Mei Lenga, Meng LI



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Supporting Information

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.

*Correspondence and requests for materials should be addressed to Jun Min Xue*, e-mail: msexuejm@nus.edu.sg

Electrochemical Testing: The as-obtained GATUF was directly used as working electrode without any other polymer binders or conductive additives. In a typical preparation of electrode, the synthesised aerogel was cut into 1 x 1 cm electrode. The 1 x 1 cm graphene aerogel electrode was then wrapped inside a 1.5 x 3 cm nickel foam. The nickel foam with the graphene aerogel was gently compressed. The cyclic voltammetry (CV) and galvanostatic charge-discharge were performed on the electrochemical analyser (Solartron S1 1287) under ambient condition. A conventional three electrodes system, with Hg/HgSO4 as the reference electrode, platinum plate as the counter electrode, and 1M KOH as the electrolyte, was used for electrochemical studies (CV and galvanostatic charge/discharge). All the calculated values were normalized with the amount of sample used.

Materials Characterization: SEM images were recorded on ZEISS SEM Supra 40 (5kV). SEM samples were prepared by dripping the sample solutions onto silicon substrate. The X-ray photoelectron spectroscopy (XPS) spectra were taken by using an Axis Ultra DLD X-ray photoelectron spectrophotometer equipped with an Al Kα X-ray source (1486.69 eV).



Figure S Photographs of (a) GATU of gelation durations 2, 4, 6, and 24hrs without pre-freezing, (b) GATU of gelation durations 0, 2, 4, 6, and 24 hours with pre-freezing, (c) GATUF and GATU prepared from 6hrs gelation after lyophilisation, (d) table shows the summary for the percentage shrinkage of graphene aerogel over 2-24hours duration, and (e) bar chart showing the densities of GATUF & GATU at different gelation durations. (f) Formation of GATUF disk using petri dish

The purpose of the 1 hour freezing before the second gelation process is to minimize the shrinkage of the sulphur functionalized graphene aerogel as shown in Figure S1. If the 1 hour freezing is not carried, a prefect graphene aerogel disc cannot be synthesized as shown in Figure S1f. Thus the 1 hour freezing is crucial to obtain a prefect GATUF disk. Without the 1 hour freezing, due to the rapid shrinkage of the aerogel with time, the disk will typically crack due to the high stress concentration.



Element

Weight %

Atomic %

C

43.42

57.86

N

15.62

17.93

O

7.14

7.17

S

33.82

16.95

Figure S2 Weight percentages and atomic percentages of C, N, O, S from EDX



Figure S3 FTIR spectrum of GATUF



Figure S4 Mechanism of sulphur-functionalized graphene oxide sheets.

Thiourea can exists as resonance structures, either in thione (C=S) or thiol (C-S-H) forms. During the gelation process, the amine groups on the thiol form can react with carboxylic acid on the graphene oxide sheet through a reaction called the thiol-carboxylic acid esterification to form thiocarboxylic acid ester. Through such a reaction, sulphur is then chemically grafted onto graphene oxide sheet via the formation of thiocarboxylic acid ester group.





Figure S5 Activation process of GATUF (cycled at 2 A g-1 for 1000 cycles).

Activation process for GATUF required a long duration as specific capacitance increased steadily with cycle number. The activation process for GATUF was conducted by cycling the electrode at 2 A g-1 for 1000 cycles. Such a long activation duration may be explained with the following hypothesis; Thiourea can be further decomposed during the electrochemical oxidation which lead to the formation of more thiocarboxylic acid ester functional group on the graphene oxide surface. Such hypothesis was supported by the Raman spectroscopy (see Figure 4e). The more the thiocarboxylic acid ester functional group, the more the sulphone functional groups are produced which leads to increased pseudocapacitance.





Figure S6 (a) CV curves of GAAAF in 1M KOH at 10 mV s-1. (b) Cyclic stability of GAAAF in 1M KOH for 500 cycles cycled at 20 A g-1.



Figure S7 (a) CV curve of GAAAF in 0.08M thiourea/1M KOH at 10 mV s-1. (b) Cyclic stability of GAAAF in 0.08M thiourea/1M KOH for 200 cycles cycled at 20 A g-1.



Figure S8 Electrochemical measurement for GATUF annealed at 400oC. (a) CV curves of GATUF annealed at scan rates 50 mV s-1, (b) Galvonstatic discharge curves at current densities 1-20 A g-1, (c) percentage retention of GATUF annealed when cycled at 20 A g-1 for 1000 cycles, and (d) Specific capacitances and percentage retention at various current densities.



Figure S9 S 2p XPS spectrums for (a) GATUF annealed before electrochemical testing, and (b) GATUF annealed after electrochemical testing.


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