Palladium and palladium–copper alloy nano particles as superior catalyst for electrochemical oxidation of methanol for fuel cell applications

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 2

Available online at www.sciencedirect.com

ScienceDirect journal homepage: www.elsevier.com/locate/he

Palladium and palladiumecopper alloy nano particles as superior catalyst for electrochemical oxidation of methanol for fuel cell applications Sreya Roy Chowdhury a, Parthasarathi Mukherjee a, Swapan kumar Bhattachrya b,* a b

Department of Chemistry, Jadavpur University, Kolkata, 700032, India Physical Chemistry Section, Department of Chemistry, Jadavpur University, Kolkata, 700032, India

article info

abstract

Article history:

Monometallic Pd, Cu and bimetallic Pd98Cu2, Pd94Cu6, Pd86Cu14 alloy nano particles with

Received 5 February 2016

small average particle diameter (6e10 nm) are synthesised by facile single pot hydro-

Received in revised form

thermal reduction with hydrazine solution at 75
C in absence of any special capping agent.

27 May 2016

Synthesized nano-materials are characterized by X-ray, electron diffraction, different

Accepted 28 May 2016

microscopic and electro-analytical studies. Graphite supported bimetallic PdxCu100-x alloy

Available online xxx

unlike mixed nano particles show synergistic and enhanced electro catalytic activity towards oxidation of methanol in alkali. The maximum specific peak current of 987 mA mg1

Keywords:

and equilibrium exchange current of 5  106 mA mg1 of Pd obtained for Pd94Cu6 alloy in

Alloy nanoparticle

the present work are much better than the corresponding values reported for PdxCu100-x

Hydrothermal process

composite and alloy materials. Cyclic voltammograms of possible intermediates like

Methanol oxidation

formaldehyde as well as sodium formate and ex-situ FTIR and chromatographic studies of

Fuel cell

reaction products reveal that Cu accelerates formation of formate rather than carbonate

Synergic electro catalytic effect

elucidating the plausible mechanism of the reaction.

Mechanistic pathway

© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Use of methanol as fuel for direct and proton exchange membrane fuel cells (DFC and PEMFC) has attracted enormous attention owing to its relatively high energy density (6.09 KWhKg1) and transport convenience [1e3]. Noble metal nanoparticles are widely used as catalysts in alcohol oxidation and oxygen reduction reactions in fuel cells and other heterogeneous catalytic processes [4,5]. But limited resources of noble metals throw challenges to reduce the dosage of their use to material scientists. Mixing of non-precious metals with

noble metals in composites and specially alloys is found to be an effective method because of their unique structure and compositions, which enhance their catalytic performances and poisoning resistance [6,7]. Regarding choice of noble metal, relatively less expensive Pd has been suggested as a good replacement of Pt because the expense of Pt-based catalysts is a major barrier in the application of fuel cell technology, since the catalyst alone accounts for ca 54% of the total stack cost [8,9]. However Pd has a much lower activity than Pt in certain fuel cells particularly acting in acid media [4]. It stimulates the need for research to design an improved

* Corresponding author. Fax: þ91 3324146584. E-mail addresses: [email protected], [email protected] (S. Bhattachrya). http://dx.doi.org/10.1016/j.ijhydene.2016.05.239 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Chowdhury SR, et al., Palladium and palladiumecopper alloy nano particles as superior catalyst for electrochemical oxidation of methanol for fuel cell applications, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.05.239

2

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 2

Pd based low cost catalyst for commercial implementation of DFC. It has been reported that the electro catalytic activity of Pd can be improved by incorporating an oxophilic metal into the electrode system, which can change the electronic and morphological properties of the binary system [10e13] and also reduce the cost. The activity of multi component catalyst is decided not only by the intrinsic properties of the metal component but also by their synergistic interactions. The changes in electronic properties of the metal-catalyst (like work function, % d-band character etc), molecular environment or ensemble, three dimensional structural characteristics (geometric) effects are responsible for such interactions. Norskov and co-workers have creatively explained the reaction activity by relating the strength of adsorbate bonding at the rate limiting step to the electronic structure of the catalyst [14,15]. In this context, electro-catalysts based on Pd nano particles have received wide spread attention because of their excellent catalytic activity in hydrogenation reaction, CeH bond activation, CeC coupling reaction, and environmental catalytic processes [16e19]. Pd has the ability to reduce proton, store and release hydrogen which can be further utilized to remove adsorbed CO formed during electro oxidation of alcohols [20e22]. In fuel cell application of bimetallic alloys of Pd with early transition metal such as PdeCo [12,23], PdeCu [24,25], PdeNi [26], PdeAg [27] etc have not only exhibited higher catalytic activities compared with monometallic Pd catalyst but also reduced the cost significantly. In our previous study monodispersed bimetallic PdeCu alloy nano particles were synthesized at room temperature to observe the anode catalytic activity in reference to oxidation of methanol [28] and ethanol [29]. In the present study, particles having lesser dimension with controlled size and composition are readily obtained by reduction with the same reducing agent, hydrazine by using slightly high temperature and reducing the number of reagents. The objectives of such study are to synthesize a better PdxCu100-x alloy catalyst of lower diameter and find out a suitable mechanism of electrooxidation of methanol in alkali. In the study, graphite supported bimetallic PdCu catalyst with composition of Pd94Cu6 alloy exhibits the highest activity indicating catalytic synergistic effect for the electro chemical oxidation of methanol. To understand the reason behind catalytic synergism obtained, the study is also extended to the mixtures of Pd and Cu nanoparticles having almost same atomic compositions with respect to that of the alloys. It has been found that Pd94Cu6 nano alloy exhibits greater electro catalytic activity than the nano particles mixtures of Pd and Cu, which gives some light on designing new bimetallic nano catalyst with high activities. In our early studies [30e32], it is observed that the presence of capping agent with the nanocatalyst often reduces the catalytic activity. Greater the concentration of the capping agent and stronger the ligand or capping agent, worse is the capability of a definite amount of the catalyst. On the other hand, nano metal catalysts and particularly nano noble metals can be obtained in absence of capping agent; the required stability comes from the solvent coordination. In such cases synthesized particles are free from unwanted adsorbed molecules and therefore show greater catalytic activity. Based on that view, PdCu alloy nanoparticles were synthesised at room temperature in absence of any capping

agent and catalytic synergism at various metallic composition was obtained.

Experimental Reagents PdCl2 (60 mass % Pd) and Nafion (10 mass %) were purchased from Arora Matthey Ltd and SigmaeAldrich respectively. CuSO4$5H2O, NaOH of analytical reagent grade from Merck, methanol from Merck and deionised water (DW) from millipore were used. Other reagents were commercially available and analytical reagent grade.

Synthesis of bimetallic nanoparticles Appropriate amount of solid PdCl2 and KCl (Merk) are taken in a 100 ml volumetric flask and are mixed (in 1:2 molar ratio) with some amount of millipore water, sonicated for 2 h and kept still for 24 h. A clean brown solution of K2PdCl4 is obtained and then the volume is made up to the mark to prepare 0.25 M K2PdCl4 solution. 5 ml of 0.25 M of K2PdCl4 (0.00125 mol) was taken in a beaker, diluted to 50 mL, heated to 75
C in an oil thermostat and 0.01 mol of hydrazine hydrate (0.486 mL) solution was added to it with constant stirring. A black precipitate was observed which after washing several times with millipore water was taken in a watch glass, then dried in a vacuum oven for 10 min at 100
C and kept in vacuum desiccators. Cu nano particles were formed in the same manner by taking 5 ml of 0.25 M CuSO4 solution. Then two nanoparticles were mixed in the appropriate atomic ratio to get bimetallic nanoparticle mixtures. In case of preparation of nanoparticles of PdCu alloy of a given binary composition, the precursor electrolyte solutions were mixed according to their respective molar ratio and then sonicated for 1 h before co-reduction with hydrazine solution at 75
C. The solution is constantly stirred for 1 h before filtration followed by washing.

Characterization of nanoparticles X-ray powder diffraction (XRD) study was carried out using a (Bruker D8 Advance) diffractometer equipped with a CuKa radiation source (l ¼ 1.5418 Å generated at 40 kV and 40 mA). The shape and size of the nanoparticles were investigated using high resolution transmission electron microscope (HRTEM) (JEOL 2010 and operating at 200 KV). Samples for TEM were prepared by irradiating the dilute solution of palladium and palladiumecopper nano alloy with ultrasonic waves, using a sonicator (D-Compact) for about 45 min. No apparent change of colour and absorbance were observed at the end of sonication. A drop of the solution was cast on a 300 mesh carbon-coated copper grid and the solvent was blotted off with filter paper after 5s of contact, followed by natural evaporation of the solvent at room temperature. The surface morphology and elemental composition were characterised by scanning electron microscopy (FEI company and model no Inspect F-50) equipped with energy dispersive spectrometer.

Please cite this article in press as: Chowdhury SR, et al., Palladium and palladiumecopper alloy nano particles as superior catalyst for electrochemical oxidation of methanol for fuel cell applications, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.05.239

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 2

3

Electrochemical measurements The electro-catalytic responses of palladium nanoparticle and palladiumecopper nanoparticle were verified using cyclic voltammetric (CV) and fixed potential chrono amperometric (CA) studies. For, this graphite carbon rod was used as a support for the electro catalyst. The mid portion of the rod was wrapped with teflon tape keeping both ends bare. Chemical deposition of palladium and palladiumecopper alloy nanoparticles was executed on one end of the rod and other end was kept bare for electrical connection. Electrodes studied, were prepared by “drop and dry” of chemical solution deposition techniques. In all cases the construction of the electrodes, 20 mg of solid sample was taken in a stoppard conical flask and then 10 ml water was added followed by sonication for at least 20 min. After sonication 5 mL of solution was taken and dropped on bare portion of a previously treated and polished disc portion of graphite carbon rod electrode and dried for about 1 h 2 ml of 5(w/v) % nafion was dropped on it covering solid deposit and dried for about 1 h. The nafion prevents the catalyst for detachment without affecting the transport of the reactants and products during the electro oxidation of methanol. Electro-catalytic responses were studied at room temperature, 27
C using an AUTOLAB electrochemical workstation along with a conventional three-electrode system. The reference electrode was Hg/HgO/OH (1 M) (MMO), whose equilibrium electrode potential was ~0.1 V with respect to standard hydrogen electrode (SHE). In each measurement, a large Pt-foil (1 cm  1 cm) was used as a counter electrode and the potential data were recorded against that of MMO.

Studies of the products Following our previous studies [33,34], a current density of 30 mA cm2 is drawn from 0.5 M methanol in 1 M NaOH solution kept in N2 atmosphere for 72 h using constructed electrodes like C/Pd, C/Pd94Cu6 and C/Pd86Cu14 as anodes in separate experiments and a large Pt electrode is used as cathode in each case. A part of the resulting anode solution was used as sample solution for chromatography study. Another part was dried in vacuum and the obtained semi solid product was used in FTIR study. Ex-situ FTIR study was carried out for semi solid products using FTIR spectrophotometer (Perkin Elmer, SN-74514, Spectrum RX1, resolution 4 cm1). Chromatographic determination of formate was executed using a high-performance liquid chromatography (HPLC) (Shimadzu Corporation, Japan). 20 mL of sample solution was injected into a Phenomenex C18 column, provided with dilute sulphuric acid (5 mM) as eluent at 30
C. Formate is detected as one of the separated compounds with a UV-absorbance detector.

Results and discussion Size and morphology of synthesized nanomaterials The crystalline structure of the Pd, Pd98Cu2, Pd94Cu6, Pd86Cu14 and CueCuO composite nanoparticles synthesized under similar experimental condition was established by XRD

Fig. 1 e XRD patterns for the synthesized (a) Pd, (b) Pd98Cu2, (c) Pd94Cu6, (d) Pd86Cu14, (e) Cu nanoparticles. Profile of PdCu mixture is presented in the inset.

studies as depicted in Fig. 1. The diffraction peaks at 40.15
, 46.62
, 68.10
and 82.22
in the profile ‘a’ assign for diffraction from the planes (111), (200), (220) and (311) of a face centered cubic (fcc) phase of pure Pd nanoparticle as reported in JCPDS, (ICDD)2003 file numbered 05-0681. Profile ‘b’ to ‘d’ of Fig. 1 show the XRD patterns of the synthesized Pd98Cu2, Pd94Cu6, Pd86Cu14 alloy nanoparticles respectively. The sharp diffraction peaks are found for all alloys which suggest the high degree of crystalline nature of the synthesized nanoparticles having diameter between 6 and 10 nm as calculated from Debye-Scherrer's equation. The occurrence of the characteristic peaks at the intermediate positions (2q) with respect to those of metallic Pd and Cu nanoparticles indicates that the synthesized PdeCu nanoparticles are composed of a PdeCu bimetallic single phase rather than a mixture of monometallic Pd and Cu nanoparticles. The X-ray diffraction peaks for Pd94Cu6 nanoparticles appear at 40.49
, 46.68
, 68.43
and 82.39
, which clearly show that the diffraction peaks of (111), (200), (220) and (311) planes for the synthesized PdeCu nanoparticles shift towards higher 2q values with increasing atom % of Cu in PdxCu100-x alloy. Similar nature of shift in 2q values for Pdx-Cu100-x alloy was found by others [28,35,36] in their syntheses of PdeCu nanoalloy. The slight shift of peak positions towards higher angle proves that the materials are PdeCu alloy with decreased d-spacing and dilation of the lattice constant, due to the incorporation of the Cu atoms into the Pd fcc lattice. The peaks of profile ‘e’ of Fig. 1, at 43.38
Cu(111) and 50.55
Cu(200) of synthesized CueCuO nano particles indicate the existence of face centered cubic copper of nanometer dimension. The sharp peaks at ca 35.72
and 38.69
of the same profile reveal the existence of (111) and (111) of end centred monoclinic CuO. Other small peaks at ca 53.61
, 61.54
and 66.08
are for (020), (113) and (311) planes respectively of CuO. Presence of Cu (II) oxide peaks in addition to peaks of Cu in profile ‘e’ indicates fresh oxidation after synthesis. Notably, Cu (II) oxide peaks are absent in XRD pattern of PdxCu100-x alloy nanoparticles indicating Cu is more stable in alloy than in the pure state. XRD pattern of mixture of Pd

Please cite this article in press as: Chowdhury SR, et al., Palladium and palladiumecopper alloy nano particles as superior catalyst for electrochemical oxidation of methanol for fuel cell applications, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.05.239

4

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 2

and Cu nanoparticle exhibits two distinct characteristic peak of Pd (111 plane of face centered cubic) and Cu nanoparticles instead of one single peak as represented in the inset of Fig. 1. The transmission electron microscope (TEM) and selectedarea electron diffraction (SAED) images further authenticate the microstructure of the synthesized PdeCu nanoparticles. Fig. 2(a) and (b) show the microstructure of the Pd and representative Pd94Cu6 nanoparticles having values of diameters, ca. 10.83 nm and 5.85 nm respectively. Fig. 2(c) and (d) represent the HRTEM image of Pd and representative Pd94Cu6 nanoparticle. Fig. 2(c) shows clear lattice fringes with lattice spacing of 2.25 Å signifying the dominance of (111) plane. This is in good agreement with the XRD results that also explain that the ratio of intensity of (111) peak is much larger than that of others. Fig. 2(d) characterize the well resolved fringes of Pd94Cu6 bimetallic phase locating between (111) plane of pure face centered cubic Pd and Cu and the spacing is 2.22 Å. A set of diffraction rings can be clearly seen in the corresponding SAED pattern of the Pd and Pd94Cu6 nanoparticles as presented by Fig. 2(e) and (f) respectively. The SAED pattern analysis reveals that the lattice spacing's obtained from for the first rings are 2.25 Å and 2.20 Å for Pd and Pd94Cu6 nanoparticles respectively, which are well consistent with the data, obtained from XRD measurements. In combination with the XRD results, the first ring can be indexed as (111) and the other

rings as reflections from different planes of the face centered cubic (fcc) Pd94Cu6 bimetallic phase. TEM, HRTEM and SAED images of Pd98Cu2 and Pd86Cu14 are presented in supporting document and the corresponding data are given in Table 1. The FESEM micrographs of the synthesized Pd, Pd94Cu6 and Pd86Cu14 nanopowder are shown in Fig. 3(a) and (b), which further authenticate the above mentioned explanation. Loosely agglomerated extremely fine particles are observed for palladium nanoparticles where as closely agglomerated particles are found for both Pd94Cu6 and Pd86Cu14 nano-alloy. EDX analyses are carried out at different zones of the Pd94Cu6 and Pd86Cu14 nanoparticles almost uniform composition (atomic percentage of Pd and Cu are 94 and 6 respectively for the former and 86 and 14 for the latter, observed throughout the sample). Two representative EDX spectra are shown in Fig. 4(a) and (b). The EDX analysis of PdeCu nanoparticles supports excellently the alloy formation as observed in XRD analysis (Fig. 1).

Electrochemical studies The steady cyclic voltammograms of the nafion-coated bare carbon electrode, carbon supported nafion coated palladium electrode, C/Pd, similar representative alloy electrode, C/ Pd94Cu6, and carbon-supported C/PdCu (Cu 6% mixture) electrode, were recorded in 1 M NaOH solution in the potential range 0.1 V to 0.6 V, at a scan rate of 50 mVs1 in Fig. 5. It is evident from the figure that no peak appears for bare carbon signifying that the base material (graphite carbon) and nafion are inactive in alkali in the potential range studied. Beside a shoulder at higher potential, two peaks for C/Pd electrode and three peaks for representative C/Pd94Cu6 electrode appear during anodic potential scan. The first peak A1 at ca 0.477 V for C/Pd and A10 at ca 0.449 V for C/Pd94Cu6 arise seemingly due to electrochemical desorption of hydrogen following the reaction:

PdH(abs/ads) þ OH / Pd þ H2O þ e

(i)

The second peak A2 at ca. (þ0.009 V) for C/Pd and similar broad peak A20 at ca 0.218 V for C/Pd94Cu6 appear for adsorption of OH ion on the electrodes following the electrochemical reaction:

Pd þ OH ¼ PdOH þ e

(ii)

Peak A30 (0.013 V)appears for C/Pd94Cu6 electrode due to the formation of Cu(OH)2 following reaction

Cuþ2OH ¼ Cu(OH)2 þ 2e

(iii)

In the next stage of oxidation, formation of PdeO takes place corresponding to the small peaks (shoulders) A3 (þ0.444 V) and A40 (þ0.367 V) following equation (iv) Fig. 2 e (a) and (b) TEM images, (c) and (d) HRTEM and (e) and (f) SAED pattern of pure Pd and Pd94Cu6 nanoparticle respectively.

PdOHads þ OH / PdO þ H2O þ e

(iv)

Please cite this article in press as: Chowdhury SR, et al., Palladium and palladiumecopper alloy nano particles as superior catalyst for electrochemical oxidation of methanol for fuel cell applications, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.05.239

5

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 2

Table 1 e Diffraction peaks, crystallite size, lattice parameters and composition of the as-prepared nanoparticles. Nano-particles Position of d-spacing ( A) d-spacing ( A) d-spacing from Crystallite Cell parameter Evaluated atomic 2q (degree) (From XRD) (From HRTEM) SAED ( A) size (nm) a ( A) % of Pd Pd Pd98Cu2 Pd94Cu6 Pd86Cu14 Cu

40.15 68.10 40.31 68.27 40.49 68.43 40.53 68.51 43.38 74.37

2.246 1.377 2.237 1.374 2.228 1.371 2.226 1.369 2.086 1.276

2.25

2.25 1.36 2.22 1.34 2.20 1.309 2.19 1.29 e e

2.24 2.22 2.21 e

10

3.890

100

8.5

3.875

98

6

3.859

94

9

3.855

86

20

3.613

0

area under the cathodic peaks corresponding to the reduction of the Pd oxide monolayer formed in the forward scan. These data (presented in cm2 mg1 within the parenthesis) reveal the order: C/PdCu mixture (Cu 15%mixture) (37.81),
Cyclic voltammetric study of methanol-oxidation

Fig. 3 e FESEM images showing the microstructure of (a) Pd (b) Pd94Cu6 (c) Pd86Cu14 nanoparticle.

The corresponding cathodic peaks C3 and C60 appear at ca 0.301 V and 0.313 V respectively due to the reaction (ii) occurring in the opposite direction during reverse scan. The reverse peak corresponds to the formation of Pd from PdeO following equation (v) [29,37,38].

PdO þ H2O þ 2e / Pd þ 2OH

(v)

The slight difference in the peak potentials of the reactions with their standard Eo values are mainly due to the kinetic factor i.e. the over-potential requirement of the respective reactions. The peak potentials are consistent with the literature Eo values [29] and with the anticipation that Cu would form M(OH) x at lower potential as Cu is more oxophilic. It is also known that the activity of a catalyst is not only controlled by the electronic and chemical properties but also by the geometrical properties. So the electrochemically active surface area (EASA) of the catalyst has been measured by the oxygen desorption method, which is applicable to metals like Pd because of its good affinity for oxygen. The mass normalized ECSAs were calculated for all electrodes by computing the

The cyclic voltammograms (CVs) for methanol oxidation has been studied in the range between 0.9 V and þ0.6 V, since it is believed that pronounced hydrogen evolution may destroy the electrodes at potential less than 0.9 V and oxygen evolution may occur above þ0.6 V. The CV profiles of C/Pd, C/ Pd98Cu2, C/Pd94Cu6, C/Pd86Cu14 alloy and C/PdCu (6% mix), C/ PdCu (15% mix) mixture are presented in Fig. 6 which resemble the typical CV of alcohol oxidation in alkaline media exhibiting two well defined peaks. The nature of the profiles is completely different from that of the blank one. The efficiency of the electro catalysts toward methanol oxidation reaction (MOR) are characterized by the forward (anodic) peak current density (iF), forward peak potentials (EF), backward peak current density (iB) and backward peak potential (EB) as presented in Table 2. Fig. 6 shows that the peaks of both forward and backward current densities (iF and iB) appear in the anodic zone indicating blocking of catalytic surface by formation of PdeO layer at higher potential and removal of it at lower potential during reverse (backward) scan [39,40]. The second anodic peak appears due to oxidation of both freshly adsorbed alcohol and the previously adsorbed poisonous carbonaceous species [41] like Pd-COads after removal of PdeO blocking at the lower potential. Alloying of Cu with Pd causes a tendency of cathodic shift of forward peak potential (EF) for MOR, for example the EF value shifts about 0.11 V when C/Pd electrode is replaced by C/ Pd86Cu14 electrode as evident in Table 2. In comparison to alloy electrodes, the current density values are small for the corresponding mixtures of Pd and Cu nanoparticles indicating alloying is beneficial for improved catalysis. For C/ Pd94Cu6 electrode, another peak is developed at ca 0.28 V seemingly due to the oxidation of poisonous intermediates like Pd-COads [32] which is observed for the best electrode

Please cite this article in press as: Chowdhury SR, et al., Palladium and palladiumecopper alloy nano particles as superior catalyst for electrochemical oxidation of methanol for fuel cell applications, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.05.239

6

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 2

Fig. 4 e EDX spectra of (a) Pd94Cu6 (b) Pd86Cu14 nanoparticles.

Fig. 5 e Cyclic voltammograms (in mA cm¡2) in 1 M NaOH on bare graphaite carbon, C/Pd, C/Pd94Cu6 alloy and C/PdCu mixture (6% Cu) electrode at room temperature. The scan rate of potential was 50 mVs¡1.

studied, C/Pd94Cu6, because of superiority over others. It is well-known that greater the work function or % d-band character in the metallic bond less will be the number of electrons available for bond formation with the other species.

Fig. 6 e The CV plot in current density (in mA mg¡1 of Pd) of (a) C/Pd (b) C/Pd98Cu2 (c) C/Pd94Cu6 (d) C/Pd86Cu14 alloy electrodes and (e) C/PdCu mixture (6% Cu) (f) C/PdCu mixture (15% Cu) for alkaline (1.0 M NaOH) oxidation of methanol (0.5 M) at room temperature. The scan rate of potential was 50 mVs¡1. Since work function of Pd (5.22e5.6 ev) > that of Cu (4.53e5.10) and % d-band character of Pd (46) > that of Cu (36), Cu can bind CO more strongly than Pd [42,43]. Thus it

Please cite this article in press as: Chowdhury SR, et al., Palladium and palladiumecopper alloy nano particles as superior catalyst for electrochemical oxidation of methanol for fuel cell applications, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.05.239

7

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 2

Table 2 e The peak potentials (EF and EB), peak current density (iF and iB) and other related parameters obtained from cyclic voltammetric studies of C/Pd, different C/PdxCu100-x alloy and C/PdCu mixture electrodes immersed in 0.5 M methanol in 1 M NaOH solution at room temperature. Electrode

EF (V)

iF (mA Cm2)

iF (mA mg1)

EB (V)

iB (mA Cm2)

iB (mA mg1)

iF/iB

C/Pd C/Pd98Cu2 C/Pd94Cu6 C/Pd86Cu14 C/PdCu (6% mix) C/PdCu (15% mix)

0.011 0.059 0.029 0.121 0.079 0.113

76.15 84.02 129.30 26.04 16.37 1.56

545.26 624.507 987.27 204.57 125.497 12.82

0.270 0.247 0.362 0.321 0.304 0.316

29.12 60.21 24.69 4.63 10.59 0.19

208.52 447.54 189.21 36.38 81.17 1.54

2.62 1.39 5.24 5.62 1.55 8.35

removes CO from Pd and Cu is more active for CO oxidation than Pd [37,44e47]. So, the presence of Cu with Pd accelerates both the adsorption of CO and the removal of CO from the catalytic surface by its oxidation thereby resisting poisoning of the C/Pd94Cu6 electrode. However, in case of electrodes constructed with mixture of Pd and Cu nano particles the electronic properties of metals are not changed and two metal crystals remain separated from each other resulting effective decrease of active sites of each metal at the surface and decrease in value for C/PdCu (6% Cu mixture). For C/PdCu (15% Cu, mixture) the change in Ef values is slightly positive with respect to that of C/Pd electrode seemingly due to merging of current peaks of reactant MeOH and intermediate CO oxidation. Very small iF value for the electrode indicates change in the mechanism with less number of available PdeCu combined active sites for the electrode. Fig. 6 depicts that iB increases with iF demonstrating that greater dehydrogenation during forward scan build up more oxidizable carbonaceous species which are oxidized during reverse scan. In the CVs of MOR, iF is mostly associated with the oxidation of freshly chemisorbed methanol molecules, so iF is normally used to measure the direct catalytic activity of electro catalysts. Inclusion of Cu into Pd matrix up to a limit, increases iF value of the electrodes. But simple addition of Cu nano particle to Pd causes decrease in iF with respective to that obtained for C/Pd, indicating change in electronic properties of the catalyst and surface plausibly the diffusion of intermediate adsorbed species are important to get improved catalytic activity. The mass normalized electro catalytic activities (presented in mA mg1of Pd within the parenthesis) of the electrodes are in the order of C/PdCu (15% Cu, mixture) (12.82)
of Pd in MOR based fuel cells as well as enhance the cell activity. Notably, the highest mass normalised peak current value obtained here is greater than that obtained at a different composition in our previous study [28]. The apparent discrepancy might be due to slight difference in the synthetic technique which results in the formation smaller nanoparticles and more active sites as compared to our previous study [28]. In the previous study [28] the synthesis of PdCu nanoalloy was made at room temperature via the formation of EDTA complex. In the present study, the initial reagents are directly reduced by hydrazine solution at relatively higher temperature (75
C). Particles formed in this method are smaller in diameter; of course at the cost of the facility of using room temperature in the synthetic protocol. The optimum concentration of Cu in PdCu nanoalloy has synergistic promotional effect which accelerates the catalytic activity. Thus, considering the comparable shape and size of the PdCu nanoalloys, the enhancement in electro catalytic activity of Pd94Cu6 nano alloy for MOR is resulted from the binding effect and synergistic effect between Pd and Cu. Table 2 reveals that the maximum values of the mass normalized iF and iB are obtained with 6 atom % of Cu. It shows that the C/Pd94Cu6 alloy electrode is the best among the electrodes studied for electrochemical MOR. The ratio of the forward anodic peak current density iF to the reverse anodic peak current density iB is utilized to illustrate the tolerance of electrocatalyst to the accumulated intermediate carbonaceous species on the surface of the electrode. Thus a greater ratio of iF/iB implies less carbonaceous residues accumulated on electrode surface indicating greater extent of reaction towards oxidation of methanol during forward scan. The iF/iB ratio is found to be negligible for C/Pd94Cu6 alloy and C/PdCu mixture (15) electrodes indicating that the accumulations of the carbonaceous poisons are negligibly small for them after the forward anodic scan. The Tafel plots are drawn using potentiodynamic pseudo steady-state polarisation data taken at the sweep rate of 2 mVs1, to compare the kinetic activities of C/Pd, Pd98Cu2,

Table 3 e Comparative Tafel slope and exchange current density data from potentiodynamic studies of C/Pd and C/ PdxCu100-x for methanol oxidation at a scan rate of 2 mV s¡1. Name of electrode C/Pd C/Pd98Cu2 C/Pd94Cu6 C/Pd86Cu14

Intercept/V

Slope/V dec1

Adj.R-Square

Exchange current density/mA mg1

0.287 0.323 0.362 0.221

0.097 0.078 0.097 0.068

0.978 0.985 0.978 0.993

0.83*106 4.98*108 5.29*106 3.96*1017

Please cite this article in press as: Chowdhury SR, et al., Palladium and palladiumecopper alloy nano particles as superior catalyst for electrochemical oxidation of methanol for fuel cell applications, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.05.239

8

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 2

Pd94Cu6, Pd86Cu14 alloy electrodes towards MOR. Tafel polarization analysis was executed following linear Tafel relation according to the equation E ¼ Ee 

2:303RT 2:303RT logi0 þ logi aF aF

The calculated equilibrium exchange current density(i0) values are presented in Table 3, the calculated Ee value is set as 1.056 V vs MMO, which represents the equilibrium potential of oxidation of methanol to carbonate after considering PH effect and potential shift between Hg/HgO and NHE in 1 M NaOH. The exchange current density obtained by extrapolating the linear fitted Tafel line to where over potential equals to zero from Fig. 7. The results (Table 3) show that the mass normalized equilibrium exchange current density towards MOR on C/Pd94Cu6 electrode (5.3  106 mA mg1) is approximately six times more than that of C/Pd catalyst (0.83  106 mA mg1).

Chronoamperometric study The relative catalytic activity and stability of the electrodes is also tested by chronoamperometry in a solution of 0.5 M MeOH in 1 M NaOH with a constant applied potential of 0.2 V (vs Hg/HgO) for 180s. Fig. 8 illustrates the profiles which reveal that the catalytic activity as measured by the constant current densities (presented within the parenthesis in mA mg1of Pd) of the electrodes varies in the order: C/Pd86Cu14 alloy (22.07), C/Pd (37.24)
Fig. 7 e Represent the Tafel behaviour of graphite supported chemically deposited (a) C/Pd, (b) C/Pd98Cu2, (c) C/Pd94Cu6, (d) C/Pd86Cu14 alloy electrode from the slowest scan rate 2 mVs¡1.

Fig. 8 e Chronoamperometric profiles for C/Pd and C/ PdxCu100-x electrodes for 0.5 M methanol in 1 M NaOH, at the potential of ¡0.2 V. The same profiles for C/Pd and C/ Pd94Cu6 catalyst in the same solution up to 2400s are presented at the inset.

To compare the nature and stability of the electrodes, C/Pd and representative C/Pd94Cu6, chronoamperometry was done for 2500 s at a fixed potential of 0.2 V and presented in the inset of Fig. 8. Both the electrodes show positive current density throughout the study, which imply that the prepared electrodes are reasonably stable.

Study of path of oxidation To elucidate the path of anodic oxidation of methanol, cyclic voltammetric study was carried out using C/Pd and C/Pd94Cu6 electrodes immersed in 1 M NaOH with and without differently concentrated (6, 12, 36, and 100 mM) methanol, formaldehyde and sodium formate fuels in the potential range of 0.9 V to þ 0.4 V at a scan rate of 50 mVs1. Fig. 9 reveals that for all the fuels there is an increment in the current values particularly in the peak positions with new characteristic features with respect to blank CV obtained for 1 M NaOH without fuels. The potential (Volt) of the most intense forward peak in the CV's of the fuels follow the order: HCHO (0.093) < CH3OH (0.073) < HCOONa (0.014) for C/Pd electrode, indicating that oxidation of HCHO is easier than others. Similar observation is also found for C/Pd94Cu6 electrode where the respective order is HCHO (0.134) < CH3OH (0.114) < HCOONa (0.037) which shows a cathodic shifts of all the peak-potentials. Comparison of most intense current peaks of HCHO and methanol for the electrodes C/Pd and C/ Pd94Cu6 reveals that methanol oxidation proceeds more through HCHO (formation) oxidation on C/Pd electrode as compared to C/Pd94Cu6 electrodes. The major difference between two sets of CV (Figs. 9 and 10) is that no peak appears for formate in CV profiles of methanol and HCHO with C/Pd electrode as evident from Fig. 9, while similar profiles with C/ Pd94Cu6 electrode show peaks for formate oxidation (zone III) (Fig. 10). This indicates Cu helps in formate formation and the subsequent oxidation of it as shown by Scheme 1.

Please cite this article in press as: Chowdhury SR, et al., Palladium and palladiumecopper alloy nano particles as superior catalyst for electrochemical oxidation of methanol for fuel cell applications, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.05.239

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 2

9

Fig. 9 e Cyclic voltammograms for C/Pd catalyst for MeOH, HCHO, HCOONa fuels each of concentration of 100 mM in 1 M aqueous NaOH. Top and bottom inset represents CVs for HCHO and HCOONa solutions respectively of concentrations 6, 12, 36, 100 mM in aqueous NaOH at scan rate 50 mVs¡1. Scheme 1 e The proposed methanol oxidation reaction pathway in alkaline medium on C/Pd, C/PdxCu100-x electrodes.

Fig. 10 e Cyclic voltammetric profiles for C/Pd94Cu6 catalyst for MeOH, HCHO, HCOONa fuels each of concentration 100 mM in 1 M aqueous NaOH. Top and bottom inset represents CV for HCOONa and HCHO solutions respectively of concentrations 6, 12, 36 and 100 mM in aqueous NaOH at scan rate 50 mVs¡1.

In case of both C/Pd and C/Pd94Cu6 electrodes, an extra peak at lower potential before the most intense peak is obtained for formaldehyde in comparison to methanol. Moreover, the onset potentials of formaldehyde (0.738 V and 0.787 V) are more negative than those of methanol (0.655 V and 0.766 V) for C/ Pd and C/Pd94Cu6 electrodes respectively. The differences in onset potentials as well as peak potentials among the two electrodes and an additional peak for C/Pd94Cu6 electrode reveal the difference in mechanisms of the electrochemical

oxidation of formaldehyde as active intermediate on the two electrodes in the process of oxidation of methanol. Again formate, besides initial peak for dehydrogenation also shows peak at higher region (peak potential ca. þ0.03 V) in the potential range studied. So formation of formate to carbonate occurs at higher potential and it also appears as intermediates in the process of complete oxidation of methanol. For C/Pd electrode the reaction does not follow significantly the electrooxidation path of conversion of formaldehyde to formate. Oxidation of methanol and formaldehyde ends with the formation of carbonate plausibly without formation of formate as intermediate for the electrode. On the other hand, in case of C/ Pd94Cu6 electrode, both the oxidations, say formaldehyde to carbonate and formate to carbonate, occur simultaneously and Cu helps in the carbonate formation.

FTIR and chromatographic studies of the products Assignments of main FTIR bands observed from spectra of the products of methanol oxidation in alkaline medium are presented in Table 4. It is observed from Fig. 11 and Table 4 that the peak(P3) at ca 872 cm1 arising due to CO2 3 ions becomes less sharp as the Cu content in the binary metal alloy (constituting the anode) is gradually increased. For each electrode the most intense broad peak (P2) appears at ca 1463 cm1 due to both and HCOO ions. Appearance of weak peak (P1) at CO2 3 1759 cm1 confirms the presence of C]O stretching (without Hbonding). These peaks indicate the presence of aldehyde (eCHO group) which reveals that methanol oxidises through the formation of formaldehyde as one of the intermediates. Moreover it is observed that the ratio of absorbance corresponding to (P3)

Please cite this article in press as: Chowdhury SR, et al., Palladium and palladiumecopper alloy nano particles as superior catalyst for electrochemical oxidation of methanol for fuel cell applications, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.05.239

10

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 2

Table 4 e Assignments of main FTIR bands observed from spectra of the products of methanol oxidation in alkaline medium. C/Pd electrode Wave number (cm1) 2979 1759 1661 1463 872 692

C/Pd94Cu6 electrode

C/Pd86Cu14 electrode

Possible assignments

Wave number (cm1)

Possible assignments

Wave number (cm1)

Possible assignments

eCH symmetrical stretching Carbonyl (C]O) stretching Formate Formate/carbonate CO3 2 CO3 2

2970

eCH symmetrical stretching Carbonyl (C]O) stretching Formate Formate/carbonate CO3 2 CO3 2

2970

eCH symmetrical stretching Carbonyl (C]O) stretching Formate Formate/carbonate CO3 2 CO3 2

1759 1652 1463 872 684

1759 1641 1463 872 692

Fig. 11 e Ex-situ FTIR profiles of MOR products for C/Pd, C/ Pd94Cu6 and C/Pd86Cu14 catalysts.

and (P1) decreases in the order (values given within the parenthesis): C/Pd(1.11)> C/Pd94Cu6(1.04)> C/Pd86Cu14(1.01) respectively. The chromatographic study [48,49] (Fig. 12) also reveals that both the maximum intensity and the area (presented within the parenthesis in minute) under the absorbanceetime curves for the different electrodes vary in the order: C/Pd(4667)< C/Pd94Cu6(5243)< C/Pd86Cu14(5582). It indicates greater formation of formate on increasing the content of Cu in PdxCu100-x alloy used as catalyst, in conformation with the IR data. These indicate that carbonate formation decreases slightly with increased atom% of Cu in the alloy nano composite constituting the electrode and this happens plausibly due to copper formate formation from intermediates of Scheme 1. So it can be suggested that methanol oxidation in alkaline medium passes through the formation of aldehyde, formate and carbonate for all the electrodes studied and the extent of formation of formate increases with increase in the content of Cu in alloy form in the electrode material.

Proposed mechanism As evident from Scheme 1, methanol oxidation occurs mainly through two paths. In path1, methanol is converted to

Fig. 12 e Absorbance versus retention time profiles for formate as an MOR product obtained using (a) C/Pd, (b) C/ Pd94Cu6 and (c) C/Pd86Cu14 electrodes. Profile (d) represents that for known concentration of formate ion, recognising the ion in the unknown samples containing products of MOR.

intermediate (A) and then intermediate (B) through formaldehyde. Intermediate (B) is subsequently converted to either MeC^O or formate ion. In path2, intermediate (C) is formed followed by formic acid and formate ion. Thus intermediate B produces both formate and carbonate anions using the parallel paths, while intermediate C produces only formate ion which again slowly oxidises to bicarbonate and carbonate ions. The intermediate (C) is possibly 5 or 6 member ring formed using PdePd and PdeCu bonds. Increased roughness helps formation of such intermediates. C/Pd electrode follows

Please cite this article in press as: Chowdhury SR, et al., Palladium and palladiumecopper alloy nano particles as superior catalyst for electrochemical oxidation of methanol for fuel cell applications, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.05.239

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 2

path1 more than path2 in comparison to C/Pd94Cu6 electrode. Cu can easily form formate than Pd due to smaller atomic radius and greater surface charge density. IR and chromatographic studies confirm the formation of more formate ion on increasing the content of copper in C/PdxCu100-x electrodes. The whole study is also consistent with the mechanism.

Conclusion In the study, electrocatalytic capabilities of synthesized different nanoalloys, PdxCu100-x are reported in reference to MOR in alkali. Careful alteration of two important experimental parameters in the protocol of synthesis of PdxCu100-x nanoalloy catalyst is explored to obtain better catalyst for oxidation of alcohol. Among the electrodes studied, C/Pd94Cu6 provides much improved electro catalytic activity in respect to oxidation of methanol, formaldehyde and sodium formate as compared to other PdeCu alloys and composites available in current literature. The electrode constructed with synthesized PdeCu mixture containing 10 atom % Cu provides less catalytic activity indicating alloying is required for better performance of the catalyst. The decrease in % d-band character of Pd in alloying with Cu helps plausibly in greater adsorption of methanol and increased formate formation on the electrode surface causing synergic effect. The study of path of oxidation, chromatography and FTIR of products, help elucidate the proposed mechanism of the reaction.

Acknowledgements The authors like to thank Jadavpur University (UGC CAS program) for all financial and instrumental support.

Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2016.05.239.

references

[1] Song C. Fuel processing for low-temperature and highetemperature fuel cell challenges and opportunities for sustainable development in the 21st century. Catal Today 2002;77:17e49. [2] Wang CY. Fundamental models for fuel cell engineering. Chem Rev 2004;104:4727e66. [3] Antolini E, Gonzalez ER. Alkaline direct alcohol fuel cells. J Power Sources 2010;195:3431e50. [4] Yu HE, Krewer U, Scott K. Principles and materials aspects of direct alkaline alcohol fuel cells. Energies 2010;3:1499e528. [5] Guo S, Wang E. Nobel metal nanomaterials: controlled synthesis and application in fuel cells and analytical sensors. Nano Today 2011;6:240e64. [6] Liu X, Wang D, Li Y. Synthesis and catalytic properties of bimetallic nanomaterials with various architectures. Nano Today 2012;7:448e66.

11

[7] Wang D, Li Y. Bimetallic nanocrystals: liquid-phase synthesis and catalytic applications. Adv Mater 2011;23:1044e60. [8] Dohle H, Schmitz H, Bewer T, Mergel J, Stotten D. Development of a compact 500W direct methanol fuel cell stack. J Power Source 2002;106:313e22. [9] Brown LF. A comparative study of fuels for on-board hydrogen production for fuel-cell-powered automobiles. Int J Hydrogen Energy 2001;26:381e97. [10] Liu H, Songe C, Zhang L, Zhang J, Wang H, Wilkinson DP. A review of anode catalyst in the direct methanol fuel cell. J Power Sources 2006;155:95e110. [11] Yin J, Shan S, Shan Ng M, Yang L, Mott D, Fang W, et al. Catalytic and electro-catalytic oxidation of ethanol over palladium-based nanoalloy catalysts. Langmuir 2013;29:9249e58. [12] Wang Y, Wang X, Li CM. Electrocatalysis of Pd-Co supported on carbon black or ball-milled carbon nanotubes towards methanol oxidation in alkaline media. Appl Catal B Environ 2010;99:229e34. [13] Chen A, Ostrom C. Palladium-based nanomaterials: synthesis and electrochemical applications. Chem Rev 2015;115:11999e2044. [14] Hammer B, Hansen LB, Norskov JK. Improved adsorption energetics within density-functional theory using revised Perdew-Burk Ernzerhof functionals. Phys Rev B 1999;59(11):7413e21. [15] Du W, Wang Qi, Saxner D, Deskins AN, Su Dong, Krzanowski JE, et al. Highly active Iridium/IridiumeTin/Tin oxide heterogeneous nanoparticles as alternative electrocatalyst for the ethanol oxidation reaction. J Am Chem Soc 2011;133:15172e83. [16] Zhang B, Yuan Y, Philippot K, Yan N. Ag-Pd and CuO-Pd nanoparticles in a hydroxyl-group functionalized ionic liquid: synthesis, characterisation and catalytic performance. Catal Sci Technol 2015;5:1683e92. [17] Maya-Cornejo J, Ortiz-Ortega E, Alvarez-Contreras L, Arjona N, Guerra-Balcazar M, Ledesma-Garcia J, et al. Copper-palladium coreeshell as an anode in a multi-fuel membraneless nanofluidic fuel cell: toward a new era of small energy conversion devices. Chem Commun 2015;51:2536e9. [18] Chen M, Zhang Z, Li L, Liu Y, Wang W, Gao J. Fast synthesis of Ag-Pd@ reduced graphene oxide bimetallic nanoparticles and their applications as carbon-carbon coupling catalysts. RSC Adv 2014;4:30914e22. [19] Liu A, Geng H, Xu C, Qiu H. A three-dimensional hierarchical nanoporous PdCu alloy for enhanced electrocatalysts and biosensing. Anal Chim Acta 2011;703:172e8. [20] Yin Z, Lin LL, Ma D. Construction of Pd-based nanocatalysts for fuel cells: opportunities and challenges. Catal Sci Technol 2014;4:4116e28. [21] Zhang Y, Huang Q, Chang G, Zhang Z, Xia T. Controllable synthesis of palladium nanocubes/reduced graphene oxide composites and their enhanced electro-catalytic performance. J Power Sources 2015;280:422e9. [22] Zhang Y, Chang G, Shu H, Oyama M, Liu X, He Y. Synthesis of Pt-Pd bimetallic nanoparticles anchord on graphene for highly active methanol electro-oxidation. J Power Sources 2014;262:279e85. [23] Wang Y, Zhao Y, Yin J, Liu M, Dong Qi, Su Y. Synthesis and electrocatalytic alcohol oxidation performance of Pd-Co bimetallic nanoparticles supported on graphene. Int J Hydrogen Energy 2014;39:1325e35. [24] Dong Qi, Zhao Y, Han X, Wang Y, Liu M, Li Ye. Pd/Cu bimetallic nanoparticles supported on graphene nanosheets: facile synthesis and application as novel electro-catalyst for ethanol oxidation in alkaline media. Int J Hydrogen Energy 2014;39:14669e79.

Please cite this article in press as: Chowdhury SR, et al., Palladium and palladiumecopper alloy nano particles as superior catalyst for electrochemical oxidation of methanol for fuel cell applications, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.05.239

12

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 2

[25] Ren Y, Zhang S, Lin R, Wei X. Electro-catalytic performance of Pd decorated Cu nanowires catalyst for the methanol oxidation. Int J Hydrogen Energy 2015;40:2621e30. [26] Hosseini MG, Abdolmaleki M, Ashrafpoor S. Methanol electro-oxidation on a nanostructured Ni/Pd-Ni electrode in alkaline media. Chin J Catal 2013;34:1712e9. [27] Yin Z, Zhang Y, Chen K, Li J, Li W, Tang P, et al. Monodispersed bimetallic PdAg nanoparticles with twinned structures: formation and enhancement for the methanol oxidation. Sci Rep 2014;4. 4288 (9 pages). [28] Mandal K, Bhattacharjee D, Roy PS, Bhattacharya SK, Dasgupta S. Room temperature synthesis of Pd-Cu nanoalloy catalyst with enhanced electrocatalytic activity for the methanol oxidation reaction. Appl Catal A General 2015;492:100e6. [29] Mukherjee P, Roy PS, Mandal K, Bhattacharjee D, Dasgupta S, Bhattacharya SK. Improved catalysis of room temperature synthesized Pd-Cu alloy nanoparticles for anodic oxidation of ethanol in alkaline media. Electrochim Acta 2015:447e55. [30] Roy PS, Bhattacharya SK. Size-controlled synthesis, characterisation and electrocatalytic behaviours of polymerprotected nickel nanoparticles: a comparison with respect to two polymers. RSC Adv 2014;4:13892e900. [31] Roy PS, Bhattacharya SK. Size-controlled synthesis and characterization of polyvinyl alcohol-coated platinum nanoparticles: role of particle size and capping polymer on the electrocatalytic activity. Catal Sci Technol 2013;3:1314e23. [32] Roy PS, Bagchi J, Bhattacharya SK. Synthesis of polymerprotected palladium nanoparticles of contrasting electrocatalytic activity: a comparative study with respect to reflux time and reducing agents. Coll Surf A Physicochem Eng Asp 2010;359:45e52. [33] Mukherjee P, Roy PS, Bhattacharya SK. Improved carbonate formation from ethanol oxidation on nickel supported Pt-Rh electrode in alkaline medium at room temperature. Int J Hydrogen Energy 2015;40:1e11. [34] Mukherjee P, Bagchi J, Dutta S, Bhattacharya SK. The nickel supported platinum catalyst for anodic oxidation of ethanol in alkaline medium. Appl Catal A General 2015;506:220e7. [35] Xu C, Lui A, Qiu H, Liu Y. Nanoporous PdCu alloy with enhanced electrocatalytic performance. Electrochem Commun 2011;13:766e9. [36] Zhang Z, Zhang C, Sun J, Kou T, Zhao C. Ultrafine nanoporous Cu-Pd alloys with superior catalytic activities towards electro-oxidation of methanol and ethanol in alkaline media. RSC Adv 2012;2:11820e8.

[37] Yin Z, Zhou W, Gao Y, Ma D, Kiely CJ, Bao X. Supported Pd-Cu bimetallic nanoparticles that have high activity for the electrochemical oxidation of methanol. Chem Eur J 2012;18:4887e93. [38] Kang WD, Wei YC, Liu CW, Wang KW. Enhancement of electrochemical properties on Pd-Cu/C electrocatalysts toward ethanol oxidation by atmosphere induced surface and structural alteration. Electrochem Commun 2011;13:162e5. [39] Bagchi J, Bhattacharya SK. Studies of the electrocatalytic activity of binary palladium ruthenium anode catalyst on Ni support for ethanol alkaline fuel cell. Trans Met Chem 2008;33:113e20. [40] Bagchi J, Bhattacharya SK. Electrocatalytic activity of binary palladium ruthenium anode catalyst on Ni-support for ethanol alkaline fuel cells. Trans Met Chem 2007;32:47e55. [41] Roy PS, Bagchi J, Bhattacharya SK. The size-dependent anode-catalytic activity of nickel-supported palladium nanoparticles for ethanol alkaline fuel cells. Catal Sci Technol 2012;2:2302e10. [42] Rochefort A, Abon M, Delichere P, Bertolini JC. Alloying effect on the adsorption properties of Pd50Cu50 {111} single crystal surface. Surf Sci 1993;294:43e52. [43] Debague Y, Abon M, Bertolini JC, Massardier J, Rochefort A. Synergistic alloying behaviour of Pd50Cu50 single crystals upon adsorption and co-adsorption of CO and NO. Appl Surf Sci 1995;90:15e27. [44] Choi KI, Vannice MA. CO oxidation over Pd and Cu catalyst IV. Pre reduced Al2O3 supported copper. J Catal 1991;131:22e35. [45] Yang L, Yan D, Liu C, Song H, Tang Y, Luo S, et al. Vertically oriented graphene oxide supported dealloyed palladiumcopper nanoparticles for methanol electro-oxidation. J Power Sources 2015;278:725e32. [46] Hammer B, Morikawa Y, Norskov JK. CO chemisorptions at metal surfaces and overlayers. Phys Rev Lett 1996;76:2141e4. [47] Hammer B, Norskov JK. Electronic factors determining the reactivity of metal surfaces. Surf Sci 1995;343:211e20. [48] Kwon Y, Koper TMM. Combining voltammetry with HPLC: application to electro-oxidation of glycerol. Anal Chem 2010;82:5420e4. [49] Santasalo-Aarnio A, Kwon Y, Ahlberg E, Kontturi K, Kallio T, Koper TMM. Comparison of methanol, ethanol and isopropanol oxidation on Pt and Pd electrodes in alkaline media studied by HPLC. Electrochem Commun 2011;13:466e9.

Please cite this article in press as: Chowdhury SR, et al., Palladium and palladiumecopper alloy nano particles as superior catalyst for electrochemical oxidation of methanol for fuel cell applications, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.05.239