Engineering Carbon Support Packing Density to Enhance Mass Transport in Fuel Cell Cathodes

To enhance the power output of proton exchange membrane fuel cells, improving the mass transport of protons and oxygen within the cathode catalyst layer is essential. The ionomer loading in the cathode catalyst layer is a key parameter governing the trade‐off between proton transport and oxygen transport: while a high ionomer loading facilitates proton conduction, it inevitably impairs oxygen transport. Herein, we demonstrate that reducing ionomer loading can be achieved by increasing the cathode catalyst layer packing density, which is controlled by rationally reducing the cavity size of hollow mesoporous carbon sphere (HMCS) supports. In situ electrochemical diagnostics reveal that a small‐cavity HMCS‐supported Pt catalyst, forming a high‐packing‐density catalyst layer, establishes a continuous ionomer network at a relatively low ionomer‐to‐carbon weight ratio of 0.6. This structure yields a low oxygen transport resistance of 0.70 s cm −1 while maintaining efficient proton conduction. With a low cathode Pt loading of 0.1 mg Pt cm −2 , the membrane electrode assembly delivers a rated power density of 1.03 W cm −2 at 0.67 V under heat‐rejection‐constrained conditions in H 2 ‐air fuel cells.