Proton Exchange Membranes from Microemulsion-Synthesized COF Nanosheets for Water Electrolysis

Proton exchange membranes (PEMs) simultaneously with high proton conductivity, strong mechanical strength and low swelling are urgently required for PEM water electrolysis. In this study, we propose a microemulsion-mediated semi-confined method for synthesis of covalent organic framework (COF) nanosheets featured by the concurrent implementation of uniform size, high crystallinity and yield, and excellent scalability, which is virtually impossible for the dominant interfacial polymerization and phase-transfer polymerization methods. The versatility of this method is demonstrated by synthesizing a range of COF nanosheets, including TpPa-SO₃H, TpBd-(SO₃H)₂, and TpTG. The resulting TpBd-(SO₃H)₂ COF nanosheets are assembled into self-standing COF membranes, achieving the highest proton conductivity (1.76 S cm^-1), high mechanical strength (92.1 MPa), and negligible swelling ratio (<5%). In practical PEM water electrolysis, the TpBd-(SO₃H)₂ COF membrane yields a current density of 3.0 A cm^-2 at 2.3 V and 60 °C, outperforming the commercial Nafion membrane (3.0 A cm^-2 at 2.7 V) under identical conditions. Our work develops an alternative platform method for COF nanosheet synthesis and marks the first application of self-standing COF membranes in PEM water electrolysis, unlocking their great potential for high-efficiency energy conversion.