UBC Theses and Dissertations
Development of the direct borohydride fuel cell anode Lam, Vincent Wai Sang
Direct borohydride fuel cells (DBFC) are a promising technology for meeting increasing energy demands of portable electronic applications. The objective of this dissertation was to contribute to the understanding of borohydride (BH₄⁻) electro-oxidation and the development of the DBFC anode; a component which can influence both the performance and cost of a DBFC system. The first part of the investigation involves the elucidation of the BH₄⁻ electro-oxidation mechanism on Pt. The BH₄⁻ electro-oxidation mechanism was studied by correlating the results obtained by the electrochemical quartz crystal microbalance technique (EQCM) and the rotating disk electrode technique (RDE) with density functional theory (DFT) calculations from the literature. It was found that BH₄⁻ electro-oxidation on Pt resulted in the adsorption of reaction intermediates, such as BH₂OHad and BOHad, which required high oxidizing potentials to desorb/ oxidize from the catalyst surface. It was also found that the BH₄⁻ oxidation mechanisms (Langmuir – Hinshelwood versus Eley - Rideal) were dictated by the availability of Pt-sites and the competitive adsorption of OH⁻ and BH₄⁻. The second part involves an investigation of the performance of three different carbon black supported anode catalysts: Pt, PtRu, and Os, with a focus on Os catalysts. Fundamental electrochemical methods combined with fuel cell experiments revealed that osmium nanoparticles are kinetically superior and stable catalysts for BH₄⁻ electro-oxidation compared to Pt and PtRu. It was also found that supported Os electrocatalysts appear to favour the direct oxidation of BH₄⁻ in comparison to Pt, and PtRu electrocatalysts. The final section of this dissertation focuses on the effect of electrocatalyst support and anode design on the performance of the DBFC anode. It was found that the Vulcan® XC-72 supported catalyst alleviated mass transfer related problems associated with hydrogen generation from BH₄⁻ hydrolysis. The most significant improvement was obtained when using the graphite substrate supported catalysts (three-dimensional anodes). Fuel cell studies revealed power densities of 103 mW cm⁻² to 130 mW cm⁻² achieved by 1.7 mg cm⁻² Os and ~1 mg cm⁻² PtRu three-dimensional electrodes respectively at 333 K, using an O₂ oxidant at 4.4 atm (abs), and a 0.5 M NaBH₄ – 2 M NaOH anolyte composition.
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