Sub-nanoconfined MXene membrane electrodes for monovalent cation-selective separation with an electrochemical ion pumping

Conventional electrosorption (ES) separates ion adsorption and release by physically switching solutions between charging and discharging half-cycles, which causes back-mixing, operational discontinuity, and energy loss. Electrochemical ion pumping (EIP) addresses this limitation by replacing solution switching with circuit switching. However, implementing EIP requires a membrane electrode that simultaneously provides electronic conduction, solution isolation, rapid ion transport, and intrinsic ion selectivity, a combination of functions that remains challenging for existing materials. Here, we demonstrate that anti-swelling free-standing Ti 3 C 2 T x MXene membrane electrodes, fabricated through heat-induced interlayer self-crosslinking and poly (vinyl alcohol)/tannic acid (PVA/TA) modification, can fulfill these combined requirements for circuit-switching EIP. The stabilized lamellar nanochannels provide static mono-/divalent cation sieving in the order NH 4 +  > K +  > Na +  > Li +  ≫ Ca 2+  > Mg 2+ , with selectivity coefficients of approximately 22 for NH 4 + /Mg 2+ and 10 for Li + /Mg 2+ . Under electrically driven EIP, the membrane electrodes maintain preferential monovalent cation transport. Short half-cycle operation (0.6 s) keeps each pumping step within a high-rate, low-capacity-per-cycle adsorption/desorption regime, increasing ion flux by approximately 9.4% and charge efficiency by approximately 28.5% relative to a 60 s half-cycle. This work demonstrates a membrane-electrode strategy for dynamic cation sieving and clarifies the structure-transport-stability trade-offs of MXene lamellar electrodes in EIP.