Enhanced Electrocatalytic Activity of Pd-Dispersed 3,4

J. Phys. Chem. C 2010, 114, 8507–8514

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Enhanced Electrocatalytic Activity of Pd-Dispersed 3,4-Polyethylenedioxythiophene Film in Hydrogen Evolution and Ethanol Electro-oxidation Reactions Rakesh K. Pandey and V. Lakshminarayanan* Raman Research Institute, C.V. Raman AVenue, Bangalore, 560080, India ReceiVed: February 17, 2010; ReVised Manuscript ReceiVed: April 2, 2010

We report in this study that a thin film of Pd nanoparticles dispersed in 3,4-polyethylenedioxythiophene (PEDOT) shows remarkable electrocatalytic activity toward hydrogen evolution reaction in acid medium and ethanol electro-oxidation reaction in alkaline medium. The method of preparation of Pd-PEDOT nanocomposite film involves the formation of Pd nanoparticles by the dissolution of Pd anode and simultaneous oxidative polymerization of 3,4-ethylenedioxythiophene followed by deposition as a thin film on the gold substrate. The highly enhanced catalytic activity of the nanocomposite film with a relatively low Pd concentration is attributed to the higher catalytic activity of nanosized Pd on gold substrate and the electrically conducting path provided by the PEDOT matrix. 1. Introduction 3,4-Polyethylenedioxythiophene (PEDOT) is one of the most studied conducting polymers. The very stable and highly conductive cationic-doped state and the low HOMO-LUMO band gap of conductive PEDOT make it a useful component of many commercial devices such as antistatic films, transparent conductors, organic light-emitting diodes, thin film transistors, and electrochromic windows.1–9 PEDOT can be prepared either by chemical or electrochemical polymerization. It is known that protic acids and a variety of Lewis acids can be used to catalyze an equilibrium reaction of 3,4-ethylenedioxythiophene (EDOT) to yield dimeric and trimeric compounds.10 However, mineral acids such as sulfuric acid, hydrochloric acid, and other strong acids are commonly used for the electrochemical synthesis of PEDOT.10 Palladium nanostructures and its nanocomposites are intensively studied because of their importance as catalytic material,11–14 hydrogen storage material,15–18 and sensors.19,20 They also have the potential to emerge as an effective alternative to Pt for electro-oxidation of small organic molecules such as methanol and ethanol in direct fuel cells. It is therefore important to prepare porous Pd nanostructures on a suitable conducting matrix to exploit the high surface area-to-volume ratio and enhanced catalytic activity. Therefore, a method of preparation of Pd nanostructures that is either solution-based or coated on suitable substrates can be of considerable interest among various research groups.21–37 Carbon-based supports are generally used for dispersing the nanoparticles and to achieve very high catalytic activity. In this respect conducting polymers can be an effective substitute to the carbon supports because of their lower ohmic drop across the electrode and the ease with which they can be functionalized with biomolecules for applications as sensors. We demonstrate in this paper excellent electrocatalytic activity of Pd nanoparticle- dispersed PEDOT film on gold surface for ethanol electro-oxidation reaction in alkaline medium and H2 evolution reaction in acidic medium. The method of preparation of Pd-PEDOT film is akin to our recent report of the Pd-PANI * To whom correspondence should be addressed. Telephone: +91-8023610122. Fax: +91-80-23610492. E-mail: [email protected].

nanocomposite film on gold substrate.38 Similarly, Li et al. have developed a method of electrophoretically depositing PANI colloids on solid surfaces.39 Whereas the reported methods in the literature generally involve multiple steps, the present method has the benefit of having just a single step to simultaneously deposit Pd nanoparticles and PEDOT to form a stable nanocomposite thin film. The method consists of in situ electrochemical generation of Pd ions in the solution by dissolving a Pd wire anode which are then reduced by EDOT present in the solution, resulting in its oxidative polymerization to PEDOT. The process is followed by the electrophoretic deposition of Pd-PEDOT nanocomposite film on the cathode. Because all the above-mentioned processes occur sequentially, the single-step method leads to a well-adhering deposit of the nanocomposite. The nanocomposite film-modified gold surface was characterized using scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDAX), inductively coupled plasma-mass spectroscopy (ICP-MS), and atomic force microscopy (AFM). Recently Mao et al. have prepared Au/PEDOT/Fe nanomaterial in aqueous solution by using a surfactant and a polymer which showed an enhanced electrocatalytic activity toward ascorbic acid oxidation.40 Kumar et al. have prepared gold nanoparticles stabilized by PEDOT using polystyrene sulfonate as a dopant in the aqueous phase.41 Vercelli et al. have studied the formation and electrocatalytic activity of mono and multilayers of Pt nanoparticles and PEDOT.42 They have observed that the catalytic activity of the composite film was comparable to that of a bare Pt substrate. Recently Harish et al. have deposited Pd from PdCl2 solution, on initially formed PEDOT film and studied the response of the film toward the dopamine and uric acid oxidation.43 Reetz and co-workers have described a method of producing size-selective Pd and Ni nanoclusters by using the respective metals as sacrificial anodes.44 In this method, for example, the dissolving Pd ions migrate toward the cathode, which are then electrochemically reduced on the surface as Pd atom clusters and subsequently stabilized in the solution by tetraalkylammonium cations. We have recently shown that, on one hand, the Pd-PANI nanofiber film on gold electrode exhibits very good electrocatalytic activity toward electro-oxidation of formic acid in acid

10.1021/jp1014687  2010 American Chemical Society Published on Web 04/20/2010

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medium and ethanol and methanol in alkaline medium.38 On the other hand, the Pd-PEDOT nanocomposite film described in the present work shows excellent catalytic activity toward hydrogen evolution reaction in acid medium and very much enhanced catalytic activity for ethanol electro-oxidation in alkaline medium, making it a credible candidate for fuel cells and other similar applications. 2. Experimental Section 2.1. Chemicals. All the chemical reagents used in this study were analytical-grade reagents. EDOT (Aldrich), ethanol (Merck), sulfuric acid (Merck), hydrochloric acid (Nice Chemicals), and sodium dodecyl sulfate (SDS) (S.D. Fine Chemicals) were used in our study. Millipore water having a resistivity of 18.2 MΩ-cm was used to prepare the aqueous solutions. 2.2. Electrochemical Setup, Substrates Preparation, and Surface Characterization Techniques. We have carried out the electrochemical characterization studies by coating PdPEDOT film on a gold disk working electrode, which was constructed by sealing a 99.99% pure gold wire (Arora Mathey) of 0.5-mm diameter with soda lime glass. The resulting electrode has a geometric area of 0.002 cm2. We have used Pd-PEDOT film deposited on a larger area gold substrate (deposited on glass by vacuum evaporation with chromium underlayer) for SEM, EDAX, and AFM studies. For galvanostatic deposition of nanocomposite film, a 0.5-mm-thick Pd wire of 99.9% purity (Advent, UK) was used as an anode and a gold electrode as the cathode. A conventional three-electrode electrochemical cell was used for hydrogen evolution and ethanol electro-oxidation studies. A platinum foil of large surface area was used as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. The cell was cleaned thoroughly before each experiment and kept in a hot air oven at 100 °C for at least 1 h before the start of the experiment. The chronoamperometry measurements were carried out by the application of a potential pulse of 10-s duration with a step size of 10 mV. Prior to the application of each potential step, the electrode surface was subjected to a potential program of 5 s each at 0.8 and -0.65 V vs SCE. Application of the positive potential ensures the removal of surface poisons and the subsequent negative potential reduces the surface oxides formed during the application of the positive potential. This applied potential program acts as a pretreatment for the surface, exposing a fresh surface for chronoamperometry studies. This method is an adaptation of the pretreatment technique reported by Herrero et al.45 The current data obtained at the end of 10 s of the potential pulse are used for the polarization plots and measurement of the Tafel slopes. Galvanostatic deposition was carried out using an EG&G potentiostat (model 263A) in constant current mode and interfaced to a PC through a GPIB card (National Instruments). The electrolyte temperature was varied using a temperature controller (JULABO Model F25). The electrochemical impedance spectroscopy studies were carried out using the EG&G lock-in amplifier (model 5210). SEM and EDAX studies were carried out using a field emission scanning electron microscope (FE-SEM, Zeiss). AFM studies were carried out using Pico Plus (Molecular Imaging) AFM in ac (tapping) mode with an n-doped silicon tip. The images obtained were raw images, which were plane-corrected using the scanning probe image processor software (Image Metrology, Denmark). 2.3. Synthesis of the Pd-PEDOT Nanocomposite on Gold Substrate. The electrolyte was prepared by dissolving 5 mg of EDOT and 0.01 M SDS in 5 mL of 0.1 M HCl. SDS was added

Pandey and Lakshminarayanan to the electrolyte solution to increase the solubility of EDOT monomer in the electrolyte. The electrostatic interaction between SDS anion and EDOT radical cation yields a strongly bonded complex between these ions.46,47 This also produces an ordered film of PEDOT on gold in the presence of SDS in aqueous medium. The present method consists of in situ electrochemical generation of Pd ions from a Pd wire anode at high current densities. The dissolved Pd ions immediately form chloropalladate complex with chloride ions present in the solution.38,48 The chloropalladate being an oxidizing agent can initiate the polymerization of EDOT, which in turn gets reduced to Pd nanoparticles. These Pd nanoparticles adhere on the PEDOT and form Pd-PEDOT nanocomposite. A similar process was reported earlier for the preparation of polyaniline-coated Au nanostructures by using chloroauric acid, a powerful oxidizing agent.49 The Pd-PEDOT nanocomposite, owing to the positively charged thiophene moieties in it,10 electrophoretically deposits on the gold substrate as a thin film, a process similar to that of the electrophoretic deposition of polyaniline colloids on gold.39 The electrolyte was continuously stirred using a magnetic stirrer during the experiment. A Pd wire of 5-mm length and 0.5-mm diameter with an area of 0.1 cm2 dipped in the electrolyte served as the anode and working electrode whereas a gold disk electrode of 0.002-cm2 geometric area acted as the cathode and counter electrode. We carried out the deposition in galvanostatic mode with different current values for an hour to study the effect of the current on the nature of coating. During the electrolysis there was an intense evolution of gases at both the electrodes because of water electrolysis. (Caution! The cell should haVe gas outlets near both the electrodes!) The solution, which was colorless in the beginning, turned light blue initially and then turned brown at the end. Simultaneously, a darkcolored thin film is formed on the cathode surface. At higher currents (60 and 70 mA) the gas evolution is rapid and the dissolution of the metal is not uniform. Hence, for all the characterization studies of the film, the specimen was deposited at a controlled anodic current of 50 mA corresponding to a current density of 0.5 A cm-2. The Pd content of the nanocomposite was evaluated using ICP-MS by dissolving the nanocomposite film in aquaregia (1:3 concentrated HNO3:concentrated HCl). The film was found to have a Pd content of 0.31 µg mm-2 of the film. 3. Results and Discussion 3.1. SEM, EDAX, and AFM Analysis of the Pd-PEDOTCoated Surface. Figure 1 shows the SEM image of the Pd-PEDOT-coated gold surface containing large clusters about 200 nm in size spread throughout the surface. There are also smaller particles on those clusters of size around 10-15 nm. Table 1 presents the results of the elemental analysis obtained from the EDAX on the Pd-PEDOT-modified surface. The weight percent of Pd is 2.68, which is quite small compared to a much larger amount of carbon (13.38%) arising from the PEDOT film. The elemental analysis also reveals the presence of S, Na, and O on the surface. The very large amount of gold (82.20 wt %) is contributed by the gold substrate, indicating the porous nature of the film. From the EDAX data, the amount of Pd present in the nanocomposite film (excluding the concentration of Au from the substrate) is calculated to be about 15%. We have carried out AFM imaging of the nanocomposite thin film to look into the three-dimensional profile of the film. Figure

Electrocatalytic Activity of Pd-PEDOT Film

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Figure 1. FE-SEM image of Pd-PEDOT nanocomposite film.

TABLE 1: EDAX Elemental Analysis Results for Pd-PEDOT Nanocomposite Film element

wt %

at. %

C O Na S Pd Au

13.40 0.92 0.50 0.34 2.68 82.20

67.71 3.50 1.27 0.64 1.53 25.36

2 shows the 5 µm × 5 µm AFM image of the Pd-PEDOTcoated surface. The features observed in the AFM image show good similarity to those seen in the SEM image. As shown in the figure, there are disklike clusters on the surface (indicated as circles) and attached to them are smaller clusters (shown by arrows). We have carried out AFM imaging in several different regions on the surface and observed similar features. The average height of the features observed in the AFM image is

16 nm with an rms roughness of 9 nm whereas the rms roughness of the unmodified gold surface was 4.5 nm, showing that the surface roughness of the coated film is not significantly higher than that of the bare substrate. 3.2. Voltammetric Response of the Pd-PEDOT-Coated Electrode in Acid Medium, Catalytic Activity toward Hydrogen Evolution Reaction. Palladium is well-known to absorb massive quantities of hydrogen in bulk to form hydrides and therefore has good technological potential as a hydrogen storage material.50 We have carried out cyclic voltammetric studies in acidic medium to study the hydrogen evolution reaction (HER) on Pd nanostructure enclosed in a PEDOT matrix. Voltammograms (a) and (b) of Figure 3A show respectively the voltammetric behavior of the Pd disk electrode and Pd-PEDOT-coated Au disk electrode in 0.5 M H2SO4 at a scan rate of 100 mV s-1. Figure 3A(b) shows the cyclic voltammogram for Pd-PEDOT nanocomposite-coated gold disk electrode in 0.5 M H2SO4 at the same scan rate. It is clear from the figures that there is a huge increase in the hydrogen evolution current for Pd-PEDOT nanocomposite film. The measured onset potential (-0.28 V) is also significantly lower than -0.50 V observed for Pd disk electrode. The large hydrogen evolution current is truly catalytic and not due to an increase in surface area of the Pd nanoparticles. This is clear when the hydrogen evolution currents are compared for unit area of the respective surfaces. For example, at a potential of -0.45 V, the currents per unit area in the case of Pd electrode and Pd-PEDOT electrodes are 7.0 and 230 mA cm-2, respectively, an enhancement by a factor of about 33. In all the cases the currents are normalized with respective true surface area or effective catalytic surface area (ECSA), which was calculated by measuring the charge under the palladium oxides reduction peak as described later in this section. According to the Heyrovsky-Volmer mechanism, the HER follows the following reaction scheme for the Pt group metals.51

H+ + e- f Hw

(1)

H+ + Hw + e- f H2

(2)

In the above reactions Hw denotes the weakly absorbed hydrogen into the bulk of the metal. In the case of Pd, as it absorbs hydrogen, a penetration reaction operates as follows:

PdsHs + Pdb f Pds + PdbHb

Figure 2. 5 µm × 5 µm AFM image Pd-PEDOT nanocomposite film. The circles and arrows show the different sizes of the clusters. The inset shows the line profile.

(3)

In the above reaction Pds, Pdb and Hs, Hb represent the surface and bulk states of Pd and hydrogen, respectively. The absorbed hydrogen atoms later diffuse to the electrode surface where electro-oxidation reaction occurs.52,53 We have carried out electrochemical impedance spectroscopy (EIS) at different potentials between -300 and -400 mV in 0.5 M H2SO4 for the Pd-PEDOT nanocomposite film. Figure 3B shows the plots of total impedance vs frequency at different potentials: (a) -300 mV, (b) -320 mV, (c) -340 mV, (d) -360 mV, (e) -380 mV, and (f) -400 mV. The corresponding values of total impedance measured at the lowest frequency of 0.1 Hz are 10, 5.6, 3.2, 1.8, 1.1, and 0.7 kΩ, respectively. This shows that the total impedance decreases systematically with the applied cathodic potential, thereby confirming the facile nature of the electron-transfer process. For comparison, the EIS

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Figure 3. (A) (a) The cyclic voltammogram of the Pd disk electrode in 0.5 M H2SO4 at a scan rate of 100 mV s-1 and (b) the cyclic voltammogram for Pd-PEDOT nanocomposite-coated gold disk electrode in 0.5 M H2SO4 at the same scan rate. (B) The total impedance plots for the Pd-PEDOT nanocomposite film in 0.5 M H2SO4 at different potentials (from (a) ) -300 mV to (f) ) -400 mV).

Figure 4. Total impedance plots for the Pd disk electrode in 0.5 M H2SO4 at different potentials (from (a) ) -300 mV to (f) ) -400 mV).

measurements carried out in 0.5 M H2SO4 for Pd disk electrode in the same conditions exhibit very much higher impedance toward the HER. For example, the total impedance at 0.1 Hz changes from 63 to 16 kΩ when the cathodic potentials are shifted from -300 to -400 mV (Figure 4). We have fitted the EIS data to an equivalent circuit that is appropriate for porous films formed by layer-by-layer assembly.54 The data obtained are presented in the Supporting Information. In addition to the charge-transfer resistance (RCT) and double-layer capacitance (Cdl), the contributions from film capacitance (CF), film resistance (RF), and Warburg impedance (Zw) are also considered. The diffusion of H+ ions to the Pd nanoclusters embedded in the polymer matrix results in the Warburg impedance in the equivalent circuit. The value of RCT and RF decreases with decreasing potential, that is, from -300 to -400 mV, which shows the increasingly facile nature of the reaction at higher cathodic potentials. The Warburg admittance (inverse of the Warburg impedance) increases with the potential, which shows the enhanced diffusion of the reactants at more negative potentials. The constant phase elements obtained by equivalent circuit fitting is analogous to the film capacitance (CF) and double-layer capacitance (Cdl) with n values 0.9 and 0.8, respectively. The variation in capacitance values with potential, however, is not very significant. Figure 5 shows the voltammetric behavior of the Pd-PEDOT-coated electrode in 0.5 M H2SO4 at a scan range of -0.3 to 1.2 V and a scan rate of 100 mV s-1. The inset shows the voltammetric behavior of the Pd disk electrode in the same electrolyte.

Figure 5. Voltammetric behavior of the Pd-PEDOT-coated disk electrode in 0.5 M H2SO4 (scan rate ) 100 mV s-1) showing the presence of two well-separated peaks for the oxidation of adsorbed hydrogen and absorbed hydrogen. Inset shows the voltammogram for Pd disk electrode in 0.5 M H2SO4.

On one hand, the CV for the Pd disk in 0.5 M H2SO4 shows a single broad anodic peak around 0.00 V. This broad hump is due to the oxidation of both the absorbed and adsorbed hydrogen on the Pd disk electrode, as reported earlier for bulk Pd electrodes.22 On the other hand, the CV for the Pd-PEDOTcoated disk electrode shows two peaks, one of which is a small shoulder peak in the negative potential region during the anodic sweep. The first peak at -0.13 V corresponds to the oxidation of the adsorbed hydrogen (Hads) whereas the subsequent peak at 0.0 V is due to the oxidation of the absorbed hydrogen (Habs) on the Pd surface.15 It is known from the literature that two well-resolved peaks for the oxidation of adsorbed and absorbed hydrogen in Pd are manifested exclusively in the case of Pd nanoparticles.15,22 This behavior is attributed to the larger number of surface sites available for adsorption on nanoparticles surface. The CV results support the above observation by exhibiting the characteristic behavior of the Pd nanoparticles on the electrode surface. In the case of Pd, the absorption of hydrogen takes place along with its adsorption and therefore the area calculated by measuring the charge under the Hads peak does not reflect the true surface area of the Pd in the nanocomposite.36 However, the electroactive surface area or the ECSA of Pd can be measured from the palladium oxide stripping analysis.22,36 The charge value under the stripping peak is measured to be 0.7 µC and with the conversion factor of 424 µC cm-2 for Pd oxides stripping current,22 the ECSA for the Pd-PEDOT film is


Figure 6. Cyclic voltammogram for the electro-oxidation of 1 M ethanol in 0.5 M NaOH for (a) Pd-PEDOT nanocomposite and (b) Pd disk electrode. (insets) (i-1) CV for ethanol oxidation on PEDOT film surface without Pd. (i-2) Zoomed portion of the CV for ethanol oxidation on Pd disk electrode.

measured to be 1.65 × 10-3 cm2. Even though the effective surface area value is relatively low for the electrode, it nonetheless exhibits a very high hydrogen evolution activity. This observation also conforms to the very low Pd content measured using EDAX and ICP-MS, which show a value of 15 wt % and 0.31 µg mm-2 Pd content in the nanocomposite film, respectively. 3.3. Catalytic Activity toward Electro-oxidation of Ethanol. 3.3.1. Cyclic Voltammetry in Alkaline Medium. Ethanol is being increasingly viewed as a better and safer alternative to methanol, in direct fuel cells, because of its lower toxicity, higher energy density, and easy availability through renewable and green chemical sources. Since the Pd nanoparticles are not directly deposited on the electrode surface and actually embedded within the polymeric matrix, the available effective active centers are higher, which makes it an ideal electrocatalytic material. We have evaluated the electrocatalytic activity of Pd-PEDOT nanocomposite-coated electrode by studying ethanol electro-oxidation as a model system in 0.5 M NaOH. The electrocatalytic oxidation of small alcohols has been intensively studied as it is the anodic reaction in direct alcohol fuel cells. Figure 6a shows the ethanol electro-oxidation voltammogram for the Pd-PEDOT-modified electrode in 1 M ethanol in 0.5 M NaOH. The onset potential for the ethanol electro-oxidation, at about 3% of the peak current, was found to be around -0.56 V and the maximum current appears at -0.21 V of 137 mA cm-2. The onset potential for the electro-oxidation observed in this work is comparable to the values reported in the literature.55,56 This current is higher than the reported values for the ethanol electro-oxidation in alkaline medium.55,56 During the reverse scan there is a secondary peak that starts at -0.18 V with the current reaching the peak at -0.27 V. The anodic current during the reverse potential scan arises due to the fact that the reduction of palladium oxides formed during the forward scan creates a partially oxide-free surface and the electro-oxidation of ethanol takes place on the freshly created Pd atoms sites. This reverse anodic peak behavior can be explained as being due to the fact that the Pd atoms are not in equilibrium with the metallic lattice after the reduction of blocking oxide film and therefore the highenergy sites possess excellent catalytic activity.25,32 The inset (i-1) in Figure 6 shows the CV for ethanol electro-oxidation on PEDOT film without Pd. It can be seen that there is no peak corresponding to the electro-oxidation of ethanol. The voltammogram for ethanol electro-oxidation conducted in 1 M ethanol in 0.5 M NaOH using a Pd disk electrode is also shown in Figure


Figure 7. The 1st and 200th cyclic voltammograms for Pd-PEDOT nanocomposite-coated Au disk electrode in 1 M ethanol in 0.5 M NaOH. Scan rate: 100 mV s-1.

6b. The peak potential in this case occurs at -0.2 V and the corresponding peak current density is 0.65 mA cm-2 (inset i-2). This can be compared to a current of about 137 mA cm-2 in the case of the Pd-PEDOT electrode. The current values are very high for a Pd loading of 0.31 µg mm-2. From these observations it is obvious that the huge enhancement in ethanol oxidation current on Pd-PEDOT electrode is brought about by the electrocatalytic effect of nanoparticles of Pd just as in the case of the hydrogen evolution reaction discussed earlier. The established mechanism for the electro-oxidation of ethanol in alkaline medium involves the oxidation of ethanol to acetate ions at the anode. The reaction can be summarized as follows:21,57

Pd + OH- f Pd-OHad + e-

(1a)

Pd-(C2H5OH)ads + 3OH- f Pd-(CH3CO)ads + 3H2O + 3e- (2a) Pd-(CH3CO)ads + Pd-OHads f Pd-CH3COOH + Pd

(rds) (3a)

The third reaction, which is the removal of the adsorbed acyl groups by the adsorbed hydroxyl groups, is the rate-limiting step.21,57 This shows that an appropriate amount of the adsorbed hydroxyl groups is required to achieve a high electro-oxidation current. In an independent experiment carried out on Pd-PEDOT nanocomposite film on Pd disk electrode (in the absence of Au), synthesized in a similar way, does not show significant catalytic activity toward ethanol electro-oxidation reaction. This probably indicates the role of gold surface in the catalysis. Because the nanocomposite film is thin and porous, we cannot completely neglect the effect of substrate (Au) on the catalytic activity, as it is known from the literature that Au and Pd alloy is a good catalytic material for ethanol electro-oxidation.58,59 The adsorption of OH- ions on gold can help in removing the poisonous CO in the form of CO2.58 To check the long-term stability of the Pd-PEDOT electrode toward the electro-oxidation of ethanol, we conducted the CV for 200 cycles. We observed very little decline (from 137 to 132 mA cm-2) in the electro-oxidation current of ethanol as can be seen in Figure 7. This shows that there is negligible

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Figure 8. Arrhenius plots (log I vs 1/T) for ethanol oxidation on Pd-PEDOT electrode at different potentials, near the foot of the cyclic voltammogram (-500 to -300 mV). The potentials and corresponding activation energies associated with each potential are labeled in the figure.

poisoning of the catalytic surface due to any of the reaction products of ethanol oxidation. This is an important advantage of Pd-PEDOT nanocomposite electrode as the electrode poisoning is a major problem in electro-oxidation reactions of alcohols. 3.3.2. Kinetics of Ethanol Electro-oxidation: ActiWation Energy (Ea) Determination and Tafel Plot Analysis for Ethanol Electro-oxidation. The Arrhenius plots (log I vs 1/T) for ethanol oxidation on Pd-PEDOT electrode at different potentials, near the foot of the cyclic voltammogram (-500 to -300 mV), are shown in Figure 8. A good linear relationship can be seen from those plots. This indicates that the essential mechanism of ethanol electro-oxidation remains the same at all the temperatures. The apparent activation energies were calculated from the slope of the curves (slope ) -Ea/2.3R), R being the gas constant. The activation energy values have been labeled adjacent to their respective plots. The average Ea value for the potential range studied is 36.7 kJ mol-1. The values obtained of Ea are comparable to the values observed for Pt-, Pd-, and Au-based56,60,61 catalytic materials and indicate the facile nature of ethanol electro-oxidation on Pd-PEDOT nanocomposite film. There may be several reasons for this low activation energy, for example, small size of the Pd nanoparticles, the presence of porous conducting polymer network, the role of underlying gold substrate, and effective removal of poisoning intermediates.57 Figure 9 shows the cyclic voltammograms for ethanol oxidation at different temperatures. It is clear from the figure that the ethanol electrocatalysis currents have increased with the temperature and also the onset potentials have shifted to the more negative values. This phenomenon can be attributed to the lower activation energy required at higher temperatures. This can be understood because of the fact that the increase in the temperature accelerates the adsorption of OH- ions to form OHads and the presence of OHads is in fact helpful in achieving higher electro-oxidation current and also in suppressing the formation of poisoning species.57,62–66 The low onset potential values at higher temperatures are probably related to the enhanced adsorption of hydroxyl ions. Consequently, the electrooxidation currents increase at higher temperatures. We find that the anodic current of reverse potential scan also increases with temperature. This current eventually attains a value higher than the anodic current of the forward potential scan.

Pandey and Lakshminarayanan

Figure 9. Cyclic voltammograms for electro-oxidation of 1 M ethanol in 0.5 M NaOH at different temperatures, scan rate ) 100 mV s-1.

Figure 10a-f shows the current-potential polarization plots for the ethanol oxidation observed at different temperatures: (a) 7, (b) 15, (c) 25, (d) 35, (e) 45, and (f) 55 °C. The plots were obtained by carrying out chronoamperometry experiment at the foot of the cyclic voltammogram, a region of interest for any realistic application of a fuel cell. The measured slope values as a function of temperature are given in Table 2. One can see from the plots that there are two slopes for all the temperatures studied. The values of the first slope (at lower overpotentials) are lower compared to those of the second slopes (at higher overpotentials). This shows that as the reaction progresses, the impurities become adsorbed on the surface, which can then be removed only at very high anodic overpotentials. The first slope value is low at lower temperatures and increases initially with the temperature and reaches a steady value at elevated temperatures. The value of the second slope, however, is smaller at lower temperatures but increases steadily with the temperature. The higher value of slopes at higher temperatures indicates that the large amounts of carbonaceous species generated during the oxidation of the ethanol adsorb at higher temperatures. The difference between the two slope values is particularly greater at lower temperatures, that is, 5, 15, and 25 °C, the first slope being very low. At elevated temperatures, that is, 35, 45, and 55 °C, the two slopes are not very distinct and the plot tends to acquire some sort of linearity. The Tafel slopes, which are the slopes of the polarization plots, generally provide significant insight into the reaction mechanism. However, in the case of the alcohol oxidation reaction, additional complications arise in the interpretation of the Tafel slopes because of the adsorption of the reaction intermediates causing surface poisoning. Because of this, the straight line between the overpotential and log I are not generally observed.66 Nevertheless, the Tafel slopes can act as reliable indicators for understanding the reaction process and its temperature dependence. In the present case, the value of the first Tafel slope (obtained at low overpotentials) increases initially with the temperature and then reaches a constant value of around 120 at higher temperatures. The values measured in this work are similar to those reported by Liang et al.65 We have also studied the effect of ethanol concentration on the reaction rate. The reaction order was calculated by using the following equations:56

I ) nFkcm



Figure 10. Tafel plots (overpotential (η vs log I) at different temperatures and the linear fittings showing two slopes in the range.

TABLE 2: Tafel Slope Values at Different Temperatures near the Foot of the Voltammogram for Pd-PEDOT Nanocomposite Film temperature (°C)

1st slope (mV dec-1)

2nd slope (mV dec-1)

7 15 25 35 45 55

64.5 82.9 106.2 117.2 122.0 115.7

139.5 150.0 167.5 170.2 173.6 174.5

log I ) log nFk + mlog c where F is Faraday’s constant, k is the reaction constant, c is the ethanol concentration, and m is the reaction order with respect to ethanol. The reaction order is obtained from log I vs log c as shown in Figure 11. The slope of the plot at constant temperature gives the apparent reaction order (m) of ethanol electro-oxidation reaction with respect to the ethanol concentration.56 We have calculated the order at two different potentials: -0.4 and -0.25 V. The values of the measured reaction orders are 1.28 and 1.4 at -0.4 and -0.25 V, respectively, which show that there is no

significant change in the reaction order with potentials and the reaction mechanism for ethanol oxidation essentially remains the same at different potentials. Conclusion We have proposed a single-step electrochemical method of synthesizing Pd-PEDOT nanocomposite films on a gold surface by the galvanostatic dissolution of Pd wire in the chloridecontaining acidic EDOT solution. A thin film of nanocomposite containing Pd nanoparticles embedded in the polymer matrix was formed on the surface. From the EDAX, ICP-MS, and Pd oxides stripping peak in cyclic voltammetry, it was found that the Pd loading in the nanocomposite is quite low. Nevertheless, the nanocomposite film exhibits very good stability and proves to be an effective electrocatalyst for the hydrogen evolution and the ethanol electro-oxidation. The activation energy calculations and Tafel plot analysis at different temperatures further confirm the excellent catalytic activity of the nanocomposite film on gold surface. Acknowledgment. The authors thank A. Dhason and V.K. William Grips, National Aerospace Laboratory (NAL), for their help in the characterization studies. Supporting Information Available: Figure and table for the fitting of the electrochemical impedance spectroscopy results for hydrogen evolution reaction. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 11. Plots of log I vs log c (ethanol concentration) at two different potentials for measuring the reaction order.

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Enhanced Electrocatalytic Activity of Pd-Dispersed 3,4

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