Molecular−Level Water Balance by Functional COFs Tailoring H─Bond Network for Boosting Proton−Oxygen Transmission in PEMFC Cathode

ABSTRACT Proton exchange membrane fuel cells (PEMFCs) are essential for sustainable energy solutions, while cathode water balance is the core key factor that limits the full realization of the performance of catalysts in the operating environment of the catalyst layer (CL). This challenge worsens under low platinum (Pt) loading and high catalyst density, as uneven hydration and local flooding simultaneously hinder proton conduction and oxygen transmission. Herein, we propose a molecular‐level water regulation strategy by integrating functionalized covalent organic frameworks (COFs) with a tailored H─bond environment into the cathode CL, enhancing proton conduction and gas transmission through a balanced bound–free water. Molecular dynamics simulations combined with spectroscopic and electrochemical analyses reveal that precise control over heteroatom type and distribution regulates the balance of bound and free water, constructing a corresponding H─bond network and reactant transmission pathway. The optimized NCOF‐based cathode maintains continuous proton transmission while suppressing local water accumulation for sufficient oxygen transmission, leading to a significant increase of 26% in peak power density under fully humidified conditions with a low Pt−loading of 0.1 mg Pt cm −2 . This work establishes a molecular‐scale structure water balance correlation and provides a general design principle for water balance in low Pt−loading PEMFCs.