Direct methanol fuel cell (DMFC) is a promising energy device for portable applications due to its high energy density and easy storage of methanol as fuel. Methanol crossover is one of the major technical issues for limiting DMFC widespread commercial applications. In this thesis, we firstly proposed and developed an integrated anode structure based on flexible graphite material to reduce methanol crossover. This new structure not only provides dual role of liquid diffusion layer and flow field plate, but also serves as a methanol and water blocker. It is found that DMFCs incorporating this new anode structure exhibit a much higher voltage than that of a conventional DMFC at high methanol concentrations. Moreover, by periodically feeding methanol and water, methanol crossover can be fu...[ Read more ]

Direct methanol fuel cell (DMFC) is a promising energy device for portable applications due to its high energy density and easy storage of methanol as fuel. Methanol crossover is one of the major technical issues for limiting DMFC widespread commercial applications. In this thesis, we firstly proposed and developed an integrated anode structure based on flexible graphite material to reduce methanol crossover. This new structure not only provides dual role of liquid diffusion layer and flow field plate, but also serves as a methanol and water blocker. It is found that DMFCs incorporating this new anode structure exhibit a much higher voltage than that of a conventional DMFC at high methanol concentrations. Moreover, by periodically feeding methanol and water, methanol crossover can be further reduced in comparison to the continuous feed way. Therefore, the new anode design simplifies the conventional anode structure and offers a promising approach in running passive-mode DMFC at high methanol feed concentrations.

Based on the investigation of mass transport of methanol through the integrated anode structure, we further developed a composite integrate anode with segmented structures for better performance of DMFCs operating at high methanol concentration. The newly composite integrated anode incorporates flow field and diffusion layer at anode with the upper half part treated by pore-forming agents with benefit for methanol mass transport and the lower half part treated with Nafion® solution with benefit for suppressing methanol crossover. The bifunctional characteristics of the composite integrated anode enables promote the peak specific power densities of DMFC by 23% at 10 M compared to the traditional one.

Finally, in order to radically suppress methanol crossover for DMFC, we prepared and characterized a composite conductive membrane made of cesium hydrogen sulfate doped with silica gel functionalized by phosphorous acid by a two-step method to be an alternative membrane to replace the traditional Nafon® membrane operated at intermediate temperature for DMFCs application. The composite membrane exhibits high proton conductivity of 2.1Χ10^-2 S cm^-1 at 160 °C, whose proton conductive mechanism predominately complies with Grotthuss mechanism. Furthermore, a single DMFC unit with the composite membrane has been successfully demonstrated with pronounced reduction of methanol crossover relative to Nafion® 115 membrane.

Keywords: Methanol crossover; Flexible graphite; Integrated anode structure; Proton conductivity; Composite membrane; Intermediate temperature; DMFC