Direct methanol fuel cells (DMFCs) are a type of electrochemical energy-conversion device that directly converts the chemical energy stored in methanol to electricity. Because of its simplicity, high energy densities, and instantaneous recharging, DMFCs have been identified as one of the most promising power sources for portable and mobile electronic devices. Although appealing, the DMFC technology is hindered by several critical technical problems, one of which is the durability of the membrane and electrode assembly (MEA) for DMFCs. Among various mechanisms that deteriorate the MEA durability, the MEA delamination and the dissolution of ruthenium (Ru) from the anode catalyst, platinum-ruthenium (Pt-Ru), are two of the most critical ones. The MEA delamination hinders the proton conduct...[ Read more ]

Direct methanol fuel cells (DMFCs) are a type of electrochemical energy-conversion device that directly converts the chemical energy stored in methanol to electricity. Because of its simplicity, high energy densities, and instantaneous recharging, DMFCs have been identified as one of the most promising power sources for portable and mobile electronic devices. Although appealing, the DMFC technology is hindered by several critical technical problems, one of which is the durability of the membrane and electrode assembly (MEA) for DMFCs. Among various mechanisms that deteriorate the MEA durability, the MEA delamination and the dissolution of ruthenium (Ru) from the anode catalyst, platinum-ruthenium (Pt-Ru), are two of the most critical ones. The MEA delamination hinders the proton conduction between the electrode and membrane, thereby resulting in a large ohmic polarization loss in cell performance, while the leaching of Ru for the PtRu catalyst leads to an increase in the activation polarization loss, reducing cell performance. This thesis work was aimed at improving the MEA durability by suppressing the interfacial delamination between the electrode and membrane and by reducing the Ru dissolution from the Pt-Ru catalyst.

Conventionally, MEAs for DMFCs are prepared by sandwiching catalyst layers, a membrane, and gas diffusion layers together under hot-pressing conditions. We studied the hot-pressing method and its effect on the MEA durability and found that an adequately longer hot-pressing duration can improve the MEA durability. To further improve the interfacial contact between the electrode and membrane and increase the MEA durability, we developed a new MEA preparation method, in which, a polymer electrolyte solution, acting as a binding agent, is introduced between a membrane and electrodes and a MEA can be formed at low pressures. We found that the MEA prepared by this glue method yielded higher cell performance and better durability than did the MEA formed by the conventional hot-pressing method. To further address the MEA delamination problem, we proposed an integrated anode structure consisting of platinum nanowires that are electrochemically deposited into a partial layer of a membrane. In this new MEA structure, as the anode electrode is integrated into the membrane, the MEA delamination becomes unlikely. We found that the integrated MEA structure gave better cell performance, as the new design can not only enable a larger electrochemically active surface area, but also reduce the rate of methanol crossover.

Another concern with respect to the MEA durability is the leaching of Ru from the anode Pt-Ru catalyst. To mitigate this issue, we proposed to incorporate the third metal, gold (Au), into the PtRu alloy catalyst, and for the first time we demonstrated that the stability of the catalyst was significantly improved, as the incorporation of gold can increase the oxidation potential of ruthenium.

In addition, we studied the kinetics of the ethanol oxidation reaction (EOR) on a palladium electrode in an alkaline medium. We found that the dehydrogenation of ethanol on the palladium electrode proceeded rather fast and the rate determining step was the removal of the adsorbed ethoxi by the adsorbed hydroxyl on the palladium electrode. For the first time, we demonstrated that the Tafel slope was 130 mV dec^-1, instead of 250 mV dec^-1 as reported in the literature. Based on the experimental data, we developed a kinetics equation, j = 0.105c_EtOH^0.24c_KOH^0.66 exp(0.40F/RT*η), for the EOR on the Pd electrode in an alkaline medium.

Keywords: Delamination; Direct Methanol Fuel Cell; Durability; Ethanol Oxidation Reaction; Interfacial Contact; Kinetics; Membrane and Electrode Assembly; Methanol Crossover; Ruthenium Crossover; Ruthenium Dissolution; Stability.