S1

Supplementary Information

Electrochemical reduction of N2 to ammonia on Co single atoms embedded

N-doped porous carbon under ambient conditions

Yanming Liu,a Qi Xu,a Xinfei Fan,b Xie Quan,*a Yan Su,c Shuo Chen,a Hongtao Yu,a Zhengjun Cai a

aKey Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education), School of

Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China.bCollege of Environmental Science and Engineering, Dalian Maritime University, Dalian 116026, China.cSchool of Physics and Opto-Electronic Technology, Dalian University of Technology, Dalian 116024,

China.

Corresponding author E-mail: quanxie@dlut.edu.cn

This file includes:Fig. S1. SEM of ZIF and TEM images of CSA/NPCs.Fig. S2. Structure characterization of CSA/NPCs.Fig. S3. Cyclic voltammograms and chronoamperometric curves.Fig. S4. Hydrazine detection.Fig. S5. 1H-NMR spectra for 14NH4

+ and 15NH4+ detection.

Fig. S6. NH3 production rates of CSA/NPC-700.Fig. S7. H2 efficiency of CSA/NPC-750.Fig. S8. SEM images before and after N2 reduction.Fig. S9. XPS spectra before and after N2 reduction.Fig. S10. Potentiostatic stability of CSA/NPC-750.Fig. S11.SEM and TEM images for NPC-750 and Co/NPC-750.Fig. S12. XPS spectra of CSA/NPCs.Fig. S13. N2 adsorption comparison.Fig. S14. Free energy for ammonia synthesis.Table S1. The EXAFS fitting results of CSA/NPC-750.Table S2. Comparison of NH3 synthesis performance between CSA/NPC-750 and electrocatalysts reported.Table S3. The N/C and Co/C ratios for CSA/NPC-750 before and after N2 reduction.Table S4. The metal and N contents of CSA/NPCs, NPC-750 and Co/NPC-750.

Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A.This journal is © The Royal Society of Chemistry 2019

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1. Supplementary Figures

5 nm

5 nm 5 nm

B

C D

500 nm

A

Fig. S1 (A) SEM image of Zn/Co bimetallic ZIF precursor, TEM images of (B) CSA/NPC-750, (C)

CSA/NPC-850 and (D) CSA/NPC-950.

0 5 10 15 20 250.0

0.1

0.2

0.3

CSA/NPC-750 CSA/NPC-850 CSA/NPC-950

dV/d

D (c

m3 g

-1 n

m-1)

Pore size (nm)20 40 60 80

C(002)

CSA/NPC-950

CSA/NPC-850

CSA/NPC-750

Inte

nsity

(a.u

.)

2 theta (deg.)

A B

Fig. S2 (A) X-ray diffraction spectra and (B) pore size distribution of CSA/NPC-750, CSA/NPC-850 and

CSA/NPC-950 (DFT method).

S3

-0.6 -0.4 -0.2 0.0 0.2 0.4-3

-2

-1

0

1

Ar N2

Curr

ent d

ensit

y (m

A cm

-2)

Potential (V vs RHE)0 1000 2000 3000 4000

-4

-3

-2

-1

0

0.0 V -0.1 V -0.2V -0.3V -0.4V

Curr

ent d

ensit

y (m

A cm

-2)

Time (s)

A B

Fig. S3 (A) Cyclic voltammograms of CSA/NPC-750 on glassy carbon in N2 or Ar saturated 0.05 M

Na2SO4 with scan rate of 20 mV s-1, (B) Chronoamperometric curves for electrochemical reduction of N2

on CSA/NPC-750 at 0.0~-0.4 V (vs RHE).

400 420 440 460 480 500

CSA/NPC-750, -0.2 V 0.05M Na2SO4

Abso

rban

ce

Wavelength (nm)

Fig. S4. UV-Vis curves of 0.05 M Na2SO4 and the sample for N2 reduction on CSA/NPC-750 (1 h, -0.2 V,

0.05 M Na2SO4) measured by p-dimethylaminobenzaldehyde method.

To find out whether hydrazine can be produced, electrochemical reduction of N2 was conducted on

CSA/NPC-750 at -0.2 V in 0.05 M Na2SO4 for 1 h and hydrazine concentration of the solution was

measured by p-dimethylaminobenzaldehyde spectrophotometric method. Details as follows: 2.0 g p-

dimethylaminobenzaldehyde, 100 mL ethanol and 10 mL HCl (12.0 mol/L) were mixed and used as color

reagent. 0.8 mL color reagent was added into a mixture of 2.0 mL solution (taken from electrochemical cell

S4

after N2 reduction for 1 h) and 2.0 mL HCl (0.24 mol/L). After mixing thoroughly, the resulting solution

sat for 20 min. Its absorbance was measured at 458 nm. The absorbance of 0.05 M Na2SO4 was used as

reference. As shown in Fig. S4, the absorbance of the sample obtained from N2 reduction on CSA/NPC-750

at -0.2 V is 0.0211, which is similar to that of 0.05 M Na2SO4 (0.0209). Therefore, hydrazine is not produced

during N2 reduction on CSA/NPC-750.

To confirm ammonia is produced from N2 reduction, 15N2 reduction has been performed on CSA/NPC-

750 at 0.05 M Na2SO4. Details as follows: Ar purge was undertaken for 30 min to remove 14N2 in the cell

and electrolyte, followed by 15N2 purge for 20 min. Electrochemical reduction of 15N2 was conducted on

CSA/NPC-750 at -0.2 V with 15N2 flow. After 15N2 reduction for 1 h, the sample was collected and adjusted

to pH 2 with HCl for ammonia quantification by H-NMR on Bruker 700 MHz spectrometer with DMSO

as internal standards. The sample was mixed with 0.2 mM DMSO (D2O was used as solvent) with ratio of

9:1. H-NMR measurement was performed for 2500 scans with water suppression. As reference, electrolysis

was also conducted under Ar on CSA/NPC-750 (-0.2 V, 1 h) and the solution was collected for 1H-NMR

measurement with the same method mentioned above. As reference, the 1H-NMR spectrum of 0.5 mM 14NH4Cl (pH 2) was measured by the same method with 250 scans. Calibration curve for 15NH4

+ was tested

using 15NH4Cl with concentrations of 0-300 uM. As shown in Fig. S5A, doublet at 6.93 ppm and 7.08 ppm

can be observed on the 1H-NMR spectrum for 15N2 reduction sample, while triplet around 6.91-7.09 ppm

appear on the 1H-NMR spectrum for 14NH4+

standard sample and no discernable peak can be observed for

electrolysis under Ar. The doublet (6.93 ppm and 7.08 ppm) for 15N2 reduction sample can be attributed to

the signal of 15NH4+,1 and the triplet (6.91-7.09 ppm) arises from 14NH4

+. The 15N labelling and Ar

electrolysis experiments indicate the detected ammonia in this work is produced from N2 reduction. To

confirm it, the 15NH4+ produced from 15N2 reduction is quantified with the calibration curve in Fig. S5B,

which shows the peak area ratio of 15NH4+/DMSO versus 15NH4

+ concentration. The obtained 15NH4+

production rate is 0.82 μmol cm-2 h-1, consistent with result of 0.86 μmol cm-2 h-1 from 14N2 reduction.

These results confirm the ammonia detected in this work is produced from N2 reduction.

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0 100 200 3000.0

0.1

0.2

0.3

0.4

S15 N

H+ 4/S

DM

SO

15NH4+ concentration (M)

y=0.00119x-0.00807

7.1 7.0 6.9

7.1 7.0 6.9

7.1 7.0 6.9

Under 15N2

14NH4+

Under Ar

H (ppm)

A B

Fig. S5 1H-NMR spectra for (A) 0.5 mM NH4+ standard (250 scans), 15N2 reduction and Ar electrolysis on

CSA/NPC-750 at -0.2 V (2500 scans), (B) calibration curve for 15NH4+: the peak area ratio of

15NH4+/DMSO versus 15NH4

+ concentration.

-0.1 -0.2 -0.30.00

0.05

0.10

0.15

0.20

0.25

NH

3 pro

duct

ion

rate

(m

ol c

m-2

h-1

)

Potential (V vs. RHE)

Fig. S6 The NH3 production rates of CSA/NPC-700 for electrochemical reduction of N2 at -0.1~-0.3 V (vs

RHE) and 0.05 M Na2SO4.

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-0.3 V -0.2 V0

20

40

60

80

100

Fara

daic

eff

icie

ncy

(%)

Potential (V vs. RHE)

Fig. S7 The H2 production efficiencies during electrochemical reduction of N2 on CSA/NPC-750 at -0.2 V

and -0.3 V.

Fig. S8 SEM images of CSA/NPC-750 electrode (A) before and (B) after electrochemical reduction of N2

at -0.2 V.

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280 282 284 286 288 290

After

In

tens

ity (a

.u.)

Binding energy (eV)

Before

770 780 790 800 810

Inte

nsity

(a.u

.)

Binding energy (eV)

After

Before

392 396 400 404

Inte

nsity

(a.u

.)Binding energy (eV)

After

Before

A B C

Fig. S9 (A) C 1s, (B) N 1s and (C) Co 2p XPS spectra of CSA/NPC-750 before and after electrochemical

reduction of N2 at -0.2 V.

0 5000 10000 15000 20000 25000-1.2

-0.9

-0.6

-0.3

0.0

Time (s)

Cur

rent

den

sity

(mA

cm

-2)

0

3

6

9

12

15

Far

adai

c ef

ficie

ncy

(%)

Fig. S10 Chronoamperometric curve and NH3 production efficiency during electrochemical reduction of

N2 on CSA/NPC-750 at -0.2 V.

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Fig. S11 SEM images of (A) NPC-750 and (B) Co/NPC-750, (C) low and (D) high resolution TEM images

of Co/NPC-750.

S9

200 400 600 800 1000

Zn

CoN

O CSA/NPC-750 CSA/NPC-850 CSA/NPC-950

Inte

nsity

(a.u

.)

Binding energy (eV)

C

396 398 400 402 404

Graphitic N

Pyrrolic N

Inte

nsity

(a.u

.)

E (eV)

Pyridinic N

396 398 400 402 404

Graphitic N

Pyrrolic N

Inte

nsity

(a.u

.)

E (eV)

Pyridinic N

396 398 400 402 404

Graphitic N

Pyrrolic N

Inte

nsity

(a.u

.)

E (eV)

Pyridinic N

C D

A B

Fig. S12 (A) XPS survey scan spectra of CSA/NPCs, N 1s XPS spectra of (B) CSA/NPC-750, (C)

CSA/NPC-850 and (D) CSA/NPC-950.

100 200 300 400 500

CSA/NPC-750 NPC-750

Inte

nsity

(a.u

.)

Temperature (ºC)

Fig. S13 N2-TPD curves of CSA/NPC-750 and NPC-750.

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DFT calculation was conducted on Vienna Ab-initio Simulation Package (VASP) with the projector

augmented wave (PAW) potentials, the generalized gradient approximation (GGA) parameterized by

Perdew, Burke and Ernzerhof (PBE) for texchange correlation functional.2,3 The Co single atoms embedded

N-doped carbon (CSA/NPC) with a 7×7 unit cell was used as model (Fig. S14). Both Co atoms coordinated

with four N atoms (Co-N4) and pyridinic N were considered in this model. The kinetic energy cutoff was

set to 400 eV for the plane-wave basis. To eliminate the interaction of periodic images of the system,

vacuum layer above the CSA/NPC plane was selected to be 20 Å. The model structures are optimized by

using thresholds for the total energy of 10−4 eV and force of 0.01 eV/Å. The free energy of possible reaction

pathways for NH3 synthesis was calculated4-6 as follows: ΔG = ΔEDFT + ΔEZPE -TΔS, where ΔEDFT is the

DFT total energy, ΔEZPE is the zero-point energy, and TΔS is the entropy difference between the initial

adsorbed state and final state. During the DFT calculation, the H, N2Hx and NHx adsorbed on CSA/NPC

are optimized by searching all the possible structures and find the lowest energy ones.

Fig. S14 The model of CSA/NPC used for DFT calculation.

2. Supplementary tablesTable S1. The EXAFS fitting results of CSA/NPC-750.

Electrocatalysts Bond type N R 2 ∆E0 (eV)CSA/NPC-750 Co-N 4.0 1.89 0.0034 -6.7

N is the coordination number, R is the distance between absorber and backscatter atoms, 2 is the Debye-Waller factor value, ∆E0 is edge-energy shift

S11

Table S2. Comparison of NH3 synthesis performance between CSA/NPC-750 and electrocatalysts reported recently

Electrocatalysts Production rate(µmol h-1 cm-2)

Efficiency(%) Conditions References

Au/CeOx-RGO 0.098 10.1% -0.2 V, pH 2 Adv. Mater. 2017, 29, 1700001

Au nanorod 0.097 8.11% -0.2 V, pH 13 Adv. Mater. 2017, 29, 1604799

Fe2O3/CNT 0.013 0.18% -1.4 V, pH 7 Angew. Chem. Int. Ed. 2017, 56, 2699

MoS2 0.29 1.17% -0.5 V, pH 7 Adv. Mater. 2018, 30, 1800191

Carbon nitride 0.95 11.59% -0.2 V, pH 1 Angew. Chem. Int. Ed. 2018, 57, 10246

CSA/NPC 0.86 10.5% -0.2 V, pH 7 This work

Table S3. The N/C and Co/C ratios obtained from XPS for CSA/NPC-750 before and after electrochemical reduction of N2 at -0.2 V.

N/C ratio Co/N ratioBefore use 0.319 0.0152After use 0.335 0.0146

XPS is used to quantify CSA/NPC-750 before and after electrochemical reduction of N2 at -0.2 V. As

shown in Figure S11 and Table S3, the atom ratios of N/C and Co/C are 0.319 and 0.0152 before use. After

electrochemical reduction of N2 for six cycles (1 h for each cycle), the N/C ratio is 0.335. The slightly

increased N content could be caused by measurement error and/or adsorbed ammonia. The Co/C ratio is

0.0146 after use, which can be considered as the same before reaction.

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Table S4. The metal and N contents of CSA/NPCs, NPC-750 and Co/NPC-750

Electrocatalysts Co content Zn content N content

CSA/NPC-750 1.4 wt% 4.3 wt% 20.6 at.%

CSA/NPC-850 1.5 wt% 1.8 wt% 15.3 at.%

CSA/NPC-950 1.8 wt% 0.03 wt% 13.0 at.%

NPC-750 0 4.2 wt% 20.7 at.%

Co/NPC-750 8.2 wt% 4.1 wt% 21.8 at.%

Metal content is measured by inductively coupled plasma atomic emission spectroscopy, N content is measured by XPS.

3. References1. F. Zhou, L. M. Azofra, M. Ali, M. Kar, A. N. Simonov, C. McDonnell-Worth, C. Sun, X. Zhang, D. R. MacFarlane, Energy Environ. Sci. 2016, 9, 2545-2549.2. J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett., 1996, 77, 3865-3868.3. G. Kresse and J. Hafner, Phys. Rev., 1993, 47, 558-561.4. Z. Geng, Y. Liu, X. Kong, P. Li, K. Li, Z. Liu, J. Du, M. Shu, R. Si and J. Zeng, Adv. Mater., 2018, 30, 1803498.5. S. Back and Y. Jung, Phys. Chem. Chem. Phys., 2016, 18, 9161-9166.6. X. Yu, P. Han, Z. Wei, L. Huang, Z. Gu, S. Peng, J. Ma and G. Zheng, Joule, 2018, 2, 1610-1622.