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

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.