Search

Primary chemomechanical impacts of confinement on hydrogen storage performance are studied using a nanolaminate structure where reduced graphene oxide (rGO) encapsulates palladium (Pd) nanoparticles. Three contributing factors are identified that can alter the reaction enthalpy: nanosizing, chemical interaction with the encapsulant, and mechanical stress induced strain from a combination of clamping force and lateral pulling force exerted on the Pd nanoparticles. The mechanical contributions are quantified by combining transmission electron microscopy, ab initio computation, and continuum elasticity theory, from which the encapsulation is found to exert an additional strain of 4.96% and 2.99% before and after hydrogen absorption, respectively, increasing the Pd and Pd hydride (PdHx) reaction enthalpy by 1.3–2.8 kJ (mol H2)−1. The effect of the chemical interaction with rGO also raises the reaction enthalpy by up to 1.6 kJ (mol H2)−1, while the nanosizing effect decreases the reaction enthalpy. The three contributing factors to the reaction enthalpy are found to be similar in magnitude, where the net effect is in agreement with the measured enthalpy increase of 3.7 kJ (mol H2)−1 from the bulk value. Hydrogen absorption kinetics and capacity also improved, which is attributed to facile nucleation of the hydrogen-rich phase enabled by the inhomogeneous strain distribution over the encapsulated PdHx nanoparticles. These results demonstrate that the chemomechanical effect can be controlled in the nanolaminate structure, providing an ideal template for tuning hydrogen storage performance.

Layered boron compounds have attracted significant interest in applications from energy storage to electronic materials to device applications, owing in part to a diversity of surface properties tied to specific arrangements of boron atoms. Here we report the energy landscape for surface atomic configurations of MgB2 by combining first-principles calculations, global optimization, material synthesis and characterization. We demonstrate that contrary to previous assumptions, multiple disordered reconstructions are thermodynamically preferred and kinetically accessible within exposed B surfaces in MgB2 and other layered metal diborides at low boron chemical potentials. Such a dynamic environment and intrinsic disordering of the B surface atoms present new opportunities to realize a diverse set of 2D boron structures. We validated the predicted surface disorder by characterizing exfoliated boron-terminated MgB2 nanosheets. We further discuss application-relevant implications, with a particular view towards understanding the impact of boron surface heterogeneity on hydrogen storage performance., Layered boron compounds attract enormous interest in applications. This work reports first-principles calculations coupled with global optimization to show that the outer boron surface in MgB2 nanosheets undergo disordering and clustering, which is experimentally confirmed in synthesized MgB2 nanosheets.

Magnesium borohydride (Mg(BH4)2, abbreviated here MBH) has received tremendous attention as a promising onboard hydrogen storage medium due to its excellent gravimetric and volumetric hydrogen storage capacities. While the polymorphs of MBH-alpha (α), beta (β), and gamma (γ)-have distinct properties, their synthetic homogeneity can be difficult to control, mainly due to their structural complexity and similar thermodynamic properties. Here, we describe an effective approach for obtaining pure polymorphic phases of MBH nanomaterials within a reduced graphene oxide support (abbreviated MBHg) under mild conditions (60-190 °C under mild vacuum, 2 Torr), starting from two distinct samples initially dried under Ar and vacuum. Specifically, we selectively synthesize the thermodynamically stable α phase and metastable β phase from the γ-phase within the temperature range of 150-180 °C. The relevant underlying phase evolution mechanism is elucidated by theoretical thermodynamics and kinetic nucleation modeling. The resulting MBHg composites exhibit structural stability, resistance to oxidation, and partially reversible formation of diverse [BH4]- species during de- and rehydrogenation processes, rendering them intriguing candidates for further optimization toward hydrogen storage applications.

An investigation is reported of possible kinetic limitations to MgB2 hydrogenation. The role of H–H bond breaking, a necessary first step in the hydrogenation process, is assessed for bulk MgB2, ball-milled MgB2, as well as MgB2 mixed with Pd, Fe and TiF3 additives. The Pd and Fe additives in the MgB2 material exist as dispersed metallic particles in the size range ~5–40 nm diameter. In contrast, TiF3 reacts with MgB2 to form Ti metal, elemental B and MgF2, with the Ti and the MgF2 phases proximate to each other and coating the MgB2 particulates with a film of thickness ~3 nm. Sieverts studies of hydrogenation kinetics are reported and compared to the rate of H–H bond breaking as measured by H-D exchange studies. The results show that H–H bond dissociation does not limit the rate of hydrogenation of MgB2 because H–H bond cleavage occurs rapidly compared to the initial MgB2 hydrogenation. The results also show that surface diffusion of hydrogen atoms cannot be a limiting factor for MgB2 hydrogenation. Instead, it is speculated that it is the intrinsic stability of the B–B extended hexagonal ring structure in MgB2 that hinders the hydrogenation of this material. This supposition is supported by B K-edge x-ray absorption measurements of the materials, which showed spectroscopically that the B–B ring was intact in the material systems studied. The TiF3/MgB2 system was examined further theoretically with reaction thermodynamics and phase nucleation kinetic calculations to better understand the production of Ti metal when TiB2 is thermodynamically favored. The results show that there exist physically reasonable ranges for which nucleation kinetics supersede thermodynamics in determining the reactive pathway for the TiF3/MgB2 system and perhaps for other additive systems as well.

Hydrogen is a long-term clean energy carrier that enables completely carbon-free energy production. However, practical implementation of hydrogen fuel technologies is restricted because of lack of ...

Magnesium has a different crystal structure from its dihydride with hydrogenation leading to a phase transition from the hexagonal closely packed Mg into a tetragonal $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Mg}{\mathrm{H}}_{2}$ rutile type structure. Such materials exhibit complex hydrogen uptake and release kinetics because hydrogen diffusivities significantly change when the crystal structure changes. To provide a foundational understanding of (de)hydrogenation kinetics that is applicable to all stages of the reaction, we performed statistically averaged molecular dynamics simulations to derive hydrogen diffusivities as a function of temperature and hydrogen content for both magnesium and magnesium hydride. Our studies confirm that hydrogen diffusivities in magnesium hydride are much lower than in magnesium, in agreement with experimental data. Additionally, we observe that in either magnesium or magnesium hydride, higher hydrogen compositions result in reduced diffusivities. The latter was not revealed by prior experiments, which were conducted at fixed hydrogen composition. Finally, we discover a non-Arrhenius behavior in magnesium hydride. The physical origin of this behavior is also discussed.

To meet the growing demand for global electrical energy storage, high‐energy‐density electrode materials are required for Li‐ion batteries. To overcome the limit of the theoretical energy density in conventional electrode materials based solely on the transition metal redox reaction, the oxygen redox reaction in electrode materials has become an essential component because it can further increase the energy density by providing additional available electrons. However, the increase in the contribution of the oxygen redox reaction in a material is still limited due to the lack of understanding its controlled parameters. Here, it is first proposed that Li‐transition metals (TMs) inter‐diffusion between the phases in Li‐rich materials can be a key parameter for controlling the oxygen redox reaction in Li‐rich materials. The resulting Li‐rich materials can achieve fully exploited oxygen redox reaction and thereby can deliver the highest reversible capacity leading to the highest energy density, ≈1100 Wh kg−1 among Co‐free Li‐rich materials. The strategy of controlling Li/transition metals (TMs) inter‐diffusion between the phases in Li‐rich materials will provide feasible way for further achieving high‐energy‐density electrode materials via enhancing the oxygen redox reaction for high‐performance Li‐ion batteries., A new key parameter for fully exploiting the oxygen redox reaction by Li/transition metals (TMs) inter‐diffusion between the two phases in Co‐free Li‐rich materials is first proposed. The resulting Li‐rich materials can achieve fully exploited oxygen redox reaction and thereby can deliver the highest reversible capacity leading to the highest energy density, ≈1100 Wh kg−1 among Co‐free Li‐rich materials.

Cell-permeable Parkin ameliorates Parkinson’s disease-like phenotypes induced by mitochondrial poisons and aggregated α-synuclein., Parkinson’s disease (PD) is a progressive neurodegenerative disorder characterized by mitochondrial dysfunction, Lewy body formation, and loss of dopaminergic neurons. Parkin, an E3 ubiquitin ligase, is thought to inhibit PD progression by removing damaged mitochondria and suppressing the accumulation of α-synuclein and other protein aggregates. The present study describes a protein-based therapy for PD enabled by the development of a cell-permeable Parkin protein (iCP-Parkin) with enhanced solubility and optimized intracellular delivery. iCP-Parkin recovered damaged mitochondria by promoting mitophagy and mitochondrial biogenesis and suppressed toxic accumulations of α-synuclein in cells and animals. Last, iCP-Parkin prevented and reversed declines in tyrosine hydroxylase and dopamine expression concomitant with improved motor function induced by mitochondrial poisons or enforced α-synuclein expression. These results point to common, therapeutically tractable features in PD pathophysiology, and suggest that motor deficits in PD may be reversed, thus providing opportunities for therapeutic intervention after the onset of motor symptoms.

Tavorite LiVPO4F has great potential as a cathode material due to its high energy density, superior thermal stability and fast kinetics. It has been controversial whether the redox reaction pathway of LiVPO4F is symmetric or not, but the origin of such symmetric/asymmetric redox reaction pathways has not been clearly understood. By combining ab initio calculations with experiments, we found that oxygen defects on fluorine sites, OF−, can be a key factor that affects the symmetry of the redox pathway of LiVPO4F1−δOδ (δ < 1). The computational results indicate that (1) the thermodynamically metastable ‘triclinic’ polymorph of VPO4F1−δOδ, which maintains the structural symmetry of LiVPO4F, can be kinetically stabilized considering its marginally higher energy than the stable ‘monoclinic’ phase; and (2) the OF− defects slightly destabilize the intermediate phase (x = 0.667) (Li0.667VPO4F versus Li0.667VPO4F0.917O0.083), inferring that the presence of OF− defects can induce the asymmetric redox pathway of LiVPO4F1−δOδ. We also experimentally found that the concentration of OF− defects is critically affected by the synthesis process of LiVPO4F, and VPO4F with lower OF− concentration can support the possible presence of the metastable (triclinic) phase, leading to the redox pathway being prone to being symmetric. Therefore, controlling oxygen defects by a synthesis processes can affect electrochemical performance via different redox reaction pathways of LiVPO4F. This understanding provides further possibilities for improving the electrochemical performance.

Knowledge and foundational understanding of phenomena associated with the behavior of materials at the nanoscale is one of the key scientific challenges toward a sustainable energy future. Size reduction from bulk to the nanoscale leads to a variety of exciting and anomalous phenomena due to enhanced surface-to-volume ratio, reduced transport length, and tunable nanointerfaces. Nanostructured metal hydrides are an important class of materials with significant potential for energy storage applications. Hydrogen storage in nanoscale metal hydrides has been recognized as a potentially transformative technology, and the field is now growing steadily due to the ability to tune the material properties more independently and drastically compared to those of their bulk counterparts. The numerous advantages of nanostructured metal hydrides compared to bulk include improved reversibility, altered heats of hydrogen absorption/desorption, nanointerfacial reaction pathways with faster rates, and new surface states capable of activating chemical bonds. This review aims to summarize the progress to date in the area of nanostructured metal hydrides and intends to understand and explain the underpinnings of the innovative concepts and strategies developed over the past decade to tune the thermodynamics and kinetics of hydrogen storage reactions. These recent achievements have the potential to propel further the prospects of tuning the hydride properties at nanoscale, with several promising directions and strategies that could lead to the next generation of solid-state materials for hydrogen storage applications.

TiCl3 and TiF3 additives are known to facilitate hydrogenation and dehydrogenation in a variety of hydrogen storage materials, yet the associated mechanism remains under debate. Here, experimental and computational studies are reported for the reactivity with hydrogen gas of bulk and ball-milled TiCl3 and TiF3 at the temperatures and pressures for which these additives are observed to accelerate reactions when added to hydrogen storage materials. TiCl3, in either the α or δ polymorphic forms and of varying crystallite size ranging from ∼5 to 95 nm, shows no detectable reaction with prolonged exposure to hydrogen gas at elevated pressures (∼120 bar) and temperatures (up to 200 °C). Similarly, TiF3 with varying crystallite size from ∼4 to 25 nm exhibits no detectable reaction with hydrogen gas. Post-exposure vibrational and electronic structure investigations using Fourier transform infrared spectroscopy and x-ray absorption spectroscopy confirm this analysis. Moreover, there is no significant promotion of H2 dissociation at either interior or exterior surfaces, as demonstrated by H2/D2 exchange studies on pure TiF3. The computed energy landscape confirms that dissociative adsorption of H2 on TiF3 surfaces is thermodynamically inhibited. However, Ti-based additives could potentially promote H2 dissociation at interfaces where structural and compositional varieties are expected, or else by way of subsequent chemical transformations. At interfaces, metallic states could be formed intrinsically or extrinsically, possibly enabling hydrogen-coupled electronic transfer by donating electrons.

Solid-state hydrogen storage materials undergo complex phase transformations in which kinetics are often limited by hydrogen diffusion that significantly changes during hydrogen uptake and release. Here we perform robust statistically-averaged molecular dynamics simulations to obtain a well-converged analytical expression for hydrogen diffusivity in bulk palladium that is valid throughout all stages of the reaction. Our studies confirm the experimentally observed dependence of the diffusivity on concentration and temperature and elucidate the underlying physics. Whereas at low hydrogen concentrations, a single dilute hopping barrier dominates, at high hydrogen concentrations, diffusion exhibits multiple hopping barriers corresponding to hydrogen-rich and hydrogen-poor local environments.

We herein describe a simple, enzyme-free, and label-free strategy for the detection of miRNA based on target-triggered catalytic hairpin assembly (CHA) and fluorescence enhancement of DNA-silver nanoclusters (DNA-AgNCs). This strategy employs two hairpin probes that are rationally designed to contain AgNCs and guanine (G)-rich DNA sequence, respectively. In principle, the two hairpin probes, which exist independently, form a hybridized complex in the presence of target miRNA through CHA reaction, thereby inducing strong fluorescence signal of AgNCs by the presence of G-rich sequence placed in close proximity. Importantly, target miRNA is released after the first CHA reaction, which promotes another CHA reaction, consequently leading to the significantly enhanced fluorescence signal of AgNCs. Based on this one-step, homogeneous, and isothermal amplification method, we successfully detected the target miRNA with high selectivity and sensitivity. In addition, the practical applicability of this strategy was also demonstrated by analyzing the target miRNA in human serum.

Robust time-averaged molecular dynamics has been developed to calculate finite-temperature elastic constants of a single crystal. We find that when the averaging time exceeds a certain threshold, the statistical errors in the calculated elastic constants become very small. We applied this method to compare the elastic constants of Pd and PdH0.6 at representative low (10 K) and high (500 K) temperatures. The values predicted for Pd match reasonably well with ultrasonic experimental data at both temperatures. In contrast, the predicted elastic constants for PdH0.6 only match well with ultrasonic data at 10 K; whereas, at 500 K, the predicted values are significantly lower. We hypothesize that at 500 K, the facile hydrogen diffusion in PdH0.6 alters the speed of sound, resulting in significantly reduced values of predicted elastic constants as compared to the ultrasonic experimental data. Literature mechanical testing experiments seem to support this hypothesis.

Two zinc-aminoclays [ZnACs] with functionalized primary amines [(-CH2)3NH2] were prepared by a simple sol-gel reaction using cationic metal precursors of ZnCl2 and Zn(NO3)2 with 3-aminopropyl triethoxysilane [APTES] under ambient conditions. Due to the facile interaction of heavy metals with primary amine sites and Zn-related intrinsic antimicrobial activity, toxicity assays of ZnACs nanoparticles (NPs) prior to their environmental and human-health applications are essential. However, such reports remain rare. Thus, in the present study, a cell viability assay of in-vitro HeLa cells comparing ZnCl2, Zn(NO3)2 salts, and ZnO (~50 nm average diameter) NPs was performed. Interestingly, compared with the ZnCl2, and Zn(NO3)2 salts, and ZnO NPs (18.73/18.12/51.49 µg/mL and 18.12/15.19/46.10 µg/mL of IC50 values for 24 and 48 h), the two ZnACs NPs exhibited the highest toxicity (IC50 values of 21.18/18.36 µg/mL and 18.37/17.09 µg/mL for 24 and 48 h, respectively), whose concentrations were calculated on Zn elemental composition. This might be due to the enhanced bioavailability and uptake into cells of ZnAC NPs themselves and their positively charged hydrophilicity by reactive oxygen species (ROS) generation, particularly as ZnACs exist in cationic NP's form, not in released Zn2+ ionic form (i.e., dissolved nanometal). However, in an in-vivo embryotoxicity assay in zebrafish, ZnACs and ZnO NPs showed toxic effects at 50–100 µg/mL (corresponding to 37.88–75.76 of Zn wt% µg/mL). The hatching rate (%) of zebrafish was lowest for the ZnO NPs, particularly where ZnAC-[(NO3)2] is slightly more toxic than ZnAC-[Cl2]. These results are all very pertinent to the issue of ZnACs’ potential applications in the environmental and biomedical fields.

Lithium-ion batteries are now reaching the energy density limits set by their electrode materials, requiring new paradigms for Li(+) and electron hosting in solid-state electrodes. Reversible oxygen redox in the solid state in particular has the potential to enable high energy density as it can deliver excess capacity beyond the theoretical transition-metal redox-capacity at a high voltage. Nevertheless, the structural and chemical origin of the process is not understood, preventing the rational design of better cathode materials. Here, we demonstrate how very specific local Li-excess environments around oxygen atoms necessarily lead to labile oxygen electrons that can be more easily extracted and participate in the practical capacity of cathodes. The identification of the local structural components that create oxygen redox sets a new direction for the design of high-energy-density cathode materials.

Li–air is a novel battery technology with the potential to offer very high specific energy, but which currently suffers from a large charging overpotential and low power density. In this work, we use ab initio calculations to demonstrate that a facile mechanism for recharging Li2O2 exists. Rather than the direct decomposition pathway of Li2O2 into Li and O2 suggested by equilibrium thermodynamics, we find an alternative reaction pathway based on topotactic delithiation of Li2O2 to form off-stoichiometric Li2–xO2 compounds akin to the charging mechanism in typical Li-ion intercalation electrodes. The low formation energy of bulk Li2–xO2 phases confirms that this topotactic delithiation mechanism is rendered accessible at relatively small overpotentials of ∼0.3–0.4 V and is likely to be kinetically favored over Li2O2 decomposition. Our findings indicate that at the Li2O2 particle level there are no obstacles to increase the current density, and point to an exciting opportunity to create fast charging Li–air...

© the Owner Societies 2017. Mg(BH4)2is a promising solid-state hydrogen storage material, releasing 14.9 wt% hydrogen upon conversion to MgB2. Although several dehydrogenation pathways have been proposed, the hydrogenation process is less well understood. Here, we present a joint experimental-theoretical study that elucidates the key atomistic mechanisms associated with the initial stages of hydrogen uptake within MgB2. Fourier transform infrared, X-ray absorption, and X-ray emission spectroscopies are integrated with spectroscopic simulations to show that hydrogenation can initially proceed via direct conversion of MgB2to Mg(BH4)2complexes. The associated energy landscape is mapped by combining ab initio calculations with barriers extracted from the experimental uptake curves, from which a kinetic model is constructed. The results from the kinetic model suggest that initial hydrogenation takes place via a multi-step process: molecular H2dissociation, likely at Mg-terminated MgB2surfaces, is followed by migration of atomic hydrogen to defective boron sites, where the formation of stable B-H bonds ultimately leads to the direct creation of Mg(BH4)2complexes without persistent BxHyintermediates. Implications for understanding the chemical, structural, and electronic changes upon hydrogenation of MgB2are discussed.

Solid-state hydrogen storage materials undergo complex phase transformations whose behavior are collectively determined by thermodynamic (e.g., Gibbs free energy), mechanical (e.g., lattice and elastic constants), and mass transport (e.g., diffusivity) properties. These properties depend on the reaction conditions and evolve continuously during (de)hydrogenation. Thus, they are difficult to measure in experiments. Because of this, past progress to improve solid-state hydrogen storage materials has been prolonged. Using PdHx as a representative example for interstitial metal hydride, we have recently applied molecular dynamics simulations to quantify hydrogen diffusion in the entire reaction space of temperature and composition. Here, we have further applied molecular dynamics simulations to obtain well-converged expressions for lattice constants, Gibbs free energies, and elastic constants of PdHx at various stages of the reaction. Our studies confirm significant dependence of elastic constants on temperature and composition. Specifically, a new dynamic effect of hydrogen diffusion on elastic constants is discovered and discussed.

In this work, we developed a novel, label-free, and enzyme-free strategy for the colorimetric detection of microRNA (miRNA), which relies on a target-catalyzed toehold-mediated strand displacement (TMSD) reaction. The system employs a detection probe that specifically binds to the target miRNA and sequentially releases a catalyst strand (CS) intended to trigger the subsequent TMSD reaction. Thus, the presence of target miRNA releases the CS that mediates the formation of an active G-quadruplex DNAzyme which is initially caged and inactivated by a blocker strand. In addition, a fuel strand that is supplemented for the recycling of the CS promotes another TMSD reaction, consequently generating a large number of active G-quadruplex DNAzymes. As a result, a distinct colorimetric signal is produced by the ABTS oxidation promoted by the peroxidase mimicking activity of the released G-quadruplex DNAzymes. Based on this novel strategy, we successfully detected miR-141, a promising biomarker for human prostate cancer, with high selectivity. The diagnostic capability of this system was also demonstrated by reliably determining target miR-141 in human serum, showing its great potential towards real clinical applications. Importantly, the proposed approach is composed of separate target recognition and signal transduction modules. Thus, it could be extended to analyze different target miRNAs by simply redesigning the detection probe while keeping the same signal transduction module as a universal signal amplification unit, which was successfully demonstrated by analyzing another target miRNA, let-7d.

The thermodynamic stability of materials can depend on particle size due to the competition between surface and bulk energy. In this Letter, we show that, while sodium peroxide (Na2O2) is the stable bulk phase of Na in an oxygen environment at standard conditions, sodium superoxide (NaO2) is considerably more stable at the nanoscale. As a consequence, the superoxide requires a much lower nucleation energy than the peroxide, explaining why it can be observed as the discharge product in some Na–O2 batteries. As the superoxide can be recharged (decomposed) at much lower overpotentials than the peroxide, these findings are important to create highly reversible Na–O2 batteries. We derive the specific electrochemical conditions to nucleate and retain Na-superoxides and comment on the importance of considering the nanophase thermodynamics when optimizing an electrochemical system.

The stoichiometry and ubiquitous observation of $p$-type conductivity of synthetic pyrite FeS${}_{2}$ thin films are investigated via first-principles computations of native (vacancies, interstitials, antisites) and extrinsic (O${}_{\mathrm{S}}$, O${}_{i}$) point defects. Native defects have high formation energies and are predicted to occur in low concentrations within the Fe- and S-rich limits, showing that pyrite should be intrinsically stoichiometric. Under sufficiently oxidizing conditions, O${}_{\mathrm{S}}$ becomes the most dominant defect type and induces $p$-type conductivity. At the experimental oxygen impurity concentration, the hole concentration is predicted to be $O({10}^{19})$ cm${}^{\ensuremath{-}3}$, in agreement with Hall measurements reported in the literature. Therefore, we attribute the unintentional $p$-type conductivity of pyrite to oxygen impurities and propose that improvements in device performance may be achieved under more reducing conditions.