Enhancing the Catalytic Activity of Co3O4 for LiâO2 Batteries through

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Enhancing the Catalytic Activity of Co3O4 for Li-O2 Battery through the Synergy of Surface/Interface/Doping Engineering Rui Gao, Zhenzhong Yang, Lirong Zheng, Lin Gu, Lei Liu, Yu Lin Lee, Zhongbo Hu, and Xiangfeng Liu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03566 • Publication Date (Web): 25 Jan 2018 Downloaded from http://pubs.acs.org on January 25, 2018

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ACS Catalysis

Enhancing the Catalytic Activity of Co3O4 for Li-O2 Battery through the Synergy of Surface/Interface/Doping Engineering

Rui Gaoa, Zhenzhong Yangb, Lirong Zhengc, Lin Gub, Lei Liua, Yulin Leed, Zhongbo Hu

and Xiangfeng Liua*

a

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

b

Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese

Academy of Sciences, Beijing 100190, P. R. China

c

Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, P. R. China

d

Department of Materials, Imperial College London, Royal School of Mines, Exhibition Road, London, SW7 2AZ – UK

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ABSTRACT Efficient bifunctional catalysts are highly desirable for Li-O2 battery to accerlerate the oxygen reduction and oxygen evolution reaction. Surface/interface regulation or doping has been used to enhance the activity of the catalysts. Herein, we propose a facile synchronous reduction strategy to fabricate yolk-shell Co3O4@Co3O4/Ag hybrid which integrates the advantages of the surface, interface and doping engineering as highly active catalyst for Li-O2 batteries. Co3O4@Co3O4/Ag based cathode shows a high initial capacity (12000 mAh·g-1@200mA·g-1), high rate capability (4700 mAh·g-1@800mA·g-1), low overpotential, and long cycle life due to the synergetic interactions of surface-, interface- and doping engineering. The underling synergetic mechanism has been uncovered by X-ray diffraction, X-ray photoelectron spectroscopy, X-ray absorption near-edge structure spectra, the aberration-corrected scanning transmission electron microscopy, electrochemical impedance

spectra

and

ex-situ

scanning

electron

microscope.

As

for

Co3O4@Co3O4/Ag, part of Ag has formed on the surface of Co3O4 shell as single atoms or clusters and a fraction of Ag has been doped into the crystal lattice of Co3O4 at the same time, which not only strengthens the Ag-Co3O4 interface binding but also tailors the valence electronic structure of Ag and Co species as well as improves the electronic conductivity. This particular architecture provides more active sites for ORR/OER and also enhances the catalytic activity. In addition, the flower-like Li2O2 forms on Co3O4@Co3O4/Ag cathode, which is more feasible to decompose than toroidal-like Li2O2. This study offers some insights into designing efficient cathode catalysts through a synergetic surface/interface/doping engineering strategy.


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ACS Catalysis

KEYWORDS ： Li-O2

battery;

Bifunctional

Surface/Interface/Doping engineering; Synergy


catalyst;

Cobalt

oxide;

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TOC


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1．INTRODUCTION Li-air battery, as the most promising post lithium battery system, has attracted extensive attention due to the super high energy density (~3500WhWh kg-1), which is about 5-10 times higher than that of the current Li-ion batteries (LIBs).

1-3

However,

due to the sluggish kinetics in oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) during the discharge/charge process Li-air batteries have suffered from large overpotential, poor cycling stability, inferior rate capability and even low coulombic efficiency, which critically limit their practical application 4-7. One of the effective methods to alleviate these issues is to develop efficient bifunctional catalysts to enhance the kinetics of both ORR and OER. 8-11

Similar to the ORR and OER process in alkaline solution, some catalysts have been reported as the cathode materials for Li-air battery. Noble metals such as platinum, gold, ruthenium, silver and other have been considered as the most appropriate catalysts for ORR and OER. 12-16 Transition metal oxides, carbide or even nitride, i.e. MnO2, TiO2, NiO and Co3O4 have also been extensively studied as stable cathode catalysts for ORR and OER due to the low cost.

17-20

Cobalt oxide is

undoubtedly one of the most special concerned catalysis for Li-air battery because of its high activity and stability. The battery catalyzed by Co3O4 provides a better compromise between the capacity retention and initial capacity. 21 However, like most of the metal oxides, the poor conductivity of bulk oxides restricts the rate capability, cycling performances and also raises the overpotential between charge and discharge


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for Li-air batteries. How to further enhance the electroctalytic performances of cobalt oxides on ORR and OER as Li-air battery cathode catalysts is still a great challenge.

A few recent studies have shown that the surface structure and the interface interactions between the catalyst and the supports both have a significant impact on the performance.

22-26

In another word, the catalytic activity can be largely enhanced

by tuning the surface atom environment and the interface binding strength. Wen et al. 24

found that some certain materials with an appropriate surface acidity can achieve a

high catalytic activity in reducing charging voltage and activation barrier. By creating the noble metal-cobalt oxides or carbon-cobalt oxides interface, the shortages of metal oxides can be alleviated to some extent and the electrocatalytic activity can be enhanced.

25

Liao et al.

26

decorated Co3O4 nanowires with Pd using a pulse

electrodeposition approach. The homogeneous Pd nanoparticles on Co3O4 could ensure the uniform growth of Li2O2 on the surface of the electrodes which leads to the small polarization and high cycling performance. Sun et al. 27 used Pt nanoparticles to modify the free-standing Co3O4 on nickel foams in order to reduce the charge overpotencial and promote the decomposition of the discharged product. In order to further enhance the activity of the catalysis and lessen the cost, noble metals were even spread on the surface of oxide in the form of atoms or cluster.

28

Generally, in

order to keep the well-defined morphology, the metal oxides usually need to be first prepared and then the noble metals are decorated on the surface of the metal oxides by physical processes or chemical reduction. 29. However, for these post-loading methods it’s hard to load the noble metal nanoparticles on the oxides surface homogeneously


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as well as obtain a strong interface interaction between the noble metal particles and oxides.

Herein, we propose a facile novel synchronous reduction strategy to fabricate yolk-shell Co3O4@Co3O4/Ag hybrid with a well-defined architecture from Prussian blue analogues (PBA) which combines the advantages of the surface, interface and doping engineering as highly active cathode catalyst for Li-O2 batteries. In this strategy, by controlling the dropping order of different nitrates, Co3[Co(CN)6]2 precursor was first formed and then AgxCoy[Co(CN)6]2·nH2O was eventually coated on

the

surface

of

the

pre-formed

Co3[Co(CN)6]2

to

form

Co3[Co(CN)6]2@AgxCoy[Co(CN)6]2·nH2O. After a thermal treatment at 500°C, the core of Co3O4 and the shell of Co3O4/Ag simultaneously formed. Through this synchronized reduction strategy, part of Ag has been doped into the crystal lattice of Co3O4 shell and part of Ag forms on the surface of Co3O4 as single atoms or clusters, which not only strengthens the Ag-Co3O4 interface binding but also tunes the electronic structure of both Ag single atoms or clusters and Co species. In compared to Ag-free

porous

Co3O4

and

the

post-loaded

Co3O4/Ag,

the

fabricated

Co3O4@Co3O4/Ag based cathode shows a higher initial capacity, lower overpotential, better rate capability and longer cycle life, which might arise from the synergetic interactions of surface-, interface- or doping engineering in Co3O4@Co3O4/Ag hybrid benefiting from the synthetic strategy. This particular surface/interface/doping engineered architecture not only provides more active sites for ORR and OER but also enhances the catalytic activity, which leads to the high electrocatalytic


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performance of Co3O4@Co3O4/Ag as Li-O2 cathode catalyst. A further study on the morphology of the discharged cathode shows that Co3O4@Co3O4/Ag based cathode tend to form a flower-like morphology, which is more active to decompose upon recharging.

2. EXPERIMENTAL SECTION Synthesis of Co3O4 and Co3O4@Co3O4/Ag Co3O4: In this experiment, 3mmol Co(NO3)2·6H2O and 10 g sodium dodecyl sulfate (SDS) were firstly dissolved in 300 mL ultrapure water to form a solution A. At the same time, 2mmol K3[Co(CN)6] was dissolved in 200mL ultrapure water to form a solution B. The solution B was then added dropwise into the solution A under strongly stirring at room temperature. Afterwards the mixed solution was stirred continuously for 12 hours. The resulting pink precipitate was collected and washed several times with absolute ethanol, and finally dried in an oven at 80 °C for 12 hours to get precursor of Co3[Co(CN)6]2 (P-1). Subsequently, the precursor was annealed at 500 °C for 3 h in air to obtain Co3O4. Co3O4@Co3O4/Ag: 3mmol Co(NO3)2·6H2O and 10 g sodium dodecyl sulfate (SDS) were first dissolved in 300 mL ultrapure water. 200mL of 2.1 mmol K3[Co(CN)6] was added dropwise into the above solution under strongly stirring at room temperature. Then 100 mL of 0.3 mmol AgNO3 was dropped into the mixture. Afterward the mixed solution was stirred continuously for 12 hours. The resulting pink precipitate was collected and washed several times with absolute ethanol, and finally dried in an oven at 80 °C for 12 hours to get precursor of


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AgxCoy[Co(CN)6]2·nH2O/Co3[Co(CN)6]2 (P-2). Subsequently, the precursor was annealed at 500 °C for 3 h in air to obtain the yolk-shell Co3O4@Co3O4/Ag. The final ratio of Ag/Co in Co3O4@Co3O4/Ag was 5.09 % measured by Inductively coupled plasma mass spectrometry (ICP-MS). Co3O4/Ag: To prepare the post-loaded Co3O4/Ag, 0.48 g as-sysnthesized Co3O4 was dispersed in 20 mL water/ ethanol mixture (1:1, v/v) containing 0.1 mmol AgNO3. The mixture was dried at 80 °C under continuous stirring. The obtained product was finally calcined at 500 °C for 1h to obtain Co3O4/Ag.

Structure characterization of the samples The powders of the samples were characterized by X-ray powder diffraction (XRD, XD-3 PERSEE) with 2θ ranging from 10 to 80°. The morphologies and EDX mapping of the samples were observed on a scanning electron microscope (SEM SU8010 HITACHI, EDX X-Flash |30 Bruker ). TEM images were obtained on a JEOL JEM-2100 (acceleration voltage, 200 kV). HRTEM images were obtained on a Philips Tecnai G2 F20 (acceleration voltage, 200 kV). X-ray photoelectron spectroscopy (XPS) was measured on Thermo escalab 250Xi. Z-contrast annular bright-field (ABF) and high-angle annular dark-field (HAADF) imaging was performed in thin specimen regions with a probe convergence angle of 25 mrad. The collection angles of them were 12-24 and 70-250 mrad, respectively. For each sample, the representative images were selected for analysis. Fourier transform infrared spectra (FTIR) was tested on a spectrometer instrument (Thermo Scientific Nicolet


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iS50, USA). X-ray-absorption fine structure (XAFS) spectra were collected on the 1W1B beamline of the Beijing Synchrotron Radiation Facility (BSRF Beijing, China). The elements analyses were performed by ICP-MS (Agilent 7500ce).

Fabrication of oxygen cathode and electrochemical measurements The electrochemical performances of the samples were measured by using a Swagelok-based Li-O2 cell, consisting of a lithium foil anode, glass filter separator (Whatsman), electrolyte (1.0M LITFSI/TEGDME solution) and a porous air electrode. The cathode and the cells were assembled as follows: 40 wt % catalyst, 10 wt % binder (PVDF) and 50wt% super P carbon were mixed with NMP to prepare catalyst slurry. The mixture was then coated on the carbon paper current collector to prepare the porous air electrode, which was then dried at 120°C under vacuum for 12 h. The loading mass of the cathodes (Super P, Catalyst and binder) are about 1.5 mg cm-2. The Swagelok-based cells were assembled in an argon-filled glove box with a water and oxygen content less than 0.1 ppm. The galvanostatic charge and discharge performance of the batteries were tested on a LAND CT2001A battery test system with the voltage between 2.0 and 4.5 V at room temperature. The cycle performance was conducted at the current density of 200 mA·g-1, and the capacity was limited to 1000 mAh·g-1. EIS (Electrochemical impedance spectra) measurements were carried out using a Autolab

PGSTAT 302N electrochemical workstation with a frequency

ranging from 0.1Hz to 1MHz. All the cells were tested under the pure dry O2


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ACS Catalysis

atmosphere at 1 atm. The capacities were all calculated based on the mass of the super P carbon loaded on the cathode.

3. RESULTS AND DISCUSSION The preparation process of Co3O4@Co3O4/Ag yolk-shell hybrids from PBA was illustrated in Figure 1. The shape-controlled Co3[Co(CN)6]2 spheres were first precipitated as the core. In this step, SDS was employed as an effective capping agent to direct the anisotropic growth of metal cyanide coordination polymer through preferential adsorption on specific crystal facets. 30 Afterwards, when Ag+ was added, the preferential capping effect of SDS could drive the Ag+ and [Co(CN)6]3− to self-assemble

on

the

surface

AgxCoy[Co(CN)6]2·nH2O.

Several

of

Co3[Co(CN)6]2

hours

later,

the

sphere

to

crystallinity

form of

AgxCoy[Co(CN)6]2·nH2O was enhanced and coated on the surface of Co3[Co(CN)6]2 sphere. After calcining at 500 °C, the core of Co3O4 and the shell of Co3O4/Ag simultaneously formed. Figure S1A show X-ray diffraction (XRD) patterns of P-1 (Co3[Co(CN)6]2) and P-2 (AgxCoy[Co(CN)6]2·nH2O/Co3[Co(CN)6]2). From Figure S1A, it can be inferred that the precursors of P-1 and P-2 can be in well agreement with the standard PDF card (JCPDS No. 72-1431) of a single face-centered cubic phase of cobalt cyanide hydrate. The weak peaks at 12.40 and 19.15° of P-2 can be ascribed to the phase of Ag3Co(CN)6 (JCPDS No. 72-1430). Figure S1B show Fourier Transform Infrared Spectroscopy (FTIR) patterns of P-1 and P-2 before and after annealing. FTIR patterns of P-1 and P-2 (Figure S1B a\b) can only reflect the surface


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functional group and the CN stretching (2170 cm−1 ) of Prussian blue analogous. We also performed FTIR of the two precursors samples after annealing (Figure S1B c\d). Through comparing FTIR of P1 and P2 before and after annealing, we found that C and N atoms were oxidized into gases and escaped, which promoted the formation of inter-connected small pores.

Figure 1. The synthesis process of yolk-shell Co3O4@Co3O4/Ag nanospheres from PBA.


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Figure 2. A, B) The XRD and XPS patterns of Co3O4 and Co3O4@Co3O4/Ag. C) The high resolution XPS spectra of Ag 3d for Co3O4@Co3O4/Ag. D) XANES spectra at Co K-edge for Co3O4@Co3O4/Ag, Co3O4 (III), and Commercial CoO (II).

Figure 2A shows the crystallographic structure of Co3O4 and Co3O4@Co3O4/Ag. The main peaks at 19.0, 31.3, 36.8, 59.3 and 65.2° could be indexed on the standard PDF card (JCPDS No. 09−0418), indicating the formation of Co3O4. In curve b, the weak peaks at 38.1° can be attributed to the peak of Ag (JCPDS No. 03−0921). The right shift of peaks in curve b indicates that the lattice parameters have changed due to the doping of Ag. The XRD patterns are also analyzed by Rietveld refinement. Based on the refinement result, the lattice parameter indeed increases from 8.0693(3) to 8.0744(6) after loading Ag. This increase of the lattice parameter further confirms that part of Ag has indeed doped into the crystal lattice of Co3O4. During the formation of Co3O4, Ag+ was self-reduced to Ag metal from AgxCoy[Co(CN)6]2 simultaneously and


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some of the Ag atoms diffused into the crystal cell of Co3O4. Figure S2 shows the XRD profile of Co3O4/Ag by a post-loading method with the same Ag amount as Co3O4@Co3O4/Ag. A relatively strong peak at 38.1° can be observed which is ascribed to Ag. The silver particles on Co3O4/Ag by the post-loading method are larger and inhomogeneous leading to the relatively strong diffraction peak.

Figure 2B and 2C present the XPS patterns of Co3O4 and Co3O4@Co3O4/Ag to identify the chemical-states and binding energies of the elements in Co3O4 and Co3O4@Co3O4/Ag. Figure 2B provides the survey profile of two samples, three elements of C, Co, and O can be found in the black curve. Four main elements can be found in pattern of Co3O4@Co3O4/Ag in the red curve. The peaks at about 366-378 eV can be ascribed to the binding energy of Ag 3d. The peaks at 367.9 eV of Ag 3d indicate the existence of Ag+, which can be partly attributed to the doped Ag. In addition to the doped Ag, the small-sized Ag can also broaden the peak and result in the energy shift, which was reported in a previous study: the Ag 3d binding energy shift and peak broadening are attributed to a combination of heterogeneous broadening and final state screening effects.

32,33

This agrees with the XRD patterns

and the Rietveld refinement (Figure S3).34 Because of the introduction of Ag on the surface, the interface and the lattice of Co3O4 shell, the electronic structure will change slightly. Figure 2D provides the X-ray absorption near-edge structure (XANES) spectra at Co K-edges. After loading Ag, Co K-edge absorption spectrum shows a slight shift toward the higher energy region (about 0.4eV), which is ascribed to the change in the local structure of cobalt coordination. The above analyses based


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on XPS and XANES indicate that the electronic structure of both Ag single atoms or clusters and Co species has been tuned due to the cooperative effect of the two factors: one derives from the doping of Ag into the lattice of Co3O4 and the other comes from the strong interaction between single Ag atoms or clusters and Co3O4 surface.

Figure 3. A) The SEM and TEM images of P-2. B) The SEM and TEM images of yolk-shell Co3O4@Co3O4/Ag nanospheres. C) The dark field images and HRTEM image of yolk-shell Co3O4@Co3O4/Ag nanospheres. D) The overlapped and separate EDS mapping images of Co, O and Ag in Co3O4@Co3O4/Ag.

Figure 3A show SEM and TEM images of the precursor of P-2. As shown in Figure 3A, P-2 precursor exhibited a truncated ball shape with an average size of 500 nm. In compared to precursor P-1 (Co3[Co(CN)6]2) (Figure S4, S5) there are some nanoparticles dotted on the surface of the precursor P-2. From the XRD we know the small particles mainly consist of AgxCoy[Co(CN)6]2. Figure 3B show SEM and TEM images of the obtained yolk-shell Co3O4@Co3O4/Ag nanospheres. From Figure 3B it


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can be induced that the spherical morphology and particle size can be well maintained during the calcination. After calcination, the C and N of [Co(CN)6]3+ decomposed into gas and escaped to form the porous structure and the surface of the sphere becomes rough.35, 36 Figure 3C show the dark field image and HRTEM image of yolk-shell Co3O4@Co3O4/Ag nanospheres. As shown in Figure 3C, some Ag nanoparticles can be found on the surface of Co3O4. The lattice spacing of 0.236nm can be ascribed to Ag(111) and the dark field images also indicates that Ag nanoparticles are uniformly decorated on Co3O4 shell, which can also be confirmed by the EDS mapping as shown in Figure 3D. The interface state can also be obtained from HRTEM in Figure 4A. Figure 4B is the magnification of the box in Figure 4A. As shown in Figure 4A and 4B three regions were divided to Co3O4, Ag species and their interface. The existence of Ag/Co3O4 heterostructure confirmed that the interface between noble metal and substrate can be strengthened through this method. The local structure was also analyzed by atomic resolution high-angle annulardark feld (HAADF) and annular bright feld (ABF)-STEM. A typical region of Co3O4 was selected to observe the surface as shown in Figure 4C and Figure 4D. In Figure 4C the dark-dot contrast in ABF-STEM images reveal the Co atom column positions as a spinel structure. In Figure 4D, the bright-dot contrast in HAADF-STEM images presents the position of Ag atom. The silver clusters can be distinguished from Co3O4 based on the brightness variations of these clusters in HAADF-STEM image. The red arrows on the figure indicate the Ag clusters on the surface, and the circles in the figure represent the single Ag atoms dotted on Co3O4. The porosity of the annealed products was


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determined by N2 adsorption–desorption measurements. On another hand, as shown in Figure S6, the pore size distribution of the annealed products are calculated by Barrett-Joyner-Halenda (BJH) method, which confirmed the existence of some pores with both small and large size in Co3O4@Co3O4/Ag. The SEM images of the post loaded Co3O4/Ag were shown in Figure S7.

Figure 4. A) The HRTEM images of Co3O4@Co3O4/Ag. B) The magnification of the seleted box in Figure 4A C) ABF-STEM images of yolk-shell Co3O4@Co3O4/Ag nanospheres. D) HAADF-STEM images of Ag atom and silver clusters dotted Co3O4 region.


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Figure 5. A) The full cycle performance and the capacity of Co3O4, Co3O4/Ag and Co3O4@Co3O4/Ag based cathodes measured at the current density of 200 mA·g-1. B) The initial capacities comparison of Co3O4 and Co3O4@Co3O4/Ag based cathodes in different current densities.

Figure 5 shows the comparison of the electrochemical performance of Co3O4, Co3O4/Ag and Co3O4@Co3O4/Ag based cathodes for Li-O2 batteries. Figure 5A show the first discharge/charge curves of Co3O4, Co3O4/Ag, and Co3O4@Co3O4/Ag at the current density of 200 mA·g-1. It is obvious that the battery cathode catalyzed by Co3O4@Co3O4/Ag exhibits a much higher initial capacity of 12000 mAh·g-1 than Co3O4 (4600 mAh·g-1) and Co3O4/Ag (7500 mAh·g-1). The cathode catalyzed by Co3O4@Co3O4/Ag has a lower overpotential than that of Co3O4 based cathode, which indicates that the modification of Ag enhances ORR and OER activity. This can be


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largely related to the effect of Ag modification on Co3O4. On one hand, single Ag atoms and clusters may play an important role for promoting ORR activity.37 On the other hand, the conductivity will be enhanced by loading silver nanoparticles and their close-knit interface.34, 38 In compared to Co3O4/Ag, Co3O4@Co3O4/Ag based cathode has a higher initial capacity, which indicate that Co3O4@Co3O4/Ag synthesized through the synchronous formation strategy provides more active sites for ORR and OER, and also enhances the catalytic activity because of the strong Ag-Co3O4 interface binding and the tuned electronic structure of both Ag single atoms or clusters and Co species as well as the improved electronic conductivity. The difference of the initial capacity for Co3O4, Co3O4/Ag and Co3O4@Co3O4/Ag based cathodes becomes larger under a high current density. Under a current density of 800 mA·g-1, the battery catalyzed by Co3O4@Co3O4/Ag can still deliver a higher initial capacity of 4700 mAh·g-1 much higher than that of Co3O4 and Co3O4/Ag (Figure 5B). As shown in Figure 6 and Figure S8, the cycle stability of Co3O4 Co3O4/Ag

and

Co3O4@Co3O4/Ag based cathode were investigated by a restriction of the capacity to 1000 mAh·g-1 at a current density of 200 mA·g-1. The yolk-shell Co3O4@Co3O4/Ag based cathode exhibit a much better cycle performance than Co3O4 or Co3O4/Ag based cathode. The discharge capacity of bare Co3O4 and Co3O4/Ag based cathodes can not reach 1000 mAh·g-1 after 39 cycles or 62 cycles.

As for Co3O4@Co3O4/Ag, the

capacity of 1000 mAh·g-1 can still be kept after 80 cycles. More importantly, the overpotential of Co3O4@Co3O4/Ag increased less than that of Co3O4 based batteries, as shown in Figure 6D. To examine the possible side reaction of the electrolyte we


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also used FTIR to detect the by-product. Figure S9 provides the FTIR curve of Co3O4@Co3O4/Ag based cathodes in different states. From the figure we can find that no obvious by-product can be detected even after 40 cycles indicating the electrolyte is relatively stable.

Figure 6. A) The cycle performances of Co3O4, Co3O4/Agand Co3O4@Co3O4/Ag based cathodes when the capacity is limited to 1000mAh·g-1 at the current density of 200 mA·g-1; B), C) The discharge/charge curves at different cycles of Co3O4@Co3O4/Ag and Co3O4 based cathodes, respectively. D) The discharge voltage flat of Co3O4, Co3O4/Ag and Co3O4@Co3O4/Ag based cathodes.

The excellent catalytic activity of Co3O4@Co3O4/Ag can be largely ascribed to the synergetic interactions of surface-, interface- or doping engineering in Co3O4@Co3O4/Ag hybrid benefiting from the synthetic strategy. First, part of Ag has been doped into the crystal lattice of Co3O4 and part of Ag forms on the surface of


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ACS Catalysis

Co3O4 as single atoms or small clusters, which not only strengthens the Ag-Co3O4 interface binding but also tunes the electronic structure of both Ag single atoms or clusters and Co species. The homogeneously distributed Ag single atoms or clusters with tuned electronic structure behave coordinatively with Co3O4 and promote ORR and OER, which lowers the overpotential and enhances the cycle stability. Therefore, this particular architecture provides more active sites for ORR and OER, and also enhances the catalytic activity. Second, the introduction of Ag also improves the electronic conductivity which also contributes to the high electrocatalytic performance. Thirdly, the porous architecture of Co3O4@Co3O4/Ag can supply more active sites for ORR and more pathways for oxygen diffusion, which is also favourable for the electrocatalytic performance. The possible mechanism has been illustrated as shown in Figure 7.

Figure 7. The possible mechanism of the ORR and OER process of Co3O4@Co3O4/Ag

In order to further figure out the role of Ag several tests were carried out to confirm the ORR and OER process of the cathode. Electrochemical impedance


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spectra (EIS) were also used to further get insights into the deterioration mechanism. Figure 8 compares the EIS patterns of Co3O4 and Co3O4@Co3O4/Ag based cathodes. From the EIS, it can be clearly observed that Co3O4@Co3O4/Ag based cathode shows a lower impedance than Co3O4 based cathode, which will be favourable to the electrocalatysis.

Figure 8. The EIS patterns of Co3O4 and Co3O4@Co3O4/Ag

To clarify the reaction process of the ORR and OER, the discharged and recharged products of Co3O4 and Co3O4@Co3O4/Ag based cathodes are analysed by scanning electron microscopy (SEM). Figure 9 shows the SEM images of Co3O4 and Co3O4@ Co3O4/Ag based cathodes under different discharge states. Before discharge, it can be clearly observed that the ball-like catalysts were uniformly mixed with Super P on the cathode. When the capacity reached to 1000 mAh·g-1, both of the catalysts were covered by a layer of irregular solid discharge products. Some toroidal and irregular products formed on the cathode catalysed by Co3O4 as shown in Figure 9B and E. In contrast, after the 1st discharge, the morphology of the discharge products of


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ACS Catalysis

Co3O4@Co3O4/Ag based cathodes were quite different from that of Co3O4 based cathode, as shown in Figure 9C and 9F. From Figure 9C, we can see the toroidal product grow gradually during the discharge process and finally deposited onto the Co3O4 based cathode. In contrary, under the effect of silver particles, the unique nanosheets with 20nm

thicknesses uniformly grew onto the

surface of

Co3O4@Co3O4/Ag based cathode and form a flower-like product. From the collected XRD patterns of the cathode, it can be confirmed that the flower-like product is Li2O2. (Figure S10) The flower-like Li2O2 can provide more surface areas and benefit to the decomposition of discharge products.15 The results indicate that Ag has an important effect on the morphology of discharged products and the modification of Co3O4 by Ag promotes the formation of the flower-like discharged product, which benefit to the OER. In order to further confirm that Li2O2 was completely decomposed XPS patterns of Co3O4@Co3O4/Ag based cathodes in different charge/discharge were supplied as shown in Figure S11. Figure S11 show XPS of Li 1s region. After discharge, the product mainly exist as Li2O2 which in well agreement with the result of XRD patterns. After 1st recharge process, no Li 1s signal can be found which means all the formed Li2O2 was decomposed.


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Figure 9. The SEM images of Co3O4 (A-C) and Co3O4@Co3O4/Ag (D-F) based cathode in different discharge states. A, D) The primitive cathode. B, E) The capacity is 1000mAh g-1. C, F) After 1st discharge. G) The probable mechanism for the electrochemical production of Li2O2 when catalysed by Co3O4. H) The probable mechanism for the electrochemical production of Li2O2 when catalysed by Co3O4@Co3O4/Ag.

Figure 9G and H illustrates the possible deposition mechanism of toroidal and flower-like Li2O2 on Co3O4 and yolk-shell Co3O4@Co3O4/Ag based cathodes during the discharge process. First, O2 bind onto the active sites of the catalysts, followed by the reduction to form O2-. Li+ was then bonded with O2- to form the intermediate


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ACS Catalysis

product LiO2 in the TEGDME based electrolytes.

39

The instable LiO2 will further

polymerize itself or bind with Li+ to form the stable Li2O2. The firstly formed product of LiO2 was accumulated in the electrolyte solution. At a critical concentration, the LiO2 will nucleate and grow on the surface sites, followed by the formation of Li2O2. [40-43]

On the surface of Co3O4@Co3O4/Ag, Ag spicies could first supply both the

active centers for the ORR and nucleation sites for Li2O2 crystallization due to its high oxygen adsorption energy.44 At the beginning of the ORR process, under the catalysis of Ag, the nucleation and crystallization of Li2O2 occurred preferentially on Ag. And the formed amorphous discharge products accumulate on the surface of the materials and coat the nanoparticles, which can be seen in Figure 9E and Figure 9H. As the discharge process goes on, with the coordinative effects of Ag and Co3O4, Li2O2 grew and crystallize into the nanosheets as shown in Figure 9F.45 However, as for bare Co3O4, both nucleation and crystallization occurred on the surface of Co3O4 and so Li2O2 grew into a typical toroidal shape as shown in Figure 9C and 9G.

Figure S12 shows the SEM images of Co3O4 (A-C) and Co3O4@Co3O4/Ag (D-F) based cathodes under different recharge states. Figure S12 A and S12 D clearly display that both of the two shapes of Li2O2 (toroidal and flower-like) began to decompose from the edge. The flower-like and edges-rich Li2O2 nanosheets are easier to accelerate the dynamics of oxidation for Li2O2 (OER), which can explain the better OER

performance

of

Co3O4@Co3O4/Ag.

46-49

After

several

cycles,

the

Co3O4@Co3O4/Ag based cathode still shows the ball-like catalyst. But as for the Co3O4 based cathode all the solid product and super P mixed irregularly. The


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accumulation of the discharged product and by-product may directly result in the loss of activity of the catalyst.

50

The above analysis can also explain the lower

overpotencial, high capacity and excellent cycling performance of Co3O4@Co3O4/Ag.

4. CONCLUSIONS In

summary,

the

surface/interface/doping engineered Co3O4@Co3O4/Ag

yolk-shell hybrid has been successfully prepared from PBA through a synchronized strategy. In compared to Ag-free porous Co3O4 and the post-loaded Co3O4/Ag, the fabricated Co3O4@Co3O4/Ag based cathode shows a much higher initial capacity, lower overpotential, better rate capability and longer cycle life, which can be largely attributed to the synergetic interactions of surface-, interface- and doping engineering in Co3O4@Co3O4/Ag hybrid benefiting from the synthetic strategy. First, part of Ag has been doped into the crystal lattice of Co3O4 and part of Ag forms on the surface of Co3O4 as single atoms or small clusters, which not only strengthens the Ag-Co3O4 interface binding but also tunes the electronic structure of both Ag single atoms or clusters and Co species, which provides more active sites for ORR and OER, and also enhances the catalytic activity. Second, the introduction of Ag also improves the electronic conductivity which also contributes to the high electrocatalytic performance. Thirdly, the porous architecture of Co3O4@Co3O4/Ag can supply more active sites for ORR and more pathways for oxygen diffusion, which is also favourable for the electrocatalytic performance. In addition, the introduction of Ag has an important effect on the morphology of discharged products, and the more easily decomposable flower-like Li2O2 forms on Co3O4@Co3O4/Ag based cathode in


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the charge process. This study also provides some new insights into designing catalysis through a synergetic surface/interface/doping engineering for high performance Li-air batteries.

ASSOCIATED CONTENT Supporting Information Supporting Information Available: FTIR, XRD, SEM, TEM, BET and XPS patterns of different samples.

AUTHOR INFORMATION *Corresponding Author: [email protected]

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Grant 11575192), the State Key Project of Fundamental Research (Grant 2014CB931900) of Ministry of Science and Technology of the People's Republic of China, and the Scientific Instrument Developing Project (Grant ZDKYYQ20170001) , International Partnership Program (Grant No. 211211KYSB20170060) and “Hundred Talents Project” of the Chinese Academy of Sciences. X-ray absorption spectroscopy characterization was performed at the Beijing Synchrotron Radiation Facility (BSRF).

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