Highly effective electrocatalysts promoting CO
reduction reaction (CO
RR) is extremely desirable to produce value-added chemicals/fuels while addressing current environmental challenges. Herein, we ...develop a layer-stacked, bimetallic two-dimensional conjugated metal-organic framework (2D c-MOF) with copper-phthalocyanine as ligand (CuN
) and zinc-bis(dihydroxy) complex (ZnO
) as linkage (PcCu-O
-Zn). The PcCu-O
-Zn exhibits high CO selectivity of 88%, turnover frequency of 0.39 s
and long-term durability (>10 h), surpassing thus by far reported MOF-based electrocatalysts. The molar H
/CO ratio (1:7 to 4:1) can be tuned by varying metal centers and applied potential, making 2D c-MOFs highly relevant for syngas industry applications. The contrast experiments combined with operando spectroelectrochemistry and theoretical calculation unveil a synergistic catalytic mechanism; ZnO
complexes act as CO
RR catalytic sites while CuN
centers promote the protonation of adsorbed CO
during CO
RR. This work offers a strategy on developing bimetallic MOF electrocatalysts for synergistically catalyzing CO
RR toward syngas synthesis.
Dynamic metal–organic frameworks (MOFs) represent a subgroup of frameworks featuring unique performance, as they are capable of adapting their pore size and/or the orientation of framework ...constituents in response to specific guest molecules such as gases or solutes and often outperform their rigid analogus in gas storage, sensing, or separation. In this review, the authors focus on recent methodical developments of advanced in situ diffraction and spectroscopic techniques for comprehensive characterization of porous frameworks. Examples for advanced instrumentation are highlighted for in situ nuclear magnetic resonance, electron paramagnetic resonance, and optical spectroscopies as well as X‐ray and neutron diffraction. Several examples of high‐resolution transmission electron microscopy (HRTEM) on MOFs are shown because HRTEM is an emerging technique for the characterization of time‐resolved structural dynamics in MOFs. These methods shed light on structural features and phase transitions of the host, its spin state, electronic structure, specific host–guest and guest–guest interactions, preferable adsorption sites, and bonding situation in the framework, focusing on the most prominent recent case studies. The synergistic development of novel in situ characterization methods and exploration of well‐defined model framework systems are crucial to advance the understanding of dynamic processes in porous materials in future.
Structural switchability, accompanied by a large breathing amplitude of the unit cell, is a unique phenomenon in flexible metal–organic frameworks (MOFs). Recent advances in the development of in situ spectroscopic (optical, nuclear magnetic and electron paramagnetic resonance) and diffraction (X‐ray, neutron and electron) techniques for monitoring the dynamic behavior of switchable MOFs upon applying external stimuli are reviewed.
Organic electrode materials are of long‐standing interest for next‐generation sustainable lithium‐ion batteries (LIBs). As a promising cathode candidate, imide compounds have attracted extensive ...attention due to their low cost, high theoretical capacity, high working voltage, and fast redox reaction. However, the redox active site utilization of imide electrodes remains challenging for them to fulfill their potential applications. Herein, the synthesis of a highly stable, crystalline 2D polyarylimide (2D‐PAI) integrated with carbon nanotube (CNT) is demonstrated for the use as cathode material in LIBs. The synthesized polyarylimide hybrid (2D‐PAI@CNT) is featured with abundant π‐conjugated redox‐active naphthalene diimide units, a robust cyclic imide linkage, high surface area, and well‐defined accessible pores, which render the efficient utilization of redox active sites (82.9%), excellent structural stability, and fast ion diffusion. As a consequence, high rate capability and ultrastable cycle stability (100% capacity retention after 8000 cycles) are achieved in the 2D‐PAI@CNT cathode, which far exceeds the state‐of‐the‐art polyimide electrodes. This work may inspire the development of novel organic electrodes for sustainable and durable rechargeable batteries.
Crystalline 2D polyarylimide integrated with carbon nanotube (2D‐PAI@CNT) is prepared via in situ synthesis and is explored as the cathode for Li‐ion batteries. The abundant redox active sites, the robust framework made from imide cores/cyclic linkage, and the well‐defined porous structure endow 2D‐PAI@CNT with ultrastable cycling stability and excellent rate capability to store Li charges, outperforming state‐of‐the‐art polyimide cathodes for rechargeable batteries.
π-Conjugated two-dimensional covalent organic frameworks (2D COFs) are emerging as a novel class of electroactive materials for (opto)electronic and chemiresistive sensing applications. However, ...understanding the intricate interplay between chemistry, structure, and conductivity in π-conjugated 2D COFs remains elusive. Here, we report a detailed characterization for the electronic properties of two novel samples consisting of Zn– and Cu–phthalocyanine-based pyrazine-linked 2D COFs. These 2D COFs are synthesized by condensation of metal–phthalocyanine (M = Zn and Cu) and pyrene derivatives. The obtained polycrystalline-layered COFs are p-type semiconductors both with a band gap of ∼1.2 eV. A record device-relevant mobility up to ∼5 cm2/(V s) is resolved in the dc limit, which represents a lower threshold induced by charge carrier localization at crystalline grain boundaries. Hall effect measurements (dc limit) and terahertz (THz) spectroscopy (ac limit) in combination with density functional theory (DFT) calculations demonstrate that varying metal center from Cu to Zn in the phthalocyanine moiety has a negligible effect in the conductivity (∼5 × 10–7 S/cm), charge carrier density (∼1012 cm–3), charge carrier scattering rate (∼3 × 1013 s–1), and effective mass (∼2.3m 0) of majority carriers (holes). Notably, charge carrier transport is found to be anisotropic, with hole mobilities being practically null in-plane and finite out-of-plane for these 2D COFs.
Dithiine linkage formation via a dynamic and self-correcting nucleophilic aromatic substitution reaction enables the de novo synthesis of a porous thianthrene-based two-dimensional covalent organic ...framework (COF). For the first time, this organo-sulfur moiety is integrated as a structural building block into a crystalline layered COF. The structure of the new material deviates from the typical planar interlayer π-stacking of the COF to form undulated layers caused by bending along the C–S–C bridge, without loss of aromaticity and crystallinity of the overall COF structure. Comprehensive experimental and theoretical investigations of the COF and a model compound, featuring the thianthrene moiety, suggest partial delocalization of sulfur lone pair electrons over the aromatic backbone of the COF decreasing the band gap and promoting redox activity. Postsynthetic sulfurization allows for direct covalent attachment of polysulfides to the carbon backbone of the framework to afford a molecular-designed cathode material for lithium–sulfur (Li–S) batteries with a minimized polysulfide shuttle. The fabricated coin cell delivers nearly 77% of the initial capacity even after 500 charge–discharge cycles at 500 mA/g current density. This novel sulfur linkage in COF chemistry is an ideal structural motif for designing model materials for studying advanced electrode materials for Li–S batteries on a molecular level.
After complete genome sequencing, the diatom Thalassiosira pseudonana has become an attractive model organism for silica biomineralisation studies. Recent progress, especially with respect to ...intracellular silicic acid processing, as well as to the natures of the biomolecules involved in diatom cell wall formation, is described. On the one hand, considerable progress has been made with respect to silicon uptake by special proteins (SITs) from the surrounding water, as well as to the storage and processing of silicon before cell division. On the other hand, the discovery and characterisation of remarkable biomolecules such as silaffins, polyamines and--quite recently--of silacidins in the siliceous cell walls of diatoms strongly impacts the growing field of biomimetic materials synthesis.
One of the main problems of gas storage in porous materials is that many molecules of interest adsorb too weakly to be retained effectively. To enhance gas storage in metal–organic frameworks (MOFs), ...we propose the use of kinetic trapping, i.e., a process where the guest gas is captured in the voids at loading conditions and not released immediately at normal conditions. In this approach, the diffusion-limiting pore size and the framework flexibility have to be matched to the gas, requiring flexible pore apertures to be smaller than the van der Waals diameter of the trapped guest. We selected the Metal–Organic Framework Ulm University-4 (MFU-4) with a pore aperture of 2.52 Å as a model coordination framework and used it for storage of xenon (with van der Waals diameter of 4.4 Å). Although xenon atoms are substantially larger than the MOF pore aperture, MFU-4 could be loaded with xenon by applying moderately high gas pressures. This is demonstrated to be due to the pore flexibility as confirmed by computational studies. The xenon loading could be tuned (from 0 wt % to more than 44.5 wt %) by changing the loading parameters such as pressure, temperature, and time, and the xenon atoms remained inside the pores upon exposing the material to air atmosphere at room temperature. To understand the material behavior, TGA, XRPD, and 129Xe NMR spectroscopy and computational studies were carried out.
We report the one-pot mechanochemical synthesis of N-doped porous carbons at room temperature using a planetary ball mill. The fast reaction (5 minutes) between calcium carbide and cyanuric chloride ...proceeds in absence of any solvent and displays a facile bottom-up strategy that completely avoids typical thermal carbonization steps and directly yields a N-doped porous carbon containing 16 wt% of nitrogen and exhibiting a surface area of 1080 m2 g-1.
Tailoring the characteristics of gating transitions in the porous network, Ni
(ndc)
dabco (ndc = 2,6-naphthalenedicarboxylate, dabco = 1,4-diazabicyclo2.2.2octane), also termed DUT-8(Ni) (DUT = ...Dresden University of Technology), was achieved by systematically adjusting the critical synthesis parameters. The impact of the starting composition and solvent mixtures in the synthesis was found to critically affect the guest-response properties of the obtained materials. A comprehensive set of physical characterization methods, namely thermal analysis,
H NMR of digested crystals, solid state
C NMR, PXRD, SEM, IR and Raman spectroscopy shows that the crystallite size is a crucial factor, determining the differing characteristics such as "gate pressure" and adsorption capacity in the guest-responsive switching behaviour of DUT-8. Crystallites smaller than 500 nm in size retain the open form after removal of the guest molecules resulting in typical "Type Ia" isotherm, whereas crystallites larger than 1 μm transform into the "closed pore" form and therefore can show a characteristic "gate opening" behaviour during gas adsorption. The particle size distribution of DUT-8(Ni) can be tailored by changing the synthesis conditions and consequently the slope of the isotherm at the "gating step" is affected. The in depth analysis of synthesis conditions and switching behaviour is an important step towards a better understanding of the fundamental principles responsible for guest responsive porosity switching in the solid state.
A new asymmetric capacitor concept is proposed providing high energy storage capacity for only one charging direction. Size‐selective microporous carbons (w<0.9 nm) with narrow pore size distribution ...are demonstrated to exclusively electrosorb small anions (BF4−) but size‐exclude larger cations (TBA+ or TPA+), while the counter electrode, an ordered mesoporous carbon (w>2 nm), gives access to both ions. This architecture exclusively charges in one direction with high rectification ratios (RR=12), representing a novel capacitive analogue of semiconductor‐based diodes (“CAPode”). By precise pore size control of microporous carbons (0.6 nm, 0.8 nm and 1.0 nm) combined with an ordered mesoporous counter electrode (CMK‐3, 4.8 nm) electrolyte cation sieving and unidirectional charging is demonstrated by analyzing the device charge‐discharge response and monitoring individual electrodes of the device via in situ NMR spectroscopy.
Size matters: By coupling a carbon molecular sieve electrode with an ordered mesoporous carbon (CMK‐3), a new asymmetric capacitor concept with rectification function and high capacitance is realized. In situ NMR spectroscopy confirms the size exclusion and unidirectional charging mechanism.
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