Bifunctional Hybrid Catalysts with Perovskite LaCo0.8Fe

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Bifunctional Hybrid Catalysts with Perovskite LaCo Fe O Nanowires and Reduced Graphene Oxide Sheets for an Efficient Li-O Battery Cathode 2

Jong Guk Kim, Youngmin Kim, Yuseong Noh, Seonhwa Lee, Yoongon Kim, and Won Bae Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14599 • Publication Date (Web): 18 Jan 2018 Downloaded from http://pubs.acs.org on January 18, 2018

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ACS Applied Materials & Interfaces

Bifunctional Hybrid Catalysts with Perovskite LaCo0.8Fe0.2O3 Nanowires and Reduced Graphene Oxide Sheets for an Efficient Li-O2 Battery Cathode Jong Guk Kim,a† Youngmin Kim,b† Yuseong Noh,c Seonhwa Lee,d Yoongon Kim,a,c and Won Bae Kim*c a

School of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), 261 Cheomdan-gwagiro, Buk-gu, Gwangju 61005, Republic of Korea

b

Carbon Resources Institute, Korea Research Institute of Chemical Technology (KRICT), 141 Gajeongro, Yuseong-gu, Daejeon 34114, Republic of Korea

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Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), 77 Cheongam-Ro, Nam-gu, Pohang, Gyeongbuk 37673, Republic of Korea

d

Department of Physics and Photon Science, Gwangju Institute of Science and Technology (GIST), 261 Cheomdan-gwagiro, Buk-gu, Gwangju 61005, Republic of Korea [†] These authors contributed equally

AUTHOR EMAIL ADDRESS Jong Guk Kim

[email protected]

Youngmin Kim

[email protected]

Yuseong Noh

[email protected]

Seonhwa Lee

[email protected]

Yoongon Kim

[email protected]

Won Bae Kim

[email protected]

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CORRESPONDING AUTHOR FOOTNOTE Prof. Won Bae Kim Tel.: +82-54-279-2397 / Fax: +82-54-279-5528 E-mail: [email protected]

ABSTRACT In this paper, bifunctional catalysts consisting of perovskite LaCo0.8Fe0.2O3 nanowires (LCFO NWs) with reduced graphene oxide (rGO) sheets were prepared for use in lithium-oxygen (Li-O2) battery cathodes. The prepared LCFO@rGO composite was explored as a cathode catalyst for Li-O2 batteries, resulted in an outstanding discharge capacity (ca. 7088.2 mAh g-1) at the first cycle. Moreover, high long stability of O2-cathode with LCFO@rGO catalyst was achieved over 56 cycles under the capacity limit of 500 mAh g-1 with a rate of 200 mA g-1, as compared to ketjen black carbon and LCFO NWs. The enhanced electrochemical performance suggests that these hybrid composites of perovskite LCFO NWs with rGO nanosheets could be a perspective bifunctional catalyst for the cathode oxygen reduction and oxygen evolution reactions in the development of next-generation Li-O2 battery cathodes.

KEYWORDS: Perovskite LaCo0.8Fe0.2O3, nanowires, graphene sheets, catalysts, Li-O2 batteries


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1. INTRODUCTION The increasing demands for rechargeable secondary batteries have triggered extensive research efforts for the development of next-generation energy storage platforms.1-4 Among the potential rechargeable secondary batteries, lithium-oxygen (Li-O2) secondary batteries have recently attracted significant research interest since the theoretical energy density (ca. 3500 Wh kg-1) of Li-O2 batteries is remarkably larger than that (ca. 400 Wh kg-1) of commercial lithium-ion secondary batteries, which makes them promising energy storage platforms for renewable energy storage and electric vehicles.5-7 However, the oxygen reduction reaction (ORR, 2Li+ + 2e- + O2 → Li2O2) and oxygen evolution reaction (OER, Li2O2 → 2Li+ + 2e- + O2) often suffer from sluggish reaction kinetics due to poor catalytic activity and low charge transfer rate. In addition, the discharge products (i.e., Li2O2) could not be recovered completely during subsequent recharge process, which leads to poor cyclability and low round-trip efficiency.8-10 In this response, several researches have been directed toward development of appropriate catalysts to decrease the overpotential, and consequently improve cycle life and round-trip efficiency of the Li-O2 batteries. Recently, various types of bifunctional ORR/OER catalysts, such as noble metals11-13 and metal oxides,14-17 have been widely studied for Li-O2 battery cathodes. In particular, perovskite oxide has been proposed as a promising cathode catalyst because of their high bifunctional catalytic activity, structural stability, and mixed conductivity through their composition variations, and relatively low cost.18-20 Unfortunately, however, the poor electronic conductivities of perovskite oxides, compared with noble metal, could be a major problem to enhance the ORR and OER catalytic kinetics.19-24 Moreover, besides the electrocatalytic activities, the configuration of bifunctional catalysts should be considered since the ORR/OER catalytic activities rely on their morphology and surface area.6,25,26 Recently, to enhance catalytic activity and cycling stability, several strategies were proposed. For instance, one-dimensional (1D) nanostructured catalysts have been of a research interest due to their high electron mobility, short Li+ transport distance, and enhanced active sites.27-31 Moreover, the 1D structured catalysts could preserve their active sites for long-term cyclings, because their properties of ACS Paragon Plus Environment

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high surface-to-volume ratios could enable to increase the Li+ and electron transport rates.32-34 Additionally, for the purpose of amelioration of the charge transport kinetics, various carbonaceous materials, such as carbon nanotube,35-37 porous carbon38,39 and graphene40-42 have been used as a supporting matrix of noble metal or metal oxide catalysts. Among the various carbon supporting materials, graphene has been intensively studied as a catalyst support due to its excellent electronic conductivity (103 – 104 S m-1), high theoretical surface area (ca, 2630 m2 g-1), and high chemical tolerance.43-47 In this sense, the perovskite LaCo0.8Fe0.2O3 nanowires (LCFO NWs) attached on the reduced graphene oxide (rGO) sheets are fabricated for a cathode catalyst in Li-O2 batteries, whose configuration could improve the electrochemical performance for the ORR/OER catalytic reactions due to the features that (i) the properties like large surface-to-volume ratio and line defects in the perovskite LCFO NWs could improve catalytic active sites,48-50 (ii) the hybridization of LCFO NWs with rGO sheets could provide efficient electron pathways, which would decrease charge/discharge polarization,6 and (iii) the 3D structure of LCFO@rGO composites can provide facile O2 diffusion path in the O2cathode during repeated cyclings.22,26 In this research, we report LCFO@rGO composites as an efficient bifunctional catalyst for the LiO2 battery cathodes. For the preparation of LCFO@rGO composites, the LCFO NWs were simply hybridized with rGO sheets. As we know, this is the first report that LCFO@rGO composites have been employed to cathode catalyst of Li-O2 batteries. The as-prepared O2-electrode with LCFO@rGO catalyst could deliver a high discharge capacity of ca. 7088.2 mAh g-1 at the initial cycle. Furthermore, the Li-O2 cell with LCFO@rGO catalyst has a high cyclablity for more 56 cyclings under a capacity limit (500 mAh g-1) together with a low overpotential of 0.98 V, in comparison to that with ketjen black (KB) and LCFO NWs. From the view of their unique structural features and high Li+ storage performances, the LCFO@rGO composites could be a promising catalyst candidate for the future Li-O2 battery cathodes.


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2. EXPERIMENTAL SECTION 2.1. Synthesis of LCFO NWs For the synthesis of the LCFO NWs, 2.0 mmol of La(NO3)3·6H2O (≥99.0 %, Aldrich), 1.6 mmol of Co(NO3)2·6H2O (≥98.0 %, Aldrich), and 0.4 mmol of Fe(NO3)3·9H2O (≥98.0 %, Aldrich) were dissolved in 40 mL of deionized water. Then, 0.07 mol of KOH (≥90.0 %, Aldrich) was putted into the above solution. After 2 h of ultrasonic stirring, the solution was sealed in a Teflon-lined autoclave and maintained at 230 oC for 48 h. The as-prepared NWs were washed thoroughly with absolute ethanol and distilled water, and followed by vacuum freeze-drying of the final product. The single crystalline LCFO NWs were obtained through subsequent heat-treatments at 400 oC for 2h and 850 oC for 2 h at a heating rate of 1 oC min-1 in an air atmosphere. Fe element was doped in the LaCoO3 NWs to improve their structural stability for the heat-treatment, as shown in Figure S1 (Supporting Information).51,52 2.2. Preparation of hybridized LCFO@rGO composites Through the modified Hummers method, graphite oxide (GO) powder was preferentially prepared from natural graphite particles.44,45 To form an exfoliated GO solution (2 mg mL-1), as-prepared GO powder was dispersed in distilled water and sonicated for 30 min. Then, l mL of 5.0 M NaOH (99.99%, Aldrich) aqueous solution and 5 mL of 0.57 M l-ascorbic acid (≥99.0 %, Aldrich) aqueous solution were added into the 20 mL of GO solution, and subsequently the mixed solution was heat-treated at 90 oC for 1 h under vigorous stirring. After natural cooling to room temperature, the mixture was purified and separated by centrifugation, and followed by freeze drying of rGO sheets. For the preparation of the hybridized LCFO@rGO, 25 mg of LCFO NWs and 5 mg of rGO sheets were separately dispersed into 10 mL of absolute acetone solvent. After 1 h of sonication of rGO solution, it was added to the LCFO NWs dispersed solution under vigorous stirring. After magnetic stirring for 12 h, the resulting material was dried through the vacuum freeze-drying method.


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2.3. Materials characterizations Scanning electron microscopy (SEM) images were taken with a JEOL JSM-7500F instrument at 5 kV. Transmission electron microscopy (TEM) images, high-resolution TEM (HRTEM) images, and energy-dispersive X-ray spectrometry (EDX) were obtained with Tecnai F30 G2 S-Twin instrument at 300 kV. Powder X-ray diffraction (XRD) patterns were recorded with Rigaku Rotalflex RU-200B diffractometer with a Ni filter by using a Cu-Kα source. Raman spectra were obtained with Renishaw Invia instrument. X-ray photoelectron spectroscopy (XPS) measurment was conducted with MultiLab 2000 by using an Al-Kα source. The Brunauer-Emmett-Teller (BET) method was used to characterize the surface areas by using N2 adsorption-desorption with a Belsorp mini II analyzer at 77 K. 2.4. Electrochemical measurements To evaluate the ORR/OER activities of the as-prepared catalysts, linear sweep voltammetry (LSV) curves were preferentially obtained in aqueous KOH solutions by using a three electrode system, which consists of a glassy carbon (GC, diameter of 5 mm) rotating disk electrode (RDE), a platinum wire counter electrode, and a KOH saturated Hg/HgO (MMO) reference electrode. For the preparation of working electrode, 6 mg of LCFO NWs or LCFO@rGO and KB (weight ratio = 1:1) was dispersed ultrasonically in a mixed solution of absolute ethanol (1 mL) and distilled water (1 mL). Then, the 10 µL of the above catalyst ink was coated onto a GC electrode. After drying the GC electrode, Nafion solution (0.01 mL, 5 wt%) was further pipetted onto the catalyst layer. The ORR and OER measurements were performed in aqueous 0.1 M KOH solutions saturated with O2 and N2, respectively. The LSV was recorded in the voltage from -0.6 to 0.1 V (versus MMO) for the ORR and from 0.3 to 1.1 V (versus MMO) for the OER. The rotating rate of RDE was 1600 rpm and a scan rate was 10 mV s-1. To evaluate the lithium storage properties of Li-O2 batteries, the O2-electrodes were prepared by painting a homogeneous slurry containing as-fabricated catalysts (40 wt%), KB (EC 600JD) (40 wt%), and polyvinylidene difluoride (PVDF) (20 wt%) onto a Toray carbon paper. The loading density (1.0 ± 0.1 mg cm-2) of the as-prepared O2-cathode was determined by weighing the electrode before and after the slurry deposition using a microbalance (Sartorius, M3P). For comparison, the O2-electrode without a 6 ACS Paragon Plus Environment

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catalyst was also fabricated with weight ratio of KB:PVDF of 80:20. After drying the electrode at 120 °C in a vacuum oven overnight, coin-type cell (CR2032) was assembled in an argon-filled glove box, using a lithium foil anode, glass fiber filter (Whatman) separator, O2-electrode, and an electrolyte consisted of 1 M lithium bis(trifluoromethane sulfonimide) (LiTFSI) in a tetra(ethylene) glycol dimethyl ether (TEGDME). For the efficient O2 transport through coin cell, the cathode side was drilled to make 33 holes (diameter of 1 mm). The assembled cell was placed in chamber filled with an ultra-pure oxygen (99.999 %) at a pressure slightly higher than 1 atm. After aging of assembled cell for 18 h, galvanostatic discharge/charge performance of the assembled Li-O2 batteries was obtained in the voltage range of 2.34.5 V with a current density of 200 mA g-1 on a WBCS 3000 battery cycler system (WonA Tech) under O2 atmosphere. The input current rates and obtained specific capacity were calculated based on the KB weight for the specific charge/discharge capacity comparisons.


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3. RESULTS AND DISCUSSION Scheme 1 illustrates the preparation steps of the LCFO@rGO catalyst system. In the first step, the LCFO NWs were prepared from the hydrothermal method and they were calcined at 400 oC for 2 h and 850 oC for 2 h in air atmosphere. In the second step, the rGO nanosheets were prepared from the GO by using an l-ascorbic acid induced reduction reaction. In the third step, the LCFO@rGO hybrid catalysts were obtained from mixing of the LCFO NWs with rGO nanosheets in an acetone solvent. The conceptual Li+ discharge/charge process (i.e., ORR/OER process) over the LCFO@rGO catalyst system is illustrated in Scheme 1d. Because the 3D hybrid structure of LCFO@rGO composites consisting of the 1D LCFO NWs and 2D rGO nanosheets can effectively facilitate Li+ diffusion and electron transfer and improve electrolyte immersion, both Li+ and O2 could migrate easily through the void space of the LCFO@rGO catalysts and then Li+ could react with O2 for the formation of a discharge product (i.e., Li2O2).6,26 Moreover, the formed Li2O2 can be readily decomposed into Li+ and O2 during recharge process. These series reactions through the LCFO@rGO catalysts can lead to high reversibility of Li-O2 batteries in this work. As shown in Figure 1, the morphology and size of the as-obtained precursor NWs, LCFO NWs, rGO nanosheets and LCFO@rGO catalysts were investigated with SEM measurements. In Figure 1a, the precursor NWs show 1D geometry with a diameter of ca. 26.0 nm and length of several microns, indicating that uniform 1D precursor NWs were formed under the hydrothermal synthetic route. The LCFO NWs were fabricated by the continuous calcination at 400 oC and 850 oC under air atmosphere (Figure 1b). The as-prepared LCFO NWs maintained their original 1D structure with some aggregated regions from the precursor NWs and revealed a diameter of ca. 52.7 nm and a length of several micrometers. It is noted that the diameter of the LCFO NWs seems to be increased during the calcination process,53 as compared with that of the precursor NWs. In Figure S2 (Supporting Information), for the experiment ranges of calcination temperature, relatively well preserved 1D LCFO NWs without forming secondary phases were obtained at a calcination temperature of 850 oC in this study. As observed in Figure 1c, the rGO has a sheet-like structure with wrinkles, which is in accordance ACS Paragon Plus Environment

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with previous studies for the completely exfoliated rGO nanosheets from the graphite powders.6,43-45 As depicted in Figure 1d, the LCFO@rGO composites were successfully fabricated via the simple dispersion of the LCFO NWs onto the rGO sheets. The as-obtained LCFO@rGO composites could have their inherent configurational merits as a bifunctional catalyst for the O2-electrode: (i) The rGO nanosheets could ensure high electrical conductivity between the LCFO NW networks for efficient ORR/OER activities.22,23 (ii) 3D hybrid structure of LCFO@rGO obtained from mixing of 1D LCFO NWs with 2D rGO nanosheets could provide enhanced catalytic sites, and supply sufficient surface areas for the storage of discharge product (see Figure S3, Supporting Information).6,26 The X-ray diffraction patterns of LCFO@rGO composites, LCFO NWs and rGO nanosheets are represented in Figure 2a. The XRD patterns of LCFO@rGO composites exhibit a single crystalline phase of rhombohedral perovskite oxide of LaCo0.8Fe0.2O3 with a space group R-3c, which is consistent with the previous reports.54,55 This is resulted from the alignment distortion of the octahedral coordination by the gradual substitution of Co ions by Fe ions.54,55 The diffraction peaks at 2θ = 23.1, 32.8, 40.5, 47.4, 58.7, and 68.8o are confirmed as those of the (012), (110), (202), (024), (214), and (220) planes, respectively, implying that the crystal structure of LaCo0.8Fe0.2O3 was not changed after the composite fabrication with rGO nanosheets. It is worth noting that a substantial re-stacking of exfoliated rGO nanosheets was prevented after the hybridization with LCFO NWs because the re-stacking related peak at 2θ = 26o was not observed.6,45 Furthermore, no impurities were detected in the XRD patterns, indicating high crystallinity and purity of the final products. For further investigating the quality of the rGO sheets and LCFO@rGO composites, Raman spectroscopic analysis was performed. As shown in Figure 2b, the peaks of LCFO@rGO composites involve the characteristic peaks from both LCFO NWs and rGO nanosheets. From the spectra of LCFO@rGO composites, the distinctive peaks were observed at 1351.1 and 1585.7 cm-1, which could be assigned to D- and G-band, respectively.43,45 The D-band is associated with a disorder degree of carbonaceous materials, and the G-band is related with a graphitization degree in the carbon.42 The D/G ratio of ca. 1.0 for the LCFO@rGO composites is much larger than that of GO (ca. 0.89), implying that rGO was successfully reduced from GO powders,43-45 ACS Paragon Plus Environment

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which is in coincide with XRD patterns. The highly ordered carbon structure could ensure a high electic conductivity, and thus facilitate electron transport during cathode reactions, which is beneficial to the enhancement of its electrochemical performance.46 Moreover, the inset of Figure 2b confirms the presence of perovskite LaCo0.8Fe0.2O3 in the wave number range from 1000 to 200 cm-1. The main band (697.1 cm-1) is assigned to the A2g mode derived from the R-3c space group of rhombohedral perovskite.54 It would arise from a higher local distortion of octahedra due to the substitution of Co by Fe cations. These peak positions are in line with those of perovskite LaCo0.8Fe0.2O3 materials reported previously,54,55 which means that a good quality of perovskite LaCo0.8Fe0.2O3 was formed in this study. The morphology of the LCFO NWs and LCFO@rGO composites was further investigated with TEM measurements. Figure 3a and 3b show representative TEM and HRTEM images of individual LCFO NWs, indicating the high crystallinity of the LCFO NWs. In Figure 3b, interplanar distance of 0.27 nm was observed from the HRTEM image, which matches well to (110) planes of the cubic perovskite type LaCo0.8Fe0.2O3. The direction of crystal growth was indicated along [110] in Figure 3b. This growth direction of LCFO NWs is also manifested in the high diffraction intensity of the (110) peaks in Figure 2a. The fast Fourier transform (FFT) pattern (inset of Figure 3b) shows bright spots, indicating well-grown single crystallites of LCFO phase.40 As highlighted by the circle in Figure 3b, there are some line defects, which can facilitate binding intermediate O22- to the catalyst surface during cathode reactions.3,48 Figure 3c represents the TEM image of the LCFO@rGO, in which the LCFO NWs are attached randomly onto the thin rGO sheets. The TEM image (Figure 3d) and the corresponding FFT pattern (inset of Figure 3d) of LCFO@rGO present graphene layers and LCFO crystallites with a (202) plane reflection. Because the LCFO NWs networks are formed on the rGO sheets, the LCFO@rGO composite could show enhanced electrical conductivity, which enables the effective electron transport through the 1D LCFO NWs and 2D rGO sheets during repeated discharge/charge cycles.6,26 The EDX elemental mapping based on a single LCFO NW further confirmed presence of the elements of La, Co, Fe, and O (Figure 3e). In addition, EDX spectra also represent the coexistence of La, Co, Fe, and O in the LCFO@rGO catalyst as shown in Figure S4 ACS Paragon Plus Environment

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(Supporting Information). From the material characterizations, it is evident that the LCFO@rGO hybridized catalyst is successfully prepared in this study. The chemical state and element composition in the LCFO@rGO composites were examined through the XPS analysis. Figure 4a shows the typical XPS survey scan of C (1s), La (3d), Co (2p), Fe (2p), and O (1s) in the LCFO@rGO composites. In Figure 4b, the C 1s spectrum showed resolved three peaks, in which the distinct peak at 284.5 eV could be related C-C bond. The deconvoluted two minor peaks at 287.9 and 285.6 eV can correspond to the C=O and C-O bond, respectively.56 As shown in Figure 4c , the La 3d spectrum displayed the two resolved peaks of La 3d5/2 at binding energy of 834.0 and 838.3 eV, and La 3d3/2 at 851.2 and 855.2 eV respectively, which is originated from the La3+ in oxide form.55,57 The binding energy difference between two major peaks is ca. 16.8 eV, whose value is in consistent with the previous literatures.57 In Figure 4d, the Co 2p emission spectrum showed two apparent photoelectron peaks at 780.2 and 795.3 eV, which can be ascribed to Co 2p3/2 and Co 2p1/2, respectively. Two main signals could be resolved into six sub-peak components: the three peaks at binding energies of 782.0, 783.2 and 797.1 eV are attributed to Co2+, while the other three peaks at 780.1, 788.8 and 795.1 eV are assigned to Co3+. The binding energy is separated by ca. 15.1 eV between the Co 2p3/2 and Co 2p1/2 peaks, which is also in line with that of LaCo0.8Fe0.2O3 materials.54,55 In the Fe 2p spectrum (Figure 4e), the peak positions at 711.6 eV, 725.0 eV are assigned to Fe 2p3/2 and Fe 2p1/2, respectively. Two major peaks could be deconvoluted into four sub-peaks: the two peaks at 710.5 and 717.1 eV are assigned to Fe2+, while the other two peaks at 712.2 and 725.0 eV are attributed to Fe3+. The binding energy separation is ca. 13.4 eV between two major peaks, which is also in consistent with the previous reports.54,55 In Figure 4f, the O 1s spectrum shows three resolved photoelectron peaks, in which the peaks at 529.1, 531.4, and 533.6 eV can be assigned to perovskite lattice oxygen, hydroxide anions, and surface-adsorbed carbonate oxygen, respectively.6,56 Note that the relative peak areas and intensity from hydroxide ions are obviously larger than that from perovskite lattice, which is beneficial to improvement of the catalyst activities for the ORR/OER.48 Furthermore, after the hybridization of LCFO NWs with rGO nanosheets, binding energy shift in the La 3d, Co 2p and Fe 2p spectrum was ACS Paragon Plus Environment

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observed as shown in Figure S5 (Supporting Information). This result means that chemical states of LCFO NWs were changed throughout the hybridization with rGO sheets, and further indicates that electron transfer between LCFO NWs and rGO sheets could be beneficial for decreasing charge/discharge overpotentials during cyclings.58-60 Based on the bifunctional catalytic activities of the LCFO@rGO composites (see Figure S6, Supporting Information) in aqueous KOH solutions, a prototype Li-O2 cell was fabricated and the charge/discharge behaviors of the LCFO@rGO composites in the O2-electrode were examined with 1 M LiTFSI in TEGDME as an electrolyte. Figure S7 (Supporting Information) shows an illustration of a prototype Li-O2 battery and a photograph of coin cell components used in this study, and represents that the assembled Li-O2 cell is well working with light emitting diodes. Figure 5a shows the initial discharge/charge voltage profiles of LCFO@rGO electrode from 2.3 to 4.5 V with a current rate of 200 mA g-1. For comparison, potential-capacity profiles of Li-O2 batteries with the KB carbon and LCFO NWs are also given under the same test conditions.6,22,42,61-63 The Li-O2 cell with LCFO@rGO catalyst delivered a discharge capacity of ca. 7088.2 mAh g-1, whose value is much larger than that with LCFO NWs (ca. 5497.1 mAh g-1) and KB carbon (ca. 5234.4 mAh g-1). Figure 5b indicates the discharge capacity-cycle number plots of Li-O2 batteries with LCFO@rGO composites, LCFO NWs, and KB carbon for 5 cycles over a voltage window of 2.3 to 4.5 V at a rate of 200 mA g-1. As can be seen, the O2-electrode with LCFO@rGO catalyst shows high discharge capacity with improved cycle durability, as compared with with KB carbon and LCFO NWs. This improved capacity retention could originate from the unique structural features of LCFO@rGO electrode, which can increase the reversibility of Li2O2 formation/decomposition and thus could improve the cycling durability. Moreover, the coulombic efficiency of the LCFO@rGO electrode, corresponding to the ratio between charge capacity and discharge capacity,26 shows ca. 92.3 % at the initial cycle, whose value is significantly larger than KB electrode (ca. 22.2 %), which would again demonstrate high reversibility of O2-electrode with LCFO@rGO catalyst.


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The cyclability of the LCFO@rGO electrode was further investigated with a restricted charge/discharge capacity of 500 mAh g-1. The restricting capacity method has been used to achieve good reversibility since the massive Li2O2 covered in the O2-electrodes during repeated cyclings could deactivate the electrocatalytic sites.6,64 Figure 5c presents the first discharge/charge profiles for the Li-O2 batteries with LCFO@rGO catalysts, LCFO NWs, and KB with a current rate of 200 mA g-1 under the restricted capacity of 500 mAh g-1. At the middle region of discharge/discharge curve, the Li-O2 cell showed a decreased overpotential (0.98 V) with the LCFO@rGO catalyst, whereas the Li-O2 cells showed an increased overpotential with LCFO NWs (1.21 V) and pure KB (1.68 V), suggesting that the LCFO@rGO catalyst was able to exhibit a high round-trip efficiency of 73.3 %, as compared with the pure KB (60.6 %) and LCFO NWs (68.6 %) at the initial cycle (see Figure S8, Supporting Information). These results again indicate that the LCFO@rGO catalyst has high catalytic activity toward the ORR/OER processes. While the LCFO@rGO catalyst was applied to the Li-O2 battery cathodes, the charge/discharge capacity with 500 mAh g-1 was stably retained over the 56 cycles, as compared with the O2-electrodes with the KB (32 cycles) and LCFO NWs (44 cycles) as shown in Figure 5d. From the synergy effect of the good ORR/OER electrocatalytic activities of LCFO NWs and high electronic conductivity of rGO sheets,46 the LCFO@rGO electrode could have low discharge/charge overpotential and improved round-trip efficiency (see Figure S9, Supporting Information). To account for the reversibility of Li-O2 batteries with the catalyst during subsequent cyclings, potential-time curves are also given in Figure S10 (Supporting information), in which the discharge potential of LCFO@rGO electrode was stable even until 400 h, indicating better cycling performance of O2-electrode with the LCFO@rGO catalysts, as compared with that with KB carbon and LCFO NWs. As shown in Figure 6, to examine the influence of the LCFO@rGO catalyst on the Li+ storage kinetics, the discharge potential curves of the LCFO@rGO and KB carbon electrode were obtained at the various discharge current rates. At the different current rates of 400, 600 and 800 mA g-1, the LCFO@rGO electrode delivered discharge capacities of 5744.6, 4336.7, and 4258.0 mAh g-1, respectively, while the KB electrode exhibited decreased discharge capacities of 4330.9, 3712.9, and ACS Paragon Plus Environment

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2781.4 mAh g-1, respectively. The discharge capacity and potential plateaus of LCFO@rGO electrode appeared to decrease as the current densities increased, however, the LCFO@rGO electrode showed distinctly higher discharge capacity and potential plateaus than the KB electrode at different current rates. As presented in Figure 6b, the capacity retention of the LCFO@rGO electrode is still 74.1 % at rate of 800 mA g-1, whose value is much larger than that of KB electrode (64.2 %). The better rate performance and higher specific capacity of the LCFO@rGO electrode can be attributed to its high ORR/OER electrocatalytic activities. To investigate the discharge/charge behaviors of KB and LCFO@rGO electrode, SEM images of each electrode before discharge, after discharge and after recharge were obtained as shown in Figure 7. Before discharge process, the KB electrode is made up of pure KB nanospheres (Figure 7a), whereas the LCFO@rGO electrode is composed of the LCFO@rGO catalysts together with KB nanospheres (Figure 7b), leading to an porous framework which could offer sufficient pathways for the O2 molecules and Li+ during repeated discharge/charge cycles.6,65 After the discharge process, the surface of the KB electrode is covered with Li2O2 (Figure 7c), while inter-voids of the LCFO@rGO electrode are well maintained (Figure 7d), Note that the inter-voids of the LCFO@rGO electrode enable to act as reservoirs of discharge products (i.e., Li2O2), which can enhance the Li2O2 decomposition for recovering its original porosity during recharge process for the improvement of rechargeability.66,67 After recharge process, most of the Li2O2 aggregates are still shown in the KB electrode (Figure 7e). Therefore, the residual Li2O2 on the KB electrode surface could further block the catalytic sites and O2 transport channels during recharge process,66 which corresponds to the XRD patterns (see Figure S11, Supporting Information). On the other hand, the Li2O2 aggregates were not observed after the recharge process for the LCFO@rGO electrode (Figure 7f), which results in the outstanding cycling performance of Li-O2 batteries, although the original morphology was not completely recovered. This might be derived from the deposition of irreversibly decomposed electrolyte or not fully decomposed Li2O2 product.6 Consequently, as-prepared Li-O2 batteries with the LCFO@rGO hybridized bifunctional catalysts in this work exhibited enhanced electrochemical performances, such as Li+ storage capacity, overpotential, ACS Paragon Plus Environment

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round-trip efficiency, cycling stability, as compared to other rGO composites and perovskite metal oxide catalysts (see Table S1, Supporting Information). These outstanding electrochemical properties with the bifunctional LCFO@rGO hybrid composites could be ascribed to their following favorable features: (i) Large aspect ratio and line defects involved in the 1D LCFO NWs could increase catalytic active sites for larger Li+ storage.48,68 (ii) 1D structure of LCFO NWs and rGO nanosheet itself could decrease the charge transfer resistance of O2-cathode for the smaller overpotential.6 (iii) Synergistic effect of the LCFO@rGO composites integrated with the 1D LCFO NWs and 2D rGO nanosheets can provide sufficient pathways for the O2 molecules through the O2-cathode.22,26


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4. CONCLUSIONS The LCFO@rGO composite catalysts were prepared via a hybridization of LCFO NWs with rGO nanosheets, in which the single crystalline perovskite LCFO NWs were distributed throughout the rGO sheet surfaces. The LCFO@rGO hybrid composites were explored as bifunctional cathode catalysts for the Li-O2 batteries which exhibited an excellent discharge capacity (ca. 7088.2 mAh g-1) at the initial cycle. Moreover, the Li-O2 cell with the LCFO@rGO could have a high reversibility over 56 cycles under a restricted charge/discharge capacity (500 mAh g-1) with a low overpotential (0.98 V) and large round-trip efficiency, as compared to that with LCFO NWs and KB carbon. These improved performances of Li-O2 batteries with the LCFO@rGO catalyst could be ascribed to the synergy effects of inherent catalytic activity of perovskite 1D LCFO NWs and high electrical conductivity of 2D rGO nanosheets. Therefore, the Li-O2 batteries with the bifunctional LCFO@rGO cathode catalyst, with the improved properties like high energy/power density and high cycle stability, could be a prospective nextgeneration secondary battery.

ACKNOWLEDGMENTS This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (NRF-2017R1A2B2012318 and NRF-2017R1A4A1015811)

Supporting Information: The Supporting Information is available free of charge on the ACS Publications website at DOI: http://pubs.acs.org. XRD patterns and SEM images of the samples at different heating temperatures, N2 adsorption/desorption isotherms, XPS spectrum, EDX spectrum and linear sweep voltammetry curves a of the samples, Picture of a prototype Li-O2 battery, electrochemical properties of the Li-O2 battery, XRD patterns of the O2-electrodes after first cycle, and Li-O2 battery performance comparison between our work and some other reports (PDF) ACS Paragon Plus Environment

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REFERENCES (1)

Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J.-M. Li-O2 and Li-S Batteries with High Energy Storage. Nat. Mater. 2012, 11, 19-29.

(2)

Julien, C.; Mauger, A.; Vijh, A.; Zaghib, K. Lithium Batteries; Science and Technology Springer International Publishing Switzerland: New York, 2016.

(3)

Oh, S. H.; Black, R.; Pomerantseva, E.; Lee, J.-H.; Nazar, L. F. Synthesis of a Metallic Mesoporous Pyrochlore as a Catalyst for Lithium-O2 Batteries. Nat. Chem. 2012, 4, 1004-1010.

(4)

Chang, Z.-W.; Xu, J.-J.; Liu, Q.-C.; Li, L.; Zhang, X.-B. Recent Progress on Stability Enhancement for Cathode in Rechargeable Non-Aqueous Lithium-Oxygen Battery. Adv. Energy Mater. 2015, 5, 1500633.

(5)

Abraham, K. M.; Jiang, Z. A Polymer Electrolyte-Based Rechargeable Lithium/Oxygen Battery. J. Electrochem. Soc. 1996, 143, 1-5.

(6)

Kim, J. G.; Kim, Y.; Noh, Y.; Kim, W. B. MnCo2O4 Nanowires Anchored on Reduced Graphene Oxide Sheets as Effective Bifunctional Catalysts for Li-O2 Battery Cathodes. ChemSusChem 2015, 8, 1752-1760.

(7)

Chang, Z.-W.; Xu, J.-J.; Zhang, X.-B. Recent Progress in Electrocatalyst for Li-O2 Batteries. Adv. Energy Mater. 2017, 1700875.

(8)

Yoon, K. R.; Jung, J.-W.; Kim, I.-D. Recent Progress in 1D Air Electrode Nanomaterials for Enhancing the Performance of Nonaqueous Lithium-Oxygen Batteries. ChemNanoMat 2016, 2, 616-634.

(9)

Yilmaz, E.; Yogi, C.; Yamanaka, K.; Ohta, T.; Byon, H. R. Promoting Formation of Noncrystalline Li2O2 in the Li-O2 Battery with RuO2 Nanoparticles. Nano Lett. 2013, 13, 46794684.

(10) Xu, J.-J.; Zhang, X.-B. Li-Air Batteries: Decouple to Stabilize. Nat. Energy 2017, 2, 17133. (11) Lu, M.; Qu, J.; Yao, Q.; Xu, C.; Zhan, Y.; Xie, J.; Lee, J. Y. Exploring Metal Nanoclusters for Lithium-Oxygen Batteries. ACS Appl. Mater. Interfaces 2015, 7, 5488-5496. ACS Paragon Plus Environment

17


Page 18 of 35

(12) Lu, M.; Chen, D.; Xu, C.; Zhan, Y.; Lee, J. Y. Enhancing the Performance of Catalytic AuPt Nanoparticles in Nonaqueous Lithium-Oxygen Batteries. Nanoscale 2015, 7, 12906-12912. (13) Xu, J.-J.; Chang, Z.-W.; Yin, Y.-B.; Zhang, X.-B. Nanoengineered Ultralight and Robust AllMetal Cathode for High-Capacity, Stable Lithium-Oxygen Batteries. ACS Cent. Sci. 2017, 3, 598604. (14) Hong, M.; Choi, H. C.; Byon, H. R. Nanoporous NiO Plates with a Unique Role for Promoted Oxidation of Carbonate and Carboxylate Species in the Li-O2 Battery. Chem. Mater. 2015, 27, 2234-2241. (15) Kim, J. G.; Noh, Y.; Kim, Y.; Lee, S.; Kim, W. B. Fabrication of Three-Dimensional Ordered Macroporous Spinel CoFe2O4 as Efficient Bifunctional Catalysts for the Positive Electrode of Lithium-Oxygen Batteries. Nanoscale 2017, 9, 5119-5128. (16) Zhang, J.; Luan, Y.; Lyu, Z.; Wang, L.; Xu, L.; Yuan, K.; Pan, F.; Lai, M.; Liu, Z.; Chen, W. Synthesis of Hierarchical Porous δ-MnO2 Nanoboxes as an Efficient Catalyst for Rechargeable LiO2 Batteries. Nanoscale 2015, 7, 14881-14888. (17) Zhang, J.; Wang, L.; Xu, L.; Ge, X.; Zhao, X.; Lai, M.; Liu, Z.; Chen, W. Porous CobaltManganese Oxide Nanocubes Derived from Metal Organic Frameworks as a Cathode Catalyst for Rechargeable Li-O2 Batteries. Nanoscale 2015, 7, 720-726. (18) Suntivich, J.; May, K. J.; Gasteiger, H. A.; Goodenough, J. B.; Shao-Horn Y. A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles. Science 2011, 334, 1383-1385. (19) Xu, J.-J.; Wang, Z.-L.; Xu, D.; Meng, F.-Z.; Zhang, X.-B. 3D Ordered Macroporous LaFeO3 as Efficient Electrocatalyst for Li-O2 Batteries with Enhanced Rate Capability and Cyclic Performance. Energy Environ. Sci. 2014, 7, 2213-2219. (20) Oh, M. Y.; Lee, J. J.; Zahoor, A.; Kumar, G. G.; Nahm, K. S. Enhanced Electrocatalytic Activity of Three-Dimensionally-Ordered Macroporous La0.6Sr0.4CoO3−δ Perovskite Oxide for Li-O2 Battery Application. RSC Adv. 2016, 6, 32212-32219. ACS Paragon Plus Environment

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Page 19 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(21) Yang, Y.; Yin, W.; Wu, S.; Yang, X.; Xia, W.; Shen, Y.; Huang, Y.; Cao, A.; Yuan, Q. Perovskite-Type LaSrMnO Electrocatalyst with Uniform Porous Structure for an Efficient Li-O2 Battery Cathode. ACS Nano 2016, 10, 1240-1248. (22) Yoon, K. R.; Kim, D. S.; Ryu, W.-H.; Song, S. H.; Youn, D.-Y.; Jung, J.-W.; Jeon, S.; Park, Y. J.; Kim, I.-D. Tailored Combination of Low Dimensional Catalysts for Efficient Oxygen Reduction and Evolution in Li-O2 Batteries. ChemSusChem 2016, 9, 2080-2088. (23) Park, H. W.; Lee, D. U.; Zamani, P.; Seo, M. H.; Nazar, L. F.; Chen, Z. Electrospun Porous Nanorod Perovskite Oxide/Nitrogen-Doped Graphene Composite as a Bi-Functional Catalyst for Metal Air Batteries. Nano Energy 2014, 10, 192-200. (24) Zhang, J.; Zhao, Y.; Zhao, X.; Liu, Z.; Chen, W. Porous Perovskite LaNiO3 Nanocubes as Cathode Catalysts for Li-O2 Batteries with Low Charge Potential. Sci. Rep. 2014, 4, 6005. (25) Qu, J.; Lu, M.; Xu, C.; Ding, B.; Zhan, Y.; Yang, J.; Lee, J. Y. Oxide-on-Metal as an Inverted Design of Oxygen Electrocatalysts for Non-Aqueous Li-O2 Batteries. Nanoscale 2014, 6, 1232412327. (26) Ryu, W.-H.; Yoon, T.-H.; Song, S. H.; Jeon, S.; Park, Y.-J.; Kim, I.-D. Bifunctional Composite Catalysts Using Co3O4 Nanofibers Immobilized on Nonoxidized Graphene Nanoflakes for HighCapacity and Long-Cycle Li-O2 Batteries. Nano Lett. 2013, 13, 4190-4197. (27) Yao, P.-J.; Wang, J.; Du, H.-Y.; Qi, J.-Q. Synthesis, Characterization and Formaldehyde Gas Sensitivity of La0.7Sr0.3FeO3 Nanoparticles Assembled Nanowires. Mater. Chem. Phys. 2012, 134, 61-67. (28) Kim, J. G.; Kim, Y.; Noh, Y.; Kim, W. B. Formation of Carbon-Coated ZnFe2O4 Nanowires and Their Highly Reversible Lithium Storage Properties. RSC Adv. 2014, 4, 27714-27721. (29) Zhao, Y.; Xu, L.; Mai, L.; Han, C.; An, Q.; Xu, X.; Liu, X.; Zhang, Q. Hierarchical Mesoporous Perovskite La0.5Sr0.5CoO2.91 Nanowires with Ultrahigh Capacity for Li-Air Batteries. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 19569-19574.


19


Page 20 of 35

(30) Lee, B.-S.; Seo, J.-H.; Son, S.-B.; Kim, S. C.; Choi, I.-S.; Ahn, J.-P.; Oh, K. H.; Lee, S.-H.; Yu, W.-R. Face-Centered-Cubic Lithium Crystals Formed in Mesopores of Carbon Nanofiber Electrodes. ACS Nano 2013, 7, 5801-5807. (31) Yin, Y.-B.; Xu, J.-J.; Liu, Q.-C.; Zhang, X.-B. Macroporous Interconnected Hollow Carbon Nanofibers Inspired by Golden-Toad Eggs toward a Binder-Free, High-Rate, and Flexible Electrode. Adv. Mater. 2016, 28, 7494-7500. (32) Liu, G.; Chen, H.; Xia, L.; Wang, S.; Ding, L.-X.; Li, D.; Xiao, K.; Dai, S.; Wang, H. Hierarchical Mesoporous/Macroporous Perovskite La0.5Sr0.5CoO3–x Nanotubes: A Bifunctional Catalyst with Enhanced Activity and Cycle Stability for Rechargeable Lithium Oxygen Batteries. ACS Appl. Mater. Interfaces 2015, 7, 22478-22486. (33) Jung, J.; Song, K.; Bae, D. R.; Lee, S. W.; Lee, G.; Kang, Y.-M. β-FeOOH Nanorod Bundles with Highly Enhanced Round-Trip Efficiency and Extremely Low Overpotential for Lithium-Air Batteries. Nanoscale 2013, 5, 11845-11849. (34) Zhang, L.; Zhang, X.; Wang, Z.; Xu, J.; Xu, D.; Wang, L. High Aspect Ratio γ-MnOOH Nanowires for High Performance Rechargeable Nonaqueous Lithium-Oxygen Batteries. Chem. Commun. 2012, 48, 7598-7600. (35) Thomas, M. L.; Yamanaka, K.; Ohta, T.; Byon, H. R. A Perfluorinated Moiety-Grafted Carbon Nanotube Electrode for the Non-Aqueous Lithium-Oxygen Battery. Chem. Commun. 2015, 51, 3977-3980. (36) Wu, G.; Mack, N. H.; Gao, W.; Ma, S.; Zhong, R.; Han, J.; Baldwin, J. K.; Zelenay, P. NitrogenDoped Graphene-Rich Catalysts Derived from Heteroatom Polymers for Oxygen Reduction in Nonaqueous Lithium-O2 Battery Cathodes. ACS Nano 2012, 6, 9764-9776. (37) Xu, J.-J.; Chang, Z.-W.; Wang, Y.; Liu, D.-P.; Zhang, Y.; Zhang, X.-B. Cathode Surface-Induced, Solvation-Mediated, Micrometer-Sized Li2O2 Cycling for Li-O2 Batteries. Adv. Mater. 2016, 28, 9620-9628.


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Page 21 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(38) Yu, X.-Y.; Feng, Y.; Guan, B.; Lou, X. W.; Paik, U. Carbon Coated Porous Nickel Phosphides Nanoplates for Highly Efficient Oxygen Evolution Reaction. Energy Environ. Sci. 2016, 9, 12461250. (39) Hu, H.; Han, L.; Yu, M.; Wang, Z.; Lou, X. W. Metal-Organic-Framework-Engaged Formation of Co Nanoparticle-Embedded Carbon@Co9S8 Double-Shelled Nanocages for Efficient Oxygen Reduction. Energy Environ. Sci. 2016, 9, 107-111. (40) Garakani, M. A.; Abouali, S.; Zhang, B.; Xu, Z.-L.; Huang, J.; Huang, J.-Q.; Heidari, E. K.; Kim, J.-K. Controlled Synthesis of Cobalt Carbonate/Graphene Composites with Excellent Supercapacitive Performance and Pseudocapacitive Characteristics. J. Mater. Chem. A 2015, 3, 17827-17836. (41) Liu, H.; Yang, W. Ultralong Single Crystalline V2O5 Nanowire/Graphene Composite Fabricated by a Facile Green Approach and Its Lithium Storage Behavior. Energy Environ. Sci. 2011, 4, 4000-4008. (42) Lim, H.-D.; Gwon, H.; Kim, H.; Kim, S.-W.; Yoon, T.; Choi, J. W.; Oh, S. M.; Kang, K. Mechanism of Co3O4/Graphene Catalytic Activity in Li-O2 Batteries Using Carbonate Based Electrolytes. Electrochim. Acta 2013, 90, 63-70. (43) Choi, S. M.; Seo, M. H.; Kim, H. J.; Kim, W. B. Synthesis of Surface-Functionalized Graphene Nanosheets with High Pt-Loadings and Their Applications to Methanol Electrooxidation. Carbon 2011, 49, 904-909. (44) Noh, Y.; Kim, Y.; Lee, S.; Lim, E. J.; Kim, J. G.; Choi, S. M.; Seo, M. H.; Kim, W. B. Exploring the Effects of the Size of Reduced Graphene Oxide Nanosheets for Pt-Catalyzed Electrode Reactions. Nanoscale 2015, 7, 9438-9442. (45) Kim, Y.; Noh, Y.; Lim, E. J.; Lee, S.; Choi, S. M.; Kim, W. B. Star-Shaped Pd@Pt Core-Shell Catalysts Supported on Reduced Graphene Oxide with Superior Electrocatalytic Performance. J. Mater. Chem. A 2014, 2, 6976-6986.


21


Page 22 of 35

(46) Lai, Y.; Chen, W.; Zhang, Z.; Qu, Y.; Gan, Y.; Li, J. Fe/Fe3C Decorated 3-D Porous NitrogenDoped Graphene as a Cathode Material for Rechargeable Li-O2 Batteries. Electrochim. Acta 2016, 191, 733-742. (47) Zhang, G.; Xia, B. Y.; Wang, X.; Lou, X. W. Strongly Coupled NiCo2O4-rGO Hybrid Nanosheets as a Methanol-Tolerant Electrocatalyst for the Oxygen Reduction Reaction. Adv. Mater. 2014, 26, 2408-2412. (48) Lu, F.; Wang, Y.; Jin, C.; Li, F.; Yang, R.; Chen, F. Microporous La0.8Sr0.2MnO3 Perovskite Nanorods as Efficient Electrocatalysts for Lithium-Air Battery. J. Power Sources 2015, 293, 726733. (49) Seo, M. H.; Park, H. W.; Lee, D. U.; Park, M. G.; Chen, Z. Design of Highly Active Perovskite Oxides for Oxygen Evolution Reaction by Combining Experimental and ab Initio Studies. ACS Catal. 2015, 5, 4337-4344. (50) Lee, J.-H.; Black, R.; Popov, G.; Pomerantseva, E.; Nan, F.; Botton, G. A.; Nazar, L. F. The Role of Vacancies and Defects in Na0.44MnO2 Nanowire Catalysts for Lithium-Oxygen Batteries. Energy Environ. Sci. 2012, 5, 9558-9565. (51) Grewe, T.; Deng, X.; Tüysüz, H. Influence of Fe Doping on Structure and Water Oxidation Activity of Nanocast Co3O4. Chem. Mater. 2014, 26, 3162-3168. (52) Cai, Y.; Fan, H.; Xu, M.; Li, Q.; Long, C. Fast Economical Synthesis of Fe-Doped ZnO Hierarchical Nanostructures and Their High Gas-Sensing Performance. CrystEngComm 2013, 15, 7339-7345. (53) Deng, J.; Zhang, L.; Dai, H.; He, H.; Au, C. T. Single-Crystalline La0.6Sr0.4CoO3-δ Nanowires/Nanorods Derived Hydrothermally without the Use of a Template: Catalysts Highly Active for Toluene Complete Oxidation. Catal. Lett. 2008, 123, 294-300. (54) Rousseau, S.; Loridant, S.; Delichere, P.; Boreave, A.; Deloume, J. P.; Vernoux, P. La(1−x)SrxCo1−yFeyO3 Perovskites Prepared by Sol-Gel Method: Characterization and Relationships


22

Page 23 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


with Catalytic Properties for Total Oxidation of Toluene. Appl. Catal. B-Environ. 2009, 88, 438447. (55) Wu, Y.; Cordier, C.; Berrier, E.; Nuns, N.; Dujardin, C.; Granger, P. Surface Reconstructions of LaCo1−xFexO3 at High Temperature during N2O Decomposition in Realistic Exhaust Gas Composition: Impact on the Catalytic Properties. Appl. Catal. B-Environ. 2013, 140-141, 151-163. (56) Kim, J. G.; Lee, S. H.; Nam, S. H.; Choi, S. M.; Kim, W. B. Standing Pillar Arrays of C-Coated Hollow SnO2 Mesoscale Tubules for a Highly Stable Lithium Ion Storage Electrode. RSC Adv. 2012, 2, 7829-7836. (57) Cheng, J.; Zhang, M.; Jiang, Y.; Zou, L.; Gong, Y.; Chi, B.; Pu, J.; Jian, L. Perovskite La0.6Sr0.4Co0.2Fe0.8O3 as an Effective Electrocatalyst for Non-Aqueous Lithium Air Batteries. Electrochim. Acta 2016, 191, 106-115. (58) Jiang, B.; Tian, C.; Pan, Q.; Jiang, Z.; Wang, J.-Q.; Yan, W.; Fu, H. Enhanced Photocatalytic Activity and Electron Transfer Mechanisms of Graphene/TiO2 with Exposed {001} Facets. J. Phys. Chem. C 2011, 115, 23718-23725. (59) Peng, D.; Qin, W.; Wu, X.; Wu, J.; Pan, Y. Improvement of the Resistance Performance of Carbon/Cyanate Ester Composites during Vacuum Electron Radiation by Reduced Graphene Oxide Modified TiO2. RSC Adv. 2015, 5, 77138-77146. (60) Cai, G.-F.; Tu, J.-P.; Zhang, J.; Mai, Y.-J.; Lu, Y.; Gu, C.-D.; Wang, X.-L. An Efficient Route to a Porous NiO/Reduced Graphene Oxide Hybrid Film with Highly Improved Electrochromic Properties. Nanoscale 2012, 4, 5724-5730. (61) Zhang, P.; Wang, R.; He, M.; Lang, J.; Xu, S.; Yan X. 3D Hierarchical Co/CoO-GrapheneCarbonized Melamine Foam as a Superior Cathode toward Long-Life Lithium Oxygen Batteries. Adv. Funct. Mater. 2016, 26, 1354-1364. (62) Black, R.; Lee, J.-H.; Adams, B.; Mims, C. A.; Nazar, L. F. The Role of Catalysts and Peroxide Oxidation in Lithium-Oxygen Batteries. Angew. Chem. 2013, 125, 410-414.


23


Page 24 of 35

(63) Yuan, M.; Lin, L.; Yang, Y.; Nan, C.; Ma, S.; Sun, G.; Li, H. In-Situ Growth of Ultrathin Cobalt Monoxide Nanocrystals on Reduced Graphene Oxide Substrates: an Efficient Electrocatalyst for Aprotic Li-O2 Batteries. Nanotechnology 2017, 28, 185401. (64) Shui, J.; Du, F.; Xue, C.; Li, Q.; Dai, L. Vertically Aligned N-Doped Coral-Like Carbon Fiber Arrays as Efficient Air Electrodes for High-Performance Nonaqueous Li-O2 Batteries. ACS Nano 2014, 8, 3015-3022. (65) Piper, D. M.; Woo, J. H.; Son, S.-B.; Kim, S. C.; Oh, K. H.; Lee, S.-H. Hierarchical Porous Framework of Si-Based Electrodes for Minimal Volumetric Expansion. Adv. Mater. 2014, 26, 3520-3525. (66) Guo, K.; Li, Y.; Yang, J.; Zou, Z.; Xue, X.; Li, X.; Yang, H. Nanosized Mn-Ru Binary Oxides as Effective Bifunctional Cathode Electrocatalysts for Rechargeable Li-O2 Batteries. J. Mater. Chem. A 2014, 2, 1509-1514. (67) Nam, D.-H.; Kim, J. W.; Lee, J.-H.; Lee, S.-Y.; Shin, H.-A-S.; Lee, S.-H.; Joo, Y.-C. Tunable Sn Structures in Porosity-Controlled Carbon Nanofibers for All-Solid-State Lithium-Ion Battery Anodes. J. Mater. Chem. A 2015, 3, 11021-11030. (68) Xu, J.-J.; Xu, D.; Wang, Z.-L.; Wang, H.-G.; Zhang, L.-L.; Zhang, X.-B. Synthesis of PerovskiteBased Porous La0.75Sr0.25MnO3 Nanotubes as a Highly Efficient Electrocatalyst for Rechargeable Lithium-Oxygen Batteries. Angew. Chem. Int. Ed. 2013, 52, 3887-3890.


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Figure Captions Scheme 1. Illustration of the preparation strategy for the LCFO@rGO composites; (a) LCFO NWs, (b) rGO nanosheets, (c) LCFO@rGO composites. (d) Hypothetical description of discharging/charging process (i.e., ORR/OER process) of as-prepared LCFO@rGO catalyst.

Figure 1. Typical SEM images of (a) precursor NWs, (b) LCFO NWs, (c) rGO nanosheets, and (d) LCFO@rGO composites.

Figure 2. (a) XRD patterns of the as-obtained rGO nanosheets, LCFO NWs, and LCFO@rGO composites. (b) Raman spectra of the rGO sheets, and LCFO@rGO composites. Inset in panel (b) shows the Raman spectra of LCFO@rGO catalysts at a low wave number.

Figure 3. (a) TEM and (b) HRTEM image of the LCFO NWs. (c and d) TEM images of LCFO@rGO composites at a different magnification. The inset images in panel (b) and (d) are corresponding FFT patterns. (e) Dark field TEM image of LCFO NWs and corresponding elemental mappings of La, Co, Fe, and O.

Figure 4. XPS spectra of (a) full scan, (b) C 1s, (c) La 3d, (d) Co 2p, (e) Fe 2p, and (f) O 1s regions of the LCFO@rGO catalyst.

Figure 5. (a) The initial discharge/charge voltage curves of prototype Li-O2 batteries with KB, LCFO NWs, and LCFO@rGO catalysts in the potential from 2.3 to 4.5 V at a rate of 200 mA g-1. (b) Cycling performance of Li-O2 batteries with KB, LCFO NWs, and LCFO@rGO catalysts for five cycles over the potential range 2.3-4.5 V. (c) The initial discharge/charge voltage curves of Li-O2 batteries with KB, ACS Paragon Plus Environment

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LCFO NWs, and LCFO@rGO catalysts at a current rate of 200 mA g-1 when cycled to limited to capacity of 500 mAh g-1. (d) Cycling performance of the Li-O2 batteries with KB, LCFO NWs, and LCFO@rGO catalysts under a capacity limitation of 500 mAh g-1 at a rate of 200 mA g-1.

Figure 6. (a) First discharge curves of Li-O2 batteries with pure KB and LCFO@rGO catalysts in the potential window from 2.3 to 4.5 V at various current densities (400, 600, and 800 mAh g-1). (b) Comparison of the first discharge capacities of Li-O2 batteries with pure KB and LCFO@rGO catalysts at different current densities.

Figure 7. Typical SEM images of pure KB electrode (a, c and e) and LCFO@rGO electrode (b, d and f) at different cycle stages: initial stage (a and b), discharge stage (c and d), and recharge stage (e and f).


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Scheme 1. ACS Paragon Plus Environment

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Figure 1.


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Figure 2.


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Figure 3. ACS Paragon Plus Environment

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Bifunctional Hybrid Catalysts with Perovskite LaCo0.8Fe

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