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How Does Nano-Confinement Boost Oxygen-Reduction Electrocatalytic Activity?

Why do biological enzymes exhibit superior catalytic activity? Well, this is mainly because they can firmly anchor reactants into catalytically active pockets using their three-dimensional structures. Inspired by this nano-confinement effect, scientists have developed “nanozymes,” a family of artificial enzymes.

A group of scientists from the University of New South Wales (Australia), University of Utah (USA), and Ruhr-Universität Bochum (Germany) investigated the interplay between the degree of nano-confinement and oxygen reduction reaction (ORR) activity. They discovered that ORR activity only scaled proportionally to the degree of confinement at low overpotentials (ORR theoretical potential: 1.23 V vs. RHE). The results have been published in Chemical Science (doi: 10.1039/C9SC05611D).

Synthesized by etching Ni from Pt-Ni alloy nanoparticles, metallic Pt nanoparticles possessing channels of different opening sizes served as the nanozymes with different confinement degrees. When the Ni content increased, the channel of the etched nanoparticles widened. Specifically, Pt nanozymes prepared from Pt-Ni nanoparticles with Pt/Ni = 1/1.5 (NZsmall) contained 69% channel openings smaller than 2 nm (Fig. 1a); Whereas those from the precursors with Pt/Ni = 1/2.5 (NZmedium) and 1/3 (NZlarge) had 52% (Fig. 1b) and 34% (Fig. 1c) <2-nm-wide channels, respectively. Therefore, the nano-confinement degrees of the three nanozymes followed the sequence of NZsmall>NZmedium>NZlarge.

Figure 1. High-resolution transmission electron microscopy images (left) and channel diameter distribution profiles (right) of (a) NZsmall, (b) NZmedium, and (c) NZlarge.

The ORR activity of the three nanozymes strongly depended on the magnitude of the overpotential. With the outer surfaces passivated by surfactants, the ORR activities of the nanozymes were only associated with O2 reduction within their channels. At low overpotentials (Fig. 2, inset), NZsmall had the highest ORR activity among all the catalysts evaluated by the authors, as indicated by its lowest kinetic current. At high overpotentials or low applied biases; however, the ORR activity of NZmedium and NZlarge rapidly increased (Fig. 2). NZmedium and NZlarge became more active than NZsmall at potentials lower than 0.82 V (vs. RHE) and 0.80 V (vs. RHE), respectively.

Figure 2. The potential (E)-dependence of kinetic current density (jk). Inset: low-overpotential (high potentials) region. Legends: solid black – NZsmall; solid blue – NZmedium; solid red – NZlarge; open black – mesoporous Pt nanoparticles without nano-channels. The consistently small absolute jk of the mesoporous nanoparticles reflected its relatively low ORR activity.

The finite element simulation revealed the underlying mechanism of the experimental results. At low overpotentials, the ORR activity was governed by the kinetics of O2-reduction. Due to high charge density, the local proton concentration inside the channels of NZsmall was the highest, leading to the fastest reaction and the highest ORR activity. At high overpotentials, the ORR activity became mass-transport limited. Nanozymes with wide channel openings, that is, NZmedium and NZlarge, allowed a large amount of O2 to diffuse into the channels, which enhanced O2 supply and augmented ORR activity.

This work unveiled the potential-dependence of the ORR catalytic activities of porous Pt nanoparticles under different degrees of nano-confinement. This insight could rationalize and further enable the design of nanozymes with tailorable ORR activities.

 

To find out more, please read:

The Importance of Nanoscale Confinement to Electrocatalytic Performance

Johanna Wordsworth, Tania M. Benedetti, Ali Alinezhad, Richard D. Tilley, Martin A. Edwards, Wolfgang Schuhmann, and J. Justin Gooding

Chem. Sci., 2019, DOI: 10.1039/C9SC05611D

 

Tianyu Liu acknowledges Zac Croft at Virginia Tech, U.S., for his careful proofreading of this post.

 

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in Chemistry from the University of California, Santa Cruz, in the United States. He is passionate about the communication of scientific endeavors to both the general public and other scientists with diverse research expertise to introduce cutting-edge research to broad audiences. He is a blog writer for Chem. Comm. and Chem. Sci. More information about him can be found at http://liutianyuresearch.weebly.com/.

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Can Al3+ Indeed Intercalate Layered Metal Oxides in Aqueous Electrolytes?

Intercalation of multivalent ions, e.g., Al3+, has received increasing attention as an energy-boosting strategy for rechargeable batteries. The charge storage process of the wide-spread Li-ion batteries relies on Li+ intercalating electrodes with layered structures. Multivalent-ion batteries could accommodate more charges than Li-ion batteries because their ions carry more charge than Li+. This concept, however, has now been challenged by a research team led by BenoÎt Limoges and Véronique Balland of Université de Paris, France. Their mechanistic investigation, published in Chemical Science, revealed that Al3+ ions were unable to intercalate electrodes in aqueous electrolytes.

The authors selected TiO2 arrays as the object of their study. These ~1 µm-high arrays were grown using the glancing angle deposition technique. By applying negative potentials to the TiO2 arrays, cations such as Al3+ could diffuse through the inter-array slits and interact with TiO2 (Figure 1).

Figure 1. Structure of the TiO2 arrays. (left) Scanning electron micrographs and (right) a cartoon illustrating ion diffusion pathways.

Electrochemistry tests elucidated that the charge-storage process of TiO2 in an Al3+-containing aqueous electrolyte correlated to proton intercalation. This conclusion was mainly based on the nearly identical cyclic voltammograms (Figure 2, top) and capacity vs. potential curves (Figure 2, bottom) of the TiO2 arrays in both AlCl3 and acetic acid aqueous electrolytes. Since acetic acid solution had no Al3+, the observed charge-storage activity could not be attributed to Al3+ intercalation. Instead, the authors argued that protons dissociated either from hydrated Al3+ cations, [Al(H2O)6]3+ or acetic acid must intercalate TiO2 and result in the observed charge-storage capacities.

Figure 2. (top) Cyclic voltammograms and (bottom) capacity vs. potential curves of TiO2 in (left) Al3+-containing and (right) acetic acid aqueous electrolytes. The electrolytes are 0.3 M KCl with different concentrations of AlCl3 or acetic acid: 0 M (black), 25 mM (blue), 50 mM (purple), 100 mM (magenta), and 250 mM (red).

The authors believe that the misconception of Al3+ intercalation is due to the overlooking of Al3+ hydration, which is inevitable when Al3+ is present in aqueous electrolytes. Removing the water shell (a prerequisite for ion intercalation) is energy costly for Al3+ because of the strong binding force between water molecules and Al3+. Additionally, even if Al3+ ions intercalate TiO2, their movement is strongly hindered by Coulombic interactions within the TiO2 lattice. The immobilized intercalated ions would then block other ions from entering the TiO2 lattice. Together, both factors prevent Al3+ from intercalating into TiO2.

In summary, this work demonstrates that the charge-storage capacity of TiO2 in Al3+-containing aqueous electrolytes is most probably due to proton intercalation. This conclusion also applies to other multivalent cations, including Zn2+ and Mn2+, as shown in this work.

 

To find out more, please read:

On the Unsuspected Role of Multivalent Metal Ions on the Charge Storage of A Metal Oxide Electrode in Mild Aqueous Electrolytes

Yee-Seul Kim, Kenneth D. Harris, BenoÎt Limoges, and Véronique Balland

Chem. Sci., 2019, doi: 10.1039/c9sc02397f

Tianyu Liu acknowledges Zachary L. Croft of Virginia Tech, the U.S., for his constructive comments on this post.

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in Chemistry from University of California, Santa Cruz in the United States. He is passionate about the communication of scientific endeavors and cutting-edge research to both the general public and other scientists with diverse research expertise. He is a blog writer for Chem. Commun. and Chem. Sci. More information about him can be found at http://liutianyuresearch.weebly.com/.

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Making Gyroid Polymer Films to Speed up Proton Conduction

Proton exchange membranes (PEMs) are essential to the functionality of fuel cells. They conduct protons in electrolytes and drive electricity generation by oxidizing fuels. Following the success of Nafion® –– a family of commercial proton-conductive fluoropolymers –– materials researchers around the globe are developing innovative PEMs with high proton conductivities and affordable prices.

A group of Japanese researchers has recently synthesized self-standing polymer films with a gyroid nanostructure. These films possess two unique characteristics that other PEMs rarely have: a high proton conductivity in the order of 10-1 S/cm and retention of the conductivity across a wide temperature range (20-120 °C). This finding has been published in Chem. Sci. (doi: 10.1039/C9SC00131J).

The authors used a tailor-made macromolecule, Diene-GZI (Figure 1a), as the building block. It had an amphipathic structure, with one end being a hydrophilic zwitterionic group and another end of a hydrophobic alkyl chain. When mixed with bis(trifluoromethanesulfonyl)imide and water, multiple Diene-GZI molecules could assemble together into a gyroid network –– an infinitely periodic minimal surface (Figure 1b). After the self-assembly, ultra-violet-irradiation-induced polymerization solidified the morphology of the gyroid nanostructure.

Figure 1. (a) The molecular structure of Diene-GZI. (b) Solidification of the self-assembled gyroid via polymerization.

The high proton conductivity of the polymer film originated from its three-dimensional gyroid structure. Since the gyroid surface was densely coated with the hydrophilic zwitterionic chains, the film could readily uptake as high as 15.6 wt.% of water at a relative humidity of 90%. The adsorbed water layers formed a three-dimensional continuous pathway along the gyroid surface, serving as proton-conduction expressways and resulting in a high conductivity in the order of 10-1 S/cm. Due to the strong binding force between water and the zwitterionic groups, heating the polymer film to 120 °C did not decrease the water content significantly, and thus, the proton conductivity remained high. Additionally, the control films with no gyroid structures were unable to compete with the gyroid film in terms of proton conductivities within the measured temperature range (Figure 2).

Figure 2. The dependence between proton conductivities and temperature. Legends: red solid circles – gyroid film; others – control samples without the gyroid nanostructure.

This work highlights the critical role of rational design of raw materials to augment the proton conductivities of PEMs. The advantage of the gyroid phase in speeding up ion diffusion could also inspire innovative materials in applications demanding ultrafast ion transport, e.g., supercapacitor electrodes.

 

To find out more, please read:

Gyroid Structured Aqua-Sheets with Sub-Nanometer Thickness Enabling 3D Fast Proton Relay Conduction

Tsubasa Kobayashi, Ya-xin Li, Ayaka Ono, Xiang-bing Zeng, and Takahiro Ichikawa

Chem. Sci., 2019, 10, 6245-6253

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in Chemistry from University of California, Santa Cruz in the United States. He is passionate about scientific communication to introduce cutting-edge research to both the general public and scientists with diverse research expertise. He is a blog writer for Chem. Commun. and Chem. Sci. More information about him can be found at http://liutianyuresearch.weebly.com/.

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A Single Nickel Site Reduces Nitrate Ions to Nitrogen Gas

Surface runoff of agricultural or landscape areas with excessive nitrate fertilizers has resulted in increasing nitrate ion concentrations in freshwater. This issue leads to eutrophication and algae blooms that significantly threaten aquatic lives and ecosystems. Eliminating nitrate ions is thus necessary to address the nitrate-borne water pollution. Nature presents a delicate yet complicated process for turning NO3 into N2 by four metalloenzymes. Is it possible to develop an artificial method with just one catalyst to drive the same process?

Lee, Baik, and coworkers at the Institute for Basic Science (IBS) and Korea Advanced Institute of Science and Technology (KAIST) answered yes. The researchers synthesized a square-planar nickel(II)-based complex, (PNP)Ni(ONO2) (PNP = N[2-P’Pr-4-methyl-C6H3]2), which converted NO3 to N2 in the presence of CO and NO. Ni2+ is the only active site. This breakthrough has been published in Chemical Science (DOI: 10.1039/C9SC00717B).

The reaction mechanism involves four successive redox reactions among (PNP)Ni(ONO2), CO, and NO (Figure 1). The first two steps consecutively transfer two oxygen atoms from (PNP)Ni(ONO2) to CO at room temperature, yielding two molecules of CO2 and a Ni-nitrosyl complex, (PNP)Ni(NO). Afterwards, the (PNP)Ni(NO) undergoes a disproportionation reaction with NO, generating (PNP)Ni(NO2) and N2O. The as-formed N2O interacts with the remaining (PNP)Ni(NO) and is eventually reduced to N2. The yield of N2 is 46% (based on the amount of N2O).

Figure 1. The schematic shows the artificial nitrate reduction with a Ni(II)-based complex, (PNP)Ni(ONO2), as the catalyst. Pr: propyl group.

Besides nitrate reduction, the (PNP)Ni(ONO2) could act as a potential alternative to the platinum-containing catalysts used in the catalytic converters of gasoline cars, because it transforms hazardous CO and NO into less toxic CO2 and N2.

To find out more please read:

One Metal is Enough: A Nickel Complex Reduces Nitrate Anions to Nitrogen Gas

Jinseong Gwak, Seihwan Ahn, Mu-Hyun Baik and Yunho Lee

Chem. Sci., 2019, 10, 4767-4774

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in Chemistry from University of California, Santa Cruz in the United States. He is passionate about scientific communication to introduce cutting-edge research to both the general public and scientists with diverse research expertise. He is a blog writer for Chem. Commun. and Chem. Sci. More information about him can be found at http://liutianyuresearch.weebly.com/.

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Learning from Nature: A Cu(II)-Porphyrin Complex Produces Oxygen Gas from Water at Ultra-Small Overpotential

Sunlight-assisted water splitting represents a sustainable way to convert solar energy into chemical energy in hydrogen and oxygen gases. Due to its high activation energy, the oxygen evolution reaction (OER) requires large overpotential for initiation. Developing suitable OER catalysts to reduce the overpotential thus becomes instrumental for the feasibility of solar energy harvesting.

Recently, a group of scientists led by Rui Cao from Renmin University of China, and Shaanxi Normal University, China, has developed a water-soluble Cu(II)-porphyrin complex as a high-performance OER catalyst. This breakthrough has been published in Chemical Science (DOI: 10.1039/C8SC04529A).

Inspired by the molecular structure of a natural OER catalyst in the photosynthesis system – photosystem II (PSII), the researchers designed a Cu2+-coordination compound with a porphyrin ligand, tetrakis(4-N-methylpyridyl)porphyrin (Figure 1a), which mimics the structure of PSII. This biomimetic Cu2+-complex exhibits outstanding catalytic OER activity in a phosphate buffer solution at pH=7.0. The current of the cyclic voltammogram of the Cu2+-complex increases sharply (due to O2 evolution) at an onset potential of 1.13 V vs. normal hydrogen electrode (Figure 1b), corresponding to an OER overpotential of 310 mV. For comparison, the cyclic voltammograms of a blank buffer solution and a CuSO4-containing buffer solution show no pronounced current enhancement (Figure 1b), indicating the electrolyte itself and the un-coordinated Cu2+ cannot generate O2 within the tested potential range. The 310 mV overpotential is approximately two times smaller than the typical values exhibited by previously reported Cu complexes.

Figure 1. (a) The molecular structure of Cu2+-tetrakis(4-N-methylpyridyl)porphyrin complex. (b) Cyclic voltammograms of 1 mM Cu2+-tetrakis(4-N-methylpyridyl)porphyrin (red), bare buffer solution (black) and buffer solution containing 1 mM CuSO4 (green). The electrode is a piece of fluorine-doped tin oxide glass slide.

The authors ascribed the ultra-small OER overpotential to the formation of an oxidized form of the Cu2+-porphyrin complex. This oxidized species is generated after the complex loses one electron, and is active for O-O bond formation and subsequent O2 evolution. The energy barrier of this one-electron-oxidation pathway is expected to be much lower than those of conventional processes involving higher-valent Cu species (e.g., Cu4+-oxo), which facilitates OER at small overpotential.

With the complete catalytic cycle of water oxidation by the Cu2+-porphyrin complex being fully revealed, OER will become more efficient and energy-saving.

To find out more please read:

Low Overpotential Water Oxidation at Neutral pH Catalyzed by A Copper(II) Porphyrin

Yanju Liu, Yongzhen Han, Zongyao Zhang, Wei Zhang, Wenzhen Lai, Yong Wang and Rui Cao

Chem. Sci., 2019, DOI: 10.1039/C8SC04529A

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in Chemistry from University of California, Santa Cruz in the United States. He is passionate about scientific communication to introduce cutting-edge research to both the general public and scientists with diverse research expertise. He is a blog writer for Chem. Commun. and Chem. Sci. More information about him can be found at http://liutianyuresearch.weebly.com/.

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Carbon Coating Promotes CO2 Reduction on Nickel Surfaces

Reducing CO2 into fuels such as CO or hydrocarbons is a promising strategy to reduce the net emission of greenhouse gases and mitigate climate change. The CO2 reduction reaction, however, is an energy-costly process. Therefore, CO2 reduction catalysts are necessary to enhance the CO2 conversion efficiency and minimize the overall energy demand.

Researchers are developing high-performance yet inexpensive CO2 reduction catalysts. Noble metals such as Au, Ag and Pd exhibit high CO2-to-CO conversion efficiency, but their scarcity restricts their large-scale practicability. Metallic Fe, Co and Ni are active in reducing CO2 and therefore, have been identified as alternatives to the noble metal catalysts.

Recently in Chem. Sci., a group of scientists led by Zhenyu Sun from Beijing University of Chemical Technology and Yousung Jung from Korea Advanced Institute of Science and Technology (KAIST) pushed the CO2-reduction activity of Ni metal to a new height. The researchers synthesized carbon-supported Ni nanocrystals via pyrolysis of Ni-based metal organic frameworks (MOFs) in argon. The resultant Ni nanoparticles had an average diameter of ~30 nm and were embedded in N-doped carbon scaffolds (Fig. 1a). Each nanoparticle was uniformly coated with a thin layer of amorphous carbon (Fig. 1b).

Figure 1. (a) The elemental mapping of the carbon-supported Ni nanoparticles. Green dots and blue regions are Ni nanoparticles and carbon matrices, respectively. (b) The scanning transmission electron microscope image showing the thin carbon coating on a Ni nanoparticle surface. (c) The CO2-to-CO conversion efficiencies at different applied potentials. Ni-NC_ATPA@C and Ni-NC_TPA@C are carbon-supported Ni nanoparticles derived from MOFs with 2-amino-terephthalic acid and terephthalic acid organic linkers, respectively. NC_ATPA@C is the Ni-free carbon powder derived from ATPA.

The CO2-to-CO conversion efficiencies of these Ni-C nanocomposites were the highest among all the reported carbon-supported Ni nanoparticles. The best Ni catalyst achieved a maximal efficiency of ~94% at an overpotential of 0.59 V, while previously reported Ni-C catalysts typically exhibited efficiencies lower than 25%. The significantly improved conversion efficiency was associated with the thin carbon coating. This coating prevented the Ni nanoparticles from directly contacting with aqueous electrolytes, and thus minimized hydrogen evolution reaction, a side reaction that decreased the conversion efficiency.

This work highlights the instrumental role of the surface carbon layers in promoting the CO2-reduction activity of Ni nanoparticles. The carbon-coating strategy could be extended to other low-cost transition metals, which may lead to a variety of cost-effective CO2-reduction catalysts.

 

To find out more please read:

Carbon-Supported Ni Nanoparticles for Efficient CO2 Electroreduction

Mingwen Jia, Changhyeok Choi, Tai-Sing Wu, Chen Ma, Peng Kang, Hengcong Tao, Qun Fan, Song Hong, Shizhen Liu, Yun-Liang Soo, Yousung Jung, Jieshan Qiu and Zhenyu Sun

Chem. Sci., 2018, 9, 8775-8780

 

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in Chemistry from University of California, Santa Cruz in the United States. He is passionate about scientific communication to introduce cutting-edge research to both the general public and scientists with diverse research expertise. He is a blog writer for Chem. Commun. and Chem. Sci. More information about him can be found at http://liutianyuresearch.weebly.com/.

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How Can We Stabilize Bismuth in Potassium-Ion Batteries? Add Salts!

The large-scale manufacturing and widespread demand of consumer electronics in modern societies call for batteries with affordable prices. Potassium-ion batteries are one of the economical alternatives to lithium-ion batteries, as the cost of potassium is much lower than lithium. However, these types of batteries have yet to be commercially available, partly due to the lack of high-performance and stable anode materials. Bismuth (Bi) is a promising anode material for potassium-ion batteries, because of its substantially higher theoretical charge-storage capacity than the conventional ones. Unfortunately, its poor durability severely hinders the applicability.

Now the drawback of Bi has been successfully addressed by increasing the electrolyte concentration. This breakthrough, demonstrated by Chuan-Fu Sun and coworkers from Chinese Academy of Sciences, China, was recently published in Chemical Science.

Sun and coworkers investigated the interplay between the electrolyte concentration and the charge-storage performance stability of Bi nanoparticles (Figure 1a). They identified the optimal concentration of the solute, potassium bis(tri-fluoromethylsulfonyl)imide, to be 5 M. Under this condition, the irreversible electrolyte reduction reaction on the Bi surface experienced the highest resistance. Impeding this unwanted reaction thus elongated the lifespan of Bi electrodes. In addition, the concentrated electrolyte resulted in thin layers of the reduction products being deposited on the Bi surface. This allowed ions in the electrolyte to easily penetrate the surface coatings and interact with the encapsulated Bi nanoparticles, maintaining the intrinsically high capacity of Bi (Figure 1b). Other concentrations either led to rapid battery failure or significantly reduced capacity.

Figure 1. (a) A scanning electron microscopy image of the Bi nanoparticles. (b) The change of the Bi electrode capacity vs. charge-discharge cycle number with different electrolyte concentrations. CE: coulombic efficiency.

This work innovates the design and development of commercially viable potassium-ion batteries. The strategy of increasing the electrolyte concentration could possibly be adapted to solve electrode instability issues associated with other rechargeable batteries.

 

To find out more please read:

Concentrated Electrolytes Stabilize Bismuth-Potassium Batteries

Ruding Zhang, Jingze Bao, Yu-Huang Wang and Chuan-Fu Sun

Chem. Sci., 2018, DOI: 10.1039/c8sc01848k

 

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in Physical Chemistry from University of California, Santa Cruz in the United States. He is passionate about scientific communication to introduce cutting-edge research to both the general public and scientists with diverse research expertise. He is a blog writer for Chem. Commun. and Chem. Sci. More information about him can be found at http://liutianyuresearch.weebly.com/.

 

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Welcoming a New Member into the Aluminum Battery Family

Yu and coworkers from The University of Queensland, Australia have introduced an aluminum-selenium (Al-Se) battery as a new member of the rechargeable Al-ion battery family. This battery reported in Chemical Science exhibited a capacity of 178 mAh per gram of Se, high discharging voltage above 1.5 V and satisfactory lifetime.

Al-ion batteries have attracted increasing attention as next-generation energy-storage devices. They are potentially more affordable and safer than Li-ion batteries, due to the natural abundance and the existence of native oxide surface layers of aluminum, respectively. One of the major challenges hindering the wide application of Al-ion batteries is the lack of feasible cathode materials. Previously investigated cathodes have drawbacks of low charge-storage capacity, low discharging voltage, poor electrical conductivity or chemical instability.

Inspired by sulfur, Yu and coworkers selected selenium as a cathode material for Al-ion batteries. Selenium has substantially higher electrical conductivity and lower ionization potential than sulfur, which is expected to improve the energy-storage capacity of batteries. However, a major drawback of selenium is that the oxidation product generated upon charging batteries, Se2Cl2, can dissolve quickly in electrolytes and lead to battery failure. Solving this problem would make selenium a promising cathode for Al-ion batteries.

To resolve this issue, the authors introduced a mesoporous carbon named CMK-3, nanorods that are capable of physically adsorbing Se2Cl2. The cathode, composed of Se nanowires and CMK-3 nanoparticles, is thus anticipated to improve the lifespan of batteries, as any Se2Cl2 that is generated will be confined inside the pores of CMK-3 (Figure 1).

Figure 1. A schematic illustrating the CMK-3’s capability of trapping Se2Cl2. The chemical equation below shows how selenium reacts with aluminum during charge and discharge processes.

As expected, the performance of these Al-Se batteries was stable. They retained more than 80% of the initial capacity after 50 consecutive charge-discharge cycles at 100 mA/g (Figure 2a). Additionally, the discharging capacity of the batteries reached 178 mAh per gram of selenium at 100 mA/g, and the discharging potential was above 1.5 V (Figure 2b).

Figure 2. (a) The specific capacity of the Al-Se batteries of each cycle at different current densities. (b) The variation of battery potential with specific capacity of the 2nd, 5th, 10th, and 30th charge-discharge cycles.

These promising Al-Se batteries could encourage future work to continue progress into the development of affordable and durable Al-ion batteries.

 

To find out more please read:

Rechargeable Aluminum-Selenium Batteries with High Capacity

Xiaodan Huang, Yang Liu, Chao Liu, Jun Zhang, Owen Noonan and Chengzhong Yu

Chem. Sci., 2018, DOI: 10.1039/C8SC01054D

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in Physical Chemistry from University of California, Santa Cruz in the United States. He is passionate about scientific communication to introduce cutting-edge research to both the general public and scientists with diverse research expertise. He is a blog writer for Chem. Commun. and Chem. Sci. More information about him can be found at http://liutianyuresearch.weebly.com/.

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Surface Charge Determines Success of Encapsulating Proteins into a ZIF-8 Metal Organic Framework

Australian scientists from The University of Adelaide and Graz University of Technology recently made a breakthrough in protein chemistry. They revealed that the key to successfully encasing proteins into metal organic frameworks (MOFs, a family of highly porous organic-metal coordination molecules) was the surface charge of the proteins. Their work was recently published in Chemical Science.

Encapsulating proteins into MOFs, a process termed “biomimetic mineralisation”, is an efficient way to protect and preserve proteins. This process is typically initiated by mixing the proteins and the precursors of a MOF. The MOF starts to grow at the protein surface and eventually fully covers the protein. As the growth of the MOF begins with nucleation on the protein surface, the surface properties play an important role in controlling the rate and quality of the encapsulation. Unfortunately, the interplay between protein surface and MOF growth has not been well understood, leading to inefficient reactions that require excess MOF precursors and long reaction times.

The authors demonstrated for the first time that the surface charge is one of the key factors that affects the possibility of biomimetic mineralisation. Specifically, they discovered that proteins with strongly negative charged surfaces, such as pepsin and bovine serum albumin, were able to be spontaneously incorporated into ZIF-8 (a benchmark MOF). Conversely, those with naturally positively charged or slightly negatively charged surfaces, including haemoglobin, were incapable of forming composites with ZIF-8. The authors further showed that changing the surface charge could allow or prohibit the encapsulation. For example, after reacting the lysine groups of haemoglobin with succinic anhydride, the surface of haemoglobin became more negative and ZIF-8 could now readily wrap around the protein (Figure 1). The surface potential threshold to induce biomimetic mineralisation was determined to be -30 mV.

Figure 1. Schematic illustrations of the biomimetic mineralisation of haemoglobin and bovine serum albumin. Haemoglobin with slightly negatively charged surface is unable to form composites with ZIF-8, but becomes active after succinylation or acetylation to make its surface strongly negatively charged. On the contrary, bovine serum albumin with a strongly negatively charged surface readily combines with ZIF-8, but loses its activity when its surface becomes less negatively charged via amination.

The mechanism of the aforementioned observations was attributed to the electrostatic attraction between the protein surface and Zn2+, one of the MOF precursors. The more negatively the surface is charged, the more easily the Zn2+ will attach to and accumulate at the protein surface. The adsorbed Zn2+ ions then serve as the nucleation sites for the MOF to grow around the protein. This hypothesis is proven by a number of control experiments and also validated by computational studies.

This study highlights the surface potential of a protein as a critical factor in its ability to induce biomimetic mineralisation with MOFs. The conclusions could potentially be extended to biomolecules other than proteins (e.g. viruses and cells) to facilitate their integration with various MOFs.

 

To find out more please read:

Protein Surface Functionalisation as A General Strategy for Facilitating Biomimetic Mineralisation of ZIF-8

Natasha K. Maddigan, Andrew Tarzia, David M. Huang, Christopher J. Sumby, Stephen G. Bell, Paolo Falcaro and Christian J. Doonan

Chem. Sci., 2018 , DOI: 10.1039/c8sc00825f

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in Physical Chemistry from University of California, Santa Cruz in United States. He is passionate about scientific communication to introduce cutting-edge research to both the general public and scientists with diverse research expertise. He is a blog writer for Chem. Commun. and Chem. Sci. More information about him can be found at http://liutianyuresearch.weebly.com/.

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Measuring the Strength of Hydrogen Bonds

For the first time, a group of scientists from University of California, San Diego in United States has quantitatively measured the strength of hydrogen bonds between two complex molecules. They also observed an abnormal trend regarding the bond strength in the absence and presence of electron transfer. This work contributes to the understanding of how the hydrogen bond strength changes, an important point that reveals the way biological systems function.

Hydrogen bonds are a type of electrostatic attraction between hydrogen atoms and certain highly electronegative atoms including N, O and F. These bonds help to bind individual water molecules together and keep water as liquid at room temperature, a critical condition for the origin of life.

The researchers, led by Prof. Kubiak, picked two ruthenium-based complexes joined by hydrogen bonds as their studying platform. As shown in Figure 1, the two-molecule system has three states depending on whether the ends are charged or not: the neutral state when both ends are not charged (left), the singly reduced state when only one end is negatively charged (middle), and the doubly reduced state when both ends are negatively charged (right). The group utilized infrared spectroscopy, UV-vis spectroscopy and electrochemical measurements to experimentally determine the strength of hydrogen bonds in these different redox states.

The results from the study found that the hydrogen bond energy of the neutral state and the doubly reduced state was in the range of 2.56-2.88 kcal/mol and 4.50-4.63 kcal/mol, respectively. Surprisingly, the hydrogen bond energy of the singly charged state did not lie between that of the neutral state and the doubly reduced state. It ranged from 7.78 kcal/mol to 8.31 kcal/mol, indicating the hydrogen bond is much stronger than for both the neutral and doubly-reduced states. The authors ascribed such an abnormality to the reinforcement brought by electron transfer i.e., the movement of the negative charge between the two ends.

This work is the first demonstration that hydrogen bond strength can be significantly enhanced by electron delocalization.

Figure 1. A schematic diagram showing the hydrogen bond strength of the singly reduced state (middle), the neutral state (left) and the doubly reduced states (right). [Note: In this figure, the lower the state lies, the stronger its hydrogen bond is. ET: electron transfer.]

To find out more please read:

Effects of Electron Transfer on the Stability of Hydrogen Bonds

Tyler M. Porter, Gavin P. Heim and Clifford P. Kubiak

Chem. Sci. DOI: 10.1039/c7sc03361c

About the blogger:

Tianyu Liu is a Ph.D. in chemistry graduated from University of California, Santa Cruz in 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 blogger 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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