Vertically Aligned Reduced Graphite Oxide Nanosheet Film and its

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Vertically Aligned Reduced Graphite Oxide Nanosheet Film and its Application in High-Speed Charge/Discharge Electrochemical Capacitor Dai Mochizuki, Ryo Tanaka, Sho Makino, Yusuke Ayato, and Wataru Sugimoto ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01478 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 14, 2019

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ACS Applied Energy Materials

Dai Mochizuki,1,2,†,* Ryo Tanaka2, Sho Makino2, Yusuke Ayato1, and Wataru Sugimoto1,2* 1 Interdisciplinary Cluster for Cutting Edge Research, Center for Energy and Environmental Science, Shinshu University, 3-15-1 Tokida, Ueda, Nagano 386-8567, Japan. 2 Faculty of Textile Science and Technology, Shinshu University, 3-15-1 Tokida, Ueda, Nagano 386-8567, Japan. † Present address: Department of Applied Chemistry, School of Engineering, Tokyo Denki University, 5 SenjuAsahi, Adachi, Tokyo 120-8551, Japan. *E-mail: [email protected] (DM), [email protected] (WS) perpendicular alignment, graphene, ice templating, electrodeposition, supercapacitor, ultrahigh rate, macroporous film

ABSTRACT: Porous electrodes with nanosheets vertically aligned to the substrate are candidates for high power energy storage applications such as rechargeable batteries and electrochemical capacitors due to the shortened ion and electron transfer pathway. Here we fabricate vertically aligned reduced graphene oxide (rGO) films by combining electrophoretic deposition and ice template methods. As deposited and freeze-dried films prepared from graphene oxide dispersion show perpendicular orientation of graphene oxide with a tilt angle of 85 ± 9° with respect to the substrate. The film thickness and pore sizes are varied from 50 to 200 μm and from 10 to 100 μm, respectively, by changing the freeze-drying conditions. The vertical alignment of nanosheets enhances the specific capacitance and charging rate behavior. At a high scan rate of 500 mV s-1, the specific capacitance was approximately 78% of that at 2 mV s-1, indicating an improvement in the rate performance due to the perpendicular orientation.

1 INTRODUCTION Graphene nanosheets have potential as an electrode material for electrochemical double-layer capacitors owing to the high surface area resulting from the high aspect ratio of graphene with 1-nm thickness and lateral size of several μm. Vertically oriented graphene electrodes with excellent diffusion and reduced resistance has been reported to show high rate performance rivaling that of aluminum electrolytic capacitors.1–5 The graphene electrode is structured in such a way that graphene stands vertically against the substrate. It is difficult, however, to manufacture vertically aligned thick films at a low cost as they are formed by the deposition of graphene on substrates by chemical vapor deposition (CVD)6,7 or by carbonization of ionic liquids which is used as a template. Therefore, it is necessary to produce vertically aligned films through an inexpensive wet process. A nanosheet thin film produced by the wet coating process can be manufactured at low cost, and the film thickness and film density can be easily controlled by the precursor solution and its coating amounts.

Nanosheets have recently attracted interest as two-dimensional anisotropic materials.8–10 The structural two-dimensional anisotropy enables electron transfer within the structure and mass transfer between the structures as a new function of the material. The anisotropy peculiar to nanosheets have been utilized for the precise fabrication of thin films by various methods such as the LangmuirBlodgett method11–14 and layer-by-layer method15,16. The nanosheet thin films, however, always horizontally align to the substrate due to their two-dimensional anisotropy, which is unsuitable for applications involving fast charge storage within pores such as electrochemical double layer capacitors. The perpendicular alignment of nanosheets with the substrate, therefore, remains a challenge.

Some attempts have been made for the vertical alignment of nanosheets using wet processes. It has been reported that nanosheets can be vertically oriented in a colloidal solution by applying magnetic or electric fields to the nanosheet suspension. When a high magnetic field is applied to a nanosheet colloid solution or a layered compound-polymer solution, the nanosheet and layers align ACS Paragon Plus Environment

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Scheme 1. Proposed formation mechanism of the vertically film by electrophoresis and freeze drying. perpendicular to the direction of the applied magnetic field in the solution.17 The nanosheets were aligned parallel to the direction of the applied magnetic field, resulting in vertical alignment of the nanosheet in the colloidal solution with the substrate. An electrophoretic deposition method has been employed for film formation by producing a nanosheet by electrophoresis under an electric field. This method is applicable to all nanosheets with surface charge and allows controlling the film thickness ranging from nanometer to micrometer scale.18–21 More specifically, if a bias voltage is applied to a negatively charged nanosheet colloid, the nanosheets become concentrated on the positively charged electrode. Additionally, the orientation of the nanosheet can be controlled by alternating electric fields.22 The nanosheets in a nanosheet colloid, which orients parallel to the alternating field, form liquid crystal nanosheets oriented in vertical direction against an electrode surface. However, the vertically oriented liquid crystal nanosheet colloid undergoes structural change and orients horizontally during the drying process. Here, we focus on freeze drying to maintain the vertical orientation of nanosheet after drying (Scheme 1). Freeze casting is an ice template method that uses ice, which is generated during freezing of colloid, as template and enables the control of pores by regulating the morphology of the ice crystal.23–26 Particularly, the macropore diameters can be extensively controlled by adjusting the freezing speed. Moreover, in the freeze casting method, since the ice crystal template grows along the freezing direction, the orientation of the macropores can be controlled. That is, by freezing from the substrate, vertical macropores can be formed from the substrate. Vertically aligned films have been formed by the freeze casting method using colloids and sols of various ceramic nanoparticles.27–29 In this work, electrophoretic deposition was adopted to nanosheet colloids followed by freeze-drying to yield a vertically oriented film. In the film forming process, the film thickness and pore diameters were controlled by the electrophoresis conditions and freezing conditions, respectively. Exfoliated graphite oxide nanosheets, or graphene

oxide (GO), forms a colloid with a negative surface charge and has the advantage of easily forming a film by various wet processing methods. A recent study reported that electrodeposition method can be successfully adopted to fabricate vertically oriented films by cathodic electrodeposition of GO to form reduced GO (rGO) and subsequent freeze-drying.30 Unfortunately, the alignment is not well controlled and it is not possible to apply cathodic EPD to other nanosheets. We will show here that anodic EPD of GO colloid and subsequent freeze drying and thermal reduction leads to rGO films oriented vertically with respect to the substrate. 2. Results and Discussion Figure 1 shows the SEM images of a deposited GO film formed by the combination of EPD and freeze-drying method. The top view shows a macroporous structure with a pore diameter of 21 ± 5 μm (Figure S1A), while the side view shows the perpendicular orientation of GO with respect to the substrate with a tilt angle of 85 ± 9° (Figure 1A and B). In the deposited film subjected to conventional drying (without freeze-drying), GO were deposited densely lying flat on the substrate (horizontal alignment), as observed from the top and side views (Figure S1c and S1d). Figure 2 shows the XRD patterns of the deposited films. In the case of conventionally deposited films, a peak with a d value of 0.87 nm is observed, which corresponds to the interlayer spacing of restacked GO and indicates the horizontal orientation of the films. However, such a peak was not observed in the case of films obtained by freeze-drying. Thus, there is no periodic structure horizontal to the substrate, in agreement with the SEM observations. These results demonstrate the successful formation of perpendicularly oriented GO films by the combination of EPD and freeze-drying. We consider that this is because the GO was concentrated with a high density on the electrode surface by EPD. In the case of direct freeze-drying of the GO colloid under zero bias (freeze-drying under open circuit condition), no film was formed on the substrate. Therefore,

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Figure 1. (A) SEM image of GO film obtained by electrophoretic deposition and freeze drying (GOns concentration: 0.1 g L-1, intermediate freezing rate, EPD duration 30min ). (B) Tilt angle distribution of the nanosheets.

Figure 2. XRD patterns of the deposited film by conventional drying (A) and the deposited film (GOns concentration: 0.1 g L-1, intermediate freezing rate, EPD duration 30min ) by freeze drying (B)

electrophoresis is a requirement for the formation of perpendicularly oriented films. Thus, it is concluded that both EPD and freeze-drying is necessary to form perpendicularly oriented GO films. During the EPD process of GO colloid, electrophoresis of negatively-charged GO toward the positive electrode occurs, and deposit on the substrate. Owing to the high anisotropic nanostructure of GO, GO migrates through the colloid and deposit on the substrate aligned vertically under applied potential. Subsequent freeze-drying leads to ice formation, pushing the GO to the grain boundaries of the ice crystals. The ice crystals acts as the template leading to macropore formation. The film processing conditions were varied in order to clarify the determining factors and control the macroporous structure of the deposited film. First, the effect of freezing speed on the structure of the deposited film was studied. It is known that the freezing speed determines the size of the ice crystals and thus the pore size of ice template.31 The pore geometry of GO films was controlled by changing the freezing speed. Figures 3A and B show the

Figure 3. SEM images of deposited film (GOns concentration:0.1 g L-1, electrophoresis duration 30min ) obtained by slowing down the freezing rate. (A: top view and B: side view images) and deposited film obtained by fast freezing rate. (C: top view and D: side view images).

SEM images of the deposited film obtained at a slow freezing rate. The cross-sectional views show that the deposited films maintained its vertical orientation against the substrate, and the top view reveals large fine pores (about 100 μm) compared to those obtained by fast immersion in liquid nitrogen. The SEM images (Figure 3C and D) of the deposited film subjected to fast freezing exhibited fine pores with 10 μm diameter. The results indicate a clear correlation between the freezing speed and the size of the fine pores; that is, the faster the freezing rate, the smaller the fine pore diameters. Therefore, the fine pores in the vertically oriented GO film resulted from the emergence of ice crystal during pre-freezing, which pushed away the nanosheet and acted as a template. The film thickness was about 200 μm at a slow freezing rate (Figure 3B), about 150 μm at an intermediate freezing rate, and about 50 μm at a fast freezing rate (Figure 3D). The slower the freezing rate, the thicker the film became. It was also confirmed from the



cross-sectional view that the pores were connected straight from the substrate to the top surface of the deposited film. Next, the effect of alternating electric field was studied, as it is known nanosheets form liquid crystal states under alternating electric field22. EPD was performed with an alternating electric voltage of ±40 V for 30 minutes at a frequency of 100 kHz with an offset potential of 4 V to induce deposition. From the SEM image of the freeze-dried film, a vertical alignment film with pore diameter of 20 μm was observed. From the cross-sectional view, it was also confirmed that GO was arranged perpendicular to the substrate. This SEM is consistent with the characteristics of deposited films fabricated using direct electric field. The above results show that the formation mechanism of the vertically oriented film presumably involves directional freezing 32 from the substrate. The ice crystal expands from the substrate in the vertical direction during prior freezing, and the nanosheet is pushed aside between the ice crystals. As the ice crystal grows, the deposited films form with GO in much wider ranges, which, in turn, rendered the films much thicker and diameters of the fine pores larger. Therefore, freezing rate is considered an important factor in deciding the structure of vertically oriented nanosheet electrode. The deposition was performed at various EPD time (tEPD) to control the thickness of the film. In the electrophoretic deposition of nanosheets without freeze drying, it was reported that the deposition amount and film thickness increased in proportion to tEPD.18 The SEM images of the deposited films obtained after EPD with different times of 5, 30, and 60 min (Figure 4) shows that the film thicknesses were about 80, 150, and 160 μm, respectively. Although thicker films were obtained for longer tEPD, there was no proportional relationship between tEPD and film thickness, and the film thickness begins to level off. It must be noted that no change in pore structure (pore diameter of ~20 μm) was observed with the different tEPD. There can be two possibilities for the lack of linear relationship between tEPD and film thickness: nanosheet deposition mechanism and limit of the nanosheet deposition amount. The electrophoresis of nanosheets in the colloid should occur with the nanosheets migrating vertically with respect to the electrode substrate, in order to minimize resistance. Thus, even if the deposition amount increases with increasing migration time, the film thickness does not change significantly. In addition, the nanosheet deposition amount approached its limit with increasing time, resulting in no increase in the film thickness.

Figure 4. SEM images of deposited film obtained by changing the EPD duration. (A: top view and B: side view image of 5min, (C: top view and D: side view images) 30 min (E: top view and F: side view images) 60min

thicknesses of the deposited films with concentrations of 0.2 and 0.4 g L-1 were the same at approximately 150 μm. The colloid concentration did not affect the pore size according to the SEM images. The above results demonstrate that the freezing rate significantly affects the pore diameter of the deposited film, and micrometer-scale pores can be formed at a fast freezing rate. The film thickness can be controlled by the EPD time, colloid concentration, and freezing rate. The film thickness can therefore be increased by increasing the EPD time, increasing the colloid concentration, or by reducing the freezing rate.

To investigate the influence of nanosheet concentration in the colloidal solution on the structure of the deposited thin film, experiments were conducted with various concentrations of nanosheet colloids. The SEM images of the deposited films for nanosheet concentrations of 0.1, 0.2, and 0.4 g L-1 obtained after freeze-drying are shown in Figure S2. The deposited film with a concentration of 0.1 g L-1 had a thickness of approximately 110 μm, whereas the


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Figure 5. Cyclic voltammograms of the deposited film by conventional drying (A) and the deposited film by freeze drying (B) by changing scan rates and the capacitance retention (C) of the deposited film by conventional drying (red circle) and the deposited film by freeze drying (blue triangle) against 2mV s-1.

The electrochemical performance resulting from the orientation in the deposited film was compared. The capacitance of the reduced graphite oxide (rGO) film was estimated by cyclic voltammetry, and the charging/discharging rate performance of the electrode was estimated by changing the potential scanning rate (Figure 5). Here, two electrodes with similar mass loading (100 μg/cm2) with different nanosheet orientation (horizontal vs vertical) are compared. In the case of horizontally oriented deposited films (nanosheets lying flat on the substrate), a large change in areal capacitance was observed upon changing the scanning rate, indicating poor charging/discharging performance. The areal capacitance was 12.2 mF cm-2 at a low scanning rate (2 mV s-1) and decreased sharply with increasing scan rate. At a high scanning rate of 500 mV s-1, the capacitance retention was 17%. The reason for this low retention is attributed to the slow diffusion of the electrolyte in the film due to the horizontal orientation of rGO nanosheet. On the other hand, the areal capacitance was higher for the perpendicularly oriented films (18.6 mF cm2) than that of the horizontally oriented deposited films at a low scanning rate. More importantly, increasing the scanning rate did not cause a large change in the capacitance; that is, at a scan rate of 500 mV s-1, the areal capacitance was approximately 78% of that at 2 mV s-1. This indicates a large improvement in the charging/discharging rate due to the perpendicular orientation. Such an improvement is most likely due to the fast charge storage within pores owing to the perpendicular orientation. In addition, rGO edges that are highly exposed on the surface may have improved electrochemical performance. The electrochemical performance of vertically oriented rGO deposit layer was studied on the influence of pore size. Figure S3a and b show the cyclic voltammograms of the deposited layer with pore sizes of 100 and 10 μm fabricated by varying the freezing rate. The capacitance retention at 500 mV s-1 was approximately 60%, which is higher than that of a horizontally oriented layer. This is sufficient proof that even different pore sizes could improve the diffusivity for vertical orientation. The volumetric capacitance decreased as the pore sizes increased. This is because with increase in

pore size, the macroporosity in the deposited layer increased, resulting in a reduced surface area. Next, we investigated the effect of film thickness of the vertically oriented rGO electrode on its electrochemical performance. We first investigated the deposited layer with film thickness varied by changing the EPD time. Figures S4a and b show the cyclic voltammograms of film with thickness of 80 μm and 160 μm fabricated with EPD time of 5 and 60 min. No significant difference in the shape of the cyclic voltammograms can be noticed. The retention rate of the capacity was approximately 70% at a fast scanning rate, which was probably an effect of the fast charge storage within pores for vertical orientation. The volumetric capacitance increased as the film thickened. Electrodes with film thicknesses changed by the colloid concentration was investigated next. Figure S5 shows the cyclic voltammograms of the deposited layers (film thicknesses: 110, 150,

Figure 6. Relationship between film thickness and capacitance per volume of each sample, pore diameter of 10 μm (blue triangle), 20 μm (red circle), and 100 μm (green square).

and 150 μm) produced with colloid concentrations of 0.1, 0.2, and 0.4 g L-1. The volumetric capacitance improved as the colloid concentration increased. In addition, this phenomenon became ascertained about the deposited layer with the same thickness for colloid concentrations of 0.2



and 0.4 g L-1. This proved that a higher colloid concentration increased the amount of deposited rGO. Figure 6 shows a plot of the relationship between film thickness and volumetric capacitance. The overall trend is that the volumetric capacitance increases with increasing film thickness with the same pore diameter. In addition, when the pore diameter are smaller, the volumetric capacitance is larger even with the same film thickness. These results indicated that the deposited film is thicker with denser nanosheet packing, and the deposited film is denser as the pore diameter is smaller. To further increase the capacitance, it is important to control both the pore diameter and the film thickness. 3. Conclusion We have developed a method for fabricating perpendicularly aligned reduced graphene oxide (rGO) electrodes by combing electrophoretic deposition of GO colloid followed by freeze-drying and thermal reduction. The newly developed method is capable of controlling film thickness and pore size, which can afford fast charge storage within pores and low resistance. The perpendicularly oriented GO electrode fabricated by this method had pores with diameters of about 20 μm and had a uniform thickness of about 150 μm, which extend vertically to the substrate. The pore size was controlled within 100 to 10 μm by controlling the freezing rate. The alignment of GO on the electrode greatly affected the capacitor response, which improved for the perpendicularly oriented electrode. This is possibly due to the improvement in the porous structure in the deposited film and reduction in the resistance by the perpendicular alignment of GO. 4. Experimental Section Preparation of GO colloids: Graphite, NaNO3, and 95 % H2SO4 were mixed and stirred for 5 min in an ice bath. Thereafter, KMnO4 was added under continuous stirring for 1 h, and the mixture was left to stand for 4 days at room temperature. Next, 98 °C ultrapure water was added and the mixture was stirred for 15 min, followed by suction filtration with the addition of 80 °C ultrapure water and 30% H2O2. The filtrate was washed with 5 wt% HCl and then washed with methanol until pH neutral. After washing, the filtrate was dried overnight in a vacuum dryer at 60 °C. The resulting product was milled using an agate mortar and pestle to yield graphite oxide. The graphite oxide powder (50 mg) dispersed in ultrapure water was irradiated with ultrasonic waves of 100 W output power and a frequency of 40 kHz for 2 h at ambient temperature. The dispersion was centrifuged at 2000 rpm for 30 min, and aliquoting the supernatant led to the production of 0.5 g L-1 graphene oxide (GO) colloid. Film formation of GO via electrophoretic deposition (EPD): A pair of electrodes (Au, 15 mm × 10 mm × 0.5 mm) affixed with NITOFLON on the back side as a sealing compound to prevent film formation was inserted to a depth of 10 mm from the liquid surface of GO colloid (7 mL) with 1

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cm distance between the electrodes. EPD was carried out by applying a voltage of 4 V at room temperature for 10 min. The negatively charge GO deposited on the positive electrode. Next, the electrode in the EPD cell (φ30 × 45 mm, glass) was immersed in liquid nitrogen to freeze. For slow freezing, the EPD cell was placed several mm from the liquid nitrogen level (Figure S6). For a fast freezing, one side of a pair of electrode substrate was affixed to the bottom surface of a syringe, while the other side was fixed in such a way that the distance between the electrodes was 1 cm. GO colloid (1.33 mL) was injected into the syringe-type EPD cell, and deposition was carried out by applying voltage. Next, the syringe-type EPD cell was immersed in liquid nitrogen for fast freezing (Figure S6). Thereafter, vacuum drying led to the formation of GO film. The GO film was treated in a mixture of flowing Ar and H2 for 2 h at a temperature of 200 °C to generate reduced GO (rGO) film. Raman spectroscopy (Hololab 5000 and Shimadzu Ltd.) was conducted to evaluate the reduction. The Raman spectrum of the sample after hydrogen reduction showed D band at around 1350 cm-1 and G band at 1583 cm-1 (Figure S8). The D/G ratio was 1.24, indicating the progress of the reduction of GO. The sample was characterized by X-ray diffraction (RINT2500HF/PC and Rigaku Ltd.) with a Cu Kα (λ = 0.15406 nm) X-ray source and tube voltage and tube current of 40 kV and 40 mA, respectively. The orientation of the rGO film was observed using a scanning electron microscope (SEM). A carbon tape was used to fix the sample to the stage, and the incident acceleration voltage was 20 kV. Electrochemical measurement: Electrochemical measurements of the rGO film was conducted with rGO film transferred onto a carbon paste coated glassy carbon (GC, 3 mm diameter) electrode. The transferred film possessed similar macropore structures as indicated by SEM image (Figure S7). GC was buff polished and oven dried and washed before use. The mass of rGO was about 100 μg/cm2. Cyclic voltammetry was conducted to evaluate the electrochemical capacitor characteristic of the electrode with a HZ-3000 (Hokuto Denko Co.) potentio/galvanostat. A beaker type three-electrode cell was used for measurements. The working, Pt mesh counter and Ag/AgCl reference electrodes were immersed in 0.5M H2SO4. The electrode potentials were converted to that of reversible hydrogen electrode (RHE) scale. Prior to measurement, with the temperature being at constant 25 °C, high-purity nitrogen was continuously flowed through in the electrolyte for 40 minutes to purge-out dissolve oxygen. High-purity nitrogen was also supplied during measurements. First, in order to obtain a clean and stable surface, the potential was scanned between 0.2-1.2V vs. RHE at a scan rate of 50mV s1 for 500 cycles. Next, the potential scans were conducted at scan rates of 500, 50, 20, 5, 2 mV s-1.


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ACS Applied Energy Materials Supporting Information. SEM images and CVs of the deposited films. This material is available free of charge via the Internet at http://pubs.acs.org.

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*Email: [email protected] *Email: [email protected]

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This work is partially supported by Advanced Low Carbon Technology Research Development Program of the Japan Science and Technology Agency (JST-ALCA, JPMJAL1008) and KAKENHI (26410235) from Japan Society for the Promotion Science (JSPS).

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Vertically Aligned Reduced Graphite Oxide Nanosheet Film and its

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