Mesoporous Hollow Nitrogen-Doped Carbon Nanospheres with

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Mesoporous Hollow Nitrogen-Doped Carbon Nanospheres with Embedded MnFe2O4/Fe Hybrid Nanoparticles as Efficient Bifunctional Oxygen Electrocatalyst in Alkaline Media Xiuju Wu, Yanli Niu, Bomin Feng, Yanan Yu, Xiaoqin Huang, Changyin Zhong, Weihua Hu, and Chang Ming Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 30 May 2018 Downloaded from http://pubs.acs.org on May 30, 2018

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ACS Applied Materials & Interfaces

Mesoporous Hollow Nitrogen-Doped Carbon Nanospheres with Embedded MnFe2O4/Fe Hybrid Nanoparticles as Efficient Bifunctional Oxygen Electrocatalyst in Alkaline Media

Xiuju Wu, Yanli Niu, Bomin Feng, Yanan Yu, Xiaoqin Huang, Changyin Zhong, Weihua Hu*, and Chang Ming Li

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

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

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ABSTRACT: Exploring sustainable and efficient electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is necessary for the development of fuel cells and metal-air batteries. Herein we report bimetal Fe/Mn-N-C material composed of spinel MnFe2O4/metallic Fe hybrid nanoparticles encapsulated in N-doped mesoporous hollow carbon nanospheres as an excellent bifunctional ORR/OER electrocatalyst in alkaline electrolyte. The Fe/Mn-N-C catalyst is synthesized via pyrolysis of bimetal ion-incorporated polydopamine nanospheres and shows impressive ORR electrocatalytic activity superior to Pt/C and good OER activity close to RuO2 catalyst in alkaline environment. When tested in Zn-air battery, the Fe/Mn-N-C catalyst demonstrates excellent ultimate performance including power density, durability and cycling performance. This work reports bimetal Fe/Mn-N-C as a highly efficient bifunctional electrocatalyst and may afford useful insights into the design of sustainable transition metal based high performance electrocatalysts.

KEYWORDS: MnFe2O4/Fe nanoparticles; mesoporous hollow carbon nanospheres; Zn-air battery; bifunctional catalyst; oxygen reduction reaction; oxygen evolution reaction


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1. INTRODUCTION The oxygen electrode reactions including cathodic oxygen reduction reaction (ORR) and anodic oxygen evolution reaction (OER) involve multiple elementary steps coupled with multi-step electron transfer and proton transfer process.1-4 The thermodynamic equilibrium potential of ORR/OER is 1.23 V vs. standard hydrogen electrode (SHE) but substantial overpotential is required to drive these two reactions due to their sluggish kinetics, which greatly hampers the efficiency of fuel cells and metal-air batteries.5-8 Noble metal platinum (Pt) and ruthenium (Ru)/iridium (Ir) based catalysts are currently the benchmark catalysts for ORR and OER, respectively.9-11 The high expense and scarcity of these catalysts urge to explore sustainable alternative electrocatalysts. Moreover, it will be of great merit if a single catalyst is able to efficiently catalyse both ORR and OER but development of such Janus catalysts with satisfying bifunctional activity is very challenging.12-15 In recent years, intensive research has been focusing on earth-abundant first-row (3d) transition metal-based oxygen electrode catalysts to achieve efficient ORR/OER electrocatalysis.16-20 For ORR catalysis, hybrid materials composed of nitrogen-doped carbon with loaded transition metal (M-N-C) were explored as a class of compelling catalysts.21-25 The real ORR catalytic site in these M-N-C catalysts, however, was not well revealed so far even though the metal coordinated by 2-4 nitrogen atoms (M-Nx) is believed to be the active centre.26-29 Recent works have further pointed to that the final ORR activity is closely associated with the simultaneous emergence of M-Nx sites and aggregated metal agglomerates such as metal, oxide, carbide or nitride nanoparticles, implying that both the aggregated metal species and M-Nx sites may contribute to the


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ORR activity.30-31 For OER, 3d transition-metal oxides and oxyhydroxides have demonstrated promising catalytic activity and good electrochemical stability under the harsh OER condition.32-33 The OER activity of unary Co, Fe, Ni and Mn oxides has been well evaluated theoretically and experimentally.34-35 Extensive research unveils that the OER performance of multiple metal-based electrocatalyst often outperforms that of the corresponding single-metal ones due to the modulation of their 3d electronic structure. Nevertheless, very few catalysts developed so far possess satisfying ORR/OER bifunctional electrocatalytic performance. In the pursuit of bifunctional oxygen electrocatalysts, transition metal spinel oxides (AB2O4) have attracted particular research interest due to their low cost, chemical versatility, and electrochemical stability under ORR/OER conditions.36 Theoretical and experimental work has screened the octahedral cation as the ORR/OER active sites in spinel and the eg occupancy of the octahedral site as the activity descriptor. Heteroatom doping, defect engineering, structural/compositional modulation, micro-nanostructure tailoring and compositing with conductive supports further offer efficient means to enhance their catalytic performance for ORR, OER, or both.37 Anyway, previous work has suggested that the ORR/OER performance of 3d transition metal spinel oxides is tightly associated with its compositional and structural properties.38-39 Careful optimization of a wide diversity of synthetic conditions is necessary before maximizing their electrocatalytic activity. In this work we report a highly efficient ORR/OER bifunctional catalyst composed of spinel MnFe2O4/Fe hybrid nanoparticles in nitrogen-doped mesoporous hollow carbon nanospheres

(named

as

Fe/Mn-N-C).

It

is

derived

from

Fe/Mn-incorporated


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polydopamine (PDA) nanospheres precursor via pyrolysis. The optimal Fe/Mn-N-C catalyst exhibits excellent ORR activity superior to 20 wt% Pt/C commercial catalyst in terms of onset potential, current density, four-electron selectivity, stability and methanol tolerance in alkaline solution. For OER, overpotential of only 360 mV is required to drive a 10 mA cm-2 current density with a low Tafel slope of 60 mV dec-1 and excellent long term durability on Fe/Mn-N-C catalyst. When tested in rechargeable Zn-air batteries, it also demonstrates higher discharging performance than commercial Pt/C catalyst as well as excellent cycling performance.

2. EXPERIMENTAL SECTION 2.1 Chemicals. Dopamine (98%), Iron (III) sulfate (Fe2(SO4)3, 21-23%), Manganese chloride (MnCl2·4H2O, 99%), RuO2 and KOH were obtained from Aladdin. Methanol (CH3OH, 99.5%), ammonia solution (NH3·H2O, 25-28% NH3 basis) and ethanol (C2H5OH, 99.7%) were bought from Chuan Dong Chemical Company, Chongqing, China. Commercial Pt/C (20 wt% on XC-72) and Nafion solution (5 wt %) were purchased from Sigma. 2.2 Synthesis of Fe/Mn-N-C electrocatalysts. FeMn-PDA nanospheres precursor was first synthesized via an aqueous polymerization of dopamine. In a typical experiment, to a 50 mL flask containing 18 mL of deionized (DI) water, 8 mL of ethanol, and 0.4 mL of ammonia solution, 100 mg dopamine hydrochloride dissolved in 2 mL of DI water was slowly added with magnetic stirring. After that, Fe2(SO4)3 (0.1 M) and MnCl2 solution (0.1 M) were slowly added into the solution to make the final concentration of Fe3+ and Mn2+ at 1.0 mM and 0.5 mM (Fe3+ /Mn2+ ratio 2:1). After magnetic stirring at room temperature for 30 h, the resulting FeMn-PDA nanospheres precursor was gathered by centrifugalized and washed three times with DI water.


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Other FeMn-PDA nanospheres were also synthesized by adjusting the concentration of Fe3+ and Mn2+ added. Fe/Mn-N-C catalyst was synthesized by processing the freezing-dried FeMn-PDA nanospheres precursor at 850 °C in inert Argon atmosphere for 3 h. Fe-N-C, Mn-N-C and N-doped carbon (N-C) were also synthesized for comparison by annealing Fe-PDA, Mn-PDA nanospheres, and PDA nanospheres (without metal ion) with the same procedure, respectively. 2.3. Electrochemical experiments. As-synthesized catalyst (2.0 mg) was mixed with Nafion solution (5 wt %, 50 µL) and absolute ethanol (1.0 mL) with the assistance of ultrasonication to gain a homogeneous suspension, 25 µL of which was spotted on a clean glassy carbon (GC) electrode (diameter 5.0 mm) or disk electrode of a glassy carbon rotating ring-disk electrode (RRDE, disk diameter 5.0 mm) or glassy carbon rotating disk electrode (RDE, diameter 5.0 mm). The catalyst loading is 0.2547 mg cm-2. The catalyst ink was naturally dried in air. The Pt/C and RuO2 were also spotted on glassy carbon electrode with the same procedure and same loading. The ORR activity was evaluated in a three-electrode cell with a Pt foil as counter electrode and a Hg/HgO (1 M KOH, 0.098V vs. RHE) as reference electrode. A potentiostat system (Autolab PGSTAT302N) equipped with a Pine rotator (AFMS-LXF) was used to collect cyclic voltammetry (CV) and linear sweep voltammetry (LSV) data in O2-saturated 0.1 M KOH electrolyte at room temperature. For OER evaluation, the graphite plate (3.0 cm×3.0 cm×2.0 mm), catalyst-loaded glassy carbon electrode (5.0 mm in diameter) and Hg/HgO (1 M KOH, 0.098V vs. RHE) were used as the counter, working and reference electrodes, respectively. 2.4. Zn-air battery testing. The Fe/Mn-N-C catalyst (7.0 mg) was blended with Super P (2.0 mg) and polyvinylidene fluoride (1.0 mg) in N-methyl-2-pyrrolidone to obtain a homogeneous ink, which was stowed onto a clean carbon cloth (0.5 cm× 0.5 cm). The catalyst loading is


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controlled to be ca. 0.5 mg cm-2 by controlling the ink volume used. The Zn-air battery was assembled using this catalyst-loaded carbon cloth as the air cathode and Zn plate (2.0 cm × 5 cm × 0.5 cm) as the anode, and 6.0 M KOH + 0.2 M Zn(Ac)2 as the electrolyte. The Zn-air battery was first allowed to discharge at 80 K ohm external resistance for 1 h under the atmosphere. After that, the electrolyte was saturated with oxygen, and the anode and cathode were connected with an electrical resistor with different resistance. The voltage across the resistor reaches a stable value, which was recorded. For charging-discharging cycling testing, the battery was subjected to charging and discharging each for 45 min at 8 mA cm-2 at 25℃ with an electrochemical workstation (CHI 760E, CH Instruments, Inc., Shanghai).40 2.5. Characterizations. The XRD patterns were tested on a powder X-ray diffraction (Shimadzu XRD-7000) with CuKα radiation. Scanning electron microscopy (SEM, JSM-7800F from JEOL, Japan) and transmission electron microscopy (TEM, JEOL 2100 from JEOL, Japan) were used to investigate the structure of the catalysts. The surface chemical states and composition of the as-prepared catalysts were characterized by X-ray photoelectron spectroscopy (XPS, collected on Escalab 250xi from Thermo Fisher Scientific). Nitrogen adsorptiondesorption isotherms were tested on Autosorb-1 (Quantachrome Instruments). The specific surface area was acquired from a multipoint BET model, and the pore size distribution was gained by the BJH model.

3. RESULTS AND DISCUSSION The synthetic procedure for Fe/Mn-C-N was shown in Figure 1. First, FeMn-PDA nanospheres were synthesized by self-polymerization of dopamine in existence of Fe3+ and Mn2+ ions.41-43 During this process, Fe3+ and Mn2+ were combined into the polymeric nanospheres by


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coordinating to pyrrolic N and catechol group of PDA. Following pyrolysis in inert atmosphere converts FeMn-PDA precursor to Fe/Mn-N-C catalyst. The structural and compositional properties of Fe/Mn-N-C were studied. The SEM image in Figure 2a clearly shows that as-synthesized Fe/Mn-N-C catalyst demonstrates nanospheres morphology with an average diameter of ca. 200 nm. Each nanosphere shows hollow structure with incomplete and porous shell, which is assembled by nanoparticles with much smaller size. The porous structure of Fe/Mn-N-C can be further observed in TEM image (Figure 2b), which is highly appreciated in ORR/OER electrocatalysis as it facilitates exposure of active sites and efficient mass transport. Notably, the structure of Fe/Mn-N-C catalyst is evidently different with the FeMn-PDA nanospheres precursor, which demonstrates smooth and solid particulate morphology according to the SEM and TEM images (Figure S1a, b). The morphological transformation is believed to be a consequence of the pyrolysis process, during which the PDA is graphitized to form graphitic carbon, accompanied by the gasification of oxygen and hydrogen containing groups in PDA to generate abundant pores and hollow interior. The MnFe2O4/Fe nanoparticles are believed to be formed by the incorporated metal ions at high temperature. They are found to be embedded in the carbon frameworks and their diameter ranges from 5 to 50 nm. Interestingly, carbon with a high degree of graphitization appears mostly around metal nanoparticles, and the carbon apart from metal nanoparticles contains more structural disorders according to the TEM image in Figure 2c, possibly due to the catalytic ability of transition metal toward graphitization of PDA. The graphitic carbon offers good conductivity and high corrosion resistance while the disordered carbon is able to accommodate abundant M-Nx active centres in the resultant Fe/Mn-N-C.44 High-resolution TEM image (Figure 2d) clearly shows lattice fringes of 0.257 and 0.202 nm, which agree well with the (311) and (110) lattice spacing of the spinel


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MnFe2O4 and metallic Fe, respectively, indicating the presence of spinel MnFe2O4 and metallic Fe phase in the catalyst. The metallic Fe phase may be formed via the reduction of Fe ions in the presence of carbon during the pyrolysis process.30 The selected-area electron diffraction (SAED) pattern of Fe/Mn-N-C in Figure 2e, shows strong ring patterns of the (311), (004), (333) (422) of spinel MnFe2O4, (110) planes of Fe crystal and (002) planes of graphitic carbon. The XRD pattern in Figure 2f further confirms the presence of spinel MnFe2O4 (PDF#38-0430), Fe (PDF#06-0696) and graphitic carbon (PDF#41-1487) in the Fe/Mn-N-C catalyst and is well consistent with the TEM, and SAED analysis. The XPS survey spectrum (Figure 3a) reveals the presence of C (84.41 at %), N (1.72 at %), O (12.98 at %), and Mn (0.24 at %) and Fe (0.65 at %) elements in the Fe/Mn-N-C catalyst. According to the C1s spectrum in Figure 3b, the carbon in Fe/Mn-N-C catalyst could be assigned to three distinct species corresponding to C=C (284.6 eV), C-O&C=N (285.7 eV), and C=O&CN (288.6 eV), respectively.45 The N 1s spectrum in Figure 3c might be befitting to three defined peaks located at 400.5, 398.5, 401.5 eV, corresponding to pyrrolic, pyridinic and graphitic N, respectively.46 The Fe 2p spectrum could be deconvoluted to six subpeaks corresponding to 2p3/2 and 2p1/2 electrons from Fe 0, Fe ℃ and Fe ℃, respectively (Figure S2a), confirming the presence of Fe with various oxidative states in the catalyst.47 The Mn 2p spectrum also indicates the presence of Mn ℃, Mn ℃ and Mn℃ in the spinel oxide (Figure S2b).48 The graphitic degree of Fe/Mn-N-C was also investigated by Raman spectrum. As shown in Figure S3, the D/G peak density ratio is 1.24, suggesting a moderate graphitic degree, which has been proved to be desirable for M-N-C catalyst as the graphitic carbon offers high conductivity and stability to the catalyst while the defective and disordered carbon is able to accommodate Fe-Nx active sites.49


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The porosity of as-prepared Fe/Mn-N-C was probed by using nitrogen adsorption isotherm measurement. A typical type IV curve is observed with a distinct hysteresis loop in P/P0 = 0.6-1 pressure region in Figure 3d.50 The specific surface area is around 125.9 m2 g-1, which is much higher than that of FeMn-PDA spheres (89.8 m2 g-1, see its BET data in Figure S4), indicating the formation of abundant pores in Fe/Mn-N-C. The pore size distribution plot is shown as inset in Figure 3d, which confirms the presence of mesopores in Fe/Mn-N-C, in line with the TEM observation. By varying the Fe3+ to Mn2+ concentration used, other catalysts were synthesized for comparison. According to their XRD patterns and SEM images shown in Figure S5 and Figure S6, it is found that all catalysts demonstrate particulate morphology. If only unary metal ion was used, the product contains metal oxide (Fe2O3 or Mn3O4). If both Fe3+ and Mn2+ are added, metallic nanoparticle, unary metal oxide and/or bimetal spinel oxide were formed, dependent on the concentration ratio used (Figure S5, S6). The dosages of Fe3+ and Mn2+ ions were first optimized by evaluating the ORR activity of resultant catalysts with CV (Figure S7) and LSV technique, as shown in Figure 4. The one synthesized with the concentration ratio of Fe3+ ions to Mn2+ ions of 2:1 (1.0 and 0.5 mM for Fe3+ and Mn2+, respectively) shows the best ORR activity in terms of onset potential and current density and thus it was chosen as the optimal dosage for Fe3+ and Mn2+ ion, respectively. It is worth noting that the Fe/Mn ratio in the Fe/Mn-PDA precursor and Fe/Mn-C-N catalyst may deviate from the dosage used due to the difference coordination ability of two metal ions in PDA. The optimal Fe/Mn-N-C shows quasi-rectangular CV curves in a N2-saturated electrolyte while demonstrates a well-defined cathodic peak at -0.123 V corresponding to ORR, as shown in


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Figure S8. Further comparison demonstrates that on this optimal Fe/Mn-N-C, the ORR starts at the potential of 0.025 V (Figure 4b), which is more positive than that of the N-C, Mn-N-C and Fe-N-C. This indicates that both Fe and Mn species contribute to the excellent ORR activity of Fe/Mn-N-C. When compared with Pt/C catalyst, the Fe/Mn-N-C shows slightly more positive onset potential and half-wave potential. At the same time, the Fe/Mn-N-C delivers a significantly higher ORR current density than Pt/C does from -0.15 to -0.75 V. The limiting current density on Fe/Mn-N-C reaches a value of -6.05 mA cm-2, much higher than that obtained on Pt/C catalyst, further exemplifying the superior ORR activity of Fe/Mn-N-C. Control experiment indicates that the Fe/Mn-N-C catalyst shows similar onset potential and slightly decreased current density after acid washing to remove the metal agglomerates, verifying the essential role of metal agglomerate to boost the ORR activity (Figure S9). The reaction selectivity of Fe/Mn-N-C was investigated by polarization curves obtained on RDE at rotating rate varying from 400 to 2500 rpm (Figure 4c). The current density increases with increased rotation speed. The corresponding Koutecky-Levich (K-L) plots are presented in Figure 4d, showing good linearity at different potentials. The average electron transfer number for ORR on Fe/Mn-N-C was calculated to be above 3.9 from -0.3 to -0.6 V using the K-L equation (Figure 4d), which is the highest one among all catalysts synthesized in present work (Figure S10 and S11), suggesting that 4-electron ORR process is predominant on this Fe/Mn-NC catalyst. To verify the ORR catalytic selectivity on Fe/Mn-N-C, RRDE measurements were performed, as shown in Figure 5a, where the ring current originates from the oxidation of peroxide species generated on disk electrode during the ORR process. Calculation indicates (Figure 5b) that less than 5% peroxide is generated. The average electron transfer number of


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ORR is ca. 3.9 in -0.2 ～ -0.7 V (Figure 5b). This agrees well with the number gained from K-L plot in Figure 4d, showing the excellent 4-electron ORR selectivity on Fe/Mn-N-C catalyst. Accelerated durability test (ADT) was carried out to evaluate the durability of Fe/Mn-N-C catalyst. As shown in Figure 6a, the polarization curve exhibits only negligible decay in onset potential and limiting current density even after 2000 and 5000 successive potential cycling from 0.4 to -0.8 V in 0.1 M KOH exposing to atmosphere, unveiling the excellent stability of Fe/MnN-C catalyst. When methanol was added into the electrolyte solution, as shown in Figure 6b, the LSV curve keeps consistent, suggesting the excellent methanol tolerance of Fe/Mn-N-C catalyst. The OER performance of Fe/Mn-N-C was further evaluated in comparison to state-of-the-art RuO2 catalyst as well as other catalysts synthesized with different Fe3+ and Mn2+ concentration, as shown as the LSV curves in Figure 7a. The Fe/Mn-N-C synthesized with Fe3+/Mn2+ concentration of 1.0/0.5 mM still demonstrates the best OER activity. It is very close to the RuO2 catalyst and outperforms other Fe/Mn based samples as well as the single-metal based catalysts (Fe-N-C and Mn-N-C). The overpotential required to support a 10.0 mA cm-2 OER current density on Fe/Mn-N-C is ca. 360 mV (Figure 7b), only slightly higher than that of RuO2 catalyst (340 mV) and lower than other catalysts synthesized, highlighting the superior OER activity of Fe/Mn-N-C. The Tafel plots of Fe/Mn-N-C and RuO2 catalyst are presented in Figure 7c. The Tafel slope of Fe/Mn-N-C and RuO2 catalyst are 60 and 54 mV dec-1, respectively. The similar Tafel slope of the Fe/Mn-N-C and the commercial RuO2 catalyst suggests high intrinsic OER activity of Fe/Mn-N-C. On Nyquist plots (Figure S12), the Fe/Mn-N-C catalyst demonstrates slightly higher charge transfer resistance than RuO2 catalyst, further confirming the high OER activity of Fe/Mn-N-C. The long-term durability of Fe/Mn-N-C catalyst for OER was investigated by chronopotentiometric measurement. As shown in Figure 7d, the potential


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required to drive a 10.0 mA cm-2 OER current density only increases ca. 10 mV even after 29 h continuous OER operation, indicating the superior stability of Fe/Mn-N-C catalyst. Considering the excellent ORR/OER activity of Fe/Mn-N-C catalyst, its electrochemical performance in Zn-air battery was further evaluated and compared to Pt/C catalyst. As shown in Figure 8a, when Fe/Mn-N-C is used, the Zn-air battery demonstrates an open-circuit voltage of ca. 1.328 V, very close to that of Pt/C-based battery, which suggests comparable ORR activity of Fe/Mn-N-C to Pt/C and is in line with the LSV comparison in Figure 4b. The cell voltage decreases at higher current density due to the polarization effect on the electrodes. Remarkably, the cell voltage of the Fe/Mn-N-C-based battery is even higher than that using Pt/C at a certain current density and thus delivers a higher power density (Figure 8a). The maximum power density of Fe/Mn-N-C-based battery reaches 37 mW cm-2, considerably higher than that of Pt/Cbased one (28 mW cm-2). This result further highlights the excellent activity of the Fe/Mn-N-C catalyst in practical application. Zn-air batteries with possess high capacity and intrinsic safety advantages are promising next-generation energy storage devices. 51-52 The Fe/Mn-N-C was subjected to further inspection in a rechargeable Zn-air battery. Figure 8b shows the charging/discharging performance of the Fe/Mn-N-C-based Zn-air battery at a current density of 8.0 mA cm-2, with an initial discharging voltage of 1.24 V and an initial charging voltage of 2.23 V. Notably, the battery shows negligible voltage change with almost constant charge and discharge voltage over a continuous recharging/discharging operation of 55 h, indicating good recharging/discharging capability. The result clearly indicates great promise of Fe/Mn-N-C catalyst for practical application.

4. CONCLUSION


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In summary, a facile strategy was developed to synthesize mesoporous Fe/Mn-N-C hollow nanospheres as an excellent Janus oxygen electrocatalyst. This catalyst contains spinel MnFe2O4/metallic Fe hybrid nanoparticles encapsulated in N-doped hollow carbon spheres and demonstrates synergistically improved ORR/OER activity. It shows ORR activity comparable to Pt/C and superb OER activity close to commercial RuO2 catalyst in alkaline environment. When tested in Zn-air battery, it demonstrates excellent performance including power density, durability and cycling. This work reports a facile route to preparation of this novel bimetal Fe/Mn-N-C for efficient bifunctional electrocatalysis and may afford useful insights into the design of sustainable transition metal based high performance electrocatalysts.

■ ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. SEM, TEM images and Nitrogen sorption isotherm plot of FeMn-PDA precursor; XRD patterns and SEM images of other catalysts synthesized with different Fe3+ to Mn2+ concentration; Fe 2p and Mn 2p XPS spectrum and Raman spectrum of Fe/Mn-N-C catalysts; CV curves and LSV curves of other catalysts; CV curves of Fe/Mn-N-C and LSV curves of Fe/Mn-N-C before and after acid washing (PDF)

■ AUTHOR INFORMATION Corresponding Author * Email: [email protected] ORCID


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Weihua Hu: 0000-0001-6278-9551 Notes The authors declare no competing financial interests.

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


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Figures and Captions

Figure 1. Schematic depiction of the preparation of Fe/Mn-N-C catalyst.


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Figure 2. SEM (a), TEM (b-d), SAED (e), and XRD pattern (f) of Fe/Mn-N-C.


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Figure 3. XPS survey spectrum (a) and high-resolution spectra of C1s (b), N1s (c) of asprepared Fe/Mn-N-C; (d) Nitrogen sorption isotherm plot (inset: BJH pore size distribution) of Fe/Mn-N-C.


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Figure 4. ORR activity of Fe/Mn-N-C catalyst. (a) LSV curves of a batch of Fe/Mn-N-C catalysts synthesized with different Fe3+/Mn2+ concentration ratio at 1600 rpm; (b) LSV curves of N-C, Fe/Mn-N-C catalyst, Fe-N-C, Mn-N-C and commercial Pt/C at 1600 rpm; (c) Rotatingdisk voltammograms of Fe/Mn-N-C catalystat different rotating rates (d) corresponding K-L plots (J-1 versus ω-1/2). Solution: O2-saturated 0.1 M KOH; scan rate: 5 mV s-1.


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Figure 5. (a) RRDE voltammograms of Fe/Mn-N-C in O2-saturated 0.1 M KOH at a scan rate of 5 mV s-1 at 1600 rpm and (b) thereof calculated peroxide yield (black line) and electron transfer number (n) (red line) at various potentials.

Figure 6. (a) LSV curves of Fe/Mn-N-C in O2-saturated 0.1 M KOH with a sweep rate of 5 mV s-1 at 1600 rpm before and after 2000 and 5000 ADT potential cycles; (b) LSV curves with a sweep rate of 5 mV s-1 at 1600 rpm in O2-saturated 0.1 M KOH with and without 20% methanol (v/v).


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Figure 7. OER activity of Fe/Mn-N-C catalyst. (a) polarization curves (without IR correction) of commercial RuO2 catalyst and a batch of Fe/Mn-N-C catalysts synthesized with different Fe3+ to Mn2+ concentration ratio in 1 M KOH solution at a scan rate of 5 mV s-1; (b) plotting the overpotential at 10 mA cm-2 OER current density on different catalysts; (c) Tafel plots of RuO2 and Fe/Mn-N-C catalyst; (d) potential curve on Fe/Mn-N-C catalyst at a constant OER current density of 10 mA cm-2.


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Figure 8. Evaluating Fe/Mn-N-C catalyst in a Zn-air battery. (a) polarization curves and power density curves of the assembled Zn-air battery with Fe/Mn-N-C or Pt/C as the cathodic catalyst; (b) Discharge/charge cycling curves of rechargeable Zn-air batteries at a current density of 8.0 mA cm-2 using the Fe/Mn-N-C-based air electrode.


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Nanoparticles Supported on B/N-Codoped Mesoporous Nanocarbon as a

Bifunctional Electrocatalyst of Oxygen Reduction/Evolution for High-Performance Zinc-Air Batteries. Acs Appl Mater Interfaces 2016, 8, 13348-13359.


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Table of Content (TOC)


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Mesoporous Hollow Nitrogen-Doped Carbon Nanospheres with

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