Polyaniline nanostructures expedient as working electrode materials in supercapacitors

Granular type polyaniline (PANi), PANi nanofibers (NFs), and PANi nanotubes (NTs) expedient as working electrode materials for supercapacitors are synthesized. The synthesis procedure used in this work facilitates not only the synthesis of solid powders of the PANi nanostructures, but also thin films constituted by the same PANi nanostructures in the same experiment. PANi NFs are found to exhibit faster electrode kinetics and better capacitance when compared to PANi NTs and granular PANi. Specific capacitance and energy storage per unit mass of PANi NFs are 239.47 Fg^?1 (at 0.5 Ag^?1) and 43.2 Wh?kg^?1, respectively. Electrical conductivity of PANi NFs is also better when compared to the other two nanostructures. Properties of the three PANi nanostructures are explicated in correlation with crystallinity, intrinsic oxidation state, doping degree, BET surface area, and ordered mesoporosity pertaining to the nanostructures.     

Electrochemical capacitance of porous NiO–CeO2 binary oxide synthesized via sol–gel technique for supercapacitor

Nanocrystalline NiO–CeO₂ binary oxide as a novel electrode material for ultracapacitor was synthesized via glycol assisted citrate sol–gel method. Unique cubic phases with aggregated crystalline microstructure of NiO–CeO₂ mixed oxides were examined by X-ray diffraction and transmission electron microscope. The observed electrochemical measurements further reveal the strong pseudocapacitance features of the mixed oxides at different current density. Binary oxide annealed at 500 °C shows an optimum specific capacitance (C _s ) of 305 Fg^?1 at the constant current density of 1 Ag^?1. The achieved C _s value undoubtedly certifies that in combination with NiO the structural stability and redox property of CeO₂ have been enhanced. Especially, increasing the calcination temperature binary oxide has shown well reversible redox features which confirm the high chemical and thermal stability of CeO₂ and it could be involve in the charge storage process effectively by their strong Ce³⁺/Ce⁴⁺ redox couples. Capacity retention and cyclic stability of the electrode was quite good, only ～5 % capacity fading was observed after 1,000 cycles. Moreover, binary oxide calcined at 700 °C exhibits a specific capacitance of 167 Fg^?1 at the constant current density of 1 Ag^?1 which states that presence of CeO₂ with NiO have controlled the grain growth and maintains their porous microstructure even at 700 °C. This facilitates to the redox process at both NiO and CeO₂ active surfaces at elevated temperature significantly.     

Sea anemone-like zinc-cobalt oxysulfides grown on Ni foam as a battery-type electrode with admirable performance

Sea anemone-like zinc-cobalt (Zn-Co) oxysulfides grown on nickel foam were synthesized by a hydrothermal treatment for the first time as a battery-type electrode for hybrid supercapacitors. Such a binder-free sea anemone-like Zn-Co oxysulfide electrode displays improved electrochemical properties, which can be attributed to the unique sea anemone-like morphology and the partly replacement of oxygen with sulfur since sulfur possesses a lower electronegativity. As a result, the Zn-Co oxysulfide electrode exhibits a specific capacity of 645.5 C g^?1 (1 A g^?1) which is much higher than 386.0 C g^?1 of Zn-Co oxide electrode at the same conditions. In addition, the Zn-Co oxysulfide electrode also demonstrates good cycling stability (76.1% capacity retention after 1000 cycles). According to these, the Zn-Co oxysulfide electrode is testified to be a promising faradaic electrode for energy storage.     

Hierarchical porous ZnMn<Subscript>2</Subscript>O<Subscript>4</Subscript> synthesized by the sucrose-assisted combustion method for high-rate supercapacitors

A simple sucrose-assisted combustion and subsequent high-temperature calcination route have been employed to prepare hierarchical porous ZnMn₂O₄ nanostructure. When used as an electrode for supercapacitor, the ZnMn₂O₄ electrode displays a high specific capacitance of 411.75 F g^?1 at a current density of 1 A g^?1, remarkable capacitance retention rate of 64.28 % at current density of 32 A g^?1 compared with 1 A g^?1, as well as excellent cycle stability (reversible capacity retention of 88.32 % after 4000 cycles). The outstanding electrochemical performances are mainly attributed to its hierarchical porous architecture, which provides large reaction surface area, fast ion and electron transfer, and good structure stability. All these impressive results demonstrate that ZnMn₂O₄ shows promise for its application in supercapacitors.     

Synthesis and characterization of nitrogen-doped graphene hollow spheres as electrode material for supercapacitors

Recently, the rapid development of graphene industry in the world, especially in China, provides more opportunities for the further extension of the application field of graphene-based materials. Graphene has also been considered as a promising candidate for use in supercapacitors. Here, nitrogen-doped graphene hollow spheres (NGHS) have been successfully synthesized by using industrialized and pre-processed graphene oxide (GO) as raw material, SiO₂ spheres as hard templates, and urea as reducing-doping agents. The results demonstrate that the content and pretreatment of GO sheets have important effect on the uniform spherical morphologies of the obtained samples. Industrialized GO and low-cost urea are used to prepare graphene hollow spheres, which can be a promising route to achieve mass production of NGHS. The obtained NGHS have a cavity of about 270 nm, specific surface area of 402.9 m² g^?1, ultrathin porous shells of 2.8 nm, and nitrogen content of 6.9 at.%. As electrode material for supercapacitors, the NGHS exhibit a specific capacitance of 159 F g^?1 at a current density of 1 A g^?1 in 6 M KOH aqueous electrolyte. Moreover, the NGHS exhibit superior cycling stability with 99.24% capacitive retention after 5000 charge/discharge cycles at a current density of 5 A g^?1.     

Novel synthesis and characterization of ZnCo<Subscript>2</Subscript>O<Subscript>4</Subscript> nanoflakes grown on nickel foam as efficient electrode materials for electrochemical supercapacitors

ZnCo₂O₄ nanoflakes were directly grown on Ni foam via a two-step facile strategy, involving cathodic electrolytic electrodeposition (ELD) method and followed by a thermal annealing treatment step. The results of physical characterizations exhibit that the mesoporous ZnCo₂O₄ nanoflakes have large electroactive surface areas (138.8 m² g^?1) and acceptable physical stability with the Ni foam, providing fast electron and ion transport sites. The ZnCo₂O₄ nanoflakes on Ni foam were directly used as integrated electrodes for supercapacitors and their electrochemical properties were measured in 2 M KOH aqueous solution. The ZnCo₂O₄ nanoflake electrode exhibits a high capacitance of 1781.7 F g^?1 at a current density of 5 A g^?1 and good rate capability (62% capacity retention at 50 A g^?1). Also, an excellent cycling ability at various current densities from 5 to 50 A g^?1 was obtained and 92% of the initial capacitance maintained after 4000 cycles. The results demonstrate that the proposed synthesis route is cost-effective and facile and can be developed for preparation of electrode materials in other electrochemical supercapacitors.     

Promising nitrogen-doped porous nanosheets carbon derived from pomegranate husk as advanced electrode materials for supercapacitors

Nitrogen-doped porous activated carbons (N-PHACs) have been successfully synthesized using pomegranate husk as carbon precursor via ZnCl₂-activation carbonization and subsequent urea-assisted hydrothermal nitrogen-doping method. The obtained N-PHACs possesses abundant mesoporous structure, high specific surface area (up to 1754.8 m² g^?1), pore volume (1.05 cm³ g^?1), and nitrogen-doping content (4.51 wt%). Besides, the N-PHACs-based material showed a high specific capacitance of 254 F g^?1 at a current density of 0.5 A g^?1 and excellent rate performance (73% capacitance retention ratio even at 20 A g^?1) in 2 M KOH aqueous electrolyte, which is attributed to the contribution of double-layer capacitance and pseudocapacitance. The assembled N-PHACs-based symmetric capacitor with a wide operating voltage range of 0–1.8 V exhibits a maximum energy density of 15.3 Wh kg^?1 at a power density of 225 W kg^?1 and superior cycle stability (only 6% loss after 5000 cycles) in 0.5 M Na₂SO₄ aqueous electrolyte. These exciting results suggest that the novel N-doping porous carbon material prepared by a green and low-cost design strategy has a potential application as high-performance electrode materials for supercapacitors.     

Synthesis of δ-MnO<Subscript>2</Subscript> film on FTO glass with high electrochemical performance

δ-Manganese dioxide (MnO₂) has been proved to own the excellent electrochemical performances for a long time. But few of studies report the electrochemical performances of δ-MnO₂ film. Here, we synthesize δ-MnO₂ film on fluorine-doped tin oxide (FTO) glass via a simple redox reaction at the room temperature. The X-ray diffraction (XRD) and Raman spectroscopy are used to confirm the physical structure, whilst cyclic voltammetry and galvanostatic charge-discharge measurements are performed to investigate the electrochemical performances. Encouragingly, δ-MnO₂ film delivers a high specific capacitance (C _s) of 350.5 F g^?1 at 100 mV s^?1 and 275.0 F g^?1 at 5 A g^?1. The capacitance retention of δ-MnO₂ film can be up to 100 % after being charge/discharge at 2 A g^?1 with 1000 cycles. This research might further indicate that δ-MnO₂ film is a promising electrode material for supercapacitors.     

Influence of graphene coating on supercapacitive behavior of sandwich-like N- and O-enriched porous carbon/graphene composites in aqueous and organic electrolytes

Polyacrylonitrile nanofiber cloth coated with graphene oxide was carbonized and activated to fabricate nitrogen- and oxygen-enriched porous carbon/graphene (NAC@Gr) sandwich-like composites. The influence of graphene coating on the microstructure, surface composition, and supercapacitive performance of the as-prepared composites was investigated. The results indicated that significantly enhanced energy storage capability can be achieved due to the high specific surface area, optimized pore structure, and surface functionality. The composites show both high gravimetric and volumetric specific capacitances, for example, 380 F g^?1 (178 F cm^?3) at 0.1 A g^?1 in 6 M KOH and 228 F g^?1 (125 F cm^?3) at 1 A g^?1 in 1 M TEABF₄/AN electrolyte. The assembled symmetric supercapacitors exhibit high energy density, high power density, excellent cycling stability, and high-rate performance.     

Facile synthesis and characterization of high-performance NiMoO<Subscript>4</Subscript> · <Emphasis Type="Italic">x</Emphasis>H<Subscript>2</Subscript>O nanorods electrode material for supercapacitors

One-dimensional NiMoO₄ · xH₂O nanorods were synthesized by a facile template-free hydrothermal method as a potential electrode material for supercapacitors. The influences of reaction temperature, reaction time, and nickel source on the properties of resultant samples were investigated. Electrochemical data reveal that the as-synthesized one-dimensional NiMoO₄ · xH₂O nanorod superstructures can deliver a remarkable specific capacitance (SC) of 1131 F g^?1 at a current density of 1 A g^?1 and remain as high as 914 F g^?1 at 10 A g^?1 in a 6 M KOH aqueous solution. Moreover, there is only 6.2 % loss of the maximum SC after 1000 continuous charge–discharge cycles at the high current density of 10 A g^?1. Such outstanding electrochemical performance may be owing to the unique one-dimensional hierarchical structures, which can facilitate the electrolyte ions and electrons to easily contact the NiMoO₄ nanorod building blocks and then allow for sufficient faradaic reactions to take place, even at high current densities.     

Roles of Al-doped ZnO (AZO) modification layer on improving electrochemical performance of LiNi<Subscript>1/3</Subscript>Co<Subscript>1/3</Subscript>Mn<Subscript>1/3</Subscript>O<Subscript>2</Subscript> thin film cathode

Al-doped ZnO (AZO) was sputtered on the surface of LiNi_1/3Co_1/3Mn_1/3O₂ (NCM) thin film electrode via radio frequency magnetron sputtering, which was demonstrated to be a useful approach to enhance electrochemical performance of thin film electrode. The structure and morphology of the prepared electrodes were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive spectrometer, and transmission electron microscopy techniques. The results clearly demonstrated that NCM thin film showed a strong (104) preferred orientation and AZO was uniformly covered on the surface of NCM electrode. After 200 cycles at 50 μA μm^?1 cm^?2, the NCM/AZO-60s electrode delivered highest discharge capacity (78.1 μAh μm^?1 cm^?2) compared with that of the NCM/AZO-120s electrode (62.4 μAh μm^?1 cm^?2) and the bare NCM electrode (22.3 μAh μm^?1 cm^?2). In addition, the rate capability of the NCM/AZO-60s electrode was superior to the NCM/AZO-120s and bare NCM electrodes. The improved electrochemical performance can be ascribed to the appropriate thickness of the AZO coating layer, which not only acted as HF scavenger to keep a stable electrode/electrolyte interface but also reduced the charge transfer resistance during cycling.     

Effect of annealing temperature on optical and electrochemical properties of chitosan–ZnO nanostructure

Chitosan–ZnO nanostructures were prepared by chemical precipitation method using different concentration of zinc chloride and sodium hydroxide solutions. Nanorod-shaped grains with hexagonal structure for samples annealed at 300 °C and porous structure with amorphous morphology for samples annealed at 600 °C were revealed in SEM analysis. X-ray diffraction patterns confirmed the hexagonal phase ZnO with crystallite size found to be in the range of ~24.15–34.83 nm. Blue shift of UV–Vis absorption shows formation of nanocrystals/nanorods of ZnO with marginal increase in band gap. Photoluminescence spectra show that blue–green emission band at 380–580 nm. The chitosan–ZnO nanostructures used on surface of a glassy carbon electrode gives the oxidation peak potential at ~0.6 V. The electrical conductivity of chitosan–ZnO composites were observed at 2.1?×?10^?5 to 2.85?×?10^?5?S/m. The nanorods with high surface area and nontoxicity nature of chitosan–ZnO nanostructures observed in samples annealed at 300 °C were suitable as a potential material for biosensing.     

Chitosan-ZnO/polyaniline ternary nanocomposite for high-performance supercapacitor

We report on the synthesis of chitosan-zinc oxide (ZnO)/polyaniline (CS-ZnO/PANI) ternary nanocomposites via in situ polymerization of aniline in the presence of CS-ZnO nanocomposite prepared by simple precipitation method. The structure, morphology, and physicochemical properties of prepared ternary composites are characterized by Fourier transform infrared, UV–visible, X-ray diffraction, SEM, EDXS, TEM, thermogravimetric/differential thermal analysis, and N₂ adsorption/desorption measurements. Their electrochemical properties are also investigated using cyclic voltammetry, galvanostatic charge–discharge tests, and electrochemical impedance spectroscopy. Electrochemical measurements show that the mesoporous CS_0.12-ZnO_2.5/PANI electrode yields larger specific capacitance (587.15 F g^?1) than the corresponding PANI-ZnO electrode without added chitosan and the capacitance retention is 80 % after 1,000 charge/discharge cycles at 175 mA cm^?2 current density in the voltage range of 0 to 0.8 V vs. SCE, due to the synergistic effect among three components which result in enhanced specific capacitance and cycling stability. The resulting composites are promising electrode materials for high-performance, environmentally friendly, and low-cost electrical energy storage devices.     

Hydrothermal synthesis of pure LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> from nanostructured MnO<Subscript>2</Subscript> precursors for aqueous hybrid supercapacitors

Pure LiMn₂O₄ samples with high crystallinity (LMO-1# and LMO-2#) were successfully synthesized by a facile hydrothermal method using δ-MnO₂ nanoflowers and α-MnO₂ nanowires as the precursors. The as-prepared samples were analyzed by XRD, SEM, and Brunauer-Emmett-Teller (BET), and their capacitive properties were investigated by cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge/discharge test. Two LiMn₂O₄ samples showed good capacitive behavior in aqueous hybrid supercapacitors. AC//LMO-1# and AC//LMO-2# delivered the initial specific capacitance of 45.4 and 40.7 F g^?1 in 1 M Li₂SO₄ electrolyte at a current density of 200 mA g^?1 in the potential range of 0～1.5 V, respectively. After 1000 cycles, the capacitance retention was 97.6% for AC//LMO-1# and 93.7% for AC//LMO-2#. Obviously, LMO-1# from δ-MnO₂ nanoflowers exhibited higher specific capacitance and better cycling performance than LMO-2#, so LMO-1# was more suitable as the positive electrode material in hybrid supercapacitors.     

Supercapacitor performances of rich nitrogen-doped mesoporous graphene fabricated by a facile template-free copyrolysis process

Rich nitrogen-doped mesoporous graphene (NDMG) with a large specific surface area of 496.8 m² g^?1 and high electrical conductivity of 327.2 S cm^?1, and suitable pore size was synthesized by a facile co-thermal annealing of pre-prepared phenolic polymer and dicyandiamide. The NDMG has a high nitrogen content (7.9 wt%) and can act as promising electroactive materials for two-electrode symmetric supercapacitors. The NDMG cells displayed a high specific capacitance of ca. 316 F g^?1 at 0.5 A g^?1, which is much higher than that of the pristine graphene devices (ca. 123 F g^?1). Moreover, compared with the capacitance drop rate of pristine graphene devices (8.9 %), the specific capacitance of NDMG cells was decreased by only 3.2 % after 2000 cycles, exhibiting a good cycling performance and reversibility. In addition, the specific capacitance of the NDMG cells can reach 251 F g^?1 at 5.0 A g^?1, revealing an excellent rate capability and implying the ability to deliver a high energy density at a high power density. The good electrochemical performances of NDMG can be attributed to its high surface area, suitable mesopore size, and high electrical conductivity.     

Growth of high-quality ZnO thin films on (11\bar{2}0) a-plane sapphire substrates by plasma-assisted molecular beam epitaxy

High-quality ZnO thin films were grown on a-plane sapphire substrates by plasma-assisted molecular beam epitaxy. X-ray diffraction and transmission electron microscopy reveal that the ZnO films have high structural quality and an atomically sharp ZnO/Al₂O₃ interface. The full width at half maximum values of the 0002 and $30\bar{3}2$ ZnO ω-rocking curves are 467.8 and 813.5 arc sec for a 600 nm thick ZnO film. A screw dislocation density of 4.35×10⁸ cm^?2 and an edge dislocation density of 3.38×10⁹ cm^?2 are estimated by X-ray diffraction. The surface of the ZnO epilayers contains hexagonal pits, which can be observed in the Zn-polar ZnO. The films have a resistivity of 0.119 Ω?cm, an electron concentration of 6.85×10¹⁷ cm^?3, and a mobility of 76.5 cm²?V^?1?s^?1 at room temperature. Low temperature photoluminescence measurements show good optical properties comparable to ZnO single crystals.     

Correlation between the mesoporous carbon sphere with Ni(OH)<Subscript>2</Subscript> nanoparticle contents for high-performance supercapacitor electrode

Ni(OH)₂ nanoparticles were decorated on mesoporous carbon spheres (MPCS) using a simple hard template method. The MPCS were derived from sodium carboxymethyl cellulose. As-prepared MPCS/Ni(OH)₂ nanocomposites were used as electrode materials for supercapacitors. These composites exhibited better electrochemical properties than a pristine mesoporous carbon sphere owing to the synergistic effect. However, the increase in Ni(OH)₂ is not proportional to the electrochemical performance improvement. The addition of an optimal amount of Ni(OH)₂, typically 1:20 by weight (MPCS:NiCl₂·6H₂O), showed an excellent specific capacitance of 1338.296 F g^?1 at a scan rate of 5 mV s^?1. These encouraging results indicate excellent potential for the development of highly capacitive energy storage devices for practical applications.     

Photocatalytic properties of zinc oxide nanorods grown by different methods

A comparison of photocatalytic properties of ZnO nanostructures fabricated by different methods was carried out. The photocatalytic properties of as grown and Ar-ion-treated ZnO materials were tested using photocatalytic degradation of an aqueous solution of methyl orange dye serving as a model water contaminant. The reaction rate constants of methyl orange photodegradation for untreated ZnO nanorods grown by the method of gas-transport reactions and hydrothermal method were equal to 5.3 × 10^?5 and 3.7 × 10^?4 s^?1, respectively, whereas for the case of the Ar-ion-treated samples they reached 1.85 × 10^?4 and 5.9 × 10^?4 s^?1, respectively. Based on the analysis of the photoluminescence spectra, it is assumed that the difference in photocatalytic activity is connected with different type of defects predominant on the surfaces of ZnO nanorods grown by the hydrothermal and gas-transport reactions methods. The experimental results show that ZnO nanostructures grown by the hydrothermal method would be promising for producing efficient catalysts.     

Liquid precipitation synthesis of Co3O4 for high-performance electrochemical capacitors

The amorphous Co₃O₄ nanostructure, which adopted sodium hexametaphosphate as structure-directing agent, has been successfully synthesized in large scale via two steps: preparation of the precursor and the calcination process. The results of X-ray diffraction indicate that the prepared materials are mainly composed of Co₃O₄; the formless Co₃O₄ nanoplate with loose structures is observed by scanning electron microscopy. Cyclic voltammetry, chronopotentiometry, and electrochemical impedance measurements are applied in a mild aqueous electrolyte (2 mol L^?1 KOH) to investigate the performance of the Co₃O₄, which show a high specific capacitance (SC) of 482.61 F g^?1 at 5 mA cm^?2. Besides, the SC degradation is only 10.05 % after 250 continuous charge–discharge cycles at 5 mA cm^?2, indicating an excellent electrochemical stability. The improved performance is reasonably ascribed to their irregular structure for ionic transport during the electrochemical reaction, which presents as promising candidates for supercapacitors.     

Fabrication of Ni-Co binary oxide/reduced graphene oxide composite with high capacitance and cyclicity as efficient electrode for supercapacitors

Nickel-cobalt binary oxide/reduced graphene oxide (G-NCO) composite with high capacitance is synthesized via a mild method for electrochemical capacitors. G-NCO takes advantages of reduced graphene oxide (RGO) and nickel-cobalt binary oxide. As an appropriate matrix, RGO is beneficial to form homogeneous structure and improve the electron transport ability. The binary oxide owns more active sites than those of nickel oxide and cobalt oxide to promote the redox reaction. Attributed to the well crystallinity, homogeneous structure, increased active sites, and improved charge transfer property, the G-NCO composite exhibits highly enhanced electrochemical performance compared with G-NiO and G-Co₃O₄ composites. The specific capacitance of the G-NCO composite is about 1750 F g^?1 at 1 A g^?1 together with capacitance retention of 79 % (900/1138 F g^?1) over 10,000 cycles at 4 A g^?1. To research its practical application, an asymmetric supercapacitor with G-NCO as positive electrode and activated carbon as negative electrode was fabricated. The asymmetric device exhibits a prominent energy density of 37.7 Wh kg^?1 at a power density of 800 W kg^?1. The modified G-NCO composite shows great potential for high-capacity energy storage.