N- and S-doped nanoporous carbon framework derived from conjugated microporous polymers incorporation with ionic liquids for efficient oxygen reduction reaction

Materials Today Energy 16 (2020) 100382

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N- and S-doped nanoporous carbon framework derived from conjugated microporous polymers incorporation with ionic liquids for efficient oxygen reduction reaction Rui Jiao, Wanli Zhang, Hanxue Sun, Zhaoqi Zhu, Zifeng Yang, Weidong Liang, An Li* College of Petrochemical Technology, Lanzhou University of Technology, Langongping Road 287, Lanzhou 730050, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 November 2019 Received in revised form 23 December 2019 Accepted 30 December 2019 Available online xxx

The rapid growth of high-performance metal-free electrocatalysts is of great significance for widespread realization of fuel cells since noble metals (e.g. platinum) used as electrocatalysts have long been considered as major bottleneck which limited their commercialization. Herein, we report the creation of a kind of novel porous carbon framework containing dual heteroatoms (N and S, named as C-CMPs-NS), which was fabricated by direct pyrolysis of ionic liquids loaded on conjugated microporous polymers, application of highly efficient metal free electrocatalyst in oxygen reduction reaction (ORR). Due to their large specific surface area, outstanding porosity and, significantly, active sites exposed own to high density doping of S and N, the catalyst showed good oxygen reduction reaction activity in alkaline electrolyte. The C-CMPs-1NP yielded a half-wave potential of 0.82 V, high onset potential (0.98 V vs. RHE), the high diffusion limiting current density of 4.2 mA cm
2 and better methanol tolerance than commercial Pt/C where nearly no ORR polarization curve shift and no change of oxygen reduction peak in cyclic voltammetry (CV) were observed in 3.0 M methanol solution. Based on these merits mentioned above, the C-CMPs-NS may hold great potentials as promising alternative of precious metal catalysts for next generation fuel cells. © 2020 Elsevier Ltd. All rights reserved.

Keywords: Metal-free electrocatalysts ORR Conjugated microporous polymers Nanoporous carbon framework N and S doping

1. Introduction As the most promising reactions for conversion technologies and electrochemical energy storage-the oxygen reduction reaction (ORR), which includes metal-air batteries and fuel cells [1e3]. Well known, the Pt-based catalysts as the noble metals catalysts have been already extensively regard as high active fractions for the ORR reaction [4e7]. However, owing to the rareness of Pt, resulting in the commercialization of fuel cells with a major obstacle that the high cost of Pt-based catalysts, and Pt have another troublesome problem under actual operating conditions, that is instability. Furthermore, in direct methanol fuel cells (DMFCs), Pt is easily poisoned due to this crossover phenomenon of methanol directly from the anode to the cathode [8e10]. Therefore, there is an urgent need to develop efficient and durable alternatives catalysts which at low cost, ideally with dual functionality for ORR [11e13]. Hence,

* Corresponding author. Fax: þ86-931-7823125. E-mail address: [email protected] (A. Li). https://doi.org/10.1016/j.mtener.2020.100382 2468-6069/© 2020 Elsevier Ltd. All rights reserved.

non-platinum/precious metal catalysts are one of the best choices to solve the above problems [14,15]. Therefore, the structure of noble metal base alloy has been proved to be one of the most effective ways to decrease the noble metal consumption, besides, it can significantly improve the comprehensive catalytic performance of noble metal catalyst in oxygen reduction reaction, including Pt-based nanostructured [16], Pt-based alloy [17] and Pt-lanthanide [18]. Comparatively, the metal-free electrocatalysts or the manufacture of non-noble metal with high performance for ORR is much more attractive with the rapid growth of science and technology makes is possible to prepare metal free electrocatalysts possess superior ORR activity or stability even than that of Pt/C. To this day, some non-noble metal or metal-free electrocatalysts such as metal-free nitrogen-doped carbon (N-doped C) [19e21], non-precious-metal oxides and carbides [21e23], transition-metalcoordinating macrocyclic compounds [24,25] and transition-metalcoordinating nitrogen-doped carbon catalysts (M-N/C) [26] have been reported as active catalysts for ORR in the most of the recent studies along this line. In particular, N-doped carbon based electrocatalysts, e.g., Metal-N-C catalysts prepared from FeeN4 and

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CoeN4 macrocycles precursors [27], N-doped graphene [28] and CNTs [29], etc., have received intensive attention because of their low cost, superior ORR activity, electrochemical stability and broader availability. The high ORR activity of those N-doped carbon are well known to be generated from the pyridine N bonded on carbon which could offer a strong affinity to oxygen atoms and in turn promotes the ORR [7], while in the case of Metal-N-C catalysts, e.g., the formation of unsaturated iron atom by N and C coordination (FeeN/C) is an important way to prepare complex ORR catalyst of FeeN/C [30,31]. In addition to the direct application of carbon based materials to this problem, many natural products [32,33] or synthetic polymers [34] have been used as substrate for production of electrocatalysts or may be conductive support material for electroctatlysts by uncomplicated carbonization add or not add dopants. In most cases, nevertheless, the discrepancy chemical make-up or formation of these mentioned precursors result in the performance of synthetic products is unpredictable or not replicable (e.g. porosity, active sites, accessible pores, etc.) after carbonization, this restricted their practical applications. Conjugated microporous polymer (CMPs) as a subclass of porous organic polymer (POP), has extended p-conjugated threedimensional (3D) network. Act as an ideal precursor of porous hard carbon materials, CMPs have a great superiority compared to those bulk polymeric compound on account of their large specific area and excellent porosity could remain after carbonization treatment owing to their rigid aromatically conjugated structure [35e38]. Such carbonized CMPs products with excellent accessible surface area and abundant porosity would be advantageous to fully adsorption of molecular oxygen, fully show of active sites and the fast transportation of oxygen species and electrons. More interestingly, the plentiful surface chemistry of CMPs (e.g., the active carbon carbon triple bond) allows they could easily be tailored chemically (for instance, introduction of N into CMPs network) or strongly bind the metal at atomic level [39], thus makes these excellent precursors for preparation of non-noble metal or metal free ORR electrocatalyst. In a continuation of our previous studies on CMPs, further improved catalytic performance by introducing heteroatoms [40], here we show a novel strategy for the creation of a kind of novel porous carbon framework containing dual heteroatoms (N and S, named as C-CMPs-NS), which was fabricated by direct pyrolysis of ionic liquids loaded on conjugated microporous polymers, for effective electrocatalytic reduction of oxygen. As a proof of concept study, the resulting N- and S-doped porous carbon framework derived from CMPs exhibits good electrocatalytic activity, high stability as well as long-term durability, and excellent methanol tolerance, rending them promising candidate as high efficiency metal-free electrocatalysts for ORR. 2. Result and discussion In this work, we synthesized five CMPs-based materials by Sonogashira-Hagihara cross-coupling reaction use 1,3,5triethynylbenzene, 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide and three different monomers. We have found that the resulting porous network structure is a threedimensional structure consisting of aromatic nodes produced by trigeminal alkynyl bonding, the simulated structure proves this as shown in Scheme 1a and b. In addition, we found that the obtained CMPs-based catalyst porous network structure consists of CMPs particles. Carbonization after adding ionic liquid, and the morphology is still keep a porous network structure in which the particles are stacked, confirmed by morphological analysis (see below), and simulated structures such as Scheme 1c Show. Among the five materials (i.e., C-CMPs-1NP, C-CMPs-2, C-CMPs-3NP, C-

CMPs-4NP and C-CMPs-5NP), we chose one of materials (C-CMPs1NP) to represent the synthesis process of C-CMPs-NP shown in Scheme 1c. It can be seen from Scheme 1c and Fig. 1a that the morphology of C-CMPs-1NP has not changed much before and after carbonization, and is a small sphere formed by particle accumulation. Due to the CMPs strict structure formed by stable p-conjugated network associating with aromatic rings, the five samples after carbonization are still in the form of nanoparticles in Fig. 1aee. The structure formed by our material satisfies main design goals, compared to those massive carbon based materials. In addition, the morphology of C-CMPs-1NP and C-CMPs-2 pile up very tightly (Fig. 1a and b), the other stack together loosely and mix with a small amount of flakes (C-CMPs-3NP~C-CMPs-5NP) in Fig. 1c, d and e. As well, from SEM wo can found the CMPs-based the catalyst were made up of many microporous balls, also from the N2 adsorptionedesorption isotherm curves we can obtained the same result (the pore size is within 2 nm, in Fig. 4). Then we in order to further confirmed the existence of the elements we introduced, from scanning electron microscopy with EDS elemental mapping analysis was utilized to determine the chemical compositions of polymers. As shown in Fig. 1fei, the polymers (C-CMPs-1NP, C-CMPs-3NP~C-CMPs -5NP) dominantly consisted of carbon (C), sulfur (S) and nitrogen (N) elements. The EDS element mapping was performed, and the result is shown in Fig. 1fei. It is clear that the N and S atoms distribute on the CMPs surface, which is worth noting that the distribution of N is more concentrate in the region of high density C, this proved the formation of CeN and is well mounted on the CMP surface. The contents of C, S and N were measured to be 97.73 at%, 0.57 and 1.70 for C-CMPs-1NP. Meanwhile, the electrochemical performance tests also performed demonstrate the successful introduction of S and N, and proved their introduction contributes to the improvement of electrochemical performance. And the uniform distribution of S and N, which was beneficial to improve electrochemical performance. Further, induced by the foreign heteroatoms can modified electronic structure and carbon defects [41]. Remarkably enough, the crossover effect of N and S has been discovered to display huge potential in improving the ORR reactivity [42]. Elemental analysis was conducted to further investigate the accurate types of elements in the CMPs, the results were summarized in Table S1, we can found there was no palladium and copper appears in any of the five polymers, so palladium and copper did not affect the electrochemical performance, the result is consistent with the XPS analysis (Table S2). To further clarify the physical structure of the CMPs-based catalyst, XRD patterns of the five catalysts showed an amorphous halo indicating that the polymer did not have a characteristic reflection of the crystalline phase are presented in Fig. 2. C-CMPs1NP shows a typical carbon pattern, similar to that of C-CMPs-2 and C-CMPs-3NP~C-CMPs-5NP. The broad peaks of 23.1 and weak peaks of 44 at 2q represent the (002) and (101) graphitic planes, showing a partial graphite structure. Moreover, with low angular offset relative to commercial graphite carbon (26.48 ) [43,44]. No significant changes were observed after doping with N, S compared with C-CMPs-1NP and CCMPs-2 in Fig. 2. In addition, the XRD pattern of C-CMPs-1NP shows no Pd and Cu particles, demonstrated that the purification process (filtered and washed with chloroform, acetone, water and methanol) has been substantially removed the Pd and Cu particles, and elemental analysis also confirmed this. As shown in Fig. 3, in the Raman spectrum, the two characteristic peaks of carbon are obvious. The D band at about 1360 cm
1, due to existence of doped N, O and S atoms consistent with the defect structure. The G band at about 1601 cm
1, correlated with the tangential vibration of the sp2 carbon atom, indicating the presence of some graphite structure.

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Scheme. 1. (a), (b) Synthesis and Simulation structure of CMPs, (c) Synthesis and Simulation structure of C-CMPs-NP (the specific simulation process was in the supporting information).

On the basis of the literature [45], the D band means an annular respiratory vibration mode of sp2-mixed carbon ring in graphene. The G band expresses an in-plane vibrational mode of the sp2hybridized carbon atoms in graphene. Wherefore, the ratio of two intensity peaks (ID/IG) expresses the extent of disorder related to the carbon structure. Evidently, the ID/IG value of C-CMPs-1NP was the largest among the five materials, presumably the result of structural distortion arising from the incorporation of N atoms [46]. Furthermore, comparing the ID/IG value of C-CMPs-1NP and CCMPs-2 carbonized at the same temperature, we found that the ID/ IG value of C-CMPs-2 is below than that of C-CMPs-1NP, this may be owing to the absence of N- and S- doping in the CMPs. Except the above, conjugated microporous polymers (CMPs) with larger surface area and excellent special performance, also can stimulate the catalysts' electrical conductivity. In Fig. 4, the N2 adsorption-desorption isotherm curves of all CMPs samples (i.e., C-CMPs-1NP, C-CMPs-2, C-CMPs-3NP, C-CMPs4NP and C-CMPs-5NP), both showed analogous I-type isotherms characteristic of microporous material (Fig. 4), which is characteristic of microporous feature of the materials according to the IUPAC [47]. The BET specific surface area of the resulting CMPs-based catalyst was calculated to be the C-CMPs-1NP (782.9 m2g-1), C-

CMPs-2 (1037.6 m2g-1), C-CMPs-3NP (533.9 m2g-1), C-CMPs-4NP (589.4 m2g-1) and C-CMPs-5NP (592.6 m2g-1) in Table S3. Exposure high activity sites is due to large surface area, indeed, the C-CMPs1NP displayed the highest activity and durability among those of the other samples. The total pore volume of single-point at P/ P0 ¼ 0.9901 was considered to be 0.259 cm3g-1 for C-CMPs-1NP, 0.266 cm3g-1 for C-CMPs-2, 0.210 cm3g-1 for C-CMPs-3NP, 0.217 cm3g-1 for C-CMPs-4NP and 0.221 cm3g-1 for C-CMPs-5NP, respectively (Table S3). From the adsorption data we obtained the pore size distribution and the isotherm is calculated by the BJH method, is descripted in Fig. 4b, d and f. The results show that most of the samples have a pore size of 1.94 nm, it was found all samples from CMPs owned the micropores structure. Compared to the CMPs support, the C-CMPs-3NP, CCMPs-4NP and C-CMPs-5NP exhibit reduced BET surface area, due to the difference between CMPs and monomer doping and pure CMP adsorption. From Fig. 4a in the nitrogen absorption (p/p0 < 0.1) for the C-CMPs-1NP revealed that the adsorption mainly occurred in micropores. Furthermore, Consistent with reduced nitrogen uptake, the micropore surface area (Smicro) is also reduced (CCMPs-3NP< C-CMPs-4NP< C-CMPs-5NP< C-CMPs-1NP < C-CMPs2). Furthermore, the special porous structure of p-converged

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Fig. 1. SEM (aee) images of C-CMPs-1NP, C-CMPs-2, C-CMPs-3NP, C-CMPs-4NP and C-CMPs-5NP, (f): EDS elemental mapping of (g) S (green), (h) N (orange), (i) C (red) in the same area of C-CMPs-1NP.

Fig. 2. XRD patterns of the C-CMPs-1NP, C-CMPs-2, C-CMPs-3NP, C-CMPs-4NP and CCMPs-5NP samples.

Fig. 3. Raman spectra of the C-CMPs-1NP, C-CMPs-2, C-CMPs-3NP, C-CMPs-4NP and CCMPs-5NP samples.

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Fig. 4. (a), (c) and (e) N2 adsorption/desorption isotherms of C-CMPs-NP (the solid circle indicates the adsorption curve and the semicircle indicates the desorption curve), (b), (d) and (f) Pore size distributions of the C-CMPs-NP calculated by BJH method.

framework be able to provide low-resistant spread channels accelerating the oxygen transport in electrolyte, it can strengthen the ORR catalytic activity [48,49]. From Fig. 5 we found the survey XPS spectrum of C-CMPs-1NP, C-CMPs-2, C-CMPs-3NP, C-CMPs-4NP and C-CMPs-5NP. It was

consisted of carbon (C 1s ¼ 284.8 eV), sulfur (S 2p ¼ 275.5 eV), oxygen (O 1s ¼ 532.9 eV) and nitrogen (N 1s ¼ 401.0 eV) [50,51]. May be due to the adsorption of atmospheric oxygen on the catalyst, resulting in the presence of oxygen. Table S2 was the composition type of several catalysts obtained by XPS test, and manifested

Fig. 5. (a) Survey XPS spectrum of C-CMPs-1NP, high-resolution (b) N 1s XPS and (c) S 2p XPS spectra, and the fitted curves of different types of N and S.

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Fig. 6. High-resolution N 1s XPS spectra of C-CMPs-3NP(a), C-CMPs-4NP (c) and C-CMPs-5NP (e), high-resolution S 2p XPS spectra (b) of C-CMPs-3NP, C-CMPs-4NP (d) and C-CMPs5NP (f).

that both N and S atoms have been successfully embedded in CMPs. The atomic percentages with respect to the catalysts' N content, were 1.70%, 2.99%, 2.83% and 2.32% for C-CMPs-1NP, C-CMPs-3NP, C-CMPs-4NP and C-CMPs-5NP, respectively. And the S content, percentages were 0.57%, 0.68%, 0.62% and 0.63% for C-CMPs-1NP, CCMPs-3NP, C-CMPs-4NP and C-CMPs-5NP, respectively. Compared with three materials (C-CMPs-3NP~C-CMPs-5NP), the electrochemical performance increases as the content of S and N increases. From this we suspected that the addition of N and S is an important

factor for the improvement of ORR performance. Also for C-CMPs1NP and C-CMPs-2, with the content of N and S increases, the catalytic performance is significantly improved. And, the deconvolution spectra of four peaks at 397.4, 399.8, 401.2 and 402.4 eV are attributable to pyridinic-N, pyrrolic-N, graphitic-N and oxidized-N, respectively (Table S4) [52,53]. Prominently, the appearance of four peaks sharply changes when the monomers are different (Fig. 6a, c and e), revealing during the carbonization of CMPs and ionic liquids produced the N

Fig. 7. CV curves of C-CMPs-1NP, C-CMPs-2, C-CMPs-3NP, C-CMPs-4NP and C-CMPs-5NP in N2-saturated (a) and O2-saturated (b) 0.1 M KOH solution, (c) CV curves of CMPs 1NP and Pt/C in N2-saturated (dotted line) and O2-saturated (solid line), (d)LSV curves of C-CMPs-1NP, C-CMPs-2, C-CMPs-3NP, C-CMPs-4NP and C-CMPs-5NP and Pt/C towards ORR, LSV voltammograms recorded in O2-saturated 0.1 M KOH at 1600 rpm.

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Fig. 8. (a) LSV curves of Pt/C at different rotation speeds in O2-saturated 0.1 M KOH solution, the inset of (a) shows KeL plots for Pt/C at different potentials, (b) LSV curves of CCMPs-1NP at different rotation speeds in O2-saturated 0.1 M KOH solution, the inset of (a) shows KeL plots for C-CMPs-1NP at different potentials.

bonding configurations which possessed the different amounts of N. It is worth noting, with the specific surface was increasing, the highest energy peak pyridine N will play a very important role, meaning that more pyridinium nitrogen atoms are embedded in the carbon network (Figs. 5a and 6). Obviously, the C-CMPs-1NP with the supreme content of pyridinic N among the five types of N has the best electrochemical energy (Table S5). Such a phenomenon is consistent with the ORR active sites of various N-doped carbon materials produced by pyridine N [7]. It is worth noting that among the four CMPs doped with ionic liquids, the content of pyridinium nitrogen of C-CMPs-1NP is the smallest, but it has the best electrochemical performance. We suggested that it may be due to the large specific surface area characteristic of C-CMPs-1NP material, resulting in more active site exposure. The extra peak at approximately 407.8 eV that emerged in the N 1s XPS spectra of C-CMPs-NP

could be allocated to the configurations of SeN bonds at the periphery of C-CMPs-NP. Fig. 5c illustrated the S 2p spectra of CCMPs-NP. Based on the literature, the binding energied at 163, 164.5 and 168.7 eV, respectively. Due to their spin-orbital coupling, the first two peaks are consistent with the 2p3/2 and 2p1/2 positions reported by thiophene-S [54,55]. The third peak should come from some oxidized sulfur [55,56]. The peak at 164.7 eV is related to sulfur, which is bound as a heteroatom in aromatics, for instance, thiophene [57]. The other peak at 168.8 eV arises from a sulfureoxygen compound. By analyzing the above XPS spectrum, we can draw conclusions that doping of 1-ethyl-3 methylimidazolium bis (trifluoromethylsulfonyl) imide can reorganized the S and N atoms and further modified the distribution of nitrogenous species. With many advantageous features in composition, large specific surface area, hard carbon structure, high active

Fig. 9. (a) The corresponding RRDE voltammograms recorded in O2-saturated 0.1 M KOH at different rotation speeds for Pt/C, (b) The corresponding RRDE voltammograms recorded in O2-saturated 0.1 M KOH at different rotation speeds for C-CMPs-1NP.

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Fig. 10. (a) Disk current curves of C-CMPs-2, C-CMPs-3NP, C-CMPs-4NP and C-CMPs-5NP, (b) Ring current curves of C-CMPs-2, C-CMPs-3NP, C-CMPs-4NP and C-CMPs-5NP, (c) H2O2 yield plots of C-CMPs-1NP, C-CMPs-2, C-CMPs-3NP, C-CMPs-4NP and C-CMPs-5NP from 0 to 0.8 V (vs RHE), (d) Electronic transfer number curves of C-CMPs-1NP.

site and superior N-doping level. When compared with the commercial Pt/C electrocatalyst benchmark test under the same test conditions, the resultant C-CMPs-1NP shows strong catalytic activity and durability as well as good resistance to methanol toxicity (Table S6). Evaluation of electrocatalytic activity in oxygen reduction reaction of CMPs-based catalyst, firstly, we performed cyclic voltammetry (CV) measurements for C-CMPs-NP in N2- and O2saturated solution (0.1 M KOH, Fig. 7). As we can discovered, the whole electrodes based on CMPs showed a clear-cut peak of ORR between 0.68 and 0.82 V (vs. reversible hydrogen electrode (RHE), the same below), but not appear in the N2-saturated solution. From Fig. 7b that each catalyst of the CV curves performed outstanding oxygen reduction peaks. Simultaneously, different ORR peak potential and peak current density are presented by different

catalysts, and the ORR peak potential followed an order: C-CMPs-1NP>C-CMPs-3NP>C-CMPs-4NP>C- CMPs-5NP>C-CMPs2 (Fig. 7b). According to the literature, we known that the catalysts possessed different intrinsic activities, with the difference peak potential in the ORR. Among them, the C-CMPs-1NP catalyst showed the strongest catalytic activity (Fig. 7c and d), the C-CMPs1NP has a more pronounced oxygen reduction peak at 0.82 V than commercial Pt/C in Fig. 7c measured under the identical conditions. In order to learn more about the linear sweep voltammetry (LSV) curve of oxygen reduction reaction kinetics, the linear sweep voltammetry recorded on the rotating ring disk electrode (RRDE) was performed at a scanning rate of 10 mV s
1 in O2-saturated 0.1 M KOH solution. In Fig. 7d we observed that the LSV curve of C-CMPs1NP is significantly smoother than that of commercial Pt/C,

Fig. 11. (a) ADT test ORR polarization curve in 0.1 M KOH solution (1600 rpm) introducing a mixture of O2 and methanol (3 M), (b) ORR polarization curve before and after 40,000 cycles in O2-saturated 0.1 M KOH solution at a rotation rate of 1600 rpm.

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Fig. 12. (a) LSV curves of C-CMPs-2 at different rotation speeds in O2-saturated 0.1 M KOH solution, the inset of (a) shows KeL plots for C-CMPs-2, C-CMPs-3NP, C-CMPs-4NP and CCMPs-5NP at different potentials, (b) Electron transfer number plots of C-CMPs-2 (vs RHE), the same characterization test for C-CMPs-3NP (c) and (d), C-CMPs-4NP for (e) and (f), CCMPs-5NP for (j) and (h).

indicated that during the whole oxygen reduction process, compared with C-CMPs-1NP, the commercial Pt/C was more prone to catalyst poisoning, thus the oxygen reduction catalytic performance of C-CMPs-1NP was better than that of commercial Pt/C.

Those signify that the electrocatalytic performance of C-CMPs1NP for ORR improved significantly due to doping the N. In contrast, the C-CMPs-2 emerges the highest diffusion limited current density (Jd) of 4.9 mA cm
2 between all the catalysts studied

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under the 1600 rmp (Fig. 7d), we suspected the main reason was that it possessed the largest surface area, more active sites are exposed. Although the diffusion limited current density of C-CMPs1NP (Jd ¼ 4.2 mA cm
2) was inferior to the C-CMPs-2, the C-CMPs1NP other aspects of electrochemical performance were superior to C-CMPs-2, very close to the commercial Pt/C (E1/2 ¼ 0.83 V, Jd ¼ 4.3 mA cm
2), it indicated that the prepared C-CMPs-NP has significant electrocatalytic activity for ORR. Compared to C-CMPs-2, C-CMPs-1NP shows significant ORR performance, this proved the catalyst's ORR activity was evidently enhanced due to the loaded of S and N. In other words, the interaction between N and S, resulted the significant improvements. In terms of N-doped carbon, previous research reported the ORR active site was produced by pyridine N, and the carbon atom with Lewis basicity is adjacent to pyridine N is the ORR active site. Considering the strong electron receptivity of this style of nitrogenatom, the carbon atom in the obtained C-CMPs-NP boundary can generate positive electrical charge, they help to adsorb oxygen, and can easily attract electrons from the anode, thereby promoting ORR [57]. What is mentioned of S- and N-doped carbon, the great majority researchers have reported, that the overwhelming majority of active site structures were formed of SeN configurations. However, catalytic performance for the C-CMPs-2 (E1/2 ¼ 0.68 V) was below the C-CMPs-1NP (E1/2 ¼ 0.82 V), C-CMPs-3NP (E1/2 ¼ 0.78 V), CCMPs-4NP (E1/2 ¼ 0.79 V) and C-CMPs-5NP (E1/2 ¼ 0.73 V) in alkaline medium (Table S7), and we speculated that there may be a shortage of CeN active sites. Thereby greatly limited the exposure of the ORR active site, also inhibited the diffusion process and adsorption process of O2 (Fig. 7b and d). In other words, the unsatisfactory electrochemical activity of C-CMPs-2 suggested that SeNx played a leading role in the high catalytic activity. To further proved the excellent performance of CMPs-based catalyst. Fig. 8 and Fig. 12 displayed the ORR curves measured at different speed of revolution rotation for C-CMPs-1NP. Form Fig. 8a and b we can saw that C-CMPs-1NP and commercial Pt/C with similar LSV curves. However, compared with expensive metals such as commercial Pt/C, the C-CMPs-1NP exhibited catalytic properties close to commercial Pt/C during oxygen reduction, the half-wave potential (E1/2) of C-CMPs-1NP was 0.82 V, the onset potential (Eonset) was 0.95 V, and the number of electron transfers was 3.96, which was infinitely close to 4. And for a commercial Pt/C that's several times more expensive than C-CMPs-1NP, it the number of electron transfers was 3.98 in the oxygen reduction process (Table S7). Therefore, the ORR catalytic performance of C-CMPs1NP was better than commercial Pt/C. Furthermore, transfer numbers measured by RRDE: C-CMPs-2, n ¼ 3.92 (Fig. 12b); CCMPs-3NP, n ¼ 3.94 (Fig. 12d); C-CMPs-4NP, n ¼ 3.90 (Fig. 12f) and C-CMPs-5NP, n ¼ 3.80 (Fig. 12h), from the 0e0.8 V. The high catalytic efficiency of the catalyst was further verified, verification by RRDE's measurement results (Fig. 10a, b and c). The C-CMPs-1NP shows a very low yield of hydrogen peroxide: 0.02e0.1 in 0.1 M KOH (Fig. 10c). Meanwhile, the number of electron transfers of the C-CMPs-1NP is 3.96 near to 4 (Pt/C) in Fig. 10d, verifying the catalytic process of C-CMPS-1NP is four electron process. Simultaneously, the yields of the hydrogen peroxide of other four catalysts in Fig. 10c, the C-CMPs-2: 0.03e0.21; C-CMPs-3NP: 0.02e0.22; CCMPs-4NP: 0.05e0.34 and C-CMPs-5NP: 0.09e0.23 (see Fig. 9). Typically, the large specific surface area and large amount of microporous properties of CMPs can contributed to the catalyst's ORR activity, It's remarkable that the surface area of C-CMPs-1NP catalyst based on CMPs is as high as 782.9 m2 g
1, micro/mesoporous structure in Fig. 4a and b, allowing the catalyst to have more active sites. In addition, there are abundant active sites on the electrochemical contact surfaces of the catalysts, at the same time due to the special structure of the CMPs material, its porous carbon

structure promoted high efficiency transmission of O2, Hþ, and H2O. Obviously, the surface area of the C-CMPs-3NP, C-CMPs-4NP and CCMPs-5NP were basically the same, as expected, the catalytic activity was relatively low. Meanwhile, according to the XPS researches (Fig. 5), It is considered that the heighten of catalytic activity of C-CMPs-NP prepared at 900  C can be attributed to the increase of pyridinic N contents. The durability of catalyst is an important factor affecting the stable operation of fuel cell, the C-CMPs-1NP displayed high methanol tolerance. It could be seen from Fig. 11a, after an addition of methanol into O2-saturated 0.1 M of KOH solution, no noticeable decay observes in the ORR current density at the CCMPs-1NP electrode. In contrast, the Pt/C catalyst emerges a remarkable drop under the same conditions, indicating the much advantage of C-CMPs-1NP than the Pt/C catalyst to methanol crossover. Therefore, the C-CMPs-1NP catalyst was good potential as a Pt substitute for practical applications, as proved by the small shifts of half-wave potential after addition of methanol. The CCMPs-1NP still possessed the half-wave potential (E1/2) of 0.82 V, the commercial Pt/C with lower the half-wave potential (E1/2) of 0.78 V compared to its initial half-wave potential (0.83 V). In order to explored durability of C-CMPs-1NP, both C-CMPS-1NP catalyst and Pt/C cycled from 0 to 1.2 V at 50 mV S
1 scan rate to evaluate their durability (Fig. 11b). After 40,000s continuous cycles, the CCMPS-1NP possessed the half-wave potential (E1/2) of 0.74 V, decreased by almost 9.7% of its initial half-wave potential (0.82 V), and the diffusion limited current density (Jd) of 3.8 mA cm
2, decreased by almost 9.5% of its initial diffusion limited current density (4.2 mA cm
2). While that of the commercial Pt/C possessed the half-wave potential (E1/2) of 0.78 V, decreased by almost 6.0% of its initial half-wave potential (0.83 V), and the diffusion limited current density (Jd) of 2.8 mA cm
2, decreased by almost 34.8% of its initial diffusion limited current density (4.3 mA cm
2). Compared to the C- CMPs-1NP, the commercial Pt/ C showed a fast decrease of the current density over 35% attenuation, the C- CMPs-1NP only decrease 9.5% attenuation. Therefore, the C-CMPs-1NP with excellent durability, good methanol tolerance and outstanding ORR activity turn it into a promising catalyst in alkaline medium. 3. Conclusion To sum up, we have proved a novel pathway for preparation of N-doped hard carbon as a new type of non-metallic catalyst derived from conjugated microporous polymers for highly efficient oxygen reduction reaction. Due to the special structure of CMPs, the prepared catalyst has super porosity and large specific surface area (782.9 m2g-1), which would greatly promotes the exposure of high active sites and the rapid transport of oxygen electrons. Besides, owning to the lone pair electrons of pyridine N have a strong affinity for oxygen atoms, the high content of pyridine N of C-CMPsNP can accelerate the Oxygen reduction reaction. In view of the intrinsic virtue of C-CMPs-NP mentioned above, the prepared CMPs-based catalyst with ionic liquid loaded on porous carbon, which has been proved to put up exceedingly good performance for ORR in 0.1 M KOH solutions (E1/2 ¼ 0.82 V, Jd ¼ 4.2 mA cm
2, n ¼ 3.96), making it a promising candidate to take the place of Ptbased catalysts for oxygen reduction reaction. Because of the recognizable adaptability of CMPs, its chemical structure and composition can be adjusted by simply changing the structure block of CMPs, it can be expected that the ORR activity, stability and methanol resistance of CMPs-based catalysts will be further enhanced, therefore, this new methodology could be used for producing multifarious high-performance CMPs-based catalyst for ORR.

R. Jiao et al. / Materials Today Energy 16 (2020) 100382

Author statement Ruin Jiao: Conceptualization, Methodology, Validation, Formal analysis, Writing- Original Draft, Visualization. Wanli Zhang: Methodology, Validation, Formal analysis. Hanxue Sun: WritingOriginal draft preparation, Investigation, Supervision, Resources. Zhaoqi Zhu: Formal analysis, Data curation, Resources. Zifeng Yang: Methodology, Formal analysis. Weidong Liang: Resources, Project administration. An Li: Conceptualization, Resources, Supervision, Project administration, Funding acquisition, Data Curation Writing- Reviewing and Editing. Declaration of Competing Interest No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. This work described was original research which has not been published previously, and not under consideration for publication elsewhere. Acknowledgment The authors are grateful to the National Natural Science Foundation of China (Grant No. 21975113, 51663012 and 51962018), the Natural Science Foundation of Gansu Province, China (Grant No. 1610RJYA001), Support Program for Hongliu Young Teachers (Q201411), Hongliu Elitist Scholars of LUT (J201401), Support Program for Longyuan Youth, Fundamental Research Funds for the Universities of Gansu Province, Project of Collaborative Innovation Team and Innovation and Entrepreneurship Talent Project of Lanzhou (2017-RC-33). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.mtener.2020.100382. References [1] T. Zhou, Y. Du, S. Yin, X. Tian, H. Yang, X. Wang, B. Liu, H. Zheng, S. Qiao, R. Xu, Nitrogen-doped cobalt phosphate@ nanocarbon hybrids for efficient electrocatalytic oxygen reduction, Energy Environ. Sci. 9 (2016) 2563e2570, https:// doi.org/10.1039/C6EE01297C. [2] Q. Liu, X. Liu, L. Zheng, J.S. hui, The solid-phase synthesis of an Fe-N-C electrocatalyst for high-Power proton-exchange membrane fuel cells, Angew. Chem. Int. Ed. 130 (2018) 1218e1222, https://doi.org/10.1002/anie.201709597. [3] X. Zeng, J. Shui, X. Liu, Q. Liu, Y. Li, J. Shang, L. Zheng, R. Yu, Single-atom to single-atom grafting of Pt1 onto Fe-N4 center: Pt1@Fe-N-C multifunctional electrocatalyst with significantly enhanced properties, Adv. Energy Mater. 8 (2018) 1701345, https://doi.org/10.1002/aenm.201701345. [4] X. Tian, J. Luo, H. Nan, H. Zou, R. Chen, T. Shu, X. Li, Y. Li, H. Song, S. Liao, R.R. Adzic, Transition metal nitride coated with atomic layers of Pt as a lowcost, highly stable electrocatalyst for the oxygen reduction reaction, J. Am. Chem. Soc. 138 (2016) 1575e1583, https://doi.org/10.1021/jacs.5b11364. [5] Y. Qin, X. Zhang, X. Dai, H. Sun, Y. Yang, X. Li, Q. Shi, D. Gao, H. Wang, N.F. Yu, S.G. Sun, Graphene oxide-assisted synthesis of Pt-Co alloy nanocrystals with high-index facets and enhanced electrocatalytic properties, Small 12 (2016) 524e533, https://doi.org/10.1002/smll.201502669. [6] N. Gupta, Y. Ding, Z. Feng, D. Su, Palladium supported on nanodiamonds as an efficient catalyst for the hydrogenating deamination of benzonitrile and related compounds, ChemCatChem 8 (2016) 22e928, https://doi.org/10.1002/ cctc.201501243. [7] D. Guo, R. Shibuya, C. Akiba, S. Saji, T. Kondo, Active sites of nitrogen-doped carbon materials for oxygen reduction reaction clarified using model catalysts, J. Nakamura Sci. 351 (2016) 361e365, https://doi.org/10.1126/ science.aad0832. [8] Y.J. Wang, N.N. Zhao, B.Z. Fang, H. Li, X.T.T. Bi, H.J. Wang, Carbon-supported Ptbased alloy electrocatalysts for the oxygen reduction reaction in polymer electrolyte membrane fuel cells: particle size, shape, and composition manipulation and their impact to activity, Chem. Rev. 115 (2015) 3433e3467, https://doi.org/10.1021/cr500519c. [9] L. Dai, Y. Xue, L. Qu, H.J. Choi, J.B. Baek, Metal-free catalysts for oxygen reduction reaction, Chem. Rev. 115 (2015) 4823e4892, https://doi.org/ 10.1021/cr5003563.

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