Development of a UiO-Type Thin Film Electrocatalysis Platform with Redox-Active Linkers

Abstract

Metal-organic frameworks (MOFs) as electrocatalysis scaffolds are appealing due to the large concentration of catalytic units that can be assembled in three dimensions. To harness the full potential of these materials, charge transport to the redox catalysts within the MOF has to be ensured. Herein, we report the first electroactive MOF with the UiO/PIZOF topology (Zr(dcphOH-NDI)), i.e., one of the most widely used MOFs for catalyst incorporation, by using redox-active naphthalene diimide-based linkers (dcphOH-NDI). Hydroxyl groups were included on the dcphOH-NDI linker to facilitate proton transport through the material. Potentiometric titrations of Zr(dcphOH-NDI) show the proton-responsive behavior via the -OH groups on the linkers and the bridging Zr-μ-OH of the secondary building units with pK values of 6.10 and 3.45, respectively. When grown directly onto transparent conductive fluorine-doped tin oxide (FTO), 1 μm thin films of Zr(dcphOH-NDI)@FTO could be achieved. Zr(dcphOH-NDI)@FTO displays reversible electrochromic behavior as a result of the sequential one-electron reductions of the redox-active NDI linkers. Importantly, 97% of the NDI sites are electrochemically active at applied potentials. Charge propagation through the thin film proceeds through a linker-to-linker hopping mechanism that is charge-balanced by electrolyte transport, giving rise to cyclic voltammograms of the thin films that show characteristics of a diffusion-controlled process. The equivalent diffusion coefficient, D, that contains contributions from both phenomena was measured directly by UV/vis spectroelectrochemistry. Using KPF as electrolyte, D was determined to be D(KPF) = (5.4 ± 1.1) × 10 cm s, while an increase in countercation size to n-BuN led to a significant decrease of D by about 1 order of magnitude (D(n-BuNPF) = (4.0 ± 2.5) × 10 cm s).

Toward Metal–Organic‐Framework‐Based Supercapacitors: Room‐Temperature Synthesis of Electrically Conducting MOF‐Based Nanocomposites Decorated with Redox‐Active Manganese

“…As revealed in Figure c and Figure d, both plots show great linearity, which suggests that the electrochemical reaction of Mn(III)/Mn(IV) occurring in the Mn‐UiO‐66 thin film is diffusion‐controlled; the diffusion behavior here represents the charge hopping between adjacent manganese sites and the diffusion of counterions within the framework. [14a], [15c], [20b] Amperometric experiments were then performed with the Mn‐UiO‐66 thin film (Figure e), and the apparent diffusivity ( D app ) can be calculated from Figure f by utilizing the Cottrell equation [Equation (1)]. [20b] where J is the current density, n is the number of electron involved in the reaction, which is 1 for Mn(III)/Mn(IV), F is Faraday constant, t is time, and C is the concentration of redox‐active Mn sites presented within the MOF thin film.…”

“…To facilitate charge transport within the Zr‐MOFs for electrochemical applications, a more common strategy is to utilize the redox hopping pathway . As the zirconium‐based nodes in Zr‐MOFs are redox‐innocent, the charge transport via redox hopping in Zr‐MOFs must be supported by the redox‐active linkers or species installed on the nodes . With the help of the redox hopping pathway, various Zr‐MOFs have been found to be electrochemically addressable and utilized in a range of electrochemical applications, including electrochromic glass, oxygen evolution,[15c], oxygen reduction, carbon dioxide reduction and electrochemical sensors .…”

“…However, it was also found that in several cases, the performances of the aforementioned applications were strongly limited by the sluggish redox hopping presented in Zr‐MOFs. [15c], , [19a], [20b] For example, the installation of redox‐active cobalt sites in a micrometer‐thick thin film of a Zr‐MOF resulted in only 1 % of electrochemically addressable cobalt sites within the entire thin film in an aqueous electrolyte. [17a] Thus, achieving a faster charge‐transport rate within Zr‐MOFs is still crucial to further enhance the resulting electrochemical performance.…”

Bulk-to-Surface Proton-Coupled Electron Transfer Reactivity of the Metal–Organic Framework MIL-125

“…20–22 In this context, several groups have developed MOFs that show PCET behavior resulting in good photo- and electro-chemical activities. 22–24 PCET has been suggested to occur on the linker, 24 the node, 25 and the linker and node combined 22,23 but has not been studied in well-known photoactive MOFs. In this work, we provide an in-depth analysis of the PCET behavior in the widely used MIL-125.…”

Artificial photosynthesis with metal and covalent organic frameworks (MOFs and COFs): challenges and prospects in fuel‐forming electrocatalysis

“…Spectroelectrochemical analysis revealed that charge transport occurred via linker‐to‐linker hopping, balanced by ion transport from the electrolyte. Changing the electrolyte cation from n ‐Bu 4 NPF 6 to KPF 6 increased the effective diffusion coefficient from 4.0 × 10 −12 cm 2 s −1 to 5.4 × 10 −11 cm 2 s −1 , highlighting the effects of ion diffusion through the film on the resultant conductivity (Johnson et al ). While these diffusion coefficients are quite low, the use of MOF and COF thin films on the order of ∼1 μm alleviates these conductivity limitations.…”