Outstanding Catalyst Performance of PdAuNi Nanoparticles for the

Oct 22, 2012 - The results show that the use of the ternary PdAuNi catalyst at the anode of an in-house fabricated DE(AEM)FC can increase the peak pow...
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Outstanding Catalyst Performance of PdAuNi Nanoparticles for the Anodic Reaction in an Alkaline Direct Ethanol (with Anion-Exchange Membrane) Fuel Cell Abhijit Dutta and Jayati Datta* Electrochemistry & Fuel Cell Laboratory, Department of Chemistry, Bengal Engineering and Science University, Shibpur, Howrah − 711 103, West Bengal, India S Supporting Information *

ABSTRACT: The present article deals with the comprehensive electrocatalytic study of the binary and ternary combinations of Ni and Au with Pd for use as the anode component of a direct ethanol fuel cell (DEFC) operating with an anion-exchange membrane (AEM). The catalysts were grown on a carbon support by chemical reduction of the respective precursors. The information on surface morphology, structural characteristics, and bulk composition of the catalyst was obtained using transmission electron microscopy, X-ray diffraction, and energy-dispersive X-ray spectroscopy. Brunauer−Emmett−Teller (BET) surface area and the pore widths of the catalyst particles were calculated by applying the BET equation to the adsorption isotherms. The electrochemical techniques like cyclic voltammetry, chronoamperometry, and impedance spectroscopy were employed to investigate the electrochemical parameters related to electro-oxidation of ethanol in alkaline pH on the catalyst surfaces within the temperature range 20−80 °C. The results show that the use of the ternary PdAuNi catalyst at the anode of an in-house fabricated DE(AEM)FC can increase the peak power density by more than 175% as compared with the use of the monometallic Pd catalyst, 108% as compared with the use of the bimetallic PdNi catalyst, and 42% as compared with the use of the bimetallic PdAu catalyst. The higher yield of the reaction products CH3CO2− and CO32− on the PdAuNi catalyst compared to its single and binary counterparts in alkaline medium, as estimated by ion chromatography, further substantiates the catalytic superiority of the PdAuNi catalyst to a remarkable extent over the other catalysts studied.

1. INTRODUCTION Among the organic fuels for fuel cells, ethanol has emerged as a green fuel, since it is eco-friendly and highly energy efficient (8.01 kWh kg−1) compared to the others.1 Bioethanol is a renewable energy source that can be easily produced in large quantities by the fermentation of sugar containing raw materials from agriculture and even from the organic fraction of municipal solid wastes.2 However, a significant challenge in the development of direct ethanol fuel cell (DEFC) technology is the need for highly active catalysts for the ethanol oxidation reaction (EOR) in an acid or alkaline environment that involve complete oxidation per ethanol molecule to CO2 releasing 12 electrons and the cleavage of the C−C bond,3 which is difficult to implement at low temperatures.4 The catalysts designed with multimetallic framework, are designated with enhanced electrochemical activity attributable to the improved functional properties of the catalyst surface and electronic effects resulting from electron transfer from the promoter element. A number of factors are responsible for enhancing the catalytic activity, for example, change in M−M interatomic distance, number of nearest neighboring metal atoms, d band vacancy, and metal content on the surface.5−10 Now, for DEFCs working in an alkaline environment, the overvoltage can be restricted by the faster kinetics of the © 2012 American Chemical Society

electrode reactions and lower alcohol permeability. Moreover the alkaline DEFCs also allow the use of inexpensive nonplatinum metal as well as the transitional metal catalysts.11,12 In particular, a great deal of interest has been focused on the use of Pd-based anode catalysts as a Pt alternative for EOR where the catalytic activity has been further increased by the addition of a second metal or metal oxide promoters.13−15 Some of the current investigations also demonstrate significant improvement in electrochemical activity by incorporation of Ni into the Pd matrix.16−19 Although some of the earlier reports on fuel cell catalysis include Pt-based trimetallic system effective in alkaline media,20−22 the work so far reported on the non-Pt ternary catalyst is absolutely scarce. Only recently, Zhao et al. reported on the synthesis of Pt-free catalysts and the electrocatalytic investigations on the single, binary, and ternary combinations for the alkaline DEFCs.23 Bambagioni et al. have also synthesized Pd-based PdNi−Zn ternary alloys for ethanol oxidation in alkaline medium and demonstrated the Pd−(Ni− Received: July 2, 2012 Revised: September 21, 2012 Published: October 22, 2012 25677

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Table 1. Composition of Catalysts and the Parameters Obtained from EDX, XRD, TEM, and Voltammetric Analysis av crystallite size (nm)

ECSA (m2/g)

electrocatalyst

Pd/Au/Ni (bath ratio)

EDAX composition (at %)

interplanar distance (Å)

lattice parameter (Å)

XRD

TEM

H2 ads/des region

reduction of adsorbed O2 region

Pd/C Au/C Ni/C Pd58Ni42/C Pd53Au47/C Pd41Au29Ni30/C

1:0:0 0:1:0 0:0:1 1:0:1 1:1:0 1:1:1

− − − Pd: 58; Ni: 42 Pd: 53; Au: 47 Pd: 41; Au: 29; Ni: 30

2.23 2.24 2.03 2.19 2.29 2.27

3.893 4.081 3.528 3.889 3.889 3.887

3.4 3.2 3.4 3.1 3.3 2.6

4.0 3.5 3.7 3.5 3.5 2.9

19.8 11.3 − 39.6 53.3 77.8

24.9 9.5 − 48.2 58.1 79.3

Cu grid were subjected to transmission electron microscopy (TEM) analysis using high-resolution TEM (HRTEM) (FEI model), STWIN operating at an accelerating voltage of 200 kV. The chemical compositions of the multimetallic catalyst layers were determined by energy-dispersive X-ray spectroscopy (EDX) using a Link ISIS EDX detector (Oxford Instruments, UK) coupled with the transmission electron microscope. The surface area of the catalysts was determined at 77 K by the BET method using nitrogen as the adsorbate in a Quantachrome Autosorb instrument (Model AS1-CT) following the usual prior treatment of samples.26 The total pore volume (Vt) and the micropore volumes (Vm) were derived from nitrogen sorption isotherm, and the pore size distribution was obtained according to Barrett−Joyner−Halenda (BJH) model calculations. 2.3. Electrochemical Studies. In the course of fabricating the electrodes, a catalyst ink was prepared by mixing 2-propanol (GR, Merck) with an appropriate amount of the catalyst in 5 wt % Nafion ionomer (Eloctrochem. Inc., USA) followed by sonication. A calculated amount of this slurry was micropipetd out onto the graphite plate (GLM grade, Graphite India Ltd.) of 0.65 cm2 surface area maintaining a constant catalyst loading of 0.77 mg cm−2. Electrochemical measurements were recorded by the help of a microprocessor controlled potentiostat/ galvanostat (PG stat 12 and FRA modules, Ecochemie BV, Netherlands) in a three electrode assembly cell consisting of a mercury/mercurous oxide (Hg/HgO) reference electrode and bright Pt-foil (10 mm × 10 mm) counter electrode and the synthesized catalysts as the working electrode. Henceforth, the potentials in this paper are expressed with respect to Hg/HgO. The working electrolyte containing 1 M ethanol (Merck, A.R) and 0.5 M NaOH was purged with N2 (XL grade from BOC India Ltd.) before each of the experiment. 2.4. Estimation of Reaction Products. The ionchromatography system (Metrohm’s Advanced Modular Ion Chromatography) composed of a L-7100 pump (Metrohm Ltd.), a conductivity detector, and Metrosep A Supp 5-250 and Metrosep A Supp 4-550 organic acid columns was employed to quantify the yields of acetate and carbonate ions produced during the electrolysis by taking aliquots from the working electrolyte after polarization at a constant potential of −300 mV for 1 h at different temperatures in the range 20−80 °C. The entire work of electrochemical characterization and the chromatographic estimation were carried out in inert atmosphere by purging N 2 into the solution before experimentation and also maintaining N2 atmosphere within the working cell set up. 2.5. Performance Study of DE(AEM)FC. Membrane electrode assemblies (MEAs) of 1 cm2 active area were fabricated by placing an anion-exchange membrane (Tokuyama

Zn)/C and Pd−(Ni−Zn−P)/C catalysts exhibited promising activity for ethanol oxidation.24,25 Thus, in pursuit of non-Pt catalyst fabrication, the present investigation centers around developing an EOR catalyst with a Pd base and incorporating either Au as the noble metal or Ni as the non-noble metal to form the binary matrix as well as combining both the cometals simultaneously with Pd to arrive at the ternary catalyst composition routed through the simultaneous chemical reduction method using NaBH4 as the reducing agent. The electrocatalytic features of nanostructured carbon supported single Pd deposits, bimetallic PdNi and PdAu, and trimetallic PdAuNi catalysts were compared. Several electrode kinetic parameters along with the apparent activation energy for EOR were derived within the temperature range 20−80 °C, and their corelation was made with the morphology and composition of the catalyst matrix. In order to reveal the extent of oxidation of the alcohols on the different catalyst surfaces, the working electrolytes were further subjected to ionchromatographic analysis to identify and estimate the products formed during the course of the oxidation reaction. Electrochemical impedance spectra for the electrode−electrolyte interface evolved the charge transfer resistance associated with the EOR. Finally, the performance testing of the DEFCs fabricated with an anion-exchange membrane (AEM) were carried out with the synthesized catalysts at a temperature of 40 °C.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Catalysts. In the course of synthesizing the catalyst particles, a 40:60 catalyst/ support ratio was maintained in all cases by taking an appropriate amount of Vulcan XC-72 carbon (Cabot India Ltd., Brunauer−Emmett− Teller (BET) surface area, 288 m2 g−1) and the metal precursors (Arora Matthey Ltd.) PdCl2·xH2O, AuCl4·xH2O, and NiCl2 in their respective proportions. Subsequent chemical reduction was done by NaBH4 (E-Merck) solution at room temperature to obtain the metallic deposit of Pd and the codeposits of PdNi, PdAu, and PdAuNi. The stoichiometry of the binary and ternary systems corresponded approximately to the nominal compositions Pd1Au1, Pd1Ni1, and Pd1Au1Ni1. 2.2. Surface Morphology and Structural Characterization. The crystalline structures of the supported catalysts were revealed through the powder X-ray diffraction (XRD) technique using a SEIFERT 2000 diffractometer operating with Cu Kα radiation (λ = 0.1540 nm) generated at 35 kV and 30 mA. Scans were recorded at 1° min−1 for 2θ values between 2° and 90°. The XRD patterns were analyzed following the JCPDS file, and Scherrer and Bragg’s formulae were used to calculate the mean diameter and the lattice parameter of each of the catalyst. The catalysts suspended on a standard carbon-coated 25678

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that of Au/C indicating the Au-rich phase.28,29 The second peak at each position almost coincides with the corresponding peak of Pd/C, a slightly lower 2θ value due to the presence of Au, indicating the Pd-rich phase, and similarly, the third one indicates the formation of a Ni-rich phase as shown in Figure 1. On the other hand, the peak corresponding to NiO (012) and Ni(OH)2 (110) is observed in the ternary catalyst as shown in Figure 1. The similar peaks are also observed with pure Ni/C nanoparticle by magnifying the corresponding region as shown in the Supporting Information (Figure S1a). The diffraction peaks of Pd, Au, and Ni can be observed in the Pd41Au29Ni30/C catalyst, indicating their coexistence in the matrices. On the other hand, in the Pd58Ni42/C catalyst, the diffraction peaks are shifted to higher 2θ values for the catalysts with respect to the corresponding peaks in the pure Pd catalyst as shown in Figure 1. Such observation indicates the alloy formation of PdNi catalysts. The average particle size for the nanocatalysts was calculated from (111) peak broadening using the Scherrer equation31 and summarized in Table 1. The lattice parameters as obtained from XRD studies for the single and multimetallic systems follow the order Au > Pd > Ni. Accordingly, Au incorporation into Pd increases the lattice parameter of Pd while Ni reduces the same although the variation is not much significant. However, for the trimetallic nanocatalyst, the lattice contraction is quite sensible. The lattice contraction is expected due to the blending of the smaller sized Au (135 pm) and Ni (135 pm) atoms with Pd (140 pm) in the crystal lattice. This is suggestive of the trimetallic nanoparticles Pd, Au, and Ni forming ensembles, composite in nature, however neither PdAuNi alloyed nor core shell structured. Figure 2a shows the typical TEM image and the particle size distribution of the Pd41Au29Ni30/C catalyst. The histogram (Figure 2a/) is obtained from an ensemble of 200 particles, the distribution ranging from 1 to 8 nm. The catalyst particles are found to be well-dispersed on the carbon support with an average size of 3.1 nm, as also reported from the XRD data. The SADP pattern shown in Figure 2a// indicates the formation of nanophases of the Pd41Au29Ni30/C catalyst with good crystallinity, and the rings are indexed for an fcc crystal structure. Figure 2b shows a HRTEM image of the Pd41Au29Ni30/C catalyst across the entire image, indicating that the prepared PdAuNi/C nanoparticles are entirely crystalline. Some well-defined fringe fingerprints were used to measure the interplanar d-spacing by line profile, shown in the insets of Figure 2b, and also with the help of the corresponding FFT pattern, shown in inset of Figure 2c. In these figures, the lattice spacing of nanoparticles are around 0.222, 0.234, and 0.201 nm, which correspond to (111) planes of Pd, (111) planes of Au, and (111) planes of Ni, respectively. The others nanoparticles of the ternary catalyst are also obtained within the same range as shown in the Supporting Information (Figure S1b−d). From Figure 2b, it is obvious that the trimetallic nanoparticles consist of tightly coupled Au, Pd, and Ni particles, possibly not existing as a perfect ternary PdAuNi alloy as has already been supported from the XRD results. Therefore, ternary nanoparticles have obvious interfaces between Au, Pd, and Ni components. A point-resolved EDX spectrum shown in Figure 2d, represents a single nanoparticle constituting the individual metallites of Pd, Au, and Ni, and the compositional consistency is maintained throughout the matrix. 3.2. Textural Properties from the Adsorption Isotherms. Textural characteristics of the mesoporous carbon

(A-006)) in between the cathode and the anode catalyst and hot pressing the assembly. The cathode was prepared by loading 1 mg cm−2 Pt/C commercial catalyst (Arroa Matthey Ltd.) onto a carbon backing layer (Torry-TGP-H-120, thickness, 0.37 mm), and similarly, the anode was also loaded with 1 mg cm−2 of the synthesized Pd-based catalysts. The electrolyte containing 1 M ethanol and 0.5 M NaOH was fed into the anode chamber at the rate of 1.0 mL min −1 while pure dry oxygen was purged into the cathode at a flow rate of 100 standard cubic centimeters per minute (sccm). MITS-pro software controlled Fuel Cell Test System (Arbin Instrument Co. USA) was employed for collecting the testing data at 40 °C.

3. RESULTS AND DISCUSSION 3.1. Electron Microscopy and X-ray Diffraction Studies. The atomic compositions of the binary and ternary catalysts are derived from EDX measurement and summarized in Table 1. Figure 1 shows the XRD patterns of Pd/C, Au/C,

Figure 1. Small-angle XRD diffractograms of Pd/C, Au/Ni, Ni/C, Pd53Au47/C, Pd58Ni42/C, and Pd41Au29Ni30/C electrocatalysts.

Ni/C, binary (Pd 58 Ni 42 /C, Pd 53 Au 47 /C), and ternary Pd41Au29Ni30/C. The XRD patterns of Pd53Au47/C and Pd41Au29Ni30/C show characteristic peaks of the face centered cubic (fcc) crystalline of Pd, Au, and Ni and exhibit diffraction peaks of (111), (200), (220), (311), and (222) facets corresponding to fcc crystal structure according to the respective JCPDS files (46-1043 for Pd, 04-0784 for Au, and 04-0850 for Ni). For Ni/C, the (311) and (222) planes appear at higher 2θ values that are not shown in the patterns. Interestingly, for all three components in the Pd41Au29Ni30/C matrix, different peak patterns are followed in all (111), (200), and (220) positions. Three distinct peaks are found at every position indicating the formation of three homogeneous trimetallic crystal phases.27,28 Xu et al. and He et al. also observed similar types of peaks on the analysis of different binary and ternary catalysts.29,30 For Pd41Au29Ni30, the three peaks corresponded to 2θ values at 38.4°, 39.5°, and 44.43° for the (111) position, 44.7°, 45.8°, and 51.81° for the (200) position, and 64.8°, 67.4°, and 76.39° for the (220) positions. The 2θ value of the first peak at every position is very close to 25679

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Figure 2. (a) TEM image of Pd41Au29Ni30/C catalyst (a/) particle size distribution, inset: SADP pattern. (b) HRTEM images and fringe pattern of a single particle having three different d-spacing. Inset: line profile analysis of fringe fingerprints for d-spacing evaluation of Pd41Au29Ni30/C. (c) FFT images of the corresponding fringe pattern. (d) Point-resolved EDX spectrum of Pd41Au29Ni30/C catalyst.

Figure 3. (a) Pore size distribution and (b) comparison of different surface area of Pd/C, Au/C, Pd58Ni42/C, Pd53Au47/C, and Pd41Au29Ni30/C catalysts.

Figure 4. Cyclic voltammograms of (a) Pd/C, Pd58Ni42/C, Pd53Au47/C, and Pd41Au29Ni30/C and (b) Pd/C, Au/C, and Ni/C catalysts in 0.5 M NaOH; scan rate = 0.05 V s−1. Insets: magnified view of oxide reduction peak region.

support (Vulcan XC-72) and the supported catalysts were obtained from the data collected on the nitrogen adsorption measured at 77 K. The BDDT classification exhibited the type IV/III nitrogen adsorption and desorption isotherm and the

hysteresis type are attributed to cylindrical pores (see Figure S2 in the Supporting Information). The adsorption isotherms are representative of the combination of micropores and mesopores as established from the PSD curves (Figure 3a) 25680

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and are used to calculate the values of BET specific surface area SBET (P/P0 varying within the range 0.05−0.2) and total pore volume Vt (at P/P0 = 0.98), where P0 is the saturation pressure. In Figure 3b, an increasing trend is observed in the total BET surface area of the catalyst surface as well as the external surface and the micropore area as Ni and Au particles get incorporated into the Pd matrix. It may be understood that Pd41Au29Ni30/C catalyst particles are not only intercalated into the micropore of the graphite support but are also grown on the external surface of the support, fairly extending the active sites. In Pd41Au29Ni30/C, the higher surface area and larger pore volume lead to better utilization of the catalyst particles, since the interconnected pore system in the narrowly dispersed matrix allow efficient transport of the reactants and products at the reaction sites. 3.3. Electrochemical Characterization. The typical electrochemical features of the surface of the catalysts are derived from cyclic voltammetry (CV) recorded in the potential region between −1000 and 800 mV in 0.5 M NaOH at a scan rate of 50 mV s−1 and shown in Figure 4a. The hydrogen region of the single Pd/C catalyst is somewhat depressed due to the penetration of large quantities of hydrogen into Pd (up to 900 times its own volume).32,33 This is a common feature with Pd and Pd-based catalysts. Also, in alkaline medium, there is fair possibility of preactivation of the surface by the OH adlayer.34 The hydrogen region32 is prominent in case of the ternary system as well as for the catalyst containing Au. On the other hand, the bare Ni and the PdNi alloy do not make significant contribution in this region (Figure 4b). Since the hydrogen adsorption method becomes less reliable, the real ECSA of the catalysts were determined by considering the columbic charges corresponding to the oxide reduction peak of the alloyed catalyst. The double layer region is highly charged spanning over 300 mV from −725 to −425 mV in the case of the PdNiAu/C catalyst, and the latter is also characterized by the largest current output throughout the anodic or cathodic sweep of the voltammograms. The ternary catalyst is also privileged with very high ECSA values corresponding to fairly large hydrogen adsorption−desorption and oxide reduction peaks, with the latter being discernible in the magnified view of Figure 4a (inset). The charges required for the reduction of PdO (qPdO-red), NiO (qNiO- red), and Au2O3 (qAu2O3-red) monolayers were taken to be 405, 430, and 400 μC cm −2 respectively, as reported in the literature.35−37 The average charge required for the oxide reduction in the case of the alloyed catalyst (Pd41Au29Ni30/C) bearing the respective atomic contents of the cometals was calculated to be 411.6 μC cm−2 and used for determining the ECSA values summarized in Table 1. The value of ECSA for Pd41Au29Ni30/C was found to be far higher than the value obtained for the pure metal catalyst and moderately higher than those for the binary catalysts reflecting the possible existence of highly active reaction centers in the ternary matrix. The higher ECSA for the Pd41Au29Ni30/C catalyst also corroborates with the smaller particle size and more uniform size distribution throughout the catalyst matrix. Figure 5a shows the stabilized voltammograms (30th cycle) of EOR on different catalysts except the bare Ni/C that does not show any typical feature in the fuel cell potential range −0.1 to 0.8 V. As shown in Figure 5a, both the bimetallic Pd58Ni42/C and Pd47Au53/C catalysts show a higher electrochemical activity than the monometallic Pd/C and Au/C catalysts, and the ternary catalyst (Pd41Au29Ni30/C) far exceeds the activity over

Figure 5. Cyclic voltammograms for the EOR on (a) single, binary, and Pt-free ternary catalysts. (b) Magnified view of onset region of cyclic voltammograms. (c) Respective oxidation potential for all the catalyst at 3.00 mA cm−2 in 0.5 M NaOH and 1.0 M ethanol at room temperature. Sweep rate: 50 mV s−1.

all of these as demonstrated by the highest peak current density and much reduced onset potential (Figure 5 a, b) for ethanol oxidation. More explicitly, a considerably reduced oxidation over voltage, − 903 mV, is observed for the ternary catalyst compared to the binary (for Pd53Au47/C, −773 mV, and for Pd58Ni42/C, −689 mV) and single Pd (−519 mV), when the potential is recorded at a current output of 3 mA cm−2 as shown in Figure 5c. In addition to this, the polarization currents of the EOR on these catalysts are also derived at a scan rate of 5 mV s−1 (Figure 6a) and are found to lie in the following order: Pd41Au29Ni30/C ≫ Pd47Au53/C > > Pd58Ni42/C > Pd at peak potential. In order to figure out the polarization pattern of the electrode surface, Tafel plots were analyzed (Figure 6 b), and the slopes are obtained within 113−195 mV dec−1 at the lower

Figure 6. Potentiodynamic polarization plots for the electrochemical oxidation of 1.0 M ethanol in 0.5 M NaOH at a slow scan rate of 1 mV s−1. Inset: Tafel plots corresponding to each of the polarization curves of the respective electrodes. 25681

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Figure 7. Cyclic voltammograms for EOR (0.5 M NaOH and 1.0 M ethanol) on (a) Pd/C, Pd58Ni42/C, Pd53Au47/C, and Pd41Au29Ni30/C at 40 °C and (b) Pd41Au29Ni30/C at 20−80 °C.

potential regime below −200 mV. The slope value of 113 mV dec−1 is indicative of a quasi-steady-state kinetic phenomenon occurring at the ternary electrode interface. However, at higher potentials, the Tafel slopes increase indicating a change in the mechanism or at least a change in the predominance of certain reaction processes. The interface gets activated by the OH coverage26,38,39 at the Pd and Ni sites facilitating the dissociation of ethanol, followed by the required oxygen supply for CO oxidation by the surface oxides of Au particles. The concerted effort by the ad-atoms in the multimetallic systems therefore accounts for the catalytic superiority of the Pd41Au29Ni30/C toward the EOR. 3.4. Temperature Effect on the Electrode Kinetics. The temperature effect on the electrocatalysis is derived within the range 20−80 °C. Although the catalysts bear the similar kind of CV profiles, the electrode kinetic phenomenon is greatly influenced by temperature on the ternary system. Figure 7a shows the comparative study of Pd and the alloyed catalysts at 40 °C while the temperature effect on Pd41Au29Ni30/C is manifested in the voltammograms shown in Figure 7b. The ethanol oxidation current is significantly raised almost 7 times on Pd41Au29Ni30/C when the temperature increases from 20 to 80 °C whereas, in the case of Pd and the binary alloys (PdAu/ C, PdNi/C), the same rise in temperature can promote the current level to a maximum of 2.6 and 3 times, respectively. Further, the activation over potential is markedly reduced with the rise in temperature as indicated by the substantial lowering of onset potential for the ternary alloy. Thus, at elevated temperature, the dissociative adsorption of ethanol become increasingly favored on the alloyed matrix due to surplus of electrochemically active reaction centers created by the facilitated OH coverage of Pd sites in the matrix.40 On the other hand, the addition of Ni to Pd matrix triggers the concurrent promotion of oxidation kinetics due to the existence of NiO/Ni (OH)2 species, on the surface.11,23,41 Further kinetic input is put forward by the addition of Au in almost the same proportion as Ni to the Pd base metal. There is always a tendency of Au in alkaline medium to form surface oxides that enable oxidation of the intermediate carbonaceous residues formed during the ethanol oxidation. Therefore, in the ternary matrix where all the metallic components exits almost in the same proportions, the role of Au become concomitant with Pd and Ni for promoting the oxidation kinetics to a very high level. The apparent activation energies Ea are derived from the Arrhenius plots (see Figure S3 in the Supporting Information)

of the oxidation current densities exhibited by the different catalysts in the potential range of −350 to 150 mV and are presented in Figure 8. The Ea values follow the order

Figure 8. Variation in activation energy (Ea) with potential for the EOR on carbon-supported Pd and different binary and ternary catalysts.

Pd41Au29Ni30/C ≪ Pd53Au47 /C < Pd58Ni42/C < Pd/C. A drastic fall in the Ea is observed from the value of 10.5 for Pd to 4.1 for Pd41Au29Ni30/C corresponding to a potential of −0.05 V. Chronoamperometry was carried out in 0.5 M NaOH containing 1.0 M ethanol on Pd/C and Pd41Au29Ni30/C at different temperatures to determine the stability of the electrodes. The ethanol oxidation current was recorded at potential −300 mV over a period of 3600 s and shown in Figure 9a. A decrease in current density was recorded at the beginning of each experiment for both electrodes. The Pd/C electrode showed rapid current decay and very low initial and final current densities at all temperatures. However, the ternary alloy proved to be the most reliable one with respect to the current decay. Diffusion is due to gradient of concentration at the interface, and it may be considered that diffusion occurs across a region parallel to the interface of effective thickness, that is, across the Nernst diffusion layer.42 In a real situation, in the study of complicated reactions, where too many 25682

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linear plots of the Cottrell equation give the Dapp values of all the catalysts in the studied range of temperature (values are given in Table S2 of the Supporting Information). Figure 9b demonstrates the comparative Dapp values at 20 and 80 °C. The diffusion coefficient is found to rise with addition of Au in the matrix, and the highest value of Dapp for ethanol is obtained with the Pd41Au29Ni30/C catalyst in the same temperature regime. The ethanol oxidation process is therefore much accelerated on the alloyed catalysts Pd41Au29Ni30/C on account of facilitated diffusion of the reactant species, and on single Pd, the oxidation kinetics is extremely sluggish, since the accumulated reaction intermediates on the surface impede the rate of diffusion of the reactant species. The relative tolerance of the different catalysts toward the carbonaceous intermediates were further justified by the deviations of the Anson’s plot from linearity (Figure 9c) due to blocking of the surface,44,45 and the investigations also allow the determination of the amount of charge involved, Q (mC cm−2), in the process of oxidation. Figure 9c demonstrates the high level of charge produced on the ternary electrode at 80 °C, and even at 40 °C, the charge remains much higher than that produced by Pd at 80 °C. A bar diagram (Figure 9d) shows the typical comparative analysis of the charges for the different catalysts recorded at 34 min at 40 °C. Figure 9d also demonstrates the high level of charge produced on the Pd41Au29Ni30/C at 80 °C, and even at 40 °C, the charge remain much higher than that produced by Pd at 80 °C. 3.5. Impedance Response. The charge transfer kinetics of the EOR on different catalysts was also investigated by means of electrochemical impedance spectroscopy (EIS) at −300 mV, in the alkaline ethanol solution within the temperature range 20−80 °C. The Nyquist plots (Figure 10a, b) of the Pd electrode show a relatively regular capacitive loop in the high frequency region while the Pd41Au29Ni30/C electrode displays a depressed capacitive loop in the high frequency region and the complex plane extends to the fourth quadrant at the low frequency end. Figure 10c shows the effect of temperature on the charge transfer resistance with addition of Au and Ni in Pd for ethanol oxidation on the catalysts. Evidently, charge transfer resistance decreases with increase in temperature in all cases, and the minimum charge transfer resistance is observed with the Pd41Au29Ni30/C formulation. This is also congruent with the results obtained with the voltammetric studies. The impedance data were analyzed using the software available with the potentiostat (PG stat 12) resulting in the respective equivalent circuits where the inductance parameter is included only for the Au-containing catalysts; this has been shown in Figure 10d and also reported in our earlier paper.17 The circuit included several impedance parameters such as Rs, CPE, and Rct representative of the solution resistance, constant phase element, and charge transfer resistance associated with the electro-oxidation of ethanol on the catalysts in general. With respect to Pd, Rs values are reduced with the addition of the Ni and Au in the Pd matrix (the related EIS parameters for all the catalysts in the temperature regime can be obtained from Table S1 in the Supporting Information). The different adsorption behavior at the material−solution interface appears to be the governing factor for the difference in Rs values of the respective systems.46 In fact, the solution resistance (Rs) implies the ohmic resistance offered by the electrolyte placed between the working and reference electrodes in the cell. In our case, the electrolyte is the same (OH− ion) for all the experiment, but electrode materials are different. The formation of the double

Figure 9. (a) Chronoamperograms and dependence of transient 1

current (inset) on t− /2 of different Pt-free catalysts in a solution containing 0.5 M NaOH and 1 M ethanol at −300 mV (vs Hg/HgO) and at 40 °C. (b) Bar diagram of respective diffusion coefficient of all 1

the catalysts calculated from transient current on t− /2 at 20 and 80 °C. 1

(c) Charge density vs t /2 plots of Pd and Pd41Au29Ni30/C at 40 and 80 °C. (d) Columbic charges obtained during the electrolysis of alkaline ethanol solution at 34 min on the set of single, binary, and ternary catalysts.

intermediates are involved as well as there are possibilities of parallel reactions to occur, there is no defined layer of definite thickness. In effect directly or indirectly, concentration gradient is governed by the electrode geometry, kinematic viscosity, and diffusion coefficient, if the solution and the electrode remain static. The apparent diffusion coefficient values Dapp (cm2 s−1) are obtained from the Cottrell equation:43 1/2 I = nFADapp Cπ −1/2t −1/2

(1)

where I is the current corresponding to the electrochemical oxidation of ethanol under diffusion controlled condition and C is the bulk concentration (mol cm−3) of ethanol. The inset of Figure 9a represents typical plots at 40 °C. The slopes of the 25683

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effect is manifested in the relative Rs values for the different catalyst system. Moreover, the capacitance values are also gradually decreased with the addition of Ni and Au in the Pd matrix due to their crystalline structure and spatial arrangements of the molecules or atoms as well as their orientation that forms its surface structure.47,48 On the other hand, the adsorption of soluble species reactant molecule and the carbonaceous residues the double layer capacitance are deviated from its simple HP configuration. The OH− ad-layer, supported by Pd, surface oxide of Au, and the formation of NiO/NiOOH promote the oxidation kinetics and might lead to a lower capacitance value for the ternary system. Moreover, presence of Ni in the ternary matrix increases the possibility of NiO, changing the atomic arrangement of the system and decreasing the micropore volume resulting in the inaccessibility for the aqueous electrolyte. There is a significant fall in the Rct values with the addition of Au and Ni individually to Pd, forming the binary matrix and simultaneously forming the ternary matrix (the related EIS parameters for all the catalysts in the temperature regime can be obtained from Table S1 in the Supporting Information). For the Au containing binary and ternary catalyst in particular, the circuit also included the inductance (L) associated with the COads oxidation. This reflects the inductive behavior of the impedance spectrum at higher temperatures with the Au-containing catalysts. The decline of L values at temperatures beyond 40 °C, for the ternary system, reflects the successful oxidative removal of carbonaceous intermediates from the surface. It is considered that CO develops on the electrocatalyst as a strongly adsorbed intermediate and the electro-oxidation of COads to CO2 becomes the rate-determining step.49−51 The diminution of Rct and L values with temperature for Pd41Au29Ni30/C transpires to the overall promotion of the electrocatalysis at higher temperatures. 3.6. Oxidation Product Analysis through Ion-Exchange Chromatography. Ethanol oxidation in alkali medium is a multistep reaction involving a complex mechanism.2526 Several reaction intermediates are expected to be formed on the catalyst surface, and particularly on the Pd electrode, acetate is reported to be the final product. This is possibly due to the ability of Pd to adsorb ethoxy ion directly in an alkaline medium and release acetate as the main product. In a recent work,51 we proposed the stepwise mechanism of the oxidation process involving electron generation in each step on the ternary matrix composed of Pd, Au, and Pt where acetate and carbonate were found to be the ultimate products. Also, in this investigation, the oxidation products, acetate and carbonate formed on the catalyst surfaces, were estimated through ion chromatographic analysis of the chronoamperometric aliquots and verified under the influence of temperature. Figure 11a represents the typical features of ion chromatograms recorded at temperatures 20 and 80 °C, and the comparative yield of the two oxidation products over all the catalysts has been shown in Figure 11b. (The estimated amounts of acetate and carbonate at all temperatures can be found from Table S2 in the Supporting Information.) The massive production of CO32− on the Pd41Au29Ni30 surfaces is a unique feature of this study particularly at higher temperatures. When compared with Pd/ C, this ternary system produces 3−4 times increase in acetate concentration and 9−12 times increase in carbonate at the respective temperatures 20 and 80 °C. It is expected that synergic existence of Ni and Au in the Pd matrix conduces to the conversion of ethanol to the ultimate product carbonate in

Figure 10. Impedance spectra for the EOR (a) on different catalysts at 80 °C, (b) on Pd41Au29Ni30/C, and (b/) on Pd electrodes at different temperatures in a mixture of 0.5 M NaOH and 1.0 M ethanol at −300 mV (vs Hg/HgO). (c) Variation of charge transfer resistance of the EOR with temperature as obtained with the equivalent circuits. (d) Equivalent circuits for modeling the electrochemical impedance response at different electrodes.

layer (dl) and surface adsorption of materials might play role in affecting the Rs values of the respective system. For example, the Pd electrode exhibits the lowest surface area and has the highest Rs value compared to the others electrodes possibly due to the excessive OH coverage on its surface. On the other hand, with the incorporation of the second and third metal in Pd matrix, there is a decrease in Rs values in the respective cases and at the same time an increase of the surface area of the binary and ternary catalysts is noted. Once the oxidation process commences, the kinetic phenomenon will be controlled by the diffusion of ions to the surface. The faster the oxidative desorption of the intermediates, the more ions will diffuse to the surface from the bulk until the steady state is reached. This 25684

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and mutimetallic system.49,51−55 Lai et al. explained that the orientation of the ethanol adsorption, prescribes the oxidation path followed on Pt(111) planes.52 Bimetallics catalysts normally give higher currents with ethanol than their single counterparts,56,57 but unlikely, the selectivity is shifted predominantly toward the acetic acid product, that is, less efficient use of the ethanol fuel. On the basis of the ion chromatographic results and our previous study,51 Scheme 1 represents the proposed EOR mechanism. To accomplish the complete oxidation of ethanol, the initial adsorption through its carbon atom has to follow two different pathways, both of which have the potential of forming the crucial π-bonded species on the catalysts surface enabling effective formation of acetate and carbonate. Moreover, the charge transfer in between the tightly coupled metallites could be a reason for the better catalytic activity of ternary nanoparticles. Eckern et al. has shown on the basis of single electron tunneling and microscopic theory that charge transfer occur between the weakly coupled metals.58 Recently, Guo et al. has also reported similar work with PdAu bimetallic nanoparticles having unalloyed nanostructure with synergistic charge transfer contribution to each other process leading to catalytic activity.59 On the basis of the above concept, the possible electronic charge transfer effects in the binary and ternary catalysts have been shown in Scheme 2. There is only one way electronic charge transfer, that is, from Pd atoms to Au atoms in PdAu alloy and Ni to Pd in PdNi alloy, since the ionization energy of Pd is smaller than that of Au and higher than Ni. The Au atoms become negatively charged in the PdAu alloy due to the electron donation from Pd atoms, and at the same time, Pd become negatively charged in the PdNi alloy, since the direction of electron transition from Ni toward Pd sites is facilitated. In the ensemble of alloyed particles of the ternary catalyst, the relatively less electronegative Ni possibly transfers charges to the other metals, which in effect can strengthen the oxygen supply of Au to accomplish the complete oxidation of ethanol ultimately prohibiting the building up of COads on this catalyst surface. In fact, theoretical computations of density functional theory parameters on the basis of optimized geometry of the multimetallic system and the charge transfer between the ad-atoms are underway in our laboratory. 3.7. Fuel Cell Performance Study. Figure 12 represents the polarization and power−density plots of the fabricated DE(AEM)FCs operating in the solution containing 1.0 M ethanol and in 0.5 M NaOH at 40 °C. In all respect, the highest performance was shown by the Pd41Ni29Au30/C anode generating an open-circuit potential (OCP) of 0.88 V and a maximum power density output of 51 mW cm−2. On an average, the OCP value for the ternary system is higher by 0.10 V and 0.17 V over the binary systems PdAu/C and PdNi/C, respectively, and by 0.26 V over the single metal. The power density generated for the ternary system is almost 42% and 108% greater than the respective binary catalysts PdAu/C and PdNi/C and 175% greater than the single metal catalyst. For the ternary system, the discharge curve extends over a longer ohmic region. This reflects a dramatic control of the poisoning effect of the ternary system with respect to the other catalysts.

Figure 11. (a) Typical ion chromatograms for acetate and carbonate produced by electro-oxidation of ethanol in a mixture of 0.5 M NaOH and 1.0 M ethanol on Pd/C, Pd58Ni42/C, Pd53Au47/C, and Pd41Au29Ni30/C electrodes at −300 mV (vs Hg/HgO) and at 20 and 80 °C. (b) Yield of acetate and carbonate estimated by ion chromatography during the electro-oxidation of ethanol on different Pt-free catalysts at 20 and 80 °C.

4. CONCLUSION In this work, mesoporous carbon supported binary and ternary nanocatalyst particles with equal concentration of Pd, Au, and Ni have been successfully synthesized by the simultaneous borohydride reduction method. The trimetallic nanoparticles

alkaline media. Several research groups have studied the mechanism of the ethanol oxidation process on the single 25685

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Scheme 1. Proposed Reaction Mechanism of Ethanol Electro-oxidation on the Ternary Catalyst Surface Ameliorated with (OH)ads in Alkaline Medium

Scheme 2. Schematic Representation of the Charge Distribution in the Pd47Au63/C, Pd58Ni42/C, and Pd41Au29Ni30/C Catalysts and the Ethanol Oxidation Reaction on the Pt-Free Ternary Catalyst Surface

have a tightly coupled structure, characterized by XRD, TEM, and EDX. The electrochemical techniques reveal that introduction of Au and Ni into Pd drastically increases the peak current density of the EOR with significant negative shift of onset potential. The oxidation current decay with temperature reflecting electro-catalytic stability is also minimal for the trimetallic system compared to the other catalysts, indicating superior CO tolerance for the oxidation reaction. The

temperature study of the product analysis by ion chromatography also indicates significant carbonate selectivity of the ternary catalyst in alkaline media. The performance screening of the catalyst materials in DE(AEM)FC demonstrates high power density and higher stability of PdAuNi/C catalyst even at low electrolyte concentrations. Finally, it may be concluded that the synergistic combination of Pd, Au, and Ni nano particles forming tightly coupled configuration of the PdAuNi/C system 25686

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Figure 12. Polarization and power−density curves of the DE (AEM)FC with Pd and different binary and ternary anode catalysts (anode: 0.5 M NaOH + 1.0 M ethanol).

can be regarded as a powerful electrocatalyst in propagating the EOR toward completion in alkaline media.

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ASSOCIATED CONTENT

S Supporting Information *

Additional data of surface morphology and electrochemistry of the synthesized nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: jayati_datta@rediffmail.com. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.D. wishes to acknowledge Senior Research Fellowship (SRF) from the Council of Scientific and Industrial Research (CSIR), New Delhi, India.

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