| Hydrogen energy is regarded as one of the most promising new energy in the 21st century for its cleanliness,efficiency and safety,it is also regarded as the right direction of mankind's strategic energy development.Storing hydrogen by solid-state material becomes more and more attractive since it allows for safe storage.In general, AB3-type hydrogen storage alloy 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 hydrogen storage alloy 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.Some research indicated that the replacement of A-side or B-side elements could well improve the reversibility,but there is few reports on the effect of A-side elements to the crystal structure and property of the AB3-type hydrogen storage alloy.In this paper,as the representive for AB3-type hydrogen storage alloy,CeNi3, LaY3Ni9 and CeY2Ni9,have been studied and compared with LaNi3.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 A-side elements to the AB3 hydrogen storage alloy in their crystal structures was discussed,based on the experimental results.The main results achieved in this paper are given as below:Effect of A-side elements on crystal structure of AB3-type compounds was investigated by means of X-ray diffraction,Rietveld structure refinement and TEM. The results indicate that LaNi3,LaY2Ni9 and CeY2Ni9 adopt a rhombohedral structure of PuNi3-type(R-3m space group),while the space group of CeNi3 is P 63/m m c.As compared to LaNi3,LaY2Ni9 shows isotropic variations in the cell parameters,with a decrease of 0.7%in a,and 2.0%in c,which leads to the cell volume reduction of 3.3%.Compared to CeY2Ni9,LaY2Ni9 shows a decrease of 0.9%in a and a increase of 2.0%in c,and an overall cell volume reduction of 1.2%.Among the two available crystallographic sites for A atom,the atom with a larger radius occupies the site 3a preferentially and the smaller one occupies the site 6c in the lattice.In the solid-H2 reaction,all the ANi3 samples underwent hydrogen-induced amorphization(HIA) to some extent,and the HIA tendency of LaNi3 is more intenser than the other three alloys Besides,decomposition into ANi5 and binary hydride AHx was found in the LaNi3 samples while ANi5 and ANi3 was found in other three samples cycled at 200℃.The substitution of element Y and element Ce could reduce the HIA.In the electrochemical experiments,the maximum discharge capacity for LaNi3, CeNi3,LaY2Ni9 and CeY2Ni9 was 177,239,292 and 208 mAh/g(discharged at 30 mA g-1),and the capacity retention after 20 cycles(S20) was 82.49%,39.33%, 54.11%and 40.38%respectively.These poor reversible capacities were due to the HIA occurring during cycling.A second discharge process could be found in the curve and it was because there were LaH2,CeH2 and YH2 in the alloys.Based on the experiments and analyses above,effect of A-side elements on crystal structure of AB3-type compounds was illuminated and the special occupancy of A-side elements was studied.Meanwhile,the A-side elements' effects to the hydrogen storage behaviors of AB3-type alloy were systematically studied and some references were put forward to future's further study. |
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