Sulphur-functionalized graphene towards high performance supercapacitor

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Sulphur-functionalized graphene towards High performance supercapacitor Wee Siang Vincent Lee, Mei Leng, Meng Li, Xiao Lei Huang, Jun Min Xue

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S2211-2855(14)00299-7 http://dx.doi.org/10.1016/j.nanoen.2014.12.030 NANOEN648

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Received date: 13 November 2014 Revised date: 10 December 2014 Accepted date: 22 December 2014 Cite this article as: Wee Siang Vincent Lee, Mei Leng, Meng Li, Xiao Lei Huang, Jun Min Xue, Sulphur-functionalized graphene towards High performance supercapacitor, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2014.12.030 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 galley proof before it is published in its final citable 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.

Sulphur-functionalized graphene towards high performance supercapacitor Wee Siang Vincent Leea, Mei Lenga, Meng Lia, Xiao Lei Huanga, Jun Min Xue*a *

Corresponding author at: National University of Singapore, Department of Materials Science and Engineering, Singapore 117573. Tel./fax +65 65164655 E-mail: [email protected] (J.M. Xue) Keywords: Supercapacitors, redox reaction, thiocarboxylic acid ester, sulphone, thiourea Abstract To enhance the energy densities, pseudocapacitive materials are usually added to electric double layer carbonaceous materials. However in the process of energy densities enhancement, power densities are usually lowered which creates a dilemma situation for researchers. Thus to obtain high energy and power densities, a new pseudocapacitive mechanism, to the best of knowledge, is introduced in this paper. High supercapacitive performance can be obtained by the redox mechanism of thiocarboxylic acid ester to sulphone with the aid of external current. The specific capacitance of the material is enhanced to as high as 1089 F g-1 at 1 A g-1, and even at high current density of 50 A g-1, it is able to discharge a respectable 833 F g-1. The material is able to exhibit high energy density of 43 Wh kg-1 at high power density of 38 kW kg-1 which validates the possibility of a harmonious coexistence between high energy density and high power density. Introduction Supercapacitor is one of the vital energy storage devices (ESD) that has received growing attention in this rapidly digitalized world due to its low maintenance, high safety, excellent cyclic stability, and more importantly high power density [1-3]. As such, it is an attractive ESD candidate in powering the next generation technologies such as electric cars, and as a grid energy storage system for clean renewable energies. However, the biggest pitfall supercapacitor faced remained to be its unsatisfactory energy density as compared to its 1

competitors. Typical energy density of supercapacitor is in the range of 5-10 Wh kg-1, which is significantly lower than the other ESD [4]. Thus to fully unleash the potential of supercapacitor in high power surge applications, energy density must be improved. Based on the energy density formula, E=1/2(CV2), the energy density of the supercapacitor is dependent on capacitance of the material and the potential window of the electrolyte. Thus, to improve the energy density in a fixed potential window, the capacity of electrode material needs to be further increased. Among the common electrode candidates [57], graphene [4,8] is highly favoured in supercapacitor research due to its exceptional mechanical, thermal, electrical properties [9,10], and its large intrinsic maximal surface area of about 2600 m2 g-1 [4,8]. However in reality, graphene is only able to discharge specific capacitance of 100 - 200 F g-1 [8,11]. To enhance its specific capacitance, synergistic combination of graphene and pseudocapactive materials is important, as specific capacitances higher than 1000 F g-1 [12-14] have been reported. Typically, transition metal oxides (TMO) such as Ni(OH)2 [14], MnO2 [12,15], and RuO2 [16,17] are widely introduced into graphene as pseudocapacitive material to achieve high specific capacitance. Other than its heavy atomic mass, high cost, and more tedious synthesis process, TMO has limited advantage in power surge application due to their poor conductivities and the slow Faradic redox reactions [18,19]. These in turn create power density bottleneck in which the supercapacitor is able to achieve. Functionalizing graphene with chemical moieties such as oxygen, nitrogen, and sulphur is an alternative method to introduce pseudocapacitance into the system. The heteroatom functional groups are able to enhance the specific capacitance of the electrode due to their pseudocapacitive reactions [20-23]. Besides the better studied nitrogen which shown enchancement in the capacitance due to the quaternary and pyridinic N oxides [24-26], sulphur is an emerging and interesting heteratom in ESD research [27-31]. Sulphur

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functionalized carbonaceous material typically shows improvement in capacitance due to the pseudocapactive reactions of sulphone and sulphoxide [30,31]. However, the power density of these sulphur functionalized systems tends to be lower. For instance, sulphurized activated carbon was reported to exhibit 35 Wh kg-1 at a relatively low power density of 2000 W kg-1 [31]. In a timely response to develop the desperately needed combination of both high energy density and high power density, a new pseudocapacitive mechanism of sulphur functionalized graphene is proposed in this paper. Other than the commonly adopted redox reaction between sulphone and sulphoxide, the new mechanism is based on the redox reaction between thiocarboxylic acid ester and sulphone. Such pseudocapacitive reaction is able to increase capacity and at the same time, power densities are not sacrificed as graphene oxide is chemically reduced during the chemical grafting process. For this new pseudocapacitive mechanism, the source of sulphur is thiourea, which is chemically grafted onto graphene oxide sheets via thiol-carboxylic acid esterification. The synthesized sulphur-functionalized graphene aerogel was able to discharge with a specific capacitance of 1089 F g-1 at 1 A g-1, and at high current density of 50 A g-1, it was still able to discharge a respectable capacitance of 833 F g-1. The material was able to retain high energy density of 42 Wh kg-1 at high power density of 38 kW kg-1 which validates the possibility of harmonious coexistence between energy density and power density Materials and methods Synthesis of sulphur functionalized graphene aerogel (GATUF) In a typical synthesis of graphene aerogel, 8 ml of graphene oxide colloidal solution (of concentration 5 mg ml-1) was prepared into a test tube. 2ml of thiourea (of concentration 150 mg ml-1) was added to graphene oxide solution. The mixture was then well vortexed and 3

bath-sonicated to obtain a well--mixed mixed graphene oxide solution with thiourea solution. The mixture was then poured into a petri-dish petri of about 5.2 cm diameter. The petri-dish dish containing the graphene oxide-thiourea thiourea mixture was placed in an 80oC oven for 30 minutes for the first gelation process. After 30 minutes, the gelated graphene oxide-thiourea oxide thiourea was then taken out of the oven and placed in the freezer for 1 hour (more details in Figure S1).. After the gelated graphene oxide-thiourea thiourea was thoroughly froze, it was then placed in the 80oC oven again for the second gelation process. The second gelation duration was varied to obtained different porosity graphene hydrogel. The graphene graphene hydrogel underwent solvent exchange with D.I water to remove any unwanted by by-products products and the remaining thiourea. After which, the graphene hydrogel underwent lyophilisation to finally obtain GATUF. Results and Discussion

Figure 1

Schematic illustration ation for the synthesis of sulphur-functionalized sulphur graphene aerogel.

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Figure 1 shows the schematic illustration of the synthesis strategy of the sulphur sulphurfunctionalized graphene aerogel. In a typical synthesis, thiourea was added to graphene oxide solution as reducing agent, and the mixture was gelated at 80oC to form graphene hydrogel. During the gelation phase, graphene oxide could be chemically reduced to graphene and sulphur functional group was chemically grafted onto pristine graphene as portrayed in Figure S4. The graphene hydrogel underwent solvent exchange with deionized water to remove excess thiourea. Lastly, graphene aerogel was obtained after lypohilization, henceforth denoted as GATUF, which was able to exhibit high conductivity of about 500 S cm-1 and surface area of about 340 m2 g-1.

Figure 2 Photographic images of (a) ( as-synthesized GATUF disk and (b) b) GATUF disk resting on small dandelion. (c) c) SEM image of cross section of 1x1 cm GATUF plate cut from the disk (inset shows the 1x1 cm

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GATUF plate used for electrochemical analysis). (d) Electron image of the selected area of GATUF for EDX mapping, and elemental mapping of (e) C, (f) S, (g) N, and h) O. (i) XPS C1s spectrum of pristine GO, and (j) XPS C1s spectrum of GATUF.

Figure 2a shows photographic image of the as-prepared graphene aerogel. An integral graphene aerogel can be formed without fragmentation. To demonstrate the feather-like property of GATUF, the synthesized product (5 cm in diameter and 4 mm in thickness) was able to rest on top of a small dandelion without collapsing as portrayed in Figure 2b, indicating its light-weight property (~ 5 mg cm-3). To facilitate the electrochemical testing process, 1 x 1 cm GATUF plate (Figure 2c inset) can be easily cut out of the GATUF disk. The structure of the 1 x 1 cm GATUF plate was then characterized with scanning electron microscopy (SEM). The 1 x 1 cm GATUF plate had a thickness of about 0.24 mm after compression, and its highly porous nature was revealed under high magnification SEM (Figure 2c), which was in favour of electrolyte infiltration and ionic migration. Energy dispersive x-ray spectroscopy (EDS) elemental mapping was conducted for the GATUF plate. Figure 2d shows the detection of carbon (Figure 2e), sulphur (Figure 2f), nitrogen (Figure 2g), and oxygen (Figure 2h) elements in the GATUF. With a closer inspection on the EDS elemental mappings, both sulphur and nitrogen were well distributed over the graphene aerogel surface and the individual element weight and atomic percentages were shown in Figure S2. Figure 2i & j show the x-ray photoelectron spectroscopy (XPS) C1s of pristine GO and GATUF, respectively. The reduction of O=C-OH proportion at around 288.4 eV for GATUF when compared to pristine GO supports the hypothesis of O=C-OH removal due to thiol-carboxylic acid esterification (sulphur functional group grafting) and at the same time it shows the effective reduction of graphene oxide by thiourea. To further analyse the structure of graphene aerogel, Fourier transform infrared (FTIR) spectroscopy (Figure S3 in S.I) was conducted. Figure S3 shows presences of primary amine C-N stretching at (1087 cm-1), C=S stretching (1190 cm-1) derived from thiourea. Interestingly, mono-substituted amides R-CONHR (1632 cm-1), and thiocarboxylic acid ester R-CO-SR (1698 cm-1) were found in 6

graphene aerogel. It was demonstrated that thiourea was chemically grafted onto graphene oxide either via the formation of mono-substituted mono amide R-CO-NHR, NHR, or thiocarboxylic acid ester, as shown in Figure S4.

Figure 3 Electrochemical measurements of GATUF. (a) a) CV curves of GATUF at scan rate of 10 mV s-1 and -1 CV curves of bare nickel foam at scan rate 10 mV s , (b) b) Galvanostatic charge/discharge curves of GATUF at 20 A g-1, (c) c) Cyclic stability of GTAUF cycled at-120 A g-1 for 10000 cycles, (d) d) Galvanostatic discharge curves of GATUF at various current densities (1-50 (1 A g ), (e) e) Specific capacitance of GATUF based on galvanostatic discharge curves, and (f) f) Energy densities and power densities for GATUF at different current densities.

Electrochemical measurements were assessed with a 3 electrodes cell setup, with platinum plate as the counter electrode in 1 M KOH electrolyte. Before the actual electrochemical measurement, GATUF was activated by cycling at 2 A g-1 for 1000 cycles and the specific capacitances of the material increases tremendously with cycle number as shown in Figure S5. Such rapid capacitance increment with cycling was due to the progressive decomposition of thiourea on graphene surface which which produced more thiocarboxylic acid ester functional groups, which were in turn oxidised to sulphone during electrochemical charging process 7

(more detail see S.I). Cyclic voltammetry (CV) was conducted for GATUF with a potential window of 0 - 0.6 V at 10 mV s-1 as shown in Figure 3a. Visible symmetrical redox peaks could be observed in the CV curves of GATUF at 0.5 V and 0.25 V, which indicated good reversibility and strong presence of pseudocapactive reaction with the reversible capacity of ca. 1000 F g-1. The nickel foam exhibited an insignificant capacitance as shown in Figure 3a, suggesting that the majority of capacity was contributed from the active electrode material. The galvanostatic charge/discharge curves at 20 A g-1 are shown in Figure 3b. The long plateaus presented in the discharge curves reinforced the presence of strong pseudocapactive behaviour as suggested in its CV curves. Figure 3c shows the excellent cyclic stability of GATUF at 20 A g-1 after electrochemical activation. Specific capacitance of the sample increases slightly with increasing number of cycles and it stabilizes at around 860 F g-1 throughout the 10000 cycles. This set of galvanostatic cycling result clearly shows the impressive electrochemical stability and high reversibility properties of the synthesized thiourea-functionalized GATUF. Figure 3d & 3e show galvanostatic discharge curves and discharge capacity of GATUF at different current densities (1-50 A g-1). The discharge plateaus can be obviously observed at various current densities due to the redox reaction. GATUF was able to discharge specific capacitance of 1089, 1030, 967, 969, 860, and 833 F g-1 at current density of 1, 2, 5, 10, 20, 50 A g-1 respectively, with about 76% capacitance retention when current density increased by 50 times, indicating that GATUF exhibited excellent electron and ionic migration performance. Energy densities and power densities were calculated based on aqueous electrolyte (1 M KOH) in 3 electrodes configuration. As tabulated in Figure 3f, GATUF was able to exhibit a maximal energy density of 55 Wh kg-1 at power density of 400 W kg-1. Even with a 9500% increment in power density, at 38 kW kg1

the sample was still able to exhibit a respectable 42 Wh kg-1, which is a mere 23.6% drop in

energy density. Even though the energy and power densities are limited by the aqueous

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electrolyte medium, from the calculations, it can be shown that GATUF was still able to retain a reasonably high energy density at high power density which is comparable to that of lithium ion battery. Electrochemical performance of non-sulphur functionalized graphene aerogel (GAAA) synthesized with ascorbic acid was shown in Figure S6, and the same sample was tested in 0.08M thiourea/ 1M KOH (Figure S7). Both samples showed inferior performance when compared to GATUF, which indicated the advantage of sulphur functionalization. Annealed GATUF was also tested for its electrochemical performance (see Figure S8). To better understand the electrochemical reaction mechanism of GATUF, ex-situ X-ray photoelectron spectroscopy (XPS) was performed and its results were shown in Figure 4. It should be noted that each deconvoluted S 2p component should have S 2p3/2 and 2p1/2 doublet with an intensity ratio of 2:1, and with an energy separation of 1.2eV. S 2p peak was measured for thiourea (Figure 4a) as a control. Only C-S species were present for thiourea at binding energies of 163.7 eV (C-S 2p3/2) and 164.9 eV (C-S 2p1/2).

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Figure 4 XPS spectrum of S 2p for (a) Thiourea, (b) GATUF, (c) c) GATUF charged, and (d) GATUF discharged.. (e) Raman spectroscopy of GATUF, GATUF charged, and GATUF after testing

Similarly, S 2p peak for GATUF (Figure ( 4b) consisted only C-S species at binding energies of 163.8 eV (C-S 2p3/2) and 165 eV (C-S (C 2p1/2). In an attempt to discern the change in sulphur species when the sample was charged, XPS was also performed on GATUF charged (i.e. sample underwent electrochemical charging and was removed for characterization before it discharges). Figure 4c revealed 2 additional sulphur doublets at 161.8 eV and 168.1 eV for the charged sample which were ascribed for S2- and sulphone (R-SO2-R) R) species respectively. S2- peaks presented were due to the H2S produced ced from thiourea decomposition, which is 10

adsorbed onto the graphene. Proportion of C-S C S doublet for the GATUF charged decreased significantly as sulphone doublet became much more dominant. Similar XPS was conducted for the annealed GATUF (Figure Figure S9) S9 and similar trend can be observed). Raman spectrum of the charged sample (Figure Figure 4e 4e)) experienced a shift in its G band to 1577 cm-1, which returned to 1587 cm-1 for the GATUF discharged. Such result suggests the increase in functional groups within the material during charging process which increases th the electron conductivity [32]. ]. Thus from the XPS and Raman results, it supports the hypothesis for the long activation ation process proposed in the earlier section.

Figure 5 Proposed oxidisation mechanism of thiourea thiourea-functionalized functionalized graphene oxide sheet during charging, and proposed oxidation of thiocarboxylic acid ester to sulphone with electrochemical oxidation with external circuit.

This phenomenon suggests that during the charging process, proportion of C-S C S in the form of thiocarboxylic acid ester (R-S-R) R) decreased relative to sulphone as S in R-S-R R R is oxidised to R-SO2-R. R. Thus there was detection of sulphone ddoublet oublet peaks for GATUF charged while sulphone doublet peaks were undetectable for GATUF. After being discharged, the sample 11

showed lower proportion of sulphone doublet and a higher proportion of C-S doublet as compared to the charged GATUF in Figure 4c, implying that R-SO2-R species were reduced to R-S-R species under discharging. Based on the relatively prominent sulphone peaks, the redox reaction might not be fully reversible. Thus under the XPS investigation, it can be affirmed that during charging process, R-S-R species were oxidised to R-SO2-R species, while R-SO2-R species were reduced to R-S-R species during the discharging process as proposed in Figure 5. The improvement in the capacitance of the material can be due to such pseudocapactive reaction of thiocarboxylic acid ester.

Conclusion Heteroatom functionalization can be one of the important strategies in enhancing the material performance as energy storage device. In particular, functionalization of graphene or carbonaceous based materials have recently attracted increasing attention in supercapacitor research. The obtained sulphur functionalized graphene was prepared with sulphur containing thiourea. GATUF was able to discharge capacitance as high as 1089 F g-1 at 1 A g-1 in 1 M KOH and it was able to exhibit a high energy density of 42 Wh kg-1 at a high power density of 38 kW kg-1. Such performance is attributed to the redox reaction of sulphur containing functional groups. Thiocarboxylic acid ester functional groups chemically grafted onto the graphene aerogel was electrochemically oxidized to sulphone, which was followed by electrochemical reduction back to thiocarboxylic acid ester. Such pseudocapacitve mechanism is noteworthy as high pseudocapacitance contribution was observed from such a mechanism, which could enhance the performance of carbonaceous materials in supercapacitor research.

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Acknowledgements The authors thank the financial support provided by Singapore MOE Tier 1 funding R-284– 000–124–112. The author would also like to thank Ms Tamie Loh Ai Jia and Mr Tan Yong Teck for their invaluable and fruitful discussion. Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http:// References [1]

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Highlights • • • •

Pesudocapacitive reaction of thiocarboxylic acid ester to sulphone is proposed which contributes to significant pseudocapacitance. XPS study proved the sulphur functional group conversion during charge/discharge process. Sulphurization of graphene oxide sheet using thiourea via a simple self-assembly process. Superior electrochemical performance displayed in sulphur functionalized graphene is noteworthy in supercapacitor research.

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Graphical abstract

Sulphur functionalized graphene aerogel was prepared such that sulphur functional group in the form of thiocarboxylic acid ester was chemically grafted onto the graphene oxide sheet. The material was able to exhibit high energy density of 43 Wh kg-1 at high power density of 38000 W kg-1 which validates the possibility of a harmonious coexistence be between high energy density and high power density.