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Enhanced performance of carbon-based supercapacitors by constructing protonic and electric dual-channels in electrodes
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 1 7 9 7 e1 8 0 2
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Short Communication
Enhanced performance of carbon-based supercapacitors by constructing protonic and electric dual-channels in electrodes Na Liang 1, Guoqiang Li 1, Yongsheng Ji, Jing Xu, Danying Zuo, Likun Huang, Hongwei Zhang* School of Materials Science and Engineering, Wuhan Textile University, WuHan, 430073, PR China
article info
abstract
Article history:
Powdery carbonaceous materials have to use binder materials when they are integrated
Received 18 June 2018
into electrodes for supercapacitors, which will results in high interfacial charge transfer
Received in revised form
resistances and reduced specific capacitance. To resolve the problem, protonic and electric
7 November 2018
dual-channels are constructed in electrodes by in situ synthesis of cesium hydrogen salt of
Accepted 13 November 2018
phosphotungstic acid on the surface of carbonaceous materials. The cesium hydrogen salt
Available online 5 December 2018
particles are confirmed by a Fourier transform infrared spectroscopy, X-ray diffractometer and an energy dispersive X-ray spectroscopy. The electrochemical properties of as-
Keywords:
fabricated electrodes are measured by cyclic voltammetry, galvanostatic charging-
Dual-channel
discharging, and impedance analysis with an electrochemical workstation. At a current
Electrode
density of 1 A g1, the electrode shows a specific capacitance of 152 F g1. Compared to the
EDLC
electrode without the cesium hydrogen salt, the value increases 25% at least. Furthermore,
Performance
the specific capacitance retention of the electrode reaches 104% of its original capacitance after 5000 charge-discharge cycles, suggesting excellent cycling stability. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction With the increasing energy demands and the worsening environmental issues, advanced energy storage and conversion devices (e.g. batteries, fuel cells, and supercapacitors) are attracting widespread attention. Among them, supercapacitors have attracted increasing interest in the past decade because of their prominent advantages of high power density, fast charge/discharge rate, high safety and long cycle
lifetime [1]. Depending on the energy storage mechanism, supercapacitors can be classed as electrical double-layer capacitors (EDLCs) and pseudo-capacitors. Compared to pseudocapacitors, EDLCs have been commercialized and even used on an Airbus A380 in emergency doors due to their high power delivery and long lifecycle [2]. However, EDLCs still suffer from low energy density, which often results in a discharge time less than a minute. To overcome the obstacle, several intensive approaches have been adopted. The first approach is exploring new carbonaceous materials with high surface area,
* Corresponding author. E-mail addresses: [email protected], [email protected] (H. Zhang). 1 The two authors contributed equally to this work. https://doi.org/10.1016/j.ijhydene.2018.11.107 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 1 7 9 7 e1 8 0 2
such as porous carbon spheres [3], mesoporous carbons [4], carbon aerogels [5], carbon nanocages [6], carbon nanotube [7], graphene [8] and graphdiyne [9]. The second approach is selecting new electrolytes to expand the operating voltages [10e12]. The third approach is constructing hybrid supercapacitors, including improving the power density and cycle life of the anode, suppressing the swelling for higher reliability and safety, improving the energy density of the cathode [13e16]. The fourth approach is engineering the interfaces to facilitate ion transport and shorten electron transfer distance, including directly growing carbonaceous materials on current collectors [17], improving wettability of carbon materials [18] and introducing an extended nanoregime interface [19]. But binder materials are still needed to integrate powdery carbonaceous materials into electrodes, which will lead to reduced specific capacitances of active materials because binder materials are often electronic and protonic insulators. In this study, we proposed a strategy to alleviate such an issue by constructing protonic and electric dual-channels in electrodes. As a proof of concept herein, cesium hydrogen salt of phosphotungstic acid (CsPW, Cs2.5H0.5PW12O40), which has strong acidity, and acetylene black (AB) were incorporated into electrodes to form protonic and electric channels, respectively. The specific capacitance of as-prepared electrodes increased to 152 F g1 from 120 F g1 of the control electrode without CsPW.
Experimental Synthesis of CsPW CsPW was synthesized by precipitation titration [20]. A typical synthesis reaction was as follows: Firstly, the aqueous solutions of cesium carbonate (Cs2CO3, 0.1 M) and phosphotungstic acid (PWA, H3PW12O40, 0.08 M) were prepared. Secondly, the solution of Cs2CO3 (20 mL) was slowly dropped in to 20 mL of PWA solution with stirring at room temperature. Thirdly, the as-obtained colloidal solution was stirred for 24 h and followed by centrifugation and freeze-thawing. Lastly, the resulting powders were dried at 110 C in a vacuum oven.
Preparation of electrodes with dual-channels In order to prepared carbon electrodes with dual-channels, activated carbon (AC, Kurary YP-50F with surface area of 1600 m2 g1) was firstly mixed with Cs2CO3 solution under magnetic stirring for 12 h. Then the PWA solution was added dropwise and stirred for another 12 h to synthesize CsPW particles on AC surfaces. Thereafter acetylene black (AB) and polytetrafluoroethylene (PTFE) were introduced under ultrasonic agitation. Subsequently, the mixed slurry was coated and pressed on a cleaned slice of stainless steel meshes as a current collector. Finally, the electrodes were dried at 80 C for 24 h. The geometric area of electrodes was 1.0 1.0 cm2 and the mass loading of the slurry on an electrode was about 6e9 mg cm2. The mass ratio of AC, CsPW þ AB and PTFE was 0.8:1.5:0.5. The mass ratio values of CsPW and AB were selected as 0, 0.5, 2/3 and 1. For the convenience of description, the corresponding electrodes were denominated as AC, 1:2, 2:3
and 1:1, respectively. As a control, the electrode without AC and with CsPW as active materials was also prepared and named as CsPW. The mass ratio of CsPW, AB and PTFE was 0.8:1.5:0.5.
Materials characterization The surface morphology of the samples was characterized by a scanning electron microscope (SEM, JEOL JSM-IT300) and the elemental mapping image was revealed by an energy dispersive X-ray spectroscopy (EDX). The Fourier Transform infrared spectroscopy (FTIR) spectra were recorded using a Nicolet 6700 FTIR spectrometer. X-ray diffractometer (X'Pert PRO) was adopted to determine the structure of CsPW and PWA particles. Cu Ka (l ¼ 1.54 Å) was used as the X-ray source at dgenerator voltage of 45 kV and current of 80 mA. Samples were scanned in 2q ranges from 10 to 90 , in steps of 0.02 and counting time 2 s per step.
Electrochemical measurements Electrochemical workstation (CHI 660E, Shanghai Chen Hua Co., Ltd) was used for all electrochemical tests. Electrochemical measurements were carried out in a two-electrode system with a 1 M H2SO4 aqueous electrolyte. The specific capacitance of the single electrode was calculated from the galvanostatic charge/discharge data by using the equation of C ¼ 4IDt/(mDV), where I is the constant current (A), m is the total mass of AC for both electrodes (g), Dt is the discharge time (s), and DV is the voltage range after the IR drop during the discharge process (V).
Results and discussion Morphology analysis The FTIR spectra of PWA, CsPW and the 2:3 electrode powders are shown in Fig. 1a. It can be found that the primary Keggin structure of PWA have been remained in its cesium salt form (CsPW). The characteristic peaks of the Keggin structure centered at 1078, 986, 890, 804 and 596 cm1 are assigned to stretching vibrations of PeO in central tetrahedral, W¼Od terminal oxygen in the Keggin structure, WeOeW corners shared bonds, WeOeW edges shared bonds and the bending vibration of OePeO, respectively [20e22]. The bands at 1617 and 3447 cm1 are respectively attributed to the deformation vibration and the stretching of hydroxyl groups of H2O [20]. Compared to PWA, the shifts of WeOeW edges shared bonds and water peaks in CsPW may indicate the interaction between Csþ cations and heteropolyacid anions [22]. Although the 10%CsPW/90%AC has a low content of CsPW, its FTIR spectrum also shows characteristic bands with low intensity of the Keggin structure at 1078, 986, 890, 804 and 596 cm1, which suggests the existence of CsPW on the AC powders. The XRD patterns of PWA, CsPW and the 2:3 electrode powders are shown in Fig. 2b. It can be seen that the diffraction peaks of CsPW become broader and shift toward slightly higher 2q angles as compared with PWA. It indicates the reduced cubic lattice constants of CsPW after the H5Oþ 2 ions in PWA is
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 1 7 9 7 e1 8 0 2
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Fig. 1 e (a) FTIR spectra (b) XRD patterns of PWA, CsPW and 2:3 electrode, SEM image of (c) AC, (d) an electrode with CsPW and (e) element mapping image of Cs in the electrode, (f) Schematic illustration for the transport of proton and electron in dual-channels.
partially replaced by smaller Csþ ions to form CsPW [22e24]. From the results of XRD and FT-IR, it can be concluded that the CsPW is successfully synthesized on AC powders. As shown in Fig. 1c, it can be observed that the AC particle shape is irregular with particle sizes ranging from several hundred nanometers to larger than 10 mm. When they are incorporated into an electrode, the space between AC particles is filled by AB particles (Fig. 1d). Furthermore, it can be
deduced from the element mapping image of Cs in Fig. 1e that the CsPW particles are mainly dispersed on the surfaces of AC particles. Based on these results, the possible mechanism of proton and electron transport in dual-channels is also illustrated in Fig. 1f. The protonic channels can increase accessibly active surface areas and reduce the diffusion distance of protons. While the electronic channels offers the fast electron transport during the charge/discharge process.
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Fig. 2 e (a) Cyclic voltammetry curves at a scan rate of 100 mV s¡1 of EDLCs with different electrodes, (b) Nyquist plots of EDLCs with different electrodes, (c) galvanostatic charge/discharge curves at a current density of 1 A g¡1 of EDLCs with different electrodes, (d) Cyclic voltammetry curves of the EDLC with the 2:3 electrodes, (e) The variation of capacitive retention with cycle numbers of the EDLC with the 2:3 electrodes (Inset: Final 10 cycles of the EDLC).
Electrochemical performance of EDLCs Electrochemical measurements were carried out in a twoelectrode system with a 1 M H2SO4 aqueous electrolyte. The electrochemical performance and comparison of the EDLCs with as-prepared electrodes are represented in Fig. 2. As shown in Fig. 2a, the CsPW has a negligible capacitance, which indicates that the AC is the dominant contributor to charge
storage in the other electrodes. Among the other electrodes which demonstrate rectangular shapes, the 2:3 electrode exhibits the highest specific capacitance because of the shortened diffusion and transport distance of protons (Fig. 1f) and reduced resistance to proton diffusion (Fig. 2b). But the further increase of CsPW in the 1:1 electrode results in a decreased specific capacitance because the reduced AB content induces a larger resistance to electronic conduction. The internal
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 1 7 9 7 e1 8 0 2
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Table 1 e Comparison of values in our work with that reported for EDLCs with AC-based electrodes. AC YF-50 YF-80 YP-50F YP-50F MSC-30c YF-50d a b c d
BET specific surface area (m2 g1)
Electrolyte
Specific capacitance (F g1)a
Reference
1600b 2347 1737 1700 3000 e
1 M H2SO4 1 M Na2SO4 6 M KOH 1 M H2SO4 0.25 M H2SO4 1 M H2SO4
152 (@ 1 A g1) 140 (@ 1 A g1) 93.7 (@1 mA cm2) ~135 (@ 1A g1) 161 (@5 mA cm2) 155 (@ 1A g1)
This work [25] [26] [27] [28] [29]
The value is for single electrode. Data from Kuraray retailer. Sulfonated poly(ether ether ketone) membrane as separator. Mass loading of active material per electrode is 0.5 mg cm2.
resistance and interfacial charge transfer of electrodes can be obtained from electrochemical impedance spectroscopies (EIS) and typical Nyquist plots of EIS are displayed in Fig. 2b. In the low frequency region, the straight lines that are nearly parallel to the imaginary axis suggest ideal capacitive charge storage behaviors in these electrodes. The interfacial charge transfer resistances (Rct) are evaluated by the diameter of the semicircles in the high frequency region. The 1:1 electrode displays the lowest Rct due to the well-constructed dualchannels, which agrees well with the analysis of Fig. 1f. The equivalent series resistances (Rs) of electrodes are provided by the intercepts of the semicircle on the real axis at high frequency. The AC electrode has the lowest Rs owing to the highest AB content. From the galvanostatic charge-discharge (GCD) curves with nearly triangular shapes in Fig. 2c, it can be calculated that the specific capacitances of AC, 1:2, 2:3 and 1:1 electrodes are 120, 143, 152 and 127 F g1 at a current density of 1 A g1, respectively. The highest specific capacitance is higher than or comparable to the values reported for AC-based supercapacitors in literature (Table 1). From Fig. 2d, it can be found that CV curves of the 2:3 electrode maintain quasi-rectangular profiles when the scan rate increases from 5 to 100 mV s1, which means that an ideal capacitive charge storage behavior and high rate capability [30]. As revealed in Fig. 2e, this electrode also displays an excellent cycling stability. In the first 100 cycles, its specific capacitance increases to 172 F g1 due to the activation of the electrode via increasing the contact area between the electrode and the electrolyte during the cycling process [31]. Thereafter, the specific capacitance slightly decreases, but it is still higher (104%) than the value of original cycle after 5000 cycles.
Conclusions In summary, protonic and electric dual-channels have been successfully constructed in electrodes by incorporating a kind of protonic conductor. The as-fabricated electrode displays an enhanced specific capacitance of 152 F g1 at a current density of 1 A g1 and remains 104% of its original specific capacitance after the cycling stability test of 5000 cycles. These results suggest it is a promising strategy to improve the electrochemical performance of electrodes with powdery AC materials.
Acknowledgments This work was supported by Hubei Provincial Natural Science Foundation of China (2018CFB267).
references
[1] Wang Y, Song Y, Xia Y. Electrochemical capacitors: mechanism, materials, systems, characterization and applications. Chem Soc Rev 2016;45:5925e50. [2] Simon P, Gogotsi Y. Materials for electrochemical capacitors. Nat Mater 2008;7:845e54. [3] Kurniawan A, Effendi C, Ong LK, Kurniawan F, Lin CX, Angkawijaya AE, et al. Preparation of nanoporous carbon microspheres by subcritical water carbonization and electrocapacitive study. Electrochim Acta 2013;111:99e107. [4] Li M, Xue J. Integrated synthesis of nitrogen-doped mesoporous carbon from melamine resins with superior performance in supercapacitors. J Phys Chem C 2014;118:2507e17. [5] Barzegar F, Bello A, Guellati O, Momodu DY, Harat A, Dangbegnon JK, et al. Effect of addition of different carbon materials on hydrogel derived carbon material for high performance electrochemical capacitors. Electrochim Acta 2015;186:277e84. [6] Xie K, Qin X, Wang X, Wang Y, Tao H, Wu Q, et al. Carbon nanocages as supercapacitor electrode materials. Adv Mater 2012;24:347e52. [7] Zheng C, Qian W, Cui C, Zhang Q, Jin Y, Zhao M, et al. Hierarchical carbon nanotube membrane with high packing density and tunable porous structure for high voltage supercapacitors. Carbon 2012;50:5167e75. [8] Lee Y-H, Chang K-H, Hu C-C. Differentiate the pseudocapacitance and double-layer capacitance contributions for nitrogen-doped reduced graphene oxide in acidic and alkaline electrolytes. J Power Sources 2013;227:300e8. [9] Krishnamoorthy K, Thangavel S, Chelora Veetil J, Raju N, Venugopal G, Kim SJ. Graphdiyne nanostructures as a new electrode material for electrochemical supercapacitors. Int J Hydrogen Energy 2016;41:1672e8. [10] Kim BC, Cho WJ, Lee WG, Kim SJ, Jalili R, Park SY, et al. Capacitive behaviour of thermally reduced graphene oxide in a novel ionic liquid containing di-cationic charge. Synth Met 2014;193:110e6. [11] Tamailarasan P, Ramaprabhu S. Carbon nanotubesgraphene-solidlike ionic liquid layer-based hybrid electrode material for high performance supercapacitor. J Phys Chem C 2012;116:14179e87.
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[12] Zaidi W, Boisset A, Jacquemin J, Timperman L, Anouti M. Deep eutectic solvents based on N-methylacetamide and a lithium salt as electrolytes at elevated temperature for activated carbon-based supercapacitors. J Phys Chem C 2014;118:4033e42. [13] Lee J-H, Lee S-H. Applications of novel carbon/AlPO4 hybridcoated H2Ti12O25 as a high-performance anode for cylindrical hybrid supercapacitors. ACS Appl Mater Interfaces 2016;8:28974e81. [14] Lee S-H, Kim JH, Yoon J-R. Laser scribed graphene cathode for next generation of high performance hybrid supercapacitors. Sci Rep 2018;8:8179. [15] Lee J-H, Kim H-K, Baek E, Pecht M, Lee SH, Lee YH. Improved performance of cylindrical hybrid supercapacitor using activated carbon/niobium doped hydrogen titanate. J Power Sources 2016;301:348e54. [16] Lee B-G, Lee S-H. Application of hybrid supercapacitor using granule Li4Ti5O12/activated carbon with variation of current density. J Power Sources 2017;343:545e9. [17] Lv P, Zhang P, Li F, Li Y, Feng Y, Feng W. Vertically aligned carbon nanotubes grown on carbon fabric with high rate capability for super-capacitors. Synth Met 2012;162:1090e6. [18] Cai T, Zhou M, Ren D, Han G, Guan S. Highly ordered mesoporous phenoleformaldehyde carbon as supercapacitor electrode material. J Power Sources 2013;231:197e202. [19] Anothumakkool B, Torris ATA, Veeliyath S, Vijayakumar V, Badiger MV, Kurungot S. High-performance flexible solidstate supercapacitor with an extended nanoregime interface through in situ polymer electrolyte generation. ACS Appl Mater Interfaces 2016;8:1233e41. [20] Amirinejad M, Madaeni SS, Rafiee E, Amirinejad S. Cesium hydrogen salt of heteropolyacids/Nafion nanocomposite membranes for proton exchange membrane fuel cells. J Membr Sci 2011;377:89e98. [21] Zhang H, Zhu B, Xu Y. Composite membranes of sulfonated poly(phthalazinone ether ketone) doped with 12phosphotungstic acid (H3PW12O40) for proton exchange membranes. Solid State Ionics 2006;177:1123e8.
[22] Dogan H, Inan TY, Unveren E, Kaya M. Effect of cesium salt of tungstophosphoric acid (Cs-TPA) on the properties of sulfonated polyether ether ketone (SPEEK) composite membranes for fuel cell applications. Int J Hydrogen Energy 2010;35:7784e95. [23] Zhang J, Sun M, Cao C, Zhang Q, Wang Y, Wan H. Effects of acidity and microstructure on the catalytic behavior of cesium salts of 12-tungstophosphoric acid for oxidative dehydrogenation of propane. Appl Catal, A 2010;380:87e94. [24] Dias JA, Caliman E, Loureiro Dias SC. Effects of cesium ion exchange on acidity of 12-tungstophosphoric acid. Microporous Mesoporous Mater 2004;76:221e32. € ckel N, Tolosa A, Presser V. Sub-micrometer [25] Kru¨ner B, Lee J, Ja novolac-derived carbon beads for high performance supercapacitors and redox electrolyte energy storage. ACS Appl Mater Interfaces 2016;8:9104e15. [26] Lee H, Kim B-H, Park Y-K, An K-H, Choi Y-J, Jung S-C. Synthesis of cobalt oxide-manganese oxide on activated carbon electrodes for electrochemical capacitor application using a liquid phase plasma method. Int J Hydrogen Energy 2016;41:7582e9. [27] Bi H, Lin T, Xu F, Tang Y, Liu Z, Huang F. New graphene form of nanoporous monolith for excellent energy storage. Nano Lett 2016;16:349e54. [28] Kim WJ, Kim D-W. Sulfonated poly(ether ether ketone) membranes for electric double layer capacitors. Electrochim Acta 2008;53:4331e5. [29] Lin T, Chen I-W, Liu F, Yang C, Bi H, Xu F, et al. Nitrogendoped mesoporous carbon of extraordinary capacitance for electrochemical energy storage. Science 2015;350:1508e13. [30] Feng D, Lv Y, Wu Z, Dou Y, Han L, Sun Z, et al. Free-standing mesoporous carbon thin films with highly ordered pore architectures for nanodevices. J Am Chem Soc 2011;133:15148e56. [31] Sun D, Aref AA, Wang B, Wang H, Qing C, Qu G, et al. Design and synthesis of hierarchical TiO2 micro-nano spheres/ Cu2O-Mn3O4 nanoflakes composite for high performance electrochemical electrodes. J Alloy Comp 2016;688:561e70.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Short Communication
Enhanced performance of carbon-based supercapacitors by constructing protonic and electric dual-channels in electrodes Na Liang 1, Guoqiang Li 1, Yongsheng Ji, Jing Xu, Danying Zuo, Likun Huang, Hongwei Zhang* School of Materials Science and Engineering, Wuhan Textile University, WuHan, 430073, PR China
article info
abstract
Article history:
Powdery carbonaceous materials have to use binder materials when they are integrated
Received 18 June 2018
into electrodes for supercapacitors, which will results in high interfacial charge transfer
Received in revised form
resistances and reduced specific capacitance. To resolve the problem, protonic and electric
7 November 2018
dual-channels are constructed in electrodes by in situ synthesis of cesium hydrogen salt of
Accepted 13 November 2018
phosphotungstic acid on the surface of carbonaceous materials. The cesium hydrogen salt
Available online 5 December 2018
particles are confirmed by a Fourier transform infrared spectroscopy, X-ray diffractometer and an energy dispersive X-ray spectroscopy. The electrochemical properties of as-
Keywords:
fabricated electrodes are measured by cyclic voltammetry, galvanostatic charging-
Dual-channel
discharging, and impedance analysis with an electrochemical workstation. At a current
Electrode
density of 1 A g1, the electrode shows a specific capacitance of 152 F g1. Compared to the
EDLC
electrode without the cesium hydrogen salt, the value increases 25% at least. Furthermore,
Performance
the specific capacitance retention of the electrode reaches 104% of its original capacitance after 5000 charge-discharge cycles, suggesting excellent cycling stability. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction With the increasing energy demands and the worsening environmental issues, advanced energy storage and conversion devices (e.g. batteries, fuel cells, and supercapacitors) are attracting widespread attention. Among them, supercapacitors have attracted increasing interest in the past decade because of their prominent advantages of high power density, fast charge/discharge rate, high safety and long cycle
lifetime [1]. Depending on the energy storage mechanism, supercapacitors can be classed as electrical double-layer capacitors (EDLCs) and pseudo-capacitors. Compared to pseudocapacitors, EDLCs have been commercialized and even used on an Airbus A380 in emergency doors due to their high power delivery and long lifecycle [2]. However, EDLCs still suffer from low energy density, which often results in a discharge time less than a minute. To overcome the obstacle, several intensive approaches have been adopted. The first approach is exploring new carbonaceous materials with high surface area,
* Corresponding author. E-mail addresses: [email protected], [email protected] (H. Zhang). 1 The two authors contributed equally to this work. https://doi.org/10.1016/j.ijhydene.2018.11.107 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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such as porous carbon spheres [3], mesoporous carbons [4], carbon aerogels [5], carbon nanocages [6], carbon nanotube [7], graphene [8] and graphdiyne [9]. The second approach is selecting new electrolytes to expand the operating voltages [10e12]. The third approach is constructing hybrid supercapacitors, including improving the power density and cycle life of the anode, suppressing the swelling for higher reliability and safety, improving the energy density of the cathode [13e16]. The fourth approach is engineering the interfaces to facilitate ion transport and shorten electron transfer distance, including directly growing carbonaceous materials on current collectors [17], improving wettability of carbon materials [18] and introducing an extended nanoregime interface [19]. But binder materials are still needed to integrate powdery carbonaceous materials into electrodes, which will lead to reduced specific capacitances of active materials because binder materials are often electronic and protonic insulators. In this study, we proposed a strategy to alleviate such an issue by constructing protonic and electric dual-channels in electrodes. As a proof of concept herein, cesium hydrogen salt of phosphotungstic acid (CsPW, Cs2.5H0.5PW12O40), which has strong acidity, and acetylene black (AB) were incorporated into electrodes to form protonic and electric channels, respectively. The specific capacitance of as-prepared electrodes increased to 152 F g1 from 120 F g1 of the control electrode without CsPW.
Experimental Synthesis of CsPW CsPW was synthesized by precipitation titration [20]. A typical synthesis reaction was as follows: Firstly, the aqueous solutions of cesium carbonate (Cs2CO3, 0.1 M) and phosphotungstic acid (PWA, H3PW12O40, 0.08 M) were prepared. Secondly, the solution of Cs2CO3 (20 mL) was slowly dropped in to 20 mL of PWA solution with stirring at room temperature. Thirdly, the as-obtained colloidal solution was stirred for 24 h and followed by centrifugation and freeze-thawing. Lastly, the resulting powders were dried at 110 C in a vacuum oven.
Preparation of electrodes with dual-channels In order to prepared carbon electrodes with dual-channels, activated carbon (AC, Kurary YP-50F with surface area of 1600 m2 g1) was firstly mixed with Cs2CO3 solution under magnetic stirring for 12 h. Then the PWA solution was added dropwise and stirred for another 12 h to synthesize CsPW particles on AC surfaces. Thereafter acetylene black (AB) and polytetrafluoroethylene (PTFE) were introduced under ultrasonic agitation. Subsequently, the mixed slurry was coated and pressed on a cleaned slice of stainless steel meshes as a current collector. Finally, the electrodes were dried at 80 C for 24 h. The geometric area of electrodes was 1.0 1.0 cm2 and the mass loading of the slurry on an electrode was about 6e9 mg cm2. The mass ratio of AC, CsPW þ AB and PTFE was 0.8:1.5:0.5. The mass ratio values of CsPW and AB were selected as 0, 0.5, 2/3 and 1. For the convenience of description, the corresponding electrodes were denominated as AC, 1:2, 2:3
and 1:1, respectively. As a control, the electrode without AC and with CsPW as active materials was also prepared and named as CsPW. The mass ratio of CsPW, AB and PTFE was 0.8:1.5:0.5.
Materials characterization The surface morphology of the samples was characterized by a scanning electron microscope (SEM, JEOL JSM-IT300) and the elemental mapping image was revealed by an energy dispersive X-ray spectroscopy (EDX). The Fourier Transform infrared spectroscopy (FTIR) spectra were recorded using a Nicolet 6700 FTIR spectrometer. X-ray diffractometer (X'Pert PRO) was adopted to determine the structure of CsPW and PWA particles. Cu Ka (l ¼ 1.54 Å) was used as the X-ray source at dgenerator voltage of 45 kV and current of 80 mA. Samples were scanned in 2q ranges from 10 to 90 , in steps of 0.02 and counting time 2 s per step.
Electrochemical measurements Electrochemical workstation (CHI 660E, Shanghai Chen Hua Co., Ltd) was used for all electrochemical tests. Electrochemical measurements were carried out in a two-electrode system with a 1 M H2SO4 aqueous electrolyte. The specific capacitance of the single electrode was calculated from the galvanostatic charge/discharge data by using the equation of C ¼ 4IDt/(mDV), where I is the constant current (A), m is the total mass of AC for both electrodes (g), Dt is the discharge time (s), and DV is the voltage range after the IR drop during the discharge process (V).
Results and discussion Morphology analysis The FTIR spectra of PWA, CsPW and the 2:3 electrode powders are shown in Fig. 1a. It can be found that the primary Keggin structure of PWA have been remained in its cesium salt form (CsPW). The characteristic peaks of the Keggin structure centered at 1078, 986, 890, 804 and 596 cm1 are assigned to stretching vibrations of PeO in central tetrahedral, W¼Od terminal oxygen in the Keggin structure, WeOeW corners shared bonds, WeOeW edges shared bonds and the bending vibration of OePeO, respectively [20e22]. The bands at 1617 and 3447 cm1 are respectively attributed to the deformation vibration and the stretching of hydroxyl groups of H2O [20]. Compared to PWA, the shifts of WeOeW edges shared bonds and water peaks in CsPW may indicate the interaction between Csþ cations and heteropolyacid anions [22]. Although the 10%CsPW/90%AC has a low content of CsPW, its FTIR spectrum also shows characteristic bands with low intensity of the Keggin structure at 1078, 986, 890, 804 and 596 cm1, which suggests the existence of CsPW on the AC powders. The XRD patterns of PWA, CsPW and the 2:3 electrode powders are shown in Fig. 2b. It can be seen that the diffraction peaks of CsPW become broader and shift toward slightly higher 2q angles as compared with PWA. It indicates the reduced cubic lattice constants of CsPW after the H5Oþ 2 ions in PWA is
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Fig. 1 e (a) FTIR spectra (b) XRD patterns of PWA, CsPW and 2:3 electrode, SEM image of (c) AC, (d) an electrode with CsPW and (e) element mapping image of Cs in the electrode, (f) Schematic illustration for the transport of proton and electron in dual-channels.
partially replaced by smaller Csþ ions to form CsPW [22e24]. From the results of XRD and FT-IR, it can be concluded that the CsPW is successfully synthesized on AC powders. As shown in Fig. 1c, it can be observed that the AC particle shape is irregular with particle sizes ranging from several hundred nanometers to larger than 10 mm. When they are incorporated into an electrode, the space between AC particles is filled by AB particles (Fig. 1d). Furthermore, it can be
deduced from the element mapping image of Cs in Fig. 1e that the CsPW particles are mainly dispersed on the surfaces of AC particles. Based on these results, the possible mechanism of proton and electron transport in dual-channels is also illustrated in Fig. 1f. The protonic channels can increase accessibly active surface areas and reduce the diffusion distance of protons. While the electronic channels offers the fast electron transport during the charge/discharge process.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 1 7 9 7 e1 8 0 2
Fig. 2 e (a) Cyclic voltammetry curves at a scan rate of 100 mV s¡1 of EDLCs with different electrodes, (b) Nyquist plots of EDLCs with different electrodes, (c) galvanostatic charge/discharge curves at a current density of 1 A g¡1 of EDLCs with different electrodes, (d) Cyclic voltammetry curves of the EDLC with the 2:3 electrodes, (e) The variation of capacitive retention with cycle numbers of the EDLC with the 2:3 electrodes (Inset: Final 10 cycles of the EDLC).
Electrochemical performance of EDLCs Electrochemical measurements were carried out in a twoelectrode system with a 1 M H2SO4 aqueous electrolyte. The electrochemical performance and comparison of the EDLCs with as-prepared electrodes are represented in Fig. 2. As shown in Fig. 2a, the CsPW has a negligible capacitance, which indicates that the AC is the dominant contributor to charge
storage in the other electrodes. Among the other electrodes which demonstrate rectangular shapes, the 2:3 electrode exhibits the highest specific capacitance because of the shortened diffusion and transport distance of protons (Fig. 1f) and reduced resistance to proton diffusion (Fig. 2b). But the further increase of CsPW in the 1:1 electrode results in a decreased specific capacitance because the reduced AB content induces a larger resistance to electronic conduction. The internal
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 1 7 9 7 e1 8 0 2
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Table 1 e Comparison of values in our work with that reported for EDLCs with AC-based electrodes. AC YF-50 YF-80 YP-50F YP-50F MSC-30c YF-50d a b c d
BET specific surface area (m2 g1)
Electrolyte
Specific capacitance (F g1)a
Reference
1600b 2347 1737 1700 3000 e
1 M H2SO4 1 M Na2SO4 6 M KOH 1 M H2SO4 0.25 M H2SO4 1 M H2SO4
152 (@ 1 A g1) 140 (@ 1 A g1) 93.7 (@1 mA cm2) ~135 (@ 1A g1) 161 (@5 mA cm2) 155 (@ 1A g1)
This work [25] [26] [27] [28] [29]
The value is for single electrode. Data from Kuraray retailer. Sulfonated poly(ether ether ketone) membrane as separator. Mass loading of active material per electrode is 0.5 mg cm2.
resistance and interfacial charge transfer of electrodes can be obtained from electrochemical impedance spectroscopies (EIS) and typical Nyquist plots of EIS are displayed in Fig. 2b. In the low frequency region, the straight lines that are nearly parallel to the imaginary axis suggest ideal capacitive charge storage behaviors in these electrodes. The interfacial charge transfer resistances (Rct) are evaluated by the diameter of the semicircles in the high frequency region. The 1:1 electrode displays the lowest Rct due to the well-constructed dualchannels, which agrees well with the analysis of Fig. 1f. The equivalent series resistances (Rs) of electrodes are provided by the intercepts of the semicircle on the real axis at high frequency. The AC electrode has the lowest Rs owing to the highest AB content. From the galvanostatic charge-discharge (GCD) curves with nearly triangular shapes in Fig. 2c, it can be calculated that the specific capacitances of AC, 1:2, 2:3 and 1:1 electrodes are 120, 143, 152 and 127 F g1 at a current density of 1 A g1, respectively. The highest specific capacitance is higher than or comparable to the values reported for AC-based supercapacitors in literature (Table 1). From Fig. 2d, it can be found that CV curves of the 2:3 electrode maintain quasi-rectangular profiles when the scan rate increases from 5 to 100 mV s1, which means that an ideal capacitive charge storage behavior and high rate capability [30]. As revealed in Fig. 2e, this electrode also displays an excellent cycling stability. In the first 100 cycles, its specific capacitance increases to 172 F g1 due to the activation of the electrode via increasing the contact area between the electrode and the electrolyte during the cycling process [31]. Thereafter, the specific capacitance slightly decreases, but it is still higher (104%) than the value of original cycle after 5000 cycles.
Conclusions In summary, protonic and electric dual-channels have been successfully constructed in electrodes by incorporating a kind of protonic conductor. The as-fabricated electrode displays an enhanced specific capacitance of 152 F g1 at a current density of 1 A g1 and remains 104% of its original specific capacitance after the cycling stability test of 5000 cycles. These results suggest it is a promising strategy to improve the electrochemical performance of electrodes with powdery AC materials.
Acknowledgments This work was supported by Hubei Provincial Natural Science Foundation of China (2018CFB267).
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