Controlled synthesis of a high-performance α-NiS/Ni3S4 hybrid by a binary synergy of sulfur sources for supercapacitor

Multi-layered heterogeneous interfaces created in Co0.85Se@Ni3S4/NF to enhance supercapacitor performances by multi-step alternating electrodeposition

Heterogeneous interface construction is of far-reaching significance to optimize the electrochemical performance of electrodes. Herein, a multi-step alternating electrochemical deposition (MAED) method is proposed to alternately deposit Co0.85Se and Ni3S4 nanosheets on a nickel foam (NF), forming a special alternate layer-by-layer structure with multi-layered heterogeneous interfaces. The creation of the multi-layered heterogeneous interfaces provides a large interfacial area for redox reactions with optimum interstitials facilitating ion diffusion, thus greatly improving the electrochemical energy storage efficiency. With the increase in the layer number, the material exhibits increasingly better energy storage performance, and 8L-Co0.85Se@Ni3S4/NF exhibits the highest specific capacitances of 2508 F g-1 and 1558 F g-1 at a scan rate of 2 mV s-1 and a current density of 1 A g-1. The 8L-Co0.85Se@Ni3S4/NF//polypyrrole (PPy)/NF asymmetric supercapacitor provides a maximum operation potential window of 1.55 V and energy densities of 76.98 and 35.74 W h kg-1 when the power densities are 775.0 and 15 500 W kg-1, respectively, superior to most of the related materials reported. Through MAED, the deposited phase and the layer number can be accurately controlled, thus providing an efficient strategy for interface construction so as to increase the electrochemical activity of the energy storage materials.

Review of NiS-Based Electrode Nanomaterials for Supercapacitors

As a new type of energy storage device, supercapacitors have the advantages of high-power densities, high safety factors, and low maintenance costs, so they have attracted widespread attention among researchers. However, a major problem with supercapacitors is that their energy densities are not high enough, which limits their application. Therefore, it is crucial to expand the application scenarios of supercapacitors to increase their energy density as much as possible without diminishing their advantages. The classification and working principles of supercapacitors are introduced in this paper. The electrochemical properties of pure NiS materials, NiS composites with carbon materials, NiS composites with sulfide materials, and NiS composites with transition metal oxides for supercapacitors are summarized. This paper may assist in the design of new electrode materials for NiS-based supercapacitors.

RGO nanosheet wrapped β-phase NiCu2S nanorods for advanced supercapacitor applications

A new integration strategy of transition metal sulfide with carbon-based materials is used to boost its catalytic property and electrochemical performances in supercapacitor application. Herein, crystalline reduced graphene oxide (rGO) wrapped ternary metal sulfide nanorod composites with different rGO ratios are synthesized using hydrothermal technique and are compared for their physical, chemical, and electrochemical performances. It is found that their properties are tuned by the weight ratios of rGO. The electrochemical investigations reveal that β-NiCu₂S/rGO nanocomposite electrode with 0.15 wt.% of rGO is found to possess maximum specific capacitance of 1583 F g^-1 at current density of 15 mA g^-1 in aqueous electrolyte medium. The same electrode shows excellent cycling stability with capacitance retention of 89% after 5000 charging/discharging cycles. The reproducibility test performed on NiCu₂S/rGO nanocomposite electrode with 0.15 wt.% of rGO indicates that it has high reproducible capacitive response and rate capability. Thus, the present work demonstrates that the β-NiCu₂S/rGO nanocomposite can serve as a potential electrode material for developing supercapacitor energy storage system.

Tremella-like 2D Nickel-Copper Disulfide with Ultrahigh Capacity and Cyclic Retention for Hybrid Supercapacitors

Two-dimensional (2D) disulfides possess unique physical and chemical properties and are widely used in electronic and photoelectric devices. Tuning the composition and optimizing the structure of the disulfides are feasible approaches to designing target sulfides for hybrid supercapacitors. This work synthesizes the tremella-like nanosheet-connected (Cu_xNi_1-x)S₂ via solvothermal and sulfur-vapor vulcanization. The 2D (Cu_xNi_1-x)S₂ electrode performs a high reversible capacity (526.0 mA h g^-1 at 1 A g^-1), decent capacity retention (75.6%) at 10 A g^-1, and prolonged cyclic retention (94.4% over 15,000 cycles), which is higher than that of (Cu_xNi_1-x)O and monometallic sulfides of NiS₂ and CuS. The elevated electrochemical properties of (Cu_xNi_1-x)S₂ are attributed to the optimized composition with increased redox reaction, enlarged lattice distance, abundant active sites, and attractive electronic and ionic conductivity. Also, (Cu_xNi_1-x)S₂ and active carbon (AC) are assembled to form a hybrid supercapacitor (HSC). The (Cu_xNi_1-x)S₂//AC HSC demonstrates a maximum specific capacitance of 231.0 F g^-1 at 1 A g^-1 and a high energy density of 82.4 W h kg^-1 at a power density of 1.82 kW kg^-1. Outstanding cyclic retentions of 94.9 and 84.5% after 8000 and 10,000 cycles are also obtained. In conclusion, this result suggests a facile routine for preparing a novel 2D layer material of (Cu_xNi_1-x)S₂ with outstanding specific capacity and cycling performance for hybrid supercapacitors.

Corrosion-controlled surface engineering improves the adhesion of materials for stable free-standing electrodes

Directly anchoring active materials on porous conductive substrates is considered an effective strategy to obtain a high-activity electrode since the direct contact between active materials and substrates benefits charge transfer, and the presence of porous structures provides more active sites. However, due to the presence of strong stress and weak adhesion, active materials loaded on the substrate are very easy to peel off during assembly and use, which can greatly shorten the lifetime of use. Herein, an ultrasonic corrosion strategy is proposed to regulate the surface of a metal substrate. We find that ultrasonic oxygen corrosion and interfacial water control play key roles in fabricating the complex electrode, which can help the surface of Cu foam to form special lamellar cross-linked CuO nanoarchitectures with strong adhesion and then overcome the defect of the deposited NiCo layered double hydroxides (NC LDH) on the stress and adhesion. The expected electrode shows more than 70% improvement in cycling stability at an ultra-high current density of 20 A g^-1, relative to the active material layer of the electrode with strong stress and weak adhesion. Meanwhile, benefiting from its lamellar cross-linked nanoarchitectures having large specific surface area and many nano-pores, it presents a high specific capacitance of 3010.8F g^-1 at 1 A g^-1 and a good rate capability of 59.3% at 50 A g^-1. It is foreseen that this finding provides a novel, universal strategy for managing the surface and interface of the metal substrate, thereby obtaining a reliable, stable electrode.

Designing a carbon nanofiber-encapsulated iron carbide anode and nickel-cobalt sulfide-decorated carbon nanofiber cathode for high-performance supercapacitors

To meet the crucial demand for high-performance supercapacitors, much effort has been devoted to exploring electrode materials with nanostructures and electroactive chemical compositions. Herein, iron carbide nanoparticles are encapsulated into carbon nanofibers (Fe₃C@CNF-650) through electrospinning and annealing methods. Nickel-cobalt sulfide nanoparticles are hydrothermally grown on electrospun carbon nanofibers (CNF@NiCoS-650). The Faradaic electrochemical reactions of transition metal compounds improve the specific capacitance of the developed electrode. Meanwhile, the electrically conductive framework of carbon nanofibers facilitates Faradic charge transport. In detail, the Fe₃C@CNF-650 anode and CNF@NiCoS-650 cathode achieve specific capacitances of 1551 and 205 F g^-1, respectively, at a current density of 1 A g^-1. A hybrid supercapacitor that is fabricated from the Fe₃C@CNF-650 anode and CNF@NiCoS-650 cathode delivers an energy density of 43.2 Wh kg^-1 at a power density of 800 W kg^-1. The designed nanostructures are promising for practical supercapacitor applications.

Nickel hydroxide/sulfide hybrids: halide ion controlled synthesis, structural characteristics, and electrochemical performance

Focusing on the synthesis of nickel-based materials (such as nickel sulfides, nickel hydroxides, and nickel oxides) is an urgent need in the fields of batteries, supercapacitors, and catalysis. However, their controlled synthesis still remains a great challenge because of the inadequate understanding of the control factor of their synthesis. A two-step solvo-/hydrothermal process with halide ion embedding/releasing was proposed to understand the effect of the halide ions on the synthesis and sulfidation of nickel hydroxy-halides. We find that the halide ions determine the formation, growth, and evolution of nickel hydroxy halides and promote them to form unique architectures and morphologies, leading to obvious differences in structural characteristics, including conductivity and electrochemical activity. Because of the presence of halide ions, a series of hybrids with multiple interfaces, which consist of hydroxides and sulfides and have various morphologies, such as flower-like balls, solid balls, porous balls, schistose, and thorny balls, with capacities ranging from 100.7 to 261.2 mA h g^-1, can be easily obtained. It is fully demonstrated that the halide anion plays a core role in the synthesis process of nickel-based materials, and this finding will provide more chances for controllably synthesizing high-activity electrode materials.

Fe and Cu dual-doped Ni3S4 nanoarrays with less low-valence Ni species for boosting water oxidation reaction

Transition metal materials with high efficiency and durable electrocatalytic water splitting activity have attracted widespread attention among scientists. In this work, two cation co-doped Ni₃S₄ nanoarrays grown on a Ni foam support were firstly synthesized through a typical two step hydrothermal process. Cu and Fe co-doping can regulate the internal electron configuration of the material, thus reducing the activation energy of the active species. Moreover, density functional theory calculations demonstrate that a low Ni²⁺ amount improves the adsorption energy of H₂O, which facilitates the formation and reaction of intermediate species in the water splitting process. The experimental results indicate that the Cu and Fe co-doped Ni₃S₄ material has superior electrochemical activity for water oxidation reaction to pure Ni₃S₄, Fe doped Ni₃S₄ and Cu doped Ni₃S₄. The Fe-Cu-Ni₃S₄ material displays a significantly enhanced electrocatalytic performance with low overpotentials of 230 mV at 50 mA cm^-2 and 260 mV at 100 mA cm^-2 for the oxygen evolution reaction under alkaline conditions. It's worth noting that when Fe-Cu-Ni₃S₄ was used as the anode and cathode, a small cell voltage of 1.59 V at 10 mA cm^-2 was obtained to achieve stable overall water splitting. Our work will afford a novel view and guidance for the preparation and application of efficient and environmentally friendly water splitting catalysts.

Carbon nanotubes interpenetrating MOFs-derived Co-Ni-S composite spheres with interconnected architecture for high performance hybrid supercapacitor

Recently, carbon nanotubes (CNT)-based interconnected architectures exhibit promising prospects in supercapacitors due to their flexibility and high electrical conductivity. Herein, a three-dimensional (3D) interconnected network structure combined with conductive carbon nanotubes interpenetrating MOFs-derived Co-Ni-S composite spheres (Co-Ni-S/CNTs) was synthesized. Such 3D interconnected architecture significantly leads to a favorable electronic structure, fast charge-transfer capacity, and more pseudocapacitive. The Co-Ni-S/CNTs-based hybrid electrode exhibits an extraordinary specific capacitance of 540.6C g^-1 at 1 A g^-1 and competitive rate performance (capacity retention rate of 69.9% when the current density increases to 10 times). Subsequently, a hybrid supercapacitor is assembled using Co-Ni-S/CNTs as the positive electrode and commercial activated carbon as negative electrode. The device delivers a high energy density of 63.5 W h kg^-1 at 800 W kg^-1 and keeps 83.0% initial capacitance retention after 10,000 cycles. The encouraging performances demonstrate the significant contribution of the 3D interconnected architecture for the future energy storage.

Coordination agent-dominated phase control of nickel sulfide for high-performance hybrid supercapacitor

The property of an active material is not only influenced by its morphology and size, but also by its crystal phase. The present phase regulation of nickel sulfide is mainly achieved by controlling the participation of sulfur source in reaction. Thus, new perspectives direct at phase control need to be explored and supplemented. Herein, we proposed a novel coordination agent-dominated phase modulation strategy assisted by a hydrothermal process. It is found that increasing the amount of coordination agent can drove the phase transformation from the initial composite of β-NiS/α-NiS/Ni₃S₄ to β-NiS/α-NiS, and then to pure β-NiS. The mechanism of phase regulation has been proposed, and the general application of this method has been demonstrated. By employing coordination agent, the size of resulted products is reduced, and the morphology is optimized. As a result, all of the pure β-NiS electrodes indicate significantly enhanced specific capacity than the pristine β-NiS/α-NiS/Ni₃S₄ composite. Notably, the sample synthesized with 3 mmol of urea (S11) shows uniform morphology and smallest size, and it gives a highest specific capacity of 223.8 mAh g^-1 at 1 A g^-1, almost 1.5 times of the original sample. The fabricated S11//rGO device delivers a high energy density of 56.6 Wh·kg^-1 at a power density of 407.5 W·kg^-1, and keeps an impressive capacity retention of 84% after 20,000 cycles. This work put forwards a new prospect for controlling the phase and composition of nickel sulfide based on coordination chemistry.

Template-free synthesis of β-NiS ball-in-ball microspheres for a high-performance asymmetrical supercapacitor

While significant advances have been made in the synthesis of core-/multi-shell materials, the synthetic process usually involves a soft/hard template and complicated procedures. In particular, it is extremely difficult to fabricate single-component core-shell structures for nickel sulfides (NSs) with a controlled phase. In this work, we demonstrate a novel facile method to synthesize a single-component β-NiS ball-in-ball microsphere. The ball-in-ball structure is easily obtained by uniquely employing 2-mercaptopropionic acid (2-MPA) as the sulfur source and ethanol as the solvent based on the Ostwald ripening process. In particular, our work demonstrates that the chemical structure of sulfur sources and solvents plays a key role in the formation of the pure β-NiS ball-in-ball structure. When used as an electrode active material, the β-NiS ball-in-ball microspheres exhibit two times stronger specific capacity and three times higher rate performance than NSs produced by a hydrothermal method. The fabricated NS-2//rGO asymmetrical supercapacitor (ASC) displays an energy density of 46.4 W h kg^-1 at a power density of 799.0 W kg^-1 and good cycling performance. Thus, this study provides a new method for controlling the phase and morphology of NSs.