Ionomer Distribution Control for Improving the Performance of Proton Exchange Membrane Fuel Cells: Insights into Structure–property Relationships

The cathode catalyst layer (CCLs) are the reaction centers in proton exchange membrane fuel cells (PEMFCs). The catalyst/carbon support and ionomer separate into a multi-length-scale mesoporous morphology that determines the PEMFCs performance. The ionomer-to-carbon (I/C) ratio is thus a key parameter in dictating morphology. The CCLs with various I/C ratios ranging from 0.50 to 0.90 are prepared, and its influence on key performance parameters, including activation overpotential (ηact), ohmic overpotential (ηohm), and concentration overpotential (ηconc), is unraveled. The findings reveal that as the I/C ratio increases, ηact decreases whereas ηohm and ηconc increases. Finally, a balanced performance is achieved at an I/C ratio of 0.75, delivering a maximum power density of 1.29 W cm−2. Furthermore, we decouple multiple factors that influence ηact, ηohm, and ηconc, establishing a comprehensive “structure–property” relationship. The ORR mass activity, dry proton accessibility, and pore structures are key parameters to determine ηact, with correlation constants exceeding 0.80. Improving ORR mass activity and dry proton accessibility, and reducing pore size and porosity in CCLs are favorable for ORR kinetics, thus reducing ηact. However, a high dry proton accessibility along with small pore sizes and low porosity lead to high ηohm and ηconc. Thus, a detailed morphology balance should be reached to obtain high-efficiency PEMFCs. Additionally, water management is another important factor determining ηconc, and its weighted importance reaches 70 %∼90 % at 2.0 A cm−2. These insights provide a quantitative understanding of the key structure features governing the PEMFCs performance, offering valuable insights for morphology design and performance optimization.