Mn3O4-decorated Co3O4 nanoparticles supported on graphene oxide: Dual electrocatalyst system for oxygen reduction reaction in alkaline medium

Author’s Accepted Manuscript Mn3O4-decorated Co3O4 nanoparticles supported on graphene oxide: Dual electrocatalyst system for oxygen reduction reaction in alkaline medium Lijun Dai, Min Liu, Ye Song, Jingjun Liu, Feng Wang www.elsevier.com/locate/nanoenergy

PII: DOI: Reference:

S2211-2855(16)30247-6 http://dx.doi.org/10.1016/j.nanoen.2016.07.007 NANOEN1382

To appear in: Nano Energy Received date: 14 April 2016 Revised date: 2 July 2016 Accepted date: 5 July 2016 Cite this article as: Lijun Dai, Min Liu, Ye Song, Jingjun Liu and Feng Wang, Mn3O4-decorated Co3O4 nanoparticles supported on graphene oxide: Dual electrocatalyst system for oxygen reduction reaction in alkaline medium, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2016.07.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Mn3O4-decorated Co3O4 nanoparticles supported on graphene oxide: Dual electrocatalyst system for oxygen reduction reaction in alkaline medium

Lijun Dai, Min Liu, Ye Song, Jingjun Liu*, Feng Wang*

State Key Laboratory of Chemical Resource Engineering; Beijing Key Laboratory of Electrochemical Process and Technology for Materials, Beijing University of Chemical Technology, Beijing 100029, P R China

[email protected] [email protected]

*

Corresponding Authors: (J. Liu). (F.Wang). Tel/Fax: +86-10-64411301; +86-10-64451996

Abstract Constructing composite materials with a smart nanostructure, by using various transition metal oxides and carbon carriers as building blocks, is of great importance to develop highly active, economical noble metal-free catalysts for oxygen reduction reaction (ORR). We have synthesized a novel ternary composite with a special 3D stacked-up nanostructure, composed of Co3O4, Mn3O4 and graphene oxide (GO), via a facile two-step aqueous synthesis without adding any structure directing agent. The composite was characterized by X-ray diffraction, scanning transmission electron 1

microscope, Raman spectroscopy, and X-ray photoelectron spectroscopy. The results revealed that Mn3O4 nanocrystals had been successfully epitaxially deposited onto the surface of Co3O4 nanoparticles to form Mn3O4-on-Co3O4 nanostructrures on surface of the graphene. In an alkaline environment, the Co3O4-Mn3O4/GO composite exhibits much better electrocatalytic activity and durbility towards ORR than individual Mn3O4/GO and Co3O4/GO catalysts. The recorded kinetic current density (JK) of O2 reduction for the composite is 2.078 mA/cm2, which is comparable to that of a commercial Pt/C (20 %) but far exceeding the sum of that obtained from the Co3O4/GO and Mn3O4/GO. The remarkably improved ORR activity is closely attributed to the enhanced synergy between these two oxides and the graphene, raised by the 3D stacked-up structure in this composite. The oxide-on-oxide heterostructure comprising Co3O4 and Mn3O4 can promote covalent electron transfer from carbon support to the oxides as a result of the interphase ligand effect between them, which facilitate the ORR kinetics. Moreover, Mn3O4 phase acting as a co-catalyst, located at the top of Co3O4 phase, also favor the chemical disproportionation of H2O2 intermediates generated by the composite during the ORR.

Keywords: dual transition metal oxides, graphene oxide, 3D stacked-up nanostructure, catalytically synergistic effect, oxygen reduction reaction

2

1. Introduction One of the major obstacles in the commercial development of renewable-energy devices such as fuel cells and metal-air batteries is the sluggish oxygen reduction reaction (ORR) occurred on the cathode side of these energy conversion and storage systems. Pt and its alloys have been regarded as the best ORR catalysts, but their large-scale application is seriously hindered by their scarcity of resources, high cost and declining activity during long-term operation.[1] Thus, tremendous efforts have been made to reduce or even replace Pt-based catalysts by developing cheap and durable non-precious metal catalysts, such as carbon-supported transition metal oxides,[2] N, B, P-doped metal-free carbon materials,[3] as well as amorphous C/N/Co or C/N/Fe catalysts.[4] Among these substitutes, as one kind of the promising alternatives to Pt-based catalysts, carbon-supported transition metal oxides, such as MnxOy,[5] CoxOy,[6] FexOy,[7] and etc., have been paid more attentions because of their excellent electro-catalytic activity and good durability during the ORR, ascribed to their inherent mixed-valence state in these transition metal oxides.

Despite these transition metal oxides and carbon materials (such as, carbon black, carbon nanotube and graphene) individually have little activity in ORR, their composite displays a surprisingly high electro-catalytic ORR activity in alkaline solutions, as suggested by many investigators.[2],[8],[9],[10] Dai and his co-workers11 reported that Co3O4/N-doped graphene hybrid displayed comparable ORR to the commercial Pt/C catalyst (E-TEK), which is superior to the N-doped carbon, free 3

Co3O4 and their physical mixture. Inspired by the fact of Co3O4 having high ORR activity, manganese oxides like Mn3O4,[12] Mn2O3,[13] MnO2[5] have been investigated as potential catalysts, applied to the ORR. Similar to the cobalt oxide, these carbon-supported manganese oxides also exhibited high ORR activity.[12] The exact origination of the ORR activity remains unclear, but the enhanced activity of these carbon-supported oxides is strongly dependent on the synergistic effects between the oxides and carbons in their hybrids, which is ascribed to the changes in mixed valence states of Mn/Co cations on exposed crystal faces in oxides and electronic states of carbon support of hybrids.[14] Some investigators further confirmed the presence of covalent C−O−M or C−N−M (M=metal) bonds formed by these metal cations and carbon atoms through the oxygen atoms as bridge at the interface between the oxides and carbons in their hybrids.[1],[14],[15] As a result, the oxidation of carbon atoms and reduction of metal cations at their interface, caused by these chemical bonds between them, can offer additional active sites for ORR. Thence, O2 can be reduced much more facilely and efficiently on the hybrid catalysts, which thereby boosting the ORR activity. These carbon-supported transition metal oxides as ORR catalysts having good electrochemical performance, yet there are still several obstacles hindering the commercialization of Co3O4/C and Mn3O4/C hybrids as promising electrocatalysts, applied to the ORR. One issue is that these hybrids have still exhibited moderate ORR activity, compared to that of state-of-the-art Pt/C catalyst (E-TEK) that has currently been considered as the best electrocatalyst by now.[16] Another issue is that the 4

relatively high catalytic activity and good stability in ORR would be decayed for these hybrid catalysts after premature aging,[17] because of the yield of the hydrogen peroxide as intermediate form ORR still increased obviously, indicating a decrease in both activity and stability.

To address these issues, one of the promising approaches is purposely to develop novel multiphase composites comprising several spinel oxides and carbon as more efficient ORR catalysts, for example Mn3O4 and Co3O4 nanoparticles on graphene, through tuning of special interfacial and nanophase structures to achieve an enhanced synergy between these electrocatalytic active oxides on carbon which may result in the much higher ORR activity and durability.[18],[19],[20],[21] Thus, the design of this kind of multiphase composites with a smart nanophase structure may be helpful to not only achieve satisfactory electrochemical performance but also deeply understand the possible synergistic effect between individual constituents in the nanocomposites. To prove this concept, therefore, we choose Co3O4 as main catalyst, Mn3O4 as co-catalyst, and graphene oxide as carrier to construct a novel ternary composite with a special nanophase nanostructure, where the Co3O4 particles are directly integrated into the graphene surfaces while the Mn3O4 nanocrystals are located only on the cobalt oxide surfaces to form a 3D stacked-up structure. In principle, this design of the composite with the unique nanophase has the following merits. First, since the Co3O4 component were directly integrated onto the graphene surface, the chemically intimate contact of them leads to the formation of the covalent C–O–Co bonds at the 5

interface, which can promote the electron transfer from the carbon carrier to the Co3O4 nanoparticles that dominante the ORR activty.[19] Moreover, the chemical bonds can also lead to lattice defect or oxygen vacancy formed in the supported Co3O4 matrix, which further contribute the ORR kinetics.[22], [23] Second, the presence of Mn3O4 component in the Co3O4-Mn3O4/graphene hybrid can remarkably promote the electron transfer between the Co3O4 and carbon as a result of interphase ligand effect between these two oxides. It may lead to further enhanced ORR activity.[24] Third, the nanoscale Mn3O4 dispersed over the Co3O4 surfaces can favor the chemical disproportionation of H2O2 intermediates generated by the Co3O4 during the ORR as a result of the ensemble effect of two oxides.[25] Owing to the special nanostructure of the composite, the trace amount of H2O2 intermediates produced over Co3O4 phase can diffuse along the oxide surfaces or through the solution to reach the adjacent Mn3O4 surfaces or Co3O4-Mn3O4 interface, where it would undergo a chemical disproportionation to generate OH− and O2 specie. Therefore, in this composite design, not only are all the desired functions of each compoment effectively utilized, but additionally a strong synergistic effect may be realized. But, the synthesis of the nanocomposite with the smart structure has not been reported and the possible synergistic effect between individual constituent has been paid less attention.

Herein, we synthesized graphene-supported Mn3O4 and Co3O4 components with a 3D stacked-up structure via a facile aqueous synthesis method without adding any structure directing agent, through coherent nucleation and crystal growth of Mn3O4 on 6

Co3O4 supported on graphene oxide (GO), based on these two oxides having the similar spinel crystal structure. The synthesized Co3O4-Mn3O4/GO composite showed much better catalytic activity and durability towards ORR than Mn3O4/GO and Co3O4/GO, but far exceeding state-of-the-art Pt/C (E-TEK) in durability. The chemically interfacial and electronic structures of the composite, raised by the coupling of integrated Co3O4 and Mn3O4 nanoparticles to the graphene, were determined in detail. The possible synergistic effect for the composite catalyzing the ORR has been proposed here. This information is helpful to deeply understand physical origin of catalytic activity of multiphase composites as efficient catalysts for ORR. 2. Experimental 2.1 Synthesis of graphene oxide-supported Co3O4-Mn3O4 composite The Co3O4-Mn3O4/GO (graphene oxide) composite was synthesized in an aqueous solution by facial two-step method. In the first step, GO-supported Co3O4 nanoparticles were firstly synthesized by chemical precipitation in aqueous solution containing ammonia (NH3·H2O) and NaBH4 at 0℃, following hydrothermal treatment at 200℃. In a typical synthesis, ultrasonic treatment was used to disperse 70 mg graphene oxide in 300 ml deionized water uniformly, obtained homogeneous GO dispersion. Then 50 ml Co(OAc)2·6H2O solution (3.01 mg/ml) was dropped slowly into this GO dispersion, and kept stirring for 30 min. 2 ml ammonia was added into the previous solution and maintained stirring the mixed solution for 1 h under air. 7

After that, slowly added 100 ml NaBH4 solution (2 mg/ml) in the mixture, keeping the reaction temperature at 0 ℃ for 3 ~ 6 h. The as-synthesized hybrid was washed by a centrifugation with deionized water 5 ~ 6 times then kept in a vial as suspension. Subsequently, 4 ml ammonia was added into 35 ml suspension (containing ~70 mg as-prepared Co3O4/GO), then transferred into 50 ml Teflon water autoclave, kept at 200 ℃ for 3 h. Through this hydrothermal reaction, the well crystallized Co3O4/GO hybrid was formed and collected in the form of suspension. In the second step, the obtained Co3O4/GO sample was used as a support to synthesize Co3O4-Mn3O4/GO composite in an aqueous solution containing KMnO4, citric acid as reducing and complexing agent through a reflux at 80℃. Prior to the addition of the Co3O4/GO into the reaction solution, 0.084 g KMnO4 and 0.192 g citric acid monohydrate were dissolved in 50 ml deionized water formed homogeneous solution in 30 min, then 24.8 ml of this solution was added to the dilution obtained by dissolved 100 mg Co3O4/GO in 200 ml deionized water. The mixture was kept at 80 ℃ with stirring for 7 h under oil bath reflux. Finally, the resulted product (Co3O4-Mn3O4/GO) was collected by centrifugation and washed with pure water. And finally, the Co3O4-Mn3O4/GO composite was solidified by lyophilization. As reference materials, the Co3O4/GO and Mn3O4/GO samples were synthesized respectively. Here, Co3O4/GO hybrid was synthesized by using the similar procedure shown in the first step mentioned above, while Mn3O4/GO hybrid was synthesized by following the second step but GO replaced Co3O4/GO as carrier. The actual weight ratio of Co3O4, 8

Mn3O4 and GO components in the hybrid catalysts was determined by ICP and TG measurements. As shown in Fig.S1 ((Supporting Information)), the actual weight ratio of Co3O4, Mn3O4 and GO components in the Co3O4-Mn3O4/GO hybrid is 51.3 %, 21.9 % and 26.8 % respectively. The actual weight ratio of Co3O4 in the Co3O4/GO is 58.1%, while the actual weight ratio of Mn3O4 in the Mn3O4/GO is 25.1%. 2.2 Physical characterizations X-ray

diffraction

(XRD)

measurements

were

performed

for

the

Co3O4-Mn3O4/GO, Co3O4/GO, and Mn3O4/GO composite, with Rigaku RINT 2200V/PC using Cu Ka radiation (l=1.5406 Å) over the range of 10-90°with a scan rate of 1° min-1. Raman spectra for the Co3O4-Mn3O4/GO composite were determined by using LabRam HR800 with a visible laser (λ=532 nm) at room temperature. The morphology of the composite was tested by transmission electron microscopy (TEM, JEOL2100F), and further investigated by an aberration-corrected scanning transmission electron microscope (STEM, JEOL ARM200F) operated at 200 kV and energy dispersive X-ray spectroscope (EDX) mapping. The electronic structures of the composite materials were characterized by X-ray photoelectron spectroscopy (XPS, ESCALAB 250). 2.3 Electrochemical activity characterizations The ORR electrocatalytic activities of the synthesized Co3O4-Mn3O4/GO, Co3O4/GO, and Mn3O4/GO samples were characterized using the rotating disk electrode in 0.1 M KOH solution. During the measurements, the rotating ring-disk 9

electrode (RRDE) was used as working electrode; a saturated calomel electrode (SCE) and Pt foil were used as reference and counter electrodes, respectively. During preparation of the working electrode, 10 mg of each catalyst sample was dispersed ultrasonically in 2 ml of alcohol and 100 µl of Nafion. Then, 20 µl of each catalyst suspension was pipetted onto the ring rotating disk electrode (S = 0.247 cm2). The scan rate used for all measurements is 5 mV s-1. 3. Results and discussion 3.1 Morphology and formation mechanism for Co3O4–Mn3O4/GO composite

Figure 1. (a) X-ray diffraction patterns for various samples including Co3O4/GO, Mn3O4/GO and Co3O4-Mn3O4/GO samples; (b) Raman spectra of Co3O4/GO, Mn3O4/GO and Co3O4-Mn3O4/GO samples, respectively. Figure 1a presents the XRD patterns for the synthesized Co3O4-Mn3O4/GO composite, Co3O4/GO, and Mn3O4/GO samples as references. For all the tested samples shown in Figure 1a, an abroad peak located at about 26° is attributed to the face-to-face stacking of the C(002) crystalline plane of the graphene oxide matrix.[24] The broad diffraction peak can be indexed to the disordered graphene oxide sheets.[26] For the synthesized Co3O4-Mn3O4/GO composite, there are eight distinct diffraction 10

peaks located at about 19.0°, 31.3°, 36.8°, 38.5°, 44.8°, 55.6°, 59.4° and 65.2° respectively, which are indexed to the (111), (220), (311), (222), (400), (422), (511) and (440) crystal facets of cubic structure Co3O4 with a spinel structure (JCPDS 43-1003).[27] Compared with those of the observed from the Co3O4/GO sample shown in Figure 1a, the recorded diffraction peaks of Co3O4 phase in the composite are strong and sharp and no obvious broadening and weakening of these characteristic peaks were observed, suggesting the formed Co3O4 phase having a well-crystallized spinel structure.[28] Moreover, the as-fabricated Co3O4-Mn3O4/GO composite also exhibits another obvious peak located at 18.0° in Figure 1a. The position of this peak is in great agreement with that of Mn3O4/GO sample, as shown in Figure 1a. So, this peak derived from Mn3O4 can be indexed to (101) plane of spinel Mn3O4 (JCPD-24-0734), which reveals the formation of separate Mn3O4 phase, apart from the Co3O4 phase in the composite. Therefore, we can conclude that the synthesized composite is consisted of both spinel Co3O4 and Mn3O4 phases, which form a multiphase structure rather than a single solid solution phase of two oxides.

To further distinguish the nature of the nanophase structure of the as-synthesized composite, Raman scatterings for the Co3O4-Mn3O4/GO, Co3O4/GO, and Mn3O4/GO samples were performed and the obtained results are shown in Figure 1b. Similar to the Co3O4/GO sample, the Raman spectrum of the Co3O4-Mn3O4/GO composite shows five prominent peaks located at 188, 470, 516, 610 and 673 cm-1, which (2) (1) 7 [29] correspond to F(3) The 2g , Eg, F2g , F2g , Oh Raman active modes of the spinel Co3O4.

11

phonon symmetries of these Raman peaks are caused by the lattice vibrations of the spinal structure, in which Co2+ and Co3+ cations are situated at tetrahedral and octahedral sites in the cubic lattice of Co3O4.[30] This provides an additional evidence of the formation of pure spinel Co3O4 phase in the composite. Compared to the spectrum of Co3O4/GO sample, however, the positions of all the recorded Raman peaks slightly shift to the higher frequencies, which should indicate the changes in agglomeration or lattice constriction of Co3O4.[31] Since the TEM analysis from Figure 2 has already confirmed that no any agglomeration of Co3O4 nanoparticles, the positively shifted Raman peaks must be related to the chemical interaction between Co3O4 and Mn3O4 phases through intimate contact of two oxides in this composite, which lead to the lattice constriction of Co3O4. Moreover, compared to the Mn3O4/GO sample in Figure 1b, the weakened even concealed Raman peaks of Mn3O4 component in the composite further confirmed the strong chemical interaction between Co3O4 and Mn3O4 phases. This strong interaction between two oxides may result from a unique nanophase configuration formed by them.

12

Figure 2. (a) HRTEM images for Co3O4-Mn3O4/GO composite; (b), (c) and (d) High magnification STEM dark field images of the composite; (e), (f) and (g) STEM-EDS elemental mapping of the composite: the green is Co while the red is Mn. Figure 2a shows the HRTEM image of the synthesized Co3O4-Mn3O4/GO composite. As observed, the gray sheets with several wrinkles are the graphene oxide, and the dark sphere-like nanoparticles anchoring on the graphene matrix should be Co3O4 or Mn3O4 phases. It can be easily recognized that these oxide nanoparticles are uniformly dispersed on the surfaces of the graphene with a narrow particle size distribution and no distinctly aggregation in Figure 2a. The average size of these nanoparticles is approximately 13.5 nm, as depicted in Figure 2b. Since different metals usually present different contrast against the background in the STEM image, Co and Mn elements in their oxides can be easily distinguished in Figure 2b. It seems that the gray is Mn3O4 only dispersed around the bright Co3O4 particles while the particle size of the Mn3O4 is far smaller than that of the Co3O4. The small Mn3O4 nanocrystals dispersed around the Co3O4 nanoparticles form an oxide-on-oxide 13

nanostructure in this composite. As evidenced by Figure 2c, the lattice spacing (d value) of an individual Co3O4 nanocrystal was measured to be 0.470 nm, very close to that (0.471nm) of the (111) lattice plane of cubic spinel Co3O4 (JCPDS 43-1003).[32] For comparison, the lattice spacing of the oxide nanocrystal adjacent to the Co3O4 nanoparticle was measured to be 0.49 nm, consisting to the (101) lattice plane of tetragonal spinel Mn3O4 (0.492 nm).[33] This image clearly shows the formation of the oxide-on-oxide nanostructures through close particle-on-particle contact between two oxides in this composite. As depicted from Figure 2d, the formation of the special nanostructure may result from the epitaxial growth of Mn3O4 along the exposed (111) lattice plane of the Co3O4 (as in Figure 2d (111) Co3O4//(101) Mn3O4). Because of Mn3O4 and Co3O4 having the similar spinel crystalline structure but the graphene oxide having a planer hexagonal structure, the spinel Mn3O4 can be rationalized to preferentially deposit on the surfaces of spinel Co3O4 nanoparticles rather than the graphene through epitaxial growth of Mn3O4. To confirm this conclusion, STEM-EDS elemental mapping was performed for the composite. As observed in Figure 2e-g, the Mn3O4 (green stands for Mn3O4) had been successfully deposited onto the surface of Co3O4 (red stands for element Co) and the formed Mn3O4-on-Co3O4 nanostructrures were well dispersed on the surface of the graphene. No Mn3O4 nanoparticles alone can be seen on the graphene matrix. The distribution of Mn is slightly larger than that of Co, indicating that the spinel Mn3O4 nanocrystals prefer to grow along the (111) plane

14

of the Co3O4 particles supported on the graphene to form the unique 3D stacked-up structure in this composite material.

Figure 3. Schematic illustration of the formation process of Co3O4-Mn3O4/GO composite

Figure 4. (a) UV absorption spectra of various cobalt ions, such as bare Co2+, [Co(NH3)6]2+ and [Co(NH3)6]3+ ions in the aqueous solution; (b) UV absorption spectra of Mn2+ ions generated in the reaction solution and citric acid chelating Mn2+ ions as reference.

15

The formation mechanism for the Co3O4-Mn3O4/GO composite with the stacked-up nanostructure can be explained as following. The composite was synthesized by a two-step process, as shown in Figure 3. In the first step, through direct growth of Co3O4 on the graphene oxide, Co3O4/GO hybrid was fabricated by chemical precipitation method in aqueous solution containing ammonia (NH3·H2O) and NaBH4 at 0℃, followed by hydrothermal treatment at 200℃. Due to the existence of epoxyl and hydroxyl functional groups on the surfaces of the graphene oxide, Co2+ ions derived from Co(OAc)2 as cobalt resource were electrostatically attracted and homogeneously anchored to the graphene surfaces.24 After addition of ammonia as a ligand into the reaction solution, it can react with Co2+ ions to generate unstable [Co(NH3)6]2+ ions,[34] which would be oxidized to stable [Co(NH3)6]3+ by dissolved O2 in the solution, as confirmed by Figure 4a. According to Equation 1, the oxidation reaction of [Co(NH3)6]2+ into [Co(NH3)6]3+ ions would take place spontaneously, because the redox potential of [Co(NH3)6]2+/[Co(NH3)6]3+ ( 0.1 V versus SHE) are significantly more negative than that of O2/OH- (0.401V versus SHE). However, the redox potential of Co2+/Co3+ ( 1.82 V versus SHE) is much positive relative to that of O2/OH- (0.401 V versus SHE), which means that the oxidation reaction from Co2+ to Co3+ ions cannot take place in absence of ammonia in this synthesis system. As NaBH4 was added into the synthesis system, it can not only facilitate partially reduce Co3+ to Co2+ ions, but also adjust the pH of the reaction solution to trigger the hydrolysis reaction of these cobalt ions at 0℃, forming small 16

amorphous Co3O4 domains on the graphene. Subsequently, the amorphous Co3O4 domains were converted to the well crystallized Co3O4 nanoparticles by hydrothermal treatment at 200℃, accompanying by the particle growth of the oxide on the graphene.

In the second step, the obtained Co3O4/GO sample was used as support to synthesize Co3O4-Mn3O4/GO composite in an aqueous solution containing KMnO4 and citric acid as reducing and complexing agent through a reflux at 80℃. Prior to the addition of the Co3O4/GO into the reaction solution, the added MnO4- ions as manganese source in the solution were firstly reduced to Mn2+ by excessive citric acid at room temperate. Because KMnO4 is a strong oxidizing agent, the reduction reaction above is very fast and almost MnO4- ions transformed into Mn2+, as confirmed by Figure 4b. Moreover, the reduced Mn2+ can combine with the residual citric acid to form divalent manganese complex in this solution. Subsequently, the Co3O4/GO synthesized above was added into the reaction solution and the mixture was refluxed at a certain temperature of 80℃. During the reflux stage, partial Mn2+ ions combined with citric acid were easily oxidized to Mn3+ ions by dissolved O2 in the aqueous solution.[35] With increasing reaction time, both Mn3+and Mn2+ ions will chemically hydrolyze and then precipitate to form crystalline Mn3O4 nanocrystals deposited on the surfaces of Co3O4, as depicted in Figure 2. During the hydrolysis and precipitation procedure, the elevated reaction temperature (80℃) is essential to promote the

17

hydrolysis precipitation of these manganese complex ions, because it is considerably difficult for the hydrolysis precipitation to take place at room temperature.

Moreover, as confirmed by the results shown in Figure 2, the produced Mn3O4 nanocrystals preferentially located onto the surfaces of the Co3O4 particles rather than the

graphene,

which

lead

to

the

oxide-on-oxide

nanostructures

in

this

Co3O4-Mn3O4/GO composite. The reason for the preferential deposition of Mn3O4 on the surfaces of Co3O4 may be attributed to the coherent nucleation of Mn3O4, since the manganese oxide features a typical spinel structure similar to that of the cobalt oxide, which would trigger the epitaxial growth along specific crystal planes of Co3O4. As evidenced by the results from Figure 2c and d, the crystalline Mn3O4 nanocrystals would grow along the (111) lattice plane of Co3O4 nanoparticles. Since the oxygen functional groups on the graphene oxide have been consumed by the deposit of the cobalt oxide, the lack of the oxygen functional remarkably may hinder the chemical deposit of the manganese oxide on the surfaces of the graphene oxide, which also contributes the formation the oxide-on-oxide nanostructures supported on the carbon in this Co3O4-Mn3O4/GO composite. Co3+ + e- → Co2+

( E0=1.82 V vs. SHE)

(1)

Co(NH3)63+ + e- → Co(NH3)62+

( E0=0.1 V vs. SHE)

(2)

O2(g) + 2H2O + 4e- → 4OH-

(E0=0.401 V vs. SHE)

18

(3)

3.2 Electronic structure of Co3O4–Mn3O4/GO composite features a stacked-up structure

Figure 5. (a) C1s and (b) O1s XPS spectra for Co3O4-Mn3O4/GO, Co3O4/GO samples and pure graphene oxide, respectively; (c) Co 2p XPS spectra of Co3O4/GO and Co3O4-Mn3O4/GO samples; (d) Mn 2p XPS spectra for Mn3O4/GO, Co3O4-Mn3O4/GO samples, respectively. The electronic structure of the Co3O4-Mn3O4/GO composite may be strongly related to its catalytic performances, yet little has been known about the nature of the electronic structure raised by the 3D stacked-up structure in this composite. Therefore, we performed XPS measurements for the Co3O4-Mn3O4/GO composite, Co3O4/GO and pure graphene oxide as references, respectively. The obtained XPS C1s and O1s spectra for these tested samples have been fitted by a Gaussian-Lorentzian model with Shirley background correlation, as depicted by 19

Figure 5a and b. As observed in Figure 5a, the C1s spectra for all the tested samples can be fitted to the mainly non-oxygenated C–C in aromatric rings (285.0 eV) and the C in C–O (286.5~286.6 eV) and C=O (287.8 eV).[36] For the Co3O4-Mn3O4/GO composite, however, the position of the peak corresponding to the C–O bonds shows a remarkably negative shift about 0.24 eV relative to that of the pure graphene oxide support. Moreover, through estimating from the corresponding peak areas in the C1s curve, the relative content of the C–O bonds in the composite is about 26.5 at%, which is significantly higher than that of the pure graphene oxide (about 17.7%). These outcomes strongly imply the formation of covalent C–O–M (M=Co or Mn) bonds at the interface between the oxides and carbon, which is responsible for the higher content of the C–O bonds in the composite. However, compared with that of the Co3O4/GO sample, the C–O bonds recorded in the composite shows no obviously negative shift in bonding energy or significant increase in its relative content, as shown in Figure 5a. It suggests that the nature of C–O–M bonds in the composite is similar to that in Co3O4/GO sample and the bonds should be C–O–Co bonds rather than C–O–Mn, because of the Mn3O4 having no directly contact with the carbon support. This conclusion is coincident with the STEM-EDS results shown in Figure 2, where Mn3O4 grows on the surface of Co3O4 rather than graphene matrix to form the oxide-on-oxide structure in this composite.

The formation of the C–O–Co bonds at the interface in the composite can also be confirmed by its O1s XPS spectrum shown in Figure 5b. As observed, the determined 20

O1s curve for the Co3O4-Mn3O4/GO composite could be fitted five peaks at about 533.3, 531.9, 530.4, 530.8 and 529.8 eV, respectively. Two peaks at about at 533.3 and 531.9 eV are assigned to C–O, –O–C=O bonds and the bonding energies of two peaks are consistent with the results available in the literature.[37] Another two peaks at about 530.3 and 529.8 eV are attributed to the Co–O[38] and Mn–O[24] bonds in each oxide matrix. The last peak at about 530.8eV may be ascribed to the C–O–M bonds formed in the composite, because the pure carbon support doesn't display this peak shown in Figure 5b. Moreover, the position and the relative content of the characteristic peak, assigned to the C–O–M bonds in the composite, are almost similar to that of the Co3O4/GO sample, suggesting the C–O–M bonds must be the C–O–Co bonds. This conclusion is well consistent with the result from the C1s spectra shown in Figure 5a. It provides an additional evidence for the formation of the predominant C–O–Co bonds in the composite.

Furthermore, compared with the peak assigned to the Co–O bonds in the Co3O4 sample in Figure 5b, the position of the corresponding peak recorded in the Co3O4-Mn3O4/GO composite slightly shifts in the direction of the higher bonding energy, indicating the chemical interaction between Co3O4 and Mn3O4 component through lattice oxygen existed in each oxide as bridge. Thus interaction may result from the unique oxide-on-oxide structure composed of two oxides through their intimate interphase contact. In order to get more information about the electronic interaction between two oxides, we performed XPS Co 2p spectrum of the 21

Co3O4-Mn3O4/GO composite material, and the obtained results are displayed in Figure 5c. From Figure 5c, it is clearly noted that the recorded Co 2p curve for the Co3O4-Mn3O4/GO composite displays one doublet with a Co 2p1/2 bonding energy of 795.7 eV and Co 2p3/2 bonding energy of 780.02 eV, which are characteristic of the oxide of Co3O4.[15] But, there is a slightly positive shift of Co 2p bonding energy, revealing that the 2p electron in Co is donated to the adjacent Mn3O4 supported on the Co3O4. To further confirm this conjecture, XPS Mn 2p spectrum of the composite was also conducted and the obtained result is shown in Figure 5d. As observed, there are two peaks at about 641.9 eV and 653.6 eV corresponded to Mn 2p3/2 and Mn 2p1/2, separated by about 11.7 eV (Figure 5c), which is in good agreement with the characteristics of manganese (II, III) oxide, Mn3O4.[39] Different from the positive shift of the Co 2p, a slightly negative shift of Mn 2p bonding energy has been observed. Thus, both positive shift of Co 2p bonding energy and negative shift of Mn 2p bonding energy can confirm the strong electric interaction between two oxides in their composite. This interaction may result in covalent electron transfer from Co3O4 to Mn3O4 components through the shared lattice oxygen atoms as bridge, which highly correlates with the enhanced ORR performance of the composite material. 3.3 The improved electrocatalytic activity of Co3O4–Mn3O4/GO composite

22

Figure 6. Comparison of ORR electro-catalytic activity for the fabricated Co3O4-Mn3O4/GO, Co3O4/GO, Mn3O4/GO catalysts and a commercial Pt/C (20wt % of Pt, relative to carbon, E-TEK). (a) ORR polarization curves for these samples in an O2-saturated 0.1M KOH solution with a rotation rate of 1600 rpm and a sweep rate of 5 mV s-1; (b) half-wave potential of these samples derived from (a); (c) and (d) the kinetic current densities (Jk) of these samples derived from (a) at -0.2V vs. saturated calomel electrode. To study the ORR electro-catalytic activity of the synthesized Co3O4-Mn3O4/GO composite, polarization plot was carried out by using a rotating disk electrode (RDE) at a fixed rotation rate of 1600 rpm in oxygen-saturated 0.1 M KOH solution at room temperature, as depicted in Figure 6a. For comparison, the polarization curves for the Co3O4/GO, Mn3O4/GO and a commercial Pt/C catalyst (E-TEK, 20%) were also performed under the same tested condition and the obtained results are shown in Figure 6a. As observed in this figure, the Co3O4-Mn3O4/GO composite shows a very high ORR performance that is comparable to that of the commercial Pt/C (E-TEK), 23

but it is much better than the individual Co3O4/GO and Mn3O4/GO samples. From Figure 6b, the electro-catalytic activity of these tested samples, evaluated from their half-wave potentials, follow the trend: Pt/C > Co3O4-Mn3O4/GO > Mn3O4/GO > Co3O4/GO, with the half-wave potential values of -187.3 mV, -223.5 mV, -265.9 mV and 312.2 mV versus a saturated calomel reference (SCE) electrode, respectively. The half-wave potential for the synthesized Co3O4-Mn3O4/GO catalyst is positively shifted by 42.5 mV and 88.7 mV respectively, relative to that obtained from Mn3O4/GO and Co3O4/GO samples. It reveals a higher electro-catalytic activity towards ORR over this composite catalyst. Since the kinetic current density (Jk) is correlated with the intrinsic activity of catalysts, the current densities of these tested samples were determined at a given potential of -0.2V (vs. SCE), where the influence of mass transport is negligible. The obtained results are depicted in Figure 6c. As expectedly, the Jk of the composite catalyst is very close to that of the commercial Pt/C catalyst but it is much higher than that obtained from the independent Co3O4/GO or Mn3O4/GO, further suggesting the superiority of the catalyst. Definitely, the recorded kinetically controlled current density (JK) on the Co3O4-Mn3O4/GO composite is 2.078 mA/cm2, which is about 2.65 and 2.22 times higher than that generated on Co3O4/GO (0.7836 mA/cm2) and Mn3O4/GO (0.9364 mA/cm2), respectively. More importantly, it is noted that the Jk generated on the composite remarkably surpasses the sum of both Co3O4/GO and Mn3O4/GO catalysts, as depicted in Figure 6d. It reveals that the remarkably enhanced ORR activity may be associated with the 24

synergistic effect between these two oxides and the graphene in this composite material. Although the nature of the synergy of the carbon-based composites remains still unclear, it is in general believed that the improved electro-catalytic performance is substantially associated with the unique chemical and electronic structures, raised by their nanophase structures. In our case, the unique 3D stacked-up nanostructure formed by two oxides and the graphene in this composite can lead to the strong covalent electron transfer at the interface between them as a result of the interphase ligand effect, as illustrated in Figure 5, which may be responsible for the improved ORR kinetics.

Figure 7. (a) Rotating ring and disk current obtained from different catalysts including Co3O4/GO, Mn3O4/GO, Co3O4-Mn3O4/GO and the commercial Pt/C in O2-saturated 0.1M KOH at 1600 rpm; (b) Percentage of peroxide formation during the ORR over these catalysts above; (c) The number (n) of electron transfer in ORR at the GC electrodes coated with these Co3O4/GO, Mn3O4/GO, Co3O4-Mn3O4/GO and Pt/C(20%) catalysts in O2-saturated 0.1M KOH. 25

Since the ORR pathway, in general, involves two-electron and four-electron processes regardless of the reaction occurred in alkaline or acidic environments, the hydrogen peroxide production is another key indicative of their ability of catalyzing ORR for catalysts. It is believed that the formation of hydrogen peroxide during ORR not only can lower oxygen utilization but also decay the durability of catalysts, because hydrogen peroxide as a strong oxidant can seriously corrode the carbon support and transition metal oxide like Co3O4, Mn3O4 and etc.[17],[21] To avoid the decay, thus, low or negligible yield of hydrogen peroxide is required for the composites as ORR catalysts from the view of practical application. Therefore, we monitored the production of hydrogen peroxide on the tested catalysts, such as Co3O4-Mn3O4/GO, Co3O4/GO, Mn3O4/GO, and the commercial Pt/C, by using a ring-rotating disk electrode (RRDE) at a fixed rotation rate of 1600 rpm in oxygen-saturated 0.1 M KOH solution with saturated O2. The resultant outcomes are illustrated in Figure 7a. The disk current (Id) presents the rate of the ORR through directly four-electron pathway to produce OH- whereas the ring current (Ir) indicates the oxidation reaction rate of H2O2 intermediate generated by the ORR via two-electron pathway. For all the tested catalysts, the hydrogen peroxide production (Y(H2O2)) and the corresponding electron transfer number (n) during the ORR were calculated from the measured Id and Ir data at a fixed cathodic polarization potential (-0.3 V), according to the following equations[40]: (

) 26

(

)

where, Id, and Ir represent the disk current and ring current, respectively. N is the current collection efficiency of the ring electrode in the RRDE apparatus and it is determined to be 0.36. In this way, the calculated hydrogen peroxide productions on these catalysts are shown in Figure 7b, while the corresponding number of transfer electrons in ORR on each catalyst is in Figure 7c. From Figure 7b, the determined yields of hydrogen peroxide intermediate during the ORR are remarkably different, remarkably dependent on these tested catalysts. The yield of H2O2 on the efficient Co3O4-Mn3O4/GO catalyst is below ~6.22%, which is slightly higher than that of the commercial Pt/C (3.95%) but far lower than those of Co3O4/GO (10.4%) and Mn3O4/GO (24.28%), respectively. It illustrates that the ORR catalyzed by the Co3O4-Mn3O4/GO catalyst may undergo via a nearly four-electron pathway to generate substantial OH- and very small amount of hydrogen peroxide. As evidenced by the results in Figure 7c, the number of transfer electrons in the ORR on the synthesized Co3O4-Mn3O4/GO catalyst is 3.90, which is close to that of Pt/C (3.94) but higher than those of Co3O4/GO (3.79) and Mn3O4/GO (3.52), This outcome provides another evidence that the ORR catalyzed by the Co3O4-Mn3O4/GO catalyst proceeds via a four-electron pathway to generate substantial OH- and negligible hydrogen peroxide.

27

Figure 8. (a) The chronopotentiometry curves in O2-saturated 0.1 M KOH at room temperature for the synthesized Co3O4/GO, Mn3O4/GO, Co3O4-Mn3O4/GO and 20% Pt/C; (b) Cyclic voltammograms of Co3O4-Mn3O4/GO and 20% Pt/C in O2-satuated 0.1M KOH solution in initial and after 1000 cycles CV in N2-satuated 0.1M KOH solution at 50 mV/s; (c) The onset potential of 20% Pt/C and Co3O4-Mn3O4/GO before and after 1000 cycles CV, derived from (b). Furthermore, the durability of Co3O4-Mn3O4/GO catalyst for ORR was evaluated by chronopotentiometry experiment in O2-satuated 0.1 M KOH solution at room temperature. For comparison, the Co3O4/GO, Mn3O4/GO and the commercial Pt/C catalyst were tested under the same condition. The experimental results obtained from these catalysts are shown in Figure 8a. As observed, the decay in the ORR activity of these catalysts over time is remarkably different. After continuous operation at a given polarization potential (-0.2 V) for over 7000s, the recorded current density of oxygen reduction over the Co3O4-Mn3O4/GO catalyst still has 94.61% of its initial 28

current density. On the contrary, the current densities obtained from the Co3O4/GO, Mn3O4/GO and Pt/C samples drop to 89.21%, 87.63% and 78.45%, respectively, suggesting the Co3O4-Mn3O4/GO composite having superior durability for ORR. Although the durability of individual Co3O4/GO and Mn3O4/Go catalyst is poor and comparable with each other shown in Figure 8a, their composite shows the excellent durability. To further verify the long-term stability of these catalysts, moreover, we also conducted a set of cyclic voltammograms (CVs) for Co3O4-Mn3O4/GO catalyst and the commercial Pt/C catalyst in N2-satuated 0.1M KOH solution, as shown in Figure 8b. The corresponding onset potentials of these two samples were determined in initial and after 1000 CV cycles. As demonstrated in Figure 8c, the onset potential of Co3O4-Mn3O4/GO composite only downshift 0.4 mV after 1000 cycles, which is much smaller than that of commercial Pt/C (60.8 mV) shown in Figure 8d. It suggests the superiority in long-term operation for the composite catalyst. The remarkably enhanced durability may originate from the 3D stacked-up structure comprising two oxides. In this composit design, because of Mn3O4 having good ability to catalyzing chemical decomposition of H2O2, it locates at the top of Co3O4 phase, can favor the chemical disproportionation of H2O2 intermediates generated by the cobalt oxide during the ORR as a result of the interphase ensemble effect between two oxides.

As shown in Fig. S2 (Supporting Information), we can observe that the kinetic rate of chemical disproportionation of H2O2 on Co3O4-Mn3O4/GO catalyst is much 29

higher than that of the other catalysts. It reveals that the hybrid catalyst has high catalytic activity towards the decomposition of H2O2 intermediate because of its unique nanophase structure. To monitor the production of hydrogen peroxide on the Co3O4-Mn3O4/GO catalyst, rotating ring-disk electrode (RRDE) measurements have been conducted in 0.1M O2-saturated KOH solution under a series of rotation rates, as shown in Fig.S3 ((Supporting Information)). As observed, under different rotation rates, these ring currents are almost similar. This shows that the kinetic rate of chemical disproportionation of H2O2 on Co3O4-Mn3O4/GO catalyst is fast, which is almost comparable to the production rate of H2O2 intermediate on the catalyst. 3.4 Analysis of origin of the enhanced activity in ORR

30

Figure 9. (a) Schematic illustrating the interfacial structures between Co3O4, Mn3O4, and graphene. (b) The schematic of the catalytic ORR pathway catalyzed by Co3O4-Mn3O4/GO catalyst. Based on the formation of the unique 3D stacked-up structure in the synthesized Co3O4-Mn3O4/GO composite, where Mn3O4 nanocrystals are located only on Co3O4 surfaces while the cobalt oxide particles are directly integrated into the graphene surfaces shown in Figure 2, therefore, the improved ORR electro-catalytic activity may be attributed to the enhanced covalent electron tranfer at the interface between these two oxides and the grapheme oxide matrix as a result of their interphase ligand and ensemble effects in this composite. In detail, the origination of the favourable ORR can be explained by several possibilities.

First, since the Co3O4 component were directly integrated onto the graphene surface, the chemically intimate contact of them leads to the formation of the covalent 31

C–O–Co bonds at the interface between the oxide and carbon, as proved by XPS results in Figure 5a and b. Thus chemical linkage can promote the covalent electron transfer from the carbon carrier to the Co3O4 nanoparticles through the the formed C– O–Co bonds where the coordinated oxygen atoms act as bridge, as depicted in Figure 9a. It would lead to the carbon atoms on graphene surfaces adjacent to the oxide nanoparticles presented substantial positive charge density.[37], [41] Some researchers have confirmed that the highly positive charge on the surfaces of carbons like carbon black and carbon nanotube can offer favorable sites for side-on oxygen surface adsorptionon onto the carbon surface; this side-on adsorption of active oxygen molecules contributes the ORR followed a direct four-electron pathway to generate OH- .[24],[42],[43]. Moreover, the chemical bonds formed at the interface between the Co3O4 and carbon can facilitate the formation of defects like cobalt vacancies that lead to an oxygen richment in the spinel Co3O4 matrix.[44],[45] Different from ideal spinel Co3O4 with an almost-perfect cubic close-packed arrangement of oxygen ions with cations distributed among tetrahedral and octahedral sites, the defected Co3O4 can further contribute the ORR kinetics of the composite catalyst, because of the presence of an excess of Co3+ that would be compensated by the cation vacancies.[15] F. Y. Cheng further confirmed that spinel oxides possessing numerous defects and abundant vacancies, show remarkable reactivity toward the ORR.[46]

Second, for the Co3O4-Mn3O4/GO composite, the presence of Mn3O4 component which only locates on the Co3O4 surfaces can remarkably impact the electronic 32

structure of the active cobalt oxide as a result of oxide-on-oxide interphase ligand between these two oxides. As evidenced in Figure 5c and d, substantial covalent eletrons in Co3O4 component can be attracted to Mn3O4 component due to the electronic affinity of Mn3O4, through the strong covalent electron tranfer from the cobalt oxide to the neighboring manganese oxide shown in Figure 9a, which would make Co3O4 lose some electrons. As a result, the electron-depleted Co3O4 on carbon, caused by the interphase interaction between two oxides, can capture more electrons from the graphene support through the C–O–Co bonds, which remarkably improves the ORR activity of the composite catalyst. This conclusion can be verified by fact that the electro-catalytic activity in ORR over the Co3O4-Mn3O4/GO catalyst is much higher than that obtained on the Co3O4/GO or Mn3O4/GO samples, as illustrated in Figure 6.

Third, the significantly improved electro-catalytic activity of the composite is also attributed to the ensemble effect at the interface in the composite shown in Figure 9. The so-called ensemble effect, where different catalyst substances can complement each other by catalyzing the different reaction steps, has been reported for the hybrids of precious metals like Pt and Ag with transition metal oxides like Mn3O4.[24] In our case, the formed oxide-on-oxide structure in the Co3O4-Mn3O4/GO composite can offer a strong synergistic effect for the multistep reaction of the ORR. As shown in Figure 9b, the Co3O4 phase serves as main catalyst on the graphene can catalyze oxygen reduction reaction via nearly 4 electron pathway to form large amount of OH33

and small amount of H2O2 via 2e pathway. The trace amount of H2O2 intermediates produced over the Co3O4 catalyst or the carbon can diffuse along the oxide surfaces or through the solution to reach the adjacent Mn3O4 surfaces or Co3O4-Mn3O4 interface, where it would undergo a chemical disproportionation to generate OH− and O2 specie. The O2 species from disproportionation of the H2O2 can again be utilized as reactants by the further ORR. Owing to the oxide-on-oxide structure formed in the Co3O4-Mn3O4/GO composite, the diffusion distance is very small, which would result in the very fast disproportionation of H2O2 intermediate. It can not only promote the decomposition of H2O2 but also advance the utilization of O2, thereby allowing for the complete utility of the active oxygen. Moreover, owing to the epitaxial growth of Mn3O4 on Co3O4 in their composite shown in Figure 2, the Mn3O4 component would form some defects in the oxide, as evidenced from the strong variation in relative intensities of XRD peaks shown in Figure 1a and the slightly lattice strain shown in Figure 2c for the spinel Mn3O4 phase in the composite. M. Casas-Cabanas et al[45] claimed that the lattice strains or the defects in spinel materials can promote the chemical decomposition of H2O2. Some investigators [24],[47] further confirmed that the adsorption of hydroxyl groups (OH-) onto the oxide surface, which are usually generated by defects, can favor the removal of H2O2 intermediate Even in extreme cases, if Co3O4 phase is inactivated due to long-range operation, it will lead to the ORR on the Co3O4-based composite undergoing a two-electron pathway and produce a large amount of H2O2. However, the ensemble effect derived 34

from the unique oxide-on-oxide structure in this composite maybe still keep excelent catalytic performance due to the specific chemical catalysis of Mn3O4 toward H2O2 decomposition, as shown in Figure 9c. In this case, a pseudo four-electron reduction process is achieved because of the good ability of Mn3O4 to chemically catalyze decomposition

of

hydrogen

peroxide.[24]

Therefore,

the

fabrication

of

Co3O4-Mn3O4/GO composite featured a smart nanophase structure may open a new way to developing efficient and cheap noble-metal free ORR catalysts for various fuel cells and metal-air batteries.

4. Conclusion In summary, we have synthesized a ternary composite with a well-defined nanostructure comprising Co3O4, Mn3O4 nanocrystals and a graphene oxide via a facile aqueous synthesis method without adding any structure directing agent, through preferential nucleation and growth of Mn3O4 nanocrystals on the surface of Co3O4 nanoparticles

rather

than

the

graphene.

These

formed

Mn3O4-on-Co3O4

nanostructrures, with a diameter of about 13.5 nm, are well dispersed on the surface of the graphene to eventually form a 3D stacked-up nanostructure in this composite. The formation machinism of the 3D stacked-up nanostructure is ascribed to the favourably coherent or epitaxial growth of Mn3O4 on the surface of Co3O4, because of the two oxides having a similar spinel structure. In an alkaline environment, the Co3O4-Mn3O4/GO composite exhibits outstanding electrocatalytic activity and 35

durbility towards ORR. The half-wave potential of the composite is about -0.2235 V, which is much more positive than those obtained from individual Mn3O4/GO and Co3O4/GO samples. The current density of oxygen reduction for the composite is 2.078 mA/cm2, which exceeds the sum of that obtained from independent Co3O4/GO and Mn3O4/GO. The remarkably improved ORR activity can be attributed to the enhanced synergy as a result of the strong ligand and ensemble effects at the interface between two oxides and the graphene, raised by the 3D stacked-up structure in the composite. In this composit design, the desired functions of each compoment are not only effectively utilized, but realized an additional synergistic effect between them. This study may provide a promising route to enhance ORR electrocatalytic activity by combining the varius active transition oxides with carbon in a cheap and noble-metal free multyphase system.

Acknowledgments This work was supported by National Natural Science Funds of China (Grant Nos. 51272018, 51432003) Appendix A. Supplementary material Supplementary

data associated with this article can be found in Supporting

Information

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Highlights 

We have synthesized a novel ternary composite with a special 3D stacked-up nanostructure by selective and epitaxial depositing of Mn3O4 nanocrystals onto surface of Co3O4 nanoparticles on a graphene matrix.



The novel Mn3O4-Co3O4/GO composite exhibits much better electrocatalytic activity, which is comparable to that of a commercial Pt/C (20 %) but far exceeding the sum of that obtained from the Co3O4/GO and Mn3O4/GO.



The remarkably improved ORR activity is closely attributed to the enhanced 39

synergy between these two oxides and the graphene, raised by the 3D stacked-up structure composed of Co3O4, Mn3O4 and graphene in this composite. 

The formed 3D stacked-up structure can promote covalent electron transfer from carbon support to the oxides as a result of the interphase ligand effect between them, which facilitate the ORR kinetics.

40