The electrokinetic effect to convert the mechanical energy from ambient has gained sustained research attention because it is free of moving parts and easy to be miniaturized for microscale applications. The practical application is constrained by the limited electrokinetic energy conversion performance. Herein, we report vertically oriented MXene membranes (VMMs) with ultrafast permeation as well as high ion selectivity, in which the permeation is several thousand higher than the largely researched horizontally stacked MXene membranes (HMMs). The VMMs can achieve a high streaming current of 8.17 A m −2 driven by the hydraulic pressure, largely outperforming all existing materials. The theoretical analysis and numerical calculation reveal the underlying mechanism of the ultrafast transport in VMMs originates from the evident short migration paths, the low energy loss during the ionic migration, and the large effective inlet area on the membrane surface. The orientation of the 2D lamella in membranes, the long-overlooked element in the existing literatures, is identified to be an essential determinant in the performance of 2D porous membranes. These understandings can largely promote the development of electrokinetic energy conversion devices and bring advanced design strategy for high-performance 2D materials.

Selective polymer membranes are one of the most promising materials to achieve large-scale and energy-efficient mass separation and energy conversion. However, their performance is subject to the selectivity–permeability trade-off. To overcome this trade-off and achieve high-performance ion separation, it is essential to fabricate thin membranes with a high density of uniformly sized nanopores with the pore size at around 1 nm. However, it has been challenging to precisely control the pore size at about 1 nm. In this Tutorial, we outline the underlying mechanism for selective ion transport through nanopores, especially the emerging mechanism as the nanopore size is tuned around 1 nm. Then, we review the recent development of the main fabrication methods of selective polymer membranes from either the top–down approach or the bottom–up approach, especially the novel techniques generating uniformly sized nanopores with nanometric or even subnanometric size. Finally, we discuss the future development of the design and fabrication of the selective polymer membranes to fulfill their potential in real applications.

Selective ion transport deteriorates in mixed salt solutions. This mixture effect originates from the increased or relatively less decreased permeability of lower permeable ions due to their reduced preferential adsorption on the charged nanotubes.