Highly flexible SCOF proton exchange membrane reinforced with PTFE to enhance fuel cell power density

Sulfonated covalent organic frameworks (SCOFs) facilitate rapid proton conduction through densely ordered sulfonic acid groups, however, the brittleness of COFs self-supporting membranes often makes potential difficulty in fuel cell assembly and limits their power density. Herein, a highly flexible SCOF proton exchange membrane is developed through in-situ growth of a continuous BD(SO₃H)₂–COF microphase within porous PTFE networks. The strong hydrogen bonding between PTFE and BD(SO₃H)₂–COF contributes to the defect-free morphology of the BD(SO₃H)₂/PTFE membrane. The reinforce of PTFE network makes the membrane extremely high flexibility, achieving an elongation at break of 124.4 % even with a remarkably high SCOF mass proportion of 90 wt% (BD(SO₃H)₂/PTFE-0.9). This allows the membrane to be folded repeatedly, even in dry state. The swelling ratio in water at 80 °C is effectively restricted to 8.6 %, even with a high ion exchange capacity of 3.6 mmol g⁻¹ and a water uptake of 68.2 %. The densely ordered sulfonic acid groups in continuous BD(SO₃H)₂–COF microphase contribute to a high proton conductivity up to 249.2 mSꞏcm⁻¹ at 80 °C, approximately 1.5 folds that of Nafion 212. As a result, the BD(SO₃H)₂/PTFE-0.9 membrane achieves a fuel cell power density of 1195.3 mWꞏcm⁻² at 80 °C, along with a high open circuit voltage of 1.01 V, surpassing the-state-of-the-art COF-based proton exchange membranes. This work provides a novel strategy to fabricate COFs into flexible and size scalable membranes, enhancing the performance of fuel cells.