(Invited) Modeling Two-Phase Transport in the Cathode Catalyst Layer of a PEM Fuel Cell with a Focus on the Ionomer Phase Structure and Its Effect on Oxygen Transport

Abstract

A hydrophobic treatment was developed to reduce the liquid water saturation level and increase its removal rate from the cathode catalyst layer (CL) of a PEM fuel cell. If this was achieved, the transport rate of oxygen to the reactive sites would be expected to increase and the fuel cell performance would be improved, especially in the high current-density mass-transport controlled region. The performance of the fuel cells with the hydrophobically treated cathode CLs tested with humidified air at 70oC shows significant improvement (> 200% increase in peak power).[1,2] The polarization curves obtained show improvements over the entire curve from the kinetic-controlled region to the mass-transport affected and mass-transport limited regions instead of only the mass-transport limited region as originally expected. A mathematical model was developed to provide insight into the causes for the observed responses with a focus on the oxygen-water two-phase transport phenomena in the catalyst layer. It suggests the hydrophobic treatment affects the CL in two ways. First, the interface of the ionomer layer exposed to the gas pores becomes more hydrophobic and facilitates less coverage and faster water drainage from the CL resulting in better performance at high current densities. Second, it reduces the hydration level in the ionomer phase which results in higher oxygen concentration in the ionomer phase on top and in the catalyst agglomerates leading to higher performance over the whole polarization curve. This presentation will discuss the major features of the new model and the findings obtained in this study.[3] These efforts show that the modeling work is not just a simulation of the experimental results using established theories and equations. It requires researching, finding, and understanding the mechanisms and phenomena that govern the physical processes in the fuel cell and result in the measured fuel cell performance. The modeling work provides insights that could not be gained from fuel cell testing.References Regis P. Dowd Jr., Cynthia S. Day, and Trung Van Nguyen, “Engineering the Ionic Polymer Phase Surface Properties of a PEM Fuel Cell Catalyst Layer,” J. Electrochem. Soc., 164 (14), F138-F146 (2017). 10.1149/2.1081702jesRegis P. Dowd, Yuanchao Li, and Trung Van Nguyen, “Controlling the Ionic Polymer/Gas Interface Property of a PEM Fuel Cell Catalyst Layer During Membrane Electrode Assembly Fabrication,” J. Appl. Electrochem., 50 (10) 993-1006 (2020). 10.1007/s10800-020-01453-wYuanchao Li and Trung Van Nguyen, “A One-Dimensional Model of the Cathode Catalyst Layer of a PEM Fuel cell Hydrophobically Treated for Water Management,” J. Electrochem. Soc., 169 114505 (2022). 10.1149/1945-7111/ac9bdf