Fast and highly selective proton transport through Ca²⁺-regularized laminar membrane for spontaneous acid recovery...

Realizing highly selective and fast proton transport is central to many applications. This work demonstrates that by simply intercalating MXene layered membranes with Ca²⁺, the proton selectivity against heavy metal ions, crucial for acidic wastewater treatment and acid recovery, can be enhanced by nearly 2 orders of magnitude, and the proton permeation rate can be enhanced by nearly 1 order of magnitude. We reveal that after intercalation, the interlayer spacings of the membrane become smaller and highly uniform, mitigating ion leakage through large spacings. Moreover, this also renders the interlayer water structure highly ordered, inducing fast proton hopping. The intercalated membrane can sieve protons with high flux (∼8 mol m^-2 h^-1) out of simulated industrial wastewater containing various heavy metal ions, showing a selectivity over 10³. Such membranes enable the harvesting of osmotic energy released during proton diffusion and acid recovery while effectively prevent the leakage of heavy metal ions, reaching a high power density of 10¹-10² W m^-2 depending on the membrane size. This work demonstrates a strategy to synergistically improve the ion flux and selectivity and will enlighten the applications of ion-selective membranes on power-generating wastewater treatment and resource utilization.