechanism of MOFs, and propose the strategies to enhance the stability of MOF-based supercapacitor electrodes.     

Modular design in metal-organic frameworks for oxygen evolution reaction

Among the cutting-edge materials for catalysis, metal-organic frameworks (MOFs) have become popular. The application has gradually shifted from organic synthesis to electrocatalysis in recent years. MOFs based on various transitions metals have exhibited excellent performance for electrocatalysis. The oxygen evolution reaction (OER) is an essential process for electrocatalysis, developing efficient OER electrocatalysts is challenging. Herein, a new model design is presented based on MOFs. By establishing the proper link between these modules, researchers may correctly and effectively synthesize hollow nanostructured MOFs, electrically conducting MOFs, hierarchically porous MOFs, functionalized MOFs and two-dimensional MOFs. The difficulties and potential of MOF-based catalysts for OER research are finally highlighted. Ultimately, the modular design concept examined in this study might be a useful tool for examining the relationships between structure and activity and accelerating the design of multi-component electrocatalysts on supply.     

Recent progress on tungsten oxide-based materials for the hydrogen and oxygen evolution reactions

As a form of clean and renewable energy, hydrogen has received much attention recently. However, industrial hydrogen production is primarily via conversion of natural gas, which consumes a large amount of energy and emits large volumes of greenhouse gases. Electrochemical water electrolysis is a promising, pollution-free method for the production of hydrogen from water. Efficient, cost-effective, stable and abundant catalysts that can drive hydrogen production in water with minimal electrical bias are a major goal towards achieving electrolysis on a large scale. Recently, tungsten oxide-based materials have emerged as one of the most promising electrocatalytic compounds, due to their activity, low cost and durability in both acid and base conditions. There are often oxygen vacancies in metal oxides, whether intentional or not, which can potentially promote the water electrolysis. In this review, we provide an overview of tungsten oxide-based materials used for electrocatalytic water splitting. In addition, mechanisms to improve the electrocatalytic activities of oxygen vacant tungsten oxide are summarized and discussed, with proposals for future research. This review article will provide a valuable resource for scientists pursuing materials for electrochemical water splitting.     

A review: Target-oriented transition metal phosphide design and synthesis for water splitting

With the scarcity of fuel energy, non-noble metal compounds assisted water of electrolysis is becoming a potential candidate for cost-effective and high-quality hydrogen production. To date, transition metal phosphide (TMP) has been considered as one of the most promising catalysts for water splitting because of its multitudinous but controlled constituent elements and stoichiometric ratios. In this review, the electronic structure analysis of TMP dialectically reveals the active derivation for catalyzing hydrogen evolution (HER) and oxygen evolution reaction (OER). And then the strategies of rationally designing the structure and composition of electrocatalyst to improve its intrinsic activity are discussed, especially interface engineering. Besides, this review also focuses on the negligible stability issue during the water splitting process. In the end, some key challenges and research orientations of TMPs are pointed out, which is instructive for developing high-efficient and stable electrocatalysts for water splitting.     

双金属Ni-Co基析氧反应电催化剂研究进展

  目的  能源消耗的持续增长和化石燃料燃烧带来的环保和能源安全问题已经引起世界各国的广泛关注。因此,发展清洁能源生产技术已成为世界范围内的主要研究重点。氢能具有无污染、比能密度高、资源丰富等特点,是最具潜力的传统化石燃料替代品之一。电催化分解水被认为是最有希望的制氢方法,但阳极上的析氧反应动力学缓慢,能量转换效率低,是大规模制氢的主要瓶颈。与稀有和昂贵的贵金属催化剂相比,镍-钴（Ni-Co）基电催化剂由于具有可调的电子结构、高导电性和低成本优势,有望在碱性溶液中实现卓越的OER活性和耐久性。  方法  文章总结并讨论了在OER中Ni-Co基电催化剂的最新研究发展。重点讨论了Ni-Co基电催化剂的设计和合成,以及在OER过程中提高其电催化性能的研究策略。  结果  为了代替钌、铱等贵金属催化剂,研究者们对Ni-Co基非贵金属催化剂进行了大量研究。包括氧化物、氢氧化物、合金、氮化物、硫化物、磷化物等在内的多种Ni-Co基催化剂通过化学结构的调控,从阳极角度提高了电催化制氢的活性。但这些催化剂又分别面临不同的缺陷,有待进一步研究克服。  结论  开发具有高OER活性的非贵金属催化剂是降低电解水制氢成本,促进氢能产业发展的重要途径。虽然仍有一些技术问题尚未解决限制了Ni-Co基催化剂替代贵金属催化剂,但作为重要的贵金属催化剂替代品,Ni-Co基催化剂的研究为新型催化剂的开发提供了重要选择。     

Metal-organic frameworks (MOFs)-based efficient heterogeneous photocatalysts: Synthesis,properties and its applications in photocatalytic hydrogen generation,CO2 reduction and photodegradation of organic dyes

Metal-organic frameworks (MOFs) are a new class of functional materials having porous structures that show extraordinary specific surface areas, and tunable surface chemistry; hence, they hold great potential as photocatalysts. This review describes the fundamentals of MOFs and possible new research directions in the area of heterogeneous MOFs that can provide enhanced photocatalytic performance, especially for hydrogen production, degradation of emerging organic pollutants, and CO₂ reduction. The role of MOFs as multifunctional photocatalysts for light-stimulated organic reactions through an effective combination of metal/ligand/guest-based photocatalysts is discussed. Recent literature is discussed critically on the design and selection of materials, with possible directions to improve their catalytic properties. Furthermore, this comprehensive review systematically discusses the current developments of various MOFs-based hybrid nanostructures as multifunctional photocatalysts from different points, including several synthetic methodologies, key features, photocatalytic mechanism, and various influencing parameters to enhance catalytic efficiency. The recent achievements are critically discussed in the designing and selection of MOFs-based functional materials, with directions to effectively improve their catalytic properties for various photocatalytic applications. The article also summarizes with challenges and future prospects for the cost-effective and large scale photocatalytic applications of MOFs-based heterostructured catalysts.     

Recent trends in MXene/Metal chalcogenides for electro-/photocatalytic hydrogen evolution reactions

Hydrogen due to high energy density and ecologically benign characteristics can become an excellent energy carrier for a sustainable energy economy and to appease the energy demand of humankind. Moreover, cost-effective and long-lasting photocatalysts can make the hydrogen generating process more economical and suitable. Recently, MXene have become one of the most sought-after composite materials for photocatalytic hydrogen generation. However, the photocstalytic performance can be further enhanced by doping with other semiconductor materials. Transition metal chalcogenides (Transition metals = Cu, Co, Ni, Zn, Cd, Mo, W)/MXene composites and mixed transition metal chalcogenide/MXene nanocomposites have been extensively investigated for the photocatalytic hydrogen generation. These materials possess unique two-dimensional layered structure that ameliorates the photocatalytic water splitting performance by increasing the light adsorption even at low photon flux density. The 2D design assists in reducing the distance necessary to transverse charge carriers to the surface. Because the layered structure tends to trap electrons in the ultrathin layers, 2D materials have unusual optoelectronic properties. In-plane covalent bonding assisted the creation of various heterojunctions and heterostructures in these 2D materials. Water splitting and hydrogen production are aided by the high surface area of these 2D materials. Due to its diverse elemental composition, unique 2D structure, good photoelectronic characteristics, large surface area, and many surface terminations. The design and production of many types of materials used as catalysts for the hydrogen evolution process are discussed in this article.     

Novel strategies to tailor the photocatalytic activity of metal–organic frameworks for hydrogen generation: a mini-review

This review provides a recompilation of the most important and recent strategies employed to increase the efficiency of metal–organic framework (MOF)-based systems toward the photocatalytic hydrogen evolution (PHE) reaction through specific strategies: tailoring the photocatalytic activity of bare MOFs and guest@MOF composites, formation of heterojunctions based on MOFs and various photocatalysts, and inorganic photocatalysts derived from MOFs. According to the data reported in this mini-review, the most effective strategy to improve the PHE of MOFs relies on modifying the linkers with new secondary building units (SBUs). Although several reviews have investigated the photocatalytic activity of MOFs from a general point of view, many of these studies relate this activity to the physicochemical and catalytic properties of MOFs. However, they did not consider the interactions between the components of the photocatalytic material. This study highlights the effects of strength of the supramolecular interactions on the photocatalytic performance of bare and MOF-based materials during PHE. A thorough review and comparison of the results established that metal–nanoparticle@MOF composites have weak van der Waals forces between components, whereas heterostructures only interact with MOFs at the surface of bare materials. Regarding material derivatives from MOFs, we found that pyrolysis destroyed some beneficial properties of MOFs for PHE. Thus, we conclude that adding SBUs to organic linkers is the most efficient strategy to perform the PHE because the SBUs added to the MOFs promote synergy between the two materials through strong coordination bonds.     

Metal-organic frameworks for CO2 photoreduction

Metal-organic frameworks (MOFs) have attracted much attention because of their large surface areas, tunable structures, and potential applications in many areas. In recent years, MOFs have shown much promise in CO₂ photoreduction. This review summarized recent research progresses in MOF-based photocatalysts for photocatalytic reduction of CO₂. Besides, it discussed strategies in rational design of MOF-based photocatalysts (functionalized pristine MOFs, MOF-photosensitizer, MOF-semiconductor, MOF-metal, and MOF-carbon materials composites) with enhanced performance on CO₂ reduction. Moreover, it explored challenges and outlook on using MOF-based photocatalysts for CO₂ reduction.     

Metal-organic-framework-derived metals and metal compounds as electrocatalysts for oxygen evolution reaction: A review

Electricity-driven oxygen evolution reaction (OER) is crucial for water dissociation because it allows for sustainable and clean energy production. However, the fast reaction should be improved for industrial applications. Therefore, numerous studies have focused on designing and synthesizing high-performance catalysts with low-cost facial processes for use in the OER. Metal-organic frameworks are hybrid materials that consist of inorganic and organic components, exemplified as ideal sacrificial templates for the fabrication of efficient anode materials to be used in water oxidation. In this account, various types of MOF-derived anode materials, including metals, metal oxides, metal phosphides, nitrides, carbides, and metal chalcogenides, are discussed. In addition, we have demonstrated the advantages of MOFs and provide studies with controverted mechanisms of the OER. Finally, the current problems and prospects for the use of MOFs in the electrochemical OER are discussed.     

Recent development in band engineering of binary semiconductor materials for solar driven photocatalytic hydrogen production

Photocatalytic hydrogen production from water splitting is a promising approach to develop sustainable renewable energy resources and limits the global warming simultaneously. Despite the significant efforts have been dedicated for the synthesis of semiconductor materials, key challenge persists is lower quantum efficiency of a photocatalyst due to charge carrier recombination and inability of utilizing full spectrum of solar light irradiation. In this review, recent developments in binary semiconductor materials and their application for photocatalytic water splitting toward hydrogen production are systematically discoursed. In the main stream, fundamentals and thermodynamic for photocatalytic water splitting and selection of photo-catalysts has been presented. Developments in the binary photocatalysts and their efficiency enhancements though surface sensitization, surface plasmon resonance (SPR) effect, Schoktty barrier and electrons mediation toward enhanced hydrogen production has been deliberated. Different modification approaches including band engineering, coupling of semiconductor catalysts, construction of heterojunction, Z-scheme formation and step-type photocatalytic systems are also discussed. The binary semiconductor materials such as TiO₂, g-C₃N₄, ZnO, ZnS, Fe₂O₃, CdS, WO₃, rGO, V₂O₅ and AgX (Cl, Br and I) are systematically disclosed. In addition, role of sacrificial reagents for efficient photocatalysis through reforming and hole-scavenger are elaborated. Finally, future perspectives for photocatalytic water splitting towards renewable hydrogen production have been suggested.     

Interface design and composition regulation of cobalt-based electrocatalysts for oxygen evolution reaction

The efficient, stable and low-cost catalyst for water splitting is a key factor especially for the industrial production of green hydrogen. Oxygen evolution reaction (OER), a critical semi-reaction, severely limits the occurrence of water splitting due to the slow kinetics of four-electron transfer. Cobalt-based materials are the well-stocked conductive catalytic feedstock, which can drive down the energy cost needed for OER via lowering the overpotential. Herein, cobalt-based materials and their derivatives in the field of OER catalysis in alkaline, acidic and neutral media are summarized. The interface design and composition regulation strategy are the key approaches for cobalt-based catalysts design in recent years. The related catalytic mechanisms and structure-function relationship have been deeply discussed and analyzed. Finally, for different media environment, a new prospect is proposed for the design of efficient cobalt OER catalytic materials and their application in industrial large-scale energy output strategies.