Symmetric Aqueous Supercapacitor Research Articles

High-performance supercapacitors based on compact graphene composite hydrogels

Because of its ultra-high specific surface area and outstanding electrochemical performance, graphene has become a study focus of electrode materials for supercapacitors. However, severe agglomeration and poor volumetric electrochemical properties greatly reduce its potential for practical applications. Here, we proposed a simple hydrothermal strategy to synthesize L-glutamic acid functionalized nanocellulose/reduced graphene oxide composite hydrogels (GNGHs) with compact porous structures and plentiful heteroatom functional groups. In the hydrothermal process, a high concentration of graphene oxide solution (GO) was employed to boost the packing density of the samples. Nanocellulose (NC) and the molecular chains of L-glutamic acid were used as spacers to modulate the porous structure of GNGHs. At the same time, NC and heteroatomic groups can also boost the surface wettability of GNGHs, thereby increasing their ion-available specific surface area. The GNGH-15 based symmetric aqueous supercapacitors (SASC) deliver high gravimetric (282.0 F g−1) and volumetric (329.9 F cm−3) specific capacitance, remarkable rate capacity, and good cycling durability. Moreover, the all-solid-state flexible supercapacitors (ASSC) assembled by GNGH-15 also exhibit prominent specific capacitance (154.7 F g−1) and good rate performance. These brilliant electrochemical properties make GNGHs a strong contender for electrode materials of energy storage devices.

From crumple conformation to gel configuration of reduced graphene oxide: The cation induction engineering

Spatial conformation of condensed state graphene is fundamentally important for its material properties, while its rational design or control is of great challenge. Herein, reduced graphene oxides with diverse crumple conformations are synthesized facilely and effectively by using electrostatic force from cation-oxidized functional group and cation-π interactions through NaCl/MgCl2/AlCl3 assisted hydrothermal reduction. A phase map for crumple conformation and gel configuration transitions of the reduced graphene oxides is established, where intramolecular effect of mono-valent cation and intermolecular effect of di-/tri-valent cations are captured and utilized to tune the crumple degree or gelation process. Desired material features, including specific surface area and surface functional groups, can be finely modulated via crumple conformation or cation interaction, and enable the reduced graphene oxides to present an outstanding rate capability of 68.8% and unparalleled areal capacitance of 98.0 μF/cm2 at 100 A/g in symmetric aqueous supercapacitors. This study broadens the topological behavior spectrum and stimulates the conformation engineering methodology of reduced graphene oxides, facilitating the development of novel graphene-based materials.

MnO2 Films deposited on CuO nanomaterials as electrode materials for supercapacitors

In this paper, CuO/MnO2 composites with different morphology were successfully fabricated by simple and economical electrodeposition and successive ionic layer adsorption and reaction (SILAR) method as electrode materials for supercapacitors (SCs). The spherical and wheat-shaped CuO/MnO2 composites were obtained by changing the deposition voltage. The wheat-shaped CuO/MnO2 composite electrodes exhibited better electrochemical performance than the spherical. Its areal capacitance is up to 261.4 mF/cm2 at a current density of 1 mA/cm2 in 1 M Na2SO4 electrolyte and the capacitance retention reaches 90% after 1000 charge-discharge cycles. The symmetric aqueous SCs was then assembled with wheat-shaped CuO/MnO2 as positive and negative electrode materials respectively. The SCs exhibited not only outstanding specific capacitance of 152.7 mF/cm2 at 1 mA/cm2, but also good cycling stability of 74.2% after 500 charge-discharge cycles. Furthermore, its operating voltage window reaches 1.6 V, which results in an outstanding energy density of 54.3 μWh/cm2 and a power density of 5040 μW/cm2. This strategy of changing the morphology of materials by changing the deposition voltage provides an ideal scheme for improving the properties.

Self-supported VO(PO3)2 electrode for 2.8 V symmetric aqueous supercapacitors

Improving the voltage window of aqueous supercapacitors beyond 2.5 V without diminishing their capacitance performance is a big challenge to achieve excellent energy density because of the undesirable water decomposition. Herein, this study developed a novel vanadium phosphate (VPO) of the VO(PO3)2 aerogel as the electrode to enlarge the device voltage, which could effectively raise HER/OER overpotentials and thus achieving a stable wide electrochemical window of −1 ∼ 1 V (vs. SCE) with remarkably improved capacitance. The VPO microstructure is organized by nanowire-stacked lamellar bricks via engineering a controllable phase-transformation using the V-P-citric acid-aerogel as the precursor. Examined as the supercapacitor electrode, in-situ X-ray powder diffraction analysis uncovers that the VO(PO3)2 undergoes a reversible phase-transformation reaction associated with Na+ intercalation/de-intercalation in Na2SO4 and a typical redox pseudocapacitance process in KOH. For the first time, a 2.8 V symmetric aqueous supercapacitor was further assembled with an energy density of 123.1 Wh kg−1 at the power density of 580 W kg−1, which is much superior to those of all reported V-base supercapacitors. The underlying mechanisms of the VO(PO3)2 on ultrahigh voltage window and charge storage are further elucidated, which provide the fundamental guidance for structuring VPO-based electrodes used for commercial energy storage devices.

Symmetric supercapacitor operating at 1.5 V with combination of nanosheet-based NiMoO4 microspheres and redox additive electrolyte

Herein, we report the preparation of interconnected nanosheet assembled microsphere-shaped NiMoO4 and its application in a symmetric aqueous supercapacitor using K3Fe(CN)6 (KFe) as redox-additive. The results show that outstanding capacity of 4648 F g−1 (516 mAh g−1) at 2 A g−1 are recorded in KOH/KFe-0.1 M mixed electrolyte, which is 3.5-fold greater than the value in KOH (1335 F g−1/148 mAh g−1). A high capacitance of 1800 F g−1 (200 mAh g−1) can be achieved at 10 A g−1 with a retention of 73.9% when 0.03 M KFe was added. The stable voltage of the symmetric device assembled by 0.03 M KFe involved electrolyte can be up to 1.5 V, higher than the decomposition voltage of water. Owing to the synergistic reaction of nanostructured NiMoO4 and KFe, the device delivers high energy (41.5 Wh kg−1) and power output (6.8 kW kg−1), outstanding rate capability (87.1%) as well as good cycling performance. These results provide an efficient strategy toward the development of high-performance metal oxide-based symmetric capacitor.

Self-standing manganese dioxide/graphene carbon nanotubes film electrode for symmetric supercapacitor with high energy density and superior long cycling stability

The low capacitance utilization and capacitance fading of manganese dioxide (MnO2) is mainly due to poor electro-conductivity and irreversible phase transform. This work proposes a new method of designing hierarchical and binder-free electrode based on MnO2 material for stable supercapacitor with high specific capacitance. Herein, we fabricated the self-standing electrode of MnO2 on nitrogen-doped graphene and single wall carbon nanotubes (SWCNTs) self-standing film (NGCF) by electrochemical deposition. As a result, as-prepared MnO2/NGCF cathode showed excellent electrochemical performance of 489.7 F g-1 at 1 A g-1. Assembled symmetric aqueous supercapacitor (SC) manifests high voltage of 2.4 V and presents excellent high energy density of 106.7 Wh kg-1 at 1200 W kg-1 and outstanding long-life stability without no decay after 10 000 charge-discharge circuits. This work proposes a new view of designing hierarchical and binder-free electrode with high energy density and long cycling stability based on MnO2 material for stable symmetric supercapacitor.

Formation of nitrogen-doped holey carbon nanosheets via self-generated template assisted carbonization of polyimide nanoflowers for supercapacitor

Holey carbon nanosheets provide unique in-plane pores and abundant interlayer channels, which facilitates the penetration and transmission of ions and significant application in the energy storage. However, rational design and fabrication of holey carbon nanosheets with optimized morphology and porosity remains a challenge. Herein, the nitrogen-doped holey carbon nanosheets (N-HCNs) have been prepared via one-step activation and self-generated template assisted carbonization route using “bottom-up” self-assembled polyimide (PI) nanoflowers as carbon precursor and ethylenediaminetetraacetic acid disodium zinc salt hydrate (EDTA-Na2Zn) as activating agent and self-generated template. The N-HCNs demonstrates an integral nanoflowers structure with intertwined and holey carbon nanosheets, which would benefit for the transport of ions and electrons, resulting in a satisfactory specific capacitance of 205 F g−1 at a current density of 0.5 A g−1. More important, a symmetric aqueous supercapacitor assembled based on N-HCNs electrodes carries a wide operating voltage of 2.0 V, possesses high energy density of 17.92 Wh kg−1 at a power density of 500 W kg−1, along with outstanding cycling stability. The present work proposes an affordable strategy to design novel porous carbon materials with controlled morphology for advanced energy storage and conversion.

Boosting operating voltage of vanadium oxide-based symmetric aqueous supercapacitor to 2 V

Vanadium oxides have great potentials in high voltage symmetric supercapacitors, benefiting from the available multiple oxidation states of vanadium. In this work, charge storage in a wide potential window of −1.1 to 0.9 V vs. SCE is achieve for vanadium oxide through vanadium valence tuning and incorporation in functionalized carbon nanowire array. The functionalized carbon nanowire array provides a good three dimensional conductive scaffold for the oxide nanowires, leading to high electron and ion transportation for the pseudo-capacitive materials. A high areal capacitance of 1.31 F cm−2 is achieved for the obtained vanadium oxide electrode (at 1 mA cm−2 in 5 M LiCl aqueous solution). It also displays good rate capability, demonstrating a quite high areal capacitance of 687 mF cm−2 at 100 mA cm−2. The assembled symmetric supercapacitor can be operate with a large voltage of 2 V and so can deliver a high volumetric energy density of 3.61 mWh cm−3 at 0.01 W cm−3. The supercapacitor also shows good stability, it can retain 91% of its capacitance after 10 000 galvanostatic charge-discharge cycles.

In Situ Doping Boron Atoms into Porous Carbon Nanoparticles with Increased Oxygen Graft Enhances both Affinity and Durability toward Electrolyte for Greatly Improved Supercapacitive Performance

AbstractSupercapacitors represent an important energy storage system for high power applications with carbon electrodes acting as the key component. Design and synthesis of advanced carbon materials with supercapacitive activity (capacitance and rate capability) and stability have been the focus of supercapacitor developments. This study reports a high‐performance carbon electrode by in situ doping boron atoms into nanoporous carbon particles, which is achieved by a continuous spraying assisted coassembly process. Boron doping leads to extra oxygen graft into carbon surface, enabling both enhanced wettability and durability for the fabricated carbon electrodes. Density functional theory calculations further suggest that boron doping enhances electrolyte ion penetration and interactions with carbon surface, leading to the improved capacitances and rate capability. The constructed symmetric aqueous supercapacitor exhibits all‐round performance improvements including high energy density (9.1 Wh kg−1) and power density (24.1 kW kg−1) as well as ultralong cycling life (100 000 cycles). This work for the first time provides insights into the role of boron doping in enhancing both supercapacitive activity and stability of carbon materials for high‐performance supercapacitors.

Rambutan-like cobalt nickel sulfide (CoNi2S4) hierarchitecture for high-performance symmetric aqueous supercapacitors

A novel hierarchical cobalt nickel sulfide (CoNi2S4) nanostructure resembling the rambutan fruit was obtained by a simple one-step hydrothermal method using ethylene glycol as the solvent. Scanning and transmission electron microscopic analyses revealed that the as-synthesized CoNi2S4 consists of numerous hairy nanorods grown radially on top of individual sphere-like core structures. The CoNi2S4 hierarchical material was applied as a potential electrode for supercapacitors in both three- and two-electrode systems. The hairy nanorods and spheres were highly interconnected to form hierarchical rambutan-like CoNi2S4 structure with increased active areas of the electrode, and this facilitate effective charge transport from the nanorods to the spherical core structure. When evaluated as an electrode material in a three-electrode system, CoNi2S4 with the hierarchical structure delivered a high specific capacitance of 1102.22Fg−1 at a current density of 1Ag−1 with excellent rate capability (68.55% capacitance retention as the current increases from 1 to 10Ag−1) and significant cycling stability (75% retention after 3000 cycles). The electrochemical properties of the hierarchical CoNi2S4 were also investigated in a symmetrical cell arrangement using 2M aqueous KOH as the electrolyte. The symmetric aqueous supercapacitor exhibited a specific capacitance of 482Fg−1 at 1Ag−1 with maximum energy density of 16.74Whkg−1 and maximum power density of 10.2kWkg−1. Furthermore, the symmetric supercapacitor exhibited excellent cycling stability, showing 92.85% capacitance retention even after 5000 cycles.

Green synthesis of ZnO/carbon (ZnO/C) as an electrode material for symmetric supercapacitor devices

With the eco friendly green chemistry technique in mind, sucrose has been used in this study as the capping agent to synthesize ZnO/C nanocomposite adapting sol-gel method. The presence of carbon in the prepared ZnO/C was confirmed through EDAX. Electrochemical investigations of ZnO/C electrode were carried out using cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy. The ZnO/C electrode exhibits a maximum specific capacitance of 820 F g−1 at a constant specific current of 1 A g−1. The symmetric aqueous supercapacitor device exhibits the specific cell capacitance of 92 F g−1 at the specific current of 2.5 A g−1. The aqueous symmetric supercapacitor device achieved an energy density of 32.61 Wh kg−1 and the power density of approximately 1 kW kg−1 at a discharge current of 1.0 A g−1. It has been found that the cells have an excellent electrochemical reversibility (92% after 400 continuous cycles) and capacitive characteristics in 1 M Na2SO4 electrolyte.

Scalable fabrication of hierarchically porous N-doped carbon electrode materials for high-performance aqueous symmetric supercapacitor

Nowadays, hierarchical porosity of electrode materials has attracted numerous attentions due to allowing the smooth mass transportation and capability of improving the rate performance of electrochemical applications. In this study, hierarchically porous N-doped carbon (HPDC) materials were fabricated in a facile and scalable way via the dehalogenation of polyvinyl dichloride and applied as electrode materials for assembling symmetric aqueous supercapacitors. The resulted supercapacitors were found having a very high specific energy of 21.5 Wh kg−1 in 1.0 M Li2SO4 with a safe operating voltage of 1.8 V. The enhanced capacitive performance was ascribed to both the HPDC’s abundant pore hierarchy with a large accessible surface area and also the applicable neutral electrolyte with a wide stable potential. Besides the potentially scalable production of hierarchically porous carbon materials with high capacitive performance, our work may also envision that other halogenated polymers such as polyvinyl chloride and its wastes can be easily converted into useful carbon materials via our developed dehalogenation strategy.

A two-volt aqueous supercapacitor from porous dehalogenated carbon

Safe/long operation of symmetric aqueous supercapacitor at 2 V was achieved using highly porous non-doped carbon from polymer dehalogenation.

Achieving battery-level energy density by constructing aqueous carbonaceous supercapacitors with hierarchical porous N-rich carbon materials

3D hierarchical porous N-rich graphitic carbon materials were prepared and further used for symmetric aqueous supercapacitors with battery-level energy density while retaining capacitor-type power density and cycling stability.

Interconnected Network of MnO2 Nanowires with a “Cocoonlike” Morphology: Redox Couple-Mediated Performance Enhancement in Symmetric Aqueous Supercapacitor

Low electronic conductivity and slow faradic processes limit the performance of MnO2 as an electrochemical pseudocapacitor with respect to cycling and power density. Herein, we report preparation of single-phase α-MnO2, composed of an interconnected nanowire network with "cocoonlike" morphology, and its application as electrode in a symmetric aqueous supercapacitor. Increased "effective" surface area, coexistence of micropores and mesopores, and enhanced electron transport in these nanowire networks result in a specific pseudocapacitance (CS) of 775 F·g(-1) in 3 M KOH, derived from cyclic voltammetry in the potential window of -1 to +1 V at a scan rate of 2 mV·s(-1), the highest reported for two-electrode symmetric configuration. Furthermore, introduction of K4Fe(CN)6 as a redox-active additive to KOH results in ∼7 times increase in energy density at a power density of ∼6000 W·kg(-1). The presence of the Fe(CN)6(4-)/Fe(CN)6(3-) redox couple provides an electron buffer source compensating for the slow faradic reactions. The results demonstrate that this simple approach might be an effective way to enhance the redox kinetics and reversibility of transition metal oxide-based pseudocapacitors.

Honeycomb porous MnO2 nanofibers assembled from radially grown nanosheets for aqueous supercapacitors with high working voltage and energy density

Honeycomb porous MnO2 nanofibers (HMONFs) have been prepared by solution reaction between KMnO4 and electrospun carbon nanofibers (CNFs). The HMONFs are entirely composed of radially grown MnO2 nanosheets with thickness about 3–7nm, which interconnect each other, forming the honeycomb pores. Formation of this unique structure occurs only at very low KMnO4 concentrations and sufficiently long reaction time. The constituting MnO2 nanosheets in the HMONFs are well separated with the sheet edges oriented on the surface, leading to excellent supercapacitive performance. Symmetric aqueous supercapacitors are assembled using the HMONFs and 1M Na2SO4 electrolyte, which exhibits a working voltage as high as 2.2V and high energy density of 41.1Wh/kg at the power density of 3.3kW/kg. The supercapacitor capacity can be retained about 76% of its initial value after 3500 cycles, which is acceptable due to its high energy density. These results indicate that the HMONFs are of high promise in developing advanced supercapacitors with high working voltage and energy density for practical applications.

Biopolymer-Assisted Synthesis of λ-MnO2Nanoparticles As an Electrode Material for Aqueous Symmetric Supercapacitor Devices

λ-MnO2 nanoparticles have been successfully synthesized using macromolecules like starch and dextrose. The X–ray diffraction pattern shows the cubic structure of the λ-MnO2. Morphological studies show the synthesized particles to be of nanospherical structure. The FTIR spectrum confirms the presence of Mn–O bonds. From the charge–discharge analysis λ-MnO2 was found to exhibit the highest specific capacitance of 384 F g–1 at a specific current discharge of 1 A g–1. The symmetric aqueous supercapacitor device exhibits the 44 F g–1 at a specific current discharge of 1 A g–1. We have obtained higher specific capacitance for λ-MnO2 in three electrode cell configuration and two electrode supercapacitor cells. This is higher than previous reports of λ-MnO2 three electrode cell and two electrode systems. All the three electrochemical measurements (cyclic voltammetry, galvanostatic charge–discharge, and impedance) proved that the λ-MnO2 was a good electrode material for a supercapacitor.