Investigating the Effects of Surface Chemistry and Pore Structure in Grafted PEM Fuel Cell Catalyst Layers

The carbon support surface chemistry strongly influences ionomer interactions, water management, and electrochemical performance in polymer electrolyte fuel cell catalyst layers; however, studies that decouple these effects from Pt particle size remain limited. Here, diazonium grafting is applied to commercial 40 wt% Pt/Vulcan and Pt/Ketjenblack catalysts to introduce five aryl functionalities (–NO 2 ,–NH 2 ,–CF 3 ,–COOH,–SO 3 H) spanning a wide range of polarities and wettabilities without altering Pt dispersion. Ex-situ characterization (X-ray photoelectron spectroscopy, thermogravimetric analysis, N 2 physisorption, contact angle, and dynamic vapor sorption) confirms successful functionalization and shows that grafting on Ketjenblack occurs preferentially at external surfaces, reducing pore accessibility. In 5 cm 2 single-cell tests, nitrogen-containing functionalities reduce cathode proton transport resistance, increasing current density at 0.7 V (1.20 vs 1.01 A cm −2 , I/C = 0.6). For hydrophilic and hydrophobic groups, optimal performance depends strongly on the ionomer-to-carbon ratio: hydrophilic functionalities enhance performance at low I/C (0.1) but become detrimental at higher I/C (0.6) due to increased water retention and unfavorable ionomer interactions. These effects are amplified for porous Ketjenblack, where pore blockage offsets gains from surface modification. These findings establish that surface functionalization and support porosity cannot be optimized independently, and suggest that next-generation mesoporous carbon supports could incorporate pore architectures compatible with surface modification.