Interconnected Hierarchically Porous Fe, N-Codoped Carbon

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Interconnected Hierarchically Porous Fe, N-Codoped Carbon Nanofibers as Efficient Oxygen Reduction Catalysts for Zn-Air Batteries Yingxuan Zhao, Qingxue Lai, Ya Wang, Junjie Zhu, and Yan Yu Liang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 24 Apr 2017 Downloaded from http://pubs.acs.org on April 24, 2017

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ACS Applied Materials & Interfaces

Interconnected Hierarchically Porous Fe, N-Codoped Carbon Nanofibers as Efficient Oxygen Reduction Catalysts for Zn-Air Batteries Yingxuan Zhao,† Qingxue Lai,† Ya Wang,† Junjie Zhu,† and Yanyu Liang†,‡,*

†

Jiangsu Key Laboratory of Materials and Technology for Energy Conversion, College of

Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, P. R. China.

‡

Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites, Nanjing

211816, P. R. China.

KEYWORDS: Electrospinning; Fe, N-codoped carbon nanofiber; Hierarchical structures; Oxygen reduction reaction; Zn-air batteries

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ABSTRACT: Developing porous carbon-based no-precious metal catalysts towards oxygen reduction reaction (ORR) is a suitable approach to significantly reduce the costs of fuel cells or metal-air

batteries.

Herein,

interconnected

hierarchically

porous

carbon

nanofibers

simultaneously doped with nitrogen and iron (HP-Fe-N/CNFs) were fabricated by facile pyrolysis of polypyrrole coated electrospun polystyrene/FeCl3 fibers. The obtained carbon nanofibers present high specific surface area (569.6 m2 g-1) and large pore volume (1.00 cm3 g-1) as well as effective doping of N and Fe. Benefiting from the improved mass transfer and utilization of active sites attributed to interconnected hierarchical porous structures, HP-Fe-N/CNFs displayed excellent ORR catalytic activity in alkaline media, with a comparable onset potential and half-wave potential, but superior selectivity, stability and tolerance against methanol to commercial 30 wt.% Pt/C. Particularly, when applied HP-Fe-N/CNFs in the assembled Zn-air battery, HP-Fe-N/CNFs outperform 30 wt.% Pt/C in power density and long-term stability, explicitly showing its promising practical application.


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1. INTRODUCTION Oxygen reduction reaction (ORR) in cathode is the key process which determined the effective of fuel cells and metal–air batteries 1-3 Due to the intrinsically sluggish kinetics of ORR, a highly efficient catalyst is strongly required.4 To data, the most efficient ORR catalysts are platinum (Pt)-based materials. However, they suffer from significant drawbacks of high price, limited reserves, poor stability and low tolerance against methanol crossover effects, which restrict their large-scale applications in ECDs.5 Consequently, it is essential to explore cheap but efficient non-precious metal catalysts as alternative materials to break through these bottlenecks. Over the past decades, tremendous achievements have been acquired in

developing

non-precious metal catalysts, including metal oxides,6 metal carbides,7 metal chalcogenides,8 non-precious metal complexes9,10 and metal-free heteroatom doped carbons.11,12 Especially, transition-metal-nitrogen codoped carbon materials (M-N/C, where M = Fe or Co) have been emerging as one of the most promising non-precious metal catalysts.13-15 Recent studies displayed plenty of high performance M-N/C catalysts and different active cites have been proposed (e.g. Fe-Nx or Fe/Fe3C@NC), but due to the complicated chemical environment ORR occurred, the authentic origin of catalytic activity in M-N/C catalysts is still in debates.16,17 What can be widely acknowledged is that the specific surface area and porous structure of catalyst evidently impact the ORR performance.18,19 Dodelet et al. found that the micropore created in pyrolysis hosted most of active sites, so the catalytic activity reflected by the microporous surface per mass of catalyst.20 However, ORR occurred in the triple-phase interface which



contains catalytic sites, oxygen and electrolyte. Catalysts dominated by micropores with poor transport efficiency of reactant (O2, OH-, H2O, etc.) also hide the activities of catalysts.21 Qiao et al. designed a series of ordered meso-/macro porous g-C3N4/C catalysts and revealed the improved catalytic performance that fast diffusion brings.22 Combined the advantages of diverse pores, hierarchically marco/meso/micro-porous structures aim at the promoted density and accessibility of active sites was much desired in the catalysts to lifting the catalytic performance Traditional methods for constructing hierarchically porous carbon materials usually employed silica template strategy.23 However, this strategy suffers from time-consuming and complex procedure which hardly meets the requirements of large-scale production. Here, a facile method is developed to fabricate interconnected hierarchically porous iron and nitrogen codoped carbon nanofibers (HP-Fe-N/CNFs). In the synthesis procedure, FeCl3 is used as the initiator for the polymerization of pyrrole upon the PS fibers, which act as one-dimensional support. After pyrolysis, carbon nanofibers with interconnected hierarchically porous structures are achieved. The high specific surface area and large pore volume promise HP-Fe-N/CNFs high-flux mass transfer and adequately utilization of active sites during ORR process. As a result, the as-synthesized catalyst displayed an excellent ORR catalytic activity in alkaline media and a parallel characteristic in assembled Zn-air battery. These results clearly demonstrate that the HP-Fe-N/CNFs as a potential alternative for Pt/C in actual application.

2. EXPERIMENTAL SECTION


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2.1. Materials Preparation. In the synthesis procedures of HP-Fe-N/CNFs, 1.2 g polystyrene (PS, Mw = 250,000) and 0.3 g Iron(III) chloride anhydrous (FeCl3) were added in 10 ml N,N-dimethylformamide (DMF) with stirring until totally dissolution to form a red solution formation. During electrospinning process, a 10 mL syringe installed with a stainless steel as the container of the mixed solution. The collect distance, applied voltage and the flow rate is 15 cm, 15 kV and 0.8 ml/h, respectively. Then the collected PS/FeCl3 fibers were dried at 60 oC for 12 h. Afterwards, PS/FeCl3 fibers and pyrrole monomer were place in a sealed vessel, the vapor phase polymerization (VPP) beginning when a pump was used to take out the air, this process was conducted at 25 oC for 12 h. The obtained dark fibers membrane was heated under nitrogen atmosphere to 900 oC for 2 h with a heating rate of 2 oC min-1. After hydrochloric acid (HCl) etching of the carbonized nanofibers and a second heat treatment under the same condition, HP-Fe-N/CNFs were achieved. For comparison, Fe and N-codpoed CNFs without hierarchical porous structures (denoted as Fe-N/CNFs) were prepared under the same conditions except that PS was replaced by ployacrylonitrile (PAN, Mw = 230000), and a preoxidized process under 250 o

C for 2 h in air before first heat treatment. 2.2. Physicochemical Characterization. Raman spectra were recorded on a Lab-RAM

HR800 (Horiba Jobin Yvon). Thermogravimetric analysis (TGA) was recorded on Thermal analytical balance (NETZSCH STA 409) with a heat rate of 10oC/min under N2. The crystal structures of the synthesized materials were analyzed by X-ray diffraction (XRD, BRUKER D8, Cu Kα). The morphology of the samples was examined by Scanning electron microscopy (SEM,



Hitachi S-4800), transmission electron microscopy (TEM, Philips Tecnai 12), high-resolution TEM (HRTEM, Philips Tecnai G2) and scanning transmission electron microscopy STEM (FEI Tecnai G2 F30 S-TWIN). Nitrogen adsorption-desorption isotherms and pore size distribution were characterized with a Micrometrics ASAP2020 analyzer at 77 K. X-ray photoelectron spectroscopy (XPS) measurements were carried out on Kratos AXIS Ultra spectrometer with a source gun of Al Kα and spot size of 400 µm. 2.3. Electrochemical Measurements. Firstly, 2.0 mg prepared catalysts was dispersed in 1.0 mg 0.05 wt.% Nafion/EtOH solution by sonication for 1 hour. Then the cover of catalysts in working electrode was through by dropping the prepared catalyst ink on a rotating disk electrode (RDE) or rotating ring-disk electrode (RRDE) electrode with loading of 0.25 mg cm-2 (0.1 mg cm-2 for 30 wt.% Pt/C). Electrochemical measurements were recorded in a three-electrode cell, with saturated calomel electrode (SCE) and a graphitic rod as the reference and counter electrodes, respectively. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) tests of RDE and RRDE were conducted in 0.1 M KOH electrolyte at room temperature on a CHI760D electrochemical workstation equipped with MSR Electrode Rotator (Pine Research Instrumentation). The tested electrode potential (E (SCE)) was calibrated to reversible hydrogen electrode potential (E (RHE)) according to the equation: E (RHE) = E (SCE) + 0.059*pH + 0.241. The electron transfer number (n) calculated from RDE test was based on the Koutecky-Levich (K-L) equation:


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1 1 1 1 1 = + = / + J J J J Bω B = 0.2nFC D / ν / Where J is the measured current density; JK and JL are the kinetic and diffusion limiting current densities, respectively; ω is the angular velocity; F is Faraday constant; C0 is the bulk concentration of O2; D0 is the diffusion coefficient of O2 and ν is the kinematic viscosity of the electrolyte. For RRDE test, the potential carried out in ring electrode is 1.4 V. The HO2- yield and electron transfer number (n) of catalysts were calculated from the following equations: IR ⁄N IR ⁄N+ID ID n= 4× IR ⁄N+ID

HO %= 200×

Where ID and IR are the disk and ring currents, respectively; N is the ring collection efficiency (0.37). 2.4. Zn-Air Battery Assembly and Measurements. Zn-air battery was evaluated in home-built electrochemical cells. For air cathode, as prepared catalyst ink was coated onto the PTFE-treated carbon fiber paper (1 cm* 1 cm) with loading of 1.0 mg cm-2 for all catalysts. A polished zinc foil and 6 M KOH were used as anode and electrolyte, respectively. Polarization curves and galvanostatic discharge tests were carried out by CHI760D electrochemical workstation and LAND testing system, respectively.

3. RESULTS AND DISCUSSION



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The fabrication process is illustrated in Scheme 1. The mixture of PS and FeCl3 dissolved in DMF was first electrospun to PS/FeCl3 fibers. Core-shell fibers (PS/FeCl3@PPy) were then obtained by exposing the fibers in pyrrole vapors under vacuum. After pyrolysis at 900oC, followed by acid leaching to remove the inactive iron particles and second pyrolysis, the Fe, N codoped carbon nanofibers with hierarchical porous structures (HP-Fe-N/CNFs) were achieved. For comparison, PS was replaced by PAN during electrospinning process to prepare Fe, N codoped carbon nanofibers (Fe-N/CNFs) under same condition. The SEM images in Figure S1A and B show the morphology of PS/FeCl3 fibers and PS/FeCl3@PPy fibers. The average diameter of PS/FeCl3 fibers is ~1.1 µm, which increased to ~1.8 µm after VPP. The surface of PS/FeCl3@PPy becomes a slight rough with a PPy shell coating. TEM images (Figure S1C, D) confirm the successful package of PPy shell with wide about 350 nm. After twice pyrolysis, the diameter of the fibers decreases sharply to 600-800 nm. Notably, HP-Fe-N/CNFs exhibit a narrow interconnected macro/meso-porous structure, as demonstrated in the SEM and TEM images in Figure 1A-C. TEM image in Figure S2A proved that the porous structure formed since PS/FeCl3@PPy fibers after first heat treatment (HP-Fe-N/CNFs-HT1). Subsequent acid leach and second heat treatment remove most of metal particles in carbon nanofibers, certified by the disappearance of major diffraction peaks of iron phase in

XRD pattern

(Figure1F,

Figure

S2B) and

preceding TEM

images

of

HP-Fe-N/CNFs-HT1 and HP-Fe-N/CNFs. The XRD pattern of HP-Fe-N/CNFs shows two typical diffraction peaks at 26o and 44o, assigned to graphite structures in HP-Fe-N/CNFs, other


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weak peaks was assigned to iron carbide.14 HRTEM images in Figure 1E and F identified the residual metal particles (Fe3C, d200=0.22 nm) with the protection of graphite shell (d002=0.352 nm) caused by iron catalysis effect.13, 24 Moreover, Raman spectra was used to examine the structure changes of carbon during second heat treatment (Figure S3). A decreased ratio of ID/IG from HP-Fe-N/CNFs-HT1 to HP-Fe-N/CNFs reflects that second heat treatment increased the graphitic degree of carbon nanofibers.25 These graphitic structures together with interconnected pores can provide an enhanced electronic conductive network and reduced diffusion length simultaneously for fast electron /mass transfer during ORR process. To gain insight into the formation of interconnected porous structures in HP-Fe-N/CNFs, TGA measurement for PS/FeCl3@PPy was carried out in N2 atmosphere. As described in Figure 2A, it can be subdivided into three weight loss regions. At the first region, the small weight loss below 374.9 oC should be attributed to the evaporation of H2O and the decomposition of pyrrole oligomers. The second region range from 374.9 oC to 462.8 oC of weight loss about 54% was mainly ascribed to the decomposition of PS.26 The third region that started at 462.8 oC and ended at 900 oC was possibly caused by the carbonization of high-molecular-weight PPy. From SEM images of PS/FeCl3@PPy heated to 300 oC, 400 oC and 500 oC in Figure 2B-D, the process of distinct morphology evolutions with no decomposition, partial decomposition and total decomposition of PS have been shown. It can be found that the fibers have no obvious pores when heated to 300 oC. But they appeared at 400 oC and turned to more open and interconnected at 500 oC. The formed unique porous structures could be attributed to the phase separation and



the complete degradation of PS during the carbonization process. When temperature raised upon the melting point of PPy and PS, relative movement occurred between polymers, leading to redistribution of the two kinds of polymer phases. After the total decomposition of PS, the interconnected channel emerged. The presumption can be also verified by replacing PS to PAN. There are no visible porous structures in Fe-N/CNF since the high carbon yield of PAN (Figure S4), reflecting the important role of PS as sacrificial polymer phase. A STEM image in Figure 2E further confirmed the hierarchical porous structures. The corresponding element mapping results revealed that nitrogen and iron were evenly distributed in the carbon matrix, indicating the successful doping of nitrogen and iron simultaneously by this synthesis strategy. The porous structure of HP-Fe-N/CNF and Fe-N/CNF was investigated by nitrogen adsorption–desorption isotherms (Figure 3A). Both samples displayed the type IV isotherm with hysteresis implied the mesoporous structures.27 Figure 3B shows the pore distribution of two samples. It is obvious that HP-Fe-N/CNFs holds identical distribution of micropores with Fe-N/CNFs, mainly attributed to the defects originated from the carbonization of PPy. However, abundant mesopores and macropores were generated in HP-Fe-N/CNFs with the assistance of PS, in accordance with TEM characterizations. On the basis of isotherms, the total specific surface areas (SSAtotal) and pore volume (Vtotal), as well as specific surface areas (SSAmicro) and pore volume (Vmicro) from micropores (calculated by density functional theory) of two carbon nanofibers were listed in Table S1. HP-Fe-N/CNFs had a higher SSAtotal of 569.6 m2 g-1 and larger pore volume of 1.00 cm3 g-1 than 443.6 m2 g-1 and 0.28 cm3 g-1 for Fe-N/CNFs,


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respectively. Besides, HP-Fe-N/CNFs exhibited a lower ratio of SSAmicro/SSAtotal than that of Fe-N/CNFs, which ensured the ORR-relevant species more opportunity to exploit the active sites that were mostly hosted in micropores.20 XPS measurement was applied to identify the chemical states of surface elements. The survey spectra show the co-existence of C, N, O and Fe in both HP-Fe-N/CNFs and Fe-N/CNFs (Figure 3C). The high resolution N1s spectra reveal four types of nitrogen dopants, assigned to pyridinic N (398.6 eV), pyrrolic N (399.7 eV), graphitic N (401.3 eV) and oxidized N (402-405 eV).28,29 It is noted that both HP-Fe-N/CNFs and Fe-N/CNFs show the dominated concentrate in the form of graphitic N and pyridinic N in total N configuration (Figure 3D). Previous researches have uncovered that pyridinic N and graphitic N could promote the ORR activity.4,30 Besides, pyridinic N could coordinate with Fe to form Fe-Nx moieties, which has been accepted as another highly effective active site.16,20 Figure 3E shows the Fe 2p spectra of two carbon nanofibers, the peaks at 710.9 eV and 714.4 eV was related to the lower energies spin-orbital (2p3/2) of Fe2+ and Fe3+ species; at the higher energies spin-orbital (2p1/2) regions, the peaks at 723.6 eV and 726.1 eV was assigned to Fe2+ and Fe3+, respectively. The peak at 718.5 eV was satellite peak.19,31 The content of N and Fe was summarized in Figure 3F, similar elements configuration were detected in all catalysts, but a little higher content of nitrogen in Fe-N/CNFs (4.17 at.%) than HP-Fe-N/CNFs (2.77 at.%) was observed. The ORR activities of the as-synthesized HP-Fe-N/CNFs catalysts in comparison with commercial 30 wt.% Pt/C and Fe-N/CNFs were evaluated by RDE and RRDE tests in 0.1 M



KOH solution. As shown in Figure 4A, HP-Fe-N/CNFs shows no reaction occurred in N2-saturated solution, while an obvious ORR peak appears at 0.80 V in O2-saturated solution. It is more positive than Fe-N/CNFs (0.69 V) and comparable to 30 wt.% Pt/C (0.82 V). Consistent results were obtained in RDE experiments, as shown in Figure 4B. The onset potential (Eonset) was 0.93 V for HP-Fe-N/CNFs while 0.85 V and 0.92 V for Fe-N/CNFs and 30 wt.% Pt/C, respectively. Besides, the half-wave potentials (E1/2) of Fe-N/CNFs, HP-Fe-N/CNFs and 30 wt.% Pt/C were 0.69 V, 0.80 V and 0.81 V, respectively. Meanwhile, the tafel slope is ~95 mV/dec for Fe-N/CNFs, ~72 mV/dec for HP-Fe-N/CNFs and ~68 mV/dec for 30 wt.% Pt/C (Figure S5A). The closed value of HP-Fe-N/CNFs and 30 wt.% Pt/C indicated the similar fast ORR kinetic process on them.32 The kinetic current density obtained by mass-transfer corrected at 0.8 V show that HP-Fe-N/CNFs with a higher instinct ORR activity than Fe-N/CNFs and comparable activity than 30 wt.% Pt/C (Figure S5B). Furthermore, the fitting lines in Koutecky−Levich (K-L) plots of two carbon nanofibers (Figure S6B, D) show well linearity and parallelity between J-1 and ω-1, implying first-order reaction kinetics relevant to the oxygen concentration.33 The electron transfer number of Fe-N/CNFs and HP-Fe-N/CNFs was ~3.5 and ~3.9 at 0.4-0.6 V according to the slopes of K-L plots, respectively. Analogical results were acquired by RRDE measurement (Figure 4C, D). The low yield of peroxide (HO2−) intermediate (3.9) between 0.3 V to 0.8 V for HP-Fe-N/CNFs, slight below 30 wt.% Pt/C but much higher than Fe-N/CNFs, coinciding well with the results from K-L plots. These results suggest a dominated four-electron pathway of ORR on HP-Fe-N/CNFs.


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Doping nitrogen will induce the uneven charge distribution of carbon materials, facilitating the ORR process through favouring the O2 adsorption and weaken the O-O bonding.4 Thus the content of N commonly is regard as a critical factor of ORR catalysts. But in this work, HP-Fe-N/CNFs with a low content of nitrogen compare to Fe-N/CNFs shows a higher ORR catalytic performance that is even comparable than 30 wt.% Pt/C. The brilliant activity is mainly originated from its unique nanoarchitecture, as illustrated in Figure 4E. On the one hand, the high aspect ratio fibers and their network inheriting from electrospinning provides a stable scaffold which could minimize diffusion and electronic conductive resistance.34,35 On the other hand, the interconnected hierarchical porous structures with a large pore volume and surface area offer a smooth transport of reactants, as well as the significantly elevated utilization of active sites.21,22 These synergetic effects as mentioned above are responsible for the high performance of HP-Fe-N/CNFs in electrochemical and Zn-air battery test (vide infra). Probing the active sites of Fe-N/C catalysts has attracted widely attention in ORR catalysis studies. Previously studies proposed that the Fe-Nx moieties or the N doped carbon activated by Fe species account for the brilliant ORR performance.7,16,36 Here further experiments were conducted to research these factor. We investigated the ORR catalytic activity of HP-Fe-N/CNFs-HT1 before and after acid leach (HP-Fe-N/CNFs-AL), as shown in Figure S7. The Eonset and E1/2 of HP-Fe-N/CNFs-AL were negative than HP-Fe-N/CNFs-HT1, That’s probably because the acid treatment may introduce some functional groups, which could block the active sits. These blocked sites could be partially repaired through a second heat



treatment.37,38 The improved ORR performance of catalyst after second heat treatment confirmed the possibility of the above assumption. However, the second heat treatment will not only change the graphite degree and porous properties, but also alter other properties such as the surface N content and states of the catalyst.25 These various changes make it difficult to distinguish the accurate contribution to the improved ORR electrocatalytic performance from metal particles, graphitic degree, surface area, and the states of surface N, respectively, during second heat treatment. In another experiments, SCN- ions were used as a electrochemical probe to detect the Fe-containing sites in HP-Fe-N/CNFs because of its high affinity with Fe element.39,40 During the RDE test (Figure S8), SCN- injection results into obvious reduced ORR catalytic activity, suggesting that the Fe ions among HP-Fe-N/CNFs in a most probable form of Fe-Nx moieties participate in the electrocatalytic process. The electrocatalytic stability was first evaluated by chronoamperometric measurements (Figure 5A). Current-time curves shows 87.8% current remain for HP-Fe-N/CNFs after 10000s, much better than that of 30 wt.% Pt/C (74.1%). The accelerated durability test (ADT) was also conducted (Figure 5C, D), only 14 mV shift of E1/2 and 0.22 mA cm-2 decreased of limiting current density for HP-Fe-N/CNFs after 1000 cycles. On the contrary, 30 wt.% Pt/C exhibits 20 mV shift and 0.36 mA cm-2 reduction in E1/2 and limiting current density, respectively. In addition, no apparent current density decay was observed for HP-Fe-N/CNFs after methanol injection, while an evident decrease of current density for 30 wt.% Pt/C occurred (Figure 5B). These results confirm the superior stability and methanol tolerance for HP-Fe-N/CNFs.


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Finally, a homemade Zn-air battery was assembled to demonstrate the cell performance. The catalysts covered on the PTFE-treated carbon fiber paper, 6 M KOH and zinc foil acted as the air cathode, electrolyte and anode, respectively. Figure 6A presents the polarization and power density curves. An open-circuit voltage of ca. 1.42 V is observed when battery is loaded with HP-Fe-N/CNFs. Notably, the peak power density of HP-Fe-N/CNFs was 135 mW cm-2 at a current density of 218 mA cm-2, higher than Fe-N/CNFs (81 mW cm-2 at current density of 112 mA cm-2) and 30 wt.% Pt/C (131 mW cm-2 at current density of 193 mA cm-2), further highlighting the key role of the interconnected hierarchical porous structures for fast electro/ion pathway and gas diffusion. HP-Fe-N/CNFs-based Zn-air battery possess specific capacity of 701 mAh gZn-1(corresponding to an energy density of 867 Wh kgZn-1) when normalized to the mass of consumed Zn at a discharge density of 5 mA cm−2 (Figure 6B). Galvanostatic discharge curves revealed that a higher voltage of HP-Fe-N/CNFs (1.23 V) than 30 wt.% Pt/C (1.21 V) after 20 h at a current density of 5 mA cm-2. Moreover, the stable voltage was 1.21 V and 1.13 V at 10 mA cm-2 and 20 mA cm-2, respectively. After the expending of Zn, we could ‘recharge’ the battery through refill Zn foil and electrolyte. As shown in Figure 6D, a long–term galvanostatic discharge test for HP-Fe-N/CNFs was carried out at 5 mA cm-2. No obvious voltage drop occurred during the test process. All above results manifest the superior catalytic performance and stability of our as-prepared catalysts in practical applications.

4. CONCLUSION



In summary, a facile strategy by using electrospinning, VPP and pyrolysis process is developed for constructing interconnected hierarchically porous carbon nanofibers. Notably, PS not only acted as a one-dimensional template, but also played an important role in generating hierarchical porous structures. The open hierarchical porous structures combined with the appropriate nitrogen and iron configurations endow HP-Fe-N/CNFs excellent ORR catalytic performance in alkaline media. Additionally, Zn-air battery assembled from HP-Fe-N/CNFs outperforms 30 wt.% Pt/C in power density and long-term stability, displaying strong practical applicability. These results demonstrate that HP-Fe-N/CNFs catalysts as a potential alternative for noble metal materials in fuel cell and metal–air batteries.

ASSOCIATED CONTENT

Supporting Information. SEM and TEM of PS/FeCl3@PPy fibers and Fe-N/CNFs; TEM image and XRD patterns of HP-Fe-N/CNFs-HT1; Raman spectra of HP-Fe-N/CNFs-HT1 and HP-Fe-N/CNFs; Tafel plot, kinetic current density, LSV curves and corresponding K-L plots of HP-Fe-N/CNFs and Fe-N/CNFs; CV and LSV curves of HP-Fe-N/CNFs-HT1 and HP-Fe-N/CNFs-AL; LSV curves of HP-Fe-N/CNFs before and after SCN- injection. Table of BET surface area and pore volume, surface atomic ratio of HP-Fe-N/CNFs and Fe-N/CNFs; Table of HP-Fe-N/CNFs compared with literatures of ORR performance.

AUTHOR INFORMATION


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Corresponding Author * [email protected] (Y.Y. Liang)

ACKNOWLEDGMENT

The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Grant No. 21273114), Natural Science Foundation of Jiangsu Province (Grant No. BK20161484), the Fundamental Research Funds for the Central Universities (Grant NO. NE2015003), the “Six Talent Peaks Program” of Jiangsu Province (Grant No. 2013-XNY-010), a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institution, and the Foundation of Graduate Innovation Center in NUAA (kfjj20160613).

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Figure Captions Scheme 1. Schematic illustration of the preparation process of HP-Fe-N/CNFs.

Figure 1. A) SEM, B) TEM and C) HRTEM images of HP-Fe-N/CNFs (insert: SAED pattern), D) and E) HRTEM images of metal particle surrounded by graphite shell in HP-Fe-N/CNFs, F) XRD patterns of HP-Fe-N/CNFs.

Figure 2. A) TGA curves of PS/FeCl3@PPy fibers in N2 atmosphere; SEM images of PS/FeCl3@PPy fibers heated to B) 300 oC, C) 400 oC and D) 500 oC; E) STEM images of HP-Fe-N/CNFs and its corresponding elements mapping images of C, N, O, and Fe.

Figure 3. A) Nitrogen adsorption–desorption isotherms and B) corresponding pore size distribution curves; C) XPS survey spectra, D) high resolution N1s spectra and E) Fe 2p spectra of Fe-N/CNFs and HP-Fe-N/CNFs; F) Nitrogen and iron contents of Fe-N/CNFs and HP-Fe-N/CNFs.

Figure 4. A) Cyclic voltammograms, B) RDE voltammograms, C) RRDE voltammograms and D) electron transfer number and peroxide yield of Fe-N-CNFs, HP-Fe-N/CNFs and 30 wt.% Pt/C in O2-saturated 0.1 M KOH solution at a rotation speed of 1600 rpm; E) Schematic illustration of interconnected hierarchical porous fibers with enhanced ORR catalytic activity.

Figure 5. A) Relative current–time (i–t) chronoamperometric responses and B) current–time (i– t) chronoamperometric responses with the addition of methanol of HP-Fe-N/CNFs and 30 wt.%



Pt/C in O2-saturated 0.1 M KOH solution at 0.8V and a rotation speed of 900 rpm. Accelerated durability test (ADT) of C) HP-Fe-N/CNFs and D) 30 wt.% Pt/C in O2-saturated 0.1 M KOH at a rotation speed of 1600 rpm for different cycles.

Figure 6. A) Polarization and power density curves of the Zn-air batteries using Fe-N/CNFs, HP-Fe-N/CNFs, 30 wt.% Pt/C as ORR catalyst (mass loading of 1 mg cm−2) and 6 M KOH solution as electrolyte; B) Specific capacities of the Zn–air batteries using HP-Fe-N/CNFs as ORR catalyst; C) Galvanostatic discharge curves of the Zn–air batteries using HP-Fe-N/CNFs and 30 wt.% Pt/C as ORR catalyst and KOH electrolyte at various current densities (5, 10 and 20 mA cm−2); D) Long–term galvanostatic discharge test using HP-Fe-N/CNFs at a current density of 5 mA cm−2. The battery is recharge by refill Zn foil and electrolyte.


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Scheme 1

Figure 1



Figure 2


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Figure 3



Figure 4


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Figure 5

Figure 6



Table of Contents


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Interconnected Hierarchically Porous Fe, N-Codoped Carbon

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