Effects of Magnesium Reduction on the Structure and Electrical Properties of Tantalum Oxide Aerogels
Date
2020-08
Authors
Aguirre, Lupita S.
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Abstract
Electrocatalyst supports that have resistance to corrosion, high surface area, high porosity, and adequate electronic conductivity are needed to improve the durability of proton-exchange membrane fuel cells (PEMFC) utilized for transportation, stationary, and portable power applications. Although platinum supported on carbon is the typical electrocatalyst used for the oxygen reduction reaction (ORR) in PEMFCs, the corrosion of carbon presents a challenge for the widespread and commercialization of this technology. Tantalum pentoxide (Ta2O5) is stable under the oxidative potentials and acidic conditions of the ORR, but prior efforts have largely focused on carbon-Ta2O5-Pt catalysts which can still result in the degradation of the support. To develop a potential Ta2O5 support, the effects of thermal treatments and magnesium reduction of tantalum oxide aerogels were evaluated in order to create a support material that meets the required characteristics typically observed in carbon-based supports. Sol-gel synthesis and supercritical drying were performed to obtain tantalum oxide (TaOx) aerogels with high surface area and porosity. Thermal treatments in air and thermal reduction using magnesium (Mg) as a reducing agent were investigated to improve the crystallinity and electronic conductivity of TaOx aerogels. The morphology and structure of TaOx aerogels were determined using scanning electron microscopy, nitrogen physisorption, and X-ray diffraction measurements. Electronic conductivity measurements were obtained using a two-point probe configuration. Results indicated that the sol-gel synthesis method was a viable approach to produce TaOx aerogels with high surface areas of 197 m2 g-1</sup> and mean pore diameters of 11 nm. Subsequent thermal treatments in air and Mg reduction modified the crystal structure, reduced the oxygen content, and improved the electronic conductivity of TaOx aerogels. Mg reduction of the as-prepared TaOx aerogel produced a material with a TaO crystal structure, a surface area of 54 m2 g-1, and an electronic conductivity of 3.7x10-3 S cm-1. Moreover, Mg reduction on the 850ÂșC-thermally treated TaOx aerogel produced a material with Ta2O crystal structure, a surface area of 11 m2 g-1, and an electronic conductivity of 3.5x10-4 S cm-1. From this study, sol-gel synthesis, thermal treatments, and Mg reduction were shown to improve the resulting structure, morphology, and electronic conductivity of TaOx aerogels making them potential alternatives to replace carbon-based supports. Understanding the effect of thermal treatments and Mg reduction in TaOx aerogels opens up new opportunities for the design and development of highly durable and stable carbon-free ORR electrocatalyst supports for fuel cells.
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Keywords
Electrocatalyst supports, Tantalum oxide
Citation
Aguirre, L. S. (2020). Effects of magnesium reduction on the structure and electrical properties of tantalum oxide aerogels (Unpublished thesis). Texas State University, San Marcos, Texas.