MOF-Derived Hierarchical CoSe2 with Sheetlike Nanoarchitectures as an Efficient Bifunctional Electrocatalyst

Low-temperature synthesis of high-entropy amorphous metal oxides (HEOs) for enhanced oxygen evolution performance

The rational design of multiple metal ions into high-entropy oxide electrode material via a single-step hydrothermal process is applicable to the evolution of oxygen molecules (O2) through simple water electrolysis. Their cost-effectiveness, high performance, and durable nature are the key factors of non-precious high-entropy multiple metal-based electrocatalysts, which can be used as replaceable catalysts instead of precious ones. This article reports a low-temperature synthesis of the cauliflower-type morphology of high-entropy amorphous metal oxides, and their electrochemical performances towards the oxygen evolution reaction (OER) are investigated. The multiple metal ion (Mn2+, Fe3+, Co2+, Ni2+, Cu2+) oxide electrode material shows an acceptable oxygen evolution reaction (OER) with an overpotential of 290 mV at a current density of 10 mA cm-2 and a lower Tafel slope value of 85 mV dec-1, respectively. Moreover, the 20 h durability test with negligible change in overpotential shows the efficacy of the modified electrode material in harsh alkaline media. The observed electrochemical results towards the OER correspond to the amorphous nature of the active material that displayed a cauliflower-type morphology, having a large specific surface area (240 m2 g-1) and providing higher electrochemical active sites as well. Consequently, post-stability characterization studies (such as PXRD, FESEM, TEM, and XPS) provide more information for understanding the post-structural and morphological results of the high-entropy amorphous metal oxide.

Structure and magnetic properties of an amine-templated one-dimensional cobalt-fluoro-sulfate containing Co4F4 cubane and hydrogen evolution reaction (HER) performance of its derived carbon-wrapped CoSe2 nanorods

Amine-templated 1D cobalt fluoro sulfate of the composition [(CH₃)₂NH₂]₂[Co₄F₄(SO₄)₃(C₃N₂H₄)₄], consisting of Co₄F₄ cubane-type secondary building unit, has been synthesized under solvothermal condition. The magnetic properties of the Co₄F₄ cubane chain exhibited a low-temperature magnetic ordering below 17 K (T_c) attributed to intra-cluster ferromagnetic coupling and did not show spin-glass freezing. The selenylation of the Co₄F₄ cubane chain leads to the formation of sphere-like CoSe₂ in the hydrothermal route (CoSe₂@HT). At the same time, nanorods of CoSe₂ encapsulated with carbon matrix were obtained in a sealed tube method (CoSe₂@ST). Moreover, CoSe₂@ST exhibited a higher hydrogen evolution reaction (HER) activity than CoSe₂@HT in an acidic medium with 177 mV overpotential to achieve the benchmark current density of 10 mA cm^-2. The promising HER performance of derived CoSe₂@ST could be attributed to an increase in the geometrical and specific activity due to the encapsulation of N-doped carbon matrix over the CoSe₂ nanorods that facilitate faster charge transfer at the electrode-electrolyte interface and higher electrochemical conductivity than the derived CoSe₂@HT. This work demonstrates a low-temperature, solvent- and reducing agent-free new synthetic approach for synthesizing framework-derived materials.

Principles of Design and Synthesis of Metal Derivatives from MOFs

Materials derived from metal-organic frameworks (MOFs) have demonstrated exceptional structural variety and complexity and can be synthesized using low-cost scalable methods. Although the inherent instability and low electrical conductivity of MOFs are largely responsible for their low uptake for catalysis and energy storage, a superior alternative is MOF-derived metal-based derivatives (MDs) as these can retain the complex nanostructures of MOFs while exhibiting stability and electrical conductivities of several orders of magnitude higher. The present work comprehensively reviews MDs in terms of synthesis and their nanostructural design, including oxides, sulfides, phosphides, nitrides, carbides, transition metals, and other minor species. The focal point of the approach is the identification and rationalization of the design parameters that lead to the generation of optimal compositions, structures, nanostructures, and resultant performance parameters. The aim of this approach is to provide an inclusive platform for the strategies to design and process these materials for specific applications. This work is complemented by detailed figures that both summarize the design and processing approaches that have been reported and indicate potential trajectories for development. The work is also supported by comprehensive and up-to-date tabular coverage of the reported studies.

Hierarchical copper-based metal-organic frameworks nanosheet assemblies for electrochemical ascorbic acid sensing

Noninvasive human health monitoring requires the development of efficient electrochemical sensors for the quantitative analysis of infinitesimal biomolecules. In this work, we reported a novel hierarchical nanosheet assemblies (HSA) of copper-based metal-organic frameworks (MOFs) as an electrochemical sensor for ascorbic acid (AA) detection. Copper 1,4-benzenedicarboxylate (CuBDC) HSA was constructed by three steps of in situ growth on stone paper, including hydrolysis, anion exchange, and heteroepitaxy growth. The monodispersed two-dimensional MOFs nanosheet units were aligned in an orderly manner and arranged into three-dimensional hierarchical assemblies. The CuBDC HSA-based AA sensor displayed a high sensitivity of 396.8 μA mM^-1 cm^-2 and a low detection limit of 0.1 μM. Excellent selectivity, stability and reproducibility were also obtained. Benefiting from the advantages of ultrathin nanosheets and nature-inspired hierarchy, this unique architecture facilitated reactant dispersion and maximized the accessible active sites and charge-transport capability and thus had superior catalytic ability for the electro-oxidation of ascorbic acid compared to bulk MOFs. Moreover, the CuBDC HSA sensor performed AA level detection in juice samples with acceptable accuracy and verified the feasibility for sweat AA sensing. This novel MOFs architecture holds great potential as an electrochemical sensor to detect AA for noninvasive human health monitoring in the future.

Recent advances in solution assisted synthesis of transition metal chalcogenides for photo-electrocatalytic hydrogen evolution

Hydrogen evolution from water splitting is considered to be an important renewable clean energy source and alternative to fossil fuels for future energy sustainability. Photocatalytic and electrocatalytic water splitting is considered to be an effective method for the sustainable production of clean energy, H₂. This perspective especially emphasizes research advances in the solution-assisted synthesis of transition metal chalcogenides for both photo and electrocatalytic hydrogen evolution applications. Transition metal chalcogenides (CdS, MoS₂, WS₂, TiS₂, TaS₂, ReS₂, MoSe₂, and WSe₂) have received intensified research interest over the past two decades on account of their unique properties and great potential across a wide range of applications. The photocatalytic activity of transition metal chalcogenides can further be improved by elemental doping, heterojunction formation with noble metals (Au, Pt, etc.), non-chalcogenides (MoS₂, In₂S₃, NiS_1-X), morphological tuning, through various solution-assisted synthesis processes, including liquid-phase exfoliation, heat-up, hot-injection methods, hydrothermal/solvothermal routes and template-mediated synthesis processes. In this review we will discuss recent developments in transition metal chalcogenides (TMCs), the role of TMCs for hydrogen production and various strategies for surface functionalization to increase their activity, different synthesis methods, and prospects of TMCs for hydrogen evolution. We have included a brief discussion on the effect of surface hydrogen binding energy and Gibbs free energy change for HER in electrocatalytic hydrogen evolution.

Metal-organic framework (MOF)-derived plate-shaped CoS1.097 nanoparticles for an improved hydrogen evolution reaction

Metal-organic framework (MOF)-derived transition metal sulfides are viewed as reliable, cost-effective, and alternative hydrogen evolution reaction (HER)-efficient electrocatalysts. They have been used to replace platinum (and their alloys) for production of renewable energy carriers such as hydrogen. Progress towards development of non-precious transition-metal sulfides through different synthetic routes to obtain unique morphological nanostructures with enhanced HER activity is challenging. We introduced a transition-metal sulfide, cobalt sulfide (CoS_1.097), derived from a cobalt MOF [Co-BPY-DDE] by following facile, one-step solvothermal sulfurization. By varying the sulfurization temperature (from 140 °C to 180 °C) during the solvothermal method, three cobalt-sulfide products were obtained: CoS_1.097-140, CoS_1.097-160, and CoS_1.097-180, respectively. Temperature variation had a vital role in optimizing the HER activity of the electrocatalyst. Besides, notable plate-shaped cobalt sulfide nanoparticles (CoS_1.097-160) required overpotential of 163 mV to deliver a current density of 10 mA cm^-2 with a low Tafel slope of 53 mV dec^-1, thereby demonstrating faster reaction kinetics during the evolution of molecular hydrogen. Furthermore, 25 h of long-term stability of the electrocatalyst reflected its practical applicability in acidic media. CoS_1.097-160 had uniform plate-shaped morphology and large electrochemical active surface area, which contributed to enhanced electrochemical performance through water electrolysis.

NiSe2 Nanoparticles Encapsulated in N-Doped Carbon Matrix Derived from a One-Dimensional Ni-MOF: An Efficient and Sustained Electrocatalyst for Hydrogen Evolution Reaction

The spherical-type NiSe₂ nanoparticles encapsulated in a N-doped carbon (NC) matrix (NiSe₂-T@NC, temperature (T) = 400-800 °C) are derived from a 1D Ni-MOF precursor of the formula [Ni(BPY)(DDE)] [(BPY = 2,2'-bipyridyl), (DDE = 4,4'-dicarboxy diphenyl ether)] via a facile solvothermal technique followed by annealing at different temperatures and selenylation strategies. The combined effect of a NC matrix and the Ni nanoparticles has been optimized during varied annealing processes with subsequent selenylation, leading to the formation of the series NiSe₂-400@NC, NiSe₂-500@NC, NiSe₂-600@NC, NiSe₂-700@NC, and NiSe₂-800@NC, respectively. The variation of annealing temperature plays a vital role in optimizing the catalytic behavior of the NiSe₂-T@NCs. Among different high-temperature annealed products, NiSe₂-600@NC shows superior electrocatalytic performance because of the unique spherical-type morphology and higher specific surface area (57.95 m² g^-1) that provides a large number of electrochemical active sites. The synthesized material exhibits a lower overpotential of 196 mV to deliver 10 mA cm^-2 current density, a small Tafel slope of 45 mV dec^-1 for better surface kinetics, and outstanding durability in an acidic solution, respectively. Consequently, the post stability study of the used electrocatalyst gives insight into surface phase analysis. Therefore, we presume that the synthesized 1D MOF precursor derived NiSe₂ nanoparticles encapsulated in a NC matrix has excellent potential to replace the noble-metal-based electrocatalyst for enhanced hydrogen evolution through simple water electrolysis.

Rapid Fabrication of Biocomposites by Encapsulating Enzymes into Zn-MOF-74 via a Mild Water-Based Approach

A zinc-based metal organic framework, Zn-MOF-74, which has a unique one-dimensional (1D) channel and nanoscale aperture size, was rapidly obtained in 10 min using a de novo mild water-based system at room temperature, which is an example of green and sustainable chemistry. First, catalase (CAT) enzyme was encapsulated into Zn-MOF-74 (denoted as CAT@Zn-MOF-74), and comparative assays of biocatalysis, size-selective protection, and framework-confined effects were investigated. Electron microscopy and powder X-ray diffraction were used for characterization, while electrophoresis and confocal microscopy confirmed the immobilization of CAT molecules inside the single hexagonal MOF crystals at loading of ∼15 wt %. Furthermore, the CAT@Zn-MOF-74 hybrid was exposed to a denaturing reagent (urea) and proteolytic conditions (proteinase K) to evaluate its efficacy. The encapsulated CAT maintained its catalytic activity in the decomposition of hydrogen peroxide (H2O2), even when exposed to 0.05 M urea and proteinase K, yielding an apparent observed rate constant (kobs) of 6.0 × 10-2 and 6.6 × 10-2 s-1, respectively. In contrast, free CAT exhibited sharply decreased activity under these conditions. Additionally, the bioactivity of CAT@Zn-MOF-74 for H2O2 decomposition was over three times better than that of the biocomposites based on zeolitic imidazolate framework 90 (ZIF-90) owing to the nanometer-scaled apertures, 1D channel, and less confinement effects in Zn-MOF-74 crystallites. To demonstrate the general applicability of this strategy, another enzyme, α-chymotrypsin (CHT), was also encapsulated in Zn-MOF-74 (denoted as CHT@Zn-MOF-74) for action against a substrate larger than H2O2. In particular, CHT@Zn-MOF-74 demonstrated a biological function in the hydrolysis of l-phenylalanine p-nitroanilide (HPNA), the activity of ZIF-90-encapsulated CHT was undetectable due to aperture size limitations. Thus, we not only present a rapid eco-friendly approach for Zn-MOF-74 synthesis but also demonstrate the broader feasibility of enzyme encapsulation in MOFs, which may help to meet the increasing demand for their industrial applications.

A novel zinc sulfide impregnated carbon composite derived from zeolitic imidazolate framework-8 for sodium-ion hybrid solid-state flexible capacitors

The pyrolysis of metal-organic frameworks (MOFs) is an easy approach to prepare metal oxides as well as nanoporous carbon with high specific surface area. In the present work, for the first time, ZIF-8 (zeolitic imidazolate framework-8) has been pyrolyzed under different conditions to derive two products, i.e., highly porous carbon (C) and zinc sulfide (ZnS) infused carbon (ZnS@C). These two materials, i.e., nanoporous C and ZnS@C, have been investigated as a negative and a positive electrode, respectively, for potential application in a hybrid asymmetrical solid-state supercapacitor device (HASD). The controlled pyrolysis approach for the preparation of ZnS@C has yielded uniformly distributed ZnS nanoparticles inside the carbon structure. A 1.8 V HASD has been assembled, which delivered an excellent energy density of 38.3 W h kg^-1 (power density of 0.92 kW kg^-1) along with the greatly desirable feature of cycling stability. The proposed selection of materials as electrodes is promising to develop futuristic hybrid capacitors.