Heterogeneous charge transfer dynamics of mono- and multi-layer films on glassy carbon and platinum single crystal electrodes

Electrochemical studies were performed on two interfacial systems where ions were adsorbed on the electrode surface to form mono- or multi-layer films. The two interfacial systems were glassy carbon/polyoxometallate anion and platinum single crystal/hydrogen ion interfaces. General electrochemical characteristics were obtained for the glassy carbon/polyoxometallate interface, while the charge transfer kinetics were studied in considerable detail for the platinum single crystal/hydrogen ion interface.The charge transfer rate constants were determined by ac impedance methods. A novel approach to correct for the cell time constant is presented in relation to the ac impedance theory developed by Laviron for rate constant determination of surface-confined redox centers. The correction method allows the determination of the charge transfer resistance and the pseudocapacitance, from which the surface coverage of the electroactive species can be found independently.The polyoxometallate of interest was the isopolymolybdate anion, (Mo$\sb7$O$\sb{24}$) $\sp{6-}$, which irreversibly adsorbed on a glassy carbon surface to give at least monolayer coverages. There were at least five reduction waves, the first three being reversible. Analysis of the pH dependence of the voltammetric response indicates that a single electron and proton is involved in each of the first three reduction waves. The rate constants were 8800, 11000, and 25000 s$\sp{-1}$ for the first, second and third reduction waves, respectively. A glassy carbon electrode with a monolayer of isopolymolybdate adsorbed on the surface appears to mediate the reduction of ferricyanide and bromate.A non-uniform increase in the quantity of material immobilized on the surface was observed when immersing the electrode in successive anion (isopolymolybdate) and cation (poly (4-vinylpryidine)) solutions. A total of seven cyclic anion/cation immersions were studied. The charge transfer kinetics were dominated by the electron transfer, and not by diffusional charge transport through the film, irrespective of the amount of material immobilized. The electron transfer rate constants for the first two reduction waves were approximately the same value, 4 s$\sp{-1}$, and decreased by approximately four orders of magnitude compared to the rate constants determined for the first two reductions that were observed for a monolayer coverage of isopolymolybdate.The rate constants were determined for all of the redox couples observed in the voltammetry of platinum single crystal electrodes in sulfuric and perchloric acids. All three low-index surfaces were examined. The surface coverage was determined from the slopes of the peak currents vs. potential scan rate, and from ac impedance measurements of the pseudocapacitance. The values determined for the surface coverages were lower when determined using the ac impedance approach. The surface coverages were similar for the known hydrogen ion and anomalous redox couples. The rate constants in sulfuric acid ranged from 2000 to 5000 s$\sp{-1}$ and in perchloric acid ranged from 200 to 800 s$\sp{-1}$. The rate constants for the known hydrogen redox couples were similar to the rate constants for the anomalous redox couples for the Pt(111) electrode.U of I OnlyETDs are only available to UIUC Users without author permissio