A hollow Co9S8 rod-acidified CNT-NiCoLDH composite providing excellent electrochemical performance in asymmetric supercapacitors

"On-The-Fly" Synthesis of Self-Supported LDH Hollow Structures Through Controlled Microfluidic Reaction-Diffusion Conditions

Layered double hydroxides (LDHs) are a class of functional materials that exhibit exceptional properties for diverse applications in areas such as heterogeneous catalysis, energy storage and conversion, and bio-medical applications, among others. Efforts have been devoted to produce millimeter-scale LDH structures for direct integration into functional devices. However, the controlled synthesis of self-supported continuous LDH materials with hierarchical structuring up to the millimeter scale through a straightforward one-pot reaction method remains unaddressed. Herein, it is shown that millimeter-scale self-supported LDH structures can be produced by means of a continuous flow microfluidic device in a rapid and reproducible one-pot process. Additionally, the microfluidic approach not only allows for an "on-the-fly" formation of unprecedented LDH composite structures, but also for the seamless integration of millimeter-scale LDH structures into functional devices. This method holds the potential to unlock the integrability of these materials, maintaining their performance and functionality, while diverging from conventional techniques like pelletization and densification that often compromise these aspects. This strategy will enable exciting advancements in LDH performance and functionality.

Promoted OH- adsorption and electron-transfer kinetics by electrospinning mono-disperse NiCo2S4 nanocrystals within porous CNFs for solid asymmetric supercapacitors

Bimetallic sulfide NiCo2S4 has been regarded as a potential supercapacitor electrode material with excellent electrochemical performance. However, the origin of its high specific capacity is little studied, and the design of a rational structure still remains a challenge to exert its intrinsic advantage. In this work, the advantage of NiCo2S4 over NiS and CoS is explained by density functional theory calculation from the aspects of energy band, density of electronic states and OH- adsorption energy. It is proved that the synergistic effect of Ni and Co in NiCo2S4 can reduce its OH- adsorption energy and provide more active electrons near the Fermi level, thus promoting electrochemical reaction kinetics in supercapacitors. Then, a simple electrospinning method is used to in-situ load mono-disperse NiCo2S4 nanocrystals within amorphous carbon nanofibers, obtaining a porous, lotus-leaf-stem-like one-dimensional nanocomposite of NiCo2S4/CNF. Ex-situ XPS characterization confirms that the proportion of metal ions involved in electrochemical reactions and the number of transferred electrons in NiCo2S4/CNF during the redox reaction are significantly higher than those in mono-metallic sulfides (NiS/CNF and CoS/CNF), verifying the calculation results. With its boosting reaction kinetics, the NiCo2S4/CNF gives the specific capacity of 757.97C g-1 at 1 A/g and the capacity retention of 95.15 % after 10,000 cycles at 5 A/g, both greater than NiS/CNF and CoS/CNF. The NiCo2S4/CNF, as the positive electrode, and activated carbon, as the negative electrode, are assembled into liquid-state and solid-state asymmetric supercapacitor (ASC) devices, and both show high power density (760.6 W kg-1 for liquid-state device and 1067.4 W kg-1 for solid-state device), high energy density (52.25 Wh kg-1 for liquid-state device and 48.54 Wh kg-1 for solid-state device) and great cycle stability. Moreover, the solid-state ASC device possesses excellent low temperature capacity and reversibility, further demonstrating the wide application potential of the NiCo2S4/CNF composite.

Controlled Synthesis of Flower-like Hierarchical NiCo-Layered Double Hydroxide Integrated with Metal-Organic Framework-Derived Co@C for Supercapacitors

Layered double hydroxides (LDHs) have come to the foreground recently, considering their unique layered structure and short ion channels when they act as electrode materials for supercapacitors (SCs). However, due to their poor rate and cycle performance, they are not highly sought after in the market. Therefore, a flower-like hierarchical NiCo-LDH@C nanostructure with flake NiCo-LDH anchored on the carbon skeleton has emerged here, which is constructed by calcination and hydrothermal reaction and applying flake ZIF-67 as a precursor. In this structure, NiCo-LDH grows outward with abundant and homogeneously distributed Co nanoparticles on Co@C as nucleation sites, forming a hierarchical structure that is combined tightly with the carbon skeleton. The flower-like hierarchical nanostructures formed by the composite of metal-organic frameworks (MOFs) and LDHs have successfully enhanced the cycle and rate performance of LDH materials on the strength of strong structural stability, large specific surface area, and unique cooperative effect. The NiCo-LDH@C electrode displays superb electrochemical performance, with a specific capacitance of 2210.6 F g^-1 at 1 A g^-1 and 88.8% capacitance retention at 10 A g^-1. Furthermore, the asymmetric supercapacitor (ASC) constructed with NiCo-LDH@C//RGO reveals a remarkable energy density of 45.02 W h kg^-1 with a power density of 799.96 W kg^-1. This project aims to propose a novel avenue to exploit NiCo-LDH electrode materials and provide theory and methodological guidance for deriving complex structures from MOF derivatives.

Improved Catalytic Activity and Stability of Co9S8 by Se Incorporation for Efficient Oxygen Evolution Reaction

Combining an excellent electrocatalytic activity with the good structural stability of Co₉S₈ remains challenging for the oxygen evolution reaction (OER). In this study, density functional theory was used to demonstrate the importance of moderate adsorption strength with *O and *OOH intermediate species on Co₉S₈ for achieving excellent electrocatalytic performances. A novel strategy was proposed to effectively optimize the *O oxidation to *OOH by introducing Se heteroatoms to adjust adsorption of the two intermediates. This process also allowed prediction of the simultaneous enhancement of the structural stability of Co₉S₈ due to the weak electronegativity of a Se dopant. The experimental results demonstrated that Se doping can regulate the charge density of Co²⁺ and Co³⁺ in Co₉S_8-xSe_x, leading to a substantially improved OER performance of Co₉S_8-xSe_x. As a result, our Co₉S_6.91Se_1.09 electrode exhibited an overpotential of 271 mV at 10 mA cm^-2 in a 1.0 M KOH solution. In particular, it also demonstrated an excellent stability (∼120 h) under a current density of 10 mA cm^-2, indicating the potential for practical applications. Overall, the proposed strategy looks promising for regulating the electronic structures and improving the electrochemical performances of sulfide materials.

Polypyrrole-coated Boron-doped Nickel-Cobalt sulfide on electrospinning carbon nanofibers for high performance asymmetric supercapacitors

Although nickel-cobalt bimetallic sulfides have been widely studied for supercapacitor electrodes, how to obtain high specific capacity and cycle stability is still an important challenge. Here, an efficient chemical redox method is used to adjust the crystal and electronic structure of cobalt-nickel sulfide (NCS) via B doping, combined with electrospinning technology and conductive polymer polypyrrole (PPy) coating to facilitate faraday redox reactions and obtain high energy density electrode materials. The resulting composite with boron-doped nickel-cobalt sulfide on electrospinned carbon nanofibers with polypyrrole-coating (PPy@B-NCS/CNF) has a high specific capacity (751.61C/g at 1 A/g) and good cycle stability (82.49 % retention after 4000 cycles at 5 A/g). With PPy@B-NCS/CNF as the positive electrode and activated carbon as the negative electrode, an asymmetric supercapacitor (ASC) is prepared. It has excellent electrochemical properties with a power density of 65.58 Wh kg^-1 and an energy density of 819.72 W kg^-1. The low-temperature performance test shows high reversibility, which provides the possibility for the development of low-temperature electrolytes. Finally, density functional theory (DFT) explains that B-doped NCS has better electrochemical properties from the energy band and state density.

Facile synthesis of mesoporous NixCo9−xS8 hollow spheres for high-performance supercapacitors and aqueous Ni/Co–Zn batteries

Porous micro/nanostructure electrode materials have always contributed to outstanding electrochemical energy storage performances. Co₉S₈ is an ideal model electrode material with high theoretical specific capacity due to its intrinsic two crystallographic sites of cobalt ions. In order to improve the conductivity and specific capacitance of Co₉S₈, nickel ions were introduced to tune the electronic structure of Co₉S₈. The morphology design of the mesoporous hollow sphere structure guarantees cycle stability and ion diffusion. In this work, Ni_xCo_9−xS₈ mesoporous hollow spheres were synthesized via a facile partial ion-exchange of Co₉S₈ mesoporous hollow spheres without using a template, boosting the capacitance to 1300 F g⁻¹ at the current density of 1 A g⁻¹. Compared with the pure Co₉S₈ and Ni-Co₉S₈-30%, Ni-Co₉S₈-60% exhibited the best supercapacitor performance, which was ascribed to the maximum Ni ion doping with morphology and structure retention, enhanced conductivity and stabilization of Co³⁺ in the structure. Therefore, Ni/Co–Zn batteries were fabricated by using a Zn plate as the anode and Ni-Co₉S₈-60% as the cathode, which deliver a high energy density of 256.5 W h kg⁻¹ at the power density of 1.69 kW kg⁻¹. Furthermore, the Ni/Co–Zn batteries exhibit a stable cycling after 3000 repeated cycles with capacitance retention of 69% at 4 A g⁻¹. This encouranging result might provide a new perspective to optimize Co₉S₈-based electrodes with superior supercapacitor and Ni/Co–Zn battery performances.

Mesoporous NiCo₉S₈-0.6 hollow spheres as a high-performance supercapacitor and aqueous Ni/Co–Zn battery.