BOROHYDRIDE OXIDATION OVER THE AU(111) AND PT(111) SURFACES: A FIRST PRINCIPLES STUDY OF THE REACTION MECHANISM
Open Access
Author:
Rostamikia, Gholamreza
Graduate Program:
Chemical Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
November 18, 2009
Committee Members:
Michael John Janik, Thesis Advisor/Co-Advisor Michael John Janik, Thesis Advisor/Co-Advisor
Keywords:
DBFC Borohydride DFT Electrocatalysis
Abstract:
Direct borohydride fuel cells (DBFCs) offer the potential for direct chemical to electrical energy conversion from a high specific energy, water soluble fuel. The lack of effective anode materials for the electrocatalysis of borohydride has been the major limitation in advancing the application of direct borohydride fuel cells. In this study, we apply electronic structure calculations to elucidate the mechanism of borohydride oxidation over the Au(111) and Pt(111) surfaces. Reaction free energies computed as a function of electrode potential are used to identify stable surface bound intermediates and likely rate limiting steps. The results suggest that the weak adsorption of BH4- over Au(111) may limit the coverage of reactive intermediates at low overpotentials. Catalytic activity of gold is also limited by high barriers for breaking B-H bonds. The mechanism of borohydride oxidation and the competing hydrolysis reaction is also examined over Pt(111) using density functional theory (DFT) methods. Adsorption over Pt(111) is dissociative and extremely exothermic at potentials of interest, leading to a high surface coverage of H* for which gaseous hydrogen evolution is competitive with oxidation. Elementary surface reactions oxidizing B-containing intermediates are favorable over Pt(111) at -0.85 V(NHE), consistent with experimental voltammetry results in the literature. The energetics of the initial adsorption step dictate the activity limitation of gold anodes and the selectivity limitation of platinum electrodes.
