Construction of Hierarchical NiCo2O4@Ni-MOF Hybrid Arrays on Carbon Cloth as Superior Battery-Type Electrodes for Flexible Solid-State Hybrid Supercapacitors

Newly Constructed NiCo2O4 Derived from ZIF-67 with Dual Mimic Enzyme Properties for Colorimetric Detection of Biomolecules and Metal Ions

Integration of novel bio-/nanostructures as effective sensing platforms is still of great significance for robust and rapid analysis. Herein, a novel metal-organic framework-derived NiCo₂O₄ was synthesized via a feasible templating method. Significantly, redox couples of both Ni³⁺/Ni²⁺ and Co³⁺/Co²⁺ provided richer oxidation-reduction reactions, thereby leading to an enhanced catalytic activity. Furthermore, NiCo₂O₄ as an enzyme mimic with peroxidase-like activity and oxidase-like activity could oxidize colorless thylbenzidine (TMB) to blue oxTMB in the absence of H₂O₂. Thus, a sensitive chromogenic sensing platform for detecting Fe²⁺, thiourea, cysteine (Cys), and epigallocatechin-3-gallate (EGCG) was proposed. The colorimetric detection methods exhibited great features of low limit of detection (LOD) and broad linear range. Owing to the complexation reaction, the chromogenic sensing system of TMB + NiCo₂O₄ + Cys achieved effective detection of Cu²⁺ and Mn²⁺ with the LODs of 0.0022 and 0.0181 mM, respectively. Developed detection methods with wide linear ranges of 0.008-0.1 mM for Cu²⁺ and 0.08-1 mM for Mn²⁺ had excellent practical potential. Similarly, the reaction system of TMB + NiCo₂O₄ + EGCG could achieve the colorimetric detection of Cu²⁺ and Fe³⁺. The great chromogenic sensing performance for detecting Cu²⁺ and Fe³⁺ with a broad linear range and a low LOD could be also realized.

Phosphate-modified Co-Ni phosphide heterostructure formed by interfacial and electronic tuning for boosted faradaic properties

Rational structural and compositional modulation endows electrode materials with unique physicochemical characteristics due to their adjustable electronic properties. Herein, a phosphate-modified hierarchical nanoarray consisting of a heterojunction with a well-aligned cobalt phosphide nanowire core and nickel phosphide nanosheet shell on flexible carbon cloth (denoted as CoP@Ni₂P-CC) is engineered. The phosphate-modulated heterogeneous phosphide with a tuned electronic structure, additional heterojunction interfaces, and high degree of covalency in the chemical bonds accelerates the reaction kinetics and enhances the energy storage performance. Due to these reasons, the as-obtained phosphide-based heterostructured CoP@Ni₂P-CC electrode delivers a capacity of 475.9 C g^-1 at 0.5 A g^-1 with a satisfying rate capability, which is greatly superior to that of its transition metal counterparts (sulfide, selenide, and oxide). After being assembled into a flexible hybrid supercapacitor (FHSC), a wide operating voltage (1.8 V), high energy/power densities (49.8 W h kg^-1/8.6 kW kg^-1), and long-term stability (85.1% capacity retention after 10 000 cycles) were achieved. This work may provide a general method from multiple strategies for designing reliable pseudocapacitive materials for flexible electronics.

Polyvinylpyrrolidone (PVP) assisted in-situ construction of vertical metal-organic frameworks nanoplate arrays with enhanced electrochemical performance for hybrid supercapacitors

Construction of two-dimensional (2D) metal-organic frameworks (MOFs) for energy storage and conversion has attracted great attention due to the synergistic advantages of 2D nanostructures and MOFs. Herein, a Co-MOF material with different 2D morphologies of vertical nanoplate arrays and faveolate nanosheets are in-situ fabricated on Ni foam with and without using polyvinylpyrrolidone (PVP) as a regulator. Toward the application in energy storage, both of two morphologies of the Co-MOF exhibit good electrochemical properties. In particular, the vertical Co-MOF nanoplate arrays deliver a high areal capacity of 8.56 C/cm² at the current density of 5 mA/cm², which is much higher than that of faveolate Co-MOF nanosheets (2.39 C/cm² at 5 mA/cm²). Moreover, a hybrid supercapacitor (HSC) device using the Co-MOF nanoplate arrays positive electrode and activated carbon (AC) negative electrode is assembled, which delivers a volumetric capacitance of 17.9 F/cm³ at 10 mA/cm², a high energy density of 7.2 mW h cm^-3 and a good cyclic stability (retaining over 88.0% of initial capacitance after 3000 cycles). These findings demonstrate that the as-fabricated 2D Co-MOFs possess a huge potential in energy storage.

Optimization of metal–organic framework derived transition metal hydroxide hierarchical arrays for high performance hybrid supercapacitors and alkaline Zn-ion batteries

A Setaria viridis-like hierarchical A-Co(OH)₂@NiCo-LDH nanoarray has been fabricated and it possesses the advantages of large surface area and stable microstructure, leading to excellent energy storage performances of HSC and AZIB.

Hierarchical core-shell 2D MOF nanosheet hybrid arrays for high-performance hybrid supercapacitors

Two-dimensional (2D) metal-organic frameworks (MOFs) with large surface area, ordered pores and ultrathin thickness have recently emerged as ideal electrode materials for supercapacitors (SCs). However, their straightforward applications are restricted by the drawbacks of self-stacking and unsatisfactory electrical conductivity. Herein, ultrathin Ni-MOF nanosheets have been grafted on zeolite imidazolate framework (ZIF-L)-derived porous Co3O4 nanosheets to form hierarchical core-shell Co3O4@Ni-MOF 2D nanosheet hybrid arrays. The porous Co3O4 "core" acts as a conductive skeleton for anchoring Ni-MOF and provides shortened ion diffusion paths. The Ni-MOF "shell" can expose large active sites. Benefiting from these merits and the synergic effects of the "core and "shell", the Co3O4@Ni-MOF/NF shows a high specific capacity (capacitance) of 225.6 mA h g-1 (1980.7 F g-1) at 1 A g-1 with decent capacitance retention (82.2% after 2000 cycles). The asymmetric two-electrode cell assembled from Co3O4@Ni-MOF/NF exhibits an energy density of 37.05 W h kg-1 at a power density of 800 W kg-1 with good long-term durability (75% capacitance retention after 10 000 cycles). Moreover, two charged cells can power a red light-emitting diode (LED) for up to 16 min, manifesting the great promise of Co3O4@Ni-MOF/NF for real energy storage devices.

ZIF-67-derived NiCo2O4@Co2P/Ni2P honeycomb nanosheets on carbon cloth for high-performance asymmetric supercapacitors

NiCo2O4@Co2P/Ni2P-CC, for the first time, was prepared using ZIF-67 as sacrificial templates through 4 steps procedure. Based on honeycomb structures, NiCo2O4@Co2P/Ni2P-CC electrode shows high areal capacitance and excellent cycle stability.

Bimetallic metal-organic frameworks anchored corncob-derived porous carbon photocatalysts for synergistic degradation of organic pollutants

Metal-organic frameworks (MOFs) are promising for photocatalysis owing to their excellent structure and performance. Unfortunately, poor stability in both aqueous solutions and high temperatures and lack of adsorption centers during reactions limit their practical applications. Herein, a bimetallic MOF anchored corncob calcined derived activated carbon (CCAC) was successfully prepared by a one-step solvothermal method. Benefiting from unique structures and synergetic effect, the porous carbon provided a high specific surface area for stable MOF support and served as an organic pollutant buffer-reservoir, which was advantageous for efficient photocatalytic degradation of organic pollutants. The optimized MOF/CCAC-5 samples possessed excellent visible light degradation rate, i.e., 100% for Rh B, more than 96% for six mixed dyes, and 98% for tetracycline. This prominent photocatalytic activity was caused by active species, including photoelectrons (e^-), photo-holes (h⁺) and superoxide free radicals (•O^2-). The transient photocurrent response and electrochemical impedance tests showed that MOF/CCAC-5 exhibited a relatively high charge separation and low carrier recombination rate. Cyclic and simulation experiments indicated high reusability, stability and universality of the composite photocatalysts. These exciting results provide new pathways for the fabrication of MOFs anchored porous carbon materials.

Pomegranate-like C@TiO2 mesoporous honeycomb spheres for high performance lithium ion batteries

Pomegranate-like C@TiO₂ mesoporous honeycomb spheres have been synthesized through two simple steps: formation of TiO₂ mesoporous honeycomb spheres and the coating of polypyrrole followed by carbonization. TiO₂ mesoporous honeycomb spheres are of large specific surface area of 153 m² g^-1 and contain abundant mesopores, which leads to high electrochemical activity and good kinetic performance of TiO₂. A layer of amorphous carbon shell with the thickness of 30-40 nm tightly encapsulates a TiO₂ mesoporous honeycomb sphere, forming a novel pomegranate-like small sphere, which significantly improves electronic conductivity and structural stability of TiO₂. Benefiting from the unique pomegranate-like structure, C@TiO₂ mesoporous honeycomb spheres exhibit high specific capacity, stable long-term cycling performance and good rate capability as an anode material for lithium ion batteries (LIBs). After 500 cycles at 1 C, the discharge capacity still reaches 184 mAh g^-1. The electrochemical performance is superior to pure TiO₂ mesoporous honeycomb spheres and most of the reported high-performance TiO₂-based composites. This work provides a new high-performance TiO₂-carbon-based composite material for LIBs as well as a new valuable research strategy.

Self-Chargeable Flexible Solid-State Supercapacitors for Wearable Electronics

Flexible supercapacitors (SCs) always face the charging issue when they are used in some special situations (e.g., wilderness island) that cannot provide electricity, which would limit the continuous energy supply for the attached wearable electronics. Herein, a self-chargeable flexible solid-state supercapacitor (FSSSC) was creatively constructed by sandwiching a piezoelectric polyvinyl alcohol/potassium hydroxide/barium titanate electrolyte between symmetric NiCo₂O₄@activated carbon cloth electrodes. By virtue of the efficient synergy of each component in the FSSSC, the device exhibits integrated merits with excellent flexibility, satisfactory electrochemical properties, and considerable self-charging capability through synchronously collecting and converting mechanical energy (e.g., repeated bending) into storable electrochemical energy in a persistent way. When the devices are serially connected and self-charged, they can be used to drive typical electronics with normal working. Such a unique material and device design enables the FSSSC with combined capabilities such as energy-harvesting and conversion and storage device for self-powered wearable electronics.

Carbon Transition-metal Oxide Electrodes: Understanding the Role of Surface Engineering for High Energy Density Supercapacitors

Supercapacitors store electrical energy by ion adsorption at the interface of the electrode-electrolyte (electric double layer capacitance, EDLC) or through faradaic process involving direct transfer of electrons via oxidation/reduction reactions at one electrode to the other (pseudocapacitance). The present minireview describes the recent developments and progress of carbon-transition metal oxides (C-TMO) hybrid materials that show great promise as an efficient electrode towards supercapacitors among various material types. The review describes the synthetic methods and electrode preparation techniques along with the changes in the physical and chemical properties of each component in the hybrid materials. The critical factors in deriving both EDLC and pseudocapacitance storage mechanisms are also identified in the hope of pointing to the successful hybrid design principles. For example, a robust carbon-metal oxide interaction was identified as most important in facilitating the charge transfer process and activating high energy storage mechanism, and thus methodologies to establish a strong carbon-metal oxide contact are discussed. Finally, this article concludes with suggestions for the future development of the fabrication of high-performance C-TMO hybrid supercapacitor electrodes.

Practical MOF Nanoarchitectonics: New Strategies for Enhancing the Processability of MOFs for Practical Applications

Over the past decades, the development of porous materials has directly or indirectly affected industrial production methods. Metal-organic frameworks (MOFs) as an emerging class of porous materials exhibit some unique advantages, including controllable composition, a large surface area, high porosity, and so on. These attractive characteristics of MOFs have led to their potential applications in energy storage and conversion devices, drug delivery, adsorption and storage, sensors, and other areas. However, powdered MOFs have limited practical applications owing to poor processability, safety hazards from dust formation, and poor recyclability. In addition, the inherent micro/mesoporosities of MOFs also reduce the accessibility and diffusion kinetics for large molecules. To improve their processability for practical applications, MOFs are often deposited as MOF layers or films (i.e., MOF-coated composites) on supporting materials or are formed into 3D structured composites, such as aerogels and hydrogels. In this article, we review recent researches on these MOF composites, including their synthetic methods and potential applications in energy storage devices, heavy metal ion adsorption, and water purification. Finally, the future outlook and challenges associated with the large-scale fabrication of MOF-based composites for practical applications are discussed.

CuO nanorods grown vertically on graphene nanosheets as a battery-type material for high-performance supercapacitor electrodes

This work reports the preparation and characterization of the CuO nanorods grown vertically on graphene nanosheets, denoted as CuO/rGO@NF. Graphene is deposited by electrostatic attraction showing the morphology of folded nanosheets, which improves the electrical conductivity of the electrode, while CuO is modified by filtered cathodic vacuum arc technology and subsequent electrochemical oxidation presenting the morphology of nanorods, which increases the contact area of active sites and shortens the ion and electronic diffusion path. The results show that the CuO/rGO@NF electrode deliver an ultrahigh specific capacity (2.51 C cm⁻² at 2 mA cm⁻²), remarkable rate performance (64.6%) and improved conductivity. A symmetrical supercapacitor is assembled by two identical electrodes, presenting the maximum energy density of 38.35 W h kg⁻¹ at a power density of 187.5 W kg⁻¹. Therefore, the CuO/rGO@NF electrode can be used as a prospective electrode for energy storage devices. In addition, the whole electrode preparation process is short in time, safe and environmentally friendly, which provides a new idea for the preparation of other electrode materials.

The CuO/rGO@NF electrode is prepared by a simple and time-saving method, which has ultrahigh area capacity and excellent rate performance.

Application of MOF-derived transition metal oxides and composites as anodes for lithium-ion batteries

Research progress of MOF-derived metal oxides and composites in lithium ion batteries has been presented based on different organic linkers.