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reduced graphene oxide as bi-functional catalyst for Li-O2 battery with improved performances
Accepted Manuscript Mesoporous CoO/reduced graphene oxide as bi-functional catalyst for Li-O2 battery with improved performances Zhonghai Song, Xue Qin, Ning Cao, Xuejiao Gao, Qiu Liang, Yanfang Huo PII:
S0254-0584(17)30799-X
DOI:
10.1016/j.matchemphys.2017.10.017
Reference:
MAC 20056
To appear in:
Materials Chemistry and Physics
Received Date: 7 January 2017 Revised Date:
20 July 2017
Accepted Date: 7 October 2017
Please cite this article as: Z. Song, X. Qin, N. Cao, X. Gao, Q. Liang, Y. Huo, Mesoporous CoO/reduced graphene oxide as bi-functional catalyst for Li-O2 battery with improved performances, Materials Chemistry and Physics (2017), doi: 10.1016/j.matchemphys.2017.10.017. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Mesoporous CoO/reduced graphene oxide as bi-functional catalyst for Li-O2 battery with improved performances Zhonghai Song a, Xue Qina *, Ning Cao a, Xuejiao Gao a, Qiu Liang a, Yanfang Huo b Department of Chemistry, School of Science, Tianjin University, and Collaborative Innovation
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a
Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China b
Tianjin Key Laboratory of Biosensing and Molecular Recognition, College of Chemistry, Nankai
University, Tianjin 300071, China
E-mail address: [email protected]
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Corresponding Author Tel.: +86 022 27403670
Abstract
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Wheat-like mesoporous CoO nanorods grown on the reduced graphene oxide (CoO/rGO) is synthesized by a simple hydrothermal method. Owing to synergistic effect between CoO and rGO, the CoO/rGO hybrid exhibits a good initial capacity of 20254 mAh g-1 along with a high coulombic efficiency (98.9%) at 200 mA g-1. In addition, the batteries show an excellent rate capability (13952 mAh g-1 at 800 mA g-1) and enhanced cycling stability (69 cycles with the
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capacity limited to 1000 mAh g-1 at 200 mA g-1). The electrochemical performance is intimately related to the unique architecture (i.e., hierarchical mesoporous structure), facilitating the reversible formation and decomposition of insoluble Li2O2. The results of electrochemical tests
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confirm that the CoO/rGO hybrid is a promising candidate for the Li-O2 batteries.
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Graphical abstract
Highlights Wheat-like mesoporous CoO nanorods are prepared and investigated in Li-O2 batteries. High initial capacity (20254 mAh g-1) and cycle stability (69 cycles) are shown. 1
ACCEPTED MANUSCRIPT Low polarization and long discharge/charge plateau are exhibited. 13952 mAh g-1 is maintained even at a current density of 800 mA g-1.
Abbreviations rGO, reduced graphene oxide; EVs, electric vehicles; ORR, oxygen reduction reaction; OER,
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oxygen evolution reaction; CoO/rGO, hierarchical mesoporous CoO nanorods grown on the reduced graphene oxide; XRD, X-ray diffraction; XPS, X-ray photoelectron spectroscopy; SEM, scanning electron microscopy; TEM, transmission electron microscopy; HRTEM, high resolution transmission electron microscopy; BET, Brunauer-Emmet-Teller adsorption; CV, cyclic
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voltammetry; GNs, graphene nanosheets.
Keywords:
1.
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Reduced graphene oxide; Mesoporous CoO nanorods; Synergistic effect; Li-O2 batteries
Introduction
The rechargeable non-aqueous Li-O2 batteries have aroused intensive research interests because of the high theoretical energy density (5200 Wh kg-1) and the property of environmental
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friendliness [1-3]. However, the Li-O2 batteries have also suffered from the sluggish kinetics of the oxygen reduction reaction (ORR, 2Li++2O2→Li2O2) and the oxygen evolution reaction (OER, Li2O2→O2+2Li+) during the discharging and charging process, resulting in the low coulombic efficiency, poor rate capability, short cycle life and high charge-discharge voltage gap [4-7], and
considerable
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thus constraining the practical applications of Li-O2 batteries. To solve the above problems, efforts
have been intensively
devoted
to design
and
synthesize
the
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bifunctional catalysts for Li-O2 batteries, including noble metals [8-9], transition metal oxides [10-12], carbonaceous materials [13-15], and a few others [16-18]. However, identifying an ideal catalyst is still a great challenge nowadays. It has been well established that the reversible formation and decomposition of insoluble Li2O2 have a significant influence on the performance of Li-O2 batteries because deposited Li2O2 can block the diffusion pathways and deactivate the catalytic sites, consequently, leading to a high overpotential and a limited discharge capacity. The reversibility of Li2O2 is associated with the architecture of O2 electrode in that the special porosity structure can favor the transportation of Li+, O2 and electron as well as provide enough areas of triple-phase boundaries (i.e., 2
ACCEPTED MANUSCRIPT oxygen-catalyst-electrolyte). Among a variety of reported catalysts, mesoporous Co-containing oxides have been considered to be the promising catalysts in view of their distinctive porous structure, which can not only facilitate the effective diffusion of oxygen and the migration of Li+ and electron, but also can prevent the accumulation of the discharge product on the surface of the
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electrode and provide enough space for the accommodation of insoluble Li2O2. Wu et al. [19] successfully synthesized CoO mesoporous nanowire array and delivered an excellent discharge specific capacity (4888 mAh g-1) and cycling efficiency (50 cycles). Gao et al. [20] reported that the oxygen-deficient CoO-A based cathode could show a higher coulombic
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efficiency and a lower charge-discharge overpotential. The results fully affirm the ORR and OER catalytic activities of mesoporous CoO.
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Except to the intrinsic catalytic activity of catalyst, the performance of Li-O2 batteries is also determined by the electroconductivity of catalyst. In order to enhance the transfer rate of charge, hybridizations of carbon materials have been utilized. Gao et al. improved the catalytic performance of CoO through the integration with the dotted carbon species, and, as a result, exhibited an enhanced performance compared to bare CoO [20,21]. Zhang et al. reported that
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CoO/BMC based cathode could show a higher initial capacity (4000 mAh g-1) and a lower overpotential [22]. Inspired by these research results, we dedicate ourselves to find a better carbon material to combine with the CoO.
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It is generally known that graphene has become a promising candidate for carbon substrate due to its extraordinary electronic conductivity, large surface area, controllable surface defects and
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easy functionalization by decorating with catalyst nanoparticles [23-26]. Graphene acting as the scaffold enormously hinders the agglomeration of metal oxide nanorods, thereby, enhancing active sites. Furthermore, the growth of metal oxide nanorods on graphene could prevent the restacking of graphene and increase the interspaces between layers, giving rise to a higher active surface area as well as larger macropores between graphene layers, thereby, to promote the diffusion of oxygen and the penetration of electrolytes [27,28]. The ideal air electrodes have the following characteristics: (1) the excellent electronic conductivity for the electrochemical reaction; (2) enough space to accommodate the insoluble discharge products of Li2O2; (3) favoring the diffusion of O2 and Li+ as well as the immersion of electrolyte. 3
ACCEPTED MANUSCRIPT Herein, we report a facile approach to synthesize highly loaded, well-dispersed CoO/rGO nanocomposites by hydrothermal method. Furthermore, this work is the first to synthesize the Wheat-like CoO nanorod, composed of a number of coadjacent nanoparticles, and disclose the influence of the structural compactness on the reversibility of a conversion electrode. Because of
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the synergistic effect between mesoporous CoO nanorod and rGO, the Li-O2 batteries shows an excellent initial discharge capacity, good cycling stability and superior rate performance. Finally, the CoO/rGO nanocomposite is employed in carbon-based oxygen electrodes, resulting in an excellent electrochemical performance, such as a significant reduction of over-potentials (up to
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320 mV) and improved cycling performance (69 cycles) in Li–O2 batteries. The results of electrochemical tests indicate that the synergistic effect between the unique structure of CoO and
2. Experimental section 2.1.
Materials preparation
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rGO endows the CoO/ rGO with a promising candidate for the Li-O2 batteries.
The CoO/rGO nanocomposite was prepared through a modified hydrothermal process [19]. Graphite oxide (GO) was synthesized via a modified Hummers’ method [29]. In detail, 30 mg of
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dried GO was firstly dissolved in 40 mL of deionized water and then sonicated for 30 min to obtain a suspension. In the second step, 0.582 g (2 mmol) of Co(NO3)2·6H2O and 0.8 g (10 mmol) of CO(NH2)2 were dissolved in the above suspension under continuous stirring for 30 min at room
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temperature. Subsequently, the prepared homogeneous solution was transferred into a 50 mL Teflon-lined stainless steel autoclave reactor and heated at 120°C for 10 h. Finally, the product
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was washed by deionized water and ethanol several times, vacuum dried and then annealed at 500°C for 2h in argon. For comparison, the pure CoO was prepared by the absence of graphite oxide with the other conditions of the experiment unchanged.
2.2.
Materials characterization
The crystal structures of pure CoO, CoO/rGO composite and discharge products were characterized by X-ray diffraction (XRD, D/MAX-2500), Raman spectroscopy (In Via Rflex). The electronic states of the samples were determined by X-ray photoelectron spectroscopy (XPS, PHI1600). The scanning electron microscopy (SEM, S4800) and transmission electron microscopy (TEM, Tecnai G2 F20) were employed to observe the morphology and detailed 4
ACCEPTED MANUSCRIPT structure of the samples as well as the lattice structure was recorded via high resolution transmission electron microscopy (HRTEM). The Brunauer-Emmet-Teller (BET) method was utilized to calculate the specific surface area of the pure CoO and CoO/rGO composite.
2.3.
Li-O2 cell assembly and electrochemical measurements
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The electrochemical properties of the samples were studied by using Swagelok-type cells, which were assembled in an argon-filled glove-box, composed of a metallic lithium anode, an oxygen cathode, a glass-fibre separator (GF/C, Whatman) and the electrolyte (1.0 M LiTFSI/TEGDME). The cathodes were prepared as follows: CoO/rGO as catalyst, binder (PVDF)
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and super P carbon as a conductive agent were mixed with NMP to manufacture a catalyst slurry. The mixed slurry was then coated on the carbon paper current collector to prepare the porous air
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electrode with a loading of about 1.2±0.05mg cm-2, which was then vacuum-dried at 120℃ for 12 h.
The cyclic voltammetry (CV) profiles were recorded using a Zahner Ennium electrochemical workstation at a scan rate of 0.1 mV s-1 between 2.0 and 4.5 V. The galvanostatic discharge-charge curves were tested at a current density of 200 mA g-1 with a restriction of the capacity to 1000
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mAh g-1. The rate capability was tested at the various current densities (200, 400, 800 mA g-1).
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3. Results and discussion
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Fig. 1. Schematic illustration of the synthesis of CoO/rGO using a two-step method as well as the possible catalytic mechanism of CoO toward ORR and OER.
Figure 1 illustrates schematically the growth of CoO on reduced graphene oxide (rGO) using a simple two-step approach. During the hydrothermal process, precursors of CoO are highly attached on the surface of graphene oxide through the interfacial interaction between functional groups and active particles. Subsequently, the graphene oxides are reduced to graphene nanosheets (GNs) during the calcination process, along with the conversion of precursors into CoO nanorods. Obviously, the CoO nanorods consisting of numerous nanoparticles are dispersed disorderly on the surface of rGO, which are in favor of accelerating the kinetics of ORR and OER.
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Fig. 2. (a) XRD patterns for the CoO/rGO and bare CoO; (b) Raman spectra for the CoO/rGO, pristine GNs and
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bare CoO; XPS spectra for the CoO/rGO composite: (c) C 1s and (d) Co 2p spectra.
The X-ray diffraction (XRD) patterns of bare CoO, CoO/rGO composite are presented in Fig. 2a. The five obvious diffraction peaks of CoO/rGO at 2θ values of 36.5° (111), 42.4° (200), 61.5°
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(220), 73.7° (311), and 77.5° (222) are in accordance with the XRD data of the cubic CoO (JCPDS Card No. 48-1719). These demonstrate that the crystal structure of CoO is not altered
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after hybridization with rGO nanosheets. The peak of the graphene at 24-28° is not observed, suggesting that graphene oxide have been reduced to the graphene nanosheets in the reaction and the stacking of graphene sheets in the CoO/rGO composite is disordered [30]. Fig. 2b shows the Raman spectrum of the as-prepared sample. The peaks from CoO/rGO nanocomposite located at 191, 594, 473 and 680 cm-1 correspond, respectively, to 2F2g, 1Eg and 1A 1g, which is consistent with that of bare CoO. Raman spectrum of CoO/rGO nanocomposite displays a G band (1596 cm−1) which is usually associated with the vibration of sp2-bonded carbon atoms in a 2-dimensional hexagonal lattice and a D band (1350 cm−1) corresponding to edge planes and disordered structures. More importantly, the D/G intensity ratio of the composite (ID/IG=1.33) is larger than that of the graphene oxide (ID/IG=0.89) [31,32], which indicates a 6
ACCEPTED MANUSCRIPT decrease in the average size of the sp2 domains upon reduction of the exfoliated graphene oxide and an increase of the amount of defect sites, which can act as nucleation sites for CoO particle growth. We also implement the X-ray photoelectron spectroscopy (XPS) analysis to further confirm the existence of Co, O and C elements and the valence state of Co. In Fig. 2c, graphene
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shows three different C 1s building energies, 284.6 eV for non-oxygenated C, 286.1 eV for carbon in C-O and 288.1 eV for carbonyl carbon (C=O), respectively [33]. The peak of C-O species is quite weak, which indicates the oxygen containing groups have been removed during the hydrothermal process and demonstrates the formation of graphene. The peaks at 780.7 eV and
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796.4 eV arise from the binding energy of Co 2p 3/2 and Co 2p 1/2, respectively, and the shake-up satellite peaks centered at 785.6 eV and 802.8 eV indicate the existence of Co2+ [34], shown in Fig.
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2d. The above-mentioned XRD, Raman and XPS physical characterizations perfectly verify the
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successful fabrication of the CoO/rGO hybrid.
Fig. 3. SEM images of bare CoO (a, b) and the CoO/rGO (c, d) at different magnifications
The morphologies and microstructures of the synthesized powders are observed by scanning electron microscopy. The SEM images (Fig. 3a and b) of bare CoO nanorods show that the special Wheat-like structures, composed of a number of coadjacent nanoparticles, have the diameters of about 60 nm and the lengths of approximately 2 micrometers. From the Fig. 3c and d, the CoO 7
ACCEPTED MANUSCRIPT nanorods constructed of nano-sized particles are distributed randomly throughout the surface of rGO. For obtained CoO/rGO composite: (1) The rGO nanosheets enhance electrical conductivity of CoO nanoparticles networks; (2) The random distribution of CoO nanorods not
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only provides more catalytically active sites but also increases the space between rGO nanosheets.
Fig. 4. (a) TEM image of CoO; (b-d) TEM images of CoO/rGO at different magnifications and the corresponding SAED pattern (inset in figure 4d).
Transmission electron microscopy is employed to further examine the detailed structures of the obtained samples. As shown in figure 4b, thanking to the role of rGO, CoO nanorods are distributed on the surface of rGO in a more disorderly state compared with the bare CoO (Fig. 4a), which can provide more active sites and sufficient reaction interfaces composed of O2-catalyst-electrotyte solution, More importantly, every typical CoO nanorod is consisted of a number of interconnected nanoparticles (about 15 nm in diameter) with some spacing, which is 8
ACCEPTED MANUSCRIPT conducive to the efficient electrolyte penetration, oxygen transport and the diffusion of Li+ as well as can store the insoluble discharge product Li2O2. The HRTEM images (Fig. 4c, d) clearly reveal that CoO nanorod anchored on rGO is comprised of many well crystallined nanoparticles with lattice spacing of about 0.246 and 0.213 nm, which are in well accordance with the (111) and (200)
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planes of the cubic CoO structure, respectively. The selected area electron diffraction (SAED) pattern (inset of figure 4d) show that the CoO/rGO is actually poly-crystalline and correspond to
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cubic CoO.
Fig. 5. Nitrogen adsorption-desorption isotherms and pore size distribution (inset) of the bare CoO (a) and the CoO/rGO hybrid (b).
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In order to further confirm the mesoporous structure, the specific surface area and pore size distribution of CoO before and after combination with graphene are characterized by N2 adsorption-desorption isotherm measurements. According to the BET analysis (Fig. 5a and b), the CoO/rGO composite possesses a larger surface area of 233 m2 g-1 compared with the bare CoO
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(92 m2 g-1), which can offer more electrochemical active sites for ORR and OER. As shown in the
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Fig. S1., the surface area of rGO is also tested and the surface area of the bare CoO is 133.2 m2 g-1, which is smaller than that of rGO in the CoO/rGO composite. More interestingly, the typical IV isotherms with a hysteresis loop are in good agreement with the mesoporous structure constructed of numerous well-crystallized nanoparticles in a nanorod [17,19]. The BJH adsorption isotherms (the insets in Fig. 5a and 5b) manifest that the pore size of CoO/rGO hybrid (7-15nm) is slightly larger than that of bare CoO (6-8nm), consistent with the results of SEM and TEM, facilitating the diffusion of oxygen and Li+ as well as the reservation of the discharge product (Li2O2). The results of SEM, TEM and BET can testify that rGO contributes to the surface parameters like surface area and pore size distribution as well as pore volume.
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Fig. 6. CV curves of three cathodes recorded in O2-saturated electrolytes at a scan rate of 0.1 mV s-1.
The cyclic voltammetry (CV) curves (Fig. 6) are utilized to examine the electrochemical catalytic activities of three cathode materials. The CoO/rGO cathode exhibits a most positive ORR
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onset potential(～3.1V) as well as a higher peak current in comparison with the graphene and CoO cathode, corroborating the excellent ORR kinetics. Compared with the electrode with the graphene or CoO, the composite shows an apparent oxidation peak and a higher current density, indicating
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the outstanding OER activity during anodic scan and the synergistic effect between CoO and rGO. These evidences confirm that CoO/rGO composite is a bi-functional catalyst for rechargeable
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Li-O2 batteries.
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Fig. 7. (a) First discharge-charge curves of Li-O2 cells catalyzed by rGO, CoO and CoO/rGO composite at a current density of 200 mA g-1; (b) Variations in discharge capacity and coulombic efficiency for Li-O2 batteries with three cathode catalysts for six cycles; (c) The discharge-charge profiles of CoO/rGO based cathode with a limited capacity of 1000 mAh g-1 at a current density of 200 mA g-1; (d) Rate performance of the Li-O2 battery with CoO/rGO at various current densities.
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The electrocatalytic activities of samples are studied in rechargeable Li-O2 batteries and the capacity are calculated on the basis of the weight of catalysts. The initial discharge-charge profiles of Li-O2 batteries assembled with CoO/rGO, CoO and rGO cathodes are evaluated at a current
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density of 200 mA g-1 with the cutoff voltage at 2.0 and 4.5V (versus Li/Li+), shown in Fig. 7a. The cathode catalyzed by CoO/rGO not only delivers a highest discharge capacity of 20254 mAh
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g-1 in comparison with bare CoO (6222 mAh g-1) and rGO (6500 mAh g-1) but also exhibits a high coulombic efficiency (98.9%), which is superior to the commercial Pt/C catalysts [27]. The reason may be that numerous accessible mesoporous of CoO and large surface areas can efficiently accommodate the insoluble discharging product (Li2O2), which prevents the discharging product from blocking the cathode, and thereby, insure the lithium to react continuously with oxygen on the cathode surface, bringing about a high capacity. In addition, most of discharge capacity is delivered above 2.55V and over half of charge capacity is delivered below 4.15V. And also importantly, the discharge plateau of CoO/rGO is 2.64V and the charge overpotential is reduced by 320mV compared with rGO, implying the higher catalytic activity for ORR and OER. This 11
ACCEPTED MANUSCRIPT may be related to the cause that the mesoporous structure can provides sufficient triple-phase boundaries to promote the rate of Li+ transport and O2 diffusion, consequently, reducing the polarization. The cyclic performances and the corresponding coulombic efficiencies of the cathodes
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catalyzed by three samples are shown in Fig. 7b and the detailed discharge-charge curves of Li-O2 batteries assembled with CoO/rGO, CoO and rGO cathodes for the first six cycles are presented in Fig. S2. The discharge capacity of CoO/rGO could maintain at above 10000 mAh g-1 with the average coulombic efficiencies＞90％ from the second cycle to the sixth cycle, indicating a good
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capacity retention, which can be attributed to the synergetic chemical coupling effects between CoO and graphene. The coupling effects is also verified with the result of CV. More importantly,
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the round trip efficiency of CoO/rGO have no significant change for the first six cycles, which can be ascribed to the unique mesoporous structure of CoO/rGO, contributing to the reversible formation and decomposition of Li2O2 ,and the result can be confirmed by that of bare CoO. The cycling stability of CoO/ rGO cathode is further tested at a current density of 200 mA g-1 with the capacity limited to 1000 mAh g-1. As displayed in figure 7c, the terminal discharge
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voltage is still above 2.48V after 69 cycles, indicating the enhanced stability of ORR catalytic activity. It should be noted that the battery have two plateaus during the charging process, which can be attributed to the hierarchical porous structure [35]. In addition, the CoO/rGO catalyst
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delivers a low overpotential (～1.47V) in the middle of the discharge-charge plateaus during the first cycle and the overpotential is only 1.66V for the 50th cycle, which is much lower than those
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of batteries with CoO or CoO-based composite [19-21]. One possible reason for this excellent cycling performance is that the interspace of graphene and the distinctive mesoporous of CoO can facilitate the transportation of oxygen and the electrolyte penetration and thus contribute to accelerate the kinetics of ORR and OER, resulting in the decrease of overpotential. Another possible may be that the distinctive mesoporous of CoO nanorod can accommodate and degrade efficiently the insoluble discharge product together with the preservation of electrochemically available surface area, and thus, could improve the rechargeability of the O2 cathode. Encouraged by the superior cycling performance, the rate capability of Li-O2 batteries using the CoO/rGO cathode catalysts is also examined at different current densities (200, 400, 800 mA g-1) and shown in Fig. 7d. Along with the increment of the current density, cathode polarization 12
ACCEPTED MANUSCRIPT becomes more and more serious, which result in the increase of overpotential and the decrease of specific discharge capacity. However, the capacities of batteries can still maintain at a level of 14879 and 13952 mAh g-1 even at a high current densities of 400 and 800 mA g-1, respectively. Even at a current density of 800 mA g-1, the discharge capacity and the overpotential hardly
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change compared with that of 400 mA g-1. As we know, the varied overpotential in charging and discharging process can arise from different O2 diffusion rates which have something to do with the different pore sizes of mesoporous. Mesoporous provided by the CoO nanorod composed of nanoparticles along with amounts of active sites can promote the adsorption and diffusion of
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oxygen so as to increase the reaction rate.
The above results clearly indicate the superior catalytic activity of CoO/graphene in catalyzing
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the reversible formation and decomposition of the discharge products, which is superior to the
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work as reported before [21,22].
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Fig. 8. SEM images of CoO/rGO cathode after the initial 1st discharge (a) and 1st charge (b).
In order to further make certain the enhanced catalytic activity of CoO/rGO in catalyzing the formation
and
decomposition
of
the
discharge
products,
the
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reversible
surface morphology and chemical composition of the CoO/rGO cathode, before and after the first cycle, are analyzed by SEM and XRD. The SEM image (Fig. 8a) shows that the discharge product are firstly deposited in the space between CoO nanoparticles instead of the surface of CoO nanoparticles, which can contribute to the continuous contact between electrolyte and electrode, thus achieving an excellent initial discharge capacity. As shown in figure 8b, the discharge products disappear and the mesoporous between CoO nanoparticles are recovered during the charge process, indicating the enhanced reversibility of Li-O2 battery assembled with CoO/rGO cathode. The good reversibility of Li-O2 batteries using CoO/GO cathodes is further proved by 13
ACCEPTED MANUSCRIPT XRD analysis (Fig. S3). The peaks corresponding to Li2O2 can be observed after the first discharging process. The characteristic Li2O2 peaks are disappeared after charging process, indicating the decomposition of Li2O2. The above results further confirm the fact that mesoporous CoO play a significant role in catalyzing the reversible formation and decomposition of the
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discharge products.
4. Conclusions
In conclusion, we demonstrate the synthesis of a hybrid of mesoporous CoO functionalized on the rGO by a facile hydrothermal growth, followed by calcination. For the hierarchical
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mesoporous of CoO, the cathode can contact efficiently with the electrolyte and oxygen as well as can provide enough space for Li2O2 storage. As a result, the initial discharge capacity can reach
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20254 mAh g-1 with a low overpotential and the capacity can maintain at 10000 mAh g-1 without obvious polarization from the second to the sixth cycle. Of course, the enhanced performance should also be intimately related to the combined effect of mesoporous CoO and graphene. In a word, we believe that the CoO/rGO will be promising electrocatalysts for accelerating the sluggish kinetics of ORR and OER in Li-O2 batteries.
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Acknowledgements
The authors gratefully acknowledge the financial support from the Open Project of Key Lab
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[15] Y.L. Li, J.J. Wang, X.F. Li, D.S. Geng, R.Y. Li, X.L. Sun, Superior energy capacity of graphene nanosheets for a nonaqueous lithium-oxygen battery, Chem. Commun. 47 (2011) 9438–9440. [16] Y. Luo, F.L. Lu, C. Jin, Y.R. Wang, R.Z. Yang, [email protected] core-shell structured nanorods as efficient electrocatalyst for Li-O2 battery with enhanced performances, J. Power Sources 319 (2016) 19–26. [17] P.X. Wang, L. Shao, N.Q. Zhang, K.N. Sun, Mesoporous CuCo2O4 nanoparticles as an efficient cathode catalyst for Li-O2 batteries, J. Power Sources 325 (2016) 506–512.
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ACCEPTED MANUSCRIPT [18] F.J. Li, D.M. Tang, Y. Chen, D. Golberg, H. Kitaura, T. Zhang, A. Yamada, H.S. Zhou, Ru/ITO: A Carbon-Free Cathode for Nonaqueous Li-O2 Battery, Nano Lett. 13 (2013) 4702–4707. [19] B.S. Wu, H.Z. Zhang, W. Zhou, M.R. Wang, X.F. Li, H.M. Zhang, Carbon-Free CoO mesoporous nanowire array cathode for high-performance aprotic Li-O2 batteries, ACS Appl. Mater. Interfaces. 7 (2015)
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[28] Y.R. Zhong, M. Yang, X.L. Zhou, Z. Zhou, Structural design for anodes of lithium-ion batteries: emerging horizons from materials to electrodes, Mater. Horiz. 2 (2015) 553–566. [29] W.S. Hummers, R.E. Offman, Preparation of Graphitic Oxide, J Am Chem Soc 80 (1958) 1339. [30] N. Cao, L.N. Wen, Z.H. Song, W. Meng, X. Qin, Li4Ti5O12/reduced graphene oxide composite as a high-rate anode material for lithium ion batteries, Electrochim. Acta 209 (2016) 235–243. [31] Y. Kim, Y. Noh, E.J. Lim, S. Lee, S.M. Choi, W.B. Kim, Star-shaped Pd@Pt core-shell catalysts supported on
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[35] J.K. Zhang, P.F. Li, Z.H. Wang, J.S. Qiao, D. Rooney, W. Sun, K.N. Sun. Three-dimensional graphene-Co3O4 cathodes for rechargeable Li–O2 batteries, J. Mater. Chem. A 3 (2015) 1504–1510.
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Biographies
Zhonghai Song is studying for Master’s degree in College of Science at Tianjin University. Xue Qin received his doctor degree (2001) from Nankai University. At present she is a professor in Department of Chemistry at Tianjin University. Her research interests are focused on synthesis and electrochemical studies of composite materials.
Ning Cao graduated from College of Chemistry and Chemical Engineering at Qufu Normal University (2013). He
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is studying for his Master’s degree in College of Science at Tianjin University.
Xuejiao Gao graduated from College of Chemistry at Hebei Normal University (2015). She will receive her Master’s degree (2018) from Tianjin University.
Qiu Liang graduated from College of Chemistry at Henan Normal University (2015). She will receive her
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Master’s degree (2018) from Tianjin University.
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Yanfang Huo is studying for PhD degree in College of Chemistry at Nankai University.
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S0254-0584(17)30799-X
DOI:
10.1016/j.matchemphys.2017.10.017
Reference:
MAC 20056
To appear in:
Materials Chemistry and Physics
Received Date: 7 January 2017 Revised Date:
20 July 2017
Accepted Date: 7 October 2017
Please cite this article as: Z. Song, X. Qin, N. Cao, X. Gao, Q. Liang, Y. Huo, Mesoporous CoO/reduced graphene oxide as bi-functional catalyst for Li-O2 battery with improved performances, Materials Chemistry and Physics (2017), doi: 10.1016/j.matchemphys.2017.10.017. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Mesoporous CoO/reduced graphene oxide as bi-functional catalyst for Li-O2 battery with improved performances Zhonghai Song a, Xue Qina *, Ning Cao a, Xuejiao Gao a, Qiu Liang a, Yanfang Huo b Department of Chemistry, School of Science, Tianjin University, and Collaborative Innovation
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a
Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China b
Tianjin Key Laboratory of Biosensing and Molecular Recognition, College of Chemistry, Nankai
University, Tianjin 300071, China
E-mail address: [email protected]
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Corresponding Author Tel.: +86 022 27403670
Abstract
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Wheat-like mesoporous CoO nanorods grown on the reduced graphene oxide (CoO/rGO) is synthesized by a simple hydrothermal method. Owing to synergistic effect between CoO and rGO, the CoO/rGO hybrid exhibits a good initial capacity of 20254 mAh g-1 along with a high coulombic efficiency (98.9%) at 200 mA g-1. In addition, the batteries show an excellent rate capability (13952 mAh g-1 at 800 mA g-1) and enhanced cycling stability (69 cycles with the
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capacity limited to 1000 mAh g-1 at 200 mA g-1). The electrochemical performance is intimately related to the unique architecture (i.e., hierarchical mesoporous structure), facilitating the reversible formation and decomposition of insoluble Li2O2. The results of electrochemical tests
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confirm that the CoO/rGO hybrid is a promising candidate for the Li-O2 batteries.
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Graphical abstract
Highlights Wheat-like mesoporous CoO nanorods are prepared and investigated in Li-O2 batteries. High initial capacity (20254 mAh g-1) and cycle stability (69 cycles) are shown. 1
ACCEPTED MANUSCRIPT Low polarization and long discharge/charge plateau are exhibited. 13952 mAh g-1 is maintained even at a current density of 800 mA g-1.
Abbreviations rGO, reduced graphene oxide; EVs, electric vehicles; ORR, oxygen reduction reaction; OER,
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oxygen evolution reaction; CoO/rGO, hierarchical mesoporous CoO nanorods grown on the reduced graphene oxide; XRD, X-ray diffraction; XPS, X-ray photoelectron spectroscopy; SEM, scanning electron microscopy; TEM, transmission electron microscopy; HRTEM, high resolution transmission electron microscopy; BET, Brunauer-Emmet-Teller adsorption; CV, cyclic
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voltammetry; GNs, graphene nanosheets.
Keywords:
1.
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Reduced graphene oxide; Mesoporous CoO nanorods; Synergistic effect; Li-O2 batteries
Introduction
The rechargeable non-aqueous Li-O2 batteries have aroused intensive research interests because of the high theoretical energy density (5200 Wh kg-1) and the property of environmental
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friendliness [1-3]. However, the Li-O2 batteries have also suffered from the sluggish kinetics of the oxygen reduction reaction (ORR, 2Li++2O2→Li2O2) and the oxygen evolution reaction (OER, Li2O2→O2+2Li+) during the discharging and charging process, resulting in the low coulombic efficiency, poor rate capability, short cycle life and high charge-discharge voltage gap [4-7], and
considerable
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thus constraining the practical applications of Li-O2 batteries. To solve the above problems, efforts
have been intensively
devoted
to design
and
synthesize
the
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bifunctional catalysts for Li-O2 batteries, including noble metals [8-9], transition metal oxides [10-12], carbonaceous materials [13-15], and a few others [16-18]. However, identifying an ideal catalyst is still a great challenge nowadays. It has been well established that the reversible formation and decomposition of insoluble Li2O2 have a significant influence on the performance of Li-O2 batteries because deposited Li2O2 can block the diffusion pathways and deactivate the catalytic sites, consequently, leading to a high overpotential and a limited discharge capacity. The reversibility of Li2O2 is associated with the architecture of O2 electrode in that the special porosity structure can favor the transportation of Li+, O2 and electron as well as provide enough areas of triple-phase boundaries (i.e., 2
ACCEPTED MANUSCRIPT oxygen-catalyst-electrolyte). Among a variety of reported catalysts, mesoporous Co-containing oxides have been considered to be the promising catalysts in view of their distinctive porous structure, which can not only facilitate the effective diffusion of oxygen and the migration of Li+ and electron, but also can prevent the accumulation of the discharge product on the surface of the
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electrode and provide enough space for the accommodation of insoluble Li2O2. Wu et al. [19] successfully synthesized CoO mesoporous nanowire array and delivered an excellent discharge specific capacity (4888 mAh g-1) and cycling efficiency (50 cycles). Gao et al. [20] reported that the oxygen-deficient CoO-A based cathode could show a higher coulombic
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efficiency and a lower charge-discharge overpotential. The results fully affirm the ORR and OER catalytic activities of mesoporous CoO.
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Except to the intrinsic catalytic activity of catalyst, the performance of Li-O2 batteries is also determined by the electroconductivity of catalyst. In order to enhance the transfer rate of charge, hybridizations of carbon materials have been utilized. Gao et al. improved the catalytic performance of CoO through the integration with the dotted carbon species, and, as a result, exhibited an enhanced performance compared to bare CoO [20,21]. Zhang et al. reported that
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CoO/BMC based cathode could show a higher initial capacity (4000 mAh g-1) and a lower overpotential [22]. Inspired by these research results, we dedicate ourselves to find a better carbon material to combine with the CoO.
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It is generally known that graphene has become a promising candidate for carbon substrate due to its extraordinary electronic conductivity, large surface area, controllable surface defects and
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easy functionalization by decorating with catalyst nanoparticles [23-26]. Graphene acting as the scaffold enormously hinders the agglomeration of metal oxide nanorods, thereby, enhancing active sites. Furthermore, the growth of metal oxide nanorods on graphene could prevent the restacking of graphene and increase the interspaces between layers, giving rise to a higher active surface area as well as larger macropores between graphene layers, thereby, to promote the diffusion of oxygen and the penetration of electrolytes [27,28]. The ideal air electrodes have the following characteristics: (1) the excellent electronic conductivity for the electrochemical reaction; (2) enough space to accommodate the insoluble discharge products of Li2O2; (3) favoring the diffusion of O2 and Li+ as well as the immersion of electrolyte. 3
ACCEPTED MANUSCRIPT Herein, we report a facile approach to synthesize highly loaded, well-dispersed CoO/rGO nanocomposites by hydrothermal method. Furthermore, this work is the first to synthesize the Wheat-like CoO nanorod, composed of a number of coadjacent nanoparticles, and disclose the influence of the structural compactness on the reversibility of a conversion electrode. Because of
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the synergistic effect between mesoporous CoO nanorod and rGO, the Li-O2 batteries shows an excellent initial discharge capacity, good cycling stability and superior rate performance. Finally, the CoO/rGO nanocomposite is employed in carbon-based oxygen electrodes, resulting in an excellent electrochemical performance, such as a significant reduction of over-potentials (up to
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320 mV) and improved cycling performance (69 cycles) in Li–O2 batteries. The results of electrochemical tests indicate that the synergistic effect between the unique structure of CoO and
2. Experimental section 2.1.
Materials preparation
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rGO endows the CoO/ rGO with a promising candidate for the Li-O2 batteries.
The CoO/rGO nanocomposite was prepared through a modified hydrothermal process [19]. Graphite oxide (GO) was synthesized via a modified Hummers’ method [29]. In detail, 30 mg of
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dried GO was firstly dissolved in 40 mL of deionized water and then sonicated for 30 min to obtain a suspension. In the second step, 0.582 g (2 mmol) of Co(NO3)2·6H2O and 0.8 g (10 mmol) of CO(NH2)2 were dissolved in the above suspension under continuous stirring for 30 min at room
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temperature. Subsequently, the prepared homogeneous solution was transferred into a 50 mL Teflon-lined stainless steel autoclave reactor and heated at 120°C for 10 h. Finally, the product
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was washed by deionized water and ethanol several times, vacuum dried and then annealed at 500°C for 2h in argon. For comparison, the pure CoO was prepared by the absence of graphite oxide with the other conditions of the experiment unchanged.
2.2.
Materials characterization
The crystal structures of pure CoO, CoO/rGO composite and discharge products were characterized by X-ray diffraction (XRD, D/MAX-2500), Raman spectroscopy (In Via Rflex). The electronic states of the samples were determined by X-ray photoelectron spectroscopy (XPS, PHI1600). The scanning electron microscopy (SEM, S4800) and transmission electron microscopy (TEM, Tecnai G2 F20) were employed to observe the morphology and detailed 4
ACCEPTED MANUSCRIPT structure of the samples as well as the lattice structure was recorded via high resolution transmission electron microscopy (HRTEM). The Brunauer-Emmet-Teller (BET) method was utilized to calculate the specific surface area of the pure CoO and CoO/rGO composite.
2.3.
Li-O2 cell assembly and electrochemical measurements
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The electrochemical properties of the samples were studied by using Swagelok-type cells, which were assembled in an argon-filled glove-box, composed of a metallic lithium anode, an oxygen cathode, a glass-fibre separator (GF/C, Whatman) and the electrolyte (1.0 M LiTFSI/TEGDME). The cathodes were prepared as follows: CoO/rGO as catalyst, binder (PVDF)
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and super P carbon as a conductive agent were mixed with NMP to manufacture a catalyst slurry. The mixed slurry was then coated on the carbon paper current collector to prepare the porous air
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electrode with a loading of about 1.2±0.05mg cm-2, which was then vacuum-dried at 120℃ for 12 h.
The cyclic voltammetry (CV) profiles were recorded using a Zahner Ennium electrochemical workstation at a scan rate of 0.1 mV s-1 between 2.0 and 4.5 V. The galvanostatic discharge-charge curves were tested at a current density of 200 mA g-1 with a restriction of the capacity to 1000
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mAh g-1. The rate capability was tested at the various current densities (200, 400, 800 mA g-1).
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3. Results and discussion
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Fig. 1. Schematic illustration of the synthesis of CoO/rGO using a two-step method as well as the possible catalytic mechanism of CoO toward ORR and OER.
Figure 1 illustrates schematically the growth of CoO on reduced graphene oxide (rGO) using a simple two-step approach. During the hydrothermal process, precursors of CoO are highly attached on the surface of graphene oxide through the interfacial interaction between functional groups and active particles. Subsequently, the graphene oxides are reduced to graphene nanosheets (GNs) during the calcination process, along with the conversion of precursors into CoO nanorods. Obviously, the CoO nanorods consisting of numerous nanoparticles are dispersed disorderly on the surface of rGO, which are in favor of accelerating the kinetics of ORR and OER.
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Fig. 2. (a) XRD patterns for the CoO/rGO and bare CoO; (b) Raman spectra for the CoO/rGO, pristine GNs and
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bare CoO; XPS spectra for the CoO/rGO composite: (c) C 1s and (d) Co 2p spectra.
The X-ray diffraction (XRD) patterns of bare CoO, CoO/rGO composite are presented in Fig. 2a. The five obvious diffraction peaks of CoO/rGO at 2θ values of 36.5° (111), 42.4° (200), 61.5°
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(220), 73.7° (311), and 77.5° (222) are in accordance with the XRD data of the cubic CoO (JCPDS Card No. 48-1719). These demonstrate that the crystal structure of CoO is not altered
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after hybridization with rGO nanosheets. The peak of the graphene at 24-28° is not observed, suggesting that graphene oxide have been reduced to the graphene nanosheets in the reaction and the stacking of graphene sheets in the CoO/rGO composite is disordered [30]. Fig. 2b shows the Raman spectrum of the as-prepared sample. The peaks from CoO/rGO nanocomposite located at 191, 594, 473 and 680 cm-1 correspond, respectively, to 2F2g, 1Eg and 1A 1g, which is consistent with that of bare CoO. Raman spectrum of CoO/rGO nanocomposite displays a G band (1596 cm−1) which is usually associated with the vibration of sp2-bonded carbon atoms in a 2-dimensional hexagonal lattice and a D band (1350 cm−1) corresponding to edge planes and disordered structures. More importantly, the D/G intensity ratio of the composite (ID/IG=1.33) is larger than that of the graphene oxide (ID/IG=0.89) [31,32], which indicates a 6
ACCEPTED MANUSCRIPT decrease in the average size of the sp2 domains upon reduction of the exfoliated graphene oxide and an increase of the amount of defect sites, which can act as nucleation sites for CoO particle growth. We also implement the X-ray photoelectron spectroscopy (XPS) analysis to further confirm the existence of Co, O and C elements and the valence state of Co. In Fig. 2c, graphene
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shows three different C 1s building energies, 284.6 eV for non-oxygenated C, 286.1 eV for carbon in C-O and 288.1 eV for carbonyl carbon (C=O), respectively [33]. The peak of C-O species is quite weak, which indicates the oxygen containing groups have been removed during the hydrothermal process and demonstrates the formation of graphene. The peaks at 780.7 eV and
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796.4 eV arise from the binding energy of Co 2p 3/2 and Co 2p 1/2, respectively, and the shake-up satellite peaks centered at 785.6 eV and 802.8 eV indicate the existence of Co2+ [34], shown in Fig.
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2d. The above-mentioned XRD, Raman and XPS physical characterizations perfectly verify the
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successful fabrication of the CoO/rGO hybrid.
Fig. 3. SEM images of bare CoO (a, b) and the CoO/rGO (c, d) at different magnifications
The morphologies and microstructures of the synthesized powders are observed by scanning electron microscopy. The SEM images (Fig. 3a and b) of bare CoO nanorods show that the special Wheat-like structures, composed of a number of coadjacent nanoparticles, have the diameters of about 60 nm and the lengths of approximately 2 micrometers. From the Fig. 3c and d, the CoO 7
ACCEPTED MANUSCRIPT nanorods constructed of nano-sized particles are distributed randomly throughout the surface of rGO. For obtained CoO/rGO composite: (1) The rGO nanosheets enhance electrical conductivity of CoO nanoparticles networks; (2) The random distribution of CoO nanorods not
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only provides more catalytically active sites but also increases the space between rGO nanosheets.
Fig. 4. (a) TEM image of CoO; (b-d) TEM images of CoO/rGO at different magnifications and the corresponding SAED pattern (inset in figure 4d).
Transmission electron microscopy is employed to further examine the detailed structures of the obtained samples. As shown in figure 4b, thanking to the role of rGO, CoO nanorods are distributed on the surface of rGO in a more disorderly state compared with the bare CoO (Fig. 4a), which can provide more active sites and sufficient reaction interfaces composed of O2-catalyst-electrotyte solution, More importantly, every typical CoO nanorod is consisted of a number of interconnected nanoparticles (about 15 nm in diameter) with some spacing, which is 8
ACCEPTED MANUSCRIPT conducive to the efficient electrolyte penetration, oxygen transport and the diffusion of Li+ as well as can store the insoluble discharge product Li2O2. The HRTEM images (Fig. 4c, d) clearly reveal that CoO nanorod anchored on rGO is comprised of many well crystallined nanoparticles with lattice spacing of about 0.246 and 0.213 nm, which are in well accordance with the (111) and (200)
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planes of the cubic CoO structure, respectively. The selected area electron diffraction (SAED) pattern (inset of figure 4d) show that the CoO/rGO is actually poly-crystalline and correspond to
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cubic CoO.
Fig. 5. Nitrogen adsorption-desorption isotherms and pore size distribution (inset) of the bare CoO (a) and the CoO/rGO hybrid (b).
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In order to further confirm the mesoporous structure, the specific surface area and pore size distribution of CoO before and after combination with graphene are characterized by N2 adsorption-desorption isotherm measurements. According to the BET analysis (Fig. 5a and b), the CoO/rGO composite possesses a larger surface area of 233 m2 g-1 compared with the bare CoO
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(92 m2 g-1), which can offer more electrochemical active sites for ORR and OER. As shown in the
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Fig. S1., the surface area of rGO is also tested and the surface area of the bare CoO is 133.2 m2 g-1, which is smaller than that of rGO in the CoO/rGO composite. More interestingly, the typical IV isotherms with a hysteresis loop are in good agreement with the mesoporous structure constructed of numerous well-crystallized nanoparticles in a nanorod [17,19]. The BJH adsorption isotherms (the insets in Fig. 5a and 5b) manifest that the pore size of CoO/rGO hybrid (7-15nm) is slightly larger than that of bare CoO (6-8nm), consistent with the results of SEM and TEM, facilitating the diffusion of oxygen and Li+ as well as the reservation of the discharge product (Li2O2). The results of SEM, TEM and BET can testify that rGO contributes to the surface parameters like surface area and pore size distribution as well as pore volume.
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Fig. 6. CV curves of three cathodes recorded in O2-saturated electrolytes at a scan rate of 0.1 mV s-1.
The cyclic voltammetry (CV) curves (Fig. 6) are utilized to examine the electrochemical catalytic activities of three cathode materials. The CoO/rGO cathode exhibits a most positive ORR
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onset potential(～3.1V) as well as a higher peak current in comparison with the graphene and CoO cathode, corroborating the excellent ORR kinetics. Compared with the electrode with the graphene or CoO, the composite shows an apparent oxidation peak and a higher current density, indicating
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the outstanding OER activity during anodic scan and the synergistic effect between CoO and rGO. These evidences confirm that CoO/rGO composite is a bi-functional catalyst for rechargeable
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Li-O2 batteries.
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Fig. 7. (a) First discharge-charge curves of Li-O2 cells catalyzed by rGO, CoO and CoO/rGO composite at a current density of 200 mA g-1; (b) Variations in discharge capacity and coulombic efficiency for Li-O2 batteries with three cathode catalysts for six cycles; (c) The discharge-charge profiles of CoO/rGO based cathode with a limited capacity of 1000 mAh g-1 at a current density of 200 mA g-1; (d) Rate performance of the Li-O2 battery with CoO/rGO at various current densities.
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The electrocatalytic activities of samples are studied in rechargeable Li-O2 batteries and the capacity are calculated on the basis of the weight of catalysts. The initial discharge-charge profiles of Li-O2 batteries assembled with CoO/rGO, CoO and rGO cathodes are evaluated at a current
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density of 200 mA g-1 with the cutoff voltage at 2.0 and 4.5V (versus Li/Li+), shown in Fig. 7a. The cathode catalyzed by CoO/rGO not only delivers a highest discharge capacity of 20254 mAh
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g-1 in comparison with bare CoO (6222 mAh g-1) and rGO (6500 mAh g-1) but also exhibits a high coulombic efficiency (98.9%), which is superior to the commercial Pt/C catalysts [27]. The reason may be that numerous accessible mesoporous of CoO and large surface areas can efficiently accommodate the insoluble discharging product (Li2O2), which prevents the discharging product from blocking the cathode, and thereby, insure the lithium to react continuously with oxygen on the cathode surface, bringing about a high capacity. In addition, most of discharge capacity is delivered above 2.55V and over half of charge capacity is delivered below 4.15V. And also importantly, the discharge plateau of CoO/rGO is 2.64V and the charge overpotential is reduced by 320mV compared with rGO, implying the higher catalytic activity for ORR and OER. This 11
ACCEPTED MANUSCRIPT may be related to the cause that the mesoporous structure can provides sufficient triple-phase boundaries to promote the rate of Li+ transport and O2 diffusion, consequently, reducing the polarization. The cyclic performances and the corresponding coulombic efficiencies of the cathodes
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catalyzed by three samples are shown in Fig. 7b and the detailed discharge-charge curves of Li-O2 batteries assembled with CoO/rGO, CoO and rGO cathodes for the first six cycles are presented in Fig. S2. The discharge capacity of CoO/rGO could maintain at above 10000 mAh g-1 with the average coulombic efficiencies＞90％ from the second cycle to the sixth cycle, indicating a good
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capacity retention, which can be attributed to the synergetic chemical coupling effects between CoO and graphene. The coupling effects is also verified with the result of CV. More importantly,
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the round trip efficiency of CoO/rGO have no significant change for the first six cycles, which can be ascribed to the unique mesoporous structure of CoO/rGO, contributing to the reversible formation and decomposition of Li2O2 ,and the result can be confirmed by that of bare CoO. The cycling stability of CoO/ rGO cathode is further tested at a current density of 200 mA g-1 with the capacity limited to 1000 mAh g-1. As displayed in figure 7c, the terminal discharge
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voltage is still above 2.48V after 69 cycles, indicating the enhanced stability of ORR catalytic activity. It should be noted that the battery have two plateaus during the charging process, which can be attributed to the hierarchical porous structure [35]. In addition, the CoO/rGO catalyst
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delivers a low overpotential (～1.47V) in the middle of the discharge-charge plateaus during the first cycle and the overpotential is only 1.66V for the 50th cycle, which is much lower than those
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of batteries with CoO or CoO-based composite [19-21]. One possible reason for this excellent cycling performance is that the interspace of graphene and the distinctive mesoporous of CoO can facilitate the transportation of oxygen and the electrolyte penetration and thus contribute to accelerate the kinetics of ORR and OER, resulting in the decrease of overpotential. Another possible may be that the distinctive mesoporous of CoO nanorod can accommodate and degrade efficiently the insoluble discharge product together with the preservation of electrochemically available surface area, and thus, could improve the rechargeability of the O2 cathode. Encouraged by the superior cycling performance, the rate capability of Li-O2 batteries using the CoO/rGO cathode catalysts is also examined at different current densities (200, 400, 800 mA g-1) and shown in Fig. 7d. Along with the increment of the current density, cathode polarization 12
ACCEPTED MANUSCRIPT becomes more and more serious, which result in the increase of overpotential and the decrease of specific discharge capacity. However, the capacities of batteries can still maintain at a level of 14879 and 13952 mAh g-1 even at a high current densities of 400 and 800 mA g-1, respectively. Even at a current density of 800 mA g-1, the discharge capacity and the overpotential hardly
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change compared with that of 400 mA g-1. As we know, the varied overpotential in charging and discharging process can arise from different O2 diffusion rates which have something to do with the different pore sizes of mesoporous. Mesoporous provided by the CoO nanorod composed of nanoparticles along with amounts of active sites can promote the adsorption and diffusion of
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oxygen so as to increase the reaction rate.
The above results clearly indicate the superior catalytic activity of CoO/graphene in catalyzing
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the reversible formation and decomposition of the discharge products, which is superior to the
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work as reported before [21,22].
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Fig. 8. SEM images of CoO/rGO cathode after the initial 1st discharge (a) and 1st charge (b).
In order to further make certain the enhanced catalytic activity of CoO/rGO in catalyzing the formation
and
decomposition
of
the
discharge
products,
the
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reversible
surface morphology and chemical composition of the CoO/rGO cathode, before and after the first cycle, are analyzed by SEM and XRD. The SEM image (Fig. 8a) shows that the discharge product are firstly deposited in the space between CoO nanoparticles instead of the surface of CoO nanoparticles, which can contribute to the continuous contact between electrolyte and electrode, thus achieving an excellent initial discharge capacity. As shown in figure 8b, the discharge products disappear and the mesoporous between CoO nanoparticles are recovered during the charge process, indicating the enhanced reversibility of Li-O2 battery assembled with CoO/rGO cathode. The good reversibility of Li-O2 batteries using CoO/GO cathodes is further proved by 13
ACCEPTED MANUSCRIPT XRD analysis (Fig. S3). The peaks corresponding to Li2O2 can be observed after the first discharging process. The characteristic Li2O2 peaks are disappeared after charging process, indicating the decomposition of Li2O2. The above results further confirm the fact that mesoporous CoO play a significant role in catalyzing the reversible formation and decomposition of the
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discharge products.
4. Conclusions
In conclusion, we demonstrate the synthesis of a hybrid of mesoporous CoO functionalized on the rGO by a facile hydrothermal growth, followed by calcination. For the hierarchical
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mesoporous of CoO, the cathode can contact efficiently with the electrolyte and oxygen as well as can provide enough space for Li2O2 storage. As a result, the initial discharge capacity can reach
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20254 mAh g-1 with a low overpotential and the capacity can maintain at 10000 mAh g-1 without obvious polarization from the second to the sixth cycle. Of course, the enhanced performance should also be intimately related to the combined effect of mesoporous CoO and graphene. In a word, we believe that the CoO/rGO will be promising electrocatalysts for accelerating the sluggish kinetics of ORR and OER in Li-O2 batteries.
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Acknowledgements
The authors gratefully acknowledge the financial support from the Open Project of Key Lab
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Biographies
Zhonghai Song is studying for Master’s degree in College of Science at Tianjin University. Xue Qin received his doctor degree (2001) from Nankai University. At present she is a professor in Department of Chemistry at Tianjin University. Her research interests are focused on synthesis and electrochemical studies of composite materials.
Ning Cao graduated from College of Chemistry and Chemical Engineering at Qufu Normal University (2013). He
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is studying for his Master’s degree in College of Science at Tianjin University.
Xuejiao Gao graduated from College of Chemistry at Hebei Normal University (2015). She will receive her Master’s degree (2018) from Tianjin University.
Qiu Liang graduated from College of Chemistry at Henan Normal University (2015). She will receive her
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Master’s degree (2018) from Tianjin University.
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Yanfang Huo is studying for PhD degree in College of Chemistry at Nankai University.
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