Halogen-driven modulation of iridium-functionalized zirconium-based metal–organic frameworks for electrocatalytic oxygen evolution in acidic media†

Abstract

Nanocrystals of a zirconium-based metal–organic framework (MOF), UiO-66, and its derivatives constructed from three distinct halogen-functionalized linkers, are synthesized. They are further subjected to post-synthetic modification to immobilize redox-active iridium ions on their nodes, where missing-linker defects are located. The crystallinity, morphology and porosity of all MOFs before and after functionalizing iridium are characterized. The number of defects in each MOF and the loading of iridium in each iridium-functionalized MOF are also quantified. Electrochemical behaviours of all iridium-functionalized MOFs and their corresponding redox-based charge-hopping processes occurring between immobilized iridium sites in the frameworks are investigated. All iridium-functionalized MOFs are employed as electrocatalysts for the oxygen evolution reaction (OER) in an acidic aqueous electrolyte containing 0.1 M of HClO₄, where Zr-based MOFs are chemically robust. Impedance experiments and further distribution of relaxation times (DRT) analysis suggest that all halogen-functionalized linkers can facilitate the OER kinetics occurring on the neighbouring iridium sites immobilized in MOFs, while the MOF with heavy iodo groups on its linkers strongly retards the mass transfer during the OER. With both facile mass transfer and fast catalytic kinetics, both iridium-functionalized MOFs with chloro and bromo groups on their linkers thus exhibit better electrocatalytic activity for the OER compared to their counterparts without halogen atoms and with iodo groups. The iridium-functionalized MOF with chloro groups can achieve 1 mA cm⁻² for the OER at an overpotential of 327 mV, outperforming that required by the iridium-functionalized UiO-66 (383 mV). Findings here highlight the importance of modulating the chemical functionality of linkers in stable MOFs in order to boost the electrocatalytic activity of the frameworks.