Rationally Designed Co3O4âC Nanowire Arrays on Ni Foam Derived

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Rationally Designed Co3O4-C Nanowire Arrays on Ni Foam Derived From Metal Organic Framework as Reversible Oxygen Evolution Electrodes with Enhanced Performance for Zn-Air Batteries Jin-Tao Ren, Ge-Ge Yuan, Chen-Chen Weng, and Zhong-Yong Yuan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03034 • Publication Date (Web): 09 Dec 2017 Downloaded from http://pubs.acs.org on December 11, 2017

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ACS Sustainable Chemistry & Engineering

Rationally Designed Co3O4-C Nanowire Arrays on Ni Foam Derived From Metal Organic Framework as Reversible Oxygen Evolution Electrodes with Enhanced Performance for Zn-Air Batteries †,‡ †,‡ †,‡ †,‡ Jin-Tao Ren, Ge-Ge Yuan, Chen-Chen Weng, and Zhong-Yong Yuan*, †National Institute for Advanced Materials, School of Materials Science and Engineering, Nankai University, Tianjin 300350, China ‡Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300350, China *Corresponding author. Email: [email protected]

Corresponding author: Professor Zhong-Yong Yuan National Institute for Advanced Materials, School of Materials Science and Engineering, Nankai University, Tongyan Road 38, Haihe Educational Park, Tianjin 300353, China E-mail: [email protected] 1

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ABSTRACT The development of high activity and stability nonprecious metal catalysts for oxygen evolution and reduction is necessary to solve the energy issues. Here, the porous nanowire arrays composed of Co3O4 nanoparticles and carbon species are prepared by a facile carbonization of the metal-organic framework materials of ZIF-67 which directly grow on Ni foam. The obtained hybrid materials possess a large surface area of 345 m2 g-1 and a high carbon content. The hierarchically interconnected nanowire arrays with porous structure strongly immobilized on Ni foam facilitate the diffusion of generated gas, shorten electrolyte diffusion distance and enhance charge transport. As the working electrode for oxygen evolution reaction without any extra modification in 1.0 M KOH, it can provide a stable current density of 10 mA cm-2 at 1.54 V (vs RHE), along with robust durability. Additionally, the Co3O4-C hybrid materials worked as oxygen reduction catalyst exhibit a positive onset potential of 0.91 V, large limiting current density, and excellent stability. When used as the air catalysts for primary Zn-air batteries, this assembled batteries deliver a large peak power density of 118 mW cm-2, and excellent operation stability. A variety of characterization results and controlled experiments demonstrate that the efficient performance of this hybrid materials towards electrocatalytic reactions originate from the unique electrode configuration, intimate distribution of active species, hierarchically porous configuration, high conductivity of Ni foam and the synergistic effect of Co3O4 and carbonaceous materials. KEYWORDS: Oxygen evolution; Oxygen reduction; Cobalt oxides; Carbon; Porosity; Zinc-air batteries

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INTRODUCTION Growing dependence of conventional fossil fuels has stimulated more interests on regenerate energy storage and conversion devices, such as metal-air batteries, water splitting system and fuel cells.1 However, the electrochemical processes occur on these devices and systems, for example, oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), are kinetically inert without the participate of catalysts.2-4 therefore, suitable catalysts become the vital issues for high-performance energy devices. Usually, the noble metals, such as IrO2, RuO2, exhibit superior electrocatalytic performance toward OER, and Pt is more suitable for ORR, whereas the scarcity and high price of them put an obstacle for practical applications.4 So, numerous efforts have been devoted to fabricate alternatives with low price and remarkable electrocatalytic activity. Transition metal-based catalysts exhibit great potential for OER and ORR due to their highly active, durable, low-cost and easily fabricable.2, 5-9 However, most catalysts are synthesized typically in the form of powder, therefore, presenting several inevitable problems.7, 9-12 First, the powder catalysts need to be coated on electrode with the assistance of conductive polymers, which increase the synthesis cost and decrease the advantages of the metal oxide catalysts.6, 13-14 Second, the aggregation of catalysts on electrode surface reduces the active surface area as well as the available active sites, thus hampering the mass/charge diffusion and affecting their catalytic performance.12, 15 Third, weak physical contact between coated catalysts and substrates always causes the exfoliation of catalysts from substrates, resulted from the diffusion of generated gas.7-8 Recently, porous metal oxides directly deposited on substrates have been considered as a promising and effective route to further boost their electrocatalytic efficiency due to the nature advantages.2, 7-9, 12 On the one hand, the porous morphology can penetrate more electrolyte as well as facilitate the diffusion of 3



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electrolyte to contact active sties and the release of generated gas bobbles; on the other hand, the advanced architecture ensures good electrical conductivity and mechanism stability of the prepared electrodes due to the self-supported configuration without the polymeric binders of active species. Noticeably, the metal oxides can be directly deposited on different substrates to form different configuration, however, the semiconductor property of metal oxides inevitably decrease their electron transport ability.3,

8, 16

In

addition, the low porosity and relative small surface area of metal oxides greatly affect the mass transport and accessible number of active sites.17-18 Therefore, further improving their electrical conductivity and introducing more accessible pores into the inner structure of metal oxides are reasonably important for improve catalytic activity. As ordered porous materials, metal-organic framework (MOF) materials have large surface area, and flexible composition.19-20 The periodic arrangement of metal cations and organic ligands in MOFs ensure them to be ideal precursors to obtain metal oxides-carbon materials after carbonization under high-temperature, wherein metal nanoparticles homogeneously in-situ encapsulate into formed carbon monolith, resulting in an intimate metal oxide-carbon configuration with high surface area and plentiful porosity.19, 21-23 The strong interaction and contact between in-situ formed metal oxides and carbonaceous species would generate promoted effect to electrocatalytic activity.16, 24-27 Ma et al. reported the hybrid Co3O4-carbon porous nanowires, and the uniform distribution of cobalt oxide nanoparticles into in-situ generated carbon monolith greatly promoted the OER catalytic performance.8 Although some groups have investigated porous carbon-metal hybrid materials derived from MOFs as electrocatalysts for electrocatalysis,3, 28-31 those catalysts are always in the form of the powder. The relatively complex and laborious steps to load powder catalysts on current collector by binder would offer inferior contact 4


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between them, affecting electrocatalytic activity for practical applications.24,

28, 32

In addition, similar

transition metal oxides/phosphides/sulfides nanoparticles derived from MOFs materials are also synthesized with good electrochemical performance.5, 24, 26, 33 However, the limited morphology of them greatly shrink their catalytic performance, especially, the post-coated strategy to fabricate electrodes further weaken the superiority of MOFs-derived carbonaceous materials. For all the above consideration, it is interesting to design and fabricate self-supported nanostructured catalysts on current collector with high activity and stability based on MOFs materials for electrocatalysis. In this work, Co3O4-carbon hybrid nanowire arrays on Ni foam derived from ZIF-67 nanowire precursor are obtained. By using facile hydrothermal process, the Ni foam skeleton is uniformly coated with dense cobalt hydroxide carbonate (Co(CO3)0.5(OH)·0.11H2O) nanowire arrays. Subsequent treatment with 2-methylimidazole (2-MeIm) in alkaline solution, the Co(CO3)0.5(OH)·0.11H2O nanowires are converted to ZIF-67 nanowire arrays. The mesoporous Co3O4-carbon nanowire arrays grown on Ni foam (Co3O4-C-NA/NF) are obtained by carbonization of the ZIF-67 nanowires under N2 atmosphere at high temperature. The periodic arrangement of MOFs ensures the uniform dispersion of metal oxide nanoparticles into the porous carbon monolith, which leads to better catalytic performance of this hybrid nanowire electrode due to their synergic effect. The innate catalyst-support and catalyst-carbon characters significantly improve adhesion and charge transfer efficiency between the active components and Ni foam. Moreover, the nanowire structure of the catalyst provides high specific surface area and abundant electrocatalytic active sites. In addition, nitrogen-doped graphitic carbon monolith derived from N-donor ligands of 2-MeIm further boost its electrocatalytic activity. As a result, this hierarchical porous three-dimension (3D) electrode shows high activity and robust stability for OER and ORR in alkaline 5



solution. When employed as air catalyst of primary Zn-air batteries, the Zn-air batteries exhibit a high open circuit voltage, large power density, and robust operation durability.

EXPERIMENTAL SECTION Synthesis of cobalt carbonated hydroxide nanowire arrays grown on Ni foam (Co-NA/NF). The cobalt carbonated hydroxide (Co(CO3)0.5(OH)·0.11H2O) nanowire arrays on Ni foam (the thickness of 1.0 mm) was obtained by a facile hydrothermal method. Firstly, the Ni foam with a 3×4 cm was treated with 2 M HCl solution with ultrasonic for 10 min to remove the surface oxide layer and washed with ethanol and water for several times and dried in vacuum overnight. Then, 5 mmol of Co(NO3)3, 10 mmol of NH4F and 25 mmol of CO(NH2)2 were dissolved in 35 mL of distilled water. Next, the resulting solution was transferred into a 50 mL Teflon-lined stainless steel autoclave and the Ni foam was immersed into the precursor solution. The autoclave was sealed and heated at 90 °C for 12 h, and then cooled to room temperature naturally. The pink-colored cobalt carbonated hydroxide nanowire arrays grown on Ni foam was obtained and named as Co-NA/NF.

Preparation of ZIF-67 nanowires on Ni foam (ZIF-67-NA/NF). The as-obtained Co-NA/NF was directly immersed into 20 mL aqueous solutions contains 1 g of 2-methylimidazole (2-MeIm) and 5 mL of triethylimine (TEA), and stabilized at room temperature for 12 h. Then, the precursor solution was heated to 75 °C for 2 h, after washed with ethanol and water, respectively, to remove the residual 2-MeIm and TEA. Finally, the ZIF-67 nanowires on Ni foam was obtained and named as ZIF-67-NA/NF.

Preparation of hybrid Co3O4-carbon nanowire arrays on Ni foam (Co3O4-C-NA/NF). The as-prepared ZIF-67-NA/NF was placed into a tube furnace and heated from room temperature to 600 °C

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with a heating rate of 2 °C min-1 for 3 h in flowing N2 atmosphere, followed by cooling down to room temperature. The obtained materials were denoted as Co3O4-C-NA/NF. By calculating the increasing mass of Ni foam after the calcined treatment, the Co3O4-C mass loading was 1.8 mg cm-2. However, when the ZIF-67-NA/NF was heated in air at 600 °C, the Co3O4-NA/NF was obtained.

Scraped Co3O4-C nanowires recoated on Ni foam (scraped Co3O4-C-NA). The as-prepared Co3O4-C-NA were scratched off from Co3O4-C-NA/NF, and then dispersed in 1 mL mixture solution of water/ethanol (1:4), following added 50 µL 5 wt% Nafion solution and sonicated for 30 min to form homogeneous ink. Finally, the obtained ink was carefully dispersed on pristine Ni foam with a same loading mass of Co3O4-C on Ni foam, and left dry in air to evaporate solvent.

Fabrication of IrO2 on Ni foam (IrO2/NF). The IrO2 coated on Ni foam was synthesized according to the reported method (J. Am. Chem. Soc., 2014, 136, 13925-13931.) with a same loading amount of Co3O4-C on Ni foam. Firstly, 0.1 g of K2IrCl6 was added to 50 mL aqueous solution containing 0.17 g of disodium hydrogen citrate sesquihydrate with magnetic stirring, and adjusted the pH to 7.5 with NaOH aqueous solution. The mixture was heated at 95 °C with continuous stirring. Then, naturally cooling down to room temperature, the pH was again adjusted to 7.5, followed by once again heating to 95 °C. Repeating this process until the pH value kept at 7.5. The obtained solution was transferred to a round-bottom flask with a reflux condenser. The solution heated at 95 °C for 2 h with bubbled O2, which was further dried at 60 °C overnight in vacuum. The final solid was heated at 300 °C in air to remove the organic compounds and washed with ethanol. Finally, the material was centrifuged and kept at 60 °C. In the end, the as-obtained IrO2 and 45 µL 5 wt % Nafion solution were dispersed in 1 mL solution of water and ethanol solution with volume ratio of 1/4 and sonicated for 30 min to obtain a homogeneous ink. Then 7



the catalyst ink was coated onto Ni foam with the same load mass of Co3O4-C on Ni foam, followed dried at room temperature overnight in air.

Physicochemical characterization. X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Focus Diffractometer with Cu-Kα radiation (λ= 0.15418 nm) operated at 40 kV and 40 mA. Scanning electron microscopy (SEM) was carried out on a Jeol JSF-7500L microscope at voltage of 5 kV. Transmission electron microscopy (TEM) was carried out on a Jeol JSM-2800F microscope at an acceleration voltage of 200 kV. The samples subjected to TEM measurements were scraped off from Ni foam and ultrasonically dispersed in ethanol and drop onto copper grids covered with a carbon film. X-ray photoelectron spectroscopy (XPS) was obtained using a Kratos Axis Ultra DLD (delay line detector) spectrometer equipped with a monochromatic Al-Kα X-ray source (1486.6 eV). All XPS spectra were recorded using an aperture slot of 300 × 700 microns, survey spectra were recorded with a pass energy of 160 eV and high resolution spectra with a pass energy of 40 eV. For SEM and XPS, the bulk electrode was directly used for characterization. N2 adsorption-desorption isotherms were obtained on a NOVA 2000e sorption analyzer (Quantachrome) at the liquid nitrogen temperature (77 K). Before measurement, the samples were degassed at 200 °C overnight. Surface areas were calculated by the multi-point Brunauer-Emmett-Teller (BET) method using adsorption date at a relative pressure range of P/P0 of 0.05 to 0.30; and total pore volumes were estimated from the volume adsorbed at a relative pressure (P/P0) of 0.99; Pore-size distribution was calculated from the adsorption branch of the isotherms by using the Non-Local Density Functional Theory (NLDFT) method. Thermogravimetric analysis (TG) was performed using a TA SDT Q600 instrument at a heating rate of 10 °C min-1 using α-Al2O3 as the reference. The synthesized nanowire array materials were scratched off from Ni foam and used for TG. 8


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Oxygen Evolution Testing. The OER tests were performed in a transitional three-electrode system, the Ag/AgCl and Pt wire were used as the reference and counter electrode, respectively, and the synthesized materials were directly used as the working electrode. The current density was normalized to the geometrical surface area and the measured potentials vs. Ag/AgCl were converted to the reversible hydrogen electrode (RHE) according to the equation: ERHE=EAg/AgCl

+

0.0591pH

+

0.205

(1) The O2 flow was maintained over the electrolyte during electrochemical measurements. The polarization curves were collected with the scan rate of 1 mV s-1 on a WaveDriver 20 Bipotentiostat/Galvanostat (Pine Research Instrumentation, USA) electrochemical workstation. Prior to measurement, the working electrodes were scanned for several times until the current was stabilized. The polarization curves were corrected for the iR compensation within the cell. The Tafel slope was calculated according to Tafel equation as below: η=

a

+

b·logJ

(2) where η was the overpotential, J was the current density and b was the Tafel slope. The overpotential was calculated as follows: η=

ERHE-1.23

(3) Electrochemical impedance spectroscopy (EIS) measurements were performed on a Zahner IM6eX (Zahner, Germany) workstation in potentiostatic mode from 0.1 to 100k Hz with an AC voltage of 5 mV. 9



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The electrical double layer capacitor (Cdl) of the as-prepared electrodes were obtained from cyclic voltammograms (CVs) in a narrow potential range of 1.131 to 1.231 V. The plot of current density (1.20 V) against scan rate obtains a linear relationship and its slope was the double layer capacitance (Cdl). The durability test was measured by both cyclic voltammograms (CVs) and chronopotentiometric response. The CV was carried out ranging from a selected potential region, and the linear sweep voltammetry (LSV) polarization curves were recorded before and after the CV testing for investigate the stability. In order to investigate the electrocatalytic oxygen evolution mechanism, the rotating ring-disk electrode (RRDE) voltammograms were carried out on a RRDE configuration (Pine Research Instrumentation, USA) including a disk electrode (glass carbon) and a ring electrode (platinum), as shown in Figure S7. Pt wire acted as counter electrode, and Ag/AgCl as the reference electrode. The Co3O4-C-NA was carefully scraped off from Co3O4-C-NA/NF, following coated onto a RRDE with the Nafion binder. A scan rate of 1 mV s-1 and a rotation rate of 1600 rpm were conducted for RRDE. To investigate the reaction pathway for OER by detecting the HO2- formation, the disk potential was stabled at 1.50 V vs RHE to oxidize possible HO2- intermediate in O2-saturated 1.0 KOH aqueous solution. On the other hand, the ring potential of the RRDE was held constantly at 0.4 V vs. RHE to reduce the O2 produced from catalyst on the disk electrode in N2-saturated electrolyte. As illustrated in Figure S7, a continuous OER (disk electrode) to ORR (ring electrode) process occurred on the RRED. The Faradaic efficiency (ε) was calculated according to the following equation: ε = Ir/(IdN)

(4)

Where the Ir and Id represents the disk current and the ring current, respectively, and the N represents the current collection efficiency of the RRDE. Since the excellent OER catalytic activity of IrO2 with nearly 10


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100% Faradaic efficiency, therefore, the IrO2 thin-film electrode was employed to calibration of the collection efficiency of the RRDE, so, the N is determined to be 0.2. Moreover, to rationally calculate the Faradaic efficiency, the electrode current is kept constantly at a small constant current of 200 µA, this current is sufficiently large to ensure an appreciable O2 production and efficiently small to minimize local saturation and bubble formation at the disk electrode (see more detailed in J. Am. Chem. Soc., 2014, 136, 13925-13931.).

Oxygen Reduction Testing. In the typical run to prepare the working electrode, 5 mg of Co3O4-C-NA carefully scraped off from Co3O4-C-NA/NF were dispersed in mixture solution containing 0.5 mL distilled water and 0.5 mL isopropanol under sonication, subsequently, coated onto the rotating disk electrode (RDE) or rotating ring-disk electrode (RRDE) (10 µL) wherein the Nafion as the binder. The resulting electrode served as a working electrode to measure the electrocatalytic properties of catalysts on a three-electrode configuration, wherein Ag/AgCl and Pt wire were used as the reference and counter electrode, respectively. All the electrochemical date were recorded using a WaveDriver 20 Bipotentiostat/Galvanostat (Pine Research Instrumentation, USA) electrochemical workstation at room temperature. For the ORR measurements, a flow of O2 was purged into the electrolyte (0.1 M KOH) during the measurements to ensure the O2 saturation. The cyclic voltammograms (CVs) and linear sweep voltammograms (LSVs) measurements were conducted at 50 and 10 mV s-1, respectively. All the electrochemical date were recorded after several CVs cycles until to obtaining stable current. On the basis of RDE test, the electron transfer number (n) involved in oxygen reduction can be calculated by the Koutechy-Levich (K-L) equation: 11



1/J

=

1/Jk

+

1/JL

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=

1/Jk

+

1/Bω1/2

(5) where J, Jk and JL are the measured current density, kinetic current and diffusion limiting current density. ω is the electrode rotating rate. B is the slope of the K-L plots based on the Levich equation as below: B = 0.2nF(DO2)2/3υ-1/6CO2

(6)

where n is the transferred electron number per oxygen molecule. F is Faraday constant (F = 96485 C mol-1). D O2 is the diffusion coefficient of O2 in 0.1 M KOH (DO2 = 1.9 × 10-5 cm2 s-1). υ is the kinetic viscosity (ν = 0.01 cm2 s-1). CO2 is the bulk concentration odium O2 (CO2 = 1.2 × 10-6 mol cm-3). The constant 0.2 is adopted when the rotation speed is expressed in rpm.

For the RRDE tests, the disk electrode was scanned cathodically at a rate of 10 mV s-1 and the ring potential was kept at 0.2 V (vs. Ag/AgCl). The electron transfer number (n) and HO2− intermediate production percentage (%HO2-) were calculated as follows: n

=

4Id/(Id

Ir/N)

+

(7) %HO2−

=

200Ir/(IdN

+

Ir)

(8) where Id and Ir are the disk and ring current, and the current collection eﬃciency (N) of the Pt ring is 0.37.. Zn-air Batteries Testing. The air cathode used for Zn-air batteries were made of hydrophobic carbon paper (CP) with the coating of active materials by drop-casting with catalyst ink with a loading mass of 1.0 mg cm-2. The catalysts ink was prepared as similar to the above procedure towards ORR. The Zn-air

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batteries tests were carried out on a home-built Zn-air instrument. A polished Zn plate was used as anode, and the electrolyte was 6.0 M KOH solution. The one side of CP allows O2 to reach the active materials which immersed into electrolyte. All the data of Zn-air batteries were measured under room atmosphere on the WaveDriver 20 Bipotentiostat/Galvanostat (Pine Research Instrumentation, USA) electrochemical workstation. Both the current density and power density were normalized to the effective surface area of air electrode. The galvanostatic discharge and discharge-charge cycling were carried out by LAND testing system.

RESULTS AND DISCUSSION

Material Synthesis and Characterization. Previously the MOFs nanoparticles were extensively fabricated, however, the nanowire morphology of MOFs on the substrate was hardly reported.3, 30, 34 This is mainly due to the slow crystallization process of MOFs, thus the nanoparticles are always obtained.25,

35

In this work, the mesoporous Co3O4-carbon

nanowire arrays directly grown on Ni foam (Co3O4-C-NA/NF) were obtained from the MOFs materials with the nanowire structure which was synthesized from cobalt hydroxide carbonate nanowire on Ni foam (Co-NA/NF) in 2-MeIm solution. The basic cobalt hydroxide carbonate would dissociate and release carbonate and hydroxide anions due to the deprotonation of 2-MeIm,36 therefore, slowly releasing the Co2+. Subsequently, the slow crystallization of Co2+ and 2-MeIm would be initiated, and replicated the original morphology of basic cobalt hydroxide carbonate nanowires, finally achieving the MOFs materials with specific morphology. By simple carbonization under N2 flow, the inorganic and organic components in MOFs could convert into Co3O4 nanoparticles and N-doped carbon monolith. More important, this 13



strategy can not only simultaneously maintain the original nanowire morphology but also generate pores in nanowires (Scheme 1).

Scheme 1. Illustration of the fabrication of Co3O4-carbon nanowires on Ni foam.

The Co-NA/NF was prepared by hydrothermal method. As displayed in Figure S1 (left), the silver Ni foam surface is coated with pink materials, and the X-ray diffraction (XRD) pattern (Figure S2a) demonstrates that the as-obtained Co-NA/NF is cobalt carbonate hydroxide (Co(CO3)0.5(OH)·0.11H2O) (Powder Diffraction File (PDF) No. 48-0083, Joint Committee on Powder Diffraction Standards (JCPDS), [2004]). Figure S2b shows the scanning electron microscopy (SEM) image of the Co-NA/NF, and it can be seen that nanowire arrays grown on the Ni foam, which has a dense form and smooth surface (Figure S2b insert). After immersed into 2-MeIm solution, the ZIF-67-NA/NF was obtained, as the color of Co-NA/NF changes from pink to purple (Figure S1 middle), and the XRD pattern (Figure S3a) reveals that the peaks of cobalt hydroxide carbonate disappear fully after treated with 2-MeIm solution, and the peaks belong to ZIF-NA/NF well match to that of the typical crystalline structure of ZIF-67 powder synthesized according to the mature method,37 without any redundant detectable peaks except the typical peaks ascribed to Ni foam, indicating cobalt hydroxide carbonate completely converts to ZIF-67 (Figure S3a).

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The magnified SEM image (Figure S3b) reveals that the ultra-small nanoparticles linked together to form the ZIF-67 nanowires. Upon high temperature carbonization, the colour of ZIF-NA/NF changes from purple to black (Figure S1 right). Low-resolution SEM image of Co3O4-C-NA/NF (Figure 1a) reveals that nanowires grow densely and integrate into arrays on 3D skeleton, which is greatly different from pristine Ni foam (Figure S4) that has a smooth surface. The high-resolution SEM images (Figure 1b and c) present that the nanowires have a length of about 2 µm, constructed from many nanoparticles of 100 nm in diameter. The obtained nanowires have similar morphology comparison to ZIF-67-NA/NF, and the smooth surface indicates that the generated Co3O4 nanoparticles are well coated by in-situ formed carbon layer and embed in Co3O4-C hybrid nanowires. The transmission electron microscopy (TEM) images (Figure 1d and e) present apparently amorphous carbon and small sized crystalline Co3O4 nanoparticles embedded in the resulting carbon monolith. The clear lattice fringe space of 0.24 nm corresponds to the (311) plane of the cubic Co3O4 spinel phase (Figure 1e). Moreover, the amorphous carbon species are also dispersed uniformly on the hybrid nanowires, as evidenced by the elemental mapping. The energy dispersive X-ray spectroscopy (EDS) elemental mapping images of Co3O4-C-NA/NF (Figure 1f) clearly display the homogeneous distribution of Co, O and C as well as N elements in selected area, consistent well with the HR-TEM results, demonstrating the homogeneous distribution of Co3O4 and carbon species.

15



Figure 1. (a), (b) and (c) SEM images, (d) TEM and (e) HR-TEM image, (f) EDS elemental mapping images of Co3O4-C-NA/NF.

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350

100

JPCDS No. 43-1003

10

20

30 40 50 60 2Theta (degree)

70

20 0

(e)

0.08

0

300

3

250

150 100

200 400 600 Temperature ( )

Co 2p

(c)

200

59.56%

40

80

(d)

67.61%

60

29.94%

Weight (%)

Intensity (a.u.)

80

Air Nitrogen

36.78%

(b)

-1

(a)

Co

50

800

(f)

Co

0 0.0

0.2 0.4 0.6 0.8 Relative pressure (P/P0)

1.0

N 1s

O

Intensity (a.u.)

0.04

0.02

C N

795.7 eV 798.4 eV

Co

780.5 eV 782.3 eV

Intensity (a.u.)

Co

0.06 dV/dD

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


Volume adsorbed (cm g STP)

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400.4 eV 398.5 eV 402.2 eV

805.1 eV 788.6 eV 802.2 eV 786.1 eV

0.00

0

5

10

15

20

Pore size (nm)

805 800 795 790 785 780 Binding energy (eV)

399.4 eV

404

402 400 398 396 Binding energy (eV)

Figure 2. (a) Wide-angle XRD pattern and (b) TG curve for Co3O4-C-NA. (c) and (d) N2 adsorption isotherms and the corresponding pore size distribution of Co3O4-C-NA/NF. (e) and (f) High-resolution XPS spectrum of the Co 2p and N 1s core level. Insert in (e): XPS survey spectra of Co3O4-C-NA/NF.

The Co3O4-C-NA/NF exhibits the cubic spinel phase of Co3O4 (PDF No. 43-1003, JCPDS, [2004]) detected by wide-angle XRD, as shown in Figure 2a. Thermogravimetric analysis (TGA, Figure 2b) indicates that the decomposition of ZIF-67-NA (scraped off from Ni foam) in different atmosphere (nitrogen and air) is of evident difference. The slight weight loss below 200 oC can be attributed to the removal of absorbed water on the surface and inner pores. In air, the oxidation of the organic ligands cause large weight loss from 200 to 400 oC temperature, and finally only about 29.94% of mass is retained. Upon further increasing the heating temperature (> 400 oC), the weight keeps stable, suggesting it totally converts to Co3O4. However, through increasing the temperature to 600 oC, it also retains 59.56% weight in nitrogen atmosphere, indicating that the final obtained Co3O4-C NA has a higher carbon content comparing with pyrolysis under air. 17



The N2 adsorption-desorption isotherms of the Co3O4-C-NA/NF are of type IV with an H3 type hysteresis loop (Figure 2b), corresponding to the mesoporous character, which is consistent with the SEM and TEM observation. Correspondingly, the pore size distribution curve obtained by NLDFT model reveals the coexistence of micro- and meso-pores (Figure 2d). The Co3O4-C-NA/NF possesses a large BET specific surface area of 345 m2 g-1 with a total pore volume of 0.224 cm3 g-1, indicating the Co3O4-C-NA/NF successfully inherits the porous morphology of the MOFs materials, which agrees well with the previous reported ZIF-67 nanoparticles after carbonization under inert atmosphere.25, 38 The large specific surface area is vital for the transport of generated gases and the electrolyte, thus boosting catalytic performance. The N2 sorption isotherms and corresponding pore size distribution of ZIF-67-NA/NF and Co3O4-NA/NF are also shown in Figure S5. The ZIF-67-NA/NF has a surface area of 543 m2 g-1 with a narrow pore size distribution, larger than that of Co3O4-NA/NF (123 m2 g-1). Those results demonstrate that the in-situ generated carbon species can efficiently enhance the specific surface area of the as-obtained Co3O4-C-NA/NF material. X-ray photoelectron spectroscopy (XPS) of Co3O4-C-NA/NF is shown in Figure 2e (insert), and the peaks corresponding to Co, C and O as well as N elements can be clearly observed. Peak deconvolution of the Co 2p core level region shows two pairs of spin-orbit doublets attributed to Co2+ and Co3+ and four shakeup satellites.8 Additionally, the energy band at 795.7 and 780.5 eV corresponded to Co 2p1/2 and Co 2p3/2, respectively, demonstrate that the generated cobalt oxide is Co3O4.8, 39 Due to the pyrolysis of ligand of 2-MeIm, the N atoms were successfully doped into obtained carbonaceous materials. Deconvolution of N 1s spectra (Figure 2f) displays the presence of pyridinic-N (398.5 eV), pyrrolic-N (399.4 eV), graphitic-N (400.4 eV) and pyridine-N-oxide groups (402.2 eV).5, 40-42 The appearance of N in hybrid 18


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materials is benefit for the electrocatalytic reaction.43-44 There is 2.78 at% N in the Co3O4-C-NA according to the XPS analysis. Therefore, the well-reserved porous structure and large surface area of the hierarchical porous Co3O4-C-NA/NF self-supported electrode are successfully obtained, which is considered to has superior electrocatalytic performance for oxygen evolution and oxygen reduction reactions.

Oxygen Evolution Activity Porous Co3O4-C nanowire array constructed from the ZIF-67 nanoparticles directly deposited on macroporous Ni foam, and it could be directly used as the working electrode for OER (Figure S6). Due to the nature ability of the Ni foam, alkaline electrolyte (1.0 KOH) is more suitable for Co3O4-C-NA/NF toward electrocatalytic oxygen evolution. Polarization curve of pristine Ni foam below 1.70 V shows a negligible current density and has a high onset potential beyond 1.57 V (Figure 3a), indicating very low activity of pristine Ni foam for OER. However, because of the oxidation effect of Co3O4, the onset potential of Co3O4-C-NA/NF is difficult to be observed. From the polarization curve recorded on Co3O4-C-NA/NF (Figure 3a), it exhibits an onset potential below 1.50 V. Additionally, the current density evidently increase along with the enhance of operated potential, demonstrating the Co3O4-C-NA/NF obtained from the pyrolysis of MOFs materials with unique structure property has significant OER catalytic activity. The IrO2 coated on Ni foam (IrO2/NF) is also synthesized with the same loading mass of Co3O4-C-NA on Ni foam. The IrO2/NF has slightly lower onset potential, however, when the operating potential increases to 1.67 V, the Co3O4-C-NA/NF exhibits a larger anodic current than that of IrO2/NF. The potential at the current density of 10 mA cm-2 (Ej=10) is a vital parameter to evaluate the catalytic

19



activity of catalysts toward OER.45-46 The Co3O4-C-NA/NF affords a Ej=10 at 1.54 V, higher than that of IrO2/NF at 1.52 V; however, this value is lower than that of many other recently developed nonprecious catalysts for OER summarized in Table S1, revealing the outstanding catalytic activity of Co3O4-C-NA/NF towards OER in alkaline media. The large specific surface area, favourable charge/mass transport in that porous nanowire structure can explain this excellent catalytic activity of Co3O4-C-NA/NF electrode. In addition, comparing with Co3O4-C-NA/NF, the ZIF-67-NA/NF has an inferior catalytic behaviour (Ej=10 at 1.68 V).

200

0.6

(a)

(b)

Co3O4-C-NA/NF

IrO2/NF

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100

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0.3

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10 15 Time(h)

20

0 1.2

25

1.3

1.4

1.5

1.6

1.7

E vs.RHE(V)

Figure 3. (a) Polarization curves of Co3O4-C-NA/NF, IrO2/NF, ZIF-67-NA/NF, and pristine Ni foam with a scan rate of 1 mV s-1. (b) Tafel plots of Co3O4-C-NA/NF and IrO2/NF. (c) Chronoamperometric 20


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response measured at a constant potential of 1.52 V (the current density of 10 mA cm-2). Inset in (c): chronopotentiometric response at a constant current density of 10 mA cm-2 of Co3O4-C-NA/NF and IrO2/NF. (d) Polarization curves of Co3O4-C-NA/NF initially and after 2000 CV cycles (scan rate of 5 mV s-1). Inset in (d): plot of the current density recorded at 1.60 V versus cycle number. All experiments were carried out in the O2-saturated 1.0 M KOH solution.

Moreover, the catalytic kinetics for oxygen evolution was evaluated by Tafel plots (Figure 3b). The Tafel slope value of Co3O4-C-NA/NF is 90 mV dec-1, indicating the oxygen evolution on Co3O4-C-NA/NF follows the Volmer−Heyrovsky mechanism. A rotating ring-disk electrode (RRDE) technique was used to investigate the reaction mechanism and estimate the Faradaic efficiency of Co3O4-C-NA towards OER, the continuous process is illustrated in Figure S7. Here, a potential of 0.4 V conducted on the Pt ring electrode in the region of oxygen reduction reaction (ORR) can ensure the immediately reducing of formed oxygen on the disk electrode. When the collection efficiency is 0.2, a constant ring current of about 40 µA can be detected under a disk current kept at 200 µA. This result implies that the Co3O4-C-NA catalysed current is the OER process,1, 8, 47 along with a higher Faradaic efficiency of 99.3% (Figure S8). Durability towards OER is of importance for actual applications. The chronoamperometric response reveals the high stability of Co3O4-C-NA/NF, showing a slight decay of current density of 5.7 % within 24 h, however, IrO2/NF displays a large current attenuation of 22.2 % which is about 4 times higher than the one of Co3O4-C-NA/NF (Figure 3c). In the chronopotentiometric response, Co3O4-C-NA/NF provides a nearly constant potential of 1.54 V to afford 10 mA cm-2 (Figure 3c inset), whereas the potential of IrO2/NF increases evidently, revealing the superior durability of Co3O4-C-NA/NF in alkaline electrolyte. Furthermore, there only about 3.3% current loss is observed for Co3O4-C-NA/NF after 2000 CVs cycles (Figure 3d), again confirming the superior stability of Co3O4-C-NA/NF to withstand accelerated degradation. Therefore, both the chronoampermetric and chronopotentiometric response show smaller 21



OER activity attenuation of Co3O4-C-NA/NF in comparison to that of IrO2/NF, demonstrating the stronger durability of the Co3O4-C-NA/NF in alkaline electrolyte. The superior electrocatalytic performance of Co3O4-C-NA/NF implies that the active species directly deposited on conductive substrates has more superiorities in comparison with the post-coated catalysts. Due to the loose contact effect between IrO2 and pristine Ni foam, the active species of IrO2 may peel off from the electrode surface during the diffusion of the generated O2 gas. Therefore, the IrO2/NF electrode exhibits inferior stability for oxygen evolution. The XRD patterns of the Co3O4-C-NA/NF after 25 h reaction show no evident change on the typical peaks in comparison with fresh Co3O4-C-NA/NF (Figure S9a), and the superstructure of nanowire is also well maintained (Figure S9b), which is important for active and stable OER catalysts, revealing the advantages of this synthesized strategy. The direct carbonization of ZIF-67 nanowires can in-situ incorporate Co3O4 nanoparticles into carbon monolith which directly grow on Ni foam skeleton, endowing Co3O4-C-NA/NF to improved conductivity and charge transfer capability as well as intimate mechanical adhesion between active catalysts and substrate. When the ZIF-67-NA/NF was heated in air at 600 °C, the Co3O4-NA/NF was obtained to discuss the effect of carbon species. However, it shows a larger onset potential beyond 1.50 V and needs an operating potential of 1.64 V to obtain 10 mA cm-2, together with high Tafel slope of 136 mV dec-1 (Figure 4a), which are both much higher than those of Co3O4- C-NA/NF, indicating its lower OER activity with inferior reaction kinetics. In addition, this self-supported morphology formed between nanowire arrays and porous substrate has proved to heighten the adhesion between them to ensure the long-term stability,8, 12 as proved by the chronoamperometric response. The Co3O4-C-NA/NF exhibits outstanding activity with a small current 22


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Page 23 of 36

attenuation of 4.8% after 20 h. However, the controllable sample is prepared by scratching off Co3O4-C nanowires from Co3O4-C-NA/NF and then post-coating it onto Ni foam, which exhibits a higher current decay within 20 h, mostly, due to the abscission of catalysts caused from the diffusion of evaluated gases during continuous operation.

100

Co3O4-C-NA/NF

(a)

(b)

Co3O4-C film

95.2%

scraped Co3O4-C-NA/NF Co3O4-NA/NF scraped Co3O4-C-NA/NF

77.9%

Co3O4-NA/NF Co3O4-C-NA/NF

0.5

60

Co3O4-C film

136 mV dec-1

0.4

114 mV dec 159 mV dec-1 90 mV dec-1

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1.5

scraped Co3O4-C-NA/NF

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10 Time (h)

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Co3O4-NA/NF

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Z"(ohm)

6

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(c)

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


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-0.2

-2 1.14

1.16 1.18 1.20 E vs.RHE (V)

0.0

1.22

1.0

1.5 Z'(ohm)

2.0

Figure 4. (a) Polarization curves of Co3O4-C-NA/NF, Co3O4-C film, Co3O4-NA/NF and scratched Co3O4-C-NA. Inset in (a): the corresponding Tafel plots of samples. (b) Chronoamperometric response curves of Co3O4-C-NA/NF and the counterpart of scratched Co3O4-C-NA at a constant potential of 1.60 V. (c) Scan rate dependence of the current densities of Co3O4-C-NA/NF at 1.20 V. (d) EIS of Co3O4-C-NA/NF and Co3O4-NA/NF recorded at 1.60 V. All experiments were carried out in the O2-saturated 1.0 M KOH solution.

23



Combining the unique structure of the Co3O4-C nanowires and the 3D porous morphology of Ni foam into the superior electrode configuration significantly promotes the reaction kinetics of OER towards Co3O4-C-NA/NF. To investigate the effect of the 3D porous configuration, the Co3O4-C-NA/NF was pressed under high pressure (> 7 MPa) to destroy its ordered macropores structure, finally obtaining the Co3O4-C film. Though they have the same loading mass of active species, the porous structure and nanowire arrays are destroyed towards Co3O4-C film. The Co3O4-C film affords a higher onset potential beyond 1.55 V and needs a larger potential of 1.64 V to provide a current density of 10 mA cm-1, which is inferior in comparison to Co3O4-C-NA/NF, as shown in Figure 4a. Also, Co3O4-C film displays a Tafel slope much higher than that of Co3O4-C-NA/NF, indicating the porous structure can cause more favourable reaction kinetics (Figure 4a inset). On the one hand, the microstructure of nanowire arrays and the macro-morphology of porous Ni foam are favourable for the release of evaluated O2 gas bubbles.7, 12 On the other hand, the porous nanowire arrays can sufficiently permit quick electrolyte diffusion, and the macroporous of the Ni foam facilitate the access of reactants in the electrolyte to the active sties.2, 6, 8-9 Furthermore, the current density of Co3O4-C-NA in the OER potential region is not sensitive to the scan rate, when the scan rate increases from 5 to 50 mV s-1, only about 3.5% attenuation (Figure S10) is obtained due to the enhanced mass transport in the porous nanowire arrays.8 Indeed, the porous structure of Co3O4-C nanowire array can provide a large active surface area, as revealed by its high electrochemical double-layer capacitance (Cdl). The Co3O4-C-NA/NF has a Cdl value of 128 mF cm-2 (Figure 4c), which is higher than that of Co3O4-NA/NF (68 mF cm-2) (Figure S11). Because of the Cdl is related to the electrochemically active surface area of catalysts,8, 13 the higher value of the Cdl implies that the porous Co3O4-C nanowire possesses large active surface area. Much exposure 24


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and effectively utilized electroactive sites of the Co3O4-C-NA/NF endows it to superior OER activity. And the in-situ generated nitrogen species in carbon monolith, as well as more electrophilic Co species, facilitate the adsorption and reaction of OH- groups and provide more active sites to enhance the OER activity of Co3O4-C-NA/NF in alkaline media.16, 22, 32, 48 Moreover, the nanowire arrays grown on the conductive Ni foam skeleton can enhance the electron transport from substrates to active catalysts.49-50 Without using any polymeric binders and extra conductive additives, Co3O4-C-NA/NF has smaller dead volume and high electroconductibility, as proved by the smaller contact and charge transfer impedance (Figure 4d). The different electrochemical impedance spectrum (EIS) of Co3O4-NA/NF and Co3O4-C-NA/NF suggest the difference between them. Much large semicircular diameter of Co3O4-NA/NF implies the larger contact and charge transfer impedance in comparison to Co3O4-C-NA/NF, demonstrating that the strong interacting between Co3O4 and carbon species can evidently improve its electrical conductivity and its catalytic performance towards OER.

Oxygen Reduction Activity and Zn-air Batteries The oxygen evolution electrode with reversible ability to efficiently catalyse the ORR is significant for practical applications, especially for those metal-air batteries.51-53 Here, taking consideration of the excellent OER catalytic ability, the ORR activity of Co3O4-C-NA was also measured. To detailed investigate its electrocatalytic ability, the Co3O4-C-NA was carefully scratched off from Co3O4-C-NA/NF, and then coated on a rotating disk electrode (RDE). Thus, the linear sweeping voltammograms (LSV) were carried out, and the obtained LSV curves were shown in Figure 5a. The Pt/C catalyst was also measured due to its superior ORR catalytic activity. In the ORR region, the Co3O4-C-NA exhibits an onset

25



potential and half-wave potential (E1/2) of 0.95 and 0.83 V, respectively, comparative to the catalytic performance of Pt/C (0.98 and 0.83 V), while much higher than those of Co3O4-NA (0.84 and 0.63 V). Importantly, the limiting current density of Co3O4-C-NA is higher than that of Pt/C when the potential below 0.22 V, greatly due to the higher surface area with hierarchical porosity providing numerous active sties, as well as the synergetic effect between Co3O4 and N doped carbonaceous materials.44, 54 This suggests that the porous structure can efficiently improve mass transport and expose more active sties.12, 15 In addition, the difference of the catalytic activity between Co3O4-C-NA and Co3O4-NA further suggests that the nature of hybrid structure can evidently enhance the electrocatalytic activity.8, 10, 55 The LSVs curves of Co3O4-C-NA under different sweeping rate were also measured and shown in Figure 5b. The limiting current density increases with the rotating speeds, due to the decrease of diffusion distance under high speed.44 And based on the Koutecky-Levich (K-L) equation on the LSV data, the calculated electron transfer number (n) of Co3O4-C-NA (3.77) closely reaches to the value of Pt/C (3.80), demonstrating the dominant of pseudo-four-electron pathway of Co3O4-C-NA towards ORR. Furthermore, the rotating ring-disk electrode (RRDE) measurements indicate that the electron transfer number of Co3O4-C-NA is 3.70 – 3.77 at the potential region of 0.20 to 0.80 V with a low HO2− production (below 15%) towards ORR process, slightly inferior to those of Pt/C, as shown in Figure 5c, further suggesting the desirable four-electron pathway towards ORR.

26


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Page 27 of 36

0 -1

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0

0

800 1600 2400 3200 Time (s)

2

4 6 Time (s)

8

10

Figure 5. (a) LSVs of the synthesized electrocatlysts on a RDE (1600 rpm, scan rate of 10 mV s-1). (b) LSVs of the as-synthesized Co3O4-C-NA on a RDE with different rotating rate. Inset in (b): K-L plots of the synthesized electrocatalysts at 0.3 V accords to the RDE curves. (c) Extent of HO2- production and the corresponding electron-transfer number. Inset in (c): The RRDE curves of catalysts. (d) The chronoamperometric response of as-fabricated Co3O4-C-NA as compared to Pt/C at 0.5 V. Inset in (d): The chronoamperometric response after methanol addition.

Moreover, the chronoamperometric measurement reveals that the Co3O4-C-NA has robust durability for ORR in alkaline medium. As shown in the chronoamperometric response (Figure 5d), the current density of Co3O4-C-NA only decrease 9.8% over 10 h under continuous operation at the voltage of 0.5 V, while 35.3% initial current lose of Pt/C. Additionally, the current density of Co3O4-C-NA shows no obvious change even after the addition of methanol which always causes the poisoning of noble metal 27



(Figure 5d inset), demonstrating its high selectivity to ORR with strong methanol tolerance ability. The cathodic current density changes evidently after adding methanol towards Pt/C, suggesting the methanol oxidation-reduction, namely, indicating the poisoning of Pt/C. Therefore, the superior activity and stronger durability of Co3O4-C-NA for ORR endow it with great potential as cathode materials in metal-air batteries and fuel cells.

(c)

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Page 28 of 36

2.1 1.8 1.5 1.2

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Cycles

120

160

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Figure 6. Zn-air batteries using Co3O4-C-NA as the air cathode catalyst. (a) Schematic of two-electrode Zn-air battery with Co3O4-C-NA as bifunctional electrocatalyst. (b) Open circuit voltage for the Co3O4-C-NA and Pt/C. Inset in (b): Photograph of the open circuit voltage for the as-assembled Zn-air barriers of CoO/CoS-NSC on coulombmeter. (c) Discharge polarization curves and corresponding power density of Zn-air batteries using Pt/C, Co3O4-C-NA, Pt/C+IrO2 and Co3O4-NA as the air cathode catalyst. (d) Charge and discharge polarization (V-i) curves of the primary Zn-air batteries using Pt/C, Co3O4-C-NA, Pt/C+IrO2 and Co3O4-NA air cathode (e) Typical charge-discharge cycling curves (charge and discharge current density are both 30 mA cm-2) of as-prepared Zn-air batteries using Pt/C, Co3O4-C-NA, Pt/C+IrO2 air cathode.

28


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Finally, the Co3O4-C-NA as air catalyst (the cathode) was paired with the zinc foil (the anode) in a home-built electrochemical cell filled with 6.0 M KOH as electrolyte to measure the Zn-air batteries performance, as illustrated in Figure 6a. Limiting the structure of the home-built electrochemical cell, the Co3O4-C-NA was scratch off from the Co3O4-C-NA/NF, then re-coated on the hydrophobic carbon paper (CP) with the assistance of Nafion polymer. During the battery tests, the oxygen was directly fed to one side of the air cathode. The formed Zn-air batteries can provide an open circuit voltage of 1.42 V, as shown on the coulombmeter (Figure 6b inset), which is slight higher than that of Pt/C (1.40 V). The polarization curve and the corresponding power density plot are both shown in Figure 6c. At the voltage of 1.2 and 1.0 V, as-prepared Zn-air batteries equipped with Co3O4-C-NA air cathode can provide the current density of 26 and 50 mA cm-2, respectively. In addition, the peak power density sustained of assembled Zn-air batteries of Co3O4-C-NA air cathode is 118 mW cm-2 (248 mA cm-2), closely reaching to the value of Pt/C (124 mW cm-2 at 219 mA cm-2), but higher than that of Pt/C+IrO2 benchmark (93 mW cm-2 at 175 mA cm-2). Furthermore, the current density and power density of Co3O4-C-NA air cathode are both better than those of Co3O4-NA, further highlighting the superiority of hierarchical porous structure and metal-carbon hybrid combination in the fast charge/mass and gas diffusion as well as the effective catalysis of reversible oxygen reduction. All these merits make as-prepared Zn-air battery with Co3O4-C-NA air cathode having superior performance in comparison to recently reported ones equipped with nonprecious metal and carbon-based air catalysts which are summarized in Table S2. The charging-discharging polarization curves of the assembled Zn-air batteries about those catalysts coated on CP as both the OER and ORR electrodes were also measured according to the galvanodynamic method. As shown in Figure 6d, the Co3O4-C-NA and Pt/C+IrO2 show outstanding rate capability 29



whatever the discharging or charging process. The charging potential of Co3O4-NA and Pt/C both suppress to Co3O4-C-NA, indicating their inferior OER electrocatalytic activity. Importantly, this batteries assembled by Co3O4-C-NA cathode also exhibits impressive long-term operation durability. The galvanostatical charge-discharge (discharging and charging for 10 min at 30 mA cm-2, respectively) of assembled Zn-air batteries with Co3O4-C-NA as air cathode was measured and shown in Figure 6e. The Co3O4-C-NA exhibits excellent rate capability, as well as the discharge and charge voltage of 1.18 and 2.11 V in the first cycle with the trip efficiency of 53.4%, respectively. And the voltage gap increases before the 20 cycles, subsequently, the gap keeps nearly stable without evidently change. The discharge and charge voltages are stabilized at 1.16 and 2.17 V after 180 cycles, respectively, which are slightly higher or lower than the value recorded at initial cycles, demonstrating the good operating stability of as-prepared Co3O4-C-NA catalyst. The Pt/C+IrO2 can also well maintain the charge and discharge potentials even cycling numbers over 130, suggesting its excellent electrocatalytic reversible. Specially, the Pt/C has an enhanced potential gap for charge and discharge along with the increase of cycling number due to its poor oxygen evolution ability. Such impressive performance towards fabricated Zn-air batteries is greatly related to the remarkable OER and ORR activity and stability of Co3O4-C-NA in alkaline electrolyte.

CONCLUSIONS The nanowire arrays constructed from carbon encapsulated Co3O4 nanoparticles directly grow on 3D porous Ni foam, and it exhibits higher OER and ORR catalytic activity, more favourable kinetics, and stronger durability in alkaline solution. The OER performance of Co3O4-C-NA/NF is comparable to IrO2

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benchmark, and better than that of most recently reported nonprecious transition-metal OER catalysts. Additionally, as ORR catalyst, the catalytic performance of Co3O4-C-NA closely reaches to that of noble Pt/C. The directly grown porous nanowire array on Ni foam, the intimate effect of Co3O4 and carbon species, and the macroporous morphology of the Ni foam as well as the appearance of nitrogen elements together endow the Co3O4-C-NA/NF with enhanced active surface area, improved mass and charge transport, favourable reaction kinetics and strong structural stability, therefore, the Co3O4-C-NA/NF exhibits superior OER and ORR catalytic activity and excellent durability. Moreover, the fabricated primary Zn-air batteries using Co3O4-C-NA air catalyst deliver a peak power density of 118 mW cm-2, along with impressive long-term durability. Thus, the outstanding performance endows this bifunctional Co3O4-C-NA with much potential in many fields, such as water-splitting devices and other emerging renewable energy conversion systems.

ASSOCAITION CONTENT Supporting Information Optical images of Co-NA/NF, ZIF-67-NA/NF and Co3O4-C-NA/NF, XRD pattern and SEM image of Co-NA/NF, XRD patterns of ZIF-67 powder and ZIF-67-NA/NF, SEM image of ZIF-67-NA/NF and pristine Ni foam, N2 sorption isotherms and corresponding pore size distribution curves of ZIF-67-NA/NF and Co3O4-NA/NF, optical images of electrocatalytic OER process, schematic illustration of the continuous OER to ORR process initiated on the RRDE, ring current of Co3O4-C-NA coated on a RRDE, XRD pattern and SEM image of Co3O4-C-NA initially and after 24 h OER reaction, polarization curves of

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Co3O4-C-NA/NF under different scan rates, scan rate dependence of the current densities of Co3O4-NA/NF at 1.20 V, ORR polarization curves of Co3O4-NA and Pt/C.

AUTHOR INFORMATION Corresponding Author *E-mail:[email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21421001, 21573115), and the Natural Science Foundation of Tianjin (15JCZDJC37100).

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Rationally Designed Co3O4âC Nanowire Arrays on Ni Foam Derived

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