Cellulose Nanocrystal Composite Membrane Enhanced with In Situ Grown Metal–Organic Frameworks for Osmotic Energy Conversion

Access to clean and renewable energy, osmotic energy from salinity gradient difference, for example, is central to the sustainability of human civilization. Despite numerous examples of nanofluidic membranes for osmotic energy conversion, one produced from abundant and renewable biomass resources remains largely unexplored. In this work, cotton-derived cellulose nanocrystals (CNCs) are employed to fabricate a membrane by self-assembly with polyvinyl alcohol (PVA) and subsequent in situ growth of metal-organic framework (MOF), UiO-66-(COOH)₂, to provide an example. The composite membrane exhibits excellent mechanical strength and toughness due to the long chains and hydrogen bonding interactions of PVA. The incorporation of UiO-66-(COOH)₂ endows the composite membrane with abundant nano- and subnano-sized ion transport channels, resulting in a 150% increase in ion conductance, while also providing superior cation selectivity through collaboration with the sulfated CNCs. The composite membrane with 27.4% MOF content can achieve an osmotic energy conversion performance of 5.10 W m^-2 under a 50-fold KCl gradient condition and a monovalent cation selectivity of ≈16 for K⁺/Mg²⁺. This work presents a solution for harvesting renewable osmotic energy by constructing nanofluidic membranes using plentiful renewable biomass materials and a simple, low-emission fabrication procedure.