Ghassemzadeh, Lida, and Shulamith Schlick
One of the main obstacles for commercializing the polymer electrolyte membrane (PEM) fuel cells is the chemical degradation of polymer electrolyte. In a fuel cell the membrane is in direct contact with an oxidizing environment at the cathode side and chemically reducing environment at the anode side. The observation of radicals in fuel cells supports the assumption of oxidative degradation of the polymer membrane via a free radical process, which under acidic condition involves H·, HO· and HOO· radicals. Attack of these radicals is considered to represent an important degradation pathway for PEMs.
In the present work, the formation of radicals at anode and cathode side of the fuel cell has been monitored by in situ ESR spin trapping spectroscopy at 300 K, in closed and open circuit voltage conditions. The formation of radicals was monitored by changing different parameters. In order to study the effect of catalyst, the fuel cell was operated with a membrane electrode assembly (MEA) with single and double side Pt electrodes. To find the effect of gas cross over, experiments has been repeated for membranes with different thicknesses. In order to find the effect of polymer structure on radical formation, perfluorosulfonic acid polymers with short and long side chain were compared. Results showed that the DMPO/OOH and DMPO/H adducts were detected both at the cathode and at the anode. However the relative intensity of DMPO/H is higher at the anode and the DMPO/OOH is higher at the cathode. The formation of these radicals in chemical reactions is controlled by gas crossover and Pt presence. When there is no catalyst, no radical was detected. The experiments with thicker membranes showed that lower rates of gas crossover in these membranes controls radical formation. For thicker membranes, DMPO/H was detected at the anode and DMPO/OOH at the cathode.