Metal-organic framework-derived metal-free highly graphitized nitrogen-doped porous carbon with a hierarchical porous structure as an efficient and stable electrocatalyst for oxygen reduction reaction

Accepted Manuscript Metal-organic framework-derived metal-free highly graphitized nitrogen-doped porous carbon with a hierarchical porous structure as an efficient and stable electrocatalyst for oxygen reduction reaction Lijuan Yang, Guancheng Xu, Jinjin Ban, Li Zhang, Gui Xu, Yan Lv, Dianzeng Jia PII: DOI: Reference:

S0021-9797(18)31195-0 https://doi.org/10.1016/j.jcis.2018.10.007 YJCIS 24162

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

5 September 2018 2 October 2018 3 October 2018

Please cite this article as: L. Yang, G. Xu, J. Ban, L. Zhang, G. Xu, Y. Lv, D. Jia, Metal-organic framework-derived metal-free highly graphitized nitrogen-doped porous carbon with a hierarchical porous structure as an efficient and stable electrocatalyst for oxygen reduction reaction, Journal of Colloid and Interface Science (2018), doi: https:// doi.org/10.1016/j.jcis.2018.10.007

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Metal-organic framework-derived metal-free highly graphitized nitrogen-doped porous carbon with a hierarchical porous structure as an efficient and stable electrocatalyst for oxygen reduction reaction Lijuan Yang, Guancheng Xu*, Jinjin Ban, Li Zhang, Gui Xu, Yan Lv, Dianzeng Jia*

Key Laboratory of Energy Materials Chemistry, Ministry of Education; Key Laboratory of Advanced Functional Materials, Autonomous Region; Institute of Applied Chemistry, Xinjiang University, Urumqi, 830046 Xinjiang, P. R. China.

* Corresponding author. E-mail: [email protected]. Tel./Fax: +86-991-8580586

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Abstract Nitrogen-doped carbon materials are promising oxygen reduction reaction (ORR) electrocatalysts owing to high performance and stability. Herein, a three-dimensional porous bio-MOF-1, Zn8(Ad)4(Bpdc)6O·2Me2NH2 (Ad = adeninate; Bpdc = biphenyldicarboxylate), was used as precursor to fabricate N-doped porous carbon materials (NPC-1000-ts, where 1000 stands for the carbonization temperature and t represents the carbonization time, t = 2, 3 and 4 h) by simple carbonization under Ar atmosphere. The porous carbon materials had different contents of graphitic N and graphitization degrees of carbon. The catalytic activities of NPCs as metal-free ORR electrocatalysts were studied. The obtained NPC-1000-4 (pyrolysis under 1000°C for 4 h) displayed outstanding ORR performance, with a positive onset potential (-0.012 V), a higher half-wave potential (E1/2) (-0.13 V) and a larger limiting current density (-5.76 mA/cm2) at -0.8 V (vs. Ag/AgCl) in KOH solution (0.1 M) than those of commercial Pt/C (20 wt%) catalyst (Eonset = -0.014 V, E1/2 = -0.14 V and -5.08 mA/cm2 at -0.8 V vs. Ag/AgCl). Obviously, the onset potential of NPC-1000-4 surpassed that of Pt/C, which was rare among currently available studies on metal-free nitrogen-doped porous carbon materials. Graphitic N significantly affected ORR catalytic performance besides graphitization degree of carbon. Meanwhile, NPC-1000-4 allowed an effective 4e--dominant ORR process, and most importantly, coupled with much higher long-term stability (89.5%) than that of commercial Pt/C (20 wt%, 65.8%) catalyst and higher resistance to methanol poisoning. The remarkable ORR activity of NPC-1000-4 can be ascribed to large surface area, considerable hierarchical porosity, high graphitization degree and synergism between enriched active sites and high portion of graphitic N. Overall, the findings guide the development of MOF-derived metal-free N-doped carbon materials as high-activity non-precious electrocatalysts for ORR. Keywords: Metal-organic framework, Oxygen reduction reaction, Metal-free nitrogen-doped carbon electrocatalyst.

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1. Introduction Oxygen electrochemistry is of immense significance to electrochemical conversion/storage devices. Especially, oxygen reduction reaction (ORR) is a vital process in different energy conversion and storage devices, such as metal-air batteries and fuel cells. [1-5] However, it has large over-potential and sluggish kinetics due to involvement of multiple steps in electron transfer (O2 + 2H2O + 4e-/4OH-, alkaline media). Therefore, it is urgent to develop an efficient and durable catalyst to facilitate ORR at a practical rate. Conventional Pt-based catalysts have been widely studied and optimized for ORR with high catalytic performance [6-8]. Nevertheless, Pt-based catalysts still suffer from high cost, CO deactivation, limited long-time stability and poor resistance to methanol. These factors evidently hinder the commercialization of Pt-based catalyst for electrochemical devices. To circumvent these issues, catalysts should be developed based on non-precious metals with superb activity and outstanding durability. Recently, non-precious metal electrocatalysts for ORR in alkaline medium have attracted considerable attention because of unique physical and chemical features, mainly including earth-abundant non-noble metal alloys [9,10], transition metal oxides [11,12], transition metal-containing nitrogen-doped carbon materials [13-15] and completely metal-free heteroatom-doped carbons [16,17]. Particularly, metal-free N-doped carbon materials have become potential substitutes for Pt-based catalysts in the electrocatalytic field [18]. To this end, researchers have endeavored to fabricate metal-free nitrogen-doped carbon catalysts with low costs and high ORR activities. For instance, He et al. synthesized nitrogen-doped hollow carbon frameworks (N-HCF-Ts, where T represents the calcination temperature) by one-step calcination of glucose and dicyandiamide. The resultant N-HCF-900 showed higher ORR activity than that of 20 wt% commercial Pt/C catalyst [19]. Typically, metal-free nitrogen-doped carbon electrocatalysts are synthesized by high-temperature calcination of nitrogen- and carbon-containing precursors (e.g. dicyandiamide, glucose, melamine, urea and CNTs) or traditional chemical doping. Nevertheless, their ORR catalytic performances are limited due to poor stability, low graphitization degree and electron conductivity as well as tedious preparation process [20-26]. Therefore, preparing suitable precursors plays a critical role in developing nitrogen-doped carbon ORR catalysts. At present, metal-organic frameworks (MOFs), which are assembled from various metal ions (or clusters) and organic ligands in a three-dimensional (3D) space, are efficient precursors for 3

synthesizing N-doped porous carbon materials [27-29]. This strategy has the following advantages: (i) organic ligands in MOFs can form carbon framework structures, and these ligands usually contain heteroatoms for preparing heteroatom-doped carbon materials by high-temperature calcination; (ii) during or after carbonization, removing metal species can provide more pores for porous carbon matrix; and (iii) MOFs facilitate the synthesis of porous carbon materials through high-temperature carbonization owing to well-defined 3D pore structure and high stability [30-32]. Up to now, several types of MOFs, such as Zn-MOF (MOF-5, ZIF-8) [33,34] and Al-PCP [35] have been identified as ideal self-sacrificial precursors for preparing highly porous carbon materials. Especially, Zn-based MOFs, as attractive precursors, have usually been used to prepare carbon materials. In the carbonization process, organic ligands can be converted into carbon, and Zn metal is acquired by carbon reducing Zn ion, giving porous carbon materials at above 908°C after Zn metal evaporation [36]. More importantly, MOF-derived nitrogen-doped porous carbon materials always have high surface areas, heteroatom doping and graphitized structures. As a result, being employed as ORR electrocatalyst, they exhibited good activity and stability [37,38]. In addition, eligible high-performance nitrogen-doped carbon ORR catalysts should at least meet the following three requirements: first, high specific surface area and abundant pore structures are necessary for oxygen and electrolyte diffusion; second, an ideal ORR catalyst should have high electronic conductivity to promote electron delivery; third, pyridinic and graphitic nitrogen atoms, as active sites, can improve the first electron transfer kinetics involved in ORR [39-41]. Thus, during the preparation of MOFs derived N-doped porous carbon materials, suitable precursors containing abundant C and N sources should be first selected, and the carbonization temperature and time should be optimized. For example, Zhang et al. fabricated NGPCs (nitrogen-doped graphitized porous carbons) by calcinating Zn-MOF (ZIF-8) precursor at high temperature. The specific surface area approached 932 m2/g, and the resulting porous carbons contained more pyridinic nitrogens. Electrochemical tests showed that as-prepared NGPC-1000-10 (calcined at 1000°C for 10 h) displayed the highest catalytic performance on ORR (half-wave and onset potentials were -0.260 and -0.089 V vs. Ag/AgCl respectively) following an effective 4e- transfer mechanism, accompanied by excellent methanol tolerance and cycling durability in alkaline solution[42]. They confirmed that the graphitization degree of carbon and porous structure are important factors for ORR. To further evaluate the effects of calcination temperature on ORR, Li 4

et al. tested the ORR catalytic activities of nitrogen-doped porous carbons which were obtained by directly carbonizing Al(OH)(Bpydc) (Bpydc represents 2,2’-bipyridine-5,5’-dicarboxylate) at different temperatures. The graphitization degree of nitrogen-doped porous carbons increased significantly with elevating pyrolysis temperature. Actually, these fabricated carbon catalysts had higher activity, stability and methanol resistance than those of commercial 10% Pt/C catalyst towards ORR [43]. Besides chemical compositions, large specific surface areas and special structures (e.g. hierarchical pores and graphitic carbon structures) have been verified as key factors for enhancing the ORR performance of nitrogen-doped carbons [44,45]. Remarkably, choosing an ideal precursor and carbonization temperature is demanded for the development of high-activity N-doped carbon ORR catalysts. As a novel adenine-based MOF, bio-MOF-1 has 3D porous structure as well as high chemical and thermal stabilities. In bio-MOF-1, adeninate ligands are rich in N, benefiting the introduction of nitrogen source and doping into carbon matrix during pyrolysis; biphenyldicarboxylate (Bpdc) contains two benzene rings that facilitate the formation of carbon in the carbonization process. Hence, we herein employed bio-MOF-1 as precursor to develop efficient and stable metal-free nitrogen-doped carbon materials as outstanding ORR electrocatalysts. First, bio-MOF-1 was obtained by a simple solvothermal reaction. Subsequently, NPC-1000-ts (t = 2, 3 and 4 h) were fabricated by carbonizing bio-MOF-1 precursor under high-purity Ar atmosphere. The influences of carbonization temperature and time on ORR activity were assessed. Electrochemical tests showed that the optimized NPC-1000-4 material was highly electrocatalytically active, also with superior cycling stability and methanol durability in alkaline media to those of commercial Pt/C (20%) catalyst, which can be attributed to high graphitization degree, porous structures, large surface area and large amounts of graphitic N. 2. Experimental section 2.1 Chemicals and Materials All chemicals and materials were available from commercial sources and used as purchased without further purification, mainly including zinc acetate dihydrate (Zn(OAC) 2∙2H2O, 99.0%, Sinopharm Chemical Reagent Co., Ltd.), adenine (Ad, 99.5%, Beijing J&K Technology Co., Ltd.), biphenyl dicarboxylic acid (H2Bpdc, 97%, Shanghai Bide Pharmaceutical Technology Co., Ltd.), N,N′-dimethylformamide (DMF) and nitric acid (Tianjin ZhiYuan Reagent Co., Ltd.). 5

2.2 Synthesis procedure 2.2.1 Synthesis of bio-MOF-1 precursor Bio-MOF-1 Zn8(Ad)4(Bpdc)6O·2Me2NH2·8DMF·11H2O crystals were synthesized according to previous literatures with some modifications [46,47]. Typically, Ad (0.1125 mmol, 15.20 mg), H2Bpdc (0.225 mmol, 54.50 mg) and zinc acetate dihydrate (0.3375 mmol, 74.08 mg) were dissolved in DMF (13.5 mL) under magnetic stirring for 30 min, into which nitric acid (1 mol/L, 1 mL) was then poured. After complete mixing, the resultant solution was added into a 25 mL Teflon-lined stainless steel autoclave, and heated at 140°C for 24 h. The produced colorless crystals were collected by centrifugation at 4000 rpm for 5 min, washed with DMF (3×10 mL) and dried in air at room temperature prior to use. 2.2.2 Synthesis of NPC-1000-ts electrocatalysts (t = 2, 3 and 4 h) In a representative carbonization process, the as-fabricated bio-MOF-1 was put into a quartz boat flatly. Then the quartz boat was transferred into a tube furnace with a programmed temperature under high-purity Ar atmosphere. The furnace was sealed airtight, heated up to 200°C with a heating rate of 5°C/min and maintained thereafter for 1 h, aiming to remove DMF and water molecules. Afterwards, the temperature was continuously increased to 1000°C at a heating speed of 5°C/min, and maintained for different carbonization times (t = 2, 3 and 4 h). Subsequently, the furnace was cooled naturally to room temperature, yielding black powders referred to as NPC-1000-2, NPC-1000-3 and NPC-1000-4 respectively. For comparison, NPC-900-2 and NPC-1000-5 were also prepared through the same experimental procedure. 3. Results and discussion Preparations and characterizations of materials

Scheme 1 Schematic illustration for synthesis of NPCs.

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A schematic diagram describing the synthesis procedure of bio-MOF-1-derived porous carbon materials is shown in Scheme 1. Bio-MOF-1 was fabricated by a modified solvothermal method in DMF using adenine, biphenyl dicarboxylic acid and zinc acetate dihydrate as reactants. Subsequently, NPCs electrocatalysts were obtained by pyrolyzing bio-MOF-1 precursor in Ar atmosphere. PXRD pattern was then conducted to identify the crystallinity and phase purity of bio-MOF-1 (Fig. S1). All XRD diffraction peaks of bio-MOF-1 in the red curve match well with the simulated ones (black curve), indicating successful fabrication of this precursor with well-defined crystallinity and phase purity. Furthermore, in the carbonization process of MOF precursor, choosing a suitable calcination temperature plays a key role in the graphitization and porosity of resultant carbon samples, and the optimization of electrochemical catalytic performance [48,49]. TGA was performed under N2 atmosphere to evaluate the thermal stability of bio-MOF-1 (Fig. S2). In stage І, the TGA curve reveals a weight loss of 20.25% at below 300°C, which can be corresponded to the loss of water and DMF molecules from bio-MOF-1, being in accordance with the theoretical value (23.08%). Afterwards, there is a small weight loss of 2.7% owing to the loss of dimethylammonium (Me2NH2+) cations in the channels. With increasing temperature, bio-MOF-1 decomposed, with a weight loss of 31.43% from 360°C to 490°C. Finally, a weight loss was observed from 490°C to 1000°C because of further carbonization along with graphitization of carbon. In accordance with TGA results, bio-MOF-1 was highly thermally stable. Subsequently, the effects of carbonization temperature and maintenance time on ORR were discussed. The electrocatalytic ORR testing results of NPC-900-2 and NPC-1000-2, NPC-1000-4 and NPC-1000-5 showed that the optimal calcination temperature and maintenance time were 1000°C and 4 h respectively (Fig. S7). Accordingly, as-prepared bio-MOF-1 crystals were directly carbonized in argon atmosphere at 1000°C for different times (2, 3 and 4 h), and referred to as NPC-1000-2, NPC-1000-3 and NPC-1000-4, respectively.

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Fig. 1 XRD patterns (a) and Raman spectra (b) of NPC-1000-2, NPC-1000-3 and NPC-1000-4. The crystallinity and graphitization degrees of NPC-1000-ts were characterized by using PXRD (Fig. 1a) and Raman spectroscopy (Fig. 1b). PXRD revealed the characteristics of amorphous carbon for all the as-prepared NPC-1000-ts carbon materials. Clearly, each carbon sample consists of only two broad peaks at around 2θ = 23° and 44°, corresponding to the (002) and (100)/(101) characteristic peaks of graphitic carbon planes respectively and confirming the formation of amorphous carbon. The broad peaks also typify porous carbon [48]. Additionally, there are no diffraction peaks of Zn metal, indicating bio-MOF-1 precursor was completely converted into carbonaceous materials after heat treatment. During pyrolysis, Zn ions were reduced into Zn metals by carbon and then vaporized at under 1000°C [50]. In contrast, Zn peaks exist in the PXRD pattern of NPC-900-2 (PDF#04-0831, Fig. S3), indicating that Zn species were not completely removed at 900°C. Thus, the catalytic activity of NPC-900-2 was inferior to that of NPC-1000-2. Meanwhile, the (002) peak in the PXRD pattern of NPC-1000-4 slightly and positively shifts, suggesting its higher graphitization degree. Fig. 1b shows the Raman spectra of the obtained NPC-1000-ts. They exhibit two distinct peaks centered at around 1346 cm-1 and 1585 cm-1, corresponding to D band (disordered carbon) and G band (graphitic carbon), respectively. Besides, ID/IG (relative intensify ratio) of NPC-1000-ts gradually decreased from 0.98 to 0.85. NPC-1000-4 had the highest degree of graphitization (ID/IG=0.85), inferring that such degree increased significantly with rising carbonization time, and fewer defects and graphitic crystallites were generated. Based on these results, graphitic carbon augmented the electronic conductivity of NPC-1000-ts, further enhancing the ORR activity.

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Fig. 2 TEM images of NPC-1000-2 (a, b), NPC-1000-3 (c, d) and NPC-1000-4 (e, f). The SEM images of NPC-1000-ts (Fig. S4a, b, c) exhibit similar graphite sheet-like layer morphologies. Actually, the multilayer graphite sheet-like structures provided sufficient effective contact areas for catalysts and reactants during ORR. TEM images (Fig. 2 and Fig. S5) reveal the inner structural properties of NPC-1000-ts. Obviously, NPC-1000-ts exhibit similar morphologies, i.e. extremely thin graphitized carbon multilayer frameworks, high-density nanosized voids and many micro-mesoporous structures, which further improved the ORR performance by enlarging the effective contact areas between catalysts and reactants [51]. With increasing carbonization time, the crystallinity and degree of graphitization were gradually elevated from NPC-1000-2 to NPC-1000-4. Similar to the XRD results, NPC-1000-4 was highly graphitized (Fig. 2f), and the lattice distance of 0.347 nm corresponded to the lattice spacing of (002) crystalline plane of carbon, verifying successful formation of graphitic carbon. Since TEM images show no zinc 9

particles, Zn vanished during carbonization, which thus elevated the porosity and surface areas of NPC-1000-ts. Moreover, graphitic carbon lattice structures with more defects played a vital role in augmenting the electrical conductivity and corrosion resistance of NPC-1000-ts, which contributed to the outstanding electrochemical ORR catalytic activity [42,52]. High-angle annular dark-field STEM (HAADF-STEM) and the corresponding element mapping further confirmed the component types of NPC-1000-4 (Fig. S4d). C and N were homogeneously dispersed throughout NPC-1000-4, but zinc species was not detected. Pore properties of NPC-1000-ts.

Fig. 3 N2 adsorption-desorption isotherms (a) and pore size distributions (b) of NPC-1000-ts derived by the DFT method. To investigate the surface areas and pore structures of as-synthesized NPC-1000-ts, the BET (Brunauer-Emmett-Teller) surface areas and pore size distributions were obtained by measuring N2 adsorption-desorption isotherms at 77 K. As shown in Fig. 3a, the isotherms of NPC-1000-ts are similar type-IV curves with obvious type H2 hysteresis loops, and the gradual absorption with increasing relative pressure from 0.45 to 0.99 proved the existence of both micro- and meso-pores, as a feature of hierarchical porous materials. By using the BET method, the surface areas of NPC-1000-ts were calculated as 667.72 (NPC-1000-2), 691.88 (NPC-1000-3) and 992.70 m2/g (NPC-1000-4). Evidently, NPC-1000-4 had the largest surface area. A larger surface area exposed more active sites, and accelerated O2 diffusion and mass transport in the ORR process, so NPC-1000-4 also exhibited the highest ORR catalytic activity. Furthermore, the pore size distributions were examined by the density functional theory (DFT) method (Fig. 3b), further proving that NPC-1000-ts mainly contained micro-pores (<2 nm) and meso-pores (4-6 nm). The pore sizes of NPC-1000-2 are approximately 0.55, 1.17, 1.62 and 4.97 nm. For NPC-1000-3, there 10

are four peaks at about 0.55, 1.18, 1.62 and 4.97 nm, and those of NPC-1000-4 are around 0.55, 0.79, 1.15, 1.61 and 4.97 nm. Obviously, the porosity of NPC-1000-4 was much superior to those of NPC-1000-2 and NPC-1000-3 in the micro/meso-pores region (0.5-6 nm) with higher peak intensity. As a result, NPC-1000-ts had hierarchical porous structures with micro/meso-pores, with micro-pores being predominant to provide active sites for ORR. The total pore volumes of NPC-1000-ts were also determined at P/P0 (0.99), varying from 0.61 (NPC-1000-2) to 0.86 (NPC-1000-4) cm3/g. With the t-plot method, the micro-pore areas and volumes of NPC-1000-ts were obtained as 408.45 and 0.16 (NPC-1000-2), 403.75 and 0.17 (NPC-1000-3), 700.71 m2/g and 0.28 cm3/g (NPC-1000-4). The textural properties (surface areas, pore sizes and volumes) of NPC-1000-ts were summarized in Table S1. The surface areas and pore volumes of NPC-1000-ts significantly increased with extended carbonization time, due to increase of porous structures and degrees of graphitization. Given large pore volume and BET surface area, various guest molecules managed to go through the inner pore channels of NPC-1000-4, thereby facilitating oxygen emission, electron transportation, and diffusion of reactants and products in the electrocatalytic reaction. Clearly, the specific surface areas and pore volumes of NPC-1000-ts can be adjusted by changing the carbonization time.

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Chemical compositions and element contents of NPC-1000-ts

Fig. 4 Overall spectrum (a), C 1s spectra (b), N 1s spectra of NPC-1000-4 (c) and schematic representation of various N types in nitrogen-doped carbon materials (d). The chemical compositions and bonding types of as-prepared NPC-1000-ts were analyzed by XPS. The surface survey XPS spectra of NPC-1000-ts (Fig. 4a and Fig. S6a, d) verify the existence of C, N and O elements, with the contents summarized in Table S2. The C 1s, N 1s and O 1s peaks are located at around 285, 400 and 532 eV respectively. The O 1s peak may originate from surface water and oxygen physically adsorbed in the test process. Additionally, the non-appearance of Zn peak from the overall spectrum confirmed that Zn metal was totally removed in the carbonization process, being in agreement with the PXRD results. Also, the carbonization temperature and time markedly affected the content of C. For NPC-1000-ts, the total C contents gradually increased with prolonged carbonization time, and the relative C percentages of NPC-1000-2, 3 and 4 were 92.33%, 93.49% and 94.45%, respectively (Table S2). The C1s spectra of NPC-1000-ts (Fig. 4b and Fig. S6b, e) present three peaks at 284.8 (C1), 286 (C2) and 287 (C3) eV, corresponding to sp2-hybridized graphite-like carbon, C-O/C-N and C=O/C=N respectively. Moreover, a slightly asymmetric signal appears at higher binding energy, as a common feature of nitrogen-doped carbon materials. In the high-resolution N 1s spectra of NPC-1000-ts (Fig. 4c and Fig. S6c, f), there are four peaks representing pyridinic nitrogen (398.4 eV), pyrrolic nitrogen (400 eV),

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graphitic nitrogen (401.2 eV) and oxidized nitrogen (402.7 eV) which are schematized in Fig. 4d. Based on N 1s spectra, the relative content percentages (%) of N-doped types were evaluated in accordance with their peak areas (Table S3). Apparently, pyridinic-N (28.04, 18.45, 16.52%) and pyrrolic-N (19.32, 18.40, 14.56%) gradually decreased with increasing carbonization time from NPC-1000-2 to NPC-1000-4, but graphitic N significantly increased. NPC-1000-4 contained more graphitic N (50.84%) than those in NPC-1000-2 (33.27%) and NPC-1000-3 (38.38%). Among all four N species, pyridinic N and graphitic N are well-established active sites for high ORR performance. Pyridinic N can promote the first electron transfer process during catalytic reaction by reducing the energy barrier on adjacent carbon atoms for O2 adsorption. Graphitic N facilitates the elevation of electronic conductivity for porous carbon materials [40,41]. In 2016, Wei et al. investigated the oxygen reduction activities of pyridinic N and graphitic N from both conductivity and intrinsic activity by using the DFT method. The N doping content was lower than 2.8% (atomic fraction), and graphitic N, as an active N moiety, better augmented the ORR catalytic activity than pyridinic N did. On the contrary, pyridine nitrogen (plane nitrogen) was the main active site in the ORR process when the N doping content exceeded 2.8% (atomic fraction). Furthermore, Wang et al reported graphitic N contributed to the ORR performance of metal-free nitrogen-doped carbons in alkaline solution by utilizing DFT calculations, because the active energies of carbon atoms neighboring to graphitic N changed much less for first-electron oxygen reduction and four-electron rate-dominating step during ORR than those of carbon atoms adjacent to pyridinic or pyrrolic N did [53-55]. The contents of doped N species in NPC-1000-ts were 3.25% (NPCF-1000-2, >2.8%), 2.52% (NPC-1000-3, <2.8%) and 2.35% (NPC-1000-4, <2.8%) respectively. Evidently, the N doping contents of NPC-1000-3 and NPC-1000-4 were lower than 2.8%, and NPC-1000-4 had maximum graphitic N (50.84%), indicating graphitic N indeed played a key role in the ORR catalytic process. Moreover, the total proportion of pyridinic and graphitic nitrogens hardly changed after different calcination times (2, 3 and 4 h) at 1000°C, suggesting that pyridinic N converted into graphitic N at this temperature. With elapsed maintenance time, the relative content of pyrrolic N gradually decreased, because it was thermally unstable and prone to conversion into graphitic N [42]. Hence, the nitrogen doping content of MOF-derived nitrogen-doped carbon catalyst can be easily controlled by tuning the carbonization temperature and time. 13

Electrocatalytic behaviors of NPCs for ORR

Fig. 5 (a) CV curves of as-prepared carbon samples for ORR at a scan rate of 50 mV s -1 in 0.1 M N2- and O2-saturated KOH solutions; (b) LSV curves of NPC-1000-ts and commercial Pt/C electrocatalysts at a rotation speed of 1600 rpm with a scan rate of 5 mV s -1 in 0.1 M O 2-saturated KOH electrolyte; (c) LSV curves of NPC-1000-4 at different rotation speeds; (d) K-L plots of NPC-1000-4 at different potentials; (e) RRDE curves of NPC-1000-4 in 0.1 M O2-saturated KOH solution at a rotation rate of 1600 rpm with a scan rate of 5 mV s-1; (f) electron transfer number (n) and H2O2 yield of NPC-1000-4 calculated by RRDE curves at various potentials. The electrocatalytic activities of NPC-1000-ts were detected in aqueous KOH solution (0.1 M) saturated by N2 or O2 at room temperature with the loading amount of 0.255 mg/cm2. For ORR experiment, pure O2 was bubbled for at least 30 min prior to measurement and maintained in the head space of the electrolyte throughout testing. CV of NPC-1000-ts was conducted within the

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potential window of -0.8 V to 0.1 V (vs. Ag/AgCl) with a sweep speed of 50 mV/s in 0.1 M N2and O2-saturated KOH solutions. As shown in Fig. 5a, there are no any reduction peaks in the three CV curves obtained in N2-saturated KOH (0.1 M) solution. In contrast, each CV curve in O2-saturated KOH electrolyte displays a clear oxygen reduction peak, and the curve of NPC-1000-4 has redox peak for ORR at -0.16 V (vs. Ag/AgCl), which was more positive than those of NPC-1000-3 (-0.17 V) and NPC-1000-2 (-0.18 V). Thus, NPC-1000-4 catalyst had the maximum ORR electrochemical catalytic activity in alkaline solution. The remarkable ORR performance of NPC-1000-4 was further demonstrated by LSV in O2-saturated 0.1 M KOH on an RDE at a rotation rate of 1600 rpm. For comparison, NPC-1000-2, NPC-1000-3 and Pt/C catalysts were also measured under the identical conditions. As presented in Fig. 5b, the limiting current densities of NPC-1000-2, NPC-1000-3, NPC-1000-4 and Pt/C are 4.15, 4.73, 5.76 and 5.08 mA/cm2 respectively at -0.8 V. The onset potential (-0.012 V) and half-wave potential (-0.13 V) vs. Ag/AgCl of NPC-1000-4 catalyst for ORR were higher than those of Pt/C (-0.014 V and -0.14 V vs. Ag/AgCl) electrocatalyst. The ORR activities of all electrocatalysts were listed in Table S4. Remarkably, NPC-1000-4 had the highest onset potential, half-wave potential and limiting current density, demonstrating that the synergistic effects of large surface area, porous structure, high degree of graphitization and graphitic N content on the ORR performance to facilitate electrolyte diffusion and charge transfer. Besides, the ORR catalytic activity of NPC-1000-4 catalyst with a higher onset potential exceeded those of most metal-free N-doped carbon catalysts in previous literatures (Table S5). To clarify the ORR kinetic mechanism and the numbers of electron delivered per oxygen molecule, NPC-1000-ts and Pt/C catalysts were subjected to RDE measurements at rotation rates from 400 to 2025 rpm (Fig. 5c and Fig. S8). The limiting current densities of these catalysts increased with elevating rotation speed owing to enhanced mass transport and reduced diffusion distance on the electrode surface in the ORR process, confirming that the current was controlled by diffusion. To assess the ORR electrocatalytic kinetics of NPC-1000-ts, Tafel plots were derived from the LSV curves of NPC-1000-ts and Pt/C electrocatalysts at 1600 rpm (Fig. S9). The calculated Tafel slope of NPC-1000-4 (79 mV/dec) was close to that of Pt/C (76 mV/dec), but lower than those of NPC-1000-2 (132 mv/dec) and NPC-1000-3 (110 mv/dec). NPC-1000-4 exhibited fast ORR electrocatalytic kinetics and high ORR catalytic activity. Furthermore, during electrochemical reaction, the electron transfer number, 15

which reflects the kinetics of a catalyst, is of great significance to the evaluation of its ORR activity. This number can be calculated through K-L plots based on the results of RDE measurements at different sweep speeds (Fig. 5d and inset of Fig. S8). All the K-L plots (J-1 vs. ω-0.5) of NPC-900-2, NPC-1000-ts and Pt/C from -0.20 V to -0.45 V are highly linear and parallel, suggesting that similar numbers of electron were transferred at these potentials and first-order reaction kinetics occurred with regard to the dissolved O2 concentration. Judging by the slopes of K-L curves and K-L equation, the transferred electron numbers (n) were calculated to be ~4.00 (NPC-900-2), 3.85 (NPC-1000-2), 4.00 (NPC-1000-3), 4.00 (NPC-1000-4) and 3.77 (Pt/C), so NPC-1000-4 underwent a four-electron ORR pathway and allowed stable catalysis. RRDE tests were further performed to determine the number of transferred electrons (n) during ORR in 0.1 M O2-saturated KOH electrolyte at a sweep rate of 1600 rpm. Fig. 5e presents that the ring current is negligible in comparison with the disk current density, due to generation of H2O2 at the disc electrode. The high degree of graphitization and microporous structure of catalysts were beneficial to reactant diffusion, generation of active sites and kinetics during ORR. As shown in Fig. 5f, the H2O2 yield of NPC-1000-4 is below 12%, and the number of transferred electrons (n) is around 4.0 over the potential window of -0.8 V to -0.2 V. Therefore, the RRDE results were consistent with those of RDE.

Fig. 6 (a) Current-time (I-t) chronoamperometric response curves of NPC-1000-4 and Pt/C catalysts at 0.3 vs. Ag/AgCl with a rotation speed of 1600 rpm in O 2-saturated KOH solution (0.1 M); (b) methanol tolerance experiments of NPC-1000-4 and Pt/C catalysts in 0.1 M O2-saturated KOH at 0.3 V vs. Ag/AgCl with a rotation speed of 1600 rpm by adding 3 mL of methanol for about 400 s.

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In addition to electrocatalytic performance, the long-term stability and methanol resistance are also two critical parameters for assessing an electrocatalyst for ORR and practical applications in fuel cells. Accordingly, the chemical stabilities of NPC-1000-4 and commercial Pt/C catalysts were tested by long-term chronoamperometric experiments at -0.3 V (vs. Ag/AgCl) in O2-saturated KOH (0.1 M) solution with a rotation rate of 1600 rpm (Fig. 6a). Under the same experimental conditions, the current density of NPC-1000-4 decreased by about 10.5% after continuous running for 18000 s, whereas that of Pt/C electrocatalyst dropped by approximately 34.2% faster. Hence, NPC-1000-4 was much more durable than commercial Pt/C toward ORR in alkaline electrolyte. Furthermore, the tolerance of cathode materials to methanol and the possible crossover effect also predominantly control the ORR process in fuel cells. Thus, NPC-1000-4 and Pt/C electrodes were also tested by the response of current density to methanol addition at 1600 rpm. During ORR, after methanol molecules (about 400 s) were introduced into KOH electrolyte, the current density of NPC-1000-4 remained almost unchanged (Fig. 6b), whereas that of Pt/C catalyst plummeted, indicating that the latter was extremely active and sensitive in terms of methanol electrooxidation and fuel crossover effect. Collectively, the as-prepared NPC-1000-4 had higher stability and resistance to the methanol crossover effect than those of Pt/C as an ORR catalyst in alkaline media. Given the high selectivity, electrocatalytic performance and stability for ORR, NPC-1000-4 may be feasible metal-free electrocatalyst in fuel cell application. Based on the above measurement results, the high electrocatalytic activity of NPC-1000-4 towards ORR can mainly be ascribed to the following aspects: (a) NPC-1000-4 had large amounts of graphitic carbon layers and large specific surface area, which ensured sufficient contact between the catalyst and reactant during ORR. Therefore, NPC-1000-4 had the highest current density among all tested NPCs and Pt/C catalysts. (b) The high graphitization degree of carbon in NPC-1000-4 augmented the electronic conductivity and reduced the energy barrier, thereby attenuating the electron transfer resistance and raising the corrosion resistance of the catalyst. (c) NPC-1000-4 had high graphitic N and moderate pyridinic N contents, as active sites that further promoted electron transfer and O2 adsorption for ORR. Moreover, XPS evidenced that graphitic N was the main active site to improve the ORR electrocatalytic performance. Therefore, the limiting current density, half-wave and onset potentials of 17

NPC-1000-4 surpassed those of Pt/C. (d) Rich porous structures of NPC-1000-4 exposed more active sites, and accelerated mass and electron transfer kinetics during ORR. In addition, more micro-pores enlarged the surface area of NPC-1000-4 and offered channels, which eventually facilitated electrolyte and oxygen diffusion. In short, the superior ORR catalytic activity of NPC-1000-4 to those of other catalysts can be attributed to the synergistic effects between unique structure features, considerable graphitic N species and high graphitization degree. Conclusions In summary, we successfully synthesized nitrogen-doped porous carbon materials (NPC-1000-ts) by simply pyrolyzing bio-MOF-1 precursor at high temperature for different maintenance times (2, 3 and 4 h). The morphologies and chemical compositions of as-obtained catalysts were characterized by XRD, Raman spectroscopy, SEM, TEM, N2 adsorption-desorption isotherms, and XPS. The catalytic activities of NPC-1000-ts for ORR were studied by CV, RDE and RRDE measurements in 0.1 M KOH solution. Notably, NPC-1000-4 exhibited the maximum electrocatalytic activity towards ORR, accompanied by much higher durability and methanol resistance than those of commercial Pt/C (20 wt%) electrocatalyst. Particularly, the onset potential of NPC-1000-4 was superior to those of Pt/C and most previously reported metal-free N-doped carbon electrocatalysts. The excellent ORR performances of NPC-1000-4 can be resulted from larger specific surface area, more micro/mesoporous structures, higher graphitization degree of carbon, and higher relative content of graphitic N atoms. Remarkably, 3D porous bio-MOF-1 can be selected as a promising precursor to generate metal-free nitrogen-doped carbon as an efficient ORR electrocatalyst in alkaline electrolyte. We provide an alternative approach for preparing highly active metal-free electrocatalysts from MOF precursor solely, and the as-prepared NPC-1000-4 carbon sample may be a qualified metal-free ORR electrocatalyst applicable to fuel cells. Supporting Information. XRD, TG, SEM, TEM, XPS, LSVs and other supplementary files of samples are available free of charge. (PDF)

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Conflicts of interest There are no conflicts of interest to declare.

Acknowledgements This work was financially supported by China National Natural Science Foundation (grant numbers 21471127, 21661029) and Xinjiang Uygur Autonomous Region Graduate Student Research Innovation Project (No. 2013711008). Thanks to Dr. Su Zhang for the valuable discussion. Notes and references [1] S.-N. Zhao, X.-Z. Song, S.-Y. Song, H.-j. Zhang, Highly efficient heterogeneous catalytic materials derived from metalorganic framework supports/precursors, Coord. Chem. Rev. 337 (2017) 80-96. [2] Y. Qian, Z. Hu, X. Ge, S. Yang, Y. Peng, Z. Kang, Z. Liu, J.Y. Lee, D. Zhao, A metal-free ORR/OER bifunctional electrocatalyst derived from metal-organic frameworks for rechargeable Zn-Air batteries, Carbon 111 (2017) 641-650. [3] A. Mahmood, W. Guo, H. Tabassum, R. Zou, Metal-Organic Framework-Based Nanomaterials for Electrocatalysis, Adv. Energy Mater. 6 (2016) 1600423. [4] S. Fu, C. Zhu, J. Song, D. Du, Y. Lin, Metal-Organic Framework-Derived Non-Precious Metal Nanocatalysts for Oxygen Reduction Reaction, Adv. Energy Mater. 7 (2017) 1700363. [5] M. Shao, Q. Chang, J. P. Dodelet, R. Chenitz, Recent Advances in Electrocatalysts for Oxygen Reduction Reaction, Chem. Rev. 116 (2016) 3594-3657. [6] L. Bu, J. Ding, S. Guo, X. Zhang, D. Su, X. Zhu, J. Yao, J. Guo, G. Lu, X. Huang, General Method for Multimetallic Platinum Alloy Nanowires as Highly Active and Stable Oxygen Reduction Catalysts, Adv. Mater. 27 (2015) 7204-7212. [7] J.X. Wang, H. Inada, L. Wu, Y. Zhu, Y.M. Choi, P. Liu, W.-P. Zhou, R.R. Adzic, Oxygen Reduction on Well-Defined Core-Shell Nanocatalysts: Particle Size, Facet, and Pt Shell Thickness Effects, J. Am. Chem. Soc. 131 (2009) 17298-17302. [8] C. Wang, F. Hu, H. Yang, Y. Zhang, H. Lu, Q. Wang, 1.82 wt.% Pt/N, P co-doped carbon overwhelms 20 wt.% Pt/C as a high-efficiency electrocatalyst for hydrogen evolution reaction, Nano Res. 10 (2017) 238-246. [9] B. You, N. Jiang, M. Sheng, W.S. Drisdell, J. Yano, Y. Sun, Bimetal-Organic Framework Self-Adjusted Synthesis of SupportFree Nonprecious Electrocatalysts for Efficient Oxygen Reduction, ACS Catal. 5 (2015) 7068-7076. [10] P. Yin, T. Yao, Y. Wu, L. Zheng, Y. Lin, W. Liu, H. Ju, J. Zhu, X. Hong, Z. Deng, G. Zhou, S. Wei, Y. Li, Single Cobalt Atoms with Precise N-Coordination as Superior Oxygen Reduction Reaction Catalysts, Angew. Chem. Int. Ed. 55 (2016) 10800-10805. [11] G. Xu, G.-C. Xu, J.-J. Ban, L. Zhang, H. Lin, C.-L. Qi, Z.-P. Sun, D.-Z. Jia, Cobalt and cobalt oxides N-codoped porous carbon derived from metalorganic framework as bifunctional catalyst

19

for oxygen reduction and oxygen evolution reactions, J. Colloid Interface Sci. 521 (2018) 141-149. [12] C. Qi, L. Zhang, G. Xu, Z. Sun, A. Zhao, D. Jia, Co@Co3O4 nanoparticle embedded nitrogen-doped carbon architectures as efficient bicatalysts for oxygen reduction and evolution reactions, Appl. Surf. Sci. 427 (2018) 319-327. [13] Z. Li, H. Sun, L. Wei, W.-J. Jiang, M. Wu, J.-S. Hu, Lamellar Metal Organic Framework-Derived Fe-N-C Non-Noble Electrocatalysts with Bimodal Porosity for Efficient Oxygen Reduction, ACS Appl. Mater. Inter. 9 (2017) 5272-5278. [14] J. Masa, W. Xia, M. Muhler, W. Schuhmann, On the Role of Metals in Nitrogen-Doped Carbon Electrocatalysts for Oxygen Reduction, Angew. Chem. Int. Ed. 54 (2015) 10102-10120. [15] J. Wei, Y. Hu, Y. Liang, B. Kong, Z. Zheng, J. Zhang, S. P. Jiang, Y. Zhao, H. Wang, Graphene oxide/core-shell structured metal-organic framework nano-sandwiches and their derived cobalt/N-doped carbon nanosheets for oxygen reduction reactions, J. Mater. Chem. A 5 (2017) 10182-10189. [16] X. Wang, J. Wang, D. Wang, S. Dou, Z. Ma, J. Wu, L. Tao, A. Shen, C. Ouyang, Q. Liu, S. Wang, One-pot synthesis of nitrogen and sulfur co-doped graphene as efficient metal-free electrocatalysts for the oxygen reduction reaction, Chem. Commun. 50 (2014) 4839-4842. [17] J. Li, Y. Zhang, X. Zhang, J. Huang, J. Han, Z. Zhang, X. Han, P. Xu, B. Song, S, N Dual-Doped Graphene-like Carbon Nanosheets as Efficient Oxygen Reduction Reaction Electrocatalysts, ACS Appl. Mater. Inter. 9 (2017) 398-405. [18] H.-X. Zhong, J. Wang, Y.-W. Zhang, W.-L. Xu, W. Xing, D. Xu, Y.-F. Zhang, X.-B. Zhang, ZIF-8 Derived Graphene-Based Nitrogen-Doped Porous Carbon Sheets as Highly Efficient and Durable Oxygen Reduction Electrocatalysts, Angew. Chem. Int. Ed. 53 (2014) 14235-14239. [19] B. He, F. Liu, S. Yan, Temperature-directed growth of highly pyridinic nitrogen doped, graphitized, ultra-hollow carbon frameworks as an efficient electrocatalyst for the oxygen reduction reaction, J. Mater. Chem. A 5 (2017) 18064-18070. [20] D. Du, P. Li, J. Ouyang, Nitrogen-Doped Reduced Graphene Oxide Prepared by Simultaneous Thermal Reduction and Nitrogen Doping of Graphene Oxide in Air and Its Application as an Electrocatalyst, ACS Appl. Mater. Inter. 7 (2015) 26952-26958. [21] M. Borghei, N. Laocharoen, E.K-Põldsepp, L.-S Johanssona, J. Campbell, E. Kauppinen, K. Tammeveski, O.J. Rojas, Porous N,P-doped carbon from coconut shells with high electrocatalytic activity for oxygen reduction: Alternative to Pt-C for alkaline fuel cells, Appl. Catal., B Environ. 204 (2017) 394-402. [22] R. Ma, Y. Zhou, P. Li, Y. Chen, J. Wang, Q. Liu, Self-Assembly of Nitrogen-doped Graphene-Wrapped Carbon Nanoparticles as an Efficient Electrocatalyst for Oxygen Reduction Reaction, Electrochim. Acta 216 (2016) 347-354. [23] H. Zhang, Y. Zhou, C. Li, S. Chen, L. Liu, S. Liu, H. Yao, H. Hou, Porous nitrogen doped carbon foam with excellent resilience for self-supported oxygen reduction catalyst, Carbon 95 (2015) 388-395. [24] L. Qin, R. Ding, H. Wang, J. Wu, C. Wang, C. Zhang, Y. Xu, L. Wang, B. Lv, Facile synthesis of porous nitrogen-doped holey graphene as an efficient metal-free catalyst for the oxygen reduction reaction, Nano Res. 10 (2017) 305-319. [25] G.A. Ferrero, A.B. Fuertes, M. Sevilla, M.-M Titirici, Efficient metal-free N-doped mesoporous carbon catalysts for ORR by a template-free approach, Carbon 106 (2016) 179-187.

20

[26] X. Li, Y. Fang, S. Zhao, J. Wu, F. Li, M. Tian, X. Long, J. Jin, J. Ma, Nitrogen-doped mesoporous carbon nanosheet/carbon nanotube hybrids as metal-free bifunctional electrocatalysts for water oxidation and oxygen reduction, J. Mater. Chem. A 4 (2016) 13133-13141. [27] K. Shen, X. Chen, J. Chen, Y. Li, Development of MOF-Derived Carbon-Based Nanomaterials for Efficient Catalysis, ACS Catal. 6 (2016) 5887-5903. [28] Z. Song, N. Cheng, A. Lushington, X. Sun, Recent Advances in Electrocatalysts for Oxygen Reduction Reaction, Catalysts 6 (2016) 116. [29] C.-J. Xuan, J. Wang, J. Zhu, D.-L. Wang, Recent Progress of Metal Organic Frameworks-Based Nanomaterials for Electrocatalysis, Acta Phys. -Chim. Sin. 33 (2017) 149-164. [30] H. Zhang, H. Osgood, X. Xie, Y. Shao, G. Wu, Engineering nanostructures of PGM-free oxygen-reduction catalysts using metal-organic frameworks, Nano Energy 31 (2017) 331-350. [31] Y. Pan, M. Xue, M. Chen, Q. Fang, L. Zhu, V. Valtchev, S. Qiu, ZIF-derived in situ nitrogen decorated porous carbons for CO2 capture, Inorg. Chem. Front. 3 (2016) 1112-1118. [32] K.J. Lee, J.H. Lee, S. Jeoung, H.R. Moon, Transformation of Metal-Organic Frameworks/Coordination Polymers into Functional Nanostructured Materials: Experimental Approaches Based on Mechanistic Insights, Acc. Chem. Res. 50 (2017) 2684-2692. [33] I.A. Khan, Y. Qian, A. Badshah, M.A. Nadeem, D. Zhao, Highly Porous Carbon Derived from MOF-5 as a Support of ORR Electrocatalysts for Fuel Cells, ACS Appl. Mater. Inter. 8 (2016) 17268-17275. [34] M. Hu, J. Reboul, S. Furukawa, N.L. Torad, Q. Ji, P. Srinivasu, K. Ariga, S. Kitagawa, Y. Yamauchi, Direct Carbonization of Al-Based Porous Coordination Polymer for Synthesis of Nanoporous Carbon, J. Am. Chem. Soc. 134 (2012) 2864-2867. [35] W. Zhang, Z.-Y. Wu, H.-L. Jiang, S.-H. Yu, Nanowire-Directed Templating Synthesis of Metal-Organic Framework Nanofibers and Their Derived Porous Doped Carbon Nanofibers for Enhanced Electrocatalysis, J. Am. Chem. Soc.136 (2014) 14385-14388. [36] X. Pei, Y. Chen, S. Li, S. Zhang, X. Feng, J. Zhou, B. Wang, Metal-Organic Frameworks Derived Porous Carbons: Syntheses, Porosity and Gas Sorption Properties, Chin. J. Chem. 34 (2016) 157-174. [37] X. Wang, X. Li, C. Ouyang, Z. Li, S. Dou, Z. Ma, L. Tao, J. Huo, S. Wang, Nonporous MOF-derived dopant-free mesoporous carbon as an efficient metal-free electrocatalyst for the oxygen reduction reaction, J. Mater. Chem. A 4 (2016) 9370-9374. [38] Z. Wang, Y. Lu, Y. Yan, T. Y. P. Larissa, X. Zhang, D. Wuu, H. Zhang, Y. Yang, X. Wang, Core-shell carbon materials derived from metal-organic frameworks as an efficient oxygen bifunctional electrocatalyst, Nano Energy 30 (2016) 368-378. [39] A. Brouzgou, S. Song, Z.-X. Liang, P. Tsiakaras, Non-Precious Electrocatalysts for Oxygen Reduction Reaction in Alkaline Media: Latest Achievements on Novel Carbon Materials, Catalysts 6 (2016) 159. [40] J. Liu, P. Song, W. Xu, Structure-activity relationship of doped-nitrogen (N)-based metalfree active sites on carbon for oxygen reduction reaction, Carbon 115 (2017) 763-772. [41] M. Kuang, G. Zheng, Nanostructured Bifunctional Redox Electrocatalysts, Small 12 (2016) 5656-5675. [42] L. Zhang, Z. Su, F. Jiang, L. Yang, J. Qian, Y. Zhou, W. Li, M. Hong, Highly graphitized nitrogen-doped porous carbon nanopolyhedra derived from ZIF-8 nanocrystals as efficient electrocatalysts for oxygen reduction reactions, Nanoscale 6 (2014) 6590-6602.

21

[43] L. Li, P. Dai, X. Gu, Y. Wang, L. Yan, X. Zhao, High oxygen reduction activity on a metal-organic framework derived carbon combined with high degree of graphitization and pyridinic-N dopants, J. Mater. Chem. A 5 (2017) 789-795. [44] W. Yuan, Y. Feng, A. Xie, X. Zhang, F. Huang, S. Li, X. Zhang, Y. Shen, Nitrogen-doped nanoporous carbon derived from waste pomelo peel as a metal-free electrocatalyst for the oxygen reduction reaction, Nanoscale 8 (2016) 8704-8711. [45] R. Zhao, W. Xia, C. Lin, J. Sun, A. Mahmood, Q. Wang, B. Qiu, H. Tabassum, R. Zou, A pore-expansion strategy to synthesize hierarchically porous carbon derived from metal-organic framework for enhanced oxygen reduction, Carbon 114 (2017) 284-290. [46] J. An, S.J. Geib, N.L. Rosi, Cation-Triggered Drug Release from a Porous Zinc-Adeninate Metal-Organic Framework, J. Am. Chem. Soc. 131 (2009) 8376-8377. [47] Y. Pan, Y. Zhao, S. Mu, Y. Wang, C. Jiang, Q. Liu, Q. Fang, M. Xue, S. Qiu, Cation exchanged MOF-derived nitrogen-doped porous carbons for CO2 capture and supercapacitor electrode materials, J. Mater. Chem. A 5 (2017) 9544-9552. [48] M. Wu, K. Wang, M. Yi, Y. Tong, Y. Wang, S. Song, A Facile Activation Strategy for an MOF-Derived Metal-Free Oxygen Reduction Reaction Catalyst: Direct Access to Optimized Pore Structure and Nitrogen Species, ACS Catal. 7 (2017) 6082-6088. [49] Q. Lai, Y. Zhao, Y. Liang, J. He, J. Chen, In Situ Confnement Pyrolysis Transformation of ZIF-8 to Nitrogen-Enriched Meso-Microporous Carbon Frameworks for Oxygen Reduction, Adv. Func. Mater. 26 (2016) 8334-8344. [50] P. Zhang, F. Sun, Z. Xiang, Z. Shen, J. Yunbc, D. Cao, ZIF-derived in situ nitrogen-doped porous carbons as efficient metal-free electrocatalysts for oxygen reduction reaction, Energy Environ. Sci. 7 (2014) 442-450. [51] S. Liu, H. Zhang, Q. Zhao, X. Zhang, R. Liu, X. Ge, G. Wang, H. Zhao, W. Cai, Metal-organic framework derived nitrogen-doped porous carbon@graphene sandwich-like structured composites as bifunctional electrocatalysts for oxygen reduction and evolution reactions, Carbon 106 (2016) 74-83. [52] M. Thomas, R. Illathvalappil, S. Kurungot, B.N. Nair, A.A. Mohamed, G.M. Anilkumar, T. Yamaguchi, U.S. Hareesh, Graphene Oxide Sheathed ZIF-8 Microcrystals: Engineered Precursors of Nitrogen-Doped Porous Carbon for Efficient Oxygen Reduction Reaction (ORR) Electrocatalysis, ACS Appl. Mater. Inter. 8 (2016) 29373-29382. [53] J. Wang, Zi.-D. Wei, Recent Progress in Non-Precious Metal Catalysts for Oxygen Reduction Reaction, Acta Phys. -Chim. Sin. 33 (2017) 886-902. [54] J. Wang, L. Li, Z.-D. Wei, Density Functional Theory Study of Oxygen Reduction Reaction on Different Types of N-Doped Graphene, Acta Phys. -Chim. Sin. 32 (2016) 321. [55] N. Wang, B. Lu, L. Li, W. Niu, Z. Tang, X. Kang, S. Chen, Graphitic N is Responsible for Oxygen Electroreduction on N-Doped Carbons in Alkaline Electrolytes: Insights from Activity Attenuation Studies and Theoretical Calculations, ACS Catal. 8 (2018) 6827-6836.

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Graphical abstract

Bio-MOF-1-derived highly graphitized N-doped porous carbon catalyst which exhibited superior ORR activity to that of commercial 20 wt% Pt/C.

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