Water potential-depending structure

Noguchi, H., Okada, T. and Uosaki, K (2008) SPG study on potential-dependent structure of water at Pt electrode/ electrolyte solution interface. Electrochim. Acta, 53, 6841-6844. 

Here, we demonstrate the usefulness of SFG spectroscopy in the study of water structure at electrode/electrolyte solution interfaces by showing the potential dependent SFG spectra in the OH-stretching vibration region at a Pt/thin film electrode/0.1 M HGIO4 solution interface in internal reflection mode. 

PEMFC)/direct methanol fuel cell (DMFC) cathode limit the available sites for reduction of molecular oxygen. Alternatively, at the anode of a PEMFC or DMFC, the oxidation of water is necessary to produce hydroxyl or oxygen species that participate in oxidation of strongly bound carbon monoxide species. Taylor and co-workers [Taylor et ah, 2007b] have recently reported on a systematic study that examined the potential dependence of water redox reactions over a series of different metal electrode surfaces. For comparison purposes, we will start with a brief discussion of electronic structure studies of water activity with consideration of UHV model systems. 

The guest molecules experience different potential depending on the nature and the spatial distribution of the ions and the structural modifications in the aluminosilicate framework associated with the Si-Al substitution. Accordingly, the diffusive process can be different [1], The efficiency of migration of guest molecules depends on several factors the Si/Al ratio, the nature of the extra framework cations, the presence of sorbed water molecules, the temperature, and the sorbate concentration [1]. 

Surface states on a semiconductor in a vacuum can sometimes be explained by means of the spare bonds that dangle from atoms on surfaces, or defects associated with dislocations. Neither of these mechanisms works at the semicon-ductor/solution interface. The dangling bonds will be expunged by adsorbed water, etc. Experiment shows that the concentration of surface states on semiconductors in solution is strongly potential dependent, and that defects in the crystal structure would not be potential dependent, at least until anodic dissolution of the substrate itself began. 

It is curious that the striking deviations of electrochemical kinetic behavior from that expected conventionally, which are the subject of this review, have not been recognized or treated in the recent quantum-mechanical approaches, e.g., of Levich et al (e.g., see Refs. 66 and 105) to the interpretation of electrode reaction rates. The reasons for this may be traced to the emphasis which is placed in such treatments on (1) quantal effects in the energy of the system and (2) continuum modeling of the solution with consequent neglect of the specific solvational- and solvent-structure aspects that can lead, in aqueous media, to the important entropic factor in the kinetics and in other interactions in water solutions. However, the work of Hupp and Weaver, referred to on p. 153, showed that the results could be interpreted in terms of Marcus theory, with regard to potential dependence of AS, when there was a substantial net reaction entropy change in the process. 

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