Cobalt Sulfide/Graphene Composite Hydrogel as Electrode for High-Performance Pseudocapacitors

Recent progress in graphene and its derived hybrid materials for high-performance supercapacitor electrode applications

Graphene, the most fascinating 2D form of carbon with closely packed carbon atoms arranged in a layer, needs more attention in various fields. For its unique electrical, mechanical, and chemical properties with a large surface area, graphene has been in the limelight since its first report. Graphene has extraordinary properties, making it the most promising electrode component for applications in supercapacitors. However, the persistent re-stacking of carbon layers in graphene, caused by firm interlayer van der Waals attractions, significantly impairs the performance of supercapacitors. As a result, many strategies have been used to get around the aforementioned problems. The utilization of graphene-based nanomaterials has been implemented to surmount the aforementioned constraints and considerably enhance the performance of supercapacitors. This review highlights recent progress in graphene-based nanomaterials with metal oxide, sulfides, phosphides, nitrides, carbides, and conducting polymers, focusing on their synthetic approach, configurations, and electrochemical properties for supercapacitors. It discusses new possibilities that could increase the performance of next-generation supercapacitors.

Eco-Friendly Cerium-Cobalt Counter-Doped Bi2Se3 Nanoparticulate Semiconductor: Synergistic Doping Effect for Enhanced Thermoelectric Generation

Metal chalcogenides are primarily used for thermoelectric applications due to their enormous potential to convert waste heat into valuable energy. Several studies focused on single or dual aliovalent doping techniques to enhance thermoelectric properties in semiconductor materials; however, these dopants enhance one property while deteriorating others due to the interdependency of these properties or may render the host material toxic. Therefore, a strategic doping approach is vital to harness the full potential of doping to improve the efficiency of thermoelectric generation while restoring the base material eco-friendly. Here, we report a well-designed counter-doped eco-friendly nanomaterial system (~70 nm) using both isovalent (cerium) and aliovalent (cobalt) in a Bi2Se3 system for enhancing energy conversion efficiency. Substituting cerium for bismuth simultaneously enhances the Seebeck coefficient and electrical conductivity via ionized impurity minimization. The boost in the average electronegativity offered by the self-doped transitional metal cobalt leads to an improvement in the degree of delocalization of the valence electrons. Hence, the new energy state around the Fermi energy serving as electron feed to the conduction band coherently improves the density of the state of conducting electrons. The resulting high power factor and low thermal conductivity contributed to the remarkable improvement in the figure of merit (zT = 0.55) at 473 K for an optimized doping concentration of 0.01 at. %. sample, and a significant nanoparticle size reduction from 400 nm to ~70 nm, making the highly performing materials in this study (Bi2-xCexCo2x3Se3) an excellent thermoelectric generator. The results presented here are higher than several Bi2Se3-based materials already reported.

Facile synthesis of MnCo2S4 nanosheets as a binder-free electrode material for high performance supercapacitor applications

This paper reported an easy synthesis of MnCo2S4 (MCS) nanosheets by a one-pot solvothermal method for high performance supercapacitor electrode material applications. The obtained MCS nanosheets with an ultrathin thickness...

Dual-electrocatalysis behavior of star-like zinc-cobalt-sulfide decorated with cobalt-molybdenum-phosphide in hydrogen and oxygen evolution reactions

Although important advances have been acquired in the field of electrocatalysis, the design and fabrication of highly efficient and stable non-noble earth-abundant metal catalysts for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) remain a significant challenge. In this study, we have designed a superior bifunctional catalyst for OER and HER in alkaline media based on the Co-Mo-P/Zn-Co-S multicomponent heterostructure. The as-prepared multicomponent heterostructure was successfully obtained via a simple three-step hydrothermal-sulfidation-electrodeposition process consisting of star-like Co-Zn-S covered with Co-Mo-P. The structure and morphology evaluation of the prepared catalysts were performed via Fourier transform infrared spectroscopy, X-ray diffraction spectroscopy, field emission scanning electron microscopy, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, and elemental mapping techniques. The electrochemical tests show that Co-Mo-P/Co-Zn-S exhibits outstanding activity toward both OER and HER with OER overpotentials of 273 mV and 312 mV to drive the benchmark current densities of 10 and 50 mA cm^-2, respectively, with a Tafel slope of 41 mV dec^-1. In addition, the HER overpotentials of 120 mV and 165 mV were required to reach the benchmark current densities of 10 and 50 mA cm^-2, respectively, with a Tafel slope of 61.7 mV dec^-1 that outperforms most other state-of-the-art catalysts. In the case of HER, the prepared catalyst required an overpotential of 202 mV to reach the current density of 200 mA cm^-2 that was much lower than the overpotential of Pt/C (286 mV) to achieve the same current density. Co-Mo-P/Co-Zn-S also exhibits a suitable stability length of 10 h for OER and HER during the chronoamperometric tests. The superior performance of the Co-Mo-P/Co-Zn-S multicomponent heterostructure toward OER and HER may be related to the large specific surface area, accelerated mass and electron transport, and synergistic effect of multiple hybrid materials. These merits suggest that Co-Mo-P/Co-Zn-S can be considered as a promising catalyst for bi-functional OER and HER, and can be offered a great promise for future applications.

A thin and multifunctional CoS@g-C3N4/Ketjen black interlayer deposited on polypropylene separator for boosting the performance of lithium-sulfur batteries

The sluggish redox kinetic and shuttle effect of polysulfides still obstruct the commercial application of lithium-sulfur (Li-S) batteries. Herein, a nanocomposite consisting of well-dispersed and lamellar-like shape CoS anchored on g-C₃N₄ nanosheets (CoS@g-C₃N₄) is prepared firstly, and then it is integrated on a polypropylene membrane combined with little conductive Ketjen black (KB) to fabricate a multifunctional and quite thin interlayer, with a thickness of only ∼ 2.1 um and areal mass loading of ∼ 0.07 mg·cm^-2. The as-prepared interlayer firstly can capture polysulfides by Li-N bond as well as Lewis acid-base interaction between CoS and polysulfide anions (S_n^2-), and more importantly, it also displays a positive effect on catalyzing the redox conversion of intermediate polysulfides. As expected, a Li-S cell assembled with this modified separator and high sulfur content cathode displays an excellent electrochemical performance, with specific capacity of ∼ 1290 mAh g^-1 at 0.2C and a low fading rate of 0.03% per cycle after 500 cycles at 1.0C. Furthermore, a high sulfur mass loading of ∼ 4.0 mg·cm^-2 electrode paired with this multifunctional separator exhibits a stable specific capacity of ∼ 600 mAh g^-1 after 250 cycles under 0.1C. This work can give some guides to rational design a quite thin and light interlayer for improving the utilization of sulfur species, with little damage to the energy density and Li ion transportation in Li-S batteries.

Mesoporous Nanostructured Composite Derived from Thermal Treatment CoFe Prussian Blue Analogue Cages and Electrodeposited NiCo-S as an Efficient Electrocatalyst for an Oxygen Evolution Reaction

Developing effective and priceless electrocatalysts is an indispensable requirement for advancing the efficiency of water splitting to get clean and sustainable fuels. Herein, we reported a feasible strategy for preparing a trimetallic (NiCoFe) superior electrocatalyst with a novel open-cage/3D frame-like structure for an oxygen evolution reaction (OER). It is prepared by consequent thermal treatments of a CoFe Prussian blue analogue frame/cage-like structure under an argon (CoFeA-TT) atmosphere and then electrochemical deposition of nickel-cobalt sulfide nanosheets as a shell layer on it. The electrochemical measurements demonstrated that the deposition of NiCo-S on CoFeA-TT (NiCo-S@CoFeA-TT) has the best catalytic performance and can drive the benchmark current density of 10 mA cm^-2 at a low overpotential of 268 mV with a Tafel slope of 62 mV dec^-1 and an excellent long-term catalytic stability in an alkaline medium. Its outstanding electrocatalytic performances are endowed from frame/cage-like structures, highly exposed active sites, accelerated mass and electron transport, and the synergistic effect of multiple hybrid components. The NiCo-S@CoFeA-TT showed a better performance than most advanced nonprecious catalysts and the noble commercial RuO₂ catalyst. This study exhibited an effective and efficient procedure to design 3D porous architecture catalysts for the energy-relevant electrocatalysis reaction.

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.

Investigation of the Nanocrystal CoS2 Embedded in 3D Honeycomb-like Graphitic Carbon with a Synergistic Effect for High-Performance Lithium Sulfur Batteries

Lithium sulfur (Li-S) batteries can offer great opportunities for the next-generation energy storage systems with tremendous energy density. However, challenges still exist in practical Li-S batteries including low sulfur utilization, and poor cycling stability and rate capability. Herein, we propose a novel hybrid catalyst structure by in situ implanting nanocrystal CoS₂ in three-dimensional honeycomb-like hierarchical porous graphitic carbon (HPGC) for high-performance Li-S batteries. A unique synergistic absorption-catalysis-functional effect is demonstrated by comprehensive experimental and theoretical analysis: strong physical and chemical co-absorption effects are originated from the large quantity of microporous HPGC and the polar surface of metallic CoS₂; the introduced nanocrystal CoS₂ with a large specific area can impose an exceptional catalytic effect on the liquid-liquid, solid-liquid, and solid-solid phase redox reactions in Li-S batteries; the reaction dynamics are further guaranteed by the multifunctional properties of the HPGC backbone, including the capabilities in polysulfide sustention, reaction product transportation, electrolyte compensation, and efficiency in assisting diverse electrochemical reaction dynamics. In this way, our results not only develop a novel CoS₂@HPGC structure, but also provide fundamental understanding on the catalytic dynamics during each reaction process. Moreover, we further propose the necessity and philosophy of the rational design of catalysts' special structure, which can fulfill the functional dynamics requirements of Li-S batteries, and can be promoted to other Li-S-related cathode design and composite catalytic structure design.

TiO2 Nanomembranes Fabricated by Atomic Layer Deposition for Supercapacitor Electrode with Enhanced Capacitance

TiO₂ is a promising environment friendly, low cost, and high electrochemical performance material. However, impediments like high internal ion resistance and low electrical conductivity restrict its applications as electrode for supercapacitor. In the present work, atomic layer deposition was used to fabricate TiO₂ nanomembranes (NMs) with accurately controlled thicknesses. The TiO₂ NMs were then used as electrodes for high-performance pseudocapacitors. Experimental results demonstrated that the TiO₂ NM with 100 ALD cycles had the highest capacitance of 2332 F/g at 1 A/g with energy density of 81 Wh/kg. The enhanced performance was ascribed to the large surface area and the interconnectivity in the case of ultra-thin and flexible NMs. Increased ALD cycles led to stiffer NMs and decreased capacitance. Moreover, one series of two supercapacitors can light up one light-emitting diode with a working voltage of ~ 1.5 V, sufficiently describing its application values.

High Mass Loading MnO2 with Hierarchical Nanostructures for Supercapacitors

Metal oxides have attracted renewed interest as promising electrode materials for high energy density supercapacitors. However, the electrochemical performance of metal oxide materials deteriorates significantly with the increase of mass loading due to their moderate electronic and ionic conductivities. This limits their practical energy. Herein, we perform a morphology and phase-controlled electrodeposition of MnO₂ with ultrahigh mass loading of 10 mg cm^-2 on a carbon cloth substrate to achieve high overall capacitance without sacrificing the electrochemical performance. Under optimum conditions, a hierarchical nanostructured architecture was constructed by interconnection of primary two-dimensional ε-MnO₂ nanosheets and secondary one-dimensional α-MnO₂ nanorod arrays. The specific hetero-nanostructures ensure facile ionic and electric transport in the entire electrode and maintain the structure stability during cycling. The hierarchically structured MnO₂ electrode with high mass loading yields an outstanding areal capacitance of 3.04 F cm^-2 (or a specific capacitance of 304 F g^-1) at 3 mA cm^-2 and an excellent rate capability comparable to those of low mass loading MnO₂ electrodes. Finally, the aqueous and all-solid asymmetric supercapacitors (ASCs) assembled with our MnO₂ cathode exhibit extremely high volumetric energy densities (8.3 mWh cm^-3 at the power density of 0.28 W cm^-3 for aqueous ASC and 8.0 mWh cm^-3 at 0.65 W cm^-3 for all-solid ASC), superior to most state-of-the-art supercapacitors.

Sacrificial-template-free synthesis of core-shell C@Bi2S3 heterostructures for efficient supercapacitor and H2 production applications

Core-shell heterostructures have attracted considerable attention owing to their unique properties and broad range of applications in lithium ion batteries, supercapacitors, and catalysis. Conversely, the effective synthesis of Bi₂S₃ nanorod core@ amorphous carbon shell heterostructure remains an important challenge. In this study, C@Bi₂S₃ core-shell heterostructures with enhanced supercapacitor performance were synthesized via sacrificial- template-free one-pot-synthesis method. The highest specific capacities of the C@Bi₂S₃ core shell was 333.43 F g^-1 at a current density of 1 A g^-1. Core-shell-structured C@Bi₂S₃ exhibits 1.86 times higher photocatalytic H₂ production than the pristine Bi₂S₃ under simulated solar light irradiation. This core-shell feature of C@Bi₂S₃ provides efficient charge separation and transfer owing to the formed heterojunction and a short radial transfer path, thus efficiently diminishing the charge recombination; it also facilitates plenty of active sites for the hydrogen evolution reaction owing to its mesoporous nature. These outcomes will open opportunities for developing low-cost and noble-metal-free efficient electrode materials for water splitting and supercapacitor applications.

Synergistically Active NiCo2 S4 Nanoparticles Coupled with Holey Defect Graphene Hydrogel for High-Performance Solid-State Supercapacitors

Nickel cobalt sulfide nanoparticles embedded in holey defect graphene hydrogel (HGH) that exhibit highly porous structures and uniform nickel cobalt sulfide nanoparticle sizes are successfully prepared by a facile solvothermal-hydrothermal method. As an electrode material for supercapacitors, the as-prepared NiCo₂ S₄ @HGH shows ultra-high specific capacitances of 1000 F g^-1 and 800 F g^-1 at 0.5 and 6 A g^-1 , respectively, owing to the outstanding electrical conductivity of HGH and high specific capacitance of NiCo₂ S₄ . After 2100 charge/discharge cycles at a current density of 6 A g^-1 , 96.6 % of the specific capacitance was retained, signifying the superb durability of NiCo₂ S₄ @HGH. Moreover, remarkable specific capacitance (312.6 F g^-1 ) and capacity retention (87 % after 5000 cycles) at 6 A g^-1 were displayed by the symmetric solid-state supercapacitor fabricated by using NiCo₂ S₄ @HGH electrodes. These auspicious supercapacitor performances demonstrate that the as-developed solvothermal-hydrothermal approach can be widely used to prepare graphene-coupled binary metal sulfides for high-performance supercapacitor applications.

Pseudocapacitive-battery-like behavior of cobalt manganese nickel sulfide (CoMnNiS) nanosheets grown on Ni-foam by electrodeposition for realizing high capacity

Hierarchical interconnected nanosheets (HIN) of cobalt manganese nickel sulfide (CoMnNiS) were synthesized on Ni foam by a simple and economical electrodeposition technique for energy storage application. Sulfonated thin nanosheets of Co, Mn and Ni provide stability of chemical activity, surface functionalization and surface reactivity to the electrode. The fabricated electrode shows a specific capacity of 257.4 mA h g⁻¹ (at 2.5 A g⁻¹), measured by galvanostatic charging–discharging (GCD). Both diffusion and capacitive mechanisms in the sulfide layer contribute to the high electrical conductivity. Asymmetric devices CoMnNiS/NiCuO and CoMnNiS/CNT (CNT = carbon nanotubes) were fabricated, providing a maximum operating voltage of 1.7 V and 1 V, specific capacity of 20.8 and 50.8 mA h g⁻¹, and energy density of 8.4 and 6.3 W h kg⁻¹ corresponding to a power density of 985 and 211 W kg⁻¹, respectively, at a current density of 0.5 and 0.63 A g⁻¹. These results demonstrate a novel material for application in energy storage devices as an electrode.

Hierarchical interconnected nanosheets (HIN) of cobalt manganese nickel sulfide (CoMnNiS) were synthesized on Ni foam by a simple and economical electrodeposition technique for energy storage application.

Charge-Transfer Analysis of 2p3d Resonant Inelastic X-ray Scattering of Cobalt Sulfide and Halides

We show that with 2p3d resonant inelastic X-ray scattering (RIXS) we can accurately determine the charge-transfer parameters of CoF₂, CoCl₂, CoBr₂, and CoS. The 160 meV resolution RIXS results are compared with charge-transfer multiplet calculations. The improved resolution and the direct observation of the crystal field and charge-transfer excitations allow the determination of more accurate parameters than could be derived from X-ray absorption and X-ray photoemission, both limited in resolution by their lifetime broadening. We derive the crystal field and charge-transfer parameters of the Co²⁺ ions, which provides the nature of the ground state of the Co²⁺ ions with respect to symmetry and hybridization. In addition, the increased spectral resolution allows the more accurate determination of the atomic Slater integrals. The results show that the crystal field energy decreases with increasing ligand covalency. The L₂ edge RIXS spectra show that the intensity of the (Coster-Kronig induced) nonresonant X-ray emission is a measure of ligand covalency.

In situ grown Ni9S8 nanorod/O-MoS2 nanosheet nanocomposite on carbon cloth as a free binder supercapacitor electrode and hydrogen evolution catalyst

Transition metal sulfide nanostructure composites have received significant attention as energy conversion and storage devices. In this work, we report a three-dimension (3D) nanostructure with the Ni₉S₈ nanorods embedded in oxygen-incorporated MoS₂ (O-MoS₂) nanosheets for supercapacitors and hydrogen evolution catalysts. The in situ grown Ni₉S₈/O-MoS₂ nanocomposite on carbon cloth can be used as a free binder supercapacitor electrode and hydrogen evolution catalyst. The Ni₉S₈/O-MoS₂ nanocomposite exhibits electrochemical behaviors with a specific capacitance of 907 F g^-1 (at 2 A g^-1) and good cycle stability after 1200 cycles due to its unique mutual embedding 3D nanostructure. Furthermore, the Ni₉S₈/O-MoS₂ nanocomposite also shows highly electrocatalytic features for hydrogen production with an onset overpotential of ∼150 mV and a low Tafel slope of ∼81 mV dec^-1. The oxygen incorporation of MoS₂ provides more active sites to participate in the catalytic process for the hydrogen evolution reaction.

Enhanced Structural Stability of Nickel-Cobalt Hydroxide via Intrinsic Pillar Effect of Metaborate for High-Power and Long-Life Supercapacitor Electrodes

Layered α-Ni(OH)₂ and its derivative bimetallic hydroxides (e.g., α-(Ni/Co)(OH)₂) have attracted much attention due to their high specific capacitance, although their insufficient cycling stability has blocked their wide application in various technologies. In this work, we demonstrate that the cycling performance of α-(Ni/Co)(OH)₂ can be obviously enhanced via the intrinsic pillar effect of metaborate. Combining the high porosity feature of the metaborate stabilized α-(Ni/Co)(OH)₂ and the improved electronic conductivity offered by graphene substrate, the average capacitance fading rate of the metaborate stabilized α-(Ni/Co)(OH)₂ is only ∼0.0017% per cycle within 10 000 cycles at the current density of 5 A g^-1. The rate performance is excellent over a wide temperature range from -20 to 40 °C. We believe that the enhancements should mainly be ascribed to the excellent structural stability offered by the metaborate pillars, and the detailed mechanism is discussed.

All-solid-state asymmetric supercapacitors based on cobalt hexacyanoferrate-derived CoS and activated carbon

All-solid-state flexible asymmetric supercapacitors fabricated using CoS and AC showed a high cell voltage, high specific capacitance, and high energy density of 5.3 W h kg⁻¹ with excellent electrochemical stability.

Simple synthesis of cobalt sulfide nanorods for efficient electrocatalytic oxidation of vanillin in food samples

Well-defined CoS nanorods (NR) were synthesized using a simple hydrothermal method, and were tested as an electrode material for electro-oxidation of vanillin. The NR material was characterized with regard to morphology, crystallinity, and electro-activity by use of appropriate analytical techniques. The resulting CoS NR@Nafion modified glassy carbon electrode (GCE) exhibited efficient electro-oxidation of vanillin with a considerable linear range of current-vs-concentration (0.5-56μM vanillin) and a detection limit of 0.07μM. Also, food samples containing vanillin were studied to test suitability for commercial applications.

High Volumetric Energy Density Asymmetric Supercapacitors Based on Well-Balanced Graphene and Graphene-MnO2 Electrodes with Densely Stacked Architectures

The well-matched electrochemical parameters of positive and negative electrodes, such as specific capacitance, rate performance, and cycling stability, are important for obtaining high-performance asymmetric supercapacitors. Herein, a facile and cost-effective strategy is demonstrated for the fabrication of 3D densely stacked graphene (DSG) and graphene-MnO₂ (G-MnO₂ ) architectures as the electrode materials for asymmetric supercapacitors (ASCs) by using MnO₂ -intercalated graphite oxide (GO-MnO₂ ) as the precursor. DSG has a stacked graphene structure with continuous ion transport network in-between the sheets, resulting in a high volumetric capacitance of 366 F cm^-3 , almost 2.5 times than that of reduced graphene oxide, as well as long cycle life (93% capacitance retention after 10 000 cycles). More importantly, almost similar electrochemical properties, such as specific capacitance, rate performance, and cycling stability, are obtained for DSG as the negative electrode and G-MnO₂ as the positive electrode. As a result, the assembled ASC delivers both ultrahigh gravimetric and volumetric energy densities of 62.4 Wh kg^-1 and 54.4 Wh L^-1 (based on total volume of two electrodes) in 1 m Na₂ SO₄ aqueous electrolyte, respectively, much higher than most of previously reported ASCs in aqueous electrolytes.

PdCo/Pd-Hexacyanocobaltate Hybrid Nanoflowers: Cyanogel-Bridged One-Pot Synthesis and Their Enhanced Catalytic Performance

Elaborate architectural manipulation of nanohybrids with multi-components into controllable 3D hierarchical structures is of great significance for both fundamental scientific interest and realization of various functionalities, yet remains a great challenge because different materials with distinct physical/chemical properties could hardly be incorporated simultaneously into the synthesis process. Here, we develop a novel one-pot cyanogel-bridged synthetic approach for the generation of 3D flower-like metal/Prussian blue analogue nanohybrids, namely PdCo/Pd-hexacyanocobaltate for the first time. The judicious introduction of polyethylene glycol (PEG) and the formation of cyanogel are prerequisite for the successful fabrication of such fascinating hierarchical nanostructures. Due to the unique 3D hierarchical structure and the synergistic effect between hybrid components, the as-prepared hybrid nanoflowers exhibit a remarkable catalytic activity and durability toward the reduction of Rhodamine B (RhB) by NaBH₄. We expect that the obtained hybrid nanoflowers may hold great promises in water remediation field and beyond. Furthermore, the facile synthetic strategy presented here for synthesizing functional hybrid materials can be extendable for the synthesis of various functional hybrid nanomaterials owing to its versatility and feasibility.

2D materials for renewable energy storage devices: Outlook and challenges

We review cost-effective, clean and durable alternative energy devices based on 2D materials.