All-natural clay–carbonate nanofluidic membranes with engineered nanochannels for high-power osmotic energy harvesting

Salinity gradient osmosis is a clean, sustainable power source via osmosic energy, however, favourable practical application through reverse electrodialysis (RED) is thwarted by the prohibitive expense and technical complexity of ion-permeable membranes. We herein introduce an eco-friendly nanofluidic membrane made of the natural montmorillonite (MMT) clay intercalated with nanoscale calcium carbonate (nano-CaCO₃) termed as the upcycled material obtained from discarded eggshells. The nano-CaCO₃ acts as a biogenic intercalant which extends the MMT spacing of the interlayer and enhances surface charge density, thereby increasing cationic flux and selectivity. The optimized composite membrane (designed as MC_0.10, with 10 wt% of CaCO₃) reaches a maximum power density of 10.28 W m⁻² under a 500-fold gradient of NaCl and outcompetes most of the conventional counterparts. The membrane is also shown to be very stable for 30 RED cycles (≈300 h) with a cation transport number t⁺ = 0.81 and a maximum power density obtained of 25.6 % of the energy conversion efficiency (η_max). Bulk structural and elemental analyses confirm the enlargement of the nanochannels and charging of the nanochannels under the inclusion of nano-CaCO₃. This all-natural membrane approach membrane approach not only enhances osmotic energy harvesting functionality but also supports sustainable resource exploitation by upcycling the waste eggshells, providing a scalable and cost-efficient application for the blue power generation.