L. A. Naslund

Spectroscopic probing of local hydrogen-bonding structures in liquid water

We have studied the electronic structure of liquid water using x-ray absorption spectroscopy at the oxygen K edge. Since the x-ray absorption process takes less than a femtosecond, it allows probing of the molecular orbital structure of frozen, local geometries of water molecules at a timescale that has not previously been accessible. Our results indicate that the electronic structure of liquid water is significantly different from that of the solid and gaseous forms, resulting in a pronounced pre-edge feature below the main absorption edge in the spectrum. Theoretical calculations of these spectra suggest that this feature originates from specific configurations of water, for which the H-bond is broken on the H-donating site of the water molecule. This study provides a fingerprint for identifying broken donating H-bonds in the liquid and shows that an unsaturated H-bonding environment exists for a dominating fraction of the water molecules.

Double Role of Water in the Fuel Cell Oxygen Reduction Reaction

Water is present during the normal operating condition of polymer electrolyte membrane fuel cells (PEMFCs) and we have, through model experiments on a platinum surface using x-ray photoelectron spectroscopy, investigated its role in two different stages in the oxygen reduction reaction (ORR). We found that the activation of oxygen to the adsorbed atomic phase occurs readily in the dry region, but not in the humid region process. In this case, water is acting to inhibit the active sites on the catalyst. On the contrary, water is found to promote the reduction of adsorbed atomic oxygen in the formation of a water-hydroxyl surface complex.