Hydrothermal Synthesis of Highly Dispersed Co3O4 Nanoparticles on

Aug 28, 2017 - Developing high-performance bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is of...
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Hydrothermal Synthesis of Highly Dispersed Co3O4 Nanoparticles on Biomass-Derived Nitrogen-Doped Hierarchically Porous Carbon Networks as an Efficient Bifunctional Electrocatalyst for Oxygen Reduction and Evolution Reactions Jianli Guan, Zhengping Zhang, Jing Ji, Meiling Dou,* and Feng Wang* State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Electrochemical Process and Technology for Materials, Beijing University of Chemical Technology, Beijing 100029, PR China S Supporting Information *

ABSTRACT: Developing high-performance bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is of vital importance in energy storage and conversion systems. Herein, we demonstrate a facile hydrothermal synthesis of highly dispersed Co3O4 nanoparticles (NPs) anchored on cattle-bone-derived nitrogen-doped hierarchically porous carbon (NHPC) networks as an efficient ORR/OER bifunctional electrocatalyst. The asprepared Co3O4/NHPC exhibits a remarkable catalytic activity toward both ORR (outperforming the commercial Pt/C) and OER (comparable with the commercial RuO2 catalyst) in alkaline electrolyte. The superior bifunctional catalytic activity can be ascribed to the large specific surface area (1070 m2 g−1), the well-defined hierarchically porous structure, and the high content of nitrogen doping (4.93 wt %), which synergistically contribute to the homogeneous dispersion of Co3O4 NPs and the enhanced mass transport capability. Moreover, the primary Zn−air battery using the Co3O4/NHPC cathode demonstrates a superior performance with an open-circuit potential of 1.39 V, a specific capacity of 795 mA h gZn−1 (at 2 mA cm−2), and a peak power density of 80 mW cm−2. This work delivers a new insight into the design and synthesis of high-performance bifunctional nonprecious metal electrocatalysts for Zn−air battery and other electrochemical devices. KEYWORDS: oxygen reduction and evolution reactions, electrocatalysts, Co3O4 nanoparticles, nitrogen-doped hierarchically porous carbon, cattle bone Therefore, there remains a big challenge to find a facile and rational route for the synthesis of Co3O4 NPs as highperformance ORR/OER bifunctional electrocatalysts. One of the important strategies to enhance the catalytic activity of Co3O4 is to increase the dispersion of Co3O4 NPs with small particle size11,12 and to promote the electrical conductivity by employing conductive support materials.13 Uniform dispersion of Co3O4 NPs on several supports as efficient bifunctional electrocatalysts has been reported previously, such as the Co3O4 NP-modified MnO2 nanotube (MnO2/Co3O4)11 and the Co3O4 NP-decorated carbon nanofiber (Co3O4−CNF).12 Recently, anchoring Co3O4 NPs on carbon materials (Co3O4/C) was considered effective to improve the activity through enhancing the dispersion of Co3O4 NPs and increasing the electrical conductivity, attributed to the increase of nucleation/anchor sites for Co3O4 NPs and decrease of electron-/mass-transfer distance.14 Additionally, as

1. INTRODUCTION The development of high-performance bifunctional electrocatalysts for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is crucial to the large-scale application of energy conversion devices (e.g., fuel cell, metal−air batteries, etc.).1,2 To date, noble metals (e.g., Pt, Ru, and Ir) and their respective oxides have been proved to be the most efficient electrocatalysts.3 Nevertheless, the high expense and limited resource of these noble-metal-based electrocatalysts are hampering their large-scale applications.4 Consequently, a wide range of nonprecious metal alternatives, including transition-metal oxides,5 transition-metal nitrides/ carbides,6 and nanocarbon materials,7,8 have been investigated as high-performance bifunctional electrocatalysts for both ORR and OER. Among them, Co3O4 nanoparticles (NPs) as one kind of the typical transition-metal oxides have been regarded as the promising candidate because of their relatively high activity, good stability, and low cost.9 Nevertheless, the pure Co3O4 NPs still exhibit an inferior catalytic activity as compared to the commercial electrocatalysts because of their intrinsic low electrical conductivity, poor dispersion, and large particle size.10 © XXXX American Chemical Society

Received: June 13, 2017 Accepted: August 18, 2017

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DOI: 10.1021/acsami.7b08533 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Scheme 1. Schematic Illustration of the Synthetic Process of the Co3O4/NHPC ORR/OER Bifunctional Electrocatalyst

2. EXPERIMENTAL SECTION

optimized supports, carbon materials with hierarchical porosity also play an important role in the enlargement of electrocatalytic active surface area of Co3O4 and the promotion of the mass transport for ORR and OER reactants.15 Another solution to boost the ORR/OER catalytic activity of Co3O4/C is to dope the carbonaceous materials with heteroatoms into the carbon matrix, which can induce a strong synergistic interaction between Co- and N-doped carbon caused by the coordination of N groups with Co cations and the modification of the electronic/chemical structure.16 On the basis of these design principles, it is evident that anchoring the Co3O4 NPs on Ndoped hierarchically porous carbon (NHPC) materials with large specific surface area (SSA) is capable of enhancing the ORR/OER bifunctional catalytic activity. As one kind of biomass wastes, animal bones (e.g., cattle bone,17 fish scale,18 crab shell,19 etc.) were widely employed as low-cost and environmentally friendly precursors to prepare the hierarchically porous carbon networks. Animal bones primarily consist of hydroxyapatite and collagen with a natural threedimensional (3-D) ordered assembly.20 As the animal bones are used as carbon precursor to be pyrolyzed, collagens can provide carbon and nitrogen sources to form heteroatom-doped carbons, whereas hydroxyapatite crystals may act as hard templates for pore structure control,17 which then result in a typical 3-D hierarchically porous structure combined with a high content of N doping. Herein, we demonstrate a facile hydrothermal synthesis of the bifunctional electrocatalyst (Co3O4/NHPC) consisting of spinel Co3O4 NPs anchored on cattle-bone-derived NHPC networks. The large SSA of the NHPC networks is beneficial for the homogeneous dispersion of Co3O4 NPs, and the high content of N doping provides numerous N−C bonds as anchor sites to attach Co3O4 NPs, resulting in a high dispersion of Co3O4 NPs that are strongly anchored on NHPC networks. The resultant Co3O4/NHPC electrocatalyst exhibited a remarkable ORR/OER catalytic performance in alkaline electrolyte, and the Zn−air battery using Co3O4/NHPC as the air electrode demonstrated a superior performance with large specific capacity and high power density.

2.1. Electrocatalyst Synthesis. NHPC was synthesized by the carbonization of dried cattle bone powder based on our previous work.17 Briefly, the cattle bone powder was precarbonized at 400 °C for 3 h in an Ar atmosphere, and then the obtained product (10 g) was mixed with KOH (4 g) and carbonized at 800 °C for 1 h in an Ar atmosphere, followed by acid washing (2 M HNO3) to remove the inorganic hydroxyapatite crystals. Finally, NHPC was obtained by rinsing with copious deionized water and drying at 80 °C under vacuum. In a typical synthesis of Co3O4/NHPC, the as-prepared NHPC (25 mg) was ultrasonically dispersed with ethanol (15 mL) and water (1 mL) for 0.5 h, followed by the addition of Co(OAc)2 (78 mg) under stirring. Afterward, ammonia (1 mL) was dropped under continuous stirring at 80 °C for 10 h and then incubated in an autoclave at 150 °C for 3 h. After that, the prepared Co3O4/NHPC was obtained by repeatedly centrifuging, washing with ethanol and water, and drying at 80 °C under vacuum. For comparison, the unsupported Co3O4 NPs (without the addition of NHPC) and the Co3O4/XC-72 electrocatalyst (adopting Vulcan XC-72 as a carbon support) were prepared via a similar procedure. 2.2. Materials Characterizations. The powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/Max-2500 diffractometer with an X-ray source of Cu Kα radiation (λ = 1.54056 Å). The scanning electron microscopy (SEM, FE-JSM-6701F, 10 kV, field emission, JEOL, Japan), high-resolution transmission electron microscopy (HR-TEM, JSM-2100, 200 kV, JEOL, Japan), and aberration-corrected high-resolution scanning transmission electron microscopy (HR-STEM, JEM-ARM200F, 200 kV, field emission, JEOL, Japan) were used to characterize the morphologies of the electrocatalysts. The thermalgravimetric (TG) analysis was conducted on a Rigaku TG-8120 instrument in an air atmosphere (heating rate: 5 °C min−1). The SSA and porosity were characterized by an N2 adsorption/desorption isotherm on a Quantachrome AUTOSORBSI instrument. Raman spectra were profiled on a confocal microscope (Horiba HR800) with an excitation laser (λ = 632.8 nm). The X-ray photoelectron spectroscopy (XPS) measurement was conducted on a spectrometer (Thermo Fisher ESCALAB 250) with Al Kα monochromatized radiation. 2.3. Electrochemical Measurements. The electrochemical workstation (CHI660E) equipped with three electrodes was employed to evaluate the electrochemical performance. The Pt wire, electrocatalyst-coated glassy carbon electrode (GCE, d = 4 mm), and saturated calomel electrode were used as the counter, working, and reference electrodes, respectively. All the potentials were reported B

DOI: 10.1021/acsami.7b08533 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces versus the reversible hydrogen electrode. To prepare a working electrode for ORR, the as-prepared electrocatalyst (5 mg) was ultrasonically dispersed in ethanol (1 mL) containing 50 μL of Nafion (5 wt %, DuPont) for 0.5 h. Then, 10 μL of the obtained homogeneous slurry was pipetted onto the prepolished GCE (electrocatalyst loading 0.38 mg cm−2). The commercial Pt/C (20 wt % of Pt, Johnson Matthey) and RuO2-coated GCEs were also prepared with a loading of 76 μgPt cm−2 and 379 μgRuO2 cm−2, respectively. For the test of OER, 20 μL of the obtained homogeneous slurry was transferred onto a carbon paper (area: 0.25 cm2) to prepare a working electrode (electrocatalyst loading 0.38 mg cm−2). The working electrodes of commercial Pt/C (76 μgPt cm−2) and RuO2 (381 μgRuO2 cm−2)-coated carbon paper were also prepared. 2.4. Battery Test. The Zn−air battery was assembled with a Zn plate as the anode, an electrocatalyst-coated carbon paper (area: 0.25 cm2) as the cathode, and an air-saturated 6.0 M KOH solution as the electrolyte. The cathode was fabricated by homogeneously casting the electrocatalyst slurry (ultrasonically mixing 5 mg of electrocatalyst in 1 mL of ethanol with 100 μL of Nafion for 0.5 h) onto the carbon paper and subsequent drying (80 °C for 0.5 h). The loading of Co3O4/ NHPC was controlled at 1.82 mg cm−2. For comparison, the commercial Pt/C cathode (Pt loading: 364 μgPt cm−2) was prepared and the corresponding Zn−air battery was also assembled. The polarization curves were recorded on a CHI660E workstation, and the discharge/charge performance was analyzed by the LAND CT 2001A instrument (Wuhan LAND Electronic Co., Ltd).

Figure 1. Representative bright-field TEM images of (a) NHPC and (b) Co3O4/NHPC (inset shows the particle size distribution of Co3O4 NPs). (c) Dark-field HR-STEM elemental mapping images of Co, O, N, and overlap. (d) Bright-field HR-TEM image of Co3O4/NHPC (inset: the high-magnification dark-field image of (d)).

3. RESULTS AND DISCUSSION 3.1. Physical Characterizations. As illustrated in Scheme 1, NHPC was first synthesized with the precarbonization of cattle bone, followed by carbonization, KOH activation, and acid pickling to remove the hydroxyapatite nanocrystals. The Co3O4/NHPC electrocatalyst was then prepared by a facile nucleation and hydrothermal crystallization process using the hydrolyzed Co(OAc)2 as the nucleus. The morphologies of NHPC and Co3O4/NHPC were observed on SEM and TEM. The as-prepared NHPC exhibited a 3-D hierarchically porous structure with interconnected mesopores and macropores (Figures S1a and 1a). After anchoring Co3O4 NPs, the resulting Co3O4/NHPC still retained the hierarchically porous structure characteristic of NHPC with well-dispersed Co3O4 NPs on the carbon networks (Figure S1b). The TEM (Figure 1b) and HRSTEM elemental mapping images (Figure 1c) further showed the homogeneously dispersed Co3O4 NPs with a small particle size of approximately 5−9 nm without any obvious aggregation. Besides, the HR-STEM elemental mapping images (Figure 1c) of Co3O4/NHPC indicated that a relatively homogeneous Ndoping structure was shown on the porous carbon networks, probably attributed to the relatively uniform pyrolysis of collagen fibrils and the improved interaction between Co3O4 NPs and NHPC derived from the coordination of N groups with Co cations.14,16 The HR-TEM image of Co3O4/NHPC (Figure 1d) showed that Co3O4 NPs possessed an obvious crystal structure with interplanar spacings of 0.23 and 0.46 nm assigned to the (311) and (111) planes of the Co3O4 crystal, respectively. The XRD pattern of Co3O4/NHPC (Figure 2a) confirmed this crystal structure with strong (311), (440), and (220) diffraction peaks corresponding to the spinel Co3O4 (JCPDS: no. 42-1467). Compared with the unsupported pure Co3O4 NPs (Figure S2), the diffraction peaks of Co3O4 in Co3O4/NHPC were obviously broadened, indicating a smaller particle size of anchored Co3O4 NPs. These results revealed that the high SSA and N-doping structure of NHPC can effectively promote the dispersion of Co3O4 NPs with a small particle size because of the abundant nucleation and anchor

sites onto the surface of NHPC support for Co3O4 NPs. The loading of Co3O4 was determined as 50 wt % by the TG analysis (Figure S3). The presence of NHPC in a Co3O4/ NHPC hybrid was confirmed by the Raman spectra with two characteristic peaks of D band (1350 cm−1) and G band (1590 cm−1) (Figure 2b). The ID/IG ratios of NHPC and Co3O4/ NHPC were determined as 1.07 and 1.11, respectively. To investigate the porous structure of NHPC and Co3O4/ NHPC, N2 adsorption/desorption measurements were conducted. The porous structure of Co3O4/NHPC was almost the same as that of NHPC, which possessed a similar type I/IV adsorption−desorption isotherm characteristic with an obvious hysteresis loop at the relative pressure more than 0.5, indicating the existence of a hierarchically micro-/mesoporous structure (Figure 2c). The pore size of Co3O4/NHPC was mainly distributed at approximately 0.1−1 and 2−4 nm (Figure 2d). Compared with NHPC, the vanished mesopores of 4−6 nm for Co3O4/NHPC were probably attributed to the loading of Co3O4 NPs with the particle size of 5−9 nm as indicated by the TEM analysis. Moreover, the SSAs of NHPC and Co3O4/ NHPC were determined as 2047 and 1070 m2 g−1, respectively. The large SSA of Co3O4/NHPC along with the uniform distribution of Co3O4 NPs was believed to be favorable to providing numerous catalytic active sites. Meanwhile, the 3-D hierarchically porous structure can lead to efficient mass transport capability toward the electrocatalytic reaction. To investigate the surface chemical compositions and element states of NHPC and Co3O4/NHPC, the XPS measurement was carried out. The XPS survey spectrum of NHPC displayed the C 1s, O 1s, and N 1s peaks with a high content of N doping (3.85 at %, Figure S4). The corresponding survey spectrum of Co3O4/NHPC (Figure S4) indicated the presence of C (79.2 at %), N (4.93 at %), O (12.7 at %), and Co (3.17 at %). The deconvoluted N 1s spectrum of Co3O4/ C

DOI: 10.1021/acsami.7b08533 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. (a) XRD patterns and (b) Raman spectra of NHPC and Co3O4/NHPC. (c) N2 adsorption/desorption isotherms and (d) pore size distributions of NHPC and Co3O4/NHPC. (e) High-resolution XPS N 1s spectra of NHPC and Co3O4/NHP. (f) High-resolution Co 2p spectra of Co3O4 and Co3O4/NHPC.

Figure 3. (a) LSV curves of NHPC, Co3O4/XC-72, Co3O4/NHPC, and the commercial Pt/C in O2-saturated 0.1 M KOH (scan rate: 5 mV s−1, electrode rotation rate: 1600 rpm) (inset shows the summary of half-wave potentials and kinetic current densities at 0.8 V). (b) Tafel plots and the corresponding slopes for these electrocatalysts. (c) LSV curves of Co3O4/NHPC at various rotating rates (inset shows the corresponding K−L plots). (d) Chronoamperometric response of Co3O4/NHPC and Pt/C in O2-saturated 0.1 M KOH at 0.8 V (electrode rotation rate: 1600 rpm).

NHPC (Figure 2e) showed the existence of pyridinic N (399 eV), pyrrolic N (400 eV), and graphitic N (401 eV). The

pyridinic N and graphitic N are considered beneficial for the catalytic activity because of the created electropositive adjacent D

DOI: 10.1021/acsami.7b08533 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. (a) LSV curves of Co3O4/XC-72, Co3O4/NHPC (inset shows the CV curve of Co3O4/NHPC), Pt/C, and RuO2 in 0.1 M KOH at a scan rate of 20 mV s−1. (b) Tafel plots and the corresponding slopes for these electrocatalysts. (c) Chronoamperometric responses of Co3O4/NHPC and RuO2 at 1.7 V. (d) LSV curves of Co3O4/NHPC, Pt/C, and RuO2 for ORR and OER in 0.1 M KOH.

carbon atoms.18 The deconvoluted Co 2p spectrum (Figure 2f) yielded six peaks ascribed to the two pairs of Co(II) and Co(III) spin−orbit doublets and their shake-up satellites.15,21 Compared with the Co 2p3/2 peak (779.9 eV) of Co3O4, an obviously negative binding energy shift of 0.7 eV was observed for Co3O4/NHPC (780.6 eV), suggesting a strong synergistic interaction between Co3O4 and NHPC.14,22,23 3.2. Electrochemical Performance of Co3O4/NHPC for ORR and OER. To evaluate the ORR activity of Co3O4/ NHPC, the linear sweep voltammetry (LSV) test was conducted in an O2-saturated 0.1 M KOH electrolyte (Figure 3a). As expected, the Co3O4/NHPC electrocatalyst exhibited a remarkable ORR activity with high onset potential (Eonset = 0.960 V), half-wave potential (E1/2 = 0.835 V), and large diffusion-limited current density (JL = 6.0 mA cm−2 at 0.3 V), significantly outperforming NHPC (Eonset = 0.881 V, E1/2 = 0.784 V, and JL = 5.40 mA cm−2 at 0.3 V), Co3O4/XC-72 (Eonset = 0.894 V, E1/2 = 0.750 V, and JL = 5.71 mA cm−2 at 0.3 V), and even the commercial Pt/C (Eonset = 0.956 V, E1/2 = 0.823 V, and JL = 5.90 mA cm−2 at 0.3 V). Furthermore, the kinetic current density (Jk) of Co3O4/NHPC was determined as 16 mA cm−2 (at 0.8 V) using the Koutecky−Levich (K−L) equation,24 which was nearly 6.2- and 1.3-fold that of NHPC and Pt/C, respectively. Moreover, the remarkable ORR activity of Co3O4/NHPC was also confirmed by the lower Tafel slope of 62 mV dec−1 at high potentials than those of NHPC (66 mV dec−1) and Pt/C (75 mV dec−1) (Figure 3b). The transferred electron number of Co3O4/NHPC was determined as 3.91 based on the K−L equation (Figure 3c), higher than that of NHPC (2.96, Figure S5) and very close to that of the commercial Pt/C (3.96, Figure S6), indicating an ideal fourelectron ORR pathway. Additionally, the Co3O4/NHPC electrocatalyst exhibited an excellent stability with only 9.3% decrease in current after the test for 10 000 s (Figure 4d), which is significantly superior than that of the commercial Pt/C (32% current decrease after 10 000 s).

The resultant Co3O4/NHPC was then subjected to the electrochemical test toward OER. The cyclic voltammetry (CV) curves of Co3O4/NHPC exhibited three redox peaks (anodic: A1, A2, and A3; cathodic: C1, C2, and C3) (Figure 4a). The A1 and A2 peaks were related to the oxidation of Co(II) to hydrous Co(III) (in the form of CoOOH) and dispersed Co(III) (referred to as the Co 3 O 4 oxide), respectively, whereas the A3 peak corresponded to the oxidation of Co(III) to Co(IV).15 It was reported that Co(IV) was the active center for OER; therefore, the elevated current density of A3 indicated a probably high OER activity of Co3O4/ NHPC. To further evaluate the OER activity of Co3O4/NHPC, LSV measurement was conducted at the potential range of 1.0− 1.8 V in 0.1 M KOH (Figure 4a). As expected, the Co3O4/ NHPC electrocatalyst exhibited a superior OER activity with a lower evolution potential than those of the commercial Pt/C and RuO2. The operating potential required for Co3O4/NHPC to deliver a current density of 10 mA cm−2 (EJ10,OER) was determined as 1.65 V, lower than those of RuO2 (1.66 V) and Pt/C (1.84 V) as well as most of the reported bifunctional electrocatalysts (Table S1), further verifying the superior catalytic activity toward OER. The Tafel slope of Co3O4/ NHPC (Figure 4b) was calculated to be 132 mV dec−1, lower than those of Co3O4/XC-72 (159 mV dec−1), RuO2 (169 mV dec−1), and Pt/C (223 mV dec−1), revealing that OER on Co3O4/NHPC was the fastest because of the high dispersion of active sites and the enhanced mass transport capability. Furthermore, the electrochemical stability of Co3O4/NHPC was also evaluated by chronoamperometry with a constant voltage of 1.7 V (Figure 4c). Unlike the commercial RuO2 (only retained 34.0% relative current), Co3O4/NHPC demonstrated a superior electrochemical stability with approximately 87.6% relative current retained after the test for 12 000 s. For the potential application of the bifunctional electrocatalyst in a Zn−air battery, the potential difference (ΔE = EJ10,OER − EJ‑3,ORR, where EJ‑3,ORR is the potential at the current density of −3 mA cm−2) was employed to evaluate the ORR/ E

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Figure 5. (a) Schematic of the basic configuration of a primary Zn−air battery. (b) Polarization and power density curves of Zn−air batteries based on Co3O4/NHPC and Pt/C cathodes, respectively. (c) Specific capacities of a Zn−air battery based on the Co3O4/NHPC cathode discharging at various current densities.

OER bifunctional catalytic activity25 (Figure 4d). A smaller ΔE value generally indicated a higher bifunctional catalytic activity for potential application as reversible oxygen cathode. Accordingly, Co3O4/NHPC exhibited a superior bifunctional catalytic activity with a smaller ΔE of 0.810 V than that of Pt/C (1.02 V), RuO2 (1.33 V) and most of the reported bifunctional electrocatalysts26−32 (Table S1), indicating that the distinguished Co3O4/NHPC can become a promising bifunctional electrocatalyst for the Zn−air battery. 3.3. Co3O4/NHPC as the Cathode in a Zn−Air Battery. The remarkable ORR/OER activity and stability of Co3O4/ NHPC prompted us to evaluate its feasibility in a primary Zn− air battery. The primary Zn−air battery was first assembled with a Zn plate as the anode and Co3O4-/NHPC-coated carbon paper as the cathode (Figure 5a). The polarization and power density curves in Figure 5b showed that the Zn−air battery using Co3O4/NHPC as the cathode exhibited a current density of 59 mA cm−2 at 1.0 V, outperforming the Zn−air battery using Pt/C (28 mA cm−2 at 1.0 V) as the cathode. The peak power density reached 80 mW cm−2 at a current density of 120 mA cm−2, which was 1.44-fold than that of the Zn−air battery using Pt/C as the cathode (55.6 mW cm−2 at 90 mA cm−2). Furthermore, the Zn−air battery using Co3O4/NHPC as the cathode also exhibited large specific discharge capacities of 795, 674, and 486 mA h gZn−1 at the current density of 2, 20, and 30 mA cm−2, respectively (Figure 5c), indicating a superior battery performance of the Co3O4/NHPC cathode. Additionally, we also carried out the rechargeable zinc−air battery test using Co3O4/NHPC as the cathode. The rechargeable zinc−air battery based on the Co3O4/NHPC cathode was subjected to more than 800 discharge/charge cycles at 2 mA cm−2 for approximately 7.5 h, demonstrating a superior rechargeability (Figure S7).

(Co3O4/NHPC) consisting of spinel Co3O4 NPs anchored on the cattle-bone-derived NHPC. The large SSA and high content of N doping of NHPC promoted the homogeneous dispersion of Co3O4 NPs on the porous carbon network. The hierarchically porous structure of NHPC was mostly preserved for Co3O4/NHPC to facilitate the mass transportation of ORR reactants. As a result, the Co3O4/NHPC electrocatalyst exhibited a remarkable ORR/OER catalytic activity and stability in alkaline electrolyte. The primary Zn−air battery based on the Co3O4/NHPC cathode demonstrated a superior performance with a peak power density of 80 mW cm−2 and a specific capacity of 795 mA h gZn−1 (at 2 mA cm−2). This work opens a new avenue for the design and synthesis of high-performance nonprecious metal ORR/OER bifunctional electrocatalysts.

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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/acsami.7b08533. Electrochemical measurement and battery test; representative SEM (secondary electron) images of NHPC and Co3O4/NHPC; XRD patterns of unsupported Co3O4 and Co3O4/NHPC; TG curve of Co3O4/ NHPC; XPS survey spectra of NHPC and Co3O4/ NHPC; LSV curves of NHPC at various rotating rates (inset shows the corresponding K−L plots); LSV curves of the commercial Pt/C at various rotating rates (inset shows the corresponding K−L plots); discharge/charge cycles of a two-electrode rechargeable zinc−air battery based on the Co3O4/NHPC cathode (at 2 mA cm−2); and summary of electrochemical performances for nonprecious ORR/OER bifunctional electrocatalysts reported in the literatures (PDF)

4. CONCLUSIONS To summarize, we have presented a facile and effective strategy for the hydrothermal synthesis of bifunctional electrocatalysts F

DOI: 10.1021/acsami.7b08533 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.D.). *E-mail: [email protected] (F.W.). ORCID

Jing Ji: 0000-0002-3834-367X Feng Wang: 0000-0002-7901-3693 Author Contributions

J.G. and Z.Z. contribute equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors appreciate the financial support from the National Natural Science Funds of China (nos. 51432003 and 51502013).

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DOI: 10.1021/acsami.7b08533 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces (31) Hardin, W. G.; Slanac, D. A.; Wang, X.; Dai, S.; Johnston, K. P.; Stevenson, K. J. Highly Active, Nonprecious Metal Perovskite Electrocatalysts for Bifunctional Metal−Air Battery Electrodes. J. Phys. Chem. Lett. 2013, 4, 1254−1259. (32) Liu, Q.; Jin, J.; Zhang, J. NiCo2S4@graphene as a Bifunctional Electrocatalyst for Oxygen Reduction and Evolution Reactions. ACS Appl. Mater. Interfaces 2013, 5, 5002−5008.

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DOI: 10.1021/acsami.7b08533 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX