Enhancing energy storage capacity of iron oxide-based anodes by adjusting Fe (II/III) ratio in spinel crystalline.

Supercapacitors, as promising energy storage candidates, are limited by their unsatisfactory anodes. Herein, we proposed a strategy to improve the electrochemical performance of iron oxide anodes by spinel-framework constraining. We have optimized the anode performance by adjusting the doping ratio of Fe (II/III) self-redox pairs. Structure and electronic state characterizations reveal that the NixFe3-xO4was composed of Fe (II/III) and Ni (II/III) pairs in lattice, ensuring a flexible framework for the reversible reaction of Fe (II/III). Typically, when the ratio of Fe (II/III) is 0.91:1 (Fe (II/III)-0.91/1), the NixFe3-xO4anode shows a remarkable electrochemical performance with a high specific capacitance of 1694 F g-1at the current density of 2 A g-1and capacitance retention of 81.58%, even at a large current density of 50 A g-1. In addition, the obtained material presents an ultra-stable electrochemical performance, and there is no observable degradation after 5000 cycles. Moreover, an assembled asymmetric supercapacitor of Ni-Co-S@CC//NixFe3-xO4@CC presents a maximum energy density of 136.82 Wh kg-1at the power density of 850.02 W kg-1. When the power density was close to 42 500 W kg-1, the energy density was still maintained 63.75 Wh kg-1. The study indicates that inherent performance of anode material can be improved by tuning the valence charge of active ions.

New Insight into the Mechanism of Multivalent Ion Hybrid Supercapacitor: From the Effect of Potential Window Viewpoint.

Multivalent ion hybrid supercapacitors have been developed as the novel electrochemical energy storage systems due to their combined merits of high energy density and high power density. Nevertheless, there are still some challenges due to the limited understanding of the electrochemical behaviors of multivalent ions in the electrode materials, which greatly hinders the large scale applications of its based hybrid supercapacitors. Herein, the long-term electrochemical behaviors of MnO2 -based electrode in the divalent Mg2+ ions electrolyte are systematically studied and linked with the morphological and electronic evolution of MnO2 by cycling at different potential windows (spanning to 1.2 V). It reveals that the different potential windows result in the different electrochemical behaviors, which can be divided into two ranges (below and above -0.2 V). And, the electrode cycled at a potential window of 0-1.2 V delivers the highest capacitance of 967 F g-1 at a scan rate of 10 mV s-1 , in which the MnO2 is transformed into a uniformly distributed and nonagglomerated nanoflake morphology promoting the intercalation and deintercalation of Mg2+ ions. This study will enrich the understanding of the charge storage mechanism of multivalent ions and provide significant guidance on the performance improvement of the hybrid supercapacitors.

Co@Co3O4 core-shell three-dimensional nano-network for high-performance electrochemical energy storage.

An alternative routine is presented by constructing a novel architecture, conductive metal/transition oxide (Co@Co3O4) core-shell three-dimensional nano-network (3DN) by surface oxidating Co 3DN in situ, for high-performance electrochemical capacitors. It is found that the Co@Co3O4 core-shell 3DN consists of petal-like nanosheets with thickness of <10 nm interconnected forming a 3D porous nanostructure, which preserves the original morphology of Co 3DN well. X-ray photoelectron spectroscopy by polishing the specimen layer by layer reveals that the Co@Co3O4 nano-network is core-shell-like structure. In the application of electrochemical capacitors, the electrodes exhibit a high specific capacitance of 1049 F g(-1) at scan rate of 2 mV/s with capacitance retention of ~52.05% (546 F g(-1) at scan rate of 100 mV) and relative high areal mass density of 850 F g(-1) at areal mass of 3.52 mg/cm(2). It is believed that the good electrochemical behaviors mainly originate from its extremely high specific surface area and underneath core-Co "conductive network". The high specific surface area enables more electroactive sites for efficient Faradaic redox reactions and thus enhances ion and electron diffusion. The underneath core-Co "conductive network" enables an ultrafast electron transport.

Intercalation chemistry engineering strategy enabled high mass loading and ultrastable electrodes for High-Performance aqueous electrochemical energy storage devices.

Aqueous electrochemical energy storage devices (AEESDs) are considered one of the most promising candidates for large-scale energy storage infrastructure due to their high affordability and safety. Developing electrodes with the merits of high energy density and long lifespan remains a challenging issue toward the practical application of AEESDs. Research attempts at electrode materials, nanostructure configuration, and electronic engineering show the limitations due to the inherent contradictions associated with thicker electrodes and ion-accessible kinetics. Herein, we propose an intercalation chemistry engineering strategy to enhance the electrolyte ion (de)intercalation behaviors during the electrochemical charge-discharge. To validate this strategy, the prototypical model of a high-mass-loading MnO2-based electrode is used with controlled intercalation of Na+ and H2O. Theoretical and experimental results reveal that an optimal content of Na+ and H2O on the MnO2-based electrode exhibits superior electrochemical performance. Typically, the resultant electrode exhibits an impressive areal capacitance of 1551 mF/cm2 with a mass loading of 9.7 mg/cm2 (at 1 mA/cm2). Furthermore, the assembled full-cell with obtained MnO2-based electrode delivers a high energy density of 0.12 mWh/cm2 (at 20.02 mW/cm2) and ultra-high cycling stability with a capacitance retention percentage of 89.63 % (345 mF/cm2) even after 100,000 cycles (tested over 72 days).

Ferroelectric-Enhanced cathode kinetics toward High-Performance aqueous Zinc-Ion batteries.

Rechargeable aqueous zinc ion batteries (AZIBs) offer promising potential for large-scale energy storage systems due to their high affordability and safety. However, their practical applications are hindered by the undesired rate capability and cycling stability of the used cathode, attributed to sluggish ions kinetics during charge-discharge process. Herein, we propose an electric field balancing strategy to regulate the electrolyte ions behavior by constructing a ferroelectric interface on the cathode surface using a prototypical of MnO2-based cathode. An appropriate thickness coating of ferroelectric materials coating (i.e., ß-PVDF) on the MnO2 surface is theoretically and experimentally demonstrated to enhance the ion kinetics due to the optimized electrical distribution during electrochemical operations. Further comprehensive electrochemical mechanism studies reveal that the ferroelectric interface on the MnO2@ß-PVDF not only promotes the diffusion of Zn2+ but also reduces the electrochemical overpotential (17.6 mV), resulting in improved electrochemical reversibility and capacity performance. The resultant MnO2@ß-PVDF cathode exhibits the highest capacity of 277.6 mAh g-1 (at 0.1 A g-1) and capacity retention of 68.6% after 120 cycles, surpassing both the pristine MnO2 and non-ferroelectric materials coated MnO2 electrodes. This success presents a new approach to enhance the overall electrochemical performance of the cathodes for the practical application of AZIBs.

Selective Center Charge Density Enables Conductive 2D Metal-Organic Frameworks with Exceptionally High Pseudocapacitance and Energy Density for Energy Storage Devices.

Conductive 2D conjugated metal-organic frameworks (c-MOFs) are attractive electrode materials due to their high intrinsic electrical conductivities, large specific surface area, and abundant unsaturated bonds/functional groups. However, the 2D c-MOFs reported so far have limited charge storage capacity during electrochemical charging and discharging, and the energy density is still unsatisfactory. In this work, a strategy of selective center charge density to expand the traditional electrode materials to the electrode-electrolyte coupled system with the prototypical of 2D Co-catecholate (Co-CAT) is proposed. Electrochemical mechanism studies and density functional theory calculations reveal that dual redox sites are achieved with the quinone groups (CAT) and metal-ion linkages (Co-O) serving as the active sites of pseudocapacitive cation (Na+ ) and redox electrolyte species (SO3 2- ). The resultant electrode delivers an exceptionally high capacity of 1160 F g-1 at 1 A g-1 and a special self-discharge rate (86.8% after 48 h). Moreover, the packaged asymmetric device exhibits a state-of-the-art energy density of 158 W h kg-1 at the power density of 2000 W kg-1 and an excellent self-discharge rate of 80.6% after 48 h. This success will provide a new perspective for the performance enhancement for the 2D-MOF-based energy storage devices.

Achieving high-rate and durable aqueous rechargeable Zn-Ion batteries by enhancing the successive electrochemical conversion reactions.

The mild electrolyte working environment of rechargeable aqueous Zn-ion batteries (AZIBs) features its promising characteristic and potential application for large-scale energy storage system. However, the poor cycling stability significantly hinders the broad application of AZIBs due to the complex electrochemical conversion reactions during charge-discharge process. Herein, we propose a strategy to improve the electrochemical performance of AZIB by enhancing the successive electrochemical conversion reactions. With a rational design of electrode, an even homogeneous electric field can be achieved in the cathode side, resulting to significantly enhanced efficiency of successive electrochemical conversion reactions. Charge storage mechanism studies reveal that the reversibility behaviors of byproducts alkaline zinc sulfate (ZHS) can dramatically determine the H+/Zn2+ de/intercalation process, and a high reversibility characteristic ensures the facilitated electrochemical kinetics. As expected, the resultant AZIB possesses outstanding electrochemical performance with a high specific capacity of 425.08 mAh⋅g-1 at 0.1 A⋅g-1, an excellent rate capacity of about 60% (246.6 mAh⋅g-1 at 1 A⋅g-1) and superior cycling stability of 93.7% after 3000 cycles (at 3 A⋅g-1). This effective strategy and thinking proposed here may open a new avenue for the development of high-performing AZIBs.

Formation of Moiré Superlattices via Surfactant/Nanosheet-Co-mediated Crystallization.

Moiré superlattices (MSLs) of two-dimensional (2D) van der Waals materials have attracted considerable attention in recent years; however, studies of bottom-up growth of twisted MSL structures via solution-processed crystallization are rarely reported. Through facile one-pot solvothermal synthesis, here we demonstrate a nonclassical surfactant/nanosheet-co-mediated crystallization pathway for formation of MSL structures with two models of SnS2 and SnSe2. Our experimental results reveal that attractive interactions between 2D inorganic building blocks and surfactant organic molecules during the initial stage of crystallization are crucial to drive surfactant-covered nanosheets to crystallize into molecule-intercalated nanosheet aggregates. Under the high-pressure condition, further crystallograpic fusion can occur if the reaction time is prolonged, which alters the interactions of adjacent layers during the coalescence of small-grain-size 2D domains due to insertion of foreign molecules, leading to interlayer rotations. This work uncovers an interesting organic-inorganic cocrystallization growth mode and provides a novel pathway for large-scale fabrication of MSLs.

Wearable and Fully Biocompatible All-in-One Structured ″Paper-Like″ Zinc Ion Battery.

A power supply with the characteristics of portability and safety will be one of the dominating mainstreams for future wearable electronics and implantable biomedical devices. The conventional energy storage devices with typical sandwich structures have complicated components and low mechanical properties, suffering from the apparent performance degradation during deformation and hindering the possibility of implanting biomedical units. Herein, a novel all-in-one structure ″paper-like″ zinc ion battery (ZIB) was designed and assembled from an electrospun polyacrylonitrile (PAN) nanomembrane (as the separator) with in situ deposited anode (zinc nanosheets) and cathode (MnO2 nanosheets), which ensures the monolith under different bending states by avoiding the relative sliding and detaching between the integrated layers. Benefiting from the well-designed all-in-one construction and electrodes, the resultant all-in-one ZIB (AZIB) features an ultrathin thickness (about 97 µm), superior specific capacity of 353.8 mAh g-1 (at 0.1 mA cm-2), and outstanding cycling stability (98.7% capacity retention after 500 cycles at 1 A cm-2). And the achieved volumetric energy density is as high as 17.5 mWh cm-3 at a power density of 116.4 mW cm-3. Impressively, the concept of wearable electronic applications of the obtained AZIB was fully demonstrated with excellent flexibility and remarkable temperature resistance under various severe conditions. Our AZIB may provide a versatile strategy for applying and developing flexible wearable electronics and implantable biomedical devices.

Boosting the Electrochemical Performance of Graphene-Based On-Chip Micro-Supercapacitors by Regulating the Functional Groups.

The on-chip system-compatible power supply shows a high demand for the rapid development of miniaturization devices, such as wireless sensors, remote detecting devices, etc. Moreover, the ever-increasing trends of multifunctionalities and long-term working conditions of such devices raise a high-performance standard for the power supply. Herein, the high-performance electrochemical energy storage micro-supercapacitors (MSCs) are obtained with a metal current collector-free symmetric graphene-based planar structure, in which the functional group of graphene was regulated extensively via fully compatible microfabrication techniques of blue-violet (BV) laser exposure and air plasma treatment. BV laser exposure enhanced the electrical conductivity by reducing the substantial functional groups. Furthermore, the wettability and active sites are tuned by air plasma treatment, thus creating a slightly functional group onto the graphene surface. The resulting reduced graphene oxide (RGO) shows a very low resistance down to 27.2 Ω sq-1, ensuring its superb electron conductivity for fast electron transfer during the electrochemical reactions. The electrochemical performance measurements reveal an areal capacitance as high as 21.86 mF cm-2, which delivers a power density of 5 mW cm-2 with an energy density of 2.49 µWh cm-2. Moreover, it shows superior long-term stability with 99% retention after 10 000 cycles, which is beyond that of most of the reported graphene-based all-solid-state MSCs.

Micromagnetic Configuration of Variable Nanostructured Cobalt Ferrite: Modulating and Simulations toward Memory Devices.

Magnetic nanostructures with flux-closure state or single-domain state have widespread application in diverse memory devices. However, an insight into the modulation of these variable states within one specific magnetic material is rarely reported but still needed. Herein, these micromagnetic configurations within prototypical cobalt ferrite (CoFe2O4) nanostructures in different size and dimension were studied by modulating the assembly of CoFe2O4 building blocks. We find that the CoFe2O4 nanowire (NW) has a multidomain structure when the diameter is about 90 nm, in which the domain walls (DWs) locate preferentially at the grain boundary and can convert to single-domain state when the diameter is reduced. Alternatively, a flux-closure domain state is obtained when the CoFe2O4 nanostructure changes from NW to nanosheet (NS), where the DWs location depends on the overall shape of NS. In addition, we further confirm that the magnetic anisotropy and magnetostatic energy are two main factors affecting the micromagnetic configuration in CoFe2O4 nanostructures by crystallographic analysis and micromagnetic simulations. Our experimental and simulation results demonstrate that the modulation of morphology and dimension are efficient to tailor the micromagnetic configuration in magnetic nanostructures.

The magnetization reversal mechanism in electrospun tubular nickel ferrite: a chain-of-rings model for symmetric fanning.

Magnetic behaviors within nanoscopic materials are being widely explored due to their intriguing performance and widespread applications. Herein, we studied the magnetization reversal mechanism in a unique tubular nickel ferrite (NiFe2O4), in which the building blocks of NiFe2O4 monocrystalline have a face-centered spinel structure and stack along the axial direction of the nanotube. We synthesized this tubular NiFe2O4 through an electrospinning method based on a phase separation process, and then investigated the magnetization reversal process and its relationship with the morphologies using the model of "chain-of-rings" from the micromagnetism theory. This model is developed based on the morphology and crystalline orientation of nanotubes, by which the symmetric fanning mechanism is demonstrated when the angle between the magnetic field and the chain is less than 45.3°. As a result, the simulated coercivity value is confirmed to be 271 Oe, which is close to the experimental value. In addition, the rationality of this model was further verified by the calculation of the effective magnetic anisotropy field. This work is significant for the application of tubular ferrite in the field of nano-devices and fundamental research.

Stabilization and Reversal of Skyrmion Lattice in Ta/CoFeB/MgO Multilayers.

Recently, magnetic skyrmion has attracted much attention due to its potential application in racetrack memory and other nanodevices. In bulk chiral magnets with non-centrosymmetric crystal structures, skyrmion lattice phase has been extensively observed. However, in film or multilayers with interfacial Dzyaloshinskii-Moriya interaction, individual skyrmion is often observed. Here, we report a short-ordered skyrmion lattice observed in [Ta(5.0 nm)/CoFeB(1.5 nm)/MgO(1.0 nm)]15 multilayer in a remnant state. The structure, stabilization, and reversal of these skyrmions are discussed. Applying a slightly tilted in-plane magnetic field caused reversal of the skyrmion lattice. This reversal came from disappearance of skyrmions and nucleation of new skyrmions in the interstitial regions of the lattice. Also, we investigated how the skyrmion lattice depended on the CoFeB thickness. Our findings provide a pathway to stabilize and reverse the skyrmions in multilayers films.

Direct Observation of Magnetocrystalline Anisotropy Tuning Magnetization Configurations in Uniaxial Magnetic Nanomaterials.

Discovering the effect of magnetic anisotropy on the magnetization configurations of magnetic nanomaterials is essential and significant for not only enriching the fundamental knowledge of magnetics but also facilitating the designs of desired magnetic nanostructures for diverse technological applications, such as data storage devices, spintronic devices, and magnetic nanosensors. Herein, we present a direct observation of magnetocrystalline anisotropy tuning magnetization configurations in uniaxial magnetic nanomaterials with hexagonal structure by means of three modeled samples. The magnetic configuration in polycrystalline BaFe12O19 nanoslice is a curling structure, revealing that the effect of magnetocrystalline anisotropy in uniaxial magnetic nanomaterials can be broken by forming an amorphous structure or polycrystalline structure with tiny grains. Both single crystalline BaFe12O19 nanoslice and individual particles of single-particle-chain BaFe12O19 nanowire appear in a single domain state, revealing a dominant role of magnetocrystalline anisotropy in the magnetization configuration of uniaxial magnetic nanomaterials. These observations are further verified by micromagnetic computational simulations.

Hydrogen atom induced magnetic behaviors in two-dimensional materials: insight on origination in the model of α-MoO3.

Atomic layered two-dimensional (2D) materials have become fascinating research topics due to their intriguing performances, but the limitation of nonmagnetic properties hinders their further applications. Developing versatile strategies endowing 2D materials with ferromagnetism is one of the main trends in current research studies. Herein, a hydrogen plasma strategy is introduced to dope hydrogen (H) atoms into the prototypical layered α-MoO3 nanosheets, by which ferromagnetic and exchange bias (EB) effects can be induced by H atom doping into α-MoO3 to form HxMoO3. These effects were interpreted by density functional theory (DFT) calculations. We find that H atom doping can introduce unoccupied states and induce a net magnetic moment localized on the d orbital of the Mo atom, because of the generated asymmetric distribution of electronic states on the Mo atom near the Fermi level. Moreover, the saturation magnetization and the EB field (He) of hydrogenated α-MoO3 are found to be tunable through altering the amount of H dopant. This work provides new perspectives for the effective manipulation of ferromagnetism and exchange interaction through H doping. We hope that the presented hydrogenation strategy is applicable for other kinds of 2D materials.

Direct observation of dynamical magnetization reversal process governed by shape anisotropy in single NiFe2O4 nanowire.

Discovering how the magnetization reversal process is governed by the magnetic anisotropy in magnetic nanomaterials is essential and significant to understand the magnetic behaviour of micro-magnetics and to facilitate the design of magnetic nanostructures for diverse technological applications. In this study, we present a direct observation of a dynamical magnetization reversal process in single NiFe2O4 nanowire, thus clearly revealing the domination of shape anisotropy on its magnetic behaviour. Individual nanoparticles on the NiFe2O4 nanowire appear as single domain states in the remanence state, which is maintained until the magnetic field reaches 200 Oe. The magnetization reversal mechanism of the nanowire is observed to be a curling rotation mode. These observations are further verified by micromagnetic computational simulations. Our findings show that the modulation of shape anisotropy is an efficient way to tune the magnetic behaviours of cubic spinel nano-ferrites.

A Coulomb explosion strategy to tailor the nano-architecture of α-MoO3 nanobelts and an insight into its intrinsic mechanism.

Tailoring the nanoarchitecture of materials is significant for the development of nanoscience and nanotechnology. To date, one of the most powerful strategies is convergent electron beam irradiation (EBI). However, only two main functions of knock-on or atomic displacement have been achieved to date. In this study, a Coulomb explosion phenomenon was found to occur in α-MoO3 nanobelts (NBs) under electron beam irradiation, which was controllable and could be used to efficiently create nanostructures such as holes, gaps, and other atomic/nanometer patterns on a single α-MoO3 NB. Theoretical simulations starting from the charging state, charging rate to the threshold time of Coulomb explosion reveal that the Coulomb explosion phenomenon should result from positive charging. The results also show that the multiple charged regions are quickly fragmented, and the monolayered α-MoO3 pieces can then be peeled off once the Coulombic repulsion is sufficient to break the Mo-O bonds in the crystalline structure. It is believed that this efficient and versatile strategy may open up a new avenue to tailor α-MoO3 NBs or other kind of transition metal dichalcogenides via the Coulomb explosion effect.

Atomic-scale imaging of the ferrimagnetic/diamagnetic interface in Au-Fe3O4 nanodimers and correlated exchange-bias origin.

Exchange-biased magnetic heterostructures have become one of the research frontiers due to their significance in enriching the fundamental knowledge in nanomagnetics and promising diverse applications in the information industry. However, the physical origin of their exchange bias effect is still controversial. A key reason for this is the lack of unequivocal observations of interface growth. In this work, we fill this gap by experimentally imaging the ferrimagnetic/diamagnetic interfaces of Au-Fe3O4 nanodimers at the atomic level. A different physical mechanism from the reported mechanisms is found based on the atomic-resolution observation of their interfacial structure and electronic states, which reveals that the antiferromagnetic and ferromagnetic interactions of the formed weak/strong ferrimagnetic bilayer are responsible for the intrinsic exchange-bias origin in Au-Fe3O4 nanodimers. The theoretical quantitative analysis of the exchange bias shift based on the observed interfacial occupation model agrees well with the experimental value for the exchange bias effect, strongly verifying the proposed exchange-bias mechanism.

Constructed uninterrupted charge-transfer pathways in three-dimensional micro/nanointerconnected carbon-based electrodes for high energy-density ultralight flexible supercapacitors.

A type of freestanding three-dimensional (3D) micro/nanointerconnected structure, with a conjunction of microsized 3D graphene networks, nanosized 3D carbon nanofiber (CNF) forests, and consequently loaded MnO2 nanosheets, has been designed as the electrodes of an ultralight flexible supercapacitor. The resulting 3D graphene/CNFs/MnO2 composite networks exhibit remarkable flexibility and highly mechanical properties due to good and intimate contacts among them, without current collectors and binders. Simultaneously, this designed 3D micro/nanointerconnected structure can provide an uninterrupted double charges freeway network for both electron and electrolyte ion to minimize electron accumulation and ion-diffusing resistance, leading to an excellent electrochemical performance. The ultrahigh specific capacitance of 946 F/g from cyclic voltammetry (CV) (or 920 F/g from galvanostatic charging/discharging (GCD)) were obtained, which is superior to that of the present electrode materials based on 3D graphene/MnO2 hybrid structure (482 F/g). Furthermore, we have also investigated the superior electrochemical performances of an asymmetric supercapacitor device (weight of less than 12 mg/cm(2) and thickness of ~0.8 mm), showing a total capacitance of 0.33 F/cm(2) at a window voltage of 1.8 V and a maximum energy density of 53.4 W h/kg for driving a digital clock for 42 min. These inspiring performances would make our designed supercapacitors become one of the most promising candidates for the future flexible and lightweight energy storage systems.

Enhanced gas sensing performance of electrospun Pt-functionalized NiO nanotubes with chemical and electronic sensitization.

Pt-functionalized NiO composite nanotubes were synthesized by a simple electrospinning method, and their morphology, chemistry, and crystal structure have been characterized at the nanoscale. It was found that the Pt nanoparticles were dispersed uniformly in the NiO nanotubes, and the Pt-functionalized NiO composite nanotubes showed some dendritic structure in the body of nanotubes just like thorns growing in the nanotubes. Compared with the pristine NiO nanotube based gas sensor and other NiO-based gas sensors reported previously, the Pt-functionalized NiO composite nanotube based gas sensor showed substantially enhanced electrical responses to target gas (methane, hydrogen, acetone, and ethanol), especially ethanol. The NiO-Pt 0.7% composite nanotube based gas sensor displayed a response value of 20.85 at 100 ppm at ethanol and 200 °C, whereas the pristine NiO nanotube based gas sensor only showed a response of 2.06 under the same conditions. Moreover, the Pt-functionalized NiO composite nanotube based gas sensor demonstrated outstanding gas selectivity for ethanol against methane, hydrogen, and acetone. The reason for which the Pt-functionalized NiO composite nanotube based gas sensor obviously enhanced the gas sensing performance is attributed to the role of Pt on the chemical sensitization (catalytic oxidation) of target gases and the electronic sensitization (Fermi-level shifting) of NiO.