Electrocatalytic water oxidation by a molecular catalyst incorporated into a metal-organic framework thin film

Sulfonate-Functionalized Metal-Organic Framework as a Porous "Proton Reservoir" for Boosting Electrochemical Reduction of Nitrate to Ammonia

The electrochemical reduction reaction of nitrate (NO3RR) is an attractive route to produce ammonia at ambient conditions, but the conversion from nitrate to ammonia, which requires nine protons, has to compete with both the two-proton process of nitrite formation and the hydrogen evolution reaction. Extensive research efforts have thus been made in recent studies to develop electrocatalysts for the NO3RR facilitating the production of ammonia. Rather than designing another better electrocatalyst, herein, we synthesize an electrochemically inactive, porous, and chemically robust zirconium-based metal-organic framework (MOF) with enriched intraframework sulfonate groups, SO3-MOF-808, as a coating deposited on top of the catalytically active copper-based electrode. Although both the overall reaction rate and electrochemically active surface area of the electrode are barely affected by the MOF coating, with negatively charged sulfonate groups capable of enriching more protons near the electrode surface, the MOF coating significantly promotes the selectivity of the NO3RR toward the production of ammonia. In contrast, the use of MOF coating with positively charged trimethylammonium groups to repulse protons strongly facilitates the conversion of nitrate to nitrite, with selectivity of more than 90% at all potentials. Under the optimal operating conditions, the copper electrocatalyst with SO3-MOF-808 coating can achieve a Faradaic efficiency of 87.5% for ammonia production, a nitrate-to-ammonia selectivity of 95.6%, and an ammonia production rate of 97 μmol/cm2 h, outperforming all of those achieved by both the pristine copper (75.0%; 93.9%; 87 μmol/cm2 h) and copper with optimized Nafion coating (83.3%; 86.9%; 64 μmol/cm2 h). Findings here suggest the function of MOF as an advanced alternative to the commercially available Nafion to enrich protons near the surface of electrocatalyst for NO3RR, and shed light on the potential of utilizing such electrochemically inactive MOF coatings in a range of proton-coupled electrocatalytic reactions.

Optimizing post-synthetic metal incorporation in mixed-linker MOFs: insights from metalation studies on bipyridine-containing UiO-67 single crystals

The postsynthetic metalation (PSM) of metal-organic frameworks (MOFs) with intrinsic metal binding sites is an intriguing strategy to introduce catalytic function into MOFs. The spatial distribution of the catalytic sites within the MOF crystal will affect the efficiency of the material, but the factors that govern depth distribution of the introduced metal sites are often not well understood. Herein, we employ Rutherford backscattering spectrometry (RBS) to investigate the metal distribution in a series of post-synthetically metalated mixed linker bpdc/BPY UiO-67 (UiO = Universitet i Oslo, bpdc = biphenyl-dicarboxylate, BPY = 2,2'-bipyridine-5,5'-dicarboxylate) single crystals as a function of linker ratio and metalation time. The RBS spectra reveal large differences in the depth distribution of inserted Ni2+ ions, and core/shell architectures are observed in high BPY materials at shorter incubation times. The incubation times to achieve uniform metal incorporation increases with increasing BPY ratios in the materials, suggesting that the presence of the BPY linkers slow down metal uptake. We propose a combination of ionic interactions and pore clogging, where coordinated ions reduce the available pore space for further ions to diffuse deeper into the framework as reasons for the observed trends. The observations are likely relevant for other mixed-linker MOF systems, and understanding the effect that linker ratios have on PSM and cation distribution will aid in future optimizations of catalytic MOFs.

Rational Design of Improved Ru Containing Fe-Based Metal-Organic Framework (MOF) Photoanode for Artificial Photosynthesis

Metal-Organic Frameworks (MOFs) recently emerged as a new platform for the realization of integrated devices for artificial photosynthesis. However, there remain few demonstrations of rational tuning of such devices for improved performance. Here, a fast molecular water oxidation catalyst working via water nucleophilic attack is integrated into the MOF MIL-142, wherein Fe3O nodes absorb visible light, leading to charge separation. Materials are characterized by a range of structural and spectroscopic techniques. New, [Ru(tpy)(Qc)(H2O)]+ (tpy = 2,2':6',2″-terpyridine and Qc = 8-quinolinecarboxylate)-doped Fe MIL-142 achieved a high photocurrent (1.6 × 10-3 A·cm-2) in photo-electrocatalytic water splitting at pH = 1. Unassisted photocatalytic H2 evolution is also reported with Pt as the co-catalyst (4.8 µmol g-1 min-1). The high activity of this new system enables hydrogen gas capture from an easy-to-manufacture, scaled-up prototype utilizing MOF deposited on FTO glass as a photoanode. These findings provide insights for the development of MOF-based light-driven water-splitting assemblies utilizing a minimal amount of precious metals and Fe-based photosensitizers.

Zirconium-based metal-organic frameworks and their roles in electrocatalysis

Due to their exceptional chemical stability in water and high structural tunability, zirconium(IV)-based MOFs (Zr-MOFs) have been considered attractive materials in the broad fields of electrocatalysis. Numerous studies published since 2015 have attempted to utilise Zr-MOFs in electrocatalysis, with the porous framework serving as either the active electrocatalyst or the scaffold or surface coating to further enhance the performance of the actual electrocatalyst. Herein, the roles of Zr-MOFs in electrocatalytic processes are discussed, and some selected examples reporting the applications of Zr-MOFs in various electrocatalytic reactions, including several studies from our group, are overviewed. Challenges, limitations and opportunities in using Zr-MOFs in electrocatalysis in future studies are discussed.

Evaluation of Ce-MOFs as Photoanode Materials for the Water Oxidation Reaction: The Effect of Doping with [Ru(bpy)(dcbpy)(H2O)2]2+ Catalyst

Artificial photosynthesis stands out as a highly effective method for harnessing sunlight to produce clean and renewable energy. The light-absorbing properties, chemical stability, and high redox activity of Ce-based metal-organic frameworks (MOFs) make them attractive materials for visible-light-driven water splitting. Currently, Ce-based MOFs remain a relatively underexplored system for photocatalytic water oxidation in acidic media. In this study, we synthesized a Ce-MOF with different linkers (1,4-benzenedicarboxylic acid, tetrafluoroterephthalic acid, 2-nitroterephthalic acid, 2,2'-bipyridine-5,5'-dicarboxylic acid, and 4,4'-biphenyldicarboxylic acid), which exhibit light-absorbing capability. Ce-based MOFs doped with [Ru(bpy)(dcbpy)(H2O)2]2+ (MOF-1 and MOF-2) water oxidation catalyst showed an enhanced photoelectrocatalytic current of ∼10-4 A·cm-2 at pH = 1, which is comparable with the [Ru(bpy)(dcbpy)(H2O)2]2+-doped MIL-126 Fe-based MOF. We also demonstrated the long-term durability of Ru-doped Ce-MOFs for photoelectrocatalytic water oxidation under acidic conditions. The as-synthesized MOFs were analyzed with powder X-ray diffraction (PXRD), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), UV-visible diffuse reflectance spectroscopy, scanning electron microscopy (SEM), and electric conductivity measurements. This study contributes to the development of cost-effective materials for sustainable photocatalytic water splitting processes.

Directing the Selectivity of Oxygen Reduction to Water by Confining a Cu Catalyst in a Metal Organic Framework

Electrocatalysis is to play a key role in the transition towards a sustainable chemical and energy industry and active, stable and selective redox catalysts are much needed. Porous structures such as metal organic frameworks (MOFs) are interesting materials as these may influence selectivity of chemical reactions through confinement effects. In this work, the oxygen reduction catalyst Cu-tmpa was incorporated into the NU1000 MOF. Confinement of the catalyst within NU1000 steers the selectivity of the oxygen reduction reaction (ORR) towards water rather than peroxide. This is attributed to retention of the obligatory H2 O2 intermediate in close proximity to the catalytic center. Moreover, the resulting NU1000|Cu-tmpa MOF shows an excellent activity and stability in prolonged electrochemical studies, illustrating the potential of this approach.

Integration of Metal-Organic Frameworks and Metals: Synergy for Electrocatalysis

Electrocatalysis is a highly promising technology widely used in clean energy conversion. There is a continuing need to develop advanced electrocatalysts to catalyze the critical electrochemical reactions. Integrating metal active species, including various metal nanostructures (NSs) and atomically dispersed metal sites (ADMSs), into metal-organic frameworks (MOFs) leads to the formation of promising heterogeneous electrocatalysts that take advantage of both components. Among them, MOFs can provide support and protection for the active sites on guest metals, and the resulting host-guest interactions can synergistically enhance the electrocatalytic performance. In this review, three key concerns on MOF-metal heterogeneous electrocatalysts regarding the catalytic sites, conductivity, and catalytic stability are first presented. Then, rational integration strategies of MOFs and metals, including the integration of metal NSs via surface anchoring, space confining, and MOF coating, as well as the integration of ADMSs either with the metal nodes/linkers or within the pores of MOFs, along with their recent progress on synergistic cooperation for specific electrochemical reactions are summarized. Finally, current challenges and possible solutions in applying these increasingly concerned electrocatalysts are also provided.

Molecular Catalysis of Energy Relevance in Metal-Organic Frameworks: From Higher Coordination Sphere to System Effects

The modularity and synthetic flexibility of metal-organic frameworks (MOFs) have provoked analogies with enzymes, and even the term MOFzymes has been coined. In this review, we focus on molecular catalysis of energy relevance in MOFs, more specifically water oxidation, oxygen and carbon dioxide reduction, as well as hydrogen evolution in context of the MOF-enzyme analogy. Similar to enzymes, catalyst encapsulation in MOFs leads to structural stabilization under turnover conditions, while catalyst motifs that are synthetically out of reach in a homogeneous solution phase may be attainable as secondary building units in MOFs. Exploring the unique synthetic possibilities in MOFs, specific groups in the second and third coordination sphere around the catalytic active site have been incorporated to facilitate catalysis. A key difference between enzymes and MOFs is the fact that active site concentrations in the latter are often considerably higher, leading to charge and mass transport limitations in MOFs that are more severe than those in enzymes. High catalyst concentrations also put a limit on the distance between catalysts, and thus the available space for higher coordination sphere engineering. As transport is important for MOF-borne catalysis, a system perspective is chosen to highlight concepts that address the issue. A detailed section on transport and light-driven reactivity sets the stage for a concise review of the currently available literature on utilizing principles from Nature and system design for the preparation of catalytic MOF-based materials.

Photoexcitation of Fe3 O Nodes in MOF Drives Water Oxidation at pH=1 When Ru Catalyst Is Present

Artificial photosynthesis strives to convert the energy of sunlight into sustainable, eco-friendly solar fuels. However, systems with light-driven water oxidation reaction (WOR) at pH=1 are rare. Broadly used [Ru(bpy)₃ ]²⁺ (bpy=2,2'-bipyridine) photosensitizer has a fixed +1.23 V potential which is insufficient to drive most water oxidation catalysts (WOCs) in acid, while Fe₂ O₃ , featuring the highly oxidizing holes, is not stable at low pH. Here, the key examples of Fe-based metal-organic framework (MOF) water oxidation photoelectrocatalysts active at pH=1 are presented. Fe-MIL-126 and Fe MOF-dcbpy structures were formed with 4,4'-biphenyl dicarboxylate (bpdc), 2,2'-bipyridine-5,5'-dicarboxylate (dcbpy) linkers and their mixtures. Presence of dcbpy linkers allows integration of metal-based catalysts via coordination to 2,2'-bipyridine fragments. Fe-based MOFs were doped with Ru-based precursors to achieve highly active MOFs bearing [Ru(bpy)(dcbpy)(H₂ O)₂ ]²⁺ WOC. Materials were analyzed with X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infra-red (FTIR) spectroscopy, resonance Raman, X-ray absorption spectroscopy, fs optical pump-probe, electron paramagnetic resonance (EPR), diffuse reflectance and electric conductivity measurements and were modeled by band structure calculations. It is shown that under reaction conditions, Fe^III and Ru^III oxidation states are present, indicating rate-limiting electron transfer in MOF. Fe₃ O nodes emerge as photosensitizers able to drive prolonged O₂ evolution in acid. Further developments are possible via MOF's linker modification for enhanced light absorption, electrical conductivity, reduced MOF solubility in acid, Ru-WOC modification for faster WOC catalysis, or Ru-WOC substitution to 3d metal-based systems. The findings give further insight for development of light-driven water splitting systems based on Earth-abundant metals.

Photoelectrochemical water oxidation by a MOF/semiconductor composite

Herein, we report the development of a MOF-semiconductor composite film active for water oxidation at a thermodynamic underpotential.

A Potential Roadmap to Integrated Metal Organic Framework Artificial Photosynthetic Arrays

Metal organic frameworks (MOFs), a class of coordination polymers, gained popularity in the late 1990s with the efforts of Omar Yaghi, Richard Robson, Susumu Kitagawa, and others. The intrinsic porosity of MOFs made them a clear platform for gas storage and separation. Indeed, these applications have dominated the vast literature in MOF synthesis, characterization, and applications. However, even in those early years, there were hints to more advanced applications in light-MOF interactions and catalysis. This perspective focuses on the combination of both light-MOF interactions and catalysis: MOF artificial photosynthetic assemblies. Light absorption, charge transport, H2O oxidation, and CO2 reduction have all been previously observed in MOFs; however, work toward a fully MOF-based approach to artificial photosynthesis remains out of reach. Discussed here are the current limitations with MOF-based approaches: diffusion through the framework, selectivity toward high value products, lack of integrated studies, and stability. These topics provide a roadmap for the future development of fully integrated MOF-based assemblies for artificial photosynthesis.

Electrocatalytic water oxidation from a mixed linker MOF based on NU-1000 with an integrated ruthenium-based metallo-linker

A novel tetratopic metallo-linker, [Ru(tda)(py(PhCOOH)₂)₂], 1, (tda = 2,2':6',2''-terpyridine-6,6''-dicarboxylate; py(PhCOOH)₂ = (4,4'-(pyridine-3,5-diyl)dibenzoic acid), that is structurally based on one of the most active molecular water oxidation catalysts has been prepared and fully characterized, including single crystal X-ray diffraction. 1 bears geometric similarities to H₄TBAPy (H₄TBAPy = 4,4',4'',4'''-(pyrene-1,3,6,8-tetrayl)tetrabenzoic acid), i.e. the native linker in NU-1000, which offers the possibility to synthesize NU-1000-Ru mixed linker MOFs solvothermally. Mixed linker MOF formation was demonstrated by powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM), and Ru linker incorporation confirmed by FT-IR, energy-dispersive X-ray (EDX) spectroscopy and inductively coupled plasma optical emission spectroscopy (ICP-OES). It was found that the Ru contents in the final mixed linker MOFs correlate with the amount of Ru linker present during solvothermal synthesis, albeit not in a linear fashion. The cyclic voltammograms (CV) of the mixed linker MOFs are largely dominated by TBAPy-based oxidations with features attributed to 1. Interestingly, Ru linkers near the crystal surface are oxidized directly by interfacial hole transfer form the electrode, while those in the crystal interior can be oxidized indirectly from oxidized TBAPy linkers at more anodic potential. Upon repeated scanning, the CVs show the appearance of new waves that arise from irreversible TBAPy oxidation, as well as from the activation of the Ru-based water oxidation catalyst. Of the materials prepared, the one with the highest Ru content, NU-1000-Ru_high, was shown to catalyze the electrochemical oxidation of water to dioxygen. The Faradaic efficiency (FE) of the construct is 37%, due to water oxidation being accompanied by oxidative transformations of the TBAPy linkers. Despite the low FE, NU-1000-Ru_high is still among the best MOF-based water oxidation catalysts, operating by a unique co-linker mediated hole-transport mechanism to supply oxidizing equivalents also to catalysts in the crystal interior.

Probing the electronic and ionic transport in topologically distinct redox-active metal-organic frameworks in aqueous electrolytes

Three topologically distinct zirconium-based metal-organic frameworks (Zr-MOFs) constructed from redox-innocent linkers, MOF-808, defective UiO-66, and CAU-24, are synthesized, and the spatially dispersed redox-active manganese sites are post-synthetically immobilized on the hexa-zirconium nodes of these Zr-MOFs. The crystallinity, morphology, porosity, manganese loading, and bulk electrical conductivity of each material are studied. The redox-hopping-based electrochemical reaction between the installed Mn(III) and Mn(IV) occurring within the thin films of these MOFs in aqueous electrolytes is investigated, in the presence of various concentrations of Na₂SO₄ in the electrolytes. Cyclic voltammetry is used to qualitatively study the redox-hopping process, and chronoamperometry is used to quantify the electrochemically active fractions of manganese sites within the MOF thin film as well as the values of apparent diffusivity for the redox-hopping process. By adjusting the concentration of Na₂SO₄ in the electrolyte, the rate-determining step for the redox-hopping process can be tuned from ionic transport to electronic transport, and the Mn-decorated MOF-808, which possesses the largest pore size, can achieve the highest value of apparent diffusivity. Findings here shed light on the selection of Zr-MOF as well as the choice of electrolyte concentration for the applications of MOFs in supercapacitors and electrocatalysis relying on such redox-hopping processes in aqueous electrolytes.

Microscopic Insights into Cation-Coupled Electron Hopping Transport in a Metal-Organic Framework

Electron transport through metal-organic frameworks by a hopping mechanism between discrete redox active sites is coupled to diffusion-migration of charge-balancing counter cations. Experimentally determined apparent diffusion coefficients, D_e^app, that characterize this form of charge transport thus contain contributions from both processes. While this is well established for MOFs, microscopic descriptions of this process are largely lacking. Herein, we systematically lay out different scenarios for cation-coupled electron transfer processes that are at the heart of charge diffusion through MOFs. Through systematic variations of solvents and electrolyte cations, it is shown that the D_e^app for charge migration through a PIZOF-type MOF, Zr(dcphOH-NDI) that is composed of redox-active naphthalenediimide (NDI) linkers, spans over 2 orders of magnitude. More importantly, however, the microscopic mechanisms for cation-coupled electron propagation are contingent on differing factors depending on the size of the cation and its propensity to engage in ion pairs with reduced linkers, either non-specifically or in defined structural arrangements. Based on computations and in agreement with experimental results, we show that ion pairing generally has an adverse effect on cation transport, thereby slowing down charge transport. In Zr(dcphOH-NDI), however, specific cation-linker interactions can open pathways for concerted cation-coupled electron transfer processes that can outcompete limitations from reduced cation flux.

Heterogenization of Molecular Water Oxidation Catalysts in Electrodes for (Photo)Electrochemical Water Oxidation

Water oxidation is still one of the most important challenges to develop efficient artificial photosynthetic devices. In recent decades, the development and study of molecular complexes for water oxidation have allowed insight into the principles governing catalytic activity and the mechanism as well as establish ligand design guidelines to improve performance. However, their durability and long-term stability compromise the performance of molecular-based artificial photosynthetic devices. In this context, heterogenization of molecular water oxidation catalysts on electrode surfaces has emerged as a promising approach for efficient long-lasting water oxidation for artificial photosynthetic devices. This review covers the state of the art of strategies for the heterogenization of molecular water oxidation catalysts onto electrodes for (photo)electrochemical water oxidation. An overview and description of the main binding strategies are provided explaining the advantages of each strategy and their scope. Moreover, selected examples are discussed together with the the differences in activity and stability between the homogeneous and the heterogenized system when reported. Finally, the common design principles for efficient (photo)electrocatalytic performance summarized.

MOF-enabled confinement and related effects for chemical catalyst presentation and utilization

A defining characteristic of nearly all catalytically functional MOFs is uniform, molecular-scale porosity. MOF pores, linkers and nodes that define them, help regulate reactant and product transport, catalyst siting, catalyst accessibility, catalyst stability, catalyst activity, co-catalyst proximity, composition of the chemical environment at and beyond the catalytic active site, chemical intermediate and transition-state conformations, thermodynamic affinity of molecular guests for MOF interior sites, framework charge and density of charge-compensating ions, pore hydrophobicity/hydrophilicity, pore and channel rigidity vs. flexibility, and other features and properties. Collectively and individually, these properties help define overall catalyst functional behaviour. This review focuses on how porous, catalyst-containing MOFs capitalize on molecular-scale confinement, containment, isolation, environment modulation, energy delivery, and mobility to accomplish desired chemical transformations with potentially superior selectivity or other efficacy, especially in comparison to catalysts in homogeneous solution environments.

Hydrogen Evolving Activity of Dithiolene-Based Metal-Organic Frameworks with Mixed Cobalt and Iron Centers

Electrocatalytic systems based on metal-organic frameworks (MOFs) have attracted great attention due to their potential application in commercially viable renewable energy-converting devices. We have recently shown that the cobalt 2,3,6,7,10,11-triphenylenehexathiolate (CoTHT) framework can catalyze the hydrogen evolution reaction (HER) in fully aqueous media with Tafel slopes as low as 71 mV/dec and near-unity Faradaic efficiency (FE). Taking advantage of the high synthetic tunability of MOFs, here, we synthesize a series of iron and mixed iron/cobalt THT-based MOFs. The incorporation of the iron and cobalt dithiolene moieties is verified by various spectroscopic techniques, and the integrity of the crystalline structure is maintained regardless of the stoichiometries of the two metals. The hydrogen evolving activity of the materials was explored in pH 1.3 aqueous electrolyte solutions. Unlike CoTHT, the FeTHT framework exhibits minimal activity due to a late catalytic onset [-0.440 V versus reversible hydrogen electrode (RHE)] and a large Tafel slope (210 mV/dec). The performance of the mixed-metal MOFs is adversely affected by the incorporation of Fe, where increasing Fe content results in MOFs with lower HER activity and diminished long-term stability and FE for H₂ production. It is proposed that the FeTHT domains undergo alternative Faradaic processes under catalytic conditions, which alter its local structure and electrochemical behavior, eventually resulting in a material with diminished HER performance.

Tunable Band Gaps in MUV-10(M): A Family of Photoredox-Active MOFs with Earth-Abundant Open Metal Sites

Titanium-based metal-organic frameworks (Ti-MOFs) have attracted intense research attention because they can store charges in the form of Ti³⁺ and they serve as photosensitizers to cocatalysts through heterogeneous photoredox reactions at the MOF-liquid interface. Both the charge storage and charge transfer depend on the redox potentials of the MOF and the molecular substrate, but the factors controlling these energetic aspects are not well understood. Additionally, photocatalysis involving Ti-MOFs relies on cocatalysts rather than the intrinsic Ti reactivity, in part because Ti-MOFs with open metal sites are rare. Here, we report that the class of Ti-MOFs known as MUV-10 can be synthetically modified to include a range of redox-inactive ions with flexible coordination environments that control the energies of the photoactive orbitals. Lewis acidic cations installed in the MOF cluster (Cd²⁺, Sr²⁺, and Ba²⁺) or introduced to the pores (H⁺, Li⁺, Na⁺, K⁺) tune the electronic structure and band gaps of the MOFs. Through the use of optical redox indicators, we report the first direct measurement of the Fermi levels (redox potentials) of photoexcited MOFs in situ. Taken together, these results explain the ability of Ti-MOFs to store charges and provide design principles for achieving heterogeneous photoredox chemistry with electrostatic control.

Cu[Ni(2,3-pyrazinedithiolate)2] Metal-Organic Framework for Electrocatalytic Hydrogen Evolution

The application of metal-organic frameworks (MOFs) as electrocatalysts for small molecule activation has been an emerging topic of research. Previous studies have suggested that two-dimensional (2D) dithiolene-based MOFs are among the most active for the hydrogen evolution reaction (HER). Here, a three-dimensional (3D) dithiolene-based MOF, Cu[Ni(2,3-pyrazinedithiolate)₂] (1), is evaluated as an electrocatalyst for the HER. In pH 1.3 aqueous electrolyte solution, 1 exhibits a catalytic onset at -0.43 V vs the reversible hydrogen electrode (RHE), an overpotential (η_10 mA/cm²) of 0.53 V to reach a current density of 10 mA/cm², and a Tafel slope of 69.0 mV/dec. Interestingly, under controlled potential electrolysis, 1 undergoes an activation process that results in a more active catalyst with a 200 mV reduction in the catalytic onset and η_10 mA/cm². It is proposed that the activation process is a result of the cleavage of Cu-N bonds in the presence of protons and electrons. This hypothesis is supported by various experimental studies and density functional theory calculations.

Mimicking the Electron Transport Chain and Active Site of [FeFe] Hydrogenases in One Metal-Organic Framework: Factors That Influence Charge Transport

[FeFe] hydrogenase (H₂ase) enzymes are effective proton reduction catalysts capable of forming molecular dihydrogen with a high turnover frequency at low overpotential. The active sites of these enzymes are buried within the protein structures, and substrates required for hydrogen evolution (both protons and electrons) are shuttled to the active sites through channels from the protein surface. Metal–organic frameworks (MOFs) provide a unique platform for mimicking such enzymes due to their inherent porosity which permits substrate diffusion and their structural tunability which allows for the incorporation of multiple functional linkers. Herein, we describe the preparation and characterization of a redox-active PCN-700-based MOF (PCN = porous coordination network) that features both a biomimetic model of the [FeFe] H₂ase active site as well as a redox-active linker that acts as an electron mediator, thereby mimicking the function of [4Fe4S] clusters in the enzyme. Rigorous studies on the dual-functionalized MOF by cyclic voltammetry (CV) reveal similarities to the natural system but also important limitations in the MOF-enzyme analogy. Most importantly, and in contrast to the enzyme, restrictions apply to the total concentration of reduced linkers and charge-balancing counter cations that can be accommodated within the MOF. Successive charging of the MOF results in nonideal interactions between linkers and restricted mobility of charge-compensating redox-inactive counterions. Consequently, apparent diffusion coefficients are no longer constant, and expected redox features in the CVs of the materials are absent. Such nonlinear effects may play an important role in MOFs for (electro)catalytic applications.

Cerium-Based Metal-Organic Framework Nanocrystals Interconnected by Carbon Nanotubes for Boosting Electrochemical Capacitor Performance

In this study, nanocrystals of a cerium-based metal-organic framework (Ce-MOF), Ce-MOF-808, are directly grown on the surface of carboxylic acid-functionalized carbon nanotubes (CNTs) by a facile one-step solvothermal synthesis method. Ce-MOF-CNT nanocomposites with various Ce-MOF-to-CNT ratios are synthesized, and their crystallinity, morphology, porosity, and electrical conductivity are examined. The redox-hopping and electrochemical behaviors of the pristine Ce-MOF in aqueous electrolytes are investigated, suggesting that the pristine Ce-MOF is electrochemically active but possesses a limited charge-transport behavior. As a demonstration, all the Ce-MOF, CNT, and nanocomposites are used as active materials for application in aqueous-based supercapacitors. The capacitive performance of the CNT can be significantly boosted with the help of redox-active Ce-MOF-808 nanocrystals.

Molecular and heterogeneous water oxidation catalysts: recent progress and joint perspectives

The recent synthetic and mechanistic progress in molecular and heterogeneous water oxidation catalysts highlights the new, overarching strategies for knowledge transfer and unifying design concepts.

Enhancing photovoltages at p-type semiconductors through a redox-active metal-organic framework surface coating

Surface modification of semiconductors can improve photoelectrochemical performance by promoting efficient interfacial charge transfer. We show that metal-organic frameworks (MOFs) are viable surface coatings for enhancing cathodic photovoltages. Under 1-sun illumination, no photovoltage is observed for p-type Si(111) functionalized with a naphthalene diimide derivative until the monolayer is expanded in three dimensions in a MOF. The surface-grown MOF thin film at Si promotes reduction of the molecular linkers at formal potentials >300 mV positive of their thermodynamic potentials. The photocurrent is governed by charge diffusion through the film, and the MOF film is sufficiently conductive to power reductive transformations. When grown on GaP(100), the reductions of the MOF linkers are shifted anodically by >700 mV compared to those of the same MOF on conductive substrates. This photovoltage, among the highest reported for GaP in photoelectrochemical applications, illustrates the power of MOF films to enhance photocathodic operation.

Photoelectrochemical performance is often hindered by sluggish charge transfer at the semiconductor interface. Here, the authors illustrate that a thin film coating made of a conductive metal-organic framework can improve the photovoltage of the underpinning semiconductors.

Synthetic strategies to incorporate Ru-terpyridyl water oxidation catalysts into MOFs: direct synthesis vs. post-synthetic approach

Incorporating molecular catalysts into metal-organic frameworks (MOFs) is a promising strategy for improving their catalytic longevity and recyclability. In this article, we investigate and compare synthetic routes for the incorporation of the potent water oxidation catalyst Ru(tda)(pyCO2H)2 (tda = 2,2':6',2''-terpyridine-6,6''-dicarboxylic acid, pyCO2H = iso-nicotinic acid) as a structural linker into a Zr-based UiO-type MOF. The task is challenging with this particular metallo-linker because of the equatorial dangling carboxylates that can potentially compete for Zr-coordination, as well as free rotation of the pyCO2H groups around the HO2CpyRupyCO2H axis. As a consequence, all attempts to synthesize a MOF with the metallo-linker directly under solvothermal conditions led to amorphous materials with the Ru(tda)(pyCO2H)2 linker coordinating to the Zr nodes in ill-defined ways, resulting in multiple waves in the cyclic voltammograms of the solvothermally obtained materials. On the other hand, an indirect post-synthetic approach in which the Ru(tda)(pyCO2H)2 linker is introduced into a preformed edba-MOF (edba = ethyne dibenzoic acid) of UiO topology results in the formation of the desired material. Interestingly, two distinctly different morphologies of the parent edba-MOF have been discovered, and the impact that the morphological difference has on linker incorporation is investigated.

Diagnosing surface versus bulk reactivity for molecular catalysis within metal-organic frameworks using a quantitative kinetic model

Metal-organic frameworks (MOFs) are becoming increasingly popular as heterogenous support matrices for molecular catalysts. Given that reactants, or potentially holes/electrons, need to diffuse into the porous framework as the reaction proceeds, the reaction can possibly take place within the bulk of the particle or be confined to a thin layer at the surface due to transport limitations. Herein, a simple steady-state reaction-diffusion kinetic model is developed to diagnose these two mutually exclusive behaviors in MOF-based systems. The oxygen evolution reaction (OER) driven by a chemical oxidant is presented as an example mechanism. Quantitative metrics for assigning either bulk or surface reactivity are delineated over a wide variety of conditions, and numerical simulations are employed to verify these results. For each case, expressions for the turnover frequency (TOF) are outlined, and it is shown that surface reactivity can influence measured TOFs. Importantly, this report shows how to transition from surface to bulk reactivity and thus identifies which experimental parameters to target for optimizing the efficiency of MOF-based molecular catalyst systems.

Transport Phenomena: Challenges and Opportunities for Molecular Catalysis in Metal-Organic Frameworks

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.

Metal-Organic Framework-Based Catalysts with Single Metal Sites

Metal-organic frameworks (MOFs) are a class of distinctive porous crystalline materials constructed by metal ions/clusters and organic linkers. Owing to their structural diversity, functional adjustability, and high surface area, different types of MOF-based single metal sites are well exploited, including coordinately unsaturated metal sites from metal nodes and metallolinkers, as well as active metal species immobilized to MOFs. Furthermore, controllable thermal transformation of MOFs can upgrade them to nanomaterials functionalized with active single-atom catalysts (SACs). These unique features of MOFs and their derivatives enable them to serve as a highly versatile platform for catalysis, which has actually been becoming a rapidly developing interdisciplinary research area. In this review, we overview the recent developments of catalysis at single metal sites in MOF-based materials with emphasis on their structures and applications for thermocatalysis, electrocatalysis, and photocatalysis. We also compare the results and summarize the major insights gained from the works in this review, providing the challenges and prospects in this emerging field.

Metal-Organic Framework-Based Engineered Materials-Fundamentals and Applications

Metal-organic frameworks (MOFs) are a fascinating class of porous crystalline materials constructed by organic ligands and inorganic connectors. Owing to their noteworthy catalytic chemistry, and matching or compatible coordination with numerous materials, MOFs offer potential applications in diverse fields such as catalysis, proton conduction, gas storage, drug delivery, sensing, separation and other related biotechnological and biomedical applications. Moreover, their designable structural topologies, high surface area, ultrahigh porosity, and tunable functionalities all make them excellent materials of interests for nanoscale applications. Herein, an effort has been to summarize the current advancement of MOF-based materials (i.e., pristine MOFs, MOF derivatives, or MOF composites) for electrocatalysis, photocatalysis, and biocatalysis. In the first part, we discussed the electrocatalytic behavior of various MOFs, such as oxidation and reduction candidates for different types of chemical reactions. The second section emphasizes on the photocatalytic performance of various MOFs as potential candidates for light-driven reactions, including photocatalytic degradation of various contaminants, CO2 reduction, and water splitting. Applications of MOFs-based porous materials in the biomedical sector, such as drug delivery, sensing and biosensing, antibacterial agents, and biomimetic systems for various biological species is discussed in the third part. Finally, the concluding points, challenges, and future prospects regarding MOFs or MOF-based materials for catalytic applications are also highlighted.

Evolution of metal organic frameworks as electrocatalysts for water oxidation

The development of metal organic framework based water oxidation catalysts is discussed here in connection with various design strategies.

Analysis of Electrocatalytic Metal-Organic Frameworks

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.

Facile Orientational Control of M2L2P SURMOFs on ⟨100⟩ Silicon Substrates and Growth Mechanism Insights for Defective MOFs

Layer-by-layer growth of Cu₂(bdc)₂(dabco) surface-mounted metal-organic frameworks (SURMOFs) was investigated on silicon wafers treated with different surface anchoring molecules. Well-oriented growth along the [100] and [001] directions could be achieved with simple protocols: growth along the [100] direction was achieved by substrate pretreatment with 80 °C piranha, while growth along the [001] direction was enabled by only rinsing silicon with absolute ethanol. Growth along the [001] direction produced more homogeneous SURMOF films. Optimization to enhance [001]-preferred orientation growth revealed that small changes in the SURMOF growth sequence (the number of rinse steps and linker concentrations) have a noticeable impact on the final film quality and the number of misaligned crystals. This new straightforward protocol was used to successfully grow other layer pillar-type SURMOFs, including the growth of Cu₂(bdc)₂(bipy) with simultaneous suppression of framework interpenetration.

Electrocatalytic Hydrogen Evolution from a Cobaloxime-Based Metal-Organic Framework Thin Film

Molecular hydrogen evolution catalysts (HECs) are synthetically tunable and often exhibit high activity, but they are also hampered by stability concerns and practical limitations associated with their use in the homogeneous phase. Their incorporation as integral linker units in metal–organic frameworks (MOFs) can remedy these shortcomings. Moreover, the extended three-dimensional structure of MOFs gives rise to high catalyst loadings per geometric surface area. Herein, we report a new MOF that exclusively consists of cobaloximes, a widely studied HEC, that act as metallo-linkers between hexanuclear zirconium clusters. When grown on conducting substrates and under applied reductive potential, the cobaloxime linkers promote electron transport through the film as well as function as molecular HECs. The obtained turnover numbers are orders of magnitude higher than those of any other comparable cobaloxime system, and the molecular integrity of the cobaloxime catalysts is maintained for at least 18 h of electrocatalysis. Being one of the very few hydrogen evolving electrocatalytic MOFs based on a redox-active metallo-linker, this work explores uncharted terrain for greater catalyst diversity and charge transport pathways.

Metal–Organic Frameworks Toward Electrocatalytic Applications

Metal–organic frameworks (MOFs) are a class of porous materials constructed from metal-rich inorganic nodes and organic linkers. Because of their regular porosity in microporous or mesoporous scale and periodic intra-framework functionality, three-dimensional array of high-density and well-separated active sites can be built in various MOFs; such characteristics render MOFs attractive porous supports for a range of catalytic applications. Furthermore, the electrochemically addressable thin films of such MOF materials are reasonably considered as attractive candidates for electrocatalysis and relevant applications. Although it still constitutes an emerging subfield, the use of MOFs and relevant materials for electrocatalytic applications has attracted much attention in recent years. In this review, we aim to focus on the limitations and commonly seen issues for utilizing MOFs in electrocatalysis and the strategies to overcome these challenges. The research efforts on utilizing MOFs in a range of electrocatalytic applications are also highlighted.

Co (II) Boron Imidazolate Framework with Rigid Auxiliary Linkers for Stable Electrocatalytic Oxygen Evolution Reaction

Metal-organic frameworks (MOFs) have significant potential for practical application in catalysis. However, many MOFs are shown to be sensitive to aqueous solution. This severely limits application of MOFs in electrocatalytic operations for energy production and storage. Here, a Co (II) boron imidazolate framework CoB(im)4(ndc)0.5 (BIF-91, im = imidazolate, ndc = 2,6-naphthalenedicarboxylate) that is rationally designed and successfully tested for electrocatalytic application in strong alkaline (pH ≈ 14) solution is reported. In such a BIF system, the inherent carboxylate species segment large channel spaces into multiple domains in which each single channel is filled with ndc ligands through the effect of zeolite channel confinement. These ligands, with strong C-H···π interaction, act as a rigid auxiliary linker to significantly enhance the structural stability of the BIF-91 framework. Additionally, the π-conjugated effect in BIF-91 stabilizes dopant Fe (III) at the atomic scale to construct Fe-immobilized BIF-91 (Fe@BIF-91). Due to the synergistic effect between Fe (III) guest and Co (II) in the framework, the Fe@BIF-91 acts as an active and stable electrocatalyst for the oxygen evolution reaction in alkaline solution.

Effects of a strong π-accepting ancillary ligand on the water oxidation activity of weakly coupled binuclear ruthenium catalysts

Significant differences were found in the proton-coupled redox chemistry and catalytic behavior of the binuclear [{Ru(H2O)(bpz)}2(tpy2ph)](PF6)4 complex [bpz = 2,2'-bipyrazine; tpy2ph = 1,3-bis(4'-2,2':6',2''-terpyridin-4-yl)benzene] as compared with the structurally analogous derivative with 2,2'-bipyridine (bpy) instead of bpz. The differences were assigned to the stronger π-accepting character of bpz relative to bpy as the ancillary ligand. The expectation of a positive shift for the Ru-centered redox potentials was confirmed for the lower oxidation state species, but that trend was reversed in the formation of the high-valence catalytic active species as shown by a negative shift of 0.14 V for the potential of the [RuIV/V[double bond, length as m-dash]O] process. Moreover, DFT calculations indicated a significant decrease of about 15% on the spin density and oxyl character of the [RuV[double bond, length as m-dash]O]3+ fragment. The significantly lower kcat(O2) for the bpz system was attributed to these combined electronic effects.

A 2,2'-bipyridine-containing covalent organic framework bearing rhenium(i) tricarbonyl moieties for CO2 reduction

The reduction of CO2 into higher energy products such as carbon-based fuels and feedstocks is an attractive strategy for mitigating the continuous rise in CO2 emissions associated with the growing global energy demand. Rhenium tricarbonyl complexes bearing 2,2'-bipyridine (2,2'-bpy) ligands are well-established molecular electrocatalysts for the selective reduction of CO2 to CO. Construction of efficient devices for this electrochemical process requires the immobilization of electrocatalysts to electrode surfaces. To integrate Re(2,2'-bpy)(CO)3 fragments into a covalent organic framework (COF), Re(5,5'-diamine-2,2'-bpy)(CO)3Cl (1) was synthesized and electrochemically investigated. Complex 1 is an active and selective electrocatalyst for the reduction of CO2 to CO with excellent faradaic efficiency (99%). The presence of the amine substituents leads to a destabilization of the π* orbital of the 5,5'-diamine-2,2'-bpy ligand with respect to the metal center. Therefore, 1 requires more negative potentials (-2.47 V vs. Fc+/0) to reach the doubly reduced catalytically active species. DFT studies were conducted to understand the electronic structure of 1, and support the destabilizing effect of the amine substituents. The Re-2,2'-bpy fragments were successfully integrated into a COF containing 2,2'-bpy moieties (COF-2,2'-bpy) via a post-metallation synthetic route to generate COF-2,2'-bpy-Re. A composite of COF-2,2'-bpy-Re, carbon black, and polyvinylidene fluoride (PVDF) was readily immobilized onto glassy carbon electrodes and electrocatalytic CO2 reduction to CO was observed at -2.8 V vs. Fc0/+, with a faradaic efficiency of 81% for CO production.

Covalent-Organic Frameworks Composed of Rhenium Bipyridine and Metal Porphyrins: Designing Heterobimetallic Frameworks with Two Distinct Metal Sites

The incorporation of homogeneous catalysts for CO₂ reduction into extended frameworks has been a successful strategy for increasing catalyst lifetime and activity, but the effects of the linkers on catalysis are underexplored. In this work, a novel rhenium bipyridine complex was synthesized for the purpose of designing a covalent-organic framework (COF) with both metalloporphyrin and metal bipyridine moieties. Investigation of the rhenium complex as a homogeneous catalyst shows a faradaic efficiency of 81(8)% for the electrocatalytic conversion of CO₂ to CO upon the addition of methanol as the proton source. Treatment of the rhenium complex with tetra(4-aminophenyl)porphyrin under Schiff base conditions produces the desired COF, as indicated by powder X-ray diffraction (PXRD) studies. Metalation of the porphyrins was accomplished through postsynthetic modification with CoCl₂ and FeCl₃ metal precursors. The retention of the PXRD peaks and appearance of new Co and Fe peaks in the corresponding X-ray photoelectron spectroscopy spectra suggest the successful incorporation of a secondary metal site into the framework. Cyclic voltammetry measurements display increases in current densities when the atmosphere is changed from N₂ to CO₂. Controlled potential electrolyses show that the cobalt-postmetalated COF has the highest activity toward CO₂ reduction, reaching a faradaic efficiency of 18(2)%.