Kelp Nanofiber‐Based Composite Membranes for Highly Efficient Osmotic Energy Conversion

Abstract The blue osmotic energy harvesting based on the nanofiber‐based layered membranes has received tremendous attention. However, the most nanofiber‐based ion‐selective membranes are suffering from uneconomical, environmentally unfriendly characteristics and a long growth cycle, as well as insufficient power density. In this work, kelp cellulose nanofibers (KCNFs) from green and inexpensive kelp with short growth cycle are compounded with hydrophilic MXene nanosheets to prepare KCNF/MXene nanofluidic membranes. Notably, the KCNF/MXene‐40 (40 is the mass fraction of KCNF) membrane with an aligned nacre‐like nanostructure achieves 3.03‐fold increase in mechanical strength compared to the pure MXene membrane. The ingenious combination of the surface charge of the MXene nanosheets and the space charge formed by negatively charged KCNFs endows the KCNF/MXene‐40 membrane with enhanced cation selectivity (≈0.85), high energy conversion efficiency of 24.04% and high power density of 66.23 W m −2 in a 500‐fold concentration gradient, which has exceeded most reported state‐of‐the‐art 2D nanofluidic membranes. More importantly, the membrane exhibits the exceptional long‐term stability with relatively unchanged power density within 6000 s. Under the conditions of the natural seawater and simulated river water, the KCNF/MXene‐based generator deliveries an output power density of ≈10.56 W m −2 .