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 (eta(act)), ohmic overpotential (eta(ohm)), and concentration overpotential (eta(conc)), is unraveled. The findings reveal that as the I/C ratio increases, eta(act) decreases whereas eta(ohm) and eta(conc) 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 eta(act), eta(ohm), and eta(conc), establishing a comprehensive "structure-property" relationship. The ORR mass activity, dry proton accessibility, and pore structures are key parameters to determine eta(act), 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 eta(act). However, a high dry proton accessibility along with small pore sizes and low porosity lead to high eta(ohm) and eta(conc). Thus, a detailed morphology balance should be reached to obtain high-efficiency PEMFCs. Additionally, water management is another important factor determining eta(conc), and its weighted importance reaches 70 %similar to 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.