O co-doped hierarchical porous carbon nanofiber self-standing film with high volumetric and gravimetric capacitance performances for aqueous supercapacitors

Journal Pre-proof B/P/N/O co-doped hierarchical porous carbon nanofiber self-standing film with high volumetric and gravimetric capacitance performances for aqueous supercapacitors Yongjun Ma, Xugang Zhang, Zhuo Liang, Chenlong Wang, Yan Sui, Bing Zheng, Yuncheng Ye, Weijing Ma, Qi Zhao, Chuanli Qin PII:

S0013-4686(20)30192-4

DOI:

https://doi.org/10.1016/j.electacta.2020.135800

Reference:

EA 135800

To appear in:

Electrochimica Acta

Received Date: 2 December 2019 Revised Date:

18 January 2020

Accepted Date: 27 January 2020

Please cite this article as: Y. Ma, X. Zhang, Z. Liang, C. Wang, Y. Sui, B. Zheng, Y. Ye, W. Ma, Q. Zhao, C. Qin, B/P/N/O co-doped hierarchical porous carbon nanofiber self-standing film with high volumetric and gravimetric capacitance performances for aqueous supercapacitors, Electrochimica Acta (2020), doi: https://doi.org/10.1016/j.electacta.2020.135800. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

B/P/N/O co-doped hierarchical porous carbon nanofiber self-standing film with high volumetric and gravimetric capacitance performances for aqueous supercapacitors Yongjun Maa,b, Xugang Zhangc, Zhuo Lianga,b, Chenlong Wanga,b, Yan Suia,b, Bing Zhenga,b, Yuncheng Yea,b, Weijing Maa,b, Qi Zhaoa,b, Chuanli Qina,b,* a

School of Chemistry and Materials Science, Heilongjiang University, Harbin

150080, China b

Key Laboratory of Chemical Engineering Process & Technology for

High-efficiency Conversion, College of Heilongjiang Province, Harbin 150080, People’s Republic of China c

Department of adhesives, Heilongjiang Institute of Petrochemistry, Harbin

150040, China

*

Corresponding authors.

E-mail addresses: [email protected]

ABSTRACT It is very challenging for carbon materials to gain high volumetric capacitance (Cv) without sacrificing gravimetric capacitance (Cg) as well as high rate and cycling performances to meet requirements for supercapacitors with limited space. Here, 1

B/P/N/O co-doped hierarchical porous carbon nanofiber self-standing film was fabricated by a facile electrospinning and one-step carbonization/activation method using polyacrylonitrile solution containing boric acid and phosphoric acid. The optimized BPNOCNF-45 has high content (24.65 at.%) of heteroatoms, especially the highest content (12.93 at.%) of active heteroatom species (N-5, N-6, B-C, O-I, P2 and P3), high bulk density (1.19 g cm-3), high meso-/macropore ratio (70%), low charge transfer and ion diffusion resistance. These contribute to its prominent Cv (395 F cm-3 at 1 A g-1) while maintaining high Cg (332 F g-1) and excellent rate performance in KOH electrolyte. By comparison, the assembled symmetric supercapacitor in Na2SO4 electrolyte delivers remarkably higher volumetric/gravimetric energy densities (21.1 Wh L-1 at 523.5 W L-1, 17.7 Wh kg-1 at 439.9 W kg-1) and capacitance retention of 90% after 10000 cycles. Furthermore, the assembled flexible supercapacitor also shows excellent capacitance performances. Such a densely co-doped porous carbon nanofiber film has great potential for applications in compact and miniaturized supercapacitors.

Keywords: B/P/N/O

co-doping;

Carbon

nanofiber

film;

Meso-/macropore

structure;

Volumetric/gravimetric capacitance performance; Supercapacitor

1. Introduction Supercapacitors are considered as the most promising energy storage devices in 21st century due to high power density, long cycle life and fast

2

charge/discharge

performance

[1-3].

With

the

rapid

development

of

miniaturized, portable and wearable electronic devices as well as electric vehicles, supercapacitors with high volumetric capacitance performance are increasingly being needed [4,5]. Carbon materials are the commercially used electrode materials for supercapacitors and considered to be the most promising due to low price, high surface area (typically larger than 2000 m2 g-1), controllable pore structure and good physical/chemical properties. Therefore, developing carbon materials with not only high gravimetric capacitance performance, but also high volumetric capacitance performance is of extreme importance for promising industrial supercapacitor application [6-9]. As we know, volumetric capacitance is closely related to gravimetric capacitance and density. High-density carbon materials always contain a small number of pores, which affects the diffusion of electrolyte ions and results in a low gravimetric capacitance. At the same time, high gravimetric capacitance requires that carbon materials are porous, resulting in a low density [10]. Thus, there still remains a challenge for carbon materials to gain high volumetric capacitance without sacrificing gravimetric capacitance. To overcome these limitations, it is essential to introduce pseudocapacitance and in the meantime tune pore structure [11,12]. Doping heteroatoms on the surface or in the bulk of carbon has been considered to be a better strategy to improve the capacitance performance mainly by introducing efficient pseudocapacitance. In particular, doping heavier heteroatoms in carbon is 3

expected to achieve high volumetric capacitance without sacrificing gravimetric capacitance because doping heavier heteroatoms can enhance the density of carbon without obviously reducing the number of pores and increase the amount of electrochemically active functional groups on the surface [13,14]. It's worth mentioning that different heteroatoms in carbon have individual effects for improving capacitance performances. For example, N and O heteroatoms can effectively contribute pseudocapacitance to the carbon surface through introducing

electron-acceptor/electron-donor

properties

and

regulating

hydrophilic property of carbon [15]. B-doping will modify the electronic structure of carbon, facilitate charge transfer and enhance the electrochemical performance [16]. P heteroatom with a lower electronegativity and large atom size poses a unique improving effect by changing the local charge density, creating more structural disorders, blocking the nonstable active oxidation sites and widening the potential window [17,18]. However, mono-heteroatom doping with limited content is less favorable for the overall electrochemical performance than multi-heteroatom doping with individual property and synergistic effect. Furthermore, not all heteroatoms, but active heteroatom species (such as N-5, N-6, B-C, O-I, P2 and P3) are beneficial for the improvement of capacitance performance [19-22]. Therefore, it can be deduced that simultaneously and effectively incorporating B/P/N/O heteroatoms into carbon could improve volumetric capacitance without sacrificing gravimetric capacitance of carbon materials. Nevertheless, to the best of our knowledge, 4

B/P/N/O co-doped carbon has not been investigated and the contributions of different B, P, N, and O species on the capacitance performance of carbon materials have seldom been reported. Tuning pore structure can also increase gravimetric capacitance and density of carbon material, thus improve volumetric capacitance performance [23]. The meso-/macropores could serve as ion-buffering reservoirs and ion transport path ways, resulting in a decreased diffusion distance, and the micropores provide a high surface area for double-layer capacitance to obtain high charge storage [24]. Regrettably, the carbon materials with high gravimetric capacitance for supercapacitors always have high proportion of micropores, low bulk density (lower than 0.8 g cm−3) and sluggish ion diffusion, which drastically decrease volumetric capacitance and rate performances [23,25]. In fact, we can deduce that the hierarchical porous carbon with high proportion of meso-/macropores promises high volumetric capacitance and rate performances. Fortunately, it has been reported that introducing boric acid and phosphoric acid in precursors could promote the formation of micropores and meso-/macropores during carbonization in the resulting carbon materials, respectively [26,27]. In other words, boric acid and phosphoric acid act as bifunctional molecules of supplying heteroatoms and tuning pores. Therefore, it is particularly meaningful and

desired

to

fabricate

the

hierarchical

porous

carbon

with

high

meso-/macropore ratio by the simply process of directly introducing moderate amount of boric acid and phosphoric acid as bifunctional molecules in 5

precursors and then carbonizating precursors. The carbon material is highly desired as an ideal candidate for highly efficient supercapacitors with high volumetric and gravimetric capacitances as well as high rate and cycling performances to meet practical requirements. Furthermore, compared to common multiple steps for doping heteroatoms and activating/tuning pores, the one-step carbonization/activation method is especially suitable for mass production and industrialization. However, the design strategy of introducing bifunctional molecules of boric acid and phosphoric acid to fabricate carbon materials with high volumetric and gravimetric capacitance performances is rarely reported. Recently, the flexible self-standing carbon nanofiber film with 3D network structure, excellent electrical conductivity, mechanical stability and bending performance has been attracting much attention in the field of supercapacitors [28-30]. The self-standing film can not only be directly assembled into supercapacitors and simplify the preparation process but also effectively avoid adding ancillary materials (conductive agent or binder) in traditional slurry-derived electrode preparation, resulting in blockage of active sites and a large contact resistance. Although porous PAN-based carbon nanofibers with high gravimetric capacitance have been fabricated via electrospinning PAN solution due to the high carbon yield, high nitrogen content and spinning ease of PAN, their high specific surface area with abundant micropores limits the volumetric capacitance and rate capability [31-34]. Based on the above considerations, it is particularly promising to introduce the facile and efficient 6

design strategy of bifunctional molecules into PAN electrospinning solution to develop B/P/N/O co-doped hierarchical porous carbon nanofiber self-standing film with high volumetric and gravimetric capacitance performances. Herein, we use PAN solution containing bifunctional molecules of boric acid and phosphoric acid to successfully fabricate B/P/N/O co-doped hierarchical porous carbon nanofiber self-standing film with superior bending performance through a facile electrospinning and one-step carbonation/activation method. In particular, the proportion of meso-/macropores in co-doped carbon nanofiber film is significantly increased by introducing phosphoric acid. As expected, the optimized BPNOCNF-45 simultaneously exhibits prominent volumetric capacitance of 395 F cm-3 at 1 A g-1 while maintaining high gravimetric capacitance of 332 F g-1, and excellent rate performance due to its high content of heteroatoms, especially the highest content of active heteroatom species (N-5, N-6, B-C, O-I, P2 and P3), high bulk density, suitable pore structure with high meso-/macropore

ratio

of

70%.

Furthermore,

both

BPNOCNF-45//BPNOCNF-45 supercapacitor and its flexible supercapacitor deliver high volumetric and gravimetric energy-power outputting performances and capacitance retention in KOH and Na2SO4 electrolytes. Thus, such a co-doped carbon nanofiber self-standing film has great potential for applications in compact and miniaturized supercapacitors. And the research also provides a facile and efficient synthesizing strategy with modifying dynamic meso-/macroporous architectures and doping multi-heteroatom for carbon 7

materials with high comprehensive performances for space-limited energy storage applications.

2. Experimental 2.1. Preparation of B/P/N/O co-doped carbon nanofiber film

The preparation process of samples is shown in Fig. 1. 2.1 g of polyacrylonitrile (PAN, Mw=150000, Shanghai Macklin Biochemical Co., Ltd.) was dissolved into 25 ml of N, N-dimethyl formamide (DMF, Tianjin Fuyu o

Fine Chemical Co., Ltd.) under 80

C and stirred for 2 h to form a

homogeneous solution. Then, different amounts of boric acid (H3BO3, Tianli Chemical Reagent Co., Ltd.) and phosphoric acid (H3PO4, Tianjin Yaohua Technology Development Co., Ltd.) in molar ratio of 3:2 were added to the solution and stirred for 3 h. PAN nanofiber film was fabricated from the mixed PAN solution by the electrospinning method. Specifically, the electrospun PAN nanofiber film was fabricated using a stainless steel needle with the inner diameter of 0.6 mm under an applied voltage of 23 kV at a flow rate of 0.5 ml h-1. The collecting aluminum foil substrate was fixed at 20 cm away from the needle. The as-collected PAN nanofiber film was firstly heated from room temperature to 180 oC at a rate of 10 oC min-1 and then pre-oxidized at 180 oC (1.5 h), 190 oC (1.5 h), 200 oC (0.6 h), 210 oC (0.6 h), 220 oC (0.6 h), 230 oC (0.6 h) and 240 oC (0.6 h). Then, the nanofiber film was heated at a rate of 10 8

o

C min-1 up to 900 oC and held for 2 h in a tubular furnace under N2 atmosphere.

Finally, the sample was boiled in deionized water at 80 oC for 6 h and dried under vacuum at 60 oC for 12 h to remove the residual boron oxides and phosphoric oxides [26,35]. The obtained B/P/N/O co-doped carbon nanofiber film was named as BPNOCNF-X, where X is the mass ratio of H3BO3 and H3PO4 to PAN. For comparison, N/O co-doped carbon nanofiber film was prepared by PAN solution without H3BO3 and H3PO4, and denoted by NOCNF. B/N/O co-doped carbon nanofiber film was prepared by PAN solution with 30 wt% H3BO3 and denoted by BNOCNF-30. (Fig. 1)

2.2. Preparation of electrodes and symmetric supercapacitors

About 2 mg of carbon nanofiber film with the size of 1.0 cm*1.0 cm was placed between two pieces of nickel-foam (Changsha Liyuan New material Co., Ltd.) and then pressed with a double roller machine to form a mechanically robust electrode. Symmetric

supercapacitors

were

assembled

by

laying

polypropene

(PPAT-CN1, Shanghai Shilong Science and Technology Co., Ltd.) battery separator or cellulose battery separator (TF 4030, NKK) between two electrodes, fixing it by organic glass plates and PTFE screws, and vacuum filling it with 6 M KOH or 1 M Na2SO4 electrolytes. Furthermore, by the same 9

process flexible supercapacitors were assembled with TF 4030 cellulose battery separator and encapsulated by polyethylene terephthalate film with 1 M Na2SO4 electrolytes. The photo of the flexible supercapacitor is shown in Fig. S5a.

2.3. Characterization

Scanning electron microscopy (SEM, Zeiss, Gemini SEM 300, Germany) and transmission electron microscopy (TEM, Hitachi, H7650, Japan) were applied to observe the morphology and microstructure of samples. Energy dispersive X-ray spectroscopy (EDS) was carried out with the same TEM. The phase structure was investigated by X-ray diffraction (XRD, Bruker, D8 ADVANCE, Germany) analysis with Cu Kα, 40 kV, 30 mA, 5°-80°. Raman spectra were analyzed by a micro-raman spectrometer (Roman, Jpbin Yvon, HR800, French) at 457.9 nm. Near-infrared diffuse reflectance spectroscopy (NIRDRS,

Bruker,

Equinox

55,

Germany)

and

X-ray

photoelectron

spectroscopy (XPS, Thermo Fisher Scientific, ESCALAB 250, UK) were applied to confirm the compositions of samples and elemental chemical states on the surface. The specific surface area and the porous structure parameters of samples were characterized by N2 adsorption/desorption measurements (Micromeritics, ASAP 2020 HD88, USA) at 77K. Brunauer-Emmett-Teller (BET) method was used to determine the specific surface area and density functional theory (DFT) method was used to analyze the pore size distributions 10

from adsorption branch isotherm with the assumption of slit-shaped pore model. The electrical conductivity of samples was investigated by Four-probe meter (RTS-9, Guangzhou four probe technology co., LTD.).

2.4. Electrochemical measurements

Electrochemical measurements were carried out in the three-electrode system and two-electrode system, respectively. In a three-electrode system, the platinum electrode, Hg/HgO electrode and measured electrode were used as the auxiliary electrode, reference electrode and working electrode, respectively. The electrochemical measurements were carried out at the room temperature in 6 M KOH electrolyte. LK98B II computer electroanalytical system was used for cyclic voltammetry (CV) tests with a potential window from -1.0 to 0 V. Electrochemical impedance spectroscopy (EIS) measurements were carried out with an electrochemical workstation (Shanghai Chenhua, CHI660E, China) at an open-circuit potential of -0.1 V and the amplitude of 10 mV in the frequency range from 10 mHz to 100 kHz. Galvanostatic charge/discharge (GCD) tests were carried out with the same electrochemical workstation as EIS and the gravimetric capacitance was calculated according to the following Eq. (1). ×∆


 = ×∆ (1)

11

where Cg (F g-1) is gravimetric capacitance of electrode materials, I (A) is the current density, ∆t (s) is the discharge time, ∆V (V) is the potential difference excluding the IR drop in the discharge process, m (g) is the mass of samples on the electrode. The volumetric capacitance was calculated based on the following Eq. (2-3) [36,37]. =

  

 

(2)


=
  (3) where ρ (g cm-3) is the density of electrode materials, Vtotal (cm3 g-1) is the total pore volume of electrode materials calculated at P/P0=0.99 by N2 isotherm, ρcarbon (2 g cm-3) is the true density of carbon materials and Cv (F cm-3) is volumetric capacitance of electrode materials. In a two-electrode system (symmetric supercapacitor), a battery programmed test instrument (Landian, CT2001A, China) was used to perform GCD measurements at different current densities. The gravimetric capacitance and volumetric capacitance of the supercapacitor were calculated according to the above Eq. (1-3). Its power density and energy density were calculated according to the following Eq. (4-7):

, =

,

=

 , =

, ×∆  .

(4)

!, ×"#$$ ∆ %, ×∆  .

(5)

(6) 12

,

=

!%, ×"#$$ ∆

(7)

where Cg,2 (F g-1) and Cv,2 (F cm-3) are the gravimetric and volumetric capacitances of the supercapacitor, Pg,2 (W kg-1) and Pv,2 (W L-1) are the gravimetric and volumetric power densities of the supercapacitor, Eg,2 (Wh kg-1) and Ev,2 (Wh L-1) are the gravimetric and volumetric energy densities of the supercapacitor.

3. Results and discussion The morphology and microstructure of samples were confirmed by SEM and TEM images (Fig. 2 and Fig. S1). As shown in Fig. 2a and Fig. S1a-f, all the samples show the 3D weaving network structure made up of overlapped nanofibers with the diameter of about 300 nm, which is advantageous to the electron transport and electrolyte infiltration. As shown in Fig. S1a, NOCNF prepared without H3BO3 and H3PO4 treatments presents smooth fiber surface. As expected, after H3BO3 or H3BO3/H3PO4 treatments the surface of fibers becomes rough and as the amount of H3BO3/H3PO4 increases, samples exhibit an increase in surface roughness with the porous architecture (Fig. S1b-e). However, BPNOCNF-50 prepared with excess amount of H3BO3/H3PO4 treatment (50 wt%) shows the broken and uneven fibers (Fig. S1f). As shown in Fig. 2b and 2c, BPNOCNF-45 prepared with 45 wt% H3BO3/H3PO4 treatment shows

the

loose

and

interconnected 13

pore

structure

with

abundant

meso-/macropores

on

the

surface

and

inside

of

nanofibers.

The

meso-/macropores can serve as ion-buffering reservoirs, giving a decreased diffusion distance. (Fig. 2)

The HRTEM image (Fig. 2d) reveals the (002) plane of graphitic carbon with interlayer spacing of 0.41 nm and uniform micropore structure composed of interconnected and worm-like micropores with short pore length, which provide a high surface area for double-layer capacitance to obtain high charge storage. These hierarchical pores are formed due to the fact that interactions among H3BO3, H3PO4 and PAN during preoxidation and carbonization result in the release of volatile compounds and subsequently removal of residual boron oxides and phosphoric oxides by washing process. For detecting the surface elemental compositions and distributions, dark-field TEM and EDS maps of BPNOCNF-45 are shown in Fig. 2e and 2f. It can be observed clearly that B/P/N/O heteroatoms are distributed uniformly on the surface of nanofibers by the facile electrospinning and one-step carbonization/activation method. Detailed porous texture parameters and heteroatom compositions of samples will be investigated by the following nitrogen adsorption/desorption and XPS measurements. XRD and Raman spectroscopy techniques were employed to confirm the microstructural change upon H3BO3 and H3PO4 treatments. As shown in XRD patterns (Fig. 3a), two broad diffraction peaks at around 22o and 43o appear in all samples, which are attributed to the (002) and (101) diffractions from the 14

order graphitic phase of their amorphous nature [38]. It shows that H3BO3 or H3BO3/H3PO4 treatments do not cause the significant change on the crystalline structure of samples. By comparison, the (002) diffraction peaks of samples prepared with H3BO3 or H3BO3/H3PO4 treatments shift to left. According to the Bragg formula the graphitic interlayer spacing (d002) values of NOCNF, BNOCNF-30,

BPNOCNF-30,

BPNOCNF-40,

BPNOCNF-45

and

BPNOCNF-50 are 0.394, 0.399, 0.401, 0.404, 0.407 and 0.421 nm, respectively. It may be due to introducing a large amount of heteroatoms into the carbon crystalline structure, causing the increase of d002. Raman spectra of all samples (Fig. 3b) show two characteristic peaks of carbon materials: D peak in 1358 cm-1 (disordered portions and defects) and G peak in 1591 cm-1 (graphitic portions). The intensity ratio of the two peaks (ID/IG) is usually used to evaluate the degree of structural order. Herein, the ID/IG values of NOCNF, BNOCNF-30, BPNOCNF-30, BPNOCNF-40, BPNOCNF-45 and BPNOCNF-50 are 0.96, 0.99, 1.00, 1.03, 1.05 and 1.09, indicating that more defects exist in samples with H3BO3 or H3BO3/H3PO4 treatments. (Fig. 3)

The functional groups and surface chemical compositions of samples were investigated by NIRDRS and XPS. As shown in NIRDRS results (Fig. S2 and Table S1), NOCNF shows five clear bands corresponding to C=O, C=N/C-N, C=C, C-C and C-O stretching vibrations. In comparison, for other three

15

samples with H3BO3 or H3BO3/H3PO4 treatments the new bands at 1018, 1360 and 776 cm-1 reveal B-O stretching vibrations, in-plane stretching vibrations and out-of-plane bending mode of hexagonal boron nitride (h-BN). Furthermore, BPNOCNF-30 and BPNOCNF-45 with H3BO3/H3PO4 treatment show bands at 718, 795-650 and 1200 cm-1 corresponding to P-O-P, P-C, and P-O-C/P=O stretching vibrations. XPS results of samples were shown in Fig. 4, Fig. S3, Table 1 and Table S2. As shown in Fig. 4a, NOCNF contains C, N, and O elements. As comparison, other five samples contain B element or B/P elements besides C, N, and O elements, demonstrating the successful co-doping of B/P/N/O heteroatoms to carbon nanofiber frameworks. The NIRDRS and XPS results are in good agreement with the result of EDS maps (Fig. 2f). Moreover, as shown in Table S2, more and more heteroatoms (B/P/N/O) are co-doped into carbon nanofiber frameworks with H3BO3 treatment or an increase in the amount of H3BO3/H3PO4. As shown in high resolution XPS spectra of BPNOCNF-45 (Fig. 4b-4e), B 1s XPS peak can be fitted with three peaks at 190.5, 191.3 and 192.0 eV, corresponding to the B-C, B-N and B-O bonds [39,40], respectively. P 2p XPS peak consists of three peaks at 132.2, 133.1 and 134.2 eV, attributed to reduced phosphorus compound as C3-P groups (P1), characteristic P atom bonded to one or two C atoms and three or two O atoms as in C-PO3 or C2-PO2 groups (P2) and CO-P type groups as (CO)3PO, (CO)2PO2 and (CO)PO3 (P3) [41], respectively. N 1s peak can be deconvoluted into five peaks at 397.9, 398.5, 16

400.0, 401.2 and 402.0 eV, belonging to N-B bond, pyridinic nitrogen (N-6) and/or N=P bond, pyrrolic nitrogen and pyridonic nitrogen (N-5), quaternary nitrogen (N-Q) and/or N-P bond, N-oxide (N-X) [4,20,35], respectively. O 1s peak is made of four peaks at 530.9, 532.4, 533.4 and 534.2 eV, assigned to C=O including carbonyl functional groups and non-bridging oxygen in the phosphate

groups

(P=O)

(O-I),

singly

bonded

oxygen

(-O-)

in

C-O-C/C-O-H/C-O-P groups (O-II), O-B bond, and carboxylic groups -COOH (O-III) [27,41,42], respectively. The high-resolution peaks for other samples are shown in Fig. S3. The schematic model of B, P, N and O species in carbon materials is shown in Fig. 4f.

(Fig. 4) (Table 1)

Apparently, it can be also verified that B or B/P heteroatoms were successfully doped with H3BO3 or H3BO3/H3PO4 treatments based on N-B/N=P/N-P peaks in the N 1s spectra (Fig. 4d and Fig. S3b-e) and O-B/O-I/O-II peaks in the O 1s spectra (Fig. 4e and Fig. S3g-j). As expected, after H3BO3 or H3BO3/H3PO4 treatments the heteroatom contents of samples become higher and as the amount of H3BO3/H3PO4 increases, samples exhibit an increase of heteroatom contents and the O heteroatom content dramatically increases, as shown in Table S2. Although a large amount of B/P/N/O heteroatoms have been doped into the carbon nanofiber frameworks, but the

17

most important species affecting energy storage performances are the active heteroatom species (N-5, N-6, B-C, O-I, P2 and P3) [19-21]. As shown in Table 1, the five samples with H3BO3 or H3BO3/H3PO4 treatments present remarkably higher active heteroatom contents than NOCNF. Especially, BPNOCNF-45 with 24.65 at.% heteroatoms (as shown in Table S2) has the highest content of active heteroatom species (12.93 at.%). Furthermore, as mentioned above, doping heavier heteroatoms in carbon is expected to achieve high volumetric capacitance without sacrificing gravimetric capacitance. This implies that BPNOCNF-45

will

exhibit

the

best

volumetric

and

gravimetric

pseudocapacitance behaviors. N2 adsorption/desorption isotherms and pore size distribution curves are shown in Fig. 5. From Fig. 5a, it is obviously observed that according to the IUPAC classification NOCNF and BNOCNF-30 show the type I sorption isotherm with the sharp increase at low relative pressure of P/P0 < 0.1 and BNOCNF-30 has a higher adsorption than NOCNF, indicating that a large quantity of micropores are introduced due to H3BO3 treatment. After H3BO3/H3PO4 treatment, BPNOCNF-30, BPNOCNF-45 and BPNOCNF-50 show a combination of type I and type IV sorption isotherms. The curves show a significant increase at relatively low pressure of P/P0 < 0.1, indicating that they have some micropores. The clear hysteresis loops in the P/P0 range from 0.4 to 1.0 and the obvious steep peaks at a higher relative pressure of P/P0 > 0.9 arise, showing the existence of abundant meso-/macropores [41,43]. The pore 18

size distribution curves (Fig. 5b) also verify that the three samples with H3BO3/H3PO4 treatment exhibit the hierarchical pore structure with micropores and considerably increased meso-/macropores. The specific surface area and pore structure parameters of samples are shown in Table 2. As expected, after H3BO3 treatment the specific surface area (SBET) and micropore volume (Vmic) of BNOCNF-30 obviously increase to 844.22 m2 g−1 and 0.39 cm3 g−1, but its meso-/macropore ratio is low (less than 30%). By further introducing H3PO4 treatment, the SBET and total pore volume (Vtotal) of BPNOCNF-30, BPNOCNF-45 and BPNOCNF-50 decrease to a certain degree, but their meso-/macropore ratios are more than 55% and twice and three times as large as those of BNOCNF-30 and NOCNF. This is due to the fact that interactions among H3BO3, H3PO4 and PAN during preoxidation and carbonization result in the release of volatile compounds. Especially, H3PO4 with absorbed water can enlarge the pores to form meso-/macropores, as shown in SEM images (Fig. 2b-d) [21]. The H3BO3/H3PO4 treatment can not only effectively increase the heteroatom content, but also enrich the hierarchical pore structure, especially meso-/macropores. The micropores, newly formed meso-/macropores will play an important role in enhancing the electrical double-layer capacitance and ion transport in the hierarchical pores. Compared to BPNOCNF-30 with almost no micropores and BPNOCNF-50 with broken and uneven fibers (Fig. S1f), BPNOCNF-45 with the uniform 3D weaving network structure, suitable specific surface area and hierarchical pore structure (379.02 m2 g-1 and 0.34 19

cm3 g-1 with high meso-/macropore ratio of 70%) is expected to gain high bulk density, simultaneously exhibit high volumetric double-layer capacitance without reducing gravimetric double-layer capacitance and low ion diffusion resistance.

(Fig. 5) (Table 2)

The electrochemical performance of samples was systematically investigated in the three-electrode system with 6 M KOH electrolyte. Fig. 6a shows the Nyquist curves of samples and fitted curves by Z–view software with the equivalent circuit depicted in the inset. The well fitted curves reflect that the electrochemical process agrees with the equivalent circuit model and the fitted values were summarized in Table S3. The Nyquist curves of Fig. 6a are divided into two parts: one is a semicircle in the high frequency region, associated with the charge transfer resistance (Rct) of the electrolyte ions and electrons, and another is a straight line in the low frequency region, associated with the diffusion and transfer of electrolyte ions in the pores. From the curves, it can be observed that all samples exhibit low solution resistance (Rs) of < 1.0 Ω. This is due to the fact that a large amount of heteroatoms existing on the surface improve the wettability of samples to electrolytes [44]. As shown in Table S3, by comparison, after H3BO3/H3PO4 treatment the Rct and Warburg diffusion resistance (WR) of four samples are lower than those of NOCNF and BNOCNF-30 mainly due to the hierarchical porous carbon nanofiber structure

20

with high proportion of meso-/macropores and abundant heteroatoms, which is especially beneficial to the charge transfer and electrolyte ion diffusion [45]. Furthermore, BPNOCNF-45 shows the smallest Rs and Rct values and low WR (0.067) and it is expected to exhibit excellent electrochemical performance. Furthermore, BPNOCNF-45 exhibits good electrical conductivity (0.27 S cm-1), as shown in Table S4, which is beneficial to the improvement of electrochemical performance. As shown in Fig. 6b, CV curve of NOCNF shows an approximately rectangular shape with slight distortion. By comparison, other samples present wider rectangular shape with obvious humps, indicating the combination of more

double-layer

capacitances

and

pseudocapacitances

coming

from

multi-heteroatoms in the carbon frameworks. It can be observed that the CV curve of BPNOCNF-45 presents the largest encircled area among samples, indicating the highest capacitance. To prove the pseudocapacitive contribution from

heteroatoms,

BPNOCNF-45-1600

was

prepared

by

heat-treating

BPNOCNF-45 under N2 at 1600 oC for 2 h to remove the B/P/N/O heteroatoms. Fig. S4 shows the CV curves of BPNOCNF-45 and BPNOCNF-45-1600 at 20 mV s-1. As expected, after high-temperature treatment, BPNOCNF-45-1600 exhibits a more regularly and narrow rectangular shape without obvious humps, confirming that most of the capacitance for BPNOCNF-45 comes from contribution of heteroatoms. As shown in Fig. 6c, CV curves of BPNOCNF-45 still exhibit the quasi-rectangular shape without obvious distraction when the 21

scanning rate increases from 1 mV s-1 to 200 mV s-1, showing its good rate performance.

(Fig. 6) GCD curves of samples at the current density of 5 A g-1 (Fig. 6d) show the obvious nonlinear characteristic due to the pseudocapacitance contribution from multi-heteroatoms in carbon frameworks. By comparison, as expected, BPNOCNF-45 presents the longest discharge time, also indicating the highest capacitance. GCD curves of BPNOCNF-45 (Fig. 6e) at different current densities display that the charge/discharge time decreases with the increase of current density. Cg and Cv values of samples calculated by Eq. (1) and Eq. (3) at different current densities are shown in Fig. 6f and Fig. 6g. As shown in Fig. 6f, BNOCNF-30 with H3BO3 treatment displays the significantly increased Cg than NOCNF due to the increased SBET and heteroatom content, especially active heteroatom species content. While its capacitance retention rate is only 55% from 1 A g-1 to 30 A g-1, attributed to its low proportion of meso-/macropores, resulting in that some of the micropores cannot be utilized enough at high current density. By comparison, BPNOCNF-45 presents the largest Cg (332 F g-1 at 1 A g-1 and 264 F g-1 at 30 A g-1) among the samples and its Cv values reach 395 F cm-3 and 314 F cm-3 at 1 A g-1 and 30 A g-1, due to the fact that BPNOCNF-45 has high content (24.65 at.%) of heteroatoms, especially the highest content (12.93 at.%) of active heteroatom species (N-5, N-6, B-C, O-I,

22

P2 and P3, as shown in Table 1), suitable surface area and pore structure with high meso-/macropore ratio of 70% (as shown in Table 2), high bulk density (1.19 g cm-3), high electrical conductivity (as shown in Table S4), low charge transfer and ion diffusion resistance (as shown in Table S3). The capacitance retention rate of BPNOCNF-45 reaches 80% from 1 A g-1 to 30 A g-1, mainly attributed to its hierarchical porous structure with high meso-/macropore ratio, which is especially beneficial for rapid infiltration of electrolyte ions into internal pores and rapid storage of charges under high charge/discharge current density. Table 3 shows Cv and Cg comparison of the reported similar carbon materials in the three-electrode system. It can be seen that Cg and Cv of BPNOCNF-45 are higher than those of many other heteroatom co-doped carbon materials, such as O/N co-doped porous carbon nanosheets, B/O co-doped carbon nanoparticles and N/P co-doped carbon nanofibers. Especially, BPNOCNF-45 shows obvious superiority in Cv due to its high Cg and relatively high bulk density compared with the common carbon (< 0.8 g cm−3), which is very important in the practical application of supercapacitors. Cycling stability of electrode materials is crucial to supercapacitors. CV curves of BPNOCNF-45 at the 1st, 2000th and 3000th cycles (Fig. 6h) show that the quasi-rectangular curves of CV do not change obviously up to 3000 cycles. Fig. 6i presents that its capacitance retention is 92% up to 3000 cycles, indicating that BPNOCNF-45 has excellent stability and cycle life. Faradaic redox reactions of

23

B, P, N, O-containing functional groups in KOH electrolyte for providing pseudocapacitance are proposed in Fig. 7 [43,46,57-60].

(Table 3)

(Fig. 7)

To further investigate the electrochemical properties of BPNOCNF-45, symmetric

supercapacitors

were

directly

assembled

with

two

same

self-standing film electrodes (about 2 mg cm-2 mass loading on one electrode) in

aqueous

electrolytes.

Fig.

8a

shows

GCD

curves

of

BPNOCNF-45//BPNOCNF-45 supercapacitor from 0.3 A g-1 to 5 A g-1 in 6 M KOH electrolyte. The GCD curves show slight nonlinearity, indicating the exist of pseudocapacitance in the charge and discharge process. The capacitance changes at different current densities (Fig. 8b) show that the capacitance retention is 76% from 0.3 A g-1 (285 F g-1 and 339 F cm-3) to 5 A g-1 (218 F g-1 and 259 F cm-3), demonstrating its good rate capability. As we know, the energy density of supercapacitors is directly related to the potential window. Na2SO4 electrolyte can display stability at high potential because the equilibrium of H+ and OH- in Na2SO4 electrolyte is unfavorable for reaction with electrodes to evolve H2 or O2 and the strong solvation of both sodium cation and sulfate anion of Na2SO4 electrolyte exists [61]. Therefore, BPNOCNF-45//BPNOCNF-45 supercapacitor was further assembled with 1 M Na2SO4 electrolyte. Fig. 8c shows CV curves of the supercapacitor under 24

different potential windows. Note that even if the potential window gets to 1.8 V, the CV curves still remain the quasi-rectangular, indicating that the supercapacitor can operate within the potential window of 1.8 V without the hydrogen or oxygen evolution. GCD curves of BPNOCNF-45//BPNOCNF-45 supercapacitor at different current densities and the current density-capacitance curves in 1 M Na2SO4 electrolyte are shown in Fig. 8d and 8e, respectively. Obviously, GCD curves in 1 M Na2SO4 electrolyte also display distinct nonlinear characteristic between 1.0 V and 1.5 V, indicating the greater pseudocapacitance contribution. Moreover, it also exhibits good rate capability with the capacitance retention of 78% when the current density increases from 0.5 A g-1 to 5 A g-1.

(Fig. 8)

Ragone plots of BPNOCNF-45//BPNOCNF-45 supercapacitor in 6 M KOH and 1 M Na2SO4 electrolytes are shown in Fig. 8f and 8g. By comparison, the supercapacitor in 1 M Na2SO4 electrolyte displays superior energy storage performances. In 1 M Na2SO4 electrolyte, the assembled symmetric supercapacitors deliver remarkably high volumetric and gravimetric energy densities (21.1 Wh L-1 at 523.5 W L-1 and 17.7 Wh kg-1 at 439.9 W kg-1). Even if the volumetric and gravimetric power densities increase to 4571.7 W L-1 and 3841.8 W kg-1, the volumetric and gravimetric energy density can still remain as high as 12.6 Wh L-1 and 10.6 Wh kg-1. The volumetric and gravimetric

25

performances of BPNOCNF-45//BPNOCNF-45 supercapacitor are significantly superior to previously reported heteroatom-doped and porous carbon-based supercapacitors. Furthermore, as shown in Fig. 8h, the supercapacitor has high capacitance retentions of 88% and 90% up to 10000 charge/discharge cycles in KOH and Na2SO4 electrolytes, demonstrating its excellent cycling stability. It is worth mentioning that the flexible carbon nanofiber self-standing film is particularly suitable for assembling the flexible supercapacitor. Fig. S5a presents

that

the

assembled

BPNOCNF-45//BPNOCNF-45

flexible

supercapacitor in 1 M Na2SO4 electrolyte displays good bendability. And as shown in Fig. S5b-e, the flexible supercapacitor can also work well within the potential window of 1.8 V and exhibit good rate performance with the capacitance retention of 78% when the current density increases from 0.5 A g-1 to 5 A g-1. As shown in Fig. S5f, the flexible supercapacitor delivers high volumetric and gravimetric energy densities (11.7 Wh L-1 and 9.8 Wh kg-1 at 4590.4 W L-1 and 3857.5 W kg-1), higher than previously reported flexible carbon-based supercapacitors. In particular, after 180o bending and the 100th bending (Fig. S5g), the CV curves still remain almost unchanged, demonstrating its remarkable bending performance and it is of great significance for practical applications. Fig. S5h shows that it also displays excellent cycling stability with the capacitance retention of 91% after 5000 charge/discharge cycles. Furthermore, a LED light is lit successfully by using two flexible supercapacitors in series (Fig. S5i). 26

In summary, the fabricated BPNOCNF-45 presents excellent energy storage performances due to possess the following features: (1) suitable surface area and pore structure (379.02 m2 g-1 and 0.34 cm3 g-1 with high meso-/macropore ratio of 70%) to provide a large number of active sites that can accommodate charges and facilitate fast ion diffusion into internal pores under high charge/discharge current density; (2) the flexible self-standing carbon nanofiber film with 3D network structure that enhances mechanical stability, electrical conductivity and bending performance; (3) doping with B/P/N/O heteroatoms, especially active heteroatom species (N-5, N-6, B-C, O-I, P2 and P3), that promotes the wettability and pseudocapacitance performance; (4) high bulk density (1.19 g cm-3) that is beneficial for achieving high volumetric capacitance performance. The fabricated BPNOCNF-45 with self-standing film structure and high volumetric/gravimetric capacitance performances has great potential for applications in compact and miniaturized supercapacitors.

4. Conclusions We demonstrate an effective and facile strategy of designing densely B/P/N/O co-doped hierarchical porous carbon nanofiber self-standing film with superior bending performance, prominent volumetric capacitance and high gravimetric capacitance. The high capacitance performances are closely associated with a suitable combination of the pseudocapacitance behavior from 27

B/P/N/O heteroatom co-doping, especially active heteroatom species co-doping, and a dense self-standing carbon nanofiber film structure with the suitable surface area and pore structure of high meso-/macropore ratio. These advantageous features have afforded considerably satisfactory energy storage performances, showing prominent volumetric capacitance of 395 F cm-3 at 1 A g-1 while maintaining high gravimetric capacitance of 332 F g-1 in a three-electrode system with 6 M KOH electrolyte. Both the assembled symmetric supercapacitor and flexible supercapacitor in 1 M Na2SO4 electrolyte deliver synergetic volumetric and gravimetric energy-power outputting performances together with high capacitance retention of around 90% after 10000 cycles, which are significantly superior to previously reported carbon-based supercapacitors. These promising performance of B/P/N/O co-doped hierarchical porous carbon nanofiber self-standing film together with the facile fabrication process may hold considerable potential to be applied in the compact and miniaturized energy storage devices.

Acknowledgements This work was supported by the National Natural Science Foundation of China (21206034 and 21601054), the Scientific Research Foundation for the Returned Overseas Chinese Scholars of Heilongjiang Province (LC2016003), Outstanding Youth Science Foundation of Heilongjiang University (JCL201202)

28

and Students Innovation and Entrepreneurship Training Project of China (201810212040).

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41

Fig. 1. Preparation process of B/P/N/O co-doped carbon nanofiber film.

Fig. 2. (a) SEM, (b) and (c) magnified SEM, (d) HRTEM, (e) dark-field TEM and (f) EDS mapping images of BPNOCNF-45.

Fig. 3. (a) XRD patterns and (b) Raman spectra of samples.

Fig. 4. (a) XPS spectra of samples; high resolution XPS spectra of: (b) B 1s, (c) P 2p, (d) N 1s, (e) O 1s in BPNOCNF-45; (f) schematic model of B, P, N and O species within carbon materials.

Fig. 5. (a) N2 absorption/desorption isotherms and (b) pore size distributions of samples.

Fig. 6. Electrochemical performances in the three-electrode system: (a) Nyquist plots; (b) CV curves at 20 mV s-1; (c) CV curves of BPNOCNF-45 from 1 to 200 mV s-1; (d) GCD curves at 5 A g-1; (e) GCD curves of BPNOCNF-45 from 1 to 30 A g-1; (f) Cg of samples at different current densities; (g) Cg and Cv of BPNOCNF-45 at different current densities; (h) CV curves of BPNOCNF-45 at 1st, 2000th and 3000th cycles at 5 mV s-1; (i) Capacitance retention of BPNOCNF-45.

42

Fig. 7. Faradaic redox reactions of B, P, N, O-containing functional groups in KOH electrolyte for providing pseudocapacitance are proposed.

Fig. 8. Electrochemical performances of BPNOCNF-45//BPNOCNF-45 supercapacitor: (a) GCD curves at different current densities in KOH electrolyte; (b) rate capabilities at different current densities in KOH electrolyte; (c) CV curves at different potential windows in Na2SO4 electrolyte; (d) GCD curves at different current densities in Na2SO4 electrolyte; (e) rate capabilities at different current densities in Na2SO4 electrolyte; (f) Ragone plots in different electrolytes, and gravimetric energy densities and power densities of other carbon-based symmetric supercapacitors; (g) Ragone plots in different electrolytes, and volumetric energy densities and power densities of other carbon-based symmetric supercapacitors; (h) cycling stability at 5 A g-1.

Table 1 Relative surface contents of N, O, B and P species obtained by N 1s, O 1s, B 1s and P 2p core level XPS spectra. Table 2 Specific surface area and pore structure parameters of samples.

Table 3 Comparison of electrochemical performances of the reported similar studies in the three-electrode system. 43

Table 1 Relative surface contents of N, O, B and P species obtained by N 1s, O 1s, B 1s and P 2p core level XPS spectra. active N configuration

O configuration

(at.%)

(at.%)

B configuration (at.%)

P configuration

heteroatom

(at.%)

species

Samples

(at.%) N-6 N-B

N-5 /N=P

NOCNF

the sum of N-5,

N-Q N-X

O- I

O- II

O-B

O-III

B-C

B-N

B-O

P1

P2

P3

/N-P

N-6, B-C, O- I, P2 and P3

1.63

1.44

2.48

2.40

1.05

2.03

0.97

3.89

BNOCNF-30

0.61

1.98

3.16

1.50

0.57

2.45

1.49

0.44

0.16

2.96

1.46

0.84

10.55

BPNOCNF-30

0.99

3.08

2.04

0.70

1.10

1.73

4.13

1.78

0.73

1.22

0.92

0.79

0.20

0.17

0.08

8.32

BPNOCNF-40

1.07

1.68

3.69

0.50

0.55

0.98

5.17

3.09

1.93

1.77

1.11

0.71

0.30

0.28

0.10

8.50

BPNOCNF-45

1.07

2.62

1.33

1.01

0.93

5.84

3.28

1.99

1.14

2.23

1.15

1.16

0.09

0.49

0.32

12.93

BPNOCNF-50

0.68

2.23

1.20

0.73

0.67

5.24

5.32

1.51

1.36

1.96

1.37

2.01

0.32

0.43

0.33

11.39

Table 2 Specific surface area and pore structure parameters of samples. SBETa

Vtotalb

Davgc

Vmicd

Vmec+mace

(m2 g-1)

(cm3 g-1)

(nm)

(cm3 g-1)

(cm3 g-1)

NOCNF

501.51

0.24

1.93

0.19

0.07

BNOCNF-30

844.22

0.39

1.84

0.34

0.11

BPNOCNF-30

219.20

0.31

5.68

0.05

0.27

BPNOCNF-45

379.02

0.34

3.60

0.11

0.24

BPNOCNF-50

586.20

0.40

2.75

0.19

0.22

Samples

a

Total specific surface area calculated by Brunaur-Emmett-Teller (BET) method.

b

Total pore volume calculated at P/P0 = 0.99.

c

Average pore diameter calculated from the equation of 4Vt/SBET..

d e

Micropore volume calculated from t-Plot micropore method.

Mesopore and macropore volume calculated from BJH method.

Table 3 Comparison of electrochemical performances of the reported similar studies in the three-electrode system. Materials

Electrolyte

Current density

Rate capability

Cg (F

Cv (F cm-3)

Ref

-1

g ) NCNFs N/P-NPCNFs-20 BDC N,S-PCNs1-1 HPC-700 1B-2CHI-WT PFC-700 PA-850 GC-700

6 M KOH 1 M H2SO4 6 M KOH 6 M KOH 6 M KOH 1 M H2SO4 6 M KOH

-1

1Ag

-1

0.5 A g

-1

1Ag

-1

0.5 A g

-1

0.5 A g

-1

1Ag

-1

0.5 A g

-1

-1

362.6

302.2

13

-1

224.9

—

41

277

150

19

298

210

20

412.5

231.0

24

62.9%, 1-20 A g 70%, 0.5-30 A g

-1

67%, 1-100 A g

-1

78.2%, 0.5-100 A g

-1

61.3%, 0.5-50 A g

-1

57.5%, 0.1-3 A g

230

253

26

-1

270

287

46

-1

75.2%, 0.5-20 A g

6 M KOH

0.2 A g

56.3%, 0.2-40 A g

302

—

47

6 M KOH

-1

-1

77%, 0.5-30 A g

325

218

48

1Ag

-1

NFMCNFs_1 : 2

1 M H2SO4

0.5 A g

—

252.6

—

49

PANDN-8

6 M KOH

0.5 A g-1

66.5%, 0.5-50 A g-1

331

107

50

PNCNFs-2-3 DGB NMHCSs-0.6-15 NCNF2-900 PSDPS-800 CoDC-0.5 BPNOCNF-45

6 M KOH 6 M KOH 6 M KOH 2 M H2SO4 6 M KOH 6 M KOH 6 M KOH

-1

1Ag

-1

1Ag

-1

0.2 A g

-1

0.5 A g

-1

1Ag

-1

1Ag

-1

1Ag

-1

67%, 1-40 A g

198

279

51

-1

235

215

52

-1

240

85

53

-1

224

200

54

253

57

55

270

262

56

332

398

This work

41.1%, 1-100 A g

67.1%, 0.2-20 A g 66.5%, 0.5-10 A g -1

94%, 1-20 A g

-1

43%, 1-100 A g

-1

80%, 1-30 A g

Fig. 1. Preparation process of B/P/N/O co-doped carbon nanofiber film.

Fig. 2. (a) SEM, (b) and (c) magnified SEM, (d) HRTEM, (e) dark-field TEM and (f) EDS mapping images of BPNOCNF-45.

Fig. 3. (a) XRD patterns and (b) Raman spectra of samples.

Fig. 4. (a) XPS spectra of samples; high resolution XPS spectra of: (b) B 1s, (c) P 2p, (d) N 1s, (e) O 1s in BPNOCNF-45; (f) schematic model of B, P, N and O species within carbon materials.

Fig. 5. (a) N2 absorption/desorption isotherms and (b) pore size distributions of samples.

Fig. 6. Electrochemical performances in the three-electrode system: (a) Nyquist plots; (b) CV curves at 20 mV s-1; (c) CV curves of BPNOCNF-45 from 1 to 200 mV s-1; (d) GCD curves at 5 A g-1; (e) GCD curves of BPNOCNF-45 from 1 to 30 A g-1; (f) Cg of samples at different current densities; (g) Cg and Cv of BPNOCNF-45 at different current densities; (h) CV curves of BPNOCNF-45 at 1st, 2000th and 3000th cycles at 5 mV s-1; (i) Capacitance retention of BPNOCNF-45.

Fig. 7. Faradaic redox reactions of B, P, N, O-containing functional groups in KOH electrolyte for providing pseudocapacitance are proposed.

Fig. 8. Electrochemical performances of BPNOCNF-45//BPNOCNF-45 supercapacitor: (a) GCD curves at different current densities in KOH electrolyte; (b) rate capabilities at different current densities in KOH electrolyte; (c) CV curves at different potential windows in Na2SO4 electrolyte; (d) GCD curves at different current densities in Na2SO4 electrolyte; (e) rate capabilities at different current densities in Na2SO4 electrolyte; (f) Ragone plots in different electrolytes, and gravimetric energy densities and power densities of other carbon-based symmetric supercapacitors; (g) Ragone plots in different electrolytes, and volumetric energy densities and power densities of other carbon-based symmetric supercapacitors; (h) cycling stability at 5 A g-1.

HIGHLIGHTS： ● Functional molecules were used to prepare B/P/N/O co-doped carbon nanofiber film. ● It displays high active heteroatom content, high meso/macropore ratio and bulk density. ● It exhibits prominent volumetric capacitance and high gravimetric capacitance. ● Symmetrical and flexible supercapacitors show excellent energy storage performances.

Yongjun Ma: Methodology, Investigation, Visualization, Validation, Writing Original Draft. Xugang Zhang: Resources, Investigation. Zhuo Liang: Data Curation, Validation. Chenlong Wang: Formal analysis, Methodology. Yan Sui: Investigation. Bing Zheng: Software. Yuncheng Ye: Writing - Review & Editing. Weijing Ma: Validation. Qi Zhao: Writing - Review & Editing. Chuanli Qin: Conceptualization, Project administration, Funding acquisition, Resources

Declaration of interests √ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: