Layer-by-layer assembled polyaniline/carbon nanomaterial-coated cellulosic aerogel electrodes for high-capacitance supercapacitor applications

Traditional layer-by-layer (LbL) assembled electrodes are mostly multilayer composites formed on two-dimensional membrane materials. In this case, the electroactive material cannot enter the interior of the substrate. With porous aerogels as the substrate, the LbL assembly of the electroactive material into the three-dimensional aerogel skeleton can be realised, greatly improving the utilisation and the electrochemical performance of the electroactive material. To create a promising aerogel electrode for high-performance energy storage devices, we herein report an aerogel based on wood pulp fibre (WPF) and cellulose nanocrystals (CNC), for use as a porous substrate for LbL assembly of nanostructural polyaniline (PANI) and graphene oxide (GO) or carboxylic multi-walled carbon nanotubes (CMCNT). Owing to the uniformly distributed multilayer nanoarchitecture, interpenetrating channels, and hydrophilic character of the cellulosic aerogel substrate, the produced electrodes of (PANI/CMCNT)₁₀ and (PANI/CMCNT)₁₀ both display high specific capacitances, favourable capacitance retention, good cycling stabilities, and structural flexibility. In the three-electrode test, their gravimetric specific capacitances are as high as 716.62 and 636.63 F g⁻¹, respectively. In addition, the assembled symmetric supercapacitors show good areal specific capacitances (1.95 and 1.49 F cm⁻²) in addition to high areal specific energies (168.64 and 113.57 mW h cm⁻², respectively). These results demonstrate that the integration of the LbL-assembled electroactive materials and porous cellulosic aerogel substrate can be a promising strategy to design high-efficiency green energy storage devices.

Cellulosic aerogel was used as a porous lightweight substrate to layer-by-layer assembly polyaniline and carbon nanomaterials for high capacitance supercapacitor applications.     

Cu-MOF derived Cu–C nanocomposites towards high performance electrochemical supercapacitors

For the development of asymmetric supercapacitors with higher energy density, the study of new electrode materials with high capacitance is a priority. Herein, the electrochemical behavior of nano copper in alkaline electrolyte is first discovered. It is found that there are two obvious reversible redox symmetric peaks in the range of −0.8–0.2 V in the alkaline electrolyte, corresponding to the conversion of copper into cuprous ions, and then converting cuprous ions into copper ions, indicating that the nanocomposite electrode has the characteristics of a pseudocapacitive reaction. It has a specific capacitance of up to 318 F g⁻¹ at a current density of 1 A g⁻¹, which remains at nearly 100% after 10 000 cycles at the same current density. When assembled with a Ni(OH)₂-based electrode into an asymmetric supercapacitor, the device shows excellent capacitive behavior and good reaction reversibility. At 0.4 A g⁻¹, the supercapacitor delivers a reversible capacity of 8.33 F g⁻¹ with an energy density of 13.5 mW h g⁻¹. This study first discovers the electrochemical behavior of nano copper, which can provide a new research idea for further expanding the negative electrodes of supercapacitors with higher energy density.

A new Cu–C nanocomposite derived from Cu-based metal–organic framework exhibits greatly improved electrochemical performance.     

Flexible and high energy density solid-state asymmetric supercapacitor based on polythiophene nanocomposites and charcoal

An asymmetric supercapacitor (ASC) was constructed using a polythiophene/aluminium oxide (PTHA) nanocomposite as an anode electrode and charcoal as a cathode electrode. The highest specific capacitance (C_sp) of the PTHA electrode was found to be 554.03 F g⁻¹ at a current density (CD) of 1 A g⁻¹ and that of the charcoal electrode was 374.71 F g⁻¹ at 1.4 A g⁻¹, measured using a three electrode system. The maximum C_sp obtained for the assembled PTHA//charcoal asymmetric supercapacitor (ASC) was 265.14 F g⁻¹ at 2 A g⁻¹. It also showed a high energy density of 42.0 W h kg⁻¹ at a power density of 735.86 W kg⁻¹ and capacitance retention of 94.61% even after 2000 cycles. Moreover, it is worth mentioning that the asymmetric device was used to illuminate a light emitting diode (LED) for more than 15 minutes. This PTHA//charcoal ASC also possesses stable electrochemical properties in different bending positions and hence finds a promising application in flexible, wearable and portable energy storage electronic devices.

A high energy density flexible solid-state asymmetric supercapacitor is fabricated using polythiophene nanocomposites and charcoal which exhibits stable electrochemical properties in different bending position.     

Facile preparation of flexible binder-free graphene electrodes for high-performance supercapacitors

A facile two-step strategy to prepare flexible graphene electrodes has been developed for supercapacitors using thermal reduction of graphene oxide (GO) and thermally reduced graphene oxide (TRGO) composite films. The tunable porous structure of the GO/TRGO film provided channels to release the high pressure generated by CO₂ gas. The graphene electrode obtained from reduced-GO/TRGO (1 : 1 in mass ratio) film showed great flexibility and high film density (0.52 g cm⁻³). Using the EMI-BF₄ electrolyte with a working voltage of 3.7 V, the as-fabricated free-standing reduced-GO/TRGO (1 : 1) film achieved a great gravimetric capacitance of 180 F g⁻¹ (delivering a gravimetric energy density of 85.6 W h kg⁻¹), a volumetric capacitance of 94 F cm⁻³ (delivering a volumetric energy density of 44.7 W h L⁻¹), and a 92% retention after 10 000 charge/discharge cycles. In addition, the solid state flexible supercapacitor with the free-standing reduced-GO/TRGO (1 : 1) film as the electrodes and the EMI-BF₄/poly (vinylidene fluoride hexafluopropylene) (PVDF-HFP) gel as the electrolyte also demonstrated a high gravimetric capacitance of 146 F g⁻¹ with excellent mechanical flexibility, bending stability, and electrochemical stability. The strategy developed in this study provides great potentials for the synthesis of flexible graphene electrodes for supercapacitors.

A supercapacitor electrode is developed with a free-standing graphene film by a facile two-step strategy. The graphene electrode achieved a gravimetric capacitance of 180 F g⁻¹ and a volumetric capacitance of 94 F cm⁻³.     

Ionic-liquid-assisted one-pot synthesis of Cu2O nanoparticles/multi-walled carbon nanotube nanocomposite for high-performance asymmetric supercapacitors

Finding earth-abundant and high-performance electrode materials for supercapacitors is a demanding challenge in the energy storage field. Cuprous oxide (Cu₂O) has attracted increasing attention due to its theoretically high specific capacitance, however, the development of Cu₂O-based electrodes with superior capacitive performance is still challenging. We herein report a simple and effective ionic-liquid-assisted sputtering approach to synthesizing the Cu₂O nanoparticles/multi-walled carbon nanotubes (Cu₂O/MWCNTs) nanocomposite for high-performance asymmetric supercapacitors. The Cu₂O/MWCNTs nanocomposite delivers a high specific capacitance of 357 F g⁻¹, good rate capability and excellent capacitance retention of about 89% after 20 000 cycles at a current density of 10 A g⁻¹. The high performance is attributed to the uniform dispersion of small-sized Cu₂O nanoparticles on conductive MWCNTs, which offers plenty of redox active sites and thus improve the electron transfer efficiency. Oxygen vacancies are further introduced into Cu₂O by the NaBH₄ treatment, providing the oxygen-deficient Cu₂O/MWCNTs (r-Cu₂O/MWCNTs) nanocomposite with significantly improved specific capacitance (790 F g⁻¹) and cycling stability (∼93% after 20 000 cycles). The assembled asymmetric supercapacitor based on the r-Cu₂O/MWCNTs//activated carbon (AC) structure achieves a high energy density of 64.2 W h kg⁻¹ at 825.3 W kg⁻¹, and long cycling life. This work may form a foundation for the development of both high capacity and high energy density supercapacitors by showcasing the great potential of earth-abundant Cu-based electrode materials.

A one-pot room-temperature-ionic-liquid-assisted sputtering approach is designed to synthesize Cu₂O/MWCNTs nanocomposite with high capacitance and long cycling life due to synergistic effects of oxygen-deficient Cu₂O and conductive MWCNTs.     

Engineering NiCo2S4 nanoparticles anchored on carbon nanotubes as superior energy-storage materials for supercapacitors

Fabricating high-capacity electrode materials toward supercapacitors has attracted increasing attention. Here we report a three-dimensional CNTs/NiCo₂S₄ nanocomposite material synthesized successfully by a facile one-step hydrothermal technique. As expected, a CNTs/NiCo₂S₄ electrode shows remarkable capacitive properties with a high specific capacitance of 890 C g⁻¹ at 1 A g⁻¹. It also demonstrates excellent cycle stability with an 83.5% capacitance retention rate after 5000 cycles at 10 A g⁻¹. Importantly, when assembled into a asymmetric supercapacitor, it exhibits a high energy density (43.3 W h kg⁻¹) and power density (800 W kg⁻¹). The exceptional electrochemical capacity is attributed to the structural features, refined grains, and enhanced conductivity. The above results indicate that CNTs/NiCo₂S₄ composite electrode materials have great potential application in energy-storage devices.

A one-step hydrothermal method was used to successfully synthesize NiCo₂S₄ nanocomposites anchored on carbon nanotubes as excellent energy storage materials for supercapacitors.     

All-solid-state supercapacitors using a highly-conductive neutral gum electrolyte

Recently, the development of safe, stable, and long-life supercapacitors has attracted considerable interest driven by the fast-growth of flexible wearable devices. Herein, we report an MnO₂-based symmetric all-solid-state supercapacitor, using a neutral gum electrolyte that was prepared by embedding aqueous sodium sulfate solution in a biopolymer xanthan gum. Resulting from the high ion conductivity 1.12 S m⁻¹, good water retention, and high structure adaption of such gum electrolyte, the presently described supercapacitor showed high electrochemical performance with a specific capacitance of 347 F g⁻¹ at 1 A g⁻¹ and an energy density of 24 μW h cm⁻² The flexible supercapacitor possesses excellent reliability and achieves a retaining capacitance of 82% after 5000 cycles. In addition, the as-prepared supercapacitor demonstrated outstanding electrochemical stability at temperatures between −15 °C to 100 °C.

A highly-conductive neutral gum electrolyte was used for all-solid-state supercapacitors with outstanding electrochemical performance at temperatures between −15 °C to 100 °C.     

Porous g-C3N4 covered MOF-derived nanocarbon materials for high-performance supercapacitors

Supercapacitors with high power density and long cycle life have shown great potential in energy storage supply for modern electronic devices. Among the component parts of supercapacitors, electrode materials with high electrical conductivity, large surface area and porosity are critical to the energy storage performances of devices. Here, we report a porous g-C₃N₄ covered MOF-derived nanocarbon material with large specific surface area and high nitrogen doping level as an electrode material for supercapacitors. The large surface area provides high capacity for ion accommodation during electrochemical processes, and the high nitrogen doping facilitates electron and ion transport with extra pseudocapacitance. The supercapacitor based on the as-synthesized material delivers a high specific capacity of 106 F g⁻¹ at current density of 1 A g⁻¹ as well as good rate capability. Furthermore, the device presents good cycling stability with capacitance retention of 91% even after 10 000 cycles at 1 A g⁻¹ under 0.8 V. This study presents a new insight into the design of nanocomposite materials with high energy storage capability and will accelerate the practical application of supercapacitors in future.

Here, we report a porous g-C₃N₄ covered MOF-derived nanocarbon material with large surface area and high nitrogen doping for supercapacitors.     

One-pot mechanochemical exfoliation of graphite and in situ polymerization of aniline for the production of graphene/polyaniline composites for high-performance supercapacitors

Graphene/polyaniline composites have attracted considerable attention as high-performance supercapacitor electrode materials; however, there are still numerous challenges for their practical applications, such as the complex preparation process, high cost, and disequilibrium between energy density and power density. Herein, we report an efficient method to produce graphene/polyaniline composites via a one-pot ball-milling process, in which aniline molecules act as both the intercalator for the exfoliation of graphite and the monomer for mechanochemical polymerization into polyaniline clusters on the in situ exfoliated graphene sheets. The graphene/polyaniline composite electrode delivered a large specific capacitance of 886 F g⁻¹ at 5 mV s⁻¹ with a high retention of 73.4% at 100 mV s⁻¹. The high capacitance and rate capability of the graphene/polyaniline composite can contribute to the fast electron/ion transfer and dominantly capacitive contribution because of the synergistic effects between the conductive graphene and pseudocapacitive polyaniline. In addition, a high energy density of 40.9 W h kg⁻¹ was achieved by the graphene/polyaniline-based symmetric supercapacitor at a power density of 0.25 kW kg⁻¹, and the supercapacitor also maintained 89.1% of the initial capacitance over 10 000 cycles.

Efficient ball-milling production of graphene/polyaniline composites as supercapacitor electrodes with enhanced capacitive contribution, rate capability, and specific capacitance.     

Hierarchical Co(OH)2@NiMoS4 nanocomposite on carbon cloth as electrode for high-performance asymmetric supercapacitors

Hierarchical Co(OH)₂@NiMoS₄ nanocomposites were successfully prepared on a carbon cloth by using a simple two-step hydrothermal method coupled with a room-temperature vulcanization method. The resulting nanocomposites were composed of large-scale uniform Co(OH)₂ nanowires fully covered with ultrathin vertical NiMoS₄ nanoflakes. Because of the synergetic effect between Co(OH)₂ and NiMoS₄, the nanocomposites exhibited good electrochemical performance as a supercapacitor electrode. In particular, a specific capacity of 2229 F g⁻¹ was achieved at a current density of 1 A g⁻¹. In addition, an asymmetrical supercapacitor fabricated using activated carbon as the negative electrode and the as-synthesised nanocomposite as the positive electrode exhibited a maximum energy density of 59.5 W h kg⁻¹ at a power density of 1 kW h kg⁻¹ and excellent cycling stability (100% capacitance retention after 5000 cycles). These results indicate that the hierarchical Co(OH)₂@NiMoS₄ nanocomposite has great potential for practical application in high-performance energy storage devices.

A hierarchical Co(OH)₂@NiMoS₄ nanocomposite was prepared on the surface of carbon cloth, which exhibited good electrochemical performance as a supercapacitor electrode.     

NH4V4O10 nanobelts vertically grown on 3D TiN nanotube arrays as high-performance electrode materials of supercapacitors

High performance supercapacitor without binders has attracted wide attention as an energy storage device. In this work, novel NH₄V₄O₁₀ nanobelts were successfully synthesized and decorated into TiN nanotube arrays by a simple hydrothermal method. The as-prepared no-binder electrode hybrids exhibited excellent electrochemical performances with a specific capacitance of 749.0 F g⁻¹ at 5 mV s⁻¹ and a capacity retention of 85.7% after 200 cycles, which makes it an appealing candidate for electrode materials of supercapacitors.

NH₄V₄O₁₀ nanobelts were synthesized and decorated into TiN nanotube arrays as supercapacitor electrode with a specific capacitance of 749.0 F g⁻¹ at 5 mV s⁻¹ and a capacity retention of 85.7% after 200 cycles.     

Ice-interface assisted large-scale preparation of polypyrrole/graphene oxide films for all-solid-state supercapacitors

In this paper, large-scale, self-standing polypyrrole/graphene oxide (PPy/GO) nanocomposite films were prepared by an environmentally friendly and easy-to-operate confined polymerization method, and were also assembled as electrode materials for symmetric all-solid-state supercapacitors. In this paper, large-scale, self-standing polypyrrole/graphene oxide (PPy/GO) nanocomposite films were prepared by an environmentally friendly and easy-to-operate confined polymerization method, and were also assembled as electrode materials for symmetric all-solid-state supercapacitors. The morphology, chemical structure and electrochemical property were characterized by field emission scanning electron microscope (FESEM), Fourier transform infrared spectroscopy (FTIR), cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS), respectively. The lamellar structure of GO and both strong interaction with ice and pyrrole could promote polymerization of pyrrole and improve the compactness of the film. With the aid of GO, the conjugation length of PPy increased, the resistance of the material decreased, and the electrochemical energy storage of the composite film was significantly enhanced. In the case of 2.5 wt% GO, the prepared PPy/GO nanocomposite supercapacitor exhibited a high area specific capacitance of 97.3 mF cm⁻² at 1 mA cm⁻². Furthermore, the PPy/GO film supercapacitor also showed excellent cycling stability and good flexibility.

A confirned polymerization method was employed to fabricate self-standing PPy/GO film for all-solid-state supercapacitor.     

Lavender-like cobalt hydroxide nanoflakes deposited on nickel nanowire arrays for high-performance supercapacitors

Hierarchical nanostructured electrodes with excellent electronic properties and high specific surface areas have promising applications in high-performance supercapacitors. However, high active mass loading and uniform structure are still crucial in fabricating such architectures. Herein, Co(OH)₂ nanoflakes were homogeneously deposited on nickel nanowire arrays (NNA) through a hydrothermal approach to form an NNA@Co(OH)₂ (NNACOH) composite electrode. The as-synthesized one dimensional (1D) system had a lavender-like structure with a high mass loading of 5.42 mg cm⁻² and a high specific surface area of 74.5 m² g⁻¹. Due to the unique electrode structure characteristics, the electrode could deliver a high specific capacitance of 891.2 F g⁻¹ at the current density of 1 A g⁻¹ (corresponding to an areal capacitance of 4.83 F cm⁻² at 5.42 mA cm⁻²). The capacitance could still maintain a high value of 721 F g⁻¹ when the current density is increased to 50 A g⁻¹. In addition, the electrode showed superior cycle stability with a capacitance retention of 89.3% after charging/discharging at the current density of 10 A g⁻¹ for 20 000 cycles. A flexible asymmetric supercapacitor (ASC) was assembled by employing NNACOH as the positive electrode and activated carbon (AC) as the negative electrode. It delivered a maximum energy density of 23.1 W h kg⁻¹ at the power density of 712 W kg⁻¹ and an energy density of 13.5 W h kg⁻¹ at the maximum power density of 14.7 kW kg⁻¹ (based on the total mass of the electrodes), showing the state-of-the-art energy storage ability of the Co(OH)₂ cathode material at device level.

Co(OH)₂ nanoflakes deposited on nickel nanowire arrays with a lavender-like architecture are prepared. Using it as cathode and activated carbon as anode, the flexible supercapacitor shows superior energy storage ability.     

Efficient coupling of MnO2/TiN on carbon cloth positive electrode and Fe2O3/TiN on carbon cloth negative electrode for flexible ultra-fast hybrid supercapacitors

Recent research and development of energy storage devices has focused on new electrode materials because of the critical effects on the electrochemical properties of supercapacitors. In particular, MnO₂ and Fe₂O₃ have drawn extensive attention because of their low cost, high theoretical specific capacity, environmental friendliness, and natural abundance. In this study, MnO₂ ultrathin nanosheet arrays and Fe₂O₃ nanoparticles are fabricated on TiN nanowires to produce binder-free core–shell positive and negative electrodes for a flexible and ultra-fast hybrid supercapacitor. The MnO₂/TiN/CC electrode shows larger pseudocapacitance contributions than MnO₂/CC. For example, at a scanning rate of 2 mV s⁻¹, the pseudocapacitance contribution of MnO₂/TiN/CC is 87.81% which is nearly 25% bigger than that of MnO₂/CC (71.26%). The supercapacitor can withstand a high scanning rate of 5000 mV s⁻¹ in the 2 V window and exhibits a maximum energy density of 71.19 W h kg⁻¹ at a power density of 499.79 W kg⁻¹. Even at 5999.99 W kg⁻¹, it still shows an energy density of 31.3 W h kg⁻¹ and after 10 000 cycles, the device retains 81.16% of the initial specific capacitance. The activation mechanism is explored and explained.

MnO₂ ultrathin nanosheet arrays and Fe₂O₃ nanoparticles are fabricated on carbon based TiN nanowires to produce binder-free and core–shell positive and negative electrodes for a flexible and ultra-fast hybrid supercapacitor.     

Facile hydrothermal synthesis of cobaltosic sulfide nanorods for high performance supercapacitors

With high reactivity, electrical conductivity, theoretical specific capacitance and well redox reversibility, transition metal sulfides are considered as a promising anode material for supercapacitors. Hence, we designed a simple two-step hydrothermal process to grow Co₄S₃ nanorod arrays in situ on flexible carbon cloth substrates. Benefited from the larger specific surface area of nanoarrays, the binder-free Co₄S₃ electrode demonstrates a higher specific capacity of 1.97 F cm⁻² at a current density of 2 mA cm⁻², while the Co₃O₄ electrode has a capacity of only 0.07 F cm⁻² at the same current density. Surprisingly, at a high scan rate of 200 mV s⁻¹, the synthesized Co₄S₃ electrode still maintains almost 100% of its initial capacitance after 5000 cycles. Moreover, when using the prepared Co₄S₃ and MnO₂ electrode as the anode and cathode, the fabricated flexible supercapacitor obtains a high volumetric energy density of 0.87 mW h cm⁻³ (power density of 0.78 W cm⁻³) and a peak power density of 0.89 W cm⁻³ (energy density of 0.50 mW h cm⁻³). The excellent electrochemical properties imply that there is a large market for the prepared materials in flexible energy storage devices.

With high reactivity, electrical conductivity, theoretical specific capacitance and well redox reversibility, transition metal sulfides are considered as a promising anode material for supercapacitors.     

Atmospheric-pressure-plasma-jet processed carbon nanotube (CNT)–reduced graphene oxide (rGO) nanocomposites for gel-electrolyte supercapacitors

This study evaluates DC-pulse nitrogen atmospheric-pressure-plasma-jet processed carbon nanotube (CNT)–reduced graphene oxide (rGO) nanocomposites for gel-electrolyte supercapacitor applications. X-ray photoelectron spectroscopy (XPS) indicates decreased oxygen content (mainly, C–O bonding content) after nitrogen APPJ processing owing to the oxidation and vaporization of ethyl cellulose. Nitrogen APPJ processing introduces nitrogen doping and improves the hydrophilicity of the CNT–rGO nanocomposites. Raman analysis indicates that nitrogen APPJ processing introduces defects and/or surface functional groups on the nanocomposites. The processed CNT–rGO nanocomposites on carbon cloth are applied to the electrodes of H₂SO₄–polyvinyl alcohol (PVA) gel-electrolyte supercapacitors. The best achieved specific (areal) capacitance is 93.1 F g⁻¹ (9.1 mF cm⁻²) with 15 s APPJ-processed CNT–rGO nanocomposite electrodes, as evaluated by cyclic voltammetry under a potential scan rate of 2 mV s⁻¹. The addition of rGOs in CNTs in the nanoporous electrodes improves the supercapacitor performance.

This study demonstrates ultrafast (15 s) atmospheric-pressure-plasma-jet (APPJ) processed CNT–rGO nanocomposite gel-electrolyte supercapacitors.     

Phyto-synthesized facile Pd/NiOPdO ternary nanocomposite for electrochemical supercapacitor applications

Sustainable and effective electrochemical materials for supercapacitors are greatly needed for solving the global problems of energy storage. In this regard, a facile nanocomposite of Pd/NiOPdO was synthesized using foliar phyto eco-friendly agents and examined as an electrochemical electrode active material for supercapacitor application. The nanocomposite showed a mixed phase of a ternary nano metal oxide phase of rhombohedral NiO and tetragonal PdO confirmed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and XPS (X-rays photoelectron spectroscopy). The optical (direct) energy value of the synthesized nanocomposite was 3.14 eV. The phyto-functionalized nanocomposite was studied for electrochemical supercapacitor properties and revealed a specific capacitance of 88 F g⁻¹ and low internal resistance of 0.8 Ω. The nanoscale and phyto organic species functionalized nanocomposite exhibited enhanced electrochemical properties for supercapacitor application.

The natural phyto bio-factories were successfully utilized for the cost-effective synthesis of facile Pd/NiOPdO ternary nanocomposite for energy storage application with enhanced electro-active site.     

A simple strategy to prepare a layer-by-layer assembled composite of Ni–Co LDHs on polypyrrole/rGO for a high specific capacitance supercapacitor

A facile and novel electrode material of nickel–cobalt layered double hydroxides (Ni–Co LDHs) layered on polypyrrole/reduced graphene oxide (PPy/rGO) is fabricated for a symmetrical supercapacitor via chemical polymerization, hydrothermal and vacuum filtration. This conscientiously layered composition is free from any binder or surfactants which is highly favorable for supercapacitors. The PPy/rGO serves as an ideal backbone for Ni–Co LDHs to form a free-standing electrode for a high-performance supercapacitor and enhanced the overall structural stability of the film. The well-designed layered nanostructures and high electrochemical activity from the hexagonal-flakes like Ni–Co LDHs provide large electroactive sites for the charge storage process. The specific capacitance (1018 F g⁻¹ at 10 mV s⁻¹) and specific energy (46.5 W h kg⁻¹ at 464.9 W kg⁻¹) obtained for the PPy/rGO|Ni–Co LDHs symmetrical electrode in the current study are the best reported for the two-electrode system for PPy- and LDHs-based composites. The outstanding performance in the prepared LBL film is a result of the LBL architecture of the film and the combined effect of redox reaction and electrical double layer capacitance.

A facile and novel electrode material of nickel–cobalt layered double hydroxides (Ni–Co LDHs) layered on polypyrrole/reduced graphene oxide (PPy/rGO) is fabricated for a symmetrical supercapacitor via chemical polymerization, hydrothermal and vacuum filtration.     

Advanced asymmetric supercapacitors with a squirrel cage structure Fe3O4@carbon nanocomposite as a negative electrode

Carbon materials have been used as negative electrodes for supercapacitor applications; nevertheless, owing to the low capacitance, they have limited ability to enhance the supercapacitor electrochemical properties. Here, we employ a facile chemical precipitation method for preparing a squirrel cage structure Fe₃O₄@carbon nanocomposite. In this architecture, the carbonized crosslinked bovine serum albumin (C) will play critical roles, serving as a skeleton for the deposition of Fe₃O₄ and a transportation pathway like “high-speed rail” for electrons, maintaining the structural stability as well as accommodating the volume expansion of Fe₃O₄ and facilitating electron transportation and the electrolyte ion diffusion. The iron oxide nanoparticles (Fe₃O₄) exhibit superior reversible redox characteristics, hence increasing the supercapacitor performance. Benefiting from a stable structure, an aqueous asymmetric supercapacitor using a CNT@Ni(OH)₂ positive electrode (cathode) and Fe₃O₄@C negative electrode (anode) has also been assembled, which presents a high energy density of 17.3 W h kg⁻¹ at a power density of 700 W kg⁻¹. The strategy for choice of Fe₃O₄@C composites will provide new opportunities for future supercapacitors with superior cyclability and high power density.

Carbon materials have been used as negative electrodes for supercapacitor applications; nevertheless, owing to the low capacitance, they have limited ability to enhance the supercapacitor electrochemical properties.     

Photo-assisted synthesis of coaxial-structured polypyrrole/electrochemically hydrogenated TiO2 nanotube arrays as a high performance supercapacitor electrode

An organic–inorganic coaxial-structured hybrid of PPy/EH-TNTAs electrode with outstanding supercapacitive performance was developed by incorporating electroactive polypyrrole (PPy) into a highly-conductive TiO₂ substrate, namely, electrochemically hydrogenated TiO₂ nanotube arrays (EH-TNTAs) through a photo-assisted potentiodynamic electrodeposition route. The as-fabricated PPy/EH-TNTAs hybrid electrode achieves a specific capacitance of up to 614.7 F g⁻¹ at 1.0 A g⁻¹ with 87.4% of the initial capacitance remaining after 5000 cycles at 10 A g⁻¹, outperforming other fabricated PPy-TNTAs hybrid electrodes. The photoelectrodeposited and electrodeposited hybrid samples as well as the EH-TNTAs-based and air–TNTAs-based hybrid samples were fully compared from electropolymerization process, morphology, structural feature and electrochemical perspectives. The results indicate that the synergy of remarkably improved conductivity and electrochemical properties of the TiO₂ substrate induced by intentionally introduced Ti³⁺ (O-vacancies) as well as the homogenous and integrated deposition of PPy triggered by light illumination enabled the outstanding supercapacitive performance of the PPy/EH-TNTAs hybrid electrode. A symmetric supercapacitor device was assembled using the PPy/EH-TNTAs hybrid as both a positive and negative electrode, respectively. It displays a high energy density of 17.7 W h kg⁻¹ at a power density of 1257 W kg⁻¹. This organic–inorganic coaxial-structured PPy/EH-TNTAs electrode will be a competitive and promising candidate for application in future energy storage devices.

An organic–inorganic coaxial-structured hybrid of PPy/EH-TNTAs electrode was developed and applied for high performance supercapacitors.