Preparation of Bi-Metallic Catalysts for Improved Fuel Cell Activity

Fuel cell performance is limited by the rate of the platinum-catalyzed, cathodic oxygen reduction reaction (ORR) in polymer electrolyte membrane (PEM) fuel cells and anodic methanol oxidation reaction (MOR) in direct methanol fuel cells (DMFC). Pt is the preferred metal since it is very resistant to corrosion and catalyzes the ORR and MOR. However, Pt is very expensive and the reactions are slow enough that it limits the overall performance of the fuel cell. Thus, there is much need to increase the ORR and MOR activity of fuel cell catalysts. Alloying secondary base metals such as Co and Ru with Pt has been shown to increase ORR and MOR activity, respectively. This project focused on the preparation and evaluation of bi-metallic catalysts using Electroless Deposition (ED) methods. Ruthenium was the first metal investigated for ED onto the Pt surface of a commercially-available Pt/XC-72 carbon catalyst. Excessive adsorption and thermal decomposition of aqueous Ru solutions using several Ru salts resulted in their dismissal as possible candidates for ED. However, one Ru salt, ruthenium hexamine chloride, (NH₃)RuCl₃, gave good stability in solution and good kinetics for electroless deposition on Pt surfaces. Cobalt salts were then tested and sodium hexanitrocobaltate(III), Na₃Co(NO₂)₆ was identified as the preferred Co salt for ED. A series of Co loadings on the Pt surface were prepared using dimethylamine borane (DMAB) as the reducing agent. These catalysts were then tested for O₂ reduction using a Rotating Disk Electrode (RDE). It was found that the deposition of approximately 0.3 monolayers of Co on Pt significantly increased the ORR activity of the catalyst and that further deposition reduced the increase in performance over the commercially-available catalyst. The optimum ratio between Pt and Co was Pt₃Co, which corresponds closely with computational studies in Xu et al. (J. Am. Chem. Soc., 126 (2004), 4717-4725).