Interest in electrifying the transportation sector has intensified as the value in reducing carbon emissions is recognized globally. Polymer-electrolyte fuel cells (PEFCs) provide an option for electrifying transportation if hydrogen is produced electrolytically from water and without carbon emissions if the electricity is provided by renewable energy resources. Though research in fuel cell powered vehicles has been ongoing, hydrogen infrastructure and concerns about hydrogen storage safety limit the technology’s uptake in the passenger vehicle market. However, fuel cells applied to heavy-duty-vehicle (HDV) transportation may address these shortcomings. HDV provide predictable routes allowing for more simple hydrogen infrastructure and larger cargo allowance in HDV allows for more secure hydrogen storage and safety. Fuel-cell-powered-HDV require higher operating temperature, pressure, and potential to realize the power-demands of the vehicle, therefore, durability requirements increase, and a better understanding of degradation is a priority. A major cause of performance degradation is due to catalyst dissolution, migration, and reprecipitation. This causes a loss of electrochemically active surface area (ECSA) which reduces the activity of the catalyst and is particularly detrimental in the cathode where the limiting factor for performance is often sluggish oxygen reduction kinetics. This loss in ECSA is due to an overall broadening of the particle size distribution (PSD) within the catalyst layer and a net loss of catalyst to the formation of a Pt band in the membrane. In this work, a Pt catalyst degradation model is developed which considers particle size dependent Pt oxidation and dissolution kinetics, Pt migration, and reprecipitation of Pt within the cell. This work interrogates the particle size dependence of these processes to explain experimentally observed loss in ECSA and changes in PSD. The degradation model is coupled with a PEFC performance model through the ECSA loss to study how the HDV-relevant operating conditions impact the cathode catalyst degradation behavior observed in PEFCs.