Bimetallic MnâCo Oxide Nanoparticles Anchored on Carbon ...

Article Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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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† †

State Key Laboratory of Solidification Processing and ‡School of Electronics and Information, Northwestern Polytechnical University, Xian 710072, China S Supporting Information *

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 a 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, the MCO/CNFs@NC catalyst exceeds the 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 (ΔE) as low as 0.88 V, suggesting its strong bifunctionality. The Zn−air battery based on MCO/CNFs@NC catalyst is found to deliver a specific capacity of 695 mA h g−1Zn and an energy density of 778 W h kg−1Zn at a current density of 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, MCO/CNFs@NC is found to display a supercapacitive nature with a good discharge capacity of 478 F g−1 at a discharge current density of 1 A g−1. KEYWORDS: oxygen reduction reaction, oxygen evolution reaction, bimetallic oxide, nitrogen-doped carbon, Zn−air battery

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INTRODUCTION

that largely impede the real-world application of these technologies, among them are the slow kinetics of ORR at the cathode side along with insufficient durability and high cost of the system, which 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

Issues such as fast growth of human population, depletion of fossil fuels and environmental concern have induced the growing demand on exploration of sustainable energy conversion and energy storage systems that are reliable, costeffective and environmentally benign. Oxygen electrochemical reactions, such as oxygen reduction reaction (ORR) and oxygen evolution reaction (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 © XXXX American Chemical Society

Received: January 18, 2018 Accepted: March 19, 2018 Published: March 19, 2018 A

DOI: 10.1021/acsaem.8b00067 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Article

ACS Applied Energy Materials

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 selfaccumulation 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 a more unique hybrid structure of the catalyst that can provide resistance toward 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 them with nitrogen-doped carbon shell. The strategy of introducing carbon nanofibers 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 toward ORR and/or OER due to the synergetic interactions (Metal− N−Carbon) between core and shell, besides 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 an electrode material for supercapacitors. 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 nanofibers, carbon nanotube, and further wrapped with protective carbon shell.

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 toward ORR and OER, their widespread 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 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 toward OER due to the variable oxidation states of transition metals.15,19−22 Consequently, the strategy of integrating carbonaceous nanomaterials and transition metalbased 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 nitrogen-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 potentials 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 a current density of 10 mA cm−2, proving the bifunctionality of the catalyst. Recently our group also reported12 spinel MnCo2O4 nanoparticles on a nitrogendoped reduced graphene oxide (MnCo2O4/NGr) hybrid for the application of hybrid Zn−air batteries. The catalyst demonstrated ORR with a half-wave potential only 30 mV lower than that of Pt/C in an alkaline environment. Furthermore, the catalyst showed the potential gap (ΔE) 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 a hybrid catalyst based on spinel CoMn2O4 nanoparticles supported on nitrogen-doped graphene aerogel. The hybrid catalyst showed activity toward ORR with a comparable onset potential of 1.06 V vs RHE and half-wave potential of 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 a multifunctional catalyst. The obtained catalyst displayed ORR activity of about 0.705 V vs RHE based on CV measurement and OER with an overpotential of about 0.66 V at current density of 10 mA cm−2. The hybrid Co3O4/N-rGO catalyst displayed about a 31.37% decline in the initial value of

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EXPERIMENTAL SECTION

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 nanofibers (CNFs) were 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 B


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Figure 1. Schematic illustration of synthetic route of MCO/CNFs@NC catalysts. MCO is MnCo2O4, CNFs is carbon nanofibers, and NC is nitrogen-doped carbon. from Xi’an Taida Chemical Reagent Co. Ltd. All the electrolyte solutions were 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 g 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 2 mL of 27% NH3 while gently stirring. The whole content was then refluxed at 80 °C for 12 h. The solution was transferred to a Teflon-lined stainless-steel autoclave and kept at 150 °C for another 12 h. Finally, the product was separated by centrifugation and freeze-dried for 24 h. 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 h at room temperature. The suspension was then washed to remove the residual polydopamine and collected by centrifugation and freeze-dried for 24 h. Finally, MCO/ CNFs@NC catalyst was obtained by carbonizing the product in a tube furnace under an Ar2 atmosphere at 800 °C for 1 h with a ramping rate of 5 °C per minute. To investigate the impact of annealing temperature, MCO/CNFs without polymer coating were also prepared at 800 °C under the same conditions. 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 highresolution 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 °C. 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 config-

uration 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 min and further at 50 °C for 5 min. All reported potentials were calibrated to the reversible hydrogen electrode (RHE). 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 min. Then the flow of either 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 a different rotation rate. The scan rate of all the measurements is 10 mV s−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 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 electrons transferred (n) based on the Koutecky−Levich equation:38

1 1 1 1 1 = + = + J JL JK JK βω1/2

(1)

β = 0.2nFC0(D0)2/3 v−1/2

(2)

JK = nFkC0

(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), C0 is the bulk concentration (solubility) of O2 in 0.1 M KOH (1.2 × 10−6 mol 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 × 10−5 cm2 s−1). For the Tafel plot, the kinetics of the current was calculated by eq 4:

JK =

J × JL (J − JL )

(4)

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. C


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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. 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 serve as the counter electrode and the reference electrode, respectively.

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, MCO/CNFs (annealed), and MCO/CNFs@ NC are shown in Figure 2a. MCO/CNFs displayed similar XRD patterns to that of spinel MnCo2O4 according to JCPDS PDF card # 23-1237. Well-defined peaks observed at 2θ values of 30.7°, 36.1°, 38.2°, 44 o, 58.1°, 63.8°, and 67.2° represent the (220), (311), (222), (400), (511), (440), and (531) facets of cubic spinel MnCo2O4, respectively. This observation is 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), that is, after being annealed at high temperature in the argon environment, beside those peaks observed in MCO/CNFs, additional peaks at 51.5° and 75.8° are observed. Furthermore, the peak observed in MCO/CNFs around 44° is also intensified. This observation may be due to the formation of some metallic Co after annealing (Fm3i (225), PDF # 15-0806). In a typical synthesis of manganese cobalt oxide nanoparticles anchored on carbon nanofibers 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 selfpolymerize in alkaline media and spontaneously form polydopamine conformal film on any surface.39 Furthermore, polydop-

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RESULTS 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 nanofibers (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 anchored on the surface of CNFs. The XPS survey spectrum of MCO/CNFs in Figure S2a confirmed the D


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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.

amine coating can serve as an adhesive layer which can immobilize the nanoparticles on the support surface thereby 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 h. 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 polydopamine 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 °C leads to the formation of MCO/CNFs@NC catalyst. The thickness of the carbon shell is another important parameter that needs to be optimized to achieve a 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 literature 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 below 1 nm under the control of the polymer coating time. On the other hand, Zhang, Z. et al.41 reported a nitrogen-doped porous carbon encapsulated bimetallic PdCo catalyst with the thickness of 5.6 nm to be highly active toward 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, 2, and 3, under the polymer coating times of 2, 3, and 4 h, respectively. As shown in Figure S6a,b, the catalyst prepared with the polymer coating time of 3 h (thickness 2) manifested the best ORR activity over those prepared at a polymer coating time of 2 h (thickness 1) and 4 h (thickness 3). The HRTEM image shown in Figure 3d indicated the thickness of the catalyst after 3 h of coating time to be about 3.8 nm. This result is in agreement with previous reports on the carbon shell encapsulated nanoparticles with 4 nm thickness which is highly preamble.39 The morphology of MCO/CNFs@NC catalyst was investigated by TEM. As shown in Figure 3a,b, nanoparticles with average size of ∼15−25 nm are uniformly distributed on the surface of carbon nanofibers without any nanoparticles E


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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.

pressures (from 0.45 to 1.0).42 MCO/CNFs@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/CNFs@NC catalyst possesses micropores and mesopores, suggesting a hierarchical porous structure. The hierarchical porosity of MCO/CNFs@ NC along with its high surface area and pore volume can favor the mass and charge transport as well as provides abundant active sites during electrocatalysis. 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/CNFs@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 in four subpeaks: 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 suggest the presence of mixed Co2+/Co3+ oxidation states which is consistent with the values reported in the literature.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).

detached from the support. Figure 3c,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 consistent with XRD results shown in Figure 2a. In the XRD pattern for MCO/CNFs@NC the peak at 44° 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 S3a,b, detached nanoparticles are observed around MCO/CNFs catalyst, but for the MCO/CNFs@NC catalyst, the particles are tightly bound around carbon nanofibers by the nitrogendoped carbon shell suggesting the significance of the shell in preventing against detachment and dissolution. Furthermore, as shown in Figure S3c,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 results in loss of catalytic activity. The surface area and porosity of the MCO/CNFs@NC catalyst were measured by nitrogen adsorption/desorption isotherms (Figure 2c,d). The catalyst displayed type IV isotherms with well-defined hysteresis loops at higher N2 F


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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 O2saturated 0.1 M KOH at different rotating rates. (d) 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).

strong peak at 0.76 V vs RHE in O2 saturated KOH electrolyte, but this peak completely vanished in N2 saturated KOH electrolyte suggesting that the observed peak 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 peak toward more negative potential than that of 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 the MCO/CNFs@NC catalyst exhibits significantly improved ORR activity, with

The total nitrogen content of the catalyst is ca. 5.44%. As shown in Figure 4d, the high-resolution N 1s 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 401.8 eV, 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 toward oxygen electrochemical reaction, the glassy carbon electrode (GCE) modified with the corresponding catalysts was used as a working electrode. The comparative cyclic voltammograms (CVs) 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 a G


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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 0.1 M 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.

shell, which is in agreement with XPS analysis result, and (ii) the unique core−shell structure of the catalyst that is strongly resistant toward self-accumulation and detachment of nanoparticles, as confirmed by TEM and durability tests. It is evident that ORR can happen either by two or four electron pathways. A four electron pathway leads to the complete reduction of O2 into OH− ions without any detectable intermediates. Whereas, the two electron pathway produces peroxide ions, which further produce radicals that are detrimental to electrodes and electrolytes.49,50 To investigate the possible ORR pathways catalyzed by MCO/CNFs@NC catalyst, the LSV curves at various rotating rates were 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

comparable onset potential (1.00 V 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, and 240 mV more negative in halfwave 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 the nitrogendoped 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 H


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ACS Applied Energy Materials coincidence, indicating first-order reaction kinetics toward 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 the 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 slope (70 mV dec−1) than that of MCO/CNFs (93 mV dec−1) and commercial Pt/C (80 mV dec−1), suggesting that the 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 kinetic-limiting 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 nitrogen-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 toward OER was also investigated. Figure 5f compares the LSV behavior of MCO/CNFs@NC, MCO/ CNFs, MCO/CNFs (annealed), MCO, CNFs, RuO2, and commercial Pt/C catalysts in the voltage range from 1.0 to 2.2 V vs RHE at 1600 rpm in O2-saturated 0.1 M 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 (ΔE) between the OER potential measured at 10 mA cm−2 current density and the ORR half-wave potential (ΔE = Ej10 − E1/2). The lower the ΔE value, the better the bifunctionality of the catalyst. The ΔE of MCO/CNFs@NC catalyst is about 0.88 V, which is the lowest among the catalysts 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 is provide in Table S2. The main problem associated with a direct methanol fuel cell (DMFC) is methanol crossover in which the methanol passes to the cathode compartment where it gets oxidized along with ORR, thus resulting poor cell performance.58 Thus, ORR

catalyst that selectively reduces oxygen but is ineffective toward 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 toward 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 3 M methanol in the electrolyte solution remained almost unchanged, implying its higher selectivity toward ORR than methanol oxidation reaction. But Pt/C (Figure 6d) displayed significant negative shift and a distinct interfering peak after the addition of 3 M methanol due to the competing methanol oxidation reaction which results in the poisoning of the catalyst.59,60 These results further suggest that the MCO/CNFs@NC catalyst can also serve as an effective cathode catalyst for alcohol fuel cells. Durability of the catalyst is another uncompromisable factor that should be taken into consideration to realize its practical application into the perspective technologies. For instance, even though Pt/C is highly active toward ORR, its real-world application is largely impeded by its instability in alkaline media.61 To investigate the long-term application of a MCO/ CNFs@NC catalyst, both accelerated durability test (ADT) and short-term stability (chronoamperometric i−t) tests 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 a MCO/CNFs@NC catalyst over that of MCO/CNFs, MCO/ CNFs (annealed), and commercial Pt/C catalysts. After 5000 s of continuous operation, the 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 decrease to 74.74, 73.66, and 86.73% of their initial state, 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 ensure the long-term application of MCO/CNFs@NC catalyst, ADT was carried out by scanning the CV within the potential range of 0.6−1 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 the MCO/CNFs@NC catalyst. I


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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 performance at 10 mA cm−2. (d) Digital images of a single-cell Zn−air battery made 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 a Zn−air battery made of MCO/CNFs@NC catalyst can operate with 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 catalystbased battery delivered a 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 the MCO/ CNFs@NC catalyst can function in a 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 MCO/CNFs@NC catalyst demonstrated by Figure 6a,e. To further explore the feasibility of the 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 consistent with the bifunctional activity and stability test. The round-trip/ voltaic efficiency (η), that is, 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 the first, 20th, and 223rd cycles, respectively. But the battery fabricated based on commercial Pt/C catalyst even though initially it exhibited a similar round-trip efficiency (η), the value quickly deteriorated after around 5 cycles. The η values of Pt/C are 58.2, 43.7, and 47.1% at the first, 20th, and 93rd cycles, respectively (Figure 8d). Supercapacitive Property. The electrochemical performance of MCO/CNFs@NC as an electrode material for a supercapacitor was also investigated in 6 M KOH aqueous solution electrolyte. Figure 9c 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 correspond 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 rates, suggesting relatively low resistance of the electrode and excellent rate capability for power delivery.63 J


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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.

Figure 9. Supercapacitive properties of MCO/CNFs@NC. (a) Galvanostatic charge/discharge curves at various current densities. (b) Specific capacitance as a function of current density. (c) CV curves at various scan rates. (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.

K


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ACS Applied Energy Materials ORCID

The galvanostatic charge−discharge curves of MCO/CNFs@ NC at different mass normalized current densities between the voltage windows of 0.0 to 0.5 V vs Hg/HgO are shown in Figure 9a. The specific capacitance (C) was calculated according to eq 5. The specific capacitance was determined to be 478, 449, 403, and 332 F g−1 at current densities of 1, 2.5, 5, and 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 a higher current density of 5 A g−1, the electrode retained as high as 87.5% of its initial capacitance after 1500 cycles. C=

I × Δt ΔV × m

Fuyi Chen: 0000-0002-2191-0930 Notes

The authors declare no competing financial interest.

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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. 192017), 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).

(5)

where I is the discharge current, Δt is the discharge time, m is the mass of the active materials, and ΔV is the voltage window.

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CONCLUSIONS In summary, we have successfully prepared bimetallic Mn−Co oxide nanoparticles anchored on carbon nanofibers and wrapped in a 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 an about 3.8 nm-thick nitrogen-doped carbon shell was 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 batteries. 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 a supercapacitive nature with a good discharge capacity of 478 F g−1 at discharge current density of 1 A g−1 and with capacitance retention of as high as 87.5% at a higher current density of 5 A g−1 after 1500 cycles.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b00067. Additional TEM, EDX, XPS, LSV and the corresponding K−L plot, EIS, and tables (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. L


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Bimetallic MnâCo Oxide Nanoparticles Anchored on Carbon ...

Bimetallic MnâCo Oxide Nanoparticles Anchored on Carbon ...

Bimetallic MnâCo Oxide Nanoparticles Anchored on Carbon ...

Bimetallic MnâCo Oxide Nanoparticles Anchored on Carbon ...