Preparation And Electrochemical Investigation On Membrane Electrode Assemblies Of Direct Methanol Fuel Cell

The membrane electrode assembly (MEA) was a key component of directmethanol fuel cell (DMFC), and the performance of the DMFC was greatly dependenton the performance of MEA. In this thesis, the MEAs of the active DMFC andair-breathing DMFC were investigated. The MEA fabrication methods were studied,and the compositions and the structures of the catalyst layers and gas diffusion layers(GDLs) of the MEAs were optimized. The mechanism of the effect of MEAhot-pressing technologies, and the mechanism of MEA activation were also analysed.Moreover, the operation parameters of active DMFC were studied.The effects of the MEA hot-pressing technologies (hot-pressing temperatures,pressures and time) on the performances and structures of the MEA were detailedlyinvestigated. The mechanism analysis of the effects of MEAhot-pressing technologieswere revealed by electrochemical impedance spectroscopies (EIS). It was concludedthat the MEA which was hot pressed at 135℃under 80 kg cm-2 for 90 s showed thehighest power density (46.0 mW·cm-2 at cell temperature of 80℃), and the MEA hadthe lower cell resistance, electrochemical reaction resistance and diffusion resistance.The fabrication methods for MEA catalyst layers were investigated. SEM andAFM analyses revealed that the scraping method was a little more profitable forimproving the cell performance due to the more flat and smooth (the surface roughnesswas lower) surface of the scraped catalyst layer. It was concluded that the surfacemorphology and roughness of the catalyst layer had less effect on the performance ofthe MEA. The optimum Nafion content in the catalyst layer was also investigated, andthe optimum Nafion content in the anode and cathode catalyst layer was 34 mass %and 28mass %, respectively. The optimum Nafion content in the catalyst layer was notrelated to the metal content of the catalyst.The compositions and the structures of anode GDL were optimized. It wasrevealed that the MEA which consisted of the untreated carbon paper and hydrophilicanode micro-porous layer (comprised carbon black and 10mass% Nafion) showed thebest performance. The evolution and diffusion characteristics of anode CO2 gasbubbles on different anode GDLs were investigated using home-made transparentsingle cell. It was observed that the hydrophobic anode GDL was not helpful for improving the CO2 gas transport in the anode GDL. It was found that the optimumPTFE content in the cathode backing layerwas 16 mass%. The MEA with much lowerPTFE content (20mass % PTFE) in the cathode MPL was more beneficial for cathodewater releasing and the long-term operation.The effects of MEA activation methods on the structures and performances of theMEAs were studied. And the MEA activation mechanism was investigated by EIStests, electrochemical surface areas tests, SEM and XRD analyses. Although theparticle sizes of the catalysts in the electrodes increased during the MEA activationprocess, the electrochemical surface areas of the catalysts were increased because thepores in the electrodes were opened after the activation. The influences of the celloperation parameters on the performances of active DMFC were also researched.Compared with anode flow rate of methanol solution, the concentration of methanolsolution was more influential on the performance of active DMFC. It was found thatthere was no need to humidify the cathode oxygen of the active DMFC.In consideration of the water flooding in the cathode catalyst layer and cathodeGDL of air-breathing DMFC, two improved"Inner-water-absorption"and"Outer-water-absorption"cathode structures were proposed. It was revealed that the"Outer-water- absorption"cathode structure with 15 mass %SiO2 in the cathode MPLwas more suitable for the improvement of performance of MEA due to the increasingof hydrophilic water transport paths in the cathode MPL, thus the water floodings inthe cathode were alleviated. Moreover, a novel"current collector inside"air-breathingMEA was proposed, in which a thin metal mesh used as the cathode current collectorwas directly embedded in the cathode catalyst layer of this novel MEA cathode. Thenovel MEA showed better cell performance (increased 10.3 %) and long-termoperation performance than the conventional one because of the lower cell resistance,enhanced oxygen transport and increased water removal rate in such a novel MEAstructure.