"Brick-and-mortar" structured nanofluidics with high cation flux employed for efficient osmotic energy harvesting

Osmotic energy, alternatively termed blue energy and salinity gradient energy, represents an essential form of green marine energy. Membrane-based reverse electrodialysis (RED) technology is deemed a promising approach to harvesting blue energy, with ion-selective nanofluidics serving as the key component. However, conventional nanofluidics suffer from deficiencies in ion selectivity and transport efficiency owing to the dearth of charge sites, thereby limiting their osmotic power output. In this study, negatively charged sodium carboxymethyl cellulose (CMC) molecular chains were introduced into MXene nanosheets to fabricate high-performance MXene@CMC nanofluidics (MC-NF). The incorporation of CMC substantially augmented the negative charge sites (both surface and space charge) within the nanofluidics, thus ensuring excellent selectivity; simultaneously, it expanded the channel size, culminating in a high cation flux. Experimental results demonstrated that MC-NF significantly surpassed pure MXene nanofluidics regarding mechanical properties and brought about a remarkable improvement in osmotic energy conversion. Under natural seawater and artificial river water conditions, MC-NF achieved an impressive power density of up to 17.10 W/m2, which is more than 10 times higher than that of pure MXene nanofluidics and 3.42 times the commercial benchmark (5 W/m2). In summary, this study provides valuable theoretical and practical guidance for designing advanced nanofluidics to extract osmotic energy from salinity gradients.