Invited Paper SS+AS-WeM9
Water at Interfaces under Realistic Conditions

Wednesday, October 30, 2013, 10:40 am, Room 202 A

I will present two classes of systems involving water at interfaces probed using ambient pressure XPS, water on BaF₂ and the interaction of oxygenated species during electrocatalysis. The structure of thin-film water on a BaF₂(111) surface under ambient conditions was studied using Auger electron detected x-ray absorption spectroscopy (XAS) from ambient to supercooled temperatures at relative humidity up to 95 %. No ice-like structure was observed in spite of the expected templating effect of the lattice-matched (111) surface. The XAS spectrum of liquid thin-film water on BaF₂ exhibits, at all temperatures, a strong resemblance to that of high-density ices for which the observed spectroscopic features correlate linearly with the density. Molecular dynamics simulations indicate that the first layer water on BaF₂(111) is indeed in a unique local structure that resembles high-density water, with a strongly collapsed second coordination shell.

The performance of fuel cells is limited by the sluggish kinetics of the oxygen reduction reaction (ORR) at the Pt cathode. An improved understanding of the catalytic steps of the ORR is thus essential for overcoming these limitations. By means of a PEM fuel cell designed to be compatible with our APXPS system, we have been able to identify the oxygenated intermediates of the ORR through their specific O 1s chemical shifts. Using XPS studies of well-defined model systems as spectroscopic references, we differentiate two types of OH intermediates whose population depends on cell voltage: hydrated and non-hydrated OH. We also establish that non-hydrated OH is the dominant surface species on a Pt cathode during ORR at high partial pressures of O₂(g). With the assistance of DFT calculations, we show that the reduction of non-hydrated OH requires less overpotential than that of hydrated OH. This indicates that tuning OH hydration through cathode or electrolyte design will be crucial for enhancing ORR activity.