MetalâOrganic Framework-Derived Reduced Graphene Oxide

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Metal-Organic Framework Derived Reduced Graphene Oxide Supported 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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08647 • Publication Date (Web): 28 Aug 2017 Downloaded from http://pubs.acs.org on August 28, 2017

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ACS Applied Materials & Interfaces

Metal-Organic Framework Derived Reduced Graphene Oxide Supported ZnO/ZnCo2O4/C Hollow Nanocages as Cathode Catalysts for Aluminum-O2 Batteries Yisi Liu a, b, Hao Jiang a, Jiayu Hao a, Yulong Liu b, Haibo Shen b, Wenzhang Lia, c, d*, Jie Li a, c a

*

School of Chemistry and Chemical Engineering, Central South University, Changsha

410083 China b

Department of Mechanical and Materials Engineering,

University of Western

Ontario,

London, Ontario N6A 5B9 , Canada c

Hunan Provincial Key Laboratory of Efficient and Clean Utilization of Manganese

Resources, Central South University, Changsha, 410083, China d

Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological

Environment Monitoring (Central South University), Ministry of Education *

Corresponding author. Tel.: +86 731 8887 9616; fax: +86 731 8887 9616.

E-mail addresses: [email protected], [email protected]

1

ACS Paragon Plus Environment


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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

towards

oxygen

reduction

reaction.

In

this

work,

a

hydrothermal-calcination approach was utilized to prepare a novel reduced graphene oxide supported hollow ZnO/ZnCo2O4 nanoparticles-embedded carbon nanocages (ZnO/ZnCo2O4/C@rGO) using a zeolitic imidazolate-frameworks (ZIF-67)/graphene oxide/zinc nitrate composite as the precursor. The ZnO/ZnCo2O4/C@rGO hybrid exhibits remarkable electrocatalytic perfomance for ORR under alkaline condition and superior stability and methonal tolerance to that of 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 condition to further investigate their catalytic performance. The coin cell with the ZnO/ZnCo2O4/C@rGO cathode catalyst displays higher open circuit voltage, discharge voltage and more sluggish potential drop than that 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 2


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ZnO/ZnCo2O4/C nanocages. Key words: metal–organic frameworks (MOFs), hollow nanocages, aluminum-air battery, oxygen reduction reaction, electrocatalysts

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 two to ten folds larger than that of state-of-the-art LIBs 1. Among the metal-air batteries, Al-air batteries hold great promise for future large-scale 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 those of magnesium (2.20 Ah g-1) and zinc (0.82 Ah g-1)

2-5

. Additionally, aluminum is an

inexpensive, abundant and environmental friendly metal with high recyclability 6. The primary Al-air battery, which 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 3



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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

non-precious

metal

electrocatalysts is a promising strategy to substitute precious metal catalysts. Recently, metal–organic frameworks (MOFs) as a sort of organic-inorganic hybrid materials self-assembled of transition metal ions and organic ligands have been applied as sacrificial templates to fabricate porous carbon-based materials transition metal compounds

10-13

and

14-18

. The MOF-derived nanostructured materials have

become greatly promising ORR electrocatalysts in virtue of their large specific surface area, robust morphology and porous characteristics. For example, Li et al.

10

designed a well-defined carbon-based network with hierarchical micro/mesoporous structure by the direct growth of MOFs (ZIF-67) on the surface of CoAl-LDHs (layered double hydroxides) followed by a subsequent pyrolysis. The resulting material shows superior electrocatalytic activity towards ORR in alkaline medium to that of 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 synthesize nitrogen and sulfur co-doped 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 condition. Dou et al.

19

successfully prepared a highly efficient bi-functional electrocatlyst (Co-embedded 4


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CNT/porous carbon) originated from ZIF-67 towards ORR and OER. 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 (DCI) 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 thaose of Pt/C catalyst under alkaline medium 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 the 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 towards ORR compared with the Co3O4 NCs and NiCo2O4 NCs. The greatly improved performance is attributed to 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 larger liquid-solid contacting area for efficient catalytic kinetics, and the hollow interior can prevent the aggregation of encapsulated transition metal oxide 5



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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 of 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. 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 Pt/C catalyst. Chen et al. 24 reported a novel nitrogen-doped graphene/cobalt-embedded porous carbon polyhedron (N/Co-doped PCP//NRGO) hybrid obtained from the ZIF-67/graphene precursor exhibited better electrocatalytic activity and durability towards ORR than that of Pt/C catalyst under alkaline condition. 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. 6


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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 towards ORR in alkaline media, and superior stability and methanol tolerance to that of commercial Pt/C catalyst. Additionally, a novel and simple primary coin Al-air battery was first fabricated to further study pratical electrocatalytic performance of the ZnO/ZnCo2O4/C@rGO and ZnO/ZnCo2O4/C hybrids.

2. Experimental 2.1 Preparation of ZIF-67 ZIF-67 was synthesized as our precious work 46 (See Supporting Information). 2.2 Synthesis of 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 ultrasonic dispersed in 20 mL of ethanol to form homogeneous dispersions, respectively. Next, the GO suspension was added into ZIF-67 solution and mixed by a magnetic stirring for three hours. The mixture was dried at 70 °C for all night to obtain ZIF-67@GO product. The Co/C@rGO hybrid was obtained by calcination of the as-prepared ZIF-67@GO at 700 ˚C in a tubular furnace under Ar atmosphere for 3 7



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h using heating rate of 2 ˚C min-1. The comparative Co/C hybrid was synthesized through same processes without adding GO. 2.3 Synthesis of reduced graphene oxide supported ZnO/ZnCo2O4/C Nanocages (ZnO/ZnCo2O4/C@rGO) hybrid In the typical synthesis of ZnO/ZnCo2O4/C@rGO hybrid, 60 mg ZIF-67@GO and 100 mg 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 for several times. Finally, the as-prepared product was put a tubular furnace and calcined in Ar under 700 ˚C for 3 h using the former heating rate. The ZnO/ZnCo2O4/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 the diameter of 10 mm, and dried under 60 ℃ 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 67µL 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 the diameter of 16 mm as the separator, and a 2 mol L-1 KOH aqueous solution as the electrolyte. 2.5 Materials characterization 8


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The crystal structure was characterized by X-ray diffractometer (D/Max2250, Japan) with nickel-filtered Cu Kα radiation. The morphologies and components of the materials were measured using Hitachi 3400N field emission scanning electron microscope. The microstructure was tested by high resolution transmission electron microscope (FEI TECNAI G2 F20) at 120 kV. The thermogravimetric (TGA) test was conducted by a SVT Q600 instrument from room temperature to 700 °C under air atmosphere. Raman spectrum was recorded by the LabRAM Hr800 confocal Raman microscopy using an excitation laser of 532 nm. A JW-BK132F 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 an Al Kα radiation. 2.6 Electrochemical measurements Electrochemical measurements were operated 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 three-electrode 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 our precious work

30

(See Supporting Information). 10 µL of catalyst ink was dropped on the rotating disk glassy carbon electrode (5 mm in diameter, pine) and dired by an infrared lamp, making loading of catalysts is about 0.45 mg cm-2. The cyclic voltammograms (CVs) and the linear sweep voltammograms (LSVs) were recorded by our precious method 30

(See Supporting Information). The electrolyte was aerated using high-purity O2 9



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for about 0.5 h before each test and O2 was maintained during the test. 2.7 The 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 constant current density of 1 mA cm-2 under ambient condition. The cut-off voltage of all cases is 0.0 V.

3. Results and discussions 3.1 The characterization of cathode materials The synthesis strategy of Co/C@rGO, and ZnO/ZnCo2O4/C@rGO hybrids were 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 Ar atmosphere at 700 °C to obtain 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 in 150 °C hydrothermal condition. In the hydrothermal procedure, the Zn2+ ions quickly react with oxygen on 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 10


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produce hollow nanocages composing with other transition mental oxides using MOFs as the self-template 31. Figure 1 shows the XRD patterns of Co/C, Co/C@rGO, ZnO/ZnCo2O4/C and ZnO/ZnCo2O4/C@rGO

hybrids.

The

characteristic

diffraction

peaks

of

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°, 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°, 74.00°, 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 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 Fm-3m(225), respectively. The peak of reduced graphene oxide at around 22° (002)

of

ZnO/ZnCo2O4/C@rGO

hybrid

is

not

observed,

indicating

that

ZnO/ZnCo2O4/C nanocages are efficiently deposited on rGO surface. As shown in Fig. S1, ZIF-67 has a regular rhombic dodecahedral morphology with the diameter of 400 ~ 500 nm. The SEM image of the ZIF-67/GO precursor (Fig. S2) presents that the ZIF-67 dodecahedrons are placing on the surface of GO and keeping the former morphology. The Co/C hybrid retains the same structure as ZIF-67 but its surface became rough and shrinks during the pyrolysis process, and the 11



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metallic cobalt nanoparticles are embedded into the carbon framework (Fig. S3). When Co/C dodecahedrons were supported on the reduced graphene oxide sheets, the particle size decreased to 100 ~ 200 nm (Fig. S4). The SEM images (Fig. 2a,b) of the ZnO/ZnCo2O4/C@rGO hybrid reveal that the ZnO/ZnCo2O4/C nanocages are uniformly grown on rGO sheets and 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, Fig. S5). The results greatly indicate that the available effect for rGO in avoiding the nanoparticles from aggregating. The EDS spectrum of the ZnO/ZnCo2O4/C@rGO hybrid (Fig. S6) shows the existence of C (83.79 At%), O (13.89 At%), Co (0.77 At%), and Zn (1.52 At%). The elemental distribution investigation was carried out by EDX mappings of the whole area (Figure 2c), which clearly reveals the coexistence and homogeneous distribution of Zn, Co, C, and O elements on rGO sheets, further demonstrating the formation of ZnO and ZnCo2O4. The low-resolution TEM images (Fig. 3a,b) show that the hollow ZnO/ZnCo2O4/C nanocages with 20 ~ 30 nm in size uniformly dispersed on reduced graphene oxide sheets. The detailed microstructure can be further observed by high-resolution TEM (HRTEM) (Fig. 3c), the hollow ZnO/ZnCo2O4/C nanocages are composed of numerous ZnO and ZnCo2O4 nanoparticles with the size of ~ 5 nm encapsulated into the carbon matrix. The HRTEM image of single ZnO/ZnCo2O4/C nanocage is exhibited as Fig. S7. The selected area electron diffraction (SAED) (Fig. 3d) indicates that the polycrystalline nature of ZnO/ZnCo2O4/C nanocages. The distinct concentric diffraction rings is ascribed to the the (101) and (103) planes of 12


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ZnO, and (220) and (511) planes of spinel ZnCo2O4, which is consistent with the XRD results. This unique structure allows facile accessibility of oxygen to contact with the effectively exposed nanoparticles to catalyze the oxygen reduction. Thermogravimetric analysis was performed in air from room temperature to 700 °C to evaluate the amount of rGO (TG-DTA, Fig. 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 can be ascribed to 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 porous characteristics of the ZnO/ZnCo2O4/C and ZnO/ZnCo2O4/C@rGO hybrids were characterized by N2 adsorption−desorption measurements. Fig. 4a shows typical IV isotherms and H4-type hysteresis loop of the materials, suggesting 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 greater 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 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 to provide effective exposure of catalytic active sites for ORR activity 32. The structural composition and the defect level of the nanomaterials were 13



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investigated by the Raman spectrum technique (Fig. 5a). Two representative Raman peaks of carbon materials at around 1390 and 1600 cm−1 are observed, corresponding to D band and G band, respectively. Generally, the intensity ratio of D band to G band, ID/IG, can be utilized for judging the degree of disorderd/ordered of 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 defect level increases caused by the introduction of rGO, which is due to the disorganized graphene edges by 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 (Fig.5b) suggests the presence of C (47.07 At%), O (28.44 At%), N (1.35 At%), Co (8.96 At%), and Zn (14.18 At%). In the high-resolution XPS spectrum of C1s (Fig. 5c), three obvious peaks can be observed, which are attributed to C-C (sp2), C-N, and C-O groups

34

. The high- resolution N1s

spectrum (Fig. 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 of nitrogen species into the ZnO/ZnCo2O4/C@rGO hybrid.

The predominant pyridinic N is known to paly positive effect in enhancing onset potentials towards ORR

36

. The high-resolution Zn2p spectrum shows two peaks at

1022.1 and 1045.1 eV，which are attributed to Zn2p3/2 and Zn2p1/2

22

. The Co2p

high-resolution spectrum (Fig. 5e) consists of two pairs of spin-orbit peaks, indicating the existence of Co2+ and Co3+

37

. The O1s spectrum (Fig. S9) shows three oxygen 14


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states, which are related to oxygen in ZnO and ZnCo2O4, and the absorbed oxygen-species or the residual oxygen-containing groups of rGO 38. 3.2 Electrocatalytic activity of the electrocatalysts for oxygen reduction reaction The ORR catalytic performance of the catalysts was first comparatively studied by CVs in O2-saturated 0.1 M KOH solution. Obvious oxygen reduction peaks between -0.1 and -0.4 V (vs Ag/AgCl) can be observed for all the catalysts in Fig. 6a, demonstrating their ORR electrocatalytic activity. The ZnO/ZnCo2O4/C@rGO hybrid shows a more positive oxygen reduction peak potential (-0.15 V) comparing with 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 the introduction of rGO plays the significant effect in promoting electrocatalytic activity. In addition, a series of linear LSVs on a rotating disk electrode (RDE) were acquired in O2-saturated 0.1 M KOH with a rotation rate of 1600 rpm to further study their ORR catalytic performance (Fig. 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), respectively. 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 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 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 15



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ORR pathway

Page 16 of 32

39

. As shown in Fig. 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), 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. Fig. 6d displays much higher ORR mass activity at -0.199 V of the ZnO/ZnCo2O4/C@rGO hybrid (0.51 A mg−1) compared with other catalysts, suggesting the optimal electrocatalytic activity. It is noted that the ORR catalytic performance of the ZnO/ZnCo2O4/C@rGO hybrid greatly surpasses those of the recently reported MOF-derived carbon based eletrocatalysts in alkaline condition (Table 1). The results highlight 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 plays 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 acts 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.

Table 1. Comparison of ORR electrocatalytic performance of the catalysts in this work with the recently reported MOF-derived carbon based electrocatalysts in alkaline solution a.

Catalyst

E1/2 (V vs Ag/AgCl)

Eonset (V vs Ag/AgCl)

Tafel slope (mV dec-1)

16


Durability

Reference

Page 17 of 32

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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

of j under -0.3 V

95.6% retention for 12000 s 99.7% retention ZnO/ZnCo2O4/C@rGO

-0.15

-0.05

46.70

of j under -0.3 V for 12000 s 75.6% retention

20% Pt/C

-0.18

-0.09

74.79

of j under -0.3 V for 12000 s

This work This work This work This work This work

93.0% retention Co-CNT/PC

undefined

-0.05

73.8

of j under 0.79 V

31

for 20000 s 96.0% retention Co-C@Co9S8 DSNCs

undefined

-0.005

undefined

of j under 0.50 V

40

for 5 h 84.0% retention

Co9S8@SNCC

-0.21

-0.12

80

of j under 0.60 V

41

for 15000 s 100% retention

Fe3C/b-NCNT

undefined

-0.005

71.2

of j under 0.78 V

42

for 70 h 70% retention of Fe3C@NC/NCS

-0.15

-0.08

undefined

j under -0.3 V

43

for 10 h 90.0% retention Co@Co3O4@C–CM

-0.15

-0.03

undefined

of j under 0.75 V

44

for 25000 s 85.6% retention N/Co-doped PCP//NRGO

undefined

-0.02

76

of j under 0.90 V

24

for 20000 s a

All the potential values here are vs. Ag/AgCl for comparison. In 0.1 M KOH electrolyte (pH=13), E(vs.

RHE)=E(vs. Ag/AgCl) + 0.1976 V + 0.059pH, converted from Ag/AgCl.

More detailed LSV studies in 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 (Fig. 6e) were carried out to further study the ORR mechanism of ZnO/ZnCo2O4/C@rGO hybrid. The corresponding Koutecky-Levich (K-L) plots at different potentials were exhibited in 17



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

Fig. 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 20 wt.% Pt/C catalyst (4.0, Fig. 7c,d), suggesting a high priority for four-electron pathway towards ORR. Furthermore, the n of ZnO/ZnCo2O4/C@rGO hybrid is much higher than that of the ZnO/ZnCo2O4/C hybrid (3.41, Fig. 7a,b), and the n of Co/C@rGO (3.93, Fig. S10a,b) is also higher than that of Co/C (3.44, Fig S10c,d), further proving the significant effect of rGO for electrocatalytic performance. The long-term stability of the ZnO/ZnCo2O4/C@rGO hybrid and the commercial Pt/C catalyst for ORR was comparatively investigated by chronoamperometric measurements (Fig. 8a). After 12000 s of reaction at 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 condition, and rGO can improve the stability. The stability of the ZnO/ZnCo2O4/C@rGO hybrid was also assessed by CVs test during 3000 continuous cycles (Fig. S11). The polarization curve shows no significant change in 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 of methanol into O2-saturated 0.1 M KOH solution during the chronoamperometric 18


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test

at

900

rpm.

Fig.

8b

shows

that

the

cathodic

ORR

current

of

ZnO/ZnCo2O4/C@rGO hybrid remains almost unchanged, whereas the current of 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 (Fig. 9a). Fig. 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 cathodic catalyst exhibits 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 great larger than that of the cell assembled with ZnO/ZnCo2O4/C. In addition, the voltage of the cell with assembled ZnO/ZnCo2O4/C decreased by ~ 0.20 V, whereas the voltage decreased by ~ 0.17 V of the cell assembled with ZnO/ZnCo2O4/C@rGO. The coin cell with ZnO/ZnCo2O4/C@rGO cathode displays 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 ZnO/ZnCo2O4/C cathode. The parameters of discharge performance are scheduled in Table 2. The discharge results further confirm better electrocatalytic 19



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Page 20 of 32

activity of the ZnO/ZnCo2O4/C@rGO hybrid, which is attributed to the motivation of rGO for the electrocatalytic activity.

Table 2. The 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 condition. 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

ZnO/ZnCo2O4/C ZnO/ZnCo2O4/C@rGO

4. Conclusions In summary, a novel reduced graphene oxide supported ZnO/ZnCo2O4 nanoparticles-embedded carbon hollow nanocages (ZnO/ZnCo2O4/C@rGO) hybrid was

synthesized

by

a

facile

hydrothermal-calcination

approach

using

a

ZIF-67/graphene oxide/zinc nitrate composite as the precursor. The nanocomposite material exhibits excellent electrocatalytic activity towards ORR, and superior stability and methanol tolerance under alkaline condition to that of 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 ORR electrocatalyst and inspires further direction in the development of reduced graphene oxide supported MOF-derived mesoporous carbon-based nanomaterials for ORR in Al-air batteries.

20


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Remarkably, the primary coin Al-air battery can be a promising electric device for practical use.

Acknowledgements 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), the open research Fund Program of Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological Environment Monitoring (Central South University), Ministry of Education.

Supporting Information 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

ZIF-67/GO

precursor,

Co/C

hybrid,

Co/C@rGO

hybrid

and

ZnO/ZnCo2O4/C hybrid, EDS spectrum of the ZnO/ZnCo2O4/C@rGO hybrid, High-resolution TEM image of the ZnO/ZnCo2O4/C nanocage, thermovimetry differential scanning calograrimetry (TG-DSC) curves and O1s 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. 21



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Figure Captions Scheme 1. Schematic illustration for the synthesis of Co/C@rGO and ZnO/ZnCo2O4/C@rGO hybrids. Figure 1. X-ray diffraction (XRD) spectrum of Co/C, Co/C@rGO, ZnO/ZnCo2O4/C and ZnO/ZnCo2O4/C@rGO hybrids, respectively. Figure 2. (a, b) FESEM images; (c) Elemental mapping images of C, O, Co and Zn in the ZnO/ZnCo2O4/C@rGO hybrid. Figure 3. (a,b,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, respectively. Figure 5. (a) Raman spectra of the Co/C@rGO, ZnO/ZnCo2O4/C and ZnO/ZnCo2O4/C@rGO hybrids, respectively. 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) C1s, (d) N1s, (e) Zn2p, and (f) Co2p, respectively. 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, respectively; (b) LSV curves of the Co/C, Co/C@rGO, ZnO/ZnCo2O4/C, ZnO/ZnCo2O4/C@rGO and 20 wt.% Pt/C electrocatalysts on 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 cures at 1600 rpm; (d) The mass activities at -0.199 V for all 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; (f) the corresponding Koutecky–Levich plots at different potentials. 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) the corresponding Koutecky–Levich plots at different potentials; (c) LSV curves of the 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; (d) the corresponding Koutecky–Levich plots at different potentials. 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 25



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O2-saturated 0.1 M KOH solution. The rotation rate is 900 rpm. The arrow indicates the introduction of 3 M methanol.

Figure 9. (a) The conformation of a coin Al-air cell; (b) The discharge curves of the coin Al-air batteries with the ZnO/ZnCo2O4/C and ZnO/ZnCo2O4/C@rGO electrocatalysts under ambient condition at a current density 1.0 mA cm−2, respectively; (c) Specific discharge capacity of the ZnO/ZnCo2O4/C and ZnO/ZnCo2O4/C@rGO hybrids. Table 1. Comparison of ORR electrocatalytic performance of the catalysts in this work with the recently reported MOF-derived carbon based electrocatalysts in alkaline solution. Table 2. The 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 condition.

Scheme 1

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Figure 1

Figure 2

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Figure 3

Figure 4

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Figure 5

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Figure 6

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Figure 7

Figure 8

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254x101mm (300 x 300 DPI)


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