Modeling Pt and Catalyst Layer Degradation in Polymer Electrolyte Fuel Cell

Cost and durability of polymer electrolyte fuel cells (PEFCs) are two most critical issues faced by commercialization of fuel cells. Cathode catalyst layer (CCL) plays an important role in PEFCs from both aspects of cost and durability. Due to the sluggish oxygen reduction reaction (ORR) at CCL, sufficient Pt-loading is needed at CCL to provide electrochemical surface area (ECA) for the ORR. Meanwhile, a micro-scale oxygen transport resistance through the ionomer film covering the Pt nanoparticles is recently found. Sufficient Pt-loading is also needed at CCL to prevent large micro-scale oxygen transport loss. The cost of the Pt metal could not be reduced by mass production and features largely in the total cost of PEFC stack. At the same time, the Pt catalysis in CCL is inevitable to degrade during the PEFC lifetime, and ECA loss and PEFC performance loss are caused consequently. So using the precious Pt wisely and reducing the Pt degradation are vital challenges to achieve long-term good performance of PEFCs. Based on this, modeling the Pt and catalyst layer degradation in PEFC is urgently needed to understand the fundamental mechanisms of CCL degradation and finding methods to mitigate the CCL degradation.;A one-dimensional model is firstly developed and validated to study Pt degradation and subsequent ECA loss through the CCL of PEFCs. The model includes two mechanisms of Pt degradation: Ostwald ripening on carbon support and Pt dissolution-re-precipitation through the ionomer phase. Impact of H2 | N2 or H2 | Air operation, operating temperature, and relative humidity (RH) on Pt degradation during voltage cycling is explored. It is shown that ECA loss is non-uniform across the CCL with a zone of exacerbated Pt degradation and hence much lower ECA found near the membrane. This non-uniform Pt degradation is caused by consumption of Pt ions by crossover H2 in both H2 | N2 and H2 | Air systems. An important consequence is that thinning the cathode electrode in a PEFC would lead to more ECA loss as a higher fraction of the thin CCL would fall in this exacerbated degradation zone. We have quantified the effect of thin CCLs on Pt degradation for the first time.;The micro-scale oxygen transport resistance significantly complicates the consequence of Pt degradation. It is found that this micro-scale oxygen resistance increases with ECA normalized current density. So as the ECA in CCL losses during the Pt degradation, it not only induces higher ORR kinetic losses, but also causes extra voltage loss by increasing the micro-scale transport resistance. To elucidate this complicated issue, the 1D physics-based Pt degradation submodel is coupled into the transient M2 model to study the non-uniform Pt degradation and its impacts on long-term PEFC performance. The performance loss of a low Pt-loading PEFC with Pt degradation, the interactions of Pt degradation with the micro-scale transport resistance, the cause and consequence of non-uniform Pt degradation, as well as a strategy of raising lower current density in current cycling test are quantified. This Pt degradation model is demonstrated to be an effective approach to better understand Pt degradation, performance loss caused by Pt degradation, and mitigation strategies to alleviate Pt degradation, all important for achieving excellent durability of PEFCs.;The non-uniform Pt degradation in the channel-land direction is well predicted by the transient physics based Pt degradation model. However, further application of this method is limited by computational cost. On the other hand, the along-channel non-uniform ECA distribution can cause non-negligible effect on end-of-life (EOL) performance of PEFC compared with uniform ECA distribution when the cathode Pt-loading of the PEFC is low. Thus capturing the along-channel non-uniform Pt degradation is essential to predict the EOL performance of PEFC. In this dissertation, an empirical model for Pt degradation is developed and integrated into M2 model to predict the along-channel non-uniform Pt degradation and the EOL performance of low Pt-loading PEFCs. This method is first applied on 3D single channel PEFC modeling case. The shift of current density distributions, and the critical role of micro-scale oxygen transport loss during the degradation for low Pt loading PEFCs are also discussed with this model. Then, this modeling method is applied on a medium scale PEFC modeling case to demonstrate its future applicability to realistic industrial PEFC regarding to EOL performance.