Addition of platinum to palladium-cobalt nanoalloy catalyst by direct alloying and galvanic displacement

Abstract

Direct methanol fuel cells (DMFC) are being investigated as a portable energy conversion device for military and commercial applications. DMFCs offer the potential to efficiently extract electricity from a dense liquid fuel. However, improvements in materials properties and lowering the cost of the electrocatalysts used in a DMFC are necessary for commercialization of the technology. The cathode electrocatalyst is a critical issue in DMFC because the state-of-the-art catalyst, platinum, is very expensive and rare, and its performance is diminished by methanol that crosses over from the anode to the cathode through the Nafion membrane. This thesis investigates the addition of platinum to a palladium-cobalt nanoalloy electrocatalyst supported on carbon black in order to improve catalyst activity for the oxygen reduction reaction (ORR) and catalyst stability against dissolution in acidic environment without significantly reducing the methanol-tolerance of the catalyst. Platinum was added to the palladium-cobalt nanoalloy catalyst using two synthesis methods. In the first method, platinum was directly alloyed with palladium and cobalt using a polyol reduction method, followed by heat treatment in a reducing atmosphere to form catalysts with 11 and 22 atom % platinum. In the second method, platinum was added to a palladium-cobalt alloy by galvanic displacement reaction to form catalysts with 10 and 22 atom % platinum. The palladium cobalt alloy was synthesized using a polyol method, followed by heat treatment in a reducing atmosphere to alloy the nanoparticles before the Pt displacement. It was found that both methods significantly improve catalyst activity and stability, with the displaced catalysts showing a higher activity than the corresponding alloy catalyst. However the alloy catalysts showed similar resistance to dissolution as the displaced catalysts, and the alloyed catalysts were more tolerant to methanol. The displaced catalyst with 22 atom % platinum (8 wt. % Pt overall) performed similar to a 20 wt. % commercial platinum catalyst in both RDE and single cell DMFC tests. The 10 and 22 atom % Pt displaced catalysts and 22 atom % Pt alloyed all showed higher Pt mass specific activities than a commercial Pt catalyst.