Supercapacitor Electrodes Made of Exhausted Activated Carbon

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Supercapacitor electrodes made of exhausted activated carbon derived SiC nanofilaments coated by graphene. Maria Sarno, Sergio Galvagno, Rosangela Piscitelli, Sabrina Portofino, and Paolo Ciambelli Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00737 • Publication Date (Web): 06 May 2016 Downloaded from http://pubs.acs.org on May 12, 2016

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Industrial & Engineering Chemistry Research

Supercapacitor electrodes made of exhausted activated carbon derived SiC nanoparticles coated by graphene. Maria Sarno1*, Sergio Galvagno2, Rosangela Piscitelli1, Sabrina Portofino2, Paolo Ciambelli1 1 Department of Industrial Engineering DIIN and Research Centre NANO_MATES, University of Salerno, Via Giovanni Paolo II, 132, 84084 Fisciano (SA), Italy 2 Department of Environment, Global Change and Sustainable Development, C.R. ENEA Portici, via Vecchio Macello loc. Granatello, 80055 Portici, Na, Italy.

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Corresponding author. Tel/Fax: +39 089 963460; +39 089964057. E-mail address: [email protected] (M. Sarno)

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Abstract Graphene has been obtained at atmospheric pressure and low temperature on exhausted activated carbon derived SiC nanoparticles. The graphene’s growth, traced by on-line analyzers, has been obtained from an external source of carbon, in particular by methane/hydrogen chemical vapour deposition (CVD). Recycled SiC has been chosen as growth substrate to carry out a convenient process and to increase the added value of the recycled, combining the favourable properties of different substances. Therefore the SiC powder and the composite material obtained have been carefully characterized by the combined use of different techniques and tested as supercapacitor electrodes. The results show a very high capacitances up to 114.7 F/g for SiC alone and three times higher in the presence of graphene with an excellent cycle stability.

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1.Introduction Carbon materials (e.g. activated carbons, carbon aerogels, and nanostructures) have been widely studied for electrical double layer capacitors (EDLCs) electrodes, because the large charge accumulation achievable during the charge/discharge process on their high surface area.1-21 In particular, graphene has shown remarkable electrochemical performance.22-28 Moreover, their low equivalent series resistances (ESR) and good electrochemical stability make of graphene an efficient substrate for pseudocapacitive28 and battery29 materials, significantly improving both energy and power capabilities of resulting composite electrodes. In particular, graphene was found able to store a capacitance of 21 µF/cm2 (550 F/g), provided the entire 2675 m2/g is fully utilised.8,30 Recently, EDLC made of semiconductors, such as: silicon,31 silicon carbide,32 titanium nitride and titanium dioxide33 nanowires have been studied. In particular, in recent years there has been increased attention to SiC, thanks to its unique electrical and thermal properties. Silicon carbide, as a promising electrode material, especially in micro-supercapacitor, has attracted considerable industrial and scientific interest

32, 34-45

due to its wide band gap, high electron mobility and many

other interesting properties, such as high hardness, excellent resistance to erosion and corrosion, etc.. In particular, Zinc blende (cubic) (3C-SiC), that exhibits outstanding electronic conductivity because possesses the smallest band gab (~2.4 eV) and one of the highest electron mobility of all the known polytypes, has shown very interesting results in 0.1M H2SO4 as a thin film supercapacitor, alone,32 on a 3-dimension porous network structure of graphite that substantially contribute to the increased electrode performance,34,36 and on a low resistivity graphitized layer avoiding energy loss through the electrode.35 Although carbon stabilizing and potentiating effects on the performance of SiC as a supercapacitor are now clear, graphene has never been studied in combination with SiC for this application at the

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best of our knowledge. Indeed, among the different methods46-48 to synthesize graphene, this can be directly prepared on SiC by thermal decomposition.49-53 The Graphitization of SiC, studied since the mid 70’s,52 has been widely explored in the last years, as a way of producing graphene films on semi-insulating substrates for fundamental studies and applications and the possibility to grow graphene on a suitable surface that doesn’t require further transfer process.49-51 The epitaxial growth method requires high temperature, ultra-high vacuum and perfectly ordered silicon carbide. Current results show that, through this method, the graphitization leads on the Si-terminated face, to the formation of a layer frequently called the buffer layer.54 Whereas on the C terminated larger domains but of multilayered rotationally disordered graphene,55 have been produced. Only recently, it has been shown that a soft annealing under hydrogen56 or air 57

allows to break the coupling at the graphene/SiC interface, transforming the buffer layer into

graphene. To improve the film quality, Emtsev et al.53 suggested to reduce the overall sublimation rate and to increase the graphitization temperature by several hundred degrees using unreactive argon at nearly ambient pressure. The key factor in achieving an enhanced growth is the higher annealing temperature (1650°C), attainable for graphene formation under argon.58-61 Following the studies of graphene formation by SiC annealing, graphene growth on SiC from external sources of carbon has been explored. The first demonstration has been done under ultra-high vacuum (UHV) environment62,63 with solid carbon source. Later, graphene growth on SiC by chemical vapour deposition (CVD), using propane64 as carbon source and argon65 or hydrogen66 as carrier gas, has been performed. It has been demonstrated67 that the graphene structure can be controlled by using propane hydrogen CVD, and that the result depends on the operating conditions, particularly on the couple carbon supply rate-growth temperature. It is worthwhile to notice that this temperature can be varied over a large range, even lower values range. By using propane under CVD conditions and hydrogen as carrier gas, either IRD structure and the 6√3×√63-R30° can be obtained on the Si4


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face66-69 depending on the growth pressure and temperature. Therefore though growth of graphene is favoured on comparable lattice structure, graphene has been recently66, 70-73 successfully grown on cheaper 3C-SiC. In particular, graphene growth was obtained by a direct carbon feeding through propane flow in a chemical vapor deposition reactor.66 A very weak interaction between the graphene layer and the substrate, crucial to preserve the astonishing intrinsic properties of graphene, has been demonstrated.70 Here we report, for the first time, the growth of graphene by methane/hydrogen CVD, on-line monitored by gas phase analyzers, on SiC, mostly 3C-SiC, nanoparticles (NPs), at atmospheric pressure and low temperature for their intrinsic advantages. SiC, derived from exhausted activated carbon, was prepared and chosen as substrate. Activated carbon is widely used as adsorbent for the removal of organics in water and wastewater treatment. The economics of the adsorption process greatly depends on the reuse of the adsorbent along several operation cycles. Regeneration refers to removal of the adsorbate and restoration of previous adsorptive capacity of the original activated carbon. The currently available regeneration methodologies which include thermal desorption, wet air oxidation, steam, infrared radiation, and solvent extraction have been extensively reviewed74 but very often the efficiency of the regeneration techniques is not high enough and decreases with the number of regenerations. So the possibility of reusing this material for a new application appear the most economical method. Recycled SiC was chosen as growth substrate to realize a convenient process and to increase the added value of the recycled, combining the favourable properties of different substances. The prepared SiC nanoparticles and the composite material (graphene on SiC) obtained were carefully characterized by the combining use of different techniques: transmission electron microscopy (TEM) coupled with an EDAX probe, scanning electron microscopy (SEM), Raman spectroscopy, thermogravimetric analysis (TG-DTG) coupled with a quadrupole mass detector, X-ray diffraction analysis and N2 adsorption–desorption isotherm.

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In particular, the aim of this paper is to provide evidence of the electrochemical performance of our SiC obtained from a recycled source, and of our SiC covered with few layers of graphene, which has even higher performance, as it emerged from the literature, and it is expected will improve the electrical transport and stabilize the contact interfaces. Moreover, the nano SiC filaments constitute a unique possibility to expose a large graphene area, which is one of the major issues raised in the realization of graphene-based electrodes. SiC nanoparticles were tested for their supercapacitor behaviour in H2SO4 solution and in a wider voltage windows (0-1V and -0.5 - 0.5V) if compared with previous published papers. The analysis of the performances of SiC alone and covered by graphene demonstrates that the conductive layer, and probably an electrical coupling between the graphene and SiC, allow to improve further the relevant performances of the electrodes. Electrodes to be used in supercapacitor applications will benefit of the SiC and graphene interesting properties, including the possibility to work in harsh environment, for example in on-chip supercapacitor by ink printing.21These results are also the proof of concept to use SiC/graphene electrodes, that can be prepared also in different geometry, for example to be implemented directly on chips. 2. Experimental 2.1 Material and methods 2.1.1 Preparation of SiC from exhausted activated carbon Silicon carbide powder was synthesized from exhausted activated carbon and inexpensive silicon dioxide, by means of a carbothermal reduction reaction which involves the following fundamental steps: SiO2(s)+C(s) → SiO(g)+CO(g)

(1)

SiO(g)+2C(s) → SiC(s)+CO(g)

(2)

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The carbon matrix was milled into particles up to 1 mm in diameter, mixed with commercial silica gel (Carlo ERBA silica gel 60 230-400 mesh ASTM) in a mixer drum for 2h, fixing the powder weight ratio SiO2/C at 1.514. After being ground, samples were maintained under ambient conditions. The reactions (1) and (2) were conducted at temperature ranging from 1500-1800 °C in a tubular furnace (Nabertherm 120/300/1800), with a residence time of nearly 1 hour, in argon atmosphere (flow rate of 50 l/h). At these experimental conditions, β-phase prevails on α-SiC and the product yields are very close to the theoretical value (nearly 35%). SiC powder was purified from the residual carbon by treating the samples at 700°C in oxidizing atmosphere and from the residual silica by dissolving the samples in hydrofluoric acid (HF) 50 wt % solution and by subsequently washing the residues with alcohol. The exploitation of solid by-products for the production of high added value materials, such as SiC powder, has been conducted in the frame of a wider research line which is devoted to the matter recovery from the waste treatment and that has got its main goals into the ceramic production. 2.1.2 Preparation of Graphene/SiC Graphene/SiC composite, in the followings G/SiC, was prepared in a continuous flow microreactor loaded with SiC and fed by pure argon, nitrogen, hydrogen, methane or their mixtures. Mass flow controllers (MFC) were used to provide constant flow rate of each gas. A K thermocouple measured the temperature of SiC bed. The reactor was heated by an electrical oven, while on-line analyzers permit the monitoring of CH4 and hydrogen concentrations in the reactor outlet gas.75-77 The substrate was previously washed in a 5% HF solution for 3h to remove SiO2 and washed until pH reached a value of 7-8. For the graphene synthesis hydrogen was used as carrier gas and methane as carbon source. SiC was annealed for 10 min under argon at 1200°C in order to uniform 7



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the surface by removing residual polishing damage. Such treatments do change the surface morphology and typically cause significant step bunching.78 SiC powder was put on a removable support inside the quartz reactor, and the gas mixture flow horizontally invests the substrate. To obtain the methane/hydrogen stream, 99.998 pure methane and 99.9990 pure hydrogen, were mixed (3000 ppm of methane in 100 cc(stp)/min total flow rate) fed to the reactor. After a temperature ramp-up to 1250°C, graphene growth was started by adding methane to hydrogen. After 5 min, the reacting gas stream was stopped, and hydrogen was fed to the reactor that cools down at ambient temperature conditions. 2.2 Characterization For the characterization: Transmission electron microscopy (TEM) images were acquired using a 200 kV FEI Tecnai microscope, equipped with an EDX probe; Scanning electron microscopy (SEM) images were obtained with a LEO 1525 microscope; Raman spectra were obtained with a micro-Raman spectrometer Renishaw inVia ( 514 nm excitation wavelength- laser power 100 mW). The laser spot diameter was about 10 µm; XRD measurements were performed with a Bruker D8 X-ray diffractometer using CuKα radiation; Thermogravimetric analysis (TG-DTG) at a 10 K/min in flowing air was performed with a SDTQ 500 Analyzer (TA Instruments) coupled with a mass spectrometer (MS); N2 adsorption–desorption isotherms at 77K, on powder samples previously outgassed in He flow at 523K for 4h, were obtained with a Thermoquest Sorptomatic 1900. Conductivity measurements of SiC and G/SiC thin films (~3 µm thick, SEM evaluation), obtained by drop-casting from ethanol powder suspensions onto mica substrates followed by a pressing, were obtained with a four-point probe surface resistance Keithley 4200 source meter. Before measurements electrical and electrochemical measurement, for SiC a dipping into 5% HF solution for 3h to remove SiO2 was performed, followed by a washing in distilled water until the pH of the leaching water reached a value of 7-8. For the electrochemical measurements, a homogeneous ink 8


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was prepared by disperding 4 mg of SiC or G/SiC in 80 µl of 5 wt.% Nafion solution. Then, the ink was drop casted onto 6 mm diameter glassy carbon electrode. Linear sweep voltammetry were obtained using a potentiostat from Amel Instruments in 0.1 M H2SO4. Saturated calomel, graphite and a loadable glassy carbon electrode as the reference, the counter and the working electrodes, respectively.8 EIS measurements were obtained in AC by applying a voltage of 5 mV amplitude in a frequency range 0.1-105 Hz at open circuit potential.

3. Results and discussion 3.1 SiC characterization The characteristic XRD pattern of the synthesized SiC is shown in Figure 1.79 The major diffraction peaks can be indexed as the (111), (200), (220), (311), and (222) reflections of 3C-SiC (cubic βphase of SiC, unit cell a = 0.4370 nm), as confirmed by the Raman analysis (Figure S1). No peaks are observed in the range 2θ=20°-30°, indicating the absence of residual carbon and SiO2 (see also Figure S2 and relative comments). Three SEM images of SiC at increasing magnification are reported in Figures 2a, 2b and 2c. The powder consists mainly of a texture of presumable curved filaments and/or nanoparticles with a diameter in the range 15-80 nm. The TEM image in Figure 2d reveals that the most powder consists of nanofilaments of about 1 µm in length and tens of nanometers in diameter; a small number of larger particles is also present. The corresponding selected-area electron diffraction (SAED) pattern (area in red of Figure 2) indicates a cubic structure of the SiC sample. The fast Fourier transform (FFT) evidences two different orientations of packed SiC with the typical 0.252 nm interlayer spacing (see the high resolution TEM image). In particular, in Figure 2 the EDS spectrum on the bottom right of the image highlighted the background of the spectrum evidencing the presence of Ca, K, Ti, Cr and Fe 9



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coming from the exhausted activated carbon. By EDS analysis it has been found that these metals correspond to less than 0,5% of the total sample weight (this level of impurities reduces the electric band gap for this SiC, up to values around the unit about the half of the typical values reported for 3C-SiC, results not shown here, this aspect is under investigation). Figure 3a shows the N2 adsorption-desorption isotherms for the SiC nanoparticles. The measured BET surface area is 500.45 m2/g, with a micropore surface area of 238,2 m2/g, the total pore volume and the microporous volume are 0.43 cm3/g of 0,083 cm3/g, respectively. The multimodal pore size distribution (BJH (Barrett-Joyner- Halenda) Desorption pore distribution) centred at 2.3, 8.1, 11,1 and 19.7 nm, due to pores between exposed surface filaments, which agree with nanoparticles packaging observed in SEM and TEM images, is shown in Figure 3b.

3.2 G/SiC physico-chemical characterization The key Raman results are displayed in Figure 4 to be compared with the reference spectrum measured on bare SiC of Figure S1. The transverse optical (TO) at 795 cm-1 and longitudinal optical (LO) between 870 and 995 cm-1 bands indicate the presence of 3C-SiC (β-SiC) phase.71,80-84 Shoulders are present at both sides of the TO and LO bands, probably due to the 6H or other SiC polytypes.69 The well isolated G, D and 2D bands near 1350, 1590 and 2700 cm-1, due to carbon, jump into the eye.87,88 In particular, we can see the so-called G band, that corresponds to an ideal graphitic lattice vibration mode with E2g symmetry,11 and the 2D band at about 2700 cm−1, that suggests the generation of graphitic domains, indeed it is a representative characteristic feature of undisturbed or highly ordered graphitic lattices.11, 16 It can be fitted with six Lorentzian lines, as it is typical of three layer graphene.88 On the other hand, the multiple components nature of the 2D Raman bands support the hypothesis of an interlayer coupling absence between graphene and the SiC surface,90 likely due not only to the SiC crystalline orientation but also to the graphene 10


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formation from carbon addition. We can also see the so-called disorder-induced D-band, that is assigned to a typical characteristic for disordered graphite or crystal defects, corresponding to a graphitic lattice vibration mode with A1g symmetry.11, 16 Figure 5 shows a TEM image of a SiC nanoparticles after graphene formation. The high resolution TEM images in the inserts of Figure 5 evidence the interface between the graphene formed and the SiC surface. Three graphene layers are visible in the image with interlayer distance of 0.35 nm. With the aim to better understand this result the evolution of the concentration profiles of H2 and CH4 during the synthesis has been recorded. Typical concentration profiles of H2 and CH4 during graphene synthesis test is shown in Figure 6. We can distinguish three temporal phases: (I) Prereaction phase: H2 and CH4 are fed to the analyzer; (II) Reaction phase: H2 and CH4 are fed to the reactor, reaching the analyzer after passing over the SiC powder; (III) Post-reaction phase: at the end of the reaction the gases, bypassing the reactor, are sent to the analyser. Considering, the methane decomposition to give carbon and hydrogen (CH4 → C + 2H2), the initial feed composition and the expansion volume factor (εCH4), we calculated the methane conversion and hydrogen yield as: CCH 4 C 0 CH 4 = ε ×C 1 + CH 4 0 CH 4 C CH 4 1−

X CH 4

RH 2 =

(1 + ε CH 4 X CH 4 ) × C H 2 0 2CCH 4

where C is the concentration and C0 the initial concentration of a single component, finding that the two values are in disagreement, indicating that the methane decomposition to carbon is not the only reaction that occurs in the reactor. This is not an unexpected result, it is well known that starting with methane, the pyrolysis is represented as occurring stepwise: CH4 → C2H6 → C2H4 → C2H2 → C

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At the highest temperature >1000°C, the ethane and ethylene are short lived. In particular, the thermal decomposition of methane, over the range of 1000°-1630°C, follows two main parallel reactions:91 2 CH4 → C2H2 + 3 H2 (1) CH4 → C + 2H2 (2) At a temperature lower than 1200°C the main reaction products of methane pyrolysis are hydrogen and carbon, with very small amounts of higher hydrocarbons. The maximum acetylene concentration in the reaction product, as well as aromatic hydrocarbons, especially benzene and naphthalene, increased with temperature. With respect to the reaction (2), short reaction times and low partial pressures of methane preferably by hydrogen dilution can limit the carbon formation.92 Obviously, one needs to distinguish between thermodynamic equilibrium conversion and conversion limited by kinetics in a finite reactor. Moreover, the methane cracking was a heterogeneous, autocatalytic reaction, with the initiation step of CH4 → CH3 +H, and with an acceleration step of CH3 → CH2 + H occurring on the carbon surface,93 so it is necessary to take into account that the deposition of successive carbon atoms is accelerated by the presence of those already deposited. Finally, in our reaction conditions, 1250 °C and high hydrogen concentration, it is expected that most of the by-product is acetylene, and can not be excluded the presence of aromatic hydrocarbons traces. Moreover, by observing the reaction chamber, we can conclude that carbon not only covers the SiC surface, but also the reactor filter that results slightly gray after synthesis. Definitely, the process is quite complex and a quantification of the deposited carbon, even more than that deposited on the SiC, is not possible, since it is necessary to know the amount of all the species involved. On the other hand, an important consideration can be done considering what happens during a blank test, performed by recording the evolution of the hydrogen and methane composition, to determine the behaviour of the system in the absence of SiC powder. The concentration of methane practically 12


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overlapped that found in the presence of SiC. In the case of hydrogen profiles, even if the hydrogen concentration variation is very small in both cases, it stays at a slightly higher values during all the test time, indicating the occurrence of an interaction between H2 and the SiC surface and/or the gas phase. This observation is a direct confirmation of what understood, downstream of numerous experimental analysis, in.67 Indeed, graphene growth on SiC has been mainly studied using sublimation methods under UHV or argon pressure conditions. In both cases the growth involves Si sublimation, that under UHV conditions happens at considerably lower temperature ~1200°C (Si sublimation begins at 1150°C) compared with the 1650°C (Si sublimation begins at 1600°C due to the argon molecules) of the atmospheric growth, that leads to more uniform and larger terraces and carbon graphitization. The situation is different in the case of the hydrocarbon/H2 CVD. H2 passivate the SiC surface, as in the detaching of the buffer layer;56 due to the presence of hydrogen, the IRD structure was grown also on the Si terminated surface under hydrogen/propane CVD.67 Moreover, back to our discussion, the presence of hydrogen determines the formation of a new number of species (SiH2, CH4, C2H2 etc…) absent in the other cases, that could led to a silicon etching together with a methane direct supply. It has been noted,67 in all the operating conditions explored, that no graphene can be observed feeding pure hydrogen, even if SiC etching occurs, but silicon and carbon released sublimate from the surface. While, in a number of operating conditions the graphene formation results by the combination of the carbon addition by the hydrocarbon and the SiC etching, the additional carbon results necessary to create a carbon overpressure and the subsequent segregation on the surface. Finally, we want to point out, that a recycling can be used considering the very high level of hydrogen and the very low conversion of methane. The thermal stability of G/SiC has been evaluated by a thermogravimetric test from room temperature to 1000°C in air flow (Figure 7). During the test, a weight loss, of about 5,3 wt.%, can 13



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be observed in the temperature range 430-750°C due to graphene oxidation, followed by a mass gain due to the formation of SiO2 by the reaction of SiC and oxygen83 (see the mass fragment m/z=44, and Figure S2a for thermogravimetric test on the synthesized SiC). Starting from 1 g of SiC nanoparticles with a mean diameter of 50 nm and the SiC density of 3.21 g/cm3 2*1014 NPs have been calculated. A deposited carbon mass of 51.87 mg can be calculate by multiplying the number of NPs for the carbon volume (1.3*10-16 cm3) and density, corresponding to the carbon weight loss in the range 430-750°C. The nitrogen adsorption-desorption isotherm of G/SiC is shown in Figure 8. The total specific surface area of G/SiC is 528.6 m2/g, with a micropore surface area of 221,2 m2/g, the total pore volume and the microporous volume are 0.45 cm3/g and 0,081 cm3/g, respectively. All the results are very similar to those of the starting SiC, also the multimodal pore size distribution (BJH (Barrett-Joyner-Halenda) Desorption pore distribution) centred at 2.3, 8.1, 11,1 and 19.7 nm (Figure 8b).

3.3 G/SiC supercapacitor performance evaluation Graphene has been used into the past for supercapacitor applications. The capacitance values increase as more is made available the graphene surface.8,

22-25

In particular, a value of 247 F/g

(electrode weight of 0.283 µg) in PVA-H3PO4 electrolyte, that behaves similarly to H2SO4,25 has been found high exposure area graphene thin film obtained through an LBL deposition. The same authors reported a normalized capacitance by geometrical area of 394 µF/cm2 (electrode area 0.08465×2.1 cm2), and a capacitance normalized by the accessible interfacial surface of 18.9 µF/cm2, quite close to the limiting double-layer capacitance. Table 1 shows results obtained using SiC into the past, it has been tested: (i) alone; (ii) in combination with carbonaceous materials; (iii) in combination with pseudocapacitive materials. In the case of electrodes designed for applications in micro-supercapacitors or planar capacitors, the 14


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reported capacitance values have been typically measured as an areal capacitance in µF/cm2. In particular, the data expressed in µF/cm2 refers to the geometrical area of the electrodes, which can be characterized by different thickness. In general, the more the electrode thickness the more the areal capacitance increases. This is particular true in liquid electrolyte due to the their ability to penetrate in the increased effective area of a thicker electrode.94 SiC capacity increases from a film of SiC aggregated nanocrystals,32 to a substrate of parallel SiC nanowires, and furtherly for SiC supported on a carbon based material. In33 the electrode is a 1 µm thick film of nanocrystalline 3C-SiC with a roughness of 30 nm, this is important to take in account in the comparison with the performance of a single graphene layer.30 It is worth noticing that: (i) because the micro-supercapacitor applications, at the best of our knowledge, the SiC capacitance has never been reported in F/g, (ii) the data showing the electrochemical performances in F/g, published to date, refer to SiC coupled with pseudocapacitive materials.37,39,40 Prior to electrochemical experiments, to evaluate the suitability of the materials prepared to be applied as electrode for electrochemical experiments, the electrical conductivities of SiC and G/SiC thin films (~3 µm thick) were measured by four-probe technique. SiC film possesses a surface resistance of 5540 Ω/sq due to defects in the films (e.g. planar defects, grain boundaries and amorphous phase),95 and probably metallic inclusions (see EDS analysis). On the other hand, it is known that 3C-SiC, that is the main component of our sample, possesses the smallest band gap (~2.4 eV) and one of the highest electron mobilities (~800 cm2/Vs).96 G/SiC film shows a lower surface resistance of 1500 Ω/sq likely due to graphene covering.97 In order to evaluate the electrochemical properties of our SiC and G/SiC nanocomposite, cycling voltammetry (CV) and galvanostatic charge/discharge tests were performed. The CV curves at 210-20-30-60-120-240 mV/s in the potential window of 0-1 V in 1M H2SO4 electrolyte are presented in Figure 9a and 9b for SiC and G/SiC, respectively. These curves, for G/SiC, subtend a larger area 15



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and are closer to a rectangular shape than for SiC alone. The specific capacitance was calculated from the discharge curves (Figure 9c and 9d). In particular, the specific capacitance for G/SiC results almost three times that exhibited by SiC at the same A/g, (40 mF/cm2 (114.7 F/g) for our SiC and 116 mF/cm2 (325.9 F/g) for G/SiC at 0.1 mA/cm2 (0.12 A/g)). The capacitances of the two electrodes at increasing current densities were shown in Figure 9e. As it is expected the capacitances of both samples decrease from 0.12 A/g to 9 A/g due to the internal resistance of the electrode. On the other hand, the capacitances values are still 232 and 87 F/g for G/SiC and SiC, respectively at 10 A/g (see inserts in Figure 9c and 9d). These results are probably due to a combination of factors, including: (i) the unique band gab observed for our SiC (ii) the highly graphene area exposed on the SiC filaments; (iii) the improved electrical transport and the contact interface stabilization due to graphene; (iv) the combination of micropores and mesopores, that permits the formation of EDLs and the electrolyte ions access;8 (v) an electrical coupling between the graphene and SiC. Figure 9f shows the long cycle stability of SiC and G/SiC as evaluated at 4 A/g. After 1.2×105 cycles SiC electrode still retains about 85% of its initial capacitance showing good stability. On the other hand, G/SiC shows an improved stability. The energy density and power densities were calculated by using the well-known equations.8 G/SiC shows very high energy density values form 91 Wh/kg to 68 Wh/kg (see Figure 10a) in correspondence of a large power density range from 122 W/Kg to 5000 W/kg. The cyclic voltammogramm (CV), in the range -0.5 - 0.5V, of G/SiC, is shows Figure 10b, giving the possibility to calculate a capacitance of 262 F/g also in this range. The cyclic voltammogramm (CV), in the range -0.5 - 0.5V, of G/SiC, is shows Figure 10b, giving the possibility to calculate a capacitance of 262 F/g also in this range. Figure 11 shows the Nyquist impedance plots of SiC and G/SiC. The Nyquist plots show a straight line in the low-frequency region revealing good electrochemical capacitive properties. The 45° lines denote the feature of ion diffusion and are ascribed to the Warburg impedance. At high frequency 16


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the diameter of the semicircles correspond to polarization resistance or charge transfer resistance of the electrode, the G/SiC electrode with good conductivity shows smaller charge transfer resistance than the SiC sample. When the ordinate tend to zero, the abscissa (real part) evidences the total internal resistance, due to the electrolyte resistance, the intrinsic resistance of the active material and the contact resistances.11 The ESR of G/SiC and SiC are 0.60 and 0.83 Ω, respectively. 4. Conclusion SiC powders were prepared from exhausted activated carbon and inexpensive silicon dioxide, by means of a carbothermal reduction reaction. The nanoparticles of SiC are about 1 µm in length and tens of nanometers in diameter, there are also a small number of larger particles. The corresponding electron diffraction pattern and the EDS spectrum confirm the cubic structure of the synthesised SiC. Graphene/SiC composite was prepared from an external sources of carbon, at atmospheric pressure, on the SiC nanoparticles. Three layers graphene were grown on the SiC surface, has evidenced by the TEM images and Raman Spectroscopy. The electrochemical characterization permits an efficient energy storage capability for both samples. In particular, the highly area exposed on the SiC nanofilament of the conductive graphene layer and probably an electrical coupling between graphene and SiC allow to improve the performances of the electrode. Supporting Information Figure S1: Raman spectrum of the synthesized SiC. Figure S2: Thermogravimetric test on the synthesized SiC: TG in green, DTG in blue (a); XRD spectrum obtained on the powder after the thermogravimetrical test (b). Acknowledgments This research was supported by TyGRe project FP7 European Union’s Seventh Programme for research, grant agreement No 226549 call FP7-ENV-2008-1 and the TRIPODE project SMARTAGS National Operative Programme PON 02_00556_3420580.

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The authors thank Dr. Filippo Giubileo for the electrical conductivity measurements.

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(81) Bosi, M.; Watts, B. E.; Attolini, G.; Ferrari, C.; Frigeri, C.; Salviati, G.; Poggi, A.; Mancarella, F.; Roncaglia, A.; Martinez, O.; Hortelano, V. Growth and Characterization of 3C-SiC Films for Micro Electro Mechanical Systems (MEMS) Applications. Cryst. Growth Des. 2009, 9, 4852. (82) Feng, Z. C.; Tin, C. C.; Hu, R.; Williams, J. Raman and Rutherford Backscattering Analyses of Cubic SiC Thin films Grown on Si by Vertical Chemical Vapor Deposition. Thin Solid Films. 1995, 266, 1. (83) Katharria, Y. S.; Kumar, S.; Prakash, R.; Choudhary, R. J.; Singh, F.; Phase, D. M. Kanjilal, D. Characterizations of Pulsed Laser deposited SiC Thin Films. J. Non-Cryst;. Solids. 2007, 353, 4660. (84) Bechelany, M.; Brioude, A.; Cornu, D.; Ferro, G.; Miele, P. A Raman Spectroscopy Study of Individual SiC Nanowires. Adv. Funct. Mater. 2007, 17, 939. (85) Windisch, C. F.; Jones, R. H., Snead, L. L. Thermogravimetric and Microscopic Analysis of SiC/SiC Materials with Advanced Interfaces. Oak Ridge National Lab.1997, 29, 296. (86) Morkoc, H.; Strite, S.; Gao, G. B., Lin, E.; Sverdlov, B.; Burns, M. A Review of Large Bandgap SiC, III-V Nitrides, and ZnSe Based II-VI Semiconductor Structures and Devices. J. Appl. Phys. 1994, 76, 1363. (87) Di Bartolomeo, A.; Sarno, M.; Giubileo, F.; Altavilla, C.; Iemmo, L.; Piano, S.; Bobba, F.; Longobardi, M; Scarfato, A; Sannino, D; Cucolo, A. M.; Ciambelli, P. Multiwalled Carbon Nanotube Films as Small-Sized Temperature Sensors. J. Appl. Phys. 2009, 105, 064518. (88) Ciambelli, P.; Sarno, M.; Gorrasi, G.; Sannino, D.; Tortora, M.; Vittoria, V. Preparation and Physical Properties of Carbon Nanotubes-PVA Nanocomposites. J. Macromol. Sci. B. 2005, 44, 779. (89) Malard, L. M.; Pimenta, M. A.; Dresselhaus, G.; Dresselhaus, M. S. Raman Spectroscopy in Graphene. Phys. Rep. 2009, 473, 51. (90) Handa, H.; Takahashi, R.; Abe, S.; Imaizumi, K.; Saito, E.; Jung, M. H.; Ito, S.; Fukidome, H.; Suemitsu, M. Transmission Electron Microscopy and Raman-Scattering Spectroscopy Observation on the Interface Structure of Graphene Formed on Si Substrates with Various Orientations. J. Appl. Phys. 2011, 50, 04DH02. (91) Guerét, C.; Daroux, M.; Billaud, F. Methane Pyrolysis – Thermodynamics. Chem. Eng. Sci. 1997, 52, 815. (92) Holmen, A.; Olsvik, O.; Rokstad, O. A. Pyrolysis of Natural Gas: Chemistry and Process Concepts. Fuel Process. Technol. 1995, 42, 249. (93) Palmer, H. B.; Hirt, T. J. The activation Energy for the Pyrolysis of Methane. J. Phys. Chem. 1963, 67, 709. (94) Kaempgen, M.; Chan, C. K.; Ma, J.; Cui, Y.; Gruner, G. Printable Thin Film Supercapacitors Using Single-Walled Carbon Nanotubes. Nano Lett. 2009, 9, 1872.

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(95) Zhuang, H.; Yang, N.; Zhang, L.; Fuchs, R.; Jiang X. Electrochemical Properties and Applications of Nanocrystalline, Microcrystalline, and Epitaxial Cubic Silicon Carbide Films. ACS Appl. Mater. Interface 2015, 9, 10886. (96) Computational Electronics: Semiclassical and Quantum Device Modeling and Simulation. Vasileska, D.; Goodnick, S.M.; Klimeck, G. CRC Press 2010. (97) Liu, J.; Notarianni, M.; Will, G.; Tiong, V.T.; Wang, H.; Motta N. Electrochemically Exfoliated Graphene for Electrode Films: Effect of Graphene Flake Thickness on the Sheet Resistance and Capacitive Properties. Langmuir 2013, 29, 13307.

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Page 26 of 38

Table 1. Comparison of the Specific Capacitance of As Prepared SiC and G/SiC with for Graphene and SiC Reported in the Literature. Sample

Electrolyte

3C-SiC film+

0.1M H2SO4

3C-SiC nanowire film on a graphite paper

0.1M H2SO4

Graphitic carbon on nitrogen-doped polycrystalline 3C-SiC

1M H2SO4

Silicon carbide nanowires on a carbon fabric

2M KCl

Silicon carbide nanowires@Ni(OH)2 on carbon fabric

1M KOH

-

Microsphere silicon carbide/nanoneedle manganese

1M Na2SO4

-

Microsphere silicon carbide-MnO2 nanoneedles

1M Na2SO4

-

SiC nanowires

3.5 KCl

SiC nanowires

3.5 KCl

SiC coated silicon nanowires

1M KCl

SiC nanoparticles

1M H2SO4

Graphene coated SiC nanoparticles

1M H2SO4

+

The film was 1 µm thick.

26


C 30-35 µF/cm2 at 100 mV/s 28000 µF/cm2 at 2mA/cm2 743 µF/cm2 at 50 mV/s 23000 µF/cm2 at 50 mV/s

400 µF/cm2

C

Ref.

-

32

-

34

-

35

-

36

1724 F/g at 2 A/g 272 F/g at 10 mV/s 59.9 F/g at 2 mV/s

37 39 40

-

41

-

42

-

43

40191 µF/cm2

114.7 F/g

at 0.1 mA/cm2

at 0.12 A/g

This paper

at 100 mV/s 240 µF/cm2 at 100 mV/s 1700 µF/cm2 at 50 mV/s

115789 µF/cm2

325.9 F/g

at 0.1 mA/cm2

at 0.12 A/g

This paper

Page 27 of 38

111

Intensity a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


220 311

200

222

20

40

60

80

2θ°

Figure 1. X-ray diffraction pattern of the synthesized SiC.

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2 SEM images of the produced SiC at increasing magnification (a,b,c). TEM image of the SiC nanoparticles (d). The inserts show: an electron diffraction patter (e) and FFT (f), taken in the red area of the TEM image; an higher-magnification image (g). Bottom of the figure: EDX spectrum collected on the all image (h, i).

28


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a

Isotherm Plot 300

250

V [cc/g]

200

150

100 50

0 0,0

0,2

0,4

0,6 p/po

0,8

1,0

dVp/dDp Desorption Pore Volume Plot

b 100

First derivative, dVp/dDp [mm3/g*nm]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 60 40 20 0 1

10 Pore diameter, Dp [nm]

100

Figure 3. Nitrogen adsorption-desorption isotherm (a) and pore-diameter distribution (b) curves of the synthesized SiC.

29



792 Intensity a.u.

2D D 1350 952

300

600

2705

G 1590

1509 1710

900 1200 1500 1800 2100 2400 2700 -1

Wavenumber cm

In ten sity a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2640

2700

2760 -1

Wavenumber cm

Figure 4. Raman spectrum of graphene SiC.

30


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.35

Figure 5. TEM image of three layers graphene on SiC.

31



1 0 0 ,0

I

III

vol. %

II H2 H 2 b la n k te s t

9 9 ,5 5

10

15

20

25

tim e (m in )

I

0 ,3 vol. %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

III

II

0 ,2

CH4 C H 4 b la n k te s t

0 ,1 5

10

15

20

25

tim e (m in )

Figure 6. Concentration profiles of H2 and CH4 during the graphene synthesis test.

32


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100

0.5 Deriv. Weight (%/min)

Heat Flow (W/g)

0.5

Weight (%)

0.0 0.3

-0.5

80 60 30

-1.0

0.1

-0.1 -1.5 60 40 30

Exo Up

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


230

430 630 Temperature (°C)

830

-0.3 Universal V4.5A TA Instruments

m/z = 44

Figure 7. TG-DTG-MS of G/SiC: TG in green; DTG in blue; DSC in maroon; MS in orange.

33



a

Isotherm Plot 300

250

V [cc/g]

200

150 100

50 0 0,0

b First derivative, dVp/dDp [mm3/g*nm]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0,2

0,4

0,6 p/po

0,8

1,0

dVp/dDp Desorption Pore Volume Plot 120 100 80 60 40 20 0 1

10 Pore diameter, Dp [nm]

100

Figure 8. Nitrogen adsorption-desorption isotherm (a) and pore-diameter distribution (b) curves of G/SiC.

34


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a

b

30 Current / Ag-1

Current / Ag-1

20

0

30 mV/sec 60 mV/sec 120 mV/sec 240 mV/sec

2 mV/sec 10 mV/sec 20 mV/sec

-20

0,0

0,2

0,4

0,6

0,8

0

-30

0,0

1,0

0,2

0,4

10 A/g

0,8

0,8

0,4

c

0,2

1,0

0,2

1000

1500

2000

2500

0

10 A/g

0,8 0,6

d

0,4 0,2 0,0

0,0

Time / s

500

0,4

0 2 4 6 8 10 12 14 16 18

0,0 0

0,6

0,6

0,0

1,0

0,12 A/g 0,19 A/g 0,62 A/g 2 A/g 4 A/g 5 A/g Voltage / V

0,6

Voltage / V

Voltage / V

0,8

0,2

0,8

1,0

Voltage / V

0,12 A/g 0,19 A/g 0,62 A/g 2 A/g 4 A/g 5 A/g 1,0

0,6

Potential (V)

1,0

0,4

30 mV/s 60 mV/s 120 mV/s 240 mV/s

2 mV/s 10 mV/s 20 mV/s

Potential (V)

0

10

20

30

Time / s

1500

Time / s

3000

40

4500

6000

Time / s

G/SiC SiC

300

200

100

e 0 0

2

4

6

8

10

Capacitance retention / %

100 Specific Capacitance / Fg-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 G/SiC SiC

60 40 20

f 0 0,0

4

5,0x10

Current density / Ag-1

5

1,0x10

5

1,5x10

5

2,0x10

5

2,5x10

Cycles

Figure 9. Cyclic voltammetry, in the range 0-1V, of SiC (a), G/SiC (b). Galvanostatic charge/discharge curves of SiC (c) and G/SiC (d). Specific capacitance of SiC and G/SiC electrodes at different scan rate (e). Capacitance retention at 4 A/g (f).

35



5

5000

20 mV/sec

G/SiC SiC

4000

Current / mAcm-2

Power density (W/kg)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 38

3000 2000

0

-5

1000

a 10

b -10 -0,6

100

-0,4

Energy density (Wh/kg)

-0,2

0,0

0,2

0,4

0,6

Potential (V)

Figure 10. Ragone Plot for G/SiC and SiC (a). Cyclic voltammetry of G/SiC in the range -0.50.5 V(b).

36


Page 37 of 38

-10 -8 Z'' (ohm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-6 -4 -2 G/SiC SiC

0 0

1

2

3

4

5

6

Z' (ohm)

Figure 11. Nyquist plots of SiC and G/SiC.

37



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Supercapacitor Electrodes Made of Exhausted Activated Carbon

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