Precise design and in situ synthesis of hollow Co9S8@CoNi-LDH heterostructure for high-performance supercapacitors

Layered double hydroxides (LDHs) and metal sulfides (MSs) have been widely used as promising electrode materials for supercapacitors, and the rational architectural design of MS/LDH heterogeneous structures is critical to optimize large energy storage. Herein, a precisely designed hollow Co9S8 nanotubes@CoNi-LDH nanosheet heterostructure on Ni foam, facilely prepared by an ingenious in situ strategy in this Co9S8 nanoarray was first used as the self-sacrificing template and metal source to in situ synthesize Co-ZIF-67 polyhedron to form the Co9S8@ZIF-67 heterostructure, and then Co9S8@ZIF-67 was in situ etched successfully using Ni2+ ions to form the final Co9S8@CoNi-LDH/NF core-shell nanoarray. This in situ synthetic strategy to fabricate the heterostructure is conducive to boosting the structural stability, modifying the electric structure and regulating the interfacial charge transfer. Due to the synergistic effect and tight heterogeneous interface, Co9S8@CoNi-LDH/NF displayed an outstanding capacitance of 9.65 F cm-2 at a current density of 2 mA cm-2 and excellent capacitance retention rate of 91.7% after 5000 cycles. In addition, the ASC device assembled with AC has an extremely high energy density of 1.0 mW h cm-2 at 2 mA cm-2 and maintains 96.9% capacitance retention after 5000 cycles. This work provides a skillful strategy for the precise design and in situ synthesis of MS/LDH heterostructures with fascinating features for electrochemical energy storage applications.

Ni3S2/Co9S8 embedded poor crystallinity NiCo layered double hydroxides hierarchical nanostructures for efficient overall water splitting

Nickel-cobalt bimetallic layered double hydroxides (NiCo LDHs) are potential electrocatalysts with high performance and stability for overall water-splitting. However, its weak conductivity limits its practical applications. Herein, a simple hydrothermal in-situ conversion strategy is employed for constructing the novel heterogeneous electrocatalyst of Ni₃S₂/Co₉S₈ embedded poor crystallinity (Pc) NiCo LDH nanosheet arrays grown on the Ni foam (Pc-NiCo LDH/ Ni₃S₂/Co₉S₈), which can improve the conductivity via regulating the crystallinity. The crystallinity of NiCo LDH is well regulated by adjusting the amount of sulfur source, and the construction of Ni₃S₂/Co₉S₈ heterostructure exposes more active sites, improves the electrical conductivity, enhances the electronic interaction between NiCo LDH and Ni₃S₂/Co₉S₈, and significantly promotes the kinetics of water splitting. The optimized Pc-NiCo LDH/Ni₃S₂/Co₉S₈ hierarchical structure as both the anode and cathode exhibit the overall water splitting performance with the cell voltage of only 1.744 V to achieve the current density of 50 mA cm^-2 in the alkaline media and shows the competitive H₂ and O₂ production rate of 6.4 and 3.1 μL s^-1, respectively, suggesting its potential practical applications. This work provides a novel idea for the design of multiphase composite electrocatalysts applied in water splitting.

Phosphate Ion-Functionalized CoS with Hexagonal Bipyramid Structures from a Metal-Organic Framework: Bifunctionality towards Supercapacitors and Oxygen Evolution Reaction

To solve energy-related environmental problems and the energy crisis, efficient electrochemical materials have been developed as alternative energy storage and conversion systems. Abundant transition metals and their sulfides are attractive electrochemical materials. Herein, we report an efficient phosphorization strategy, which improves the overall electrochemical performance of metal sulfides. In detail, CoS hexagonal bipyramids were synthesized through simple calcination combined with in situ sulfurization of a cobalt-based metal-organic framework template, and then phosphate ion-functionalized CoS (P-CoS) was prepared through a phosphorization reaction. P-CoS exhibited outstanding electrochemical activity as both supercapacitor electrode and oxygen evolution reaction (OER) catalyst. Supercapacitors based on CoS and P-CoS as the electrodes had high specific capacitances of 304 and 442 F g^-1 , respectively, and remained stable for over 10 000 cycles at 5 A g^-1 . For OER, P-CoS showed a current density of 10 mA cm^-2 at an overpotential of 340 mV, with a small Tafel slope. In conclusion, functionalizing CoS with phosphate ions is a promising method for enhancing chemical reactivity and accelerating ion and electron transfer.

2D nanosheet/3D cubic framework Ni-Co sulfides for improved supercapacitor performance via structural engineering

The construction of multi-dimensional structured battery-type electrode materials is a promising strategy to develop high performance electrodes for supercapacitors. Herein, a series of battery-type Ni3S2@Co3S4 electrodes with different morphologies are synthesized by controlling the hydrothermal reaction time. Owing to the unique structure with independent but interconnected 2D nanosheets and 3D cubic frameworks, NCS-60 displays high conductivity, numerous active sites and good wettability behavior. It can deliver a high specific capacity of 388.9 mA h g-1 (3500 F g-1) at 1 A g-1, an outstanding rate capacity of maintaining 88.6% at 10 A g-1 and long cycle stability. The battery-type supercapacitor hybrid (BSH) device with active carbon (AC) as the negative electrode delivers an energy density of 41.8 W h kg-1 at the power density of 800 W kg-1. This study provides a feasible route for regulating the morphologies of in situ growth materials that improve the electrochemical performance of supercapacitors.

Enhanced water splitting performance of GaN nanowires fabricated using anode aluminum oxide templates

Highly ordered GaN nanowires were fabricated using an anodic aluminum oxide (AAO) template. Compared to planar GaN, the GaN nanowires significantly increased the light absorption, and the saturated photocurrent increased by a factor of 5 from 0.075 to 0.38 mA cm⁻². The photocurrent increase with the GaN nanowires is not only due to their increased surface to volume ratio and reduction in the distance for photo-generated carriers to reach the electrolyte, but also the built-in electric field, which mainly contribute to the enhancement in their water splitting ability. The GaN nanowires can lead to band bending due to their surface states and the formation of a polarized electric field to accelerate the separation of photo-generated carriers. We also established a theoretic model to simulate the band bending in the nanowires. The results showed that when the nanowire diameters are equal or bigger than the full width of depletion region, the nanowires have the maximum electric field, which improves their water splitting performance significantly. These results provide a cost-effective way for highly efficient water splitting.

Highly ordered GaN nanowires were fabricated using an anodic aluminum oxide (AAO) template.

α-Ni(OH)2/NiS1.97 heterojunction composites with excellent ion and electron transport properties for advanced supercapacitors

It is recognized that an effective strategy to promote the industrialization of supercapacitors is to enhance the ion and electronic conductivities of electrode materials. In this work, it is demonstrated that the NO/NS-8 heterojunction material obtained via an epitaxial growth method based on ion exchange can be used as an outstanding electrode material for supercapacitors. The construction of heterojunctions between α-Ni(OH)2 and NiS1.97 allows the components to provide each other with ion or electron transport paths and endows NO/NS-8 with excellent ion and electron transport properties; this leads to a high utilization rate of active materials and an unprecedented high specific capacitance (up to 2375.8 F g-1 at 1 mV s-1 in a three-electrode system). Using the as-prepared NO/NS-8 heterojunction material as an electroactive material, an asymmetric supercapacitor with long cycle life (62.8% capacitance retention after 10 000 cycles at a current density of 5 A g-1) and high energy and power densities (128.4 W h kg-1 at a power density of 402.9 W kg-1 and 63.8 W h kg-1 at 7662.7 W kg-1) is finally demonstrated. This work provides a novel strategy for developing unique heterojunction materials for energy storage.

Bio-inspired nano-engineering of an ultrahigh loading 3D hierarchical Ni@NiCo2S4/Ni3S2 electrode for high energy density supercapacitors

Energy density has become a critical barrier in supercapacitor engineering and improvement of the electrode-loading is urgently demanded. However, there is conflict between the high loading and good electrochemical properties of supercapacitors. Herein, ultrahigh loading (10.33 mg·cm-2) 3D hierarchical NiCo2S4/Ni3S2 on Ni foam with outstanding performance is obtained via bio-inspired nano-engineering, which contains compact nanowire arrays catching urchin-like micro-particles. Using this high-loading material as a binder-free electrode achieves excellent areal capacitances with 16.90 F·cm-2 at 10.33 mA·cm-2 and 1.17 F·cm-2 at 5.17 mA·cm-2 in a three-electrode system and asymmetric supercapacitor device, respectively. The device also exhibits a high energy density of 4.69 W h m-2 (power density of 10.33 W·m-2) and an outstanding stability of 91.4% after 8000 cycles (20.66 mA·cm-2). Its excellent performance is attributed to the well-designed structure and composition: (i) a large contact area with the electrolyte raises the utilization efficiency of the active material, therefore guaranteeing the high capacitance of the active materials; (ii) the high electronic conductivity network constructed through NiCo2S4 and the short diffusion length boost its rate performance; (iii) the reserved space in the hierarchical structure could hold the volume change and enhance the cycling performance of the electrode in the charge/discharge cycles. Thus, this work not only provides a method for the construction of a high-loading and high-performance electrode for asymmetric supercapacitors, but could also shed light on the design of compact nano-materials for other energy storage systems.

Bi-component synergic effect in lily-like CdS/Cu7S4 QDs for dye degradation

CdS has attracted extensive attention in the photocatalytic degradation of wastewater due to its relatively narrow bandgap and various microstructures. Previous reports have focused on CdS coupled with other semiconductors to reduce the photocorrosion and improve the photocatalytic performance. Herein, a 3D hierarchical CdS/Cu₇S₄ nanostructure was synthesized by cation exchange using lily-like CdS as template. The heterojunction material completely inherits the special skeleton of the template material and optimizes the nano-scale morphology, and achieves the transformation from nanometer structure to quantum dots (QDs). The introduction of Cu ions not only tuned the band gap of the composites to promote the utilization of solar photons, more importantly, Fenton-like catalysis was combined into the degradation process. Compared with the experiments of organic dye degradation under different illumination conditions, the degradability of the CdS/Cu₇S₄ QDs is greatly superior to pure CdS. Therefore, the constructed CdS/Cu₇S₄ QDs further realized the optimization of degradation performance by the synergic effect of photo-catalysis and Fenton-like catalysis.

CdS has attracted extensive attention in the photocatalytic degradation of wastewater due to its relatively narrow bandgap and various microstructures.

A facile synthetic route to tungsten diselenide using a new precursor containing a long alkyl chain cation for multifunctional electronic and optoelectronic applications

Single source precursors for coating and subsequent thermal decomposition processes enable a large-scale, low-cost synthesis of two-dimensional transition metal dichalcogenides (TMDs). However, practical applications based on two-dimensional TMDs have been limited by the lack of applicable single source precursors for the synthesis of p-type TMDs including layered tungsten diselenide (WSe₂). We firstly demonstrate the simple and facile synthesis of WSe₂ layers using a newly developed precursor that allows improved dispersibility and lower decomposition temperature. We study the thermal decomposition mechanism of three types of (Cat⁺)₂[WSe₄] precursors to assess the most suitable precursor for the synthesis of WSe₂ layers. The resulting chemical and structural exploration of solution-processed WSe₂ layers suggests that the (CTA)₂[WSe₄] may be a promising precursor because it resulted in the formation of high-crystalline WSe₂. In addition, this study verifies the capability of WSe₂ layers for multifunctional applications in optoelectronic and electronic devices. The photocurrent of WSe₂-based photodetectors shows an abrupt switching behavior under periodic illumination of visible or IR light. The extracted photoresponsivity values for WSe₂-based photodetectors recorded at 0.5 V correspond to 26.3 mA W⁻¹ for visible light and 5.4 mA W⁻¹ for IR light. The WSe₂-based field effect transistors exhibit unipolar p-channel transistor behavior with a carrier mobility of 0.45 cm² V⁻¹ s⁻¹ and an on-off ratio of ∼10.

Single source precursors for coating and subsequent thermal decomposition processes enable a large-scale, low-cost synthesis of two-dimensional transition metal dichalcogenides (TMDs).

Hierarchical CuCo2O4@NiMoO4 core–shell hybrid arrays as a battery-like electrode for supercapacitors

Hierarchical CuCo₂O₄@NiMoO₄ core–shell hybrid arrays exhibit a high specific capacitance, good rate capability and excellent cycling stability.