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Spark discharge generation was used to synthesize Mg-Ti nanocomposites consisting primarily of a metastable body-centered-cubic (bcc) alloy of Mg and Ti. The bcc Mg-Ti alloy transformed upon hydrogenation into the face-centered-cubic fluorite Mg1-yTiyHx phase with favorable hydrogen storage properties. Both metal and metal hydride nanocomposites showed a fractal-like porous morphology, with a primary particle size of 10-20 nm. The metal content of 70 atom % (at %) Mg and 30 at % Ti, consistently determined by XRD, TEM-EDS, and ICP-OES, was distributed uniformly across the as-prepared sample. Pressure-composition isotherms for the Mg-Ti-H nanocomposites revealed large differences in the thermodynamics relative to bulk MgH2, with a much less negative enthalpy of formation of the hydride as small as -45 ± 3 kJ/molH2 as deduced from van't Hoff plots. The plateau pressures of hydrogenation were substantially higher than those for bulk MgH2 in the low temperature range from 150 to 250 °C. The reaction entropy was simultaneously reduced to values down to 84 ± 5 J/K mol H2, following a linear relationship between the enthalpy and entropy. Plausible mechanisms for the modified thermodynamics are discussed, including the effect of lattice strains, the presence of interfaces and hydrogen vacancies, and the formation of excess free volume due to local deformations. These mechanisms all rely on the finely interdispersed nanocomposite character of the samples which is maintained by grain refinement.

Carbon dioxide (CO2) is currently considered as a waste material due to its negative impact on the environment. However, it is possible to create value from CO2 by capturing and utilizing it as a building block for commodity chemicals. Electrochemical conversion of CO2 has excellent potential for reducing greenhouse gas emissions and reaching the Paris agreement goal of zero net emissions by 2050. To date, Carbon Capture and Utilization (CCU) technologies (i.e. capture and conversion) have been studied independently. In this communication, we report a novel methodology based on the integration of CO2 capture and conversion by the direct utilization of a CO2 capture media as electrolyte for electrochemical conversion of CO2. This has a high potential for reducing capital and operational cost when compared to traditional methodologies (i.e. capture, desorption and then utilization). A novel mixture of chemical and physical absorption solvents allowed for the captured CO2 to be converted to formic acid with faradaic efficiencies up to 50 % and with carbon conversion of ca. 30 %. By increasing the temperature in the electrochemical reactor from 20 °C to 75 °C, the productivity towards formic acid increased by a factor of 10, reaching up to 0.7 mmol•m-2 •s-1. The direct conversion of captured CO2 was also demonstrated for carbon monoxide formation with faradaic efficiencies up 45 %.

Mg is an attractive hydrogen storage material not only because of its high gravimetric and volumetric hydrogen capacities but also because of it low material costs. However, the hydride of MgH 2 is too stable to release hydrogen under moderate conditions. We demonstrate that the formation of nanometer-sized clusters of Mg reduces the stability of MgH 2 by the interface energy effect in the immiscible Mg−Ti system. Ti-rich Mg x Ti 1−x (x < 0.5) thin films deposited by magnetron sputtering have a hexagonal close packed (HCP) structure, which forms a face-centered cubic (FCC) hydride phase upon hydrogenation. Positron Doppler broadening depth profiling demonstrates that after hydrogenation, nanometer-sized MgH 2 clusters are formed which are coherently embedded in an FCC TiH 2 matrix. The P (pressure)−T (optical transmission) isotherms measured by hydrogenography show that these MgH 2 clusters are destabilized. This indicates that the formation of nanometer-sized Mg allows for the development of a lightweight and cheap hydrogen storage material with a lower desorption temperature. ■ INTRODUCTIONMg is one of the typical lightweight metals with a density of 1.74 g cm −3 , which is much less than that of many transition and rare earth metals. 1 Mg has a hexagonal close-packed (HCP) structure, and that forms the hydride phase MgH 2 by hydrogenation. MgH 2 has a rutile-type body-centered tetragonal (BCT) structure (α-MgH 2 ) at moderate temperatures and pressures. 2,3 The gravimetric hydrogen capacity is 7.6 mass %, which is higher than most metal hydrides. The volumetric hydrogen capacity of 109 g-H 2 l −1 corresponds to 2.9 times the gaseous hydrogen concentration under a pressure of 70 MPa and 1.6 times the one of liquid hydrogen at 20 K. 4 Mg is one of the most attractive hydrogen storage materials not only because of the high capacity but also the low material costs. However, MgH 2 is too stable to desorb hydrogen at moderate temperatures and pressures and thus unsuited for practical applications. The enthalpy for hydride formation is −75 kJ mol −1 -H 2 , 2,3 which corresponds to a dehydrogenation temperature of around 550 K under a hydrogen pressure of 0.1 MPa. Furthermore, the slow reaction kinetics for hydrogenation and dehydrogenation is a disadvantage for using it as hydrogen storage material. The diffusion of hydrogen in both Mg and MgH 2 has been studied, 2,5,6 and the activation energy for hydrogen diffusion in MgH 2 of 140 kJ mol −1 is particularly high. 6 This value is around 2−5 times those in hydrides of LaNi 5 7 and V. 8 A possible way to destabilize MgH 2 and enhance the reaction kinetics is to reduce the size of Mg. It has been calculated that MgH 2 is destabilized by decreasing the size on a nanometer scale. 9−11 The destabilization is remarkable for a MgH 2 cluster with a diameter below 1.3 nm, corresponding to 19 Mg atoms or less. 11 On the basis of these calculations, the dehydrogenation temperature for a MgH 2 cluster of 0.9 nm would be more than 100 K lower than for bulk MgH 2 under a given hydr...

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