Sub-two-micron ultrathin proton exchange membrane with reinforced mechanical strength

Proton exchange membranes (PEM) serve as indispensable components in fuel cells, flow batteries, and other electrochemical applications. Over the past six decades, PEMs of diverse compositions and structures have been developed for various industrial scenarios. Among all parameters, the thickness is pivotal in affecting the performance, cost, and stability of PEM. Yet for either pure or composite PEM, there is a lower boundary of thickness if the PEM is still targeted to practical utilization. As there is a trade-off between thickness and mechanical stability for a freestanding membrane. We selected ultra-high molecular weight polyethylene (UHMWPE) for its outstanding processability and high modulus intrinsically to breakthrough this trade-off. By biaxially stretching UHMWPE into ∼300 nm thin porous membranes and impregnating Nafion interiorly, with careful interfacial engineering, a type of ∼1.6-μm thin composite PEM was prepared. As the thinnest freestanding PEM with practical use ever reported, surficial, cross-sectional, and interfacial structures were rigorously characterized to explain its performance. In the end, this thin PEM demonstrated a membrane power density of up to 663.69 mW cm⁻² for H₂–O₂ fuel cell set-up. This work provides new insight into the structure-performance correlation of PEM with a near-sub-micron thickness. And it may also inspire future research to further address the performance-stability paradox of PEM with thin/ultrathin thickness range.