Multiheteroatom-Doped Porous Carbon Catalyst for Oxygen

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Multiheteroatom-Doped Porous Carbon Catalyst for Oxygen Reduction Reaction Prepared using 3D Network of ZIF-8/Polymeric Nanofiber as a Facile-Doping Template Peimin Huang,† Haodong Li,† Xiayun Huang,* and Daoyong Chen* State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Fudan University, Shanghai 200433, China S Supporting Information *

ABSTRACT: We report a facile and versatile method for fabrication of multiheteroatom-doped hierarchically porous carbon with a large specific surface area, using the 3D network constructed by ZIF-8 coated wormlike micelles as template. The uniform and highly pure wormlike micelles developed in our laboratory is essential, because they not only are responsible for the formation of hierarchical porosity, but also play as a versatile platform for multiheteroatoms doping. In a primary experiment, S, N, B, and P heteroatoms were doped conveniently and the resultant porous carbons have the excellent oxygen reduction reaction performance comparable to the commercial 20% Pt/C. KEYWORDS: ZIF-8, wormlike micelles, hierarchical porous carbon, multiheteroatoms doping, oxygen reduction reaction

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template because of its high specific surface area, and tailorable porosity.15 Moreover, the organic ligands in the MOF architecture serve as the doping precursor and were then converted to doped heteroatoms (i.e., N doped) by direct pyrolysis, which were homogeneously dispersed in the porous carbon.16 However, because of the limited elements existed in the MOF ligands, the current MOF-derived porous carbon particles are mainly single heteroatom doped.17 To achieve multiheteroatoms doping, the post modification of ligand was utilized to introduce various types of heteroatoms precursors,18 which limited the scalable applications. Besides, Dai et al.19 fabricated the N, S codoped porous nanocarbon by encapsulating the urea and dimethyl sulfoxide inside the MOF-5 crystals. It showed a high ORR activity due to the synergistic effect of N and S. But only limited doping precursors were small enough to be encapsulated inside the pores of MOF and prohibited the development of the multiheteroatoms doping as a versatile method. Moreover, the carbonization of those MOF nanocrystals above-mentioned resulted in the close-packed structures and led to the reduction of the effective surface area.16 Although the extra combined doping methods, such as mechanical mixing, could be applied to obtain the multiheteroatoms doped porous carbon,20 it would result in the inhomogeneous distribution of the heteroatoms. Therefore, the homogeneously multiheteroatom doped porous carbons are largely unexplored, highly desired

orous carbon materials have attracted great attention in the fields of gas separation, water purification, catalyst supports, batteries, supercapacitors, and fuel cells because of their high specific surface area, good stability, excellent electricity conductance, and low cost. 1−5 Among these applications, the oxygen reduction reaction (ORR) catalyst in the fuel cells is one of promising applications for porous carbon materials. It is well-known that one of the technical challenges for the fuel cells is to enhance the sluggish kinetics of ORR.6 To date, platinum and its alloys are considered as the effective ORR catalyst due to their high catalytic activities.7 But both the fabrication and the resulting catalysts were involved with the noble metal (i.e., platinum), which suffered from high cost, limited supply, and intolerance to fuel crossover.8 Nowadays, metal-free porous carbons have been widely employed as the Pt-alternative catalyst, which reduces the cost remarkably.9 To enhance the ORR performance of porous carbon material, the generation of the hierarchical pores and the introduction of multiheteroatoms (i.e., B, N, P, S) into the porous carbon are two effective approaches. The hierarchical pores can improve the effective surface area and facilitate mass transportation,2 whereas the homogeneous multiheteroatoms doping can improve the overall catalytic activity and conductivity.10 Thus, we believe that the ideal porous carbon material for ORR catalyst requires low cost, high specific surface area, hierarchical pores, and the homogeneous multiheteroatoms doping. To obtain porous carbon, various synthetic methods were utilized, such as physical or chemical activation,11 chemical vapor decomposition,12 nanocasting,13 template carbonization,1 and pyrolysis.14 In particular, metal−organic frameworks (MOFs) has been recently investigated as the porous carbon © 2017 American Chemical Society

Received: May 8, 2017 Accepted: June 15, 2017 Published: June 15, 2017 21083

DOI: 10.1021/acsami.7b06427 ACS Appl. Mater. Interfaces 2017, 9, 21083−21088

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ACS Applied Materials & Interfaces

crossover tolerance due to the synergistic effect of multiheteroatoms. Briefly, polymeric wormlike micelles with uniformity and high purity were fabricated in a large scale based on the method we reported.27 They resulted from the self-assembly of poly(ethylene glycol)-block-poly(4-vinylpyridine) (PEG113-bP4VP118) micelles and DNA. After cross-linking by 1, 4dibromobutane, the polymeric wormlike micelles have the PEG as shell and cross-linked quaternized (positively charged) P4VP as core. The ZIF-8 coated nanofibrous network was then prepared by in situ crystallization of ZIF-8 on the surface of polymeric wormlike micelles.26 The 3D ZIF-8 nanofibrous network consists of the internal positively charged P4VP core, and the ZIF-8 nanocrystals layer (Scheme 1).26 Such ZIF-8-

and still remain a big challenge in the large-scale fabrication from the MOF-derived porous carbon system.21 Compared with the porous carbon particles derived from the MOF nanocrystals, 1D MOF-derived nanofibrous carbon with a 3D interconnected architecture can easily produce the abundant hierarchical pores with high effective surface area, which not only shortens the mass transport pathways but also facilitates reactants contact with active sites as well as electron transfer.22 To obtain such 1D MOF-derived carbon nanofibrous network, we employed the nanofibers, including polymeric electrospun nanofibers23,24 and tellurium nanowires (TeNWs),20 as the templates. However, it is still difficult for the electrospinning approach to achieve homogeneous multiheteroatoms doping. The relative stable MOFs crystals were embedded in the spun nanofibers, leading to inhomogeneous MOFs dispersion in the nanofibers,25 therefore limiting the versatile multiheteroatoms homogeneous doping. Besides, Yu et al.20 synthesized 1D ZIF-8 hybrid nanofibers using the TeNWs as template. After pyrolysis, the hybrid material transformed into the 1D ZIF-8derived N-doped carbon nanofibers with high effective surface area and hierarchical pores. The further P doping was conducted by reannealing with triphenylphosphine in nitrogen atmosphere. The resulting N, P codoped porous carbon nanofiber could outperform the ORR performance of the commercial 20% Pt/C catalyst. However, TeNWs are costly and poisonous which limit its large-scale fabrication. And the reannealing method might not generate the homogeneous distribution (in a molecular level) of multiheteroatoms in the network.20 Herein, we successfully fabricate multiheteroatoms doped hierarchically porous carbon nanofibrous networks, using the 3D network as template, which constructed from ZIF-8/ wormlike micelle hybrid nanofibers. All of their properties fulfill the requirements of the ideal porous carbon materials as ORR catalysts. In our previous work, we reported that the 3D network constructed from hybrid nanofibers with a uniform width was formed by an integrated ZIF-8 nanocrystal layer coating on the surface of polymeric wormlike micelles.26 The polymeric wormlike micelles have a poly(ethylene glycol) (PEG) shell and a cross-linked poly(4-vinylpyridine) (P4VP) core.27 And the 3D network of hybrid nanofibers induced various hierarchical pores, such as micropores, mesopores and macropores. In the present work, we found that the 3D ZIF-8 hybrid nanofibrous network could efficiently absorb the anions (i.e., B4O72−, HPO42−, and SO42−), which was confirmed as the facile and versatile multiheteroatoms doping precursors of porous carbon. After carbonization, the porous carbon nanofibrous network remained such large amount of hierarchical pores and achieved high specific surface area. What is more, with adjusting the adsorption species of the anions (B4O72−, HPO42−, or SO42−), various multiheteroatoms were homogeneously doped in porous carbon nanofibrous networks. The polymeric wormlike micelles inside the precursors not only act as the skeleton of the network and become micropores, mesopores and macropores after carbonization, which are responsible for the formation of the hierarchically porous structure with a large specific surface area, but also play as a versatile platform for the homogeneous multiheteroatoms doping. Preliminary results showed that both the ternary heteroatoms (N, B, P) doped and the quaternary heteroatoms (N, B, P, S) doped porous carbon exhibited excellent ORR activity, outstanding durability, and methanol

Scheme 1. Illustration of the Synthesis of MultiheteroatomDoped Porous Carbon Nanofibrous Network

coated nanofibers have a diameter of ∼98 nm (Figure 1a, e), containing ∼20 wt % polymeric nanofibers calculated from TGA (Figure S1). Because of the crystal growth of the ZIF-8, some of the coated ZIF-8 tends to fuse and interconnect together to form the cross-linking points, which was confirmed by our previous work.26 Thus, the 3D cross-linked nanofibrous network was obtained. Such network has a loosely packed “empty” structure, which contains a variety of hierarchical porosity including micropores, mesopores, and macropores (Figure 1a).26 The PXRD pattern also shows that 3D ZIF-8 nanofibrous network was identical to the pristine ZIF-8 with the similar high crystallinity (Figure S2a). As both the quaternized P4VP core in the 3D ZIF-8 nanofibrous network and ZIF-8 nanocrystals are positively charged, they can actively absorb anions via electrostatic interaction. Taking saturated absorption of HPO42− for example, 5 mg of the 3D nanofibrous network could absorb 0.64 mg of HPO42− in 20 mL of HPO42− aqueous solution (concentration was fixed at 0.1 mg/mL). Notably, the anion could reach inside P4VP cores through the gaps between ZIF-8 nanocrystals, but not the pores of ZIF-8 nanocrystals. The pores of ZIF-8 are too small to intake the large size anion. Here, we choose B4O72−, HPO42−, and SO42− as the B-, P-, S-doped carbon precursors, respectively. At the same time, N doping was provided from both the 2-methylimidazole ligand of ZIF-8 nanocrystals and the pyridine units of P4VP core. It can be expected that such anion adsorption was a suitable homogeneous multiheteroatom-doped procedure for porous carbon network. After absorbing different kinds of anion, including B4O72−, HPO42−, or SO42−, the lyophilization method was utilized to 21084

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Figure 1. Morphology of porous nanofibrous network. FESEM and TEM images of (a, e) porous ZIF-8-coated nanofibrous network; (b, f) N-doped carbonized nanofibrous network, carbon N; (c, g) N-, B-, P-doped carbonized nanofibrous network, carbon NBP; and (d, h) N-, B-, P-, S-doped carbonized nanofibrous network, carbon NBPS, respectively.

Figure 2. (a) TEM and (b) HAADF-STEM images of carbon NBPS; (c−h) representative C, N, O, P, S, and overlay elemental mappings. The scale bar is 40 nm.

(Figure 1b−d, f−h and Figures S3−S5) because of the pyrolysis of both ZIF-8 layer and the polymeric wormlike micelles. As shown in the SEM and TEM images, it contains a large number of mesopores and macropores in the network (Figure 1b−d, f− h and Figures S3−S5). The EDS analysis suggests that N, P, and S are successfully doped into the resulted hierarchically porous carbon nanofibrous network of carbon NBPS, and disperse homogeneously (Figure 2). As calculated, the ZIF-8 layer has ∼30 nm in thickness. During carbonization, the multiheteroatoms due to the absorption of anions inside the P4VP cores diffuse outward in a short distance, achieving homogeneous heteroatoms doping. As the content of doped B element was too little to be detected in the EDS, the XPS investigation was performed to demonstrate the existence of the doped B element in the nanofibrous network. Because the quaternized 4VP units disperse homogeneous in the core of nanofibers and interact with anions, B elements were speculated to be doped in the carbon fiber homogeneously.

prevent the collapse of the hierarchical pores in the interconnected 3D nanofibrous network. It is reported that the reduced Zn metal from ZIF-8 can vaporized during carbonization along with Ar gas flow at 1000 °C, leaving heteroatoms doped porous carbon.28 Here, all the 3D nanofibrous network precursors were carbonized at 1000 °C, resulting in a variety of single, dual, ternary and quaternary heteroatom (N, B, P, S) doped porous carbon nanofibrous network (Scheme 1). For convenience, carbon X was used to denote for the different multiheteroatom-doped nanofibrous porous carbon network and X represents the doped heteroatoms. For example, carbon NBPS denotes for B, P, and S heteroatom-doped porous carbon nanofibrous network derived from 3D ZIF-8-coated nanofibrous network. After pyrolysis, multiheteroatoms doped porous carbon nanofibrous network maintain the similar 3D nanofibrous network with abundant numbers of hierarchical pores. The diameter of porous carbon nanofibers shrunk to ∼40 nm 21085

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ACS Applied Materials & Interfaces Note here, such analysis of EDS mapping of light element doping in porous carbon reaches the commercial instrument limit. To the best of our knowledge, our result was one of few clear and highly magnified element mappings by EDS. PXRD patterns of the multiheteroatom-doped hierarchically porous carbon nanofibrous network showed only two distinct peaks at around 25 and 44° (Figure S2b), indicating the graphitization of all the doped porous carbons. No any other peaks were discovered, suggesting there was no other impurity and the reduced Zn metal was vaporized along with Ar gas flow successfully. ICP data (Table S1) also confirm that Zn metals in all the porous carbons have been successfully removed with less than 0.74% (wt %) remaining. The Raman spectra (Figure S6) shows that the ID/IG values of carbon N are smaller than for any other doped hierarchically porous carbon samples, meaning the versatile elements have been incorporated into the SP2 lattice of graphite successfully.21 The investigation of the XPS spectra of C 1s, O 1s, N 1s, B 1s, S 2p3, and P 2p3 suggested that the covalent incorporation of heteroatoms with porous carbon network (Figures S7−S11, Table S2). The EDS results confirmed that heteroatoms disperse in the carbon nanofibrous network homogeneously, so the surface atom contents of porous carbon fibers represent for the overall atom content of the network. The surface atom content of all the heteroatomdoped carbon samples are listed in the Table S3. Taking carbon NBPS as an example, it contains C, 85.75%; O, 10.46%; N, 1.80%; B, 1.33%; P, 0.18%; S, 0.46% (atom %). Therefore, the anion adsorption is a sufficient way in our system to actively obtain the multiheteroatoms doped precursors, and then correspondingly carbonized to achieve the multiheteroatom doped hierarchically porous carbon nanofibrous network. To evaluate the porosity and the pore size distribution in the hierarchically porous carbon nanofibrous network, N2 adsorption−desorption isotherms were conducted. Before carbonization (Figure S12), the Brunauer−Emmett−Teller (BET) surface area and the total pore volume of as-prepared 3D ZIF-8 nanofibrous network was 1310.06 m2/g (1.57 cm3/g). It displays both the type I isotherm at low relative pressures (P/P0 < 0.1), which indicated the existence of micropores of ZIF-8, and a type-H4 hysteresis loop at medium relative pressures (0.4 < P/P0 < 0.8), meaning the existence of slitlike mesopores, which are the gaps between each ZIF-8 crystal.26 After carbonization, multiheteroatom-doped hierarchically porous carbon nanofibrous networks have the BET surface area and total pore volume of 963 m2/g (2.45 cm3/g), 1050.56 m2/g (2.26 cm3/g), and 1106.79 m2/g (2.78 cm3/g) for the carbon N, carbon NBP, and carbon NBPS, respectively (Figure 3 and Table S4). The slight decrease in the BET surface area after the direct carbonization of the as-prepared 3D ZIF-8 nanofibrous network (carbon N) was due to the collapse of carbon nanofibrous network. Moreover, the multiheteroatom-doped hierarchically porous carbons, carbon NBP, and carbon NBPS have the higher surface area compared with carbon N, suggesting this facile doping method is successful.19 Besides, all the heteroatom doped porous carbon networks studied in our system exhibit a pronounced hysteresis loop and the typeIV curve that is similar to type I curves, suggesting both the micropores and slitlike mesopores of as-prepared ZIF-8 nanofibrous network retained. The extra mesopores probably resulted from the pyrolysis of internal polymeric wormlike micelles. At medium relative pressures (0.4 < P/P0 < 0.8), carbon N only shows a slight increase, whereas carbon NBP and carbon NBPS present much obvious increase, indicating

Figure 3. N2 adsorption−desorption isotherms of carbon N, carbon NBP, and carbon NBPS.

that the pyrolysis of anions created more mesopores during carbonization. In addition, the micropores distribution by NonLocal Density Functional Theory (NLDFT) and mesopores distribution by Barret-Joyner-Halenda (BJH) model further demonstrate the existence of micropores and mesopores (Figure S12). At the high relative pressure region (P/P0 > 0.8), the quick increase of the isotherm represented the existence of the macropores, which was attributed by the loosely packed nanofibrous network. With regard to carbon NBPS, the hierarchical porous network contains 0.18 cm3/g for micropores (Vmicro), 1.74 cm3/g for mesopores (Vmeso), and 0.86 cm3/g for macropores (Vmacro), implying its hierarchical porous structure. For its sluggish kinetics, ORR has become the bottleneck for improving fuel cells performance.6 Heteroatom-doped metalfree MOF-derived carbon was considered as promising catalyst for ORR performance enhancement.20,21 Hence, we choose ORR as the catalysts demo application for the multiheteroatoms doped hierarchically porous carbon nanofibrous network. Carbon N, carbon NBP, and carbon NBPS were chosen as the representative system of single-, ternary- and quaternary-heteroatom-doped hierarchically porous carbon nanofibrous network, respectively. The ORR performance of other carbon samples were also characterized (Figures S13− 18). It is well-known that the activity, durability and methanol crossover tolerance are the key parameters to evaluate the ORR performance. Cyclic voltammetry (CV) and rotating disk electrode (RDE) were carried out in a three-electrode system. A significant peak occurred in the O2-saturated 0.1 mol/L KOH solution, whereas featureless peaks were observed in the N2saturated 0.1 mol/L KOH solution, indicating the existence of ORR catalytic activity (Figure 4a, Figure S13). Linear sweep voltammograms (LSVs) results showed that the onset potential of carbon NBPS (−0.086 V) is the highest among the porous carbons in our system and comparable to the commercial 20% Pt/C (−0.074 V). Remarkably, Figure 4c, d showed that the durability and methanol crossover tolerance of carbon NBPS are obviously superior to commercial 20% Pt/C, thus behaving outstanding ORR performance. Besides, the diffusion-limited current density was another aspect to evaluate ORR activity. In particular, the diffusion-limited current density of carbon NBP is equivalent to that of 20% Pt/C (5.22 mA/ cm2), which is lower than the theoretical value for a 4-electron transfer catalyst at 1600 rpm (∼6 mA/cm2) on the basis of the 21086

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derived from 3D network of ZIF-8/wormlike micelle hybrid nanofibers. The polymeric wormlike micelles play an important role because they can not only be the versatile platform to provide the homogeneous multiheteroatoms doping but also provide the nanofibrous network skeleton and hierarchical porosity, which contributed to the large specific surface area after carbonization. By controlling the absorption of B4O72−, HPO42−, or SO42−, we can easily attain single, dual, ternary, and quaternary heteroatoms doping, suggesting a potential broad tunable doping capability. The hierarchical pores with abundant micropores, mesopores, and macropores facilitated the fast reactants mass transportation and the extensive contact between reactants and active sites. The 1D carbon nanofibrous network also induced the electron transfer and improved electrochemical performance. Preliminary results showed that carbon NBP and carbon NBPS exhibit outstanding ORR activities. Besides, its relative low cost, long-term durability and excellent methanol tolerance even outperform commercial 20% Pt/C, implying great potential as the catalyst for ORR. Moreover, it provides a facile and versatile pathway to largescale fabrication of multiheteroatom-doped hierarchically porous carbon-based catalysts, which mainly shows a great potential in the energy application.

Figure 4. (a) CV curves of carbon N, carbon NBP, carbon NBPS, and commercial 20% Pt/C in O2-saturated 0.1 mol/L KOH solutions with a scan rate of 50 mV s −1; (b) LSVs of carbon N, carbon NBP, carbon NBPS, and commercial 20% Pt/C in an O2-saturated 0.1 mol/L KOH solution at a rotation rate of 1600 rpm and a scanning rate of 10 mV s−1; (c) current−time (I−t) chronoamperometric responses of carbon NBP, carbon NBPS, and commercial 20% Pt/C in O2-saturated 0.1 mol/L KOH at −0.6 V and a rotating rate of 1600 rpm in 25000 s; (d) current−time (I−t) chronoamperometric responses for carbon NBP, carbon NBPS and commercial 20% Pt/C at −0.6 V and a rotating rate of 1600 rpm in 10000 s with the addition of 3 mL of methanol at ∼1000 s.

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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/acsami.7b06427. Materials and experimental details, TGA, XRD, FESEM, TEM, Raman, XPS, BET analysis of the 3D ZIF-8 nanofibrous network precursors and all the carbon samples; CVs, LSVs, and K-L Plots of corresponding multiheteroatom-doped porous carbon nanofibrous network; mechanism discussion of the ORR activities; comparison with other references’ ORR activities (PDF)

Levich equation because of the existence of the Nafion layer. And its long durability and methanol crossover tolerance also outperform these of 20% Pt/C (Figure 4c, d). It can be concluded that carbon NBP also has the excellent ORR performance. For further investigation of the ORR pathway, calculated by Koutecky−Levich (K−L) plots, electron transfer numbers for carbon NBP and carbon NBPS is 3.70 and 3.48, suggesting a major four-electron pathway for ORR (Figures S17 and S18). Thus, both carbon NBP and carbon NBPS exhibit outstanding ORR performance mainly due to the synergistic effect of multiheteroatoms, high specific surface area, and the hierarchical pore property. In addition, the carbon NBP and NBPS show the ORR activities comparable to the best ORR activities reported for metal-free carbons (Table S5). Besides, the ORR performances of other kinds of multiheteroatom (including N, B, P, and S) doped porous carbons were also investigated. The ORR activities followed the trend as shown below: 20% Pt/C > carbon NBPS > carbon NBP > carbon NBS = carbon NPS > carbon NS > carbon NB > carbon NP > carbon N, suggesting multiheteroatoms doping enhances the ORR catalyst activity (Figure S13). However, this trend is neither corresponding with the trend of the overall content of pyridinic-N and graphitic-N that are favorable for ORR29 (carbon NPS > carbon N > carbon NB > carbon NBP > carbon NBS > carbon NBPS > carbon NP > carbon NS, Table S2), indicating that the synergistic effect of multiheteroatoms plays an important role in the ORR performance. Here, we speculate that the synergistic effect of the multiheteroatoms might be related to the strengthened disturbance to the homogeneous electronic structures of carbon lattice.30 Further discussion of the mechanism is shown in the Supporting Information 19. In summary, we discover a facile and versatile approach to fabricate a variety of a multiheteroatom (N, B, P, S) doped hierarchically porous carbon nanofibrous network. It was

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xiayun Huang: 0000-0003-3053-7354 Daoyong Chen: 0000-0001-6776-6332 Author Contributions †

P.H. and H.L. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support from the programs of NSFC (21334001 and 21574025), MOST (2016YFA0203302), STCSM (16JC1400702), and the open project of state key laboratory of supramolecular structure and materials (sklssm201737). We are also thankful to FEI Company APR Nanoport for support with EDS characterizations.

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REFERENCES

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DOI: 10.1021/acsami.7b06427 ACS Appl. Mater. Interfaces 2017, 9, 21083−21088