Flexible, Porous, and Metal–Heteroatom-Doped Carbon Nanofibers as Efficient ORR Electrocatalysts for Zn–Air Battery
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Doping and porosity generation were completed simultaneously.
Metal–heteroatom-doped carbon nanofibers are flexible, porous, and well dispersed.
Results include excellent oxygen reduction reaction and enhanced Zn–air battery performance.
KeywordsElectrospinning Zn/Co-ZIFs Carbon nanofibers Flexible porous structure ORR Zn–air battery
New energy technology has become an optimal solution for the energy crisis and environmental pollution caused by the rapid depletion of fossil resources [1, 2]. Recently, sustainable energy conversion and storage systems, such as supercapacitors, fuel cells, and batteries, have been developed rapidly. Among these various new energy devices, fuel cells and metal–air batteries have received increasing attention because of their low contribution to pollution. However, the oxygen reduction reaction (ORR), one of the key reactions of fuel cells and metal–air batteries, has sluggish intrinsic electrode kinetics, hampering the practical application of fuel cells and metal–air batteries . Up to now, Pt-based materials are known as the best catalytic materials for ORR. However, these materials suffer from the prohibitive cost, severe scarcity, serious intermediate tolerance, and poor stability . As an alternative, non-precious metal-doped carbon-based materials with various nanostructures, such as porous/hollow carbon nanoparticles [5, 6, 7], porous/core–shell carbon nanofibers [8, 9, 10], porous/sandwich-type graphene nanosheets [11, 12, 13], and porous graphene aerogels [14, 15], have emerged and attracted great attention. Porous carbon materials, containing catalytic active metal nanoparticles for effective catalysis, have been regarded as crucial supporting materials, owing to their high specific surface area, highly porous structure, and excellent electrical conductivity.
Metal–organic frameworks (MOFs), constructed by bridging metal ions and organic functional ligands into three-dimensional (3D) ordered crystal frameworks with rich micropores and high surface areas, provide a good platform for designing metal–heteroatom-doped carbon catalysts [16, 17]. The first MOF used as a template for porous carbon synthesis was reported by Xu et al. . Among the different types of MOFs, zeolitic imidazole frameworks (ZIFs), a subclass of MOFs, are the most-studied candidates because of their high content of nitrogen and metal ions . Moreover, such complex units consisting of nitrogen and metal ions (MN4) are easy to form active sites for ORR. Thus, N-rich ZIFs (e.g., ZIF-8, ZIF-67, bimetallic Zn/Co-ZIFs) were used as self-sacrificing templates and precursors to construct electrocatalysts with high surface areas, uniform N doping, and Co–Nx active sites by the high-temperature carbonization . However, there remain some problems associated with the obtained ZIF-derived electrocatalysts, such as poor electrical conductivity, aggregation of loaded metal nanoparticles, and poor mechanical stability, which may affect their practical applications.
Recently, combining ZIFs with low-dimensional materials has gotten an increasing amount of attention. Tellurium nanowire-directed templating synthesis of ZIF-8 nanofibers has been demonstrated by Wang Zhang et al. . After carbonization, the as-obtained ZIF-8 nanofibers can be easily converted into highly porous carbon nanofibers with complex network structures, hierarchical pores, and high surface areas, which are beneficial to the improvement of electrochemical properties. Ahn et al. [22, 23] reported a similar synthesis of one-dimensional (1D) hierarchically porous N- and Co-doped carbon nanotubes for efficient ORR by combining a 1D tellurium nanotube as the main template for the carbon nanotube backbone, with an anchored Zn/Co-ZIF as a sub-template for the carbon framework. The porous carbon derived from the bimetallic composites of ZIF-8 and ZIF-67, with a proper ratio, generates synergistic effects, such as a high degree of graphitic carbon, a formation of Co–Nx active sites, and a high surface area. To improve the inter-particle conductivity of the electrocatalysts, multiwall carbon nanotubes (MWCTs) were used in ZIF synthesis, which interconnect the nanoparticles and provide electron conducting highways. Zhang et al.  introduced MWCNTs to increase the electronic conductivity and mass transport of ORR catalysts derived from bimetallic Zn/Fe-ZIFs. ZnO nanorods and nanowires were also used as facile self-sacrifice templates to fabricate hierarchically porous carbon nanotubes from core–shell ZnO@ZIF-8 nanorods and ZnO@Zn/Co-ZIFs nanowires [25, 26]. The in situ reduction and evaporation of ZnO effectively resolved the aggregation issue during carbonization and therefore formed hierarchical pores without using any extra template. 1D carbon nanofiber materials have been paid extensive attention due to their excellent conductivity and flexibility, which are beneficial to improving their catalytic performance and designing flexible electronic devices [27, 28]. Moreover, carbon nanofibers not only solve the above-mentioned waste of inorganic templates but also provide longer electron transport channels. Electrospinning is a simple and efficient method for the preparation of nanofibers [29, 30]. Direct carbonization of electrospun precursor nanofibers is a fast and efficient method for preparing carbon nanofibers [31, 32]. Carbon nanofibers obtained via electrospinning followed by a subsequent carbonization have several advantages: (1) high electrical conductivity through the connection between the nanofibers, (2) fast mass transmission from the network structure and high surface area, and (3) cost effectiveness from the simple preparation procedure . In our previous work, Zn/Co-ZIFs/PAN core–shell nanofibers were well-designed and prepared through Zn/Co-ZIFs grown in situ on the surface of electrospun nanofibers . The results showed that electrochemical performance was improved. However, the electrochemical performance was still worse than that of the commercial catalyst, which may be due to its small surface area. Recently, Liu et al.  developed a novel N, Co-contained MOF-based hierarchical carbon nanofiber as an ORR catalyst, which was synthesized by incorporating Zn/Co-ZIFs with electrospun Co2+/PAN nanofibers, followed by carbonization and acid-leaching treatment. However, the size of the prepared ZIF nanocrystals is ultra-small in their study, and additional metal ions (Co2+) were required for the electrospinning process. Based on these studies, we investigated combining electrospun PAN nanofibers with just as-prepared Zn/Co-ZIFs in different contents, as a precursor for flexible, porous, and well-dispersed metal–heteroatom-doped carbon nanofiber catalysts.
Herein, we report a facile approach to prepare well-dispersed metal (Zn/Co) and heteroatom (N) co-doped porous carbon nanofibers (Zn/Co–N@PCNFs) film based on electrospun Zn/Co-ZIFs/PAN nanofibers. During the process, Zn/Co-ZIF nanocrystals with a larger size (~ 900 nm) and different contents were loaded onto electrospun PAN nanofiber without any additional metal ions. Such a facile method not only can yield a hierarchical porous structure but can also achieve a good distribution of metal active sites in the porous carbon nanofibers, which is important for ORR. Zn/Co–N@PCNFs-800 (carbonization temperature is 800 °C) exhibited an excellent ORR performance. In addition, the suitability and durability of Zn/Co–N@PCNFs-800 were tested as the oxygen cathode for primary and rechargeable Zn–air batteries, showing relatively good electrochemical properties.
2 Experimental Section
Polyacrylonitrile (PAN, Mw = 150,000 g mol−1), zinc nitrate hexahydrate (Zn(NO3)2·6H2O), cobalt nitrate hexahydrate (Co(NO3)2·6H2O), 2-methylimidazole (C4H6N2, MIM), methanol (MeOH), ethanol (EtOH, ≥ 99.7%), potassium hydroxide (KOH, 98%), and N,N-dimethylformamide (DMF) were all purchased from Aladdin Chemical Reagent Co. Nafion solution (5 wt%) was purchased from DuPont Co. Common commercial 20 wt% Pt/C catalyst and RuO2 were bought from Johnson Matthey Co. All chemicals were of analytical grade and used without further purification.
2.2 Preparation of the Samples
2.2.1 Preparation of Zn/Co-ZIF Nanocrystals
The preparation of Zn/Co-ZIF nanocrystals was based on a previous procedure with modifications . Typically, 5.0 mmol Zn(NO3)2·6H2O and 10.0 mmol Co(NO3)2·6H2O were dissolved into 150 mL methanol to form a clear solution. The molar ratio of Zn2+/Co2+ was set to 1/2. A mixture of 60 mmol 2-methylimidazole with 50 mL methanol was added to the above solution with 12 h incubation at room temperature. The product was separated by centrifugation and then washed thoroughly with methanol three times, and finally dried overnight at 60 °C under a vacuum oven.
2.2.2 Preparation of the Zn/Co-ZIFs/PAN Precursor Nanofibers
The Zn/Co-ZIFs/PAN precursor nanofibers were prepared by electrospinning . In a typical experiment, 0.5 g PAN powder was dissolved into 4.5 g DMF solvent . The blended solution was continuously stirred for 6 h at 40 °C. Then, 1.0 g Zn/Co-ZIF nanoparticles were added into above solution and stirred for another 6 h at 40 °C. Afterward, the electrospinning process was carried out with a high voltage of 20 kV and an extrusion rate of 0.6 mL h−1. The obtained nanofibers were collected on aluminum foil (~ 15 × 15 cm2). The collect distance between the nozzle and the aluminum foil was 15 cm. The Zn/Co-ZIFs/PAN nanofiber film was easily peeled off from the collector and put into a vacuum oven overnight at a temperature of 80 °C to remove the residual solvents.
2.2.3 Preparation of the Zn/Co–N@PCNF Electrocatalysts from Zn/Co-ZIFs/PAN Nanofibers
The obtained Zn/Co-ZIFs/PAN nanofiber film was pre-oxidized at 280 °C for 2 h at a heating rate of 2 °C min−1 under air atmosphere. The obtained pre-oxidized nanofiber film was then directly carbonized at the target temperatures (500, 600, 700, 800, 900, and 1000 °C) for 2 h at a heating rate of 5 °C min−1 in N2 atmosphere and then naturally cooled to room temperature to obtain the flexible, porous, and well-dispersed metal–heteroatom-doped carbon nanofibers. (The samples were named as Zn/Co–N@PCNFs-T, where T is the target carbonization temperature.)
2.3 Physical Characterizations
The microstructure and surface morphology of the obtained samples were observed by scanning electron microscopy (SEM, S-4700, Hitachi, Japan). The internal structure and graphitic structure were investigated by transmission electron microscopy (TEM, Tecnai G2 T20, FEI, USA) and high-resolution transmission electron microscopy (HR-TEM, JEM 3010, JEOL, Japan). Scanning transmission electron microscopy (STEM) and color mapping were employed to distinguish the elemental dispersion in these samples by HR-TEM. The thermal decomposition behavior of the precursor nanofibers was determined by thermal gravimetric analysis (TGA, Q500, TA Instruments, USA). Fourier transform infrared (FT-IR) spectra of the samples were measured by spectrometer (Nicolet-is5 IR, Thermo Fisher Scientific, USA). The crystal structure of the samples was evaluated on a powder X-ray diffraction (XRD, D8 Advance, Bruker, Germany) system with Cu-Kα radiation. Raman spectra analysis was conducted on a Raman spectrometer (Invia Reflex, Renishaw, British) at 514 nm. X-ray photoelectron spectroscopy (XPS, Thermal Scientific K-Alpha XPS spectrometer) was employed to analyze the chemical composition of these samples. Nitrogen absorption/desorption isotherms were obtained on a Quantachrome Autosorb-iQ gas sorptometer via the conventional volumetric technique, and the corresponding surface areas were determined by using the Brunauer–Emmett–Teller (BET) method.
2.4 Electrochemical Measurements
All electrochemical measurements were performed in a three-electrode system on an electrochemical workstation (CHI 760E, Shanghai Chenhua, China) in 0.1 M KOH electrolyte. A glassy carbon (GC) rotating disk electrode (RDE, ALS, Japan) of 4.0 mm in diameter was used as a working electrode. Before use, the working electrode was polished carefully with 50 nm Al2O3 powders to obtain a mirror-like surface and then washed with deionized water and ethanol and allowed to dry. A platinum wire and Ag/AgCl (3.0 M KCl) electrode were used as the counter and reference electrodes, respectively. The electrochemical measurements were carried out in a 0.1 mol L−1 KOH aqueous electrolyte at the temperature of 298 K. To prepare the working electrode, 5.0 mg of the catalyst was dispersed in a solution consisting of 1.0 mL of absolute ethanol and 100 µL of 5 wt% Nafion, and then sonicated for 1 h to form a well-dispersed black catalyst ink. For the catalyst ink, 5.0 µL was drop-cast onto the glassy carbon surface (~ 0.18 mg cm−2 loading) and dried at room temperature for electrochemical testing. The working electrodes were scanned for about 50 cycles until the signals were stabilized, and then, the data were collected. Before testing, a continuous N2/O2 flow was bubbled into the electrolyte for 30 min. The cyclic voltammetry (CV) experiments were cycled in 0.1 M N2- and O2-saturated KOH electrolyte solutions with a sweep rate of 50 mV s−1. The RDE tests were measured in 0.1 M O2-saturated KOH electrolyte solutions with a sweep rate of 10 mV s−1 and different speed rates (400–2500 rpm). For comparison, 20 wt% Pt/C was used in the same electrochemical tests.
Zn–air battery assembly and test: the rechargeable Zn–air battery performance was tested using a homemade Zn–air battery. To assemble the Zn–air battery, a polished zinc plate (0.3 mm of thickness) was used as the anode; an air electrode coated by 100 μL catalyst ink of Zn/Co–N@PCNFs-800 or a mixture of 20 wt% Pt/C + RuO2 (1:1 in a mass ratio) onto carbon paper (electrode area: 0.8 cm in diameter; catalyst loading: 1.2 mg cm−2), dried naturally to form a uniform catalyst layer, was used as the cathode; and 6.0 M KOH solution served as the electrolyte. The potential–current polarization curves for the batteries were recorded on a CHI 760e workstation. The discharge/charge performance and stability for the batteries were analyzed by a Lanhe-CT2001A testing system at room temperature .
3 Results and Discussion
Doping is an important factor affecting electrochemical properties. XPS analyses were carried out to further characterize the change in elemental composition and chemical status of these carbonized samples. As shown in Fig. S5a, b, the elemental content of Zn, Co, and N decreased with increase of carbonization temperature, ranging from 500 to 1000 °C. The C/O ratio also increases with carbonization temperature, indicating that the conductivity gradually improved. The XPS and EDS results are consistent, which indicates that the composition of the material is uniform. The XPS high-resolution spectra of elemental Zn gradually disappeared when the temperature was above 900 °C, as elemental Zn in ZIFs easily evaporates (~ 900 °C), resulting in porous carbon structures during high-temperature treatment. The elemental content of cobalt increased first and then decreased with increase of carbonization temperature. This is due to the gradual doping of cobalt from the inside to the surface of nanofibers, which then evaporated at higher temperatures. The change in the content of elemental cobalt is consistent with the change in the nitrogen content, which imply the existence of Co–Nx–C species. At the temperature of 800 °C, the sample has a relatively high content of cobalt and nitrogen. N atoms could incorporate into the graphene layers to replace carbon atoms at different sites during the carbonization process (above 700 °C), and in doing so, they were split into various binding energies in the XPS spectra: pyridinic-N 398.7 ± 0.3 eV, pyrrolic-N 400.4 ± 0.3 eV, and graphitic-N 401.4 ± 0.3 eV. It is worthy to note that carbons with pyridinic-N and pyrrolic-N at the edges of the graphene layers show higher charge mobility and better donor–acceptor properties than carbons with graphitic-N do .
In summary, flexible, porous, and well-dispersed metal–heteroatom-doped carbon nanofibers were prepared by a direct high-temperature carbonization approach using electrospun Zn/Co-ZIFs/PAN nanofibers as the precursor. The flexible porous bimetal–heteroatom-doped carbon nanofibers exhibited the excellent ORR electrocatalytic activity, superior stability, and methanol tolerance under 0.1 M KOH solution, which can be ascribed to the synergistic effect of Co–Nx species, uniform dispersions of Co nanoparticles and N dopants, high surface area, distinct conductive curving of carbon nanofibers, as well as the hierarchical pore structure. The excellent ORR performance was also demonstrated in a homemade rechargeable zinc–air battery. In addition, this Zn/Co–N@PCNFs-800 film exhibited good flexibility, which could be applied to flexible devices. Our work illustrates the great potentials of hybrid porous carbon nanofiber materials as ORR and OER electrocatalysts. We hope that this work can spark interests in developing multi-functional electrocatalysts toward application in renewable energy technologies.
The authors would like to thank the Natural Science Foundation of Jiangsu Province (Grant No. BK20171200) for their financial support. The authors also wish to acknowledge the support provided by the Excellent PhD International Visit Program of Beijing University of Chemical Technology. The authors also wish to acknowledge the Zn–air battery tests by Jinhe Shu from Beijing University of Chemical Technology. We are also sincerely grateful to my friends (Yige Zhao; Nannan Guo; Yongzheng Shi; they are all from Beijing University of Chemical Technology) for the advices and help to my experiments.
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