Bimetallic Mn-Co oxide nanoparticles anchored on carbon nanofibers

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Bimetallic Mn-Co oxide nanoparticles anchored on carbon nanofibers wrapped in nitrogen doped carbon for application in Zn-air batteries and Supercapacitors Tesfaye Tadesse Gebremariam, Fuyi Chen, Qiao Wang, Jiali Wang, Yaxing Liu, Xiaolu Wang, and Adnan Qaseem ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00067 • Publication Date (Web): 19 Mar 2018 Downloaded from http://pubs.acs.org on March 20, 2018

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ACS Applied Energy Materials

Bimetallic Mn–Co oxide nanoparticles anchored on carbon nanofibers wrapped in nitrogen doped carbon for

application

in

Zn–air

batteries

and

Supercapacitors Tesfaye Tadesse Gebremariam,† Fuyi Chen,*,†,‡ Qiao Wang,† Jiali Wang,† Yaxing Liu,†,‡ Xiaolu Wang,† and Adnan Qaseem† † State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xian, 710072, China. ‡ School of Electronics and Information, Northwestern Polytechnical University, Xi’an 710072, China. KEYWORDS: oxygen reduction reaction, oxygen evolution reaction, bimetallic oxide, nitrogen doped carbon, Zn–air battery.

1 Environment ACS Paragon Plus

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ABSTRACT: The exploration and rational design of cost–effective, highly active and durable catalysts for oxygen electrochemical reaction is crucial to actualize the prospective technologies such as metal air batteries and fuel cells. Herein manganese cobalt oxide nanoparticles anchored on carbon nanofibers and wrapped in nitrogen doped carbon shell (MCO/CNFs@NC) is successfully prepared. Benefiting from the synergistic effect between the core nanoparticles and nitrogen doped carbon shell, MCO/CNFs@NC catalyst exhibits oxygen reduction reaction (ORR) activity with comparable onset potential (1.00 V vs. RHE) and half–wave potential (0.76 V vs. RHE) which is only about 40 mV lower than that of the state of art Pt/C catalyst. Furthermore, MCO/CNFs@NC catalyst exceeds Pt/C catalyst by a great margin in terms of stability in alkaline media. Additionally, MCO/CNFs@NC catalyst is strongly tolerant to methanol crossover, promising its applicability as cathode catalyst in alcohol fuel cells. Moreover, MCO/CNFs@NC catalyst exhibits the oxygen evolution reaction (OER) activity with low overpotential of 0.41 V at the current density of 10 mA cm–2 and ORR/OER potential gap (∆E) as low as 0.88 V suggesting its strong bifunctionality. The Zn–air battery based on MCO/CNFs@NC catalyst is found to deliver specific capacity of 695 mA h g–1Zn and an energy density of 778 W h kg–1Zn at current density 20 mA cm–2. The mechanically rechargeable Zn–air battery based on MCO/CNFs@NC catalyst is also found to function continually by only reloading the consumed Zn anode and electrolyte. Furthermore, the electrically rechargeable battery based on MCO/CNFs@NC catalyst is found to function for more than 220 cycles with negligible loss of voltaic efficiency. Moreover, the MCO/CNFs@NC is found to display supercapacitive nature with a good discharge capacity of 478 F g–1 at discharge current density of 1 A g–1.


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INTRODUCTION Issues such as fast growth of human population, depletion of fossil fuel and environmental concern have induced the growing demand on exploration of sustainable energy conversion and energy storage systems that are reliable, cost effective and environmentally benign. Oxygen electrochemical reactions, such as ORR and OER, are the bases of various prospective technologies for sustainable and renewable energy conversion and storage devices like fuel cells and rechargeable metal air batteries.1,

2

However, there are numerous challenges that largely

impede the real–world application of these technologies. Among, the slow kinetics of ORR at the cathode side along with insufficient durability and high cost of the system is the most challenging one.3 Additionally, the practical applications of electrically rechargeable metal air batteries require the air electrode that is bifunctional and durable which can work under the harsh conditions of repetitive discharge and charge in alkaline media. However, the bifunctionality of the air electrode for the realization of high power performance is challenged by the considerable overpotential observed by both the ORR and OER.4 Therefore, the use of highly efficient bifunctional catalyst is compulsory to boost the kinetics of both ORR and OER. Although noble metals such as Pt, Ir and Ru–based catalysts have shown the desired catalytic activity towards ORR and OER, their wide spread application was impeded by their scarcity, high cost and electrochemical instability.5, 6 Alternatively, great efforts have been paid on the exploration and rational design of nonprecious catalysts that can lower the cost, increase the activity and improve the durability of the catalysts. The hybrid structure of transition metal oxides and carbonaceous nanomaterials have been widely reported as efficient bifunctional catalysts in alkaline medium.7-12 Due to their appealing characteristics such as high structural stability, large surface area, low electric resistance and



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rapid mass transfer, the carbonaceous nanomaterials are often introduced into the catalyst to improve conductivity and structural stability.13-16 Furthermore, carbonaceous nanomaterials such as carbon nanotubes, graphene, nanoporous carbons and carbon nanofibers doped with heteroatoms are considered as promising candidates for ORR in alkaline media.17, 18 On the other hand, the transition metal–based oxides nanoparticles are reported to be active towards OER due to the variable oxidation states of transition metals.15,

19-22

Consequently, the strategy of

integrating carbonaceous nanomaterials and transition metal–based oxides nanoparticles is an efficient way to develop bifunctional oxygen catalysts for both ORR and OER.23, 24 Recently, Moni Prabu et. al

25

prepared bimetallic Co–Mn oxides/carbon hybrid based on

CoMn2O4 anchored onto reduced graphene oxide (CMO/rGO) and N–doped reduced graphene oxide (CMO/N–rGO) via a hydrothermal method for the application in Zn–air batteries. The obtained CMO/rGO and CMO/N–rGO catalysts have displayed ORR half–wave potential of 110 mV and 60 mV lower than that of the state of art Pt/C catalyst respectively. CMO/N–rGO catalyst further offered an OER activity with overpotential of about 0.43 V at current density of 10 mA cm–2 proving the bifunctionality of the catalyst. Recently our group also reported 12 spinel MnCo2O4 nanoparticles on nitrogen–doped reduced graphene oxide (MnCo2O4/NGr) hybrid for the application of hybrid Zn–air batteries. The catalyst demonstrated ORR with half–wave potential only 30 mV lower than that of Pt/C in alkaline environment. Furthermore, the catalyst showed the potential gap (∆E) of 0.91 V proving its potential applicability as a bifunctional catalyst. However, the catalyst displayed a loss of about 17 mV of its initial half–wave potential after being subjected to 1000 cycles of accelerated durability test. Yisi Liu et. al.26 reported hybrid catalyst based on spinel CoMn2O4 nanoparticles supported on nitrogen–doped graphene aerogel. The hybrid catalyst showed activity towards ORR with comparable onset potential of


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(1.06 V vs. RHE) and half–wave potential (0.74 V vs. RHE) which is about 50 mV lower than that of 20% Pt/C. Tingting Zhang et.al.27 reported Co3O4 nanoparticles anchored on nitrogen– doped reduced graphene oxide (Co3O4/N–rGO) as multifunctional catalyst. The obtained catalyst displayed ORR activity of about 0.705 V vs. RHE based on CV measurement and OER with overpotential of about 0.66 V at current density of 10 mA cm–2. The hybrid Co3O4/N–rGO catalyst displayed about 31.37% decline in the initial value of current after being subjected to 2000 s in amperometric test at constant potential of 0.665 V vs. RHE. However, transition metal oxides and their hybrid structure with carbonaceous nanomaterials still suffer from the low catalytic activities, durability and conductivity.28, 29 The self–accumulation of nanoparticles into less active large particles and detachment of the nanoparticles from the support are the major cause of low catalytic activity and durability. Most metallic oxides nanoparticles are also intrinsically insulators which result in low conductivity. So, the rational design of more unique hybrid structure of the catalyst that can provide resistance towards self– accumulation and detachment of nanoparticles and offer enhanced conductivity is desirable. Herein we have devised a novel approach by supporting nanoparticles of bimetallic oxides onto carbon nanofibers and then wrapping it with nitrogen doped carbon shell. The strategy of introducing carbon nanofiber is expected to enhance the conductivity related issue because of their good electronic transport properties, which can further facilitate the kinetics of the electrochemical reactions.30,

31

The encapsulation of transition metal oxides into the nitrogen

doped carbon shell can augment the electrocatalytic activities of the catalyst towards ORR and/or OER due to the synergetic interactions (Metal–N–Carbon) between core and shell, beside, the wrapping of the nanoparticles within carbon shell is an effective way in preventing self– accumulation and detachment.32-34 To demonstrate our approach, we have selected manganese



cobalt oxide (MCO) as bimetallic oxide nanoparticles. Mixed transition metal oxides such as MCO are potential candidates for bifunctional catalyst due to their abundance, ease of preparation and reasonably good redox stability in aqueous alkaline solutions. The presence of multiple valences of the cations and structural flexibility in such mixed transition metal oxide systems is helpful to fine–tune their catalytic properties.35, 36 Dopamine is chosen as the source of nitrogen doped carbon shell owing to its multifunctional coating ability and ease of control over the thickness of the shell

37

. The optimized MCO/CNFs@NC catalyst manifested reasonably

high ORR and OER along with remarkable durability and methanol tolerance. The shell effectively prevented the nanoparticle from self–accumulation and detachment from the surface of the support thereby enhancing the activity and stability. Considering its activity and stability, practical application of MCO/CNFs@NC catalyst was also demonstrated in a custom–built Zn– air battery under ambient condition. Moreover, MCO/CNFs@NC was also found to serve as electrode materials for supercapacitor. This design concept can be extended to fabricate other novel, active and stable catalysts like nanoparticles of transition metals or their oxides supported on carbonaceous nanomaterials such as carbon nanofiber, carbon nanotube and further wrapped with protective carbon shell.

EXPERIMENTAL Chemicals: Cobalt (II) acetate tetrahydrate (Co(CH3COO).4H2O, > 99.5%) and potassium hydroxide (KOH) were purchased from Guangdong chemicals. Manganese acetate tetrahydrate (Mn(CH3COO).4H2O, > 99 %), ammonia solution (25–27 %) and Ethanol (C2H5OH, >99.7 %) were purchased from Tianjin Fuchen. Platinum on carbon (Pt/C, 20 wt.%) was obtained from Johnson Matthey fuel cells. Nafion ionomer solution (5 %) was purchased from Dupont. Carbon


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nanofibers (CNFs) obtained from Aladdin Co. Ltd. Trizma® base (tris–(hydroxymethyl) aminomethane) and dopamine hydrochloride were purchased from Sigma–Aldrich. Argon (Ar), nitrogen (N2) and oxygen (O2) (99.99 %) gases were used as received from Xi’an Taida Chemical Reagent Co. Ltd. All the electrolyte solution was prepared with ultra–pure distilled water (18.25 MΩ.cm –1). Synthesis of MCO anchored on carbon nanofibers: First manganese cobalt oxide nanoparticles (MCO) anchored on carbon nanofibers (MCO/CNFs) was prepared as a precursor. 0.2 gram of CNFs was dispersed into 80 ml of ethanol under sonication to unravel the bundles. Then 10 mL aqueous solution containing 0.36 mmol of manganese acetate and 0.72 mmol of cobalt acetate was dropwise added into the suspension followed by 2mL of 27 % NH3 while gently stirring. The whole content was then refluxed at 80 oC for 12 hrs. The solution was transferred to a Teflon–lined stainless–steel autoclave and kept at 150 oC for another 12 hrs. Finally, the product was separated by centrifugation, and freeze dried for 24 hrs. Synthesis of MCO/CNFs@NC catalyst: First, 0.1 g of assynthesized MCO/CNFs was dispersed in a solution containing 0.1 g of Trizma® base (tris–(hydroxymethyl) aminomethane) and 80 mL of distilled water. Then, 0.1 g of dopamine hydrochloride was added rapidly into the suspension with vigorous stirring. The suspension was gently stirred for 3 hrs at room temperature. The suspension was then washed to remove the residual polydopamine and collected by centrifugation and freeze dried for 24 hrs. Finally, MCO/CNFs@NC catalyst were obtained by carbonizing the product in a tube furnace under an Ar2 atmosphere at 800 °C for 1 hr with a ramping rate of 5 oC per minute. To investigate the impact of annealing temperature, MCO/CNFs without polymer coating was also prepared at 800 °C under the same condition.



Characterizations: The crystal structures of the catalysts were examined with X–ray diffraction spectroscopy (XRD) (PANalytical X'Pert Pro MPD) using Cu K radiation. The working potential and current employed were 40 kV and 40 mA, respectively. The morphology and structure of the catalysts were examined using transmission electron microscope (TEM, FEI Tecnai F30) and high–resolution transmission electron microscopes with an accelerating voltage of 200 kV in each case. Energy–dispersive X–ray (EDX) spectra were obtained by field–emission scanning electron microscopy (FESEM, FEI NovaSEM 450). Nitrogen adsorption/desorption isotherms were measured on an accelerated surface area and porosimetry system (MicroActive for ASAP 2460 2.01) at –196 oC. The specific surface area of the MCO/CNFs@NC catalyst was calculated by applying the Brunauer–Emmett–Teller (BET) method. The pore sizes and pore volumes were evaluated by Barrett–Joyner–Helenda (BJH) model. The surface composition of the catalysts was investigated by X–ray photoelectron spectra (XPS, ESCALAB 250) with a monochromatic Al Kα X–ray source (E=1486.6 eV). Electrochemical measurements: All electrochemical measurements were performed with a conventional three–electrode configuration using rotating disc electrode (RDE) and a CHI 660C electrochemical workstation. The measurements were carried out using modified glassy carbon (GC) electrode, Pt wire, and Hg/HgO (1 M KOH) as working, counter, and reference electrode, respectively. The modified GC electrode was prepared by casting 5 µL of catalyst ink made by mixing 5 mg sample dispersed in 1 mL of ethanol and water mixture (1:1 v/v) and 10 µL of 5 wt% Nafion onto the GC, followed by drying at room temperature for 10 minutes and further at 50 oC for 5 min. All reported potentials were calibrated to the reversible hydrogen electrode (RHE).


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Cyclic voltammetry (CV) measurements were done in both N2 and O2 saturated 0.1 M KOH. Prior to recording of data, N2 or O2 was bubbled through the electrolyte for 20 minutes. And then the flow of either of the gas was maintained over the electrolyte during the recording of CV to guarantee the continued saturation. Linear Sweep voltammetry (LSV) was performed in O2 saturated 0.1 M KOH solution using rotating disk electrode (RDE) at different rotation rate. The scan rate of all the measurements is 10 mVs−1 unless and otherwise stated. To further examine the

reaction

mechanism

of

the

catalysts,

the

working

electrode

was

scanned cathodically with varying the speed of RDE from 400 rpm to 2500 rpm. Then the least– square fitted slopes of Koutecky–Levich plots (J– 1 vs. ω– 1/2) were used to calculate the number of electron transferred (n) based on the Koutecky–Levich equation.38 1 1 1 1 1 = + = 1 +

= 0.2 2 = 3 where J is the measured current density, JK is the kinetic limiting current and JL is diffusion limiting current densities, ω is the angular velocity, n is transferred electron number, F is the Faraday constant (96,485 C mol−1), Co is the bulk concentration (solubility) of O2 in 0.1 M KOH (1.2 × 10−6mol cm−1), v is the kinematic viscosity of the electrolyte (0.01 cm2 s−1) and D0 is the diffusion coefficient of O2 in 0.1 M KOH (1.9 × 10−5cm2 s−1). For the Tafel plot, the kinetics of the current was calculated by equation (4).

=

× 4 − 9 Environment ACS Paragon Plus


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Electrochemical impedance spectra (EIS) of MCO, MCO/CNFs, MCO/CNFs (Annealed) and MCO/CNFs@NC catalysts were measured in O2 saturated 0.1 M KOH within the frequency range of 100 kHz to 0.1 Hz using a sinusoidal signal with amplitude of 5 mV. Zn–air battery: The custom–built Zn–air battery was made with a zinc plate anode and the catalyst containing air cathode. The gap between the two electrodes was filled with an electrolyte containing 6 M KOH and 0.2 M zinc acetate. The catalyst ink was made by sonicating a mixture of active materials and Vulcan XC–72 in 2:1 mass ratio dispersed in ethanol with 40 µL of 5% Nafion ionomer. The air cathode was then prepared by dropping the above solution onto the carbon paper and drying for 30 min at 70 °C to achieve a loading of 1.2 mg cm–2. The performance of the Zn–air batteries was measured by CT 2001A (LANHE Company) battery testing system in ambient atmosphere at room temperature. Galvanostatic discharge and charge cycling performance of rechargeable Zn–air batteries was measured at a constant current density of 5 mA cm–2 with 15 min discharge and 15 min charge. For comparison, the Zn–air battery performance based on 20 wt % Pt/C catalyst was also tested under the same conditions. Supercapacitor Test: The working electrode for supercapacitor was prepared by mixing the active

material

(i.e.,

MCO/CNFs@NC),

acetylene

black,

and

polymer

binder

poly(tetrafluoroethylene) (PTFE) in a weight ratio of 80:10:10. The slurry was then pasted onto commercial Ni foam and then pressed at 10 MPa followed by drying overnight at 60 °C. The electrochemical tests were conducted with a CHI 660C electrochemical workstation in an aqueous 6.0 M KOH electrolyte with a three–electrode cell, where Pt wire and Hg/HgO electrode serves as the counter electrode and the reference electrode respectively.


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Figure 1. The schematic illustration of synthetic route of MCO/CNFs@NC catalysts, MCO is MnCo2O4, CNFs is nanofibers, and NC is N–doped carbon. RESULT AND DISCUSSION Preparation and structural characterization of the catalyst: Figure 1 illustrates the typical synthesis procedure of the MCO/CNFs@NC catalyst. First manganese cobalt oxide nanoparticles anchored on carbon nanofiber (MCO/CNFs) as a precursor was synthesized by a combined process of refluxing followed by solvothermal method as explained in the experimental section. As shown in the typical TEM image (Figure S1a), the obtained MCO/CNFs displayed plentiful nanoparticles being anchored on the surface of CNFs. The XPS survey spectrum of MCO/CNFs in Figure S2a confirmed the presence of definite peaks corresponding to the binding energies of Mn, Co, O, and C elements. The EDX measurement results (Figure S1d) further assured the presence of the stated elements in the product. The ratio of Co to Mn measured by XPS as well as EDX is ∼ 2 which is in good agreement with the starting precursor ratio. The XRD patterns of MCO/CNFs and MCO/CNFs (Annealed), MCO/CNFs@NC are shown in Figure 2a. MCO/CNFs 11 Environment ACS Paragon Plus


Figure 2. (a) XRD patterns of MCO/CNFs@NC, MCO/CNFs, MCO/CNFs (Annealed) and CNFs (b) EDX spectra of MCO/CNFs@NC (c, d) N2 adsorption/desorption isotherms and the corresponding pore size distribution of MCO/CNFs@NC catalyst, respectively. displayed similar XRD patterns to that of spinel MnCo2O4 according to JCPDS PDF card # 231237. Well defined peaks observed at 2θ values of 30.7o, 36.1o, 38.2o, 44 o, 58.1o63.8o and 67.2o represent the (220), (311), (222), (400), (511), (440) and (531) facets of cubic spinel MnCo2O4, respectively. This observation is in consistent with selected area electron diffraction (SAED) (Figure S1b) and the high resolution TEM (HRTEM) shown in Figure S1c. The peak at 25.6 o is attributed to (002) planes of graphitic carbon. Therefore, all the elemental, structural and morphology characterizations witnessed the complete formation of crystalline MnCo2O4 nanoparticles onto the surface of carbon nanofibers. In the case of the XRD patterns for MCO/CNFs (Annealed) i.e. after being annealed at high temperature in the argon environment,


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beside those peaks observed in MCO/CNFs, additional peaks at 51.5o and 75.8o are observed. Furthermore the peak observed in MCO/CNFs around 44o is also intensified. This observation may be due to the formation of some metallic Co after annealing (Fm3m (225), PDF#15-0806). In a typical synthesis of manganese cobalt oxide nanoparticles anchored on carbon nanofiber embedded into the nitrogen doped carbon matrix (MCO/CNFs@NC), dopamine is used as a source of carbon shell. Dopamine is a biomolecule with catechol and amine functional group that can self–polymerize in alkaline media and spontaneously form polydopamine conformal film on any surface.39 Furthermore, polydopamine coating can serve as an adhesive layer which can immobilize the nanoparticles on the support surface there by preventing self–accumulation and detachment.32, 40 Moreover, the pyrolysis of polydopamine at appropriate temperature can form nitrogen doped carbon. The schematic route and the details of specific preparation methods for MCO/CNFs@NC catalyst are illustrated in Figure 1 and the experimental section, respectively. In brief, as–prepared MCO/CNFs precursors were dispersed in a solution containing tris– (hydroxymethyl) aminomethane (pH 8.5). Then dopamine hydrochloride was added into the solution and stirred at room temperature for 3 hrs. The color of the supernatant leftover after centrifugation of the content with and without dopamine hydrochloride clearly shows the polymerization of dopamine (Figure S5). The color of the content with dopamine hydrochloride added was changed into dark brown owing to the polymerization of dopamine into poly dopamine in alkaline condition. The resulting suspension was collected by centrifugation, washed with deionized water and then freeze dried. Then thermal annealing of the obtained polydopamine coated MCO/CNFs at 800 oC leads to the formation of MCO/CNFs@NC catalyst. The thickness of the carbon–shell is another important parameter needs to be optimized to achieve core–shell catalyst with good catalytic performance. The carbon–shell needs to be



permeable for the reactant molecules to reach the surface of the nanoparticles. In the literatures it has been documented that the thickness of the in situ formed carbon shell can be precisely controlled by varying the polymer coating time.37, 41 The thickness of the polydopamine coating has been reported to be approximately proportional to polymer coating time. Recently, Chung, D. Y et.al.32 managed to prepare the ORR catalyst with high activity by optimizing the thickness of carbon–shell bellow 1 nm under the control of time of polymer coating. On the other hand, Zhang, Z. et.al.41 reported, N–doped porous carbon encapsulated bimetallic PdCo catalyst with the thickness of 5.6 nm to be highly active towards ORR. Herein, we have taken the advantage of the polydopamine coating time into consideration to indirectly control the thickness of the carbon shell. We have prepared MCO/CNFs@NC catalysts with three variable thicknesses Thickness 1, Thickness 2 and Thickness 3 under the polymer coating time of 2, 3 and 4 hrs, respectively. As shown in Figure S6 (a, b) the catalyst prepared with the polymer coating time of 3 hrs (Thickness 2) manifested the best ORR activity over those prepared at polymer coating time of 2 hrs (Thickness 1) and 4 hrs (Thickness 3). The HRTEM image shown in Figure 3d indicated the thickness of the catalyst after 3 hrs of coating time to be about 3.8 nm. This result is in agreement with previous report on the carbon shell encapsulated nanoparticles with 4 nm thickness which is highly preamble.39


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Figure 3. (a, b) Bright Field TEM images of MCO/CNFs@NC catalyst. (c, d) HRTEM for nanoparticles on the nanofiber and a selected core-shell nanostructure of MCO/CNFs@NC catalyst. The morphology of MCO/CNFs@NC catalyst was investigated by TEM. As shown in Figure 3 (a, b), nanoparticles with average size of ∼15–25 nm are uniformly distributed on the surface of carbon nanofibers without any nanoparticles detached from the support. Figure 3 (c, d) clearly shows a continuous and uniform nitrogen doped carbon shell with a thickness of ∼3.8 nm bound around carbon nanofibers and well–spread nanoparticles of manganese cobalt oxide are fully embedded in the stated shell. As shown in Figure 3d the lattice fringes of the nanoparticle were about 0.21 nm corresponding to the (400) plane of manganese cobalt oxide which is in



consistence with XRD results shown in Figure 2a. In the XRD pattern for MCO/CNFs@NC the peak at 44o is also intensified similar to that of MCO/CNFs (Annealed), which may be due to the formation of some metallic Co beside bimetallic oxide during annealing in the presence of carbon. Furthermore, as evidenced by comparative TEM image shown in Figure S3 (a, b) detached nanoparticles are observed around MCO/CNFs catalyst, but for that of MCO/CNFs@NC catalyst the particles are tightly bound around carbon nanofibers by the nitrogen doped carbon shell suggesting the significance of the shell in preventing against detachment and dissolution. Furthermore, as shown in Figure S3 (c, d) aggregated nanoparticles are observed on the surface of carbon nanofibers in the case of MCO/CNFs catalyst but the nanoparticles of MCO/CNFs@NC catalyst are well–spread suggesting the importance of nitrogen doped carbon shell in effectively preventing self–accumulation associated with metal oxide nanoparticles. On the other hand as shown in Figure S4b the nanoparticles of MCO/CNFs (Annealed) are interconnected in to a large aggregate due to agglomeration at elevated temperature which result in loss of catalytic activity. The surface area and porosity of the MCO/NFs@NC catalyst were measured by Nitrogen adsorption/desorption isotherms (Figure 2c and d). The catalyst displayed type–IV isotherms with well–defined hysteresis loops at higher N2 pressures (from 0.45 to 1.0).42 MCO/NFs@NC catalyst exhibited high BET surface area of about 100.2 m2 g–1. Furthermore as shown in Figure 2d, the pore–size distributions calculated by the (BJH) model shows MCO/NFs@NC catalyst possesses micropores and mesopores suggesting a hierarchical porous structure. The hierarchical porosity of MCO/NFs@NC along with its high surface area and pore volume can favors the mass– and charge–transport as well as provides abundant active sites during electrocatalysis.


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Figure 4. XPS spectra of MCO/CNFs@NC catalyst. (a) Survey spectrum (b) High–rsolution Co 2p spectrum (c) High–resolution Mn 2p spectrum (d) High–resolution N 1s spectrum along with peak deconvolution. The XPS analysis was performed to investigate the surface elemental composition and oxidation states of the constituents of the as prepared catalysts. As shown in Figure 4a, the survey spectrum of MCO/CNF@NC catalyst displayed the presence of definite peaks corresponding to the binding energies of Mn, Co, O, N, and C elements. The high–resolution spectrum of Mn 2p shows two main peaks at 652.67 and 641.17 eV separated by spin–orbit splitting of 11.5 eV which are attributed to 2p1/2 and 2p3/2, respectively (Figure 4c). Deconvolution of these two main peak results four sub–peaks: the pair at 641.5 and 652.8 eV corresponds to the binding energy of Mn2+, while the other pair at 644.0 and 654.6 eV is attributed to the binding energy of



Mn3+.43-45 Similarly, high resolution spectrum of Co 2p shows two main peaks at 796.37 and 780.47 eV separated by spin–orbit splitting of 15.9 eV along with evident satellite peaks at 786.07 and 802.37 eV which correspond to Co 2p1/2 and Co 2p3/2, respectively (Figure 4b). Furthermore, the deconvoluted peak results of Co 2p suggests the presence of mixed Co2+/Co3+ oxidation states which is in consistence with the values reported in the literatures.46, 47 The ratio of Co to Mn in MCO/CNFs@NC catalysts as measured by XPS is ∼ 2 which is in good agreement with the starting precursor ratio and EDX result (Figure 2b). The total nitrogen content of the catalyst is ca. 5.44%. As shown in Figure 4d the high resolution N1s spectrum can be further deconvoluted into three types of N species corresponding to pyridinic N (39.4 %), pyrrolic N (43.5 %), and graphitic N (17.2 %) structures located at the binding energies of 398.1, 400.1, and 401.8eV, respectively. The presence of the graphitic N is believed to increase ionic conductivity while the pyridinic N and pyrrolic N are expected to improve the charge storage by increasing surface redox reaction. 26, 48 Electrochemical Test: To investigate the performance of the as–synthesized catalysts towards oxygen electrochemical reaction, the glassy carbon electrode (GCE) modified with the corresponding catalysts was used as a working electrode. The comparative cyclic voltammograms (CV) of the as–synthesized catalysts in O2 and N2 saturated 0.1 M KOH electrolyte are shown in Figure 5a. MCO/CNFs@NC catalyst displayed strong peak at 0.76 V vs. RHE in O2 saturated KOH electrolyte but this pick completely vanished in N2 saturated KOH electrolyte suggesting that, the observed pick is attributed to the reduction of molecular oxygen on the surface of the catalyst. The CV of Pt/C catalyst is 0.80 V vs. RHE which is only about 40 mV more positive than MCO/CNFs@NC catalyst. On the other hand, CV for the other as– synthesized catalysts exhibited the ORR pick towards more negative potential than that of


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Figure 5. (a) CV curves of MCO/CNFs@NC, MCO/CNFs, MCO/CNFs (Annealed), MCO, CNFs and Pt/C catalysts in N2 (broken lines) and O2–saturated (solid lines) alkaline solutions (b) LSV curves of MCO/CNFs@NC, MCO/CNFs, MCO/CNFs (Annealed), MCO, CNFs and Pt/C catalysts in an O2–saturated 0.1 M KOH solution at electrode rotation speed of 1600 rpm. (c) LSV curves of MCO/CNFs@NC catalyst in O2–saturated 0.1 M KOH at different rotating rates. (d) Koutecky Levich (K–L) plots at various potentials. (e) Tafel slope values at low overpotential regions for MCO/CNFs@NC, MCO/CNFs and Pt/C catalysts. (f) OER polarization curves for MCO/CNFs@NC, MCO/CNFs, MCO/CNFs (Annealed), CNFs, MCO, RuO2 and Pt/C catalysts in an O2–saturated 0.1 M KOH solution at 1600 rpm and overpotentials derived from OER polarization curves at 10 mA cm–2 (inset). MCO/CNFs@NC catalyst. The order of catalytic activity of the catalysts based on CV results is MCO/CNFs@NC > MCO/CNFs > MCO/CNFs (Annealed) > CNFs > MCO. The ORR activity



of MCO/CNFs@NC catalyst was further evaluated by linear sweep voltammetry (LSV) in comparison to MCO/CNFs, MCO/CNFs (Annealed), MCO, CNFs and commercial Pt/C. Figure 5b shows MCO/CNFs@NC catalyst exhibits significantly improved ORR activity, with comparable onset potential (1.00V vs. RHE) to that of Pt/C catalyst (1.03 V vs. RHE). The half– wave potential of MCO/CNFs@NC catalyst is 0.76 V vs. RHE which is only about 40 mV lower than that of the state of art Pt/C (0.80 V vs. RHE). On the other hand, MCO/CNFs, MCO/CNFs (Annealed), CNFs and bare MCO displayed poor ORR catalytic activity with about 80, 140, 180, 240 mV more negative in half–wave potential than that of MCO/CNFs@NC catalyst. The order of catalytic activity of the catalysts based on LSV results is MCO/CNFs@NC > MCO/CNFs > MCO/CNFs (Annealed) > CNFs > MCO which is in accord with the CV results. The electrocatalytic parameters of the catalysts are summarized in Table S1. The observed electrochemical test results of MCO/CNFs@NC catalyst suggest the importance of nitrogen doped carbon shell to push the ORR activity of the wrapped catalyst closer to the benchmark Pt/C relative to unwrapped one. The enhanced ORR activity of MCO/CNFs@NC catalyst is attributed to i) the synergetic interaction (M–N–C) between the core nanoparticles and nitrogen doped carbon shell which is in agreement with XPS analysis result. ii) the unique core–shell structure of the catalyst that is strongly resistant towards self–accumulation and detachment of nanoparticles, as confirmed by TEM and durability test. It is evident that ORR can happen either by two or four electrons pathways. Four electrons pathway leads to the complete reduction of O2 into OH– ions without any detectable intermediates. Whereas, two electrons pathway produces peroxide ions, which further produce radicals that are detrimental to electrodes and electrolytes.49, 50 To investigate the possible ORR pathways catalysed by MCO/CNFs@NC catalyst the LSV curves at various rotating rates were


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measured (Figure 5c) and converted to the corresponding Koutechy–Levich (K–L) plots (Figure 5d). The LSV curves at various rotational speeds displayed the same onset potential while the current densities were enhanced directly with increasing rotational speed owing to the increased mass transport further suggesting the kinetically controlled process of ORR.51-54 Furthermore, the K–L plots showed a good linearity and coincidence, indicating first–order reaction kinetics towards the concentration of dissolved oxygen and similar n values per oxygen molecule for ORR.55,

56

The electron transfer number of MCO/CNFs@NC catalyst calculated from K–L

equation is 4, suggesting a four–electron pathway for ORR process. This n value is larger than that of MCO/CNFs (3.92), MCO/CNFs (Annealed) (3.68) and CNFs (2.04) (Figure S7), indicating the enhanced catalytic activity of MCO/CNFs@NC catalyst. To further analyze the kinetic properties of ORR, the Tafel slopes were obtained from the linear plots of LSV (1600 rpm) at low overpotentials where the ORR rate was dependent on the surface reaction rate on the electrocatalyst.57 As shown in Figure 5e MCO/CNFs@NC demonstrated the lower Tafel slop (70 mV dec–1) than that of MCO/CNFs (93 mV dec–1) and commercial Pt/C (80 mV dec–1), suggesting that MCO/CNFs@NC catalyst owns a faster electron transfer rate. Furthermore, the observed significant difference between the Tafel slope of MCO/CNFs@NC catalyst and that of MCO/CNFs catalyst suggests the benefit of encapsulation in facilitating electron transfer rate. Additionally, the mass activities of the catalyst were calculated by normalizing the kineticlimiting current to the mass of active materials. As shown in Figure 6f the mass activity of MCO/CNFs@NC catalyst was also found to be about 2.7, 14.5 and 115 times larger than that of MCO/CNFs, MCO/CNFs (Annealed) and CNFs catalysts, respectively suggesting the favorable kinetics and high intrinsic ORR activity of MCO/CNFs@NC catalyst.



EIS was conducted to examine the kinetic properties of MCO, MCO/CNFs, MCO/CNFs (Annealed) and MCO/CNFs@NC catalysts. As shown by the Nyquist plots (Figure S8) the smallest semicircle is observed in the case of MCO/CNFs@NC catalyst suggesting its charge transfer resistance is much smaller than that of MCO, MCO/CNFs and MCO/CNFs (Annealed). This result further assures the significance of N–doped carbon shell in enhancing the electronic conductivity and allows much easier charge transfer at the electrode/ electrolyte interface. To assess the bifunctionality of MCO/CNFs@NC catalyst, its catalytic activity towards OER was also investigated. Figure 4f compares the LSV behavior of MCO/CNFs@NC, MCO/CNFs, MCO/CNFs (Annealed), MCO, CNFs, RuO2and commercial Pt/C catalysts in the voltage range from 1.0 to 2.2 V vs. RHE at 1600 rpm in O2–saturated 0.1M KOH. The MCO/CNFs@NC catalyst exhibited the lower overpotential of 0.41 V which is only about 20 mV higher than that of the state of art RuO2 at the current density of 10 mAcm–2, suggesting its great potential as an efficient OER catalyst for sustainable energy applications. The performance of bifunctionality of the catalyst can be further evaluated by determining the potential gap (∆E) between the OER potential measured at 10 mA cm−2 current density and the ORR half–wave potential (∆E = Ej10 − E1/2). The lower the ∆E value, the better the bifunctionality of the catalyst. The ∆E of MCO/CNFs@NC catalyst is about 0.88 V which is the lowest among the catalyst mentioned here in Table S1. A summary of comparison of the electrochemical performances of the electrocatalysts for bifunctional oxygen catalysis in this work and other related literature are provide as Table S2 in supporting information. The main problem associated with direct methanol fuel cell (DMFC) is methanol crossover in which the methanol passes to the cathode compartment where it gets oxidized along with ORR and thus resulting poor cell performance.58 Thus, ORR catalyst that selectively reduces oxygen


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but ineffective towards methanol oxidation is of great interest. Herein, the effect of methanol crossover was investigated in the presence of methanol by chronoamperometric measurements at 0.7 V vs. RHE. As shown in Figure 6b when 3 M methanol was injected into the O2– saturated 0.1 M KOH no apparent change in normalized current density was observed in the case MCO/CNFs@NC catalyst indicating its strong tolerance towards methanol oxidation while maintaining its original ORR performance. In contrast, the commercial Pt/C catalyst experienced a sharp decrease in normalized current density, suggesting a conversion of the dominated oxygen reduction reaction to the methanol oxidation reaction. As shown in Figure 6c LSV curves of MCO/CNFs@NC catalyst before and after the addition of 3M methanol in the electrolyte solution remained almost unchanged implying its higher selectivity towards ORR than methanol oxidation reaction. But Pt/C (Figure 6d) displayed significant negative shift and a distinct interfering peak after the addition of 3M methanol due to the competing methanol oxidation reaction which results in the poisoning of the catalyst.59,

60

These results further suggest that

MCO/CNFs@NC catalyst can also serve as an effective cathode catalyst for alcohol fuel cells. Durability of the catalyst is another uncompromisable factors that should be taken into consideration to realize its practical application into the perspective technologies. For instance, even though Pt/C is highly active towards ORR, its real–world application is largely impeded by its instability in alkaline media.61 To investigate the long–term application of MCO/CNFs@NC catalyst both accelerated durability test (ADT) and short–term stability (chronoamperometric i–t) test were conducted.

As shown in Figure 6a the short–term stability test evaluated by

chronoamperometric measurements under a constant cathodic voltage of 0.7 V vs. RHE in the O2–saturated 0.1 M KOH solution displayed the superior stability of MCO/CNFs@NC catalyst over that of MCO/CNFs, MCO/CNFs (Annealed) and commercial Pt/C catalysts. After 5000 s of



Figure 6. (a) Chronoamperometric responses of MCO/CNFs@NC, MCO/CNFs, MCO/CNFs (Annealed) and Pt/C catalysts at 0.7 V vs. RHE and 1600 rpm in an O2–saturated 0.1 M KOH solution. (b) Chronoamperometric responses of MCO/CNFs@NC and Pt/C catalysts at 0.7 V vs. RHE and 1600 rpm in an O2–saturated 0.1 M KOH solution with the addition of methanol at 300s. (c) and (d) LSV for MCO/CNFs@NC and Pt/C electrode in O2–saturated 0.1 M KOH and 0.1M KOH + 3 M methanol. (e) LSV for ADT at initial and after each of the mentioned cycles in the legend. (f) Mass activity of the catalysts measured at 0.80 V vs. RHE continuous operation MCO/CNFs@NC catalyst retained around 96.03 % of its initial current density. On the other hand, the current density of MCO/CNFs, MCO/CNFs (Annealed) and commercial Pt/C catalysts fall down to 74.74, 73.66 and 86.73 % of their initial state, 24 Environment ACS Paragon Plus

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respectively. The observed instability of MCO/CNFs is strongly consistent with the observation of TEM image in Figure S3 whereby the detached and aggregated nanoparticles were observed in the case of MCO/CNFs catalyst. On the other hand MCO/CNFs (Annealed) showed large continuously interconnected particles (Figure S4b) due to the agglomeration of the nanoparticles at higher annealing temperature which decreases the activity of the catalyst and results instability as well. Further to assure the long–term application of MCO/CNFs@NC catalyst, ADT was carried out by scanning the CV within the potential range of 0.6 to 1 V vs. RHE. The LSV curve is recorded periodically as indicated in Figure 6e. MCO/CNFs@NC catalyst demonstrated the superior long–term durability with slight activity loss of only about 13 mV after being subjected to 5000 cycles. As shown in Figure S4a, the TEM image of MCO/CNFs@NC catalyst conducted after 5000 cycles of ADT retained the same morphology indicating the strong resistance of the carbon shell against detachment and agglomeration of the nanoparticle from the support. This observation is in accord with all electrochemical tests confirming the electrochemical stability of MCO/CNFs@NC catalyst.



Performance of Zn–air battery: To prove the practical application of MCO/CNFs@NC catalyst in primary as well as rechargeable Zn–air batteries, the custom–built batteries were assembled as shown in Figure 7d and described in the experimental section. The galvanodynamic charge/discharge along with power density profiles are shown in Figure 7a. The batteries based on MCO/CNFs@NC catalyst demonstrated open circuit potential of 1.55 V, which is close to the theoretical value of 1.65 V and delivered the maximum power density of 75 mW cm−2 at the current density of 100 mA cm–2. As shown in Figure 7b, the galvanostatic discharge plot demonstrated that primary Zn–air battery made of MCO/CNFs@NC catalyst can operate with

Figure 7. Performance of Zn–air batteries based on MCO/CNFs@NC catalyst (a) Galvanodynamic charge/discharge profiles (left) and power density curves (right). (b) Galvanostatic discharge curves of the primary Zn–air batteries. (c) Mechanically rechargeable Zn–air battery (c) Mechanically rechargeable Zn–air battery performance at 10 mA cm–2. (d) Digital images of single–cell Zn–air battery made of MCO/CNFs@NC catalyst.


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stable voltages of 1.2, 1.16 and 1.12 V versus Zn at discharge current density of 5, 10 and 20 mA cm–2 respectively. Furthermore, the MCO/CNFs@NC catalyst based battery delivered specific capacity of about 742, 718 and 695 mAh g–1Zn corresponding to the specific energy densities of 890, 834 and 778 Wh kg–1Zn at 5, 10 and 20 mA cm–2, respectively. Moreover, we have proved that MCO/CNFs@NC catalyst can function in mechanically rechargeable battery by only refueling the consumed zinc anode and electrolytes at the end of each discharge. Figure 7c shows that mechanically rechargeable Zn–air battery based on MCO/CNFs@NC catalyst demonstrated stable discharge profile and the same specific capacity for each of the three consecutive anode materials. These results are in accord with the superior stability and durability performance of

Figure 8. Comparative galvanostatic charge–discharge cycling performance of rechargeable Zn–air batteries based on MCO/CNFs@NC and commercial Pt/C catalysts at 5 mA cm–2. (a) Voltage versus time, (b) Voltage versus cycle number (c) and (d) Comparative voltaic/round-trip efficiency of MCO/CNFs@NC and Pt/C catalysts, respectively.



MCO/CNFs@NC catalyst demonstrated by Figure 6 (a, e). To further explore the feasibility of MCO/CNFs@NC catalyst for rechargeable Zn–air batteries galvanostatic charge–discharge cycling was performed as shown in Figure 8. The Zn–air battery based on MCO/CNFs@NC catalyst demonstrated stable galvanostatic charge–discharge cyclic performance, which is in consistent with the bifunctional activity and stability test. The round–trip/voltaic efficiency (η) i.e., the ratio between charge and discharge voltage plateau experienced negligible decrement after more than 220 cycles (Figure 8c). The η values of MCO/CNFs@NC catalyst are 58.2, 57.1 and 55.7 % at 1st and after 20th and 223rd cycles respectively. But the battery fabricated based on commercial Pt/C catalyst even though initially it exhibited similar round–trip efficiency (η) the value quickly deteriorated after around 5 cycles. The η value of Pt/C are 58.2, 43.7 and 47.1 % at 1st and after 20th and 93rd cycles respectively (Figure 8d). Supercapacitive property: The electrochemical performance of MCO/CNFs@NC as an electrode material for supercapacitor was also investigated in 6 M KOH aqueous solution electrolyte. Figure 9a shows the typical cyclic voltammetry (CV) curves at various scanning rates ranging from 5 to 50 mV s−1 in the potential window of 0.0 to 0.7 V vs Hg/HgO. The shape of the CV curves apparently reveals pseudocapacitive characteristics of MCO/CNFs@NC. As clearly evidenced from the figure a pair of well–defined redox peaks are observed at all scanning rates which corresponds to the Faradaic redox reactions related to M–O/M–O–OH (M refers to Mn or Co) associated with OH− anions in the electrolyte.62 The voltage gap between redox peaks found at around 0.22 and 0.36 V of slow scan rate of 5 mV s−1 tends to be wider proportional to the scan rate due to the increased current response. But the shape of the curves remains the same with different scan rate suggesting relatively low resistance of the electrode and excellent rate capability for power delivery.63 The galvanostatic charge–discharge curves of MCO/CNFs@NC


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Figure 9. Supercapacitive properties of MCO/CNFs@NC. (a) CV curves at various scan rates. (b) Galvanostatic charge/discharge curves at various current densities. (c) Specific capacitance as a function of current density. (d) Cycling stability of MCO/CNFs@NC at current density of 5 A g−1. Inset is the profile of portion of charge/discharge cycles. at different mass normalized current densities between the voltage windows of 0.0 to 0.5 V vs. Hg/HgO are shown in Figure 9b. The specific capacitance (C) was calculated according to equation (5). The specific capacitance was determined to be 478, 449, 403, and 332 F g−1 at current densities of 1, 2.5, 5, 10 A g−1 respectively. Long cyclic stability of the electrode is crucial for its practical application. The cycling stability of MCO/CNFs@NC was evaluated at constant current densities of 5 A g−1, as shown in Figure 9d. At higher current density of 5 A g−1 the electrode retained as high as 87.5% of its initial capacitance after 1500 cycles.

=

×∆! ∆"×#

5



where I is the discharge current, ∆t is the discharge time, m is the mass of the active materials, and ∆V is the voltage window.

CONCLUSSIONS In summary, we have successfully prepared bimetallic Mn–Co oxide nanoparticles anchored on carbon nanofibers and wrapped into nitrogen doped carbon shell (MCO/CNFs@NC). First manganese cobalt oxide nanoparticles anchored on carbon nanofibers as precursor were prepared by a combined process of refluxing followed by solvothermal method. Then about 3.8 nm thick nitrogen doped carbon shell were in situ formed on the surface of MCO/CNFs through thermal annealing of polydopamine. The nitrogen–doped carbon shell not only pushes the ORR activity closer to the benchmark Pt/C but also grants superior stability to the catalyst by preventing the nanoparticles from falling apart from the support.

Furthermore, MCO/CNFs@NC catalyst

catalyzed the ORR path way with an electron transfer number of 4.00 and the lowest Tafel slope of 70 mV dec–1. MCO/CNFs@NC catalyst is also found to catalyze OER with low overpotential of 0.41 V at the current density of 10 mA cm–2. The suitability of MCO/CNFs@NC catalyst as a positive electrode catalyst was also demonstrated in the primary as well as rechargeable Zn–air battery. The Zn–air batteries based on MCO/CNFs@NC catalyst delivered the specific capacity of 695 mA h g–1Zn and the energy density of 778 W h kg– 1Zn at 20 mA cm−2. The mechanically rechargeable Zn–air battery based on MCO/CNFs@NC catalyst is also found to function continually by only replacing the consumed zinc anode and electrolyte. The electrically rechargeable battery based on MCO/CNFs@NC catalyst is found to function for more than 220 cycles with negligible loss of voltaic efficiency. Moreover, the MCO/CNFs@NC is found to display supercapacitive nature with a good discharge capacity of 478 F g−1 at discharge current


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density of 1 A g−1 and with capacitance retention of as high as 87.5% at higher current density of 5 A g−1 after 1500 cycles. ASSOCIATED CONTENT The following files are available free of charge on the ACS Publications website at DOI: xxxxx Additional TEM, EDX, XPS, LSV and the corresponding K–L plot, EIS and Tables (PDF). AUTHOR INFORMATION Corresponding Author *E–mail: [email protected] (F. C.). ORCID Fuyi Chen: 0000–0002–2191–0930 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (grant nos. 51271148 and 50971100), the Research Fund of State Key Laboratory of Solidification Processing in China (grant no. 150–ZH–2016), the Aeronautic Science Foundation Program of China (grant no.2012ZF53073), the Project of Transformation of Scientific and Technological Achievements of NWPU (grant no. 19–2017), the Doctoral Fund of Ministry of Education of China (grant no.20136102110013), and the Open Fund of State Key Laboratory of Advanced



Technology for Materials Synthesis and Processing (Wuhan University of Technology grant no. 2018–KF–18).

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TOC Figure


Bimetallic Mn-Co oxide nanoparticles anchored on carbon nanofibers

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