Low Density Three-Dimensional Metal Foams as Significant

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Low Density Three Dimensional Metal Foams as Significant Electrocatalysts toward Methanol Oxidation Reaction Divya Catherin Sesu, Indrajit M. Patil, Moorthi Lokanathan, Haridas Babaso Parse, Phiralang Marbaniang, and Bhalchandra A. Kakade ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03480 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 23, 2017

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ACS Sustainable Chemistry & Engineering

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Low Density Three Dimensional Metal Foams as Significant Electrocatalysts

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toward Methanol Oxidation Reaction

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Divya Catherin Sesua,b, Indrajit Patila,c, Moorthi Lokanathana,b, Haridas Parsea,c , Phiralang

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Marbanianga,c and Bhalchandra Kakade*,a,c a

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b

SRM Research Institute, SRM University, Kattankulathur – 603 203, Chennai (India)

Department of Physics and Nanotechnology, SRM University, Kattankulathur – 603 203, Chennai (India).

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c

Department of Chemistry, SRM University, Kattankulathur – 603 203, Chennai (India) Fax: (+91) 44-2745 6702; Tel: (+91) 44-2741 7920.

9 *

Corresponding author E-mail: [email protected]

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Abstract

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Transition metals have been emerged as highly active catalysts for methanol oxidation reaction.

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The development of low density metallic foams is exceedingly intriguing for various

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applications. Here we report a systematic design of three dimensional (3D) porous

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nanocomposites (foam) of transition metals like Cu and Ni with reduced graphite oxide (Cu-

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Ni@rGO) using simple self-propagation combustion method, where Cu-Ni foam structures are

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wrapped around reduced graphite oxide. The field emission scanning electron microscopy

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(FESEM) and transmission electron microscopy (TEM) show nanoporous structural morphology.

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X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) show presence of different

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oxidation states of metal ions and phase formation respectively. The electrochemical studies of

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Cu-Ni@rGO nanocomposites exhibit interesting methanol electrooxidation properties with

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exciting current density of 280 mA/cm2, which further retain 95% of their activity even after 600

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s. In addition, these, Cu-Ni@rGO structures also reveal negligible poisoning effects during

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methanol electrooxidation. More interestingly, Cu-Ni@rGO nanocomposites show remarkable

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electrochemical CO oxidation to form CO2 and the evidences support the Eley-Rideal

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mechanism of CO oxidation, where presence of oxygen does not affect the oxidation process.

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Key words: Methanol oxidation, Self-propagation, Transition metal, and Eley-Rideal

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mechanism

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Introduction

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Fuel cells, in particular direct methanol fuel cells (DMFC) represent a new and renewable energy

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technology with potential applications in the power demanding areas such as portable electronic

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devices, and distributed stationary power sources.1-4 The methanol oxidation reaction (MOR) is

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the primary component in DMFCs, where the electrochemical conversion of methanol into

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products like CO25 or intermediates like formaldehyde6 and its derivatives provides

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energy.7Besides, the breakthrough of low density metallic foams (a class of materials

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having pores that are connected to each other and form an interconnected network which is

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relatively soft) is highly fascinating for various applications, especially in energy conversion and

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storage applications, thanks to their incredible mechanical, electrical, thermal and chemical

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properties.8 It is commonly known that out of all the pure metals, platinum has the highest

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catalytic activity for the oxidation of methanol in both acidic and alkaline media.9,10 In spite of

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its catalytic activity, the most important restrictive factor for Pt to be used commercially is its

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property to get easily poisoned by adsorbed CO intermediates,11 which are formed during the

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oxidation of methanol, which in turn blocks the Pt active sites. Interestingly, Pt based alloy

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catalysts like RuPt makes an important category of the electrocatalysts for improved

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thermodynamics of MOR on such surfaces.12 Because of the cost and less abundance of Pt, its

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usage is inadequate in practical applications.13 Therefore, transition metals have been focused as

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an alternative metal for maintaining the high catalytic activity along with better resistance to

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poisoning.14 Nickel (Ni) is being one of the non-precious metal catalysts for fuel cells, super

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capacitors and rechargeable batteries due to its interesting redox properties and excellent

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electrochemical performance.15-17 Generally, Ni based electrocatalysts favor oxygen evolution or

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MOR than oxygen reduction reaction because of the distinctive performance in carbon coupling

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reactions.18 Specifically, in a typical MOR in presence of Ni based electrocatalysts, formation of

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NiOOH is the rate determining step.19 Copper (Cu), also being a non-precious metal, is relatively

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low cost with high electrochemical stability and high resistance to poisoning. Usually, Cu

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surface undergoes partial oxidation to form Cu(OH)2 and further to CuO; formation of CuO

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makes the system more suitable for MOR. Thus, the improved catalytic efficiency with lessening 2 ACS Paragon Plus Environment

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of poisoning effect could be achieved after alloying nickel with copper, resulting in a suitable

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electrocatalyst for MOR in alkaline medium.20-22

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In the present study, we report low density three dimensional foam structures of Cu-

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Ni@rGO to enhance the MOR. The presence of Cu with Ni metal plays a vital role in the

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oxidation of methanol, where Cu soothes the surface of the Ni to produce the more porous

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structure.23-25 while wrapping with the graphite oxide sheets, the extent of porosity increases

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further in the Cu-Ni@rGO structures due to the interaction of nitrate precursors with the

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graphene sheets. The role of reduced graphite oxide in the foam is not only to increase the

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activity and the oxidation of methanol but it also sustains the stability, which is revealed by the

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chronoamperometric studies. The poisoning effect has been reduced due to the formation of

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supermethoxy structure, followed by a reaction, not leading to the formation of CO, which is the

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important component for the reduced poisoning effect. More interestingly, Cu-Ni@rGO also

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shows exciting electrochemical CO oxidation to form CO2 and the evidences support the Eley-

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Rideal mechanism of CO oxidation.26

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Experimental section

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Preparation of Foams

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Copper nitrate (Cu(NO3)2.3H2O) was acquired from Hi media and nickel nitrate (Ni

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(NO3)2.6H2O), sulphuric acid (H2SO4), hydrochloric acid (HCl) and potassium permanganate

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(KMnO4) were acquired from Rankem. Graphite powder (synthetic conducting grade, 325

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meshes, 99.99%) was acquired from Alfa-Aesar. Hydrogen peroxide (H2O2, 30% w/v) was

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procured from Fisher scientific and potassium hydroxide (KOH) was purchased from Merk

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Specialties Pvt. Ltd. Methanol was procured from Finar and ethylene glycol was purchased SRL.

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All the chemicals were used without any further purification. Millipore water (18 MΩ) was used

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throughout all the experiments and electrochemical property measurements.

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Preparation of Cu-Ni@rGO nanocomposite

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A simple self-propagated combustion method has been employed for the preparation of 3D low

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density Cu-Ni@rGO nanocomposite foam structures. Upon comparison, the self-propagation

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combustion method is very simple, easy and inexpensive process than that of wet chemical,27 3 ACS Paragon Plus Environment


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electrospinning28 and hydrothermal method29 etc. In a typical self-propagation combustion

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method, 0.72 g of Ni(NO3)2.6H2O, 1.03 g of Cu(NO3)2.3H2O and 20 mg of graphite oxide

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(synthesized using an improved Hummer’s method)30 were mixed in 1.8 mL of ethylene glycol

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and the mixture was sonicated for one hour. Then the solution was poured into petridish, which

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was previously heated to the temperature of 250 ºC. The rigorous combustion reaction occurs

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with the effervescence converting the reactants into the product (composite of Cu, Ni and

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reduced graphite oxide as Cu-Ni@rGO) in the form of foam. The resultant composite of Cu-

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Ni@rGO was collected after one hour after cooling down to room temperature and designated as

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Cu60Ni40@rGO. Similar procedure has been repeated for the preparation of composite without

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rGO, which is designated as Cu60Ni40. In addition, other composites with various compositions

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of Cu and Ni were prepared for the comparison.

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Electrochemical Studies:

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In order to investigate the MOR and carbon monoxide (CO) oxidation reaction, the

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electrochemical studies were carried out using three electrodes system on a Bipotentiostat

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CHI760E (CH Instruments, Inc. USA). The electrode system consists of platinum wire as an

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auxiliary electrode and Ag/AgCl as a reference electrode. A modified glassy carbon (GC)

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electrode was used as a working electrode, which was polished with 0.05 mm alumina slurry.

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The electrode was coated with the ink made up of 5 mg of catalyst and 1 ml mixture of solution

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containing 400 µL of Nafion, 20 ml of iso-propyl alcohol (IPA) and 79.6 ml of deionized water.

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The slurry was sonicated for half an hour in an ultra-sonication bath to make the homogeneous

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dispersion. 4 µL of the dispersed solution was drop casted on the GC electrode surface and the

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electrode was kept for drying for 5 h at room temperature. All of the electrochemical

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measurements were carried out in an aqueous 1 M KOH solution at room temperature. Prior to

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electrochemical measurements, nitrogen (N2) gas was bubbled for at least 30 mins to ensure the

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saturation. This catalyst-loaded working electrode was then electrochemically cleaned by cycling

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between 0.1 V to -0.9 V versus Ag/AgCl in an N2 saturated 1 M KOH solutions. For CO

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oxidation, the CO and O2 gas were bubbled for at least 10 mins to ensure the saturation.

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Characterization:

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The structural analysis of Cu-Ni composites was carried out by XRD with XpertPro

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diffractometer, PANalytical using CuKα line (λ=1.5406 Å, 40 kV, 40 mA) in the 2θ range of 104 ACS Paragon Plus Environment

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90˚ with scan rate of 5˚/min. The morphological studies were carried out on a quantum 200 FEG

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FESEM. The X-ray photoelectron spectroscopic (XPS) analysis was performed using a

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Shimadzu ESCA 3400 instrument. All individual spectra were deconvoluted and fitted using

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Shirley software and standardized by C 1s peak (binding energy = 285 eV). Inductively coupled

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plasma – mass spectrometry (ICP-MS) has been performed using ICP-MS; Perkin Elmer,

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Optima 5300 DV to understand the composition of the Cu60Ni40@rGO nanocomposites. The

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composition of Cu60Ni40@rGO nanocomposite shows the presence of 61.8 wt% of Cu and 38.2

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wt% of Ni and the same data has been listed in Table S2.

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Results and discussion:

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X-ray Diffraction studies:

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Figure 1 shows the comparative XRD patterns of Cu-Ni composites with and without addition of

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rGO i.e. Cu60Ni40@rGO (red curve) and Cu60Ni40 (black curve) recorded in the 2θ range of

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20o – 80o. XRD pattern for Cu60Ni40@rGO depicts the formation of oxides such as CuNiO2

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(JCPDS No: 00-006-0720), CuO (JCPDS No: 00-048-1548), NiO (JCPDS No: 00-004-0835),

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and other oxide impurities to higher extent rather than formation of CuNi (JCPDS No: 00-047-

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1406) alloy. We believe that the formation of oxides is favored over CuNi alloy formation due to

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addition of rGO. Whereas, XRD pattern of Cu60Ni40 shows highly intense (111) and (200)

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peaks of CuNi alloy compared to the oxides.31 We believe that synergistic effect of CuNiO2/rGO

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is the reason for reduction in poisoning effect and thus enhancement in the current density during

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MOR (as discussed later). Though the addition of GO has very apparent effect on the nature of

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the products formed, the presence of rGO has not been observed in the XRD pattern of

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Cu60Ni40@rGO, probably, due to its very low concentration, which is below the detection limit

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of XRD. In addition, XRD data for a series of composite foams prepared with different

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compositions of Cu and Ni along with rGO is given in Figure S1 in ESI.

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Surface Morphology

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Figure 2 (a & b) reveal the low magnification FESEM images of Cu60Ni40 and

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Cu60Ni40@rGO nanocomposites respectively, where porous spongy structures resembling

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honey comb structure is observed.32-33 It is apparent from the Fig. 2(a) that the wrapping of

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graphite oxide sheet into the metal increases the number of pores in the composite and thus in 5 ACS Paragon Plus Environment


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turn increases the surface area of the nanocomposite foam. This is due to the fact that during

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combustion (self-propagation) process the evolution of nitrate gas further increases the porosity

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in the micron-sized graphene sheets. On the other hand, Cu60Ni40 shows less porous structure

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due to absence of rGO, as shown in Figure 2(b). In addition, the pore size in case of

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Cu60Ni40@rGO is lower than that of Cu60Ni40 composite, whereas the number of pores in case

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of former is higher, again thanks to induced porosity developed in presence of rGO (clearly seen

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in Fig. 2(a & c). Also, FESEM images of Cu60Ni40@rGO nanocomposite with different

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magnifications are given in Figure S2 in ESI for further details. FESEM images of series of

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nanocomposites with different compositions of Cu and Ni with rGO are given in Figure S3 in

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ESI.

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Figure 3 a shows TEM image of Cu60Ni40@rGO nanocomposite foam at lower

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magnification, where a nanoporous structure is evident. HRTEM image in Fig. 3b also reveals

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the fractal-like growth of metal or metal oxides on a substrate as a heterogeneous catalytic

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growth of anisotropic nanoparticles. A plenty of nanoneedles ranging from 50 – 70 nm have been

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decorated on the surface. Selected area electron diffraction (SAED) pattern in inset of Fig. 3b

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also suggests a polycrystalline nature of the foam structure including CuO, NiO, and CuNi.

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Further, Fig. 4 shows high angle annular dark field-scanning transmission electron microscopy

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(HAADF-STEM) image of Cu60Ni40@rGO foam structure, where intertwined porous nature of

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the foam has been clearly seen. Different color contrast in the elemental mapping also confirms

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the presence of Cu, Ni, Carbon and oxygen in the foam structure. In addition, in order to

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understand the presence of rGO (in the form of carbon), EDS has been provided, as shown in

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Figure S4 in ESI. EDS analysis also confirms the presence of Cu, Ni, and O in the foam

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structure.

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XPS has been extensively used to study the surface composition, for the presence of

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metals, alloys, oxides and hydroxides. Accordingly, Fig. 5 (a-d) shows deconvoluted spectra of

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core levels of Cu 2p, Ni 2p, C 1s and O 1s respectively. The deconvoluted spectrum of Cu 2p

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(Fig. 5a) shows three major contributions from CuO, Cu(OH)2 and Cu-Ni alloy corresponding to

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the binding energies of 934.8 eV, 933.7 eV and 932.6 eV respectively.34 Ni 2p spectrum (shown

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in Fig. 5b) has been deconvoluted into four different peaks corresponding to species Ni(OH)2,

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NiO, NiOOH and Ni(0) at their binding energies of 856.8 eV, 855.6eV, 854.8 eV and 853.2eV 6 ACS Paragon Plus Environment

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respectively.35 The deconvoluted C1s spectrum (Figure 5c) also corroborate the presence of

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oxidized surface of carbon due to the addition of rGO into the foam structure and closely

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matches with the presence of carbonyl and carboxyl groups at 286.13 eV and 288.92 eV

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respectively.36 Besides, the O 1s spectrum indicates the various contributions from CuO, NiO,

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Cu(OH)2, Ni(OH)2. It is difficult to distinguish between the contribution of O 1s from CuO and

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NiO as well as from Cu (OH)2 and Ni(OH)2 due to their closely matching binding energies.

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Therefore, roughly two distinct contributions corresponding to their oxides (529.6 eV) and

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hydroxides (531.5 eV) have been deconvoluted in O 1s spectrum. Minor contributions of C-OH,

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and –C=O have not been considered to get best possible fitting in the present case.37 Usually, Ni

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and Cu undergo oxidation to form Ni(OH)2 to NiOOH and Cu(OH)2 to CuO which enhance the

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formation of supermethoxy structure. In the present case, formation of Ni(OH)2 and Cu(OH)2, is

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clearly evident form XPS studies. As shown in Fig. S5, the specific surface area of the Cu-

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Ni@rGO

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adsorption/desorption

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Barrett−Joyner−Halenda (BJH) method (as shown in Fig. S5 in ESI). The Cu-Ni@rGO

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composite sample exhibits IV type of adsorption isotherm, due to presence of nanoporosity

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(average pore size ∼ 3 nm) in the sample. In addition, the Cu-Ni@rGO nanofoam shows surface

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area of 6.966 m²/g, which is actually an insignificant value.

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Methanol Oxidation Reaction:

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Figure 6 summarizes the electrocatalytic oxidation of methanol using Cu60Ni40@rGO as well as

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Cu60Ni40 foam composites, which was performed in alkaline solution (1 M KOH) in the

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potential range of 0.8 to 2.0 V versus RHE. Comparative MOR measurements for

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Cu60Ni40@rGO and Cu60Ni40 have been carried out at the scan rate of 100 mV/s and 1600

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rpm in the presence of 1 M methanol (Fig. 6a). Cu60Ni40@rGO shows a better onset potential

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of 1.315 V vs RHE and current density of 280 mA/cm2 at 1.998 V compared to Cu60Ni40.

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Cu60Ni40 shows MOR at an onset potential of 1.415 V with lower current density of 150

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mA/cm2 at 1.998 V. This lower activity of Cu60Ni40 is perhaps due to the less conducting

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network because of absence of rGO in the foam. Moreover, both of the foam composites in

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absence of methanol (red and black curves for Cu60Ni40@rGO and Cu60Ni40 respectively in

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Figure 6a) show an onset potential of 1.698 V vs RHE and a current density of 40 - 45 mA/cm2

was

measured and

using the

pore

Brunauer–Emmett–Teller size

distribution

has

analysis been

by

nitrogen

determined

using



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and this background current could perhaps be due to the oxygen evolution reaction (OER).

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Similar MOR measurements have been performed in static conditions (without rotation of

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working electrode) and the voltammograms have been shown in Figure S4 in ESI. The data

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clearly indicates higher OER activity by the Cu60Ni40@rGO with diffusion limited

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mechanism.38-42 The absence of curve crossing in the cathodic scan also shows a better tolerance

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factor (ratio of anodic peak current to cathodic peak current) in all foam structures. The results of

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present studies are also compared with previous reports and summarized in Table S1 in ESI,

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showing improved performance of Cu60Ni40@rGO nanofoam structures. In addition, a series of

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composites with different compositions of Cu and Ni were employed along with GO to prepare

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various foams and tested for OER in similar way (Fig. S7 in ESI) and it is found that

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Cu60Ni40@rGO foam shows a best MOR activity and onset potential, of all compositions. As

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shown in Fig. 6(b), the volcano type of plot indicates a linear increase in the current density with

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the addition of copper up to 60 wt% followed by a decrease in the current density with further

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addition of Cu.

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Mechanism:

16

() + () → () + ()

(1)

17

+ () → ()

18

2 () → 2() +

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Though there exist various contradictions over the better catalytic sites of Cu surface (whether

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Cu, CuO or Cu2O), our present data shows the presence of CuO in the foam structures is very

21

important for methanol oxidation (inset of Fig. 6a). According to theoretical results, CuO (110)

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makes an important site for the methanol oxidation with negligibly small poisoning effect, thanks

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to the absence of formation of CO intermediate.43 Interestingly, Cu60Ni40@rGO foam structures

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provide a synergistic effect between CuO and NiO structures, thanks to formation of CuNiO2,

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where actually the superior MOR activity of Ni@rGO (rich in NiO) eclipses the activity of

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Cu@rGO (rich in CuO). Thus, presence of CuNiO2 (being an active component) makes the

27

overall system important for better methanol oxidation reaction. A separate comparison of MOR

28

voltammograms for Ni@rGO and Cu@rGO has been shown in the supporting information (Fig.

(2) ( )

(3)


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S8 in ESI), which reveals the triggering of methanol oxidation on CuO surface due to presence

2

of NiO in the Cu60Ni40@rGO foam composite. A partial oxidation of methanol (CH3OH) to

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formaldehyde (H2CO) takes place on CuO (110), without forming CO intermediate.44 In the

4

initial stage, the methanol diffuses to catalytic surface and produces methoxy species,

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specifically by Cu (110) plane. The supermethoxy structure undergoes the sequential steps to

6

produce dioxymethylene (H2COO) molecule, which further forms the formate (HCOO) as a rate

7

limiting process.45 Thus, Cu60Ni40@rGO foam provides a better catalytic surface with zero

8

poisoning effects due to CO free reaction mechanism.

9

Besides,

Fig.

6(c) exhibits

the comparative chronoamperometric curves

for

10

Cu60Ni40@rGO and Cu60Ni40 foams, performed in 1 M KOH in the presence of 1 M methanol

11

at 0.1 V vs Ag/AgCl. The data shows that both samples show initial drop in the current density

12

but reach saturation after 50 s. However, Cu60Ni40@rGO shows retention of almost 95% of its

13

current density even after 600 s, which is much better than Cu60Ni40 (retention of 50% of the

14

current density). This indicates the stability of Cu60Ni40@rGO foam has been improved due to

15

presence of GO in its backbone.46 The results are also compared with that of the commercial

16

Pt/C electrocatalyst (20 wt%; Fuel cells Technology, USA), indicating much better activity,

17

tolerance and stability of Cu60Ni40@rGO nanocomposite foam than Pt/C (Fig. S9 in ESI).

18

CO electro-oxidation

19

Moreover, the oxidation of carbon monoxide (CO) to carbon dioxide (CO2) is one of the

20

most studied reactions in heterogeneous catalysis.47 Accordingly, Fig.7 shows the

21

electrochemical CO → CO2 oxidation curves performed in 0.1 M KOH using Cu60Ni40@rGO

22

foam structure, indicating a potential catalytic efficiency of the foam structure. Surprisingly, the

23

peak at 1.548 V versus RHE (corresponding to β-NiOOH and CuOOH) has become distinct

24

during CO oxidation, revealing a possible interaction between CO molecules and β-NiOOH and

25

CuOOH surface sites. Figure 7 also reveals an onset potential of 1.398 V vs RHE and current

26

density of 28 mA/cm2, which is found to be superior to previous reports.48-49 Basically, the CO

27

oxidation on such surfaces follow Eley-Rideal mechanism in which the CO species (not

28

adsorbed) reacts with previously adsorbed O2 species.50Similar mechanism has been observed in

29

the present case, where the presence of excess O2 molecules does not change the behavior of the



Page 10 of 24

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voltammmogram. Thus, Cu60Ni40@rGO foam makes an important category of noble metal-free

2

electrocatalysts for MOR and CO oxidation; of course further systematic studies might take this

3

technology to the next level.

4 5

Conclusion

6

A novel, simple and cost effective synthesis procedure to design low density three dimensional

7

nanocomposite foam structures for MOR is discussed. Among various compositions,

8

Cu60Ni40@rGO shows excellent MOR activity. It has been showed the electrocatalytic current

9

density of 280 mA/cm2 has been obtained with an onset potential of 1.315 V vs RHE. About

10

95% current retention has been achieved by Cu60Ni40@rGO foam structure, showing its

11

exciting stability towards MOR. The performance has been improved by the addition of graphite

12

oxide and is mainly caused by the intercalation of nitrate precursors to induce more pores even

13

inside the sheets. Further, Cu60Ni40@rGO foam structures show exciting CO → CO2

14

conversion indicating a possible Eley-Rideal mechanism. Owing to its structural and

15

electrochemical properties, this foamy nanostructure also has significant potential as a high

16

performance electrode material for DMFCs.

17 18

Conflicts of interest

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There are no conflicts of interest to declare.

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Acknowledgement

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The authors thank the Department of Science and Technology - Science and Engineering

23

Research Board, India (DST-SERB; No.SB/FT/CS-120/2012) and (EMR/2016/004689) for

24

financial support. Authors also acknowledge SRM University for providing research

25

infrastructure.

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Synopsis:

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The whole script describes about alternative energy for current scenario. So we are uploading

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this manuscript to your journal

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Supporting Information


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1

Figure S1: XRD patterns of nanocomposite foams with different compositions of copper and

2

nickel.

3

Figure S2: FESEM images of Cu60Ni40@rGO with different magnifications (a), (b), (c) and

4

(d).

5

Figure S3: FESEM images of Cu-Ni nanocomposites with different compositions of copper and

6

nickel.

7

Figure S4: EDAX for C, O, Cu and Ni of Cu60Ni40@rGO foam.

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Figure S5: (a) Nitrogen (N2) adsorption-desorption isotherms and (b) corresponding BJH pore

9

size distributions (PSDs) of Cu60Ni40@rGO foam.

10

Figure S6: Cyclic voltammograms (CVs) for Cu60Ni40@rGO in presence (blue curve) and

11

absence (black curve) of methanol as well as for Cu60Ni40, in presence (pink curve) and

12

absence (red curve) of methanol recorded in 1 M KOH at scan rate of 100 mV/s without 1600

13

rpm.

14

Figure S7: The cyclic voltammograms of Cu-Ni nanocomposites with different compositions of

15

copper and nickel in 1 M KOH in the presence of methanol at 1600 rpm.

16

Figure S8: Cyclic voltammograms of Ni, Ni/rGO, Cu and Cu/rGO recorded in 0.1 M KOH in

17

the presence of methanol at 1600 rpm.

18

Figure S9: (a) MOR activity for commercial Pt/C electrocatalyst (20 wt% Pt loading) recorded

19

in 0.1 M KOH at 1600 rpm with a scan rate of 100 mV/s; (b) the chronoamperometric curves for

20

Pt/C in 1 M methanol solution in 0.1M KOH solution at 0.848 V vs RHE.

21 22

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Figure 1

1 2

3 4 5

Figure 1. XRD pattern of nanocomposite of Cu60Ni40 (black curve) and Cu60Ni40@rGO (red

6

curve).

7 8 9 10 11 12 13 17 ACS Paragon Plus Environment


Page 18 of 24

1 2 3 4

Figure 2

5

6 7 8

Figure 2. FESEM images of Cu60Ni40@rGO at lower (a), higher (c) magnification and

9

Cu60Ni40 at lower (b), and higher (d) magnification respectively.


Page 19 of 24 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


1 2 3

Figure 3

4 5

Figure 3. (a, b) TEM images of Cu60Ni40@rGO foam recorded at different magnifications.

6

Inset shows the selected area electron diffraction (SAED) pattern from image 3b.

7 8



Page 20 of 24

Figure 4

1

2 3

Figure 4. HAADF-STEM image and representative EDS elemental maps for C, O, Cu and Ni of

4

Cu60Ni40@rGO foam.

5 6 7 8


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Figure 5

1

2 3

Figure 5. XPS core level spectra of (a) Cu 2p, (b) Ni 2p, (c) C 1s and (d) O 1s recorded from

4

Cu60Ni40@rGO film deposited by drop casting method. The solid lines are non-linear least

5

square fits to the data.

6 7 8 9



1

Page 22 of 24

Figure 6

2

3 4 5 6 7 8 9 10 11 12

Figure 6.(a) Cyclic voltammograms (CVs) for Cu60Ni40@rGO in presence (blue curve) and absence (black curve) of methanol as well as for Cu60Ni40, in presence (pink curve) and absence (red curve) of methanol recorded in 1 M KOH at scan rate of 100 mV/s with 1600 rpm; inset showing a schematic representation of the electrocatalytic conversion of methanol to formic acid by the Cu-Ni@rGO nanocomposite foam (b) the volcano plot assimilated from CVs for different compositions of CuNi@rGO nanofoam in a 1M KOH solution at 100 mV/s and in the presence of 1 M methanol (c) the chronoamperometric curves for Cu60Ni40@rGO nanocomposite foam (black curve) and Cu60Ni40 nanocomposite foam (red curve) in a 1 M KOH solution at 100 mV/s.

13 14 15


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1


Figure 7

2 3

Figure 7. Superimposed CVs for CO oxidation using Cu60Ni40@rGO nanocomposite in 0.1 M

4

KOH solution recorded at scan rate of 100 mV/s with 1600 rpm in the presence (red curve) and

5

absence (black curve); inset shows Eley-Rideal mechanism for CO → CO2 oxidation reaction.

6 7 8 9 10



1

For Table of Contents Use Only

2

Graphical Abstract

Page 24 of 24

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4 5

An efficient and CO tolerant noble metal-free CuNi@rGO nanocomposite foam has been

6

reported

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Low Density Three-Dimensional Metal Foams as Significant

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