Pt-based Catalysts for Electrochemical Oxidation of Methanol

Abstract

This research mainly focuses on low temperature fuel cells. In detail, this research can be divided into following parts: i) Research on synthesis of low Pt loading catalyst, ii) Design of a new support material, iii) Development of one-step synthesis of Pt/graphene catalyst.

Part 1. Synthesis of Low Pt Loading Catalyst Pt-based electrodes for proton exchange membrane fuel cells have been prepared by various methods such as polyol, impregnation, microemulsion. However, inactive catalyst sites are always present in the catalyst layer prepared by the conventional method. These inactive sites are not available for fuel cell reaction because the electrochemical reaction is limited only at the interface between the polymer electrolyte and the catalyst that is exposed to the reactant, known as the triple-phase reaction zone. In order to overcome this limitation, the electrodeposition method was applied on a surface of a substrate. Low loading Pt-Co cathode catalyst on a Nafion(Na+)-bonded carbon layer was fabricated by using galvanostatic pulse technique to show the advantage of electrodeposition for high utilization of catalyst in proton exchange membrane fuel cell. The chemical composition of the electrodes could be controlled by varying the concentration of Co precursor. The Pt-Co catalysts evenly exist on the surface on the surface of carbon electrode and its thickness was about 5.8 m, which is four times thinner than conventional Pt/C. From XPS studies, Pt 4f spectrum of catalyst indicated the presence of oxidation states Pt2+ in addition to metallic Pt. The Pt-Co catalyst with a ratio of 3.2:1 was found to provide the highest single cell performance among the compositions studies. Therefore, the electrodeposition technique offers not only enhanced catalyst utilization but also simplification of preparation. Part 2. A New Support Material Carbon is an ideal material for supporting nano-sized metallic particles in a catalyst for low temperature polymer fuel cells. No materials other than carbon have the essential properties of electronic conductivity, corrosion resistance, surface properties, and low cost requied for the commercialization of fuel cells. However, carbon supports are relatively easy to be corroded under harsh fuel cell operation conditions, which would cause a rapid decay of electrochemical performance. To circumvent this shortcoming, the rational construction of carbon-based electrode materials with more advanced architectural design has been identified as the major solution. In this part, we designed a well-arranged structure of graphene-Vulcan carbon composite to prepare highly dispersed 40 wt.% PtRu electrocatalyst. Vulcan carbon was added as a nano spacer to enhance the utilization and electrochemical activites of graphene-based materials. The results show that a PtRu catalyst loaded onto the graphene-Vulcan carbon (3:1 w/w) composite exhibits high electrocatalytic activity and high stability toward methanol electrochemical oxidation owing to the special structure of the graphene-Vulcan carbon composite. Also, we investigated the feasibility of a chemically activated graphene-supported electrocatalyst. The modified graphene sheets were synthesized by chemical activation using KOH. The chemically activation of graphene resulted in the creation of pores of various sized and increased the number of edge sites. The chemically activated graphene-supported PtRu catalyst shows a high electrocatalytic activity and stability toward methanol oxidation because of its three-dimensional and porous structure that facilitates the efficient access of reactants. These results indicate that a graphene-Vulcan carbon composite and chemically activated graphene can be used as a catalyst support in fuel cells. Part 3. One-Step Synthesis of Pt/graphene Catalyst Graphene-metal nanoparticle catalysts have typically been prepared by chemical or thermal reduction of mixture of GO (or graphene) and metallic precursors. These methods involve hazardous chemical, or high temperatures and complicated procedures. In this part, we demonstrate the one step co-electrodeposition synthesis of the Pt/graphene catalyst using a galvanostatic pulse technique. Pt/graphene catalyst was uniformly deposited on a glassy carbon substrate, which facilitated the simultaneous electrochemical reduction of graphene oxide and formation of Pt nanoparticles. Compared to the commercial Pt/C catalyst, the electrochemically reduced Pt/graphene catalyst exhibited improved electrocatalytic activity for methanol oxidation due to the synergistic effects of an increase in the number of catalytic reaction sites and an enhancement of the charge transfer rate.