High-voltage Asymmetric Supercapacitor Research Articles

Boosting the Energy Density of Bowl-Like MnO2@Carbon Through Lithium-Intercalation in a High-Voltage Asymmetric Supercapacitor with "Water-In-Salt" Electrolyte.

Highly concentrated "'water-in-salt"' (WIS) electrolytes are promising for high-performance energy storage devices due to their wide electrochemical stability window. However, the energy storage mechanism of MnO2 in WIS electrolytes-based supercapacitors remains unclear. Herein, MnO2 nanoflowers are successfully grown on mesoporous bowl-like carbon (MBC) particles to generate MnO2/MBC composites, which not only increase electroactive sites and inhibit the pulverization of MnO2 particles during the fast charging/discharging processes, but also facilitate the electron transfer and ion diffusion within the whole electrode, resulting in significant enhancement of the electrochemical performance. An asymmetric supercapacitor, assembled with MnO2/MBC and activated carbon (AC) and using 21m LiTFSI solution as the WIS electrolyte, delivers an ultrahigh energy density of 70.2Whkg-1 at 700W kg-1, and still retains 24.8Whkg-1 when the power density is increased to 28kWkg-1. The ex situ XRD, Raman, and XPS measurements reveal that a reversible reaction of MnO2 + xLi+ + xe-↔LixMnO2 takes place during charging and discharging. Therefore, the asymmetric MnO2/MBC//AC supercapacitor with LiTFSI electrolyte is actually a lithium-ion hybrid supercapacitor, which can greatly boost the energy density of the assembled device and expand the voltage window.

Improved electrochemical performance of sandwich-structured N-rich C@MnO2@C electrodes

This paper reports the capacitive behavior of N-enriched carbon nanofiber/MnO2/carbon nanofibers with a triple layer structure (N-rich C@MnO2@C) prepared by continuous electrospinning. N-functional groups were introduced into the upper (or lower) or middle layer of the triplex and the structures and electrochemical properties of each are compared. N-functional groups introduced into the lower (or upper) layer induce a high surface area, numerous pyrrolic-N, high electrical conductivity, and low contact angle in N-rich C@MnO2@C composites. The optimized material provides a specific capacitance of 201 Fg−1 (1 mAcm−2) with more than 84% specific capacitance retention in a symmetric two-electrode cell configuration using a 6 M KOH electrolyte. In addition, it shows a maximum energy density of 24.8 and 13.41 Whkg−1 at power densities of 400 and 10,000 Wkg−1 with excellent cycling stability (95% capacity retention after 10,000 cycles at 1 mAcm−2). An assembled asymmetric device, consisting of optimized N-rich C@MnO2@C as the positive electrode and C@MnO2@C as a negative electrode, delivers a high energy density of 41 Whkg−1 at a power density of 400 Wkg−1. This material has excellent potential as an electrode for constructing high-voltage asymmetric supercapacitors in aqueous electrolytes.

Oxygen vacancies refilling and potassium ions intercalation of δ-manganese dioxide with high structural stability toward 2.3 V high voltage asymmetric supercapacitors

Oxygen vacancies occupation and coordination environment modulation of the transition-metal based electrodes are effective strategies to improve the structural stability and electrochemical performance. In this work, the 2-methylimidazole (2-MI) doped manganese dioxide (MnO2) anchored on carbon cloth (CC) is fabricated via a simple method (MI-MnO2-x/CC), where the oxygen defects on/inside the K+ doped δ-MnO2 nanosheets are in-situ created by reductive ethanol/Mn2+ and occupied by 2-MI ligands. With the pre-embedded K+ ions and abundant ligand-refilled defects, the electronic coordination structure, structural stability and electron/ion diffusion efficiency can be effectively enhanced. Therefore, the MI-MnO2-x/CC reveals a remarkable specific capacitance of 721.2 mF cm−2 with excellent cycle durability (capacitance retention of 93.4% after 10,000 cycles) under 1.3 V operation potential window. In addition, an asymmetric supercapacitor assembled by MI-MnO2-x/CC and activated mechanical exfoliated graphene oxide yields a maximum energy density of 57.0 Wh kg−1 and a highest power density of 23.0 kW kg−1 under 2.3 V. This effective oxygen defect stabilization strategy by ligands refilling can be extended to various metal oxide-based electrodes for energy storage and conversion.

A novel TiO2/CuSe based nanocomposite for high-voltage asymmetric supercapacitors

TiO2/CuSe nanocomposites were synthesized using a straightforward wet chemical without any sophisticated template. A variety of characterization techniques, including XRD, Raman spectra, UV-Vis spectroscopy, X-ray photoelectron spectroscopy (XPS), and SEM/EDS analysis were used to elucidate the structure, morphology, and optical properties of the TiO2/CuSe nanocomposite. In addition, CV, GCD, and EIS were used to study the supercapacitive performance of the TiO2/CuSe based electrodes in detail. Three-electrode cells made of TiO2/CuSe with 0.56 g weight ratio of CuSe content (denoted as ZT-3) exhibited a maximum specific capacitance of around 184 F g−1, low Rs and Rct values (2.99 Ω/6.21 Ω) among samples investigated, facilitating the charge transport and efficient utilization of ion diffusions during the electrochemical activities. Based on the outstanding performance of the ZT-3 electrode, we further assembled an asymmetric supercapacitor (ASC) composed of ZT-3//AC and successfully expanded a voltage window (2.2 V) for the first time, which yielded a specific capacitance of 40.5 F g−1 at 1 A g−1. More importantly, the ASC exhibited a high specific energy of 27 Wh kg−1 and maintained 11.4 Wh kg−1 at a high specific power of 7125.5 W kg−1 even at a high current density of 20 A g−1 with excellent cycling stability (90% retention) after 20,000 cycles.

(Invited) Design Principles of High Voltage Asymmetric Supercapacitors Based on Electrode Energetics

As the global appetite for clean energy is growing like never before, energy storage devices like supercapacitors are attracting considerable attention. With the advent of nanotechnology and the development of advanced materials, many researchers are focused on developing new nanomaterials and nanostructures to improve the energy storage capacity of these interesting devices. However, a major limitation for its widespread application as an energy storage device is its inferior energy density compared to Li-ion batteries, though their power density is much higher. An approach to achieve high energy density is to increase the voltage output of supercapacitors. Developing high-performing asymmetric supercapacitors is a viable direction to achieve this goal. To develop asymmetric supercapacitors, two different materials are used as anode and cathode. Currently, clear guidelines to select these two different materials are missing. A reason for this void is the complex involvement of multiple factors related to electrodes and electrolytes that determine the voltage window. In this talk, the importance of the work-function of electrodes in designing a high voltage asymmetric supercapacitor will be discussed. This would help understand the energetics of electrode-electrolyte interaction, which governs the output voltage window while the supercapacitor is in operation. It enables to better design supercapacitors with higher voltage window and thus increased energy density. We will also talk about the electrode-electrolyte interaction and the influence of hydrogen and oxygen evolution overpotential on the voltage of asymmetric supercapacitors.

High Voltage Asymmetric Supercapacitors Developed by Engineering Electrode Work Functions

High Voltage Asymmetric Supercapacitors Developed by Engineering Electrode Work Functions

Core–shell-structured MnO2@carbon spheres and nitrogen-doped activated carbon for asymmetric supercapacitors with enhanced energy density

Asymmetric supercapacitors have potential applications in renewable-energy technology owing to their remarkable electrochemical properties. A high-voltage asymmetric supercapacitor was developed based on a core–shell-structured MnO2@carbon sphere composite (MnO2@CS) as the cathode, nitrogen-doped activated carbon as the anode and a neutral aqueous Na2SO4 solution as the electrolyte. MnO2@CS was successfully fabricated by hydrothermally growing MnO2 on the surface of carbon spheres. A nitrogen-containing benzoxazine resin was adopted as a precursor to produce in situ nitrogen-doped activated carbon. Such an aqueous electrolyte-based asymmetric supercapacitor can be cycled reversibly in the high-voltage region of 0–1.9 V and exhibits a superior energy density of 8 Wh kg−1 at an ultrahigh power density of 9627 W kg−1 owing to the matching of MnO2@CS and porous nitrogen-doped activated carbon. Moreover, the asymmetric supercapacitor presents acceptable cycling performance, with 74.4% retention after 1000 cycles at 1 A g−1, and a charge–discharge efficiency of the electrode of almost 100%. A high-voltage asymmetric supercapacitor was developed based on a core–shell-structured MnO2@carbon sphere composite as the cathode, and in situ nitrogen-doped activated carbon as the anode. It exhibits a superior energy density of 8 Wh kg−1 at an ultrahigh power density of 9627 W kg−1.

An Asymmetric Supercapacitor with Mesoporous NiCo2O4 Nanorod/Graphene Composite and N-Doped Graphene Electrodes

In the present work, mesoporous NiCo2O4 nanorod/graphene oxide (NiCo2O4/GO) composite was prepared by a facile and cost-effective hydrothermal method and meanwhile, N-doped graphene (N-G) was fabricated also by a hydrothermal synthesis process. NiCo2O4/GO composite and N-G were used as positive and negative electrodes for the supercapacitor, respectively, which all displayed excellent electrochemical performances. The NiCo2O4/GO composite electrode exhibited a high specific capacitance of 709.7 F g−1 at a current density of 1 A g−1 and excellent rate capability as well as good cycling performance with 84.7% capacitance retention at 6 A g−1 after 3000 cycles. A high-voltage asymmetric supercapacitor (ASC) was successfully fabricated using NiCo2O4/GO composite and N-G as the positive and negative electrodes, respectively, in 1 M KOH aqueous electrolyte. The ASC delivered a high energy density of 34.4 Wh kg−1 at a power density of 800 W kg−1 and still maintained 28 Wh kg−1 at a power density of 8000 W kg−1. Furthermore, this ASC showed excellent cycling stability with 94.3% specific capacitance retained at 5 A g−1 after 5000 cycles. The impressive results can be ascribed to the positive synergistic effects of the two electrodes. Evidently, our work provides useful information for assembling high-performance supercapacitor devices.

MnO2/thermally reduced graphene oxide composites for high-voltage asymmetric supercapacitors

We report on the facile fabrication method of MnO2/reduced graphene oxide (MnO2/rGO) composites for supercapacitor applications using the hydrothermal approach. The rGO obtained by thermal exfoliation/reduction of graphite oxide with negligible amount of oxygen (0.8wt%) served as a highly stable, conductive support for porous MnO2-nanoneedle-like particles, providing a proper rate capability to the MnO2/rGO electrode. The MnO2/rGO (3:1) was used as the positive electrode in an asymmetric supercapacitor operating at an outstanding cell voltage of 2.1V in 1molL−1 Na2SO4. Activated carbon was used as the negative electrode. The calculated energy densities reached remarkable values of 21 and 24Whkg−1 (at 100Wkg−1) for the asymmetric device charged up to 2.0 and 2.1V, respectively. The cyclability test followed by post-mortem scanning electron microscopy (SEM) studies of the electrode material revealed excellent electrochemical stability of the MnO2/rGO electrode after cycling up to 2.0V, as reflected by no loss of capacitance after 5000 cycles and preservation of the pristine morphology of the electrode material. The small decrease in capacitance (8%) after 5000 cycles of charge/discharge for the asymmetric system with an operating cell voltage of 2.1V was the result of the MnO2 morphology transformation.

Morphology controlled preparation of ZnCo2O4 nanostructures for asymmetric supercapacitor with ultrahigh energy density

Porous ZnCo2O4 nanostructures with hexamethylenetetramine (HMT) as adscititious alkali are fabricated via a facile hydrothermal route and heat treatment process. The morphology of the materials can be easily tuned from nanowires to nanobelts by simple varying the hydrothermal temperature. Due to the superiority of the porous nanostructure and the convenient ion transport, the obtained ZnCo2O4 nanostructures are further applied as electrode materials for supercapacitors and exhibit noticeable pseudocapacitive performance with high specific capacitance of 776.2 F g−1 at 1 A g−1 and good cycle stability (84.3% capacity retention at 3 A g−1). Moreover, a high-voltage asymmetric supercapacitor using the ZnCo2O4 as the anode assembled with the freeze-dried reduced graphene oxide (F-RGO) cathode displays superior electrochemical performance with an ultrahigh energy density (84.48 Wh kg−1 at 0.4 kW kg−1), which reveals a great promise for practical application in electrochemical devices.

High-voltage asymmetric supercapacitor based on MnO2 nanotubes//active carbon-multiwalled carbon nanotubes

Active carbon (AC) composited with multiwall carbon nanotubes (CNTs) is synthesized through a high-temperature process for application as a negative electrode material of capacitors. Highly porous interconnected CNTs are uniformly coated onto the surface of AC sheet with a large area that reversibly absorbed hydrogen, resulting in a supercapacitor with larger working voltage and higher pseudocapacitance. A MnO2 nanotubes//AC–CNTs asymmetric electrochemical capacitor is assembled within a wide voltage region of 0–2.4 V in 2.0 M Li2SO4 aqueous electrolyte. Moreover, high energy density and power density, as well as superior cycling stability with >92 % capacitance retention over 6000 cycles, are observed. The reversible hydrogen absorption of the negative electrode may have practical applications in supercapacitors with high energy density and power density. The AC–CNTs composite was synthesized through a high-temperature process for application as a negative electrode which electrically sorbed hydrogen that not only enlarge the potential window, but also improve the pseudocapacitance, which having excellent energy density and power density.

Electrochemical Capacitor of MnFe2O4 with Organic Li-Ion Electrolyte

Compared with batteries, electrochemical capacitors have the potential for providing higher power density and good cyclability. Pseudocapacitance results from either superficial or multi electron-transfer faradaic reactions with fast charge/discharge properties. An increasing number of metal oxides exhibiting pseudocapacitive behaviors have been discovered in recent years. We have previously discovered a new nanocrystalline oxide material having the formula of MnFe2O4that have so far been the only oxide electrode showing pseudocapacitance in both aqueous and organic Li-ion electrolytes. The discovery of this material has opened up the possibility of setting up a high voltage asymmetric supercapacitors of substantially enhanced energy density over the state-of-the-art EDLC organic supercapacitors. In this study, MnFe2O4 spinel oxide electrodes was synthesized and characterized for supercapacitor applications using organic Li-ion electrolyte. The change in structure, valence state and local atomic arrangement upon charging/discharging are investigated with in operando X-ray analyses. The operation voltage window is carefully determined in view of leakage current for the upper voltage limit. The lower voltage window is selected to maximize the energy density of the spinel oxide electrode. For the purpose of increasing the energy density of cell up for the demand of electronic devices, asymmetric capacitor is assembled using the spinel oxide as positive electrode and pre-lithiated silicon coated graphite (SGF) as negative electrode. It is shown that the asymmetric capacitor has greatly increased the energy density by asymmetric mass matching between anode and cathode and by extending the overall operation voltage window. Figure 1

Fabrication of manganese dioxide nanoplates anchoring on biomass-derived cross-linked carbon nanosheets for high-performance asymmetric supercapacitors

In this paper, MnO2 nanoplates loading on biomass-derived cross-linked carbon nanosheets have been prepared by a two-step synthesis. At first, the cross-linked carbon nanosheets derived from willow catkin are synthesized by one-step pyrolysis and activation method, then the MnO2 anchored cross-linked carbon nanosheets is prepared via in-situ hydrothermal deposition. The asymmetric supercapacitor with terrific energy and power density is assembled by employing the MnO2 anchored cross-linked carbon nanosheets as the positive electrode and the cross-linked carbon nanosheets as the negative electrode in a 1 M Na2SO4 electrolyte. The asymmetric supercapacitor displays a high energy density of 23.6 Wh kg−1 at a power density of 188.8 W kg−1 within a wide voltage rage of 0–1.9 V. In addition, the asymmetric supercapacitor exhibits excellent cycling stability with only 1.4% capacitance loss after 10000 cycles at 1 A g−1. These discoveries open up the prospect of biomass/biowaste derived carbon-based composites for high-voltage asymmetric supercapacitors with superb energy and power density performance.

High-performance aqueous asymmetric supercapacitor based on spinel LiMn2O4 and nitrogen-doped graphene/porous carbon composite

Spinel LiMn2O4 has been prepared by a facile hydrothermal procedure and the prepared LiMn2O4 exhibits superior electrochemical performance, such as high specific capacitance (409Fg−1 at 2mVs−1) and excellent rate capability (85% retention at 100mVs−1). In order to achieve high energy and power densities, a high-voltage asymmetric supercapacitor (ASC) is successfully assembled using the as-prepared LiMn2O4 as the positive electrode and nitrogen-doped graphene/porous carbon composite as the negative electrode in a Li2SO4 aqueous electrolyte. The optimized ASC delivers a high specific energy of up to 44.3Whkg−1 at a specific power of 595Wkg−1 with an output voltage of 1.8V based on the total weight of the active electrode materials. This novel strategy provides a promising route to design high-performance supercapacitors with high energy density for next-generation storage systems.

Hierarchical mesoporous NiCo2O4@MnO2 core–shell nanowire arrays on nickel foam for aqueous asymmetric supercapacitors

We demonstrate the design and fabrication of hierarchical mesoporous NiCo2O4@MnO2 core–shell nanowire arrays on nickel foam via a facile hydrothermal and electrodeposition process for supercapacitor applications. In order to increase the energy density and voltage window, a high-voltage asymmetric supercapacitor based on hierarchical mesoporous NiCo2O4@MnO2 core–shell nanowire arrays on nickel foam as the positive electrode and activated carbon (AC) as the negative electrode was successfully fabricated. The as-fabricated asymmetric supercapacitor device achieved a specific capacitance of 112 F g−1 at a current density of 1 mA cm−2 with a stable operational voltage of 1.5 V and a maximum energy density of 35 W h kg−1. The present NiCo2O4@MnO2 core–shell nanowire arrays with remarkable electrochemical properties could be considered as potential electrode materials for next generation supercapacitors in high energy density storage systems.

High voltage asymmetric supercapacitor based on MnO2 and graphene electrodes

Asymmetric supercapacitor cells based on MnO2 as positive electrode and graphene as negative electrode have been constructed with 1M Na2SO4 aqueous electrolyte. Amorphous MnO2 nanoparticles were prepared by reducing KMnO4 in N,N-dimethylformamid (DMF). Graphene was prepared by hydrothermal reduction of exfoliated graphite oxide sheets. The MnO2/graphene asymmetric supercapacitor could operate reversibly at a high cell voltage of 2.0V, showing an energy density of 25.2Whkg−1 at a power density of 100Wkg−1, which is much higher than those of symmetric supercapacitors based on MnO2/MnO2 (4.9Whkg−1) and graphene/graphene (3.6Whkg−1). Moreover, the asymmetric system exhibits capacitance retention of 96% after 500 cycles.

High-voltage asymmetric supercapacitors operating in aqueous electrolyte

The performance of supercapacitors based on different materials with pseudocapacitive properties such as several conducting polymers (ECPs), amorphous manganese dioxide (a-MnO2), and activated carbon is reported. Composite electrodes of high resiliency and good electronic conductivity were obtained by mixing the active materials with carbon nanotubes. The various limitations of all the above-mentioned materials, when used as negative and positive electrodes in traditional symmetric systems, are shown. It is demonstrated that a successful application of ECPs and a-MnO2 in supercapacitor technologies is possible only in an asymmetric configuration, i.e. with electrodes of different nature for positive and negative polarizations. Several types of asymmetric capacitors were developed by combining ECPs, a-MnO2, and activated carbon and characterized in aqueous electrolyte by galvanostatic charge–discharge, cyclic voltammetry, and impedance spectroscopy. The best device considering the specific energy and power is the asymmetric supercapacitor using a-MnO2 and poly(3,4-ethylenedioxythiophene) (PEDOT) for the positive and negative electrodes, respectively. It has an operating voltage of 1.8 V, which is attributed to different operating potentials of both electrodes, and good electrochemical stability in neutral aqueous electrolyte. According to the voltage value, the energy density of the asymmetric capacitor at a current density of 250 mA/g is found to be 13.5 W h/kg, which is about ten times more than for a symmetric capacitor based on PEDOT in an aqueous medium. The asymmetric capacitor provides two times higher power than a symmetric capacitor based on activated carbon in organic electrolyte, and is thus extremely promising for the development of environmentally friendly systems.