Supporting Information

Transcription

1 Supporting Information Oxygen Reduction on Graphene-Carbon Nanotube Composites Doped Sequentially with Nitrogen and Sulfur Drew C. Higgins, Md Ariful Hoque, Fathy Hassan, Ja-Yeon Choi, Baejung Kim, Zhongwei Chen* Department of Chemical Engineering, Waterloo Institute for Nanotechnology, Waterloo Institute of Sustainable Energy, University of Waterloo, 200 University Ave. W, Waterloo, ON, N2L 3G1, Canada.

2 1.0 Additional experimental details 1.0 Synthesis of graphene-carbon nanotube nanocomposite: GO was prepared using an improved Hummer s method reported previously. 1 Using this technique, 2.2 g of graphite was added to a 400 ml mixture of H 2 SO 4 /H 3 PO 4 in a 9:1 ratio. KMnO 4 (18 g) was then added, and the mixture was heated to 50 o C where it was allowed to react for 16 hours. Following this, the solution was cooled down to 10 o C using an ice bath, and 400 ml of distilled de-ionized water was added dropwise, followed by 15 ml of 30 % hydrogen peroxide. The solid was then separated by centrifugation, washed thoroughly with water, ethanol and hydrochloric acid, and then collected by freeze drying. Commercial CNTs were oxidized (O- CNTs) by refluxing in 6 M nitric acid, 2 followed by washing with DDI water, collection by centrifugation and drying at 60 o C in an oven overnight. The prepared GO and O-CNT (5:1 weight ratio) were then physically mixed and ultrasonicated in acetone for 8 hours to achieve complete, uniform dispersion, while maintaining the temperature at ca. 10 o C by periodically adding ice to the sonicator water bath. Following this, the acetone was allowed to evaporate under ambient conditions and the graphene-carbon nanotube (GC) nanocomposite was collected for further processing. 1.1 Sequential nitrogen and sulfur doping of graphene-carbon nanotube composite: GC powder was collected and loaded into a quartz tube for pyrolysis in ammonia for nitrogen doping. A sealed tube furnace was used, and pyrolysis was carried out in a mixture of ammonia (60 sccm) and argon (140 sccm) at 500 o C for 5 hours, employing a heating rate of 5 o C/min. For comparative purposes, GC-NH was prepared in a similar fashion, however using a relatively higher pyrolysis temperature of 900 o C. Samples prepared by annealing in ammonia at 500 o C

3 and 900 o C are denoted as GC-NL and GC-NH, respectively (L = lower temperature, H = higher temperature). The collected GC-N materials were then physically mixed with phenyldisulfide (PDS) in a 1:5 ratio by mechanically grinding in a mortar and pestle. The powder was then placed in a quartz tube again and pyrolyzed under pure argon at 900 o C for 5 hours using a heating rate of 5 o C/min. This step-wise doping process resulted in GC nanocomposite materials doped with both nitrogen and sulfur (GC-NLS and GC-NHS) and subsequently investigated for their ORR catalysis properties under alkaline conditions. Physicochemical characterization: Scanning electron microscopy (SEM) was carried out on a LEO FESEM Transmission electron microcscopy (TEM) was carried out on a JEOL 2010F. Raman spectroscopy was carried out on a Bruker Senterra Raman Microscope operating with a wavelength of 532 nm. X-ray photoelectron spectroscopy (XPS) was carried out using a monochromatic Al Kα X-ray source (Thermal Scientific). Electrochemical characterization: The prepared GC materials were subjected to ring disc electrode (RDE) testing in order to evaluate their ORR activity. 4 mg of catalyst was ultrasonically dispersed in 1 ml of ethanol/ddi water (75:25 vol. %) ink containing 0.05 wt. % Nafion and 2 mg Ketjen EC-600J carbon. 20 ul of this blended ink mixture was then deposited on a cm 2 electrode and allowed to dry, leading to an overall GC catalyst loading of 80 ug. All electrochemical testing was carried out in 0.1 M KOH at room temperature employing a saturated calomel electrode (SCE) as a reference, which was later converted to RHE. ORR performance was evaluated by linearly sweeping the electrode potential under oxygen saturation from 1.05 to 0.1 V vs RHE at a scan rate of 10 mvs -1 at various electrode rotation speeds. Background currents obtained under

4 nitrogen saturated electrolyte conditions were subtracted in order to eliminate capacitative contributions. ORR polarization curves were collected at various rotation speeds (100, 400, 900, 1600 and 2500 rpm). Koutecky-Levich analysis 3 of GC-NLS was carried out using these data sets according to equation (1): = + (1) Here, i is the measured current density, i k is the kinetically limited current density and i L is the diffusion limited current density and can be further broken down according to equation (2): / =0.2 / / (2) Here, n is the average number of electrons transferred per molecule of oxygen, F is Faraday s constant (96,485 Cmol -1 ), C O2 is the concentration of oxygen in solution (1.2 x 10-6 molcm -3 ), D O2 is the diffusion coefficient of oxygen in solution (1.0 x 10-5 cm 2 s -1 ), v is the solution viscosity (0.01 cm 2 s -1 ) and w is the electrode rotation rate. Therefore, after generating a plot of i -1 versus w -0.5 (Koutecky-Levich plot) and calculating the slope, this value can be used in correspondence with the aforementioned constants to calculate the average number of electrons transferred per molecule of oxygen at varying electrode potentials.

5 2.0 Supplementary figures Figure S1: SEM image of GC-NHS.

6 Figure S2: High resolution S 2p spectrum of GC-S.

7 Figure S3: SEM images of (a,b) C-NLS and (c,d) G-NLS.

8 Intensity / a.u. C-NLS G-NLS Raman shift / cm -1 Figure S4: Raman spectra of C-NLS and G-NLS.

9 Figure S5: ORR polarization curves for GC-NLS obtained at various electrode rotation rates.

10 Figure S6. ORR polarization curves of commercial Pt/C (15 ug Pt cm -2 ) before and after ADT obtained at 900 rpm electrode rotation and 10 mvs -1 potential scan rate in 0.1 M KOH.

11 Figure S7: ORR polarization curves of GC-NL, GC-NLS and GC-NL900 at 1600rpm.

12 Figure S8: ORR polarization curves for GC-NL collected at various electrode rotation rates.

13 Figure S9: ORR polarization curves of GC-NL, GC-NLS and GC-NLB at 1600rpm.

14 Figure S10: ORR polarization curves for G-NLS collected at various electrode rotation rates.

15 Figure S11: ORR polarization curves for C-NLS collected at various electrode rotation rates.

16 Figure S12: ORR polarization curves for GC-NL collected at various electrode rotation rates.

17 Figure S13: ORR polarization curves for GC-NH collected at various electrode rotation rates.

18 Figure S14: ORR polarization curves for GC-NHS collected at various electrode rotation rates.

19 Figure S15: ORR polarization curves for GC-NL900 collected at various electrode rotation rates.

20 Figure S16: ORR polarization curves for GC-NLB collected at various electrode rotation rates. 3.0 References (1) Kim, B. J.; Lee, D. U.; Wu, J.; Higgins, D.; Yu, A.; Chen, Z. J. Phys. Chem. C 2013, 117, (2) Higgins, D. C.; Choi, J.-Y.; Wu, J.; Lopez, A.; Chen, Z. J. Mater. Chem. 2012, 22, (3) Lin, Z.; Waller, G.; Liu, Y.; Liu, M.; Wong, C.-P. Adv. Energy Mater. 2012, 2,