Preparation And Applications Of Carbon Supported Pt-Based Anode Electrocatalysts For Direct Methanol Fuel Cells

Direct methanol fuel cell(DMFC) is classified as low temperature fuel cells, which is a variant of proton exchange membrane fuel cell(PEMFC). In DMFC, methanol is utilized directly as fuel, without the need of converting into hydrogen. The development of DMFCs as commercial products has received widespread attention due to the following advantages: abundant source of fuel, relative safety(in comparison with hydrogen) of storage and relatively mature technology. However, several issues remained to be solved from fundamental research and development to commercialization in the market. Among them, the sluggish anode catalyst for methanol oxidation reaction was considered to be the most important problems.This thesis was then aimed for the development and applications of highly active and stable Pt-based anode electrocatalysts for DMFC. Development of fabrication technique for Pt nanoparticles with optimized sizes and synthesis of composite carbon support were two main focuses of this work. The as-prepared composite support was used to load 40 wt.% Pt nanoparticles(Pt/CS) as an electro-catalyst for DMFC.In this work, a modified citrate reduction method assisted by inorganic salt stabilization for the preparation of very stable and highly dispersed Pt hydrosols(2.0 nm) was developed. The addition of inorganic salt(stabilizing Pt hydrosols through a thicker electrostatic double layer) significantly reduced the citrate amount needed(Cyt3-/Pt4+=1:1), therefore the conventionally used post heat treatment to remove excess citrate was unnecessary for carbon-supported Pt electrocatalysts. The open circuit potentials of the Pt precursors before reduction were measured, and most probably the first time used to account for the Pt hydrosol stability and correlated with zeta potential measurement and transmission electron microscopy(TEM) imaging. The carbon supported Pt electrocatalysts prepared in this work exhibited high mass activity toward methanol oxidation(as a probe reaction) relative to commercially available Pt/C electrocatalysts. Highly ordered PtxRuy nanocomposites are electrostatically self assembled when the oppositely charged Ru and Pt colloids were mixed at room temperature. CO stripping voltammograms of PtxRuy/C showed that the CO stripping peak and the on-set oxidation potential were highly dependent on Pt/Ru ratio which was mostly probably due to the different interstitial space between Pt and Ru in the self-assembled patterns.Motivated by addressing the support corrosion or stability issue for electrocatalysts, a carbon composite support was prepared by wet-impregnation of nano-sized Ti O2 and cobalt(II) acetate tetrahydrate together with XC-72 carbon black, followed by thermal treatment at 900 oC in a reductive atmosphere. Decomposition of cobalt acetate under thermal treatment yielded Ti O2-Co3O4-C composite support(CS), as was confirmed by X-ray diffraction analysis. The composite support was found to be more corrosion resistant and stable in acidic solution than that of the conventionally used carbon black, as was witnessed by the chronoamperometric response. This composite support was then used to load 40.3 wt.% Pt nanoparticles(Pt/CS) as an electrocatalyst for proton exchange membrane fuel cells. It was found that 27.9% of the electrochemical surface area(ECSA) retained in an accelerated stability test(AST) in 0.5 M H2SO4 for Pt/CS, compared with 8.4% for 39.5 wt.% Pt catalyst supported on XC-72 carbon black. The TEM image after AST for Pt/CS also confirmed its improved resistance to agglomeration. The Pt/CS catalyst showed higher mass-normalized activity for methanol oxidation in 0.5 M CH3 OH + 0.5 M H2SO4 and a slower activity decay relative to that of the benchmarked Pt catalyst on carbon.