Geometrical engineering of nearly fully cation-selective 2D angstrom-scale ionic diode membranes for highly efficient osmotic energy conversion

Achieving a membrane with perfect ion selectivity, high energy conversion efficiency, and high ionic flux is crucial towards ultrahigh osmotic energy generation, but still challenging due to the inherent tradeoff between membrane's selectivity and permeability. Herein, we propose the strategy of asymmetric sub-nanoconfinement by designing a two-dimensional (2D) lamellar sub-nanofluidic MXA membrane using Ti<inf>3</inf>C<inf>2</inf>T<inf>x</inf> MXene and highly space charged aramid nanofibers. By employing geometric engineering and integrating the membrane into an epoxy-acrylic device with in-plane orientation, the asymmetric MXA membrane exhibits a strong ionic diode effect with a rectification ratio up to 37-fold. Remarkably, the synergy of surface and space charges in 2D sub-nanofluidic channels renders the MXA membrane nearly fully cation-selective, independent of the applied concentration gradient. Benefiting from these fantastic features, an ultrahigh power of 9.7 W m⁻² along with an ultrahigh energy conversion efficiency of ∼49.8% (approaching the theoretical upper limit of 50%) can be achieved under a 500 mM/10 mM NaCl gradient, surpassing that of the existing 2D sub-nanoscale osmotic energy generators. Moreover, the proposed device can exhibit exceptional long-term structural and performance stability for over 140 h. This study presents an approach for creating a 2D angstrom-scale ionic diode membrane with enhanced ionic rectification, selectivity, efficiency, and stability for highly efficient osmotic energy harvesting.