Engineering Robust MXene Nanosheet/Cotton Nanofiber Composite Membranes for Efficient Osmotic Energy Harvesting

Abstract MXene membranes, composed of tightly packed nanosheets, face challenges such as uncontrolled and narrow slit‐shaped pores between the nanosheets, inadequate surface charges within the channels, and limited mechanical strength, which compromise their ability to capture “blue energy” when exploiting salinity gradients. In this study, cotton nanofibers (CNFs) are used as bridging units to develop a robust MXene/CNF composite membrane (MCM) with enhanced osmotic energy harvesting performance. The addition of CNFs transforms the densely packed lamellar structure of the stacked MXene nanosheets into a more loosely stacked configuration, creating enlarged transport pathways with reduced mass transfer resistance. The abundant functional groups of MXene and CNFs provide a high negative space charge density, facilitating an efficient cation separation. Moreover, the interaction between fibers and nanosheets significantly enhances the tensile strength of the pristine MXene membrane (≈22 MPa) by 7 times for the reinforced MCM (≈145 MPa). The output power density of MCMs reaches up to ≈9.7 W m −2 with a high energy conversion efficiency of 28.6% when mixing synthetic seawater (0.5 m NaCl) and river water (0.01 m NaCl). This study provides a universal strategy for assembling nanosheets into high‐performance membranes.