Comparison of C14- and C15-Predomiated AB2 Metal Hydride Alloys for Electrochemical Applications
Herein, we present a comparison of the electrochemical hydrogen-storage characteristics of two state-of-art Laves phase-based metal hydride alloys (Zr 21.5 Ti 12.) prepared by induction melting and hydrogen decrepitation. The relatively high contents of lighter transition metals (V and Cr) in the first composition results in an average electron density below the C14/C15 threshold ( e/a ∼ 6.9) and produces a C14-predominated structure, while the average electron density of the second composition
... second composition is above the C14/C15 threshold and results in a C15-predominated structure. From a combination of variations in composition, main phase structure, and degree of homogeneity, the C14-predominated alloy exhibits higher storage capacities (in both the gaseous phase and electrochemical environment), a slower activation, inferior high-rate discharge, and low-temperature performances, and a better cycle stability compared to the C15-predominated alloy. The superiority in high-rate dischargeability in the C15-predominated alloy is mainly due to its larger reactive surface area. Annealing of the C15-predominated alloy eliminates the ZrNi secondary phase completely and changes the composition of the La-containing secondary phase. While the former change sacrifices the synergetic effects, and degrades the hydrogen storage performance, the latter may contribute to the unchanged surface catalytic ability, even with a reduction in total volume of metallic nickel clusters embedded in the activated surface oxide layer. In general, the C14-predominated alloy is more suitable for high-capacity and long cycle life applications, and the C15-predominated alloy can be used in areas requiring easy activation, and better high-rate and low-temperature performances. earth-based AB 5 alloys, which have a capacity of approximately 330 mAh·g -1 . Other performance comparisons between these two MH alloy families are available in an earlier review article  . Different from the single CaCu 5 crystal structure in the AB 5 MH alloys, the main phase in the AB 2 MH alloys can be C14, C15, or a mixture of two, which provides additional freedom in composition design to address various requirements, such as ultra-low temperature performance, high-temperature storage, and overcharge performance  . C14 and C15 are two Laves structures and form the largest intermetallic compound group  . The difference between these two structures originates from the different types of packings in two types of metal nets, Kagome 6363 nets formed by B atoms and containing hexagons and triangles, and A 2 B buckled nets formed by both A and B atoms  . There are 6 types of these nets, depending of their orientation along the  direction of the hexagonal/trigonal unit cells, as shown in Figure 1 ; A, B, and C nets for the Kagome 6363 nets and a, b, and c nets for the A 2 B buckled nets. The packing of these nets creates AcBc 2-layer stacking, resulting in a hexagonal C14 type Laves type structure, or 3-layer stacking (AcBaCb), resulting in a face-centered cubic (fcc) C15 Laves type structure, both with AB 2 stoichiometry. As shown in Figure 1 , atoms in the A layer form a triangular net and there are two possible arrangements for the next layer-atoms in the B or C position. If the stacking of the triangular nets follows the sequence A-B-A-B, as shown in Figure 1b , a hexagonal crystal structure is formed. In the case of another stacking sequence, A-B-C-A-B-C, the structure is fcc with the same packing density as for the hexagonal one ( Figure 1c ). For the Laves phases, the triangular net is replaced by an A 4 B 8 slab with an A 2 B-B 3 -A 2 B-B 3 structure, and C14 and C15 are formed following the A-B-A-B and A-B-C-A-B-C stacking sequences, respectively. Another member of the Laves phases, hexagonal C36, has the same building slabs, but they are stacked in a different sequence, AbCaBaCb. However, the C36 type of structure is much less abundant than C14 and C15 , and we will not discuss it further in this work.