Synthesis of Pt and Pt-based electrocatalysts for direct liquid fuel cells

Abstract

The natural abundance of platinum (Pt), a commonly used precious metal in fuel cell devices, is limited and the price of Pt is rising in recent years. It poses great challenge for the commercialization of fuel cells, which can be a promising candidate for environmentally benign energy supply in the future. Intensive effort has been made to the development of highly efficient Pt containing catalysts for fuel cells with low Pt loading over the past decades. In view of this, the purpose of the research described in this thesis is to develop Pt-based catalysts with excellent electrochemical performance as well as enhanced Pt utilization. The key idea for the catalyst design is a core-shell structure construction with the Pt on the surface since only surface atoms take part in the electrocatalysis. Au@Pt/MWCNTs composite was synthesized by ions adsorption - in situ electrochemical reduction approach. Electrochemical analysis was used for the first time to prove that the Au@Pt/MWCNTs composite consisted of a submonolayer Pt structure on the surface of Au nanoparticles. The coverage of Pt can readily be tuned by changing the concentration of the Pt source or by repeating the ions adsorption process after each electrochemical reduction. The as prepared Au@Pt/MWCNTs catalyst displays facile electrochemical properties. Au@Pt/MWCNTs with low Pt coverage, namely below 40%, was inactive for methanol whereas it can electroxidize formic acid through a direct pathway effectively. Also, it was proved to be an effective catalyst for methanol tolerant oxygen reduction reaction (ORR). On the other hand, Au@Pt/MWCNTs of high Pt coverage exhibits both direct and indirect oxidation of formic acid, in addition to gradual development of methanol oxidation ability. Its specific area activity toward methanol has seen as much as a 2 fold enhancement when compared to commercially available Pt/C catalyst. PtAg hollow nanospheres were synthesized by an approach involving galvanic displacement. Ag nanoparticle precursors were first prepared as the template for the construction of the PtAg hollow structure. The thickness of the nanoshell can be easily controlled by changing the Pt source avaliable in the galvanic replacement reaction. Both the XRD and XPS analysis confirmed that the hollow spheres consisted of a PtAg alloy. Electrochemical studies proved that the PtAg alloy possessed better electrochemical performance compared to the commercially available Pt black for both methanol and formic acid oxidations. The enhancement in the electrocatalytic capability can be ascribed to the better utilization of Pt as well as the electronic effect induced by the incorporation of Ag. Pd@PdPt/MWCNTs composite was fabricated by a two-step method. Pd/MWCNTs precursor was first prepared by the hydrolysis of PdCl₄²⁻ in the presence of MWCNTs in an aqueous medium at 60°C prior to its reduction by hydrogen. XRD was used to confirm the identity of the as formed PdO/MWCNTs intermediate. The surfactant free Pd/MWCNTs composite shows even size distribution of Pd nanoparticles without the formation of aggregates. The coating of Pd nanoparticles by PdPt alloy was realized through galvanic replacement between Pd and PtCl₄²⁻ at 90°C. Electrochemical investigations revealed that the resulting Pd@PdPt/MWCNTs display improved activity toward methanol and ethanol oxidations as well as better durability.