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Conjugated molecule functionalized graphene films for energy storage devices with high energy density
Journal Pre-proof Conjugated molecule functionalized graphene films for energy storage devices with high energy density Liheng Wang, Xingke Ye, Yucan Zhu, Hedong Jiang, Jianxing Xia, Ziyu Yue, Zhongquan Wan, Chunyang Jia, Xiaojun Yao PII:
S0013-4686(20)30196-1
DOI:
https://doi.org/10.1016/j.electacta.2020.135804
Reference:
EA 135804
To appear in:
Electrochimica Acta
Received Date: 28 August 2019 Revised Date:
27 January 2020
Accepted Date: 28 January 2020
Please cite this article as: L. Wang, X. Ye, Y. Zhu, H. Jiang, J. Xia, Z. Yue, Z. Wan, C. Jia, X. Yao, Conjugated molecule functionalized graphene films for energy storage devices with high energy density, Electrochimica Acta (2020), doi: https://doi.org/10.1016/j.electacta.2020.135804. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier Ltd.
L. Wang and C. Jia conceived the idea and designed the experiments. L. Wang carried out the synthesis and material characterizations. X. Ye, Y. Zhu, H. Jiang, Z. Yue, Z. Wan helped with the electrochemical tests, data analysis and discussion. J. Xia and X. Yao helped with the theoretical calculation and data analysis. L. Wang wrote the manuscript.
and C. Jia
Conjugated molecule functionalized graphene films for energy storage devices with high energy density Liheng Wanga, Xingke Yea, Yucan Zhua, Hedong Jianga, Jianxing Xiaa, Ziyu Yuea, Zhongquan Wana, Chunyang Jiaa,∗, Xiaojun Yaob a
State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Electronic
Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China. b
State Key Laboratory of Applied Organic Chemistry, School of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China
Graphical abstract
∗ Corresponding author. Tel.: +86 28 83201991; Fax: +86 28 83202569. Email: [email protected] (C. Y. Jia)
Conjugated molecule functionalized graphene films for energy storage devices with high energy density Liheng Wanga, Xingke Yea, Yucan Zhua, Hedong Jianga, Jianxing Xiaa, Ziyu Yuea, Zhongquan Wana, Chunyang Jiaa,∗, Xiaojun Yaob a
State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Electronic
Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China. b
State Key Laboratory of Applied Organic Chemistry, School of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China
Abstract The design of electrode materials is critical to improve electrochemical performance
of
supercapacitors.
Here,
3,4,9,10-perylene-tetracarboxylicacid-dianhydride (PTCDA) molecule functionalized reduced graphene oxide (rGO) self-supporting film with fluffy and porous structure is built by simple and green processes. Due to the synergistic effect between PTCDA molecule with fast Faradaic reaction and fluffy rGO skeleton with excellent ion transport performance, the as-fabricated PTCDA/rGO film electrode exhibits significantly improved electrochemical performance compared with rGO electrode in the symmetric supercapacitor and zinc-ion hybrid supercapacitor, including gravimetric capacitance of 242.9 F g-1 at 2 A g-1 in Li2SO4 aqueous electrolyte and ∗ Corresponding author. Tel.: +86 28 83201991; Fax: +86 28 83202569. Email: [email protected] (C. Y. Jia)
1
discharge capacity of 162.8 mAh g-1 at 1 A g-1 in ZnSO4 aqueous electrolyte. Besides, the symmetric supercapacitor based on PTCDA/rGO film electrodes delivers a gravimetric energy density of 19.7 Wh kg-1 at an ultra-high gravimetric power density of 45 kW kg-1, and the zinc-ion hybrid supercapacitor based on PTCDA/rGO cathode delivers an extremely high gravimetric energy density of 120.5 Wh kg-1 at a gravimetric power density of 740.1 W kg-1. This work provides practical strategy to construct the synergistic relationship between the structure and composition of electrode materials, and explores the influence of this strategy on the electrochemical performance of symmetric and hybrid supercapacitors. Keywords: PTCDA, rGO, synergistic effect, high energy density, zinc-ion hybrid supercapacitor
2
1. Introduction With the depletion of fossil fuels and the increase of environmental pollution, the development and utilization of pure electric vehicles are receiving more and more attention. As the core component of pure electric vehicles, energy storage system plays an important role in electric vehicle technology, which leads to the rapid development of electrical energy storage devices such as batteries, supercapacitors and supercapacitor-battery hybrid devices in recent years [1-5]. Lithium-ion battery, as the mainstream device for energy storage, has been broadly applicated in practical life owing to their high energy densities (150-200 Wh kg-1) [6]. However, due to the various resistance losses caused by sluggish electron and ion transport, the large amount of heat released by lithium-ion battery at high power bring serious potential safety hazard [7]. Compared with batteries, supercapacitors exhibit unique advantages including fast charge/discharge rate, long service life, high power density and greater safety performance at high power, thus they are considered as promising candidates for powering electric vehicle. But commercial supercapacitors often show a relatively poor energy density (5-10 Wh kg-1) limited by the charge storage mechanism of ion adsorption/desorption on the surface of electrodes [2,5,6]. Therefore, how to improve the energy density of the device has become a huge challenge in the field of supercapacitor [8]. The synergistic design of the morphology and composition for electrode material is consider to be one of the ways to improve the energy storage performance of the supercapacitor. By combining the materials with rapid Faradaic reaction and the morphology with fast ion transfer kinetics into the electrode materials, 3
the energy storage performance of symmetric supercapacitor can be enhanced through occurrence of redox reaction while maintaining excellent rate performance. Moreover, due to the different energy storage mechanisms (battery-type Faradaic process and capacitive energy storage mechanism) used on two electrodes, metal-ion hybrid supercapacitors, such as lithium-ion [9-11], sodium-ion [10], zinc-ion [12,13] and magnesium-ion [14] hybrid supercapacitors, inherit the advantages of batteries (high energy density) and supercapacitors (high power density), and make up for their shortcomings. Therefore, changing device structure is also considered to be effective in improving energy storage performance of supercapacitor. Among these hybrid supercapacitors, the zinc-ion hybrid supercapacitors have attracted widespread attention in recent years due to zinc-metal high theoretical anode capacity (820 mAh g−1) [15], low cost (USD $2 kg−1) [16], high electrical conductivity and nontoxicity. Graphene, as a star on the horizon of materials science, has a wide application in many fields [9,17-19]. The fluffy and porous reduced graphene oxide (rGO) hydrogel/aerogel are considered to be ideal electrode material due to excellent electrical
conductivity,
good
chemical
stability
and
the
interconnected
three-dimensional (3D) network structure [20]. However, the pure rGO electrodes often exhibit poor energy density in supercapacitor owing to the limitation of energy storage mechanism. In order to boost energy density of the rGO electrode, combining various energy storage mechanisms on electrodes has been regarded as an effective way. The current major researches focus on adding transition metal oxides/sulfides [21-24] and conductive polymers [25-27] with the fast Faradaic reactions into rGO 4
skeleton. Due to high theoretical capacitance, cheapness and easy to chemical modification, the small organic molecules with redox activity are also considered to be ideal active ingredients of electrode materials and have received attention in recent years [28,29]. As one of them, 3,4,9,10-perylene-tetracarboxylicacid-dianhydride (PTCDA), known as Red 224, was widely reported in lithium-ion, magnesium-ion, sodium-ion and hydronium-ion batteries as organic cathode due to nontoxicity, cheapness and the high theoretical specific capacity (273 mAh g-1) [30-36]. But the low power capability caused by its low electrical conductivity limit its application in field of supercapacitors [37]. Therefore, relying on the excellent morphological structure of rGO hydrogel, non-covalently attaching PTCDA molecules as active materials to rGO skeleton provides a useful strategy to improve electrochemical performance of the electrode materials. Herein, we reported a self-supporting PTCDA/rGO film electrode for the symmetric supercapacitor and the zinc-ion hybrid supercapacitor. Through green and simple aqueous reduction process, a fluffy and porous structure is obtained while the PTCDA molecules are attached to rGO skeleton by π-π stacking. Based on synergistic effect between fast Faradaic reaction of PTCDA molecules and excellent ion transport performance of rGO skeleton, the PTCDA/rGO film electrode can deliver a gravimetric capacitance (CM) of 242.9 F g-1 at 2 A g-1 in Li2SO4 aqueous electrolyte with a potential window of 1.8 V and excellent rate capability (capacitance retention of 72.1% at 100 A g-1). The symmetric supercapacitor based on the PTCDA/rGO film electrodes is assembled to deliver an excellent gravimetric energy density (EM, SC) of 5
19.7 Wh kg-1 at a high gravimetric power density (PM, SC) of 45 kW kg-1. Furthermore, the zinc-ion hybrid supercapacitor based on the PTCDA/rGO film as cathode exhibits an excellent discharge capacity of 162.8 mAh g-1 at 1 A g-1 in aqueous electrolyte. This device can deliver an outstanding gravimetric energy density (EM, ZHS) of 120.5 Wh kg-1 at a gravimetric power density (PM, ZHS) of 740.1 W kg-1 at current density of 1 A g-1 and capacity retention rate of ~91% after 5000 charge/discharge cycles at 20 A g-1. Moreover, in order to match the practical demand, the flexible solid-state devices based on PTCDA/rGO films were successfully assembled, and exhibited superior bending performance confirmed by the capacitance retention rate of 98.4% after bending 500 times. These excellent results demonstrate that our work provides a practical strategy for supercapacitors to break through the bottleneck of energy density and fit the needs of practical application.
2. Materials and experimental section 2.1. Materials Graphite powder and PTCDA (≥99.0%) were purchased from Aladdin. Polyvinyl alcohol (PVA, 98-99% hydrolyzed, medium molecular weight) was purchased from Alfa Aesar. Other chemical reagents of reagent grade were purchased from China and used without further purifications. 2.2. Preparation of PTCDA/rGO films Graphite oxide (GO) was prepared via oxidative exfoliation of natural graphite powder by modified Hummer’s method [38]. The concentration of GO suspensions is 6
~14 mg mL-1. The PTCDA powder was added into the GO suspensions (the weight ratio of PTCDA and GO is 1:10), and then dispersed by sonication for 3 hours. After adding PTCDA powder, the color of the slurry changes from golden yellow to blood red. Next, the PTCDA/GO slurry was coated onto Teflon plate by scraper and dried at 40 °C overnight. After PTCDA/GO film was completely dried, it was peeled off from the substrate. Finally, the PTCDA/GO film was reduced in ascorbic acid aqueous solution (4 mg mL-1) at 90 °C for 2.5 hours to produce PTCDA/rGO film. The rGO film was prepared under the same conditions without adding PTCDA powder. 2.3. Preparation of PTCDA electrodes The PTCDA powder, carbon black and polyvinylidene fluoride in a weight ratio of 7:2:1 were mixed by grinding in 1-methyl-2-pyrrolidone solvent. Next, the paste was evenly coated on aluminum foil and then dried at room temperature to prepare the PTCDA electrode. 2.4. Preparation of PVA-Li2SO4 gel electrolyte 1 g PVA powder was added into 10 mL deionized water and heated to 90 °C for 1 hour with slight stirring until the solution becomes transparent and clear. Finally, 1 g 2 M Li2SO4 aqueous solution was dropped into the PVA solution with strong stirring. 2.5. Fabrication of devices The symmetric sandwich-type supercapacitors based on PTCDA electrodes, rGO films and PTCDA/rGO films (1×1 cm2) were fabricated by using 2 M Li2SO4 as the electrolyte. Platinum foils and filter paper were used as current collectors and separator, respectively. The flexible solid-state supercapacitor (FSSC) was fabricated 7
using Au coated nylon paper as current collectors and PVA-Li2SO4 gel as electrolyte and separator, respectively. The PTCDA/rGO films were cut rectangular electrodes with 1×2 cm2, and then placed on Au paper. Subsequently, the gel electrolyte was evenly coated on the surface of the electrodes and dried at room temperature. After excess water was evaporated, two PTCDA/rGO electrodes were tightly pressed together and encapsulated in polyethylene. Zinc-ion hybrid supercapacitors were fabricated by rGO film and PTCDA/rGO film with 1×1 cm2 as cathodes and zinc foil as anodes in 2 M ZnSO4 aqueous electrolyte. 2.6. Characterization methods The morphologies of all samples were verified by scanning electron microscope (SEM, FEI Inspect F50). The elemental content of all samples was investigated by X-ray photoelectron spectrometry (XPS, ThermoFisher Escalab 250Xi). The structure of all films was investigated via Raman spectroscope (Renishaw Invia) using 532 nm wavelength excitation laser. The interlayer spacing and crystal structure was studied by X-ray diffraction (XRD, X’ Pert PRO). The water contact angle was investigated by drop shape analyzer (Kruss DSA 100). The sheet resistance (RS, Ω sq-1) of all films was tested by standard four-point probe measurement (Laresta-GP). The electrochemical performance of the symmetric supercapacitors, the FSSCs and the zinc-ion hybrid supercapacitors were evaluated by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) in a two-electrode system by electrochemical working station (CHI660 E). The gravimetric specific capacitance (CM) and volume specific capacitance (CV) of 8
electrodes in symmetric supercapacitor were calculated by GCD curves based on equations (1) and (2), respectively. The specific gravimetric capacity (C) of electrodes in zinc-ion hybrid supercapacitor was calculated by equation (3). The volume specific capacitance (CV, device) of FSSC was calculated from GCD curves based on equation (4), the volumetric energy density (EV, device) and power density (PV, device) of FSSC were calculated based on equations (5) and (6), respectively. The gravimetric energy density (EM,
SC)
and the gravimetric power density (PM,
SC)
of symmetric
supercapacitor were calculated by equation (7) and equation (8), respectively. The gravimetric energy density (EM, ZHS) and the gravimetric power density (PM, ZHS) of zinc-ion hybrid supercapacitor were calculated by equation (9) and equation (10), respectively. = =
2 2
1 =
2
ℎ 3 , ! =
4 3.6 !
#, ! =
' , ! $%& 3600#, !
)ℎ * 5 ,, ! =
) * 6 7.2
' $%& 3600#,./
)ℎ 1 7 ,,./ =
) 1 8 28.8 45 3600#,23.
)ℎ 1 9 ,,23. =
) 1 10 = 3.6
#,./ = #,23.
Where M is the mass of single electrode (g), V0 is the volume of single electrode (cm3), Vdevice is the volume of FSSC that includes electrodes, current collectors and gel electrolyte (cm3), the I is the discharge current (A), △t is the discharge time (s), △V is the potential window (V), Vmax is the operation potential during discharge process (V), VAVG is the average voltage scan range (V).
9
3. Results and discussion 3.1. Design concept and preparation process of electrodes In order to solve the poor energy storage performance of rGO films and meet the needs of practical applications, one of the effective improvement ways is to boost the Faradaic charge storage performance of electrode materials by adding redox species as active ingredient. Some design concepts for the construction of electrode materials are listed below. Firstly, the selected redox species should have excellent electrochemical reactivity and fast reaction kinetics [39-42]. Secondly, the designed electrode materials should possess long-term stability (i.e., strong anti-degradation ability and no irreversible side reactions during redox reactions) and outstanding ion transport performance in specific electrolyte [42-44]. Thirdly, electrode materials should have the ability to store charge for long periods of time via suppressed cross diffusion and redox shuttling [45-48]. Finally, simply preparation processes and mild preparation methods are also required for practical production process. Fig. 1 is here According to the above viewpoints, rGO hydrogel film with interconnected 3D network structure was selected as electrode skeleton. This stable structure can not only improve the ion transport performance, but also anchor abundant redox materials, which can prevent that the active materials leak from the matrix during the continuous redox processes [20,49]. Due to excellent electrochemical activity, multi-electron redox process and high redox stability [32,50,51], the commercially available PTCDA molecule was selected as redox material together with the fluffy rGO hydrogel 10
skeleton to prepare electrodes. A PTCDA molecule with planar π-conjugated structure consists of a perylene ring with two conjugated anhydride groups attached on opposite sides. This special structure is conducive to the formation of the π-π stacking between PTCDA molecule and rGO sheet. Thus, the PTCDA molecules are adsorbed on the GO sheets by π-π stacking to prepare slurry via simple the assistance of ultrasound. The above mixed slurry is uniformly scraped into PTCDA/GO films and then reduced to PTCDA/rGO films under mild conditions (Fig. 1). 3.2. Material characterizations SEM is used to analyze the morphology of all samples, the PTCDA powder is composed of a large number of one-dimensional rods with a diameter ranging from hundreds of nanometers to 5 µm and a length ranging from hundreds of nanometers to 20 µm (Fig. S1a, b). After GO films and PTCDA/GO films are reduced and freeze-dried, the cross-section SEM images demonstrate that the rGO film has a fluffy and porous structure fabricated by the mild aqueous reduction method (Fig. 2a). Because ascorbic acid is converted to oxalic acid and gluonic acid in the reduction process, the agglomeration of rGO sheets is effectively prevented by hydrogen bonding between these acids and residual oxygen-containing functional groups on rGO sheets [52,53]. Due to the reduced agglomeration and the residual hydrophilic oxygenated groups on rGO sheets, the rGO sheets can encapsulate plenty of water in the process of reduction [54], thus the interconnected 3D network structure was constructed in rGO film. The PTCDA/rGO film completely preserves the fluffy and porous structure, and the fluffy structure is orderly distributed in the film (Fig. 2b-d). 11
Besides, no obvious block-like PTCDA appears in the SEM image of the PTCDA/rGO film, indicating that the PTCDA molecules were uniformly dispersed on the rGO sheets. Furthermore, by folding the films multiple times, the as-fabricated PTCDA/rGO film shows excellent flexibility, indicating its potential application in flexible electronic devices (Fig. S2). Fig. 2 is here XRD was utilized to analyze the crystal structure of PTCDA powder and the interlayer spacing of GO film, rGO film and PTCDA/rGO film (Fig. 3a). The GO film shows a peak at ~10.1° corresponding to the characteristic reflection of (001) and a wide interlayer spacing of 8.75 Å. After reduction by ascorbic acid, the characteristic peak of GO completely disappeared in XRD pattern of rGO film owing to the removal of lots of oxygen-containing functional groups, and (002) face of rGO appear in ~24.8°, which prove that GO sheets were successfully reduced under the mild conditions. Besides, the characteristic (002) reflection of rGO slightly shift from ~24.8° to ~25.1° after the addition of the PTCDA molecules, and the inter-layer spacing of rGO sheets in freeze-dried rGO film and PTCDA/rGO film are calculated to be 3.59 Å and 3.55 Å, respectively. The small to negligible difference in inter-layer spacing indicates that the agglomeration between rGO sheets is not aggravated by the addition of PTCDA molecules. The diffraction peaks of the PTCDA/rGO film at 9°,10.3° and 12.7° are identical with the peaks of (011), (002) and (012) faces in the XRD pattern of the PTCDA powder, and the diffraction peaks of (110) and (102) faces at 25.4° and 27.7° of the PTCDA powder do not show in XRD pattern of 12
PTCDA/rGO film due to being covered by the characteristic (002) peak of rGO [55]. This result certificates that the PTCDA molecule has high structural stability, and the reduction process of GO sheets do not affect the chemical properties of PTCDA molecule. XPS was employed to analyze chemical composition of all samples (Fig. 3b). The carbon-to-oxygen ratio (C/O) of PTCDA power, GO film, rGO film, PTCDA/rGO film are 5.18, 1.98, 4.46 and 4.91, respectively. The rGO film and PTCDA/rGO film exhibit obvious increase in C1s peak while decreased O1s peak compared with GO film. This result also indicates that the most of oxygen-containing functional groups were successfully removed from GO sheets in the reduction process. C/O of PTCDA/rGO film is slightly higher than rGO film due to the addition of PTCDA molecules. The C1s peaks were deconvoluted into C-C/C=C (284.114 eV), C-O (286.12 eV), C=O (287.21 eV) and O-C=O (288.31 eV) (Fig. S3a-d) [38]. The peak intensity of each oxygen-containing bonds is significantly reduced after the reduction process, which further indicates that the oxygen-containing functional groups are removed from GO sheets. The disordered carbon peak (D-peak) at 1353 cm-1 and the graphitized carbon peak (G-peak) at 1581 cm-1 of all films (Fig. 3c) are detected by Raman spectroscopy [56]. The peak intensity ratio (ID/IG) of GO film, rGO film and PTCDA/rGO film are calculated via peak intensity of D-peak divided by peak intensity of G-peak, which are 0.86, 1.07 and 1.00, respectively. The disorder degree of the sheets is increased owing to the oxygen-containing functional groups detached from GO sheets with the 13
process of reduction reaction. Therefore, the ID/IG of rGO film and PTCDA/rGO film are higher than GO film. The almost identical ID/IG of rGO film and PTCDA/rGO film demonstrate that the disordered degree of rGO sheets is almost unaffected by the PTCDA molecule. Besides, according to previous reports [57], the peak appears near D-peak of PTCDA/rGO film, which is considered to be a superposition of Raman information from PTCDA molecules. Fig. 3 is here Besides, because the PTCDA molecules are absorbed on rGO sheets to prepare electrodes by π-π stacking, without destroying the sp2 hybrid orbitals of rGO sheets. the PTCDA/rGO film exhibits a satisfactory value of sheet resistance (81.3 Ω sq-1) compared with rGO film (62.1 Ω sq-1). The water contact angles of rGO film and PTCDA/rGO film are 86.3° and 77.9° (Fig. S4), respectively. Therefore, the PTCDA/rGO film exhibit better wettability than rGO film owing to the addition of PTCDA, which lead to the entire electrode penetrated by the aqueous electrolyte more easily. 3.3. Electrochemical performance of PTCDA/rGO films in the symmetric supercapacitor CV, GCD and EIS are used to verify the excellent supercapacitor performance of PTCDA/rGO film in Li2SO4 electrolyte. Compared with acidic and alkaline electrolytes, the lower H+ and OH- contents of Li2SO4 neutral electrolyte increase the overpotentials of hydrogen and oxygen evolution reactions, thus the symmetric supercapacitor exhibits a wide voltage window of 1.8 V (CV curves at different 14
electrochemical windows in Fig. S6a) [58,59]. Fig. 4a records the CV curves of PTCDA powder, rGO film and PTCDA/rGO film at scan rate of 50 mV s-1. The pure PTCDA powder hardly exhibits supercapacitor performance, and the curve of rGO film exhibits a nearly rectangular shape, revealing the pronounced electric double layer (EDL) capacitive characteristic and fast current response in neutral electrolyte, but the ion storage capacity of the rGO film is poor limited by energy storage mechanism. After adding PTCDA molecules into rGO skeleton, the area of the CV loop is significantly expanded under the premise of maintaining fast current response during voltage reversal (the CV shape without significant deformation at scan rate of 1 V s-1 in Fig. 4b), and two pairs of redox peaks clearly appear in this curve, which are caused by typical chemical bond reaction between the two conjugated anhydride groups on PTCDA and lithium ions (Scheme 1) [36]. The addition of PTCDA molecules increases the number of Faradaic active sites in the film electrode, resulting in the occurrence of redox reactions in process of charging and discharge [60-62]. Therefore, the area of CV curve is expanded and the energy storage performance is boosted. To further explore the root cause of performance improvement, the binding energy (Eb) values of PTCDA-Li, PTCDA-Li2 and graphene-Li obtained by density functional theory (DFT) calculations are -46.804 kcal/mol, -46.521 kcal/mol and -27.287 kcal/mol, respectively (Fig. S5). This result proves that the chemical adsorption performance of PTCDA molecules on lithium ions is much better than that of graphene. Besides, it was proved by almost the same Eb values of PTCDA-Li and PTCDA-Li2 that the two conjugated anhydride bonds on the same PTCDA can 15
simultaneously anchor lithium ions. Scheme 1 is here In GCD curves at current density of 2 A g-1 (Fig. 4c, the GCD curves of PTCDA/rGO film at different current density are recorded in Fig. S6c, d), the discharge time of PTCDA/rGO film is significantly longer than rGO film, and the CM of PTCDA/rGO film and rGO film are 242.9 F g-1 and 107.2 F g-1 at 2 A g-1, respectively. When the current density is increased to 100 A g-1 (Fig. 4d), the CM of PTCDA/rGO film and rGO film still maintains 175.2 F g-1 and 54.8 F g-1, respectively, and the lower voltage drops of both PTCDA/rGO and rGO films confirm that the excellent ion and electron transport performance of fluffy rGO skeleton is retained in the PTCDA/rGO film. Furthermore, because of the improvement of hydrophilicity, the fluffy 3D structure of the rGO/PTCDA film can contact with the electrolyte more effectively, which leads to the enhancement of ion transport, thus the slightly increasing rate performance of PTCDA/rGO film [63,64]. Besides, the specific volume capacitance (CV) of PTCDA/rGO film (48.6 F cm-3) is also higher than the CV of rGO film (22.9 F cm-3) at current density of 2 A g-1. Owing to a wide potential window and high CM of the PTCDA/rGO film in aqueous Li2SO4 electrolyte, the symmetric supercapacitor based on the PTCDA/rGO films can deliver an excellent EM, SC of
27.3 Wh kg-1 at a PM, SC of 899.2 W kg-1, and can still maintain an EM, SC of 19.7
Wh kg-1 at a high PM, SC of 45 kW kg-1 (Fig. 4e). These values are higher than most carbon material electrodes containing organic materials reported in supercapacitors (Table 1). 16
Table 1. Comparison of energy density (EM) and power density (PM) of electrode materials in supercapacitor Electrode materials
EM (Wh kg-1)
PM (kW kg-1)
References
Th–GA//Th–GA
17.7
12.8
[45]
TN-NPCs//AQ-NPCs
23.5
0.7
[65]
HPCF4//HPCF4
9.1
3.5
[66]
DT-RGN//DT–RGN
8.4
27.5
[67]
NPCs/AQ//NHCSs/PQ-2
8.34
0.6
[68]
AZ–SGHs//AZ–SGHs
18.2
0.7
[69]
PTCDA/rGO//PTCDA/rGO
27.3 19.7
0.9 45.0
This work
To further analyze the reasons for the improvement of the electrochemical performance, EIS of the symmetric supercapacitor based on rGO film and PTCDA/rGO film are obtained in the frequency range from 10-2 Hz to 105 Hz at the open circuit voltage, and the Nyquist plot are represented in Fig. 4f. Based on an equivalent circuit (inset of Fig. 4f), the specific values of the different circuit elements are fitted in Table S1. The internal resistance (Rs) values of the symmetric supercapacitor based on rGO film and PTCDA/rGO film are 1.133 Ω and 1.069 Ω, respectively. Because the same aqueous electrolyte (2 M Li2SO4) is used in these symmetric supercapacitors, the devices exhibit almost the same Rs values. The interfacial charge transfer resistance (Rct) is slightly smaller for PTCDA/rGO film (0.355 Ω) compared than rGO film (0.538 Ω). The small Rct of PTCDA/rGO film indicates the interconnected 3D network structure and excellent wettability are conducive to the rapid ion transfer in the interface of electrolyte and electrode. These 17
results sufficiently confirm that the PTCDA/rGO film has the excellent ions diffusion and transmission behavior without being affected by adding PTCDA molecules, thereby leading to the emergence of rapid reaction kinetics and outstanding electrochemical activity. Besides, the stable support structure of rGO skeleton and the strong π-π interaction between PTCDA molecules and rGO sheets can effectively prevent the structural collapse of the electrode materials during continuous redox process[70-72], thus the PTCDA/rGO film exhibits excellent cycling stability in Li2SO4 electrolyte and maintains ~93% of the initial specific capacitance value after 5000 cycling at 1 V s-1 (Fig. 4g). Fig. 4 is here In order to respond actual demand, FSSC based on the PTCDA/rGO films was assembled. The CV curves of the device at 20 mV s-1 to 200 mV s-1 (Fig. 5a) exhibit satisfactory current response in gel electrolyte. But compared with aqueous electrolyte, because the transmission of ions is restricted by gel electrolytes, the peak intensity of redox peaks decreases and the peak position shift in the CV curves of FSSC [73]. In PVA-Li2SO4 gel electrolyte, the PTCDA/rGO film also shows a satisfactory CM of 180.4 F g-1 evaluated by the GCD curve at 2 A g-1, and the CM still retains 75.7% of its initial value at current density of 20 A g-1 (Fig. 5b shows the GCD curves at different current densities). The FSSC based on the PTCDA/rGO electrodes can deliver a high EM, SC of 20.3 Wh kg-1 at a PM, SC of 900 W kg-1, and when the PM, SC increases to 9 kW kg-1, the device still maintains an EM, SC of 15.4 Wh kg-1. Meanwhile, for a more comprehensive evaluation of device performance, we calculate the EV, device and PV, 18
device
of the device. The volume of device includes the volume of the two electrodes,
two current collectors and gel electrolyte. The FSSC based on the PTCDA/rGO electrodes shows EV, device of 2.88 Wh L-1 at PV, device of 127.68 W L-1 and EV, device of 2.18 Wh L-1 at PV, device of 1277.56 W L-1 in PVA gel electrolyte (Fig. 5c), which is better than the recently reported FSSC device base on rGO electrodes (Table 2). Table 2. Comparison of EV, device and PV, device of FSSCs Electrode materials
Electrolyte
CV, device (F cm-3)
EV, device (Wh L-1)
PV, device (W L-1)
References
rGO/PW12// rGO/PW12
PVA/ H2SO4
2.95
1.05
11.5
[74]
cellulose/rGO/AgNPs//cellulos e/rGO/AgNPs
PVA/KCl
8.56
1.19
31.25
[75]
rGO/MMNNBs// rGO/MMNNBs
PVA/H3PO4
15.4
1.05
35
[76]
CC@rGO//PPy
PVA/H3PO4
12.3
1.10
10
[77]
Co2.18Ni0.82Si2O5(OH)4//rGO
PVA/KOH
1.15
0.496
38.8
[78]
N-rGO/Mn//AC
PVA/LiCl
4.49
2.02
180
[79]
Ni/GF/H-CoMoO4//Ni/GF/H-F e 2O 3
PVA/KOH
3.6
1.13
150
[80]
ErGO/NCA// ErGO/NCA
PVA/Na2SO4
1.72
0.15
4
[81]
PPDA-HGF// PPDA-HGF
PVA/H2SO4
19.6
2.72
146.5
[82]
PTCDA/rGO//PTCDA/rGO
PVA/Li2SO4
6.40 4.85
2.88 2.18
127.68 1277.56
This work
The Nyquist plot (Fig. 5d) of FSSC exhibits small diameter of the semicircle at the high frequency followed by a near-vertical line at low frequency, which confirms that the PTCDA/rGO film still maintains good ion transport performance and ideal capacitive behavior in gel electrolyte. To demonstrate excellent flexibility of the FSSC, Fig. 5e and 5f show the capacitance retention under various bending angles and after bending 500 times, respectively. Compared with non-bending devices, the 19
maximum increase and decrease in specific capacitance of FSSC at different bending angles are 4.8% and 2.7%, respectively, and the capacitance value of FSSC still retains 98.4% of the initial specific capacitance value after bending 500 times. These excellent electrochemical and bending performances fully demonstrate the commercial application prospects of FSSC based on PTCDA/rGO film electrodes in the field of electric vehicles and flexible electronics. Fig. 5 is here 3.4. Electrochemical performance of PTCDA/rGO films in zinc-ion hybrid supercapacitor To further broaden the application range of this film and explore the synergistic design concept of electrode materials in hybrid supercapacitors, a zinc-ion hybrid supercapacitor based on PTCDA/rGO film as cathode and zinc foil as anode was assembled in 2 M ZnSO4 electrolyte. The reversible deposition/stripping of zinc ion and capacitive energy storage process occur on the anode and cathode of the zinc-ion hybrid supercapacitor, respectively (the schematic illustration of PTCDA/rGO//Zn zinc-ion hybrid supercapacitor in Fig. 6a). Compared with the symmetric supercapacitor, The CV curve of the zinc-ion hybrid supercapacitor exhibits a completely different shape by the different energy storage mechanism of anode material. Because large numbers of redox active sites brought by PTCDA molecules appear in the cathode of the zinc-ion hybrid supercapacitor, redox process with good reversibility occur in the near surface region of the electrode. Accordingly, the CV curves at 50 mV s-1 (Fig. 6b) show that the PTCDA/rGO film has better charge 20
storage capacity and faster current response compared with the rGO film under a voltage window of 0.2 ~ 1.8 V. At high scan rate of 100 mV s-1 (the CV curves at scan rate from 10 to 100 mV s-1 in Fig. 6c), the CV curve is no obvious deformation, which indicates that the device based on PTCDA/rGO film cathode has fast Faradaic reaction process and excellent ion transport performance. Besides, a pair of redox peaks appear in the CV loop after adding the PTCDA molecules to rGO skeleton, which results from enolization process of PTCDA molecules. The carbonyl groups (C=O) on the conjugated anhydride groups of PTCDA are converted into enolate groups (C-O-) and combined with zinc ions to form enolates [33]. Fig. 6 is here The GCD curves of PTCDA/rGO film at different current densities are represented in Fig. S7. Because no conspicuous voltage platform appears in these GCD curves, the zinc-ion hybrid supercapacitor based on PTCDA/rGO cathode shows excellent capacitance performance. Calculated by the GCD curves (GCD curves at 1 A g-1 in Fig. 6d), the discharge capacity of PTCDA/rGO film and the rGO film are 162.8 mAh g-1 and 104.1 mAh g-1, respectively. When the current density is increased to 20 A g-1, the PTCDA/rGO film still maintains high discharge capacity of 104.6 mAh g-1 (the discharge capacity at different current densities are recorded in Fig. 6f). The improvement of capacity is attributed to the occurrence of the redox reactions and the increase of the active sites for ion adsorption/desorption on the cathode by addition of PTCDA molecules. Furthermore, due to the synergistic effect between excellent ion/electron-transfer performance of fluffy rGO skeleton and outstanding ion 21
adsorption/desorption performance of PTCDA molecules, this device based on PTCDA/rGO film as cathode delivers an EM, ZHS up to 120.5 Wh kg-1 at a PM, ZHS of 740.1 W kg-1 at current density of 1 A g-1, and an EM, ZHS of 77.06 Wh kg-1 at a PM, ZHS of 14.7 kW kg-1 at current density of 20 A g-1. Compared with recently published papers about aqueous zinc-ion hybrid energy storage devices (Table 3), the zinc-ion hybrid supercapacitor based on PTCDA/rGO film as cathode exhibits excellent energy storage performance and power performance, shown in the Ragone plot (Fig. 6g). Furthermore, the zinc-ion hybrid supercapacitor based on PTCDA/rGO cathode shows excellent capacity retention rate of ~91% after 5000 charge/discharge cycles at current density of 20 A g-1 and Coulombic efficiency of ~100% during the charge/discharge cycle (Fig. 6h). To further demonstrate the potential of application, the packaged zinc-ion hybrid supercapacitors based on PTCDA/rGO films (single electrode area: 1×1.5 cm2) as cathode and zinc foils as anode are assembled, and the LED board with 25 green LED (drive voltage: 3.2 V) lighted up by two devices connected in series (Fig. S8). These results strongly suggest that the concept of synergistic design for symmetric supercapacitor electrode is also suitable for designing cathode materials of the zinc-ion hybrid supercapacitor to improve their performance. Table 3. Comparison of EM and PM of aqueous zinc-ion hybrid energy storage devices Hybrid energy storage devices
EM (Wh kg-1)
PM (kW kg-1)
References
rGO-VO2//Zn
65.0
7.8
[83]
AC//Zn
30
14.9
[84]
22
Cu-HCF//Zn
33.8
0.47
[85]
MXene-rGO//Zn
34.9
0.28
[86]
HPC//Zn
63.7
3.16
[87]
MnO2–CNTs//MXene
29.7
2.48
[88]
PPy//Zn-graphite
64.0
11.7
[89]
ACC//Zn
94
0.07
[90]
RTEG-MnO2//Zn
50.12
15.26
[91]
Zn3[Fe(CN)6]2//Zn
46
1.7
[92]
PTCDA/rGO//Zn zinc-ion hybrid supercapacitor
120.5 77.1
0.7 14.7
This work
4. Conclusions In summary, we report a green and simple strategy to construct PTCDA/rGO film electrode with outstanding performance in the symmetric supercapacitor and the zinc-ion hybrid supercapacitor. Due to fluffy rGO skeleton with excellent ion diffusion performance and PTCDA molecules with rapid reaction kinetics, the symmetric supercapacitor based on PTCDA/rGO electrodes can deliver a satisfactory EM, SC of 19.7 Wh kg-1 at an ultra-high PM, SC of 45 kW kg-1. the PTCDA/rGO film as cathode in the zinc-ion hybrid supercapacitor exhibits the discharge capacity of 162.8 mAh g-1 at 1 A g-1 and can deliver an extremely high EM, ZHS of 120.5 Wh kg-1 at a PM, ZHS
of 740.1 W kg-1 at current density of 1 A g-1. The PTCDA/rGO electrodes
demonstrate outstanding performance in both the symmetric supercapacitor and the zinc-ion hybrid supercapacitor, which broadens the scope of application of the electrode, and may provide inspiration for traditional supercapacitor materials applied in the field of hybrid supercapacitors via analyzing structural characteristics and 23
design concept of this electrode materials. Specifically, the organic molecules with different kinds and quantities of redox active groups are introduced into the microstructure with fast ion transport performance, so as to build a synergistic relationship between structure and composition, and improve the electrochemical performance of electrode materials.
Acknowledgements We are grateful to the National Key R@D Program of China (No. 2017YFB0702802), National Natural Science Foundation of China (Grant Nos. 51773027 and 21572032) and the Fundamental Research Funds for the Central Universities of China (No. ZYGX2019Z007) for financial support.
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27
Figure captions Fig. 1. Schematic illustration of preparing PTCDA/rGO film. Fig. 2. The cross-section SEM of (a) rGO film and (b-d) PTCDA/rGO film. Fig. 3. (a) XRD patterns, (b) XPS spectra and (c) Raman spectra. Scheme 1. The proposed electrochemical reaction of PTCDA in Li2SO4 electrolyte during the discharge/charge process. Fig. 4. Comparison of electrochemical performance of the symmetric supercapacitors based on rGO films and PTCDA/rGO films. (a) CV curves of PTCDA, rGO film and PTCDA/rGO film at 50 mV s-1. (b) CV curves of PTCDA/rGO film at 100 mV s-1 to 1000 mV s-1. (c) GCD curves at 2 A g-1. (d) GCD curves at 100 A g-1. (e) Ragone plot. (f) Nyquist plot (inert shows magnified high frequency region and the equivalent circuit). (g) Cycling stability of PTCDA/rGO film. Fig. 5. Electrochemical and bending performance of FSSC based on PTCDA/rGO films. (a) CV curves at various scan rates. (b) GCD curves from 2 A g-1 to 20 A g-1. (c) Ragone plot for FSSCs reported in the literature and our work. (d) Nyquist plot (inset is the magnified plot). Capacitance retention of (e) different bending angles (inert is Schematic illustration of FSSC) and (f) bending times. Fig.
6.
Comparison
of
electrochemical
performance
of
zinc-ion
hybrid
supercapacitors based on rGO film and PTCDA/rGO film. (a) Schematic illustration of zinc-ion hybrid supercapacitor based on PTCDA/rGO film. (b) CV curves at 50 mV s-1. (c) CV curves of PTCDA/rGO film at 10 mV s-1 to 100 mV s-1. (d) GCD curves at 1 A g-1. (e) Nyquist plot. (f) The discharge capacity at different current 28
densities. (g) Ragone plot. (h) Capacitance retention and Coulombic efficiency.
29
Figures
Fig. 1
Fig. 2 30
(b)
Intensity (a.u.)
Intensity (a.u.)
C1s peak
PTCDA/rGO rGO GO G-peak
D-peak
O1s peak C/O=4.91
C/O=4.46
C/O=1.98 (110) (102)
(011) (002)
(012)
10
(c)
PTCDA/rGO rGO GO PTCDA
Intensity (a.u.)
PTCDA/rGO rGO GO PTCDA
(012) (011) (002)
(a)
20
C/O=5.18
30
40
2θ θ (o)
50
60
70
200
400
Binding energy (eV)
Fig. 3
Scheme 1
31
600
1000
1200
1400
Wave number (cm-1)
1600
200
Current density (A g-1)
Current density (A g-1)
150 100
4
0
2.0
100 mV s-1 200 mV s-1 500 mV s-1 700 mV s-1 1000 mV s-1
(b)
PTDCA/rGO rGO PTCDA
8
(c)
PTCDA/rGO rGO
1.5
Potential (V)
(a)
50
1.0
0
-50
-4
0.5
-100
50 mV s-1
-8
-150
2 A g-1 0.0
0.4
0.8
1.2
1.6
0.0
2.0
0.4
0.8
1.6
2.0
0
(e)
PTCDA/rGO rGO
IR
Energy density (Wh kg-1)
1.2
0.8
0.4
150
200
250
Time (s) (f)
PTCDA/rGO rGO
This work
25
8
40
Ref. 65 20
6
Ref. 69
-Z'' (Ω Ω)
(d) 1.6
100
50
30
IR
50
Voltage (V)
Voltage (V) 2.0
Voltage (V)
1.2
30
Ref. 45
15
-Z'' (Ω Ω)
0.0
4
20 10
Ref. 66
Ref. 68
2
Ref. 67 10
5
0 1
100 A g-1 0.0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
103
Time (s)
Capacitance retention (%)
2
3
4
Z' (Ω Ω)
5
6
7
8
0
0 104
0
105
-1
Power density (W kg )
10
20
30
Z' (Ω Ω)
40
50
(g) 100
80
60 0
1000
2000
3000
4000
5000
Cycle number
Fig. 4
2 A g-1 5 A g-1 7 A g-1 10 A g-1 20 A g-1
(b) 1.6
Potential (V)
20
Current density (A g-1)
2.0
20 mV s-1 50 mV s-1 70 mV s-1 100 mV s-1 200 mV s-1
10
1.2
0
0.8
-10
0.4
3.5
(c) 3.0
Energy density (Wh L-1)
30
(a)
Th is w o rk
Ref. 82 2.5
Ref. 79
2.0
1.5
Ref. 77
1.0
Ref. 74
0.5
Ref. 75 Ref. 76
Ref. 80
Ref. 78
-20
Ref. 81
0.0 0.0
0.4
0.8
1.2
1.6
2.0
0.0
0
50
100
150
200
110
102
Power density (W L-1)
103
110
(e)
(d) 100
101
Time (s)
Voltage (V)
(f) Capacitance retention (%)
Capacitance retention (%)
10
100
8
-Z'' (Ω )
75
-Z'' (Ω )
6
50
4
2
25
100
90
80
70
90
80
Bending 70
0 0
5
10
Z' (Ω)
0
60 0
25
50
Z' (Ω)
75
100
60
0
60
120
Bending angle (°)
Fig. 5
32
180
0
100
200
300
Bending number
400
500
45
(b)
(c)
PTCDA/rGO rGO
30
Current density (A g-1)
Current density (A g-1)
20
0
10 mV s-1 20 mV s-1 50 mV s-1 70 mV s-1 100 mV s-1
15
0
-15
-20
-30
50 mV s-1 -45
0.0
0.4
0.8
1.2
1.6
2.0
0.0
Voltage (V) 1.8
100
(e)
PTCDA/rGO rGO
1.6
0.8
1.2
1.6
Voltage (V)
(f)
PTCDA/rGO rGO
1.4
2.0
PTCDA/rGO rGO
160
Discharge capacity (mAh g -1 )
(d)
0.4
Voltage (V)
120
-Z'' (Ω Ω)
1.2
50
1.0 0.8 0.6 0.4
80
40
-1
1Ag
0
0.2 0
50
100
150
Discharge capacity (mAh g-1)
200
30
60
Z' (Ω)
90
120
0
5
10
15
20
Current density (A g-1)
25
110
(g)
105
Ref. 90 80
Ref. 83 Ref. 89
Ref. 87
60
Ref. 92
Ref. 91
40
Ref. 86
Ref. 85
C ap acity reten tion (% )
100
This work
100
90
100
80 95 70 90 60 85
50
Ref. 84
Ref. 88
40
20 102
103
Power density (W kg-1)
104
80 0
1000
Fig. 6
33
2000
Cycle number 3000
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5000
C oulom b ic efficien cy (% )
(h)
120
Energy density (Wh kg -1 )
0 0
250
Conjugated molecule functionalized graphene films for energy storage devices with high energy density Liheng Wanga, Xingke Yea, Yucan Zhua, Hedong Jianga, Jianxing Xiaa, Ziyu Yuea, Zhongquan Wana, Chunyang Jiaa,∗, Xiaojun Yaob a
State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Electronic
Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China. b
State Key Laboratory of Applied Organic Chemistry, School of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China
Highlights 1. PTCDA molecules functionalized fluffy hydrogel structure is built in rGO film. 2. The synergistic effect between structure and composition is explored for performance. 3. PTCDA/rGO film exhibits good capacitance and rate performance in neutral electrolyte. 4. PTCDA/rGO//Zn hybrid supercapacitor shows the excellent capacity and energy density.
∗ Corresponding author. Tel.: +86 28 83201991; Fax: +86 28 83202569. Email: [email protected] (C. Y. Jia)
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0013-4686(20)30196-1
DOI:
https://doi.org/10.1016/j.electacta.2020.135804
Reference:
EA 135804
To appear in:
Electrochimica Acta
Received Date: 28 August 2019 Revised Date:
27 January 2020
Accepted Date: 28 January 2020
Please cite this article as: L. Wang, X. Ye, Y. Zhu, H. Jiang, J. Xia, Z. Yue, Z. Wan, C. Jia, X. Yao, Conjugated molecule functionalized graphene films for energy storage devices with high energy density, Electrochimica Acta (2020), doi: https://doi.org/10.1016/j.electacta.2020.135804. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier Ltd.
L. Wang and C. Jia conceived the idea and designed the experiments. L. Wang carried out the synthesis and material characterizations. X. Ye, Y. Zhu, H. Jiang, Z. Yue, Z. Wan helped with the electrochemical tests, data analysis and discussion. J. Xia and X. Yao helped with the theoretical calculation and data analysis. L. Wang wrote the manuscript.
and C. Jia
Conjugated molecule functionalized graphene films for energy storage devices with high energy density Liheng Wanga, Xingke Yea, Yucan Zhua, Hedong Jianga, Jianxing Xiaa, Ziyu Yuea, Zhongquan Wana, Chunyang Jiaa,∗, Xiaojun Yaob a
State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Electronic
Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China. b
State Key Laboratory of Applied Organic Chemistry, School of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China
Graphical abstract
∗ Corresponding author. Tel.: +86 28 83201991; Fax: +86 28 83202569. Email: [email protected] (C. Y. Jia)
Conjugated molecule functionalized graphene films for energy storage devices with high energy density Liheng Wanga, Xingke Yea, Yucan Zhua, Hedong Jianga, Jianxing Xiaa, Ziyu Yuea, Zhongquan Wana, Chunyang Jiaa,∗, Xiaojun Yaob a
State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Electronic
Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China. b
State Key Laboratory of Applied Organic Chemistry, School of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China
Abstract The design of electrode materials is critical to improve electrochemical performance
of
supercapacitors.
Here,
3,4,9,10-perylene-tetracarboxylicacid-dianhydride (PTCDA) molecule functionalized reduced graphene oxide (rGO) self-supporting film with fluffy and porous structure is built by simple and green processes. Due to the synergistic effect between PTCDA molecule with fast Faradaic reaction and fluffy rGO skeleton with excellent ion transport performance, the as-fabricated PTCDA/rGO film electrode exhibits significantly improved electrochemical performance compared with rGO electrode in the symmetric supercapacitor and zinc-ion hybrid supercapacitor, including gravimetric capacitance of 242.9 F g-1 at 2 A g-1 in Li2SO4 aqueous electrolyte and ∗ Corresponding author. Tel.: +86 28 83201991; Fax: +86 28 83202569. Email: [email protected] (C. Y. Jia)
1
discharge capacity of 162.8 mAh g-1 at 1 A g-1 in ZnSO4 aqueous electrolyte. Besides, the symmetric supercapacitor based on PTCDA/rGO film electrodes delivers a gravimetric energy density of 19.7 Wh kg-1 at an ultra-high gravimetric power density of 45 kW kg-1, and the zinc-ion hybrid supercapacitor based on PTCDA/rGO cathode delivers an extremely high gravimetric energy density of 120.5 Wh kg-1 at a gravimetric power density of 740.1 W kg-1. This work provides practical strategy to construct the synergistic relationship between the structure and composition of electrode materials, and explores the influence of this strategy on the electrochemical performance of symmetric and hybrid supercapacitors. Keywords: PTCDA, rGO, synergistic effect, high energy density, zinc-ion hybrid supercapacitor
2
1. Introduction With the depletion of fossil fuels and the increase of environmental pollution, the development and utilization of pure electric vehicles are receiving more and more attention. As the core component of pure electric vehicles, energy storage system plays an important role in electric vehicle technology, which leads to the rapid development of electrical energy storage devices such as batteries, supercapacitors and supercapacitor-battery hybrid devices in recent years [1-5]. Lithium-ion battery, as the mainstream device for energy storage, has been broadly applicated in practical life owing to their high energy densities (150-200 Wh kg-1) [6]. However, due to the various resistance losses caused by sluggish electron and ion transport, the large amount of heat released by lithium-ion battery at high power bring serious potential safety hazard [7]. Compared with batteries, supercapacitors exhibit unique advantages including fast charge/discharge rate, long service life, high power density and greater safety performance at high power, thus they are considered as promising candidates for powering electric vehicle. But commercial supercapacitors often show a relatively poor energy density (5-10 Wh kg-1) limited by the charge storage mechanism of ion adsorption/desorption on the surface of electrodes [2,5,6]. Therefore, how to improve the energy density of the device has become a huge challenge in the field of supercapacitor [8]. The synergistic design of the morphology and composition for electrode material is consider to be one of the ways to improve the energy storage performance of the supercapacitor. By combining the materials with rapid Faradaic reaction and the morphology with fast ion transfer kinetics into the electrode materials, 3
the energy storage performance of symmetric supercapacitor can be enhanced through occurrence of redox reaction while maintaining excellent rate performance. Moreover, due to the different energy storage mechanisms (battery-type Faradaic process and capacitive energy storage mechanism) used on two electrodes, metal-ion hybrid supercapacitors, such as lithium-ion [9-11], sodium-ion [10], zinc-ion [12,13] and magnesium-ion [14] hybrid supercapacitors, inherit the advantages of batteries (high energy density) and supercapacitors (high power density), and make up for their shortcomings. Therefore, changing device structure is also considered to be effective in improving energy storage performance of supercapacitor. Among these hybrid supercapacitors, the zinc-ion hybrid supercapacitors have attracted widespread attention in recent years due to zinc-metal high theoretical anode capacity (820 mAh g−1) [15], low cost (USD $2 kg−1) [16], high electrical conductivity and nontoxicity. Graphene, as a star on the horizon of materials science, has a wide application in many fields [9,17-19]. The fluffy and porous reduced graphene oxide (rGO) hydrogel/aerogel are considered to be ideal electrode material due to excellent electrical
conductivity,
good
chemical
stability
and
the
interconnected
three-dimensional (3D) network structure [20]. However, the pure rGO electrodes often exhibit poor energy density in supercapacitor owing to the limitation of energy storage mechanism. In order to boost energy density of the rGO electrode, combining various energy storage mechanisms on electrodes has been regarded as an effective way. The current major researches focus on adding transition metal oxides/sulfides [21-24] and conductive polymers [25-27] with the fast Faradaic reactions into rGO 4
skeleton. Due to high theoretical capacitance, cheapness and easy to chemical modification, the small organic molecules with redox activity are also considered to be ideal active ingredients of electrode materials and have received attention in recent years [28,29]. As one of them, 3,4,9,10-perylene-tetracarboxylicacid-dianhydride (PTCDA), known as Red 224, was widely reported in lithium-ion, magnesium-ion, sodium-ion and hydronium-ion batteries as organic cathode due to nontoxicity, cheapness and the high theoretical specific capacity (273 mAh g-1) [30-36]. But the low power capability caused by its low electrical conductivity limit its application in field of supercapacitors [37]. Therefore, relying on the excellent morphological structure of rGO hydrogel, non-covalently attaching PTCDA molecules as active materials to rGO skeleton provides a useful strategy to improve electrochemical performance of the electrode materials. Herein, we reported a self-supporting PTCDA/rGO film electrode for the symmetric supercapacitor and the zinc-ion hybrid supercapacitor. Through green and simple aqueous reduction process, a fluffy and porous structure is obtained while the PTCDA molecules are attached to rGO skeleton by π-π stacking. Based on synergistic effect between fast Faradaic reaction of PTCDA molecules and excellent ion transport performance of rGO skeleton, the PTCDA/rGO film electrode can deliver a gravimetric capacitance (CM) of 242.9 F g-1 at 2 A g-1 in Li2SO4 aqueous electrolyte with a potential window of 1.8 V and excellent rate capability (capacitance retention of 72.1% at 100 A g-1). The symmetric supercapacitor based on the PTCDA/rGO film electrodes is assembled to deliver an excellent gravimetric energy density (EM, SC) of 5
19.7 Wh kg-1 at a high gravimetric power density (PM, SC) of 45 kW kg-1. Furthermore, the zinc-ion hybrid supercapacitor based on the PTCDA/rGO film as cathode exhibits an excellent discharge capacity of 162.8 mAh g-1 at 1 A g-1 in aqueous electrolyte. This device can deliver an outstanding gravimetric energy density (EM, ZHS) of 120.5 Wh kg-1 at a gravimetric power density (PM, ZHS) of 740.1 W kg-1 at current density of 1 A g-1 and capacity retention rate of ~91% after 5000 charge/discharge cycles at 20 A g-1. Moreover, in order to match the practical demand, the flexible solid-state devices based on PTCDA/rGO films were successfully assembled, and exhibited superior bending performance confirmed by the capacitance retention rate of 98.4% after bending 500 times. These excellent results demonstrate that our work provides a practical strategy for supercapacitors to break through the bottleneck of energy density and fit the needs of practical application.
2. Materials and experimental section 2.1. Materials Graphite powder and PTCDA (≥99.0%) were purchased from Aladdin. Polyvinyl alcohol (PVA, 98-99% hydrolyzed, medium molecular weight) was purchased from Alfa Aesar. Other chemical reagents of reagent grade were purchased from China and used without further purifications. 2.2. Preparation of PTCDA/rGO films Graphite oxide (GO) was prepared via oxidative exfoliation of natural graphite powder by modified Hummer’s method [38]. The concentration of GO suspensions is 6
~14 mg mL-1. The PTCDA powder was added into the GO suspensions (the weight ratio of PTCDA and GO is 1:10), and then dispersed by sonication for 3 hours. After adding PTCDA powder, the color of the slurry changes from golden yellow to blood red. Next, the PTCDA/GO slurry was coated onto Teflon plate by scraper and dried at 40 °C overnight. After PTCDA/GO film was completely dried, it was peeled off from the substrate. Finally, the PTCDA/GO film was reduced in ascorbic acid aqueous solution (4 mg mL-1) at 90 °C for 2.5 hours to produce PTCDA/rGO film. The rGO film was prepared under the same conditions without adding PTCDA powder. 2.3. Preparation of PTCDA electrodes The PTCDA powder, carbon black and polyvinylidene fluoride in a weight ratio of 7:2:1 were mixed by grinding in 1-methyl-2-pyrrolidone solvent. Next, the paste was evenly coated on aluminum foil and then dried at room temperature to prepare the PTCDA electrode. 2.4. Preparation of PVA-Li2SO4 gel electrolyte 1 g PVA powder was added into 10 mL deionized water and heated to 90 °C for 1 hour with slight stirring until the solution becomes transparent and clear. Finally, 1 g 2 M Li2SO4 aqueous solution was dropped into the PVA solution with strong stirring. 2.5. Fabrication of devices The symmetric sandwich-type supercapacitors based on PTCDA electrodes, rGO films and PTCDA/rGO films (1×1 cm2) were fabricated by using 2 M Li2SO4 as the electrolyte. Platinum foils and filter paper were used as current collectors and separator, respectively. The flexible solid-state supercapacitor (FSSC) was fabricated 7
using Au coated nylon paper as current collectors and PVA-Li2SO4 gel as electrolyte and separator, respectively. The PTCDA/rGO films were cut rectangular electrodes with 1×2 cm2, and then placed on Au paper. Subsequently, the gel electrolyte was evenly coated on the surface of the electrodes and dried at room temperature. After excess water was evaporated, two PTCDA/rGO electrodes were tightly pressed together and encapsulated in polyethylene. Zinc-ion hybrid supercapacitors were fabricated by rGO film and PTCDA/rGO film with 1×1 cm2 as cathodes and zinc foil as anodes in 2 M ZnSO4 aqueous electrolyte. 2.6. Characterization methods The morphologies of all samples were verified by scanning electron microscope (SEM, FEI Inspect F50). The elemental content of all samples was investigated by X-ray photoelectron spectrometry (XPS, ThermoFisher Escalab 250Xi). The structure of all films was investigated via Raman spectroscope (Renishaw Invia) using 532 nm wavelength excitation laser. The interlayer spacing and crystal structure was studied by X-ray diffraction (XRD, X’ Pert PRO). The water contact angle was investigated by drop shape analyzer (Kruss DSA 100). The sheet resistance (RS, Ω sq-1) of all films was tested by standard four-point probe measurement (Laresta-GP). The electrochemical performance of the symmetric supercapacitors, the FSSCs and the zinc-ion hybrid supercapacitors were evaluated by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) in a two-electrode system by electrochemical working station (CHI660 E). The gravimetric specific capacitance (CM) and volume specific capacitance (CV) of 8
electrodes in symmetric supercapacitor were calculated by GCD curves based on equations (1) and (2), respectively. The specific gravimetric capacity (C) of electrodes in zinc-ion hybrid supercapacitor was calculated by equation (3). The volume specific capacitance (CV, device) of FSSC was calculated from GCD curves based on equation (4), the volumetric energy density (EV, device) and power density (PV, device) of FSSC were calculated based on equations (5) and (6), respectively. The gravimetric energy density (EM,
SC)
and the gravimetric power density (PM,
SC)
of symmetric
supercapacitor were calculated by equation (7) and equation (8), respectively. The gravimetric energy density (EM, ZHS) and the gravimetric power density (PM, ZHS) of zinc-ion hybrid supercapacitor were calculated by equation (9) and equation (10), respectively. = =
2 2
1 =
2
ℎ 3 , ! =
4 3.6 !
#, ! =
' , ! $%& 3600#, !
)ℎ * 5 ,, ! =
) * 6 7.2
' $%& 3600#,./
)ℎ 1 7 ,,./ =
) 1 8 28.8 45 3600#,23.
)ℎ 1 9 ,,23. =
) 1 10 = 3.6
#,./ = #,23.
Where M is the mass of single electrode (g), V0 is the volume of single electrode (cm3), Vdevice is the volume of FSSC that includes electrodes, current collectors and gel electrolyte (cm3), the I is the discharge current (A), △t is the discharge time (s), △V is the potential window (V), Vmax is the operation potential during discharge process (V), VAVG is the average voltage scan range (V).
9
3. Results and discussion 3.1. Design concept and preparation process of electrodes In order to solve the poor energy storage performance of rGO films and meet the needs of practical applications, one of the effective improvement ways is to boost the Faradaic charge storage performance of electrode materials by adding redox species as active ingredient. Some design concepts for the construction of electrode materials are listed below. Firstly, the selected redox species should have excellent electrochemical reactivity and fast reaction kinetics [39-42]. Secondly, the designed electrode materials should possess long-term stability (i.e., strong anti-degradation ability and no irreversible side reactions during redox reactions) and outstanding ion transport performance in specific electrolyte [42-44]. Thirdly, electrode materials should have the ability to store charge for long periods of time via suppressed cross diffusion and redox shuttling [45-48]. Finally, simply preparation processes and mild preparation methods are also required for practical production process. Fig. 1 is here According to the above viewpoints, rGO hydrogel film with interconnected 3D network structure was selected as electrode skeleton. This stable structure can not only improve the ion transport performance, but also anchor abundant redox materials, which can prevent that the active materials leak from the matrix during the continuous redox processes [20,49]. Due to excellent electrochemical activity, multi-electron redox process and high redox stability [32,50,51], the commercially available PTCDA molecule was selected as redox material together with the fluffy rGO hydrogel 10
skeleton to prepare electrodes. A PTCDA molecule with planar π-conjugated structure consists of a perylene ring with two conjugated anhydride groups attached on opposite sides. This special structure is conducive to the formation of the π-π stacking between PTCDA molecule and rGO sheet. Thus, the PTCDA molecules are adsorbed on the GO sheets by π-π stacking to prepare slurry via simple the assistance of ultrasound. The above mixed slurry is uniformly scraped into PTCDA/GO films and then reduced to PTCDA/rGO films under mild conditions (Fig. 1). 3.2. Material characterizations SEM is used to analyze the morphology of all samples, the PTCDA powder is composed of a large number of one-dimensional rods with a diameter ranging from hundreds of nanometers to 5 µm and a length ranging from hundreds of nanometers to 20 µm (Fig. S1a, b). After GO films and PTCDA/GO films are reduced and freeze-dried, the cross-section SEM images demonstrate that the rGO film has a fluffy and porous structure fabricated by the mild aqueous reduction method (Fig. 2a). Because ascorbic acid is converted to oxalic acid and gluonic acid in the reduction process, the agglomeration of rGO sheets is effectively prevented by hydrogen bonding between these acids and residual oxygen-containing functional groups on rGO sheets [52,53]. Due to the reduced agglomeration and the residual hydrophilic oxygenated groups on rGO sheets, the rGO sheets can encapsulate plenty of water in the process of reduction [54], thus the interconnected 3D network structure was constructed in rGO film. The PTCDA/rGO film completely preserves the fluffy and porous structure, and the fluffy structure is orderly distributed in the film (Fig. 2b-d). 11
Besides, no obvious block-like PTCDA appears in the SEM image of the PTCDA/rGO film, indicating that the PTCDA molecules were uniformly dispersed on the rGO sheets. Furthermore, by folding the films multiple times, the as-fabricated PTCDA/rGO film shows excellent flexibility, indicating its potential application in flexible electronic devices (Fig. S2). Fig. 2 is here XRD was utilized to analyze the crystal structure of PTCDA powder and the interlayer spacing of GO film, rGO film and PTCDA/rGO film (Fig. 3a). The GO film shows a peak at ~10.1° corresponding to the characteristic reflection of (001) and a wide interlayer spacing of 8.75 Å. After reduction by ascorbic acid, the characteristic peak of GO completely disappeared in XRD pattern of rGO film owing to the removal of lots of oxygen-containing functional groups, and (002) face of rGO appear in ~24.8°, which prove that GO sheets were successfully reduced under the mild conditions. Besides, the characteristic (002) reflection of rGO slightly shift from ~24.8° to ~25.1° after the addition of the PTCDA molecules, and the inter-layer spacing of rGO sheets in freeze-dried rGO film and PTCDA/rGO film are calculated to be 3.59 Å and 3.55 Å, respectively. The small to negligible difference in inter-layer spacing indicates that the agglomeration between rGO sheets is not aggravated by the addition of PTCDA molecules. The diffraction peaks of the PTCDA/rGO film at 9°,10.3° and 12.7° are identical with the peaks of (011), (002) and (012) faces in the XRD pattern of the PTCDA powder, and the diffraction peaks of (110) and (102) faces at 25.4° and 27.7° of the PTCDA powder do not show in XRD pattern of 12
PTCDA/rGO film due to being covered by the characteristic (002) peak of rGO [55]. This result certificates that the PTCDA molecule has high structural stability, and the reduction process of GO sheets do not affect the chemical properties of PTCDA molecule. XPS was employed to analyze chemical composition of all samples (Fig. 3b). The carbon-to-oxygen ratio (C/O) of PTCDA power, GO film, rGO film, PTCDA/rGO film are 5.18, 1.98, 4.46 and 4.91, respectively. The rGO film and PTCDA/rGO film exhibit obvious increase in C1s peak while decreased O1s peak compared with GO film. This result also indicates that the most of oxygen-containing functional groups were successfully removed from GO sheets in the reduction process. C/O of PTCDA/rGO film is slightly higher than rGO film due to the addition of PTCDA molecules. The C1s peaks were deconvoluted into C-C/C=C (284.114 eV), C-O (286.12 eV), C=O (287.21 eV) and O-C=O (288.31 eV) (Fig. S3a-d) [38]. The peak intensity of each oxygen-containing bonds is significantly reduced after the reduction process, which further indicates that the oxygen-containing functional groups are removed from GO sheets. The disordered carbon peak (D-peak) at 1353 cm-1 and the graphitized carbon peak (G-peak) at 1581 cm-1 of all films (Fig. 3c) are detected by Raman spectroscopy [56]. The peak intensity ratio (ID/IG) of GO film, rGO film and PTCDA/rGO film are calculated via peak intensity of D-peak divided by peak intensity of G-peak, which are 0.86, 1.07 and 1.00, respectively. The disorder degree of the sheets is increased owing to the oxygen-containing functional groups detached from GO sheets with the 13
process of reduction reaction. Therefore, the ID/IG of rGO film and PTCDA/rGO film are higher than GO film. The almost identical ID/IG of rGO film and PTCDA/rGO film demonstrate that the disordered degree of rGO sheets is almost unaffected by the PTCDA molecule. Besides, according to previous reports [57], the peak appears near D-peak of PTCDA/rGO film, which is considered to be a superposition of Raman information from PTCDA molecules. Fig. 3 is here Besides, because the PTCDA molecules are absorbed on rGO sheets to prepare electrodes by π-π stacking, without destroying the sp2 hybrid orbitals of rGO sheets. the PTCDA/rGO film exhibits a satisfactory value of sheet resistance (81.3 Ω sq-1) compared with rGO film (62.1 Ω sq-1). The water contact angles of rGO film and PTCDA/rGO film are 86.3° and 77.9° (Fig. S4), respectively. Therefore, the PTCDA/rGO film exhibit better wettability than rGO film owing to the addition of PTCDA, which lead to the entire electrode penetrated by the aqueous electrolyte more easily. 3.3. Electrochemical performance of PTCDA/rGO films in the symmetric supercapacitor CV, GCD and EIS are used to verify the excellent supercapacitor performance of PTCDA/rGO film in Li2SO4 electrolyte. Compared with acidic and alkaline electrolytes, the lower H+ and OH- contents of Li2SO4 neutral electrolyte increase the overpotentials of hydrogen and oxygen evolution reactions, thus the symmetric supercapacitor exhibits a wide voltage window of 1.8 V (CV curves at different 14
electrochemical windows in Fig. S6a) [58,59]. Fig. 4a records the CV curves of PTCDA powder, rGO film and PTCDA/rGO film at scan rate of 50 mV s-1. The pure PTCDA powder hardly exhibits supercapacitor performance, and the curve of rGO film exhibits a nearly rectangular shape, revealing the pronounced electric double layer (EDL) capacitive characteristic and fast current response in neutral electrolyte, but the ion storage capacity of the rGO film is poor limited by energy storage mechanism. After adding PTCDA molecules into rGO skeleton, the area of the CV loop is significantly expanded under the premise of maintaining fast current response during voltage reversal (the CV shape without significant deformation at scan rate of 1 V s-1 in Fig. 4b), and two pairs of redox peaks clearly appear in this curve, which are caused by typical chemical bond reaction between the two conjugated anhydride groups on PTCDA and lithium ions (Scheme 1) [36]. The addition of PTCDA molecules increases the number of Faradaic active sites in the film electrode, resulting in the occurrence of redox reactions in process of charging and discharge [60-62]. Therefore, the area of CV curve is expanded and the energy storage performance is boosted. To further explore the root cause of performance improvement, the binding energy (Eb) values of PTCDA-Li, PTCDA-Li2 and graphene-Li obtained by density functional theory (DFT) calculations are -46.804 kcal/mol, -46.521 kcal/mol and -27.287 kcal/mol, respectively (Fig. S5). This result proves that the chemical adsorption performance of PTCDA molecules on lithium ions is much better than that of graphene. Besides, it was proved by almost the same Eb values of PTCDA-Li and PTCDA-Li2 that the two conjugated anhydride bonds on the same PTCDA can 15
simultaneously anchor lithium ions. Scheme 1 is here In GCD curves at current density of 2 A g-1 (Fig. 4c, the GCD curves of PTCDA/rGO film at different current density are recorded in Fig. S6c, d), the discharge time of PTCDA/rGO film is significantly longer than rGO film, and the CM of PTCDA/rGO film and rGO film are 242.9 F g-1 and 107.2 F g-1 at 2 A g-1, respectively. When the current density is increased to 100 A g-1 (Fig. 4d), the CM of PTCDA/rGO film and rGO film still maintains 175.2 F g-1 and 54.8 F g-1, respectively, and the lower voltage drops of both PTCDA/rGO and rGO films confirm that the excellent ion and electron transport performance of fluffy rGO skeleton is retained in the PTCDA/rGO film. Furthermore, because of the improvement of hydrophilicity, the fluffy 3D structure of the rGO/PTCDA film can contact with the electrolyte more effectively, which leads to the enhancement of ion transport, thus the slightly increasing rate performance of PTCDA/rGO film [63,64]. Besides, the specific volume capacitance (CV) of PTCDA/rGO film (48.6 F cm-3) is also higher than the CV of rGO film (22.9 F cm-3) at current density of 2 A g-1. Owing to a wide potential window and high CM of the PTCDA/rGO film in aqueous Li2SO4 electrolyte, the symmetric supercapacitor based on the PTCDA/rGO films can deliver an excellent EM, SC of
27.3 Wh kg-1 at a PM, SC of 899.2 W kg-1, and can still maintain an EM, SC of 19.7
Wh kg-1 at a high PM, SC of 45 kW kg-1 (Fig. 4e). These values are higher than most carbon material electrodes containing organic materials reported in supercapacitors (Table 1). 16
Table 1. Comparison of energy density (EM) and power density (PM) of electrode materials in supercapacitor Electrode materials
EM (Wh kg-1)
PM (kW kg-1)
References
Th–GA//Th–GA
17.7
12.8
[45]
TN-NPCs//AQ-NPCs
23.5
0.7
[65]
HPCF4//HPCF4
9.1
3.5
[66]
DT-RGN//DT–RGN
8.4
27.5
[67]
NPCs/AQ//NHCSs/PQ-2
8.34
0.6
[68]
AZ–SGHs//AZ–SGHs
18.2
0.7
[69]
PTCDA/rGO//PTCDA/rGO
27.3 19.7
0.9 45.0
This work
To further analyze the reasons for the improvement of the electrochemical performance, EIS of the symmetric supercapacitor based on rGO film and PTCDA/rGO film are obtained in the frequency range from 10-2 Hz to 105 Hz at the open circuit voltage, and the Nyquist plot are represented in Fig. 4f. Based on an equivalent circuit (inset of Fig. 4f), the specific values of the different circuit elements are fitted in Table S1. The internal resistance (Rs) values of the symmetric supercapacitor based on rGO film and PTCDA/rGO film are 1.133 Ω and 1.069 Ω, respectively. Because the same aqueous electrolyte (2 M Li2SO4) is used in these symmetric supercapacitors, the devices exhibit almost the same Rs values. The interfacial charge transfer resistance (Rct) is slightly smaller for PTCDA/rGO film (0.355 Ω) compared than rGO film (0.538 Ω). The small Rct of PTCDA/rGO film indicates the interconnected 3D network structure and excellent wettability are conducive to the rapid ion transfer in the interface of electrolyte and electrode. These 17
results sufficiently confirm that the PTCDA/rGO film has the excellent ions diffusion and transmission behavior without being affected by adding PTCDA molecules, thereby leading to the emergence of rapid reaction kinetics and outstanding electrochemical activity. Besides, the stable support structure of rGO skeleton and the strong π-π interaction between PTCDA molecules and rGO sheets can effectively prevent the structural collapse of the electrode materials during continuous redox process[70-72], thus the PTCDA/rGO film exhibits excellent cycling stability in Li2SO4 electrolyte and maintains ~93% of the initial specific capacitance value after 5000 cycling at 1 V s-1 (Fig. 4g). Fig. 4 is here In order to respond actual demand, FSSC based on the PTCDA/rGO films was assembled. The CV curves of the device at 20 mV s-1 to 200 mV s-1 (Fig. 5a) exhibit satisfactory current response in gel electrolyte. But compared with aqueous electrolyte, because the transmission of ions is restricted by gel electrolytes, the peak intensity of redox peaks decreases and the peak position shift in the CV curves of FSSC [73]. In PVA-Li2SO4 gel electrolyte, the PTCDA/rGO film also shows a satisfactory CM of 180.4 F g-1 evaluated by the GCD curve at 2 A g-1, and the CM still retains 75.7% of its initial value at current density of 20 A g-1 (Fig. 5b shows the GCD curves at different current densities). The FSSC based on the PTCDA/rGO electrodes can deliver a high EM, SC of 20.3 Wh kg-1 at a PM, SC of 900 W kg-1, and when the PM, SC increases to 9 kW kg-1, the device still maintains an EM, SC of 15.4 Wh kg-1. Meanwhile, for a more comprehensive evaluation of device performance, we calculate the EV, device and PV, 18
device
of the device. The volume of device includes the volume of the two electrodes,
two current collectors and gel electrolyte. The FSSC based on the PTCDA/rGO electrodes shows EV, device of 2.88 Wh L-1 at PV, device of 127.68 W L-1 and EV, device of 2.18 Wh L-1 at PV, device of 1277.56 W L-1 in PVA gel electrolyte (Fig. 5c), which is better than the recently reported FSSC device base on rGO electrodes (Table 2). Table 2. Comparison of EV, device and PV, device of FSSCs Electrode materials
Electrolyte
CV, device (F cm-3)
EV, device (Wh L-1)
PV, device (W L-1)
References
rGO/PW12// rGO/PW12
PVA/ H2SO4
2.95
1.05
11.5
[74]
cellulose/rGO/AgNPs//cellulos e/rGO/AgNPs
PVA/KCl
8.56
1.19
31.25
[75]
rGO/MMNNBs// rGO/MMNNBs
PVA/H3PO4
15.4
1.05
35
[76]
CC@rGO//PPy
PVA/H3PO4
12.3
1.10
10
[77]
Co2.18Ni0.82Si2O5(OH)4//rGO
PVA/KOH
1.15
0.496
38.8
[78]
N-rGO/Mn//AC
PVA/LiCl
4.49
2.02
180
[79]
Ni/GF/H-CoMoO4//Ni/GF/H-F e 2O 3
PVA/KOH
3.6
1.13
150
[80]
ErGO/NCA// ErGO/NCA
PVA/Na2SO4
1.72
0.15
4
[81]
PPDA-HGF// PPDA-HGF
PVA/H2SO4
19.6
2.72
146.5
[82]
PTCDA/rGO//PTCDA/rGO
PVA/Li2SO4
6.40 4.85
2.88 2.18
127.68 1277.56
This work
The Nyquist plot (Fig. 5d) of FSSC exhibits small diameter of the semicircle at the high frequency followed by a near-vertical line at low frequency, which confirms that the PTCDA/rGO film still maintains good ion transport performance and ideal capacitive behavior in gel electrolyte. To demonstrate excellent flexibility of the FSSC, Fig. 5e and 5f show the capacitance retention under various bending angles and after bending 500 times, respectively. Compared with non-bending devices, the 19
maximum increase and decrease in specific capacitance of FSSC at different bending angles are 4.8% and 2.7%, respectively, and the capacitance value of FSSC still retains 98.4% of the initial specific capacitance value after bending 500 times. These excellent electrochemical and bending performances fully demonstrate the commercial application prospects of FSSC based on PTCDA/rGO film electrodes in the field of electric vehicles and flexible electronics. Fig. 5 is here 3.4. Electrochemical performance of PTCDA/rGO films in zinc-ion hybrid supercapacitor To further broaden the application range of this film and explore the synergistic design concept of electrode materials in hybrid supercapacitors, a zinc-ion hybrid supercapacitor based on PTCDA/rGO film as cathode and zinc foil as anode was assembled in 2 M ZnSO4 electrolyte. The reversible deposition/stripping of zinc ion and capacitive energy storage process occur on the anode and cathode of the zinc-ion hybrid supercapacitor, respectively (the schematic illustration of PTCDA/rGO//Zn zinc-ion hybrid supercapacitor in Fig. 6a). Compared with the symmetric supercapacitor, The CV curve of the zinc-ion hybrid supercapacitor exhibits a completely different shape by the different energy storage mechanism of anode material. Because large numbers of redox active sites brought by PTCDA molecules appear in the cathode of the zinc-ion hybrid supercapacitor, redox process with good reversibility occur in the near surface region of the electrode. Accordingly, the CV curves at 50 mV s-1 (Fig. 6b) show that the PTCDA/rGO film has better charge 20
storage capacity and faster current response compared with the rGO film under a voltage window of 0.2 ~ 1.8 V. At high scan rate of 100 mV s-1 (the CV curves at scan rate from 10 to 100 mV s-1 in Fig. 6c), the CV curve is no obvious deformation, which indicates that the device based on PTCDA/rGO film cathode has fast Faradaic reaction process and excellent ion transport performance. Besides, a pair of redox peaks appear in the CV loop after adding the PTCDA molecules to rGO skeleton, which results from enolization process of PTCDA molecules. The carbonyl groups (C=O) on the conjugated anhydride groups of PTCDA are converted into enolate groups (C-O-) and combined with zinc ions to form enolates [33]. Fig. 6 is here The GCD curves of PTCDA/rGO film at different current densities are represented in Fig. S7. Because no conspicuous voltage platform appears in these GCD curves, the zinc-ion hybrid supercapacitor based on PTCDA/rGO cathode shows excellent capacitance performance. Calculated by the GCD curves (GCD curves at 1 A g-1 in Fig. 6d), the discharge capacity of PTCDA/rGO film and the rGO film are 162.8 mAh g-1 and 104.1 mAh g-1, respectively. When the current density is increased to 20 A g-1, the PTCDA/rGO film still maintains high discharge capacity of 104.6 mAh g-1 (the discharge capacity at different current densities are recorded in Fig. 6f). The improvement of capacity is attributed to the occurrence of the redox reactions and the increase of the active sites for ion adsorption/desorption on the cathode by addition of PTCDA molecules. Furthermore, due to the synergistic effect between excellent ion/electron-transfer performance of fluffy rGO skeleton and outstanding ion 21
adsorption/desorption performance of PTCDA molecules, this device based on PTCDA/rGO film as cathode delivers an EM, ZHS up to 120.5 Wh kg-1 at a PM, ZHS of 740.1 W kg-1 at current density of 1 A g-1, and an EM, ZHS of 77.06 Wh kg-1 at a PM, ZHS of 14.7 kW kg-1 at current density of 20 A g-1. Compared with recently published papers about aqueous zinc-ion hybrid energy storage devices (Table 3), the zinc-ion hybrid supercapacitor based on PTCDA/rGO film as cathode exhibits excellent energy storage performance and power performance, shown in the Ragone plot (Fig. 6g). Furthermore, the zinc-ion hybrid supercapacitor based on PTCDA/rGO cathode shows excellent capacity retention rate of ~91% after 5000 charge/discharge cycles at current density of 20 A g-1 and Coulombic efficiency of ~100% during the charge/discharge cycle (Fig. 6h). To further demonstrate the potential of application, the packaged zinc-ion hybrid supercapacitors based on PTCDA/rGO films (single electrode area: 1×1.5 cm2) as cathode and zinc foils as anode are assembled, and the LED board with 25 green LED (drive voltage: 3.2 V) lighted up by two devices connected in series (Fig. S8). These results strongly suggest that the concept of synergistic design for symmetric supercapacitor electrode is also suitable for designing cathode materials of the zinc-ion hybrid supercapacitor to improve their performance. Table 3. Comparison of EM and PM of aqueous zinc-ion hybrid energy storage devices Hybrid energy storage devices
EM (Wh kg-1)
PM (kW kg-1)
References
rGO-VO2//Zn
65.0
7.8
[83]
AC//Zn
30
14.9
[84]
22
Cu-HCF//Zn
33.8
0.47
[85]
MXene-rGO//Zn
34.9
0.28
[86]
HPC//Zn
63.7
3.16
[87]
MnO2–CNTs//MXene
29.7
2.48
[88]
PPy//Zn-graphite
64.0
11.7
[89]
ACC//Zn
94
0.07
[90]
RTEG-MnO2//Zn
50.12
15.26
[91]
Zn3[Fe(CN)6]2//Zn
46
1.7
[92]
PTCDA/rGO//Zn zinc-ion hybrid supercapacitor
120.5 77.1
0.7 14.7
This work
4. Conclusions In summary, we report a green and simple strategy to construct PTCDA/rGO film electrode with outstanding performance in the symmetric supercapacitor and the zinc-ion hybrid supercapacitor. Due to fluffy rGO skeleton with excellent ion diffusion performance and PTCDA molecules with rapid reaction kinetics, the symmetric supercapacitor based on PTCDA/rGO electrodes can deliver a satisfactory EM, SC of 19.7 Wh kg-1 at an ultra-high PM, SC of 45 kW kg-1. the PTCDA/rGO film as cathode in the zinc-ion hybrid supercapacitor exhibits the discharge capacity of 162.8 mAh g-1 at 1 A g-1 and can deliver an extremely high EM, ZHS of 120.5 Wh kg-1 at a PM, ZHS
of 740.1 W kg-1 at current density of 1 A g-1. The PTCDA/rGO electrodes
demonstrate outstanding performance in both the symmetric supercapacitor and the zinc-ion hybrid supercapacitor, which broadens the scope of application of the electrode, and may provide inspiration for traditional supercapacitor materials applied in the field of hybrid supercapacitors via analyzing structural characteristics and 23
design concept of this electrode materials. Specifically, the organic molecules with different kinds and quantities of redox active groups are introduced into the microstructure with fast ion transport performance, so as to build a synergistic relationship between structure and composition, and improve the electrochemical performance of electrode materials.
Acknowledgements We are grateful to the National Key R@D Program of China (No. 2017YFB0702802), National Natural Science Foundation of China (Grant Nos. 51773027 and 21572032) and the Fundamental Research Funds for the Central Universities of China (No. ZYGX2019Z007) for financial support.
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Figure captions Fig. 1. Schematic illustration of preparing PTCDA/rGO film. Fig. 2. The cross-section SEM of (a) rGO film and (b-d) PTCDA/rGO film. Fig. 3. (a) XRD patterns, (b) XPS spectra and (c) Raman spectra. Scheme 1. The proposed electrochemical reaction of PTCDA in Li2SO4 electrolyte during the discharge/charge process. Fig. 4. Comparison of electrochemical performance of the symmetric supercapacitors based on rGO films and PTCDA/rGO films. (a) CV curves of PTCDA, rGO film and PTCDA/rGO film at 50 mV s-1. (b) CV curves of PTCDA/rGO film at 100 mV s-1 to 1000 mV s-1. (c) GCD curves at 2 A g-1. (d) GCD curves at 100 A g-1. (e) Ragone plot. (f) Nyquist plot (inert shows magnified high frequency region and the equivalent circuit). (g) Cycling stability of PTCDA/rGO film. Fig. 5. Electrochemical and bending performance of FSSC based on PTCDA/rGO films. (a) CV curves at various scan rates. (b) GCD curves from 2 A g-1 to 20 A g-1. (c) Ragone plot for FSSCs reported in the literature and our work. (d) Nyquist plot (inset is the magnified plot). Capacitance retention of (e) different bending angles (inert is Schematic illustration of FSSC) and (f) bending times. Fig.
6.
Comparison
of
electrochemical
performance
of
zinc-ion
hybrid
supercapacitors based on rGO film and PTCDA/rGO film. (a) Schematic illustration of zinc-ion hybrid supercapacitor based on PTCDA/rGO film. (b) CV curves at 50 mV s-1. (c) CV curves of PTCDA/rGO film at 10 mV s-1 to 100 mV s-1. (d) GCD curves at 1 A g-1. (e) Nyquist plot. (f) The discharge capacity at different current 28
densities. (g) Ragone plot. (h) Capacitance retention and Coulombic efficiency.
29
Figures
Fig. 1
Fig. 2 30
(b)
Intensity (a.u.)
Intensity (a.u.)
C1s peak
PTCDA/rGO rGO GO G-peak
D-peak
O1s peak C/O=4.91
C/O=4.46
C/O=1.98 (110) (102)
(011) (002)
(012)
10
(c)
PTCDA/rGO rGO GO PTCDA
Intensity (a.u.)
PTCDA/rGO rGO GO PTCDA
(012) (011) (002)
(a)
20
C/O=5.18
30
40
2θ θ (o)
50
60
70
200
400
Binding energy (eV)
Fig. 3
Scheme 1
31
600
1000
1200
1400
Wave number (cm-1)
1600
200
Current density (A g-1)
Current density (A g-1)
150 100
4
0
2.0
100 mV s-1 200 mV s-1 500 mV s-1 700 mV s-1 1000 mV s-1
(b)
PTDCA/rGO rGO PTCDA
8
(c)
PTCDA/rGO rGO
1.5
Potential (V)
(a)
50
1.0
0
-50
-4
0.5
-100
50 mV s-1
-8
-150
2 A g-1 0.0
0.4
0.8
1.2
1.6
0.0
2.0
0.4
0.8
1.6
2.0
0
(e)
PTCDA/rGO rGO
IR
Energy density (Wh kg-1)
1.2
0.8
0.4
150
200
250
Time (s) (f)
PTCDA/rGO rGO
This work
25
8
40
Ref. 65 20
6
Ref. 69
-Z'' (Ω Ω)
(d) 1.6
100
50
30
IR
50
Voltage (V)
Voltage (V) 2.0
Voltage (V)
1.2
30
Ref. 45
15
-Z'' (Ω Ω)
0.0
4
20 10
Ref. 66
Ref. 68
2
Ref. 67 10
5
0 1
100 A g-1 0.0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
103
Time (s)
Capacitance retention (%)
2
3
4
Z' (Ω Ω)
5
6
7
8
0
0 104
0
105
-1
Power density (W kg )
10
20
30
Z' (Ω Ω)
40
50
(g) 100
80
60 0
1000
2000
3000
4000
5000
Cycle number
Fig. 4
2 A g-1 5 A g-1 7 A g-1 10 A g-1 20 A g-1
(b) 1.6
Potential (V)
20
Current density (A g-1)
2.0
20 mV s-1 50 mV s-1 70 mV s-1 100 mV s-1 200 mV s-1
10
1.2
0
0.8
-10
0.4
3.5
(c) 3.0
Energy density (Wh L-1)
30
(a)
Th is w o rk
Ref. 82 2.5
Ref. 79
2.0
1.5
Ref. 77
1.0
Ref. 74
0.5
Ref. 75 Ref. 76
Ref. 80
Ref. 78
-20
Ref. 81
0.0 0.0
0.4
0.8
1.2
1.6
2.0
0.0
0
50
100
150
200
110
102
Power density (W L-1)
103
110
(e)
(d) 100
101
Time (s)
Voltage (V)
(f) Capacitance retention (%)
Capacitance retention (%)
10
100
8
-Z'' (Ω )
75
-Z'' (Ω )
6
50
4
2
25
100
90
80
70
90
80
Bending 70
0 0
5
10
Z' (Ω)
0
60 0
25
50
Z' (Ω)
75
100
60
0
60
120
Bending angle (°)
Fig. 5
32
180
0
100
200
300
Bending number
400
500
45
(b)
(c)
PTCDA/rGO rGO
30
Current density (A g-1)
Current density (A g-1)
20
0
10 mV s-1 20 mV s-1 50 mV s-1 70 mV s-1 100 mV s-1
15
0
-15
-20
-30
50 mV s-1 -45
0.0
0.4
0.8
1.2
1.6
2.0
0.0
Voltage (V) 1.8
100
(e)
PTCDA/rGO rGO
1.6
0.8
1.2
1.6
Voltage (V)
(f)
PTCDA/rGO rGO
1.4
2.0
PTCDA/rGO rGO
160
Discharge capacity (mAh g -1 )
(d)
0.4
Voltage (V)
120
-Z'' (Ω Ω)
1.2
50
1.0 0.8 0.6 0.4
80
40
-1
1Ag
0
0.2 0
50
100
150
Discharge capacity (mAh g-1)
200
30
60
Z' (Ω)
90
120
0
5
10
15
20
Current density (A g-1)
25
110
(g)
105
Ref. 90 80
Ref. 83 Ref. 89
Ref. 87
60
Ref. 92
Ref. 91
40
Ref. 86
Ref. 85
C ap acity reten tion (% )
100
This work
100
90
100
80 95 70 90 60 85
50
Ref. 84
Ref. 88
40
20 102
103
Power density (W kg-1)
104
80 0
1000
Fig. 6
33
2000
Cycle number 3000
4000
5000
C oulom b ic efficien cy (% )
(h)
120
Energy density (Wh kg -1 )
0 0
250
Conjugated molecule functionalized graphene films for energy storage devices with high energy density Liheng Wanga, Xingke Yea, Yucan Zhua, Hedong Jianga, Jianxing Xiaa, Ziyu Yuea, Zhongquan Wana, Chunyang Jiaa,∗, Xiaojun Yaob a
State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Electronic
Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China. b
State Key Laboratory of Applied Organic Chemistry, School of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China
Highlights 1. PTCDA molecules functionalized fluffy hydrogel structure is built in rGO film. 2. The synergistic effect between structure and composition is explored for performance. 3. PTCDA/rGO film exhibits good capacitance and rate performance in neutral electrolyte. 4. PTCDA/rGO//Zn hybrid supercapacitor shows the excellent capacity and energy density.
∗ Corresponding author. Tel.: +86 28 83201991; Fax: +86 28 83202569. Email: [email protected] (C. Y. Jia)
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: