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Ultrafast Molecular Stitching of Graphene Films at the Ethanol/Water Interface for High Volumetric Capacitance Gang Lian,*,†,‡ Chia-Chi Tuan,‡ Liyi Li,‡ Shilong Jiao,† Kyoung-Sik Moon,‡ Qilong Wang,§ Deliang Cui,† and Ching-Ping Wong*,‡ †

State Key Lab of Crystal Materials, Shandong University, Jinan 250100, P. R. China School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States § Key Lab for Special Functional Aggregated Materials of Education Ministry, School of Chemistry & Chemical Engineering, Shandong University, Jinan 250100, P. R. China ‡

S Supporting Information *

ABSTRACT: Compact graphene film electrodes with a high ionaccessible surface area have the promising potential to realize highdensity electrochemical energy storage (or high volumetric capacitance), which is vital for the development of flexible, portable, and wearable energy storage devices. Here, a novel, ultrafast strategy for stitching graphene sheets into films, in which p-phenylenediamine (PPD) molecules are uniformly intercalated between the graphene sheets, is simply constructed at the ethanol/water interface. Due to uniformly interlayer spacing (∼1.1 nm), good wettability, and an interconnected ion transport channel, the binder-free PPD− graphene film with a high packing density (1.55 g cm−3) delivers an ultrahigh volumetric capacitance (711 F cm−3 at a current density of 0.5 A g−1), high rate performance, high power and energy densities, and excellent cycling stability in aqueous electrolytes. This interfacial stitching strategy holds new promise for the future design of enhanced electrochemical energystorage devices. KEYWORDS: Supercapacitors, volumetric capacitance, graphene films, PPD, interfacial stitching

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low ion-accessible surfaces and high diffusion resistance of electrolytes.15 Impaired ion transport becomes more pronounced in subnanometer pores.15 Because ionic diffusion is crucial for charging of electrical double-layer capacitors, an effective solution to this issue is to open the interlayer spacing between the sheets. Therefore, porous graphene foams and hydrogel films have been prepared as electrodes to offer large void spaces for electrolyte storage, contributing to high Cwt. Regrettably, these structures only resulted in mediocre Cvol because of the ultralow packing densities.19−21 Thus, graphenebased supercapacitors currently suffer from low Cvol and volumetric energy densities (Evol) and are hard to be scaled up for portable applications. Therefore, there should be a tradeoff between a spacing of separated graphene sheets and high packing density of graphene films. Molecular spacers, intercalated between the graphene sheets, can create a uniform spacing for electrolyte ion diffusion and increase the accessible surfaces of graphene films. For instance, amino groups in aromatic amine molecules enable the spacers to covalently bond to graphene oxide (GO) through nucleophilic substitution and amide formation reaction.22 The

he supercapacitor, one of the most important electrochemical energy storage devices, has attracted dramatic attention for the extensive needs of hybrid electronic vehicles, mobile electronics, clean energy, and portable, flexible, and wearable electronic devices,1−3 due to their fast charge− discharge rate, high power density, and long cycle life.4−6 Twodimensional (2D) materials are of particular interest in supercapacitors in terms of their large electrochemically accessible surfaces.7−10 For example, 2D titanium carbide (Ti3C2),8 with a high conductivity, has shown promise as a capacitor electrode with a specific volumetric capacitance (Cvol) of ∼900 F cm−3. High Cvol values of ∼700 F cm−3 have been achieved with MoS2 nanosheets of metallic 1T phase in aqueous electrolyte.10 Such excellent performance is important for portable and mobile power sources, which are in need of as much energy storage as possible in rather limited space. As a typical 2D material, graphene presents a unique 2D structure, high intrinsic electrical conductivity, and ultrahigh theoretical surface area and is a potentially promising candidate for use in supercapacitors.7,11 Unfortunately, graphene film electrodes only show the capacitance under 500 F cm−3 in aqueous electrolytes.11−18 Improvement in Cvol requires a gravimetric capacitance (Cwt) and a density (ρ) to be maximized (Cvol = Cwtρ). Compact graphene easily leads to graphite-like behavior of high density but limited Cvol due to its © 2017 American Chemical Society

Received: September 27, 2016 Revised: December 18, 2016 Published: January 30, 2017 1365

DOI: 10.1021/acs.nanolett.6b04035 Nano Lett. 2017, 17, 1365−1370

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Figure 1. (a) Optical image showing the formation of GO film at the ethanol/water interface. (b) Schematic illustration of the formation process of PPD−GO film. (c) Optical image of PPD−GO film. (d) Optical image of PPD−graphene film.

Figure 2. (a) Optical image of PPD−graphene film (12PE). Inset: The flexible character of film. (b) Low- and (c) high-magnified SEM images of PPD−graphene film (12PE). (d) SEM image and corresponding EDX mapping of N atom. (e) XRD patterns of GO, PPD−GO, and PPD− graphene films. (f) XPS spectra of GO, PPD−GO, and PPD−graphene films. (g) High-resolution N1s spectra.

tion. Figure 1b schematically illustrates the interfacial stitching process for the formation of a PPD−GO film. It is known that GO sheets are negatively charged due to the existence of many functional groups (e.g., carboxyl and hydroxyl) on the basal surfaces and edges (Figures S2 and S3). When the PPD/ ethanol solution contacted the GO suspension, an electrostatic interaction between the negatively charged groups on GO and positively charged groups of PPD immediately extracted GO sheets from the aqueous solution to the ethanol−water interface (Figure 1a). Then the adjacent GO sheets were stitched together by the amino groups on the para-position of

formation of covalent bonds can also ensure high durability during charge/discharge cycles of the supercapacitors. Meanwhile, to improve the packing density of the graphene films, large-size pores should be removed as much as possible. In this work, a novel, ultrafast strategy was sought to stitch GO sheets using p-phenylenediamine (PPD) molecules to form PPD-mediated GO film at the ethanol/water interface (Figure 1a and S1), which is different from the traditional oil/water interfacial methods.23−25 This is the first time to achieve the stitching of GO sheets at an interface of miscible solvents, and the involved solvents are easily removed by simple volatiliza1366

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Figure 3. (a) CV curves and (b) CD curves of PPD−graphene film (12PE) at various scan rates and current densities. (c) Cycling stability of PPD− graphene film (12PE) at a current density of 5 A g−1. Inset: The CD curves before and after cycling. (d) EIS spectrum. Inset: High-magnified EIS spectrum at high frequency. (e) CV curves of the samples at a scan rate of 10 mV s−1. Black curve: PPD−graphene film (12 PE); red curve: PPD− graphene film (E); blue curve: pristine graphene film. (f) Specific volumetric capacitance of samples measured from CV. (g) Specific volumetric capacitance of samples measured from CD. The correspondence between line’s colors and samples is the same as panel e. (h) Gravimetric capacitance as a function of areal mass loading of PPD−graphene film at the current density of 0.5 A g−1. Inset: SEM images of films (5.5 μm and 30 μm in thickness). (i) Ragone plots of Evol and Pvol of graphene film and PPD−graphene film (E and 12PE, against the volume of the electrode only).

amount of PPD molecular “bridges” integration. The obtained film was named the PPD−graphene film (12PE). Details are shown in the Experimental Section and Table S1. In terms of versatility, the interfacial stitching method is also compatible with other amine molecules, e.g., monoamine (oleylamine and amino-terminated silane), diamine (ethylenediamine and hexamethylenediamine), triamine (1,2,4-triaminobenzene), and tetramine (1,2,4,5-benzentetramine). The thickness and transmittance of films could be controlled by the concentrations of amine and GO, and film-forming time. Besides thin films, thick GO paper, foam, and hollow spheres with layer-bylayer structures were also obtained by this general stitching strategy, which will be explored in future studies. The resulting PPD−graphene films presented substantial flexibility and excellent tensile property after drying thoroughly (Figures 2a and S5). They could be directly used as binderless electrodes. More importantly, the binder-free, compact graphene film created uniformly expanded spacing (∼1.1 nm) and a continuous ion transport network that led to an exceptionally high Cvol. The structure and interlayer spacing of PPD−graphene film (12PE) were verified by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The cross-section images revealed the film with rather uniform thickness and layer-by-layer structures (Figure 2b,c), which was also confirmed by the observation of nitrogen (N) atoms distribution in the film. The

aromatic ring to form uniform films. When PPD molecules were intercalated/adsorbed in GO films, the molecules served as nanoscale spacers between GO sheets. Furthermore, only 10 s was required for the formation of PPD−GO films (5−6 um thickness, Figure 1c) via the ultrafast interfacial assembly. In the film-forming process, only two glass slides and a container are needed, so a much larger film can be expected, compared to that obtained via vacuum filtration. Although, only a small amount of PPD molecules were actually intercalated in the separately layered films to create molecular channels (see below), we were able to enhance the PPD intercalation density by immersing the PPD−GO films in a PPD/ethanol solution for 12 h. Because ethanol showed better wetting behavior toward the PPD−GO film than water (Figure S4), the immersion and reduction procedures all proceeded in the PPD/ethanol solution (10 mg mL−1) for adequate diffusion and intercalation. After reduction of the film in an ethanol− thermal reaction, the damp graphene films were swollen in the thickness direction and should be heavily compressed by capillary and controlled mechanical forces to increase the packing density through the removal of volatile solvent trapped in the films (Figure 1d). Upon the removal of volatile solvents, the thickness of film was obviously reduced, while its basal dimension showed a negligible change. The graphene sheets in the flat film were stacked in a nearly face-to-face fashion, so the packing density was up to ∼1.55 g cm−3, combined with a large 1367

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generally conducive to the supercapacitance performance, so the robust film was expected to be an ideal candidate for binder-free electrodes (Figure S9). The electrochemical energy storage of graphene film electrodes was performed in 1 M H2SO4 with potential ranging from 0 to 0.8 V. Cyclic voltammetry (CV) experiments and charge/discharge (CD) behaviors of PPD−graphene film (12PE) were performed at scan rates ranging from 2 to 50 mV s−1 and current densities ranging from 0.5 to 20 A g−1 (Figure 3a,b). The nearly rectangular CV curves and triangular charge/discharge curves indicated an ideal efficient electrical double-layer capacitor (EDLC).26 The rectangular shape was still maintained when the scan rate increased to 50 mV s−1, indicating fast charging/ discharging kinetics. The slight deviation from ideal CV curves was primarily caused by the minor contribution from the redox reaction of oxygen-containing groups and physically adsorbed amine.12,18 Therefore, the capacitance mainly originated from the highly accessible surface area due to uniformly expanded spacing. We also studied the cycling performance of PPD− graphene film (12PE) electrode. First, the SEM images showed no obvious difference in thickness and morphology of films over 1000 charge/discharge cycles at a current rate of 5 A g−1 (Figure S10). Second, capacitive retention was in excess of 93% even after 5000 cycles (Figure 3c), demonstrating impressive stability of the electrodes. The inset exhibited the first and 5000th cycles with a very small time difference. The Coloumbic efficiency was over 91%, confirming that the outstanding performance was not due to parasitic reactions.8 Electrochemical impedance spectroscopy (EIS) was used to probe the electron transfer and ion transport properties over the frequency range from 10 mHz to 100 kHz at an open-circuit potential with an AC perturbation of 10 mV (Figure 3d). The PPD−graphene film (12PE) electrode had a very low equivalent series resistance of ∼0.23 ohms, which was obtained from the X-intercept of the Nyquist plot. This might be attributed to good wettability, suggesting good ion response.4 The nearly perpendicular line was shown at the low-frequency region, which indicated a nearly ideal capacitive behavior with a high conductivity and rapid ion diffusion.12 For determining the effect of PPD intercalation, three graphene film electrodes were prepared to present the difference of electrochemical performance, including a pristine graphene film, PPD−graphene film (E), and PPD−graphene film (12PE) (Figure S11 and Table S1). Figure 3e showed the CV curves of pristine graphene film and PPD−graphene films (E and 12PE) at the scan rate of 10 mV s−1. These near-rectangular characters of CV curves implied a very low contribution of pseudocapacitance, indicating it is close to the ideal EDLC. It was obvious that PPD-mediated graphene films exhibited much improved electrochemical performance compared to pristine graphene film. The volumetric capacitances showed exceptionally high values of 721 F cm−3 at scan rates of 2 mV s−1 and 711 F cm−3 at a current density of 0.5 A g−1 (Figure 3f,g), which are much higher than those of other two films and reported graphenebased EDLC in aqueous electrolytes (Table S2). A high capacitance of 672 F cm−3 was still maintained even at the current density up to 20 A g−1, presenting high stability at a large range of current density. The high capacitive rate performance is noteworthy and demonstrates great potential for applications as electrodes. The ultrahigh volumetric capacitive performance of film should be derived from the large accessible surface area, dense structure, interconnected molecular channels, and acceptable conductivity, guaranteeing

energy-dispersive X-ray spectroscopy (EDX) mapping of N atoms showed their uniform intercalation inside the film to separate the dense stacking of graphene sheets (Figure 2d). The thickness of film is about 5.5 um. The expanded spacing of (002) plane was also supported by XRD analysis (Figure 2e). The XRD pattern of GO film showed a peak at 10.6°, corresponding to an interlayer spacing of 0.83 nm. After PPD intercalation into the GO film, the spacing was slightly expanded to 0.9 nm. Because the ratio of PPD in the GO film was relatively low (Figure 2f) and most of the adsorbed molecules only lay on the surfaces of GO sheets, the interlayer spacing was slightly expanded, compared to the pristine GO film. As is known, the damp graphene film was obviously swollen along the thickness direction in the immersion and reduction process, so the adequate intercalation of PPD molecules was allowed to form chemical bonds between PPD and graphene. The appearance of strong peak at ∼8° suggested an additional separation between the graphene layers in the PPD−graphene film (12PE). The spacing of (002) plane was expanded to ∼1.1 nm, derived from the inclusion of abundant PPD molecules between the graphene layers. The spacing also suggests that the PPD molecules are vertically situated between graphene layers. The formed chemical bonds stitch the adjacent layers and further expand the interlayer spacing. However, the peak at ∼25° from compact pristine graphene nearly disappeared, indicating that the majority of graphene sheets did not stack back to graphite. The uniformly expanded spacing between the graphene layers is certainly a benefit for ion diffusion parallel to the graphene planes. Meanwhile, the interconnected channels could be favorable for ion diffusion into adjacent channels and speed up ion transport across the entire film (inset in Figure 2e). X-ray photoelectron spectroscopy (XPS) was performed to investigate the chemical composition of the graphene films. The PPD−graphene film (12PE) exhibited a very high nitrogen content of ∼14 at % (Figures 2f and S6). The calculated ratio of six-carbon rings from graphene and PPD molecule, respectively, was close to 3.7, which implied a high-density intercalation of PPD molecules in-between the graphene layers. Meanwhile, it showed much lower oxygen content. The C/O ratio increased from 2.42 for GO to 8.38 for the PPD−graphene film (12PE, deducting the contribution from PPD), indicating the effective reduction of GO in an amine solution. Deconvoluted N1s XPS contained two characteristic peaks at 399 and 401 eV, corresponding to −NH2 and −NH groups,22 respectively (Figure 2g). The chemical bonds expected for amino groups and graphene were more evident from the higher ratio of −NH group to −NH2 group after reduction, compared to the N1s signal of PPD−GO film. It obviously showed that most of the primary amino groups were covalently bonded to carbon atoms in graphene sheets. The chemical bonds were further confirmed by Fourier transform infrared spectroscopy (Figure S3), and then a schematic illustration shows the chemistry reaction process between the PPD molecules and functional groups on GO sheets (Figure S7). Due to the good solubility of PPD molecules in water, the modification of PPD also enhanced the surface wettability of graphene film. The contact angle of water on the composite film was measured to be ∼57.4° (Figure S8), which was lower than that of the pristine graphene films (∼85.1°). The good wettability is beneficial for the diffusion of aqueous electrolytes. These improved properties, including uniformly expanded spacing, good wettability, and pore interconnectivity, are 1368

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film electrodes with ultrahigh volumetric capacitance provide promising applications in portable electronic devices. Furthermore, the interfacial stitching strategy promotes new opportunities for binderless graphene films as high-performance supercapacitors and other energy-storage devices.

fast electron transfer and ion transport. Furthermore, the obtained graphene film breaks the traditional consideration that a compact graphene film was believed to restrict ion transport and possesses a limited capacitance. Because the thickness of film could be readily tuned by changing the concentration of amine or GO, or film-forming time, the interfacial stitching provides a general strategy to allow films of the required thickness. Interestingly, unlike the 2D Ti3C2 electrode,10 the capacitance did not pronouncedly degrade with increased thickness (or areal mass loading), which could be attributed to uniform alignment of sheets and efficient 2D ion transport channels that allowed ion to reach all accessible surface area (Figure 3h and S12). To eliminate the influence of press-dry treatment for the removal of large-size pores in the films, we obtained two PPD−graphene film (12PE) electrodes via different drying routes after reduction: press-dry and freeze-dry methods. Details of preparation are included in the Experimental Section and Table S1. Compared to the press-dry graphene film, the freeze-dry film was much thicker and had abundant honeycomb-like pores in it (Figure S13). The enormous difference of microstructures, however, conducted nearly comparative Cwt values (Figure S14). Because proton is the smallest cation and can easily diffuse into the modified graphene films, the removal of large-size honeycomblike pores did not limit the electrochemical energy storage performance in the aqueous electrolyte. Therefore, pore interconnectivity, not pore size, may play the dominant role in the advanced performance. Nevertheless, the fluffy structures pronouncedly reduced the density of film, resulting in a limited Cvol value, which further indicated the necessity of press-dry treatment to increase the Cvol value of PPD−graphene films. The volumetric energy density, Evol, and power density, Pvol, are two important values to characterize the electrochemical performance of supercapacitors. We calculated an Evol of 15.4 W h L−3 at a current density of 0.5 A g−1, giving a Pvol of 380 W L−3, which are much higher than those of PPD−graphene film (E) and pristine graphene film (Figure 3i). These values were sequentially improved at a larger voltage range (0−1 V). The calculated Evol and Pvol were as high as 24 W h L−3 and 500 W L−3 at 0.5 A g−1, respectively, which are higher than most of previously reported, high-performance graphene-based supercapacitors in aqueous electrolytes.10,12−14,27,28 For supercapacitor applications, these properties, combined with high Cvol, high rate performance, and good cyclic stability, are notable, thus making this PPD-intercalated graphene film supercapacitor potentially competitive against other electrochemical energy-storage devices. In summary, we have demonstrated a novel, ultrafast strategy for stitching GO sheets via PPD molecules into thicknesscontrolled PPD-intercalated GO films at the ethanol/water interface. This process is based on electrostatic interaction of negatively charged GO and positively charged PPD at the interface. After the reduction of the films in a PPD/ethanol solution, binder-free, compact PPD−graphene films (density: 1.55 g cm−3) are created with uniformly expanded interlayer spacing, good wettability, and interconnected molecular channels. The films can be directly applied as electrodes, and these structural features allow fast electron transfer and efficient ion transport. The graphene electrodes present extraordinary electrochemical properties, including ultrahigh volumetric capacitance (711 F cm−3), high rate performance, high volumetric power, and energy densities in aqueous electrolyte. Combined with a long cycling stability, the modified graphene

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b04035. Information on experimental section, zeta potential, FTIR spectra, contact angles, elemental compositions, SEM images, and electrochemical performance of graphene samples (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Ching-Ping Wong: 0000-0003-3556-8053 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We want to express our sincere thanks to Dr. Xiaoliang Zeng for the tensile test of films. We are thankful for the financial support from Advanced Research Project Agency-Energy (Grant DE-AR0000303), the NSFC (Grant 51102151, 51372143, 50990061, 21073107), Natural Science Foundation of Shandong Province (ZR2011EMQ002, 2013GGX10208), and Independent Innovation Foundation of Shandong University (2012GN051).

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REFERENCES

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