Revealing the Trade-Off between Catalytic Activity and Mass Transport in Fuel Cells: Perspectives from Solvent and Ionomer Coverage State

In response to the conflicting challenges between enhancing polarization performance and optimizing platinum catalyst utilization in proton exchange membrane fuel cells (PEMFCs), this study systematically deciphers the formation mechanism of triple-phase interfaces (TPIs) in catalytic layers (CLs) and develops a multidimensional CL construction strategy guided by solvent properties. The cooperative regulation of hydrogen bonding and polarity in mixed solvent systems on ionomer self-assembly behavior is revealed, and the dynamic coupling mechanism between Pt/C-ionomer agglomeration and CL microstructure evolution is elucidated. The ∼500 nm ionomer aggregates were induced to form by sulfonic group (-SO₃^-)-hydrogen-bond networks in strong hydrogen bonding/polarity solvent systems (water-based), further forming shear-thinning slurry. The synergistic effects are demonstrated between hierarchical porosity and locally covered TPIs in the resultant CL architecture, achieving enhanced mass transport efficiency despite elevated ohmic resistance. In contrast, the weak hydrogen bonding/polarity system (ethyl propionate-based) generates ∼100 nm uniformly coated ionomer structures with eliminated shear-thinning characteristics. Although achieving reduced internal resistance, oxygen transport is deteriorated due to dense ionomer coverage in this system, leading to significant performance decay at high current densities. Notably, the weak hydrogen bonding/strong polarity system (dimethyl sulfoxide-based) produces oversized aggregates (>1000 nm), creating Pt-exposed interfaces with insufficient ionomer coverage. Although achieving minimal ohmic loss, this structure causes a surge in oxygen transport resistance; therefore, the performance is degraded across all current ranges. This work establishes a cross-scale design paradigm spanning from molecular self-assembly to mesoscopic structure evolution for fuel cell electrode engineering. It provides fundamental insights into solvent-mediated ionomer distribution strategies for synergistic optimization of catalytic activity and mass transport capability.