Cathodic deposition enables the formation of metal–organic framework (MOF) films, yet precise control of surface building-unit concentrations remain challenging. By finely tuning these concentrations, metastable ZIF-L membranes are obtained with high gas separation performance. This approach establishes a general strategy for regulating building units under negative bias, applicable to other surface-sensitive MOFs.

Abstract

Cathodic deposition is an emerging technique for the preparation of metal–organic framework (MOF) films. However, monitoring and controlling the concentrations of MOF building units (i.e., metal nodes and ligands) on the electrode surface remains challenging and has yet to be reported. This limitation greatly hinders the application of this technique to the synthesis of MOFs sensitive to building unit concentrations, such as the well-known metastable ZIF-L. Here, using the cathodic deposition of ZIF-L films as an example, we demonstrate how the surface metal node and ligand concentrations influence ZIF-L film generation by utilizing in situ pH measurements and theoretical calculations. Also, we propose a strategy to regulate the concentrations of these building units, enabling the straightforward and rapid cathodic deposition of thin ZIF-L films. With polydimethylsiloxane (PDMS) post-modification, the electrochemically deposited ZIF-L films on commercial anodized alumina oxide (AAO) substrate exhibit good gas separation performance, achieving a H₂/N₂ selectivity of 19.4 and a H₂ permeance exceeding 360 GPU (1 GPU = 3.35 × 10⁻¹⁰ mol m^{− 2} s⁻¹ Pa⁻¹).