Efficient and Durable Bifunctional Oxygen Catalysts Based on NiFeO

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Efficient and Durable Bifunctional Oxygen Catalysts Based on NiFeO@MnOx Core-Shell Structures for Rechargeable Zn-air Batteries Yi Cheng, Shuo Dou, Jean-Pierre Veder, Shuangyin Wang, Martin Saunders, and San Ping Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16180 • Publication Date (Web): 16 Feb 2017 Downloaded from http://pubs.acs.org on February 17, 2017

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ACS Applied Materials & Interfaces

Efficient and Durable Bifunctional Oxygen Catalysts Based on NiFeO@MnOx Core-Shell Structures for Rechargeable Zn-air Batteries Yi Chenga, Shuo Doub, Jean-Pierre Vederc, Shuangyin Wangb, Martin Saundersd, San Ping Jianga* a

Fuels and Energy Technology Institute & Department of Chemical Engineering, Curtin University, Perth, WA 6102, Australia

b

State Key Laboratory of Chem/Bio-Sensing and Chemometrics, College of

Chemistry and Chemical Engineering, Hunan University, Changsha 410082, Hunan, China c

John de Laeter Centre, Curtin University, Perth, WA 6102, Australia

d

Centre for Microscopy, Characterization and Analysis (CMCA), The University of Western Australia, Clawley, WA 6009, Australia

*

Corresponding author: [email protected] (SP Jiang)

Abstract:

Rechargeable Zn-air battery is limited by the sluggish kinetics and poor durability of the oxygen catalysts. In this paper, a new bifunctional oxygen catalyst has been developed through embedding the ultrafine NiFeO nanoparticles (NPs) in a porous amorphous MnOx layer, in which the NiFeO-core contributes to the high activity for the oxygen evolution reaction (OER) and the amorphous MnOx-shell functions as active phase for the oxygen reduction reaction (ORR), promoted by the

1

ACS Paragon Plus Environment


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synergistic effect between the NiFeO core and MnOx shell. The electrocatalytic activity and durability of NiFeO@MnOx depends strongly on the NiFeO:MnOx ratio. NiFeO@MnOx with NiFeO:MnOx weight ratio of 1:0.8 shows the best performance for reversible ORR and OER, with a potential gap (∆E) of 0.792 V to achieve a current density of 3 mA cm-2 for ORR (EORR=3) and 5 mA cm-2 for OER (EOER=5) in 0.1 M KOH solution. The high activity of the NiFeO@MnOx(1:0.8) has been demonstrated

in

a

Zn-air

battery.

Zn-air

battery

fabricated

using

the

NiFeO@MnOx(1:0.8) oxygen electrode shows similar initial performance with that of Pt-Ir/C oxygen electrode but a much better durability under charge and discharge cycles as the result of the structure confinement effect of amorphous MnOx. The results demonstrate NiFeO@MnOx as an effective bifunctional oxygen catalysts for rechargeable metal-air batteries.

Keywords: NiFeO@MnOx core-shell catalysts; Bifunctional oxygen catalysts; Synergistic effect; Structure confinement; Rechargeable Zn-air batteries.

1 Introduction

The urgent need to develop devices that can store more energy to power the new generation portable devices and electric vehicles has stimulated wide interest to develop energy storage and conversion devices with high energy density and good stability. Current prevailing batteries such as rechargeable Li batteries are still insufficient for electric vehicles and new generation of portable devices.1 Among various energy storage and conversion devices, Zn-air batteries have been proved of great potential due to their 2


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low-cost, high safety and reasonable high energy density.1-6 However, the development of rechargeable Zn-air batteries depends strongly on the bifunctional oxygen electrode, which should be not only efficient for both OER and ORR, but also chemically and electrochemically stable over a wide range of potentials experienced during charge and discharge.2-5, 7 Presently, reversible bifunctional oxygen catalysts for ORR and OER rely on precious metals-based catalysts, such as RuO2/IrO2 and Pt.8 However, their high-cost and poor stability under reversible OER and ORR limits their full-scale applications.9-11 Transition-metal-oxides are gaining increasing attention in the development of efficient oxygen catalysts due to their high activity, stability and low-cost in alkaline solution,2 and have been recognized as a class of efficient bifunctional oxygen catalysts for Zn-air batteries.2, 4-5, 12-14 Among the transition-metal-oxides, NiFe-based oxides have been known as the most efficient catalysts for OER with activity comparable to Ir-based materials in alkaline conditions.15-18 Louie and Bell conducted a detailed investigation on the electrochemical activity of electrodeposited Ni-Fe film for OER in alkaline solution, and Ni-Fe film with Fe in the range of 20-50% exhibited OER activity that is two orders of magnitude higher than that of a freshly deposited Ni film and three orders of magnitude higher than that of a Fe film.16 However, NiFe-based oxides are inefficient for ORR. Manganese oxide, on the other hand, is known as one of the best non-noble metal catalysts for ORR19-23 and has been applied as oxygen electrode in metal-air batteries because it is abundant, environmentally benign and has diversified structures.24-25 In order to improve the performance for

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reversible OER and ORR, manganese based oxides such as CaMn4Ox25 and cobalt-manganese-based spinels26-28 has been developed. However, their activity still cannot meet the requirement for the efficient rechargeable Zn-air batteries. Here, a novel NiFeO@MnOx core-shell structure supported on graphene via embedding ultrafine NiFeO in a continuous porous amorphous MnOx film has been developed as bifunctional oxygen electrocatalysts for reversible OER and ORR for rechargeable Zn-air batteries. This novel core-shell structure shows outstanding activity and stability for both ORR and OER due to the fact that the porous amorphous MnOx shell not only exhibits high ORR activity, but also shows an excellent structural confinement effect on the NiFeO nanoparticle core. The best results were obtained on NiFeO@MnOx with a weight ratio of 1:0.8. 2 Experimental 2.1 Materials Graphene used in this study was obtained from the Graphene Supermarket (1.6 nm Flakes, USA) and purified following the established procedure with HCl acid (30 wt %).29 Nitric acid (65%, Fluka), potassium hydroxide (Sigma-Aldrich), ethanol (Sigma-Aldrich), nickel(II) acetylacetonate (Ni(acac)2, Sigma-Aldrich), Iron(III) acetylacetonate (Fe(acac)3, Sigma-Aldrich), potassium permanganate (KMnO4, Sigma-Aldrich), ethylene glycol (Sigma-Aldrich), ethanol (Sigma-Aldrich), nafion solution (5% in isoproponal and water), poly(ethyleneimine) (PEI, molecular weight ~1300, Sigma-Aldrich) were used without further purification. 2.2 Preparation of NiFeO@MnOx catalysts supported on graphene

4


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The purified graphene was functionalized with PEI as follows:30 50 mg graphene was sonicated in 400 mL Milli-Q water in the presence of 0.5 wt% PEI for 2 h, then the dispersion was stirred overnight before it was filtrated using a nylon membrane (0.2 µm) and washed for several times to remove the excess PEI. The as-prepared solid was dried in vacuum oven for 24 h at 71 oC. PEI functionalized graphene (20 mg) was then ultrasonicated in 100 mL EG solution for 1 h, followed by the addition of Ni(acac)2 and Fe(acac)3. The positively charged PEI functionalized graphene allows the self-assembly of the negatively charge acetylacetonate groups and the positively charge metal ions through the electrochemical force.31 The mixture was ultrasonicated for 15 min, stirred for 1 h before being heated in a microwave oven (1000 W) for 4 min. The dispersion was then filtered using a nylon filter membrane and washed for several times. The loading of NiFeO on graphene was 25 wt%, and the as-prepared catalysts were denoted as NiFeO. NiFeO (15 mg) was dispersed in 40 mL Milli-Q water under ultrasonic for 30 min and certain amount of KMnO4 solution (1 mg mL-1) was added. The dispersion was stirred at room temperature for 30 min followed by the addition of 40 mL ethanol. The mixture was then refluxed at 80 oC in an oil bath for 2 h and filtered. The solids obtained were washed for several times with ethanol. The as-prepared materials were denoted as NiFeO@MnOx. NiFeO@MnOx with different NiFeO:MnOx ratio was synthesized with the MnOx loading of 10, 15, 20, 25 and 30 wt% by controlling the volume of KMnO4 solution, which resulted in a NiFeO to MnOx weight ratios of 1:0.4,

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1:0.6, 1:0.8, 1:1 and 1:1.2, and were denoted as NiFeO@MnOx(1:0.4), NiFeO@MnOx(1:0.6),

NiFeO@MnOx(1:0.8),

NiFeO@MnOx(1:1)

and

NiFeO@MnOx(1:1.2), respectively. Figure 1 shows the procedures of synthesis of NiFeO@MnOx core-shell structured catalysts. For comparison, 25wt% MnOx on PEI-functionalized graphene was prepared in the same way and the product was denoted as MnOx. NiFeO and MnOx was also mixed with a weight ratio of 1:0.8 and was denoted as NiFeO+MnOx(1:0.8). 2.3 Characterization

The catalysts were identified with X-ray diffractometer (XRD, Rigaku D/MAX RINT 2500) operated at 40 kV and 30 mA with Cu Ka (l = 1.5406 A˚) in the range of 20-90o. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Kratos Axis Ultra DLD spectrometer using a monochromatic AlKα (1486.6 eV) irradiation source operated at 225 W. The electron binding energy scale was calibrated for each sample by setting the main line of the C 1s spectrum to 284.5 eV. XPS spectra were collected with pass energy of 160 eV for the survey spectra and 40 eV for the high-resolution spectra. Each high-resolution spectrum was fitted with a Gaussian-Lorentzian (70%-30%) line shape with the full-width half maximum (FWHM) constrained to values considered reasonable for each element. Fitting of Mn 2p and Ni 2p spectra, which possess extended multiplet structures, was achieved using specified empirical fitting parameters previously derived from standard samples.32-33 For each of the fitted manganese and nickel species, binding energy differences and area ratios were constrained for each species, whilst the absolute 6


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binding energy values were allowed to vary by ±0.3 eV to accommodate any error associated with charge referencing. Distributions and morphology of the materials on graphene was studied by transmission electron microscopy (TEM) and high angle annular dark field STEM (HAADF-STEM) with elemental mapping on Titan G2 60-300 at 80 kV. BET surface area was measured using a Gemini 2360 surface analyzer. The

electrochemical

measurements

were

conducted

with

a

standard

electrochemical cell using a Princeton potentiostat (Versastat3，USA). The catalysts (2 mg) were ultrasonically mixed in 4 mL of ethanol Nafion mixture (with Ethnol: Nafion 9:1) to form a homogeneous ink, followed by pipeting catalyst ink onto the surface of glassy carbon disc electrode with diameter of 5 mm. The catalyst loading was 0.1 mg cm-2. Pt foil (3.0 cm2) and saturated calomel electrode (SCE) with electrolytic bridge were used as the counter and reference electrodes, respectively. The potentials in the present study were given versus RHE reference electrode (E = ESCE + 0.247 + 0.059pH, here 0.247 V is the potential for SCE at 20 oC). Linear scan voltammetrys (LSV) were conducted to study the ORR activity at a scan rate of 10 mV s-1 in O2-saturated 0.1 M KOH solution at 1600 rpm. Rotating ring-disk electrode (RRDE) experiments were conducted in the same three-electrode system by using a 5.6 mm outer diameter glassy carbon disk electrode and a Pt ring (inner diameter 6.5 mm, and outer diameter 7.5 mm) with collection efficiency of 26 % (Standard E5 Series Tip, Pine Instrument Company) at a scan rate of 10 mV s-1 in O2-saturated 0.1 M KOH solution with rotating rate of 1600 rpm. The

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electrochemical data were collected with a multichannel potentiostat (Gamry, Interface1000). The OER activities were also characterized by LSV at scan rate of 10 mV s-1 in 0.1 M KOH solution with rotating rate of 1600 rpm. The IR-corrected Tafel plots were recorded at a scan rate of 1 mV s-1 with the electrode initially conditioned at current density of 0.5 mA cm-2 for 5 min.34 The stability of the oxygen catalysts for reversible OER and ORR was tested using chronoamperometry at current density of 3 mA cm-2 for ORR and 5 mA cm-2 for OER in O2-saturated 0.1 M KOH solution with catalyst loading of 0.2 mg cm-2. For comparison, a Ir/C were prepared by mixing Ir black (Hesen) with XC-72 with weight ratio of 1:1 to form a 50% Ir/C. Pt/C (20%, Johnson Matthey) and Ir black composite with weight ratio of Pt:Ir black = 1:1 (Pt-Ir/C) was also prepared and investigated for both ORR and OER under identical conditions. Zn-air batteries were assembled on a two-electrode system by using Zn plate as anode, oxygen catalysts supported on carbon paper as air electrode, glass fiber filter (Whatman, GF/D) as the separator, and 6 M KOH as the electrolyte. To prepare carbon paper supported electrode, 70% electrocatalyst, 20% Super P, and 10% pol(vinylidene fluride) (PVDF) were mixed in N-Methyl-2-pyrrolidone (NMP) under stirring to form a slurry, coated on a hydrophobic carbon paper (Toray, TGP-H-060) with a catalyst loading ~0.25 mg cm-2 based on active material and dried at 50 °C for 12 h. A reference Zn-air battery by using Pt-Ir/C composite catalyst with an active material loading of 0.25 mg cm-2 as electrocatalyst for ORR and OER was also fabricated in the same way. The Zn-air battery performance was evaluated through

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galvanostatic pulse cycling method at 2 mA cm-2 for 100 cycles on a CT2001A battery work station. 3 Results and discussions 3.1 NiFeO@MnOx structure Figure

2

shows

the

typical

STEM

and

TEM

images

of

NiFeO,

NiFeO@MnOx(1:0.4), NiFeO@MnOx(1:0.8) and NiFeO@MnOx(1:1.2). The fine NiFeO NPs with the average size of ~2.2 nm were homogenously deposited on graphene sheet with high density (Fig.2A). An increase of size was observed when the ultrafine NiFeO NPs were covered by a layer of manganese oxide (Fig.2B). The increase of the MnOx loading leads to an increase of the thickness of the MnOx layer, forming continuous core-shell structure in the case of NiFeO:MnOx ratios with 1:0.8 and 1:1.2 (Fig.2C and D). The selected-area electron diffraction (SAED) patterns show that NiFeO and NiFeO@MnOx(1:0.8)

exhibits

similar

diffraction

patterns

corresponding

to

graphite(100) (0.21nm) and graphite(110) (0.12nm),35 indicating that the NiFeO and NiFeO@MnOx are probably amorphous. This is also supported by the XRD patterns (Fig. 3A), where no obvious peaks were obtained for NiFeO and MnOx. And the only peak obtained at 26.6o is attributed to the multilayered graphene. The nitrogen sorption isotherm profiles of NiFeO and NiFeO:MnOx(1:1) show a shape similar with type-IV with a hysteresis loop associated with slit-shaped pores (Fig. 3B), revealing the existence of microporous and mesoporous structure in NiFeO@MnOx. The BET surface area is 107.1 m2 g-1 for NiFeO and increases to 332.3 m2 g-1 for

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NiFeO:MnOx(1:1). The increase of the BET surface area is likely resulted from the porous amorphous manganese oxide

36

and the increased size of nanostructure on

graphene after deposition of MnOx may inhibit the stack of graphene sheet. The elemental mapping of the NiFeO and NiFeO@MnOx catalysts with different NiFeO:MnOx ratios was obtained using EDS coupled with STEM (Figure 4). The EDS mapping indicates that most of Ni and Fe are overlapped, indicating the formation of NiFeO oxide with homogenous distribution for both Ni and Fe. This is critical for their activity for OER.15 In the case of NiFeO@MnOx, all three elements, Mn, Ni and Fe, are overlapped, which suggests the coverage of ultrafine NiFeO NPs with a layer of amorphous MnOx and possible formation of core-shell structure, considering the fact that NiFeO@MnOx was prepared through a two-step self-assembly method (Fig.1). In order to confirm the formation of core-shell structure, we synthesized a structure with large sized NiO particles following the same procedure for preparation of NiFeO@MnOx. The STEM-EDS mapping results clearly show a NiO-core MnOx-shell structure (Fig. S1, supporting information). The mapping results of NiFeO@MnOx did not show a typical core-shell structure likely due to the fact that the homogenous distribution of the ultrafine NiFeO NPs makes it hard to differentiate the core and the shell. With the increase of the MnOx loading, the EDS peak intensity significantly increases, which indicates an increase of the thickness of MnOx (Fig. 4E). Figure 5 displays the XPS core-level spectra of Mn 2p, Ni 2p and Fe 2p. Fitting of the Mn 2p spectra was achieved through the use of empirical fitting parameters

10


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derived from standard samples reported in literature.33 These spectra were most adequately fitted with peaks corresponding to Mn2O3 (Fig. 5A). Hence, MnOx exists as Mn2O3 in the NiFeO@MnOx samples. Mn 2p core-level spectra display two characteristic peaks at binding energy (BE) of 642.1 and 653.7 eV, corresponding to Mn 2p3/2 and Mn 2p1/2 spin-orbit peaks for NiFeO@MnOx(1:0.8), 0.4 eV higher than BEs of 641.7 and 653.4 eV for MnOx. And the BE of Mn 2p3/2 and Mn 2p1/2 spin-orbit peaks for NiFeO@MnOx(1:0.8) is 642.0 and 653.6 eV, about 0.3 eV higher than those for MnOx. The BE of Ni 2p3/2 and Ni 2p1/2 is 855.8 eV and 873.7 eV for NiFeO, respectively, 0.2-0.7 eV higher than 855.6 eV and 873.3 eV for NiFeO@MnOx(1:0.4), 855.6 eV and 873.2 eV for NiFeO@MnOx(1:0.8) and 855.5 eV and 873.0 eV for NiFeO@MnOx(1:1.2). Fitting of the Ni 2p spectra according to empirical fitting parameters defined in literature32-33 suggests that the NiFeO material is present as a mixture of Ni(OH)2 and NiFe2O4 in a ratio of 2.3:1 (Fig. 5B). Notably, this ratio is preserved across all NiFeO@MnOx samples, independent of MnOx loading. Similar downshift of BE of Fe 2p in NiFeO@MnOx was also observed (Fig. 5C). However, the Fe 2p spectra have not been deconvoluted due to complications arising from spectral overlaps with Ni LMM Auger peaks which introduces an inherent degree of error in fitting and interpretation. The negative BE shift of Ni 2p and Fe 2p and the positive BE shift of Mn 2p for NiFeO@MnOx core-shell structure clearly indicates the electron donation from MnOx shell to NiFeO NP core. Ni(OH)2 and NiFe2O4 NP cores were covered by a layer of amorphous Mn2O3.

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3.2 ORR activity Figure 6A is the ORR activity of NiFeO, MnOx and NiFeO@MnOx. NiFeO shows a very low activity for ORR with an onset potential of 0.88 V and a half-wave potential of 0.675 V. MnOx shows a better ORR activity with an onset potential of 0.89 V and half-wave potential of 0.74 V. The ORR activity is significantly improved for the NiFeO@MnOx, and shows a volcano-type dependence on the NiFeO:MnOx ratio. NiFeO@MnOx exhibits similar onset potentials at 0.94 V regardless the NiFeO:MnOx ratio, but the half-wave potentials increase from 0.769 V for NiFeO@MnOx(1:0.4) to 0.809 V for NiFeO@MnOx(1:0.8) as the MnOx loading increases. The ORR activity is optimized at NiFeO@MnOx with weight ratios of 1:0.8 to 1:1. With further increase of the ratio to 1:1.2, NiFeO@MnOx(1:1.2), the half-wave potential decreases again to 0.784 V. The good activity of NiFeO@MnOx(1:0.8) is also supported by the high current densities obtained at 0.8 and 0.7 V (Fig. 6B). For example, the current densities of NiFeO@MnOx(1:0.8) are 2.66 and 4.71 mA cm-2 at 0.8 and 0.7 V, respectively, higher than that of 2.02 and 3.64 mA cm-2 for NiFeO@MnOx(1:0.6), and significantly higher than 0.74 and 2.16 mA cm-2 for NiFeO and 1.42 and 2.99 mA cm-2 for MnOx. On the other hand, NiFeO+MnOx(1:0.8) exhibits a half-wave potential of 0.667 V, 139 mV lower than the value of NiFeO@MnOx(1:0.8)

(Fig.

6C).

The

significant

better

ORR

activity

of

NiFeO@MnOx as compared to MnOx, NiFeO and NiFeO+MnOx(1:0.8) mixture reveals the synergistic effect between MnOx shell and NiFeO core, consistent with XPS results. NiFeO@MnOx(1:0.8) also shows a better ORR activity than Ir/C (Fig.

12


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6C). It is notice that the half-wave potential of NiFeO@MnOx(1:0.8) is only 46 mV more negative as compared with Pt/C, indicating the promising potential of NiFeO@MnOx(1:0.8) as non-noble metal ORR catalyst. The high ORR activity for NiFeO@MnOx(1:0.8) is also supported by the relatively lower ring current density (jR) obtained from the RRDE study, indicating the low H2O2 yield during ORR process (Fig. 6D). The current density at the ring electrode for NiFeO increases from 0.060 to 0.465 mA cm-2 as the potential decrease from 0.8 to 0.5 V, then decreases to 0.195 mA cm-2 as the potential reaches 0.2 V. The jR for NiFeO@MnOx(1:0.8) increases from 0.060 to 0.120 mA cm-2 as the potential decreases from 0.8 to 0.2 V, which is significantly lower than that of NiFeO and only slightly higher than 0.025 mA cm-2 for Pt/C and 0.075 mA cm-2 for Ir/C catalysts. The H2O2 yield and the electron transfer number were calculated according to the formulas:37

H 2O2 % =

n=

200

IR N

(1)

I ( R + ID ) N

4I D I N ( R + ID ) N

(2)

where ID is the disk current, IR is the ring current, N is the collection efficiency and n is the electron transfer number. The H2O2 yield for the ORR on NiFeO@MnOx(1:0.8) is less than 10.0% at the potential range of 0.2-0.8 V, significantly lower than 50% for NiFeO at the potential range of 0.5-0.8V, and close to 5.0% for Pt/C and 5.0-8.0% for Ir/C (Fig. 6E). The electron transfer number for NiFeO is ~3 in the potential range of 13



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0.4-0.8 V, revealing that the ORR on NiFeO is a mixed 2- and 4-electron transfer processes. The electron transfer number for NiFeO@MnOx(1:0.8) is 3.8 in the potential range of 0.2-0.8 V, close to 3.9 for the reaction on Pt/C and Ir/C (Fig.6E), indicating that the ORR on NiFeO@MnOx(1:0.8) mainly proceeds through a 4-electron process. 3.3 OER activity Figure 7A is the LSV of NiFeO, MnOx and NiFeO@MnOx for OER in 0.1 M KOH solution. MnOx exhibits very low activity for OER with an onset potential of 1.70 V and a potential of 1.95 to achieve a current density of 5 mA cm-2 (EOER=5 = 1.95 V). On the other hand, NiFeO NPs show very high activity for OER, initiating at 1.485 V and EOER=5=1.580 V. The OER activities of NiFeO@MnOx slightly decrease as the increases of loading of MnOx. For example, NiFeO@MnOx(1:0.4), NiFeO@MnOx(1:0.6), NiFeO@MnOx(1:0.8) and NiFeO@MnOx(1:1) show similar onset potential at 1.54 V and EOER=5=1.60 V, 60 and 20 mV higher than that of NiFeO, respectively. The slightly decrease of OER activity for NiFeO@MnOx is likely due to coverage of active NiFeO surface by less active MnOx. These results indicate that the OER activity depends on the embedded NiFeO. Interestingly, MnOx+NiFeO(1:0.8) mixture shows similar onset potential with that of NiFeO@MnOx(1:0.8), but slightly higher EOER=5 of 1.613 V. Pt/C catalyst shows an EOER=5 of 1.95 V for OER, 0.315 V lower than that of NiFeO@MnOx(1:0.8). The-state-of-the-art OER catalysts, Ir/C exhibits an EOER=5 of 1.581 V (Fig. 7B), slightly better than that of NiFeO@MnOx(1:0.8).

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Figure 7C shows the Tafel slopes for NiFeO@MnOx, NiFeO, MnOx and Ir/C electrodes measured in 0.1 KOH solutions. The Tafel slope for NiFeO is 58 mV/dec and 123 mV/dec for MnOx. The Tafel slope for NiFeO@MnOx is in the range of 37-46 mV/dec, close to that of NiFeO, but significantly lower than that for the reaction on MnOx. The high Tafel slope for the OER on MnOx indicates that the reaction is determined by the first electron discharge upon the absorption of OH-. While for the NiFeO@MnOx core-shell structured catalysts, the OER reaction is determined by the second electron transfer, similar to that on transitional metal oxides.38-39 The overall activity of the oxygen electrocatalysts for reversible ORR and OER were evaluated by the potential gap between EOER=5 and EORR=3 (∆E=EOER=5 -EORR=3), as shown in Figure 7D. EORR=3 is highest at NiFeO:MnOx of 1:0.8 and 1:1, and EOER=5 slightly

decreases

with

the

increase

of

MnOx

loading.

Consequently,

NiFeO@MnOx(1:0.8) and NiFeO@MnOx(1:1) shows the lowest ∆E of 0.789 V and 0.795 V, respectively. The ∆E of NiFeO@MnOx(1:0.8) is significantly lower than 1.1 V of Pt/C, 0.847 V of Ir/C, 0.919 V of NiFeO+MnOx(1:0.8) and 1.15 V of MnOx. The results demonstrate that NiFeO@MnOx is a promising candidate for bifunctional oxygen catalysts. The performance of NiFeO@MnOx electrode shows a much higher activity as compared with other manganese based bifunctional oxygen electrode reported in the literatures (see Table 1).

25, 40-47

For example, MnCo2O4 spinel exhibits a △E of 0.98

V in 0.1 M KOH solution with catalysts loading of 0.5 mg cm-2.41 Du et al. developed

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a Co3O4 NPs functionalized MnO2 nanotubes for bifunctional air catalysts, the hybrid shows a △E of 1.5 V with catalysts loading of 0.1 mg cm-2.48 CoMn2O4 NPs anchored

on nitrogen-doped graphene nanosheets displays a △E of 0.84 V at catalysts loading

of 1.14 mg cm-2.42 Yu et al synthesized MnNiCoO4 NPs on nitrogen-doped multi-wall

carbon nanotubes (N-CNTs) and achieved a low △E of 0.72 V in 0.1 M KOH solution with catalysts loading of 0.26 mg cm-2, due to the synergistic effect of

MnNiCoO4 NPs and N-CNTs.43 This indicates that the performance of the NiFeO@MnOx electrocatalysts could be further improved if N-doped carbon materials were used as supports. 3.4 The stability for reversible OER and ORR The stability of oxygen catalysts under reversible OER and ORR process is of vital importance in the development of efficient bifunctional oxygen catalysts for rechargeable Zn-air Batteries.49 Before the catalysts were applied to rechargeable Zn-air

batteries,

the

stability

of

the

NiFeO@MnOx

was

studied

using

chronoamperometry at current density of 5 mA cm-2 for OER and 3 mA cm-2 for ORR (Fig. 8). The initial △E is 0.852 V, 0.816 V, 0.792 V, 0.798 V and 0.826 V for NiFeO@MnOx with NiFeO:MnOx ratio of 1:0.4, 1:0.6, 1:0.8, 1:1 and 1:1.2, respectively. The best stability was obtained on NiFeO@MnOx(1:0.8) and NiFeO@MnOx(1:1) with final ∆E values of 0.881 V and 0.892 V after 300 cycles which are significantly lower than that observed on MnOx (∆E was 1.152 V at the 1st cycle and increased to 2.21 V after 11 cycles) and on NiFeO+MnOx(1:0.8) (∆E was 0.919 V at the 1st cycle and increased to 1.046 V after 300 cycles).

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Pt/C shows very low cycling durability, with the significant increase of ∆E from 0.94 V at the 1st cycle to 2.5 V at the 8th cycle (Fig. 8), consistent with the reported results.50-51 The poor stability of the Pt/C is likely due to the oxidation of Pt under OER condition52 and the aggregation of Pt NPs. Ir/C catalyst is also not stable, showing an increase of 0.552 V after 300 cycles. NiFeO@MnOx(1:0.8) and NiFeO@MnOx(1:1) show a much better stability as compared to that of the state-of-the-art bifunctional catalysts, Pt-Ir/C, despite its initial high performance. The microstructure of NiFeO and NiFeO@MnOx(1:0.8) after 300 reversible ORR-OER cycles was examined by STEM and the results are shown in Fig. 9. NiFeO NiFeO experiences a significantly agglomeration and its particle size increased from 2.1 nm to 12.3 nm after 300 reversible ORR-OER cycles (Fig.9A). In the case of of NiFeO@MnOx(1:0.8), the average particle size is 3.3 nm after 300 ORR-OER cycles (Fig.9B). This is considered to be very close to the original particle size, considering that the size of NiFeO NP core is ~2.1 nm. EDS mapping shows the uniform distribution of Ni, Fe and Mn (Fig.9C), indicating that ultrafine NiFeO NPs were confined within the amorphous MnOx shell. The result is consistent with that reported for NiO@MnOx core-shell structures for supercapacitors,31 indicating that the porous amorphous MnOx layer plays an important role in enhancement of the stability of embedded NiFeO NPs via the structure confinement effect. 3.5 Zn-air batteries The best performed NiFeO@MnOx(1:0.8) and Pt-Ir/C bifunctional oxygen catalysts were used to fabricate rechargeable Zn-air battery to evaluate the catalysts

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performance under realistic operating conditions (Fig. 10A). Fig. 10B exhibits the discharge voltage at different current densities and the corresponding power density. For the Zn-air battery fabricated using the NiFeO@MnOx(1:0.8) and Pt-Ir/C oxygen catalysts, the discharge voltages decrease from 1.32 V and 1.29 V at current density of 2 mA cm-2 to 0.81 V and 0.79 V at current density of 120 mA cm-2 respectively. The results reveal that the Zn-air battery using NiFeO@MnOx(1:0.8) oxygen electrode shows slightly better performance compared with that using Pt-Ir/C. The power density increase with the increase of current density, and the highest power density was 81 and 79 mW cm-2 obtained at a current density of 100 mA cm-2 for NiFeO@MnOx(1:0.8) and the Pt-Ir/C electrodes, respectively. This is significantly higher than 55 mW cm-2 at 70 mA cm-2 reported by Zhang et al. using the nitrogen and phosphorus co-doped mesoporous carbon foam.53 The charge voltages increase with the increase of current density and are 1.87 V, 1.97 V, 2.00 V and 2.06 V at current density of 2, 10, 20 and 50 mA cm-2, respectively, for the Zn-air battery using NiFeO@MnOx(1:0.8) as the oxygen electrode, which are slightly higher than that with Pt-Ir/C oxygen electrode (Fig.10C). The potential gap for charge and discharge is 0.60, 0.70 and 0.76 V at current density of 2, 5 and 10 mA cm-2 for the Zn-air battery with NiFeO@MnOx(1:0.8) oxygen electrode, similar with 0.61, 0.68 and 0.74 V obtained for the Pt-Ir/C oxygen electrode, and lower than that reported on oxygen electrodes such as Co3O4 nanowire,54 Ag-Cu Alloy,55 N, P-doped carbon foam53 and N-doped three-dimensional graphene nanoribbon networks.56 Table 2 compares the performance of Zn-air battery using NiFeO@MnOx(1:0.8) oxygen electrode with

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typical results reported. 49, 54-55, 57-58 The

cycling

stability

of

the

rechargeable

Zn-air

batteries

using

NiFeO@MnOx(1:0.8) and Pt-Ir/C as its oxygen electrode are also investigated. A significant deteriorating performance was observed for the Zn-air battery using Pt-Ir/C as the oxygen electrode during the cycling test (Fig.10E). The discharge potential decreases from 1.28 V to 0.76 V after 100 cycles, and the △E for the charge and discharge is increased significantly from 0.54 V to 1.09 V mainly due to the loss of catalytic activity for ORR because the oxidation of Pt under the charge conditions and the aggregation of NPs.59 Much better cycling stability was obtained on Zn-air battery using the NiFeO@MnOx(1:0.8) as the oxygen electrode (Fig.10D). The initial discharge and charge potentials are 1.325 V and 1.885 V, respectively, and the △E for charge and discharge is 0.56 V, close to 0.54 for the Zn-air battery with Pt-Ir/C oxygen electrode, but lower than that reported in literature.27,

51, 54-55, 57, 60

The discharge potential

decreased slightly from 1.325 V to 1.20 V and remained stable after ~40 cycles. Thus the △E increased from 0.56 V to 0.69 V, an increase of 0.130 V, much smaller than 0.555 V on the Zn-air battery using the Pt-Ir/C oxygen electrode. This again demonstrates that the NiFeO@MnOx is a promising noble-metal-free oxygen catalyst with high activity and excellent stability for metal-air applications. NiFeO exhibits high activity for OER, but the ORR activity is limited by its poor ability for O-O bond cleavage.61 The ORR activity is significantly improved by the NiFeO@MnOx core-shell structures due to the synergetic effect between the

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amorphous MnOx layer and the embedded NiFeO NPs. As shown by the XPS study, NiFeO draws the electron from MnOx-shell, which causes a decrease of the affinity and adsorption energy of oxygen species on MnOx, promoting the kinetics of the ORR process.62-64 The significantly enhanced ORR activity is also supported by a 4-electron process and significant decrease of Tafel slope for ORR on NiFeO@MnOx. For NiFeO@MnOx core-shell structures, the nitrogen sorption isotherm profiles display the microporous property of the amorphous MnOx (Fig. 3B). These micropores or tunnels are the pathways for fast mass transfer such as OH- or O2.65-66 This allows the NiFeO NPs embedded in the amorphous MnOx layer as primarily active phase for OER activity, and also explains the close OER activity between NiFeO@MnOx and NiFeO. Consequently, the NiFeO@MnOx core-shell structure exhibits efficient bifunctional properties with significantly high activity for both OER and ORR. The excellent durability of NiFeO@MnOx is clearly due to the structure confinement effect of highly porous and amorphous MnOx layer as shown in Fig.11. These results reveal that ultrafine NiFeO NPs embedded in a thin and amorphous MnOx layer are highly efficient and very stable bifunctional oxygen catalysts for rechargeable Zn-air battery. 4 Conclusions In this study, a NiFeO@MnOx core-shell structure supported on graphene sheet was developed through an easy and scalable self-assembly method. This core-shell structured catalyst shows excellent bifunctional activity for OER and ORR because it allows the utilization of the amorphous MnOx shell for ORR, and the embedded

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ultrafine NiFeO for OER. The optimum performance of the NiFeO@MnOx was obtained at NiFeO:MnOx ratio of 1:0.8 and 1:1. NiFeO@MnOx(1:0.8) shows the highest activity and durability for reversible OER and ORR, achieving a △E of 0.792 V at the initial cycle and a ∆E of 0.88 V after 300 ORR-OER cycles, significantly lower than that of Pt/C, Ir/C and Pt-Ir/C catalysts. The high performance of the bifunctional oxygen catalysts was also demonstrated in the rechargeable Zn-air batteries as an air electrode. The highest power density obtained is 81 mW at current density of 100 mA cm-2, close to 79 mW obtained on the rechargeable Zn-air batteries using Pt-Ir/C oxygen electrode. More importantly, the Zn-air batteries using NiFeO@MnOx(1:0.8) oxygen electrode shows a significantly better cycling durability, and the ∆E between charge and discharge only increased by 0.130 V after 100 charge and discharge cycles, significantly lower than that of 0.555 V for the Zn-air battery using Pt-Ir/C oxygen electrode. This study indicates that ultrafine NiFeO NPs embedded in amorphous MnOx layer are a promising bifunctional oxygen catalyst for rechargeable metal-air batteries.

Acknowledgment

This research was supported by the Australian Research Council under Discovery

Project

Funding

Scheme

(project

number:

DP150102044

and

DP150102025). The authors acknowledge the facilities, and the scientific and technical assistance of the National Imaging Facility at the Centre for Microscopy, Characterization & Analysis, the University of Western Australia, a facility funded by

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the University, State and Commonwealth Governments. The authors would also like to acknowledge the technical assistance of Dr Kane O’Donnell with XPS analysis and the WA X-Ray Surface Analysis Facility, funded by an Australian Research Council LIEF grant (LE120100026).

Supporting Information Available: TEM images and STEM-EDS mapping of NiO@MnOx core-shell structured electrocatalysts.

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Phys. Chem. Lett. 2013, 4 (8), 1254-1259. 47. Jung, K. N.; Hwang, S. M.; Park, M. S.; Kim, K. J.; Kim, J. G.; Dou, S. X.; Kim, J. H.; Lee, J. W., One-Dimensional Manganese-Cobalt Oxide Nanofibres as Bi-Functional Cathode Catalysts for Rechargeable Metal-Air Batteries. Sci. Rep. 2015, 5, 7665-7675. 48. Du, G.; Liu, X.; Zong, Y.; Hor, T. S. A.; Yu, A.; Liu, Z., Co3o4 Nanoparticle-Modified Mno2 Nanotube Bifunctional Oxygen Cathode Catalysts for Rechargeable Zinc-Air Batteries. Nanoscale 2013, 5 (11), 4657-4661. 49. Li, Y.; Gong, M.; Liang, Y.; Feng, J.; Kim, J.-E.; Wang, H.; Hong, G.; Zhang, B.; Dai, H., Advanced Zinc-Air Batteries Based on High-Performance Hybrid Electrocatalysts. Nat. Commun. 2013, 4, 1805. 50. Ge, X.; Liu, Y.; Goh, F. W. T.; Hor, T. S. A.; Zong, Y.; Xiao, P.; Zhang, Z.; Lim, S. H.; Li, B.; Wang, X.; Liu, Z., Dual-Phase Spinel Mnco2o4 and Spinel Mnco2o4/Nanocarbon Hybrids for Electrocatalytic Oxygen Reduction and Evolution. ACS Appl. Mater. Interfaces 2014, 6 (15), 12684-12691. 51. Prabu, M.; Ramakrishnan, P.; Ganesan, P.; Manthiram, A.; Shanmugam, S., Lati0.65fe0.35o3−Δ Nanoparticle-Decorated Nitrogen-Doped Carbon Nanorods as an Advanced Hierarchical Air Electrode for Rechargeable Metal-Air Batteries. Nano Energy 2015, 15, 92-103. 52. Gilroy, D.; Conway, B. E., Surface Oxidation and Reduction of Platinum Electrodes: Coverage, Kinetic and Hysteresis Studies. Canadian J. Chem. 1968, 46 (6), 875-890. 53. Zhang, J.; Zhao, Z.; Xia, Z.; Dai, L., A Metal-Free Bifunctional Electrocatalyst for Oxygen Reduction and Oxygen Evolution Reactions. Nat. Nano 2015, 10, 444-452. 54. Lee, D. U.; Choi, J. Y.; Feng, K.; Park, H. W.; Chen, Z. W., Advanced Extremely Durable 3d Bifunctional Air Electrodes for Rechargeable Zinc-Air Batteries. Adv. Energy Mater. 2014, 4 (6). 55. Jin, Y.; Chen, F.; Lei, Y.; Wu, X., A Silver-Copper Alloy as an Oxygen Reduction Electrocatalyst for an Advanced Zinc-Air Battery. ChemCatChem 2015, 7 (15), 2377-2383. 56. Yang, H. B.; Miao, J.; Hung, S.-F.; Chen, J.; Tao, H. B.; Wang, X.; Zhang, L.; Chen, R.; Gao, J.; Chen, H. M.; Dai, L.; Liu, B., Identification of Catalytic Sites for Oxygen Reduction and Oxygen Evolution in N-Doped Graphene Materials: Development of Highly Efficient Metal-Free Bifunctional Electrocatalyst. Sci. Adv. 2016, 2 (4). 57. Liu, X.; Park, M.; Kim, M. G.; Gupta, S.; Wu, G.; Cho, J., Integrating Nico Alloys with Their Oxides as Efficient Bifunctional Cathode Catalysts for Rechargeable Zinc–Air Batteries. Angew. Chem. Intern. Ed. 2015, 54 (33), 9654-9658. 58. Prabu, M.; Ramakrishnan, P.; Nara, H.; Momma, T.; Osaka, T.; Shanmugam, S., Zinc–Air Battery: Understanding the Structure and Morphology Changes of Graphene-Supported Comn2o4 Bifunctional Catalysts under Practical Rechargeable Conditions. ACS Appl. Mater. Interfaces 2014, 6 (19), 16545-16555.

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59. Li, L.; Liu, C.; He, G.; Fan, D.; Manthiram, A., Hierarchical Pore-in-Pore and Wire-in-Wire Catalysts for Rechargeable Zn- and Li-Air Batteries with Ultra-Long Cycle Life and High Cell Efficiency. Energy Environ. Sci. 2015, 8 (11), 3274-3282. 60. Zhan, Y.; Du, G.; Yang, S.; Xu, C.; Lu, M.; Liu, Z.; Lee, J. Y., Development of Cobalt Hydroxide as a Bifunctional Catalyst for Oxygen Electrocatalysis in Alkaline Solution. ACS Appl. Mater. Interfaces 2015, 7 (23), 12930-12936. 61. Mistry, H.; Varela, A. S.; Kühl, S.; Strasser, P.; Cuenya, B. R., Nanostructured Electrocatalysts with Tunable Activity and Selectivity. Nat. Rev. Mater. 2016, 16009. 62. Han, B. H.; Xu, C. X., Nanoporous Pdfe Alloy as Highly Active and Durable Electrocatalyst for Oxygen Reduction Reaction. Int. J. Hydrog. Energy 2014, 39 (32), 18247-18255. 63. Kang, Y. S.; Choi, K. H.; Ahn, D.; Lee, M. J.; Baik, J.; Chung, D. Y.; Kim, M. J.; Lee, S. Y.; Kim, M.; Shin, H.; Lee, K. S.; Sung, Y. E., Effect of Post Heat-Treatment of Composition-Controlled Pdfe Nanoparticles for Oxygen Reduction Reaction. J. Power Sources 2016, 303, 234-242. 64. Wang, D. L.; Lu, S. F.; Kulesza, P. J.; Li, C. M.; De Marco, R.; Jiang, S. P., Enhanced Oxygen Reduction

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Figure 1. Synthesis of NiFeO@MnOx core-shell structured catalysts on PEI-functionalized graphene.

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2. TEM micrographs of A) NiFeO, B) NiFeO@MnOx(1:0.4), C) NiFeO@MnOx(1:0.8) and D) NiFeO@MnOx (1:1.2). Inserted in (A) and (C) are corresponding histogram and diffraction patterns.

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Figure 3. A) XRD patterns for NiFeO, NiFeO@MnOx(1:0.4) and NiFeO@MnOx(1:1.2), and B) nitrogen sorption isotherms of NiFeO and NiFeO@MnOx(1:1).

29



E) Mn

Intensity/ (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Cu

Fe

Ni

Cu

d) c) b) a)

5

6

7

8

Energy/ KeV

9

10

Figure 4. HAADF-STEM and EDS mapping of A) NiFeO, B) NiFeO@MnOx(1:0.4), C) NiFeO@MnOx(1:0.8), D) NiFeO@MnOx(1:1.2), and E) EDS spectra of the mapping area for a) NiFeO, b) NiFeO@MnOx(1:0.4), c) NiFeO@MnOx(1:0.8) and d) NiFeO@MnOx(1:1.2).

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Page 31 of 40

A)

Intensity/ (a.u.)

MnOx

NiFeO@MnOx(1:1.2)

NiFeO@MnOx(1:0.8)

NiFeO@MnOx(1:0.4)

660

B)

655

650

645

Binding Energy/eV

640

Intensity/ (a.u.)

NiFeO@MnOx(1:1.2)

NiFeO@MnOx(1:0.8)

NiFeO@MnOx(1:0.4)

NiFeO

870

865

860

855

Binding Energy/eV

850

C)

Intensity/ (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NiFeO NiFeO@MnOx(1:0.4) NiFeO@MnOx(1:0.8) NiFeO@MnOx(1:1.2)

750

740

730

720

710

Binding Energy/eV

700

Figure 5. Deconvoluted high resolution XPS core-level spectrum of A) Mn 2p, B) Ni 2p and C) Fe 2p for MnOx, NiFeO and NiFeO@MnOx.

31



Electron transfer number/ n

E) 100

4

80

H2O2 yield /%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3

60

2

40

1

20 0 0.2

0.4

0.6

0 0.8 0.2

NiFeO NiFeO@MnOx(1:0.8) 50% Ir/C 20% Pt/C

0.4

Potential/ Vvs RHE

0.6

0.8

Figure 6. A) LSV curves of NiFeO@MnOx, B) plots of half-wave potential and current density at 0.7 and 0.8 V for NiFeO@MnOx as a function of NiFeO:MnOx ratios, C) LSV curves of NiFeO+MnOx(1:0.8), Pt/C, Ir/C and NiFeO@MnOx(1:0.8) electrodes, D) Ring and disk current densities during the ORR process, and E) H2O2 yield and electron transfer number during the ORR process. The tests were carried out in O2-saturated 0.1 M KOH at scan rate of 10 mV s-1 and rotating rate of 1600 rpm. Catalysts loading was 0.1 mg cm-2.

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Figure 7. A) LSV curves for OER on NiFeO@MnOx, B) LSV curves for OER on NiFeO+MnOx(1:0.8), Pt/C, Ir/C and NiFeO@MnOx(1:0.8), C) plots of Tafel curves of the NiFeO@MnOx and Ir/C, and D) plots of EORR=3, EOER=5 and △E of Ir/C, Pt/C and NiFeO@MnOx as a function of NiFeO:MnOx weight ratios. The tests were carried out in 0.1 M KOH solution at scan rate of 10 mV s-1 and rotating speed of 1600 rpm. Catalyst loading was 0.1 mg cm-2.

33



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 8. A) Plots of potentials measured during the reversible OER at 5 mA cm-2 and ORR at 3 mA cm-2 on NiFeO@MnOx, MnOx, NiFeO+MnOx(1:0.8), Pt/C, Ir/C and Pt-Ir/C oxygen catalysts in O2-saturated 0.1 M KOH with catalyst loading of 0.2 mg cm-2, and B) plots of △E of the catalysts at 1st and 300th cyclers.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 9. A) STEM image and histogram of NiFeO, B) STEM image histogram of NiFeO@MnOx(1:0.8) and C) STEM-EDS mapping of NiFeO@MnOx(1:0.8) after 300 reversible ORR-OER cycles.

35



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 10. A) Schematic structure of the Zn-air battery, B) Polarization and power density curves of the Zn-air batteries using Pt-Ir/C and NiFeO@MnOx(1:0.8) as oxygen electrodes, C) Charge and discharge polarization curves, D) Discharge/charge cycling curves of Zn-air batteries at a current density of 2 mA cm-2 using the NiFeO@MnOx(1:0.8) air electrode, E) Discharge/charge cycling curves of Zn-air batteries at a current density of 2 mA cm-2 using the Pt-Ir/C air electrode. The catalysts loading is 0.25 mg cm-2 and the electrolyte is 6 M KOH.

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Figure 11. Scheme showing the bifunctional activity and structural confinement of NiFeO@MnOx core-shell structured catalysts for OER and ORR.

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Page 38 of 40

Table 1. List of potential gap of selected manganese based oxygen electrocatalysts reported in the literature. Test condition

ORR

OER

EjORR=3

EjOER=5

References

△E*

Electrolyte

Loading (mg

(KOH)

cm-2)

1.0

1M

0.5

0.691

1691

44

MnO2/graphene/CNT

1.27

0.1 M

0.068

0.770

2.0

45

MnCo2O4 spinel

0.98

0.1 M

0.40

0.659

1.646

41

Co3O4/MnO2 nanotubes

1.362

0.1 M

0.10

0.520

1.882

43

CoMn2O4/N-G

0.84

0.1 M

1.14

0.792

1.632

42

MnOx thin ﬁlm

0.92

0.760

1.68

20% Pt/C

1.16

1.8

Materials (α-MnO2)2-(LaNiO3)3/CNTs

0.1 M

0.028

0.860

25

20% Ir/C

0.9

0.700

1.6

LaNiO3

0.88

0.1 M

0.05

0.700

1.58

46

MnNiCoO4/N-MWCNT

0.72

1M

0.26

0.880

1.6

43

Mn-Co oxide nanofibres

1.01

0.1 M

0.22

0.700

1.71

47

NiFeO+MnOx

0.946

0.1 M

0.1

0.667

1.613

This work

NiFeO@MnOx(1:0.8)

0.788

0.1 M

0.805

1.593

20% Pt/C

1.1

0.1 M

0.850

1.95

50% Ir/C

0.847

0.1 M

0.738

1.585

0.1

∆E was obtained based on the linear scan voltammetry data.

*

38


This work

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Table 2. The performance of Zn-air battery in this study in comparison with selected oxygen electrodes reported in the literature. Materials

Loading

EDischarge

△E*

N, P-doped carbon foam

0.5

1.1

1.05

53

N-doped 3D graphene nanoribbon

0.5

1.2

0.90

56

CoO/N-CNT

1

1.3

0.70

49

Co3O4 nanowire

1.5

1.05

0.80

54

Ag-Cu Alloy

-

1.18

0.74

55

NiCoOx/N-CNTs

0.53

1.15

0.65

57

CoMn2O4/ N-G

1.18

1.08&

0.70

58

NiFeO@MnOx(1:0.8)

0.25

1.25

0.70

This work

Reference

EDischarge was obtained at discharge current density of 10 mA cm-2 and ∆E* is the potential gap to achieve a charge and discharge current density of 10 mA cm-2 for Zn-air batteries.

39



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154x140mm (150 x 150 DPI)


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Efficient and Durable Bifunctional Oxygen Catalysts Based on NiFeO

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