Effect of vanadium doping on MXene-based supercapacitor

The two-dimensional titanium carbide MXene (Ti₃C₂T_x) acts as a promising pseudocapacitive material for supercapacitor electrodes. In this paper, the properties of vanadium-doped titanium carbide MXene (Ti₃C₂T_x) are tuned using a simple hydrothermal method to intercalate the alkali metal adsorbates (K⁺) into the electrode material. The synthesis of the supercapacitor device is carried on glass substrate as well as on a flexible graphite sheet. The X-ray diffraction and scanning electron microscopy are conducted to observe the change in structural properties of vanadium-doped MXene. The cyclic voltammetry and galvanostatic charge–discharge are carried out on Metrohm autolab workstation. The ratio of ammonium vanadate and MXene has been varied from 0.025:0.1 to 0.1:0.1 with a step size of 0.025 to obtain the capacitance results. The results depict that the ratio of 0.025:1 shows the highest capacitance of 258.07 mF/cm² and 1107 mF/cm² in 6 M KOH (20 mV/s) on glass and graphite substrate, respectively. This is mainly because the ratio of 0.025:1 provides the maximum exfoliation which allows electrolyte ions to penetrate in the active material and thus, facilitates fast electron transport resulting in high-performance supercapacitors. Further, this paper also discusses the successful fabrication of the supercapacitor devices on a flexible graphite sheet for the first time. The results show that the capacitance value on flexible substrate is at par with that of the glass substrate. To further understand the increased capacitive properties of vanadium-doped MXene, the processes involving charge transfer and mass transport are investigated by performing electrochemical impedance spectroscopy (EIS). The radius on the EIS plot of vanadium-doped MXene is smaller than that of the undoped DMSO MXene, which indicates that the vanadium doping made the charge transfer easier. Moreover, the capacitance retention of 92.7% and 82.2% is achieved on graphite as well as glass substrate after 3000 cycles.

     

Supercapacitance of ruthenium oxide deposited on titania and titanium substrates

Titanium planar sheet formed by a chemical polishing process and titania nanotube array formed by an electrochemical anodization process are used as electrode substrates, on which electroactive ruthenium oxides are deposited by an electroreduction and electrooxidation process for supercapacitor applications. Morphological characterization and electrochemical properties of the electrode substrates and ruthenium oxide electrodes have been investigated. Crystalline titania nanotube array shows a much higher electric double layer capacitance than titanium planar sheet due to its high surface area of nanotube walls. Additionally, the well-defined ruthenium oxide–titania/titanium nanotube array electrode exhibits a much higher redox supercapacitance and a lower capacitance decay than ruthenium oxide/titanium planar film electrode. Such a superior energy-storage performance of ruthenium oxide–titania/titanium is ascribed to highly accessible nanotube channels for the reversible redox reaction of ruthenium oxide. The modification strategy of ruthenium oxide electrode by introducing highly ordered nanotube array structure instead of planar film structure can significantly improve specific capacitance as well as cyclic charge-discharge stability.     

Activated carbon from agave wastes (agave tequilana) for supercapacitors via potentiostatic floating test

To prepare an efficient supercapacitor, an activated carbon from agave wastes was prepared and their electrochemical performance was evaluated as a novel electrode for supercapacitor. The carbon was prepared by two thermal pyrolysis processes under nitrogen atmosphere. The first pyrolysis was achieved at 500 °C until the charring of the bagasse; in the second pyrolysis step, the char was impregnated with different mass ratios of KOH (1:2–1:4) and thermally treated at 800 or 900 °C, for 1 h under N₂ flow. The textural analysis showed that the activated carbon had a specific surface area of 1462 m² g^?1 and depicted a type I isotherm (IUPAC) characteristic of a microporous carbon. Raman spectroscopy and XRD measurements confirm that the activated carbon contains a small graphitization degree and a disordered structure. The electrochemical study of the symmetric carbon supercapacitor was carried out in 1 M Li₂SO₄ solution as the electrolyte. The electrochemical performance of the coin cell supercapacitor was evaluated under an accelerated aging floating test consisting of potentiostatic steps at different voltages (1.5, 1.6 and 1.8 V) for 10 h followed by galvanostatic charge/discharge sequences, and the overall procedure summarized a floating time up to 200 h. The highest capacitance was observed at a floating voltage of 1.5 V, with a large initial specific capacitance of 297 F g^?1.

     

Supercapacitor Electrodes Based on Hierarchical Mesoporous MnOx/Nitrided TiO2 Nanorod Arrays on Carbon Fiber Paper

Metal nitride nanoarrays are attractive to electrochemical energy storage and in this work, hierarchical mesoporous manganese oxide (MnO_x) nanoflakes and nitrided TiO₂ nanorod arrays (NTNA) are prepared on carbon fiber paper (CFP) by hydrothermal synthesis and electrodeposition. The MnO_x/NTNA/CFP electrode delivers outstanding electrochemical performance such as high areal capacitance of 327 mF cm⁻² at a current density of 0.25 mA cm⁻² and good cycling stability with 96% retention after 5000 cycles. Compared to the MnO_x/TiO₂/CFP and MnO_x/CFP electrodes, the MnO_x/NTNA/CFP electrode possesses better electrochemical properties such as higher areal capacitance, better electrochemical activity, and cycling life. The enhanced performance can be attributed to the nitrided TiO₂ nanorod arrays with higher conductivity offering low electrochemical impedance and fast ion/electron transfer. The MnO_x/NTNA/CFP electrode is a promising candidate in high‐performance supercapacitor applications.     

Rational Construction of Hollow Core‐Branch CoSe2 Nanoarrays for High‐Performance Asymmetric Supercapacitor and Efficient Oxygen Evolution

Metal selenides have great potential for electrochemical energy storage, but are relatively scarce investigated. Herein, a novel hollow core‐branch CoSe₂ nanoarray on carbon cloth is designed by a facile selenization reaction of predesigned CoO nanocones. And the electrochemical reaction mechanism of CoSe₂ in supercapacitor is studied in detail for the first time. Compared with CoO, the hollow core‐branch CoSe₂ has both larger specific surface area and higher electrical conductivity. When tested as a supercapacitor positive electrode, the CoSe₂ delivers a high specific capacitance of 759.5 F g^?1 at 1 mA cm^?2, which is much larger than that of CoO nanocones (319.5 F g^?1). In addition, the CoSe₂ electrode exhibits excellent cycling stability in that a capacitance retention of 94.5% can be maintained after 5000 charge–discharge cycles at 5 mA cm^?2. An asymmetric supercapacitor using the CoSe₂ as cathode and an N‐doped carbon nanowall as anode is further assembled, which show a high energy density of 32.2 Wh kg^?1 at a power density of 1914.7 W kg^?1, and maintains 24.9 Wh kg^?1 when power density increased to 7354.8 W kg^?1. Moreover, the CoSe₂ electrode also exhibits better oxygen evolution reaction activity than that of CoO.     

Titanium Disulfide Coated Carbon Nanotube Hybrid Electrodes Enable High Energy Density Symmetric Pseudocapacitors

While electrochemical supercapacitors often show high power density and long operation lifetimes, they are plagued by limited energy density. Pseudocapacitive materials, in contrast, operate by fast surface redox reactions and are shown to enhance energy storage of supercapacitors. Furthermore, several reported systems exhibit high capacitance but restricted electrochemical voltage windows, usually no more than 1 V in aqueous electrolytes. Here, it is demonstrated that vertically aligned carbon nanotubes (VACNTs) with uniformly coated, pseudocapacitive titanium disulfide (TiS₂) composite electrodes can extend the stable working range to over 3 V to achieve a high capacitance of 195 F g^?1 in an Li‐rich electrolyte. A symmetric cell demonstrates an energy density of 60.9 Wh kg^?1—the highest among symmetric pseudocapacitors using metal oxides, conducting polymers, 2D transition metal carbides (MXene), and other transition metal dichalcogenides. Nanostructures prepared by an atomic layer deposition/sulfurization process facilitate ion transportation and surface reactions to result in a high power density of 1250 W kg^?1 with stable operation over 10 000 cycles. A flexible solid‐state supercapacitor prepared by transferring the TiS₂–VACNT composite film onto Kapton tape is demonstrated to power a 2.2 V light emitting diode (LED) for 1 min.     

Capacitive behavior studies on electrical double layer capacitor using poly (vinyl alcohol)–lithium perchlorate based polymer electrolyte incorporated with TiO2

Electric double layer capacitors (EDLCs) based on activated carbon electrodes and poly (vinyl alcohol)–lithium perchlorate (PVA–LiClO₄)-nanosized titania (TiO₂) doped polymer electrolyte have been fabricated. Incorporation of TiO₂ into PVA–LiClO₄ system increases the ionic conductivity. The highest ionic conductivity of 1.3 × 10⁻⁴ S cm⁻¹ is achieved at ambient temperature upon inclusion of 8 wt.% of TiO₂. Differential scanning calorimetry (DSC) analyses reveal that addition of TiO₂ into polymer system increases the flexibility of polymer chain and favors the ion migration. Scanning electron microscopy (SEM) analyses display the surface morphology of the nanocomposite polymer electrolytes. The electrochemical stability window of composite polymer electrolyte is in the range of −2.3 V to 2.3 V as shown in cyclic voltammetry (CV) studies. The performance of EDLC is evaluated by electrochemical impedance spectroscopy (EIS), CV and galvanostatic charge–discharge technique. CV test discloses a nearly rectangular shape, which signifies the capacitive behavior of an ELDC. The EDLC containing composite polymer electrolyte gives higher specific capacitance value of 12.5 F g⁻¹ compared to non-composite polymer electrolyte with capacitance value of 3.0 F g⁻¹ in charge–discharge technique. The obtained specific capacitance of EDLC is in good agreement with each method used in this present work. Inclusion of filler into the polymer electrolyte enhances the electrochemical stability of EDLC.     

A new asymmetric supercapacitor based on λ-MnO2 and activated carbon electrodes

Lithium-ion intercalated compound λ-MnO₂ was used as positive electrode in asymmetric supercapacitor with activated carbon used as negative electrode in 1 mol L^− 1 Li₂SO₄ aqueous electrolyte solution. Phase composition, morphology and particle sizes of λ-MnO₂ were studied by powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). Electrochemical capacitive performance of the asymmetric supercapacitor was tested by cyclic voltammetry and galvanostatic charge-discharge tests. The results show that the asymmetric supercapacitor has electrochemical capacitance performance within wide potential range of 0-2.2 V. The specific capacitance is 53 F g^− 1 at a constant current density of 10 mA cm^− 2. The energy density is 36 W h kg^− 1 with a power density of 314 W kg^− 1. It is obvious that λ-MnO₂ is a potential electrode material for asymmetric supercapacitor.     

A structural supercapacitor based on activated carbon fabric and a solid electrolyte

ABSTRACT

This paper presents investigations to create a structural supercapacitor with activated carbon fabric electrodes and a solid composite electrolyte, consisting of organic liquid electrolyte 1?M TEABF₄ in propylene carbonate and an epoxy matrix where different compositions were considered of 1:2, 1:1 and 2:1 w/w epoxy:liquid electrolyte. Vacuum-assisted resin transfer moulding was used for the impregnation of the electrolyte mixture into the electrochemical double layer capacitor (EDLC) assembly. The best electrochemical performance was exhibited by the 1:2 w/w epoxy: liquid electrolyte ratio, with a cell equivalent-in-series resistance of 160?Ω?cm² and a maximum electrode-specific capacitance of 101.6?mF?g^?1 while the flexural modulus and strength were 0.3?GPa and 29.1?MPa, respectively, indicating a solid EDLC device.     

Flexible Wire‐Shaped Supercapacitors in Parallel Double Helix Configuration with Stable Electrochemical Properties under Static/Dynamic Bending

Wire‐shaped flexible supercapacitors (SCs) have aroused much attention due to their small size, light weight, high flexibility, and deformability. However, the previously reported wire‐shaped SCs usually involve complex assembly processes, encounter potential structural instabilities, and the influence of dynamic bending on the electrochemical stability of wire‐shaped SCs is also not clear. Here, a parallel double helix wire‐shaped supercapacitor (PDWS) protocol has been developed with two symmetric titanium@MnO₂ fiber electrodes winded on a flexible nylon fiber by a simple and reliable process. The PDWSs show an operate voltage of 0.8 V, a high capacitance of 15.6 mF cm^–2 and an energy density of 1.4 µWh cm^–2. Due to rational structure design, the PDWSs demonstrate excellent mechanical and electrochemical stability under both static and dynamic deformations. Over 3500 bending cycles, 88.0% of the initial capacitance can still be retained. In terms of dynamic bending, it is found that the cyclic voltammetry curves show periodically fluctuations simultaneously with the bending frequency and the intensity of fluctuation increases with higher bending frequency, while the dynamic capacitance is almost not affected. With extraordinary mechanical flexibility and excellent electrochemical stability, the high performance PDWS is considered to be a promising power source for wearable electronics.     

A self-healing hydrogel electrolyte towards all-in-one flexible supercapacitors

The self-healing electrolytes play an essential role in self-healing supercapacitors. Herein, poly (vinyl alcohol)/sulphuric acid (PVA/H₂SO₄) hydrogel electrolytes with self-healing properties are prepared, which has been achieved by dynamic hydrogen bonds between PVA chains. The obtained PVA hydrogel displays fast self-healing capability, reliable mechanical performance (stress at 0.29 MPa after stretching to 238%) and high ionic conductivity (57.8 mS cm^?1). Based on these excellent properties, an all-in-one self-healing supercapacitor is assembled by in situ polymerization of aniline on the surface of PVA/H₂SO₄ hydrogel electrolyte. The assembled all-in-one supercapacitor shows outstanding capacitance performance (specific capacitance 504 mF cm^?2 at current density of 0.2 mA cm^?2 and energy density 35 μWh cm^?2 at power density 100 μW cm^?2), good cycle stability (after 5000 cycles of charging and discharging, the capacitance retention rate is 77%), excellent flexibility and considerable self-healing performance (69% capacitance retention rate after the fifth self-healing cycle). This self-healing supercapacitor will promote the development of self-healing energy storage devices in wearable electronics.

     

Vertically Aligned Graphene–Carbon Fiber Hybrid Electrodes with Superlong Cycling Stability for Flexible Supercapacitors

Graphene electrode–based supercapacitors are in high demand due to their superior electrochemical characteristics. A major bottleneck of using the supercapacitors for commercial applications lies in their inferior electrode cycle life. Herein, a simple and facile method to fabricate highly efficient supercapacitor electrodes using pristine graphene sheets vertically stacked and electrically connected to the carbon fibers which can result in vertically aligned graphene–carbon fiber nanostructure is developed. The vertically aligned graphene–carbon fiber electrode prepared by electrophoretic deposition possesses a mesoporous 3D architecture which enabled faster and efficient electrolyte‐ion diffusion with a gravimetric capacitance of 333.3 F g^?1 and an areal capacitance of 166 mF cm^?2. The electrodes displayed superlong electrochemical cycling stability of more than 100 000 cycles with 100% capacitance retention hence promising for long‐lasting supercapacitors. Apart from the electrochemical double layer charge storage, the oxygen‐containing surface moieties and α‐Ni(OH)₂ present on the graphene sheets enhance the charge storage by faradaic reactions. This enables the assembled device to provide an excellent gravimetric energy density of 76 W h kg^?1 with a 100% capacitance retention even after 1000 bending cycles. This study opens the door for developing high‐performing flexible graphene electrodes for wearable energy storage applications.     

A Flexible Ionic Liquid Gelled PVA‐Li2SO4 Polymer Electrolyte for Semi‐Solid‐State Supercapacitors

A novel high‐performance flexible gel polymer electrolyte (FGPE) for supercapacitors is prepared by a freeze‐drying method. In the presence of 1‐butyl‐3‐methylimidazolium chloride (BMIMCl) ionic liquid, Li₂SO₄ can easily be added into poly(vinyl alcohol) (PVA) aqueous solution over a large concentration range. The resultant FGPE demonstrates considerably high ionic conductivity (37 mS cm⁻¹) and a high fracture strain at 100% elongation at the optimal weight ratio of PVA:BMIMCl:Li₂SO₄ = 1:3:2.2. The supercapacitor fabricated with the resultant FGPE and activated carbon electrodes shows an electrode‐specific capacitance of 136 F g⁻¹ with a stable operating voltage of 1.5 V, a maximum energy density of 10.6 Wh kg⁻¹, and a power density of 3400 W kg⁻¹. Double supercapacitors in series can efficiently drive a light emitting diode (LED) bulb for over 5 min and the retention of the specific capacitance reaches 90% even after 3000 charge–discharge cycles. The ionic conductivity and charge–discharge behaviors of the resultant FGPE are not affected by bending up to 180°. The flexible supercapacitor device shows only a small capacitance loss of 18% after 1000 cycles of 135° bending.     

The effects of surface modification on the supercapacitive behaviors of carbon derived from calcium carbide

The capacitive behaviors of calcium-carbide-derived carbon (CCDC) before and after nitric acid (HNO₃) modification are investigated. The structure and morphology of the HNO₃-modified CCDC (M-CCDC) are examined by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The performances of the supercapacitor using M-CCDC as electrode active material are studied by cyclic voltammetry, galvanostatic charge/discharge, electrochemical impedance spectroscopy, and cycle life measurements. The results show that the capacitance of the supercapacitor increases from 154.7 to 196.5 F g⁻¹ and the capacitance decay is only 1.3% over 10,000 cycles for the M-CCDC, which exhibits higher capacitive performance than the pristine CCDC electrode in the aqueous electrolyte solution. The superiority of the M-CCDC in capacitance properties is caused by the variations of surface wettability and the interstitial pore structure of CCDC, which results from the introduction of polar oxygen functional groups onto the CCDC surface by HNO₃ modification.     

Development of 3D printing technology for the manufacture of flexible electric double-layer capacitors

This study presents a novel process and manufacturing system for the fabrication of Electric Double-Layer Capacitors (EDLCs) as energy storage devices. It shows an approach for printing multilayer EDLC components using 3D printing technology. This process allows layers of activated carbon (AC) slurry, gel electrolyte, and composite solid filaments to be printed with high precision. The study describes the detailed process of deposition of the AC and gel electrolyte using the dual nozzle system. The performance of the flexible EDLCs manufactured by 3D printing in a rectilinear infill pattern has been investigated. It describes the energy storage performance of the printed supercapacitors in relation to the differences in thickness of the AC printed layers and the differences in density of gel electrolyte. A supercapacitor based on printed AC and composite materials displays a specific capacitance of 38.5?mF?g^?1 when measured at a potential rate change of 20?mV?s^?1 and a current density of 0.136?A?g^?1. The highest energy density value for the flexible EDLC was 0.019?Wh?kg^?1 and power density of 165.0?W?kg^?1 in 1.6?M H₂SO₄/PVA gel electrolyte.     

A non-aqueous nickel–manganese sulfide on rGO wrapped Ni-foam as a supercapacitor advanced electrode material

Development of sulfide-based electrodes for non-aqueous electrolytes is a promising research area in supercapacitor applications. In this present work a novel and safe non-aqueous electrolyte can increase the specific capacitance in a wider range of potential window for electrochemical double layer capacitance (EDLCs). Herein, pristine NiS, MnS, and ternary metal composites of NiMnS/rGO-0.05 wt%, NiMnS/rGO-0.1 wt% and NiMnS/rGO-0.15 wt% are synthesized via one-step hydrothermal route. The fabricated supercapacitor electrodes performance was assessed using cyclic voltammetry, galvanostatic charge and discharge, and electrochemical impedance spectroscopy with 1 M tetraethylammonium tetrafluoroborate in acetonitrile. The as-prepared NiMnS/rGO-0.15 wt% electrode achieves an excellent specific capacitance of 352 F g^?1 at a scan rate of 5 mV s^?1 with capacitance retention of 82% through 10,000 successive cycles. Remarkably, the NiMnS/rGO-0.15 wt% composite indicates its superior electrochemical behavior as an electrode material in a non-aqueous electrolyte.

     

Ag-doped NiO porous network structure on Ni foam as electrode for supercapacitors

Ag-doped NiO porous network structure grown on Ni foam has been synthesized through a facile hydrothermal method. Then network structure is assembled by numerous interconnected superfine nanowires, and Ag is uniformly distributed in the body of NiO network. The unique porous network structure doped by conductive Ag and the direct integration of electrode materials on Ni foam current collector provide efficient pathways for electron transport and electrolyte ions diffusion. The electrochemical results demonstrate that the Ag-doped NiO electrode exhibits a specific capacitance of 570.7 F g^?1 and excellent cycling stability. The Ag-doped NiO electrode with relatively high electrochemical performance is a promising candidate for the supercapacitor electrodes. These results can also provide strategies to develop advanced electrode materials supercapacitor applications.     

Intertwined Nanocarbon and Manganese Oxide Hybrid Foam for High‐Energy Supercapacitors

Rapid charging and discharging supercapacitors are promising alternative energy storage systems for applications such as portable electronics and electric vehicles. Integration of pseudocapacitive metal oxides with single‐structured materials has received a lot of attention recently due to their superior electrochemical performance. In order to realize high energy‐density supercapacitors, a simple and scalable method is developed to fabricate a graphene/MWNT/MnO₂ nanowire (GMM) hybrid nanostructured foam, via a two‐step process. The 3D few‐layer graphene/MWNT (GM) architecture is grown on foamed metal foils (nickel foam) via ambient pressure chemical vapor deposition. Hydrothermally synthesized α‐MnO₂ nanowires are conformally coated onto the GM foam by a simple bath deposition. The as‐prepared hierarchical GMM foam yields a monographical graphene foam conformally covered with an intertwined, densely packed CNT/MnO₂ nanowire nanocomposite network. Symmetrical electrochemical capacitors (ECs) based on GMM foam electrodes show an extended operational voltage window of 1.6 V in aqueous electrolyte. A superior energy density of 391.7 Wh kg^?1 is obtained for the supercapacitor based on the GMM foam, which is much higher than ECs based on GM foam only (39.72 Wh kg^?1). A high specific capacitance (1108.79 F g^?1) and power density (799.84 kW kg^?1) are also achieved. Moreover, the great capacitance retention (97.94%) after 13 000 charge–discharge cycles and high current handability demonstrate the high stability of the electrodes of the supercapacitor. These excellent performances enable the innovative 3D hierarchical GMM foam to serve as EC electrodes, resulting in energy‐storage devices with high stability and power density in neutral aqueous electrolyte.     

Binder-free polyaniline interconnected metal hexacyanoferrates nanocomposites (Metal = Ni,Co) on carbon fibers for flexible supercapacitors

Improvement of the electrical conductivity, specific capacitance and binder-free polyaniline (PANI) interconnected with metal(II) hexacyanoferrate(III) (MHCF) nanocomposites (M?=?Ni, Co) on flexible carbon fibers (CF) were designed in our present research goal. PANI/MHCF/CF nanocomposites were prepared by one-step co-polymerization method. Electrochemical studies like cyclic voltammetry, galvanostatic charge–discharge and electrochemical impedance spectroscopy were analyzed. Under the optimized conditions, the nanocomposites demonstrated remarkable electrochemical performances as supercapacitor electrode with outstanding specific capacitances of ~725 F g^?1 at a current density of 1 A g^?1, and retained ~325 F g^?1 even at a high current density of 20 A g^?1 in 0.5 M H₂SO₄?+?0.5 M Na₂SO₄ solution. The excellent cycling stability with capacitance retention of 80% after 1000 cycles may be a potential electrode material for future supercapacitor when its cycling stability and rate performance are addressed.     

Highly conductive supercapacitor based on laser-induced graphene and silver nanowires

One of the foremost necessary desires of energy systems has been the existence of efficient, flexible, transportable, and eco-friendly devices. Among all the energy storage systems, supercapacitors have attracted plenty of attention thanks to their distinctive properties. Among all capacitor technologies, laser-induced graphene (LIG)-based capacitors are within the spotlight nowadays due to their high flexibility and simple manufacture. The most downside with LIG-based capacitors is their low conductivity and low charge capacity. During this work, to overcome this problem, the surface of LIG is covered with silver nanowires (AgNWs) and LIG/AgNWs composite is employed to form supercapacitor. In this study, all the electrochemical properties of the prepared composite were investigated, and therefore the results showed that AgNWs could increase the electrical conductivity of LIG by about 2.25 times, improve electrode–electrolyte interaction, and increase areal capacitance by 1.3 times. Additionally, the synthesized supercapacitor shows stable cyclic behavior and retention capacity equal to 78% after 1000 charge–discharge cycles. A singular increase in LIG conductivity and improved in its cyclic performance. Furthermore, galvanostatic charge/discharge curves indicated acceptable charge capacity of the LIG/AgNWs supercapacitor.