PERFORMANCE AND LIMITATIONS IN LOW TEMPERATURE FUEL CELLS

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Abstract

The recent wave of interest in sustainability has brought the benefits of fuel cells into the public sphere. Fuel cells are viewed as viable power sources for many applications, including ground transport, distributed power generation and portable electronics. The commercial breakthrough of fuel cells is hindered by the high price of fuel cell components. Lower prices will be achieved by developing new materials and improving performance. For that it is necessary to understand and minimize degradation of fuel cell components. This thesis addressed these questions with an experimental approach. The emphasis of this work was on the effect of operating conditions effect on the 8 cells-stack built by SRE company, with a post-mortem analysis to show how they influenced the life-time of the stack and which components were subject to more severe degradation.In the first part of this work, after the pre-conditioning a series of tests was done, under various conditions. The stack stability at open-circuit revealed a high degradation over the first hour. Galvanostatic tests were done and the last cells of the stack always displayed negative potential values, indicating a fuel starvation. Hydrogen flow rate, hydrogen pressure, air flow rate, and hydrogen inlet temperature effects were studied. The increase of hydrogen flow rate did not provide any advantage in terms of power. The effect of hydrogen pressure on the fuel cell performance reported in the literature was confirmed. The increment on hydrogen inlet temperature showed an opposite effect to that expected, with maximum performance obtained at room temperature. Therefore, the main conclusion to be drawn from this part was related to the water management, limiting the operation of the stack to room temperatures. Modifications on the anode flow field channels should be done in order to overcome this limitation.The best performance was obtained with 0.4 L min-1 H2 at 500 mbar and 7.56 L min-1 of air, at room temperature.After 1500h operation, a performance decrease of 34% was achieved and the polarization curve showed the existence of limitation in the activation and mass transport regions. A post-mortem analysis of some cells by SEM, TEM and EDS provided reasons for the voltage loss in these two regions. Loss of PTFE ionomer in both catalytic layers; morphological changes in the catalyst surfaces such as, loss of porosity and platinum aggregation, deformation on the MEA components (anode, cathode and membrane) and carbon corrosion were some of them. Others, like delamination and cracking were also detected. Catalyst migration and agglomeration on the interface of the electrodes was observed at cells 2, 4, 6 and 7. A platinum band was also detected on the membrane at 2 m apart from the anode of cell 4. In some cases, dissolution occurred with re-deposition of the platinum particles with faceted shape; in other cases, migration and agglomeration occurred with the original spherical shape maintained. Regarding carbon corrosion, the obtained data from thickness variations of anode and cathode layers were not in agreement with the EDS analysis, hindering precise conclusions as to where occurred the carbon corrosion. More work is necessary in this area.

Keyword(s)

Degradation; PEM Fuel Cells; Performance; SEM; TEM

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