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Advanced Composites and Hybrid Materials

, Volume 2, Issue 3, pp 456–461 | Cite as

Carbon electrodes with double conductive networks for high-performance electrical double-layer capacitors

  • Guoqiang Li
  • Yongsheng Ji
  • Danying Zuo
  • Jing Xu
  • Hongwei ZhangEmail author
Original Research
  • 283 Downloads

Abstract

Tremendous efforts have been devoted to develop the active materials for supercapacitor electrodes, but very few are focused on the improvement of the electrode material/electrolyte interfaces via protonic conductors. In this study, double conductive networks are constructed in powdery carbon electrodes by using sulfated zirconia as protonic conductor and acetylene black as electric conductor. The synergetic effect of protonic and electric double transport pathways enhances the electrode material/electrolyte interfaces, reduces diffusion distance of protons, and increases the electrode specific capacitance. Compared to the control electrode without protonic conductor, the electrode shows a specific capacitance with 21.0% increment. The proposed strategy may be a promising avenue for preparation of powdery carbon electrodes used in electrical double-layer capacitors.

Graphical abstract

Double conductive networks in powdery carbon electrodes constructed by sulfated zirconia as protonic conductor and acetylene black as electric conductor

Keywords

Carbon electrode Protonic conductor Double conductive network EDLC Specific capacitance 

1 Introduction

In the last decades, the increasing concerns of the energy crisis and environmental pollutions have motivated significant interests in developing new energy storage and conversion systems [1, 2]. Among them, carbon-based electrical double-layer capacitors (EDLCs) have attracted considerable attention because of their outstanding properties, such as high power density, long-term cycling stability, quick charging–discharging ability, and environmental friendliness [2, 3]. Although carbon-based EDLCs have been commercialized, they still suffer from relatively low energy density compared with batteries [3]. Many strategies have been adopted to enhance the energy density, such as synthesizing new carbon materials with high specific surface area [4, 5, 6], doping heteroatoms into carbon matrix [7, 8], absorbing redox species [9, 10, 11], constructing binder-free electrodes [12, 13, 14], using nonaqueous electrolytes with wide voltage range [15, 16, 17], and forming asymmetric supercapacitors [18, 19]. But the electrodes based on the powdery carbon materials cannot avoid the use of binders and conductive additives, which is unfavorable for improving the electrode material/electrolyte interfaces. For conventional electrodes based on the powdery carbon materials, there is only an electron-conducting network in which electrons can transfer through the binder coatings on the surfaces of carbon particles. On the contrary, ions cannot transfer through such binder coatings owing to inert binders.

As a consequence, it is necessary to enhance ionic diffusion through the electrode material/electrolyte interfaces by interface engineering. Herein, we take advantage of sulfated zirconia (SZrO2), which is recognized as the strongest solid superacid [20], as a proton conductor to form a proton-conducting network. In this work, we investigate the effect of the proton-conducting network on the performance of as-prepared electrodes. The specific capacitance of as-prepared electrodes increases from 124 to 150 F g−1 of the control electrode without SZrO2.

2 Experimental

SZrO2 was prepared by immersing zirconium oxide (ZrO2) nano-particles (< 100 nm, Sinopharm Chemical Reagent Co., Ltd.) in an aqueous 0.5 M H2SO4 solution with magnetic stirring for 60 min. Thereafter, the wet powder was separated from the liquid and dried in a vacuum oven at 110 °C for 12 h, followed by calcining at 600 °C for 3 h under atmospheric conditions to obtain SZrO2 particles [21].

Fourier transform infrared spectroscopy (FT-IR) spectra and crystallographic analysis of SZrO2 particles were performed by Nicolet 6700 FT-IR spectrometer and X-ray diffractometer (X’Pert PRO) equipped with Cu Kα radiation (λ = 1.54 Å) at d-generator voltage of 45 kV with a step size of 0.02°, respectively. The surface morphologies of the electrode and SZrO2 were examined by a scanning electron microscope (SEM, JEOL JSM-IT300) and the elemental mapping image was revealed by an energy-dispersive X-ray spectroscopy (EDX).

All electrochemical measurements were carried out in a two-electrode configuration cell with 1 M H2SO4 aqueous solution as the electrolyte at room temperature. The electrodes were prepared by blending activated carbon (AC, Kurary YP-50F with surface area of ~ 1600 m2 g−1) with SZrO2, acetylene black (AB), and polytetrafluoroethylene (PTFE) under ultrasonic agitation, followed by coating the obtained slurry on a cleaned slice of stainless steel meshes and dried at 80 °C for 24 h. The mass content of four components in the electrode is listed in Table 1. The cyclic voltammetry (CV), galvanostatic charging–discharging (GCD) curves, and electrochemical impedance spectra (EIS) were measured using a Chen Hua electrochemical workstation (CHI660E). The cycling stabilities were conducted on a LAND CT2001A battery test system.
Table 1

The mass content of four components in the electrode

Electrode

AC

SZrO2

AB

PTFE

E1

80%

8%

8%

4%

E2

80%

5.3%

10.7%

4%

E3

80%

3.2%

12.8%

4%

SZrO2

0

80%

16%

4%

AC

80%

0

16

4%

3 Results and discussion

The structures of SZrO2 particles were investigated by FT-IR and XRD. As shown in Fig. 1a, the absorption bands at 3412 and 1634 cm−1 are contributed by the H–O–H bending and O–H stretching of the water adsorbed on the surface of SZrO2 because of their hygroscopic nature, respectively [22]. The peaks at 1235, 1140, and 1045 cm−1 are attributed to the asymmetric stretching vibrations of S=O and S–O binding via two oxygen atoms to the Zr4+ cations [21]. The peak at 742 cm−1 is assigned to the characteristic vibrations of ZrO2 [21]. These results confirm the formation of SZrO2. The XRD patterns of ZrO2 and SZrO2 are shown in Fig. 1b. It reveals that there exist the typical diffraction peaks centered at 2θ angles of 24.04°, 28.16°, 31.46°, 34.24°, and 50.20°, which are indexed as (110), (− 111), (111), (200), and (002) crystalline facets of ZrO2 crystal (JCPDS PDF No. 37-1484), respectively [23]. Furthermore, the SZrO2 has nearly same diffraction peaks as the ZrO2, indicating unchanged crystalline structure during the preparing of SZrO2. The SEM images of ZrO2 and SZrO2 particles (Fig. 2a, b) also exhibit similar morphology with sizes of less than 100 nm, which is consistent with the XRD analysis. Figure 2c displays the morphology of the E3 electrode. It is observed that the powdery AC, AB, and SZrO2 are bound into integration by PTFE. In addition, SEM elemental mapping analysis illustrates the existence of C, O, and Zr elements in the E2 electrode, suggesting the successful incorporation of SZrO2 which is uniformly dispersed on the surfaces of AC particles.
Fig. 1

a FT-IR spectrum of the SZrO2 particles. b XRD patterns of ZrO2 and SZrO2 particles

Fig. 2

SEM images of a ZrO2 and b SZrO2 particles and c E3 electrode; elemental mapping images of E2 electrode: d C–K edge, e O–K edge, and f Zr–K edge

The electrochemical performance of these electrodes was tested in a two-electrode system using 1 M H2SO4 as an aqueous electrolyte. Figure 3a exhibits the GCD curves of E1, E2, E3, and AC electrodes at a current density of 1 A/g. The GCD curves display nearly symmetrical triangular shapes with a slight potential drop, suggesting a fairly good capacitive charge storage behavior. From the GCD curves, the specific capacitance (C) can be calculated according to Eq. (1):
$$ C=\frac{4 I\varDelta t}{m\varDelta V} $$
(1)
where I is the constant current (A), m is the total mass of AC for both electrodes (g), Δt is the discharge time (s), and ΔV is the voltage range after the IR drop during the discharge process (V). The C values of E1, E2, E3, and AC electrodes are 105, 136, 150, and 124 F/g, respectively. These values also agree with the CV curves of these electrodes in Fig. 3b. The C value of E3 electrode is higher than or comparable to the reported values of AC-based supercapacitors in literatures (Table 2). The increased C values of E2 and E3 electrodes are 9.7% and 21.0% compared with the C value of AC electrode, respectively. The increment of the E3 electrode is comparable to that of the reduced graphene oxide (rGO)-bound electrode without PTFE, whose C value is 27.3% higher than the conventional PTFE-bound electrode [29]. Since the SZrO2 nearly contributes no capacitance (Fig. 3b), the result may be attributed to the enhanced protonic conductivity derived from SZrO2. The high mass ratio of SZrO2 and AB in the E1 electrode leads to unwell-developed double conductive networks; the optimum mass ratio of SZrO2 and AB is 1:4 in the E3 electrode. As a consequence, the IR drops of E1, E2, E3, and AC electrodes are 0.058, 0.034, 0.028, and 0.052 V, respectively. The synergetic effect of protonic and electric double conductive networks in electrodes is illuminated in Fig. 4, which not only can remain high electric conductivity but also can shorten the diffusion and transport distance of protons owing to improved AC/electrolyte interfaces. Hence, the well-developed double conductive networks in electrodes can result in a rapid I–V response and minimal resistance.
Fig. 3

a GCD curves at a current density of 1 A g−1, b CV curves at a scan rate of 100 mV s−1, c Nyquist plots, and d the cycling life test of the E3 electrode

Table 2

Comparison of C values in our work with that of EDLCs with AC-based electrodes in literature

AC

BET specific surface area (m2 g−1)

Electrolyte

Specific capacitance (F g−1)a

Reference

YF-50

1600b

1 M H2SO4

150 (@ 1 A g−1)

This work

YF-80

2347

1 M Na2SO4

140 (@ 1 A g−1)

[24]

YP-50F

1737

6 M KOH

93.7 (@ 1 mA cm−2)

[25]

YP-50F

1700

1 M H2SO4

~ 135 (@ 1 A g−1)

[26]

MSC-30c

3000

0.25 M H2SO4

161 (@ 5 mA cm−2)

[27]

YF-50d

1 M H2SO4

155 (@ 1 A g−1)

[28]

aThe value is for single electrode

bData from Kuraray retailer

cSulfonated poly(ether ether ketone) membrane as separator

dMass loading of active material per electrode is 0.5 mg cm−2

Fig. 4

The double conductive networks in the electrode

The improvement effect of double conductive networks can be also confirmed by EIS of the four electrodes (Fig. 3c). The nearly vertical lines at low frequency mean ideal capacitive charge storage behaviors in these electrodes [30]. The diameter of the semicircles in the high-frequency region on the real axis reflects the interfacial charge transfer resistances (Rct) of electrodes [31]. The Rct of E3 electrode is much smaller than the other electrodes, suggesting faster charge transport due to well-developed double conductive networks. The intercepts of the semicircle on the real axis at high frequency indicate the equivalent series resistances (Rs) of electrodes. All Rs values are very similar, but Rs values of E2 and E3 electrodes are still slightly less than that of the AC electrode. The cycling stability test of the E3 electrode is shown in Fig. 3d. After 2000 cycles at a current density of 1 A g−1, the C value decreases from 150 to 138 F/g, displaying a good cycling stability.

4 Conclusions

The protonic and electric double transport pathways are successfully constructed in powdery carbon-based electrodes using SZrO2 and AB. Compared with the AC electrode without SZrO2, the newly prepared electrode exhibits a 21.0% increase of C value. The cycling life of 92% capacitance retention is achieved after 2000 cycles. The significant enhancement can be attributed to the unique double conductive networks. This study presents a potential strategy to fabricate high-performance powdery carbon electrodes for supercapacitors.

Notes

Funding

This work was supported by the Hubei Provincial Natural Science Foundation of China (2018CFB267).

Compliance with ethical standards

Conflict of interest

The authors declare that they have no competing interest.

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© Springer Nature Switzerland AG 2019

Authors and Affiliations

  1. 1.School of Materials Science and EngineeringWuhan Textile UniversityWuhanPeople’s Republic of China

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