CoFe2O4 Nanoparticles Supported on N

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One-Pot Synthesis of Co/CoFe2O4 Nanoparticles Supported on NDoped Graphene for Efficient Bifunctional Oxygen Electrocatalysis Yanli Niu, Xiaoqin Huang, Lei Zhao, Weihua Hu, and Chang Ming Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03888 • Publication Date (Web): 02 Feb 2018 Downloaded from http://pubs.acs.org on February 3, 2018

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ACS Sustainable Chemistry & Engineering

One-Pot Synthesis of Co/CoFe2O4 Nanoparticles Supported on N-Doped Graphene for Efficient Bifunctional Oxygen Electrocatalysis Yanli Niu, Xiaoqin Huang, Lei Zhao, Weihua Hu,* Chang Ming Li *

Institute for Clean energy & Advanced Materials, Faculty of Materials & Energy, Southwest University; Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energies, 2 Tiansheng Road, BeiBei, Chongqing 400715, People’s Republic of China

* Corresponding author. E-mail: [email protected] (W. H. Hu); [email protected] (C. M. Li)

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ABSTRACT: We herein report a facile strategy to synthesize transition metal/spinel oxide nanoparticles coupled with nitrogen-doped graphene (Co/CoFe2O4@N-graphene) as an efficient bifunctional electrocatalyst toward oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). This approach involves a spontaneous solution-polymerization of polydopamine (PDA) film on graphene oxide (GO) sheets in the presence of Fe3+, Co2+ to form Fe/Co-PDA-GO precursor, followed by pyrolysis at 800℃ in argon (Ar) atmosphere. During the calcination process, Co/CoFe2O4 nanoparticles are in situ formed via high-temperature solid state reaction and are further entrapped by the PDA-derived N-doped carbon layer. As-prepared Co/CoFe2O4@N-graphene exhibits highly efficient catalytic activity and excellent stability for both ORR and OER in alkaline solution. This work reports a facile synthetic approach to develop highly active electrocatalysts while offering great flexibility to tailor their components and morphologies, and thus provides a useful route to the design and synthesis of a broad variety of electrocatalysts.

KEYWORDS: transition metal/spinel oxide; N-doped carbon layer; polydopamine; oxygen reduction reaction; oxygen evolution reaction.


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Introduction The electrochemistry of molecular oxygen including oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) has been intensively investigated for more than one century. 1-3 Unfortunately, both reactions still possess as primary bottleneck for the commercial applications of some renewable energy conversion/storage devices such as fuel cell, metal–air battery and water electrolyzer. Noble metal platinum (Pt) and ruthenium (Ru)/iridium (Ir) oxides have been recognized as the state-of-the-art ORR and OER catalysts, respectively, but their high cost and limited reserves severely hinder their large-scale implementation in these devices.4,

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Consequently, various earth-abundant first-row transition metal oxides attracted considerable attention as they demonstrate compelling potential to activate ORR and OER. 6-10 Amongst a wide variety of transition metal oxides investigated, bimetal spinel oxides AxB3-xO4 (A and B stands for transition metal such as Co, Fe, Ni, Zn, Mn and so on, 1≤x≤2) are particularly interesting due to their high abundance, mixed valence, outstanding redox stability, and tunable micro-nanostructures.

11-15

The cation in octahedral site has recently been identified

as active sites for the ORR/OER of spinel oxides and the eg occupancy in the octahedral site is proposed as the activity descriptor.

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Diverse spinel-based materials are explored as promising

electrocatalysts for ORR, OER, or both. Monodisperse MxFe3−xO4 (M = Fe, Cu, Co, Mn) nanoparticles are synthesized and their ORR activity is compared after being loaded on carbon support.

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Hollow CoFe2O4 nanospheres with hierarchical porosity are found to demonstrate

considerable ORR/OER activity and stability, and thus offer great potential for application in lithium-air battery. 17 Hybridization with conducting carbon nanomaterials such as graphene and carbon nanotube (CNT) is an effective strategy to fulfill the catalytic potential of spinels. The carbon materials assure fast electron exchange and also facilitate the convenient mass transport


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of reactants. At the same time, the unique structural and chemical properties of the carbon supports offer great possibility to tailor the nanostructure of the spinels for high performance electrocatalysis. Spinel-based nanocomposites such as CoFe2O4/CNTs, Ni-doped CoFe2O4 hollow

nanospheres,

CoFe2O4/PANI

composite,

CoFe2O4/graphene,

MnCo2O4/carbon,

CoFe2O4/carbon, spinel CoFe2O4 on rod-like ordered mesoporous carbon have been reported as ORR/OER bifunctional electrocatalysts.

14, 15, 18-22

Nitrogen-doped carbons are further explored

as highly efficient supporting materials to boost the ORR/OER catalytic activity.

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The

synergistically enhanced activity of this type of hybrid catalysts is believed to be associated with the strong coupling between the spinel oxide and doped carbon. The dopants offer active sites for the nucleation/growth of metal oxide and improve the catalyst stability due to the high binding energy of N-doped carbon with metal species. The N-doping also helps enhance the catalytic performance via interfacial charge transfer between metal oxide and carbon support. 24 It has been recognized that the catalytic performance of spinel-based catalysts depends heavily on their morphology, dimension, crystallographic orientation, and the way the spinels couple to the supporting materials. Therefore, it is of essential importance to explore appropriate synthetic strategy to enable rational design and controllable synthesis. The approaches used so far dominantly are based on solution-reaction to grow spinels on carbon supports by, using e.g., solgel processing, co-precipitation, or hydrothermal/solvothermal approaches.25-27 These methods, unfortunately cannot offer sufficient control over the anchoring of spinels as a consequence of the intrinsic mismatch between the oxide and carbon lattice, resulting in unsatisfying stability and insufficient charge transfer ability and in turn poor catalytic activity and/or durability. Moreover, the N-doping of carbon, which is beneficial to the catalytic performance, cannot be achieved in most cases unless tedious doping procedure is applied before the spinel growth.


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In this work we report a facile strategy to synthesize metallic cobalt/spinel CoFe2O4 nanoparticles supported on nitrogen-doped graphene (Co/CoFe2O4@N-graphene) as an efficient bifunctional oxygen electrocatalyst. This approach involves a spontaneous solutionpolymerization of polydopamine (PDA) film on graphene oxide (GO) sheets in the presence of Fe3+, Co2+ to form Fe/Co-PDA-GO precursor, followed by pyrolysis at 800℃ in argon (Ar) atmosphere (Figure 1). During the calcination process, Co/CoFe2O4 nanoparticles are in situ formed via solid-state reaction and are entrapped by the PDA-derived N-doped carbon layer. It enables simultaneous N-doping of graphene and in situ growth and entrapment of metal/spinel oxide nanoparticles with intimate contact. After careful optimization of the initial Fe/Co concentration and other experimental parameters, the resulting Co/CoFe2O4@N-graphene exhibits highly efficient catalytic activity and excellent stability for both ORR and OER in alkaline solution.

Experimental Chemicals: Graphite powder (99.95%), Nafion (5%), Pt/C (20%), RuO2, cobalt nitrate hexahydrate and dopamine were supplied from Sigma-Aladdin. Hydrogen peroxide solution (H2O2, 30%), methanol (CH3OH, 99.5%), hydrochloric acid (HCl, 37%), sodium nitrate (NaNO3) and sulfuric acid (H2SO4, 98%) were obtained from Chongqing Chuan Dong Chemical Company. All chemicals were of analytical grade and used as received without further purification. All the solutions were prepared with deionized (DI) water from a Millipore water system. Synthesis of electrocatalysts: GO was synthesized according to the modified Hummers’ method. 6

In a typical reaction, natural graphite (2 g), NaNO3 (1.2 g), and concentrated H2SO4 were added


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to a 250 mL flask, which was placed in an ultrasonic bath and magnetically stirred in an ice bath for 15 min. Next, 8.8 g of KMnO4 was added slowly to this flask. Once mixed, the ice bath was removed and the suspension was stirred at room temperature for 12 h. After adding 72 mL of DI water dropwise, the suspension was further stirred at 50 ℃ for 12 h and 35 ℃ for 12 h. After that, 100 mL of water was added, followed by the slow addition of 22 mL of H2O2 and the mixture was stirred for another 3 h at 35 ℃ . The resulting precipitate was collected by centrifugation and repeatedly washed with 5 wt. % HCl and DI water in sequence for several times until the PH of supernatant was close to 6. Finally, the GO suspension was obtained via mild sonication of the precipitate. To prepare the Fe/Co-PDA-GO precursor, 20 mg of GO were dispersed in 10 mM Tris-buffer (pH = 8.5) under ultrasonication for 30 min to form a uniform solution. Thereafter, 0.1 mM of FeCl3 and 0.9 mM of Co (NO3)2·6H2O and dopamine (2 mg mL-1) were added to the above solution. The mixture was magnetically stirred at ambient atmosphere for 24 h. The Fe/Co-PDAGO product was collected by centrifugation and washed three times with DI water to remove any impurities, followed by freeze-drying overnight. The final Co/CoFe2O4@N-graphene catalyst was obtained by annealing of the Fe/Co-PDA-GO precursor at 800℃ for 3 h in Ar atmosphere with a heating rate of 5℃ min-1. For comparison, various materials were also prepared with the same procedure but using different Fe3+ and Co2+ concentrations. Electrochemical Experiment: The working electrodes for electrocatalytic measurements were prepared according to the following procedure. 2 mg of as-prepared catalyst was dispersed in a solution containing 1 mL of ethanol and 50 µL of Nafion (5%) dispersion. The mixture was ultrasonicated for 30 min until a homogeneous ink was obtained. Then the well-dispersed suspension was loaded onto the a clean glassy carbon electrode (GC, 3 mm diameter, 5 µL


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suspension applied) or glass carbon rotating ring disk electrode (RRDE, 5 mm diameter, 25 µL suspension applied), respectively. Finally, the electrode was dried in ambient air before use. The electrocatalytic activity of the catalyst toward OER and ORR was evaluated in a conventional three-electrode electrochemical cell using an Autolab potentiostat (PGSTAT302N) system coupled with a Pine rotator (AFMS-LXF). An Hg/HgO electrode and Pt foil as the reference electrode and the counter electrode, respectively. Before the OER linear sweep voltammetry (LSV) tests, cyclic voltammograms (CVs) were performed in the potential range between 0 V and 1.0 V vs. Hg/HgO at a scan rate of 50 mV s-1 in 1 M KOH for 20 cycles. For OER stability testing, the possible bubbles previously generated on the electrode were carefully removed by gentle stirring in the electrolyte solution. The ORR LSV was collected at different rotation speeds in O2-saturated 0.1M KOH electrolyte. Electrochemical impedance spectroscopy (EIS) was recorded at onset potential with an amplitude of 5 mV in the frequency range of 106 Hz ~ 1 Hz. Characterizations: The morphology of the catalyst was characterized by using transmission electron microscopy (JOEL, JEM-2100) and field emission scanning electron microscopy (FESEM, JOEL, JSM-7800F). X-ray diffraction (XRD) pattern was examined by an X-ray diffraction instrument (RIGAKU, D/MAX 2550 VB/PC). The surface chemical composition was determined by X-ray photoelectron spectrum (XPS, ESCALAB 250Xi, Thermo Fisher). The Brunauere-Emmette-Teller (BET) surface area was obtained by the N2 adsorption/desorption measurement using a NoVa 1200e system.

Results and discussion


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The nanostructure and morphology of Co/CoFe2O4@N-graphene were first investigated. As shown in Figure 2a, the SEM images of Co/CoFe2O4@N-graphene reveal that dense nanoparticles with a size range of 30-50 nm are uniformly anchored on the graphene sheets. From the TEM image in Figure 2b, it is found that nanoparticles are covered by a flocculent carbon thin layer, which should be formed from PDA film after the carbonization process. The morphology of Co/CoFe2O4@N-graphene with lower and higher Co2+/Fe3+ molar ratio is shown in Figure S1. The high-resolution TEM image on the marked region further discloses the structural features of Co/ CoFe2O4@N-graphene. As in Figure 2c, the crystal fringes are clearly displayed and the d-spacing between the adjacent lattice is determined to be 0.25 nm, corresponding to the spacing between the (113) plane of spinel CoFe2O4. 28 The lattice with a distance of 0.2 nm could be indexed to the (111) inter-planar spacing of metallic cobalt. 29 These data suggest the presence of metallic cobalt clusters as well as spinel CoFe2O4 domains in the nanoparticles. The metal/oxide interface formed is believed to be favorable for heterogeneous catalysis as it provided dual active sites and/or strongly influences the physio-chemical properties of catalyst. 30, 31 The crystal structure of Co/CoFe2O4@N-graphene was examined by XRD. As shown in Figure 2d, the strong peak at about 25.5° in the XRD pattern corresponds to the (002) facets of graphite carbon. The diffraction peak located at 35.4º is in good agreement with the spinel oxides CoFe2O4 (JCPDS file No. 22-1086), corresponding to facets of (311). 32 The peaks at 44.2º, 51.5º and 75.9º are well indexed to (111), (200) and (220) facets of metallic cobalt (JCPDS file No. 15-0806). 33 The XRD pattern confirms the presence of metallic cobalt and spinel CoFe2O4 in the sample, which is in line with the TEM observation in Figure 2c. CoFe2O4 has an inverse spinel structure belonging to Fd3m space group with 8 tetrahedral sites occupied by Co and 16


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octahedral sites by Fe. It possesses considerable electrical conductivity and surface redox centers due to the electron hopping between different valence states of metals in Oct-sites.

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The XRD

patterns of other Co/CoFe2O4@N-graphene catalysts synthesized with lower and higher Co2+/Fe3+ molar ratio are shown in Figure S2 for comparison. The porosity of Co/CoFe2O4@N-graphene was analyzed by nitrogen adsorption-desorption isotherm, as displayed in Figure 3a. A type-IV isotherm is observed for Co/CoFe2O4@Ngraphene, suggesting its mesoporous structure. 34 The pore size distribution (inset in Figure 3a) suggests that the pore diameter of Co/CoFe2O4@N-graphene is dominantly about 19 nm. The Brunauer-Emmett (BET) surface area of Co/CoFe2O4@N-graphene is determined to be 193.728 m2 g−1. The larger surface area is anticipated to provide more electrocatalytic active sites and efficient channels for mass transport during the ORR and OER catalytic process. The oxidation state and chemical composition of Co/CoFe2O4@N-graphene were analyzed by XPS measurements. The survey spectrum indicates the existence of C, N, O, Fe and Co in the catalyst (Figure 3b). The content of N, Co and Fe element in the Co/CoFe2O4@N-graphene is 2.61 at%, 1.31 at% and 2.12 at%, respectively. Notably, the Co/Fe ratio in the Co/CoFe2O4@Ngraphene catalyst is inconsistent with that used for synthesis of Fe/Co-PDA-GO precursor, which may be due to the difference chelation ability of PDA to Co/Fe ions. The high-resolution N 1s spectrum shown in Figure 3c could be deconvoluted into three peaks from pyridinic-N (398.3 eV), pyrrolic-N (400.3 eV) and graphitic-N (401.3 eV), respectively, suggesting successful doping of nitrogen in the carbon frameworks. Both theoretic calculation and experimental result suggest that the pyridinic N atoms could activate molecular oxygen with their lone pair electrons and the graphitic N atoms are capable of enhancing the ORR by promoting electron transfer and facilitating the dissociation of molecular oxygen on neighboring C atoms.

35-38

The Fe 2p


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spectrum (Figure 3d) shows two peaks at binding energy of 710.9 eV and 724.2 eV, which could be assigning to Fe2p3/2 and Fe2p1/2, respectively. The peak at 717.8 eV is a satellite peak of transition metal Fe.

32

The Co 2p spectrum (Figure 3e) shows two characteristic peaks at 781.8

eV and 797.7 eV, corresponding to the Co2+ 2p3/2 and Co2+2p1/2 spin-orbit split peaks, respectively, with two satellite peaks for Co2+. Notably, the week signal of Co0 is observed at 796.0 eV, further demonstrating the presence of metallic Co nanoparticles on the Co/CoFe2O4@N-graphene.

36, 39

The high-resolution XPS spectrum of the O 1s in Figure 3f

shows a large peak at 531.8 eV, which is attributed to the lattice oxygen (M-O-M) in the metaloxygen framework. 40 The ORR activity of as-synthesized Co/CoFe2O4@N-graphene catalyst was first examined. As presented in Figure 4a, compared to thermally reduced GO and N-doped graphene (synthesized by carbonization of PDA-GO), Co/CoFe2O4@N-graphene shows essentially enhanced ORR activity, featuring a much more positive ORR peak potential at -0.16V in CV curves in O2saturated 0.1 M KOH aqueous solution. Similar trend is observed in rotating disk electrode (RDE) measurements. The linear sweeping voltammograms (LSV) of Co/CoFe2O4@N-graphene at a rotation rate of 1600 rpm presents a positive onset potential of -0.06 V and a current density 4.3 mA cm-2 at -0.75 V, close to those of commercial Pt/C catalysts, as shown in Figure 4b. Remarkably, the concentration of Co2+ and Fe3+ for the synthesis of Fe/Co-PDA-GO precursor shows a significant influence on the ORR activity of resulting catalysts. As shown in Figure S3 and S4, the catalyst shows best ORR activity when a Co2+ /Fe3+ molar ratio of 9:1 was used, which was then chosen as the optimal concentration ratio. LSV measurement was performed on RDE the under different rotation speeds to investigate the ORR kinetics processes of Co/CoFe2O4@N-graphene. As shown in Figure 4c, the current


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density is enhanced with increasing rotation rate from 400 to 2500 rpm. The inset displays the corresponding Koutecky–Levich plots (J-1 vs. ω-1/2) at different potentials, where the linearity of the K-L plots and near parallelism of the fitting lines imply the first-order reaction kinetics toward the concentration of dissolved oxygen (Figure 4d). 41 The peroxide species (H2O-) generated during the ORR process was monitored by using RRDE. The electron transfer number (n) during the ORR was further calculated based on the disk and ring currents (Id and Ir) and ring collecting efficiency (0.37). As shown in Figure 4f, the measured HO2- yield is below 5% over the potential of -0.8 ~ -0.2V for Co/CoFe2O4@Ngraphene. The average electron transfer number is calculated to be ~3.9, suggesting a dominant 4e reduction pathway in Co/CoFe2O4@N-graphene catalyzed ORR. The Co/CoFe2O4@N-graphene catalyst also exhibits excellent ORR stability and methanol tolerance, as demonstrated by the accelerated test (ADT) in Figure 5a.

42

There is no obvious

shift in the E1/2 on the LSV curves of Co/CoFe2O4@N-graphene before and after 10000 potential cycles. Only negligible decrease in limiting current density is observed, indicating superb stability of the catalyst. Moreover, there is no evident deterioration in catalytic activity upon the addition of methanol according to the LSV curves (Figure 5b). The excellent durability and methanol tolerance are also confirmed by the chronoamperometric measurements at -0.3 V, as shown in Figure 5c, d. OER activity of as-prepared Co/CoFe2O4@N-graphene in alkaline solution was evaluated by LSV. The concentration of Co2+ and Fe3+ for the synthesis of Fe/Co-PDA-GO precursor was also optimized in term of OER activity, and an optimal 9:1 Co2+/Fe3+ molar ratio was chosen, as shown in Figure S3. As shown in Figure 6a, the onset potential on Co/CoFe2O4@N-graphene is slightly more positive comparable to those of the RuO2 catalyst. The overpotential (η) required


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for achieving a current density of 10 mA cm-2 is 400 mV for Co/CoFe2O4@N-graphene catalyst without IR compensation, which is very close to that for RuO2 catalyst (10 mA cm-2@ 370 mV overpotential). Remarkably, the slope of current density vs. η for Co/CoFe2O4@N-graphene is higher than that of RuO2 catalyst in the high current density region. At overpotential of 432 mV, the current density on Co/CoFe2O4@N-graphene reaches an equal value of 15 mA cm-2 with the RuO2 catalyst. At higher overpotential, the current density on Co/CoFe2O4@N-graphene surpasses that obtained on RuO2 catalyst, which may originate from the higher surface area and active site density of Co/CoFe2O4@N-graphene. As shown in the Figure 6b, the Tafel slope is found to be 66 and 81 mV dec-1 for Co/CoFe2O4@N-graphene and RuO2, respectively, further suggesting that Co/CoFe2O4@N-graphene is a highly efficient OER catalyst. The kinetics of the catalytic processes on the Co/CoFe2O4@N-graphene was examined by electrical impedance spectroscopy (EIS; Figure 6c). At onset potential, the charge transfer resistance (Rct) of OER on Co/CoFe2O4@N-graphene slightly bigger (ca. 38 ohm) than that on RuO2, further illustrating the OER activity of Co/CoFe2O4@N-graphene. The long-term stability of Co/CoFe2O4@N-graphene catalyst was also investigated. As shown in Figure 6d, the polarization curve of Co/CoFe2O4@N-graphene after 1000 potential cycles (scanning rate of 50 mV s-1) retains almost same to the initial curve, suggesting stable OER catalytic activity of Co/CoFe2O4@N-graphene in alkaline condition. The morphology and crystalline structure of Co/CoFe2O4@N-graphene were well preserved after 1000 potential cycles, as shown as the SEM image and XRD pattern in Figure S5, further implying the excellent stability of this catalyst for OER.

The ORR and OER catalytic performance of as-prepared Co/CoFe2O4@N-graphene catalyst is compared with that of other spinel-based bifunctional catalysts reported to date, as in Table 1. It


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is found that the Co/CoFe2O4@N-graphene catalyst outperforms most bifunctional catalysts in terms of onset potential, half-wave potential, overpotential required for a 10 mA cm-2 OER current density, and/or Tafel slope. Therefore, it holds great potentials for practical applications. Its excellent catalytic performance could be attributed to the rational combination of its structural and compositional merits. The Fe cations on octahedral site in the spinel CoFe2O4 may be the ORR/OER active sites for this Co/CoFe2O4@N-graphene catalyst according to previous research.16 At the same time, the Co/CoFe2O4 agglomerates on N-doped carbon with intimate contact are believed to be capable of boosting the ORR activity of neighboring carbon and possible Fe-Nx active sites; the amorphous Fe-Co bimetal hydroxide layer generated upon anodic potential on the spinel oxide nanoparticles are also suggested to be possibly responsible for the high OER activity of Co/CoFe2O4@N-graphene. 43-46

Conclusions

In summary, we reported Co/CoFe2O4@N-graphene as a bifunctional catalyst for ORR and OER. It exhibits highly efficient catalytic activity and excellent stability for both ORR and OER in alkaline solution. Remarkably, the synthetic approach involves a simple one-pot solution reaction with subsequent thermal treatment, and also provides great flexibility to manipulate the components and nanostructures of the transition metal (oxides), and thus may offer useful route to the design and synthesis of a broad variety of electrocatalysts.

ASSOCIATED CONTENT Supporting Information.


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SEM, TEM images (Figure S1) and XRD patterns (Figure S2) of Co/CoFe2O4@N-graphene synthesized with different Co/Fe molar ratio; CV curves (Figure S3) and polarization curves (without IR correction, Figure S4) of Co/CoFe2O4@N-graphene synthesized with different Co2+/Fe3+ molar ratio; SEM image and XRD pattern of Co/CoFe2O4@N-graphene after 1000cycle CV for OER process (Figure S5). Corresponding Author * Corresponding author. E-mail: [email protected] (W. H. Hu); [email protected] (C. M. Li)

ACKNOWLEDGMENT We would like to gratefully acknowledge the financial support from National Natural Science Foundation of China (No. 21273173), Natural Science Foundation Project of CQ CSTC (cstc2016jcyjA0493),

Fundamental

Research

Funds

for

the

Central

Universities

(XDJK2015B014), and Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energies.


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Figures and Captions

Figure 1. Schematic depiction of the preparation of Co/CoFe2O4@N-graphene.

Figure 2. SEM (a), TEM images (b, c) and the XRD pattern (d) of Co/CoFe2O4@N-graphene.


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Figure 3. Nitrogen absorption and desorption isotherms of Co/CoFe2O4@N-graphene (a, inset shows the corresponding BJH pore distribution); XPS survey spectrum (b) and high-resolution XPS spectra of N1s (c), Fe2p (d), Co2p (e) and O1s (f) of Co/ CoFe2O4@N-graphene.


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Figure 4. (a) CV curves of reduced GO, N-doped graphene, Co/CoFe2O4@N-graphene at a scan rate of 50 mV s-1; (b) LSV curves of reduced GO, N-doped graphene, Co/CoFe2O4@N-graphene and commercial Pt/C at a scan rate of 5 mV s-1 at 1600 rpm; (c) Rotating-disk voltammograms of Co/CoFe2O4@N-graphene at a scan rate of 5 mV s-1 at different rotating rates and (d) corresponding K-L plots (J-1 versus ω -1/2); (e) RRDE voltammograms of Co/CoFe2O4@Ngraphene at a scan rate of 5 mV s-1 at 1600 rpm and (f) thereof calculated peroxide yield (black


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line) and electron transfer number (n) (red line) at various potentials. Electrolyte solution: O2saturated 0.1 M KOH.

Figure 5. LSV curves of Co/CoFe2O4@N-graphene at a rotation rate of 1600 rpm in O2saturated 0.1 M KOH, sweep rate of 5 mV s-1. Before and after 10000-cycle ADT (a); with and without 20% methanol (v/v) (b); Amperometric curves of Co/CoFe2O4@N-graphene at 0.3 V (vs. Hg/HgO) in O2-saturated 0.1 M KOH solution. J0 defines the initial current density (c); Amperometric curves of Co/CoFe2O4@N-graphene in O2-saturated 0.1 M KOH solution. The arrows indicate the addition of 20% (v/v) methanol into the electrolyte (d).


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Figure 6. (a) The polarization curves (without IR correction) of Co/CoFe2O4@N-graphene and commercial RuO2 at 1600 rpm with a scan rate of 5 mV s-1 in 0.1 M KOH solution and (b) the corresponding Tafel plots; (c) Nyquist plots of Co/CoFe2O4@N-graphene and RuO2 electrocatalysts with inset showing the electrical equivalent circuit used to simulate the Nyquist plots, where Rs is the electrolyte resistance, Rct is the charge-transfer resistance, and Cdl represents the double-layer capacitance; (d) Stability evaluation of the Co/CoFe2O4@N-graphene by polarization curves at 1600 rpm before and after CV cycling.


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Table 1. ORR/OER performance overview of reported bifunctional spinel catalysts. (All potentials are listed vs. Relative hydrogen electrode, RHE).

CoFe2O4

EORR onset (V) 0.72

EORR half (V) 0.58

EOER (V) @10 mA cm-2 1.69

Tafel OER (mV dec-1) 74

CoFe2O4

0.71

0.61

1.56

75

CoFe2O4

0.66

0.59

1.67

83

NiFe2O4

0.54

0.42

1.57

68

MnCo2O4

0.73

0.59

1.76

85

FeCo2O4

0.75

0.64

1.71

87

NiMn2O4

0.72

0.56

1.61

83

Co/CoFe2O4

0.77

0.63

1.62

66

Spinels

Feature

References

on graphene on mesoporous carbon coupled with CNTs crossed link MWCNTs on nitrogenenriched carbon nanofiber on hollow graphene spheres ilmenite/spinel hybird on N-doped graphene

14 12 18 10 9 31 29 this work


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Table of Content (TOC)

Highly active and sustainable ORR/OER bifunctional electrocatalyst is synthesized via a facile synthetic approach, offering a useful route to development of a broad variety of electrocatalysts for clean energy.


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CoFe2O4 Nanoparticles Supported on N

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