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Continuously hierarchical nanoporous graphene film for flexible solid-state supercapacitors with excellent performance
Author’s Accepted Manuscript Continuously hierarchical nanoporous graphene film for flexible solid-state supercapacitors with excellent performance Kaiqiang Qin, Jianli Kang, Jiajun Li, Enzuo Liu, Chunsheng Shi, Zhijia Zhang, Xingxiang Zhang, Naiqin Zhao www.elsevier.com/locate/nanoenergy
PII: DOI: Reference:
S2211-2855(16)30075-1 http://dx.doi.org/10.1016/j.nanoen.2016.04.019 NANOEN1224
To appear in: Nano Energy Received date: 28 January 2016 Accepted date: 13 April 2016 Cite this article as: Kaiqiang Qin, Jianli Kang, Jiajun Li, Enzuo Liu, Chunsheng Shi, Zhijia Zhang, Xingxiang Zhang and Naiqin Zhao, Continuously hierarchical nanoporous graphene film for flexible solid-state supercapacitors with excellent performance, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2016.04.019 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.
Continuously Hierarchical Nanoporous Graphene Film for Flexible Solid-State Supercapacitors with Excellent Performance Kaiqiang Qina, Jianli Kangb,c*, Jiajun Lia, Enzuo Liua,d, Chunsheng Shia, Zhijia Zhangb,c, Xingxiang Zhangb,c, Naiqin Zhaoa,d*
a
School of Materials Science and Engineering and Tianjin Key Laboratory of Composites and Functional Materials, Tianjin University, Tianjin 300072, China b
State Key Laboratory of Separation Membranes and Membrane Processes, Tianjin Polytechnic University, Tianjin 300387, China
c
School of Materials Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, China
d
Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China
[email protected] (J. Kang) [email protected] (N. Zhao) *
Corresponding authors.
Abstract Continuously hierarchical nanoporous graphene (hnp-G) films are synthesized by a combination of low-temperature CVD growth of hydrogenated graphite (HG) coating on nanoporous copper (NPC) and rapid catalytic pyrolysis of HG at high temperature. Lowtemperature growth of HG coating on NPC can obviously delay the coarsening evolution of NPC at high temperature, providing the precondition to obtain hnp-G with small pore size (1150 nm) by catalytic pyrolysis at high temperature. The high specific surface area (1160 m2/g) 1
of hnp-G are mainly originated from the external surface (954.7 m2/g), resulting in fully accessible channels for ion transport. More importantly, the continuously 3D hierarchical nanoporous structure and fully wettability of the hnp-G with gelled electrolyte not only effectively prevent the restacking of graphene even under dramatic squeezing but also guarantee the continuous and short electron/ion diffusion pathway in the whole electrodes, resulting in ultrahigh specific capacitance (38.2 F/cm3 based on the device) and excellent rate performance. The symmetric SC offers ultrahigh energy density (2.65 mW h/cm3) and power density (20.8 W/cm3) and exhibits almost identical performance at various curvatures and excellent lifetime (94 % retention after 10000 cycles), suggesting its wide application potential in powering wearable/miniaturized electronics.
Keywords: Hierarchical nanoporous graphene; hydrogenated graphite; solid-state supercapacitor; nanoporous copper; chemical vapor deposition
Introduction The fast development of soft portable electronic devices (PEDs) puts forward new challenges for the compatible energy storage devices [1-6]. Flexible solid-state supercapacitors (SCs), as a new class of energy storage devices, attracted considerable attention in recent years due to their small size, low weight, ease of handling, high power density and excellent reliability [712]. They can be coupled with PEDs as power sources. As a fundamental two-dimensional (2D) materials, graphene sheets are promising to be used as basic building blocks to construct graphene-based structures for use as SC electrodes [13-16]. However, from a practical viewpoint for use in devices, it is essential to construct the 2
2D materials in 3D configurations with preservation of their intrinsic properties due to the parallel restacking of graphene sheets to form graphite-like powders/films and greatly reduce their active surface area [17-19]. To date, a number of synthetic methods for 3D graphene configurations, based on the strategies of either self-assembly, template-assisted preparation or direct chemical vapor deposition (CVD) [20-22], have been developed. The highly porous electrodes based on the binder-free 3D graphene can boost specific surface area and favor ion/electron diffusion for high gravimetric capacitance but usually suffer from low volumetric capacitance due to its low packing density [23-25]. For some practical applications, especially in PEDs that require small in size, it is necessary to achieve high capacitance within a limited area or volume [4,26,27]. Recently, some condensed/compressed reduced graphene oxide films with 3D meso/micro-channels for ion diffusion were developed as electrodes with high volumetric capacitance in liquid electrolyte [28-30]. However, there are few reported literatures about such condensed/compressed graphene for solid-state devices and the rate capacity or power density is not satisfactory due to the uncontinuous structure and electron/ion pathway [31,32]. Although some researchers tried to prepare 3D continuous graphene with relative small pore size by chemical vapor deposition (CVD) using nanoporous metal [33-35], the average pore size of the obtained graphene film by traditional CVD is still larger than 200 nm due to the dramatic coarsening of nanoporous metal at high temperature and no works about such 3D continuous graphene with pore size less than 100 nm for solid state SCs are reported still now as far as we know. Therefore, fabricating 3D continuous graphene based solid-state flexible SCs with excellent volumetric performance is still challenging. Herein, a continuously hierarchical nanoporous graphene (hnp-G) film with high specific surface area was prepared by rapid catalytic pyrolysis of hydrogenated graphite (HG) using nanoporous copper (NPC) as catalyst. It is known that nanoporous metal is inclined to coarsen at high temperature[36] and thus the resulted 3D graphene by traditional CVD have large 3
pores and low density [33-35]. To control the pore size of the grown nanoporous graphene (np-G) with higher density, we propose a two-step method to prepare 3D continuous graphene with hierarchical nanopores, which combines coating a thin layer of HG by CVD on NPC at low temperature (200 oC) and then rapid catalytic pyrolysis of HG at high temperature (Figure 1A). Low temperature uniform coating of HG on NPC can obviously delay the coarsening evolution of NPC at high temperature, providing the precondition to obtain np-G with small pores by catalytic pyrolysis at high temperature. Furthermore, the obtained hnp-G films are continuous and flexible, which can be directly used as binder-free electrodes for flexible solid state SCs. As shown in Figure 1B, the two pieces of hnp-G films are dipped into sulfuric acid/polyvinyl alcohol (H2SO4/PVA) solution and then partially extracted and dried. The as-obtained electrodes with thin electrolyte coating layers are squeezed to form a micrometer-thin solid state device, sandwiched by gelled electrolyte between. BrunauerEmmett-Teller (BET) analysis reveals that the high specific surface area (1160 m2/g) of hnpG are mainly originated from the external surface (954.7 m2/g), resulting in fully accessible channels for electrolyte/ion transport. Furthermore, the continuously 3D hierarchical nanoporous structure and good wettability of the hnp-G with gelled electrolyte not only effectively prevent the restacking of graphene even under dramatic compression but also guarantee the continuous and short electron/ion diffusion pathway in the whole device, resulting in ultrahigh device capacitance (38.2 F/cm3) and excellent rate performance. Experimental Section Preparation of nanoporous copper catalyst Cu40Mn60 ingots were prepared by melting pure Cu and Mn (> 99.9 at%) using an Arprotected arc melting furnace. After annealing at 850 oC for 24 h for microstructure and composition homogenization, the ingots were cold-rolled to a thickness of ~ 100 μm at room temperature. Nanoporous copper (NPC, size with 1 cm × 0.8 cm) were prepared by chemical
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de-alloying in 0.05 M HCl aqueous solution for 30 min at room temperature under 100 Torr pressure. After de-alloying, the samples were rinsed thoroughly with water and dried. Synthesis of three-dimensional continuous hnp-G structure The as-prepared NPC was transferred into the center of the horizontal tube furnace and directly heated to the reaction temperature of HG under Ar (500 sccm) and H2 (200 sccm) flow. The HG growth was performed at 200 oC with a mixture of C2H2 (5 sccm), Ar (500 sccm) and H2 (200 sccm) for 10 min. Then, the samples in the tube was pulled out of the hot zone under H2 (200 sccm) flow and heating the furnace to 800 oC. After that, the samples in the tube was pull back into the center of the furnace for 1 - 30 min and then pull out to the cold zone, which should be rapidly cooled down to room temperature with a fan. The NPC substrates were dissolved by 5 g FeCl3+10 mL HCl+100 mL H2O solution for 12 h and then immersed in concentrated nitric acid solution for 1 h at room temperature. Finally, the hnp-G films were washed by water rinsing and standby. Fabrication of the flexible solid-state supercapacitors The gel electrolyte was prepared as follows. Polyvinyl alcohol (PVA) (molecular weight: 75 000-80 000 g/mol; Aladdin Chemicals) and H2SO4 (Analytical grade) were used as received. First, 10 g H2SO4 was mixed with 100 mL of deionized water, and 10 g of PVA was added. The whole mixture was heated steadily up to 85 oC under vigorous stirring until the solution became clear. Then the solution was kept at 85 oC without stirring. Second, two pieces of the as-prepared hnp-G films supported on Au coated polyethylene terephthalate (PET) were soaked in the hot gel electrolyte for few seconds, and then picked out. The redundant electrolyte was removed by filter paper. After that, the electrode with a thin solution coating layer was left in the fume hood at room temperature for 4 h to vaporize the excess water. Then the two pieces of electrodes were squeezed together for 5 times to assemble solid-state SC. Characterization
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The structure of the hnp-G was characterized by scanning electron microscopy (SEM, Hitachi S4800) and transmission electron microscopy (TEM, PhilpsTecnai G2 F20). The elemental analysis was measured using Elementar (Vario EL cube). A micro-Raman spectrometer (Renishaw, Invia microscope) with a 325 nm and 532 nm laser were used in the Raman study. The CV, galvanostatic charge/discharge, and EIS with frequency ranging from 0.01 Hz to 100 kHz were conducted with an electrochemical station (CHI 660E). Nitrogen adsorption isotherms of hnp-G were measured at 77 K using an AutosorbiQ instrument (Quantachrome U.S.). The total surface area was calculated with the BET method, and the pore size distribution data were calculated using the BJH and DFT method based on the adsorption and desorption data. The functional group on the surface of the hnp-G was characterized by FT-IR (Thermo Scientific Nicolet 6700) and X-ray photoelectron spectroscopy (XPS, Axis Ultra DLD) with an Al Kα (mono) anode at an energy of 120 W in a vacuum of 5 × 10-9 Torr. Result and Discussion In a typical experiment, the nanoporous copper (NPC) with a pore size of ~ 40 nm and thickness of ~ 100 μm were prepared by chemical dealloying of single-phase Cu40Mn60 (at.%) foil in 0.05 M HCl solution (Figure S1). The as-prepared NPC was then used as catalyst in a low-temperature CVD process (200 oC). As shown in Figure 2A, the pore size of the NPC has no obvious changes. High resolution transmission electron microscopy (TEM) images show that the ligaments of NPC are uniformly coated by a thin layer of graphite-like carbon with an interlayer spacing of about 0.4 nm (Figure 2B), which is much larger than that of graphite. Raman spectrum of the sample (Figure S2A) displays four typical peaks of hydrogenated graphite (HG) around 2920, 1600, 1350 and 980 cm-1, where 2920 cm-1 is corresponding to sp3 C-Hx stretching modes, 1600, 1350 and 980 cm-1 are labeled as the G, D and T (detected only in UV excitation) peaks, respectively [37,38]. Infrared spectrum (Figure S2B) and XPS spectra (Figure S3B) also confirm the existence of C-H. Consequently, it is reasonable to conclude that the low-temperature synthesized carbon is HG. The elemental 6
analysis (Figure S2C) shows that the HG contains ~ 29.1 at.% of hydrogen. To investigate its dehydrogenation evolution, HG was annealed at different temperatures for 1 minute. It is obvious that HG is gradually dehydrogenated with temperature increasing and almost fully converted to pure carbon (99.5 %) at 800 oC (Figure S2C). Further increasing the temperature, carbon content has no obvious change. Raman analysis shows that the ID/IG rates of the samples annealed without NPC catalyst increase obviously from 0.78 to 1.31 after full dehydrogenation (Figure S4), inferring that HG is converted to amorphous carbon, consistent with that reported by Ferrari et. al. [37]. To convert HG to high-quality graphene, catalytic pyrolysis of HG by NPC is performed at 800 oC. It is well known that the ligaments and pores of NPC are inclined to coarsen at high temperature because of the high surface activity [36]. To comprehensively understand the catalytic evolution of the HG coated NPC, pure NPC is annealed at 800 oC for different time. It is found that the coarsening of NPC is not an uniform process but initiates from some nucleation sites to coarsen as particle-linked structures and then transfer to uniform ones by diffusion (Figure S5). Figure S6 shows the SEM images of the HG coated NPC annealed at 800 oC for different times. It is obvious that NPC breaks through the bound of HG coating and follows the similar coarsening evolution rule as that of pure NPC, although HG coating obviously delays the diffusion rate of NPC. After annealing for 1min, the graphene coated NPC becomes particle-linked structure with a ligament size of 30 - 150 nm. With prolonging the annealing time, graphene coated NPC is inclined to coarsen to uniform network with larger pores. It is more interesting that many erected graphene sheets are filled in the pores of NPC, meaning that the coarsening of NPC catalyzes the conversion of HG coating layer to continuously hnp-G with a lot of suspended edges. The scanning electron microscopy (SEM) image (Figure 2C), scanning transmission electron microscopy (STEM) images (Figure 2D) and TEM images (Figure 2E) confirm that many ultra-thin graphene nanosheets erect from the surface of np-G, which directly coated on NPC. High-resolution TEM images indicate that 7
the erected nanosheets are mainly mono- and bilayer graphene while the np-G coated on the NPC mainly consists of more layers (Figure 2F). Raman analysis indicates that the ID/IG ratios of the obtained hnp-G are much lower than that of ones without NPC catalyst (Figure S7), confirming that HG catalyzed by NPC is defective graphene instead of amorphous carbon during dehydrogenation [37,39]. The I2D/IG rates of hnp-G reduce while ID/IG rates increase with annealing time, inferring that both stacked layers and defects (suspended edges) of graphene reduce (Figure S7B). Brunauer-Emmett-Teller (BET) and Barrett-JoynerHalenda (BJH) analyses (Figure S8) reveal that the NPC@hnp-G obtained at 800 oC for 1 min has a specific area of ~ 43.1 m2/g and the pore size mainly lies in 1 - 100 nm. After removing NPC, the sample (hnp-G) exhibits a high specific surface area of ∼ 1160 m2/g, which mainly comes from the external surface (~ 954.7 m2/g). If comparing the pore size distribution before and after removing the NPC, we will find that the large pores (100 - 150 nm) are obviously increased after removing the NPC, indicating the large pores are mainly created by removing the NPC template. More interestingly, the reduction of specific surface area of hnp-G with annealing time mainly come from the loss of external surface. Combined with Raman analysis and weight loss of hnp-G with annealing time (Figure S8F), it is reasonable to conclude that the reduction of specific surface area of hnp-G should be mainly due to the hydrogen etching of suspended graphene as well as the coarsened pores of np-G. Figure 2G shows the optical image of the bended hnp-G film after removing the NPC substrate, inferring the film is highly flexible and robust. Because of the highest specific surface area and mesoporous volumes, the flexible hnp-G films, obtained by catalytic annealing for 1 min, were used as electrode candidates for flexible solid-state SCs. To further increase the wettability and active sites of hnp-G with electrolyte, the hnp-G film was treated by concentrated nitric acid for 1h at room temperature before assembling, which introduced abundance of -NO2, -NH2, C-O, and C=O groups on the surface of the hnp-G (Figure S2B, Figure S3). Electrochemical test confirmed that acid-treated hnp8
G produced obvious pseudo-capacitance as well as double layer capacitance (Figure S9), which was believed to be caused by N-doping groups.[9] A symmetric solid-state SC was assembled by attaching two pieces of hnp-G films onto each side of a H2SO4-PVA gel membrane. Macroscopically, the entire device shows the super mechanical property of flexibility, which can be folded without any cracking (Figure 3A). SEM image indicated that two pieces of hnp-G films varnished with electrolyte were dramatically squeezed to 30 μm without any cracks, which was sandwiched by a thin electrolyte membrane (~ 6 μm) between (Figure 3B). High-magnification SEM image (Figure 3C) confirmed that the hnp-G were fully penetrated by the gelled electrolyte, inferring that the whole hnp-G was well contacted with the electrolyte. Figure 4A shows the cyclic voltammetry (CV) curves of the flexible solid-state device at various scan rates. The rectangular shapes of the CVs, even at high scan rate of 300 mV/s, infer the fast diffusion of ions in the hnp-G electrodes and the very rapid current response to voltage reversal. Furthermore, charge/discharge curves in Figure 4B indicate that the charging curves are symmetrical with their discharging counterpart as well as good linear voltage-time profiles, demonstrating a good capacitive performance. The volumetric capacitance of the device (including electrodes, separator and electrolyte) is 38.2 F/cm3 at 0.1 A/g (305 F/g and 95.4 F/cm3 based one electrode) (Figure S10), which is substantially higher than those of the previously reported graphene-based solid-state devices [40,41]. The capability of retaining high capacitance during ultrafast charging/discharging is critical for high-performance supercapacitor devices. In our case, the specific capacitance of the treated hnp-G electrode decreases very slowly and smooth with the increase of current density (Figure S10 and 4C). When the current density increases from 1 to 20 A/g, it achieves a remarkable capacitance retention rate of 83 %, which is much higher than the best reported rate capability of carbon-based solid-state supercapacitors [8, 42].
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The volumetric power and energy densities against the device stack, as well as a more comprehensive comparison with the previously reported SCs, are plotted in the Ragone plot in Figure S11. The maximum energy density is up to 2.65 mW h/cm3, which is among the highest values of most recently reported electric double-layer capacitors and pseudocapacitors. The maximum volumetric power density is 20.8 W/cm3, which is comparable to that of pCNFs/G [8], almost 1 order of magnitude higher than that of Ti/Co3O4//CNF/G [43] and (MnO2/PEDOT:PSS)/AC [44], 2 orders of magnitude higher than that of H-TiO2@MnO2//HTiO2@C [45], CNT/PPy-based SCs [46] and 3 orders of magnitude higher than that of activated carbon cloths-based SCs [47]. In order to demonstrate the flexibility of the wearable devices, we measured the CV curves under various bending angles. No significant deviation of the CV curves (Figure S12) was observed when the angle changed from 0o to 135o under bending, displaying excellent capacitance stability at various curvatures. We also performed electrochemical impedance spectroscopy (EIS) measurements (Figure S13). The quasivertical profile in the low-frequency region indicates that the device has nearly ideal supercapacitor behavior [47]. Additionally, the absence of a semicircle in the high-frequency region suggests the ionic conductivity at the electrode and electrolyte is excellent [42,48]. Furthermore, the solid state device demonstrates good cycling stability (Figure 4D), with only a 6 % reduction of the capacitance after 10000 cycles, indexing excellent long-term performance durability. Even in a bending state, the device shows similar behavior, which is attributed to the high mechanical flexibility of the electrodes as well as the interpenetrating network between the hnp-G electrodes and the gelled electrolyte. To comprehensively evaluate the overall performance of the as-prepared SCs, a radar plot summarizing the combination property in this work and in the recent literatures was drawn (Figure S14). The larger area encompassed within the radar plot indicated the better overall performance of the hnp-G based SCs.
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In general, the total energy that can be stored in a single SC is too low for most practical application. Thus, depending on the application, SCs in series and/or parallel combinations are necessary to form a ‘bank’ with a specific voltage and capacitance rating [49]. The adaptability of the hnp-G assembled devices in serial/parallel combination is demonstrated by connecting three ones together both in series and parallel configurations (Figure 5A and B). In the parallel assembly, the output current increase by a factor of 3, and thus the discharge time was three times that of a single device when operated at the same current density. As expected, when the three SCs are combined in series, the output voltage increases by a factor of 3, which can power a light-emitting diode (LED) (Figure S15). Conclusion In summary, a continuously hnp-G film with ultrahigh external surface area (∼ 954.7 m2/g) was prepared by catalytic pyrolysis of HG, which was synthesized by CVD using NPC catalyst. Due to the ultrahigh external surface area and flexibility, two pieces of hnp-G films were assembled using H2SO4-PVA gel as solid-state electrolyte to form a symmetric flexible SC with a total thickness of ~ 30 μm. The symmetric SC exhibits excellent rate performance, 83 % reduction of specific capacitance when the current density increases from 1 to 20 A/g, due to the continuously 3D hierarchical nanoporous structure and good wettability of the hnpG with gelled electrolyte. Significantly, the symmetric SC offers ultrahigh energy density (2.65 mW h/cm3) and power density (20.8 W/cm3) based on the device and exhibits almost identical performance at various curvatures and excellent lifetime, which suggests its wide application potential in powering wearable/miniaturized electronics. Acknowledgements This work is sponsored by the Key Program of the National Natural Science Foundation of China (No. 51531004), National Natural Science Foundation of China (No. 51472177), Tianjin Research Program of Application Foundation and Advanced Technology (Nos. 14JCYBJC20900, 14JCYBJC19600), China-EU Science and Technology Cooperation Project 11
(SQ2013ZOA100006), and Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT) of the Ministry of Education of China (No. IRT13084).
References [1]
P. Yang, W. Mai, Nano Energy 8 (2014) 274-290.
[2]
D. Dubal, J. Kim, Y. Kim, R. Holze, C. Lokhande, W. Kim, Energy Technol. 2 (2014) 325-341.
[3]
A. Tyagi, K. Tripathi, R. Gupta, J. Mater. Chem. A 3 (2015) 22507.
[4]
C. Zhang, W. Lv, Y. Tao, Q. Yang, Energy Environ. Sci. 8 (2015) 1390-1403.
[5]
L. Li, Z. Wu, S. Yuang, X. Zhang, Energy Environ. Sci. 7 (2014) 2101-2122.
[6]
X. Wang, X. Lu, B. Liu, D. Chen, Y. Tong, G. Shen, Adv. Mater. 26 (2014) 4763-4782.
[7]
D. Yu, K. Goh, Q. Zhang, L. Wei, H. Wang, W. Jiang, Y. Chen, Adv. Mater. 26 (2014) 6790-6797.
[8]
K. Qin, J. Kang, J. Li, C. Shi, Y. Li, Z. Qiao, N. Zhao, ACS Nano 9 (2015) 481-487.
[9]
J. Zhao, S. Chen, S. Xu, M. Shao, Q. Zgabgm F, Wei, J. Ma, M. Wei, D. Evans, X. Duan, Adv. Funct. Mater. 24 (2014) 2938-2946.
[10] X. Lu, M. Yu, G. Wang, Y. Tong, Y. Li, Energy Environ. Sci. 7 (2014) 2160-2181. [11] Z. Sun, C. Yang, B. Xie, Z. Lin, Z. Zhang, J. Liu, B. Li, F. Kang, C. P. Wong, Energy Environ. Sci. 7 (2014) 2652-2659. [12] H. Xu, X. Hu, H. Yang, Y. Sun, C. Hu, Y. Huang, Adv. Energy Mater. 5 (2015) 1401882 (7pp). [13] D. Pham, T. Lee, D. Luong, F. Yao, A. Ghosh, V. Le, T. Kim, B. Li, J. Chang, Y. Lee, ACS Nano 9 (2015) 2018-2027. [14] R. Raccichini, A. Varzi, S. Passerini, B. Scrosati, Nat. Mater. 14 (2015) 271-279. [15] Z. Wu, Z. Liu, K. Parvez, X. Feng, K. Mullen, Adv. Mater. 27 (2015) 3669-3675.
12
[16] Z. Wu, K. Parvez, A. Winter, H. Vieker, X. Liu, S. Han, A. Turchanin, X. Feng, K. Mullen, Adv. Mater. 26 (2014) 45524558. [17] X. Peng, L. Peng, C. Wu, Y. Xie, Chem. Soc. Rev. 43 (2014) 3303-3323. [18] J. Shi, W. Du, Y. Yin, Y. Guo, L. Wan, J. Mater. Chem. A 2 (2014) 10830-10834. [19] Y. Ito, Y. Tanabe, H. Qiu, K. Sugawara, S. Heguri, N. Tu, K. Huynh, T. Fujita, T. Takahashi, K. Tanigaki, M. Chen, Angew. Chem. Int. Ed. 53 (2014) 4822-4826. [20] Y. Shao, M. El-Kady, L. Wang, Q. Zhang, Y. Li, H. Wang, M. Mousavi, R. Kaner, Chem. Soc. Rev. 44 (2015) 3639-3665. [21] S. He, W. Chen, Nanoscale 7 (2015) 6957-6990. [22] S. Zhou, J. Xu, Y. Xiao, N. Zhao, C. Wong, Nano Energy 13 (2015) 458-466. [23] P. Yu, X. Zhao, Z. Huang, Y. Li, Q. Zhang, J. Mater. Chem. A 2 (2014) 14413-14420. [24] Y. He, W. Chen, X. Li, Z. Zhang, J. Fu, C. Zhao, E. Xie, ACS Nano 7 (2013) 174-182. [25] Z. Chen, W. Ren, L. Gao, B. Liu, S. Pei, H. Cheng, Nat. Mater. 10 (2011) 424-428. [26] M. Beidaghi, Y. Gogotsi, Energy Environ. Sci. 7 (2014) 867-884. [27] Y. Xu, K. Sheng, C. Li, G. Shi, ACS Nano 4 (2010) 4324-4330. [28] X. Yang, C. Cheng, Y. Wang, L. Qiu, D. Li, Science 341 (2013) 534-537. [29] T. Kim, G. Jung, S. Yoo, K. Suh, R. Ruoff, ACS Nano 7 (2013) 6899-6905. [30] J. Huang, J. Wang, C. Wang, H. Zhang, C. Lu, J. Wang, Chem. Mater. 27 (2015) 21072113. [31] X. Cai, M. Peng, X. Yu, Y. Fu, D. Zou, J. Mater. Chem. C 2 (2014) 1184-1200. [32] Y. Xu, Z. Lin, X. Huang, Y. Liu, Y. Huang, X. Duan, ACS Nano 7 (2013) 4042-4049. [33] Y. Ito, Y. Tanabe, H. Qiu, K. Sugawara, S. Heguri, N. Tu, K. Huynh, T. Fujita, T. Takahashi, K. Tanigaki, M. Chen, Angew. Chem. Int. Ed. 53 (2014) 4822-4826. [34] Y. Ito, H. Qiu, T. Fujita, Y. Tanabe, K. Tanigaki, M. Chen, Adv. Mater. 26 (2014) 41454150.
13
[35] Y. Ito, W. Cong, T. Fujita. Z. Tang, M. Chen, Angew. Chem. Int. Ed. 54 (2015) 21312136. [36] T. Fujita, P. Guan, K. McKenna, X. Lang, A. Hirata, L. Zhang, T. Tokunaga, S. Arai, Y. Yamamoto, N. Tanaka, Y. Ishikawa, N. Asao, Y. Yamamoto, J. Erlebacher, M. Chen, Nat. Mater. 11 (2012) 775-780. [37] A. Ferrari, J. Robertson, Phys. Rev. B 64 (2001) 075414 (13pp). [38] J. Robertson, E. O’Reilly, Phys. Rev. B 35 (1987) 2946-2957. [39] N. Conway, A. Ferrari, A. Flewitt, J. Robertson, W. Milne, A. Tagliaferro, W. Beyer, Diamond Relat. Matter. 9 (2000) 765-770. [40] J. Miller, R. Outlaw, B. Holloway, Science 329 (2010) 1637-1639. [41] D. Pech, M. Brunet, H. Durous, P. Huang, V. Mochalin, Y. Gogotsi, P. Taberna, P. Simon, Nat. Nanotechnol. 5 (2010) 651-654. [42] M. El-Kady, V. Strong, S. Dubin, R. Kaner, Science 335 (2012) 1326-1330. [43] X. Wang, B. Liu, R. Liu, Q. Wang, X. Hou, D. Chen, R. Wang, G. Shen, Angew. Chem. Int. Ed. 126 (2014) 1880-1884. [44] Z. Su, C. Yang, C. Xu, H. Wu, Z. Zhang, T. Liu, C. Zhang, Q. Yang, B. Li, F. Kang, J. Mater. Chem. A 1 (2013) 12432-12440. [45] X. Lu, M. Yu, G. Wang, T. Zhai, S. Xie, Y. Ling, Y. Tong, Y. Li, Adv. Mater. 25 (2013) 267-272. [46] Y. Chen, L. Du, P. Yang, P. Sun, X. Yu, W. Mai, J. Power Sources 287 (2015) 68-74. [47] G. Wang, H. Wang, X. Lu, Y. Ling, M. Yu, T. Zhai, Y. Tong, Y. Li, Adv. Mater. 26 (2014) 2676-2682. [48] Z. Wu, K. Parvez, X. Feng, K. Mullen, Nat. Commun. 4 (2013) 2487 (8pp). [49] M. El-Kady, R. Kaner, Nat. Commun. 4 (2013) 1475 (9pp).
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Figures
(A)
(B) Figure 1 (A), (B) Schematic diagram of the fabrication process of the hnp-G and flexible solid state supercapacitors.
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Figure 2 (A) SEM image of the NPC coated with HG, the insert is TEM image. (B) HRTEM image of the HG after remove the NPC. (C), (D) SEM and HAADF-STEM images of the hnp-G after treated by HNO3. (E) High-magnification TEM image of ultra-thin graphite nanosheets located onto the surface of hnp-G. (F) High-magnification TEM images of graphene sheets with different number of layer in hnp-G. (G) Photograph of hnp-G film, demonstrating its integrity and flexibility.
Figure 3 (A) Digital picture of the solid-state device (size ~ 1 cm × 0.8 cm) under folded condition. (B) SEM image of the cross section of the ultrathin solid-state device, which was obtained by cracking the sample in liquid nitrogen (N2). (C) High-magnification SEM image of the local area in B, indicating that the hnp-Gs were well bonded with the solid-state H2SO4-PVA gelled electrolyte.
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Figure 4 (A) Typical cyclic voltammograms of the flexible solid-state device at different scan rates from 5 - 300 mV/s. (B) Galvanostatic charge/discharge curves at different current density. (C) The specific capacitances versus current density. (D) Cycling stability of the device at a current density of 2 A/g.
Figure 5 (A) The CV curves of three devices in series and parallel at the scan rate of 50 mV/s. (B) The galvanostatic charge/discharge curves of three devices in series and parallel at 2 A/g.
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Highlights
Free-standing Continuously hierarchical nanoporous graphene (hnp-G) film with high specific surface area (1160 m2/g) and small pore size (1 - 150 nm) was successfully fabricated by a simple method. An ultrathin symmetric solid-state SCs was fabricated by two pieces of hnp-G films using H2SO4/PVA gel as solid-state electrolyte. The symmetric SC offers ultrahigh energy density (2.65 mW h/cm3) and power density (20.8 W/cm3) and exhibits almost identical performance at various curvatures and excellent lifetime (94 % retention after 10000 cycles).
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PII: DOI: Reference:
S2211-2855(16)30075-1 http://dx.doi.org/10.1016/j.nanoen.2016.04.019 NANOEN1224
To appear in: Nano Energy Received date: 28 January 2016 Accepted date: 13 April 2016 Cite this article as: Kaiqiang Qin, Jianli Kang, Jiajun Li, Enzuo Liu, Chunsheng Shi, Zhijia Zhang, Xingxiang Zhang and Naiqin Zhao, Continuously hierarchical nanoporous graphene film for flexible solid-state supercapacitors with excellent performance, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2016.04.019 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.
Continuously Hierarchical Nanoporous Graphene Film for Flexible Solid-State Supercapacitors with Excellent Performance Kaiqiang Qina, Jianli Kangb,c*, Jiajun Lia, Enzuo Liua,d, Chunsheng Shia, Zhijia Zhangb,c, Xingxiang Zhangb,c, Naiqin Zhaoa,d*
a
School of Materials Science and Engineering and Tianjin Key Laboratory of Composites and Functional Materials, Tianjin University, Tianjin 300072, China b
State Key Laboratory of Separation Membranes and Membrane Processes, Tianjin Polytechnic University, Tianjin 300387, China
c
School of Materials Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, China
d
Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China
[email protected] (J. Kang) [email protected] (N. Zhao) *
Corresponding authors.
Abstract Continuously hierarchical nanoporous graphene (hnp-G) films are synthesized by a combination of low-temperature CVD growth of hydrogenated graphite (HG) coating on nanoporous copper (NPC) and rapid catalytic pyrolysis of HG at high temperature. Lowtemperature growth of HG coating on NPC can obviously delay the coarsening evolution of NPC at high temperature, providing the precondition to obtain hnp-G with small pore size (1150 nm) by catalytic pyrolysis at high temperature. The high specific surface area (1160 m2/g) 1
of hnp-G are mainly originated from the external surface (954.7 m2/g), resulting in fully accessible channels for ion transport. More importantly, the continuously 3D hierarchical nanoporous structure and fully wettability of the hnp-G with gelled electrolyte not only effectively prevent the restacking of graphene even under dramatic squeezing but also guarantee the continuous and short electron/ion diffusion pathway in the whole electrodes, resulting in ultrahigh specific capacitance (38.2 F/cm3 based on the device) and excellent rate performance. The symmetric SC offers ultrahigh energy density (2.65 mW h/cm3) and power density (20.8 W/cm3) and exhibits almost identical performance at various curvatures and excellent lifetime (94 % retention after 10000 cycles), suggesting its wide application potential in powering wearable/miniaturized electronics.
Keywords: Hierarchical nanoporous graphene; hydrogenated graphite; solid-state supercapacitor; nanoporous copper; chemical vapor deposition
Introduction The fast development of soft portable electronic devices (PEDs) puts forward new challenges for the compatible energy storage devices [1-6]. Flexible solid-state supercapacitors (SCs), as a new class of energy storage devices, attracted considerable attention in recent years due to their small size, low weight, ease of handling, high power density and excellent reliability [712]. They can be coupled with PEDs as power sources. As a fundamental two-dimensional (2D) materials, graphene sheets are promising to be used as basic building blocks to construct graphene-based structures for use as SC electrodes [13-16]. However, from a practical viewpoint for use in devices, it is essential to construct the 2
2D materials in 3D configurations with preservation of their intrinsic properties due to the parallel restacking of graphene sheets to form graphite-like powders/films and greatly reduce their active surface area [17-19]. To date, a number of synthetic methods for 3D graphene configurations, based on the strategies of either self-assembly, template-assisted preparation or direct chemical vapor deposition (CVD) [20-22], have been developed. The highly porous electrodes based on the binder-free 3D graphene can boost specific surface area and favor ion/electron diffusion for high gravimetric capacitance but usually suffer from low volumetric capacitance due to its low packing density [23-25]. For some practical applications, especially in PEDs that require small in size, it is necessary to achieve high capacitance within a limited area or volume [4,26,27]. Recently, some condensed/compressed reduced graphene oxide films with 3D meso/micro-channels for ion diffusion were developed as electrodes with high volumetric capacitance in liquid electrolyte [28-30]. However, there are few reported literatures about such condensed/compressed graphene for solid-state devices and the rate capacity or power density is not satisfactory due to the uncontinuous structure and electron/ion pathway [31,32]. Although some researchers tried to prepare 3D continuous graphene with relative small pore size by chemical vapor deposition (CVD) using nanoporous metal [33-35], the average pore size of the obtained graphene film by traditional CVD is still larger than 200 nm due to the dramatic coarsening of nanoporous metal at high temperature and no works about such 3D continuous graphene with pore size less than 100 nm for solid state SCs are reported still now as far as we know. Therefore, fabricating 3D continuous graphene based solid-state flexible SCs with excellent volumetric performance is still challenging. Herein, a continuously hierarchical nanoporous graphene (hnp-G) film with high specific surface area was prepared by rapid catalytic pyrolysis of hydrogenated graphite (HG) using nanoporous copper (NPC) as catalyst. It is known that nanoporous metal is inclined to coarsen at high temperature[36] and thus the resulted 3D graphene by traditional CVD have large 3
pores and low density [33-35]. To control the pore size of the grown nanoporous graphene (np-G) with higher density, we propose a two-step method to prepare 3D continuous graphene with hierarchical nanopores, which combines coating a thin layer of HG by CVD on NPC at low temperature (200 oC) and then rapid catalytic pyrolysis of HG at high temperature (Figure 1A). Low temperature uniform coating of HG on NPC can obviously delay the coarsening evolution of NPC at high temperature, providing the precondition to obtain np-G with small pores by catalytic pyrolysis at high temperature. Furthermore, the obtained hnp-G films are continuous and flexible, which can be directly used as binder-free electrodes for flexible solid state SCs. As shown in Figure 1B, the two pieces of hnp-G films are dipped into sulfuric acid/polyvinyl alcohol (H2SO4/PVA) solution and then partially extracted and dried. The as-obtained electrodes with thin electrolyte coating layers are squeezed to form a micrometer-thin solid state device, sandwiched by gelled electrolyte between. BrunauerEmmett-Teller (BET) analysis reveals that the high specific surface area (1160 m2/g) of hnpG are mainly originated from the external surface (954.7 m2/g), resulting in fully accessible channels for electrolyte/ion transport. Furthermore, the continuously 3D hierarchical nanoporous structure and good wettability of the hnp-G with gelled electrolyte not only effectively prevent the restacking of graphene even under dramatic compression but also guarantee the continuous and short electron/ion diffusion pathway in the whole device, resulting in ultrahigh device capacitance (38.2 F/cm3) and excellent rate performance. Experimental Section Preparation of nanoporous copper catalyst Cu40Mn60 ingots were prepared by melting pure Cu and Mn (> 99.9 at%) using an Arprotected arc melting furnace. After annealing at 850 oC for 24 h for microstructure and composition homogenization, the ingots were cold-rolled to a thickness of ~ 100 μm at room temperature. Nanoporous copper (NPC, size with 1 cm × 0.8 cm) were prepared by chemical
4
de-alloying in 0.05 M HCl aqueous solution for 30 min at room temperature under 100 Torr pressure. After de-alloying, the samples were rinsed thoroughly with water and dried. Synthesis of three-dimensional continuous hnp-G structure The as-prepared NPC was transferred into the center of the horizontal tube furnace and directly heated to the reaction temperature of HG under Ar (500 sccm) and H2 (200 sccm) flow. The HG growth was performed at 200 oC with a mixture of C2H2 (5 sccm), Ar (500 sccm) and H2 (200 sccm) for 10 min. Then, the samples in the tube was pulled out of the hot zone under H2 (200 sccm) flow and heating the furnace to 800 oC. After that, the samples in the tube was pull back into the center of the furnace for 1 - 30 min and then pull out to the cold zone, which should be rapidly cooled down to room temperature with a fan. The NPC substrates were dissolved by 5 g FeCl3+10 mL HCl+100 mL H2O solution for 12 h and then immersed in concentrated nitric acid solution for 1 h at room temperature. Finally, the hnp-G films were washed by water rinsing and standby. Fabrication of the flexible solid-state supercapacitors The gel electrolyte was prepared as follows. Polyvinyl alcohol (PVA) (molecular weight: 75 000-80 000 g/mol; Aladdin Chemicals) and H2SO4 (Analytical grade) were used as received. First, 10 g H2SO4 was mixed with 100 mL of deionized water, and 10 g of PVA was added. The whole mixture was heated steadily up to 85 oC under vigorous stirring until the solution became clear. Then the solution was kept at 85 oC without stirring. Second, two pieces of the as-prepared hnp-G films supported on Au coated polyethylene terephthalate (PET) were soaked in the hot gel electrolyte for few seconds, and then picked out. The redundant electrolyte was removed by filter paper. After that, the electrode with a thin solution coating layer was left in the fume hood at room temperature for 4 h to vaporize the excess water. Then the two pieces of electrodes were squeezed together for 5 times to assemble solid-state SC. Characterization
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The structure of the hnp-G was characterized by scanning electron microscopy (SEM, Hitachi S4800) and transmission electron microscopy (TEM, PhilpsTecnai G2 F20). The elemental analysis was measured using Elementar (Vario EL cube). A micro-Raman spectrometer (Renishaw, Invia microscope) with a 325 nm and 532 nm laser were used in the Raman study. The CV, galvanostatic charge/discharge, and EIS with frequency ranging from 0.01 Hz to 100 kHz were conducted with an electrochemical station (CHI 660E). Nitrogen adsorption isotherms of hnp-G were measured at 77 K using an AutosorbiQ instrument (Quantachrome U.S.). The total surface area was calculated with the BET method, and the pore size distribution data were calculated using the BJH and DFT method based on the adsorption and desorption data. The functional group on the surface of the hnp-G was characterized by FT-IR (Thermo Scientific Nicolet 6700) and X-ray photoelectron spectroscopy (XPS, Axis Ultra DLD) with an Al Kα (mono) anode at an energy of 120 W in a vacuum of 5 × 10-9 Torr. Result and Discussion In a typical experiment, the nanoporous copper (NPC) with a pore size of ~ 40 nm and thickness of ~ 100 μm were prepared by chemical dealloying of single-phase Cu40Mn60 (at.%) foil in 0.05 M HCl solution (Figure S1). The as-prepared NPC was then used as catalyst in a low-temperature CVD process (200 oC). As shown in Figure 2A, the pore size of the NPC has no obvious changes. High resolution transmission electron microscopy (TEM) images show that the ligaments of NPC are uniformly coated by a thin layer of graphite-like carbon with an interlayer spacing of about 0.4 nm (Figure 2B), which is much larger than that of graphite. Raman spectrum of the sample (Figure S2A) displays four typical peaks of hydrogenated graphite (HG) around 2920, 1600, 1350 and 980 cm-1, where 2920 cm-1 is corresponding to sp3 C-Hx stretching modes, 1600, 1350 and 980 cm-1 are labeled as the G, D and T (detected only in UV excitation) peaks, respectively [37,38]. Infrared spectrum (Figure S2B) and XPS spectra (Figure S3B) also confirm the existence of C-H. Consequently, it is reasonable to conclude that the low-temperature synthesized carbon is HG. The elemental 6
analysis (Figure S2C) shows that the HG contains ~ 29.1 at.% of hydrogen. To investigate its dehydrogenation evolution, HG was annealed at different temperatures for 1 minute. It is obvious that HG is gradually dehydrogenated with temperature increasing and almost fully converted to pure carbon (99.5 %) at 800 oC (Figure S2C). Further increasing the temperature, carbon content has no obvious change. Raman analysis shows that the ID/IG rates of the samples annealed without NPC catalyst increase obviously from 0.78 to 1.31 after full dehydrogenation (Figure S4), inferring that HG is converted to amorphous carbon, consistent with that reported by Ferrari et. al. [37]. To convert HG to high-quality graphene, catalytic pyrolysis of HG by NPC is performed at 800 oC. It is well known that the ligaments and pores of NPC are inclined to coarsen at high temperature because of the high surface activity [36]. To comprehensively understand the catalytic evolution of the HG coated NPC, pure NPC is annealed at 800 oC for different time. It is found that the coarsening of NPC is not an uniform process but initiates from some nucleation sites to coarsen as particle-linked structures and then transfer to uniform ones by diffusion (Figure S5). Figure S6 shows the SEM images of the HG coated NPC annealed at 800 oC for different times. It is obvious that NPC breaks through the bound of HG coating and follows the similar coarsening evolution rule as that of pure NPC, although HG coating obviously delays the diffusion rate of NPC. After annealing for 1min, the graphene coated NPC becomes particle-linked structure with a ligament size of 30 - 150 nm. With prolonging the annealing time, graphene coated NPC is inclined to coarsen to uniform network with larger pores. It is more interesting that many erected graphene sheets are filled in the pores of NPC, meaning that the coarsening of NPC catalyzes the conversion of HG coating layer to continuously hnp-G with a lot of suspended edges. The scanning electron microscopy (SEM) image (Figure 2C), scanning transmission electron microscopy (STEM) images (Figure 2D) and TEM images (Figure 2E) confirm that many ultra-thin graphene nanosheets erect from the surface of np-G, which directly coated on NPC. High-resolution TEM images indicate that 7
the erected nanosheets are mainly mono- and bilayer graphene while the np-G coated on the NPC mainly consists of more layers (Figure 2F). Raman analysis indicates that the ID/IG ratios of the obtained hnp-G are much lower than that of ones without NPC catalyst (Figure S7), confirming that HG catalyzed by NPC is defective graphene instead of amorphous carbon during dehydrogenation [37,39]. The I2D/IG rates of hnp-G reduce while ID/IG rates increase with annealing time, inferring that both stacked layers and defects (suspended edges) of graphene reduce (Figure S7B). Brunauer-Emmett-Teller (BET) and Barrett-JoynerHalenda (BJH) analyses (Figure S8) reveal that the NPC@hnp-G obtained at 800 oC for 1 min has a specific area of ~ 43.1 m2/g and the pore size mainly lies in 1 - 100 nm. After removing NPC, the sample (hnp-G) exhibits a high specific surface area of ∼ 1160 m2/g, which mainly comes from the external surface (~ 954.7 m2/g). If comparing the pore size distribution before and after removing the NPC, we will find that the large pores (100 - 150 nm) are obviously increased after removing the NPC, indicating the large pores are mainly created by removing the NPC template. More interestingly, the reduction of specific surface area of hnp-G with annealing time mainly come from the loss of external surface. Combined with Raman analysis and weight loss of hnp-G with annealing time (Figure S8F), it is reasonable to conclude that the reduction of specific surface area of hnp-G should be mainly due to the hydrogen etching of suspended graphene as well as the coarsened pores of np-G. Figure 2G shows the optical image of the bended hnp-G film after removing the NPC substrate, inferring the film is highly flexible and robust. Because of the highest specific surface area and mesoporous volumes, the flexible hnp-G films, obtained by catalytic annealing for 1 min, were used as electrode candidates for flexible solid-state SCs. To further increase the wettability and active sites of hnp-G with electrolyte, the hnp-G film was treated by concentrated nitric acid for 1h at room temperature before assembling, which introduced abundance of -NO2, -NH2, C-O, and C=O groups on the surface of the hnp-G (Figure S2B, Figure S3). Electrochemical test confirmed that acid-treated hnp8
G produced obvious pseudo-capacitance as well as double layer capacitance (Figure S9), which was believed to be caused by N-doping groups.[9] A symmetric solid-state SC was assembled by attaching two pieces of hnp-G films onto each side of a H2SO4-PVA gel membrane. Macroscopically, the entire device shows the super mechanical property of flexibility, which can be folded without any cracking (Figure 3A). SEM image indicated that two pieces of hnp-G films varnished with electrolyte were dramatically squeezed to 30 μm without any cracks, which was sandwiched by a thin electrolyte membrane (~ 6 μm) between (Figure 3B). High-magnification SEM image (Figure 3C) confirmed that the hnp-G were fully penetrated by the gelled electrolyte, inferring that the whole hnp-G was well contacted with the electrolyte. Figure 4A shows the cyclic voltammetry (CV) curves of the flexible solid-state device at various scan rates. The rectangular shapes of the CVs, even at high scan rate of 300 mV/s, infer the fast diffusion of ions in the hnp-G electrodes and the very rapid current response to voltage reversal. Furthermore, charge/discharge curves in Figure 4B indicate that the charging curves are symmetrical with their discharging counterpart as well as good linear voltage-time profiles, demonstrating a good capacitive performance. The volumetric capacitance of the device (including electrodes, separator and electrolyte) is 38.2 F/cm3 at 0.1 A/g (305 F/g and 95.4 F/cm3 based one electrode) (Figure S10), which is substantially higher than those of the previously reported graphene-based solid-state devices [40,41]. The capability of retaining high capacitance during ultrafast charging/discharging is critical for high-performance supercapacitor devices. In our case, the specific capacitance of the treated hnp-G electrode decreases very slowly and smooth with the increase of current density (Figure S10 and 4C). When the current density increases from 1 to 20 A/g, it achieves a remarkable capacitance retention rate of 83 %, which is much higher than the best reported rate capability of carbon-based solid-state supercapacitors [8, 42].
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The volumetric power and energy densities against the device stack, as well as a more comprehensive comparison with the previously reported SCs, are plotted in the Ragone plot in Figure S11. The maximum energy density is up to 2.65 mW h/cm3, which is among the highest values of most recently reported electric double-layer capacitors and pseudocapacitors. The maximum volumetric power density is 20.8 W/cm3, which is comparable to that of pCNFs/G [8], almost 1 order of magnitude higher than that of Ti/Co3O4//CNF/G [43] and (MnO2/PEDOT:PSS)/AC [44], 2 orders of magnitude higher than that of H-TiO2@MnO2//HTiO2@C [45], CNT/PPy-based SCs [46] and 3 orders of magnitude higher than that of activated carbon cloths-based SCs [47]. In order to demonstrate the flexibility of the wearable devices, we measured the CV curves under various bending angles. No significant deviation of the CV curves (Figure S12) was observed when the angle changed from 0o to 135o under bending, displaying excellent capacitance stability at various curvatures. We also performed electrochemical impedance spectroscopy (EIS) measurements (Figure S13). The quasivertical profile in the low-frequency region indicates that the device has nearly ideal supercapacitor behavior [47]. Additionally, the absence of a semicircle in the high-frequency region suggests the ionic conductivity at the electrode and electrolyte is excellent [42,48]. Furthermore, the solid state device demonstrates good cycling stability (Figure 4D), with only a 6 % reduction of the capacitance after 10000 cycles, indexing excellent long-term performance durability. Even in a bending state, the device shows similar behavior, which is attributed to the high mechanical flexibility of the electrodes as well as the interpenetrating network between the hnp-G electrodes and the gelled electrolyte. To comprehensively evaluate the overall performance of the as-prepared SCs, a radar plot summarizing the combination property in this work and in the recent literatures was drawn (Figure S14). The larger area encompassed within the radar plot indicated the better overall performance of the hnp-G based SCs.
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In general, the total energy that can be stored in a single SC is too low for most practical application. Thus, depending on the application, SCs in series and/or parallel combinations are necessary to form a ‘bank’ with a specific voltage and capacitance rating [49]. The adaptability of the hnp-G assembled devices in serial/parallel combination is demonstrated by connecting three ones together both in series and parallel configurations (Figure 5A and B). In the parallel assembly, the output current increase by a factor of 3, and thus the discharge time was three times that of a single device when operated at the same current density. As expected, when the three SCs are combined in series, the output voltage increases by a factor of 3, which can power a light-emitting diode (LED) (Figure S15). Conclusion In summary, a continuously hnp-G film with ultrahigh external surface area (∼ 954.7 m2/g) was prepared by catalytic pyrolysis of HG, which was synthesized by CVD using NPC catalyst. Due to the ultrahigh external surface area and flexibility, two pieces of hnp-G films were assembled using H2SO4-PVA gel as solid-state electrolyte to form a symmetric flexible SC with a total thickness of ~ 30 μm. The symmetric SC exhibits excellent rate performance, 83 % reduction of specific capacitance when the current density increases from 1 to 20 A/g, due to the continuously 3D hierarchical nanoporous structure and good wettability of the hnpG with gelled electrolyte. Significantly, the symmetric SC offers ultrahigh energy density (2.65 mW h/cm3) and power density (20.8 W/cm3) based on the device and exhibits almost identical performance at various curvatures and excellent lifetime, which suggests its wide application potential in powering wearable/miniaturized electronics. Acknowledgements This work is sponsored by the Key Program of the National Natural Science Foundation of China (No. 51531004), National Natural Science Foundation of China (No. 51472177), Tianjin Research Program of Application Foundation and Advanced Technology (Nos. 14JCYBJC20900, 14JCYBJC19600), China-EU Science and Technology Cooperation Project 11
(SQ2013ZOA100006), and Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT) of the Ministry of Education of China (No. IRT13084).
References [1]
P. Yang, W. Mai, Nano Energy 8 (2014) 274-290.
[2]
D. Dubal, J. Kim, Y. Kim, R. Holze, C. Lokhande, W. Kim, Energy Technol. 2 (2014) 325-341.
[3]
A. Tyagi, K. Tripathi, R. Gupta, J. Mater. Chem. A 3 (2015) 22507.
[4]
C. Zhang, W. Lv, Y. Tao, Q. Yang, Energy Environ. Sci. 8 (2015) 1390-1403.
[5]
L. Li, Z. Wu, S. Yuang, X. Zhang, Energy Environ. Sci. 7 (2014) 2101-2122.
[6]
X. Wang, X. Lu, B. Liu, D. Chen, Y. Tong, G. Shen, Adv. Mater. 26 (2014) 4763-4782.
[7]
D. Yu, K. Goh, Q. Zhang, L. Wei, H. Wang, W. Jiang, Y. Chen, Adv. Mater. 26 (2014) 6790-6797.
[8]
K. Qin, J. Kang, J. Li, C. Shi, Y. Li, Z. Qiao, N. Zhao, ACS Nano 9 (2015) 481-487.
[9]
J. Zhao, S. Chen, S. Xu, M. Shao, Q. Zgabgm F, Wei, J. Ma, M. Wei, D. Evans, X. Duan, Adv. Funct. Mater. 24 (2014) 2938-2946.
[10] X. Lu, M. Yu, G. Wang, Y. Tong, Y. Li, Energy Environ. Sci. 7 (2014) 2160-2181. [11] Z. Sun, C. Yang, B. Xie, Z. Lin, Z. Zhang, J. Liu, B. Li, F. Kang, C. P. Wong, Energy Environ. Sci. 7 (2014) 2652-2659. [12] H. Xu, X. Hu, H. Yang, Y. Sun, C. Hu, Y. Huang, Adv. Energy Mater. 5 (2015) 1401882 (7pp). [13] D. Pham, T. Lee, D. Luong, F. Yao, A. Ghosh, V. Le, T. Kim, B. Li, J. Chang, Y. Lee, ACS Nano 9 (2015) 2018-2027. [14] R. Raccichini, A. Varzi, S. Passerini, B. Scrosati, Nat. Mater. 14 (2015) 271-279. [15] Z. Wu, Z. Liu, K. Parvez, X. Feng, K. Mullen, Adv. Mater. 27 (2015) 3669-3675.
12
[16] Z. Wu, K. Parvez, A. Winter, H. Vieker, X. Liu, S. Han, A. Turchanin, X. Feng, K. Mullen, Adv. Mater. 26 (2014) 45524558. [17] X. Peng, L. Peng, C. Wu, Y. Xie, Chem. Soc. Rev. 43 (2014) 3303-3323. [18] J. Shi, W. Du, Y. Yin, Y. Guo, L. Wan, J. Mater. Chem. A 2 (2014) 10830-10834. [19] Y. Ito, Y. Tanabe, H. Qiu, K. Sugawara, S. Heguri, N. Tu, K. Huynh, T. Fujita, T. Takahashi, K. Tanigaki, M. Chen, Angew. Chem. Int. Ed. 53 (2014) 4822-4826. [20] Y. Shao, M. El-Kady, L. Wang, Q. Zhang, Y. Li, H. Wang, M. Mousavi, R. Kaner, Chem. Soc. Rev. 44 (2015) 3639-3665. [21] S. He, W. Chen, Nanoscale 7 (2015) 6957-6990. [22] S. Zhou, J. Xu, Y. Xiao, N. Zhao, C. Wong, Nano Energy 13 (2015) 458-466. [23] P. Yu, X. Zhao, Z. Huang, Y. Li, Q. Zhang, J. Mater. Chem. A 2 (2014) 14413-14420. [24] Y. He, W. Chen, X. Li, Z. Zhang, J. Fu, C. Zhao, E. Xie, ACS Nano 7 (2013) 174-182. [25] Z. Chen, W. Ren, L. Gao, B. Liu, S. Pei, H. Cheng, Nat. Mater. 10 (2011) 424-428. [26] M. Beidaghi, Y. Gogotsi, Energy Environ. Sci. 7 (2014) 867-884. [27] Y. Xu, K. Sheng, C. Li, G. Shi, ACS Nano 4 (2010) 4324-4330. [28] X. Yang, C. Cheng, Y. Wang, L. Qiu, D. Li, Science 341 (2013) 534-537. [29] T. Kim, G. Jung, S. Yoo, K. Suh, R. Ruoff, ACS Nano 7 (2013) 6899-6905. [30] J. Huang, J. Wang, C. Wang, H. Zhang, C. Lu, J. Wang, Chem. Mater. 27 (2015) 21072113. [31] X. Cai, M. Peng, X. Yu, Y. Fu, D. Zou, J. Mater. Chem. C 2 (2014) 1184-1200. [32] Y. Xu, Z. Lin, X. Huang, Y. Liu, Y. Huang, X. Duan, ACS Nano 7 (2013) 4042-4049. [33] Y. Ito, Y. Tanabe, H. Qiu, K. Sugawara, S. Heguri, N. Tu, K. Huynh, T. Fujita, T. Takahashi, K. Tanigaki, M. Chen, Angew. Chem. Int. Ed. 53 (2014) 4822-4826. [34] Y. Ito, H. Qiu, T. Fujita, Y. Tanabe, K. Tanigaki, M. Chen, Adv. Mater. 26 (2014) 41454150.
13
[35] Y. Ito, W. Cong, T. Fujita. Z. Tang, M. Chen, Angew. Chem. Int. Ed. 54 (2015) 21312136. [36] T. Fujita, P. Guan, K. McKenna, X. Lang, A. Hirata, L. Zhang, T. Tokunaga, S. Arai, Y. Yamamoto, N. Tanaka, Y. Ishikawa, N. Asao, Y. Yamamoto, J. Erlebacher, M. Chen, Nat. Mater. 11 (2012) 775-780. [37] A. Ferrari, J. Robertson, Phys. Rev. B 64 (2001) 075414 (13pp). [38] J. Robertson, E. O’Reilly, Phys. Rev. B 35 (1987) 2946-2957. [39] N. Conway, A. Ferrari, A. Flewitt, J. Robertson, W. Milne, A. Tagliaferro, W. Beyer, Diamond Relat. Matter. 9 (2000) 765-770. [40] J. Miller, R. Outlaw, B. Holloway, Science 329 (2010) 1637-1639. [41] D. Pech, M. Brunet, H. Durous, P. Huang, V. Mochalin, Y. Gogotsi, P. Taberna, P. Simon, Nat. Nanotechnol. 5 (2010) 651-654. [42] M. El-Kady, V. Strong, S. Dubin, R. Kaner, Science 335 (2012) 1326-1330. [43] X. Wang, B. Liu, R. Liu, Q. Wang, X. Hou, D. Chen, R. Wang, G. Shen, Angew. Chem. Int. Ed. 126 (2014) 1880-1884. [44] Z. Su, C. Yang, C. Xu, H. Wu, Z. Zhang, T. Liu, C. Zhang, Q. Yang, B. Li, F. Kang, J. Mater. Chem. A 1 (2013) 12432-12440. [45] X. Lu, M. Yu, G. Wang, T. Zhai, S. Xie, Y. Ling, Y. Tong, Y. Li, Adv. Mater. 25 (2013) 267-272. [46] Y. Chen, L. Du, P. Yang, P. Sun, X. Yu, W. Mai, J. Power Sources 287 (2015) 68-74. [47] G. Wang, H. Wang, X. Lu, Y. Ling, M. Yu, T. Zhai, Y. Tong, Y. Li, Adv. Mater. 26 (2014) 2676-2682. [48] Z. Wu, K. Parvez, X. Feng, K. Mullen, Nat. Commun. 4 (2013) 2487 (8pp). [49] M. El-Kady, R. Kaner, Nat. Commun. 4 (2013) 1475 (9pp).
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Figures
(A)
(B) Figure 1 (A), (B) Schematic diagram of the fabrication process of the hnp-G and flexible solid state supercapacitors.
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Figure 2 (A) SEM image of the NPC coated with HG, the insert is TEM image. (B) HRTEM image of the HG after remove the NPC. (C), (D) SEM and HAADF-STEM images of the hnp-G after treated by HNO3. (E) High-magnification TEM image of ultra-thin graphite nanosheets located onto the surface of hnp-G. (F) High-magnification TEM images of graphene sheets with different number of layer in hnp-G. (G) Photograph of hnp-G film, demonstrating its integrity and flexibility.
Figure 3 (A) Digital picture of the solid-state device (size ~ 1 cm × 0.8 cm) under folded condition. (B) SEM image of the cross section of the ultrathin solid-state device, which was obtained by cracking the sample in liquid nitrogen (N2). (C) High-magnification SEM image of the local area in B, indicating that the hnp-Gs were well bonded with the solid-state H2SO4-PVA gelled electrolyte.
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Figure 4 (A) Typical cyclic voltammograms of the flexible solid-state device at different scan rates from 5 - 300 mV/s. (B) Galvanostatic charge/discharge curves at different current density. (C) The specific capacitances versus current density. (D) Cycling stability of the device at a current density of 2 A/g.
Figure 5 (A) The CV curves of three devices in series and parallel at the scan rate of 50 mV/s. (B) The galvanostatic charge/discharge curves of three devices in series and parallel at 2 A/g.
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Highlights
Free-standing Continuously hierarchical nanoporous graphene (hnp-G) film with high specific surface area (1160 m2/g) and small pore size (1 - 150 nm) was successfully fabricated by a simple method. An ultrathin symmetric solid-state SCs was fabricated by two pieces of hnp-G films using H2SO4/PVA gel as solid-state electrolyte. The symmetric SC offers ultrahigh energy density (2.65 mW h/cm3) and power density (20.8 W/cm3) and exhibits almost identical performance at various curvatures and excellent lifetime (94 % retention after 10000 cycles).
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