Electrochemical study of proton exchange membrane fuel cells, and, Development of platinum based catalysts for electro-oxidation of ethanol.

摘要

This research includes three related parts: Investigation of the effects of Nafion loading in the cathode catalyst layer on the performance of hydrogen fuel cells, designing a DHE (dynamic hydrogen electrode) reference electrode to resolve cell performance into individual electrode behaviors, and developing Pt based binary and ternary catalysts for electro-oxidation of ethanol.; It was found that Nafion content and distribution in the cathode catalyst layer had a strong influence on cell performance. There was an optimum Nafion loading for the best cell performance, which is due to the fact that a balance has to be made between ionic conductivity and O2 transport resistance.; By simulating experimental impedance data, reasonable conductivity profiles and resistance values for the cathode catalyst layer have been extracted, and they provide insights into the understanding of cathode behavior.; A novel DHE reference electrode has been designed to analyze cell performance losses. The special feature of this DHE reference electrode is that it can be conveniently fitted into a commercial cell without the need of any modifications. Polarization measurements and impedance spectroscopy have demonstrated that it was quite stable during measurements.; With the aid of the DHE reference electrode, we have separated cell performance losses into anode performance losses and cathode performance losses. For hydrogen fuel cells, it was found that at low current densities, the cell performance was mainly determined by cathode performance, while at high current densities, the anode performance contributed significantly to the cell performance. For methanol fuel cells, it was found that at low current densities, cell performance losses were due to both cathode performance losses and anode performance losses, while at high current densities, the rapid decrease of cell performance was mainly due to cathode performance losses. It was also found that the anode performance was improved at a higher concentration of methanol, whereas the cathode performance decreased significantly at a higher concentration of methanol.; In order to develop direct ethanol fuel cells, a variety of Pt based binary and ternary catalysts have been prepared for electro-oxidation of ethanol. It was found that carbon supported Pt/Sn (4:1) and carbon supported Pt/Ru/Pb (1:1:0.15) catalysts have significantly enhanced catalytic activities for electro-oxidation of ethanol relative to Pt. Preliminary fuel cell tests have shown these two catalysts to be quite promising.; Finally, a simplified equivalent circuit has been built to simulate the impedance behavior of PEMFC electrodes. The excellent fit between the experimental data and simulation data indicates that the circuit is reasonable. Meaningful resistance values have been extracted by simulation, and these values shed light on understanding cell performance losses under different operating conditions.