Ionomer-free Pt Microenvironment in Bimodal Carbon Scaffolds Generates Exceptional Oxygen Reduction Mass Activity in PEM Fuel Cells

Pt nanoparticles (NPs) on carbon are widely used as catalysts in polymer exchange membrane (PEM) fuel cell cathodes. However, the Nafion ionomer/binder can poison Pt and reduce its electrocatalytic activity. To solve this dilemma, a bimodal ball-and-stick carbon scaffold (BCS) was developed, consisting of spheres containing 3-D-interconnected primary mesopores (12 nm) housing Pt NPs, with Nafion found only on the sphere surfaces, creating an ionomer-free micro-environment that protects Pt from Nafion poisoning. These catalysts exhibit exceptionally high mass activities for oxygen reduction (ORR) that are retained after accelerated durability testing in a membrane-electrode-assembly, even demonstrating performance improvement. The transport of protons to Pt NPs occurs via water on the hydrophilic mesopore surfaces, verified by the pronounced effect of RH on ORR kinetics and excellent ORR activity even without any Nafion added. These findings establish a clear link between ionomer poisoning of Pt and ORR activity and highlight the effectiveness of combining specific carbon morphologies and oxygen-rich surfaces to produce ultra-high performance.