Supporting information. Free Standing Graphene Oxide Paper: New Polymer Electrolyte for Polymer Electrolyte Fuel Cells. Ravikumar, Keith Scott

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1 Supporting information Free Standing Graphene Oxide Paper: New Polymer Electrolyte for Polymer Electrolyte Fuel Cells Ravikumar, Keith Scott School of chemical Engineering and advanced Materials, Newcastle University Address, Newcastle Upon Tyne, United Kingdom, NE1 7RU Experimental Section SI 1. Synthesis of graphite oxide Graphite oxide was synthesized from natural flake graphite by the modified Hummers method 1. In brief, 200 mg of Graphite particles were immersed in Concentrated sulfuric acid (46 ml) and KMnO 4 (6 g) was added slowly in small quantities in which temperature was maintained between the 0 and 5 o c using an ice bath, after the complete oxidation of KMnO 4, the mixture was heated to 37 o c and kept at this temperature about 30 min. Then 12 ml of distilled water added slowly to this mixture then temperature of mixture is raised to 95 o c and it is maintained for about 15 min, this mixture was further diluted with 280 ml of water and later 20 ml of 30% H 2 O 2 was added and left for 5 min, then solid was filtered off and washed with 5% HCl until filtrate was free from sulphate ions. GO thus obtained was further washed thoroughly with water and dried in air for 24 hours. SI 2. a) Synthesis of Sulfonated graphite oxide through aryl diazonium reaction of sulfanilic acid The aryl diazonium salt used for sulfonation was prepared as following: To a 100 ml beaker placed 5 ml NaOH (2%) and 0.05 g SA, and allowed SA to dissolve in warm water bath. To the above solution 0.02 g NaNO 2 was added at RT, after NaNO 2 dissolved the mixed solution was added into 10 ml ice water and 1 ml of concentrated HCl under stirring, the temperature was kept at 0 for 15 min and diazonium salt was formed. Then the diazonium salt solution was added drop wise into 50 ml of GO (1 mg/ml) solution and the mixed solution was stirred strongly for 4 h in an ice water bath. After centrifuging and washing with water for several times, the obtained SGO was dispersed in water and stored at RT for use 2.

2 SI 2. b) Formation of free standing sulfonated graphene oxide paper Preparation and characterisation of graphene oxide paper was reported by D.A. Dikin et al. Nature, 2007, 448, and they proposed a mechanism for formation of free standing graphene oxide by vacuum filtration. The same mechanism applies to the formation of sulfonated graphene oxide (SGO) paper in our case graphene oxide is modified by adding a sulfonic acid group. During the filtration, SGO sheets are assembled onto the surface of Anodisc membrane (Cellulose acetate ester. As the deposition of SGO sheets on top of each other increases and the water flow decreases. The filtration time depends on amount of SGO solution used for desired thickness normally takes from about 2 days. After drying, membrane can be peel off from the filter and it can be folded like normal paper or membrane. SGO is hydrophilic, but a piece of SGO left in water for several hours does not disintegrate and maintain its shape. If the paper is handled while it is still wet, it will disintegrate but it will regain its mechanical strength when it is dried and it can handled. This behaviour indicates that no covalent bonding was achieved between the SGO sheets. Stiffness and flexibility is result of unique interlocking-tile arrangement of nanoscale SGO sheets 3.

3 SI 3 Membrane Electrode Assembly Preparation To prepare the fuel cell membrane electrode assemblies, the membrane was sandwiched between 20% Pt/C anode and 50% Pt/C cathode (0.4 mg cm -2 ; from Alfa Aesar) electrodes made as follows. The catalyst ink was made by ultrasonicating with 2-propanol and 20 wt% Nafion ionomer, and the ink was sprayed onto gas diffusion electrodes (carbon paper) with a wet proofed micro-porous layer (H2315 T10AC1), purchased from Freudenberg (FFCCT, Germany). The MEA was hot pressed at 50 o C and a pressure of 40 kg cm -2 for 3 minutes.

4 SI 4 Conductivity measurements AC impedance measurements were carried out between frequencies of 20 khz and 1 khz to measure the in-plane and through plane conductivities of the composite membranes using a four-point probe method with a Frequency Response Analyzer (Voltech TF2000, UK). The method involves four equally spaced probes in contact with the measured material: two of the probes were used to source current whilst the other two were used to measure the voltage drop. The cell was operated at 100% RH at ambient pressure and with a variable temperature. Durability of the SGO paper conductivity was performed by holding at 50 o C and collecting the resistance value for every one hour up to ten hours. In plane conductivity data shows that after 1 h slight conductivity drops of about 1 mscm -1 and remains unchanged, in case of through plane the drop in conductivity is about 3 mscm -1 after 2 h and remains unchanged up to 10 h. In our previous report SGO/PBI composite membrane for high temperature PEMFC, SGO exhibits good stability at 150 o C 4. These results indicate that SGO paper is very stable under electrochemical environment Conductivity Scm SGO paper Inplane conductivity at 50 o C 100% RH Throughplane conductivity at 50 o C and 100% RH Time/h Figure S1, Durability of SGO paper conductivity at 50 o C and 100% RH

5 SI 5 Ion Exchange Capacity (IEC) The ion exchange capacity (IEC) of the membranes was measured with the classical titration technique. The samples were soaked in a large volume of 0.1 mol L -1 HCl solutions, washed With distilled water to remove excess HCl, and then immersed in1 M NaCl aqueous solution to release protons from the membrane. The released H + was back titrated with a 0.01 M NaOH aqueous solution using phenolphthalein as an indicator. The IEC value (in meq g -1 ), which is defined as milli equivalents (meq.) of sulfonic groups per gram of dried sample, is obtained from following equation. IEC = V NaOH X C NaOH / W dry Where V NaOH is the volume of NaOH consumed, C NaOH is the concentration of NaOH and W dry is the weight of the dry membrane (g).

6 SI 6 FT-IR analysis of Sulfonated graphite oxide, Graphite oxide and Graphite Transmittance % Sulfonated graphite oxide Graphite Oxide Wavenumber cm -1 Figure S2, FTIR spectra of graphite oxide and sulfonated graphite oxide FTIR studies confirmed the successful oxidation of graphite to graphite oxide and sulfonated graphite oxide, as shown in figure. The presence of different types of oxygen functionalities in GO and SGO at cm -1 (O H stretching vibrations) and at cm -1 (stretching vibrations from C=O) and SGO shows a peak at 1346 cm -1 for SO 3 H. Hydrogen bonded O-H stretching vibrations observed at 3183 cm -1 for GO and SGO.

7 SI 7 EDX data of GO and SGO Material Carbon Oxygen Sulphur Wt% At% Wt% At% Wt% At% GO SGO Table S1, EDX data of graphite oxide and sulfonated graphite oxide obtained from ESEM The elemental analysis of GO and SGO paper was performed by ESEM on different areas of paper and an average of wt% data is presented in Table S1. The Sulphur content in SGO is about wt%, therefore the degree of sulfonation is ~ 11%. (The degree of sulfonation is directly proportional to the Sulphur content. Functinalisation of MCNTs by Kumar et al. Nanaoscale Research Letters 2011, 6:583 reported that the degree of sulfonation is directly proportional to sulfur content 4 )

8 SI 8 XRD spectrum of SGO, GO and Graphite Intensity GO Graphite SGO theta Figure S3, X-ray diffraction of graphite oxide, sulfonated graphite oxide and graphite XRD spectra of graphite oxide, Sulfonated graphite oxide and graphite powder. The peak observed at 26.45º in the graphite sample corresponds to the interplanar distance between the different graphene layers. Chemical oxidation of graphite disturbs ordering of layer and introduces a variety of functional groups (epoxide, carboxyl, etc) as the C-C bonds are broken during the oxidation process 4. The functional groups increases the interplanar distance between the sheets this larger distance between graphene oxide sheets shifts the XRD peak to smaller angles, thus the appearance of broader peak at around 11.26º in GO. After sulfonation, the disruption of GO structure and increased the attractive interaction between the GO layers. This was confirmed by X-ray diffraction pattern of SGO, the shift of 2θ towards higher angle was observed in SGO.

9 SI 9 Mechanical stability of SGO paper Mechanical stability measurements of all SGO paper samples were performed on Instron 4505 machine and measurements have been repeated for three samples (20mm X 11mm; 35µm) for reproducibility. Figure shows a typical plot of stress Vs strain for SGO paper. The maximum tensile stress was MPa and Young s modulus was 1.59 GPa. These results indicate that SGO paper is stiff and brittle. 20 (a) SGO paper 20 (b) SGO paper Strain (MPa) 10 Stress (Mpa) Stress (%) Strain (%) Figure S4, a) Young s modulus and b) stress strain curve of SGO paper

10 SI 10 Hydrogen Crossover of SGO paper MEA 5 4 H 2 cross over of SGO Paper MEA at 40 o C Current density ma cm Potential mv Figure S5, Hydrogen crossover plot of SGO paper MEA Hydrogen crossover test of SGO paper MEA was performed by passing hydrogen on cathode and argon gas was fed with cathode. Hydrogen crossover current was measured by linear sweep voltammetry. The crossover current was 4.3 ma cm -2, which was more than that of Nafion based membranes. Therefore the OCV of SGO MEA is reduced (0.72 V).

11 References [1] William S. Hummers Jr. and Richard E. Offeman, J. American Chemical Society 1958, 80, 1339 [2] Jung-Hwan Jung, Jin-Han Jeon, Vadahanambi Sridhar, Il-Kwon Oh, Carbon, 2011, 149, [3] Dmitriy A. Dikin, Sasha Stankovich, Eric J. Zimney, Richard D. Piner, Geoffrey H.B. Dommett, Guennadi Evmenenko, SonBinh T. Nguyen, and Rodney S. Ruoff, Nature, 2007, 448, [4] Xu CX, Cao YC, Kumar R, Wu X, Wang X, Scott K, Journal of Materials Chemistry 2011, 21(30), [5] Pawan Kumar, Jin- Soo Park, Prabhasharan Randhawa, Sandeep Sharma, Mun-Sik Shin and Satpal Singh Sekhon, Nanoscale Research Letters, 2011, 6:583 [5] Brian Seger, Prashant V. Kamath, J. Phys Chem. C, 2009; 113,