NiCo2S4 Nanotrees Directly Grown on the Nickel NP-Doped Reduced Graphene Oxides for Efficient Supercapacitors

In this work, we report a feasible fabrication of NiCo2S4 nanotree-like structures grown from the Ni nanoparticle (NP)-doped reduced graphene oxides (Ni-rGO) by a simple hydrothermal method. It is found that the presence of Ni NPs on the surface of the rGOs initiates growth of the NiCo2S4 nanotree flocks with enhanced interfacial compatibility, providing excellent cyclic stability and rate performance. The resulting NiCo2S4/Ni-rGO nanocomposites exhibit a superior rate performance, demonstrating 91.6% capacity retention even after 10,000 cycles of charge/discharge tests

Effect of Potassium Ions on the Formation of Mixed-Valence Manganese Oxide/Graphene Nanocomposites

One-pot synthesis of mixed-valence manganese oxide (MnOx)/potassium ion-doped reduced graphene oxide (rGO) composites for efficient electrochemical supercapacitors is introduced. Using manganese nitrate and potassium permanganate as co-precursors for the MnOx and by directly annealing the rGO without tedious purification steps, as described herein, MnOx/rGO composites with a high specific capacitance of 1955.6 F g−1 at a current density of 1 A g−1 are achieved. It is found that the presence of potassium ions helps in the development of mixed-valence MnOx on the surface of the rGO

Electrical bridging effects of dual-carbon microsphere frameworks in Si-based composite anodes for high-performance Li-ion batteries

Herein, the successful fabrication of a high-performance Si-based composite Li-ion battery (LIB) anode, comprising a dual-carbon framework of reduced graphene oxide (r-GO) and oxidized single-walled carbon nanohorns (o-NHs), was demonstrated using a simple and scalable spray-drying process followed by heat treatment (h-s-GO/Si/NH). The r-GO nanosheets in the h-s-GO/Si/NH anode acted as a robust spherical framework that facilitated the mechanical and electrical connection between the carbon-coated Si (c-Si) nanoparticles, homogeneous dispersion of c-Si and o-NH nanoparticles, and suppression of the volume expansion and pulverization that occur during lithiation/delithiation. Additionally, the o-NH nanoparticles incorporated in the h-s-GO/Si/NH composite served as electrical bridges between the r-GO nanosheets, resulting in enhanced electrical conductivity and effortless Li-ion shuttling. The h-s-GO/Si/NH composite anode exhibited high electrochemical performance with a very high initial gravimetric charge capacity (2961 mAh g−1 at 0.1 A g−1), stable initial Coulombic efficiency (80.6% at 0.2 A g−1), and high cycling stability (983 mAh g−1 at 0.2 A g−1 after 50 cycles). This study highlights the importance of the effective design of electrically conductive three-dimensional frameworks in Si-based composite anodes, which may contribute to the development of high-performance LIB anode materials. © 2023 The AuthorsTRU

Optimization of Chemi‐adsorption, EDLC, and Redox Capacitance Through Electro‐precipitation Synthesis of Fe3O4/NiO@rGO/h‐BN for the Development of Hybrid Supercapacitor

3D Fe3O4/NiO was grafted on to the 2D rGO/h‐BN by electro‐precipitation method. Nitrogen of h‐BN moiety and oxygen functional groups of rGO played the role of negative active site to trap the metallic cations. Electrochemical charge storage mechanism was optimized by controlling the stoichiometry and defect contents of Fe3O4/NiO@rGO/h‐BN. Stoichiometry of the electro‐precipitated samples was tailored in presence of negative active sites of rGO/h‐BN and applied D.C. bias of the electrochemical bath. In addition, the nucleation and growth of metal oxides were influenced by the stacking and vacancy defects of rGO/h‐BN sheets. High specific capacitance (1328 F g−1) of Fe3O4/NiO@rGO/h‐BN was attributed to the synergistic effect of electrochemical double layer capacitance of rGO, chemi‐adsorption of –OH ions on Lewis acid (boron of h‐BN moiety) and redox capacitance of Fe3O4/NiO in alkaline medium. In addition, the presence of pyrrolic defect at the rGO/h‐BN stacking region acted as the nucleation site and provided additional redox capacitance by shifting the Fermi level towards the valance band. An asymmetric supercapacitor (ASC) was constructed using Fe3O4/NiO@rGO/h‐BN and thermally reduced GO as positive and negative electrode, respectively. ASC showed high energy (82 W h Kg−1) and power density (5600 W Kg−1) along with low relaxation time constant (2.2 ms) and high stability (79%) after 10,000 charge discharge cycles

Effect of the Solvent Ratio (Ethylene Glycol/Water) on the Preparation of an Iron Sulfide Electrocatalyst and Its Activity towards Overall Water Splitting

The polyol method is an efficient procedure for metal sulfide preparation where polyol not only acts as a solvent but also as reducing and morphology‐modulating agent. Herein, iron sulfide particles were prepared via a modified polyol method by changing the ethylene glycol (EG) : water (H2O) ratio in the mixed solvent. Analytical techniques and electronic microscopy studies confirmed that the change in EG : H2O ratio modulated the crystal structure, morphology, and electronic structure of the prepared iron sulfide particles. The electrocatalytic activity of iron sulfide changed owing to these modulations. EG helped in the formation of a sheet‐like structure – a morphology that favours a higher accessibility to the catalytically active sites. As evidenced form electrochemical impedance studies, an increased electron density near the Fermi level, a faster substrate adsorption‐desorption rate at the active sites, and a faster charge transfer at the electrode‐electrolyte interface were the key factors for the amplification in catalytic activity. The prepared iron sulfide particles showed an overall water splitting efficiency that is comparable to that of the state‐of‐the‐art RuO2‐Pt/C couple in alkaline medium. This study shows the potential of the polyol method in the preparation and catalytic‐activity modulation of Fe−S‐based electrocatalysts

Effect of Ion Diffusion in Cobalt Molybdenum Bimetallic Sulfide toward Electrocatalytic Water Splitting

The electrocatalyst comprising two different metal atoms is found suitable for overall water splitting in alkaline medium. Hydrothermal synthesis is an extensively used technique for the synthesis of various metal sulfides. Time-dependent diffusion of the constituting ions during hydrothermal synthesis can affect the crystal and electronic structure of the product, which in turn would modulate its electrocatalytic activity. Herein, cobalt molybdenum bimetallic sulfide was prepared via hydrothermal method after varying the duration of reaction. The change in crystal structure, amount of Co–S–Mo moiety, and electronic structure of the synthesized materials were thoroughly investigated using different analytical techniques. These changes modulated the charge transfer at the electrode–electrolyte interface, as evidenced by electrochemical impedance spectroscopy. The Tafel plots for the prepared materials were investigated considering a less explored approach and it was found that different materials facilitated different electrocatalytic pathways. The product obtained after 12 h reaction showed superior catalytic activity in comparison to the products obtained from 4, 8, and 16 h reaction, and it surpassed the overall water splitting activity of the RuO2–Pt/C couple. This study demonstrated the ion diffusion within the bimetallic sulfide during hydrothermal synthesis and change in its electrocatalytic activity due to ion diffusion

Cobalt Sulfide/Nickel Sulfide Heterostructure Directly Grown on Nickel Foam: An Efficient and Durable Electrocatalyst for Overall Water Splitting Application

Fabrication of high-performance noble-metal-free bifunctional electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in water is a promising strategy toward future carbon-neutral economy. Herein, a one-pot hydrothermal synthesis of cobalt sulfide/nickel sulfide heterostructure supported by nickel foam (CoSx/Ni3S2@NF) was performed. The Ni foam acted as the three-dimensional conducting substrate as well as the source of nickel for Ni3S2. The formation of CoSx/Ni3S2@NF was confirmed by X-ray diffraction and X-ray photoelectron spectroscopy. The formation of CoSx/Ni3S2@NF facilitated easy charge transport and showed synergistic electrocatalytic effect toward HER, OER, and overall water splitting in alkaline medium. Remarkably, CoSx/Ni3S2@NF showed catalytic activity comparable with that of benchmarking electrocatalysts Pt/C and RuO2. For CoSx/Ni3S2@NF, overpotentials of 204 and 280 mV were required to achieve current densities of 10 and 20 mA cm–2 for HER and OER, respectively, in 1.0 M KOH solution. A two-electrode system was formulated for overall water splitting reaction, which showed current densities of 10 and 50 mA cm–2 at 1.572 and 1.684 V, respectively. The prepared catalyst exhibited excellent durability in HER and OER catalyzing conditions and also in overall water splitting operation. Therefore, CoSx/Ni3S2@NF could be a promising noble-metal-free electrocatalyst for overall water splitting application

Hierarchical Cobalt Sulfide/Molybdenum Sulfide Heterostructure as Bifunctional Electrocatalyst towards Overall Water Splitting

The development of non‐noble metal based electrocatalysts for overall water splitting is a potent strategy towards a carbon‐neutral and clean energy economy. Herein, hierarchical CoSx@MoS2 was synthesized via a one‐pot solvothermal process. Formation of the heterostructure was confirmed by electron microscopy and spectroscopic techniques. CoSx@MoS2 showed competent electrocatalytic activity towards both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline medium. Superior electrocatalytic activity was attributed to the increase in number of active sites, betterment in charge transfer and facilitation of H‐ and O‐ containing active species adsorption‐desorption at the active sites. Overall water splitting efficiency of CoSx@MoS2 was found to be superior in comparison to the state‐of‐the‐art RuO2‐Pt/C couple. Along with efficiency the heterostructure also exhibited long‐term operational durability. Thus, hierarchical CoSx@MoS2 is a potential non‐noble metal based bifunctional electrocatalyst towards overall water splitting