Semiflexible spiro-branched membrane with adjustable sub-nanoscale channels and high positive charge for acid recovery

•Rigid contorted branching nodes induce a semiflexible 3D network structure with abundant sub-nanoscale microporous channels.•The size of these channels can be precisely adjusted between the hydration diameters of acid anions and metal cations.•Microporous channels with well-tuned size and positive charges selectively transport acid anions and protons.•Acid recovery devices based on reported membranes achieve excellent H+ dialysis coefficients and good selectivity. Acid recovery from industrial wastewater based on membrane separation is an essential technology to improve the economic efficiency and environmental friendliness. However, the low recovery efficiency and trade-off between acid permeability and selectivity of membrane materials have hindered the widespread application of this technology. In this study, we developed a spiro-branched membrane with semiflexible three-dimensional (3D) network structure by utilizing rigid contorted units as branching nodes. The semiflexible 3D network structure led to inefficient chain packing and the creation of numerous sub-nanoscale microporous ion channels, with sizes that can be finely adjusted. Furthermore, this 3D network structure allows the membrane to maintain low swelling and excellent stability while being enriched with positively charged groups. Ultimately, the optimally tuned membrane separator demonstrated an outstanding H+ dialysis coefficient (up to 108.6 × 10−3 m h−1), good selectivity (42.2) and high stability, surpassing both commercial membranes and those reported recently.