Synthetic polymer membranes are enabling components in key technologies at the water–energy nexus, including desalination and energy conversion, because of their high water/salt selectivity or ionic conductivity. However, many applications at the water–energy nexus require ion selectivity, or separation of specific ionic species from other similar species. Here, the ion selectivity of conventional polymeric membrane materials is assessed and recent progress in enhancing selective transport via tailored free volume elements and ion–membrane interactions is described. In view of the limitations of polymeric membranes, three material classes—porous crystalline materials, 2D materials, and discrete biomimetic channels—are highlighted as possible candidates for ion‐selective membranes owing to their molecular‐level control over physical and chemical properties. Lastly, research directions and critical challenges for developing bioinspired membranes with molecular recognition are provided.

Minimizing the energy consumption of desalination processes is an important goal for augmenting freshwater production and mitigating water scarcity. Chemical, civil, mechanical, and environmental engineering students can derive and analyze the energy consumption of desalination processes by applying engineering fundamentals such as thermodynamics, transport phenomena, and process design. We explore the fundamental thermodynamic limits of the most prominent desalination technologies in a format designed for engineering students and instructors. Two thermodynamically reversible processes for reverse osmosis (RO) and electrodialysis (ED) are developed to demonstrate that reversible processes consume the theoretical minimum energy, which is the Gibbs free energy of separation. We then quantify the practical minimum energy consumption for RO and ED, showing that the energy consumption of these processes approaches the minimum thermodynamic limit with increased process staging.

Despite decades of dominance in separation technology, progress in the design and development of high-performance polymer-based membranes has been incremental. Recent advances in materials science and chemical synthesis provide opportunities for molecular-level design of next-generation membrane materials. Such designs necessitate a fundamental understanding of transport and separation mechanisms at the molecular scale. Molecular simulations are important tools that could lead to the development of fundamental structure–property–performance relationships for advancing membrane design. In this Perspective, we assess the application and capability of molecular simulations to understand the mechanisms of ion and water transport across polymeric membranes. Additionally, we discuss the reliability of molecular models in mimicking the structure and chemistry of nanochannels and transport pathways in polymeric membranes. We conclude by providing research directions for resolving key knowledge gaps related to transport phenomena in polymeric membranes and for the construction of structure–property–performance relationships for the design of next-generation membranes.

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