NiCo2O4 Research Articles

Fluorine Induced In Situ Formation of High Valent Nickel Species for Ultra Low Potential Electrooxidation of 5-Hydroxymethylfurfural.

The Nickel-based catalysts have a good catalytic effect on the 5-hydroxymethylfurfural electrooxidation reaction (HMFOR), but limited by the conversion potential of Ni2+ /Ni3+ , 1.35V versus RHE, the HMF electrooxidation potential of nickel-based catalysts is generally greater than 1.35V versus RHE. Considering fluorine has the highest Pauling electronegativity and similar atomic radius of oxygen, the introduction of fluorine into the lattice of metal oxides might promote the adsorption of intermediate species, thus improving the catalytic performance. F is successfully doped into the lattice structure of NiCo2 O4 spinel oxide by the strategy of hydrothermal reaction and low-temperature fluorination. As is confirmed by in situ electrochemical impedance spectroscopy and Raman spectroscopy, the introduction of F weakens the interaction force of metal-oxygen covalent bonds of the asymmetric MT -O-MO backbone and improves the valence of Ni in tetrahedra structure, which makes it easier to be oxidized to higher valence active Ni3+ under the action of electric field and promotes the adsorption of OH- , while the decrease of Co valence enhances the adsorption of HMF with the catalyst. Combining the above reasons, F-NiCo2 O4 shows superb electrocatalytic performance with a potential of only 1.297V versus RHE at a current density of 20mA cm-2 , which is lower than the most catalyst.

NiCo2 O4 /Hollow Mesoporous Carbon Nanosphere Hybrids Enabling Super-Hydrophobicity, Thermal Insulation, and Highly Efficient Microwave Absorption.

The combination of 2D magnetic nanosheets and mesoporous carbon with unique interfaces shows considerable prospects for microwave absorption (MA). However, traditional assembly procedures make it impossible to accurately manage the assembly of magnetic nanosheets in carbon matrices. Herein, a reverse strategy for preparing complex magnetic nanosheet cores inside carbon-based yolk-shell structures is developed. This innovative approach focuses on controlling the initial crystallite formation sites in a hydrothermal reaction as well as the inflow and in situ growth behavior of 2D NiCo-layered double hydroxide precursors based on the capillary force induced by hollow mesoporous carbon nanospheres. Accordingly, the as-prepared YS-CNC-2 absorber exhibits remarkable MA performances, with an optimal reflection loss as low as -60.30dB at 2.5mm and an effective absorption bandwidth of 5.20GHz at 2.0mm. The loss of electromagnetic waves (EMW) depends on natural resonance loss, dipole polarization relaxation, and multiple scattering behavior. Ontopof that, the functionalized super-hydrophobic MA coating is produced in spraying and curing processes utilizing YS-CNC-2 nanoparticles and fumed silica additives in the polydimethylsiloxane matrix. The excellent thermal insulation, self-cleaning capability, and durability in diverse solutions of the coating promise potential applications for military equipment in moist situations.

Underlying Mechanisms and Tunability of the Anomalous Hall Effect in NiCo2 O4 Films with Robust Perpendicular Magnetic Anisotropy.

Their high tunability of electronic and magnetic properties makes transition-metal oxides (TMOs) highly intriguing for fundamental studies and promising for a wide range of applications. TMOs with strong ferrimagnetism provide new platforms for tailoring the anomalous Hall effect (AHE) beyond conventional concepts based on ferromagnets, and particularly TMOs with perpendicular magnetic anisotropy (PMA) are of prime importance for today's spintronics. This study reports on transport phenomena and magnetic characteristics of the ferrimagnetic TMO NiCo2 O4 (NCO) exhibiting PMA. The entire electrical and magnetic properties of NCO films are strongly correlated with their conductivities governed by the cation valence states. The AHE exhibits an unusual sign reversal resulting from a competition between intrinsic and extrinsic mechanisms depending on the conductivity, which can be tuned by the synthesis conditions independent of the film thickness. Importantly, skew-scattering is identified as an AHE contribution for the first time in the low-conductivity regime. Application wise, the robust PMA without thickness limitation constitutes a major advantage compared to conventional PMA materials utilized in today's spintronics. The great potential for applications is exemplified by two proposed novel device designs consisting only of NCO films that open a new route for future spintronics, such as ferrimagnetic high-densitymemories.

Low Temperature Epitaxial LiMn2O4 Cathodes Enabled by NiCo2O4 Current Collector for High-Performance Microbatteries.

Epitaxial cathodes in lithium-ion microbatteries are ideal model systems to understand mass and charge transfer across interfaces, plus interphase degradation processes during cycling. Importantly, if grown at <450 °C, they also offer potential for complementary metal-oxide-semiconductor (CMOS) compatible microbatteries for the Internet of Things, flexible electronics, and MedTech devices. Currently, prominent epitaxial cathodes are grown at high temperatures (>600 °C), which imposes both manufacturing and scale-up challenges. Herein, we report structural and electrochemical studies of epitaxial LiMn2O4 (LMO) thin films grown on a new current collector material, NiCo2O4 (NCO). We achieve this at the low temperature of 360 °C, ∼200 °C lower than existing current collectors SrRuO3 and LaNiO3. Our films achieve a discharge capacity of >100 mAh g-1 for ∼6000 cycles with distinct LMO redox signatures, demonstrating long-term electrochemical stability of our NCO current collector. Hence, we show a route toward high-performance microbatteries for a range of miniaturized electronic devices.

Peculiar thickness dependence of perpendicular magnetic anisotropy in epitaxial NiCo2O4 spinel films

We reported a peculiar thickness dependence of perpendicular magnetic anisotropy (PMA), stronger in thinner films, in spinel NiCo2O4 (NCO) films deposited epitaxially on MgAl2O4 (001) single crystalline substrates. The effective PMA field is about 3 T in 23.0 nm-thick films but increases dramatically to about 7 T in 4.5 nm-thick films. X-ray absorption spectra reveal an inhomogeneous distribution of Co2+ cations along the film's normal with more Co2+ adjacent to the film/substrate interface. X-ray linear dichroism spectra reveal that the PMA in NCO films is associated with electrons occupying the x2–y2 orbital of Co cations at the tetrahedral sites. This work demonstrates the potential of ultrathin NCO films in all-oxide spintronic devices.

Redox kinetics promoted by flower-like La2CoMnO6 @NiCo2O4 heterojunctions for high-performance supercapacitors

The double perovskite oxide La2CoMnO6 (LCMO) is suitable for supercapacitor applications owing to its fast oxygen anion conduction and strong electron conductivity. However, the poor electron transfer of spinel oxide electrode materials weakens the intensity of surface redox reactions, thereby hindering the further improvement of energy density of the supercapacitors. In this study, flower-like heterogeneous La2CoMnO6 @NiCo2O4 materials were in-situ grown on a nickel foam (LCMO@NCO/Ni) via interface engineering and morphological engineering for the first time. The as-constructed heterogeneous materials benefit from the high electron conductivity of LCMO for the rapid transfer of electrons in NiCo2O4 (NCO), resulting in increased surface redox reaction rates. Interestingly, these successfully constructed heterogeneous LCMO and NCO structures act as electron donor and acceptor, respectively. Such a system promotes the increase in the electron cloud density around NCO, leading to an enhanced intensity of the redox reactions. The unique nanoflower-like structure and strong interfacial interaction of the developed heterogeneous LCMO@NCO/Ni material result in an electrode material with excellent supercapacitor performance with a high specific capacitance of 598 F·g−1 at a current density of 1 A·g−1. The asymmetric supercapacitor assembled with activated carbon and as-prepared material possesses a high energy density of 39.9 Wh·kg−1 at 1600 W·kg−1, proving suitable for practical applications. Overall, the proposed synthesis strategy is promising for the fabrication of novel heterogeneous electrode materials for supercapacitors with improved energy density.

A porous TiC supported nanostructured complex metal oxide ceramic electrode for oxygen evolution reaction

Developing low-cost and high-performance electrodes with a customized size is greatly demanded to satisfy the industrial requirement of electrochemical water splitting. To meet this challenge, a novel binder-free electrode consisted of perovskite Sr2Fe1.4Ni0.1Mo0.5O6-δ (SFNMO) nanofibers or spinel NiCo2O4 (NCO) nanosheets tightly bonded on a porous TiC substrate has been developed to catalyze the oxygen evolution reaction in alkaline water electrolysis. The optimal SFNMO/TiC electrode requires a stable overpotential of ∼380 mV to achieve a current density of 30 mA cm−2 within 20 h of operation. The competitive OER performance of such electrode can be attributed to its unique large finger-like straight pore structure, which favors the mass transfer of electrolyte solution as well as produced oxygen bubbles. Furthermore, the strong adhesion between metal oxide catalysts and TiC substrate is derived from the interfacial reaction (form a TiCxOy solid solution), thus lowering the contact resistance of catalyst/substrate interface and promoting the charge transfer kinetics of electrodes, which leads to an outstanding catalytic performance.

Confining NiCo2O4 nanoparticles in nitrogen doped carbon nanotubes embedded in 3D porous graphene aerogels for superior lithium storage

The huge volume expansion and poor electrical conductivity have seriously hindered the further development of redox-based electrode materials such as NiCo2O4 (NCO). Herein, we designed a novel NiCo2O4@nitrogen-doped carbon nanotube@3D graphene (NCO@CT@3DG) aerogel through hydrothermal assembly and high-temperature carbonization. In this aerogel, graphene nanosheets constructed the framework of the aerogel and NCO nanoparticles were deposited on the surface of nitrogen-doped carbon nanotubes which served as active materials for Lithium-ion batteries. The NCO decorated carbon nanotubes connecting the graphene sheets served as the scaffold to support the 3D cross-linked graphene network. As anode material of lithium-ion batteries, the optimized NCO@CT@3DG exhibited excellent electrochemical properties with high capacity as well as superior cycle and rate performance, which could maintain the discharge specific capacity of 810.3 mAh/g at the current density of 1.0 A/g after 500 cycles. The excellent properties are ascribed to the synergistic effect of each individual component and the unique graphene aerogels hybrid structure. This study demonstrates the great application prospects of 3DG aerogel hybrid structure in improving electrochemical properties of active materials with high volume expansion and poor conductivity.

Interfacial and surface magnetism in epitaxial NiCo2O4(001)/MgAl2O4 films

NiCo2O4 (NCO) films grown on MgAl2O4 (001) substrates have been studied using magnetometry and x-ray magnetic circular dichroism based on x-ray absorption spectroscopy and spin-polarized inverse photoemission spectroscopy with various thicknesses down to 1.6 nm. The magnetic behavior can be understood in terms of a layer of optimal NCO and an interfacial layer (1.2 ± 0.1 nm), with a small canting of magnetization at the surface. The thickness dependence of the optimal layer can be described by the finite-scaling theory with a critical exponent consistent with the high perpendicular magnetic anisotropy. The interfacial layer couples antiferromagnetically to the optimal layer, generating exchange-spring styled magnetic hysteresis in the thinnest films. The non-optimal and measurement-speed-dependent magnetic properties of the interfacial layer suggest substantial interfacial diffusion.

Fabrication of Ce-doped Hollow NiCo2 O4 Nanoprisms with Heterointerface from MOF-Engaged Strategy for Oxygen Evolution Reaction.

The reasonable design and controlled synthesis of efficient and hollow nanocatalysts with plentiful heterointerface and fully exposed active sites to accelerate the electron transfer and mass transfer process for oxygen evolution reaction (OER) is highly desirable for water splitting by electrolysis. Herein, a metal-organic framework (MOF)-engaged strategy is developed to prepare Ce-doped hollow mesoporous NiCo2 O4 nanoprisms (NiCo2 O4 /CeO2 HNPs) for enhanced OER. Due to the advanced synthesis strategy generating a large number of interfaces between NiCo2 O4 and CeO2 , as well as modulated electrons of the active center by the synergistic action of multi-metals, the obtained catalyst exhibits excellent OER performance with a small overpotential of 290 mV at current density (J) of 10 mA cm-2 . Spinel/Perovskite hollow nanoprisms synthesized by a similar way demonstrates the versatility of our strategy. This work may provide new insights into the development of rare earth-doped hollow polymetallic spinel oxide catalysts.

Temperature Ramp Strategy for Regulating Oxygen Vacancies in NiCo2O4 Nanoneedles Towards Enhanced Electrocatalytic Water Splitting.

The oxide-based systems often suffer from higher overpotentials compared to transition metal sulphides and phosphides for the electrochemical hydrogen evolution reaction. Interestingly, the generation of oxygen vacancy has been seen as strategy for further activating transition metal oxides (NiCo2O4 as a model system) for electrochemical watersplitting. Herein, we employ the temperature ramp strategy (ambient air calcination) for the generation of oxygen vacancies in NiCo2O4 (NCO) towards tuning of electrocatalytic enhancements. The NiCo2O4 synthesized at temperature ramp rates of 2 (NCO-2), 5 (NCO-5), and 10ºC/ min (NCO-10) depicts contrasting structural features and varying Ni:Co:O surface composition. The decrease in the crystallite size and converse trend in the particle strain were observed from NCO-2 to NCO-10. Interestingly, the surface Ni:Co:O ratios of 1:0.78:3.6, 1:0.81:3.3, and 1:0.69:2.8 for NCO-2, NCO-5, and NCO-10, respectively, were observed. The reduced relative oxygen ratio in the latter implies generation of ample amount of oxygen vacancy defects. HER performance depicts a consistent trend with enhanced oxygen defect concentration with the overpotential requirement of 700, 647, and 597 mV for NCO-2, NCO-5, and NCO-10, respectively, for the generation of a cathodic current of 25 mA cm-2. The same trend in an electrocatalytic enhancement is observed for other cathodic currents.

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.

Mixed Phase Nickel‐Cobaltite for High Energy Density Asymmetric Supercapacitor using Cathodic Overpotential Synthesis Route

AbstractFor flexible solid‐state electronics, it is essential to accelerate the transition to renewable materials and environmentally friendly manufacturing procedures. Symmetric supercapacitors (SCs) have a limited voltage window and capacity, hampering their use in practical applications such as portable electronics, hybrid vehicles, and transportation. To improve the energy density of supercapacitors, the strategy of using an advanced pseudo‐capacitive electrode in an asymmetric device configuration is feasible and effective. Here, we report on the design and synthesis of hierarchical honeycomb‐like morphology with controlled Ni/Co molar ratios of NiCo2O4 (NCO) coatings on flexible stainless‐steel substrates utilizing high cathodic overpotential deposition. The NCO electrode retains 90.5% of the three‐electrode capacitance (911 F g−1, or 594 C g−1) even after increasing the current density to 10 A g−1 (∼10 times). Furthermore, we have fabricated a 1.5 V asymmetric supercapacitor (ASC, NCO//AC); such a device delivers maximum specific energy and specific power of 76.8 Wh kg−1 and 16858 W kg−1 in 2 M KOH electrolyte and excellent cycling stability (∼88% retention after 5000 cycles).

A novel template processing method for preparing spinel oxides with various morphologies to assemble high-performance supercapacitors

With the increasing complexity of the application scenarios of nanomaterials, transition metal oxides (TMOs) have shown excellent development potential. However, the synthesis of TMOs with customized morphologies for different application environments is still challenging. Herein, an acid-alcohol treatment based on the different intensities of the redox reactions between nitric acid and ethanol was developed to prepare different carbon templates. Through impregnation and annealing of the templates, NiCo2O4 (NCO) materials with hollow spherical (NCO-HS), multi-shell spherical (NCO-MS), and lamellar (NCO-L) morphologies were obtained. The characterization was performed through different analytical methods, which revealed various structures, morphologies, and surface chemistry of the materials. NCO-HS, NCO-MS, and NCO-L were tested for supercapacitor applications and exhibited a high specific capacitance of 402.6, 1037.1, and 531.4 F g−1, respectively. Overall, the proposed method can be used to customize the synthesis of functional materials with various morphologies for future advanced applications.

Surface antibacterial properties and mechanism of NiCo2O4 against E. Coli

Bacterial infections seriously affect and threaten human health. Antibacterial agents, including inorganic metal-based materials, have widely been explored owing to their effective properties in killing pathogens. Among these, NiCo2O4 (NCO) is a semiconductor material with anti-spinel structure and antibacterial properties. Herein, porous rod-like NCO was synthesized for use as antibacterial material against E. coli. The antibacterial properties and bactericidal mechanism under light and dark conditions revealed NCO with large specific surface area (51.707 m2/g), positive Zeta potential (17.3 eV), and more surface oxygen species. The bactericidal rate of 0.3 mg/mL NCO against E. coli under light and dark conditions reached 98.0% and 94.0%, respectively. The antibacterial mechanism of NCO under dark conditions illustrated the involvement of reactive metal Lewis acid active sites on its surface toward bacteria to produce lipid peroxidation, thereby inactivating bacteria. Under visible light irradiation, the main antibacterial ability was caused by the joint action of metal Lewis acid active site and reactive oxygen species. Overall, cyclic changes of valence states of metal elements and surface oxygen species in materials can be used for bacterial oxidative damage, relevant for bacterial disinfection.

Evaluating the role of current collectors in supercapacitor electrodes with NiCo2O4 nanospheres

Supercapacitor as an electrochemical energy storage device is well known for delivering high power density than batteries but faces the challenge of lower electrical energy density. To overcome this limitation, researchers are exploring new and advanced nanostructured electrode materials, particularly metal oxides, and composites. Most of the supercapacitor research is focused on investigating electrode materials, which are important in determining the device's electrochemical performance. Along with this premise, this study says that a current collector (CC) plays an essential part in an electrode's electrochemical performance and is just as important as the electrode material. It is crucial to investigate the role of current collectors in supercapacitor electrode electrochemical performance. In this study, four different current collectors are used to deposit NiCo2O4 (NCO) as the active electrode material: nickel foam (Ni), flexible graphite sheet (FG), copper foil (Cu), and stainless-steel foil (SS). The electrochemical tests are performed with and without active material loaded on different CCs and investigated for the contribution made by each CC. Results indicate that Ni foam possesses its own specific capacity of 128 Fg-1 and as an electrode, NCO–Ni delivered the highest specific capacity of 960 Fg-1 at a current density of 1 Ag-1, compared with NCO-FG (237 Fg−1), NCO–Cu (169 Fg−1), and NCO-SS (64 Fg−1) electrodes.

Bioinspired Hierarchical Self-Assembled Nanozyme for Efficient Antibacterial Treatment.

Along with the rapid development and ever-deepening understanding of nanoscience and nanotechnology, nanomaterials hold promise to mimic the highly evolved biological exquisite nanostructures and sophisticated functions. Here, inspired by the ubiquitous antibacterial nanostructures on the wingsurfacesofsome insects, a NiCo2 O4 nanozyme with self-adaptive hierarchical nanostructure is developed that can capture bacteria of various morphotypes via the physico-mechanical interaction between the nanostructure and bacteria. Moreover, the developed biomimetic nanostructure further exhibits superior peroxidase-like catalytic activity, which can catalytically generate highly toxic reactive oxygen species that disrupt bacterial membranes and induce bacterial apoptosis. Therefore, the mechano-catalytic coupling property of this NiCo2 O4 nanozyme allows for an extensive and efficient antibacterial application, with no concerns of antimicrobial resistance. This work suggests a promising strategy for the rational design of advanced antibacterial materials by mimicking biological antibiosis.

Amorphous Oxysulfide Reconstructed from Spinel NiCo2S4 for Efficient Water Oxidation

The progress of effective and durable electrocatalysts for oxygen evolution reaction (OER) is urgent, which is essential to promote the overall efficiency of green hydrogen production. To improve the performance of spinel cobalt-based oxides, which serve as promising water oxidation electrocatalysts in alkaline electrolytes, most researches have been concentrated on cations modification. Here, an anionic regulation mechanism is employed to adopt sulfur(S)anion substitution to supplant NiCo2 O4 by NiCo2 S4 , which contributed to an impressive OER performance in alkali. It is revealed that the substitution of S constructs a sub-stable spinel structure that facilitates its reconstruction into active amorphous oxysulfide under OER conditions. More importantly, as the active phase in the actual reaction process, the hetero-anionic amorphous oxysulfide has an appropriately tuned electronic structure and efficient OER electrocatalytic activity. This work demonstrates a promising approach for achieving anion conditioning-based tunable structure reconstruction for robust and easy preparation spinel oxide OER electrocatalysts.

Manipulate the magnetic and electronic states in NiCo2O4 films through protonation

Element doping and substitution have been widely used for the engineering of the electronic structure and macroscopic magnetic properties of various materials. However, traditional doping process does not allow further optimization after preparation of the samples. Hydrogen annealing-induced protonation has emerged as a powerful method for the efficient control of uniaxial lattice expansion to modulate the physics properties of the otherwise inaccessible novel correlated phases in oxide thin-films. Here, we report the modulation of the magnetic moment, magneto-transport and electrical-transport in inverse-spinel NiCo2O4 (NCO) by protonation. Strong dependence of saturation magnetic moment and electrical conduction on deposition temperature and proton doping parameters has been systematically investigated. The magnetoresistance ratio of the protonated NCO could be increased by 400% by engineering of the doping condition and thus the phase configuration. These findings indicate that protonation is one effective strategy for regulation and optimization of NCO-based magnetic and electric devices.

Unraveling Photothermal-Enhanced bulk charge transport and surface oxygen reactions in TiO2 photoanodes for highly efficient photoelectrochemical water oxidation

Titanium dioxide (TiO2) has been extensively used as an important photoanode material for photoelectrochemical (PEC) water splitting, but its activity is greatly hindered by the low charge transport and sluggish oxygen evolution reaction (OER) kinetics. Herein, we demonstrate spinel oxide NiCo2O4 (NCO) acted as both photothermal material and oxygen evolution cocatalyst could significantly promote the PEC performance of TiO2 photoanodes, and a high photocurrent of 2.34 mA cm−2 at 1.23 V vs. reversible hydrogen electrode has been achieved by NCO/TiO2 photoanodes assisted by the photothermal effect. Importantly, a mechanism is proposed to explain the enhanced bulk charge transport and surface OER activity by an integrated various operando characterizations and density functional theory (DFT) study. Intensity-modulated photocurrent spectroscopy (IMPS) analysis and DFT calculations imply the relatively high temperature induced by the photothermal effect could accelerate small polaron hopping in the bulk of TiO2, which is verified by the electron diffusion coefficient variation of TiO2 with oxygen vacancies at different temperatures. Operando cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) reveal that photothermal conversion facilitates the oxidation of Ni2+ at lower potential and promotes the adsorption of *OH and deprotonation of *OOH intermediates. This work provides deep insight into the photothermal-enhanced PEC performance by observing the bulk small polaron hopping and surface dynamic evolution during the PEC process, which may underpin future advances in promoting PEC and photocatalytic performances of small-polaron-type semiconductors.