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Nano Research
Nano Res
DOI
10.1007/s12274-014-0604-y
Uniform MnO2 nanostructures supported on
hierarchically porous carbon as efficient
electrocatalyst for rechargeable Li-O2 batteries
Xiaopeng Han, Fangyi Cheng(), Chengcheng Chen, Yuxiang Hu and Jun Chen()
Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0604-y
http://www.thenanoresearch.com on October 8 2014
© Tsinghua University Press 2014
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TABLE OF CONTENTS (TOC)
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Uniform
MnO2
nanostructures
supported
on
hierarchically porous carbon as efficient electrocatalyst
for rechargeable Li-O2 batteries
Xiaopeng Han, Fangyi Cheng*, Chengcheng Chen,
Yuxiang Hu and Jun Chen*
Key Laboratory of Advanced Energy Materials Chemistry
(Ministry
of
Education),
College
of
Chemistry,
Collaborative Innovation Center of Chemical Science and
Engineering, Nankai University, Tianjin 300071, China.
MnO2 nanostructures supported on hierarchically porous carbon was
synthesized and exhibited much enhanced Li-O2 battery performance,
owning to the improved catalytic activity and favorable transportation
of ions, oxygen and electrons.
7
8
Provide the authors’ webside if possible.
9
Author 1, webside 1
10
Author 2, webside 2
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Nano Research
DOI (automatically inserted by the publisher)
Research Article
1
2
3
Uniform
MnO2
nanostructures
supported
on
hierarchically
porous
carbon
as
efficient
electrocatalyst for rechargeable Li-O2 batteries
Xiaopeng Han, Fangyi Cheng(), Chengcheng Chen, Yuxiang Hu and Jun Chen()
4
5
6
Received: day month year
ABSTRACT
Revised: day month year
Through in situ redox deposition and growth of MnO2 nanostructures on
hierarchically porous carbon (HPC), a MnO2/HPC hybrid was synthesized and
employed as cathode catalyst for non-aqueous Li-O2 batteries. Owning to the
mild synthetic condition, MnO2 was uniformly distributed on the surface of the
carbon support, without destroying the hierarchical porous nanostructure. As a
result, the as-prepared MnO2/HPC nanocomposite exhibits much enhanced
Li-O2 battery performance, including low charge overpotential, good rate
capacity and long cycle stability up to 300 cycles with controlling capacity of
1000 mAh g-1. The combination of multi-scale porous network of the
shell-connected carbon support and the high-dispersion MnO2 nanostructure
benefits the transportation of ions, oxygen and electrons and contributes to the
superior electrode performance.
Accepted: day month year
(automatically inserted by
the publisher)
© Tsinghua University Press
and Springer-Verlag Berlin
Heidelberg 2014
KEYWORDS
Lithium-oxygen batteries,
manganese
oxide,
nanocomposite, catalyst,
oxygen electrochemistry
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1
Introduction
Rechargeable lithium-oxygen (Li-O2) batteries have
12 stimulated extensive interest due to their high
13 theoretical specific energy far exceeding that of
14 current
lithium-ion batteries [1-4]. However,
15 formidable challenges need to be addressed before
16 the
Li-O2 systems could realize commercial
17 deployment [5, 6]. One of the most important
11
parameters that critically determine the battery
performance
is
the
reversible
20 formation/decomposition of discharged products in
21 the oxygen electrodes. The severe sluggish kinetics of
22 both the oxygen reduction reaction (ORR) and
23 oxygen evolution reaction (OER) not only decrease
24 the energy efficiency but also limit the rate capability.
25 In addition, the irregular precipitation of insoluble
26 and insulating discharge species (mainly Li2O2) is
18
19
10
Address correspondence to Fangyi Cheng, [email protected]; Jun Chen, [email protected]
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Nano Res.
prone to block the pores of the cathode, thus
resulting in low capacity and poor cycling stability.
3 Therefore, to build advanced Li-O2 batteries, it is
4 desirable to develop efficient cathode that allows fast,
5 reversible redox reactions between gaseous oxygen
6 and solid peroxide. An ideal cathode should possess
7 several critical properties: high activity for catalyzing
8 dual ORR/OER, large surface area for providing
9 abundant active sites, high electrical conductivity for
10 fast electron transfer, robust porous structure to
11 accommodate
discharge products and facilitate
12 transportation of oxygen and electrolyte [7-12].
13
Recent studies on the catalysts materials of oxygen
14 electrodes are mainly focused on noble metals or
15 alloys [13-19], metal oxides [20-29], sulfides [30] and
16 nanostructured carbonaceous materials [7, 31-34].
17 Among the nonprecious catalysts, manganese oxides
18 are attractive because of many advantages such as
19 low
cost,
natural
abundance,
considerable
20 electrocatalytic
activity
and
environmental
21 friendliness [20-26]. However, manganese oxides
22 suffer from low electronic conductivity. Although
23 mixing with carbon additive is a common strategy to
24 enhance electron conduction in metal oxide-based
25 electrode, the side reaction of carbon with Li 2O2 or
26 electrolyte will form Li 2CO3 and degenerate the
27 electrochemical
performance [35, 36]. Peculiar
28 architectured materials with a metal oxide shell and
29 carbon core (e. g., carbon nanotubes@RuO2 [28])
30 could alleviate the unwanted reaction while combine
31 the merits of two components. By taking advantages
32 of carbon and cheap metal oxides and bearing in
33 mind the above-discussed rational design of air
34 cathode, herein we develop an oxygen electrocatalyst
35 that consists of homogeneous MnO 2 nanostructures
36 supported on hierarchically porous carbon matrix
37 (designated as MnO2/HPC), which, to the best of our
38 knowledge, has never been employed in Li-O2
39 system. The MnO2/HPC composite materials are
40 fabricated by in situ self-sacrificed template method
41 and provide several virtues, as illustrated in Fig. 1.
42 Along with the numerous exposed MnO 2 catalytic
43 sites, the multiscale (micro/meso/macro) pores offer
44 sufficient space and interface for O 2 diffusion, Li2O2
45 accommodation, and electrolyte immersion, while
46 the interconnected carbon matrix serves as electron
47 conducting network. Otherwise, the MnO2 wrapped
48 HPC hybrid hinders the reduced oxygen species (O 2-)
directly contact with the carbon, resulting in
suppressed
side
reactions.
Expectedly,
the
51 MnO2/HPC hybrid endows the Li-O2 battery with
52 low overpotential, good rate capability, and stable
53 cyclability.
1
49
2
50
54
55
56
57
Figure 1 Schematic illustration of the synthesis of MnO2/HPC
(HPC: hierarchically porous carbon) nanocomposite and its
merits as an oxygen electrode in a Li-O2 battery.
58
59
2
Experimental
60
61
2.1 Materials Synthesis
62
The honeycomb-like structure hierarchically porous
carbon (HPC) was firstly prepared via a
65 self-assembly template strategy. Typically, 0.5 g of
66 phenolic resin was ground into powder and added to
67 20 mL of absolute ethanol at 50 C. After completely
68 dissolved, 2 mL of tetraethylorthosilicate (TEOS) was
69 added and stirring for 15 min to form a transparent
70 light yellow solution. 50 mL mixture of 25 wt%
71 aqueous ammonia and ethanol (2:1, volume ratio)
72 was quickly poured into the solution and then
73 reacted at 50 C for 3h. The mixed slurry was
74 evaporated at 60 C and the obtained solid was
75 carbonized at 700 C for 2h in an argon atmosphere.
76 The black product was treated with 10 wt % HF to
77 remove the silica, then washed with distilled water
78 and dried to obtain HPC spheres.
79
The MnO2/HPC nanocomposite was synthesized
80 via a direct redox reaction between the obtained
81 porous carbon and the KMnO4 solution. In this case,
82 the hierarchically carbon acts not only as a sacrificial
83 reducing agent but also as a substrate for the growth
84 of MnOx. In brief, 30 mg as-prepared porous carbon
85 was firstly ultrasonically dispersed in 30 mL KMnO 4
86 solution (0.05 M) in an ice bath. Then, the mixed
87 solution was reacted for 90 min at 50 C and the final
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product was obtained by centrifugation and washed
with distilled water before drying in vacuum at 80 C
3 overnight. Pure MnO2 was obtained by prolonging
4 the reaction time to 3 h.
5
For comparison, the composites of loading MnO2
6 on other types of carbon (Vulcan carbot XC-72 and
7 Super P, designated as MnO2/Vc-72 and MnO2/Super
8 P,
respectively) were prepared through similar
9 procedures using the corresponding carbon supports.
homogenous ink of 90 wt% catalyst materials and 10
wt% polyvinylidene fluoride (PVdF) onto the Ni
51 foam current collector with a catalyst mass loading of
52 about 0.8 mg cm-2. After completing assembly, the
53 cells were placed in a sealed container filled with 1
54 atm high-purity oxygen.
55
Electrochemical tests were performed at room
56 temperature after 5 h rest on a Land-CT2001A battery
57 system. All the specific capacity and current density
58 were calculated based on the total amount of the
59 catalyst. The cyclic voltammograms (CVs) were
60 conducted
within 2.0-4.6 V on Parstat 263A
61 workstation (AMTECT). Electrochemical impedance
62 spectroscopy (EIS) was tested using Parstat 2273
63 potentiostat/galvanostat
workstation within a
64 frequency range of 100 kHz to 100 mHz.
1
49
2
50
10
11
2.2 Materials Characterization
12
The structures and morphologies of the as-prepared
14 samples were respectively characterized by powder
15 X-ray diffraction (XRD, Rigaku/MiniFlex600 with Cu
16 Kα
radiation),
Raman
microscope
(DXR,
17 Thermo-Fisher
Scientific at 532 nm excitation),
18 field-emission scanning electron microscopy (SEM,
19 JEOL
JSM7500F) and transmission electron
20 microscopy (TEM, Philips Tecnai-F20). The carbon
21 content
of the composite was determined by
22 elemental analyser (Elementar, vario El cube) and
23 energy dispersive X-ray spectroscopy (EDS). X-ray
24 photoelectron
spectroscopy (XPS) results were
25 collected on a Perkin Elmer PHI 1600 ESCA system.
26 X-ray absorption (XAS) was performed on Shanghai
27 Synchrotron Radiation Facility (SSRF). Surface area
28 analysis
was measured by using the N2
29 adsorption/desorption
isotherms at 77 K on a
30 BELsorp-Mini instrument. Thermogravimetric (TG)
31 tests were performed using a Netzsch STA 449 F3
32 Jupiter analyzer. Fourier Transform Infrared (FTIR)
33 spectroscopies
were performed on a FTIR-650
34 spectrometer (Tianjin Gangdong) at a resolution of 2
35 cm-1.
13
36
37
38
2.3
Cell
assembly
measurements
and
electrochemical
39
The electrochemical performance of non-aqueous
41 Li-O2 cell was analyzed by a 2023-type coin cell. All
42 the cells were fabricated in an argon-filled glove box
43 (Mikrouna Universal 2440/750) using a lithium metal
44 anode, a glass fiber separator, a catalyst contained
45 oxygen cathode and an electrolyte of 1 M LiTFSI
46 (lithium
bis-(trifluoromethanesulfonyl) -imide) in
47 TEGDME (tetraethylene glycol dimethyl ether). The
48 oxygen
cathodes were prepared by casting a
40
65
66
3
Results and discussions
67
Fig. 2a displays the typical scanning electron
microscopy (SEM) image of MnO2/HPC. The hybrid
70 maintains the morphology of the honeycomb-like,
71 shell-connected hollow HPC microspheres (Fig. S1 in
72 the
Electronic Supplementary Material (ESM)).
73 Transmission electron microscopy (TEM) image (Fig.
74 2b)
clearly reveals that uniform MnOx is
75 homogenously supported on the porous carbon. The
76 high-resolution TEM imaging (Fig. 2c) shows lattice
77 fringes with interlayer distance of 0.7 nm, which
78 agrees with the neighbouring spacing of the (001)
79 plane
of birnessite-type MnO2. The texture of
80 MnO2/HPC
is
analyzed
by
nitrogen
81 adsorption-desorption isotherms. The determined
82 Brunauer-Emmett-Teller (BET) specific surface area is
83 95.7 m2/g (Fig. 2d). The corresponding pore size
84 distribution data calculated by the BJH method (inset
85 in Fig. 2d) shows hierarchical pore features with
86 micropores
diameter below 2.0 nm, mesopores
87 centered at 4 nm and macropores located in a wide
88 range of 20–100 nm. In our synthesis, it is noted that
89 a
higher reaction temperature and a longer
90 deposition time would lead to overconsumption of
91 the
carbon substrate and destruction of the
92 hierarchically porous nanostructure (Fig. S2 in the
93 ESM).
94
The formation of birnessite-type MnO2 in
95 MnO2/HPC is confirmed by X-ray diffraction (XRD,
96 Fig. S3 in the ESM). Broad peaks imply poorly
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crystallized property, which may originate from the
small particle size and amorphous nature of the
3 powders as a result of the mild synthesis condition
4 [37]. Raman spectra (Fig. S4 in the ESM) indicate the
5 presence of both carbon and MnO 2. Beside the two
6 characteristic
signals of carbon, two peaks are
7 observed at 570 cm-1 and 640 cm-1 in the low
8 wavelength region, which can be attributed to the
9 Mn-O
stretching and the symmetric stretching
10 vibration
of the MnO6 groups, respectively. In
11 addition, the peak splitting width of 4.85 eV in Mn 3s
12 X-ray photoelectron spectroscopy (XPS, Fig. S5a in
13 the ESM) and the X-ray absorption near edge
structure (XANES, Fig. S5b in the ESM) suggest the
presence of oxygen vacancies and a multiple valence
16 (+3 and +4) of Mn, which could contribute to
17 enhanced
electrochemical activity [24, 38, 39].
18 Furthermore,
elemental mapping evidences the
19 homogenous dispersion of MnO 2 in the carbon
20 framework matrix (Fig. 2e). The weight percentage of
21 manganese oxide is determined to be about 46% in
22 the
synthesized MnO2/HPC nanocomposite by
23 combining energy dispersive spectroscopy (EDS),
24 element analysis, and thermogravimetric analysis
25 (TGA) in air (Fig. S6 in the ESM).
1
14
2
15
26
27
28
Figure 2 (a) SEM and (b, c) TEM images of the synthesized MnO2/HPC composite materials. (d) Nitrogen adsorption-desorption
isotherms and pore size distribution (inset) of MnO2/HPC. (e) Elemental mapping of MnO2/HPC.
The synthesized MnO2/HPC was employed as an
30 ORR/OER bifunctional cathode catalyst in Li-O2
31 batteries. Fig. 3a shows the first discharge-charge
32 voltage profiles of the Li-O2 cells assembled with
33 MnO2/HPC
and the comparative HPC and
34 conventional Super P carbon with the restricting
35 capacity of 1000 mAh g-1. The composite-based cell
36 exhibits
a significantly lowered charge plateau
37 voltage and slightly increased discharge plateau
38 voltage, resulting in an overpotential of 0.68 V. This is
39 ca. 0.75 V and 1.01 V smaller than those of the cells
29
made with HPC and Super P cathodes, respectively.
41 The round-trip efficiency (the ratio of discharge to
42 charge
voltages) of MnO2/HPC cell attains a
43 remarkably high value of about 80%. The kinetics of
44 the three electrodes were also investigated by cyclic
45 voltammetry (CV) in TEGDME (tetraethylene glycol
46 dimethyl
ether)-based electrolyte (Fig. 3b). The
47 MnO2/HPC
hybrid shows a higher ORR onset
48 potential (~ 2.75 V), a much lower OER peak
49 potential (~ 3.80 V), and much larger cathodic/anodic
50 peak currents compared with HPC and Super P.
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Nano Res.
These results indicate superior electrocatalytic
activity of MnO2/HPC.
3
The rate capability of prepared catalysts was tested
4 at various current densities, as shown in Fig. 3c and
5 Fig. S7 in the ESM. The Li-O2 cell with MnO2/HPC
6 exhibits higher discharge capacity than those with
7 HPC and Super P to the terminal discharge voltage of
8 2.0 V. Specially, even at a high current density of 5000
9 mA g-1, the MnO2/HPC-based battery can deliver a
10 discharge capacity of 2260 mAh g -1, which is 4- and
11 75-fold larger than those of HPC (554 mAh g -1) and
12 Super P (30 mAh g-1), respectively. At a current
13 density of 500 mA g-1, the Li-O2 battery with
14 MnO2/HPC maintains a specific capacity of
>2000
15 mAh g-1 for 60 cycles over a wide voltage window of
16 2.0–4.4 V (Fig. S8 in the ESM). In contrast, the
discharge capacity of HPC or Super P rapidly
decreases to < 150 mAh g-1 after 10 cycles. When
19 restricting a cycling capacity of 1000 mAh g -1, the
20 MnO2/HPC
cell sustains 300 cycles with the
21 discharge terminal above 2.0 V while the HPC and
22 Super P cells can merely afford 66 and 34 cycles,
23 respectively (Fig. 3d and Fig. S9 in the ESM). The
24 performance of MnO2/HPC is also much better than
25 pure
MnO2, MnO2/Vc-72 and MnO2/Super P
26 counterparts (Fig. S10 in the ESM), suggesting the
27 advantages
of the hierarchically porous carbon
28 support. The battery performance presented here is
29 among the best results reported in Li-air systems
30 with manganese oxides electrocatalysts (Table S1 in
31 the ESM) [20-23].
1
17
2
18
32
33
34
35
36
Figure 3 (a) First discharge-charge curves of Li-O2 batteries with Super P, HPC and MnO2/HPC at a current density of 100 mA g-1. (b)
CV profiles at a scan rate of 0.1 mV s-1. (c) Galvanostatic profiles of MnO2/HPC electrode at different rates. (d) Cyclability and terminal
discharge voltages of three electrodes at a current density of 350 mA g-1. The current density and specific capacity are calculated on the
basis of the total catalyst weight.
The cathodes of Li-O2 cells were disassembled and
38 characterized at discharged and charged states.
39 Among the discharged electrodes, there is much
40 difference in morphology. The precipitate on Super P
41 (Fig. 4a) presents typical toroid shape [8, 40]. In
42 comparison, mossy species composed of nanosheets
43 uniformly covers the surface of MnO2/HPC (Fig. 4b).
37
TEM imaging (Fig. S11a in the ESM) reveals porous
45 structure
of the discharged mossy solid. The
46 concentric rings in electron diffraction pattern (Fig.
47 S11b in the ESM) are unambiguously indexed to
48 Li2O2 and indicate polycrystalline nature of the
49 generated Li2O2. After recharged, the super P-based
50 electrode
contains aggregated particles and is
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clogged by a dense layer (Fig. 4c) whereas the mossy
discharged
species
disappears
and
the
3 honeycomb-like hierarchically porous morphology is
4 fully regained for MnO2/HPC electrode (Fig. 4d and
5 Fig. S11c, d in the ESM). XRD analysis (Fig. 4e)
6 clearly indicates the formation of Li 2O2, which
7 disappears
in the recharged cathodes. Fourier
8 Transform Infrared (FTIR) spectroscopies (Fig. 4f) of
9 the
recharged electrodes further evidence the
10 presence
of Li2CO3, HCO2Li (Li formate) or
11 CH3CO2Li (Li acetate). Especially, the formation of
12 carbonate
and carboxylate lithium on Super P
13 electrode is much more obvious than the case of
MnO2/HPC hybrid after the same 30 cycles. The
accumulation of these species upon cycling would
16 retard the mass transportation and charge transfer,
17 leading
to increased electrode polarization and
18 performance degradation of the cell. This conjecture
19 is
verified by the electrochemical impedance
20 spectroscopy (Fig. S12 and Table S2 in the ESM) that
21 show
gradual increase of electrode resistance.
22 Compared with neat Super P, the increase rate of
23 charge transfer resistance is much slower in the
24 hybrid electrode. Thus, the MnO 2 wrapped HPC
25 hybrid can effectively suppress the side reaction of
26 carbon.
1
14
2
15
27
28
29
30
31
32
Figure 4 Characterization of the discharged/charged electrodes after 5 cycles at terminal capacity of 1000 mAh g -1 and constant current
density of 350 mA g-1. SEM images of discharged Super P (a) and MnO2/HPC (b) electrodes. SEM images of charged Super P (c) and
MnO2/HPC (d) electrodes. (e) XRD patterns of MnO2/HPC, Super P electrodes and the reference Li2O2. The peaks of MnO2 and Super P
are marked with * and #, respectively. (f) FTIR of the 5th-cycle and 30th-cycle MnO2/HPC, Super P cathodes and the standard lithium
carbonates and carboxylate.
The above results clearly indicate the superior
activity of MnO2/HPC in catalyzing the reversible
35 formation/decomposition
of Li 2O2 during the
36 discharge/charge process of Li-O2 cells. Possibly, the
37 remarkable performance of MnO2/HPC could be
38 attributed to the combined benefits of MnO x and
39 HPC.
The interconnected hierarchically porous
40 structure of carbon network not only favors electron
41 conduction,
oxygen diffusion and electrolyte
42 permeation, but also provides abundant sites and
43 larger
room for Li2O2 precipitation (Fig. 1).
Manganese oxide itself is known to have
considerable activity for the ORR/OER [38, 41-45].
46 Also, the metal oxide surfaces have been shown to be
47 less “sticky” than pure carbon surfaces riddled with
48 dangling
bonds [46], which could weaken the
49 binding
of the intermediate superoxide to the
50 substrate and facilitating the transport of superoxide
51 species,
leading to a thin and uniform
52 nucleation/crystallization of discharged product (i.e.,
53 Li2O2) on the hybrid electrode. These two factors
54 might contribute to the homogeneous distribution of
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peroxides. Compared to the dense toroid-shaped
2 Li2O2 (Fig. 4a), the formed porous and loose solid
3 (Fig. 4b) could provide more exposed surfaces and
4 interfaces,
enabling
easy
decomposition of
5 discharged
product on recharging (Fig. 4d).
6 Consequently, the Li-O2 batteries assembled with
7 MnO2/HPC manifest high round-trip efficiency and
8 good rechargeability.
50
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59
1
10
4
Conclusions
[2]
51
52
[3]
53
54
55
[4]
56
57
58
[5]
60
61
11
In summary, hierarchically porous MnO2/HPC
13 nanocomposite was synthesized and catalytically
14 investigated as cathode electrocatalyst in aprotic
15 Li-O2 batteries. Compared with neat HPC and
16 conventional Super P carbon electrodes, the obtained
17 MnO2/HPC electrodes exhibit lower overpotential (~
18 0.68 V at 100 mA g-1), better rate capability (2260
19 mAh g-1 at 5 A g-1) and longer operational life (~ 300
20 cycles with a fixed terminal capacity of 1000 mAh g -1).
21 The remarkable performance of MnO 2/HPC was
22 ascribed
to a combination of positive effects
23 including intrinsic ORR/OER catalytic activity, the
24 unique architecture, the favorable morphology of
25 Li2O2, and the reduced side reactions. Our results
26 would
enlighten
the
rational
design
of
27 multifunctional porous carbon/metal oxide hybrid
28 materials
as
cheap
yet
efficient
cathode
29 electrocatalysts for advanced Li-O2 batteries.
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[6]
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[8]
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[9]
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Acknowledgements
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85
This work was supported by the National 973
34 (2011CB935900), NSFC (21322101 and 21231005), and
35 MOE (ACET-13-0296 and B12015).
33
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[11]
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Electronic
Supplementary
Material:
38 Supplementary
material
(additional
material
39 characterizations and electrochemical tests, such as
40 XRD, Raman, SEM, TEM, XPS, TGA, EIS results etc)
41 is available in the online version of this article at
42 http://dx.doi.org/10.1007/s12274-***-****-*
43 (automatically inserted by the publisher).
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31
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1
Electronic Supplementary Material
2
3
4
5
Uniform
MnO2
nanostructures
supported
on
hierarchically
porous
carbon
as
efficient
electrocatalyst for rechargeable Li-O2 batteries
Xiaopeng Han, Fangyi Cheng(), Chengcheng Chen, Yuxiang Hu and Jun Chen()
Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)
6
7
8
Figure S1 SEM (a, b) and TEM (c, d) images of HPC. e) Elemental mapping and EDS spectra of HPC. SEM
images display the macroporous honeycomb-like monolith morphology, which is made up of shell-connected
11 carbon hemispheres. The inset of (b) shows the diameter distribution of carbon spheres (diameter about 410
12 nm). TEM images reveal the macroporous inner structure and the inter-connected carbon framework of the
13 as-prepared HPC. No obvious Si signal was observed from the EDS spectra, indicating the silica template has
14 been completely removed.
9
10
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Nano Res.
1
2
3
Figure S2 SEM images of MnO2/HPC samples with different preparation procedures. (a) reaction temperature
50 C, deposition time 180 min; (b) reaction temperature 65 C, deposition time 90 min.
4
Figure S3 XRD patterns of synthesized pure HPC, MnO2/HPC samples. The broad diffraction peaks at 2
6 around 11.8, 37.2 and 66.0 in the MnO2/HPC composite can be readily indexed to the birnessite-type MnO2
7 (Joint Committee on Powder Diffraction Standards, JCPDS card No. 42-1317).
5
8
9
Figure S4 Raman spectra of prepared HPC and MnO2/HPC.
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Nano Res.
1
Figure S5 (a) Mn 3s XPS spectra and (b) Mn K-edge XANES of synthesized MnO2/HPC composite. Mn K-edge
3 XANES of β-MnIVO2 and MnII,III3O4 are shown as reference standards. The Mn 3s XPS splitting peaks energies
4 locate at 88.5 eV and 83.65 eV, respectively, with separation of 4.85 eV, which indicates a mixed valence of Mn
5 between +3 and +4 [1, 2]. This result is also confirmed by comparing the K-edge absorption position to those of
6 β-MnIVO2 and Mn3O4 reference standards.
2
7
Figure S6 EDS spectra (a) and TG profile (b) of as-prepared MnO2/HPC hybrid at heating rate of 5 C min-1 in
9 air flow at speed of 50 mL min -1. In EDS spectra, the signal of potassium (K) element can be found. The
10 presence of K cations in the formed birnessite-type MnO2 originates from the KMnO4 precursor, which could
11 stabilize the lattice structure of manganese oxides [3, 4]. In TG profile, the mass loss before 150 C can be
12 attributed to the removal of physi-adsorbed water. The following rapid mass loss, peaked at 380 C, correspond
13 to the carbon combustion in the MnO 2/HPC composite.
8
14
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Nano Res.
1
Figure S7 Rate performance of (a) HPC and (b) Super P electrodes.
2
3
Figure S8 Cycling performance of three catalysts at 500 mA g-1 over a wide voltage window from 2.0 to 4.4 V.
4
Figure S9 Discharge-charge curves of the Li-O2 batteries with three cathode catalysts at selected cycles at a
6 current density of 350 mA g-1 with controlling cycling capacity of 1000 mAh g -1. (a): MnO2/HPC hybrid, (b):
7 pure HPC, (c): conventional Super P carbon.
5
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Nano Res.
1
Figure S10 TEM images of prepared MnO2/Vc-72 (a) and MnO2/Super P (b). (c) First discharge/charge profiles
at 100 mA g-1 and (d) Cyclability and terminal discharge voltages at a current density of 350 mA g -1 based on
4 pure MnO2, MnO2/Vc-72 and MnO2/Super P. To overcome the poor electronic conductivity, the pure MnO 2
5 electrode was prepared by mixing pure MnO 2 with the Vulcan carbot XC72 (weight ratio, 3:7). As for
6 MnO2/Vc-72 and MnO2/Super P, MnO2 nanowires were grown on the carbon supports. The morphologies of
7 deposited MnO2 on Vc-72 and Super P are much different from the case of HPC that confines homogenous
8 surface growth of MnO2. With the cut-off capacity of 1000 mAh g -1, the charge/discharge overpotentials are 1.56
9 V, 1.36 V and 1.20 V and the cycling number are 33, 64 and 74 for pure MnO 2, MnO2/Super P and MnO2/Vc-72
10 cathodes, respectively. The superior performance of MnO 2/HPC can be clearly seen, indicating the advantages
11 of the hierarchically porous carbon support which not only facilitates the transportation of oxygen and
12 electrolyte but also offers empty space to accommodate the discharge product, thus leading to enhanced
13 electrode performance.
2
3
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Nano Res.
1
Figure S11 (a) TEM image of discharged MnO2/HPC cathode and (b) the corresponding electron diffraction
pattern. (c) TEM image of recharged MnO2/HPC cathode. (d) Raman spectra of pristine MnO2/HPC cathode
4 and the electrodes after five cycles. Raman results indicate that the Li 2O2 generates after discharging and
5 disappears in the recharged electrode, whereas there is no signal showing structural change of the MnO2/HPC
6 catalyst before and after cycling.
2
3
7
Figure S12 Electrochemical impedance spectroscopies (EIS) of (a) MnO 2/HPC and (b) Super P of
discharged/recharged cathodes of pristine, the 5th and 30th cycles. At 5th discharged state, the increase extent
10 of charge transfer resistance (Rct) of MnO2/HPC electrode (72.8  to 144.1 ) is slightly lower than that of Super
11 P (192.1  to 571.2 ), which is in consistent with the different case of deposited discharged products and the
12 restrained ability towards the side reaction. After 5th recharging, the resistance of MnO 2/HPC-based batteries
13 could recover to that of the fresh batteries to a large extent (96.0  vs. 72.8 ), whereas the Super P electrodes
14 cannot recover absolutely (441.5  vs. 192.1 ). This phenomenon becomes much more apparent for extended
8
9
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Nano Res.
30th cycle (see Table S2 for details). The EIS results here again suggest a relatively reversible reaction for Li 2O2
formation/decomposition during the repeated discharge/charge process over the MnO2/HPC cathode as well as
3 the suppressed side reaction with the carbon.
1
2
4
Table S1 Summary of electrochemical performance of Li-O2 batteries based on manganese oxides catalysts.
5
catalyst
maximum
capacity based
on total mass
/ mAh g-1
rate capability
/ mAh g-1
cycle performance
/controlling capacity
α-MnO2 [5]
730
----
10 cycles/2000 mAh g-1
α-MnO2/Graphene [6]
2304
----
25 cycles/3000 mAh g-1
-MnO2 [7]
7000
1900 at 800 mA g-1
120 cycles/>1000 mAh g-1
3D-Graphene-MnO2 [8]
3660
770 at 387 mA g-1
132 cycles/1000 mAh g−1
MnCo2O4/Graphene [9]
3780
2743 at 800 mA g-1
40 cycles/1000 mAh g-1
La0.75Sr0.25MnO3 [10]
7333
6666 at 133 mA g-1
124 cycles/1000 mAh g−1
manganese oxide
nanowires [11]
~5000
----
47 cycles/2000 mAh g-1carbon
our sample
9283
2260 at 5000 mA g-1
300 cycles/1000 mAh g−1
6
7
8
Table S2 Comparison of the fitted charge transfer resistance (Rct, ) at different discharged/recharged states with
prolonged cycles based on MnO2/HPC and Super P cathodes.
9
States
Pristine
5th discharge
5th recharge
30th discharge
30th recharge
MnO2/HPC
72.8
144.1
96.0
188.5
119.0
Super P
192.1
571.2
441.5
755.3
605.0
Catalyst
10
11
Supplementary References
[1] Toupin, M.; Brousse, T.; Bélanger, D. Charge storage mechanism of MnO2 electrode used in aqueous Electrochemical
capacitor. Chem. Mater. 2004, 16, 3184-3190.
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Toupin, M.; Brousse, T.; Bélanger, D. Influence of microstucture on the charge storage properties of chemically
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synthesized manganese dioxide. Chem. Mater. 2002, 14, 3946-3952.
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Xiao, W.; Wang, D. L.; Lou, X. W. Shape-Controlled Synthesis of MnO2 Nanostructures with Enhanced
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Electrocatalytic Activity for Oxygen Reduction. J. Phys. Chem. C 2009, 114, 1694-1700.
18 [4]
Liu, J. P.; Jiang, J.; Cheng, C. W.; Li, H. X.; Zhang, J. X.; Gong, H.; Fan, H. J. Co 3O4 Nanowire@MnO2 Ultrathin
19
Nanosheet Core/Shell Arrays: A New Class of High-Performance Pseudocapacitive Materials. Adv. Mater. 2011, 23,
20
2076-2081.
21 [5]
Débart, A.; Paterson, A. J.; Bao, J. L.; Bruce, P. G. α-MnO2 nanowires: a catalyst for the O2 electrode in rechargeable
22
lithium batteries. Angew. Chem. Int. Ed. 2008, 47, 4521-4524.
23 [6]
Cao, Y.; Wei, Z. K.; He, J.; Zang, J.; Zhang, Q.; Zheng, M. S.; Dong, Q. F. α-MnO2 nanorods grown in situ on
24
graphene as catalysts for Li-O2 batteries with excellent electrochemical performance. Energy Environ. Sci. 2012, 5,
25
9765-9768.
26 [7]
Hu, X. F.; Han, X. P.; Hu, Y. X.; Cheng, F. Y.; Chen, J. -MnO2 nanostructures directly grown on Ni foam: a cathode
12
13
Address correspondence to Fangyi Cheng, [email protected]; Jun Chen, [email protected]
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Nano Res.
catalyst for rechargeable Li-O2 batteries. Nanoscale 2014, 6, 3522-3525.
Liu, S. Y.; Zhu, Y. G.; Xie, J.; Huo, Y.; Yang, H. Y.; Zhu, T. J.; Cao, G. S.; Zhao, X. B.; Zhang, S. C. Direct growth of
3
flower-like δ-MnO2 on three-dimensional graphene for high-performance rechargeable Li-O2 batteries. Adv. Energy
4
Mater., in press, DOI: 10.1002/aenm.201301960.
5 [9]
Wang, H. L.; Yang, Y.; Liang, Y. Y.; Zheng, G. Y.; Li, Y. G.; Cui, Y.; Dai, H. J. Rechargeable Li–O2 batteries with a
6
covalently coupled MnCo2O4–graphene hybrid as an oxygen cathode catalyst. Energy Environ. Sci. 2012, 5,
7
7931-7935.
8 [10] Xu, J. J.; Xu, D.; Wang, Z. L.; Wang, H. G.; Zhang, L. L.; Zhang, X. B. Synthesis of perovskite-based porous
9
La0.75Sr0.25MnO3 nanotubes as a highly efficient electrocatalyst for rechargeable lithium–oxygen batteries. Angew.
10
Chem. Int. Ed. 2013, 52, 3887-3890.
11 [11] Oh, D.; Qi, J. F.; Lu, Y.-C.; Zhang, Y.; Shao-Horn, Y.; Belcher, A. M. Biologically enhanced cathode design for
12
improved capacity and cycle life for lithium-oxygen batteries. Nat. Commun. 2013, 4, 2756-2763.
1
2
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