- Email: [email protected]
Mesopore-dominant activated carbon aerogels with high surface area for electric double-layer capacitor application
Author’s Accepted Manuscript Mesopore-dominant activated carbon aerogels with High surface area for Electric double-layer capacitor application Chengfei Li, Xiaoqing Yang, Guoqing Zhang www.elsevier.com
PII: DOI: Reference:
S0167-577X(15)30514-0 http://dx.doi.org/10.1016/j.matlet.2015.09.003 MLBLUE19519
To appear in: Materials Letters Received date: 1 July 2015 Revised date: 9 August 2015 Accepted date: 1 September 2015 Cite this article as: Chengfei Li, Xiaoqing Yang and Guoqing Zhang, Mesoporedominant activated carbon aerogels with High surface area for Electric doublelayer capacitor application, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2015.09.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. 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.
Mesopore-dominant activated carbon aerogels with high surface area for Electric double-layer capacitor application Chengfei Li, Xiaoqing Yang*, Guoqing Zhang School of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, PR China
Abstract: A very important yet challenging issue to address is how to greatly increase the surface area of porous carbons while keeping a developed mesoporosity for ion diffusion/transfer in Electric double-layer capacitor application. Herein we report the fabrication of a new class of mesopore-dominant activated carbon aerogel (ACA-P) with high surface area based on the pore formation and widening effect of H3PO4. ACA-P shows a specific capacitance of 201 F g-1 at 0.1 A g-1, which stays over 90% (180 F g-1) as the current density increases to 5 A g-1. This excellent supercapacitive behavior is ascribed to the high surface area of 2477 m2 g-1 donated by the extremely high mesopore ratio of 99%, which can guarantee rapid diffusion/transfer of electrolyte. Keywords: Carbon materials; Mesopore; Electric double-layer capacitors; Carbon aerogel; Microstructure; 1. Introduction Electric double-layer capacitors (EDLCs) have attracted widespread interest as promising energy storage because of their high power density, long cycle life and fast charge-discharge capability [1-5]. From a material science point of view, the performance of EDLCs depends on the nanostructure of the electrode materials [5, 6]. Porous carbon material (PCM) is now the most widely used electrode material due to its large surface area and high porosity, which can offer abundant active sites for electric double-layer formation [1, 4, 7].
* Corresponding author: Xiaoqing Yang Tel: +86-020-39322570. Fax: +86-020-39322570 E-mail addresses: [email protected]
In general, tremendous ion-accessible surface area is considered necessary for EDLCs electrode material with high energy density. Thus many efforts have been made to increase the surface area by donating microporosity to further enhance the capacitance, such as ZnCl2 [1] , KOH [8] and NH3 [9] activation. However, illimitable introduction of micropores will undoubtedly cause sharp reduction of mesoporosity and enhanced internal resistance, hindering rapid transfer/diffusion of electrolyte and electron throughout the entire surface and carbon skeleton, respectively, and thereby reduce the specific capacitance at high current density. For example, Fang et al [10] prepared a kind of activated carbon aerogel (ACA) by KOH activation for EDLCs application. It presented a much higher capacitance compared with the non-activated carbon aerogel (CA, 130 vs. 50 F g-1) but a much lower capacitance retention (46 % vs. 80 %) since the mesopore ratio (Vmes%) of CA decreased from 63% to 24% after activation. Therefore, the most important yet really challenging issue to address is how to greatly increase the surface area of PCMs without sacrificing the mesoporosity. Herein, we develop a class of mesopore-dominant ACA with high surface area based on the pore formation and widening of CA using H3PO4 as the porogen, and highlight its superior ability for energy storage in EDLCs. It has been proved that H3PO4 can not only act like a porogen to introduce micropores on PCM, but also continue etching the obtained micropores to small mesopores [11, 12], thus obtaining high mesoporous surface area. 2. Experimental. 2.1 Preparation of ACA. ACA was prepared via a microemulsion-templated method with some modifications [6]. Briefly, resorcinol (R), formaldehyde (F), deionized water and cetyltrimethylammonium bromide (CTAB) were mixed with a R/F/CTAB molar ratio of 125:250:1 under magnetic stirring and then transferred into a glass vial. The vial was sealed and put into a water bath (85 oC) to cure for 3 days. After curing, the as-obtained organic aerogel (OA) was dried in an oven at 100 oC for over 8 h. Subsequently, approximately 2 g of the as-prepared OA was mixed with 4 mL of 85% H3PO4. The mixture was dried at 110 oC and then carbonized at 900 oC for 3 h under N2 atmosphere. The obtained ACA was denoted as ACA-P.
For comparison, another sample of non-activated CA was prepared through the same procedure without adding H3PO4. 2.2 Structure Characterization. The microstructure of the samples was observed by a JEOL JSM-6330F Scanning Electron Microscope (SEM) and a JEOL JEM-2010 transmission electron microscope (TEM). N2 adsorption/desorption isotherms were taken using an ASAP 2020 surface area and porosity analyzer (Micrometrics Instrument Corporation). Brunauer-Emmett-Teller (BET) method was utilized to calculate the BET surface area (SBET). Micropore volume (Vmic), mesopore volume (Vmes) and pore size distribution (PSD) curves of the samples were analyzed by t-plot, Barrett-Joyner-Halendar and density functional theory, respectively. 2.3 Electrochemical measurements The electrochemical performance of the samples was measured in 1 M H2SO4 using a sandwich-type two-electrode testing cell. Composite electrode comprising active mass (80 wt.%), acetylene black (10 wt.%) and poly(vinylidene difluoride) (10 wt.%) binder was used as working electrodes. Galvanostatic charge-discharge tests were executed using a Xinwei battery test equipment (CT2001A). Cyclic voltammetry (CV) was also recorded using the same cell. For comparison, a commercially available activated carbon (AC-YP) for supercapacitors application from Kuraray Chemical Co. LTD was also tested and served as a reference. The specific capacitance Cg (in F g-1) can be calculated from the discharge curves by the formula C g =
I ⋅ ∆ t m 1 + m 2 , where I, △t, △U, m and m are the discharge current, ⋅ 1 2 ∆U m1 ⋅ m 2
discharge time, discharge voltage, mass of the positive and negative active electrode materials, respectively. 3. Results and discussions Figure 1 shows the SEM and TEM images of the AC-YP and as-prepared CA/ACA-P samples. CA presents a three-dimensional network consisting of numerous interconnected nanoparticles with uniform size of 20 nm, leading to a continuous mesoporous structure. After activation, these nanoparticles aggregate to larger carbon bulks because the etching effect of H3PO4 reduces the strength of the carbon skeleton. These nanobulks themselves contain numerous
small-sized nanopores (Figure 1d). On the other hand, carbon blocks without any meso/macroporosity can be found on commercial AC-YP (Figure 1c).
Figure 1. SEM images of a) CA, b) ACA-P and c) AC-YP; d) TEM image of ACA-P N2 adsorption-desorption isotherms were performed to analyze the nanostructure of the as-prepared samples quantitatively (Figure 2a). The uptakes at high and medium relative pressure (P/P0) of CA and ACA-P indicate that they are both typical mesoporous PCMs [7, 10]. The mesopore constructed by the interconnected nanoparticles of CA is centered at 18 nm (Figure 2b), which disappears after activation, consisting with the SEM result of aggregation phenomenon. Instead, because of the aforementioned pore formation and widening effect of H3PO4, ACA-P shows uniform small mesopore centered at 2.5 nm. Such small mesopores are beneficial to constructing large ion-accessible surface area. As a result, ACA-P presents a much larger SBET of 2477 m2 g-1 than CA (635 m2 g-1), while simultaneously keeping an extremely developed Vmes% of 99%. This surface area is much higher than that of most mesopore-dominant carbons. Different to ACA-P, AC-YP shows a micropore-dominant nanostructure and a surface area of 1417 m2 g-1
(Figure 2 and Table 1). We prospect that such a large surface area arisen from the high mesoporosity will make ACA-P particularly enticing for energy storage in EDLCs, because it can facilitate better diffusion and transport of electrolyte. Thus, we carried out electrochemical measurements for the as-prepared samples.
a)
800
ACA-P CA AC-YP
3 -1 Differential Pore Volume (cm g )
3 Quantity Adsorbed (cm /g STP)
1000
600 400 200 0 0.0
0.2
0.4
0.6
0.8
1.0
3.0
ACA-P CA AC-YP
b)
2.5 2.0 1.5 1.0 0.5 0.0 1
Realative presure (P/P0)
10
100
Pore width (nm)
Figure 2. a) Nitrogen adsorption-desorption isotherms at 77 K of the samples and b) their corresponding PSD curves. Table 1 Pore parameters of the as-prepared samples Sample
Vmes%*
SBET
Vmic
Vmes
(m2 g-1)
(cm3 g-1)
(cm3 g-1)
CA
635
0.18
0.85
83%
ACA-P
2477
0.01
1.21
99%
AC-YP
1417
0.48
0.21
30%
* Vmes%=Vmes/(Vmes+Vmic)×100% Figure 3a provides the galvanostatic charge-discharge curves of the as-prepared samples at a current density of 0.1 A g-1. Owing to the highest surface area of 2477 m2 g-1, ACA-P shows the longest discharge time, demonstrating that it has a much higher Cg than CA and commercial AC-YP. The calculated Cg of ACA-P, CA and AC-YP are 201, 87 and 161 F g-1, respectively. Meanwhile, as we mentioned above, the extremely high mesoporosity will facilitate better diffusion/transfer of electrolyte, thus resulting in high capacitance retention at higher charge-discharge rates. As shown in Figure 3b, with increasing the current density, ACA-P only shows slight decrease of the capacitance. For example, the
capacitance retention stays above 90% (180 F g-1) when the current density increases by 50 times (5 A g-1), whereas that of AC-YP decreases sharply to 74%, indicative of much more inaccessible surface area of AC-YP at high current density.
Potential(V)
0.8 0.6
ACA-P CA AC-YP
0.4 0.2
b)
100
Capacitance retention (%)
a)
1.0
80
ACA-P AC-YP
60
40
20
0.0 0
500
1000
1500
2000
2500
0
1
Time (s)
c)
ACA-P CA AC-YP
1.5
3
-1
4
5
1.0 0.5 0.0
-0.5 -1.0
d)
120
Capacitance retntion %
-1 Current density (A g )
2.0
2
Current density (A g )
ACA-P
100 80 60 40 20 0
0.0
0.2
0.4
0.6
0.8
Voltage(V)
1.0
0
1000
2000
3000
4000
5000
Cycle number
Figure 3. a) Galvanostatic charge–discharge curves at a current density of 0.1 A g−1, b) capacitance retention at different current densities, c) CV curves at a scan rate of 5 mV s-1 and d) Cycle stability of ACA-P at 1 A g-1 This excellent mass transfer capability of ACA-P can be confirmed by CV measurements (Figure 3c). Generally, the rectangular shape of the CV curves can be used to estimate the ability of ion diffusion/transfer within the nanocarbon structure [13]. It can be observed from the CV curves that ACA-P shows near-rectangular shape, as compared to AC-YP with a distorted shape, implying the important role of developed mesoporosity in diffusion/transfer of the electrolyte. Furthermore, the cycling performance of ACA-P was investigated by repeating the charge/discharge test at 1 A g-1 for 5000 cycles. Figure 3d shows extraordinarily high capacitance retention of 100% during the cycles, demonstrating the excellent stability of its supercapacitive behavior.
4. Conclusions. A mesopore-dominant ACA-P with high surface area was prepared based on the pore formation and widening effect of H3PO4. A large number of small mesopores whose size is large enough for rapid diffusion/transfer of electrolyte are constructed during H3PO4 activation, providing large interfaces for efficient electric-double layer formation. As a result, ACA-P shows much higher Cg of 201 F g-1 and capacitance retention of 90% when the current density increases by 50 times compared with commercial AC-YP (161 and 74%). It can be concluded that the as-prepared ACA-P with large surface area and developed mesoporous structures is very promising for use as electrodes materials in EDLCs with high power density and energy density.
Acknowledgements This research was financially supported by the Start-up funding for “One-Hundred Young Talents” of Guangdong University of technology (220413521). References [1] Zhang ZJ, Dong C, Ding XY, Xia YK. A generalized ZnCl2 activation method to produce nitrogen-containing nanoporous carbon materials for supercapacitor applications. Journal of Alloys and Compounds. 2015;636:275-81. [2] Xiong G, Hembram KPSS, Reifenberger RG, Fisher TS. MnO2-coated graphitic petals for supercapacitor electrodes. Journal of Power Sources. 2013;227:254-9. [3] Xiong G, Meng C, Reifenberger RG, Irazoqui PP, Fisher TS. A Review of Graphene-Based Electrochemical Microsupercapacitors. Electroanalysis. 2014;26:30-51. [4] Yang C, Li D. Flexible and foldable supercapacitor electrodes from the porous 3D network of cellulose nanofibers, carbon nanotubes and polyaniline. Materials Letters. 2015;155:78-81. [5] Xiong G, Meng C, Reifenberger RG, Irazoqui PP, Fisher TS. Graphitic Petal Electrodes for All-Solid-State Flexible
Supercapacitors. Advanced Energy Materials. 2014;4:n/a-n/a. DOI: 10.1002/aenm.201300515 [6] Wang J, Yang X, Wu D, Fu R, Dresselhaus MS, Dresselhaus G. The porous structures of activated carbon aerogels and their effects on electrochemical performance. Journal of Power Sources. 2008;185:589-94. [7] Zheng DF, Jia MQ, Xu B, Zhang H, Cao GP, Yang YS. The simple preparation of a hierarchical porous carbon with high surface area for high performance supercapacitors. New Carbon Materials. 2013;28:151-4. [8] Wang J, Kaskel S. KOH activation of carbon-based materials for energy storage. Journal of Materials Chemistry. 2012;22:23710. [9] Kim ND, Kim SJ, Kim GP, Nam I, Yun HJ, Kim P, et al. NH3-activated polyaniline for use as a high performance electrode material in supercapacitors. Electrochimica Acta. 2012;78:340-6. [10] Fang B, Binder L. A modified activated carbon aerogel for high-energy storage in electric double layer capacitors. Journal of Power Sources. 2006;163:616-22. [11] Yu HR, Cho S, Jung MJ, Lee YS. Electrochemical and structural characteristics of activated carbon-based electrodes modified via phosphoric acid. Microporous and Mesoporous Materials. 2013;172:131-5. [12] Nahil MA, Williams PT. Pore characteristics of activated carbons from the phosphoric acid chemical activation of cotton stalks. Biomass and Bioenergy. 2012;37:142-9. [13] Zhong H, Xu F, Li ZH, Fu RW, Wu DC. High-energy supercapacitors based on hierarchical porous carbon with an ultrahigh ion-accessible surface area in ionic liquid electrolytes. Nanoscale 2013;5:1-5.
Highlights
Activated carbon aerogel (ACA) with developed mesoporosity is prepared.
ACA presents extremely high surface area donated by high mesopore ratio of 99%.
ACA shows better supercapacitive behavior compared to commercial activated carbon.
PII: DOI: Reference:
S0167-577X(15)30514-0 http://dx.doi.org/10.1016/j.matlet.2015.09.003 MLBLUE19519
To appear in: Materials Letters Received date: 1 July 2015 Revised date: 9 August 2015 Accepted date: 1 September 2015 Cite this article as: Chengfei Li, Xiaoqing Yang and Guoqing Zhang, Mesoporedominant activated carbon aerogels with High surface area for Electric doublelayer capacitor application, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2015.09.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. 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.
Mesopore-dominant activated carbon aerogels with high surface area for Electric double-layer capacitor application Chengfei Li, Xiaoqing Yang*, Guoqing Zhang School of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, PR China
Abstract: A very important yet challenging issue to address is how to greatly increase the surface area of porous carbons while keeping a developed mesoporosity for ion diffusion/transfer in Electric double-layer capacitor application. Herein we report the fabrication of a new class of mesopore-dominant activated carbon aerogel (ACA-P) with high surface area based on the pore formation and widening effect of H3PO4. ACA-P shows a specific capacitance of 201 F g-1 at 0.1 A g-1, which stays over 90% (180 F g-1) as the current density increases to 5 A g-1. This excellent supercapacitive behavior is ascribed to the high surface area of 2477 m2 g-1 donated by the extremely high mesopore ratio of 99%, which can guarantee rapid diffusion/transfer of electrolyte. Keywords: Carbon materials; Mesopore; Electric double-layer capacitors; Carbon aerogel; Microstructure; 1. Introduction Electric double-layer capacitors (EDLCs) have attracted widespread interest as promising energy storage because of their high power density, long cycle life and fast charge-discharge capability [1-5]. From a material science point of view, the performance of EDLCs depends on the nanostructure of the electrode materials [5, 6]. Porous carbon material (PCM) is now the most widely used electrode material due to its large surface area and high porosity, which can offer abundant active sites for electric double-layer formation [1, 4, 7].
* Corresponding author: Xiaoqing Yang Tel: +86-020-39322570. Fax: +86-020-39322570 E-mail addresses: [email protected]
In general, tremendous ion-accessible surface area is considered necessary for EDLCs electrode material with high energy density. Thus many efforts have been made to increase the surface area by donating microporosity to further enhance the capacitance, such as ZnCl2 [1] , KOH [8] and NH3 [9] activation. However, illimitable introduction of micropores will undoubtedly cause sharp reduction of mesoporosity and enhanced internal resistance, hindering rapid transfer/diffusion of electrolyte and electron throughout the entire surface and carbon skeleton, respectively, and thereby reduce the specific capacitance at high current density. For example, Fang et al [10] prepared a kind of activated carbon aerogel (ACA) by KOH activation for EDLCs application. It presented a much higher capacitance compared with the non-activated carbon aerogel (CA, 130 vs. 50 F g-1) but a much lower capacitance retention (46 % vs. 80 %) since the mesopore ratio (Vmes%) of CA decreased from 63% to 24% after activation. Therefore, the most important yet really challenging issue to address is how to greatly increase the surface area of PCMs without sacrificing the mesoporosity. Herein, we develop a class of mesopore-dominant ACA with high surface area based on the pore formation and widening of CA using H3PO4 as the porogen, and highlight its superior ability for energy storage in EDLCs. It has been proved that H3PO4 can not only act like a porogen to introduce micropores on PCM, but also continue etching the obtained micropores to small mesopores [11, 12], thus obtaining high mesoporous surface area. 2. Experimental. 2.1 Preparation of ACA. ACA was prepared via a microemulsion-templated method with some modifications [6]. Briefly, resorcinol (R), formaldehyde (F), deionized water and cetyltrimethylammonium bromide (CTAB) were mixed with a R/F/CTAB molar ratio of 125:250:1 under magnetic stirring and then transferred into a glass vial. The vial was sealed and put into a water bath (85 oC) to cure for 3 days. After curing, the as-obtained organic aerogel (OA) was dried in an oven at 100 oC for over 8 h. Subsequently, approximately 2 g of the as-prepared OA was mixed with 4 mL of 85% H3PO4. The mixture was dried at 110 oC and then carbonized at 900 oC for 3 h under N2 atmosphere. The obtained ACA was denoted as ACA-P.
For comparison, another sample of non-activated CA was prepared through the same procedure without adding H3PO4. 2.2 Structure Characterization. The microstructure of the samples was observed by a JEOL JSM-6330F Scanning Electron Microscope (SEM) and a JEOL JEM-2010 transmission electron microscope (TEM). N2 adsorption/desorption isotherms were taken using an ASAP 2020 surface area and porosity analyzer (Micrometrics Instrument Corporation). Brunauer-Emmett-Teller (BET) method was utilized to calculate the BET surface area (SBET). Micropore volume (Vmic), mesopore volume (Vmes) and pore size distribution (PSD) curves of the samples were analyzed by t-plot, Barrett-Joyner-Halendar and density functional theory, respectively. 2.3 Electrochemical measurements The electrochemical performance of the samples was measured in 1 M H2SO4 using a sandwich-type two-electrode testing cell. Composite electrode comprising active mass (80 wt.%), acetylene black (10 wt.%) and poly(vinylidene difluoride) (10 wt.%) binder was used as working electrodes. Galvanostatic charge-discharge tests were executed using a Xinwei battery test equipment (CT2001A). Cyclic voltammetry (CV) was also recorded using the same cell. For comparison, a commercially available activated carbon (AC-YP) for supercapacitors application from Kuraray Chemical Co. LTD was also tested and served as a reference. The specific capacitance Cg (in F g-1) can be calculated from the discharge curves by the formula C g =
I ⋅ ∆ t m 1 + m 2 , where I, △t, △U, m and m are the discharge current, ⋅ 1 2 ∆U m1 ⋅ m 2
discharge time, discharge voltage, mass of the positive and negative active electrode materials, respectively. 3. Results and discussions Figure 1 shows the SEM and TEM images of the AC-YP and as-prepared CA/ACA-P samples. CA presents a three-dimensional network consisting of numerous interconnected nanoparticles with uniform size of 20 nm, leading to a continuous mesoporous structure. After activation, these nanoparticles aggregate to larger carbon bulks because the etching effect of H3PO4 reduces the strength of the carbon skeleton. These nanobulks themselves contain numerous
small-sized nanopores (Figure 1d). On the other hand, carbon blocks without any meso/macroporosity can be found on commercial AC-YP (Figure 1c).
Figure 1. SEM images of a) CA, b) ACA-P and c) AC-YP; d) TEM image of ACA-P N2 adsorption-desorption isotherms were performed to analyze the nanostructure of the as-prepared samples quantitatively (Figure 2a). The uptakes at high and medium relative pressure (P/P0) of CA and ACA-P indicate that they are both typical mesoporous PCMs [7, 10]. The mesopore constructed by the interconnected nanoparticles of CA is centered at 18 nm (Figure 2b), which disappears after activation, consisting with the SEM result of aggregation phenomenon. Instead, because of the aforementioned pore formation and widening effect of H3PO4, ACA-P shows uniform small mesopore centered at 2.5 nm. Such small mesopores are beneficial to constructing large ion-accessible surface area. As a result, ACA-P presents a much larger SBET of 2477 m2 g-1 than CA (635 m2 g-1), while simultaneously keeping an extremely developed Vmes% of 99%. This surface area is much higher than that of most mesopore-dominant carbons. Different to ACA-P, AC-YP shows a micropore-dominant nanostructure and a surface area of 1417 m2 g-1
(Figure 2 and Table 1). We prospect that such a large surface area arisen from the high mesoporosity will make ACA-P particularly enticing for energy storage in EDLCs, because it can facilitate better diffusion and transport of electrolyte. Thus, we carried out electrochemical measurements for the as-prepared samples.
a)
800
ACA-P CA AC-YP
3 -1 Differential Pore Volume (cm g )
3 Quantity Adsorbed (cm /g STP)
1000
600 400 200 0 0.0
0.2
0.4
0.6
0.8
1.0
3.0
ACA-P CA AC-YP
b)
2.5 2.0 1.5 1.0 0.5 0.0 1
Realative presure (P/P0)
10
100
Pore width (nm)
Figure 2. a) Nitrogen adsorption-desorption isotherms at 77 K of the samples and b) their corresponding PSD curves. Table 1 Pore parameters of the as-prepared samples Sample
Vmes%*
SBET
Vmic
Vmes
(m2 g-1)
(cm3 g-1)
(cm3 g-1)
CA
635
0.18
0.85
83%
ACA-P
2477
0.01
1.21
99%
AC-YP
1417
0.48
0.21
30%
* Vmes%=Vmes/(Vmes+Vmic)×100% Figure 3a provides the galvanostatic charge-discharge curves of the as-prepared samples at a current density of 0.1 A g-1. Owing to the highest surface area of 2477 m2 g-1, ACA-P shows the longest discharge time, demonstrating that it has a much higher Cg than CA and commercial AC-YP. The calculated Cg of ACA-P, CA and AC-YP are 201, 87 and 161 F g-1, respectively. Meanwhile, as we mentioned above, the extremely high mesoporosity will facilitate better diffusion/transfer of electrolyte, thus resulting in high capacitance retention at higher charge-discharge rates. As shown in Figure 3b, with increasing the current density, ACA-P only shows slight decrease of the capacitance. For example, the
capacitance retention stays above 90% (180 F g-1) when the current density increases by 50 times (5 A g-1), whereas that of AC-YP decreases sharply to 74%, indicative of much more inaccessible surface area of AC-YP at high current density.
Potential(V)
0.8 0.6
ACA-P CA AC-YP
0.4 0.2
b)
100
Capacitance retention (%)
a)
1.0
80
ACA-P AC-YP
60
40
20
0.0 0
500
1000
1500
2000
2500
0
1
Time (s)
c)
ACA-P CA AC-YP
1.5
3
-1
4
5
1.0 0.5 0.0
-0.5 -1.0
d)
120
Capacitance retntion %
-1 Current density (A g )
2.0
2
Current density (A g )
ACA-P
100 80 60 40 20 0
0.0
0.2
0.4
0.6
0.8
Voltage(V)
1.0
0
1000
2000
3000
4000
5000
Cycle number
Figure 3. a) Galvanostatic charge–discharge curves at a current density of 0.1 A g−1, b) capacitance retention at different current densities, c) CV curves at a scan rate of 5 mV s-1 and d) Cycle stability of ACA-P at 1 A g-1 This excellent mass transfer capability of ACA-P can be confirmed by CV measurements (Figure 3c). Generally, the rectangular shape of the CV curves can be used to estimate the ability of ion diffusion/transfer within the nanocarbon structure [13]. It can be observed from the CV curves that ACA-P shows near-rectangular shape, as compared to AC-YP with a distorted shape, implying the important role of developed mesoporosity in diffusion/transfer of the electrolyte. Furthermore, the cycling performance of ACA-P was investigated by repeating the charge/discharge test at 1 A g-1 for 5000 cycles. Figure 3d shows extraordinarily high capacitance retention of 100% during the cycles, demonstrating the excellent stability of its supercapacitive behavior.
4. Conclusions. A mesopore-dominant ACA-P with high surface area was prepared based on the pore formation and widening effect of H3PO4. A large number of small mesopores whose size is large enough for rapid diffusion/transfer of electrolyte are constructed during H3PO4 activation, providing large interfaces for efficient electric-double layer formation. As a result, ACA-P shows much higher Cg of 201 F g-1 and capacitance retention of 90% when the current density increases by 50 times compared with commercial AC-YP (161 and 74%). It can be concluded that the as-prepared ACA-P with large surface area and developed mesoporous structures is very promising for use as electrodes materials in EDLCs with high power density and energy density.
Acknowledgements This research was financially supported by the Start-up funding for “One-Hundred Young Talents” of Guangdong University of technology (220413521). References [1] Zhang ZJ, Dong C, Ding XY, Xia YK. A generalized ZnCl2 activation method to produce nitrogen-containing nanoporous carbon materials for supercapacitor applications. Journal of Alloys and Compounds. 2015;636:275-81. [2] Xiong G, Hembram KPSS, Reifenberger RG, Fisher TS. MnO2-coated graphitic petals for supercapacitor electrodes. Journal of Power Sources. 2013;227:254-9. [3] Xiong G, Meng C, Reifenberger RG, Irazoqui PP, Fisher TS. A Review of Graphene-Based Electrochemical Microsupercapacitors. Electroanalysis. 2014;26:30-51. [4] Yang C, Li D. Flexible and foldable supercapacitor electrodes from the porous 3D network of cellulose nanofibers, carbon nanotubes and polyaniline. Materials Letters. 2015;155:78-81. [5] Xiong G, Meng C, Reifenberger RG, Irazoqui PP, Fisher TS. Graphitic Petal Electrodes for All-Solid-State Flexible
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Highlights
Activated carbon aerogel (ACA) with developed mesoporosity is prepared.
ACA presents extremely high surface area donated by high mesopore ratio of 99%.
ACA shows better supercapacitive behavior compared to commercial activated carbon.