Hierarchical architecture of polyaniline nanoneedle arrays on electrochemically exfoliated graphene for supercapacitors and sodium batteries cathode

Materials and Design 188 (2020) 108440

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Hierarchical architecture of polyaniline nanoneedle arrays on electrochemically exfoliated graphene for supercapacitors and sodium batteries cathode Yingjuan Sun a, Liansheng Jiao b, Dongxue Han c, Faxing Wang d, Panpan Zhang d, Hongyan Li a,⁎, Li Niu c,⁎ a

Department of Materials Science and Engineering, College of Chemistry and Materials Science, Jinan University, Guangzhou 510632, China Department of Chemistry and Chemical Engineering, Hebei Normal University for Nationalities, Chengde 067000, China c Center for Advanced Analytical Science, c/o School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou 510006, China d Department of Chemistry and Food Chemistry and Center for Advancing Electronics Dresden (cfaed), Dresden University of Technology, 01062 Dresden, Germany b

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• The polyaniline nanoneedles can act as active material and function as spacer between the graphene sheets. • The composite can be used in sodium ion full cell and shows remarkable rate capability (64 mAh g-1) and cycling stability. • Quantitative kinetics analysis for sodium battery has validated the capacitive- and diffusion-controlled mechanism. • The composite played an excellent role as a supercapacitor electrode (specific capacitance is 736 F g-1).

a r t i c l e

i n f o

Article history: Received 1 September 2019 Received in revised form 17 December 2019 Accepted 17 December 2019 Available online 19 December 2019 Keywords: Electrochemical exfoliation of graphene Needle-like polyaniline Cathode Sodium batteries Supercapacitor

a b s t r a c t This study reports an efficient and straightforward method to construct hierarchical architectures for electrochemical energy storage. The composites, which are based on needle-like polyaniline (PANI) and high quality electrochemical exfoliation graphene (EG), can be a potential candidate for sodium batteries cathode due to favorable structural and morphological properties. Kinetics analysis of cyclic voltammetry has evidenced the diffusion controlled and pseudocapacitive controlled contributions of charge storage in the EG-PANI composite cathode. For the first time, the full sodium cell based on the EG-PANI composite was constructed and the rate capacities reached up to 64 mA h g−1 at the current densities of 100 mA g−1. In addition, the readily available composites bearing high conductivity and high-quality contact interface also enable the design of efficient supercapacitor. Specifically, the specific capacitance of EG-PANI composites electrode is 736 F g−1 at 0.2 A g−1, and the specific capacitance can be achieved to 410 F g−1 under 10 A g−1 (high current density). The synergistic effect of EG and PANI provides a remarkable storage capacity. Our work has inspired the search for large scale and hierarchical architectures materials for electrochemical energy storage. © 2019 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

⁎ Corresponding authors. E-mail addresses: [email protected] (H. Li), [email protected] (L. Niu).

https://doi.org/10.1016/j.matdes.2019.108440 0264-1275/© 2019 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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1. Introduction Nowadays, under the continuing development of smart devices and electric products, the requirement for up-and-coming energy storage devices with high rate performance and long cycle life are urgently needed [1,2]. Apart from the conventional lithium-ion technology, sodium-ion batteries (SIBs) have attracted a great deal of interests because of similar physical and chemical properties, and a higher natural abundance of Na compared with Li [2–6]. However, Na+ ion has large radius and heavy mass which are unfavorable for electrochemical charge-discharge kinetics [7,8]. Many efforts have been focused on exploitation of appropriate anode materials for high performance SIBs [9–12]. Unfortunately, they suffer from low initial coulombic efficiency, poor cycle stability and rate performance [13]. Na metal is the optimal choice with the lowest redox potential (−2.71 V vs. SHE) and the highest specific capacity (1166 mAh g−1) among all anode materials of Na batteries [2]. As the battery capacity is a function of capacity of both cathode and anode materials, thus using a battery technology based on Na metal as anode facilitates the design of storage systems with high energy density. Consequently, preparing appropriate cathode materials with improved performances towards practical utilization is highly desired. Meanwhile, as a next generation energy storage devices, the supercapacitors with short-term energy storage, burst-mode power delivery and ultra-long cycling stability have been well defined [14–16]. Due to the simple synthesis method, protruding environmental stability, high pseudocapacitance and lots of interesting chemical, physical and optical properties, conductive polymers could be considered as promising electrode materials for solar cells, supercapacitors and transistors [17–19]. Among them, polyaniline (PANI) is one of the typical conductive polymer materials. It can be used in rechargeable batteries, supercapacitors, electrochromic display devices, sensors and information storage [20–23]. Cellulose/PANI microspheres have been developed as carbon-based anodes for high performance sodium-ion batteries [13]. Cao's group reported the Co3S4@polyaniline nanotubes as anode material for sodium ion batteries [24]. Zhang et al. reported the SnO2/ polyaniline composite for sodium-ion batteries anode [25]. However, few reports have considered single designed material in both supercapacitors and full sodium cell cathode at the same time [26,27]. In this work, we reported the composite of needle-like PANI array on high quality electrochemically exfoliated graphene (EG) through a green chemical process. High quality and high-yield EG has been achieved and used in previous studies. Further we have successfully fabricated the flexible supercapacitors from the EG materials [15]. In this work, the as-obtained EG-PANI composite materials exhibited favorable electrochemical performance when it used as supercapacitors electrode and Na metal batteries cathode. For sodium cathode side, the obtained composite showed remarkable rate capability. Specifically, reversible capacities of 73, 58, 52, 49 and 45 mAh g−1 were retained when discharge current densities increased from 100 mA g−1 to 150 mA g−1, 200 mA g−1, 300 mA g−1 and 500 mA g−1. Meanwhile, kinetics analysis of cyclic voltammetry has evidenced the diffusion controlled and pseudocapacitive controlled contributions of charge storage in the EGPANI hybrid materials. In addition, the contribution of capacitivecontrolled in the whole charge stored was 28.9% at 2 mV s−1 (low scan rate) and 53.4% at 8 mV s−1 (high scan rate). Then, for the first time, Na ion full cell was prepared and tested in order to further explore the practical application of the EG-PANI composite. The full cell was tested in the voltage window of 2.4–4.4 V at the various current densities of 100, 150, 200, 300 and 500 mA g−1. The corresponding rate capacities will achieve at 64, 52, 41, 36 and 26 mAh g−1. Also, the composite exhibited remarkable capacity retention during cycling 550 cycles at the current density of 300 mA g−1. For the supercapacitor test, at the current density of 0.2 A g−1 (low current density), the specific capacitance of EG-PANI composites is 736 F g−1, and even at current density of 10 A g−1 (high current density), the specific capacitance is 410 F g−1, which is much higher than the specific

capacitance of active carbon. We hope through this work open the novel opinion for developing economic and efficient energy storage materials. 2. Experimental 2.1. Chemicals Graphite foil (0.13 mm, 99.8%, Alfa Aesar), isopropanol (IPA), ammonium sulfate ((NH4)2SO4), Polyvinyl alcohol (PVA), ((2,2,6,6tetramethylpiperidin-1-yl)oxyl) (TEMPO), Aniline, Ammonium persulfate (APS), PVDF, Whatman glass paper separator, Acetylene black (AB), NMP (N-Methyl pyrrolidone) and ethanol (N99.9%) were purchased from Sigma-Aldrich. All aqueous solutions were prepared with ultrapure water from a Milli-Q Plus system. 2.2. Preparation of electrochemically exfoliated graphene The detailed preparation process has been described in our previous report [15]. Typically, graphite foil exfoliation was performed in a homemade two electrode system, whereby graphite foils were used as working anode and Pt foils as counter electrode. The electrolyte for the graphite exfoliation was prepared by dispersing ammonium sulfate (0.05 M) and TEMPO (50 mg) in DI water. The electrochemical exfoliation of graphite foil was carried out by applying constant positive voltage (+10 V) on the working electrode. The EG powder was first collected with cellulose filters and washed repeatedly with DI water and ethanol by vacuum filtration. The washing process was repeated for several times to clear any chemical residues. Then, the EG powder was re-dispersed into isopropanol (IPA) solution by sonication for 5 h under ice bath. The dispersion was kept for 24 h for the precipitation of unexfoliated graphite flakes. The concentration of the upper dispersion was determined using the vacuum filtration method, and it could be used further for characterization and device fabrication. In this work, the TEMPO acted as an eliminating agent to defend the attack of oxidative radicals. The intercalation of sulfate ions leads to complete exfoliation. More importantly, such electrochemical exfoliation can produce over 10 g of high-quality EG in 1 h (Fig. 1a–b). According to the XPS characterization in Fig. 1c, it can be clearly seen that some oxygen groups were found on the surface of EG. 2.3. Preparation of the EG-PANI composite The ethanol (99.9%, 13 mL) and HCl aqueous solution (1 M, 10 mL) were mixed as mixture. Then the EG thin nanosheets (10 mg) were dispersed into the above mixture by sonication. Hereafter, aniline (42 μL) was also injected into the above solution. After 10 min stirring of this mixture and then HCl aqueous solution (1 M, 10 mL) with APS (68.5 mg) was put into it. Under the ice-bath, the final solution was stirred gently for 24 h (called EG-PANI architectures). The obtainedproducts were gathered and washed with DI-water and ethanol three times. 2.4. Electrochemical measurements 2.4.1. Fabrication of sodium battery cathode For the electrode fabrication, the as-obtained composite, acetylene black, and poly(vinylidene difluoride) (PVDF) (mass ratio of 80:10:10) were mixed into a homogeneous slurry with mortar and pestle in NMP. Then the obtained slurry was casted onto current collector of Al foil using the doctor blade technique. The electrolyte consisted of a solution of NaPF6 (1 M) in diethylene glycol dimethyl ether. The coin cells were assembled in an Ar-filled glove box and the glass fiber as the separator. The pure sodium

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Fig. 1. a–b) The digital photographs of the EG and the schematic illustration of the mechanism of electrochemical exfoliation. c) The XPS survey spectra of EG.

metal foil was used as counter electrode. Galvanostatic charge and discharge process were processed using the LAND-CT2011A battery testing instrument under different current density within a voltage range of 2.0–4.4 V vs. Na/Na+ at room temperature. Other electrochemical experiments were recorded with a CHI 760E electrochemical workstation.

2.4.2. Fabrication of supercapacitors device All electrochemical measurements were carried out in a conventional three electrode system with SCE as the reference and a Pt plate as the counter electrode in 6 M KOH solution. For the electrode fabrication, the as-obtained composite, acetylene black, and poly(tetrafluoroethylene) (PTFE) (mass ratio of 80:10:10) were mixed into a homogeneous slurry with mortar. Then the obtained slurry was casted onto current collector of carbon paper and dried at 120 °C for 12 h (the loading amount of active materials was 1 mg). The cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge-discharge tests were selected to show the electrochemical performances of the prepared electrodes. The electrochemical behavior

was recorded within a potential window of 0.0 V to 1.0 V. The specific capacitance was calculated according to the following discharge process equation:
Cs F g−1 ¼ I t=E m

ð1Þ

where I is the current (A), t is the discharge time (s), E is the potential change during the discharge process (V), and m is the mass of active material (g). 3. Results and discussion 3.1. Structural characterization The preparation procedure of EG-PANI composite is schematically described in Fig. 2. Firstly, aniline was added into the stable dispersion of EG and HCl solution by sonication. Then, APS was introduced into the above dispersion solution to perform the polymerization reaction of aniline. Finally, the powder of EG-PANI composite will obtain through washing and ice drying. The mechanism of nanoneedle-like PANI

Fig. 2. Schematic illustration of the synthesis steps for the composites of EG-PANI.

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formation on the surfaces of EG can be explained as followed. Due to electrostatic forces, the aniline molecules will be attracted by numerous active sites on the surface of EG. This situation will help PANI grow at the beginning of aniline monomer polymerization. Noteworthy, the seed template on the surface of EG would trigger the polymerization process. As soon as APS is added into the reaction system, aniline ions begin to polymerize in these active sites, forming many tiny PANI protuberances on the EG surface. These protuberances reduce the interfacial energy barrier between EG and the other aniline ions, and also act as “seeds” for further polymerization leading to the formation of nanoneedle-like PANI [17]. The exfoliated state of the graphene was further ascertained by the SEM and TEM characterizations (Fig. 3a and b). It can be clearly shown that the thickness of the graphene flakes was significantly decreased

compared with bulk graphite, which indicates the graphite has been successfully exfoliation. From Fig. 3a, the EG nanosheets were observed with flat and wrinkling feature. Moreover, a typical six-fold symmetric diffraction pattern was shown in selected area electron diffraction (SAED, inset in Fig. 3a). Among the SAED, the diffraction for the (1210) plane is stronger than the diffraction of (0-110) plane, which means the bilayer graphene sheet with high crystallinity [15]. The morphology and microstructure of the as-prepared EG-PANI composites also were demonstrated by SEM and TEM after polymerization reaction of aniline. In Fig. 3c and e, the numerous needle-like polyaniline arrays were clearly found and highly distributed on the surface of graphene sheets. Those nanoneedles were measured around 40 nm in length and 15 nm in diameter from high-resolution SEM image (Fig. 3d). From the high-resolution TEM (HRTEM) analysis (Fig. 3f), the

Fig. 3. a–b) TEM and SEM images of EG. Insets of a) is SAED image and elements analysis of EG. Inset of b) is the photograph of the EG dispersion. c–d) SEM images of needle-like PANI on the surface of EG. e) TEM image of EG-PANI. f) High resolution lattice structure of composite of EG-PANI. g–i) The elements mapping of EG-PANI composite, g) C1s, h) N 1s and i) O 1s. The scale bar is 500 nm.

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crystalline EG sheets are well coupled with the amorphous PANI nanoneedles in the EG-PANI composite. The elemental mapping images of C, N, and O of the EG-PANI were investigated and shown in Fig. 3g–i. The PANI and the EG-PANI composite were also tested by XRD measurements as shown in Fig. 4a. The well-defined peaks at 2θ = 15.1, 20.3, and 25.1° are similar to those for crystal polyaniline [28,29]. The FTIR spectra of the PANI and the EG-PANI composite are shown in Fig. 4b. The peaks at 1570 belong to the C_C stretching of the quinoid ring and the peaks at 1489 cm−1 belong to the benzenoid ring. In addition, the peaks at 1295 and 1130 cm−1 are attributed to C\\N stretching of the secondary aromatic amine, and the peak at 801 cm−1 is ascribed to the out-of-plane bending of C\\H on the 1,4-disubstituted ring. These characteristics are in well agreement with previous report of PANI [28]. The XPS characterization of PNAI and EG-PANI composite (Fig. 4c and d) shows the presence of C1s (~285 eV), O1s (~532 eV) and N1s (~399 eV), which also confirms the occurrence of PANI [30,31]. More characterization analysis of EG, such as XRD, XPS and AFM were shown in Fig. S4 (Supporting information). 3.2. Electrochemical performances The electrochemical properties of the EG-PANI composite electrode were characterized and shown in Fig. 5. Typically, cyclic voltammetry (CV) is employed to get further insight into the kinetics analysis of sodium storage behaviors regarding to the EG-PANI composite material. As shown in Fig. 5a, it demonstrated the CV curves of the EG-PANI/Na half cells for first five cycles in the voltage range of 2.0–4.4 V at a scan rate of 2–8 mV s−1.

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As shown in Fig. 5b, the CV features of the EG-PANI composites appeared as two pairs of well-defined redox peaks located at 3.7/3.5 V and 3.1/2.9 V (vs. Na/Na+). Those redox peaks are very similar to the CV patterns of PANI material in electrolytes. The redox reaction in the EG-PANI can be attributed to the reversible insertion/extraction of the ionizable Na+ for the charge counterbalance. According to the above analysis, the EG-PANI composite electrode may serve as a high voltage and high capacity cathode for constructing rechargeable sodium batteries with the charge-discharge mechanism as followed [32]: Cathode : PANI þ PF− 6 ↔PANIþ PF− 6 þ e

ð2Þ

Anode : Naþ þ e↔Na ðorNaCx Þ

ð3Þ

Overall reaction : PANI þ PF− 6 þ Naþ ↔PANIþ PF− 6 þ Na ðorNaCx Þ ð4Þ Furthermore, kinetics analysis of charge storage mechanism of the capacitive effects (k1v) and diffusion controlled (k2v0.5) contributions regarding to the voltage and response current, which can be acquired by CV curves analysis (Fig. 5b) according to the following Equation [33], iðVÞ ¼ k1 v þ k2 v0:5

ð5Þ

or iðVÞ=v0:5 ¼ k1 v0:5 þ k2

ð6Þ

where ν is the scan rate (mV s−1), i is the current (A), and k1, k2 are constants. According to the Eq. (5) in above, k1 represents the slope of

Fig. 4. a) XRD patterns, b) FTIR spectra and c-d) XPS spectra of the PANI and EG-PANI composite.

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Fig. 5. a) CV curves of the obtained composite in the potential window of 2.0–4.4 V at various scan rates. b–f) The kinetics analysis of the pseudocapacitive and diffusion part under different scan rate. g) Contribution ratios of capacitances from the pseudocapacitive controlled reaction and diffusion controlled reaction at various scan rates. h) Comparison of the CV curves of the as-obtained EG-PANI composite and EG in the 2.0–4.4 V potential window at 2 mV s−1. i) Electrochemical impedance spectrum of EG-PANI electrodes.

fitting line, and the k2 represents the intercept. Fig. 5b–f showed two parts capacity depicted of the CV curves at different scan rates of 2 mV s−1, 3 mV s−1, 5 mV s−1, 7 mV s−1 and 8 mV s−1 according to the quantitative calculation analysis. Among the contributions, the red is diffusion-controlled region and the blue is capacitive-controlled region. Therefore, the contribution of capacitive-controlled in the whole charge stored was 28.9% at 2 mV s−1 (low scan rate) and 53.4% at 8 mV s−1 (high scan rate) by calculating the blue enclosed area. It is shown that the proportions of capacitive contribution increased gradually from the low sweep rate to high sweep rate (Fig. 5g). Upon cycling, the redox peaks remained almost unchanged, demonstrating the good reversibility of Na ions influx/deflux in the composite and the high electrochemical performance [34]. In Fig. 5h, CV curve of EG exhibit a relatively rectangular shape compared with CV curves of EG-PANI, which shows the electric double layer capacitance (EDLC) characteristic. However, the CV curve of EG-PANI demonstrates the typical CV characteristic of PANI with two pairs of redox peaks. The two peaks is attributing to the redox transition of PANI between a semiconducting state (leucoemeraldine form) and a conducting state (polaronic emeraldine form) and transformation of emeraldine-pernigraniline. Fig. 5i shows

the impedance spectrum of the EG-PANI based electode. Charge transfer resistance (electrolyte and electrode) and ion diffusion are the important steps in electrochemical processes. The semicircle corresponds to the charge transfer resistance and the slope line is related to the Na+ diffusion into the electrodes [2,33]. It is found that the EG-PANI composite electrode exhibits a low charge transfer resistance value around 20 Ω as well as a shorter and steeper sloping linear range, which indicating the lower diffusion resistance. As displayed in Fig. 6a and b, the rate capability of as-obtained composite perform well. At the same time, when the discharge current densities increased from 100 mA g−1 to 150 mA g−1, 200 mA g−1, 300 mA g−1 and 500 mA g−1, the reversible capacities will achieve at 73, 58, 52, 49 and 45 mAh g−1. In Fig. 6c, the stability percentage of EG and EG-PANI are 91% and 82%, respectively. Also, the EG-PANI composite exhibited much improved capacity retention during cycling (Fig. 6c) compared with bare EG. The remarkable electrochemical performances of the EG-PANI composite electrode could be attributed to the high conductivity of EG and the synergistic effect between EG and PANI. This needle-like structure of PANI is beneficial to contact the electrolyte and conducive to electrolyte transportation. In Fig. 6d, the EG-

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Fig. 6. a) Typical charge-discharge curves of the composite cycled between 2.2 V and 4.4 V at different current densities. b) Reversible capacities at various current densities. c) Cycling performance at a constant current density of 100 mA g−1. d) Optical image of a LED powered using the EG-PANI composite based sodium battery.

PANI composite based sodium battery is able to light a red lightemitting diode around 7 min with the working voltage around 1.9–2.2 V after being fully charged. In order to further explore the practical application of the EG-PANI composite, the Na ion full cell was prepared and tested. In Fig. S2a and b (Supporting information), the full cell was tested in the voltage window of 2.4–4.4 V at various current densities of 100, 150, 200, 300 and 500 mA g−1. The corresponding rate capacities achieved up to 64, 52, 41, 36 and 26 mAh g−1. Also, the composite exhibited remarkable capacity retention during cycling 550 cycles at the current density of 300 mA g−1 (Fig. S2c). In general, the EG-PANI composite based battery exhibit high reversible capacity, good cycling and rate performance, which can be ascribed to the following reasons [35,36]: i) EG-PANI nanocomposite with a large interlayer space and more active sites for Na+ storage; ii) the thin EG nanosheets as a conductive substrate can enhance the conductivity of the composite; iii) the Na+ ions can effective intercalate into the PANI nanoneedles; iv) the EG-PANI composite is beneficial for relieving the structure strain caused by the Na+ ion insertion/extraction. As a result, the EG-PANI architectures electrode based battery achieves the high rate capability and long cycling stability in the real test. This further proves that the designed electrode structure is essential for improving battery performance. The high-quality contact interface and high conductivity of EG-PANI composite also can act as an obviously electrode material for supercapacitors [37]. Fig. 7a shows the CV curves of EG-PANI composite at different scan rates from 5 mV s−1 to 200 mV s−1. It can be found that the cathodic peaks and anodic peaks shifted to the high voltages and low voltages at higher scan rate, respectively. The peak current has a

proportional relationship with the scan rate, the peak current densities increased with increase of scan rates. The GCD curves of EG-PANI composites were performed at different current densities (Fig. 7b). From the GCD curves, it can be clearly seen that the shape of all curves is nearly symmetric and linear, which shown the good capacitance of electrode materials [26,38]. The specific capacitance of EG-PANI nanocomposites based electrode was calculated according to Eq. (1) using the discharge curves and shown in Fig. 7c. Specifically, the specific capacitance of EGPANI composites electrode is 736 F g−1 at current density of 0.2 A g−1, and the specific capacitance can be achieved to 410 F g−1 under a high current density of 10 A g−1. Obviously, the specific capacitance of 552 F g−1 (1 A g−1) of EG-PANI composites exhibited a much higher value than specific capacitance of 188 F g−1 (1 A g−1) of bare EG. This phenomenon is benefits from the novelty design of the composites structure and a faster ion diffusion rate. The Nyquist plots of supercapacitors for EG-PANI composites electrodes are shown in Fig. 7d, which shows a straight line in the low frequency region and semicircular in the high frequency region. More specifically, the straight line (near-vertical) indicates a good capacitive behavior, representative of fast ion diffusion in the EG-PANI electrode material. The resistance can be estimated about 2 Ω from the x-intercept of EG-PANI, which could be attributed to high conductivity of EG in the composite electrode. Simultaneously, the EG-PANI composite based in-plane all solid state micro-supercapacitor was fabricated by spray coating the solution-processable ink onto a Capton substrate through a mask (Fig. 7e). The stability of this micro-supercapacitor with PVA/KOH electrolyte was measured at a scan rate of 200 mA g−1 for 2000 cycles and was shown in Fig. 7f. In the long term stability test, it will maintain at

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Fig. 7. a) CV curves of the EG-PANI based supercapacitor recorded at different scan rates of 5, 10, 20, 50,100, and 200 mV s−1. b) Galvanostatic charge/discharge (GCD) curves of EG-PANI based supercapacitor recorded at different current densities of 0.2, 0.5, 1, 2, 5 and 10 A g−1. c) Specific capacitance of EG and EG-PANI based supercapacitors. d) Nyquist plot of EG-PANI composites electrode. The Z′ is real impedance and Z″ is imaginary impedance. e) Digital photographs of EG-PANI based microsupercapacitor device. f) Long term stability of microsupercapacitor at a current density of 200 mA g−1.

2.4 F g−1 (the initial capacitance is 3 F g−1) after 500 cycles and 2.175 F g−1 after 2000 cycles. In addition, different contents of PANI will have significant effects the electrochemical performance. Hence, the 4.2 μL, 42 μL (Selected content in this paper) and 420 μL of aniline precursor were selected to investigate supercapacitor performance (Fig. S3). From Fig. S3a, it cannot be clearly observed the PANI after the small amount (4.2 μL) of aniline precursor was injected into the reaction. On the contrary, the PANI will obviously gather together after excessive amount (420 μL) of aniline precursor was injected into the reaction (Fig. S3b). Only after adding a quantitative aniline precursor (42 μL) into the reaction, we can find the needle-like polyaniline arrays on the surface of graphene sheets (Fig. S3c). From the GCD curves upon different ratio PANI based composite, we found that the capacitance of supercapacitor based on excessive amount and small amount of PANI in EG-PANI composite are much lower than appropriate amount of PANI in EG-PANI composite (Fig. S3d). The comparison table for the supercapacitor capacitance, sodium ion battery cathode rate, full sodium ion battery rate and full sodium ion battery cycles of the EGPANI composite with state-of-the-art material is shown in Table S1. At last, from the electrochemical measurements, it can be found that the EG-PANI composites could be the promising candidate material for battery and supercapacitor applications.

composite used as electrode materials in supercapacitors, the composite material exhibited enhanced charge-discharge performance in comparison with control sample. Significantly, in the hybrid architecture, PANI needles on the surface of the graphene layers not only act as active material, but also function as a conducting spacer, preventing the irreversible π–π stacking between the graphene sheets and enhancing electrolyte shuttling as well as electric conductivity. In summary, we hope this work encourage further research for economic and efficient materials used for electrochemical energy-storage systems.

4. Conclusions

This research received no specific grant from any funding agency, commercial or not-for-profit sectors. The authors have no disclosures to make.

Composites of high quality graphene and needle-like PANI were synthesized by an electrochemical exfoliation of graphene and follow-up low temperature polymerization method. The synthesis process was cost-effective, simple and easy to scale-up. As-prepared material can be used well in full cell and supercapacitor. On one hand, when the EG-PANI composite used as cathode materials in sodium batteries, the composite material exhibited enhanced rate capability and cycling stability in comparison with bare EG. In addition, kinetics analysis of cyclic voltammetry has evidenced the diffusion controlled and pseudocapacitive controlled contributions of charge storage in the EGPANI composite cathode. On the other hand, when the EG-PANI

Credit author statement Y. Sun: Design the experiment, Collect the data, data analysis, Writing - original draft, review & editing. L. Jiao: Data analysis. D. Han: Data analysis. F. Wang: Data analysis. P. Zhang: Data analysis. H. Li: Supervision, Funding acquisition. L. Niu: Supervision. Funding sources/disclosures

Declaration of competing interest None. Acknowledgements H. Li acknowledges support by the Start-up Funding of Jinan University (88016105 and 55800001) and the Fundamental Research Funds for the Central Universities (12819023).

Y. Sun et al. / Materials and Design 188 (2020) 108440

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