Supercapacitor In Aqueous Electrolyte Research Articles

Electrostatic self-assembly of MXene and edge-rich CoAl layered double hydroxide on molecular-scale with superhigh volumetric performances

Abstract It is highly desirable to design and synthesize two-dimensional nanostructured electrode materials with high electrical conductivity, large electrolyte-accessible surface area and more exposed active sites for energy storage applications. Herein, MXene/CoAl-LDH heterostructure has been prepared through electrostatic ordered hetero-assembly of monolayer MXene and edge-rich CoAl-LDH nanosheets in a face-to-face manner on molecular-scale for supercapacitor applications. Benefiting from the unique structure, strong interfacial interaction and synergistic effects between MXene and CoAl-LDH nanosheets, the electrical conductivity and exposed electrolyte-accessible active sites are significantly enhanced. The as-prepared MXene/CoAl-LDH-80% (ML-80) film exhibits high volumetric capacity of 2472 C cm−3 in 3 M KOH electrolyte with high rate capability of 70.6% at 20 A g−1. Notably, to the best of our knowledge, the high volumetric capacity is the highest among other previously reported values for supercapacitors in aqueous electrolytes. Furthermore, our asymmetric supercapacitor device fabricated with ML-80 and MXene/graphene composite as cathode and anode, respectively, exhibits impressive volumetric energy density of 85.4 Wh L−1 with impressive cycling stability of 94.4% retention ratio after 30,000 continuous charge/discharge cycles.

Hierarchical Mn3 O4 Anchored on 3D Graphene Aerogels via C-O-Mn Linkage with Superior Electrochemical Performance for Flexible Asymmetric Supercapacitor.

Flexible asymmetric supercapacitors are more appealing in flexible electronics because of high power density, wide cell voltage, and higher energy density than symmetric supercapacitors in aqueous electrolyte. In virtues of excellent conductivity, rich porous structure and interconnected honeycomb structure, three dimensional graphene aerogels show great potential as electrode in asymmetric supercapacitors. However, graphene aerogels are rarely used in flexible asymmetric supercapacitors because of easily re-stacking of graphene sheets, resulting in low electrochemical activity. Herein, flower-like hierarchical Mn3 O4 and carbon nanohorns are incorporated into three dimensional graphene aerogels to restrain the stack of graphene sheets, and are applied as the positive and negative electrode for asymmetric supercapacitors devices, respectively. Besides, a strong chemical coupling between Mn3 O4 and graphene via the C-O-Mn linkage is constructed and can provide a good electron-transport pathway during cycles. Consequently, the asymmetric supercapacitor device shows high rate cycle stability (87.8 % after 5000 cycles) and achieves a high energy density of 17.4 μWh cm-2 with power density of 14.1 mW cm-2 (156.7 mW cm-3 ) at 1.4 V.

High-voltage aqueous asymmetric pseudocapacitors based on methyl blue-doped polyaniline hydrogels and the derived N/S-codoped carbon aerogels

The energy density of aqueous-electrolyte supercapacitors based on nanostructured carbon materials is usually limited by relatively narrow operating voltage window (about 1 V in acid aqueous electrolyte), due to the existence of overpotential. We report a novel N/S-codoped carbon aerogel (NS-CAG) derived from a methyl blue-doped polyaniline hydrogel (MB-PHG) with 3D interconnected supramolecular architecture through polymerization and carbonization. The obtained NS-CAG shows a high active nitrogen content along with simultaneous interlayer expansion, probably attributed to the MB-enabled supramolecular architecture of MB-PHG precursor, in favor of improving its pseudocapacitance and electrolyte ion diffusion. The assembled asymmetric pseudocapacitors based on the graphite fiber-supported NS-CAG and MB-PHG electrodes, can break through the overpotential barrier to an ultrahigh operating voltage window of 2.0 V in 1 M H2SO4, resulting in the excellent energy density of 59.7 Wh kg−1 at 1 kW kg−1. This high-energy asymmetric pseudocapacitor can open up an exciting route to extend the operating voltage window for aqueous-electrolyte energy storage.

Nitrogen, oxygen and sulfur co-doped hierarchical porous carbons toward high-performance supercapacitors by direct pyrolysis of kraft lignin

The O-N-S co-doped hierarchical porous carbons are prepared by direct pyrolysis of kraft lignin which is the byproduct from papermaking black liquor. The proposed preparation method is extremely facile, green-environmental, and low-cost without any additional activating agents, additives or templates. The kraft lignin-derived carbon materials possess large specific surface areas (338−1307 m2 g−1), hierarchical porous structures and abundant multi-heteroatoms co-doping (9.84–19.91 wt%). Benefiting from above synergistic advantages, the as-fabricated symmetric supercapacitor in aqueous electrolyte delivers a high specific capacitance of 244.5 F g−1 at 0.2 A g−1, excellent rate-capability (81.8% retention of initial capacitance at 40.0 A g−1), and outstanding cycling stability (91.6% retention over 10000 cycles). Importantly, this device in aqueous electrolyte delivers an energy density of 8.5 W h kg−1 at a power density of 100 W kg−1. Furthermore, a remarkable energy density of 66.8 W h kg−1 at a power density of 1.75 kW kg−1 has been achieved and 32.2 W h kg−1 is still maintained even at an ultrahigh power density of 40.0 kW kg−1 when ionic liquid serves as electrolyte. This study demonstrates the successful conversion of low-valued natural biomass derivative into sustainable high-performance supercapacitor electrode materials with a simple, low-cost, and green-environmental production process.

The excellent capacitive capability for N,P-doped carbon microsphere/reduced graphene oxide nanocomposites in H2SO4/KI redox electrolyte

In order to ameliorate the capacitance and energy density of supercapacitors in aqueous electrolytes, nitrogen- and phosphorus-codoped carbon microspheres/reduced graphene oxide nanocomposites are obtained by hydrothermal treatment of graphite oxide and N,P-doped carbon microsphere. The as-prepared nanocomposites display a high specific surface area of 604.3 m2 g−1 and hierarchical pore structure composed of micropores, mesopores, and macropores. N,P-codoped carbon microspheres/reduced graphene oxide are used as electrode materials in the redox electrolyte of H2SO4 and KI aqueous solution, exhibiting a excellent capacitance performance of up to ~ 654 F g−1 at 2 A g−1, corresponding to the energy density of 14.53 Wh kg−1 at power density of 402 W kg−1. Even at a high current of 20 A g−1, the electrode can keep a capacitance of 318 F g−1, showing an energy density of 7 Wh kg−1 at power density of 3984 W kg−1. This study indicates the potential of nitrogen- and phosphorus-codoped carbon microspheres/reduced graphene oxide in high energy density supercapacitor.

3D macro-micro-mesoporous FeC2O4/graphene hydrogel electrode for high-performance 2.5 V aqueous asymmetric supercapacitors

Distinguished from commonly used Fe2O3 and Fe3O4, a three-dimensional multilevel macro-micro-mesoporous structure of FeC2O4/graphene composite has been prepared as binder-free electrode for supercapacitors. The as-prepared materials are composed of macroporous graphene and microporous/mesoporous ferrous oxalate. Generally, the decomposition voltage of water is 1.23 V and the practical voltage window limit is about 2 V for asymmetric supercapacitors in aqueous electrolytes. When FeC2O4/rGO hydrogel was used as the negative electrode and a pure rGO hydrogel was used as the positive electrode, the asymmetrical supercapacitor voltage window raised to 1.7 V in KOH (1.0 mol/L) electrolyte and reached up to 2.5 V in a neutral aqueous Na2SO4 (1.0 mol/L) electrolyte. Correspondingly it also exhibits a high performance with an energy density of 59.7 Wh/kg. By means of combining a metal oxide that owns micro-mesoporous structure with graphene, this work provides a new opportunity for preparing high-voltage aqueous asymmetric supercapacitors without addition of conductive agent and binder.

Synchronously boosting gravimetric and volumetric performance: Biomass-derived ternary-doped microporous carbon nanosheet electrodes for supercapacitors

Porous carbon materials are the most widely used electrode materials in supercapacitors. Numerous works focus on increasing specific surface area and engineering hierarchical porosity as well as doping heteroatoms to improve the gravimetric performance for porous carbon electrodes. However, highly porous structures will cause low density for porous carbon electrodes, resulting in poor volumetric performance. The complex synthesis methods also restrict the wide applications of the resultant carbons. In this work, we demonstrate a sustainable and efficient strategy to synthesize ternary-doped microporous carbon nanosheets via treating fish skin with KOH. Notably, KOH firstly turns the fish skin into water-soluble gelatin, then KOH-derived potassium compounds simultaneously act as templates and activators to form micropore-dominant carbon nanosheets. The precursor (fish skin) and reagent (KOH) are low-cost, and potassium compound in the resultant carbons (K2CO3) is low-toxicity and easy-disposal. The relatively low pyrolysis temperature (600 °C) reserves numerous O, N and S heteroatoms derived from fish skin and leads to rational specific surface area, pore volume and compacting density of the porous carbon nanosheets. Beneficial from the unique nanostructure and rich surface heteroatoms, the resultant carbon shows integrated high gravimetric and volumetric performance as electrode for supercapacitor in aqueous electrolyte.

Temperature effect on the synthesis of lignin-derived carbons for electrochemical energy storage applications

Herein, we present a detailed study by N2 sorption and Small Angle X-ray Scattering (SAXS) of the carbonization and KOH activation of lignin for its application as active material for electrochemical energy storage. It has been observed that i) the carbonization of lignin above 700 °C leads to a hard carbon with a large amount of bulk (buried) fine structure microporosity and a good performance as Na-ion negative electrode, ii) when KOH activation is done after complete carbonization it is mainly increasing the accessibility of the initial bulk microporosity, leading to a carbon with good performance as symmetric supercapacitor in aqueous electrolyte and iii) when carbonization and KOH activation are done simultaneously, a distinct pore structure is generated with a large amount of mesopores, suitable for symmetric supercapacitor in organic electrolyte. By combining SAXS, which is sensitive to bulk as well as surface porosity, and N2 sorption which probes surface porosity, it has been possible to follow the intricate mechanism of microporosity development. Finally, it is believed that these results can be extrapolated to various biomass based precursors.

High-Potential Metalless Nanocarbon Foam Supercapacitors Operating in Aqueous Electrolyte.

Light-weight graphite foam decorated with carbon nanotubes (dia. 20-50 nm) is utilized as an effective electrode without binders, conductive additives, or metallic current collectors for supercapacitors in aqueous electrolyte. Facile nitric acid treatment renders wide operating potentials, high specific capacitances and energy densities, and long lifespan over 10 000 cycles manifested as 164.5 and 111.8 F g-1 , 22.85 and 12.58 Wh kg-1 , 74.6% and 95.6% capacitance retention for 2 and 1.8 V, respectively. Overcharge protection is demonstrated by repetitive cycling between 2 and 2.5 V for 2000 cycles without catastrophic structural demolition or severe capacity fading. Graphite foam without metallic strut possessing low density (≈0.4-0.45 g cm-3 ) further reduces the total weight of the electrode. The thorough investigation of the specific capacitances and coulombic efficiencies versus potential windows and current densities provides insights into the selection of operation conditions for future practical devices.

Carbon cloth@T-Nb2O5@MnO2: A rational exploration of manganese oxide for high performance supercapacitor

Numerous efforts have been made to investigate manganese oxides as active materials in the field of energy storage. To further explore the full use of MnO2 for supercapacitor in aqueous electrolyte, a facile two-step hydrothermal method are developed to synthesis T-Nb2O5@MnO2 composites supported by carbon cloth (CC). With the hierarchical nanostructure and rational synergistic effect, CC@T-Nb2O5@MnO2 exhibits a high specific capacitance (319Fg−1 at 0.2Ag−1) and a good cycling stability (85.7% retention after 2000 cycles). An asymmetric supercapacitor with CC@T-Nb2O5@MnO2 nanocomposite as positive electrode and activated graphene oxide as negative electrode yielded an energy density of 31.76Whkg−1 and a maximum power density of 2250Wkg−1. These delightful findings demonstrate CC@T-Nb2O5@MnO2 to be a nice electrode material, and meanwhile provide an idea to design novel structure with synergic mechanism for high-performance supercapacitors.

Spatial Charge Storage within Honeycomb‐Carbon Frameworks for Ultrafast Supercapacitors with High Energy and Power Densities

AbstractCarbon‐based supercapacitors store charge through the adsorption of electrolyte ions onto the carbon surface. Therefore, it would be more attractive for the enhanced charge storage if the locations for storing charge can be extended from carbon surface to space. Here, a novel spatial charge storage mechanism based on counterion effect from Fe(CN)63− ions bridged by oxygen groups and confined into honeycomb‐carbon frameworks is presented, which can provide additionally spatial charge storage for electrical double‐layer capacitances in a negative potential region and pseudocapacitances from Fe(CN)63−/Fe(CN)64− in a positive potential region. More importantly, an ultrafast supercapacitor based on this novelty carbon can be charged/discharged within 0.7 s to deliver both high specific energy of 15 W h kg−1 and ultrahigh specific power of 79.1 kW kg−1 in 1 m Na2SO4 electrolyte, much higher than those of previously reported asymmetric supercapacitors in aqueous electrolytes, as well as excellent cycling stability. These features suggest a new generation of ultrafast asymmetric supercapacitors as novel high‐performance energy storage devices.

Powering electrodes for high performance aqueous micro-supercapacitors: Diamond-coated silicon nanowires operating at a wide cell voltage of 3 V

In this study, we report originally the excellent electrochemical performance of a micro-supercapacitor based on diamond-coated silicon nanowire (SiNW) electrodes using an aqueous electrolyte (0.1M LiClO4). The deposition of a nanometric boron-doped diamond coating on SiNWs allowed the enlargement of the electrochemical window up to 3V maintaining an extraordinary capacitive electrochemical response due to its high overvoltage. Thereby, the device exhibited an areal capacitance of 0.4 mF cm-2, a high power density of 50 mW cm-2 as well as an outstanding cycling stability after 2·106 galvanostatic charge-discharge cycles at a high current density of 10mA cm-2. In addition, an ultra-fast charge-discharge rate was determined to be 1.5ms, demonstrating the supercapacitor's enormous ability to deliver high power values at very rapid pulse times (Pmax: 321 mW cm-2). These results also evidence the potential of diamond coating to overcome the main drawbacks of silicon (e.g. rapid silicon oxidation due to presence of water) to be employed in aqueous electrolyte supercapacitors. Consequently, the role of diamond paves the way to high performance SiNW-based micro-supercapacitor development at large electrochemical windows in aqueous electrolytes, which are at present limited to 1V.

Flexible, aqueous-electrolyte supercapacitors based on water-processable dioxythiophene polymer/carbon nanotube textile electrodes

Water soluble-solvent resistant approach for aqueous supercapacitors.

“Brick-and-mortar” sandwiched porous carbon building constructed by metal-organic framework and graphene: Ultrafast charge/discharge rate up to 2 V s−1 for supercapacitors

Supercapacitors based on microporous carbons face some contradictory and competitive challenges such as relatively high specific capacitance and low charge rates. Previously, the mesopores in these carbons can effectively enhance the ion transport, but their charge and discharge rates are often less than 1Vs−1. Here, we have demonstrated “brick-and-mortar” sandwiched porous carbon building by using MOF-5 derived porous carbon film as “mortar” and the graphene nanosheet as “brick”. The confined mesoporous channels between-in graphene sheets provide efficient ion transport pathways for fast ion diffusion, and the existence of graphene can effectively maintain the conductivity and structural stability of the carbon building. As a result, the obtained porous carbon building exhibits fast frequency response with an ultrahigh rate up to 2Vs−1, high specific capacitance of 345Fg−1 at 2mVs−1, and outstanding cycling stability of 99% capacitance retention after 10,000 cycles. More importantly, the as-assembled symmetric supercapacitor in aqueous electrolyte can deliver a high energy density of 30.3Whkg−1 at a power density of 137Wkg–1 and superior cycling life (94% capacitance retention after 10,000 cycles). Even at a high power density of 11.9kWkg–1, it still remains an energy density of 10.6Whkg–1, higher than those of previously reported carbon-based symmetric supercapacitors and other asymmetric supercapacitors.

Cycling Performance of Ppy Derived Carbon Based Symmetric Supercapacitors in Aqueous Electrolyte

Electrochemical capacitors (ECs) also known as supercapacitors have attracted significant interest for the development of next generation of energy storage systems. ECs are characterized by their high power density and low energy density when compared with batteries [1,2]. ECs are suitable for transient energy saving application such as energy capture during braking in vehicles, in construction equipment such as cranes and for opening of doors in the A380 Jumbo jet, portable and flexible electronics [1]. However, the performance of ECs devices for most of these applications is exclusively dependent on the physicochemical properties of the electrode materials. Electrode materials with diverse dimensionalities such as 1D, 2D and 3D nanostructures have been explored to improve supercapacitive properties by taking advantages of the unique ability of the different nanostructure for ion propagation and charge storage [3,4]. The stability of supercapacitors is a very important feature for characterization of ECs devices. The traditional method of evaluating the stability is via the constant current charge-discharge (CCCD) cycling over several thousands of cycles which in many cases does not show degradation to the ECs devices. As an alternative to this, voltage holding or floating test has been proposed as a reliable technique to test the stability of electrode materials [5,6]. Here we report on the performance (specific capacitance) of symmetric capacitors based on mesoporous PPY derived carbons and the stability of the symmetric capacitors based on voltage holding (floating conditions) operating in aqueous electrolytes evaluated by cyclic constant current charge-discharge tests. Variation in the specific capacitance values with the floating conditions shows a good cycling life performance in alkaline electrolyte for ~190 h with slight degradation observed after 140 h.

Transition metal sulfides grown on graphene fibers for wearable asymmetric supercapacitors with high volumetric capacitance and high energy density.

Fiber shaped supercapacitors are promising candidates for wearable electronics because they are flexible and light-weight. However, a critical challenge of the widespread application of these energy storage devices is their low cell voltages and low energy densities, resulting in limited run-time of the electronics. Here, we demonstrate a 1.5 V high cell voltage and high volumetric energy density asymmetric fiber supercapacitor in aqueous electrolyte. The lightweight (0.24 g cm−3), highly conductive (39 S cm−1), and mechanically robust (221 MPa) graphene fibers were firstly fabricated and then coated by NiCo2S4 nanoparticles (GF/NiCo2S4) via the solvothermal deposition method. The GF/NiCo2S4 display high volumetric capacitance up to 388 F cm−3 at 2 mV s−1 in a three-electrode cell and 300 F cm−3 at 175.7 mA cm−3 (568 mF cm−2 at 0.5 mA cm−2) in a two-electrode cell. The electrochemical characterizations show 1000% higher capacitance of the GF/NiCo2S4 as compared to that of neat graphene fibers. The fabricated device achieves high energy density up to 12.3 mWh cm−3 with a maximum power density of 1600 mW cm−3, outperforming the thin-film lithium battery. Therefore, these supercapacitors are promising for the next generation flexible and wearable electronic devices.

Preparation and characterization of porous carbon from expanded graphite for high energy density supercapacitor in aqueous electrolyte

In this work, we present the synthesis of low cost carbon nanosheets derived from expanded graphite dispersed in Polyvinylpyrrolidone, subsequently activated in KOH and finally carbonized in Ar/H2 atmosphere. Interconnected sheet-like structure with low concentration of oxygen (9.0 at.%) and a specific surface area of 457 m2 g−1 was obtained. The electrochemical characterization of the carbon material as supercapacitor electrode in a 2-electrode configuration shows high specific capacitance of 337 F g−1 at a current density of 0.5 A g−1 as well as high energy density of 37.9 Wh kg−1 at a power density of 450 W kg−1. This electrical double layer capacitor electrode also exhibits excellent stability after floating test for 120 h in 6 M KOH aqueous electrolyte. These results suggest that this activated expanded graphite (AEG) material has great potential for high performance electrode in energy storage applications.

Ultrafine nickel–copper carbonate hydroxide hierarchical nanowire networks for high-performance supercapacitor electrodes

In this work, ultrafine nickel–copper carbonate hydroxide nanowire arrays on copper foam are synthesized via a facile hydrothermal method without any adscititious surfactant and binder. The fabricated nickel–copper carbonate hydroxide hierarchical nanowire networks electrodes for supercapacitors in aqueous electrolyte exhibit a significantly enhanced specific capacitance (971Fg−1 at 1Ag−1) and energy density (48.55Whkg−1 at 541Wkg−1). Meanwhile, the nickel–copper carbonate hydroxide hierarchical nanowire networks as electrode materials have excellent long-life cycling stability, retaining 91.5% of the initial capacitance after 2000 cycles. Thereby, the nickel–copper carbonate hydroxide hierarchical nanowire networks are promising electrode materials for high-energy-density long-life cycling supercapacitor application.

Analysis of impedance spectroscopy of aqueous supercapacitors by evolutionary programming: Finding DFRT from complex capacitance

Electrochemical impedance spectroscopy (EIS) of aqueous-electrolyte supercapacitors can shed light on the physical processes occurring within the cell, given a proper analysis technique. Impedance Spectroscopy Genetic Programming (ISGP) is a novel technique to analyze EIS data. ISGP finds a functional form of the distribution of relaxation times utilizing genetic algorithm and offers a detailed quantitative analysis.In this work, EIS data of aqueous-electrolyte supercapacitors were analyzed by transforming the data to complex capacitance representation. A distribution function of relaxation times (DFRT) model was obtained, comprised of two peaks, each correlated with a different physical process (diffusion and EDLC formation). Two cell configurations were examined and the differences regarding capacitance distribution and diffusion rates are presented.

(Invited) Flexible and Conductive Mxene/Carbon Nanotube Composite Paper for Energy Storage

Carbon nanomaterials, such as carbon nanotubes (CNTs), are attracting much attention as candidates for electrodes of energy storage devices due to their large open surfaces and high conductivity. Recently, MXenes, a new family of 2D carbides, have shown their great promise for electrodes in supercapacitors and Li-ion batteries. [1, 2] Herein, our new results show that the incorporation of CNTs into MXene films results in composite papers with much improved performance. [3] Free-standing sandwich-like MXene/CNT composite papers composed of alternating layers of MXene and CNTs were fabricated by vacuum filtration method. The resulting “papers” have good flexibility, high electrical conductivity and a structure accessible to electrolyte ions. They exhibit much better performance than pure MXene films and the randomly mixed MXene/CNT composite papers. When employed as electrodes for supercapacitors in aqueous electrolyte, capacitances of ~390 F/cm3 were achieved. Besides, the composites showed ~75% increase in rate performance compared to pure MXene films and exhibited no capacity degradation after 10,000 cycles. The MXene/CNT composite papers can also serve as stable anodes for high-rate Li-ion batteries. Highly reversible and stable capacity of ~250 mAh/g was yielded by the titanium-based MXene/CNT composites at 0.5 C (2 hrs charge/discharge), about 10 times of that for pure MXene films. The niobium-based MXene/CNT composites achieved a high reversible capacity of ~650 mAh/g at 0.1 C (10 hrs charge/discharge). At 10 C (6 min charge/discharge), a reversible capacity of ~300 mAh/g was retained for >300 cycles, indicating excellent rate performance and cycling stability. These conductive, flexible and mechnaically strong papers are promising materials that can be used in wearable or structural electrochemical energy storage and conversion systems. [1] M. Naguib, et al., MXenes: A New Family of Two-Dimensional Materials, Advanced Materials, 26, 992-1005 (2014) [2] M. R. Lukatskaya, et al., Cation Intercalation and High Volumetric Capacitance of Two-dimensional Titanium Carbide, Science, 341, 1502-1505 (2013) [3] M. Q. Zhao, et al. Flexible MXene/Carbon Nanotube Composite Paper with High Volumetric Capacitance, Advanced Materials, in press (2014)