Cation-selective Mo2TiC2Tx MXene membrane for osmotic energy harvesting

Abstract Though promising in renewable energy, osmotic energy is limited by the unsatisfactory conversion performance caused by defective ion transport and selectivity of semipermeable membranes. As an emerging family of two-dimensional (2D) materials, MXenes have been attracting extensive interests for constructing osmotic membrane due to its natural 2D nanoconfined space, hydrophilicity and abundant surface terminations. The regulation of the surface charge density of MXenes plays an important role for the improvement of osmotic energy conversion. Herein, we systematically investigate Mo2TiC2Tx MXene membranes for osmotic energy harvesting. Benefitting from the improved surface negative-charged density treated by alkali solution, and the 2D nanoconfined space, the Mo2TiC2Tx MXene membrane shows improved cation selectivity and permeability performance. The osmotic voltage (Vos) increases to 83 mV with an improved cation transference number (t+) of 0.95 at 0.5M/0.01M alkali KCl solution (pH=9), while osmotic voltage (Vos) is 74 mV with a cation transference number (t+) of 0.9 at 0.5M/0.01M alkali KCl solution (pH=7). The output power density (Pmax) reaches up to 13.1 W m-2 with an energy conversion efficiency (ηmax) of 40.5% at 0.5M/0.01M alkali KCl solution (pH=9), which is superior to many of other 2D osmotic membranes. The modification method of surface charge density for Mo2TiC2Tx MXene osmotic membrane has shown great perspective in renewable osmotic energy harvesting.