Controllable Hortensia-like MnO2 Synergized with Carbon nanotubes

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Energy, Environmental, and Catalysis Applications

Controllable Hortensia-like MnO2 Synergized with Carbon nanotubes as an Efficient Electrocatalyst for Long-term Metal-air Batteries Nengneng Xu, Qi Nie, Lingyiqian Luo, Chen-Zhong Yao, Qiaojuan Gong, Yuyu Liu, Xiaodong Zhou, and Jinli Qiao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b15047 • Publication Date (Web): 07 Dec 2018 Downloaded from http://pubs.acs.org on December 8, 2018

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ACS Applied Materials & Interfaces

Revised version: ID: am-2018-15047n.R3

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Controllable Hortensia-like MnO2 Synergized with Carbon nanotubes as an Efficient Electrocatalyst for Long-term Metal-air Batteries

4

Nengneng Xua,b,c, Qi Nieb, Lingyiqian Luoc, Chenzhong Yaoa, Qiaojuan Gonga*, Yuyu Liud,

1 2

5

Xiao-Dong Zhouc, Jinli Qiaoa,b*

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aDepartment

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Cheng 04400, China

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bState

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of Environmental Science and Engineering, Donghua University, 2999 Ren’min North

of Applied Chemistry, Yuncheng University, 1155 Fudan West Street, Yun

Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College

10

Road, Shanghai 201620, China

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cDepartment

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70504

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dInstitute

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China

of Chemical Engineering, University of Louisiana at Lafayette, Lafayette, LA

of Sustainable Energy, Shanghai University, 99 Shangda Road, Shanghai 200444,

15 16

KEYWORDS. MnO2/CNTs, oxygen evolution reaction, oxygen reduction reaction,

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stability, metal-air battery

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ABSTRACT. The exploitation of a high-activity and low-cost bifunctional catalyst for

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oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) as cathode

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catalyst is a research priority in metal-air batteries (MeABs). Herein, a novel bifunctional

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hybrid catalyst of hortensia-like MnO2 synergized with carbon nanotubes (CNTs) (MnO2/CNTs)

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is controllably synthesized by reasonably designing the crystal structure and morphology as well

ACS Paragon Plus Environment

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as electronic arrangement. Based on these strategies, the hybrid accelerates the reaction kinetics

2

and avoids the change of structure. As expected, MnO2/CNTs exhibit remarkable ORR and OER

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activity (low ORR Tafel slope: 71 mV dec-1, low OER Tafel slope: 67 mV dec-1, and small

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potential difference (ΔE): 0.85 V) and long-term stability, which should be attributed to its

5

unique morphology, K+ ions in the 2×2 tunnels, and synergistic effect between MnO2 and CNTs.

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Notably, in zinc-air batteries (ZABs), aluminum-air batteries (AABs) and magnesium-air

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batteries (MABs), the composite shows high power density (ZABs: 243 mW cm-2, AABs: 530

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mW cm-2 and MABs: 614 mW cm-2) and large specific capacities (793 mAh gZn-1, 918 mAh gAl-1

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and 878 mAh gMg-1). Importantly, the rechargeable ZABs reveal small charge/discharge voltage

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drop (0.81 V) and strong cycle durability (500 hours), which are better than the noble-metal

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Pt/C+IrO2 mixture catalyst (the voltage drop: 1.15 V at initial and 2 V after 100 hours). These

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superior performances together with the simple synthetic method of the hybrid reveal great

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promise in large-power energy storage and conversion equipment.

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INTRODUCTION

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In response to the challenges of environment pollution and energy crisis, the immediate

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imperative is to develop renewable clean energy instead of using traditional fossil fuels.1-4

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Among these energies, metal-air batteries (MeABs), possess lower cost, higher power density,

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larger energy density, and better environmental compatibility, attracting tremendous interest.5-7

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However, their widespread application is still limited by their limitations in the sluggish kinetics

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of oxygen reduction and evolution reactions (ORR/OER).4,

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electrochemical processes, determine the performance of MeABs.10-11 In this case, exploiting

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high-performance electrocatalysts has become the all-important opportunity for accelerating

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ORR and OER, and finally boosting the overall performance of MeABs. 2


8-9

ORR and OER, as core

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Among various materials, noble metals, carbon materials and transition metal oxides have

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been widely investigated because of the decent electrochemical performance.9 However, the

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unsatisfactory durability and high cost of noble metal and their alloy directly hinder their large-

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scale applications.12-14 Thus, it is urgent to develop highly efficient catalyst for ORR and OER in

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MeABs. Compared with other alternatives for noble metal catalysts, MnO2 is the most widely

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studied ORR catalyst for primary MeABs because of its unique advantages, like low cost, low

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toxicity and environmental friendliness.15-18 Meanwhile, MnO2 exists in     and  phrases.

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Among all crystallographic forms, -MnO2 shows the highest performance owing to its unique

9

layered structure and 2×2 tunnel structure.19 Especially, the 2×2 tunnel structure formed corner-

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or edge- sharing [MnO6] octahedral to improve ions transfer in the lattice framework, further

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promoting the ORR rate.20 Furthermore, -MnO2 surface has abundant hydroxyl groups and

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defects, which benefits the dissociation of O-O double bond and the adsorption/desorption of

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O2.21 However, the low electron transfer rate further limits the catalytic activity of -MnO2 due

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to poor electric conductivity.22 Carbon nanotubes (CNTs) herein were added into MnO2 because

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of its high conductivity and large surface area.10,

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coatings, and introducing conductive substrates to improve the catalytic performance of MnO2

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(high-cost and complex process), the electrochemical behaviors of metal oxide catalysts can be

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quickly enhanced by combining with CNTs. The method of adding CNTs endows the composites

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with excellent activity and high durability by accelerating the electron transfer, providing larger

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reaction area, and thus improving the contact between electrode and electrolyte.24-26 Although -

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MnO2 and carbon composites possess ideal ORR catalytic activity, its OER activity is very poor.

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Encouragingly, the catalytic behaviors of MnO2 could be dramatically improved by controllable

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strategies through the optimization of surface configuration, morphological architecture and

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electronic state.27-29 According to the aforementioned discussion, it inspires researchers to

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deliberate whether it is the method to enhance the electrochemical performances (particularly,

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Compared with cations doping, metal

3



1

OER) of MnO2 by the rational morphology design and the combination of CNTs. In addition, the

2

corresponding analyses have not been studied in terms of the effects of crystal structures and

3

morphology of the composites for the ORR/OER catalytic activity/stability.

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Based on these considerations, hortensia-like MnO2, which prepared by adjusting the

5

parameter of first-step hydrothermal reaction, interweaved with CNTs (denoted as MnO2/CNTs)

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have been controllably designed by simple chemical reaction in this work. As expected, the

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MnO2/CNTs composite shows high ORR/OER activity (0.85 V) and superior stability (2000

8

cycles). Meanwhile, the MeABs based on the hybrid catalyst exhibit high power density

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(aluminum-air batteries (AABs): 530 mW cm-2, magnesium-air batteries (MABs): 614 mW cm-2

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and zinc-air batteries (ZABs): 243 mW cm-2), strong durability, and large specific capacities,

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which are better than the state-of-the-art Pt/C+IrO2 catalyst. Most importantly, the rechargeable

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ZABs display extremely small voltage difference of 0.81 V and superior stability without

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decaying after 500 hours. Such excellent ORR/OER activities and battery performances of the

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MnO2/CNTs composite can be attributed to the 2×2 tunnel structure, unique interconnected

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conducting network and the synergistic effect between MnO2 and CNTs. Particularly, these

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advantageous morphological and structural characterization as well as the synergistic effect

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efficiently afford high density reaction surface and expose abundant actives sites, benefiting

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contact/activation of O2 and OH-. The unique network provides more electron transport highway

19

that improves electron transfer and facilitates the transport of electrolytes and oxygen, and act as

20

an effective support and buffer layer to hinder the structural conversion/ collapse/ aggregation.

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Additionally, the K+ ions in the tunnels are beneficial for the structural stability and activity of

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the catalyst.

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EXPERIMENTAL SECTION Preparation of catalyst. 0.79 g of potassium permanganate (KMnO4, >99.5%, 4


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Sinopharm chemical reagent Co., Ltd, China) and 0.20 g melamine (C3N3(NH2)3, >99.5%,

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Sinopharm chemical reagent Co., Ltd, China) were added into 50 mL of deionized water.

3

Then, the obtained solution (pH = 8.2) was transferred into a steel autoclave kept at 140 oC for

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different times, such as 1, 2, 4, 8, 12 and16 hours. Finally, the precursor powder was collected by

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five centrifuge-wash cycles and was annealed in a furnace at 350oC for 1 hours. These final

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samples were denoted as MnO2-xh (MnO2-1h, MnO2-2h, MnO2, MnO2-8h, MnO2-12h and MnO2-16h,

7

respectively.) MnO2/CNTs are synthesized by further hydrothermal methods. In detail, 0.5 g

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precursor MnO2-4h and 0.025 g CNTs were added into 15 mL NH4OH (pH = 12.8), then

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transferred into a steel autoclave kept at 160 oC for 6 hours. Then, the hybrid was washed with

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DI water and followed by drying at 70 oC for 6 hours. The collected powder was heated at 350

11

oC

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CNTs (MnO2+CNTs) was also prepared by directly mixing the 0.5g MnO2 powder with 0.025g

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CNTs (weight ratio: 1:0.05) in ethanol, followed by drying in an oven at 70oC for 1 hour. To

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explore the effects of CNTs, the different ratio hybrids were obtained by adjusting the additive

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weight of CNTs. In detail, 0.5 g precursor MnO2 and CNTs (the weight ratio of CNT to MnO2

16

varied from 0.01, 0.025, 0.1 and 0.2 respectively) were added into 15 mL NH4OH, and

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transferred into a steel autoclave kept at 160 oC for 6 hours. Then, the precursor was washed with

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DI water and followed by drying at 70 oC for 6 hours. The collected powder was heated at 350

19

oC

for 1 hour (denoted as MnO2/CNTs-0.01, MnO2/CNTs-0.025, MnO2/CNTs-0.1 and MnO2/CNTs-

20

0.2,

respectively).

for 1 hour (denoted as MnO2/CNTs). For comparison, the mixture of MnO2 powder and

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In order to discuss the effect of the K+ ions, the as-synthesized MnO2 was soaked in

22

concentrated HNO3 accompanied by continuously stirring at 60 oC for 3 days.30-31 Then, the

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MnO2 was washed with DI water until the pH of the solution was 7. Finally, for removing the

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residual water in the 2×2 tunnels, the sample was heated at 280 oC in air.

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Physical characterization. X-ray diffraction (XRD, Philips PW3830) was performed to

26

analyze the crystallite phase and structure of the obtained samples over the 2θ range from 5o to

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80o. X-ray photoelectron spectroscopy (XPS, Thermo Scientific) was carried out on a Theta 5



1

Probe electron spectrometer with a monochromatic Al Ka source (1,486.7 eV) to characterize the

2

chemical states and compositions of the samples. Scanning electron microscopic (SEM, Hitachi

3

S3600 instrument, 15 kV), Transmission electron microscopy (TEM, JEOL 2010F, 200 kV

4

accelerating voltage) and high-resolution TEM (HRTEM) were used to investigate the

5

morphology and structure of the samples. The Brunauer-Emmett- Teller (BET, Quantachrome

6

Instruments NOVA 1200e) method was utilized to calculate the specific surface areas. Initially,

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all samples were degassed on the instrument degassing station at 180 oC.

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Electrode preparation and Electrochemical measurements. Catalyst ink was prepared by

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mixing 5 mg of sample with 8 L 5 wt% nafion and 1 mL ethanol. Then, 8 L ink was loaded

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onto the clean rotating disk electrode (RDE). Notably, the mixture of commercial Pt/C (20%)

11

and IrO2 (the ratio of weight: 1:1) was also coated onto the RDE as benchmark electrode. The as-

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prepared RDEs were used as the working electrode. The saturated Hg/HgO electrode and Pt wire

13

were used as the reference electrode and the counter electrode, respectively. Especially, all

14

obtained potentials were converted to reversible hydrogen electrode (RHE) by Nernst equation.

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Before ORR/OER activity test, all working electrodes were carried out cyclic voltammetry (CV).

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Then, liner scanning voltammetry (LSV) was performed with the potential range of 0.2 V -1.0 V

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for ORR at 5 mV s-1and a rotation rate of 300 rpm, 600 rpm, 900 rpm, 1200 rpm, 1500 rpm and

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the potential range of 1.0 V - 2 V for OER at 5 mV s-1and a rotation rate of 1600 rpm without Ir

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compensation. The transferred electron number of ORR was calculated from the slopes of

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Koutecky-Levich (K-L) plots.32 The electrical double layer capacitor (Cdl) of sample was

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measured from double-layer charging curves using CV at different scan rates from 2 to 10 mV s-

22

1

23

rate is the Cdl.

in a potential range of 1.10-1.15 V. The slope of plot of current density at 1.14 V against scan

24

MeABs assembly and measurement. Catalyst sprayed on the gas diffusion layer was used

25

as air-cathode. AABs were fabricated using polished aluminum plate as anode and 4 M KOH

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solution as electrolyte. MABs were fabricated using polished magnesium plate as anode and 15

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wt% NaCl solution as electrolyte. ZABs were fabricated using clean zinc plate as anode and 6

28

M KOH solution as electrolyte. ZABs, AABs and MABs were tested under ambient atmosphere. 6


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Notably, the mixture of 6 M KOH and 0.2 M ZnCl2 as the cycling electrolyte was used in the

2

rechargeable ZABs, which is continuously pumped to the system. The discharge polarization

3

curve and power density plots were obtained using a Galvanodynamic method. The charge-

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discharge cycling was performed by a galvanic pulse method at constant current density.

5 6

RESULT AND DISCUSSION

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To expound the growth mechanism of hortensia-like MnO2, their morphologies synthesized

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under 1, 2, 4, 8, 12, and 16 hours were studied by SEM (Figure 1). Based on the MnO2

9

morphology evolution, their morphologies are time dependent. In the initial synthesized stage (1

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hour, Figures 1a and b), the irregular surface of MnO2 grows many initial crystal nuclei by the

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redox reaction. Then, these crystal nuclei will tend form large secondary structures following

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long reaction time because of the thermodynamic instability (2 hours, Figures 1c and d). During

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the reaction progresses, the primary crystal shows different prior growth orientation due to the

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electronic structure of Mn ion, resulting in the formation of 1D nanosheets of self-assembled

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robust hortensia-like microspheres (4 hours, Figures 1e and f). As the reaction proceeds, these

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hortensia-like microspheres gradually grow nanowires on the surface, and further tend to

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aggregate together forming the coexistence of microspheres and nanowires (8 and 12 hours,

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Figures 1g and j). However, as the reaction time increased to 16 hours (Figures 1k and l), the

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hortensia-like structures are completely destroyed owing to the ultrahigh surface energy. As

20

expected, the optimal MnO2 exhibits unique hortensia-like morphology and remarkable

21

ORR/OER activity (Figure S1), and thus is chosen as a precursor to further design and

22

synthesize the final sample of MnO2/CNTs.

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As shown in Figure 2a, the final sample of MnO2/CNTs was synthesized by further simple

24

hydrothermal reaction. The crystalline phase of MnO2, CNTs and MnO2/CNTs are disclosed by

25

the XRD spectrum (Figure 2b). The crystalline phase of MnO2/CNTs is similar to that of MnO2-

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4h.

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0141, Figure 1c), in exception to one small peaks at 26.4° attributed to the CNTs, suggesting

28

successful conversion of MnO2 into MnO2/CNTs. Meanwhile, the representative low-

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magnification TEM images (Figures 2d and 2f) shows that the MnO2/CNTs and MnO2 have

Their diffraction peaks are attributed to the tunnel structure -MnO2 (JCPDS Card no. 44-

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similar hortensia-like morphology. Especially, the hortensia-like nanostructure is completely

2

maintained after second hydrothermal reaction process, suggesting prominent structural and

3

morphological stability. Evidently, the hortensia-like MnO2 is well anchored on the surface

4

CNT, and interweaved with CNTs forming an interconnected network, which could effectively

5

enlarge the electroactive region, improve the conductivity of the hybrid, shortening the ion

6

diffusion pathway, provide a direct electron transfer pathway, preventing the aggregation of

7

hortensia-like MnO2 while avoiding the collapse of hortensia-like morphology during the ORR

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and OER process, further improving the structure stability and enhancing the catalytic activity.

9

The HRTEM images (Figures 2e and g) indicate that the detailed morphologies and crystal

10

structures of MnO2 into MnO2/CNTs. Figure 2e reveals two lattice-spacing of 0.69 and 0.49 nm,

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corresponding to the (110) and (200) lattice planes of -MnO2, respectively. Furthermore, the

12

HRTEM image of MnO2/CNTs also shows two lattice-spacing of 0.24 nm and 0.21 nm which is

13

corresponded to (211) and (301) lattice planes respectively. The result further indicates the 2×2

14

tunnel nature of MnO2/CNTs is in agreement with XRD result. As mentioned above, compared

15

with MnO2, the MnO2/CNTs hybrid form a transport network (Figure S2), effectively enlarging

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the electroactive region of the hybrid and preventing the aggregation of metal oxide, which can

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be proved by nitrogen adsorption-desorption isotherms. As shown in Figure 2h and its inset,

18

MnO2 and MnO2/CNTs display a type-IV isotherm. Notably, the Brunauer-Emmett-Teller (BET)

19

specific surface area for the MnO2/CNTs is 345 m2 g-1, which is larger than that of MnO2 (97 m2

20

g-1) and MnO2+CNTs (127 m2 g-1), and is closed to the CNTs (431 m2 g-1, Figure S3, Supporting

21

Information). The phenomenon demonstrates the MnO2 synergized with CNTs can provide larger

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active surface area and more active sites for electrochemical process, which is further evaluated

23

by the electrochemical active surface area (ECSA) discussed later.33 Undoubtedly, the high

24

specific

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adsorption/desorption of reactant and product, and accelerate the transport of ions and electrons,

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thus advantageous for the ORR and OER activity.34 In addition, the crystal chemistry of -MnO2

27

prepared from redox reaction of potassium permanganate always contains a residual fraction of

28

K+ ions in the 2×2 tunnels. As confirmed by both the TEM with energy-dispersive X-ray

29

spectroscopy (TEM-EDS, Figure S4) and the XRD with Rietveld refinement (Table S1), the

surface

can

effectively

afford

more

triple

8


phase

regions,

improve

the

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1

compositions of a-MnO2 and MnO2/CNTs were thus deduced to be K0.091MnO2 and

2

K0.078MnO2/CNTs, respectively, indicating that the little K+ ions probably lost into the NH4OH

3

solution during the second hydrothermal process.

4

To probe the chemical states of MnO2 and MnO2/CNTs, XPS measurements are performed

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and the corresponding plots are revealed in Figures 2i-j and Figure S5. Two obvious peaks

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locate at 642.35 and 654.03 eV, corresponding to the Mn 2P3/2 and Mn 2P1/2, respectively.35-36

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Whereas the peaks of MnO2/CNTs (Mn 2P3/2: 641.90 eV and Mn 2P1/2: 653.58 eV) shift

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negatively compared with pore MnO2 which stem from the interaction between MnO2 and CNTs.

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Figure 2j displays that the O1s spectrum of MnO2 and MnO2/CNTs were deconvoluted into four

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sharp peaks (O1, O2, O3 and O4), which corresponds to the metal-oxygen bonds, the hydroxyl

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groups at surface, the number of defect sites, and the physi/chemisorbed water at the surface.37-38

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The compound MnO2/CNTs reveals higher spectral area value of O3 (42%) than that of MnO2

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(38%), indicating MnO2/CNTs with more defect sites due to the existence of CNTs.39 Moreover,

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the four peaks of MnO2/CNTs show more closed spectral area value than MnO2, further

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suggesting that the change of electronic structure resulted from the synergistic effect between

16

MnO2 and CNTs. Based on the discussion, the possible ion diffusion pathway and electron

17

transfer pathway can be demonstrated in Figure 2k: first of all, unique hortensia-like

18

morphology affords high density reaction surface, benefiting contact/activation of O2 and OH-;

19

Then, the synergistic effect between MnO2 and CNTs directly accelerate the ORR/OER kinetics;

20

Additionally, the interconnected conducting network not only acts as the electron transport

21

highway achieving rapid electron transfer in the entire electrochemical process; but also

22

facilitating the transport of electrolytes and oxygen. Hence, the hybrid will vastly improve the

23

electrochemistry and battery performance by taking advantage of unique hortensia-like

24

morphology and transport network as well as the synergistic effect between MnO2 and CNTs.

25

As expected, the MnO2/CNTs hybrid shows remarkable ORR and OER performance. As

26

shown in Figure 3a and Figure S6a, the hybrid exhibits the largest current density at 0.2 V

27

compared with those of MnO2, CNTs, MnO2+CNTs, MnO2/CNTs-0.01, MnO2/CNTs-0.025,

28

MnO2/CNTs-0.1, MnO2/CNTs-0.2 and Pt/C+IrO2, which is caused by the high BET surface area of

29

the hybrid and the synergistic effect between MnO2 and CNTs. Moreover, the Ej=3 of 9



1

MnO2/CNTs, which is the potential under the ORR current density of 3 mA cm-2, is only 20 mV

2

more negative than Pt/C+IrO2, suggesting that the hybrid has excellent ORR activity.

3

Encouragingly, the MnO2/CNTs hybrid displays greater advantages in onset potential, half-wave

4

potential and current density than CNTs and MnO2, which can be attributed to the synergistic

5

effect between MnO2 and CNTs. In addition, the strong ORR kinetics of the hybrid is further

6

proved by its small Tafel slope of 71 mV dec-1 compared with MnO2 (94 mV dec-1), CNTs (187

7

mV dec-1) and Pt/C+IrO2 (69 mV dec-1, Figure 3b), indicating the quick transport of reactant

8

species. Hence, the high ORR activity of MnO2/CNTs is also benefit from the unique structure

9

by providing more active sites and highly efficient transport pathways. To investigate the

10

electrons transfer number (n) of the ORR, RDEs were carried out at 300, 600, 900, 1200, 1500

11

rpm (Figure S7). The corresponding Koutecky-Levich plots of MnO2/CNTs reveal good

12

linearity. The n of the hybrid catalyst is calculated to be 3.99 which is larger than that of MnO2

13

(3.61 mV dec-1) and CNTs (3.14) and is the same with Pt/C+IrO2 (3.99 mV dec-1, Figure 3c),

14

suggesting the four-electron pathway of the ORR process for MnO2/CNTs (O2(g)+2H2O(l) +4e-

15

=4OH-(l)). In detail, the ORR mechanism on MnO2/CNTs can be described as the following24, 40:

16

MnO2(s)+H2O(l)+e- = MnOOH(s)+OH-(l)

17

2MnOOH(s)+O2(g) = (MnOOH)2 • • • O2(s)

18

(MnOOH)2 • • • O2(s)+e-=MnOOH • • • O(s)+OH-(l)+MnO2(s)

19

MnOOH • • • O(s)+e-=MnO2(s)+OH-(l)

20

Besides ORR, the OER activity of MnO2, CNTs, MnO2/CNTs, MnO2+CNTs, MnO2/CNTs-

21

0.01,

22

M KOH (Figure 3d and Figure S6b). MnO2/CNTs also shows the largest current density at final

23

potential. Meanwhile, the potential under the OER current density of 10 mA cm-2 (Ej=10) of

24

MnO2/CNTs displays is 1.65 V, which is very close to the value of Pt/C+IrO2 and obviously

25

smaller than all samples of MnO2 and CNTs. Notably, the MnO2/CNTs shows the lowest Tafel

26

slope value of 67 mV dec-1 than those of MnO2 (144 mV dec-1), CNTs (283 mV dec-1) and

27

Pt/C+IrO2 (71 mV dec-1, Figure 3e), also indicating its favorable OER kinetics. Similar to ORR,

28

the OER mechanism on MnO2/CNTs can be described as the following41:

29

MnO2/CNTs-0.025, MnO2/CNTs-0.1, MnO2/CNTs-0.2 and Pt/C+IrO2 were also performed in 0.1

MnO2(s)+OH-(l)-e- = MnOOH • • • O(s) 10


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1

MnOOH • • • O(s)+OH-(l)+MnO2(s)-e- = (MnOOH)2 • • • O2(s)

2

(MnOOH)2 • • • O2(s)=2MnOOH(s)+O2(g)

3

MnOOH(s)+OH-(l)-e- = MnO2(s)+H2O(l)

4

As reported, the potential difference (ΔE = Ej=10 - Ej=3) is used to evaluate the ORR and

5

OER activity. The smaller the ΔE is, the better the catalytic activity of the bifunctional catalyst.

6

As shown in Figure 3f, the ΔE value of MnO2/CNTs displays 0.850 V, which is close to that of

7

Pt/C+IrO2 (0.838 V), and obviously smaller than that of CNTs (> 2V) and MnO2 (1.267 V).

8

Moreover, the ΔE value of MnO2/CNTs is also smaller or comparable to other high-performance

9

bifunctional materials reported (Table S2, Supporting Information), directly confirming that the

10

high bifunctional activity toward ORR and OER of the composite catalyst. To further understand

11

the superior bifunctional activity, the ECSA of samples were tested and determined by

12

electrochemical double-layer capacitance (Cdl, Figure S8). As shown in Figure 3g, the obtained

13

Cdl of MnO2/CNTs is 18.7 mF cm-2, which is larger than those of CNTs (5.6 mF cm-2), MnO2

14

(9.6 mF cm-2) and Pt/C+IrO2 (16.7 mF cm-2). The result shows that the composite catalyst has

15

the largest electrochemical active surface area, corresponding to its large BET surface area. The

16

high ECSA of MnO2/CNTs can improve the reaction interface toward ORR and OER by

17

providing more efficient triple phase regions of oxygen, catalyst and the electrolyte.42

18

A high-performance bifunctional catalyst used for MeABs should not only shows the

19

remarkable catalytic activity, but also exhibits the strong durability towards ORR/OER. As

20

shown in Figure 3h, the ORR and OER performance of MnO2 reveals obvious decrease after

21

1000 cycles. Whereas, the MnO2/CNTs hybrid catalyst shows excellent stability. Specifically,

22

LSV curves of initial and after 2000 cycles for MnO2/CNTs are almost identical. Meanwhile, the

23

LSV curves of after 2000 cycles and until 6000 cycles are cycle dependent. Notably, LSV curves

24

of after 6000 cycles and 8000 cycles for MnO2/CNTs are almost identical again, indicating that

25

the ORR and OER performance of the hybrid drops to a minimum level. In a word, these results

26

display the MnO2/CNTs has better durability than MnO2. To gain insight into the primary reason

27

behind the attenuation principle and stability, the crystal structures and morphologies of

28

MnO2/CNTs and MnO2 after the stability test are further assessed by XRD, SEM and TEM

29

characterizations. As shown in Figure 4a, the most peaks of MnO2 after 1000 cycles correspond 11



1

well with the rutile structure -MnO2, suggesting unfortunate conversion of -MnO2 into -

2

MnO2 after stability test. -MnO2 shows more denser 1×1 structure the 2×2 tunnel structure of

3

-MnO2, which limits the electrochemical activity due to its low ions transfer way (Figure 4b).

4

In addition, the morphology of MnO2 displays huge change at durability (Figures 4c-e). The

5

hortensia-like structure of MnO2 was destroyed and agglomerated into large irregular particles.

6

The change of crystal structures and morphology for MnO2 results in the attenuation of

7

performance. The OER and the ORR involve a standard redox reaction (ORR:

8

O2(g)+2H2O(l)+4e- = 4OH-(l), and OER: 4OH-(l)-4e- = O2(g)+2H2O(l)). In this redox reaction,

9

MnO2 as a bifunctional catalyst carried out a series of chemical reactions to accelerate the

10

process. On the basis of the current understanding, Mn3+ is critical for enhancing both the OER41

11

and the ORR43 activities. Meanwhile, the Mn4+/Mn3+ species act as reaction mediator for oxygen

12

reduction and evolution and might assist the charge transfer. Especially, the electron transfer

13

reactions is likely to be accelerated because of the unique MnO6 octahedron of alpha MnO2. In

14

the MnO6 octahedron, however, Mn3+ was unstable due to its unpaired single electrons in the eg

15

band. Therefore, it was likely caused by the phase change from alpha MnO2 to beta MnO2. In

16

addition, three other possible driving forces can induce the phase transition. The first mechanism

17

was Jahn-Teller distortion. The second was the ORR and OER. For instance, Mn3+ ions will be

18

stabilized due to the low absorbed oxygen in ORR process, leading that the electrons of Mn3+

19

ions transfer to absorbed oxygen. But due to its low conduction, the electron transfer is very

20

slow. The third mechanism was a disproportional reaction (2Mn3+=Mn4++Mn2+). The

21

generated Mn2+ ions will be dissolved into 0.1M KOH solution, which might also be a cause of

22

performance degradation by resulting in material loss. Hence, the phase change from alpha

23

MnO2 to beta MnO2 was possibly due to the unstable Mn3+ ions, Jahn-Teller distortion,

24

electrochemical reaction and disproportional reaction.41, 43 For MnO2/CNTs, the hybrid catalyst

25

shows almost unchanged crystal structure and morphology with the basic character of 2×2 tunnel

26

structure and hortensia-like hierarchical microsphere after 2000 cycles (Figures 4f-h). Based on

27

these findings and compared with MnO2, the excellent stability of the hybrid catalyst should be

28

attributed to the synergistic effect between MnO2 and CNTs. However, the crystal structure of 12


Page 12 of 27

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1

hybrid catalyst completely converts into rutile structure after 6000 cycles and 8000 cycles, which

2

is similar to MnO2 (Figure 4f and i-n). In addition, one interesting finding is that the weight of

3

CNTs is negatively correlated to the cycle times, whereas CNTs directly affect the conductivity

4

and structure stability of the hybrid.24 Another worthwhile mentioned phenomenon is that the

5

hortensia-like structure not only agglomerates into large irregular particles but also forms

6

abundant nanowires after 4000 cycles. Herein, the degradation of ORR and OER performance

7

after cycles test should be due to the change of structures. The unique interconnected network

8

effectively remits the degradation of the hybrid catalyst, which could serve as an effective

9

support and buffer layer to hinder the structural conversion/collapse/aggregation.44 Moreover, the

10

effect of K+ ions was studied. Notably, it can be seen that the phase and morphology of -MnO2

11

are well maintained during the acid treatment as no extra peak generation or elimination is

12

observed (deduced to be K0.026MnO2, Figures S9 a-c and Table S1). Figure S9d displays the

13

ORR/OER activity and stability of MnO2 with HNO3 treatment. After HNO3 treatment, the

14

K0.026MnO2 shows similar catalytic activity, instead, the activity has obvious decrease after 600

15

cycles. These findings reveal that the K+ ions in the 2×2 tunnels have a slight positive effect on

16

the catalytic activity, and greatly preserve the catalytic stability of MnO2. Moreover, the phase

17

and the morphology of MnO2 with HNO3 treatment after 600 cycles have huge changes, further

18

indicating that the K+ ions are beneficial for the structural stability of the sample (Figure S9 c

19

and e).

20

Based on the above remarkable electrochemical activity and stability of the MnO2/CNTs

21

catalyst, AABs, MABs and ZABs are assembled using the hybrid as the catalyst of air electrode

22

to investigate its practical application potential. All air electrodes are constructed by the same

23

method to ensure the reliability and comparability. As expected, in Figure 5a, the open-circuit

24

voltage (OCV) and the maximum power density of the AABs with the hybrid air electrode are

25

2.1 V and 530 mW cm-2 at 0.95 V, respectively, even superior to the state-of-the-art Pt/C+IrO2

26

(1.8 V and 188 mW cm-2 at 0.85 V) and those of reported catalysts (Table S2, Supporting

27

Information). The AABs with MnO2/CNTs air electrode also shows excellent durability with

28

stable discharge voltage (1.72 V) for 140 hours during the galvanostatic discharging process at

29

50 mA cm-2, which is much higher than that with Pt/C+IrO2 (1.45V for 68 hours, Figure 5b). In 13



1

addition, the MABs based on MnO2/CNTs air electrode shows higher peak power density of 614

2

mW cm-2, exceeding that of the Pt/C+IrO2 catalyst (213 mW cm-2, Figure 5c). The galvanostatic

3

discharge curves at 20 mA cm-2 of the MABs based on the hybrid air electrode presents a good

4

working voltage of 1.27 V and a very small fluctuation in the following 70 hours, which

5

precedes the Pt/C+IrO2 (1.12 V for 38 hours, Figure 5d). Similarly, the ZABs exhibits a

6

maximum peak power density as high as 243 mW cm-2, which is also larger than the reference

7

(154 mW cm-2, Figure 5e). As expected, the ZABs also displays ideal durability with stable

8

output voltage of 1.26V for 190 hours at 20 mA cm-2, better than the Pt/C+IrO2 (1.21 V for 65

9

hours, Figure 5f). Notably, the specific capacities normalized to the mass of Al, Mg and Zn for

10

AABs, MABs and ZABs with based on MnO2/CNTs cathode reach as high as 918 mAh gAl-1,

11

878 mAh gMg-1 and 793 mAh gZn-1, respectively. Moreover, the MnO2/CNTs as bifunctional

12

catalyst and the mixture of 6 M KOH and 0.2 M ZnCl2 as cycling electrolyte are used in

13

rechargeable ZABs. As shown in Figure 5g, the galvanostatic charge-discharge technique was

14

carried out for evaluating the cycle stability of the rechargeable ZABs. For Pt/C+IrO2, the

15

voltage difference is 1.15 V at first cycle, whereas the voltage difference increases to 2 V after

16

100 hours. For MnO2/CNTs, the initial discharge and charge voltage plateaus are 1.29 V and

17

2.10 V at 10 mA cm-2, respectively. Notably, there is no visible change of the discharge and

18

charge voltage during the whole charge-discharge cyclic test for 500 hours. The voltage

19

difference is only 0.81 V and the corresponding coulomb efficiency is up to 100%. Such high

20

battery performance of the hybrid catalyst should be attributed to the excellent ORR and OER

21

activity and stability. Excitingly, MeABs with high performance will inspire its application

22

prospect.

23 24

CONCLUSIONS

25

In summary, a bifunctional composite catalyst of hortensia-like MnO2 interweaved with

26

CNTs (MnO2/CNTs) has been controllably synthesized by simple two-step hydrothermal

27

reaction. The hybrid catalyst exhibits a 2×2 tunnel structure, unique hortensia-like morphology,

28

large BET surface area (345 m2 g-1) and strong interconnected conducting network. In addition, 14


Page 14 of 27

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1

the MnO2/CNTs hybrid catalyst shows superior activity (ΔE: 0.850 V) and durability (2000

2

cycles) towards ORR and OER, which makes it an ideal air electrode catalyst candidate for

3

MeABs. As expected, the composite used in MeABs reveals higher maximum power density,

4

higher working voltage and excellent discharge stability (ZABs: 243 mW cm-2/ 1.26 V for 190

5

hours, AABs: 530 mW cm-2/ 1. 72 V for 140 hours and MABs: 614 mW cm-2/ 1.27 V for 70

6

hours) over the commercial Pt/C+IrO2 catalyst (ZABs: 154 mW cm-2/ 1.21 V for 65 hours,

7

AABs: 188 mW cm-2/ 1. 45 V for 68 hours and MABs: 213 mW cm-2/ 1.12 V for 38 hours).

8

Importantly, the rechargeable ZABs also display excellent cycle stability (500 hours) and small

9

charge-discharge voltage gap (0.81 V). The improved electro-catalytic performance of the

10

composite catalyst could be attributed to the synergistic effect between MnO2 and CNTs, and the

11

unique advantages of each component. First, unique hortensia-like morphology affords high

12

density reaction surface (Cdl 18.7 mF cm-2), improving the contact of O2 and OH-; Second, the

13

high specific surface can effectively afford more triple phase regions, inspire the

14

adsorption/desorption of reactant and product. Especially, the interconnected conducting network

15

and the synergistic effect further shorten the electron transfer pathway, greatly improve the

16

conductivity, and effectively impede the structural conversion/ collapse/ aggregation. In addition,

17

the K+ ions in the 2×2 tunnels have a positive effect on the catalytic activity and stability of

18

MnO2. This work opens a new route to reasonably design high-performance bifunctional electro-

19

catalyst for MeABs.

20 21

ASSOCIATED CONTENT

22

Supporting Information.

23

AUTHOR INFORMATION

15



1

Corresponding Author

2

*Corresponding author. Tel: +86-21-67792379. Fax: +86-21-67792159.

3

E-mail: [email protected] and [email protected].

4

Notes

5

The authors declare no competing financial interest.

6

ACKNOWLEDGMENTS

7

This work is financially supported by the National Natural Science Foundation of China

8

(U1510120) ， the Fundamental Research Funds for the Central Universities (CUSF-DH-D-

9

2018075) and Shanghai Tongji Gao Tingyao Environmental Science & Technology

10

Development Foundation (STGEF).

16


Page 16 of 27

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1


a

c

b

1m

e

500 nm

500 nm

1m

500 nm

h

1m

k

j

i

1m

g

f

1m

d

500 nm

500 nm

l

1m

500 nm

Figure 1 SEM images of (a and b) MnO2-1h. (c and d) MnO2-2h. (e and f) MnO2-4h. (g and h) MnO2-8h. (i and j) MnO2-12h. (k and l) MnO2-16h.

17



Page 18 of 27

1 a

c

b

e

d

h

f

i

j

g

k

Figure 2 (a) Schematic diagram for the formation process of MnO2/CNTs. (b) XRD patterns of MnO2, CNTs and MnO2/CNTs. (c) Crystal structure of tunnel MnO2. (d) TEM image of MnO2. (e) TEM image of MnO2. (f) TEM image of MnO2/CNTs. (g) TEM image of MnO2/CNTs. (h) N2 adsorption isotherms and inset: the corresponding surface areas of MnO2 and MnO2/CNTs. XPS spectra of MnO2/CNTs. (i) Mn 2p; (j) O 1s. (k) Schematic reaction mechanism of the OER and ORR processes catalyzed by MnO2/CNTs.

18


-3

20 mV

-4

CNT MnO2

-5

MnO2/CNTs

-6

1.0 MnO2/CNTs -1 71 mV dec

Pt/C+IrO2

0.2

0.4 0.6 0.8 1.0 Potential / V (vs RHE)

e Potential / V (vs RHE)

Current density / mA cm

35 30 25 20

CNT MnO2

15

MnO2/CNTs

10 5

Pt/C+IrO2

OER

0

g2.4

1.2 1.4 1.6 1.8 Potential / V (vs RHE)

2.0

CNT MnO2

1.6

Pt/C+IrO2

2.0

1.6 Pt/C+IrO2

0.8

-1

69 mV dec MnO2

0.6

-1

94 mV dec CNTs -1 187 mV dec

0.4 0.2

-2

d 40

c -1

-2

1.2

At 0.35 V CNTs n=3.14 MnO2 n=3.61

1.2

2

-1

b

2.2 2.1 2.0 1.9

0.0

CNTs -1 283 mV dec MnO 2

-1

71 mV dec

144 mV dec

MnO2/CNTs

1.7

-1

67 mV dec

1.6

h

f

Pt/C+IrO2 -1

0 10 20 30-2 Current density / mA cm

n=2

MnO2/CNTs n=3.99 Pt/C+IrO2 n=3.99

0.4

n=4

-1.5 -3.0 -4.5 -2 Current density / mA cm

1.8

1.5

-1

ORR

J / cm mA


-2

a0

Potential / V (vs RHE)

1

J / mA cm-2

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


Potential / V (vs RHE)

Page 19 of 27

40

0.0 0.000 2.0 1.5

0.015

0.030 0.045 ω-1/2 / rpm-1/2

0.060

OER polarization at Ej=10 mA cm

>2 V

1.267 V

-2

0.850 V

0.838 V

1.0 0.5 0.0

ORR polarization at Ej=-3 mA cm

CN

Ts

Mn

O2 M

/CN nO 2

-2

Ts

C+

P t/

MnO2/CNTs

1.2 0.8 0.4 0.0

2

4 6 8 Scan rate / mV s-1

10

Figure 3 (a) ORR polarization curves, (b) Tafel plots of ORR and (c) K-L plots at different potential at 0.35 V with various rotation rates of Pt/C+IrO2, MnO2, CNTs and MnO2/CNTs. (d) OER polarization curves and (e) Tafel plots of OER of Pt/C+IrO2, MnO2, CNTs and MnO2/CNTs. (f) Comparison of OER and ORR bifunctional activities (ΔE) of samples in this work. (g) A plot of the current density at 1.14 V vs. the scan rate to determine the double layer capacitance (Cdl) of MnO2, CNTs and MnO2/CNTs. (h) The durability of MnO2 and MnO2/CNTs.

19


IrO 2


1

a

Page 20 of 27

b

c

d

e

f

g

h

i

j

k

l

m

n

Figure 4 (a) XRD patterns of MnO2 at initial and after 1000 cycles, (b) the corresponding crystal structure, (c and d) SEM images and (e) TEM image of MnO2 after 1000 cycles. (a) XRD patterns of MnO2/CNTs at initial and after 2000, 4000, 6000 and 8000 cycles. (g) SEM image and (h) TEM image of MnO2/CNTs after 2000 cycles. (i-k) SEM images and (l-n) TEM 20 cycles, respectively. images of MnO2/CNTs after 4000, 6000 and 8000


2000

AABs

MnO2/CNTs Pt/C+IrO2

Voltage / mV

1600

600

1200 400 800 200

400 0

1600

MnO2/CNTs

0 150 300 450 600-2 750 Current density / mA cm 800 MnO2/CNTs MABs

Pt/C+IrO2

2.0 1.5 1.0 0.5 0.0

d

0 2.5

20

40

60 80 Time / h

800

200

0 300 600 900 -2 1200 Current density / mA cm 1500 300 MnO2/CNTs ZABs Pt/C/IrO2 240 1200 0

1.5 1.0 0.5 0.0

f

0

20

2.5

40 Time / h

600

120

300

60

0

g

0

3.0

200 400 -2 Current density / mA cm

0 600

Pt/C+IrO2

2.0 Voltage / V

180

Power density / mW cm

900

60 MnO2/CNTs

-2

e

400

400 0

600

Pt/C+IrO2

2.0 Voltage / V

Voltage / mV

1200

Power density / mW cm

Pt/C+IrO2

100 120 140 MnO2/CNTs

-2

c

0

b 2.5

800 -2

a

Voltage / mV

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


Power density / mW cm Voltage / V

Page 21 of 27

1.5 1.0 0.5 0.0

0

40

80 120 Time / h

160

200

MnO2/CNTs Pt/C+IrO2

2.5 2.0 1.5 1.0 0.5

0

100

200

300

400

500

Figure 5 The polarization curve and corresponding power density plot, and the long-time galvanostatic discharge curve at a current density of 50 mA cm-2 of (a and b) AABs. The polarization curve and corresponding power density plot, and the long-time galvanostatic discharge curve at a current density of 20 mA cm-2 of (c and d) MABs, and (e and f) ZABs. (d) the C-D cycling curves of the rechargeable ZABs. 21



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properties

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alpha-MnO(2)

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Journal of Materials Chemistry A 2014, 2 (35), 14493-14501.

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BRIEFS. A novel bifunctional hybrid catalyst of hortensia-like MnO2 synergized with carbon nanotubes (CNTs) (MnO2/CNTs) is controllably synthesized. As expected, the hybrid catalyst exhibits the superior catalytic kinetics and durability towards ORR and OER. Notably, the MeABs based on the MnO2/CNTs bifunctional catalyst reveal larger peak power density, and shows excellent stability toward both discharge and charge-discharge processes. SYNOPSIS (Word Style “SN_Synopsis_TOC”). -2

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OER

e-

ORR

e-


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40 35 30 25 20 15 10 5 0 -5

MnO2intial MnO2 after 1000 cycles MnO2/CNTs intial MnO2/CNTs after 2000 cycles MnO2/CNTs after 4000 cycles MnO2/CNTs after 6000 cycles MnO2/CNTs after 8000 cycles

E= 1.23 V

0.4

0.8 1.2 1.6 Potential / V (vs RHE)

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2.0

Controllable Hortensia-like MnO2 Synergized with Carbon nanotubes

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