Non‑aqueous energy storage devices using graphene .... Non...mance electrochemical energy storage devices with long cycleability and high power density in aqueous media which predominantly

This document is downloaded from DR‑NTU (https://dr.ntu.edu.sg)Nanyang Technological University, Singapore.

Non‑aqueous energy storage devices usinggraphene nanosheets synthesized by green route

Mhamane, Dattakumar; Suryawanshi, Anil; Banerjee, Abhik; Aravindan, Vanchiappan;Ogale, Satishchandra; Srinivasan, Madhavi

2013

Mhamane, D., Suryawanshi, A., Banerjee, A., Aravindan, V., Ogale, S., & Srinivasan, M. (2013).Non‑aqueous energy storage devices using graphene nanosheets synthesized by greenroute. AIP Advances, 3(4).

https://hdl.handle.net/10356/100325

https://doi.org/10.1063/1.4802243

© 2013 The Authors. This paper was published in AIP Advances and is made available as anelectronic reprint (preprint) with permission of The Authors. The paper can be found at thefollowing official DOI: [http://dx.doi.org/10.1063/1.4802243]. One print or electronic copymay be made for personal use only. Systematic or multiple reproduction, distribution tomultiple locations via electronic or other means, duplication of any material in this paperfor a fee or for commercial purposes, or modification of the content of the paper isprohibited and is subject to penalties under law.

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Non-aqueous energy storage devices using graphene nanosheetssynthesized by green routeDattakumar Mhamane, Anil Suryawanshi, Abhik Banerjee, Vanchiappan Aravindan, Satishchandra Ogale et al. Citation: AIP Advances 3, 042112 (2013); doi: 10.1063/1.4802243 View online: http://dx.doi.org/10.1063/1.4802243 View Table of Contents: http://aipadvances.aip.org/resource/1/AAIDBI/v3/i4 Published by the AIP Publishing LLC. Additional information on AIP AdvancesJournal Homepage: http://aipadvances.aip.org Journal Information: http://aipadvances.aip.org/about/journal Top downloads: http://aipadvances.aip.org/most_downloaded Information for Authors: http://aipadvances.aip.org/authors

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AIP ADVANCES 3, 042112 (2013)

Non-aqueous energy storage devices using graphenenanosheets synthesized by green route

Dattakumar Mhamane,1 Anil Suryawanshi,1 Abhik Banerjee,1

Vanchiappan Aravindan,2,a Satishchandra Ogale,1,b

and Madhavi Srinivasan2,3,c1Centre of Excellence in Solar Energy, National Chemical Laboratory (CSIR-NCL),Dr Homi Bhabha Road, Pune, 411 008, India and Network Institute of Solar Energy(CSIR-NISE), New Delhi, India2Energy Research Institute @ NTU (ERI@N), Nanyang Technological University,Research Techno Plaza, 50 Nanyang Drive, 637553, Singapore3School of Materials Science and Engineering, Nanyang Technological University,639798 Singapore

(Received 8 March 2013; accepted 1 April 2013; published online 12 April 2013)

In this paper we report the use of triethylene glycol reduced graphene oxide (TRGO)as an electrode material for non-aqueous energy storage devices such as supercapaci-tors and Li-ion batteries. TRGO based non–aqueous symmetric supercapacitor is con-structed and shown to deliver maximum energy and power densities of 60.4 Wh kg–1

and 0.15 kW kg–1, respectively. More importantly, symmetric supercapacitor showsan extraordinary cycleability (5000 cycles) with over 80% of capacitance retention.In addition, Li-storage properties of TRGO are also evaluated in half-cell config-uration (Li/TRGO) and shown to deliver a reversible capacity of ∼705 mAh g–1with good cycleability at constant current density of 37 mA g–1. This result clearlysuggests that green-synthesized graphene can be effectively used as a prospectiveelectrode material for non-aqueous energy storage systems such as Li-ion batteriesand supercapacitors. Copyright 2013 Author(s). All article content, except whereotherwise noted, is licensed under a Creative Commons Attribution 3.0 UnportedLicense. [http://dx.doi.org/10.1063/1.4802243]

I. INTRODUCTION

Supercapacitors (SC) and Lithium-ion batteries (LIB) are the key solutions for today’s hugeenergy storage demands and expected to power hybrid electric vehicles (HEV) and electric vehicles(EV) in near future.1–6 SC or electric double layer capacitors (EDLC) are important high perfor-mance electrochemical energy storage devices with long cycleability and high power density inaqueous media which predominantly contain carbonaceous electrodes.7, 8 However, energy densityis highly limited due to the restricted (due to water splitting issue) operating potential (∼1.23 V) ofsuch aqueous electrolyte solutions.9, 10 Hence, the concept of making asymmetric SC has emergedto suppress the negative and positive polarization and thereby widening the operating potentialin an aqueous medium up to ∼2 V.11 However, still the energy density of such asymmetric SCsystem is far below than the desired level to power HEV and EV, poor cycleability being anotherimportant concern. Another approach to enhance the energy density is employing non-aqueous elec-trolytes without compromising power density according to the equation, E = 1/2 CV2. Since theelectrochemical stability window for organic solutes is wider (∼3 V) than that of aqueous solutions(∼1.23 V), higher voltages can be accessed and thus higher energy density can be realized. While

aCorresponding author: [email protected] author: [email protected] author: [email protected]

2158-3226/2013/3(4)/042112/9 C© Author(s) 20133, 042112-1

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http://dx.doi.org/10.1063/1.4802243http://dx.doi.org/10.1063/1.4802243http://dx.doi.org/10.1063/1.4802243mailto: [email protected]: [email protected]: [email protected]://crossmark.crossref.org/dialog/?doi=10.1063/1.4802243&domain=pdf&date_stamp=2013-04-12

042112-2 Mhamane et al. AIP Advances 3, 042112 (2013)

employing such non-aqueous electrolyte solutions, cycleability of SC is severely affected as com-pared to aqueous solutions.10, 12–16 Poor cycleability in non-aqueous media results in the case ofEDLC capacitors irrespective of carbon allotropes used.7, 8 Among the various forms of carbon,graphene is found to be a more attractive candidate to study the supercapacitive behaviour in non-aqueous media, however no extensive work has yet been carried out in such medium.17–19 Takingthe overwhelming advantages of graphene-like unique two-dimensional structure made up of hon-eycomb lattice of sp2 hybridized carbon atoms, superior electrical conductivity, high specific surfacearea and high thermal and chemical stability, we made an attempt to employ it as an electrode mate-rial for non-aqueous SC applications.20, 21 Previously, Ruoff and co-workers17, 18, 22 have reported theutilization of such graphene-based materials as electrode material for non-aqueous SC applicationsby employing ionic liquids as electrolyte solutions. Unfortunately, no cycleability study has beenreported, hence we set out to investigate the electrochemical performance of such graphene-basedsymmetric SC using conventional Li-ion battery electrolytes up to 5000 galvanostatic cycles.

In addition to the supercapacitive properties of graphene, the possibility of using the same asanode material for LIBs has also been explored in half-cell configurations.23–25 Generally, highenergy density LIB requires good performance anodes with capacity more than commerciallyavailable graphitic anodes (∼372 mAh g–1). Due to the unique two dimensional structures ofgraphene nanosheets capable of adsorbing Li+ ions on both sides turbostatically, it provides amaximum reversible capacity of ∼744 mAh g–1.17, 24 Further, graphitic anodes are involved in Li-insertion/extraction reaction, and under high current operations such anodes endure the problem ofLi-plating issues and poor cycleability, which in turn provide a poor power density of Li-ion powerpacks. Whereas in graphene nanosheets pseudocapacitive storage mechanism certainly increases thepower density as well without compromising energy density and this makes graphene nanosheets asan attractive anode material for LIB applications.26–28

From the applications standpoint scalable synthesis of graphene nanosheets is very crucialfor high performance SC and LIB applications.9 Most solution processed graphene nanosheetpreparations involve the usage of environmentally hazardous reluctants such as hydrazine hydrate,NaBH4, hydroquinone, sodium azide, dimethyl hydrazine etc., to reduce graphite oxide (GO) inwater medium, since it is heavily loaded with oxygen containing functionalities such as (-COOH,-OH, -C=O etc.).20, 29 Hence, cost-effective and eco-friendly approach is warranted for the synthesisgraphene nanosheets. At the same time, it is important to evaluate whether the quality of such green-synthesized graphene nanosheets is adequate for realistic charge storage applications. In this work wehave evaluated the performance of triethylene glycol reduced graphene oxide (TRGO) nanosheets29

for non-aqueous energy storage applications and show that such sheets exhibit excellent performancein these application domains.

II. EXPERIMENTAL

High purity 99.8% graphite powder was purchased from Alfa Aeser. For GO and TRGOsynthesis KMnO4, NaNO3, Conc. H2SO4, Conc. HCl, NH3, 30% H2O2, and trigol were purchasedfrom Merck and used as such. Modified Hummers method was used for the synthesis of GO.29

It mainly involves concentrated H2SO4 mediated oxidation of graphite powder in the presence ofstrong oxidizing agent KMnO4. Briefly, in an ice-water bath 5 g of graphite powder was mixed with3.75 g of NaNO3. To this 375 mL of Conc. H2SO4 was added slowly. The solution was stirred forgetting homogeneous mixture and subsequently 22.5 g of KMnO4 was added slowly. The slurrythus obtained was cooled for additional 2 h in the same ice-water bath. At this stage the ice bathwas removed and the slurry was further stirred at room temperature for ∼5 days yielding a brownishslurry. This slurry was then diluted with 700 ml 5% H2SO4 solution at 98 ◦C. Finally, 15 ml 30%H2O2 was added to the above solution. The GO solution was then subjected for purification toremove inorganic impurities. Reduction of GO was then carried out in non-aqueous medium and isdescribed in details elsewhere.29 Briefly, sufficient quantity i.e. 100 mg of purified GO was subjectedto exfoliation in 100 ml. trigol for 1 h in sonicator bath followed by reduction by heating in argonatmosphere at ∼278 ◦C. The obtained product was kept for drying after washing several times withdeionised water and methanol.

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042112-3 Mhamane et al. AIP Advances 3, 042112 (2013)

Powder XRD pattern was recorded using Philips X’Pert PRO. Raman spectroscopy was carriedout using confocal micro-Raman spectrometer LabRAM ARAMIS Horiba Jobin-Yvon apparatuswith laser excitation wavelength of 532 nm. Brunauer-Emmett-Teller (BET) surface area measure-ments were conducted using Quantachrome (Nova 3200 e) Surface area & Pore size analyzer.Morphological features of TRGO were analyzed by FE-SEM (Hitachi S-4200) and HR-TEM (HR-TEM, FEI Tecnai 300). All the electrochemical measurements were conducted in two electrodecoin-cell (CR2016) assembly. For the fabrication of LIB anode, composite electrode was made up of5 mg of TRGO, 1 mg of Super-p and 1 mg of conductive binder (Teflonized acetylene black, TAB-2).Then, it was pressed on a 200 mm2 stainless steel mesh, which served as the current collector, anddried at 60 ◦C for 4 h under vacuum before conducting cell assembly under Ar filled glove box. Thetest cells (half-cell) were composed of TRGO as an anode and metallic lithium as cathode, separatedby microporous glass fiber separator (Whatman, Cat. No. 1825 047, UK). One molar (1 M) LiPF6was dissolved in ethylene carbonate (EC) and diethyl carbonate (DEC) 1:1 wt.% mixture (obtainedfrom DAN VEC) which was used as electrolyte. In the text it is referred to as EC:DEC. For thesupercapacitor assembly, composite electrodes were made consisting of 4 mg of TRGO, 2 mg ofSuper-p and 1 mg of TAB-2 loading. SC were assembled with two symmetric electrodes and sepa-rated by Whatman paper and filled with the above electrolyte solution. Pictorial presentation of cellsunder study is shown in electronic supplementary information (ESI) FIG S1.30 Cyclic voltammetric(CV) studies were carried out using Solartron, 1470E and SI 1255B Impedance/gain-phase analyzercoupled with a potentiostat. Galvanostatic cycling profiles were recorded using Arbin battery tester(BT 2000) at ambient temperature conditions.

III. RESULTS AND DISCUSSION

Fine synthesis of GO from graphite (oxidation) and its conversion in the presence of trigol toTRGO (reduction) was characterized by XRD, UV-vis absorption spectroscopy, Raman spectroscopy,XPS, thermo gravimetric analysis (TGA) as partly discussed in our previous work.29 The key XRDand Raman data for three samples (graphite powder, GO and TRGO) are shown in Figure 1(a) and1(b) respectively. The substantial elimination of oxygen containing functionalities from GO wasfurther confirmed by FTIR spectroscopy. Figure 1(c) shows the FT-IR spectra for GO and TRGO.Three main stretching frequencies of GO at ∼1616 cm–1 (O-H bending, epoxide group or ringvibrations), 1722 cm–1 (C=O) and ∼3350 cm–1 (O−H) are completely eliminated and new bandin olefinic (i.e. C=C) region is clearly observed. The HR-TEM analysis was carried out for thinand sheets-like morphological features of TRGO. Figure 1(d) (i) and (ii) show representative FE-SEM and HR-TEM image for TRGO respectively.29 Additional HR-TEM images are shown in the(ESI) FIG S2.30 The thickness of the as-prepared TRGO was observed by atomic force microscopy(AFM). The images (FIG S3, ESI)30 confirm presence of few layers of graphene nanosheets; theaverage measured height for TRGO being ∼8-10 nm). The BET surface area of TRGO is found to be62 m2 g–1.

Cyclic voltammetry (CV) was used to study the supercapacitive behaviour of TRGO nanosheetsin symmetric two electrode configuration between 0-3 V. Figure 2(a) represents the CV traces ofsymmetric supercapacitor cycled at different scan rates from 1 to 50 mV s–1. Rectangular shapewith mirror like CV signatures indicates a purely capacitive behaviour of TRGO nanosheets. It isobvious to notice that increasing sweep rate leads to a decrease in net charge under the curve. Athigher scan rates the surface of the active material (TRGO) is only involved in the electrochemicalreaction rather than bulk which results in a decrease in specific capacitance.31 As per the energystorage mechanism, during charging process both anions (PF6–) and cations (Li+) are adsorbed on thegraphene nanosheets, whereas the process is reversed during discharge.32 Further, the electrochemicalperformance of hydrazine reduced graphene is also given in figure 2(b) for comparison under thesame testing conditions. A small variation in the electrochemical performance of such systemsis known to occur due to the specifics of the synthetic procedures employed for the preparationof such graphene nanosheets.33 It is worth noticing that the observed current responses for bothgraphene based systems are found to be comparable to each other, which clearly indicates that trigol

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042112-4 Mhamane et al. AIP Advances 3, 042112 (2013)

FIG. 1. (a) X-ray diffraction pattern of graphite, graphite oxide (GO) and triethylene glycol reduced graphene oxide (TRGO)(b) Raman spectra and (c) FT-IR spectra (d) i) FE-SEM image of and triethylene glycol reduced graphene oxide (TRGO) andii) HR-TEM image of TRGO.

reduction is beneficial for the synthesis of high performance supercapacitor electrode componentwith environmental friendliness.

Galvanostatic charge-discharge studies were conducted for the symmetric configuration andtypical charge-discharge curves at various current densities are obtained as given in Figure 3.

The specific capacitance (Cs) values are calculated according to the following equation,

Cs = 2 × Im dv/dt

where, Cs is the specific capacitance (F g–1), I is current applied, m is the mass of single TRGOelectrode pressed on stainless steel mesh, dV/dt is the value of slope obtained from the dischargecurve. The Cs of 97, 80, 67, 52 and 25 F g–1 are noted for current density values of 0.1, 0.5, 1, 2 and5 A g–1, respectively. The observed values are highly comparable to the previous reports by Ruoff andco-workers,17, 18 in which the authors used ionic liquids (1 M tetraethylammonium tetrafluoroborate

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042112-5 Mhamane et al. AIP Advances 3, 042112 (2013)

FIG. 2. Cyclic voltammetric traces of (a) triethylene glycol reduced graphene oxide (TRGO) and (b) hydrazine reducedgraphene in symmetric supercapacitor configuration with 1 M LiPF6 in EC:DEC electrolyte solution recorded between0-3 V at various scan rates.

(TEA BF4) in acetonitrile or propylene carbonate) as electrolytes in single electrode configurationand such a system delivered a specific capacitance of ∼112 F g–1. Further, the cycleability ofsymmetric supercapacitor was evaluated between the 0-3 V at a current density of 2 A g–1 up to5000 cycles and the data are shown in Figure 4.

It is clear that symmetric supercapacitor delivers a good cycleability with capacitance retentionof over ∼80% even after 5000 cycles. Generally, Li-ion containing organic electrolytes are reactivewith the carbonaceous electrodes via side reactions. As a result, unwanted mass (inorganic by-products) builds up over the surface of the electrode, reducing the active area for the electrochemicalreaction and increasing the cell resistance. Therefore, specific capacitance value decreases somewhatunder the prolonged cycling.

Unfortunately, there is no extended cycleability of non-aqueous symmetric supercapacitors tobe able to compare with the present values. However, recently Kim et al.34 have reported 67%capacitance retention in the ionic liquid medium after 2000 cycles in the case of vertically alignedcarbon nanotube (CNT) based non-aqueous supercapacitor. Further, such an excellent performance

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042112-6 Mhamane et al. AIP Advances 3, 042112 (2013)

FIG. 3. Typical galvanostatic cycling profiles of triethylene glycol reduced graphene oxide (TRGO) in symmetric superca-pacitor assembly with 1 M LiPF6 in EC:DEC electrolyte solution recorded between 0-3 V at various current densities

FIG. 4. Cycling profiles of triethylene glycol reduced graphene oxide (TRGO) in symmetric supercapacitor assembly with1 M LiPF6 in EC:DEC electrolyte solution recorded between 0-3 V at current density of 2 A g–1. Inset: Ragone plot ofnon-aqueous symmetric supercapacitor. The energy and power densities were calculated by the data obtained from figure 3.

is mainly due to the usage of conventional Li-ion battery electrolytes, wherein the anionic size is alsoclose to BF4– anions.16 Figure 4 inset shows the plot of specific energy vs. specific power densitiesand the values are calculated based on the equation given in our previous reports.32, 35 TRGO basedsupercapacitor is capable of delivering maximum energy and power densities of 60.4 Wh kg–1 and0.15 kW kg–1, respectively. The observed energy density values are higher than the aqueous systemcomprising graphene based electrodes and other carbon based systems as well.6–9, 14

Li-storage properties of TRGO nanosheets as anode were evaluated in half-cell configurations(Li/TRGO) by both potentiostatic and galvanostatic measurements. First, cyclic voltammetric traceswere recorded between 0.005-3 V vs. Li at scan rate of 0.1 mV s–1 and the result is given inFigure 5. In the first cathodic sweep, decomposition of electrolyte is apparently evident at lowerpotentials (

042112-7 Mhamane et al. AIP Advances 3, 042112 (2013)

FIG. 5. Cyclic voltammogram of triethylene glycol reduced graphene oxide (TRGO) in half-cell configuration (Li/TRGO)and cycled between 0.005-3 V vs. Li, in which metallic lithium serves as both counter and reference, electrodes.

graphene nanosheets, which mainly comprises of polyethylene oxide, polycarbonates and someinsoluble inorganic by-products.25, 36 However, in the subsequent anodic sweep and rest ofthe cycles, no noticeable reduction/oxidation curves are apparent. In addition, almost rectangu-lar shape of CV traces clearly indicates that the Li-ions are adsorbed on the both sides of thegraphene nanosheets or in other words psudocapacitive storage.24, 26 Reduction of area under thecurve during the cycling up to 10 cycles corresponds to the decrease in the specific capacity values.

Galvanostatic charge-discharge profiles were conducted between 0.005-3 V vs. Li at currentdensity of 37 mA g–1 in ambient temperature conditions. The cell showed the initial dischargecapacity of ∼1178 mAh g–1 (Figure 6(a)) with reversible capacity of ∼704 mAh g–1. Irreversiblecapacity is found ∼474 mAh g–1 (40%) and this huge irreversible capacity loss is common in thecase of all carbonaceous materials during first cycle except graphite. In the second cycle, TRGOhalf-cell displayed the discharge capacity of 600 mAh g–1 with columbic efficiency of ∼95%.

As mentioned earlier, huge irreversible capacity loss is expected for all the cases of anodematerials except insertion type which is mainly attributed to the decomposition of solvent moleculesand subsequent formation polymeric films. The polymeric films are the main components in thesolid electrolyte interphase (SEI) formed over the surface of the electrode.25, 36

The plot of discharge capacity vs. cycle number is given for 30 cycles in figure 6(b). Capacityfading is observed during the cycling and capacity of 365 mAh g–1 is noted after 30 cycles, which is52% of reversible capacity. Yoo et al.23 reported that chemically synthesized graphene nanosheetsshow reversible capacity of 540 mAh g–1 and retains 54% of its capacity after 20 cycles. In theirwork on graphene nanoribbons Bhardwaj et al.37 showed a reversible capacity of ∼820 and∼520 mAh g–1 for first and 14th cycles, respectively.

Very recently, Vargas et al.28 have summarized the Li-storage properties of graphene nanosheetsand concluded that, “The quality and structure of the graphene nanosheets can differ from batch tobatch, which can make devices behave inconsistently.” Further, it is apparent to notice that graphenenanosheets exhibited capacity fading in half-cell configurations, irrespective of the synthesis condi-tions employed with different reversible capacity values.33 However, further studies are in progressto suppress the capacity fade during cycling. Such pseduocapacitive storage mechanism in graphenenanosheets certainly improves the power density of Li-ion power packs and thereby enabling andability to power the HEV and EV.38 Rate capability study was conducted for TRGO nanosheets withvarious current densities and presented in figure 6(c). The cell delivered the reversible capacities of∼677, 282, 152, 92 and 54 mAh g–1 for current density of 0.037, 0.5, 1, 2 and 4 A g–1, respectively.

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042112-8 Mhamane et al. AIP Advances 3, 042112 (2013)

0 5 10 15 20 25 300

400

800

1200(b)

Cap

acit

y (m

Ah

g-1)

0 10 20 30 40 50 600

400

800

1200 Charge Discharge

0.2 A g-10.5 A g-1

1 A g-1 2 A g-1

0.2 A g-1

(c)

37 mA g-1

Cycle number

0 200 400 600 800 1000 12000.0

0.5

1.0

1.5

2.0

2.5

3.0

2

2 1

1

(a)

Irreversible capacity

Pot

enti

al (

V v

s. L

i)

Capacity (mAh g-1)

FIG. 6. (a) Galvanostatic discharge-charge profiles of triethylene glycol reduced graphene oxide (TRGO) in half-cellconfiguration (Li/TRGO) at current density of 37 mA g–1 between 0.005-3 V vs. Li (integer number represents cycle number)(b) Plot of discharge capacity vs. number of cycles and (c) Rate capability studies of Li/TRGO cell at various current densities.

It is apparent to notice that the test cells are capable of delivering stable capacities at relatively highcurrent rates. Such high current performance of TRGO nanosheets is also noticed in the symmetricsupercapacitor studies as well. This clearly indicates that the TRGO nanosheets can be effectivelyused for high power applications irrespective of the non-aqueous storage devices used.

IV. CONCLUSIONS

To summarize, graphene nanosheets prepared by eco-friendly trigol assisted reduction processare evaluated for charge storage applications in non-aqueous media. Excellent cycleability was notedfor TRGO in non-aqueous symmetric supercapacitor assembly with over ∼80% capacity retention

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042112-9 Mhamane et al. AIP Advances 3, 042112 (2013)

after 5000 cycles. The symmetric supercapacitor delivered the maximum energy and power densitiesof 60.4 Wh kg–1 and 0.15 kW kg–1, respectively. The Li-storage properties of TRGO nanosheetswere also evaluated in half-cell configuration (Li/TRGO). The cell delivered the reversible capacityof ∼705 mAh g–1 at current density of 37 mA g–1 with noticeable amount of capacity fade. At highcurrent rates, stable cycling profiles were noted at ambient temperature conditions.

ACKNOWLEDGMENTS

DM acknowledges fellowship support from CSIR, Govt. of India. This work is a part of CSIR’sTAPSUN initiative. VA and SM thank National Research foundation (NRF, Singapore) for financialsupport through Competitive Research Programme (CRP) (Grant no. NRF-CRP4-2008-03) andClean Energy Research Project (CERP) (Grant no. NRF-2009-EWT-CERP001-036).

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