Supporting Information for

Metal-free N-doped carbon blacks as excellent electrocatalysts for

oxygen reduction reactions

Junghoon Oh,a Sunghee Park,a Dawoon Jang, Yunseok Shin, Donggyu Lim, Sungjin Park*

WCSL (World Class Smart Lab) Green Energy Battery Lab, Department of Chemistry and

Chemical Engineering, Inha University, 100 Inha-ro, Nam-gu, Incheon, 22212, Republic of

Korea

a These authors contributed equally to this work

Email: sungjinpark@inha.ac.kr

Experimental

Table S1. Summary of production conditions.

Pre-heating temperature (°C)

Post-heating temperature (°C) Melamine

N-CB-600-1000 600 1000 O

N-CB-700-1000 700 1000 O

N-CB-750-1000 750 1000 O

CB-750-1000 750 1000 X

CB-1000 One-time heating at 1000 °C X

N-CB-1000 One-time heating at 1000 °C O

Preparation of N-CB-1000

A mixture of melamine (5 g, 99%, Sigma-Aldrich) powder and CB (200 mg) was loaded into

a quartz crucible and placed in the center of a quartz tube. The furnace was heated at 1000 °C

under N2 flow (150 sccm) for 2 h. The sample was subsequently cooled to room temperature

to afford a black powder.

Preparation of CB-1000

CB (200 mg) was loaded into a quartz crucible and placed in the center of a quartz tube. The

furnace was heated at 1000 °C under N2 flow (150 sccm) for 2 h. The sample was

subsequently cooled to room temperature to afford a black powder.

Preparation of CB-750-1000

A mixture of melamine (5 g, 99%, Sigma-Aldrich) powder and CB (200 mg) was loaded into

a quartz crucible and placed in the center of a quartz tube. The pre-heating process was

performed at designated temperatures (Table 1) under air flow for 2 h. The furnace was

cooled to room temperature then post-heating process was performed at designated

temperatures (Table 1) under N2 flow for 2 h. Finally, the furnace was cooled to room

temperature to afford a black powder.

Electrochemical measurements

Electrochemical measurements were performed using an electrochemical analyzer

(BioLogic, VSP, France; RRDE-3A ALS, Japan) and an O2-saturated 0.1 M KOH electrolyte

with a three-electrode system at room temperature. A glassy-carbon ring disk electrode

(RDE) was used as the working electrode (011169, ALS Co., Ltd., GC diameter: 3.0 mm)

with a graphite rod counter electrode and Hg/HgO (013592, ALS Co., Ltd./ 1 M NaOH

filling solution) reference electrode. The RDE was polished using 0.3- and 0.05-µm alumina

suspensions before catalyst casting. To prepare the catalyst ink, 5 mg of catalyst, 0.2 mL of

distilled water, 0.8 mL of anhydrous ethanol, and 0.05 mL of Nafion (5 wt% in isopropanol,

Aldrich) were mixed [for Pt/C catalyst ink, 4 mg of 20 wt% Pt/C (HiSPEC 3000, Johnson-

Matthey) was used], and the slurry was sonicated for 30 min using an ultrasonic cleaner

(Powersonic 410). The catalyst ink (5 µL) was deposited onto the glassy carbon electrode of

the RDE (3 mm in diameter) using a micropipette, then dried at room temperature for 30 min.

All potentials were reported with respect to the reversible hydrogen electrode (RHE). Before

electrochemical measurements, the surface of the catalyst was cleaned by cycling the

potential between 0.05 and 1.20 V (vs RHE) 100 times at a scan rate of 100 mV·s-1. Cyclic

voltammetry (CV) was performed over a voltage range of 0.05–1.2 V (vs RHE) at a scan rate

of 20 mV·s-1 in N2-saturated 0.1 M KOH electrolyte solutions. The ORR activity of the

catalysts was measured using linear sweep voltammetry (LSV) from 1.1 to 0.2 V (from 0.2 to

1.1 V for the Pt/C catalyst) at a scan rate of 20 mV·s-1 and rotating speeds of 100, 400, 900,

1600, and 2500 rpm in an O2-saturated electrolyte with oxygen purging.

The number of electrons transferred (n) was calculated from the slopes of Koutecky-Levich

plots. The kinetic parameters were analyzed based on the Koutecky-Levich equations:

Where i is the measured current density; ik and il are the kinetic- and diffusion-limiting

current densities; is the angular velocity of the disk (=2πN, where N is the linear

rotation speed); n is the overall number of electrons transferred in the ORR; F is the Faraday

constant (96485 C·mol-1); C0 is the bulk concentration of O2 in 0.1 M KOH (1.2 × 10-6

mol·cm-3); D0 is the diffusion coefficient of O2 in 0.1 M KOH (1.9 × 10-5 cm2·s-1); and ν is the

kinematic viscosity of the electrolyte (1.0 × 10-2 cm2·s-1).

To examine the long-term durability of the catalysts, chronoamperometric response was

measured in an O2-saturated 0.1 M KOH solution. CV was performed over a voltage range of

0.05–1.2 V (vs RHE) at a scan rate of 100 mV·s-1 with a rotation speed of 1600 rpm for 1000,

5000, and 10000 cycles. The measured current was normalized with respect to its initial

current. To assess the methanol-tolerance of the catalysts, methanol (3 mL, 99.8%, Sigma-

Aldrich) was added after 600 s in an O2-saturated 0.1 M KOH solution.

Instrumentation

High resolution scanning electron microscopy (HR-SEM) images were obtained using a field

emission gun (SU8010, Hitachi, Japan). The morphologies of the samples were characterized

using transmission electron microscopy (TEM) images obtained with a field emission gun

transmission electron microscope (JEM2100F, JEOL, Japan) operating at 200 kV. Energy

dispersive X-ray spectroscopy (EDX) spectra were recorded using a JEM-2100 F (JEOL)

instrument at 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were

performed using an angle-resolved X-ray photoelectron spectrometer (Theta probe, Thermo

Fisher Scientific, UK). X-ray diffraction (XRD) patterns were obtained using a DMAX-2500

instrument (Rigaku, Tokyo, Japan). Brunauer-Emmett-Teller (BET) surface area

measurements (Tristar, ASAP 2020, Micromeritics, USA) were performed using nitrogen

adsorption isotherms.

Table S2. BET surface areas and pore size data of the samples.

SampleSurface area

(m2/g)

Pore diameter

(nm)

Pore volume

(cm3/g)

Ketjen black 1320 6 2.3

N-CB-600-1000 729 12 2.3

N-CB-700-1000 954 11 3.0

N-CB-750-1000 838 13 3.1

Table S3. Atomic composition of the prepared samples determined by XPS.

Sample C (at%) N (at%) O (at%)

N-CB-750-1000 96.9 0.7 2.4

N-CB-700-1000 97.5 0.5 2.0

N-CB-600-1000 97.1 1.2 1.7

CB-750-1000 98.1 - 2.0

N-CB-1000 98.2 0.9 1.0

CB-1000 89.8 - 10.2

Table S4. Summary of ORR electrocatalytic performance of efficient catalysts previously

reported

SampleOnset potential

(V vs RHE)

Half wave potential

(V vs. RHE)

Current density

(at 0.2 V)

Ref

Fe-doping

N-CB-750-1000 0.97 0.82 -6.09This work

FNCT800-100 0.941 0.703 -5.319 [1]

Fe3O4/N-C-900 0.909 0.784 -5.12 [2]

Fe-N-C-700 0.828 0.656 -3.5 [3]

Fe-PANI/C-Mela 0.98 ca. 0.8 -5.9 [4]

Fe2N-NGC1-1000 0.83 ca.0.7 -4.5 [5]

N-doped Fe/Fe3C@C/RGO 1 0.93 - [6]

Fe/Co-NpGr 0.93 ca. 0.8 -4.3 [7]

Fe-N-CNT-OPC 0.914 ca. 0.73 -5.9 [8]

Fe-N-CC 0.94 0.83 -4.85 [9]

FP-Fe-TA-N-850 0.98 0.78 -5.0 [10]

Co-doping

Co15-N-C800 0.97 0.82 -3.3 [11]

Co-N/C-A 0.95 0.83 -3.5 [12]

Co3O4@N-C 0.95 0.70 -2.1 [13]

Co@Co3O4@CCM 0.93 0.81 -4.1 [14]

Co-g-C3N4@rGO 0.93 0.82 -5.0 [15]

Metal Free N,P-GCNS 1.01 0.86 -5.56 [16]

N, P-MC 0.95 0.84 -5.4 [17]

NPMC-1000 0.94 0.85 -4.3 [18]

PCN-CFP 0.94 0.67 - [19]

N-graphene/CNT 0.88 0.68 -4.8 [20]

CNT/HDC-1000 0.92 0.82 -5.5 [21]

Table S5. ID/IG ratios calculated from D and G bands in Raman spectra.

Sample ID/IG

Kejen black 1.30

N-CB-600-1000 1.26

N-CB-700-1000 1.23

N-CB-750-1000 1.18

N-CB-1000 1.17

Table S6. Charge transfer resistances (Rct) calculated from Nyquist plots of various samples.

Sample Rct ()

Ketjen black 3088

N-CB-600-1000 2019

N-CB-700-1000 582

N-CB-750-1000 260

CB-1000 2105

CB-750-1000 1160

N-CB-1000 1966

Figure S1. SEM images of (a) N-CB-600-1000, (b) N-CB-700-1000, and (c) N-CB-750-

1000.

Figure S2. TEM images of (a) N-CB-600-1000 and (b) N-CB-700-1000.

Figure S3. TEM image of N-CB-750-1000 for elemental mapping (left) and elemental

mapping of N (right).

Figure S4. BJH pore diameter of Ketjen black, N-CB-600-1000, N-CB-700-1000, and N-

CB-750-1000.

Figure S5. LSV curves at various rotating speeds. (a) Ketjen black, (b) N-CB-600-1000, (c)

N-CB-700-1000, and (d) N-CB-750-1000.

Figure S6. Raman spectra of Ketjen black, N-CB-600-1000, N-CB-700-1000, N-CB-750-

1000, N-CB-1000.

Figure S7. Nyquist plots obtained with a GC electrode at -0.13 V and 1600 rpm in O2-

saturated 0.1 M KOH.

Figure S8. XPS spectra of Ketjen black, N-CB-750-1000, CB-750-1000, CB-1000, and N-

CB-1000; (a) survey scan, (b) N1s, and (c) deconvoluted C1s.

Figure S9. LSV curves of CB-750-1000, N-CB-1000, and CB-1000 at various rotating

speeds.

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