(4bt) Membrane and Catalyst Degradation in Polymer Electrolyte Fuel Cells

Components of polymer electrolyte fuel cell (PEFC) membrane electrode assemblies (MEAs) deteriorate during long-term operation. Two of the most serious issues are the deterioration of perfluorinated ionomer membrane used as the solid electrolyte and electrocatalyst dissolution.

In the case of membrane degradation, it has been shown that reactive oxygen species (ROS) such as HO? and HO2? radicals (1-7) produced at the anode and cathode of the fuel cell as well as within the electrolyte membrane are responsible for the degradation reactions. The formation of hydrogen peroxide during fuel cell operation has been confirmed and degradation by-products have been detected in product water. Three approaches can be used to minimize the effect of ROS in a PEFC: (i) the use of free radical scavengers (8-10); (ii) the use of dispersed peroxide decomposition catalysts within the electrolyte (11) and (iii) the use of dispersed peroxide decomposition catalysts within the electrodes (12-14). The present study is aimed at validating these approaches by: 1) incorporating hydrogen peroxide decomposition catalysts (MnO2; WO3) in the electrodes; and 2) incorporating radical scavengers (CeO2; MnO2; metal nanoparticles) in the electrolyte. The efficacy of the hydrogen peroxide decomposition catalysts is evaluated in terms of: 1) their ability to mitigate the generation of H2O2; and 2) their ability to lower the rate of electrolyte membrane degradation. The former is estimated through rotating ring disk electrode (RRDE) experiments by estimating the percentage of H2O2 detected at the ring electrode during the oxygen reduction reaction. The latter is estimated through MEA studies, wherein the amount of fluoride ions released by the membrane is estimated after a specific operating period in an accelerated test. The efficacy of the radical scavengers incorporated within the electrolyte was evaluated through accelerated durability testing of MEAs.

In the case of Pt dissolution, it was revealed that the major cause of active Pt surface area loss is Pt coarsening (Pt dissolution and redeposition as in Ostwald ripening; 15-17) and Pt diffusion away from the cathode (18). Cathode catalyst degradation models were developed examining the effect of relative humidity (RH), hydrogen crossover and reactant gas partial pressure on Pt dissolution (19-22). RH has a drastic effect on Pt degradation; an approximate three- and two-fold increase of Pt mass and active surface area loss respectively at 100% RH (compared to 50% RH) was observed (20). Hydrogen crossover increases Pt mass loss from the particle surface resulting to an enhanced Pt surface area loss (22), while reactant gas partial pressure does not affect catalyst degradation rate (20). Despite these modeling attempts, there is partial information about the role of Pt particle size distribution (23), Pt+2 ions diffusivity and platinum oxide chemical dissolution reaction on catalyst degradation rate. This work aims to address the role of particle size distribution on Pt stability. A theoretical model is built based on modified Butler-Volmer equations (19, 21) to describe the electrochemical reactions of Pt dissolution and oxidation considering the size dependency of particle stability (Kelvin equation) and platinum oxide surface coverage. The introduction of volume-average particle radius equations in the model allows the use of different Pt particle size distributions in the model, examining their distribution trend and effect on Pt stability.

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