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Capacitive Behavior of Azide Functionalized Graphene Oxide as Electrode Materials

Utkan Şahintürk, Ahmet Ekerim

Yıldız Technical University - Türkiye

Abstract

Graphene based electrode materials for supercapacitors have been reported with the specific capacitance. Here GO was prepared by a modified Hummer’s method. We introduce a synthetic path to functionalize GO in a controlled manner through selectively functionalizing the basal plane of GO with azide in freeze-dried solids. This product azide-functionalized grapheneoxide (GO-N3) provides a versatile reactive motif for further chemical reactions such as electrochemical reactions on this electroactive sites. Based on this consideration, nickel foam (NF) coated as working electrode with GO and GO-N3 ultrasonically. Prepared GO-N3 electrode was reduced electrochemically after 200 cycles. Capacitive behaviour of GO-N3 and RGO-N3 have been investigated.

The microstructure and surface functionalities of GO-N3 were determined by scanning electron microscopy (SEM), energy dispersive (EDX) and Fourier Transform Infrared (FT-IR) spectroscopies. The electrochemical properties of the GO and GO-N3 materials compared by cyclic voltammetry (CV) through modification of NF.

1. Introduction

Graphene is a single layer of carbon atoms with a hexagonal arrangement in a twodimensional lattice. Its high thermal conductivity, high specific surface area, excellent electronic conductivity, and huge theoretical surface area (2630 m2 g 1) make it promising for potential applications in supercapacitor electrodes.Graphene sheets derived from exfoliation of the graphite oxide formed by chemical oxidization of graphite are intrinsically decorated by abundant active sites, such as functional groups (mainly –OH, –COOH, –C=O, C–O–C–) and lattice defects (atom vacancy, distortion, dangling bonds) on the lateral surface and at their edges. The presence of functional groups on graphene is essential with respect to many active species such as metal oxide have been introduced to improve the pseudocapacitance (PC) as well as the whole performance of graphene-based supercapacitor electrodes. However, such hybrids usually deliver poor rate capability owing to the large volumetric change or degradation

during charge/discharge process. Therefore, it is still a big challenge to explore the basic building blocks of graphene with large double-layer capacitance (EDLC) and PC due to fast and reversible surface redox processes between the electrolyte and various electroactive species on graphene electrode surface. Generally, heteroatom modification has been proven to be the most promising method for enhancing the capacity, surface wettability of materials, and electronic conductivity, while maintaining a good cycling performance for instance, superior supercapacitor performance was available on heteroatom-modified nanocarbon. Modification of graphene structure with hetero atoms can tremendously change the electronic properties with can enhance its performance in the diversity fields of applications Commonly used heteroatom namely S, N, P and B-doped graphene. Among them, recently N-doped graphene has attracted attentions due to is various applications.The doping of nitrogen (N) atom in graphene structure can enhance the electron mobility which leads to a larger capacitance because of its strong valance bond and atomic size[1-6].

2. Experimental Procedure

2.1. Chemicals and equipment

The experimental studies were carried on with high puritychemicals. Ultra-pure water ( 18 M , Milli-Q, Millipore) were used as solvents. Electrochemical grade potassium hydroxide (KOH) were purchased from Aldrich and used without further purification. Graphite powder was purchased from Alfa Aesar (Graphite flake, natural, 325 mesh, 99.8%metal basis), potassium permanganate (KMnO4), sodium nitrate (NaNO3), sulfuric acid (H2SO4) were obtained from Merck andhydrogen peroxide (H2O2), sodium azide (NaN3) was from Sigma-Aldrich, and used as received. Freeze-drying was accomplished on a Christ Alpha 1-2 LD from Martin Christ, Germany. For centrifugation a Nuve NF800 centrifuge was used. The FT-IR spectra were recorded in the 4000–400 cm 1 range on Perkin Elmer Spectrum One (ATR sampling accessory) spectrophotometers. The electronic spectra and absorbance measurements were recorded on Agile 8453 UV–visspectrophotometer.

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2.2 Synthesis of graphene oxide (GO) and azido graphene oxide (GO-N3)

GO in Fig.1 were prepared according to previously reported procedures with minor changes and characterized by comparing their spectraldata to those reported earlier [7,8].

Fig.1 Oxidation of graphite. Graphite powder (0.5g) and sodium nitrate (0.5 g) were placed in concentrated H2SO4 (95–98 %, 23 mL). The ingredients were mixed in an ice-bath that had beencooled to 0 °C. KMnO4 (3.0 g) was added gradually with stirring and cooling and the mixture was stirred at 35 °C for 60 min. Then, water(40 mL) was added slowly to an increase in temperature to go upand was held with stirring at 90 °C for 30 min. Again, H2O2 solution (30%, 3 mL) on ice-water (100 mL) were added until gas evolutionwas completed. Upon this treatment the suspension turned in colorbrown to yellow. The suspension was filtered while it was still warm. After washing with water (200 mL). The residue solid wasdispersed in water and then centrifuged at 8000 rpm for 15 min two times. Finally, the resultant pure GO was obtained after it wasdried at 40 °C for 24 in vacuum. GO was functionalized with sodium azide to product azide-functionalized grapheneoxide (GO-N3) in Fig.2-3-4 and characterized by comparing their spectral data to those reported earlier [9].

Fig.2 Azide functionalization of grapheneoxide.

Fig.3 Different forms of N and O functionalities in carbon materials.

Fig.4 Electrochemically reduced graphene oxide.

2.3 Electrode preparation

4.0 mg GO and GO-N3 and were dissolved in 4.0 mL ultra-pure DMF under ultrasonic bath for 30 min. Then 50 μL the solutions was casted on NF ultrasonically and was dried in vacuum at 50 °C for 120 min (Fig.5).

Fig.5 Prepared working electrodes on NF. Finally the electrodes rinsed several times with water and KOH. A platin (Pt) wire, and saturated calomel electrode (SCE) (E=+0.241 V) separated

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from the bulk of the solution by a double bridge were used as counter and reference electrodes respectively. KOH in ultra-pure water was the supporting electrolyte at a concentration of 6 mol dm 3. Repeating 10 CV cycles between 1.00 V and + 0.00 V in KOH/H2O electrolyte system were employed to reduce GO-N3 and to form NF/RGO-N3 at 50 mVs-1 scan rate. Then NF/RGO-N3 electrode was dried in vacuumat 50 °C for 120 min. NF/RGO-N3 rinsed several times with water and KOH used as the working electrode (Fig.6).

Fig.6 Electrochemical Cell 2.4 Electrochemical measurements

The electrochemical applications and measurements wereperformed with a potentiostat (GAMRY Instruments, Reference 1000 Potentiostat/ Galvanostat/ ZRA) utilizing a three-electrode cell configuration at 25°C. For cyclic voltammetry (CV) the working electrodes were a bare and modified nickel foams with GO-N3 and RGO-N3 same surface area of 1 cm2. High purity N2 was used to remove dissolved O2 for at least 15 min prior to each run and to maintain a nitrogen atmosphere over the solution during the measurements.

2.5 Capacitance Calculation

The specific capacitance of the electrode (C in Fg 1) was calculated from CV curves by the following equation:

𝑉 𝑉 𝑖𝑉𝑑𝑉 𝑉 𝑉 𝑚𝑣

where 𝑣 in V s 1 is the scan rate, m in g is the mass of active materials in the electrode, V in V is the potential window, V0 in V is the initial value of

potential window, and i(V) in A is the real time current during scanning.

3. Results and Discussion

GO-N3 was hydrolytically stable at room temperature and further proved the azide functionalization with the FTIR absorption at 2049 and 2113 cm 1. GO and GO-N3 FTIR spectra are compared as shown in Fig. 7.

Fig.7 FTIR spectra of GO and GO-N3

To give further evidence ofthe controlled mild functionalization of GO with azide, we used the EDX spectroscopy experiment. The composition of this azide modified structure is confirmed by the SEM-EDX spectroscopy whichreveals the presence of peak for N along with carbon and oxygen peaks (Fig. 8-a-b).

Fig.8.a EDS Spectra of GO

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Fig.8.b EDS Spectra of GO-N3

The morphology of these two materials (GO and GO-N3) was observed by SEM. From the SEM images (Fig.9), it can be seen that the GO sheets was found more stacking of sheets, but the GO-N3 sheets were more separated flake like a good lamellar structure, and rich wrinkles structures on the surface for the usual N-doped graphene[31].

Fig.9 SEM images of GO and GO-N3 respectively.

Fig.10 Cyclic voltammogram of NF and NF/GO-N3.

Fig.11 Electrochemical reduction of NF/GO-N3

Table 1. Specific capacitance of Ni foam at 50 mVs-1 scan rates in 6 M KOH.

Working Electrode

Specific capacitance from CV (F/g)

Bare NF 1,48 NF/GO-N3 115,35

NF/RGO-N3 209,6 4.Conclusion

In this work, hierarchically azidated grapheneoxide have been successfully prepared graphite oxide and subsequent azide-functionalization by chemical modification of the 2D basic building block graphene. Ion diffusion improvement through structural optimization of 3D honeycomb nanostructure, the functional 3D assembly of RGO-N3 shows high specific capacitance by cylic voltammetry measurements. In prospect, this indicates a promising way for hierarchical graphene honeycombs with tunable surface chemistry and mediated porous structure. The integration of graphene into hierarchical structures provides a maximum utilization of the excellent properties of graphene and could lead to promising 3D macrostructures. Such porous hierarchical architectures will benefit applications especially energy storage devices, such as supercapacitors, which require fast mass transfer through mesoporous, reactant reservoirs, and tunable surface chemistry. 5. Acknowledgment

This research has been supported by Y ld z Technical University Scientific Research Projects

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Coordination Department. Project Numbers: 2014-07-02-DOP01 and 2014-07-02-KAP01. The authors would like to thank Dr. Bahadir Keskin for the FT-IR measurements.

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