Porous Perovskite La0.6Sr0.4Co0.8Mn0.2O3 Nanofibers Loaded with

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Porous Perovskite La0.6Sr0.4Co0.8Mn0.2O3 Nanofibers Loaded with RuO2 Nanosheets as an Efficient and Durable Bifunctional Catalyst for Rechargeable Li-O2 Batteries Xiuling Zhang, Yudong Gong, Shaoqing Li, and Chunwen Sun ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02153 • Publication Date (Web): 06 Oct 2017 Downloaded from http://pubs.acs.org on October 8, 2017

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ACS Catalysis

Porous Perovskite La0.6Sr0.4Co0.8Mn0.2O3 Nanofibers Loaded with RuO2 Nanosheets as an Efficient and Durable Bifunctional Catalyst for Rechargeable Li-O2 Batteries

Xiuling Zhang,1,3 Yudong Gong,1,3 Shaoqing Li,1,3 and Chunwen Sun1,2*

1

Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing, 100083, P. R. China. 2 CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology (NCNST), Beijing, 100190, China. 3 University of Chinese Academy of Sciences, Beijing 100049, People's Republic of China * Corresponding authors. Tel.: +86-10-82854648, fax: +86-10-82854648.Email: [email protected], (C. Sun)

ABSTRACT: The design and synthesis of efficient electrocatalysts are important for electrochemical energy conversion and storage technologies. Poor electrocatalytic activities of the cathode catalysts toward both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are still two major challenges facing Li-O2 batteries. Here, we report ultra-long porous perovskite La0.6Sr0.4Co0.8Mn0.2O3 nanofibers (LSCM NFs) loaded with RuO2 nanosheets (RuO2@LSCM NFs) used as a promising catalyst for Li-O2 batteries. The LSCM nanofibers were synthesized via an electrospinning technique followed by heat treatment. RuO2 nanosheets were loaded by a wet impregnation method. Compared with that of the pristine LSCM NFs, the cell with RuO2@LSCM NFs catalyst exhibits good performances toward ORR and OER with a higher specific discharge capacity (12741.7 mA h g-1), improved cyclability and rate capability as well as low voltage gap. Moreover, the results of LSV indicate that LSCM NFs can efficiently catalyze the decomposition of the reaction side product Li2CO3 while RuO2@LSCM NFs is capable of decomposing LiOH. The enhanced cell performances are attributed to the merits of high catalytic activity and its porous structure of RuO2@LSCM NFs catalyst.

KEYWORDS:

electrospinning;

perovskite;

La0.6Sr0.4Co0.8Mn0.2O3

nanosheets; Li-O2 batteries. 1

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nanofibers;

RuO2

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INTRODUCTION With an ever increasing on automatic vehicles and the resultant pressure on energy and environment, a growing demand for clean energy storage and conversion systems has increased rapidly.1-3 In recent years, more and more attentions have been paid to the rechargeable lithium-oxygen (Li-O2) batteries, due to their super-high specific energy density.4-6 However, before its commercialization, there are numerous challenges to be solved, such as high overpotential, low rate capability and poor cycling performance.7-10 In recent years, extensive research efforts on various electrocatalysts have been made to address

these problems, including metal

oxides,11-13 metal nanoparticles,14,15 metal phosphates,16 metal nitrides,17,18 and organometallic compounds.19 Although great progresses have been made to enhance the electrochemical activities toward the oxygen-reduction reaction (ORR) corresponding to the formation of Li2O2 during discharge and oxygen-evolution reaction (OER) corresponding to the decomposition of Li2O2 during charge, highly efficient and durable bifunctional electrocatalysts are still highly desired for designing high-performance Li-O2 batteries.20 Furthermore, the discharge product oxide Li2O2 is prone to react with organic electrolyte and carbon electrode, further generating the byproducts of carbonates and carboxylates, such as Li2CO3 and CH3CO2Li.20-22 Moreover, the water in O2 atmosphere is highly reactive toward the discharge product Li2O2 and metal Li, leading to the formation of the side product LiOH.23 These side products with poor conductivity accumulated on the surface of electrodes cause an increase of overpotential during the following charge/discharge process, thus the rechargeability of Li-O2 battery.24 These results emphasize the necessary to catalyze the decomposition of the side products Li2CO3 and LiOH formed on the surface of cathode during cell cycling. Perovskite oxides with a general formula ABO3 are known as promising bifunctional catalysts for both the ORR and the OER, mainly owing to their distinct advantages

of

low

lost,

high

ionic/electronic

conductivity

and

superior

electrochemical stability.25-30 In addition, the specific structure of perovskite oxides 2


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allows substituting elements at A and/or B sites in a wide composition range. Recently, Shao-Horn et al.31 experimentally identified the charge-transfer energy, the relative energies of transition-metal (TM) 3d and O 2p valence electronic states, as a key electronic descriptor that interrelates the bulk electronic structure and surface properties of many perovskite oxides. Sunarso et al. and Zhu et al. illustrated that LaCoO3 exhibited better catalytic activities and rechargeable stability as a catalyst for metal-air batteries.32,33 Gyenge et al. reported that MnO2-LaCoO3 mixed materials have synergistic catalytic effect towards ORR and OER, and the insertion of potassium is beneficial for decreasing the overpotentials.34,35 There are two obvious advantages for A-site partial substitution with low-valence metal ions, leading to increasing oxygen vacancies and the proportion of the B-site transition-metal ions to unstable oxidation states (B3+/B4+ redox couple).36,37 Xu et al. prepared porous La0.75Sr0.25MnO3 nanotubes with high surface area via an electrospinning technique, they reported that the La0.75Sr0.25MnO3 nanotubes can improve long cycling life and lower the overvoltage of lithium-oxygen batteries.38 Since the B-site is commonly considered as the active site, the substitution of B site with a large amount of other ions with redox couples can enhance oxygen mobility.39,40 Moreover, compared to nanoparticles or two dimensional (2D) platelets morphology, the nanofibers with high aspect ratio and porosity usually show superior performance for the ORR/OER reaction since it can maximize the catalytic sites and facilitate the diffusion of electrons and reactants.38 Herein, we prepared porous ultra-long La0.6Sr0.4Co0.8Mn0.2O3 nanofibers (denoted as LSCM NFs) via an electrospinning method combined with subsequent heating treatment for the first time. Then, RuO2 nanoparticles were deposited on the surface of La0.6Sr0.4Co0.8Mn0.2O3 nanofibers by a simple chemical impregnation method reported previously,41 as schematically described in Figure 1. In this work, we demonstrate the decomposition effect of lithium carbonate and lithium hydroxide species on this composite catalyst in the Li-O2 cell, by coupling RuO2 nanosheets with La0.6Sr0.4Co0.8Mn0.2O3 nanofibers (LSCM NFs) (denoted as LSCM NFs@RuO2 3


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electrode), for the first time. Although previous reports have shown that RuO2 has superior electronic conductivity and better catalytic activity towards ORR and OER, and could also promote the decomposition of lithium hydroxide, the catalytic effect of LSCM NFs on decomposing the lithium carbonate and the enhanced function of LSCM NFs@RuO2 on enhancing the decomposition of the side products of lithium carbonate and lithium hydroxide have not been demonstrated yet. Then, the electrochemical performances of LSCM NFs@RuO2 electrode in pure oxygen were investigated systematically.

Figure 1．Schematic illustration of the synthesis processes of LSCM NFs and RuO2@LSCM NFs.

RESULTS AND DISCUSSION Characterization of Materials. Figure 2a shows XRD patterns of the pristine LSCM NFs and RuO2@LSCM NFs samples. All the diffraction peaks of the as-electrospun product can be indexed to well-crystallized perovskite-type oxide similar to La0.6Sr0.4CoO3 (JCPDS No. 89-5718). The peaks marked with diamonds in the pattern of the RuO2@LSCM NFs 4


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can be assigned to the RuO2 (JCPDS No. 87-0726). Field-emission scanning electron microscope (FESEM) was used to examine the morphology and structure of the as-synthesized samples. Figure 2c shows the as-electrospun product exhibits a nanofiber morphology with a diameter of about 500 nm and smooth surface. Interestingly, after calcination at 650 oC for 3 h, ultra-long porous nanofibers with grooves are obtained (Figure 2d and 2e). Compared with the as-electrospun precursor fibers, the LSCM NFs show slight shrinkage in diameter, about 200-300 nm. Moreover, some small pores on the nanofibers wall are formed due to release of decomposed gases during calcination process. Figure 2f displays that the RuO2 nanosheets are well grown on the surface of the LSCM NFs. The RuO2 nanosheets exhibit high aspect ratios with a thickness of about 10 nm. The weight ratio of RuO2 is determined to 8.4wt% by ICP analyses. The porous structure and pore size are investigated by nitrogen adsorption-desorption measurements, as presented in Figure 2b. The specific surface areas of the LSCM NFs and RuO2@LSCM NFs are 17.5 m2 g-1 and 50.88 m2 g-1, respectively. The average pore diameter of the LSCM NFs and RuO2@LSCM NFs are 22.6 nm and 38.9 nm, respectively.

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Figure 2.（a）XRD patterns of LSCM NFs and RuO2@LSCM NFs samples. The peaks marked with diamonds can be assigned to RuO2, (b) Nitrogen adsorption-desorption isotherms and pore size distribution (inset of b) of the two samples, (c-f) SEM images of the samples: (c) the as-electrospun product, (d, e) LSCM NFs after calcination at 650 oC, and (f) RuO2@LSCM NFs.

Figures 3a displays a SEM image of RuO2@LSCM NFs, showing the overall morphologies. Figure 3b shows an overall elemental mapping of the RuO2@LSCM 6


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NFs. The respective elemental distribution mappings of La, Sr, Co, Mn, Ru and O in the RuO2@LSCM NFs are further shown in Figures 3c-h, indicating they are homogeneously distributed in the RuO2@LSCM NFs.

Figure 3. (a) SEM image of RuO2@LSCM NFs, (b) The overall element mapping image in the selected region, and (c-h) The element mapping images of La, Sr, Co, Mn, Ru and O of RuO2@LSCM NFs.

TEM was used to characterize the microstructure of the obtained samples. It can be seen that there are many pores inside the LSCM NFs marked with red circles, as shown in Figure 4a. The LSCM NFs are consisted of nanoparticles. The marked interplanar spacing in Figure 4c is about 0.273 nm, which corresponds to the (110) lattice planes of LSCM NFs. The low magnification TEM image (Figure 4b) shows the surface of LSCM NFs is coated with RuO2 nanosheets. The marked interplanar spacing in the HRTEM TEM image of the RuO2@LSCM NFs (Figure 4d) is 0.280 nm, which corresponds to the (111) lattice planes of RuO2. After the chemical impregnation, the RuO2 nanosheets grow in the pores among different nanoparticles, 7


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resulting in a decrease of external and internal pores.

Figure 4．TEM and HRTEM images of the samples: LSCM NFs (a, c) and RuO2@LSCM NFs (b, d).

Electrochemical Performance. The electrocatalytic activities for ORR and OER of the LSCM NFs and RuO2@LSCM NFs electrodes were investigated by means of CV in the potential range of 2.0-4.2 V (vs. Li+/Li). As shown in Figure 5a, both the anodic and cathodic currents of the cell with the RuO2@LSCM NFs electrode are increased, while the overpotentials of the cells are decreased. Compared with the cell with LSCM NFs catalyst, the cathodic current of the cell with the RuO2@LSCM NFs catalyst is increased an order of magnitude, which may be related to the superior electrocatalytic of the RuO2 nanoparticles toward ORR.42,43 During anodic scan, the decomposition voltage of the discharge product starts at 3.50 V and shows a higher anodic current density, indicating better OER catalytic activity. Hence, the RuO2@LSCM NFs 8


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electrode shows enhanced electrocatalytic performance toward ORR and OER, which is attributed to the loading of RuO2 nanosheets and consistent with the previously reported results.42,44-46 Figure 5b shows the discharge and charge curves of Li-O2 cells with the LSCM NFs and RuO2@LSCM NFs catalysts respectively at a current density of 50 mA g-1. The cell with the LSCM NFs catalyst shows a discharge voltage plateau at 2.69 V and capacities of 8675.9 mA h g-1 and 4820 mA h g-1 if the mass of the Super P and the total mass of Super P and catalyst are taken into consideration, respectively. In comparison, the cell with the RuO2@LSCM NFs cathode shows a longer discharge voltage plateau at 2.70 V and a much higher capacities of 12741.7 mA h g-1 and 7078.7 mA h g-1 if the mass of the Super P and the total mass of Super P and catalyst are taken into consideration, respectively. However, the charge voltage plateaus of the cells with the LSCM NFs and RuO2@LSCM NFs catalysts are 4.20 and 4.03 V, respectively. Compared with that of the cell with the LSCM NFs catalyst, the polarization of the cell with the RuO2@LSCM NFs catalyst is reduced by 0.17 V. The rate performance of the cells with the LSCM NFs and RuO2@LSCM NFs catalysts at various current densities ranging from 50 to 300 mA g-1 with a fixed capacity of 1000 mA h g-1 are shown in Figures 5c and 5d, respectively. The voltage plateau and charge capacity are displayed in Table S1 (Supporting Information). It can be seen that the LSCM NFs electrode is more sensitive to the current density than the RuO2@LSCM NFs electrode, and the charge capacity of the cell with the LSCM NFs catalyst cannot reach the cut-off capacity of 1000 mA h g-1 with increasing the current densities to more than 200 mA g-1. It indicates that the catalytic activity towards OER was improved due to loading RuO2 nanosheets. When the current density goes back to 50 mA g-1, the discharge/charge voltages of the cell with the RuO2@LSCM NFs catalyst are almost the same as the initial ones while there is a bigger difference for the cell with the LSCM NFs catalyst, indicating a good stability of the RuO2@LSCM NFs based electrode.

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Figure 5. (a) Typical CV curves of the cells with the LSCM NFs and RuO2@LSCM NFs catalysts in the potential range of 2.0-4.2 V (vs. Li+/Li ) at a scan rate of 1 mV s-1, (b) The discharge-charge -1

profiles of Li-O2 batteries with two different cathodes at a current density of 50 mA g , (c, d) The discharge-charge profiles of the cells with the LSCM NFs and RuO2@LSCM NFs catalysts -1

respectively at various current densities ranging from 50 to 300 mA g at a fixed capacity of 1000 -1

mA h g .

Figure 6a and 6b show the galvanostatic discharge/charge curves of the Li-O2 cells with the LSCM NFs and RuO2@LSCM NFs catalysts with a fixed capacity of 500 mA h g-1. From the results of LSV curves shown in Figure S1, it can be seen that the testing cut-off voltages of the cells with the LSCM NFs and RuO2@LSCM NFs catalysts are 4.3 V and 4.4 V respectively at a current density of 50 mA g-1. For the cell with LSCM NFs cathode, the first discharge voltage plateau is about 2.70 V, and the charge terminal voltage reaches 4.10 V. In comparison, the discharge voltage plateau and the charge terminal voltage of the cell with RuO2@LSCM NFs catalyst is 2.73 V 10


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and 4.05 V respectively. In addition, with increasing the cycling number, the overpotentials of these two cells increase slowly. Notably, the cell with RuO2@LSCM NFs catalyst shows a lower overpotential and the capacity can still retain at 500 mA h g-1 after 100 cycles. For the cell with LSCM NFs catalyst, it cannot reach the cut-off capacity of 500 mA h g-1 after 100 cycles. The discharge terminal voltage and charge terminal voltage are displayed in Figure 6d, respectively. It can be seen that the discharge terminal voltage of the cell with the RuO2@LSCM NFs catalyst is always higher than that of the cells with the LSCM NFs catalyst. Meanwhile, the cell with the RuO2@LSCM NFs catalyst shows lower charge terminal voltage. The LSCM NFs based cathode has a higher catalytic activity toward ORR while the RuO2 nanosheets show enhanced effect on ORR and OER. We also investigated the Li-O2 cell performance with a higher limited capacity of 2000 mA h g-1 with the LSCM NFs catalyst and RuO2@LSCM NFs catalyst at a current density of 100 mA g-1, shown in Figure S2. It indicates that the cell with the LSCM NFs can be reversibly cycled for 17 cycles (Figure S2c). However, the cell with the RuO2@LSCM NFs catalyst can cycle well for 25 cycles. Compared with that of the cell with the the LSCM NFs catalyst, the cell with the RuO2@LSCM NFs catalyst shows lower overpotentials even at a higher current density and limited capacity (Figure S2d). These results indicate that the RuO2@LSCM NFs based cathode has a higher catalytic activity toward ORR and OER.

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Figure 6. Discharge-charge profiles of the Li-O2 cells with the LSCM NFs catalyst (a) and the RuO2@LSCM NFs catalyst (b) at the current density of 50 mA g-1 with a limited capacity of 500 mA h g-1, (c) Cycling performance of the cells with the LSCM NFs and RuO2@LSCM NFs catalyst, and (d) Discharge /charge terminal voltages of the cells with two different catalysts.

The phase composition and the morphology of LSCM NFs and RuO2@LSCM NFs based cathodes under different discharge/charge states are characterized by XRD, FT-IR and SEM. Figure 7a shows the XRD patterns of the RuO2@LSCM NFs based cathodes in different charge/discharge states. In Figure 7a, the peaks of Li2O2 are observed after the first discharge for the cell with the RuO2@LSCM NFs electrode, indicating that Li2O2 is the main discharge product. The peaks of Li2O2 disappear after the first charge, which suggests that the Li2O2 product forming during discharge can be decomposed completely during charge. The XRD patterns of the LSCM NFs electrode are similar to that of the RuO2@LSCM NFs, as presented in Figure S2a. FT-IR spectra were used to further demonstrate the decomposition of the products, as displayed in Figure 7b. The absorbance peak of Li2O2 at 510 cm-1 is observed for the 12


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RuO2@LSCM NFs electrode under discharge state, as shown in Figure 7b. Besides, the absorbance peaks are observed at about 860 cm-1 and 1410 cm-1, which are corresponded to Li2CO3 and LiOH, respectively.21 The peaks at ~1130 and 1590 cm-1 can be assigned to LiTFSI and CH3COOLi, respectively. It is noted that all the products peaks vanish after charge, suggesting that the RuO2@LSCM NFs are capable to decompose Li2O2, Li2CO3, LiOH and even CH3COOLi. The FTIR spectra of the LSCM NFs electrode are similar to that of the RuO2@LSCM NFs electrode, as displayed in Figure S2b. These results are consistent with the LSV results (Figure 8).

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Figure 7. XRD patterns of RuO2@LSCM NFs based cathode of the Li-O2 cells after the first discharge and charge (a), FTIR spectra of the RuO2@LSCM NFs based cathode after the first discharge and charge (b), SEM images of the RuO2@LSCM NFs cathode: before discharge (c), when the discharge capacity reaches 4000 mA h g-1 (d), at a full discharge state of the first discharge (e), and at a full charge state in the first charge (f).

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The morphology evolution of the RuO2@LSCM NFs based cathode under different discharge/charge states were characterized by SEM. As performed in Figure 7c, the RuO2@LSCM NFs and super P particles can be clearly observed in the original electrode. When the discharge capacity reaches 4000 mA h g-1 (Figure 7d), discharge products with a flake shape form on the surface of the cathode. After the full discharge during cycle, the products grow bigger gradually and they present toroid shape, as shown in Figure 7e. According to the before mentioned XRD results, it can be concluded that the toroid-like product is Li2O2, which may block oxygen diffusion and electron transport.47,

48

The morphology evolution of the LSCM NFs based

cathode shows a similar result to those of the RuO2@LSCM NFs based cathode during discharge process, as shown in Figure S2 (c-f). Figure 6f shows the SEM image of the RuO2@LSCM NFs electrode after the first charge, toroid-like Li2O2 vanishes, and the RuO2@LSCM NFs and super P particles can be identified clearly, indicating that the RuO2@LSCM NFs can catalyze the decomposition of Li2O2 completely during charge, which is corroborated by XRD results. EIS was also used to investigate the resistance of LSCM NFs and LSCM NFs@RuO2 based cathodes in the pure O2 condition. As shown in Figure S3, the results indicate that LSCM NFs@RuO2 has a lower resistance and the electrode can be recovered after charging. To verify that Li2O2 is oxidized during charge for the cells with the LSCM NFs and RuO2@LSCM NFs based cathodes, we prepared electrodes by adding bulk Li2O2 powders. The cells were charged from open circuit potential to 4.8 V by a LSV testing at a sweep rate of 1 mV s-1. This control experiment can exclude the influence factors of discharge products, like morphological or compositional differences caused by different cathode catalysts. Figure 8a shows the LSV curves of the cells with Li2O2-LSCM NFs and Li2O2-RuO2@LSCM NFs. The ratio of Li2O2 is about 10wt%. The first oxidation peak of the cell with the RuO2@LSCM NFs (red curve) occurs at 4.05 V, approximately 150 mV lower than that of the LSCM NFs (black curve). The onset potential is decreased to 3.63 V compared with that of LSCM NFs (3.85 V). The second oxidation peak for the cell with the LSCM NFs appears at 4.76 V, about 230 15


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mV higher than that of the RuO2@LSCM NFs. The onset potential of the cell with the LSCM NFs catalyst is 4.48 V, higher than that of the cell with the RuO2@LSCM NFs at 4.28 V. Based on the results shown in Figure S1, the peaks with an onset potential at 4.53 V and 4.76 V are attributed to the electrolyte decomposition. Figure 8d shows the charge profiles of the Li-O2 cell with the LSCM NFs and RuO2@LSCM NFs. In comparison with LSCM NFs, both the onset potential and overpotential of the RuO2@LSCM NFs based cathode are reduced. The result further demonstrates that the RuO2 nanosheets have a higher OER activity than that of LSCM NFs since the charge process corresponds to the oxidation of Li2O2.42,44

Figure 8. Anodic LSV curves of the cells with LSCM NFs and RuO2@LSCM NFs based 16


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electrodes from open potential to 4.8 V at a scan rate of 1 mV s-1 to show the oxidation of Li2O2 (a), Li2CO3 (b), LiOH (c); Charge profiles of the Li-O2 cells with the LSCM NFs and RuO2@LSCM NFs based cathodes at a current density of 25 mA g-1 by adding Li2O2 (d), Li2CO3 (e), LiOH (f). The weight ratio of super P, LSCM NFs or RuO2@LSCM NFs, binder, and Li2O2, Li2CO3 or LiOH is 4:4:1:1.

Previous studies report that the carbon tends to react with Li2O2 during charge to form an interfacial layer of Li2CO3. Furthermore, electrolyte decomposition can also produce Li2CO3, which may increase the OER overpotential and drive the charging voltage to well above 4.0 V.21,49,50 It is the major challenge in developing high-performance rechargeable Li-air batteries. Nazar et al. found that superoxide species de-hydrofluorinates PVDF to produce H2O2 as a side product catalyzed by the MnO2 catalyst, which then is decomposed and yielded H2O.51 Then the product Li2O2 easily reacts with H2O to form LiOH.52 So it is important to develop catalysts, which can decompose the reaction side products of Li2CO3 and LiOH. To examine the feasibility of electrocatalytic decomposition of the byproducts Li2CO3 and LiOH during discharge/charge cycles, the following anodic LSV measurements were carried out, as shown in Figure 8b and 8c. The onset potential of the cell with Li2CO3-LSCM NFs electrode is about 4.08 V, approximately 0.11 V lower than that of the cell with the RuO2@LSCM NFs catalyst. For the electrode containing LiOH, the onset potential of the cell with the RuO2@LSCM NFs catalyst occurs at 4.16 V, which is lower than that of the cell with the LSCM NFs catalyst. Since the TEGDME electrolyte is stable with the LSCM NFs and RuO2@LSCM NFs based cathodes below 4.51 V and 4.37 V, respectively. The charge profiles of the cells with the LSCM NFs and RuO2@LSCM NFs mixed with 10 wt% Li2CO3 and LiOH respectively are shown in Figure 8e and 8f. In Figure 8e, compared with the cells with the RuO2@LSCM NFs electrode, both the onset potential and overpotential of the cells with the LSCM NFs-Li2CO3 based cathodes are all reduced. Meanwhile the specific capacity is higher. On the other hand, for the cell with the RuO2@LSCM NFs 17


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mixed with LiOH electrode, it has a higher capacity and lower charging voltage than those of the cell with LSCM NFs electrode, shown in Figure 8f. The results demonstrate that the LSCM NFs have good electrocatalytic activities toward the oxidation of Li2CO3, while the RuO2@LSCM NFs have a favorable effect on decomposing LiOH product. Table 1 presents the electrochemical performance of the Li-O2 batteries with the RuO2@LSCM NFs based cathode, which is also compared with those recent results of other catalysts reported in the literatures.

28,40,46,50,53-59

The electrode with the

RuO2@LSCM NFs catalyst has many better properties, including high initial capacity (12742 mA h g-1), a low voltage range (2.6-4.3 V) and long cycling stability (≥100 cycles). The enhanced performances of the obtained porous LSCM NFs and RuO2@ LSCM NFs nanocomposite towards ORR and OER can be attributed to the following factors: (i) the nanofibers and nanosheets with porous morphology are not only beneficial for electrolyte/electrode contact but also provide more pores facilitating the transport of O2; (ii) the specific structure of the RuO2@ LSCM NFs with high surface area can provide more active sites for ORR/OER as a catalyst for Li-O2 batteries; (iii) the RuO2 nanosheets could further improve the conductivity of LSCM NFs. In addition, La0.6Sr0.4Co0.8Mn0.2O3 nanofibers can improve the oxygen reduction reactions, and also boost the decomposition of lithium carbonate as the tenacious product to be removed in the air cathode. More importantly, RuO2 nanosheets can promote the decomposition of lithium hydroxide, meanwhile have higher activities towards both oxygen reduction and evolution reactions and decrease the overpotential compared with that of La0.6Sr0.4Co0.8Mn0.2O3 nanofibers. From Table 1, it can be seen clearly that our two materials as catalysts for Li-O2 batteries show better specific discharge and recharge capacities, good cyclability as well as decomposition capability for byproducts.

Table 1 The performance comparison of Li-O2 cells with different catalysts 18


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ACS Catalysis

Catalyst

Electrolyte

Initial

Limited

Cycle

Voltage

capacity

number

range (V

Reference

capacity (mAh g-1carbon)

(mAh/g)

vs. Li+/Li)

Pyrochlore

LiPF6

6920

1000

26

2.0-4.5

54

Super P

LATP

14192

500

10

2.0-4.5

55

La0.5Sr0.5CoO3

LiTFSI

5799

500

50

1.5-4.4

28

CoO/C

LiTFSI

7011

1000

50

2.3-4.5

57

Carbon

PFSA-Li

~5000

1500

25

2.5-4.5

56

RuO2

LiClO4

1110

1000

50

2.3-4.0

53

KB carbon

LiTFSI

10600

1000

10

2.2-4.4

50

CNT@ RuO2

LiTFSI

4350

300

100

2.3-4.7

46

La0.8Sr0.2Co0.8Fe0.2O3

LiTFSI

11026

500

50

2.2-4.5

30

[email protected]

LiTFSI

7992

1000

80

2.2-4.4

58

LaNiO3

LiCF3SO3

3407

1000

24

2.0-4.3

59

LaNi0.25Co0.75O3

LiTFSI

6620

1000

49

2.3-4.25

40

RuO2@LSCM NFs

LiTFSI

12742

500

100

2.6-4.3

This

Pb2[Ru1.7Pb0.3]O6.5

work

CONCLUSIONS In summary, RuO2@LSCM NFs have been prepared by an electrospinning technique combined with a wet impregnation method. As a catalyst for Li-O2 batteries, 19


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it shows good electrochemical performance in terms of the high initial capacity, long cyclability, and rate capability. Importantly, the overpotential is decreased greatly. The enhanced ORR/OER performances are attributed to the good property of LSCM NFs with its specific morphology toward ORR and the high catalytic activity of RuO2 nanosheets toward OER. Moreover, LSCM NFs is capable of decomposing the byproduct of Li2CO3 to some extent. Besides enhancing the electronic conductivity of LSCM NFs, RuO2 nanosheets greatly decrease the overpotential of the decomposition of Li2O2 and LiOH. We believe that this work provides a promising bifunctional catalyst to advance the practical applications of metal air batteries.

EXPERIMENTAL METHODS Synthesis of porous La0.6Sr0.4Co0.8Mn0.2O3 nanofibers and RuO2 nanoparticles decorated La0.6Sr0.4Co0.8Mn0.2O3 nanofibers The porous La0.6Sr0.4Co0.8Mn0.2O3 nanofibers were synthesized by an electrospinning method combined with subsequent heating treatment. In a typical process, stoichiometric

La(NO3)3·6H2O,

Sr(NO3)2,

Co(CH3COO)2·4H2O

and

Mn(CH3COO)2·4H2O were weighted with a total amount of metal ions is 4 mmol. Then, the reagents were added into 15 mL of N,N-dimethylformamide (DMF) under vigorous stirring

to

form

a

homogeneous

solution.

After

that,

2g

of

polyvinylpyrrolodone (PVP) were added and the resulting solution was stirred for 12 h at room temperature. Then the obtained solution was placed in a plastic syringe equipped with a 19-gauge metal nozzle made of stainless steel. The feed rate of the solution was kept at a flow rate of 0.2 mL h-1. The distance between the collector and the needle tip was 20 cm. Electrospinning experiments were performed in air with a relative humidity of 20~30%. The applied voltage was fixed to 20.0 kV and the electrospinning fibers were collected on a piece of aluminum foil. The as-spun nanofibers were collected and dried at 70 ºC for 10 h under vacuum, and then calcined at a heating rate of 2 ºC min -1 in air and maintained at 650 ºC for 3 h. The obtained 20


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ACS Catalysis

product was denoted as LSCM NFs. To load RuO2 nanoparticles on the surfaces of LSCM NFs, we used a method reported before.60 In brief, 1g RuCl3·xH2O solution was dissolved into 100 mL deionized water. 0.1 M KOH solution was dropped into the above 1 mL RuCl3·xH2O solution until the pH value was adjusted to approximately 10.0, then the suspension of Ru(OH)3 was obtained. After the above solution was stirred for 1 h, it was washed with deionized water and anhydrous ethanol until the pH value was about 7. After that, the Ru(OH)3 particles were then re-dispersed in 6 mL ethanol by ultrasonic treatment for 10 minutes. The obtained Ru(OH)3 precursor was mixed with 40 mg La0.6Sr0.4Co0.8Mn0.2O3 nanofibers by further ultrasonic treatment for 5 mins. After drying at 40 oC for four days, the sample was calcined in air at 300 °C for 4 h and then cooled

to

room

temperature.

Finally,

RuO2

nanoparticles

decorated

La0.6Sr0.4Co0.8Mn0.2O3 nanofibers were obtained, and it was donated as RuO2@ LSCM NFs.

Materials characterization Powder X-ray diffraction (XRD) analysis was characterized on a PANalytical X’Pert3 Powder diffractometer with Cu Kα radiation (λ = 1.5418, 40 kV, 40 mA) in the 2θ range of 10-80o. Morphology was characterized by transmission electron microscope (TEM, Tecnai G2 F20) and field-emission scanning electron microscope (FESEM, NOVA NANOSEM 450). Nitrogen sorption isotherms were measured at 77 K with a Micromeritcs ASAP-2046 analyzer. Before measurements, the samples were degassed in a vacuum at 150 °C for 6 h. The pore volume and pore diameter distribution were derived from adsorption branches of the isotherms by the Barrett-Joyner-Halenda (BJH) model. The RuO2 loading was analyzed by induced coupled plasma (ICP, Thermo Electron Corporation). A Fourier-transform infrared (FTIR) test was conducted on a spectrometer (Vertex 80, Bruker) in the frequency range of 400-2000 cm-1 with a resolution of 4 cm-1 and 32 scans at room temperature.

21


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Electrochemical Measurement The Li-O2 batteries were assembled with typical 2032 coin cells with a few of holes on one side in an argon-filled glove box with water and oxygen levels less than 0.5 ppm. The oxygen cathodes were prepared by casting a homogenous ink mixture consisting of 40 wt% LSCM NFs or RuO2@LSCM NFs, 50 wt% super P, and 10 wt% polyvinylidene fluoride (PVDF) onto a carbon paper current collector with a diameter of 12 mm and carbon loading is 0.8±0.1 mg. After that, the carbon papers were dried in an oven at 80 °C for 2 h, then dried at 120 °C for 12 h under vacuum. 1 M lithium bis-(trifluoromethanesulfony)-imide (LiTFSI) in tetraethlene glycol dimethyl ether (TEGDME) was used as electrolyte and glass fiber papers were acted as separators. Lithium foils were used as anode. The Li-O2 cells were tested in a dry plastic box filled with pure oxygen atmosphere. Galvanostatic discharge-charge tests were carried out in the voltage range of 2.2~4.2or 4.4 V (vs. Li+/Li) after 4 hours rest period using a battery testing system (LANDIAN, BTS3000) at room temperature. The specific capacity of the electrodes were calculated based on the mass of Super P. Cyclic voltammetry (CV) and linear sweep voltammograms (LSV) measurements were carried out with a CHI 604E electrochemical workstation at a scan rate of 1 mV s-1. Electrochemical impedance spectra (EIS) measurements were performed on Zahner electrochemical workstation in the frequency range from 0.1 Hz to 106 Hz.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website at DOI: 10.1021/acscatal. The rate performance of the cells with LSCM NFs and RuO2@LSCM NFs catalysts, LSV curves, XRD patterns, SEM images of LSCM NFs based cathode and electrochemical impedance spectra of Li-O2 cells with the LSCM NFs or RuO2@LSCM NFs catalysts.

22


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ACS Catalysis

Acknowledgements The authors acknowledge the financial support of National Key R & D Project from Ministry of Science and Technology, China (2016YFA0202702) and the National Science Foundation of China (Nos. 51372271 and 51672029). This work was also supported by the Thousands Talents Program for the pioneer researcher and his innovation team in China.

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Table of Contents (TOC) RuO2 [email protected] nanofibers were prepared by an electrospinning technique followed by heat treatment and used as an excellent catalyst for Li-O2 batteries. The remarkable electrochemical performance can be attributed to the excellent property of LSCM NFs with unique morphology toward ORR and the superior catalytic activity of RuO2 nanosheets toward OER.

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Porous Perovskite La0.6Sr0.4Co0.8Mn0.2O3 Nanofibers Loaded with

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