[Thesis]. Manchester, UK: The University of Manchester; 2011.
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.