Reduced graphene oxide supported chromium oxide hybrid as high efficient catalyst for oxygen reduction reaction

Reduced graphene oxide supported chromium oxide hybrid as high efficient catalyst for oxygen reduction reaction

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Reduced graphene oxide supported chromium oxide hybrid as high efficient catalyst for oxygen reduction reaction Changlan Wen a,1, Xueping Gao b,1, Taizhong Huang a,*, Xiaoying Wu a, Luping Xu c, Jiemei Yu a, Haitao Zhang a, Zhaoliang Zhang a,**, Jitian Han d, Hao Ren a a

Shandong Provincial Key Laboratory of Fluorine Chemistry and Chemical Materials, School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, China b School of Materials Science and Engineering, Shandong University, Jinan 250061, China c Xi'an Modern Chemistry Research Institute, Xi'an, 710065, China d School of Energy and Power Engineering, Shandong University, Jinan 250061, China

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abstract

Article history:

Reduced graphene oxide (rGO) supported nano-chromium oxide (Cr2O3/rGO) catalyst for

Received 8 February 2016

oxygen reduction reaction (ORR) has been successfully synthesized by the pyrolysis of

Received in revised form

chromium-urea coordination compound. The structure and morphology of the hybrid are

8 May 2016

investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and high-

Accepted 8 May 2016

resolution transmission electron microscopy (HRTEM) tests. XRD tests reveal that the

Available online xxx

Cr2O3 with hexagonal structure is obtained. SEM and TEM tests show that the nano-Cr2O3 is supported by rGO sheet. The cyclic voltammetry, tafel, linear scanning voltammetry and

Keywords:

current-time chronoamperometric tests prove that the obtained Cr2O3/rGO hybrid has a

Oxygen reduction reaction

remarkable catalytic activity and good stability for oxygen reduction. Both the rotating disc

Electrocatalysis

electrode and rotating ring disc electrode tests approve that the ORR major happens

Reduced graphene oxide

through 4-electron reaction style. The Cr2O3/rGO hybrid is a promising low cost and high

Chromium oxides

performances catalyst for ORR of alkaline electrolyte. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Catalysts for oxygen reduction and evolution reactions are key factors to novel-energy technologies, such as fuel cells, water splitting and etc. Despite tremendous efforts have been made, developing oxygen reduction catalysts with high activity and low cost still remains a great challenge for scientists.

Traditionally, Pt-based catalysts are regarded as the most promising catalysts for oxygen reduction reaction (ORR). Unfortunately, rare resources, high cost, and sluggish oxygen reduction catalytic activity limited the large scale applications of Pt-based catalysts [1e3]. To eliminate the dependence on the Pt-based catalysts for oxygen reduction, great efforts are being taken to develop precious metal free catalysts with high

* Corresponding author. Tel.: þ86 531 89736103; fax: þ86 531 82765969. ** Corresponding author. E-mail addresses: [email protected] (T. Huang), [email protected] (Z. Zhang). 1 These authors are equally contributed to the paper. http://dx.doi.org/10.1016/j.ijhydene.2016.05.051 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Wen C, et al., Reduced graphene oxide supported chromium oxide hybrid as high efficient catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.051

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cost effectiveness and comparable or even better catalytic performances [4,5]. Metal oxide is one of the most promising alternatives for Pt-based catalysts among all the being investigated catalysts. The catalytic performances of Mn2O3, CoO and spinel-type LiCoO2 for ORR have been investigated [6e8]. The benefits of abundant in sources, low cost, high chemical and electrochemical stability make the metal oxide based catalysts to be hopeful alternatives for Pt-based precious metal catalysts for ORR [9e11]. Investigations on transitional metal oxides for ORR have been conducted. Results showed that the non-precious transitional-metal oxides, including binary and mixed oxides, had great potential to be alternative catalysts with high activity and stability for ORR [12,13]. Liang et al. reported that the rGO supported Co3O4 nano-crystals was a high-performance catalyst for both ORR and oxygen evolution reaction (OER) [14]. Similarly, the Cu2O/rGO composite also exhibited remarkable electrocatalytic activity for ORR in alkaline fuel cells [15]. It is believed that the metal oxides, such as titanium oxide, tungsten oxide, molybdenum oxide, etc., could be adopted as independent electro-catalysts for ORR [16]. But, in fact, each kind of metal oxide has some drawbacks for practical applications. More efforts are being taken to develop high efficiency non-precious transitional metal oxide based catalysts for ORR. Chromium oxides, Cr2O3, a trivalent chromium (Cr(III)) oxide, is an important industrial material that has been adopted as abrading agents and pigments [17]. Cr2O3 is chemically stable and insoluble in both acidic and alkaline medium [18]. But the conductivity of Cr2O3 is rather weak, which inhibits its application in electrochemical research fields. Graphene, an ultra thin two-dimensional carbonaceous material, has attracted tremendous attention in the scientific community due to its excellent electronic conductivity and mechanical properties [19]. Graphene supported non-precious metal oxides based catalysts have showed great promise for ORR [20e22]. Reduced graphene oxide (rGO) supported metal oxide for fuel cells had been investigated [23e25]. The rGO based materials also adopted in sensors with the aim of enhancing conductivity [26e29]. But the structure and catalytic characteristics of graphene supported Cr2O3 have never been reported. In this work, we synthesized the rGO supported nano-Cr2O3 (Cr2O3/rGO) catalysts for ORR by the pyrolysis of the graphene oxide supported chromium-urea coordination compound. The obtained Cr2O3/rGO showed surprisingly high catalytic activity and electrochemical stability for ORR in alkaline electrolyte. The peak current intensity was above three times higher than that of Pt/C catalyst. To the best of our knowledge, this is the first report on the rGO supported chromium oxide as catalyst for ORR. This research explores a new kind of high-performance, low-cost, and rich sources catalyst for ORR.

previously [30]. All the other chemicals are analytical reagents and used as received without any further treatment. 1 g Cr(NO3)39H2O was dissolved in 50 ml absolute ethanol and 16.0 ml graphene oxide (GO) (3 mg/ml in DI water). And the mixture was ultra-sonicated 30 min. Then saturated urea/ ethanol solution was added dropwise into the solution at 75e80  C with violent stirring until the final mole ratio of Cr3þ to urea reached 1:6 [31]. Finally we got the mixture of GO and green precipitation of Cr-urea coordination compound. The mixture was filtered and the solid was dried at 80  C. The obtained metal coordination had also been adopted as catalysts for ORR [32]. The coordination compound was moved into a quartz boat and heated in nitrogen atmosphere with the flowing rate of 0.5 L/min at 600  C for 2 h with a quartz tube furnace. The hybrid of Cr2O3/rGO was obtained. And the loading content of Cr2O3 was 3.9 mg per 1 mg rGO. The heating temperature was determined according to the thermal gravimetric and differential scanning calorimetry (TG-DSC) tests using Perkin Elmer STA6000. The TG-DSC test results are shown in Fig. 1. It was clearly observed that there were no peaks appeared anymore when the temperature above 500  C, which meant that the structure of the obtained material reached a stable state. To ensure the material's structure, 600  C was adopted to prepare the catalyst.

Structure and electrochemical tests The morphology of the as-prepared catalyst, the selected area electron diffraction (SAED) pattern, the energy-dispersive Xray spectroscopy (EDS), and the elemental mapping were obtained by using a Tecnai 20 U-TWIN transmission electron microscope (TEM) and a Hitachi (S-4800) scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectroscopy analyzer. Powder X-ray diffraction (XRD) test was performed on a Bruker D8-advance diffractometer using Cu-Ka1 radiation. The data were collected between the diffraction angles (2 theta) of 10 and 80 . Based on the XRD test results, the corresponding cell parameters of Cr2O3 were calculated by using the Jade software. X-ray photoelectron spectroscopy (XPS) tests were conducted using a Perkin Elmer PHI5300 spectrometer with a Mg Ka source. The electro-chemical performances tests were conducted by using a CHI 760D electrochemical workstation (CHI Inc.,

Experimental Preparation of Cr2O3/rGO catalyst The graphene oxide was prepared from graphite by using improved Hummer's method that had been reported

Fig. 1 e TG-DSC tests of Cr-urea coordination compound.

Please cite this article in press as: Wen C, et al., Reduced graphene oxide supported chromium oxide hybrid as high efficient catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.051

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USA) with a three-electrode system consisting of a glassy carbon disk as working electrode, a Pt wire as counter electrode and an Ag/AgCl as reference electrode. A RRDE-3A electrode was adopted for rotating disc electrode (RDE) tests and rotating ring disc electrode (RRDE) tests. The working electrodes were prepared as follows. Typically, catalyst dispersions were prepared by mixing 5 mg of the catalyst powder in 50 ml Nafion solution (5.0% Nafion in ethanol) and 450 ml DI water, followed by 30 min ultrasonication. The glassy carbon disk electrodes (3 mm diameter, 0.07068 cm2 surface area) served as the substrate and were polished to a mirror surface. 50 ml of the prepared catalyst suspension was pipetted onto the glassy carbon disk electrodes and then fully dried. The loading of Cr2O3/rGO on the electrode was o.5 mg, which meant that the loading density was7.07 mg/cm2. The working electrode was first cycled between 0.2 and 0.8 V at a sweeping rate of 100 mV s1 in an Ar-saturated KOH solution (0.1 M) at room temperature until reproducible cyclic voltammetry (CV) results were obtained. Then, the CV measurements of oxygen reduction were conducted by cycling the potential between 0.2 and 0.8 V in oxygen saturated 0.1 M KOH electrolyte. The linearly sweeping voltammetry (LSV) tests were conducted from 0.1 V to 0.4 V with the sweeping rate of 0.005 V/s. The electrochemical impedance spectroscopy (EIS) test data were collected at half wave potential of oxygen reduction from 1 to 105 Hz. The polarization of oxygen reduction with catalysis of Cr2O3/rGO was examined by tafel tests from 0.4 to 0.1 V. The long-time running stability test was recorded 5000 s at the potential of 0.32 V versus Ag/AgCl reference electrode. For comparison, some catalytic characteristics of the Pt/C (Hispec 3000) catalyst, which was purchased from Alfa Aesar (Ward Hill, MA, USA), were also examined.

Result and discussions The morphology and crystal structure of Cr2O3/rGO are illustrated in Fig. 2. Fig. 2a showed the SEM tests of Cr2O3/rGO. It was clearly revealed that some nano-particles are uniformly deposited on the surface of rGO sheets. Energy-dispersive Xray spectroscopy (EDS) tests were conducted to investigate the compositional elements and corresponding elemental distribution mapping of Cr2O3/rGO [33]. The EDS tests proved the coexistence of Cr, O and C elements. On the other hand, it was also proved that the distribution of Cr, O and C elements were consistent with each other. The signal of silicon came from the substrate of the tests. Based on the EDS test, the mass content of Cr, O and C were also obtained. The mass contents of Cr, O and C in the synthesized materials were about 48%, 45%, and 7%, respectively. X-ray powder diffraction (XRD) patterns of the synthesized material are illustrated in Fig. 2b. It was easily indexed that the patterns could be attributed to Cr2O3 with hexagonal structure. The corresponding JCPDS No. is 38-1479. The diffraction peak at 2q ¼ 26 could be attributed to rGO nano-sheet. And the other peaks are mainly aroused by Cr2O3. Based on the elemental mapping tests of Cr, O and C of Cr2O3/rGO in Fig. 2a, it could be deduced that the Cr2O3 were evenly distributed on the rGO sheet. On the basis of the XRD patterns, the parameters of Cr2O3 were obtained. The value of

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a and c of the hexagonal structure were 0.497 nm and 1.34 nm, respectively. The distribution of Cr2O3 on rGO was also proved by TEM tests. Fig. 2c clearly revealed that the Cr2O3 particles were uniformly distributed on rGO sheet. The high-resolution transmission electron microscopy (HRTEM) in Fig. 2d showed the crystal lattice of Cr2O3. The crystal lattice distance was 0.267 nm, which was corresponding to the (104) facet of Cr2O3. Both the XRD and HRTEM tests confirmed the successful synthesis of Cr2O3/rGO catalyst. The inset in Fig. 2d was the selected area electron diffraction (SAED) tests of Cr2O3/rGO catalyst. The diffraction patterns of the SAED could be attributed to the hexagonal structure of Cr2O3. This also affirmed the successful synthesis of Cr2O3/rGO catalyst. The successful synthesis of Cr2O3/rGO was also confirmed by the X-ray photoelectron spectroscopy (XPS) tests. The XPS spectra in Fig. 3a revealed the coexistence of Cr, O, and C elements, which was consistent with the results of EDX tests. The high resolution XPS of Cr2p in Fig. 3b clearly presented that the chromium combined with oxygen and formed CreO bond, which approved the successful synthesis of Cr2O3, too. The binding energies of 586.1 and 576.2 eV were corresponding to Cr 2p1/2 and Cr 2p3/2, respectively [34]. The deconvolved peaks of 530.3 eV of Fig. 3c was related to O1s of Cr2O3 [35]. This was in accordance with the Cr2p test results. On the other hand, the deconvolved peaks of 531.4 eV and 534.4 eV should be assigned to O1s and C]O bonds, respectively [36,37]. Fig. 3d showed the high resolution XPS of C1s. The deconvolved peaks of C1s and C]O bonds were corresponding to binding energy 284.7 eV and 287.6 eV, separately [38]. The XPS tests also proved the successful synthesis of Cr2O3/rGO catalyst. Fig. 4a showed the CV tests of Cr2O3/rGO between the potential of 0.8 V and 0.2 V at a sweeping rate of 0.05 V1 in oxygen and argon saturated 0.1 M KOH electrolyte, separately. Results showed that there was no peak observed in the argon saturated electrolyte, which meant that the catalyst of Cr2O3/ rGO was stable in the alkaline electrolyte. In comparison, an apparent current peak was detected in the oxygen-saturated electrolyte. This should be attributed to the catalytic performance of Cr2O3/rGO for oxygen reduction [39]. Fig. 4b showed the tafel tests of Cr2O3/rGO for ORR. It could be seen that the polarization potential approximately linearly changed with the lgJ. The ORR kinetic parameters could be extracted according to the following tafel equations (1) and (2):  h ¼ E0  E ¼ blg J0 þ blgðJk Þ



2:303RT ð1  aÞna F

(1)

(2)

where E is the measured experimental potential, E0 is the equilibrium potential for ORR, b is the tafel slope, J0 is the exchange current density, a is the electron transfer coefficient in the rate-determining step (RDS) of the ORR, na is the electron transfer number, R is the universal gas constant (8.314 J/mol$K), T is the temperature (K), and F is the Faraday's constant (96485 C/mol) [40,41]. The calculated tafel slope of the Cr2O3/rGO was 111 mV dec1, which was very close to that of Pt/C catalyst (~119 mV dec1) [42]. Based on the tafel tests, it was also obtained that the exchange current density J0 was 6:02  104 . The result was higher than the reported J0 of

Please cite this article in press as: Wen C, et al., Reduced graphene oxide supported chromium oxide hybrid as high efficient catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.051

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Fig. 2 e (a) SEM images, elemental mapping and EDS tests of Cr2O3/rGO catalysts. (b) XRD pattern of Cr2O3/rGO (The corresponding JCPDS No. is 38-1479). (c) TEM images and (d) high-resolution TEM images of Cr2O3/rGO. The inset is the corresponding selected area electron diffraction (SAED) patterns. The arrow in (c) pointed to Cr2O3 phase and the substrate was reduced graphene oxide.

Pt/C catalyst [43]. The inset in Fig. 4b illustrated the LSV tests of oxygen reduction with the catalysis of Cr2O3/rGO and Pt/C, respectively. It was clearly revealed that the onset potential of ORR of Cr2O3/rGO was 0.27 V (versus Ag/AgCl), which was very close to that of commercial Pt/C catalyst (0.1 V versus Ag/AgCl). The inset of Fig. 4b also showed that the peak current density (J) of Cr2O3/rGO reaches up to 1.58 mA/cm2. But the peak current intensity of Pt/C was only about 0.52 mA/cm2, which was only one third of that of Cr2O3/rGO catalyst. Researches on rGO supported FeeNeC catalyst also found that the peak current intensity of ORR was above that of Pt/C catalyst [44]. The improved catalytic performance should be

attributed to the coupling effect between Cr2O3 and rGO, which was similar to the effect of CoS2 and N,S-doped graphene oxide [45]. The long-term running stability is also one of the most important factors for excellent electrocatalysts [46]. Fig. 4c showed the long-term running stability tests of Cr2O3/rGO and Pt/C catalyst for ORR by using chronoamperometric measurements in oxygen saturated 0.1 M electrolyte. Results showed that, after 5000 s continuous running, the current density of Cr2O3/rGO only decreased 15.4%. In contrast, the current density of the commercial Pt/C catalyst decreased about 84.2%. In the research of PMO/rGO, it was also found

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Fig. 3 e XPS spectra of Cr2O3/RGO full spectra (a), high resolution of Cr element (b), O element (c) and C element (d).

that the catalytic activity decreased with the continuous CV tests [47]. This confirmed that the Cr2O3/rGO hybrid had a good stability for catalyzing ORR in 0.1 M KOH. The stability of Cr2O3/rGO hybrid is superior to that of Pt/C catalyst. The better stability of the Cr2O3/rGO catalyst could be attributed to strong

affinity between the Cr2O3 and rGO sheets, which prevented the agglomeration or lost of Cr2O3 [48,49]. The good long-term running stability makes the Cr2O3/rGO a promising catalyst for fuel cells.

Fig. 4 e (a) Cyclic voltammetry (CV) tests of the Cr2O3/rGO in the Ar and O2 saturated 0.1 M KOH electrolyte at a sweeping rate of 0.05 V s¡1. (b) Tafel plots of ORR with the catalysis of Cr2O3/rGO. The inset is the LSV tests of Cr2O3/rGO and Pt/C catalysts at a sweeping rate of 5 mV s¡1. (c) Currentetime (iet) chronoamperometric tests of Cr2O3/rGO and the Pt/C catalyst at ¡0.32 V. (d) EIS Nyquist diagrams of Cr2O3/rGO for an open-circuit voltage of ¡0.32 V. The inset is the simulated equivalent circuit of ORR. (e) CV tests for Cr2O3/rGO at different sweeping rates. (f) The correlation of square root of sweeping rate and peak current intensities. (g) CV tests of the bare Cr2O3 catalyzing oxygen reduction. Please cite this article in press as: Wen C, et al., Reduced graphene oxide supported chromium oxide hybrid as high efficient catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.051

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Fig. 4d showed a representative Nyquist diagram of EIS test for ORR with the catalysis of Cr2O3/rGO catalyst in the 0.1 M KOH solution. The inset was the module equivalent circuit. The Nyquist plots of ORR showed one half circle in the high frequency region that associated with the reaction resistance. And the linearly change in the low frequency region was related to Warburg component of semi-infinite diffusive manner of oxygen reduction. This semi-infinite diffusive character was related to the adsorption of reactants and the diffusion of intermediate products on the electrode surface [42,50]. Based on the simulated equivalent circuit, it could be obtained that the ohmic resistance Rs is 4.3 U and reaction resistance Rt of oxygen reduction is 76.9 U. Xia and his coworkers investigated the EIS tests of Pt/Vulcan catalyst, which revealed that both ohmic resistant and the reaction resistant were lower than that of Cr2O3/rGO [51]. This should be attributed to that the conductivity of Pt was higher than that of Cr2O3. Fig. 4e illustrated the CV tests of oxygen reduction with different sweeping rates. It was clearly showed that the peak current intensity increased with increasing sweeping rate. The correlation of the peak current density on the square root of sweeping rate was illustrated in Fig. 4f. It could be seen that the peak current intensity linearly increased with the increasing square root of sweeping rate, which indicated that the Cr2O3/rGO catalyzed ORR on the electrode surface was a diffusion-controlled process [52]. This result was consistent with the EIS test results. Fig. 4g showed the CV tests of bare Cr2O3. It was clearly showed that the current of oxygen reduction was lower than that of Cr2O3/rGO. The synergistic effect and the high conductivity of rGO enhanced the catalytic performance of Cr2O3. The ORR mechanism of Cr2O3/rGO was also investigated by rotating disc electrode (RDE) tests. Fig. 5a showed the RDE tests of ORR at different rotating speeds. It was obviously illustrated that the current intensity increased with increasing rotation speed at the same potential, which should be induced by the increasing diffusion of oxygen on the electrode surface. The kinetics of ORR was analyzed according to the KouteckyLevich equation (3) [14]: 1 1 1 1 1 ¼ þ ¼ þ J JL JK Bu1=2 JK

(3)

where J is the measured current density, JK and JL are the kinetic- and the diffusion-limiting current densities at a particular potential, u is the electrode rotation speed in rad/s (u ¼ 2p N/60, N is the linear rotation speed); B is given by the following equation (4): B ¼ 0:62nFC0 ðD0 Þ2=3 w1=6

(4)

where n is the electron transfer number, F is the Faraday constant, D0 is the diffusion coefficient of O2 in the electrolyte, w is the kinematic viscosity, and C0 is the bulk concentration of O2 in the electrolyte [10,53]. Based on the RDE tests, the dependence of the current intensity on the rotation speed at different potential are illustrated in Fig. 5b. Calculation showed that the value of n were 3.4, 3.5, and 3.7 corresponding to the potential of 0.5 V, 0.6 V, and 0.7 V (v.s. Ag/AgCl), respectively. It could be deduced according to the value of n

that 4-electron an 2-electron ORR coexisted in the system. The 4-electron transfer ORR pathway forms OH, thus, the 2electron ORR involves the formation of H2O2 intermediate [33]. The obtained value of n was little lower than that of CrN/ carbon nitride containing graphitic carbon nanocapsule hybrid [54]. The differences should be attributed to the nitrogen doped graphene support [55,56]. The current intensity of Cr2O3/rGO is a little lower than that of Pt/C catalyst in acidic electrolyte. The current intensity of Pt/C catalyst reaches up to 6e7 mA/cm2 [57]. In order to further understand the reaction kinetics, RRDE tests were also performed and the results are showed in Fig. 5c. The dependence of the electron transfer number and the percentage ratio of H2O2 on the electrode potential were calculated according to equations (5) and (6). The calculated results are displayed in Fig. 5d. n¼

4  Id ðId þ Ir =NÞ

H2 O2 % ¼ 200 

(5) Ir =N ID þ Ir =N

(6)

where Id is the disk current, Ir is the ring current, and N is the geometric factor of the RRDE known as current collection efficiency of the Pt ring, which was determined to be 0.39 [11,33]. The RRDE measurement further proved that there was negligible ring current, and n, calculated from RRDE curves in Fig. 5d, was about 3.4e3.5 over the potential range from 0.25 to 0.6 V versus Ag/AgCl reference electrode. This result is consistent with the results of RDE tests. Fig. 5d also showed that the percentage of H2O2 slightly increase with the decrease of electrode potential. The slight difference between the RDE and RRDE in the potential range of 0.1 V to 0.4 V should be attributed to the unstable oxygen reaction state. The 4electron and 2-electron ORR coexisted in the system, which happened as the following reactions [58,59]: O2 þ2H2 O þ 4e 44OH ðmajor reactionÞ

(7)

O2 þ2H2 O þ 2e 42OH þH2 O2 ðminor reactionÞ

(8)

H2 O2 þ2e 42OH ðminor reactionÞ

(9)

Both RDE and RRDE test results demonstrated that the electron transfer number was very approach to 4. It could be deduced that the 4-electron process was the dominating pathway for ORR on the working electrode, which should be attributed to the catalytic characteristics of Cr2O3/rGO. The Cr2O3/rGO catalyst was a promising catalyst for ORR with high efficiency. The catalytic mechanism of oxygen reduction has been schematically illustrated in Fig. 5e. It could be seen that the oxygen was reduced to H2O2 and OH with different electron transfer number.

Conclusions In this paper, we investigated the structure and catalytic performance for ORR of rGO supported Cr2O3. XRD, SEM, TEM and XPS all proved that the Cr2O3/rGO catalyst was

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Fig. 5 e (a) Rotating disk electrode (RDE) tests and (b) corresponding K-L line of Cr2O3/rGO. (c) Rotating ring-disk electrode (RRDE) tests and (d) corresponding transfer electron number, n, of ORR with the catalysis of Cr2O3/rGO at 1600 rpm. (e) The mechanism of oxygen reduction with the catalysis of Cr2O3/rGO.

successfully synthesized. The synthesized Cr2O3/rGO showed high catalytic performance for ORR. The peak current intensity of ORR was above three times higher than that of Pt/C catalyst. On the other hand, the long-term running stability of Cr2O3/rGO catalyst was much better than that of Pt/C catalyst. RDE and RRDE tests confirmed that the 4-electron reaction was the dominating pathway for ORR with the catalysis of Cr2O3/rGO catalyst. This research explored a novel way to developing high performance catalyst from transitional metal oxides.

Acknowledgments Financial support for this work was provided by the China National Natural Science Foundation (Grant No. 51302022, 51376110), Science Development Project of Shandong Provincial (No. 2014GGX104004, 2015GSF116005), Shandong Provincial Natural Science Foundation, China (No. ZR2015EM044),

the Specialized Research Fund for the Doctoral Program of Higher Education of China (No.2013013110006) and the Major International (Regional) Joint Research Project of the National Natural Science Foundation of China (No. 61320106011).

references

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Please cite this article in press as: Wen C, et al., Reduced graphene oxide supported chromium oxide hybrid as high efficient catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.051