From Salinity Gradients to Sustainable Power: A Paradigm Shift in Materials, Thermodynamics, and System Design in Next‐Generation Osmotic Energy Harvesting

Osmotic energy, arising from the Gibbs free energy of mixing between solutions of differing salinity, is an entropy-driven resource that can, in principle, deliver continuous baseload power at river-sea interfaces, desalination brine outlets, and wastewater discharges. Despite this attractive thermodynamic ceiling, practical deployment of pressure-retarded osmosis, reverse electrodialysis, and emerging osmotic architectures has long been constrained by membrane resistance, concentration polarization, fouling, and limited operational stability. This Review traces the evolution of osmotic energy conversion through the lens of materials science, from early polymeric and ion-exchange membranes to contemporary highly selective ion pathways. By correlating pore size, surface charge density, interfacial chemistry, and hierarchical morphology with ion selectivity, power density, and durability under realistic salinity gradients, we distil general design principles that reconcile the classical permeability selectivity trade-off and mitigate fouling and internal resistance. We further discuss the integration of molecular dynamics and multiscale transport modelling to rationalize ion migration in confined geometries and to guide the atomic-scale engineering of nanochannels. Finally, we assess emerging directions, including scalable fabrication routes, and data-driven optimization of membrane and module design that define a materials-centered roadmap for translating osmotic energy from laboratory prototypes to technologically and economically relevant blue energy infrastructures.