Nitrogen, Sulfur Co-doped Carbon Derived from Naphthalene-Based

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Article Cite This: ACS Appl. Energy Mater. 2018, 1, 161−166

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Nitrogen, Sulfur Co-doped Carbon Derived from Naphthalene-Based Covalent Organic Framework as an Efficient Catalyst for Oxygen Reduction Chenghang You,*,†,‡ Xiaowei Jiang,† Xianghui Wang,† Yingjie Hua,*,† Chongtai Wang,† Qiang Lin,† and Shijun Liao*,‡ †

School of Chemistry and Chemical Engineering, Hainan Normal University, Haikou 571158, China The Key Laboratory of Fuel Cell Technology of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, China

‡

S Supporting Information *

ABSTRACT: Developing highly efficient and low-cost electrocatalysts toward oxygen reduction reaction (ORR) is crucial for novel electrochemical energy conversion systems. In this work, we fabricated an efficient metal-free ORR catalyst with high porosity and surface area (1116 m2 g−1) from a naphthalene-based covalent organic framework. The catalyst exhibits a superior ORR performance to the commercial Pt/C catalyst (with its half-wave potential 30 mV more positive) and outperforms most recently reported metal-free ORR catalysts. In addition to high ORR performance, our catalyst also demonstrates excellent methanol tolerance, outstanding stability, and high catalytic efficiency (nearly 100% four-electron path selectivity). By studying catalysts’ structures and compositions, we found that the sulfur introduction can not only increase the total and ORR active N contents effectively but also grant catalysts much higher surface areas, all of which, we suggest, should be the reasonable origins for our catalyst’s outstanding catalytic performance. KEYWORDS: oxygen reduction, carbon catalysts, S, N co-doped, covalent organic framework, naphthalene

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INTRODUCTION Exploring low-cost electrocatalysts to replace Pt to drive oxygen reduction reaction (ORR) effectively is a key task for the utilizations of advanced energy systems.1−12 Doped carbons have been regarded as attractive candidates due to their low cost and high ORR performances. However, these catalysts still fail to meet the demand of practical applications to date. Thus, developing carbons with better ORR performance and stability is still of great importance for those advanced energy systems’ commercialization. For carbons, doping with heteroatoms (e.g., N, S, and so on) is an effective way for ORR performance enhancements, since heteroatoms can modify carbons’ charge distribution effectively. Especially, co-doping of heteroatoms with different electronegativity can further improve the ORR performance because of the synergistic effects between heteroatoms.13−15 And among those heteroatoms, sulfur has successfully attracted © 2017 American Chemical Society

intensive attention due to its ability to replace carbon atoms and the strong synergistic effects with N dopants.16,17 Covalent−organic frameworks (COFs), composed by small moleculars through strong covalent bonds, have attracted increasing attention recently,18,19 since their compositions and porosity can be precisely controlled through careful designations. And these compositions and porosity are also expected to be maintained during the annealing procedures due to the high strength of covalent bonds,20 making COFs promising precursors for carbons. Friedel−Crafts (F−C) reaction is an effective strategy for preparing COFs.21−23 With the help of Lewis acid, small aromatic moleculars can be cross-linked by stable bridges (e.g., Received: October 19, 2017 Accepted: December 13, 2017 Published: December 13, 2017 161

DOI: 10.1021/acsaem.7b00045 ACS Appl. Energy Mater. 2018, 1, 161−166

Article

ACS Applied Energy Materials

Figure 1. Schematic demonstration of catalysts’ preparation and Fourier transform infrared spectra (FTIR).

carbonyl bridges), during which hierarchically porous structures can be formed as a result of the stacking of small moleculars (Figure 1). Meanwhile, the as-formed carbonyl bridges can be further substituted by heteroatoms (e.g., N, S, and so on) during the annealing procedure, which facilitates the heterodoping. All of these can benefit the enhancement of carbons’ ORR performance. Inspired by these factors, we developed a N and S co-doped metal-free ORR catalyst with high porosity and surface area (1116 m2 g−1) from a naphthalene-based COF (organized by the F−C reaction). The catalyst exhibits a superior ORR performance to the commercial Pt/C catalyst (with its halfwave potential 30 mV more positive) and outperforms most recently reported metal-free ORR catalysts. In addition to high ORR performance, our catalyst also demonstrates excellent methanol tolerance and outstanding stability, as well as high catalytic efficiency (nearly 100% four-electron path selectivity).

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Figure 2. SEM image: (a) CPN-N; (b) CPN-S; (c) CPN-NS; (d) magnification of CPN-NS’s SEM image; (e) TEM image of CPN-NS; (f) STEM image of CPN-NS and corresponding EDS mapping images.

EXPERIMENTAL SECTION

Preparation of Catalysts. The typical preparation of catalysts was illustrated in Figure 1. Briefly, 0.5 g of aluminum chloride (AlCl3), 0.5 mL of carbon tetrachloride (CCl4), and 0.25 g of SBA-15 were dispersed in 100 mL of carbon dichloride (CH2Cl2) under vigorous stir. The suspension was then refluxed for 1 h before 0.32 g of naphthalene was added. After that, the mixture was refluxed for another 24 h to undergo the F−C reaction, followed by filtrating and rinsing with hydrochloride acid−ethanol solution and deionized (DI) water. The obtained residue was dried at 110 °C to form stable carbonyl bridges between naphthalene molecular layers, which can be confirmed by the FTIR spectra illustrated in Figure 1. The obtained polymerized naphthalene was named as PN. A 1.0 g amount of PN powder and 5.0 g of sulfur were milled and mixed thoroughly in a mortar. Then the blend was programmed, heated, and annealed at 900 °C in a mixed flow of N2 and NH3, followed by leaching with hydrofluoric acid and drying in vacuum. The obtained catalyst was named as CPN-NS, where “NS” implies that the catalyst contains both N and S. For comparison, catalysts with different dopants were prepared through similar procedures, which were then denoted as CPN, CPN-S, and CPN-N, respectively.

also confirmed by its transmission electron microscopic (TEM) image (Figure 2e). Figure 2f exhibits the scanning transmission electron microscopic (STEM) and energy dispersive spectra (EDS) mapping images of CPN-NS, which suggests that N and S have been homogeneously doped into the carbon lattice. Figure 3a exhibits catalysts’ N2 adsorption−desorption isotherms and BET surface areas. The type IV isotherms, with hysteresis in the medium- and high-pressure regions, suggest the existence of both micro- and mesoporous structures in these catalysts. And among the four catalysts, CPN-NS has the highest surface area of 1116 m2 g−1, while CPN has the lowest of 404 m2 g−1, vs 686 and 793 m2 g−1 for CPN-S and CPN-N (inserted in Figure 3a), respectively, suggesting that N and S introduction can effectively increase catalysts’ surface areas. Figure 3b shows the pore-size distributions of different catalysts. It can be found that the micropore volumes in catalysts increased after S was introduced, indicating that S addition can facilitate the formation of micropores, which should be attributed to the S doping and the decomposition of carbon lattice, especially S containing structures. Meanwhile, N doping helps increase the number of mesopores (Figure 3b). And among the four catalysts, CPN-NS, co-doped by S and N, has the highest volumes of both micro- and mesopores, which, we suggest, should be the reasonable origins for its high surface areas (Figure 3a).

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RESULTS AND DISCUSSION Morphology, Structure, and Composition. Figure 2 shows the scanning electron microscopic (SEM) images of various materials. It can be found that all of the catalysts shared similar rod-like morphologies (Figure 2a−c), indicating the same origination of such morphologies. From the magnification of CPN-NS’s SEM images (Figure 2d), one can find that the surface of CPN-NS is rather rough, with numerous grooves, which can be 162


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Figure 3. (a) N2 adsorption−desorption isotherms; (b) pore-size distributions.

Figure 4. (a) N 1s spectra for CPN-N; (b) S 2p spectra for CPN-S; (c) N 1s spectra for CPN-NS; (d) S 2p for CPN-NS; (e) N compositions in CPN-N and CPN-NS; (f) S compositions in CPN-S and CPN-NS.

Table 1 summaries the atomic compositions of various catalysts from their X-ray photoelectron spectra (XPS). It can be seen that CPN-N, without any S addition, has a N content of 1.93 at. %. When S was introduced, the N content increased to 4.02 at. %. Obviously, the S addition can significantly facilitate maintaining N contents. Figure 4 illustrates the XPS spectra of N 1s and S 2p in different catalysts, and parts e and f of Figure 4 summarize their atomic compositions, respectively. One can also find that S addition can not only increase the total N content (Table 1) but also tune the N compositions. After sulfur was introduced, the oxidized N content decreased drastically from 26.79 to 1.55 at. % (Figure 4e). Generally,

graphic, pyrrolic, and pyridinic N are recognized as the ORR active species.24−26 The higher contents of these N species in CPN-NS might also imply that it has a better catalytic performance, which will be confirmed by its results of electrochemical measurements later. Figure 4f is the S compositions in CPN-S and CPN-NS. Obviously, when codoped with N, the oxidized S increases slightly from 15.63 to 19.51 at. %. Considering the difference in N compositions (Figure 4c), the reduction of oxidized N, we suggest, should be a proper reason for this. Electrocatalytic Activity for ORR. Figure 5a shows the linear sweep voltammetry (LSV) curves and the half-wave potentials of various catalysts. It is interesting that CPN, without any N and S contents, exhibits a moderate ORR catalytic activity, with a half-wave potential of 0.74 V (vs RHE), which, we suggest, can be attributed to the oxygen containing groups,27 its unique porous structures, and the defects in the structure.28,29 After N and S were introduced, the catalysts’ performances were obviously enhanced, with half-wave potentials shifting positively by 102 and 40 mV, respectively, suggesting the important role of S and N in enhancing a catalyst’s

Table 1. Surface Atomic Composition of Various Catalysts atomic content (at. %) CPN-N CPN-S CPN-NS

C

O

N

94.24 89.18 86.42

3.65 3.65 5.12

1.93 4.02

S

Si

6.94 4.21

0.19 0.24 0.22 163


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Figure 5. (a) LSV curves in O2-saturated 0.1 M KOH solution and the half-wave potentials of different catalysts; (b) Tafel plots; (c) results of rotating ring-disk electrode (RRDE) measurements; (d) peroxide yields and electron transfer number based on the RRDE measurements.

performance. Among the as-prepared catalysts, CPN-NS has the highest ORR performance, with a half-wave potential (E1/2) of 0.868 V (vs RHE), which can be more active than a commercial Pt/C catalyst (E1/2 = 0.838 V, vs RHE) and outperform most recently reported metal-free ORR catalysts (Supporting Information Table S1). Figure 5b shows the Tafel plots derived from the LSV curves. CPN-NS’s lowest Tafel slope of 47 mV dec−1 (vs 110.8, 94.3, and 64.4 mV dec−1 for CPN, CPN-S, and CPN-N, respectively) implies it has the lowest overpotential among the catalysts, which confirms its superior ORR performance again. To get insight into the ORR procedure occurring on our catalysts, we conducted RRDE measurements and calculated the peroxide yields and electron transfer number of every catalyst (Figure 5c,d). As illustrated in Figure 5d, CPN-NS has the lowest peroxide yields and the highest electron transfer number approaching 4, indicating it can catalyze the ORR through an almost entirely four-electron path. That is, CPNNS has a high selectivity toward a four-electron path. Meanwhile, the lowest electron transfer number, combining with the highest peroxide yield of CPN, implies that CPN can only catalyze the ORR through a two-electron path. In addition to high performances, catalysts’ resistance toward crossover effect and stability is also important for practical applications. Thus, we evaluated the methanol tolerance and stability of CPN-NS and commercial Pt/C catalyst by using the I−t curves. From the current−time curves in Figure 6a, it can be found that CPN-NS has an excellent methanol tolerance. When methanol was added, the performance of CPN-NS was hardly influenced. By contrast, Pt/C suffered a significant drop under the same condition. After a 20000 s ORR, CPN-NS still maintained 97.5% of its initial performance (Figure 6b); however, Pt/C catalyst lost

Figure 6. Current−time (I−t) curves of CPN-NS and Pt/C catalysts: (a) methanol introduced after 100 s; (b) 20000 s continuous I−t curves.

nearly 20% of its performance, which confirms CPN-NS’s outstanding stability.

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CONCLUSIONS In summary, we fabricated an efficient N and S co-doped metal-free ORR catalyst with high porosity and surface area (1116 m2 g−1) from a naphthalene-based COF. The catalyst exhibits a superior ORR performance to the commercial Pt/C catalyst (with its half-wave potential 30 mV more positive), and outperforms most recently reported metal-free ORR catalysts. In addition to high ORR performance, our catalyst also demonstrates excellent methanol tolerance, outstanding stability, and high catalytic efficiency (nearly 100% fourelectron path selectivity). By studying catalysts’ structures and compositions, we found that the sulfur introduction can not only increase the total and ORR active N contents effectively but also grant catalysts more micropores and much higher surface areas, all of which, we suggest, should be the reasonable origins for our catalyst’s outstanding ORR performance. 164


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.7b00045. Details of characterization and electrochemical measurements; comparison of catalytic activities of metal-free ORR catalysts recently reported (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(S.L.) Tel.: +86 20-87113586. E-mail: [email protected]. *(C.Y.) E-mail: +86 898-65888762. [email protected]. *(Y.H.) E-mail: +86 898-65888762. [email protected]. ORCID

Chenghang You: 0000-0001-8232-2262 Shijun Liao: 0000-0003-2481-0377 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (NSFC Project No. 21606061), the Natural Science Foundation of Hainan Province (Project No. 20162022), the Program of Hainan Association for Science and Technology Plans to Youth R & D Innovation (Project No. 201502), and Haikou Key Scientific and Technological Projects (Project Nos. 2016032 and 2016030).

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Nitrogen, Sulfur Co-doped Carbon Derived from Naphthalene-Based

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