A New Architecture Design for Observing Lithium Oxide Growth-Evolution Employing Geometric Catalyst Positioning

Abstract

A common route to overcoming the large overpotentials and poor reversibility in current Li-O2 batteries has been to develop various solid-state catalysts (i.e. noble metals, metal oxides) decorated on the oxygen electrodes.[1-2] Although these approaches have yielded incremental improvements, catalytic sites are easily deactivated by the continuous precipitation of solid products.[3] Understanding the mechanisms of discharge product formation and evolution and optimizing the catalyst systems are necessary to guide future Li-O2 cell design.[4] In our previous report,[5] we demonstrated that dispersing catalyst particles in an insulating membrane over the oxygen electrode helps to maintain their function by reducing passivation of the active surfaces during lithium peroxide formation. The major benefit of this approach is that the oxygen evolution reaction (lithium peroxide dissolution) is facilitated by these isolated catalytic sites. While the catalytic membrane architecture is quite effective in practical cells, it is important to investigate the exact chemistry and morphology of products far from the electrode surface and thereby elucidate the remarkable function of these electrically isolated sites. In this work we demonstrate a new architecture using anodic aluminum oxide (AAO) membranes as a porous scaffold with Pd nanoparticles dispersed throughout as the catalytic sites. This approach allows us to directly observe lithium-oxide product formation and evolution both in the presence of catalysts and without to better understand the mechanistic influence of catalyst positioning. We show that AAO membranes can be readily cross-sectioned (without destroying the well-defined pore structure), and can incorporate the formation of integrated lithium-oxide products. As a result, we clearly observe a planar-to-particulate morphological transition in Li-O2 products during cell discharge and the reverse during charge. The Pd-AAO interlayer facilitates the uniform growth of discharge products as well as the evolution reaction, minimizing the buildup of residues seen in the absence of Pd. In this talk, we will provide evidence for improved charge transfer and reduced overpotentials as a result of catalyst positioning inside the AAO membrane pores. The novelties of this work demonstrate how the catalyst positioning is important and how insolated catalyst sites can function separately from electrical contact to the electrode for improved columbic round trip efficiency for next generation Li-O2 cell designs. [1] Ryu, W. H.; Yoon, T. H.; Song, S. H.; Jeon, S.; Park, Y. J.; Kim, I. D. Nano Lett 2013,13, 4190-7. [2] Gittleson, F. S.; Ryu, W. H.; Schwab, M.; Xiao Tong, Andre D. Taylor, Chem Commun 2016, In press. DOI: 10.1039/C6CC01778A [3] Gittleson, F. S.; Ryu, W. H.; Taylor, A. D. ACS Appl Mater Interfaces 2014,6, 19017-19025. [4] Gittleson, F. S.; Yao, K. P. C.; Kwabi, D. G.; Sayed; Ryu, W.-H.; Shao-Horn, Y.; Taylor, A. D., ChemElectroChem 2015, 2, 1446-1457. [5] Ryu, W. H.; Gittleson, F. S.; Schwab, M.; Goh, T.; Taylor, A. D., Nano Lett 2015, 15, 434-441