Oxygen Reduction for Fuel Cells: Mechanistic Studies and the Design of New Catalysts

Abstract

In this talk, we discuss aspects of our work related to the structure and reactivity of electrode surfaces. We report on our efforts directed at establishing the mechanism of oxygen and peroxide reduction on many different metal surfaces. First, we evaluate Pt-Pt bond strains developing during the oxygen reduction reaction (ORR) as determined by in situ EXAFS measurements obtained with and without the presence of O2 The change in Pt-Pt bond distance in carbon-supported nanoclusters under O2 is found to be a maximum of 0.23 (±0.15) % to 0.40 (±0.20) % larger than under a N2 atmosphere. The Pt-Pt surface bond strains are discernible at macroscopic scales in cantilever-based bending measurements of Pt thin films under O2 and Ar over the same potential range. EXAFS - measured Pt-Pt bond strains correspond to a stress thickness and are well matched to the experimentally determined values obtained using the cantilever bending method. The electrochemical stress method is useful in evaluating reactivity in other contexts, such as the oxygen evolution reaction (OER).The understanding derived from the mechanistic work provides directions for synthesis of advanced catalysts for oxygen reduction. In particular, we have synthesized a series of bio-insipired metal coordination polymers exhibiting oxygen reduction activity. Complexes exhibit modest activity in acid, but have good activity in base where the per metal activity at maximum power is greater than that afforded by Pt. Extension of our initial synthetic work shows that a µ-η1-η1 motif for oxygen coordination gives a lower overpotential relative to the more structurally defined µ-η2-η2 geometry, probably because dioxygen coordinated in the former way is more susceptible to nucleophilic attack. Recent efforts directed at mimicking the trinuclear site in the multicopper oxidases will be described.Finally, we use the catalyst motif developed for ORR studies in a novel proton transfer switch, the operation of which will be described.