Interfacial processes in energy storage and conversion devices

Abstract

My research has been focused on characterization and control of the interfacial processes in advanced electrochem. systems presented by solid acid fuel cells (SAFC) and Li ion batteries. The performance of these electrochem. systems is closely related to the processes occurring at the interface of electrodes and electrolytes. The electrocatalysis at the interface of the super-protonic (at 245 °C) CsH_2PO_4 solid acid and electrodes occurs at active sites which are also accessible to the humidified gases. Pt remains the best catalyst for both H_2 oxidn. and O_2 redn. reactions in the anodes and cathodes of the solid acid fuel cells. Since the electrocatalysis of the H_2 → H+ is an efficient process at 245 °C, the structure of the interface plays a major role in overall performance of the anodes. 3D nanostructures of carbon nanotubes decorated carbon fibers coated with Pt provide an efficient contact between electrolyte, Pt coatings, and H_2 gas. Interestingly, the activity of such structures correlates with the film thickness, where thinner films show higher activity. On the cathode side, because of the strong O=O bond, oxygen redn. is a slow reaction even on a Pt surface at 245 °C, requiring high loadings of the precious Pt catalyst. Solid state TiO_2 catalysts are an attractive alternative. The activity of the TiO_2 films grown on Ti metal under slightly oxidizing environment at elevated temp. (600-900 °C) is controlled by the phase and thickness of the films. Thinner films with more rutile phase show higher activity. In Li ion batteries, our anal. shows high mol. wt. oligomerized species are formed as a part of solid electrolyte interphase (SEI). Nature of the formed SEI and degree of oligomerization can be controlled by using electrolyte additives such as vinylene carbonate and Vinyl ethylene carbonate. Interfacial and bulk mech. effects also play a major role in capacity retention, and battery performance. Advanced Sn based anodes, depending on their SnO_x content, exhibit significant changes in compressive and tensile surface stresses which directly influences their performance. In graphite anodes, our anal. shows that the mech. effects scale with the charging rate, which explains the chemo-mech. degrdn. effects common during fast charging of batteries.

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