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 acidfunctionalized 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 solutionphase 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. Postelectrocatalysis characterizations of the nanocomposite after water oxidation are also performed.

Defective Metal–Organic Framework Nanocrystals as Signal Amplifiers for Electrochemical Dopamine Sensing

“…The uniform morphologies of all UiO-66 materials can be found in low-magnification SEM images as well (see Figure S2). TEM images in Figure S3 reveal that the UiO-66-RT is composed of nanocrystals with a smooth surface, which is consistent with the finding reported previously for defective UiO-66 …”

“…TEM images in Figure S3 reveal that the UiO-66-RT is composed of nanocrystals with a smooth surface, which is consistent with the finding reported previously for defective UiO-66. 56 Nitrogen gas adsorption and desorption were performed to probe the porosity of each material (see Figure 2b). Brunauer− Emmett−Teller (BET) theory was further utilized to estimate the specific surface areas of all Zr-MOFs, and the resulting values are also listed in Figure 2b.…”

Electrocatalytic Water Oxidation by Hydrolytically Stable Metal‐Organic Frameworks at Both Neutral and Alkaline Medium: Inverse Relation of Dimensionality with Catalytic Activity

“…[19][20][21] Recently, Kung and co-workers have shown that the presence of red-ox active sites can enhance the electrical conductivity through red-ox hopping process by doping Ir in a Zr-based MOF and the overall composite shows high electrochemical water oxidation in acidic medium. [22] Coordination environment of the catalytically active metal centres is also an important parameter for the electrocatalysis. Different mechanistic studies have explored that the presence of a coordinated water/hydroxy or two water molecules at the geminal position facilitate water splitting behaviour enormously.…”

“…[32] Finally, the MOFs should possess its structural integrity during electrochemical oxidation and reduction. [22,24,33] It is well-established that the 2D MOFs are better electrocatalyst than their 3D analogues based on the number of active sites, facile mass transfer and electron transport, [34,35] but such comparison with 1D analogues have not been performed with the consideration of relatively weak stability and durability. Zhang et al, have demonstrated that 1D materials like metal oxides, metal sulfides, metal phosphides etc.…”

“…are developed [19–21] . Recently, Kung and co‐workers have shown that the presence of red‐ox active sites can enhance the electrical conductivity through red‐ox hopping process by doping Ir in a Zr‐based MOF and the overall composite shows high electrochemical water oxidation in acidic medium [22] . Coordination environment of the catalytically active metal centres is also an important parameter for the electrocatalysis.…”