MNi4.8Sn0.2(M=La, Nd)-supported multi-walled carbon nanotube composites as hydrogen storage materials

Abstract

Two composites LaNi 4.8 Sn 0.2 /CNTs and NdNi 4.8 Sn 0.2 /CNTs were prepared by an impregnation-reduction method. Their hydrogen storage capacity could reach up to 2.96 wt% and 2.88 wt% respectively at room temperature and 1.0 MPa pressure. These values, which might result from the synergetic effect between the alloy nanoparticles and the pretreated CNTs, were three times higher than those of the unsupported MNi 4.8 Sn 0.2 (M=La, Nd) alloys under the same conditions. XRD and TEM revealed that the alloy particles were uniformly dispersed on the CNTs and the average particle size was ca. 30 nm. The composites also showed good stability and the hydrogen storage capacity decreased by less than 6% after 100 adsorption-desorption cycles. Moreover, no noticeable change in crystalline structure was observed for the composites.hydrogen storage, multi-walled carbon nanotubes, rare earth element, nanoparticle AB 5 alloys such as MNi 5 (M=La, Nd) have been studied widely and used successfully as hydrogen storage materials in nickel-hydride batteries. With the development of fuel cell and hydrogen energy, and the investigation of large-scale storage of hydrogen, these materials have attracted more and more attention recently [1][2][3][4][5] . It was found that partial substitution of the Ni atoms was an efficient way to improve the thermodynamic properties of the alloy, and thus enhanced its stability and hydrogen storage capacity [6] . When a fraction of nickel atoms was substituted by a group IVA element, especially by Sn, the plateau pressure of the isotherm was lowered and also the cycling lifetime of the hydride was improved in nickel-metal hydride batteries [7,8] .It was reported that LaNi 5−x Sn x metal hydride with x = 0.2-0.25 had a reduced PCT hysteresis and an enhanced stability in thermal cycling [9] and thermal aging [10] . The alloys with x<0.2 showed a broadening X-ray diffraction pattern after cycling, but those with x ≥0.2 did not [9] . It was interesting that the alloys disintegrated into fine particles after cycling when x<0.2, whereas the size of the alloys particles remained unchanged for samples with x>0.2. According to the report of Bowman et al. [11] , LaNi 4.8 Sn 0.2 lost only 10% of its initial capacity after 1500 thermal cycles and less than 15% after 10000 cycles. The outstanding stability could be ascribed to the higher binding energy between the substitution atoms and the nickel atoms, which reduced the mobility of the atoms and prevented the disproportionation.Although the AB 5 alloys, such as MNi 4.8 Sn 0.2 (M = La, Nd), showed very good performance and stability, their hydrogen storage capacity, about 1 wt% at room temperature and 1.0 MPa pressure, was still very low, especially for large-scale hydrogen storage application. We speculated that their large grain sizes (>10 μm) might be one of the reasons causing their low storage capacity.Quite recently, it was reported that hydrogen uptake in metal alloy-assisted carbon nanotubes (CNTs) was feasible [12][13][14][15][16] . In our previous paper ...