A review of performance optimization of MOF‐derived metal oxide as electrode materials for supercapacitors

Metal‐organic frameworks (MOFs), as new class of porous materials, are constructed by inorganic metal centers and bridging organic links. Recently, MOFs have been proved to be effective templates for preparing metal oxides with large surface areas and controlled shape by directly annealing in air. There are lots of reports about metal‐organic framework‐derived metal oxides as electrode materials for supercapacitors. Metal‐organic framework‐derived metal oxides can offer higher capacitances compared with that prepared by other synthetic methods, likely attribute to high surface areas and optimal pore sizes. However, at present, the specific capacitances of MOF‐derived metal oxides received are far lower than theoretical values, and the cycle numbers could not meet practical demands. Accordingly, much effort has been made to improve the performance by further modifying MOFs. Thus, this paper focused on the advances in performance optimization of MOF‐derived metal oxide as electrode materials for supercapacitors as follows:

1. Dual metal MOF‐derived binary metal oxides. Metal oxides with 2 metal cations possess better electrical conductivity and richer redox active sites than single metal oxides.
2. Metal‐organic framework‐derived carbon/metal oxide composites (MO@C) or graphene/MOF‐derived graphene/metal oxide composites. Doping carbon not only facilitate transportation of electrodes but also contribution to extra double‐layer capacitance.
3. Hybrid MOF‐derived metal oxide composites (MO@MO). Metal oxide composites can produce some synergistic effects that the individuals cannot provide.
4. Metal‐organic framework‐derived metal oxides with a hollow structure. The Hollow structure could shorten ion diffusion distance and adapt to volume expansion generated during the ion intercalated/extracted process.

     

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 advancements in MOFs synthesis and their green applications

Metal organic frameworks (MOFs) are crystalline, hybrid, materials containing metal ions and organic molecules; components that together form three-dimensional structures. At the present, the large-scale production necessary for the commercialization of MOFs based industrial processes seems to be the greatest challenge, mainly due to the fact that MOFs require harsh operating conditions and large amounts of toxic solvents during their synthesis which result in added production costs and raise safety and environmental related concerns. This review provides an overview on the recent developments in the synthesis of MOFs and their green applications. The currently used solvothermal processes for the synthesis of MOFs necessitate the use of hazardous and non-renewable solvents such as such as dimethylformamide (DMF), dimethylformamide (DEF), methanol, ethanol, and acetonitrile. Green and economical routes for MOFs synthesis that are non-solvent based have been rigorously under study to facilitate the commercialization of synthesis processes that are safe and environmentally benign. Such processes include water-based MOF synthesis, mechanochemical MOFs synthesis and the use of super-critical fluids and ionic liquids as solvents to replace the conventionally used ones. The paper also discusses the recent developments in the applications of MOFs in innovative and vital applications such as hydrogen production, fuel cells, and water desalination. Challenges and corrective actions attributed to the use of MOFs in these applications are also presented and discussed.     

Metal organic frameworks for hydrogen purification

High purity hydrogen is one of the key factors in determining the lifetime of proton exchange membrane (PEM) fuel cells. However, the current industrial processes for producing high purity hydrogen are not only expensive, but also come with low energy efficiencies and productivity. Finding more cost-effective methods of purifying hydrogen is essential for ensuring wider scale deployment of PEM fuel cells. Among various hydrogen purification methods, adsorption in porous materials and membrane technologies are seen as two of the most promising candidates for the current industrial hydrogen purification methods, with metal organic frameworks (MOF) being particularly popular in research over the last decade. Despite many available reviews on MOFs, most focus on synthesis and production, with few reports focused on performance for hydrogen purification. This review describes the working principle and performance parameters of adsorptive separations and membrane materials and identifies MOFs that have been reported for hydrogen purification. The MOFs are summarised and their performance in separating hydrogen from common impurities (CO₂, N₂, CH₄, CO) is compared systematically. The challenges of commercial application of MOFs for hydrogen purification are discussed.     

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.     

Three-dimensional flower-like Co-based metal organic frameworks as efficient multifunctional catalysts towards oxygen reduction,oxygen evolution and hydrogen evolution reactions

Design and development of cost-efficient multifunctional three-dimensional (3D) metal organic frameworks (MOFs) towards oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are very significant for green energy devices. Herein, a scalable one-pot solvothermal method is developed to obtain a series of multifunctional 3D flower-like MOFs. In addition, systematic studies are also conducted on the effects of various metal cations and N-containing ligands on the structures, compositions, and multifunctional performance of the obtained MOFs. As a result, 3D flower-like Co-MOFs using Co²⁺ as a metal cation and 2,2’:6′,2″-terpyridine as a N-containing ligand exhibit the highest multifunctional performance towards ORR, OER and HER. The scalable method provides a new prospect to design and develop other MOFs-based multifunctional catalysts.     

Review on processing of metal–organic framework (MOF) materials towards system integration for hydrogen storage

Development of safe and effective hydrogen storage systems is critical for further implementation of hydrogen in fuel cell technologies. Amongst the various approaches to improve the performance of such systems, porous materials‐based adsorptive hydrogen storage is envisaged as a long‐term solution because of the excellent reversibility, good kinetics and the possibility to store hydrogen at low pressures. Metal–organic frameworks (MOFs) have attracted much attention as porous hydrogen storage materials in the transition from the laboratory to commercial applications. However, MOF materials are often obtained as loose powders with low packing densities and low thermal conductivities. Therefore, to facilitate this transition and enable the MOF materials to form part of a practical hydrogen storage system, knowledge of the ‘processing’ techniques to improve the properties of the powders is essential. However, the processing routes of MOF materials towards system integration are rarely reviewed in the literature although this is of great significance in their proper assessment and potential use for hydrogen storage on a commercial scale. In this review, we begin by introducing the general requirements of an MOF materials‐based hydrogen storage system and present how these requirements translate into desired characteristics for further processing. Then, an overview of MOF materials processing towards system integration is provided with an emphasis on improving selected properties including (i) structural stability, (ii) thermal conductivity and (iii) hydrogen storage properties. Copyright © 2014 John Wiley & Sons, Ltd.     

Characterisation of metal organic frameworks for adsorption cooling

Silica gel/water adsorption cooling systems suffer from size, performance and cost limitations. Therefore, there is a need for new adsorbent materials that outperform silica gel. Metal organic frameworks (MOFs) are new micro-porous materials that have extraordinary porosity and uniform structure. Due to the lack of published data that characterise MOF/water adsorption, this paper experimentally investigates the adsorption characteristics of HKUST-1 (Cu-BTC (copper benzene-1,3,5-tricarboxylate), C₁₈H₆Cu₃O₁₂) and MIL-100 (Fe-BTC (Iron 1,3,5-benzenetricarboxylate), C₉H₃FeO₆) MOFs compared to silica gel RD-2060. The adsorption characteristics of Silica gel RD-2060, HKUST-1 and MIL-100 were determined using an advanced gravimetric dynamic vapour sorption analyser (DVS). Results showed that HKUST-1 performed better than silica gel RD-2060 with an increase of water uptake of 93.2%, which could lead to a considerable increase in refrigerant flow rate, cooling capacity and/or reducing the size of the adsorption system. However, MIL-100 MOF showed reduced water uptake comparable to silica gel RD-2060 for water chilling applications with evaporation at 5 ⁰C. These results highlight the potential of using MOF materials to improve the efficiency of water adsorption cooling systems.     

Flower-like MOF-derived Co–N-doped carbon composite with remarkable activity and durability for electrochemical hydrogen evolution reaction

Electrochemical hydrogen evolution reaction (HER) is one of the most economical, sustainable, and attractive methods to produce hydrogen. Contemporarily, it is still a challenge to develop low-cost catalysts with high activity and durability for HER. Herein, we report a simple strategy to develop a Co–N-doped carbon electrocatalyst derived from a new cobalt metal-organic framework (MOF). The new flower-like 2D→3D MOF {[Co(BIPA)(5-OH-bdc)](DMF)}_n (1) was constructed based on bis(4-(1H-imidazol-1-yl)phenyl)amine (BIPA) and 5-hydroxyisophthalic acid (5-OH-H₂bdc). After that, the Co–N-doped carbon composite Co-MOF-800 was prepared via calcination of MOF 1. Interestingly, Co-MOF-800 exhibited excellent electrocatalytic activity and durability for HER. The onset potential (0.12 V) and E_j=10 value (0.193 V) of the Co-MOF-800 electrode were comparable to that of the most active non-precious metal HER electrocatalysts derived from other MOFs. The HER performance of Co-MOF-800 was stable without degradation even after long-term cycling.     

Synthesis and characterization of porous HKUST-1 metal organic frameworks for hydrogen storage

Hydrogen storage capacity has been investigated on a copper-based metal organic framework named HKUST-1 with fine structural analyses. The crystalline structure of HKUST-1 MOF has been confirmed from the powder X-ray diffraction and the average particle diameter has been found about 15–20 μm identified by FE-SEM. Nitrogen adsorption isotherms show that HKUST-1 MOF has approximately type-I isotherm with a BET specific surface area of 1055 m²g⁻¹. Hydrogen adsorption study shows that this material can store 0.47 wt.% of H₂ at 303 K and 35 bar. The existence of Cu (II) in crystalline framework of HKUST-1 MOF has been confirmed by pre-edge XANES spectra. The sharp feature at 8985.8 eV in XANES spectra represents the dipole-allowed electron transition from 1s to 4p_xy. In addition, EXAFS spectra indicate that HKUST-1 MOF structure has the Cu–O bond distance of 1.95 Å with a coordination number of 4.2.     

Metal-organic framework as a photocatalyst: Progress in modulation strategies and environmental/energy applications

Progress in the design, synthesis, and modification of metal-organic frameworks (MOFs) has immensely helped expand their applications in a wide variety of research fields. Such developments offered great opportunities for upgrading their efficiencies in diverse photocatalytic applications (e.g., N₂/CO₂ reduction, H₂ generation, organic synthesis, and environmental remediation) through enhanced conversion/storage of solar energy. The MOF-based photocatalytic platforms are, nonetheless, subject to many practical problems (e.g., inapplicability for industrial upscaling and thermodynamic instability under environmental conditions). In this review, the effects of synthesis/modification strategies on MOF photocatalysis have been discussed with respect to the type of inorganic nodes, the modulation of organic ligands, and the pre-/post-synthesis modification in MOF networks (i.e., MOF-based composite). Particular emphasis was placed on the technical advances achieved in the photoelectronic/catalytic performances of MOFs in multiple energy/environmental (redox) reactions based on both experimental and theoritical analyses. Further, the technical merits/disadvantages of MOF photocatalysts (in terms of structural defects, light absorption, active sites, and kinetic/thermodynamic stability) have been evaluated in relation to quantum efficiency and charge transfer mechanisms in various photo-redox reactions. The pursuit of strategies for enhanced kinetic stability of MOFs have also been highlighted based on the nature/strength of coordination modes, the inertness of metal centers, and the functionality of ligand types. Lastly, the current limitations of MOF-based photocatalysts are addressed with respect to their practical applications at industrial scales along with a discussion on their future use.     

Stacked nano rods of cobalt and nickel based metal organic frame work of 2–amino benzene dicarboxylic acid for photocatalytic hydrogen generation

Stacked nanorods of cobalt and nickel based hetero bimetallic organic frameworks (MOFs) of 2–amino benzene dicarboxylic acid are developed as photocatalyst for hydrogen evolution reaction. The ratio of metals in the catalyst is tuned to achieve a narrow band gap, and the MOF with optimized Ni to Co ratio of 1: 0.5 (1Ni0.5Co@NH₂BDC) exhibited the lowest band gap (2.2 eV) and electron–hole recombination rate. The catalyst exhibits enhanced photocatalytic activity for hydrogen evolution reaction due to the absorption of photons by 2–amino benzene dicarboxylic acid and excitation of electrons from HOMO to LUMO of the organic linker. The excited electrons relay to the cluster of the framework and reduce the protons gather around the cluster to hydrogen. The holes in the HOMO state are occupied by the electrons from the sacrificial agents and it supports the photocatalytic hydrogen evolution by avoiding electron–hole recombination. The stability of MOF catalyst in water splitting medium even with sacrificial agents confirms its competency with the state–of–the–art photocatalytic materials.     

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.     

Heterofullerene-linked metal–organic framework with lithium decoration for storing hydrogen and methane gases

Hydrogen and methane are considered as the promising fuels in the future, however, one of the obstacles to their utilizations is the lack of efficient storage and safe transportation materials. In this theoretical work, a novel kind of metal–organic framework (MOF) is designed using heterofullerene as linker from density functional theory calculations and first-principles molecular dynamics simulations. Based on grand canonical Monte Carlo simulations, we explore the adsorption performances of H₂ and CH₄ in the proposed porous MOF materials, which exhibit spectacular capacities for hydrogen storage as well as for methane storage after Li doping, both achieving the targets set by U.S. Department of Energy at workable conditions.     

Estimation of system-level hydrogen storage for metal-organic frameworks with high volumetric storage density

Metal organic framework (MOF) materials have emerged as the adsorbent materials with the highest H₂ storage densities on both a volumetric and gravimetric basis. While measurements of hydrogen storage at the material level (primarily at 77 K) have been published for hundreds of MOFs, estimates of the system-level hydrogen storage capacity are not readily available. In this study, hydrogen storage capacities are estimated at the system-level for MOFs with the highest demonstrated volumetric and gravimetric H₂ storage densities. System estimates are based on a single tank cryo-adsorbent system that utilizes a type-1 tank, multi-layer vacuum insulation, liquid N₂ cooling channels, in-tank heat exchanger, and a packed MOF powder inside the tank. It is found that with this powder-based system configuration, MOFs with ultra-high gravimetric surface areas and hydrogen adsorption amounts do not necessarily provide correspondingly high volumetric or gravimetric storage capacities at the system-level. Meanwhile, attributes such as powder packing efficiency and system cool-down temperature are shown to have a large impact on the system capacity. These results should shed light on the material properties that must to be optimized, as well as highlight the important design challenges for cryo-adsorbent hydrogen storage systems.     

Current trends in structural development and modification strategies for metal-organic frameworks (MOFs) towards photocatalytic H2 production: A review

Photocatalytic H₂ generation using semiconductor photocatalysts is considered as a cost-effective and eco-friendly technology for solar to energy conversion; however, the present photocatalysts have been recognized to depict low efficiency. Currently, porous coordination polymers known as metal-organic frameworks (MOFs) constituting flexible and modifiable porous structure and having excess active sites are considered to be appropriate for photocatalytic H₂ production. This review highlights current progress in structural development of MOF materials along with modification strategies for enhanced photoactivity. Initially, the review discusses the photocatalytic H₂ production mechanism with the concepts of thermodynamics and mass transfer with particular focus on MOFs. Elaboration of the structural categories of MOFs into Type I, Type II, Type III and classification of MOFs for H₂ generation into transition metal based, post-transition metal based, noble-metal based and hetero-metal based has been systematically discussed. The review also critically deliberate various modification approaches of band engineering, improvement of charge separation, efficient irradiation utilization and overall efficiency of MOFs including metal modification, heterojunction formation, Z-scheme formation, by introducing electron mediator, and dye based composites. Also, the MOF synthesized derivatives for photocatalytic H₂ generation are elaborated. Finally, future perspectives of MOFs for H₂ generation and approaches for efficiency improvement have been suggested.     

Ni nanoparticles embedded in N doped carbon nanotubes derived from a metal organic framework with improved performance for oxygen evolution reaction

Recently, to improve the catalytic activity of oxygen evolution reaction (OER) electrocatalysts, some design strategies, such as the decrease of the catalyst particle size, the formation of the porous structure and the couple of carbon-based materials, are receiving increased attention in energy-related systems. Herein, based on metal organic framework (MOF), we develop an effective strategy to synthesize Ni nanoparticles embedded in N doped carbon nanotubes (Ni NPs@N-CNTs) catalyst. In consequence, the Ni NPs@N-CNTs integrates the advantageous features of NPs and N-CNTs towards OER, such as more catalytic sites, large surface area, pore-rich structure and good electrical conductivity. Benefiting from the favorable features, the Ni NPs@N-CNTs exhibits a better OER performance than commercial RuO₂ in alkaline medium, which includes a lower onset potential (1.49 V), a smaller Tafel slope (106 mV dec^?1). The present work opens a new window for the construction of the coupling materials between NPs and carbon-based materials to increase the electrocatalytic activity of transition metal catalysts.     

Hydrogen storage in Ca-decorated,B-substituted metal organic framework

The feasibility to store hydrogen in calcium-decorated metal organic frameworks (MOFs) is explored by using first-principles electronic structure calculations. We show that substitution of boron atoms into the benzene ring of the MOF linker substantially enhances the Ca binding energy to the linker as well as the H₂ binding energy to Ca. The Kubas interaction between H₂ molecules and Ca added in the MOF gives rise to a large number of bound H₂'s (8H₂'s per linker) with the binding energy of 20 kJ/mol, which makes the system suitable for reversible hydrogen storage under ambient conditions.     

MOFs in photoelectrochemical water splitting: New horizons and challenges

Growing energy consumption with the augmentation in universal population to more than nine billion by 2050 and exhausting fossil fuel reserves necessitates a harsh revolution from non-renewable energy reservoirs to renewable energy reservoirs with zero carbon emission. In the present scenario, solar energy prompted photoelectrochemical (PEC) water splitting or “Artificial Photosynthesis” via light gripping semiconductor material, originates out as the most promising methodology in accomplishing the global energy crisis. Recent studies have amply demonstrated the potential of metal-organic frameworks (MOF) towards PEC applications. They are porous crystalline coordination polymers assembled through an appropriate choice of metal ions and multidentate organic ligands. Owing to their structural regularity and synthetic tunability, MOFs integration with PEC is considered in terms of enhancing and broadening light absorption, providing active sites and directing charge transfer dynamics. Here, we have explored MOFs role in PEC and classified them into different categories such as photosensitizers, co-catalysts, counter electrode, template and also for imparting additional stability to the electrode system. MOFs mediated PEC water splitting is promising but is still rare and in its infancy. Therefore, it is pertinent and timely to take stock of the advancements made and develop insight on the use of MOFs, as an emerging solution for the problems encountered in PEC. This review covers the basics of MOF & mainly describes various case studies done during last 10 years and providing adequate impetus to researchers for critically assessing the recent advances and challenges that are faced by scientists and researchers at large.     

Room-temperature hydrogen storage performance of metal-organic framework/graphene oxide composites by molecular simulations

Metal-organic framework/graphene oxide (MOF/GO) composites have been regarded as potential room-temperature hydrogen storage materials recently. In this work, the influence of MOF structural properties, GO functional group contents and different amounts of doped lithium (Li⁺) on hydrogen storage performance of different MOF/GO composites were investigated by grand canonical Monte Carlo (GCMC) simulations. It is found that MOF/GO composites based on small-pore MOFs exhibit enhanced hydrogen storage capacity, whereas MOF/GO based on large-pore MOFs show decreased hydrogen storage capacity, which can be ascribed to the novel pores at MOF/GO interface that favors the enhanced hydrogen storage performance due to the increased pore volume/surface area. By integrating the small-pore MOF-1 with GO, the hydrogen storage capacity was enhanced from 9.88 mg/go up to 11.48 mg/g. However, the interfacial pores are smaller compared with those in large-pore MOFs, resulting in significantly reduced pore volume/surface area as well as hydrogen storage capacities of large-pore MOF/GO composite. Moreover, with the increased contents of hydroxyl, epoxy groups as well as carboxyl group modification, the pore volumes and specific surface areas of MOF/GO are decreased, resulting in reduced hydrogen storage performance. Furthermore, the room-temperature hydrogen storage capacities of Li⁺ doped MOF/GO was improved with increased Li⁺ at low loading and decrease with the increased Li⁺ amounts at high loading. This is due to that the introduced Li⁺ effectively increases the accessible hydrogen adsorption sites at low Li⁺ loading, which eventually favors the hydrogen adsorption capacity. However, high Li⁺ loading causes ion aggregation that reduces the accessible hydrogen adsorption sites, leading to decreased hydrogen storage capacities. MOF-5/GO composites with moderate Li⁺ doping achieved the optimum hydrogen storage capacities of approximately 29 mg/g.