Ab <sub> 3 </ Sub> Alloy Hydrogen Storage Characteristics

Storing hydrogen by solid-state material is a very attractive manner since it allows for safe storage.However,the gravimetric hydrogen density of this manner is too low for mobility and transport applications,many efforts have therefore been made to develop new materials with higher energy density and better kinetic behavior.For instance,in search for new metal hydrides,recent attention has been dedicated to the family of AB₃-type intermetallics.In general,AB₃-type intermetallics crystallize either in the PuNi₃-type rhombohedral structure(R-3m space group) or in the hexagonal CeNi₃-type structure(P6₃/mmc space group),but with the difference in the long-range stacking arrangement only.The PuNi₃-type rhombohedral structure can be considered as alternating stacking of AB₅ (CaCu₅,Haucke phase) and AB₂(MgZn₂,Laves phase) subunits.Nevertheless,the experimental studies on AB₃-type intermetallics reveal that the reversibility in hydrogen storage is rather poor.The underlying mechanism responsible for this is not yet well understood,but indispensable for further improvement in performance. Considering the structural feature,we assume that these may be related to the AB₂ subunit in AB₃-type crystal structure.To prove this assumption,in this paper,the hydrogen storage behaviors of AB₃-type intermatallics were systematically studied and compared with those of AB₂-type ones. First,various AB₃- and AB₂-type intermatallics were designed and prepared by changing the element for the A-side from La,Ce to Y,while that for the B-side was fixed as Ni(called as ANi₃ and ANi₂ samples hereafter).These samples were then subjected to the hydrogen absorption/desorption cycling under hydrogen gas and the electrochemically hydrogen charging/discharging in alkaline solution,respectively. Finally,the relevance of hydrogen storage properties of ANi₃ intermetallics to the ANi₂ subunits in their crystal structures was discussed,based on the experimental results. The main results achieved in this paper are given as below:The ANi₃ and ANi₂ samples with nominal compositions,LaNi₃,CeNi₃,YNi₃, LaY₂Ni₉,CeY₂Ni₉,LaNi₂,CeNi₂ and YNi₂,were prepared by induction melting.The phase components of each sample were checked by X-ray diffraction(XRD),and the diffraction patterns were further analyzed by the Rietveld refinement method to obtain the lattice and structural parameters of the main phases.The results showed that the as-prepared ANi₃ samples crystallized in the PuNi₃-type structure(R-3m space group), except CeNi₃ in the CeNi₃-type structure(P6₃/mmc space group).For the ternary intermetallics LaY₂Ni₉,most of the La atoms lay on site 3a and Y atoms on site 6c; while CeY₂Ni₉ had a randomly distribution of Ce and Y over the two sites.In the solid-H₂ reaction,all the ANi₃ samples underwent hydrogen-induced amorphization(HIA) to some extent,and the tendency of HIA could be described in the order YNi₃/LaY₂Ni₉＜CeNi₃＜LaNi₃.Interestingly,this order coincides with that of the corresponding ANi₂ ones.Besides,decomposition into ANi₅ and binary hydride AH_x was found in the ANi₃ samples cycled at 200℃.In the electrochemical experiments,the maximum discharge capacity for LaNi₃, CeNi₃,YNi₃,LaY₂Ni₉ and CeY₂Ni₉ was 175,240,170,277 and 205 mAh/g (discharged at 30 mA g^-1),and the capacity loss after 20 cycles was 17%,62%,46%, 45%,59%,respectively.These poor reversible capacities were due to the HIA occurring during cycling.Further more,the ANi₃ and corresponding ANi₂ intermetallics exhibited similar discharge behaviors.In summary,the hydrogen-induced amorphization(HIA) is mainly responsible for the poor reversibility of ANi₃ intermetallics.The comparison with the corresponding ANi₂ ones reveals that the HIA as observed in ANi₃ intermetallics is clearly relevant to the ANi₂ subunit in their crystal structure.Modification of ANi₂ subunit is imperative in order to enhance the cycling stability of ANi₃ intermetallics.One should choose ANi₂ subunits with the radius ratio r_A/r_B＜1.37,exp.,substituting the B-side element in AB₃-type intermetallics by an atom with larger radii,that is replacement of Ni by Al, Mn and other transition metals.