Because of the complexity in the chemistry and electronic structure of perovskite and related oxides, multiple physical descriptors have been proposed for predicting the kinetics of oxygen reduction on them. These include the availability and mobility of oxygen defects that mediate the reaction, and the ease of electron transfer at the surface. The process is further complicated by the detrimental segregation and phase separation of constituents at the surface. Interfaces between dissimilar oxides are attracting significant interest for their potential role in accelerating charge transport and surface reaction kinetics. If well understood and controlled, they can provide a new way to enable high-performance solid-oxide fuel cells, separation membranes as well as fast switching memristors. For example, recent studies have demonstrated that cobaltite hetero-interfaces exhibit orders of magnitude faster oxygen reduction kinetics compared with either single phase. To reveal the mechanisms behind this unusually high performance, we have investigated the local electronic structure and defect chemistry at nanometer resolution in model multilayer superlattices and vertical nanostructures that are made of dissimilar cobaltites. Our results indicate that the electronic and chemical interactions between the two phases are the likely mediators behind such an unprecedented enhancement. To accomplish this, we used a novel combination of in-situ scanning tunneling spectroscopy, focused ion beam milling and high energy X-ray photoelectron spectroscopy. We found that the higher reactivity of these dissimilar interfaces to the oxygen reduction reaction is due to enhanced electron availability for charge transfer and the suppression of detrimental cation segregation. Furthermore, we have put forth elastic strain to be an important driver of the kinetics of surface reactions and diffusion in functional oxides, and demonstrated these concepts on fluorite, perovskite and Ruddlesden Popper structures. There remains still a large amount of open questions on how dissimilar oxide interfaces oxygen reduction at the surfaces. However, these recent results are encouraging for an improved understanding of oxide hetero-interfaces at elevated temperatures and could enable new interfaces with fast oxygen transport and oxygen reduction kinetics.

Oxygen reduction mechanisms on perovskite oxides and the effects of dissimilar interfaces

249th National Meeting (2015)

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