Degradation Mechanism against Hydrogenation Cycles in Mg_2–xPr_xNi₄ (x = 0.6 and 1.0)

Abstract

A long cycle life of metal hydrides is paramount for applications. We present an investigation of the degradation mechanism against hydrogenation cycles in Mg2–x Pr x Ni4 (x = 0.6 and 1.0). Mg1.0Pr1.0Ni4 shows significant degradation and loss of capacity after only a few cycles. In stark contrast, Mg1.4Pr0.6Ni4 did not show any reduction of hydrogen storage capacity until 100 cycles at 313 K. The peak broadening of X-ray diffraction (XRD) patterns and accumulation of lattice strain were observed concomitantly with an increase of hydrogenation cycles only in Mg1.0Pr1.0Ni4. These changes were not observed in Mg1.4Pr0.6Ni4. In pair distribution function (PDF) patterns, r-dependent peak broadening was observed and it became significant with an increasing number of cycles in Mg1.0Pr1.0Ni4, suggesting an increase of the dislocation density. Mg1.4Pr0.6Ni4 showed higher hardness and more pulverization upon hydrogenation than Mg1.0Pr1.0Ni4. These results suggest that the dominant mechanism, the stress induced by hydrogen occupation release, must change from the formation of dislocations to pulverization by increasing the Mg content of the alloy. We discuss how this leads to better cycling properties.

Recent advances in the application of total scattering methods to functional materials

“…Fits with a structural model containing dislocations gave so far the best results as reported in Figure 10 39 indicating that the increase in the concentration of such defects is the reason for the reduction in the reversible hydrogen storage capacity of vanadium-titanium alloy system. A similar approach was then adopted by the same research group in Sakaki et al 40 to solve the structure-properties correlation problem for a different material for hydrogen storage, i.e. the Mg 2-x Pr x Ni 4 system.…”

Development of an in situ synchrotron X-ray total scattering setup under pressurized hydrogen gas

“…Sample Lattice constant (Å ) dimension, while the other parameters did not show size dependence. However, the increase of U was much smaller than the change caused by hydrogenation (Kim et al, 2013;Sakaki et al, 2014). The lattice strain of Ni in the ex situ capillary estimated using U and Y was 0.46 and 0.06%, respectively, while that in the in situ one was 0.35 and 0.06%, respectively.…”

“…When dislocations were introduced upon hydrogenation, Q broad increased more drastically. For example, the change in Q broad during cyclic hydrogenation was more than 0.035 in V 1Àx Ti x and Mg 2Àx Pr x Ni 4 (Kim et al, 2013;Sakaki et al, 2014). Therefore, this increment in Q broad with the capillary dimension is sufficiently small to allow us to observe the change in the dislocation density upon hydrogenation.…”

Hydrogen induced amorphization of LaMgNi4 phase in metal hydride alloys

“…Same report also pointed out a spatial framework is formed by connected the NiH 4 tetrahedral and MgH 6 octahedral by sharing H-vertices [10]. Among other phases such as AB 2 , AB 3 , A 2 B 7 , A 5 B 19 , AB 4 , and AB 5 , AA'B 4 phase is also found when a single rare earth (RE) element was used instead of a mischmetal in the A-site [11,12] [18,23], PrMgNi 4 [18,24], NdMgNi 4 [17,18,25], GdMgNi 4 [15,19], and SmMgNi 4 [26,27], have been studied, and some of these alloys have shown reasonable cycle stability, while others went through HIA when hydrided at a relatively higher temperature. When approximated for pseudo AB 2 stoichiometry, the LaMgNi 4 has a R A /R B of 1.39 using the 12-coordinate metallic radii, which is slightly higher than the threshold for HIA (1.37) [2]; HIA for LaMgNi 4 has been reported only when hydrided at 5 MPa H 2 and 100 C [19].…”