Title

Anode electrocatalysts for direct borohydride fuel cells.

Date of Award

2006

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Electrical and Computer Engineering

Keywords

Engineering, Materials Science.

Rights

CC BY-NC-ND 4.0

Abstract

Colloidal Os and Os-alloys, Pt and Pt-alloys, Au and Au alloys, Ir and Ir-alloys, and Pd and Pd alloys (1:1 atomic ratio for all alloys) supported on Vulcan XC-72 (20 wt% metal) were prepared according to the Bonneman method and investigated for their electrocatalytic activity with respect to methanol and borohydride oxidation for Os and with respect to borohydride oxidation only for Pt, Au, Ir, and Pd for fuel cell applications. Voltammetry on static and rotating electrodes, chronoamperometry and chronocoulometry were performed on the colloidal catalysts immobilized on glassy carbon with the help of a NafionRTM 117 solution acting as the ionic conductor and binder of the catalyst layer. Insights were gained with regard to the BH4- electro-oxidation mechanism, and apparent kinetic parameters were determined such as Tafel slopes, exchange current densities, heterogeneous rate constants, and the total number of electrons involved. The electrolytes were: 0.5 M H2SO4 for methanol oxidation and 2M NaOH for borohydride oxidation, respectively. The fundamental studies were followed by direct borohydride fuel cell experiments using a 2 M NaOH - 2 M NaBH4 solution on the anode side, 5 mg cm -2 colloidal anode catalyst load and NafionRTM 117 membrane. The cathode was a conventional O2 gas diffusion electrode with 4 mg cm-2 Pt. Voltammetry results showed that the Os-based materials were catalytically inactive with respect to methanol oxidation in acid media. It was found that all three pure colloidal Os (10, 20 and 30 %wt) materials showed catalytic activity in cyclic voltammetry experiments toward the oxidation of borohydride. Chronopotentiometry experiments with a current step of 25 mA cm-2 showed that the 20 %wt Os gave the lowest anodic overpotential, about 0.3 V, of the Os-based materials. On Pt and its alloys, cyclic voltammetry (CV) results show that colloidal Pt and colloidal Pt-alloys were electrochemically active toward borohydride oxidation with oxidation potentials ranging between -0.85 and +0.3 V vs. a mercury/mercury oxide reference electrode (MOE). Of the investigated colloidal catalysts, the Pt-Ir alloy gave the highest voltammetric BH4 - oxidation current densities at potentials more negative than about -0.2 V vs. MOE, which is ultimately the domain of interest for borohydride fuel cells. (Abstract shortened by UMI.)Dept. of Electrical and Computer Engineering. Paper copy at Leddy Library: Theses & Major Papers - Basement, West Bldg. / Call Number: Thesis2006 .A89. Source: Dissertation Abstracts International, Volume: 67-07, Section: B, page: 4038. Thesis (Ph.D.)--University of Windsor (Canada), 2006.