Endogenous Asymmetry in Heterogeneous Smectite Membrane Enhancing Salinity Gradient Energy Conversion

To mitigate the concentration polarization caused by non-ohmic resistance, asymmetric ion exchange membranes (IEMs) with nanochannels of comparable Debye length are frequently employed in salinity gradient energy conversion. Conventional asymmetric IEMs always rely on exogenous interventions to tailor their asymmetries as employing polyelectrolytes or modifying substrate porous membranes. Herein, the endogenous asymmetry of the natural smectite family is leveraged to develop composite membranes comprising structurally asymmetric montmorillonite and saponite sheets (MMT-SAP). The fabricated MMT-SAP membrane demonstrates superior ion selectivity and outstanding stability, making it an exceptional salinity gradient energy generation device. Mounted on artificial seawater and river water at 50-fold salinity gradient, the MMT-SAP demonstrates a cation selectivity of 0.96 and a high-power generation output 4.5 W m^-2. The maximum power output is found to reach 8.46 W m^- ² at the pH = 11.0. The theoretical study of MMT-SAP reveals that a potential ion migration mechanism within layered mineral channels is determined by two crucial factors: channel size and the heterogeneous structure distribution in the smectite-based membrane.