Junwen Zhou

Rational design of a metal–organic framework host for sulfur storage in fast, long-cycle Li–S batteries

Unlike an intercalation cathode, which has an intrinsic host structure made of redox metal sites allowing the transport of Li+/e−, sulfur as a conversion cathode requires an additional host to store and immobilize the mobile redox centers, polysulfides. Metal–organic frameworks (MOFs) as a class of highly porous and well-defined crystalline materials are a promising platform to search for an effective host through rational design. With the appropriate selection of an electrolyte and a cutoff voltage range, sulfur stored in an appropriate MOF host can take advantage of both intercalation (fast and stable) and conversion (high energy density) cathodes. Herein, we describe a fast cathode with long cycle life based on sulfur and ZIF-8 nanocrystals. With 30 wt% sulfur loading in the electrode, it achieves remarkable discharge capacities of 1055 mA h g−1 (based on sulfur) at 0.1 C and 710 mA h g−1 at 1 C. The decay over 300 cycles at 0.5 C is 0.08% per cycle, prominent in long-cycle Li–S batteries. By comparing with another three distinct MOFs, MIL-53 (Al), NH2-MIL-53 (Al) and HKUST-1, as well as two sets of ZIF-8 with particle sizes in the micrometer range, it reveals that (i) the small particle size of the MOF host is appreciable to achieve a high capacity and (ii) small apertures, associated with functionalities in the open framework that have affinity with the polysulfide anions, can help achieve a stable cycling. We believe that the findings are general and applicable for the rational design of new hosts for sulfur in other porous material families to produce more effective and stable Li–S batteries.

MOF derived catalysts for electrochemical oxygen reduction

Developing noble metal free catalysts for the oxygen reduction reaction (ORR) is of critical importance for the production of low cost polymer electrolyte membrane fuel cells. In this paper, metal organic frameworks (MOFs) are used as precursors to synthesize ORR catalysts via pyrolysis in an inert atmosphere. The ORR performance is found to be closely associated with the metal/ligand combination in MOFs. The Co-imidazole based MOF (ZIF-67) derived catalyst exhibits the best ORR activity in both alkaline and acidic electrolytes. The Co cations coordinated by the aromatic nitrogen ligands in ZIF-67 may assist the formation of ORR active sites in the derived catalyst. The best ORR performance is obtained when the porosity of the derived catalyst is maximized, by optimizing the pyrolysis temperature and the acid leaching process. The performance of the best MOF derived catalyst is comparable to that of Pt/C in both alkaline and acidic electrolytes.

Preparation of Nanofibrous Metal–Organic Framework Filters for Efficient Air Pollution Control

Environmental challenges especially air pollution (particulate matter (PM) and toxic gases) pose serious threats to public health globally. Metal-organic frameworks (MOFs) are crystalline materials with high porosity, tunable pore size, and rich functionalities, holding the promise for poisonous pollutants capture. Here, nanocrystals of four unique MOF structures are processed into nanofibrous filters (noted as MOFilter) with high MOF loadings (up to 60 wt %). The MOFilters show high PM removal efficiencies up to 88.33 ± 1.52% and 89.67 ± 1.33% for PM2.5 and PM10, respectively, in the hazy environment, and the performance remains largely unchanged over 48 h of continuous filtration. For the first time, the interactions between such porous crystalline material and particulate pollutants were explored. These thin MOFilters can further selectively capture and retain SO2 when exposed to a stream of SO2/N2 mixture, and their hierarchical nanostructures can easily permeate fresh air at high gas flow rate with the pressure drop <20 Pa.

Exfoliation of Covalent Organic Frameworks into Few-Layer Redox-Active Nanosheets as Cathode Materials for Lithium-Ion Batteries

Covalent organic frameworks (COFs) have attracted growing interest by virtue of their structural diversity and tunability. Herein, we present a novel approach for the development of organic rechargeable battery cathodes in which three distinct redox-active COFs were successfully prepared and delaminated into 2D few-layer nanosheets. Compared with the pristine COFs, the exfoliated COFs with shorter Li+ diffusion pathways allow a significant higher utilization efficiency of redox sites and faster kinetics for lithium storage. Unlike diffusion-controlled manners in the bulk COFs, the redox reactions in ECOFs are mainly dominated by charge transfer process. The capacity and potential are further engineered by reticular design of COFs without altering the underlying topology. Specifically, DAAQ-ECOF exhibits excellent rechargeability (98% capacity retention after 1800 cycles) and fast charge-discharge ability (74% retention at 500 mA g-1 as compared to at 20 mA g-1). DABQ-ECOF shows a specific capacity of 210 mA h g-1 and a voltage plateau of 2.8 V.

Challenges and recent advances in MOF–polymer composite membranes for gas separation

Membrane technology has attracted tremendous attention in the field of gas separation due to its low cost and energy consumption. Polymer membranes are used in some industrial-scale gas separation processes, however, they often suffer a trade-off between permeability and selectivity. To overcome this limitation, porous materials with molecular sieve properties have been combined with polymers to give membranes with enhanced gas separation performance. Metal–organic frameworks (MOFs) are nanoporous materials possessing ultrahigh porosity, large surface area, structural diversity and rich functionalities, which make them promising candidates for gas separation. This review primarily focuses on the fabrication methods of MOF–polymer composite membranes including MOF-based mixed-matrix membranes (MMMs) and polymer supported MOF membranes. Recent progress in MOF membrane fabrication, incorporating the challenges and difficulties faced, are presented. Furthermore, corresponding solutions and strategies are given in detail to offer instructions to fabricate membranes with ideal morphology and performance.

Partitioning MOF-5 into Confined and Hydrophobic Compartments for Carbon Capture under Humid Conditions

Metal-organic frameworks (MOFs), by virtue of their remarkable uptake capability, selectivity, and ease of regeneration, hold great promise for carbon capture from fossil fuel combustion. However, their stability toward moisture together with the competitive adsorption of water against CO2 drastically dampens their capacity and selectivity under real humid flue gas conditions. In this work, an effective strategy was developed to tackle the above obstacles by partitioning the channels of MOFs into confined, hydrophobic compartments by in situ polymerization of aromatic acetylenes. Specifically, polynaphthylene was formed via a radical reaction inside the channels of MOF-5 and served as partitions without altering the underlying structure of the framework. Compared with pristine MOF-5, the resultant material (PN@MOF-5) exhibits a doubled CO2 capacity (78 vs 38 cm(3)/g at 273 K and 1 bar), 23 times higher CO2/N2 selectivity (212 vs 9), and significantly improved moisture stability. The dynamic CO2 adsorption capacity can be largely maintained (>90%) under humid conditions during cycles. This strategy can be applied to other MOF materials and may shed light on the design of new MOF-polymer materials with tunable pore sizes and environments to promote their practical applications.

In Situ Growth of MOFs on the Surface of Si Nanoparticles for Highly Efficient Lithium Storage: Si@MOF Nanocomposites as Anode Materials for Lithium-Ion Batteries

A simple yet powerful one-pot strategy is developed to prepare metal-organic framework-coated silicon nanoparticles via in situ mechanochemical synthesis. After simple pyrolysis, the thus-obtained composite shows exceptional electrochemical properties with a lithium storage capacity up to 1050 mA h g(-1), excellent cycle stability (>99% capacity retention after 500 cycles) and outstanding rate performance. These characteristics, combined with their high stability and ease of fabrication, make such Si@MOF nanocomposites ideal alternative candidates as high-energy anode materials in lithium-ion batteries.

Shaping of Metal–Organic Frameworks: From Fluid to Shaped Bodies and Robust Foams

The applications of metal-organic frameworks (MOFs) toward industrial separation, catalysis, sensing, and some sophisticated devices are drastically affected by their intrinsic fragility and poor processability. Unlike organic polymers, MOF crystals are insoluble in any solvents and are usually not thermoplastic, which means traditional solvent- or melting-based processing techniques are not applicable for MOFs. Herein, a continuous phase transformation processing strategy is proposed for fabricating and shaping MOFs into processable fluids, shaped bodies, and even MOF foams that are capable of reversible transformation among these states. Based on this strategy, a cup-shaped Cu-MOF composite and hierarchically porous MOF foam were developed for highly efficient catalytic C-H oxidation (conv. 76% and sele. 93% for cup-shaped Cu-MOF composite and conv. 92% and sele. 97% for porous foam) with ease of recycling and dramatically improved kinetics. Furthermore, various MOF-based foams with low densities (<0.1 g cm(-3)) and high MOF loadings (up to 80 wt %) were obtained via this protocol. Imparted with hierarchically porous structures and fully accessible MOFs uniformly distributed, these foams presented low energy penalty (pressure drop <20 Pa, at 500 mL min(-1)) and showed potential applications as efficient membrane reactors.

Roll‐to‐Roll Production of Metal‐Organic Framework Coatings for Particulate Matter Removal

A powerful roll-to-roll hot-pressing strategy for mass production of metal-organic framework (MOF)-based filters (MOFilters) using various MOF systems with ranges of substrates is presented. Thus-obtained MOFilters show superior particulate matter removal efficiency under desired working temperatures. Such versatile MOFilters can be scaled up and purposely designed, which endows MOFilters with great potentials in both residential and industrial pollution control.

The impact of the particle size of a metal–organic framework for sulfur storage in Li–S batteries

The particle size of an electrode material is known to play an essential role in its electrochemical performance in Li-ion batteries. In Li–S batteries, porous host materials are applied to store sulfur and suppress the escape of polysulfides; yet the particle size of the host as an important parameter remains largely unexplored. Herein we chose ZIF-8, a metal–organic framework (MOF) proved promising for sulfur storage, as the proof-of-concept prototype, and systematically synthesized five sets of ZIF-8 samples of different particle sizes (from 1 μm), using them as S@MOF cathodes. The results show that sulfur utilization increases monotonically with the decrease of ZIF-8 particle size ( 950 mA h g−1 at 0.5 C), while the best cycling stability (75% over 250 cycles at 0.5 C) is achieved with a moderate size (∼200 nm).