Supporting Information

Carbon Nitride Embedded with Transition Metals for Selective

Electrocatalytic CO2 Reduction

Chunmei Ding,a, # Chengcheng Feng,b, # Yuhan Mei,c Fengyuan Liu,d Hong Wang,a Michel

Dupuis,a,c Can Lia,*

a State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy

of Sciences; Dalian National Laboratory for Clean Energy, Dalian 116023, China.

b School of Chemistry and Materials Science, University of Science and Technology of China,

Hefei 230026, China

c Department of Chemical and Biological Engineering and Computation and Data-Enabled

Science and Engineering Program, University at Buffalo, State University of New York,

Buffalo, New York, USA.

d State Key Laboratory of Fine Chemicals, Zhang Dayu School of Chemistry, Dalian

University of Technology, Dalian 116023, China

1

1. Synthesis of Materials

All chemicals were analytical grade and were used as purchased without further

purification. The details are as follows: NiCl2·6H2O (98%, Tianjin Kemiou Chemical Reagent

Co., China), CuCl2·2H2O (98%, Sinopharm, China), MnCl2·4H2O (98%, Sinopharm, China),

dicyandiamide (Sinopharm, China), CO2 gas (> 99.999%, Guangming, Dalian, China).

Multiwall carbon nanotubes (CNT, TIME NANO, Chengdu, China) were pretreated

before use. Typically, 4 g CNT was dispersed in 200 mL HNO3 (65~68 wt.%) and refluxed at

140 oC for 13 h. After cooling down, the above mixture was transfered to 250 mL water, and

then cooled to room temperature. Then the suspension was filtered and washed to neutral pH.

The product was dried at 65 oC and grinded.

Metal-C3N4-CNT catalysts with various transition metals were synthesized with a

method modified from that previously reported.[1] Based on theoretical predictions, one

triazine (C6N8) or two g-C3N4 moieties can coordinate one metal atom. Therefore, it was

expected that 3 mol dicyandiamide can coordinate 1 mol metal ions. Based on this

assumption, firstly, 25 mg of treated CNT was dispersed in 100 mL water. Then 50 mg

dicyandiamide and 0.1 mmol of metal precursor were added and stirred for 20 h at 50 oC.

Note that the amount of dicyandiamide was doubled to ensure coordination efficiency and to

also account for melamine sublimation during the polymerisation of g-C3N4 from

dicyandiamide. The mixture was then dried in vacuum at 55 oC and the collection was

annealed at 600 oC in N2 with a flow rate of 50 mL min-1. The product was subsequently

stirred in 1 M HCl for 10 h to remove the metal and metal oxides, and then washed with large

amount of water several times. The C3N4-CNT was prepared by the same procedure without

the addition of metal precursor, and pristine C3N4 was synthesized by annealing

dicyandiamide precursor directly in N2. Metal-CNT was synthesized similarly without

dicyandiamide. Metal-N/CNT was prepared by annealing the metal-C3N4-CNT at 900 oC in N2

for 2 h.

2

The electrode was prepared by drop casting the catalyst ink on C paper (Toray, H060).

The ink was a mixture of 2 mg catalyst, 60 μL water, 90 μL ethanol and 50 μL nafion solution

(1 wt.% in ethanol), ultrasonically dispersed for 30 min. The loading amount of the catalyst

was 1 mg cm-2.

2. Physical characterizations

The morphologies were examinzed by transmission electron microscopy (TEM, Hitachi

HT7700) and high resolution transmission electron microscopy (HRTEM, JEOL JEM-

2000EX). The high angle annular dark field scaning transmission electron microscopy

(HAADF-STEM) images were taken on a probe-corrected JEM-ARM200F electron

microscope. Compositional maps were acquired with an energy dispersive X-ray spectrometer

(EDS) fitted on the microscope. X-ray photoelectron spectroscopy (XPS) measurements were

performed using a Thermofisher ESCALAB 250Xi spectrometer with Monochromated Al Kα

excitation (1486.6 eV, 15 kV, 10.8 mA). The Raman spectra was detected on a RENISHAW

inVia Raman Microscope with 532 nm laser (2.25 mW) with a scan time of 60 s.

For the elemental ICP measurements (ICP-OES, PerkinElmer, 7300DV), 25 mg sample was

put into a quartz boat and treated in air at 700 oC for 6 h in a muffle oven. The solid collection

was dissolved by 4 mL of concentrated acid.

3. Electrochemical measurements

Electrochemical measurements were conducted in a three-electrode setup with Pt counter

electrode (2 cm × 3 cm) and SCE reference electrode. Otherwise mentioned, all potentials

were controlled by a potentiostat (CHI 660e) without iR compensation and were converted to

reversible hydrogen electrode (RHE) scale using E (V vs. RHE) = E (V vs. Ag/AgCl) +

0.1976 V + 0.059* pH. The pH values of 0.5 M KHCO3 saturated with Ar and CO2 are 8.3

and 7.5, respectively. The selectivity and reaction rate of CO2RR were evaluated by

controlled-potential electrolysis in an H-type cell containing 0.5 M KHCO3 electrolyte with a

Nafion-117 membrane. Electrolytes were pre-saturated and bubbled with CO2 during

3

measurements. CO evolution rate was determined by an online gas chromatograph (GC,

Agilent 7890, Ar carrier and TDX-01 packed column) equipped with TCD (for H2 detection),

FID detectors and a methanizer (for CO detection). The gas flows though the cell at a constant

flow and was vented into the sampling loop of the GC. A video of CO2 reduction at -0.8 V vs.

RHE with NiCu-C3N4-CNT catalyst was recorded (Movie S1).

The relative electrochemical surface area is determined by the double-layer capacitances

calculated from the plot of discharging current densities vs. scan rates.[2, 3] The cyclic

voltammetry was measured in 0.5 M KHCO3 saturated with CO2 within a none-faradaic

potential region (0.8 V to 0.5 V vs. RHE). Three cycles are recorded at each scan rate and the

current values were taken during the second cathodic scan at 0.65 V. To note that, the relative

electrochemical surface area estimated from the double layer capacity current includes the

contribution of the exposed C substrate.

4. Computational details

4.1. Methods and models

We performed density functional theory (DFT) calculations using a basis set of plane waves

to describe the electron density in conjunction with the projector augmented wave (PAW)

pseudopotentials to represent the 1s core electrons of C and N, and the 1s, 2s, 2p, 3s core

electrons of the transition metal atoms, and with the PBE functional as the energy functional

of the electron density, as implemented in the Vienna ab initio Simulation Package (VASP).

[4-10] The plane wave energy cutoff was set to 550 eV for all metal-C3N4 systems. To note

that, the C3N4 model for DFT calculation is single layer for simplicity. A Gaussian smearing

of 0.01 eV for molecules and 0.2 eV for metal-C3N4 systems was applied for total energy

computations during geometry optimization. The convergence criterion for the self-

consistent-field (SCF) energy calculations was set to be 10-7 eV, and that 0.01 eV/ Å for

geometry optimization. The Bloch representation of the periodic one-electron states were

calculated using the Monkhorst−Pack k-point grid of 2 × 2 × 1 for metal-C3N4 system. For

4

molecules, only the gamma point was sampled. Supercells for energy calculations were

constructed with a 20 Å vacuum layer separation in the z direction to prevent interactions

between the periodic images for metal-C3N4 systems. For molecules, a 20 Å × 20 Å × 20 Å

box was used for simulations. Solvation effects were considered for all systems by simulating

the aqueous environment via a polarizable continuum (with a dielectric constant ε = 80 for

water) surrounding the molecule or the surface atoms as an ‘implicit’ model of solvation as

implemented in VASPsol.[11, 12]

4.2. Energy calculation CO2 activation

The catalytic activity of the carbon-based structures was determined from the binding

energies of the reaction intermediates (i.e., *H, *COOH) to the active site in their lowest

energy conformation following the methodology of Cheng et al.[13] And the overpotential of

the CO2 reduction reaction in these systems determined from the first hydrogenation step to

form *COOH or *OCHO.[14] Thus we investigated here the adsorption and activation energy

of CO2 on different metal-C3N4 structures (Table S3). As mentioned above, CO2 is adsorbed

on the first layer of the C3N4 for the calculation. The reaction process was shown as equation

(1) with ‘*’ denoting the bare system with the active site:

* + H+ + e- + CO2 → *COOH (1)

To interpret the energy data calculated for the various intermediates, we used the

computational standard hydrogen electrode (SHE) model proposed by Nørskov et al.[15] to

calculate potential (E) and pH-dependent free-energies. We report here the data for E = 0 V

versus SHE and pH = 0. The reaction energies of the intermediates were calculated and

corrected for zero-point energy (ZPE), enthalpy correction, entropy correction, and solvation

effects to obtain the reaction free energies (ΔG) according to equation (2) and (3). We used

the ZPE and thermodynamic corrections for the metal-C3N4 system reported by Cheng et al.

for their metal-porphyrin-graphene systems[13] as shown in Table S5.

ΔE = E(ads*) -E(ads) - E(*) (2)

5

ΔG = ΔEsol + ΔH + ΔZPE – TΔS (3)

292 288 284 280

Raw data Peak sum O-C=O 291 C-OH 285.8 C=C 284.6 Background

Cou

nts/

s

Eb / eV

NiMn-CNT C 1s

292 288 284 280

Raw data Peak sum O-C=O 291 C-OH 285.8 C=C 284.6 background

Cou

nts/

s

Eb / eV

NiCu-CNT C 1s

292 288 284 280

Raw Intensity Peak sum O-C=O 291 N=C-N 288.7 C-OH 285.8 C=C 284.6 Background

Cou

nts/

s

Eb / eV

NiMn-C3N4-CNT C 1s

292 288 284 280

Raw data Peak sum O-C=O 291 N=C-N 288.7 C-OH 285.8 C=C 284.6 Background

Cou

nts/

sEb / eV

NiCu-C3N4-CNT C 1s(a) (b)

(c) (d)

Figure S1. High resolution C 1s XPS spectra of (a) NiMn-C3N4-CNT, (b) NiCu-C3N4-CNT,

(c) NiMn-CNT and (d) NiCu-CNT catalysts. All signals are calibrated by C 1s (284.6 eV).

Typical C species of triazine rings (N=C-N, 288.7 eV) can be observed on samples with

C3N4. All the samples show signals of C=C (284.6 eV) from CNTs, oxidized carbon species

C-OH (285.8 eV) and O-C=O (291 eV).

6

408 404 400 396

NiMn-C3N4-CNT Raw data Peak sum C=N-C triazine 398.6 N-(C)3 399.8 N-H 401.3 Background

Cou

nts/

s

Eb / eV

N 1s

408 404 400 396

NiCu-C3N4-CNT Raw Intensity Peak sum C=N-C triazine 398.6 N-(C)3 399.8 N-H 401.3 Background

Cou

nts/

s

Eb / eV

N 1s

(c) (d)

408 404 400 396

C3N4-CNT Raw data Peak sum C=N-C triazine 398.6 N-(C)3 399.8 N-H 401.3 Background

Cou

nts/

s

Eb / eV

N 1s

540 520 400 360 320 280

NiCu-CNT

NiCu-C3N4-CNT

C 1s

N 1s

Cou

nts/

s

Eb /eV

O 1s

NiMn-C3N4-CNT

NiMn-CNT

(a) (b)

Figure S2. (a) The C 1s, N 1s, and O 1s XPS spectra of (NiMn, NiCu)-C3N4-CNT and

(NiMn, NiCu)-CNT catalysts. The high resolution N 1s XPS spectra of (b) C3N4-CNT, (c)

NiMn-C3N4-CNT and (d) NiCu-C3N4-CNT catalysts. The binding energies are calibrated by C

1s (284.6 eV).

The N 1s signals are only observed on samples containing C3N4, which display three typical

N species of g-C3N4 (C=N-C triazine rings at 398.6 eV, N-(C)3 tertiary nitrogen at 399.8 eV,

and N-H amino groups at 401.3 eV). The modification of C3N4-CNT with metal species in the

triazine matrix may have little influence on the texture property of C3N4-CNT. As emphasized

in the main text, there is indeed chemical interaction between N and metal atoms, forming M-

Nx structures. Comparing the N 1s XPS spectra of C3N4-CNT and (NiMn, NiCu)-C3N4-CNT,

the binding energies of N 1s show no changes after metal modification, that’s because the

7

amount of metal species may be too low to cause obvious shift of the binding energy of N

species.

1200 1400 1600 1800 2000

C3N4-CNT

NiCu-C3N4-CNT

NiMn-C3N4-CNT

NiCu-CNT

CNT

NiMn-CNT

1.55

1.18

1.28

1.21

1.35

1.38

1607

1576 G

Inte

nsity

/a.u

.

Raman shift/ nm-1

1343 D

ID/IG

Figure S3. Raman spectra of CNT, C3N4-CNT, NiMn-CNT, NiCu-CNT, NiMn-C3N4-CNT

and NiCu-C3N4-CNT catalysts.

During the pretreatment of CNTs under strong acid condition, there will be many defects on

oxidized CNTs, the Raman spectra shows that the ratio of D-band (disorder of the graphite

carbon) and G-band (presence of crystalline graphitic carbon) is obvious decreased, indicating

that disorder and defects of the graphite carbon is restored during the polycondensation

process of g-C3N4. In addition, there is little change in the ratio of ID/IG, indicating the

introduction of metal species show negligible influence on the texture property of C3N4-CNT.

8

(a) (b)

(c) (d)

100 nm 100 nm

100 nm 100 nm

Figure S4. The TEM images of NiMn-C3N4-CNT (a: without washing; b: washed in 1 M HCl

for 12 h) and NiCu-C3N4-CNT (c: without washing; d: washed in 1 M HCl for 12 h) catalysts

There are many nanoparticles before being washed in acid. However, there are negligible

metal nanoparticles in the catalyst after acid treatment, indicating the metal or metal oxide

nanoparticles are removed. Thus, the CO2RR activity is contributed by metal species

embedded in C3N4-CNT instead of metal or metal oxide nanoparticles.

9

Figure S5. The element mapping results of NiCu-C3N4-CNT catalyst (a) microscopic

image, (b) C element, (c) O element, and (d) Ni element.

Although the background noise of Ni is strong, the element mapping images show that the

Ni element is uniformly distributed on the surface of CNT. Higher density of Ni element

signals can be observed in areas with C and O elements, indicated by the yellow lines.

Table S1. The loading amount of metal elements of (NiCu, NiMn)-C3N4-CNT samples (all

have been washed by acid) determined by ICP measurements

SamplesNi

(wt.%)

Cu

(wt.%)

Mn

(wt.%)

Ni

(μmol cm-2)

Cu

(μmol cm-2)

Mn

(μmol cm-2)

NiCu-C3N4-CNT 6.657 2.079 1.119 0.3271

NiMn-C3N4-CNT 9.188 2.175 1.565 0.3959

10

potentiostat

CE WE

CO

CO2

H+

RE

CO2 CO

Nafion-117stir

Figure S6. A schematic description of the H-type CO2 reduction electrochemical cell

-1.0 -0.8 -0.6 -0.4 -0.2 0.0-40

-30

-20

-10

0

Cu-C3N4-CNT

Ar CO2

J/ (m

A c

m-2)

E vs. RHE/ V-1.0 -0.8 -0.6 -0.4 -0.2 0.0

-50

-40

-30

-20

-10

0

Ni-C3N4-CNT

Ar CO2

J/ (m

A c

m-2)

E vs. RHE/ V

(a) (b)

(c)

-1.0 -0.8 -0.6 -0.4 -0.2 0.0

-40

-30

-20

-10

0

Ar CO2

Mn-C3N4-CNT

J/ (m

A c

m-2)

E vs. RHE/ V

-1.0 -0.8 -0.6 -0.4 -0.2 0.0-80

-60

-40

-20

0

Co-C3N4-CNT

Ar CO2

J/ (m

A c

m-2)

E vs. RHE/ V

-1.0 -0.8 -0.6 -0.4 -0.2 0.0-40

-30

-20

-10

0

Fe-C3N4-CNT

Ar CO2

J/ (m

A c

m-2)

E vs. RHE/ V

(d)

(e)

Figure S7. The LSV scans of M-C3N4-CNT (M = (a) Ni, (b) Cu, (c) Mn, (d) Fe, and (e) Co)

in 0.5 M KHCO3 saturated with Ar (dash) or CO2 (solid)

11

-1.0 -0.9 -0.8 -0.7 -0.6 -0.50

100

200

300

400NiMn-C

3N

4-CNT

Ar

P

rodu

cts/

(m

ol c

m-2 h

-1)

H2 CO

E vs. RHE/ V-1.0 -0.9 -0.8 -0.7 -0.6 -0.5

0

200

400

600

Evs. RHE/ V

NiCu-C3N

4-CNT

Ar

Pro

duct

s/ (

mol

cm

-2 h

-1)

H2 CO

(a) (b)

-0.6 -0.7 -0.8 -0.9 -1.00

20

40

60

80

100

NiMn-C3N

4-CNT

CO2

total CO H

2 F /%

E vs. RHE/ V-0.6 -0.7 -0.8 -0.9 -1.0

0

20

40

60

80

100

NiCu-C3N

4-CNT

CO2

total CO H

2

F/ %

E vs. RHE/ V

(c) (d)

Figure S8. The activity of (a) NiMn-C3N4-CNT and (b) NiCu-C3N4-CNT catalysts in 0.5 M

KHCO3 saturated with Ar. The ηF of CO and hydrogen evolution of (c) NiMn-C3N4-CNT and

(d) NiCu-C3N4-CNT at different potentials in 0.5 M KHCO3 saturated with CO2.

12

-1.0 -0.9 -0.8 -0.7 -0.6 -0.540

60

80

100

NiCu 4:1 NiCu 9:1 NiCu 1:1

F(C

O)/

%

E vs. RHE/ V

-1.0 -0.9 -0.8 -0.7 -0.6 -0.50

200

400

NiCu 4:1 NiCu 9:1 NiCu 1:1

CO

/ (m

ol c

m-2 h

-1)

E vs. RHE/ V

(a) (b)

(c) (d)

-1.0 -0.9 -0.8 -0.7 -0.6 -0.540

60

80

100

NiMn 4:1 NiMn 9:1 NiMn 1:1

F(C

O)/

%

E vs. RHE/ V

-1.0 -0.9 -0.8 -0.7 -0.6 -0.50

100

200

300

400

NiMn 4:1 NiMn 9:1 NiMn 1:1

CO

/ (m

ol c

m-2 h

-1)

E vs. RHE/ V

-1.0 -0.8 -0.6 -0.4 -0.2 0.0-40

-30

-20

-10

0

NiCu 4:1-Ar NiCu 4:1-CO2

NiCu 1:1-Ar NiCu 1:1-CO2

NiCu 9:1-Ar NiCu 9:1-CO2

J/ (m

A c

m-2)

E vs. RHE/ V-1.0 -0.8 -0.6 -0.4 -0.2 0.0

-40

-30

-20

-10

0

NiMn 4:1-Ar NiMn 4:1-CO2

NiMn 1:1-Ar NiMn 1:1-CO2

NiMn 9:1-Ar NiMn 9:1-CO2

J/ (m

A c

m-2)

E vs. RHE/ V

(e) (f)

Figure S9. The (a, b) ηF(CO) and (c, d) CO evolution rates of NiMn-C3N4-CNT and NiCu-

C3N4-CNT catalysts with different Ni:Mn and Ni:Cu ratios in 0.5 M KHCO3 saturated with

CO2. (e, f) The LSV currents of NiMn-C3N4-CNT and NiCu-C3N4-CNT catalysts with

different Ni:Mn and Ni:Cu ratios in 0.5 M KHCO3 saturated with CO2 and Ar.

The highest ηF(CO) and CO evolution rate were obtained with the ratios of Ni:Mn and

Ni:Cu of around 4:1. The ηF(CO) firstly increases with the increase of overpotential and then

decreases, demonstrating the highest ηF(CO) at around -0.8 V vs. RHE. The CO evolution

rates increase with the applied bias.

13

0 20 40 60 80 1000

1

2

3 Ni-C3N4-CNT Mn-C3N4-CNT Cu-C3N4-CNT NiCu-C3N4-CNT NiMn-C3N4-CNT C3N4-CNT

J (c

apac

itive

)/ (m

A c

m-2)

Scan rate/ (mV s-1)

(a)

0 20 40 60 80 1000

1

2

3 NiCu-C3N4-CNT-without washing NiMn-C3N4-CNT-without washing NiCu-C3N4-CNT NiMn-C3N4-CNT

J/ (m

A c

m-2)

Scan rate/ (mV s-1)

(b)

Figure S10 The capacitive current density of different catalysts in 0.5 M KHCO3 as a

function of scan rate over a non-faradic potential range.

Table S2 The relative electrochemical active surface area calculated from the slope of

capacitive current density vs. scan rate in Figure S10.

Relative electrochemical active surface area

Ni-C3N4-CNT 3.9

Cu-C3N4-CNT 3.8

Mn-C3N4-CNT 4.3

NiCu-C3N4-CNT 1.0

NiMn-C3N4-CNT 1.4

C3N4-CNT 2.7

NiCu-C3N4-CNT-without acid treatment 2.0

NiMn-C3N4-CNT-without acid treatment 3.6

14

(d)

(a’) (b’) (c’)

(d’)

(a) (b) (c)

(a’’) (b’’) (c’’)

(d’’)

(e)

(e’)

(e’’)

1.921.91

2.19

1.881.96

2.052.14

2.07

1.881.94

2.082.00

1.97

2.97 3.362.74

2.93 2.11

2.76 2.312.342.16

2.35 2.76

2.31

2.25

2.24

2.29

2.572.82

Figure S11. The optimized structures of mono-metal (a, a’, a’’) Ni-C3N4, (b, b’, b’’) Cu-C3N4,

(c, c’, c’’) Mn-C3N4, bi-metal (d, d’, d’’) NiMn-C3N4 and (e, e’, e’’) NiCu-C3N4 units from

different views. Coloring scheme: blue (N atoms), gray (C atoms), yellow (Ni atoms), green

(Cu atoms) and purple (Mn atoms).

Different coordination structures are formed because the different electronic properties of

metal atoms. For bimetallic systems, the metal atoms seem to be out of the g-C3N4 planar

because of the repulsing interaction of two metal atoms.

15

Table S3. The first-step reaction free energies of (H+ + e- + CO2) forming *COOH on

different structures obtained by DFT calculation

Structure ΔG*COOH

Mn-C3N4 0.97

Ni-C3N4 0.61

Cu-C3N4 0.37

NiCu-C3N4 -0.77

NiMn-C3N4 -1.16

Table S3 shows that the reaction free energies change ΔG*COOH for (Ni, Cu, Mn)-C3N4 are

all positive value, but the values are negative for (NiCu, NiMn)-C3N4 bimetallic catalysts.

That means the reaction (H+ + e- + CO2) forming *COOH changes from endothermic reaction

for mono-metal catalysts to exergonic reaction for bimetallic catalysts.

16

880 870 860 850

10000

15000

20000Ni 2p 855.2

855.3

872.8

872.7

Cou

nts/

s

Eb/ eV

872.5

855.1

Ni-C3N4-CNT

NiCu(4:1)-C3N4-CNT

NiCu(1:1)-C3N4-CNT

880 870 860 8508000

16000

24000 Ni 2p

Ni-C3N4-CNT

NiMn(1:1)-C3N4-CNT

855.3

855.4

872.9

872.8

Cou

nts/

s

Eb/ eV

NiMn(4:1)-C3N4-CNT

872.5 855.1

960 950 940 9308000

10000

12000Cu 2p

932.3

932.3

952.3

952.3

Cou

nts/

s

Eb/ eV

951.3

932.1

NiCu(4:1)-C3N4-CNT

NiCu(1:1)-C3N4-CNT

Cu-C3N4-CNT

660 650 640

10000

12000

14000Mn 2p

643

643

Cou

nts/

s

Eb/ eV

641.7

Mn-C3N4-CNT

NiMn(4:1)-C3N4-CNT

NiMn(1:1)-C3N4-CNT

(a) (b)

(c) (d)

Figure S12. The (a, b) Ni 2p, (c) Cu 2p, and (d) Mn 2p XPS spectra of (NiMn, NiCu)-C3N4-

CNT and (Ni, Cu, Mn)-CNT catalysts. Eb is calibrated with C 1s (284.6 eV).

The binding energies of Ni 2p, Mn 2p and Cu 2p in (NiMn, NiCu)-C 3N4-CNT are shifted to

higher values by about 0.1 eV ~ 0.3 eV compared with those in mono-metal materials,

because the introduction of a second metal element may influence the electronic structures of

the neighboring metal centers.

17

-1.0 -0.8 -0.6 -0.4 -0.2 0.0

-30

-20

-10

0

J/ (m

A c

m-2)

E vs. RHE/ V

CNT-ArCNT-CO2

NiMn-CNT-ArNiMn-CNT-CO2

NiCu-CNT-ArNiCu-CNT-CO2

-1.0 -0.8 -0.6 -0.4 -0.2 0.0-30

-20

-10

0

C3N4-CNT

J/ (m

A c

m-2)

E vs. RHE/ V

ArCO2

(a) (b)

Figure S13. The LSV scans of the (a) C3N4-CNT electrodes and (b) those without C3N4 in 0.5

M KHCO3 saturated with Ar (dash) or CO2 (solid).

-0.6 -0.7 -0.8 -0.9 -1.00

20

40

60

80

100

F C

O /

%

E vs. RHE/ V

C3N4-CNT Ni-C3N4-CNT Mn-C3N4-CNT Cu-C3N4-CNT

C3N4-CNT

Ni-C3N4-CNT

Mn-C3N4-CNT

Cu-C3N4-CNT0

100

200

300

COH

2

Pro

duct

s/ (

mol

cm

-2 h

-1)

(a) (b)

Figure S14. (a) The Faradic efficiency (ηF) of CO evolution at different potentials, (b)

hydrogen and CO evolution rates at -0.8 V vs. RHE on C3N4-CNT and M-C3N4-CNT (M = Ni,

Mn, Cu) catalysts in 0.5 M KHCO3 saturated with CO2.

18

(a) (b)

-1.0 -0.8 -0.640

60

80

100

NiMn-washedNiMn-without washingNiCu-washedNiCu-without washing

F(C

O)/

%

E vs. RHE/ V

0

100

200

300

NiMnwithoutwashing

NiMnwashed

COH

2

Pro

duct

s/ (m

ol c

m-2 h

-1)

NiCuwashed

NiCuwithoutwashing

Figure S15. (a) The hydrogen and CO evolution rates at -0.8 V vs. RHE and (b) the Faradic

efficiency (ηF) of CO evolution at different potentials in 0.5 M KHCO3 saturated with CO2 on

(NiCu, NiMn)-C3N4-CNT catalysts and those without being washed in acid.

(a) (b)

0.0 0.5 1.0 1.50

50

100

150

e/2 CO H

2

-0.5V

Pro

duct

s/(

mol

h-1cm

-2)

Time/h

-0.6VNiMn-C

3N

4-CNT

0.0 0.5 1.0 1.50

50

100

150

e/2 CO H

2

NiCu-C3N4-CNT

-0.5 V

-0.8V

Pro

duct

s/(

mol

h-1cm

-2)

Time/h

-0.6 V

Figure S16. (c) The time-course CO and H2 evolution rates, and the corresponding amount of

e/2 calculated from the current with (c) NiMn-C3N4-CNT and (d) NiCu-C3N4-CNT catalysts at

-0.5 V and -0.6 V vs. RHE and -in 0.5 M KHCO3 saturated with CO2.

19

Table S4 Comparison of the results of this work and those in literatures.

MaterialsMaximu

m ηF(CO)

JCO/ (mA cm-2) at potentials

(vs. RHE) of maximum ηF(CO)

Author, Ref.

No. in main text

(NiCu, NiMn)-C3N4-CNT 90% 15.8 at -0.8 V This work

Ni-N-C 85% 8 at -0.75 V Wen, [36]

single atom Ni-NSG 97% 22 at -0.72 V Yang, [40]

Ni-N-C 93% 3.5 at -0.67 V Hu, [41]

single atom Ni -NCNTs 91.3% 23.5 at -0.7 V Cheng, [49]

Coordinatively

unsaturated Ni-N-C98% 57.5 at -0.83 V Yan, [50]

Co-N2-C 94% 17 at -0.63 V Wang, [52]

Co-N5-C 99% 4.5 at -0.73 V Pan, [53]

Fe-N-C 93% 2.8 at -0.58V Pan, [54]

Co single atom-N/C 82% 10.8 at -0.8 V Geng, [55]

Ni single atom-NCNTs 95% 54 at -1 V Lu, [56]

Ni single atom-GO 92% 26.8 at -0.7 V Cheng, [57]

Ni-N-C 96% 7.5 at -0.75 V Pan, [58]

Table S5. ZPE and thermodynamic corrections in eV’s for CO2RR species and intermediates

Species ΔH + ZPE T×∆S

CO2 0.43 0.66

*COOH 0.72 0.24

*OCHO 0.71 0.24

* 0.00 0.00

20

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