Enhancing the Catalytic Activity of Co3O4 Nanosheets for Li-O2

6 days ago - Synopsis. We propose to boost the catalytic activity of Co3O4 nanosheets for Li-O2 batteries by the incoporation of an oxygen vacancy wit...
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Enhancing the Catalytic Activity of Co3O4 Nanosheets for Li‑O2 Batteries by the Incoporation of Oxygen Vacancy with Hydrazine Hydrate Reduction Rui Gao,†,∥ Zhaoru Shang,†,∥ Lirong Zheng,‡ Junkai Wang,† Limei Sun,*,§ Zhongbo Hu,† and Xiangfeng Liu*,†

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†

Center of Materials Science and Optoelectronics Engineering, College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, P. R. China ‡ Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China § Department of Nuclear Physics, China Institute of Atomic Energy, Beijing 102413, China S Supporting Information *

ABSTRACT: Li-O2 battery attracts great interest because of the high energy density. But the poor kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) have blocked the practical application. Designing the efficient bifunctional cathode catalysts is of great importance for the Li-O2 battery. Tuning the electronic and surface structure of the catalysts plays an important role. Herein, we propose to enhance the catalytic performance of Co3O4 nanosheets for rechargable LiO2 batteries by hydrazine hydrate-induced oxygen vacancy formation. The hydrazine hydrate reduction not only introduces oxygen vacancies into Co3O4 nanosheets and modulates the electronic structure but also roughens the surface, which all contribute to the enhancement of ORR and OER activity, especially the activity and stability for OER. Li-O2 cells catalyzed by the oxygen defects-enriched Co3O4 ultrathin nanosheets exhibit much better electrochemical performances in terms of the high initial capacity (∼11 000 mAh g−1), the lower overpotential (∼1.1 V), and the longer cycle life (150 cycles@200 mA g−1). This can be largely attributed to the synergy of the enriched oxygen vacancies and the roughened surface of Co3O4 nanosheets, which not only improves the electron and Li+ conductivity but also provides more active sites and reaction spots. The proposed facile strategy may also be applied to modify other oxides based catalysts for Li-O2 batteries or other fields.

1. INTRODUCTION

The incorporation of appropriate vacancies into oxides also has an important effect on the catalytic activity.24−28 Ruetschi et al.24 found that the cation vacancies in MnO2 have a positive influence on the electrochemical reactivity. Shao25 et al. enhanced ORR and OER activity of perovskite oxides by tuning cation deficiency. Chen et al.26 reported that vacanciesbearing MnO2 showed a large improvement of ORR and OER activity. Wang et al.29 prepared Co3O4 nanosheets with oxygen vacancies by plasma engraving, and the introduced oxygen vacancies improved the electrocatalytic performance. Lei et al.30 found that, after chemically etching, mixed-valence MnOx can be obtained with enhanced ORR activity. Zhang et al.31 enhanced ORR activity of MnO2 by using facile hydrogenation of the surface. Therefore, it is highly desirable to achive the high catalytic performances for a Li-O2 battery by tuning the electronic and surface structure of the catalysts. In this study, we propose a facile hydrazine hydratereduction strategy to synthesize the surface-roughened and oxygen vacancies-enriched porous Co3O4 ultrathin nanosheets

Rechargeable Li-O2 batteries have received much attention due to their environmental friendliness and extremely high energy density (up to 5200 Wh kg−1).1−6 However, the sluggish kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), and the resultant large polarization, poor cycling life, and low rate capability restricted their practical applications.7,8 Numerous electrocatalysts, i.e., noble metals, carbonaceous materials, and transition metal oxides, have been designed to accelerate both ORR and OER and improve the electrochemical performance of Li-oxygen batteries.9−15 Transition metal oxides, especially cobalt oxides based catalysts, have attracted much more attention because of not only their rich morphology but also their low cost and high activity as well.16−23 For examples, Dai et al. reported that graphene-Co3O4 nanocomposites had efficient and stable ORR and OER catalytic activities.18 Sun et al. proved that grapheneCo3O4 nanocomposites can be well used in Li-O2 batteries as bifunctional catalyst.19 Liu et al. proved that the (111) crystal plane of Co3O4 exhibits better ORR and OER activities than other planes.22 © XXXX American Chemical Society

Received: January 3, 2019

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DOI: 10.1021/acs.inorgchem.9b00007 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Preparation Process of Def-Co3O4-NS and the Formation of Oxygen Vacancies

Figure 1. (a) The XRD patterns of Co3O4-NS and Def-Co3O4-NS. (b) The XANES patterns of Co3O4-NS and Def-Co3O4-NS. (c) Co 2p XPS spectra of pristine Co3O4-NS and Def-Co3O4-NS. (d) Raman spectra of Co3O4-NS and Def-Co3O4-NS. (XPS) was carried out on a Thermo ESCALAB 250Xi with an Al monochromatic X-ray source (Kα hυ = 1486.6 eV) to determine the atomic composition and surface functional groups of samples. All spectra were normalized using 284.6 eV of C 1s. Raman spectra were obtained from the RENISHAW inVia with an excitation wavelength of 532 nm. X-ray-absorption fine structure (XAFS) spectra was carried out on Beijing Synchrotron Radiation Facility (BSRF Beijing, China) 1W1B XAFS-beamline. The morphology of the catalysts and the cathode in different charge/discharge states was observed by a scanning electron microscope (FE-SEM SU8010 HITACHI) and transmission electron microscope (TEM) using an FEI Tecnai G2 F20 S-TWIN (acceleration voltage, 200 kV). The acceleration voltage of FE-SEM was below 5 kV, 10 μA with the work distance of 8 mm. Atomic force microscope (AFM) images were collected on an NTMDT Prima. The surface area of the powder was calculated by the Brunauer−Emmett−Teller method on a Micromeritics ASAP2460. 2.3. Fabrication of Oxygen Cathode and Electrochemical Measurements. The electrochemical performances of Li-O 2 batteries with different catalysts were measured by using a kind of 2032 coin cell with 28 holes. The cell consisiting of a lithium metal anode, a Whatsman glass filter separator, and a carbon paper-based air electrode was assembled, and LITFSI/TEGDME electrolyte (1.0 M) was used. Co3O4 catalyst (40%), PVDF binder (20%), and super P carbon (40%), with some NMP added, were used to prepare the calalyst slurry. The mixture was then coated on the carbon paper current collector to obtain the porous air electrode. The cathodes were first dried at 80 °C for 3 h, followed by 120 °C under vacuum for 12 h. The loading mass of super P carbon and Co3O4 catalyst is about 0.5 mg·cm2. The coin cells were assembled in a high-purity argonfilled glovebox (LAB2000, Etelux). Galvanostatic discharge/recharge cycles of the batteries were measured by a battery test system (LAND CT2001A, 5 V, 5 mA) in the voltage range of 2.0−4.5 V (vs Li metal).

(Def-Co3O4-NS), which combines the positive effects of the ultrathin nanosheets, oxygen vacancies, and rough surface on the catalytic activity. Def-Co3O4-NS shows much higher electrochemical activity, especially in the OER process, and Li-O2 cells with Def-Co3O4-NS exhibit much better electrochemical performances

2. EXPERIMENTAL SECTION 2.1. Synthesis of Materials. In this experiment, we first fabricated an ultrathin Co3O4 nanosheet through a hydrothermal method as previously reported.32 In detail, 0.2 g of polyethylene oxide−polypropylene oxide−polyethylene oxide (PEO20−PPO70− PEO20, Pluronic P123) was first dissolved in 3 mL of ethanol, 2.5 mL of H2O, and 12 mL of ethylene glycol to form a homogeneous solution. Next, 0.125 g of Co(Ac) 2 ·4H 2 O and 0.07 g of hexamethylenetetramine were added into the mixed solution under vigorous stirring for 30 min to get a pink solution. After that, the solution was transferred into a 45 mL autoclave and kept at 170 °C for 15 h. Then, the precursor was washed with water and ethanol in turn for 3 times and calcinated at 400 °C for 1 h in air to get ultrathin Co3O4 nanosheets. Then we introduced different oxygen vacancies into the Co3O4 nanosheets. 70 mg Co3O4 nanosheets were dipped in 25 mL of 1 M hydrazine hydrate for 1 h (sample 4) under stirring. Afterward, the obtained Co3O4 was washed with water and ethanol and dried at 80 °C to obtain the Def-Co3O4-NS. 2.2. Structure Characterization of the Samples. Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku Smartlab powder diffractometer with 9 kW Cu Kα radiation (λ = 0.154 nm). 2θ scans ranging from 20 to 80°. X-ray photoelectron spectroscopy B

DOI: 10.1021/acs.inorgchem.9b00007 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. (a, b) EXAFS patterns of Co3O4-NS and Def-Co3O4-NS in R-space and k-space. (c, d) Wavelet transform of bulk Co3O4 and Def-Co3O4NS. The capacity-limited cycling of 500 mAh·g−1 was tested at the current density of 100 mA·g−1. The electrochemical impedance spectra (EIS) were colected on a multichannel electrochemical workstation (PARSTAT MC2000, Princeton). The cells were measured in a dried oxygen box (∼1.0 atm). The total mass of the loaded super P and the catalyst was applied to calculate the capacity and current density.

become weakened at long distances (more than 3 Å) in qspace (Figure 2a) and the amplitude is weakened in k-space (Figure 2b), which further confirm the structure of the ultrathin nanosheets. In the structure of nanosheets, a dimension was limited and few atoms at a long distance can be detected. Wavelet transform and Fourier transform from EXAFS can effectively reflect the backscattering atoms that provides not only a radial distance resolution but also the resolution in the k-space as shown in Figure 2c,d.37,38 From Figure 2c,d, we can find a similar distribution profile of bulkCo3O4 and D-Co3O4-NS, and there are two main shells of Co atoms representing Co-O and Co-Co coordination, which means the two Co3O4 exhibit a similar coordination structure and similar environment. At a long distance, the distribution profile of Co-Co and Co-O is largely weakened as for the DCo3O4-NS as marked in Figure 2c,d, which means fewer atoms appear at the higher shell. In the structure of nanosheets, more atoms will be extended along the 2D direction. More Co atoms will be exposed on the surface when the nanosheets are thin, which will result in the lower coordination number of surface Co atoms.39,40 In order to further analyze the surface state of Co3O4 nanosheets, X-ray photoelectron spectroscopy (XPS) and Raman spectrum were applied (Figure 1c,d). Figure 1c provides the XPS patterns of Co3O4 nanosheets. Two main peaks and two satellite peaks are observed at about 779.5 and 794.7 eV, which can be ascribed to Co 2p3/2 and Co 2p1/2, respectively·41,42 After hydrazine hydrate etching, the region of Co 2p for Def-Co3O4-NS shifts to a lower bonding energy, and the Co3+/Co2+ ratio decreases while the Co/O ratio increases, indicating the formation of some oxygen vacancies29 (Figure S2). This was further confirmed by Raman spectra (Figure 1d). In Figure 1d, there are three characteristic peaks at 474, 517,

3. RESULT AND DISCUSSION Scheme 1 shows the preparation process of Def-Co3O4-NS. In this process, hydrazine hydrate not only promotes the formation of oxygen vacancies as a reducing media but also roughens the surface of Co3O4 nanosheets and produces more pores. The rough surface and enriched oxygen vacancies can not only improve the electron and Li+ conductivity but also provide more active sites.30,33−35 X-ray diffraction, X-ray absorption spectra, XPS, and Raman have been applied to characterize the structures of Co3O4-NS and Def-Co3O4-NS as shown in Figure 1. Figure 1a shows the XRD patterns of Co3O4-NS and Def-Co3O4-NS, and all the diffraction peaks can be indexed in the standard PDF card: No. 74-2120, indicating that, after hydrazine hydrate reduction, the main phase of Co3O4 can be maintained. Figure S1 shows the refinement of the XRD patterns with the Rietveld method. After introducing the oxygen vacancy, the lattice parameters decreased from 8.08826 to 8.08338 Å, which means the absence of oxygen can largely result in the lattice distortion. Figure 1b provides the X-ray absorption near edge structure (XANES) patterns of standard Co3O4 bulk, Co3O4-NS, and Def-Co3O4-NS. All the peaks show a similar shape, which also indicates that, after hydrazine hydrate reduction, the main chemical environment of Co has not been largely changed.36 As for the k3-weighted extended X-ray absorption fine structure (EXAFS) patterns in Figure 2a,b, the peaks of nanosheets C

DOI: 10.1021/acs.inorgchem.9b00007 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 3. SEM and TEM images of Co3O4-NS and Def-Co3O4-NS. (a, b) The FE-SEM images of Co3O4-NS and Def-Co3O4-NS. The inset figures are details on the surface. (c, d) The TEM images of Co3O4-NS and Def-Co3O4-NS. (e, f) HRTEM images of Co3O4-NS and Def-Co3O4-NS. The inset figures show the lattice spacing of them.

Figure 4. (a) Galvanostatic discharge−charge profiles of Li-O2 batteries with two different cathodes at current density of 100 mA·g−1. (b) The cell capacity for Co3O4-NS and Def-Co3O4-NS at different discharge current densities of 100, 200, and 500 mA·g−1. (c) The first capacity cutoff discharge/charge curve. (d) The cycle performance and discharge stability of Co3O4-NS and Def-Co3O4-NS based battery at the current density of 200 mA·g−1.

and 691 cm−1 which can be indexed to Raman-active modes of the crystal Co 3 O 4 corresponding to Eg, 1F 2g , and A 1g, respectively.43 After engraved by hydrazine hydrate, the peak of 691 cm−1 shifts to a lower wavenumber of 685 cm−1, which reflects the change of surface electronic structure.39,44 Figure 3a−d shows the scanning electron microscopy (SEM) and transmission electron microscope (TEM) images of Co3O4 before and after hydrazine hydrate etching. Both of the samples show a morphology of nanosheets with a porous structure. However, after hydrazine hydrate reduction, the nanosheets display a discontinuous surface and the quantity of pores also increases. As shown in SEM images, the surface of

Co3O4 becomes much rougher after etching by hydrazine hydrate, which was also confirmed by atomic force microscopy (AFM) images (Figure S3). Figure 3e,f shows the high resolution-TEM of Co3O4-NS and Def-Co3O4-NS. In Figure 3e,f, the crystal planes (111) and (220) of Co3O4 nanosheets can be clearly indexed with the lattice spacing of 0.466 and 0.286 nm. There is no obvious change to the crystal structure of Co3O4 after etching. However, by analyzing the HRTEM images, we can also find the lattice distortion and rough surface of Def-Co3O4-NS. As shown in Figure S4, the red cycles show the micropore and the yellow arrows clearly indicate the island defects and lattice defects, which is in agreement with the D

DOI: 10.1021/acs.inorgchem.9b00007 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 5. (a) A typical CV curve for Li-O2 battery with Co3O4-NS and Def-Co3O4-NS as catalysts in 1.0 M LiTFSI/TEGDME as electrolyte at a scan rate of 2 mV s−1. (b) The polarization curves of OER on pristine Co3O4-NS and Def-Co3O4-NS at 1600 rpm on GC cathode.

Figure 6. Nyquist plots of the cell catalyzed by Co3O4-NS and Def-Co3O4-NS at different discharge state. (a) Before discharge, (b) after first discharge for 10 h at the current density of 100 mA·g−1.

To further prove that the roughened and oxygen vacanciesenriched surface can accelerate both ORR and OER processes in Li-O2 batteries, cyclic voltammetry (CV) was performed to investigate the reaction during charge and discharge.45 CV obtained on a coin cell battery with 1.0 M LiTFSI/TEGDME as electrolyte at a scan rate of 2 mV/s is shown in Figure 5a. Both of the two electrodes show ORR performance after 2.9 V, while the Def-Co3O4-NS shows a lower overpotential. As for the OER process, when the low potential was cut off at 2.5 V, an obvious oxidation peak at about 4.0−4.2 V results from the decomposition of Li2O2.46 Compared with 4.2 V of Co3O4-NS, the potential of the Def-Co3O4-NS based cell reduced to 4.1 V, which indicates a faster dynamic process of chemical reaction in the Def-Co3O4-NS based cell. This agrees with the conclusion in Figure 4c. After introducing oxygen vacancies and the roughened surface, the activity of OER promoted is largely enhanced. We further conduct the linear sweep voltammetry (LSV) and CV experiments of pristine Co3O4NS and Def-Co3O4-NS in 0.1 M KOH electrolyte. This may be not identical with the reaction of the Li-O2 batteries, but to some degree, this can evaluate the performance of a catalyst for ORR and OER. The results are shown in Figure 5b, and Figures S7 and S8, respectively. The Co3O4 nanosheets were first mixed with super P and Nafion and then decorated on a glassy carbon (GC) electrode with a diameter of 5 mm. All potentials of Figure 5b and Figures S7 and S8 were referenced to a reversible hydrogen electrode (RHE). Figure S7 illustrates the CV curves of Co3O4-NS and Def-Co3O4-NS loaded on a GC electrode in an oxygen saturated 0.1 M KOH solution with a scan rate of 20 mV s−1. From the patterns, an obvious positive shift of the reduction peak can be found for DefCo3O4-NS compared with pristine Co3O4-NS. The result is also in well agreement with LSV curves at 1600 rpm (Figure

result of XRD refinement. Moreover, Def-Co3O4-NS exhibits a more obvious mesoporous structure and the surface area increases to 39.1 m2 g−1 from 29.4 m2 g−1 (Co3O4-NS) after etching (Figures S5 and S6). In order to investigate the electrocatalytic performance of Def-Co3O4-NS as cathode catalyst, a special designed 2032 coin cell was used to test the performance of a Li-O2 battery at 1 atm oxygen atmosphere. Figure 4 shows the initial capacity of the batteries by using Co3O4-NS and Def-Co3O4-NS as cathode catalysts. In Figure 4a, it can be found that the initial capacity of the Def-Co3O4-NS based battery is 11 000 mAh·g−1 higher than that of Co3O4-NS. It means that the roughened surface can enhance the ORR performance and increase the discharge capacity. Figure 4b shows rate capability at different current densities, and the results indicate that the Def-Co3O4NS based battery shows much better rate capability than Co3O4-NS. To study their cycle performance, a capacity-cutoff experiment was conducted. The capacity of all the batteries was limited to 500 mAh·g−1 under the current density of 200 mA· g−1. Figure 4c shows the comparison of the first discharge/ charge curve for the battery catalyzed by Co3O4-NS and DefCo3O4-NS. It is obvious that Def-Co3O4-NS shows excellent OER performance and delivers a charge flat less than 4.0 V. In contrast, the Co3O4-NS based battery shows a higher overpotential with a charge flat of 4.2 V. This shows that Def-Co3O4-NS has a better OER activity. Figure 4d clearly illustrates the cycling stability of the two kinds of cells. The cell using Co3O4-NS shows a low discharge flat and reaches no more than 90 cycles at a current density of 200 mA·g−1. In contrast, the battery based on Def-Co3O4-NS maintained a stable capacity of 500 mAh·g−1 and the discharge voltage shows no tendency of declining after 150 cycles or more. E

DOI: 10.1021/acs.inorgchem.9b00007 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 7. SEM images of Def-Co3O4-NS based cathodes under different discharge/recharge states. (a) Pristine cathode before discharge; (b) after the first discharge to 1000 mAh·g−1 at the current density of 100 mA·g−1; (c) after the first recharge process; (d) after 20 cycles.

bulk resistance and electronic resistance of the current collector, and the second one relates to the parallel combination of the charge-transfer resistance or kinetic resistance relates to slow lithium ion transfer coupled with the double layer capacitance of cathodes.51 After discharge for the same time, the impedance can reflect the whole condition of the cell including catalyst, cathode, anode, electrolyte, and even the discharge product. With the same amount of discharge product, the cell catalyzed by Def-Co3O4-NS can get a lower impedance. This means, under the catalysis by the Def-Co3O4-NS, both ion and electron conductivity are increased, which is beneficial not only for ORR but also for the OER. A rapid Li+ conductivity of the discharge product can make a discharge product oxidation at a lower potential so as to enhance the OER activity. Ex situ SEM and XRD were applied to analyze the formation and decomposition of the discharge product. Figure 7 shows the SEM images of Def-Co3O4-NS based cathodes at different discharge/recharge states. Figure 7a represents the morphology of the Def-Co3O4-NS based cathode before discharge. Figure 7b shows the morphology of the Def-Co3O4-NS based cathode after the first discharge to 1000 mAh·g−1 at the current density of 100 mA·g−1. As shown in Figure 7b, the toroid-like Li2O2 appears on the surface of the cathode. After the first recharge, the toroid-like Li2O2 disappears, reflecting a better OER performance as shown in Figure 7c. After 20 cycles, Li2O2 can be decomposed and Def-Co3O4-NS can be also clearly found as shown in Figure 7d, which further confirms the high OER performance of Def-Co3O4-NS. These results were also in well agreement with ex situ XRD analysis as shown in Figure S9. After the first discharge to 1000 mAh·g−1, the discharge product of Li2O2 is clearly detected. After the first recharge, no Li2O2 is detected, indicating the decomposition of Li2O2. After 20 cycles, no other diffraction peaks of Li2O2 can be observed on the charged electrode. The above results further indicate the high electrocatalytic performance of DefCo3O4-NS.

S8), in which the scan rate is 20 mV/s. Compared with Co3O4NS, Def-Co3O4-NS exhibits better ORR catalytic performances. As shown in Figure S8, Def-Co3O4-NS shows a lower overpotential and higher limiting current density of 3.8 mA cm−2 than that of Co3O4-NS (3.2 mA cm−2). As for OER performance, we discover an elevation of higher limiting current density and obvious oxidation peaks of Co(III) (1.53 V) after hydrazine hydrate etching. As shown in Figure 5b, when the current density reaches 10 mA/cm2, vacancy-free Co3O4 needs a potential of 1.70 V, while the Def-Co3O4-NS only needs 1.66 V. And after 10 cycles, the potential of Co3O4NS and Def-Co3O4-N increases to 1.75 and 1.67 V, respectively, which clearly shows that Def-Co3O4-NS exhibits a better OER activity and stability. These results indicate a better electrochemical activity of Def-Co3O4-NS, especially on OER activity and OER stability. On the one hand, the rough surface can provide more active sites and benefit the adsorption of the discharge product.29,30 On the other hand, the introduction of oxygen vacancies can largely improve the electron and Li+ conductivity and enhance the activity and stability of the catalyst.34 The electrochemical impedance spectroscopy (EIS) can provide a direct evidence of the electron/Li+ conductivity and the interface between discharge product and cathode materals.47,48 Figure 6 shows the EIS curves of Def-Co3O4NS and Co3O4-NS based batteries before discharge. It can be seen that all impedance spectra before discharge exhibit a high frequency and medium frequency depressed semicircle in the region of 50 kHz and 20 Hz, and a linear part at the very low frequency side below 20 Hz.49 It is obvious that the battery catalyzed by Def-Co3O4-NS shows a smaller diameter of the semicircle in the EIS than Co3O4-NS, reflecting a rapid electron and Li+ conductivity. Figure 6b shows the EIS curves of the two batteries after discharge at high and medium frequency. The curves exhibit a high frequency depressed semicircle roughly between 50 kHz and 1 kHz, and a medium frequency depressed semicircle in the region of 1 kHz and 2 Hz, which is caused by the dispersion of two different processes.50 The first semicircle corresponds to the electrolyte F

DOI: 10.1021/acs.inorgchem.9b00007 Inorg. Chem. XXXX, XXX, XXX−XXX

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(6) Wang, K.-X.; Zhu, Q.-C.; Chen, J.-S. Strategies toward HighPerformance Cathode Materials for Lithium-Oxygen Batteries. Small 2018, 14, 1800078. (7) Lu, Y.-C.; Shao-Horn, Y. Probing the Reaction Kinetics of the Charge Reactions of Nonaqueous Li−O2 Batteries. J. Phys. Chem. Lett. 2013, 4, 93−99. (8) Girishkumar, G.; McCloskey, B.; Luntz, A. C.; Swanson, S.; Wilcke, W. Lithium−Air Battery: Promise and Challenges. J. Phys. Chem. Lett. 2010, 1, 2193−2203. (9) Chang, Z.-W.; Xu, J.-J.; Liu, Q.-C.; Li, L.; Zhang, X.-B. Recent Progress on Stability Enhancement for Cathode in Rechargeable NonAqueous Lithium-Oxygen Battery. Adv. Energy Mater. 2015, 5, 1500633. (10) Chang, Z.; Xu, J.; Zhang, X. Recent Progress in Electrocatalyst for Li-O2 Batteries. Adv. Energy Mater. 2017, 7, 1700875. (11) Xie, J.; Yao, X.; Madden, I. P.; Jiang, D. E.; Chou, L. Y.; Tsung, C. K.; Wang, D. Selective Deposition of Ru Nanoparticles on TiSi2 Nanonet and Its Utilization for Li2O2 Formation and Decomposition. J. Am. Chem. Soc. 2014, 136, 8903−8906. (12) Li, A.; Zhang, X.; Xie, Z.; Chang, Z.; Zhou, Z.; Bu, X. PAN@ ZIF-67-Derived “Gypsophila”-Like CNFs@Co-CoO Composite as a Cathode for Li-O2 Batteries. Inorg. Chem. 2018, 57, 14476−14479. (13) Lu, J.; Lee, Y. J.; Luo, X.; Lau, K. C.; Asadi, M.; Wang, H. H.; Brombosz, S.; Wen, J.; Zhai, D.; Chen, Z.; Miller, D. J.; Jeong, Y. S.; Park, J. B.; Fang, Z. Z.; Kumar, B.; Salehi-Khojin, A.; Sun, Y. K.; Curtiss, L. A.; Amine, K. A Lithium-Oxygen Battery Based on Lithium Superoxide. Nature 2016, 529, 377−382. (14) Gao, R.; Zhou, Y.; Liu, X.; Wang, J. N-Doped Defective Carbon Layer Encapsulated W2C as a Multifunctional Cathode Catalyst for High Performance Li-O2 Battery. Electrochim. Acta 2017, 245, 430− 437. (15) Yang, W.; Salim, J.; Ma, C.; Ma, Z.; Sun, C.; Li, J.; Chen, L.; Kim, Y. Flowerlike Co3O4 Microspheres Loaded with Copper Nanoparticle as An Efficient Bifunctional Catalyst for Lithium-Air Batteries. Electrochem. Commun. 2013, 28, 13−16. (16) Han, L.; Dong, S.; Wang, E. Transition-Metal (Co, Ni, and Fe)Based Electrocatalysts for the Water Oxidation Reaction. Adv. Mater. 2016, 28, 9266−9291. (17) Xiao, Z.; Wang, Y.; Huang, Y.-C.; Wei, Z.; Dong, C.-L.; Ma, J.; Shen, S.; Li, Y.; Wang, S. Filling the Oxygen Vacancies in Co3O4 with Phosphorus: An Ultra-Efficient Electrocatalyst for Overall Water Splitting. Energy Environ. Sci. 2017, 10, 2563−2569. (18) Liang, Y.; Li, Y.; Wang, H.; Zhou, J.; Wang, J.; Regier, T.; Dai, H. Co3O4 Nanocrystals on Graphene as A Synergistic Catalyst for Oxygen Reduction Reaction. Nat. Mater. 2011, 10, 780−786. (19) Sun, C.; Li, F.; Ma, C.; Wang, Y.; Ren, Y.; Yang, W.; Ma, Z.; Li, J.; Chen, Y.; Kim, Y.; Chen, L. Graphene−Co3O4 Nanocomposite as An Efficient Bifunctional Catalyst for Lithium−Air Batteries. J. Mater. Chem. A 2014, 2, 7188−7196. (20) Dou, S.; Wang, X.; Wang, S. Rational Design of Transition Metal-Based Materials for Highly Efficient Electrocatalysis. Small Methods 2019, 3, 1800211. (21) Zeng, Y.; Wang, Y.; Huang, G.; Chen, C.; Huang, L.; Chen, R.; Wang, S. Porous CoP Nanosheets Converted from Layered Double Hydroxides with Superior Electrochemical Activity for Hydrogen Evolution Reactions at Wide Ph Ranges. Chem. Commun. 2018, 54, 1465−1468. (22) Gao, R.; Zhu, J.; Xiao, X.; Hu, Z.; Liu, J.; Liu, X. FacetDependent Electrocatalytic Performance of Co3O4 for Rechargeable Li−O2 Battery. J. Phys. Chem. C 2015, 119, 4516−4523. (23) Liu, S.; Zhang, W.; Chen, N.; Sun, C. Porous Urchin-like Co3O4 Microspheres as an Efficient Bifunctional Catalyst for Nonaqueous and Solid-State Li-O2 Batteries. ChemElectroChem 2018, 5, 2181−2185. (24) Ruetschi, P.; Giovanoli, R. Cation Vacancies in MnO2 and Their Influence on Electrochemical Reactivity. J. Electrochem. Soc. 1988, 135, 2663−2669. (25) Zhu, Y.; Zhou, W.; Yu, J.; Chen, Y.; Liu, M.; Shao, Z. Enhancing Electrocatalytic Activity of Perovskite Oxides by Tuning

4. CONCLUSION The surface-roughened and oxygen vacancies-enriched porous Co3O4 ultrathin nanosheets have been prepared by a facile hydrazine hydrate-reduction strategy. The obtained DefCo3O4-NS combines the advantages of the ultrathin nanosheets structure, oxygen vacancies, and rough surface, which largely enhanced both ORR and OER activity as a cathode catalyst for a Li-O2 battery. Def-Co3O4-NS based Li-O2 cells showed a higher initial capacity, lower overpotential, and longer cycle life. This study offers some insights into designing efficient bifunctional electrocatalysts for Li-O2 batteries.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00007. The XRD pattern and XPS patterns of Co3O4-NS and Def-Co3O4-NS, AFM images of Co3O4-NS and DefCo3O4-NS. The compared BET data of Co3O4-NS and Def-Co3O4-NS. CV curves of Co3O4-NS and DefCo3O4-NS loaded on GC electrode. The polarization curves of ORR on pristine Co3O4-NS and Def-Co3O4NS. Ex situ XRD patterns of Def-Co3O4-NS based cathodes at different discharge/recharge states (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +86 10 8825 6840 (X.L.). *E-mail: [email protected] (L.S.). ORCID

Xiangfeng Liu: 0000-0001-9633-7721 Author Contributions ∥

These two authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Beijing Natural Science Foundation (Grant No. 2182082), the National Natural Science Foundation of China (Grants 11575192 and 11675267), the Scientific Instrument Developing Project (Grant ZDKYYQ20170001), the International Partnership Program (Grant Nos. 211211KYSB20170060 and 211211KYSB20180020), and “Hundred Talents Project” of the Chinese Academy of Sciences. Allocation of beamtime at 1W1B beamline, BSRF, Beijing, China, is gratefully acknowledged.

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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) Lim, H.-D.; Lee, B.; Bae, Y.; Park, H.; Ko, Y.; Kim, H.; Kim, J.; Kang, K. Reaction Chemistry in Rechargeable Li−O2 Batteries. Chem. Soc. Rev. 2017, 46, 2873−2888. (3) Lu, J.; Li, L.; Park, J. B.; Sun, Y. K.; Wu, F.; Amine, K. Aprotic and Aqueous Li-O2 Batteries. Chem. Rev. 2014, 114, 5611−5640. (4) Zhang, P.; Zhao, Y.; Zhang, X.-B. Functional and Stability Orientation Synthesis of Materials and Structures in Aprotic Li-O2 Batteries. Chem. Soc. Rev. 2018, 47, 2921−3004. (5) Grey, C. P.; Tarascon, J. M. Sustainability and in Situ Monitoring in Battery Development. Nat. Mater. 2017, 16, 45−56. G

DOI: 10.1021/acs.inorgchem.9b00007 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Cation Deficiency for Oxygen Reduction and Evolution Reactions. Chem. Mater. 2016, 28, 1691−1697. (26) Cheng, F.; Zhang, T.; Zhang, Y.; Du, J.; Han, X.; Chen, J. Enhancing Electrocatalytic Oxygen Reduction on MnO2 with Vacancies. Angew. Chem., Int. Ed. 2013, 52, 2474−2477. (27) Huang, L.; Chen, R.; Xie, C.; Chen, C.; Wang, Y.; Zeng, Y.; Chen, D.; Wang, S. Rapid Cationic Defect and Anion Dual-Regulated Layered Double Hydroxides for Efficient Water Oxidation. Nanoscale 2018, 10, 13638−13644. (28) Wang, Y.; Qiao, M.; Li, Y.; Wang, S. Tuning Surface Electronic Configuration of NiFe LDHs Nanosheets by Introducing Cation Vacancies (Fe or Ni) as Highly Efficient Electrocatalysts for Oxygen Evolution Reaction. Small 2018, 14, No. 1800136. (29) Xu, L.; Jiang, Q.; Xiao, Z.; Li, X.; Huo, J.; Wang, S.; Dai, L. Plasma-Engraved Co3O4 Nanosheets with Oxygen Vacancies and High Surface Area for the Oxygen Evolution Reaction. Angew. Chem., Int. Ed. 2016, 55, 5277−5281. (30) Lei, K.; Han, X.; Hu, Y.; Liu, X.; Cong, L.; Cheng, F.; Chen, J. Chemical Etching of Manganese Oxides for Electrocatalytic Oxygen Reduction Reaction. Chem. Commun. 2015, 51, 11599−11602. (31) Zhang, T.; Cheng, F.; Du, J.; Hu, Y.; Chen, J. Efficiently Enhancing Oxygen Reduction Electrocatalytic Activity of MnO2 Using Facile Hydrogenation. Adv. Energy Mater. 2015, 5, 1400654. (32) Dou, Y.; Xu, J.; Ruan, B.; Liu, Q.; Pan, Y.; Sun, Z.; Dou, S. X. Atomic Layer-by-Layer Co3O4/Graphene Composite for High Performance Lithium-Ion Batteries. Adv. Energy Mater. 2016, 6, 1501835. (33) Gao, R.; Liu, L.; Hu, Z.; Zhang, P.; Cao, X.; Wang, B.; Liu, X. The Role of Oxygen Vacancies in Improving the Performance of CoO as a Bifunctional Cathode Catalyst for Rechargeable Li-O2 Batteries. J. Mater. Chem. A 2015, 3, 17598−17605. (34) Yan, D.; Li, Y.; Huo, J.; Chen, R.; Dai, L.; Wang, S. Defect Chemistry of Nonprecious-Metal Electrocatalysts for Oxygen Reactions. Adv. Mater. 2017, 29, 1606459. (35) Zhang, J.-J.; Wang, H.-H.; Zhao, T.-J.; Zhang, K.-X.; Wei, X.; Jiang, Z.-D.; Hirano, S.-I.; Li, X.-H.; Chen, J.-S. Oxygen Vacancy Engineering of Co3O4 Nanocrystals through Coupling with Metal Support for Water Oxidation. ChemSusChem 2017, 10, 2875−2879. (36) Wang, H. Y.; Hung, S. F.; Chen, H. Y.; Chan, T. S.; Chen, H. M.; Liu, B. Operando Identification of Geometrical-Site-Dependent Water Oxidation Activity of Spinel Co3O4. J. Am. Chem. Soc. 2016, 138, 36−39. (37) Fei, H.; Dong, J.; Arellano-Jiménez, M. J.; Ye, G.; Kim, N. D.; Samuel, E. L.G.; Peng, Z.; Zhu, Z.; Qin, F.; Bao, J.; Yacaman, M. J.; Ajayan, P. M.; Chen, D.; Tour, J. M. Atomic Cobalt on Nitrogendoped Graphene for Hydrogen Generation. Nat. Commun. 2015, 6, 8668. (38) Funke, H.; Scheinost, A. C.; Chukalina, M. Wavelet Analysis of Extended X-ray Absorption Fine Structure Data. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 71, 094110. (39) Gao, S.; Jiao, X.; Sun, Z.; Zhang, W.; Sun, Y.; Wang, C.; Hu, Q.; Zu, X.; Yang, F.; Yang, S.; Liang, L.; Wu, J.; Xie, Y. Ultrathin Co3O4 Layers Realizing Optimized CO2 Electroreduction to Formate. Angew. Chem., Int. Ed. 2016, 55, 698−702. (40) Sun, Y.; Gao, S.; Lei, F.; Liu, J.; Liang, L.; Xie, Y. AtomicallyThin Non-Layered Cobalt Oxide Porous Sheets for Highly Efficient Oxygen-Evolving Electrocatalysts. Chem. Sci. 2014, 5, 3976. (41) Ma, T. Y.; Dai, S.; Jaroniec, M.; Qiao, S. Z. Metal-organic Framework Derived Hybrid Co3O4-Carbon Porous Nanowire Arrays as Reversible Oxygen Evolution Electrodes. J. Am. Chem. Soc. 2014, 136, 13925−13931. (42) Gong, Y.; Ding, W.; Li, Z.; Su, R.; Zhang, X.; Wang, J.; Zhou, J.; Wang, Z.; Gao, Y.; Li, S.; Guan, P.; Wei, Z.; Sun, C. Inverse Spinel Cobalt−Iron Oxide and N-Doped Graphene Composite as an Efficient and Durable Bifuctional Catalyst for Li−O2 Batteries. ACS Catal. 2018, 8, 4082−4090. (43) Varghese, B.; Teo, C. H.; Zhu, Y.; Reddy, M. V.; Chowdari, B. V. R.; Wee, A. T. S.; Tan, V. B. C.; Lim, C. T.; Sow, C. H. Co3O4

Nanostructures with Different Morphologies and their Field-Emission Properties. Adv. Funct. Mater. 2007, 17, 1932−1939. (44) Wang, J.; Gao, R.; Zhou, D.; Chen, Z.; Wu, Z.; Schumacher, G.; Hu, Z.; Liu, X. Boosting the Electrocatalytic Activity of Co3O4 Nanosheets for a Li-O2 Battery through Modulating Inner Oxygen Vacancy and Exterior Co3+/Co2+ Ratio. ACS Catal. 2017, 7, 6533− 6541. (45) Sun, C.; Li, F.; Ma, C.; Wang, Y.; Ren, Y.; Yang, W.; Ma, Z.; Li, J.; Chen, Y.; Kim, Y.; Chen, L. Graphene-Co3O4 Nanocomposite as an Efficient Bifunctional Catalyst for Lithium−air Batteries. J. Mater. Chem. A 2014, 2, 7188. (46) Peng, Z.; Freunberger, S. A.; Hardwick, L. J.; Chen, Y.; Giordani, V.; Barde, F.; Novak, P.; Graham, D.; Tarascon, J. M.; Bruce, P. G. Oxygen Reactions in A Non-Aqueous Li+ Electrolyte. Angew. Chem., Int. Ed. 2011, 50, 6351−6355. (47) Bardenhagen, I.; Yezerska, O.; Augustin, M.; Fenske, D.; Wittstock, A.; Bäumer, M. In Situ Investigation of Pore Clogging During Discharge of a Li/O2 Battery by Electrochemical Impedance Spectroscopy. J. Power Sources 2015, 278, 255−264. (48) Huang, J.; Tong, B. Probing the Reaction Interface in Li-O2 Batteries Using Electrochemical Impedance Spectroscopy: Dual Roles of Li2O2. Chem. Commun. 2017, 53, 11418−11421. (49) Mirzaeian, M.; Hall, P. J. Characterizing Capacity Loss of Lithium Oxygen Batteries by Impedance Spectroscopy. J. Power Sources 2010, 195, 6817−6824. (50) Mohazabrad, F.; Wang, F.; Li, X. Influence of the Oxygen Electrode Open Ratio and Electrolyte Evaporation on the Performance of Li-O2 Batteries. ACS Appl. Mater. Interfaces 2017, 9, 15459− 15469. (51) Nelson, R.; Weatherspoon, M. H.; Gomez, J.; Kalu, E. E.; Zheng, J. P. Investigation of a Li-O2 Cell Featuring a Binder-Free Cathode via Impedance Spectroscopy and Equivalent Circuit Model Analysis. Electrochem. Commun. 2013, 34, 77−80.

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DOI: 10.1021/acs.inorgchem.9b00007 Inorg. Chem. XXXX, XXX, XXX−XXX