Construction of self-supported hierarchical NiCo-S nanosheet arrays for supercapacitors with ultrahigh specific capacitance

Polyvinyl alcohol/sodium alginate-based conductive hydrogels with in situ formed bimetallic zeolitic imidazolate frameworks towards soft electronics

Bimetallic zeolitic imidazolate frameworks (BZIFs) have received enormous attention due to their unique physi-chemical properties, but are rarely reported for electrically conductive hydrogel (ECH) applications arising from low intrinsic conductivity and poor dispersion. Herein, we propose an innovative strategy to prepare highly conductive and mechanically robust ECHs by in situ growing Ni/Co-BZIFs within the polyvinyl alcohol/sodium alginate dual network (PZPS). 2-methylimidazole (MeIM) ligands copolymerize with pyrrole monomers, enhancing the electrical conductivity; meanwhile, MeIM ligands act as anchor points for in-situ formation of BZIFs, effectively avoiding phase-to-phase interfacial resistance and ensuring a uniform distribution in the hydrogel network. Due to the synergism of Ni/Co-BZIFs, the PZPS hydrogel exhibits a high areal capacitance of 630.3 mF·cm-2 at a current density of 0.5 mA·cm-2, promising for flexible energy storage devices. In addition, PZPS shows excellent mechanical strength and toughness (with an ultimate tensile strength of 405.0 kPa and a toughness of 784.2 kJ·m-3 at an elongation at break of 474.0 %), a high gauge factor of up to 4.18 over an extremely wide stress range of 0-42 kPa when used as flexible wearable strain/pressure sensors. This study provides new insights to incorporating highly conductive and uniformly dispersed ZIFs into hydrogels for flexible wearable electronics.

Controlled synthesized of ternary Cu-Co-Ni-S sulfides nanoporous network structure on carbon fiber paper: a superior catalytic electrode for highly-sensitive glucose sensing

BACKGROUND

Efficient monitoring of glucose concentration in the human body necessitates the utilization of electrochemically active sensing materials in nonenzymatic glucose sensors. However, prevailing limitations such as intricate fabrication processes, lower sensitivity, and instability impede their practical application. Herein, ternary Cu-Co-Ni-S sulfides nanoporous network structure was synthesized on carbon fiber paper (CP) by an ultrafast, facile, and controllable technique through on-step cyclic voltammetry, serving as a superior self-supporting catalytic electrode for the high-performance glucose sensor.

RESULTS

The direct growth of free-standing Cu-Co-Ni-S on the interconnected three-dimensional (3D) network of CP boosted the active site of the composites, improved ion diffusion kinetics, and significantly promoted the electron transfer rate. The multiple oxidation states and synergistic effects among Co, Ni, Cu, and S further promoted glucose electrooxidation. The well-architected Cu-Co-Ni-S/CP presented exceptional electrocatalytic properties for glucose with satisfied linearity of a broad range from 0.3 to 16,000 μM and high sensitivity of 6829 μA mM- 1 cm- 2. Furthermore, the novel sensor demonstrated excellent selectivity and storage stability, which could successfully evaluate the glucose levels in human serum. Notably, the novel Cu-Co-Ni-S/CP showed favorable biocompatibility, proving its potential for in vivo glucose monitoring.

CONCLUSION

The proposed 3D hierarchical morphology self-supported electrode sensor, which demonstrates appealing analysis behavior for glucose electrooxidation, holds great promise for the next generation of high-performance glucose sensors.

Electric field-assisted laser ablation fabrication and assembly of zinc oxide/carbon nanocomposites into hierarchical structures for supercapacitor electrodes

One of the major challenges in the field of electrochemical energy storage device performance improvement is the development of suitable synthetic materials for electrodes that can provide high power and high energy density features combined with their long-term stability. Here, we have developed a novel two-step approach based on DC glow discharge plasma pre-treatment of a carbon cloth substrate followed by electric field-assisted laser ablation for the synthesis of ZnO/C nanocomposites in a liquid and their simultaneous assembly into hierarchically organized nanostructures onto the pre-processed carbon cloth to produce a supercapacitor electrode. To form such nanostructures, a processed carbon cloth was included in the electrical circuit as a cathode during laser ablation of zinc in water, while a zinc target served as an anode. A series of studies have been performed to explore the structure, morphology, composition and electrochemical characteristics of the synthesized ZnO/C nanocomposites. Application of the external field provided additional possibilities for tuning the particle morphology. The parameters of the obtained nanostructures were shown to depend on the direction of the applied electric field and liquid composition. SEM studies revealed a nanoflower-like morphology of the prepared nanomaterial having potential in supercapacitor applications due to a large surface area. The ZnO/C nanoflowers, deposited onto a carbon cloth substrate, were tested for energy storage by cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) analysis. The results showed a pseudocapacitor behavior with a maximum specific capacitance of about 3045 F g-1 (at a scan rate of 1 mV s-1). These results demonstrate a promising storage efficiency of the synthesized ZnO/C nanocomposite as a material for supercapacitors.

Facile Route to Achieve a Hierarchical CuO/Nickel-Cobalt-Sulfide Electrode for Energy Storage

Herein, a novel self-supporting CuO/nickel-cobalt-sulfide (NCS) electrode was designed in a two-step electrodeposition technique followed by a calcination process. Three-dimensional copper foam (CF) was exploited as the current collector and spontaneous source for the in situ preparation of the CuO nanostructures, which ensured sufficient deposition space for the subsequent NCS layer, thus forming abundant electrochemical active sites. Such a hierarchical structure is conducive to providing a smooth path for promoting electronic transmission. Therefore, the optimized CuO/NCS electrode exhibits outstanding energy storage capability with extremely superior specific capacitance (Cs) of 7.08 F cm-2 at 4 mA cm-2 and coulombic efficiency of up to 94.83%, as well as excellent cycling stability with capacitance retention of 83.33% after 5000 cycles. The results presented in this work extend our horizons to fabricate novel hierarchical structured electrodes applied to energy storage devices.

Road Map for In Situ Grown Binder-Free MOFs and Their Derivatives as Freestanding Electrodes for Supercapacitors

Among several electrocatalysts for energy storage purposes including supercapacitors, metal-organic frameworks (MOFs), and their derivatives have spurred wide spread interest owing to their structural merits, multifariousness with tailor-made functionalities and tunable pore sizes. The electrochemical performance of supercapacitors can be further enhanced using in situ grown MOFs and their derivatives, eliminating the role of insulating binders whose "dead mass" contribution hampers the device capability otherwise. The expulsion of binders not only ensures better adhesion of catalyst material with the current collector but also facilitates the transport of electron and electrolyte ions and remedy cycle performance deterioration with better chemical stability. This review systematically summarizes different kinds of metal-ligand combinations for in situ grown MOFs and derivatives, preparation techniques, modification strategies, properties, and charge transport mechanisms as freestanding electrode materials in determining the performance of supercapacitors. In the end, the review also highlights potential promises, challenges, and state-of-the-art advancement in the rational design of electrodes to overcome the bottlenecks and to improve the capability of MOFs in energy storage applications.

MOF-Derived Ultrathin NiCo-S Nanosheet Hybrid Array Electrodes Prepared on Nickel Foam for High-Performance Supercapacitors

At present, binary bimetallic sulfides are widely studied in supercapacitors due to their high conductivity and excellent specific capacitance (SC). In this article, NiCo-S nanostructured hybrid electrode materials were prepared on nickel foam (NF) by using a binary metal-organic skeleton as the sacrificial template via a two-step hydrothermal method. Comparative analysis was carried out with Ni-S and Co-S in situ on NF to verify the excellent electrochemical performance of bimetallic sulfide as an electrode material for supercapacitors. NiCo-S/NF exhibited an SC of 2081 F∙g^-1 at 1 A∙g^-1, significantly superior to Ni-S/NF (1520.8 F∙g^-1 at 1 A∙g^-1) and Co-S/NF (1427 F∙g^-1 at 1 A∙g^-1). In addition, the material demonstrated better rate performance and cycle stability, with a specific capacity retention rate of 58% at 10 A∙g^-1 than at 1 A∙g^-1, and 75.7% of capacity was retained after 5000 cycles. The hybrid supercapacitor assembled by NiCo-S//AC exhibited a high energy density of 25.58 Wh∙kg^-1 at a power density of 400 W∙kg^-1.

Defect-Induced Atomic Arrangement in CoFe Bimetallic Heterostructures with Boosted Oxygen Evolution Activity

Three CoFe-bimetallic oxides with different compositions (termed as CoFeO_x -A/N/H) are prepared by thermally treating metal-organic-framework (MOF) precursors under different atmospheres (air, N_2, and NaBH₄ /N₂ ), respectively. With the aid of vast oxygen vacancies (O_v ), cobalt at tetrahedral sites (Co²⁺ (Th)) in spinel Co₃ O₄ is diffused into interstitial octahedral sites (Oh) to form rocksalt CoO and ternary oxide CoFe₂ O₄ has been induced to give the unique defective CoO/CoFe₂ O₄ heterostructure. The resultant CoFeO_x -H exhibits superb electrocatalytic activity toward water oxidation: overpotential at 10 mA cm^-2 is 192 mV, which is 122 mV smaller than that of CoFeO_x -A. The smaller Tafel slope (42.53 mV dec^-1 ) and higher turnover frequency (785.5 h^-1 ) suggest fast reaction kinetics. X-ray absorption spectroscopy, ex situ characterizations, and theoretical calculations reveal that defect engineering effectively tunes the electronic configuration to a more active state, resulting in the greatly decreased binding energy of oxo intermediates, and consequently much lower catalytic overpotential. Moreover, the construction of hetero-interface in CoFeO_x -H can provide rich active sites and promote efficient electron transfer. This work may shed light on a comprehensive understanding of the modulation of electron configuration of bimetallic oxides and inspire the smart design of high-performance electrocatalysts.

Cycling stability of Fe2O3 nanosheets as supercapacitor sheet electrodes enhanced by MgFe2O4 nanoparticles

The Fe2O3 material is a common active material for supercapacitor electrodes and has received much attention due to its cheap and easy availability and high initial specific capacitance. In the present study, we prepared adhesive-free Fe2O3 sheet electrodes for supercapacitors by growing Fe2O3 material on nickel foam by hydrothermal method. The sheet electrode exhibited a high initial specific capacitance of 863 F g-1, but we found that the sheet lost its specific capacitance too quickly through cyclic stability tests. To solve this problem, Fe2O3/MgFe2O4 composites were grown on nickel foam (NF). It was found through testing that the cycling stability of the sheet electrode gradually increased as the content of MgFe2O4 material increased. When the molar ratio of Fe2O3 to MgFe2O4 material was 1 : 1, the initial specific capacitance of the sheet electrode was 815 F g-1 and the capacitance remained at 81.25% of the initial specific capacitance after 1000 cycles. The better cycling stability results from the more stable structure of the composite, the synergistic effect leading to better reversibility of the reaction.

Achieving Self-Supported Hierarchical Cu(OH)2/Nickel-Cobalt Sulfide Electrode for Electrochemical Energy Storage

Herein, nickel-cobalt sulfide (NCS) nanoflakes covering the surface of Cu(OH)₂ nanorods were achieved by a facile two-step electrodeposition strategy. The effect of CH₄N₂S concentration on formation mechanism and electrochemical behavior is investigated and optimized. Thanks to the synergistic effect of the selected composite components, the Cu(OH)₂/NCS composite electrode can deliver a high areal specific capacitance (Cs) of 7.80 F cm^-2 at 2 mA cm^-2 and sustain 5.74 F cm^-2 at 40 mA cm^-2. In addition, coulombic efficiency was up to 84.30% and cyclic stability remained 82.93% within 5000 cycles at 40 mA cm^-2. This innovative work provides an effective strategy for the design and construction of hierarchical composite electrodes for the development of energy storage devices.

Sandwich-Structured Hybrid of NiCo Nanoparticles-Embedded Carbon Nanotubes Grafted on C3N4 Nanosheets for Efficient Photodehydrogenative Coupling Reactions

Exploring cheap and efficient hybrid catalysts offers exciting opportunities for enhancing the performance of photocatalysts in the green organic synthesis field. Herein, a facile and effective approach is designed for the synthesis of a sandwich-structured hybrid in which NiCo bimetallic nanoparticles are embedded in the tip of nitrogen-doped carbon nanotubes (N-CNTs) grafted on both sides of a nitrogen deficient C₃N₄ (N_v-C₃N₄) nanosheet for photodehydrogenative coupling reactions. Such a brand-new type of sandwich-structured hybrid comprises N_v-C₃N₄ nanosheets and surrounding N-CNTs embedded with NiCo nanoparticles at their tips. Remarkably, the resultant hybrid exhibits integrated functionalities, abundant active sites, enhanced visible light absorption, and excellent interfacial charge transfer ability. As a result, the optimized NiCo@N-CNTs@N_v-C₃N₄ photocatalyst shows significantly improved photodehydrogenative coupling performance of amines to imines compared to the control single-metal-based catalysts (Ni@N-CNTs@N_v-C₃N₄ and Co@N-CNTs@N_v-C₃N₄). The mechanistic investigation through experimental and computational study demonstrates that, compared with single-metal-based hybrids, the NiCo bimetallic hybrid exhibits stronger amine adsorption and weaker photogenerated hydrogen atom adsorption, thus promoting the dehydrogenative activation of primary amines and fast generation of imines. This work presents a promising insight for designing and preparing efficient photocatalysts to trigger organic synthesis in high yields.

Cooperative effect of bimetallic MOF-derived CoNi(OH)2@NiCo2S4nanocomposite electrocatalysts with boosted oxygen evolution activity

Highly efficient and inexpensive electrocatalysts for oxygen evolution reaction (OER) are extensively studied for water splitting. Herein, a unique bimetallic nanocomposite CoNi(OH)₂@NiCo₂S₄nanosheet arrays derived from metal-organic-frameworks (MOFs, CoNi-ZIF) is simply fabricated on Ni foam, endowing large specific surface area and outstanding electrical conductivity. Compared with their single-metallic counterparts, the bimetallic composite displays dramatically low overpotential and small Tafel slope as well as outstanding catalytic stability. The overpoptential at 20 mA cm^-2for CoNi(OH)₂@NiCo₂S₄is only 230 mV in comparison with Ni(OH)₂@Ni₃S₂(266 mV), Co(OH)₂@Co₃S₄(294 mV) and RuO₂(η = 302 mV). First-principle calculations based on density functional theory (DFT) are carried out and reveal that the introduction of Ni in Co(OH)₂helps lowered the energy difference of ΔG_OOH*-ΔG_O*, and thereby boosting the OER reactivity. This study provides an effective approach for the rational construction of low-cost metal hybrids.

Self-supported metal-organic framework-based nanostructures as binder-free electrodes for supercapacitors

Metal-organic frameworks (MOFs), an interesting class of functional inorganic materials, have recently emerged as suitable electrode materials or templates/precursors of electrode materials for supercapacitors (SCs). The key in utilizing MOF-based electrode materials is to address the low electronic conductivity and poor stability issues. Therefore, the rational design and fabrication of self-supported binder-free electrodes is considered the most promising strategy to address these challenges. In this review, we summarize the recent advances in the design and manufacture of self-supported MOF-based nanostructures and their use as binderless electrodes for SCs, especially over the last five years. The synthesis strategies for constructing pristine MOFs, MOF composites and MOF derivative arrays are overviewed. By highlighting the advantages and challenges of each class of electrode materials, we hope that this review will provide some insights into the rational design of MOF-based electrode materials to promote the future development of this highly exciting field.

MOF-derived hierarchical core-shell hollow Co3S4@NiCo2O4 nanosheet arrays for asymmetric supercapacitors

Transition metal sulfides (TMSs) have been widely explored as electrode materials for supercapacitors (SCs). Nevertheless, the application of single TMS is limited due to its lattice expansion and dissolution in...

Bimetallic sites and coordination effects: electronic structure engineering of NiCo-based sulfide for 5-hydroxymethylfurfural electrooxidation

The electrocatalytic performance of 5-hydroxymethylfurfural (HMF) oxidation was improved by NiCo-based bimetallic-sites construction and S coordination species tuning.

CuCo LDHs Coated CuCoTe Honeycomb-Like Nanosheets as a Novel Anode Material for Hybrid Supercapacitors

Metal-organic frameworks derived metal chalcogenides as a new class of active materials can abolish the existing challenges in supercapacitors with their large electroactive sites and enhanced electrochemical conductivities. With its adequate conductivity and electrochemical properties, tellurium based metal chalcogenide electrodes can deliver better electrochemical performances than other chalcogenides. Herein, CuCoTe honeycomb-like nanosheets are grown on nickel foam (CuCoTe HNSs/NF) and then CuCo layered double hydroxides are successively coated on them (CTC HLSs/NF). The CTC HLSs/NF electrode exhibits tremendous performance with its high specific capacity of 399 mAh g^-1 at 7 A g^-1 of current density and good capacity retention (81.3%) after 3000 cycles. Finally, CTC HLSs/NF electrode is utilized for the hybrid supercapacitor (HSC) assembly along with activated carbon coated nickel foam in an aqueous electrolyte. The fabricated HSC shows high energy density (214.7 Wh kg^-1 ) and power density (40 kW kg^-1 ). Moreover, the device retains 96.3% of its capacitance at the end of the 5000th cycle, showing its high stability. Owing to their unique morphology and superior electrochemical properties, the present method of fabrication and selected materials can address the issues faced by electrochemical capacitors.

Recent Developments of Transition Metal Compounds-Carbon Hybrid Electrodes for High Energy/Power Supercapacitors

Due to their rapid power delivery, fast charging, and long cycle life, supercapacitors have become an important energy storage technology recently. However, to meet the continuously increasing demands in the fields of portable electronics, transportation, and future robotic technologies, supercapacitors with higher energy densities without sacrificing high power densities and cycle stabilities are still challenged. Transition metal compounds (TMCs) possessing high theoretical capacitance are always used as electrode materials to improve the energy densities of supercapacitors. However, the power densities and cycle lives of such TMCs-based electrodes are still inferior due to their low intrinsic conductivity and large volume expansion during the charge/discharge process, which greatly impede their large-scale applications. Most recently, the ideal integrating of TMCs and conductive carbon skeletons is considered as an effective solution to solve the above challenges. Herein, we summarize the recent developments of TMCs/carbon hybrid electrodes which exhibit both high energy/power densities from the aspects of structural design strategies, including conductive carbon skeleton, interface engineering, and electronic structure. Furthermore, the remaining challenges and future perspectives are also highlighted so as to provide strategies for the high energy/power TMCs/carbon-based supercapacitors.

A high-energy-density supercapacitor with multi-shelled nickel-manganese selenide hollow spheres as cathode and double-shell nickel-iron selenide hollow spheres as anode electrodes

Thanks to the attractive structural characteristics and unique physicochemical properties, mixed metal selenides (MMSes) can be considered as encouraging electrode materials for energy storage devices. Herein, a straightforward and efficient approach is used to construct multi-shelled nickel-manganese selenide hollow spheres (MSNMSeHSs) as cathode and double-shell nickel-iron selenide hollow spheres (DSNFSeHSs) as anode electrode materials by tuning shell numbers for supercapacitors. The as-designed MSNMSeHS electrode can deliver a splendid capacity of ∼339.2 mA h g-1/1221.1 C g-1, impressive rate performances of 78.8%, and considerable longevity of 95.7%. The considerable performance is also observed for the DSNFSeHS electrode with a capacity of 258.4 mA h g-1/930.25 C g-1, rate performance of 75.5%, and longevity of 90.9%. An efficient asymmetric apparatus (MSNMSeHS||DSNFSeHS) fabricated by these two electrodes depicts the excellent electrochemical features (energy density of ≈112.6 W h kg-1 at 900.8 W kg-1) with desirable longevity of ≈94.4%.

Self-templated formation of (NiCo)9S8 yolk-shelled spheres for high-performance hybrid supercapacitors

Rational materials design for the synthesis of desirable hollow micro- and nanostructures has recently revealed the remarkable potential for high-performance energy storage and conversion devices. Owing to their unique "core-void-shell" structural configurations, yolk-shell-structured electrode materials can achieve intimate contact with the electrolyte and alleviate the volume expansion issue during electrochemical cycling, which is therefore poised to further boost the electrochemical properties of hybrid supercapacitors. Herein, a facile self-templated strategy, consisting of a hydrothermal step and a high-temperature sulfurization process, has been developed for the construction of yolk-shell (NiCo)9S8 spheres in situ coated by graphite carbon ((NiCo)9S8/GC) due to the non-equilibrium thermal treatment of alkali metal alkoxides. The as-synthesized yolk-shelled sphere exhibits a high specific capacitance of 1434.4 F g-1 (179.3 mA h g-1) at a current density of 1 A g-1, and good rate capability and cycling stability with 83.1% capacitance retention at 8 A g-1 over 5000 cycles. To further demonstrate its practical application, a hybrid supercapacitor device was assembled using (NiCo)9S8/GC as the battery-type positive electrode and activated carbon (AC) as the capacitive-type electrode. The as-fabricated device can reach a wide voltage window of up to 1.6 V, deliver a high energy density of 55.6 W h kg-1 at a power density of 800.3 W kg-1 and maintain 90.2% of specific capacitance after 3000 cycles.

The Application of Metal-Organic Frameworks and Their Derivatives for Supercapacitors

Supercapacitors (SCs), one of the most popular types of energy-storage devices, present lots of advantages, such as large power density and fast charge/discharge capability. Being the promising SCs electrode materials, metal-organic frameworks (MOFs) and their derivatives have gained ever-increasing attention due to their large specific surface area, controllable porous structure and rich diversity. Herein, the recent development of MOFs-based materials and their application in SCs as the electrode are reviewed and summarized. The preparation method, the morphology of the materials and the electrical performance of various MOFs and their derivatives (such as carbon, metal oxide/hydroxide and metal sulfide) are briefly discussed. Most of recent works concentrate on Ni-, Co- and Mn-MOFs and their composites/derivatives. Conclusions and our outlook for the researches are also given, which would be a valuable guideline for the rational design of MOFs materials for SCs in the near future.