Hydrogen Energy Materials

Hydrogen is an ideal secondary energy source，with high energy density，zero carbon emission，and a wide range of sources. Efficient and safe hydrogen storage materials are one of the keys to enabling hydrogen energy applications. Rare earth-based hydrogen storage alloys have the advantages of high volumetric hydrogen storage density，superior kinetic，and moderate temperature for hydrogen absorption and desorption. Due to the structural limitations，the actual hydrogen storage capacity of the commercialized AB₅ type alloy is close to the theoretical hydrogen storage capacity of 1.4%. The hydrogen storage capacity of rare earth alloys should be further improved to meet practical application needs. In recent years，rare earth AB₂ type hydrogen storage alloys with large theoretical hydrogen storage capacity （>2.0%） are a class of promising hydrogen storage materials. However，the structures of rare earth AB₂ alloys are prone to occur hydrogen-induced amorphization and disproportionation after hydrogen absorption and desorption. Compared with other rare earth AB₂ alloys，YNi₂ alloy has a smaller relative molecular mass and a smaller atomic radius ratio （R_A/R_B），but YNi₂ alloy can only form crystalline Y_0.95Ni₂H_2.6 （1.27%）. Hydrogen-induced amorphization can occur when more hydrogen atoms are absorbed，and adjusting the stoichiometric ratio of alloys is a common method to improve the hydrogen storage properties of the rare earth hydrogen storage alloys. By this method，the unit cell volume of the alloys will change due to the change of the atomic ratio and a second phase may also be formed，which can improve the hydrogen storage capacity and plateau characteristics of the alloys. At present，the effect of stoichiometric ratio on the structure and hydrogen storage performance of the yttrium-nickel-based alloys has not been investigated. In this study，YNi_x （x=1.9，2.1，2.3，2.5） alloys were prepared by magnetic induction melting under Ar atmosphere from high purity metals Y （99.5%） and Ni （99.9%）. After melting，the alloys were wrapped in tantalum foil and then placed in an argon filled quartz tube for step annealing. The annealing process was carried out at 600 ℃ for 6 h，750 ℃ for 6 h，850 ℃ for 12 h，750 ℃ for 60 h then cooled down with the furnace. The heating rates were 5 ℃·min⁻¹ below 600 ℃·and 1 ℃·min⁻¹ above 600 ℃. The influence of stoichiometric ratio x on the phase structure and hydrogen storage properties of the alloys was investigated by X-ray diffractometer （XRD），electron probe micro analysis （EPMA） and Sievert's methods. It was found that with the increase of the stoichiometric ratio x from 1.9 to 2.5，YNi phase in the alloy disappeared，the abundance of Y_0.95Ni₂ phase gradually decreased，and YNi₃ phase appeared and gradually increased. When x=2.5，the abundance of YNi₃ phase reached to 92.85%. The hydrogen storage performance showed that with the increase of the stoichiometric ratio，the hydrogen absorption rate of the alloy was gradually accelerated due to the increase of Ni content and phase boundary，which could improve the diffusion rate of hydrogen on the surface and interior of the alloy. With the increase of the stoichiometric ratio x，the capacity decay in the first four cycles of the alloys gradually decreased because the abundance of Y_0.95Ni₂ phase that was prone to occur amorphization and disproportionation in the alloys gradually decreased. When x=2.5，the abundance of YNi₃ phase in the alloy were greater than 90%，and the hydrogen storage capacity did not decay. The stable hydrogen absorption capacity of the alloys increased also due to the decrease of the Y_0.95Ni₂ phase abundance. YNi_2.5 alloy showed the maximum stable hydrogen absorption capacity of 1.716%. Pressure-capacity-temperature （PCT） curves of the alloys showed that the reversible hydrogen absorption and desorption performance of the alloys were improved by increasing the stoichiometric ratio. When x=1.9 and 2.1，the PCT curves of the alloy did not show plateau for hydrogen absorption and desorption. When x=2.3 and 2.5，PCT curves exhibited a double plateau for hydrogen absorption and desorption. Since the main phase in the alloy was YNi₃ phase，the plateau characteristic of PCT curves were like that of YNi₃ alloy. The plateau pressures of the hydrogen absorption curve were 0.0035 and 0.50 MPa，respectively. The structural changes of the alloys before and after hydrogen absorption and desorption show that the capacity attenuation of YNi_x （x=1.9，2.1，2.3，2.5） alloys were mainly due to the disproportionation of Y_0.95Ni₂ phase to form YNi₃ phase YH₂ phase. YNi₃ phase structure did not change after hydrogen absorption and desorption，and could stably absorb and desorb hydrogen. The above results indicated that the increase of the stoichiometric ratio could significantly improve the hydrogen absorption and desorption capacity and the reversible hydrogen storage capacity of yttrium-nickel-based alloys. In the following research，it will be continued to increase the stoichiometric ratio and optimize alloy composition in order to further increase the hydrogen storage capacity and improve the plateau characteristics.