Cross-layer alternating paired charge distribution to boost proton conductivity of COF lamellar membrane

Covalent organic frameworks (COFs) have attracted great interest for the development of proton exchange membranes (PEMs) in the field of hydrogen fuel cells. However, the Grotthuss mechanism of proton transfer fails to discriminate the preferential pathways on the complicated hydrogen-bond network, thus limiting cell performance. Herein, a heterocharged COF lamellar membrane with cross-layer alternating paired charge distribution was fabricated by alternately assembling the positively charged TpEB and negatively charged TpPa-SO₃H nanosheets. We demonstrated that the TpEB nanosheet drives the proton to directional transfer along the hydrogen-bond network on the sulfonic acid periphery of TpPa-SO₃H by significantly reducing ineffective motions in branched hydrogen-bond network for both through-plane and in-plane directions. Meanwhile, it provides a low absorption energy for proton transfer from the sulfonic acid shell. This assembly effect occurs within the three nanosheet layers during each alternate film formation (A₃B₃) and contributes to the long-term stability of the proton conductivity for TpEB@TpPa-SO₃H. Notably, it achieves a maximum power density of 223 mW cm⁻² at 60 °C and 100 % RH, which is superior to those of 83 mW cm⁻² and 22 mW cm⁻² for the homocharged TpPa-SO₃H and TpEB membranes, respectively. This work provides new insights into the design of high-conductivity PEMs from engineered COF membranes.