Identification of Potentially Promising Metal Hydrides for Hydrogen Storage Applications

Abstract

Light metal hydrides are excellent candidates for hydrogen storage. They can store hydrogen effectively, safely， and reversibly under moderate conditions. Among them, perovskite hydrides and lithium borohydride (LiBH4) have received special interest due to the low cost, availability, and high gravimetric and volumetric hydrogen density. However, the poor thermodynamic in dehydrogenation process prevents their practical applications. It is important challenge to facilitate hydrogen release in these metal hydrides. In this work, using density functional theory (DFT) calculations, a series of perovskite hydrides with a general formula ABH3 comprising alkali metals of A, where A is Li, Na, K, Rb, or Cs, and alkaline-earth metals of B, where B is Be, Mg, Ca, Sr, or Ba were screened and the most favorable dehydrogenation pathways were obtained. The thermodynamic properties of these perovskite hydrides, especially for NaCaH3 and KMgH3 for hydrogen storage were mainly investigated. The structural and dehydrogenation properties of lithium borohydride surface were also checked. To tune hydrogen release processes, the destabilization approaches with adaptive dopants have been employed on NaCaH3, KMgH3, and LiBH4. Besides, the effects of pressure/strain on these metal hydrides were examined. Furthermore, the electronic properties such as density of states (DOS) and Bader charges were analyzed to find the reaction mechanism of hydrogen release under these improvements. Firstly, perovskite hydrides (ABH3) were studied to screen the highest potential hydrides for hydrogen release. Herein, the ground state structures of all available systems were obtained. We then investigated the most favorable dehydrogenation pathway for each ABH3 system and found that NaCaH3 was the most attractive ABH3 system. Analysis was performed to determine the influence of the alkali dopants (at the A-site) and alkaline-earth dopants (at the B-site) on hydrogen release from NaCaH3. For this analysis, we calculated the reaction enthalpies of a NaCaH3 system for hydrogen release with different dopants and pathways. Cs was the best dopant for improving hydrogen release with the lowest reaction enthalpy. However, no clear effect from B-site doping on the dehydrogenation was found. Mg-Based metal hydrides have attracted considerable attention for hydrogen storage. The perovskite hydride KMgH3 with an ideal cubic structure has been viewed as a promising material to store hydrogen. To make a further study on the improvement of hydrogen release, M-doped KMgH3 (M = Li, Na, Rb, or Cs) were examined for the structural stability and dehydrogenation properties. The reaction enthalpies (∆H) of the four possible dehydrogenation reaction pathways were calculated using the doped structures with different phases (Pm¯3m, Pnma, R3c). The most favorable reaction pathway among these four pathways was found. Among the alkali metal dopants M investigated, the most promising dopant for this reaction was Li. In addition, the pressures of 0, 0.5, 1.0, 1.5, and 2 GPa were applied to dopant-free and doped KMgH3. It was found that pressure is useful for tuning the reaction enthalpies of the dehydrogenation reactions. For most hydrides, the surface property is an important factor responsible for hydrogen release. Lithium borohydride (LiBH4) has excellent properties for hydrogen storage due to its high lightweight (21.78 g/mol) and the gravimetric/volumetric hydrogen densities. For LiBH4(010) surface, the structural and chemical effects on dehydrogenation were considered using the strain (-3% − +3%) and five dopants (M = Na, K, Al, F, or Cl). The hydrogen release energies of a hydrogen molecule decreased with increasing tensile strain on the LiBH4(010) surface. The tensile strain was useful for promoting the dehydrogenation process by weakening the B-H interactions. Among the dopants examined, the most favorable dopant to enhance dehydrogenation was Al. The ranking of dopants for promoting hydrogen release was Al > Cl > F > Na > K. Remarkably, co-doping of Al and Cl was more effective for hydrogen release than the single doping of Al or Cl with the lowest hydrogen desorption energy. These methods that destabilize metal hydrides are practical for tuning the hydrogen release of LiBH4 hydrides. It is very significant to understand the dehydrogenation mechanism of metal hydrides for hydrogen storage. A first-principle approach is a valuable and powerful tool in the design of alloy systems. Our work shows that both dopant/co-doping and pressure/strain can facilitate the release of hydrogen from NaCaH3, KMgH3, and LiBH4 hydrides by reducing the reaction enthalpies. These results can be utilized as efficient means for the design of highly promising hydride-based hydrogen storage materials.