MnO2 Nanofilms on Nitrogen-Doped Hollow Graphene Spheres as a

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MnO2 Nanofilms on Nitrogen-Doped Hollow Graphene Spheres as a High Performance Electrocatalyst for Oxygen Reduction Reaction Qiangmin Yu, Jiaoxing Xu, Chuxin Wu, Jianshuo Zhang, and Lunhui Guan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11870 • Publication Date (Web): 07 Dec 2016 Downloaded from http://pubs.acs.org on December 8, 2016

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ACS Applied Materials & Interfaces

MnO2 Nanofilms on Nitrogen-Doped Hollow Graphene Spheres as a High Performance Electrocatalyst for Oxygen Reduction Reaction Qiangmin Yu,[a][b][c] Jiaoxing Xu,[a][b] Chuxin Wu,*[a][b] Jianshuo Zhang[a][b][c] and Lunhui Guan*[a][b] [a]

Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Institute of

Research on the Structure of Matter, Chinese Academy of Sciences. Fuzhou 350108, China [b]

Fujian Provincial Key Laboratory of Nanomaterials, Fujian Institute of Research on the

Structure of Matter, Chinese Academy of Sciences. Fuzhou 350108, China [c]

University of Chinese Academy of Sciences, Beijing 100049, China.

Keywords: Nitrogen-doped graphene; oxygen reduction reaction; synergistic effect; Zn-air battery.

Abstract: Platinum is commonly chosen as electrocatalyst used for oxygen reduction reaction (ORR). In this study, we report an active catalyst composed of MnO2 nanofilms grown directly on nitrogen-doped hollow graphene spheres, which exhibits high activity toward ORR with positive onset potential (0.94 V vs RHE), large current density (5.2 mA cm-2) and perfect

ACS Paragon Plus Environment

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stability. Significantly, when it was used as catalyst for air electrode, a Zinc–air battery exhibited a high power density (82 mW cm-2) and specific capacities (744 mAh g-1), comparable to that with Pt/C (20 wt%) as air cathode. The enhanced activity is ascribed to the synergistic interaction between MnO2 and the doped hollow carbon nanomaterials. This easy and cheap method paves a way of synthesizing high-performance electrocatalysts for ORR.

1. Introduction As for fuel cells and metal-air batteries, high performance electrocatalysts for oxygen reduction reaction (ORR) are crucial. Platinum (Pt) exhibits the best performance on the cathode among the electrocatalysts for ORR.1-3 However, the high price of precious Pt metal prevents the Pt-based catalysts from widely using in commercial markets.4-5 In addition, the simultaneous oxidation of fuel, such as methanol, at cathode, ascribed to the crossover effect, reduce the ORR performance of Pt-based cathode catalyst.6 Thus, it is emergent to increase the activity and reduce the costs of the ORR catalysts. A lot of works have been devoted to designing high-efficient Pt-free catalysts for ORR. For instance, non-precious metal oxides and carbon nanomaterials, including carbon nanotubes and grapheme, have been actively studied as electrocatalysts thanks their fruitful resource and environmental friendness.7-10 Among the metal oxides, lower crystallinity manganese oxides (MnO2) with a porous microstructure, high surface area and fast ion transportation have been widely developed for ORR catalyst.11-13 Nevertheless, MnO2 exhibit unsatisfied ORR activities due to its low electrical conductivity. It is desirable to increase the catalytic activity of MnO2 for ORR by combining with high conductive materials. Recently, various forms of carbon were used


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as electrode materials to support the metal oxides due to their unique structure and extraordinary electrical conductivity.14-15 However, the loose interaction between metal oxides and nanocarbon materials decreased the durability of the electrocatalysts.

Therefore, it has become a crucial

issue to design an optimal structure of MnO2 closely supported on carbon materials to improve the catalytic performance.16-18 In this study, we fabricated an active catalyst based on MnO2 nanofilms directly grown on nitrogen-doped hollow graphene spheres (denoted as MnO2/NHGSs) through a mild oxidation reaction on nitrogen-doped hollow graphene spheres (N-HGSs). The catalyst exhibits superior catalytic performance for ORR, such as the positive half-wave potential, high current density, and superb stability in alkaline media. Furthermore, the Zn–air batteries by using MnO2/N-HGS as catalyst for air cathode presented high performance comparable to that with the commercial Pt/C (20 wt%) catalyst. The excellent ORR activity of MnO2/N-HGSs is from the synergetic effect between MnO2 and N-HGS.

Scheme 1. Schematic presentation showing the formation processes of MnO2/N-HGS catalyst. 2. Experimental section 2.1. Synthesis of MnO2/N-HGSs and reference samples


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MnO2/N-HGSs nanocomposites were prepared via template method and mild oxidation process, as shown in Scheme 1. Firstly, silica template spheres with mean diameter of 300 nm were introduced into the suspension of graphene oxide, which were prepared by the Hummer’s method and stirred for 3h. After standing, discarding the upper solution and drying the lower suspension overnight at 80℃, the dried products were heated at 850℃ for 2 h under flowing nitrogen, forming SiO2@rGO. Subsequently, the hollow graphene spheres (HGSs) were obtained by removal of SiO2 templates with HF (10 wt %) solutions etching for 12 h at room temperature. Secondly, the N-HGSs were synthesized through doping HGSs with nitrogen. The HGS suspension was first mixed with 3 ml NH4OH (30% solution). The mixtures were treated by hydrothermal reaction in an autoclave at 180 ℃ for 8 h. After filtration, the obtained samples were annealed at 800℃ under nitrogen atmospheres for 2 h. Finally, low concentration of KMnO4 (0.012 mol L-1) was gradually dripped into N-HGS dispersion at 60℃ for 2 h. The obtained solid sample was annealed at 800℃ under nitrogen atmosphere for 2h, forming the MnO2/N-HGS hybrid. Another reference sample MnO2/HGSs was synthesized under the same steps by gradually dripped low concentration of KMnO4 into HGS dispersion. 2.2. Preparation of the electrode 5 mg of MnO2/N-HGS catalyst and 100 µL of Nafion solution (DuPont Corp., 5 wt %) were dispersed in 1.9 mL of isopropanol alcohol and mixed to form a uniform ink by an ultrasonic bath. Then, 30 µL of the catalysts ink were loaded onto a neat glass carbon electrode (GCE) surface with a diameter of 5 mm. The commercial Pt/C (20 wt%) and other catalysts were also measured using the procedure. 2.3. Preparation of Zn-air batteries and battery test


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The MnO2/N-HGS nanocomposites were used as the air electrode for a Zinc-air battery. Homogeneous catalyst ink consisting of catalysts, ionomer (Nafion solution, 5 wt %) and ethanol was sprayed onto a gas diffusion layer with a catalyst loading amount of 1.0 mg cm-2. For comparison, the commercial Pt/C (20 wt%) catalysts with the same loading were prepared using the same method. To evaluate the performance of the Zn-air batteries, a cell with volume of 40 mL were assembled. The anode was a polished zinc plate and the electrolyte was 6 M KOH.. The effective area of the air electrode is 3.14 cm2 and allows O2 from ambient air to reach the catalyst sites. 2.4. Characterizations and Electrochemical Measurements The microstructures and elemental mapping of the MnO2/N-HGS catalyst were observed by using field-emission transmission electron microscopy (TEM, FEI, JEM-2010) and scanning electron microscopy (SEM, SU8010). Surface areas of the materials were measured by using N2 adsorption-desorption isotherms at 77 K on Autosorb-iQ2-XR (Quantachrome Instruments). Xray photoelectron spectroscopy (XPS) tests were performed on a VG Scientific ESCALAB 250Xi. Thermogravimetric (TG) measurements were performed on a NETZSCH STA449 F3 Jupiter analyzer. The electrochemical characterizations were performed on a CHI 760D electrochemical analyzer (CH Instruments, Inc., Shanghai, China), coupled with a RDE and RRDE system. A three-electrode cell was used with a gas flow system. A GCE was used as the working electrode, a Pt wire was used as the counter electrode, a standard/AgCl electrode was used

as

the

reference

electrode.

All

the

potentials

were

calibrated to RHE, ( ERHE = EAg/AgCl + 0.20 V + 0.0591pH). All the tests were performed at ambient environment. The electrochemical performance of the catalysts in an aqueous solution of 0.1 M KOH was explored by cyclic voltammetry (CV) and linear sweeping voltammetry


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(LSV) measurements. CV curves were obtained at a scan rate of 50 mV s-1. LSV was performed at rotating rates of 500, 900, 1200, 1600, 2000, and 2500 rpm from positive to negative with a sweeping rate of 10 mV s-1. The KOH solution was bubbled with pure N2 (99.999%) or O2 (99.999%) for 30 min before the electrochemical measurements. Chronoamperometric tests were performed at 0.77 V in alkaline medium for 12 h with a speed of 1600 rpm. Koutecky-Levich (K-L) equations were chosen to analyze the data from RDE19:

= / +

(1)

= 0.62 ( )/ / = nFk

(2) (3)

Where, j is the measured current density, ω is the electrode rotation rate, F is the Faraday constant (F = 96485 C cm-1), C0 is the bulk concentration of O2 (C0 = 1.2×10-3 mol L-1), D0 is the diffusion coefficient of O2 (D0 = 1.9×10-5 cm s-1) in 0.1 M KOH, v is the kinetic viscosity of the electrolyte (0.01 cm2 s-1), and k is the electron-transfer rate constant. B can be determined from the slope of the K-L equation. "#$%&'

=

#$%&' (

)*%+, -

. / % =

(4)

#*%+, /1 #$%&' (

)*%+, -

(5)

Where N is the collection efficiency (37%), IDisk and IRing are the voltammetric currents at the disk and ring electrodes, respectively.20 3. Results and discussion The pore distribution of MnO2/N-HGSs was first analyzed by N2 adsorption-desorption isotherms with HGS as comparison (Figure 1a). The Brunauer-Emmett-Teller (BET) specific surface area of the MnO2/N-HGSs is 302 m2 g-1, similar with 318 m2 g-1 of the HGSs, suggesting


6

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that the diameter distribution of the pores are from 2 to 35 nm. The total pore volume of MnO2/N-HGS is 1.8 cm3 g-1, little higher than that of HGS (1.6 cm3 g-1), which is benefit to mass transfer. These data demonstrates that the whole structure of HGSs was barely influenced by KMnO4 mild oxidation. The morphology of MnO2/N-HGSs was characterized by SEM and TEM. Figure 1b and 1c indicate that the composite possesses an unbroken sphere structure. Figure 1d reveals that the MnO2 nanofilms are supported on the transparent N-HGSs homogeneously. Moreover, HR-TEM image illustrates the low

crystallinity of the MnO2

nanofilms on the N-HGS surface (inset in Figure 1d), The HR-TEM image reveals weak lattice fringes with a distance of 0.14 nm and 0.24 nm, ascribed to the (020) and (111) plane of MnO2, consistent with the XRD results (Figure S2). The elemental mapping of the MnO2/N-HGS indicated that C, N, O and Mn atoms were distributed uniformly in the MnO2/N-HGS (Figure 1e).


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Figure 1. (a) Nitrogen sorption isotherm, of the HGS and the MnO2/N-HGS , the inserted image shows the pore size distribution of the materials; (b) SEM images of MnO2/N-HGS (c) and (d) TEM images with different magnification. (e) STEM image of the MnO2/N-HGS, together with the elemental mapping images of C, N, O and Mn. XPS analysis (Figure 2a) reveals that the catalyst is mainly composed of C (84.9 at %), N (2.3 at %), O (8.7 at %), and Mn (4.1 at %), confirming that N have been successfully doped into the MnO2/N-HGS composite. The content of MnO2 was also investigated by thermogravimetric analysis (Figure S1), which is basically in accordance with the analysis of XPS. The high resolution spectrum of N1s (Figure 2b) is divided into four peaks, which can be assigned as graphitic N (centered at 401.1eV), pyrrolic N (centered at 399.8 eV), pyridinic N (centered at 398.6 eV) and N-oxides respectively.21-23 The graphitic N and pyridinic N are more ORR active


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than other forms of N.24 The O 1s peak centered at 530.5 eV is ascribed to the oxygen bonded with manganese in MnO2 (Figure 2c). Another peak centered at 532.7 eV is assigned to C-O in graphene.

25

The Mn2p spectra is divided into two prominent peaks centered at 653.2 and 641.8

eV, ascribed to the Mn 2p1/2 and Mn 2p3/2 spin–orbit peaks of MnO2, respectively.26 The energy separation of Mn 3s keeps the value of 4.9 eV (Figure 2e), indicating that in the composite, Mn4+ ions are dominant.27 The XPS results are in qualitative agreement with the XRD results shown in Figure S2.

Figure 2. (a) The XPS spectrum surveyed from 50 to 700 eV of MnO2/N-HGS and the atomic % of each of element; high-resolution (b) N 1s spectrum; (c) O 1s spectrum;(d) Mn 2p spectrum;(e) Mn 3s spectrum.


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The ORR activity of the MnO2/N-HGS in the alkaline electrolyte (0.1 M KOH) saturated by N2 or O2 was measured in a typical three-electrode system. As shown in Figure 3a, in the O2saturated 0.1 M KOH electrolyte, there appears a clear cathodic current with a peak centered at 0.82 V , which is absent in the N2-saturated electrolyte, implying the occurrence of efficient ORR of MnO2/N-HGS catalyst. The anodic peak in Figure 3a was assigned to the transformation of manganese ions from low valent stateto high valent state.28 RDE and RRDE measurements were also conducted to check the catalytic performance of the MnO2/N-HGS together with the reference samples in O2-saturated 0.1 M KOH with a sweep rate of 10 mV s-1 at 1600 rpm. In RDE measurement (Figure 3b), the MnO2/N-HGS catalyst exhibited a prominent positive onset potential (0.94 V) than 0.83 V of the N-HGS () and 0.88 V of the MnO2/HGS catalysts. The halfwave potential for MnO2/N-HGS (0.84 V) was similar to 0.85 V of Pt/C, and larger than that of N-HGS (0.71 V) and MnO2/HGS (0.75 V). The MnO2/N-HGS catalyst also showed a quite stable diffusion limiting current of 5.2 mA cm-2, which was close to the 5.4 mA cm-2 of the commercial Pt/C (20 wt%)). The ORR onset potential of the N-HGS catalyst showed a much positive shift after MnO2 films was grown on the surface. Besides, the current density of MnO2/N-HGS catalyst increased drastically than that of N-HGS catalyst. This result demonstrated that the active sites of catalyst were increased by MnO2 anchored on N-HGS. RDE tests at different rotating speeds were performed to obtain the kinetic-limiting current density (jk) andelectron transfer number (n) . When the rotating speeds increased, the current density at the MnO2/N-HGSelectrode enhanced, becaused at high speeds the diffusion distance shortened (Figure 3c). The corresponding K-L plots showed perfect linearity and parallelism to the case of Pt/C (Figure 3d). The electron transfer number (n) of the MnO2/N-HGS electrode was calculated to be 3.65-3.85 (Figure 3f), only a little less than 3.98-3.90 of the Pt/C electrode at the potential


10

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value from 0.77 to 0.47 V, suggesting a nearly 4e oxygen reduction process. The results were consistent with the RRDE measurements. As shown in Figure 3e, in the low overpotential scope of 0.77 V to 0.47 V, the H2O2 yield was below 10%, and the n was calculated to be 3.68-3.53 (Figure 3f,), slight lower than 3.82-3.73 of Pt/C. Apparently, the performance of MnO2/N-HGS was greatly enhanced compared with N-HGS and MnO2/HGS catalyst. Moreover, the catalytic activity of MnO2/N-HGS was one of the best catalytic performances for MnOx composites reported by previous results (Table S1 in supporting information). The superior electrocatalysis performance of MnO2/N-HGS for ORR contributes from the synergistic effects of MnO2 and the N-HGS associated with a unique porous structure.


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Figure 3. (a) CV curves of the MnO2/N-HGS electrode at 50 mV s-1 in 0.1 M KOH solutions saturated with N2 or O2. (b) LSVcurves of the HGS, N-HGS, MnO2/HGS, MnO2/N-HGS and Pt/C catalysts in 0.1 M KOH saturated with O2 at 10 mV s-1 at 1600 rpm. (c) LSV curves of the MnO2/N-HGS electrode at different rotation speeds in 0.1 M KOH saturated with O2 at 10 mV s-1. (d) The K–L plot of the Pt/C and the MnO2/N-HGS electrodes calculated from Figure (c) and Figure S3 at different potentials. (e) RRDE voltammogram curves of MnO2/N-HGS and Pt/C electrodes at 1600 rpm. (f) The n values of the Pt/C and the MnO2/N-HGS electrodes calculated on RDE and RRDE at different electrode potentials. We further measured the stability and tolerance to methanol performance of the catalysts, which was tested by adding 1 M methanol into the O2-saturated 0.1 M KOH solution. The current density of the Pt/C electrode shows a sharp decrease when methanol was introduced (figure 4a), while the current density on the MnO2/N-HGS electrode exhibits a small decay when methanol was introduced. These results demonstrate clearly that the active functional groups on the MnO2/N-HGS catalyst are much steadier than those on the commercial Pt/C. The long-term stability of the catalysts was studied using the current–time (i–t) chronoamperometric method, which was carried out at a fixed potential of 0.77 V for 12 h in 0.1 M KOH solution saturated with O2 at 1600 rpm (Figure 4b). The MnO2/N-HGS catalyst presents superior stability to Pt/C catalyst, with small decay (~15%) in ORR activity over 12 h of successive operation. On the contrary, the Pt/C catalyst exhibited ~26% decrease in activity, indicating that the long-term ORR currents of Pt/C catalyst were lower than that of the MnO2/N-HGS catalyst.


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Figure 4. (a) Current–time (i–t) chronoamperometric responses at 0.67 V in 0.1 M KOH solution saturated with N2 for the MnO2/N-HGS and Pt/C electrodes at the rotation speed of 1600 rpm then introducing O2 and adding methanol (1 M). (b) Current–time chronoamperometric responses at 0.77 V of the commercial Pt/C and the MnO2/N-HGS electrodes over 12 h in 0.1M KOH solution saturated with O2 at 1600 rpm. To evaluate the utility of MnO2/N-HGS catalyst in actual energy conversion devices, a Zn-air battery was fabricated. For comparison, the commercial Pt/C catalysts were also measured under the identical conditions. The open-circuit voltage of MnO2/N-HGS was 1.48 V with a peak power density of 82 mW cm-2, which is comparable to that of Pt/C catalysts (1.49 V and 94 mW cm-2) in the Zn-air batteries (Figure 5a). The galvanostatic discharge curves (Figure 5b) indicated that the discharge potentials plateaus decreased when the current densities increased. The battery with the MnO2/N-HGS catalyst showed voltage plateaus of ≈1.31, ≈1.26 and ≈1.14 V at the discharge current densities of 5, 10 and 25 mA cm-2, respectively. These values are higher than other previous results.29-31 In the galvanostatic discharge curves (Figure 5c), the voltage of MnO2/N-HGS in long-term discharge after 24 h didn’t dropped, highlighting the excellent stability of the MnO2/N-HGS catalyst toward ORR. The specific capacity of the Zn–air battery


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assembled with a MnO2/N-HGS air cathode was 744 mA h g-1 (only considering the mass of Zn consumed) at 10 mA cm-2, comparable with 757 mA h g-1 of Pt/C as air cathode.

Figure 5. (a) Polarization and power density curves of the Zn-air batteries with MnO2/N-HGS (red lines) and Pt/C (black lines) as catalysts; (b) The galvanostatic discharge curves of a Zn–air batterie with MnO2/N-HGS as catalysts at various current densities (5, 10, and 25 mA cm-2); (c) Long-term galvanostatic discharge curves of Zn–air batteries with MnO2/N-HGS and Pt/C catalysts at a discharging current density of 10 mA cm-2. 4. Conclusions In summary, we have constructed a nitrogen-doped hollow graphene spheres with MnO2 loading. The new nanocomposites were utilized to catalyze ORR. The enhanced ORR activity is ascribed to the synergistic effect between the unique Nitrogen-doped hollow spheres structure and MnO2


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nanofilms anchored. The catalyst shows good catalytic activity and long durability, as well as better practicability, which make it an ideal candidate ORR catalyst in energy conversion devices. Supporting Information TGA curves of MnO2/N-HGS and HGS; XRD patterns of MnO2/N-HGS and HGS; LSV curves of the Pt/C electrode at different rotation rates in O2-saturated 0.1 M KOH at 10 mV s-1 (; and electrochemical performance of different electrocatalysts for ORR. Corresponding Author * Correspondence to: Chuxin Wu, Lunhui Guan, [email protected]; Telephone/Fax: 86-591-63173550.

E-mail:

[email protected];

Acknowledgements This research was supported by the Science and Technology Planning Project of Fujian Province (Grant No. 2014H2008), the strategic Priority Research Program of the Chinese Academy of Sciences (No. XDA09010402) and National Natural Science Foundation of China (Grant no.21501174) . References (1). Meier, J. C.; Galeano, C.; Katsounaros, I.; Topalov, A. A.; Kostka, A.; Schuth, F.; Mayrhofer, K. J. J., Degradation Mechanisms of Pt/C Fuel Cell Catalysts under Simulated StartStop Conditions. Acs Catalysis 2012, 2 (5), 832-843. (2). Kang, Y. J.; Ye, X. C.; Chen, J.; Cai, Y.; Diaz, R. E.; Adzic, R. R.; Stach, E. A.; Murray, C. B., Design of Pt-Pd Binary Superlattices Exploiting Shape Effects and Synergistic Effects for Oxygen Reduction Reactions. J Am Chem Soc 2013, 135 (1), 42-45. (3). Proch, S.; Wirth, M.; White, H. S.; Anderson, S. L., Strong Effects of Cluster Size and Air Exposure on Oxygen Reduction and Carbon Oxidation Electrocatalysis by Size-Selected Pt-n (n

MnO2 Nanofilms on Nitrogen-Doped Hollow Graphene Spheres as a

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