Efficient and Durable Oxygen Reduction Electrocatalyst Based on

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An Efficient and Durable Oxygen Reduction Electrocatalyst Based on CoMn Alloy Oxide Nanoparticles Supported Over N-doped Porous Graphene Santosh K. Singh, Varchaswal Kashyap, Narugopal Manna, Siddheshwar N. Bhange, Roby Soni, Rabah Boukherroub, Sabine Szunerits, and Sreekumar Kurungot ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b01983 • Publication Date (Web): 23 Aug 2017 Downloaded from http://pubs.acs.org on August 23, 2017

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An Efficient and Durable Oxygen Reduction

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Electrocatalyst Based on CoMn Alloy Oxide

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Nanoparticles Supported Over N-doped Porous

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Graphene

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Santosh K. Singh,†,‡ Varchaswal Kashyap,†,‡ Narugopal Manna,†,‡ Siddheshwar N. Bhange,†,‡

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Roby Soni,†,‡ Rabah Boukherroub, § Sabine Szunerits,§ Sreekumar Kurungot†,‡*

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†Physical and Materials Chemistry Division, National Chemical Laboratory (CSIR), Dr. Homi

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Bhabha Road, Pashan, Pune 411 008, India

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‡

Academy of Scientific and Innovative Research, Anusandhan Bhawan, 2 Rafi Marg, New Delhi

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110 001, India §

Univ. Lille, CNRS, Centrale Lille, ISEN, Univ. Valenciennes, UMR 8520 - IEMN, F-59000

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Lille, France.

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KEYWORDS: Microwave synthesis, oxygen reduction reaction, porous N-doped graphene,

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cooperativity effect, anion exchange membrane fuel cell, Zn–air battery.

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ABSTRACT: Transition metal oxide derived materials are very important for various

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applications, such as electronics, magnetism, catalysis, electrochemical energy conversion, and

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storage. Development of efficient and durable catalysts for the oxygen reduction reaction (ORR),

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an important reaction in fuel cells and metal–air batteries, is highly desirable. Moreover, the

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futuristic catalysts for these applications need to be cost–effective in order to ensure a

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competitive edge for these devices in the energy market. This article describes the synthesis of a

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cost–effective and efficient electrocatalyst for ORR. It is based on supporting CoMn alloy oxide

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nanoparticles on N–doped porous graphene through a simple and scalable microwave irradiation

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method. Microwave irradiation was found to be very crucial for the fast creation of pores in the

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graphene framework with a concomitant formation of the CoMn alloy oxide nanoparticles. A

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series of catalysts have been synthesized by varying the Co:Mn ratio, among which, the one with

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the Co:Mn ratio of 2:1 (designated as CoMn/pNGr(2:1)) displayed remarkably higher ORR

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activity in 0.1 M KOH solution. It showed a ~60 mV potential shift with a low Tafel slope of 74

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mV/decade, which is comparable to that derived from the commercial Pt/C catalyst. This high

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activity of CoMn/pNGr(2:1) has been credited to the co–operative effect arising from the metal

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entities and the defects present in the N-doped porous graphene. Finally, real system–level

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validations of the use of CoMn/pNGr(2:1) as cathode catalyst could be performed by fabricating

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and testing single–cells of an anion exchange membrane fuel cell (AEMFC) and a primary Zn–

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air battery, which successfully demonstrated the efficiency of the catalyst to facilitate ORR in

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real integrated systems of the single–cell assemblies.

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1. INTRODUCTION

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Design and synthesis of advanced electrocatalysts with improved catalytic activity for the

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oxygen reduction reaction (ORR) have become extremely important in achieving improved

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performances of fuel cells and metal–air batteries, some of the next generation energy devices.1-3

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ORR is one of the most studied reactions in electrochemistry. Still, the sluggish ORR kinetics at


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the cathode is considered a major limiting factor for enhancing the energy–conversion efficiency

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in fuel cells.4,5 So far, Pt–based electrode materials are mainly used as the electrocatalysts for

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facilitating ORR in real systems. However, scarcity, high cost and low durability of many of

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these catalysts restrict their wider use for these energy devices.4,6,7 In view of these limitations,

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numerous studies have been dedicated to the advancement of catalysts with ultra–low Pt content

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by modifying the morphological and electronic properties with the aim in enhancing ORR

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activity.8-10 These catalysts still suffer from the Pt scarcity in nature, high cost, low stability, and

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in some cases to electrode poisoning.4,11,12

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To overcome these issues, the development of more efficient and durable ORR catalysts have

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shifted towards the use of non–Pt group metals (non–PGM) or metal–free electrocatalysts as

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alternatives. Non–PGM (Co, Fe, Mn, Mo etc.) based electrocatalysts are extensively studied by

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different research groups.13-18 In many cases, the ORR catalysts synthesized from the non–PGMs

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have generally performed well in the half–cell reaction even though they often suffer from issues

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related to low conductivity and stability under operating conditions.19-22 Supporting non–PGMs

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over the surface of conducting carbon materials with proper anchoring sites has resulted in

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largely improved conductivity.14,17,22 Recently, our group has reported spherical (SP) Co3O4

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nanoparticles supported over N–doped graphene (Co3O4-SP/NGr) catalyst for ORR in alkaline

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medium.23 The importance of N–doping of graphene towards the overall enhancement in the

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activity and stability of the catalyst was pointed out in this study, and the importance of N sites

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for the anchoring of Co3O4 nanoparticle was underlined.23 In addition, hetero–atom doping in the

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graphene framework breaks its electro–neutrality, thereby enhancing the overall catalytic

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activity.23,24 Similarly, the introduction of defects in the graphene framework also changes the

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electronic properties which is reflected in the catalytic performance.25,26 Among the non–PGMs,


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Co3O4 nanoparticles are one of the most promising candidates for ORR in alkaline medium.

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Their mixed valency (Co2+, Co3+) allows faster electron movement during the catalytic reactions

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through the hopping mechanism.27-29 Moreover, precise tuning of the morphological and

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electronic properties can further enhance the electrocatalytic characteristics of the Co3O4

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nanoparticles.30-32 The enhanced activity of the Co3O4 catalyst can be achieved by alloying with

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different transition metals.18,33-36 Typically, alloying of two transition metals induces favorable

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changes in intrinsic properties by altering the density of state (DOS) at the Fermi level of the

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metallic sites and also amends the catalytic Gibbs free energy, which plays an essential role in

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determining their enhanced catalytic activity.37-39

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Accomplishing good metal alloy oxide nanoparticle dispersion over a conducting carbon with

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resulting increased catalytically active sites through an affordable and efficient method is a

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challenging task. Reduced grpahene oxide (rGO) based nansotructures are one of the most

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successful candidates in the battle of catalyst support materials because of their high and tunable

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surface area, mechanical strength and conductivity.40,41 Reduced graphene oxide shows however

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very low intrinsic catalytic activity. By doping with heteroatoms and/or creating defective sites

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in the carbon framework results in enhanced electrocatalytic activity in rGO based materials.42,43

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We report here on a simple and scalable microwave irradiation based route for the synthesis of

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CoMn alloy oxide nanoparticle supported on porous N–doped reduced graphene oxide as a

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potential ORR electrocatalyst. It will be shown that the prepared catalyst outperforms other

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catalysts by taking advantage of N–doping, defective sites in graphene, increased number of

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catalytic sites due to the introduction of pores during the synthesis process, and the higher

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intrinsic activity of the CoMn alloy oxide nanoparticles compared to Co3O4 nanoparticles. The

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direct microwave irradiation method is in addition more advantageous over other techniques44,45


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such as solvothermal, hydrothermal or high–temperature calcination methods which are

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energetically expensive and time–consuming processes.9,13,34,46 In the microwave irradiation

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method, the eddy currents generated over the surface of the adsorbed metal nanoparticles on the

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NGr heats the metal particles in a directed manner up to their melting temperature. This

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subsequently causes fusion of the metal nanoparticles along with the dissolution of the carbon

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support which is oxidized to CO2 when the irradiation is performed in air.45,47 This dissolution

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process of the supported N–doped graphene creates pores in the system which finally turns into

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the N–doped porous graphene (pNGr). The graphite dissolution into the molten metal is already

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reported in the literature.45,47 The highly active catalyst was achieved by using a simple two–step

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synthesis process in which N–doped graphene was prepared by using a high–temperature

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treatment method followed by the microwave irradiation of the metal ions adsorbed with the

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intervention of the functional groups and N active sites. The obtained hybrid catalyst shows more

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positive onset potential and better kinetics for ORR reaction in comparison to the Co3O4 and

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Mn3O4 nanoparticles supported catalysts.

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2. EXPERIMENTAL SECTION

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2.1 Materials: Graphite powder, phosphoric acid (H3PO4), sulfuric acid (H2SO4), potassium

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permanganate (KMnO4), cobalt (II) acetate tetrahydrate (Co(OAc)₂·4 H₂O), manganese (II)

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acetate (Mn(OAc)2), potassium hydroxide (KOH), zinc powder and melamine were procured

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from Sigma–Aldrich. Ethanol (EtOH) and hydrochloric acid (HCl) were procured from Thomas

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Baker. The state-of-the-art Pt/C catalyst was purchased from Alfa Aesar. All the chemicals were

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used as such without any further purification. For the fabrication of AEMFC, the gas diffusion

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layer (carbon paper) was procured from Graftech. The anion exchange membrane was purchased

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from FuMA–Tech GmbH.


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Figure 1. Schematic presentation of the synthesis of the CoMn alloy oxide nanoparticles

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supported on pNGr (CoMn/pNGr) electrocatalyst and stepwise fabrication of the single cell of

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the anion exchange membrane fuel cell by using CoMn/pNGr as the cathode material.

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2.2 Synthesis of Graphene Oxide (GO): For the synthesis of GO, improved Hummer’s

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method was followed.48 Briefly, 3 g of graphite powder was mixed with 18 g of KMnO4 by using

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a mortar and pestle. The mixed powder was added slowly to a mixture of HNO3:H2SO4 (1:9).

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The obtained solution, after the complete addition of the mixture, was kept under stirring at 60

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o

C for 24 h. The obtained reaction mixture was slowly poured into the 3% H2O2 containing ice

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cooled water. This leads to a yellow solution, washed with copious amount of water by

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centrifugation at 10,000 rpm. The obtained solution was further washed with 30% HCl for the


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removal of any unreacted metal impurities. Lastly, a dark chocolate colored viscous solution was

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obtained after several times washing with DI water, which was further washed with ethanol and

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acetone followed by drying at room temperature.

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2.3 Synthesis of N–doped Graphene (NGr): A mixture of GO and melamine powder (1:5)

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was dispersed in DI water by ultrasonication for 10 min. The mixture was subjected to stirring at

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80 oC until a well mixed dried powder was achieved. The obtained GO–melamine powder was

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calcined at 900 oC for 4 h under an argon atmosphere. The powder collected after cooling to

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room temperature is designated as NGr.

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2.4

Synthesis

of

CoMn

Alloy

Oxide

Nanoparticle

Supported

Porous

NGr

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(CoMn/pNGr(2:1)) Catalyst: For the synthesis of the CoMn/pNGr(2:1) catalyst, cobalt (II)

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acetate tetrahydrate and manganese (II) acetate with the molar ratio of 2:1 were dispersed in

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EtOH:H2O (3:2) solution. Then NGr was added by maintaining the metal salt to NGr ratio as

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1:1. The resulting mixture was ultrasonicated for 30 min. by giving a pulse of 30 sec. ON and 20

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sec. OFF. Finally, the obtained well–dispersed mixture of the metal salt and NGr was kept on

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stirring for overnight at room temperature. During the stirring process, the metal ions which are

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positively charged are anchored and nucleated over the functional groups present over the NGr

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by electrostatic interaction. The solution was filtered and washed with EtOH:H2O mixture and

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the filtered material was dried under an IR–lamp. The resulting metal:NGr mixture powder was

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subjected to microwave treatment (700 Watt) for 3 min. with 6–pulses of 30 sec. ON and 2 min.

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OFF. A different texture has been observed for the final obtained catalyst, which was fluffier

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than the parent metal:NGr mixture powder. The obtained catalyst was named as

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CoMn/pNGr(2:1), where the numbers correspond to the Co to Mn molar ratio.


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Similarly, other catalysts were also prepared by changing the molar ratio of Co:Mn by keeping

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the same ratio of the metal salt to NGr. The obtained catalysts with different metal ratios were

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named as Co3O4/pNGr, CoMn/pNGr(1:1), CoMn/pNGr(1:2) and Mn3O4/pNGr.

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Characterization: The high resolution transmission electron microscopy (HRTEM) analysis

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was carried out by dispersing 1 mg of the powdered sample in 5 mL isopropyl alcohol (IPA).

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From this solution, 5 µL was drop coated on a 200 mesh Cu grid. The material coated Cu grid

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was dried under IR lamp and analyzed using an FEI, TECNAI G2 F20 instrument operated at an

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accelerated voltage of 200 kV (Cs = 0.6 mm, resolution 1.7 Å). Atomic force microscopy (AFM)

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analysis of the prepared catalyst was performed by using the Bruker instrument in tapping mode.

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For performing the powder X-ray diffraction (PXRD) analysis, Bruker D8 Advanced X-ray

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diffractometer at room temperature using Cu Kα radiation (λ = 1.5406 Å) was used. The

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scanning of the samples was performed between the 2θ ranges from 10 to 80 with a scan speed

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of 0.5° min−1 and a step size of 0.01° in 2θ. Thermogravimetric analysis (TGA) was carried out

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by using a SDTQ600 TG–DTA analyzer. The temperature ramp for the TGA measurement was

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kept at 10 oC

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analysis was carried out using a VG Microtech Multilab ESCA 3000 spectrometer attached with

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a Mg Kα-X-ray source (hν = 1.2536 keV) instrument. XPS data were analyzed by using the

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CasaXPS software. Raman spectroscopic analysis was performed by using an HR 800 Raman

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spectrometer (Jobin Yvon, Horiba, France) equipped with a 632 nm red laser (NRS 1500 W).

min-1 under oxygen environment. X–ray photoelectron spectroscopy (XPS)

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Electrochemical Characterization: A conventional 3–electrode set–up consisting of a glassy

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carbon electrode (GCE) coated with the active catalyst as a working electrode (WE), a graphite

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rod as a counter electrode (CE) and an Hg/HgO as a reference electrode (RE) was used for

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performing all the electrochemical measurements. All the potential measurements obtained vs.


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Hg/HgO electrode were converted to a reversible hydrogen electrode (RHE) scale by calibration

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(Figure S1, Spporting Information). The instrument used for performing cyclic voltammetry

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(CV), linear sweep voltammetry (LSV), and rotating ring disc electrode (RRDE) was a VMP–3

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model Bio-Logic potentiostat. The obtained current was normalized with the geometrical surface

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area of the working electrode to get the current density. Prior to the preparation of the working

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electrode, the glassy carbon electrode (GCE) was polished with 0.05 and 0.3 µm alumina

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slurries. On the polished GCE surface, 10 µL of the catalyst slurry prepared by mixing 5 mg of

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the active catalyst and 40 µL, 0.05 % Nafion® solution dispersed in 1 mL IPA:H2O (3:2)

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sonicated for an hour was drop–coated. The catalyst coated GCE was dried under air and used as

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the working electrode for all the electrochemical measurements. 0.1 M KOH was used as an

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electrolyte for the electrochemical measurements.

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Membrane Electrode Assembly (MEA) Fabrication and Single Cell Test: The ORR active

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catalyst was tested as a fuel cell cathode and its activity in the real system was evaluated by

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MEA analysis in the anion exchange membrane fuel cell (AEMFC) in a single cell mode. For

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making the cathode, the active catalyst (CoMn/pNGr (2:1) and commercial Fumion® (10 wt. %)

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ionomer were dispersed in IPA through ultrasonication for 20 min. Subsequently, the well–

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dispersed catalyst slurry was brush coated on an active area of 2x2 cm2 of the gas diffusion layer

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(GDL), where the catalyst loading was maintained as 2.0 mg cm-2. The comparison has been

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performed with an MEA derived from Pt/C, where commercial 40 wt % Pt/C (Johnson Matthey)

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was used as the anode as well as the cathode catalysts with a loading of 0.8 mg cm −2. For the

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MEA prepared by using CoMn/pNGr(2:1) catalyst as the cathode, the same anode with the Pt/C

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loading of 0.8 mg cm-2 was used. The Fumion® to catalyst ratio was maintained at 0.4. The MEA

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was prepared by sandwiching the KOH doped commercial Fumapem FAA–3 membrane


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between the anode and cathode by pressing under an applied pressure of 1500 psi for 1 min. at

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room temperature. Finally, the assembled MEA was tested in a single cell fixture, which consists

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of serpentine flow–field channels in the monopolar graphite plates (the fixture was procured

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from Fuel Cell Technologies Inc, USA). The steady–state polarization of the fabricated MEA

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were performed at 50 °C with ambient pressure by maintaining a flow of humidified (100% RH)

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H2 and O2 at the flow rates of 50 sccm and 100 sccm, respectively. A fuel cell test station (Fuel

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Cell Technologies Inc, USA) was used for the measurement.

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Primary Zn–air Battery Testing: The primary Zn–air battery (ZAB) was constructed by

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using a commercial split test cell (EQ-STC-MTI-Korea). The cathode (air electrode) was

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fabricated with a catalyst loading of 1 mg cm-2 over the GDL. A slurry of the active catalyst was

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prepared by dispersing required amount of the catalyst in IPA:H2O (3:2) and sonicating for 1 h.

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After getting a well–dispersed solution, 10 wt. % Fumion® solution was added and the mixture

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was further sonicated for 1 h. Finally, the obtained slurry was brush coated on the GDL and dried

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at 60 °C. Zinc powder was used as the negative electrode material. Finally, the ZAB system was

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fabricated by assembling the Zn powder and air cathode using a Celgard® membrane and 6.0 M

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KOH as a separator and electrolyte, respectively, in the standard split test cell. The assembled

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battery was tested at room temperature by recording steady–state polarization at 5 mV s-1.

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3. RESULTS AND DISCUSSION:

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Figure 1 shows a diagrammatic representation of the synthesis of CoMn/pNGr catalyst which

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involves a two–step process. The first step involves the synthesis of NGr by annealing a 1:5

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powder mixture of GO:melamine at 900 oC under Ar environment followed by anchoring of Co2+


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and Mn2+ ions over the functional groups of NGr. The metal ions anchored over NGr were

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subjected to microwave irradiation to get the final metal alloy oxide nanoparticles supported on

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Figure 2. HRTEM images of pristine NGr and the as–prepared CoMn/pNGr(2:1): (a) the TEM

5

image showing a transparent sheet of pristine NGr, (b) TEM image of CoMn/pNGr(2:1), at low

6

magnification, shows the porous nature of the pNGr sheet, (c and d) the images of

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CoMn/pNGr(2:1) at higher magnification. Dark spots over the surface of pNGr correspond to the

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metal alloy oxide nanoparticles and the marked nanoparticles with circle indicated by red colored


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arrows are the metal alloy oxide nanoparticles in the size range of 2–3 nm. The diffused ring

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pattern of SAED reveals the nano–crystalline nature of the metal alloy oxide nanoparticles and

3

the periphery of the concentric rings corresponds to the different diffraction planes, (e) HAADF–

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STEM image showing uniformly distributed metal alloy nanoparticles, (f–j) EELS images

5

showing the elemental mapping for (f) Carbon, (g) Nitrogen, (h) Oxygen, (i) Cobalt, and (j)

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Manganese.

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pNGr catalyst. The morphology of the supported metal alloy oxide nanoparticles and created

8

pores over the surface of the conducting support material was investigated by using high

9

resolution transmission electron microscopy (HRTEM) analysis. Figure 2a shows a transparent

10

sheet, which corresponds to the pristine NGr. After microwave irradiation of NGr, ion–

11

exchanged N–doped graphene with the metal moieties (Co2+ and Mn2+) displays uniformly

12

distributed pores in the N–doped graphene sheet which can be clearly observed in Figure 2b.

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The mechanism for the generation of heat and the formation of metal nanoparticles has already

14

been discussed by Ananikovin et al.47 In short, during the microwave irradiation, the eddy

15

current is generated over the metal, which causes melting of the metal nanoparticles and leads to

16

the fusion of Co and Mn metals to form the alloy nanoparticles; irradiation in air causes the

17

formation of metal alloy oxide nanoparticles. Moreover, during the melting process, the

18

supporting carbon was quickly oxidized to CO2, thereby, creating pores in NGr. The pores

19

generated in the NGr are about 3–4 nm in diameter (Figure 2). The porous nature of the

20

supported pNGr is further confirmed by the pore size distribution analysis. The CoMn alloy

21

oxide nanoparticles supported over the pNGr surface are in a nano size regime, not visible at low

22

magnification (Figure 2b). Figure 2c and 2d depict the magnified images of the CoMn alloy

23

oxide nanoparticles supported over pNGr. The particle size observed for the CoMn alloy oxide


12

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nanoparticles was found to be in the range of 2–3 nm diameter. The TEM images in Figure 2c

2

and 2d correspond to CoMn/pNGr(2:1); they exhibit a good contrast difference for some of the

3

CoMn alloy oxide nanoparticles with pNGr. However, in some cases, the contrast difference is

4

not very much distinct. One possible reason for the fuzziness in the visibility can be the metal

5

particles lying on the back side of the pNGr sheet.24 The observed particle size from the TEM

6

analysis was found to be in the nano size regime, which is further reflected in the selected area

7

electron diffraction (SAED) pattern shown in the inset of Figure 2d. The SAED pattern of the

8

CoMn alloy oxide nanoparticles exhibits well–defined diffraction rings, suggesting the

9

polycrystallinity of the nanoparticles. The observed rings correspond to the (111), (200), (311),

10

and (400) planes of the CoMn alloy oxide nanoparticles.16

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To check the composition and uniformity in the distribution of the metal alloy nanoparticles,

12

electron energy–loss spectroscopy (EELS) analysis of the CoMn/pNGr(2:1) sample was

13

performed. Figure 2e shows the high–angle annular dark field scanning TEM (HAADF–STEM)

14

image of the CoMn/pNGr(2:1) which reveals a uniform distribution of the alloy oxide

15

nanoparticles over the surface of pNGr. The EELS signal intensity maps in Figure 2f–j exhibit

16

the elemental mapping of C, N, O, Co and Mn, respectively. Figure 2g shows the uniformly

17

distributed N–doping in the porous graphene framework. Similarly, the color contrast in Figure

18

2i and 2j corresponds to the distribution of Co and Mn metals and reveals that the both the

19

metals are lying together. From the color distribution and its intensity, it is clear that the

20

stoichiometric ratio of Co and Mn follows the similar trend as we have taken during the

21

synthesis, which is further reflected in the energy dispersive X–ray spectrometry (EDAX)

22

(shown in Figure S2, Supporting Information) and XPS analysis which is going to be

23

discussed in a later section. EELS analysis was performed for pristine NGr as well to check the


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distribution of doped–N sites in the graphene framework. Figure S3 (Supporting Information)

2

depicts the EELS analysis of porous N–doped graphene where N sites are uniformly distributed.

3

Similarly, elemental mapping of the

4 5

Figure 3. AFM analysis of CoMn/pNGr(2:1): a) topographic image of the sample; b)

6

topographic image of the sample displaying different dark and light spots due to the pores of

7

pNGr and the metal nanoparticles, respectively; c) line scan of the selected part of

8

CoMn/pNGr(2:1) and d) the corresponding height profile diagram.

9

other control samples (Co3O4/pNGr and Mn3O4/pNGr) was performed (Figures S4 and S5,

10

Supporting Information). The color distribution for the Co and Mn follows a similar kind of

11

uniformly distributed pattern as observed in the case of CoMn/pNGr(2:1). The observed pattern

12

of the metal distribution in the controlled samples could be attributed to the adoption of similar

13

reaction conditions. However, the perceived difference in the catalytic activity of the different


14

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1

samples can be ascribed to the altered intrinsic properties due to changes in metal compositions

2

and inter–digitation during the synthesis process.

3

The surface properties of the supported CoMn alloy oxide nanoparticles over pNGr and the

4

porosity of pNGr were further verified by atomic force microscopy (AFM) analysis (Figure 3).

5

A monolayer of the sample was loaded over the completely flat surface of a silica substrate and

6

the analysis was performed in the tapping mode. Figure 3a shows the topographic image of

7

CoMn/pNGr(2:1) loaded over the silica substrate which is displaying the graphene sheet along

8

with the metal alloy oxide nanoparticles. Figure 3b depicts the presence of pores in the

9

supported pNGr substrate and the supported CoMn alloy oxide nanoparticles. The pores created

10

during the microwave treatment are clearly identified as the dark spots in the image, whereas the

11

light spots in the sample resemble to the supported metal nanoparticles. Figure 3c corresponds to

12

the line scan of the selected part of the CoMn/pNGr(2:1) image. The corresponding height

13

profile diagram is presented in Figure 3d, which displays the trough and crests due to the created

14

pores in pNGr and the supported alloy nanoparticles.

15

Furthermore, the loading of the metal alloy oxide nanoparticles was quantified with the help of

16

thermogravimetric (TGA) analysis in an oxygen environment. The TGA profile (Figure S6,

17

Supporting Information) displays an abrupt loss in weight of the sample starting at 500 oC

18

assigned to the burning of carbon in the oxidative environment. The residue obtained after the

19

decomposition of carbon provides an idea about the loading of the metal alloy oxide. The

20

observed loading of the metal alloy oxide is ~18 % in the case of the CoMn/pNGr(2:1) sample.

21

Also, during the analysis, phase change was not observed, inferring to the complete formation of

22

the metal alloy oxide nanoparticles during the synthesis process. The comparative pore size

23

distribution profiles of pristine NGr and CoMn/pNGr(2:1) (Figure 4a) are in good agreement


15

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1

with the TEM result where CoMn/pNGr(2:1) is showing a pore diameter in the range of 3–4 nm.

2

The defective sites and high porosity allow the facile and fast diffusion of the electrolyte ions

3

during catalysis and this advantage is a decisive factor in accomplishing the higher

4

electrocatalytic activity in the present case.

5 6

Figure 4. Physical characterizations of the prepared materials: a) the comparative pore size

7

distribution profiles of NGr and CoMn/pNGr(2:1); b) the comparative XRD spectra of NGr,

8

Co3O4/pNGr, CoMn/pNGr(2:1) and Mn3O4/pNGr (the dotted coloured lines show the shifting of

9

the peaks); c) the comparative Raman spectra of NGr, Co3O4/pNGr, CoMn/pNGr(2:1) and

10

Mn3O4/pNGr; d) the comparative XPS survey scan spectra of Co3O4/pNGr, CoMn/pNGr(2:1)


16

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ACS Catalysis

1

and Mn3O4/pNGr; e) the comparative Co 2p XPS spectra of Co3O4/pNGr and CoMn/pNGr(2:1),

2

f) the comparative Mn 2p XPS spectra of Mn3O4/pNGr and CoMn/pNGr(2:1); g) deconvoluted

3

N 1s spectrum of NGr; h) deconvoluted N 1s spectrum of CoMn/pNGr(2:1).

4

In order to understand the crystal planes and the phases of the prepared NGr and the metal

5

oxide nanoparticles supported over pNGr, powder XRD analysis was performed (Figure 4b).

6

The powder XRD spectra of the prepared catalysts are showing broad reflection peaks which

7

reveal the nanocrystallinity of the supported metal oxide nanoparticles. Moreover, the observed

8

diffraction pattern for CoMn/pNGr(2:1) sample is assigned to the cubic-MnCo2O4 spinel phase.16

9

Figure 4b shows all the major diffraction peaks at the 2 values of 18.55°, 30.54°, 35.99°,

10

43.76°, 57.91 and 63.62° (blue curve), which can be indexed to the diffraction from (111), (220),

11

(311), (400), (511) and (440) planes of face-centered cubic MnCo2O4 (PDF No. 00-023-1237).16

12

The diffraction peaks corresponding to Co3O4 and Mn3O4 are present in the control samples, viz.

13

Co3O4/pNGr and Mn3O4/pNGr. The observed peaks for Co3O4/pNGr correspond to the presence

14

of the face-centered cubic Co3O4 nanoparticles (PDF No. 01-074-1657).49 Simillarly, for the

15

Mn3O4/pNGr, the XRD peaks are resembles to the body-centered tetragonal lattice of the

16

supported Mn3O4 nanoparticles (PDF No. 00-001-1127).50 Moreover, by looking carefully at the

17

XRD spectra, a small shift in the 2θ value in the diffraction pattern of the bimetallic system can

18

be observed, which is again inferring to the formation of the metal alloy oxide phase. The

19

observed shift in the diffraction peaks can be assigned to the change in the fringe width due to

20

the alloy formation. The perceived diffraction peaks for the CoMn/pNGr(2:1) sample are in good

21

agreement with the result obtained in the SAED pattern as already mentioned in a previous

22

section. A broad diffraction peak seen in the XRD profile of the samples at 26o is attributed to


17

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1

the (002) graphitic plane (G) of the supported pNGr. The graphitic peak is present in all the

2

samples.51

3

The introduced defects in the porous N–doped graphene support have been further investigated

4

with the help of Raman analysis. Figure 4c displays the Raman spectra of NGr, Co3O4/pNGr,

5

CoMn/pNGr(2:1) and Mn3O4/pNGr samples. The Raman spectra exhibit the typical defective

6

(D) and graphitic (G) peaks at ~1350 and ~1600 cm-1, respectively. The D peak at 1350 cm-1

7

corresponds to the graphitic lattice vibration mode with the A1g symmetry, whereas, the G peak

8

at 1600 cm-1 is due to the ideal graphitic lattice vibration mode with the E2g symmetry.26,51 The

9

D–band represents the introduced defects in the graphene framework and the G–band describes

10

the orderliness in the graphene.52,53 In order to check the extent of defects, the ID/IG ratio of the

11

samples were calculated; CoMn/pNGr(2:1) was found to possess the highest ID/IG ratio of 1.31.

12

This higher ID/IG ratio stands out as a substantiating evidence on the presence of high density of

13

the defective sites in the system which are responsible for its improved catalytic activity.

14

The systems were analyzed by XPS in order to assess the N–doping in the graphene

15

framework, composition of the alloy oxide nanoparticles, oxidation states of Co and Mn and the

16

interaction of the alloy oxide nanoparticles with pNGr. Figure 4d shows the survey XPS scan

17

spectra of pristine NGr, Co3O4/pNGr, CoMn/pNGr(2:1) and Mn3O4/pNGr. The XPS bands of C

18

1s, N 1s and O 1s can be clearly seen in all the samples. In the case of Co 3O4/pNGr and

19

Mn3O4/pNGr, extra peaks due to Co 2p and Mn 2p are observed, respectively. The XPS

20

spectrum of CoMn/pNGr(2:1) depicts peaks corresponding to both Co 2p and Mn 2p, indicating

21

the presence of both elements in the system. The atomic % ratio of Co:Mn is found to be ~2:1,

22

validating the result obtained from EDAX analysis (Figure S2, Supporting Information). The

23

Co 2p high core spectra of Co3O4/pNGr and CoMn/pNGr(2:1) (Figure 4e) comprise


18

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ACS Catalysis

1

two prominent peaks at 779.7 eV and 796.0 eV attributed to Co 2p3/2 of Co2+ and Co3+ as

2

expected for Co3O4 with mixed valence states. The Mn 2p core level spectra of Mn3O4/pNGr and

3

CoMn/pNGr(2:1) consist of a broad band at 641.4 (Mn 2p3/2) and a smaller one at 652.6 (Mn

4

2p1/2) due to Mn2+ or Mn3+ (Figure 4f). The absence of a satellite peak at 647 eV rules out the

5

presence of Mn3O4. In the case of Co3O4/pNGr and Mn3O4/pNGr samples (Figure 4e and 4f),

6

comparable features are observed. A small shift of the binding energy of Co and Mn by (~0.76

7

eV for Co and ~0.64 eV for Mn) is observed in the bimetallic CoMn/pNGr(2:1) sample in

8

comparison to their counterpart monometallic systems. The observed shift in the metallic peaks

9

can be accounted for the change in the electron density because of the interaction established

10

between the two metallic entities and the functional groups (doped-N and oxygen functional

11

groups) present in the conducting support.23,24,54,55

12

The atomic percentage of nitrogen in the case of pristine NGr is found to be 5.20 %. A very

13

small difference in the nitrogen content is observed between the NGr and CoMn/pNGr(2:1) (4.83

14

%) samples as evidenced by the survey spectra presented in Figure 4d. Similarly, from the XPS

15

survey spectrum of CoMn/pNGr(2:1), the atomic ratio of Co to Mn is determined to be ~2:1. The

16

N 1s high resolution XPS spectra of NGr and CoMn/pNGr(2:1) are seen in Figure 4g and 4h. In

17

the case of NGr, the spectrum can be deconvoluted into bands at 396.97, 398.65, and 400.41 eV

18

corresponding to pyridinic–N (48.9 wt. %), pyrrolic–N (31.2 wt. %), and quaternary–N (19.9 wt.

19

%), respectively.56-58 The N1s spectrum of CoMn/pNGr(2:1) is comparable and can be

20

deconvoluted into bands with binding energies of 397.45, 399.06, and 400.82 eV, respectively,

21

which are assigned to the pyridinic–N (45.8 wt.%), pyrrolic–N (31.8 wt. %), and quaternary–N

22

(22.4 wt. %), respectively. Moreover, a small shift observed in the binding energies of the

23

deconvoluted N1s bands could be credited to the mutual interaction between the doped-N and the


19

ACS Catalysis

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1

supported metal oxide nanoparticles.24,59 In addition, the percentage of pyridinic–N is found to

2

be higher in comparison to pyrrolic–N and graphitic–N. The superior catalytic activity of the

3

pyridinic–N containing carbon materials is well documented in the literature.60,61 The higher

4

percentage of the doped pyridinic–N in the supported catalyst contributes to the higher

5

performance of the catalyst.

6

Electrochemical Activity: The effect of the modulated properties of the catalyst was

7

evaluated by measuring their electrochemical performance. The electrochemical activity was

8

determined by cyclic voltammetry (CV), rotating disc electrode (RDE) and rotating ring disc

9

electrode (RRDE) modes. All the electrochemical analyses were carried out by using a

10

conventional 3–electrode system containing an active material coated glassy carbon electrode

11

(GCE) as the working electrode (WE), graphite rod as a counter electrode (CE) and Hg/HgO as a

12

reference electrode (RE) in 0.1 M KOH solution. The recorded potentials with respect to

13 14

Figure 5. Electrochemical analysis of the samples for ORR activity: a) the comparative LSV

15

profiles of Pt/C, Co3O4/pNGr, CoMn/pNGr(2:1), CoMn/pNGr(1:1), CoMn/pNGr(1:2) and


20

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ACS Catalysis

1

Mn3O4/pNGr recorded at a scan rate of 10 mV s-1 and an electrode rotation rate of 1600 rpm; b)

2

K–L plots generated for the different samples to calculate the electron transfer number during the

3

ORR process; c) the comparative Tafel plots of Pt/C and CoMn/pNGr(2:1); d) the plots showing

4

the percentage of H2O2 monitored at the ring electrode under different applied disk potentials for

5

ORR during the RRDE measurements; e) the number of electrons transferred during the ORR

6

reaction as calculated from the RRDE analysis (Inset: the H2O2 % and the number of electrons

7

calculated at 0.6 V); f) LSV plots recorded for the CoMn/pNGr(2:1) catalysts before and after

8

ADT.

9

Hg/HgO have been calibrated to RHE (experimental details are given in Supporting

10

Information). The electrocatalytic activity of the catalyst was with that of the commercial state-

11

of-the-art (Pt/C) catalyst for a better understanding of its performance.

12

The CV profiles of the prepared catalysts, including Pt/C, have been recorded under N 2

13

saturated and O2 saturated 0.1 M KOH solutions (Figure S9, Supporting Information). The

14

onset potential afforded for ORR by CoMn/pNGr(2:1) (0.94 V vs. RHE) is lower than that of

15

Pt/C (1.0 V), but higher than the other catalysts (0.82, 0.85, 0.91, 0.92, 0.84 and 0.86 V for

16

pNGr, Co3O4/pNGr, CoMn/pNGr(1:1), CoMn/pNGr(1:2), Mn3O4/pNGr, MnCo2O4/NGr

17

respectively). To get a more quantitative and insightful understanding of the intrinsic activity of

18

the prepared catalysts, RDE analysis was performed. Figure 5a and Figure S10, Supporting

19

Information, show the comparative linear sweep voltammograms (LSVs) recorded for the

20

different

21

CoMn/pNGr(1:2), Mn3O4/pNGr) and MnCo2O4/NGr at a scan rate of 10 mV s-1 in oxygen

22

saturated 0.1 M KOH solution with the electrode rotation rate of 1600 rpm. From the LSV plots,

23

it becomes clear that the CoMn/pNGr(2:1) catalyst shows superior activity, considering the onset

samples

(Pt/C,

pNGr,

Co3O4/pNGr,

CoMn/pNGr(2:1),

CoMn/pNGr(1:1),


21

ACS Catalysis

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Page 22 of 36

1

potential, half–wave potential (E1/2) and limiting current density. Furthermore, CoMn/pNGr(2:1)

2

displays a LSV profile which matches well with that of Pt/C catalyst. The registered onset

3

potential for CoMn/pNGr(2:1) catalyst is 0.94 V, which is 60 mV high in comparison to that of

4

Pt/C. However, in terms of the half–wave potential (E1/2), there exists a fairly close matching

5

performance between MnCo2O4/pNGr(2:1) and Pt/C. This is a clear indication of the ability of

6

CoMn/pNGr(2:1) to display better activity under higher current demand conditions. The E 1/2

7

values calculated for the different catalysts are in the order of Pt/C (0.822 V) > CoMn/pNGr(2:1)

8

(0.791 V) > CoMn/pNGr(1:2) (0.734 V) > CoMn/pNGr(1:1) (0.726 V) > MnCo2O4/NGr (0.648)

9

> Co3O4/pNGr (0.656 V) > Mn3O4/pNGr (0.571 V) > pNGr (0.556). The improved activity of

10

the CoMn/pNGr(2:1) catalyst in comparison to the counterpart materials (Co3O4/pNGr and

11

Mn3O4/pNGr) attributes to its modulated intrinsic properties caused by the alloying of the two

12

different metal entities which involves the cooperativity effect along with synergism from the

13

heteroatom doped porous conducting support, pNGr. In addition, the obtained higher activity of

14

the in-situ grown CoMn/pNGr(2:1) catalyst in comparison to the ex-situ supported MnCo2O4

15

nanoparticles over non-oxidized NGr catalyst suggests the importance of strong coupling

16

existing between the CoMn alloy oxide nanoparticles and pNGr. Apart from this, the nanopores

17

present in pNGr might also have served as a deciding factor in establishing seamless reactant

18

distribution at the active sites especially when the current drag increases. In the experiment, it

19

has been found that the material obtained by simple mixing of the metal salt and NGr shows

20

lower activity in comparison to the microwave treated samples (shown in Figure S11,

21

Supporting Information), which reveals the important role of the microwave treatment towards

22

the formation of the alloy oxide nanoparticles and the concomitant creation of porosity in NGr.

23

Furthermore, to check the contribution of the supported metal oxide nanoparticles towards the


22

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ACS Catalysis

1

ORR, mass activity of the prepared catalyst was calculated by normalizing the current at 0.80 V

2

with the loaded weight of the metal oxide active centers (Table S1, Supporting Information).

3

To get a better understanding towards the ORR reaction kinetics, the LSVs of

4

CoMn/pNGr(2:1) catalysts were recorded at different rotation rates (400, 900, 1200, 1600, 2000

5

rpm) of the working electrode (Figure S12, Supporting Information). The obtained LSVs at

6

different rotations have been used for the derivation of Koutecky−Levich (K−L) plots (Figure

7

5b). The limiting current density (j) of the LSVs was found to increase with increasing the

8

rotation rate of the working electrode, which could be assigned to the better mass transfer to the

9

working electrode due to the hydrodynamic effect. Figure 5b shows the K−L plots obtained by

10

plotting the inverse of the square root of the current density (j−1/2) vs. inverse of the square root

11

of the angular rotation rate of the electrode (ω−1/2) at 0.27 V vs. RHE. Using the K–L plots, the

12

number of electron (n) transfer during the ORR process on the catalysts has been calculated from

13

the slope of the respective plots (details of the calculation provided in Supporting

14

Information). Accordingly, CoMn/pNGr(2:1) displays an electron transfer number of 3.98

15

which indicates a 4–electron transfer, much closer to that of Pt/C (4.09) (Figure 5b). The

16

electron transfer number during the ORR process calculated from the K–L plot follows the order:

17

Pt/C (4.09) > CoMn/pNGr(2:1) (3.98) > Co3O4/pNGr (3.70) > CoMn/pNGr(1:1) (3.66) >

18

CoMn/pNGr(1:2) (3.46) > Mn3O4/pNGr (3.08). Further information on the ORR kinetics has

19

been gained from Tafel analysis. Figure 5c depicts comparative Tafel plots for Pt/C and

20

CoMn/pNGr(2:1). CoMn/pNGr(2:1) shows a very low Tafel slope of 74 mV/dec, which

21

indicates faster kinetics involved in the ORR process. For comparison, the Tafel slope was

22

calculated for the standard commercial catalyst (Pt/C); a value of 62 mV/dec was determined,

23

which is very close to that of the prepared CoMn/pNGr(2:1) catalyst. The fairly close matching


23

ACS Catalysis

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Page 24 of 36

1

values of the Tafel slope in both the systems authenticate the efficiency of the active sites present

2

on CoMn/pNGr(2:1) to facilitate ORR with a comparable rate as that can be achieved on Pt/C.

3

This might be affected by the porous nature of pNGr and the highly active catalytic centers

4

generated by CoMn alloy oxides along with the defects in pNGr and N–doping.

5

The parasitic 2–electron transfer ORR process, if it happens, can generate H2O2 as one of the

6

side products along with the H2O generated from the preferred 4–electron reduction process.

7

Hence, in order to measure the extent of the peroxide (HO2¯) formation during ORR, RRDE

8

analysis was performed. The obtained RRDE plots for the prepared catalysts are shown in

9

Figure S13, Supporting information, where the disc current corresponds to the reduction of O2

10

and the corresponding ring current indicates the generation of H2O2 during the ORR process. The

11

percentage of H2O2 generated with respect to the applied reduction potential is shown in Figure

12

5d. The percentage of H2O2 collected over the ring electrode during ORR was calculated for the

13

different catalysts at 0.60 V vs. RHE and it is significantly higher for Mn3O4/pNGr (~40 %)

14

followed by Co3O4/pNGr (16 %) > CoMn/pNGr(1:1) (15 %) > CoMn/pNGr(1:2) (13 %) >

15

CoMn/pNGr(2:1) (8.5 %) > Pt/C (3 %). Thus the lower percentage of H2O2 observed in case of

16

CoMn/pNGr(2:1) further substantiates the dominance of the 4–electron transfer mechanism. The

17

number of electrons transferred during the ORR process is further calculated from RRDE

18

(Figure 5e), which gives fairly good matching with those values calculated previously through

19

RDE analysis.

20

In addition to the electrochemical activity of the catalyst in terms of the low overpotential,

21

faster kinetics and improved E1/2, the electrochemical stability under the operating condition is

22

an important criterion to judge the feasibility of the system for real applications. This has been

23

validated by performing a comparative accelerated durability test (ADT) of both Pt/C and


24

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ACS Catalysis

1

CoMn/pNGr(2:1) by cycling the potential in the range of 1.10 V to 0.70 V at a scan rate of 100

2

mV s-1 in O2 saturated 0.1 M KOH solution. ADT data of CoMn/pNGr(2:1) presented in Figure

3

5f clearly indicate that the catalyst experiences only a negligible change of ~11 mV in the half–

4

wave potential (E1/2) after 5000 continuous cycling. On the other hand, Pt/C has shown ~35 mV

5

change in E1/2 (Figure S14, Supporting Information), which validates the better structural

6

integrity of the developed catalyst upon exposure to rigorous electrochemical conditions.

7

Normally, coalescence, migration and even detachment of Pt nanoparticles from the surface of

8

the carbon substrate during the electrochemical process are cited as the inherent issues associated

9

with the state-of-the-art Pt/C catalyst. The better stability of CoMn/pNGr(2:1) is believed to be

10

due to the effectual interaction between the CoMn–alloy oxide nanoparticles and the pNGr

11

framework. The small–order defects and the nanopores generated during the microwave

12

irradiation process are believed to create effective anchoring sites for the alloy nanoparticles.

13

Catalysts for PEMFC applications need to display good fuel tolerance since the impurities in

14

the fuel or decomposition of the hydrocarbon fuels can poison the active sites present on the

15

cathode catalyst. Hence, methanol tolerance has been investigated for Pt/C and CoMn/pNGr(2:1)

16

in the chronoamperometric mode by using oxygen–saturated 0.1 m KOH solution as the

17

electrolyte and by applying a constant potential at the working electrode of 0.822 V vs. RHE for

18

200 s with the rotation at 1000 rpm (Figure S15, Supporting Information). During the course

19

of the chronoamperometric analysis, 2 mL of 3 M methanol was admitted in the cell. The

20

presence of methanol triggered oxidation on the Pt/C catalyst followed by a drop in the oxygen

21

reduction current. This clearly indicates that the Pt surface is poisoned with the byproduct

22

formed during methanol decomposition. On the other hand, CoMn/pNGr(2:1) is unaffected by


25

ACS Catalysis

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Page 26 of 36

1

methanol addition, indicating that the catalyst is completely resistant towards methanol

2

oxidation.

3 4

Figure 6. The realistic validation of CoMn/pNGr(2:1) as a cathode catalyst in an alkaline

5

exchange membrane fuel cell (AEMFC) system: a) schematic presentation of the AEMFC

6

assembly; b) the comparative I–V polarization plots generated on the single–cells made by using

7

the commercial 40% Pt/C and CoMn/pNGr(2:1) as the cathode catalysts.

8

Finally, a system level demonstration is performed to validate how well the prepared

9

CoMn/pNGr(2:1) electrocatalyst can work as a cathode electrode in an anion exchange

10

membrane fuel cell (AEMFC). The performance has been evaluated in a fabricated single cell by

11

using CoMn/pNGr(2:1) and Pt/C as the cathode and anode catalysts, respectively. The

12

commercially available FuMA Tech–FAA membrane was used as the anion exchange electrolyte

13

membrane. Figure 6b shows the comparative I–V polarization plots of single cells prepared by

14

two 2×2 cm2 membrane electrode assemblies (MEAs) based on Pt/C and CoMn/pNGr(2:1) as the

15

cathode catalysts. The MEA fabricated with the CoMn/pNGr(2:1) catalyst displayed an open

16

circuit voltage (OCV) of 0.92 V with a maximum achieved power density of 35.20 mW cm-2 at

17

67.62 mA cm-2 current density (Figure 6b). However, the MEA fabricated with Pt/C exhibited


26

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an OCV of 0.96 V with a maximum power density of 58.96 mW cm-2 attained at 129.62 mA cm-

2

2

3

CoMn/pNGr(2:1) based system, as an MEA with completely Pt–free cathode, the performance

4

attained on the cathode derived from the developed catalyst is remarkable. Positioning of the

5

performance characteristics of the present system in AEMFC has been made by comparing the

6

literature data available on similar Pt–free systems (Table S2, Supporting Information). For

7

ensuring a better performance of AEMFC, the role of the anion exchange membrane towards the

8

OH¯ conduction is very important. A better membrane along with an optimized protocol for

9

making MEAs from the Pt–free catalysts are expected to further improve the cell performance.

current density (Figure 6b). Even though the Pt/C based single–cell outperformed the

10 11

Figure 7. Demonstration of CoMn/pNGr(2:1) as a cathode catalyst for a Zn–air battery system:

12

a) schematic presentation of the Zn–air battery assembly, and b) the comparative I–V

13

polarization plots for the batteries fabricated by using Pt/C and CoMn/pNGr(2:1) as the air

14

electrode materials.

15

To validate the problem associated with the MEA preparation in the AEMFC where the low

16

OH- conducting membrane is the limiting factor for the high performance, the Zinc-air battery

17

testing is conducted by using the prepared catalyst. Compared to AEMFC, demonstration of

18

CoMn/pNGr(2:1) as a cathode catalyst for a primary Zn–air battery (ZAB) by using KOH as an


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electrolyte has given a close matching performance characteristics with a similar system derived

2

from Pt/C as the cathode catalyst (Figure 7b). For the ZAB testing, a Teflon–coated carbon

3

paper modified with CoMn/pNGr(2:1) was used as the cathode and zinc powder as the anode

4

material. Moreover, for comparison purpose, we fabricated a ZAB unit by employing Pt/C as the

5

cathode material. The observed open–circuit voltages (OCVs) for the systems derived from

6

CoMn/pNGr(2:1) and Pt/C are 1.50 and 1.52 V, respectively (Figure 7b). The observed OCVs

7

are consistent with the values reported in the literature for ZABs. The comparative steady–state

8

cell polarization plots of both the constructed cells exhibit peak power densities of 210 and 230

9

mW m-2 for the electrodes derived respectively from CoMn/pNGr(2:1) and Pt/C. This close

10

matching performance between the two systems clearly validates the efficiency of the developed

11

catalyst for its realistic application.

12

4. CONCLUSION

13

A highly efficient and easy route for the preparation of a non-precious metal alloy oxide

14

nanoparticles supported on porous N–doped graphene electrocatalyst for the oxygen reduction

15

reaction (ORR) application has been demonstrated. The synthesis of the catalyst is achieved by

16

the scalable microwave irradiation method. Among a series of catalysts prepared, the one with

17

the designation CoMn/pNGr(2:1) displayed a uniform distribution of the CoMn alloy

18

nanoparticles over pNGr as confirmed by TEM and elemental mapping analyses.

19

CoMn/pNGr(2:1) clearly outperformed its counterpart catalysts such as pNGr, Co3O4/pNGr,

20

CoMn/pNGr(1:1), CoMn/pNGr(1:2), Mn3O4/pNGr, and MnCo2O4/NGr for electrochemical

21

ORR. This has been credited to a set of interrelating factors in the system such as the effective

22

formation of nano sized bimetallic alloy nanoparticles, their strong interaction with the substrate

23

through the doped nitrogen, oxide functional groups and unsaturated carbon centers along the


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nanopores, better reactant distribution affected by the nanoporous nature of the support etc.

2

CoMn/pNGr(2:1) also displayed notable structural integrity in comparison to the state-of-the-art

3

Pt/C catalyst as evidenced from its survivability during potential cycling under triggered

4

conditions. The present study also demonstrates the interest of CoMn/pNGr(2:1) for real

5

applications by employing it as the cathode electrode material for anion exchange membrane fuel

6

cell (AEMFC) and primary zinc air battery (ZAB). In the case of AEMFC, the performance of

7

the CoMn/pNGr(2:1) system is lower than the Pt/C based system. On the other hand, in the case

8

of ZAB, both the CoMn/pNGr(2:1) and Pt/C based systems displayed close matching

9

performance characteristics.

10

ASSOCIATED CONTENT

11

Electronic Supplementary Information (ESI) available: [experimental details for the

12

electrochemical analysis for RDE, RRDE, TGA, comparative CV of PtC, pNGr, Co3O4/pNGr,

13

CoMn/pNGr(2:1, CoMn/pNGr(1:1), CoMn/pNGr(1:2), Mn3O4/pNGr and MnCo2O4/NGr

14

catalysts, Hg/HgO reference electrode calibration, methanol crossover test, C 1s and O 1s, XPS

15

spectra of NGr, Co3O4/pNG,CoMn/pNGr(2:1) and Mn3O4/pNGr, elemental mapping of the

16

catalysts, EDAX analysis of CoMn/pNGr(2:1). See DOI: 10.1039/c000000x/ “This material is

17

available free of charge via the Internet at http://pubs.acs.org.

18

AUTHOR INFORMATION

19

* Corresponding Author

20

*E-mail: [email protected].

21

Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory, Pune 411008,

22

Maharashtra, India.


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1

Author Contributions

2

The manuscript was written through the contributions of all authors. All authors have given

3

approval to the final version of the manuscript.

4

Funding Sources

5

The present study is financially supported by Council of Scientific and Industrial Research

6

(CSIR), New Delhi, India, by project funding (TLP003526) to SK and research fellowship to

7

SKS.

8

ACKNOWLEDGMENT

9 10

Page 30 of 36

SKS and KS acknowledge CSIR for the research fellowship to SKS and project funding (TLP003526) to KS.

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Table of Content An Efficient and Durable Oxygen Reduction Electrocatalyst Based on CoMn Alloy Oxide Nanoparticles Supported Over N–doped Porous Graphene Santosh K. Singh,†,‡ Varchaswal Kashyap,†,‡ Narugopal Manna,†,‡ Siddheshwar N. Bhange,†,‡ Roby Soni,†,‡ Rabah Boukherroub, § Sabine Szunerits,§ Sreekumar Kurungot†,‡*


Efficient and Durable Oxygen Reduction Electrocatalyst Based on

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