Pure NiCo2O4 Research Articles

Importing Atomic Rare‐Earth Sites to Activate Lattice Oxygen of Spinel Oxides for Electrocatalytic Oxygen Evolution

Spinel oxides have emerged as highly active catalysts for the oxygen evolution reaction (OER). However, due to covalency competition, the OER process on spinel oxides often follows an arduous adsorbate evolution mechanism (AEM) pathway. Herein, we propose a novel rare‐earth sites substitution strategy to tune the lattice oxygen redox of spinel oxides and bypass the AEM scaling relationship limitation. Taking NiCo2O4 as a model, the incorporation of Ce into the octahedral site induces the formation of Ce‐O‐M (M: Ni, Co) bridge, which triggers charge redistribution in NiCo2O4. The developed Ce‐NiCo2O4 exhibits remarkable OER activity with a low overpotential, satisfactory electrochemical stability, and good practicability in anion‐exchange membrane water electrolyzer. Theoretical analyses reveal that OER on Ce‐NiCo2O4 surface follows a more favorable lattice oxygen mechanism (LOM) pathway and non‐concerted proton‐electron transfers compared to pure NiCo2O4, as further verified by pH‐dependent behavior and in situ Raman analysis. 18O‐labeled electrochemical mass spectrometry directly demonstrates that oxygen originates from the lattice oxygen of Ce‐NiCo2O4 during OER. It is discovered that electron delocalization of Ce 4f states triggers charge redistribution in NiCo2O4 through the Ce‐O‐M bridge, favoring antibonding state occupation of Ni‐O bonding in [Ce‐O‐Ni] site, thereby activating lattice oxygen redox of NiCo2O4 in OER.

Importing Atomic Rare-Earth Sites to Activate Lattice Oxygen of Spinel Oxides for Electrocatalytic Oxygen Evolution.

Spinel oxides have emerged as highly active catalysts for the oxygen evolution reaction (OER). However, due to covalency competition, the OER process on spinel oxides often follows an arduous adsorbate evolution mechanism (AEM) pathway. Herein, we propose a novel rare-earth sites substitution strategy to tune the lattice oxygen redox of spinel oxides and bypass the AEM scaling relationship limitation. Taking NiCo2O4 as a model, the incorporation of Ce into the octahedral site induces the formation of Ce-O-M (M: Ni, Co) bridge, which triggers charge redistribution in NiCo2O4. The developed Ce-NiCo2O4 exhibits remarkable OER activity with a low overpotential, satisfactory electrochemical stability, and good practicability in anion-exchange membrane water electrolyzer. Theoretical analyses reveal that OER on Ce-NiCo2O4 surface follows a more favorable lattice oxygen mechanism (LOM) pathway and non-concerted proton-electron transfers compared to pure NiCo2O4, as further verified by pH-dependent behavior and in situ Raman analysis.18O-labeled electrochemical mass spectrometry directly demonstrates that oxygen originates from the lattice oxygen of Ce-NiCo2O4 during OER. It is discovered that electron delocalization of Ce 4f states triggers charge redistribution in NiCo2O4 through the Ce-O-M bridge, favoring antibonding state occupation of Ni-O bonding in [Ce-O-Ni] site, thereby activating lattice oxygen redox of NiCo2O4 in OER.

Structural, morphological, optical, and magnetic properties of NiO-added NiCo2O4 electrode materials synthesized by sol-gel, Co-precipitation and ultrasonication methods and their electrochemical supercapacitor response

In this study, NiO-added NiCo2O4 composite samples were prepared using three distinct synthesis methods to study the impact of synthesis technique and NiO addition on the electrochemical performance of NiCo2O4. 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 NiCo2O4. 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 NiCo2O4. Adding NiO phase to the NiCo2O4 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.

Ultrasonic-Assisted Electrodeposition of Mn-Doped NiCo2O4 for Enhanced Photodegradation of Methyl Red, Hydrogen Production, and Supercapacitor Applications

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.

Nanoflower-like NiCo2O4 Composite Graphene Oxide as a Bifunctional Catalyst for Zinc-Air Battery Cathode.

Developing efficient bifunctional catalysts for nonprecious metal-based oxygen reduction (ORR) and oxygen evolution (OER) is crucial to enhance the practical application of zinc-air batteries. The study harnessed electrostatic forces to anchor the nanoflower-like NiCo2O4 onto graphene oxide, mitigating the poor inherent conductivity in NiCo2O4 as a transition metal oxide and preventing excessive agglomeration of the nanoflower-like structures during catalysis. Consequently, the resulting composite, NiCo2O4-GO/C, exhibited notably superior ORR and OER catalytic performance compared to pure nanoflower-like NiCo2O4. Notably, it excelled in OER catalytic activity of the OER relative to the precious metal RuO2. As a bifunctional catalyst for ORR and OER, NiCo2O4-GO/C displayed a potential difference of 0.88 V between the ORR half-wave potential and the OER potential at 10 mA·cm-2, significantly lower than the 1.08 V observed for pure flower-like NiCo2O4 and comparable to the 0.88 V exhibited by precious metal catalysts Pt/C + RuO2. The NiCo2O4-GO/C-based zinc-air battery demonstrated a discharge capacity of 817.3 mA h·g-1, surpassing that of precious metal-based zinc-air batteries. Moreover, charge-discharge cycling tests indicated the superior stability of the NiCo2O4-GO/C-based zinc-air battery compared to its precious metal-based counterparts.

Core-shell-structured NiCo2O4 composite with GO/C to enhance its electrocatalytic performance as a cathode catalyst in zinc-air batteries

This study employed a simple and effective method to synthesize a composite material comprising core-shell NiCo2O4 and GO/C. The combination of core-shell NiCo2O4 and GO effectively improved the poor conductivity of NiCo2O4 and prevented the aggregation of NiCo2O4 microspheres. In electrochemical tests, compared to pure core-shell NiCo2O4, NiCo2O4@GO/C exhibits improved catalytic performance in both oxygen reduction (ORR) and oxygen evolution reactions (OER). Its OER catalytic performance is notably superior to that of noble metal catalysts. The NiCo2O4@GO/C-based liquid-state zinc-air battery exhibited a stable cycling efficiency of 51% over 200 h of charge-discharge cycles, significantly outperforming precious metal-based batteries. Additionally, a flexible quasi-solid-state zinc-air battery, assembled with the NiCo2O4@GO/C composite material as the cathode catalyst, exhibited an outstanding cycling efficiency of 58% in charge-discharge cycles. These results indicate that the NiCo2O4@GO/C composite material holds great potential as a bifunctional electrocatalytic catalyst for zinc-air batteries.

Construction of a highly reactive Zn/NiCo2O4 surface and analysis of its antimicrobial properties

By incorporating Zn into NiCo2O4, a rod-shaped Zn/NiCo2O4 nanocomposite with an increased number of active sites on its surface was constructed in this study. The antimicrobial activity of this material against Escherichia coli was investigated, revealing a significant improvement compared to pure NiCo2O4 under both dark conditions and light irradiation, with an increase in the antimicrobial rate from 45.71 % to 99.06 % and 51.48 %–88.13 % at a dosage of 0.1 mg/mL, respectively. This enhancement can be attributed to the increased formation of oxygen vacancies on the material surface after Zn doping. These vacancies lead to greater exposure of high-valence metal active sites and enhanced redox reactions. Additionally, introducing defects improves the energy level distribution, resulting in a stronger reduction ability at the conduction band position. Consequently, the generation of reactive oxygen species increases, enhancing both contact sterilization and photocatalytic sterilization. Furthermore, Zn/NiCo2O4 demonstrated excellent cyclic stability, maintaining an antibacterial rate of up to 95.13 % after five cycles of antibacterial activity. In conclusion, the use of this material is not restricted to light conditions, and the addition of Zn greatly enhances its antibacterial effectiveness. This study demonstrates that Zn/NiCo2O4 has significant potential for applications in the field of antibacterial materials.

Highly selective xylene gas detection based on NiCo2O4 multilayer hexagonal structures

Xylene is a flammable, toxic and harmful inert gas. Achieving highly selective detection of xylene gas is essential for protecting the environment and health, but is full of challenges. Herein, NiCo2O4 with multilayer hexagonal structure has been synthesized through a combination strategy of hydrothermal and annealing, and is applied to detect xylene gas. The effect of composition, surface area, crystallinity and porous degree on gas sensing performance has also been investigated. NiCo2O4 multilayer hexagonal structures annealed at 400 °C (NCO MHs-400) exhibit a better gas sensing selectivity toward xylene gas compared to NiCo2O4 multilayer hexagonal structures annealed at 500 °C and at 600 °C (NCO MHs-500 and NCO MHs-600), which predominantly originates from the catalytic oxidation of cobalt component to xylene in the as-prepared pure phase NiCo2O4. Furthermore, NCO MHs-400 presents a superior stability and reliability, well moisture resistance, and a detection limit of 1 ppm toward xylene at a working temperature of 170 °C.

A Facile Synthetic Approach for Zn2TiO4@NiCo2O4 Core‐Shell Nanoparticles Using Layered Double Hydroxide Precursors and Studies on Their Peroxidase‐Like Activity

AbstractIn the present work, a facile synthetic approach has been developed for Zn2TiO4@NiCo2O4 core‐shell nanoparticles (NPs) using layered double hydroxide (LDH) as precursors. The Zn2TiO4 microspheres (ZTS) constitute the core and the shell consists of NiCo2O4 nanoparticles. X‐ray diffraction (XRD) results confirm presence of both Zn2TiO4 and NiCo2O4 phases in Zn2TiO4@NiCo2O4 core‐shell nanoparticles. Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) analyses indicate formation of hierarchical porous shell structure of NiCo2O4 with varying thickness (132±20 nm to 171±35 nm) on the Zn2TiO4 spheres. Optical studies indicate bandgap of ~2.2 eV corresponding to NiCo2O4 nanoparticles in the Zn2TiO4@NiCo2O4 core‐shell nanoparticles. The Zn2TiO4@NiCo2O4 core‐shell nanoparticles possess high surface area (124 m2/g to 160 m2/g) as measured from Brunauer‐Emmett‐Teller (BET) analysis. Magnetic studies of Zn2TiO4@NiCo2O4 core‐shell nanoparticles indicate soft ferromagnetic and hard ferromagnetic behavior at 300 K and 5 K, respectively. The Zn2TiO4@NiCo2O4 core‐shell nanoparticles act as nanozyme and they exhibit better peroxidase‐like activity as compared to pure NiCo2O4 NPs and natural horseradish peroxidase (HRP).

Grapefruit juice containing rich hydroxyl and oxygenated groups capable of transforming 1D structure of NiCo2O4 into 0D with excessive surface vacancies for promising energy conversion and storage applications

Fabrication of an efficient electrode material is highly demanded for obtaining high energy conversion electrochemical system. In this study, we describe a green and innovative approach of using grape fruit juice as a high carrier of reducing and surface modifying agents for manipulating the surface properties of nickel‑cobalt bimetallic oxide NiCo2O4 nanostructures. Different amounts of grape fruit juice were used to discern its role on the morphological and surface vacancies of NiCo2O4 nanostructures during oxygen evolution reaction (OER). Importantly, the physical characterization revealed the morphological transformation of NiCo2O4 nanostructures from 1D nanorods to short range 0D nanoparticles. Furthermore, the surface studies have shown that NiCo2O4 nanostructures prepared with grapefruit juice have high density of surface vacancies and higher amount of Ni2+ and a Co2+ metallic ions on the surface of NiCo2O4 compare to pure NiCo2O4 nanostructures. The OER of electrodes modified with NiCo2O4 nanostructures, particularly (sample-2, 2 mL of grape fruit juice) gave rise to an overpotential value of 260 mV at 10 mAcm−2 in 1.0 M KOH aqueous solution along with high durability/stability for 40 h. Additionally, the obtained electrochemical impedance spectroscopy value of 80 Ω and active surface area of 18.1 μFcm−2 attested for the excellent OER performance of NiCo2O4 nanostructures (sample-2). Eventually, the proven enhancement in surface vacancies, alternation in the mixed oxidation states of both Ni and Co together with the morphological transformation that have been accomplished via the green, simple, and low-cost strategy of using grape juice make it a suitable option for large scale production of wide range of multifunctional metal oxides and related nanostructured materials for plethora of energy conversion and storage applications.

NiCo2O4 with changed catalytic process and enhanced catalytic property as mimic enzyme by introducing Mo6 +

The modification of transition metal oxide by introducing with heteroatoms is a valid strategy to enhance artificial enzyme activities benefited from the change of the catalytic process and electronic properties. Herein, a simple hydrothermal method has been used to fabricate Mo-NiCo2O4 (MNCO) via introducing Mo6+ into NiCo2O4 (NCO). Compared with pure NiCo2O4, the acquired NiCo2O4 (MNCO) with the introduction of Mo exhibits better catalytic activity of peroxidase mimic enzyme. More interesting, only·O2- and h+ can be captured during catalytic process when MNCO as artificial enzyme, while·OH,·O2- and h+ can be found in the presence of NCO. Meanwhile, after the introduction of Mo, electron transfer has been drastically accelerated which contributed to the enhanced peroxidase-like activity. The Km value of H2O2 of NCO is 49-fold higher than that of MNCO, indicating that better affinity between MNCO and H2O2. And the LOD (detection limit) of MNCO is 22 times lower than that of NCO in the detection of H2O2. Based on excellent peroxidase-like active, a sensing platform for detecting hydroquinone in seawater is designed, confirming the great practicability in sensing fields.

Insights in Utilizing NiCo2O4/Co3O4 Nanowires as Anode Material in Lithium‐Ion Batteries

AbstractIn this study, a facile and cost‐effective hydrothermal approach is employed to synthesize a mesoporous NiCo2O4/Co3O4 nanocomposite with nanowire morphology by using polyvinyl pyrrolidone as structure‐directing agent. The obtained NiCo2O4/Co3O4 nanocomposite shows better electrochemical performance than pure NiCo2O4 due to mainly two reasons: i) a strong synergistic effect between NiCo2O4 and Co3O4, which enhances the Li+ diffusion rate as well as lower the charge‐transfer resistance, and ii) the involvement of Co3O4 to contribute to the total capacity due to its high electrochemical activity. However, the performance of a NiCo2O4/Co3O4 nanocomposite electrode starts degrading after 400 cycles while pure NiCo2O4 maintains steady performance. Since the NiCo2O4/Co3O4 nanocomposite sample shows high porosity, it is believed that the obtained nanowire morphology cannot tolerate volume variations, which are generally triggered off during repeated Li+ (de‐)insertion at long‐term cycling. Therefore, the obtained results bring new insights in terms that there is a sweet spot between Li+ diffusion and high porosity in utilizing Co3O4 within a nanocomposite. This study is of guidance to shed the light on the research of ternary transition metal oxide nanocomposite materials for lithium‐ion batteries.

Ternary metal oxide nanorods (Ni0.5Cu0.5Co2O4) as efficient positive materials for non-enzymatic glucose sensing and fuel cell application

Ternary metal oxide nanorods Ni0·5Cu0·5Co2O4 with high activities for glucose oxidation were prepared through a simple hydrothermal process. The as prepared Ni0.5Cu0.5Co2O4 NRs with uniform aspect ratio were composed of many nanoparticles with diameter less than 10 nm Ni0.5Cu0.5Co2O4 NRs modified glassy carbon electrodes displayed highly improved non-enzymatic glucose sensing (NEGS) performance compared to pure phase NiCo2O4 NRs and CuCo2O4 NRs. The sensitivity of Ni0.5Cu0.5Co2O4 NRs for glucose detection in the lower (5–3000 μM) and higher regional (3000–7000 μM) is 859.31 μA mM−1 cm−2 and 528.81 μA⋅mM−1⋅cm−2, respectively. The limit of detection (LOD) was estimated to be 1.67 μM (S/N = 3). Moreover, the direct glucose fuel cells (DGFCs) equipped with Ni0.5Cu0.5Co2O4 NRs anode also presented much higher power density (413.7 μW cm−2), which is 2 times higher than that of CuCo2O4 NRs (200.2 μW cm−2) and 4.7 times than NiCo2O4 NRs (87.4 μW cm−2). The improved electrochemical performance of Ni0.5Cu0.5Co2O4 NR may benefit from the highly increased charge transfer, which is due to the incorporation of third metal atoms into the spinel lattice of CuCo2O4 or NiCo2O4. The results suggested that Ni0.5Cu0.5Co2O4 NR may be a promising catalyst for application in both NEGS and DGFCs.

Enhanced photoelectrocatalytic performance of porphyrin-modified nickel cobaltite for methanol oxidation under visible light

Rational design of efficient and durable non-precious metal electrocatalysts for methanol oxidation reaction (MOR) is a challenge of current research. In this work, the photosensitive organic molecule 5,10,15,20-tetrakis (4-carboxyphenyl) porphyrin (TCPP) has been used to modify nickel cobaltite (NiCo2O4) nanomaterials. Under visible light irradiation, its catalytic performance for MOR is better than that of pure NiCo2O4. The photoelectrocatalytic mechanism of TCPP/NiCo2O4 is attributed to the effective inhibition of photoelectron and hole recombination and then accelerated electron transfer, which are verified by photoluminescence (PL) emission spectra, impedance spectroscopy (EIS) and photocurrent response curves. Notably, the mass activity of TCPP/NiCo2O4 for MOR under visible light irradiation is 299.2 mA mg−1, higher than that of pure NiCo2O4 under dark (51.2-fold) and under visible light (36.5-fold). This study is very meaningful for the development of efficient noble-metal free photoelectrocatalysts for MOR.

Insight into improved oxygen evolution reaction on electronic modulation of phosphorus doped NiCo2O4

In this work, we report the fabrication of phosphorus doped NiCo2O4 (P-NiCo2O4) to achieve the development of oxygen evolution reaction (OER) catalysts. The obtained 1% P-NiCo2O4 exhibited better OER activity than that of pure NiCo2O4 with an overpotential of 370 mV at 10 mA cm−2, a high Cdl value of 41.8 mF cm−2, a small Tafel slope value of 95 mV dec−1, as well as remarkable stability. The superior electrochemical performances could be attributed to the formed mipor layer, resulting in the acceleration of the diffusion speed of electrolyte or ions on the interface between liquid and electrode. The correlation of experiments and DFT calculation results revealed that due to P doping, the electron density of Co and Ni around replaced O atom increased, leading to the higher ratio of Co3+/Co2+ and Ni3+/Ni2+, which is beneficial to the rate-determining step (RDS) of transformation of HO* to O*. The calculated maximum ΔG of RDS are 1.68 eV and 1.54 eV for NiCo2O4 and P-NiCo2O4, respectively, demonstrating the better OER performance of P-NiCo2O4.

Fabrication of ultrathin lily-like NiCo2O4 nanosheets via mooring NiCo bimetallic oxide on waste biomass-derived carbon for highly efficient removal of phenolic pollutants

Rational design of photocatalysts with the controllable structure to expose active sites is crucial to degrade phenolic pollutants. Herein, we report the in-situ synthesis of lily-like NiCo2O4 nanosheets (0.95 nm) grown on waste biomass-derived carbon (abbreviated as NCC) for the highly efficient photocatalytic removal of phenolic pollutants through the integration of experiment and theory calculation. These as-fabricated NCC photocatalysts (2.29 nm) exhibit superior performance for the fast removal of phenolic pollutants (such as 4-Acetamidophenol (AP), phenol, and aspirin) in comparison with their counterpart and previously reported candidates. By using AP as a model molecule for mimicking pollutants, the photocatalytic degradation performance of NCC nanosheets is ∼ 1.3 and 1.9 times higher than that of pure NiCo2O4 (NC) and porous carbon, respectively. Various factors that influenced the degradation process such as peroxymonosulfate (PMS) concentration, catalyst dosage, AP concentration, and initial pH are thoroughly investigated. The free radical quenching experiments and electron paramagnetic resonance (EPR) spectra further demonstrate the existence of SO4−·, ·OH, ·O2–, and 1O2 in the reaction system and the ·O2– radical play a major role, which is also confirmed by the density functional theory (DFT) calculation. Several degradation pathways (e.g., demethylation, deacylation, hydroxylation, and ring-opening) are then proposed based on the detected intermediates. Finally, the high stability and good reusability of NCC nanosheets are confirmed via the degradation efficiency of over 85% even after five successive cycles.

Hierarchical NiCo2O4 nanostructured arrays decorated over the porous Ni/C as battery-type electrodes for supercapacitors

The rational construction of hierarchical electrode materials has always been an attractive strategic development in energy storage. In this paper, the porous Ni/C (NC) substrate, which is derived from Ni metal–organic frameworks (Ni-MOFs), is served as the conductive skeleton on Ni foam (NF) for supporting two different morphologies NiCo2O4 nanostructure arrays (NiCo2O4/NC). The hierarchical layered nanomaterial utilizesthe porous NC substrate as “favorable skeleton” for electron transfer and is coupled to vertical well-organized NiCo2O4 nanoarrays to improve specific capacitance of composites. Benefits from the advantages of hierarchical layered structure, the electrochemical properties of as-constructed NiCo2O4/NC with two different morphologies were better than those of pure NiCo2O4 prepared under the same conditions. Moreover, the optimized NiCo2O4 nanowires arrays on NC substrate (NiCo2O4NWs/NC) electrode acquire prominent specific capacitance of 1945.9 F g−1 (270.3 mAh g−1) at 1 A g−1 and good rate performance. Additionally, the assembled asymmetric device by NiCo2O4NWs/NC and carbon nanotube (CNT) displays a high energy density of 58.31 Wh kg−1 at a power density of 749.7 W kg−1, as well as good cyclic stability. The remarkable properties manifest that the hierarchical NiCo2O4NWs/NC electrode has a good prospect in electrochemical applications.

Effect of different synthesis methods on morphology and electrochemical behavior of spinel NiCo2O4 nanostructures as electrode material for energy storage application

In the present study, four different methods viz. Hydrothermal, Sol-Gel, Solid-state, Co-precipitation methods are used to successfully synthesize the nickel cobaltite (NiCo2O4) nanostructures. The product obtained from the hydrothermal method shows an intermediate behavior between battery-type and pseudocapacitor-type, whereas other methods yield pure battery-type NiCo2O4 nanostructures. Scanning electron microscopy (SEM) examination evaluated the distinct morphology acquired from each methodology, which had a massive effect on the electrochemical performance of the materials as formulated. To assess the electrochemical performance in 2 M KOH electrolyte, techniques such as Galvanostatic charge–discharge (GCD) and cyclic voltammetry (CV) have been used. The greatest specific capacity recorded in the case of sol–gel synthesized product was 617.22C/g at a current density of 1.0 A/g, but significantly enhanced rate-performance is observed in the case of Co-precipitation produced product (568.20C/g at 1.0 A/g and 374.26C/g at 10.0 A/g). The obtained results verified the effect of different methods of synthesis on morphology and electrochemical performance of the same electrode material prepared under identical environmental conditions.

Hierarchical NiCo / NiO / NiCo 2 O 4 composite formation by solvothermal reaction as a potential electrode material for hydrogen evolutions and asymmetric supercapacitors

Electrochemical energy storage and conversion mechanisms are highly viable routes in the current scenario, and hence supercapacitor and electrolysis of water splitting have gained significant attention in recent times. Thus, transition metal nickel cobalt oxides are widely inspected as promising electrode materials for electrocatalysis and supercapacitors. However, the pure NiCo2O4 was highly hindered by its widespread uses due to low electronic conductivity and deficient active edges. This study is aimed to fabricate the NiCo/NiO/NiCo2O4 composite using a solvothermal process with different ratios of solvents. The inherent NiCo/NiO/NiCo2O4 composite holds plentiful active edges and improved electrical conductivity than the pure. Interestingly, the electrocatalytic results revealed that the NiCo/NiO/NiCo2O4 (NC) composite prepared by a solvent ratio of (double distilled water (DDW) to N, N-Dimethylformamide (DMF)) 2:1 (NC1) displayed the small overpotentials of 144 and 142 mV to realize 10 mA cm−2 with the robust durability during a 24 hours hydrogen evolution reaction (HER) test in the acid and alkaline electrolyte, respectively. Moreover, the designed NC1 composite as a supercapacitor electrode exhibited considerable capacitance of 638 F g−1 at a current density of 1 A g−1. In comparison, the analog DDW to DMF ratio 1:1 (NC2) and 1:2 (NC3) structures showed 514 F g−1 and 415 F g−1, respectively. Further, a dual-electrode device was fabricated using the NC1 composite and activated carbon, exhibited an energy density of 28.14 W h kg−1 at 414.45 W kg−1 power density with excellent device stability (89.4%) for 10 000 charge/discharge cycles.

Size-controllable porous flower-like NiCo2O4 fabricated via sodium tartrate assisted hydrothermal synthesis for lightweight electromagnetic absorber

Although the performance of NiCo2O4-based absorbers with multiple components has made great progress, the property of pure NiCo2O4 is still far from the requirements of high-performance electromagnetic wave absorbers. It is recognized that morphology control is an effective strategy to improve electromagnetic absorbing capacity of absorbers. Herein, this work reported the fabrication of porous flower-like pure NiCo2O4 via simple hydrothermal reaction with the assistant of sodium tartarate in where tartaric acid served as a structure-directing agent. It was demonstrated that size distribution and electromagnetic absorbing capacity of the obtained NiCo2O4 could be modulated easily by controlling addition of sodium tartrate. It was verified that dipole polarization originated from lattice defect and oxygen vacancy as well as interfacial polarization ascribing to adjacent and interconnected flakes are responsible for the excellent electromagnetic absorbing performance. The obtained porous flower-like NiCo2O4 exhibited broad absorption bandwidth at thin thickness as well as proper dissipation ability. This work offers a new strategy to fabricate size-controllable porous flower-like NiCo2O4 electromagnetic absorber. It is believed the obtained NiCo2O4 will be a promising candidate as a lightweight electromagnetic absorbing material.