Bifunctional Hybrid Catalysts with Perovskite LaCo0.8Fe0.2O3

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 5429−5439

Bifunctional Hybrid Catalysts with Perovskite LaCo0.8Fe0.2O3 Nanowires and Reduced Graphene Oxide Sheets for an Efficient Li− O2 Battery Cathode Jong Guk Kim,†,⊥ Youngmin Kim,§,⊥ Yuseong Noh,∥ Seonhwa Lee,‡ Yoongon Kim,†,∥ and Won Bae Kim*,∥ †

School of Materials Science and Engineering and ‡Department of Physics and Photon Science, Gwangju Institute of Science and Technology (GIST), 261 Cheomdan-gwagiro, Buk-gu, Gwangju 61005, Republic of Korea § Carbon Resources Institute, Korea Research Institute of Chemical Technology (KRICT), 141 Gajeong-ro, Yuseong-gu, Daejeon 34114, Republic of Korea ∥ Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), 77 Cheongam-Ro, Nam-gu, Pohang, Gyeongbuk 37673, Republic of Korea S Supporting Information *

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, resulting in an outstanding discharge capacity (ca. 7088.2 mAh g−1) at the first cycle. Moreover, a high stability of the O2-cathode with the 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 the Ketjenblack 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 catalysts, such as noble metals11−13 and metal oxides,14−17 have been widely studied for the Li−O2 battery cathodes. In particular, perovskite oxide has been proposed as a promising cathode catalyst because of its high bifunctional catalytic activity, structural stability, and mixed conductivity through its composition variations, and a relatively low cost.18−20 Unfortunately, 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 because the ORR/OER catalytic activities rely on their morphology and surface area.6,25,26 Recently, to enhance the catalytic activity and cycling stability, several strategies have been proposed. For instance, one-dimensional (1D) nanostructured catalysts have been of 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

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 because the theoretical energy density (ca. 3500 Wh kg−1) of the Li−O2 batteries is remarkably larger than that (ca. 400 Wh kg−1) of the commercial lithium-ion secondary batteries, making 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, leading to a poor cyclability and a low round-trip efficiency.8−10 In this response, several researches have been directed toward the development of appropriate catalysts to decrease the overpotential and, consequently, improve the cycle life and round-trip efficiency of the Li−O2 batteries. Recently, various types of bifunctional ORR/OER © 2018 American Chemical Society

Received: September 26, 2017 Accepted: January 18, 2018 Published: January 18, 2018 5429

DOI: 10.1021/acsami.7b14599 ACS Appl. Mater. Interfaces 2018, 10, 5429−5439

Research Article

ACS Applied Materials & Interfaces Scheme 1. Illustration of the Preparation Strategy for the LCFO@rGO Compositesa

a

(a) LCFO NWs, (b) rGO nanosheets, and (c) LCFO@rGO composites; (d) hypothetical description of discharging/charging process (i.e., ORR/ OER process) of the as-prepared LCFO@rGO catalyst.

comparison to that with the Ketjenblack (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.

sites for long-term cyclings because their properties of 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 carbon,38,39 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 the catalytic active sites,48−50 (ii) the hybridization of LCFO NWs with rGO sheets could provide efficient electron pathways, which would decrease the charge/ discharge polarization,6 and (iii) the three-dimensional (3D) structure of the LCFO@rGO composites can provide a facile O2 diffusion path in the O2-cathode during repeated cyclings.22,26 In this research, we report the LCFO@rGO composite as an efficient bifunctional catalyst for the Li−O2 battery cathodes. For the preparation of the 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 O2electrode 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 the LCFO@rGO catalyst has a high cyclability for more 56 cyclings under a capacity limit (500 mAh g−1) together with a low overpotential of 0.98 V, in

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 put into the above solution. After 2 h of ultrasonic stirring, the solution was sealed in a Teflon-lined autoclave and maintained at 230 °C for 48 h. The asprepared NWs were washed thoroughly with absolute ethanol and distilled water, followed by vacuum freeze-drying of the final product. The single-crystalline LCFO NWs were obtained through subsequent heat treatments at 400 °C for 2 h and 850 °C for 2 h at a heating rate of 1 °C 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), the as-prepared GO powder was dispersed in distilled water and sonicated for 30 min. Then, 1 mL of 5.0 M NaOH (99.99%, Aldrich) aqueous solution and 5 mL of 0.57 M Lascorbic acid (≥99.0%, Aldrich) aqueous solution were added into 20 mL of GO solution, and subsequently, the mixed solution was heattreated at 90 °C for 1 h under vigorous stirring. After natural cooling to room temperature, the mixture was purified and separated by centrifugation, followed by freeze-drying of the rGO sheets. For the preparation of hybridized LCFO@rGO, 25 mg of LCFO NWs and 5 mg of rGO sheets were separately dispersed in 10 mL of absolute acetone solvent. After 1 h of sonication, the rGO solution was added to the LCFO NWs dispersed solution under vigorous stirring. 5430

DOI: 10.1021/acsami.7b14599 ACS Appl. Mater. Interfaces 2018, 10, 5429−5439

Research Article

ACS Applied Materials & Interfaces

Figure 1. Typical SEM images of (a) precursor NWs, (b) LCFO NWs, (c) rGO nanosheets, and (d) LCFO@rGO composites. fiber filter (Whatman) separator, O2-electrode, and an electrolyte consisting of 1 M lithium bis(trifluoromethane sulfonimide) (LiTFSI) in a tetra(ethylene)glycol dimethyl ether (TEGDME). For the efficient O2 transport through the coin cell, the cathode side was drilled to make 33 holes (diameter of 1 mm). The assembled cell was placed in a chamber filled with ultrapure oxygen (99.999%) at a pressure slightly higher than 1 atm. After aging the assembled cell for 18 h, the galvanostatic discharge/charge performance of the assembled Li−O2 batteries was obtained in the voltage range of 2.3−4.5 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 the obtained specific capacity were calculated based on the KB weight for the specific charge/discharge capacity comparisons.

After magnetic stirring for 12 h, the resulting material was dried through the vacuum freeze-drying method. 2.3. Materials’ Characterizations. The scanning electron microscopy (SEM) images were taken with a JEOL JSM-7500F instrument at 5 kV. The transmission electron microscopy (TEM) images, high-resolution TEM (HRTEM) images, and energy-dispersive X-ray spectrometry (EDX) were obtained with a Tecnai F30 G2 S-Twin instrument at 300 kV. The powder X-ray diffraction (XRD) patterns were recorded with a Rigaku Rotaflex RU-200B diffractometer with a Ni filter by using a Cu Kα source. The Raman spectra were obtained with a Renishaw inVia instrument. The X-ray photoelectron spectroscopy (XPS) measurement was conducted with a MultiLab 2000 using an Al Kα source. The Brunauer−Emmett−Teller 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 (vs MMO) for the ORR and from 0.3 to 1.1 V (vs MMO) for the OER. The rotating rate of RDE was 1600 rpm and the scan rate was 10 mV s−1. To evaluate the lithium storage properties of the Li−O2 batteries, the O2-electrodes were prepared by painting a homogeneous slurry containing the as-fabricated catalysts (40 wt %), KB (EC 600JD) (40 wt %), and poly(vinylidene difluoride) (PVDF) (20 wt %) onto a Toray carbon paper. The loading density (1.0 ± 0.1 mg cm−2) of the asprepared 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 catalyst was also fabricated with the weight ratio of KB/PVDF of 80:20. After drying the electrode at 120 °C in a vacuum oven overnight, a coin-type cell (CR2032) was assembled in an argon-filled glovebox, using a lithium foil anode, glass

3. RESULTS AND DISCUSSION Scheme 1 illustrates the preparation steps of the LCFO@rGO catalyst system. In the first step, the LCFO NWs are prepared from the hydrothermal method and calcined at 400 °C for 2 h and 850 °C for 2 h in air atmosphere. In the second step, the rGO nanosheets are prepared from the GO by using an L-ascorbic acid-induced reduction reaction. In the third step, the LCFO@ rGO hybrid catalysts are obtained by mixing the LCFO NWs with the 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 the LCFO@rGO composites consisting of the 1D LCFO NWs and two-dimensional (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 the recharge process. These series reactions through the LCFO@ rGO catalysts can lead to a high reversibility of the Li−O2 batteries in this work. As shown in Figure 1, the morphology and size of the asobtained precursor NWs, LCFO NWs, rGO nanosheets, and LCFO@rGO catalysts are investigated with SEM measurements. In Figure 1a, the precursor NWs show a 1D geometry with a 5431

DOI: 10.1021/acsami.7b14599 ACS Appl. Mater. Interfaces 2018, 10, 5429−5439

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

continuous calcination at 400 and 850 °C under air atmosphere (Figure 1b). The as-prepared LCFO NWs maintain their original 1D structure with some aggregated regions from the precursor

diameter of ca. 26.0 nm and the length of several microns, indicating that uniform 1D precursor NWs are formed under the hydrothermal synthetic route. The LCFO NWs are fabricated by 5432

DOI: 10.1021/acsami.7b14599 ACS Appl. Mater. Interfaces 2018, 10, 5429−5439

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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.

ORR/OER activities.22,23 (ii) 3D hybrid structure of LCFO@ rGO obtained from the mixing of 1D LCFO NWs with 2D rGO nanosheets can 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 the LCFO@rGO composites, LCFO NWs, and rGO nanosheets are represented in Figure 2a. The XRD patterns of the LCFO@rGO composites exhibit a single crystalline phase of a rhombohedral perovskite oxide of LaCo0.8Fe0.2O3 with a space group R3̅c, which is consistent with the previous reports.54,55 This is a result of 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 68.8° 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

NWs and reveal 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 increase 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 are obtained at a calcination temperature of 850 °C in this study. As observed in Figure 1c, the rGO has a sheetlike structure with wrinkles, which is in accordance 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 are 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 O2electrode: (i) The rGO nanosheets can ensure a high electrical conductivity between the LCFO NW networks for efficient 5433

DOI: 10.1021/acsami.7b14599 ACS Appl. Mater. Interfaces 2018, 10, 5429−5439

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ACS Applied Materials & Interfaces

Figure 5. (a) Initial discharge/charge voltage curves of the 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, 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.

(697.1 cm−1) is assigned to the A2g mode derived from the R3̅c space group of the rhombohedral perovskite.54 It will arise from a higher local distortion of the octahedra due to the substitution of Co by Fe cations. These peak positions are in line with those of the perovskite LaCo0.8Fe0.2O3 materials reported previously,54,55 which means that a good quality perovskite LaCo0.8Fe0.2O3 was formed in this study. The morphology of the LCFO NWs and LCFO@rGO composites is further investigated with the TEM measurements. Figure 3a,b shows the representative TEM and HRTEM images of individual LCFO NWs, indicating the high crystallinity of the LCFO NWs. In Figure 3b, the interplanar distance of 0.27 nm is observed from the HRTEM image, which matches well with (110) planes of the cubic perovskite type LaCo0.8Fe0.2O3. The direction of crystal growth is indicated along [110] in Figure 3b. This ⟨110⟩ growth direction of the 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 the LCFO phase.40 As highlighted by the circle in Figure 3b, there are some line defects, which can facilitate the binding intermediate O22− to the catalyst surface during cathode reactions.3,48 Figure 3c represents the TEM image of LCFO@ rGO, in which the LCFO NWs are attached randomly onto the

nanosheets. It is worth noting that a substantial re-stacking of the exfoliated rGO nanosheets is prevented after hybridization with the LCFO NWs because the re-stacking-related peak at 2θ = 26° is not observed.6,45 Furthermore, no impurities are detected in the XRD patterns, indicating a high crystallinity and purity of the final products. For further investigating the quality of the rGO sheets and the LCFO@rGO composites, Raman spectroscopic analysis is performed. As shown in Figure 2b, the peaks of the LCFO@rGO composites involve the characteristic peaks from both LCFO NWs and rGO nanosheets. From the spectra of the LCFO@rGO composites, the distinctive peaks are observed at 1351.1 and 1585.7 cm−1, which could be assigned to the 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 to a graphitization degree in 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 is successfully reduced from the GO powders,43−45 which coincides with the XRD patterns. The highly ordered carbon structure could ensure a high electric 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 the perovskite LaCo0.8Fe0.2O3 in the wave number range from 1000 to 200 cm−1. The main band 5434

DOI: 10.1021/acsami.7b14599 ACS Appl. Mater. Interfaces 2018, 10, 5429−5439

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aqueous KOH solutions, a prototype Li−O2 cell is fabricated and the charge/discharge behaviors of the LCFO@rGO composites in the O2-electrode are 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 working well with light-emitting diodes. Figure 5a shows the initial discharge/charge voltage profiles of the LCFO@rGO electrode from 2.3 to 4.5 V, with a current rate of 200 mA g−1. For comparison, the potential−capacity profiles of the Li−O2 batteries with the KB carbon and the LCFO NWs are also given under the same test conditions.6,22,42,61−63 The Li−O2 cell with the LCFO@rGO catalyst delivers a discharge capacity of ca. 7088.2 mAh g−1, a value much larger than that obtained 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 the Li−O2 batteries with the LCFO@rGO composites, LCFO NWs, and KB carbon for five cycles over a voltage window of 2.3−4.5 V at a rate of 200 mA g−1. As can be seen, the O2-electrode with the LCFO@rGO catalyst shows a high discharge capacity with improved cycle durability, as compared with the KB carbon and LCFO NWs. This improved capacity retention could originate from the unique structural features of the 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 the charge capacity and the discharge capacity,26 is ca. 92.3% at the initial cycle, a value significantly larger than the KB electrode (ca. 22.2%), which would again demonstrate a high reversibility of the O2-electrode with the LCFO@rGO catalyst. The cyclability of the LCFO@rGO electrode is further investigated with a restricted charge/discharge capacity of 500 mAh g−1. The restricting capacity method has been used to achieve good reversibility because 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 the 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. In the middle region of the discharge/discharge curve, the Li−O2 cells show a decreased overpotential (0.98 V) with the LCFO@rGO catalyst, whereas the Li−O2 cells show an increased overpotential with the LCFO NWs (1.21 V) and pure KB (1.68 V), suggesting that the LCFO@rGO catalyst is 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 a high catalytic activity toward the ORR/OER processes. While the LCFO@rGO catalyst is applied to the Li−O2 battery cathodes, the charge/discharge capacity with 500 mAh g−1 is 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 the LCFO NWs and a high electronic conductivity of the rGO sheets,46 the LCFO@rGO electrode could have a low discharge/ charge overpotential and an improved round-trip efficiency (see Figure S9, Supporting Information). To account for the reversibility of the Li−O2 batteries with the catalyst during subsequent cyclings, potential−time curves are also given in Figure S10 (Supporting Information), in which the discharge

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 can 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 confirms the presence of the elements La, Co, Fe, and O (Figure 3e). In addition, the EDX spectra also represent the coexistence of La, Co, Fe, and O in the LCFO@rGO catalyst, as shown in Figure S4 (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 are 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 shows three resolved peaks, where the distinct peak at 284.5 eV can be related to the C−C bond. The deconvoluted two minor peaks at 287.9 and 285.6 eV can correspond to the CO and C−O bonds, respectively.56 As shown in Figure 4c, the La 3d spectrum displays the two resolved peaks of La 3d5/2 at the binding energy of 834.0 and 838.3 eV and La 3d3/2 at 851.2 and 855.2 eV, respectively, which originates from La3+ in an oxide form.55,57 The binding energy difference between two major peaks is ca. 16.8 eV, consistent with the previous literatures.57 In Figure 4d, the Co 2p emission spectrum shows 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 subpeak components: the three peaks at the binding energies of 782.0, 783.2, and 797.1 eV are attributed to Co2+, whereas 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 and 725.0 eV are assigned to Fe 2p3/2 and Fe 2p1/2, respectively. Two major peaks could be deconvoluted into four subpeaks: the two peaks at 710.5 and 717.1 eV are assigned to Fe2+, whereas 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 consistent with the previous reports.54,55 In Figure 4f, the O 1s spectrum shows three resolved photoelectron peaks, where the peaks at 529.1, 531.4, and 533.6 eV can be assigned to the perovskite lattice oxygen, hydroxide anions, and surfaceadsorbed carbonate oxygen, respectively.6,56 Note that the relative peak areas and the intensity from the hydroxide ions are obviously larger than that from the perovskite lattice, which is beneficial for the improvement of the catalyst activities for the ORR/OER.48 Furthermore, after the hybridization of LCFO NWs with the rGO nanosheets, the binding energy shift in the La 3d, Co 2p, and Fe 2p spectra is observed, as shown in Figure S5 (Supporting Information). This result means that the chemical states of the LCFO NWs change throughout the hybridization with rGO sheets, and further indicates that the electron transfer between the LCFO NWs and rGO sheets could be beneficial for decreasing the charge/discharge overpotentials during cyclings.58−60 Based on the bifunctional catalytic activities of the LCFO@ rGO composites (see Figure S6, Supporting Information) in 5435

DOI: 10.1021/acsami.7b14599 ACS Appl. Mater. Interfaces 2018, 10, 5429−5439

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ACS Applied Materials & Interfaces

Figure 6. (a) First discharge curves of the 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 mA g−1). (b) Comparison of the first discharge capacities of Li−O2 batteries with pure KB and LCFO@rGO catalysts at different current densities.

To investigate the discharge/charge behaviors of the KB and LCFO@rGO electrode, the SEM images of each electrode before discharge, after discharge, and after recharge are obtained as shown in Figure 7. Before the discharge process, the KB electrode is made up of pure KB nanospheres (Figure 7a), whereas the LCFO@rGO electrode is composed of LCFO@ rGO catalysts together with KB nanospheres (Figure 7b), leading to a porous framework that 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), whereas the intervoids of the LCFO@rGO electrode are well maintained (Figure 7d). Note that the intervoids 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 the 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 the O2 transport channels during the recharge process,66 which corresponds to the XRD patterns (see Figure S11, Supporting Information). On the other hand, the Li2O2 aggregates are not observed after the recharge process for the LCFO@rGO electrode (Figure 7f), which results in the outstanding cycling performance of the Li− O2 batteries, although the original morphology is not completely recovered. This might be derived from the deposition of irreversibly decomposed electrolyte or not fully decomposed Li2O2 product.6 Consequently, the as-prepared Li−O2 batteries with the LCFO@rGO hybridized bifunctional catalysts in this work exhibit enhanced electrochemical performances, such as Li+ storage capacity, overpotential, round-trip efficiency, and 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 a larger Li+ storage.48,68 (ii) 1D structure of the LCFO NWs and rGO nanosheet itself could decrease the charge transfer resistance of the O2-cathode for a 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 O2cathode.22,26

curves of the LCFO@rGO and KB carbon electrode are obtained at various discharge current rates. At the different current rates of 400, 600, and 800 mA g−1, the LCFO@rGO electrode delivers discharge capacities of 5744.6, 4336.7, and 4258.0 mAh g−1, respectively, whereas the KB electrode exhibits decreased discharge capacities of 4330.9, 3712.9, and 2781.4 mAh g−1, respectively. The discharge capacity and the potential plateaus of the LCFO@rGO electrode appears to decrease as the current densities increase; however, the LCFO@rGO electrode shows 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 the rate of 800 mA g−1, whose value is much larger than that of the KB electrode (64.2%). A better rate performance and a higher specific capacity of the LCFO@rGO electrode can be attributed to its high ORR/OER electrocatalytic activities.

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 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 a large round-trip efficiency, as compared to that with LCFO NWs and KB carbon. These improved performances of the Li−O2 batteries with the LCFO@rGO catalyst could be ascribed to the synergy effects of the inherent catalytic activity of perovskite 1D LCFO NWs and a high electrical conductivity of 2D rGO nanosheets. Therefore,

potential of the LCFO@rGO electrode is stable even until 400 h, indicating a better cycling performance of the 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

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

Author Contributions

the Li−O2 batteries with the bifunctional LCFO@rGO cathode catalyst, with improved properties like high energy/power density and high cycle stability, could be a prospective nextgeneration secondary battery.

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⊥

Notes

The authors declare no competing financial interest.

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ASSOCIATED CONTENT

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 (NRF2017R1A2B2012318 and NRF-2017R1A4A1015811).

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b14599. 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 of the samples, pictures 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)

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J.G.K. and Y.K. are contributed equally to this work.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +82-54-279-2397. Fax: +82-54-279-5528. ORCID

Youngmin Kim: 0000-0002-6893-5270 Won Bae Kim: 0000-0002-1251-9681 5437

DOI: 10.1021/acsami.7b14599 ACS Appl. Mater. Interfaces 2018, 10, 5429−5439

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