Electro-oxidation of isopropanol on to Pt loaded carbon felt surface modified by polyaniline.
Advances in the field of electrically conducting polymers have led to development of materials with wide application potential [1-2]. There are various polymeric films, such as polypyrrole (PPY), polyaniline (PANI), poly (3-methylethiophene) (PMT), poly(3,4-ethylenedioxythiophene) (PEDOT) etc. A thin film of a conducting polymer (CP) improves the interfacial properties between the electrode and the electrolyte. Conducting polymers can allow a facile flow of electronic charge during the electrochemical oxidation of alcohol on Pt. An electrochemically deposited conducting polymer developes three dimensionally on a substrate . Therefore, it introduces roughness and a high porosity, which generate a large surface area for electrochemical reactions. Polyaniline (PANI) is most interesting material among the various conducting polymers because of its moderately high conductivity, reversible redox system, easy preparation and possible applications as electro-catalyst towards various electro-oxidation reactions.
Direct alcohol fuel cells (DAFCs) attract tremendous attention as power sources in numerous applications at low operating temperature. Among several alcohols, which can be used in a direct alcohol fuel cell (DAFC) are methanol, ethanol and 2PrOH. Electro-oxidation of higher carbon atom (longchain) alcohols involves more intermediates and products than that of methanol electro-oxidation and thus more efficient electro-catalysts are needed at lower temperature. Platinum is the best choice as an electro-catalyst, but its cost and surface poisoning phenomenon inhibit it from large scale applications.
The most vital problems of fuel cells (Direct alcohol fuel cells, DAFCs) are poisoning of the electro-catalyst even with Pt as the catalyst and its crossover from anode to cathode compartment. A major bottleneck for commercial operation of platinum catalysed alcohol oxidation is catalyst deactivation: catalyst over-oxidation, catalyst poisoning, and corrosion. The summation of all mechanisms decreased the number of active catalytic sites. The reaction is generally assumed to take place on reduced (zero-valent) platinum sites. On exposure to oxygen the platinum surface is oxidised, resulting in the formation of an inactive surface platinum oxide. This is also referred to as over oxidation and sub-surface oxygen formation. Various surface intermediates and by-products are formed during alcohol electro-oxidation. Some of these intermediates are not readily oxidisable and remain strongly adsorbed on to the catalyst surface. Consequently, they prevent fresh alcohol molecules from adsorbing and undergoing further electro-oxidation reaction. Thus electro-oxidation of intermediates is the rate limiting step. This poisoning of the catalyst surface seriously slows down the electro-oxidation reaction. Besides, a small percentage of the intermediates desorbs before being oxidized to CO2 and hence reduce fuel efficiency. The electro-catalytic activity may be increased using conducting polymers. It has been reported that modified polyaniline (PANI) electrodes have catalytic properties for organic fuel oxidation. Polyaniline is usually used as matrix to incorporate noble metal catalysts in the application for electro-oxidation of small molecules such as hydrogen, methanol and formic acid, etc. The reason for incorporating metallic particles into porous matrixes is to increase the specific area of these materials and thus improve catalytic efficiency. Another reason is the higher tolerance of the platinum particles to poisoning effect due to adsorption of CO species, in comparison with the serious problem of poisoning effect on bulk platinum electrodes. On the other hand, it is of great importance to investigate the catalytic activity of low loading platinum catalysts and to determine the lowest metal loading necessary to reach the practical performances in terms of current density and power density because of relatively high cost of catalytic material.
2 RELATED WORK
Conducting polymers are usually used as matrix to incorporate noble metal catalysts in the application for electro-oxidation of small molecules such as hydrogen, methanol and formic acid. Kitani et al.  prepared Pt-modified PANI on carbon electrodes by electrochemical deposition and studied catalytic activity for methanol electro-oxidation. From cyclic voltammetry and chronoamperometry studies, they showed greater catalytic activity for Pt/PANI/C electrodes than Pt/C electrodes itself. The remarkable effect of PANI was attributed to a greater dispersion of Pt particles on PANI. Liu et al.  studied the effect of preparation conditions of PANI on the dispersion of Pt for methanol oxidation. Galvanostatically prepared PANI consisted of porous and interconnected nanowires, and therefore, Pt particles were dispersed more uniformly than on potentiostatically prepared PANI. The former electrode was shown to have greater catalytic activity for methanol oxidation. The electro-deposition of platinum micro particles into polyanlllne (PANI) flims on glassy carbon (GC) electrodes and their catalytic activity for the reduction of hydrogen and the oxidation of methanol were described by Kost et al. . Electrodeposited platinum micro particles are dispersed in a three-dimensional array in fibril-type PANI film electrodes as evidenced by scanning electron microscope photomicrographs. These Pt/ PANI /GC electrodes exhibit good activity with respect to the catalytic reduction of hydrogen and the catalytic oxidation of methanol.
Moreover, it has been recently reported that the polyaniline (PANI) film coated electrodes have been used as electro-catalyst for alcohol oxidation. It has been recently reported by Prasad et al.  that the PANI film, electrodeposited onto Pt without added Pt particles, is able to oxidise methanol at concentrations above 1 M. At lower concentrations, adsorption of methanol occurs on PANI resulting in a reduction in intrinsic voltammetric peak currents of PANI in sulphuric acid.
Methanol oxidation and CO adsorption were studied on Pt electrodes modified by electro-deposited PANI films (PANI/Pt) . Differential electrochemical mass spectrometry (DEMS) and Fourier transform infrared spectroscopy (FTIRS) were used to provide evidence of the existence of a free metal surface under electrodeposited PANI films. The electro-catalytic properties of platinum nano particles incorporated into PANI films were investigated for oxidation of ethanol in sulphuric acid solutions by Fungaro et al. The effect of the formation conditions of platinum-modified PANI films (as cycle numbers of CV, sweep rates) and alcohol concentration were evaluated. The platinum-modified PANI films formed by the optimization conditions can be used as a conducting substrate for the ethanol electro-oxidation.
Above studies show that both methanol oxidation and CO adsorption occur on Pt electrodes modified by electro-deposited PANI films (Pt/PANI) in the absence of Pt particles. The literature for 2-PrOH electro-oxidation onto PANI coated electrode is hardly found. In view 2-PrOH as the potential fuel, fundamental insight for the electro-oxidation of these alcohols is important. Moreover, one of the common approaches to increase energy and power density of an electrode is to use light weight electrode materials with maximum active material utilization factor. Again conducting polymers are promising electrode materials for a large number of electrochemical cells. They offer low cost, light weight and can easily be moulded into any shape and size.
In the present work, the application of PANI in modification of catalytic activity of Pt loaded C-felt electrode towards 2-Propanol electro oxidation has been reported. We are particularly interested in the study of oxidation kinetics in terms of anodic peak current with the variation of thickness of the deposited PANI onto Pt loaded C-felt surface. The thickness has been controlled by controlling the deposition time. We have studied the electro oxidation of 2-PrOH onto these surfaces by cyclic voltammetry and amperomtry technique.
3 MATERIALS AND METHODS
[H.sub.2]S[O.sub.4] (Merck), 2-Propanol (Merck), [H.sub.2]Pt[Cl.sub.6]6[H.sub.2]O (Arrora Matthey Limited ) were used as supplied. Aniline (Merck) was distilled .The distillate was collected rejecting head and tail fractions. 2-PrOH (Merck) was distilled before use. Carbon felt (Alpha-Aesar) was used as anode support with geometrical area 2 cm2. Electrochemicxal measurements were performed using a potentiostat- galvanostat( PAR Versastat TM II ).All experiments were performed at 32[degrees]C.
3.2 Electrode preparation
Pt particles were deposited on the working electrode under galvanostatic condition using +2.5mA/[cm.sup.2] current for 30 minutes from chloroplatinic acid solution.
Polyaniline has been prepared using standard electrochemical technique. Electrolyte solution was 0.5 M aniline in 1M H2SO4 solution. The solution was taken in a one compartment cell in which Pt deposited C-felt (or C-felt) was used as working electrode and counter and reference electrode merged to Pt wire. 1mA/[cm.sup.2] current under galvanostatic condition was passed for 100 seconds ,300 seconds and 600 seconds to deposit a film of polyaniline and the corresponding electrodes are designated as E1,E2 and E3. After polymerization the polymer coated electrodes were washed with distilled water and used for electrochemical studies.
3.3 Electrochemical measurement
The catalytic activity of all the electrodes in 2-Propanol oxidation was measured by cyclic voltammetry. A three electrode setup was constructed for this study, where carbon supported electrodes (1cm x 1 cm) were the working electrodes, the counter electrode was the Pt foil (1 cm2) while a saturated calomel electrode (SCE) served as reference electrode .The electrolyte was a 0.5 M H2SO4 solution, either with or without 2-Propanol. We investigated the cyclic voltammetry of PANI coated electrodes in H2SO4 solution and found that within the potential limits, the response changed on cycling . After about the five cycles, the PANI film shows a steady response and then it was used as an anode in 2-PrOH fuel cell. Amperometry measurements were done at 0.45 V for 1800s.
3.4 Electrode characterization
The morphology of surfaces of anode was investigated using scanning electron microscope (SEM, Hitachi S-3000N). The scanning electron microscope (SEM) is a type of electron microscope that images the sample surface by scanning it with a high-energy beam of electrons. The electrons interact with the atoms that make up the specimen sample producing signals that contain information about the sample's surface topography and other properties.
4 RESULTS AND DISCUSSIONS
Figure 1 and figure 2 show the SEM images of the C-felt and C-felt /Pt-PANI (E1) electrode surfaces, respectively. The Hitachi S-3000N is a variable pressure scanning electron microscope and has a tungsten electron source. The Hitachi S3000N is able to accept specimens up to 150mm in diameter and 20mm high. Specimens were imaged in SEM without any further specimen preparation as the specimens were conductive. From the figure 2, it is clear that each fibre of the C-felt electrode is covered with a uniform layer of PANI with some cracks in a non uniform manner.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
Figure 3 shows typical CVs of 2-Propanol electro-oxidation onto Pt loaded C-felt support and modified electrochemically prepared PANI at different thickness at scan rate 50 mV.[s.sup.-1]. The loading of the electro-active metals onto carbonfelt has been calculated considering 100% coulombic efficiency. Calculated Pt loading is found to be 4.546.mg.
[FIGURE 3 OMITTED]
The real surface area can not be determined so geometric area is to be considered here. It is interesting to note that typical oxidation peaks of 2-Propanol which are well reported onto Pt surface are also clearly shown in the presence of PANI . The peak at about 0.55V attributed to dehydrogenation of 2-Propanol and the second peak is due to the overall oxidation of 2-Propanol. From the CV, it is clearly seen that Pt loaded C-felt enhance electro oxidation significantly when it is modified with thin film of PANI but the observed enhancement gradually decreases with thickness of the polyaniline. The variation of Ip1 (first anodic oxidation peak current) and Ip2 (second anodic oxidation peak current) of this system are shown in figure 4 and 5.
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
To compare the catalytic activity, we have compared the current values at 0.5 V and this is shown in figure 6.
[FIGURE 6 OMITTED]
So at any particular applied potential, the current values are significantly enhanced when surface is modified by PANI, current increases with the increase in thickness of the films and then decreases showing a maximum. The maximum current is observed for the films deposited for at about 150 seconds. The catalytic activity of the electrode towards 2-PrOH oxidation decreases, if the deposition is carried out for more than 150 seconds, which is probably due to the blocking of active sites of the electrode as well as the slow diffusion of 2-PrOH molecules through the thick PANI layer. A thin deposited PANI exhibits highest catalytic activity. It should be noted that with increasing thickness of the conducting polymer coating the catalytic activity of the electrode towards 2-Propanol oxidation decreases. The improved catalytic activity of PANI modified electrode towards 2Propanol oxidation may be explained in terms of better adsorption of 2-PrOH molecules onto the nano pockets of PANI surface. The sizes of the pockets are of 5 to 20 nm . PANI may adsorb some of the intermediates which block the active sites. A thick coating over the C-felt/Pt surface with PANI film decreases the oxidation current by blocking the diffusion of 2-Propanol molecules. Again, with increasing thickness, the number density of porous site onto the surface gradually decreases hindering adsorption of alcohol molecules. The data agree with previous observations using spectroscopic ellipsometry that PANI film porosity diminishes with increasing film thickness up to a constant value [12-13]. Figure 7 shows that the tolerance power of the modified electrodes is high at all scan rates.
[FIGURE 7 OMITTED]
A higher [I.sub.f] /[I.sub. b] ratio indicates better oxidation during anodic scan and less accumulation of oxidisable species on the catalyst surface. PANI may adsorb some of the intermediates which block the active sites . The adsorption prevents the dispersed Pt particles from becoming deactivated.
Amperometry studies (Figure 8) show that the catalytic activity of E1 and E2 are better than that of C-felt/Pt for 2-Propanol oxidation.
The currents Ip of peak1 increase with the square root of scan rate. Plots of the anodic peak currents of 2-PrOH oxidation (Ip) versus scan rates (V) ranging from 30 to 70 mV [s.sup.-1] gave straight lines (figure 8) and thus obeys the following relationship.
Ip =2.985*[10.sup.5] n [[(1-[alpha])[n.sub.[alpha]].sup.-1/2] A [D.sup.1/2] C [V.sup.1/2] (1)
Where [Ip.sub.a] is the anodic peak current (mA), n is the number of electrons involved in the oxidation, A is the area of electrode ([cm.sup.2]), V is the scan rate ([Vs.sup.-1]), C is the concentration of the electro active species in bulk solution (mol [cm.sup.-3]). Thus 2-Propanol electro-oxidation is an irreversible one.
[FIGURE 8 OMITTED]
[FIGURE 9 OMITTED]
This work shows an interesting application of modification of electrode surface by a thin film of conducting polymer, polyaniline (PANI). It may be concluded that C-felt/Pt-PANI (E1 electrode, thin film) electrode exhibits better performance in its electro-catalytic activity towards electro-oxidation of 2-PrOH in [H.sub.2]S[O.sub.4] medium. This has been explained in terms of presence of nano pockets onto thin PANI surfaces and thus better adsorption of 2-PrOH. It should be noted that with increasing thickness of the conducting polymer coating the catalytic activity of the electrode towards 2-PrOH electro-oxidation decreases which is probably due to the blocking of active sites of the electrode surface by thick deposited polymer layer. The variation of catalytic activity towards 2-Propanol oxidation with thickness of PANI shows that a thin film enhances catalytic activity but a thick film deactivates.
Authors acknowledge the immense help received from the authors whose articles are cited and included in references of this manuscript. The authors are also grateful to Prof S Shome (GSI, India) for SEM study.
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Abhik Chatterjee (1), Moitrayee Chatterjee (2), Susanta Ghosh (2), I Basumallick (2)
(1) Department of Chemistry, Raiganj College (University College), Raiganj-733134,India.
(2) Department of Chemistry, Visva-Bharati University, Santiniketan-731235, India.
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|Author:||Chatterjee, Abhik; Chatterjee, Moitrayee; Ghosh, Susanta; Basumallick, I.|
|Publication:||International Journal of Emerging Sciences|
|Date:||Mar 1, 2012|
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