Water‐Retentive Covalent Organic Framework Membranes for Efficient Proton Conduction in PEMFCs

Abstract Well‐ordered proton conduction pathways and effective water retention in proton exchange membranes (PEMs) are essential for achieving high proton conductivity, especially under low‐humidity conditions. Here, β ‐ketoenamine‐linked covalent organic framework (COF) membranes with subnanometer channels and meta‐substituted diamine linkers containing proton carriers (−NH ₂ , −CO ₂ H, −SO ₃ H), and a sulfonated COF with conventional hexagonal nanometer channels are reported. The structural design of COF‐SO ₃ H integrates spatial confinement within concave subnanometer channels and locally enriched, highly hydrophilic −SO ₃ H groups, significantly enhancing water retention and promoting hydrogen‐bond network formation. The membrane achieves high proton conductivities of 96.2 mS cm ⁻¹ at 20% RH (90 °C) and 220.1 mS cm ⁻¹ at 98% RH (80 °C). Density functional theory (DFT) calculations and experiments reveal that at low RH, the locally enriched −SO ₃ H group strengthens −SO ₃ H∙∙∙H ₂ O interactions and promotes water clustering. At high RH, these interactions suppress excessive water diffusion and stabilize extended hydrogen‐bond networks, facilitating continuous low‐barrier proton transport. In H ₂ /O ₂ PEM fuel cells (PEMFC), COF‐SO ₃ H delivers high peak power densities of 368.6 mW cm ⁻² under 20% RH and 695.1 mW cm ⁻² under 98% RH at 80 °C, and good durability over 100 h of open‐circuit voltage (OCV) testing at 20% RH and 80 °C.