Anal. Chem. 1988, 60, 2379-2384
LITERATURE CITED Slbbald, A. J. Mol. Electron. 1986, 2 , 51-63. Czaban. J. D. Anal. Chem. 1985, 57, 345-350A. Nylander. C. J. J. Phys. E: Sci. Instrum. 1985, 18, 736-750. Pickup, J. C. Lancet 1985, 617-620. North, J. R. Trends Biotechnol. 1985, 3 . 180-187. Sutherland. R. M.; Dahne, C.; Place. J. F.; Rlngrose, A. S. Clin. Chem. (Winston-Salem. N . C . ) 1984, 3 0 , 1533-1538. Aizawa, M.; Ikarlyama, Y.; Emoto. K. Proc. Int. Meet. Chem. Sens., 2nd 1986, 6-30, 622-625. Alzawa, M.; Kato, S.; Suzuki, S. J. Memb. Scl. 1977, 2 , 125-132. Umezawa, Y. Proc. Int. Meet. Chem. Sens. 1983, F.109, 705-710. Keatlng, M. Y.; Rechnltz, G. A. Anal. Chem. 1984. 56, 801-806. Collins, S.; Janata. J. Anal. Chim. Acta 1982. 736, 93-99. Janata, J.; Blackburn, G. F. Ann. N . Y . Acad. Sci. 1984, 428, 288-292. Katsube, T.; Hara, M. Proc. Int. Conf. So/.-State Sens. Actuators, 4th; 1987, 816. Ho, M. H. Proc. Int. Meet. Chem. Sens., 2nd 1986, 6-35, 639-643. Bastiaans. G. J.; Gocd, C. M. Prm. Int. Meet. Chem. Sens., 2nd 1988, 6-29, 616-621. Thompson, M.; Dhallwal, 0. K.; Arthur, C. L.; Caiabrese, G. S. IEEE Trans. Sonics UiYrason. 1987, SU-34, 127-135. Newman, A. L.; Hunter, K. W.; Stanbro, W. D. Roc. Int. Meet. Chem. Sens.. 2nd 1986, 6-23, 596-598. Mandrand. B.; Colin, B.; Martelet, C.; Jaffrezlc, N. European Patent No. 87401000. Kijhier, G.; Milstein. C. Nature (London) 1975, 244, 42-43. Gobet, J.; Kovats, E. Adsorpt. Sci. Techno/. 1984, 7 , 77-92. Antakll, S. C.; Serpinet, J. Chromatographle 1987, 2 3 , 767-769.
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(22) Weetall, H. H. Immobilized Enzymes, Antigens, Antibodies and Peptides; Dekker: New York, 1975; p 525. (23) Gaget, C.; Morel, D.; Traore, M.; Serplnet, J. Analusis 1984, 72, 386-392. (24) Morel, D.; Serplnet, J. J. Chromatogr. 1980, 200, 95-104. (25) Szabo, K.; Le Ha, P.; Schneider, P.; Zekner, P.; Kovats, E. Helv. Chim. Acta 1984, 6 7 , 2128-2142. (26) Robinson, P. J.; Dunnill, D.; Lllly, M. D. Biochim. Biophys. Acta 1971, 242, 659-661. (27) Batalilard, P.; ClOchet, P.; Jaffrezlc-Renautt, N.; Kong, X. G.; Martelet, C. Sens. Actuators 1987, 72, 245-254. (28) Diot, J. L.; Joseph, J.; Martin, J. R.; CIOchet, P. J. Eiectroanai. Chem. Interfacial Electrochem . 1985. 793, 75-88. (29) Martin, J. R.; Jalaguier, P. LPI UA CNRS 404 F69131 Ecully, personal communication, 1986. (30) Biophysical Chemistry Part 11: Techniques for a Study of Bioiogical Structure and Functlon; Freeman: San Francisco, CA, 1980; p 584. (31) Pluedemann, E. P. Chemicaiv Modified Surfaces Vol. 7 ; Leyden, D. E., Ed.; Gordon & Breach: 1986. (32) Jaffrezlcdenault, N. LPI UA CNRS 404 F69131 Ecuily, personal communication 1987. (33) Karch, K.; Sebestian, I.; Haiaz, I. J. Chromatogr. 1976, 722, 3-16.
RECEIVEDfor review November 1, 1987. Accepted June 6, 1988. Financial support of this research was provided by Biomerieux. This research was presented in a preliminary form at the Journ6e d’Etude Capteurs Chimiques et Biochimiques, Ecully, France, May 1987.
Electrodeposition of Platinum Microparticles into Polyaniline Films with Electrocatalytic Applications Kent M. Kost and Duane E. Bartak* Department of Chemistry, University of North Dakota, Grand Forks, North Dakota 58202
Beth Kazee and Theodore Kuwana University of Kansas, Center for Bioanalytical Research, 2095 Constant Avenue, Lawrence, Kansas 66046
The electrodeposltion of platinum microparticles into polyanlllne (PA) fllms on glassy carbon (gc) electrodes and their catalytic actlvlty for the reductlon of hydrogen and the 0x1datlon of methanol are descrlbed. Electrodeposltedplatinum mlcropartlclesare dispersed In a three-dimensional array in Hbrll-type polyanlllne flhn electrodes as evldenced by scanning electron microscope photomicrographs. These Pt/PA/gc electrodes exhlblt good actlvlty with respect to the catalytic reductlon of hydrogen and the catalytic oxidation of methanol. Slnce pdyanlilne Is a conducting polymer at potentials posnlve of 0.2 V vs Ag/AgCI, the PA fllms contribute a substantlal amount of charge during the oxidation of methanol at 0.6 V. I n addition, they also offer a protecting matrix for the Pt mlcropartlcles against partlcle loss and contamination from the bulk solutlon. The electrodes exhibited excellent longterm stabiilty In the acldlc methanol solutlons.
Metal microparticles dispersed in polymer modified electrodes have been recently recognized to have potential applications in electrocatalysis. Wrighton and co-workers first described the deposition of Pt and Pd into a viologen-based polymer to improve hydrogen evolution on semiconductor electrodes (1,2).Wrighton has more recently demonstrated 0003-2700/86/0360-2379$01.50/0
that Rh or Pd deposition in a cobaltocenium redox polymer on a p-type photocathode resulted in improved hydrogen generation (3). Kao and Kuwana dispersed Pt into poly(vinylacetic acid) (PVAA) with applications for the electrocatalytic generation of hydrogen and the reduction of oxygen (4). Our laboratory recently described the electrodeposition of Pt microparticles into poly(4-vinylpyridine) (PVP) which had been electrochemically polymerized or cross-linked and obtained excellent stability and good activity with respect to hydrogen evolution (5). In addition, Itaya and co-workers electrodeposited Pt microparticles on a Nafion-coated electrode and studied the hydrogen evolution with respect to the available active Pt surface area (6). Thus, several different types of polymers including those that can act as ionomers (e.g. PVP and PVAA) and those that contain redox groups (e.g. viologen) have been used as matrices for metal electrodeposition. Electrocatalytic applications, which utilize conducting polymers, should offer a potential significant increase in efficiency. Recently, there has been a considerable effort in the electrochemical preparation and study of several conducting polymers including polypyrrole, polythiophene, and polyaniline (7). Polyaniline, in particular, is an interesting conducting polymer with a wide range of conductivity (15 orders of magnitude) including conductivity values that approach 5 S/cm (8). The conduction mechanism of polyaniline has 0 1988 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 60,
NO. 21,
NOVEMBER 1, 1988
been shown to include the exchange of both electrons and protons (8). Furthermore, PA is generally homogeneous, strongly adherent to the electrode surface, and chemically stable (9, 10). Finally, a number of research groups have looked a t electrochemically polymerized polyaniline with respect to impedance (1I ) , pH-potential relationships (9, 12, 14), “protonic acid doping” (8,13),and the redox mechanism (9, 10, 12, 15). The goals of this study are to (i) deposit stable polyaniline films on polished glassy carbon electrodes, (ii) deposit metal microparticles in a three-dimensional array into the polyaniliie matrix, and (iii) evaluate the electrodes with respect to hydrogen evolution and methanol oxidation. Although Wrighton and co-workers have electrodeposited P d in polyaniline for possible applications as chemical sensors, they did not characterize the f i s or their catalytic activity (16). We herein report on the dispersion and characterization of platinum microparticles in polyaniline and the application of the resulting films for hydrogen evolution and the oxidation of methanol. EXPERIMENTAL SECTION Chemicals. Reagent-grade aniline and spectro-grade methanol (Fisher Scientific) and potassium hexachloroplatinate (Aldrich) were used as received. Water was distilled and subsequently passed through a Millipore Q water purification system prior to use. Aqueous solutions were prepared daily with the Millipore Q water. Glassware was cleaned in a 2-propanol/KOH bath followed by concentrated HN03 and thoroughly rinsed with Millipore Q water. Electrodes, Electrochemical Cells, and Instrumentation. The electrochemical cell used in this work has been previously described (17). A silver/silver chloride (saturated KC1) electrode was used as the reference electrode and potentials are reported with respect to this reference. The working electrodes were cut from glassy carbon (gc) plates (approximately 1.5 cm2,GC-20 and GC-30 grade, Tokai). The geometric area exposed to the solution, as defined by a Kalrez (Du Pont) gasket was 0.385 cm2, The gc electrodes were hand-polished successively with 600 grit, 1.0-, 0.3-, and 0.05-pm alumina slurries with copious rinsing with Millipore Q water and ultrasonicated between subsequent polishing stages. The auxiliary electrode was a Pt wire, which was isolated by a frit. A platinum foil, which was used in the hydrogen evolution experiments, was polished in the same manner as the gc electrodes. Bulk Pt for the oxidation of methanol was obtained by reducing KzPtCIBfrom 0.5 M H2S04on a gc electrode until solid Pt was observed on the exposed surface (50.3 mC/cm2). The available surface area for this bulk Pt electrode was quantitated by measurement of the charge for the hydrogen adsorption waves. All electrochemical measurements were performed with either a PAR Model 723 or an IBM EC 225 potentiostat. Data were acquired with a HP 7035 B X-Y recorder (cyclic voltammetry) or a Houston Omniscribe recorder (chronoamperometry). The polymer/Pt films were examined by an Hitachi Model S-570 scanning electron microscope (SEM). This SEM is equipped with an extra secondary electron (SE)detector located above the objective lens, which, unlike conventional SE detectors, produces a pure SE image in which there is no contribution from backscattered electrons (BSE). Therefore, photographs taken using this SE detector contain only topographic information. BSE emission, however, is strongly dependent on the atomic number of the emitter; BSEs also have much higher energies, and therefore greater escape depths, than SEs. Thus, BSE images were used to obtain compositional and subsurface information and to produce a high level of contrast between polymer and particles. All specimens were examined without any coatings or other preparation. In all SEM photomicrographs, the numbers at the bottom are, from left to right, photo sequence number, acceleration voltage, magnification, and value of dotted scale bar. Qualitative elemental analyses were carried out with a Princeton Gamma-Tech System 4 energy-dispersive X-ray spectrometer (EDS). During the latter part of this work, digital imaging and feature analysis capabilities were added to the EDS and were used to determine Pt particle size distributions.
Preparation of the Polyaniline Films. The polyaniliie films were produced by potential sweeping continuously in 0.1 M aniline and 0.5 M sulfuric acid or 1.0 M perchloric acid solution between -0.1 and 1.0 V until approximately 90-120 mC of charge was consumed on an electrode with a geometrical area of 0.385 cm2. It has been reported that 80 mC/cm2 is required to deposit a 1 pm thick film (18);therefore, the films prepared in this study were approximately 3 to 4 pm thick. The thickness was verified by a cross-sectional SEM analysis (vide infra). The film-coated electrodes were subsequently rinsed with a copious amount of Millipore-pure water and remounted for metal depositions. Such films were extremely stable and adhered very strongly to the electrode surface. Dispersion of Platinum Microparticles in the Polyaniline Films. Platinum was electrodeposited into the polyaniline films by three different potentiostatic techniques: (i) single-potential step chronoamperometry, (ii) double-potential step chronoamperometry, and (iii) constant-potential exhaustive electrolysis. The polymer electrodes were allowed to remain in contact with a 5 mM K2PtC16/0.5M HzSOl solution for at least 15 min prior to deposition. It was determined by SEM/EDS analysis of the Pt/PA films that solution contact times of 15 min were sufficient to saturate the film with anionic hexachloroplatinate. The penetration of the hexachloroplatinate into the polymer was due to the fibrillar nature of the polymer and because the polymer was at least partially protonated in 0.5 M H2S04.Single-potential step chronoamperometry was carried out by stepping from the rest potential to -0.20 V at variable times and concurrently measuring the charge passed, which was used to estimate the amount of deposited platinum. Double-potential step chronoamperometry utilized a potential waveform of 0.60 to -0.40 V (400ms) to 0.10 V. The deposition time at 0.10 V was also varied to obtain the desired amount of electrodepositedPt microparticles. Due to the reduction of the conducting form of the polymer at these potentials, quantitation of the amount of deposited platinum obtained by using coulometric measurements could not be carried out. However, upper-value estimates were made on the loading levels of Pt with the above data. Exhaustive reductions were accomplished at -0.20 V by the electrolysis of a known amount of hexachloroplatinate in a 5-mL acidic solution under bubbling argon. Evaluation of the Platinum Microparticle/Polyaniline Films. The catalytic activities of the Pt microparticle/polyaniline films were evaluated with regard to the generation of hydrogen from 0.5 M sulfuric acid solutions by slow scan linear sweep voltammetry (2 mV/s) under continuous purging by argon gas. Oxidation of methanol was examined in 1M methanol/l M H2S04 solution by constant potential polarization at +0.600 V under an argon purge. RESULTS AND DISCUSSIONS Preparation a n d Characterization of Polyaniline Films. The electrochemical synthesis of polyaniline (PA) in the form of emeralidine has been previously described by Adams and co-workers in 1962 (19). More recently, there have been several reports in which a variety of electrochemical techniques and conditions were used to prepare polyaniline films (8-15,ZO-22). Electropolymerization offers the possibility of controlling the thickness and homogeneity of the film on the electrode surface. Figure 1 shows a cyclic voltammogram for the polymerization of PA prepared from 0.5 M sulfuric acid/O.l M aniline solution. The first cycle shows an oxidative peak of aniline at approximately 0.9 V. Subsequent potential cyclings indicate a regular growth of the polymeric deposit as previously described (12). If the PA film is removed from the 0.1 M aniline/0.5 M H2S04,rinsed with water, placed in a 0.5 M H2S04, and cycled between -0.1 and 0.86 V, a cyclic voltammogram as illustrated in Figure 2 results. Four redox couples are readily apparent with Epa.valuesof 0.23, 0.53, 0.60, and 0.80 V. Upon repeated potential cycling, the peak currents associated with the processes a t 0.23 and 0.80 V decrease in magnitude while the peak currents for the redox couples a t 0.53 and 0.60 V increase in magnitude. MacDiarmid (9)and,
ANA1.YTICAL CHEMISTRY, VOL. 60. NO. 21, NOVEMBER 1. I988
I
I
I o
I
+O.X) C0.50 t0.30 tO.10 t1.10
t0.90 t0.70 t0.50
E
VL.
t0.M
tO.10
-0.10
Ag/AgCI,V
Fbure 1. Cycllc voltammogram for ltm ox!datlve polymerization of poiyaniline from a solutbn mntalnlng 0.1 M aniline with 0.5 M sulfuric acid. The sweep rate was 20 mVls on a 0.385 om2 glassy carbon substrate.
E
VI.
2381
-0.10
Ag/AgCI, V
FWn 3. Cydic voltammogam of Wan-
film, WMdr was prepared by oxllative pdymerizatii (CV, 5 scam. -0.1 to 1.0 V, 20 mvls), han 0.1 M anilinel0.5 M sulfuric acid. The scan rate was 20 mVls on a
0.385-cm2glassy carbon substrate
,Vf, t0.90 +0.70 t0.50 t O . 3 0 +0.10 E
V.I
-0.10
AglAgC1.V
Fgure 2. Cycllc voltam-m of the resulting polymeric film described In Fbue l, which was placed In 0.5 M suiiurk acid ekbotj'b. The scan rate was 20 mVls for a total of 17 scans.
more recently, Buttry (12)have propased that the wave at 0.23 V represents an initial oxidative proceas in which the polymer is converted from an insulator to a conductor. Figure 3 shows cyclic voltammograms of more than 50 consecutive cycles in which the peak current for the wave a t 0.23 V remains relatively constant, indicating stability for that couple if the positive potential is limited to values less than 0.60 V. The decrease in peak current for this process, when the anodic potential window is set a t 0.85 V, is consistent with a degradation of the polymer occurring due to potential cycling through the redox couple a t 0.80 V. It has heen previously reported that complete oxidation of the polymer to the imine form at potentials positive of 0.80 V results in hydrolysis of the imine nitrogen-carbon bond to a quinone form (22). Thus the couples a t 0.53 and 0.60 V probably represent a quinone-imine and quinone couple that results from this hydrolysis reaction. The shapes of the voltammograms for these two couples are relatively sharp with a rapid decrease in current a t the back side of the waves, indicating that the quinone remains as a surface-confined species. It should be pointed out that potentials more negative than 0.6 V were avoided to prevent polymer degradation (vide infra). Platinum Miemparticle Deposition in the Polyaniline Films. Electrochemical deposition of platinum in the polyaniline films was accomplished by either potential-step chronoamperometry or exhaustive electrolysis a t constant potential using an acidic hexachloroplatinate solution. In the potential-step chronoamperometric experiments, integration
Figure 4. SEM photomicrograph of platinum microparticles in a polyaniline film. The polyaniline lilm was prepared by me electrochemical oxidation of 0.5 M aniline in 1 M HCIO, using a constant current of 2 mA/cm2for 90 s on a glassy carbon electrode (Gc30). The deposition of PI was by single-potential step chronoamperometry (rest potential to -0.20 V (20s)) from 5 mM K,PtCI,lO.5 M H,SO,.
of the current was carried out to determine the loading level of platinum in the f h . However, due to considerable charging of the conducting film and concurrent hydrogen evolution, the platinum loading levels calculated by using this technique should be regarded as upper-value estimates. A SEM photomicrograph of a Pt/PA film in which the Pt was deposited by the potential-step chronoamperometric technique is shown in Figure 4. A fibrillar morphology of the polyaniline can clearly be observed with a fibril diameter of approximately lo00 A, which is similar to that reported by MacDiarmid (9). I t is not clear whether there is a compact microspheriod underlayer, which is beneath the fibrils as reported by MacDiannid (9) and Bard (23).A relatively small amount of platinum (less than 2 pg/cm*) was electrodeposited in this film. The platinum can be seen as particles with diameters less than 1000 8, on the surface of the fibrils. It
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 21. NOVEMBER 1. 1988
photomicrograph of a polyaniline film containing electrodeposited platinum particles. The poiyaniline film was prepared by cyclic voltammetry (six scans, 20 mVh, -0.1 to 1.0 V) (net charge of 260 mClcm'). The platinum was deposited by doubk-potentiil step chronoamperometry (0.6 V (10 s) to -0.4 V (600 ms) to 0.1 V (30 s)) from 5 mM K,FtCI,IO.S M H,SO1. Flgure 5.
SEM
also appears that the particles may he partially embedded in the fibrils. A SEM photomicrograph of Pt particles at a higher loading level of platinum (less than 4 pg/cm2) is shown in Figure 5. The platinum was electrodeposited by the double-potential step technique with the particles on and in the fibrils. The particle size appears to he less uniform (approximately 2W1000 A) when electrodeposited by the double-potential step as opposed to the single-step method. Figure 6 is a SEM photomicrograph of a polyaniline film containing 20 pg/cm2 of Pt electrodeposited by constant-potential exhaustive electrolysis, which permits a more accurate determination of the platinum deposited. In this case, agglomeration of the platinum has occurred with clusters of particles in the polyaniline film. However due to the fibrillar nature of the polymer, there appears to be open "pockets" in the general morphology of these films. The catalytic activities of Pt/PA electrodes with loading levels in the range of 1 W O pg/cm2 of Pt produced by the exhaustive electrolysis technique were tested with regard to hydrogen evolution and methanol oxidation (vide post). In order to more clearly ascertain if the platinum was dispersed in a uniform manner, a cross-sectional SEM view of the polyaniline film was taken. Figure 7 shows secondary (SE) and backscattered P S E ) electron images of a Pt/PA/gc electrode in which the Pt has been deposited by the singlepotential step technique. The electrode was carefully cleaved and mounted to obtain a cross-sectional view of the polymer film on the gc substrate. At these loading levels, there is not a significant increase in the amount deposited at the polymer-glassy carbon interface as was noted by Itaya in Naiion films (6). The BSE image with its higher level of atomic number contrast clearly shows that the platinum particles are three-dimensionally dispersed in tbe PA film without a significantly larger accumulation of metal on the glassy carbon surface relative to the polymer matrix. The SE image shows
SEM photomicrograph of a polyaniline film Containing ektrodeposited platinum. The poiyaniline film was prepared by cyclic voltammetry (four scans, -0.1 to 1.0 V, 20 mV/s. net charge of 187 mC/cm2). The platinum (20 pglcm') was electrodeposited by exhaustive electrolysis at -0.2 V (2 h with nitrogen bubbling) from 5 mM
Flgure 6.
K,FtCI,IO.5 M H,SO,.
Table I. Summary of the Hydrogen Evolution Activities on Pt/Polyaniline Electrodes'
electrode glassy carbon (gc) polyaniline/gc GC-20-A polyaniline GC-20-H polyaniline GC-20-B polyaniline GC-20-G polyaniline GC-20-3 polyaniline Pt foild
Pt loading level,' pg
excbange current density (mA/cm?:
H2
cmp
generation
0
0.05
0
0.01
13 15
0.5
23 36
0.8
51
0.7 0.9
1.2 0.9
"The activity was determined by linear scan voltammetry at 2 mV/s in a 0.5 M H&O, solution with nitrogen bubbling. 'All loading levels were calculated assuming a 100% efficiency for the reduction of Pt(1V) to Pt(0) or with the assumption that all of the available Pt in solution is reduced (exhaustive electrolysis). 'Exchange current densities were based upon the geometric area of the carbon substrate. dThe Pt foil active-surface area was calcuby integration of the hydrogen-absorption related to be 7.5 os duction
WSYPS.
that the polymer layer is about 5 pm thick and that the fibrillar morphology is maintained at all distances from the gc surface. Catalytic Activity. Hydrogen Evolution. The catalytic activity of Pt/PA/gc electrodes was evaluated by measuring the current for hydrogen evolution. Exchange current densities based upon the gc geometric area were found to be dependent on the Pt loading levels (Table I). Exchange current densities, determined from Tafel plots of the current in the region of overpotential less than 50 mV were consistent with those obtained for bulk Pt (24). The exchange currents were similar to those reported previously for platinum particles that had been electrodepo-
ANALYTICAL CHEMISTRY, VOL. 60,NO. 21. NOVEMBER 1. 1988 2383
Flgue 7. SEM photomicrographs with a cross-sectional view of a polyanliine film containing Pi microparticles on a glassy carbon electrode (-0) wkh detection 01 (A) backscattered electrons and (E) secondary electrons. The polyaniline film was prepared by cyclic vowammetry (six scans, -0.1 to 1.0 V, 20 mV/s. net charge of 310 mCI. The pt was deoosited by single-potential step chronoamperometry (rest potential lo -0.2 V) from 5 mM K*RCIe/0.5 M H,SO,.
sited into ionomers (e.g., PVP and PVAA) (4,5). Polyaniline, due to ita conducting properties, offers several avenues for electrocatalysis with metal particles. However, the conductivity of polyaniline decreases substantially a t the more negative potentials necessary for hydrogen evolution in 1 N acid solution (14,16). Exchange current densities of 0.5-1.2 mA/cm2based upon the carbon substrate area were obtained in this potential region of diminished polymer conductivity. Although these exchange current densities are reasonable based upon the geometric area of the carbon substrate, it would appear that a significant portion of the highly dispersed platinum particles are not active at the potentials (4.1to 4 . 4 V vs Ag/AgCl) necessary for hydrogen evolution. At these relatively negative potentials, the polyaniline is probably not conductive enough to allow all of the dispersed platinum particles to he electroactive. However, it should be noted that deposition of the platinum microparticles occurs in a relatively homogeneous manner throughout the polymer matrix (see Figure 7). Oxidation of Methanol. Platinum, which has been activated hy anodic pretreatment, has been reported to show a high initial catalytic activity for methanol oxidation (25). However, the activity of the platinum electrode decreases considerably during the course of polarization. The decrease in activity with time has been attrihuted to C-OH strongly adsorbed on the Pt making the reaction to produce COPextremely difficult. However, recent in situ IR spectroelectrochemical studies indicate that the adsorbed CO species exist at high coverages on platinum during the oxidation of methanol a t potentials of 0.134.95 V vs NHE (26,27). Ara m a h has reported on the deposition of platinum on Nation and Neosepta, which are a cation-exchange pertluorosulfonate polymer and a anion-exchange quaternary ammonium polymer, respectively (28,ZS). Both types of Pt/polymer membranes showed higher catalytic activity than bulk platinum for the methanol oxidation over longer (e.g., 20 h) periods of time. It has been proposed that the polymer matrix stabilizes
the platinum in different oxidation states (29). These oxidation states have been proposed to he responsible for the increased activity of platinum-tin oxide electrodes toward the catalysis of methanol oxidation (30). However, it should be noted that Janssen and Moolhuysen have p r o p w d that the enhancement of methanol oxidation on platinum-tin catalyst was due to a “liiand’ effect or an adsorption mechanism (31). The conducting ability of the plyaniline fiis can he most favorably exploited in the potential range of 0.24.6 V without causing extensive degradation of the polymer (vide ante). Utilization of the polyaniline film in this potential range should permit facile charge transport to wcur through the molecular polymer structure rather than hy ion transport, which occurs in the ionomer-type films such as PVP. Therefore, the electrocatalytic oxidation of methanol was studied hy using the polyaniline films that contained the platinum microparticles. T h e Pt/PA/gc electrodes were thus evaluated in regard to the oxidation of methanol a t 0.60 V vs Ag/AgCI. Figure 8 illustrates the current-time behavior for the oxidation of methanol on the platinum/polyaniline film electrode vs hulk platinum (i.e., Pt electrodeposited on glassy carbon). Note that these current-time curves are the log of the current density over a time period of 20 h. The bulk platinum shows a steady decrease throughout the course of polarization at 0.600 V vs Ag/AgCI. This is apparently the result of poisioning of the platinum surface due to either -CO or 50 pg of Pt/cm2) are necessary in order t o observe significant changes in the voltammogram. At lower Pt loading levels (e.g. 20 pg of Pt/cm2),relatively long-time experiments are necessary as in the polarization experiment (see Figure 8) in order to observe the steady-state currents resulting from methanol oxidation relative to the charging currents for the polyaniline. All of the electrodes showed a remarkable stability (Le., film integrity and catalytic activity was maintained) in the acidic methanol solutions. No visible degradation of the polymer was observed after more than 30 h of continuous exposure to these solutions. Further evidence that the Pt microparticles
Platinum microparticles can be electrodeposited as a three-dimensional array in a polyaniline film. More importantly, these electrodes show high activity with respect to hydrogen evolution and methanol oxidation. They also show remarkable catalytic and mechanical stability in acidic media a t working potentials negative of 0.6 V vs Ag/AgCl. More extensive work on multimetal depositions and their catalytic properties, particularly with other conducting polymers, are currently in progress. Registry No. PA, 25233-30-1; gc, 7440-44-0;Pt, 7440-06-4;H2, 1333-74-0; H+, 12408-02-5;aniline, 62-53-3; methanol, 67-56-1.
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RECEIVED for review December 9, 1987. Resubmitted June 1, 1988. Accepted July 28, 1988. This work was partially supported by the National Science Foundation through an EPSCoR Grant. Purchase of the scanning electron microscope and the energy dispersive X-ray spectrometer with funds from the National Science Foundation is gratefully acknowledged.
