Carbon Shell‐Encapsulated PtFe Catalysts in High Temperature Polymer Electrolyte Membrane Fuel Cells: Balancing Molecular Sieving and Diffusion for Enhanced Reactant Access

ABSTRACT High‐temperature polymer electrolyte membrane fuel cells (HT‐PEMFCs) operating at 160°C on phosphoric acid‐doped polybenzimidazole membranes address the water management and CO poisoning challenges faced by conventional PEMFCs. However, phosphate anions adsorb strongly onto Pt active sites, hindering oxygen reduction kinetics (“phosphate poisoning”) and limiting long‐term performance. To address these challenges, we present PtFe nanoparticles encapsulated within a porous carbon shell engineered to impart molecular sieving of phosphate anions and protect against Pt degradation. Previously, the molecular‐sieving effect of carbon shell coatings has been studied in half‐cell evaluations; however, the reaction mechanism of carbon shell‐encapsulated catalysts remains under debate. Herein, we integrate the catalysts into membrane electrode assemblies and demonstrate high HT‐PEMFC performances (peak power density of 0.94 W·cm −2 and stability for 200 h). Furthermore, scanning electrochemical microscopy measurements demonstrate that O 2 diffuses through the engineered porous carbon shell to the PtFe core rather than via uncontrolled defects, emphasizing the need for optimized mass transport. This single‐cell performance represents a significant step toward practical, high‐performance HT‐PEMFCs and underscores the importance of catalyst architectures that mitigate phosphate poisoning while maintaining catalytic activity under harsh, high‐temperature conditions.