Terminal-carboxyl-engineered 3D PLA nanochannels via ionic liquid templating for high osmotic energy conversion

Ion exchange membranes (IEMs) are crucial for osmotic energy harvesting via reverse electrodialysis, yet face a fundamental trade-off: increasing charge carriers boosts conductivity but reduces permselectivity due to swelling. This stems from poor control over ionic group distribution in the IEMs. Herein we demonstrate terminal-carboxyl-engineered 3D polylactic acid (PLA) nanochannels fabricated via ionic liquid templating for high-performance osmotic energy conversion. As a crystalline polymer, PLA's carboxyl terminals cannot be incorporated into the crystalline lattice due to structural mismatch, leading to their exclusion from the crystalline regions. To fabricate a 3D interconnected nanoporous structure, we employed an ionic liquid as a templating agent, which has noncovalent interaction with the carboxyl terminals. Under the synergistic effect of crystallization-driven exclusion and noncovalent interaction with ionic liquid, the carboxyl groups preferentially accumulate at nanochannel interfaces. This unique architecture enables exceptional cation selectivity and conductivity, achieving high power density (42.9 W/m2 under 500-fold gradient) among scaled IEMs while maintaining >93 % performance over 9 days. The controlled single-carboxyl-per-chain distribution establishes a new design concept for energy harvesting.