Applied Surface Science 252 (2006) 7964–7969 www.elsevier.com/locate/apsusc
Platinum particles dispersed poly(diphenylamine) modified electrode for methanol oxidation P. Santhosh a, A. Gopalan a,*, T. Vasudevan a, Kwang-Pill Lee b b
a Department of Industrial Chemistry, Alagappa University, Karaikudi, India Department of Chemistry Graduate School, Kyungpook National University, Daegu 702-701, South Korea
Received 1 December 2004; received in revised form 26 March 2005; accepted 6 October 2005 Available online 28 November 2005
Abstract A modified potentiostatic method, termed the ‘pulse pontentiostatic method’ (PPSM) was used to get nano fibrillar poly(diphenylamine) (PDPA) film on Indium tin oxide (ITO) coated glass electrode and also for making modified electrode with platinum particles dispersed in PDPA. Platinum clusters were electrodispersed under constant potential on PDPA films to obtain catalytic electrodes for methanol oxidation. Energy dispersive analysis of X-rays (EDAX) results showed that the Pt microparticles are deposited into PDPA film. Scanning electron micrograph, SEM images show that the deposition results spherical catalytic particles. X-ray photoelectron spectroscopy (XPS) results inform that the net electronic charge on carbon atom and also the imine/amine ratio was not affected by Pt loadings. The modification of electrode surface by nano fibular PDPA improves the electrocatalytic activity for methanol oxidation. # 2005 Published by Elsevier B.V. Keywords: Polydiphenylamine; Platinum; Modified electrode; Morphology; Methanol oxidation
1. Introduction The expensive catalytic materials, such as platinum, and relatively low electrocatalytic efficiency for electrochemical reactions of the fuel are drawbacks to be circumvented for commercial applications. Anodic methanol oxidation at a platinum catalyst is a self-poisoning reaction that blocks the active electrocatalyst surface through strong adsorption of reaction intermediates such as CO. As a result, poor mass specific power densities are obtained for the cell [1,2]. To improve both the oxidation rate and electrode stability, considerable efforts have been applied to the study of electrode materials for the direct electrochemical oxidation of methanol [3–13]. The electrocatalytic activity of platinum could be improved by making composite electrodes for methanol oxidation. At this juncture, recent advances have shown that electronic conducting polymers such as polypyrrole [14,15], polythiophene [16] and polyaniline [17–19] can serve as porous
* Corresponding author. Tel.: +91 4565 228836; fax: +91 4565 225202. E-mail address:
[email protected] (A. Gopalan). 0169-4332/$ – see front matter # 2005 Published by Elsevier B.V. doi:10.1016/j.apsusc.2005.10.002
supports to disperse the platinum particles and the resulted composites have excellent properties. Polyaniline (PANI) has been applied extensively to support platinum catalysts. Platinum can be dispersed in such a support [17–19] that leads to a decrease in the amount of expensive noble metal used. Also, such composites have improved catalytic activity for the oxidation of methanol via a better utilization of the platinum crystallites and in decreasing the poisoning effect [20]. Polydiphenylamine (PDPA) could be grown as stable and adherent film on electrode surface under potentiodynamic conditions from aqueous solutions [21]. The polymer is a conductor in its partially oxidized state, which occurs in the potential range where most organic fuels oxidize [22,23]. Hence, dispersion of metallic particles inside PDPA can provide electrocatalytically active PDPA. It has also been reported that the use of a rigid matrix of conductive polymers allows a better dispersion of electrocatalytic particles through larger portion of the availability of surface to participate in the dispersion and thus prevent agglomeration of metallic particles [24]. The present work presents the preparation of a novel polymeric electrode by dispersing the Pt particles in PDPA as matrix under periodical potential perturbation. The resulting
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modified electrode was characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and EDAX, XPS and cyclic voltammetry were used to evaluate the electroactivity with respect to the oxidation of methanol.
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X-ray photoelectron spectroscopy (XPS) was performed with Microlab 310D spectrometers. XPS spectra employed Mg Ka (hn = 1253.6 eV) irradiation as the photon source, with a primary tension of 12 kV and as emission current of 20 mA. The pressure of the analysis chamber during the scan was about 1010 mbar.
2. Experimental details 2.5. Evaluation of the Pt/PDPA electrode 2.1. Materials Diphenylamine, DPA (E-Merck) and H2PtCl6 (Aldrich) were used without additional treatment. All other reagents were of analytical grade. All solutions were prepared with double distilled water.
The Pt/PDPA film was evaluated with regard to the electrosorption of methanol in 0.5 M H2SO4. It consists of adsorbing methanol at a fixed potential (Eads = +180 mV). The adsorbed species are formed using a single sweep of potential at a scan rate about of 50 mV/s to minimize further readsorption of methanol species.
2.2. Electrodeposition of PDPA film 3. Results and discussions Films of PDPA were deposited by the galvanostatic method (GSM) or the pulse potentiostatic method (PPSM) using a three-electrode cell assembly from 40 mM DPA in 4 M sulfuric acid aqueous solutions kept at room temperature using BAS 100 BW Electrochemical Analyzer. The working electrode was an Indium tin oxide (ITO) coated glass electrode of resistance 10 V/&. The working electrode was cleaned with acetone before the deposition of PDPA film. A platinum foil and Ag/AgCl were used as counter and reference electrode respectively. The film thickness was determined from the anodic charge, which was involved in the anodic scan of the cyclic voltammogram run in 0.5 M H2SO4 solution [25]. 2.3. Dispersion of platinum particles in polymer films Platinum was potentiostatically electrodeposited into the PDPA films. The reduction of Pt particles were accomplished at either 200 mV or +200 mV in the solution of 0.5 mM H2PtCl6 and 0.5 M H2SO4 solution under nitrogen atmosphere. The charge utilized for the deposition was used to estimate the amount of platinum included in the PDPA matrix. 2.4. Characterization of Pt/PDPA electrode SEM observations were carried out with Au coated vacuum ion sputter by liquid hydrogen method using Cambridge Scanning Microscope with energy dispersive analysis of X-rays (EDAX). The rotating system was attached in the instrument, utilized for accurate measurements for various magnifications.
3.1. Morphology of PDPA synthesized by PPSM method and GSM method Electrochemical polymerization provides the possibility of controlling the thickness and homogeneity of the polymer film on the electrode surface. It has been reported that conducting polymer films with dispersed Pt microparticles has a good electrocatalytic activity [17–19], though the polymer morphology depends on the conditions of electropolymerization. Polymer films with different microstructure can be obtained by different methods. Polydiphenylamine, PDPA, has been synthesized by galvanostatic method (GSM), cyclicvoltammetry (CV) or a pulse potentostatic method (PPSM) [21]. On comparision of the stability and adherence of the polymer film deposited by the above mentioned methods, PPSM provides superior film and offers advantages. Strong activation of growth of PANI has been reported when the pulse programme was applied [21,22,26,27]. Different reasons were assigned for the activated deposition by PPSM. Continuous redistribution of redox centers in polymeric nuclei, periodic stretching and relaxing of polymer chains allowing monomer molecules to diffuse into it, the possibility of charged oligomeric species align close to the surface of the electrode making higher electroactive molecules ready for deposition during the periodic pulses, were considered for the activated growth. Besides, the discontinuous potential process reduces the concentration polarization of electrode/solution interface. In the present work, the PDPA films were synthesized by both PPSM and GSM methods. Fig. 1(a) shows the surface
Fig. 1. Surface morphologies of PDPA films prepared by PPSM (a) and GSM (b).
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micrograph of the PDPA films synthesized by PPSM. From Fig. 1(a), PDPA films with nano fibular structure can be observed. For comparison, PDPA films were synthesized by galvanostatic method (GSM). The corresponding SEM figure is given in Fig. 1(b). A typical granular PDPA structure with individual granules having different diameter is observed. Compared with granular PDPA film, nano fibular PDPA film has relatively higher specific surface area. The available larger area is beneficial for Pt dispersion. Furthermore, the smaller transfer resistance of conductive particles in fibular PDPA film decreases the catalyst poison [28]. This means that nano fibular PDPA film can be used as a better catalyst support.
3.2. Platinum micro particles deposition in the polymer film Electrochemical deposition of platinum in PDPA films was potentiostatically performed using an acidic hexachloroplatinate solution. The SEM morphologies of Pt/PDPA films are shown in Fig. 2. Fig. 2(a) shows a SEM micrograph of 200 mg cm2 deposited of Pt at 200 mV. It is apparent from the SEM figure that agglomeration of platinum appears with clusters of particles in the PDPA film. SEM micrographs of 500 mg cm2 and 600 mg cm2 Pt deposition at +200 mV for 5 h were presented in Fig. 2(b and c), respectively. Here, heavy agglomeration of platinum occurs with a cluster of particles. In the spherical particles of 1–5 mm in diameter, the deposition of platinum particles seems to be uniform (Fig. 2(b) than Fig. 2(c)). Owing to the high standard potential of platinum reduction, the platinum particles can be readily deposited at both 200 mV and +200 mV. A perusal of the micrographs of Pt deposited PDPA at different potential shows that the size of the Pt particles depends on the potential applied. The particles size deposited at 200 mV (Fig. 2(a)) appears to be much smaller than that of one deposited at +200 mV (Fig. 2(b and c)). This may be due to the fact that hydrogen adsorption along with the Pt deposition is more pronounced when the potential is at 200 mV than +200 mV, and consequently, the competitive hydrogen adsorption inhibits
the growth of nuclei resulting in deposition of smaller particle size. The possible reactions are [29]: Nucleation of Pt particles: PtCl2 6 þ 2e
! !
PtCl2 4 þ 2Cl ðþ2e þ PDPAÞ Pt=PDPA þ 6Cl
(1) Crystal growth of Pt particles: PtCl2 6 þ 2e
! PtCl2 4 þ 2Cl ðþ2e þ PtÞ ! Pt=PDPA þ 6Cl Adsorption of H on the Pt dispersed nuclei:
(2)
Hþ þ e þ Pt , PtH H2 evolution reaction:
(3)
2PtH ! 2Pt þ H2 or
(4)
PtH þ Hþ þ e ! Pt þ H2
(5)
Nucleation is more pronounced for Pt deposition at +200 mV than at 200 mV. In the deposition process of Pt on PDPA as substrate, the hydrogen ions could also be competitively adsorbed onto the electrode surface. It may ultimately result the formation of a reversible covalent bond (Eq. (3)). The reaction mentioned in Eq. (3) may significantly occur at 200 mV than at +200 mV. The other possible reaction after the reaction in Eq. (2) is mentioned as Eq. (4) or Eq. (5), wherein the adsorbed protons directly converted into hydrogen gas. The role of adsorbate (H+) may be visualized in two ways; the hydrogen ion can compete with the electroactive species for reaction sites (Pt) on the electrode surface, thereby lowering the concentration of reactant at the site of electron transfer. Alternatively, the adsorbate (H+) may cover the surface and increase the electron hoping between Pt ions and Pt particles as difficult. As a result, at +200 mV, reactions as represented by Eqs. (1) and (2) may be dominant, On the other hand, at 200 mV reactions in Eqs. (1), (3) and (4) or Eq. (5),
Fig. 2. SEM images of ITO/PDPA-fibular/Pt. Pt loading 200 mg cm2, Pt deposited at 200 mV (a), Pt loading 500 mg cm2 (b) 600 mg cm2 (c), Pt deposited at +200 mV for 5 h.
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Fig. 3. EDAX plot for Pt dispersed PDPA, Pt loading 600 mg cm2, Pt deposited at 200 mV for 5 h. Accelerating voltage 20 keV.
may be predominant. As a result, the size of Pt particles deposited at +200 mV is larger than that deposited at 200 mV. The EDAX plot for Pt deposited PDPA electrode with Pt loading of about 600 mg cm2 deposited at 200 mV for 5 h is shown in Fig. 3. The EDAX confirms the presence of Pt on the modified electrode. The presence of other elements such as S and Cl, are due to the composition of the working solutions.
Fig. 5. XPS N1s core spectra of PDPA and Pt dispersed PDPA.
3.4. Electrosorption of methanol on Pt/PDPA electrode 3.3. XPS studies XPS was employed to analyze the core-level spectra of C, N and Pt. The C1s and N1s core-level spectra of PDPA and Pt/ PDPA are shown in Figs. 4 and 5, respectively. The close comparison of C1s core-level spectra (Fig. 4), reveals that the C1s spectra for PDPA and Pt/PDPA are virtually identical. The net electronic charge residing on the carbon atoms of PDPA are not influenced by the presence of Pt. The core-level spectra of N1s for PDPA and Pt deposited PDPA were also identical (Fig. 5). These results inform that the imine and amine nitrogen present in PDPA were not changed by the deposition of Pt particles. The Pt core-level spectrum of Pt/PDPA is shown in Fig. 6. The figure represents Pt/PDPA film prepared with Pt loading of about 600 mg cm2, deposited at 200 mV for 5 h. The spectrum confirms the presence of Pt particles in the PDPA film. Also, Pt is present mainly as Pt(0) not in Pt(IV) state as reported by Hable and Wringhton [30].
Fig. 4. XPS C1s core spectra of PDPA and Pt dispersed PDPA.
The Pt/PDPA electrode was prepared by depositing Pt at 200 mV and with a 600 mg cm2 loading of Pt particles. The voltommogram of a Pt/PDPA electrode in 0.5 M H2SO4 is shown in Fig. 7 (solid line). The CV recorded for PDPA in 1 M H2SO4 is also given in Fig. 7 (dotted line) for comparison. An adsorption time of 30 s and adsorption potential of about +180 mV (Eads) was chosen to be suitable for methanol adsorption. It is important to note that the redox peaks of PDPA are present even after the Pt loading. On the basis of the above results, it can be concluded that the electrode is suitable for the study of electrosorption. Fig. 8 shows the CVs of the electrosorption/electrooxidation of the previously adsorbed methanol at Eads = +180 mV for two adsorption times, 10 s and 1 min with 1 M CH3OH in the supporting electrolyte. The peak at +600 mV (Fig. 8) corresponds to the oxidation of the species arising from preadsorbed methanol at +180 mV. The oxidation peak current
Fig. 6. XPS data of Pt dispersed PDPA, Pt loading 600 mg cm2, Pt deposited at 200 mV for 5 h.
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Fig. 7. Cyclic voltammograms of PDPA electrode (- - -), Pt dispersed PDPA electrode (—) in 0.5 M H2SO4 (vs. Ag/AgCl) Scan rate 50 mV/s.
for the methanol oxidation increases with the increasing the adsorption time (Fig. 8). The appearance of the peak during the reverse scan may be due to the adsorption of methanol from the solution containing 1 M CH3OH and 0.5 M sulfuric acid. For confirming the assignment of the cathodic peak for the adsorption of methanol from the solution, CV of the Pt/PDPA electrode was recorded in a blank electrolyte (0.5 M H2SO4). The inset figure shows the CV for the preadsorbed methanol oxidation measured in a 0.5 M H2SO4 (blank electrolyte) solution. A sharp peak is observed during the anodic sweep without having methanol in the electrolyte solution. The peak observed at +600 mV is due to the oxidation of methanol, preadsorbed at +180 mV. It is important to note that there is no peak observed in the reverse scan (Fig. 8, inset). Hence, it can be concluded that the cathodic peak observed in CVs
Fig. 8. Cyclic voltammograms of CH3OH oxidation on the Pt dispersed PDPA electrode for various adsorption time in 1 M CH3OH + 0.5 M H2SO4 (Pt loading 600 mg cm2), Eads = 180 mV; (- - -) CV of Pt dispersed PDPA electrode in 0.5 M H2SO4 (vs. Ag/AgCl). Scan rate 50 mV/s (Inset: Cyclic voltammogram of preadsorbed methanol oxidation measured in 0.5 M H2SO4).
Fig. 9. Plot of anodic peak current as a function of Platinum loading (a), adsorption time (b) and thickness of PDPA film (c).
corresponding to the electrolysis of solution containing 1 M CH3OH arises from the adsorption of methanol from the solution. Further, it can be noticed from Fig. 8 that peak positions of Pt/PDPA electrode changed after the adsorption of methanol on the electrode surface. This may be due to the change of conducting PDPA layer into an insulating one as a result of incorporation of methanol into it. The anodic peak current associated with the methanol oxidation with Pt/PDPA modified electrodes having different Pt loadings is presented in Fig. 9(a). The anodic peak current increases with increase in the Pt loading and reaches saturation after a Pt loading of about 500 mg cm2. The variation of the anodic current as a function of adsorption time for methanol oxidation is given in Fig. 9(b). The anodic current increases significantly with increasing adsorption time. The decreasing trend in peak current is due to the saturation of active sites at the electrode surface. The influence of the thickness of PDPA film on electrocatalytic activity of methanol oxidation was
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investigated and the corresponding results are shown in Fig. 9(c). Under the condition of constant Pt loading (600 mg cm2), the anodic current rises dramatically for the film thickness up to 5 mm and drops afterwards. This implies that the electrocatalysis of methanol oxidation is sensitive to thickness of the PDPA. The increase in the peak current for the thickness of PDPA up to 5 mm may be due to the occupation of Pt particles in the pores of PDPA with the real sizes. The decrease in the anodic peak current for the methanol oxidation beyond 5 mm may be due to the lessening of real surface area of Pt particles as a result of masking the part of the real surface of pt particles by the excessive presence of PDPA on the surface of the electrode. 4. Conclusions Polydiphenylamine (PDPA) film with nano fibular structure was obtained with pulse potentiostatic deposition. Pt microparticles could be dispersed into PDPA film under 200 mV deposition potential. There is no noticeable interaction between Pt particles and the amine or imine groups in PDPA. The modification of electrode surface by nano fibular PDPA with dispersed Pt particles improves the electrocatalytic activity for methanol oxidation. References [1] M. Goetz, H. Wendt, J. Appl. Electrochem. 31 (2001) 81. [2] A.M. Castro-Luna, J. Appl. Electrochem. 30 (2000) 1137. [3] M. Watanabe, S. Saegua, J. Electroanal. Chem. 271 (1989) 213.
[4] [5] [6] [7] [8] [9] [10] [11] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]
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W.Z. Li, C.H. Liang, J.S. Qiu, Carbon 40 (2002) 787. Z.L. Liu, X.H. Lin, J.Y. Lee, Langmuir 18 (2002) 4054. J.H. Ye, P.S. Fedkiw, Electrochim. Acta 41 (1996) 221. A.A. Mikhaylova, O.A. Khazova, V.S. Bagotzky, J. Electroanal. Chem. 480 (2000) 225. K. Shimazu, R. Inada, H. Kita, J. Electroanal. Chem. 284 (1990) 523. M.P. Hogarth, J. Munk, A.K. Shukla, A. Hamnett, J. Appl. Electrochem. 24 (1994) 85. X.H. Xia, T. Iwasita, F. Ge, W. Vielstich, Electrochim. Acta 41 (1996) 711. W.F. Lin, J.T. Wang, R.F. Savinell, J. Electrochem. Soc. 144 (1997) 1917. W.R. Williams, C.M. Marks, L.D. Schmidt, J. Phys. Chem. 96 (1992) 5922. I. Becerk, S. Suzer, F. Kadirgan, J. Electroanal. Chem. 502 (2001) 118. K. Bouzek, K.M. Mangold, K. Juttner, Electrochim. Acta 46 (2000) 661. A. Yassar, J. Roncali, F. Garnier, J. Electroanal Chem. 225 (1988) 53. H. Laborde, J.M. Leger, C. Lamy, J. Appl. Electrochem. 24 (1994) 1019. K.M. Kost, D.E. Bartak, Anal. Chem. 60 (1988) 2379. A.A. Mikhaylova, E.B. Molodkina, O.A. Khazova, V.S. Bagotzky, J. Electroanal. Chem. 509 (2001) 119. K. Rajendra Prasad, N. Munichandriah, J. Power Sources 103 (2002) 300. V. Rajendrana, A. Gopalan, T. Vasudevan, T.-C. Wen, J. Electrochem. Soc. 147 (2000) 3014. C.-Y. Chung, T.-C. Wen, A. Gopalan, Electrochim. Acta 47 (2001) 423– 431. M.-S. Wu, T.-C. Wen, A. Gopalan, J. Electrochem. Soc. 148 (2001) D65– D73. P. Kulesza, M. Matczak, A. Wolkiewicz, B. Grzybowska, M. Galkowski, M.A. Malik, A. Wieckowski, Electrochim. Acta 44 (1999) 2131. D.E. Stilwell, S.M. Park, J. Electrochem. Soc. 135 (1988) 2491. V. Tsakova, A. Milchev, Electrochim. Acta 36 (1991) 1579. V. Tsakova, A. Milchev, J. Electroanal. Chem. 346 (1993) 85. L.T. Cai, H.Y. Chen, J. Appl. Electrochem. 28 (1998) 161. D. Pletcher, A First Course in Electrode Processes, Alresford Press Ltd., Hans, England, 1991, p. 32. C.T. Hable, M.S. Wrighton, Langmuir 7 (1991) 1309.
