RESEARCH ARTICLE

A novel and simple nitrogen-doped carbon/polyanilineelectrode material for supercapacitors

Liangshuo LI1, Lin QIN1, Xin FAN (✉)1, Xinyu LI2, and Ming DENG3

1 Guangxi Key Laboratory of Optical and Electronic Materials and Devices, Key Laboratory of New Processing Technology forNonferrous Metal and Materials (MOE), College of Materials Science and Engineering, Guilin University of Technology,

Guilin 541004, China2 College of Sciences, Guilin University of Technology, Guilin 541004, China

3 College of Information Science and Engineering, Guilin University of Technology, Guilin 541004, China

© Higher Education Press 2021

ABSTRACT: We demonstrated a simple and environment-friendly method in thepreparation of N-doped carbon/PANI (NCP) composite without binder. The structureand the property of NCP have been characterized by XPS, IR, XRD, SEM, CV, GCD and EIS.The results reveal that NCP has high capacitance performance of up to 615 F$g-1 at 0.6A$g-1. Additionally, the asymmetric NCP300//carbon supercapacitor delivers a highcapacitance (111 F$g-1 at 1 A$g-1) and a capacity retention rate of 82% after 1200 cyclesat 2 A$g-1. The ASC cell could deliver a high energy density of 39.1 W$h$kg-1 at a powerdensity of 792.6 W$kg-1.

KEYWORDS: polyaniline; electrodeposition; carbonization; supercapacitor

Contents

1 Introduction2 Experimental

2.1 Materials2.2 Synthesis of NC and NCP2.3 Characterization and electrochemical tests

3 Results and discussion3.1 Morphology of NC and NCP samples3.2 Structure of NC and NCP samples3.3 Electrochemical properties

4 ConclusionsAcknowledgementsReferences

1 Introduction

The climatic variation and the restricted availability ofmineral fuel have immensely affected the world economyand society. With a rapid growing market for wearableelectronic devices and the further development of hybridelectric vehicles, there has been a rapid increase and urgentdemand for environment-friendly high-power energyresources. In order to alleviate and cope with the globalenergy crisis, people are committing to manufactureefficient energy storage and low energy-loss materials,such as lithium ion batteries, supercapacitors, and solarcells [1–2]. Among them, supercapacitor has been widelyconcerned due to extremely high power density, good cyclestability and lower maintenance cost against conventionalcapacitors and batteries [3–6]. Supercapacitor is dividedinto two categories, depending on the charge-storagemechanism. One is electrical double-layer capacitor(EDLC), such as carbon-based materials, where charges

Received September 10, 2020; accepted October 26, 2020

E-mail: xfan@glut.edu.cn

Front. Mater. Sci. 2021, 15(1): 147–157https://doi.org/10.1007/s11706-021-0535-y

are stored at the interface between electrodes andelectrolytes [7]. The other is pseudo-capacitive super-capacitor, such as metal oxides, hydroxides and conductivepolymers [8–9].Polyaniline (PANI), as a conductive polymer, is widely

used as the electrode material of supercapacitor because ofits easy preparation, low cost, good chemical stability,excellent electrical conductivity, redox activity andpseudo-capacitive performance [10–11]. However, PANIundergoes large volumetric swelling and shrinking owingto the ion doping/de-doping during the charge/dischargeprocess [11–12]. In addition, the diffusion rate of ionsinside PANI is slow and the discharge rate is relatively lowin the electrochemical reaction process. Therefore,mechanical failure occurs in charging and dischargingprocesses due to the expansion and the contraction ofPANI, resulting in poor cycle stability [10]. To overcomethis shortcoming, PANI was usually combined with othercapacitive materials to form binary and ternary composites[11,13]. Besides, it is difficult to control the morphology ofPANI in the polymerization process [14].Carbon is one of the most abundant materials in nature.

Carbon materials are widely used as the electrode materialsof supercapacitors due to their advantages of good physicaland chemical stability, high natural abundance, diversestructures, large specific surface area and excellentelectrical conductivity [15–16]. However, the specificcapacitor of carbon materials is relatively low. Carbonmaterials are often used as electrode materials in combina-tion with other materials such as metal hydroxides andconductive polymers [17–18]. In previous reports, thecomposite of PANI with conductive carbon materials, suchas active carbon, carbon nanofibers, carbon nanotubes andgrapheme, had been extensively investigated [16,19–21].The composite of conductive carbon material with PANIcan greatly improve its specific capacitance.PANI can evenly disperse on the surface of these carbon

materials, thus improving the electrochemical activity. Thestable framework of carbon materials reduces the volumechange of PANI in the charging/discharging process,consequently improving the cyclic stability of the materi-als. In addition, due to its large specific surface area, carbonmaterials are also conductive to the full contact betweenelectrode materials and electrolyte, so as to improveelectrochemical activity and rate capacity. However, asmost of PANIs prepared at present exist in powder form, itis necessary to use traditional abrasive coatings to preparesupercapacitor electrode materials [20,22–23]. So it will

lead to an increase of the contact resistance in the electrodematerial due to the addition of binder, and the electrodematerial cannot fully react, which results in dead volumeand limits the improvement of its capacitance. Therefore,the direct growth of aniline on carbon materials to generatePANI will greatly improve the utilization rate of materials,reducing their contact resistance, and thereby improvingthe electrochemical properties.In this paper, hierarchical nitrogen-doped carbon (NC)

and hierarchical nitrogen-doped carbon/PANI composite(NCP) are prepared by electrodeposition of PANI oncarbon materials.The composite exhibits the capacitance performance of

as high as 615 F$g–1 at the current density of 0.6 A$g–1.Additionally, the asymmetric supercapacitor (ASC)composed of NCP//carbon displays a high capacitance(111 F$g–1 at 1 A$g–1) and an excellent capacity retentionrate of 82% after 1200 cycles at 2 A$g–1. The ASC shows ahigh energy density of 39.1 W$h$kg–1 at a power density of792.6 W$kg–1 and a maximum power density of 8 kW$kg–1

at an energy density of 17.8 W$h$kg–1 with excellentstability.

2 Experimental

2.1 Materials

Aniline was used after purification, and sulfuric acid, nitricacid, and perchloric acid sodium sulfate were used withoutfurther purification.

2.2 Synthesis of NC and NCP

Firstly, the carbon cloth was cut into 1 cm � 2 cm, whichwas in turn cleaned by anhydrous ethanol and distilledwater for 15 min. Then it was placed in a bowl of 60 mLsulfuric acid and 20 mL nitric acid, standing for 24 h at85 °C. After that, the product was washed to neutral bydistilled water, and then dried in vacuum at 60 °C. NC wasprepared by potentio-static electro-deposition method forvarious deposition times. In a typical experiment, carboncloth, platinum sheet and saturated calomel electrode(SCE) were used as working electrode, opposite electrodeand reference electrode, respectively. The electrolyteconsisted of 1 mol$L–1 perchloric acid and 0.5 mol$L–1

distilled aniline monomer, and the voltage is 0.75 V. Aftercarbonization under Ar atmosphere at 700 °C for 2 h, thenitrogen-enriched carbon nanowires were prepared on the

148 Front. Mater. Sci. 2021, 15(1): 147–157

carbon cloth. The deposition time are 600, 900, 1200 and1500 s, respectively, and the as-synthesized NC nanowireswere denoted as NC600, NC900, NC1200 and NC1500,respectively. The as-synthesized NC nanowires were usedas the working electrode to synthesize NCP according tothe previous potentio-static electro-deposition method. Thelengths of deposition time are 100, 300 and 500 s,respectively, and then the product was dried and weighed.The mass loading of the electro-active material wasdetermined to be about 3–6 mg on each electrode byweighing the mass difference between as-fabricatedelectrodes and the original carbon cloth. These as-synthesized samples were denoted as NCP100, NCP300and NCP500, respectively.

2.3 Characterization and electrochemical tests

The morphologies of NC and NCP samples were observedusing scanning electron microscopy (SEM, Hitachi S-4800, Tokyo, Japan). The X-ray diffraction (XRD) patternswere performed in the range of 2θ = 5°–80° by stepscanning (PANalytical X′Pert Pro X-ray diffractometer).The structures of the samples were studied by Fouriertransform infrared spectroscopy (FTIR, Nicolet Impact400, Nicolet Instrument Technologies Inc., Madison,USA). X-ray photoelectron spectroscopy (XPS) resultswere acquired using an Omicron 5000 spectrophotometervia Al Kα at 1486.6 kV and the Raman spectrum wasobtained using LabRam Aramis (Horiba Jobin-Yvon,France). The electrochemical properties of NC and NCPsamples as well as ASC were examined by cyclicvoltammetry (CV), galvanostatic charge/discharge(GCD), electrochemical impedance spectroscopy (EIS)and charge/discharge cycling stability on an electrochemi-cal workstation (CHI660E, China). For NC and NCPsamples, the electrochemical performance was measured ina three-electrode system using Pt foil as the counterelectrode, Ag/AgCl electrode as the reference electrode and1 mol$L–1 H2SO4 as electrolyte, and the electrochemicalperformance of ASC was measured in a two-electrodesystem in 1 mol$L–1 Na2SO4 electrolyte.The specific capacitance (Cg, F$g

–1) is calculated fromthe charge/discharge curve by using the followingequation:

Cg ¼ IΔtmΔV

(1)

where I (A) is the current loaded, Δt (s) is the discharge

time, ΔV (V) is the potential change during dischargeprocess, and m (g) is the mass of active material in a singleelectrode.The charge balance between the two electrodes needs to

follow the relationship of qþ ¼ q – , and the optimal massratio between the positive electrode and the negativeelectrode can be obtained according to Eq. (2):

mþm –

¼ C –ΔV –

CþΔVþ(2)

where C+ and C – (F$g–1) are the specific capacitancevalues of NCP and NC electrodes, respectively; m (g) isthe mass loading of each electrode, and ΔV (V) refers to thepotential window. The asymmetric supercapacitor wasfabricated using NC as the anode electrode and thesynthesized NCP as the cathode electrode in 1 mol$L–1

H2SO4 solution.The specific energy E (W$h$kg–1) and the specific power

P (kW$kg–1) were respectively calculated by followingequations:

E ¼ CV 2

7:2(3)

P ¼ 3600E

t(4)

3 Results and discussion

3.1 Morphology of NC and NCP samples

The SEM images of NC samples were shown in Fig. 1. Itcan be observed that the NC nanowires became graduallypacked with the increase of deposition time, and the poresize of the NC sample decreased with the increase ofdeposition time, which was unfavorable for charge transferand electrolyte ions [24–25]. So, the porous structure in theNC900 sample is suitable for charge transfer and electrolyteions to rapidly access into the internal active sites,providing high structural stability and excellent electro-chemical properties in redox reaction process. The elementmapping analysis of the NC900 sample was shown inFig. 1(e), which illustrates the carbon, nitrogen and oxygenelements evenly distributed on the nanowires in thematerial. Doping of heteroatoms (N, O) into carbon is aneffective method to improve the surface wettability andelectrical conductivity, and provide additional pseudo-capacitance due to a reversible redox reaction [26–27]. The

Liangshuo LI et al. A novel and simple NCP electrode material for supercapacitors 149

SEM images of NCP samples were shown in Fig. 2. It canbe observed that the NCP nanowires became graduallypacked with the increase of the deposition time. When thedeposition time is 500 s, the deposition layer is too thick,resulting in the covering of pores formed after carboniza-tion.

3.2 Structure of NC and NCP samples

The XRD characterization of PANI, NC and NCP sampleswere shown in Fig. 3(a). The XRD pattern of PANInanotubes showed typical diffraction peaks at 2θ = 20.8°and 24.8°, in which the peak at 2θ = 20.8° may be ascribedto periodicity parallel to the polymer chain, while the peakat 2θ = 24.8° may be caused by the periodicityperpendicular to the polymer chain, revealing the amor-phous structure of PANI. The XRD pattern of the NCsample showed two broad diffraction peaks at about 2θ =24° and 43.6°, describing a high level of graphitization,while the XRD pattern of the NCP sample showed onlybroad diffraction peaks at about 2θ = 24°, possibly due tothe fact that the deposited PANI layer weakens thediffraction peak at 2θ = 43.6°, proving the successfulcombination of NC and PANI.

Figure 3(b) shows FTIR spectra of PANI, NC and NCPsamples, exhibiting the molecular structure and chemicalbonds of PANI, NC and NCP. For pure PANI nanowires,the broad band at 3442 cm–1 was attributed to the stretchingvibration N –H of an aromatic amine as well as thestretching vibration of absorbed water. Two bands at 1608and 1488 cm–1 were ascribed to the C = C stretchingvibrations of quinoid (Q) ring and benzenoid ring (quinoidring and benzenoid ring are the basic molecular units ofPANI), respectively [28]. The band at 1307 and 1252 cm–1

belonged to the C –N stretching mode of an aromaticamine. The typical N = Q = N stretching band of PANI wasat 1120 cm–1 [29]. The single peak at 821 cm–1

corresponded to the C –H out of plane bending mode,which has been used as a key to identify the type ofsubstituted benzene [30]. The characteristic peaks of PANIdecreased significantly after carbonization, and the broadband at 3440 cm–1 was attributed to the stretching vibrationN –H of an aromatic amine [28]. The peak at 1617 cm–1

was attributed to C –N stretching modes [31–32], while theother sharp peaks at 1384 and 1121 cm–1 corresponded tothe characteristic absorbance of single C –N bonds [33].For re-deposition in NC sample to form NCP layers, thecharacteristic peaks obtained were roughly the same as

Fig. 1 SEM images of NC samples at different deposition time: (a) 600 s; (b) 900 s; (c) 1200 s; (d) 1500 s. (e) EDX mappings of C, Oand N in the NC900 sample.

Fig. 2 SEM images of NCP samples at different deposition time: (a) 100 s; (b) 300 s; (c) 500 s.

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those obtained by primary deposition PANI, suggestingthat PANI deposited in the second time completely coveredthe surface of NC sample.The Raman spectra of PANI, NC and NCP samples were

shown in Fig. 3(c). The spectrum of pure PANI showed aband at 1169 cm–1 corresponding to the C –H bending ofthe quinoid ring, the bands at 1335 and 1405 cm–1 whichwere owing to the carrier vibrations of the C –N+ polaronicstructure [34], a band at 1494 cm-1 which was attributed toC = N of the quinoid protonated diimine units [35–37], andthe bands at 1562 and 1599 cm–1 which were due to theN –H bending of the bipolaronic structure, proving thepresence of the doped PANI structure [38–39]. Ramanspectroscopy can distinguish the types of various chemicalbonds between carbon atoms and between carbon atomsand hybrid atoms (N, O). Therefore, Raman spectroscopyplays an important role in the study and characterization ofgraphite materials, especially carbon materials obtainedthrough high temperature pyrolysis [40]. After carboniza-tion at 700 °C, the Raman spectra of the NC samples werecomposed of two bands centered at 1572 cm–1 (G band —

the graphitic band) and 1349 cm–1 (D band — the disorderinduced band), which proved that NC samples were typicalgraphitized carbon materials. The stretching vibration ofC –C bond in the graphitic material gave rise to the G-bandfeature, which is common to all sp2 carbon systems [41]. Inthe Raman spectra, the quantifying of disorder in a carbonmaterial was usually made by analyzing the ID/IG intensityratio between the disorder-induced D band and thegraphitic G band [42]. The ID/IG ratio, determined fromthe integrated peak areas, was found to be 0.9. Therefore,the NC sample synthesized at 700 °C displayed feweramounts of defects and impurities. Based on the abovedata, the NC sample could serve as a highly conductive andporous substrate, which facilitates ion and electronictransportation and also provides sufficient loading ofactive materials [42]. From the Raman spectrum analysis

of the NCP sample, showing similar results with which ofPANI, it could be demonstrated that the NC samples hadbeen completely covered by PANI.XPS was used to investigate the surface electronic state

and the doping level of PANI, NC and NCP samples. Thefull XPS survey spectra of PANI, NC and NCP sampleswere presented in Fig. 4(a), and the XPS measurementspectra of PANI and NCP samples confirmed the existenceof C, N, O and Cl elements, which is in agreement withprevious reports [43–44]. NC samples only contained C, Nand O elements, indicating the purity of the preparedsamples without impurities. The high-resolution XPSspectrum of N 1s presented in Fig. 4(b) showed thatPANI fitted into three peaks, the benzenoid amine( –NH – ) with the binding energy at 399.9 eV, the quinoidamine ( –N =) with the binding energy at 399.0 eV, and thenitrogen cationic radical (N+) with the binding energy at401.5 eV [45–48]. The presence of a Cl peak indicated thatPANI was successfully doped with Cl–. For the N 1sspectrum of NC samples (Fig. 4(c)), the main peak isdivided into two peaks corresponding to various electronicstates: the nitrogen cationic radical at 400.2 eV (N+), andthe quinoid amine at 399.5 eV [49]. Figure 4(d) showedthat the N 1s envelop of the NCP sample were fitted intothree peaks. The peaks centered at 398.6 and 397.3 eVwere characteristic peaks of di-imine nitrogen ( –N =) [49–50]. In addition, the peak at 400.5 eV was ascribed to theN+ state [50].

3.3 Electrochemical properties

The performances of the supercapacitor cells based on theNC sample were evaluated by CV, GCD and EISconducted in three-electrode systems, in which NC wasused as the working electrode, and a platinum sheet and anAg/AgCl electrode as the counter electrode and thereference electrode, respectively. 1 mol$L–1 H2SO4 aqu-

Fig. 3 (a) XRD patterns, (b) FTIR spectra and (c) Raman spectra of PANI, NC and NCP samples.

Liangshuo LI et al. A novel and simple NCP electrode material for supercapacitors 151

eous solution was used as the electrolyte. The CV curves ofdifferent composite electrodes with a potential windowfrom 0 to 0.8 V (vs. Ag/AgCl) were shown in Fig. 5(a). Itcan be seen that the CV curve of the NC900 sample showedthe largest area, corresponding to specific capacitance inthe electrochemical performance. It may be due to theconnected pore structure in the NC900 sample (Fig. 1(b)).Furthermore, the CV curve of the NC900 electrode showedthe nearly reversible redox peak shape, indicating excellentelectrochemical reversibility and double layer capacitance,originating from the doped N elements [51].The representative GCD curves at a current density of

1 A$g–1 were shown in Fig. 5(b). The average specificcapacitance values, Cg of the samples were calculated fromthe discharge process according to Eq. (1).Obviously, the voltage drop was small over a wide

potential range of 0–0.4 V (vs. Ag/AgCl) in GCD curves ofNC samples, indicating a significant pseudo-capacitancecontribution resulted from the complex and overlappedredox reactions of the N-doped samples. The capacitancedetermined by galvanostatic measurements at highercurrent densities increased by the same order as previouslyobtained data from CV, and the specific capacitance valueswere 126, 180, 158 and 54 F$g–1 at a current density of

1 A$g–1 for NC600, NC900, NC1200 and NC1500, respec-tively.The conductivity and interfacial charge transfer process

at the electrode/electrolyte interface were studied by EIS,and the electrochemical impedance spectra of NC sampleswere presented in Fig. 5(c). With Z′ against Z″, the EIS plotwas recorded in the frequency range of 0.01 Hz to 100 kHz.No apparent semicircle was observed in the EIS graph ofNC900, which meant lower charge transfer resistanceattributed to strong electrical conductivity. The equivalentseries resistance (ESR) for NC900 was 1.33 Ω, indicatinglow internal resistance of the materials. Warburg impe-dance represented by the straight line appearing at the low-frequency region indicated diffusion resistance of electro-lyte on the electrode surface [52]. As shown in Fig. 5(c),the slope of the straight line for NC900 was the highest,revealing the high capacitive nature of the compositematerial. It is also seen that the line for NC900 is shorterthan that of the other NC samples, which indicated that theion diffusion path through the electrode was relativelyshorter, so it is beneficial to improve the capacitance of theelectrode materials. The CV curves (Fig. 5(d)) of the NC900

electrode were measured at different scan rates and theirGCD profiles (Fig. 5(e)) were tested at different current

Fig. 4 (a) XPS spectra of PANI, NC and NCP. High-resolution XPS spectra of N 1s for (b) PANI, (c) NC and (d) NCP.

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densities. Accordingly, quasi-rectangular CV curves andnear triangle GCD curves could prove the supercapacitivebehavior of the NC samples. Also, CV curves and GCDprofiles of NC900 were linear and symmetric, implying highcoulombic efficiency and good reversibility, respectively.All CV curves in Fig. 6(a) exhibited rectangular shapes

with sharp redox peaks, suggesting EDLC properties of thecarbon electrode and the pseudo-capacitive nature of theconducting polymer active material [29,53]. The threedistinctive redox peaks were associated with the transitionbetween different states of PANI. Specifically, the firstredox pair was attributed to the transition between semi-conductive and conductive states of PANI. The second onewas associated with other structure such as thebenzoquinone–hydroquinone couple. Lastly, the third onewas associated with the transition between pernigranilineand quinone diamine [54–55]. In addition, it is clear thatthe area of CV for the NCP300 electrode was much largerthan those for the others, suggesting larger specificcapacitance of the NCP300 electrode. In Fig. 6(b), thenearly symmetrical nonlinearities with a sloped potentialplateaus in GCD curves exhibited the electrochemicalreversibility and pseudo-capacitive behavior of the flexibleelectrode [56]. Furthermore, EIS investigation was con-ducted in a frequency range from 0.01 Hz to 100 kHz to

understand the intrinsic ohmic resistance, charge exchangeresistance and ionic resistance. Typical Nyquist plot wasshown in Fig. 6(c). The intersection of impedance responseto the X-axis was expressed with Rs, which is the sum of thesolution and internal electrode resistance [57–58]. The Rs

values for NCP100, NCP300 and NCP500 electrodes were1.328, 1.385 and 1.257 Ω, respectively, indicating thesimilar electrical conductivity and ion response. However,the straight line at low-frequency region indicated lowerdiffusion resistance of the electrolyte on the electrodesurface for the NCP300 electrode, which is conducive forelectrolyte diffusion into the electrode material [53].Figure 6(d) showed CV curves of the NCP300 electrode

at different scan rates in 1 mol$L–1 H2SO4. The peakcurrents of the NCP300 electrode increased significantlywith the increase of the scan rate, suggesting excellent ratecapability of the NCP300 electrode. Figure 6(e) displayedGCD curves of the NCP300 electrode at different currentdensities from 1 to 20 A$g–1. The specific capacitancescalculated using Eq. (1) were 615, 550, 550, 455, 400 and362.5 F$g–1 at 0.6, 1, 2, 4, 10 and 20 A$g–1, respectively.In order to respond the practical application of electrode

materials and characterize the excellent electrochemicalproperties of electrode materials, the ASC devices had beenassembled by using the NCP300 electrode and the NC900

Fig. 5 Electrochemical performance of different NC samples: (a) CV curves at 20 mV$s–1; (b) GCD profiles at 1 A$g–1; (c) EIS resultswith the inset showing enlarged parts of Nyquist plots. Electrochemical performance of the NC900 electrode: (d) CV curves at differentscan rates from 5 to 100 mV$s–1; (e) GCD profiles at different current densities from 0.5 to 20 A$g–1.

Liangshuo LI et al. A novel and simple NCP electrode material for supercapacitors 153

electrode as a positive electrode and a negative electrode,respectively, in 1 mol$L–1 Na2SO4. Stable CV curves(Fig. 7(a)) were obtained in the potential windows of – 0.8to 0 V for the NC900 electrode, showing the ideal electricdouble-layer capacitor behavior and from 0 to 0.8 V for theNCP300 electrode, revealing pseudo-capacitive behavior,which is also consistent with reports in Ref. [59].According to CV data of the NCP300 electrode and theNC900 electrode in a three-electrode system in 1 mol$L–1

Na2SO4 solution at 20 mV$s–1 (Fig. 7(a)), the correspond-ing specific capacitances for the NCP300 electrode and theNC900 electrode were 132 and 333 F$g–1, respectively. Tobalance the charge storage in this asymmetric cell, theoptimized mass ratio between the NCP300 electrode and theNC900 electrode was calculated to be 0.396. As shown inFig. 7(b), the assembled ASC could remain stable within avoltage window of 1.6 V, and the calculated gravimetriccapacitances for the ASC cell (Fig. 7(b)) was 122 F$g–1 at20 mV$s–1. NCP300//NC900 ASC exhibited the combina-tion of both pseudo-capacitive and double-layer capacitivebehavior as shown in CV curves of Fig. 7(c). As the currentdensity increased, the current increased gradually, whilethe shape of the CV pattern did not change significantly at ascan rate up to 100 mV$s–1. Figure 7(d) displayed GCD

plots of the ASC cell at various current densities in thepotential window of 0–1.6 V. There was no obvious IRdrop in the GCD plots, and the symmetrical charge/discharge curves represented good capacitive character-istic. The Casy was determined by the total weight ofNC900 negative electrode and NCP300 positive electrode.The calculated gravimetric capacitances for ASC cell(Fig. 7(d)) was 111 F$g–1 at a current density of 1 A$g–1,which is higher than other reported ASC based on PANImaterials (61–107 F$g–1). Even at a high current density of10 A$g–1, the ASC device still had a high specificcapacitance of 50 F$g–1. Superior cycling stability wasrevealed from the result of long-term cycling measure-ments in Fig. 7(e), which showed that 82% of the initialcapacitance was maintained after 1200 cycles at 2 A$g–1.Calculated from the GCD curves, the Casy values wereillustrated in Fig. 7(f). The results showed that the Casy

value of NCP300 was 615 F$g–1 at 0.6 A$g–1. When thecurrent density reached 20 A$g–1, the capacitance wasstill 362.5 F$g–1, indicating the capacitance retentionof 59%. The ASC showed a high energy density of39.1 W$h$kg–1 at a power density of 792.6 W$kg–1 and amaximum power density of 8 kW$kg–1 at an energy densityof 17.8 W$h$kg–1 with excellent stability.

Fig. 6 Electrochemical performance of different NCP samples: (a) CV curves at 20 mV$s–1; (b) GCD profiles at 1 A$g–1; (c) EIS resultswith the inset showing enlarged parts of Nyquist plots. Electrochemical performance of the NCP300 electrode: (d) CV curves at differentscan rates from 5 to 100 mV$s–1; (e) GCD profiles at different current densities from 0.5 to 20 A$g–1.

154 Front. Mater. Sci. 2021, 15(1): 147–157

4 Conclusions

In conclusion, we have successfully prepared the NCPcomposite material. The NCP300 electrode exhibits largespecific capacitance of 615 F$g–1 at 0.6 A$g–1 and capacityretention rate of 59% at the current density of 20 A$g–1.The ASC cell was assembled with the NCP300 electrode(positive electrode) and the NC900 electrode (negativeelectrode). The operating voltage of the ASC cell reachesup to 1.6 V and exhibits excellent electrochemicalperformance with a specific capacitance of 111 F$g–1.Additionally, the ASC cell can deliver a highenergy density of 39.1 W$h$kg–1 at a power density of792.6 W$kg–1 and a maximum power density of 8 kW$kg–1

at an energy density of 17.8 W$h$kg–1 with excellentstability.

Acknowledgements This research was funded by the Natural ScienceFoundation of Guangxi Province (2020GXNSFAA159015), the GuangxiKey Laboratory of Optical and Electronic Materials and Devices (20KF-20),the Open Funds of Key Laboratory of New Processing Technology forNonferrous Metal and Materials of Ministry of Education (19AA-18), thePostgraduate Joint Cultivation Base of the Education Department ofGuangxi, and the Opening Project of Guangxi Key Laboratory of CalciumCarbonate Resources Comprehensive Utilization (Hezhou University)(HZXYKFKT201903).

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