Solvent-Controlled Morphology of Zinc-Cobalt Bimetallic Sulfides for Supercapacitors

Bimetallic sulfides offer high theoretical specific capacitance and good stability as electrode materials due to their diverse redox reactions, larger specific surface areas, and better conductivity. The morphology of the electrode material is an important influencing factor for the electrochemical properties. Herein, a series of ZnCoS electrode materials with different morphologies were prepared by varying the solvent in the solvothermal reaction, and the effects of different microstructures on the electrochemical properties of ZnCoS were investigated. The ratio of water and ethanol in the solvent was controlled to modulate the microstructure of the as-prepared ZnCoS materials. XRD and XPS revealed the physical and chemical structure of the ZnCoS materials. SEM and TEM observations showed that the microstructure of ZnCoS transformed from one-dimensional wires to two-dimensional sheets with increasing amounts of ethanol. The maximum specific capacitance of the as-prepared ZnCoS materials is 6.22 F cm-2 at a current density of 5 mA cm-2, which is superior to that of most previously reported bimetallic sulfides. The enhanced electrochemical performance could be ascribed to its sheet-assembled spherical structure, which not only shortens the path of ion diffusion but also increases the contact between surface active sites and the electrolyte. Moreover, the spherical structure provides numerous void spaces for buffering the volume expansion and penetration of the electrolyte, which would be favorable for electrochemical reactions. Furthermore, the ZnCoS electrodes were coupled with activated carbon (AC) electrodes to build asymmetric supercapacitors (ASCs). The ASC device exhibits a maximum energy density of 0.124 mWh cm-2 under a power density of 2.1 mW cm-2. Moreover, even under a high-power density of 21 mW cm-2, the energy density can still reach 0.055 mWh cm-2.

Dual-ZIF-derived "reassembling strategy" to hollow MnCoS nanospheres for aqueous asymmetric supercapacitors

Construction of delicate nanostructures with a facile, mild-condition and economical method is a key issue for building high-performance electrode materials. We demonstrate a facile and novel "reassembling strategy" to hollow MnCoS nanospheres derived from dual-ZIF for supercapacitors. The spherical shell's surface structure, thickness and Mn distribution were controlled by regulating the solvothermal reaction time. The chemical composition, phases, specific surface areas and microstructure were studied and the electrochemical performances were systematically estimated. As the unique low-crystalline and optimized hollow nanosphere structure contributes to increasing active sites, MnCoS nanospheres exhibit excellent electrochemical performance. The test results show that the specific capacitance increases with increasing solvothermal time, and the MCS with a 5 h reaction time exhibits optimal electrochemical properties with a high specific capacity of 957 C g-1 (1 A g-1). Furthermore, an MCS-5//AC asymmetric supercapacitor device delivers a specific energy as high as 36.9 W h kg-1 at a specific power of 750 W kg-1.

Influence of Transition Metals (Cu and Co) on the Carbon-Coated Nickel Sulfide Used as Positive Electrode Material in Hybrid Supercapacitor Device

Herein, a facile, environment-friendly and cost-effective approach was followed for the preparation of metal sulfide-based supercapacitor electrodes. The effect of transition metal interrogation on the morphology and electrochemical performance of carbon-coated nickel sulfide composite electrode was investigated. Physicochemical characterization showed that the enhancement in electrical conductivity and electrochemical reaction sites with the introduction of copper (Cu) and cobalt (Co) was due to the variation in morphology. Fast ionic transformation and improvement in the number of redox active sites might improve the supercapacitor performance. The electrochemical experiment showed that the NCoSC electrode exhibited the highest capacitance value of ~760 F g−1 at 2 A g−1 current density as compared to the NCuSC and NSC electrodes. Therefore, a hybrid supercapacitor (HSC) device was fabricated by using NCoSC as the positive electrode and thermally reduced graphene oxide (TRGO) as the negative electrode. The fabricated device demonstrated maximum energy density of ~38.8 Wh Kg−1 and power density of 9.8 kW Kg−1. The HSC device also showed ~89.5% retention in specific capacitance after 10,000 charge–discharge cycles at 12 A g−1 current density. So, the tuning of electronic and physical properties by the introduction of Cu and Co on nickel sulfide improved the supercapacitor performance.

MoO3 thin layers on NiCo2S4 substrate for efficient electrochemical charge storage

Ternary oxides/sulfides have long been investigated as promising electrode materials for charge storage applications. However, it is important to rationally design nanostructured hybrid composites for superior charge storage performance as electrodes in devices. In this work, MoO₃@NiCo₂S₄ hybrid composites materials are synthesized by the hydrothermal method followed by annealing at different temperatures. The charge storage properties of these materials are tested by cyclic voltammetry, galvanostatic charge-discharge curves and electrochemical impedance spectroscopy. It is found that the structure of the hybrid composite material not only assists electron and charge transportation but also precisely control the volume expansion during redox reactions, contributing to superior electrochemical behavior. Among all the electrodes, the electrode fabricated with MoO₃@NiCo₂S₄ composite material annealed at 400 °C (MoO₃@NiCo₂S₄-400) is the best for charge storage applications. At 400 °C, MoO₃ spreads as a thin layer of surface polymeric molybdates on NiCo₂S₄ as seen in the XRD pattern. Significantly, it delivers the highest capacitance of 1622 F g^-1 at 1 A g^-1 in 2 M aqueous KOH electrolyte compared to other hybrid composite electrodes, NiCo₂S₄ (962 F g^-1), MoO₃@NiCo₂S₄-500 (1412 F g^-1) and MoO₃@NiCo₂S₄-600 (970 F g^-1), under the same measurement conditions. Furthermore, the MoO₃@NiCo₂S₄-400 hybrid electrode shows better cyclic stability with 93% capacitance retention after 3000 charge-discharge cycles at 8 A g^-1. The synergistic effect of two components and annealing temperature plays important role in enhancing the charge storage performance. This work shows the importance of the synthesis temperature on the functional character of ternary sulfide/oxide composite materials for charge storage applications.

Bimetallic metal-organic frameworks and their derivatives

Bimetallic metal-organic frameworks (MOFs) have two different metal ions in the inorganic nodes. According to the metal distribution, the architecture of bimetallic MOFs can be classified into two main categories namely solid solution and core-shell structures. Various strategies have been developed to prepare bimetallic MOFs with controlled compositions and structures. Bimetallic MOFs show a synergistic effect and enhanced properties compared to their monometallic counterparts and have found many applications in the fields of gas adsorption, catalysis, energy storage and conversion, and luminescence sensing. Moreover, bimetallic MOFs can serve as excellent precursors/templates for the synthesis of functional nanomaterials with controlled sizes, compositions, and structures. Bimetallic MOF derivatives show exposed active sites, good stability and conductivity, enabling them to extend their applications to the catalysis of more challenging reactions and electrochemical energy storage and conversion. This review provides an overview of the significant advances in the development of bimetallic MOFs and their derivatives with special emphases on their preparation and applications.

Hydrothermal Synthesis of Cobalt Ruthenium Sulfides as Promising Pseudocapacitor Electrode Materials

In this paper, we report the successful synthesis of cobalt ruthenium sulfides by a facile hydrothermal method. The structural aspects of the as-prepared cobalt ruthenium sulfides were characterized using X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy. All the prepared materials exhibited nanocrystal morphology. The electrochemical performance of the ternary metal sulfides was investigated by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy techniques. Noticeably, the optimized ternary metal sulfide electrode exhibited good specific capacitances of 95 F g−1 at 5 mV s−1 and 75 F g−1 at 1 A g−1, excellent rate capability (48 F g−1 at 5 A g−1), and superior cycling stability (81% capacitance retention after 1000 cycles). Moreover, this electrode demonstrated energy densities of 10.5 and 6.7 Wh kg−1 at power densities of 600 and 3001.5 W kg−1, respectively. These attractive properties endow proposed electrodes with significant potential for high-performance energy storage devices.

Facile Dual-Ligand Modulation Tactic toward Nickel-Cobalt Sulfides/Phosphides/Selenides as Supercapacitor Electrodes with Long-Term Durability and Electrochemical Activity

The use of high electrochemical active binary nickel-cobalt sulfides/phosphides/selenides (Ni-Co-X, X = S, P, Se) as electrochemical energy storage materials still has a space for improvement because they become electrochemically unstable during long-term use. Herein, a facile and cost-effective dual-ligand synergistic modulation tactic is described to substantially improve the durability of Ni-Co-X (X = S, P, Se) at the atomic level by partially substituting S, P, and Se ligands into the nickel-cobalt hydroxide precursor, respectively. Remarkably, the dual-ligand electrodes on Ni-foam achieve superior durability and high electrochemical activity when used as positive electrodes in supercapacitors. Impressively, the density functional theory calculations demonstrate that the OH ligand in NiCo₂(MOH)_x (M = S, P, Se) could attract electrons from metal-S/metal-P/metal-Se bonds to the metal-O bond, enhancing the binding energy of metal-S/metal-P/metal-Se bonds and improving the long-term durability of Ni-Co-X (X = S, P, Se) in alkaline electrolytes. Moreover, OH and S/P/Se ligands could effectively alter the electron structure and result in favorable electrochemical activity. Overall, this tactic could offer an exciting avenue to achieve long-term durability and electrochemical activity of supercapacitor electrodes simultaneously.

Hollow core–shell NiCo2S4@MoS2 dodecahedrons with enhanced performance for supercapacitors and hydrogen evolution reaction

Hollow core–shell NiCo₂S₄@MoS₂ heterostructures were fabricated using zeolitic imidazolate frameworks as templates and exhibited enhanced electrochemical performance for supercapacitors and hydrogen evolution reaction.