Preparation of a Carbon-Platinum-Polyaniline Support for Atomic Metal Deposition

All chemicals were used as received. Potassium tetrabromoplatinate, K₂PtBr₄ (99%) and tetrafluroboric acid, HBF₄ (48%) from Strem Chemicals, perchloric acid (70%) from Macron Fine Chemicals, aniline (99.5+%), 1-propanol (99.5+%) and propionic acid (99%) from Aldrich, sulfuric acid (95-98%) from BDH Chemicals, potassium hydroxide from J. T. Baker, propionaldehyde (99+%) from Acros Organics and 2-propanol (70%) from EMD.

Electrochemical experiments were done with an electrochemical workstation (CH Instruments 406, Austin, TX) in a three-electrode cell. The double-junction reference electrode was Ag/AgCl in 1 M KCl//1 M KNO₃. All potentials mentioned in this work are referred to this electrode (E = 0.235 V vs. SHE). Thin synthetic graphite plates from Advanced Carbon Engineered Solutions (North Bay, Ontario) with bulk density of 1.88 g/cm³ and specific resistivity of 10 μΩm were used as counter and working electrodes. Before use, each 3.5 cm × 1.0 cm × 0.05 cm electrode was polished with alumina lapping pads (30, 12 and 0.05 μm), obtained from Allied High Tech Products Inc. and was thoroughly rinsed with 2-propanol. The working electrode was then encased in Coleflex heat shrinkable tubing, and a geometric area of 0.5 cm × 0.7 cm was cut out. In order to ensure proper wetting of the exposed porous carbon material, each working electrode was held under vacuum in plating solutions for approximately 20 minutes, or until air bubbles were no longer seen forming on the surface of the electrode. Immediately after this step, either platinum was electropolymerized from 0.001 M or 0.0085 M K₂PtBr₄ in 1 M HClO₄ or polyaniline was deposited from solutions of either 0.25 M aniline/1 M HClO₄ or 0.4 M aniline/2 M HBF₄. Platinum depositions were carried out at 60°C to decrease deposition time. Unlike the chloroplatinate salts, potassium tetrabromoplatinate solutions are more stable in acid and do not hydrolyze.²³ The K₂PtBr₄ stability in 1 M HClO₄ was confirmed by UV absorbance measurements before use. Platinum was deposited on carbon, C/Pt, or on PANI, C/PANI-Pt, at a constant potential of −0.1 V, as specified in the Results. The polyaniline was either electropolymerized directly on carbon, C/PANI, or on platinum, C/Pt-PANI, at constant potential of 0.8 V until a charge density of 1.43 C/cm² was reached and was kept constant throughout this work. After each deposition, the electrode was rinsed with deionized water and cyclic voltammograms (CVs) were recorded in 1 M HClO₄ at 20 mV/s between +0.9 V and −0.2 V.

Cyclic voltammograms were recorded in solutions of 0.5 M 1-propanol in 1 M KOH at 50 mV/s using Pt foil as counter electrode. All CVs were recorded between +0.4 V and −0.7 V until a stable voltammogram was obtained.

HPLC measurements were performed on Agilent 1100 Series HPLC system (Agilent Technologies, USA) with HC-75 (H+) column, 7.8 × 305 mm, particle size 9 μm (Hamilton, USA). The mobile phase for isocratic elution was 5 mM H₂SO₄ in water at a flow rate of 0.7 ml/min. The separation column was thermostated to 80°C and a variable wavelength detector was used to measure the absorbance at 190 nm. For system control and data evaluation, Agilent ChemStation software, rev. B.04.02 SP1, (Agilent Technologies, USA) was employed. Volume of sample injected on the column was 100 μl. Peaks were identified using standard addition method. Sensitivity of the method to 1-propanol (0.27 AU·L/mol) was significantly lower than sensitivity to propionic acid (7.97 AU·L/mol). Sensitivity to unknown product of aldol condensation as a marker of propionaldehyde was approximately 85 AU·L/mol, i.e. ten times higher than sensitivity to propionic acid.

Electrooxidation of 1 M 1-propanol in 1 M KOH was performed in a U-type glass cell with a frit separating the working and auxiliary electrode compartments of 3 and 15 ml, respectively. Platinum mesh (6.25 cm²) was used as auxiliary electrode in the larger compartment. Both compartments were filled with the same solution. Oxidation of 1-propanol for HPLC analysis was conducted by applying potential step sequence described in the Results. This sequence of pulses was repeated 360 times making the cumulative time at the oxidation potential, −0.26 V, a total of one hour. Samples were analyzed after 12 hours of storage at room temperature.

Further characterization of the electrodes was carried out with scanning electronic microscope (SEM) Hitachi SU8230 cold field-emission SEM (Tokyo, Japan).

It has been generally accepted that the physico-chemical properties of the supporting carbon material and size and distribution of metal particles are important characteristics. Prior to the Pt-deposition the carbon electrodes were characterized by SEM and cyclic voltammetry. Figure 2A shows the SEM image of the polished synthetic graphite plate (see Experimental section). It shows open surface structure that governs the low resistance and low double layer capacity observed in the CV in Figure 2C curve 1. The SEM image obtained after Pt deposition demonstrates that the growth of platinum agglomerates is uneven with asymmetric, irregular edges (Figure 2B). In agreement with the accepted nucleation and growth theory, the irreversible growth of the nanocrystals is achieved through atomic addition until the reaction is terminated.²⁹ Figure 2C curve 2, shows a sharp peak corresponding to the underpotential deposition of protons on the Pt surface at potentials ranging from −0.25 to 0 V. Before the hydrogen evolution, on the reverse scan of the same potential range, the oxidative desorption of protons from the Pt surface generates a broad peak. The anodic region bound by E > 0.8 V and the single peak at ∼0.5 V correspond to oxidation and reduction of Pt respectively. The CV shown as curve 3 changed profoundly after an electrochemical annealing treatment was applied in steps (of 10 cycles) between −1.0 and +1.0 V with a scan rate of 1 V/s. From the SEM micrograph shown in Figure 2D, it appears that during this treatment the nanoclusters coalesce, resulting in larger hemispherical (1.0 μm) platinum agglomerates. The corresponding CV is shown in Figure 2C, curve 3. It agrees with a voltammogram of the polycrystalline platinum electrode.²⁵ The distinct peaks due to strong hydrogen adsorption at 0.1 V and weaker hydrogen adsorption at −0.1 V indicate a high surface quality of deposited Pt.

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In order to examine the effect of the deposited platinum on the electrochemical surface area (ESA), voltammograms were recorded after the electrochemical annealing step, Fig. 3. The inset in this figure shows a CV recorded after annealing, with the shaded region indicating the area of H-atom adsorption peaks. The integration of the charge within this region followed the method applied by Stevens and Dahn.³⁰ Figure 3 shows the dependency of the ESA upon the applied deposition charge densities, Q_Pt, from 0.07 to 7.14 C/cm².

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With the increased Pt loading, the ESA begins slowly to level off. The diminishing increase of the ESA may result from an interplay between primary and the secondary nucleation centers (on the surface of the electrodeposited Pt nanoparticles) leading to complex nanograined deposits. The latter limits the number of specific crystal faces that can be formed on the platinum during sulfate ion adsorption.³¹

In order to examine the effect of Pt-loading on PANI activity, the polymerization of aniline was always carried out from the same electrolyte at constant potential E = 0.8 V and controlling the thickness of the deposition using a charge density of 1.42 C/cm². It was observed that the shape of the CV shown in Figure 1 as curve B does not undergo fundamental changes by varying the Pt-loading. The average charge density under the oxidation peak at 0.15 V for all the different Pt-loadings was determined to be 90.0 ± 1 mC/cm². This reveals the expected consistency for PANI deposition.

The platinum loaded carbon support in the absence and presence of PANI in 1 M KOH was compared in Figure 4A and 4C. The hydrogen adsorption/ desorption peak potentials were shifted in the alkaline medium toward the more negative potentials as compared to Fig. 1. The pH change provides more insights into the Pt-loading effect on the carbon support; at the higher Pt-content the shape of the CV curves is better resolved. The electrode surface becomes more sensitive to other electrochemical reactions such as the oxygen reduction around −0.3 V that may coincide with the hydrogen peroxide oxidation/reduction. The C/Pt-PANI electrode also mirrors that behavior. Peaks in the same potential range that can likely be assigned to the formation of Pt(OH)_x and PtO_x,³² are also seen on the reverse scan. Oxidation of 0.5 M n-propanol in 1 M KOH on C/Pt and C/Pt-PANI electrodes is compared in Fig. 4B and 4D. It is observed that with increased platinum loading, there is an increase in activity toward 1-propanol oxidation with the potential remaining around −0.2 V. Furthermore, at a higher charge density of 4.5 C/cm², the peaks corresponding to hydrogen adsorption on platinum can also be seen between −0.4 and −0.7 V (curve 4 of Fig. 4B and 4D). Comparing the absence and presence of PANI (Fig. 4B and 4D), having a PANI film present tends to slightly suppress the peak currents, but does not disturb the general trend due to platinum loadings.

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A most direct test of the electrocatalytic efficiency is the electrolysis and analysis of the products of 1-propanol oxidation on C/Pt and C/Pt-PANI electrodes. For that purpose an HPLC analysis of the electrolyte containing oxidation products from 1 M 1-propanol in 1 M KOH was developed. The characteristic feature of oxidation of propanol in strongly alkaline medium is the blockage of the working electrode with oxidation product resulting in greatly reduced current efficiency and characteristic cyclic voltammograms (insets of Figure 5A and 5B). The peak around −0.2 V observed during the forward scan indicates the catalytic oxidation of 1-propanol. The products of the oxidation are adsorbed on the electrode surface in the region of 0.0 to 0.4 V and block further oxidation. A periodic application of "cleaning" pulse removes the blocking products and restores the electrolysis. Thus, electrolysis was carried out using pulsing regime shown in Figure 5C. The repetitive pulses were initiated at −0.26 V (for 10 s) then stepped to 0.4 V (for 0.1 s) and terminated at −0.6 V (for 0.2 s). The resulting chronoamperometric response for the last applied oxidation pulse at −0.26 V in the series is shown in Figure 5A and 5B. Electrocatalytic activity for C/Pt-PANI electrodes is generally somewhat lower than that for the bare C/Pt. That can be explained by the limited mass transport of propanol due to the layer of PANI. At the same time, electrodes with higher Pt loading tend to show higher catalytic activity as can be seen in Fig. 5, curves 1–4.

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Figure 6 shows HPLC chromatograms of samples taken after one hour of pulsed electrooxidation using C/Pt and C/Pt-PANI electrodes. As a result of 1-propanol (peak 2) oxidation, propionic acid (peak 1) was detected. Another expected product, propionaldehyde, could not be solely identified due to the competing aldol condensation reaction.³³ However, the presence of peak 3 and several minor peaks prove propionaldehyde being one of the products. These peaks were also found in chromatograms of samples prepared by adding propionaldehyde to 1 M KOH and 1 M 1-propanol (data not shown). The peak 1 (propionic acid) is approximately two times higher for C/Pt than for C/Pt-PANI electrode. That indicates that PANI layer on the Pt electrode does not contribute to catalytic activity of the oxidation. The ratio of peak 1 to peak 3 is roughly the same for both electrodes indicating that oxidation on either electrode is governed by the same mechanism.

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