Thermochemistry of Alane Complexes for Hydrogen Storage: A Theoretical and Experimental Investigation.

Knowledge of the relative stabilities of alane (AlH(3)) complexes with electron donors is essential for identifying hydrogen storage materials for vehicular applications that can be regenerated by off-board methods; however, almost no thermodynamic data are available to make this assessment. To fill this gap, we employed the G4(MP2) method to determine heats of formation, entropies, and Gibbs free energies of formation for 38 alane complexes with NH(3-n)R(n) (R = Me, Et; n = 0-3), pyridine, pyrazine, triethylenediamine (TEDA), quinuclidine, OH(2-n)R(n) (R = Me, Et; n = 0-2), dioxane, and tetrahydrofuran (THF). Monomer, bis, and selected dimer complex geometries were considered. Using these data, we computed the thermodynamics of the key formation and dehydrogenation reactions that would occur during hydrogen delivery and alane regeneration, from which trends in complex stability were identified. These predictions were tested by synthesizing six amine-alane complexes involving trimethylamine, triethylamine, dimethylethylamine, TEDA, quinuclidine, and hexamine and obtaining upper limits of ΔG° for their formation from metallic aluminum. Combining these computational and experimental results, we establish a criterion for complex stability relevant to hydrogen storage that can be used to assess potential ligands prior to attempting synthesis of the alane complex. On the basis of this, we conclude that only a subset of the tertiary amine complexes considered and none of the ether complexes can be successfully formed by direct reaction with aluminum and regenerated in an alane-based hydrogen storage system

Entropy of Li intercalation in LixCoO2

The entropy of lithiation of LixCoO2 for 0.5 < x less than or equal to 1.0 was determined from measurements of the temperature dependence of equilibrated voltages of electrochemical cells. Measured changes in the entropy of the lithiation reaction were as large as 9.0 k(B)/atom, and as large as 4.2 k(B)/atom within the "O3" layered hexagonal structure of LixCoO2. Three contributions to the entropy of lithiation for the O3 phase were assessed by experiment and calculation. The phonon entropy of lithiation was determined from measurements of inelastic neutron scattering. Phonon entropy can account for much of the negative entropy of lithiation, but its changes with lithium concentration were found to be small. Electronic structure calculations in the local density approximation gave a small electronic entropy of lithiation of the O3 phase. The configurational entropy from lithium-vacancy disorder was large enough to account for most of the compositional trend in the entropy of lithiation of the O3 phase if ordered structures exist at lithium concentrations of x=1/2 and x=5/6. The electrochemical measurements showed the existence of a two-phase region in the composition range between x=5/6 and 0.95. Electronic structure calculations gave evidence that these phases were metallic and insulating, respectively. Changes of the electronic and configurational entropy were found to be of comparable importance for this metal-insulator transition

Thermochemistry of Alane Complexes for Hydrogen Storage: A Theoretical and Experimental Comparison

Knowledge of the relative stabilities of alane (AlH3) complexes with electron donors is essential for identifying hydrogen storage materials for vehicular applications that can be regenerated by off-board methods; however, almost no thermodynamic data are available to make this assessment. To fill this gap, we employed the G4(MP2) method to determine heats of formation, entropies, and Gibbs free energies of formation for thirty-eight alane complexes with NH3-nRn (R = Me, Et; n = 0-3), pyridine, pyrazine, triethylenediamine (TEDA), quinuclidine, OH2-nRn (R = Me, Et; n = 0-2), dioxane, and tetrahydrofuran (THF). Monomer, bis, and selected dimer complex geometries were considered. Using these data, we computed the thermodynamics of the key formation and dehydrogenation reactions that would occur during hydrogen delivery and alane regeneration, from which trends in complex stability were identified. These predictions were tested by synthesizing six amine-alane complexes involving trimethylamine, triethylamine, dimethylethylamine, TEDA, quinuclidine, and hexamine, and obtaining upper limits of delta G for their formation from metallic aluminum. Combining these computational and experimental results, we establish a criterion for complex stability relevant to hydrogen storage that can be used to assess potential ligands prior to attempting synthesis of the alane complex. Based on this, we conclude that only a subset of the tertiary amine complexes considered and none of the ether complexes can be successfully formed by direct reaction with aluminum and regenerated in an alane-based hydrogen storage system.Comment: Accepted by the Journal of Physical Chemistry

White Lines and 3d-Occupancy for the 3d Transition-Metal Oxides

Electron energy-loss spectrometry was employed to measure the white lines at the L23 absorption edges of the 3d transition-metal oxides and lithium transition-metal oxides. The white-line ratio (L3/L2) was found to increase between d^0 and d^5 and decrease between d^5 and d^10, consistent with previous results for the transition metals and their oxides. The intensities of the white lines, normalized to the post-edge background, are linear for the 3d transition-metal oxides and lithium transition-metal oxides. An empirical correlation between normalized white-line intensity and 3d occupancy is established. It provides a method for measuring changes in the 3d-state occupancy. As an example, this empirical relationship is used to measure changes in the transition-metal valences of Li_{1-x}Ni_{0.8}Co_{0.2}O_2 in the range of 0 < x < 0.64. In these experiments the 3d occupancy of the nickel ion decreased upon lithium deintercalation, while the cobalt valence remained constant.Comment: 6 pages, 7 figure

Electrolyte-Assisted Hydrogen Storage Reactions

Use of electrolytes, in the form of LiBH_4/KBH_4 and LiI/KI/CsI eutectics, is shown to significantly improve (by more than a factor of 10) both the dehydrogenation and full rehydrogenation of the MgH_2/Sn destabilized hydride system and the hydrogenation of MgB_2 to Mg(BH_4)_2. The improvement revealed that interparticle transport of atoms heavier than hydrogen can be an important rate-limiting step during hydrogen cycling in hydrogen storage materials consisting of multiple phases in powder form. Electrolytes enable solubilizing heavy ions into a liquid environment and thereby facilitate the reaction over full surface areas of interacting particles. The examples presented suggest that use of electrolytes in the form of eutectics, ionic liquids, or solvents containing dissolved salts may be generally applicable for increasing reaction rates in complex and destabilized hydride materials

Electrolyte-Assisted Hydrogen Storage Reactions

Use of electrolytes, in the form of LiBH_4/KBH_4 and LiI/KI/CsI eutectics, is shown to significantly improve (by more than a factor of 10) both the dehydrogenation and full rehydrogenation of the MgH_2/Sn destabilized hydride system and the hydrogenation of MgB_2 to Mg(BH_4)_2. The improvement revealed that interparticle transport of atoms heavier than hydrogen can be an important rate-limiting step during hydrogen cycling in hydrogen storage materials consisting of multiple phases in powder form. Electrolytes enable solubilizing heavy ions into a liquid environment and thereby facilitate the reaction over full surface areas of interacting particles. The examples presented suggest that use of electrolytes in the form of eutectics, ionic liquids, or solvents containing dissolved salts may be generally applicable for increasing reaction rates in complex and destabilized hydride materials

Power estimation for non-standardized multisite studies

AbstractA concern for researchers planning multisite studies is that scanner and T1-weighted sequence-related biases on regional volumes could overshadow true effects, especially for studies with a heterogeneous set of scanners and sequences. Current approaches attempt to harmonize data by standardizing hardware, pulse sequences, and protocols, or by calibrating across sites using phantom-based corrections to ensure the same raw image intensities. We propose to avoid harmonization and phantom-based correction entirely. We hypothesized that the bias of estimated regional volumes is scaled between sites due to the contrast and gradient distortion differences between scanners and sequences. Given this assumption, we provide a new statistical framework and derive a power equation to define inclusion criteria for a set of sites based on the variability of their scaling factors. We estimated the scaling factors of 20 scanners with heterogeneous hardware and sequence parameters by scanning a single set of 12 subjects at sites across the United States and Europe. Regional volumes and their scaling factors were estimated for each site using Freesurfer's segmentation algorithm and ordinary least squares, respectively. The scaling factors were validated by comparing the theoretical and simulated power curves, performing a leave-one-out calibration of regional volumes, and evaluating the absolute agreement of all regional volumes between sites before and after calibration. Using our derived power equation, we were able to define the conditions under which harmonization is not necessary to achieve 80% power. This approach can inform choice of processing pipelines and outcome metrics for multisite studies based on scaling factor variability across sites, enabling collaboration between clinical and research institutions

Mapping disparities in education across low- and middle-income countries

Analyses of the proportions of individuals who have completed key levels of schooling across all low- and middle-income countries from 2000 to 2017 reveal inequalities across countries as well as within populations. Educational attainment is an important social determinant of maternal, newborn, and child health(1-3). As a tool for promoting gender equity, it has gained increasing traction in popular media, international aid strategies, and global agenda-setting(4-6). The global health agenda is increasingly focused on evidence of precision public health, which illustrates the subnational distribution of disease and illness(7,8); however, an agenda focused on future equity must integrate comparable evidence on the distribution of social determinants of health(9-11). Here we expand on the available precision SDG evidence by estimating the subnational distribution of educational attainment, including the proportions of individuals who have completed key levels of schooling, across all low- and middle-income countries from 2000 to 2017. Previous analyses have focused on geographical disparities in average attainment across Africa or for specific countries, but-to our knowledge-no analysis has examined the subnational proportions of individuals who completed specific levels of education across all low- and middle-income countries(12-14). By geolocating subnational data for more than 184 million person-years across 528 data sources, we precisely identify inequalities across geography as well as within populations.Peer reviewe

Electronic Environments and Electrochemical Properties in Lithium Storage Materials

The local electronic environments and energy storage properties of lithium electrodes are investigated through inelastic electron scattering and electrochemical measurements. Experimental and computational methods are developed to characterize the electronic structure of lithiated compounds during electrochemical cycling. An electrochemical investigation of new lithium alloys has led to a better understanding of the thermodynamics, kinetics, and mechanical properties of nanostructured materials. These studies have also inspired the development of new anode materials for rechargeable lithium batteries. One of the large controversies regarding lithium cathodes concerns the arrangement of the local electronic environments in the host material and how these environments are affected by lithium intercalation. To investigate this issue, the core edges of the 3d transition-metal oxides were studied using electron energy-loss spectrometry. A number of techniques were developed to better understand how characteristics of the electronic structure are reflected in the core edge and near-edge structure of metal oxides. An empirical relationship is established between the transition-metal L23 white line intensity and the transition-metal 3d occupancy. In addition, the near-edge structure of the oxygen K-edge was used to investigate the 2p electron density about the oxygen ions. The results of these investigations were used to study charge compensation in lithiated transition-metal oxides (e.g., LiCoO2 and LiNi0.8Co0.2O2) during electrochemical cycling. These results show a large increase in state occupancy of the oxygen 2p band during lithiation, suggesting that much of the lithium 2s electron is accommodated by the anion. Ab initio calculations of the oxygen 2p partial density of states curves confirm the increase in unoccupied states that accompany lithium extraction. In contrast with the large changes observed in the oxygen K-edge, much smaller changes were observed in the transition-metal L23 white lines. Surprisingly, for layered LiCoO2 and Li(Ni, Co)O2, the transition-metal valence changes little during the charge compensation accompanying lithiation. These results have led to a better understanding of intercalation hosts and the role of oxygen in these layered structures. Recent demand for alternatives to graphitic carbon for lithium anodes motivated an investigation into novel binary lithium alloys. The large volume expansions associated with lithium insertion is known to generate tremendous microstructural damage, making most alloys unsuitable for rechargeable lithium batteries. Electrodes of nanostructured lithium alloys were prepared in an attempt to mitigate the particle decrepitation that occurs during cycling and to shorten diffusion times for lithium. Anodes of silicon and germanium were prepared in thin film form as nanocrystalline particles (10 nm mean diameter) and as continuous amorphous thin films (60-250 nm thick). These nanostructured materials exhibited stable capacities up to six times larger than what is found in graphitic carbons, which are currently the industry standard. In addition, these electrodes do not suffer from particle decrepitation and therefore exhibit excellent cycle life. Nanocrystalline electrodes of silicon and germanium were found to transform into a glassy phase via an electrochemically driven solid-state amorphization during the initial alloying. The disordered structure is believed to assuage strains of intercalation by bypassing multiple crystallographic phases. However, the primary reason for the improved reversibility in these electrodes is attributed to the nanoscale dimensions, which circumvent conventional mechanisms of mechanical deterioration. Nanostructured Li-Si and Li-Ge exhibit the highest reversible electrochemical capacities yet reported for an alloy electrode. Future investigations of the local electronic environments in cathodes could be extended to include more complicated systems such as Li(Ni, Mn)O2 and Li(Fe, X)PO4. Our results suggest that the electronic stability of the metal ion is necessary to maintain a prolonged cycle life. Therefore, an understanding of charge compensation in these complex oxides will be important for understanding new cathode materials. The electronic environments will also be a critical component in the development of alternative anodes, such as binary and ternary lithium alloys. Chemical and valence maps will be used to determine how the lithium is distributed and how its chemical potential varies throughout the electrode. In addition, a better understanding of the thermodynamics, kinetics, and mechanical properties of lithium hosts will be necessary for the development of lithium electrodes with high capacities and high rate capabilities.</p