Hydrogen spillover inspired bifunctional Platinum/Rhodium Oxide-Nitrogen-Doped carbon composite for enhanced hydrogen evolution and oxidation reactions in base.

The poor activity of Pt-based-catalysts for alkaline hydrogen oxidation/evolution reaction (HOR/HER) encourages scientific society to design an effective electrocatalyst to develop alkaline fuel cells/electrolyzers. Herein, platinum/rhodium oxide-nitrogen-doped carbon (Pt/Rh2O3-CNx) composite is prepared for alkaline HER and HOR inspired by hydrogen spillover. The HER performance of Pt/Rh2O3-CNx is âˆ¼ 6 times higher than Pt/C. In HOR, Pt/Rh2O3-CNx possesses an exchange current density of 657.60 mA/mgmetal, which is âˆ¼ 3.4 times higher than Pt/C. Hydrogen and hydroxyl binding energy (HBE and OHBE) contribute equally to alkaline HOR/HER. The experimental and theoretical evidence suggests that the enhanced HER and HOR activity of Pt/Rh2O3-CNx may be due to hydrogen spillover from Pt to Rh2O3. Small work function difference [0.08 eV] of the system suggested hydrogen-spillover is feasible, which has been justified by reaction-free energy calculations. We proposed that the dissociation of hydrogen (H2) and water (H2O) occurs at Pt to form Pt-adsorbed hydrogen species (Pt-Had). Then, some Had moves to Rh2O3 through hydrogen spillover and reacts with neighboring Had or adsorbed hydroxyl species (OHad) to form H2 or H2O, which enhances the HER and HOR activity, respectively. The role of water-metal-hydroxyl species in the electrical double layer was also demonstrated on alkaline HOR/HER. This work may help to design the hydrogen-spillover-based catalysts for several renewable energy technologies.

Hydrogen-Bonded Metal-Organic Framework Nanosheet as a Proton Conducting Membrane for an H2 /O2 Fuel Cell.

Proton-conducting metal-organic frameworks (MOFs) have attracted attention as potential electrolytes for fuel cells. However, research progress in utilizing MOFs as electrolytes for fuel cells has been limited, mainly due to challenges associated with issues such as the fabrication of MOF membranes, and hydrogen crossover through the MOF's pores. Here, we report proton conductivity and fuel cell performance of a self-standing membrane prepared from of a bismuth subgallate MOF nanosheets with non-porous structure are reported. The fabricated MOF nanosheet membrane with no binding agent exhibitsed structural anisotropy. The proton conductivity in the membrane thickness direction (4.4 × 10-3  S cm-1 ) at 90 °C and RH 100% is observed to be higher than that in the in-plane direction of the membrane (3.3 × 10-5  S cm-1 ). The open circuit voltage (OCV) of a fuel cell with ≈120 µm proton conducting membrane is 1.0 V. The non-porous nature of the MOF nanosheets contributes to the relatively high OCV. A fuel cell using ≈40 µm membrane as proton conducting electrolyte records a maximum of 25 mW cm-2 power density and a maximum of 109 mA cm-2 current density with 0.91 V OCV at 80 °C in humid conditions.

Hydrogen spillover enhances alkaline hydrogen electrocatalysis on interface-rich metallic Pt-supported MoO3.

Efficient and cost-effective electrocatalysts for the hydrogen oxidation/evolution reaction (HOR/HER) are essential for commercializing alkaline fuel cells and electrolyzers. The sluggish HER/HOR reaction kinetics in base is the key issue that requires resolution so that commercialization may proceed. It is also quite challenging to decrease the noble metal loading without sacrificing performance. Herein, we report improved HER/HOR activity as a result of hydrogen spillover on platinum-supported MoO3 (Pt/MoO3-CNx-400) with a Pt loading of 20%. The catalyst exhibited a decreased over-potential of 66.8 mV to reach 10 mA cm-2 current density with a Tafel slope of 41.2 mV dec-1 for the HER in base. The Pt/MoO3-CNx-400 also exhibited satisfactory HOR activity in base. The mass-specific exchange current density of Pt/MoO3-CNx-400 and commercial Pt/C are 505.7 and 245 mA mgPt-1, respectively. The experimental results suggest that the hydrogen binding energy (HBE) is the key descriptor for the HER/HOR. We also demonstrated that the enhanced HER/HOR performance was due to the hydrogen spillover from Pt to MoO3 sites that enhanced the Volmer/Heyrovsky process, which led to high HER/HOR activity and was supported by the experimental and theoretical investigations. The work function value of Pt [Φ = 5.39 eV) is less than that of ß-MoO3 (011) [Φ = 7.09 eV], which revealed the charge transfer from Pt to the ß-MoO3 (011) surface. This suggested the feasibility of hydrogen spillover, and was further confirmed by the relative hydrogen adsorption energy [ΔGH] at different sites. Based on these findings, we propose that the H2O or H2 dissociation takes place on Pt and interfaces to form Pt-Had or (Pt/MoO3)-Had, and some of the Had shifted to MoO3 sites through hydrogen spillover. Then, Had at the Pt and interface, and MoO3 sites reacted with H2O and HO- to form H2 or H2O molecules, thereby boosting the HER/HOR activity. This work may provide valuable information for the development of hydrogen-spillover-based electrocatalysts for use in various renewable energy devices.

Infusion of Variable Chemical Structure to Tune Stacking among Metal-Organic Layers in 2D Nano MOFs.

Invited for the cover of this issue is the group of Biplab Manna at the University of Kumamoto. The image depicts various kinds of stacking between the metal-organic layers of MOFs. Read the full text of the article at 10.1002/chem.202201665.

Infusion of Variable Chemical Structure to Tune Stacking among Metal-Organic Layers in 2D Nano MOF.

Thickness of two-dimensional (2D) metal-organic frameworks (MOFs) govern their intriguing functionalities. Primarily this thickness is controlled by the stacking between the metal-organic layers (MOL). It is observed that until now such modulating factors for stacking efficiency of MOL are not well studied. Here, we report a fundamental hypothesis to comprehend regulation of stacking efficiency among MOLs as a function of chemical structure of organic ligands (dicarboxylic acids and pillar linkers). This basically involves a series of isostructural three-dimensional (3D) MOFs which contain linkers of variable chemical nature that could be depillared to generate 2D stacked MOFs of different thickness. Depending on the linkers, we encountered the formation of single MOL to stacked multiple MOLs as evidenced from atomic force microscopic and other experimental analysis. The present study gives a concrete correlation between the stacking within 2D MOFs (from monolayer to multilayers), and their 3D counter parts, which may provide a thickness tuning pathway for 2D MOFs.

Base-Resistant Ionic Metal-Organic Framework as a Porous Ion-Exchange Sorbent.

A systematic approach has been employed to obtain a hydrolytically stable cationic metal-organic framework (MOF). The synthesized two-dimensional Ni(II)-centered cationic MOF, having its backbone built from purely neutral N-donor ligand, is found to exhibit uncommon resistance over wide pH range, particularly even under highly alkaline conditions. This report presents a rare case of a porous MOF retaining structural integrity under basic conditions, and an even rarer case of a porous cationic MOF. The features of stability and porosity in this ionic MOF have been harnessed for the function of charge- and size-selective capture of small organic dye through ion-exchange process across a wide pH range.

Ultrahigh Ionic Conduction in Water-Stable Close-Packed Metal-Carbonate Frameworks.

Utilization of the robust metal-carbonate backbone in a series of water-stable, anionic frameworks has been harnessed for the function of highly efficient solid-state ion-conduction. The compact organization of hydrophilic guest ions facilitates water-assisted ion-conduction in all the compounds. The dense packing of the compounds imparts high ion-conducting ability and minimizes the possibility of fuel crossover, making this approach promising for design and development of compounds as potential components of energy devices. This work presents the first report of evaluating ion-conduction in a purely metal-carbonate framework, which exhibits high ion-conductivity on the order of 10-2 S cm-1 along with very low activation energy, which is comparable to highly conducting well-known crystalline coordination polymers or commercialized organic polymers like Nafion.

Toxic Aromatics Induced Responsive Facets for a Pore Surface Functionalized Luminescent Coordination Polymer.

A luminescent coordination polymer was synthesized based on a linker prefunctionalization-based design principle coupled with an appropriate template selection protocol adopted during crystallization. Luminescent linker derived photoluminescence emission signature together with the reversibly dynamic host polymer exhibited a unique response toward environmentally toxic aromatics in the solid state, arguably crucial for the designed development of toxin-responsive solid materials.

An Ultrahydrophobic Fluorous Metal-Organic Framework Derived Recyclable Composite as a Promising Platform to Tackle Marine Oil Spills.

Derived from a strategically chosen hexafluorinated dicarboxylate linker aimed at the designed synthesis of a superhydrophobic metal-organic framework (MOF), the fluorine-rich nanospace of a water-stable MOF (UHMOF-100) exhibits excellent water-repellent features. It registered the highest water contact angle (≈176°) in the MOF domain, marking the first example of an ultrahydrophobic MOF. Various experimental and theoretical studies reinforce its distinctive water-repellent characteristics, and the conjugation of superoleophilicity and unparalleled hydrophobicity of a MOF material has been coherently exploited to achieve real-time oil/water separation in recyclable membrane form, with significant absorption capacity performance. This is also the first report of an oil/water separating fluorinated ultrahydrophobic MOF-based membrane material, with potential promise for tackling marine oil spillages.

Harnessing Lewis acidic open metal sites of metal-organic frameworks: the foremost route to achieve highly selective benzene sorption over cyclohexane.

π-Complexation triggered Lewis acid-base interactions between open metal sites (OMS) of metal-organic frameworks (MOFs), and π-e(-) rich adsorptive benzene (Bz) is exploited to establish M-MOF-74 as the best Bz-selective MOF sorbent, marking the first report of utilizing OMS behind benzene/cyclohexane separation; a key advance from the energy-economy standpoint of industrial separation.

A Water-Stable Cationic Metal-Organic Framework as a Dual Adsorbent of Oxoanion Pollutants.

A three-dimensional water-stable cationic metal-organic framework (MOF) pillared by a neutral ligand and with Ni(II)  metal nodes has been synthesized employing a rational design approach. Owing to the ordered arrangement of the uncoordinated tetrahedral sulfate (SO4 (2-) ) ions in the channels, the compound has been employed for aqueous-phase ion-exchange applications. The compound exhibits rapid and colorimetric aqueous-phase capture of environmentally toxic oxoanions (with similar geometries) in a selective manner. This system is the first example of a MOF-based system which absorbs both dichromate (Cr2 O7 (2-) ) and permanganate (MnO4 (-) ) ions, with the latter acting as a model for the radioactive contaminant pertechnetate (TcO4 (-) ).

Neutral N-donor ligand based flexible metal-organic frameworks.

This short review gives a focussed discussion on metal-organic frameworks (MOFs) made of neutral N-donor ligands which show structural flexibility under various exogenous stimuli. Chemical stimuli such as presence of anions, free guests, coordinated guests and physical stimuli (light, heat and so on) render structural flexibility in MOFs. Single-crystal-to-single-crystal transformation studies have attracted a lot of attention for the understanding of such flexible MOF materials. Such a dynamic structural behavior with proper host-guest interactions gives very interesting functions such as chemical separation, sensing and magnetic properties and so on.

A π-electron deficient diaminotriazine functionalized MOF for selective sorption of benzene over cyclohexane.

A diaminotriazine functionalized novel MOF (DAT-MOF-1) has been synthesized stemming out of a π-electron-deficient pore-surface functionalization based linker-design principle, which results in efficient selectivity of benzene sorption over its aliphatic analogue cyclohexane, crucial from the industrial standpoint.

Coherent fusion of water array and protonated amine in a metal-sulfate-based coordination polymer for proton conduction.

A new function of metal-sulfate-based coordination polymer (CP) for proton conduction was investigated through rational integration of a continuous water array and protonated amine in the coordination space of the CP. The H-bonded arrays of water molecules along with nitrogen-rich aromatic cation (protonated melamine) facilitate proton conduction in the compound under humid conditions. Although several reports of metal-oxalate/phosphate-based CPs showing proton conduction are known, this is the first designed synthesis of a metal-sulfate-based CP bearing water arrays functioning as a solid-state proton conductor.

Exploiting Framework Flexibility of a Metal-Organic Framework for Selective Adsorption of Styrene over Ethylbenzene.

The separation of styrene and ethylbenzene mixtures is industrially important and is currently performed in highly energy-intensive vacuum distillation columns. The primary objective of our investigation is to offer an energy-efficient alternative for selective adsorption of styrene by a flexible metal-organic framework, DynaMOF-100. The structural transformation of DynaMOF-100 is specifically triggered on inclusion of styrene within the framework; this structural transformation is reversible. The styrene/ethylbenzene adsorption selectivity, originated as an outcome of the framework flexibility, is found to be much superior to the only two MOFs yet reported, serving styrene/ethylbenzene separation purpose.

An Amide-Functionalized Dynamic Metal-Organic Framework Exhibiting Visual Colorimetric Anion Exchange and Selective Uptake of Benzene over Cyclohexane.

A novel porous metal-organic framework (MOF) architecture is formed by a neutral amide-functionalized ligand and copper(II). Upon desolvation, this compound undergoes a dynamic structural transformation from a one-dimensional (1D) porous phase to a two-dimensional (2D) non-porous phase that shows selective uptake of benzene over cyclohexane. The as-synthesized compound also acts as a visual colorimetric anion sensor for thiocyanate.

Aqueous phase nitric oxide detection by an amine-decorated metal-organic framework.

Selective and sensitive aqueous phase nitric oxide (NO) detection has been demonstrated by implementing an unsophisticated approach of ligand modulation in a porous, robust metal-organic framework (MOF). The detection is achieved through deamination by NO in an amine-decorated luminescent MOF. This is the first report of employing a physiologically stable, functionalized MOF as a NO sensor.

Selective anion exchange and tunable luminescent behaviors of metal-organic framework based supramolecular isomers.

Owing to the conformational (cis or trans) flexibility of a N-donor ligand, the combinations of the same and Cd(ClO4)2 under variable solvent templates afforded two supramolecular isomers based on two-dimensional metal-organic frameworks. Both compounds contain weakly coordinating ClO4(-) anions attached to the metal centers. Both frameworks showed facile anion exchange behaviors with various kinds of foreign anions. Moreover, both frameworks showed anion-driven structural dynamism and exhibited the preferential uptake of strongly coordinating anions over others. Anion-regulated modulation in luminescent behaviors was also observed in both cases.

Dynamic metal-organic framework with anion-triggered luminescence modulation behavior.

A three-dimensional cationic framework based on a flexible neutral nitrogen-donor ligand was synthesized and undergoes guest-driven structural dynamics in a reversible way. Size-selective anion-exchange and tunable luminescent behavior of the framework has been explored.

Guest-responsive function of a dynamic metal-organic framework with a π Lewis acidic pore surface.

A 3D dynamic coordination framework with an electron-deficient pore surface has been synthesized by using Zn(II) (having a variable coordination number) and a predesigned flexible π-electron-deficient core-based ligand, exhibiting chemical separations based on pore surface functionalization (π Lewis acidic pore surfaces and open metal sites) and framework flexibility, giving rise to a unique smart guest-responsive material.