The review of MXenes for osmotic energy harvesting: Synthesis and properties

Osmotic energy harvesting has been achieving extensive interests due to the promising potential as an emerging renewable energy in the past decades. Both reverse electrodialysis (RED) and pressure-retarded osmosis (PRO) are two general strategies to transforming the osmotic energy to the electric energy, in which the output power and energy conversion efficiency are two significant parameters. Utilization of semipermeable membranes assembled from laminar two-dimensional (2D) nanosheets is regarded as a promising way to realizing high output power and energy conversion efficiency in osmotic energy harvesting due to the interconnected channels, where the interlayer spacing close to Debye length can effectively facilitate the counterion transport. MXenes, as an emerging family of 2D materials, have shown huge potential in the field of energy storage, conversion and harvesting due to their physicochemical properties including hydrophilicity, controllable negative surface terminations and processibility. In this review, we first discuss the basic principles and parameters of osmotic energy harvesting, and look back its development history. Then the introduction of the basic structures, synthesis and membrane processing of MXenes is systematically summarized. Notably, the dispersibility of MXenes shows a significant effect on obtaining semipermeable membranes with uniform structures and good permeability. Subsequently, the application of the pristine, modified and composited MXene membranes used for osmotic energy harvesting is reviewed in detail. In the end, the outlooks for future investigations and the application of MXenes in osmotic harvesting are presented.