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Clarifying the Controversial Catalytic Performance of Co(OH)2 and Co3O4 for Oxygen Reduction/Evolution Reactions Toward Efficient Zn-Air Batteries Zhishuang Song, Xiaopeng Han, Yida Deng, Naiqin Zhao, Wenbin Hu, and Cheng Zhong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 23 May 2017 Downloaded from http://pubs.acs.org on May 24, 2017

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ACS Applied Materials & Interfaces

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Clarifying the Controversial Catalytic Performance

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of

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Reduction/Evolution Reactions Toward Efficient

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Zn-Air Batteries

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† † † † †‡ †‡ Zhishuang Song, Xiaopeng Han,*, Yida Deng, Naiqin Zhao, Wenbin Hu, and Cheng Zhong*,

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†

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and Engineering, ‡Key Laboratory of Advanced Ceramics and Machining Technology (Ministry

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of Education), Tianjin University, Tianjin 300072, China.

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KEYWORDS: Zn-air battery, electrochemical deposition, ORR/OER, Co(OH)2, Co3O4,

Co(OH)2

and

Co3O4

for

Oxygen

Tianjin Key Laboratory of Composite and Functional Materials, School of Materials Science

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bifunctional catalysts

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ABSTRACT: Cobalt-based nanomaterials have been widely studied as catalysts for the oxygen

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reduction reaction (ORR) and oxygen evolution reaction (OER) due to their remarkable

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bifunctional catalytic activity, low cost and easy available. However, controversial results

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concerning OER/ORR performance exist between different types of cobalt-based catalysts,

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especially for Co(OH)2 and Co3O4. To address this issue, we develop a facile electrochemical

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deposition method to grow Co(OH)2 directly on the skeleton of carbon cloth, and further Co3O4

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was obtained by post thermal treatment. The entire synthesis strategy removes the use of any

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binders and also avoids the additional preparation process (e.g. transfer and slurry-coating) of

ACS Paragon Plus Environment

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final electrodes. This leads to a true comparison of the ORR/OER catalytic performance between

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Co(OH)2 and Co3O4, eliminating uncertainties arising from the electrode preparation procedures.

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The surface morphologies, microstructures and electrochemical behaviors of prepared Co(OH)2

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and Co3O4 catalysts were systemically investigated by scanning electron microscopy,

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transmission electron microscopy, atomic force microscopy and electrochemical characterization

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methods. The results revealed that the electrochemically deposited Co(OH)2 was in the form of

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vertically-aligned nanosheets with average thickness of about 4.5 nm. After the thermal

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treatment in air atmosphere, Co(OH)2 nanosheets were converted into mesoporous Co3O4

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nanosheets with remarkably increased electrochemical active surface area (ECSA). Although the

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ORR/OER activity normalized by the geometric surface area of mesoporous Co3O4 nanosheets is

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higher than that of Co(OH)2 nanosheets, the performance normalized by the ECSA of the former

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is lower than that of the latter. Considering the superior apparent overall activity and durability,

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the Co3O4 catalyst has been further evaluated by integrating it into a Zn–air battery prototype.

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The Co3O4 nanosheets in-situ supported on carbon cloth cathode enables the assembled Zn-air

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cells with large peak power density of 106.6 mW cm−2, low charge and discharge overpotentials

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(0.67 V), high discharge rate capability (1.18 V at 20 mA cm-2), and long cycling stability (400

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cycles), which is comparable or even superior to the mixture of state-of-the-art Pt/C and RuO2

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cathode.

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INTRODUCTION

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With the surging global energy crisis and environmental problem, the demands for

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regenerative, clean, and sustainable energy supplements are unprecedented in recent years.

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Metal-air batteries systems, especially in the rechargeable form, represent one of the most

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promising renewable energy storage technologies mainly due to their extremely high theoretical


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energy densities.1-3 However, proceeding complex multi-electron redox processes, the key

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oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) taken places at the air-

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cathode suffer from intrinsic sluggish kinetics and large overpotentials, which results in limited

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energy densities, poor efficiencies, and short cycle lifetimes.4-6 Consequently, bifunctional

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catalysts with highly efficient ORR and OER activities are required to promote the rechargeable

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metal-air battery technologies. At present, Pt and its alloys are recognized as the best ORR

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electrocatalysts, but are not efficient for OER catalysis.7,

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excellent OER activity, but exhibit poor ORR activity.9,

10

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scarcity of these precious metals unfortunately impede their wide range commercialization.11

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Therefore, it is urgently needed to develop earth-abundant, durable, and highly efficient

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bifunctional catalysts, which can efficiently expedite the slow kinetics of oxygen reduction and

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evolution processes to construct advanced metal-air battery systems.

In contrast, Ir and RuO2 possess Additionally, the high cost and

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Substantial research efforts have been devoted to exploring and establishing efficient, cost-

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effective, and precious-metal-free candidates to address the oxygen involved redox reactions,

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including transition metals,12,

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catalysts,9,

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particularly, cobalt-based catalysts (e.g., layered Co(OH)2 and spinel Co3O4), have received

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tremendous interests because of their advantageous features including inexpensive,

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environmental friendliness, easy availability, and considerable bifunctional performance.16, 21, 22

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The electrochemical properties of Co-based catalysts have been greatly enhanced through

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nanostructured engineering, doping with another metal element and compositing with carbon

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nanomaterials et al,6, 23-25 but in most cases underperform those of noble-metal-based materials.

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More importantly, an examination of these literatures demonstrates that controversial results

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their hydroxides,6,

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oxides,15-17 sulfides,18,

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and metal-free

along with diverse micro-nanostructures. Among them, 3d transition metal,


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concerning the OER/ORR performance exist in two typical types of cobalt-based catalysts

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between Co(OH)2 and Co3O4.21, 26-28 Specifically, Lee’s group reported that cobalt hydroxide

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displayed greater activity than cobalt oxides (CoO and Co3O4) for ORR and OER in alkaline

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conditions;21 Liu et al revealed that compared with Co3O4/rGO, Co(OH)2/rGO shows comparable

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OER performance but with less mass loading.26 However, other literatures revealed an entirely

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contrary conclusion. For instance, Zhou et al reported the ultrathin porous Co3O4 nanoplate

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exhibited more efficient OER performance than Co(OH)2;27 Switzer’s group revealed a much

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larger double layer capacitance of Co3O4 as compared to Co(OH)2, which indicates a larger

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electrochemical active surface area of the former.28 To the best of our little knowledge,

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irrespective of material surface properties induced by different synthesis strategies, one of the

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most important reason leading to different results mainly contributes to the diverse preparation

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procedures of electrodes used for electrochemical evaluation, which not only requires the

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employment of polymer binders (e.g., Nafion) and carbon black additive, but also needs

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additional dipping/slurry coating process on conductive substance, thereby, bringing inevitable

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uncertainties to the final experimental comparisons. Thus, in addition to deriving from the

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intrinsic catalytic properties of the compounds, the apparent ORR/OER performance difference

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of these catalysts may originate from the diverse preparation processes of electrodes, which

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complicates valuable comparisons of the catalytic capability and durability from different

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catalyst systems. Therefore, it’s highly essential to develop reliable and standard methods to

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prepare and evaluate the catalytic performance of cobalt-based catalysts, which could extremely

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eliminate the additional impact on OER and ORR in order to further clarify the controversial

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results, thus providing meaningful guidance to the further rational development of new oxygen-

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related transition metal-based materials.


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The aim of the present work is to clarify the existing controversial ORR and OER catalytic

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performance between Co(OH)2 and Co3O4 by using an in-situ, clean and one-step

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electrodeposition route in the absence of any organic and inorganic additives and without further

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sample transferring and coating operations. The Co(OH)2 nanosheets were in-situ directly grown

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on the conductive carbon cloth (CC) via a facile electrochemical deposition method (designated

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as Co(OH)2 NS/CC). The CC supported mesoporous Co3O4 nanosheets were then synthesized by

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a post-annealing treatment in air atmosphere (designated as Co3O4 NS/CC). The designed

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synthesis strategy is highlighted by two features. First, the whole electrodeposition process is

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extremely clean since it avoids the use of any surfactants, binders, reducing and capping agents,

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which may increase the electron transfer resistance and cover the surface exposed active sites.

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Second, the absence of additional catalyst transferring and smearing processes allows a true

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comparison of OER and ORR properties of the synthesized electrocatalysts and avoids any

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uncertainties induced by the electrode preparation and transfer procedures.29-30 Followed

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electrochemical tests reveal that although the ORR/OER activities normalized by the geometric

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area of mesoporous Co3O4 NS/CC is much higher than that of Co(OH)2 NS/CC, the performance

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normalized by the electrochemical active surface area (ECSA) of the former is lower than that of

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the latter, indicating the superior intrinsic catalytic activity of the layered Co(OH)2. As for the

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catalytic durability, Co3O4 nanosheets are remarkably electrochemical reversible while the α-

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Co(OH)2 is irreversibly oxidized to CoOOH after the OER process. Furthermore, considering the

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superior apparent catalytic performance and stability, the as-prepared Co3O4 NS/CC has been

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further employed as an efficient electrode in practical Zn-air batteries (ZABs), which exhibits

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large power density, low discharge/recharge overpotential, and long cycling life time.

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EXPERIMENTAL SECTION


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Reagents and Materials. Cobalt (II) nitrade hexahydrate (Co(NO3)2⋅6H2O, 99%) was purchased

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from Damas-beta. The carbon cloth (WOS1002) was purchased from Phychemi Company

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Limited. Potassium hydroxide (KOH, 99.99%), ethanol (CH3CH2OH, 99.8%), Zinc acetate

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dehydrate (C4H6O4Zn·2H2O，99%) and acetone (CH3COCH3, 99.5%) were purchased from

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Beijing Chemicals (Beijing, China). The deionized water (18.2 MΩ·cm) used was obtained via

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an ultrapure water system (Millipore). All aqueous solution was freshly prepared with deionized

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(DI) water. High purity oxygen (O2, 99.99%) gas was used to deaerate the 0.1 M KOH aqueous

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solution. All reagents were of analytical grade and used as received without further purification.

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Materials Synthesis. The Co(OH)2 NS/CC was prepared by a facile cathodic electrochemical

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deposition method. The electrodeposition was performed in a standard three-electrode system at

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room temperature. The carbon cloth with the size of 1 cm × 2 cm was used as the working

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electrode and the reference and counter electrode were saturated calomel electrode (SCE) and Pt

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plate, respectively. Before using, the carbon cloth substrate was ultrasonically washed by

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acetone, absolute ethanol and finally deionized water with each step lasts for 10 min,

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respectively, and then dried in 60 °C under ambient atmosphere. The cathodic electrodeposition

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was carried out on a CHI 630b electrochemical workstation (CHI Instrument, China) at -1.0 V vs

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SCE for 1200 seconds. The obtained precursor coated carbon cloth was taken out from the

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electrolyte and rinsed with the DI water for three times. The deposition of cobalt hydroxide on

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the CC substrate surface can be expressed by the following reactions:31

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NO3− + H2O + 2e−→ NO2− + 2OH− Co2+ + 2OH− → Co(OH)2

(1) (2)


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Subsequently the as-precursor Co(OH)2 (mass loading: ~2.0 mg cm-2) was thermally heated at

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200 ºC, 300 ºC and 400 ºC in the furnace for 2 h at a heating rate of 2 ºC min−1 and then cooled

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down naturally in air, yielding the Co3O4 spinels (designated as Co3O4-200 ºC, Co3O4-300 ºC,

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and Co3O4-400 ºC) with the average mass loading of 1.5, 1.0, and 0.8 mg cm-2 , respectively. The

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oxidation conversion from Co(OH)2 to Co3O4 follows the reaction below:32

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6Co(OH)2 + O2 → 2Co3O4 + 6H2O

(3)

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Materials Characterization. The phase structures and surface morphologies were characterized

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by powder X-ray diffraction (XRD, RIGAKU D/Max 2500 diffractometer using Cu Kα radiation,

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40 kV and 40 mA), scanning electron microscopy (SEM, s4800 Hitachi, 30 kV) equipped with

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energy dispersive X-ray (EDX), transmission electron microscopy (TEM, JEOL JEM-2100F,

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200 kV) and atomic force microscopy (AFM, Agilent afm5500). In order to perform TEM

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experiment, the samples were detached from CC substrate and dispersed in ethanol by ultrasonic

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treatment for 30 min, then dipped on the cupper grid. Thermogravimetric experiment was carried

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out in air atmosphere from 50 ºC to 500 ºC at 10 ºC min-1 by Netzsch STA449F3 Jupiter

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analyzer. Raman spectrum was conducted on a confocal Raman microscope (DXR, Thermo-

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Fisher Scientific).

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Electrocatalytic Measurements. Electrocatalytic properties were evaluated using cyclic

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voltammetry and linear sweep voltammetry on a CHI 660D electrochemistry workstation (CHI

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Instrument, China) with a standard three electrode system in O2-saturated KOH aqueous

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solutions at room temperature. The as-prepared Co(OH)2 NS/CC and Co3O4 NS/CC were

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directly employed as the working electrode. The Ag/AgCl (sat-KCl) and Pt foil were used as the

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reference electrode and counter electrode, respectively. Prior to testing, the supporting 0.1 M

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KOH electrolyte was bubbled with pure O2 for at least 30 min and then maintained into the


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headspace over the electrolyte during the whole experiments. The electrochemical active surface

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area (ECSA) was estimated from the electrochemical double-layer capacitance. Electrochemical

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impedance spectroscopy (EIS) was conducted on a CHI 660D electrochemistry workstation

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(Chen Hua Instrument, Shanghai, China) with an AC voltage with 10 mV amplitude over a

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frequency range from 5 mHz to 100 kHz at 1.56 V vs RHE The electrochemical stability

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measurements were conducted on a IviumStat workstation with a standard three electrode system

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at 1.66 V for OER and 0.56 V for ORR. Unless stated, all the potential has been calibrated with

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respect to the reversible hydrogen electrode (RHE) according to ERHE = EAg/AgCl + 0.059 × pH +

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0.197 V.

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Zinc-Air Battery Fabrication and Testing. The performance of ZABs was assessed by a home-

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built electrochemical two electrode using Zn-foil as anode electrode, electrochemically deposited

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Co3O4-400 °C NS/CC as cathode electrode, 6.0 M KOH and 0.2 M Zn(AC)2 mixed solution as

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the electrolyte, respectively. For comparison, Pt/C + RuO2 mixture electrode was prepared by

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ultrasonicating 5 mg Pt/C and 5 mg RuO2 in 1mL isopropyl alcohol for 30 min. Afterwards, 35

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µL of 5% Nafion solution was added into the above mixed solution, followed by continuous

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sonication for another 30 min. The as-prepared homogeneous ink was drop-coating on carbon

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cloth and dried at 50 °C for 5h. The mass loading is about 0.74 mg cm-2. Plastic ZABs were

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comprised by one plastic bag filled with a Zn plate anode, a glass fiber separator, and the Co3O4

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NS/CC air cathode. Many air holes were punched on the air side of the plastic bag to facilitate

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the air pass. The discharge-charge and cycling performance of assembled ZABs were tested on

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Land-CT2001A devices with oxygen pre-saturated electrolyte and continuously fed during the

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measurements. The voltage-current polarization curves were collected on IviumStat workstation


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at a scanning rate of 5 mV s-1. The cycling performance of ZABs was conducted at a current

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density of 5 mA cm-2 with periods of 2 min discharge and 2 min charge per cycle

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RESULTS AND DISCUSSION

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Figure 1. (a) X-ray diffraction (XRD) pattern of electrodeposited Co(OH)2 NS/CC and Co3O4-

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200 °C, Co3O4-300 °C and Co3O4-400 °C, respectively. Crystal structure models of (b)

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hydrotalcite-like α-Co(OH)2 and (c) spinel Co3O4, respectively.

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The crystal phase and purity of the as-synthesized Co(OH)2 NS/CC and Co3O4 NS/CC were

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investigated by powder X-ray diffraction (XRD), as shown in Figure 1a. The peak at around 2θ ≈

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26.5° in all products is the typical signal of carbon from the carbon cloth support. Before thermal

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treatment, the XRD patterns of electrodeposited sample on the carbon cloth show six obvious

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diffraction peaks, which locate at 2θ = 11.5 º , 33.1 º , 34.3 º , 43.1 º , 59.2 º , and 60.2 º ,

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corresponding to the (003), (101), (012), (018), (110), and (113) reflections of hydrotalcite-like

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α-Co(OH)2 crystal phase, respectively.33, 34 The considerable broadening of the XRD diffraction

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peaks indicates the poor crystallinity and small crystallite size of formed layered α-Co(OH)2


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precursor.35, 36 The basal plane spacing of the as-synthesized α-Co(OH)2 is ca. 8.0 Å, which is

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much larger than that of β-Co(OH)2 (4.6 Å), due to the intercalated water molecules and NO3-.35,

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36

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observed in the XRD pattern, indicating the high-purity of as-synthesized α-Co(OH)2 precursor.

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The typical diffraction peaks of three samples annealed at different temperatures are centered at

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2θ = 19º, 31.3º, 36.9º, 44.8º, 59.4º and 65.3º, which can be indexed to (111), (220), (311),

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(400), (511), and (440) crystal planes, respectively, of face-centered spinel Co3O4 phase (Figure

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1c, space group: Fd-3m, JCPDS card no. 74-2120), in addition to an extra peak located at 43° in

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Co3O4-200 °C. This peak belongs to (018) of Co(OH)2. The XRD results indicate an incomplete

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oxidation from α-Co(OH)2 to Co3O4 after thermal treatment at 200 °C and a successful phase

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conversion at 300 °C and 400 °C in air atmosphere. The relative intensities of the diffraction

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peaks increase with increased annealing temperature in Co3O4 series, which implies that the

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higher temperature leads to better crystalline degree of final products. It should be noted that the

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calcination treatment has no influence on the carbon cloth substrate, as revealed by the zero

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weight loss determined by thermogravimetric (TG) measurements (Supporting Information

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Figure S1).

The crystal structure model is illustrated in Figure 1b. No peak from other phases was


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Figure 2. SEM images of electrodeposited Co(OH)2 NS/CC (a, b) and Co3O4 NS/CC (c-f) after

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thermal treatment at different temperatures in air atmosphere for 2 h. (c, d) 200 °C, (e) 300 °C,

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and (f) 400 °C, respectively.

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Figure 2 displays the typical scanning electron microscopy (SEM) images of electrodeposited

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Co(OH)2 and Co3O4 on CC support after thermal treatment at 200, 300, and 400 ºC for 2h,

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respectively. As can be seen from the SEM images in Figure 2a and 2b, the Co(OH)2 nanosheet

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arrays are uniformly and vertically stand on the substrate (Figure S2), forming an intercrossed

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nanowall network. After annealing, the Co3O4 nanosheets almost completely retain the previous

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morphology of Co(OH)2 precursor and lie perpendicular on the CC support (Figure 2c-f),

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suggesting the remarkable structural stability of the nanosheet nanostructures. The average

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thicknesses of the Co(OH)2 and Co3O4-400 ºC are approximately 4.5 and 8.5 nm, respectively,

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as determined by the high-resolution atomic force microscopy (AFM) (Figure 3). SEM elemental

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mappings in Figure 4 further demonstrate the homogeneous distribution of Co(OH)2 and Co3O4-

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400 ºC decorated on carbon fiber support. Moreover, the Co3O4 nanosheets are interconnected

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with each other, with mesopores formed on the nanosheets surface owning to the continuous


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release and loss of H2O molecules and the volume shrinkage of the crystal structure from

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Co(OH)2 to Co3O4 after the thermal oxidation treatment,32, 37 integrating into a three-dimensional

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hierarchical net-like porous structure (Figure 2d-f). The porous nanostructure gradually becomes

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more evident with the increased calcination temperatures. The observed porous structure in

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Co3O4-400 °C is also verified by the pore distribution calculated by the Barrett-Joyner-Halenda

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(BJH) method and the corresponding Brunauer-Emmett-Teller (BET) specific surface area is

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42.9 m2 g-1, as determined by N2 adsorption/desorption measurements (Figure S3). The

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fabricated highly open and porous nanoarchitecture is favorable to the penetration and

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accessibility for electrolyte and oxygen, enabling a low interfacial resistance for gas and ions

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transportation.

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Figure 3. AFM image of (a) Co(OH)2 NS and (b) Co3O4-400 ºC NS. The corresponding

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thickness measurement data of (c) Co(OH)2 NS and (d) Co3O4-400 ºC NS.


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Figure 4. SEM mapping of Co and O elements of Co(OH)2 NS/CC (a-c) and Co3O4-400 ºC

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NS/CC (d-f).

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The detailed morphological and structural information of as-prepared α-Co(OH)2 NS/CC

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and the annealed Co3O4-400 ºC NS/CC were further characterized by transmission electron

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microscopy (TEM). The as-prepared Co(OH)2 and Co3O4 nanosheets were scraped down from

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the CC support and ultrasonically dispersed for TEM observation. TEM image confirms the

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ultrathin nanosheet of synthesized Co(OH)2 (Figure 5a, b)and interlinked porous nanosheets of

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Co3O4 (Figure 5c, d). The size of numerous pores ranges from 10 to 20 nm in Co3O4 nanosheets,

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forming a mesoporous nanostructure. From the high resolution TEM (HR-TEM) in Figure 5e and

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5f, the measured interplanar spacing of 0.27 nm and 0.21 nm corresponds to (012) and (018)

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lattice planes of α-Co(OH)2 nanocrystals while the observed clear lattice spacing of 0.28 nm and

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0.24 nm are in good agreement with the standard value of (220) and (311) lattice planes of Co3O4

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spinel, respectively. Additionally, the collected well-defined selected area electronic diffraction

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(SAED) patterns of the nanoflakes coincide with the corresponding diffraction of layered α-

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Co(OH)2 (inset in (e)) and cubic Co3O4 (inset in (f)), confirming the XRD analysis.


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Figure 5. (a, b) TEM images of electrochemically deposited Co(OH)2 nanosheet and (c, d)

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Co3O4 nanosheet; (e) HRTEM of Co(OH)2 and SAED pattern (inset); (f) HRTEM of Co3O4 and

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SAED pattern (inset).

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To evaluate the electrochemical ORR and OER catalytic activities of synthesized samples, the

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half-cell tests were performed using three-electrode system in O2- or N2-saturated 0.1 M KOH

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electrolyte. The prepared Co-based nanosheets covered CC, Ag/AgCl (Sat’d KCl), and Pt foil

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were used as the working electrode, reference electrode, and counter electrode, respectively. The

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ORR polarization curves of electrodeposited α-Co(OH)2 NS/CC and annealed Co3O4 NS/CC are

10

shown in Figure 6a. It can be seen that the CC support exhibits poor electrocatalytic ORR

11

activity. After electrochemically depositing Co(OH)2, the onset potential positively shifted and

12

the reduction current density increased, indicating the enhanced ORR performance of deposited

13

Co(OH)2 nanosheet. After thermal treatment, the ORR performance is remarkably further


14

Page 15 of 40

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1

improved with much more positive onset potential of Co3O4-200 ºC at 0.88 V, Co3O4-300 ºC at

2

0.90 V, and Co3O4-400 ºC at 0.91 V, respectively. In addition, the corresponding reduction

3

current densities of calcined Co3O4 samples at 0.2 V achieve 2.25, 2.42 and 2.79 mA cm-2,

4

respectively, which are nearly 3-fold larger than that of Co(OH)2/CC electrode (0.84 mA cm-2).

5

The cathodic reduction current of Co3O4 slightly increases with increasing calcination

6

temperatures, which is ascribed to the richer pore nanostructures introduced by thermal treatment

7

at higher temperature, as also evidenced by SEM characterizations (Figure 2d-f). Combining

8

with the mass loading, our results further reveal the difference of mass activities in Co3O4 series.

9

The reduction peaks appear at around 0.80 V of Co3O4 nanosheet in Figure 6a contribute to

10

catalyzing ORR, which is confirmed by the comparison of CV curves in O2- and N2-saturated

11

electrolyte, although the peak potential shifts to negative value owning to applied larger scanning

12

rate (Figure S4). In a metal-air battery, ORR occurs in the discharge process while OER

13

corresponds to the recharge process, which plays a critical role in determining the

14

discharge/charge energy efficiency as well as the cycling life in rechargeable battery systems.

15

Thus, the OER activities of bare CC, prepared Co(OH)2, and Co3O4 treated at different

16

temperatures are also investigated in this study (Figure 6b). Since Co(OH)2 is irreversibly

17

oxidized to CoOOH during the activation processes,38, 39 the oxidation peak at around 1.46 V for

18

Co(OH)2 NS/CC in Figure 6b is attributed to the electrochemical oxidation of CoOOH to CoO2.

19

This is also verified by the CV cycles of Co(OH)2 NS/CC (Figure S5), where a larger oxidation

20

peak and only a minor reduction process on the reverse sweep are observed on the first cycle

21

indicating the irreversible oxidation of Co(OH)2.38,

22

symmetrical redox peaks centered at about 1.50 V and 0.75 V is observed, which is associated

23

with the reversible transformation between CoOOH and CoO2 (CoOOH + OH- → CoO2 + H2O +

40

On the following cycles, a pair of


15


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Page 16 of 40

1

e-). This phenomenon is more obvious in 1 M KOH media (Figure S6), similar to the results in

2

reference.40 For Co3O4 series, the first oxidation peaks (Figure 6b and S7) at lower potential (~

3

1.26 V) are assigned to the oxidation from CoII to CoIII and the second peaks at higher potential

4

(~ 1.56 V) are attributed to their further oxidation from CoIII to CoIV, respectively.41,

5

anodic current recorded on Co3O4/CC electrode surpasses that of Co(OH)2/CC while in case of

6

Co3O4/CC series, again the higher temperature treated Co3O4/CC affords the superior

7

performance in the operating potential above 1.71 V, following an order of Co3O4-400 ºC >

8

Co3O4-300 ºC > Co3O4-200 ºC. Therefore, the porous Co3O4 nanosheet supported on carbon

9

cloth treated at 400 ºC exhibits the superior apparent bifunctional activity for dual ORR and

10

OER in alkaline media, rendering it as a promising and low cost electrocatalyst for application in

11

rechargeable metal-air batteries and regenerative fuel cells.

42

The


16

Page 17 of 40

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1 2

Figure 6. LSVs for ORR (a) and OER (b) of electrochemically deposited α-Co(OH)2 and

3

thermally treated Co3O4 at different temperature at a scanning rate of 5 mV s-1 in O2-saturated

4

0.1 M KOH. (c) Electrochemical impedance spectroscopy measured at 1.56 V. Inset shows the

5

simplified Randles equivalent circuit used for fitting. Rs, Rct, and CPE represent the electrolyte

6

resistance, charge transfer resistance, and double layer capacity, respectively. (d) Double-layer

7

capacitance measurements for determining electrochemically active surface area of prepared

8

catalysts.

9

As a useful technique to characterize the electrode interface and reaction kinetics,

10

electrochemical impedance spectroscopy (EIS) was performed from 100 kHz to 5 mHz to

11

analyze the performance disparity of prepared Co(OH)2/CC and Co3O4/CC electrodes. As


17


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

Page 18 of 40

1

presented in Figure 5c, the results show that the dramatic difference in performance is clearly not

2

ascribed to the difference in the charge transfer resistance (Rct). The EIS data was fitted by using

3

a simplified Randles equivalent circuit (inset in Figure 6c) and Rct values of 2.5, 3, 5, 20 Ω were

4

determined for the Co(OH)2, Co3O4-200 °C, Co3O4-300 °C, and Co3O4-400 °C samples,

5

respectively. Thus, these Rct values do not match the trend of increasing activity. In an attempt to

6

gain a deeper understanding about the apparent performance discrepancy, the capacitive currents

7

versus scan rates for the same material were measured to demonstrate the double-layer

8

capacitance (Cdl) of the tested electrodes (Figure 6d and S8). The Cdl value can be employed to

9

estimate the electrochemical active surface area (ECSA) for solid-liquid-gas tri-phase active

10

interface. Consequently, the Cdl values are measured to be 0.48, 1.19, 1.45, and 4.43 mF cm-2 for

11

Co(OH)2, Co3O4-200 ºC, Co3O4-300 ºC, and Co3O4-400 ºC, respectively. This comparison

12

reveals that the meseporous Co3O4 nanosheet treated at 400 °C displays an about ten times

13

effective active surface area as compared to Co(OH)2 nanosheet. Thus, although the ORR and

14

OER activity normalized by the geometric surface area of mesoporous Co3O4 nanosheets is

15

higher than that of Co(OH)2 nanosheets, the activity normalized by the ECSA of the former is

16

lower than that of the latter, indicating that the superior intrinsic electrocatalytic activity of

17

Co(OH)2 than that of Co3O4.


18

Page 19 of 40

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1 2

Figure 7. (a) OER polarization linear curves of Co(OH)2/CC obtained at 5 mV s-1 in O2-

3

saturated 0.1 M KOH at various cycles. (b) XRD patterns of Co(OH)2 before and after 200 OER

4

cycles. Inset shows the digital image of the electrode before and after 200 OER cycles. (c) SEM

5

of Co(OH)2 after 200 OER cycles. (d) Chronoamperometric responses of Co3O4-400 ºC NS/CC

6

for OER at 1.66 V and ORR at 0.56 V in O2-saturated 0.1 M KOH. (e) XRD patterns of Co3O4-


19


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

Page 20 of 40

1

400 ºC NS/CC before and after 200 OER cycles. Inset shows the digital image of the electrode

2

before and after 200 OER cycles. (f) SEM of Co3O4-400 ºC after polarization at 1.76 V for 17.5

3

h.

4

The long-term operational durability of Co(OH)2/CC and Co3O4/CC electrode was tested by

5

cycled linear sweep voltammetry and chronoamperometric method. Figure 7a shows the OER

6

polarization profile of electrodeposited α-Co(OH)2 at different cycles. The first LSV curve has a

7

larger anodic peak than the following cycles, indicating the irreversible oxidation of Co(OH)2 to

8

CoOOH.38 XRD and Raman analysis further confirm the original Co(OH)2 has been oxidized to

9

be CoOOH after OER processes (Figure 7b and S9),43-45 which is consistent with the results from

10

previous reports that Co(OH)2 can be irreversibly oxidized to CoOOH.38 The electrochemical

11

conversion was also confirmed by the color change after 200 OER cycles (inset in Figure 7b).

12

Since no noticeable morphological change is observed for Co(OH)2 NS/CC electrode after OER

13

cycles (Figure 7c), therefore, the huge difference of OER curves is attributed to the structural

14

transformation

15

chronoamperometric responses of Co3O4-400 ºC NS/CC for OER at 1.66 V and ORR at 0.56 V

16

in O2-saturated 0.1 M KOH electrolyte. After 17.5 h continuous polarization period, the current

17

retention is about 91.7 % for OER and 93.8 % for ORR, indicating its remarkable bifunctional

18

catalytic durability in alkaline media. This is further evidenced by the similar LSVs before and

19

after 200 cycles (Figure S10). In addition, the XRD patterns (Figure 7e) and SEM image (Figure

20

7f) of Co3O4-400 ºC NS/CC before and after 200 OER cycles reveals that the spinel phase and

21

morphology of Co3O4 keep remarkable stable, which is also verified by the same surface color of

22

the electrode (inset in Figure 7e). These results demonstrated the excellent electrochemical

23

reversibility of Co3O4 nanosheet throughout the whole polarization period whereas the α-

from

layered

Co(OH)2

to

spinel

CoOOH.

Figure

7d

shows

the


20

Page 21 of 40

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1

Co(OH)2 was oxidized to CoOOH after the OER process and then kept unchanged. In both cases,

2

the nanosheet morphology and surface configuration are remarkably invariable, suggesting the

3

superior structural durability of the interconnected nanosheet architecture.

4 5

Figure 8. (a) Schematic configuration electrode for evaluating the performance of ZABs. (b)

6

Polarization plot and corresponding power density curve of ZABs based on Co3O4-400 ºC

7

NS/CC air cathode. (c) Galvanostatic discharge curves of ZABs catalyzed by Co3O4-400 ºC

8

NS/CC and Pt/C at various current densities. (d) Discharge/charge cycles of rechargeable ZABs


21


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

Page 22 of 40

1

with Co3O4-400 ºC NS/CC and Pt/C+RuO2 cathodes at a current density of 5 mA cm-2. (e) A

2

practical packaging of home-made plastic ZABs based on Co3O4 NS/CC cathode and a LED

3

light lit by assembled ZABs.

4

Considering the superior apparent overall bifunctional catalytic capability and stability through

5

half reaction measurements, the Co3O4-400 ºC NS/CC integrated electrode is further

6

electrochemically evaluated in realistic primary and rechargeable prototype of ZABs. As

7

illustrated in Figure 8a, the electrode configuration consisted of a polished Zn plate (anode), the

8

prepared Co3O4 NS/CC (air cathode) and 6.0 M KOH with 0.2 M Zn(AC)2 (electrolyte),

9

respectively. The polarization plot and corresponding power density of assembled two electrode

10

ZABs are shown in Figure 8b. The in-situ fabricated Co3O4-400 ºC NS/CC cathode delivers a

11

high open circuit voltage (OCV) of 1.45 V (Figure S11), a large power density of 107 mW cm-2

12

and a maximal current density of 205 mA cm-2. Galvanostatic discharge measurements reveal

13

that the Co3O4 NS/CC exhibited a discharge platform of 1.28, 1.25, 1.23, and 1.18 V at current

14

densities of 2, 5, 10, 20 mA cm-2, respectively, whereas the values for commercial Pt/C are 1.24,

15

1.20, 1.13 and 0.99 V, indicating the superior rate capability of Co3O4 NS/CC than that of Pt/C

16

electrode (Figure 8c). Moreover, at a discharge current density of 2 mA cm-2, the battery with

17

Co3O4 NS/CC eventually operated for 53 h after merely refilling the Zn plate two times (Figure

18

S12), suggesting the mechanical rechargeability. By normalized to the consumed mass of Zn

19

metal, the specific discharge capacities are estimated to be 661 and 535 mA h g-1 (Figure S13),

20

giving high energy densities of 840 and 670 Wh kg-1 at 5 and 10 mA cm-2, respectively.

21

Furthermore, the Co3O4 NS/CC bifunctional electrode enables the fabricated rechargeable Zn-air

22

battery with outstanding cycling stability, as evidenced by 400 cycles at a shadow

23

charge/discharge state (Figure 8d). The Co3O4 NS/CC cathode exhibits a discharging potential of


22

Page 23 of 40

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1

1.24 V and a charging potential of 1.94 V at 5 mA cm-2, which is comparable to the state-of-the-

2

art precious metal-based catalysts of Pt/C+RuO2 mixture (1.22 V and 1.95 V for discharge and

3

charge). After 400 cycles, the variation of discharging and charging potential of Co3O4 NS/CC-

4

based cell is negligible, whereas the average voltage of discharge platform of Pt/C+RuO2

5

electrode decreases by 6.5 %, signifying the better rechargeability and higher efficiency of the

6

former. Compared to the case in N2 atmosphere, the CV curve of assembled ZABs in O2-

7

saturated electrolyte exhibits obvious reduction current, suggesting the discharge behavior

8

contributes to catalyzing ORR (Figure S14). To further demonstrate the practicability of the

9

Co3O4 NS/CC cathode, a home-made “plastic Zn-air battery” is assembled using Zn plate anode

10

and glass fiber separator (Figure 8e). Expectably, the battery could deliver a discharge capacity

11

of 124 mA h at a current density of 10 mA cm-2, corresponding to the energy density of 406 Wh

12

kg-1 based on the cell and 367 Wh kg-1 based on the cell and consumed O2 (Figure S15). The

13

energy density could be further improved with advanced optimization of the cell design and

14

electrode structure in next research stage. The presented electrochemical properties and cycle life

15

here is remarkably compared to other primary or rechargeable ZABs system recent reported in

16

publications (Table S1 and S2). These achieved advancements contribute to the high apparent

17

activity of Co3O4 spinel, enhanced electrochemical surface active area, porous nanosheets

18

morphology as well as the in-situ grown feature on conductive carbon cloth. The cost

19

effectiveness, bifunctional activity, and respectable rechargeability make the synthesized Co3O4

20

NS/CC highly promising as air-cathode for the practical applications in commercial rechargeable

21

metal-air batteries.

22

CONCLUSION


23


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Page 24 of 40

1

In conclusion, aim at clarifying the present controversial electrocatalytic behaviors of Co(OH)2

2

and Co3O4 for ORR/OER, in this work, a clean and facile electrodepositing strategy was

3

developed to prepare Co(OH)2 and Co3O4 active catalysts in-situ grown on carbon cloth in the

4

absence of any binders and surfactants and without further transferring or coating processes to

5

fabricate air cathodes. The synthesized α-Co(OH)2 and Co3O4 are interconnected nanosheets

6

with mesopores observed on Co3O4 surface owning to the thermal treatment process.

7

Comparison of electrochemical properties reveal that although the ORR/OER activity

8

normalized by the geometric area of mesoporous Co3O4 NS/CC is higher than that of Co(OH)2

9

NS/CC, the performance normalized by the ECSA of the former is lower than that of the latter.

10

As an example practical device application, the as-prepared Co3O4-400 ºC NS/CC electrode

11

enables the assembled primary or rechargeable ZABs with large discharge peak power density

12

(106.6 mW cm−2), low overpotential (0.67 V), high rate capability (1.18 V at 20 mA cm-2) and

13

long cycling life (400 cycles at 5 mA cm-2), which are comparable or even superior to state-of-

14

the-art commercial Pt/C and RuO2 mixed cathode. This work would provides deeper

15

fundamental understanding about the apparent and intrinsic activity of transition metal hydroxide

16

and oxide, which could shed light on the future development of advanced electrocatalysts for

17

oxygen-related clean energy storage and conversion technologies such as metal-air batteries and

18

fuel cells.

19

ASSOCIATED CONTENT

20

Supporting Information.

21

Additional TG, SEM, BET data, CV curves in O2 and N2-saturated 0.1 M KOH, Raman and LSV

22

after 200 OER cycles, and electrochemical performance of Zn-air batteries etc, are included in


24

Page 25 of 40

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1

Supporting Information. This material is available free of charge via the Internet at

2

http://pubs.acs.org.

3

AUTHOR INFORMATION

4

Corresponding Author

5

*E-mail: [email protected] (X.H.);

6

*E-mail: [email protected] (C.Z.)

7

Notes

8

The authors declare no competing financial interest.

9

ACKNOWLEDGMENT

10

This work was supported by the National Natural Science Foundation of China (51602216 and

11

51472178), Joint Funds of the National Natural Science Foundation of China, and Guangdong

12

Province (U1601216), National Key Research and Development Program (2016YFB0700205),

13

and Tianjin Natural Science Foundation (16JCYBJC17600).

14

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Y.; Wu, H. Y. Synthesis of r-cobalt hydroxides with Different Intercalated Anions and

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Figure 1. (a) X-ray diffraction (XRD) pattern of electrodeposited Co(OH)2 NS/CC and Co3O4-200 °C, Co3O4300 °C and Co3O4-400 °C, respectively. Crystal structure models of (b) hydrotalcite-like α-Co(OH)2 and (c) spinel Co3O4, respectively. 160x82mm (300 x 300 DPI)


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Figure 2. SEM images of electrodeposited Co(OH)2 NS/CC (a, b) and Co3O4 NS/CC (c-f) after thermal treatment at different temperatures in air atmosphere for 2 h. (c, d) 200 °C, (e) 300 °C, and (f) 400 °C, respectively. 160x74mm (300 x 300 DPI)



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Figure 3. AFM image of (a) Co(OH)2 NS and (b) Co3O4-400 ºC NS. The corresponding thickness measurement data of (c) Co(OH)2 NS and (d) Co3O4-400 ºC NS. 155x88mm (300 x 300 DPI)


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Figure 4. SEM mapping of Co and O elements of Co(OH)2 NS/CC (a-c) and Co3O4-400 ºC NS/CC (d-f). 119x65mm (300 x 300 DPI)



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Figure 5. (a, b) TEM images of electrochemically deposited Co(OH)2 nanosheet and (c, d) Co3O4 nanosheet; (e) HRTEM of Co(OH)2 and SAED pattern (inset); (f) HRTEM of Co3O4 and SAED pattern (inset). 99x101mm (300 x 300 DPI)


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Figure 6. LSVs for ORR (a) and OER (b) of electrochemically deposited α-Co(OH)2 and thermally treated Co3O4 at different temperature at a scanning rate of 5 mV s-1 in O2-saturated 0.1 M KOH. (c) Electrochemical impedance spectroscopy measured at 1.56 V. Inset shows the simplified Randles equivalent circuit used for fitting. Rs, Rct, and CPE represent the electrolyte resistance, charge transfer resistance, and double layer capacity, respectively. (d) Double-layer capacitance measurements for determining electrochemically active surface area of prepared catalysts. 160x121mm (300 x 300 DPI)



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Figure 7. (a) OER polarization linear curves of Co(OH)2/CC obtained at 5 mV s-1 in O2-saturated 0.1 M KOH at various cycles. (b) XRD patterns of Co(OH)2 before and after 200 OER cycles. Inset shows the digital image of the electrode before and after 200 OER cycles. (c) SEM of Co(OH)2 after 200 OER cycles. (d) Chronoamperometric responses of Co3O4-400 ºC NS/CC for OER at 1.66 V and ORR at 0.56 V in O2saturated 0.1 M KOH. (e) XRD patterns of Co3O4-400 ºC NS/CC before and after 200 OER cycles. Inset shows the digital image of the electrode before and after 200 OER cycles. (f) SEM of Co3O4-400 ºC after polarization at 1.76 V for 17.5 h. 160x174mm (300 x 300 DPI)


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Figure 8. (a) Schematic configuration electrode for evaluating the performance of ZABs. (b) Polarization plot and corresponding power density curve of ZABs based on Co3O4-400 ºC NS/CC air cathode. (c) Galvanostatic discharge curves of ZABs catalyzed by Co3O4-400 ºC NS/CC and Pt/C at various current densities. (d) Discharge/charge cycles of rechargeable ZABs with Co3O4-400 ºC NS/CC and Pt/C+RuO2 cathodes at a current density of 5 mA cm-2. (e) A practical packaging of home-made plastic ZABs based on Co3O4 NS/CC cathode and a LED light lit by assembled ZABs. 160x156mm (300 x 300 DPI)



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Clarifying the Controversial Catalytic Performance ... - ACS Publications

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