Eco‐Friendly Synthesis and Mechanistic Exploration of Multifunctional Cu/Cr Self‐Assemblies for Durable and High‐Performance Fuel Cell Composite Membranes

ABSTRACT Designing durable and high‐performance proton exchange membranes (PEMs) remains a critical challenge for advancing polymer electrolyte membrane fuel cells (PEMFCs). In this study, we exploit the distinct coordination chemistries of Cu(II) and Cr(III) to construct multifunctional macromolecular self‐assemblies of aspartic acid (L‐AA) via a greener route, which were further incorporated (2 wt.%) into sulfonated poly(phenylene oxide) (SPPO) to yield composite membranes (SPCu and SPCr). The dual functionality of L‐AA‐Cr(III) (─COOH and ─NH 2 groups) enabled stronger H‐bonding interactions with ─SO 3 H groups, enhanced proton conduction, and water uptake compared to the L‐AA‐Cu(II) (─NH 2 only). Consequently, SPCr achieved a peak power density of 512.4 mW cm −2 and a current density of 718.3 mA cm −2 at 0.6 V, outperforming pristine SPPO by 88.1% and SPCu by 21%. Linear sweep voltammetry confirmed substantially lower H 2 crossover (0.96 mA cm − 2 ), while accelerated degradation tests for 100 h at 80°C and 30% RH revealed only 19% OCV decay (1.7 mV h −1 ) for SPCr. Post‐ADT analysis further demonstrated 85.96% power retention for SPCr, establishing its superior long‐term durability. These findings highlight the efficacy of multifunctional fillers in enhancing proton conduction, acid‐base interactions, fuel cell performance, and long‐term stability of PEMFC membranes.