Iridium‐Functionalized Metal‐Organic Framework Nanocrystals Interconnected by Carbon Nanotubes Competent for Electrocatalytic Water Oxidation

Abstract Nanocrystals of a zirconium‐based metal‐organic framework (MOF), UiO‐66, are grown on the surface of the carboxylic acid‐functionalized multi‐walled carbon nanotubes (CNT) at room temperature to synthesize the UiO‐66‐CNT nanocomposites with tunable MOF‐to‐CNT ratios. The porosity, crystallinity, morphology, and electrical conductivity of the nanocomposite are characterized. Spatially dispersed iridium sites are thereafter installed on the defect sites presented within the entire UiO‐66 crystals in these nanocomposites by a self‐limiting solution‐phase approach. The resulting Ir‐functionalized UiO‐66, CNT, and UiO‐66‐CNT nanocomposites are served as the electrocatalysts for water oxidation in acidic aqueous solutions. By utilizing the redox‐hopping pathways to transport electrons within the Ir‐functionalized MOF crystals as well as the electronic conduction between MOF crystals provided by CNT, the nanocomposite with the optimal MOF‐to‐CNT ratio can outperform both the Ir‐functionalized UiO‐66 and CNT. Electrochemical impedance spectroscopy (EIS) is utilized to probe the reaction kinetics occurring on the decorated iridium sites and the transporting behaviors of electrons/ions within these catalytic thin films. For the first time, the electrocatalytic kinetics and the transporting limitations within the MOF‐based thin film can be decoupled with the help of EIS technique. Post‐electrocatalysis characterizations of the nanocomposite after water oxidation are also performed.