Ordered, Scalable Heterostructure Comprising Boron Nitride and

Oct 11, 2016 - For a potential application, the h-BN/graphene hybrid films are fabricated supercapacitor's electrodes revealing high volumetric capaci...
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Ordered, Scalable Heterostructure Comprising Boron Nitride and Graphene for High-Performance Flexible Supercapacitors Segi Byun,† Joon Hui Kim,† Sung Ho Song,‡ Minku Lee,§ Jin-Ju Park,§ Gyoungja Lee,§ Soon Hyung Hong,*,† and Dongju Lee*,§ †

Department of Material Science and Engineering, Graphene Research Center (GRC), and KAIST Institute for the NanoCentury, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea ‡ Division of Advanced Materials Engineering, Kongju National University, Chungnam 32588, Republic of Korea § Nuclear Materials Development Division, Korea Atomic Energy Research Institute, 989-111 Daedeok-daero, Yuseong-gu, Daejeon 34057, Republic of Korea S Supporting Information *

ABSTRACT: Heterostructures based on combining two-dimensional (2D) crystals in one stack have unusual physical properties and allow the creation of novel devices. Although this method of mechanically transferring individual 2D crystals is required for precise control, it is not scalable. Large-scale fabrication of heterostructures remains a key challenge for practical applications. Here, we provide a simple solution-based method using electrostatic interaction assembly of boron nitride (h-BN) and graphene to produce hybrid films with van der Waals heterostructures. The hybrid films prepared by this fabrication method tend to be alternately stacked and provide compact structured films. For a potential application, the h-BN/graphene hybrid films are fabricated supercapacitor’s electrodes revealing high volumetric capacitance, superior rate capability, a permanent life cycle, and high flexibility due to their synergistic effects. We anticipate that the hybrid films are useful as scalable flexible electrodes in supercapacitors, and our solution-based method has great potential for application in energy storage and electronics.

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continuous, high-quality h-BN thin films in this manner, and they typically require high temperatures and a complex procedure for the transfer of the resulting h-BN films from the growth substrate.11 An alternative approach is a simple wet chemistry route, which uses liquid-phase exfoliation to produce dispersions of 2D crystals. Liquid-phase exfoliation has several advantages: it is a simple and low-cost process that is readily applicable to different 2D crystals and provides crystal properties that are similar to those of 2D crystals obtained by micromechanical cleavage.12,13 This approach has been used to prepare heterostructures of various 2D crystals by low-cost fabrication techniques such as drop- and spray-coating, vacuum filtering, and inkjet printing.7,11 This synthetic method provides a generic approach for the fabrication of a variety of hybrid layered materials containing atomic layers of various compositions and properties.14 Nevertheless, the strategy is impeded by certain limitations of the liquid-phase exfoliation process such as low yield, poor solubility (surfactants are needed), and the relatively small size of the liquid-exfoliated 2D crystals. To overcome these deficiencies, we recently developed a hydroxide-assisted ball milling process, which can produce

INTRODUCTION Two-dimensional hexagonal boron nitride (h-BN), consisting of sp2-bonded boron and nitrogen atoms, has attracted intense research interest because of its several advantages, including superior mechanical strength, high thermal conductivity, chemical and thermal stability, excellent dielectric properties, and an atomically smooth surface with few dangling bonds or charge traps.1,2 Atomically thin h-BN is used as a nanofiller for composites,3,4 in oxidation-resistant coatings,5 and as a substrate for high-quality graphene electronics.6 The remarkable properties of h-BN make it an ideal candidate for fabricating van der Waals heterostructures by combining it with other two-dimensional (2D) materials such as graphene and transition metal dichalcogenides. This class of materials covers a very broad range of properties; the obtained heterostructures can be tuned for exceptional properties or used for specific applications.7 Most heterostructures have been produced to date by micromechanical cleavage of bulk layered crystals followed by multistage stacking of 2D crystal layers using a dry transfer method.2,8 While this technique provides extremely highquality stacks,9 it cannot be scaled up for practical applications. Another approach used for the large-scale fabrication of heterostructures is the direct growth of h-BN by chemical vapor deposition.10 However, it is challenging to produce © 2016 American Chemical Society

Received: July 19, 2016 Revised: October 9, 2016 Published: October 11, 2016 7750

DOI: 10.1021/acs.chemmater.6b02947 Chem. Mater. 2016, 28, 7750−7756

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Chemistry of Materials

mL) in a round flask was mixed with 360 mL of an ammonia solution (28 wt % in water), and 300 mL of deionized water was added. The mixture was heated at 90 °C for 7 h with reflux. After being cooled to room temperature, the mixture was washed and collected. The material was redispersed in deionized water (3 mg mL−1) by sonication to make the amine-modified GO suspension (NH2-GO). DMF [9:1 (v/v) DMF/H2O] was added to provide a NH2-GO concentration of 0.3 mg mL−1. Chemical reduction of the suspension of NH2-GO sheets was performed with hydrazine monohydrate (1 μL/3 mg of NH2-GO) for 12 h at 80 °C while the mixture was being stirred, creating a homogeneous black suspension. After being cooled to room temperature, the suspension was diluted by adding DMF to the suspension [9:1 (v/v) DMF/suspension mixture].17 Exfoliation of h-BN by Hydroxide-Assisted Ball Milling To Form OH-BNNPs. A horizontal planetary mill (Fritsch Pulverisette 5) was used for the exfoliation. Micrometer-sized h-BN powder (2 g) and a 2 M aqueous NaOH solution were loaded into a steel grinding bowl with an 8 mm diameter steel balls at a ball:powder ratio of 50:1. The rotational speed of the planetary mill was set to 200 rpm, and the mixture was milled for 24 h. The milled product was collected and rinsed with aqueous HCl to remove the remaining residual Fe ion and repeatedly washed with deionized water until the pH was close to neutral. The samples were dried in a vacuum oven, dispersed in IPA at an initial concentration of 0.5 mg mL−1, and sonicated for 1 h. The dispersed BN solution was centrifuged at 2000 rpm for 30 min to remove any thick flakes and aggregates. Detailed descriptions of all of the experimental procedures are given in our previous research.18 Fabrication of the BN/rGO Hybrid Films and Flexible Supercapacitors. The OH-BNNP and NH2-rGO dispersions were filtered through preweighed PVDF membrane filters (47 mm in diameter, 0.22 μm pore size, Millipore). The membranes were dried, and the mass of the deposited material was used to calculate the concentration of the dispersions. The NH2-rGO solution and the OHBNNP suspension were diluted with IPA as required to provide the same concentration and were mixed together. The total mass of the hybrid films was ∼5 mg after drying, and the OH-BNNP:NH2-rGO mass ratio was varied from 0.25:4 to 2:4 by adjusting the volume ratio of the NH2-rGO and OH-BNNP dispersions. The mixture was sonicated for 60 min (Branson Sonifier S-450A, 400 W). The BN/ rGO hybrid films were then produced by vacuum filtration of the composite suspension through a PVDF membrane filter. During filtration, the pH of the mixed solution was adjusted to 4. This pH change makes both 2D craystals oppositely charged and thus stimulates the electrostatic interactions. OH-BNNP and NH2-rGO were assembled by electrostatic interaction between positively charged NH2-rGO and negatively charged OH-BNNP. The surface charges of OH-BNNP and NH2-rGO were examined by a ζ potential instrument (Figure S5). The BN/rGO hybrid film thus obtained was washed with IPA to remove any DMF residue and dried in a vacuum oven to remove the remaining solvent. To remove residual water and hydroxyl groups from the rGO, the hybrid film was then subsequently heattreated at 200 °C for 1 h in air at a heating rate of 1 °C min−1. The PVA/KOH gel electrolyte was prepared by mixing 1 g of PVA (Mw = 146000−186000; Sigma-Aldrich) with 1 g of KOH in 10 mL of deionized water and then heating it at 85 °C for several hours until the solution became transparent. To fabricate a flexible supercapacitor, the annealed hybrid film was cut into a circle 13 mm in diameter, and two pieces of the film were assembled onto a flexible PET film using the PVA/KOH polymer gel electrolyte as the separator and electrolyte. Then, Pt foil (50 μm thick) was adhered to one side of the hybrid film (∼3 mm) to provide an external electrical connection for the electrochemical measurements; finally, the film was carefully sealed with Kapton tape to maintain the cell structure. Characterization. The morphologies of OH-BNNP and NH2rGO were investigated by AFM (Seiko Instruments, Inc., Chiba, Japan) in tapping mode under ambient conditions. TEM (JEOL JEM2200FS and FEI Titan Cubed G2 60-300 at the KARA analysis center) analyses were also conducted. EELS measurements were performed with the transmission electron microscope operating at 200 kV. Fieldemission scanning electron microscopy (FE-SEM) (Hitachi S4800)

hydroxyl-functionalized h-BN nanoplatelets with advantages such as a relatively large flake size (∼1.5 μm), only slightly damaged in-plane structures, and high dispersibility in various solvents. We have adapted our recently reported hydroxide-assisted ball milling process to produce a BN/rGO hybrid film having a van der Waals heterostructure. The simple solution-based method employs the electrostatic interaction assembly of a hydroxyl-functionalized h-BN nanoplatelet (OH-BNNP) with amine-modified reduced graphene oxide (NH2-rGO). The OHBNNP and NH2-rGO tended to be alternately stacked by this approach. It is potentially scalable laterally and in the vertical direction through simple vacuum filtration. The heterostructure provided a compact structured free-standing film because of its 2D geometry and homogeneous dispersion in the hybrid film. High-resolution transmission electron microscopy (HR-TEM), electron energy loss spectroscopy (EELS), and energydispersive X-ray spectroscopy (EDS) were used to confirm the alternatively stacked structure. Additionally, the h-BN and graphene composition of the hybrid film was tuned by varying the volume ratio of the dispersions to provide films with different properties. The heterostructures made with various 2D layered materials have been studied for various applications. Among them, recent theoretical and experimental studies of hBN/graphene alternating structure show intriguing electrochemical performances without an increase in the dimensions of the structures because h-BN acts as a spacer with a lattice constant close to that of graphene.15,16 To the best of our knowledge, however, the work described here represents the first demonstration of applying BN/rGO heterostructures for flexible supercapacitors by h-BN as a 2D structured support and electrolyte channel in an rGO film. The OH-BNNP acted as both supports and proton channels to the NH2-rGO film, and in the hybrid film, they enhanced the tensile strength of the film as a mechanical support and permitted a high packing density (∼1.6 g cm−3) to be achieved. Furthermore, the optimized proportion of OH-BNNP in the hybrid film functioned as a 2D binder in a supercapacitor and provided improved electrochemical performances such as a high volumetric capacitance (242 F cm−3 at 10 mV s−1 in a KOH electrolyte), superior rate capability, and a permanent cycle life (99% retention after 10000 cycles) compared with those of a neat NH2-rGO film. The potential application of the hybrid film in a flexible supercapacitor was further demonstrated, revealing 100% initial capacitance retention even after 1000 bending cycles.

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EXPERIMENTAL SECTION

Materials. h-BN powder and graphite powder were purchased from Kojundo Korea Co., Ltd., and Bay Carbon, Inc., respectively. Sodium hydroxide (NaOH), hydrochloric acid (HCl), sulfuric acid (H2SO4), potassium permanganate (KMnO4), hydrogen peroxide (H2O2), aqueous ammonia, hydrazine (N2H4), potassium hydroxide (KOH), and dimethylformamide (DMF) were purchased from Junsei Chemical Co., Ltd. Isopropanol (IPA) was purchased from Merck. All chemicals were used without further purification. Preparation of Amine-Modified Reduced Graphene Oxide (NH2-rGO). GO was made by the modified Hummers method. Graphite (1 g) and H2SO4 (40 mL) were mixed in an ice bath. KMnO4 (3.5 g) was slowly added as an oxidizing agent to the mixed solution. Once the solution was homogeneous, deionized water was added as the oxygen source and H2O2 (10 mL) was added to remove Mn ions. The mixture was filtered and washed with aqueous HCl to remove residual salts. The resulting solid was dried in a vacuum oven without heating and dispersed in deionized water to make a GO dispersion (0.1 mg mL−1). The homogeneous GO dispersion (300 7751

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Chemistry of Materials was used to evaluate the thickness of the hybrid films. The surface functional groups were measured by Fourier transform infrared (FTIR) spectroscopy (Jasco FT/IR-4100 type A, ATR mode) and X-ray photoelectron spectroscopy (Sigma Probe, Al Kα; Thermo Scientific, Hudson, NH). The response to tensile loading was measured using a universal testing machine (Instron 8848 Microtester) with a crosshead speed of 0.2 mm min−1. The test specimens consisted of strips of uniform width (6 mm) and thickness (0.01 mm). All electrochemical measurements were performed using an electrochemical instrument (BiStat, Biologic Science Instruments, Grenoble, France) in a two-electrode configuration in 6 M KOH. The active material was placed onto a gold current collector, and it was assembled with a KOH-soaked separator (glass fiber filter paper, Whatman). The electrochemical impedance spectra were recorded at a potential of 0 V in the frequency range of 100 kHz to 10 mHz with a 10 mV sinusoidal wave. Calculation of the Capacitances, Energies, and Power Densities. The specific (gravimetric and volumetric) capacitances were calculated using the following equations:19

Cg =

2 mvΔV

ρ=

m Vol.

(2)

Cv = Cgρ

(3)

∫ I dV

Figure 1. Fabrication procedure for synthesizing BN/rGO hybrid films having heterostructure from OH-BNNP and NH2-rGO. (a) Schematic illustration of the synthesis of a BN/rGO hybrid film. (b) Digital images of the hybrid film showing the flexibility of the film. (c) Crosssectional high-resolution SEM images showing the morphology of the hybrid film. The average thickness of the hybrid film was ∼2.5 μm.

(1)

Figures S6 and S7 show the top view and corresponding crosssectional view of BN/rGO free-standing films. The hybrid films were ∼2.5 μm thick with a homogeneous distribution of OHBNNPs. TEM images of cross-sectional specimens extracted from the hybrid films are shown in Figure 2 and Figure S8. TEM

where m is the mass of the active electrode, v is the scan rate, I is the discharge current, ΔV is the potential window, Vol. is the volume of the active electrode, and ρ is the packing density of the film. The energy and power densities of the film were calculated using the following equations:19

Eg =

1 Cg ΔV 2/3600 8

(4)

Ev = Eg ρ

(5)

Pg = I ΔV /2m

(6)

Pv = Pgρ

(7)

where Eg and Pg are the gravimetric energy and power density and Ev and Pv are the volumetric energy and power density, respectively.

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RESULTS AND DISCUSSION Figure 1 schematically illustrates the procedure for synthesizing hybrid films from OH-BNNP and NH2-rGO. To synthesize the BN/rGO hybrid films, an ammonia solution was used to introduce amine groups onto the graphene oxide (GO) surfaces, and then the amine-modified graphene oxide was chemically reduced using hydrazine. The amine groups introduced on the rGO sheets led to a positively charged stable rGO suspension, which was necessary for the electrostatic interaction assembly and to improve the electrical conductivity.20 Also, OH-BNNPs are produced by hydroxideassisted ball milling.18 The microstructural and spectroscopic analyses of the NH2-rGO and OH-BNNP are further elaborated in Figures S1−S4. The BN/rGO hybrid films were prepared by mixing and vacuum-filtering dispersions of NH2rGO in DMF and OH-BNNP in isopropanol (IPA) (Figure 1a). It was crucial to produce a homogeneous suspension of rGO and BNNP to obtain a uniform multilayer architecture. Figure 1a schematically illustrates the ideal hybrid film structure, and Figure 1b presents a digital image of a film prepared by the experimental technique. The digital image shows the outstanding structural flexibility of the specimens. Scanning electron microscopy (SEM) images in Figure 1c and

Figure 2. Material characterization of the BN/rGO hybrid films. Cross-sectional bright field HR-TEM micrographs of (a) the NH2rGO film and (d) the 0.5:4 BN/rGO film. HAADF-STEM image of the cross section of (b) the NH2-rGO film and (e) the 0.5:4 BN/rGO film. Elemental profiles of the carbon and boron K-edges extracted from the EELS spectra of (c) the NH2-rGO film and (f) the 0.5:4 BN/ rGO film along the white line shown in panels b and e.

specimens were prepared using focused ion beam (FIB) milling and the lift-out method.9 In our fabrication process, the sheets of OH-BNNP and NH2-rGO were preferentially oriented parallel to the surface of the substrate as the leaves spread across the substrate surface. TEM images of the cross section of the hybrid film (Figure 2) exhibited a relatively high degree of ordering of the BN/rGO multilayers. Moreover, the thickness of the OH-BNNP layer was 3−5 nm, in good agreement with the atomic force microscopy (AFM) results. Detailed analysis of the BN distribution in the hybrid films was performed using EELS line scanning (Figure 2 and Figure S8) and EDS elemental mapping (Figure S9). The acquired cross-sectional 7752

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Chemistry of Materials EELS images also allowed for identification of the approximate locations of the OH-BNNP layers. Indeed, ELLS plots of the fluctuation along the white line shown in high-angle annular dark field scanning transmission electron microscopy (HAADFSTEM) images showed a decrease in the intensity of the boron signal and a simultaneously increased intensity of the carbon signal, which indicated an alternating layered structure of h-BN and graphene. The distance between the OH-BNNP layers, meaning the interlayer thickness of rGO between the OHBNNP layer, tended to decrease with an increase in the OHBNNP:NH2-rGO ratio. The average distances between the OH-BNNP layers in the 0.5:4, 1:4, and 2:4 BN/rGO samples were 156 ± 132, 95 ± 87, and 44 ± 37 nm, respectively. It is also noted that all hybrid films are considered h-BN/graphene heterostructures comprising different interlayer thicknesses. To enhance the electrochemical performance of the restacked graphene film, inhibiting restacking of graphene sheets is crucial for achieving high specific capacitance as reported previously.21,22 The specific capacitance severely decreases when restacking between graphene sheets occurs, which originates mainly from irreversible restacking of the individual reduced graphene sheets during the reduction and drying processes, which makes substantial surfaces of reduced graphene unavailable for charge storage. We expect that OHBNNP in the hybrid film increases the electroactive surface area of rGO film by acting as an efficient electrolyte channel. Therefore, various BN/rGO hybrid films with different mass loading levels of OH-BNNPs were prepared to investigate the potential application of OH-BNNPs as a 2D structured support and an electrolyte channel in a graphene-based film. The mass loading levels were adjusted from 0.25:4 to 2:4 (OHBNNP:NH2-rGO) by varying the relative volume ratios of the solutions. The neat NH2-rGO and the hybrid films were examined using cyclic voltammetry (CV) in 6 M KOH at various scan rates from 10 mV s−1 to 10 V s−1. At a scan rate of 1 V s−1, the CV curve (Figure 3a) of the 0.5:4 BN/rGO hybrid film had a large integrated area even at a very fast scan rate. This indicated that the hybrid film had a volumetric capacitance higher than that of and a rate capability better than that of a neat NH2-rGO film. Interestingly, although the volumetric capacitance values of the hybrid films were calculated by considering the total volume of the electrode, which included the OH-BNNP thickness, the volumetric capacitance of the 0.5:4 BN/rGO hybrid film (242 F cm−3 at 10 mV s−1) exceeded that of the neat NH2-rGO film (221 F cm−3 at 10 mV s−1). In addition, the capacitive behavior was explored by measuring the volumetric capacitances as a function of scan rate for the neat NH2-rGO and various hybrid films (Figure 3b). The rate capability of the hybrid film was enhanced with increasing amounts of added OH-BNNPs. However, because the OH-BNNPs did not contribute to the volumetric capacitances, the maximal capacitances of the hybrid films at a slow scan rate of 10 mV s−1 decreased with increase in OHBNNP loading (Figure 3b). Furthermore, as more OH-BNNPs were incorporated into the hybrid film (for instance, >20 wt % BNNPs in the film), its electrical conductivity dramatically decreased because of the insulating nature of h-BN. The volumetric capacitances at 10 mV s−1 of the 1:4 and 2:4 BN/ rGO hybrid films were 196 and 131 F cm−3, respectively. Interestingly, the OH-BNNPs can enhance either the volumetric capacitance or the rate capability of the rGO film if the optimal amount of OH-BNNPs is added. The optimal

Figure 3. Electrochemical properties of the BN/rGO hybrid films. I(a) Volumetric CV curves of the neat NH2-rGO and 0.5:4 BN/rGO films at a scan rate of 1 V s−1. (b) Trend of the volumetric capacitance of all of the films at scan rates of 10 mV s−1 to 10 V s−1. (c) Cyclic performances of the neat NH2-rGO and 0.5:4 BN/rGO films after 10000 cycles at 300 mV s−1. The inset shows identical CV curves for the 0.5:4 BN/rGO film at the first and last cycle. (d) Nyquist plots of the neat NH2-rGO and hybrid films. The inset is the Nyquist plot magnified in the high-frequency region, which reveals that the optimized amount of OH-BNNPs in the hybrid film effectively reduced the ESR value of the film.

amount of OH-BNNPs was ∼11 wt % in the 0.5:4 BN/rGO hybrid film. This result suggests that OH-BNNP can create electrolyte channels in the hybrid films that act as an effective proton channel and assist fast diffusion of electrolyte ions such as K+ and OH− between the NH2-rGO sheets, because the monolayer h-BN nanosheet exhibits an extremely high proton conductivity of ∼100 mS cm−2 at 300 K, which results in the enhanced rate capability of the hybrid film.23 Furthermore, because of the 2D sheet morphology of the OH-BNNPs, which was flakes only a few nanometers in thickness (3−5 nm) with a large width (∼1.5 μm), the assembly of flakes led to the formation of compact structured and mechanically robust BN/ rGO hybrid films but an ion accessible area larger than that of the neat rGO film. For accurate measurement of capacitance and the examination of reproducibility, 0.5:4 BN/rGO hybrid films with different mass loadings of the active materials were prepared and tested, where detailed analysis of the result was further elaborated in Figure S10. Furthermore, to examine whether the optimized hybrid film acted as a negative or positive electrode, a half-cell potential measurement for the 0.5:4 BN/rGO film was conducted and is shown in Figure S11. The optimized hybrid film of a 0.5:4 BN/rGO film exhibited a high volumetric capacitance with superior rate performance because it was a synergistic effect of OH-BNNP/rGO while a high packing density that was as high as that for the neat NH2rGO film was maintained (both had a density of ∼1.6 g cm−3). The material properties of each film are listed in Table S1. Furthermore, in repeated cyclic performance tests at a scan rate of 300 mV s−1 (Figure 3c), the optimized 0.5:4 BN/rGO film retained ∼99% of its initial capacitance even after 10000 cycles, 7753

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Chemistry of Materials while the neat NH2-rGO film kept only ∼94%, revealing a superior cyclic stability. Additionally, the charge/discharge curves of the optimized 0.5:4 BN/rGO hybrid film (Figure S12) had a linear and symmetrical triangular shape without IR drops from 0.6 to 5.6 A g−1 at various current densities. This indicated either a low internal resistance or ideal capacitive behavior. Additionally, for practical applications, it is important to evaluate the leakage current and self-discharge characteristics of the optimized hybrid film-based supercapacitor, and the results are shown in Figure S13. To improve our understanding of the kinetics of electrolyte diffusion in the films, the BN/rGO films were studied by electrochemical impedance spectroscopy (EIS) in 6 M KOH over a frequency range from 100 kHz to 10 mHz. The Nyquist plots (Figure 3d) show that the neat NH2-rGO and hybrid films all displayed the typical characteristics of electrochemical double-layer capacitors (EDLCs). The Nyquist plot of an ideal EDLC consists of a straight line at low frequencies; therefore, all of the films were considered to behave as ideal capacitors. At high frequencies (inset of Figure 3d), the Nyquist plots were all semicircular with a straight line connecting the ends of the curve. The semicircle relates to the charge transfer resistance (Rct) involving either a surface property of the porous electrode or the electroactive surface area, and therefore, the Rct decreases with an increase in the electroactive surface area.24 The Rct value of the films dramatically decreased from 2.43 Ω (for neat NH2-rGO) to 0.43 Ω (for the 0.5:4 BN/rGO film) when the optimal amount of OH-BNNPs was present in the hybrid film. When OH-BNNPs are added as a restacking inhibitor in the rGO film to an optimal amount (e.g., 0.5:4 BN/rGO film), the resulting film should have an effective surface area that is larger than that of the bare restacked rGO film, because OH-BNNP acts as restacking inhibitors without a significant decrease in the electrical conductivity of hybrid films. However, the addition of more OH-BNNPs to the rGO/BN hybrid film can significantly reduce the electrical conductivity of the film, resulting in degradation of capacitance, due to the insulating nature of BN. To examine the ion accessible surface area of the BN/rGO hybrid film, electrochemical active surface area (ECSA) analysis was adapted (Figure S14 and Table S2).25−29 On the basis of the result, the ECSAs of NH2-rGO and 0.5:4 BN/rGO films were calculated to be 321.25 and 468.75 cm−2, respectively. From this analysis, we concluded that the ECSA value of the BN/rGO hybrid film was higher than that of the bare NH2rGO film. This means that BNNP can act as an effective proton transport for inhibiting restacking of the rGO film, and hence, the rGO film with the optimal amount of BNNPs had more ion accessible surface area but was densely packed. Furthermore, Xray diffraction (XRD) spectra and Brunauer−Emmett−Teller (BET) surface area measurement for hybrid films supported the increase in rGO interlayer spacing via the introduction of OHBNNPs in the hybrid films (Figures S15 and S16, respectively). The electrochemical results of the optimized BN/rGO film suggest that OH-BNNPs can act as an effective electrolyte channel in graphene-based films for applications in high-density supercapacitors and provide intriguing properties such as the formation of compact structured films and the maintenance of high packing densities and volumetric capacitances. This resulted in enhancement of electrochemical features such as the cyclic stability and rate performance. The volumetric energy and power densities of the neat NH2-rGO and 0.5:4 BN/rGO films were calculated from their volumetric capacitances to compare their electrochemical performances. Figure S17 shows

the volumetric Ragone plot of various carbon materials in the aqueous electrolyte system. In addition to areal and gravimetric data, electrochemical performances for the NH2-rGO and hybrid films are provided in Figures S18 and S19. Because of the high packing densities of the NH2-rGO and hybrid films, their volumetric energy densities were also high, i.e., 7.68 and 8.41 mWh cm−3, respectively, at a scan rate of 10 mV s−1. Although the energy density of the hybrid film was below that of highly porous graphene, it exceeded the values of other carbon materials such as porous graphitic carbon, activated carbon, graphene powder, and laser-scribed graphene film.30−32 Furthermore, with its high energy density, the 0.5:4 BN/rGO film delivered an ultrahigh power density of 129 W cm−3, while the NH2-rGO film provided only 104 W cm−3 at a fast scan rate of 10 V s−1. From these performance comparison data, it is expected that the optimized BN/rGO hybrid film could be used as an electrode in high-density flexible supercapacitors to provide enhanced electrochemical properties such as superior power performance and semipermanent cycle life. For a potential application, the hybrid film in an electrode of a flexible supercapacitor was further demonstrated. A flexible supercapacitor prototype device was fabricated using an optimized hybrid film (Figure 4a). For use in flexible supercapacitors, an electrode material must be free-standing and mechanically robust with good mechanical strength and have superior electrochemical performance.33−37 Mechanical testing revealed that the tensile strength of the neat NH2-rGO film was enhanced by adding OH-BNNPs, and the optimal

Figure 4. BN/rGO hybrid film-based flexible supercapacitor. (a) Schematic diagram of an all-solid-state BN/rGO hybrid film-based flexible supercapacitor and a digital photograph showing the flexibility of the device. (b) Electrochemical properties of the optimized hybrid film (0.5:4 BN/rGO)-based flexible supercapacitor. CV curves of the hybrid film-based device before and after 1000 bending cycles, which shows almost the same integrated area after the bending cycles. (c) Charge/discharge curve of the film at a current density of 2.9 A g−1 without any significant IR drop. (d) Mechanical properties of the neat NH2-rGO and hybrid films, which demonstrates that the optimal amount of BN in the hybrid film acted as a mechanical nanofiller and enhanced the tensile strength of the film. 7754

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loading was 11 wt % for the 0.5:4 BN/rGO hybrid film (Figure S20). The tensile strength of that film was ∼36 MPa, which exceeded that of the neat NH2-rGO film (30 MPa). This indicated that the OH-BNNPs in the hybrid film functioned as nanofiller and enhanced its mechanical strength. This is attributed to the high quality of OH-BNNPs, which were damaged only slightly during the fabrication processes, while NH2-rGO has a low crystallinity damaged from a severe chemical exfoliation and reduction process. This results in differences in mechanical properties between OH-BNNP and NH2-rGO where the former should retain the intrinsic properties (crystallinity, strength, etc.) and the latter should have degraded mechanical properties (mechanical properties of graphene are significantly affected by its crystallinity).38 However, OH-BNNP should be added only with a certain optimal amount in the hybrid film to enhance its mechanical property. This is due to an increase in defect concentration associated with nanofiller embedment in the hybrid films,39 which results in degradation of its mechanical strength (Figure 2 and Figure S8, HRTEM image). Thus, the optimized hybrid film is suitable as an electrode for a flexible supercapacitor because of its favorable mechanical strength and superior electrochemical performance. The results of tensile testing are summarized in Table S3. Figure 4b shows that the CV curve of the hybrid film-based flexible supercapacitor exhibited a volumetric capacitance of ∼95 F cm−3 at 50 mV s−1 and also revealed stable device operation even after bending. Detailed electrochemical data of the flexible device, such as charge/ discharge curve, EIS data, and its volumetric capacitance as a function of scan rate, are presented in Figure 4c and Figure S21. Moreover, even after 1000 bending tests (Figure 4d), the device exhibited 100% capacitance retention, although some capacitance fluctuations were measured during the prolonged test. This indicated good long-term stability and flexibility of the hybrid film-based flexible supercapacitor. The electrochemical performance of the device could be likely further enhanced by optimizing experimental conditions such as the device design and electrolyte concentration.

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Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b02947. Experimental methods used for amine-modified reduced graphene oxide (NH2-rGO), exfoliation of h-BN, and preparation of BN/rGO hybrid films and flexible supercapacitors and equipment and characterization techniques; chemical compositions of rGO; HRTEM and AFM images of OH-BNNP and NH2-rGO; microstructures of BN/rGO heterostructures; and electrochemical and mechanical properties of BN/rGO heterostructures (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by the Korea Atomic Energy Research Institute (KAERI) R&D Program and also by the Center for Advanced Soft-Electronics funded by the Ministry of Science, ICT, and Future Planning as Global Frontier Project (2013M3A6A5073173).

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REFERENCES

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CONCLUSION

In summary, we have demonstrated a novel approach to building scalable heterostructures by stacking 2D layered materials. The solution-based method using electrostatic interaction assembly of graphene and h-BN reported here provided ordered stacks with a scalable heterostructure; the layers were ordered and stacked in the vertical direction but randomly stacked in the lateral direction. This synthetic process is versatile and inexpensive, uses simple technology, and can be applied to other layered materials. The potential use of the BN/ rGO hybrid film in a high-performance flexible supercapacitor was also investigated. The hybrid film displayed enhanced electrochemical performance, i.e., high volumetric capacitance, superior rate capability, and permanent cycle life, because the OH-BNNPs served as mechanical supports and an electrolyte channel to the rGO film. The optimized hybrid film was quite suitable as an electrode for a flexible supercapacitor. The results of this research suggest that our solution-based method used to achieve scalable h-BN/graphene heterostructures could be extended to other layered materials for various applications in energy storage and electronics. 7755

DOI: 10.1021/acs.chemmater.6b02947 Chem. Mater. 2016, 28, 7750−7756

Article

Chemistry of Materials

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DOI: 10.1021/acs.chemmater.6b02947 Chem. Mater. 2016, 28, 7750−7756