Role of Graphene Oxide Liquid Crystals in Hydrothermal Reduction

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The Role of Graphene Oxide Liquid Crystals in Hydrothermal Reduction and Supercapacitor Performance Bin Wang, Jinzhang Liu, Yi Zhao, Yan Li, Wei Xian, Mojtaba Amjadipour, Jennifer M Macleod, and Nunzio Motta ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05779 • Publication Date (Web): 16 Aug 2016 Downloaded from http://pubs.acs.org on August 16, 2016

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ACS Applied Materials & Interfaces

The Role of Graphene Oxide Liquid Crystals in Hydrothermal Reduction and Supercapacitor Performance Bin Wang,† Jinzhang Liu,*，† Yi Zhao,† Yan Li, † Wei Xian,‡ Mojtaba Amjadipour,§ Jennifer MacLeod,§ and Nunzio Motta§ †

School of Materials Science and Engineering, Beihang University, Beijing 100191, China

‡

Siansonic Technology Co. Ltd., Beijing 101111, China

§

School of Chemistry, Physics, and Mechanical Engineering, Queensland University of

Technology, Brisbane 4001, QLD, Australia

ABSTRACT

The formation of liquid crystal (LC) phases in graphene oxide (GO) aqueous solution is utilized to develop high-performance supercapacitors. To investigate the effect of LC formation on the properties of subsequently reduced GO (rGO), we compare films prepared through blade-coating of viscous LC-GO solution and ultrasonic spray-coating of diluted GO aqueous dispersion. After hydrothermal reduction under identical conditions, the films show different morphology, oxygen content, and specific capacitance. Trapped water in the LC GO film plays a role in preventing restacking of sheets and facilitating the removal of oxygenated groups during the reduction process. In device architectures with either liquid or polymer electrolyte, the specific capacitance

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of the blade-coated film is twice as high as that of the spray-coated one. For a blade-coated film with mass loading of 0.115 mg/cm2, the specific capacitance reaches 286 F/g in aqueous electrolyte and 263 F/g in gelled electrolyte, respectively. This study suggests a route to pilotscale production of high-performance graphene supercapacitors through blade-coated LC-GO films.

KEYWORDS: graphene, supercapacitor, liquid crystal, energy storage, coating

1.INTRODUCTION

Energy storage is a key component in electrical energy systems. Supercapacitors store energy by physical adsorption of ions and have properties superior to conventional batteries that store charge through chemical reactions. Ionic batteries normally take hours for a full charge, and the capacity would remarkably decay after hundreds of charge-discharge cycles. Also, high current output is dangerous to battery as it may damage the device. Supercapacitors can be charged/discharged over one million cycles, and have the ability of fast charging and discharging. The high power density of supercapacitors makes them complementary to batteries in applications that require high power output. Furthermore, supercapacitors can be made very environmentally friendly due to the use of carbon materials for electrodes and non-toxic electrolytes. Commercial supercapacitors are based on activated carbon, with specific energy ranging from 1 Wh/kg to 10 Wh/kg, 10-50 times less than Li-ion batteries.1 The emergence of graphene brings new possibilities for high-performance supercapacitors that have a competitive edge over ionic batteries. Graphene, a robust and atomically-thin carbon layer, is conductive and has a very high specific surface area (2675 m2/g), giving a theoretical capacitance of 550 F/g.2 Although the high-yield production of graphene oxide (GO) using graphite powder and oxidizing


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agents has opened new opportunities for industrial applications of graphene, research on supercapacitors based on reduced GO (rGO) has not yet spawned a mature technology in terms of scalable production and satisfactory device performance. Many methods have been developed to reduce GO,3 but the quality of rGO is still not comparable to that of pristine graphene, due to defects and the insufficient removal of oxygen. When used in supercapacitors, the major problem is that rGO sheets suffer from π-π interactions to restack, forming graphite as well as decreasing the specific surface area of graphene electrodes.2 This behavior dictates the main challenge in using GO in supercapacitors: to remove the maximum number of oxygenated groups from GO sheets while simultaneously preventing the sheets from graphitizing during the reduction process.

GO sheets in water at high concentration spontaneously align to form liquid crystals (LCs),4-7 making the solution viscous. This is a slow process at room temperature, requiring at least one week for GO sheets to be divided into LC domains. This type of LC-GO solution has previously been used to make graphene fibers,8-10 capitalizing on the alignment of GO LCs during extrusion from a needle, where they become aligned due to the sheer force. Here, we use a viscous LC-GO solution in conjunction with the blade-coating technique,11 which resembles the standard procedure for making films used in Li-ion batteries or commercial supercapacitors. In industrial production, the active material in powder form is normally mixed with a polymer binder and organic solvent to form a slurry, which is roll-to-roll printed onto metal foils. In our work, the use of a viscous LC-GO solution obviates the need for this polymer binder, because the LC-GO film tightly adheres to the substrate. Blade coating provides the advantage of being a highly-scalable technique; in principle the blade-coated GO films can be fabricated at meter


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scale, depending on the machine. However, an appropriate reduction method would be crucial for high specific capacitance following the conversion from LC-GO to rGO.

The hydrothermal reduction of GO using only water is a green method compared to others that consume reduction agents. This method has conventionally been used for preparing 3D graphene aerogel.12,13 Generally, a GO dispersion with a concentration of 1-5 mg/ml is sealed in an autoclave, and heated until the rGO sheets are inter-connected to form a 3D network structure. In the present work, we have translated this process to our GO films, with particular attention to comparison of the efficacy of hydrothermal reduction for blade-coated and spray-coated films. In the spray-coating process, diluted GO solution is atomized by an ultrasonic nozzle to form droplets with uniform size around 40 µm. Through this process, the formation of LC GO domains on the substrate is largely prohibited. In this study, we systematically compare hydrothermally-reduced GO films formed through blade coating, which retains LCs, and spray coating, which suppresses them, to unravel the relationship between LCs and performance of rGO films in supercapacitors.

2. EXPERIMENTAL SECTION 2.1 Preparation of Graphene film. GO was synthesized by a two-step oxidation process reported by Xu et al,5 using expanded graphite (EG, 50 mesh, Qingdao Jingrilai Graphite Co., Ltd) as the precursor. Viscous LC-GO (6.8 mg/ml) solution was coated onto Au/Polyimide substrates using an automatic blade coater. The gap between the blade and substrate was set at different values, as 200 µm, 400 µm, 600 µm, and 800 µm, respectively, to produce films with different thicknesses. After standing for 3 min, the blade-coated GO gel film was immersed into acetone for coagulation, and then dried in air, as illustrated in Figure S1. Water between LC


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domains was extracted by acetone, while water molecules inside LCs remain trapped. The polymer substrate is not soluble in acetone. Spray coating was done using an ultrasonic coating system from Siansoni Technology Co. Ltd. and an aqueous solution of GO (2.5 mg/ml). During the spray-coating process, the substrate was kept at 60 oC to ensure efficient evaporation of water, and the coating was repeated to obtain the desired film thickness. For hydrothermal reduction, the GO films were sealed in teflon-lined autoclaves filled with deionized water that were heated to 150 oC in an oven and held for 3 h. In previous work, the optimal hydrothermal reduction temperature for achieving maximum capacitance of graphene hydrogel was reported to be 180 oC.14 We first attempted to reduce the blade-coated LC GO film at 180 oC using the hydrothermal method, however it led to severe contraction of the rGO film, detaching it from the substrate (Figure S2a). By testing different temperatures for hydrothermal reaction (Figure S2b), we determined to use 150 oC as a reduction parameter in this comparison study, because the obtained rGO film remains attached to the substrate after reduction, which is important for making solid-state devices using gelled electrolyte, and shows good capacitance. 2.2 Fabrication of supercapacitors. To make solid-state devices, first 1 g of polyvinyl alcohol powder (PVA, molecular weight ~89,000-98,000, Sigma Aldrich), 1 g concentrated H2SO4, and 10 ml deionized water were mixed. The mixture was steadily heated to 90 oC under constant stirring until it became totally transparent. Second, the viscous PVA-H2SO4 solution was dropped onto the rGO film and spread by the blade coater with the gap of 100 µm, followed by air drying at room temperature for 4 h. This process was repeated twice to make the gelled electrolyte thick enough to avoid any short circuits in the device. The two electrode films were then pressed face-to-face to form a sandwich-structured device, and the PVA polymer gel with thickness approx. 25 µm between two rGO films also works as binder. For making


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supercapacitors with aqueous electrolyte, two rGO films and a porous separator film were immersed into 1 M H2SO4 aqueous solution for 5 h. Afterwards, the rGO films were stacked with the separator in between and vacuum sealed into a plastic bag. For each electrode a thin copper wire was bound to the current collector using silver epoxy glue. 2.3 Characterizations. A field-emission scanning electron microscopy (FE-SEM, Zeiss Sigma VP) was employed to study the morphology of the rGO films. Raman spectra were recorded using a Jobin-Yvon HR800 Raman spectrometer using 0.27 mW laser with wavelength of 632.8 nm. X-ray photoelectron spectroscopy (XPS) analysis was performed using an Omicron Scienta XPS with a 125 mm hemispherical electron energy analyzer. The X-ray source was a Mg Kα (1253.6 eV) at 300 W, incident at 65o to the sample surface. A four-probe resistance tester was used to measure the sheet resistance of the rGO films. For thermogravimetric analysis (TGA), the GO solution was freeze-dried and the obtained GO foam was pressed to fill the Pt crucible, which was heated in Ar from room temperature to 600 oC at an increasing rate of 5 oC/min. 2.4 Electrochemical Performance Characterization. All electrochemical experiments were carried out using a two-electrode system (CorrTest CS 310). The area of the supercapacitor is 1×1 cm2, corresponding to the overlapped area of two electrodes. Electrochemical impedance spectroscopy (EIS) measurements were carried out with an amplitude of 5 mV over a frequency range from 0.01 Hz to 1 MHz. Cycling stability tests were conducted by galvanostatic charge/discharge (CD) at a current density of 1 A/g for ~4000 cycles. The gravimetric specific capacitances (Cg) and areal specific capacitances (Ca) derived from CD curves were calculated based on the following equations: Cg = (I∆t)/(m∆V)

(1)


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Ca = (I∆t)/(S∆V)

(2)

where I is the discharge current, ∆t is the discharge time, m is the mass load of one electrode, ∆V is the potential change after a full discharge, and S is the area of the electrode. The energy density based on active materials (Em), stacked device (Estack) and their corresponding power density (P) were calculated by using the following equations: Em = (1/2)Cg∆V2

(3)

Estack= (1/2d)Ca∆V2

(4)

P = E/∆t

(5)

where C is the specific capacitance, ∆V is the potential change for a full discharge, d is the gap between two opposite electrodes, and ∆t is the time for a full discharge. 3. RESULTS AND DISCUSSION Figure 1a illustrates the blade-coating and spray coating of GO films onto Au-coated polyimide substrates. In principle, the lateral size of the GO film formed by either technique can be on the order of metres, depending on the size of plastic substrate. For our optical observations using polarized-light optical microscopy (POM), which can reveal details of any present LC phases, we also coated GO films onto glass slides. The blade-GO film was immersed into acetone for coagulation, then naturally dried. The spray-GO film was dried at 60 oC during the coating process. On the POM stage the sample is laid between two polarizers. Figures 1b and 1d are POM images of blade-coated and spray-coated GO films, respectively, with the two polarizers crossed. In this configuration, virtually no light is transmitted if the isotropic-nematic phase transition does not occur in the sample. In Figure 1b, the bright texture is caused by birefringence of GO LCs, and the direction of blade sliding can be tracked as indicated by dashed lines. Figure


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1c was taken with the two polarizers parallel, showing that bright ridges in Figure 1b appear even over relatively thick regions in the film. In Figure 1d, which shows the spray-coated film, small bright dots are randomly distributed across the film. These dots turn dark when the two polarizers are parallel, as seen in Figure 1e. During the spray coating process, birefringent LC domains could be formed where the concentration of GO is locally high during drying, resulting in the bright dots in Figure 1d. However, these LC features are much smaller and occupy less area than in the blade-coated film, where LC features are dominant. FE-SEM observation reveals the morphology of the rGO films. Plane-view SEM images show that the spray-rGO film is rough in surface, likely due to the random deposition of droplets during the coating process (Figure S3). Figure 2 shows cross-sectional images of rGO films. Blade-rGO and spray-rGO films with thicknesses around 1 µm are shown in Figures 2a and 2b, respectively. Both images show a qualitatively similar structure of thick layers separated by gaps. Each layer consists of stacked rGO sheets. However, the blade-rGO film comprises sheets that appear somewhat rumpled, whereas those in the spray-rGO film are relatively flat. It is known that rGO sheets suffer from strong π-π interaction and tend to restack to form graphite when in contact, which reduces the specific surface area as well as the capacitance. When piled up, rumpled graphene sheets have less contacting area compared to flat ones. The restackinginhibiting effect of crumpled graphene has been widely studied in previous work.16,17 In Figure 2c, the type of buckling and bending of stacked layers in the thick blade-rGO film is a known characteristic of GO LCs, and is commonly found in the cross-section of rGO fibers made by injecting LC-GO solution into a coagulation bath.10 This rumpled feature of rGO sheets in the blade-coated film could play a role in increasing the specific surface area as well as the capacitance.


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Figure 1. (a) Illustration of the two approaches for coating GO films onto Au/polyimide substrates: blade coating of viscous LC-GO solution and ultrasonic spray coating of diluted GO solution. The LC-GO solution in a glass tube is bright when paced between two crossed polarizers with a light source behind. (b) and (c) POM images of a blade-coated GO film on glass with two polarizers crossed and parallel, respectively. (d) and (e) POM images of a spraycoated GO film on glass with two polarizers crossed and parallel, respectively.


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XRD analysis of the films shows that the (002) peak of the blade-rGO films is wider than that of the spray-rGO film (Figure S4). Broadening of XRD peaks can arise from size effects, with the height of stacked domains being inversely proportional to the full width at half maximum of (002) peak, and from variability in the spacing along the stacking direction. The broadening of the blade-rGO peak suggests that the stacking is weaker in the blade-rGO films than in the spray-rGO films. SEM images reveal that the gaps between aggregated layers in the spray-rGO are wider than in the blade-rGO film. This difference is more obvious in thicker rGO films prepared by the two methods, as shown in Figures 2d and 2f, which correspond to the marked areas in Figures 2c and 2e, respectively. Based on the (002) interlayer spacing given by XRD, each layer visible in the cross-sectional images consists of dozens of rGO sheets stuck together. Before reduction, GO sheets were stacked tightly without gaps. After reduction, gaps and voids were formed in the film. The wider the gap between layers, the thicker the layer consisting of stacked rGO sheets. Taken together, the weaker restacking within the sheets and the smaller gap between the sheets in the blade-rGO film suggest that these films are more homogenous than the spray-rGO films.


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Figure 2. FE-SEM images showing the cross-sections of rGO films. (a) and (b) blade-rGO and spray-rGO thin films, respectively. (c) and (d) A blade-rGO film about 4 µm in thickness. Bending and buckling of the stacked layers are related to the LC feature. (e) and (f) A relatively thick spray-rGO film at low and high magnifications.

Raman spectroscopy and XPS measurements were used to examine the quality and chemistry of the rGO films. Figure 3a shows the Raman spectra of GO, blade-rGO, and spray-


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rGO films. Each spectrum contains two bands assigned as D and G. The G band is one of the two E2g modes which is caused by stretching vibrations in the basal plane (sp2) of graphene, and the D band usually arises from disorder and imperfection of the carbon crystallites. For the GO film, the ratio of ID/IG is 0.77, whereas the ratios for blade-rGO and spray-rGO films are 1.27 and 1.26, respectively, indicating that the removal of oxygen from the graphene oxide sheets leads to more defects. The chemistry of the rGO sheets was studied using XPS. Figures 3b, 3c, and 3d show high-resolution XPS C1s spectra of GO, blade-rGO, and spray-rGO, respectively. Deconvolution of the C1s peak reveals contributions from C-C, C-O, C=O, and COOH bonds.15 The area under each fitted peak reflects the content of the corresponding bond. Therefore, from the fitting of the C 1s XPS regions we obtain the relative concentrations of surface functional groups, as summarized in Table S1. Approximately 54% of carbon atoms in GO sheets are chemically bound to oxygen, indicating a high oxidation degree. After the hydrothermal reduction, the atomic percentage of oxygenated carbon was reduced to 28% for the blade-rGO and 36% for the spray-rGO. Four-probe resistance measurements show that the sheet resistance of blade-rGO is 2.42 kΩ/sq, lower than that of spray-rGO (2.82 kΩ/sq). According to XPS survey spectra, the bladerGO contains 13.8 at.% oxygen, while the oxygen content in spray-rGO is 22.9 at.% (Table S1). Thus the reduction in oxygen content corresponds to an increase in conductivity of the rGO film, as expected. The O1s peaks of XPS spectra of the three samples can be deconvolved to reveal CO and C=O species (Figure S5). Our TGA result shows that at 150 oC the loss of GO weight in Ar is 7 % (Figure S6), implying that heating plays a role in the reduction of GO in our 150 oC hydrothermal reduction. However, the removal process of oxygenated groups from GO in water is not only by heating.


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Figure 3. (a) Raman spectra of GO, blade-rGO, and spray-rGO films. (b) Deconvolved XPS spectrum of the C1s region of GO. (c and d) Deconvolved XPS spectra of the C1s region of blade-rGO and spray-rGO films, respectively. The original blade-GO film lost more oxygen through the reduction process. In the blade-coated LC-GO hydrogel film, after a mild drying process water molecules may remain trapped between stacked flakes, as illustrated in Figure 4a. This can be inferred from the birefringence feature of naturally dried LC-GO film, since the liquid crystallinity will not be apparent without water between GO sheets. The negatively charged groups in the GO surface will lead to an electrostatic interaction with water polar molecules trapped between aligned GO sheets in LC domains. Under the conditions for hydrothermal reduction, the retained water


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molecules are thermally agitated and can facilitate the removal of oxygen through kinetic effects, as illustrated in Figure 4b. In principle, water could penetrate into the spray-GO film during hydrothermal reduction, but our C1s XPS data suggests otherwise (Table S1), since the absence of water molecules lowers the probability for breaking carbon-oxygen bonds during this process. As a result, the spray-rGO film contains more oxygen than the blade-rGO film. To further verify this hypothesis, we prepared two blade-coated LC-GO films: a film naturally-dried at room temperature, and one dried at 60 oC and aged for three weeks at room temperature. Both films were sealed in an autoclave and hydrothermally reduced at 150 oC for 3 h. XPS analysis shows that the naturally-dried film has fewer oxygenated groups after the reduction process (Figure S7 and Table S1), supporting our argument that trapped water in the LC GO sheets plays an important role in facilitating hydrothermal reduction.

Figure 4. (a) Illustration of the naturally-dried LC GO film that contains water molecules trapped between layers. (b) During the hydrothermal process, agitated water molecules bombard the oxygenated groups to facilitate the breaking of C-O and C=O bonds. Red arrows with random directions indicate the motions of H2O molecules to impact oxygenated groups.


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We fabricated symmetric supercapacitors using both blade-coated and spray-coated GO films to compare their performance. First, both flexible devices with PVA-H2SO4 gelled electrolyte were studied. Figures 5a and 5b are current-voltage (CV) curves for two devices based on blade-rGO and spray-rGO films, respectively, measured at different voltage scan rates ranging from 10 mV/s to 200 mV/s. The electrode films are similar in density, ~0.23 mg/cm2 for the blade-rGO film and ~0.27 mg/cm2 for the spray-rGO film. Galvanostatic charge-discharge (CD) curves of the two devices at different current densities are shown in Figures 5c and 5d, respectively. In CV loops, the variation of current with voltage is related to ion diffusion and adsorption/desorption processes at the surfaces of graphene flakes. At a fixed voltage scan rate, the wider the CV loop, the higher the capacitance. These results show that the blade-rGO device has a higher capacitance than the spray-rGO device. This is also supported by CD curves, where the longer the period of a CD cycle at constant current, the higher the capacitance. Both devices show an extremely low internal resistance (IR drop) in CD curves, which contributes to high power densities. The low internal resistance of our devices is also confirmed by impedance measurements (Figure S8). Overall, the equivalent series resistance of the blade-rGO film is lower than that of the spray-rGO film, which is in agreement with our sheet resistance measurements and the XPS analysis which shows that the blade-rGO is more reduced. We made a second set of devices using 1 M H2SO4 aqueous solution as the electrolyte, instead of PVA-H2SO4. For each type of rGO film, the device resistance is reduced by replacing the gelled polymer electrolyte with H2SO4 aqueous electrolyte. The equivalent series resistance of devices using blade-rGO films is in the range of 7.5 Ω to 16 Ω, while for the device based on spray-rGO film the values are 17.5 Ω (H2SO4) and 23 Ω (H2SO4-PVA).


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The shapes of the CV curves of the blade-coated and spray-coated solid-state devices are different (Figure 5 and Figure S9). Comparison between the two sets of devices allows us to assess whether this is related to the chosen electrolyte. The distorted shape of the CV loop measured from spray-rGO device does not appear to be related to the electrolyte. For the bladerGO film in aqueous electrolyte, the CV curve is closer to a quasi-square shape, implying ideal capacitive behavior. We attribute the distorted oval shape of the spray-rGO CV loops to the unstable capacitance that varies with swept voltages. According to the governing equation, I=CdV/dt, the current should remain constant in the CV measurement, corresponding to a rectangular loop, if the capacitance C is constant. However, the capacitance of the rGO films is related to the total area of rGO sheets participating in ion adsorption. The variation in capacitance of the spray-rGO films could be related to their structural inhomogeneity, which results in an increased voltage being necessary to drive ions to access the densely packed bundles of rGO sheets. Thus, the capacitance increases as the voltage is scanned from low to high. On the contrary, with decreasing the voltage, the capacitance drops nonlinearly, making the CV loop asymmetric. The dependence of specific capacitance (F/g) on current density for blade-rGO and sprayrGO films is compared in Figures 5e and 5f. In aqueous electrolyte, the blade-rGO film has a specific capacitance up to 250 F/g, whereas the maximum capacitance of the spray-rGO film is ~125 F/g (Figure 5e). In the literature, three methods have been reported to increase the capacitance of graphene electrodes: making graphene sheets in a three-dimensional (3D) structure,18,19 etching the graphene sheets to make nanopores,20,21 and nitrogen doping of graphene.22,23 Xu et al pressed 3D graphene hydrogel film onto an Au-coated plastic film to made flexible supercapacitors, achieving 196 F/g when using 1 M H2SO4 liquid electrolyte;18 Xu


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et al used a wet-etching method to make holey graphene, and measured the capacitance as 283 F/g for 3D hydrogel film and 209 F/g for a compact film, at 1 A/g in 1 M H2SO4 aqueous electrolyte;21 Sui et al prepared N-doped graphene aerogels and obtained 223 F/g at 0.2 A/g, with 1 M H2SO4 aqueous electrolyte.23 These results suggest that the capacitance of our blade-rGO film, which is already superior to most of the published values, could be further improved by means of nitrogen doping or etching nanopores in graphene sheets. The blade-coated film in liquid H2SO4 shows much higher capacitance than in H2SO4-PVA gel, whereas for the spraycoated film the capacitance shows minor difference when swapping the two types of electrolytes. Normally, the liquid electrolyte has better infiltration into electrodes, and gives rise to higher capacitance compared to the gelled one. However, the spray-coated rGO film that exhibits low capacitance has a large portion of stacked sheets, indicated by thick layers in Figure 2f. Therefore, whether liquid or gelled electrolytes filled in interspaces between layers, ions are blocked to penetrate into the layer consisting of stacked rGO sheets. As a result, changing the electrolyte from solid to liquid caused no much increase of capacitance for the spray-coated film. The blade-rGO film shows higher capacitance than the spray-rGO film, indicating that the presence of large LCs correlates favorably with capacitance. First, water trapped in GO LC domains facilitates the removal of oxygen during the hydrothermal reduction process. The higher the reduction degree, the more ions could be adsorbed in rGO sheets when charged. Second, the LC formation leads to rumpled feature of rGO sheets, inhabiting restacking of rGO sheets and increasing the specific surface area. For supercapacitors with a liquid electrolyte, the goal is to pursue a high specific capacitance per mass (F/g). For solid-state supercapacitors, the electrode film is required to be thin, and the relevant measurement is specific capacitance per area (mF/cm2). We prepared blade-coated GO films with different thicknesses by adjusting the gap


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between blade and substrate during the coating process. For spray-coated GO films, different thicknesses were obtained by controlling the coating times. Figure 6a shows the relationship of areal capacitance (mF/cm2) with the mass loading of rGO film (mg/cm2). The capacitance was calculated from galvanostatic CD curves at 100 µA/cm2. Overall, increasing the mass of rGO sheets leads to a higher areal capacitance, however the increase is not linear, and approaches a saturation value with mass loading. This is likely because as more rGO sheets pile up, those at the bottom make a decreasing contribution to the capacitance. The capacitance of spray-coated film saturates faster with additional thickness, due to the cumulative effects of the structural inhomogeneity of the films, which would act to decrease ion diffusion. By increasing the mass loading of spray-rGO film from 0.27 mg/cm2 to 0.55 mg/cm2, the areal specific capacitance measured in 1 M H2SO4 aqueous electrolyte was increased by only 1.2 times. Figure 6b shows the dependence of mass-normalized specific capacitance, deduced from galvanostatic CD curves at 1 A/g, against the mass loading of rGO sheets. Overall, the thinner the electrode film, the higher the mass-normalized specific capacitance (F/g). The maximum value is 286 F/g from the blade-rGO film with 0.115 mg/cm2 mass loading in 1 M H2SO4 aqueous solution. We believe that by etching holes in GO sheets and make them to form LCs, the specific capacitance of blade-rGO films would remain high with increasing the mass loading over 1 mg/cm2, which is critical for practical application. The specific capacitance of spray-rGO film drops below 100 F/g when the mass loading exceeds 0.3 mg/cm2, not competitive compared to activated carbon used in commercial supercapacitors.


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Figure 5. Comparison of the electrochemical performance of two supercapacitors incorporating blade-rGO (0.23 mg/cm2) and spray-rGO (0.27 mg/cm2) films, respectively. (a) and (b) CV curves of two solid-state devices based on blade-rGO and spray-rGO films, respectively. (c) and (d) Galvanostatic CD curves of the two devices using gelled electrolyte. (e) Dependence of the specific capacitance against discharge current densities, for blade-rGO and spray-rGO films in aqueous electrolyte. (f) The relationship of specific capacitance with discharge current density for two solid-state devices based on blade-rGO and spray-rGO films, respectively.


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Figure 6. The variation of specific capacitance of blade-rGO films with increasing mass loading. (a) The areal specific capacitance deduced from CD curves at 100 µA/cm2. (b) The mass specific capacitance deduced from CD curves at current density of 1 A/g.

Compressed rGO hydrogels with a 3D network structure have been used as supercapacitor electrodes, normally at tens of micrometers in thickness. Our rGO films derived from LC GO are more compact and have an advantage in terms of large size and specific capacitance. Hence, the blade-coated rGO film with a thickness less than 10 µm is more suitable for fabricating flexible and solid-state supercapacitors. Figure 7a shows a photograph of a solid-state device with a length of 9 cm. The CV loops at different bending angles are shown in Figure 7b. These nearly identical CV curves indicate stable capacitive behavior, which we attribute to the mechanical robustness of the graphene film. In addition, the device shows excellent lifetime stability (Figure S10a) and cycle stability (Figure S10b). The galvanostatic CD curves in Figure 7c show that although the time for a full discharge is slightly extended due to good infiltration of gelled electrolyte in graphene films and evaporation of water, the capacitance was high even after one


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month. The capacitance retention of the device remains 90 % after repeatedly charging and discharging the device at 1 A/g over 4000 cycles. Ragone plots for the solid-state device based on blade-rGO films with 0.23 mg/cm2 mass loading are shown in Figure 7d and 7e, respectively. In this device, two rGO films with thicknesses of a few micrometers were face-to-face bound by a thin layer of gelled electrolyte, forming a flexible device. The high capacitance and low resistance of the rGO film, together with the narrow gap between two electrodes, contribute to high volumetric energy and power densities. In Figure 7d, the energy density of our device is comparable to that of Li-ion thin film battery. For comparison, the energy and power densities of other supercapacitors based gelled electrolyte are shown, with values taken from the literature. Various electrode materials were used in those devices, including 3D compressed graphene foam,24 graphene/MWCNT mixture film,25 laser-scribed graphene film26 and CNT film.27 Figure 7e shows the high gravimetric energy and power densities of our device, in comparison to those based on N-doped carbon nanofibers,28 3D graphene hydrogel,18 single-wall CNTs (SWCNTs),29 SWCNT-graphene composite aerogel,30 P-doped graphene,31 holey graphene hydrogel,21 N and S co-doped porous carbon nanosheets,32 and vertically aligned CNTs on carbon nanofibers.33 Our blade-coated rGO film outperforms those 3D structured graphene electrodes in the form of hydrogel or aerogel.


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Figure 7. (a) Photograph of a flexible solid-state supercapacitor based on blade-rGO films. (b) CV curves of the device at 20 mV/s for different bending angles. (c) Galvanostatic CD curves at 1 A/g for the device taken at different time durations. (d) and (e) Ragone plot for power and energy densities in volume and mass, respectively. In (d), values for devices based on 3D graphene foam (Ref. 24), graphene-MWCNT mixture film (Ref. 25), Li-ion thin film battery (Ref. 26), and CNT film (Ref.27) are marked for comparison. In (e), values for devices based on holey graphene hydrogel film (Ref. 21), N-doped carbon fiber (Ref. 28), SWCNT film (Ref. 29), P-doped graphene (Ref. 30) N, S co-doped carbon nanosheets (Ref. 32, marked by the red open ellipse), CNT on carbon nanofibers (Ref. 33, marked by the green open ellipse) and are shown.


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4. CONCLUSIONS We have prepared GO films by two scalable methods: the blade coating of viscous LC GO solution and the ultrasonic spray coating of diluted GO aqueous solution. Both films were hydrothermally reduced with identical conditions in order to correlate the effect of LC formation with supercapacitor performance. Structural analysis by SEM and chemical analysis by XPS reveal that the rGO sheets in the blade-coated film are rumpled and contain fewer oxygenated components than those in the spray-coated film. Through control experiments, we attribute the chemical difference to trapped water in the LC GO film, which facilitates oxygen removal from GO sheets during the reduction process. Second, by comparing supercapacitors made using PVA-H2SO4 gelled and aqueous H2SO4 electrolytes, we found that blade-rGO films consistently exhibit better electrochemical performance than the spray-rGO films. The specific capacitance of the blade-coated graphene films is about twice as high as that of the spray-coated films. For a thin blade-rGO film with mass loading of 0.125 mg/cm2, the specific capacitance reaches 286 F/g in aqueous electrolyte and 262 F/g in gelled electrolyte. Increasing the mass loading can improve the areal capacitance, but decreases the mass specific capacitance. The energy and power densities for a typical solid-state supercapacitor based on blade-rGO film compare favorably with a range of examples from the literature. Our study suggests that the liquid crystallinity in GO sheets can be favorable for developing high-performance graphene supercapacitors. Moreover, the combination of blade-coating technique and viscous LC GO solution promises a scalable way to make large-size devices. AUTHOR INFORMATION Corresponding Author


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*E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOLEDGEMENT This work was supported by ‘The Fundamental Research Funds for Central Universities’ through Beihang University. The authors thank Dr Chenlu Bao from Nanyang Technological University, Singapore, for valuable discussions. Some characterizations were performed at the Central Analytical Research Facility (CARF) operated by the Institute for Future Environments, QUT.

Supporting Information Available: Experimental details and additional figures. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES 1.

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Table of content 31x11mm (300 x 300 DPI)


Role of Graphene Oxide Liquid Crystals in Hydrothermal Reduction

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