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In this study, NiO-added NiCo 2 O 4 composite samples were prepared using three distinct synthesis methods to study the impact of synthesis technique and NiO addition on the electrochemical performance of NiCo 2 O 4. The chosen methods, sol-gel, co-precipitation, and ultrasonication, represent a range of approaches that yield different particle structures and morphologies. The structural and morphological studies of composite samples were investigated using X-ray diffraction (XRD) and field emission scanning electron microscope (FESEM), respectively. XRD analysis revealed the crystalline composition, revealing the presence of two distinct phases: NiO and NiCo 2 O 4. The Rietveld analysis of the composite samples supports the dual phases in the samples. FESEM analysis confirms the change in size and shape of the particles with the synthesis procedure. Optical properties, measured via UV–visible absorbance spectra, demonstrated that the energy gap varied with the synthesis method. The sol-gel samples exhibited the highest energy gap at 2.20 eV. The magnetic properties were measured using the vibration sample magnetometer (VSM), and the results confirm that the synthesis procedure influences magnetic loops and coercivity. The electrochemical performance of the samples in a 3 M potassium hydroxide electrolyte revealed that the co-precipitation method produced the highest capacitance of 398 Fg-1 at a 10 mV/s sweep rate, indicating superior energy storage capacity compared to pure NiCo 2 O 4. Adding NiO phase to the NiCo 2 O 4 matrix is believed to enhance electrochemical performance by creating additional charge at the grain boundaries, leading to a synergistic effect and improved electron and ion transport. The exceptional electrochemical performance of these composite materials suggests that the synthesis methods and the addition of NiO play a critical role in determining the overall efficiency and applicability of battery-type supercapacitors. These findings pave the way for further research into optimizing synthesis parameters to enhance energy storage capacity and inspire the development of advanced spinel metal oxide composite electrodes for supercapacitors and other energy storage applications. [ABSTRACT FROM AUTHOR]

A simple solid-state method was developed for large-scale growth of spinel nickel cobaltite (NiCo2O4) nanoparticles on the surface of N-doped reduced graphene oxide nanosheet (N-rGO) as the enhanced performance electrode for electrochemical capacitors. The synthesis involves the room-temperature solid state reaction of a bimetallic (Ni, Co) hydroxide precursor and subsequent thermal transformation to NiCo2O4 nanoparticles on the N-rGO support. Morphological analyses reveal that, with loading rate 49 wt%, NiCo2O4 nanoparticles with an average size of around 12 nm are formed and uniformly dispersed on the surface of N-rGO nanosheets (NiCo2O4/N-rGO). Electrochemical measurements of NiCo2O4/N-rGO composites display a high specific capacitance of 1135 F g− 1 (based on NiCo2O4) at a current density of 1 A g− 1, and excellent cycling performance (100% capacitance retention after 2000 cycles) even at a very high current density of 8 A g− 1, which are significantly higher than that of NiCo2O4 nanoparticles prepared without using N-rGO. This much enhanced electrochemical performance is enabled by both the well-dispersed NiCo2O4 nanoparticles that offer large accessible surface area and the mesoporous structure of NiCo2O4/N-rGO composites that facilitate electrons/ions transport. The facile two-step synthesis process developed in this work is promising to prepare electrode materials for high-performance electrochemical capacitors. [ABSTRACT FROM AUTHOR]

Due to their high energy density and cost‐effectiveness, lithium–sulfur batteries (LSBs) are considered highly promising for the next generation of energy storage technologies. However, the soluble lithium‐polysulfides (LiPSs) notorious for causing the shuttle effect and the sluggish redox kinetics have hindered their practical commercialization. To tackle these challenges, a heterostructural catalyst featuring NiS‐NiCo2O4 interfaces is developed, which serves as an interlayer for LSBs. These interfacial sites leverage the advantages of polar NiCo2O4 and conductive NiS, enabling smooth Li+ diffusion, rapid electron transport, and effective immobilization of LiPSs. This synergistic approach promotes the conversion of sulfur species, resulting in a high discharge capacity of 526 mAh g−1 at 3 C for cells with the NiS‐NiCo2O4 interlayer. Additionally, remarkable cycling stability is achievable with an areal sulfur loading of ≈5.0 mg cm−2. It is believed that this research paves the way for practical applications of LSBs. [ABSTRACT FROM AUTHOR]

[Display omitted] The efficiency of photocatalytic hydrogen evolution can be significantly enhanced while maintaining cost-effectiveness through the synergistic effect of defect surface engineering and multi-component heterojunctions. The structure and properties of NiCo 2 O 4 nanorods were modified by inducing oxygen vacancies at different temperatures in this study, resulting in improved optical properties and electron adsorption capacity. The presence of oxygen vacancies leads to a reduction in the band gap of NiCo 2 O 4 , thereby enhancing electron transport efficiency through band gap engineering. Simultaneously, surface properties undergo changes, and vacancy defects serve as electron trapping centers, facilitating an increased participation of electrons in the hydrogen evolution reaction process. The dodecahedron KMP with a cavity structure is additionally introduced to form an S-scheme heterojunction with NiCo 2 O 4. This establishes a novel mechanism for electron transport, which effectively enhances the separation of electron-hole pairs and improves the redox capacity of the photocatalytic system. The adsorption of intermediates in the hydrogen production process is enhanced through synergistic regulation of d-band centers via surface defect engineering and S-scheme heterojunction. Additionally, this approach improves the separation efficiency of electron-hole pairs and accelerates electron transfer dynamics, significantly enhancing hydrogen production efficiency. [ABSTRACT FROM AUTHOR]

Fuel cells are promising fossil fuel alternatives that are both environmentally friendly and powerful. Direct methanol fuel cells are particularly suitable for lightweight cars and portable electronics. Developing efficient, cost-effective, and eco-friendly catalysts for energy storage and conversion devices is vital for long-term energy security. Numerous studies have explored using metal oxides and modifications to mixed transition metal oxide (MTMO)-based catalysts to enhance the methanol oxidation process. Coupling transition metals (Mn, Co, Ni, Fe, Zn, etc.) in MTMOs improves redox processes and electronic conductivity, advancing electrochemical applications. Among these, nickel cobaltite (NiCo₂O₄) stands out due to its material stability and higher electronic conductivity compared to single-component metal oxides NiO and Co₃O₄. NiCo₂O₄ shows great potential as a composite-based material for fuel cell applications. This paper reviews studies on NiCo₂O₄ as composite-based materials for electrocatalysts, including various methods for synthesizing nanostructures suitable for direct methanol fuel cells. It also highlights the challenges and potential benefits of using NiCo₂O₄ in practical energy storage systems, offering valuable perspectives and insights for early-stage researchers focused on future research and development in composite-based materials for energy applications. [ABSTRACT FROM AUTHOR]

To address the shortcomings of traditional intumescent flame retardants (IFRs), a novel four-in-one IFR (M [APP/NiCo2O4]) was constructed for the first time. It integrates acid source, gas source, carbon source (crosslinking β-CD), and synergistic agent (NiCo2O4 nanoparticles) into a single entity by a co-encapsulation technology. Under the loading of 20 wt%, its flame retardant performance was significantly better than that of simple mixed flame retardants, and it could increase the limiting oxygen index of polypropylene (PP) from 18 to 30.2 and pass the UL-94 V-0 rating. At the same time, it could reduce the peak of heat release rate, total heat release, and peak of CO production of PP by 77%, 22%, and 80%, respectively, showing excellent flame retardant performance. The excellent flame retardant performance is mainly because the synergist NiCo2O4 nanoparticles in the four-in-one flame retardant can easily combine and efficiently interact with the acid source, carbon source, and gas source in the intumescent flame retardant to form a more stable protective char residue. In addition, the crosslinking β-CD shell enhances both the water resistance and initial thermal decomposition temperature of PP containing the four-in-one IFR compared to PP with simple mixed control samples. After 72 h of water resistance test, it can still maintain the UL-94 V-0 rating. The presence of the crosslinking β-CD shell also improves the compatibility of the four-in-one flame retardant in PP, and its adverse effects on the mechanical properties of PP are also significantly smaller than those of the mixed control sample.

This paper presents a complete analysis of the electrical and dielectric characteristics of N i C o 2 O 4 (NCO) bulk ceramic synthesized by the co-precipitation process as a future electronic device material. The structural analysis following the synthesis process revealed the formation of cubic-phase spinel NCO in the shape of nanospheres, with an approximate diameter of 25 nm. The present study aimed to examine the relationship between capacitance and conductance for the NCO under investigation, focusing on their frequency and temperature dependencies. The frequency range of interest ranged from 100 Hz to 1.5 MHz, while the temperature range considered was from 300 K to 450 K. The C/f plot has exhibited the phenomenon of negative capacitance, which may be attributed to the augmentation of polarization and the incorporation of additional minority carriers inside the structure. The dielectric parameters, such as permittivity, dielectric loss, alternating current conductance, and electrical modulus, were determined by capacitance and conductance measurements, taking into account their frequency and temperature reliance. All dielectric calculations were influenced by negative capacitance measurements. The negative capacitance feature of N i C o 2 O 4 is attributed to the ferroelectric properties of the Ni and Co elements in its composition. In conclusion, this paper has made a valuable contribution to the existing body of literature by enhancing our comprehension of the negative capacitance behavior and dielectric characteristics of the NCO. [Display omitted] [ABSTRACT FROM AUTHOR]

An NH4F-assisted chemical synthesis was carried out to develop NiCo2O4 nanostructures for supercapacitor (SC) electrode application. The study reveals that the presence of NH4F in the precursor during the chemical bath deposition (CBD) process changes the morphology of NiCo2O4 nanostructures from interconnected nanosheets to nanoform arrays on stainless steel (SS) substrates. This morphological transformation significantly influences the electrochemical charge storage properties of the SC electrode. Various structural characterizations further supported this conversion process. The nanoform arrays of NiCo2O4 exhibited an enhanced capacity, showing 548 mC cm−2 (913 C g−1) at j = 0.25 mA cm−2, more than the capacity of 488 mC cm−2 (813 C g−1) observed for interconnected nanosheets. This improvement is likely attributed to the enhanced electronic conduction and ionic diffusion pathways facilitated by the mesoporous electrode surface. In addition, an aqueous asymmetric hybrid SC was assembled using activated carbon on SS as the negative electrode and a nanoform NiCo2O4 on SS as the positive electrode. The hybrid SC exhibited a maximum capacity of 46 mC cm−2 at 5 mV s−1, accompanied by a maximum energy density of 7.70 μWh cm−2 and a maximum power density of 9751.61 μW cm−2, while maintaining stability at 59% over 10000 GCD cycles. This study provides insight into the influence of additives during CBD process on the electrochemical storage performance of SC electrodes. [ABSTRACT FROM AUTHOR]

Energy storage technology plays a critical role in integrating variable energy sources into the grid and ensuring energy consistency. Electrochemical supercapacitors are one of the most promising energy storage devices, as they present multiple advantages of high power density, rapid charge/discharge characteristics, and long‐term cycle stability. Herein, the NiCo2O4/molybdate nanocomposites are developed as electrode materials for supercapacitor applications. The NiCo2O4/molybdate nanocomposites are synthesized by a facile single‐pot hydrothermal method and are coated on a carbon cloth substrate to form flexible supercapacitor electrodes. The structures, chemical compositions, morphologies, and textural properties of these materials are carefully studied by X‐Ray diffraction, X‐Ray absorption spectroscopy, scanning electron microscopy/energy‐dispersive X‐Ray spectroscopy mapping, and N2 adsorption–desorption isotherms. The formation of spinel NiCo2O4 nanorods decorated with molybdate (AMoO4, A = Co, Ni) particles is confirmed for all samples. The NiCo2O4/CoMoO4 electrode exhibits pseudocapacitive behavior and provides the highest specific capacitance (287.28 F g−1 at current density 6 A g−1), about 5.5 times as high as that of NiCo2O4, with excellent cycle stability (107% specific capacitance retention after 1000 charge/discharge cycles at 1 A g−1). Therefore, the NiCo2O4/CoMoO4 composites can be considered as a promising pseudocapacitor electrode material. [ABSTRACT FROM AUTHOR]

Herein, the single‐phase Ni‐based bimetallic spinel oxide (NiM2O4, M = Mn, Fe, Co) nanoparticles are successfully synthesized by sol–gel method and coated on a carbon cloth substrate to form flexible electrodes for supercapacitor applications. The effects of transition metal activity, calcination temperature, surface, and textural properties on the electrochemical properties of spinel electrode materials are demonstrated. It is found that lowering the calcination temperature results in a decrease in crystallite size and particle size, leading to an increase in surface area and pore volume. X‐Ray absorption spectroscopy reveals the presence of Mn2+/3+, Fe2+/3+, and Co2+/3+ in materials. According to the electrochemical studies, NiMn2O4 electrode in Na2SO4 electrolyte exhibits electrical double‐layer capacitance behavior, while NiFe2O4 and NiCo2O4 electrodes in KOH electrolyte exhibit pseudocapacitive behavior. All electrode materials have low solution resistance and charge transfer resistance. NiCo2O4 provides the highest specific capacitance (85.60 F g−1 at 2.5 A g−1) followed by NiFe2O4 and NiMn2O4. It seems likely that the high electrochemical activity of Co2+/3+ and small particle size of NiCo2O4 nanoparticles play an important role in improving the redox process and charge transfer, which may enhance the electrochemical performance of this electrode material. [ABSTRACT FROM AUTHOR]

This paper presents a novel ultrasonic-assisted electrodeposition process of Mn-doped NiCo2O4 onto a commercial nickel foam in a neutral electroplating bath (pH = 7.0) under an ultrasonic power of 1.2 V and 100 W. Different sample properties were studied based on their crystallinity through X-ray diffraction (XRD), morphology was studied through scanning electron microscopy (SEM), and photodegradation was studied through ultraviolet–visible (UV–Vis) spectrophotometry. Based on the XRD results, the dominant crystallite phase obtained was shown to be a pure single NiCo2O4 phase. The optical properties of the photocatalytic film showed a range of energy band gaps between 1.72 and 1.73 eV from the absorption spectrum. The surface hydroxyl groups on the catalytic surface of the Mn-doped NiCo2O4 thin films showed significant improvements in removing methyl red via photodegradation, achieving 88% degradation in 60 min, which was approximately 1.6 times higher than that of pure NiCo2O4 thin films. The maximum hydrogen rate of the composite films under 100 mW/cm2 illumination was 38 μmol/cm2 with a +3.5 V external potential. The electrochemical performance test also showed a high capacity retention rate (96% after 5000 charge–discharge cycles), high capacity (260 Fg−1), and low intrinsic resistance (0.8 Ω). This work concludes that the Mn-doped NiCo2O4 hybrid with oxygen-poor conditions (oxygen vacancies) is a promising composite electrode candidate for methyl red removal, hydrogen evolution, and high-performance hybrid supercapacitor applications. [ABSTRACT FROM AUTHOR]

This study explores the synthesis of nickel-cobalt bimetallic precursors using phytic acid (PA) as a phosphorus source and their application as supercapacitor electrodes. Through high-temperature annealing, these precursors were converted into phosphorus-doped nickel-cobalt bimetallic oxides (NCO) with certain defects. The morphologies and particle size of NCO can be easily adjusted by the amount of PA. Compared to the undoped nickel-cobalt bimetallic oxides, the phosphorus-doped materials exhibited significantly enhanced electrochemical activity and specific capacitance. The optimized electrode exhibits the highest specific capacitance of 1075 F·g−1 at a current density of 1 A·g−1. The asymmetric supercapacitor assembled together with activated carbon can deliver a relatively high energy density of 39.8 Wh·kg−1 at a power density of 696 W·kg−1. Additionally, this paper provides a detailed discussion on the impact of phosphorus doping on the performance of supercapacitor electrodes and the potential of PA as an eco-friendly phosphorus source in the synthesis of high-performance electrode materials.

The paper reports the development of Cu-doped NiCo 2 O 4 ternary inverse spinel oxides through microwave-assisted hydrothermal method as effective catalysts for alkaline hydrogen evolution reaction (HER). The effective and controlled doping of Cu to NiCo 2 O 4 lattice by the partial replacement of Ni2+/Ni3+ with Cu+/Cu2+ results in the fine-tuning of structural, morphological, and electronic characteristics, favourable for electrocatalytic HER. The study effectively correlates the influence of variation in the percentages of Cu-doping on the material properties and thereby the electrocatalytic characteristics. The experimental results indicate that Cu 0.10 Ni 0.90 Co 2 O 4 electrode having optimum Cu-doping (10%) as the best electrocatalyst with the lowest onset potential for hydrogen evolution (242.7 mV) and reduced overpotential (72.5 mV) values by following a Volmer – Heyrovsky mechanism during HER. The improvement in HER performance after Cu-doping is due to the generation of greater number of active sites and reduction in electronic band gap. The present work advantageously utilises the scope of microwave-assisted transition metal doping strategy to fine tune the physicochemical and electronic characteristics, favourable for electrocatalytic HER, and this may pave a new way for tuning the electrochemical properties of metal oxides for various applications. • Microwave-assisted hydrothermal synthesis is used effectively to develop Cu-doped NiCo 2 O 4. • Cu-doping to NiCo 2 O 4 lattice creates more oxygen vacancies and improves electronic conductivity. • Cu 0.10 Ni 0.90 Co 2 O 4 with 10% Cu-doping exhibits promising electrocatalytic activity towards HER. • Cu-doped NiCo 2 O 4 follows Volmer – Heyrovsky mechanism during alkaline HER. [ABSTRACT FROM AUTHOR]

In the quest for the development of thermally stable, highly active and low-cost catalysts for use in catalyzed diesel particulate filter, nano-composites are new areas of research. Therefore, we reported the easy synthesis of spinel NiCo 2 O 4 /perovskite LaCoO 3 nano-composite, and its individual oxides NiCo 2 O 4 and LaCoO 3 for comparison. The detailed insights into the physio-chemical characteristics of formed NiCo 2 O 4 / LaCoO 3 nano-composite were done based on various characterization analysis such as X-ray diffraction (XRD), Fourier transform infrared (FT-IR), N 2 physiosorption, scanning electron microscopy-energy dispersive spectroscopy (SEM-EDX), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The characterization analysis of NiCo 2 O 4 /LaCoO 3 revealed the successful formation of a chemical interface possessing strong interfacial interaction, resulting in desirable physicochemical characteristics such as small crystallite size, abundant mesoporosity, high specific surface area and activation of surface lattice oxygen. Owing to the desirable characteristics, the activity results over NiCo 2 O 4 /LaCoO 3 nano-composite showed the excellent CO oxidation performance and high soot oxidation activity, recyclability and thermal stability. This work mainly attempts to emphasize the effectiveness of the facile, inexpensive and conventionally used precipitation method for the successful formation of highly efficient nano-composites. [ABSTRACT FROM AUTHOR]

The intercalation pseudocapacitance mechanism holds the potential to significantly narrow the gap between supercapacitors and lithium-ion batteries, particularly in terms of energy density and power density properties. Herein, 1D nanorod and 2D nanoflake mixed structures of nickel cobaltite (NiCo2O4) were prepared for asymmetric supercapacitor application. For comparison, the NiO and Co3O4 materials were also prepared, and examined their supercapacitor properties. The NiCo2O4 store charges through the diffusion-controlled process and delivers the specific capacity of 347 C g−1 (694 F g−1) at 1 A g−1 and it withstands 96% of initial capacity after 5000 cycles at a current density of 10 A g−1. Furthermore, the asymmetric supercapacitor device (NiCo2O4//AC) exhibits the specific capacity of 141 C g−1 (88 F g−1) at 1 A g−1, with an excellent energy and power density of 31.3 Wh kg−1 and 800 W kg−1 respectively. The device retained 86% of its initial capacity after 5000 GCD cycles at 10 A g−1. Therefore, the 1D/2D mixed morphologies with diffusion-controlled charge storage process are guaranteed to achieve superior electrochemical performance. [ABSTRACT FROM AUTHOR]

The magnetic spinel cobalt ferrite (CoFe 2 O 4) powders had the high microwave absorption performance at the large thicknesses on account of the low permittivity. Combining the CoFe 2 O 4 powders with other dielectric loss materials had been suggested to improve their impedance matching and microwave absorption performance. In this work, the microwave absorption performance of the CoFe 2 O 4 powders was enhanced by adding the various amounts of the NiCo 2 O 4 nanoparticles as dielectric loss material. The CoFe 2 O 4 -x NiCo 2 O 4 composite powders (x = 0, 12.5, 25, and 50 wt %) were prepared by a facile two-pot solution combustion synthesis method. Although the X-ray diffraction peaks of NiCo 2 O 4 phase were absent, but the decrease of saturation magnetization from 61 to 30 emu/g was related to the amount of NiCo 2 O 4 material. Furthermore, the NiCo 2 O 4 nanoparticles were nucleated and grown on the surface of spongy CoFe 2 O 4 powders because the synthesized CoFe 2 O 4 powders were added to the precursor solution of NiCo 2 O 4 material containing the nickel and cobalt nitrates as oxidants and glycine as organic fuel. The microwave absorption performance was analyzed by measuring the permittivity and permeability spectra which were applied for evaluating the reflection loss versus frequency and thickness. The CoFe 2 O 4 -25 wt % NiCo 2 O 4 composite powders had the higher microwave absorption performance including the high reflection loss of −40 dB and broad effective bandwidth of 5.6 GHz of Ku band (12–18 GHz) at a small matching thickness of 1.9 mm. The high microwave absorption performance of the x = 25 wt % sample was attributed to its better impedance matching via a proper combination of the permittivity and permeability values. [ABSTRACT FROM AUTHOR]

Transition bimetal oxides are widely used in the field of electrochemical sensing because of their variable valence and bimetallic synergies. In this work, we demonstrate the facile synthesis of snow-cake like NiCo2O4 nanostructures and its application as a working electrode modification material in enzyme-free glucose electrochemical sensing field. In a three-electrode system, the electrocatalytic oxidation activity for glucose of NiCo2O4 nanostructures is tested. The snow-cake like NiCo2O4 nanostructures are endowed with superior electron-transfer capability, large surface area, and abundant intrinsic redox couples of Ni2+/Ni3+ and Co2+/Co3+ ions. The modified electrode exhibits excellent glucose-sensing properties, such as detection of wide range of glucose concentrations with good linearity and high sensitivity of 983 μA mM−1 cm−2 at 0.45 V operating potential. Moreover, the interference from the commonly interfering species such as ascorbic acid (AA), sodium chloride (NaCl) and uric acid (UA) can be effectively avoided. [ABSTRACT FROM AUTHOR]

The phenomenon of poor cycle stability can be effectively solved by using metal–organic frameworks (MOFs) as precursor to obtain oxides. In this work, a layered NiO@NiCo2O4 nanocomposite electrodes were designed from bimetallic (Ni/Co) based bimetallic metal–organic framework (Ni/Co-MOF) by simple hydrothermal method followed by subsequent controlled calcination under air atmosphere at various temperatures for the transition process. XRD and FE-SEM studies were substantiating the formation of MOF derived NiO@NiCo2O4 nanocomposite with layered structure. Interestingly, electrochemical studies shows that MOF derived NiO@NiCo2O4 nanocomposite achieved at the calcination temperature at 300 °C and 400 °C demonstrates high specific capacitance of 2461 F/g and 2180 F/g at the current density of 1 A/g, which keeps the precursor Ni/Co-MOF with high specific capacitance peculiarity of 2250 F/g at 2 A/g. In terms of cycle retention rate, the NiO@NiCo2O4 nanocomposite obtained at 400 °C still has a high retention rate of 78% under the current density of 15 A/g for 4500 cycles. It significantly improved the precursor Ni/Co-MOF low cycle retention rate of only 56% after 1000 cycles. The results show that the metal oxide nanocomposites calcined with MOFs as sacrificial template not only retain the high specific capacitance characteristics of MOFs, but also improve its low cycle stability, which has great potential in electrochemical supercapacitor electrode materials and provides a certain useful reference for the direction of global energy demand. [ABSTRACT FROM AUTHOR]

In this study, we investigated the phase transition of cobalt spinel (Co3O4) nanoparticles into Co3-xNixO4/CoO heterostructures by introducing varying amounts of nickel (x = 0.0–0.16) and subjecting the particles to high annealing temperatures of 1000 ℃. X-ray diffraction (XRD) analysis confirmed the Co3-xNixO4CoO structure for all samples. Transmission electron microscopy (TEM) provided further insights into the phase or heterostructure of the samples after annealing, revealing the arrangement of the two phases. Fourier-transform infrared spectroscopy measurements demonstrated a band shift around 537 cm-1 with increasing Ni content, while ultraviolet-visible (UV-Vis) measurements indicated the energy band (Eg). Significant morphological changes were observed in scanning electron microscope (SEM) measurements at 0.16 Ni, displaying irregular agglomerates. Our findings suggest that introducing Ni into the Co3O4 structure and increasing the annealing temperature to 1000 ℃ can lead to the formation of a heterostructured system. Furthermore, our study's significance is highlighted by the streamlined synthesis of NiCo2O4/CoO using the sol-gel method followed by calcination. This departure from complex techniques provides an efficient route to acquiring the NiCo2O4/CoO system, a promissory material for advancing supercapacitor research.

Purposes In-depth research on electrode materials and preparation processes to develop new electrochemical sensors for heavy metal ion detection is of great importance. Methods The porous NiCo2O4 array grown on Ni foam (NiCo2O4/NF) was synthesized by hydrothermal method. Then, the composition, morphology, and electrochemical properties of the sample were characterized by X-ray diffraction, scanning electron microscopy, and electrochemical testing. Findings The results show that the NiCo2O4 array is uniformly loaded on the skeleton of Ni foam with a highly open nanowire array structure. The NiCo2O4/NF composite material is directly applied as the electrode for electrochemical determination of Hg2+ solutions with different concentrations. In 0.125~2.5 μmol/L and 2.5~35 μmol/L concentration range, the detection sensitivities are 0.025 and 0.003 4 μA/μmol/L, respectively. Meanwhile, the electrode shows good stability and reproducibility. In addition, when the composite electrode is also used to detect mixed heavy metal ion solutions of different concentrations, it also shows high sensitivity and discrimination. Conclusions The research results in this paper provid a new method for designing and synthesizing high-performance electrode materials, for conveniently detecting heavy metal ions by electrochemical method.

In this work, the one-dimensional (1D) Ni–Co–Se nanowire arrays with rich grain-boundaries were prepared through the solvothermal method and gas-phase selenizaiton. The results showed that the structure and crystallization of the Ni–Co–Se nanowire arrays could be modulated through the optimization of selenizaiton time. The optimal Ni–Co–Se electrode sample displayed an area specific capacitance of 242.6 μ Ah cm−2 at 30 mA cm−2 with a current retention rate of 68.34%. The assembled Ni–Co–Se/Active carbon (AC) electrode-based asymmetric supercapacitor (ASC) showed the area specific capacitances of 329.2 μ Ah cm−2 and 225.8 μ Ah cm−2 at 3 mA cm−2 and 30 mA cm−2, respectively. A 73.33% retention rate of capacitance was observed after 8000 charge/discharge cycles. Besides, the further fabricated all-solid ASC delivered the power densities of 342.94 W kg−1 and 3441.33 W kg−1 at the energy densities of 37.62 Wh kg−1 and 25.81 Wh kg−1, respectively. Those results suggested the potentials of the obtained Ni–Co–Se nanowire arrays as electrode material for the high-performance pseudocapacitors. [ABSTRACT FROM AUTHOR]

The lithium-air battery is a new type of secondary battery technology that is currently receiving a lot of attention in the field of power storage technology. These batteries are known to offer high energy densities and potentially longer driving ranges. In this study, NiCo2O4 and CNTs were used to create a composite for use as the cathode of a Li-air battery. Improving the 3D needl-like structure that provides extensive transport channels for electrolyte infiltration and numerous sites facilitated charge transfer reactions and the synergistic effect of highly electrocatalytic NiCo2O4 with pronounced activity and high conductive CNTs, with the synthesized NiCo2O4@CNTs composites exhibiting active catalytic performance for both OER and ORR reactions. It also showed improved cycle performance at high current densities. NiCo2O4@CNTs composites were successfully fabricated using a hydrothermal method together with a sequential annealing treatment. The components of the completed composite were confirmed using TGA, XRD, and SEM, and the specific surface area was analyzed using BET. The composite was performed for over 120 cycles at a current density of 200 mA∙g−1, and 500 mA∙g−1 was achieved under the capacity limiting condition of 500 mAh∙g−1. The charging/discharging characteristics were compared under various current densities, exhibiting stable cyclability. The high catalytic activity of NiCo2O4 oxide supports its potential use as a cathode in Li-air batteries. [ABSTRACT FROM AUTHOR]

Developing an efficient electrocatalyst for the oxygen electrode is a significant challenge in the field. In this study, Co–N co-doped carbon (denoted as CZIF-67) is used as modifier to enhance the catalytic activity of the spinel oxide NiCo2O4. Among all the catalysts synthesized, the composite catalyst containing 10% CZIF-67 (where CZIF-67 constitutes 10% of the total mass) exhibits the highest catalytic activity for both the oxygen evolution reaction and the oxygen reduction reaction. This is evidenced by the largest current densities of 275.6 mA·cm−2 and 235.6 mA·cm−2 at the cutoff voltage of 1.9268 V (vs. RHE) and 0.3268V (vs. RHE). XPS characterization showed the surface element composition and valence analysis of 10% CZIF-67, which revealed the synergistic reaction between CZIF-67 and NiCo2O4 to a certain extent. Electrochemical impedance spectroscopy (EIS) results indicate that the charge transfer resistance of 10% CZIF-67 catalyst is the lowest, suggesting a faster charge transfer rate. It can be attributed to the synergistic effects between appropriate amount of CZIF-67 and the spinel oxide NiCo2O4. Additionally, the large specific surface and the porous structure of CZIF-67 can provide more active sites for the reactions. [ABSTRACT FROM AUTHOR]

Electromagnetic pollution could harm sensitive electronic equipment due to the rising use of electronic devices and communication infrastructure. The supercapacitor's electrochemical performance should be enhanced, and electromagnetic damage should be prevented. This study proposes NiCo2O4/CF composites for supercapacitors and microwave absorption. They are made by combining hydrothermal and annealing processes. Dense NiCo2O4 nanoneedles were uniformly grown on the outer layer of carbon foam (CF) as a growth skeleton, preventing the agglomeration of NiCo2O4. The composite had a specific capacitance of 537.5 F/g at 1 A/g. When the current density was set to 1 A/g, the supercapacitor that used NiCo2O4/CF as the cathode had a specific capacitance of 70.7 F/g, and when the current density was increased to 10 A/g, the original specific capacitance of 87.2 % could still be maintained after 5000 charge‐discharge cycles. At a power density of 3695.5 W/kg, an energy density of 22.1 Wh/kg could be maintained. Furthermore, we performed a microwave absorption test and determined its reflection loss curve for various sample thicknesses. Recombination enhanced the composite material's microwave absorption capability by greatly reducing the dielectric loss and the magnetic loss. [ABSTRACT FROM AUTHOR]

NiCo2O4, as a low‐cost and high‐performance binary transition metal oxide material, has been widely used in the fabrication of supercapacitor electrodes. However, the low energy density of binary metal oxides limits their large‐scale practical applications in supercapacitors, so it is highly desirable to design the structure of the NiCo2O4‐based supercapacitor. Therefore, in this paper, NiCo2O4 is compounded with polyaniline (PANI) to improve its electrochemical performance by exploiting the synergistic effect. The NiCo2O4@PANI composite electrode exhibits a specific capacitance of 561.2 F/g at 10 mV/s. In this study, asymmetric supercapacitors are assembled with NiCo2O4@PANI as the positive electrode and MXene as the negative electrode. The NiCo2O4@PANI‐CC//MXene‐CC supercapacitor has a specific capacitance of 31.95 F/g at a scan rate of 10 mV/s and a good cycle retention capacity of 86.2% after 3000 cycles. The excellent electrochemical performances are attributed to the dynamic equilibrium and fast redox kinetics of the two electrodes. [ABSTRACT FROM AUTHOR]

Radiation exposure induces defects in the crystal structure, leading to alterations in the structural, optical, and morphological properties of metal oxides. These irradiated metal oxides have found extensive use in various household applications. In this particular investigation, carbon-coated nickel cobalt oxide/rGO nanocomposites were synthesized through ultrasonication. Subsequently, these nanocomposites were subjected to three different doses of gamma radiation using a 60Co source. Both un-irradiated and irradiated samples were subjected to characterization using FTIR, SEM, XRD, and UV-Vis spectroscopy. The FTIR analysis confirmed the presence of a spinel-type structure in NiCo2O4. SEM analysis revealed the presence of crumbled sheets of rGO, and the irradiation process caused a transformation of these stacked sheets into thinner sheets. XRD analysis indicated a decrease in crystalline size and an increase in dislocation density with higher doses of gamma irradiation. UV-Vis spectroscopy demonstrated a reduction in both direct and indirect optical band gaps as a result of gamma irradiation. Furthermore, it was observed that Urbach energy and refractive index increased with gamma irradiation. Based on these findings, it can be concluded that gamma irradiation has a significant impact on the morphological, structural, and optical properties of NiCo2O4/rGO@C nanocomposites. [ABSTRACT FROM AUTHOR]

Among the many methods of hydrogen production, the electrolysis of water has a broad prospect. In this experiment, NiCo 2 O 4 nanosheets with different degrees of oxidation were synthesized on nickel foam by the hydrothermal method, air calcination and reduction steps. The experimental results show that reduction treatment significantly improved the catalytic activity of NiCo 2 O 4 materials under alkaline medium conditions of 1.0 mol/L KOH. V–NiCo 2 O 4 -4 shows better catalytic activity with an overpotential of 285 mV and 340 mV for the oxygen evolution reaction (OER) at 10 mA cm−2 and 50 mA cm−2, respectively. The better catalytic activity of V–NiCo 2 O 4 -4 is mainly assigned to the flower-like structure of the catalyst surface and the moderate reduction of NaBH 4 , resulting in more oxygen vacancies and active sites. Density functional theory calculation also indicates that the V–NiCo 2 O 4 -4 material has a stronger water absorption energy (0.75 eV) compared to the V–NiCo 2 O 4 -2 and V–NiCo 2 O 4 -6 materials, which provides a new foundation for the construction of binder-free high-performance electrodes for water splitting. The V–NiCo 2 O 4 -4 showed better results at 50 mA cm−2, which required an overpotential of 340 mV for oxygen evolution reaction (OER). [Display omitted] • A series of NiCo 2 O 4 nanosheets were synthesized by hydrothermal method. • This V–NiCo 2 O 4 -4 exhibits enhanced activity for OER. • DFT show that the V–NiCo 2 O 4 -4 exhibits the stronger water absorption energy. • This V–NiCo 2 O 4 -4 material exhibits relatively good stability for OER. [ABSTRACT FROM AUTHOR]

In this study, we investigated the phase transition of cobalt spinel (Co3O4) nanoparticles into Co3−xNixO4/CoO heterostructures by introducing varying amounts of nickel (x = 0.0–0.16) and subjecting the particles to high annealing temperatures of 1000 ℃. X-ray diffraction (XRD) analysis confirmed the Co3−xNixO4CoO structure for all samples. Transmission electron microscopy (TEM) provided further insights into the phase or heterostructure of the samples after annealing, revealing the arrangement of the two phases. Fourier-transform infrared spectroscopy measurements demonstrated a band shift around 537 cm−1 with increasing Ni content, while ultraviolet-visible (UV-Vis) measurements indicated the energy band (Eg). Significant morphological changes were observed in scanning electron microscope (SEM) measurements at 0.16 Ni, displaying irregular agglomerates. Our findings suggest that introducing Ni into the Co3O4 structure and increasing the annealing temperature to 1000 ℃ can lead to the formation of a heterostructured system. Furthermore, our study’s significance is highlighted by the streamlined synthesis of NiCo2O4/CoO using the sol-gel method followed by calcination. This departure from complex techniques provides an efficient route to acquiring the NiCo2O4/CoO system, a promissory material for advancing supercapacitor research. [ABSTRACT FROM AUTHOR]

The mechanical and electronic properties of NiCo2O4 crystal under high pressure are explored based on the density functional theory method. The pressure in the range from 0 to 50GPa is applied to NiCo2O4 and the behaviour concerning electronic properties is studied. The energy band gap of NiCo2O4 gradually decreases from 2.196 eV (0GPa) to 1.785 eV (50GPa). The elastic constants for cubic NiCo2O4 are calculated and bulk, shear and Young’s modulus are further computed. Furthermore, Cauchy’s pressure, Pugh’s criterion and Pois-son’s ratio of NiCo2O4 are calculated and reported under high pressure. The NiCo2O4 exhibits ductile property under high pressure, which is evident from Pugh’s criterion. In addition, hardness and anisotropy factor is also calculated and reported.

Developing the high-capacity anode materials such as conversion-type metal oxides which possess both Li and Na storage activity is very practical for the high-energy Li-ion battery (LIB) and Na-ion battery (LIB). Herein, we use NiCo2O4 anodes as a model to investigate the morphology evolution which accounts for the poor cycling performance and understand the effect of structure optimization on the electrochemical performance. Three NiCo2O4 samples with different morphologies of microspheres, nanospheres and nanosheets are synthesized. Firstly, the serious structural degradation of NiCo2O4 microspheres is observed whether it works as a LIB or SIB anode. In addition, a significant difference between the lithiation and sodiation capacity of NiCo2O4 materials reveals Na+ ions only partially intercalated in NiCo2O4 and the conversion reaction limited by the strain. Next, NiCo2O4 nanosheets on Ni foam as a binder-free anode for the LIB are investigated which suggest the positive effect of 3D nanostructures on the morphology stability. As a result, NiCo2O4 nanosheets deliver a high lithiation capacity of 1092 mAh g−1 after 100 cycles at 0.5 A g−1 and an excellent rate capacity of 643 mAh g−1 at 4 A g−1. Finally, NiCo2O4 nanospheres are evaluted as a SIB anode which indicate the smaller particle size of active materials is beneficial to the release of stress and structure stability during discharge–charge processes. Relatively speaking, the nano sheet structure is with the best electrochemical performance based on the capacity retention. A rational design of the electrode' architecture is very important for the conversion-type 3d transition metal oxide anodes for the advanced LIB and SIB. [ABSTRACT FROM AUTHOR]

Exploring highly efficient electrocatalysts (OER) is critical for oxygen evolution reaction. Herein, for the first time, P-doped NiCo 2 O 4 nanosheet arrays generated on nickel foam (namely P–NiCo 2 O 4 /NF) were constructed via a feasible protocol. Notably, the etching treatment can open the internal structure of ZIF-67 to offer more electrocatalytically active surface area, and obtained NiCo 2 O 4 showed great electrocatalytic performance in OER. The introduction of P can effectively reduce the charge transfer resistance of NiCo 2 O 4 and boost OER kinetics. More importantly, doping P not only improved the hydrophilicity of NiCo 2 O 4 , but also reduced its surface potentials, which provided convenience for the adsorption of OH− over P–NiCo 2 O 4 /NF electrode. As expected, the P–NiCo 2 O 4 /NF electrode with good long-term durability displayed excellent performance for OER in alkaline solution, and an overpotential of 121.6 mV was achieved at a current density of 10 mA/cm2. To sum up, current work can provide a meaningful reference to prepare electrocatalyst for efficient OER. [Display omitted] • The P-doped NiCo 2 O 4 nanosheet array grown on nickel foam was first constructed. • An overpotential of 121.6 mV for OER was achieved at a current density of 10 mA/cm2. • Doping P improved affinity of NiCo 2 O 4 for OH− and reduced charge transfer resistance. • As-prepared P–NiCo2O4/NF electrode showed excellent long-term durability for OER. [ABSTRACT FROM AUTHOR]

[Display omitted] Flexible supercapacitors have received considerable attention for their potential application in flexible electronics, but usually suffer from relatively low energy density. Developing flexible electrodes with high capacitance and constructing asymmetric supercapacitors with large potential window has been considered as the most effective approach to achieve high energy density. Here, a flexible electrode with nickel cobaltite (NiCo 2 O 4) nanowire arrays on nitrogen (N)-doped carbon nanotube fiber fabric (denoted as CNTFF and NCNTFF, respectively) was designed and fabricated through a facile hydrothermal growth and heat treatment process. The obtained NCNTFF-NiCo 2 O 4 delivered a high capacitance of 2430.5 mF cm−2 at 2 mA cm−2, a good rate capability of 62.1 % capacitance retention even at 100 mA cm−2 and a stable cycling performance of 85.2 % capacitance retention after 10,000 cycles. Moreover, the asymmetric supercapacitor constructed with NCNTFF-NiCo 2 O 4 as positive electrode and activated CNTFF as negative electrode exhibited a combination of high capacitance (883.6 mF cm−2 at 2 mA cm−2), high energy density (241 μW h cm−2) and high power density (80175.1 μW cm−2). This device also had a long cycle life after 10,000 cycles and good mechanical flexibility under bending conditions. Our work provides a new perspective on constructing high-performance flexible supercapacitors for flexible electronics. [ABSTRACT FROM AUTHOR]

The three-dimensional (3D) nanostructured binary metal oxides have a massive prospect in energy-related applications owing to their outstanding electrochemical performance and stability. 3D hierarchical tremella-like nickel cobaltite (NiCo 2 O 4) has been successfully synthesized by a hydrothermal method coupled with an annealing treatment process. The effects of temperature and reaction time in hydrothermal process on crystal structure, morphology, particles size, surface area and pore size distribution were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), Brunauer–Emmett–Teller analysis (BET) and Barrett-Joyner-Halenda (BJH) analysis. The results showed that 3D hierarchical nanoporous structures composed of interconnected thin nanoflakes with thickness of about 20 nm began to form when the mixture was heated for 6 h at 180 °C with considerably high specific surface area of 169.61 m2/g. On the other hand, the highest surface area (203.21 m2/g) and narrowest pore size distribution (9.13 nm) are achieved by the sample heated at a temperature of 160 °C for 12 h. These findings indicated that temperature and reaction time parameters in hydrothermal process play a major role in producing different quality and altered morphological structures of NiCo 2 O 4 powders which later will affect the electrochemical performance of fuel cell. • 3D nanostructures of NiCo 2 O 4 was successfully synthesized by hydrothermal method. • Hydrothermal temperature and reaction time affected the morphologies of NiCo 2 O 4. • The desired nanostructures began to form when sample was heated for 6 h at 180 °C. • The highest specific surface area was obtained by sample heated at 160 °C for 12 h. [ABSTRACT FROM AUTHOR]

Supercapacitors can be called electrostatic double-layer capacitors (EDLCs) or pseudo-capacitors based on their energy storage mechanism. Pseudo-capacitors with metal oxide electrodes that transmit electric charge offer great application power but low stability. This study aims to characterize trisodium citrate (TSC) modified NiCo2O4/rGO electrodes as a preliminary study for the NiCo2O4 nanoparticle pseudo-capacitor. The materials were characterized using SEM (Scanning Electron Microscopy), XRD (X-Ray Diffractometer), FT-Raman (Fourier Transform Raman Spectroscopy), and FTIR (Fourier Transform Infrared Spectroscopy). All of these analyses confirmed that NiCo2O4 was manufactured successfully using TSC. At XRD, the angles of 2????, namely 45.04 and 53.34°, matched the plane of the cubic phase nanorod crystals of NiCo2O4 with values of 400 and 511, respectively. In the Raman examination, the presence of rGO, which increased the crystallinity of NiCo2O4 nanoparticles, was confirmed. FTIR analysis indicated that the 1550-1600 cm−1 corresponds to a C=C functional group in an aromatic ring or a C=O ????-???? structure, and the wavenumber of 2300 cm−1 corresponds to the OH group in TSC, an alkanoate derivative. SEM analysis determined that NiCo2O4 nanoparticles with 1 mmol TSC are the ideal material for supercapacitor electrodes, as the structure is the most uniform, soft, and tidy. Further analysis is needed to confirm the stability of the modified material.

Abstract The binary metal oxides show advantages in energy storage devices. Specifically, nickel cobaltite (NiCo2O4) materials showed promising pseudocapacitive properties, high electrical conductivity and large surface area by virtue of their effective porous structure. NiCo2O4 nanosheets were hydrothermally grown in this work over flexible tapes of Aluminum (Al) and Copper (Cu). A nanosheets structure obtained of NiCo2O4 as confirmed by SEM and AFM images. The measured thickness by 3D profilometer of NiCo2O4 nanosheets based Al framework found to be 4.3 µm compared to 8.4 µm thick of film based-Cu framework. Asymmetric supercapacitor prepared from graphite and NiCo2O4 electrodes separated by filter paper. Acidic aqueous electrolyte of H2SO4 and basic aqueous electrolyte of KOH were employed to verify the cyclic activity and electrochemical reaction of asymmetric prepared supercapacitor devices. The basic KOH electrolyte shows a high stability and better charge transfer/ionic diffusion compared to the acidic H2SO4 electrolyte in particular for NiCo2O4 film-based Cu framework. The energy density and power density values were 0.9 W h kg−1 and 66.45 W kg−1, respectively. The highest specific capacity (in F.g−1) = 10.09 coincides with NiCo2O4/Cu supercapacitor in the basic KOH electrolyte. The charge storage in the supercapacitor system of NiCo2O4 and graphite can be ascribed in the form of Faradic charge transfer and capacitive non-faradic double layer, respectively.

This paper presents a novel ultrasonic-assisted electrodeposition process of Mn-doped NiCo2O4 onto a commercial nickel foam in a neutral electroplating bath (pH = 7.0) under an ultrasonic power of 1.2 V and 100 W. Different sample properties were studied based on their crystallinity through X-ray diffraction (XRD), morphology was studied through scanning electron microscopy (SEM), and photodegradation was studied through ultraviolet–visible (UV–Vis) spectrophotometry. Based on the XRD results, the dominant crystallite phase obtained was shown to be a pure single NiCo2O4 phase. The optical properties of the photocatalytic film showed a range of energy band gaps between 1.72 and 1.73 eV from the absorption spectrum. The surface hydroxyl groups on the catalytic surface of the Mn-doped NiCo2O4 thin films showed significant improvements in removing methyl red via photodegradation, achieving 88% degradation in 60 min, which was approximately 1.6 times higher than that of pure NiCo2O4 thin films. The maximum hydrogen rate of the composite films under 100 mW/cm2 illumination was 38 μmol/cm2 with a +3.5 V external potential. The electrochemical performance test also showed a high capacity retention rate (96% after 5000 charge–discharge cycles), high capacity (260 Fg−1), and low intrinsic resistance (0.8 Ω). This work concludes that the Mn-doped NiCo2O4 hybrid with oxygen-poor conditions (oxygen vacancies) is a promising composite electrode candidate for methyl red removal, hydrogen evolution, and high-performance hybrid supercapacitor applications.

Metal-organic frameworks (MOFs) exhibit many structural advantages, such as large specific surface area and abundant active sites, which make them promising candidates as electrode materials for non-enzymatic glucose sensors. In this work, using MOFs as templates, MOFs derived Co 3 O 4 nanoparticles (NPs) were in-situ synthesized on the surface of NiCo 2 O 4 nanowires (NWs), forming a self-supporting electrode based on nickel foam (NF), which was directly used for the detection of glucose. Because of the presence of MOFs derivatives and self-supporting structures, NF/NiCo 2 O 4NWs @Co 3 O 4NPs electrode exhibits good structural stability and high conductivity, which ensure the excellent electrocatalytic performance. The NF/NiCo 2 O 4NWs @Co 3 O 4NPs electrode shows prominent electrocatalytic performance toward glucose with a wide testing range (0.001–1.7 mmol/L), high sensitivity (8163.2 μA cm−2 mM−1), fast response time (within 1 s), high anti-interference ability and long-term stability. Further, the electrode was used to test different glucose levels in live mouse serum samples, showing acceptable reliability and high accuracy comparing with blood glucose meter test (medical test method). Thus, NF/NiCo 2 O 4NWs @Co 3 O 4NPs electrode with self-supporting core-shell nanostructure will be a promising candidate applied in non-enzymatic glucose sensors. [ABSTRACT FROM AUTHOR]

Urchin-like NiCo2O4 was synthesized onto flexible carbon felt (CF) substrate via facile hydrothermal process as a high performed electrode for supercapacitors. The hydrothermal temperature was varied from 140 to 200 °C in order to investigate its effect on the morphological, structural, and electrochemical properties of the as-synthesized NiCo2O4@CF. Moreover, X-ray diffraction analysis revealed the formation of NiCo2O4 composite with polycrystalline nature. The SEM images demonstrate the development of NiCo2O4 structure from nanowires to a combination between nanowires and urchin like during the increasing of the hydrothermal temperature. This combination is extremely advantageous in accomplishing an exceptional electrochemical capacity by virtue of the high surface area of interaction with electrolyte and the decrease in the ion transport path. Indeed, the double nanostructured NiCo2O4 synthesized at 200 °C allowed to obtain excellent electrochemical performance with high specific capacity of 243.1 mAh.g−1(0.278 mAh.cm−2) at current value of 2 mA and capacity retention of ~ 91.3% after 5000 cycles in 6-M KOH aqueous electrolyte. As a consequence, the obtained double nanostructured NiCo2O4@CF flexible electrode is well suited for use in hybrid devices and flexible electronic devices. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Cellulose has a wide range of applications in many fields due to their naturally degradable and low-cost characteristics, but few studies can achieve cellulose-nanofibers by conventional electrospinning. Herein, we demonstrate that the freestanding cellulose-based carbon nanofibers are successfully obtained by a special design of electrospinning firstly, pre-oxidation and high-temperature carbonization (1600 °C), which display a superior electrical conductivity of 31.2 S·cm−1 and larger specific surface area of 35.61 m2·g−1 than that of the polyacrylonitrile-based carbon nanofibers (electrical conductivity of 18.5 S·cm−1, specific surface area of 12 m2·g−1). The NiCo2O4 nanoflake arrays are grown uniformly on the cellulose-based carbon nanofibers successfully by a facile one-step solvothermal and calcination method. The asprepared cellulose-based carbon nanofibers/NiCo2O4 nanoflake arrays are directly used as electrodes to achieve a high specific capacitance of 1010 F·g−1 at 1 A·g−1 and a good cycling stability with 90.84% capacitance retention after 3000 times at 10 A·g−1. Furthermore, the all-solid-state symmetric supercapacitors assembled from the cellulose-based carbon nanofibers/NiCo2O4 deliver a high energy density of 62 W·h·kg−1 at a power density of 1200 W·kg−1. Six all-solid-state symmetric supercapacitors in series can also power a 'DHU' logo consisted of 36 light emitting diodes, confirming that the cellulose-based carbon nanofiber is a promising carbon matrix material for energy storage devices. [ABSTRACT FROM AUTHOR]

Herein, hierarchical Ni/Co-layered double hydroxide decorated NiCo 2 O 4 electrocatalyst (NiCo-LDH@NCO) on nickel foam (NF) was synthesized using a facile hydrothermal method with electrochemical reconstruction. By converting the already existing NCO surface to hierarchical NiCo-LDH, a larger surface area and high electrical conductivity are obtained. Both structure and morphology of fabricated electrocatalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and field emission scanning electron microscopy (FESEM) techniques. The electrochemical reconstruction process is accompanied by changes in the morphology of the NCO that significantly increase accessibility to the electrolyte and active surface area. Therefore, the as-obtained electrocatalyst exhibits excellent oxygen evolution reaction (OER) activity and long-term stability for 35 h continuous reaction, superior-performing the reported other OER catalysts. In addition, this work opens a new path for the development of highly efficient and innovative low-cost OER electrocatalysts able to replace Pt-based metals for energy conversion devices. [Display omitted] • NiCo-LDH@NCO was synthesized via a facile two-step method. • Synthesized electrocatalyst shows the lowest Tafel slope value (61 mVdec−1). • Synergistic effect of NiCo-LDH and NCO increases the OER activity. [ABSTRACT FROM AUTHOR]