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Manganese/Cobalt Bimetal Nanoparticles Encapsulated in Nitrogen-Rich Graphene Sheets for Efficient Oxygen Reduction Reaction Electrocatalysis Fawang Wu, Yanli Niu, Xiaoqin Huang, Yihong Mei, Xiuju Wu, Changyin Zhong, and Weihua Hu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b01890 • Publication Date (Web): 08 Jul 2018 Downloaded from http://pubs.acs.org on July 9, 2018

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ACS Sustainable Chemistry & Engineering

Manganese/Cobalt Bimetal Nanoparticles Encapsulated in Nitrogen-Rich Graphene Sheets for Efficient Oxygen Reduction Reaction Electrocatalysis Fawang Wu, Yanli Niu, Xiaoqin Huang, Yihong Mei, Xiuju Wu, Changyin Zhong, and Weihua Hu*

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

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

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ABSTRACT: It is of vital importance to search for non-precious metal based sustainable and efficient oxygen reduction reaction (ORR) electrocatalyst for next generation of energy conversion and storage technology. We herein report a hybrid bimetal material composed of MnO/Co nanoparticles encapsulated in nitrogen-rich graphene nanosheets (MnO/Co-N-G) as a high performance ORR catalyst in alkaline electrolyte. The MnO/Co-N-G catalyst is derived from Mn2+, Co2+ incorporated polydopamine (PDA) coated graphene oxide (GO) sheets via carbonization process. The morphology, structure, and composition properties of as-prepared MnO/Co-N-G catalyst are systematically investigated. Electrochemical measurements show that the MnO/Co-N-G catalyst exhibits excellent ORR activity superior to commercial Pt/C, featuring higher limiting current density, better methanol resistance and excellent long term durability in alkaline solution. The bimetal nanoparticles are believed to responsible to the impressive ORR activity of the catalyst.

KEYWORDS: MnO/Co-N-G; polydopamine; oxygen reduction reaction; MnO/Co nanoparticle; N-rich graphene

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INTRODUCTION The slow kinetics of oxygen reduction reaction (ORR) is a key bottleneck that affects the power density and energy conversion efficiency of regenerative fuel cells and rechargeable metal-air batteries.1-3 Efficient ORR electrocatalysts are required to address this problem. Carbon supported precious metal (namely, Pt) nanoparticles are the best ORR catalyst, but the scarcity and high cost of Pt limit the large-scale application of this kind of electrocatalyst.4-5 Alternatively, composite materials composed of earth-abundant transition metals (Fe, Co, Mn, etc.) anchored on nitrogen doped carbon scaffold (M-N-C) have been intensively investigated as promising sustainable ORR catalysts since the encouraging discovery of the impressive ORR activity of cobalt phthalocyanine.6-12 It is widely believed that the excellent ORR activity of these M-N-C catalysts originates from the atomic metal-nitrogen centers (M-Nx, x from 2 to 4).13-14 The metal agglomerates formed in the pyrolysis process are less active.15-16 On the other hand, recent in-depth research has indicated unambiguously that these metal agglomerates incorporated in N-doped carbon is able to enhance the ORR activity.17-18 In same cases, the Fe-N-C electrocatalyst without direct Fe-N coordination shows even better ORR performance than Pt/C catalyst in alkaline media.19-20 It may suggest that both the M-Nx centers and aggregated metal nanoparticles synergistically contribute to the ORR activity.21-23 Therefore, the composition and structure of M-N-C catalyst should be carefully optimized to synergistically combine the merits from M-Nx sites and metal agglomerates before maximizing the ORR activity. 24-25 3



Unary transition metal-based M-N-C catalysts such as Fe-N-C and Co-N-C with various configurations and morphologies have been intensively examined for ORR electrocatalysis

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. Incorporating binary or multiple metals into the doped carbon

affords a promising means to further improve the ORR activity as it allows for modulation of the electronic structure and microstructure of the resulting materials. In this regard, a myriad of bimetal M-N-C materials such as perovskite and spinel oxides have been developed and demonstrated to be competent ORR catalysts.29 Compared to the one containing single-crystalline-phase bimetal nanoparticles, binary M-N-C catalysts containing multiple crystalline-phase such as oxide/metal may be advantageous as the abundant defects and interfaces formed are potentially highly active sites for ORR.30 However, very limited research has been devoted to this area. In this work, we reported a promising bimetallic ORR catalyst composed of MnO/Co bimetallic nanoparticles encapsulated in N-doped graphene (denoted MnO/Co-N-G). It is synthesized via a simple pyrolysis of Mn2+, Co2+ incorporated polydopamine (PDA) coated graphene oxide (GO) sheets (Mn/Co-PDA-GO precursor) at 800 °C in Ar atmosphere, as shown in Figure 1. The optimal MnO/Co-N-G catalyst exhibits an excellent ORR performance in alkaline medium with higher limiting current density, better methanol resistance and excellent long term durability compared to commercial Pt/C. Most importantly, as-prepared MnO/Co-N-G catalyst outperforms its analogous based on unary metal. Meanwhile, it displays better performance than commercial Pt/C catalysts as a cathodic catalyst in a zinc-air primary cell.

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EXPERIMENTAL SECTION

Chemicals. Graphite powder (99.95%), Co (NO3)2·6H2O, dopamine, hydrogen peroxide solution (H2O2, 30 %), methanol (CH3OH, 99.5%), hydrochloric acid (HCl, 37 %), sodium nitrate (NaNO3) and sulfuric acid (H2SO4, 98%) were purchased from Aladdin Company. 20 wt % Pt/C and 5 wt% Nafion solution were obtained from Sigma. All reagents were used as received without further purification. Synthesis of electrocatalysts. GO was first prepared by a typical Hummers method with minor modifications.31 As-obtained GO suspension (1.33 mL, ca. 15.0 mg mL-1) and dopamine (40 mg) were mixed in 20 mL of Tris-buffer (pH = 8.5) under energetic stirring, to which MnCl2 solution (180 µL, 0.1 M) and Co(NO3)2 solution (20 µL, 0.1 M) were slowly added. The solution was stirred at room temperature for 24 h. The product was collected by centrifugation and washed several times with DI water before freeze-drying. The freeze-dried powder was processed at 800 ℃under Ar atmosphere for 3 h. For comparison, MnO-N-graphene (MnO-N-G), Co-N-graphene (Co-N-G) and N-doped graphene (N-G) were also synthesized by the similar procedure. Electrochemical measurements. To prepare the working electrode, 1.0 mg of the catalyst (MnO/Co-N-G, Mn-N-G, Co-N-G, N-G or commercial 20 wt% Pt/C) was dispersed in 1.00 mL of 0.05 wt % Nafion ethanol solution and sonicated for about 45 min. The catalyst-loaded electrode was prepared by dropping 5.0 µL of the above-prepared ink on a glassy carbon (GC, diameter 3.0 mm) electrode and dried under room temperature. For rotating ring-disk electrode (RRDE, diameter 5.0 mm), 5



25 µL ink solution was applied. Prior to the experiment, the electrolyte was bubbled with O2 via a bubbler for 15 min. Cyclic voltammetry (CV), linear sweep voltammetry (LSV) were measured on an Autolab potentiostat (PGSTAT302N) system coupled with a Pine rotator (AFMS-LXF) in a three-electrode cell. A Pt foil and Hg/HgO/6 M KOH were used as counter electrode and reference electrode, respectively. Zn-air battery testing. To evaluate the ORR performace of as-prepared catalyst, the catalyst was mixed with Super P and polyvinylidene fluoride (PVDF) at a weight ratio of 7:2:1 in N-methyl-2-pyrrolidone (NMP) under stirring. The ink was further pasted onto carbon fiber paper (1 × 1 cm2) with a loading of ca. 0.5 mg cm−2 . After drying in air, such catalyst-loaded carbon fiber paper was used as the air cathode. The electrolyte is 6.0 M KOH solution containing 0.2 M Zn(Ac)2. A zinc plate (2 × 5 × 0.5 cm3) was used as the anode. This Zn–air battery was allowed to discharge for 1 h at an external loading of 80 K ohm under the atmosphere. During measurment, the electrolyte was first saturated with oxygen, and an electrical resistor was connected between the anode and cathode, and the stable voltage across the resistor was recorded. Characterizations. Transmission electron microscopy (TEM) and field emission scanning electron microscopy (FE-SEM) images are collected on a JEOL JEM-2100 (200kV) and JSM-7800F with energy dispersive X-ray spectroscopy (EDS) from JOEL, respectively. The X-ray photoelectron spectroscopy (XPS) measurement was conducted on an ESCALAB 250Xi system equipped with a monochromated Al Kα

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source from Thermo Fisher. The powder X-ray diffraction (XRD) patterns were carried out on a D/Max2550 VB/PC diffractometer (40 kV, 200 mA) using Cu Kα as the radiation. Inductively coupled plasma optical emission spectrometry (ICP-OES) analysis was performed on Model ARCOS FHS12 (SPECTRO Analytical Instruments Inc., Germany).

RESULTS AND DISCUSSION XRD pattern of the as-prepared MnO/Co-N-G hybrid was shown in Figure 2(a). The six diffraction peaks at 34.9°, 40.5°, 58.7°, 70.2°, 73.8° and 44.2° correspond to (111), (200), (220), (311), (222) planes of MnO (PDF #07-0230), and (111) facet of metal cobalt (PDF#15-0806), respectively32-33, indicating the presence of crystalline MnO and Co nanoparticles in the catalyst. The broad peak at 26.2° is due to the presence of graphitized carbon. No impurity peaks were observed. SEM and TEM measurements were used to investigate the morphology of as-prepared MnO/Co-N-G. Figure 2(b) displays the SEM images of graphene sheets with wrinkles. Dense nanoparticles with tens nanometer diameter are uniformly loaded on the nanosheets. Notably, the nanoparticles are not simply attaching on graphene sheet but being encapsulated by a thin carbon layer, which is believed to be derived from the PDA film during the pyrolysis. This protective layer is able to prevent the nanoparticles from being detached or dissolved. Figure 2c further confirms that the nanoparticles are encapsulated on the graphitic carbon layers, implying good electrochemical stability of this catalyst. The size distrubution is also shown in Figure 2c. The nanoparticles demonstrate a diamter of 45.6±18.1 nm 7



according to the statistical analysis of total 135 nanoparticles. The high-resolution TEM image in Figure 2d demonstrates the well-defined crystalline lattice spacing of 0.25 nm and 0.218 nm, which matches well with the (111) plane of MnO phase and the (111) plane of metallic Co phase, and is consistent with the XRD pattern shown in Figure 2(a)

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. Remarkably, it is revealed in Figure 2d that the MnO domain and

metallic Co were well mixed in a single nanoparticle rather than to form individual particles with different crytalline phases. This unique structure contains abundant interfaces between different crystall domains and may facilitate the electrocatalysis. The XRD patterns, SEM and TEM images of the samples synthesized with solely Mn2+ or Co2+ are shown in Figure S1, which exhibits similar morphology with MnO or metallic Co crystalline phase, respectively. The element distribution on MnO/Co-N-G was investigated with EDS element mapping, as shown in Figure 2(e-i). The N element was uniformly dispersed on the whole surface of graphene sheets, as in Figure 2f and Figure 2g, suggesting uniform N-doping on the graphene. Figure 2h and Figure 2i show that Mg and Co elements are maibly distributed on the MnO/Co nanoparticles, implying the hybrid structure of the nanoparticles. XPS measurements were conducted to analyse the doping pattern and bonding configuration of MnO/Co-N-G. The survey spectrum demonstrates the presence of C (92.17 at %), N (1.91 at %), O (5.25 at %), Mn (0.34 at %) and Co (0.32 at %) elements without other element in Figure S2. The weight percentages of Mn2+ and Co2+ measured by ICP-OES are 1.51 and 1.59 % in MnO/Co-N-G catalyst,

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respectively, in line with the XPS analysis. As revealed in Figure 3a, the high-resolution XPS C1s spectrum is composed of three subpeaks corresponding to C=C (284.6 eV), C-O&C=N (285.8 eV), and C=O&C-N (288.7eV), respectively. It indicates the successful doping of heteroatoms on the carbon lattice.35-36 The high-resolution N 1s spectrum in Figure 3b could be well-fitted to four related peaks at 398.3, 400.1, and 401.2 eV, which are assigned to pyridinic-N, pyrrolic-N, and graphitic-N respectively, suggesting that nitrogen was successfully incorporated into the carbon framework.37-38 Accordingly, the Mn 2p spectrum in Figure 3c shows that the signals from Mn 2p1/2 (653.4 eV) and Mn 2p3/2 (641.6 eV).39-40 The Co 2p spectrum in Figure 3d shows two peaks at 781.1 eV and 796.2 eV, corresponding to the Co 2p3/2 and Co 2p1/2.41-42 The concentration of Co2+ and Mn2+ used was optimized. The MnO/Co-N-G catalyst synthesized with 0.9 mM of Mn2+ and 0.1 mM of Co2+ (Mn:Co=9:1) demonstrates best ORR performance according to the linear scanning voltammetric (LSV) curves shown in Figure S3. Thus this sample was chosen as the optimal one for further investigation. The ORR catalytic performance of MnO/Co-N-G was first studied by CV. As shown in Figure 4a, a pronounced cathodic ORR peak at -0.136 V with a current density of -2.88 mA cm-2 was observed on MnO/Co-N-G. This ORR peak is more positive than that of the N-doped graphene (synthesized by carbonization of PDA-GO), Co-N-G and MnO-N-G. The remarkably improved ORR activity of MnO/Co-N-G suggests possible synergistic effect of MnO and metallic Co nanoparticles.30

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The ORR catalytic activity of MnO/Co-N-G was further investigated using Rotating disk electrode (RDE) measurements. As shown as the LSV curves in Figure 4b, the onset potential on MnO/Co-N-G is close to that of Pt/C catalyst and is much more positive that of the N-C, Co-N-C and Mn-N-C catalyst. The result is consistent with the trend observed from CV in Figure 4a. The half wave potential of MnO/Co-N-G is ca. -0.116 V, which is even slightly more positive than that of commercial Pt/C (-0.136 V). At the same time, the ORR limiting current density on MnO/Co-N-G (-5.55 V) is very close to that of Pt/C (-5.48 V) from -0.12 to -0.75 V, demonstrating excellent ORR performance of the MnO/Co-N-G. RDE tests were performed to examine the ORR reaction selectivity of MnO/Co-N-G (Figure 4c). It is observed that the current density increases with increased rotational speed because the dissolved oxygen diffuse to electrode faster at higher rotation speed. The Koutecky-Levich (K-L) plots at different potentials in Figure 4d show good linearity with stable slope. The average electron transfer number on MnO/Co-N-G is calculated to be 3.89-3.98 at 0.4-0.7V according to the K-L equation, suggesting that ORR is proceeding predominently via favourable four-electron pathway on MnO/Co-N-G. In order to confirm the ORR catalytic selectivity on MnO/Co-N-G, RRDE measurements were performed to detect the formation of peroxide species during the ORR, as shown in Figure 5a. The measured HO2- yield is less than 11.5% over the potential range of -0.25 ～-0.75 V, and the electron transfer number is calculated to

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be ca. 3.9 (Figure 5b), which agrees well with the results obtained from K-L plot in Figure 4d, suggesting the excellent selectivity of four-electron ORR on MnO/Co-N-G. The stability of MnO/Co-N-G catalyst was examined by accelerated durability test (ADT), where the potential is continuously swept bewteen -0.3 to -0.8 V. Figure 6a shows that there is no significant deterioration in ORR activity accroding to the polarization curves before and after 5000-cycle ADT. Only very slight decrease in the limiting current density is observed. This indicates the excellent stability of MnO/Co-N-G for ORR, which is further confirmed by the amperometric measurements, as shown in Figure 6b. After 10 h continuous ORR operation, current density on MnO/Co-N-G only decreases for 10%, which is much lower than the loss on Pt/C. The MnO/Co-N-G catalyst also is highly tolerant to methanol. As shown in Figure 6c, the LSV curve of MnO/Co-N-G keeps almost unchanged after adding the methanol to the solution. Meanwhile, the amperometric measurement in Figure 6d indicates the excellent tolerance of MnO/Co-N-G catalyst to methanol. The excellent ORR catalytic activity of MnO/Co-N-G encourages us to construct a Zn–air battery to evaluate its practical application, in which the MnO/Co-N-G catalyst loaded on carbon paper served as the air cathode. It is observed that the MnO/Co-N-G based battery delivers an open-circuit voltage of 1.42 V (Figure 7a), which is very close to that using Pt/C catalyst (Figure 7b). The polarization curve and the power density curves of the Zn–air battery are shown in Figure 7b. At a same discharging current, the cell voltage of MnO/Co-N-G based battery is slightly lower than that of Pt/C based one. However, at high current density region, the

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MnO/Co-N-G catalyst-based battery possesses higher voltage, and delivers higher maximum power density, which is calculated to be 34.5 mW cm−2 at 41.5 mA cm−2. The role of MnO/Co nanoparticles in the ORR electrocatalysis is further investigated. As shown in Figure S4, the onset-potential of the resulted catalyst remains unchanged, but the limiting current density sightly decreases after the removel of MnO/Co nanoparticles from the MnO/Co-N-G catalyst via acid pickling. It suggests that the as-prepared MnO/Co-N-G catalyst contains highly active M-Nx (M=Co, Mn) sites to facilitate the ORR and the MnO/Co nanoparticles are able to enhance the ORR acitivity, which is consistent with previous observations.43-45 At the same time, the ORR acitivity of MnO/Co-N-G catalyst was measured in acidic solution, as shown in Figure S5. It demonstrates reasonable activity but is inferior to the Pt/C in terms of either half-wave potential or limiting current density.

CONCLUSIONS In summary, a highly efficient MnO/Co-N-G catalyst was reported with Pt/C-comparable ORR activity in alkaline media. The impressive ORR activity of as-prepared MnO/Co-N-G catalyst is tightly associated with its compositional and structural properties. The active atomic M-Nx (M= Co, Mn) sites and the encapsulated MnO/Co nanoparticles are present in the catalyst to synergistically boost the ORR activity. The graphene sheets offer huge surface areas to accommodate the active sites and also afford excellent conductivity for smooth electron transfer and mesoporous structure for efficient mass transport. This synthetic method is very promising for the development of various transition metal-based electrocatalysts. 12


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ASSOCIATED CONTENT

Supporting Information.

The Supporting Information is available free of charge on the ACS Publications website at DOI: XPS survey spectrum, XRD, SEM, TEM and electrocatalytic performance data (PDF)

Corresponding Author * Corresponding author. E-mail: [email protected] (W. H. Hu)

ACKNOWLEDGEMENTS We would like to gratefully acknowledge the financial support from Natural Science Foundation Project of CQ CSTC (cstc2016jcyjA0493), Fundamental Research Funds for the Central Universities (XDJK2018B001), and Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energies.

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Figures and Captions

Figure 1. Schematic illustration of the synthesis of MnO/Co-N-G catalyst.

Figure 2. XRD pattern with standard cards (a), SEM image (b), TEM images (c-d) and EDS element mapping of C (f), N (g), Mn (h) and Co (i) of MnO/Co-N-G catalyst. 14


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Figure 3. High-resolution XPS spectra of C 1s (a), N 1s (b) Mn 2p (c) and Co 2p (d) of MnO/Co-N-G catalyst.

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Figure 4. (a) CV curves of MnO/Co-N-G, MnO-N-G, Co-N-G and N-G at a scan rate of 50 mV s-1; (b) LSV curves of N-G, Co-N-G, MnO-N-G, MnO/Co-N-G, and Pt/C at a scan rate of 5 mV s-1 at 1600 rpm; (c) RDE voltammograms of MnO/Co-N-G at a scan rate of 5 mV s-1 at different rotating rates and corresponding K-L plots (J-1 versus ω-1/2). Electrolyte solution: O2-saturated 0.1 M KOH.

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Figure 5. (a) RRDE voltammograms of MnO/Co-N-G in O2-saturated 0.1 M KOH at a scan rate of 5 mV s-1 at 1600 rpm; (b) peroxide yield (black line) and the electron transfer number (n) (red line) of the MnO/Co-N-G at various potentials derived from the RRDE data.

Figure 6. (a) LSV curves of MnO/Co-N-G in O2-saturated 0.1 M KOH before and after 5000-cycle ADT. Sweep rate 5 mV s-1, rotation rate: 1600 rpm; (b) amperometric measurement at -0.3V on MnO/Co-N-G and Pt/C electrode in O2-saturated 0.1 M KOH; (c) LSV curves of MnO/Co-N-G in O2-saturated 0.1 M KOH with and without

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20% methanol (v/v). Sweep rate 5 mV s-1, rotation rate: 1600 rpm; (d) amperometric measurement at -0.3V on MnO/Co-N-G and Pt/C electrode in O2-saturated 0.1 M KOH with and without 20% methanol (v/v).

Figure 7. (a) Photograph showing the MnO/Co-N-G-based Zn–air battery with an open-circuit voltage of 1.42 V; (b) polarization curves and the power density curves of Zn–air battery with Pt/C or MnO/Co-N-G as cathodic catalyst.

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Table of Content (TOC)

Bimetal material MnO/Co-N-G was reported as a high performance ORR catalyst, and it exhibits excellent ORR activity superior to Pt/C in alkaline solution, and bimetal nanoparticles are found to responsible to the impressive ORR activity.

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Cobalt Bimetal Nanoparticles ... - ACS Publications

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