Facet-Dependent Electrocatalytic Performance of Co3O4 for

Subscriber access provided by SELCUK UNIV

Article

The Facet-dependent Electrocatalytic Performance of Co3O4 for Rechargeable Li-O2 Battery Rui Gao, Jinzhen Zhu, Xiaoling Xiao, Zhongbo Hu, Jianjun Liu, and Xiangfeng Liu J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 11 Feb 2015 Downloaded from http://pubs.acs.org on February 11, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 24

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

The Journal of Physical Chemistry

The Facet-dependent Electrocatalytic Performance of Co3O4 for Rechargeable Li-O2 Battery Rui Gaoa, Jinzhen Zhub, Xiaoling Xiaoa, Zhongbo Hub, Jianjun Liub*, Xiangfeng Liua** a

College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, China b

State Key Laboratory of High Performance Ceramics and Superfine Microstructure , Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China.

*Corresponding Author: [email protected]. Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China. Tel. +86 21 5241 1087 **Corresponding Author: [email protected]. University of Chinese Academy of Sciences, Beijing 100049, China. Tel. +86 10 8825 6840

ACS Paragon Plus Environment

1


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

Page 2 of 24

ABSTRACT: The facet-dependent performance has aroused great interest in the fields of catalyst, lithium ion battery and electrochemical sensor. In this study, the well-defined Co3O4 cubes with exposed (001) plane and octahedrons with exposed (111) plane have been successfully synthesized and the facet-dependent electrocatalytic performance of Co3O4 for rechargeable Li-O2 battery has been comprehensively investigated by the combination of experiments and theoretical calculations. The Li-O2 battery cathode catalyzed by Co3O4 octahedron with exposed (111) plane shows much higher specific capacity, cycling performance and rate capability than Co3O4 cube with exposed (001) plane, which may be largely attributed to the richer Co2+ and more active sites on (111) plane of Co3O4 octahedrons. The DFT-based first-principles calculations further indicate that Co3O4 (111) has a lower activation barrier of O2 desorption in oxygen evolution reaction (OER) than Co3O4 (001), which is very important to refresh active sites of catalyst and generate a better cyclic performance. Also, our calculations indicate that Co3O4 (111) surface has a stronger absorption ability for Li2O2 than Co3O4 (001), which may be an explanation for a larger initial capacity in Co3O4 (111) plane by experimental observation.

Keywords: Co3O4; Facet-dependent performance; Li-O2 battery; Electrocatalysis; DFT calculations


2

Page 3 of 24

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


1. INTRODUCTION Recently, rechargeable lithium-oxygen batteries have attracted considerable attention owing to the super high energy density in comparison with lithium ion battery or other chemical batteries.1-5 But the sluggish kinetics of the oxygen reduction reaction (ORR: 2Li++O2→Li2O2) and oxygen evolution reaction (OER: Li2O2→2Li++O2) on the cathode, and the resultant huge polarization during the charging/discharging, low efficiency, poor cycling performance and low rate capability seriously restricted their practical application in electric vehicles (EVs).6-8 Noble metals, carbonaceous materials, transition-metal oxides, and metal nitrides have been extensively explored as electrocatalysts to enhance the reaction kinetics of both ORR and OER .9-13 Among the transition-metal oxides based catalysts, Co3O4 draws great interest because of the good compromise between the initial capacity and the capacity retention.14,15 Bruce et al. compared the catalytic activities of some catalysts and found that the initial capacity of Co3O4 was 2000mAh·g-1 and its capacity retention was 6.5% per cycle indicating a better compromise between the initial capacity and the capacity retention.16 Some strategies have been taken to further improve the electrocatalytic activity of Co3O4. For examples, Cui et al. demonstrated that the three-dimensional cubic mesoporous Co3O4 showed a much higher catalytic reactivity than the two-dimensional hexagonal mesoporous Co3O4.17 Park et al. reported the higher capacity and lower overpotential of carbon spheres dotted homogeneously with Co3O4 nanoparticles.18 Sun et al. reported that graphene-Co3O4 nanocomposites had efficient and stable ORR and OER catalytic activities.19 In addition to the above mentioned methods, the crystal plane effect has aroused great interest in some fields such as catalysts, electrochemical energy storage and electrochemical detection of heavy metal ions.20-24 The materials with the same chemical compositions but with different exposed well-defined facets show quite different performance because of their different atom arrangement and electronic structures. Xie et al. reported the importance of morphology control on the catalytic activity and found that Co3O4 nanorod with more (110) exposed facets showed surprisingly high catalytic activity for low-temperature CO oxidation due to the surface richness of active Co3+ sites.25 Wang et al. explored the catalytic activities of Co3O4 by using H2O2 and different organic substrates as the substrates of peroxidase mimics. It shows that the peroxidase-like catalytic activities of Co3O4 nanomaterials are crystal plane-dependent and follow the order: (112) >> (110) > (100).26,27


3


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

Page 4 of 24

Huang et al. reported the facet-dependent electrochemical properties of Co3O4 toward heavy metal ions and found that the (111) facet of Co3O4 nanoplates has better electrochemical sensing performance than that of the (001) facet of Co3O4 nanocubes..28 Liu et al. reported the high performance lithium storage of Co3O4 nanocages with highly exposed (110) facets.29 In 2012, we reported the facet-dependent electrochemical properties of Co3O4 as anode material for Li-ion battery and the Co3O4 octahedron with exposed (111) plane has the highest reversible capacity and the best rate capability compared to Co3O4 cube with exposed (001) plane and Co3O4 truncated octahedron with exposed (001) and (111) planes.30 Interfacial electrocatalysis has become an attractive point in a rechargable Li-O2 battery research in order to improve cyclic performance and reduce overpotential in oxygen evolution reaction (OER). The exposed crystal plane of Co3O4 might also have a significant effect on the electrochemical performances (specific capacity, cycling performance and rate capability, et al.) as the bifunctional electrocatalyst for rechargeable Li-O2 battery. Very recently, Su et al. synthesized single crystalline Co3O4 nanocrystals (cubic, pseudo octahedral, nanosheets and nanoplatelets ) with different exposed crystal planes and reported the reducing charge-discharge overpotential with crystal planes of Co3O4 and established an order of catalytic activity: (111)>(110)>(112)>(100).31 However, the mechanisms of Co3O4 catalyzing ORR and OER and some other facet-dependent electrochemical performances such as rate capability and cycle stability were not discussed in details. Based on first-principles calculations, unravelling underlying physical mechanism of catalytic activity of different crystal plane structures is also very important to develop the novel catalyst in Li-O2 battery. In this work, we combined experimental and theoretical studies to elucidate exposed cystal planeperformance relationship of Co3O4 as bifunctional electrocatalyst for rechargeable Li-O2 battery. As a result, a joint experimental and theoretical study to reveal facet-dependent catalytic mechanism is the essence of this paper. The well-defined Co3O4 cubes and octahedrons have been successfully synthesized and their electrocatalytic performances have been characterized. The Li-O2 battery cathode catalyzed by Co3O4 octahedron with exposed (111) plane shows much higher specific capacity, cycling performance and rate capability than Co3O4 cube with exposed (001) plane, which may be largely attributed to the richer Co2+ and more active sites on (111) plane of Co3O4 octahedron. The DFT-based first-principles calculations indicate that Co3O4 (111) has a lower activation barrier of O2 desorption in OER than Co3O4 (001), which is very important


4

Page 5 of 24

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


to refresh active sites of catalyst and generate a better cyclic performance. Also, our calculations indicate that Co3O4 (111) surface has a stronger absorption ability for Li2O2 than Co3O4 (001), which may be an explanation for experimental observation of a larger initial capacity in Co3O4 (111).

2. EXPERIMENTAL SECTION 2.1. Synthesis of Co3O4 Co3O4 cube and Co3O4 octahedron were synthesized according to our previous work.30 In this experiment, 0.04 mol cobalt nitrate hexahydrate and 0.01 mol sodium hydroxide were dissolved in 40 mL deionized water to prepare the Co3O4 cube. The mixed reactants were then transferred to a 50 mL hydrothermal synthesis reactor and heated at 180 °C for 5 h. The reactor was allowed to cool to room temperature. The obtained products were washed with deionized water and ethanol several times, subsequently dried at 60 °C overnight. To prepare the Co3O4 octahedron with the same method, the amount of Co(NO3)2·6H2O and NaOH were 0.2 mol and 0.05 mol. All the samples were finally calcined at 500 °C for 3 h in air. 2.2. Materials Characterization The powder of the two kinds of Co3O4 were characterized by X-ray powder diffraction (XRD, XD-3 PERSEE ) with 2θ ranging from 10° to 80°. The morphologies of the samples were observed on a scanning electron microscope (SEM, S-4800 HITACHI). The surface area of the powder was calculated by the Brunauer−Emmett−Teller (BET, Gemini V) method. 2.3. Preparation of cathode The electrochemical performances of Li-O2 battery was measured by using a Swagelok cell, which was assembled in an Ar-filled glovebox with a water and oxygen content less than 0.1 ppm. The cell consists of a lithium mental anode, Whatsman glass filter separator, electrolyte and an oxygen electrode. The electrolyte is composed of 0.5 M LITFSI (bistrifluoromethanesulfonimide lithium salt) dissolved in TEGDME (tetraethylene glycol dimethyl ether). 20 wt % Co3O4 catalyst, 20 wt % binder (PVDF) and 60 wt % super P carbon were mixed with NMP to prepare the slurry. The mixture was then coated on the nickel foam current collector to prepare the oxygen electrode, which was then dried at 120 °C under vacuum overnight. 2.4. Electrochemical measurements


5


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

Page 6 of 24

The galvanostatic charge and discharge performance of the cells were tested on a LAND CT2001A battery tester system with the voltage between 2.0 and 4.6 V at the current density of 100 mA·g-1, 200 mA·g-1 and 400 mA·g-1. The cycle performance was conducted at the current density of 100 mA·g-1. And the capacity was limited to 1000 mAh·g-1. All the cells were tested under the pure O2 atmosphere. The current density and capacities were calculated based on the mass of the carbon loaded on the cathode. 2.5. Computational Methods First principles calculations were performed by using density functional theory (DFT) with the generalized gradient approximation (GGA) for the exchange-correlation function as formulated by Perdew, Burke, and Ernzerhof (PBE). The valence electron-ion interaction was modeled by the projector augmented wave (PAW) potential as implemented in the Vienna ab initio simulation package (VASP).32,33 The plane wave basis set with a cut-off energy of 450 eV was used. Electron correlation within the d states significantly affects the electronic structure and energetic properties of transition metal oxides. Based on the previous reports, the GGA+U (U=2.0V) approach was used in our calculations.34 The band gap of bulk Co3O4 is calculated to be 1.51 eV by DFT + U which is consistent with the experimental value (1.44~1.52 eV).35-38 In order to avoid overbinding of DFT-calculated O2 molecule, the energy of O2 molecule is determined by the formula of H (T = 0 K , O2 ) = 2 H (T = 0 K , O) − ∆E exp tl ,39 where ΔEexptl (5.12 eV) is the binding energy of O2,40 H (T=0, X) is the calculated ground state of oxygen atom (X = O) or oxygen molecular (X = O2). Based on experimentally thermodynamic data of bulk Li2O2 and Li,40 the open-circuit potential of 2Li++O2+2e↔Li2O2 is calculated as 2.98 V, which is close to the experimental value (2.96 V). The formation enthalpy and Gibbs energy of Li2O2 from calculation is -6.57 eV and -5.96 eV, respectively, which are in good agreement with the experimental data (-6.57 eV and -5.92 eV). 3. RESULTS AND DISCUSSION 3.1. Structural characterization of Co3O4


6

Page 7 of 24

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


Figure 1. a) The XRD patterns of Co3O4 samples. (1) standard; (2) cube; (3) octahedron. b) The BET curve of Co3O4 cubes and Co3O4 octahedrons. The X-ray powder diffraction (XRD) patterns of the two Co3O4 samples are shown in Figure 1(a). All the diffraction peaks could be indexed on the standard PDF card (JCPDS No. 09-0418). This means both Co3O4 cube and Co3O4 octahedron belong to the same space group. Figure 1(b) shows the BET curves of Co3O4 cubes and Co3O4 octahedrons, respectively. The specific surface areas of Co3O4 cube and Co3O4 octahedron are 3.44m2·g-1 and 1.83m2·g-1, respectively. The SEM images of the two Co3O4 samples are shown in Figure 2. Figure 2a and 2b display SEM images of Co3O4 cubes under low and high magnifications, respectively. The size of Co3O4 cube is about 500 nm. Figure 2c and 2d represent the images of Co3O4 octahedrons and the size is about 700 nm. According to the structure model of cobaltosic oxide (face-centered cubic nanocrystal), the Co3O4 particle with a cube shape exposes six (001) planes, while the octahedron exposes eight (111) planes.


7


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

Page 8 of 24

Figure 2. The SEM images of Co3O4 cubes (a, b) and octahedrons (c, d) 3.2. Electrocatalytic performance

Figure 3. The cycle performance and the capacity of the cathode catalyzed by Co3O4 cubes and octahedrons measured at the current density of 100mAh/g . Figure 3 shows the cycle performance and the full capacity of the cathode catalyzed by Co3O4 cubes and octahedrons,respectively, measured at the current density of 100 mA·g-1. Co3O4 octahedrons catalyzed cathode show a larger initial capacity than Co3O4 cubes. In addition, the capacities of both the two samples drop after 6 cycles but the cathode catalyzed by Co3O4 cubes shows a much faster reduction. Co3O4 octahedron can maintain above 2100mAh·g-1 and the capacity retention rate is about 85%. In contrast, the capacity of Co3O4 cubes is only about 1200 mAh·g-1 and the retention rate drops to 66% after 6 cycles. This indicates that the


8

Page 9 of 24

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


Co3O4 octahedron catalyzed cathode has larger reverible capacity and capacity retention in comparison with Co3O4 cubes catalyzed cathode.

Figure 4. The initial charge/discharge capacity of electrode catalyzed by Co3O4 cubes (a) and octahedrons (b), measured at a variety of current densities of 100 mA·g-1, 200 mA·g-1and 400 mA·g-1,respectively. Figure 4 shows the initial charge/discharge capacity of electrode catalyzed by Co3O4 cubes (Figure 4a) and octahedrons (Figure 4b), measured at different current densities: 100 mA·g-1, 200 mA·g-1 and 400 mA·g-1. The first discharge specific capacities of the cathode catalyzed by Co3O4 cubes at the current densities of 100 mA·g-1, 200 mA·g-1 and 4 00mA·g-1 are shown in Figure 4 (a), which illstrate the capacity of 1787 mAh·g-1 1412 mAh·g-1, and 908 mAh·g-1, respectively. In contrast, Figure 4 (b) shows the first discharge specific capacity of the cathode catalyzed by Co3O4 octahedrons at the same current densities. The first discharge specific capacity of the cathode catalyzed by Co3O4 octahedrons at the current densities of 100 mA·g-1, 200 mA·g-1 and 4 00mA·g-1 increases to 2463mAh·g-1, 1890mAh·g-1 and 1253mAh·g-1, respectively. In comparison with Co3O4 cubes the capacity of the cathode catalyzed by Co3O4 octahedrons at the different current densities increases by 37.8, 33.9, and 38.0%, respectively. The results indicate that the spcific capacity


9


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

Page 10 of 24

of cathode catalyzed by Co3O4 octahedrons is much higher than that of Co3O4 cubes at different current densities. On one hand, the result shows that both of the oxygen electrodes are sensitive to the current density and the specific capacity decreases with the increase of the current density, which maybe result from the low electrical conductivity of the reaction products. The reaction products cover the surface of the electrode and limit the reaction kinetics, which result in the decrease of the specific capacity with the increasing current densities

41-44

. On the other hand, the Li-O2 battery catalyzed by Co3O4 octahedron shows a much higher

capacity than the cubes indicating that Co3O4 octahedron has a much better rate capability than Co3O4 cube. Moreover, it should be noted that, at a high current density (400 mA·g-1), the electrode catalyzed by Co3O4 octahedron has a lower polarization than the Co3O4 cube. A voltage gap between the charge and discharge progress of Co3O4 cube is 2.28 V at the current density of 400mA·g-1. But the voltage gap of Co3O4 octahedron catalyzed cathode drops to 2.00 V under the same condition.

Figure 5. The cycle performances of the cathode catalyzed by Co3O4 cubes (a) and octahedrons (b), measured at the current density of 100 mAh·g-1 when the capacity is limited to 1000 mAh·g-1.


10

Page 11 of 24

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


Figure 5 show the cycle performance of the cathode catalyzed by Co3O4 cubes (Figure 5a) and octahedrons (Figure 5b) measured at the current density of100 mAh·g-1 when the capacity is limited to 1000 mAh·g-1. The restricted capacity method is usually used to avoid a large depth of discharge and achieve better reversibility45. The cell taking Co3O4 cubes as the catalyst cannot keep stable and the capacity drops sharply. From Figure 5 (a), it can be seen that after 10 cycles the capacity cannot reach 1000mAh·g-1, and after 20 cycles the capacity is less than 200mAh·g-1. Figure 5 (b) shows cycle performance of the cell with the cathode catalyzed by Co3O4 octahedrons, which exhibits excellent capacity after 20 cycles and the curves of the charge/discharge keep stable. The above analysis indicates that the cathode catalyzed by Co3O4 octahedrons exhibits much higher charge/discharge capacity, lower loss of capacity and a high cycling stability. The large difference of electrochemical performance between the cathode catalyzed by Co3O4 cubes and octahedrons can be largely attributed to the differences of morphology with differently exposed crystal planes and atom arrangements on the surface.46-48The possible reason will be further discussed in the followings. The above results show that the cathode catalyzed by Co3O4 octahedron with exposed (111) planes exhibits much higher charge/discharge capacity, rate capability and cycling stability than Co3O4 cube with exposed (001) planes. The reason may be various in different catalysts. For exapmle, Surface area is usually an important factor affecting the activity of catalyst materials. It is well accepted that the materials with large surface areas can provide a high catalytic activity.17, 49 However, in this study, the specific surface area of Co3O4 octahedrons (1.83 m2·g-1) with better catalytic activity is much less than Co3O4 cubes (3.44 m2·g-1) with poorer catalytic activity, which indicates that the difference of the electrocatalytic activity does not result from the specific surface areas.50 The difference of the electrocatalytic activity for Co3O4 octahedrons and cubes can be largely attributed to the “crystal plane effect”. The Co3O4 octahedron exposed (111) can provide more active catalysis sites, which definitely facilitate the ORR and the OER progress. In an ORR progress, it is well accepted that Co2+ (3d54s2) on the surface has a better catalytic activity than the Co3+ (3d54s1). During the reaction, Co2+ tends to transfer electrons to the absorbed O2 molecules and weaken or assist breaking the O-O bond, and Co2+ themselves are oxidized to Co3+ intermediate product.51 Co3O4 octahedron with exposed (111) crystal plane can offer more Co2+. Figure 6 directly shows the unit cell of Co3O4, (001) and (111) crystal planes, and the polyhedron with different planes.


11


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

Page 12 of 24

By calculating the Co2+ of the plane, we can find that the (001) plane contains only one Co2+, while (111) plane contains 1.875 Co2+, which is almost twice of the former. A few previous publications also report that the exposed (001) and (111) crystal planes only have Co2+.25,30 This means Co3O4 octahedron has more active sites to catalyze ORR and OER and exhibits a higher electrocatalytic activity. The similar conclusion can also be proved by using CoO as catalyst for the Li-O2 batteries and its better performance.52-54 This is why the cathode catalyzed by Co3O4 octahedron with exposed (111) plane has much higher electrochemical performances than Co3O4 cube with exposed (001) planes.

Figure 6. Schematic illustration of atomic configurations for Co3O4 cube with exposed (001) planes and Co3O4 octahedron with exposed (111) planes. 3.3. The DFT-based first-principles calculations

Figure 7. The Gibbs surface energies as a function of ∆µO for the various possible structures of Co3O4 (111) and (001) planes.


12

Page 13 of 24

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


The experimental studies indicated that Co3O4 (111) plane generated a higher electrochemical performances such as initial capacity and cyclic stability than Co3O4 (001), which can be largely attributed to a higher catalytic activity of Co3O4 (111) for ORR and OER in Li-O2 battery than Co3O4 (001). In order to reveal catalytic discrepancy of planes, the DFT-based calculations were carried out to explore interaction mechanisms of Co3O4 planes with Li2O2 in the ORR and OER processes. It is well-known that the spinel Co3O4 exhibits rich surface structures even existing different possible structures under one index. As a result, the surface energies as a function of O2 chemical potential are calculated to determine the possible structure of Co3O4 (111) and (001) planes. As shown in Figure 7, the O-rich Co3O4 (111)C may be more stable than Co/Ocoexisted Co3O4 (001)A under a high O2 concentration. In discharging, nanostructured Li2O2 is generated on active sites of Co3O4 planes.55 Due to a highly exothermic process of Li++O2→Li2O2, the active barrier height of O2 in ORR may not be determinant for initial capacity. Therefore, the adsorption energies of Li2O2 molecule on Co3O4 planes are calculated to simply estimate their different initial capacities. In fact, the initial capacity of Li-O2 battery is mainly affected by O2 adsorption amount on surface. Since O2 is not directly dissociated on the clean Co3O4 (001) and (111) planes, the adsorption capacity of O2 can be estimated by the adsorption energy of Li2O2. The calculated adsorption energies of 0.85 and 0.63 eV on Co3O4 (111) and (001) surface show that the Co3O4 (111) has a stronger adsorption ability for Li2O2 than Co3O4 (001). It may be a possible explanation for a higher initial capacity for Co3O4 (111) than Co3O4 (001). Because the limited active sites of catalyst are covered by nanoscale Li2O2 formed during discharging, the OER efficiency directly affects cyclic performance and charging overpotential of Li-O2 battery. By calculating catalytic mechanism of Li2O2→Li++O2 on different planes, the charging voltage can be quantitatively estimated. Based on these results, the cyclic performance in experiment can be explained. During charging, Li+ ions under an electromotive force are desorbed from the interface of Li2O2/Co3O4 and finally migrate to anode via electrolyte. Such an electromotive force corresponds to the charging voltage. Thermodynamically, the charging process can be described as

∆G =  E − E0 + ∆N Li ( µ Li − eU ) + ∆NO2 µO2 


(1)

13


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

Page 14 of 24

where E is the total energy of the slab; E0 is the total energy of the initial slab; ∆NLi and ∆NO2 are the number of the Li and O2 removed; and µLi and µO2 represent the chemical potentials of Li bulk and O2, respectively. Therefore, the U value is used to describe the charging voltage.56-58 The calculated energy profiles of stepwise Li+ and O2 desorption of Li2O2 on Co3O4 (001) and (111) planes are presented in Figure 8(a) and (b), respectively. As shown in Figure 8(a), the charging voltage on Co3O4(111) surface is 2.69 V for Li+→Li+→O2. More importantly, the rate-determinant step (O2 desorption) in this reaction path requires overcoming 1.27 eV activation barrier. In contrast, the charging voltage and ratedeterminant barrier of OER on Co3O4 (001) are 3.69V and 1.92 eV, respectively, as shown in Figure 8(b). Co3O4 (111) can significantly reduce charging overpotential than Co3O4 (001), which is in qualitative agreement with our experimental measurement. In comparison, Co3O4 (111) has a higher catalytic activity for OER than Co3O4 (001). Our Bader charge calculations indicate that 0.76 e- of Li2O2 is transferred to Co3O4 (111), while only 0.016 e- of Li2O2 is transferred to Co3O4 (001). It indicates that electron-withdrawing ability of catalyst may be determinant for OER kinetic rate.

Figure 8. Energy Profiles of OER paths Li+→Li+→O2 and Li+→O2→ Li+ of Li2O2 on Co3O4 (111) (a) and (001) surfaces (b). The reaction free energies of all intermediates in the reaction paths are negative under listed charging voltages. The structural evolutions of intermediates of these reaction paths are displayed in Figure 9. Figure 9a shows the structural evolution of Li+→Li+→O2 and Li+→O2→Li+ OER paths of Li2O2 on Co3O4 (111). Figure


14

Page 15 of 24

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


9b shows the structural evolution of Li+→Li+→O2 and Li+→O2→Li+ OER paths of Li2O2 on Co3O4 (001). Two reaction paths, Li+→Li+→O2 and Li+→O2→Li+, are considered based on our previous calculations and Ceder et al’s work57, 58. It is noted that the present reaction paths only consider thermodynamic desorption of Li+ and O2 by calculating the energy difference of initial and final states. Since electrostatic interaction between Li+ and O2 is dominated in our Li2O2/Co3O4 system, the calculated desorption energy can be considered as activation barrier. The calculation procedure has been applied in the similar systems.56-59

Figure 9. Structural evolution of Li+→Li+→O2 and Li+→O2→Li+ OER paths of Li2O2 on Co3O4(111) (a) and Co3O4(001) planes (b). Furthermore, we estimate the kinetic rates of OER on two planes based on the calculated activation energies. In our calculations, the overall activation energies along the lowest energy path on a crystal plane are


15


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

Page 16 of 24

approximated by the activation barrier of rate-determinant step, which does not consider other possible rate limitation such as electron conductivity and Li+ transport. The calculated activation barriers for O2 desorption steps are 1.27 and 1.92 eV for the Co3O4 (111) and (001) surfaces, respectively. Given that the rate of OER corresponds to the current density on the cathodes surface, we can further evaluate surface current density by i=2e(Nsite/S)Aexp(-∆Gact/kT)

(2)

where Nsite/S is the number of Li2O2 per surface area, A is the prefactor.52 Given that A and Nsite/S are similar on two surfaces, the surface current density on Co3O4 (111) is ~107 times larger than that of Co3O4 (001). Based on the analysis of the above experimental and the calculated results a schematic illustration of the facet-dependent electroctalalytic mechanism of ORR and OER on Co3O4 (001) and Co3O4 (111), respectively, is presented, as shown in Figure 10. Co3O4 (111) surface exposes more active sites and it has a stronger absorption ability for Li2O2 than Co3O4 (001) in ORR and a lower activation barrier of O2 desorption in OER than Co3O4 (001).

Figure 10. Schematic illustration of the facet-dependent electroctalalytic mechanism of ORR and OER on Co3O4 (001) and Co3O4 (111), respectively. Co3O4 (111) surface exposes more active sites and it has a stronger absorption ability for Li2O2 than Co3O4 (001) in ORR and a lower activation barrier of O2 desorption in OER than Co3O4 (001). 4. CONCLUSIONS


16

Page 17 of 24

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


In summary, well-defined Co3O4 cubes and octahedrons have been successfully synthesized and the crystal plane effect of Co3O4 as the bifunctional cathode catalyst for rechargeable Li-O2 battery was discovered. The Li-O2 battery cathode catalyzed by Co3O4 octahedron with exposed (111) plane shows much higher specific capacity, cycling performance and rate capability than Co3O4 cube with exposed (001) plane. This can be largely attributed to the richer Co2+ on (111) plane of Co3O4 octahedron, which supplies more active sites for ORR and OER. The DFT-based first-principles calculations were performed to reveal that Co3O4 (111) has a lower activation barrier of O2 desorption in OER than Co3O4 (001), which is very important to refresh active sites of catalyst. Also, our calculations indicate that Co3O4 (111) surface has a stronger absorption ability for Li2O2 than Co3O4 (001), which may be an explanation for experimental observation of a larger initial capacity in Co3O4 (111).

ACKNOWLEDGMENT This work was supported by the State Key Project of Fundamental Research (2014CB931900 and 2012CB932504), the Chinese Academy of Sciences (“Hundred Talents Project”), NSFC (21201177) and NSFC(51432010).

REFERENCES (1)

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. (2)

Kwabi, D.G.; Ortiz-Vitoriano, N.; Freunberger, S.A.; Chen, Y.; Imanishi, N.; Bruce, P.G.; Shao-Horn,

Y. Materials Challenges in Rechargeable Lithium-air Batteries. MRS Bulletin 2014, 39, 443-452. (3)

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.


17


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

(4)

Page 18 of 24

Freunberger, S. A.; Chen, Y.; Peng, Z.; Griffin, J. M.; Hardwick, L. J.; Barde, F.; Novak, P.; Bruce, P.

G. Reactions in the Rechargeable Lithium-O2 Battery with Alkyl Carbonate Electrolytes. J. Am. Chem. Soc. 2011, 133, 8040-8047. (5)

Li, F.; Zhang, T.; Zhou, H. Challenges of Non-Aqueous Li–O2 Batteries: Electrolytes, Catalysts, and

Anodes. Energy Environ. Sci. 2013, 6, 1125-1141. (6)

Wu, D.; Guo, Z.; Yin, X.; Pang, Q.; Tu, B.; Zhang, L.; Wang, Y. G.; Li, Q. Metal-Organic

Frameworks as Cathode Materials for Li-O2 Batteries. Adv. Mater. 2014, 20, 3258-3262. (7)

Black, R.; Adams, B.; Nazar, L. F. Non-Aqueous and Hybrid Li-O2 Batteries. Adv. Energy Mater.

2012, 2, 801-815. (8)

Park, M.; Sun, H.; Lee, H.; Lee, J.; Cho, J. Lithium-Air Batteries: Survey on the Current Status and

Perspectives Towards Automotive Applications from a Battery Industry Standpoint. Adv. Energy Mater. 2012, 2, 780-800. (9)

Li, F.; Tang, D. M.; Jian, Z.; Liu, D.; Golberg, D.; Yamada, A.; Zhou, H. Li-O2 Battery Based on

Highly Efficient Sb-Doped Tin Oxide Supported Ru Nanoparticles. Adv. Mater. 2014, 26, 4659-4665. (10) Lu, J.; Cheng, L.; Lau, K. C.; Tyo, E.; Luo, X.; Wen, J.; Miller, D.; Assary, R. S.; Wang, H.-H.; Redfern, P.; et al. Effect of the Size-Selective Silver Clusters on Lithium Peroxide Morphology in LithiumOxygen Batteries. Nat. Commun. 2014, 5, 4895. (11) Guo, Z.; Zhou, D.; Dong, X.; Qiu, Z.; Wang, Y.; Xia, Y. Ordered Hierarchical Mesoporous/ Macroporous Carbon: A High-Performance Catalyst for Rechargeable Li-O2 Batteries. Adv. Mater. 2013, 25, 5668-5672. (12) Cao, R.; Lee, J.-S.; Liu, M.; Cho, J. Recent Progress in Non-Precious Catalysts for Metal-Air Batteries. Adv. Energy Mater. 2012, 2, 816-829.


18

Page 19 of 24

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


(13) Oh, S. H.; Nazar, L. F. Oxide Catalysts for Rechargeable High-Capacity Li-O2 Batteries. Adv. Energy Mater. 2012, 2, 903-910. (14) 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. (15) Black, R.; Lee, J. H.; Adams, B.; Mims, C. A.; Nazar, L. F. The Role of Catalysts and Peroxide Oxidation in Lithium-Oxygen Batteries. Angew. Chem. Int. Ed. 2013, 52, 392-398. (16) Débart, A.; Bao, J.; Armstrong, G.; Bruce, P. G. An O2 Cathode for Rechargeable Lithium Batteries: The Effect of a Catalyst. J. Power Sources 2007, 174, 1177-1182. (17) Cui, Y.; Wen, Z.; Sun, S.; Lu, Y.; Jin, J. Mesoporous Co3O4 with Different Porosities as Catalysts for the Lithium–Oxygen Cell. Solid State Ionics 2012, 225, 598-603. (18) Park, C. S.; Kim, K. S.; Park, Y. J. Carbon-Sphere/Co3O4 Nanocomposite Catalysts for Effective Air Electrode in Li/Air Batteries. J. Power Sources 2013, 244, 72-79. (19) Sun, C.; Li, F.; Ma, C.; Wang, Y.; Ren, Y.; Yang, W.; Ma, Z.; Li, J.; Chen, Y.; Kim, Y.; et al. Graphene–Co3O4 Nanocomposite as an Efficient Bifunctional Catalyst for Lithium–Air Batteries. J. Mater. Chem. A 2014, 2, 7188-7196. (20) Cheng, F.; Chen, J. Metal-Air Batteries: From Oxygen Reduction Electrochemistry to Cathode Catalysts. Chem. Soc. Rev. 2012, 41, 2172-2192. (21) Si, R.; Flytzani-Stephanopoulos, M. Shape and Crystal-Plane Effects of Nanoscale Ceria on the Activity of Au-CeO2 Catalysts for the Water–Gas Shift Reaction. Angew. Chem. 2008, 120, 2926-2929. (22) Hu, L.; Peng, Q.; Li, Y. Selective Synthesis of Co3O4 Nanocrystal with Different Shape and Crystal Plane Effect on Catalytic Property for Methane Combustion. J. Am. Chem. Soc. 2008, 130, 16136-16137.


19


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

Page 20 of 24

(23) Cheng, F.; Shen, J.; Ji, W.; Tao, Z.; Chen, J. Selective Synthesis of Manganese Oxide Nanostructures for Electrocatalytic Oxygen Reduction. ACS Appl. Mater. Interfaces 2009, 1, 460-466. (24) Zhou, K.; Li, Y. Catalysis Based on Nanocrystals with Well-Defined Facets. Angew. Chem. Int. Ed. 2012, 51, 602-613. (25) Xie, X.; Li, Y.; Liu, Z. Q.; Haruta, M.; Shen, W. Low-Temperature Oxidation of Co Catalysed by Co3O4 Nanorods. Nature 2009, 458, 746-749. (26) Mu, J.; Zhang, L.; Zhao, M.; Wang, Y. Catalase Mimic Property of Co3O4 Nanomaterials with Different Morphology and Its Application as a Calcium Sensor. ACS Appl. Mater. Interfaces 2014, 6, 70907099. (27) Mu, J.; Zhang, L.; Zhao, G.; Wang, Y., The Crystal Plane Effect on the Peroxidase-Like Catalytic Properties of Co3O4 Nanomaterials. Phys. Chem. Chem. Phys. 2014, 16, 15709-15716. (28) Yu, X. Y.; Meng, Q. Q.; Luo, T.; Jia, Y.; Sun, B.; Li, Q. X.; Liu, J. H.; Huang, X. J., Facet-Dependent Electrochemical Properties of Co3O4 Nanocrystals toward Heavy Metal Ions. Sci. Rep. 2013, 3, 2886. (29) Liu, D.; Wang, X.; Wang, X.; Tian, W.; Bando, Y.; Golberg, D. Co3O4 Nanocages with Highly Exposed {110} Facets for High-Performance Lithium Storage. Sci. Rep. 2013, 3, 2543. (30) Xiao, X.; Liu, X.; Zhao, H.; Chen, D.; Liu, F.; Xiang, J.; Hu, Z.; Li, Y. Facile Shape Control of Co3O4 and the Effect of the Crystal Plane on Electrochemical Performance. Adv. Mater. 2012, 24, 5762-5766. (31) Su, D.; Dou, S.; Wang, G. Single Crystalline Co3O4 Nanocrystals Exposed with Different Crystal Planes for Li-O2 Batteries. Sci. Rep. 2014, 4, 5767. (32) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-energy Calculations Using a Plane-wave Basis Set. Phys. Rev. B 1996, 54, 11169. (33) Kresse, G.; Furthmuller, J. Efficiency of Ab-initio Total Energy Calculations for Metals And Semico Nductors Using A Plane-wave Basis Set. Comput. Mater. Sci. 1996, 6, 15-50.


20

Page 21 of 24

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


(34) Jiang, D.-e.; Dai, S. The Role of Low-coordinate Oxygen on Co3O4(110) in Catalytic CO Oxidation. Phys. Chem. Chem. Phys. 2011, 13, 978-984. (35) Ren, Y.; Ma, Z.; Qian, L. P.; Dai, S.; He, H. Y.; Bruce, P. G. Ordered Crystalline Mesoporous Oxides as Catalysts for CO Oxidation. Catal. Lett. 2009, 131, 146-154. (36) Razdobarov, V. A.; Sadykov, V. A.; Veniaminov, S. A.; Bulgakov, N. N.; Kovalenko, O. N.; Pankratiev, Y. D.; Popovskii, V. V.; Kryukova, G. N.; Tikhov, S. F. Nature of The Active Oxygen of Co3O4. React. Kinet. Catal. Lett. 1988, 37, 109-114. (37) Pollard, M. J.; Weinstock, B. A.; Bitterwolf, T. E.; Griffiths, P. R.; Newbery, A. P.; Paine, J. B. A Mechanistic Study of The Low-temperature Conversion of Carbon Monoxide to Carbon Dioxide over a Cobalt Oxide Catalyst. J. Catal. 2008, 254, 218-225. (38) Petitto, S. C.; Langell, M. A. Surface Composition and Structure of Co3O4(110) and The Effect of Impurity Segregation . J Vac. Sci. Tech. A 2004, 22, 1690-1696. (39) Radin, M. D.; Rodriguez, J. F.; Tian, F.; Siegel, D. J. Lithium Peroxide Surfaces Are Metallic, While Lithium Oxide Surfaces Are Not. J. Am. Chem. Soc. 2012, 134, 1093-1103. (40) Chase, M. W. American Institute of Physics: Melville, NY 1998. (41) Guo, X.; Zhao, N. The Role of Charge Reactions in Cyclability of Lithium-Oxygen Batteries. Adv. Energy Mater. 2013, 3, 1413-1416. (42) Fan, W.; Guo, X.; Xiao, D.; Gu, L. Influence of Gold Nanoparticles Anchored to Carbon Nanotubes on Formation and Decomposition of Li2O2 in Nonaqueous Li–O2 Batteries. J. Phy. Chem. C 2014, 118, 73447350. (43) Xu, J. J.; Wang, Z. L.; Xu, D.; Zhang, L. L.; Zhang, X. B. Tailoring Deposition and Morphology of Discharge Products Towards High-Rate and Long-Life Lithium-Oxygen Batteries. Nat. Commun. 2013, 4, 2438.


21


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

Page 22 of 24

(44) Gerbig, O.; Merkle, R.; Maier, J. Electron and Ion Transport in Li2O2. Adv. Mater. 2013, 25, 31293133. (45) Ma, S.; Sun, L.; Cong, L.; Gao, X.; Yao, C.; Guo, X.; Tai, L.; Mei, P.; Zeng, Y.; Xie, H.; et al. Multiporous MnCo2O4 Microspheres as an Efficient Bifunctional Catalyst for Nonaqueous Li–O2 Batteries. J. Phy. Chem. C 2013, 117, 25890-25897. (46) Wang, J.; Li, Y.; Sun, X. Challenges and Opportunities of Nanostructured Materials for Aprotic Rechargeable Lithium–Air Batteries. Nano Energy 2013, 2, 443-467. (47) Dong, S.; Wang, S.; Guan, J.; Li, S.; Lan, Z.; Chen, C.; Shang, C.; Zhang, L.; Wang, X.; Gu, L.; et al. Insight into Enhanced Cycling Performance of Li–O2 batteries Based on Binary CoSe2/CoO Nanocomposite Electrodes. J. Phy. Chem. Lett. 2014, 5, 615-621. (48) Truong, T. T.; Liu, Y.; Ren, Y.; Trahey, L.; Sun, Y. Morphological and Crystalline Evolution of Nanostructured MnO2 and Its Application in Lithium–Air Batteries. ACS nano 2012, 6, 8067-8077. (49) Kim, K. S.; Park, Y. J. Catalytic Properties of Co3O4 Nanoparticles for Rechargeable Li/Air Batteries. Nanoscale Res. Lett. 2012, 7, 47. (50) Song, K.; Jung, J.; Heo, Y. U.; Lee, Y. C.; Cho, K.; Kang, Y. M., Alpha-MnO2 Nanowire Catalysts with Ultra-High Capacity and Extremely Low Overpotential in Lithium-Air Batteries through Tailored Surface Arrangement. Phys. Chem. Chem. Phys. 2013, 15, 20075-20079. (51) Xiao, J.; Kuang, Q.; Yang, S.; Xiao, F.; Wang, S.; Guo, L. Surface Structure Dependent Electrocatalytic Activity of Co3O4 Anchored on Graphene Sheets toward Oxygen Reduction Reaction. Sci. Rep. 2013, 3, 2300. (52) Guo, S.; Zhang, S.; Wu, L.; Sun, S., Co/CoO Nanoparticles Assembled on Graphene for Electrochemical Reduction of Oxygen. Angew. Chem. Int. Ed. 2012, 51, 11770-11773.


22

Page 23 of 24

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


(53) Liang, Y.; Wang, H.; Diao, P.; Chang, W.; Hong, G.; Li, Y.; Gong, M.; Xie, L.; Zhou, J.; Wang, J.; et al. Oxygen Reduction Electrocatalyst Based on Strongly Coupled Cobalt Oxide Nanocrystals and Carbon Nanotubes. J. Am. Chem. Soc. 2012, 134, 15849-15857. (54) Sun, B.; Liu, H.; Munroe, P.; Ahn, H.; Wang, G. Nanocomposites of CoO and a Mesoporous Carbon (Cmk-3) as a High Performance Cathode Catalyst for Lithium-Oxygen Batteries. Nano Res. 2012, 5, 460-469. (55) Mitchell, R. R.; Gallant, B. M.; Shao-Horn, Y.; Thompson, C. V. Mechanisms of Morphological Evolution of Li2O2 particles During Electrochemical Growth. J. Phy. Chem. Lett. 2013, 4, 1060-1064. (56) Rossmeisl, J.; Nørskov, J. K.; Taylor, C. D.; Janik, M. J.; Neurock, M. Calculated Phase Diagrams for the Electrochemical Oxidation and Reduction of Water over Pt(111). J. Phys. Chem. B 2006, 110, 2183321839. (57) Mo, Y.; Ong, S. P.; Ceder, G. First-Principles Study of the Oxygen Evolution Reaction of Lithium Peroxide in the Lithium-Air Battery. Phys. Revi. B 2011, 84, 205446. (58) Hummelshoj, J. S.; Luntz, A. C.; Norskov, J. K. Theoretical Evidence for Low Kinetic Overpotentials in Li-O2 Electrochemistry. J. Chem. Phys. 2013, 138, 034703. (59) Ren, X.; Zhu, J.; Du, F.; Liu, J.; Zhang, W. B-Doped Graphene as Catalyst to Improve Charge Rate of Lithium–Air Battery. J. Phy. Chem. C 2014, 118, 22412-22418.


23


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

Page 24 of 24

Table of Contents


24

Facet-Dependent Electrocatalytic Performance of Co3O4 for

Recommend Documents