Structural and electronic aspects of IrO 2 films prepared by electrodeposition on Au substrates were investigated by in situ L III -edge X-ray absorption and surface enhanced Raman spectroscopies in both acid and alkaline aqueous solutions. Linear correlations were found between the extent of oxidation of Ir 3+ in the films determined from a statistical fit of the white line, which includes contributions from each of the sites differing by a single electron, and from coulometric analysis of the voltammetric curves. Analysis of the extended X-ray absorption fine structure (EXAFS) yielded Ir-O bond lengths decreasing in the sequence 2.02, 1.97, and 1.93 Å for Ir 3+ , Ir 4+ , and Ir 5+ sites, respectively. Whereas SERS provided evidence for the presence of crystalline IrO 2 in the highly hydrated films, the lack of intense shells in the Fourier transform of the EXAFS function beyond the nearest oxygen neighbors indicates that the films do not display long-range order.

The microgravimetric characteristics of electrodeposited nickel and composite Ni/Co hydrous oxide films on Au electrodes in KOH electrolytes were examined in situ with a combination of quartz crystal microbalance (QCM) and voltammetric techniques. In the case of freshly prepared a-Ni(OH)2 hydrous oxide films, denoted as a-Ni(OH)2(hyd), the mass increased during oxidation and then decreased upon subsequent reduction. As the redox cycling was continued, however, this behavior gradually reversed, i.e., the mass decreased upon oxidation and then increased following further reduction. This unique effect could be correlated with a shift in the position of the oxidation peak maximum (E°°)in the voltammogram toward more positive potentials, which has been ascribed to the transformation of a-Ni(O)2(hyd) into 3-Ni(OH)2(hyd). Based on these findings, it can be concluded that the mechanism of ionic charge transport in these two types of lattices is markedly different. In contrast, composite Ni/Co (9:1) hydrous oxide films displayed qualitatively the same behavior as pure freshly prepared Ni(OH)2(hyd) in the same alkaline media regardless of the extent of cycling. This observation provides evidence that the incorporation of cobalt into Ni(OH)2(hyd) modifies the structural properties of the lattice during the oxidation/reduction process.

Modifications in electronic and structural aspects of RuO 2 films electroprecipitated onto Au electrodes induced by changes in the applied potential have been examined in situ in aqueous 0.50 M H 2 SO 4 by Ru K-edge X-ray absorption spectroscopy (XAS). The Fourier transform of the k 3 -weighted extended X-ray absorption fine structure (EXAFS), k 3 χ(k), for the film polarized at +1.20V vs RHE is characterized by two shells attributed to Ru-O and Ru-Ru interactions with average distances of 1.94(1) and 3.12(2) Å, respectively, in agreement with results obtained ex situ for Ru 4+ in hydrous RuO 2 by other groups. In contrast, films in the reduced state, i.e., +0.40 V vs RHE, yielded only a single shell ascribed to a Ru-O interaction at 2.02(1) Å with no evidence for a distant Ru-Ru shell. The long Ru-O distance is in agreement with that reported earlier for the hydrous Ru 3+ ion [Ru-(OH 2 ) 6 ] 3+ in the solid state. Moreover, the difference between the average Ru-O bond lengths for the reduced and oxidized films is consistent with the difference in the ionic radii of Ru 3+ and Ru 4+ . On this basis it has been suggested that films in the reduced state contain Ru 3+ sites, consistent with the electrochemical results, in a phase with apparently less order beyond the Ru-O coordination sphere than for hydrous RuO 2 .

The electrocatalytic activity of selenium-, and sulfur-modified polycrystalline Pt electrodes for O 2 reduction in acid media has been examined using rotating ring-disk electrode techniques. The results obtained indicated that, within a rather narrow range of coverages, both Se-, and S-modified Pt surfaces promote O 2 reduction via a two-electron pathway to yield hydrogen peroxide at close to 100% faradaic efficiency over a wide potential region. Also presented in this work is an experimental procedure for Se-modification of high area, unsupported Pt particles based on fluidized packed bed reactor principles. Such a strategy could be readily scaled up opening new prospects for the development of large scale hydrogen peroxide generation in acid media, including electrochemically based, room temperature, Nafion-based oxygen concentrators.The reduction of dioxygen in aqueous electrolytes, including Nafion, 1-5 has received extraordinary attention over the past few decades. 6,7 Much of the research impetus has been driven by the need to improve the performance of gas permeable cathodes for fuel cell applications, including stationary energy generation, 8 vehicular propulsion, 9 and, more recently, portable electronics. 10,11 Not surprisingly, most of the emphasis, from a fundamental viewpoint, has focused on the search for electrocatalysts displaying both high activity and selectivity for the four-electron reduction of O 2 to yield water as the product. Yet another electrochemical process that involves O 2 as a reactant is the electrosynthesis of hydrogen peroxide, 12-16 an environmentally benign chemical currently being considered for use in diverse technological areas, ranging from the industrial bleaching of paper 17,18 and wastewater treatment, 19 to oxidation of hazardous chemicals and bioremediation. 17 Although certain types of carbon are effective electrocatalysts for the 2e Ϫ reduction of O 2 in alkaline media, in terms of potency and high selectivity, 6 materials displaying equally desirable properties in acid electrolytes are quite rare. Notable exceptions are provided by certain transition metal macrocycles of the phthalocyanine 20 and porphyrin types 21-24 and by a few simpler inorganic and organic species, either in solution phase or immobilized on the surface of otherwise inert electrodes by spontaneous adsorption, chemical derivatization, and other means. Unfortunately, many of these molecular electrocatalysts also promote decomposition of hydrogen peroxide 25 generating radicals that attack their often delicate structures rendering products devoid of significant activity. Direct generation of H 2 O 2 in acid media is particularly advantageous, as it would meet pH conditions required for Fenton's reagent applications 26,27 both in chemical synthesis and chemical degradation without further processing. In addition, it could impact markedly the development of lightweight, Nafion-based, roomtemperature electrochemical oxygen concentrators for medical and other applications. 28 The approach currently unde...