Nitrogen-doped carbon networks derived from the electrospun [email protected] polyethylenimine nanofibers as flexible supercapacitor electrodes

Journal of Alloys and Compounds 808 (2019) 151737

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Nitrogen-doped carbon networks derived from the electrospun polyacrylonitrile@branched polyethylenimine nanofibers as flexible supercapacitor electrodes Xinwei Zhao a, 1, Guangdi Nie b, *, 1, Yaxue Luan a, Xiaoxiong Wang a, Shiying Yan a, **, Yun-Ze Long a, *** a b

Collaborative Innovation Center for Nanomaterials & Devices, College of Physics, Qingdao University, Qingdao, 266071, PR China State Key Laboratory of Bio-Fibers and Eco-Textiles, College of Textiles and Clothing, Qingdao University, Qingdao, 266071, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 January 2019 Received in revised form 5 August 2019 Accepted 6 August 2019 Available online 6 August 2019

Nitrogen-doped carbon nanofibers (NCNFs) are important electrode materials that have received tremendous attention in terms of their capacity to store energy. At present, it is still desirable to fabricate high-performance NCNFs for supercapacitors. Here, for the first time, polyacrylonitrile@branched polyethylenimine (PAN@bPEI) composite nanofibers are proposed as precursors to prepare freestanding NCNFs, which can then be used directly in flexible supercapacitors. The single electrode based on the resultant NCNFs with unique chemical composition exhibits electrochemical properties that are superior to those of the polyacrylonitrile-derived CNFs electrode. In addition, a quasi-solid-state symmetric supercapacitor assembled without the use of any adhesive or conductive agent also shows a comparable energy density, favorable cycling durability, and mechanical stability, indicating that bPEI is an effective nitrogen source with which to construct nitrogen-doped carbon fibers for energy storage applications. © 2019 Elsevier B.V. All rights reserved.

Keywords: Branched polyethylenimine Nitrogen-doped Carbon nanofibers Flexible electrodes Supercapacitors

1. Introduction There has recently been an urgent demand for clean and renewable power sources because of the ever-increasing environmental pollution and fossil fuel consumption [1,2]. Supercapacitors, which are one of the most promising types of energy storage devices, have attracted extensive attention due to their high power density, fast charge/discharge rate, long cycling durability, good safety record, and low cost; these devices can thus fill the gap between the conventional dielectric capacitors and rechargeable batteries [3e5]. It is reported that electrode materials play a pivotal role in determining the electrochemical performance of supercapacitors [6,7]. Among the various candidate electrode materials, such as carbons, transition metal oxides, and conducting polymers, especially the carbonaceous species including graphene, activated

* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (G. Nie), [email protected] (S. Yan), [email protected] (Y.-Z. Long). 1 These two authors contributed equally to this work. https://doi.org/10.1016/j.jallcom.2019.151737 0925-8388/© 2019 Elsevier B.V. All rights reserved.

carbon, carbon nanotubes (CNTs), and carbon nanofibers (CNFs) with ultralight weight, unique structural characteristics, and favorable electrical and mechanical properties, are considered as the ideal ones for supercapacitor electrodes [8e10]. It is universally acknowledged that carbon materials store energy based on the so-called electrochemical double-layer capacitance (EDLC) mechanism, which relies heavily on the accessible area at electrode/electrolyte interface [11,12]. Therefore, numerous efforts have been dedicated to constructing porous carbon nanostructures for high-performance supercapacitors over the past couple of years. For example, Kim et al. [13] prepared graphene nanomeshes with controlled pore size and pore density distribution through a scalable catalytic carbon gasification process. Duan et al. [14] designed carbon nanofiber webs composed of wellconnected porous ultrathin carbon nanobubbles using electrospun ZnO nanofibers as hard templates and ethanol as a carbon source. The porous carbons with enlarged specific surface areas indeed have an enhanced capacitance compared to their solid counterparts [13,14]. However, in the ideal case in which the entire surface (2630 m2 g
1) is involved, the theoretical capacity of graphene is only around 550 F g
1 [15]. It is found that doping with heteroatoms (e.g., B, N, S, P) can

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X. Zhao et al. / Journal of Alloys and Compounds 808 (2019) 151737

tailor the surface chemical functionalities and electronic structure of carbon materials for charge storage [16e20]. The incorporation of nitrogen, a typical heteroatom, into carbons can contribute to the additional faradaic pseudocapacitance (which is dominated by reversible surface redox reactions) and improve the electrical conductivity and wettability of the electrode; this in turn would elevate the electrochemical performance of supercapacitors [21,22]. Song et al. [23] demonstrated that nitrogen-enriched porous CNFs possessed a higher specific capacitance, a better rate capability, and a more excellent cycling stability than the corresponding undoped sample. Besides, Shao et al. [24] reported that the as-obtained freestanding nitrogen-doped porous CNFs (NPCNFs) derived from the electrospun polyacrylonitrile/polyaniline (PAN/PANi) core-shell nanofibers had superior electrochemical properties over the pristine CNFs as flexible supercapacitor electrodes. In general, nitrogen-doped carbons are fabricated via posttreatment or in-situ methods [25]. In the post-treatment approach, nitrogen-containing groups like ammonia, urea, and nitrogen plasma usually need to be attached to carbons at the final stage, leading to an uneven nitrogen distribution and a relatively low nitrogen content on the surface of carbon substrates [26e28]. For the in-situ method, the frequently-used nitrogen sources such as PAN, PANi, and polypyrrole (PPy) are directly converted to nitrogen-doped carbon materials with the homogeneous doping of nitrogen atoms through a simple high-temperature pyrolysis process. This is a more facile technique compared to the posttreatment approach because it can use various easily-available nitrogen precursors [29e31]. In addition to the above PAN, PANi, and PPy, branched polyethylenimine (bPEI) is an alternative nitrogen precursor that is used in the in-situ method owing to the high level of nitrogen atoms in its backbone. Moreover, bPEI can be grafted onto PAN via the reactions between the amino groups (-NH2) from bPEI and the cyano (-C^N) or ester groups (-C (¼O)eOCH3) from PAN, which generates amidino (-C (¼NH)eNH-) and amide bonds (-C (¼O)eNH-) [32,33]. This will result in the formation of high-quality carbon materials with more nitrogen doping and uniform nitrogen distribution after thermal treatment. However, to the best of our knowledge, there has been no report on the preparation of nitrogen-doped carbon fibers using bPEI as a nitrogen source to date for supercapacitor electrodes. Herein, for the first time, the electrospun PAN@bPEI composite nanofibers are chosen as precursors to fabricate nitrogen-doped CNFs (NCNFs) that can be utilized directly in flexible supercapacitors. 2. Experimental 2.1. Materials and methods Polyacrylonitrile (PAN, Mw ¼ 150000) and branched polyethylenimine (bPEI, Mw ¼ 25000) were purchased from SigmaAldrich. N,N-dimethylformamide (DMF) and ethylene glycol were provided by Sinopharm Chemical Reagent Co., Ltd. All the chemical reagents were used as bought or received without any further purification or treatment. The production process of NCNFs is illustrated in Scheme 1. First, the PAN@bPEI composite nanofibers were fabricated through electrospinning and post-treatment methods [33,34]. A spinning solution was obtained by dissolving 0.5 g of PAN powder in 4.5 g of DMF under vigorous stirring at 60  C for ~3 h. The typical conditions used for electrospinning PAN nanofibers are listed as follows: applied voltage 18 kV, collecting distance 18 cm, and solution feed rate 12 mL min
1. The resultant PAN fibrous membranes (~60 mg) were then immersed in 20 mL of ethylene glycol solution containing 200 mg of bPEI, which was refluxed in an oil bath with magnetic

Scheme 1. Schematic of the preparation process for nitrogen-doped carbon nanofibers.

stirring at 140  C for 8 h. The yellow PAN@bPEI nonwovens were washed with deionized water and absolute ethanol for several times before dried in an oven (40  C). In order to prepare NCNFs, PAN@bPEI composites were subjected to a pre-oxidation (air, 230  C, 2 h) and carbonization procedure (Ar, 800  C, 2 h) in a tube furnace. 2.2. Characterization The morphology and nanostructure of the samples were observed using scanning electron microscopy (SEM; Phenom Germany Pro SEM), field-emission SEM (FESEM; FEI Nova NanoSEM), and transmission electron microscopy (TEM; FEI Tecnai G2 F20). Xray diffraction (XRD) analysis was performed with a powder diffractometer (PANalytical B.V. Empyrean) using Cu Ka radiation. A Horiba LabRAM HR Evolution apparatus with a 633 nm laser as the excitation source was involved in recording the Raman spectra. The chemical composition and surface electronic state of the CNFs and NCNFs were further confirmed by an X-ray photoelectron spectroscopy (XPS; Thermo Scientific ESCALAB 250XI). 2.3. Electrochemical measurements A conventional three-electrode system composed of a working electrode, a platinum foil counter electrode and a Hg/HgO reference electrode was adopted to evaluate the electrochemical performance of the samples, which was measured in 3 M KOH aqueous electrolyte with a CHI 660e electrochemical workstation (Shanghai Chenhua instrument Co., Ltd.). The single working electrode was prepared by coating and pressing an ethanol slurry containing active material (~8 mg), carbon black and polytetrafluoroethylene (PTFE) at a mass ratio of 8:1:1 onto the nickel foam current collector (1  1 cm2), and then it was dried at 40  C for 10 h in an oven. The cyclic voltammetry (CV) curves were acquired at various scan rates spreading from 10 to 200 mV s
1, and the galvanostatic charge/ discharge (GCD) experiments were carried out by changing the current density from 1.0 to 20.0 A g
1 in the potential window of
0.8e0.2 V. The electrochemical impedance spectra (EIS) were tested over the frequency range of 105e0.01 Hz with an amplitude of 5 mV. For the fabrication of sandwich-like symmetric supercapacitor, first, the NCNFs membranes were cut into small squares of 1  1 cm2 and then directly pressed onto nickel foam to form binder-free electrodes. A poly (vinyl alcohol) (PVA)/KOH sol electrolyte was chosen here. In detail, 1 g of PVA and 1 g of KOH were mixed into 10 mL of deionized water under constant stirring at

X. Zhao et al. / Journal of Alloys and Compounds 808 (2019) 151737

85  C until the solution became clear. After the polymer sol electrolyte was cooled down to room temperature, we poured a portion of this solution into a petri dish where the excess water was allowed to evaporate, generating a solidified PVA/KOH gel that was tailored into pieces with the same size as electrodes. The two binder-free NCNFs electrodes were immersed in the remaining sol electrolyte overnight and then assembled face-to-face with the above PVA/KOH gel as separator. The electrochemical behavior of the symmetric device, including specific capacity, energy density, power density, and cycling durability was investigated through GCD measurements with a potential window of 0e1.2 V using both the CHI 660e workstation and Land Battery workstation (Wuhan Land Instrument Company, China). The specific capacitance (C) was determined by the GCD profiles according to the following equation (1):

C ðF g
1 Þ ¼

I Dt mDV

(1)

where I (A), Dt (s), DV (V), and m (g) are the applied current, discharge time, potential window, and the total mass of the electrode active materials, respectively. The energy density (E) and power density (P) of the supercapacitor were calculated from equations (2) and (3) below:

E ðWh kg
1 Þ ¼ P ðW kg
1 Þ ¼

Ccell DV 2 2  3:6

3600E Dt

(2)

(3)

where Ccell (F g
1) represents the specific capacitance of the twoelectrode device calculated from equation (1), DV (V) is the operating voltage, and Dt (s) is the discharge time of the assembled symmetric cell. 3. Results and discussion Figs. S1A and S1B show the scanning electron microscopy (SEM)

3

images of PAN and PAN@bPEI nanofibers. By comparison, a smoother surface is observed for pure PAN fibers. The typical SEM images of CNFs and NCNFs (Fig. 1AeB) demonstrate that the resultant NCNFs with rough surface possess a larger fiber diameter compared to the CNFs due to the coating of nitrogen-doped carbon layer derived from bPEI, which can also be deduced from the diameter distributions of their precursor fibers (Figs. S1C and S1D). Both the two carbonized nanofibers are randomly oriented and uniform in size. As the inset of Fig. 1B exhibits, the NCNFs membrane can be bent mechanically without any breakage, suggesting that it has good flexibility [35,36]. The stress-strain curves (Fig. S3) also indicate that NCNFs have a higher tensile strength and a larger elongation at break than the pristine CNFs. The transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images of NCNFs (Fig. 1CeD) show that the spacing between the adjacent lattice fringes is 0.34 nm wide, which corresponds to the standard value of d002 for graphite carbon. The well-defined diffraction ring in the selected area electron diffraction (SAED) pattern (Fig. 1E) can be indexed to the characteristic (002) plane of NCNFs. X-ray diffraction (XRD) analysis (Fig. 2A) was implemented to verify the crystal natures of the carbon-based samples. Both CNFs and NCNFs reveal two broad diffraction peaks centered at around 24 and 44 , which are attributed to the (002) and (100) faces of graphite carbon, respectively; this is in complete agreement with the relevant HRTEM and SAED data. Fig. 2B describes the Raman spectra of the two materials. Double dominating peaks located at 1330 and 1588 cm
1 are associated with the structure defeatinduced D band and crystalline G band. In contrast, much sharper D and G peaks coupled with an obvious 2D band at ~2710 cm
1 are detected for NCNFs, demonstrating its slightly higher degree of graphitization [37]. To further study the element composition and electronic state on the surface of CNFs and NCNFs, we performed X-ray photoelectron spectroscopy (XPS) on the samples, and the results are depicted in Figs. 3 and S2. Fig. 3A is the XPS full spectrum of NCNFs illustrating binding peaks of C, N, and O, which indicates the successful integration of nitrogen atoms into the carbon backbone. The nitrogen content on the surface of NCNFs is estimated to be

Fig. 1. SEM images of (A) CNFs and (B) NCNFs. Inset: optical photograph of NCNFs membrane. (C) TEM, (D) HRTEM, and (E) SAED patterns of NCNFs.

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X. Zhao et al. / Journal of Alloys and Compounds 808 (2019) 151737

Fig. 2. (A) XRD patterns and (B) Raman spectra of the as-prepared CNFs and NCNFs.

Fig. 3. XPS spectra of NCNFs: (A) wide-scan spectrum, (B) C 1s, (C) N 1s, and (D) O 1s regions.

~5.07 at. %, much higher than those on the PAN-derived CNFs carbonized at 800  C under N2 (2.87 at. %) or NH3 (3.3 at. %) flows [38,39]. This signifies that bPEI is an effective nitrogen source to use in the production of nitrogen-doped carbon materials. The highresolution C 1s spectra in Figs. 3B and S2A can be resolved into three peaks at 284.8, 286.0, and 288.8 eV, belonging to CeC/C]C/ CeH, CeO/CeN, and C]O [40], respectively. In general, there are four nitrogen doping forms in the graphitic structure: pyridinic N (398.0 eV), pyrrolic N (399.8 eV), graphitic N (400.9 eV), and oxidized N (402.3 eV) [41], which are all observed in the N 1s fine spectra (Figs. 3C and S2B) via peak-differentiating and imitating analysis. The nitrogen species can not only serve as the faradaic active sites but also facilitate electron transfer in the carbon lattices [42]. In the O 1s region (Figs. 3D and S2C), the three Gaussian peaks are assigned to C]O (531.0 eV), O]CeO (532.3 eV), and surfaceabsorbed water (533.6 eV) [43]. Compared with the core level

spectra of NCNFs, only the N 1s region of CNFs makes a difference, that is, the reduced content of graphitic N, which is responsible for the electron transfer in the carbon lattices [16]. The current-voltage (IeV) characteristics were employed to compare the conductivity of the samples (Fig. S4). The electrochemical performance of CNFs and NCNFs was initially assessed in the three-electrode system by cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) tests. It is quite clear that the integral area of the CV curves (Fig. 4A) for the NCNFs electrode is larger than that for CNFs, manifesting that NCNFs possess a higher specific capacitance at the scan rate of 10 mV s
1 [44]. This conclusion is also validated by the corresponding GCD profiles (Fig. 4B) at a current density of 1.0 A g
1. Specifically, according to equation (1), the capacitance of the NCNFs electrode is calculated to be 192.5 F g
1, which is superior to that of CNFs electrode (93.8 F g
1) thanks to the nitrogen doping-induced

X. Zhao et al. / Journal of Alloys and Compounds 808 (2019) 151737

5

Fig. 4. (A) CV curves at a scan rate of 10 mV s
1 and (B) GCD profiles at a current density of 1.0 A g
1 for the CNFs and NCNFs electrodes. (C) Scan rate-dependent CV curves and (D) current density-dependent GCD profiles for the NCNFs electrode. (E) Changes in specific capacitance and capacitance retention along with the discharge current density. (F) Nyquist plots recorded at an open-circuit voltage. Insets: a magnified view at high frequency and the equivalent circuit diagram.

extra faradaic reaction and rapid electron transfer during the charge/discharge processes. This value is also comparable with or even higher than those of porous nitrogen-doped CNFs (198 F g
1, 1.0 A g
1, 6 M KOH), PANi-based nitrogen-containing CNTs (163 F g
1, 0.1 A g
1, 30 wt% KOH), hierarchical nitrogen-doped porous carbon (198 F g
1, 1.0 A g
1, 6 M KOH), monodispersed nitrogen-doped carbon nanospheres (191.9 F g
1, 0.1 A g
1, 1 M H2SO4), nitrogen-doped carbon microfibers (196 F g
1,

5 mV s
1, 6 M KOH), freestanding porous CNFs (104.5 F g
1, 0.2 A g
1, 0.5 M H2SO4), nitrogen-doped hollow CNFs (197 F g
1, 0.2 A g
1, 6 M KOH), and cross-linked nitrogen-enriched porous CNFs (163 F g
1, 1.0 A g
1, 6 M KOH), etc. (Table 1) [29,45e52], revealing that our NCNFs may have good potential for application in supercapacitors. The CV curves at various scan rates and GCD profiles at different current densities were provided to gain a better understanding of

Table 1 Comparison of specific capacitance with other reported nitrogen-doped carbon materials. Materials Porous nitrogen-doped CNFs PANi-based nitrogen-containing CNTs Hierarchical nitrogen-doped porous carbon Nitrogen-doped carbon nanospheres Nitrogen-doped carbon microfibers Freestanding porous CNFs Nitrogen-doped hollow CNFs Cross-linked nitrogen-enriched porous CNFs Nitrogen-doped activated carbon NCNFs

Testing method 3-electrode 3-electrode 3-electrode 3-electrode 3-electrode 3-electrode 3-electrode 3-electrode 3-electrode 3-electrode

Electrolyte 6 M KOH 30 wt% KOH 6 M KOH 1 M H2SO4 6 M KOH 0.5 M H2SO4 6 M KOH 6 M KOH 2 M KOH 3 M KOH

Current density
1

1.0 A g 0.1 A g
1 1.0 A g
1 0.1 A g
1 5 mV s
1 0.2 A g
1 0.2 A g
1 1.0 A g
1 0.5 A g
1 1.0 A g
1

C (F g
1)

Ref.

198 163 198 191.9 196 104.5 197 163 156.4 192.5

[29] [45] [46] [47] [48] [49] [50] [51] [52] This work

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X. Zhao et al. / Journal of Alloys and Compounds 808 (2019) 151737

the electrochemical behaviors of NCNFs. As shown in Fig. 4C, all the CV curves exhibit rectangle-like shapes in the potential window of
0.8e0.2 V, which remains almost unchanged with a 20-fold increase in the scan rate from 10 to 200 mV s
1; this indicates the excellent rate capability of the NCNFs electrode. The related current density-dependent GCD profiles (Fig. 4D) are nearly linear with the discharge branches symmetrical to their charge counterparts, further testifying the ideal capacitive feature and good electrochemical reversibility of the electrode. Fig. 4E displays the specific capacitance and capacitance retention of CNFs and NCNFs electrodes at diverse current densities. It can be seen that approximately 42.9% of the initial capacity is retained for the NCNFs electrode when the discharge current density is increased 20 times from 1.0 to 20.0 A g
1; this is higher than the value obtained for the CNFs electrode (37.5%). The improved rate capability of NCNFs may be owing to the nitrogen doping and the rough surface of the fiber, which guarantee the rapid electron transfer and easy ion diffusion, respectively [53,54]. The electrochemical impedance spectra (EIS) simulated with an equivalent circuit are present in Fig. 4F, where Rs, Rct, W, C1, and C2 stand for the inherent series resistance, charge-

transfer resistance, Warburg impedance, EDLC and pseudocapacitance, respectively. The linear part of the CNFs and NCNFs electrodes are nearly coincident in the low-frequency region, suggesting that the two systems have the same diffusion resistances [55]. It is worth noting that the internal resistance of the NCNFs electrode is slightly lower than that of CNFs as evidenced by the point that intersects with the real axis at high frequency; this implies a faster electron transfer and matches well with the rate capability. With the aim of developing flexible supercapacitors for practical application, we assembled a quasi-solid-state symmetric device using PVA/KOH gel electrolyte to separate two binder-free NCNFs electrodes that were prepared by pressing the NCNFs membranes directly onto nickel foams (Fig. 5A). The optimal working voltage window was confirmed to be 0e1.2 V by slowly extending the range of the scanning voltage until a spike appeared at the boundary of the curve (Fig. 5B). As depicted in Fig. 5CeD, both the rectangle-like CV profiles at various scan rates (10e200 mV s
1) and the nearly triangular GCD curves at diverse current densities (0.5e10.0 A g
1) demonstrate the capacitive behavior and good electrochemical

Fig. 5. (A) Schematic of the sandwich-type symmetric device, (B) CV curves (50 mV s
1) at different operating potential windows, (C) CV curves at diverse scan rates (10e200 mV s
1) and (D) GCD profiles at various current densities (0.5e10.0 A g
1) with a potential window of 0e1.2 V, (E) Ragone plot, inset: a red LED indicator lit by two supercapacitors in series, and (F) cycling performance of the assembled two-electrode cell at 2.0 A g
1. The inset shows the first and last 5 cycles of the GCD curves. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

X. Zhao et al. / Journal of Alloys and Compounds 808 (2019) 151737

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Fig. 6. Flexibility of the assembled supercapacitor: (A) CV profiles (10 mV s
1) under various bending states and (B) specific capacitance and capacitance retention as functions of bending angles. The inset illustrates the GCD curves (1.0 A g
1) at different bending levels.

reversibility of the NCNFs device. The specific capacitance of the two-electrode cell is as high as ~40.0 and 37.1 F g
1 at discharge current densities of 0.5 and 1.0 A g
1, respectively. Fig. 5E is the Ragone plot describing the relationship between energy density and power density. The maximum energy density of the fabricated supercapacitor can reach up to ~8.0 W h kg
1 at a power density of 300 W kg
1, which should be further improved, but it has been enough to illuminate a commercial light-emitting diode (LED) indicator through two devices (1  1 cm2) connected in series (inset in Fig. 5E). Long-term cycling property is another important aspect for supercapacitors [54,56,57]. After 1000 charge/discharge cycles at a current density of 2.0 A g
1, the specific capacitance remains nearly constant, and ~84.5% of the capacitance retention is observed for the device, reflecting its favorable electrochemical stability. In order to evaluate the capability of our supercapacitor to withstand harsh bending or shape deformation, we carried out a series of flexibility tests, bending it at angles ranging from 45 to 180 . As can be found from the relevant CV and GCD curves in Fig. 6AeB, there is almost no significant change in the capacitive behavior when the device is gradually bent from its normal state to different angles. It is calculated that the assembled supercapacitor only delivers a slight capacitance decrease of 8.1% at a high bending angle of 180 , revealing its remarkable mechanical flexibility.

4. Conclusions In summary, we demonstrated, for the first time, a simple method by which to prepare flexible NCNFs using PAN@bPEI composite nanofibers as the carbon precursor and nitrogen source. The obtained NCNFs electrode exhibits a higher specific capacitance of 192.5 F g
1 (1.0 A g
1) and better rate capability than the conventional individual CNFs electrode due to the nitrogen dopinginduced additional faradaic reaction and rapid electron transfer. A quasi-solid-state symmetric supercapacitor was assembled without any adhesive or conductive agent, which shows satisfactory electrochemical performance as well as good mechanical stability. All these are indicative of the fact that bPEI could have great potential for application as an effective nitrogen source in energy storage devices.

Acknowledgements This work was supported by the research grants from China Postdoctoral Science Foundation (2018M630745), Natural Science Foundation of Shandong Province, China (ZR2019BEM001), and National Natural Science Foundation of China (51673103).

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