Supplementary Information. Unusual High Oxygen Reduction Performance in All-Carbon Electrocatalysts

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1 Supplementary Information Unusual High Oxygen Reduction Performance in All-Carbon Electrocatalysts Wei Wei 1, 4,, Ying Tao 1, 4,, Wei Lv 2,, Fang-Yuan Su 2, Lei Ke 2, Jia Li 2, Da-Wei Wang 3, *, Baohua Li 2, Feiyu Kang 2 1, 2, 4, * and Quan-Hong Yang 1 Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin, , China 2 Engineering Laboratory for Functionalized Carbon Materials, Graduate School at Shenzhen, Tsinghua University, Shenzhen, , China 3 School of Chemical Engineering, The University of New South Wales, Sydney, NSW 2033, Australia 4 The Synergistic Innovation Center of Chemistry and Chemical Engineering of Tianjin, Tianjin, , China These authors contributed equally to this work. Correspondence and requests for materials should be addressed to Q.-H.Y. ( qhyangcn@tju.edu.cn) and D.-W. W. ( da-wei.wang@unsw.edu.au) 1 / 11

2 Electrochemical Measurements Cyclic voltammetry (CV). The catalyst (1 mg) and Nafion solution (0.02 ml 5%) were dispersed in water/isopropanol mixed solvent (0.8 ml of 3:1 v/v) by sonication to form a homogeneous ink. Then the catalyst ink (2 μl) was loaded onto a pre-polished glassy carbon electrode of 2 mm in diameter for 4 times. The ink was dried slowly in air, obtaining a uniform catalyst distribution across the electrode surface. CV (using the Autolab pontentiostat from Metrohm) was conducted in an electrochemical cell using an Ag/AgCl electrode as the reference electrode, a Pt wire as the counter electrode and the sample modified glassy carbon electrode as the working electrode. Electrolyte (0.1 M KOH aqueous solution) was saturated with O 2 by bubbling O 2 prior to the start of each experiment for at least 40 min. The working electrode was cycled 5 times, and then data were collected from 0.2 to -0.8 V at a scan rate of 5 mv s -1. CV measurements were also performed in N 2 -saturated electrolyte by bubbling N 2 to the electrolyte at least 40 min. Rotating disk electrode (RDE) measurement. For the RDE measurements, catalyst inks were prepared by the same method as that of CV. The ink (8 μl) was loaded on a glassy carbon rotating disk electrode of 4 mm in diameter (Pine Instruments) for 4 times. The working electrode was cycled 5 times, and then the working electrode was swept at a rate of 5 mv s 1 from 0.2 to -1.2 V (for Pt/C, from 0.2 to -0.8 V) with varying rotating speeds from 400 to 2025 rpm. 2 / 11

3 Koutecky Levich plots (J -1 vs. ω -1/2 ) were analysed at V. The slopes of their best linear fit lines were used to calculate the number of electrons transferred (n) on the basis of the Koutecky-Levich equation. 1/J=1/J L +1/J K =1/(Bω (1 2) )+1/J K B=0.62nFC 0 D (2 3) 0 v (-1 6) Where J is the measured current density, J K and J L are the kinetic- and diffusionlimiting current densities, ω is the angular velocity, n is transferred electron number, F is the Faraday constant, C 0 is the bulk concentration of O 2, v is the kinematic viscosity of the electrolyte. Rotating ring-disk electrode (RRDE) measurement. For the RRDE measurements, catalyst ink was prepared by the same method as that of CV. The ink (6 μl) was loaded on a glassy carbon rotating ring-disk electrode of 4 mm in diameter (Pine Instruments) for 4 times. The working electrode was cycled 5 times, and then the working electrode was swept at a rate of 5 mv s 1 from 0.2 to -1.2 V with the rotating speed of 1600 rpm, and the ring potential was constant at 0.6 V. The amount of H 2 O 2 and the electron transfer number (n) were determined by the followed equations: % HO - 2 =200 (I r /N)/(I d +I r /N) n=4 I d /(I d +I r /N) Where I d is disk current, I r is ring current and N is current collection efficiency of the Pt ring. 3 / 11

4 ORR activities on nickel foams For measurements on nickel foams, the working electrode was prepared by loading G-CNT 100/20 catalyst ink with a density of about 0.75 mg cm -2 on a nickel foam. For the aqueous system, CVs with different scan rate were conducted in an electrochemical cell using an Ag/AgCl electrode as the reference electrode and a Pt sheet as the counter electrode. The electrolyte (0.1 M KOH aqueous solution) was saturated with O 2 by bubbling O 2 at least 40 min prior to the start of each experiment. For the non-aqueous system, CVs with different scan rate were conducted in an electrochemical cell (EL-CELL, Germany) using a lithium foil as the reference electrode and the counter electrode. The electrolyte (1M LiCF 3 SO 3 in TEGDME) was saturated by continuously bubbling O 2 during the tests. 4 / 11

5 Figures and Tables Figure S1. Photo of G-CNT 100/20 hydrogel after a hydrothermal self-assembly process. Figure S2. Low-magnification SEM image of G-CNT 100/20. 5 / 11

6 Figure S3. (a) XPS spectra of G-CNT 100/20, G-CNT 100/5, graphene monolith and CNT; (b) Element mapping of G-CNT 100/20. These results demonstrate that G-CNT is not doped by metal or dopants. (a) (b) Figure S4. Microscopy imaging of a commercial Pt/C catalyst. (a) SEM image; (b) TEM image. These images demonstrate uniform size distribution of Pt nanoparticles on Pt/C. (a) (b) Figure S5. (a) SEM image and (b) TEM image of a graphene monolith. 6 / 11

7 Figure S10. (a) SEM image and (b) TEM image of a GN monolith. Figure S6. SEM image of a simple mixture of graphene and CNT. Figure S7. (a) LSV curves of Pt/C in O 2-saturated electrolytes at different rotation rates (in rpm); (b) comparison between LSV curves of G-CNT 100/20 and Pt/C at 1600 rpm. 7 / 11

8 Figure S8. (a,b) The CV curves of catalyst (G-CNT 100/20) loaded electrode in O 2-saturated 0.1 M KOH solution; (c) The plots of peak current vs. v 1/2 (v denotes scan rate). The linear relationships exist between the peak current and v 1/2, indicating that such an ORR reaction is a diffusion-controlled process in 0.1 M KOH solution. Figure S9. (a,b) The CV curves of catalyst (G-CNT 100/20) loaded electrode in O 2-saturated 1M LiCF 3SO 3 in TEGDME; (c) The plot of peak current vs. v 1/2 (v denotes scan rate). The linear relationships exist between the peak current and v 1/2, indicating that such an ORR reaction is a diffusion-controlled process in 1M LiCF 3SO 3 in TEGDME. Figure S10. Charge/discharge plots of (a) G-CNT 100/20 and (b) Super-P in 1M LiCF 3SO 3 in TEGDME. 8 / 11

9 Intensity (a.u.) G-CNT100/20 G-CNT 100/10 G-CNT 100/5 G-CNT 100/3.3 G-CNT 100:2 Graphene monolith Wavenumber (cm -1 ) Figure S11. Raman spectra of G-CNTs and a graphene monolith. (a) (b) (c) (d) Figure S12. (a, b) SEM images and (c,d) TEM images of G-CNT 100/5. 9 / 11

10 Table S1. ORR performance (electron transfer number) of several all-carbon materials measured under the same conditions as that for G-CNTs Samples n Super-P (a commercial carbon black) 1.6 CNT 2.1 Graphene 2.5 A mixture of graphene and CNT 2.9~3.3 Graphene monolith 2.87 G-CNT 100/ All-carbon catalysts Doped Graphenes Graphenesupported catalysts Table S2. ORR performance of the reported graphene-based materials. Note that materials in light blue shading are doped graphenes, and materials in light green shading are graphene-supported metal or metal oxide catalysts. Sources Scan rate mv s -1 Loading density mg cm -2 Ep (v s.rhe) V Ip, O2- Ip, Inert gas ma cm -2 ma mg -1 This Work Nat. Mater. 2011, 10, Nat. Mater. 2011, 10, ~ Adv. Mater. 2012, 24, Angew. Chem. Int. Ed. 2012, 51, Angew. Chem. Int. Ed. 2011, 50, Angew. Chem. Int. Ed. 2012, 51, ~ ~ 3.6 Adv. Energy Mater. 2012, 2, J. Am. Chem. Soc. 2012, 134, ~ ~ 3.91 Nat. Nanotechnol. 2012, 7, ~ Nat. Mater. 2011, 10, Nat. Mater. 2011, 10, J. Am. Chem. Soc. 2012, 134, Angew. Chem. Int. Ed. 2011, 50, J. Am. Chem. Soc. 2012, 134, ~ ~ n 10 / 11

11 Simulation The simulation for the charge transfer between a graphene and a CNT A brief simulation was performed with the program package DMol3 in the Materials Studio (Accelrys Inc.) software. For simplification, a sheet of 40 six-membered rings and a single-wall carbon nanotube are chosen as the model for simulated graphene (denoted sg) and simulated carbon nanotube (denoted scnt), respectively. A combined system of sg and scnt is used to simulate the graphene-carbon nanotube hybrid (denoted sg-cnt). After the calculation, the original flat sg warps a little due to the interaction with the scnt (Figures S16a and b), which in agreement with the TEM images (Figures 2b and c). The calculation results further support the change of charge density. Based on the atomic charge data of carbon atoms in sg, it is found that the carbon atoms on sg adjacent to the scnt exhibit positive charge density compared to those of the sg free of nanotubes (Figures S11c and d). Furthermore, the total atomic charge of sg in the sg-cnt is 0.232, and for sg free of nanotube the total atomic charge is 0. This also demonstrates that scnt receives electron from sgn, which is in agreement with the KPFM and Raman results. A detailed simulation is ongoing to investigate the adsorption of a single oxygen molecule and the following oxygen reduction reaction on graphene-carbon nanotube hybrid. (a) (b) Å (c) (d) Figure S12. a) and b) Image of the sg-cnt after simulation; the calculated charge density distribution of some selected carbon atoms for c) sg free of nanotubes and d) the sg in the sg-cnt after calculation. 11 / 11