Supporting Information

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1 Supporting Information Nitrogen-Doped Carbon Nanoparticle Carbon Nanofiber Composite as an Efficient Metal-Free Cathode Catalyst for Oxygen Reduction Reaction Gasidit Panomsuwan, *,, Nagahiro Saito,,, and Takahiro Ishizaki, Department of Materials Science and Engineering, Faculty of Engineering, Shibaura Institute of Technology, Toyosu, Koto-ku, Tokyo , Japan Department of Materials, Physics and Energy Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya , Japan Social Innovation Design Center (SIDC), Institute of Innovation for Future Society, Nagoya University, Furo-cho, Chikusa-ku, Nagoya , Japan Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency (JST), Honcho, Kawaguchi, Saitama , Japan Present address: NU-PPC Plasma Chemical Technology Laboratory, The Petroleum and Petrochemical College, Chulalongkorn University, Soi Chulalongkorn 12, Phayathai Road, Pathumwan, Bangkok 10330, Thailand * S1

2 Figure S1. Schematic illustration of the solution plasma process system used for the preparation of NCNP CNF composite. S2

3 Optical Emission Spectroscopy Measurement Optical emission spectrum of the plasma generated in 2-cyanopyridine without CNFs was recorded with an Avantes AvaSpec-3648 fiber optic spectrometer system in the wavelengths from 300 to 800 nm. The spectrum was recorded by averaging 3 scans with a 200 ms integration time. The optical probe was set at about 15 mm above the plasma zone. Figure S2. Optical emission spectrum of the plasma generated in 2-cyanopyridine without CNFs. The optical emission spectrum shows the dominant emissions of Swan bands originating from diatomic carbon (C 2 ) (transition d 3 g a 3 u ; sequences = 2, 1, 0, +1, +2) at the wavelength range of nm. S1 The emission peaks at 487 and 656 nm are associated with Balmer atomic hydrogen H and H, respectively. S2 Additionally, the emission peaks corresponding to the CN violet system (transition B 2 + X 2 + sequence = 1, 0, +1) are also observed at the shorter wavelengths of nm. S3,S4 S3

4 Figure S3. (a) FESEM and (b) TEM images of pristine CNFs. The corresponding SAED pattern of CNFs in the inset of (b) presents a well-defined concentric ring, indicating good crystallinity. Figure S4. (a) FESEM and (b) TEM images of NCNPs. The corresponding SAED pattern of NCNPs in the inset of (b) shows a broad and diffuse ring, suggesting a dominant amorphous phase. Table S1. Textural parameters of CNFs, NCNPs, and NCNP CNF composite obtained from the N 2 adsorption desorption isotherms. Specific surface area (m 2 g 1 ) Pore volume (cm 3 g 1 ) Mean pore size (nm) CNFs NCNPs NCNP CNF S4

5 Table S2. Surface and bulk elemental compositions of CNFs, NCNPs, and NCNP CNF composite obtained from XPS and EA measurements, respectively. Surface composition, XPS (atom % / wt %) Bulk composition, EA (wt %) C O N C H N Surface/Bulk ratio (N) CNFs 91.63/ / NCNPs 91.69/ / / NCNP CNF 91.87/ / / Table S3. Percentages and absolute contents of various nitrogen states for NCNPs and NCNP CNF composite obtained from the deconvolution of XPS N 1s peaks. Percentage (%)/Absolute content (atom %) Pyridinic N Pyrrolic N Graphitic N Pyridinic N-oxide NCNPs 20.7/ / / / 0.15 NCNP CNF 23.6/ / / / 0.14 S5

6 Figure S5. High-resolution XPS C 1s spectra of NCNPs and NCNP CNF composite. The XPS C 1s peak can be deconvoluted into six peaks. The most pronounced peak at ev (C I ) is evidence that the carbons are present in the sp 2 graphite structural form. The other five peaks at the tail of asymmetric XPS C 1s peak can be assigned to the presence of sp 3 bond (C II : ev), C O/C=N (C III : ev), C=O/C N (C IV : ev), C=O bonds of different types (carbonyl, aldehyde, etc.) (C V : ev), and interactions (C VI : ev). S5 S8 Figure S6. High-resolution XPS O 1s spectra of NCNPs and NCNP CNF composite. The XPS O 1s peak can be deconvoluted into five peaks, including quinines (O I : ev), C=O (O II : carbonyl and carboxyl; ev), C O (O III : epoxy and hydroxyl, ev), OH (O IV : hydroxyl, ev), and adsorbed molecular water (O V, ev). S8,S9 S6

7 Rotating Disk Electrode (RDE) Measurement Rotating ring disk (RDE) measurement was performed to verify the ORR pathway evaluated from the RRDE measurement. A series of LSV curves was recorded at various rotation speeds from 225 to 2500 rpm (Figure S7). Notably, the current density in the diffusion-controlled region increases with an increase in rotation speed, which can be explained by faster oxygen diffusion through the electrode surface. On the basis of the LSV curves recorded at various rotation speeds, the n values per O 2 molecule involved in the ORR process can be determined by the Koutecky Levich (K L) equation given as below: S , (S-1) 1/ 2 J Jk Jd Jk B B nfv C D 1/ 6 2 /3 0.62, (S-2) O2 O2 where J is the measured, kinetic density, J k is the kinetic current density, J d is the diffusion-limiting current density, is the angular velocity of the disk in rad s 1 ( = 2 N/60, N = rotation speed in rpm), F is the Faraday constant (96485 C mol 1 ), DO 2 is the diffusion coefficient of O 2 in the electrolyte ( cm 2 s 1 ), v is the kinematic viscosity of the electrolyte (0.01 cm 2 s 1 ), and concentration of O 2 in the electrolyte ( mol cm 3 ). CO 2 is the bulk S7

8 Figure S7. A series of LSV curves for (a) CNFs, (b) NCNPs, (c) NCNP CNF composite, and (d) Pt/C in an O 2 -saturated 0.1 M KOH solution at various rotation speeds from 225 to 2500 rpm with a scan rate of 10 mv s 1 (after background current subtraction). S8

9 Figure S8. The K L plots of (a) CNFs, (b) NCNPs, (c) NCNP CNF composite, and (d) Pt/C in the potential range from 1.0 to 0.4 V derived from a series of LSV curves at various rotation speeds in Figure S7. Figure S9. Electron transfer numbers (n) of various catalysts calculated from the slopes of linear fit lines in Figure S8 in the potential range from 1.0 to 0.4 V. S9

10 Electrical Conductivity Measurement Electrical conductivity measurement was carried out by a two-probe method at room temperature. The catalysts were loaded into a hollow Teflon cylinder (inner diameter: 50 mm) and then compressed between two copper pistons as the electrodes in ambient air at a pressure of 0.6 MPa. The direct current (1 10 ma) was applied using a Metronix 692A DC constant power supply and the voltage between electrodes was measured simultaneously using a Keithley 2001 digital multimeter. The electrical conductivity ( ) of all catalysts can be determined from the following equation: S11,S12 l li RA VA, (S-3) where l is the distance between the two copper pistons in cm, R is the electrical resistance, I is the applied DC current, V is the measured voltage, and A the area of the piston surface in cm 2. In the measurement, the voltages were recorded at various applied currents in order to confirm the ohmic behavior. Table S4. Electrical conductivity of CNFs, NCNPs, and NCNP CNF composite. Electrical conductivity (S cm 1 ) CNFs NCNPs NCNP CNF S10

11 Electrocatalytic Activity of NCNP/CNF Mixture To confirm the unique properties of NCNP CNF composite prepared by the solution plasma process in this study, a mixture of NCNPs and CNFs was prepared by a physical mixing method for comparison. The certain amount of NCNPs and CNFs with the corresponding weight ratio of 2:1 was physically mixed in an agate mortar and pestle. After that, the mixture of NCNPs and CNFs was dispersed in ethanol and sonicated for 1 h to obtain a homogeneous dispersion. The homogeneous mixture was filtrated and dried in an oven at 100 C. The resulting product is denoted as NCNP/CNF mixture. Furthermore, the catalyst ink of NCNP/CNF mixture was prepared using the same procedure with other catalysts and dropped onto a GC electrode for the electrochemical measurements. S11

12 Figure S10. Electrocatalytic activity for the ORR of NCNP/CNF mixture: (a) CV curves in N 2 and O 2 -saturated 0.1 M KOH solution at a scan rate of 50 mv s 1. (b) LSV curves measured on an RRDE in O 2 -saturated 0.1 M KOH solution (after background current subtraction) at a rotation speed of 1600 rpm and a scan rate of 10 mv s 1. (c) Electron transfer number (n) and (d) HO 2 yield derived from the disk and ring currents at the potential range from 1.0 to 0.3 V. (e) A series of LSV curves at various rotation speeds from 225 to 2500 rpm. (f) The K L plots (J 1 versus 1/2 ) derived from the LSV curves at various rotation speeds in Figure S10e. The n values of NCNP/CNF mixture calculated from the slopes of the K L plots at the potential range from 1.0 to 0.4 V are shown in the inset. References (S1) Harilal, S. S.; Issac, R.C.; Bindhy, C.V.; Nampoori, V. P. N.; Vallaban, C. P. G. Optical Emission Studies of Species in Laser-Produced Plasma from Carbon, J. Phys. D: Appl. Phys., 1995, 30, (S2) Tatarova, E.; Dias, F. M.; Ferreira, C. M. Hot Hydrogen Atoms in a Water-Vapor Microwave Plasma Source, Int. J. Hydrogen Energy, 2009, 34, (S3) Lefohn, A. E.; Mackie, N. M.; Fisher, E. R. Comparison of Films Deposited from Pulsed and Continuous Wave Acetonitrile and Acrylonitrile Plasmas, Plasmas Polym., 1998, 3, (S4) Stillahn, J. M.; Fisher, E. R. Gas Phase Energetics of CN Radicals in Radio Frequency Discharges: Influence on Surface Reaction Probability During Deposition of Carbon Nitride Films, J. Phys. Chem. A, 2010, 114, (S5) Wang, H.; Maiyalagan, T.; Wang, X. Review on Recent Progress in Nitrogen-Doped Graphene: Synthesis, Characterization, and Its Potential Applications, ACS Catal. 2012, 2, (S6) Zhang, Y.; Ge, J.; Wang, L.; Wang, D.; Tao, X.; Chen, W. Manageable N-doped Graphene for High Performance Oxygen Reduction Reaction. Sci. Rep. 2013, 3, (S7) Valázquez-Palenzuela, A.; Zhang, L.; Wang, L.; Cabot, P. L.; Brillas, E.; Tsay, K.; Zhang, J. Carbon-Supported Fe N x Catalysts Synthesized by Pyrolysis of the Fe(II) 2,3,5,6-Tetra (2- pyridyl) pyrazine Complex: Structure, Electrochemical Properties, and Oxygen Reduction Reaction Activity. J. Phys. Chem. C 2011, 115, (S8) Chen, C.-M.; Huang, J.-Q.; Zhang, Q.; Gong, W.-Z.; Yang, Q.-H.; Wang, M.-Z.; Yang, Y.-G. Annealing a Graphene Oxide Film to Produce a Free Standing High Conductive Graphene Film. Carbon, 2012, 50, (S9) Datsynk, V.; Kalyva, M.; Papagelis, K.; Parthenios, J.; Tasis, D.; Siokou, A.; Kallitsis, I.; Galiotis, C. Chemical Oxidation of Multi-walled Carbon Nanotubes. Carbon 2008, 46, S12

13 (S10) Xing, W.; Yin, G.; Zhang, J. Rotating Electrode Methods and Oxygen Reduction Electrocatalysts; Elsevier: Amsterdam, 2014, Chapter 6. (S11) Sánchez-González, J.; Macías-García, A.; Alexandre-Franco, M. F.; Gómez-Serrano, V. Electrical Conductivity of Carbon Blacks under Compression, Carbon, 2005, 43, (S12) Celzard, A.; Marêché, J. F.; Payot, F.; Furdin, G. Electrical Conductivity of Carbonaceous. Powders, Carbon, 2002, 40, S13