reduced graphene oxide as an efficient electrocatalyst for oxygen reduction reaction

Accepted Manuscript One-pot hydrothermal synthesis of Zinc ferrite/reduced graphene oxide as an efficient electrocatalyst for oxygen reduction reaction Wei Hong, Lingzhi Li, Ruinan Xue, Xiaoyang Xu, Huan Wang, Jingkuo Zhou, Huilin Zhao, Yahui Song, Yu Liu, Jianping Gao PII: DOI: Reference:

S0021-9797(16)30255-7 http://dx.doi.org/10.1016/j.jcis.2016.04.035 YJCIS 21225

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

8 March 2016 20 April 2016 21 April 2016

Please cite this article as: W. Hong, L. Li, R. Xue, X. Xu, H. Wang, J. Zhou, H. Zhao, Y. Song, Y. Liu, J. Gao, Onepot hydrothermal synthesis of Zinc ferrite/reduced graphene oxide as an efficient electrocatalyst for oxygen reduction reaction, Journal of Colloid and Interface Science (2016), doi: http://dx.doi.org/10.1016/j.jcis. 2016.04.035

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One-pot hydrothermal synthesis of Zinc ferrite/reduced graphene oxide as an efficient electrocatalyst for oxygen reduction reaction Wei Honga, Lingzhi Lia, Ruinan Xuea, Xiaoyang Xua, Huan Wanga, Jingkuo Zhoua, Huilin Zhaoa, Yahui Songa, Yu Liua,b,*, Jianping Gaoa,b,* a

School of Science, Tianjin University, Tianjin 300072, PR China

b

Collaborative Innovation Center of Chemical Science and Engineering Tianjin 30072, PR China

Abstract Fabrication of low-cost and efficient electrocatalyst for oxygen reduction reaction (ORR) is highly desirable. Herein, Zinc ferrite/reduced graphene oxide (ZnFe2O4/rGO) is prepared by a quite simple and environmentally benign approach and applied as a high performance ORR electrocatalyst for the first time. The surface morphology and chemical composition of ZnFe2O4/rGO are characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, thermogravimetric analysis and Fourier transform infrared spectroscopy. Cyclic

voltammetry, linear sweep voltammetry and

chronoamperometry are used to evaluate the electrochemical activities and stabilities of ZnFe2O4/rGO catalysts in alkaline media. Among ZnFe2O4/rGO with different mass ratios, the catalyst with 69.8 wt% ZnFe2O4 (called ZnFe2O4/rGO (3)) has the best catalytic activities and it shows much superior methanol tolerance and better durability than the commercial Pt/C catalyst. Due to the synergistic effect, the ZnFe2O4/rGO (3) nanohybrid exhibits high ORR catalytic performance and durability, which follows a desirable four electron transfer mechanism in alkaline media.

Therefore, it may be a highly competitive catalyst for fuel cells and metal-air batteries. Keyword: ZnFe2O4 ; reduced graphene oxide; eletrocatalyst; oxygen reduction reaction; nanohybrid; fuel cells 1. Introduction With the growing demand for fossil fuels and increasing environmental problems from burning fossil fuels, finding clean and new energies has become a hot topic. Due to the low or zero emissions, fuel cells have attracted enormous attention. The oxygen reduction reaction (ORR) plays an important role in fuel cells. However, the ORR kinetics at the cathode is very slow, so a electrocatalyst is needed to accelerate the kinetics [1]. As we know, Pt and Pt-based materials are considered the most reliable ORR electrocatalysts [2]. Nevertheless, the high-cost and element scarcity of Pt hinder the practical application of fuel cells and metal-air batteries [3]. In this regard, it turns out to be a focal task to reduce Pt content or explore high performance electrocatalysts at low cost for ORR that can replace the commercial Pt/C. Many efforts are now mainly devoted to the metal-free [4] and non-precious metal [5] catalysts, including B [6], N [7], I [8] and S [9] doped carbon materials, transition metal chalcogenides [10], transition metal carbides [11] and transition metal oxides [12]. Among these non-precious catalysts, transition metal-based oxide materials have gained special interest. Due to the prominent advantages of low price, high activity and stability as well as environmental friendliness [13], the mixed valence oxides of transition metals with

a spinel structure (AB2O4) have been widely reported in lithium ion batteries [14], supercapacitors [15] and eletrocatalysts [16]. As non-precious metal ORR catalysts, they also exhibited good performance with superior methanol tolerance and better durability in alkaline medium [13]. Therefore, investigation of these catalyst for ORR are becoming more and more intrigued [17-19]. However, the electrocatalytic activities of Zinc ferrite (ZnFe2O4) for the ORR are still unreported. In order to obtain good catalytic activity, spinel oxides are usually supported on conducting surface to ensure fast electron transport since this type of oxide is a semiconductor. When carbon nanotubes and carbon black are used as support materials, the catalysts on their external surface easily peel off from the surface of carbon nanotubes or carbon black due to the low interaction between the catalysts and carbon supports. Good chemical, high electrical conductivity, large surface area and open pores of graphene sheets [20-22] makes it an ideal substrate for spinel oxides [23, 24]. To the best of our knowledge, no paper involving ZnFe2O4/rGO nanohybrid as a catalyst for ORR has been reported. In this work, we demonstrate the synthesis of ZnFe2O4/rGO nanohybrids by a one-pot hydrothermal strategy at different mass ratios by using rGO as the support. The electrocatalytic activities of the as-synthesized ZnFe2O4/rGO nanohybrids for the ORR in alkaline medium have been studied using a rotating disk electrode (RDE) and cyclic voltammetry (CV) technique. Compared with ZnFe 2O4 and rGO, the ZnFe2O4/rGO (3) nanohybrid exhibited the enhanced electrocatalytic activities for ORR.

2. Experimental 2.1. Materials 20% Pt/C powder was purchased from Alfa Aesar, and Nafion (5 wt%) was from DuPont. Natural graphite powder was from Qingdao Graphite Factory. Potassium permanganate, sodium nitrate, concentrated sulfuric acid, hydrogen peroxide (30%), hydrochloric acid, ethylene glycol (EG), iron nitrate, zinc nitrate, sodium acetate anhydrous were all from Sigma. All other chemicals were purchased from Tianjin Chemical Reagent Co. All reagents are analytical grade and they were used as received. 2.2. Synthesis of ZnFe2O4/rGO GO was prepared from purified natural graphite by a modified Hummer ,s method [25]. The concentration of the final GO aqueous dispersion used for ZnFe2O4/rGO preparation was 2.6 mg/mL. For a typical synthetic process of ZnFe2O4/rGO nanohybrids, 2 mmol Fe(NO3)3·9H2O and 1 mmol Zn(NO3)3·6H2O were dispersed into 10 mL of ethylene glycol by stirring for 30 min, and then 1.35 g sodium acetate (NaAc) was added and stirred for another 30 min at room temperature. After that, 32 mL of GO dispersion was added to the above mixture solution and stirred for 30 min. Then, the solution was transferred to a 100 mL Teflon-lined stainless steel autoclave and heated at 180 °C for 12h under autogenous pressure. After being cooled down in air to room temperature, the black precipitate was centrifuged, washed with distilled water and ethanol for several times, and finally dried at 50 °C for 12h. The weight ratio of ZnFe2O4 to GO can be adjusted by changing the ratios of Zn-Fe precursors to

GO, and the resultant ZnFe2O4/rGO nanohybrids are designated as ZnFe2O4/rGO (1), ZnFe2O4/rGO (2), ZnFe2O4/rGO (3) and ZnFe2O4/rGO (4) when ZnFe2O4/GO theoretical weight ratios are 0.91, 1.84, 2.75 and 3.67, respectively. For comparison, the ZnFe2O4 nanoparticles were synthesized under the same conditions without addition of GO while rGO was prepared without using any salts. 2.3. Characterization 2.3.1. X-ray diffraction analysis The X-ray diffraction (XRD) patterns of the samples were measured via an X-ray diffractometer (Rigaku D/Max 2200PC) instrument: CuKα radiation (λ=0.15418 nm) with the voltage and electric current setted at 28 kV and 20 mA at room temperature. The samples were measured in the scattered angle of 10° to 80° (2θ) with steps of 4 °/min. 2.3.2. Thermogravimetric analysis and differential scanning calorimetry analysis Prior to thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), the samples were dried in a vacuum at 40 °C for 3 days. TGA and DSC were performed with a Rigaku-TD-TDA analyzer in air atmosphere with a heating rate of 10 °C/min. 2.3.3. Fourier transform infrared spectroscope analysis Fourier transform infrared (FT-IR) spectra of the samples were recorded on a Perkin-Elmer Paragon-1000 FT-IR spectrometer in the range of 400-4000 cm-1. 2.3.4. Transmission Electron Microscopy and Scanning Electron Microscope analysis Transmission electron microscopy (TEM) was performed using a Philips Tecnai

G2F20 microscope at 200 KV. The surface morphology of the samples was observed by scanning electron microscope (SEM, S-4800, HITACHI, Japan). In order to get the real morphology, the samples were dispersed in ethanol by sonication at the first, and then they were deposited on a silicon wafer (for SEM) or copper grids coated with a carbon film (for TEM), respectively. 2.3.5. X-ray photoelectron spectroscope analysis Elemental analysis was performed on an X-ray photoelectron spectrometer (XPS) with a Mg Kα anode (PHI1600 ESCA System, PERKIN ELMER, US). 2.4. Electrochemical measurements The electrochemical measurements were carried out on a PINE instrument using a CHI 660D electrochemical station (CH Instruments, Inc, Shanghai) at 25 °C. A three-electrode configuration was used, a Pt mesh and a Hg/HgO (1.0 M KOH) electrode were used as counter and reference electrodes, respectively. The working electrode was prepared by dropping 10 μL of the catalyst ink onto the rotating disk electrode with geometric surface area of 0.196 cm2. The ink was prepared by ultrasonically mixing 4 mg of catalyst with 1.9 mL of ethanol and 0.1 mL of Nafion solution for about 1 h. The electrolyte was 0.1 M KOH or 0.1 M KOH/0.5 M CH3OH. The solutions were bubbled with pure N2 or O2 for 20 min. 3. Results and discussion Fig. 1 shows the XRD patterns of the ZnFe2O4/rGO (3), ZnFe2O4 and rGO. The characteristic peaks in ZnFe2O4/rGO (3) can be indexed as cubic spinel structure (PDF#65-3111) except for the peak at around 24° that is attributed to (002) peak of

rGO. The characteristic peaks observed at 2θ of 18.2°, 29.9°, 35.2°, 36.8°, 42.9°, 53.2°, 56.5°, 62.2°, 70.4°, 73.3°, 74.6° and 78.4° are attributed to the crystal planes of (111), (220), (311), (222), (400), (422), (511), (440), (620), (533), (622) and (444), respectively. Furthermore, the strong intensity diffraction peaks suggest that both pure

（ 444）

（ 533） （ 622）

（ 620）

(440)

(511)

(422)

(400)

（ 222）

（ 444）

（ 533） （ 622）

（ 620）

(511)

C(100)

(422)

(400)

(440)

ZnFe2O4/rGO (3)

（ 222）

(311)

(220) (220)

(111)

Intensity

(111)

C(002)

(311)

ZnFe2O4 and ZnFe2O4 in ZnFe2O4/rGO nanohybrid are well crystallized.

ZnFe2O4

rGO

10

20

30

40

50

60

70

80

(degree)

Fig. 1. XRD patterns of ZnFe2O4, rGO and ZnFe2O4/rGO (3) nanohybrid. The surface morphology and structure of the as-prepared ZnFe2O4/rGO were examined by SEM, TEM and high-resolution TEM (HRTEM). Fig. 2a shows the SEM image of ZnFe2O4/rGO (3) nanohybrid. As can be seen, ZnFe2O4 nanoparticles are densely dispersed on rGO nanosheets. Fig. 2b shows the TEM image of pure rGO, indicating the wrinkle textures. As shown in Fig. 2c, ZnFe2O4 nanoparticles show a diameter of 6-20 nm and are uniformly dispersed on the rGO surface without serious aggregation. The high-resolution TEM image of an individual ZnFe2O4 nanoparticle,

presented in Fig. 2d, exhibits a clear crystal lattice with a spacing of 0.25 nm, corresponding to the interplanar spacing of (311) plane of ZnFe 2O4 cubic structure.

Fig. 2. SEM image of ZnFe2O4/rGO (3) nanohybrid (a); TEM images of pure rGO (b) and ZnFe2O4/rGO (3) nanohybrid (c); High-resolution TEM (d) of ZnFe2O4/rGO (3)

nanohybrid. FTIR spectra of GO, rGO and ZnFe2O4/rGO (3) nanohybrid are recorded and shown in Fig. 3. In the spectrum of GO, the characteristic peaks at 3410 cm-1, 1740 cm-1, 1610 cm-1, 1415 cm-1, 1240 cm-1 and 1065 cm-1 are assigned to stretching bonds of O-H, C=O, aromatic C-C, carboxyl C-O, epoxy C-O and alkoxy, respectively, which are in good agreement with earlier report [26]. For rGO and ZnFe2O4/rGO (3), peaks for oxygen function groups of rGO are markedly weakened or completely disappeared; and meanwhile new bands at around 544 cm-1 and 423 cm-1 appeared for ZnFe2O4/rGO (3). The band at 544 cm-1 can be identified to tetrahedral Zn2+ (Zn-O mode) stretching vibration, and the band observed at 423 cm-1 can be attributed to the octahedral Fe3+ (Fe-O mode) stretching vibration [27,28]. The above FTIR results demonstrated effective reduction of the GO and formation of a composite containing ZnFe2O4 nanoparticles and rGO sheets.

ZnFe2O4/rGO (3)

Zn-O

Transmittance

rGO

Fe-O

GO C=O aromatic C-C

alkoxy epoxy C-O carboxyl C-O

O-H

4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm )

Fig. 3. FTIR spectra of GO, rGO and ZnFe2O4/rGO (3) nanohybrid. The content of ZnFe2O4 in the ZnFe2O4/rGO nanohybrid was investigated by TG and DSC (Fig. 4). It can be observed that there are two steps of weight decrease as the temperature increases. The weight loss of 2.8% at the first stage from 30 to 150 °C is attributed to the evaporation loss of physically absorbed water in the ZnFe2O4/rGO nanohybrids [29]. The second stage with weight loss of 27.4% from 150 to 800 °C should be ascribed to the burning of rGO in the nanohybrid. A big exothermic peak at about 400 °C in DSC curve corresponds to the reaction between rGO and O 2. Therefore, based on the above analysis, ZnFe2O4 content in the ZnFe2O4/rGO (3) nanohybrid is about 69.8%.

100 12

95 10

Weight (%)

8

85 6

80 4

DSC (mW/mg)

90

75 2

70 0

65 0

100

200

300

400

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700

800

Temperature (C)

Fig. 4. TG and DSC curves of the ZnFe2O4/rGO (3) nanohybrid in air atmosphere. XPS measurements were performed to study the surface element composition and cation oxidation state of ZnFe2O4/rGO (3) nanohybrid. The signals of Fe, Zn, C and O can be identified from the survey spectrum (Fig. 5a), and no obvious peaks for other elements have been observed. The Fe 2p3/2 and Fe 2p1/2 peaks (Fig. 5b) at 711.1 eV and 725.1 eV binding energy are due to the Fe3+ at octahedral sites [30]. The peaks at binding energy of 1021.6 eV and 1044.9 eV (Fig. 5c) can be attributed to Zn 2p3/2 and Zn 2p1/2 of Zn2+ [31]. The spectrum of O 1s with an additional shoulder can be divided into three main peaks centered at 530.1 eV, 531.5 eV and 532.7 eV, respectively. The peaks at 530.1 eV should be attributed to oxygen atoms in the oxides of ZnFe2O4, while the other two fitting peaks can be assigned to chemisorbed or dissociated oxygen or OH species on the surface of ZnFe 2O4 [32, 33].

Intensity (a.u.)

1000

730 800

725

720 600

Zn 3s Zn 3p Fe 3p

C 1s

O 1s Zn LMM

Fe 2p1/2 Fe 2p3/2

Intensity (a.u.)

Zn 2p1/2 Zn 2p3/2 O KLL

a

400

b

715

Binding energy (eV) 200

710

0

Binding Energy (eV)

Fe 2p3/2

Fe 2p1/2

705

c

Intensity (a.u.)

Zn 2p3/2

Zn 2p1/2

1055

1050

1045

1040

1035

1030

1025

1020

Binding Energy (eV)

d

Intensity (a.u.)

O 1s 530.1 eV

O 1s 532.7 eV

540

538

536

534

O 1s 531.5 eV

532

530

528

526

Binging Energy (eV)

Fig. 5. XPS spectra of the ZnFe2O4/rGO (3) nanohybrid: survey spectrum (a); Fe 2p spectrum (b); Zn 2p spectrum (c); and O 1s spectrum (d). To evaluate the electrocatalytic performance of these ZnFe2O4/rGO nanohybrids

for ORR at different mass ratio, the liner-sweep voltammetry (LSV) curves of the ZnFe2O4/rGO electrodes are shown in Fig. 6. It can be observed that ZnFe2O4/rGO (3) shows superior activity than other ZnFe2O4/rGO nanohybrids in Fig. 6 and Table S1. The enhanced electrocatalysis of ZnFe2O4/rGO (3) is mainly due to the small size and excellent dispersibility of ZnFe2O4. On one hand, with the increase of ZnFe2O4 content, the active materials have been increased, which contributes to enhancing the activity. On the other hand, ZnFe2O4 particles tend to agglomerate to reduce the surface energy when concentrations of ZnFe2O4 precursors are too high, which will form particles with large size and poor dispersion. This trend is against the improvement of electrocatalytic activity.

-1

-2

Current Density (mAcm )

0

-2

-3

ZnFe2O4/rGO (1)

-4

ZnFe2O4/rGO (2) ZnFe2O4/rGO (3)

-5

ZnFe2O4/rGO (4)

-0.8

-0.6

-0.4

-0.2

0.0

0.2

Potential (V vs.Hg/HgO)

Fig. 6. linear-sweep voltammetry curves of ZnFe2O4/rGO with different ZnFe2O4 contents in oxygen-saturated 0.1 M KOH at a scan rate of 10 mV s -1 and a rotation rate of 1600 rpm.

The ORR electrocatalytic activity of ZnFe2O4, rGO and ZnFe2O4/rGO (3) were investigated by CV. As shown in Fig. 7a-c, all the CV curves in the N2-saturated atmosphere show no ORR peaks. The CV of ZnFe2O4 in the O2-saturated electrolyte shows a reduction peak at -0.344 V, and rGO shows a reduction peak at around -0.258 V. As for ZnFe2O4/rGO (3) catalyst, the reduction peak is observed at -0.232 V, which is more positive than these of ZnFe2O4 (-0.344 V) and rGO (-0.258 V). The positive reduction potential and large reduction current observed at ZnFe2O4/rGO (3) indicates that ZnFe2O4/rGO (3) catalyst has better ORR activity than ZnFe2O4 and rGO.

a 0.06 0.04

-2

Current Density (mAcm )

0.02 0.00 -0.02 -0.04 -0.06 -0.08 -0.10 -0.12

N2

-0.14

O2

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Potential (V vs.Hg/HgO)

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Current Density (mAcm )

0.2

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N2 O2

-1.0 -0.8

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Potential (V vs.Hg/HgO)

c

0.4 0.2

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Current Density (mAcm )

0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 N2

-1.4

O2

-1.6 -0.8

-0.6

-0.4

-0.2

0.0

0.2

Potential (V vs.Hg/HgO)

Fig. 7. CV curves of ZnFe2O4 (a), rGO (b) and ZnFe2O4/rGO (3) (c) modified electrodes in O2 and N2-saturated 0.1 M KOH solution with a scan rate of 50 mV s -1. The LSV curves of the rGO, ZnFe2O4 and ZnFe2O4/rGO (3) in the

oxygen-saturated 0.1 M KOH, which was measured at a rotation rate of 1600 rpm, are displayed in Fig. 8. It can be observed that the onset potential of ZnFe 2O4/rGO (3) nanohybrid is -0.08 V, which is more positive than ZnFe2O4 (-0.20 V) and rGO (-0.09 V). Moreover, the ZnFe2O4/rGO (3) nanohybrid shows the maximum peak current density among these three electrodes. So the ZnFe2O4/rGO (3) nanohybrid outperforms the ZnFe2O4 and rGO in electrocatalytic activity for ORR, which is in agreement with the CV results. Due to two-dimensional structure of rGO, the O2 can easy access to the active sites from both sides for ORR. Meanwhile, the high electric conductivity of rGO can ensure ZnFe2O4/rGO (3) having high electric conductivity, results in increasing of ORR activity. Obviously, the high electrocatalytic activity of ZnFe2O4/rGO (3) could be attributed to the synergy between the two components.

1

-2

Current Density (mAcm )

0

-1

-2

-3

-4 ZnFe2O4/rGO (3) rGO

-5

ZnFe2O4

-0.8

-0.6

-0.4

-0.2

0.0

0.2

Potential (V vs.Hg/HgO)

Fig. 8. linear-sweep voltammetry curves of ZnFe 2O4, rGO and ZnFe2O4/rGO (3) in oxygen-saturated 0.1 M KOH at a scan rate of 10 mV s -1 and a rotation rate of 1600

rpm. Fig. 9a-c reveals the LSV curves of ZnFe2O4, rGO and ZnFe2O4/rGO (3) at different rotating speeds in a potential window of 0.20 to -0.8 V. With increasing rotation speeds, an increase in the limiting current density could be seen. The Koutecky-Levich (K-L) equation is often used to identify the mechanism of the ORR [34].

1 1 1   J J k B 0.5

(1)

J k  nFkCO2

(2)

Where J and Jk are the measured disk and kinetic current density, respectively; B is related to the diffusional current density; F is the Faraday constant (96485 C·mol-1); ω is the electrode rotation speed; k is the electron-transfer rate constant. B could be expressed using the following equation: 2

B  0.2nFD 3

1

6

CO2

(3)

Where υ represents the kinematic viscosity of the electrolyte (1.13×10 -2 cm-2 s-1); n is the number of electron exchanged during ORR process; CO2 is the bulk concentration of oxygen in electrolyte (1.2×10-6 mol cm-3); D is the diffusion coefficient of oxygen (1.9×10-5 cm2 s-1) [35]. The K-L plot of J-1 vs ω-1/2 at a potential of -0.5 V is shown in Fig. 9d. We can see a good linear relationship between J-1 and ω-1/2. The electron transfer number n is calculated from equation (3) based on the slope (1/B) of the K-L plot. The electron transfer number for ZnFe2O4, rGO and ZnFe2O4/rGO (3) are 3.5, 3.73 and 3.89 respectively, which clearly indicate that ZnFe2O4/rGO (3) catalyst follows a desirable four-electron pathway of the ORR.

a

0.0

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Current Density (mAcm )

-0.5

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-1.5 400 800

-2.0

1200 1600 2000

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2400

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Potential (V vs.Hg/HgO)

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800 1200 1600 2000

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d 1.0

n=3.5 0.9

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J (cm mA )

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-1

0.6

n=3.7

0.5

n=3.9 0.4

ZnFe2O4/rGO (3) rGO ZnFe2O4

0.3 0.020

0.025

0.030

0.035 -1/2



(rpm

0.040

0.045

0.050

-1/2

)

Fig. 9. LSV curves of ZnFe2O4 (a), rGO (b) and ZnFe2O4/rGO (3) (c) catalysts in O2 saturated 0.1 M KOH (scan rate: 10 mV s-1) recorded at various rotation rates (400-2400 rpm). K-L plots of ZnFe2O4, rGO and ZnFe2O4/rGO (3) at -0.5 V (d).

Fig. 10 shows CV curves of ZnFe2O4/rGO (3) and Pt/C in oxygen-saturated 0.1 M KOH in the presence (red line) or absence (black line) of 0.5 M methanol, in order to investigate the ability of the catalysts against methanol crossover. The CV curves for ZnFe2O4/rGO (3) show no peak of methanol oxidation and the peak position of oxygen reduction has only a slight shift in 0.5 M methanol, demonstrating that ZnFe2O4/rGO (3) catalyst shows a good methanol tolerance for ORR (Fig. 10a). In contrast, the commercial Pt/C catalyst has an obvious peak of methanol oxidation (Fig. 10b), indicating that the ability of Pt/C catalyst against methanol crossover is poor. Compared with the commercial Pt/C catalyst, ZnFe2O4/rGO (3) has a better methanol tolerance which suggests that ZnFe2O4/rGO (3) may be used as methanol-tolerant catalysts for ORR.

a

0.4 0.2

-2

Current Density (mAcm )

0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4 -1.6 -0.8

-0.6

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Potential (V vs.Hg/HgO)

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0.2

b

-2

Current Density（ mAcm )

3

2

1

0

-1

-2 -0.8

-0.6

-0.4

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Potential (V vs.Hg/HgO)

Fig. 10. CV curves of ZnFe2O4/rGO (3) (a) and Pt/C (b) in oxygen-saturated 0.1 M KOH (black line) and in the presence of 0.5 M methanol (red line). Scan rate: 50 mV s-1. In addition, the durability of ZnFe2O4/rGO (3) for the ORR was evaluated by the chronoamperometric technique under the potential of -0.4 V for 7200 s in oxygen-saturated 0.1 M KOH at 1600 rpm. As seen in Fig. 11, the current density of the Pt/C catalyst decreases faster than that of ZnFe2O4/rGO (3) catalyst. The result reveals that ZnFe2O4/rGO (3) catalyst is more stable than the commercial Pt/C catalyst.

100

90

i/i0 (%)

80

70

60 Pt/C ZnFe2O4/rGO (3)

50 0

1000

2000

3000

4000

5000

6000

7000

Time (s)

Fig. 11. Chronoamperometric curves of ZnFe2 O4/rGO (3) and Pt/C in oxygensaturated at 1600 rpm. 4. Conclusions In summary, we have demonstrated the synthesis of ZnFe2O4/rGO nanohybrids with different mass ratios by a facile one-pot hydrothermal strategy. Among these ZnFe2O4/rGO nanohybrids, the catalyst with 69.8 wt% ZnFe2O4 has the best electrocatalytic activities. The results of the electrochemical measurements demonstrate that the ZnFe2O4/rGO (3) catalyst follows a desirable four-electron pathway of the ORR. At the same time, it can be found that the ZnFe2O4/rGO (3) catalyst shows better durability and methanol tolerance than the commercial Pt/C catalyst. The good catalytic activity, durability and methanol tolerance of ZnFe2O4/rGO (3) catalyst are attributed to the strong coupling and synergistic effect between ZnFe2O4 and rGO. As a result, our work provides a new choice for

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