Platinum Electrode: How its chemical comp osition affects electrochemical reactions.

Pt is a noble metal that is very stable and resistant to corrosion. Because of its stability, like Au, it is often used to make jewelry. As an electrode, it is used to oxidize organics or to generate hydrogen or reduce oxygen. You can order Pt as a wire of various diameters or foil with different thicknesses. Cypress/ESA sells Pt disk electrodes encased in glass or PEEK polymer with tip diameters of 1 mm and 10 um. These contain polycrystalline Pt, not single crystals, so the atoms are not arranged with a defined crystalline structure. Often, Pt electrodes contain a small amount of rhodium to give rigidity, especially when the electrode is a thin foil.

Figure 1 shows the cyclic voltammogram of a smooth Pt electrode in 0.5 M H₂SO₄. Usually, initial CVs do not show such well-defined peaks or waves as seen in this figure. After many repetitive cycles, the peaks will begin to emerge as the surface is cleaned, and will go to a steady-state voltammogram. In the potential region of 0.4 to 0.8 (vs. NHE), the Pt surface is "clean" and the current flows to charge the double layer. As the potential goes more positive, the current is attributed to the formation of adsorbed oxygen or platinum oxide. The potential indicated by "2" is the start of bulk oxygen evolution. As the potential scan is reversed, the cathodic current is attributed to the reduction of surface platinum oxide. If there is dissolved oxygen in the solution, it will be reduced in this potential region concurrently with the reduction of platinum oxide. Scanning through the double layer region, the peaks due to adsorbed H-atoms are seen, where the shape, size and number of peaks depends on the crystal faces of the Pt surface. Bulk hydrogen evolution begins at "1." These surface processes are pH dependent with the cyclic voltammogram shifting by -60 mV per unit pH. The integrated charge under the H-atom adsorption peaks is said to be a good indicator of how many "clean" Pt atoms are on the surface and thus, a determinant of the surface area of active Pt [ref. 4]. Reference #4 is an excellent source to learn about chemisorption processes at solid electrodes.

Figure 1. Cyclic voltammogram for a smooth Pt electrode in 0.5 M H₂SO₄. The various regions are discussed in the text. The shape, number and size of the peaks depend on the pretreatment of the electrode, solution impurities and supporting electrolyte. Reprinted with permission from A. J. Bard and L. Faulkner in Electrochemical Methods: Fundamentals and Applications, 2^nd Ed., J. Wiley & Sons. Copyright J. Wiley & Sons 2001.

Surface cleanliness is a major issue with all electrodes. Pt is unique in that the surface impurities can be removed by repetitive scanning through a potential region where the impurities can be oxidized or removed into solution as platinum oxide is reduced. In the double-layer region, the high surface activity of Pt, due to its atoms having unfilled d-orbitals, can be used advantageously to detect analytes eluted in liquid chromatography utilizing a technique called "pulse amperometric detection (PAD)" [ref 5]. PAD is a pretty neat idea. Analytes (e.g. aliphatic amines, carbohydrates and alcohols) that normally are not considered electroactive are adsorbed onto the clean Pt surface while the applied potential is held in the double layer region. The potential is then stepped to a value in the Pt oxide region and there is concurrent oxidation of the adsorbed species and Pt oxide formation. The difference in the current or the integrated charge can be related to the amount of analyte, after subtraction of the background, which is obtained from an experiment without the presence of the analyte in solution. The potential is then stepped to the H-atom region to clean the electrode; i.e., to reduce the Pt oxide and remove any impurities that may be on the surface from the oxidation of the analyte. The process is then repeated to get some statistics on the process. The PAD analysis relies on the amount of analyte adsorbed being proportional to the concentration in solution, and the analyte not catalytically oxidized (dehydrogenated) by clean Pt. In the latter case, Johnson and LaCourse have developed innovative PAD procedures to accommodate the different mechanisms seen with various analytes, particularly with Au electrodes [ref. 6, 7].

PAD is one illustration of how the surface chemistry plays an important role and can be advantageously utilized for electro analytical purposes.