438661 Interfacial Processes in Energy Storage and Conversion Devices

Sunday, November 8, 2015
Exhibit Hall 1 (Salt Palace Convention Center)
Hadi Tavassol, Material Science, Caltech, Pasadena, CA; Material Science, Northwestern University, Evanston, IL

My research has been focused on characterization and control of the interfacial processes in advanced electrochemical systems presented by solid acid fuel cells (SAFC) and Li ion batteries. The performance of these electrochemical 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) CsH2PO4 solid acid and electrodes occurs at active sites which are also accessible to the humidified gases. Pt remains the best catalyst for both H­2 oxidation and O2 reduction reactions in the anodes and cathodes of the solid acid fuel cells. Although the electrocatalysis of the H2 → 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 H2 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 reduction is a slow reaction even on a Pt surface at 245 °C, requiring high loadings of the precious Pt catalyst.  Solid state TiO2 catalysts are an attractive alternative. The activity of the TiO2 films grown on Ti metal under slightly oxidizing environment at elevated temperature (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 analysis shows that high molecular weight 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 mechanical effects also play a major role in capacity retention, and battery performance. Advanced Sn based anodes, depending on their SnOx content, exhibit significant changes in compressive and tensile surface stresses which directly influences their performance. In graphite anodes, our analysis shows that the mechanical effects scale with the charging rate, which explains the chemo-mechanical degradation effects common during fast charging of batteries.

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