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Elucidating the Stability of Two Metal-Organic Frameworks toward Carbon Dioxide Sorption: A Comparative Study

Metal-organic frameworks (MOFs) are coordination networks consisting of organic ligands and metal cores. They possess crystalline structures with metal complexes as the basic building blocks. These complexes assemble together and extend periodically to form the MOF structures. MOFs represent a family of highly porous materials with ultrahigh surface area (typically >1000 m2 g-1). Other attractive characteristics for MOFs are abundant active metal cores and unique porous structures with tunable pore width, useful for gas storage applications.

Capturing carbon dioxide has evolved into an intriguing research area, mainly due to environmental concerns triggered by high levels of greenhouse gas emissions. Some MOFs have already been explored as carbon dioxide storage materials and exhibited storage capability exceeding that of conventional absorbents (e.g. amines). Aside from the absorption capacity of carbon dioxide, the performance stability over prolonged operation periods is another figure of merit for MOF-based absorbents. However, there are limited studies in this area. Now for the first time, research groups led by Zeng and Zhao from National University of Singapore compared the performance stability of two representative MOFs, HKUST-1 and UiO-66(Zr). The unit cell of the two MOFs are shown in the inset of Figure a.

The two aforementioned MOFs were subjected to 500 carbon dioxide absorbing and desorbing cycles (Figure a). The carbon dioxide uptake amount of the two MOFs was gauged at specific cycle numbers (Figure b). Whilst HKUST-1 displayed a consistent decreasing storage capacity with increasing cycle number, the capacity of UiO-66(Zr) fluctuated but remained relatively constant. The results clearly indicate that HKUST-1 is more vulnerable and instable than UiO-66(Zr) during long-term working cycles.

The authors then investigated the mechanisms associated with the different stability performances. They first observed that the surface area of HKUST-1 decreased 24% to 1270 m2 g-1 after the stability test, whereas that of UiO-66(Zr) remained relatively intact. Moisture-induced structural collapse was excluded as a possible reason by carrying out a control experiment with ultra-pure and dry hydrogen gas. The authors then exploited multi-frequency atomic force microscopy and concluded that the difference in elastic modulus of the two MOF crystals played an important role in determining the corresponding MOF durability. UiO-66(Zr) has an elastic modulus (ca. 28 GPa) much higher than that of HKUST-1 (ca. 19 GPa), meaning that the former is more elastic than the latter. The high elasticity of UiO-66(Zr) can efficiently buffer the volumetric deformation caused by carbon dioxide absorption and desorption, preventing UiO-66(Zr) crystals from structural failure.

Figure. (a) Illustration of one cycle of the carbon dioxide absorption-desorption test. The inset shows where one carbon dioxide molecule resides in the corresponding MOFs. (b) The evolution of carbon dioxide uptake capacity (blue) and surface area (black) of HKUST-1 and UiO-66(Zr).

This work is expected to provide general guidelines on studying the structural stability of other MOFs with applications associated with gas storage and separation.

 

To find out more please read:

Structure Failure Resistance of Metal-organic Frameworks toward Multiple-cycle CO2 Sorption

Zhigang Hu, Yao Sun, Kaiyang Zeng, and Dan Zhao

DOI: 10.1039/c7cc04313a

About the author:

Tianyu Liu is a Ph.D. in chemistry graduated from University of California-Santa Cruz. He is passionate about scientific communication to introduce cutting-edge researches to both the general public and the scientists with diverse research expertise. He is a web writer for the Chem. Commun. and Chem. Sci. blog websites. More information about him can be found at http://liutianyuresearch.weebly.com/.

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Dissolving and Stabilizing the Precursor of Graphene in Organic Solvents

Graphene, a two-dimensional single-layer graphite sheet, has aroused worldwide attention since the last decade. Its ultrahigh electrical and thermal conductivities, high mechanical stiffness and unique band structure have attracted extensive research efforts to develop graphene-based electronics, photonics, printing materials etc. Currently, among various strategies, the wet-chemical method still remains the most practical protocol for large-scale production of graphene in laboratories. This process in general involves two steps: the oxidative exfoliation of graphite, a.k.a. Hummers’ method, followed by reduction of the oxidized graphite sheets. Graphite oxide (GO), possessing a layered structure analogous to graphite but with rich oxygen functionalities (such as hydroxyl and carboxyl groups) anchored on each layer, is the product of the first step and thus serves as a precursor of graphene.

As the aforementioned wet chemical method is usually carried out in water, GO is primarily stored as aqueous-based colloidal dispersions. However, GO is reported to be chemically unstable in water since water molecules can react with electropositive carbons of GO. Though the reaction is not rapid, it partially removes the oxygen functionalities and breaks the carbon matrix, which eventually forces GO to precipitate and reduces the shelf life of the GO precursor.

Recently, Shi and coworkers from Tsinghua University have successfully prolonged the lifetime of GO by dispersing it in organic solvents. During the last purification step of the Hummers’ method, instead of using de-ionized water, anhydrous ethanol was utilized to rinse the GO product and obtain ethanol-wetted GO. X-ray diffraction revealed that ethanol molecules existed in the inter-layer space between adjacent layers. The ethanol-wetted GO could be readily dissolved in propylene carbonate, an organic solvent, for concentrations ranging from 0.1 mg mL-1 to 40 mg mL-1 (Figures a and b). More importantly, GO could be stored in propylene carbonate for at least a month without a colour change, whilst the colour of aqueous GO dispersion discernibly darkened (Figure c). Spectroscopic studies indicated that the colour change was attributed to the loss of oxygen functionalities. The results unambiguously prove that GO in propylene carbonate is much more stable than GO in water.

Figure. (a) Dissolution of ethanol-wetted GO in propylene carbonate. (b) GO colloidal dispersions with various concentrations. (c) Color evolution of GO dispersions (1 mg mL-1) with water and propylene carbonate as solvents before and after storing for 28 days under ambient conditions.

Aside from propylene carbonate, dimethyl sulfoxide, ethylene glycol and N,N-dimethylformamide are solvents that can dissolve the ethanol-wetted GO. The successful stabilization of GO colloidal dispersions could ensure the steady production of graphene in laboratories, as well as reveal new opportunities to develop GO-based devices.

To find out more please read:

Organic Dispersions of Graphene Oxide with Arbitrary Concentrations and Improved Chemical Stability

Wencheng Du, Mingmao Wu, Miao Zhang, Guochuang Xu, Tiantian Gao, Liu Qian, Xiaowen Yu, Fengyao Chi, Chun Li and Gaoquan Shi

DOI: 10.1039/c7cc04584k

About the author:

Tianyu Liu is a Ph.D. in chemistry graduated from University of California-Santa Cruz. He is passionate about scientific communication to introduce cutting-edge researches to both the general public and the scientists with diverse research expertise. He is a web blog writer for the Chem. Commun. and Chem. Sci. blog websites. More information about him can be found at http://liutianyuresearch.weebly.com/.

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Releasing A Pungent Anti-cancer Reagent with A Trisulfide Linker Inspired by Garlic

People who love the taste of garlic are often annoyed by its lingering smell. While there are various tips to get rid of this unpleasant odor, have you ever thought that this garlic aroma brings you chemical compounds that are potent anti-cancer reagents?

Diallyl trisulfide, one of the natural occurring components rendering the flavor of garlic, is able to release hydrogen sulfide (H2S) upon contacting with thiol compounds (i.e., organic molecules with –SH functional groups). H2S is a pungent gas that one might never forget after sniffing a rotten egg. However, this “notorious” gas, when at low concentrations, is reported to be friendly to our bodies. It relaxes vascular smooth muscle, reduces blood pressure, lowers risk associated with cancer as well as protects gastrointestinal, nervous and immune systems. All the aforementioned benefits of H2S have aroused worldwide efforts in developing H2S-releasing and bio-compatible materials that mimic the natural products for pharmaceutical applications.

Davis, Quinn and co-workers from Monash University, Australia and University of Warwick, United Kingdom, recently published a paper in Chemical Communications that reports a trisulfide-linked organic polymer capable of releasing H2S when meets –SH groups. As shown in the scheme below, the synthesized polymer is composed of three parts: a polyethylene glycol (PEG) unit on the left (in blue), a cholesterol (CHOL) group on the right (in orange), and a linker (in black) joining the two ends. PEG and CHOL are chosen mainly due to their bio-compatibility. By changing the structure of the middle linker, the authors obtained three types of polymers that behave differently when mixing with thiol compounds. The trisulfide linker (denoted as T) enables release of H2S gas and initiates polymer degradation. The disulfide linker (denoted as D) allows polymer degradation only. The amide linker (denoted as C) containing no sulfide atoms is inert to the thiol exposure.

Scheme. The chemical structure of the synthesized polymers with different linkers.

Experiments showed that the T-linked polymers are capable of releasing H2S both in vitro and in vivo.

A fluorescent probe, which can be reduced by H2S and becomes fluorescent, is applied to detect the existence of H2S. As shown in Figure a, the trisulfide linked polymers tested in vitro exhibited the highest fluorescence when mixing with L-cysteine (a thiol compound to trigger H2S generation). For the in vivo measurements, the authors incubated HEK293 cells with the polymers and the probe. Similar as the in vitro results, the fluorescence intensity of the cells containing the T-linked polymers is the highest (Figure b). Both the in vitro and in vivo results unambiguously proved that the presence of the T-linker was responsible for generating H2S. Additionally, another set of tests using Nile Red confirmed the biodegradability of the T-linked polymers.

Figure. (a) Fluorescence spectra collected from different systems in vitro. The inset shows the chemical reaction between the probe (SF4) and H2S that displays fluorescence. (b) Fluorescence intensity of different polymers over time in HEK293 cells.

The developed tri-sulfide linker may allow the mimicry of endogenous biosynthesis, the initiation of discrete signaling events and the synthesis of next-generation pharmaceutical excipients.

 

To find out more please read:

Garlic-inspired Trisulfide Linkers for Thiol-stimulated H2S Release
Francesca Ercole, Michael R. Whittaker, Michelle L. Halls, Ben J. Boyd, Thomas P. Davis and John F. Quinn
DOI: 10.1039/c7cc03820h

About the author:

Tianyu Liu is a Ph.D. in chemistry graduated from University of California-Santa Cruz. He is passionate about scientific communication to introduce cutting-edge researches to both the general public and the scientists with diverse research expertise. He is a web writer for the Chem. Commun. and Chem. Sci. blog websites. More information about him can be found at http://liutianyuresearch.weebly.com/.

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A Promising Cathode Material for Magnesium-ion Batteries Has Been Identified

Research associated with batteries is gaining increasing attention and extensive efforts in recently decades, partly due to the development of sustainable energy to combat a series of problems including fossil fuel depletion, environmental pollution and global warming. Batteries are indispensable energy storage devices for the utilization of sustainable energy (e.g., solar and wind energy). One of the battery’s cutting-edge research topics is to achieve novel batteries with higher capacity (a figure-of-merit to measure how much electrical energy a battery can store) and better reliability than the lithium-ion batteries that currently dominate the battery market.

In the past decade, batteries based on magnesium ions, termed as magnesium-ion batteries, are emerging. The magnesium-ion batteries possess at least two advantages over lithium-ion batteries. Firstly, their typical anode material, magnesium metal, has a theoretical capacity of 3833 mAh/cm3. This value is much higher than that of graphite, a conventional anode material for lithium-ion batteries. Secondly, the formation of metal dendrite on anode surface can be avoided by replacing lithium metal with magnesium metal. Metal dendrites grow from anodes can eventually touch cathodes, causing electric short circuits and triggering fire and explosion. Therefore, magnesium-ion batteries are safer than lithium-ion batteries. However, nothing can be perfect. The limited mobility of Mg2+ of cathode materials greatly reduces the capacity (particularly at fast charging rates) and practicability of the magnesium-ion batteries.

Now Rong et al. has published an article in Chemical Communications stating that a promising cathode material capable of fast conducting Mg2+ for magnesium-ion batteries has been identified. The material is a molybdenum phosphate compound with a chemical formula of Mo3(PO4)3O. It is composed of several edge-sharing MoO6 octahedra, corner-sharing MoO5 trigonal bipyramids, MoO4 tetrahedra, and PO4 tetrahedra. Using advanced simulation and computation techniques (i.e., the first-principles density functional theory), the authors first proved that Mg2+ can stably reside in some interstitial sites among the aforementioned polyhedra, indicating the identified compound is active for Mg2+ storage. In addition, the authors plotted two possible pathways for Mg2+ diffusion during charge and discharge processes (shown in the Figure). As illustrated in Figure a1, the first one is an inner-channel path along the b-axis. The second one is an inter-channel path along the c-axis.

The most striking feature of the path #1 is its ultra-low activation barrier (i.e., the highest potential energy that a Mg2+ need to overcome when diffusing) of only ~80 meV (Figure a2). Such a low diffusion barrier is expected to allow facile Mg2+ diffusion within the bulk of Mo3(PO4)3O, which can boost the capacity of the magnesium-ion batteries particularly at elevated charging rates. On the contrary, the activation barrier of the path #2 is as high as ~1200 meV. The authors claimed that the Mg2+ diffusion along the path #2 “should be ~1018 times less frequent than” the path #1.

 

 

Figure (a1) schematic of the Mg2+ diffusion path #1 and (a2) its corresponding diffusion potential barrier distribution along the way. (b1) Schematic of the Mg2+ diffusion path #2 and (a2) its corresponding diffusion potential barrier distribution along the way.

 

At last, the authors estimated the theoretical average potential that Mo3(PO4)3O can reach is 1.98 V, corresponding to a promising energy density of 173 Wh/kg. Although the proposed phosphate is hypothetical, the investigation of its stability reveals the possibility that this material can be experimentally synthesized.

To find out more please read:

Fast Mg2+ Diffusion in Mo3(PO4)3O for Mg Batteries
Ziqin Rong, Penghao Xiao, Miao Liu, Wenxuan Huang, Daniel C. Hannah, William Scullin, Kristin A. Persson and Gerbrand Ceder
DOI: 10.1039/c7cc02903a

About the author:

Tianyu Liu is a Ph.D. in chemistry graduated from University of California-Santa Cruz. He is passionate about scientific communication to introduce cutting-edge researches to both the general public and the scientists with diverse research expertise. He is a web writer for the Chem. Commun. and Chem. Sci. blog websites. More information about him can be found at http://liutianyuresearch.weebly.com/.

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A “Cube-in-tube” Carbon-Metal Oxide Lithium-ion Battery Composite Electrode with Outstanding Capacity and Durability

Lithium-ion batteries are indispensable for powering a number of electronics (e.g. cell phones, laptops and even electric vehicles) used in the modern society. The key components of a lithium-ion battery are its two electrodes (anode and cathode) because they largely dictate the amount of electrical energy (measured by a parameter called “capacity”) the battery can hold. Increasing the capacity as well as prolonging the lifetime of lithium-ion batteries are practically desirable. Material scientists worldwide are searching for electrode materials to achieve the goal.

In the past decade, a diverse array of metal oxides has been developed as lithium-ion battery anodes with promising performance. These anodes have exhibited considerably higher capacity (~1000 mAh/g) than the current commercial anode material, graphite (372 mAh/g). However, two major drawbacks of metal oxides, namely the limited electrical conductivity and the short lifetime, impede their feasibility for practical applications. While the poor electrical conductivity is an intrinsic physical property of most metal oxides, their short life time is caused by the large volumetric deformation during charge and discharge processes. The deformation will eventually trigger pulverization of electrodes and lead to loss of capacity. Therefore, developing novel strategies that manage to turn metal oxides to viable electrode candidates with satisfying lifetime becomes necessary.

Now writing in Chemical Communications, a research group led by Professor Liqiang Mai and Professor Qi Li from Wuhan University of Technology, China demonstrated a metal oxide-carbon composite anode that exhibited both high capacity and super-long lifetime. The structure of this composite is a “cube-in-tube” configuration (Figure 1): the manganese oxide nanoparticle-embedded carbon “tubes” encapsulate the CoSnO3 (a binary metal oxide) “cubes”. This unique composite electrode delivered a maximal capacity of 960 mAh/g at a current density of 0.1 A/g (Figure 2a), around three times higher than the theoretical capacity of graphite. More impressively, as shown in Figure 2b, the electrode displayed outstanding stability with ~99% of capacity retained after 1500 consecutive charge and discharge cycles (roughly equivalent to four years’ use), much higher than those of the current commercial products and other laboratory-developed composites.

Figure 1. Schematic of the synthesis strategy and the morphology of the “cube-in-tube” metal oxide-carbon composite lithium-ion battery electrode.

Figure 2. (a) Plot of capacity at different current densities; (b) The stability performance evaluated at 2 A/g.

The authors attributed the electrode’s excellent durability to two reasons. Firstly, the hollow structures (both the “tube” and the “cube”) provide adequate empty space to accommodate volumetric change of metal oxides. Secondly, the soft nature of carbon renders its ability to serve as a mechanical buffer layer. Both aspects reduce the possibility of structural pulverization and promote long lifetime.

To find out more please read:

Facile Electrospinning Formation of Carbon-confined Metal Oxide Cube-in-tube Nanostructures for Stable Lithium Storage

Ziang Liu, Ruiting Guo, Jiashen Meng, Xiong Liu, Xuanpeng Wang, Qi Li, Liqiang Mai

DOI: 10.1039/C7CC0327A

About the author:

Tianyu Liu is a Ph.D. in chemistry graduated from University of California-Santa Cruz, United States. He is passionate about scientific communication to introduce cutting-edge researches to both the general public and the scientists with diverse research expertise. He is a web writer for the Chem. Commun. and Chem. Sci. blog websites. More information about him can be found at http://liutianyuresearch.weebly.com/.

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