N, F-Codoped Microporous Carbon Nanofibers as Efficient Metal-Free Electrocatalysts for ORR
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A new and facile method to synthesize N, F-codoped microporous carbon nanofiber (N, F-MCF) electrocatalysts via electrospinning, hydrothermal process, and thermal treatment.
Polyvinylidene fluoride is applied as a fluorine source in oxygen reduction reaction (ORR) catalysis for the first time in literature.
N, F-MCFs exhibit distinguished electrocatalytic activity, stability, and methanol tolerance for ORR in alkaline media.
KeywordsMetal-free catalyst Oxygen reduction reaction N, F-codoped Carbon nanofiber Polyvinylidene fluoride
Oxygen reduction reaction (ORR) electrocatalysts for proton exchange membrane fuel cells (PEMFCs) have gained significant attention because of their sluggish kinetic process [1, 2, 3, 4, 5]. To date, platinum (Pt)-loaded carbon is considered as the most effective ORR electrocatalyst [6, 7, 8, 9]. However, Pt-based catalysts still have several shortcomings, such as poor durability, limited reserves, high cost, and carbon monoxide (CO) poisoning [10, 11]. Currently, metal-free heteroatom-doped carbons are widely considered as promising catalysts to replace Pt-based carbon catalysts in the near future because of their high electrocatalytic activity toward ORR, cost effectiveness, long-term cycling stability, and excellent tolerance to methanol and CO oxidations [12, 13]. Among these materials, nitrogen (N)-doped carbons are extensively studied because the electronegativity of N (3.04) induces charge redistribution of adjacent atoms in an N-doped carbon surface layer, which greatly enhances the ORR activity of carbon electrocatalysts [14, 15, 16, 17, 18]. Besides N, other nonmetal atoms with different electronegativities, such as boron (B) [19, 20], sulfur (S) [21, 22], phosphorus (P) [23, 24], and fluorine (F) [25, 26, 27, 28, 29], can enhance the ORR activity of carbon catalysts.
In addition, the largest electronegativity is observed in F atoms (4.0). Ishizaki et al. reported that F atoms bonded to ionic and semi-ionic C atoms can act as electron acceptors, which promote charge transfer between the F and C atoms, thereby resulting in higher conductivity and modification of the electronic properties of pristine carbons . Moreover, Lu et al. reported that F doping can improve the wettability of the catalyst surface, thereby facilitating both electrolyte and O2 transportation within porous frameworks . Therefore, F doping is advantageous for ORR activities. Moreover, N and F atoms can enhance ORR activities by a synergetic effect. N, F-codoped carbon electrocatalysts, such as carbon black , mesoporous carbon [26, 32], graphdiyne , porous carbon , carbon nanoparticles , graphene , and graphite nanofibers , are widely prepared, and they exhibited excellent properties as ORR electrocatalysts in alkaline media. It is difficult to dope F atoms into carbon matrix; thus, a large number of F sources are required. Currently, NH4F is the most commonly used F source. However, the facile decomposition property of NH4F increases the synthesis difficulty of F-doped carbons. Further, the F-doped content in obtained carbon samples is less than 1 at%, while the mass amount of NH4F used is 20 times more than that of the carbon source [25, 27, 33]. Thus, it is important to develop new F sources and highly efficient F-doping methods.
Besides heteroatom doping, carbon morphology is another key factor that affects catalyst activities. High surface areas along with suitable micropores or mesopores can increase the number of active sites for ORR and facilitate O2 transportation during ORR. In this study, we present a facile in situ method to synthesize N, F-codoped microporous carbon nanofibers (N, F-MCFs) as electrocatalysts with high Brunauer–Emmett–Teller (BET) surface area via electrospinning polyacrylonitrile/polyvinylidene fluoride/polyvinylpyrrolidone (PAN/PVDF/PVP) tricomponent polymers followed by a hydrothermal process and thermal treatment. PVDF is used as a source of F and C atoms. PAN acts as a source of N and C atoms. PVP, which is removed by the hydrothermal process, is applied as a porogen for N, F-MCFs. The as-synthesized N, F-MCFs are characterized systematically, and their ORR activities and stabilities are investigated. Benefitted from the N, F-codoped effect and unique nanofiber structure with microporous pore walls, N, F-MCFs exhibit both highly catalytic activity and stabilities for ORR in alkaline solutions. The catalytic activity of N, F-MCFs in acidic solutions is also investigated preliminarily.
2 Experimental Methods
2.1 Materials and Chemicals
PAN (Mw = 150,000 g mol−1) and PVP (Mw = 10,000 g mol−1) were purchased from J&K Scientific Ltd. PVDF (Solef 5130) was obtained from Solvay. 5% Nafion® solution (Nafion 117) was obtained from E. I. DuPont Company. Other chemicals, such as N,N-dimethylformamide (DMF), potassium hydroxide (KOH), and ethanol, were purchased from Sinopharm Chemical Reagent CO., Ltd. and used as received. Deionized water was used throughout the experiments.
2.2 Electrospinning of PAN/PVP/PVDF Membranes
The tricomponent PAN/PVP/PVDF nanofibrous membranes were prepared via facile single-nozzle electrospinning. The PAN (3 wt%), PVP (3 wt%), and PVDF (3 wt%) membranes were mixed in a sealed glass bottle with DMF and magnetically stirred for 24 h at room temperature as a precursor solution. Further, this precursor solution was loaded into a 3-mL plastic syringe connected with a stainless needle of 0.5 mm inner diameter. During the electrospinning process, the operating voltages were 12 kV, the flow rate was 0.2 mL h−1, and the collecting distance was 14 cm. The electrospun PAN/PVP/PVDF membranes were peeled off from the aluminum foil.
2.3 Preparation of N, F-MCFs
The electrospun PAN/PVP/PVDF membranes were transferred into a 100-mL Teflon stainless autoclave with deionized water and hydrothermally treated under 110 °C for 6 h to remove PVP. Further, the hydrothermal-treated membranes were washed with deionized water and dried under 100 °C in a blast oven to obtain PAN/PVDF fibrous membranes.
The peroxidation and carbonization of these PAN/PVDF fibers were performed in an electric heating tube furnace. First, the dried PAN/PVDF fibrous membranes were sealed in a graphite boat covered by a carbon paper. Subsequently, the samples were preoxidized in an air atmosphere under 220 °C for 2 h at a heating rate of 2 °C min−1. Further, the samples were carbonized in an N atmosphere under 1000 °C for 2 h at a heating rate of 2 °C min−1. The as-prepared samples (N, F-MCFs) were cooled down to the room temperature.
2.4 Physical and Electrochemical Characterization
The morphology of N, F-MCFs was characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) using Nova NanoSEM 450 and Talos F200X (both from FEI Company, USA), respectively. X-ray photoelectron spectroscopy (XPS, Kα) analyses were performed on an AXIS UltraDLD X-ray photoelectron spectrometer system equipped with Al radiation as a probe, and the analysis spot size was 400 μm in diameter. Raman spectra were collected by DXR Micro-Raman Spectroscopy (Thermo Fisher Scientific, USA), equipped with a holographic grating of 1800 lines mm−1 and a He–Ne laser (532 nm) as the excitation source. BET measurements were performed on an ASAP 2460 surface area and porosimetry analyzer (Micromeritics Instrument Corp., USA).
3 Results and Discussion
3.1 Structure and Morphology of N, F-MCFs
The pore structure of N, F-MCFs was investigated by adsorption–desorption isotherms of N2 at − 196 °C. As shown in Fig. 2c, d, both isotherms are between type I isotherms with type H4 hysteresis loops following IUPAC classification, thereby indicating the existence of micropores. The BET surface areas of N, F-MCFs-A and N, F-MCFs-B are 709.78 and 621.46 m2 g−1, respectively. The corresponding pore size distributions of the two samples are calculated by Barrett–Joyner–Halenda (BJH) desorption, and their average pore widths are 2.37 and 2.77 nm. The large BET surface area and the existence of micropores are advantageous for high ORR activities.
3.2 Electrocatalytic Activities Toward ORR of N, F-MCFs
To further estimate the ORR reaction kinetics of N, F-MCFs, a series of LSV tests were carried out with various rotation speeds from 400 to 2000 rpm in an O2-saturated 0.1 M KOH electrolyte (Fig. 4c, e). On the basis of LSV curves at different rotations, the Koutecky–Levich plots and the electron transfer numbers (n) for ORR were obtained from the K–L equations. The great linearity and parallelism of the K–L plots (Fig. 4d, f) suggest a direct four-electron pathway for better ORR efficiency. The average n value of N, F-MCFs-A and N, F-MCFs-B is 4.0 and 4.1, which suggested that the complete reduction of O2 to OH− over N, F-MCFs by a four-electron transfer process in 0.1 M KOH. For further confirmation of 4e− selectivity, the yield of H2O2 was measured via rotating ring-disk electrode (RRDE) in Fig. S3. The percentage of H2O2 of N, F-MCFs-A is below 10%, and n is about 3.75 in the potential range from 0 to 0.8 V versus RHE (Fig. 4g) in 0.1 M KOH, which indicates the low peroxide formation and promising ORR activity. The poisoning experiment with CN−, which can strongly bond to active metal sites, was performed to identify the active sites of the prepared catalysts . As shown in Fig. 4h, although the limiting current density drops a little, the onset potential and half-wave potential of N, F-MCFs-A almost unchanged with and without CN− in 0.1 M KOH. That means the catalytic active sites in N, F-MCFs-A are primarily derived from the F-doped and N-doped carbon sites rather than from other metallic coordination sites.
In summary, we demonstrated a facile method to synthesize N, F-MCF electrocatalysts via electrospinning PAN/PVDF/PVP tricomponent polymers followed by the hydrothermal process and thermal carbonization. N, F-MCFs exhibited distinguished electrocatalytic activity for ORR in alkaline media, including higher onset potential (0.94 V vs. RHE), half-wave potential (0.81 V vs. RHE), and electron transfer number (4.0) because of their unique nanofiber structure with microporous pore walls and synergistic effect of high doped F and N content. In acidic media, the N, F-MCFs also exhibited a four-electron transfer pathway for ORR. In addition, the N, F-MCFs showed outstanding tolerance to methanol and superior stability (95.7%) compared to commercial Pt/C catalysts. As a result of all the superior electrocatalytic performance, this work provides an efficient pathway of in situ synthesis of N, F-MCFs as a highly active metal-free ORR electrocatalyst in the further application of fuel cells.
We gratefully acknowledge funding for this work provided by the National Nature Science Foundation of China (51573090), National Key R&D Program of China (2016YFB0302000) and Open Foundation from State Key Laboratory of Fluorinated Functional Membrane Material.
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