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).

The introduction of a simple methyl substituent on the bipyridine ligand of [Ru(tBu3 tpy)(bpy)(NCCH3 )](2+) (tBu3 tpy=4,4',4''-tri-tert-butyl-2,2':6',2''-terpyridine; bpy=2,2'-bipyridine) gives rise to a highly active electrocatalyst for the reduction of CO2 to CO. The methyl group enables CO2 binding already at the one-electron reduced state of the complex to enter a previously not accessible catalytic cycle that operates at the potential of the first reduction. The complex turns over with a Faradaic efficiency close to unity and at an overpotential that is amongst the lowest ever reported for homogenous CO2 reduction catalysts.

A molecular water oxidation catalyst, [Ru(tpy)(dcbpy)(OH)](ClO) (tpy = 2,2':6',2''-terpyridine, dcbpy = 2,2'-bipyridine-5,5'-dicarboxylic acid) [1], has been incorporated into FTO-grown thin films of UiO-67 (UiO = University of Oslo), by post-synthetic ligand exchange. Cyclic voltammograms (0.1 M borate buffer at pH = 8.4) of the resulting UiO67-[RuOH]@FTO show a reversible wave associated with the Ru couple in the anodic scan, followed by a large current response that arises from electrocatalytic water oxidation beyond 1.1 V vs. Ag/AgCl. Water oxidation can be observed at an applied potential of 1.5 V over the timescale of hours with a current density of 11.5 μA cm. Oxygen evolution was quantified in situ over the course of the experiment, and the Faradaic efficiency was calculated as 82%. Importantly, the molecular integrity of [1] during electrocatalytic water oxidation is maintained even on the timescale of hours under turnover conditions and applied voltage, as evidenced by the persistence of the wave associated with the Ru couple in the CV. This experiment highlights the capability of metal organic frameworks like UiO-67 to stabilize the molecular structure of catalysts that are prone to form higher clusters in homogenous phase.

Metal–organic frameworks (MOFs) are appealing heterogeneous support matrices that can stabilize molecular catalysts for the electrochemical conversion of small molecules. However, moving from a homogeneous environment to a porous film necessitates the transport of both charge and substrate to the catalytic sites in an efficient manner. This presents a significant challenge in the application of such materials at scale, since these two transport phenomena (charge and mass transport) would need to operate faster than the intrinsic catalytic rate in order for the system to function efficiently. Thus, understanding the fundamental kinetics of MOF-based molecular catalysis of electrochemical reactions is of crucial importance. In this Perspective, we quantitatively dissect the interplay between the two transport phenomena and the catalytic reaction rate by applying models from closely related fields to MOF-based catalysis. The identification of the limiting process provides opportunities for optimization that are uniquely suited to MOFs due to their tunable molecular structure. This will help guide the rational design of efficient and high-performing catalytic MOF films with incorporated molecular catalyst for electrochemical energy conversion.

The electrochemical analysis of molecular catalysts for the conversion of bulk feedstocks into energy-rich clean fuels has seen dramatic advances in the last decade. More recently, increased attention has focused on the characterization of metal-organic frameworks (MOFs) containing well-defined redox and catalytically active sites, with the overall goal to develop structurally stable materials that are industrially relevant for large-scale solar fuel syntheses. Successful electrochemical analysis of such materials draws heavily on well-established homogeneous techniques, yet the nature of solid materials presents additional challenges. In this tutorial-style review, we cover the basics of electrochemical analysis of electroactive MOFs, including considerations of bulk stability, methods of attaching MOFs to electrodes, interpreting fundamental electrochemical data, and finally electrocatalytic kinetic characterization. We conclude with a perspective of some of the prospects and challenges in the field of electrocatalytic MOFs.

Mechanistic studies of electrocatalytic reactions play a crucial role in developing efficient electrocatalysts and solar-fuel devices. The foot of the wave analysis (FOWA) for cyclic voltammetry, recently developed by Saveánt and Costentin, provides a powerful means to evaluate the performance of molecular electrocatalysts. However, there is a considerable amount of confusion in the community on how to interpret FOWA in multielectron electrochemical reactions. Herein, we further expand their earlier models from the Nernstian region to all scenarios (i.e., including non-Nernstian behavior) and systematically examine individual parameters, such as formal potentials and reaction rate constants, to explore deeper insights and limitations. Detailed analysis from in silico voltammograms based on different mechanistic models reveals characteristic features of FOWA traces for different kinetic phenomena, which is useful to diagnose kinetic profiles and elucidate the limits of FOWA. The lessons learned from these analyses are further used to reconcile the discrepancy of rate constants determined by FOWA versus other methods, such as time-resolved spectroscopy, for molecular electrocatalysts that catalyze proton reduction or the reduction of CO 2 to CO. Such reconciliation demonstrates that electrochemical methods along with FOWA can serve as an alternative tool to determine kinetic information and probe mechanistic insights, which otherwise may be challenging and complicated to achieve by conventional methods. In addition, general guidelines and warnings are also presented to avoid potential errors or mishandling when using FOWA.

A novel class of materials that self-assemble in water into equilibrium network structures with a well-defined mesh size consist of polyethylene glycols (PEG's) end-capped with micelle-forming fluorocarbon tails. These micellar systems form flowable aqueous gel-like networks that permit electrophoretic DNA sequencing in capillary columns. The gels have unusual rheological properties, including network breakdown under shear, resulting in plug flow that allows columns refill with complete ejection of byproducts of the previous sequencing analysis. In this system, DNA fragment electrophoretic mobilities are unaffected by the hydrophobicity of the polymer tails. Low molecular weight (M) PEG chains (M 8000) show catastrophic resolution loss for DNA fragments larger than 100 bases due to band broadening. For a longer PEG segment (M 35000) separating the end groups, band broadening occurs for DNA fragments larger than 300 bases, implying that the PEG segment length controls the mesh size in the equilibrium network structure. Optimum sequencing results were obtained from a 6% solution of a 1:1 mixture of C6F13 end-capped- and C8F17 end-capped PEG 35,000. The resolution limit of fluorescent-dye-labeled sequencing products in this formulation was 450 bases in 75 microns capillaries at 200 V/cm.

Interpolation algorithms can be developed to size unknown single-stranded (ss) DNA fragments based on their electrophoretic mobilities, when they are compared with the mobilities of standard fragments of known sizes; however, sequence-specific anomalous electrophoretic migration can affect the accuracy and precision of the called sizes of the fragments. We used the anomalous migration of ssDNA fragments to optimize denaturation conditions for capillary electrophoresis. The capillary electrophoretic system uses a refillable polymer that both coats the capillary wall to suppress electro-osmotic flow and acts as the sieving matrix. The addition of 8 M urea to the polymer solution, as in slab gel electrophoresis, is insufficient to fully denature some anomalously migrating ssDNA fragments in this capillary electrophoresis system. The sizing accuracy of these fragments is significantly improved by the addition of 2-pyrrolidinone, or increased capillary temperature (60 degrees C). the effect of these two denaturing strategies is additive, and the best accuracy and precision in sizing results are obtained with a combination of chemical and thermal denaturation.