Initial Iron‐Phthalocyanine Covalent Organic Polymer Based Three‐Dimensional Porous Cathode Catalysts Layer Directly for Proton‐Exchange Membrane Fuel Cells

ABSTRACT Covalent organic polymers (COPs), with their well‐defined and tunable structures, show promising prospects as molecular electrocatalysts for the oxygen reduction reaction (ORR). However, the conjugated two‐dimensional structures assemble through direct π–π stacking into molecularly dense layers that severely obstruct the multidimensional oxygen transport pathways, thus compromising performance in proton exchange membrane fuel cells (PEMFCs). Herein, we constructed a three‐dimensional nitrogen‐doped graphene scaffold for the in situ growth of an iron‐phthalocyanine‐based covalent organic polymer, yielding the composite material denoted as COP Fe @3D‐NG. By leveraging nondestructive x‐ray computed tomography and Avizo processing, we reconstructed their realistic porous structures. The further pore‐scale multiphysics simulations demonstrated that the 3D porous network significantly enhances mass transport with an increase of oxygen diffusion coefficient compared to the 2D structure. As a result, the PEMFCs fabricated with COP Fe @3D‐NG as the cathode catalyst demonstrated a ∼2.7‐fold increase of higher peak power density than the 2D analogue. These findings highlight a fundamental principle for next‐generation cathode design that engineering 3D porosity to create multidimensional mass transport pathways is crucial for accelerating oxygen transfer and achieving high‐performance PEMFCs.