Functionalization of graphene with nitrogen using ethylenediaminetetraacetic acid and their electrochemical energy storage properties

Abstract

Recently there has been a considerable focus on the synthesis of nitrogen functionalized graphene for energy storage and conversion. Herein, we report a simple, economical and facile process for the synthesis of nitrogen containing graphene composite which can be scaled up for mass production by using a nitrogen containing organic compound, ethylenediaminetetraacetic acid (EDTA) and graphene oxide as precursors. From the XRD studies, the increase in the interlayer distance between the graphene sheets confirms the functionalization of graphene sheets and the FT-IR spectroscopic analysis revealed the presence of N-containing functional groups in N-doped graphene sheets. XPS analysis confirms the chemical nature of N-containing functional groups, and TG analysis showed the amount of EDTA loaded on the graphene sheets. This composite exhibits a large specific capacitance of 290 F g⁻¹ at 0.1 A g⁻¹ with a capacitance retention of 67% and 58% at high current densities of 10 and 20 A g⁻¹, respectively, thereby showing superior rate capability. In addition, it showed long-term electrochemical stability through 6000 charge–discharge cycles even at a high current density of 5 A g⁻¹ with a specific capacitance loss of 2%.

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Introduction

With the rapid increase in the energy consumption levels in today's economy there has arisen an inevitable necessity for an eco-friendly, high performance energy storage system. Li-ion batteries and supercapacitors are widely used for electrochemical energy storage applications.¹ For application in electric vehicles (EVs) and hybrid electric vehicles (HEVs), high power density is required in addition to high energy density and long life.² To meet such demands, supercapacitors are emerging as reliable energy storing devices with high cycle stabilities (>100 000 cycles),³ high power capability, low maintenance and fast dynamics of charge propagation.^4–7 As illustrated in the ‘Ragone plot’,⁸ which shows a comparison between various energy storage devices, supercapacitors occupy a vital position in specific power (10 kW kg⁻¹) as well as energy density (5 W h kg⁻¹).⁹ With a higher power capability than batteries and fuel cells and a larger energy density compared to conventional capacitors, supercapacitors are believed to be expedient to meet the increasing power demands of energy storage systems in the twenty first century. It is known that depending upon the charge storage mechanism, supercapacitors are classified into 3 categories namely electrical double layer capacitors (EDLC), pseudo-capacitors and hybrid capacitors.^8,10 Capacitance in EDLC's is attributed to the electrostatic (non-faradaic) charge accumulation at the electrode/electrolyte surface and therefore is strongly dependent on the surface area of the electrode materials accessible to the electrolyte ions.¹¹ Pseudocapacitance is faradaic in origin which involves electrosorption, intercalation and fast, reversible redox reactions between the electroactive species on the working electrode surface and the electrolyte. Hybrid capacitors aim at exploiting the relative advantages and alleviating the relative disadvantages of EDLC's and pseudocapacitors to achieve better performance. The use of both faradaic and non-faradaic mechanisms for charge storage has enabled hybrid capacitors to achieve high energy and power densities than EDLC's without sacrificing their cycling stability and affordability.

Although pseudocapacitors exhibit higher specific capacitance values than EDLC's, they lack cycling stability and rate capability due to redox reactions at the electrode interface.¹² Recent research focuses on enhancing energy densities of EDLC's by modifying electroactive material surfaces with oxygen,^13,14 nitrogen,^15–21 phosphorus²² containing functional groups. Graphene based materials are gaining popularity and are being extensively used as supercapacitor electrode materials exhibiting specific capacitance values ranging from 120 F g⁻¹ to 180 F g⁻¹.^23,24 Graphene nanocomposites with pseudocapacitive metal oxides and polymers are attempted to increase the specific capacitance of graphene.^25–27 Additionally, recent research has indicated that doping nitrogen into carbons in graphene helps in retaining cycling durability of materials with the simultaneous enhancement of specific capacitance.¹⁸ Nitrogen can be incorporated into graphene through various physical methods like chemical vapour deposition method,²⁸ hydrogen arc discharge method²⁹ and supercritical processes.³⁰ These processes however are expensive and require advanced equipments and cannot be employed for mass production. To combat these shortcomings there are many chemical methods which have been reported recently.¹⁶ And, nitrogen doping in graphene nanosheets showed a great enhancement in specific capacitance.³¹ The chemical nature and amount of nitrogen doping in graphene plays a vital role in supercapacitor performance.³¹ Nitrogen doped porous carbons have been prepared in the past from biomass derivatives such as glucosamine,³² chitosan,³² and gelatin.³³ To date many nitrogen containing organic precursors have been used as supercapacitor electrode materials with varying capacitance values. Some of them being acetonitrile,³⁴ ammonia,³⁵ hydrazine,³⁶ hexamethylenetetramine,³⁷ and urea.³⁸ Also, nitrogen containing polymers such as polyaniline,³⁹ polypyrrole,⁴⁰ and urea-formaldehyde⁴¹ have been employed to prepare nitrogen doped composites. EDTA functionalized graphene and carbon nanotubes haves been used for electro-oxidation of formic acid.^42,43 Here, we report a simple, facile and effective method to synthesize nitrogen functionalized graphene using ethylenediaminetetraacetic acid (EDTA) as the precursor. To the best of our knowledge, there have been no reports about functionalizing nitrogen into graphene using pure EDTA.

Experimental

Graphene oxide (GO) synthesized from natural graphite (Sigma Aldrich) by modified Hummer's method was exfoliated in water by ultrasonication.^44,45 The concentration of GO in the dispersion was found to be 2 mg ml⁻¹. 20 mg EDTA was dissolved in 75 ml of water at a pH = 9 (by addition of ammonia solution) under constant stirring condition for 2 h. Graphene–EDTA composite (GED) was prepared by reducing a homogeneous solution of GO and EDTA (1 : 1 wt ratio) using 250 μl hydrazine monohydrate. The obtained solution was washed with hot water several times in order to remove impurities. This was followed by reflux at 225 °C in a silicone oil bath for 4 h under vigorous stirring condition. After refluxing, the resultant product was naturally cooled to room temperature and the resulting black solution was filtered using a membrane filter (47 mm diameter, 0.45 micron pore size) and washed with water for several times. The black residue was collected and dried in a vacuum oven for 4 h at 60 °C.

Materials characterization

X-ray diffraction (XRD) patterns were acquired using powder X-ray diffraction measurements using a PAN Analytical X′ Per PRO Model X-ray Diffractometer with CuK_α radiation (α = 1.5418 Å) from 10–80° at 0.02° step and a count time of 0.2 s. Transmission electron microscopy (TEM) images were taken with a Tecnai 20 G2 (FEI, Netherlands) working at an accelerating voltage of 200 kV. The morphology and surface nature of the materials was characterized using scanning electron microscopy (SEM) using Hitachi, Japan (Model S-3000H) with an acceleration voltage of 5–30 kV. Fourier transform infrared spectroscopy (FT-IR) spectra were obtained using a Nexus 670 spectrometer (Thermo Electron Corporation make). The chemical nature of the prepared materials was characterized using a laser Raman system (RENISHAW Invia laser Raman microscope) equipped with a semiconducting laser with a wavelength of 633 nm. Thermogravimetric analyses (TGA) of the GED was carried out using TGA/DTA instruments (Model SDT Q 600) from room temperature to 800 °C with a heating rate of 5 °C min⁻¹ in air. The CHNS analysis was carried out to determine the composition of GED using Vario EL III IP20 (Germany make). The Energy dispersive X-ray Spectroscopy (EDX) and UV-Visible spectroscopy results was acquired using Carl Zeiss AG (Supra 55VP) and Agilent G11038, respectively. X-ray photoelectron spectroscopy (XPS) characterization was employed on MULTILAB 2000 Base system with X-ray with Twin Anode Mg/Al (300/400 W) as X-ray Source.

Evaluation of electrochemical properties

Typically, 2.5 ± 0.5 mg of the active material was kneaded with acetylene black (2 : 1 wt%) and dispersed in 5 drops of the binder (a homogenous dispersion prepared by ultrasonication of water and nafion (20 wt%) for 15 minutes). The working electrode was prepared by casting the above electroactive ink onto a Pt current collector. The prepared electrode was allowed to dry overnight at ambient temperature and pressure before conducting the electrochemical measurements. All electrochemical measurements were performed in a three electrode cell in which GED/Pt, Ag/AgCl electrode and a platinum plate were employed as the working electrode, reference and counter electrodes, respectively. 0.5 M H₂SO₄ served as the electrolyte at ambient temperature for all the electrochemical characterization experiments. Cyclic Voltammetry (CV) and galvanostatic charge–discharge measurements of the half cells were performed using Autolab MCUR70130 (Eco Chemie) in the potential range −0.35 V to +0.45 V vs. Ag/AgCl. Electrochemical impedance spectroscopy (EIS) measurements were carried out in the frequency range of 1 MHz to 1 mHz at open circuit potential (OCP). The specific capacitance values were calculated from the galvanostatic charge–discharge curves.

Results and discussion

Fig. 1a reveals the XRD patterns of GO, RGO and GED. A strong and sharp peak at 2θ = 11.86° observed for GO sample disappeared after reduction with hydrazine and a broad peak is obtained at 2θ = 24.6° for RGO. The peak for GED is observed at 2θ = 24.1° and the shift in 2θ value towards low angle for GED compared to RGO indicates increase in interlayer distance ‘d’ of GED due to presence of EDTA functional groups between the graphene layers. To confirm the EDTA functionalization on graphene sheets, FT-IR spectra of RGO, EDTA and GED were recorded and shown in Fig. 1b. RGO shows the presence of many oxygen functionalities and a broad absorption band in the range of 3300–3500 cm⁻¹ corresponds to the O–H groups, the carbonyl stretching was observed at 1700 cm⁻¹, the peak at 1630 cm⁻¹ is indicative of C=C stretching, O–H bending was observed at 1365 cm⁻¹, epoxy group vibrations are observed at 1100 cm⁻¹ and peroxide vibrations at 1023 cm⁻¹ are observed.³⁷ On the contrary, most of these oxygen containing functional groups are absent in the GED spectrum. In addition, GED shows peaks at 1026 cm⁻¹ and 1400 cm⁻¹ which correspond to the C–N stretching in the fingerprint region.³⁷ These peaks were inherited from nitrogen functionalization of RGO by EDTA. In addition to the functionalization, the quality of graphene nanosheets also plays a major role in supercapacitors. Thus, it is essential to understand the chemical nature of graphene nanosheets. It is known that Raman spectroscopy has been widely used to distinguish sp² and sp³ carbon in carbon materials.^46,47 FT-Raman spectroscopy of RGO and GED were recorded and shown in Fig. 1c. Both RGO and GED samples show two broad D-band and G-band corresponding to the A₂g phonon and e₂g phonon of graphene, respectively.⁴⁸ In comparison with RGO a small shift was observed in the D-band and G-band of GED indicating functionalization. The D and G band intensity ratio (I_D/I_G) indicates the defective nature of graphene nanosheets and the observed I_D/I_G ratio of GED (1.124) is greater than that of RGO (1.0178) indicating more number of defects in GED compared to RGO. Functionalization in GED was further confirmed from CHNS and FE-SEM EDX analysis, which indicated the presence of ∼4% of nitrogen in the GED.

Fig. 1 (a) XRD pattern of GO, RGO and GED, (b) FT-IR spectra of EDTA, GED and RGO, (c) Raman spectra of RGO and GED.

X-ray photoelectron technique is a standard analytical tool to identify and confirm various elements in the materials even at diminutive quantity. Also, it is widely used to identify the nature (oxidation state, chemical environment) of elements. Here, we used XPS to confirm the nitrogen doping/functionalization in GED and its bonding nature to carbon atoms in the graphene network. It approves the presence of nitrogen in GED arising from the stable nitrogen in the precursor which has endured reflux. In Fig. 2a, the XPS survey spectra of N-doped graphene shows three distinguished peaks at 284.6, 399.6 and 532.3 eV corresponding to C1s, N1s of the doped N, and O1s, respectively.⁴¹ On deconvoluting the N1s spectrum, three peaks are obtained at 399.3 eV, 400.6 eV and 402.3 eV due to the presence of pyridinic-N, pyrrolic-N and quaternary-N,¹⁶ respectively (Fig. 2b). From the peak intensities it is concluded that pyridinic-N content is higher than the other two forms. It is worthy to note here that among the three type of N-functionalization/doping, pyridinic type nitrogen showed better supercapacitance contribution. Similarly, the deconvolution of C1s peak results three peaks at 284.6, 285.8 and 289 eV (Fig. 2c), the sharp peak centered at 284.6 eV represents the sp² carbon in the N-doped graphene sheets.¹⁶ The peak at 285.8 eV confirms the existence of N-sp² C originating from the substitutional doping of N atom in GNS.^49–51 The peak observed at 289 eV is ascribed to the C–O bonding configurations in the N-doped GNS,⁵¹ indicates the existence of unreduced oxygen containing functional groups in GED. The broad peak observed from 530 to 536 eV confirms the presence of oxygen containing functional group in the GED nanocomposite (Fig. 2d). From the XPS spectra of GED, the presence of nitrogen was detected and this indicated that EDTA can be used as an effective agent to functionalize graphene oxide with nitrogen.

Fig. 2 (a) XPS survey spectrum of GED and XPS spectrum of (b) N1s, (c) C1s and (d) O1s levels.

The morphology, size and quality of prepared RGO and GED samples were investigated by FE-SEM and TEM analysis. Fig. 3a shows the TEM image of RGO and Fig. 3c and d shows the TEM images of GED. It can be clearly seen from the TEM images that the RGO surface is highly uniform, large and homogenous as compared to GED surface which has a highly irregular and modified surface which asserts the EDTA functionalization on the graphene nanosheets surface. Whereas, when the RGO amount is increased in the GED (2 : 1) the surface of graphene sheets becomes smooth as like pure RGO nanosheets (Fig. 3e and f). Fig. 3b shows the SEM image of GED which is indicative of the formation of thick nanosheets with winkled morphology that is consistent with the above TEM images.

Fig. 3 TEM images of (a) RGO, (c and d) GED electrode materials (1 : 1), (e and f) GED electrode materials (2 : 1), (b) SEM image of GED (1 : 1).

TG analysis was employed in air atmosphere to study the decomposition behavior of EDTA, RGO and GED, and estimate the weight loading of EDTA on RGO in GED (Fig. 4a). As seen from the TG profile, EDTA decomposes at 249 °C and this is represented by the rapid weight loss of 84%, and the residue was decomposed completely within 500 °C. Whereas, the TG profile of GED shows three kind of weight loss profile, initial weight loss of 11% which occurred from ambient temperature to 175 °C is attributed to physically and chemically adsorbed water molecules. Further increase in temperature results in gradual decomposition of EDTA from RGO surface. It is worthy to note here that unlike pure EDTA, the EDTA in the GED decomposes gradually due to the different kinds of interaction or bonding between the EDTA and functional groups in RGO nanosheets. A residue of 60% is obtained at 480 °C beyond which decomposition of RGO and residue of GED takes place. It can be clearly seen by comparing the TG profile of RGO and GED, the un-functionalized RGO decompose much faster than the GED. From the weight loss observed from the TG analysis it is concluded that GED contains EDTA and RGO in ∼1 : 2 weight ratio. It is noteworthy that EDTA and RGO were taken in 1 : 1 wt% during the preparation of GED. Lower amount of EDTA in GED as compared to RGO affirms the removal of unbounded EDTA molecules from the RGO surface during the washing process. This clearly indicates that the EDTA molecules in RGO are strongly bounded to the carbon networks. The existence of EDTA in GED was further confirmed using UV-visible spectroscopic analysis (Fig. 4b). UV-visible spectra of RGO shows an absorption peak at λ = 231 nm whereas it shifts to a higher wavelength, λ = 262 nm for GED, the observed red shift in the absorption peak confirms the EDTA functionalization of graphene surface in GED.

Fig. 4 (a) TG profile of EDTA, RGO and GED at 5 °C min⁻¹ in air, (b) UV-visible spectra of RGO and, GED.

To understand and estimate the charge storage behavior of RGO and GED as electrode materials for supercapacitors, they were subjected to cyclic voltammetry, galvanostatic charge–discharge and impedance analysis in 0.5 M H₂SO₄ medium (Fig. 5). Fig. 5a illustrates the CV curves of RGO and GED at a scan rate of 25 mV s⁻¹, it can be clearly seen that both the curves are rectangular in shape which indicates EDLC behavior of the prepared electrodes. As, the integrated area under the CV curve denotes the specific capacitance, it is evident from the CV curves that GED electrode has a greater specific capacitance compared to RGO. The GED electrodes were further studied at different scan rates as shown in Fig. 5b, and at all the scan rates GED showed a prominent rectangular shape curve indicating the stability of the GED electrodes. The specific capacitance was further investigated using galvanostatic charge–discharge at various current densities ranging from 0.1 A g⁻¹ to 20 A g⁻¹ (Fig. 5c and d). On investigating the charge–discharge curves it was observed that even at a high current density of 20 A g⁻¹ the charge–discharge curve is very stable exhibiting favorable symmetry and linearity. The specific capacitance value calculated from the galvanostatic charge–discharge curves at 0.1, 1, 2, 5, 10, 15 and 20 A g⁻¹ are 290, 232, 225, 213, 195, 178 and 168 F g⁻¹, respectively. On increasing the current density from 0.1 to 20 A g⁻¹, 58% of the specific capacitance value was retained resulting in high rate capacitance of 168 F g⁻¹ which is still higher than the specific capacitance of the prepared RGO. To investigate the electrochemical stability of the GED electrode, galvanostatic charge–discharge cycles were performed for 6000 cycles at a current density of 5 A g⁻¹ (Fig. 5e). Interestingly, we observed a very stable capacitance of 210 F g⁻¹ (about 98% of original capacitance) after 6000 cycles of charge discharge indicating long-term electrochemical cycling ability. As, long cycle life is a vital parameter, the prepared GED electrode is promising materials for the electrochemical supercapacitor applications. The ratio of RGO and EDTA in GED materials was optimized to attain maximum specific capacitance, GED with three different weight ratios of RGO and EDTA (1 : 2, 1 : 1, 2 : 1) were prepared. Fig. S1† shows the galvanostatic charge–discharge profile of GED with different weight ratio of RGO and EDTA at 0.1 A g⁻¹ current density. The electrodes containing 1 : 2, 1 : 1 and 2 : 1 ratio of RGO and EDTA with 10 wt% of acetylene black showed a specific capacitance of 97, 230 and 255 F g⁻¹, respectively. The effect of acetylene black loading was also studied by preparing the working electrode (GED, 1 : 1) with 10, 20, 33 wt% of acetylene black and GED. From the galvanostatic charge–discharge measurements (Fig. S2†), it was observed that the samples with 10, 20 and 33 wt% of acetylene black loading showed a specific capacitance of 230, 260 and 290 F g⁻¹.

Fig. 5 (a) Cyclic voltammetry of RGO and GED at 25 mV s⁻¹ scan rate, (b) cyclic voltammetry of GED electrode at different scan rates, (c) galvanostatic charge–discharge curve of GED electrode at 0.1 A g⁻¹ current density, (d) galvanostatic charge–discharge curve of GED at different current densities, (e) specific capacitance of GED electrode for 6000 cycles of charge–discharge at 5 A g⁻¹ current density and (f) Nyquist plot of RGO and GED electrodes in 0.5 M H₂SO₄ aqueous electrolyte, (inset: Nyquist plot of RGO and GED electrodes in high frequency region).

Electrochemical impedance spectroscopic (EIS) analysis was carried out for RGO and GED electrodes in the frequency range of 1 MHz to 1 mHz at OCP and the resulting Nyquist plots are shown in Fig. 5f. The presence of a distinctively small semicircle (Fig. 5f, inset) in the case of GED is indicative of a very low charge transfer resistance (R_ct) at the electrode/electrolyte interface which favors faster ion transfer kinetics.⁵² Low charge transfer resistance implies that the electrolyte ions can easily permeate into the pores of the electrode material and access the surface of active electrode material. The equivalent series resistance (ESR) of the material was calculated from the x intercept of the Nyquist plot and was found to be 2.5 Ω for RGO while it showed an interestingly lower value of 1.5 Ω for GED which suggests that GED is a promising electrode material for supercapacitor applications. The slope of the 45° portion of the curve is called as the Warburg resistance and is considered to be a result of the frequency dependence of ion diffusion in the electrolyte/electrode surface.⁵³ The short Warburg curve is an indication of a short ion diffusion path which will allow easy access of electrolyte ions to the electrode surface. The more vertical the curve, the more closely the electrode behaves as an ideal capacitive electrode. As seen in Fig. 5f, the more vertical shape observed at lower frequencies for GED electrode is an indication of pure capacitive behavior.⁵² The knee frequency is an important factor which determines the rate performance of an electrode material.⁵⁴ The knee frequency values of RGO and GED were found to be 215 mHz and 3.72 Hz respectively, and it is known that higher knee frequency corresponds to higher rate performance.⁵⁴

Conclusions

We have demonstrated a simple method for the synthesis of N-functionalized graphene using pure EDTA and RGO. The increase in ‘d’ value revealed the functionalization of RGO. Raman spectroscopic analysis and UV-visible absorption studies confirmed the functionalization. The FT-IR spectrum of GED demonstrated the introduction of N-containing functional groups in RGO. From the TG profile, it is estimated that the ratio of EDTA and RGO in GED is ∼1 : 2 wt%. The CHNS analysis and FE-SEM EDX analysis confirm a 4% N functionalization in GED. Cyclic voltammetry analysis showed better charge storage behavior for GED than RGO. Galvanostatic charge–discharge experiments on GED showed a higher specific capacitance of 290 F g⁻¹ at 0.1 A g⁻¹ current density and an excellent cyclic stability of 6000 cycles at a very high current density of 5 A g⁻¹. From the above studies it is believed that the prepared GED will be a promising material for high performance supercapacitor applications.

Acknowledgements

M. Sathish thanks CSIR-CECRI for financial support through OLP 77 & MULTIFUN (CSC0101) project.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra02756f

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