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Structure-Controlled, Vertical Graphene-Based, BinderFree Electrodes from Plasma-Reformed Butter Enhance Supercapacitor Performance Dong Han Seo, Zhao Jun Han, Shailesh Kumar, and Kostya (Ken) Ostrikov*

uninterruptible power supplies, pacemakers, consumer electronics, to hybrid electric vehicles and heavy load levelling.[3] However, for practical usages supercapacitors also need to satisfy a few important criteria, including high specific capacitance, stable charge and discharge property, large capacitance retention, as well as cost-efficient and environmentally-friendly fabrication.[4] These criteria to a large extent are determined by the structure, morphology, reactivity and binding of electrode materials in the fabrication process. Recently, vertical graphene nanosheets (VGNS) showed great promise as supercapacitor electrodes due to their excellent electrical conductivity, large surface area and in particular, an inherent threedimensional (3D), open network structure with graphene flakes oriented perpendicularly to the electrode surface, in contrast to the horizontal graphene.[5] Such structural and morphological advantages are expected to significantly enhance the capacitive mechanism of charge storage through increased ion diffusivity and ion accessibility. Indeed, previous studies have shown that VGNS-based electrodes had exceptionally high power density and stable performance that are desirable for applications such as miniaturized electronics and alternating current (ac) line filtering.[5–7] However, so far VGNS electrodes have not materialised their promises due to the low specific capacitance, which was only a few times larger than that of the commercial aluminum electrolytic capacitors of a similar size.[5] VGNS are commonly produced using hazardous, expensive purified hydrocarbon gases in a high-temperature environment (700–1000 °C) with long fabrication time (20–45 min). It remains unclear on how to control the capacitance by adjusting the structural and morphological features of VGNS in these processes. Moreover, non-conductive polymeric binders are often needed to integrate VGNS and other nano-carbon structures,[8] which inevitably results in increased electrical resistance and reduced power and energy densities. On the other hand, performance of carbon-based supercapacitor electrodes can be further improved by integrating metal oxide nanoparticles which store electrical charge through fast and highly reversible surface redox (Faradaic) reactions.[9]

Vertical graphene nanosheets (VGNS) hold great promise for high-performance supercapacitors owing to their excellent electrical transport property, large surface area and in particular, an inherent three-dimensional, open network structure. However, it remains challenging to materialise the VGNSbased supercapacitors due to their poor specific capacitance, high temperature processing, poor binding to electrode support materials, uncontrollable microstructure, and non-cost effective way of fabrication. Here we use a single-step, fast, scalable, and environmentally-benign plasma-enabled method to fabricate VGNS using cheap and spreadable natural fatty precursor butter, and demonstrate the controllability over the degree of graphitization and the density of VGNS edge planes. Our VGNS employed as binder-free supercapacitor electrodes exhibit high specific capacitance up to 230 F g−1 at a scan rate of 10 mV s−1 and >99% capacitance retention after 1,500 chargedischarge cycles at a high current density, when the optimum combination of graphitic structure and edge plane effects is utilised. The energy storage performance can be further enhanced by forming stable hybrid MnO2/VGNS nano-architectures which synergistically combine the advantages from both VGNS and MnO2. This deterministic and plasma-unique way of fabricating VGNS may open a new avenue for producing functional nanomaterials for advanced energy storage devices.

1. Introduction Owing to continuous increase in the demand for energy stoarge, supercapcitors have attracted strong attention due to their distinct advantages such as high power density, long lifetime, and fast charge and discharge capability.[1,2] Supercapacitor is an ideal energy storage device to complement or replace batteries and fuel cells in various applications ranging from

D. H. Seo, Dr. Z. J. Han, Dr. S. Kumar, Prof. K. Ostrikov Plasma Nanoscience CSIRO Materials Science and Engineering P.O. Box 218, Lindfield, New South Wales 2070, Australia E-mail: [email protected] D. H. Seo, Prof. K. Ostrikov Plasma Nanoscience@Complex Systems School of Physics The University of Sydney New South Wales 2006, Australia

DOI: 10.1002/aenm.201300431

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Because of extra charge into the nanoparticles, metal oxide-based electrodes usually offer a higher capacitance (known as the pseudocapacitance) than those based on pure carbon; for instance, the theoretical capacitance of ruthenium oxide (RuO2) and manganese oxide (MnO2) can achieve ∼1000 and ∼1370 F g−1 respectively.[3] However, the synergistic and stable integration of metal oxide nanoparticles into carbon-based electrodes is challenging, mainly due to their poor electrical conductivity and weak adhesion between carbon and metal oxide nanostructures, which often lead to structural instability during the electrochemical redox reactions.[10,11] Figure 1. Growth of vertical graphene nanosheets (VGNS) using natural precursor butter on In this work, we demonstrate a simple, the flexible, porous Ni foam. (a-c) Illustration of how butter is transformed into VGNS strucfast (less than 10 min), single-step and cost- ture using the plasma-enabled process: (a) photograph of butter pasted on porous Ni foam, efficient method to directly fabricate VGNS (b) radio-frequency (RF) inductively-coupled plasma in operation with Ar/H2 gas mixture, and on porous nickel (Ni) foam without non-con- (c) the grown VGNS. (d) Photograph of bent VGNS on Ni foam, demonstrating the flexibility ductive polymeric binders in a low tempera- of the material. SEM images of (e) pristine porous Ni foam and (f) VGNS grown with 80% H2, ture environment (400 °C). Instead of expen- showing that dense graphene nanosheets uniformly covered on the top of Ni foam after the plasma-assisted growth process. sive and hazardous hydrocarbon gases, the synthesis of VGNS uses cheap, spreadable and environmentally-benign natural precursor butter (mostly parameters such as the plasma pretreatment, power, bias, gas consisting of butterfat, milk proteins and water) in a plasmacomposition and flow rate, a variety of morphology and conassisted chemical vapor deposition (CVD) process. We show ductivity of VGNS could be produced using methane gas as that the 3D and open structure of VGNS gives rise to high the precursor.[12,13] Here, we extend the fabrication of VGNS capacitance and excellent cycle stability of the electrodes (>99% by using spreadable and environmentally-benign natural precapacitance retention after 1,500 cycles). The highest specific cursor, butter (see Experimental Section). This single-step, capacitance attained reaches 230 F g−1 at a scan rate of 10 mV s−1 efficient process of transforming solid-state butter into VGNS in low electrolyte concentration of 0.1 M sodium sulfate arises from the unique ability of the plasma,[14] which can break (Na2SO4), which is among the highest values of similar strucdown virtually any hydrocarbon-containing precursors in a diftures reported before. The capacitance can further be conferent state of matter (i.e., solid, liquid, or gas) and re-build trolled by tailoring the sp2 graphitic content and the thickthem into functional nanostructures.[15] More importantly, no ness of the edge planes in the VGNS. Moreover, stable and metal catalyst was required in the synthesis, thus avoiding the synergistic integration of metal oxide nanostructure to VGNS multi-step process of separating or removing impurities prior is achieved using electrochemical deposition of MnO2 nanoto device fabrication. flowers. The specific capacitance is enhanced up to 280 F g−1 The synthesis is schematically illustrated in Figures 1a-c. A through the formation of hybrid MnO2/VGNS structure and thin layer of butter was firstly pasted on the porous Ni foam the excellent cycle stability is retained (>97% capacitance retenwith a porosity of ∼95% and then loaded directly into the tion after 1,000 cycles). We also compare the electrochemical radio-frequency (RF) inductively-coupled plasma chamber. properties of conventional VNGS produced from purified The porous Ni foam provided a large volume to accommomethane and other readily available natural precursors such as date a high density of VGNS and an inter-connected conduchoney and milk. Our results show that the VGNS derived from tive network to ensure good current-collecting property.[16,17] butter gives the most reliable and promising performance. The During the growth, a gas mixture of Ar and H2 was fed into presented deterministic and plasma-unique way of fabricating the chamber at a constant pressure of 2.5 Pa and RF power VGNS may thus open a new avenue for fabricating VGNS of 1000 W. No external substrate heating was applied yet the with controllable characteristics for future VGNS-based energy temperature could reach 400–450 °C due to the plasma-heating storage devices. effect.[12] The VGNS was subsequently obtained and the vertical orientation was a result of the electric field-guided growth in the plasma sheath.[14] The area of VGNS can be easily pre-deter2. Results and Discussion mined by the size of butter pasted on the Ni foam (Figure 1c). In addition, the VGNS show excellent flexibility with retained 2.1. Plasma-Enabled Growth of VGNS Using Butter structural integrity after numerous cycles of bending (Figure 1d).[18] VGNS composed of graphene nanosheets with 3–15 graphitic Figures 1e-f show the typical scanning electron microscopy layers are synthesized in a plasma-assisted CVD process (SEM) images of VGNS grown with 80% H2, where dense (Figure 1). Previously we showed that by adjusting the growth and uniform structure of VGNS was observed to cover the

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entire Ni foam. The 3D, binder-free VGNS residing on the highly-conductive, porous Ni foam possess good electrical conductivity and large surface area with a minimal tortuosity. The strong binding of VGNS to Ni foam also helps avoiding the agglomeration of graphene nanosheets commonly occurred in wet chemical processing, thus enabling a good ion diffusivity and a fast and stable electric double layer (EDL) formation.[19] The fabrication process can be easily scaled up with excellent controllability and reproducibility. Importantly, the equivalent thermal processes failed to transform butter into the VGNS structures. We have conducted thermal annealing of butter at both high (900 °C) and low (450 °C) temperatures with the same gas composition as in the plasma-assisted CVD processes. However, either carbon fibres or porous carbon were obtained at low temperatures and amorphous carbon was obtained at high temperatures, with no characteristic features related to the graphene nanosheets (Supplemen- Figure 2. Controllable graphitic structure and edge planes in VGNS. (a) SEM image of VGNS tary Figure S1). This is not surprising as grown at 80% H2 showing a high density of thin edge planes transparent to the electron beam. thermal process could only decompose the (b) Raman spectra of VGNS grown at (1) 40% and (2) 80% H2. The Raman active D, G, and 2D bands are labeled in the spectra. (c-d) XPS C 1s spectra of VGNS grown at (c) 40% and (d) 80% carbon-containing precursors into the basic H . Three peaks in each spectrum are assigned to the sp2 carbon, sp3 carbon, and the energy 2 carbonaceous building blocks at a very high loss “shake-up” feature. Difference between the raw data and the fitted peaks are also plotted. energy cost, and is unable to re-organize these carbon building blocks into the vertical orientation in the absence of catalyst nanoparticles.[14] The was obtained, i.e, a higher sp2 carbon content (Supplementary results from these complimentary thermal processes thus conFigure S2). The ID/IG ratios were 2.1 and 1.1 for H2 concentrafirmed the efficiency and unique advantages of the plasma in tion at 40% and 80%, respectively. Further increasing the H2 producing these functional nanostructures. concentration to 90%, however, tended to increase the disorder in VGNS (ID/IG = 1.37). Moreover, the thickness of the exposed edge planes also followed a similar trend, i.e., they gradually became thinner at up to 80% H2 and then turned to be thicker 2.2. Controllable sp2 Content and Edge Planes in VGNS at 90% H2 (Supplementary Figure S2). The above Raman analysis of sp2- and sp3-bonded carbon in We also varied the H2 concentration in the plasma-assisted the VGNS was confirmed by X-ray photoelectron spectroscopy process to tailor both the graphitic structure and morphology (XPS) measurements, as shown in Figures 2c-d. In the XPS of VGNS. Figure 2a shows the high-resolution SEM image of C1s spectra, two peaks centered at binding energies of 284.5 VGNS synthesized at 80% H2, where a high density of edge and 285.3 eV were assigned to the sp2 and sp3 carbon content, planes was identified. These edge planes were thin enough respectively, and a broad peak at binding energy of ∼290 eV was to be transparent to the low-energy electron beam. Figure 2b due to the energy loss “shake-up” features.[22] The ratio of XPS compares the Raman spectra of VGNS grown at both 40% peak intensity between sp2- and sp3-bonded carbon was signifand 80% H2. Three characteristic peaks of VGNS, named D, icantly increased in VGNS grown from 40% to 80% H2 conG and 2D, were clearly observed in both spectra. The D-peak centration, indicative of a better graphitic ordering. Since no appeared at ∼1350 cm−1 is due to the A1g mode breathing vibraoxygen was detected in either spectrum (the detection limit of tion of hexagonal sp2-carbons and becomes Raman active after XPS was ∼1 at.%; Supplementary Figure S3), it is implied that neighboring sp2-carbons are converted into sp3-hybridization. the type of defect was likely to be the amorphization of carbon The G-peak at ∼1585 cm−1 is associated with the E2g in-plane structure.[21,23] vibrational mode of sp2-bonded carbon; and the 2D-peak at We therefore demonstrated the systematic control of the ∼2690 cm−1 is a second order vibration caused by the scattering graphitic structure and edge planes in VGNS by varying the of phonons at the zone boundary.[20] Typically, the ratio of ID/ H2 concentration in the plasma-assisted CVD process. H2 has IG is used to determine the defect level in graphitic structure been shown to play competing roles in the nucleation and of carbonaceous materials while the ratio of I2D/IG is used to growth of many carbon-based nanostructures.[24,25] Under the reflect the thickness of individual graphene sheets.[12,21] As present plasma-assisted conditions, carbon building units are shown in Figure 2b, we observed that by varying the concenfirst extracted from butter and then re-form into the hexagonal tration of H2 up to 80%, better graphitic ordering of VGNS



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carbon-rings, which are the basic elements for constructing graphene nanosheets. H2 at a low pressure can enhance the surface flux of these building units through the recombination-mediated energy dissipation on the surface,[25] thereby facilitating the nucleation and growth of VGNS.[26,27] As the H2 concentration is increased, this constructive effect becomes stronger and the graphitic quality becomes higher. At an even higher H2 concentration, however, the etching effect of H2 emerges and becomes dominant,[28] which can create defects and etch away the graphitic structures in the grown VGNS. As a result, the graphitic ordering of VGNS is reduced and only thick edge planes are produced at a high H2 concentration. This interplay of the constructive and etching effects of H2 thus enables one to obtain the VGNS with the optimum combination of graphitic structure and thin edge planes. 2.3. Electrochemical Performance of VGNS Electrode The supercapacitive performance of controlled VGNS structures is then evaluated in a three-electrode cell configuration (see Experimental Section). Figures 3a-b show Figure 3. Electrochemical performance of VGNS derived from butter. Cyclic voltammetry (CV) the cyclic voltammetry (CV) curves of two curves of VGNS grown at (a) 40% and (b) 80% H2 at varied scan rates. Galvanostatic charge/ VGNS electrodes grown at 40% and 80% H2 discharge curves of VGNS grown at (c) 40% and (d) 80% H2 at a current density of 1.5 A/g. (e) in 0.1 M Na2SO4 aqueous electrolyte, respec- The Nyquist plots for VGNS grown at 40% (black) and 80% (red) H2. Inset shows the magnified tively. One can clearly notice that both elec- high-frequency region. (f) The specific capacitance and cycle stability of VGNS grown at 40% trodes showed a nearly rectangular shape (black) and 80% (red) H2. Current values above zero in (b) are positive. in CVs at varied scan rates, suggesting the formation of electric double layer and fast ion transport in the VGNS structures.[19] Figures 3c-d show the The specific capacitance C shown in Figure 3f was calculated charge and discharge curves of the two electrodes at a constant from the CV curves by integrating the discharge current against current of 1.5 A g−1, respectively. A linear dependence between the potential V.[33] While the specific capacitance for VGNS the discharge potential and time was identified, implying the grown at 40% H2 was only ∼20 F g−1 at a scan rate of 100 mV s−1, [ 29 ] absence of major Faradaic processes. it increased drastically to 112 F g−1 for VGNS grown at 80% Figure 3e shows the electrochemical impedance spectra (EIS) H2. Furthermore, the specific capacitance for VGNS grown for VGNS, where the frequency-dependent impedance is given at 80% H2 could notably reach 230 F g−1 at a scan rate of as real (Z′) and imaginary (Z″) components in the Nyquist plot. 10 mV s−1 (Supplementary Figure S2). This value is among A vertical curve was featured at the low frequencies, indicating the best of various 3D graphene-based electrodes (exfoliated or a nearly ideal capacitive behaviour.[22] In the high-frequency reduced),[34,35] which have the reported high-end capacitance of range (inset of Figure 3e), a semicircle intersected with the real ∼200 F g−1 at similar scan rates (Supplementary Table S1). It is (Z′) axis in both spectra, which was attributed to charge transfer also an order of magnitude larger than that of VGNS obtained at the electrode-electrolyte interface.[30] As the intersection was with uncontrolled graphitic structure and edge planes.[5] In lower in the VGNS grown at 80% H2, the VGNS with a higher addition, the cycling tests in Figure 3f indicated that both samples retained >99% of their initial capacitance after 1,000 sp2 graphitic content implied a lower charge transfer resistcycles at the scan rate of 100 mV/s. The charge-discharge tests ance. These results are consistent with the small potential drop were also performed and a consistently high capacitance retenobserved at the top of discharge curves (Figures 3c-d), which tion (>99%) was obtained after 1,500 cycles even at a high was an indication of a low equivalent series resistance (ESR). current density (see Supplementary Figure S4). These results Notably, this range of electrical resistance is similar to other thus demonstrated the reliable long-term performance of the graphene-based electrodes processed by the common thermal electrodes. or chemical methods.[7,31,32]

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While the three-electrode cell configuration is commonly used for determining the electrochemical properties of a particular electrode material, we also measured the capacitance of VGNS in a two-electrode cell configuration, which provides better comparison with other materials in applications.[36] As shown in Supplementary Figure S5, the specific capacitance measured from the two-electrode cell configuration was approximately half of the capacitance from the threeelectrode cell configuration for the respective electrodes, which were in an excellent agreement with the theoretical analysis.[36] 2.4. Enhanced Capacitance by MnO2 Deposition We further coated MnO2 nanoflowers on top of the VGNS surfaces to combine the advantageous features from both VGNS and metal nanoparticles for supercapacitors (see Experimental Section). MnO2 was chosen because of its high capacitance, natural abundance, relatively low cost, safety and environmental friendliness.[37] Figure 4a shows the SEM image of MnO2 nanoflowers uniformly decorated on VGNS. These MnO2 nanoflowers contain many platelet-like edges, which may facilitate fast ion kinetics and also show no significant structural re-arrangements during the electrochemical redox reactions.[38] Figure 4b shows the Raman spectra of the hybrid structures based on VGNS grown at 40% and 80% H2. One can see that while the peaks associated with MnO2 nanoflowers emerged in the range of 400–600 cm−1,[39] the D, G and 2D peaks, as well as the ID/IG ratio of bulk VGNS remained unchanged. These results indicate that the structure of VGNS Figure 4. Enhanced performance of the hybrid MnO2/VGNS structures. (a) Typical SEM image was not altered after the electrodeposition. Figures 4c-d show the corresponding CV of MnO2 nanoflowers deposited on the VGNS. (b) Raman spectra of MnO2 deposited on the VGNS grown at (1) 40% and (2) 80% H2. Inset shows the characteristic Raman peaks of curves of hybrid MnO2/VGNS electrodes at MnO2. CV curves of the hybrid MnO2/VGNS grown at (c) 40% and (d) 80% H2 at varied scan varied scan rates. It is noticeable that the CV rates. Galvanostatic charge/discharge curves of the hybrid MnO /VGNS grown at (e) 40% 2 curves deviated from the rectangular shape, and (f) 80% H2 at a current density of 1.5 A/g. (g) The Nyquist plots for MnO2/VGNS hybrids evidencing the pseudocapacitive behaviour grown at 40% (black) and 80% (red) H2. Inset shows the magnified high-frequency region. due to the presence of MnO2.[40] Figures 4e-f (h) The specific capacitance and cycle stability of MnO2/VGNS hybrids grown at 40% (black) show the charge and discharge curves of both and 80% (red) H2. electrodes at a constant current of 1.5 A g−1. The curves showed a stable and linear potential-time relationafter the deposition of MnO2. The capacitance had increased to ship similar to those of bulk VGNS. Moreover, Figure 4g shows ∼122 and ∼146 F g−1 for MnO2 decorated on VGNS grown at the Nyquist plots for the MnO2/VGNS structures. It was found 40% and 80% H2, respectively. At a scan rate of 10 mV s−1, the that the semicircle intersecting with the Z’-axis was smaller for highest capacitance achieved was ∼280 F g−1, as measured from the MnO2/VGNS grown at 80% H2 (inset of Figure 4g), indithe hybrid MnO2/VGNS grown at 80% H2. Given the density of cating that a better charge transfer was induced. MnO2 in the hybrid structure (see Supplementary Figure S6b), Figure 4h shows the specific capacitance and cycling perthis corresponded to an estimated specific capacitance of formance of the MnO2/VGNS electrodes. As compared with 1060 F g−1 for MnO2, close to its theoretical value.[37] More pure VGNS, the capacitance of both electrodes was enhanced importantly, both electrodes retained a remarkable >97% of the

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2.5. Role of sp2 Content and Edge Planes Let us now discuss why the capacitance of VGNS-based electrodes can be controlled by the graphitic content and morphology of the VGNS. Given that C = εA/d (where ε is the electrolyte dielectric constant, A is the electrode surface area accessible to the electrolyte ions, and d is the effective thickness of the double layer), electrode materials with a larger surface area therefore give higher capacitance. In the case of VGNS, the edge planes were exposed and could be directly accessible to electrolyte ions, providing a large surface area for charge storage. It was also shown that the edge planes had a much larger specific capacitance (50–70 μF cm−2) as compared with that of the basal plane surface (∼3 μF cm−2).[5] As a result, a high density of sharp edge planes in the open, uniform 3D structure of our VGNS structures greatly increased the specific capacitance. The sp2 graphitic structure of VGNS also plays a major role in enhancing their charge storage capability. Indeed, sp2bonded carbon is preferred over sp3-bonded carbon in graphene-based supercapacitor electrodes, as the latter increases the charge transfer resistance and could hardly contribute to charge storage. For example, Zhu et al. reported a continuous 3D network of reduced graphene oxide electrode with 98% sp2 bonding, which yielded high values of gravimetric capacitance.[22] El-Kady et al. also demonstrated a high capacitance of laser-scribed highly-porous reduced graphene oxide with excellent conductivity.[19] These studies confirmed that a large fraction of highly-porous carbon with a high sp2 bonding, a high C/O atomic ratio and a low H content or dangling bonds, was desirable for high capacitance. It was also shown that the 3D graphene sheets could have a theoretical capacitance of ∼550 F/g if the entire sp2-bonded carbon surface area is exposed.[19] Next, we discuss the relationship between the structure of graphene-based electrodes and the specific capacitance. It is known that a good electrical conductivity is favored for supercapacitor electrodes.[42,43] However, a certain degree of structural disorder (such as mesoporous and microporous structures) is also needed for the eletrochemically active materials. Such disorder could provide a higher surface area for the enhanced surface ion adsorption, leading to a higher capacitance.[32,44,45] Thus, a trade-off between the structural quality and the specific capacitance of graphene-based electrodes is necessary for achieving high-performance supercapacitors. The Raman ID/IG ratios in the present VGNS had a range similar to the graphene nanosheets produced by the thermal or microwave methods.[22,46] It is, however, slightly higher as compared to that produced using methane (CH4) as the precursor.[5] Methane is known as a purified carbon precursor for graphitic nanostructures due to its high thermal stability. The

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relatively high quality and good electrical conductance of VGNS produced from CH4 have enabled an excellent rate capability of the electrodes.[5] In contrast, we used less stable butter as the natural precursor, which inevitably led to a slightly higher Raman ID/IG ratio. However, the VGNS produced from butter showed tunable edge planes and controllable sp2 graphitic content. This level of control, to the best of our knowledge, has not been reported in the literature. We thus believe that the VGNS produced from butter can find applications for a variety of supercapacitor devices.

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initial capacitance after 1,000 cycles (Figure 4h). This excellent cycle stability obtained in the present hybrids was attributed to the 3D and open structure of VGNS and the strong interactions between the MnO2 nanoflowers and the underneath edge planes of the VGNS, which allowed the highly reversible reaction of MnO2 with little lattice distortions, phase transformations, or dissolution of Mn ions into the electrolyte.[41]

2.6. Comparison with Other Precursors We also fabricated VGNS on porous Ni foam using purified methane gas as the precursor. SEM images showed that the number density of thin edge planes was significantly lower for VGNS produced from methane as compared to those from butter (Supplementary Figures S7-S8). Moreover, it was found that by varying the H2 concentration, the highest capacitance of VGNS produced from methane was ∼40 F g−1 at a scan rate of 100 mV s−1. This capacitance was significantly lower than that for VGNS produced from butter, demonstrating the efficiency of using naturally-derived butter for high-performance electrode materials. In addition, we also compared the supercapacitive performance of VGNS produced from other readily available natural precursors such as honey and milk.[15] Our results showed that the VGNS derived from butter had the highest capacitance and the best cycle stability. In contrast, the VGNS produced from honey and milk showed poor adhesion with the Ni foam support, which caused the nanostructures to be unstable during the electrochemical tests (Supplementary Figure S9).

3. Conclusion In summary, we have demonstrated a green, single-step, plasma-assisted route to produce VGNS supercapacitor electrode structures directly on porous Ni foam using natural precursor butter. We found that along with the increase in H2 concentration (up to the optimum value), the graphitic ordering in the VGNS increased and the thickness of edge planes decreased. The controllable VGNS structures showed great promise as binder-free electrode materials for supercapacitor applications. A high specific capacitance of 230 F g−1 was obtained at a low concentration of electrolyte, which is one of the highest values reported using pure carbon-based nanostructures. Decoration of the VGNS with MnO2 further demonstrated that the hybrid nano-architecture could enhance the capacitance while retaining the cycle stability. Our VGNS structures thus provide a set of desirable features as the platform for next-generation supercapacitors, which include: i) the open, uniform 3D structure with large surface area enables the accessibility of electrolyte and fast ion kinetics; ii) the high-density of thin edge planes with a high sp2 graphitic content provides a good electrical conductivity and superior charge storage capability; iii) the strong adhesion between the MnO2 and the edge planes of VGNS provides the stability for



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the hybrid nano-architecture and accommodates the structural changes during pseudocapacitive charge storage. Competitive advantages of performance of our VGNS-based supercapacitor electrodes compared to other recent reports are summarized in Supplementary Table S1. Our fabrication process of VGNS involved only inexpensive and environmentally-benign precursors and is easily scalable. These results may advance the utilization of graphenes in high-performance supercapacitors and contribute to a better control of the charge storage processes at the molecular and atomic levels, which are critical for the development of smaller and more powerful energy storage devices.

configuration used symmetric VGNS electrodes in a split flat cell test module (EQ-STC, MTI Corp.). Cyclic voltammetry (CV), galvanostatic charge-discharge and electrochemical impedance spectroscopy (EIS) measurements were conducted using BioLogic VSP 300 potentiostat/ galvanostat. CV tests were swept in the potential range of 0 V to 0.8 V. Galvanostatic charge-discharge curves were obtained at a constant current density of 1.5 A g−1. EIS measurements were performed in the frequency range of 0.01 Hz to 100 kHz. The specific capacitance C was calculated from the CV curves by integrating the discharge current I against the potential V according to C = IdV/v m V , where v is the scan rate (V s−1), m is the mass of VGNS, and ΔV is the operating potential window (0.8 V).

Supporting Information 4. Experimental Section Synthesis of vertical graphene nanosheets (VGNS): The deposition of VGNS was carried out in a radio-frequency (RF) inductively coupled plasma CVD system. Porous Ni foam with a porosity of ∼95% was used as the substrate without any catalyst. The Ni foam was firstly pasted with butter prior to be loaded in the reactor. We chose butter as the natural precursor for VGNS because it is cheap, readily available, environmentally friendly, and easily spreadable on any substrate surface. A gas mixture of Ar and H2 was then fed into the system for growing VGNS. The power of RF plasma was at 1000 W and the growth lasted for 9 min. The samples were placed in the centre of the plasma generation zone. No external substrate heating was used during the growth process yet the substrate temperature was estimated to be 400–450 °C due to the plasma-heating effects. To investigate the effect of H2 concentration, we varied the concentration from 40% to 90% by keeping the pressure constant at 2.5 Pa. For other natural precursors such as honey and milk, the Ni foam was firstly dipped into the liquid precursor solutions. Identical plasma-assisted growth conditions were then adopted. The VGNS was also grown using purified methane gas as the precursor. A gas mixture of Ar, CH4 and H2 at a constant pressure of 4 Pa and RF plasma at a constant power of 1000 W was implemented for the growth of VGNS. MnO2 flower deposition: MnO2 flower nanostructures were deposited on VGNS using the electrodeposition method, where the electrolyte was 25 mL of 1 M Na2SO4 (Sigma Aldrich) mixed with 25 mL of 1 M MnSO4 (Sigma Aldrich) solution. When the VGNS and a Pt reference electrode were immersed in the electrolyte, the voltage was scanned in the potential window of 0–1.2 V at a rate of 100 mV/s for 50 cycles. After the scan was finished, the hybrid MnO2/VGNS structures were dried in air before characterizations and electrochemical measurements. Equivalent thermal chemical vapor deposition process: A conventional single-zone horizontal quartz tube furnace with an inner diameter of 50 mm and a length of 90 cm was used with the working pressure fluctuating between 150–160 Pa. After loading the butter/Ni samples, the temperature of furnace was raised to 450 °C and 900 °C at a gas composition of 50 sccm Ar and 50 sccm H2. The conversion time was kept constant at 9 min. No VGNS were observed from these processes. Microscopy and microanalysis: Field-emission scanning electron microscopy (FE-SEM) was a Zeiss Auriga microscope operated at 5 keV electron beam energy with an InLens secondary electron detector. Raman spectroscopy was performed using a Renishaw inVia spectrometer with laser excitation at 514 nm (Ar laser) and a probing spot size of ∼1 μm2. X-ray photoelectron spectroscopy (XPS; Specs SAGE 150) used the Mg Kα excitation at 1253.6 eV. Both survey and narrow scans of C 1s and O 1s were obtained. The C 1s peak analysis was performed using the free software XPSPEAK 4.1 (http://www.uksaf.org/software.html). Electrochemical measurements: The electrochemical measurements were performed in 0.1 M Na2SO4 aqueous electrolyte at room temperature. The three-electrode cell configuration used the VGNSbased samples as a working electrode, a Pt foil as a counter electrode and an Ag/AgCl reference electrode; while the two-electrode cell

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Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements We thank I. Levchenko, A. E. Rider, J. H. Fang, and H. Y. Yang for technical assistance and insightful discussions. D. H. S acknowledges support via an Australian Postgraduate Award (APA) and CSIRO’s OCE Postgraduate Scholarship Program. Z. J. H. acknowledges support from ARC DECRA Fellowship, CSIRO’s OCE Postdoctoral Fellowship and Sensors and Sensor Network (SSN) TCP Program. K. O. acknowledges support from ARC Future Fellowship and CSIRO’s OCE Science Leader Program. Received: April 23, 2013 Revised: May 7, 2013 Published online: July 3, 2013

[1] D. Pech, M. Brunet, H. Durou, P. Huang, V. Mochalin, Y. Gogotsi, P.-L. Taberna, P. Simon, Nat. Nanotech. 2010, 5, 651. [2] L.-Q. Mai, F. Yang, Y.-L. Zhao, X. Xu, L. Xu, Y.-Z. Luo, Nat. Comm. 2011, 2, 381. [3] J. R. Miller, P. Simon, Science 2008, 321, 651. [4] P. Simon, Y. Gogotsi, Phil. Trans. R. Soc. A 2010, 368, 3457. [5] J. R. Miller, R. A. Outlaw, B. C. Holloway, Science 2010, 329, 1637. [6] K. Sheng, Y. Sun, C. Li, W. Yuan, G. Shi, Sci. Rep. 2012, 2, 247. [7] Z. Bo, Z. Wen, H. Kim, G. Lu, K. Yu, J. Chen, Carbon 2012, 50, 4379. [8] N. Wang, C. Wu, J. Li, G. Dong, L. Guan, ACS Appl. Mater. Interfaces 2011, 3, 4185. [9] P. Simon, Y. Gogotsi, Nat. Mater. 2008, 7, 845. [10] L. Hu, M. Pasta, F. La Mantia, L. Cui, S. Jeong, H. D. Deshazer, J. W. Choi, S. M. Han, Y. Cui, Nano Lett. 2010, 10, 708. [11] Y. Wang, S. F. Yu, C. Y. Sun, T. J. Zhu, H. Y. Yang, J. Mater. Chem. 2012, 22, 17584. [12] D. H. Seo, S. Kumar, K. Ostrikov, Carbon 2011, 49, 4331. [13] D. H. Seo, S. Kumar, K. Ostrikov, J. Mater. Chem. 2011, 21, 16339. [14] K. Ostrikov, Rev. Mod. Phys. 2005, 77, 489. [15] D. H. Seo, A. E. Rider, S. Kumar, L. K. Randeniya, K. Ostrikov, Carbon 2013, 60, 221. [16] J. Chen, K. Sheng, P. Luo, C. Li, G. Shi, Adv. Mater. 2012, 24, 4569. [17] Z. Yan, L. Ma, Y. Zhu, I. Lahiri, M. G. Hahm, Z. Liu, S. Yang, C. Xiang, W. Lu, Z. Peng, Z. Sun, C. Kittrell, J. Lou, W. Choi, P. M. Ajayan, J. M. Tour, ACS Nano 2013, 7, 58. [18] Y. He, W. Chen, X. Li, Z. Zhang, J. Fu, C. Zhao, E. Xie, ACS Nano 2013, 7, 174.


Adv. Energy Mater. 2013, 3, 1316–1323


Adv. Energy Mater. 2013, 3, 1316–1323

[32] D. Mhamane, A. Suryawanshi, S. M. Unni, C. Rode, S. Kurungot, S. Ogale, Small 2013, DOI: 10.1002/smll.201202670. [33] M. F. El-Kady, R. B. Kaner, Nat. Comm. 2013, 4, 1475. [34] M. D. Stoller, S. Park, Y. Zhu, J. An, R. S. Ruoff, Nano Lett. 2008, 8, 3498. [35] W. Lv, D.-M. Tang, Y.-B. He, C.-H. You, Z.-Q. Shi, X.-C. Chen, C.-M. Chen, P.-X. Hou, C. Liu, Q.-H. Yang, ACS Nano 2009, 3, 3730. [36] M. D. Stoller, R. S. Ruoff, Energy Environ. Sci. 2010, 3, 1294. [37] X. Lang, A. Hirata, T. Fujita, M. Chen, Nat. Nanotech. 2011, 6, 232. [38] M. Toupin, T. Brousse, D. Belanger, Chem. Mater. 2004, 16, 3184. [39] L. Mao, K. Zhang, H. S. O. Chan, J. Wu, J. Mater. Chem. 2012, 22, 1845. [40] A. L. M. Reddy, M. M. Shaijumon, S. R. Gowda, P. M. Ajayan, J. Phys. Chem. C 2010, 114, 658. [41] C. J. Hung, J. H. Hung, P. Lin, T. Y. Tseng, J. Electrochem. Soc. 2011, 158, A942. [42] M. Zhi, C. Xiang, J. Li, M. Li, N. Wu, Nanoscale 2013, 5, 72. [43] J. Li, X. Cheng, A. Shashurin, M. Keidar, Graphene 2012, 1, 1. [44] Z. Fan, Q. Zhao, T. Li, J. Yan, Y. Ren, J. Feng, T. Wei, Carbon 2012, 50, 1699. [45] V. H. Luan, H. N. Tien, L. T. Hoa, N. T. M. Hien, E.-S. Oh, J. Chung, E. J. Kim, W. M. Choi, B.-S. Kong, S. H. Hur, J. Mater. Chem. A 2013, 1, 208. [46] C.-M. Chen, Q. Zhang, M.-G. Yang, C.-H. Huang, Y.-G. Yang, M.-Z. Wang, Carbon 2012, 50, 3572.



FULL PAPER

[19] M. F. El-Kady, V. Strong, S. Dubin, R. B. Kaner, Science 2012, 335, 1326. [20] S. Niyogi, E. Bekyarova, M. E. Itkis, H. Zhang, K. Shepperd, J. Hicks, M. Sprinkle, C. Berger, C. N. Lau, W. A. de Heer, E. H. Conrad, R. C. Haddon, Nano Lett. 2011, 10, 4061. [21] A. Eckmann, A. Felten, A. Mishchenko, L. Britnell, R. Krupke, K. S. Novoselov, C. Casiraghi, Nano Lett. 2012, 12, 3925. [22] Y. Zhu, S. Murali, M. D. Stoller, K. J. Ganesh, W. Cai, P. J. Ferreira, A. Pirkle, R. M. Wallace, K. A. Cychosz, M. Thommes, D. Su, E. A. Stach, R. S. Ruoff, Science 2011, 332, 1537. [23] C.-C. Chang, C.-C. Chen, W.-H. Hung, I.-K. Hsu, M. A. Pimenta, S. B. Cronin, Nano Res. 2012, 5, 854. [24] Z. J. Han, B. K. Tay, C. M. Tan, M. Shakerzadeh, K. Ostrikov, ACS Nano 2009, 3, 3031. [25] K. Ostrikov, E. C. Neyts, M. Meyyappan, Adv. Phys. 2013, 62, 113. [26] Y.-D. Lim, D.-Y. Lee, T.-Z. Shen, C.-H. Ra, J.-Y. Choi, W. J. Yoo, ACS Nano 2012, 6, 4410. [27] K. Ostrikov, H. Mehdipour, J. Am. Chem. Soc. 2012, 134, 4303. [28] Z. Luo, S. Lim, Y. You, J. Miao, H. Gong, J. Zhang, S. Wang, J. Lin, Z. Shen, Nanotechnology 2008, 19, 255607. [29] P. L. Taberna, P. Simon, J. F. Fauvarque, J. Electrochem. Soc. 2003, 150, A292. [30] D. Aurbach, M. D. Levi, E. Levi, H. Telier, B. Markovsky, G. Salitra, U. Heider, L. Hekier, J. Electrochem. Soc. 1998, 145, 3024. [31] X. Ling, L. Xie, Y. Fang, H. Xu, H. Zhang, J. Kong, M. S. Dresselhaus, J. Zhang, Z. Liu, Nano Lett. 2010, 10, 553.

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