Preparation of Hierarchical Spinel NiCo2O4 Nanowires for High-Performance Supercapacitors

Structural modulation of tin nickelate nanostructures embedded in reduced graphene oxide for high-performance asymmetric supercapacitors

Development of a new, cost-effective, advanced energy storage material via a simple method is a great challenge among researchers. In this study, we have synthesized efficient spinel tin nickelate nano-popcorns, SnNi2O4 (SNNPs), via a solvothermal process. Furthermore, to enhance their charge transfer characteristics, SNNPs were impregnated on reduced graphene oxide (rGO) nanosheets through ultrasonication to obtain SnNi2O4@rGO (SNNPR). By varying the percentage load ratio of SNNPs and rGO, six different nanocomposites, namely, SNNPR-1, SNNPR-2, SNNPR-3, SNNPR-4, SNNPR-5 and SNNPR-6, were produced. They were thoroughly characterized using spectroscopic and microscopic techniques. The electrochemical analysis of all the SNNP-based electrode materials was performed using cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS). Among these six electrode materials, SNNPR-3 produces a maximum specific capacitance (Csp) of 225 mA h g-1 (1624 F g-1) in a three-electrode assembly at 1 A g-1 and retains a cycle stability of 94% up to 1000 cycles. Based on the superiority of SNNPR-3, an asymmetric supercapacitor (ASC) was fabricated with SNNPR-3 as the cathode and activated carbon (AC) as the anode (SNNPR-3//AC). Exhibiting a thorough electrochemical performance, the present ASC yielded a specific capacitance, Csp of 264 F g-1, high energy density of 62.3 W h kg-1 and power density of 2600 W kg-1. The device exhibited 80.02% of retention capacitance even after 5000 cycles. Also, SNNPR-3//AC was able to illuminate a green light emitting diode. Therefore, this asymmetric energy storage device has enormous potential for practical applications in the future.

Lattice strain induced d-band centre engineering enabled pseudocapacitive energy storage in 2D hypo-hyper electronic V-NiCo2O4 for asymmetric supercapacitors

Understanding the role of fundamental structural engineering of materials in unravelling the underlying rudimentary electronic structure-dependent charge storage mechanisms is crucial for developing new strategic approaches toward high-performance electrochemical energy storage devices. Here, we demonstrate the role of strain engineering by V doping-induced lattice contraction in NiCo2O4 for increasing the energy density and power density of aqueous asymmetric hybrid supercapacitors. For application in energy storage, we demonstrate the influence of electron-deficient V4+/5+ doping in electron-rich Ni2+ sites, which has been found to result in the formation of a hypo-hyper electronically coupled cation pair causing a shift in the d-band and O 2p band centres and distortion of CoO6 octahedra. Optimization of V doping to 3 mol%, achieved by a binder-free one-step hydrothermal method, has yielded a 96% increase in specific capacitance of up to 2316 F g-1 from 1193 F g-1 in pristine materials at 1 A g-1 in a three-electrode configuration with a coulombic efficiency (η%) of 94% and a 24% increase in rate capacity. A two-fold increase in specific capacitance in the pouch cell device, fabricated with a functionalized carbon nanosphere positive electrode, has been observed for the V-doped samples at 1 A g-1 with a η% of 97% and a maximum energy density of 96.3 W h g-1 and a maximum power density of 8733.6 W g-1 which are 41% and 24.3% higher than the pristine device, respectively. Excellent cycling stability of 95.4% capacitance retention has been observed after 6000 cycles. DFT calculations have been carried out to understand the previously unexplored effect of lattice strain on charge transport and quantum capacitance, and ultimately its effect on the transition state kinetics of energy storage faradaic reaction mechanisms. The aim of this work is to establish a fresh perspective on developing a deep understanding of the fundamental electronic and structural properties of materials by drawing in concepts from descriptor models in electrocatalysis to reveal the role of lattice strain and d-band centre tailoring in enabling pseudocapacitive energy storage.

Chitosan Supports Boosting NiCo2O4 for Catalyzed Urea Electrochemical Removal Application

Currently, wastewater containing high urea levels poses a significant risk to human health. Else, electrocatalytic methodologies have the potential to transform urea present in urea-rich wastewater into hydrogen, thereby contributing towards environmental conservation and facilitating the production of sustainable energy. The characterization of the NiCo2O4@chitosan catalyst was performed by various analytical techniques, including scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Furthermore, the activity of electrodes toward urea removal was investigated by several electrochemical techniques. As a function of current density, the performance of the modified NiCo2O4@chitosan surface was employed to remove urea using electrochemical oxidation. Consequently, the current density measurement was 43 mA cm-2 in a solution of 1.0 M urea and 1.0 M KOH. Different kinetic characteristics were investigated, including charge transfer coefficient (α), Tafel slope (29 mV dec-1), diffusion coefficient (1.87 × 10-5 cm2 s-1), and surface coverage 4.29 × 10-9 mol cm-2. The electrode showed high stability whereas it lost 10.4% of its initial current after 5 h of urea oxidation.

Microplotter Printing of a Miniature Flexible Supercapacitor Electrode Based on Hierarchically Organized NiCo2O4 Nanostructures

The hydrothermal synthesis of a nanosized NiCo₂O₄ oxide with several levels of hierarchical self-organization was studied. Using X-ray diffraction analysis (XRD) and Fourier-transform infrared (FTIR) spectroscopy, it was determined that under the selected synthesis conditions, a nickel-cobalt carbonate hydroxide hydrate of the composition M(CO₃)_0.5(OH)·0.11H₂O (where M-Ni²⁺ and Co²⁺) is formed as a semi-product. The conditions of semi-product transformation into the target oxide were determined by simultaneous thermal analysis. It was found by means of scanning electron microscopy (SEM) that the main powder fraction consists of hierarchically organized microspheres of 3-10 μm in diameter, and individual nanorods are observed as the second fraction of the powder. Nanorod microstructure was further studied by transmission electron microscopy (TEM). A hierarchically organized NiCo₂O₄ film was printed on the surface of a flexible carbon paper (CP) using an optimized microplotter printing technique and functional inks based on the obtained oxide powder. It was shown by XRD, TEM, and atomic force microscopy (AFM) that the crystalline structure and microstructural features of the oxide particles are preserved when deposited on the surface of the flexible substrate. It was found that the obtained electrode sample is characterized by a specific capacitance value of 420 F/g at a current density of 1 A/g, and the capacitance loss during 2000 charge-discharge cycles at 10 A/g is 10%, which indicates a high material stability. It was established that the proposed synthesis and printing technology enables the efficient automated formation of corresponding miniature electrode nanostructures as promising components for flexible planar supercapacitors.

Electrochemical Performance of Iron-Doped Cobalt Oxide Hierarchical Nanostructure

In this study, hydrothermally produced Fe-doped Co3O4 nanostructured particles are investigated as electrocatalysts for the water-splitting process and electrode materials for supercapacitor devices. The results of the experiments demonstrated that the surface area, specific capacitance, and electrochemical performance of Co3O4 are all influenced by Fe3+ content. The FexCo3-xO4 with x = 1 sample exhibits a higher BET surface (87.45 m2/g) than that of the pristine Co3O4 (59.4 m2/g). Electrochemical measurements of the electrode carried out in 3 M KOH reveal a high specific capacitance of 153 F/g at a current density of 1 A/g for x = 0.6 and 684 F/g at a 2 mV/s scan rate for x = 1.0 samples. In terms of electrocatalytic performance, the electrode (x = 1.0) displayed a low overpotential of 266 mV (at a current density of 10 mA/cm2) along with 52 mV/dec Tafel slopes in the oxygen evolution reaction. Additionally, the overpotential of 132 mV (at a current density of 10 mA/cm2) and 109 mV with 52 mV/dec Tafel slope were obtained for x = 0.6 sample towards hydrogen evolution reaction (HER). According to electrochemical impedance spectroscopy (EIS) measurements and the density functional theory (DFT) study, the addition of Fe3+ increased the conductivity at the electrode–electrolyte interface, which substantially impacted the high activity of the iron-doped cobalt oxide. The electrochemical results revealed that the mesoporous Fe-doped Co3O4 nanostructure could be used as potential electrode material in the high-performance electrochemical capacitor and water-splitting catalysts.

Electrochemical Performance of Aluminum Doped Ni1−xAlxCo2O4 Hierarchical Nanostructure: Experimental and Theoretical Study

For electrochemical supercapacitors, nickel cobaltite (NiCo2O4) has emerged as a new energy storage material. The electrocapacitive performance of metal oxides is significantly influenced by their morphology and electrical characteristics. The synthesis route can modulate the morphological structure, while their energy band gaps and defects can vary the electrical properties. In addition to modifying the energy band gap, doping can improve crystal stability and refine grain size, providing much-needed surface area for high specific capacitance. This study evaluates the electrochemical performance of aluminum-doped Ni1−xAlxCo2O4 (0 ≤ x ≤ 0.8) compounds. The Ni1−xAlxCo2O4 samples were synthesized through a hydrothermal method by varying the Al to Ni molar ratio. The physical, morphological, and electrochemical properties of Ni1−xAlxCo2O4 are observed to vary with Al3+ content. A morphological change from urchin-like spheres to nanoplate-like structures with a concomitant increase in the surface area, reaching up to 189 m2/g for x = 0.8, was observed with increasing Al3+ content in Ni1−xAlxCo2O4. The electrochemical performance of Ni1−xAlxCo2O4 as an electrode was assessed in a 3M KOH solution. The high specific capacitance of 512 F/g at a 2 mV/s scan rate, 268 F/g at a current density of 0.5 A/g, and energy density of 12.4 Wh/kg was observed for the x = 0.0 sample, which was reduced upon further Al3+ substitution. The as-synthesized Ni1−xAlxCo2O4 electrode exhibited a maximum energy density of 12.4 W h kg−1 with an outstanding high-power density of approximately 6316.6 W h kg−1 for x = 0.0 and an energy density of 8.7 W h kg−1 with an outstanding high-power density of approximately 6670.9 W h kg−1 for x = 0.6. The capacitance retention of 97% and 108.52% and the Coulombic efficiency of 100% and 99.24% were observed for x = 0.0 and x = 0.8, respectively. First-principles density functional theory (DFT) calculations show that the band-gap energy of Ni1−xAlxCo2O4 remained largely invariant with the Al3+ substitution for low Al3+ content. Although the capacitance performance is reduced upon Al3+ doping, overall, the Al3+ doped Ni1−xAlxCo2O4 displayed good energy, powder density, and retention performance. Thus, Al3+ could be a cost-effective alternative in replacing Ni with the performance trade off.

Design and Synthesis of Novel 2D Porous Zinc Oxide-Nickel Oxide Composite Nanosheets for Detecting Ethanol Vapor

The porous zinc oxide-nickel oxide (ZnO-NiO) composite nanosheets were synthesized via sputtering deposition of NiO thin film on the porous ZnO nanosheet templates. Various NiO film coverage sizes on porous ZnO nanosheet templates were achieved by changing NiO sputtering duration in this study. The microstructures of the porous ZnO-NiO composite nanosheets were investigated herein. The rugged surface feature of the porous ZnO-NiO composite nanosheets were formed and thicker NiO coverage layer narrowed the pore size on the ZnO nanosheet template. The gas sensors based on the porous ZnO-NiO composite nanosheets displayed higher sensing responses to ethanol vapor in comparison with the pristine ZnO template at the given target gas concentrations. Furthermore, the porous ZnO-NiO composite nanosheets with the suitable NiO coverage content demonstrated superior gas-sensing performance towards 50-750 ppm ethanol vapor. The observed ethanol vapor-sensing performance might be attributed to suitable ZnO/NiO heterojunction numbers and unique porous nanosheet structure with a high specific surface area, providing abundant active sites on the surface and numerous gas diffusion channels for the ethanol vapor molecules. This study demonstrated that coating of NiO on the porous ZnO nanosheet template with a suitable coverage size via sputtering deposition is a promising route to fabricate porous ZnO-NiO composite nanosheets with a high ethanol vapor sensing ability.

One-dimensional hierarchical nanostructures of NiCo2O4, NiCo2S4 and NiCo2Se4 with superior electrocatalytic activities toward efficient oxygen evolution reaction

Oxygen evolution reaction (OER), a sluggish multistep process in electrochemical water splitting, is still a challenging issue to achieve with cheap, earth-abundant non-precious and non-polluting materials. In this work, three different electrocatalysts, specifically NiCo₂O₄, NiCo₂ S₄, and NiCo₂ Se_4, synthesized by simple hydrothermal process, show excellent OER activity. This report not only projects OER performances but also demonstrates a modified method for the transformation of NiCo₂O₄ to NiCo₂ S₄ and NiCo₂ Se₄ via sulfidation and selenization reactions. The well crystalline, porous nature of NiCo₂O₄, NiCo₂ S₄, and NiCo₂Se₄ electrocatalysts with one dimensional (1D) structural morphology affords overpotentials of 346 mV, 309 mV and 270 mV at current density of 10 mA cm^-2 in 1 M KOH. In particular, NiCo₂Se₄ exhibits a low overpotential as well as a smaller Tafel slope of 63 mV dec^-1, leading to robust stability in alkaline conditions. The abundant active sites, large mass and size of NiCo₂Se₄ enhances the performance of the OER. This type of selenide-based material with low toxicity is also an advantage for eco-friendly applications.

High performance supercapacitor electrodes based on spinel NiCo2O4@MWCNT composite with insights from density functional theory simulations

In this work, we demonstrated the supercapacitor performance of pristine and composites of spinel NiCo₂O₄ with a multi-walled carbon nanotube (MWCNT) assembled in a two-electrode cell configuration. Spinel NiCo₂O₄ and NiCo₂O₄@MWCNT composites were synthesized via a facile hydrothermal method. The supercapacitive performance of as-synthesized NiCo₂O₄ and NiCo₂O₄@MWCNT fabricated on Ni-foam was studied in a 0.5M K₂SO₄ electrolyte using electrochemical measurement techniques. The symmetric cell configuration of NiCo₂O₄@MWCNT delivers high specific capacitance (374 F/g at 2 A/g) with high energy density and power density (95 Wh/kg and 3 964 W/kg, respectively) compared to that of pristine NiCo₂O₄ electrodes (137 F/g at 0.6 A/g). Furthermore, the energy storage performance of the asymmetric cells of NiCo₂O₄//MWCNT and NiCo₂O₄@MWCNT//MWCNT was studied to enhance cycling stability (retention of 74.85% over 3000 cycles). We have also theoretically studied the supercapacitance performance of pristine NiCo₂O₄ and NiCo₂O₄@SWCNT hybrid structures through its structural and electronic properties using density functional theory predictions. The higher specific capacitance of the NiCo₂O₄@SWCNT hybrid system with high power density and energy density is supported by the enhanced density of states near the Fermi level and increased quantum capacitance of the hybrid structure. We have theoretically computed the diffusion energy barrier of K⁺ ions of the K₂SO₄ electrolyte in the NiCo₂O₄ layer and compared it with the diffusion barrier for Na⁺ ions. The lesser diffusion energy barrier for K⁺ ions in the NiCo₂O₄ layer contributes toward higher energy storage capacity. Thus, owing to superior electrochemical performance of NiCo₂O₄ composites with MWCNTs, it can serve as a high-performance electrode material for supercapacitor applications.

A MoNiO4 flower-like electrode material for enhanced electrochemical properties via a facile chemical bath deposition method for supercapacitor applications

Schematic representation of the synthesis of MoNiO₄ flower-like nanostructures.

Application of micro-impinging stream reactors in the preparation of Co and Al co-doped Ni(OH)2 nanocomposites for supercapacitors and their modification with reduced graphene oxide

A micro-impinging stream reactor (MISR) consisting of a commercial T-junction and steel capillaries, which is of intensified micromixing efficiency as compared with traditional stirred reactors (STR), was applied for the preparation of Co and Al co-doped Ni(OH)2 nanocomposites and their modification with reduced graphene oxide (RGO). The co-precipitation preparation process was conducted under precisely controlled proportions and concentrations of reactants in the MISR. Therefore, element analysis showed a higher uniform distribution of metal ions within the nanocomposites obtained through the MISR. The structural characterization and electrochemical measurements also showed that the MISR-prepared metal-doped nanocomposites were of more uniform dispersion and superior electrochemical performance than those prepared with STR. In addition, by modifying with RGO in the MISR, the electrochemical performance of Co and Al co-doped Ni(OH)2 nanocomposites could be further improved. The Co and Al co-doped Ni(OH)2/RGO prepared under optimal conditions achieved an ultrahigh specific capacitance of 2389.5 F g-1 at the current density of 1 A g-1 and displayed an excellent cycling stability with 83.7% retention of the initial capacitance after 1000 charge/discharge cycles in 6 M KOH aqueous solution.

HMT-Controlled Synthesis of Mesoporous NiO Hierarchical Nanostructures and Their Catalytic Role towards the Thermal Decomposition of Ammonium Perchlorate

In this work, mesoporous nickel oxide (NiO) hierarchical nanostructures were synthesized by a facile approach by hydrothermal reaction and subsequent calcination. The phase structure, microstructure, element composition, surface area, and pore size distribution of the as-prepared products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and the Brunauer–Emmett–Teller (BET) technique. The precursor of Ni3(NO3)2(OH)4 nanosheet, Ni3(NO3)2(OH)4 microsphere, and Ni(HCO3)2 sub-microsphere was obtained by hydrothermal reaction at 160 °C for 4 h when the ratio of Ni2+/HMT (hexamethylenetetramine) was 2:1, 1:2, and 1:3, respectively. After calcination at 400 °C for 2 h, the precursors were completely transformed to mesoporous NiO hierarchical nanosheet, microsphere, and sub-microsphere. When evaluated as additives of the thermal decomposition of ammonium perchlorate (AP), these NiO nanostructures significantly reduce the decomposition temperature of AP, showing obvious catalytic activity. In particular, NiO sub-microsphere have the best catalytic role, which can reduce the high temperature decomposition (HTD) and low temperature decomposition (LTD) temperature by 75.2 and 19.1 °C, respectively. The synthetic approach can easily control the morphology and pore structure of the NiO nanostructures by adjusting the ratio of Ni2+/HMT in the reactants and subsequent calcination, which avoids using expensive templates or surfactant and could be intended to prepare other transition metal oxide.

Solvent-Tuned Synthesis of Mesoporous Nickel Cobaltite Nanostructures and Their Catalytic Properties

In this paper, we prepared mesoporous nickel cobaltite (NiCo2O4) nanostructures with multi-morphologies by simple solvothermal and subsequent heat treatment. By adjusting the solvent type, mesoporous NiCo2O4 nanoparticles, nanorods, nanowires, and microspheres were easily prepared. The as-prepared products were systematically characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Brunauer–Emmett–Teller (BET) method. Furthermore, the catalytic activities towards the thermal decomposition of ammonium perchlorate (AP) of as-prepared NiCo2O4 nanostructures were investigated.

Temperature-Dependent Electrical Transport Properties of Individual NiCo2O4 Nanowire

Understanding the electrical transport properties of individual nanostructures is of great importance to the construction of high-performance nanodevices. NiCo₂O₄ nanowires have been investigated widely as the electrodes in electrocatalysis, supercapacitors, and lithium batteries. However, the exact electrical transport mechanism of an individual NiCo₂O₄ nanowire is still ambiguous, which is an obstacle for improving the performance improvement of energy storage devices. In this work, NiCo₂O₄ nanowires were prepared successfully by thermal transformation from the CoNi-hydroxide precursors. The electrical transport properties of an individual NiCo₂O₄ nanowire and its temperature-dependent conduction mechanisms were studied in detail. The current-voltage characteristics showed that an ohmic conduction in a low electrical field (< 1024 V/cm), Schottky emission in a middle electric field (1024 V/cm < E < 3025 V/cm), and Poole-Frenkel conduction at a high electric field (> 3025 V/cm). A semiconductive characteristic is found in the temperature-dependent conductivity in the NiCo₂O₄ nanowire; the electrical conduction mechanism at low temperature (T < 100 K) can be explained by Mott's variable range hopping (VRH) model. When the temperature is greater than 100 K, electrical transport properties were determined by the VRH and nearest neighbor hopping (NNH) Model. These understandings will be helpful to the design and performance improvement of energy-storage devices based on the NiCo₂O₄ nanowires.

Enhanced electrochemical performance of nanoplate nickel cobaltite (NiCo2O4) supercapacitor applications

Well-ordered, unique interconnected nanostructured binary metal oxides with lightweight, free-standing, and highly flexible nickel foam substrate electrodes have attracted tremendous research attention for high performance supercapacitor applications owing to the combination of the improved electrical conductivity and highly efficient electron and ion transport channels. In this study, a unique interconnected nanoplate-like nickel cobaltite (NiCo2O4) nanostructure was synthesized on highly conductive nickel foam and its use as a binder-free material in energy storage applications was assessed. The nanoplate-like NiCo2O4 nanostructure electrode was prepared by a simple chemical bath deposition method under optimized conditions. The NiCo2O4 electrode delivered an outstanding specific capacitance of 2791 F g-1 at a current density of 5 A g-1 in a KOH electrolyte in a three-electrode system as well as outstanding cycling stability with 99.1% retention after 3000 cycles at a current density of 7 A g-1. The as-synthesized NiCo2O4 electrode had a maximum energy density of 63.8 W h kg-1 and exhibited an outstanding high power density of approximately 654 W h kg-1. This paper reports a simple and cost-effective process for the synthesis of flexible high performance devices that may inspire new ideas for energy storage applications.