Excellent Supercapacitive Performance Research Articles

Excellent Supercapacitor Performance of Robust Nickel–Organic Framework Materials Achieved by Tunable Porosity, Inner-Cluster Redox, and in Situ Fabrication with Graphene Oxide

Metal–organic frameworks have shown promising applications as electrode materials for supercapacitors because of the high porosity and tunable structures, but their poor water stability and conductivity limit their capacitance and efficiency. To demonstrate how to overcome these drawbacks, three isostructural Ni–organic frameworks with [NiII2NiIII(μ3-OH)(COO)6] trinuclear building blocks are selected. Taking advantage of high-connected architectures, absence of open metal sites, and effective inner-cluster redox process, three Ni–organic frameworks all are stable in KOH electrolyte and exhibit a pseudo-capacitor behavior with high specific capacitances up to 394, 426, and 465 F g–1. The increasing porosity facilitates the diffusion of metal ions and electrons and thus increases their electrochemical performance. Furthermore, we demonstrate that the in situ fabrication of metal–organic frameworks with graphene oxide can effectively promote their supercapacitor performance. With a given 3% graphene oxide do...

Synthesis of Honeycomb-Like Co₃O₄ Nanosheets with Excellent Supercapacitive Performance by Morphological Controlling Derived from the Alkaline Source Ratio.

Honeycomb-like Co3O4 nanosheets with high specific surface area were successfully synthesized on porous nickel foam by the facile hydrothermal method followed by an annealing treatment (300 °C), which were used as high-performance supercapacitor electrodes. The effects of the mole ratio of hexamethylenetetramine (HMT) and Co(NO3)2 (1:1, 2:1, 3:1, 4:1, 5:1 and 6:1) as the reactants on the morphological evolution and electrochemical performance of the electrodes were investigated in detail. X-ray diffractometry (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) were applied to characterize the structure and morphology of the products. The electrochemical performance was measured by cyclic voltammetry (CV) and galvanostatic charge/discharge. The mole ratio of HMT and Co(NO3)2 produced a significant effect on the morphological evolution of Co3O4. The morphological evolution of Co3O4 with the increase in the mole ratio was followed: the nanosheets accompanied with a large number of spherical nanoparticles → the formation of some strip-like particles due to the agglomeration of spherical nanoparticles → the formation of new nanosheets resulting from the growth of strip-like particles → the formation of coarse flower-like particles owing to the connection among the nanosheets → the nanosheets gradually covered with flower-like particles. Accompanied with the change, the specific surface area was increased firstly, and then decreased. A maximum was obtained at a HMT and Co(NO3)2 mole ratio of 4:1. The evolution in morphology of Co3O4 was responsible for the change in electrochemical performance of the electrode. The specific capacitance value of the electrode prepared at a HMT and Co(NO3)2 mole ratio of 4:1 was highest (743.00 F·g−1 at 1 A·g−1 in the galvanostatic charge/discharge test). The similar result was also observed in the CV test with a scanning rate of 5 mV·s−1. Moreover, the electrode also demonstrated an excellent cyclic performance, in which about 97% of the initial specific capacitance remained at 1 A·g−1 for 500 cycles in the galvanostatic charge/discharge test. This excellent electrochemical performance was ascribed to high specific surface area of Co3O4 nanosheets that provide added channels and space for the ions transportation.

Facile synthesis of high-surface area mesoporous biochar for energy storage via in-situ template strategy

A novel synthetic strategy was successfully developed to fabricate mesoporous biochar with high-surface area from crab shell as carbon precursor and its natural CaCO3 as in-situ template. The influences of pyrolysis temperature on the pore structure were investigated via N2 adsorption-desorption isotherms and scanning/transmission electron microscope. The as-prepared biochar at pyrolysis temperature of 900 °C possessed high surface area of 634 m2/g and large mesopore percentage of 70.80%. The maximum specific capacitance of biochar electrode reached 220 F/g. The excellent supercapacitor performance of this supercapacitor was strongly attributed to the thermal decomposition of CaCO3, producing high surface area and mesoporous structure for raw biochar.

Ni Doped V2O5 Nanocomposite Based Asymmetric Supercapacitor for Future Energy Storage Applications

The growing demand for enormous transportable power in our modern generation has enthralled more concern in developing high performance advanced energy storage devices like batteries, supercapacitors etc. for practical applications. Among various energy storage devices, supercapacitors have engrossed prevalent attention since last 20 years due to its excellent advantages, such as higher power density than batteries, higher energy density than conventional capacitors, long cycle life, wide range of operating temperature etc. Conversely, regardless of their outstanding industrial success, these supercapacitors are still open to improvements. Energetic research is ongoing on all aspects of supercapacitors, i.e., electrode materials, electrolytes, and cell construction. The major factors off-putting their wider practical application remain cost and safety. Many advance electrode materials with their high performance electrochemical capacitance value have already been reported in several reputed journals but its commercialization is still beyond the horizon. Lots of improvements are yet to be done for practical purpose. Innovative improvements are taking place to address these limitations. Among the oxide materials for application in electrochemical supercapacitors, ruthenium oxide (RuO2) and iridium oxide (IrO2) have accomplished much attention. RuO2 has a high double-layer and pseudo-capacitance (up to ~ 700 F g-1) and are stable in aqueous acid and alkaline electrolytes. The capacitance sensitively depends on the method of preparation. Tactlessly, disadvantage of RuO2 such as the high cost of the raw material and toxicity is lagging its great application in supercapacitors device applications. Consequently, in recent years great efforts have been commenced to find new and cheaper materials. Development of advance multifunctional electrode materials with porous, high surface area as well as good electrically interconnection is essential to improve the electrochemical performance of a hybrid supercapacitor. Several metal oxides and hydroxides, for example, those of nickel (Ni), cobalt (Co), Iron (Fe), vanadium (V), and manganese (Mn) are being studied extensively. Among various transition metal oxides, V2O5 have already been established to be a promising electrode material for pseudocapacitor compared to carbon-based electrode materials but relatively poor electrical conductivity seems to be a great challenge for their practical energy storage devices application. It has been observed that incorporation of one metal element into other metal oxide can improve the electrical conductivity as well as electrochemical performance.1-3 In this work, we have synthesized Ni doped V2O5 nanocomposite by facile easy hydrothermal process. The overall electrochemical studies along with different spectroscopic analysis have been carried out to investigate the various crucial factors for better improvement. This nanocomposite based asymmetric supercapacitor (ASC) performance has been studied extensively using two electrode set up for practical applications. Activated carbon (AC) based electrode and Ni doped V2O5 composite based electrode have been used as negative and positive electrode, respectively. Various electrochemical parameter such as cyclic voltammetry (CV), Galvanostatic charge discharge (GCD), electrochemical impedance (EIS) etc. have been calculated using well known equations.1 The maximum specific capacitance is obtained 82 Fg-1 in 1 M Na2SO4 gel electrolyte from this fabricated asymmetric supercapacitor device. The two electrode measurement and as fabricated ASC device application are shown in Fig. 1. The synthesized Ni doped V2O5 nanocomposite demonstration excellent asymmetric supercapacitor performance with good stability. The overall studies on the electrochemical properties as well as device performance of this material propose that it can be proved as promising supercapacitor applications in future. Author A. Ray and S. Das (IFA13-PH-71) wish to thank Department of Science and Technology (D.S.T), INSPIRE, Govt. of India for financial support. REFERENCES A. Ray et al., Appl. Surf. Sci. 443, 581 (2018).B. E. Conway, J. Electrochem. Soc., 138, 1539 (1991).A. Ray et al., Electrochim. Acta , 266, 404 (2018). Figure 1

Preparation of metal-organic framework-derived porous carbon and study of its supercapacitive performance

Metal-organic frameworks are formed via the connection of transition metals with oxygen- or nitrogen-containing organic ligands; the organic ligands are rich in carbon and contain heteroatoms such as N and P. Herein, NMFC-T porous carbon was prepared with (Ni3(BTC)2(Me2NH)3) as the carbon precursor using the strategy of pre-carbonization and KOH activation. Influence of carbonization temperature on properties of NMFC-T porous carbon was investigated, and the electrochemical performance of NMFC-800 porous carbon was studied using a three-electrode system. Results revealed that NMFC-800 porous carbon has a high specific surface area of 1143 m2g-1 with a mesopore proportion over 80%. Using KOH solution as an electrolyte, the specific capacitance was as high as 211 F g−1 at a current density of 1 A g−1, and the capacitance retention was 81.04% after 5000 cycles. NMFC-800 porous carbon has excellent supercapacitive performance and good cycle stability.

Excellent durable supercapacitor performance of hierarchical porous carbon spheres with macro hollow cores

Hierarchical porous carbon spheres (HPCS) with 500 nm hollow cores and 100 nm shells have been prepared using hard template method. The synthesized HPCS has specific surface area up to 2489 m2 g−1 and pore volume as high as 1.45 cm3 g−1. The durable supercapacitor performance of HPCS is excellent with capacitance retention up to 99.7% after 10,000 charge/discharge tests at 10 A g−1 from three-electrode testing and 97.1% after 10,000 cycles at 10 mA cm−2 from two-electrode testing in 6 M KOH solution. HPCS also presents good supercapacitor performance with capacitance of 152 F g−1 at scan rate of 2 mV s−1 and keeps this capacitance of 123 F g−1 at 100 mV s−1. From the charge/discharge tests, HPCS displays capacitance up to 167 F g−1 and 132 F g−1 at current density of 0.2 A g−1 and 20 A g−1, respectively. From two-electrode testing, the capacitances of HPCS is 34.2 m F cm−2 at 2 mV s−1 and 37.4 m F cm−2 at 0.5 mA cm−2.

Facile one-step synthesis of agaric-like cobalt–manganese oxide nanosheets for supercapacitor with excellent long-term stability

A facile one-step method is developed to synthesize homogenous agaric-like cobalt–manganese oxide nanosheets with excellent supercapacitive performance in this study. The optimal sample shows morphologies of equiaxed spheres with diameter of 300 ± 30 nm, which consists of agaric-like nanosheets with thickness of approximate 3 nm. The active material is composed of MnO2 and Co3O4 which is identified by the results of high resolution transmission electron microscopy, X-ray diffraction pattern and X-ray photoelectron spectroscopy spectrum. The as-prepared sample owns a high specific capacitance of 683 F g−1 at a scan rate of 5 mV s−1, while the value still retains 64% at a fast scan rate of 100 mV s−1 (436 F g−1). Moreover, the sample exhibits a remarkable long-term durability which can keep 83.5% capacitance retention over 20,000 charge/discharge cycles at a current density of 5 A g−1. These results suggest that the thin agaric-like nanosheets plays an important role in the excellent performance for supercapacitor. The investigation offers a simple way to prepare agaric-like cobalt–manganese oxide which has potential applications in supercapacitors with excellent long-term stability.

Preparation and Supercapacitive Performance of Lead Dioxide Electrodes with Three-Dimensional Porous Structure

The three-dimensional porous structure PbO2 electrodes (3D-PbO2 electrodes) were prepared in the lead nitrate solution by potentiostatically electrodeposition method using oxygen bubble as dynamic template, which can be used as the positive materials of the supercapacitors. The morphology and structure of 3D-PbO2 electrodes were characterized by scanning electron microscope (SEM) and X-ray diffraction (XRD). The supercapacitive performance of 3D-PbO2 electrodes were investigated by cyclic voltammetry (CV), galvanostatic charge-discharge tests (GCD) and electrochemical impedance spectroscopy (EIS). The specific capacitance of 3D-PbO2 electrodes can reach 195.6 F g–1 at 0.2 A g–1, which is 2.8 times higher than that of Flat-PbO2 electrodes (68.8 F g–1). The charge transfer resistance (Rct) of 3D-PbO2 electrodes (8.21 Ohm cm–2) is lower than that of Flat-PbO2 electrodes (21.32 Ohm cm–2). The excellent supercapacitive performance of 3D-PbO2 electrodes can be attributed to the three-dimensional porous structure, which can enlarge the active surface area of lead dioxide electrodes and promote the electrolyte diffusion and electrons propagation.

3D interconnected Bi2S3 nanosheets network directly grown on nickel foam as advanced performance binder-free electrode for hybrid asymmetric supercapacitor

In this paper, we report the successful synthesis of “rock-candy” shape Bi2S3 nanocrystals and 3D interconnected Bi2S3 nanosheets network on Ni foam by a facile hydrothermal method. The structure and composition of the as-synthesized products were characterized by XRD, XPS, SEM and SEM analyses. The unique structure makes them have high electrochemical properties. At current density of 0.5 A g−1, the binder-free Bi2S3/Ni foam working electrode with high specific area possesses high specific capacity (500.8 mAh g−1) compared to pristine Bi2S3 (245.5 mAh g−1). In the actual application of Bi2S3/Ni foam, the assembled Bi2S3/Ni foam//AC hybrid asymmetric supercapacitor exhibits excellent supercapacitive performances, such as high specific capacitance (148.7 F g-1 at 1 A g−1), high energy density (45.1 Wh Kg−1 at 3202 W Kg−1), excellent cycling stability (capacitance retention is about 98.7% after 2000 cycles at a current density of 3 A g−1) and low Warburg and charge transfer resistances.

Flexible self-charging supercapacitor based on graphene-Ag-3D graphene foam electrodes

A flexible three-dimensional porous graphene foam-based supercapacitor (GFSC) is presented here for energy storage applications. With a novel layered structure of highly conductive electrodes (graphene-Ag conductive epoxy–graphene foam), forming an electrochemical double layer, the GFSC exhibits excellent electrochemical and supercapacitive performance. At a current density of 0.67 mA cm−2, the GFSCs show excellent performance with areal capacitance (38 mF cm−2) about three times higher than the values reported for flexible carbon-based SCs. The observed energy and power densities (3.4 µW h cm−2 and 0.27 mW cm−2 respectively) are better than the values reported for carbon-based SCs. Analyzed under static and dynamic bending conditions, the GFSCs are stable with up to 68% capacitance retention after 25000 charge–discharge cycles. The light-weight, cost-effective fabrication and no self-heating make the GFSCs a promising alternative to conventional source of energy in the broad power density ranging from few nW cm−2 to mW cm−2. In this regard, GFSC was integrated with a flexible photovoltaic cell resulting in a flexible self-charging power pack. This pack was successfully utilized to power continuously a wearable CuO nanorod based chemi-resistive pH sensor.

Facile synthesis of porous CuCo2O4 composite sheets and their supercapacitive performance

Abstract The synthesis of metal oxide composites with porous structures for supercapacitor application has drawn much attention owing to their high surface area and easy access of the electrolyte ions to the electrode surface through the pores of the active materials. A facile hydrothermal approach is suggested for the synthesis of porous CuCo2O4 composite sheets and their application as an active electrode material for supercapacitor application. Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) measurements show the formation of porous CuCo2O4 composite sheets. BET surface area measurements show that the porous CuCo2O4 composite sheet has 69.44 m2 g−1 surface area, which is 4.7 times higher than quasi-spherical CuCo2O4 nanoparticles. The porous CuCo2O4 composite sheet delivered 1037 C g−1 specific capacity at 5 mV s−1. Additionally, the porous CuCo2O4 composite sheet retained 94% of its initial specific capacity after 5000 charge-discharge cycles at 10 A g−1 indicating an excellent cyclic stability. This excellent supercapacitive performance is attributed to the high surface area and enhanced ion transport through the pores of the CuCo2O4 sheets. This high specific capacity and excellent cyclic stability of the porous CuCo2O4 composite sheets prove to be a promising candidate for supercapacitor application.

Prepared multifunctional aerogel for high performance supercapacitors and effective adsorbents

Energy and environment as the current research hots pot, how to effectively combine the two is very important and full of challenge. In this research, we design a new type multifunctional aerogel material, which not only can applied for supuercapacitors, but also acted as adsorbents for adsorb organic pollutant. This multifunctional aerogel was prepared by one-pot hydrothermal and freeze-drying method based on elongated TiO2 nanotubes (eTNTs) and graphene. During the hydrothermal process, the graphene nanosheets and eTNTs self-assembled into three-dimensional (3D) interconnected networks, in which the eTNTs with uniform size are intercalate the pore interconnected framework. The results show that the as-prepared eTNTs/graphene aerogel (TGA) exhibits a high specific surface area of 343.2 m2 g−1. The highly porous and interconnected 3D nanostructure provided efficient migration of electrolyte ions and electrons, and thus the TGA exhibited excellent electrochemical performance for supercapacitors. The binder free TGA electrode possessed high performance electrochemical properties with an excellent specific capacitance (476.8 F g−1 at scan rate of 5 mV s−1), and outstanding cycle stability (92% capacitance retention after 5000 cycles). In addition, the TGA also showed admirable adsorption capacity for organic pollutant of bisphenol A, which reached 523.5 mg g−1. The excellent electrochemical and adsorption capacities suggest the TGA to be the promising materials application in energy storage and environmental remediation.

Supercapacitance of core-shell hierarchical porous nitrogen-doped carbon in non-aqueous electrolyte

Abstract Uniform nitrogen-doped carbon sphere with a 250 nm diameter macro hollow core and 75 nm thick shell structure containing 8 nm mesopores is synthesized from a silica hard template. The synthesized nitrogen doped hollow core mesoporous shell carbon (Fe-N-HCMS) presents excellent supercapacitor performance with a capacitance of 352 F g−1 (0.5 A g−1) in 0.5 M H2SO4 and a capacitance of 189.4 F g−1 (0.5 A g−1) in 1 M TEABF4/ACN. The capacitance retention of Fe-N-HCMS carbon is as high as 92% after tests of 8000 repeating cycles. The improved high-rate performance of Fe-N-HCMS carbon is ascribed to the high surface area (1788 m2 g−1), large pore volume (3.98 cm3 g−1), well-defined mesopores, leading to lower transport resistance and a thin diffusion layer. The enhanced durability of Fe-N-HCMS carbon also results from improved mechanical strength and better electrical conductivity with a higher graphitic content.

(Invited) Fullerene Nanostructures: Preparation and Application

During the past two decades, the self-assembly of spherical fullerene molecules has attracted intensive attention for their definite structures, high electronic affinity and three-dimensional (3D) electron transporting property. We report herein the systematical investigation of the self-assembly behavior of C60 molecules to form various polyhedron microstructures and well-aligned arrays together with their extraordinary optical properties.[ 1 ] Furthermore, a solution-processible co-assembly strategy taking advantage of charge-transfer interaction is developed to prepare C70/porphyrin supramolecular architectures, which possess excellent electrical conductivity and high thermal resistance, and charge separation characteristics.[ 2,3 ] More practically, porous carbon materials with abundant functional moieties are readily obtained by KOH activation of C70 microtubes, which exhibit excellent supercapacitor performances.[ 4 ] References [1] S. Zheng, et al., Carbon 201 8, DOI:10.1016/j.carbon.2017.10.012. [2] S. Zheng, et al., Nano Res. 201 8, DOI:10.1007/s12274-017-1809-7. [3] B. Wang, S. Zheng, et al., J. Am. Chem. Soc. 2017, 139, 10578. [5] S. Zheng, H. Ju, X. Lu, Adv. Ener. Mater. 2015, 1500871.

Lamellar Oxygen-Enriched Graphene Hydrogel with Linking-up Network Porous Structure for High-Performance Supercapacitors

Here, we report a lamellar holey oxygen-enriched graphene hydrogel (LGH) consisting of graphene sheets with a large amount of oxygen functional groups and in-plane pores and confirm its excellent supercapacitive performances. LGH is quite different from the traditional graphene hydrogel in terms of both the morphology and porous structure. Importantly, the in-plane pores on the graphene sheets link up the out-of-plane interval between the graphene sheets to form a linking-up network porous structure. The in-plane pores result in more micropores and the additional oxygen functional groups on graphene sheets. The LGH single electrode exhibits an ultrahigh capacitance (337.8 F g–1 at 1 A g–1) and excellent rate capability (the specific capacitance at 100 A g–1 retains 77% relative to 1 A g–1). The asymmetric supercapacitor is assembled by using the LGH as positive electrode material and the graphene hydrogel that has adsorbed anthraquinone as negative electrode material, respectively. The device exhibits a h...

Nitrogen Graphene: A New and Exciting Generation of Visible Light Driven Photocatalyst and Energy Storage Application.

The synthesis of nitrogen, boron, and nitrogen–boron-codoped graphenes was attained via mixing solutions of GO with urea, boric acid, and a mixture of both, respectively, followed by drying in vacuum and annealing at 900 °C for 10 h. These materials were thoroughly characterized employing XRD, TEM, FTIR, Raman, UV–vis, XPS, IPCE%, and electrical conductivity measurements. The nitrogen-doped graphene (NG) showed an excellent supercapacitor performance with a higher specific capacitance (388 F·g–1 at 1 A·g–1), superior stability, and a higher power density of 0.260 kW kg–1. This was mainly due to the designated N types of doping and most importantly N–O bonds and to lowering charge transfer and equivalent series resistances. The NG also indicated the highest photocatalytic performance for methylene blue (MB 20 ppm, power = 160 W, λ > 420 nm) and phenol (5 ppm) degradation under visible light illumination with rate constants equal 0.013 min–1 and 0.04 min–1, respectively. The photodegradation mechanism was proposed via determining the energy band potentials using the Mott–Schottky measurements. This determined that photoactivity enhancement of the NG is accounted for by acquisition of nitrogen-oxy-carbide phases that shared in inducing a higher IPCE% (60%) and a lower band gap value (1.68 eV) compared to boron and nitrogen–boron-codoped graphenes. The achieved photodegradation mechanism relied on scavengers performance suggesting that •OH and electrons were the main reactive species responsible for the MB photodegradation.

Facile synthesis of graphene-AgVO3 nanocomposite with excellent supercapacitor performance

The graphene-AgVO3 nanocomposites (GA30) were synthesized by facile one-step chemical-bath method. SEM and TEM images of the synthesized nanocomposites indicate that this method is possible to anchor the AgVO3 nanoparticles onto the graphene surface. Galvanostatic charge-discharge (GCD) shows that specific capacitance, cycle stability, and rate capability of the supercapacitor device can be enhanced with AgVO3 incorporation. The GA30 shows a high energy density of 10 Wh/kg at a power density of 25 W/kg as well as a high power density of 2045 W/kg at an energy density of 6 Wh/kg, which are much higher than those of graphene/activated carbon composites (GC30) and previous reported graphene-based composites. The improvement in specific capacitance, cycle stability, energy density and powder density of the graphene would be attributed to the special microstructure with high-rate transportation of both electrolyte ions and electrons due to the incorporation of AgVO3.

Hierarchical micro/nanostructured Co3O4@MnCo2O4 core-shell nanowire arrays on Ni foam for electrochemical energy storage

In this work, hierarchical micro/nanostructured Co3O4@MnCo2O4 nanowire arrays supported on nickel foam were prepared via a facile hydrothermal process for the first time, in which the hydrothermally synthesized Co3O4 nanowire arrays served as the scaffold, anchoring the MnCo2O4 nanosheets. When evaluated as binder-free electrodes for supercapacitors, the optimized Co3O4@MnCo2O4 hybrid electrodes exhibited a remarkable electrochemical performance with a high specific capacitance of 736.5 F g−1 at a current density of 1 mA cm−2, nearly two times than that of the pristine Co3O4 electrode (370.67 F g−1). Moreover, the Co3O4@MnCo2O4 hybrid electrode exhibited superior cycling stability with 76.95% retention over 3000 cycles at a current density of 10 mA cm−2. Since the excellent supercapacitive performance of Co3O4@MnCo2O4 hybrid electrode could be attributed to the unique core-shell architecture and the synergistic effect of the Co3O4 nanowires and the ultrathin MnCo2O4 nanosheets, Co3O4@MnCo2O4 nanowire arrays hold great potentials for the applications in the field of electrochemical energy storage.

Hexamethylenetetramine-assisted hydrothermal synthesis of octahedral nickel ferrite oxide nanocrystallines with excellent supercapacitive performance

Octahedral nickel ferrite oxide (NiFe2O4) nanocrystals with average sizes of 81, 69, 63 and 46 nm were fabricated using hexamethylenetetramine as adscititious alkali via a facile hydrothermal route at various temperatures. The formation mechanism of octahedral nickel ferrite oxide nanocrystals was discussed in detail. Interestingly, the nanocrystalline size decreased with the increase in the hydrothermal reaction temperature. We studied the influence of hydrothermal temperatures on the evolution of the nanocrystalline and analyzed the relationship between the sizes of the nanocrystalline and their capacitive properties. Compared to the large-sized counterpart (81, 69 and 63 nm), the small-sized nanocrystals (46 nm) presented a maximum specific capacitance (562.1 F g−1) and remarkable cycling stability (80.3% capacity retention after 1500 cycles) at 4 A g−1. The excellent performance of the NiFe2O4 nanocrystals (46 nm) was mainly attributed to the unique octahedral nanostructures with a small size (fully exposing more electroactive sites and providing more sufficient expressways for rapid charge transfer) and their compositional advantages of nickel and cobalt (multiple oxidation states for redox reactions and relatively desirable electroconductivity). More remarkably, an asymmetric supercapacitor composed of NiFe2O4 (as the positive electrode) and activated carbon (as the negative electrode) displayed an ultrahigh energy density (34.91 Wh kg−1 at 1100 W kg−1) and an advanced cycling stability (84.5% capacity retention after 1000 cycles), which suggested that the decreased crystal size played a pivotal role in size-dependent capacitive performance enhancement.

Supercapacitor and photocatalytic performances of hydrothermally-derived Co3O4/CoO@carbon nanocomposite

Hydrothermally-derived Co3O4/CoO@carbon nanocomposite with spike-like cobalt oxide anchored on the conductive carbon network showed excellent supercapacitor and photocatalytic performances.