Insight on the effect of surface modification by carbon materials on the Ionic Liquid Electric Double Layer Charge Storage properties

Accepted Manuscript Title: Insight on the effect of surface modification by carbon materials on the Ionic Liquid Electric Double Layer Charge Storage properties Author: Renata Costa Carlos M. Pereira A.Fernando Silva PII: DOI: Reference:

S0013-4686(15)30058-X http://dx.doi.org/doi:10.1016/j.electacta.2015.06.142 EA 25267

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

22-4-2015 30-6-2015 30-6-2015

Please cite this article as: Renata Costa, Carlos M.Pereira, A.Fernando Silva, Insight on the effect of surface modification by carbon materials on the Ionic Liquid Electric Double Layer Charge Storage properties, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.06.142 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Insight on the effect of surface modification by carbon materials on the Ionic Liquid Electric Double Layer Charge Storage properties Renata Costa, Carlos M. Pereira, A. Fernando Silva Faculdade de Ciências da Universidade do Porto, Departamento de Química e Bioquímica, CIQUP-Physical Analytical Chemistry and Electrochemistry Group Rua do Campo Alegre, s/n 4169 – 007 Porto, Portugal Email: [email protected]; Fax: +351 220402659; Tel: +351 220402613

Graphical abstract

Highlights  Graphene modified glassy carbon electrode dramatically increases the capacitive current at electrode/ionic liquid interface.  The increase of the capacitive current is proportional to the amount of graphene.  The exfoliation process, before the reduction of graphite oxide, is remarkably important in order to achieve higher double layer capacitances.  The imidazolium cation seems to preferably interact with graphene via their alkyl side chains promoting the hydrophobic interactions with the imidazolium ring being slightly tilted.

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Abstract Development of new and better performing energy storage devices rely on the use of innovative materials. The combination of nanostructured materials with high specific area, such as graphene, and conventional electrode materials such as Glassy Carbon (GC) and the use of the composite electrode in ionic liquids are considered to be a promising strategy for improved energy storage devices. Following our previous studies of electrochemical interfaces involving 1-butyl3-methylimidazolium

( tris(pentafluoroethyl)trifluorophosphate)

([C4 MIM][FAP]) ionic liquid and Hg, Au, Pt and GC we extended the search for better electrochemical performance by preparing GC electrode surfaces modified with different carbon materials (reduced graphene oxide, reduced graphite oxide and graphite). Cyclic voltammetry of these electrode surfaces in [C4 MIM][FAP] ionic liquid shows a 100 fold increase in the capacitive current of the composite electrode when compared with plain GC electrode.

Introduction

The interaction of ionic liquids with charged surfaces is still relevant for the understanding of the specific interactions on the molecular arrangement near surface region [1-4]. Ionic liquids enclose chemical and structural diversity with multiple molecular interactions. The complexity of ionic liquids at electrified interface stems from on the complex structure and nature of the ions [5], its purity [6] and is particularly dependent on the specific interactions between the electrode material and the IL [7,8]. The relationship between differential capacitance curves behavior with the EDL structure has become the centerpiece to many IL EDL models [9-14]. Several

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theoretical works has been performed in order to evaluate the influence of electrode roughness on the double layer formation in ionic liquids [15-18]. From the experimental point of view, it has been demonstrated that the nature of the electrode surface significantly affect the overall shape of the C(E) curves, and consequently, the arrangement of the ions at the electrified surface [19]. Very recently, a study aimed to assess the effect of the electrode surface structure roughness on EDL was published [20]. The authors compared a oriented surface of thin Au (111) film on mica with a polycrystalline Au disc surface in contact with the non-aromatic pyrrolidinium (1-butyl-1-methylpyrrolidinium bis(trifluoromethane)sulfonimide) ionic liquid and found that the electrode surface structure and roughness have weak effect on the fast process occurring on the double layer formation while having a strong influence on the slower processes. Despite the various studies reported in the literature, the influence of the electrode surface properties on the Electric Double Layer (EDL) structure is still only partially understood [2,21]. Models that do not accommodate the molecular-scale effects of ion geometry (e.g. finite volume of ions) and also the impact of the electrode surface nature and structure on the C(E) curves, will fail to give a realistic theoretical picture of the RTIL/electrode interface. Controlling the electrode nature and structure (topography and geometry) can be a route to obtain relevant information on the EDL as indicated by its fundamental property such as differential capacitance. Over the past decade, ionic liquids and graphene type materials have received a great deal of attention. Many experimental and theoretical works emerged focusing on the dispersion of carbon nanomaterials in ILs and on the importance of the intermolecular interactions between graphene and the IL [22].

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Graphene is composed of a honeycomb arrangement of π-electron-rich carbon material, sp2 – hybridized with two-dimensional nanostructure [23,24]. Graphene properties and characteristics have been widely described [25-32]. Recently, Bianco et al. [33] proposed a nomenclature to a more precise description of the graphene based subject materials so that confusion and inconsistency would be minimized in order to get a better systematization of terms associated with materials based on graphene. The effect of surface modification with high specific area materials such graphene is commonly discussed in the literature and is of great importance for systems which require an electrode with high and more accessible surface area such as supercapacitors [34,35]. The microscopic interaction between graphene nanomaterial and ILs can be explained by short-range interactions involving ILs cation- π interaction of carbon nanomaterials and by the long range dispersion interactions. Very recently, Fedorov and Lynden-Bell have been probing the mechanisms of the interfacial layer formation of the neutral graphene/1,3-dimethyimidazolium chloride (ionic liquid) interface by molecular dynamics simulation [36]. Ivaništšev et al. [37] published a very interesting approach using a molecular dynamics-based simulation to obtain the free energy profiles in order to evaluate the influence of two ionic liquid solvents (1-butyl 3-methylimidazolium tetrafluoroborate and 1,3-dimethylimidazolium chloride) on the interaction of dissolved ions with a graphene electrode surface. The capacitance-potential curves of the EDL obtained from a computational study performed by Paek et al. [38] on the interfacial structure of graphene and [C4MIM][PF6] IL were found to exhibit convex- or bellshape. The MD simulations clearly demonstrate the distinct alternative layering of and

in the vicinity of an electrified graphene surface with the authors

assigning particular importance of the quantum capacitance contribution to the total interfacial capacitance between graphene and [C4MIM][PF6]. 4

In line with a previous work centered on the role of the electrode material on the IL EDL structure, this work was outlined in order to provide a relatively inexpensive way of improving the energy storage properties of electrochemical devices through the modification of a glassy carbon electrode with carbon materials in contact with an ionic liquid ([C4MIM][FAP]). Previous experience on the use of [C4MIM][FAP] for studies on electrified interfaces [19] indicate the suitability of this liquid since it demonstrates to have excellent hydrolytic stability, low viscosity and high electrochemical and thermal stability. The reported results underline the importance of combining high specific area materials such as reduced graphene oxide modified electrodes with distinctive ILs to achieve higher density currents and higher capacitances. Detailed experimental molecular information on the surface chemistry of graphene is limited, however this work contributes to highlight the importance of ionic liquid- reduced graphene oxide interactions at a charged and planar electrode. This may be very important to develop applications stemmed from surface interactions of high specific area carbon materials using ionic liquids as electrolytes for advanced electrochemical storage devices.

Experimental The hydrophobic ionic liquid [C4MIM][FAP] was purchased to Merck with the highest purity grade available (higher than 99 %). Graphite (Aldrich, 1-2 micron), hydrazine hydrate (Aldrich, 98 %), methanol (Merck, 99.9 %), sulfuric acid (Aldrich, 95-97 %), potassium chloride (Merck, 99.5 %), potassium permanganate (Merck), dimethyl formamide (DMF, Merck, 99.8 %) were all used without further purification. Graphite oxide was prepared from graphite powders by following a method described by Hummers and Offeman [39]. Briefly, 2.0 g of graphite powder was first added into

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100 mL concentrated H2SO4 at room temperature. Under stirring, the mixture was cooled to 5 °C using an ice bath, and the temperature of the mixture was kept below 5 °C for 30 min. KMnO4 (8.0 g) was then added gradually under stirring and cooling. 100 mL distilled water was added into the mixture, stirred for 1 h, and further diluted to approximately 300 mL with distilled water. After that, 20 mL of 30% H2O2 was added to the mixture to reduce the residual KMnO4. The solid was filtered, washed with 5% HCl aqueous solution (800 mL) to remove metal ions and with ultrapure water until the pH was 6. The resulting graphite oxide was dried at 30 °C for 24 h. The chemical reduction of graphite oxide was adapted from reference [40]. The chemically reduced graphene oxide was prepared by were prepared by dispersing the graphite oxide with ultrapure water (3 mg/mL) to yield a yellow-brown suspension followed by sonication of this mixture in 1 L ultrasonic bath (Bandelin Sonorex digitec) for 3 h. The sonicated yellow-brown suspension was then treated with hydrazine hydrate, and the mixture was heated in an oil bath at about 95 °C in a water-cooled condenser for about 12 h. The reduced graphene oxide was obtained as a black solid and was filtered and washed with deionized water (5 × 100 mL) and methanol (5 × 20 mL). Following this, the precipitate was dried at room temperature for about 24h. The immobilization of graphene was preceded by the preparation of a dispersion of 10 mg of graphene particles in 1 mL of DMF followed by ultrasonication for about 6 h to facilitate the complete dispersion of rGO. Different volumes (0.5, 1.0 and 1.5 L) of this dispersion were then spread on the glassy carbon electrode surface using a micropipette. The solvent evaporation was performed at room temperature followed by a more thorough drying step placing the electrode in a high vacuum line for a period of 12 h. The glassy carbon electrode was then ready to be immersed in the ionic liquid.

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IL purification procedure included the washing of the ionic liquid several consecutive times using ultra-pure water under stirring in order to dissolve and remove water soluble impurities. To reduce the water content to an acceptable minimum level, the IL was heated for several hours at 70-80 °C under vacuum (≈ 10 Pa) and under continuous stirring. The final water content, according to Karl-Fisher titration (831 KF coulometer Metrohm) was below 30 ppm. Before each experiment all glass material was washed with concentrated sulfuric acid followed by abundant washing with ultra-pure water and finally with boiling ultra-pure water. The ultra-pure water was obtained by filtration through the purification system Milli-Q deionized water with a volume resistivity of not less than 18.2 Mcm The X-ray diffraction patterns were obtained in a Siemens D5000 X-ray difractometer and were carried out with Cu Kα radiation (=1.54056) using an operating voltage of 40 kV, a step size of 0.01°. This equipment consists of a theta/2theta diffraction instrument operating in the reflection geometry, focused by a primary Ge crystal monochromator. The detector is a standard scintillation counter. Surface area was measured using the BET Brunauer-Emmett-Teller (BET) Analyser Micromeritics Tristar II analyzer through nitrogen gas adsorption–desorption isotherms at 77.3 K. Prior to measurements, the sample was out gassed at 80º C for 1 h and at 100 °C for 5 h. Electrochemical experiments were carried out in a water jacketed three-electrode cell and the temperature was kept constant by the use of a thermostated bath. The experiments were performed inside a N2-filled glove box to prevent oxygen and moisture interference. The working electrode used was a GC electrode (Metrohm), the counter electrode was made of glassy carbon, and a silver wire was used as a quasireference electrode connected to an Autolab 302N. During the experiments, N2 was kept

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over the electrolyte without disturbing cell configuration. Capacitance and current intensities were normalized using the GC working electrode geometrical area (0.0314 cm2). Before each study, the electrode was polished with 1 micron diamond paste Buehler during 2 minutes and washed with abundant Millipore water to remove any diamond paste trace remaining in the electrode surface.

Results Carbon materials characterization

Figure 1 a) shows a SEM image of a flake of exfoliated hydrazine-reduced graphene oxide with a transparent aspect and a slight thin wrinkled structure typically observed in graphene sheets [41]. The wrinkled and rippled structure may be a result of deformation upon the exfoliation process. SEM images in Figure 1 b,c and d) were obtained after surface immobilization of the reduced graphene oxide flakes and point to the possibility of rGO flakes being overlapped or aggregated. Under this hypothesis the immobilization of rGO on the glassy carbon electrode will maintain a high fraction of electrode surface non-covered by the carbon material thus in direct contact with the electrolyte. The structural properties of graphite and the reduced material rGO (with and without exfoliation) were characterized by X-ray diffraction (XRD) and the XRD patterns are presented in Figure 2. For the graphite powder, the XRD profile presents a sharp and intense peak at 2θ = 26.6º which has been attributed to the (002) plane and is in agreement with the value found in the literature [42].

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By analyzing the XRD pattern of rGO, after chemical reduction by hydrazine, the sharp (002) peak observed for graphite powder disappeared to give place to a broad peak with markedly lower intensity. For the rGO sample the intensity of the broad peak is lower than the reduced graphite oxide sample. This broad peak has been ascribed to the partial restacking of the graphene layers and may indicates disorder created during synthesis by modified Hummer’s method [43]. The BET surface area was obtained from the nitrogen adsorption–desorption isotherms shown in Figure 3. The sample has a BET surface area of 30.6 ± 0.2 m²/g and a pore volume of 0.00214 cm³/g measured by nitrogen adsorption at 77 K. The adsorption isotherm shows a slight adsorption in the low pressure region (<0.1 P/P0) and a steep adsorption step at pressures higher than 0.9 P/P0, with a hysteresis loop in the desorption branch. The hysteresis loop and low pressure adsorption also indicate the formation of mesopores and a small amount of micropores, respectively [44]. The presence of microporosity, mesoporosity, and some macroporosity characteristics is evidenced by the shape of the isotherms and corroborated by the average pore diameter ranging between 1.86 and 54.84 nm. The BET surface area of 52 ± 5 m2/g is significantly lower than the theoretical surface area of 2630 m2/g for individual graphene sheets[45] or 456 m2/g for graphene oxide [46]. Although this can be considered a low surface area value, it should be stressed that the BET surface area of the starting material (graphite 1-2 m) is very low (10 ± 1 m2/g) and the increase in surface area (5.2 fold increase) is of the same order of magnitude found in the literature for the formation of reduced graphene oxide from carbon raw materials [46]. Furthermore a reduced value of BET surface area can be the consequence of a significantly amount of surface area that is not accessible to nitrogen adsorption due to the overlapping (re-stacking) as suggested above [45] or/and by the

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agglomeration of the exfoliated graphene layers. This may result from the van der Waals interactions occurring between the adjacent graphene sheets. The electrochemical characterization of graphene surface was initially assessed by CV using the redox probes [Fe(CN)6]3-/4- and [Ru(NH3)6]3+ in 1.0 M KCl aqueous solution and is described in Figure S1 and S2 in the supplementary information section. Increasing the amount of the immobilized reduced graphene oxide on the GC surface resulted in an increase in the redox peak current, while keeping nearly constant the value of ΔEp for the redox probe (difference between the peak potential of the oxidation and reduction processes). This may reflect the presence of similar microstructural features independent of the mass of rGO on the electrode. The effect of the presence of reduced graphite oxide and reduced graphene oxide on the polarization response of the GC/[C4MIM][FAP] interface was followed by cyclic voltammetry (Figure 4). From the analysis of Figure 4 it is clear that the exfoliation process, before the reduction with hydrazine undoubtedly enhance the electrochemical performance of graphene, namely an increase in the charging/discharging current and an increase in the operative voltage window up to 3.8 V when compared to reported values in the range of 2.3–2.7 V [47] for activated carbon (AC) in contact with a quaternary ammonium salt dissolved in propylene carbonate (PC) or acetonitrile (ACN). Figure 5 shows the results of a systematic study carried out in order to evaluate the effect of increasing the amount of rGO on CVs. Similar studies were carried out using glassy carbon modified with graphite powder dispersed (10 mg/mL) in DMF (the starting material on graphene synthesis). The results shown in Figure 5 supports the proposal that large surface area of the immobilized carbon material lead to a significant increase in the capacitance at glassy

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carbon-rGO modified electrode/[C4MIM][FAP] interface. In addition, since its active surface forms effectively a large flat structure, charge transport in reduced graphene oxide seems to be much more efficient than in glassy carbon without modification. Figure 5 also reveals the enhancement of the rectangular shape on the voltammetry characteristics, reflecting an increase on the ideal capacitance behavior of the carbon material electrode as a result of the systematic increasing of rGO amount on the GC electrode. For the graphite powder, the capacitive current is slightly higher when compared with GC surface without modification, however in the same order of magnitude of that found at a bare glassy carbon electrode. The capacitive response of the three systems composed with different amounts of rGO was evaluated by cycling between -2.0 and 1.8 V vs. Ag varying the scan rate from 10 mV/s to 100 mV/s. The results are also presented as supplementary information (Figure S4 (a, b and c), supplementary information section). The difference between the structured reduced graphene oxide and the structureless glassy carbon electrodes is manifested mainly by the increase in capacitive current. It is remarkable the observed regular increase of the capacitive current with the increasing amount of rGO on the surface and with the increase of the scan rate. The rising of the capacitive currents observed in the presence of rGO on the glassy carbon surface seems to be the result of higher electroactive area and enhanced surface wettability (which is evident in analyzing contact angle values that can be found in figure S5 as supplementary information). The attempts to obtain differential capacitance values from EIS (Electrochemical Impedance Spectroscopy) data were not successful since it proved to be impossible to fit the data with a suitable equivalent circuit. Therefore, we followed a different approach to obtain capacitance values, although we are aware that techniques such as

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CV, long-term potentiostatic and galvanostatic tests, and EIS can give very different capacitance values. The specific capacitance was calculated by integration of the CV using equation 1 [48].

(1) where Q represents the charge and U the potential interval. Figure 6 displays the effect of the amount of graphene on the capacitance extracted from the CV using equation 1. The potential dependence of specific capacitance extracted from CVs shows a maximum at negative potentials and a minimum at positive potentials. The specific capacitance systematically increases with the increase of the amount of rGO at the electrode surface which are in line with experimental and simulation work recently published. The work of Ghatee et al. revealed a strong interaction and a preferred orientation of the ion pair to the graphene solid surface [49]. The results obtained by sum frequency generation vibrational spectroscopy and contact angle measurements suggest a significant interaction of the alkyl chains of the cation with graphene, which are extended parallel to the surface and the imidazolium ring is slightly tilted to the surface plane of graphene promoting the possibility for π–π interactions between imidazolium ring and the surface [50]. Similar trend was found in a Molecular Dynamics simulations performed at [C4MIM][PF6]/neutral hydrophobic graphene surface interface. That results point to the possibility of the imidazolium ring of

cations lying flat on

the graphene surface, with its methyl and butyl side chains elongated along the graphene surface. Studies carried out by Fedorov et al. [36] using atomistic molecular dynamics

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simulations reveal significant enrichment of cations at the neutral graphene surface with cation imidazolium rings being parallel to the surface. Despite the relevant scientific and practical importance of understanding the effect of temperature on the electrochemical performance of the ionic liquid/electrode interface, the available data is still scarce especially in the case of interfaces involving graphene. Furthermore, in most cases, the available data concerns mixtures of ionic liquids with molecular solvents such as acetonitrile [51].The interface GC-rGO/[C4MIM][FAP] with 1.5 L of rGO was selected to study the effect of temperature on the electrochemical behavior and the results of which are presented in Figure 7 a). The effect of varying the temperature on the electrochemical window of the GC-rGO/ionic liquid indicates that increasing the temperature reduces both potential limits and decreases the amplitude of the intensity current of the voltammogram. The effect of temperature on the capacitance is illustrated in Figure 7 b). Three potentials regions of effect of temperature can be identified in Figure 7 b): A large negative potential up to the maxima, the effect of increasing temperature is very small. The potential of maxima and minimum shifts in the positive direction with increasing of temperature. The second potential region comprehends the potential range between the maxima and the minimum where the temperature coefficient is negative. At far positive potentials, the specific capacitance rises with temperature and both potential region are apparently separated by an isosbestic point. Generally, as described in the literature and from our previous experience, the capacitance increases with increasing temperature, however, sometimes the magnitude of the increase is not significant (essentially for the smooth Hg surface) and it is not uniform throughout the potential window. This effect seems to be more pronounced in the maximum and minimum curves of C(E) and near the potential limits. Similar trend

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is also observed in molten salts, however, there is no generally accepted explanation to describe this behavior [52]. For molten salts, some authors based their interpretation on the assumption that the rise in temperature creates an increase in the number of holes in the structure of the molten salt [53,54]. For ionic liquids, Lockett et al.[55] suggested that growth and change in the profile of potential-capacity curves with increasing temperature results from a decrease in the association of the ions in the double layer. The increase in C(E) curves with rising temperature was also reported by Silva et al. [56] for [C4MIM][PF6] in contact with Hg, Pt and GC. The authors put forward a similar reason, i.e. there is an increase in the concentration of ions in the interface as simple consequence of the breakdown of aggregates when the temperature increases. The theoretical models developed for the double layer of molten salts and RTILs suggests that this phenomenon may be associated with a probable reduction in the electrostatic interactions between the ions with increasing temperature [57]. By contrast, negative temperature coefficients of the differential capacitance have also been experimentally obtained [58] and predicted by molecular dynamics simulations [59]. C(E) curves with negative and positive coefficients have been obtained experimentally for dicationic ionic liquids [8] and also were predicted by density functional theory (DFT) [60], Monte Carlo simulations [61] and the mean spherical approximation (MSA) theory of concentrated electrolytes [57]. Reorientation of the cation at the rGO surface with increasing temperature apparently from a tilted position as observed for graphite to a more parallel position would decrease the thickness of the EDL leading to an increase in the capacitance. Experimentally the opposite is observed which points to the existence of other factors which are affecting the capacitance. It may be suggested that increasing the temperature

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affects the substrate-rGO interaction and eventually causing removal of the carbon material from the electrode surface. The observed effect is certainly the result of a complex interplay of the effect of temperature on the surface properties and on the ionic liquid heterogeneities.

Conclusion

Reduced graphene oxide modified GC electrodes in contact with [C4MIM][FAP] ionic liquid show a very stable electrochemical behavior, in particular they display an outstanding increase in capacitive current making this system suitable candidate for electrode material application for supercapacitor devices using ionic liquids as electrolytes. Even a small amount of rGO (0.5 L) can increase the capacitance approximately 100 % when compared with GC without modification. Graphite composite electrodes/[C4MIM][FAP] interface also display higher current densities, however they are of the same order of magnitude of those obtained at the bare glassy carbon electrode pointing out to the outstanding properties of rGO materials. These preliminary results were obtained with non-optimized rGO materials. Therefore, improving the purification steps and preventing rGO from re-stacking may play a key role in further improvement of the electrochemical performance of carbon materialsbased materials, namely by maximizing the double layer capacitance per unit area. The capacitance curves extracted from the CVs are asymmetric and exhibit a local maximum (-0.75 V (vs. Ag, 60 ºC)) and a local minimum at approximately + 0.65 V (vs. Ag, 60 ºC). The effect of temperature on the electrochemical stability of the system was also evaluated yielding simultaneously two opposite temperature coefficients separated by an isobestic point.

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Acknowledgements

This research was carried out with financial support of FCT – CIQUP – Physical Analytical Chemistry and Electrochemistry group (PEST-C/QUI/UI0081/2013) and Renata Costa acknowledges a Pos-Doc scholarship awarded by FCT with reference SFRH/BPD/89752/2012 under the QREN - POPH - Advanced Training, subsidized by the European Union and national MEC funds.

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Figure Error! Main Document Only.– Scanning electron microscopy (SEM) images: a) sheet of r educed gr aphene oxide, b, c and d) immobilization of 0.5 L of r educed gr aphene oxide on the GC electrode.

400

200

graphite

0 0

5

10

15

20

25

30

35

15

20

25

30

35

15

20

25

30

35

Intensity (a.u.)

15 10

reduced graphite oxide

5 0 0

5

10

15 10

reduced graphene oxide

5 0 0

5

10

2/degree

Figure Error! Main Document Only. - XRD patterns of graphite, reduced graphite oxide and r educed gr aphene oxide samples.

21

Quantity adsorbed (cm3/cm STP)

60

40

20

N2 Adsorption 0 0.0

N2 Desorption 0.2

0.4

0.6

0.8

1.0

o

Relative pressure (p/p )

Figure Error! Main Document Only. - Nitrogen adsorption–desorption isotherms of reduced graphene oxide obtained at 77.3 K.

1500

1000

j/A.cm

-2

500

0

-500

-1000

-1500 -3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

E/V vs Ag

Figure Error! Main Document Only. - Effect of exfoliation of carbon material in hydrazine reduction process on electrochemical performance of glassy carbon modified electrode at [C4MIM][FAP] interface (black: without modification, red: reduced graphite oxide and green reduced gr aphene oxide (0.5 L)).

22

4000

a)

j/A.cm

-2

2000

0

A

-2000

-4000 -3 100

-2

-1

0

1

2

3

0

1

2

3

b)

75

j/A.cm

-2

50 25 0 A

-25 -50 -75 -100 -3

-2

-1

E/V vs Ag

Figure Error! Main Document Only. – a) Effect of different carbon materials (graphite, reduced graphene oxide, reduced graphite oxide and graphite) modification on CV´s measured at GC/[C4MIM][FAP] interface, b) Zoom of the region A presented in Figure 5 a).

600

C/F.cm

-2

400

200

VrGO/L 0 0.5 1.0 1.5

0 -3

-2

-1

0

1

2

3

E/V vs. Ag

Figure Error! Main Document Only. - Capacitance obtained at GC-rGO/[C4MIM][FAP] interface with different quantities of rGO.

23

2000

a)

1500 1000

j/A.cm

-2

500 0 -500 -1000 -1500 -2000 -3

-2

-1

0

1

2

3

800

b) 700

C/F.cm

-2

600 500 400

Temperature/ºC 20 30 40 50 60

300 200 -3

-2

-1

0

1

2

3

E/V vs. Ag

Figure Error! Main Document Only. - Effect of temperature on capacitance measured at GCrGO/[C4MIM][FAP] interface (1.5 L rGO).

24