First-Principles Screening of Complex Transition Metal Hydrides for High Temperature Applications

Abstract

Metal hydrides with enhanced thermodynamic stability with respect to the associated binary hydrides are useful for high temperature applications in which highly stable materials with low hydrogen overpressures are desired. Though several examples of complex transition metal hydrides (CTMHs) with such enhanced stability are known, little thermodynamic or phase stability information is available for this materials class. In this work, we use semiautomated thermodynamic and phase diagram calculations based on density functional theory (DFT) and grand canonical linear programming (GCLP) methods to screen 102 ternary and quaternary CTMHs and 26 ternary saline hydrides in a library of over 260 metals, intermetallics, binary, and higher hydrides to identify materials that release hydrogen at higher temperatures than the associated binary hydrides and at elevated temperatures, T > 1000 K, for 1 bar H2 overpressure. For computational efficiency, we employ a tiered screening approach based first on solid phase ground state energies with temperature effects controlled via H2 gas alone and second on the inclusion of phonon calculations that correct solid phase free energies for temperature-dependent vibrational contributions. We successfully identified 13 candidate CTMHs including Eu2RuH6, Yb2RuH6, Ca2RuH6, Ca2OsH6, Ba2RuH6, Ba3Ir2H12, Li4RhH4, NaPd3H2, Cs2PtH4, K2PtH4, Cs3PtH5, Cs3PdH3, and Rb2PtH4. The most stable CTMHs tend to crystallize in the Sr2RuH6 cubic prototype structure and decompose to the pure elements and hydrogen rather than to intermetallic phases.

Ternary Ruthenium Complex Hydrides for Ammonia Synthesis

“…Ba2RuH6 is found to be even more stable than Li4RuH6 (Supplementary Table 1 and Figs. 3c-d) 25 . These experimental results show that the bulk Li4RuH6 and Ba2RuH6 phases will remain stable under the ammonia synthesis conditions applied in this study (H2 partial pressure from 0.25 to 7.5 bar, operation temperature from 323 K to 573 K).…”

Complex hydrides as thermal energy storage materials: characterisation and thermal decomposition of Na2Mg2NiH6

“…With regards to the use of CTMHs as TES materials, it has been shown that the inclusion of sodium, or other metals with high vapour pressures, will inhibit the operating temperatures of the system in order to avoid evaporation and capacity loss. Despite this, a variety of CTMHs have been identied as possible TES materials, 4,8,10 although, to date, their thermal properties have not been fully explored.…”

“…[4][5][6] Their thermal stability and hydrogen storage capacity have made this class of materials applicable for implementation as hydrogen storage materials, thermal energy storage (TES) materials [7][8][9] and possibly as neutron moderators or nuclear fuel components in nuclear reactors. 10 Furthermore, the rich and diverse chemistry made possible by the variety of cations (i.e. alkali, alkali-earth and rare-earth metals) 4,11,12 able to stabilise the transition metal hydride anion may prove useful in the development of multifunctional materials.…”

Structural, electronic, optical and elastic properties of the complex K2PtCl6-structure hydridesARuH6(A= Mg, Ca, Sr and Ba): first-principles study

“…Especially, some of the so-called complex transition metal hydrides (CTMHs) such as the A 2 TH 6 hydrates, where A = Mg, Ca, Sr, Ba, Eu, Sm; T = Fe, Ru, Os, which have the highest hydrogen density among the known condensed materials, are stable even at high temperature. Due to their high decomposition temperature T d , which is generally greater than 550 K, the CTMHs have been largely ignored for ambient temperature applications such as hydrogen solid-state storage in fuel cell vehicles [31]. However, the high stabilities and high hydrogen density of these hydrides make them potentially attractive for high-temperature applications, such as the thermochemical storage of heat for solar thermal plants or excess industrial heat for which hydrogen discharge temperature exceeding 700 K are desired [23,31,32].…”

“…Due to their high decomposition temperature T d , which is generally greater than 550 K, the CTMHs have been largely ignored for ambient temperature applications such as hydrogen solid-state storage in fuel cell vehicles [31]. However, the high stabilities and high hydrogen density of these hydrides make them potentially attractive for high-temperature applications, such as the thermochemical storage of heat for solar thermal plants or excess industrial heat for which hydrogen discharge temperature exceeding 700 K are desired [23,31,32]. On other hand, the high temperature stabilities and semiconducting character of some of these materials led the researchers to the idea that these hydrides can be useful in semiconductor electronics [33,34], photovoltaics [35] and optoelectronic devices [36].…”