Inducing in operando chloride ion filtration through an electrocatalytic film architecture integrated with ZIF-8 nanosheets En route to seawater electrolysis

• ZIF-8 nanosheets cut CER selectivity by 95% at 500 mM and low current density. • Nanosheet integration increases OER current and reduces CER without kinetic changes. • Diffusional control mechanism confirmed for chloride ion sieving in MOF films. Ameliorating direct seawater electrolysis to produce green hydrogen requires an advancement in the conception of material designs and electrocatalytic film architectures. Indeed, to complement the hydrogen evolution reaction (HER) at the cathode, oxygen must be produced thanks to the oxygen evolution reaction (OER) at the anode. Nevertheless, this kinetically slow reaction is in competition with the kinetically facile chlorine evolution reaction (CER). In this work, the entropic selectivity of the ZIF-8 MOF molecular sieving material and the high aspect ratio of a nanosheet morphology was probed to induce the in situ selective filtration of the undesired chloride ion towards the electrocatalyst surface, namely iridium nanoparticles interdispersed on Vulcan carbon. Cross-sectional analysis revealed a homogeneous dispersion of the nanosheets within the electrocatalytic film. This composite catalyst enabled a ∼65% drop in CER selectivity upon increasing the Zn/Ir molar ratio when evaluated at a disk current density of 3 mA/cm 2 at a chloride ion concentration of 10 mM. Further evaluating the CER selectivity at lower current densities along the disk indicated a near 95% drop in CER selectivity at 500 mM chloride ion concentration, shown through rotating ring disk electrode experiments. A competitive absorbing mechanism was indicated and the unchanged kinetics of the CER suggests a decrease of the CER current due to an induced diffusional control. The development of a composite architecture enabling the in operando filtration of chloride ions seems promising towards producing hydrogen by direct seawater electrolysis. The integration of ZIF-8 nanosheets within an electrocatalytic film architecture demonstrates effective chloride ion filtration, leading to a substantial decrease in chlorine evolution reaction selectivity and promoting the oxygen evolution reaction for efficient seawater electrolysis.