High-Yield Continuous-Flow Synthesis of Spheroidal C60@Graphene Composites as Supercapacitors.

Graphene spheres confining fullerene C60 are quantitatively formed under high-shear and continuous-flow processing using a vortex fluidic device (VFD). This involves intense micromixing a colloidal suspension of graphite in DMF and an o-xylene solution of C60 at room temperature in the absence of surfactants and other auxiliary substances. The diameters of the composite spheres, C60@graphene, can be controlled with size distributions ranging from 1.5 to 3.5 μm, depending on the VFD operating parameters, including rotational speed, flow rate, relative ratio of C60 to graphite, and the concentration of fullerene. An electrode of the composite material has high cycle stability, with a high areal capacitance of 103.4 mF cm-2, maintaining its capacitances to 24.7 F g-1 and 86.4 mF cm-2 (83.5%) at a high scan rate of 100 mV s-1.

Introduction

Carbon nanomaterials have emerged as key materials for future technology, for developing complex functional structures devoid of potentially toxic metals and metals, which are likely to have supply chain issues in the near future.[1] Graphene, as a single 2D planar sheet of graphite arranged in a hexagonal lattice,[2] is one of the newest forms of such nanomaterials. It has remarkable chemical, physical, and electrical properties, with high conductivity in the absence of defects and wrinkles, large specific surface area, and excellent mechanical flexibility,[1,3] and features in a growing number of applications. These include batteries, supercapacitors, catalyst supports, drug delivery, electrode materials for energy storage devices, membranes, biomedical devices, and coatings, and these relate to its multifarious and interdisciplinary properties.[1,3−7]

Graphene can be transformed into other structures including 0D nanoparticles (carbon dots), 1D nanofibers (carbon nanofibers and nanotubes), 2D nanosheets (graphene oxide), 3D nanostructures (hollow carbon nanospheres), fullerenes, and composites of these.[8−11] Among these different carbon nanomaterials, graphene spheres show promise in a wide range of applications covering energy storage including lithium ion batteries, separation systems, oxygen-reduction, catalyst supports, nanoreactors, and adsorption.[12−21]

The synthesis of graphene spheres with smooth surfaces and uniform sizes uses soft template processing,[22] hard templating (silicon template),[23] solvothermal techniques,[24] sol pyrolysis,[25] microemulsion polymerization,[26] hydrothermal reduction,[27,28] and gelation.[29] Related to this are extended oxidation strategies to alter the morphology and surface topography of graphene oxide (GO) nanosheets, as a potential route to build up 3D graphene architectures.[30,31] However, the synthetic strategies for gaining access to such architectures are limited by the high cost of production, the use of harsh chemicals or surfactants, high temperature processing as a high-energy penalty, and limited scope for scaling up the process.[5] A challenge is to develop a scalable, low-cost method that is high in green chemistry or sustainability metrics, for gaining access to 3D graphene-based materials, including graphene spheres and composites thereof.

To this end, we have explored the use of continuous-flow processing to prepare graphene spheres and graphene-fullerene composite spheres directly from graphite and fullerene C60, at room temperature, avoiding the use of auxiliary reagents. This involved the use of the in-house developed continuous-flow vortex fluidic device (VFD), which was developed early this decade.[4] It is a versatile microfluidic platform containing a dynamic thin film, in contrast to traditional channel based microfluidics, which can suffer from clogging. Mechanoenergy is delivered into the liquid in the VFD in a controlled way.[4] Under continuous-flow VFD processing, jet feeds deliver solution into an inclined rapidly rotating tube, opened at one end and usually housing a glass or quartz tube of 20 mm outside diameter (OD) and 16.000 ± 0.013 mm internal diameter (ID) (Figure ),[32] with the liquid whirling up and out of the tube. In the confined mode of operation of the VFD, a finite volume of liquid is processed in the tube, and this is effective in processing small volumes of liquid as well as a proven method for exploring the operating parameter space of the VFD for a particular process prior to extending the process into continuous flow.[33]

Figure 1

Schematic illustration of the overall method for fabricating the composite spheres under shear stress in a vortex fluidic device (VFD), using a 1:1 mixture of (a) an o-xylene solution of C60 and (b) a suspension of graphite in DMF, with the optimized conditions at a rotational speed (ω) = 4k rpm under continuous-flow conditions, (c) flow rate (ν̇) = 0.5 mL min–1, θ = 45°, concentration of graphite in DMF 1 mg mL–1, and concentration of C60 in o-xylene 0.5 mg mL–1, and (d) the resulting composite spheres.

The versatile VFD has a number of remarkable applications, encompassing probing the structure of self-organized systems, material processing, controlling chemical reactivity and selectivity, including enhancing enzymatic reactions, and more.[34] The first publication reporting the VFD was in 2012, describing the exfoliation of graphite along with hexagonal boron nitride, using the confined mode of operation of the device.[35] Since then, other forms of carbon have been fabricated in the device, including toroidal structures of self-assembled single walled carbon nanotubes,[36] laser aided slicing of single and multiwalled carbon nanotubes,[37] the formation of carbon dots (from multiwalled carbon nanotubes),[33] controlling the self-assembly of fullerene C60 molecules into nanotubules using water as an the antisolvent,[32] and the fabrication of fullerene C60 cones in a 1:1 mixture of o-xylene and DMF.[38] The VFD is also effective in controlling the nucleation and growth of palladium nanoparticles on graphene,[39] decorating palladium nanoparticles on carbon nano-onions,[40] and transforming graphene oxide into graphene oxide scrolls.[41]

We report a facile room temperature one-step synthesis of graphene spheres confining fullerene C60, directly from graphite dispersed in DMF and an o-xylene solution of the fullerene. Remarkably, the composite material, C60@graphene, is formed quantitatively with respect to graphite, with the ability to tune the size of the spheres while avoiding using auxiliary substances such as surfactants, which can affect surface properties of the resulting nanomaterials. To demonstrate an application of C60@graphene, its supercapacitor performance was evaluated using the spheres as electrodes, which delivered a gravimetric capacitance of 29.5 F g–1 at a scan rate of 5 mV s–1. While other types of graphene can reach such capacitance up to 250 F g–1,[42−45] the C60@graphene electrode has an areal capacitance of 103.4 mF cm–2, which is higher than values for other carbon derivatives, for example, in GF-CNT@Fe2O3 (53.56 mF/cm2 at 10 mA/cm2).[46] Moreover, the device reported herein can maintain its capacitances to 24.7 F g–1 and 86.4 mF cm–2 (83.5%) at a high scan rate of 100 mV s–1, establishing a high rate capability of the graphene sphere electrode and potential of the material in the next-generation energy storage devices.

Experimental Section

Materials and Chemicals

Graphite flakes of 99% purity were suspended in 99.5% dimethyformamide (DMF), and fullerene C60 (99.5% purity) was dissolved in o-xylene, with both materials and solvents purchased from Sigma Aldrich.

Sample Preparation and Materials Synthesis

Suspensions of graphite in DMF were prepared at different concentrations, namely, 1, 1.5, 2, and 2.5 mg mL–1, and then sonicated for 15 min followed by centrifugation for 30 min to remove undispersed graphite. Solutions of fullerene C60 were prepared in o-xylene at different concentrations, namely, 0.5, 1, 1.5, and 2 mg mL–1. Initially, as-received fullerene was added to o-xylene, and the mixture was allowed to stand at room temperature for 24 h, whereupon it was filtered (60 μm filter paper) to remove undissolved particles before mixing with graphite dispersed in DMF in the VFD using different jet feeds (Figure ). Operating parameter space for the VFD was systematically explored, in particular the rotational speed of the rapidly rotating glass tube, concentrations of graphite and fullerenes, flow rate, and ratio of the two solvents. After VFD processing, the resulting carbon material was collected via centrifugation at g = 1.751 rfc for 20 min.

Characterization

Samples of C60@graphene on silicon wafers were prepared by drop casting followed by evaporation under ambient conditions. The morphology, size, and shape of the particles and their properties were studied using a number of complementary techniques including scanning electron microscopy (SEM) with an accelerating voltage of 5 kV, operating at 10 mm working distance, transmission electron microscopy (TEM) conducted on a TECNAI 20 microscope operated at 120 and 200 kV, and X-ray diffraction (XRD) with the data collected using a Bruker ADVANCE D8 ECO, Co Kα, at an operating wavelength of 1.7988 Å with 2θ varied from 10 to 80°. Samples of the as-prepared material were formed by drying them in nitrogen at 55 °C for 3 days. They were then stored and kept at room temperature for 48 h. Raman spectroscopy was used to confirm structure integrity of the as-prepared carbon materials, with the spectra recorded using a Horiba XploRA apparatus at a fixed wavelength of 532 nm.

Electrochemical Characterization and Supercapacitor Fabrication

To fabricate the supercapacitors, the active material was mixed with carbon black and polyvinylidene difluoride (PVDF) in a mass ratio of 80:10:10. The resulting paste was pressed and attached to platinum foils as the current collectors, between which a piece of filter paper was sandwiched as a separator and 1.0 M H2SO4 was used as the electrolyte. All electrochemical tests including cyclic voltammetry (CV), galvanostatic charge/discharge (CD), and electrochemical impedance spectroscopy (EIS) were carried out using a two-electrode cell configuration by a CHI 760C electrochemical workstation.

Results and Discussion

Fabrication of C60@Graphene Spheres

Conventional methods for fabricating graphene/carbon spheres require complex multistep procedures, which are typically nonscalable. We find that a graphene-fullerene C60 composite is readily prepared in the VFD in the absence of other reagents and importantly in quantitative yield relative to graphite with the processing illustrated in Figure a. Under continuous-flow mode of operation in the VFD, one jet feed delivered an o-xylene solution of C60, with another jet feed delivering a suspension of graphite in DMF, both to the hemispherical base of the rapidly rotating glass tube. The operating parameters of the VFD were systematically varied to form the optimal product, as the most uniform in terms of morphology, size, and shape distributions of the spheres and highest yield. For this, the rotational speed was 4k rpm, the tilt angle of the glass tube θ was 45°, the concentration of C60 in o-xylene was 0.5 mg mL–1, the concentration of graphite in DMF was 1 mg mL–1, with a 1:1 ratio of the two solvents, and the flow rate for both liquids was 0.5 mL min–1. The tilt angle of the tube in the VFD was fixed at 45°, where there is unique complex fluid dynamics, which is the optimized angle for a plethora of processing applications of the device.[4]

Mechanism of Formation of the Composite Spheres

Intense micromixing DMF with o-xylene in a 1:1 ratio in the absence of graphite results in the formation of cones composed of approximately 0.5 to 2.5 μm diameter self-assembled particles of C60, also under continuous-flow conditions and with the optimal rotational speed also at 4k rpm.[38] As in the present study, the DMF acts as an antisolvent, resulting in the formation of nanoparticles of the fullerene, which are likely to adhere to the surface of the graphite flakes (Figure a), facilitating exfoliation by leveraging the graphene layers, which then wrap up the fullerene particles under high shear (Figure b). Then the process starts again on the resulting exposed graphene surface once a certain threshold of fullerene particles adheres to the surface and so on. This accounts for the fullerene particles being primarily confined within an outer layer of graphene sheets (Figure c). Processing in the absence of C60 (for a 1:1 mixture of o-xylene and DMF) affords exfoliated graphene, whereas the same processing for C60 in the absence of graphite results in the formation of particles of the fullerene <100 nm (Figure S1), which importantly are similar to the size of the fullerene particles present within the spheres. These were determined by sonicating the isolated spheres in DMF for 30 min (Figure d), with the resulting ruptured spheres containing particles of C60 in the range of 5 to 100 nm. These findings give credence to the proposed mechanism of the formation of particles of C60 adhering to the exposed graphene surface of graphite, presumably favored by π–π interactions between the two components. These are then transformed into spheres comprising at most a few layers of graphene, with the number of sheets of stacked graphene limited by the mechanical energy imparted in the dynamic thin film in the device.

Figure 2

(a–c) Proposed mechanism of the VFD synthesis of graphene spheres confining particles of self-assembled C60 and (d) SEM image of a fragmented sphere formed during sonication after VFD processing; scale bar 2 μm.

Morphology of the C60@Graphene Spheres

SEM images of different magnification of graphene sphere composites, formed under the above optimized conditions, are shown in Figure , along with a size distribution (inset, centered at 3.5 μm). The particles are regular spheres with smooth surfaces and are not highly aggregated. The spheres were also examined using TEM (Figure a,b), which revealed small particles of C60 ∼10 to 50 nm in diameter attached to the surface of the composite materials and other nanoparticles of comparable size unattached, with C60 particles on the inside. The presence of C60 particles on the surface of the spheres and inside the spheres is in accordance with the observation by SEM (Figure ). The high-resolution TEM image (Figure c,d) reveals that the graphene spheres have some C60 particles on the surface of the spheres as well as inside them.

Figure 3

(a–e) SEM images (from lower magnification to higher magnification) of C60@graphene, with a size distribution (inset), formed in o-xylene and DMF, under continuous-flow mode at a 1:1 ratio (ω = 7.5k rpm, concentration of C60 in o-xylene 0.5 mg mL–1, concentration of graphite in DMF 1 mg mL–1, flow rate ν̇ = 0.5 mL min–1 for both liquids entering the rotating tube in the VFD, and θ = 45°).

Figure 4

(a,b) TEM images of graphene spheres, (c,d) HRTEM images of graphene spheres formed in o-xylene and DMF, under continuous-flow mode at a 1:1 ratio at 4k rpm, concentration of fullerenes C60 in o-xylene 0.5 mg mL–1, concentration of graphite in DMF 1 mg mL–1, flow rate 0.5 mL min–1 for both liquids entering the rotating tube in the VFD, and θ = 45°.

We found that the morphology, size, and shape of the spheres varied on changing the rotational speed, flow rate of both liquids, and changing the concentrations of graphite and fullerene, as established using SEM. The optimized speed for generating smooth spheres was 4k rpm (Figure a–e). At much higher speed, 7.5k rpm, with the other parameters unchanged, the surfaces of the spheres are textured, but remarkably, the spheres now have a hole connecting the outer surface and inner surface of the tube (Figure S5). At 6k and 9k rpm, with the other parameters unchanged, the surfaces of the spheres are also textured and nonuniform (Figures S4 and S6). The ratio of the two solutions delivered to the VFD tube was initially set at 1:1, which was based on the success of this ratio in forming the fullerene cones[38] of comparable diameter to the spheres in the present study, the only difference being the absence of graphite. The initial flow rate of both solutions was fixed at 0.5 mL min–1, which was subsequently found to be optimal. Increasing the flow rates of both solutions shuts down the formation of spheres (Figure S3). Increasing the flow rate of DMF containing graphite up to 2.5 mL min–1 and the flow rate of the fullerene o-xylene solution to 2 mL min–1 produced dissimilar structures, with irregular spheroidal structures (Figure S2).

XRD patterns of the spheres are consistent with the material composed of graphite/graphene and fullerene C60[32] (Figure a–c). The spheres were collected immediately after VFD processing by centrifugation for 30 min, washed twice with hexane, and dried at 55 °C for 3 days. The major peaks of fcc fullerene C60 are at 2θ = 10.8, 17.7, and 20.8°, corresponding to the (111), (220), and (311), respectively, with the major peaks at 30.2, 10.8, and 60.8° corresponding to the (002) and (004) planes of graphite. Clearly, the XRD data establishes the formation of a hybrid composite material composed of fcc C60 and graphene. From the Scherrer equation, the crystal domains of fullerenes C60 were estimated to be 6.5 nm in diameter. The presence of graphene as one or a limited number of stacked sheets is consistent with shifts in the graphite diffraction pattern,[47] as well as consideration of the energetics of bending a finite number of sheets, as discussed above. Moreover, the absence of some graphite peaks can be attributed to assembled fullerene C60 on the surface of graphene.[47] Raman spectra (532 nm excitation) for the spheres prepared at the optimized conditions and as-received graphite and fullerene C60 before processing, for comparison, were recorded (Figure d–f). Graphite itself has three main peaks, namely, the D band, G band, and 2D band, at 1349, 1579, and 2717 cm–1, respectively,[48,49] and pristine C60 has the pentagonal pinch mode [Ag(2)] at 1469 cm–1 and additional Hg modes at 1420 and 1570 cm–1.[50,51] In contrast, the spheres have two Raman peaks at 1364 and 1600 cm–1, which can be attributed to the D band and G band, respectively, of graphene, in agreement with the literature.[50,52,53]

Figure 5

XRD patterns of (a) spheres prepared in the VFD, (b) graphite as-received, and (c) fullerene C60 as-received. Raman spectra of (d) spheres, prepared in the VFD, under continuous flow at a 1:1 ratio, 4k rpm rotational speed, concentration of fullerene 0.5 mg mL–1, graphite concentration 1 mg mL–1, flow rate 0.5 mL min–1 for both liquids, and tilt angle 45°, (e) as-received graphite, and (f) as-received C60.

Electrochemical Performance

The electrochemical performance of the composite material as a symmetrical supercapacitor is summarized in Figure . A C60@graphene electrode shows cyclic voltammetry (CV) with a nearly rectangular shape (Figure a), indicating an ideal capacitive behavior similar to commercial electrochemical capacitors. Figure a,b illustrates the CV curves at different scan rates (5 to 1000 mV s–1), revealing the stability and ability of the electrode to maintain the ideal rectangular CV shapes during operation over a wide range of scan rates. The response anodic and cathodic currents (recorded from the CV curves at 0.5 V) showed a linear relationship at different scan rates (Figure c) with R2 values of 0.9975 and 0.9979 for the charge and discharge curves, respectively.

Figure 6

(a) Cyclic voltammogram (CV) curves of graphene sphere composites formed using the VFD, under continuous flow at a 1:1 ratio, 4k rpm rotational speed, concentration of fullerene 0.5 mg mL–1, graphite concentration 1 mg mL–1, flow rate 0.5 mL min–1 for both liquids, and tilt angle 45°, at different scan rates from 5 to 100 mV s–1, (b) CV curves at different scan rates from 200 to 1000 mV s–1, (c) response anodic and cathodic currents, (d) charge/discharge curves of graphene sphere composites at different current densities, (e) gravimetric capacitance and areal capacitance values versus scanning rate calculated from the CV curves of the graphene sphere composites, and (f) Nyquist plot of graphene sphere composites.

The galvanostatic charge/discharge (CD) curves at different current densities (Figure d) show nearly triangular shapes of the CD curves due to the efficient EDLC nature of the graphene sphere electrode and fast kinetics of the electrolyte ion transport through the porous carbon electrode structure. Additionally, the low equivalent series resistance (ESR) of the fabricated device can be predicted from a small voltage drop (IR) of 0.0075 V at the start of the discharge curve measured at a current density of 1 A g–1. Figure e displays the calculated gravimetric and areal capacitances for the graphene sphere electrode at different scan rates, delivering a gravimetric capacitance of 29.5 F g–1 at a scan rate of 5 mV s–1, compared to different types of graphene spheres.[54] This value is lower due to the absence of residual oxygen-containing functional groups than reduced graphene oxide.[55,56] However, the graphene sphere electrode has an areal capacitance of 103.4 mF cm–2, which is much higher than reported values for other carbon derivatives.[46,57] Moreover, this device can maintain its capacitances to 24.7 F g–1 and 86.4 mF cm–2 (83.5%) at a high scan rate of 100 mV s–1, confirming the high rate capability of the graphene sphere electrode.

A Nyquist plot of the graphene sphere electrode (Figure f) shows a vertical curve at lower frequencies and no observed semicircle higher-frequency region, indicating a nearly ideal capacitive behavior of the cell. The device shows also a lower ESR of 0.5 Ω owing to the low internal resistance of the designed electrode material. The impedance phase angle versus frequency plot for the graphene sphere electrode is illustrated in Figure a. The device provides a phase angle of −83.8° (close to −90°) at low frequencies, showing an ideal capacitive behavior. Furthermore, the time constant τ0 at a phase angle of −45° is found to be 0.16 s; this fast frequency response of the graphene spheres is directly related to a significant ion transport rate of the large accessible surface area of the material. The cyclic stability of the composite electrode was examined by cyclic voltammetry at a scan rate of 100 mV s–1 for 5000 cycles (Figure b), showing a capacity retention of 97.5%. The nearly identical CV curves for first and last cycles (Figure b, inset) confirm the good stability and high reversibility of the C60@graphene electrode.

Figure 7

(a) Impedance phase angle versus frequency for a C60@Graphene sphere electrode, with the material formed using the VFD, under continuous flow at a 1:1 ratio of liquids, 4k rpm rotational speed, concentration of fullerene 0.5 mg mL–1, graphite concentration 1 mg mL–1, flow rate 0.5 mL min–1 for both liquids, and tilt angle 45°. (b) Cyclic stability of the composite electrode at a scan rate of 100 mV s–1; the inset is the nearly identical CV curves for first and last cycles of the electrode.

Conclusions

A simple and effective method to fabricate all carbon spheres built of fullerene C60 and graphene, under continuous-flow mode of operation of the VFD microfluidic platform, has been developed. The structure and the morphology of the spheres were evaluated by SEM, TEM, HRTEM, XRD, and Raman spectroscopy. The spheres are rather uniform in size and shape. Importantly, the yield of graphene spheres is high, and the processing is scalable. The method has potential for the synthesis of spheroidal composite materials of fullerenes with other 2D materials. In addition, the use of the composite material of assembled spheres in an electrochemical device and in supercapacitance, with high capacitance maintained at a high scan rate, has potential in developing all carbon energy storage materials.