Decoupling the Impact of Surface Functionalization over Pt at the Cathode on Kinetics and Mass Transport in PEMFCs.

Proton exchange membrane fuel cells (PEMFCs) are critical for clean energy conversion, yet their performance is often constrained by the Pt-ionomer interface at the cathode. The sulfonate groups in Nafion, the most widely used ionomer, poison Pt active sites and form a dense layer that impedes oxygen transport. Here, we present an interface-specific molecular modulation strategy using 7-ethyltheophylline (ETP) to address both limitations. An "ETP-first" preparation method ensures preferential ETP adsorption on Pt before ionomer addition, enabling controlled surface functionalization. We demonstrate that ETP competitively displaces poisoning sulfonate groups while intrinsically activating Pt sites. Crucially, we uncover a decoupling between reaction kinetics and mass transport: peak mass activity (0.33 A mgPt-1, ∼50% enhancement) occurs at low ETP loadings (0.15%), whereas pressure-independent oxygen transport resistance (RNP) decreases monotonically with increasing ETP loading up to 1.0%. This divergence leads to distinct optima for H2-O2 versus H2-air performance─the former primarily governed by kinetics, the latter by kinetics at high voltages and by mass transport at high current densities. These findings reveal that molecular modifiers can independently tune kinetics and transport at the Pt-ionomer interface, offering a new dimension for PEMFC optimization beyond conventional catalyst design.