Nanointerface-regulated cerium confinement via crown ether coordination in proton exchange membranes

Cerium (Ce)-based free radical scavengers, including soluble Ce³⁺ species, have been widely investigated to enhance the chemical durability of proton exchange membranes (PEMs) owing to their rapid and regenerative redox cycling. However, the high mobility of soluble Ce³⁺ ions in hydrated membranes leads to severe leaching, disruption of ion-cluster nanostructures, and degradation of proton exchange membrane fuel cell (PEMFC) performance. Regulating the behavior of cerium species within the nanophase-separated environment of perfluorosulfonic acid (PFSA) membranes remains a critical challenge. Herein, we report a nanointerface-regulated cerium confinement strategy enabled by crown ether coordination. A model organometallic complex (Ce/HMCRE) is constructed using 2-hydroxymethyl-15-crown-5 ether, in which host-guest coordination and secondary hydrogen bonding interactions cooperatively modulate cerium distribution at polymer nanointerfaces. This coordination-mediated nanoconfinement effectively suppresses direct Ce³⁺-sulfonate interactions while preserving the intrinsic ion-cluster morphology of PFSA membranes. As a result, the Ce/HMCRE complex exhibits significantly enhanced cerium retention (3.76-fold higher than free Ce³⁺) together with sustained radical scavenging activity. The corresponding membrane electrode assembly delivers a low open circuit voltage decay rate of 0.45 mV h^-1 and retains 83.4% of its maximum power density after 150 hours of accelerated degradation testing. This work highlights the importance of nanointerface engineering and confined microenvironments in regulating redox-active species within ionomer membranes, providing new insights into the design of durable electrochemical energy materials.