Abstract Although promising in renewable energy, osmotic energy is hindered by the unsatisfactory conversion performance caused by the limited 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 Mo₂TiC₂T _x MXene membranes for osmotic energy harvesting. Benefitting from the improved surface negative-charged density treated by alkali solution, and the 2D nanoconfined space, the Mo₂TiC₂T _x MXene membrane shows improved cation selectivity and permeability performance. The osmotic voltage (V _os) increases to 83 mV with an improved cation transference number (t ₊) of 0.95 at 0.5 M/0.01 M alkali KCl solution (pH = 9), while osmotic voltage (V _os) is 74 mV with a cation transference number (t ₊) of 0.9 at 0.5 M/0.01 M alkali KCl solution (pH = 7). The output power density (P _max) reaches up to 13.1 W m⁻² with an energy conversion efficiency (η _max) of 40.5% at 0.5 M/0.01 M alkali KCl solution (pH = 9), which is superior to many of other 2D osmotic membranes. The modification of surface charge density for Mo₂TiC₂T _x MXene membrane may pave a way for improving the performance of MXene based osmotic energy harvesting.