Preparation of Porous Graphene@Mn3O4 and Its Application in the

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Preparation of porous graphene@Mn3O4 and its application in oxygen reduction reaction and supercapacitor Tian Wang, Qiujian Le, Xiaolong Guo, Ming Huang, Xiaoying Liu, Fan Dong, Jintao Zhang, and Yu Xin Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04447 • Publication Date (Web): 11 Oct 2018 Downloaded from http://pubs.acs.org on October 13, 2018

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ACS Sustainable Chemistry & Engineering

Preparation of porous graphene@Mn3O4 and its application in oxygen reduction reaction and supercapacitor Tian Wang,† Qiujian Le,† Xiaolong Guo,‡ Ming Huang,*,§ Xiaoying Liu,# Fan Dong,# Jintao Zhang,⊥ and Yu Xin Zhang*,† † College

of Materials Science and Engineering Chongqing University Chongqing 400044, P.R.

China. E-mail: [email protected] (Y. X. Zhang) ‡

College of Aerospace Engineering Chongqing University Chongqing 400044, P.R. China

§ School

of Materials Science and Engineering Ulsan National Institute of Science and

Technology (UNIST) Ulsan 44919, Republic of Korea. E-mail: [email protected]; [email protected] (M. Huang) #

Engineering Research Center for Waste Oil Recovery Technology and Equipment of Ministry of Education, College of Environment and Resources Chongqing Technology and Business University Chongqing, 400067, China

⊥

Key Laboratory for Colloid and Interface Chemistry, Ministry of Education, School of

Chemistry and Chemical Engineering Shandong University Jinan, 250100, China

ACS Paragon Plus Environment

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KEYWORDS: Porous graphene, Mn3O4, Electrocatalysis, Supercapacitor

ABSTRACT: Porous graphene has been recognized as a promising material for use in electrochemical applications. Engineering the porous graphene based hierarchical and hybrid structures is a promising way to further improve the electrochemical performances. Here we reported a rational design of porous graphene@Mn3O4 (PGM) structure for the application in both oxygen reduction reaction and supercapacitor. Thanks to the efficient porous graphene substrate and rational decoration of Mn3O4, the catalytic performance of as-prepared PGM is comparative to that of Pt/C when used as electrocatalysts for oxygen reduction reaction, showing relatively positive onset and half-wave potential (0.89 and 0.81 V) and large diffusionlimiting current density (5.85 mA cm-2). In addition, PGM also shows good specific capacitance (208.3 F g-1), cycle stability and rate performance when used in the supercapacitor electrodes and asymmetric device (maximum energy density of 30.1 Wh kg-1 and power density of 9500 W kg-1).

INTRODUCTION Graphene, a novel two-dimensional material, has fascinating electrical, mechanical and thermal properties and thus offers various possibilities for fabricating electronic and electrochemical devices.1,2 Up to now, various methods have been developed to synthesize graphene, which can be briefly divided into three categories: mechanical exfoliation,3 chemical vapor deposition (CVD)4 and wet chemical methods.5 Among


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them, CVD method holds the best controllability towards morphology and size of asprepared graphene by tuning the CVD growth condition and the substrate engineering. Moreover, porous graphene with unique structural and electronic characteristics, has opened up opportunities in various fields, such as supercapacitors,6 gas separation,7 lithium-air battery,8 spintronics9,10 and hydrogen storage.11 When used as electrode material, porous graphene can greatly enhance its wettability and accelerate the electrolyte diffusion compared with perfect graphene. Besides, in the case of CVDderived porous graphene, some defects and additional edges can be induced during the CVD deposition and the template removal processes. It is believed that these defects could lead to the unbalanced charge distribution and can serve as additional active sites in electrochemical reactions,12 which is crucial for the use of electrochemical active materials. Thus, porous graphene acquired from CVD method is a promising platform since the interconnected graphene units can serve as efficient electron pathways and abundant meso-/micro-pores can be employed as ion diffusion channels. With the benefits from the porous graphene, many efforts have been made to investigate the coupling/combination of graphene with other transition metal oxides (TMOs), such as Fe2O3, Co3O4, NiO, MnOx, to fully achieve their synergistic effect.13-21 Among these TMOs mentioned above for the combination with porous graphene, manganese oxides were believed as the most promising candidates due to their low cost, high electrochemical activity, and environmentally friendly nature. In addition, the abundant valences (+2, +3, +4, +6 and +7) of Mn endowed MnOx with more possibility for the use in electrochemical fields. As the +2 and +3 valence states of Mn both exist in Mn3O4 phase, it is believed that such mixed valences will highly promote the


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electrochemical activities. Although there have been some papers focused on the bifuctional applications in electrochemical fields.22-24 As far as we know, there were almost no works have reported the use of porous graphene@Mn3O4 hybrid materials as bi-functional active materials in both ORR and supercapacitor applications. Herein, we reported the preparation of the porous graphene@Mn3O4 (PGM) hybrid structure. With the synergistic effect of the conductive and porous graphene and the uniformly distributed Mn3O4, the PGM hybrid shows outstanding ORR and supercapacitor properties. The as-prepared PGM delivered relatively positive onset and half-wave potential (0.89 and 0.81 V vs RHE) and large diffusion-limiting current density (5.85 mA cm-2), which are comparative to those of commercial Pt/C. When used as supercapacitor electrode materials, the PGM achieved a specific capacitance of 208.3 F g1

with capacitance retention of 86% after 2000 cycles. An asymmetric supercapacitor with

PGM as the positive electrode and activated graphene as the negative electrode in 1 M Na2SO4 electrolyte had a gravimetric energy density of 30.1 Wh kg-1 at power density of 475 W kg-1. These results suggest that the potential use of the as-prepared PGM in future ORR catalyst and supercapacitor electrodes. EXPERIMENTAL SECTION Synthesis of magnesium oxide @graphene. The magnesium oxide (MgO) was applied as the template to grow porous graphene using a chemical vapor deposition method as described previously.25 Typically, magnesium oxide powder was loaded in a quartz boat, which was placed in the center of a quartz tube in the CVD system. The system was first pumped down to ~0.3 mTorr followed by two purges in pure Ar, and then a flow of an Ar gas (60 sccm) was introduced to achieve a pressure of 200 Torr. The system was then heated to 1030 °C in 1 h and


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Figure 1 Schematic illustration of the synthesis of porous graphene@Mn3O4.

then a flow of an CH4/H2 (50 sccm/30 sccm) mixture was then introduced into the system for the graphene growth for 1.5 hours. Synthesis of porous graphene (PG). 1 g MgO@graphene was added into 150 mL HCl aqueous (1 M) and followed with a constant stirring for 24 h to etch away the MgO template. The porpus graphene (PG) was collected by filtration, washing with deionized water and ethanol for several times, and dried at 80 °C. Synthesis of porous graphene@Mn3O4 。 Typically, 15 mg of as-prepared PG and 0.192 g of Mn(CH3COO)2·4H2O were mixed in 30 mL ethanol. The mixture was heated in a water bath (60 °C) followed by adding 2.61 mL of KOH-C2H5OH solution (2 M) dropwisely, and was maintained for several hours (5-8 hours) under vigorous stirring. After washing, porous graphene@Mn3O4 was dried at 60 oC, and was labeled as PGM-5, PGM-6, PGM-7 and PGM-8 respectively, according to their reaction time. Characterization and electrochemical measurements. The detail information can be found in Supporting Information. RESULTS AND DISCUSSION


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Material characteristics. The fabrication process for porous graphene@Mn3O4 is shown in Fig. 1. The porous MgO powders used as templates for the graphene synthesis were prepared by the calcination of the commercially available magnesium hydroxide (Purchased from Sigma Aldrich). Using the chemical vapor deposition (CVD) method, graphene thin layers encapsulate the MgO templates to form MgO@graphene core-shell structure. After the removal of the MgO templates in hydrochloric acid, porous graphene (PG) is obtained. The as-prepared PG is further used as the template for the deposition of Mn3O4 to form the PG@Mn3O4 composite, which can be described as the following equation: 6 Mn2++O2+12OH−

2Mn3O4+6H2O

In Fig. 2a, the diffraction peaks of the as-prepared sample matched the standard XRD pattern of Hausmannite-Mn3O4 (JCPDS 80-0382). The characteristic diffraction peak of graphite at 26.4° was indiscernible, indicating the few-layer nature of deposited graphene. The Raman spectra of porous graphene (Fig. 2b) displays characteristic D peak (~1340 cm−1), G peak (~1580 cm−1), and 2D peak (~2700 cm−1), which are representative Raman peaks of graphene-based structures.26-29 The intensity ratio of D and G bands (ID/IG) the obtained PG is about 2, which

(a)

(b)

Figure 2. (a) XRD patterns of porous graphene@Mn3O4, (b) Raman spectra of porous graphene and porous graphene@Mn3O4.


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indicated high-density defects on the templated graphene grown herein. After deposition of Mn3O4, we can clearly observe a typical peak located at around 650 cm-1 which is characteristic Eg mode of crystalline Mn3O4.20,30 Both XRD and Raman spectroscopy results corroborate the successful combination of the Mn3O4 with the porous graphene. To verify the chemical states of Mn in the composite, XPS was performed and the result is shown in Fig. 3. The XPS full-survey-scan demonstrates the existence of Mn, O, C elements in the as-prepared composite. Mn3s spectrum can be fitted into four peaks which are located at 82.4, 83.5, 88.2 and 89.0 eV, respectively. The peaks at 83.5 and 89.0 eV with a spin energy separation of 5.5 eV is in good agreement with the Mn2p1/2

(a)

(b)

(c)

(d)

Figure 3. XPS spectra of porous graphene@Mn3O4. (a) XPS survey scan, (b) Mn3s spectrum, (c) O1s spectrum and (d) C1s spectrum.


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and Mn2p3/2 of Mn3+ and the other two peaks at the binding energy of 82.4 eV and 88.2 eV (with an energy separation of 5.8 eV) were assigned to the Mn2p1/2 and Mn2p3/2 of Mn2+. The co-existence of Mn2+ and Mn3+ in the as-prepared manganese oxide indicates the Mn3O4 phase which agrees well with our XRD data. The Mn2p spectrum (Fig. S1) further confirmed the Mn3O4 phase. In O1s spectrum, the apparent binding energies at 533.6, 531.2 and 529.7 eV can be assigned to the characteristic peaks of H-O-H, Mn-OH

(a)

(b)

and Mn-O-Mn respectively. The XPS peak of H-O-H may result from the water molecules absorbed on the surface of samples and the Mn-OH could be attributed to the bond between Mn and residual hydroxyl during the formation of Mn3O4. The XPS peak at 529.7 eV indicates the Mn-O-Mn bond. The above XPS results demonstrate the successful deposition of Mn3O4 on the surface of porous graphene. The morphologies of the as-prepared PG and PG@Mn3O4 were characterized by scanning electron microscope (SEM). As presented in Fig. 4a-b, MgO@graphene shows hexagonal shape with average size of around 500 nm. After the acid treatment, the hexagonal structure was maintained without the support of MgO. Nitrogen adsorption-desorption results (Fig. S2a and c) indicated that after MgO removal, an obvious increase of BET surface area appeared (from 60.3 m2 g-1 to 1327.1 m2 g-1) with a pore volume of 1.75 cm3 g-1. The pore size distribution obtained

(e)

(f)

from the adsorption branch by the Barrett–Joyner–Halenda (BJH) method indicated that after acid treatment, the hexagonal structure of template was maintained without the support of MgO. Besides, Fig. S2d clearly indicated that the porous graphene showed concentrated distribution of pores at around 3–20 nm, which can be attributed to the porous MgO template. The high BET specific surface area and the mesoporous structure can offer abundant active sites and efficient transport for electrons and ions, leading to a high electrochemical property. The surface area and


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(a)

(b)

(c)

(d)

(e)

(f)

Figure 4. SEM images of (a-b) MgO@graphene, (c-d) porous graphene and (e-f) porous graphene@Mn3O4 obtained at 6 h (PGM-6). porosity results indicate that the as-prepared porous graphene could be a promising support or platform for the deposition of pseudo-capacitive materials (such as transition metal oxides), leading to a better contact with electrolyte and hence promote the ions transfer during electrochemical process. To have a more clear understanding of the mass loading effect to the electrochemical performances, we have prepared a series of PG@Mn3O4 with different reaction


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(a)

(b)

(c)

(d)

(e)

(f)

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Figure 5. TEM images of (a-c) porous graphene and (d-f) porous graphene@Mn3O4.

time (Fig. 4e-f and Fig. S3). As the reaction time prolonged, the size of Mn3O4 nanoparticles was slightly increased. Among the four samples, PGM-6 exhibits a more uniform size and fluffy feature of the deposited Mn3O4. Specifically, when the reaction time was not enough (PGM-5), there was too little Mn3O4 decorated on the surface of porous graphene, indicating that porous graphene played dominant role in the electrochemical properties of PGM-5, which is far than enough. While the Mn3O4 shell becomes more dense and aggregated after 6 hours, these deposited Mn3O4 might block the tunnels for electrons and ions transport in the electrochemical test (Fig. S3c-d). In addition, under this circumstances, Mn3O4 made the major contribution to overall electrochemical performances, which will suffer from its inferior conductivity. Thus, it is assumed that PGM-6 would possess the best performance since the Mn3O4 nanoparticles are in


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uniform size and are well separated, which highly increase the utilization of the electroactive materials. Transmission electron microscope (TEM) images (Fig. 5a-c) show that PG has hexagonal shape with some transparency. The enlarged TEM image indicates its porous structure. After the deposition of Mn3O4, the structure was maintained but with more flaky surface morphology and porous feature. The TEM results verify the successful synthesis of such porous graphene@Mn3O4 core-shell hybrid structures. Catalytic properties in oxygen reduction reaction. The porous graphene@Mn3O4 was used in oxygen reduction reaction (ORR) to estimate its catalytic performance (Fig. 6). As shown in Fig. 6a, there was a much stronger peak (located at ~0.89 V) in the CV curve measured in O2saturated solution when compared with the CV curve measured in N2-saturated electrolyte, indicating its apparent ORR activity. To further figure out the influence of reaction time (a series of products prepared with different reaction time) on ORR activities, polarization curves of different types of PGM and commercial Pt/C were presented in Fig. 6b. Among all samples, PGM-6 delivers the best performance. Specifically, the onset potential (∼0.89 V vs RHE) and half-wave potential (~0.81 V vs RHE) of PGM-6 is much more positive than PGM-5, PGM-7 and PGM-8 samples, which are comparative to those of Pt/C (∼0.91 V and ~0.85 V vs RHE, respectively). Moreover, in the aspect of diffusion-limiting current density, the value of PGM-6 (5.85 mA cm-2) almost shows no difference with that of Pt/C (5.89 mA cm-2). These electrochemical results directly confirm the above-mentioned speculation about the influence of Mn3O4 loading on ORR activities. Specifically, when Mn3O4 loading is too low, little active sites can be offered to catalyse the O2 reduction in alkaline electrolyte. Meanwhile, overfull Mn3O4 could aggregate with each other and block tunnels for electrons and ions transport. In addition, the overall conductivity will also be highly damaged due to the poor conductivity of Mn3O4.


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(a)

(b)

(c)

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Figure 6. Oxygen reduction reaction properties of porous graphene@Mn3O4 (PGM). (a) CV curves in 0.1 M KOH solution saturated with N2 and O2, respectively.

(b)

Polarization

curves

of

commercial

Pt/C

and

porous

graphene@Mn3O4 samples prepared at different reaction time (5, 6, 7 and 8 h). (c) Polarization curves of PGM-6 at various rotation speeds. (d) K-L plots for PGM prepared at 5, 6, 7 and 8 h, (e) Electron transfer number of PGM prepared at 5, 6, 7 and 8 h. (f) Chronoamperometric profiles commercial Pt/C and PGM-6.


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Thus, we focused more on PGM-6 and investigated its overall catalytic activity in the next section. Polarization at various rotation speeds of PGM-6 were recorded to explore its electrocatalytic kinetics characteristics (Fig. 6c). Obviously, all curves show an ideal shape and indicate a uniform increase of diffusion-limiting current density with the increase of rotation speeds. This can be explained by the shortened diffusion length of O2. It’s apparent that PGM-6 shows superior current densities than all the other three samples at all rotation speeds. After deducting the current background in N2-saturated solution, the corresponding Koutecky-Levich (K-L) plots of PGM-5, PGM-6, PGM-7 and PGM-8 are shown in Fig. 6d. Typical first-order reaction kinetics of ORR is proved by the good linearity of these curves. According to the inverse relation between slope of K-L plot and electron transfer number (n), the calculated n at 0.3, 0.4 and 0.5 V is given in Fig. 6e. PGM-6 shows an average electron transfer number of 3.91, which is larger than that of PGM-5 (3.76), PGM-7 (3.52) and PGM-8 (3.02). These results indicate that the ORR catalyzed by PGM-6 is an efficient dominant four-electron reaction. Furthermore, the durability of PGM-6 was also measured via chronoamperometric method. After testing for 5 h, PGM-6 electrode still remained 89.3 % of initial current, while Pt/C electrode only held 77.6 % of initial current in the same test condition (Fig. 6f). These superior catalytic performances can be explained by rational design of nanostructures. On one hand, porous graphene obtained by CVD methods provides not only ideal substrate for electron and ion transfer, but also a stable template for the deposition of Mn3O4 nanoparticles, which offer enough active sites to catalyse the reduction of oxygen. On the other hand, appropriate Mn3O4 loading keeps a great balance between offering active sites


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and affecting the conductivity of PGM. To conclude, the relatively positive onset and half-wave potential, large diffusion current density, dominate four-electron reaction and outstanding durability all demonstrate the great potential of PGM-6 for use in the ORR catalyst. Electrochemical properties of supercapacitor. Apart from the measurement of ORR activity, we have also tested the capacitive performance of PGM-6 in order to show its general superiority in electrochemical fields. CV curves at various scan rates (5 to 200 mV s-1) of PGM-6 were

(a)

(b)

(c)

(d)

Figure 7. Capacitive properties of porous graphene@Mn3O4 electrode (PGM-6) in 1 M Na2SO4 solution. (a) CV curves at different scan rates. (b) Charge/discharge curves at different current densities. (c) Specific capacitance with different current densities and (d) Cycling performance.


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examined (Fig. 7a) which shows a gradual increase of the current with the increase of the scan rates. In order to evaluate the rate capability and specific capacitance of PGM-6, charge/discharge curves at current density of 0.5-10 A g-1 are displayed in Fig. 7b, through which the specific capacitances are calculated as 208.3, 178.7, 154.4, 126.0, 110.4, 107.0 F g-1 at 0.5, 1, 2, 5, 8, 10 A g-1, respectively. The capacitance retention is around 51.4% with a 20 times increase of the current density, showing a good rate capability (Fig. 7c). Figure 7d shows the long-term cycling performance of the PGM-6 for consecutive 2000 cycles. The specific capacitance of the electrode maintained about 86% of its initial value after 2000 cycles, demonstrating the good stability of the PGM as a supercapacitor's electrode. Additionally, the electrochemical impedance spectrums of the electrode before and after 2000 cycles are also presented (Fig. S4). After 2000 cycles, only a slight increase of the internal resistance from 1.2 to 2.9 Ω was observed. An asymmetry supercapacitor (ASC) based on PGM-6 was assembled to further investigate its practical application. CV curves (Fig. S5a) at different potential windows

(b)

(a)

(c)

Figure 8 Capacitive performances of an asymmetric supercapacitor assembled with PGM-6 as the positive electrode and active graphene (AG) as the negative electrode.(a) CV curves measured at different scan rates . (b) Charge-discharge curves at different current densities. (c) Ragone plot of the asymmetric supercapacitor.


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were tested to determine the most appropriate potential window. According to the CV test, 0-1.9 V is chosen as the potential window for the further tests. The CV profiles of the asymmetric cell remained relatively rectangular at a high scan rate of 200 mV s−1, which demonstrated good charge/discharge properties and rate capability of the asymmetric supercapacitor (Fig. 8a). As shown in Fig. 8b, the specific capacitance of the cell is calculated to be 60.1 F g-1 based on the total mass of active materials in the two electrodes at a current density of 0.5 A g-1. In addition, the capacitance was maintained more than 50% even with the increase of current density for 20 times, demonstrating the good rate capability of the ASC device (Fig. S5b). The device retained 82% of its initial specific capacitance after 2000 cycles (Fig. S5c). A maximum gravimetric energy density of 30.1 Wh kg-1 at power density of 475 W kg-1 and maximum power density of 9500 W kg-1 at energy density of 15.83 Wh kg-1 are obtained, respectively. These values are much higher than those of other related Mn3O4-based asymmetric supercapacitors (see details in the Ragone plots (Fig. 8c)).31-37 These results show our PGM//AG asymmetric supercapacitor device is promising in practical applications. CONCLUSIONS In summary, a porous graphene@Mn3O4 product has been prepared and used as electrochemical active materials. The porous and hierarchy structure allows better contact with electrolyte, resulting in good electrochemical performances in ORR and supercapacitors due to the synergistic effect of both the components. The as-prepared PGM delivered relatively positive onset and half-wave potential (0.89 and 0.81 V vs RHE) and large diffusion-limiting current density (5.85 mA cm-2), which are comparable to those of commercial Pt/C. For the use of electrodes in supercapacitor, PGM reached a


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specific capacitance of 208.3 F g-1 in the three-electrode system, and retained 86% capacitance after 2000 cycles. The assembled ASC device can be reversibly charged and discharged at an operation voltage of 1.9 V in 1.0 M Na2SO4 aqueous electrolyte, delivering a gravimetric energy density of 30.1 Wh kg-1 at power density of 475 W kg-1. The results indicated that the porous graphene@Mn3O4 is a promising candidate in for electrochemical application, and the proposed synthetic methodology would open new opportunities of other porous graphene@transition metal oxides hybrids for of the use as high performance electroactive materials. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: xx.xxxx/acssuschemeng.xxxxxxx Detailed characterization and electrochemical measurements. Additional XPS spectra, BET, SEM, Nyquist plots and other supercapacitor measurements. (PDF) AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Y. X. Zhang) *E-mail: [email protected]; [email protected] (M. Huang) Notes The authors declare no conflict of interest.


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ACKNOWLEDGMENT The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (Grant No. 21576034 and 21503025), the Innovative Research Team of Chongqing (CXTDG201602014) and State Education Ministry and Fundamental Research

Funds

for

the

Central

Universities

(106112017CDJQJ138802,

106112017CDJSK04XK11 and 106112016CDJZR135506). The authors also thank Electron Microscopy Center of Chongqing University for material characterizations.

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TOC

Porous graphene@Mn3O4 exhibited excellent performances as bi-functional active materials in oxygen reduction reaction catalysis and supercapacitor.


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Graphical abstract


Preparation of Porous Graphene@Mn3O4 and Its Application in the

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