Graphene Oxide for Superior

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Electronspun Mat of Polyvinyl Alcohol/Graphene Oxide for Superior Electrolyte Performance Qin Pan, Ningjun Tong, Nanfei He, Yixin Liu, Eunkyoung Shim, Behnam Pourdeyhimi, and Wei Gao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14498 • Publication Date (Web): 09 Feb 2018 Downloaded from http://pubs.acs.org on February 11, 2018

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ACS Applied Materials & Interfaces

Electronspun Mat of Polyvinyl Alcohol/Graphene Oxide for Superior Electrolyte Performance

Qin Pan†‡, Ningjun Tong‡, Nanfei He‡, Yixin Liu‡, Eunkyoung Shim†‡, Behnam Pourdeyhimi†‡, and Wei Gao†‡*

† The Nonwovens Institute, North Carolina State University, Raleigh, North Carolina 27606, United States ‡ Department of Textile Engineering, Chemistry & Science, College of Textiles, North Carolina State University, Raleigh, North Carolina 27695, United States

KEYWORDS: electrospinning, solid-state electrolyte, monolithic supercapacitors, graphene oxide, polyvinyl alcohol

*

E-mail: (W. G.) [email protected]

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ABSTRACT

Here we describe an electrospun mat of polyvinyl alcohol (PVA) and graphene oxide (GO) as a novel solid-state electrolyte matrix, which offers better performance retention upon drying after infiltrated with aqueous electrolyte. The PVA-GO mat overcomes the major issue of conventional PVA-based electrolytes, which is the ionic conductivity decay upon drying. After exposure to 45 ± 5% relative humidity at 25 ℃ for 1 month, its conductivity decay is limited to 38.4%, whereas that of pure PVA mat is as high as 84.0%. This mainly attributes to the hygroscopic nature of GO and the unique nanofiber structure within the mat. Monolithic supercapacitors have been derived directly on the mat via a well-developed laser-scribing process. The as-prepared supercapacitor offers an areal capacitance of 9.9 mF cm-2 at 40 mV s-1 even after 1 -month of aging in ambient conditions, with high device-based volumetric energy density of 0.13 mWh cm-3 and power density of 2.48 W cm-3 respectively, demonstrating great promises as a more stable power supply for wearable electronics.


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Introduction Supercapacitors have been widely used in hybrid electric vehicles, back-up power for consumer electronics, impulsive energy harvesting etc., thanks to its high specific capacity, high power density and long cycle life.1-2 Recently, research on all-solid-state supercapacitors has dramatically expanded, mainly due to their potential to solve the leakage issue of liquid electrolyte, as well as their flexibility favoring the incorporation into various shaped devices.3-4 Polymer gel electrolyte are widely used in these supercapacitors, among which polyvinyl alcohol (PVA) is a popular water-based candidate, owing to its environmental friendliness, nontoxicity, and relatively low cost.5-6 A variety of PVA-based electrolytes have been developed, including acidic PVA-H2SO4, PVA-H3PO4, alkaline PVA-KOH, and neutral PVA-LiCl.7-9 However, an existing challenge for all gel electrolytes is their diminishing ionic conductivity upon drying, especially when used in a wide range of temperatures.10-11

Sung et al. first discussed the effects of solvent evaporation on the conductivity decay of polyvinyl chloride (PVC)-based electrolyte in Li-ion battery.12 One possible solution they demonstrated is to co-polymerize PVC with polyvinyl acetate (PVAC), which requires complex processes and harsh reaction conditions. Mohamad et al. observed similar conductivity evolution in PVA-KOH gel electrolyte over long-term storage.13 The PVA-KOH (40 wt% KOH) showed low conductivity of 8.5 × 10 S cm-1 after stored at 24% RH, 25 ℃ for 30 days, which even decreased to 1.3 × 10 S cm-1 after 100 days. They pointed out water content is crucial to the ionic conductivity, due to its significant influence on ion dissociation that produces charge carriers for ionic transport. However, no effective 3 ACS Paragon Plus Environment


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strategy was proposed. Lu et al. successfully extended the storage time of poly (vinylidene fluoride-hexafluoro propylene (P(VDF-HFP))-based electrolyte by replacing the organic solvent with nonvolatile ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4).10 Yet ionic liquids are synthetically challenging and less cost-effective, some of which are even moisture-sensitive.10, 14 In addition, covering a layer of Kapton tape or PDMS film over the gel electrolyte is also a feasible way to stabilize it, which however will increase the thickness and weight of the whole device, resulting in lower flexibility and energy density.15-16

Graphene oxide (GO), a proton conductor with plenty of oxygenated groups attached to the carbon plane functioning as proton-hopping sites, can act as an effective dopant in electrolyte to facilitate ion transport.17-18 Huang et al. and Yang et al. reported low content of GO can increase the ionic conductivity by c.a. 70.9% and c.a. 200% in PVA-KOH-GO hydrogel and P(VDF-HFP)-EMIMBF4-GO gel electrolyte, respectively.19-20 Other two-dimensional nanoflakes such as boron nitride, titanium carbide have also been reported to improve the ionic conductivity of polymer gel electrolytes.21-23 However, besides ionic conductivity enhancement, the role of GO on moisture management in polymer electrolyte is rarely investigated. The hygroscopic nature of GO allows it to spontaneously absorb water via hydrogen bonding formed between the water molecules and the oxygenated groups. The escape of such physisorbed water is energetically less flavored as compared to that of free water.24-25 Therefore, GO is expected to be capable of maintaining high moisture contents in gel electrolytes and improving the stability over long-term storage. In addition, GO can be easily converted to graphene by laser scribing process, which is controllable in porosity and 4 ACS Paragon Plus Environment

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chemistry, due to the localized heating capability of the laser beam. The laser reduced graphene (LGO) with porous structure and large surface area, is widely used in micro-supercapacitors, which show high energy and power density, good flexibility and easy integration into circuits.5, 15, 26-27

Electrospun mat, due to the unique nanofiber and porous structure with high specific surface, has demonstrated wide applications in electrodes and separators in energy storage systems.28-29 So far, many reports have studied the electrospinning of PVA-GO composite, with focuses varying from process optimization, structure-property relation, mechanical reinforcement, to biomedical applications.30-33 But to our best knowledge, the PVA-GO electrospun mat as polymer electrolyte has rarely been investigated. In this study, we prepared PVA-GO electrospun mats and further derived solid supercapacitors on them, via drop casting of GO onto the mat surfaces followed by a well-developed laser-writing process.5,

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The PVA-GO electrospun mat saturated with 1 M H2SO4 works as a novel

polymer electrolyte, which effectively overcomes the disadvantage of conductivity loss upon drying in the conventional PVA-gel electrolytes. This improvement is mainly due to the hygroscopic nature of GO trapping water molecules via hydrogen binding, and also the nanofibers and porous mat structure with large surface area improving the electrolyte and moisture uptake during the long-term storage. After being stored at 25 ℃, 45 ± 5% RH for over 1 month, the ionic conductivity decay of PVA-GO mat is well restricted within 38.4%, whereas that of pure PVA mat is as high as 84.0%. The electrospun mat structure also helps improve the cycling stability significantly, since the porous structure facilitates the ionic diffusion during the charge/discharge process. The mat-based sandwich supercapacitor shows 5 ACS Paragon Plus Environment


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an areal capacitance of 9.9 mF cm-2 with an energy density of 0.13 mWh cm-3 and a power density of 2.48 W cm-3, superior among the similarly laser-scribed GO based EDLCs reported in the literature.

Results and Discussion Fabrication of PVA-GO Electrospun Mat for Monolithic Supercapacitors. PVA-GO and pure PVA aqueous solution was electrospun into mats with thickness of c.a. 65 µm. Figure 1 (a), (b) and (c) shows the SEM images of pure PVA and PVA-GO electrospun mats (25: 1 w/w and 10: 1 w/w, respectively), with insets of their photographic images showing the color of mats darkening with increasing GO loading. The morphology evolution with introducing more GO is clearly indicated, from pure PVA mat showing smooth and straight fibers, to PVA-GO (25: 1, w/w) mat showing non-uniform fibers with wrinkled surface and scattered lumps, eventually to PVA-GO (10: 1, w/w) mat in which large spindle-like lumps formed and fibers deformed even further, since the high-content GO increased the surface tension of spinning-dope droplets, leading to poorer spinnability (see Figure S1 in the supporting information for more details).31 The fiber diameter for pure PVA is 156 ± 36 nm, while that for PVA-GO (25: 1, w/w) is a bit smaller of 121 ± 65 nm but with wider diameter distribution. GO homogeneously dispersed in the PVA matrix at the molecular level and formed PVA-GO composite fibers during electrospinning, which is confirmed in the X-ray diffraction pattern where no characteristic peaks for GO was observed (see Figure S2). Since the hierarchical fibers/lump structure in the PVA-GO (10: 1, w/w) mat lowers the


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mechanical strength of the mat, the PVA-GO (25: 1, w/w) mat was used for further supercapacitor fabrication, owing to its reasonable GO loading, strength, and flexibility.

Figure 1 SEM images of (a) pure PVA, (b) PVA-GO (25:1, w/w), (c) PVA-GO (10:1, w/w) electrospinning mat, with insets of their photographic images; (d) cross-sectional SEM image of monolithic supercapacitor with sandwich geometry on PVA-GO mat; (e) laser reduced GO (LGO) region; (f) photographic images of PVA-GO supercapacitors with concentric circular (CC) and sandwich geometries.

Supercapacitors with different geometries were directly fabricated onto the PVA-GO mats via laser patterning as illustrated in Scheme 1. Since both PVA and GO is soluble/dispersible in water, a thermal treatment at 180 ℃ for 15 mins was used for partial crosslinking and higher crystallinity of PVA, leading to the transition to water-insoluble mat for the subsequent supercapacitor fabrication. GO/water dispersion (10 wt%) was drop casted onto the mat surface, forming a GO layer with thickness of c.a. 15 µm on both sides, serving as the precursor for laser scribing. A programmed CO2 laser patterning process allows us to 7 ACS Paragon Plus Environment


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trigger the thermal decomposition of GO confined at well-defined region, resulting in various geometries including sandwich and planar concentric-circular (CC) ones (Figure 1 (f)). The cross-section of a sandwich supercapacitor was shown in Figure 1 (d). PVA-GO nanofibers closely packed with each other, and the whole mat sandwiched between the laser-scribed GO (LGO) at the top and bottom surfaces, acting as a highly porous separator and the polymer skeleton for solid-state electrolyte. As two electrodes, the LGO layers exhibited fluffy and porous morphology (Figure 1 (e)), indicating dramatically increased surface area which leads to high areal capacitance and enhanced contact with electrolyte. Several drops of 1 M H2SO4 was simply dropped onto the device until saturation, and rapidly absorbed into the mat by capillary force. After dried at 25 ℃, the PVA-GO mat functions as solid-state electrolyte, with a large number of ions and water molecules absorbed on it. The whole device was then stored at 25 ℃, 45 ± 5% relatively humidity, for further electrochemical analysis.

Scheme 1 Schematic illustration of electrospinning of PVA-GO mat and the subsequent fabrication of supercapacitors via laser patterning processes.


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Chemical Structure. Figure 2 (a) and (b) shows the FT-IR and Raman spectra of the PVA-GO and PVA mat. No obvious difference can be seen from the FT-IR spectra, owing to the similar chemical composition of PVA and GO. From the Raman spectra, apart from the PVA peaks, the PVA-GO shows G peak at 1580 cm-1 from a primary in-plane vibration of the graphitic plane, and D peak at 1350 cm-1, from the defects on the graphitic structure, which confirms the presence of GO.34



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Figure 2 (a) FT-IR spectra and (b) Raman spectra of PVA-GO and PVA electrospun mats.

Evolution of Electrolyte Performance of PVA-GO Mat. One of the major problems for PVA-gel electrolyte is its resistance increases over time due to water evaporation, resulting in inferior ion diffusion and lower capacitance. Our PVA-GO electrospun mat can overcome this issue effectively. To investigate the role of GO, we fabricated the same sandwich supercapacitors on the as-prepared PVA-GO mat (PVA-GO-m) and pure PVA mat (PVA-m) under the same condition, and tracked their performance evolution over 1 month when stored in the same environment (25 ℃, 45 ± 5% RH). Figure 3 compared their chronological evolution of cyclic voltammetry (CV) curves and the electrochemical impedance spectra (EIS).


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Figure 3 Evolution of device performance with PVA-GO-m and PVA-m over 1-month storage at 25 ℃, 45%±5% RH. Cyclic voltammetry curves and Nyquist plots from 1 M Hz to 0.01 Hz, with insets showing the zoom-in region at the high frequency domain, for PVA-GO-m (a, b), and PVA-m (c, d). D means the number of storage days, e.g. D0 represents as-made.

Over the 1-month monitoring, the CV curves of PVA-GO-m maintained good box-like shapes, indicating excellent ionic diffusion. Upon saturation, it showed an areal capacitance of 5.5 mF cm-2, higher than reported LGO sandwich supercapacitors with the typical capacitance of 1~3 mF cm-2 due to our optimized laser patterning process.6, 26 The areal capacitance was more related to the surface area of LGO than the electrolyte, which was 11 ACS Paragon Plus Environment


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greatly influenced by our optimized laser scribing process. The capacitance increased slowly, since the ion concentration increased with moisture evaporation, from 5.7 mF cm-2 after 1 day, to 7.5 mF cm-2 after 2 days, 9 mF cm-2 after 6 days and eventually reaches 9.9 mF cm-2 after 36 days. It showed a highest device-based volumetric energy density of 0.13 mWh cm-3 and power density of 2.48 W cm-3 respectively, comparable with the reported LGO EDLCs.5-6 In contrast, the PVA-m showed a more rapid increase in capacitance (Figure 3 (c)). Its capacitance after 3 days is almost twice of that in initial, indicating significantly increased ion concentration and much faster water evaporation. This is also in agreement with the resistance increase which will be discussed later. After 1 month, the shape of CV curves for PVA-m showed a transition trend to spindle-like shape, indicating sluggish ionic diffusion over time. The capacitance of PVA-m increased from 2.5 mF cm-2 (beginning), to 4.3 mF cm-2 (3 days), 5.3 mF cm-2 (10 days), 5.4 mF cm-2 (14 days) and eventually reached 5.8 mF cm-2 (36 days).

In addition, the resistance evolution was investigated via EIS analysis, which can be calculated by the intercept of the curve with real axis. An equivalent circuit (see Figure S7) is used to fit each EIS spectrum, and the quantitative analysis results were shown in Table S1. In Figure 3 (b), upon saturation, PVA-GO-m showed the lowest ESR of 5.5 Ω, since at this stage the water content was considered to be 100%, and the ions transported in the liquid water phase. When stored at 25 ℃ overnight, the free water evaporated and only the water molecules absorbed and bonded in the PVA-GO-m via hydrogen bonding can act as the ion hopping sites. Consequently, the resistance was expected to increase. However, Figure 3 (b) shows only slightly change between 5~6 Ω in resistance within 1 week, indicating water 12 ACS Paragon Plus Environment

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evaporation was effectively suppressed due to presence of GO. On the contrary, the resistance of PVA-m increased by c.a. 400%, from the beginning 4.1 Ω to 20.3 Ω after stored for only 3 days under the same condition, as shown in Figure 3 (d). After 36 days, the PVA-GO-m shows resistance of 9.3 Ω, while that of pure PVA-m was as high as 25.7 Ω. The increase of ESR for PVA-GO-m is well restricted within 69.1%, but the resistance of PVA-m increases by 526.8%. The ionic conductivity was calculated by eq. (4). The conductivity decay over 1-month for PVA-GO-m is significantly alleviated (38.4%) compared with that of pure PVA mat (84.0%). The reasons for the observed impact of GO include: 1) the oxygenated groups in GO can absorb water molecules from air and form hydrogen bonding to intercalate them between interlayers. Unlike free water molecules, the evaporation of bound water is not energy-favored, therefore it stayed stable in PVA-GO-m for relatively long term and can serve as pathways for ion transport; 2) incorporating GO into PVA fibers via electrospinning will lead to lower crystallinity (See X-ray diffraction patterns in Figure S2).35 Lower polymer crystallinity will improve moisture adsorption, since the amorphous region provides more sites for water adsorption than crystal region. Thus, the PVA-GO-m has higher capability to absorb water from air, leading to improved moisture management; 3) GO itself is a proton conductor, with the oxygenated groups as hopping sites. With more hopping sites, the PVA-GO-m showed better ion diffusion during the long-term storage. The similar phenomenon was also observed in the supercapacitors with concentric-circular (CC) configuration (See Figure S3).

Advantages of Electrospun Mat Structure. Apart from the role of GO, the unique structure of electrospun mat is also the key to the excellent energy-storage performance. For 13 ACS Paragon Plus Environment


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comparison, we prepared PVA-GO gel films with the same GO loading and pure PVA gel films via solution cast, and fabricated sandwich supercapacitors on them by the same procedure. Figure 4 (a) compares the EIS spectra of the PVA-GO and PVA cast film (PVA-GO-f, PVA-f respectively), PVA-GO and PVA mat (PVA-GO-m, PVA-m respectively) right upon saturated with 1 M H2SO4. The two electrospun mats exhibited lower resistivity than the two cast films. The ionic conductivity was 2.57 mS cm-1 for PVA-GO-m, 3.51 mS cm-1 for PVA-m, 0.90 mS cm-1 for PVA-GO-f and 1.29 mS cm-1 for PVA-f, respectively. With GO, the conductivity went lower in both mats and films, probably due to the steric effects of GO sheets at the GO loading (3.8 wt%) higher than 1 wt%.19, 36 The reasons that the electrospun mats offer higher initial ionic conductivity include: 1) the good infiltration of H2SO4 throughout the electrode through the intra- and inter-fiber void space of the mat; 2) the nanofibers with larger surface area can adsorb more ions.


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Figure 4 (a) Nyquist plots from 1 M Hz to 0.01 Hz of PVA-GO-m, PVA-m, PVA-GO-f and PVA-f upon saturated with 1 M H2SO4; inset shows the zoom-in region at the high frequency domain; the evolution of EIS spectra (b) and CV curves (c) of PVA-GO-f and PVA-f after 1-day storage; (d) the cycling performance of PVA-GO-m and PVA-GO-f at the current density of 1 mA cm-2.

Figure 4 (b) shows the resistivity of both PVA-f and PVA-GO-f increased significantly only after 1 day, and the slope of the rightmost tilted line also decreased, indicating the inferior ionic diffusion of the both films. The PVA-f shows even higher resistivity of 69.1 kΩ∙cm as compared with that of PVA-GO-f (12.0 kΩ∙cm), probably because GO can also lock some water molecules via hydrogen bonding even in the film form. Therefore, compared with conventional film structures, the porous electrospun mat with nanofibers is also a crucial 15 ACS Paragon Plus Environment


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factor for long-term storage stability, owing to: 1) the nanofiber with large surface area providing more sites for water adsorption; 2) the porous structure enhancing the initial electrolyte uptake as well as the following moisture adsorption; 3) the voids between fibers providing more space than the dense film to bind water molecules. The areal capacitance of PVA-GO-f decreases from 4.4 mF cm-2 to 2.0 mF cm-2, while that for PVA-f decreases from 4.3 mF cm-2 to 3.7 mF cm-2. Unlike the electrospun mats, the box-like shape of CV curves cannot even be retained after 1-day of storage, which changed to a spindle-like shape, indicating rather poor ionic diffusion within the device. The performance of PVA-GO-m in alkaline medium (6 M KOH) has also been investigated. It initially shows an ionic conductivity of c.a. 2.5 mS cm-1 and capacitance of 5.4 mF cm-2 upon saturation, comparable with that in 1 M H2SO4, but only maintains the performance for 1 day, after which the resistance increased dramatically (see Figure S4 in supporting information). One possible reason for the inferior stability is that the strong alkali condition triggers the decomposition of GO, leading to the loss of oxygenated groups that are crucial in water retention. In addition, we investigated the influence of the electrospun mat structure on the cycling performance. Apart from solid-state electrolyte, the porous mat as the separator also benefits ionic diffusion during cycling. Figure 4 (d) compares the capacitance retention of PVA-GO-m and PVA-GO-f at the same current density of 1 mA cm-2 during continuously charged/discharged for 1000 cycles. The PVA-GO-m shows much better cycling stability with capacitance retention of 88.8% than that of PVA-GO-f (63.7%), which is also superior among reported PVA-H2SO4 electrolyte systems, thanks to the improved ionic diffusion through the porous separator.3, 19, 37 16 ACS Paragon Plus Environment

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Conclusion

In summary, thanks to the synergetic effect of hygroscopic GO and the electrospun nanofiber structure, the multifunctional PVA-GO electrospun mat as solid-state electrolyte, separator, and backbone for the monolithic supercapacitors can offer excellent energy storage performance even during long-term storage. It showed a minimal conductivity decay of 38.4%, after stored at 25 ℃, 45 ± 5% RH, for over 1 month, overcoming the instability issues of PVA-based electrolyte. GO absorbs and locks water in the mat via hydrogen bonding, while the nanofibers provide plenty of anchor sites, leading to effective alleviation of water evaporation. As a porous separator, it also facilitates the ion transport, resulting in high ionic conductivity and good cycling performance. As a result, the sandwich supercapacitor on PVA-GO mat can offer a capacitance density of 9.9 mF cm-2 at the scan rate of 40 mV s-1 even after 1-month of storage, with excellent device energy density of 0.13 mWh cm-3 and power density of 2.48 W cm-3 respectively, potential to be used as a stable power source for flexible electronics.

Experimental Section

Preparation of PVA-GO Solution and Electrospinning: GO was prepared by modified Hummer’s method as reported in the literature.38 Graphite was purchased (Micro 850) from Asbury Graphite Mills Inc., Kittanning, PA. PVA ( M = 89,000~98,000 , 99 + % 17 ACS Paragon Plus Environment


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hydrolyzed) was supplied by Aldrich Chemistry. The synthesis procedure for a typical well-dispersed PVA-GO solution (PVA:GO= 25: 1, w/w) was as follows: GO (20 mg) was added to distilled water (50 mL) and then sonicated vigorously for 1 h until homogenous dispersion was formed. PVA (500 mg) was dissolved in the GO aqueous dispersion under stirring at 90 ℃ for 0.5 h to obtain a homogeneous suspension, and then cooled to room temperature. The PVA-GO solution was electrospun using a syringe pump at a constant voltage of 16 kV, with needle inner diameter of 0.7 mm and a feeding rate of 1 mL h-1. An aluminum collector was connected to the ground was placed 10 cm from the tip of the needle to obtain nanofiber mats. The obtained mat was dried overnight at 25 ℃. The pure PVA mats was prepared via the same procedure.

Preparation of monolithic supercapacitors on PVA-GO and PVA mats: For the purpose of supercapacitor fabrication, the PVA-GO and PVA mats were stabilized via heated in the oven at 180 ℃ for 15 mins. GO dispersion (10 wt%) was cast coated onto mat surfaces, as the precursor of the following laser writing process. The as-prepared mats were laser reduced by Epilog Laser Mini (18 inch×12 inch, 40 W). The laser process was controlled by the power and scanning speed. Based on our optimization, the power was set to be 4 W and the scanning speed to be 15% to treat once on each side of sandwich supercapacitor. Several drops of 1 M H2SO4 was directly dropped onto the mats and got rapidly absorbed, until the whole mat was saturated with H2SO4. Gold-coated vinyl films were used as current collectors. For comparison, the same supercapacitors fabricated on the PVA and PVA-GO gel cast film were prepared via the same process, except for replacing the mats with gel films. The typical process for gel film preparation was as following: the well-dispersed PVA-GO aqueous 18 ACS Paragon Plus Environment

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solution (PVA:GO= 25: 1, w/w) was drop casted onto a PTFE non-sticky film (for easy peel-off) and dried in an oven at 60 ℃ for 15 mins. The process was repeated several times until the film thickness reaches ~ 100 µm. Pure PVA gel film was prepared under the same process.

Materials characterization: Scanning electron microscopy (SEM) pictures were collected with FEI XHR-Verios 460L field emission SEM. Samples were sputter coated with palladium for 45 seconds before use. Sample cross-section was prepared in the liquid nitrogen using blade. FT-IR spectra (ATR) were collected with a Thermo Nicolet Nexus 470 instrument. Raman spectra were obtained with a Bayspec 3 in 1 nomadic Raman microscope by exciting a 532 nm laser.

Electrochemical characterization: Cyclic voltammograms (CV) and electrochemical impedance spectroscopy (EIS) measurements were conducted with an Autolab workstation (Metrohm, USA). CV was recorded at a typical scan rate of 40 mV s-1 in the window of 0~1 V. EIS was measured in the frequency range from 0.01 Hz to 1 M Hz with an amplitude of 10 mV. The specific areal capacitance ( ! ) was calculated according to the equation:

"#$%&# 'F)

=

* 01. '1) '+, ⁄+- )/

where i refers to the mean current in CV curves, dV/dt is the scan rate, S is the area of the whole device, which is 1 cm2 for sandwich supercapacitor and 0.45 cm2 for CC supercapacitor.



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The energy density E was calculated also based on the volume of the whole device,

E=

× Δ: ; , 01. '2) 2 × 3600 '

Graphene Oxide for Superior

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