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3D Nanofibrous Air Electrode Assembled With Carbon Nanotubes Bridged Hollow Fe2O3 Nanoparticles for High Performance Lithium-Oxygen Batteries Ji-Won Jung, Ji-Soo Jang, Tae Gwang Yun, Ki Ro Yoon, and Il-Doo Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15421 • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on January 30, 2018

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ACS Applied Materials & Interfaces

3D Nanofibrous Air Electrode Assembled With Carbon

Nanotubes

Bridged

Hollow

Fe2O3

Nanoparticles for High Performance LithiumOxygen Batteries Ji-Won Jung,†,a Ji-Soo Jang,†,a Tae Gwang Yun,a Ki Ro Yoona and Il-Doo Kim*,a a

Department of Materials Science and Engineering, Korea Advanced Institute of Science and

Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon, 34141, Republic of Korea

* E-mail: [email protected] †J. W. Jung and J.-S. Jang contributed equally to this work.

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ABSTRACT Lithium-oxygen batteries (LOBs) have been considered as one of the most viable energy source options for electric vehicles (EVs) due to their high-energy density. However, they are still faced with technical challenges such as low round-trip efficiency and short-cycle life; these mainly originate from cathode part in battery. In this work, we designed 3D nanofibrous air electrode consisted of hierarchically structured CNT bridged hollow Fe2O3 nanoparticles (H-Fe2O3/CNT NFs). Composite nanofibers consisted of hollow Fe2O3 NPs anchored by multiple CNTs offered enhanced catalytic sites (interconnected hollow Fe2O3 NPs) and fast charge transport highway (bridged CNTs) for facile formation and decomposition of Li2O2, leading to outstanding cell performance: (1) Swagelok cell exhibited highly reversible cycling characteristics for 250 cycles with a fixed capacity of 1000 mAh g-1 at a current density of 500 mA g-1. (2) A module composed of two pouch-type cells stably powered a LED lamp operated at 5.0 V.

Keywords: lithium-air batteries; iron oxide; carbon nanotubes; hierarchical porous structure; cathode


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1. INTRODUCTION Lithium-oxygen (Li-O2) batteries (LOBs) have recieved great attention because of exceptionally high energy density (3505 Wh kg-1) far exceeding the state-of-art Li-ion batteries (LIBs).1,2 However, LOBs still suffer from technical problems such as short-cycle life and high overpotentials resulting in low round-trip efficiency; these are primarily determined by O2-breathing porous cathode where Li2O2 is formed and decomposed.3,4 While discharge-charge reactions are ongoing, pores of cathode can be easily clogged and deactivated by insulating Li2O2 that is not sufficiently removed, limiting electron transfer and O2 diffusion in porous cathode. This induces rapid increases of overpotentials, especially for charge process (called oxygen evolution reaction, OER).5 So far, a number of efficient catalysts have been suggested for improved OER, which can lead to facile decomposition of Li2O2. Among them, conductive metals such as cobalt (Co), ruthenium (Ru), and gold (Au) have been introduced as efficient and stable catalysts.6-8 However, these catalysts are expensive, thus their use might be limited in practical level of Li-O2 cell fabrication. As one of alternatives to these, iron (Fe)-based catalysts have been employed as an O2 cathode material because they are cost-effective and abudant in earth’s crust.9,10 Lv et al. synthesized mace-like Fe-based oxide, and used it as porous cathode material for LOBs.11 However, intrinsic electronic conductivityof iron-based oxide is very low, leading to considerable loss of energy density in LOBs; therefore, it should be mixed with inexpensive conducting carbon materials (e.g., CNTs, graphene, and CNFs).12-14 For examples, Wu et al. synthesized Fe2O3 nanoparticles (NPs) supported by Vulcan XC-72 carbon as LOB cathode and suggested that Fe2O3 coupled with conductive carbon can effectively reduce charge overpotential.9 Feng and co-workers reported that a multi-layered Fe2O3/graphene composite, which was produced through thermal casting approach, offers a lot of catalytically active sites and alleviates unwanted parasitic side reactions on graphene.13 3 ACS Paragon Plus Environment


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Although these results indicated that Fe2O3 is a prospective catalyst, it has rarely been studied, particularly in long cycling cathodes for LOBs. Moreover, technical challenges remain to realize desirable architecture with an optimized hybridization of Fe2O3 catalyst and conductive carbon for highly reversible formation and decomposition of Li2O2. Ideally combined Fe2O3 catalyst and conductive carbon can remarkably reduce overpotentials and maximize discharge capacity of LOBs. In this work, we synthesized hierarchically structured composite nanofibers consisted of hollow Fe2O3 NPs anchored by multiple CNTs (hereafter, H-Fe2O3/CNT NFs) as an air cathode through electrospinning of Fe precursor/co-polymer (PAN/PMMA) NFs and subsequent controlled 2-step calcinations; (i) first step in reducing atmosphere for in-situ growth of CNTs on Fe3C NPs loaded carbon nanofibers (CNFs) and (ii) second step in air for conversion of Fe3C NPs to hollow Fe2O3 NPs through “Kirkendall effect”.15,16 The anchoring of CNTs onto hollow Fe2O3 NPs, which forms H-Fe2O3/CNT composite NFs, brings multifold benefits for improving capacity, potential efficiency, and stability of Li-O2 cells. First, hollow Fe2O3 NPs with many mesopores up to tens of nanometers provide efficient mass transport, maximized active sites, and sufficient reservoirs for Li+, O2 and Li2O2. Second, CNTs tangled among hollow Fe2O3 NPs serve as nanobridges for fast electron transport. These combined features lead to superior air cathode outperforming KB cathode in Li-O2 cell. Moreover, pouch-type cell employing enlarged H-Fe2O3/CNT NFs electrode was fabricated to power 5 V LED and their electrochemical properties were discussed.


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2. EXPERIMENTAL SECTION Materials Iron (II) acetate (Fe(CO2CH3)2), poly(methyl methacrylate) (PMMA, Mw = 1,300,000 g mol1

), Polyacrylonitrile (PAN, Mw = 150,000 g mol-1), and N,N-dimethylformamide (DMF, 99.8

%) were purchased from Sigma-Aldrich (St. Louis, USA). All chemicals were used without further purification. Preparation of H-Fe2O3/CNT NFs To prepare the electrospinning solution, 0.3 g of Iron (II) acetate, 0.3 g of PAN, and 0.3 g of PMMA were homogeneously dissolved in 6 g of DMF. The composite solution was mixed by vigorous stirring at 60 oC for 12 h. Then, single-spinneret (25 gauge) electrospinning was carried out to prepare Fe precursor/co-polymer (PAN/PMMA) NFs in the following conditions; a voltage of 12 kV, distance of 20 cm between 25 gauge needle and the metallic collector, and feeding rate of 0.1 ml min-1. The as-spun nanofibers (NFs) were firstly stabilized in a box furnace at 200 oC for 1 h and subsequently annealed in a tube furnace at 750 oC for 6 h with reducing atmosphere (H2/N2, 4 %/96 %, v/v). The annealed product was designated as Fe3C/CNT-CNFs. Finally, Fe3C/CNT-CNFs were additionally annealed in a box furnace at 400 oC for 1 h with heating rate of 5 oC min-1. The final product was denoted as H-Fe2O3/CNT NFs. Characterization The morphologies of Fe3C/CNT-CNFs and H-Fe2O3/CNT NFs were observed by using scanning electron microscopy (SEM, XL-30 SFEG, Philips) and transmission electron microscopy (FETEM, Tecnai G2 F30 S-Twin, FEI). The specific surface areas of HFe2O3/CNT NFs and H-Fe2O3/CNT NFs were measured using the Brunauer-Emmett-Teller theory (BET, ASAP2020, Micromeritics). The pore size distribution of prepared samples was 5 ACS Paragon Plus Environment


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obtained from N2 desorption branch using the Barrett-Joyner-Halenda (BJH). The crystal structures of the prepared samples were analyzed by powder X-ray diffraction (PXRD, D/MAX-RC 12kW, Rigaku) with Cu Kα radiation (λ =1.54 Å). The chemical bonding state and elements of prepared samples were evaluated by X-ray photoelectron spectroscopy (XPS, K-alpha, Thermo Scientific) and Raman spectrometer with a 514 nm laser (FT-Raman, Bruker). Elemental analysis (EA) was conducted by using an element analyzer (Flash 2000, Thermo Scientific). Electrochemical measurements For evaluation of the Li-O2 cells with H-Fe2O3/CNT NFs as air cathode, we utilized Swagelok-type cells according to our previous work. Prior to cell assembly, cathode was prepared as follows: 60 mg of H-Fe2O3/CNT NFs, 30 mg of KB, and 10 mg of polyvinylidene difluoride (PVdF) (4 wt%) were dissolved in N-methyl-2-pyrrolidone (NMP) solvent and mixed together using agate mortal for slurry; the KB electrode without catalyst was composed of KB and PVDF binder in the weight ratio of 9:1. The slurry was casted on Ni mesh (pore diameter=100 µm, thickness=50 µm) and carbon cloth with micron-sized diameter (HCP330, WizMAC). After then they were dried in vacuum oven for overnight. The mass loading of H-Fe2O3/CNT NFs (or only KB) was approximately 0.5 ~ 0.6 mg cm-2. All of the Li-O2 cells were assembled in Ar-filled glovebox (water content was less than 0.5 ppm). The Li-O2 cells consisted of Li metal foil as anode, glass fiber (Watman GF/F) as separator, 1 M LiTFSI in TEGDMEas electrolyte, the as-made cathode, and carbon paper with 5 wt% of PTFE (CNL Energy, GDL 10 BC) as gas diffusion layer (GDL). The used gel polymer electrolyte was a poly(methyl methacrylate) (PMMA) (Sigma Aldrich)-based gel electrolyte of synthesized with LiClO4 salt (Sigma Aldrich) dissolved in acetonitrile (ACN) (Sigma Aldrich) solvent, and propylene carbonate solution as the plasticizer. The ratio of ACN : PC : PMMA : LiClO4 was 70 : 20 : 7 : 3 weight percent. The combined solution was 6 ACS Paragon Plus Environment

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continuously stirred at 80°C for 3 h and stirred overnight at room temperature. To understand electrochemical reactions in cathode with H-Fe2O3/CNT NFs, cyclic voltammetry (CV) was performed at a scan rate of 0.05 mV s-1 within a range of 2.0 – 4.3 V using battery testing device (WBCS3000 device by WonATech). Other electrochemical performances were investigated by carrying out galvanostatic charge-discharge experiments with different capacities at various current densities between voltage range of 4.8 and 2.0 versus Li/Li+ with a battery testing system (Multicycling Battery Test System) under 1 atm O2 gas flowing.

3. RESULTS AND DISCUSSION Figure 1 indicates the synthetic process of hierarchical composite NFs consisted of CNTs anchored hollow Fe2O3 NPs (H-Fe2O3/CNT NFs). Firstly, one dimensional (1D) as-spun NFs comprising of Fe precursor (Fe(AC)2) and co-polymer (PAN/PMMA) composite NFs were formed by single nozzle electrospinning. Due to the immiscible nature between PAN and PMMA polymers in electrospinning solution, the composite NFs undergo phase separation of PAN and PMMA inside the composite NFs along the longitudinal direction.17 Interestingly, first high-temperature calcination of PAN and PMMA under H2/N2 (4%/96%, v/v) reducing atmosphere leads to thermal decomposition of PMMA, while PAN was carbonized during this heat treatment. During the heat treatment, numerous pores in the carbon nanofibers (CNFs) were generated by the removal of PMMA while PAN is formed into 1D carbon backbone. At the same time, carbon and H2 gas are combined to form C2H2 that is carbon source for growth of CNTs.18 Meanwhile, the phase of Fe(Ac)2 precursor was transformed into Fe3C nanoparticles (NPs) which play critical role as catalytic sites, where CNTs were grown by consuming C2H2 gas. According to previous research, precipitaion rate of carbon is faster than diffusion rate, thus it induces the metal-carbon composite supersaturated condition, which results in growth of CNTs on metallic NPs.19 Accordingly, when pyrolysis of PMMA 7 ACS Paragon Plus Environment


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occurred close to Fe3C NPs in this work, carbon sources can be rapidly delivered toward catalytic Fe3C NPs. Thus CNTs were more preferrentially precipitated on Fe3C NPs. To effectively provide ORR and OER sites in Li-O2 cell, second calcination of CNTs anchored porous Fe3C based CNFs (hereafter, Fe3C/CNT-CNFs) was carried out at 400 oC in ambient air to synthesize H-Fe2O3/CNT NFs as final product through phase change of Fe3C NPs to hollow Fe2O3 NPs and thermal decomposition of CNF backbones. During the calcination, respective Fe3C NPs were oxidized to hollow Fe2O3 NPs owing to “Kirkendall effect“ in Fe NPs.20,21 Because inward diffusion rate of oxygen anions is slower than outward diffusion rate of Fe cations, oxidation of Fe cations occurred on the surface of Fe NPs; this results in hollow Fe2O3 NPs with inner cavities, which are assembled along 1D-longitudinal direction (Figure 2(d,e)) (See details in Figure S2(a,b)). As shown in Figure 2(a) and 2(b), Fe3C/CNT-CNFs with bumpy surface morphology were formed after first carbonization in reducing atmosphere of the as-spun Fe precursor/PAN/PMMA composite NFs. The size of exposed Fe3C NPs on CNFs was under 50 nm, employed as the effective catalyst for the growth of multi-walled CNTs (MWCNTs).19 Through transmission electron microscopy (TEM) image of Fe3C/CNT-CNFs (Figure 2(c)), we observed that hierachically interconnected CNT bundles were formed on amorphous CNF backbone with numerous pores, which were randomly mixed with Fe3C NPs. Some Fe3C NPs were larger than 50 nm, allowing formation of nanoscale graphitic layers, given descriptive name “nano-onion“, on Fe3C NPs (Figure S1). As displayed in TEM image of H-Fe2O3/CNT NFs (Figure 2(f)), unlike Fe3C/CNT-CNFs, only two components (Hollow Fe2O3 NPs and short CNT bundles) except CNFs appeared in porous 1D architecture due to thermal decomposition of amorphous backbone carbon of CNFs during the second calcination; meanwhile, CNTs remained due to relative low annealing temperature (400 °C). Entangled graphitic CNTs among hollow Fe2O3 NPs can serve as nanobridges for electron 8 ACS Paragon Plus Environment

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transfer (Figure S2(c-f)). The size distribution of hollow Fe2O3 NPs is exhibited in Figure S3, and mean particle size of Fe2O3 was about 63.9±23.07 nm, which is sufficient value for stably accomodating Li2O2 in consideration of 23.6 nm-thick wall of hollow Fe2O3 NPs. To investigate surface area, pore size, and volume characteristics, Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) analyses were performed for both Fe3C/CNT-CNFs and H-Fe2O3/CNT composite NFs (Figure S4). Fe3C/CNT-CNFs had larger surface area of 259.85 m2 g-1 compared with H-Fe2O3/CNT NFs (166.92 m2 g-1) and larger pores (avg. pore size=7.52 nm) were predominantly formed in H-Fe2O3/CNT NFs compared with Fe3C/CNTCNFs (avg. pore siz=4.58 nm). The differences in surface area and pores are mainly caused by the thermal decomposition of amorphous CNF in Fe3C/CNT-CNFs during second calcination. Due to the decompositon of amorphous CNFs, larger pores can be formed on the H-Fe2O3/CNT NFs, thus it is thought that Fe3C/CNT-CNFs are much densely packed compared with H-Fe2O3/CNT NFs, which may not be suitable to store Li2O2 inside. Moreover, exposed Fe3C is not stable in highly oxidative atmosphere and is fully converted to oxidized species during second calcination step.22,23 These are reasons that we selected HFe2O3/CNT NFs as the target material for LOBs instead of Fe3C/CNT-CNFs. Atomic distribution of carbon (C), iron (Fe), and oxygen (O) in H-Fe2O3/CNT NFs was confirmed by scanning TEM-energy dispersive X-ray spectroscopy (STEM-EDS) elemental mapping analysis (Figure 2(g)). STEM image of H-Fe2O3/CNT NFs showed distinctive porous 1D nanostucture assembled of hollow Fe2O3 NPs. Fe (yellow) and O (red) appeared at almost same position, indicating the formation of iron oxide phases. In particular, it is obviously found that carbon element (green) was detected in entire region of the NFs, which demonstrates that the CNTs percolated in whole amorphous CNFs scaffold were homogeneously distributed. Amount of carbon (~34 wt%) in H-Fe2O3/CNT NFs was



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determined by TGA data (Figure S5); the thermal decomposition of CNFs and partial decomposition of CNTs occurred during calcination. To verify crystal structures of Fe3C/CNT-CNFs and H-Fe2O3/CNT NFs, powder Xray diffraction (PXRD) analysis was carried out (Figure 3(a)). Multiple peaks (red line) in PXRD data correspond to crystal phase of Fe3C in Fe3C/CNT-CNFs(JCPDS file no. 652411).24 Meanwhile, the peaks of blue line reveal hematite Fe2O3 phases with (012), (104), and (110) planes (rhombohedral hematite, JCPDS file no. 33-0664), which are formed by the oxidization of Fe3C after second calcination. The PXRD results clearly demonstrate phase transformation from Fe3C to Fe2O3 according to our two-step calcinations. Raman spectroscopy using 514 nm excitation also confirmed the phase conversion (Figure 3(b) and Figure S6). Obvious peaks at ~1,300 cm-1 and ~1,600 cm-1 correspond to D-band and G-band of CNTs, which indicate presence of disordered and graphitized carbon in CNTs; high temperature oxidation may induce many defects and disorders in the H-Fe2O3/CNTs, which were further verified by evaluating high intensity ratio of the D-band to the G-band (ID/IG = 1.40). Other sharp peaks related with CNTs were shown at 2,700 cm-1and 2,950 cm-1. These two sharp peaks in Raman data are assigned to 2D-band and D+G-band that are obvious evidences for formation of high purity CNTs, respectively.24 Furthermore, the five strong resonant peaks appeared at 221, 287, 401, 497, and 601 cm-1, which were associated with A1g and Eg modes of hematite Fe2O3 crystallites.25 Chemical states of H-Fe2O3/CNTs NFs were further corroborated by X-ray photoelectron spectroscopy (XPS) for O 1s, Fe 2p, and C 1s (Figure 3(c)). For O 1s, the peaks at 529 and 531 eV were observed, which are related to Fe-O of Fe2O3 and C-O binding, respectively. The XPS Fe 2p spectra can be deconvoluted into three peaks at 711, 714 and 719 eV, which should be related to Fe3+ in hematite, Fe2+ in Fe3O4, and satellite phase, respectively;26 these peaks clearly differ from XPS Fe 2p of Fe3C in Fe3C/CNTs-CNFs 10 ACS Paragon Plus Environment

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(Figure S7). According to the phase diagram of Fe-O in previous research, hematite Fe2O3 and Fe3O4 can co-exist in iron oxides synthesized in air ambient at 400 oC, similar to our case.26 However, Fe3O4 phase was not oberved by PXRD analysis, and thus it can be anticipated that low concentration of Fe3O4 might be present on the surface of H-Fe2O3/CNTs. For C 1s, the deconvoluted peaks can be attributed to C-N (288 eV), C-O (285.8 eV), C-C (284.8 eV), C=C (283.6 eV), and C=Fe (282.4 eV) binding energy, respectively. Specific electrochemical reactions of Li-O2 cells including H-Fe2O3/CNTs NFs and KB (reference sample) were examined by cyclic voltammetry (CV) with voltage window of 2.0 – 4.3 V at a scan rate of 0.05 mV s-1 (Figure 4(a) and 4(b)). All evaluations were conducted under identical conditions in terms of size of oxygen window and electrolyte (1 M LiTFSI in TEGDME without additive). For both H-Fe2O3/CNTs NFs and KB electrodes, under Ar atmosphere, no cathodic and anodic peaks were observed. While the cells are filled with pure O2, there are redox peaks, and peak currents of H-Fe2O3/CNT NFs electrode were much higher than those of KB electrode; this reuslt indates that ORR (E1,c) and OER (E1,a and E2,a) are much reversible in H-Fe2O3/CNT NFs compared with KB electrode. In particular, distinctive peaks located at different potentials on charging appeared in H-Fe2O3/CNTs electrode, suggesting stepwise oxidation (E1,a and E2,a) of Li2O2 generated on H-Fe2O3/CNT NFs. Previous reports have demostrated that oxidation of Li2O2 can be realized by different pathways as follows.27,28 Li2-xO2 → (1-x)Li+ + (1-x)e- + LiO2

(1)

Li2O2 → Li+ + e- + LiO2

(2)

LiO2 + LiO2 → Li2O2 + O2

(3)

Li2O2 → 2Li+ + 2e- + O2

(4)



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E1,a and E2,a peaks may be attributed to removal of superoxide-like species (Li2-xO2) located at outer surfaces of Li2O2 (Equation (1)). In general, peak at about 3.7 V can be assigned to decomposition of Li2O2 through single-electron trasnfer pathway (Equation (2) and Equation (3)), but such peak was not observed. On the other hand, the strong peak (above 4.0 V) associated with oxidation of Li2O2 via two-electron transfer pathway (Equation (3)) was observed. It is important to note that currents of the peaks below 3.5 V were much higher and dominant for Li-O2 cell with H-Fe2O3/CNT NFs compared with KB; these results indicate that H-Fe2O3/CNT NFs electrode is advantageous in terms of thermodynamics of decomposition of Li2O2. First discharge-charge profiles were investigated with a limited capacity of 5000 mAh g-1 at a current density of 500 mA g-1 (cell operating time is 10 h for discharge and charge processes) (Figure 4(c)). In the case of the cell with H-Fe2O3/CNT NFs, discharge and charge overpotentials were 0.17 and 0.91 V (voltage gap is ~ 1.0 V), whereas the cell with KB exhibits ORR and OER overpotentials of 0.23 and 1.24 V (voltage gap is ~ 1.47 V); this result demonstrates that hierarchically structured Fe2O3/CNT hybrid NFs provide numerous active sites and facile electron pathway for Li2O2 when high specific capacity is delivered; gradual growth of Li2O2 on H-Fe2O3/CNT NFs up to 5000 mAh g-1 is represented in Figure S8, showing concurrent formation of disc-like and smooth Li2O2 becoming larger as the specific capacity increases.29 Possibly, smooth Li2O2 rather than large disc-like Li2O2 tends to be formed in numerable nanopores acting as active sites for nucleation of amorphous discharge products (e.g., LiO2 and Li2O2).30 Rate-capability was further investigated in terms of discharge-charge voltage curves varied by current density with a capacity of 5000 mAh g-1 (Figure 4(d)). Up to 2500 mAh g-1, all of the cells showed almost identical voltage gaps (~1.0 V). Before delivering 2500 mAh g-1, interestingly, the OER overpotential of the cell operated at 1000 mA g-1 was the lowest. However, the charge overpotential at 1000 mA g-1 increased rapidly compared with that at other current densities 12 ACS Paragon Plus Environment

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after 2500 mAh g-1; nevertheless, its terminal charge voltage was below 4.1 V at a capacity of 5000 mAh g-1. To study potential characteristics, galvanostatic discharge-charge profiles of HFe2O3/CNT NFs were evaluated upon cycling (Figure 5). With a capacity limitation of 1000 mAh g-1 at 500 mA g-1, voltage curves of ORR and OER were remarkably reversible (Figure 5(a)). Discharge and charge terminal voltages are shown in Figure 5(b), which exhibit that all of terminal voltages were higher than 2.75 V (close value to theoretical voltage - ORR) and lower than 4.0 V (OER) for 250 cycles. The O2-breathing cell with H-Fe2O3/CNT NFs running at 500 mA g-1 stably operated for over 900 h (=37.5 days). Meanwhile, for reference KB electrode, discharge-charge profiles showed that ORR and OER overpotentials increased very quickly; all of charges were completed at voltage above 4.0 V and the cell with KB electrode was degradaded for less than 80 cycles (Figure S9). These results imply that HFe2O3/CNT NFs have much enhanced ORR and OER catalytic activities than KB due to rationally combined CNTs bridged Fe2O3 catalysts; Fe2O3 could be in favorable of adsorption of LiO2 and Li2O2 and CNTs could facilitate charge transfer (Figure S10). A comparison of performance of H-Fe2O3/CNT NFs with state-of-art Fe-based carbon composite cathodes is shown in Table S1. Interestingly, in spite of a high capacity restriction at 2000 mAh g-1, which means much severe condition for formation and decomposition of Li2O2, the battery cell with H-Fe2O3/CNT NFs showed very low overpontentials with voltage gap (1.2 V) between plateaus for first ORR and OER (Figure 5(d)). Although first reactions were favorable for cell operation, discharge and charge potentials declined and rose slowly, respectively; however, it was clearly observed that those voltage curves showed no visible changes after 20 cycles. Discharge and charge terminal voltages appeared at approximately 2.7 V and 4.0 V for 100 cycles, respectively; cell operating time was 800 h, which is comparable to 100 cycles (Figure 5(e,f)). 13 ACS Paragon Plus Environment


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To check morphology and phases of discharge products, we conducted ex-situ SEM and XPS analyses (Figure 6). As shown in Figure 6(a), disc-like Li2O2 is manifested in HFe2O3/CNT NFs cathode, which appears as densely filled 1D NFs with solid Li2O2; this feature was supported by TEM images (Figure S11), exhibiting Li2O2-infilled H-Fe2O3/CNT NFs. Although a small amount of Li2O2 (indicated by dotted arrow) remained undecomposed after recharge, almost all of Li2O2 were removed (Figure 6(b)). Reversible formation and decomposition of Li2O2 occurring on H-Fe2O3/CNT NFs were further confirmed by XRD patterns (Figure 6(c)). The peaks at about 33o, 35o and 58.5o appeared after first full discharge, which are obviously attributed to generation of Li2O2. Moreover, Fe2O3 phases were also detected, meaning that hollow Fe2O3 NPs maintained their phase upon discharging. After first recharge, there are no peaks for Li2O2, indicating that H-Fe2O3/CNT NFs can afford excellent Li2O2 storage. After 50th recharge, small amount of disc-like Li2O2 residue was irreversbly present (Figure 6(d)). However, longitudinal pores of H-Fe2O3/CNT NFs were still present, which means that H-Fe2O3/CNT NFs can persistingly provide catalytic sites for Li2O2. To further investigate chemical states of specific discharge products, i.e., parasitic side products such as Li2CO3 and LiRCO3, we carried out XPS analysis with H-Fe2O3/CNT NFs (Figure 6(e) and 6(f)).31 For C 1s regions that are observed after the first discharge, peaks at 287.9 and 288.9 eV are associated with carboxylate (O-C=O) and Li2CO3; however, intensities of such deconvoluted peaks are low. Accordingly, discharge products including Li2O2, Li2CO3, and LiF were also identified in XPS Li 1s. After recharge, all discharge peaks disappeared for Li 1s, whereas the peak related to Li2CO3 was still detected for C 1s, implying that electrolyte decomposition might happen in intial cycle. Considering XPS data for KB electrode, parasitic side reactions originating from interfacial reaction between carbon and Li2O2 were suppressed for H-Fe2O3/CNT NFs compared with KB electrode (Figure S12 for comparison);32 this may be due to low susceptibility of graphitic CNTs toward O2- attack, 14 ACS Paragon Plus Environment

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compared with KB carbon.33 Stability of metallic Fe phase in H-Fe2O3/CNT NFs was confirmed by ex-situ XPS data after 1st cycle and 50th cycle (Figure S13). Comparing with Fe 2p XPS data in bare H-Fe2O3/CNT NFs (Figure 3(c)), metallic Fe peak disappeared after 1st cycle and 50th cycle, indicating that metallic Fe is not stable and easily converted to metal oxide species. Meanwhile, Fe oxidation peaks of Fe2O3 were not changed, implying high stability of Fe2O3 in Li-O2 cell. We schematically illustrated the most plausible reaction mechanism in HFe2O3/CNTs NFs as shown in Figure 7(a,b). In discharging step, both hollow Fe2O3 NPs with abundant pores and conductive CNT nanobridges for facile pathways for electron can improve the ability to form disc-like and smooth Li2O2 in complementary manner. For decomposition of Li2O2, through synergistic effect of both catalytic Fe2O3 and conductive CNTs branches, suitable release of Li+, easy O2 diffusion, and fast electron transport are accelerated; Li+ and O2 released from suface of disc-like Li2O2 can be promoted on Fe2O3, whereas electron movement can be accelerated along conductive CNTs interconnections.29,34 Based on good electrochemical performance of H-Fe2O3/CNT NFs as air cathode, we fabricated large-area pouch-type Li-air cells (Figure S14) using H-Fe2O3/CNT NFs-coated flexible carbon cloth (Figure S15) to confirm potential application in practical electronic devices (See experimental details in Experimental Section). As shown in Figure 7(c), the pouch-type Li-air battery cell was stacked according to the following sequence: gel polymer electrolyte (electrochemically stable LiClO4/PMMA) was located between Li anode and air cathode, all of which were encapsulated by holeless Al pouch and O2-breathing Al pouch for anode and cathode, respectively.35 Long-term behaviors of pouch-cell with H-Fe2O3/CNT NFs were confirmed by time (h) vs. voltage graph as exhibited in Figure 7(d). In initial cycles, some activation happened; then, the cell was stably operated for 250 h (=125 cycles) with a fixed capacity of 1000 mAh g-1 at a current density of 500 mA g-1. The corresponding voltage 15 ACS Paragon Plus Environment


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profiles were shown in Figure 7(e). As can be seen, ORR and OER potentials were highly reversible for 100 cycles; potential gap between ORR and OER at 500 mAh g-1 is barely 1.46 V. Two pouch cells with H-Fe2O3/CNT NFs were paired to make series connection, which was used to operate light emitting diode (LED) ramp with operating voltage of 5.0 V (Figure 7(f)). Open circut voltage (OCV) of the cell series was 5.55 V, which was sufficient to turn on the light of the LED ramp as shown in the digital image (on/off behaviors of LED (KAIST logo) powered by the pouch cells were supported by video in supporting data); this result demonstrates H-Fe2O3/CNT NFs based air electrode’s potential feasibility as a stable power source for electronic devices.

4. CONCLUSION In summary, we have successfully hybridized hollow Fe2O3 NPs and CNTs in porous 1D architecture by Kirkendall effect and in-situ growth of CNTs with autonomous provision of carbon sources. Conductive CNT branches effectively interwinded the hollow Fe2O3 NP catalysts along 1D longitudinal direction, which greatly redeems intrinsically inferior electronic conductivity of Fe2O3. This hierachically structured nanocomposite with large surface area offers high electronic conductivity and numerous active sites, improving efficiency for formation and decomposition of Li2O2; mixture of disc-like and smooth particle-like Li2O2 was stably formed on H-Fe2O3/CNT NFs with sufficient room to house the discharge products, which were favorable to be decomposed. The H-Fe2O3/CNT NFs delivered exceptionally long-cycle life with low ORR and OER overpotentials in Li-O2 battery cells. Feasibility of novel electrode was confirmed by powering LED (5.0 V) with pouch-type LOBs.


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The rational nanoengineering, i.e., immiscible polymer electrospinning followed by heating control for synthesizing the catalyst/CNTs hybrid materials, can further deliver the new-class of catalytic materials.

SUPPORTING INFORMATION Supporting Information Available: SEM and TEM images, size distribution of hollow Fe2O3 nanoparticles, BET and BJH data, TGA, Raman, XPS data, characteristics of Li2O2, electrochemical performance of KB electrode, Ex-situ data and comparison for Li-O2 cell performance for Fe-based carbon composite catalysts. CORRESPONDING AUTHOR *E-mail: [email protected] ORCID Il-Doo Kim: 0000-0002-9970-2218 Ji-Won Jung: 0000-0001-7044-4764 Ji-Soo Jang: 0000-0001-6018-7231 Tae Gwang Yun: 0000-0003-2756-9013 Ki Ro Yoon: 0000-0003-3532-7911 NOTES The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by a Korea CCS R&D Center (KCRC) grant funded by the Korean government (Ministry of Science, ICT & Future Planning) (No. NRF-2014M1A8A1049303) 17 ACS Paragon Plus Environment


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and by the Wearable Platform Materials Technology Center (WMC), funded by a National Research Foundation of Korea (NRF) Grant of the Korean Government (MSIP) (No. 2016R1A5A1009926). This work was also supported by Global Ph.D Fellowship Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. NRF-2016H1A2A1907718).

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Figure 1. Schematic illustration of synthesis of porous H-Fe2O3/CNT NFs via simple electrospinning and two-step heat treatments for in-situ growth of CNT and hollow Fe2O3 NPs.



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Figure 2. (a,b) SEM and (c) TEM images of Fe3C/CNT-CNFs. (d,e) SEM and (f) TEM images of H-Fe2O3/CNT NFs. (g) STEM-EDS elemental mapping images of H-Fe2O3/CNT NFs for Fe (yellow), O (red), and C (green).


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(a)

(b)

Fe3C/CNT-CNFs

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Intensity (a. u.)

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Figure 3. Phase characterizations. (a) XRD data of Fe3C/CNT-CNFs and H-Fe2O3/CNT NFs. (b) Raman spectrum of H-Fe2O3/CNT NFs. (c) XPS data of H-Fe2O3/CNT NF surfaces for O 1s, Fe 2p, and C 1s.



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Figure 4. Cyclic voltammograms of (a) H-Fe2O3/CNT NFs and (b) KB. (c) Galvanostatic discharge-charge profiles of H-Fe2O3/CNT NFs and KB on first cycle with a restricted capacity of 5000 mAh g-1 running at a current density of 500 mA g-1. (d) Comparison of overpotentials of H-Fe2O3/CNT NFs with a capacity limit of 5000 mAh g-1 running at different rates (250, 500 and 1000 mA g-1). All data were evaluated under same conditions such as 1.0 M LiTFSI in TEGDME electrolyte, oxygen window, and separator.


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Figure 5. Electrochemical performance. (a) Voltage profiles of H-Fe2O3/CNT NFs with a specific capacity of 1000 mAh g-1 at a current density of 500 mA g-1. (b) Galvanostatic discharge-charge terminal voltages and (c) curves (times vs. voltage) of H-Fe2O3/CNT NFs, corresponding to (a). (d) Voltage profiles of H-Fe2O3/CNT NFs with a specific capacity of 2000 mAh g-1 at a current density of 500 mA g-1. (e) Galvanostatic discharge-charge terminal voltages and (f) curves (times vs. voltage) of H-Fe2O3/CNT NFs, corresponding to (d).



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Figure 6. Ex-situ data. SEM images of H-Fe2O3/CNT NFs after (a) first discharge and (b) recharge. (c) Ex-situ XRD data of H-Fe2O3/CNT NFs. (d) SEM images of H-Fe2O3/CNT NFs after 50th cycle. XPS spectra of H-Fe2O3/CNT NFs after (e) first discharge and (f) recharge for C 1s and Li 1s.


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Figure 7. (a) scheme of H-Fe2O3/CNT NFs. (b) Proposed reaction mechanism of HFe2O3/CNT NFs with Li+ and O2 in LOB cell. (c) Stacking sequence of battery components in pouch-type cell with H-Fe2O3/CNT NFs. (d) Cycling performance and (e) voltage profile data of pouch-type cell with H-Fe2O3/CNT NFs. (f) Digital images of series connection of two pouch-type cells with H-Fe2O3/CNT NFs, showing open circuit voltage (OCV) and operation of light emitting diode (LED) (operating voltage: 5.0 V).



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