A lithium-oxygen battery based on lithium superoxide.

Batteries based on sodium superoxide and on potassium superoxide have recently been reported. However, there have been no reports of a battery based on lithium superoxide (LiO2), despite much research into the lithium-oxygen (Li-O2) battery because of its potential high energy density. Several studies of Li-O2 batteries have found evidence of LiO2 being formed as one component of the discharge product along with lithium peroxide (Li2O2). In addition, theoretical calculations have indicated that some forms of LiO2 may have a long lifetime. These studies also suggest that it might be possible to form LiO2 alone for use in a battery. However, solid LiO2 has been difficult to synthesize in pure form because it is thermodynamically unstable with respect to disproportionation, giving Li2O2 (refs 19, 20). Here we show that crystalline LiO2 can be stabilized in a Li-O2 battery by using a suitable graphene-based cathode. Various characterization techniques reveal no evidence for the presence of Li2O2. A novel templating growth mechanism involving the use of iridium nanoparticles on the cathode surface may be responsible for the growth of crystalline LiO2. Our results demonstrate that the LiO2 formed in the Li-O2 battery is stable enough for the battery to be repeatedly charged and discharged with a very low charge potential (about 3.2 volts). We anticipate that this discovery will lead to methods of synthesizing and stabilizing LiO2, which could open the way to high-energy-density batteries based on LiO2 as well as to other possible uses of this compound, such as oxygen storage.

A high throughput dynamic method for characterizing thermodynamic properties of catalyzed magnesium hydrides by thermogravimetric analysis.

The use of the conventional pressure-composition-temperature (PCT) method to determine the thermodynamics of metal hydrides is a time-consuming process. This work presents an efficient method based on thermogravimetric analysis (TGA), to characterize the thermodynamic parameters. Through cycling catalyzed magnesium hydride in a TGA apparatus under a flowing gas with a constant hydrogen partial pressure, TGA curves can be used to determine absorption/desorption equilibrium temperatures. Based on the van't Hoff analysis, the reaction enthalpies and entropies can be derived from the equilibrium temperatures as a function of hydrogen pressure. Using the results of this work we calculated the hydrogenation and dehydrogenation enthalpies, which are 79.8 kJ per mol per H2 and 76.5 kJ per mol per H2, respectively. These values are in good agreement with those reported values using the PCT method. These results demonstrate that the TGA can be an efficient and effective method for measuring thermodynamic parameters of metal hydrides.

Thermodynamic Destabilization of Ti-O Solid Solution by H2 and Deoxygenation of Ti Using Mg.

Reactive metals including Ti, Zr, Hf, and V, among others, have a strong chemical affinity to oxygen, which makes them difficult to produce and costly to use. It is especially challenging to produce pure or metal alloy powders of these elements when extremely low oxygen content is required, because they have high solubility for oxygen, and the solid solution of these metals with oxygen is often more stable thermodynamically than their oxides. We report a novel thermochemical approach to destabilize Ti(O) solid solutions using hydrogen, thus enabling deoxygenation of Ti powder using Mg, which has not been possible before because of the thermodynamic stability of Ti(O) solid solutions relative to MgO. The work on Ti serves as an example for other reactive metals. Both analytical modeling and experimental results show that hydrogen can indeed increase the oxygen potential of Ti-O solid solution alloys; in other words, the stability of Ti-O solid solutions is effectively decreased, thus increasing the thermodynamic driving force for Mg to react with oxygen in Ti. Because hydrogen can be easily removed from Ti by a simple heat treatment, it is used only as a temporary alloying element to destabilize the Ti-O systems. The thermodynamic approach described here is a breakthrough and is applicable to a range of different materials. This work is expected to provide an enabling solution to overcome one of the key scientific and technological hurdles to the additive manufacturing of metals, which is emerging rapidly as the future of the manufacturing industry.

Seeding ductile nanophase in ceramic grains.

Transgranular brittle fracture is the dominant failure mode of brittle materials, including ceramics and ceramic matrix composites. However, strengthening these materials without sacrificing their toughness has been a big challenge. In this study, an innovative approach is proposed to achieve coordinated strengthening and toughening of ceramics-based composites by introducing specific ductile coherent nanoparticles into ceramic grains. As an example, the WC-Co cemented tungsten carbides were used to demonstrate how this brittle material can achieve ultrahigh strength without losing toughness by seeding metallic nanoparticles inside WC grains. The mechanisms for inducing the formation and modulating the amount, size, and distribution of such nanophase within the ceramic grains were disclosed. The fraction of transgranular ruptures of the brittle ceramic phase was reduced significantly due to the presence of the ductile coherent in-grain nanoparticles. Both the strength and strain limit of the cemented carbides were remarkably increased compared to their counterparts reported in the literature. The coordinated strengthening and toughening strategy proposed in this work is applicable to a broad range of ceramics and ceramic matrix composites to obtain superior comprehensive mechanical properties.

Thermodynamic and kinetic destabilization of magnesium hydride using Mg-In solid solution alloys.

Efforts to thermodynamically destabilize magnesium hydride (MgH2), so that it can be used for practical hydrogen storage applications, have been a difficult challenge that has eluded scientists for decades. This letter reports that MgH2 can indeed be destabilized by forming solid solution alloys of magnesium with group III and IVB elements, such as indium. Results of this research showed that the equilibrium hydrogen pressure of a Mg-0.1In alloy is 70% higher than that of pure MgH2. The temperature at 1 bar hydrogen pressure (T1bar) of Mg-0.1In alloy was reduced to 262.9 °C from 278.9 °C, which is the T1bar of pure MgH2. Furthermore, the kinetic rates of dehydrogenation of Mg-0.1In alloy hydride doped with a titanium intermetallic (TiMn2) catalyst were also significantly improved compared with those of MgH2.

A new, energy-efficient chemical pathway for extracting Ti metal from Ti minerals.

Titanium is the ninth most abundant element, fourth among common metals, in the Earth's crust. Apart from some high-value applications in, e.g., the aerospace, biomedicine, and defense industries, the use of titanium in industrial or civilian applications has been extremely limited because of its high embodied energy and high cost. However, employing titanium would significantly reduce energy consumption of mechanical systems such as civilian transportation vehicles, which would have a profound impact on the sustainability of a global economy and the society of the future. The root cause of the high cost of titanium is its very strong affinity for oxygen. Conventional methods for Ti extraction involve several energy-intensive processes, including upgrading ilmenite ore to Ti-slag and then to synthetic rutile, high-temperature carbo-chlorination to produce TiCl4, and batch reduction of TiCl4 using Mg or Na (Kroll or Hunter process). This Communication describes a novel chemical pathway for extracting titanium metal from the upgraded titanium minerals (Ti-slag) with 60% less energy consumption than conventional methods. The new method involves direct reduction of Ti-slag using magnesium hydride, forming titanium hydride, which is subsequently purified by a series of chemical leaching steps. By directly reducing Ti-slag in the first step, Ti is chemically separated from impurities without using high-temperature processes.

Hydrogenation of nanocrystalline Mg at room temperature in the presence of TiH(2).

Magnesium and magnesium-based alloys are considered attractive candidates as rechargeable hydrogen storage materials because of their high hydrogen storage capacities (theoretically up to 7.6 wt %), reversibility, and low cost. In this work, the hydrogenation of nanocrystalline magnesium at room temperature in the presence of TiH(2) was studied. The magnesium was derived by dehydrogenation of nanostructured MgH(2)-0.1TiH(2) prepared by using an ultra-high-energy and high-pressure planetary milling technique. Significant uptake of hydrogen by magnesium at room temperature was observed. The results demonstrate that the nanostructured MgH(2)-0.1TiH(2) system is superior to undoped nano- or micrometer-scaled MgH(2) with respect to the hydrogenation properties of magnesium at room temperature. This finding is potentially useful for a range of energy applications including mobile or stationary hydrogen fuel cells, cooling medium in electricity generation, and differential pressure compressors.

Hydrogen storage properties of nanosized MgH2-0.1TiH2 prepared by ultrahigh-energy-high-pressure milling.

Magnesium hydride (MgH(2)) is an attractive candidate for solid-state hydrogen storage applications. To improve the kinetics and thermodynamic properties of MgH(2) during dehydrogenation-rehydrogenation cycles, a nanostructured MgH(2)-0.1TiH(2) material system prepared by ultrahigh-energy-high-pressure mechanical milling was investigated. High-resolution transmission electron microscope (TEM) and scanning TEM analysis showed that the grain size of the milled MgH(2)-0.1TiH(2) powder is approximately 5-10 nm with uniform distributions of TiH(2) among MgH(2) particles. Pressure-composition-temperature (PCT) analysis demonstrated that both the nanosize and the addition of TiH(2) contributed to the significant improvement of the kinetics of dehydrogenation and hydrogenation compared to commercial MgH(2). More importantly, PCT cycle analysis demonstrated that the MgH(2)-0.1TiH(2) material system showed excellent cycle stability. The results also showed that the DeltaH value for the dehydrogenation of nanostructured MgH(2)-0.1TiH(2) is significantly lower than that of commercial MgH(2). However, the DeltaS value of the reaction was also lower, which results in minimum net effects of the nanosize and the addition of TiH(2) on the equilibrium pressure of dehydrogenation reaction of MgH(2).

Amorphous TiCu-Based Additives for Improving Hydrogen Storage Properties of Magnesium Hydride.

Magnesium hydride has long been regarded as a promising candidate material for hydrogen and heat storage due to its high hydrogen capacity, reversibility, and low cost. Catalytic doping has been demonstrated as one of the most effective methods to improve hydrogen storage properties of MgH2. In this study, amorphous Ti45Cu41Ni9Zr5 and Ti40Cu47Zr10Sn3 alloys are used as additives for MgH2. Nanostructured MgH2 doped with amorphous or crystalline TiCu-based alloys are prepared by using a high-energy mechanochemical synthesis method. Results show that the amorphous TiCu additives provide enhanced catalytic effects compared to crystalline alloys of the same composition. Doping MgH2 using an amorphous Ti45Cu41Ni9Zr5 alloy yielded improved dehydrogenation kinetics compared to using crystalline Ti40Cu47Zr10Sn3 alloy. The analysis using transmission electron microscopy reveals that there are nanostructured catalytic particles uniformly distributed in the amorphous TiCu-catalyzed MgH2. The MgH2 system catalyzed by amorphous TiCu-based alloy shows little degradation during hydrogenation and dehydrogenation cycling at 300 °C. The amorphous TiCu-based catalysts are thermally stable at temperatures up to 360 °C. Heating the amorphous Ti45Cu41Ni9Zr5-catalyzed MgH2 to temperatures above 360 °C led to disproportionation of the catalyst alloy and the formation of MgCu2 and Ti2Cu. In addition, PCI analysis of the amorphous Ti45Cu41Ni9Zr5-catalyzed MgH2 shows a slight increase in hydrogen equilibrium pressure, resulting in a reaction enthalpy of -78.7 kJ/mol·H2 and an entropy of 145.0 J/K·mol·H2. The entropy calculated from this study is approximately 10 J/K·mol·H2 higher than values previously reported for undoped and catalyzed Mg-H systems.

Hydrogen-enabled microstructure and fatigue strength engineering of titanium alloys.

Traditionally, titanium alloys with satisfactory mechanical properties can only be produced via energy-intensive and costly wrought processes, while titanium alloys produced using low-cost powder metallurgy methods consistently result in inferior mechanical properties, especially low fatigue strength. Herein, we demonstrate a new microstructural engineering approach for producing low-cost titanium alloys with exceptional fatigue strength via the hydrogen sintering and phase transformation (HSPT) process. The high fatigue strength presented in this work is achieved by creating wrought-like microstructures without resorting to wrought processing. This is accomplished by generating an ultrafine-grained as-sintered microstructure through hydrogen-enabled phase transformations, facilitating the subsequent creation of fatigue-resistant microstructures via simple heat treatments. The exceptional strength, ductility, and fatigue performance reported in this paper are a breakthrough in the field of low-cost titanium processing.

A new Li-Al-N-H system for reversible hydrogen storage.

Complex metal hydrides are considered as a class of candidate materials for hydrogen storage. Lithium-based complex hydrides including lithium alanates (LiAlH(4) and Li(3)AlH(6)) are among the most promising materials owing to its high hydrogen content. In the present work, we investigated dehydrogenation/rehydrogenation reactions of a combined system of Li(3)AlH(6) and LiNH(2). Thermogravimetric analysis (TGA) of Li(3)AlH(6)/3LiNH(2)/4 wt % TiCl(3)-(1)/(3)AlCl(3) mixtures indicated that a large amount of hydrogen (approximately 7.1 wt %) can be released between 150 degrees C and 300 degrees C under a heating rate of 5 degrees C/min in two dehydrogenation reaction steps. The results also show that the dehydrogenation reaction of the new material system is nearly 100% reversible under 2000 psi pressure hydrogen at 300 degrees C. Further, a short-cycle experiment has demonstrated that the new combined material system of alanates and amides can maintain its hydrogen storage capacity upon cycling of the dehydrogenation/rehydrogenation reactions.

Dehydrogenation of a combined LiAlH4/LiNH2 system.

Although there have been numerous materials systems studied as potential candidates for hydrogen storage applications, none of the materials known to date has demonstrated enough hydrogen capacity or efficiency at required operating temperature ranges. There are still considerable opportunities for discovery of new materials or material systems that could lead to advances in science as well as commercial technologies in this area. LiAlH(4) is one of the most promising materials owing to its high hydrogen content. In the present work, we investigated dehydrogenation properties of the combined system of LiAlH(4) and LiNH(2) under atmospheric argon. Thermogravimetric analysis (TGA) of 2LiAlH(4)/LiNH(2) mixtures without any catalysts indicated that a large amount of hydrogen (approximately 8.1 wt %) can be released between 85 and 320 degrees C under a heating rate of 2 degrees C/min in three dehydrogenation reaction steps. It is found that LiNH(2) effectively destabilizes LiAlH(4) by reacting with LiH during the dehydrogenation process of LiAlH(4).

A dehydrogenation mechanism of metal hydrides based on interactions between Hdelta+ and H-.

This paper describes a reaction mechanism that explains the dehydrogenation reactions of alkali and alkaline-earth metal hydrides. These light metal hydrides, e.g., lithium-based compounds such as LiH, LiAlH4, and LiNH2, are the focus of intense research recently as the most promising candidate materials for on-board hydrogen storage applications. Although several interesting and promising reactions and materials have been reported, most of these reported reactions and materials have been discovered by empirical means because of a general lack of understanding of any underlying principles. This paper describes an understanding of the dehydrogenation reactions on the basis of the interaction between negatively charged hydrogen (H-, electron donor) and positively charged hydrogen (Hdelta+, electron acceptor) and experimental evidence that captures and explains many observations that have been reported to date. This reaction mechanism can be used as a guidance for screening new material systems for hydrogen storage.