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Temperature dependence on methanol oxidation and product formation on Pt and Pd modified Pt electrodes in alkaline medium
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Temperature dependence on methanol oxidation and product formation on Pt and Pd modified Pt electrodes in alkaline medium S.S. Mahapatra, A. Dutta, J. Datta* Department of Chemistry, Bengal Engineering & Science University, Shibpur, Howrah-711 103, West Bengal, India
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abstract
Article history:
The effect of temperature on the catalytic oxidation of methanol on electrodeposited
Received 21 August 2010
platinum and platinum-palladium alloy were carried out for a temperature range of
Received in revised form
293e353 K. The morphology of the catalyst surface was studied using scanning electron
14 November 2010
microscopy, where as the structure and bulk composition of the PtePd/C catalyst were
Accepted 21 November 2010
determined by X-ray diffraction and energy dispersive X-ray spectroscopy. Cyclic vol-
Available online 31 December 2010
tammetry, chronoamperometry and electrochemical impedance spectroscopy were used to investigate the electrochemical parameters related to electro-oxidation of methanol
Keywords:
under the influence of temperature. Apparent activation energies of the oxidation reac-
Methanol electro-oxidation
tions on Pt/C and PtePd/C were determined at different potentials within the same
PtePd/C catalyst
temperature range. A pronounced influence of temperature towards methanol oxidation
Electrode poisoning
was observed on the PtePd/C catalyst as compared to Pt alone. The incorporation of Pd into
Temperature dependence
Pt decreases the charge transfer resistance and activation energy of the methanol oxida-
Product analysis
tion substantially which ensures greater tolerance of this catalyst towards methanolic residues. Quantitative analysis of the oxidation reaction product by ion chromatography further substantiates the much better performance of the PtePd electrode than Pt alone for electrocatalytic oxidation of methanol in alkaline medium. Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The Direct Alcohol Fuel Cells (DAFCs), particularly those fuelled by methanol [1e3] and ethanol [4e6] has now emerged as one of the prospective power sources. Liquid fuels, such as low molecular weight alcohols have advantages, compared to pure hydrogen, because they can easily be handled, stored and transported using the present gasoline infrastructure with only slight modifications and these can be used directly without the necessity of reforming. Among the different fuel candidates, methanol has been considered as the most appropriate fuel for the DAFCs because of its low molecular
weight, simplest structure, high energy density (6.1 kWh kg1) and can easily be oxidized over suitable Pt-based catalysts. The main problems of DAFCs are poor performance of anode catalysts, especially at lower temperatures and the severe fuel crossover from anode to cathode, which leads to poisoning of cathode catalyst [7,8]. The improvement in anode catalysis is also essential to control the fuel permeation. Pt has been demonstrated as the only active and stable single noble metal for alcohol oxidation, particularly in acid medium. However, pure platinum is readily poisoned by CO-like intermediates formed during methanol electro-oxidation [9]. The high cost of the platinum also limits its use. The major challenges in the
* Corresponding author. Tel.: þ91 33 2668 4561-63x514; fax: þ91 33 2668 2916. E-mail address: [email protected] (J. Datta). 0360-3199/$ e see front matter Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.11.085
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Direct Methanol Fuel Cell (DMFC) development are to reduce the usage of expensive Pt metal at the same time improve the alcohol oxidation kinetics. One approach to counter these problems is to use of potential Pt-based alloys for minimizing the Pt loading. Another effective approach is to increase the alcohol electro-oxidation rate at elevated temperatures. A number of catalyst systems have been investigated for their suitability as methanol oxidation catalysts in acid medium including Pt alloy catalysts PteRu [10e12], PteSn [13,14], PtePd [15e17], PteRh [18,19]. Making alloys with a second or third metal modify electrocatalytic properties of Pt in order to overcome poisoning effects of adsorbed CO [20]. The use of Pd is of interest as it is at least 50 times more abundant on the earth than Pt and it can act as good electrocatalyst for organic fuel oxidation [21e23]. It is also reported that the release of hydrogen occluded in palladium may provide a viable route for lowering the surface concentration of adsorbed CO [24]. The PtePd binary system also exhibits a high resistance against CO poisoning from the oxidation of formic acid [25]. Although much effort has been taken for the development of DAFCs in acid medium, studies on alkaline fuel cells using small organic molecules are relatively less. However, in recent times interest in the alkaline fuel cell has arisen in view of its low cost, wider selection of possible electrocatalysts and expected faster kinetics of the proton transfer reactions as well as for the oxygen reduction [26e28]. The application of alkaline electrolytes to the DMFC could lead to the reduction in catalyst loading, as well as allow the use of less expensive non-precious metal catalysts. The facile adsorption of the organic molecules on Pt surface and simultaneous activation of Pd surfaces by the hydroxyl species in alkaline medium are expected to produce synergic effect in improving the kinetics of methanol oxidation in PtePd binary system. Furthermore the rate and extent of alcohol oxidation in a fuel cell environment can be substantially controlled by regulation of cell temperature [29e31]. In this paper, the temperature effect on methanol oxidation has been investigated with Pt/C and PtePd/C electrodes in alkaline medium. Different electrochemical techniques like cyclic voltammetry (CV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS) were employed for the study. Apparent activation energies (Ea(app)) for methanol oxidation reaction were determined at different potentials within the temperature range of 293 Ke353 K. In alkali media the electro-oxidation of methanol proceeds through intermediate formation of the anions like formate and ion exchange chromatography was employed to quantify these reaction products at different temperatures and to assess the degree of electro-catalysis on the PtePd/C electrodes.
2.
Experimental
2.1.
Materials
Methanol was purified by using standard procedure [32] and was stored over a Type 4A molecular sieve beads. The other reagents such as NaOH and HCl were of GR grade purity (Merck) and H2PtCl6$6H2O and PdCl2 were obtained from Arora Matthey
Ltd. The graphite block (saw cut finish grade) used as the catalyst substrate was procured from Graphite India Limited. All the solutions were freshly prepared with Milli-Q water.
2.2.
Preparation of Pt/C and PtePd/C electrocatalysts
The graphite block as received was cut into pieces polished and mild acid treated and finally boiled in water vigorously for 10 min. The graphite coupons were dried in hot air oven at 110 C for 2 h and used as the substrate for electrodeposition. Electrochemical deposition of Pt or co-deposition of Pt and Pd were made on coupons of graphite samples with surface area of 0.65 cm2. Both H2PtCl6$6H2O and PdCl2 solutions were prepared in 2.0 M HCl. Electrodeposition of Pt was carried out using 0.05 M H2PtCl6$6H2O solutions. For electrochemical co-deposition of Pt and Pd, an equimolar mixture of 0.05 M H2PtCl6$6H2O and 0.05 M PdCl2 was used. For each case electrodeposition was performed at room temperature under galvanostatic control by applying charges of 3 coulombs (5 mA cm2 current for 10 min) with the help of computer controlled PG Stat (AUTOLAB 12). Electrodes containing catalyst loading of 0.5 mg cm2 were selected for the present investigation.
2.3. Physico-chemical characterization of the electrocatalysts Bulk composition of the PtePd/C electrocatalyst was determined by energy dispersive X-ray spectroscopy (EDX) while scanning electron microscopy (SEM) was employed to reveal the surface morphology. All these measurements were done with a JSM-6700F FESEM at an accelerating potential of 5 kV. In order to obtain information about the surface and bulk structure of the catalyst, X-ray diffraction (XRD) study was carried out with the help of Philips PW 1140 parallel beam X-ray diffractometer with Bragg-Bretano focusing geometry ˚ ). and monochromatic Cu Ka radiation (l ¼ 1.54 A
2.4.
Electrochemical measurements
Electrochemical measurements like cyclic voltammetry and chronoamperometry were carried out in the standard threeelectrode assembly cell consisting of the synthesized catalyst as working electrode, mercury-mercuric oxide (MMO) as reference electrode and platinum foil (2 1 cm2) as counter electrode at different temperature with the help of computer controlled AUTOLAB 12 Potentiostat/Galvanostat from Ecochemie B.V., The Netherlands. A two compartment H-type cell was used for conducting electrochemical measurements as shown in Fig. 1. The cell temperature was thermostated within the range of 293e353 K. The working solutions were deaerated by purging with nitrogen (XL grade). Electrochemical impedance spectroscopy studies were conducted using the same AUTOLAB 12 PGSTAT coupled with a frequency response analyzer (FRA) module. EIS was performed with amplitude of 5 mV for frequencies ranging from 40 kHz to 100 mHz. Each scan contained about 100 data points (20 data points per decade). The impedance spectra were fitted to an equivalent circuit model using a non-linear fitting program. All the potential in this paper are referred to the reference hydrogen electrode (RHE) at cell operating temperature and
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Fig. 1 e Schematic diagram of the electrochemical cell setup.
the current is normalized to the geometrical surface area. The temperature correction for Hg/HgO reference electrode in alkaline medium was made [33].
2.5.
graphite surface both the metals were grown together on the same sphere making an assembly of ad-atoms in the form of agglomerated particles. The size of the particles does not show much variation and were found to remain within the range of (80e100 nm). The Pt and Pd nanoparticles co-electrodeposited on graphite surface exhibited an XRD pattern of a typical face-centeredcubic lattice structure as shown in Fig. 3. The strong diffraction peaks at the Bragg angles of 40.080, 46.670, 68.020, 82.080 and 86.450 correspond to the (111), (200), (220), (311) and (222) facets of PtePd crystal. Alloying of Pt and Pd does not change the diffraction pattern. With the incorporation of Pd into the fcc structure of Pt, the diffraction peaks were shifted to higher values of 2q, which is indicative of contraction of lattice. No characteristic peaks of Pt or Pd oxides were detected. The (111) peak was used to calculate the particle size of the PtePd crystal according to the DebyeeScherrer equation. The average particle size in the PtePd/C catalyst matrix was found to be 6.5 nm. However, the morphology indicated agglomeration of smaller particles throughout the matrix. The bulk compositions of electrocatalysts were investigated using energy dispersive Xray spectroscopy (EDX) and atomic ratio of Pt to Pd was found to be 1:2.
Ion-chromatographic analysis 3.2.
Aliquots from the working electrolyte were analyzed by ion chromatography after polarization at constant potential for 2 h in the selected temperature range 293e353 K. Metrohm’s Advanced Modular Ion Chromatography systems and a Metrosep A Supp 5e250 column were used for analyzing the formate and carbonate ions. A 1 ml sample of electrolyte solution was diluted with 19 ml of filtered Milli-Q water and 25 ml of this sample was injected into the ion chromatograph.
3.
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Results and discussion
3.1. Structural characterization of Pt/C and Pt-Pd/C electrocatalyst The morphologies of the catalyst surface were studied using scanning electron microscopy. PtePd particles show better dispersion than Pt on the graphite substrate shown in Fig. 2. As platinum and palladium were co-electrodeposited on the
Effect of temperature on methanol oxidation
Fig. 4a shows background behavior of Pt/C and PtePd/C electrodes in 0.5 M NaOH at scan rate of 50 mV s1 at room temperature. The cyclic voltammogram of Pt/C shows three potential peaks during the positive-going sweep as shown in the inset of the Fig. 4a. Peak centering at 568 mV is due to the oxidation of the adsorbed hydrogen. Peak emerging above 350 mV, can be attributed to the adsorption of OH on the surface. The formation of Pt oxide layer emerges above ca. 190 mV. Peak centering at 215 mV, can be attributed to the reduction of the Pt(II) oxide and a small hydrogen adsorption peak appears at 625 mV with the Pt/C electrode during the cathodic sweep. The feature of the cyclic voltammogram of Pt-Pd catalyst demonstrated both the characteristics of Pd/C and Pt/C. However the shift in peak positions for the voltammogram of PtePd catalyst indicates the formation of an alloy of these metals. The x-ray analysis also supports this fact. For the
Fig. 2 e Scanning electron micrographs of Pt/C and PtePd/C electrocatalysts.
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Fig. 3 e X-ray diffraction (XRD) pattern of Pt/C and PtePd/C electrocatalyst.
PtePd/C alloyed electrode, the hydrogen desorption as well as OH adsorption peaks overlaps and appears as a single peak at 340 mV. The OH adsorption clearly dominates in this region and starts around 730 mV. The presence of Pd facilitates the adsorption of OH and Suppresses the hydrogen desorption. The board peak in the potential region 730 and 170 mV can be attributed to the hydrogen desorption and OH adsorption. The double layer region for PtePd/C is more compressed and oxide formation starts at lower potential ca. 165 mV. The appearance of single oxide formation as well as reduction peak is in accordance with alloyed behavior of the PtePd. The PtePd/ C electrodes deliver oxide reduction peak at ca. 395 mV. The electrochemical surface area (ECSA) of the Pt/C catalyst was determined by measuring the charge collected in the hydrogen adsorption/desorption region after double layer correction and assuming a value of 210 mC cm2 charge needed for oxidation of a single layer of hydrogen on a smooth Pt surface [34]. For PtePd/C catalyst the ECSA was determined by considering the columbic charges corresponding to oxide reduction peak of the alloyed catalyst. However, it is not possible to measure ECSA value accurately by using this method because the reduction peaks may be ascribed to the reduction of the oxides of Pd and Pt formed on the surface of the PtePd/C catalyst during the positive scan [35]. The charges required for the reduction of PdO (qPdO-red) and PtO (qPtO-red) monolayer were taken to be 405 and 420 mC cm2 respectively as reported in literature [26,34]. The mean value of charges required for the oxide reduction of the alloyed catalyst having
Fig. 4 e Cyclic voltammograms of (a) Pt/C and PtePd/C electrodes in 0.5 M NaOH solution at a scan rate of 50 mV sL1 and 298 K, (b) Pt/C electrode in 0.5 M NaOH and 1.0 M methanol and (c) PtePd/C electrode in 0.5 M NaOH and 1.0 M methanol at different temperatures at a scan rate of 50 mV sL1.
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Pt to Pd ratio 1:2, was calculated to be 410 mC cm2 and used for evaluating the ECSA of the alloyed catalyst. The value of ECSA for PtePd/C was found to be far high (51.4 m2 g1), about 15 times more than the value obtained for Pt/C (3.4 m2 g1) reflecting higher catalytic surface area for the alloyed catalysts. Fig. 4b,c shows stabilized (after 30 cycles) cyclic voltammograms for the methanol oxidation reaction on Pt/C and PtePd/C electrodes respectively in a solution containing 0.5 M NaOH and 1.0 M methanol with increasing temperature from 293 to 353 K. It is observed from Figures that although the overall nature remains the same, significant changes occurs in the features of voltammogram profiles for the Pt/C and PtePd/ C catalysts, particularly at higher temperatures. Considering Fig. 4b for Pt/C electrodes, the oxidation current in the forward sweep (peak ‘a’) rises by a factor of 2.5 on increase of temperature from 293 to 353 K. This indicates that temperature has a positive effect on methanol oxidation leading to faster reaction kinetics and reduced deactivation of the catalyst surface by CO poisoning. Another effect of temperature on Pt/C for methanol oxidation is the almost vertical increase of reverse oxidation peak (‘b’) which increases with temperature. Since the renewed oxidation peak is related to the reduction of adsorbed OH species, the vertical increase indicates that the reduction of the adsorbed OH occurs very fast at high temperatures on Pt. Lower H2O binding energy of Pt could be one of the reasons for this behavior [36]. The forward sweep is characterized by the typical peak current, which declines due to the formation of strongly bonded Pt-oxides/hydroxide as surface species. In the reverse sweep, the progressive reduction of these oxides initiates the renewed oxidation of the methanolic species at low potentials. With increase in temperature, the potential differences between ‘a’ and ‘b’ peaks are reduced. Since the peak potential corresponding to ‘a’ are significantly related to OH production by H2O dissociation, the reduced potential difference suggests that OH production on Pt/C is activated at higher temperature. The voltammograms of PtePd/C, Fig. 4c, obtained with the same temperature range, exhibit a current increase by 2.45 times at 353 K compared to that at 293 K. The extent of increase is almost the same with Pt/C electrodes. However the significant difference of the two electrodes is the appearance of a second oxidation peak ‘a2’at the elevated temperatures 333 K and 353 K with PtePd/C. This may be attributed to further oxidation of the intermediates. With increase in temperature, the peak potentials for methanol oxidation particularly on Pt/C electrode shift positively with a broadening of the peak width. This behavior reflects the simultaneous formation of less strongly bonded Pt-oxides at higher temperature along with the methanol oxidation. The positive shift of the peak potential (Ep) may also result from catalyst deactivation due to faster intermediate formation. This phenomenon is also supported by the extended plateau current for methanol oxidation at higher potentials as also at high temperature. The peak broadening for methanol oxidation on PtePd/C electrode also suggests that the peak currents ‘a1’ and ‘a2’ are independent of OH adsorption [26]. Both Pt/C and PtePd/C shows linear increase of peak current with temperature as shown in the inset of Fig. 5a. The pronounced influence of temperature on the electro-catalysis of PtePd/C system as compared to Pt/C is further clarified from
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Fig. 5 e Variation of (a) IF/IB and (b) anodic peak current, ip with temperature for methanol oxidation on Pt/C and PtePd/C electrodes obtained from Fig. 4b,c.
the higher value of IF/IB ratio for PtePd/C catalyst as shown in Fig. 5a. On the other hand, a distinct positive shift of the reoxidation peaks is indicative of the reversibility of the oxide/ hydroxide formation in alkaline medium facilitated on the Pd containing catalyst surface. As observed in Fig. 6a, the onset potential (Eonset) at the Pt/C electrode shifts in the negative direction with a slope of 1.96 mV/degree from 293 to 353 K. The Eonset at PtePd/C electrode also shifts in the negative direction with a constant slope of 2.32 mV/degree and exhibits slightly higher temperature dependence than that of Pt/C electrode. Fig. 6b shows the influence of temperature on the forward peak positions (a1) in the voltammograms of the methanol oxidation reaction. It is observed that Eps are shifted to more anodic potentials almost in regular intervals with the increase of temperature while, negligible positive shift is observed with the binary PtePd system indicating attainment of the pseudo-steady state kinetics even at low temperatures. The long-term stability of the Pt/C and PtePd/C electrodes was derived from chronoamperometric experiments carried out in a solution containing 0.5 M NaOH and 1.0 M methanol at different temperatures. The methanol oxidation current was recorded at potential 60 mV (vs. RHE) over a period of 1000 s, as shown in Fig. 7a,b. A decrease of current density was recorded at the beginning of each experiment for both Pt/C and PtePd/C electrodes at each temperature. Pt/C electrode showed rapid decay of current density for all temperatures. This current decay is attributed to a poisoning effect by methanolic residues or to the formation of Pt-oxides [27]. For PtePd/C electrode, current decay was comparatively less. Thus, PtePd/C electrode exhibits greater tolerance to methanolic residues particularly at higher temperatures 333 K and 353 K respectively. The long-term poisoning rates are summarized in Table 1, where from it is evident that the longterm poisoning rate for methanol oxidation on Pt/C decreases with the increase of temperature while on PtePd/C electrode, the same increases with the increase of temperature.
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Fig. 6 e Variation of (a) onset potential, Eonset and (b) peak potential, Ep with temperature for methanol oxidation on Pt/C and PtePd/C electrodes obtained from Fig. 4b,c.
The PtePd/C electrode has the slower rate of poisoning than that of the Pt/C electrode at all temperature except at 353 K. It may be inferred that the methanol oxidation starts with a faster rate on PtePd/C than on Pt/C with the formation of methanolic intermediates, which are not removed or desorbed with a similar rate. The reaction is thereby impeded.
This is in agreement with the results of the cyclic voltammetric experiments. The current corresponding to the electrochemical oxidation of methanol under diffusion controlled condition is described by Cottrell equation [37]: 1=2 Cp1=2 t1=2 I ¼ nFADapp
(1)
Fig. 7 e Chronoamperograms of (a) Pt/C and (b) PtePd/C electrodes in a solution containing 0.5 M NaOH and 1.0 M methanol at L60 mV & different temperatures. Dependence of transient current on tL1/2 for (c) Pt/C and (d) PtePd/C electrodes derived from the data of chronoamperograms.
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Table 1 e Temperature dependence of long-term poisoning rate (d), diffusion coefficient (Dapp) and different EIS circuit parameters for methanol oxidation on Pt/C and PtePd/C electrodes at L60 mV. Pt/C
d (% per sec) Dapp 105 (cm2 s1) Rs/ohm Rct/ohm CPE 103/F cm2 n R3/ohm L1
PtePd/C
293 K
313 K
333 K
353 K
293 K
313 K
333 K
353 K
0.026 5.44 5.62 3.66 2.9 0.98 14.20 10.92
0.024 3.60 3.61 2.12 3.4 0.94 18.67 4.00
0.020 3.60 1.98 1.04 3.7 0.93 9.99 0.66
0.018 3.60 1.36 0.73 5.8 0.85 8.19 0.073
0.0052 3.34 5.00 1.93 6.0 0.94 e e
0.010 2.56 3.16 1.58 7.8 0.91 e e
0.014 2.56 1.71 1.01 9.0 0.93 e e
0.024 2.56 1.38 0.60 10.0 0.83 e e
where, Dapp and C are the apparent diffusion coefficient (cm2 s1) and the bulk concentration (mol cm3) of methanol, respectively. The plot of I versus t1/2 is linear, and from its slope (Fig. 7c,d), the value of Dapp of methanol was calculated and presented in Table 1. The diffusion coefficient values of methanol are more or less comparable for both the electrodes. The slight lowering of Dapp values at higher temperatures, seemingly represents the sluggishness in the oxidation kinetics at temperature as low as 293 K and the effective oxidation rate is attained at temperature near 313 K when the accumulation of reaction intermediates adsorbed at the surface impede the rate of diffusion of the reactant species. Fig. 8 shows the pseudo-Arrhenius plots for the current densities of the methanol oxidation on the Pt/C and PtePd/C electrodes at different potentials. Almost linear relationships between log i and 1/T is maintained in all cases, indicating similar reaction mechanism at each potential and the apparent activation energies (Ea(app)) were calculated from the slope of these plots. The activation energy for both the electrodes when plotted against potential in Fig. 9, are found to be potential dependent. Reduction in the activation energy (from ∼17 kJ mol1 to ∼13 kJ mol1), is fairly linear in the potential range of 60 to 60 mV for both the electrodes. Thereafter the potential does not make much difference on the Pt/C electrode where as further decrease of Ea to ∼10.2 kJ mol1 is attained for PtePd system at high potential. The slightly larger value of Ea(app) for methanol oxidation on PtePd/C electrode at low potential could indicate that the rate determining is the adsorption step, where as the lower activation energy on Pt/C
at this potential ascribed to heterogeneous electrocatalytic process with mixed activation-adsorption control reaction [28,38].
3.3.
Effect of temperature on the impedance response
Electrochemical impedance spectroscopy (EIS) was used to investigate reaction process and kinetics of methanol oxidation on Pt/C and PtePd/C electrodes at different temperatures and at a fixed potential of 60 mV (vs. RHE). The Nyquist plot for methanol oxidation in a solution containing 0.5 M NaOH and 1.0 M methanol at temperature 293e353 K are shown in Fig. 10. The equivalent circuit elements as derived from Nyquist plot for methanol oxidation on Pt/C and PtePd/C electrodes at 60 mV (vs. RHE) are summarized in Table 1. The diameter of the semicircle decreased as temperature increased for both Pt/C and PtePd/C electrodes. The equivalent circuit compatible with the impedance data for both the catalysts are presented in the inset of the corresponding plots Fig. 10 (a,b). In this circuit Rs, CPE, Rct represent solution resistance, constant phase element and charge transfer resistance associated with the methanol oxidation primarily involving the dissociative adsorption of methanol on the surface. The charge transfer resistance (Rct) decreases with the increase in temperature of the solution for both Pt/C and PtePd/C electrodes indicating faster reaction kinetics of methanol oxidation at elevated temperature. The Rct values for methanol oxidation on the PtePd/C electrode is lower
Fig. 8 e Arrhenius plots for methanol oxidation at different potentials on (a) Pt/C and (b) PtePd/C electrodes in a mixture of 0.5 M NaOH and 1.0 M methanol.
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(-4)max to high frequency value for Pt/C compared to PtePd/C indicates the transformation of capacitive to the resistive behavior of the electrodeeelectrolyte interface. The contribution of several impedance parameters may further be substantiated by the evaluation through graphical method which however is not undertaken in this study [39,40].
3.4. Estimation of oxidation products found during methanol electro-oxidation
Fig. 9 e Dependence of apparent activation energy for methanol oxidation with potentials on Pt/C and PtePd/C electrodes.
than that on the Pt/C electrode at lower temperature, however Rct values are comparable for both the electrodes particularly at higher temperature and suggests improved methanol oxidation kinetics on PtePd/C catalyst at lower temperature in alkaline medium. The overall low value of Rct for both the electrodes at this temperature range indicates faster rate of charge transfer in alkaline medium. The increase in solution conductivity with rise in temperature is reflected in the gradual decrement in Rs values with temperature. The CPE impedance coefficients for both the Pt/C and PtePd/C catalysts were increased with rise in temperature. The comparatively higher values exhibited for PtePd/C, translates to the double layer charging at the (OH)ads activated surface, facilitated in presence of Pd. The electro-oxidation of methanol shows inductive behavior at all temperatures only on Pt surfaces which is indicative of the presence of reaction intermediates being formed at the catalyst surface and getting chances for faster oxidation at higher temperatures. The circuit for the Pt/ C catalyst includes inductive resistance R0 and the inductance, L. Interestingly a drastic fall in the values is observed with the rise of temperature from 293 to 353 K, which is attributed to a greater temperature influence on the methanol oxidation kinetics on Pt/C electrodes. This is furthermore elucidated with the Bode plots (Inset). The distinct shift in
The effect of varying temperature on the rate of product formation for oxidation of methanol was studied over a range of temperatures (293e353 K) on Pt/C and PtePd/C electrodes respectively. The concentration of the end products were determined through ion-chromatographic analysis of the chronopotentiometric aliquots taken at 60 mV after 2 h. The overall oxidation reaction equation for the methanol in alkaline medium is supposed to be
CH3OH þ6OH / CO2 þ 5H2O þ 6e
(A)
Complex process of methanol electro-oxidation in alkaline medium involving 6-electron transfer and several intermediate organic species is suggested to proceed through the following paths:
The presence of formate and CO2 (as carbonate) as the main soluble reaction products and CO as the main poisoning species have been detected in alkaline medium [9,41,42]. The (CO)ads formation follows pathway ‘b’ or ‘c’ and responsible for current decay during methanol oxidation. The selectivity of product formation largely depends on the potential as well as temperature. The first step of the oxidation process is the dissociative adsorption of methanol followed by subsequent dehydrogenation, water activation, noble metal oxide formation at higher potentials, its reduction and finally desorption or oxidation of the organic species by reactivation.
Fig. 10 e Impedance spectra for methanol oxidation at different temperatures on (a) Pt/C and (b) PtePd/C electrodes in a mixture of 0.5 M NaOH and 1.0 M methanol at L60 mV.
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Hydroxide ions get adsorbed on the electrode surface and initiate the formation of surface hydroxide and oxide species. The associated electrochemical reaction occurring at the electrodeeelectrolyte interface is as follows [43,44] M þ OH # M-(OH)ads þ e
(B)
M þ 6 OH # [M(OH)6] 2sol þ 4e
(C)
MeOH þ MeOH / MeO þ H2O þ M
(D)
A low level of hydrous oxide may be formed at the metal sites and surface defects due to the repetitive formation and reduction of thin compact layer via the above reactions. This hydrous oxide supplies the active oxygen atom to oxidize the organic species. The OH ions required for the equilibrium are mainly supplied by the solution OH ion at higher potential. Through the Pd incorporation into the Pt matrix, the hydrogen adsorption and desorption characteristics is significantly improved at much lower potential than that on the Pt alone. The desorption of H particularly at the Pd sites plays important role in the anodic potential region in alkali medium and activate the catalyst surface promoting deprotonation. The surface hydroxides are more readily formed in presence of Pd in the catalyst matrix. Thus the poisoning effect of (CO)ads is efficiently reduced due the synergic effect of Pt and Pd in the matrix. Dissociative chemisorption of methanol molecules on Pt proceeds through the following steps: M þ (CH3OH)sol # M-(CH3OH)ads
(E)
M-(CH3OH)ads þ OH / M-(CH3O)ads þ H2O þ e
(F)
M-(CH3O)ads þ OH / M-(CH2O)ads þ H2O þ e
(G)
M-(CH2O)ads þ OH / M-(CHO)ads þ H2O þ e
(H)
M-(CHO)ads þ OH / M-(CO)ads þ H2O þ e
(I)
M-(CO)ads þ OH / M-(COOH)ads þ e
(J)
M-(COOH)ads þ OH / M þ CO2 þ H2O þ e
(K)
(where M stands for PtePd) In presence of adequate alkali the oxidation of M-(CHO)ads and M-(CO)ads may proceed directly through the steps (L) and (M) to the ultimate production of CO2. M-(CHO)ads þ 3OH / M þ CO2 þ 2H2O þ3e M-(CO)ads þ 2OH (on M) / M þ CO2 þ H2O þ 2e
(L)
Fig. 11 e Effect of temperature on (a) carbonate and (b) formate production during methanol oxidation on Pt/C and PtePd/C electrodes in a mixture of 0.5 M NaOH and 1.0 M methanol at L60 mV for 2 h.
species may not be possible at low potential as they strongly bind to the electrode surface. However, at higher potential, highly active oxygen atoms again become available and the poisonous PtCO or Pd2CO species are oxidized via the reaction (J) or (M). Fig. 11 (a,b) shows the yield of carbonate and formate ions on both PtePd/C and Pt/C electrodes throughout the temperature range studied in the present investigation. The formate and carbonate yield on both the catalysts increased approximately linearly with temperature. Extensive production of formate is observed (Fig. 11b) with PtePd/C electrode which further rise with temperature. On the other hand, the appreciably high yield of carbonate ions at high temperatures (Fig. 11a) with the Pt/C electrode is due to favorable oxidation of (CO)ads getting oxidized at such temperatures. The higher yield of formate ions for methanol oxidation reaction on PtePd/C in the present investigation indicates that the formate is produced following the path ‘a’ and ‘c’. The presence of Pd in the alloy facilitates the adsorption of OH ion on the surface so that the (OH)ads reacts with the (CO)ads and methanol oxidation reaction proceeds through path ‘c’ (Equation J).
(M)
The strength of the bonding of (CHO)ads on the surface probably determines the entire rate of the reaction. In general the chemisorbed bonding of (CHO)ads on Pt group of metals in alkali is weak, such that further oxidation takes place without much difficulty, i.e. without irreversibly blocking the electrode’s active sites. The several intermediate organic species have been identified by FTIR spectroscopic study which revealed that the CO species are found to be linearly bonded to Pt sites (PtCO) and bridge bonded to Pd sites [Pd2C]O] [44]. Complete removal of CO
4.
Conclusion
The PtePd/C catalyst exhibits better catalytic activity for methanol oxidation in comparison to Pt/C, however following different pathway, particularly at higher temperatures. For methanol oxidation, the onset potentials on PtePd/C catalyst shows negative shift with temperature and the values are lower than that obtained with Pt/C catalyst. The decay in oxidation current density with time is nominal for the PtePd/
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C catalyst at all temperature while the Pt/C catalyst shows appreciable current decay indicating higher rate of catalyst poisoning. Pd addition reduces the activation energies and the charge transfer resistance and promote the electrocatalytic activity towards the methanol oxidation. The rapid kinetics of the dissociative adsorption in alkali media followed by facile oxidation of the intermediates species on the alloyed surface is demonstrated by the favorable kinetic parameters of the charge transfer reaction. The beneficial effect of catalysis by Pd addition to Pt is ascribed to the facile adsorption of OH on this surface as furnished in the suggested mechanism and is further reflected by the high yield of formate as well as low yield of carbonate obtained during the oxidation of methanol on PtePd/C catalyst. The overall impression is that Pt nanocatalyst can be comfortably replaced by its alloyed form with Pd, whereby the Pt loading is substantially reduced without compromising the catalytic output
Acknowledgement
[12]
[13]
[14]
[15]
[16]
[17]
The authors gratefully acknowledge MHRD, New Delhi, Govt. of India for the instrumental facility provided to the department.
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Temperature dependence on methanol oxidation and product formation on Pt and Pd modified Pt electrodes in alkaline medium S.S. Mahapatra, A. Dutta, J. Datta* Department of Chemistry, Bengal Engineering & Science University, Shibpur, Howrah-711 103, West Bengal, India
article info
abstract
Article history:
The effect of temperature on the catalytic oxidation of methanol on electrodeposited
Received 21 August 2010
platinum and platinum-palladium alloy were carried out for a temperature range of
Received in revised form
293e353 K. The morphology of the catalyst surface was studied using scanning electron
14 November 2010
microscopy, where as the structure and bulk composition of the PtePd/C catalyst were
Accepted 21 November 2010
determined by X-ray diffraction and energy dispersive X-ray spectroscopy. Cyclic vol-
Available online 31 December 2010
tammetry, chronoamperometry and electrochemical impedance spectroscopy were used to investigate the electrochemical parameters related to electro-oxidation of methanol
Keywords:
under the influence of temperature. Apparent activation energies of the oxidation reac-
Methanol electro-oxidation
tions on Pt/C and PtePd/C were determined at different potentials within the same
PtePd/C catalyst
temperature range. A pronounced influence of temperature towards methanol oxidation
Electrode poisoning
was observed on the PtePd/C catalyst as compared to Pt alone. The incorporation of Pd into
Temperature dependence
Pt decreases the charge transfer resistance and activation energy of the methanol oxida-
Product analysis
tion substantially which ensures greater tolerance of this catalyst towards methanolic residues. Quantitative analysis of the oxidation reaction product by ion chromatography further substantiates the much better performance of the PtePd electrode than Pt alone for electrocatalytic oxidation of methanol in alkaline medium. Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The Direct Alcohol Fuel Cells (DAFCs), particularly those fuelled by methanol [1e3] and ethanol [4e6] has now emerged as one of the prospective power sources. Liquid fuels, such as low molecular weight alcohols have advantages, compared to pure hydrogen, because they can easily be handled, stored and transported using the present gasoline infrastructure with only slight modifications and these can be used directly without the necessity of reforming. Among the different fuel candidates, methanol has been considered as the most appropriate fuel for the DAFCs because of its low molecular
weight, simplest structure, high energy density (6.1 kWh kg1) and can easily be oxidized over suitable Pt-based catalysts. The main problems of DAFCs are poor performance of anode catalysts, especially at lower temperatures and the severe fuel crossover from anode to cathode, which leads to poisoning of cathode catalyst [7,8]. The improvement in anode catalysis is also essential to control the fuel permeation. Pt has been demonstrated as the only active and stable single noble metal for alcohol oxidation, particularly in acid medium. However, pure platinum is readily poisoned by CO-like intermediates formed during methanol electro-oxidation [9]. The high cost of the platinum also limits its use. The major challenges in the
* Corresponding author. Tel.: þ91 33 2668 4561-63x514; fax: þ91 33 2668 2916. E-mail address: [email protected] (J. Datta). 0360-3199/$ e see front matter Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.11.085
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Direct Methanol Fuel Cell (DMFC) development are to reduce the usage of expensive Pt metal at the same time improve the alcohol oxidation kinetics. One approach to counter these problems is to use of potential Pt-based alloys for minimizing the Pt loading. Another effective approach is to increase the alcohol electro-oxidation rate at elevated temperatures. A number of catalyst systems have been investigated for their suitability as methanol oxidation catalysts in acid medium including Pt alloy catalysts PteRu [10e12], PteSn [13,14], PtePd [15e17], PteRh [18,19]. Making alloys with a second or third metal modify electrocatalytic properties of Pt in order to overcome poisoning effects of adsorbed CO [20]. The use of Pd is of interest as it is at least 50 times more abundant on the earth than Pt and it can act as good electrocatalyst for organic fuel oxidation [21e23]. It is also reported that the release of hydrogen occluded in palladium may provide a viable route for lowering the surface concentration of adsorbed CO [24]. The PtePd binary system also exhibits a high resistance against CO poisoning from the oxidation of formic acid [25]. Although much effort has been taken for the development of DAFCs in acid medium, studies on alkaline fuel cells using small organic molecules are relatively less. However, in recent times interest in the alkaline fuel cell has arisen in view of its low cost, wider selection of possible electrocatalysts and expected faster kinetics of the proton transfer reactions as well as for the oxygen reduction [26e28]. The application of alkaline electrolytes to the DMFC could lead to the reduction in catalyst loading, as well as allow the use of less expensive non-precious metal catalysts. The facile adsorption of the organic molecules on Pt surface and simultaneous activation of Pd surfaces by the hydroxyl species in alkaline medium are expected to produce synergic effect in improving the kinetics of methanol oxidation in PtePd binary system. Furthermore the rate and extent of alcohol oxidation in a fuel cell environment can be substantially controlled by regulation of cell temperature [29e31]. In this paper, the temperature effect on methanol oxidation has been investigated with Pt/C and PtePd/C electrodes in alkaline medium. Different electrochemical techniques like cyclic voltammetry (CV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS) were employed for the study. Apparent activation energies (Ea(app)) for methanol oxidation reaction were determined at different potentials within the temperature range of 293 Ke353 K. In alkali media the electro-oxidation of methanol proceeds through intermediate formation of the anions like formate and ion exchange chromatography was employed to quantify these reaction products at different temperatures and to assess the degree of electro-catalysis on the PtePd/C electrodes.
2.
Experimental
2.1.
Materials
Methanol was purified by using standard procedure [32] and was stored over a Type 4A molecular sieve beads. The other reagents such as NaOH and HCl were of GR grade purity (Merck) and H2PtCl6$6H2O and PdCl2 were obtained from Arora Matthey
Ltd. The graphite block (saw cut finish grade) used as the catalyst substrate was procured from Graphite India Limited. All the solutions were freshly prepared with Milli-Q water.
2.2.
Preparation of Pt/C and PtePd/C electrocatalysts
The graphite block as received was cut into pieces polished and mild acid treated and finally boiled in water vigorously for 10 min. The graphite coupons were dried in hot air oven at 110 C for 2 h and used as the substrate for electrodeposition. Electrochemical deposition of Pt or co-deposition of Pt and Pd were made on coupons of graphite samples with surface area of 0.65 cm2. Both H2PtCl6$6H2O and PdCl2 solutions were prepared in 2.0 M HCl. Electrodeposition of Pt was carried out using 0.05 M H2PtCl6$6H2O solutions. For electrochemical co-deposition of Pt and Pd, an equimolar mixture of 0.05 M H2PtCl6$6H2O and 0.05 M PdCl2 was used. For each case electrodeposition was performed at room temperature under galvanostatic control by applying charges of 3 coulombs (5 mA cm2 current for 10 min) with the help of computer controlled PG Stat (AUTOLAB 12). Electrodes containing catalyst loading of 0.5 mg cm2 were selected for the present investigation.
2.3. Physico-chemical characterization of the electrocatalysts Bulk composition of the PtePd/C electrocatalyst was determined by energy dispersive X-ray spectroscopy (EDX) while scanning electron microscopy (SEM) was employed to reveal the surface morphology. All these measurements were done with a JSM-6700F FESEM at an accelerating potential of 5 kV. In order to obtain information about the surface and bulk structure of the catalyst, X-ray diffraction (XRD) study was carried out with the help of Philips PW 1140 parallel beam X-ray diffractometer with Bragg-Bretano focusing geometry ˚ ). and monochromatic Cu Ka radiation (l ¼ 1.54 A
2.4.
Electrochemical measurements
Electrochemical measurements like cyclic voltammetry and chronoamperometry were carried out in the standard threeelectrode assembly cell consisting of the synthesized catalyst as working electrode, mercury-mercuric oxide (MMO) as reference electrode and platinum foil (2 1 cm2) as counter electrode at different temperature with the help of computer controlled AUTOLAB 12 Potentiostat/Galvanostat from Ecochemie B.V., The Netherlands. A two compartment H-type cell was used for conducting electrochemical measurements as shown in Fig. 1. The cell temperature was thermostated within the range of 293e353 K. The working solutions were deaerated by purging with nitrogen (XL grade). Electrochemical impedance spectroscopy studies were conducted using the same AUTOLAB 12 PGSTAT coupled with a frequency response analyzer (FRA) module. EIS was performed with amplitude of 5 mV for frequencies ranging from 40 kHz to 100 mHz. Each scan contained about 100 data points (20 data points per decade). The impedance spectra were fitted to an equivalent circuit model using a non-linear fitting program. All the potential in this paper are referred to the reference hydrogen electrode (RHE) at cell operating temperature and
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Fig. 1 e Schematic diagram of the electrochemical cell setup.
the current is normalized to the geometrical surface area. The temperature correction for Hg/HgO reference electrode in alkaline medium was made [33].
2.5.
graphite surface both the metals were grown together on the same sphere making an assembly of ad-atoms in the form of agglomerated particles. The size of the particles does not show much variation and were found to remain within the range of (80e100 nm). The Pt and Pd nanoparticles co-electrodeposited on graphite surface exhibited an XRD pattern of a typical face-centeredcubic lattice structure as shown in Fig. 3. The strong diffraction peaks at the Bragg angles of 40.080, 46.670, 68.020, 82.080 and 86.450 correspond to the (111), (200), (220), (311) and (222) facets of PtePd crystal. Alloying of Pt and Pd does not change the diffraction pattern. With the incorporation of Pd into the fcc structure of Pt, the diffraction peaks were shifted to higher values of 2q, which is indicative of contraction of lattice. No characteristic peaks of Pt or Pd oxides were detected. The (111) peak was used to calculate the particle size of the PtePd crystal according to the DebyeeScherrer equation. The average particle size in the PtePd/C catalyst matrix was found to be 6.5 nm. However, the morphology indicated agglomeration of smaller particles throughout the matrix. The bulk compositions of electrocatalysts were investigated using energy dispersive Xray spectroscopy (EDX) and atomic ratio of Pt to Pd was found to be 1:2.
Ion-chromatographic analysis 3.2.
Aliquots from the working electrolyte were analyzed by ion chromatography after polarization at constant potential for 2 h in the selected temperature range 293e353 K. Metrohm’s Advanced Modular Ion Chromatography systems and a Metrosep A Supp 5e250 column were used for analyzing the formate and carbonate ions. A 1 ml sample of electrolyte solution was diluted with 19 ml of filtered Milli-Q water and 25 ml of this sample was injected into the ion chromatograph.
3.
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Results and discussion
3.1. Structural characterization of Pt/C and Pt-Pd/C electrocatalyst The morphologies of the catalyst surface were studied using scanning electron microscopy. PtePd particles show better dispersion than Pt on the graphite substrate shown in Fig. 2. As platinum and palladium were co-electrodeposited on the
Effect of temperature on methanol oxidation
Fig. 4a shows background behavior of Pt/C and PtePd/C electrodes in 0.5 M NaOH at scan rate of 50 mV s1 at room temperature. The cyclic voltammogram of Pt/C shows three potential peaks during the positive-going sweep as shown in the inset of the Fig. 4a. Peak centering at 568 mV is due to the oxidation of the adsorbed hydrogen. Peak emerging above 350 mV, can be attributed to the adsorption of OH on the surface. The formation of Pt oxide layer emerges above ca. 190 mV. Peak centering at 215 mV, can be attributed to the reduction of the Pt(II) oxide and a small hydrogen adsorption peak appears at 625 mV with the Pt/C electrode during the cathodic sweep. The feature of the cyclic voltammogram of Pt-Pd catalyst demonstrated both the characteristics of Pd/C and Pt/C. However the shift in peak positions for the voltammogram of PtePd catalyst indicates the formation of an alloy of these metals. The x-ray analysis also supports this fact. For the
Fig. 2 e Scanning electron micrographs of Pt/C and PtePd/C electrocatalysts.
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Fig. 3 e X-ray diffraction (XRD) pattern of Pt/C and PtePd/C electrocatalyst.
PtePd/C alloyed electrode, the hydrogen desorption as well as OH adsorption peaks overlaps and appears as a single peak at 340 mV. The OH adsorption clearly dominates in this region and starts around 730 mV. The presence of Pd facilitates the adsorption of OH and Suppresses the hydrogen desorption. The board peak in the potential region 730 and 170 mV can be attributed to the hydrogen desorption and OH adsorption. The double layer region for PtePd/C is more compressed and oxide formation starts at lower potential ca. 165 mV. The appearance of single oxide formation as well as reduction peak is in accordance with alloyed behavior of the PtePd. The PtePd/ C electrodes deliver oxide reduction peak at ca. 395 mV. The electrochemical surface area (ECSA) of the Pt/C catalyst was determined by measuring the charge collected in the hydrogen adsorption/desorption region after double layer correction and assuming a value of 210 mC cm2 charge needed for oxidation of a single layer of hydrogen on a smooth Pt surface [34]. For PtePd/C catalyst the ECSA was determined by considering the columbic charges corresponding to oxide reduction peak of the alloyed catalyst. However, it is not possible to measure ECSA value accurately by using this method because the reduction peaks may be ascribed to the reduction of the oxides of Pd and Pt formed on the surface of the PtePd/C catalyst during the positive scan [35]. The charges required for the reduction of PdO (qPdO-red) and PtO (qPtO-red) monolayer were taken to be 405 and 420 mC cm2 respectively as reported in literature [26,34]. The mean value of charges required for the oxide reduction of the alloyed catalyst having
Fig. 4 e Cyclic voltammograms of (a) Pt/C and PtePd/C electrodes in 0.5 M NaOH solution at a scan rate of 50 mV sL1 and 298 K, (b) Pt/C electrode in 0.5 M NaOH and 1.0 M methanol and (c) PtePd/C electrode in 0.5 M NaOH and 1.0 M methanol at different temperatures at a scan rate of 50 mV sL1.
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Pt to Pd ratio 1:2, was calculated to be 410 mC cm2 and used for evaluating the ECSA of the alloyed catalyst. The value of ECSA for PtePd/C was found to be far high (51.4 m2 g1), about 15 times more than the value obtained for Pt/C (3.4 m2 g1) reflecting higher catalytic surface area for the alloyed catalysts. Fig. 4b,c shows stabilized (after 30 cycles) cyclic voltammograms for the methanol oxidation reaction on Pt/C and PtePd/C electrodes respectively in a solution containing 0.5 M NaOH and 1.0 M methanol with increasing temperature from 293 to 353 K. It is observed from Figures that although the overall nature remains the same, significant changes occurs in the features of voltammogram profiles for the Pt/C and PtePd/ C catalysts, particularly at higher temperatures. Considering Fig. 4b for Pt/C electrodes, the oxidation current in the forward sweep (peak ‘a’) rises by a factor of 2.5 on increase of temperature from 293 to 353 K. This indicates that temperature has a positive effect on methanol oxidation leading to faster reaction kinetics and reduced deactivation of the catalyst surface by CO poisoning. Another effect of temperature on Pt/C for methanol oxidation is the almost vertical increase of reverse oxidation peak (‘b’) which increases with temperature. Since the renewed oxidation peak is related to the reduction of adsorbed OH species, the vertical increase indicates that the reduction of the adsorbed OH occurs very fast at high temperatures on Pt. Lower H2O binding energy of Pt could be one of the reasons for this behavior [36]. The forward sweep is characterized by the typical peak current, which declines due to the formation of strongly bonded Pt-oxides/hydroxide as surface species. In the reverse sweep, the progressive reduction of these oxides initiates the renewed oxidation of the methanolic species at low potentials. With increase in temperature, the potential differences between ‘a’ and ‘b’ peaks are reduced. Since the peak potential corresponding to ‘a’ are significantly related to OH production by H2O dissociation, the reduced potential difference suggests that OH production on Pt/C is activated at higher temperature. The voltammograms of PtePd/C, Fig. 4c, obtained with the same temperature range, exhibit a current increase by 2.45 times at 353 K compared to that at 293 K. The extent of increase is almost the same with Pt/C electrodes. However the significant difference of the two electrodes is the appearance of a second oxidation peak ‘a2’at the elevated temperatures 333 K and 353 K with PtePd/C. This may be attributed to further oxidation of the intermediates. With increase in temperature, the peak potentials for methanol oxidation particularly on Pt/C electrode shift positively with a broadening of the peak width. This behavior reflects the simultaneous formation of less strongly bonded Pt-oxides at higher temperature along with the methanol oxidation. The positive shift of the peak potential (Ep) may also result from catalyst deactivation due to faster intermediate formation. This phenomenon is also supported by the extended plateau current for methanol oxidation at higher potentials as also at high temperature. The peak broadening for methanol oxidation on PtePd/C electrode also suggests that the peak currents ‘a1’ and ‘a2’ are independent of OH adsorption [26]. Both Pt/C and PtePd/C shows linear increase of peak current with temperature as shown in the inset of Fig. 5a. The pronounced influence of temperature on the electro-catalysis of PtePd/C system as compared to Pt/C is further clarified from
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Fig. 5 e Variation of (a) IF/IB and (b) anodic peak current, ip with temperature for methanol oxidation on Pt/C and PtePd/C electrodes obtained from Fig. 4b,c.
the higher value of IF/IB ratio for PtePd/C catalyst as shown in Fig. 5a. On the other hand, a distinct positive shift of the reoxidation peaks is indicative of the reversibility of the oxide/ hydroxide formation in alkaline medium facilitated on the Pd containing catalyst surface. As observed in Fig. 6a, the onset potential (Eonset) at the Pt/C electrode shifts in the negative direction with a slope of 1.96 mV/degree from 293 to 353 K. The Eonset at PtePd/C electrode also shifts in the negative direction with a constant slope of 2.32 mV/degree and exhibits slightly higher temperature dependence than that of Pt/C electrode. Fig. 6b shows the influence of temperature on the forward peak positions (a1) in the voltammograms of the methanol oxidation reaction. It is observed that Eps are shifted to more anodic potentials almost in regular intervals with the increase of temperature while, negligible positive shift is observed with the binary PtePd system indicating attainment of the pseudo-steady state kinetics even at low temperatures. The long-term stability of the Pt/C and PtePd/C electrodes was derived from chronoamperometric experiments carried out in a solution containing 0.5 M NaOH and 1.0 M methanol at different temperatures. The methanol oxidation current was recorded at potential 60 mV (vs. RHE) over a period of 1000 s, as shown in Fig. 7a,b. A decrease of current density was recorded at the beginning of each experiment for both Pt/C and PtePd/C electrodes at each temperature. Pt/C electrode showed rapid decay of current density for all temperatures. This current decay is attributed to a poisoning effect by methanolic residues or to the formation of Pt-oxides [27]. For PtePd/C electrode, current decay was comparatively less. Thus, PtePd/C electrode exhibits greater tolerance to methanolic residues particularly at higher temperatures 333 K and 353 K respectively. The long-term poisoning rates are summarized in Table 1, where from it is evident that the longterm poisoning rate for methanol oxidation on Pt/C decreases with the increase of temperature while on PtePd/C electrode, the same increases with the increase of temperature.
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Fig. 6 e Variation of (a) onset potential, Eonset and (b) peak potential, Ep with temperature for methanol oxidation on Pt/C and PtePd/C electrodes obtained from Fig. 4b,c.
The PtePd/C electrode has the slower rate of poisoning than that of the Pt/C electrode at all temperature except at 353 K. It may be inferred that the methanol oxidation starts with a faster rate on PtePd/C than on Pt/C with the formation of methanolic intermediates, which are not removed or desorbed with a similar rate. The reaction is thereby impeded.
This is in agreement with the results of the cyclic voltammetric experiments. The current corresponding to the electrochemical oxidation of methanol under diffusion controlled condition is described by Cottrell equation [37]: 1=2 Cp1=2 t1=2 I ¼ nFADapp
(1)
Fig. 7 e Chronoamperograms of (a) Pt/C and (b) PtePd/C electrodes in a solution containing 0.5 M NaOH and 1.0 M methanol at L60 mV & different temperatures. Dependence of transient current on tL1/2 for (c) Pt/C and (d) PtePd/C electrodes derived from the data of chronoamperograms.
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Table 1 e Temperature dependence of long-term poisoning rate (d), diffusion coefficient (Dapp) and different EIS circuit parameters for methanol oxidation on Pt/C and PtePd/C electrodes at L60 mV. Pt/C
d (% per sec) Dapp 105 (cm2 s1) Rs/ohm Rct/ohm CPE 103/F cm2 n R3/ohm L1
PtePd/C
293 K
313 K
333 K
353 K
293 K
313 K
333 K
353 K
0.026 5.44 5.62 3.66 2.9 0.98 14.20 10.92
0.024 3.60 3.61 2.12 3.4 0.94 18.67 4.00
0.020 3.60 1.98 1.04 3.7 0.93 9.99 0.66
0.018 3.60 1.36 0.73 5.8 0.85 8.19 0.073
0.0052 3.34 5.00 1.93 6.0 0.94 e e
0.010 2.56 3.16 1.58 7.8 0.91 e e
0.014 2.56 1.71 1.01 9.0 0.93 e e
0.024 2.56 1.38 0.60 10.0 0.83 e e
where, Dapp and C are the apparent diffusion coefficient (cm2 s1) and the bulk concentration (mol cm3) of methanol, respectively. The plot of I versus t1/2 is linear, and from its slope (Fig. 7c,d), the value of Dapp of methanol was calculated and presented in Table 1. The diffusion coefficient values of methanol are more or less comparable for both the electrodes. The slight lowering of Dapp values at higher temperatures, seemingly represents the sluggishness in the oxidation kinetics at temperature as low as 293 K and the effective oxidation rate is attained at temperature near 313 K when the accumulation of reaction intermediates adsorbed at the surface impede the rate of diffusion of the reactant species. Fig. 8 shows the pseudo-Arrhenius plots for the current densities of the methanol oxidation on the Pt/C and PtePd/C electrodes at different potentials. Almost linear relationships between log i and 1/T is maintained in all cases, indicating similar reaction mechanism at each potential and the apparent activation energies (Ea(app)) were calculated from the slope of these plots. The activation energy for both the electrodes when plotted against potential in Fig. 9, are found to be potential dependent. Reduction in the activation energy (from ∼17 kJ mol1 to ∼13 kJ mol1), is fairly linear in the potential range of 60 to 60 mV for both the electrodes. Thereafter the potential does not make much difference on the Pt/C electrode where as further decrease of Ea to ∼10.2 kJ mol1 is attained for PtePd system at high potential. The slightly larger value of Ea(app) for methanol oxidation on PtePd/C electrode at low potential could indicate that the rate determining is the adsorption step, where as the lower activation energy on Pt/C
at this potential ascribed to heterogeneous electrocatalytic process with mixed activation-adsorption control reaction [28,38].
3.3.
Effect of temperature on the impedance response
Electrochemical impedance spectroscopy (EIS) was used to investigate reaction process and kinetics of methanol oxidation on Pt/C and PtePd/C electrodes at different temperatures and at a fixed potential of 60 mV (vs. RHE). The Nyquist plot for methanol oxidation in a solution containing 0.5 M NaOH and 1.0 M methanol at temperature 293e353 K are shown in Fig. 10. The equivalent circuit elements as derived from Nyquist plot for methanol oxidation on Pt/C and PtePd/C electrodes at 60 mV (vs. RHE) are summarized in Table 1. The diameter of the semicircle decreased as temperature increased for both Pt/C and PtePd/C electrodes. The equivalent circuit compatible with the impedance data for both the catalysts are presented in the inset of the corresponding plots Fig. 10 (a,b). In this circuit Rs, CPE, Rct represent solution resistance, constant phase element and charge transfer resistance associated with the methanol oxidation primarily involving the dissociative adsorption of methanol on the surface. The charge transfer resistance (Rct) decreases with the increase in temperature of the solution for both Pt/C and PtePd/C electrodes indicating faster reaction kinetics of methanol oxidation at elevated temperature. The Rct values for methanol oxidation on the PtePd/C electrode is lower
Fig. 8 e Arrhenius plots for methanol oxidation at different potentials on (a) Pt/C and (b) PtePd/C electrodes in a mixture of 0.5 M NaOH and 1.0 M methanol.
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(-4)max to high frequency value for Pt/C compared to PtePd/C indicates the transformation of capacitive to the resistive behavior of the electrodeeelectrolyte interface. The contribution of several impedance parameters may further be substantiated by the evaluation through graphical method which however is not undertaken in this study [39,40].
3.4. Estimation of oxidation products found during methanol electro-oxidation
Fig. 9 e Dependence of apparent activation energy for methanol oxidation with potentials on Pt/C and PtePd/C electrodes.
than that on the Pt/C electrode at lower temperature, however Rct values are comparable for both the electrodes particularly at higher temperature and suggests improved methanol oxidation kinetics on PtePd/C catalyst at lower temperature in alkaline medium. The overall low value of Rct for both the electrodes at this temperature range indicates faster rate of charge transfer in alkaline medium. The increase in solution conductivity with rise in temperature is reflected in the gradual decrement in Rs values with temperature. The CPE impedance coefficients for both the Pt/C and PtePd/C catalysts were increased with rise in temperature. The comparatively higher values exhibited for PtePd/C, translates to the double layer charging at the (OH)ads activated surface, facilitated in presence of Pd. The electro-oxidation of methanol shows inductive behavior at all temperatures only on Pt surfaces which is indicative of the presence of reaction intermediates being formed at the catalyst surface and getting chances for faster oxidation at higher temperatures. The circuit for the Pt/ C catalyst includes inductive resistance R0 and the inductance, L. Interestingly a drastic fall in the values is observed with the rise of temperature from 293 to 353 K, which is attributed to a greater temperature influence on the methanol oxidation kinetics on Pt/C electrodes. This is furthermore elucidated with the Bode plots (Inset). The distinct shift in
The effect of varying temperature on the rate of product formation for oxidation of methanol was studied over a range of temperatures (293e353 K) on Pt/C and PtePd/C electrodes respectively. The concentration of the end products were determined through ion-chromatographic analysis of the chronopotentiometric aliquots taken at 60 mV after 2 h. The overall oxidation reaction equation for the methanol in alkaline medium is supposed to be
CH3OH þ6OH / CO2 þ 5H2O þ 6e
(A)
Complex process of methanol electro-oxidation in alkaline medium involving 6-electron transfer and several intermediate organic species is suggested to proceed through the following paths:
The presence of formate and CO2 (as carbonate) as the main soluble reaction products and CO as the main poisoning species have been detected in alkaline medium [9,41,42]. The (CO)ads formation follows pathway ‘b’ or ‘c’ and responsible for current decay during methanol oxidation. The selectivity of product formation largely depends on the potential as well as temperature. The first step of the oxidation process is the dissociative adsorption of methanol followed by subsequent dehydrogenation, water activation, noble metal oxide formation at higher potentials, its reduction and finally desorption or oxidation of the organic species by reactivation.
Fig. 10 e Impedance spectra for methanol oxidation at different temperatures on (a) Pt/C and (b) PtePd/C electrodes in a mixture of 0.5 M NaOH and 1.0 M methanol at L60 mV.
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Hydroxide ions get adsorbed on the electrode surface and initiate the formation of surface hydroxide and oxide species. The associated electrochemical reaction occurring at the electrodeeelectrolyte interface is as follows [43,44] M þ OH # M-(OH)ads þ e
(B)
M þ 6 OH # [M(OH)6] 2sol þ 4e
(C)
MeOH þ MeOH / MeO þ H2O þ M
(D)
A low level of hydrous oxide may be formed at the metal sites and surface defects due to the repetitive formation and reduction of thin compact layer via the above reactions. This hydrous oxide supplies the active oxygen atom to oxidize the organic species. The OH ions required for the equilibrium are mainly supplied by the solution OH ion at higher potential. Through the Pd incorporation into the Pt matrix, the hydrogen adsorption and desorption characteristics is significantly improved at much lower potential than that on the Pt alone. The desorption of H particularly at the Pd sites plays important role in the anodic potential region in alkali medium and activate the catalyst surface promoting deprotonation. The surface hydroxides are more readily formed in presence of Pd in the catalyst matrix. Thus the poisoning effect of (CO)ads is efficiently reduced due the synergic effect of Pt and Pd in the matrix. Dissociative chemisorption of methanol molecules on Pt proceeds through the following steps: M þ (CH3OH)sol # M-(CH3OH)ads
(E)
M-(CH3OH)ads þ OH / M-(CH3O)ads þ H2O þ e
(F)
M-(CH3O)ads þ OH / M-(CH2O)ads þ H2O þ e
(G)
M-(CH2O)ads þ OH / M-(CHO)ads þ H2O þ e
(H)
M-(CHO)ads þ OH / M-(CO)ads þ H2O þ e
(I)
M-(CO)ads þ OH / M-(COOH)ads þ e
(J)
M-(COOH)ads þ OH / M þ CO2 þ H2O þ e
(K)
(where M stands for PtePd) In presence of adequate alkali the oxidation of M-(CHO)ads and M-(CO)ads may proceed directly through the steps (L) and (M) to the ultimate production of CO2. M-(CHO)ads þ 3OH / M þ CO2 þ 2H2O þ3e M-(CO)ads þ 2OH (on M) / M þ CO2 þ H2O þ 2e
(L)
Fig. 11 e Effect of temperature on (a) carbonate and (b) formate production during methanol oxidation on Pt/C and PtePd/C electrodes in a mixture of 0.5 M NaOH and 1.0 M methanol at L60 mV for 2 h.
species may not be possible at low potential as they strongly bind to the electrode surface. However, at higher potential, highly active oxygen atoms again become available and the poisonous PtCO or Pd2CO species are oxidized via the reaction (J) or (M). Fig. 11 (a,b) shows the yield of carbonate and formate ions on both PtePd/C and Pt/C electrodes throughout the temperature range studied in the present investigation. The formate and carbonate yield on both the catalysts increased approximately linearly with temperature. Extensive production of formate is observed (Fig. 11b) with PtePd/C electrode which further rise with temperature. On the other hand, the appreciably high yield of carbonate ions at high temperatures (Fig. 11a) with the Pt/C electrode is due to favorable oxidation of (CO)ads getting oxidized at such temperatures. The higher yield of formate ions for methanol oxidation reaction on PtePd/C in the present investigation indicates that the formate is produced following the path ‘a’ and ‘c’. The presence of Pd in the alloy facilitates the adsorption of OH ion on the surface so that the (OH)ads reacts with the (CO)ads and methanol oxidation reaction proceeds through path ‘c’ (Equation J).
(M)
The strength of the bonding of (CHO)ads on the surface probably determines the entire rate of the reaction. In general the chemisorbed bonding of (CHO)ads on Pt group of metals in alkali is weak, such that further oxidation takes place without much difficulty, i.e. without irreversibly blocking the electrode’s active sites. The several intermediate organic species have been identified by FTIR spectroscopic study which revealed that the CO species are found to be linearly bonded to Pt sites (PtCO) and bridge bonded to Pd sites [Pd2C]O] [44]. Complete removal of CO
4.
Conclusion
The PtePd/C catalyst exhibits better catalytic activity for methanol oxidation in comparison to Pt/C, however following different pathway, particularly at higher temperatures. For methanol oxidation, the onset potentials on PtePd/C catalyst shows negative shift with temperature and the values are lower than that obtained with Pt/C catalyst. The decay in oxidation current density with time is nominal for the PtePd/
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C catalyst at all temperature while the Pt/C catalyst shows appreciable current decay indicating higher rate of catalyst poisoning. Pd addition reduces the activation energies and the charge transfer resistance and promote the electrocatalytic activity towards the methanol oxidation. The rapid kinetics of the dissociative adsorption in alkali media followed by facile oxidation of the intermediates species on the alloyed surface is demonstrated by the favorable kinetic parameters of the charge transfer reaction. The beneficial effect of catalysis by Pd addition to Pt is ascribed to the facile adsorption of OH on this surface as furnished in the suggested mechanism and is further reflected by the high yield of formate as well as low yield of carbonate obtained during the oxidation of methanol on PtePd/C catalyst. The overall impression is that Pt nanocatalyst can be comfortably replaced by its alloyed form with Pd, whereby the Pt loading is substantially reduced without compromising the catalytic output
Acknowledgement
[12]
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[14]
[15]
[16]
[17]
The authors gratefully acknowledge MHRD, New Delhi, Govt. of India for the instrumental facility provided to the department.
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