Microwave-mediated synthesis for improved morphology and pseudocapacitance performance of nickel oxide

Influence of Annealing Temperature on the OER Activity of NiO(111) Nanosheets Prepared via Microwave and Solvothermal Synthesis Approaches

Earth-abundant transition metal oxides are promising alternatives to precious metal oxides as electrocatalysts for the oxygen evolution reaction (OER) and are intensively investigated for alkaline water electrolysis. OER electrocatalysis, like most other catalytic reactions, is surface-initiated, and the catalyst performance is fundamentally determined by the surface properties. Most transition metal oxide catalysts show OER activities that depend on the predominantly exposed crystal facets/surface structure. Therefore, the design of synthetic strategies to obtain the most active crystal facets is of significant research interest. In this work, rock salt NiO OER catalysts with (111) predominantly exposed facets were synthesized by a solvothermal (ST) method either heated under supercritical or microwave-assisted (MW) conditions. Particular emphasis was placed on the influence of the post annealing temperature on the structural configuration and OER activity to compare their catalytic performances. The as-prepared electrocatalysts are pure α-Ni hydroxides which were converted to rock salt NiO (111) nanosheets with hexagonal pores after heat treatment at different temperatures. The OER activity of the electrodes has been evaluated in 0.1 M KOH using geometric and intrinsic current densities via normalization by the disk area and BET area, respectively. The lowest overpotential at a geometric current density of 10 mA/cm2 is found for samples pretreated by heating between 400 and 500 °C with a catalyst loading of 115 μg/cm2. Despite the very similar nature of the catalysts obtained from the two methods, the ST electrodes show a higher geometric and intrinsic current density for 500 °C pretreatment. The MW electrodes, however, achieve an optimal geometric current density for 400 °C pretreatment, while their intrinsic current density requires pretreatment over 600 °C. Interestingly, pretreated electrodes show consistently higher OER activity as compared to the poorly crystalline/less ordered hydroxide as-prepared electrocatalysts. Thus, our study highlights the importance of the synthesis method and pretreatment at an optimal temperature.

Role of synthetic process parameters of nano-sized cobalt/nickel oxide in controlling their structural characteristics and electrochemical energy performance as supercapacitor electrodes

The synthesis of nano-sized bimetallic Cobalt/Nickel oxides (Ni1.5Co1.5O4) with a 1:1 Co/Ni atomic ratio has been achieved using a surfactant-free co-precipitation/hydrothermal process. The growth mechanism of Cobalt/Nickel oxides Ni1.5Co1.5O4 is elucidated by tuning the synthesis process parameters, including co-precipitation pH and hydrothermal time. The formation of Cobalt/Nickel oxides Ni1.5Co1.5O4 oxide began with the nucleation of cobalt nickel hydroxide nanoplates through the co-precipitation process, followed by dissolution-recrystallization, stacked hexagonal nano-flakes, and a flower-like microstructure. The electrochemical performances of the oxides were evaluated, with the largest surface area observed at pH 9 being the main factor for the best super-capacitive performance. As hydrothermal time increased, the structural directing growth forward, resulted in the formation of a nano-flower structure with a larger surface area. The as-prepared cobalt nickel oxide exhibited a maximum specific capacitance value of 525.5 F g-1 at a current density of 1 A g-1 and energy and power densities of 88.2 WhKg-1 and 606 WKg-1, respectively.

Graphitic Carbon Nitride-Induced Multifold Enhancement in Electrochemical Charge Storage of CoS-NiCo2S4 for All-Solid-State Hybrid Supercapacitors

In order to improve the electro-microstructural physiognomics of electrode materials for applications in better efficiency supercapacitors, herein graphitic carbon nitride (GCN)-heterostructurized CoS-NiCo₂S₄ is designed using a controlled material growth synthesis procedure. The developed CoS-NiCo₂S₄/GCN possesses ample hydrophilicity, possible charge transfer between GCN and CoS-NiCo₂S₄, uniform phase distribution, and distinctive microstructural characteristics. The preliminary electrochemical studies in the three-electrode setup show GCN-induced lower charge transfer resistance and very unique Warburg profile corresponding to extremely low diffusion resistance in CoS-NiCo₂S₄/GCN as compared to pristine CoS-NiCo₂S₄. Furthermore, GCN is found to significantly induce surface-controlled (capacitive-type) charge storage and frequency-independent specific capacitance up to 10 Hz in CoS-NiCo₂S₄. Furthermore, the CoS-NiCo₂S₄||N-rGO and CoS-NiCo₂S₄/GCN||N-rGO all-solid-state hybrid supercapacitor (ASSHSC) devices were fabricated using N-rGO as the negative electrode material, and the inducing effect of GCN on the supercapacitive charge storage performance of the devices is thoroughly studied. Results demonstrate that the mass specific capacitance and areal capacitance of CoS-NiCo₂S₄/GCN||N-rGO are ∼2 and ∼4 times more than those of the CoS-NiCo₂S₄||N-rGO ASSHSC device, respectively. Furthermore, the CoS-NiCo₂S₄/GCN||N-rGO offers more energy density, rate energy density, and additional charge-discharge durability (over ∼10,000 cycles) than the CoS-NiCo₂S₄||N-rGO ASSHSC device. The multifold performance improvement of CoS-NiCo₂S₄ with GCN heterostructurization is ascribed to GCN-induced supplemented porosity and pore widening, ionic nonstoichiometry (Ni^2±δ, Co^2±δ, and Co^3±δ), wettability, integrated enhancement in the conductivity, and electroactive-ion accessibility in the CoS-NiCo₂S₄/GCN heterocomposite. The present study offers vital physicoelectrochemical insights toward the future development of low cost and high-performance electrode materials, and their implementation in high-rate and operationally stable all-solid-state hybrid supercapacitor devices, for application in the next-generation front-line technologies.

An Overview of the Emerging Technologies and Composite Materials for Supercapacitors in Energy Storage Applications

Energy storage is one of the challenges currently confronting the energy sector. However, the invention of supercapacitors has transformed the sector. This modern technology's high energy capacity, reliable supply with minimal lag time, and extended lifetime of supercapacitors have piqued the interest of scientists, and several investigations have been conducted to improve their development. However, there is room for improvement. Consequently, this review presents an up-to-date investigation of different supercapacitor technologies' components, operating techniques, potential applications, technical difficulties, benefits, and drawbacks. In addition, it thoroughly highlights the active materials used to produce supercapacitors. The significance of incorporating every component (electrode and electrolyte), their synthesis approach, and their electrochemical characteristics are outlined. The research further examines supercapacitors' potential in the next era of energy technology. Finally, concerns and new research prospects in hybrid supercapacitor-based energy applications that are envisaged to result in the development of ground-breaking devices, are highlighted.

Hierarchical Nickel-Cobalt Hydroxide Composite Nanosheets-Incorporated Nitrogen-Doped Carbon Nanotubes Embedded with Nickel-Cobalt Alloy Nanoparticles for Driving a 2 V Asymmetric Supercapacitor

A class of electrode materials with favorable structures and compositions and powerful electrochemical (EC) properties are needed to boost the supercapacitor capacity significantly. In this study, an inventive technique was established to produce a well-aligned nickel-cobalt alloy nanoparticles-encapsulated N-doped carbon nanotubes with porous structure and good conductivity on carbon cloth (NiCo@NCNTs/CC) as a substrate. Then, nanosheets of nickel-cobalt layered double hydroxide (NiCo-LDH) were grown on NiCo@NCNTs/CC via a simple EC deposition method to construct a self-supported monolithic hierarchical nanosheets/nanotubes composite electrode of NiCo-LDH/NiCo@NCNTs/CC. In such a composite electrode, the NiCo@NCNTs can act as a good conductor and structural scaffold to grow NiCo-LDH nanosheets with a three-dimensional open and porous structure, which helps to improve the electron/ion-transfer performance, increase the number of exposed reactive sites, and inhibit the aggregation of NiCo-LDH nanosheets, thereby boosting the capacitance and stability. As a positive electrode, the NiCo-LDH/NiCo@NCNTs/CC hierarchical nanosheets/nanotubes electrode displays 1898 mF cm^-2 (1262 A g^-1) of high capacitance, long-term stability with a capacitance retention of around 100% after 8000 cycles, and nearly 103% Coulombic efficiency. After assembling into an asymmetric supercapacitor with a Co(OH)₂/NiCo@NCNTs/CC negative electrode, 2 V of operating voltage with 73.1 μW h cm^-2 (52.8 W h kg^-1) of energy density was achieved. Our investigation gives a potential approach for constructing the integrated composite electrode of transition-metal compounds-carbon materials for high-performance supercapacitors.

Influence of the Molar Ratio of Co and V in Bimetallic Oxides on Their Pseudocapacitive Properties

Bimetallic oxides have significant attraction as supercapacitor electrode materials due to their highly reversible redox processes, which are commonly associated with their surface chemistry and morphological features. Here, we report the synthesis, characterization, and electrochemical evaluation of bimetallic oxides with different molar compositions of Co and V (Co_0.6V_0.4, Co_0.64V_0.36, Co_0.68V_0.32, and Co_0.7V_0.3 denoted as S1, S2, S3, and S4 samples, respectively). The materials were synthesized by a modified solvothermal method using glycerol as a stabilizing agent, characterized by X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, scanning electron microscopy-energy-dispersive X-ray spectroscopy, X-ray fluorescence spectroscopy, N₂ adsorption isotherms, cyclic voltammetry, and galvanostatic charged/discharged in a three-electrode cell. The role of the CoV oxide compositions on the pseudocapacitive properties was studied through the analysis of the energy storage mechanism following the power law and Dunn's methodology to obtain the b values. An important finding of this work is that CoV oxides exhibited electrochemical characteristics of a pseudocapacitive electrode material even though the charge storage occurs in bulk. This behavior is consistent with the pseudocapacitance generated by redox processes, showing b values of 0.67, 0.53, 0.75, and 0.84, with a capacitive current contribution of 74, 74, 63, and 70% analyzed at a scan rate of 1 mV s^-1, for S4, S3, S2, and S1 samples, respectively. Co_0.7V_0.3 (S4) oxide presented the highest specific capacitance of 299 F g^-1 at 0.5 A g^-1 with a Coulombic efficiency of 93% tested at 4 A g^-1. The better electrochemical performance of this sample was attributed to the synergistic effect of the Co and V atoms since a minimum amount of V in the structure may distort the crystal lattice and improve the electrolyte diffusion, in addition to the formation of several oxidation states due to reduction of V⁵⁺, including V³⁺ and V⁴⁺ as well as to the formation of the metastable V₄O₉.

Critical Aspects of Various Techniques for Synthesizing Metal Oxides and Fabricating Their Composite-Based Supercapacitor Electrodes: A Review

Supercapacitors (SCs) have attracted attention as an important energy source for various applications owing to their high power outputs and outstanding energy densities. The electrochemical performance of an SC device is predominantly determined by electrode materials, and thus, the selection and synthesis of the materials are crucial. Metal oxides (MOs) and their composites are the most widely used pseudocapacitive SC electrode materials. The basic requirements for fabricating high-performance SC electrodes include synthesizing and/or chemically modifying unique conducting nanostructures, optimizing a heterostructure morphology, and generating large-surface-area electroactive sites, all of which predominantly rely on various techniques used for synthesizing MO materials and fabricating MO- and MO-composite-based SC electrodes. Therefore, an SC’s background and critical aspects, the challenges associated with the predominant synthesis techniques (including hydrothermal and microwave-assisted syntheses and chemical-bath and atomic-layer depositions), and resulting electrode electrochemical performances should be summarized in a convenient, accessible report to accelerate the development of materials for industrial SC applications. Therefore, we reviewed the most pertinent studies on these synthesis techniques to provide insight into the most recent advances in synthesizing MOs and fabricating their composite-based SC electrodes as well as to propose research directions for developing MO-based electrodes for applications to next-generation SCs.

Efficient Oxidation of Cyclohexane over Bulk Nickel Oxide under Mild Conditions

Nickel oxide powder was prepared by simple calcination of nickel nitrate hexahydrate at 500 °C for 5 h and used as a catalyst for the oxidation of cyclohexane to produce the cyclohexanone and cyclohexanol—KA oil. Molecular oxygen (O₂), hydrogen peroxide (H₂O₂), t-butyl hydrogen peroxide (TBHP) and meta-chloroperoxybenzoic acid (m-CPBA) were evaluated as oxidizing agents under different conditions. m-CPBA exhibited higher catalytic activity compared to other oxidants. Using 1.5 equivalent of m-CPBA as an oxygen donor agent for 24 h at 70 °C, in acetonitrile as a solvent, NiO powder showed exceptional catalytic activity for the oxidation of cyclohexane to produce KA oil. Compared to different catalytic systems reported in the literature, for the first time, about 85% of cyclohexane was converted to products, with 99% KA oil selectivity, including around 87% and 13% selectivity toward cyclohexanone and cyclohexanol, respectively. The reusability of NiO catalyst was also investigated. During four successive cycles, the conversion of cyclohexane and the selectivity toward cyclohexanone were decreased progressively to 63% and 60%, respectively, while the selectivity toward cyclohexanol was increased gradually to 40%.

Ribbon-like Nickel Cobaltite with Layer-by-Layer-Assembled Ordered Nanocrystallites for Next-Generation All-Solid-State Hybrid Supercapatteries

In the context to develop ultra-efficient electrode materials with good physicoelectrochemical and electrostructural properties, for their application in high-performance supercapatteries, herein, a facile tartrate-mediated inhibited crystal growth method is reported to engineer thoroughly uniform ribbon-like nickel cobaltite (NiCo₂O₄) microstructure with unique layer-by-layer-assembled nanocrystallites. This material demonstrates significant kinetic reversibility, good rate efficiency and bulk diffusibility of the electroactive ions, and a predominant semi-infinite diffusion mechanism during the redox-based charge storage process. This material also shows bias-potential-independent equivalent series resistance, very low charge-transfer resistance, and diagonal Warburg profile, corresponding to the ion diffusion occurring during the electrochemical processes in supercapacitors and batteries. Further, the fabricated NiCo₂O₄-based all-solid-state supercapattery (NiCo₂O₄||N-rGO) delivers excellent rate-specific capacity, very low internal resistance, good electrochemical and electrostructural stability (∼94% capacity retention after 10,000 charge-discharge cycles), energy density (31 W h kg^-1) of a typical rechargeable battery, and power density (13,003 W kg^-1) of an ultra-supercapacitor. The ultimate performance of the supercapattery is ascribed to low-dimensional crystallites, ordered inter-crystallite and channel-type bulk and boundary porosity, multiple reactive equivalents, enhanced electronic conductivity, and "ion buffering pool" like behavior of ribbon-like NiCo₂O₄, supplemented with enhanced electronic and ionic conductivities of N-doped rGO (negative electrode) and PVA/KOH gel (electrolyte separator), respectively.

Stoichiometry-grain size-specific capacitance interrelationships in nickel oxide

Nickel oxide exhibits almost the highest theoretical specific capacitance (C s), which includes contributions from non-faradaic double layer charging and faradaic OH- adsorption. However, the realistic and tangible C s is due to the faradaic process, which can be influenced by chemical (i.e. stoichiometry) and structural (i.e. grain size) changes. Hence, it is necessary to investigate the interrelationships among chemical and structural features and charge storage capacity. Here, a non-stoichiometric nickel oxide (Ni x O) containing Ni2+ and Ni3+ was synthesized by a sol-gel method at 620, 720 and 920 °C using Ni(NO3)2·6H2O and citric acid. The grain size as estimated from X-ray diffraction increases from 55 to 194 nm with increase in the synthesis temperature. The stoichiometry measured through Ni2+ (or Ni3+) fraction from X-ray photoelectron spectroscopy also increases from 70.3 to 99.2 atom% with synthesis temperature. The C s due to faradaic OH- adsorption was estimated from cyclic voltammetry in 2 M KOH within -0.05 to +0.60 V vs. Hg/Hg2Cl2/KCl (sat. in water). This C s increases from 7.5 to 92.4 F g-1 with a decrease in the grain size and stoichiometry (increase in Ni3+) due to possibly the increased conductivity and NiOOH formation through OH- adsorption. The deviation from stoichiometry at lower grain size mainly stems from nickel vacancy accommodation, according to the thermodynamic model proposed here. The interrelationships among stoichiometry, grain size and the specific capacitance of nickel oxide are investigated.

Electrochemical charge storage performance of mesoporous MoO3@Co3O4nanocomposites as electrode materials

Constructing a novel nanocomposite structure based on Co₃O₄is of the current interest to design and develop efficient electrochemical capacitors. The capacitive performance of MoO₃@Co₃O₄nanocomposite is compared with pristine Co₃O₄nanoparticles, both of them being synthesized by hydrothermal technique. A BET surface area of ∼41 m²g^-1(almost twice that of Co₃O₄) and average pore size of 3.6 nm is found to be suitable for promoting Faradaic reactions in the nanocomposite. Electrochemical measurements conducted on both samples predict capacitive behavior with quasi-reversible redox reactions. MoO₃@Co₃O₄nanocomposite is capable of delivering a superior specific capacitance of 1248 F g^-1at 0.5 A g^-1along with notable stability of 92% even after 2000 cycles of charge-discharge and Coulombic efficiency approaching 100% at 10 A g^-1. The outstanding results obtained in this work assure functional adequacy of MoO₃@Co₃O₄nanocomposite in fabricating high-performance electrochemical capacitors.

Core-shell Ni1.5Sn@Ni(OH)2 nanoflowers as battery-type supercapacitor electrodes with high rate and capacitance

Poor conductivity and aggregation of two-dimensional Ni(OH)₂ nanosheets hinder their extensive applications in supercapacitors. In the current study, a core-shell nanoflower composite is successfully synthesized using a high conductivity Ni_1.5Sn alloy and Ni(OH)₂ nanosheets via a facile two-step hydrothermal reaction. The alloy material enhances the conductivity of the sample and promotes electron transport for Ni(OH)₂. The as-prepared core-shell structure effectively restrains the clustering of nanosheets and improves the specific surface area of active materials. The optimized NS@NL-3 displays an outstanding specific capacitance (1002.2C g^-1 at 1 A g^-1) and satisfactory capacitance retention rate (80.63% at 20 A g^-1) by adjusting the coating amount of Ni(OH)₂ nanosheets, which is significantly higher compared with the performance of pure Ni(OH)₂ (609.6C g^-1 at 1 A g^-1 and 55.64% at 20 A g^-1). The all-solid-state hybrid supercapacitor (HSC) is fabricated with activated carbon (AC) as the negative electrode and NS@NL-3 as the positive electrode, which shows a high energy density of 57.4 Wh kg^-1 at 803.6 W kg^-1 as well as a superior cycling stability (88.45 % after 10,000 cycles). Experiment shows that 42 LEDs are effortlessly lit by two series-wound solid-state HSC devices, which indicates its high potential for practical applications.

Microwave-Enabled Size Control of Iron Oxide Nanoparticles on Reduced Graphene Oxide

Nanoparticle-functionalized 2D material networks are promising for a wide range of applications, but in situ formation of nanoparticles is commonly challenged by rapid growth. Here, we demonstrate controlled synthesis of small and dispersed iron oxide nanoparticles on reduced graphene oxide (rGO) networks through rapid localized heating with microwaves with low-cost iron nitrate as the precursor. The strong coupling of the microwave radiation with the rGO network rapidly heats the network locally to decompose the iron nitrate and generate iron oxide nanoparticles, while cessation of microwaves leads to rapid cooling to minimize crystal growth. Small changes in the microwave reaction time (<1 min) led to very large changes in the iron oxide morphology. The solid-state microwave syntheses produced narrower nanoparticle size distribution than conventional heating. These results illustrate the potential of solid-state microwave syntheses to control the nanoparticle size on 2D materials through rapid localized heating under the microwave process conditions, which should be extendable to a variety of transition metal oxide-rGO systems.

An enhancement on supercapacitor properties of porous CoO nanowire arrays by microwave-assisted regulation of the precursor

A microwave-assisted hydrothermal approach with a follow up thermal treatment was employed to prepare 1D porous CoO nanowires, which is constructed by numerous high crystallinity nanoparticles. A significant change in crystal structure of the precursor were observed, as position shift and absence of some diffraction peaks, which was induced by the microwave-assistance during hydrothermal process. Moreover, the precursor's purity was also effectively improved. As a result, the as-synthesized CoO annealed from the microwave-assisted precursor exhibited a morphology and phase structure significantly different from that of without microwave involvement. Benefiting from the 'microwave effect', the microwave-assisted as-fabricated porous CoO nanowires showed an enhanced specific capacitance (728.8 versus 503.7 F g^-1 at 1 A g^-1 ), strengthened rate performance (70.0% versus 53.2% maintenance at 15 A g^-1), reduced charge transfer resistance (1.06 Ω versus 2.39 Ω), enlarged window voltage (0.85 versus 0.7 V) and enhanced cycle performance (82.3% versus 76.5% retention after 5000 cycles at 15 A g^-1), compared with that of sample without microwave assistance. In addition, the corresponding electrochemical properties are also higher than those reported CoO sample prepared by solvothermal method. In conclusion, this work provides a practical way for enhancing electrochemical properties of supercapacitor materials through adjusting the precursor by microwave assistance into hydrothermal process.

Cost-Effective Synthesis of Efficient CoWO4/Ni Nanocomposite Electrode Material for Supercapacitor Applications

In the present study, the synthesis of CoWO4 (CWO)-Ni nanocomposites was conducted using a wet chemical method. The crystalline phases and morphologies of the Ni nanoparticles, CWO, and CWO-Ni composites were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDAX). The electrochemical properties of CWO and CWO-Ni composite electrode materials were assessed by cyclic voltammetry (CV), and galvanostatic charge-discharge (GCD) tests using KOH as a supporting electrolyte. Among the CWO-Ni composites containing different amounts of Ni1, Ni2, and Ni3, CWO-Ni3 exhibited the highest specific capacitance of 271 F g-1 at 1 A g-1, which was greater than that of bare CWO (128 F g-1). Moreover, the CWO-Ni3 composite electrode material displayed excellent reversible cyclic stability and maintained 86.4% of its initial capacitance after 1500 discharge cycles. The results obtained herein demonstrate that the prepared CWO-Ni3 nanocomposite is a promising electrode candidate for supercapacitor applications.

MoO3 thin layers on NiCo2S4 substrate for efficient electrochemical charge storage

Ternary oxides/sulfides have long been investigated as promising electrode materials for charge storage applications. However, it is important to rationally design nanostructured hybrid composites for superior charge storage performance as electrodes in devices. In this work, MoO₃@NiCo₂S₄ hybrid composites materials are synthesized by the hydrothermal method followed by annealing at different temperatures. The charge storage properties of these materials are tested by cyclic voltammetry, galvanostatic charge-discharge curves and electrochemical impedance spectroscopy. It is found that the structure of the hybrid composite material not only assists electron and charge transportation but also precisely control the volume expansion during redox reactions, contributing to superior electrochemical behavior. Among all the electrodes, the electrode fabricated with MoO₃@NiCo₂S₄ composite material annealed at 400 °C (MoO₃@NiCo₂S₄-400) is the best for charge storage applications. At 400 °C, MoO₃ spreads as a thin layer of surface polymeric molybdates on NiCo₂S₄ as seen in the XRD pattern. Significantly, it delivers the highest capacitance of 1622 F g^-1 at 1 A g^-1 in 2 M aqueous KOH electrolyte compared to other hybrid composite electrodes, NiCo₂S₄ (962 F g^-1), MoO₃@NiCo₂S₄-500 (1412 F g^-1) and MoO₃@NiCo₂S₄-600 (970 F g^-1), under the same measurement conditions. Furthermore, the MoO₃@NiCo₂S₄-400 hybrid electrode shows better cyclic stability with 93% capacitance retention after 3000 charge-discharge cycles at 8 A g^-1. The synergistic effect of two components and annealing temperature plays important role in enhancing the charge storage performance. This work shows the importance of the synthesis temperature on the functional character of ternary sulfide/oxide composite materials for charge storage applications.

Organic template-based ZnO embedded Mn3O4 nanoparticles: synthesis and evaluation of their electrochemical properties towards clean energy generation

To deal with fossil fuel depletion and the rise in global temperatures caused by fossil fuels, cheap and abundant materials are required, in order to fulfill energy demand by developing high-performance fuel cells and electrocatalysts. In this work, a natural organic agent has been used to synthesize nano-structured ZnO/Mn₃O₄ with high surface area and enhanced electrocatalytic performance. Upon pre-annealing treatment, mixed metal oxide precipitates are formed due to the complex formation between a metal oxide and organic extract. The thermally annealed mixed oxide ZnO/Mn₃O₄ was characterized by XRD diffractometer, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX). Gas chromatography-mass spectrometry (GC-MS) identified methyldecylamine as a major stabilizing agent of the synthesized nanomaterial. Using a Tauc plot, the calculated band energy for the synthesized ZnO/Mn₃O₄ mixed metal oxide was 1.65 eV. Moreover, we have demonstrated the effects of incorporated organic compounds on the surface chemistry, morphology and electrochemical behavior of ZnO/Mn₃O₄. The phyto-functionalized ZnO/Mn₃O₄ was deposited on Ni-foam for electrocatalytic studies. The fabricated electrode revealed good performance with low over-potential and Tafel slope, suggesting it to be suitable as a potential catalyst for water splitting application, in particular for the oxygen evolution reaction (OER). The overall findings of the current study provide a cost-effective and efficient organic template for functionalization and sustainable fabrication of ZnO/Mn₃O₄ nanomaterial for application as an electrocatalyst.

To deal with fossil fuel depletion and the rise in global temperatures caused by fossil fuels, cheap and abundant materials are required, in order to fulfill energy demand by developing high-performance fuel cells and electrocatalysts.

Three-Dimensional Core-Branch α-Fe2O3@NiO/Carbon Cloth Heterostructured Electrodes for Flexible Supercapacitors

A convenient and scalable hydrothermal method was developed for the fabrication of the core-branch Fe₂O₃@NiO nanorods arrays directly grown on flexible carbon cloth (denoted as Fe₂O₃@NiO/CC). Such a unique architecture was applied as an electrode of the supercapacitors. As a result, the Fe₂O₃@NiO/CC exhibited a high areal capacitance ~800 mF cm^-2 at 10 mA cm^-2, which was about 10 times increase with respect to Fe₂O₃ nanorods array grown on carbon cloth (Fe₂O₃/CC). The Fe₂O₃@NiO/CC also had the long life cycle (96.8 % capacitance retention after 16,000 cycles) and remarkable rate capability (44.0 % capacitance loss at a very large current density of 100 mA cm^-2). The superior performance of the Fe₂O₃@NiO/CC should be ascribed to the reduction of the contact resistance and the free-standing structure of the flexible electrode. This study provides a novel strategy to construct high-performance flexible electrode materials with unique core-branch structure by incorporating two different pseudocapacitive materials.

Highly ordered mesoporous flower-like NiO nanoparticles: synthesis, characterization and photocatalytic performance

Nanostructured flower-like NiO microspheres with flower size ∼2–5 μm and nanoflake size ∼10–100 nm have been successfully synthesized with and without CTAB, and fully characterized, and their photocatalytic performance has been investigated.

Study of “Ni-doping” and “open-pore microstructure” as physico-electrochemical stimuli towards the electrocatalytic efficiency of Ni/NiO for the oxygen evolution reaction

Added “Ni-doping” and “open-pore microstructure” act as physico-electrochemical stimuli towards enhanced electrocatalytic efficiency and electromechanical stability of Ni/NiO for the low-overpotential oxygen evolution reaction in alkaline medium.

High-performance spinel NiMn2O4 microspheres self-assembled with nanosheets by microwave-assisted synthesis for supercapacitors

Spinel NiMn₂O₄ microspheres self-assembled with nanosheets directly grown on a 3D nickel foam were successfully prepared by a facile microwave-assisted hydrothermal process.

A high-performance histidine-functionalized MWCNT-GONR/Co–Ni LDH flower cluster structural composite via a microwave synthesis for supercapacitors

A flower cluster structural histidine-functionalized multi-walled carbon nanotube-graphene oxide nanoribbon/Co–Ni LDH (His-MW/LDH) composite was synthesized via the microwave method.