Synthesis of Fe3O4 nanocubes anchored in MgCo2O4 nanoneedle arrays for flexible all-solid-state asymmetric supercapacitor

The design of novel heterostructure with multifunctional characteristics is of great technical significance for the development of new energy storage devices. However, the lower conductivity of metal oxides and the accumulation caused by irreversible phase transition after multiple cycles are the main reasons for the low specific capacitance and cycle life. Herein, we synthesized bimetallic oxide MgCo₂O₄ nanoneedles with a spinel structure, and firmly anchored Fe₃O₄ nanocubes on MgCo₂O₄ nanoneedles by ion-exchange strategy. Thanks to the constructed heterostructure of nanoneedles/nanocubes, the introduction of Fe₃O₄ effectively improves the electron transport path in MgCo₂O₄ during repeated charging and discharging, and increases the effective activation sites involved in electron transfer. As a result, a higher specific capacitance of 1648 F g^?1 at 1 A g^?1 and an ultra-long cycle life of 78.6% capacitance retention after 6000 continuous charge/discharge cycles are obtained. A flexible all-solid-state asymmetric supercapcitor assembled with MgCo₂O₄-Fe₃O₄ as positive electrode and AC as negative electrode can deliver an ultra-high energy density of 78 Wh kg^?1 and maximum power density of 1.2 kW kg^?1, as well as extraordinary capacitive retention of 75.2% after 10,000 cycles. These excellent properties reveal the potential and application value of MgCo₂O₄-Fe₃O₄ in the development of high-performance supercapacitors.     

CoFe2O4@methyl cellulose core-shell nanostructure and their hybrids with functionalized graphene aerogel for high performance asymmetric supercapacitor

Recently, the use of asymmetric supercapacitors (ASC) has attracted much attention due to their optimum storage of energy and a high range of voltage. Here, we have indicated the design and fabrication of a unique ASC based on metal-spinel core-shell nanocomposite (CoFe₂O₄@MC) as a positive electrode and a p-phenylenediamine (PPDA)-graphene aerogel composite (AP) as a negative electrode in aqueous KOH electrolyte solution. The CoFe₂O₄@MC nanocomposite was prepared by the chemical deposition method. The AP was also effortlessly organized using the hydrothermal method. Considering the incorporation of methylcellulose carbohydrate polymer (MC) into the CoFe₂O₄ nanomaterial and consequently having a porous structure, a specific capacitance of 433.3 F g^?1 was obtained at the current density of 1 A g^?1 with the configuration of three electrodes. The CoFe₂O₄@MC//AP-ASC operates in the voltage range up to 2.3 V and provides a specific capacitance of 99 in 1 A g^?1. It presents an impressive energy density and power density of ~73 W h Kg^?1 and 1056 W kg^?1, respectively which prove its quality. The most important feature seems to be good cycling stability and capacity retention of 89% after 2000 cycles. These splendid outcomes show that CoFe₂O₄@MC nanocomposite possibly seems to be a satisfying choice for the next generation of devices with the capability of energy storage.     

Microwave-assisted hydrothermal synthesis of crystalline WO3-WO3·0.5H2O mixtures for pseudocapacitors of the asymmetric type

Crystalline tungsten oxide mixtures, WO₃-WO₃·0.5H₂O, prepared by microwave-assisted hydrothermal (MAH) synthesis at 180 °C for various periods, show capacitive-like behavior at 200 mV s⁻¹ and C_S ≈ 290 F g⁻¹ at 25 mV s⁻¹ in 0.5 M H₂SO₄ between −0.6 and 0.2 V. Oxide rods can be obtained via the MAH process even when the synthesis time is only 0.75 h while WO₃·0.5H₂O sheets with poor capacitive performances are obtained by a normal hydrothermal synthesis process at the same temperature for 24 h. The aspect ratio of tungsten oxide rods is found to increase with prolonging the MAH time while all oxides consist of WO₃ and WO₃·0.5H₂O. The oxide mixtures prepared by the MAH method with annealing in air at temperatures ≤400 °C show promising performances for electrochemical capacitors (ECs). Due to the narrow working potential window of the oxide mixtures, an aqueous EC of the asymmetric type, consisting of a WO₃-WO₃·0.5H₂O anode and a RuO₂·xH₂O cathode, with a potential window of 1.6 V is demonstrated in this work, which shows the device energy and power densities of 23.4 W kg⁻¹ and 5.2 kW kg⁻¹, respectively.     

Flexible all-solid-state asymmetric supercapacitor based on three-dimensional MoS2/Ketjen black nanoflower arrays

High electrochemical properties of negative electrode materials are highly desirable for flexible asymmetric supercapacitors (ASCs). Although benefiting from the unique structure and broad operation potential, molybdenum disulfide (MoS₂) has caused concern as a negative electrode material because its low electrochemical stability and poor conductivity hinder the exploitation of its application in flexible ASCs. Here we investigated a facile two-step hydrothermal approach to fabricate MoS₂/Ketjen black (KB) composites on flexible carbon cloth. Following the construction of flower-like MoS₂ on carbon cloth, KB nanospheres were embedded in MoS₂ via a secondary hydrothermal route. The as-prepared MoS₂/KB electrode presents a high capacitance of 429 F g⁻¹ at a current specific of 1 A g¹. In addition, the hybrid ASC device of NiCo₂O₄//MoS₂/KB was built, which delivers a high energy density of 25.7 Wh kg⁻¹ and power density of 16 kW kg⁻¹. These results are ascribed to the favorable structure of MoS₂ and inherently superior conductivity of KB, which improves wettability, structural stability and electronic conductivity. In brief, the proposed all-solid-state ASC device offers potential application in future portable electronics and flexible energy storage devices.     

Electrochemical profile of an asymmetric supercapacitor using carbon-coated LiTi2(PO4)3 and active carbon electrodes

In this work, we reported an asymmetric supercapacitor in which active carbon (AC) was used as a positive electrode and carbon-coated LiTi₂(PO₄)₃ as a negative electrode in 1 M Li₂SO₄ aqueous electrolyte. The LiTi₂(PO₄)₃/AC hybrid supercapacitor showed a sloping voltage profile from 0.3 to 1.5 V, at an average voltage near 0.9 V, and delivered a capacity of 30 mAh g⁻¹ and an energy density of 27 Wh kg⁻¹ based on the total weight of the active electrode materials. It exhibited a desirable profile and maintained over 85% of its initial energy density after 1000 cycles. The hybrid supercapacitor also exhibited an excellent rate capability, even at a power density of 1000 W kg⁻¹, it had a specific energy 15 Wh kg⁻¹ compared with 24 Wh kg⁻¹ at the power density about 200 W kg⁻¹.     

Characterization of MnFe2O4/LiMn2O4 aqueous asymmetric supercapacitor

A new type of asymmetric supercapacitor containing a MnFe₂O₄ negative electrode and a LiMn₂O₄ positive electrode in aqueous LiNO₃ electrolyte has been synthesized and characterized. The nanocrystalline MnFe₂O₄ anode material has a specific capacitance of 99 F g⁻¹ and the LiMn₂O₄ cathode a specific capacity of 130-100 mAh g⁻¹ under 10-100 C rate. The cell has a maximum operating voltage window of ca. 1.3 V, limited by irreversible reaction of MnFe₂O₄ toward reducing potential. The specific power and specific energy of the full-cell increase with increasing anode-to-cathode mass ratio (A/C) and saturate at A/C ∼4.0, which gives specific cell energies, based on total mass of the two electrodes, of 10 and 5.5 Wh kg⁻¹ at 0.3 and 1.8 kW kg⁻¹, respectively. The cell shows good cycling stability and exhibits significantly slower self-discharge rate than either the MnFe₂O₄ symmetric cell or the other asymmetric cells having the same cathode but different anode materials, including activated carbon fiber and MnO₂.     

Synthesis of CuCo2S4 nanosheet arrays on Ni foam as binder-free electrode for asymmetric supercapacitor

Recently more and more concerns have been paid on ternary metal sulfides for use in supercapacitors because of their better electrochemical performances compared with binary counterparts. In this work, CuCo₂S₄ nanosheet arrays on Ni foam were prepared by a sequential ion-exchange strategy under hydrothermal conditions, where Co₃O₄ was converted into Co₄S₃ by an anion-exchange reaction between Co₃O₄ and S^2? ions, subsequently the Co₄S₃ was transformed into CuCo₂S₄ through a cation-exchange reaction with Cu²⁺ ions. The as-prepared CuCo₂S₄ was characterized by powder X-ray diffraction, high-resolution X-ray photoelectron spectroscopy, field emission scanning electron microscopy and transmission electron microscopy. The CuCo₂S₄ arrays were composed of interconnected thin nanosheets with thickness of about 10 nm. The CuCo₂S₄ nanosheet arrays on Ni foam were directly employed as a binder-free electrode showing a high specific capacitance of 3132.7 F g^?1 at a current density of 1 A g^?1. Besides, an asymmetric supercapacitor based on this synthesized CuCo₂S₄ electrode as positive electrode and active carbon as negative electrode can deliver a high energy density of 46.1 Wh kg^?1 at a power density of 991.6 W kg^?1, and exhibits good rate capability and cycling stability.     

Adjustment of electrodes potential window in an asymmetric carbon/MnO2 supercapacitor

The electrodes mass ratio of MnO₂/activated carbon supercapacitors has been varied in order to monitor its influence on the potential window of both electrodes and consequently to optimize the operating voltage. It appeared that the theoretical mass ratio (R = 2), calculated considering an equivalent charge passed across both electrodes, is underestimated. It was demonstrated that R values of 2.5-3 are better adapted for this system; the extreme potential reached for each electrode is close to the stability limits of the electrolyte and active material, allowing a maximum voltage to be reached. During galvanostatic cycling up to 2 V, the best performance was obtained with R = 2.5. The specific capacitance increased from 100 to 113 F g⁻¹ during the first 2000 cycles, then decayed up to 6000 cycles and finally stabilized at 100 F g⁻¹. SEM images of the manganese based electrode after various numbers of thousands cycles exhibited dramatic morphological modifications. The later are suspected to be due to Mn(IV) oxidation and dissolution at high potential values. Hence, the evolution of specific capacitance during cycling of the asymmetric capacitor is ascribed to structural changes at the positive electrode.     

Mesoporous MnO2 fibers as an efficient bifunctional absorber for high-performance lithium-sulfur batteries

A novel morphology of mesoporous MnO₂ fibers (MOF) are successfully prepared for the first time as host materials for lithium-sulfur (LiS) batteries. The as-prepared mesoporous MnO₂ fibers can restrain the polysulfides dissolution via chemical bonding and physical trapping at the same time. As a result, the mesoporous MnO₂ fibers sulfur (MOF/S) composites exhibit excellent cycle performance. The MOF/S composite electrodes deliver a high initial capacity of 1015 mAh g^?1 and maintain 815 mAh g^?1 after 200 cycles at 0.1 C.     

One step synthesis of rGO-Ni3S2 nano-cubes composite for high-performance supercapacitor electrodes

In the last decade, supercapacitors possessing high power density and cyclic stability have attracted great interests in various applications. Graphene-based composite electrodes are known as a promising candidate for supercapacitors due to synergistic effects. For the first time, in this work, we develop a simple one-step hydrothermal synthesis of graphene wrapped Ni₃S₂ nanocubes (rGO-Ni₃S₂) composite for high-performance and low-cost supercapacitor electrodes. The rGO-Ni₃S₂ electrode exhibits an ultrahigh specific capacity of 616 C g^?1 at the current density of 1 A g^?1 with excellent cycling durability of 92.7% after 5000 cycles, which is much better when compared with the counterpart without graphene (pure Ni₃S₂). We attribute the remarkable performance of the rGO-Ni₃S₂ electrode to the synergistic effects of the graphene as the conductive support and Ni₃S₂ cubics as the pseudocapacitive material. This work constitutes a step forward towards the development of low-cost and high-performance supercapacitors for the next generation of portable electronics.     

Hierarchically structured TiO2@MnO2 nanowall arrays as potential electrode material for high-performance supercapacitors

We have reported a facile route for the fabrication of TiO₂@MnO₂ core–shell nanostructures for use as an electrode material, using a simple hydrothermal process for supercapacitor applications. Field-emission scanning electron microscopy and transmission electron microscopy studies confirmed the formation of a MnO₂ nanowall shell structure on the core of TiO₂ nanorod surfaces. The nanostructured TiO₂@MnO₂ core–shell was used as an electrode material, which exhibited excellent electrochemical properties with an improved areal capacitance of 22.19 mF cm⁻² (TM-3) at a scan rate of 5 mV s⁻¹ in a 1-M Na₂SO₄ electrolyte solution. Moreover, the electrode material demonstrated excellent performance with long term cycling stability, by retaining 85% of its initial capacitance after 4000 cycles.     

Industrial scale modelling of the thermochemical energy storage system based on CO2 + 2NH3 ↔ NH2COONH4 equilibrium

The thermochemical storage of energy by the system carbon dioxide, ammonia and ammonium carbamate is studied in detail. In particular, the kinetics and the thermodynamics of the reversible reaction is studied. We give two industrial models for the operation of this system. In the first, the separation of the gases NH₃ and CO₂ is achieved by compression and liquefaction of NH₃. In the second, a method of separation of the gases is proposed which is based on the solubility of NH₃ in ethanol while CO₂ is practically insoluble. The operation of this system is examined both in closed form and in the case in which CO₂ is rejected in the atmosphere, and it is taken from alcoholic fermentation or from the combustion gases of power plants burning lignite. The mass and energy balance is given, for each case, and the amount of energy losses by the use of this storage system is calculated. Finally, we give some estimates for the area of solar collectors and the amount of chemicals which are required in order to cover the energy needs of a community.     

Long-term cycling behavior of asymmetric activated carbon/MnO2 aqueous electrochemical supercapacitor

Activated carbon–MnO₂ hybrid electrochemical supercapacitor cells have been assembled and characterized in K₂SO₄ aqueous media. A laboratory cell achieved 195,000 cycles with stable performance. The maximal cell voltage was 2 V associated with 21 ± 2 F g⁻¹ of total composite electrode materials (including activated carbon and MnO₂, binder and conductive additive) and an equivalent serie resistance (ESR) below 1.3 Ω cm². Long-life cycling was achieved by removing dissolved oxygen from the electrolyte, which limits the corrosion of current collectors. Scaling up has been realized by assembling several electrodes in parallel to build a prismatic cell. A stable capacity of 380 F and a cell voltage of 2 V were maintained over 600 cycles. These encouraging results show the interest of developing such devices, including non-toxic and safer components as compared to the current organic-based devices.     

Study on the WO3 dry lithiation for all-solid-state electrochromic devices

In this report, a simple WO₃ dry lithiation is proposed for fabrication of all-solid-state electrochromic devices and characterized completely by X-ray photoelectron spectroscopy and electrochemical method. Lithiation is carried out by electron-beam evaporation of metal lithium, and the lithiated films have different components and electrochromic properties with different lithiation degrees. It is found that if Li/W ratio is less than 0.25, tungsten bronze Li_xW0₃ is formed and the lithiated by wet method. Finally, a lithium-based all-solid-state electrochromic device with proper lithiation degree is fabricated using this dry method.     

LiBH4 as solid electrolyte for Li-ion batteries with Bi2Te3 nanostructured anode

The present study highlights the first-ever application of fastest lithium (Li) ion conducting complex hydride containing cluster anions, namely lithium borohydride (LiBH₄) into an all-solid-state Li-ion battery having Bi₂Te₃ as anode material. Bi₂Te₃ nanostructures were prepared by the simple wet chemical method and characterized by their crystal structure, morphology and electronic structure using X-ray diffraction (XRD), scanning electron microscopy (SEM), Transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). SEM and TEM experiments revealed the dimensions as 20–60 nm for nanoparticles and 30–90 nm for nanosheets. The formation of Bi₂Te₃ nanostructures along with Bi₂O₃ as the residual phase is confirmed by XRD analysis. The crystallite size of nanoparticles and nanosheets are calculated as 19 nm and 39 nm respectively from XRD profile. The XPS study also confirms the formation of nanostructured Bi₂Te₃ along with Bi₂O₃. Finally, the electrochemical performance of these nanostructures is observed using the galvanostatic charge-discharge curve at 0.1C and 0.5C.     

Redox active MoO3-Polyaniline hybrid composite for hydrogen evolution reaction and supercapacitor application

A simple oxidative polymerization approach has been used to synthesize MoO₃/PANI hybrid composite for energy conversion and storage application. In this carbon-free energy conversion process, as-developed electrocatalyst (MoO₃/PANI hybrid composite) has significantly improved the molecular hydrogen generation via electro-reduction of protons. The hydrogen evolution kinetics parameters, overpotential at 10 mA cm⁻² is ∼110 mV and Tafel slope 132 mV/dec have been obtained for the as-developed catalyst which suggests the reaction is controlled by Volmer-limited reaction step. Further the as-developed material has been tested for energy storage application and exhibited specific capacitance of 680 F. g⁻¹.     

Catalytic effect of monoclinic WO3, hexagonal WO3 and H0.23WO3 on the hydrogen sorption properties of Mg

The H sorption properties of mixtures Mg + WO₃ (having various structures) and Mg + H_0.23WO₃ are reported. First, the higher conversion of Mg into MgH₂ during reactive mechanical grinding (under 1.1 MPa of H₂) for higher WO₃ content is due to the improvement of the milling efficiency. Then, it is shown that the hydrogen absorption properties are almost independent of the crystal structure of the catalyst and that only the particles' size and the specific surface play a major role. Finally, for the desorption process, it appears that the chemical composition and structure of the catalyst, together with the particle size and specific surface have an effect.     

Li3V2(PO4)3/C composite as an intercalation-type anode material for lithium-ion batteries

The carbon coated monoclinic Li₃V₂(PO₄)₃ (LVP/C) powder is successfully synthesized by a carbothermal reduction method using crystal sugar as the carbon source. Its structure and physicochemical properties are investigated using X-ray diffraction (XRD), scanning electron microscopy, high-resolution transmission electron microscopy and electrochemical methods. The LVP/C electrode exhibits stable reversible capacities of 203 and 102 mAh g⁻¹ in the potential ranges of 3.0-0.0 V and 3.0-1.0 V versus Li⁺/Li, respectively. It is identified that the insertion/extraction of Li⁺ undergoes a series of two-phase transition processes between 3.0 and 1.6 V and a single phase process between 1.6 and 0.0 V. The ex situ XRD patterns of the electrodes at various lithiated states indicate that the monoclinic structure can still be retained during charge-discharge process and the insertion/deinsertion of lithium ions occur reversibly, which provides an excellent cycling stability with high energy efficiency.     

Modeling the optical properties of WO3 and WO3-SiO2 thin films

The optical properties and surface morphology of sol-gel spin coated WO₃ and WO₃-SiO₂ composite films annealed at 250 and are investigated. For the purpose of extracting the optical parameters of the films, a novel form for the dielectric function is introduced, consisting of two Tauc-Lorentz oscillators and an Urbach tail component, which is suited for amorphous multi-transition materials with substantial subgap absorption. The evolution of the refractive indices, transmittances, and band gaps with doping is marked by sizable shifts at 2.0-2.5% SiO₂ doping for the films, and 4.0-4.5% doping for the films. In addition, pronounced changes in the surface roughness of the films occur at these doping values.     

Convenient storage of concentrated hydrogen peroxide as a CaO2·2H2O2(s)/H2O2(aq) slurry for energy storage applications

Prior investigations have proposed, and successfully implemented, a stand-alone supply of aqueous hydrogen peroxide for use in fuel cells. An apparent obstacle for considering the use of aqueous hydrogen peroxide as an energy storage compound is the corrosive nature of the nominally required 50 wt.% maximum concentration. Here we propose storage of concentrated hydrogen peroxide in a high weight percent solid slurry, namely the equilibrium system of CaO₂·2H₂O₂(s)/H₂O₂(aq), that mitigates much of the risk associated with the storage of such high concentrations. We have prepared and studied surrogate slurries of calcium hydroxide/water that are assumed to resemble the peroxo compound slurries. These slurries have the consistency of a paste rather than a distinct two-phase (liquid plus solid) system. This paste-like property of the prepared surrogates enable them to be contained within a 200 lines-per-inch. (LPI) nickel mesh screen (33.6% open area) with no solids leakage, and only liquid transport driven by an adsorbent material is placed in physical contact on the exterior of the screen. This hydrogen peroxide slurry approach suggests a convenient and safe mechanism of storing hydrogen peroxide for use in, say, vehicle applications. This is because fuel cell design requires only aqueous hydrogen peroxide use, that can be achieved using the separation approach utilizing the screen material here. This proposed method of storage should mitigate hazards associated with unintentional spills and leakage issues arising from aqueous solution use.