A Study On Amorphous Ce-Mg Hydrogen Storage Electrode Alloys

Recently, Ln-Mg-based hydrogen storage alloys (Ln=La, Ce, Misch-metal) have been regarded as the third new generation of promising hydrogen storage alloys because of their high hydrogen storage capacity, abundant resources and light weight, and have become the study hotspot at home and abroad. However, the polycrystalline Ln-Mg-based hydrogen storage alloys can hardly electrochemically charge/discharge at room temperature, which can be only accomplished by nanocrystalline or amorphous alloys prepared by ball-milling with Ni powder. It was found that amorphous Ln-Mg alloys prepared by ball-milling with Ni show very high electrochemical discharge capacities.but the related mechanisms are not yet fully understood. Therefore, it is of scientific and engineering interest to study the formation mechanism amorphous microstructure and influence on the electrochemical properties of Ln-Mg-based hydrogen storage alloys.In this thesis, the microstructure and electrochemical properties of the amorphous CeMg₁₁Ni + xwt.%Ni composites prepared by ball-milling the as-cast CeMg₁₁Ni alloy powder with certain amount of Ni powder were systematically investigated from the point of Ni amount and ball-milling time. And in order to improve the cycling stability of the ball-milled CeMg₁₁Ni + 200wt.%Ni composite, we also investigate the effect of microencapsulation with Ti powder and carbon nanopowder by ball milling technique. The results can be concluded as follows:(1) The as-cast CeMgnNi alloy has a very low discharge capacity of 23.8mAh/g, and no significant improvement in discharge performance is obtained even after ball-milling for 100h without addition of Ni powder. But mechanical grinding with Ni powder can greatly improve the maximum discharge capacity of the composite, and the CeMg₁₁Ni + 200wt.%Ni composite ball-milled for 25h exhibits the highest maximum discharge capacity of 1050.3 mAh/g. This is due to the fact that Ni powder promotes the formation of amorphous structure, and on the other hand, Ni powder itself has a good electrocatalytic activity and can act as the active sites for the hydrogen redox reaction. For the composite with a certain Ni content, there is an optimal ball-milling time for the accomplishment of the highest maximum dischargecapacity, and extending the ball-milling time has a negative effect on the maximum discharge capacity. In the initial ball-milling stage, the amorphorization degree increases with the ball-milling time, thus leading to the increment of discharge capacity. But if we prolong the ball-milling time beyond the optimal one, the electrochemical discharge capacity is mainly controlled by the particle size, because the amorphorization degree increases very little.(2) In spite of the high discharge capacity of the amorphous CeMguNi + xwt.%Ni composite, it shows very poor cycling stabiltiy. After mechanical microencapsulation with Ti powder, the cycling stability improves with the increase of Ti amount, but the maximum discharge capacity and high-rate dischargeability decline to some extent. This is due to the fact that a compact passive film of Ti oxide is formed on the alloy surface, which can prevent the invasion of the eletrolyte into the inner alloy, thus leading to the improvement of cycling stability. But at the same time, the passive film has a negative effect on the charge-transfer reaction and hydrogen diffusion process on the alloy surface, leading to the worsening of the maximum discharge capacity and high-rate dischargeability.(3) The microencapsulation with carbon nanopowder degrades the overall electrochemical properties of the ball-milled composites. And the maximum discharge capacity and cycling stability decrease with the increase of carbon nanopowder content. The decrease of maximum discharge capacity may be due to the bad electric conductivity of carbon nanopowder, which decreases the electrochemical activity of alloy surface. The worsening of cycling stability may be due to the dissolution of carbon nanopowder from the electrode into the electrolyte, exposing much more fresh alloy surface to the corrosive electrolyte and increasing the opportunity of electrolyte coming into contact with the inner alloy, thus leading to the rapid degradation of the cycling capacity.