N-doped carbon nanofibers arrays as advanced electrodes for supercapacitors

Journal Pre-proof N-doped carbon nanofibers arrays as advanced electrodes for supercapacitors Ganguo Pan, Feng Cao, Yujian Zhang, Xinhui Xia

PII:

S1005-0302(19)30416-5

DOI:

https://doi.org/10.1016/j.jmst.2019.10.004

Reference:

JMST 1813

To appear in:

Journal of Materials Science & Technology

Received Date:

27 August 2019

Revised Date:

15 September 2019

Accepted Date:

5 October 2019

Please cite this article as: Pan G, Cao F, Zhang Y, Xia X, N-doped carbon nanofibers arrays as advanced electrodes for supercapacitors, Journal of Materials Science and Technology (2019), doi: https://doi.org/10.1016/j.jmst.2019.10.004

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Research Article

N-doped carbon nanofibers arrays as advanced electrodes for supercapacitors Ganguo Pan 1,*, Feng Cao 1, Yujian Zhang 1, Xinhui Xia 2,* 1 2

Department of Materials Chemistry, Huzhou University, Huzhou 313000, China State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China

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* Corresponding author.

E-mail address: [email protected] (G. Pan).

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Graphical abstract

N-doped carbon nanofibers arrays are prepared by electrodeposition process and demonstrated with large surface area and high

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electrical conductivity leading to enhanced rate capability and high supercapacitor performance

Highlights

> Highly porous activated N-doped carbon nanofibers arrays are constructed. > N-doped carbon nanofibers are proven with high supercapacitor performance > N-doped carbon porous structure is favorable for fast ion/electron transfer

[Received 27 August 2019; revised 15 September 2019; accepted 5 October 2019]

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The advancement of supercapacitors largely relies on the innovation of electrode materials with high-rate performance and ultra-long cycling stability. In this work, unique N-doped nanofibers on carbon cloth (N-CNFs/CC) are prepared by an electrodeposition-annealing method for application in supercapacitors. The as-prepared N-doped nanofibers (N-CNFs) show diameters of 100-150 nm and cross-link with each other forming porous conductive network. Due to enhanced conductivity and reinforced structural stability, the N-CNFs/CC arrays are demonstrated with better electrochemical performance than CNFs/CC counterpart, including higher specific capacitance (195.2 F g-1 at a current density of 2.5 A g-1), excellent rate capability (80.5% capacity retention as the rate increases from 2.5 to 20 A g-1) and good cycling stability (99.5% retention after 10000 cycles). These reinforced

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electrochemical properties are attributed to N-doped conductive architecture with faster ion/electron transfer paths and more active sites. Our findings may offer a new way for construction of advanced high-rate electrodes for energy storage.

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Key words: Supercapacitors; Carbon fibers; Electrode; Porous materials; Electrodeposition

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1. Introduction

High power devices such as supercapacitors (SCs) have been the research hotspot in the past

a

consensus

that

the

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decade due to their combined advantages of high power density and excellent cycling life[1-4]. It is performance

of

SCs

is

mainly

determined

by

the

physicochemical/electrochemical properties of electrode materials[5-8]. It is well accepted that

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there are two kinds of SCs: electric double layer capacitors (EDLCs) and pseudocapacitors [9]. Though pseudocapacitors have been widely studied in recent years, they still have huge barriers to the market due to relatively poor cycling stability and high cost as compared to EDLCs. Given all that, EDLCs are still the mainstream in the present market [10]. Up to now, carbonaceous

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materials such as activated carbon, graphene, and carbon nanotubes, are the most popular electrode materials for EDLCs[11]. To further improve their performance, binder-free design is another important strategy, which does not need to use insulating polymer binders for the subsequent electrode test, and thereby reduce inner resistance and power performance [12].

In such a context,

it is believed that developing binder-free carbon nanoarrays is crucial to achieve good electrochemical performance with high power density and large energy storage capacity [13, 14]. Of the explored carbon electrodes, carbon fibers are considered as good candidates because of excellent flexibility, high electrical conductivity, lightweight, and good mechanical stability and

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low cost[15-17]. However, there are still many challenges that should be settled for construction of high-performance carbon fiber electrodes[11, 18]. Common micron level carbon fiber materials (such carbon fibre cloth) do not have satisfactory EDLC performance owing to their relatively low specific surface area, large size and small porosity [15]. Therefore, it is of great importance to increase the electrical conductivity, intrinsic activity and surface area availability for carbon nanofiber. Meanwhile, nanoscale design is also indispensable because nanostructure can provide fast ion/electron transport path and offer large surface/interface. In this regard, it is verified that heteroatom (e.g., N) doping could not only improve the electronic conductivity of carbon nanofiber[19, 20], but also decrease the energy barrier of ion adsorption/desorption [21-24]. Inspired by the above considerations, it would be very interesting to design and fabricate binder-free N-doped

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carbon nanofibers (N-CNFs) electrodes and explore their EDLCs performance.

In this work, we report an electrodeposition-annealing method for rational synthesis of N doped carbon nanofiber arrays directly on flexible carbon cloth (N-CNFs/CC) and demonstrate their excellent EDLC performance. Our designed N doped carbon fibers (100-200 nm in diameter)

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are firmly anchored on the surface of carbon cloth forming unique binder-free N-CNFs/CC arrays with highly open porous structure and good electrical conductivity. Benefiting from binder-free

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design, large active area and good conductivity, the as-prepared N-CNFs/CC arrays show excellent supercapacitor performance with a high specific capacity of 195.2 F g -1 at 2.5A g-1, excellent rate

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capability (157.1 F g -1 at 20 A g-1) and ultra-stable cycling life with 99.5% retention after 10000 cycles, much better than common CNFs/CC counterparts. Our research provides a new route for

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construction of advanced carbon electrodes for energy storage. 2. Experimental

2.1. Materials synthesis

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2.1.1. Synthesis of N-CNFs/CC electrodes First, N-CNFs are grown on clean carbon cloth substrate by electrochemical polymerization

plus annealing treatment. The electrolyte (100 mL) contained Na 2HPO4 (7.7 g), p-toluenesulfonyl (1.9 g) and pyrrole (0.7 mL). The clean carbon cloth was used as the working electrode. A Pt plat and a saturated calomel electrode (SCE) were used as the counter and reference electrode, respectively. The electro-polymerization was carried out by using an electrochemical workstation with a typical current of 2 mA for 2 h. The electro-polymerization was used to prepare polypyrrole precursor on CC substrate, which was then annealed at 800 °C in Ar atmosphere for 2 h to form

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N-CNFs/CC electrodes. The load mass of carbon nanofibers was about 2.0 mg cm -2. The weight of carbon cloth substrate was about 12.2 mg cm-2. 2.1.2. Synthesis of CNFs/CC arrays For comparison, the common CNFs/CC arrays were fabricated by using a CVDelectrophoresis method. First, the copper tartrate was used as the CVD catalyst for the growth of carbon nanofibers. The copper tartrate was prepared by mixing 30 mL of 0.05 M CuCl 2 and 30 mL of 0.05 M sodium-potassium tartrate to produce the copper tartrate, which was then transferred into the CVD tube. The CVD growth of carbon nanofibers reaction was conducted at in argon with flowing acetylene at 250 oC for 2 h under atmospheric pressure. Then, the samples were collected

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and immersed in solution consisting of 1.5 M FeCl 3 and 1 M HCl for 9 h to completely dissolve the copper catalyst. Then, the sample was annealed at 850 oC for 3 h in argon to form carbon nanofibers. Carbon nanofibers were coated on the commercial carbon cloth by using a simple electrophoretic deposition (EPD) method. The EPD electrolyte was composed of 100 mg carbon

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nanofibers and 50 mg Mg(NO 3)2 in 100 mL isopropyl alcohol. The EPD was conducted in a twoelectrode cell (25 oC) with commercial carbon cloth as the working electrode and a Pt electrode as

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the counter electrode. The distance between two electrodes was 1 cm. The EPD was carried out

2.2. Materials characterization

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with a voltage of 60 V for 2 min. The load mass of carbon nanofibers was about 1.8 mg cm -2.

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Scanning electron microscopy (SEM, Hitachi S-4700) was employed to observe the morphologies of samples. Transmission electron microscopy (TEM, FEI Tecnai G2 F20 at 200 kV) served for demonstrating the microstructure of samples. Powder X-ray diffraction (XRD, Rigaku D/max 2550 PC, Cu Kα) and Raman spectra under laser excitation at 514 nm (LabRamHRUV) were used to identify the crystalline structures and composition of samples. The

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surface chemistry of samples was performed by X-ray photoelectron spectroscopy (XPS, eSCALAB250Xi). Moreover, Brunauer-Emmett-Teller (BET) surface area was obtained by a BET analyzer (JW-BK112). 2.3. Electrochemical measurement The electrochemical performances of CC/CNFs arrays were investigated in 1 M Na 2SO4 solution using an electrochemical workstation (CHI 660E). N-CNFs/CC electrode was used as the working electrode, and platinum plate worked as the counter electrode along with saturated calomel electrode (SCE) as the reference electrode. For both CV and charge/discharge test, the 4

voltage was controlled in the range of 0-0.9 V. The current densities of 2.5, 5.0, 7.5, 10, and 20 A g−1 were used for the charge/discharge test. electrochemical impedance spectroscopy (EIS) test was carried at room temperature with the frequency ranges from 0.01 to 10 5 Hz. The specific capacitance (Cm) is calculated according to the following equation [25, 26]: Cm=It/mV where I, m, Δt and ΔV was the discharge current (A), weight (g) of active materials, discharge time (s), and discharge potential window (V), respectively. 3. Results and discussion

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The schematic fabrication process of N-CNFs/CC arrays and morphology characterization are shown in Fig. 1. It is seen that the carbon cloth (CC) substrate has clean surface and its fibres have diameters of 8-10 m (Fig. 1(a) and (b)). After electrodeposition (ED) plus thermal treatment, N-doped carbon nanofibers (N-CNFs) are uniformly grown on the CC substrate forming binder-

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free N-CNFs/CC arrays (Fig. 1(d)-(g)). Notice that the N-CNFs show diameters of 100-150 nm and are interconnected with each other forming a uniform porous conductive network. Without

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any additional insulating binders and additives, the as-obtained N-CNFs/CC can be directly used as a binder-free electrode for supercapacitors. For comparison, carbon nanofibers without N

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doping are also grown on CC substrate to produce CNFs/CC electrode by a CVD-electrophoretic deposition (EPD) process. As shown in Fig. 2(a) and (b), the CNFs/CC counterparts display similar morphology to that of N-CNFs/CC except for the rougher surface of carbon nanofibers. Further

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structural features of N-CNFs and CNFs were elucidated via transmission electron microscop y (TEM) and high-resolution transmission electron microscopy (HRTEM). Note that the average diameter of N-CNF and CNF is in the range from 100 to 150 nm (Fig. 3(a)-(b) and Fig. 4(a)-(b)). Moreover, no well-defined lattice fringes are observed for N-CNFs (Fig. 3(c)) and CNFs (Fig. 4(c)), revealing their amorphous nature, also confirmed by the weak diffraction rings in their

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selected area electron diffraction (SAED) patterns (Fig. 3(d) and inset image in Fig. 4(a)). Elemental mapping images of N-CNFs also clearly demonstrate the uniform distribution of C, N and O in the N-doped carbon nanofibers (Fig. 3(e)). The element O arises form the oxygencontaining groups such as -OH on the surface of N-CNFs. The phases and crystal structures of NCNFs/CC and CNFs/CC samples were studied by X-ray diffraction (XRD). As shown in Fig. 5(a), a strong diffraction peak at 2θ = 25.9° is obviously detected and indexed with the (002) plane of graphitic carbon[27]. Another two weak peaks located at 43.9 o and 53.7o correspond to the (101) and (004) crystal planes of carbon, respectively [5]. Though the XRD pattern of N-CNFs is similar 5

to that of CNFs, their difference can be detected by Raman results. Fig. 5(b) displays the Raman spectra of N-CNFs and CNFs. Obviously, both of N-CNFs/CC and CNFs/CC display peaks at 1339 cm−1 (D-band) and 1598 cm−1 (G-band), characteristics of graphitic carbon [25]. However, the NCNFs show stronger peaks at D and G-bands than those of CNFs/CC. Moreover, the intensity ratio of G-band to D-band (ID/IG) is higher for N-CNFs/CC, demonstrating its higher graphitization feature. Furthermore, the existence of N-doped carbon nanofibers is further characterized by XPS. The broad peak of N 1s (Fig. 5(c)) could be resolved into three peaks at 398.4, 399.5 and 400.8 eV, corresponding to the pyridinic N, pyrrolic N and graphitic N in the N-CNFs, respectively[28, 29]

. Furthermore, the C-N bond located at ~ 285.7 eV also could be detected in C 1s spectrum for

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N-CNFs (Fig. 5(d)). In addition, C=O (288.5 eV) and C=C bond (284.5 eV) could also be found [30]. As for the C 1s spectrum of CNFs arrays, two peaks at 284.7 and 286.3 eV are ascribed to the C=C and C-O-H bonds, respectively[29]. Based on the results above, it is reasonable that the N-CNFs/CC arrays can be successfully prepared by our united electrodeposition plus thermal treatment process.

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BET measurements were carried out to examine the porous nature and determine the surface area of the samples. The N2 adsorption/desorption isotherm curves of N-CNFs/CC and CNFs/CC are

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shown in Fig. 5(e). The as-prepared N-CNFs exhibit a specific surface area of 262 m 2 g−1, larger than that of CNFs/CC (235 m 2 g−1), indicating that the N-CNFs possess in larger surface areas,

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which is beneficial for exposing more active sites to achieve better performance. The electrochemical performances of the N-CNFs/CC and CNFs/CC electrodes were measured in a three-electrode cell with a Pt plate as the counter electrode, a saturated calomel

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electrode (SCE) as the reference electrode and 1 M Na 2SO4 as the electrolyte. Fig. 6(a) shows the cyclic voltammetry (CV) profiles of the N-CNFs/CC at different scan rates, from which rectangular patterns of CV curves confirm an ideal EDLC behavior. Moreover, the shape of the CV curve has not altered much with the increase of scan rate, revealing remarkable mass transport of electrons and ions. Moreover, the area of N-CNFs/CC at different scan rates are larger than the

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CNFs/CC at the same scan rates (Fig. 6(c)), which suggests that the N-CNFs/CC electrode has better capacitive reactivity. Additionally, the nearly symmetrical triangle shapes of the charge/discharge curves of both electrodes are shown in Fig. 6(b) and (d)). According to the charge/discharge curves, reversible capacities of 195.2, 191.5, 183.3, 171.1 and 157.1 F g -1 can be achieved for the N-CNFs/CC electrode at 2.5, 5, 7.5, 10 and 20 A g -1, respectively, superior to the CNFs/CC counterpart (170.2, 161.1, 146.6, 124.2 and 100.8 F g -1 at 2.5, 5, 7.5, 10 and 20 A g-1, respectively) (Fig. 7(a)). Even at a high current density of 20 A g -1, the N-CNFs/CC electrode still remains a capacity of 157.1 F g-1 with 80.5% retention of the original capacitance, revealing its 6

outstanding high rate performance. In order to further understand the electrochemical properties of the as-prepared electrodes, EIS analysis was carried out at room temperature to provide information on electrochemical processes (Fig. 7(b)). The semicircle in the high-frequency range is associated with the charge transfer resistance (Rct), while the straight line in the low-frequency region corresponds to ion diffusion in the electrode material[31]. Obviously, the smaller semicircle at the high-frequency region reveals that N-CNFs/CC electrode exhibits lower inner resistance and faster charge transfer at the electrode-electrolyte interface[32]. Meanwhile, at the low frequency region, the more vertical straight line implies better and faster ion diffusion paths of the N CNFs/CC electrode[33]. More importantly, excellent high-rate cycling stability is achieved for the CC/CNFs electrode. The long-term cycling test was conducted at a current density of 2.5 A g -1

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(Fig. 7(c)). After 10000 cycles at 2.5 A g -1, the N-CNFs/CC electrode shows a discharge capacity of ~195 F g−1, with a capacity retention rate of ~99.5% compared to the first cycle (195.9 F g −1), demonstrating its excellent cycle stability. SEM images of N-CNFs electrode after 10000 cycles at 2.5 A g-1 is shown in Fig. 8. It is seen that the whole electrode structure is still well preserved,

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demonstrating its excellent mechanical stability. In contrast, the CNFs/CC counterpart exhibits reversible capacities of 165 F g −1 (capacity retention of 97%). The integrated N-doped arrays

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structure is responsible for the notable capacitor performance. (1) Binder-free characteristics ensures low inner resistance and N doping is significantly beneficial for exposure of more active

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sites[34]. (2) Highly porous structure offers short diffusion paths for ions and better soaking s pace between N-CNFs and electrolyte[35, 36]. (3) The integrated conductive carbon network provides fast transportation paths for electron, and thereby resulting in fast reaction kinetics and high utilization

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of active materials [37-39]. (4) The twined core-branch structure is favorable to keep the structure stable and good long-term cycles[40]. 4. Conclusion

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In conclusion, we have successfully synthesized unique N-CNFs/CC arrays by a simple and effective electrodeposition method. The approach could open up the possibility for synthesizing high-quality N-CNFs on conductive substrates. The as-prepared N-CNFs are uniformly decorated on carbon fiber forming 3D conductive network. The enhanced electrochemical properties can be credited to the porous conductive architecture and N doping. Consequently, the N-CNFs/CC electrode exhibits a high specific capacity (195.2 F g -1 at a current density of 2.5 A g -1), remarkable rate capability (80.5% capacity retention as the rate increases from 2.5 to 20 A g -1) and excellent cycling stability. It is believed that the N-CNFs/CC arrays are promising electrodes for high-rate

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supercapacitors. Acknowledgements This work is supported by Zhejiang Provincial Natural Science Foundation of China (Grant No. LY17E040001), National Natural Science Foundation of China (Grant No. 51772272, 51728204), Fundamental Research Funds for the Central Universities (Grant No. 2018QNA4011), Qianjiang Talents Plan D (QJD1602029), and Startup Foundation for Hundred-Talent Program of Zhejiang University. References

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Figure and table captions

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Adv. Sci., 6 (2019) 1900151.

Fig. 1 (a) Schematic illustration of preparation process of N-CNFs/CC. (b-e) SEM images of N-

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CNFs/CC.

Fig. 2 (a-b) SEM images of CNFs/CC electrodes (inset images: magnified CNFs and low-

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magnification image).

images of N-CNFs: C, N, O.

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Fig. 3 (a-c) TEM-HRTEM images and (d) SAED pattern of N-CNFs. (e) Elemental mapping

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Fig. 4 (a-c) TEM-HRTEM image of CNFs (SAED pattern in inset).

Fig. 5 (a) XRD patterns and (b) Raman spectra of N-CNFs/CC and CNFs/CC samples. XPS spectra of N-CNFs/CC and CNFs/CC arrays: (c) N 1s and (d) C 1s. (e) Adsorption-desorption isotherm

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curves of N-CNFs and CNFs samples.

Fig. 6 Electrochemical characterization of N-CNFs/CC and CNFs/CC electrodes: (a, c) CV curves at different scan rates; (b, d) Charge/discharge curves at different current densities. Fig. 7 Electrochemical performance of N-CNFs/CC and CNFs/CC electrodes: (a) Rate performance; (b) Nyquist plots; (c) Cycling performance at 2.5 A g -1.

Fig. 8 SEM images of N-CNFs electrode after 10000 cycles at 2.5 A g -1. 10

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Figure list:

Fig. 1 (a) Schematic illustration of preparation process of N-CNFs/CC. SEM images of (b-c) carbon cloth

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(CC) substrate and (d-g) N-CNFs/CC electrode.

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Fig. 2 (a-b) SEM images of CNFs/CC electrodes (inset images: magnified CNFs and low-magnification

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image).

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Fig. 3 (a-c) TEM-HRTEM images and (d) SAED pattern of N-CNFs. (e) Elemental mapping images of N-

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CNFs: C, N, O.

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Fig. 4 (a-c) TEM-HRTEM image of CNFs (SAED pattern in inset).

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b (002)

a

D

Intensity (a.u.)

(004)

(101)

Intensity (a.u.)







CNFs/CC

G

CNFs

N-CNFs N-CNFs/CC

30

40

50

60

70

2 (degree)

500

d

N 1s Pyridiniac N

Intensity (a.u.)

80

Pyrrolic N

Intensity (a.u.)

c

20

Graghitic N N-CNFs

CNFs

750

C 1s C-C

C-N C=O

N-CNFs

C-C

C-OH

398

399

400

401

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CNFs

397

402

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100 0 0.0

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288

290

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3

Quantity Adsorbed (cm /g STP)

N-CNFs CNFs

400 300

284

Binding Energy (eV)

Binding Energy (eV) 500

1000 1250 1500 1750 2000

Raman Shift (cm-1)

ro of

10

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/P0)

Jo

Fig. 5 (a) XRD patterns and (b) Raman spectra of N-CNFs/CC and CNFs/CC samples. XPS spectra of NCNFs/CC and CNFs/CC arrays: (c) N 1s and (d) C 1s. (e) Adsorption-desorption isotherm curves of NCNFs and CNFs samples.

16

b 1.0

a -1

50 mV s -1 200 mV s

100 mV s -1 300 mV s

CC at 50 mV s

0

-50

N-CNFs/CC

0.8 0.6 0.4 0.2

N-CNFs/CC

-100

0.0 0.0

0.2

0.4

0.6

0.8

1.0

Potential (V vs. SCE)

d

0

20

40

80

100

120

Voltage (V)

-1

0

-50

0.8 0.6 0.4

0.0

0.4

0.6

0.8

1.0

-p

CNFs/CC

0.2

2.5 A g -1 5.0 A g -1 7.5 A g -1 10 A g -1 20 A g

CNFs/CC

0.2

0.0

160

-1

-1

100 mV s -1 300 mV s

50

140

Time (s)

ro of

-1

50 mV s -1 200 mV s

-100

60

1.0

100

Current density (A g )

-1

50

c

-1

2.5 A g -1 5.0 A g -1 7.5 A g -1 10 A g -1 20 A g

-1

Voltage (V)

-1

Current density (A g )

100

0

20

40

60

80

100

120

140

Time (s)

re

Potential (V vs. SCE)

Fig. 6 Electrochemical characterization of N-CNFs/CC and CNFs/CC electrodes. (a, c) CV curves at

Jo

ur na

lP

different scanning rates; (b, d) Charge/discharge curves at different current densities.

17

b

a

10

N-CNFs/CC CNFs/CC

-1

Specific capacitance (F g )

250

CNFs/CC N-CNFs/CC

8

-Z '' ()

200 150 100

6 4 2

50 0

0

5

10

15

-1

20

25

Current density(A g )

0

0

1

2

3

4

5

6

7

8

9

10

Z ()

200

150

100

N-CNFs/CC CNFs/CC

0

2500

5000

ro of

-1

Specific capacity (F g )

c

7500

-p

Cycle number

10000

re

Fig. 7 Electrochemical performance of N-CNFs/CC and CNFs/CC electrodes. (a) Rate performance; (b)

Jo

ur na

lP

Nyquist plots; (c) Cycling performance at 2.5 A g-1.

Fig. 8 SEM images of N-CNFs electrode after 10000 cycles at 2.5 A g -1.

18