Three-Dimensional Metal-Organic Framework Graphene Nanocomposite as a Highly Efficient and Stable Electrocatalyst for the Oxygen Reduction Reaction in Acidic Media

Molecular Catalysis of Energy Relevance in Metal-Organic Frameworks: From Higher Coordination Sphere to System Effects

The modularity and synthetic flexibility of metal-organic frameworks (MOFs) have provoked analogies with enzymes, and even the term MOFzymes has been coined. In this review, we focus on molecular catalysis of energy relevance in MOFs, more specifically water oxidation, oxygen and carbon dioxide reduction, as well as hydrogen evolution in context of the MOF-enzyme analogy. Similar to enzymes, catalyst encapsulation in MOFs leads to structural stabilization under turnover conditions, while catalyst motifs that are synthetically out of reach in a homogeneous solution phase may be attainable as secondary building units in MOFs. Exploring the unique synthetic possibilities in MOFs, specific groups in the second and third coordination sphere around the catalytic active site have been incorporated to facilitate catalysis. A key difference between enzymes and MOFs is the fact that active site concentrations in the latter are often considerably higher, leading to charge and mass transport limitations in MOFs that are more severe than those in enzymes. High catalyst concentrations also put a limit on the distance between catalysts, and thus the available space for higher coordination sphere engineering. As transport is important for MOF-borne catalysis, a system perspective is chosen to highlight concepts that address the issue. A detailed section on transport and light-driven reactivity sets the stage for a concise review of the currently available literature on utilizing principles from Nature and system design for the preparation of catalytic MOF-based materials.

Graphene-Based Metal-Organic Framework Hybrids for Applications in Catalysis, Environmental, and Energy Technologies

Current energy and environmental challenges demand the development and design of multifunctional porous materials with tunable properties for catalysis, water purification, and energy conversion and storage. Because of their amenability to de novo reticular chemistry, metal-organic frameworks (MOFs) have become key materials in this area. However, their usefulness is often limited by low chemical stability, conductivity and inappropriate pore sizes. Conductive two-dimensional (2D) materials with robust structural skeletons and/or functionalized surfaces can form stabilizing interactions with MOF components, enabling the fabrication of MOF nanocomposites with tunable pore characteristics. Graphene and its functional derivatives are the largest class of 2D materials and possess remarkable compositional versatility, structural diversity, and controllable surface chemistry. Here, we critically review current knowledge concerning the growth, structure, and properties of graphene derivatives, MOFs, and their graphene@MOF composites as well as the associated structure-property-performance relationships. Synthetic strategies for preparing graphene@MOF composites and tuning their properties are also comprehensively reviewed together with their applications in gas storage/separation, water purification, catalysis (organo-, electro-, and photocatalysis), and electrochemical energy storage and conversion. Current challenges in the development of graphene@MOF hybrids and their practical applications are addressed, revealing areas for future investigation. We hope that this review will inspire further exploration of new graphene@MOF hybrids for energy, electronic, biomedical, and photocatalysis applications as well as studies on previously unreported properties of known hybrids to reveal potential "diamonds in the rough".

The Effect of an External Magnetic Field on the Electrocatalytic Activity of Heat-Treated Cyanometallate Complexes towards the Oxygen Reduction Reaction in an Alkaline Medium

This work focuses on the development of an electrocatalytic material by annealing a composite of a transition metal coordination material, iron hexacyanoferrate (Prussian blue) immobilized on carboxylic-acid-functionalized reduced graphene oxide. Pyrolysis at 500 °C under a nitrogen atmosphere formed nanoporous core–shell structures with efficient activity, which mostly included iron carbide species capable of participating in the oxygen reduction reaction in alkaline media. The physicochemical properties of the iron-based catalyst were elucidated using transmission electron microscopy, X-ray diffraction, Mössbauer spectroscopy, and various electrochemical techniques, such as cyclic voltammetry and rotating ring–disk electrode (RRDE) voltammetry. To improve the electroreduction of oxygen over the studied catalytic material, an external magnetic field was utilized, which positively shifted the potential by ca. 20 mV. The formation of undesirable intermediate peroxide species was decreased compared with the ORR measurements without an external magnetic field.

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.

A historical perspective on porphyrin-based metal-organic frameworks and their applications

Porphyrins are important molecules widely found in nature in the form of enzyme active sites and visible light absorption units. Recent interest in using these functional molecules as building blocks for the construction of metal-organic frameworks (MOFs) have rapidly increased due to the ease in which the locations of, and the distances between, the porphyrin units can be controlled in these porous crystalline materials. Porphyrin-based MOFs with atomically precise structures provide an ideal platform for the investigation of their structure-function relationships in the solid state without compromising accessibility to the inherent properties of the porphyrin building blocks. This review will provide a historical overview of the development and applications of porphyrin-based MOFs from early studies focused on design and structures, to recent efforts on their utilization in biomimetic catalysis, photocatalysis, electrocatalysis, sensing, and biomedical applications.

Porphyrin-based frameworks for oxygen electrocatalysis and catalytic reduction of carbon dioxide

The recent progress made on porphyrin-based frameworks and their applications in energy-related conversion technologies (e.g., ORR, OER and CO₂RR) and storage technologies (e.g., Zn–air batteries).

Heterometallic coordination polymers as heterogeneous electrocatalysts

Heterometallic coordination polymers have been rapidly developed as heterogeneous electrocatalysts. This review highlights the synthesis strategies of these polymers and the relationships between structures and electrocatalytic performances.

Applications of Metal-Organic-Framework-Derived Carbon Materials

Carbon materials derived from metal-organic frameworks (MOFs) have attracted much attention in the field of scientific research in recent years because of their advantages of excellent electron conductivity, high porosity, and diverse applications. Tremendous efforts are devoted to improving their chemical and physical properties, including optimizing the morphology and structure of the carbon materials, compositing them with other materials, and so on. Here, many kinds of carbon materials derived from metal-organic frameworks are introduced with a particular focus on their promising applications in batteries (lithium-ion batteries, lithium-sulfur batteries, and sodium-ion batteries), supercapacitors (metal oxide/carbon and metal sulfide/carbon), electrocatalytic reactions (oxygen reduction reaction, oxygen evolution reaction, and hydrogen evolution reaction), water treatment (MOF-derived carbon and other techniques), and other possible fields. To close, some existing problem and corresponding possible solutions are proposed based on academic knowledge from the reported literature, along with a great deal of experimental experience.

Klonopin assay using modified electrode with multiwalled carbon nanotubes and poly melamine nanocomposite

Developing of cheap, sensitive and stable sensors plays a significant role in pharmaceutical and clinical applications. Considering the effective role of Klonopin (KNP) in the treatment of epilepsy, KNP quantification in its production process for dose adjustments and checking the purity and also after its usage by patents for bioavailability testing and effectiveness assay is vital. In present work, an efficient electrochemical sensor based on poly melamine and multiwalled carbon nanotubes nanocomposite (PMela/CNTs) was constructed which displayed effective electrochemical response toward KNP. Electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and square-wave voltammetry (SWV) experiments were applied for performance evaluation of the PMela/CNTs modified electrode and electrochemical redox behavior of KNP. Distinguish synergetic effect was observed between CNTs and poly melamine in response to KNP electrochemical redox reaction. A linear detection range of 0.05 to 10 μM with the detection limits of 63 nM was achieved for KNP analysis. The practical application of the PMela/CNTs modified electrode revealed satisfactory results for quantification of KNP in biological fluids.

Hierarchically porous Fe–N–C nanospindles derived from a porphyrinic coordination network for oxygen reduction reaction

NaCl-assisted pyrolysis of an Fe porphyrinic coordination network combined with concentrated-sulfuric-acid post-activation results in a hierarchically porous Fe–N–C nanospindle catalyst.

Study of Electrocatalytic Properties of Metal-Organic Framework PCN-223 for the Oxygen Reduction Reaction

A highly robust metal-organic framework (MOF) constructed from Zr₆ oxo clusters and Fe(III) porphyrin linkers, PCN-223-Fe was investigated as a heterogeneous catalyst for oxygen reduction reaction (ORR). Films of the framework were grown on a conductive FTO substrate and showed a high catalytic current upon application of cathodic potentials and achieved high H₂O/H₂O₂ selectivity. In addition, the effect of the proton source on the catalytic performance was also investigated.

Insights into the mechanism of electrocatalysis of the oxygen reduction reaction by a porphyrinic metal organic framework

Insights into the mechanism of electrocatalysis of the oxygen reduction reaction by a porphyrinic metal organic framework.