advances.sciencemag.org/cgi/content/full/2/4/e1501122/DC1

Supplementary Materials for

Identification of catalytic sites for oxygen reduction and oxygen

evolution in N-doped graphene materials: Development of highly

efficient metal-free bifunctional electrocatalyst

Hong Bin Yang, Jianwei Miao, Sung-Fu Hung, Jiazang Chen, Hua Bing Tao, Xizu Wang, Liping Zhang,

Rong Chen, Jiajian Gao, Hao Ming Chen, Liming Dai, Bin Liu

Published 22 April 2016, Sci. Adv. 2, e1501122 (2016)

DOI: 10.1126/sciadv.1501122

This PDF file includes:

Experimental Section

fig. S1. Molecular structures of precursors.

fig. S2. Temperature and time profile of pyrolysis and carbonization process.

fig. S3. FESEM images of the sample after pyrolysis at different temperatures.

fig. S4. Low-magnification FESEM image of the N-GRW.

fig. S5. FESEM images of samples prepared with different melamine–to–L-

cysteine ratios.

fig. S6. XPS spectra of C3N4 and S-doped C3N4.

fig. S7. TEM and FESEM images of C3N4 and S-doped C3N4.

fig. S8. Nitrogen adsorption isotherms of C3N4 and S-doped C3N4.

fig. S9. TGA and heat flow curves of S-doped C3N4.

fig. S10. XPS spectra of S-doped C3N4 and S-doped C3N4 carbonized at 800°C.

fig. S11. Nitrogen adsorption isotherms of S-doped C3N4 and S-doped C3N4

carbonized at 800°C.

fig. S12. S2p core XPS spectra of S-doped C3N4 and S-doped C3N4 carbonized at

800° to 1000°C.

fig. S13. FESEM and TEM images of N-HGS and N-GS.

fig. S14. Nitrogen adsorption isotherms and pore size distribution of N-doped

graphene samples.

fig. S15. C1s, N1s, and O1s core level high-resolution XPS spectra of the N-

GRW, N-HGS, and N-GS.

fig. S16. XRD patterns and Raman spectra of the N-GRW, N-HGS, and N-GS.

fig. S17. LSV of N-doped graphene samples at different rotation speeds in the

ORR region.

fig. S18. Linear sweeping voltammograms and Koutecky-Levich plots of different

catalysts in the ORR region.

fig. S19. Cyclic voltammograms of N-doped graphene catalysts and Pt/C (20%) in

Ar- and O2-saturated 1 M KOH.

fig. S20. Peroxide yield and electron transfer number of N-doped graphene

catalysts.

fig. S21. Tafel plots of N-doped graphene catalysts in ORR region.

fig. S22. Cyclic voltammograms of Pt/C and the N-GRW electrode in O2-

saturated 0.1 M KOH filled with methanol.

fig. S23. Current-time response of Pt/C and N-GRW for ORR with/without

introducing CO into the electrolyte.

fig. S24. Cycling durability of Pt/C and N-GRW.

fig. S25. Linear sweeping voltammograms and Tafel plots of N-doped graphene

catalysts in the OER region.

fig. S26. RRDE measurements for the detection of H2O2 and O2 generated during

the OER process.

fig. S27. OER Tafel plots of N-doped graphene catalysts.

fig. S28. Cyclic voltammograms of the N-GRW in the OER region at different

scan rates.

fig. S29. Faraday efficiency of OER measurement.

fig. S30. Mott-Schottky plots of N-doped graphene catalysts in two potential

regions.

fig. S31. XPS valence band spectra of the N-GRW.

fig. S32. Nyquist plots of N-doped graphene samples in the ORR and OER

regions.

fig. S33. The assembly processes for preparation of hybrid air cathode.

fig. S34. The assembly processes for the fabrication of a rechargeable zinc-air

battery.

fig. S35. ORR and OER performances of air cathode tested in half-cell

configuration.

fig. S36. Charge/discharge profiles and power density curves of zinc-air batteries

assembled from mixed Pt/C + Ir/C air electrode.

fig. S37. Open-circuit voltage profiles of zinc-air batteries.

fig. S38. Charging/discharging cycling curves of the N-GRW–loaded electrode at

charging/discharging current densities of 2 mA cm−2.

fig. S39. Charging/discharging cycling curves of Pt/C and Ir/C electrodes at

charging/discharging current densities of 20 mA cm−2.

fig. S40. XRD patterns of materials detached from Ti foil after long time

charging.

note S1. XPS study of C3N4 and S-doped C3N4 samples.

note S2. Deconvolution of C1s, N1s, and O1s XPS spectra of N-doped graphene

samples.

note S3. Average crystallite size of the sp2 domains of N-doped graphene samples

from Raman spectra.

note S4. Brief explanation of ac impedance of N-doped graphene samples.

table S1. Structural and compositional parameters of nitrogen-doped graphene

catalysts.

table S2. Surface nitrogen and oxygen species concentrations of nitrogen-doped

graphene catalysts.

table S3. Comparison of ORR and OER performances of our N-doped graphene

nanoribbon networks with the recently reported highly active bifunctional

catalysts.

Legends for videos S1 to S3

Other Supplementary Material for this manuscript includes the following:

(available at advances.sciencemag.org/cgi/content/full/2/4/e1501122/DC1)

video S1 (.mp4 format). The evolution of O2 bubbles at potentials from 1.6 to 1.8

V versus RHE.

videos S2 and S3 (.mp4 format). The demonstration of water splitting driven by a

single zinc-air battery.

Experimental Section Materials

All chemicals: melamine (99%), L-cysteine (98%), L-serine (99%), L-alanine (99%),

potassium hydroxide (99%), zinc chloride (≥ 98%) and zinc foil (thickness: 0.20 mm,

purity: 99.9%) were purchased from Sigma-Aldrich and used without further

purification. Commercial noble metal catalysts Ir/C (20% Ir on Vulcan XC-72) and Pt/C

(20% Pt on Vulcan XC-72) were purchased from Premetek.

Synthesis of nitrogen-doped graphene networks

In a typical synthesis of nitrogen-doped graphene nanoribbons (N-GRW) with

interconnected three dimensional (3D) network architecture, a mixture of melamine

and L-cysteine with a mass ratio of 4 : 1 was firstly ground into a homogeneous

precursor in a ZrO2 mortar. Subsequently, the fine powder mixture was undergone a

pyrolysis and carbonization process in a tubular furnace (Carbolite, UK) under argon

atmosphere. Detailed temperature and time profiles for the pyrolysis and

carbonization processes are shown in Fig. 2. After cooling down to room

temperature under argon atmosphere, the final product was collected for

subsequent characterization. For the synthesis of holey nitrogen-doped graphene

sheets (N-HGSs) and nitrogen-doped graphene sheets (N-GS), the L-cysteine

precursor was replaced by L-alanine and L-serine while keeping the same molar ratio

to melamine as in the synthesis of the N-GRW. Control experiments with different

ratios of melamine to L-cysteine were conducted under the same experimental

conditions for investigating the role of L-cysteine on the structural evolution of the

N-GRW.

Characterization

Crystal structure and morphology of the N-doped graphene based catalysts were

examined by X-ray diffraction (XRD, Bruker AXS D8 Advance) with Cu Kα radiation (λ

= 1.5406 Å), field-emission scanning electron microscope (FESEM, JEOL JSM-6700F),

and transmission electron microscope (TEM, JEOL JEM-3000F). The thickness of the

samples was measured by atomic force microscopy (Nanoman, Veeco, Santa Barbara,

CA) using tapping mode. Raman spectroscopy was performed in backscattering

mode on a Renishaw inVia Raman microscope using a 514.5 nm laser. The laser

power at the exit was 5 mW and the beam was focused on the sample using a 50X

objective len. Spectra recorded in the range of 600–2000 cm−1 were analysed by

fitting with four Lorentzian peaks. Detailed chemical compositions of the samples

were analyzed by X-ray photoelectron spectroscopy (XPS) on an ESCALAB 250

photoelectron spectrometer (Thermo Fisher Scientific) using a monochromatic Al Kα

X-ray beam (1486.6 eV). All binding energies were referenced to the C 1s peak (284.6

eV). Work function of the N-doped graphene samples were examined with

ultraviolet photoelectron spectroscopy (UPS, VG ESCALAB Mk II) using He I (21.2 eV)

resonance line. Surface area was measured based on BET method using Autosorb 6B

at liquid-nitrogen temperature.

Electrochemical measurements

All electrochemical measurements were carried out on a rotating electrode system

(pine Inc.) with a CHI 760e bipotentiostat. A three-electrode cell configuration was

employed with a working electrode of glassy carbon rotating disk electrode (RDE) 5

mm in diameter or rotating ring-disk electrode (RRDE) (disk outer diameter, ring

inner diameter, ring outer diameter are 5 mm, 6.5 mm and 7.5 mm, respectively), a

counter electrode of platinum foil, and a reference electrode of Ag/AgCl in 3 M KCl.

All potentials were calculated with respect to reversible hydrogen electrode (RHE)

scale according to the Nernst equation (ERHE = EAg/AgCl + 0.059 × pH + 0.197 V, at 25 °C).

The electrolyte used was 0.1 and 1 M KOH aqueous solution. The linear sweep

voltammograms (LSVs) were collected at a scan rate of 5 mV s-1. The Tafel slope was

calculated based on Tafel equation (ƞ=b*log(j/jo)), where ƞ is the overpotential, b is

the Tafel slope, j is the current density, and jo is the exchange current density. The

onset potentials were determined based on the starting of the linear region in Tafel

plots. All data were corrected for an ohmic drop (~6 Ω and ~22 Ω in 1 and 0.1 M KOH,

respectively). Electrochemical impedance spectra were recorded at DC potential of -

0.86 V (for ORR), 1.6 V (for OER) vs. RHE, and an AC potential frequency range of

100000 – 1 Hz with an amplitude of 10 mV in 1 M KOH. Mott-Schottky analysis was

carried out at a DC potential range of 0 – 1.2 V vs. RHE (scan from 1.2 to 0 V, anodic

scan) and 1.0 – 2.0 V vs. RHE (scan from 1.0 to 2.0 V, cathodic scan) with an AC

potential frequency of 10 kHz in 0.1 M KOH. The amplitude of AC potential was 10

mV.

To prepare catalyst ink for the ORR and OER testing, 5 mg of catalyst and 25 μL of 5%

Nafion 117 solution (DuPont) were introduced into 975 μL of water-isopropanol

solution with equal volume of water and isopropanol and sonicated for 3 hours.

Same recipe was used to prepare the Pt/C (20 %) and Ir/C (20 %) catalysts ink. For

the ORR test, an aliquot of 24 μL of the catalyst ink was applied onto a glassy carbon

RDE or RRDE and allowed to dry in air, giving a catalyst loading of 0.6 mg cm-2. The

loading of Pt/C (20 %) on glassy carbon RDE or RRDE is 150 μg/cm2 (6 μL). An O2-

saturated electrolyte was prepared by purging O2 (99.999%) into the electrolyte

solution for 30 minutes, and a flow of O2 was maintained over the electrolyte during

electrochemical measurements. For comparison, CV measurements were also

performed in an Ar (99.99%)-saturated electrolyte. For RDE measurements, the

working electrode was scanned at a rate of 5 mV s-1 at various rotation speed (400,

625, 900, 1225, 1600, and 2025 rpm) (Pine Instrument Co.). For accelerated

degradation test (ADT), the working electrode was swept at a scan rate of 50 mV s-1.

The Koutecky-Levich equation was used to analyze the number of electrons

transferred

1

J=

1

𝐽𝑘𝑖𝑛+

1

𝐽𝑑𝑖𝑓𝑓=

1

𝐽𝑘𝑖𝑛+

1

B ∗ √𝜔 , 𝐵 = 0.62𝑛𝐹𝐷2 3⁄ ∙ 𝜐−1 6⁄ ∙ 𝐶 (1)

where J, Jdiff, Jkin are the measured current density, the diffusion-limiting current

density, and the kinetic-limiting current density, respectively; ω is the rotation speed

in rpm, F is the Faraday constant (96485 C mol−1), D is the diffusion coefficient of

oxygen in 0.1 M KOH (1.9 × 10−5 cm2 s−1), υ is the kinetic viscosity (0.01 cm2 s−1),

and C0 is the bulk concentration of oxygen (1.2 × 10−6 mol cm−3). Based on Equation 1,

the number of electrons transferred (n) and JKin can be obtained from the slope and

intercept of the Koutecky–Levich plots, respectively. Total electron-transfer number

(n) and hydrogen peroxide yield (%H2O2) were determined by RRDE approach using

n =4𝐼𝑑𝑖𝑠𝑘

𝐼𝑟𝑖𝑛𝑔 𝑁⁄ + 𝐼𝑑𝑖𝑠𝑘 (2)

%𝐻2𝑂2 =2𝐼𝑟𝑖𝑛𝑔/𝑁

𝐼𝑟𝑖𝑛𝑔 𝑁⁄ + 𝐼𝑑𝑖𝑠𝑘× 100% (3)

where Idisk and Iring are the voltammetric currents at the disk and ring electrode,

respectively. N is the RRDE collection efficiency, which was determined to be 0.26.

For the OER test of N doped graphene materials, a catalyst loading of 0.3 mg cm-2

was applied onto on glassy carbon RDE or RRDE. The loading of Ir/C (20 %) is 150

μg/cm2 (6 μL). RRDE measurements were conducted at 25 °C in an oxygen-saturated

KOH solution at a scan rate of 5 mV s−1 and a rotation speed of 1600 rpm. (Pine

Instrument Co.). The catalyst loaded carbon cloth with 0.25 mg cm-2 was used as the

working electrode for studying the OER stability. The carbon cloth was pre-treated in

a mixed solution of sulfuric acid and hydrogen peroxide (3 : 1 by volume) for 2 hours

at room temperature to remove organic contaminants and hydroxylate the surface

before loading the catalyst.

Faraday efficiency was estimated using volumetric method. The evolved oxygen gas

on catalyst loaded carbon cloth electrode (2.7 cm2) was collected in a 50 ml

graduated tube, which was filled with the electrolyte. Current controlled electrolysis

was carried out at both mild and large currents (5 and 25 mA cm-2) for about 5 and

2.5 h under ambient conditions (25°C, 1 atm). The time at each 1 ml of oxygen gas

for mild current density and 5 ml for large current density of collected gas were

recorded. Meanwhile, the accumulated charges passing through the working

electrode were calculated by current x time. To determine the composition and

purity of the product, the collected gas was sampled with a Hamilton syringe and

then analyzed using gas chromatograph (Agilent GC 490). From the GC spectra, O2

was the only detected gaseous product throughout the test without any other

impurities.

The calculation of turnover frequency (TOF) (s-1) for ORR and OER:

To calculate the TOF for ORR and OER, the following formula was used

TOF (𝑠−1) =number of oxygen turnover 𝑐𝑚2geometric area ⁄

the active sites 𝑐𝑚2geometric area ⁄ (4)

TOF (s-1) = (number of oxygen turnover)/the active sites = (J/4F)/n, J is the current

density for ORR or OER at a given overpotential, n is the number of active sites. F is

the faraday constant (96485.3 s A mol-1). (J/4F) represents the total oxygen turnover

in ORR or OER.

Catalyst loading on glassy carbon RRDE of N-GRW for ORR and OER are 0.6 mg/cm2

and 0.3 mg/ cm2, respectively.

Current density for ORR (at 0.8 V vs. RHE) and OER (overpotential 360 mV) from LSV

polarized curves in 1 M KOH are 3.2 mA/cm2 and 10 mA/cm2, respectively.

The number of oxygen turnover for ORR and OER were calculated from the current

density according to

number of oxygen turnover

= (j𝑚𝐴

𝑐𝑚2) (

1𝐶 𝑠⁄

1000𝑚𝐴) (

1 𝑚𝑜𝑙 𝑒−

96485.3C ) (

1 𝑚𝑜𝑙 𝑂2

4 𝑚𝑜𝑙 𝑒−) ∗ 6.02 ∗ 1023

For ORR: (3.2𝑚𝐴

𝑐𝑚2) (1𝐶 𝑠⁄

1000𝑚𝐴) (

1 𝑚𝑜𝑙 𝑒−

96485.3C ) (

1 𝑚𝑜𝑙 𝑂2

4 𝑚𝑜𝑙 𝑒−) ∗ 6.02 ∗ 1023 = 5.00 ∗ 1015 (1 𝑠⁄

𝑐𝑚2)

For OER: (10𝑚𝐴

𝑐𝑚2) (1𝐶 𝑠⁄

1000𝑚𝐴) (

1 𝑚𝑜𝑙 𝑒−

96485.3C ) (

1 𝑚𝑜𝑙 𝑂2

4 𝑚𝑜𝑙 𝑒−) ∗ 6.02 ∗ 1023 = 1.56 ∗ 1016 (1 𝑠⁄

𝑐𝑚2)

Compositional parameters of N-GRW:

In atomic ratio: 89.3 (C-at%), 5.9 (N-at%) and 4.7 (O-at%)

In weight ratio: 87.4 (C-wt%), 6.7 (N-wt%) and 6.2 (O-wt%).

For ORR: the active sites are the carbon atoms adjacent to the quaternary-N (1.65

wt%)

For OER: the active sites are the carbon atoms near the pyridinic-N (3.18 wt %) and

ketonic (C=O) group (0.63 wt%):

The active site density for OER and ORR are:

For ORR: {number of quaternary-N} x 3

= {(0.6 mg/cm2)*0.00165*(mmol/14mg)*6.02*1020} x 3 = 6.39*1016 sites cm-2

For ORR: {number of pyridinic-N + ketonic} x 2 = {(0.3 mg/cm2)*[0.00318*(mmol/14mg) + 0.00063*(mmol/16mg)] *6.02*1020} x 2 = 4.8*1016 sites cm-2

TOF(s-1) for ORR: 5.00∗1015(

1 𝑠⁄

𝑐𝑚2)

6.39∗1016(𝑠𝑖𝑡𝑒𝑠

𝑐𝑚2 )= 0.08 𝑠−1

TOF(s-1) for OER: 1.56∗1016(

1 𝑠⁄

𝑐𝑚2)

4.8∗1016(𝑠𝑖𝑡𝑒𝑠

𝑐𝑚2 )= 0.33 𝑠−1

X-ray absorption near edge structure (XANES)

The K-edge X-ray absorption spectra of C and N were measured in total electron

yield mode at room temperature using BL-20A at National Synchrotron Radiation

Research Center (NSRRC, Hsinchu, Taiwan) in which the electron storage ring was

operated at 1.5 GeV with a beam current of 300 mA. The ORR/OER bifunctional

catalyst (N-GRW) was supported on microporous support layer (nickel foam) under

10 Ton pressure. Before XANES measurements, the samples were reacted in an O2-

saturated 1 M KOH for one hour at 0.55 V and 1.65 V vs. RHE for ORR and OER,

respectively. After ORR and OER, samples were soaked dry in vacuum, and then

subjected to an ultrahigh vacuum chamber (1 x 10-9 torr) for the total electron yield

X-ray absorption spectra (TEY-XAS) collection. For C and N K-edge absorption, the

data was collected at the 6 m high-energy spherical grating monochromator (HSGM)

beamline with 10 × 10 μm opening slits, corresponding to ~0.08 eV energy resolution.

Simultaneously, the signal from a gold grid located upstream in the X-ray path was

recorded, and the spectra were normalized using the incident beam intensity, I0,

keeping the area under the spectra fixed over an energy range of 280 to 302 eV for C

K-edge and 395 to 416 eV for N K-edge.

Rechargeable zinc-air battery

Rechargeable zinc-air battery in two-electrode configuration was assembled

according to the following procedure: first, the air electrode was made by dipping a

pre-treated carbon cloth substrate (1 × 2 cm2, vide supra) into a bottle filled with 5

ml catalyst ink (2 mg of catalysts and 50 μL 5% Nafion solution were dispersed in 950

μL of a 1:1 (v/v) water/isopropanol), and being shaken gently for 1 h, followed by

drying in air. This process was repeated once to reach a catalyst loading of about 0.5

mg cm-2. Subsequently, the catalyst loaded carbon cloth was attached to a gas

diffusion layer (GDL, AvCarb P75T, Fuel Cell Store) to form a carbon cloth/GDL hybrid

electrode for half-cell testing and rechargeable zinc-air battery assembly. The

detailed procedure for the preparation of carbon cloth/GDL hybrid electrode and

assembly of rechargeable Zn-air battery in two-electrode configuration were shown

in figs. S33 and S34. The electrochemical evaluations of catalyst (ORR and OER) in

half cell were performed in a three-electrode configuration using an Hg/HgO

electrode (filled with 6 M KOH) as the reference, a graphite rod as the counter

electrode and a carbon cloth/GDL hybrid electrode as the working electrode (fig.

S35a). All zinc-air batteries were tested in air at room temperature. The electrolyte

used was 6 M KOH filled with 0.2 M ZnCl2 (dissolved in KOH to form zincate,

Zn(OH)42−) to ensure reversible zinc electrochemical reactions at the anode. The

assembled zinc-air batteries were cycled at both low (2 mA cm-2) and high (20 mA

cm-2) charging/discharging current densities.

Supplementary Figures

fig. S1. Molecular structure of three different precursors used to synthesize N-doped graphene catalysts.

fig. S2. Temperature and time profile of pyrolysis and carbonization process.

0 200 400 600 8000

400

800

1200

2 oC/min

2h@1000 oC

Te

mp

era

ture

(oC

)

Time (mins)

2h@600 oC

2.5 oC/min

In Argon

fig. S3. FESEM images of the samples prepared from a mixture of melamine and L-cysteine with a

mass ratio of 4 : 1 after pyrolysis and carbonization process at (a) 700 oC, (b & c) 800 oC for 2 hours.

Scale bar is 1 µm in all images.

fig. S4. Low magnification FESEM image of the N-GRW (pyrolyzed and carbonized at 1000 oC for 2 hours). Scale bar is 1 µm.

fig. S5. FESEM images of samples prepared with different melamine to L-cysteine ratios: (a & b) 19 : 1

and (c & d) 7 : 3. All images are obtained from samples pyrolyzed and carbonized at 1000 oC for 2

hours. Scale bar is 1 µm in all images.

fig. S6. XPS spectra of C3N4 and S-doped C3N4 samples: (a) XPS survey spectra, high resolution spectra of (b) C 1s, (c) N 1s, and (d) S2p, (e) schematic diagram of structures of C3N4 and S-doped C3N4.

note S1. The C1s spectrum of C3N4 sample can be deconvoluted into three peaks

located at 284.6 eV (C-C), 285.8 eV (C=N), and 288.0 eV (N-C=N). The peak at 398.4

eV in N1s is assigned to the aromatic N bonded to two C (C=N-C) in the triazine or

heptazine rings. The peak at 399.4 eV is ascribed to the sp2 hybridized N bonded to

three atoms (C–N(–C)–C or C–N(–H)–C), and the peak at 400.9 eV is attributed to the

sp3 hybridized terminal N (N–H) of the heptazine rings. The peak at 163.5 eV in S2p is

due to the formation of C-S-C. The percentage of C-N=C and N-C=N reduced while

that of C-S-C increased with the incorporation of L-cysteine during the synthesis,

indicating that the formation of C-S-C could impede the extension of carbon nitride

plane by hindering the formation of the lateral C-N bonds.

fig. S7. TEM and FESEM images of (a) & (c) melamine (C3N4) and (b) & (d) mixture of melamine and L-

cysteine (S-doped C3N4) with a mass ratio of 4 : 1 pyrolyzed at 600 oC for 2 hours. Scale bar is 1 µm in

(c) and (d).

fig. S8. Nitrogen adsorption isotherms of C3N4 and S-doped C3N4 measured at 77 K. BET surface area of S-doped C3N4 is 78 m2 g-1, larger than that of C3N4 (31 m2 g-1).

fig. S9. TGA and heat flow curves of S-doped C3N4 measured from 25 to 900 °C in nitrogen

atmosphere at a heating rate of 10°C min-1.

200 400 600 800 10000

20

40

60

80

100

Temperature (oC)

We

ight

(%)

-5

0

5

10

15

He

at flo

w (m

W)

fig. S10. XPS spectra of the samples prepared from melamine and L-cysteine at a mass ratio of 4 : 1

pyrolyzed and carbonized at 600 oC (S-doped C3N4) and 800 oC for 2 hours.

fig. S11. (a) Nitrogen adsorption isotherms of the samples prepared from melamine and L-cysteine at

a mass ratio of 4 : 1 pyrolyzed and carbonized at 600 oC (S-doped C3N4) and 800 oC for 2 hours, and (b)

the corresponding pore size distribution calculated using the BJH method. The specific surface area

increases from 78 to 480 m2 g-1 after pyrolyzation at 800 oC.

fig. S12. S2p core level high-resolution XPS spectra of the sample prepared from melamine and L-

cysteine with a mass ratio of 4 : 1 pyrolyzed and carbonized at 600 oC to 1000 oC for 2 hours. The

atomic ratio of S is 0.7 0.61, 0.22 and 0.10 at % for samples pyrolyzed and carbonized at 600 oC, 800 oC, 900 oC and 1000 oC.

From the XPS spectra of C3N4 and S-doped C3N4 samples (fig. S6), it could be seen that the formation

of C-S-C could impede the extension of carbon nitride plane by hindering the formation of the lateral

C-N bonds. Because of the low bonding energy of C-S (272 kJ mol-1), the C-S bond would break at the

high temperature stage, leading to dramatic reduction in the S concentration from 0.7 % to 0.1 %. In

the whole construction process of N-GRW, most of the S finally would be released, which worked just

as an intermediate for the formation of N-GRW.

172 170 168 166 164 162 160

Inte

nsity (

a.u

.) 1000 oC

900 oC

800 oC

Binding energy (eV)

600 oC

S2p

C-S-CC-SO3-C

C-SO2-C

fig. S13. FESEM and TEM images of N-HGS (a, b and c) and N-GS (d, e and f). Scale bar: 1 µm in (a) and (d), Scale bar: 100 nm in (b) and (e). Substitution of L-cysteine with amino acids bearing other functional groups, e.g., methyl (L-alanine) and hydroxyl (L-Serine) groups during the synthesis would lead to the formation of holey N-doped graphene sheets (N-HGS) with sizes less than 500 nm and N-doped graphene sheets (N-GS) up to a few micrometers.

fig. S14. (a) Nitrogen adsorption isotherms of three N-doped graphene samples measured at 77 K and (b) the pore size distribution calculated using the BJH method. The specific surface area of the N-GRW, N-HGS and N-GS are 530, 480 and 460 m2 g-1, respectively.

fig. S15. C1s, N1s and O1s core level high-resolution XPS spectra of the N-GRW (a, b and c), N-HGS (d, e and f) and N-GS (g, h and i).

note S2. For the deconvolution of C1s, N1s and O1s spectra, the FWHM and energy

of the peak were constrained for the same element state in all samples. Gaussian

Lorentzian shape GL(30) curves were used for curve fitting. The C1s spectra were

deconvoluted into six peaks: sp2 C-C (284.0 eV), sp3 C-C (284.6 eV), C=N, C-O (285.3

eV), C=O, C-N (286.2 eV), -COO (288.7 eV), and π-π* (291 eV). The N1s spectra were

deconvoluted into four peaks (pyridinic-N, 397.8 eV; quaternary-N, 400.8 eV;

pyrrolic-N, 398.9 eV and oxidized N, 402.0 eV). Among them, the pyridinic-N and

quaternary-N are the dominant species in nitrogen-doped graphene samples. The

O1s spectra were deconvoluted into three peaks C=O (530.3 eV), C-O (531.8 eV) and

C-OH (533.4 eV).

fig. S16. (a) X-ray diffraction patterns and (b) Raman spectra of the N-GRWs N-HGS, and N-GS.

note S3. Raman spectra in the range of 600–2000 cm−1 of three N-doped graphene

were analysed by fitting with four Lorentzian peaks: graphitic (G), disorder (D),

amorphous (Am), and disorder sp3 (P peaks). In general, G band is the characteristic

feature of graphitic layers, which corresponds to the in-plane vibration of sp2 chains

associated with the E2g symmetry (J. Chem. Phys. 53, 1126-1130 (1970), Carbon 33,

1561-1565 (1995), Solid State Commun. 143, 47-57 (2007)). The D band displays a

doublet, which stems from: (i) imperfect sp2 chains and superimposed stretching

mode of sp3, and (ii) weakly coupled vibration modes of polycyclic carbon rings,

corresponding to disordered carbon or defective graphitic structures (Solid State

Commun. 143, 47-57 (2007)). The ratio of ID/IG is calculated to be 3.34, 2.34 and 2.10,

respectively for the N-GRW, N-HGS and N-GS, which is the characteristic value for

defective graphite structures, due to the low content of sp2 graphite carbon as well

as the presence of substantial vacancies or defects. Larger ID/IG ratio usually is

associated with more disordered carbon structure. ID/IG is inversely proportional to

the in-plane coherence length (La), which is the mean average crystallite size of the

sp2 domains in the nanographite system, calculated from La = C(λ)*(ID/IG)-1, C(λ) =

43.5 Å for 514 nm (J. Chem. Phys. 53, 1126-1130 (1970), Phys. Rev. B 59, R6585-

R6588 (1999), Carbon 53, 130-136 (2013)),. The calculated La are 1.3, 1.8 and 2.1 nm

for the N-GRW, N-HGS and N-GS, respectively, showing that the size of crystalline

domain in the N-GRW is distinctly smaller than that of N-HGS and N-GS, which is

consistent with the results obtained from the SEM and TEM images (fig. S13).

Smaller size of the N-GRW suggests that the N-GRW possesses more edge sites,

confirmed by lower sp2/sp3 ratio from high resolution C1s spectra (0.36, 0.58, 0.68

for the N-GRW, N-HGS and N-GS, respectively, (fig. S15).

fig. S17. LSV on rotating disk electrode in O2-saturated 0.1 M KOH at different rotation speed from 400 to 2025 rpm at a scan rate of 5 mV s−1 in ORR region. (a) N-GRW, (b) N-HGS, (c) N-GS, and (d) Pt/C.

fig. S18. (a) Linear sweeping voltammograms on rotating disk electrode in O2-saturated 0.1 M KOH at

a rotation speed of 1600 rpm and a scan rate of 5 mV s−1, (b) Koutecky–Levich plots of different

catalysts at 0.7 V vs. RHE. The Koutecky–Levich plots at different electrode potentials display good

linearity, and the electron transfer number (n) of N-GRW and N-HGS are calculated to be close to 4,

suggesting direct reduction of oxygen to OH-. Graphene sheets (GS) were prepared by thermal

reduction of graphene oxide at 1000 oC in Argon for 120 min.

fig. S19. Cyclic voltammograms of N-doped graphene catalysts and Pt/C (20%) on rotating disk electrode in Ar (dotted lines) and O2 (solid lines) saturated 1 M KOH, scan rate: 50 mV/s. N-GRW showed much more positive ORR onset potential and higher cathodic current than N-GS. The ORR activity trends are in the order of N-GRW > N-HGS > N-GS, consistent with the results from LSV scan.

fig. S20. Peroxide yield and electron transfer number (n) of N-doped graphene and commercial Pt/C catalysts at various potentials based on the corresponding RRDE data as shown in Fig. 3a. Catalyst loading amount: 0.6 mg cm-2 for N-doped graphene catalyst and 0.15 mg cm-2 for commercial Pt/C catalyst.

fig. S21. Tafel plots of N-doped graphene catalysts in ORR region on rotating disk electrode in O2-saturated 1 M KOH electrolyte at rotation speed of 1600 rpm.

fig. S22. Cyclic voltammograms on rotating disk electrode (RDE) for Pt/C electrode (catalyst loading

amount: 60 g cm-2) (top) and the N-GRW electrode (catalyst loading amount: 0.3 mg cm-2) (bottom) in O2-saturated 0.1 M KOH filled with 3 M methanol.

fig. S23. Chronoamperometric (current-time) responses of Pt/C and N-GRW for ORR with/without

introducing CO into the electrolyte, at 0.7 V vs. RHE in O2-saturated 0.1 M KOH, at a rotation speed

of 900 rpm.

fig. S24. Cyclic voltammograms of Pt/C and N-GRW in O2 saturated 1 M KOH in the cycling durability test. Pt/C experienced a dramatic loss (35 %) in electrochemical active surface area (ECSA) after 2000 consecutive cycles; on the other hand, no loss of ECSA in N-GRW was noticed.

fig. S25. (a) Linear sweeping voltammograms (LSVs) of different samples. Graphene sheets (GS) were prepared by thermal reduction of graphene oxide at 1000 oC in Argon for 120 min. (b) Tafel plots, of N-doped graphene catalysts in OER region on rotating disk electrode in O2-saturated 0.1 M KOH at a scan rate of 5 mV s-1 and rotation speed of 1600 rpm. The OER activity of N-doped graphene catalysts showed the same trend as measured in 1 M KOH in the order of N-GRW > N-HGS > N-GS.

fig. S26. (a) Photograph of N-GRW loaded rotating disk electrode after LSV scan in the OER region. (b) Rotating ring disk electrode measurements for the detection of H2O2 and O2 generated from N-GRW catalyst during the OER process in 1 M KOH electrolyte. The ring electrode potential was controlled at 1.50 V vs. RHE for monitoring H2O2 generation and the disc electrode potential was fixed at -0.6 V vs. RHE for monitoring O2 generation during OER scan (rotation speed: 1600 rpm; catalyst loading amount: 0.05 mg/cm2). Very low Pt ring current (purple line) at 1.5 V vs. RHE indicates negligible HO2

- formation and 4e OER pathway for N-GRW in alkaline medium. The ORR onset potential of Pt ring is

1.5 V vs. RHE, the same as the OER onset potential for N-GRW.

fig. S27. OER Tafel plots of N-doped graphene catalysts on rotating disk electrode in O2-saturated 1 M KOH at a rotation speed of 1600 rpm. The scan rate is 1 mV s-1.

fig. S28. Cyclic voltammograms of the N-GRW loaded carbon cloth for OER at scan rates from 1 - 20 mV s-1. It can be concluded that the contribution from double layer capacitance of electrode towards the OER current is negligible.

fig. S29. Comparison of evolved oxygen vs. the amount of consumed electrons during the course of

electrolysis. The slopes are 0.248 and 0.249 for the data collected under mild and large currents,

indicating ~98% faraday efficiency.

fig. S30. Mott-Schottky plots at frequency of 1000 Hz. Catalyst loading: 0.25 mg cm-2 on glassy carbon electrode; 0.1 M Ar saturated KOH was used as the electrolyte. The plots of CscL

−2 vs. E/V show positive (a) and negative (b) slopes for the N-GRW, N-HGS and N-HGS in two potential regions, confirming n and p types doping by N, indicating bipolar characteristics of our N-doped graphene samples.

fig. S31. XPS valence band spectra of the N-GRW.

14 12 10 8 6 4 2 0

Inte

nsity (

a.u

.)

Binding Energy (eV)

2p

2s

hyb 2p

2p

N lone pair

Ef

C-N

fig. S32. Nyquist plots of N-doped graphene samples on the rotating disk electrode at (a) 0.86 V vs. RHE for ORR and (b) 1.60 V vs. RHE for OER in 1 M KOH.

note S4. Smallest semicircle of N-GRW as compared to N-HGS and N-GS in the

Nyquist plots obtained in ORR and OER regions, which are regarded as an activation

loss in electrochemical reactions, indicates the best ORR and OER performance for N-

GRW. The semicircle diameter increases in the order of N-GRW < N-HGS < N-GS for

ORR and N-GRW < N-GS < N-HGS for OER, indicating ORR activity increases in the

order of N-GS < N-HGS < N-GRW and OER activity increases in the order of N-HGS <

N-GS < N-GRW, consistent with the LSV results.

fig. S33. Photographs showing the assembly processes to load catalyst onto carbon cloth/gas diffusion layer for the preparation of hybrid air cathode. The diameter of hybrid air cathode is 10 mm.

fig. S34. Photographs of the assembly processes for the fabrication of a rechargeable zinc-air battery. A cellulose acetate membrane soaked with aqueous 1M KOH electrolyte as the separator.

fig. S35. ORR and OER performances of air cathode tested in half-cell configuration. (a) Photograph of the electrochemical cell for the measurement. (b) ORR and OER polarization curves. (c) Galvanostatic pulse cycling of air cathode. Catalyst loaded carbon cloth/GDL hybrid electrode (catalyst loading amount: 0.5 mg cm-2) was applied as the working electrode in O2-saturated 6 M KOH, using graphite foil and Hg/HgO (filled with 6 M KOH) as counter and reference electrode, respectively. The larger ORR and OER current densities of the N-GRW electrode stem from a number of combinational effects of larger specific surface area of carbon cloth as compared to glassy carbon electrode and more efficient O2 diffusion assisted by the gas diffusion layer.

fig. S36. Galvanodynamic charge/discharge profiles and power density curves of zinc-air batteries assembled from mixed Pt/C + Ir/C (1 : 1 by weight) air-electrode, catalyst loading amount: 0.5 mg cm-2 for Ir/C + Pt/C.

fig. S37. Open circuit voltage (OCV) profiles of zinc-air batteries assembled from N-GRW, commercial Pt/C (20 %), Ir/C (20 %) and Pt/C (20 %)+ Ir/C (20 %): (catalysts loading are 0.25 mg cm-2 for Ir/C and Pt/C) as air cathode, Catalyst loading amount is 0.5 mg cm-2 for all batteries.

fig. S38. Charging/discharging cycling curves of the N-GRW loaded electrode at charging/discharging current densities of 2 mA cm-2. Battery was tested in pure O2 ambient and 6 M KOH + 0.2 M ZnCl2 electrolyte. Catalyst loading amount is 0.5 mg cm-2 for all batteries.

fig. S39. Charging/discharging cycling curves of Pt/C (black line) and Ir/C (red line) loaded electrodes

at charging/discharging current densities of 20 mA cm-2. All batteries were tested in air and 6 M KOH

+ 0.2 M ZnCl2 electrolyte. Catalyst loading amount is 0.5 mg cm-2 for all batteries.

0 5 10 15 20

0.5

1.0

1.5

2.0

2.5

3.0

Voltage (

V)

Time (hour)

Pt/C

Ir/C

fig. S40. (a) Photographs of Ti foil as metal cathode after being charged at 2 and 20 mA cm-2 for 1800 s (left image), showing the deposition of metallic zinc on Ti foil. After discharging, the deposited zinc on Ti foil could be completely consumed (right image). (b) XRD patterns of materials detached from Ti foil after long time charging at 20 mA cm-2.

Supplementary Tables

table S1. Structural and compositional parameters of nitrogen-doped graphene catalysts.

table S2. Surface nitrogen and oxygen species concentrations of nitrogen-doped graphene catalysts (obtained from deconvoluted high resolution N1s and O1s XPS spectra).

table S3. Comparison of ORR and OER performances of our N-doped graphene nanoribbon networks

with the recently reported highly active metal-free carbon and transition-metal oxide bifunctional

catalysts.

Supplementary Videos

video S1. This video shows the evolution of O2 bubbles at potentials from 1.6 to 1.8

V vs. RHE. No peeling of catalysts was observed, showing firm attachment of catalyst

on carbon cloth.

videos S2 and S3. Videos show the demonstration of water splitting driven by a

single zinc-air battery. The evolution of O2 and H2 gas bubbles are distinctly observed

on NiFe layer double hydroxide and Pt/C electrodes in 0.1 M KOH.