Recent progress in pristine MOF-based catalysts for electrochemical hydrogen evolution, oxygen evolution and oxygen reduction

Robust High-performance Bifunctional Porous Cobalt MOF-Based Catalysts for Overall Water Splitting

MOF-based materials, as bifunctional catalysts for electrocatalytic water splitting, play an important role in the application and development of clean fuel hydrogen energy. This study presents a series of novel 3D Co-based MOFs with layered networks, including [Co(4,4'-bipy)0.5(aip)(CH3OH)·H2O]n (Co-MOF 1), [Co2(1,3'-bit)(aip)2(CH3OH)·H2O]n (Co-MOF 2), [Co(4,4'-bipb)(aip)]n (Co-MOF 3), and [Co2(4,4'-bipe)(aip)2·1.5H2O]n (Co-MOF 4). Their single-crystal structures of Co-MOFs 1-4 are characterized and analyzed before being applied in alkaline solutions for water decomposition (OER and HER). The electrocatalytic tests indicate that Co-MOFs 1-4 exhibit a good performance. Notably, Co-MOF 4 exhibits great behavior which has low overpotentials of 94 and 188 mV (OER) as well as 185 and 352 mV (HER) at the currents of 10 and 100 mA cm-2, respectively. In comparison with Co-MOFs 1-3, Co-MOF 4 has the lowest Tafel slopes, highest ECSA, and smallest resistance. The immanent qualities, such as distinct interwoven long chain layered structure, unsaturated coordination modes, and synergistic catalytic qualities among Co ions, contribute to explaining the results. The fundamentals provide valuable information for the investigation of innovative MOF-based bifunctional electrocatalysts for overall water splitting.

A mixed-ligand Co metal-organic framework and its carbon composites as excellent electrocatalysts for the oxygen evolution reaction in green-energy devices

Oxygen evolution reaction (OER) electrocatalysts are frequently made from noble metal-based oxides like ruthenium/iridium oxides. However, because of their scarcity and high price, researchers are now focusing on creating innovative OER catalysts based on affordable transition metals that have improved electrical conductivity and accessibility to active sites. Metal-organic frameworks (MOFs), a unique class of inorganic materials with excellent physical and chemical properties, have witnessed significant progress in promising green energy systems. In this work, a novel mixed-ligand metal-organic framework [Co(μ-1κN,2κN'-BDP)(μ3-1κoo',2κo''2κo'''-BTC)]n·nH2O (BDP = boron-dipyrromethene or BODIPY; BTC = benzene tricarboxylate) denoted as CoBDPMOF has been synthesized, and its composites with different carbon materials have been designed. Compared to the pristine MOF, the composites showed enhanced electrocatalytic activity toward the oxygen evolution reaction (OER) in alkaline media. In addition, the CoBDPMOF with activated carbon showed the highest OER performance with a low Tafel slope (82 mV dec-1) and the highest j600 (59.8 mA cm-2), outperforming noble metal IrO2, the OER benchmark electrocatalyst. This study presents new insights into the design and application of CoBDPMOF-based materials for energy conversion and suggests promising avenues for further research and development in electrocatalysis.

A comparison between 2D and 3D cobalt-organic framework as catalysts for electrochemical CO2 reduction

Electrocatalytic CO2 reduction, as an effective way to reduce the CO2 concentration, has gained attention. In this study, we prepared ZIF-67 nanoparticles and nanosheets and investigated them as electrocatalysts for CO2 reduction. It was found that ZIF-67 nanosheets, because of their two-dimensional morphologies, provide more under-coordinated cobalt nodes and have lower overpotentials for both hydrogen evolution and CO2 reduction reactions. Also, the rate-determining step for hydrogen evolution changes from Volmer for ZIF-67 nanoparticles to Hyrovsky for ZIF-67 nanosheets. Also, the presence of Mg2+ ions in solution causes more facile CO2 reduction, especially for ZIF-67 nanosheets.

Synthesis and Oxygen Evolution Reaction Application of a Co-Cd Based Bimetallic Metal-Organic Framework

In the realm of renewable energy technologies, the development of efficient and durable electrocatalysts is paramount, especially for applications like electrochemical water splitting. This research focuses on synthesizing a novel bimetallic metal-organic framework (BMMOF11) using earth-abundant elements, cobalt (Co) and cadmium (Cd). BMMOF11 showcases a distinctive structure with distorted octahedral chains of CoO and CdO, linked by benzene tricarboxylic acid (BTC). Our study primarily investigates the electrocatalytic efficiency of BMMOF11, particularly in water oxidation reactions. For practical analysis, BMMOF11 was anchored onto nickel foam, forming BMMOF11/NF, to evaluate its electrocatalytic properties. Electrochemical testing revealed that BMMOF11/NF begins water oxidation at an onset potential of 1.62 V versus RHE, demonstrating high activity with a lower overpotential of 0.4 V to achieve a current density of 10 mA/cm2 . Moreover, BMMOF11/NF maintained stable water splitting performance, sustaining a current density of approximately 70 mA/cm2 under a voltage of 1.9 V relative to RHE. These findings indicate that BMMOF11/NF is a promising candidate for large-scale electrochemical water splitting, offering a blend of high activity and stability.

Metal-Organic-Framework-Based Nanoarrays for Oxygen Evolution Electrocatalysis

The development of highly active and stable electrode materials for the oxygen evolution reaction (OER) is essential for the widespread application of electrochemical energy conversion systems. In recent years, various metal-organic frameworks (MOFs) with self-supporting array structures have been extensively studied because of their high porosity, abundant metal sites, and flexible and adjustable structures. This review provides an overview of the recent progress in the design, preparation, and applications of MOF-based nanoarrays for the OER, beginning with the introduction of the architectural advantages of the nanoarrays and the characteristics of MOFs. Subsequently, the design principles of robust and efficient MOF-based nanoarrays as OER electrodes are highlighted. Furthermore, detailed discussions focus on the composition, structure, and performance of pristine MOF nanoarrays (MOFNAs) and MOF-based composite nanoarrays. On the one hand, the effects of the two components of MOFs and several modification methods are discussed in detail for MOFNAs. On the other hand, the review emphasizes the use of MOF-based composite nanoarrays composed of MOFs and other nanomaterials, such as oxides, hydroxides, oxyhydroxides, chalcogenides, MOFs, and metal nanoparticles, to guide the rational design of efficient OER electrodes. Finally, perspectives on current challenges, opportunities, and future directions in this research field are provided.

A viologen-based Cd(II) coordination polymer as a multifunctional platform for photochromism, chemochromism and a broad range of fluorescence pH sensing

A multi-responsive Cd(II) coordination polymer (1) has been constructed by introducing a viologen derivative as both the framework backbone and ligand side pendant. Notably, compound 1 exhibits intriguing properties, including photochromism, methanol-assisted photochromism and chemochromism to ammonia. Furthermore, compound 1 also displays fluorescence pH sensing ability in a wide pH range.

Interfacial engineering and chemical reconstruction of Mo/Mo2C@CoO@NC heterostructure for promoting oxygen evolution reaction

Chemical reorganization and interfacial engineering in hybrid nanomaterials are promising strategies for enhancing electrocatalytic performance. Herein, MoO₃@zeolitic imidazolate framework-67 (ZIF-67) heterogeneous nanoribbons are designed through coordination assembly. By following heat treatment, a Mo/Mo₂C@CoO@NC heterostructure with nitrogen-doped carbon-encapsulated CoO hexagons (CoO@NC) anchored on the Mo/Mo₂C jag matrix was fabricated. Notably, through controllable experimental optimization, the as-prepared Mo/Mo₂C@CoO@NC heterostructure exhibits numerous active centers (e.g. Mo, Mo₂C, CoO, and NC), fully exposed active sites (numerous pores and jagged structures), and abundant heterointerfaces (Mo/Mo₂C, Mo₂C/CoO@NC, Mo₂C/amorphous, and CoO@NC/amorphous), and exhibits good conductivity (localized single-crystal behavior, graphitized carbon). As a result, the as-developed Mo/Mo₂C@CoO@NC heterostructures inherit impressive oxygen evolution reaction (OER) performance with an overpotential of only 215 mV at 10 mA cm^-2. Furthermore, Mo/Mo₂C@CoO@NC heterostructures exhibit excellent stability with a current density retention of 98.4% after 20 h chronoamperometry. This work provides deep insights into chemical reconstructions and tuning heterointerfaces to efficiently enhance the OER activity of heterostructure-based electrocatalysts.

Eu(III)-Functionalized MOF-Based Dual-Emission Ratiometric Sensor Integrated with Logic Gate Operation for Efficient Detection of Hippuric Acid in Urine and Serum

With the introduction of Eu³⁺ ions as the secondary fluorescent signal reporter and sensing active sites, a dual-emission ratiometric sensor of Eu³⁺@NiMOF (Eu³⁺ functional NiMOF) for hippuric acid (HA) detection in urine and serum was fabricated via the postsynthetic encapsulating strategy. Based on the two emission signals at 441 nm (turn-on) and 628 nm (turn-off), the produced Eu³⁺@NiMOF ratiometric sensor provided enhanced sensitivity, higher selectivity, and 9.7 times lower limits of detection (LOD) for the detection of HA (2.38 μM, 0.42 μg·mL^-1) than that of the pristine NiMOF. Considering the high sensitivity and visualization results, further exploration of intelligent applications in the HA sensing process was carried out by constructing a tandem combinational logic gate to improve the practicability and convenience with the help of a smartphone. This work provides a promising approach for developing MOF-based ratiometric sensors to detect biomarkers.

Recent advances in the metal-organic framework-based electrocatalysts for trifunctional electrocatalysis

The sustainable energy technology is in great demand due to the depletion and the risks associated with the use of fossil fuels. Various energy technologies like regenerative fuel cells, zinc-air batteries, and overall water-splitting devices have a huge scope in the growth of green energy. The efficiency of these devices is reliant upon the multifunctional electrocatalysts, which include both bifunctional and trifunctional electrocatalysts. Among the different categories of the materials used for such multifunctional electrocatalysis, metal-organic-frameworks (MOFs) occupy a very consolidated place because of their high surface area, porosity, and many other unique physicochemical properties. However, the use of MOFs for the trifunctional electrocatalytic applications is in the budding phase and needs to be explored more. Further, most of these MOF-based trifunctional electrocatalysts are derived by pyrolyzing MOFs at high temperatures. Therefore, there is a need to develop more conductive MOFs which can be directly utilized for the trifunctional applications. In this frontier article, we present the latest reports on the MOF-based materials for trifunctional applications. The material design strategies of the MOF-based materials for trifunctional electrocatalysis have been discussed. The progressive improvements made with MOFs in electrocatalytic applications have been provided with emphasis on the structural, active site and compositional requirements. Finally, the challenges and viewpoints on the future development of the MOF-based materials for trifunctional electrocatalysis have been provided.

Facile fabrication of self-supporting porous CuMoO4@Co3O4 nanosheets as a bifunctional electrocatalyst for efficient overall water splitting

Research shows that redox complementarity and synergism among the ingredients of heterogeneous catalysts can enhance the performance of the catalyst. In this research, a porous CuMoO₄@Co₃O₄ nanosheet electrocatalyst is prepared, which is uniformly decorated on nickel foam (NF) by hydrothermal reactions and the impregnation method. The CuMoO₄@Co₃O₄ is an efficient bifunctional catalyst with prominent electrocatalytic activity and durability. It requires overpotentials of only 54 and 251 mV to obtain current densities of 10 and 50 mA cm^-2 for the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER) in 1.0 mol L^-1 KOH, corresponding to Tafel slope values of 98.8 and 87.4 mV dec^-1, respectively. Furthermore, the CuMoO₄@Co₃O₄ shows excellent stability of 120 h chronopotentiometry at a current density of 100 mA cm^-2 for the HER/OER. Notably, an alkaline electrolyzer (with CuMoO₄@Co₃O₄ as the HER and OER electrodes) can deliver a current density of 10 mA cm^-2 at a low voltage of 1.51 V. The catalytic activity of CuMoO₄@Co₃O₄ can be attributed to the structure of the porous nanosheets and the synergistic effect between CuMoO₄ and Co₃O₄.

Molecular Regulation Based on Functional Trimetallic Metal-Organic Frameworks for Efficient Oxygen Evolution Reaction

Metal-organic frameworks (MOFs) as classic crystalline porous materials have attracted great interest in the catalytic field. However, how to realize molecular regulation of the MOF structure to achieve a remarkable oxygen evolution reaction (OER) electrocatalyst is still a challenge. Herein, we designed several series of special MOF materials to explore the relationship between the structure and properties as well as the related reactive mechanism. First, various metal centers, including Fe, Co, Ni, Zn, and Mg, were utilized to construct the first series of trimetallic MOF materials, namely, M₃-MOF-BDC, where BDC = 1,4-benzenedicarboxylic acid, also known as terephthalic acid. Among of them, Fe₃-MOF-BDC shows the best OER performance and only needs an overpotential of 312 mV at 10 mA cm^-2. Then, functional BDC-X ligands (X = NH₂, OH, NO₂, DH) with various characteristic groups were selected to construct a new series, namely, Fe₃-MOF-BDC-X, to further improve its OER electrocatalytic performance. As expected, Fe₃-MOF-BDC-NH₂ exhibited a greatly enhanced OER performance with ultralow Tafel slopes of 45 mV dec^-1 and overpotentials of 280 mV at 10 mA cm^-2 when the BDC-NH₂ ligand was adopted, even superior to commercial IrO₂ (323 mV) and most of the reported pristine MOFs as OER electrodes. Much higher structural stability was proven. The detailed structure-property relationship and mechanism are discussed. In a word, this work provides a very important theoretical basis for the design and exploration of new MOF electrocatalysts.

Highly Effective OER Electrocatalysts Generated from a Two-Dimensional Metal-Organic Framework Including a Sulfur-Containing Linker without Doping

Metal-organic frameworks (MOFs) with different topologies formed by the self-assembly of sulfur-containing inorganic ligands, cobalt ions, and large ligands can be used to prepare electrocatalysts for water splitting in order to fully explore the advantages of MOFs in terms of structural tailoring and quantitative assembly. It is possible to avoid using an extradoped sulfur source to reduce waste as well as to disperse Co and sulfur elements evenly and controllably throughout the final material to maximize the overall synergistic effect. In this work, different kinds of bimetallic MOF materials containing sulfur can be synthesized very conveniently by using an economical and practical diffusion method. These materials are directly used as OER electrocatalysts, and the bimetallic MOFs have the best electrocatalytic performance when the ratio of Co to Fe is 6:4. The overpotential at a current density of 10 mA cm^-2 was 260 mV, with a Tafel slope of 56 mV dec^-1 and good stability. It was assembled with 20% commercial Pt/C material into a two-electrode system for all-water decomposition, and the decomposition voltage at 10 mA cm^-2 was 1.81 V. From the electronic configuration microscopic point of view, the introduction of iron ions changed the original synergistic effect for Co-S-Co, which more easily led to the formation of high-valence Co³⁺ and finally produced highly active electrocatalytic sites. From a macroscopic point of view, the material produced in situ during the electrochemical reaction process not only retains the original 2D layered structure but also utilizes bubbles to produce a loose structure with defective sites. These structural features are advantageous because they provide not only an abundance of active sites and permeable channels but also the necessary interfaces and electron-transport channels for the formation of electrostatic charge-separation layers, making it easier to intercalate and delaminate the hydroxide ions. Furthermore, the changed hydroxyl ions and nitrogen and sulfur atoms on the channel surface may operate as interaction sites, increasing the surface characteristics, facilitating electron transfer, and reducing electron-transfer resistance. To summarize, the rational design of sulfur-containing layered MOF materials directly as water-splitting catalysts is a crucial next step in developing cost-effective, environmentally friendly, and low-energy-consumption electrocatalysts based on the findings of this study.

MOF/COF hybrids as next generation materials for energy and biomedical applications

The rapid increase in the number and variety of metal organic frameworks (MOFs) and covalent organic frameworks (COFs) has led to groundbreaking applications in the field of materials science and engineering. New MOF/COF hybrids combine the outstanding features of MOF and COF structures, such as high crystallinities, large surface areas, high porosities, the ability to decorate the structures with functional groups, and improved chemical and mechanical stabilities. These new hybrid materials offer promising performances for a wide range of applications including catalysis, energy storage, gas separation, and nanomedicine. In this highlight, we discuss the recent advancements of MOF/COF hybrids as next generation materials for energy and biomedical applications with a special focus on the use of computational tools to address the opportunities and challenges of using MOF/COF hybrids for various applications.

MOF/COF hybrids combine the outstanding features of MOF and COF structures, such as high crystallinities, large surface areas, high porosities, and improved chemical and mechanical stabilities.

A highly active oxygen evolution electrocatalyst derived from Co/Ni-succinic acid framework under mild conditions

By utilizing a Co-MOF consisting of succinic acid, Co/Ni/B derivatives were synthesized under mild conditions to act as efficient OER electrocatalysts.

Electronically modulated nickel boron by CeOx doping as a highly efficient electrocatalyst towards overall water splitting

Exploiting economic, efficient and durable non-noble metal electrocatalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is promising, but still faces enormous challenges. Herein, the strategy of doping a metal boride with a rare earth metal oxide has been explored to develop a highly efficient bifunctional electrocatalyst. The novel electrocatalyst CeO_x-NiB consists of CeO_x-doped NiB supported on nickel foam, and was fabricated by a one-step mild electroless plating reaction. Remarkably, the CeO_x-NiB@NF electrode delivers a current density of 10 mA cm^-2 at overpotentials of only 19 mV and 274 mV for the HER and OER, respectively. Two-electrode electrolyzers with the CeO_x-NiB@NF electrode require only 1.424 V to deliver 10 mA cm^-2 for overall water splitting in 1.0 M KOH, outperforming the Pt-C/NF∥IrO₂/NF electrolyzer. Meanwhile, the electrode also has good stability (can work for 100 hours at 10 mA cm^-2) and industrial-grade current density. This work provides a new idea for the development of efficient and durable non-precious metal catalysts.

Sulfur doped FeOx nanosheet arrays supported on nickel foam for efficient alkaline seawater splitting

Developing economical, efficient and stable bifunctional catalysts for hydrogen production from seawater is of great significance for hydrogen utilization. Herein, sulfur doped iron oxide nanosheet arrays supported on nickel foam (FeO_x-Ni₃S₂@NF) are prepared by a one-pot solvothermal reaction. Owing to the high intrinsic activity of FeO_x-Ni₃S₂, the large catalytic specific surface area of nanosheet arrays and the fast charge transportation capability achieved by the self-supporting configuration, the FeO_x-Ni₃S₂@NF electrode delivers excellent catalytic performance in alkaline simulated seawater (1 M KOH + 0.5 M NaCl). Impressively, a low overpotential of 120 mV at 50 mA cm^-2 with a Tafel slope of 57 mV dec^-1 for the hydrogen evolution reaction and an overpotential of 470 mV at 200 mA cm^-2 with a Tafel slope of 62 mV dec^-1 for the oxygen evolution reaction are achieved. More importantly, the voltage is only 1.5 V at 50 mA cm^-2 for continuous overall water splitting for 100 h at 200 mA cm^-2 with negligible decay in alkaline simulated seawater with almost 100% Faraday efficiency. This work provides a simple and universal strategy to prepare highly efficient bifunctional catalytic materials, promoting the development of Earth-abundant materials to catalyse seawater splitting to produce high-purity hydrogen.

Synthesis of ultrafine Co/CoO nanoparticle-embedded N-doped carbon framework magnetic material and application for 4-nitrophenol catalytic reduction

An ultrafine Co/CoO nanoparticle-embedded N-doped porous carbon framework magnetic material was successfully synthesized based on a designed Co-MOF. Co/CoO@NC has good catalytic activity and recyclability for the 4-NP reduction reaction.

Anion exchange strategy to improve electrocatalytic hydrogen evolution performances in cationic metal–organic frameworks

Through an anion exchange strategy, the post-synthetic cationic metal–organic frameworks present better electrocatalytic performances during the hydrogen evolution reaction process than the pristine one.