Uniform Nitrogen and Sulfur Co-doped Carbon Bowls for the

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Uniform nitrogen and sulfur co-doped carbon bowls for the electrocatalyzation of oxygen reduction Xiaobao Li, Xiaohong Gao, Peiyan Xu, Chenghang You, Wei Sun, Xianghui Wang, Qiang Lin, and Shijun Liao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00126 • Publication Date (Web): 11 Mar 2019 Downloaded from http://pubs.acs.org on March 12, 2019

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ACS Sustainable Chemistry & Engineering

Uniform nitrogen and sulphur co-doped carbon bowls for the electrocatalyzation of oxygen reduction Xiaobao Lia, Xiaohong Gaoa, Peiyan Xua, Chenghang Youa*, Wei Suna*, Xianghui Wanga, Qiang Lina, Shijun Liaob aKey

Laboratory of Tropical Medicinal Plant Chemistry of Ministry of Education,

School of Chemistry and Chemical Engineering, Hainan Normal University, Haikou, 571158, China, E-mail: [email protected], [email protected] bThe

Key Laboratory of Fuel Cell Technology of Guangdong Province, School of

Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510641, China, E-mail: [email protected]

KEYWORDS: nitrogen and sulphur co-doped; uniform carbon bowls; oxygen reduction reaction; polyacrylonitrile

ABSTRACT Developing highly efficient and low-cost catalyst to replace platinum (Pt) is a key task for the commercialization of advanced energy systems, e.g. proton exchange membrane fuel cells, direct methanol fuel cells, metal-air batteries, and etc.. In this work, we fabricated hollow carbon bowls doped with nitrogen and sulphur by using

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polyacrylonitrile and sulphur as the precursors through a pyrolysis procedure. The obtained catalyst exhibited high ORR performance, with a half-wave potential comparable to that of Pt/C catalyst in an alkaline medium. Our catalyst also demonstrated outstanding methanol tolerance, stability, as well as high selectivity towards four-electron path. Combining with the characterization results, we suggest, the following factors should be the proper origins for our catalyst’s outstanding performance: (1) the high contents of active species, including graphitic/pyridinic N species and sulfide S (-C-S-C-), which are believed to modify the surface charge distribution, create defects and active sites; (2) the high surface area (1146 m2 g-1), which can supply more exposed active sites; (3) bowl-like morphologies, which makes the inner surface of carbon shell more accessible to electrolyte and O2.

INTRODUCTION The rapid growing consumption of tradition fuels and the global climate change have stimulated extensive efforts to develop efficient and green energy technologies1-10, e.g. fuel cells11-12, metal-air batteries13-17, and etc.. Among these advanced energy systems, oxygen reduction reaction (ORR) plays a key role, which strongly dependent on the efficiency and stability of the electrocatalysts applied. Although platinum (Pt) based catalysts have been known to be efficient for ORR, the high cost and limited reserve of Pt have seriously precluded those energy

technologies

from

commercialization18.


Therefore,

developing

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alternatives to Pt is the key to the applications of those advanced energy technologies. Nitrogen doped carbons (NC), have successfully attracted great attention due to their low cost, environmental friendliness and relative high ORR electrocatalytic performance19-24. By codoping with other hetero atoms25-27, such as B, S, P, F, and etc., NCs’ performance can be significantly improved owing to the possible synergistic effect between different heteroatoms on the catalytic activity. Among the codoping heteroatoms, the combination of N and S is interesting. On one hand, the structural defects induced by sulphur atom’s large radius can facilitate the charge dislocation and oxygen adsorption28; on the other hand, the two lone pair electrons of sulphur atom may also contribute to the interaction between doped carbon and molecular oxygen29-30. Apart from hetero doping, rationally designed morphology and porous structures are also favourable for obtaining enhanced ORR performance, which are expected to provide better electrolyte permeability, faster electron transfer and mass transport31-34. Hollow carbon spheres (HCS) with controlled nanostructures have received rapidly increasing interest due to their low density, high surface area and abundant pores located on the spherical walls30, 35. Despite of these advantages, hollow structures with excessive hollow interior space would also greatly decrease the packing density of electrode materials36 and weaken the electrolyte permeability for the surface of the cavity inside, making the internal surface cannot be fully utilized. Thus, carbon bowls, possessing the



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advantages of HCS and overcoming HCS’s limitation, can be more attractive. And, N, S co-doped carbon bowls, with well-designed porous structures, can be expected to achieve exceptional ORR performance. Based on these considerations, we designed and synthesized uniform N and S codoped carbon nanobowls by using polyacrylonitrile and sublimated sulphur as the precursors. The catalyst exhibited good electrocatalytic activity towards the ORR in alkaline medium, with its half-wave potential comparable to that of Pt/C catalyst. Significantly, the catalyst also showed outstanding stability, excellent methanol tolerance and high selectivity of four-electron path.

EXPERIMENTAL SECTION Preparation of doped carbon bowls The preparation of N and S co-doped carbon bowls is illustrated in Fig. 1. Firstly, silica nanospheres (SN) of ~200 nm were prepared through a modified Stöber method35. Briefly, 184 ml alcohol was first mixed with 8 mL deionized (DI) water and 8 mL ammonium hydroxide homogeneously in a three-neck flask, and then, 8 mL tetraethoxysilane (TEOS) was added. The mixture was stirred at 50 oC for 8 h to obtain a

white

suspension

(SN

suspension).

Subsequently,

2

mL

methacryloxypropyltrimethoxy-silane (MPS) was added and then the mixture was stirred at 50 oC for another 12 h to afford a suspension of surface-modified silica nanospheres (MSN). After that, the temperature was raised to 70 oC and N2 was introduced to exhaust the air in the flask, followed by injecting 10 mL


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acrylonitrile monomer. After a trace amount of azobisisoheptonitrile (AIBN) was added, the polymerization was continued for 6 h before the mixture was filtered, washed with DI water, and dried in a vacuum. The obtained white powder was named as “MSN/PA”. For comparisons, SN/PA was also prepared through the same procedures without MPS.

Fig. 1 Scheme illustration of N, S carbon bowls preparation

The precursor (MSN/PA) was first carbonized in a tube furnace at 600 oC and 900 oC under the protection of N2 for 4 and 1 h subsequently, followed by cooling down naturally to room temperature. 1.0 g of the obtained powder was then thoroughly mixed with certain amount of sublimed sulphur (0, 1, 3, 5 g) in a mortar and pyrolyzed at 900 oC in a N2 flow, followed by cooling down naturally, leaching with hydrofluoric acid, rinsing with DI water and drying in a vacuum. The obtained catalysts were then named as CB, CB-1S, CB-3S and CB-5S



respectively, where the “S” indicates that the catalyst contains sulphur and the number implies the amount of sulphur used in the precursor. In order to study the formation process of carbon bowls, we also prepared “CM” from MSN/PA through the same procedures to CB’s except the hydrofluoric leaching. And the catalyst “PAC” was also prepared only through the first heat treatment.

Results and discussion We first characterized the intermediates and catalysts by using the SEM. As illustrated in Fig. 2a and b, SN and MSN share the same nanospherical morphologies, suggesting that the introduction of MPS can hardly affect the morphologies of SN. After the polymerization, the former smooth surface of MSN became rough (Fig. 2c), indicating the PA has been successfully coated on the MSN, while serious aggregation has happened in SN/PA (Fig. 2d), suggesting the MPS is important for the homogeneous coating of PA. After the pyrolysis procedure, the obtained CM exhibited the same integrate nanospherical morphology to that of MSN (Fig. 2e). Their similar radius indicates the as-formed carbon shell would be rather thin. After CM was leached with hydrofluoric acid, it is interesting that the previous carbon shells turn to carbon bowls (Fig. 2f and S1). Obviously, these nano carbon bowls should originate from the fracture of the as-formed carbon shells during the removal of silica templates.


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a

b

c

d

200 nm

e

f

200 nm

Fig. 2 SEM images: (a) SN; (b) MSN; (c) MSN/PA; (d) SN/PA; (e) CM; (f) CB3S. Fig. 3a shows the N2 adsorption and desorption isotherms of CB-3S, the type IV isotherms suggest that there exists both micro- and mesopores in CB-3S. From the pore size distributions and the cumulative pore volumes illustrated in



Fig. 3b and S2, it can be found that the obtained four catalysts have high density around 5 nm. As the sulphur usage increases, the pore density around 5 nm increased first, and CB-3S has the highest pore density. When sulphur usage was further raised, the meso pore density decreased, however, which should be attributed to the carbon loss induced by sulphur addition (Table 1). Considering the BET surface areas, catalysts’ surface areas increased from 498 (CB) to 1146 (CB-3S) m2 g-1. When the mass ratio between sulphur and CM was higher than 3, catalyst’s surface declined, which is similar to the results derived from the pore size distribution. And the carbon loss (Table 1 and S1) induced by sulphur addition, we suggest, should also be the proper reasons. Fig. 4a illustrates the XRD patterns of the obtained catalyst. One can find two obvious diffraction peaks centered at ~24o and ~43o in all the four obtained catalysts, which are corresponding to the (002) and (100) reflections of carbon, respectively. When sulphur was introduced into the precursor, the diffraction peaks become weaker. And, the more sulphur usage is, the broader and weaker the diffraction peaks are, suggesting the carbon structure becomes more disordered after S doping. Their Raman spectra also confirmed these structural defects, where the ratio between G-band and D-band (ID/IG) increases from 1.04 to 1.17 as sulphur usage rises.


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0.7

a

b

0.6 Adsorption Desorption

2400 2000 1600 1200 800 400 0

Cumulative Pore Volume (cm3 g-1)

2800

dV/dD (cm3g-1nm-1)

3200

0.5 0.4 0.3 0.2 0.1

CB-3S

2.0

1.5

1.0

0.5

0.0

0

50

0.0

0.2

0.4

0.6

0.8

1.0

0

50

BET surface Areas (m2 g-1)

Relative Pressure (P/P0) 1600 1400

100

150

200

Pore Size (nm)

0.0

100

150

200

Pore Size (nm)

c

1146

1200 1000 789

800 600

764

498

400 200 0

CB

CB-S

CB-3S

CB-5S

Fig. 3 (a) N2 adsorption and desorption isotherms of CB-3S; (b) pore-size distribution and cumulative pore volume (inserted) of CB-3S, calculated by using the Barrett–Joyner–Halenda (BJH) methods; (c) BET surface areas of different catalysts.

a

b

CB CB-1S CB-3S CB-5S

C (002)

D-band G-band

Intensity

C (100)

Intensity

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 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Volume Adsorped (STP, cm3g-1)

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CB CB-1S CB-3S CB-5S ID/IG=1.17

ID/IG=1.12 ID/IG=1.06 ID/IG=1.04

10

20

30

40

50

60

2 (degree)

70

80

800

1000

1200

1400

1600

Raman Shift (cm-1)

1800

2000

Fig. 4 (a) XRD patterns of obtained catalysts; (b) Raman spectra.

To determining the elemental composition of the as-prepared catalysts, we conduct XPS and EA measurements and summary the data in Table 1, S1 and Fig. S3a. From the survey spectra (Fig. S3a), C1s, N1s, O1s and S2p signals can



be observed in all the catalysts derived from sulphur-containing precursors, which confirms that these catalysts (CB-1S, CB-3S, CB-5S) were mainly composed by C, N, O and S. And among the four catalysts, CB has the highest C content of 91.04 at%. As sulphur usages rise, the C contents declines, which can be also confirmed by their EA results illustrated in Table S1. The declined carbon contents also suggest that sulphur introduction facilitates the carbon loss during the pyrolysis procedures, which is consistent with the results derived from their XRD and Raman measurements. And these defects induced by sulphur addition, we suggest, should be also the proper reason for the more abundant mesopores and higher surface areas of catalysts containing sulphur (Fig. 3). Fig. 5a and b exhibit the high resolution N1s XPS spectra and the relative deconvolution results for CB and CB-3S. Four different N species can be found in both CB and CB-3S, corresponding to oxidized (~401.8 eV), graphitic (~400.9 eV), pyrrolic (~399.7 eV) and pyridinic (~398.3 eV) N species, respectively. The same N species can be also found in CB-1S and CB-5S (Fig. S3 b and c). Generally, graphitic and pyridinic N species are regarded as the active N species for ORR. Thus, the atomic contents of these two N species will be important for ORR catalysts. Based on the deconvolution results, we calculated the atomic contents of various N species and illustrated these data in Fig. 5c. Among the four catalysts, CB-1S has the highest graphitic N content of 34 at%, while CB-5S has the lowest of 26 at%, vs. 30 and 33 at% for CB and CB-3S, respectively. For pyridinic N species, CB-5S has the highest of 31 at%, while CB


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has the lowest of 19 at%, vs. 24 and 30 at% for CB-1S and CB-3S, respectively, which might suggest that sulphur addition can facilitate the formation of pyridinic N in catalysts. And among the four catalysts, CB-3S has the highest total amount of active N species of 63 at%. Fig. 5d shows the high resolution S2p spectra of CB-3S, where both oxidized S (-C-SOx-C-) and sulphide S (-C-S-C-) can be found. And the atomic content of sulphide S can be as high as 92 at%, which is supposed to be active for ORR31, 37-40. The highest active N species and sulphide S contents of CB-3S, we suggest, should be also the proper reasons for CB-3S’s highest ORR performance among the as-prepared catalysts, which will be discussed in the next section.

Table 1 surface compositions of various catalysts from their XPS spectraa Species Concentration (at%) Catalysts

a

C1s

O1s

S2p

Si2p

N1s

CB

91.04

4.70

--

0.38

3.88

CB-1S

89.65

4.62

2.02

0.53

3.17

CB-3S

85.61

7.89

2.81

0.51

3.17

CB-5S

85.26

6.89

3.68

0.48

3.70

Hydrogen is not taken into account for the calculations



a

b

Graphitic

Pyridinic

50 45 40 35 30 25

10

400

398

396

Pyrrolic

404

402

Binding Energy (eV)

c 35

26 18

d 30 31

30 26

26 26 16 17

400

398

396

Binding Energy (eV)

CB CB-S CB-3S CB-5S

34 33

20 15

Intensity 402

Oxidized

24 19

Intensity

Intensity

Oxidized

404

Graphitic

Pyridinic

Pyrrolic

Atomic Contents (%)

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 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-C-S-C-

-SOx-

11

5 0

Oxidized N Graphitic N Pyrrolic N

Pyridinic N

174

172

170

168

166

164

162

Binding Energy (eV)

Fig. 5 (a) N1s spectra of CB; (b) N1s spectra of CB-3S; (c) Atomic contents of N species in different catalysts; (d) S2p spectra of CB-3S.

To evaluate the catalytic performance of our catalysts, we first conducted LSV measurements under the rotation rate of 1600 rpm, as illustrated in Fig. 6a. It can be observed that CB, without any sulphur addition, demonstrated moderate catalytic activity towards ORR, with its half wave potential 72 mV lower than that of commercial Pt/C (0.765 vs. 0.837 V). Compared with PAC, prepared through only one-pyrolysis procedure, CB exhibits a much higher current density but a similar half-wave potential to the one of PAC (0.765 vs. 0.761 V), suggesting the second heat-treatment can improve catalysts’ ORR performance to some extent. After sulphur was introduced, catalysts’ performances were significantly improved. And among the obtained catalysts, CB-3S shows the highest performance, with the half-wave potential shifted positively to 0.845 V


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(vs. RHE), which is comparable with that of Pt/C catalyst. Obviously, sulphur plays a rather important role in enhancing catalysts’ ORR performance. However, when sulphur usage was further increased, catalyst’s performance decline, with half-wave potential decreased by 12 mV (0.833 vs. 0.845 mV), implying that only appropriate sulphur can enhance catalysts’ performance. Based on the characterization results, we suggest, the higher sulphur usage induced the lower surface area (Fig. 3), less porous structures (Fig. 3 and S2), lower contents of active N and S species (Fig. 5), as well as more serious carbon loss (Table 1 and

0.833 0.837

0.95

0.761 0.765

-2 -3

PAC

CB

CB-1S CB-3S CB-5S

PAC CB CB-1S CB-3S CB-5S Pt/C

Pt/C

-4 -5 -6 0.1 0.2

0.6 0.4

0.3 0.4 0.5

0.6 0.7 0.8

Potential (V) vs. RHE

0.0

0.0 -0.2

-0.1

-0.4 -0.6

CB CB-1S CB-3S CB-5S PAC

-0.8 -1.0 -1.2

0.2

0.4

0.6

0.8


1.0

-0.2 -0.3

b=54.5 mV dec-1 b=54.6 mV dec-1 b=56.5 mV dec-1

0.80

0.70

80 60

b=74.8 mV dec-1

PAC CB CB-1S CB-3S CB-5S

0.75

100

0.1

b=50.2 mV dec

0.85

0.9 1.0

d

-1

0.90

0.65

0.2

2

b

-0.6

e

-0.4

-0.2

0.0

log Ik

0.2

0.4

0.6

0

c

-1 -2

0.24

-3

0.615 V 0.565 V 0.515 V 0.465 V 0.415 V 0.365 V 0.315 V

0.20

0.16 0.05

0.06

0.07

0.08

0.09

0.10

n=4.03

0.11

-1/2 (rps-1/2)

-4

900 1200 1600 2000 2500 3000

-5 -6 -7 0.1

4.0

CB CB-1S CB-3S CB-5S PAC

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Potential (V vs. RHE)

0.9

1.0

f

3.5 3.0

40

2.5

20 0

1

J -1 (mA-1cm2)

0.845 0.818

Current Density (mA cm-2)

-1

a

Electron Transfer Number

0


1

Ring Current (mA) Peroxide Yields (%)

Current Density (mA cm-2)

S1), should be the proper reasons.

Disk Current (mA)

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 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.2

0.4

0.6


2.0

PAC CB CB-1S CB-3S CB-5S

0.2

0.4


0.6

Fig. 6 (a) LSV curves under the rotation of 1600 rpm and half-wave potentials of obtained catalysts; (b) Tafel plots; (c) LSV curves of CB-3S under various rotations; (d) The results of RRDE measurements; (e) Peroxide yields calculated from the RRDE measurements results; (f) Electron transfer number. The LSV and RRDE measurements were performed in an oxygen saturated 0.1 M KOH aqueous solution.



To shed light on the origin of our catalysts’ various performance, Tafel analysis was performed and the results were listed in Fig. 6b. It can be found that in the surface reaction kinetics control region, CB-3S has the lowest Tafel slope of 50.2 mV dec-1, followed by CB-5S (54.5 mV dec-1), CB-1S (54.6 mV dec-1), CB (56.5 mV dec-1) and PAC (74.5 mV dec-1), revealing its lowest overpotential among the obtained catalysts. The lower Tafel slopes of sulphur containing catalysts also proved that sulphur introduction can facilitate catalysts’ performance. To get insight into the kinetics of the ORR on our catalysts, we recorded their LSV curves under different rotation rates ranging from 900 to 3000 rpm and calculated the electron transfer number by using the K-L equations (Fig. 6c and S4). Among the four catalysts, CB-3S has the highest electron transfer number around 4 (Fig.5c and S4), indicating that the ORR occurs on it follows a fourelectron mechanism, which is similar to Pt/C catalysts. From the electron transfer number of catalysts with different sulphur contents, one can also find that the electron transfer numbers of sulphur-containing catalysts are much higher than that of CB, suggesting sulphur addition can enhance carbons’ selectivity towards four-electron catalytic path. To confirm the kinetics and the catalytic pathway, we further conducted RRDE measurements, as shown in Fig. 6d. CB-3S shows the lowest peroxide yield over the potential range of 0.125-0.65 V (vs. RHE), corresponding to the highest


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electron transfer number (n) of ~3.7 among the obtained catalysts. These results well agree with those derived from the K-L analysis, which further confirms that the ORR occurred on CB-3S primarily follows a four-electron mechanism. Based on the characterization results and electrochemical measurements, we suggest, the following factors should be the proper origins for CB-3S outstanding ORR performance: (1) the high contents of active species, including graphitic/pyridinic N species and sulphide S (-C-S-C-), which are believed to modify the surface charge distribution, create defects and active sites30, 38, 41-42; (2) the high surface area, which can supply more exposed active sites43-44; (3) bowl-like morphologies, which makes the inner surface of carbon bowls more accessible to electrolyte and O2. Apart from high catalytic performance, the methanol tolerance and long-term stability are also important for their practical applications, especially for methanol related fuel cells. Thus, we recorded the i-t curves of CB-3S and Pt/C catalyst, and illustrated these data in Fig. 7. 120

a

100

Methanol addition

80 60 40

CB-3S P/C

20 0

0

100

200

300

400

500

Relative Current (%)

120

Relative Current (%)

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 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100

b

96.8%

80

80.7%

60 40

CB-3S Pt/C

20 0

5000

Time (s)

10000

15000

20000

Time (s)

Fig. 7 (a) Methanol crossover measurements for the CB-3S and Pt/C catalysts; (b) Stability evaluation of CB-3S and Pt/C at a rotating rate of 1600 rpm. The



methanol crossover measurements and stability evaluation were conducted in an oxygen saturated 0.1 M KOH aqueous solution. As shown in Fig. 7a, as methanol was introduced, CB-3S exhibited no obvious current decay after 400 sec of exposure to methanol. By contrast, the Pt/C catalyst suffer a cliff decline under the same conditions, suggesting CB-3S has superior methanol tolerance to that of Pt/C catalyst, which might also make it promising in methanol-based fuel cells. Fig. 7b reveals the i-t curves of CB-3S and Pt/C catalysts after a 20000 sec test. After the test, CB-3S maintained 96.8% of its initial performance, while nearly 20% of Pt/C’s performance was lost under the same measurements, suggesting CB-3S is rather stable.

Conclusions In summary, we fabricated a hollow carbon bowls doped with nitrogen and sulphur by using polyacrylonitrile as the precursors through a pyrolysis procedure. The obtained catalyst exhibited high ORR performance, with a halfwave potentials comparable to that of Pt/C catalyst in an alkaline medium. Our catalyst also demonstrated outstanding methanol tolerance, stability, as well as high selectivity towards four-electron path. Combining with the characterization results, we suggest, the following factors should be the proper origins for our catalyst’s outstanding performance: (1) the high contents of active species, including graphitic/pyridinic N species, sulphide S (-C-S-C-), which are believed to modify the surface charge distribution, create defects and active sites; (2) the


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high surface area, which can supply more exposed active sites; (3) bowl-like morphologies, which makes the inner surface of carbon bowls more accessible to electrolyte and O2.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Details of characterization and electrochemical measurements; Comparison of catalytic activities of metal-free ORR catalysts recently reported.

AUTHOR INFORMATION Corresponding Author E-mail: [email protected], [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (NSFC Project No. 21606061, 31360069 ， 21662012), the Development Program for Innovative Research Team in Ministry of Education (Grant No. IRT-16R19) ， the Research Project of Hainan Provincial Department of Education (Hnky2017-21), the



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Natural Science Foundation of Hainan Province (Project No. 218MS044), the Research Fund Program of Key Laboratory of Fuel Cell Technology of Guangdong Province.

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Table of Contents Graphic

Nitrogen and sulfur co-doped nano carbon bowls with high electrocatalytic performance towards oxygen reduction




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Uniform Nitrogen and Sulfur Co-doped Carbon Bowls for the

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