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DOI: 10.1039/D0NR00511H (Paper) Nanoscale, 2020, 12, 8785-8792

Bottom-up preparation of hierarchically porous MOF-modified carbon sphere derivatives for efficient oxygen reduction†‡

Lulu Chai§ ab, Qi Huang§ a, Han Cheng a, Xian Wang a, Linjie Zhang bc, Ting-Ting Li d, Yue Hu *a, Jinjie Qian *ab and Shaoming Huang *ae
aKey Laboratory of Carbon Materials of Zhejiang Province, College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou, 325000, China. E-mail: jinjieqian@wzu.edu.cn; jinjieqian@foxmail.com
bState Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350002, China
cChimie du solide et de l’énergie-Collège de France 11 Place Marcelin Berthelot, Paris, 75005, France
dSchool of Materials Science and Chemical Engineering, Ningbo University, Ningbo, 315211, China
eSchool of Materials and Energy, Guangdong University of Technology, Guangzhou, 510006, China

Received 18th January 2020 , Accepted 20th March 2020

First published on 23rd March 2020

Abstract

The rational design and controllable preparation of carbon-based catalysts for oxygen reduction reactions (ORRs) are at the core of key technologies for fuel cells and chargeable batteries in the field of advanced energy conversion and storage. In the present study, a Co species was synthesized by tuning the Zn dopant content in a bimetallic zeolitic-imidazolate framework functionalized with carbon spheres (CoZn-ZIF/CS). Using CoZn-ZIF/CS as the precursor, Co nanoparticles on N-doped carbon spheres were generated at 1000 °C (CoZn-ZIF/CS-1000). These systems were bottom-up synthesized and extensiv ely investigated for their ORR performance. The Brunauer–Emmett–Teller surface area and total pore volume with values of 586 m2 g−1 and 0.39 cm3 g−1, respectively, for the CoZn-ZIF/CS-1000 are appropriate compared to those of the porous ZIF precursor. As expected, it exhibited high-activity ORR performance in an alkaline medium with a half-wave potential of 0.82 V vs. RHE and the diffusion-limited current density is 5.11 mV cm−2. Meanwhile, the obtained CoZn-ZIF/CS-1000 electrocatalyst shows better electrochemical stability and methanol tolerance than the commercial Pt/C. Therefore, our discovery opens up a new way to regulate the catalytic performance of carbon material templates and MOF derivatives, which could be further applied in the development of highly active catalysts for applications in chemical energy utilization.

1. Introduction

Nowadays, the extensive use and large-scale production of new stable carbon-free energy sources can be regarded as the most promising way to reduce or even replace fossil fuels for a greener environment.1–3 In this context, an effective method for exploring a clean renewable energy is an electrochemical energy conversion technology,4 which requires the development of high-efficiency electrocatalysts for oxygen reduction reaction (ORR) at cathodes to play a key role in the commercialization of various new clean energy technologies, including renewable fuel cells, rechargeable metal–air batteries, and overall water splitting.5–7 Although Pt-based materials have long been identified as the best ORR electrocatalysts, the widespread utilization of Pt-based materials are significantly hindered by factors, such as high cost, low durability, and poor methanol tolerance.8,9 Therefore, low cost, durable, and highly efficient non-platinum group metal-based electrocatalysts as alternatives are being developed and a great effort is concentrated on replacing the high-cost Pt-based materials with high performance materials.

Recently, carbon-based materials including carbon nanotubes (CNT),10 graphene,11 and nanocarbons12 have been widely used in many applications due to their practicality. Among these, nanoporous carbon is an intriguing hierarchically porous carbon material with large porosity and is considered a new frontier in multifunctional materials because of its remarkable characteristics such as large specific surface areas, high porosity, excellent conductivity, and controlled nanostructure generation.13–15 Meanwhile, as a template, these carbon materials not only increase electrical conductivity for target composites but also prevent the aggregation of active components and provide enough space to resist volume expansion for improved performance. On the other hand, metal–organic frameworks (MOFs) have emerged as a subclass of crystalline and microporous materials. They are structurally assembled from organic linkers and metal cations.16,17 With an appropriate thermal and/or chemical treatment, these fascinating hybrid MOFs can be conveniently converted into a variety of high-energy storage materials, including porous carbon and metal hydroxides/oxides/carbides/nitrides. In addition, due to their ordered porous and tunable structure, electroactive materials with porous, hollow or even more complex structures can be easily obtained using MOF precursors. Recently, a subclass of MOFs, zeolitic imidazolate frameworks (ZIFs), with a high specific surface area and adjustable structure and composition were developed as self-templates and successfully converted into nitrogen-doped carbon composites for energy conversion and storage due to abundant carbon and nitrogen species.18–22 For example, Jin et al. found a novel hybrid sulfur host composed of hollow Co3S4-NBs inserted/threaded with interlaced CNTs using ZIF-67 as a precursor, and this structure was found to be highly efficient when applied in high-rate and heat-resistant lithiusulfur batteries.23 Liu et al. reported hierarchical Co3ZnC/carbon nanotube-inserted nitrogen-doped carbon concave-polyhedrons synthesized via the direct pyrolysis of bimetallic zeolitic imidazolate framework precursors and were used for both high-performance rechargeable Li-ion and Na-ion batteries.24 Zhang et al. designed and prepared the pyrolysis precursor of a cobalt-coordinated framework porphyrin with graphene hybridization to fabricate single-atom Co–Nx–C electrocatalysts, whose excellent electrochemical performances were realized for both bifunctional oxygen electrocatalysis and rechargeable Zn-air batteries.25 Though MOF-derived carbon materials with abundant reaction sites, researchers have studied some composites composed of multiple carbon-based substrates such as CNTs and carbon cloth to improve their conductivity for electrochemical reactions.26,27 However, it remains a huge challenge to develop new hybrid materials derived from porous MOF precursors that have advantages in all areas of capacity, rate performance, and cycle stability.

Herein, we have demonstrated the design and synthesis of a cobalt species by tuning the zinc dopant content in a bimetallic zeolitic-imidazolate framework functionalized with carbon sphere precursors (CoZn-ZIF/CS) and generated Co nanoparticles (NPs) on the as-calcined N-doped carbon spheres. As illustrated in Scheme 1, these pretreated functionalized CSs are used as templates and polydopamine (PDA) is further employed to modify the carbon materials with a thin polymer layer using a one-pot polymerization (Step I), thus leading to the formation of PDA/CS. In Step II, the PDA/CS is decentralized in a Co(II) and/or Zn(II) ion methanol solution, in which the PDA-coated CSs can electrostatically pre-adsorb metal cations due to their abundant hydroxyl groups, which are further coordinated with 2-MeIm ligands to successfully obtain CoZn-ZIF/CS, Co-ZIF/CS, and Zn-ZIF/CS. Then, the core–shell CoZn-ZIF/CS was subjected to pyrolyzation under an Ar atmosphere by a thermal treatment to conveniently transform these precursors into Co NPs embedded in N-doped CSs (CoZn-ZIF/CS-1000) in Step III. In other two control samples, Zn-ZIF/CS-1000 and CoZn-ZIF/CS-1000, Zn atoms with a low boiling point (907 °C) may evaporate at 1000 °C and the reduction in Co atoms was in situ carbonized by organic components at a high temperature. Among them, Co-ZIF/CS-1000 displays a superior ORR activity with excellent methanol tolerance and stability in the following ORR test.


image file: d0nr00511h-s1.tif
Scheme 1 Synthetic illustration for the preparation process of Zn-ZIF/CS-1000, Co-ZIF/CS-1000, and CoZn-ZIF/CS-1000 catalysts.

2. Experimental details

2.1 Chemicals and materials

All chemicals were of reagent grade and used without processing unless otherwise mentioned. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 99%, Aladdin), zinc acetate dihydrate (Zn(Ac)2·2H2O, 99.0%, Aladdin), cobalt nitrate hexahydrate (Co(NO3)2·6H2O, 99%, Aladdin), 1,3,5-benzenetricarboxylic acid (H3BTC, 98%, Aladdin), 2-methylimidazole (2-MeIm, 98%, Aladdin), dopamine hydrochloride (98%, Aladdin), tris(hydroxymethyl)aminomethane (THAM, 99%, Aladdin), platinum on carbon (Pt/C, 20%, Johnson Matthey), N,N-dimethylacetamide (DMA, ≥99.8%, Aladdin), N,N-dimethylformide (DMF, ≥99%, Aladdin), methanol (MeOH, 99.5%, Aladdin), ethanol (EtOH, 95%, Aladdin), nitric acid (HNO3, 70%, Aladdin), de-ionized water (DI H2O, 18 MΩ, MiliQ), and Nafion solution (5 wt%, DuPont) were used.

2.2 Syntheses

Preparation of carbon spheres (CSs). The facile synthesis of CSs is a slight modification of reported methods.28 In general, Zn(NO3)2·6H2O (186 mg, 0.63 mmol) and the ligand of H3BTC (90 mg, 0.42 mmol) were dissolved in a 100 mL pressure glass tube containing 50 mL of DMA (Beijing Synthware Glass, Inc.) and the mixture was heated to 150 °C for 30 minutes. Then, the as-obtained samples denoted as Zn-BTC were centrifuged and washed with DMF and EtOH several times and further dried at 85 °C overnight. Further, these as-obtained Zn-BTC spheres were placed in a quartz boat and transferred into a quartz tube and carbonized at the designated temperature of 1000 °C for 1 hour under an Ar steam (100 sccm). Moreover, the CVD furnace was allowed to cool to room temperature and the products were harvested as black powders. Finally, 100 mg of the carbonized product was inserted into a solution containing 20 mL of HNO3 for 3 hours at room temperature for acid functionalization. The product was obtained after centrifugation with DI H2O at a rotational speed of 1200 rpm and named as the functional CSs.
Preparation of PDA/CS. Typically, the CS template (60 mg) was homogeneously dispersed in a DI H2O (90 mL) and EtOH (120 mL) solution via strong sonication for 60 minutes, followed by the addition of dopamine hydrochloride (1.26 mmol, 240 mg) and stirring for 3 hours. Then, 60 Ml of a THAM aqueous solution (25 mM, 180 mg) was added under violent magnetic stirring for 2 hours at room temperature. After that, the resultant product was filtered and rinsed with DI water three times, followed by drying in a vacuum oven at 85 °C for 12 hours. The product was denoted as PDA/CS.
Preparation of core–shell CoZn-ZIF/CS, Co-ZIF/CS and Zn-ZIF/CS. First, 40 mg PDA/CS, Co(II) and/or Zn(II) ions (Zn(Ac)2 (22 mg) + Co(NO3)2 (30 mg), Co(NO3)2 (52 mg), and Zn(Ac)2 (44 mg)) were dissolved in 20 mL MeOH in a 100 mL pressure glass tube with sufficient ultrasonication for 1 hour to form the A solution. However, the B solution consisted of 2-MeIm (5.40 mmol, 444 mg) dissolved in 20 Ml of the MeOH solvent. Then, a mixture of the A and B solution was stirred for 15 minutes at 70 °C and the precipitate was collected by centrifugation (9500 rpm, 3 minutes) using EtOH three times. Finally, the samples were placed in a vacuum oven and dried at 85 °C overnight, and are denoted as CoZn-ZIF/CS, Co-ZIF/CS and Zn-ZIF/CS. CoZn-ZIF/CS (∼85 mg), Co-ZIF/CS (∼70 mg) and Zn-ZIF/CS (∼70 mg) were obtained in ca. 67%, 57%, and 68% yields based on Co(NO3)2 and/or Zn(Ac)2.
Preparation of core–shell CoZn-ZIF/CS-1000, Co-ZIF/CS-1000, and Zn-ZIF/CS-1000. The above-synthesized CoZn-ZIF/CS, Co-ZIF/CS, and Zn-ZIF/CS were used as precursors and transferred into a tube furnace, calcined to 1000 °C for 1 hour at a ramp rate of 20 °C min−1 under an Ar atmosphere flow. After naturally cooling down to room temperature, these products were collected and marked as CoZn-ZIF/CS-1000, Co-ZIF/CS-1000, and Zn-ZIF/CS-1000, respectively, according to the same calcination conditions.

3. Results and discussion

The detailed synthesis of CoZn-ZIF/CS-1000 and the two control catalysts is depicted in Scheme 1. First, we selected poly-dispersed Zn-BTC-derived CSs with an average particle size of 1.5 μm as templates (Fig. S1a and 1b‡). Second, a PDA layer was further attached to the CSs, as shown in Fig. S1c.‡ Scanning electron microscopy (SEM) images (Fig. S1d–1f‡) show that the obtained PDA/CSs are well dispersed in a MeOH solution and a layer of MOF precursors (CoZn-ZIF, Co-ZIF, and Zn-ZIF) grew on the external surface of the CSs. According to our strategy, PDA molecules are rich in functional hydroxyl groups. Therefore, these functionalized PDA/CSs provide abundant nucleation sites and selectively attract Co(II) and Zn(II) ions, further promoting the rapid nucleation of ZIF NPs with the ligand of 2-MeIm.29 Compared to pure Co-ZIF/CS, the additional introduction of a certain amount of Zn(II) dopant can replace the original Co(II) centers in CoZn-ZIF/CS. Thus, the spatial isolation of the cobalt species is effectively achieved at the atomic level.30,31

As shown in Fig. 1 and S2, S3,‡ the ZIF-derived composites were successfully obtained using the direct carbonization treatment. Fig. 1a–c exhibits SEM and transmission electron microscopy (TEM) images of the synthesized CoZn-ZIF/CS-1000, in which it can be clearly observed that the spherical CSs and dispersed Co NPs are formed in situ. Meanwhile, the high-resolution TEM (HR-TEM) images of the CoZn-ZIF/CS-1000 display two unique fringe spacings with d1 = 0.20 nm and d2 = 0.33 nm, corresponding to the Co (111) and graphitic carbon (002) lattice planes.32 In the selected area electron diffraction (SAED) mode, the polycrystalline nature of CoZn-ZIF/CS-1000 is confirmed, which can be indexed to the diffraction rings of the Co and carbon phases (Fig. 1f), and is also in good accordance with the HR-TEM image. Furthermore, energy dispersive X-ray spectroscopy (EDS), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and the corresponding elemental mappings of CoZn-ZIF/CS-1000 exhibit the dispersion of the chemical elements C, N, O, Co, and Zn in the single CS (Fig. 1g and h). In comparison with the other two catalysts Co-ZIF/CS-1000 and Zn-ZIF/CS-1000, the Zn atoms in CoZn-ZIF/CS-1000 can evaporate at high temperatures causing the zinc components to act as pore-forming agents to increase the distance between adjacent Co species in space and promote the exposure of active sites, which would be beneficial for the following electrochemical reactions.


image file: d0nr00511h-f1.tif
Fig. 1 SEM (a), TEM (b and c) images, and HR-TEM (d and e) images of the CoZn-ZIF/CS-1000 microspheres; the corresponding SAED pattern (f) and EDS spectrum (g); HAADF-STEM image and the corresponding mappings(h) of C, N, O, Co, and Zn for CoZn-ZIF/CS-1000.

We have demonstrated the crystal structure and phase of these ZIF-decorated CSs and their derivatives at high temperatures using powder X-ray diffraction (PXRD). As depicted in Fig. 2a, the PXRD pattern confirmed that the functional CSs were a type of structurally amorphous carbon. After the rapid nucleation of Co/Zn-based ZIFs, CoZn-ZIF/CS, Co-ZIF/CS, and Zn-ZIF/CS showed the characteristic peaks of ZIF-8/67, indicating that the core–shell composites of CSs and ZIFs were successfully formed. Meanwhile, from the PXRD patterns of CoZn-ZIF/CS-1000, Co-ZIF/CS-1000, and Zn-ZIF/CS-1000, we can observe that strong diffraction peaks of the ZIF-based precursors totally disappear, while new diffraction peaks appear at 2θ = 44.2°, 51.5°, and 75.8° for CoZn-ZIF/CS-1000 and Co-ZIF/CS-1000, which are attributed to the Co (111), (200), and (220) lattice planes (Co_PDF#15-0806), respectively. In addition, an obvious but broad peak is present from the (002) plane (C_PDF#41-1487), indicating that all these carbon-based materials maintain well-formed graphitic structures. The results indicate that Co-ZIF could be completely converted into Co NPs in CoZn-ZIF/CS-1000 and that the CS structure is well retained during the high temperature carbonization process.


image file: d0nr00511h-f2.tif
Fig. 2 The PXRD patterns of (a) CoZn-ZIF/CS, Co-ZIF/CS, and Zn-ZIF/CS and (b) CoZn-ZIF/CS-1000, Co-ZIF/CS-1000 and Zn-ZIF/CS-1000.

N2 sorption isotherms and pore size distribution are used to analyze the surface properties and porosity of the systems, as shown in Fig. 3a and b and other related data listed in Table S1.‡ In Fig. S4, the Zn-BTC-derived CSs have a high Brunauer–Emmett–Teller (BET) specific surface area (SBET = 1336 m2 g−1) and microporous characteristics, which can provide sufficient interface for mass transfer to promote the electrochemical absorption and electron release. When a layer of PDA is attached to CS, the specific surface area of PDA/CSs decreases sharply due to the tight polymer coating of the solvent accessible pore volume. Moreover, all CoZn-ZIF/CS, Co-ZIF/CS, and Zn-ZIF/CS show type-I isotherms and maintain the microporous structure of MOFs (Fig. S5‡). Compared with Zn-ZIF/CS-1000 (SBET = 638 m2 g−1, Vmeso+macro/Vmicro = 1.34) and Co-ZIF/CS-1000 (311 m2 g−1, 6.08), CoZn-ZIF/CS-1000 possesses a moderate BET specific surface area with an increased volume of mesopores and macropores (586 m2 g−1, 2.44) due to zinc evaporation (Fig. 3a and b). The highest peaks of dV/dlogD for CoZn-ZIF/CS-1000 (3.05 cm3 g−1), Co-ZIF/CS-1000 (5.12 cm3 g−1), and Zn-ZIF/CS-1000 (1.20 cm3 g−1) occur within a narrow range of 3.5 to 4.7 nm (Fig. S6‡), which is beneficial for fast ion diffusion and the reduced interfacial resistance between these hierarchically porous carbon particles derived from CoZn-ZIFs. In general, Raman spectroscopy is an effective technique to study the composition and structural properties of carbon materials. In particular, two significant peaks are detected at 1345 and 1580 cm−1 in the Raman spectra, corresponding to the D and G bands, respectively. In Fig. 3c, CoZn-ZIF/CS-1000 (ID/IG = 0.81) shows a better degree of graphitization as compared to Co-ZIF/CS-1000 (ID/IG = 0.97) and Zn-ZIF/CS-1000 (ID/IG = 0.99). Doping with zinc atoms is not only conducive to the formation of hierarchical pores, including micro/meso/macropores, but also can promote graphitization.33,34 The above results prove that high porosity and a high degree of graphitization would increase mass transport and electrical conductivity, thereby promoting ORR activity.


image file: d0nr00511h-f3.tif
Fig. 3 (a) N2 adsorption–desorption isotherms, (b) the corresponding pore size distribution curves, and (c) Raman spectra for CoZn-ZIF/CS-1000, Co-ZIF/CS-1000, and Zn-ZIF/CS-1000.

The bond types and valence states in CoZn-ZIF/CS-1000, Co-ZIF/CS-1000, and Zn-ZIF/CS-1000 materials are verified via X-ray photoelectron spectroscopy (XPS) measurements (Fig. 4 and S7–S9‡). For the XPS survey spectra, C, N, O, Zn, and Co elements can be detected on the surface of CoZn-ZIF/CS-1000, which is consistent with the EDS analysis and elemental mapping (Fig. 4a). For the deconvoluted C 1s spectrum, several peaks can be highly fitted in Fig. 4b at 284.5 eV, 285.4 eV, 286.3 eV, and 287.7 eV and are indexed to the formation of the sp2 C[double bond, length as m-dash]C, sp3 C–C, C–N, and C–O bonds, respectively.35 For the high-resolution N 1s spectrum (Fig. 4c), four types of subpeaks are fitted with binding energies of 398.1 eV (pyridine-N), 399.4 eV (pyrrolic-N), 400.7 eV (pyridine-N), and 401.7 eV (oxidized-N) and where a high percentage of pyridine-N and pyridine-N suggests that these active nitrogen species can create a large number of chemisorption sites for ORR. We have further compared the N dopant proportions of three as-calcined materials (CoZn-ZIF/CS-1000, Zn-ZIF/CS-1000, and Co-ZIF/CS-1000) in Table S2.‡ According to the high-resolution O 1s curve (Fig. 4d), three subpeaks with binding energies of 530.6, 531.9, and 533.0 eV are assignable to the signals of metal oxides (Co–O and Zn–O), C[double bond, length as m-dash]O, and C–OH/C–O–C, respectively.36 Furthermore, the deconvoluted Co 2p graph includes two regions in Fig. 4e, in which the Co 2p3/2 peaks of 778.6, 780.6, 782.7, and 786.5 eV, and the Co 2p1/2 peaks of 794.9, 797.1, 800.9, and 803.1 eV are indexed to metallic Co, oxidized Co(II)/Co(III), a Co–N bond, and Co satellite peaks, respectively.37 Finally, the Zn 2p spectrum of CoZn-ZIF/CS-1000 are fitted well with two obvious but broad spin–orbit peaks of 2p3/2 and 2p1/2, which are assigned to metallic Zn(0), as shown in Fig. 4f and S10.‡[thin space (1/6-em)]38


image file: d0nr00511h-f4.tif
Fig. 4 (a) Full survey XPS spectrum, the corresponding deconvoluted XPS spectra of (b) C 1s, (c) N 1s, (d) O 1s, (e) Co 2p, and (f) Zn 2p for CoZn-ZIF/CS-1000.

Inspired by the CS templates, unique pore features, high graphitization degree, and rich N doping, the ORR performance of CoZn-ZIF/CS-1000 was evaluated in a 0.1 M KOH solution with a three-electrode cell. We have also investigated the electrochemical properties of Co-ZIF/CS-1000, Zn-ZIF/CS-1000 and, for comparison, 20 wt% Pt/C based electrodes. The cyclic voltammetry (CV) curves in an N2-/O2-saturated solution at 10 mV s−1 at room temperature are exhibited in Fig. 5a, in which the observed oxygen reduction peak of CoZn-ZIF/CS-1000 (0.83 V) shifts to a higher positive potential as compared to Co-ZIF/CS-1000 (0.81 V) and Zn-ZIF/CS-1000 (0.77 V). The linear sweep voltammograms (LSV) are shown in Fig. 5b, and CoZn-ZIF/CS-1000 has the diffusion limited current density (JL) of 5.11 mV cm−2 at 0.2 V vs. RHE. This clearly reveals a higher ORR activity than Co-ZIF/CS-1000 (JL = 4.76 mV cm−2) and Zn-ZIF/CS-1000 (JL = 3.92 mV cm−2). When compared to previous reports, the CoZn-ZIF/CS-1000 catalyst demonstrates a competitive ORR performance, as summarized in Table S3.‡Fig. 5c and S11a–11c‡ show the LSV curves of the rotating disk electrode at different speeds from 100 to 2500 rpm, which were recorded to better understand the ORR mechanism of the catalyst and their corresponding Koutecky–Levich (K–L) curves exhibit almost linear and parallel fitting lines at different potentials from 0.3–0.7 V. According to the K–L equation, the number of electrons transferred (n) is calculated to be 3.82 for CoZn-ZIF/CS-1000, confirming a pseudo 4e mechanism, as shown in Fig. 5c inset and S11d–11f.‡ The other electrocatalysts display similar n values and the H2O2 yields are shown in Fig. 5d and S12.‡ In Fig. 5e, the Tafel slopes for CoZn-ZIF/CS-1000 and Co-ZIF/CS-1000 are calculated to be 75.39 and 67.53 mV dec−1, which are compared to Zn-ZIF/CS-1000 (106.37 mV dec−1) and Pt/C (87.66 mV dec−1) and suggest faster kinetics during oxygen reduction.


image file: d0nr00511h-f5.tif
Fig. 5 (a) CV curves of all catalysts in N2-(dashed) and O2-saturated (solid) 0.1 M KOH solutions; (b) LSV curves at a rotation rate of 1600 rpm; (c) LSV curves at different rotation speeds at 5 mV s−1 for CoZn-ZIF/CS-1000, inset is the corresponding K–L plots at different potentials; (d) electron transfer numbers; (e) Tafel slope curves; (f) Cdl values as a function of scan rate; and (g) Nyquist plots of CoZn-ZIF/CS-1000, Co-ZIF/CS-1000, Zn-ZIF/CS-1000, and Pt/C catalysts.

In this case, the CV-based electrochemical double layer capacitance measurements (Cdl) of CoZn-ZIF/CS-1000 reflect the large electrochemically active surface area from 20–100 mV s−1 (Fig. S13† and Fig. 5f), suggesting more active sites. These results confirm the synergistic effect of active Co NP dispersion and pore-forming zinc coordination on the CS template, which improves ORR activity. In the electrochemical impedance spectroscopy (EIS) data, the diameter in the high frequency and the vertical straight line in the low frequency are attributed to charge-transfer and electrolyte diffusion at the electrode/electrolyte interface, respectively, which are used for evaluating the reaction interface and kinetics.39,40 The equivalent electrical circuit in the Fig. 5g inset is composed of the solution resistance (Rs), charge transfer resistance (Rct), and constant phase element of the catalyst/electrolyte interface (CPE). It can be seen from Fig. 5g that the Rct value for CoZn-ZIF/CS-1000 is 10.45 Ω, compared to Co-ZIF/CS-1000 (12.03 Ω), Zn-ZIF/CS-1000 (17.40 Ω), and Pt/C (14.04 Ω), which indicates that our active material possesses good conductivity with faster electron transport during ORR.

For practical applications of ORR catalysts, their durability is judged to be an important parameter. To evaluate the long-term durability of CoZn-ZIF/CS-1000, a constant current polarization measurement at 0.6 V vs. RHE was also performed at a cathode current density and a rotation rate of 1600 rpm (Fig. 6a). After 10 hours, a slight loss in current density with a 95.78% retention rate compared to the retention rate of 86.64% for Pt/C was observed, which implies that the as-obtained CoZn-ZIF/CS-1000 electrocatalyst exhibits excellent stability towards ORR. Furthermore, the CoZn-ZIF/CS-1000 electrocatalyst also displays a relatively stable performance after ORR tests, where morphology was confirmed by SEM and TEM images (Fig. S14‡). Meanwhile, the characteristic PXRD peaks of CoZn-ZIF/CS-1000 hardly changed after the ORR stability test in Fig. S15,‡ indicating that the target material has good stability. In addition, 18 mL of CH3OH was added quickly to the 0.1 M KOH electrolyte solution using i–t technology to determine the methanol tolerance of both CoZn-ZIF/CS-1000 and Pt/C (Fig. 6b). In addition, based on the above-mentioned good ORR electrocatalytic activity and stability, we further applied the CoZn-ZIF/CS-1000 material to homemade Zn-air batteries. An air cathode, CoZn-ZIF/CS-1000, and Pt/C electrodes were also studied for better comparison. In Fig. S16,‡ the CoZn-ZIF/CS-1000 catalyst provides an open circuit voltage (OCV) of 1.44 V, which is close to the OCV of Pt/C (1.47 V). Fig. 6c shows the polarization and power density curves of the self-made Zn-air battery, from which the CoZn-ZIF/CS-1000 displays a maximum current density of 158.8 mA cm−2 and a peak power density of 113.5 mW cm−2, which are inferior to those of Pt/C (249.5 mA cm−2, 125.5 mW cm−2). Moreover, compared to a Pt/C-based battery (654.3 mA h g−1), the CoZn-ZIF/CS-1000-based primary battery generates a high specific capacity of 595.6 mA h g−1, as shown in Fig. 6d. From the above results, it can be concluded that CoZn-ZIF/CS-1000 has a high catalytic ORR performance with satisfying stability and resistance in alkaline media due to its highly conductive carbon skeleton, hierarchically porous structure, and well-dispersed active sites.


image file: d0nr00511h-f6.tif
Fig. 6 (a) Chronoamperometric response at 0.6 V for 10 h. (b) Chronoamperometric response at 0.6 V after the introduction of 3.0 M MeOH for CoZn-ZIF/CS-1000; (c) polarization and power density curves of CoZn-ZIF/CS-1000 and Pt/C based Zn-air batteries; (d) galvanostatic discharge curves of theses Zn-air batteries at 10 mA cm−2.

4. Conclusions

In summary, we successfully prepared Co NPs on N-doped carbon spheres by means of a bimetallic ZIF-functionalized CS precursor. By tuning the Zn dopant content in CoZn-ZIF/CS, CoZn-ZIF/CS-1000 can be conveniently obtained and Zn evaporation leads to the formation of large void space in the carbon spheres and abundant active sites. The synthesized CoZn-ZIF/CS-1000 exhibits a hierarchically microporous and meso-/macro-porous structure, which is conducive to the exposure of active sites and high-throughput mass transfer. Therefore, the ORR performance of CoZn-ZIF/CS-1000 is higher than that of the two control catalysts (Co-ZIF/CS-1000 and Zn-ZIF/CS-1000) in terms of JL, durability and methanol resistance. In addition, the current method can be further generalized to obtain other transition metal and hetero-atom doped carbon materials for energy applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by National Nature Science Foundation of China (51972237, 51920105004, 51672193, 51420105002), the State Key Laboratory from Structural Chemistry, the Chinese Academy of Sciences (20190008) and the Graduate Scientific Research Foundation of Wenzhou University (3162018030, 2019R429048). We also acknowledge the financial support of the Excellent Youth Cultivation Project from Wenzhou University.

Notes and references

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Footnotes

† Dedicated to the completion of 60 years of Fujian Institute of Research on the Structure of Matter.
‡ Electronic supplementary information (ESI) available: Fig. S1–S11 and Tables S1 and S2. See DOI: 10.1039/d0nr00511h
§ These authors contributed equally to this work.
This journal is © The Royal Society of Chemistry 2020
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