Anal. Chem. 1994,66, 1781-1783
Quantitative Determination of Ion Migration in a Thin-Layer Electrochemical FT-IR Cell Mahesh G. Samant,' KeiJIKunlmatsu,t and Hajime Seki IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, California 95 120-6099
This paper examines ion migration in thin-layer geometry used in a typical infrared cell designed for probing the electrode/ electrolyte interface. The system used is perchloric acid on a polycrystalline platinum. Previously obtained IR data have been explained qualitatively on the basis of ion migration into the thin-layer electrochemical cell. Here we have measured the extinction coefficient of perchlorate ions in solution and used it to convert IR intensities into a change in concentration of ions in the thin solution layer as a function of electrode potential. The agreement between the IR results and the coulometric data is good. These data are consistent with the lack of specific adsorption of perchlorate anions on a Pt electrode. Infrared spectroscopy (IR) is used extensively to probe the solution side of the electrode/electrolyte interface.14 Most measurements are made with reflection geometry using a p-polarized IR beam. Typically, the angle of incidence of the IR beam is 60-70' with respect to the surface normal. The background absorption of the IR beam by the electrolyte is minimized by having only a thin layer (about 1 pm) of electrolyte on the electrode surface during the measurements. This is achieved by bringing the electrode in close proximity to the IR-transparent window. The requisite sensitivity to submonolayer/monolayer coverages is obtained by computing the difference spectrum, which represents a ratio of spectra obtained at reference and sample potentials. An assumption is made that at the reference potential there is no or minimal adsorption of the electrolyte species. If this assumption is invalid, the interpretation of the IR difference spectra may become ~omplicated.~ Another implicit assumption is that there is no significant migration of ions into and out of the thin electrolyte layer to the contacting bulk solution. If there is a reasonable separation between IR bands corresponding to surface and solution species, then ion migration does not complicate interpretation of the difference spectra. Some recent studies have detailed systems in which the IR results are best interpreted on the basis of ion migration into the thin-layer spectroelectrochemical cell."9 t
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(1) Seki, H. In Electrochemical Surface Science; Soriaga, M. P., Ed.; American Chemical Society: Washington, DC, 1988. (2) Ashley, K.; Pons, S. Chem. Rev. 1988.88, 673. ( 3 ) Bewick, A.; Pons, S.In Advances in Infrared andRamanSpectroscopy;Clark, R. J., Hester, H. E., Eds.; Wiley-Heyden: London, 1985; Vol. 12. (4) Stole, S.M.; Popenoe, D. D. Porter, M. D. In Electrochemical Interfaces; Abruna, H. D., Ed.; VCH Publishers: New York, 1991. ( 5 ) Parry, D. B.; Samant, M. G.; Melroy, 0. R. Appl. Specrrosc. 1991, 45,999. (6) Iwasita, T.; Nart, F. C. J . Electroanal. Chem. 1990, 295, 215. (7) Bae, I. T.; Xing, X.; Scherson, D.; Yeager, E. B. Anal. Chem. 1989,61,1164. (8) Bae, I. T.; Scherson, D.;Yeagar, E. B. Anal. Chem. 1990, 62, 45. 0003-2700/94/0366-1781$04.50/0
0 1994 American Chemical Society
Typically, in systems where the pH of the thin-layer cell is substantially modified as a function of the electrode potential, ion migration can play a significant role. This sensitivity to the presence of ions in the thin layer of the electrolyte results in spite of the use of p-polarized IR radiation. The wavelength of IR radiation at 1000-1250 cm-l is 8-10 pm. At such a wavelength the p-polarized radiation has an intensity maximum of a little less than twice the original intensity at the electrode surface, which then decreases slowly to the window surface. For a 1-pm-thick electrolyte layer, the intensity of the p-polarized radiation decays to about 70% of the maximum."J Hence p-polarized radiation is sensitive to the entire layer of the electrolyte. In this study we have chosen HC104 on a Pt electrode since earlier reports have demonstrated that ion migration plays a major role in the IR resu1ts.w Furthermore, the electrochemists have always assumed that C104- ions do not specifically adsorb on the electrode surface; though there is a lot of evidence supporting this assumption, conclusive evidence is still lacking. In this study, we wish to understand and separate the contribution to the total IR signal from migration of solution species from the bulk solution and the contribution from species adsorbing on the electrode surface. We have measured the extinction coefficient of C104ion in solution and obtained FT-IR difference spectra in the presence of a Pt electrode.
EXPERIMENTAL SECTION Thespectroelectrochemicl cell with a beveled BaF2 window used in this study has been described elsewhere.'' The Pt electrode was a 25-mm disk polished to a mirror finish by sequential polishing with 1,0.3, and 0.05-pm alumina powder (Buehler). The 0.1 M HC104 electrolytesolution was prepared with Nanopure 18-MQ water and doubly distilled perchloric acid. The cell was deaerated by bubbling ultrapure nitrogen (99.99%, Air Products), and the Pt electrode underwent repeated electrochemical cycling while in contact with bulk electrolyte until stable voltamogramms were obtained. The Pt electrode was then pressed against the BaFz window to achieve a thin-layer configuration. A Ag/AgCl (3 M KCl) was used as a reference electrode, and all potentials are reported with respect to this reference. A Fourier transform IR spectrometer (IBM Instruments IR/98) was used with a narrow-band liquid nitrogen cooled mercury
