CNT nanocomposites at industrial scale current density

Journal Pre-proof Efficient electroreduction of CO2 to CO by Ag decorated S-doped g-C3N4/CNT nanocomposites at industrial-scale current density Jianan Chen, Zhenyu Wang, Hyeonseok Lee, Jianjun Mao, Craig A. Grimes, Chang Liu, Mingyang Zhang, Zhouguang Lu, Yue Chen, Shien-Ping Feng PII:

S2542-5293(19)30156-7

DOI:

https://doi.org/10.1016/j.mtphys.2019.100176

Reference:

MTPHYS 100176

To appear in:

Materials Today Physics

Received Date: 27 October 2019 Revised Date:

17 December 2019

Accepted Date: 20 December 2019

Please cite this article as: J. Chen, Z. Wang, H. Lee, J. Mao, C.A. Grimes, C. Liu, M. Zhang, Z. Lu, Y. Chen, S.-P. Feng, Efficient electroreduction of CO2 to CO by Ag decorated S-doped g-C3N4/CNT nanocomposites at industrial-scale current density, Materials Today Physics, https://doi.org/10.1016/ j.mtphys.2019.100176. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Ltd. All rights reserved.

Credit Author Statement

J.N.C. and S.P.F. developed the concept and designed the experiments. J.N.C., Z.Y.W., H.S.L., and M.Y.Z performed the experiments. J.N.C., Z.Y.W., and Z.G.L. contributed to the material characterizations. J.J.M., J.N.C., and Y.C. contributed to the DFT calculations. J.N.C., H.S.L., C.A.G., J.J.M., Y.C., Z.G.L., and S.P.F. contributed to the interpretation of the results. J.N.C., Z.Y.W., and C.L. contributed to the construction of the experimental platform. J.N.C., H.S.L., and S.P.F. co-wrote the manuscript.

Graphical Abstract

Ag decorated sulfur-doped C3N4/CNT nanocomposites were synthesized as a highly active and selective eCO2RR catalyst. The resulting nanocomposites exhibit excellent performance in eCO2RR to CO, yielding a high current density of -21.3 mA cm-2 at -0.77 VRHE and maximum CO Faradaic efficiency over 90% in H-cell. In addition, when combining with flow cell configuration, the obtained catalyst delivers the best eCO2RR performance among C3N4-derivatives, with a current density larger than 200 mA cm-2 and great CO Faradaic efficiency over 80% in a wide potential window.

1 2

Efficient electroreduction of CO2 to CO by Ag

3

decorated S-doped g-C3N4/CNT nanocomposites at industrial-scale current density

4 a,†

5 6 7 8 9 10 11 12 13 14 15

Jianan Chen , Zhenyu Wanga,b,†, Hyeonseok Leea,c†, Jianjun Maoa,†, Craig A. Grimesd, Chang Liua, Mingyang Zhanga, Zhouguang Lub,*, Yue Chena,*, Shien-Ping Fenga,* a Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Rd., Pokfulam 999077, Hong Kong b Department of Materials Science and Engineering, Southern University of Science and Technology, China c Department of Photonics, National Sun Yat-Sen University, Kaohsiung, 80424, Taiwan d Flux Photon Corporation, 5950 Shiloh Road East, Alpharetta, Georgia 30005, United States

16

Corresponding Authors: *E-mail for Shien-Ping Feng: [email protected]

17

*E-mail for Yue Chen: [email protected]

18

*E-mail for Zhouguang Lu: [email protected]

19

Abstract

20

In recent years, the application of graphitic carbon nitride (g-C3N4) for electrochemical CO2 reduction reaction (eCO2RR) has aroused strong interest. However, this material is still facing severe activity issue towards eCO2RR so far, and studies on its catalytic mechanism have not been sufficiently implemented either.

21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

Herein, we report an Ag decorated sulfur-doped graphitic carbon nitride/carbon nanotube nanocomposites (Ag-S-C3N4/CNT) for efficient eCO2RR to CO. The resulting Ag-S-C3N4/CNT catalyst exhibits a notable performance in eCO2RR, yielding a high current density of -21.3 mA cm-2 at -0.77 VRHE and maximum CO Faradaic efficiency over 90% in H-type cell. Strikingly, when combining with flow cell configuration, the fabricated nanocomposites permit an industrial-scale and cost-effective eCO2RR, showing a current density larger than 200 mA cm-2 and the Faradaic efficiency of CO over 80% in a wide potential window, delivering the best eCO2RR performance among the C3N4-derivatives. Moreover, the catalytic mechanism of this nanocomposites has been further explored through density functional theory (DFT) and electrochemical methods carefully. Our work not only sheds light on industrial-scale eCO2RR to CO but also leads to new insights on the 1

1

application of C3N4-based composite materials in electrocatalytic processes.

2

KEYWORDS: CO2 reduction; electrocatalyst; C3N4; flow cell.

3

1. Introduction

4

The concentration of atmospheric carbon dioxide (CO2) has continued to rise

5

sharply from 260 ppm to more than the deadly 400 ppm mark since the first industrial

6

revolution.[1, 2] The accumulation of the atmospheric CO2 is deemed to be the culprit

7

of many environmental problems, such as global warming, and erratic weather

8

pattern.[3, 4] To alleviate these climate challenges, the electrochemical CO2 reduction

9

reaction (eCO2RR) for the production of value-added products (e.g., CO, CH4),

10

through solar and wind-derived renewable electricity, provides a near-perfect solution

11

to curb CO2 emission while producing useful chemicals and storing energy.[5, 6]

12

However, despite certain breakthrough has been made in eCO2RR, the practical

13

applications are still limited by the low production rate, poor selectivity, and high cost

14

of catalysts.

15

The eCO2RR current density (j) signifies the production rate and activity of the

16

catalysts. Considering the techno-economic factors, a gross margin model proposed in

17

2016 has pointed out the importance of high eCO2RR current density (j > 200 mA

18

cm-2) for practical applications.[7-9] However, in addition to the intrinsic activity of

19

the catalysts, an overwhelming percentage (>95%) of current researches on eCO2RR

20

to CO are conducted in H-type cell,[8] where the low CO2 solubility in aqueous

21

solution restricts the j of eCO2RR to ~35 mA/cm2,[10-13] and few of catalysts have

22

demonstrated the capability for industrial-rate production.[14] Additionally, the liquid

23

reaction environment in H-type cells also facilitates the competitive hydrogen

24

evolution reaction (HER) against the eCO2RR.[14] To overcome these limitations and

25

achieve an industrial-scale current density, the flow cell architecture has been

26

introduced to the eCO2RR research field recently because of its independency from

27

CO2 solubility and unique triple-phase boundary formed at the catalyst-electrolyte

28

interfaces.[15] However, although the utilization of flow cells is widely investigated

29

in fuel cells and water electrolysis, the studies of applying this technique in eCO2RR 2

1

are still limited so far.[12]

2

In addition to the reaction rate issue, the eCO2RR is a complicated process with a

3

wide variety of gas (e.g., CO, CH4, C2H4, etc.) and liquid (e.g., ethanol, formic acid,

4

n-propanol, etc.) products.[16-20] Among the various products, CO is one of the most

5

valuable products and the conversion of CO2 to CO holds several unique advantages.

6

Firstly, CO is generally more selective and convenient to be separated from the

7

electrolyte.[21] Secondly, since the direct conversion of CO2 into multi-carbon (C2+)

8

products is not effective, an alternative two-step strategy, where CO2 is firstly reduced

9

to CO and sequentially reduced to C2+ has been demonstrated to be productive.[19, 22]

10

Thirdly, the economic cost analysis evidence that CO owns the best market

11

compatibility and the highest net present value.[23] However, satisfactory selectivity

12

requires suitable binding strength with the reaction intermediates, which relies on the

13

well-designed composition and structure of the catalysts.[24]

14

In the past few decades, carbon-based nanomaterial has been widely developed

15

as electrocatalysts.[25-27] Among the various carbon materials, the low-dimensional

16

graphitic carbon nitride (g-C3N4) consisting of a prototypical 2D graphitic structure,

17

has shown promising performance in the recent reported electrocatalytic processes.[28,

18

29] Besides its low cost, stable, easily synthesizable, and environmentally friendly

19

merits, the g-C3N4 could serve as an ideal molecular scaffold for eCO2RR because of

20

several precious advantages: (1) the abundant pyridinic nitrogen atoms incorporated

21

in g-C3N4 can lead a strong CO2 affinity, and the selective adsorption of CO2 is

22

helpful for accelerating the eCO2RR and competing with other competitive

23

reactions.[30, 31] (2) the carbon atoms in g-C3N4 exhibit a high oxophilicity, which is

24

beneficial for adsorbing the complex oxygenated intermediates (eg., *COOH, *CO)

25

during multiple eCO2RR steps.[32] Nevertheless, to date, the application of g-C3N4

26

derivatives in eCO2RR has been rarely reported as it is restricted by intrinsic poor

27

conductivity and limited exposed active sites, and the eCO2RR mechanism on g-C3N4

28

still has not been fully investigated. Some strategies have been proposed to overcome

29

the innate disadvantages of g-C3N4. Heteroatoms (like sulfur, boron, etc.) doping has

30

been demonstrated as an effective approach in narrowing the g-C3N4 band gap and 3

1

increasing its conductivity to tune the electrical properties of g-C3N4 for oxygen

2

reduction reaction (ORR) and hydrogen evolution reaction (HER).[33, 34] In addition,

3

R. Amal et al. have combined multiwall carbon nanotubes (MWCNTs) with g-C3N4 to

4

improve the conductivity for eCO2RR, but the current density is still lower than −8

5

mA cm−2 at −0.8 VRHE and maximum Faradaic efficiency for CO (FE(CO)) is only

6

60%.[35] X.H. Li and J.S. Chen have synthesized a 2D polarized g-C3N4 with a large

7

reactive area for CO production, the catalyst achieves a maximum FE(CO) around 80%

8

at −1.1 VAg/AgCl, whereas the reaction current density is less than −5 mA cm−2.[36]

9

Besides these works, although ternary Au-carbon dots-C3N4[37] and Cu decorated

10

C3N4[31] have been developed toward eCO2RR, the activity and selectivity issues still

11

haven’t been well addressed.

12

In this work, we synthesized Ag nanoparticles decorated sulfur-doped C3N4/CNT

13

nanocomposites (Ag-S-C3N4/CNT) with excellent activity and selectivity towards

14

eCO2RR to CO. When measuring in the traditional H-type cell, the Ag-S-C3N4/CNT

15

showed a remarkable high current density of −21.3 mA cm−2 at −0.77 VRHE and the

16

maximum FE (CO) of 91.4 ± 0.01% at −0.8 VRHE. Moreover, to achieve an

17

industrial-scale and cost-effective CO production, the Ag-S-C3N4/CNT was coupled

18

with flow cell configuration, demonstrating a remarkable maximum current density of

19

330 mA cm−2 at a cell voltage of 3 V with a 93% maximum FE (CO) at 2.8 V. Our

20

synthesized Ag-S-C3N4/CNT represents the best performance of eCO2RR to CO with

21

great scalability and stability among all the reported C3N4-derivatives.

22 23 24

2. Results and discussion 2.1 g-C3N4 based nanocomposites

25

The TEM image and HR-TEM image of the Ag-S-C3N4/CNT nanocomposites

26

are shown in Figure 1. The C3N4 matrix is anchored on the surface of the interlaced

27

CNTs, and Ag nanoparticles are decorated on the surface of C3N4 matrix uniformly

28

(Figure 1a). The HR-TEM image of the Ag nanoparticle is shown in Figure 1b, where

29

the d spacing of 0.233 nm is consistent with the (111) lattice plane of Ag. Figure 1c 4

1

presents the schematic of the Ag-S-C3N4/CNT nanocomposites system. The HAADF

2

image (Figure 1d) confirms the Ag nanoparticles are successfully decorated on

3

S-C3N4/CNT. The EDS elemental mapping patterns for C, N, Ag, and S (Figure 1e-i)

4

reveal the uniform distribution of S-C3N4 on the CNT. The detailed synthesis

5

procedures can be found in the experimental section.

6

The XPS of the Ag-S-C3N4/CNT nanocomposites is shown in Figure S2. The

7

high-resolution C 1s spectrum is shown in Figure S2a, the major peak at 284.8 eV is

8

assigned to the sp2 C=C bond.[38] Another two peaks located at 288.0 eV and 291.3

9

eV can be ascribed to the N-C=N of triazine rings in C3N4 and the residual COOH

10

functional group on CNT, respectively.[35, 39] The N 1s spectrum (Figure S2b) can

11

be fitted into 3 peaks at 397.2 eV, 398.1 eV, and 399.8 eV. The major peak at 397.2 eV

12

indicates the presence of hybridized aromatic C=N-C group. Peaks at 398.1 eV and

13

399.8 eV correspond to the tertiary N (N-(C)3) and amino groups (N-H),

14

respectively.[40] The high resolution of the S 2p spectrum (Figure S2c) consists of

15

peak S 2p2/3 and S 2p1/2, and the Ag3d5/2 and Ag3d3/2 peaks (Figure S2d) are found at

16

binding energies of 368.0 eV and 374.0 eV, respectively. [41, 42] In addition, the XPS

17

element measurement was also taken to analyze the element proportion. The C is

18

44.11%, N is 54.88%, S is 0.49%, and Ag is 0.52%.

19

The XRD patterns of CNT, g-C3N4, C3N4/CNT, and Ag-C3N4/CNT are compared

20

in Figure S3 to further prove the formation of C3N4 in the nanocomposites. The

21

diffraction peaks at around 2θ = 27.6° and 13.0° in g-C3N4, C3N4/CNT, and

22

Ag-C3N4/CNT pattern are the characteristic peaks of g-C3N4. [43] Fourier transform

23

infrared

24

nanocomposites are shown in Figure S4. The broad peaks between 3000 cm-1 to 3500

25

cm-1, and 1000 cm−1 to 1700 cm−1 correspond to amine group and characteristic

26

stretching modes of CN heterocycles, respectively. The sharp peak at around 810 cm-1

27

is attributed to a typical breathing mode of triazine units. [44] The peak centered at

28

~1700 cm-1 can be ascribed to the carboxyl groups on the surface of the CNTs. [45]

(FTIR)

spectra

of

Ag-S-C3N4/CNT,

29 30 5

S-C3N4/CNT,

and

S-C3N4

1

2.2 eCO2RR in H-type cell

2

Figure 2a shows the linear sweep voltammetry (LSV) results of the C3N4/CNT,

3

S-C3N4/CNT, and Ag-S-C3N4/CNT nanocomposites in CO2-saturated 0.1M KHCO3

4

solution. As control samples, bare carbon fiber paper (CFP) and CNT coated CFP

5

electrodes only contribute a negligible low current density. The doping of sulfur is

6

beneficial in improving the eCO2RR reactivity, and the S-doped C3N4/CNT

7

nanocomposites with Ag decoration exhibit the highest current density among the

8

measured samples. By decorating Ag nanoparticles, the S-C3N4/CNT achieves a

9

noticeably 70% current density improvement from -12.5 mA cm-2 to -21.3 mA cm-2 at

10

-0.77 VRHE. The eCO2RR performances of C3N4-derivatives without CNT were also

11

investigated and shown in Figure S7 and 8. Obviously, the modification of C3N4 and

12

S-doped C3N4 by CNT could boost up the reaction current densities (Figure S8a and

13

Figure 2a). In addition, the potentiostatic tests (Figure S8c) further shows the

14

decorated Ag nanoparticle leads to a selective CO production.

15

The LSV curves of Ag-S-C3N4/CNT and bare CNT at 10 mV s-1 in 0.1M KHCO3

16

saturated with CO2 or Ar are displayed in Figure 2b and Figure S9. For

17

Ag-S-C3N4/CNT, the significant current density enhancement in a CO2-rich

18

environment demonstrates its high eCO2RR ability. In contrast, the electrochemical

19

behaviors of bare CNT show almost no difference in CO2 and Ar saturated electrolyte.

20

Potentiostatic tests were performed applying the potential ranged from -0.6 VRHE to

21

-0.9

22

chronoamperograms at different potentials are recorded in Figure 2c, and the gaseous

23

products under each bias were quantitatively analyzed by using GC online detection.

24

The FE(CO) & FE(H2) are found to be dependent to the applied bias and their total FE

25

is always over 90%, suggesting CO and H2 are the major products for this eCO2RR

26

(Figure 2d). At a low working potential of -0.6 VRHE, the FE(CO) and FE(H2) of

27

Ag-S-C3N4/CNT are calculated to be 69.5 ± 0.01% and 23.5 ± 0.05%, respectively. In

28

contrast, bare CNT shows a H2 favorable reaction pathway at this potential, with

29

FE(H2) as high as 82.4 ± 0.07% and FE (CO) is only 12.1 ± 0.02%. The FE(CO) of

30

Ag-S-C3N4/CNT and bare CNT rises with increasing the working potentials, and all

VRHE

to

study

the

selectivity

6

behavior

of

Ag-S-C3N4/CNT.

The

1

peaks at -0.8 VRHE, which is determined to be 91.4 ± 0.01% and 79.0 ± 0.06%,

2

respectively. In brief, the eCO2RR on Ag-S-C3N4/CNT becomes more selective to CO

3

at a broad potential window. Furthermore, the activity of the catalyst remains intact

4

even after 60d storage, with FE(CO) around 89% and FE(H2) around 11%. (Figure

5

S10)

6

However, it should be noted that during the Ag nanoparticle decoration process,

7

some Ag may be settled on the CNT where S-C3N4 sheets are uncovered. To

8

investigate the origin of the high eCO2RR activity of Ag-S-C3N4/CNT, CNT modified

9

by Ag nanoparticle (Ag-CNT) was synthesized in a similar manner. The eCO2RR

10

performance of Ag-CNT was recorded in Figure S11. The results indicate the

11

decoration of Ag nanoparticles can slightly improve the current density and FE(CO)

12

of CNT, but this performance is far worse than Ag-S-C3N4/CNT. These tests exclude

13

the eCO2RR catalytic contribution from Ag-CNT interface and demonstrate the high

14

eCO2RR ability is mainly generated from the synergetic effect of Ag nanoparticle,

15

S-doped g-C3N4, and CNTs.

16 17

2.3 Proposed mechanism of eCO2RR on Ag-S-C3N4/CNT nanocomposites

18

A series of studies were carried out to understand the synergetic effect of Ag

19

nanoparticles, S-doped C3N4, and CNT for eCO2RR performance. As mentioned in

20

the introduction above, the current density signifies the activity of eCO2RR and

21

excellent electrical properties of catalytic systems such as the distribution of carrier

22

concentration and carrier mobility are highly required for excellent eCO2RR.

23

The Ag nanoparticles, sulfur atoms, and C3N4 scaffold constitute the outermost

24

catalytic surface, on which CO2 is directly converted into CO. Density functional

25

theory (DFT) calculations were firstly performed to investigate their electronic

26

interaction and effects on tuning the eCO2RR catalytic ability. The simulated band gap

27

of C3N4 (1.19 eV in Figure 3a) via the DFT calculation is reduced after S-doping

28

(1.09 eV for S-C3N4 in Figure 3b). Further modified band structure (0.23 eV for

29

Ag-S-C3N4 in Figure 3c) and density of states (DOS) (Figure 3d) are observed with

30

the decoration of Ag nanoparticles. Moreover, Bader charge analysis shows that 7

1

apparent electron transfers occur both from the S atom (1.47e) to the C and N atoms,

2

and from the Ag nanoparticle (0.27e) to the S-C3N4 scaffold, leading to an

3

electron-rich region on the Ag-S-C3N4 interface (Figure 3e and 3f).

4

Possible electron accumulation is also found at the C3N4/CNT interface

5

evidenced by work functions measurement via Kelvin probe (Supplementary Note 1).

6

The electron accumulation to C3N4 can be occurred at the C3N4/CNT interface owing

7

to the larger work function of C3N4 (4.907 eV) than CNT (4.849 eV), and more

8

accumulation can be achieved at S-C3N4/CNT interface due to the larger work

9

function of S-C3N4 (4.993 eV). In addition, the electron transfer from Ag to S-C3N4 is

10

also evidenced by the smaller work function of 4.696 eV measured at Ag-S-C3N4

11

interface. All these electron accumulation behaviors are compatible with the DFT

12

results, and details of the proposed mechanism are further provided in the

13

supplementary note 2. Therefore, with combining excellent charge transport property

14

of CNT and, materials properties of S-C3N4, the electron accumulations at Ag-S-C3N4

15

and S-C3N4/CNT interfaces enhance the electrical properties of the Ag-S-C3N4/CNT

16

nanocomposites together, which is a favorable structure for achieving high eCO2RR

17

performance.

18

The thermodynamic barriers for eCO2RR to CO on the various nanocomposites

19

were then calculated by DFT to reveal the origin of the high catalytic activity of

20

Ag-S-C3N4/CNT. The mechanism of eCO2RR is considered to be complicated along

21

with multiple intermediates formation. The overall eCO2RR to CO can be described

22

as a three-state diagram comprising the following elementary steps:[46]

23

CO2 (g) + H+ + * + e-‐

24

*COOH + H+ + e-‐

25

*CO

‐CO(g) + *

‐*COOH

(1)

‐*CO + H2O (l)

(2) (3)

26

Where * represents an adsorption site. The computational free-energy profiles for the

27

three nanocomposites at experimental conditions (298 K, 1 atm, and -0.55 VRHE) are

28

shown in Figure 3g. Overall, two exergonic processes (downhill in the energy)

29

corresponding to the formation of *COOH and *CO intermediates are observed on all

30

the catalysts. However, for the C3N4 and S-doped C3N4, the release of CO is all 8

1

endothermic, resulting the reaction (3) (CO desorption) becomes the rate-determining

2

step.[47] In contrast, owing to the proper binding energy of *COOH, the release of

3

CO on Ag-S-C3N4 is still exergonic, which is a clear indication of its excellent

4

eCO2RR performance from the thermodynamic point of view.[48, 49] In brief, there

5

are two effects by using Ag nanoparticles. The interfacial electron transfer from Ag to

6

S-C3N4 contributes to the conductivity for the catalytic system. Besides, the

7

decoration of Ag nanoparticles tunes the *COOH binding energy and leads to a high

8

selectivity of CO production.

9

Electrochemical impedance spectroscopy (EIS) was further conducted at

10

-0.8VRHE to elucidate the excellent reactivity against CO2 and intermediate species of

11

the

12

Ag-S-C3N4/CNT, S-C3N4/CNT, and C3N4/CNT are compared in Figure 4a. Compared

13

with the C3N4/CNT and S-C3N4/CNT nanocomposites, an apparent smaller charge

14

transfer resistance (Rct) is observed on the Ag-S-C3N4/CNT, indicating the decoration

15

of Ag nanoparticles induces an enhanced electrocatalytic ability and a faster electron

16

exchange between the catalysts and CO2. In addition, the EIS Bode plots (Figure S12)

17

reveal the S-C3N4/CNT possesses a smaller resistance than the C3N4/CNT at the

18

high-frequency region, which further proves the conductivity improvement originated

19

from the sulfur doping effect. Figure 4b shows the Tafel plots, the large Tafel slope of

20

pristine C3N4 (504mV/dec) is decreased to 460mV/dec after S doping and further

21

down to 254mV/dec with Ag decoration, suggesting more favorable kinetics for CO

22

production.[50] However, after combining the CNT, the Tafel slope decrease

23

drastically to 158 mV/dec for S-C3N4/CNT, and 136mV/dec for Ag-S-C3N4/CNT,

24

which means the CNT plays a vital role in enhancing the eCO2RR ability. Owing to

25

the excellent electrical properties and reactivity against CO2 and intermediate species,

26

the eCO2RR to CO catalytic performances of our synthesized Ag-S-C3N4/CNT is at

27

the forefront comparing with the recent reported carbon-based composite

28

nanomaterials,[31, 35-37, 43, 51-56] and this partial CO current density also

29

represents the breakthrough performance among all the C3N4-derivatives (Figure 4c

30

and Table S2). [31, 35-37]

Ag-S-C3N4/CNT nanocomposite.

The

9

corresponding

Nyquist

plots

of

1

In summary, the doping of the sulfur element not only increases the conductivity

2

of C3N4 but also leads to a down-shifted band structure of C3N4. When C3N4 forms a

3

junction between Ag and CNT that possess a relatively smaller work function, the

4

Ag-S-C3N4, and S-C3N4/CNT interfaces can accumulate electrons from Ag and CNT,

5

making the catalytic system more conductive. In addition, the decoration of Ag

6

nanoparticles enhances the selectivity of CO production by facilitating the release of

7

CO on the surface of Ag-S-C3N4. Basically, the composite material promotes the

8

electron transfer inside the catalyst and stabilizes the intermediates during the

9

eCO2RR process.

10

2.4 eCO2RR in the flow cell

11

The electrocatalytic CO2 reduction to useful products is an efficient approach to

12

alleviate the greenhouse effect. Here, to achieve an industrial-scale eCO2RR current

13

density (j > 200 mA cm-2), a homemade flow cell was employed for continuous CO

14

generation (Figure 5a, b, and S13) at ambient pressure and room temperature. The

15

operation and frameworks of this eCO2RR flow cell are described in the experimental

16

section. As depicted in Figure 5b, Ag-S-C3N4/CNT nanocomposites are coated on the

17

porous and superhydrophobic gas diffusion electrode (GDE) to form a

18

well-engineered catalysts-electrolyte interface, where gaseous CO2 is fed on directly.

19

The porous GDE structure guarantees efficient CO2/CO transportation in/out the

20

active triple phase boundary, providing opportunities to boost eCO2RR current density

21

to industrial level in normal temperature and pressure conditions.[19] In addition, by

22

connecting the CO2 flow cell in parallel to form electrolyzer stacks, the CO

23

production capacity can be controlled flexibly.[7]

24

Figure 5 is the eCO2RR performance using Ag-S-C3N4/CNT nanocomposites in

25

the flow cell. With the increase of cell voltage, the overall and partial current densities

26

of eCO2RR increase accordingly and exceed 200 mA cm-2 at 2.9 V. The FE (CO)

27

varies with voltages, but always maintains above 80% and peaks at 2.8 V with the

28

maximum value ~93%. To the best of our knowledge, this eCO2RR performance

29

demonstrates the highest reaction rate achieved so far with C3N4-based

30

nanocomposites, and it is also one of the best results compared with the reported 10

1

literature.[57] Besides, the electricity-to-CO (ETC) efficiency of Ag-S-C3N4/CNT in

2

flow cell was calculated to stay above 40% in a wide potential range, which is

3

comparable to the excellent works reported recently.[57-59] The long-term stability

4

test demonstrates a stable eCO2RR performance without severe decay in both current

5

density and FE (CO) at the cell voltage of 2.9V for 24 hours (Figure 5e).

6 7

3. Conclusions

8

In summary, highly active and selective Ag-S-C3N4/CNT nanocomposites were

9

synthesized by an economic method for efficient eCO2RR. The Ag-S-C3N4/CNT not

10

only achieved a remarkable eCO2RR performance in H-type cell but also

11

demonstrated an industrial-scale current density in flow cell configuration with

12

excellent selectivity and stability, and this performance represents the best eCO2RR

13

performance by using C3N4-based materials so far. The experimental results and DFT

14

calculations reveal the efficient eCO2RR performance comes from the synergetic

15

effect of Ag nanoparticles, sulfur elements, C3N4 scaffold, and CNT supports, where

16

the enhanced intrinsic electrical properties and CO2 reactivity have promoted the

17

electron transfer and stabilized the reaction intermediates. This study has led to a

18

deeper understanding of the eCO2RR mechanism on the C3N4-derivates, which will

19

also be instructive for other electrocatalytic applications. In brief, the highly efficient

20

Ag-S-C3N4/CNT shows a potential of industrial-scale CO production and paves the

21

way for a closing anthropogenic carbon cycle.

22 23 24

4. Experimental Section

25

4.1 Materials

26

Silver nitrate (AgNO3, 99.8%) was provided by Showa. Thiourea (99%) and urea

27

(99%) were purchased from Alfa Aesar. Sodium borohydride (NaBH4, 98.0%),

28

potassium bicarbonate (KHCO3, 99.7%), sodium citrate (99.0%), potassium

29

hydroxide (KOH, 99%) and Nafion perfluorinated ion-exchange membrane (0.002 in) 11

1

were supplied by Sigma-Aldrich. Ruthenium oxide (RuO2, 99.9%) was from Macklin.

2

Nafion perfluorinated resin solution (5.0 wt %), carbon fiber paper (0.21 mm) and gas

3

diffusion electrode (GDE) (HCP120) were purchased from Hesen Electric Co., Ltd..

4

High purity CO2 (99.99%), Ar (99.99%) and He (99.99%) were supplied by Linde

5

HKG Ltd. Carbon Nanotube-COOH (GCM340 >98 wt %, multi-walled, -COOH

6

content: 3.86 wt %) was provided by Carbon nanotubes plus. DI water generated by a

7

Millipore Direct Q-5 purification system was used to prepare solutions and rinse

8

electrodes.

9

4.2 Characterizations

10

High resolution of TEM, HAADF images, energy dispersive spectroscopy (EDS)

11

spectrum and selected area electron diffraction (SAED) were carried out by

12

transmission electron microscope (FEI Tecnai F30). X-ray photoelectron spectroscopy

13

(XPS) was obtained by KRATOS. All electrochemical tests were conducted by CHI

14

660E electrochemical workstation (CH Instruments, Inc., U.S.A.). The flow cell was

15

fabricated by laser cutter from Tech-Labs (VLS 2.30). The flow rate of gas and

16

electrolyte were controlled by a high-accuracy mass flow controller (MFC YJ-700

17

Konxin) and syringe pump (NE-100 NewEra). Gas products were analyzed by gas

18

chromatograph (GC, SRI instruments 8610C) equipped with helium ionization

19

detector (HID) and thermal conductivity detector (TCD). The work function values

20

were measured by Scanning Kelvin Probe System (SKP5050 KP Technology).

21

4.3 Preparation of the nanocomposites

22

The pristine g-C3N4 (S-doped g-C3N4) was synthesized by heating urea (thiourea)

23

as a precursor in a covered crucible bowl at 550 ℃ for 4 h at a ramping rate of 3 ℃

24

min-1 and then cooled down to room temperature naturally. For the preparation of

25

C3N4/CNT (S-C3N4/CNT), urea (thiourea) precursor and carboxyl-functionalized

26

MWCNTs (CNT-COOH) were dispersed in DI water (10 mg/mL) with 1:1 mass ratio,

27

the resultant suspension was firstly ultra-sonicated for 0.5 h and then stirred

28

vigorously for another 0.5 h at room temperature. Herein, CNT-COOH was used as

29

the catalyst supporting network to increase electron transport; the carboxyl groups

30

could improve the dispersion of CNT in DI water, allowing effective contact between 12

1

CNT-COOH and precursor. [60] The stirring temperature was then raised to 70 ℃

2

until the water was completely evaporated so that the urea (thiourea) precursor would

3

be anchored on the CNT-COOH. During this process, the existence of carboxyl

4

groups could provide better interfacial binding strength, which is helpful for the

5

combination of precursor and CNT-COOH. [61, 62] After an overnight vacuum

6

drying procedure, urea (thiourea) was loaded on CNT-COOH (Figure S1). The

7

C3N4/CNT (S-C3N4/CNT)

8

(thiourea)/CNT-COOH composite powder to the covered crucible in a tube furnace

9

and then heating at 550 ℃ for 4 h at a ramping rate of 3 ℃ min-1 under Ar atmosphere,

10

where the precursor was decomposed to g-C3N4 (S-doped g-C3N4) while CNT-COOH

11

was reduced to CNT.

was

finally

obtained

by

transferring

the

urea

12

For preparing the Ag-C3N4, Ag-S-C3N4, Ag-C3N4/CNT, Ag-S-C3N4/CNT, and

13

Ag/CNT nanocomposites (Ag, 5 wt %), similar approaches were employed to

14

decorate Ag nanoparticles on the prepared C3N4, S-C3N4, C3N4/CNT, S-C3N4/CNT,

15

and bare CNT, respectively. First, 100 mg resultant powder was dissolved in 100mL

16

DI water with the aid of sonication, after which 0.59 g sodium citrate and 7.9 mg

17

AgNO3 were added in the aqueous solution with 1 h stir at room temperature.

18

Afterward, freshly prepared 20mL 0.1M NaBH4 was dripping into the solution to

19

reduce the Ag, then stirred for another 1h. The products were collected by vacuum

20

filtration and washed with DI water several times. The catalysts were finally obtained

21

after drying at 80 ℃ in a vacuum oven for 12 h.

22 23

4.4 Electrochemical measurements

24

4.4.1 Electrocatalytic activity tests

25

All electrocatalytic tests were performed in a standard three-electrode system by

26

using CHI 660E. A gas-tight H-type electrochemical cell with two compartments

27

separated by ion exchange membrane was used in these tests. A platinum plate was

28

invoked as a counter electrode, a saturated calomel electrode (SCE) was used as the

29

reference electrode and catalysts modified carbon fiber paper (CFP) was used as a

30

working electrode. Catalyst inks were prepared by dispersing 5 mg catalyst powder in 13

1

900 µL a mixture of DI water and ethanol (1:1 v/v), and then 100 µL 0.5 wt% of

2

Nafion solution was added with the assistance of sonication. 100 µL of the

3

homogeneous catalyst ink was then dropped cast onto the CFP with an area of 1 cm2

4

(0.5 mg cm-2). The catalysts/CFP electrode was cyclic scanned until the curves

5

become stable before eCO2RR measurements. Linear sweep voltammetry (LSV) tests

6

were performed in CO2-saturated (pH = 6.8) or Ar-saturated (pH = 8.8) 0.1 M KHCO3

7

electrolyte. Chronoamperometry measurements were carried out in CO2-saturated

8

0.1M KHCO3. High purity CO2 was bubbled into the solution continuously at 20

9

sccm by using MFC, and the electrolyte was stirred vigorously to ensure the sufficient

10

mass transport to and from the working electrode. All potentials were measured by

11

SCE and then converted to RHE by the following equation: ERHE = ESCE + 0.242 V +

12

0.059 × pH. IR compensation was always performed before each test to assess

13

accurate electrode potential.

14 15

4.4.2 eCO2RR scaling up in the flow cell

16

The scaling up of eCO2RR to CO was performed in a homemade three-channel

17

flow cell. The eCO2RR cathode and OER anode were prepared by drop-casting the

18

homogeneous catalyst and RuO2 ink (commercial OER catalysts) onto the GDE to a

19

loading amount 1 mg cm-2. The cathode and anode were separated by ion exchange

20

membrane and the distance was set about 4 mm. The catholyte (CO2 saturated 1.0M

21

KHCO3) and anolyte (1.0M KOH) were delivered by syringe pumps at 3 mL min-1.

22

CO2 gas flow was controlled at 20 sccm by MFC.

23

The electricity-to-CO (ETC) efficiency (η) of the flow cell system was defined as

24

the ratio of the chemical energy to the applied electrical energy, which can be

25

calculated based on equation (1):[58, 59]

26

η (%) =

E chem E 0 × FE(CO) = CO × 100 Eapplied Vapplied

(1)

27

Where the Echem represents the chemical energy stored in the eCO2RR to CO

28

process and Eapplied represents the input electrical energy. E0CO is the equilibrium

29

potential for eCO2RR to CO which equals to 1.34 V. Vapplied stands for the working 14

1

voltage. FE (CO) is the Faradaic efficiency of CO.

2 3

4.5 Electrochemical data analysis:

4

Gas products were quantified by using GC (SRI instruments 8610C) equipped

5

with HID and TCD. High purity helium (99.99%) was used as the carrier gas. The

6

partial current densities of CO and H2 production were calculated from the GC peak

7

areas according to the equation (2) and (3). The Faradaic efficiency (FE) of products

8

was calculated by the equation (4):[5]

9

jCO =

10

jH 2 =

Peak area

α Peak area

β

× Flow rate × (

2 Fp ) × (Electrode area) −1 RT

(2)

× Flow rate × (

2 Fp ) × (Electrode area)−1 RT

(3)

FE (%) =

11

nFx×flow rate × 100 jtotal

(4)

12

Where α and β are conversion factors for CO and H2 determined with the

13

calibration results of GC by standard gases, p =1.013 bar, and T =298.15K. In

14

equation (3), n is the number of electrons changed (n = 2 for eCO2RR to CO), F is the

15

Faraday’s constant (96485 C mol-1), x is the mole fraction of a specific product and

16

jtotal is the total current.

17 18 19

4.6 Density functional theory calculations Density functional theory (DFT) calculations were performed with the

20

generalized

gradient

approximation

(GGA)

21

Perdew-Burke-Ernzerhof (PBE) using the plane-wave basis Vienna Ab initio

22

Simulation Package (VASP) [63, 64]. The project-augmented wave (PAW) method

23

was used to treat the core and valence electron interaction [65]. A plane wave cutoff

24

energy of 500 eV was used in all DFT calculations. The Brillouin zone integrations

25

were performed with a k-point density of 2π ×0.03 Å−1 adopting the Γ-centered

26

Monkhorst-Pack scheme. A Gaussian smearing of 0.20 eV was applied during the

27

geometry optimization, while for accurate density of states (DOS) computations, a 15

in

the

parametrization

of

1

tetrahedron method with Blöchl correction was employed. The convergence criterion

2

of the electronic self-consistent iteration was set to 10-5 eV. We used a 2×2 supercell

3

of graphitic carbon nitride (g-C3N4) to perform the simulations. The height of slab

4

was set to 15 Å to ensure negligible interaction between adjacent slabs and dipole

5

corrections were employed. One N atom was replaced by S atoms in the g-C3N4

6

monolayer to simulate S-doped C3N4 (S-C3N4). To simulate the Ag-S-C3N4 composite,

7

a Ag nanoparticle (Ag8) [66] was stacked atop the S atom on S-C3N4 . Our simulation

8

models are shown in Figure SN2 of the Supporting Information (SI). The dispersion

9

effects have been included by adopting a damped vdW correction (PBE-D2) proposed

10

by Grimme [67]. The atomic charge and the electron transfer were calculated using

11

the Bader charge analysis code developed by the Henkelman group [68].

12 13 14

ASSOCIATED CONTENT

15

Supporting Information.

16

The following files are available free of charge.

17

CV, SEM, EDX, TEM, SAED, and XPS

18

AUTHOR INFORMATION

19

Author Contributions

20

All authors have given approval to the final version of the manuscript. †These authors

21

contributed equally. J.N.C. and S.P.F. developed the concept and designed the

22

experiments. J.N.C., Z.Y.W., H.S.L., and M.Y.Z performed the experiments. J.N.C.,

23

Z.Y.W., and Z.G.L. contributed to the material characterizations. J.J.M., J.N.C., and

24

Y.C. contributed to the DFT calculations. J.N.C., H.S.L., C.A.G., J.J.M., Y.C.,

25

Z.G.L., and S.P.F. contributed to the interpretation of the results. J.N.C., Z.Y.W., and

26

C.L. contributed to the construction of the experimental platform. J.N.C., H.S.L., and

27

S.P.F. co-wrote the manuscript.

28 16

1 2

Acknowledgments

3

The authors acknowledge the constructive discussions with Prof. Z. G. Lu (Southern

4

University of Science and Technology) and Dr. Craig A. Grimes (Flux Photon

5

Corporation). The authors also acknowledge the financial support of the General

6

Research Fund of the Research Grants Council of Hong Kong Special Administrative

7

Region, China under Award Number 17204516 and 17206518, and Environment and

8

Conservation Fund (ECF 49/2017). This work was also partially supported by

9

HKU-Zhejiang Institute of Research and Innovation (HKU-ZIRI). JM and YC are

10

grateful for the research computing facilities offered by ITS, HKU.

11 12 13 14

Conflict of Interest

15

The authors declare no conflict of interests.

16 17

References

18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

[1] G. Pearman, D. Etheridge, F. De Silva, P. Fraser, Evidence of changing concentrations of atmospheric CO2, N2O and CH4 from air bubbles in Antarctic ice, Nature, 320 (1986) 248. [2] S.C. Peter, Reduction of CO2 to Chemicals and Fuels: A Solution to Global Warming and Energy Crisis, ACS Energy Letters, 3 (2018) 1557-1561. [3] S.J. Davis, K. Caldeira, H.D. Matthews, Future CO2 emissions and climate change from existing energy infrastructure, Science, 329 (2010) 1330-1333. [4] S. Chu, Y. Cui, N. Liu, The path towards sustainable energy, Nature materials, 16 (2017) 16. [5] X. Zhang, Z. Wu, X. Zhang, L. Li, Y. Li, H. Xu, X. Li, X. Yu, Z. Zhang, Y. Liang, Highly selective and active CO 2 reduction electrocatalysts based on cobalt phthalocyanine/carbon nanotube hybrid structures, Nature communications, 8 (2017) 14675. [6] E. Anagnostou, E.H. John, K.M. Edgar, G.L. Foster, A. Ridgwell, G.N. Inglis, R.D. Pancost, D.J. Lunt, P.N. Pearson, Changing atmospheric CO 2 concentration was the primary driver of early Cenozoic climate, Nature, 533 (2016) 380. [7] S. Verma, B. Kim, H.R.M. Jhong, S. Ma, P.J. Kenis, A Gross

Margin Model for Defining

Technoeconomic Benchmarks in the Electroreduction of CO2, ChemSusChem, 9 (2016) 1972-1979. [8] T. Burdyny, W.A. Smith, CO 2 reduction on gas-diffusion electrodes and why catalytic performance 17

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44

must be assessed at commercially-relevant conditions, Energy & Environmental Science, (2019). [9] J.T. Song, H. Song, B. Kim, J. Oh, Towards Higher Rate Electrochemical CO2 Conversion: From Liquid-Phase to Gas-Phase Systems, Catalysts, 9 (2019) 224. [10] W. Zhang, Y. Hu, L. Ma, G. Zhu, Y. Wang, X. Xue, R. Chen, S. Yang, Z. Jin, Progress and perspective of electrocatalytic CO2 reduction for renewable carbonaceous fuels and chemicals, Advanced Science, 5 (2018) 1700275. [11] K. Jiang, S. Siahrostami, T. Zheng, Y. Hu, S. Hwang, E. Stavitski, Y. Peng, J. Dynes, M. Gangisetty, D. Su, Isolated Ni single atoms in graphene nanosheets for high-performance CO 2 reduction, Energy & Environmental Science, 11 (2018) 893-903. [12] D.M. Weekes, D.A. Salvatore, A. Reyes, A. Huang, C.P. Berlinguette, Electrolytic CO2 reduction in a flow cell, Accounts Chem Res, 51 (2018) 910-918. [13] B.A. Rosen, A. Salehi-Khojin, M.R. Thorson, W. Zhu, D.T. Whipple, P.J. Kenis, R.I. Masel, Ionic liquid–mediated selective conversion of CO2 to CO at low overpotentials, Science, 334 (2011) 643-644. [14] T. Zheng, K. Jiang, N. Ta, Y. Hu, J. Zeng, J. Liu, H. Wang, Large-Scale and Highly Selective CO2 Electrocatalytic Reduction on Nickel Single-Atom Catalyst, Joule, 3 (2019) 265-278. [15] D.T. Whipple, E.C. Finke, P.J. Kenis, Microfluidic reactor for the electrochemical reduction of carbon dioxide: the effect of pH, Electrochemical and Solid-State Letters, 13 (2010) B109-B111. [16] K.P. Kuhl, T. Hatsukade, E.R. Cave, D.N. Abram, J. Kibsgaard, T.F. Jaramillo, Electrocatalytic conversion of carbon dioxide to methane and methanol on transition metal surfaces, Journal of the American Chemical Society, 136 (2014) 14107-14113. [17] Z. Weng, Y. Wu, M. Wang, J. Jiang, K. Yang, S. Huo, X.-F. Wang, Q. Ma, G.W. Brudvig, V.S. Batista, Active sites of copper-complex catalytic materials for electrochemical carbon dioxide reduction, Nature communications, 9 (2018) 415. [18] C.-T. Dinh, T. Burdyny, M.G. Kibria, A. Seifitokaldani, C.M. Gabardo, F.P.G. de Arquer, A. Kiani, J.P. Edwards, P. De Luna, O.S. Bushuyev, CO2 electroreduction to ethylene via hydroxide-mediated copper catalysis at an abrupt interface, Science, 360 (2018) 783-787. [19] M. Jouny, W. Luc, F. Jiao, High-rate electroreduction of carbon monoxide to multi-carbon products, Nature Catalysis, 1 (2018) 748. [20] J. Wu, S. Ma, J. Sun, J.I. Gold, C. Tiwary, B. Kim, L. Zhu, N. Chopra, I.N. Odeh, R. Vajtai, A metal-free electrocatalyst for carbon dioxide reduction to multi-carbon hydrocarbons and oxygenates, Nature communications, 7 (2016) 13869. [21] A. Wiheeb, Z. Helwani, J. Kim, M. Othman, Pressure swing adsorption technologies for carbon dioxide capture, Separation & Purification Reviews, 45 (2016) 108-121. [22] J.M. Spurgeon, B. Kumar, A comparative technoeconomic analysis of pathways for commercial electrochemical CO 2 reduction to liquid products, Energy & Environmental Science, 11 (2018) 1536-1551. [23] M. Jouny, W. Luc, F. Jiao, General techno-economic analysis of CO2 electrolysis systems, Industrial & Engineering Chemistry Research, 57 (2018) 2165-2177. [24] S. Zhu, Q. Wang, X. Qin, M. Gu, R. Tao, B.P. Lee, L. Zhang, Y. Yao, T. Li, M. Shao, Tuning Structural and Compositional Effects in Pd–Au Nanowires for Highly Selective and Active CO2 Electrochemical Reduction Reaction, Advanced Energy Materials, 8 (2018) 1802238. [25] L. Dai, Carbon-based metal-free catalysts: design and applications, John Wiley & Sons2018. [26] F. Yu, L. Yu, I. Mishra, Y. Yu, Z. Ren, H.J.M.T.P. Zhou, Recent developments in earth-abundant and non-noble electrocatalysts for water electrolysis, Materials Today Physics, 7 (2018) 121-138. 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44

[27] Y.R. Poudel, W.J.M.T.P. Li, Synthesis, properties, and applications of carbon nanotubes filled with foreign materials: a review, Materials Today Physics, 7 (2018) 7-34. [28] Y. Zheng, Y. Jiao, Y. Zhu, L.H. Li, Y. Han, Y. Chen, A. Du, M. Jaroniec, S.Z. Qiao, Hydrogen evolution by a metal-free electrocatalyst, Nature communications, 5 (2014) 3783. [29] M. Green, Z. Liu, R. Smedley, H. Nawaz, X. Li, F. Huang, X.J.M.T.P. Chen, Graphitic carbon nitride nanosheets for microwave absorption, Materials Today Physics, 5 (2018) 78-86. [30] Y. Jiao, Y. Zheng, S.C. Smith, A. Du, Z. Zhu, Electrocatalytically switchable CO2 capture: first principle computational exploration of carbon nanotubes with pyridinic nitrogen, ChemSusChem, 7 (2014) 435-441. [31] Y. Jiao, Y. Zheng, P. Chen, M. Jaroniec, S.-Z. Qiao, Molecular scaffolding strategy with synergistic active centers to facilitate electrocatalytic CO2 reduction to hydrocarbon/alcohol, J Am Chem Soc, 139 (2017) 18093-18100. [32] Y. Zheng, Y. Jiao, J. Chen, J. Liu, J. Liang, A. Du, W. Zhang, Z. Zhu, S.C. Smith, M. Jaroniec, Nanoporous graphitic-C3N4@ carbon metal-free electrocatalysts for highly efficient oxygen reduction, J Am Chem Soc, 133 (2011) 20116-20119. [33] Z. Pei, J. Zhao, Y. Huang, Y. Huang, M. Zhu, Z. Wang, Z. Chen, C. Zhi, Toward enhanced activity of a graphitic carbon nitride-based electrocatalyst in oxygen reduction and hydrogen evolution reactions via atomic sulfur doping, Journal of Materials Chemistry A, 4 (2016) 12205-12211. [34] C. Xu, Q. Han, Y. Zhao, L. Wang, Y. Li, L. Qu, Sulfur-doped graphitic carbon nitride decorated with graphene quantum dots for an efficient metal-free electrocatalyst, Journal of Materials Chemistry A, 3 (2015) 1841-1846. [35] X. Lu, T.H. Tan, Y.H. Ng, R. Amal, Highly selective and stable reduction of CO2 to CO by a graphitic carbon nitride/carbon nanotube composite electrocatalyst, Chemistry–A European Journal, 22 (2016) 11991-11996. [36] B. Zhang, T.-J. Zhao, W.-J. Feng, Y.-X. Liu, H.-H. Wang, H. Su, L.-B. Lv, X.-H. Li, J.-S. Chen, Polarized few-layer gC 3 N 4 as metal-free electrocatalyst for highly efficient reduction of CO 2, Nano Research, 11 (2018) 2450-2459. [37] S. Zhao, Z. Tang, S. Guo, M. Han, C. Zhu, Y. Zhou, L. Bai, J. Gao, H. Huang, Y. Li, Enhanced Activity for CO2 Electroreduction on a Highly Active and Stable Ternary Au-CDots-C3N4 Electrocatalyst, ACS Catalysis, 8 (2017) 188-197. [38] Y. Wang, R. Ou, H. Wang, T. Xu, Graphene oxide modified graphitic carbon nitride as a modifier for thin film composite forward osmosis membrane, Journal of Membrane Science, 475 (2015) 281-289. [39] G. Gao, M. Pan, C.D. Vecitis, Effect of the oxidation approach on carbon nanotube surface functional groups and electrooxidative filtration performance, Journal of Materials Chemistry A, 3 (2015) 7575-7582. [40] L. Cao, R. Wang, D. Wang, Synthesis and characterization of sulfur self-doped g-C3N4 with efficient visible-light photocatalytic activity, Materials Letters, 149 (2015) 50-53. [41] Q. Xu, P. Pu, J. Zhao, C. Dong, C. Gao, Y. Chen, J. Chen, Y. Liu, H. Zhou, Preparation of highly photoluminescent sulfur-doped carbon dots for Fe (III) detection, Journal of Materials Chemistry A, 3 (2015) 542-546. [42] J. Feng, D. Fan, Q. Wang, L. Ma, W. Wei, J. Xie, J.J.C. Zhu, S.A. Physicochemical, E. Aspects, Facile synthesis silver nanoparticles on different xerogel supports as highly efficient catalysts for the reduction of p-nitrophenol, 520 (2017) 743-756. [43] L. Zhang, F. Mao, L.R. Zheng, H.F. Wang, X.H. Yang, H.G.J.A.C. Yang, Tuning Metal Catalyst with 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44

Metal–C3N4 Interaction for Efficient CO2 Electroreduction, 8 (2018) 11035-11041. [44] Q. Liang, Z. Li, X. Yu, Z.H. Huang, F. Kang, Q.H.J.A.M. Yang, Macroscopic 3D porous graphitic carbon nitride monolith for enhanced photocatalytic hydrogen evolution, 27 (2015) 4634-4639. [45] T.Y. Ma, S. Dai, M. Jaroniec, S.Z.J.A.C.I.E. Qiao, Graphitic carbon nitride nanosheet nanotube three

dimensional porous composites as high

carbon

performance oxygen evolution

electrocatalysts, 53 (2014) 7281-7285. [46] L. Zhang, F. Mao, L.R. Zheng, H.F. Wang, X.H. Yang, H.G. Yang, Tuning Metal Catalyst with Metal– C3N4 Interaction for Efficient CO2 Electroreduction, ACS Catalysis, 8 (2018) 11035-11041. [47] P. Su, K. Iwase, T. Harada, K. Kamiya, S. Nakanishi, Covalent triazine framework modified with coordinatively-unsaturated Co or Ni atoms for CO 2 electrochemical reduction, Chemical Science, 9 (2018) 3941-3947. [48] H.A. Hansen, J.B. Varley, A.A. Peterson, J.K. Nørskov, Understanding trends in the electrocatalytic activity of metals and enzymes for CO2 reduction to CO, The journal of physical chemistry letters, 4 (2013) 388-392. [49] J.T. Feaster, C. Shi, E.R. Cave, T. Hatsukade, D.N. Abram, K.P. Kuhl, C. Hahn, J.K. Nørskov, T.F. Jaramillo, Understanding selectivity for the electrochemical reduction of carbon dioxide to formic acid and carbon monoxide on metal electrodes, Acs Catalysis, 7 (2017) 4822-4827. [50] H. Yang, Y.w. Hu, J.j. Chen, M.S. Balogun, P.p. Fang, S. Zhang, J. Chen, Y. Tong, Intermediates Adsorption Engineering of CO2 Electroreduction Reaction in Highly Selective Heterostructure Cu Based Electrocatalysts for CO Production, Advanced Energy Materials, (2019) 1901396. [51] Z. Chen, K. Mou, X. Wang, L. Liu, Nitrogen

Doped Graphene Quantum Dots Enhance the Activity

of Bi2O3 Nanosheets for Electrochemical Reduction of CO2 in a Wide Negative Potential Region, Angewandte Chemie International Edition, 57 (2018) 12790-12794. [52] W. Bi, X. Li, R. You, M. Chen, R. Yuan, W. Huang, X. Wu, W. Chu, C. Wu, Y. Xie, Surface Immobilization of Transition Metal Ions on Nitrogen

Doped Graphene Realizing High

Efficient and

Selective CO2 Reduction, Adv Mater, 30 (2018) 1706617. [53] Z. Ma, C. Lian, D.F. Niu, L. Shi, S. Hu, X. Zhang, H. Liu, Enhancing CO2 electroreduction with Au/pyridine/carbon nanotubes hybrid structures, ChemSusChem, (2019). [54] J. Choi, P. Wagner, S. Gambhir, R. Jalili, D.R. Macfarlane, G.G. Wallace, D.L. Officer, Steric Modification of a Cobalt Phthalocyanine/Graphene Catalyst to Give Enhanced and Stable Electrochemical CO2 Reduction to CO, ACS Energy Letters, (2019). [55] Z. Chen, K. Mou, S. Yao, L. Liu, Zinc

Coordinated Nitrogen

Codoped Graphene as an Efficient

Catalyst for Selective Electrochemical Reduction of CO2 to CO, ChemSusChem, 11 (2018) 2944-2952. [56] F. Pan, B. Li, X. Xiang, G. Wang, Y. Li, Efficient CO2 Electroreduction by Highly Dense and Active Pyridinic Nitrogen on Holey Carbon Layers with Fluorine Engineering, ACS Catalysis, (2019). [57] Z. Yin, H. Peng, X. Wei, H. Zhou, J. Gong, M. Huai, X. Li, G. Wang, J. Lu, L. Zhuang, Alkaline Polymer Electrolyte CO2 Electrolyzer Operated with Pure Water, Energy & Environmental Science, (2019). [58] C. Zhao, Y. Wang, Z. Li, W. Chen, Q. Xu, D. He, D. Xi, Q. Zhang, T. Yuan, Y. Qu, Solid-Diffusion Synthesis of Single-Atom Catalysts Directly from Bulk Metal for Efficient CO2 Reduction, Joule, (2018). [59] Y. Wang, J. Liu, Y. Wang, Y. Wang, G. Zheng, Efficient solar-driven electrocatalytic CO 2 reduction in a redox-medium-assisted system, Nature communications, 9 (2018) 5003. [60] Y. Wang, Z. Iqbal, S. Mitra, Rapidly functionalized, water-dispersed carbon nanotubes at high concentration, Journal of the American Chemical Society, 128 (2006) 95-99. [61] C. Gao, Z. Guo, J.-H. Liu, X.-J. Huang, The new age of carbon nanotubes: an updated review of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14

functionalized carbon nanotubes in electrochemical sensors, Nanoscale, 4 (2012) 1948-1963. [62] Y. Xing, L. Li, C.C. Chusuei, R.V. Hull, Sonochemical oxidation of multiwalled carbon nanotubes, Langmuir, 21 (2005) 4185-4190. [63] G. Kresse, J. Furthmüller, Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set, Comp Mater Sci, 6 (1996) 15-50. [64] G. Kresse, J. Furthmüller, Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set, Physical review B, 54 (1996) 11169. [65] P.E. Blöchl, Projector augmented-wave method, Physical review B, 50 (1994) 17953. [66] V. Bonačić-Koutecky, V. Veyret, R. Mitrić, Ab initio study of the absorption spectra of Ag n (n= 5–8) clusters, The Journal of Chemical Physics, 115 (2001) 10450-10460. [67] S. Grimme, Semiempirical GGA

type density functional constructed with a long

range

dispersion correction, J Comput Chem, 27 (2006) 1787-1799. [68] G. Henkelman, A. Arnaldsson, H. Jónsson, A fast and robust algorithm for Bader decomposition of charge density, Comp Mater Sci, 36 (2006) 354-360.

15

16 17 18 19 20

Figure 1. (a) TEM, (b) HR-TEM images of the Ag-S-C3N4/CNT nanocomposites. (c) Schematic of eCO2RR to CO on Ag-S-C3N4/CNT nanocomposites. (d) Annular dark-field scanning transmission microscope (HAADF-STEM) image and EDS mapping of the (e) overlap elements, (f) C, (g) N, (h) S, and (i) Ag, respectively, of 21

1 2 3 4

the Ag-S-C3N4/CNT nanocomposites.

5 6 7 8 9 10 11 12 13

Figure 2. eCO2RR performance of Ag-S-C3N4/CNT nanocomposites. (a) LSV curves comparison, scan rate=10mV/s. (b) LSV at 10mV/s in Ar- and CO2-saturated 0.1M KHCO3 solution. (c) Chronoamperograms at -0.6V,-0.7V, -0.8V and -0.9V versus RHE. (d) Faradaic efficiencies comparison with bare CNT. All the measurements are carried out in CO2 saturated 0.1M KHCO3 solution.

22

1 2 3 4 5 6 7 8 9 10 11 12

Figure 3. Band structures of (a) C3N4, (b) S-C3N4, and (c) Ag-S-C3N4 nanocomposites; (d) density of states (DOS) of C3N4, S-C3N4, and Ag-S-C3N4; Fermi energy locates at 0 eV. Charge density difference (CDD) between the adsorbed system and those of the separated entities for (e) S-C3N4 and (f) Ag-S-C3N4, where yellow and cyan isosurfaces represent electron accumulation and depletion, respectively; the isosurface value is 0.0004 e Å−3. (g) The calculated Gibbs free-energy diagram for CO2RR to CO catalyzed by C3N4, S-C3N4, and Ag-S-C3N4; Brown, light blue, yellow, and light grey balls represent C, N, S, and Ag atoms, respectively.

23

1 2 3 4 5 6 7 8 9

Figure 4. (a) EIS Nyquist plots of Ag-S-C3N4/CNT, S-C3N4/CNT, and C3N4/CNT nanocomposites, the inset shows the corresponding Randles equivalent circuit. EIS data were collected at -0.8V vs. RHE. (b) Tafel plots for C3N4, S-C3N4, Ag-S-C3N4, S-C3N4/CNT, and Ag-S-C3N4/CNT nanocomposites at different applied potentials. All the measurements are carried out in CO2 saturated 0.1M KHCO3 solution. (c) CO partial current density comparison with other recently reported carbon-based nanocomposites for eCO2RR to CO. Blue area represents the C3N4-based materials.

24

1 2 3 4 5 6 7

Figure 5. (a) Photograph and (b) schematic representation of the CO2 flow cell electrolyzer used in this study. (c) Overall and partial current densities, (d) the corresponding FEs and ETC efficiency of Ag-S-C3N4/CNT nanocomposites on gas diffusion electrode in the CO2 flow electrolyzer. (e) Long-term electrolysis tests under a cell voltage of 2.9 V.

8

25

Highlights:
C3N4-based nanomaterial has been developed as an efficient catalyst towards electrochemical CO2 reduction reaction
Systematical studies were carried out to understand the catalytic mechanism of the C3N4-derivates.
The best electrochemical CO2 reduction performance among the C3N4-based materials was achieved by using the flow cell configuration.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: