Symmetric Supercapacitor Cell Research Articles

The preparation of liquefied bio-stalk carbon nanofibers and their application in supercapacitors.

Carbon nanofibers (CNFs) have been widely used in electrochemical energy storage devices because of their excellent conductivity, extremely large surface area and structural stability. Herein, we obtained a viscous, liquefied bio-stalk carbon via the simple chemical treatment of biomass, and mixed it with polyacrylonitrile to prepare a spinning solution. Subsequent electrospinning and high temperature activation resulted in the successful preparation of liquefied lignin-based activated carbon nanofibers. The as-prepared liquefied bio-stalk carbon nanofibers exhibited an outstanding electrochemical performance (specific capacitance of 273 F g−1 at 0.5 A g−1 current density), and a capacitance retention of 210 F g−1 even under a large current density of 10 A g−1. Besides its high specific capacitance and outstanding rate capability, the symmetrical supercapacitor cell based on the liquefied carbon-based nanofiber electrodes also exhibited an excellent cycling performance with 92.76% capacitance retention after 5000 charge–discharge cycles. This study provides a new strategy for the future development of supercapacitor electrode materials and enhances the development of biomass energy.

Iron molybdate and manganese dioxide microrods as a hybrid structure for high-performance supercapacitor applications

FeMoO4@MnO2 microrod binary hybrid structure is synthesized by a simple two-step hydrothermal method. The phase formation and morphology of FeMoO4@MnO2 were analyzed using X-ray diffraction (XRD) patterns and scanning electron microscopy (SEM) respectively. High-resolution transmission electron microscopy (HRTEM) analysis of the sample confirms its microrod morphology and polycrystalline nature. The electrochemical measurements revealed the supercapacitive behavior by encompassing both double layered capacitance as well as pseudocapacitance at lower and higher scan rates respectively. A symmetric fabrication method was employed using FeMoO4 @MnO2 as both negative and positive electrode for electrochemical analysis. The symmetric supercapacitor cell yields a specific capacitance of 839.29 F g−1 at a current density of 1 A g−1 in addition to higher cyclic stability and performance. The sandwiched symmetric supercapacitor device displays a high specific energy density of 56.95 W h kg−1 at a power density of 418.46 W kg−1. In addition to that, the device showed a promising capacity retention of 87% with an efficiency 80% of after 10,000 cycles. The electrochemical results show that FeMoO4@MnO2 could present itself as a promising electrode material for the next generation supercapacitors applications.

Redox additive enhanced capacitance: Multi-walled carbon nanotubes/polyaniline nanocomposite based symmetric supercapacitors for rapid charge storage

In the present study, we report all-solid-state high energy density symmetric supercapacitors based on partially exfoliated multi-walled carbon nanotubes (Px-MWCNT)/Polyaniline (PANI) nanocomposites with redox-active polymer gel as an electrolyte/separator. Fabrication of mechanically stable polymer gel electrolyte with high ionic conductivity, excellent compatibility with the active material, and long cycle life is pivotal to obtain superior electrochemical performance for flexible solid-state supercapacitor devices. An organically reversible redox-active (hydroquinone-HQ) non-covalent approach adopted to achieve the significant enhancement in specific capacitance. The highest specific capacitance of 186.1, 809.6 and 644.4 F g−1 was achieved at 25 mV s−1 for Px-MWCNT, Px-MWCNT/PANI and PANI based symmetric supercapacitor cells, respectively. The enhancement in capacitance was found to be increased in the order of 82.5%, 129% and 34.3% for Px-MWCNT, Px-MWCNT/PANI, and PANI respectively when compared with devices lacking in redox-active HQ. Moreover, incorporation of redox-active HQ transforms the pseudocapacitance to battery-type behavior. The polymer grafted morphology of nanocomposite (Px-MWCNT/PANI) in 1:1 wt ratio as an electrode material with redox-active gel polymer electrolyte exhibits synergistic effect in specific capacitance enhancement with excellent charge-discharge rate capability and electrochemical stability (78% capacitive retention after 2000 cycles) than pure PANI (69.4% capacitive retention after 2000 cycles) based supercapacitor.

Electrophoretic deposition of manganese oxide and graphene nanoplatelets on graphite paper for the manufacture of supercapacitor electrodes

The preparation of coatings of manganese oxide and graphene nanoplatelets onto graphite paper by aqueous electrophoretic deposition is studied. This method is a fast and environmental friendly procedure for the manufacture of electrodes with enhanced specific capacitance for supercapacitors. The electrodes are prepared either sequentially, depositing consecutively the suspension of each material or simultaneously, from suspensions of mixtures of the two materials. Some of the electrodes show specific capacitances as high as 422 F g−1. The effects of (i) the electrophoretic deposition route chosen and (ii) the composition and load of the active electrode material deposited on the supercapacitor performance are studied. The symmetric supercapacitor cells show specific energy up to 9.2 W h kg−1 and specific power up to 1.8 kW kg−1 for 0.1 and 10 A g−1, respectively. The highest specific energy values are obtained in cells built with electrodes prepared by depositing first a layer of graphene nanoplatelets and then, manganese oxide.

Cellulose-derived hierarchical porous carbon for high-performance flexible supercapacitors

Hierarchical porous carbon was synthesized using the spray drying method with cellulose as the carbon source. It was found that thermal treatment temperature had a significant influence on the microstructure of the carbon. A sample obtained at thermal treatment temperature of 1000 °C displayed a high specific surface area of 1096 m2/g, a large pore volume (1.59 cm3/g), and an appropriate pore system suitable for aqueous sodium chloride electrolyte. A flexible symmetric supercapacitor cell fabricated with this carbon as both electrode exhibited a high specific capacitance (194 F/g at current density 1 A/g), a good rate performance (137 F/g at 10 A/g) and long cycle life (only 4.8% capacitance loss after 5000 cycles). The cell delivered a specific energy of 15.2 Wh/Kg at a specific power of 751 W/Kg, and maintain its stable electrocapacitive performance and good flexibility at the bending condition.

Fast and controllable reduction of graphene oxide by low-cost CO2 laser for supercapacitor application

Direct reduction of graphene oxide has been regarded as the economically viable route for large-scale synthesis of graphene. However, the currently known methods suffer from either poor reduction efficiency or involve multi-step and energy-intensive reduction processes. Here, we demonstrate a remarkably fast, single step as well as highly efficient reduction technique to produce high-quality multilayer graphene film using a compact and low-cost CO2 laser pyrolysis. Thanks to the intrinsically high absorptivity of graphene oxide in the near- and mid-infrared regions, the irradiation of CO2 laser generates instantaneous and strong localized heating on it and thus burst apart the oxygen functional groups from the graphene oxide layers. The extent of reduction in the synthesized multilayer graphene films can be fruitfully controlled by variation of laser processing parameters such as laser intensity, scanning speed and shifting pitch. To prove the worth of this method, the graphene films were used as the binder-free and self-standing electrode for symmetric supercapacitor cell. The electrochemical performance data shows that specific capacitance and cyclic stability has a contrasting relation with the reduction efficiency. We believe that this CO2 laser-based reduction method could guarantee a high outturn of multilayer graphene and its composites for innumerable applications.

Novel nanocomposite of MnFe2O4 and nitrogen-doped carbon from polyaniline carbonization as electrode material for symmetric ultra-stable supercapacitor

In this work, direct carbonization of polyaniline manganese ferrite (Mn-PANI) nanocomposite was employed to prepare a novel N-doped carbon material decorated with manganese ferrite (Mn-CPANI) and implemented as an ultra-stable symmetric supercapacitor electrode material. The surface morphology of as-prepared samples is investigated with field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). Also, uniform distribution of manganese ferrite (MnFe2O4) on PANI surface and the N-doped carbon material is confirmed through EDX analysis. Carbonized nanocomposite contains about 8 wt% of nitrogen. The obtained Mn-CPANI nanocomposite shows a high specific capacitance of 329 F g−1 and exhibits excellent capacitance retention of 83.2% from 1 to 10 A g−1, which is more stable compared to Mn-PANI nanocomposite. Moreover, the symmetric Mn-CPANI supercapacitor cell possesses a specific capacitance of about 246 F g−1 (at 1 A g−1) and an excellent stable cyclability (only 3% of specific capacitance decreases after 10000 cycles). The excellent enhanced electrochemical performance of Mn-CPANI nanocomposite could be originated from the combination and synergism of N-doped carbon material as an electrical double-layer capacitor with pseudocapacitive MnFe2O4. As a result, a novel electrode material is developed for high-performance ultra-stable energy storage devices.

Effective pseudocapacitive charge storage/release by hybrids of poly(3,4-ethylenedioxypyrrole) with Fe3O4 nanostructures or Co3O4 nanorods

The synergy between two redox materials, the high electrical conductivity and the chemically robust durable structure of poly(3,4-ethylenedioxypyrrole) (PEDOP) combined with the high theoretical capacities, cost-effectiveness and abundance of M3O4 (M: Fe or Co) type oxides is exploited. Hybrid films of PEDOP@Co3O4 nanorods (NRs) and PEDOP@Fe3O4 nanostructures (NSs) are fabricated over flexible carbon (C)-fabric substrates for the very first time. A symmetric supercapacitor cell based on the PEDOP@Fe3O4 NSs hybrid outperforms the remaining cells based on pristine oxides, polymer and even PEDOP@Co3O4 NRs. The PEDOP@Fe3O4 NSs hybrid based cell delivers a high specific capacitance of 673 F g−1 at 1 A g−1 with 83% retention after 5000 cycles. Additionally, it exhibits a wide voltage window of 1 V and an extraordinarily high energy density of 93 Wh kg−1 at a power density of 0.5 kW kg−1. Conducting atomic force microscopy studies reveal that the nanoscale (high) current flowing domains are uniform and almost seamless across the film of PEDOP@Fe3O4 NSs. In contrast, the insulating or low current flowing domains predominate the surfaces of PEDOP and PEDOP@Co3O4 NRs. The nano-level electrical conductivity for the PEDOP@Fe3O4 NSs film is also seven-fold times higher than that of PEDOP and PEDOP@Co3O4 NRs films. The reasons for enhanced electrochemical responses of the hybrids compared to the pristine oxides are explained. The porous morphology, enhanced electrical conduction, and lower ion-diffusion resistance offered by the PEDOP@Fe3O4 NSs film enables improved charge transfer and transport, thus manifesting in a good rate response, high energy density, and acceptable endurance parameters. A real time application of an illumination of green light emitting diode with three such cells also highlights the practical viability of this hybrid.

Synthesis and characterization of polyaniline-grafted CNT as electrode materials for supercapacitors

Polyaniline (PANI) was grafted with carbon-nanotubes (CNT) via azobenzene (Azo) to increase the pseudo-capacitance of PANI. CNT was functionalized with azobenzene and used in the in-situ polymerization of aniline using emulsion polymerization. X-ray diffraction result supports the formation of composite materials. Morphology of grafted composites showed a higher uniform growth in PANI/ CNT-Azo composite than PANI/CNT composite. Electrochemical performance was evaluated by symmetric super-capacitor cells using cyclic voltammetry and chargedischarge studies. PANI/CNT-Azo showed a higher specific capacitance value than pristine PANI. The results show grafting of PANI onto CNT via Azo helps to achieve higher capacitance and thermal stability than non-grafted PANI/CNT composite materials. Open image in new window

One-step synthesis of PEDOT-PSS●TiO2 by peroxotitanium acid: a highly stable electrode for a supercapacitor

Pseudo capacitors can reserve high energy and power densities with a high efficiency, and a long life period. In this work, a hybrid material of poly (3,4-ethylenedioxythiophene) (PEDOT)-poly(4-styrene sulfonic acid) (PSS)-titanium dioxide (TiO2) was synthesized by the facile one-pot method by novel and low-cost oxidant peroxotitanium acid (PTiA) for the first time. PTiA acts as both oxidant and source for TiO2. XRD, EDAX, and XPS analysis supported the formation of the hybrid material. The hybrid material showed bundles of nanofibers morphology and stable up to 315 °C. This hybrid material was explored as a stable electrode material in symmetric supercapacitor cell configuration. This device showed a specific capacitance (SC) of 162 F g−1 at a current density of 0.25 A g−1. Subjected the call for 38,000 charge–discharge (CD) cycles at a higher current density of 1.25 A g−1 and it showed 52% retention of initial capacitance (125 F g−1).

Ag/Biochar composite for supercapacitor electrodes

A new composite material was obtained using a simple activation process of biochar (BC) and high performance supercapacitor (SC) electrodes were fabricated. BC was attained by carbonization of infested ash tree residue at 700 °C and was then chemically activated in an Ag2SO4/HNO3 solution. The physical properties of the activated – Ag/BC composite product and the electrochemical properties of electrodes fabricated with this composite material were investigated and compared with properties of electrodes fabricated with activated biochar (a-BC). The a-BC was obtained by activation of BC in HNO3. Experimental results with the Ag/BC composite based electrodes show favourable electrochemical performance. A high specific capacitance of 494 F g−1 was achieved in an aqueous LiN((SO2CF3)2) electrolyte, which is almost 31.4% higher than that of the a-BC based electrodes (376 F g−1) and 42% higher than was previously reported (335 F g−1 [Kouchachvili et al. (2015)]) using the identical aqueous electrolyte. Also, high cycling stability with a specific capacitance retention of 98.6% was demonstrated after 2000 cycles. Symmetric SC cells were therefore assembled with the Ag/BC composite and the a-BC based electrodes. The former cell has since shown an excellent energy density of 12.8 Wh kg−1 that is 37% higher than the energy density of the later SC cell (8 Wh kg−1), whereas the power density of both SC cells remained the same (366 W kg−1 at a charge/discharge current of 1.3 mA).

Environment-benign synthesis of rGO/MnOx nanocomposites with superior electrochemical performance for supercapacitors

Chemical oxidation synthesis of graphene oxide (GO) through modified Hummers methods has been widely employed for producing graphene- or reduced GO (rGO)-based advanced functional materials such as rGO/MnOx nanocomposites. However, the manganese species in GO colloids are usually washed out during GO synthesis through modified Hummers methods, causing manganese waste and environmental risk. In this paper, we report preparation of rGO/MnOx nanocomposites, in which MnOx is composed of Mn2O3, Mn3O4, and MnO2 components, through anneal treatment of the precursor counterparts obtained by simple pH tuning of GO colloids. The rGO/MnOx nanocomposites exhibit superior electrochemical performance for supercapacitors. rGO/MnOx-5, derived from GO colloids with pH 5, exhibits a high gravimetric discharge capacitance (Cdis) of 191 F g−1 at 20 A g−1 and a high capacitance retention (82.9%) relative to Cdis at 1 A g−1. Furthermore, typical symmetric supercapacitor cells made from rGO/MnOx-5 show a high areal capacitance (172 mF cm−2) and excellent capacitance retention (96.6%) at 2 A g−1 (10 mA cm−2) for 20,000 cycles, holding great potential for practical applications. The superior electrochemical performance of rGO/MnOx nanocomposites is attributed to multiple charge storage mechanisms in association with the coexistence of mixed-valent manganese oxides.

Supercritical fluid processing of N-doped graphene and its application in high energy symmetric supercapacitor

A simple one-pot methodology is developed for the synthesis of nitrogen doped graphene via supercritical fluid (SCF) processing using glycine as a nitrogen precursor. The presence of various N-containing functional groups was determined by FT-IR and the amount of N-doping in the graphene was found to be 4.5 wt% using the elemental analysis and X-ray photoelectron spectroscopy. The electrochemical capacitance measurements are performed using cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy. The nitrogen doped graphene exhibited enhanced capacitive performance with a maximum specific capacitance of 270 F/g at 0.5 A/g current density with high specific capacitance retention of 90% over 10,000 cycles at 10 A/g current density. The fabricated symmetric supercapacitor cell showed a high energy density of 4.1 and 36 Wh/kg in aqueous and ionic liquid electrolyte, respectively. The high energy density obtained in ionic liquid is promising for their potential application in electrochemical energy system.

Unrivaled Combination of Surface Area and Pore Volume in Micelle-Templated Carbon for Supercapacitor Energy Storage

We created Immense Surface Area Carbons (ISACs) by a novel heat treatment that stabilized the micelle structure in a biological based precursor prior to high temperature combined activation – pyrolysis. While displaying the morphology akin to commercial activated carbon, ISACs contain an unparalleled combination of electrochemically active surface area and pore volume (up to 4051 m2 g-1, total pore volume 2.60 cm3 g-1, 76% small mesopores). The carbons also possess the benefit of being quite pure (combined O and N: 2.6 – 4.1 at.%), thus allowing for a capacitive response that is primarily EDLC. Tested at commercial mass loadings (~ 10 mg cm-2) ISACs demonstrate exceptional specific capacitance values throughout the entire relevant current density regime, with superior rate capability primarily due to the large fraction of mesopores. In the optimized ISAC, the specific capacitance (Cg) is 540 F g-1 at 0.2 A g-1, 409 F g-1 at 1 A g-1 and 226 F g-1 at a very high current density of 300 A g-1 (~0.15 second charge time). At intermediate and high currents such capacitance values have not been previously reported for any carbon. Tested with a stable 1.8 V window in a 1M Li2SO4 electrolyte, a symmetric supercapacitor cell yields a flat energy – power profile that is fully competitive with organic electrolyte systems: 29 Wh kg-1 at 442 W kg-1 and 17 Wh kg-1 at 3940 W kg-1. The cyclability of symmetric ISAC –cells is also exceptional due to the minimization of faradaic reactions on the carbon surface, with 80% capacitance retention at over 100,000 cycles in 1M Li2SO4 and 75,000 cycles in 6M KOH.

Lithium and sodium ion capacitors with high energy and power densities based on carbons from recycled olive pits

Abstract In this work, we are presenting both lithium and sodium ion capacitors (LIC and NIC) entirely based on electrodes designed from recycled olive pit bio-waste derived carbon materials. On the one hand, olive pits were pyrolized to obtain a low specific surface area semigraphitic hard carbon to be used as the ion intercalation (battery-type) negative electrode. On the other hand, the same hard carbon was chemically activated with KOH to obtain a high specific surface area activated carbon that was further used as the ion-adsorption (capacitor-type) positive electrode. Both electrodes were custom-made to be assembled in a hybrid cell to either build a LIC or NIC in the corresponding Li- and Na-based electrolytes. For comparison purposes, a symmetric EDLC supercapacitor cell using the same activated carbon in 1.5 M Et4NBF4/acetonitrile electrolyte was also built. Both LIC and NIC systems demonstrate remarkable energy and power density enhancement over its EDLC counterpart while showing good cycle life. This breakthrough offers the possibility to easily fabricate versatile hybrid ion capacitors, covering a wide variety of applications where different requirements are demanded.

Superior supercapacitors based on nitrogen and sulfur co-doped hierarchical porous carbon: Excellent rate capability and cycle stability

Abstract Vastly improving the charge storage capability of supercapacitors without sacrificing their high power density and cycle performance would bring bright application prospect. Herein, we report a nitrogen and sulfur co-doped hierarchical porous carbon (NSHPC) with very superior capacitance performance fabricated by KOH activation of nitrogen and sulfur co-doped ordered mesoporous carbon (NSOMC). A high electrochemical double-layer (EDL) capacitance of 351 F g −1 was observed for the reported NSHPC electrodes, and the capacitance remains at 288 F g −1 even under a large current density of 20 A g −1 . Besides the high specific capacitance and outstanding rate capability, symmetrical supercapacitor cell based on the NSHPC electrodes also exhibits an excellent cycling performance with 95.61% capacitance retention after 5000 times charge/discharge cycles. The large surface area caused by KOH activation (2056 m 2 g −1 ) and high utilized surface area owing to the ideal micro/mesopores ratio (2.88), large micropores diameter (1.38 nm) and short opened micropores structure as well as the enhanced surface wettability induced by N and S heteroatoms doping and improved conductivity induced by KOH activation was found to be responsible for the very superior capacitance performance.

Hierarchical Polyaniline Spikes over Vegetable Oil derived Carbon Aerogel for Solid-State Symmetric/Asymmetric Supercapacitor

Herein, we report an environment friendly methodology for the synthesis of carbon aerogel (CA) using bio-mass (mustard oil) as precursor. The synthesis process is highly reproducible and economical yet devoid of any sophisticated procedure and hazardous organic/inorganic chemicals. Exfoliated graphitic planes of carbon beads with enhanced inter-planer spacing are observed in synthesized CA with low bulk density (0.144gcm−3) and high specific surface area (1032m2g−1). The CA comprises of meso/microporosity with open pore microstructure. Symmetric supercapacitor cell (CA║CA) is fabricated to test the capacitive performance of CA in H2SO4-polyvinyl alcohol (H2SO4-PVA) gel as electrolyte. The CA║CA cell exhibits nearly symmetrical voltammograms up to working potential window of 2 V and impedance response demonstrates phase angle ∼80° indicating highly capacitive behavior. Polyaniline (PANI) spikes are further synthesized over CA using chemical oxidative polymerization route to introduce pseudocapacitive characteristics in addition to double layer capacitance. Solid-state symmetric (CA-PANI║CA-PANI) and asymmetric (CA║CA-PANI) supercapacitor cells are fabricated to examine the electrochemical performance. Fabricated CA║CA-PANI supercapacitor cell exhibits remarkable high power density (7.9kWkg−1) and high energy density (55.6 Wh kg−1). The CA║CA-PANI cell is capable to power a light-emitting diode (LED) bulb effectively for 3.5min by assembling two supercapacitors (1×1cm2).

Oxygen-containing hierarchically porous carbon materials derived from wild jujube pit for high-performance supercapacitor

Hierarchically porous carbon (HPC) materials with rich oxygen-containing groups at the surface were prepared by a facile one-pot approach using wild jujube pit (WJP) as the biomass precursor. Effect of annealing temperature on the texture and electrochemical properties of the obtained HPC materials was investigated. After the WJP is annealed at 800°C, the resulting HPC sample (WJC-800) contains 6.7% oxygen element and displays an ultra-high specific surface area (2438m2g−1) and a high pore volume (1.00cm3g−1). The WJC-800 sample possesses interconnected micro-, meso- and macro-pores, and such a structure is beneficial to enhance its electrochemical properties. In a three-electrode configuration, the WJC-800-derived electrode can deliver a specific capacitance of 398Fg−1 (95.5Fcm−3) at 0.5Ag−1, and retain 345Fg−1 (82.8Fcm−3) (86.7% of that at 0.5Ag−1) even at 20Ag−1. The capacitance retention of the assembled symmetric supercapacitor cell remains 97% and 87% in the 6M KOH and 1M Na2SO4 electrolyte after 10,000 cycles at 5Ag−1, respectively. The symmetric supercapacitor cells using the 1M Na2SO4 electrolyte attains a high energy density of 13.33Whkg−1 at the power density of 16kWkg−1. The results indicate that the WJP can act as a novel and appropriate biomass precursor to prepare the low-cost HPC materials for high-performance supercapacitor.

Electrochemical Cycling Behavior of Pyrrolidinium Ionic Liquid Tethered TiO2 Nanoparticle-Hybrid Electrolytes: Influence of Grafting Density

Exploring new electrolyte materials is important to comprehend high performance secondary charge storage devices for effective utilization of renewable energy. Ionic liquids with broad electrochemical voltage window and wide range of thermal stability are prospective materials for the same. As an improved version of this class of materials, we report herein, on the synthesis of novel pyrrolidinium based ionic liquid tethered nanoparticle hybrid electrolytes and compared their conductivity and electrochemical cycling behaviors. Hybrid electrolytes are prepared by covalently grafting N-Methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl) imide to the surfaces of pre-synthesized TiO2 nanoparticles at three different core volume fractions, ranging from 0.11 to 0.46. Uniform dispersion of the core nanoparticle within the matrix of the electrolytes improves their mechanical properties. With increasing core volume fractions, the electrical conductivity slightly decreases due to increasing viscosity. Nevertheless, nanoparticle grafting significantly improves the electrochemical performance of the electrolytes. The electrolytes show very good capacitance retentivity (>90%) and long-term cycling stability above room temperature even at the lowest grafting density within symmetric stainless steel electrodes. Impressive room temperature electrochemical performance of these materials within symmetric supercapacitor cell containing CMS graphite as electrodes further demonstrates the applicability of these hybrid materials as promising electrolytes for secondary charge storage devices.

Supercapacitor electrode materials with hierarchically structured pores from carbonization of MWCNTs and ZIF-8 composites.

Due to their high specific surface area and good electric conductivity, nitrogen-doped porous carbons (NPCs) and carbon nanotubes (CNTs) have attracted much attention for electrochemical energy storage applications. In the present work, we firstly prepared MWCNT/ZIF-8 composites by decoration of zeolitic imidazolate frameworks (ZIF-8) onto the surface of multi-walled CNTs (MWCNTs), then obtained MWCNT/NPCs by the direct carbonization of MWCNT/ZIF-8. By controlling the reaction conditions, MWCNT/ZIF-8 with three different particle sizes were synthesized. The effect of NPCs size on capacitance performance has been evaluated in detail. The MWCNT/NPC with large-sized NPC (MWCNT/NPC-L) displayed the highest specific capacitance of 293.4 F g-1 at the scan rate of 5 mV s-1 and only lost 4.2% of capacitance after 10 000 cyclic voltammetry cycles, which was attributed to the hierarchically structured pores, N-doping and high electrical conductivity. The studies of symmetric two-electrode supercapacitor cells also confirmed MWCNT/NPC-L as efficient electrode materials that have good electrochemical performance, especially for high-rate applications.