Polymer electrolyte membrane fuel cells (PEMFCs) running on hydrogen hold great promise to provide a source of clean energy and reduce our dependence on foreign oil, but fuel cells must become more durable before they can be implemented. The effects of cationic contamination, the process of foreign cations replacing protons in the ionomer phase, reduce fuel cell durability.
The purpose of this study was to experimentally determine and subsequently model the effects of cationic contamination on PEMFCs. This was accomplished using a three branched approach. The first branch included experiments evaluating the performance of fuel cell systems. The second branch consisted of theory based models to explain experimental observations by postulating the mechanisms of cationic contamination. The final branch used alternative fuel cell configurations to validate the mechanistic postulates.
This study shows the drastic effects that cationic contamination can have on PEMFCs. These effects were quantified by contaminating PEMFCs to known amounts and then assessing fuel cell performance as a function of level of contamination. The maximum power that can be harnessed from contaminated cells decreases proportionally to the percentage of protons that are replaced. This decrease is due to a decrease in electrode kinetics, cell thermodynamics, and in the limiting current of the system.
The transport of protons and contaminant ions was modeled to show why these limitations occur. When current is drawn in a contaminated system the cationic contaminants will accumulate at the cathode side of the membrane. This can decrease kinetics since fewer protons are available for reaction. This will also decrease the thermodynamic cell potential since a concentration gradient forms across the electrodes. Finally, the fuel cell limiting current will decrease because the fuel cell will become limited by the transport of protons to the cathode electrode.
A nontraditional fuel cell configuration was employed to separate kinetic and thermodynamic effects in the system by increasing the time scale and length scale in which the protons must travel. This strip cell configuration was used to prove the existence of cationic contaminant concentration gradients across the cell. Additionally, both thermodynamic and kinetic mechanisms of performance degradation were confirmed.