In-situ Growth of Ultrathin ZIF-67 Nanosheets on Conductive Ti@TiO 2 /CdS Substrate for High-efficient Electrochemical Catalysis

Insight into the Coupling of HgS and CuO with Metal-Organic Frameworks Support in Electrocatalytic Oxygen Evolution Reaction

This study investigates the coupling of mercury sulfide (HgS) and copper oxide (CuO) nanoparticles with metal-organic frameworks (MOFs) as a support material for enhancing the electrocatalytic oxygen evolution reaction (OER). The integration of HgS and CuO into the MOF framework aims to leverage the unique electronic and structural properties of both the nanoparticles and the MOFs to improve catalytic performance. Metal-organic frameworks (MOFs), particularly ZIF-67, are investigated for their potential to catalyze water-splitting reactions due to their high porosity and large specific surface areas. The strategic incorporation of HgS and CuO into ZIF-67 significantly enhances its electrocatalytic properties, resulting in remarkable performance metrics: a low overpotential of 246 mV at 10 mA/cm2, a Tafel slope of 123 mV/dec, an expanded electrochemical active surface area (ECSA) of 23.56 cm2, and a reduced charge transfer resistance of 34.86 Ω. This integration enhances porosity and increases active surface area, which is crucial for improved catalytic performance. This investigation introduces an innovative methodology for fabricating highly efficient electrocatalysts, positioning HgS/CuO/ZIF-67 as a promising candidate for oxygen evolution reactions in alkaline media. The findings highlight the potential of this novel nanocomposite in future clean energy applications, particularly in the realm of water-splitting technologies.

High-Valence Co Stabilized by In-Situ Growth of ZIF-67 on NiCo-LDH for Enhanced Performance in Oxygen Evolution Reaction

The application of metal-organic frameworks (MOFs) in the electro-catalysis of heterogeneous structures is limited by the problems of low electrical conductivity and poor mechanical strength due to the complex synthesis process, although their high specific surface area and controllable structure. In this study, a method involving metal precipitation and ligand reaction is used during the electrochemical corrosion of hydroxides/oxy-hydroxides to obtain ZIF-67 in situ. The in situ growth technology not only effectively addresses the bonding strength and material conductivity challenges in the heterostructure between MOFs and the substrate but also enhances the catalyst's surface area and activity. Additionally, the exposure and protection of Co4+ by ZIF-67 contribute to the electrocatalyst's performance, demonstrating a low overpotential (η100) of 293 mV, a Tafel slope of 25.8 mV dec-1, and a charge transfer resistance of 3.9 Ω, with long-term robustness proven in continuous stability test exceeding 75 000 s under the superhigh current density of 500 mA cm-2. This work on binder-free in situ growth of MOFs not only provides relevant theoretical insights and experimental experience for cost-effective and controllable production of MOF-based catalysts but also offers ideas for the development of future electrocatalysts by exploring the exposure and protection of active site using MOFs materials.

Electrochemical-Based Sensing Platforms for Detection of Glucose and H2O2 by Porous Metal-Organic Frameworks: A Review of Status and Prospects

Establishing enzyme-free sensing assays with great selectivity and sensitivity for glucose and H₂O₂ detection has been highly required in biological science. In particular, the exploitation of nanomaterials by using noble metals of high conductivity and surface area has been widely investigated to act as selective catalytic agents for molecular recognition in sensing platforms. Several approaches for a straightforward, speedy, selective, and sensitive recognition of glucose and H₂O₂ were requested. This paper reviews the current progress in electrochemical detection using metal-organic frameworks (MOFs) for H₂O₂ and glucose recognition. We have reviewed the latest electrochemical sensing assays for in-place detection with priorities including straightforward procedure and manipulation, high sensitivity, varied linear range, and economic prospects. The mentioned sensing assays apply electrochemical systems through a rapid detection time that enables real-time recognition. In profitable fields, the obstacles that have been associated with sample preparation and tool expense can be solved by applying these sensing means. Some parameters, including the impedance, intensity, and potential difference measurement methods have permitted low limit of detections (LODs) and noticeable durations in agricultural, water, and foodstuff samples with high levels of glucose and H₂O₂.

Recent advancements in metal-organic frameworks composites based electrochemical (bio)sensors

Metal-organic frameworks (MOFs) are a novel class of crystalline materials which find widespread applications in the field of microporous conductors, catalysis, separation, biomedical engineering, and electrochemical sensing. With a specific emphasis on the MOF composites for electrochemical sensor applications, this review summarizes the recent construction strategies on the development of conductive MOF composites (post-synthetic modification of MOFs, in situ synthesis of functional materials@MOFs composites, and incorporating electroactive ligands). The developed composites are revealed to have excellent electrochemical sensing activity better than their pristine forms. Notably, the applicable functionalized MOFs to electrochemical sensing/biosensing of various target species are discussed. Finally, we highlight the perspectives and challenges in the field of electrochemical sensors and biosensors for potential directions of future development.

High-activity daisy-like zeolitic imidazolate framework-67/reduced grapheme oxide-based colorimetric biosensor for sensitive detection of hydrogen peroxide

Colorimetric biosensors, based on enzyme-like nanomaterials, have come into the spotlight in virtue of their visual detection. Herein, a daisy-like zeolitic imidazolate framework-67/reduced grapheme oxide (ZIF-67/rGO) nanozyme with unique 3D hierarchical structures has been designed to realize visual detection of hydrogen peroxide (H₂O₂) that is recognized as a strong oxidizing agent or reactive oxygen species associated with oxidative stress in biological systems. The daisy-like ZIF-67/rGO is prepared by a facile one-step liquid-phase method conducted under room temperature. The successful introduction of rGO endows the daisy-like ZIF-67/rGO nanozyme with abundant porous structure, high specific surface area, and good charge transfer capability, which significantly accelerates the adsorbability and recognition towards the substrates and the oxidation rate of TMB-H₂O₂ reaction, and thus improving the nanozyme activity observably. It is conductive to nanozyme-modulated H₂O₂ determination. The established colorimetric biosensor platform based on ZIF-67/rGO nanozyme exhibits remarkable sensitivity and high specificity for the application in visual detection of H₂O₂. The detection limit of ZIF-67/rGO-based biosensor platform is as low as 3.81 μM, which is nearly 8 times lower than that of ZIF-67-based biosensor platform. Moreover, its potential applicability as an ideal platform for colorimetric biosensors is demonstrated by testing the concentration of H₂O₂ in milk samples, which sheds light on the promising application of the proposed biosensing system in point-of-care detection.

Onion-Ring-like Carbon and Nitrogen from ZIF-8 on TiO2/Fe2O3 Nanostructure for Overall Electrochemical Water Splitting

The carbon and nitrogen derived from ZIF-8 embedded in TiO₂/Fe₂O₃ (i.e., C,N-ZIF/TiFe) nanostructures exhibit superior electrocatalytic performance toward oxygen evolution reactions (OER), hydrogen evolution reactions (HER), and overall H₂O splitting. The results showed that the C,N-ZIF/TiFe nanostructure was the best catalyst in comparison to ZIF/TiFe and TiFe nanostructures toward HER and OER. These results revealed that combining the highly active carbon and nitrogen from ZIF-8 with a TiO₂/Fe₂O₃ semiconductor enriched the overall H₂O splitting. A possible OER mechanism is attributed to some groups that support the surface active site of the catalyst and adsorbent intermediate species. Finally, this inexpensive electrocatalyst was synthesized without noble metals and showed superior electrocatalytic activity and great stability with the potential to achieve ground-breaking and novel applications in fuel cells.

Metal-Organic Frameworks and Metal-Organic Gels for Oxygen Electrocatalysis: Structural and Compositional Considerations

Increasing demand for sustainable and clean energy is calling for the next-generation energy conversion and storage technologies such as fuel cells, water electrolyzers, CO₂ /N₂ reduction electrolyzers, metal-air batteries, etc. All these electrochemical processes involve oxygen electrocatalysis. Boosting the intrinsic activity and the active-site density through rational design of metal-organic frameworks (MOFs) and metal-organic gels (MOGs) as precursors represents a new approach toward improving oxygen electrocatalysis efficiency. MOFs/MOGs afford a broad selection of combinations between metal nodes and organic linkers and are known to produce electrocatalysts with high surface areas, variable porosity, and excellent activity after pyrolysis. Some recent studies on MOFs/MOGs for oxygen electrocatalysis and their new perspectives in synthesis, characterization, and performance are discussed. New insights on the structural and compositional design in MOF/MOG-derived oxygen electrocatalysts are summarized. Critical challenges and future research directions are also outlined.

Liquid-Phase Deposition Synthesis of ZIF-67-Derived Synthesis of Co3O4@TiO2 Composite for Efficient Electrochemical Water Splitting

In this article, a novel Co3O4@TiO2 composite is synthesized by applying two-step methods. ZIF-67 is synthesized and used as a template for the synthesis of the composite. The composite is designed by using the effective photocatalytic properties of Co3O4 and TiO2. The resulting synthesized composite is supposed to offer superior properties compared to their counterparts. The synthesized Co3O4@TiO2 composite is characterized by powder X-ray diffraction (PXRD), Brunauer–Emmet–Teller (BET), atomic force microscopy (AFM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Electrochemical water splitting, including hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) studies on the Co3O4@TiO2 composite, is evaluated by cyclic voltammetry (CV) and linear sweep voltammetry (LSV) analysis in a 2M aqueous KOH electrolyte. The current generation stability of these samples is deliberated by chronoamperometric measurements. It is observed, from LSV results at a 1 mV/s scan rate, that metal oxides incorporated on other metal oxides have a higher current density and lower onset potential as compared to pure metal oxides. From the obtained results, it has become evident that synthesized studies on the Co3O4@TiO2 composite possess significant potential for electrochemical water splitting with the lowest onset potential, highest current density, better OER, and HER activity.

Advanced non-noble materials in bifunctional catalysts for ORR and OER toward aqueous metal-air batteries

The catalyst in the oxygen electrode is the core component of the aqueous metal-air battery, which plays a vital role in the determination of the open circuit potential, energy density, and cycle life of the battery. For rechargeable aqueous metal-air batteries, the catalyst should have both good oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalytic performance. Compared with precious metal catalysts, non-precious metal materials have more advantages in terms of abundant resource reserves and low prices. Over the past few years, great efforts have been made in the development of non-precious metal bifunctional catalysts. This review selectively evaluates the advantages, disadvantages and development status of recent advanced materials including pure carbon materials, carbon-based metal materials and carbon-free materials as bifunctional oxygen catalysts. Preliminary improvement strategies are formulated to make up for the deficiency of each material. The development prospects and challenges facing bifunctional catalysts in the future are also discussed.

One-Step Electrodeposition of Silver Nanostructures on 2D/3D Metal-Organic Framework ZIF-67: Comparison and Application in Electrochemical Detection of Hydrogen Peroxide

Metal-organic frameworks (MOFs) have been widely used as supporting materials to load or encapsulate metal nanoparticles for electrochemical sensing. Herein, the influences of morphology on the electrocatalytic activity of Co-containing zeolite imidazolate framework-67 (ZIF-67) as supporting materials were studied. Three types of morphologies of MOF ZIF-67 were facilely synthesized by changing the solvent because of the influence of the polar solvent on the nucleation and preferential crystal growth. Two-dimensional (2D) ZIF-67 with microplate morphology and 2D ultrathin ZIF-67 nanosheets were obtained from pure H₂O (H-ZIF-67) and a mixed solution of dimethylformamide and H₂O (D-ZIF-67), respectively. Three-dimensional ZIF-67 with rhombic dodecahedron morphology was obtained from pure methanol (M-ZIF-67). Then, one-step electrodeposition of silver nanostructures on ZIF-67-modified glassy carbon electrode (Ag/ZIF-67/GCE) was performed for the reduction of hydrogen peroxide (H₂O₂). Cyclic voltammetry can be used to investigate the electrocatalytic activity of Ag/ZIF-67/GCE, and Ag/H-ZIF-67/GCE displayed the best electrocatalytic property than Ag/D-ZIF-67/GCE and Ag/M-ZIF-67/GCE. The electrochemical H₂O₂ sensor showed two wide linear ranges of 5 μM to 7 mM and 7 to 67 mM with the sensitivities of 421.4 and 337.7 μA mM^-1 cm^-2 and a low detection limit of 1.1 μM. In addition, the sensor exhibited good selectivity, high reproducibility, and stability. Furthermore, it has been utilized for real-time detection of H₂O₂ from HepG2 human liver cancer cells. This work provides a novel strategy for enhancing the detection performance of electrochemical sensors by changing the crystalline morphologies of supporting materials.

Structural Engineering of Low-Dimensional Metal-Organic Frameworks: Synthesis, Properties, and Applications

Low-dimensional metal-organic frameworks (LD MOFs) have attracted increasing attention in recent years, which successfully combine the unique properties of MOFs, e.g., large surface area, tailorable structure, and uniform cavity, with the distinctive physical and chemical properties of LD nanomaterials, e.g., high aspect ratio, abundant accessible active sites, and flexibility. Significant progress has been made in the morphological and structural regulation of LD MOFs in recent years. It is still of great significance to further explore the synthetic principles and dimensional-dependent properties of LD MOFs. In this review, recent progress in the synthesis of LD MOF-based materials and their applications are summarized, with an emphasis on the distinctive advantages of LD MOFs over their bulk counterparties. First, the unique physical and chemical properties of LD MOF-based materials are briefly introduced. Synthetic strategies of various LD MOFs, including 1D MOFs, 2D MOFs, and LD MOF-based composites, as well as their derivatives, are then summarized. Furthermore, the potential applications of LD MOF-based materials in catalysis, energy storage, gas adsorption and separation, and sensing are introduced. Finally, challenges and opportunities of this fascinating research field are proposed.

Self-Sacrificial Template-Directed Vapor-Phase Growth of MOF Assemblies and Surface Vulcanization for Efficient Water Splitting

Direct use of metal-organic frameworks (MOFs) with robust pore structures, large surface areas, and high density of coordinatively unsaturated metal sites as electrochemical active materials is highly desirable (rather than using as templates and/or precursors for high-temperature calcination), but this is practically hindered by the poor conductivity and low accessibility of active sites in the bulk form. Herein, a universal vapor-phase method is reported to grow well-aligned MOFs on conductive carbon cloth (CC) by using metal hydroxyl fluorides with diverse morphologies as self-sacrificial templates. Specifically, by further partially on-site generating active Co₃ S₄ species from Co ions in the echinops-like Co-based MOF (EC-MOF) through a controlled vulcanization approach, the resulting Co₃ S₄ /EC-MOF hybrid exhibits much enhanced electrocatalytic performance toward the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), with overpotentials of 84 and 226 mV required to reach a current density of 10 mA cm^-2 , respectively. Density functional theory (DFT) calculations and experimental results reveal that the electron transfer between Co₃ S₄ species and EC-MOF can decrease the electron density of the Co d-orbital, resulting in more electrocatalytically optimized adsorption properties for Co. This study will open up a new avenue for designing highly ordered MOF-based surface active materials for various electrochemical energy applications.

Pristine Metal-Organic Frameworks and their Composites for Energy Storage and Conversion

Metal-organic frameworks (MOFs), a new class of crystalline porous organic-inorganic hybrid materials, have recently attracted increasing interest in the field of energy storage and conversion. Herein, recent progress of MOFs and MOF composites for energy storage and conversion applications, including photochemical and electrochemical fuel production (hydrogen production and CO₂ reduction), water oxidation, supercapacitors, and Li-based batteries (Li-ion, Li-S, and Li-O₂ batteries), is summarized. Typical development strategies (e.g., incorporation of active components, design of smart morphologies, and judicious selection of organic linkers and metal nodes) of MOFs and MOF composites for particular energy storage and conversion applications are highlighted. A broad overview of recent progress is provided, which will hopefully promote the future development of MOFs and MOF composites for advanced energy storage and conversion applications.