MetalâOrganic Framework-Derived Reduced Graphene Oxide

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Metal−Organic Framework-Derived Reduced Graphene OxideSupported ZnO/ZnCo2O4/C Hollow Nanocages as Cathode Catalysts for Aluminum−O2 Batteries Yisi Liu,†,∥ Hao Jiang,† Jiayu Hao,† Yulong Liu,∥ Haibo Shen,† Wenzhang Li,*,†,‡,§ and Jie Li*,†,‡ †

School of Chemistry and Chemical Engineering, ‡Hunan Provincial Key Laboratory of Efficient and Clean Utilization of Manganese Resources and §Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological Environment Monitoring, Ministry of Education, Central South University, Changsha 410083, China ∥ Department of Mechanical and Materials Engineering, University of Western Ontario, London, Ontario N6A 5B9, Canada S Supporting Information *

ABSTRACT: Aluminum−air battery is a promising candidate for large-scale energy applications because of its low cost and high energy density. Remarkably, tremendous efforts have been concentrated on developing efficient and stable cathode electrocatalysts toward the oxygen reduction reaction. In this work, a hydrothermal-calcination approach was utilized to prepare novel reduced graphene oxide (rGO)-supported hollow ZnO/ZnCo2O4 nanoparticle-embedded carbon nanocages (ZnO/ZnCo2O4/C@rGO) using a zeolitic imidazolate framework (ZIF-67)/graphene oxide/zinc nitrate composite as the precursor. The ZnO/ZnCo2O4/C@rGO hybrid exhibits remarkable electrocatalytic performance for oxygen reduction reaction under alkaline conditions and superior stability and methanol tolerance to those of the commercial Pt/C catalyst. Furthermore, novel and simple Al−air coin cells were first fabricated using the hybrid materials as cathode catalysts under ambient air conditions to further investigate their catalytic performance. The coin cell with the ZnO/ZnCo2O4/C@rGO cathode catalyst displays a higher open circuit voltage and discharge voltage and more sluggish potential drop than those of the cell with the ZnO/ZnCo2O4/C cathode catalyst, which confirms that rGO can enhance the electrocatalytic activity and stability of the catalyst system. The excellent electrocatalytic performance of the ZnO/ZnCo2O4/C@rGO hybrid is attributed to the prominent conductivity and high specific surface area resulting from rGO, the more accessible catalytic active sites induced by the unique porous hollow nanocage structure, and synergic covalent coupling between rGO sheets and ZnO/ZnCo2O4/C nanocages. KEYWORDS: metal−organic frameworks (MOFs), hollow nanocages, aluminum−air battery, oxygen reduction reaction, electrocatalysts environmentally friendly metal with high recyclability.6 The primary Al−air battery is composed of an aluminum anode, air cathode, and a suitable electrolyte, typically such as sodium hydroxide (NaOH), potassium hydroxide (KOH), and sodium chloride (NaCl) solutions.7 The oxygen reduction reaction (ORR) is the central cathodic process in metal−air batteries. Owing to the sluggish ORR kinetics, developing highly efficient cathode electrocatalysts has been the main challenge to promote the electrode performance in metal−air batteries. Precious metals and their alloys are well-known as excellent ORR electrocatalysts,8,9 but the high price and lack severely hamper their extensive applications in metal−air battery systems. Therefore, exploring highly efficient nonprecious metal electrocatalysts is a promising strategy to substitute precious metal catalysts.

1. INTRODUCTION Modern society is currently in a transitional phase from the fossil-fuel-based economy to the clean energy alternatives required to minimize environmental pollution. Therefore, it is important to search for safe, reliable, and efficient energy storage technologies to be used in large-scale applications. Recently, metal−air batteries, such as lithium (Li)−air, sodium (Na)−air, potassium (K)−air, zinc (Zn)−air, magnesium (Mg)−air, and aluminum (Al)−air batteries, have been extensively studied, exhibiting high theoretical energy densities ranging between 2- and 10-fold larger than those of state-ofthe-art lithium-ion batteries (LIBs).1 Among the metal−air batteries, Al−air batteries hold great promise for future largescale energy applications due to their lowest cost, high theoretical voltage (2.7 V), and high theoretical specific capacity of 2.98 Ah g−1, which is the second highest only to that of lithium (3.86 Ah g−1) and much higher than that of magnesium (2.20 Ah g−1) and zinc (0.82 Ah g−1).2−5 Additionally, aluminum is an inexpensive, abundant, and © 2017 American Chemical Society

Received: June 15, 2017 Accepted: August 28, 2017 Published: August 28, 2017 31841

DOI: 10.1021/acsami.7b08647 ACS Appl. Mater. Interfaces 2017, 9, 31841−31852

Research Article

ACS Applied Materials & Interfaces

Hence, combining MOFs with rGO to form a hybrid material is a significant strategy to improve the overall electrocatalytic performance from their respective advantages. For instance, Zhou et al.23 designed and synthesized a composite of Co3O4 hollow nanoparticles and Co−organic complexes supported on N-doped graphene, which exhibited superior electrocatalytic performance as a bifunctional cathode catalyst to that of the Pt/ C catalyst. Chen et al.24 reported a novel nitrogen-doped graphene/cobalt-embedded porous carbon polyhedron (N/Codoped PCP//NRGO) hybrid obtained from the ZIF-67/ graphene precursor, which exhibited better electrocatalytic activity and durability toward ORR than that of the Pt/C catalyst under alkaline conditions. However, it is still in the primary stage of exploiting the composite materials consisting of graphene and MOF-derived carbon-based materials to greatly improve catalytic activity and durability for ORR. Herein, we designed a novel ORR electrocatalyst of reduced graphene oxide-supported ZnO/ZnCo2O4/C hollow nanocages derived from a template of Co-based MOFs, ZIF-67, and graphene oxide. Via optimized hydrothermal and pyrolysis processes, the hollow MOF nanocage and mesoporous structure was maintained. Nanostructured ZnO and ZnCo2O4 spinel have been developed as electrocatalysts for ORR owing to their low price, environmental compatibility, and intrinsic catalytic activity.25−28 The ZnO/ZnCo2O4/C@rGO hybrid exhibits excellent electrocatalytic performance toward ORR in alkaline media and superior stability and methanol tolerance to those of the commercial Pt/C catalyst. Additionally, a novel and simple primary coin Al−air battery was first fabricated to further study the practical electrocatalytic performance of the ZnO/ZnCo2O4/C@rGO and ZnO/ZnCo2O4/C hybrids.

Recently, metal−organic frameworks (MOFs), as a sort of organic−inorganic hybrid materials self-assembled from transition metal ions and organic ligands, have been applied as sacrificial templates to fabricate porous carbon-based materials10−13 and transition metal compounds.14−18 The MOFderived nanostructured materials have become greatly promising ORR electrocatalysts by virtue of their large specific surface area, robust morphology, and porosity characteristics. For example, Li et al.10 designed a well-defined carbon-based network with a hierarchical micro/mesoporous structure by the direct growth of MOFs (ZIF-67) on the surface of CoAllayered double hydroxides (LDHs) followed by a subsequent pyrolysis. The resulting material shows superior electrocatalytic activity toward ORR in alkaline medium to that of the Pt/C catalyst and excellent long-time stability by virtue of the versatility of MOFs and LDHs. Zhu et al.12 reported an innovative approach to synthesizing nitrogen- and sulfurcodoped honeycomb-like porous carbon immobilizing Co9S8 nanoparticles using an aluminum-based MOF (MIL-101-NH2). The resulting Co9S8@CN900 exhibits highly efficient catalytic activity for ORR, high durability, and methanol tolerance, even outperforming Pt/C catalysts under alkaline conditions. Dou et al.19 successfully prepared a highly efficient bifunctional electrocatalyst (Co-embedded carbon nanotube (CNT)/ porous carbon) originated from ZIF-67 toward ORR and oxygen evolution reaction. The rechargeable Zn−air battery assembled with the catalyst shows good performance and cycling stability. In another paper, a lamellar MOF from N-rich dicyanoimidazole and iron acetate was developed as a precursor to synthesize a Fe−N−C-based ORR catalyst with desirable porosity and high surface area, exhibiting better catalytic activity and durability than those of the Pt/C catalyst under alkaline conditions.20 Lately, great attention has been focused on building novel hollow transition metal oxide nanocages derived from MOF templates for electrocatalysis. For instance, Lou et al.21 first synthesized Co3O4/NiCo2O4 double-shelled nanocages (DSNCs) involving the formation of ZIF-67/Ni−Co LDH yolk-shelled structures with a polyhedral shape and subsequent thermal annealing in air. The Co3O4/NiCo2O4 DSNCs exhibit much better electrocatalytic activity toward ORR compared to that of the Co3O4 NCs and NiCo2O4 NCs. The greatly improved performance is attributed to a much higher surface area and driving force for catalytic reactions derived from the complex double-shelled nanocage structure and the better conductivity and more active sites resulted by the incorporation of Ni2+. Yin et al.22 reported hierarchical mesoporous ZnO/ ZnFe2O4/C nanocages prepared by an optimized pyrolysis process from MOF. The ZZFC cathode exhibited high specific capacity and excellent stability in lithium−oxygen batteries due to the unique porous framework producing more active sites and faster mass/electron transportation. In general, the high surface area of hollow nanocages can provide more active sites and a larger liquid−solid contacting area for efficient catalytic kinetics and the hollow interior can prevent the aggregation of encapsulated transition metal oxide nanoparticles. Moreover, anisotropic nanocages with uniform size, low density, large void space, and shell permeability are strongly beneficial for electrocatalysis. However, the reports on the ORR catalysis by hollow transition metal oxide nanocages are limited. Reduced graphene oxide (rGO) has been a popular material to enhance the electrical conductivity, dispersion, and chemical stability of composite materials owing to its excellent electrical properties, large surface area, and good mechanical strength.

2. EXPERIMENTAL SECTION 2.1. Preparation of ZIF-67. ZIF-67 was synthesized as in our precious work45 (see the Supporting Information). 2.2. Synthesis of a Reduced Graphene Oxide-Supported Cobalt-Embedded Carbon Polyhedron (Co/C@rGO) Hybrid. Graphene oxide (GO) was prepared from natural graphite by modified Hummer’s method.29 The Co/C@rGO hybrid was synthesized by a facile pyrolysis approach derived from ZIF-67 and GO. First, 60 mg of ZIF-67 and 20 mg of GO were ultrasonically dispersed in 20 mL of ethanol to form homogeneous dispersions, respectively. Next, the GO suspension was added into ZIF-67 solution and mixed by magnetic stirring for 3 h. The mixture was dried at 70 °C overnight to obtain the ZIF-67@GO product. The Co/C@rGO hybrid was obtained by calcination of as-prepared ZIF-67@GO at 700 °C in a tubular furnace under an Ar atmosphere for 3 h using a heating rate of 2 °C min−1. The comparative Co/C hybrid was synthesized through same processes without adding GO. 2.3. Synthesis of a Reduced Graphene Oxide-Supported ZnO/ZnCo2O4/C Nanocage (ZnO/ZnCo2O4/C@rGO) Hybrid. In a typical synthesis of the ZnO/ZnCo2O4/C@rGO hybrid, 60 mg of ZIF-67@GO and 100 mg of Zn(NO3)2·6H2O were uniformly mixed in 20 mL of ethanol and 20 mL of deionized water by ultrasonication. The solution was then transferred into a 50 mL Teflon-lined autoclave and heated at 150 °C for 3 h. The precipitate was collected and purified by centrifuging with deionized water and ethanol several times. Finally, the as-prepared product was put in a tubular furnace and calcined in Ar under 700 °C for 3 h using the former heating rate. The ZnO/ZnCo 2 O4 /C nanocages were synthesized by the same procedures without adding GO. 2.4. Fabrication of Aluminum−Air Coin Batteries. The air electrode was fabricated by coating catalyst ink on a carbon paper (Toray, TGP-H-60, Alfa Aesar) with a diameter of 10 mm and dried under 60 °C for 3 h to make the loading of about 1.5 mg cm−2. The catalyst paste is composed of 10 mg of the catalyst dispersed in 67 μL 31842


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ACS Applied Materials & Interfaces Scheme 1. Schematic Illustration for the Synthesis of Co/C@rGO and ZnO/ZnCo2O4/C@rGO Hybrids

of 5 wt % Nafion (D-521, Alfar Aesar) and 1.0 mL of isopropyl alcohol mixture. The Al−air coin cell was fabricated with a 1 cm × 1 cm aluminum foil (99.997%, 0.25 mm, Alfa Aesar) as the anode, a piece of glass microfiber (Whatman, Grade 934-AH) with a diameter of 16 mm as the separator, and a 2 mol L−1 KOH aqueous solution as the electrolyte. 2.5. Material Characterization. The crystal structure was characterized by an X-ray diffractometer (D/Max2250, Japan) with nickel-filtered Cu Kα radiation. The morphologies and components of the materials were analyzed using a Hitachi 3400N field-emission scanning electron microscope. The microstructure was tested by a high-resolution transmission electron microscope (FEI TECNAI G2 F20) at 120 kV. The thermogravimetric test was conducted by a SVT Q600 instrument from room temperature to 700 °C under an air atmosphere. The Raman spectrum was recorded by LabRAM Hr800 confocal Raman microscopy using an excitation laser of 532 nm. A JWBK132F analyzer was used to analyze Brunauer−Emmett−Teller (BET) specific surface area and pore size characteristics. The surface elemental analysis was conducted on a Thermo Fisher Scientific 1063 X-ray photoelectron spectrometer with Al Kα radiation. 2.6. Electrochemical Measurements. Electrochemical measurements were performed by rotating disk electrode (RDE) voltammetry on an electrochemical work station (Zennium Zahner, Germany) at room temperature in 0.1 M KOH solution, using a conventional threeelectrode system with a rotating disk glassy carbon electrode, a platinum foil counter electrode, and an Ag/AgCl reference electrode. The catalyst ink was prepared as in our precious work30 (see the Supporting Information). The catalyst ink (10 μL) was dropped on the rotating disk glassy carbon electrode (5 mm in diameter, pine) and dried by an infrared lamp, making loading of catalysts of about 0.45 mg cm−2. The cyclic voltammograms (CVs) and linear sweep voltammograms (LSVs) were recorded by our previous method30 (see the Supporting Information). The electrolyte was aerated using highpurity O2 for about 0.5 h before each test and O2 was maintained during the test. 2.7. Performance Testing of Al−Air Coin Batteries. The electrocatalytic performance of catalysts was further investigated in an Al−air coin cell system at room temperature. The discharge tests of the cells with ZnO/ZnCo2O4/C@rGO and ZnO/ZnCo2O4/C hybrids were conducted on a multichannel testing system (LANHE CT2001A) with a constant current density of 1 mA cm−2 under ambient conditions. The cutoff voltage in all cases is 0.0 V.

3. RESULTS AND DISCUSSION 3.1. Characterization of Cathode Materials. The synthesis strategies of Co/C@rGO and ZnO/ZnCo2O4/C@ rGO hybrids are schematized in Scheme 1. First, the dodecahedral ZIF-67 nanoparticles were adhered to the surface of GO due to electrostatic interactions. Then, the ZIF-67/GO powders were heated under an Ar atmosphere at 700 °C to obtain the Co/C@rGO nanohybrid. The reduced graphene oxide-supported ZnO/ZnCo2O4 nanocages are formed by using ZIF-67/GO as a sacrificial template reacting with Zn(NO3)2 under 150 °C hydrothermal conditions. In the hydrothermal procedure, the Zn2+ ions quickly react with oxygen on the ZIF-67 surface to form ZnO nanoparticles. Because the ionic radius of Zn 2+ ions (74 pm) is larger than that of Co2+ ions (72 pm), the inner organic framework is gradually dissolved and Co2+ ions release and react with Zn2+ to form ZnCo2O4 nanoparticles. Finally, the hollow ZnO/ ZnCo2O4/C nanocages supported on rGO are fabricated by calcination under Ar at 700 °C for 3 h. Significantly, this strategy can be extended to produce hollow nanocages composed with other transition mental oxides using MOFs as the self-template.31 Figure 1 shows the X-ray diffraction (XRD) patterns of Co/ C, Co/C@rGO, ZnO/ZnCo2O4/C, and ZnO/ZnCo2O4/C@ rGO hybrids. The characteristic diffraction peaks of the ZnO/ ZnCo2O4/C@rGO hybrid are at 2θ = 31.77, 34.42, 36.25, 47.54, 56.60, 62.86, 66.38, 67.96, 69.10, 72.56, and 76.95°, which correspond to the (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), and (202) lattice planes of ZnO (JCPDS card No. 21−1486) with a space group F63mc(186), respectively. The characteristic diffraction peaks at 18.96, 31.21, 36.80, 38.49, 44.74, 55.57, 59.28, 65.15, and 77.22° can be ascribed to the (111), (220), (311), (222), (400), (422), (511), (440), and (533) lattice planes of ZnCo2O4 (JCPDS card No. 23−1390) with a space group Fd3m(227), respectively. Additionally, the characteristic diffraction peaks of the Co/C@rGO hybrid at 44.26, 51.56, and 75.90° can be indexed to the (111), (200), and (220) planes of Co (JCPDS card No. 15-0806) with a space group Fm3̅m(225), respectively. The peak of reduced graphene oxide at around 22° (002) of the ZnO/ZnCo2O4/C@rGO hybrid is 31843


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ZnCo2O4/C nanocages are uniformly grown on rGO sheets and that the particle size of nanocages is around 20−30 nm, which is much smaller than that of the ZnO/ZnCo2O4/C hybrid (40−80 nm, Figure S5). The results greatly indicate that the available effect for rGO in avoiding the nanoparticles from aggregating. The energy-dispersive system (EDS) spectrum of the ZnO/ZnCo2O4/C@rGO hybrid (Figure S6) shows the existence of C (83.79 atom %), O (13.89 atom %), Co (0.77 atom %), and Zn (1.52 atom %). The elemental distribution investigation was carried out by energy-dispersive X-ray mappings of the whole area (Figure 2c), which clearly reveals the coexistence and homogeneous distribution of the Zn, Co, C, and O elements on rGO sheets, further demonstrating the formation of ZnO and ZnCo2O4. The low-resolution transmission electron microscopy (TEM) images (Figure 3a,b) show that the hollow ZnO/ZnCo2O4/C nanocages with 20−30 nm in size are uniformly dispersed on reduced graphene oxide sheets. The detailed microstructure can be further observed by high-resolution TEM (HRTEM) (Figure 3c); the hollow ZnO/ZnCo2O4/C nanocages are composed of numerous ZnO and ZnCo2O4 nanoparticles with a size of ∼5 nm encapsulated into the carbon matrix. The HRTEM image of a single ZnO/ZnCo2O4/C nanocage is shown in Figure S7. The selected-area electron diffraction (SAED) (Figure 3d) indicates the polycrystalline nature of ZnO/ZnCo2O4/C nanocages. The distinct concentric diffraction rings are ascribed to the (101) and (103) planes of ZnO and the (220) and (511) planes of spinel ZnCo2O4, which is consistent with the XRD results. This unique structure allows facile accessibility of oxygen to contact the effectively exposed nanoparticles to catalyze oxygen reduction. Thermogravimetric analysis was performed in air from room temperature to 700 °C to evaluate the amount of rGO (thermogravimetry differential thermal analysis (TG-DTA), Figure S8). The weight loss between 20 and 100 °C can be ascribed to the evaporation of water (0.06 wt %). The gradual weight loss from 250 to 400 °C

Figure 1. X-ray diffraction (XRD) spectra of Co/C, Co/C@rGO, ZnO/ZnCo2O4/C, and ZnO/ZnCo2O4/C@rGO hybrids.

not observed, indicating that ZnO/ZnCo2O4/C nanocages are efficiently deposited on the rGO surface. As shown in Figure S1, ZIF-67 has a regular rhombic dodecahedral morphology with a diameter of 400−500 nm. The scanning electron microscopy (SEM) image of the ZIF67/GO precursor (Figure S2) presents that the ZIF-67 dodecahedrons are placed on the surface of GO and keeping the former morphology. The Co/C hybrid retains the same structure as that of ZIF-67, but its surface becomes rough and shrinks during the pyrolysis process, and the metallic cobalt nanoparticles are embedded into the carbon framework (Figure S3). When Co/C dodecahedrons were supported on the reduced graphene oxide sheets, the particle size decreased to 100−200 nm (Figure S4). The SEM images (Figure 2a,b) of the ZnO/ZnCo2O4/C@rGO hybrid reveal that the ZnO/

Figure 2. (a, b) Field-emission scanning electron microscopy images and (c) elemental mapping images of C, O, Co, and Zn in the ZnO/ZnCo2O4/ C@rGO hybrid. 31844


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Figure 3. (a−c) TEM and HRTEM images and (d) selected-area electron diffraction (SAED) pattern of the ZnO/ZnCo2O4/C@rGO hybrid.

Figure 4. (a) N2 adsorption−desorption isotherm and (b) BJH desorption pore size distribution of the ZnO/ZnCo2O4/C and ZnO/ZnCo2O4/C@ rGO hybrids.

distributions of the ZnO/ZnCo2O4/C@rGO and ZnO/ ZnCo2O4/C hybrids are acquired by the Barret−Joyner− Halenda (BJH) method. The average pore width of the ZnO/ ZnCo2O4/C@rGO hybrid is 9.54 nm, which is smaller than that of the ZnO/ZnCo2O4/C hybrid (11.41 nm). These mesopores are helpful in providing effective exposure of catalytic active sites for ORR activity.32 The structural composition and the defect level of the nanomaterials were investigated by the Raman spectrum technique (Figure 5a). Two representative Raman peaks of carbon materials at around 1390 and 1600 cm−1 are observed, corresponding to the D band and G band, respectively. Generally, the intensity ratio of the D band to G band, ID/IG, can be utilized for judging the degree of disorder/order of

can be ascribed to the oxidation of rGO in this hybrid. The total weight loss is 28.0 wt %; therefore, the content of rGO in the hybrid is 27.94 wt %. The porosity characteristics of the ZnO/ZnCo2O4/C and ZnO/ZnCo2O4/C@rGO hybrids were characterized by N2 adsorption−desorption measurements. Figure 4a shows typical IV isotherms and H4-type hysteresis loop of the materials, suggesting that the mesoporous structures are maintained. The calculated BET specific surface area of the ZnO/ZnCo2O4/C@ rGO hybrid is 112.54 m2 g−1, which is much higher than that of the ZnO/ZnCo2O4/C hybrid (54.19 m2 g−1), indicating that graphene can increase the specific surface area. The higher specific surface area is available for supplying more active sites, resulting in better electrocatalytic activity. The pore size 31845


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Figure 5. (a) Raman spectra of the Co/C@rGO, ZnO/ZnCo2O4/C, and ZnO/ZnCo2O4/C@rGO hybrids. The laser excitation wavelength is 532 nm. (b) XPS spectra of the ZnO/ZnCo2O4/C@rGO hybrid. High-resolution XPS spectra of ZnO/ZnCo2O4/C@rGO: (c) C 1s, (d) N 1s, (e) Zn 2p, and (f) Co 2p, respectively.

carbon nanomaterials.33 The ID/IG values are 1.54, 1.03, and 1.29 for ZnO/ZnCo2O4/C@rGO, ZnO/ZnCo2O4/C, and Co/ C@rGO, revealing the increase of the defect level caused by the introduction of rGO, which is due to the disorganized graphene edges by the elimination of functional groups of graphene oxide nanosheets. Additionally, the prominent peaks detected in the range of 400 and 700 cm−1 can be ascribed to the functional

groups containing zinc and cobalt. The survey scan spectrum of the ZnO/ZnCo2O4/C@rGO hybrid (Figure 5b) suggests the presence of C (47.07 atom %), O (28.44 atom %), N (1.35 atom %), Co (8.96 atom %), and Zn (14.18 atom %). In the high-resolution X-ray photoelectron spectrum (XPS) of C 1s (Figure 5c), three obvious peaks can be observed, which are attributed to C−C (sp2), C−N, and C−O groups.34 The high31846


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Figure 6. (a) Cyclic voltammograms of the Co/C, Co/C@rGO, ZnO/ZnCo2O4/C, ZnO/ZnCo2O4/C@rGO, and 20 wt % Pt/C electrocatalysts in O2-saturated 0.1 M KOH aqueous solution; (b) LSV curves of the Co/C, Co/C@rGO, ZnO/ZnCo2O4/C, ZnO/ZnCo2O4/C@rGO, and 20 wt % Pt/C electrocatalysts on a RDE electrode in O2-saturated 0.1 M KOH solution at a rotation rate of 1600 rpm with a scan rate of 5 mV s−1; (c) Tafel slopes from LSV curves at 1600 rpm; (d) mass activities at −0.199 V for all of the catalysts; (e) LSV curves of the ZnO/ZnCo2O4/C@rGO hybrid in O2-saturated 0.1 M KOH solution at various rotation rates with a scan rate of 5 mV s−1; and (f) corresponding Koutecky−Levich plots at different potentials.

shows two peaks at 1022.1 and 1045.1 eV, which are attributed to Zn 2p3/2 and Zn 2p1/2.22 The Co 2p high-resolution spectrum (Figure 5e) consists of two pairs of spin−orbit peaks, indicating the existence of Co2+ and Co3+.37 The O 1s spectrum (Figure S9) shows three oxygen states, which are related to oxygen in ZnO and ZnCo2O4 and the absorbed oxygen species or the residual oxygen-containing groups of rGO.38

resolution N 1s spectrum (Figure 5d) can be divided into three peaks located at 398.4, 399.8, and 401.3 eV, which can be assigned to pyridinic N, pyrrolic N, and quaternary N, respectively,35 indicating the doping of nitrogen species into the ZnO/ZnCo2O4/C@rGO hybrid. The predominant pyridinic N is known to play a positive role in enhancing onset potentials toward ORR.36 The high-resolution Zn 2p spectrum 31847


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Table 1. Comparison of the ORR Electrocatalytic Performance of the Catalysts in This Work with That of the Recently Reported MOF-Derived Carbon-Based Electrocatalysts in Alkaline Solutiona catalyst

E1/2 (V vs Ag/AgCl)

Eonset (V vs Ag/AgCl)

Tafel slope (mV dec−1)

durability

reference

Co/C

−0.20

−0.10

78.25

undefined

Co/C@rGO

−0.17

−0.07

71.25

undefined

ZnO/ZnCo2O4/C

−0.25

−0.14

87.39

95.6% retention of j under −0.3 V for 12 000 s

ZnO/ZnCo2O4/C@rGO

−0.15

−0.05

46.70


20% Pt/C

−0.18

−0.09

74.79


Co-CNT/PC Co-C@Co9S8 DSNCs Co9S8@SNCC Fe3C/b-NCNT Fe3C@NC/NCS Co@Co3O4@C−CM N/Co-doped PCP//NRGO

undefined undefined −0.21 undefined −0.15 −0.15 undefined

73.8 undefined 80 71.2 undefined undefined 76

93.0% retention of j under 0.79 V for 20 000 96.0% retention of j under 0.50 V for 5 h 84.0% retention of j under 0.60 V for 15 000 100% retention of j under 0.78 V for 70 h 70% retention of j under −0.3 V for 10 h 90.0% retention of j under 0.75 V for 25 000 85.6% retention of j under 0.90 V for 20 000

−0.05 −0.005 −0.12 −0.005 −0.08 −0.03 −0.02

s s

s s

this work this work this work this work this work 31 40 41 42 43 44 24

a

All of the potential values here are versus Ag/AgCl for comparison. In 0.1 M KOH electrolyte (pH = 13), E (vs RHE) = E (vs Ag/AgCl) + 0.1976 V + 0.059 pH, converted from Ag/AgCl.

Figure 7. (a) LSV curves of the ZnO/ZnCo2O4/C hybrid in O2-saturated 0.1 M KOH solution at various rotation rates with a scan rate of 5 mV s−1, (b) corresponding Koutecky−Levich plots at different potentials, (c) LSV curves of 20 wt % Pt/C in O2-saturated 0.1 M KOH solution at various rotation rates with a scan rate of 5 mV s−1, and (d) corresponding Koutecky−Levich plots at different potentials.

performance of the catalysts was first comparatively studied by CVs in O2-saturated 0.1 M KOH solution. Obvious oxygen

3.2. Electrocatalytic Activity of the Electrocatalysts for the Oxygen Reduction Reaction. The ORR catalytic 31848


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Figure 8. (a) Current−time chronoamperometric response of Pt/C, ZnO/ZnCo2O4/C, and ZnO/ZnCo2O4/C@rGO electrocatalysts at −0.30 V in O2-saturated 0.1 M KOH solution; (b) chronoamperometric response of Pt/C and ZnO/ZnCo2O4/C@rGO electrocatalysts at −0.30 V in O2saturated 0.1 M KOH solution. The rotation rate is 900 rpm. The arrow indicates the introduction of 3 M methanol.

reduction peaks between −0.1 and −0.4 V (vs Ag/AgCl) can be observed for all of the catalysts in Figure 6a, demonstrating their ORR electrocatalytic activity. The ZnO/ZnCo2O4/C@ rGO hybrid shows a more positive oxygen reduction peak potential (−0.15 V) compared to that of ZnO/ZnCo2O4/C (−0.21 V), N/C@rGO (−0.19 V), and Co/C (−0.21 V) but is negative than that of commercial Pt/C (−0.12 V), suggesting that the introduction of rGO plays a significant role in promoting electrocatalytic activity. In addition, a series of linear LSVs on a rotating disk electrode (RDE) were acquired in O2saturated 0.1 M KOH with a rotation rate of 1600 rpm to further study their ORR catalytic performance (Figure 6b). Remarkably, the ZnO/ZnCo2O4/C@rGO hybrid shows an onset potential (Eonset) of −0.05 V, which is more positive than that of ZnO/ZnCo2O4/C (−0.14 V), Co/C@rGO (−0.07 V), and Co/C (−0.10 V). The half-wave potential (E1/2) of ZnO/ ZnCo2O4/C@rGO is −0.15 V, which is more positive than that of other catalysts including the Pt/C catalyst of −0.18 V (Table 1), demonstrating fast ORR kinetics of the ZnO/ZnCo2O4/ C@rGO hybrid. The more positive Eonset and E1/2 represent a higher activity. Furthermore, the stable and wide current plateau from −0.4 to −0.8 V of the ZnO/ZnCo2O4/C@rGO hybrid was observed, representing a diffusion-controlled process corresponding to the efficient four-electron-dominated ORR pathway.39 As shown in Figure 6c, the ZnO/ZnCo2O4/ C@rGO hybrid exhibits a Tafel slope of 46.70 mV dec−1, which is much smaller than that of ZnO/ZnCo2O4/C (87.39 mV dec−1), Co/C@rGO (71.25 mV dec−1), and Co/C (78.25 mV dec−1) and even outperforming that of Pt/C (74.79 mV dec−1), indicating that the ZnO/ZnCo2O4/C@rGO hybrid has a faster reaction kinetic. Figure 6d displays a much higher ORR mass activity at −0.199 V of the ZnO/ZnCo2O4/C@rGO hybrid (0.51 A mg−1) compared to that of the other catalysts, suggesting an optimal electrocatalytic activity. It is noted that the ORR catalytic performance of the ZnO/ZnCo2O4/C@rGO hybrid greatly surpasses that of the recently reported MOFderived carbon-based eletrocatalysts under alkaline conditions (Table 1). The results highlight the improved ORR activity of the ZnO/ZnCo2O4/C@rGO hybrid, which is ascribed to the high specific surface area, unique mesoporous hollow framework, more accessible catalytic active sites, and small amount of N-doping. Remarkably, rGO acts as a conductive layer to

promote the transfer and arrival to active sites of electrons. The mesoporous hollow nanocages originated from ZIF-67/Zn(NO3)2 act as the catalytic site, where the fast transport of reactants and exposure of active sites are facilitated. In addition, the synergistic effect between the ZnO/ZnCo2O4/C nanocages and rGO actually enhances the ORR catalytic activity. More detailed LSV studies in an O2-saturated 0.1 M KOH electrolyte at a scan rate of 5 mV s−1 with different rotation rates from 400 to 1600 rpm (Figure 6e) were carried out to further study the ORR mechanism of the ZnO/ZnCo2O4/C@ rGO hybrid. The corresponding Koutecky−Levich (K−L) plots at different potentials are shown in Figure 6f. The electron transfer numbers (n) of ORR can be calculated by K−L equations (see Supporting Information). The n of the ZnO/ ZnCo2O4/C@rGO hybrid is calculated to be about 3.95, which is comparable to that of the 20 wt % Pt/C catalyst (4.0, Figure 7c,d), suggesting a high priority for the four-electron pathway toward ORR. Furthermore, the n of the ZnO/ZnCo2O4/C@ rGO hybrid is much higher than that of the ZnO/ZnCo2O4/C hybrid (3.41, Figure 7a,b) and the n of Co/C@rGO (3.93, Figure S10a,b) is also higher than that of Co/C (3.44, Figure S10c,d), further proving the significant effect of rGO on electrocatalytic performance. The long-term stabilities of the ZnO/ZnCo2O4/C@rGO hybrid and the commercial Pt/C catalyst for ORR were comparatively investigated by chronoamperometric measurements (Figure 8a). After 12 000 s of reaction at a constant voltage of −0.3 V, the ZnO/ZnCo2O4/C@rGO hybrid exhibits a remarkable durability with 99.7% retention of the current density toward ORR, whereas the ZnO/ZnCo2O4/C hybrid shows nearly 95.6% retention and the Pt/C catalyst undergoes a faster current decline (75.6% retention). The results demonstrate that the ZnO/ZnCo2O4/C@rGO hybrid possesses better ORR stability than that of ZnO/ZnCo2O4/C and Pt/C under alkaline conditions and rGO can improve the stability. The stability of the ZnO/ZnCo2O4/C@rGO hybrid was also assessed by the CV test during 3000 continuous cycles (Figure S11). The polarization curve shows no significant change in the half-wave potential and a slight change in the limiting current after continuous 3000 cycles, suggesting high stability of the catalyst. Furthermore, the methanol crossover effect was studied by adding 3 M methanol into O2-saturated 31849


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Figure 9. (a) Conformation of a coin Al−air cell, (b) discharge curves of the coin Al−air batteries with the ZnO/ZnCo2O4/C and ZnO/ZnCo2O4/ C@rGO electrocatalysts under ambient conditions at a current density of 1.0 mA cm−2, and (c) specific discharge capacity of the ZnO/ZnCo2O4/C and ZnO/ZnCo2O4/C@rGO hybrids.

Table 2. Summary of Discharge Performance of the Coin Cells Assembled with ZnO/ZnCo2O4/C and ZnO/ZnCo2O4/C@rGO at a Current Density of 1.0 mA cm−2 under Ambient Conditions ZnO/ZnCo2O4/C ZnO/ZnCo2O4/C@rGO

open circuit voltage (V)

flat plateau (V)

specific capacity (mAh g−1)

power density (mW cm−2)

1.39 1.53

1.16 1.29

31.5 42.6

1.26 1.41

oxide/zinc nitrate composite as the precursor. The nanocomposite material exhibits excellent electrocatalytic activity toward ORR and superior stability and methanol tolerance under alkaline conditions to those of the commercial Pt/C catalyst, which is ascribed to the unique hollow nanocage structure and the synergistic effect between the ZnO/ ZnCo2O4/C nanocages and rGO. The discharge performance of the coin Al−air cell demonstrates the important role of rGO. Our work implies the great potential of the hybrid material as the ORR electrocatalyst and inspires us to further directing the development of reduced graphene oxide-supported MOFderived mesoporous carbon-based nanomaterials for ORR in Al−air batteries. Remarkably, the primary coin Al−air battery can be a promising electric device for practical use.

0.1 M KOH solution during the chronoamperometric test at 900 rpm. Figure 8b shows that the cathodic ORR current of the ZnO/ZnCo2O4/C@rGO hybrid remains almost unchanged, whereas the current of the Pt/C catalyst significantly decreases owing to incomplete methanol oxidation on the electrode surface, indicating that the ZnO/ZnCo2O4/C@rGO hybrid possesses better tolerance of crossover effect against methanol. 3.3. Al−Air Coin Cell Discharge Studies. The aforementioned discussions have intensely suggested the potentiality of the ZnO/ZnCo2O4/C@rGO hybrid to be used as an electrocatalyst for ORR. To prove the practical catalytic performance of the catalyst, a novel coin Al−air cell was assembled (Figure 9a). Figure 9b exhibits the discharge behaviors of coin cells using the ZnO/ZnCo2O4/C@rGO and ZnO/ZnCo2O4/C hybrids as cathodic catalysts at a constant current density of 1.0 mA cm−2, consuming air under room temperature. The coin cell assembled with ZnO/ ZnCo2O4/C@rGO as the cathodic catalyst exhibits a higher open circuit voltage of 1.39 V than that of the coin cell with ZnO/ZnCo2O4/C (1.53 V). Obviously, the flat plateau of the coin cell assembled with ZnO/ZnCo2O4/C@rGO is larger than that of the cell assembled with ZnO/ZnCo2O4/C. In addition, the voltage of the cell assembled with ZnO/ZnCo2O4/C decreased by ∼0.20 V, whereas the voltage of the cell assembled with ZnO/ZnCo2O4/C@rGO decreased by ∼0.17 V. The coin cell with the ZnO/ZnCo2O4/C@rGO cathode displays a higher specific capacity of 42.6 mAh g−1 (Figure 9c) and higher power density of 1.41 mW cm−2 than those of the coin cell with the ZnO/ZnCo2O4/C cathode. The parameters of discharge performance are scheduled in Table 2. The discharge results further confirm better electrocatalytic activity of the ZnO/ZnCo2O4/C@rGO hybrid, which is attributed to the motivation of rGO for the electrocatalytic activity.

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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.7b08647. Detailed synthesis method of ZIF-67, electrochemical measurements, K−L equation to calculate the electron transfer number (n), SEM and TEM images of ZIF-67, SEM images of the ZIF-67/GO precursor, the Co/C hybrid, the Co/C@rGO hybrid, and the ZnO/ ZnCo2O4/C hybrid, EDS spectrum of the ZnO/ ZnCo2O4/C@rGO hybrid, high-resolution TEM image of the ZnO/ZnCo2O4/C nanocage, thermogravimetry differential scanning calorimetry (TG-DSC) curves and O 1s high-resolution XPS spectra of the ZnO/ZnCo2O4/ C@rGO hybrid, LSV curves of Co/C and Co/C@rGO hybrids in O2-saturated 0.1 M KOH solution at various rotation rates with a scan rate of 5 mV s−1, and the durability test of the ZnO/ZnCo2O4/C@rGO hybrid (PDF)

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4. CONCLUSIONS In summary, a novel reduced graphene oxide-supported ZnO/ ZnCo2O4 nanoparticle-embedded carbon hollow nanocage (ZnO/ZnCo2O4/C@rGO) hybrid was synthesized by a facile hydrothermal-calcination approach using a ZIF-67/graphene

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +86 731 8887 9616. Fax: +86 731 8887 9616 (W.L.). 31850


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ACS Applied Materials & Interfaces *E-mail: [email protected] (J.L.).

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ORCID

Wenzhang Li: 0000-0002-6801-4105 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the National Nature Science Foundation of China (No. 51474255), the Open-End Fund for the Graduate Student Research Innovation Project of Hunan Province (No. 150140008), the Open-End Fund for the Graduate Student Independent Exploration and Innovation Project of Central South University (No. 2017zzts018), and the open research Fund Program of Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological Environment Monitoring (Central South University), Ministry of Education.

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