Rationally Designed Co3O4–C Nanowire Arrays on Ni Foam Derived

Dec 9, 2017 - Porous Co3O4−C nanowire arrays directly grown on Ni foam exhibit ... hybrid materials possess a large surface area of 345 m2 g–1 and...
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Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 707−718

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

ACS Sustainable Chem. Eng. 2018.6:707-718. Downloaded from pubs.acs.org by MOUNT ROYAL UNIV on 08/07/18. For personal use only.

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S Supporting Information *

ABSTRACT: The development of high activity and stability nonprecious metal catalysts for oxygen evolution and reduction is necessary to solve energy supply issues. Here, 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, the 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 material toward electrocatalytic reactions originates from the unique electrode configuration, intimate distribution of active species, hierarchically porous configuration, high conductivity of Ni foam, and synergistic effect of Co3O4 and carbonaceous materials. KEYWORDS: Oxygen evolution, Oxygen reduction, Cobalt oxides, Carbon, Porosity, Zinc−air batteries

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INTRODUCTION Global growing dependence on conventional fossil fuels has stimulated more interest in renewable energy storage and conversion devices, such as metal−air batteries, water splitting systems, and fuel cells.1 However, the electrochemical processes occuring in these devices and systems, for example, oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), are kinetically inert without the participation of catalysts.2−4 Therefore, suitable catalysts have become a vital issue for high-performance energy devices. Usually, noble metals, such as IrO2 and RuO2, exhibit superior electrocatalytic performance toward OER, and Pt is more suitable for ORR; however, the scarcity and high price of them create obstacles 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 high activity, durability, low-cost, and easy fabrication.2,5−9 However, most catalysts are synthesized typically in the form of powders, therefore presenting several inevitable problems.7,9−12 First, the powder catalysts need to be coated on an electrode with the assistance of conductive polymers, which © 2017 American Chemical Society

increase the synthesis cost and decrease the advantages of the metal oxide catalysts.6,13,14 Second, the aggregation of catalysts on the 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, resulting 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 electrocatalytic efficiency due to their natural advantages.2,7−9,12 On the one hand, the porous morphology can penetrate the electrolyte better as well as facilitate the diffusion of electrolyte to contact active sites and the release of generated gas bubbles; on the other hand, the advanced architecture ensures good electrical conductivity and mechanism stability of the prepared electrodes due to the selfsupported configuration without the polymeric binders of Received: August 31, 2017 Revised: November 1, 2017 Published: December 9, 2017 707

DOI: 10.1021/acssuschemeng.7b03034 ACS Sustainable Chem. Eng. 2018, 6, 707−718

Research Article

ACS Sustainable Chemistry & Engineering

further boosts its electrocatalytic activity. As a result, this hierarchical porous three-dimensional (3D) electrode shows high activity and robust stability for OER and ORR in alkaline solution. When employed as the air catalyst of primary Zn−air batteries, the Zn−air batteries exhibit a high open circuit voltage, large power density, and robust operation durability.

active species. Noticeably, the metal oxides can be directly deposited on different substrates to form different configurations; however, the semiconductor properties of metal oxides inevitably decrease their electron transport ability.3,8,16 In addition, the low porosity and relatively 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 improved 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 ensures them to be ideal precursors to obtain metal oxide−carbon materials after carbonization at high-temperature, wherein metal nanoparticles homogeneously in situ encapsulate into formed carbon monoliths, 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 better electrocatalytic activity.16,24−27 Ma et al. reported 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 collectors by binder would offer inferior contact between them, affecting electrocatalytic activity for practical applications.24,28,32 In addition, similar transition metal oxide/phosphide/sulfide nanoparticles derived from MOF materials are also synthesized with good electrochemical performance.5,24,26,33 However, the limited morphology of them greatly shrinks their catalytic performance capability; especially, the postcoat strategy to fabricate electrodes further weakens the superiority of MOF-derived carbonaceous materials. Due to all of the above considerations, it is interesting to design and fabricate self-supported nanostructured catalysts on current collectors with high activity and stability based on MOF materials for electrocatalysis. In this work, Co3O4−carbon hybrid nanowire arrays on Ni foam derived from ZIF-67 nanowire precursor are obtained. By using a facile hydrothermal process, the Ni foam skeleton is uniformly coated with dense cobalt hydroxide carbonate (Co(CO3)0.5(OH)·0.11H2O) nanowire arrays. After 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 characteristics 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

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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. First, the Ni foam, 3 cm × 4 cm, was treated with 2 M HCl solution with ultrasonic for 10 min to remove the surface oxide layer, and it was washed with ethanol and water 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 pinkcolored cobalt carbonated hydroxide nanowire arrays grown on Ni foam were obtained and named 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 washing 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 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 ZIF67-NA/NF was heated in air at 600 °C, 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 addition of 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 the same loading mass of Co3O4−C on Ni foam and left dry in air to evaporate the solvent. Fabrication of IrO2 on Ni Foam (IrO2/NF). The IrO2 coated on Ni foam was synthesized according to the reported method8 with the same loading amount of Co3O4−C on Ni foam. First, 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 the pH was adjusted 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. This process was repeated until the pH value was maintained at 7.5. The obtained solution was transferred to a round-bottom flask with a reflux condenser. The solution was 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 asobtained 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 the catalyst ink was coated onto Ni foam with the same load mass of Co3O4−C on Ni foam, followed by drying at room temperature overnight in air. 708

DOI: 10.1021/acssuschemeng.7b03034 ACS Sustainable Chem. Eng. 2018, 6, 707−718

Research Article

ACS Sustainable Chemistry & Engineering 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 JSF7500L 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 dropped 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 μm × 700 μm, survey spectra were recorded with a pass energy of 160 eV, and high resolution spectra were reccorded 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 multipoint 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. 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:

E RHE = EAg/AgCl + 0.0591pH + 0.205

(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, followed by coating onto the RRDE with the Nafion binder. A scan rate of 1 mV s−1 and a rotation rate of 1600 rpm were used 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 )

Where, Ir and Id represent the disk current and the ring current, respectively, and N represents the current collection efficiency of the RRDE. Because of the excellent OER catalytic activity of IrO2 with nearly 100% Faradaic efficiency, therefore, the IrO2 thin-film electrode was employed to calibrate of the collection efficiency of the RRDE; so, N is determined to be 0.2. Moreover, to rationally calculate the Faradaic efficiency, the electrode current is kept constant at a small value of 200 μA; this current is sufficiently large to ensure appreciable O2 production and efficiently small to minimize local saturation and bubble formation at the disk electrode (see more details in ref 8). 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 was dispersed in a mixture solution containing 0.5 mL distilled water and 0.5 mL isopropanol under sonication and, subsequently, coated onto the rotating disk electrode (RDE) or rotating ring-disk electrode (RRDE) (10 μL) with Nafion as the binder. The resulting electrode served as a working electrode to measure the electrocatalytic properties of catalysts in a three-electrode configuration, wherein the Ag/AgCl and Pt wire were used as the reference and counter electrode, respectively. All the electrochemical dates 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 CV and LSV measurements were conducted at 50 and 10 mV s−1, respectively. All the electrochemical dates were recorded after several CV cycles until a stable current was obtained. On the basis of the RDE test, the electron transfer number (n) involved in oxygen reduction can be calculated by the Koutechy−Levich (KL) equation:

(1)

The O2 flow was maintained over the electrolyte during electrochemical measurements. The polarization curves were collected with a 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 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 log J

1/J = 1/Jk + 1/JL = 1/Jk + 1/Bω1/2

(2)

where η was the overpotential, J was the current density, and b was the Tafel slope. The overpotential was calculated as follows: η = E RHE − 1.23

(4)

(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 KL plots based on the Levich equation as below:

(3)

B = 0.2nF(DO2)2/3υ−1/6CO2

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. The electrical double layer capacitor (Cdl) of the as-prepared electrodes were obtained from cyclic voltammograms (CVs) in a narrow potential range of 1.131−1.231 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 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 to investigate the stability. In order to investigate the electrocatalytic oxygen evolution mechanism, the rotating ring−disk electrode (RRDE) voltammograms were carried out (Pine Research Instrumentation, USA) including a disk electrode (glass carbon) and a ring electrode

(6)

where n is the transferred electron number per oxygen molecule. F is the Faraday constant (F = 96 485 C mol−1). DO2 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 repetitions per minute. 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:

709

n = 4Id /(Id + Ir /N )

(7)

%HO2− = 200Ir /(IdN + Ir)

(8) DOI: 10.1021/acssuschemeng.7b03034 ACS Sustainable Chem. Eng. 2018, 6, 707−718

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ACS Sustainable Chemistry & Engineering Scheme 1. Illustration of the Fabrication of Co3O4−Carbon Nanowires on Ni Foam

Figure 1. (a−c) SEM, (d) TEM, and (e) HR-TEM images and (f) EDS elemental mapping images of Co3O4−C-NA/NF. 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 Battery Testing. The air cathode used for Zn−air batteries was 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 similar to the above procedure toward ORR. The Zn−air battery tests were carried out on a home-built Zn−air instrument. A polished Zn plate was used as the anode, and the electrolyte was 6.0 M KOH solution. The one side of CP allows O2 to reach the active materials immersed into the electrolyte. All the data of Zn−air batteries were measured at room atmosphere on the WaveDriver 20 bipotentiostat/galvanostat 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 with a LAND testing system.

carbonate nanowires 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 2MeIm would be initiated and replicate the original morphology of basic cobalt hydroxide carbonate nanowires, finally achieving MOF 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 importantly, this strategy can not only simultaneously maintain the original nanowire morphology but also generate pores in nanowires (Scheme 1). The Co-NA/NF was prepared by a 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 CoNA/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 inset). After immersion into the 2-MeIm solution, ZIF-67-NA/NF was obtained, as the

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RESULTS AND DISCUSSION Material Synthesis and Characterization. Previously, MOF nanoparticles have been extensively fabricated; however, the nanowire morphology of MOFs on a substrate has hardly been reported.3,30,34 This is mainly due to the slow crystallization process of MOFs; thus, 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 MOF materials with nanowire structure which were synthesized from cobalt hydroxide 710

DOI: 10.1021/acssuschemeng.7b03034 ACS Sustainable Chem. Eng. 2018, 6, 707−718

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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. (e inset) XPS survey spectra of Co3O4−C-NA/NF.

evidenced by the elemental mapping. The energy dispersive Xray 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, very consistent with the HR-TEM results, demonstrating the homogeneous distribution of Co3O4 and carbon species. The Co3O4−C-NA/NF exhibits the cubic spinel phase of Co3O4 (PDF no. 43-1003, JCPDS [2004]) detected by wideangle XRD, as shown in Figure 2a. Thermogravimetric analysis (TGA, Figure 2b) indicates that the decomposition of ZIF-67NA (scraped off from Ni foam) in different atmospheres (nitrogen and air) is of evident difference. The slight weight loss below 200 °C 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 °C temperature, and finally only about 29.94% of mass is retained. Upon further increasing the heating temperature (>400 °C), the weight stays stable, suggesting that it totally converts to Co3O4. However, through increasing the temperature to 600 °C, it also retains 59.56% weight in a nitrogen atmosphere, indicating that the final obtained Co3O4−C NA has a higher carbon content compared with pyrolysis under air. The N2 adsorption−desorption isotherms of the Co3O4−CNA/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 mesopores (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 that Co3O4−C-NA/NF successfully inherits the porous morphology of the MOF materials, which

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 treatment with 2-MeIm solution, and the peaks belonging to ZIF-NA/NF match well with 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). The magnified SEM image (Figure S3b) reveals that the ultrasmall nanoparticles linked together to form the ZIF-67 nanowires. Upon high temperature carbonization, the color of ZIF-NA/ NF changes from purple to black (Figure S1 right). The lowresolution SEM image of Co3O4−C-NA/NF (Figure 1a) reveals that nanowires grow densely and integrate into arrays on 3D skeleton, which is much different from pristine Ni foam (Figure S4) that has a smooth surface. The high-resolution SEM images (Figure 1b and c) show 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 an in situ formed carbon layer and embed in Co3O4−C hybrid nanowires. The transmission electron microscopy (TEM) images (Figure 1d and e) show an apparently amorphous carbon and small sized crystalline Co3O4 nanoparticles embedded in the resulting carbon monolith. The clear lattice fringe spacing 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 711

DOI: 10.1021/acssuschemeng.7b03034 ACS Sustainable Chem. Eng. 2018, 6, 707−718

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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 response measured at a constant potential of 1.52 V (the current density of 10 mA cm−2). (c inset) 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). (d inset) 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.

self-supported electrode are successfully obtained, which are considered to have 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. The 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 the very low activity of pristine Ni foam for OER. However, because of the oxidation effect of Co3O4, the onset potential of Co3O4−CNA/NF is difficult to observe. 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 activity of catalysts toward OER.45,46 The Co3O4−C-NA/NF affords an Ej=10 at 1.54 V, higher than that of IrO2/NF at 1.52 V; however, this value is lower than

agrees well with the previously reported ZIF-67 nanoparticles after carbonization under an 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 (inset), 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 corresponding to Co 2p1/2 and Co 2p3/2, respectively, demonstrates 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 the 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 materials is beneficial for the electrocatalytic reaction.43,44 There is 2.78 at % N in 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 712

DOI: 10.1021/acssuschemeng.7b03034 ACS Sustainable Chem. Eng. 2018, 6, 707−718

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Figure 4. (a) Polarization curves of Co3O4−C-NA/NF, Co3O4−C film, Co3O4-NA/NF, and scratched Co3O4−C-NA. (a inset) 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.

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 that 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 an alkaline electrolyte. Furthermore, there is only about 3.3% current loss is observed for Co3O4−CNA/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 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 greater superiority in comparison with the postcoated 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 of reaction show no evident change on the typical peaks in comparison with fresh Co3O4−

that of many other recently developed nonprecious catalysts for OER summarized in Table S1, revealing the outstanding catalytic activity of Co3O4−C-NA/NF toward OER in alkaline media. The large specific surface area, favorable charge/mass transport in that porous nanowire structure can explain this excellent catalytic activity of the Co3O4−C-NA/NF electrode. In addition, compared with Co3O4−C-NA/NF, the ZIF-67NA/NF has inferior catalytic behavior (Ej=10 at 1.68 V). 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 toward OER, and 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 ORR can ensure the immediate reduction 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 catalyzed current is the OER process,1,8,47 along with a higher Faradaic efficiency of 99.3% (Figure S8). Durability toward OER is of importance for actual applications. The chronoamperometric response reveals the high stability of Co3O4−C-NA/NF, showing a slight decay of 713

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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 rates. (b inset) KL 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. (c inset) RRDE curves of catalysts. (d) Chronoamperometric response of as-fabricated Co3O4−C-NA as compared to Pt/C at 0.5 V. (d inset) Chronoamperometric response after methanol addition.

kinetics of OER toward 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 toward 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 favorable reaction kinetics (Figure 4a inset). On the one hand, the microstructure of nanowire arrays and the macro-morphology of porous Ni foam are favorable 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 macropores 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 the Cdl is related to the electrochemically active surface area of catalysts,8,13 the higher value of the Cdl implies

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 a 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 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 postcoating 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. 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 714

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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. (b inset) Photograph of the open circuit voltage for the asassembled 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, and Pt/C+IrO2 air cathodes.

that the porous Co3O4−C nanowire possesses large active surface area. Much exposure and effectively utilized electroactive sites of the Co3O4−C-NA/NF endows it 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 proven 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 suggests the difference between them. The much larger 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 electrical conductivity and catalytic performance toward OER. Oxygen Reduction Activity and Zn−Air Batteries. The oxygen evolution electrode with reversible ability to efficiently catalyze ORR is significant for practical applications, especially for metal−air batteries.51−53 Here, taking consideration of the excellent OER catalytic ability, the ORR activity of Co3O4−CNA 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 LSVs 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 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 (KL) 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 pseudofour-electron pathway of Co3O4−C-NA toward ORR. Furthermore, the 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%) toward ORR 715

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charge−discharge (discharging and charging for 10 min at 30 mA cm−2, respectively) of assembled Zn−air batteries with Co3O4−C-NA as the air cathode was measured and is shown in Figure 6e. The Co3O4−C-NA exhibits excellent rate capability, as well as discharge and charge voltages of 1.18 and 2.11 V in the first cycle with a trip efficiency of 53.4%, respectively. And the voltage gap increases before the 20 cycles; subsequently, the gap stays nearly stable without evident 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 asprepared Co3O4−C-NA catalyst. The Pt/C+IrO2 can also maintain the charge and discharge potentials well even cycling numbers over 130, suggesting that its excellent electrocatalysis is 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 toward fabricated Zn−air batteries is greatly related to the remarkable OER and ORR activity and stability of Co3O4−C-NA in alkaline electrolyte.

process, slightly inferior to those of Pt/C, as shown in Figure 5c, further suggesting the desirable four-electron pathway toward ORR. 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 (Figure 5d inset), demonstrating its high selectivity to ORR with strong methanol tolerance ability. The cathodic current density changes evidently after adding methanol toward 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. 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 scratched off from the Co3O4−C-NA/NF, then recoated onto 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 mean that the as-prepared Zn−air battery with a Co3O4−C-NA air cathode has superior performance in comparison to recently reported ones equipped with nonprecious metals and carbonbased 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 whatever the discharging or charging process. The charging potential of Co3O4−NA and Pt/C are both suppressed compared to that of Co3O4−C-NA, indicating their inferior OER electrocatalytic activity. Importantly, these batteries assembled by the Co3O4−C-NA cathode also exhibit impressive long-term operation durability. The galvanostatical

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CONCLUSIONS Nanowire arrays are constructed from carbon encapsulated Co3O4 nanoparticles directly grown on 3D porous Ni foam. They exhibit higher OER and ORR catalytic activity, more favorable kinetics, and stronger durability in alkaline solution. The OER performance of Co3O4−C-NA/NF is comparable to the IrO2 benchmark and better than those of most recently reported nonprecious transition-metal OER catalysts. Additionally, as an ORR catalyst, the catalytic performance of Co3O4−C-NA closely reaches that of noble Pt/C. The directly grown porous nanowire array on Ni foam, the intimate effect of Co 3 O 4 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, favorable 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 for water-splitting devices and other emerging renewable energy conversion systems.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b03034. Optical images of Co-NA/NF, ZIF-67-NA/NF, and Co3O4−C-NA/NF; XRD pattern and SEM image of CoNA/NF; XRD patterns of ZIF-67 powder and ZIF-67NA/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 Co3O4NA/NF; optical images of electrocatalytic OER process; schematic illustration of the continuous OER to ORR process initiated on the RRDE; ring current of Co3O4− 716

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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 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 (PDF)

AUTHOR INFORMATION

Corresponding Author

*Mailing address: 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]. ORCID

Zhong-Yong Yuan: 0000-0002-3790-8181 Notes

The authors declare no competing financial interest.

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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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DOI: 10.1021/acssuschemeng.7b03034 ACS Sustainable Chem. Eng. 2018, 6, 707−718

Research Article

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DOI: 10.1021/acssuschemeng.7b03034 ACS Sustainable Chem. Eng. 2018, 6, 707−718