A Study On The Microstructure Modulation And Surface Modification Of V_2.1TiNi_0.4Zr_0.06Mn_0.05 Hydrogen Storage Electrode Alloy

In the thesis,previous research of the hydrogen storage electrode alloys and modifying methods of V-Ti-Ni based hydrogen storage electrode alloys with dual-phases have been reviewed.Based on previous research of our group,V_2.1TiNi_0.4Zr_0.06Mn_0.05 alloy with a higher discharge capacity of 462 mA·h·g^-1 was chosen to modify.Quenching treatment was adopted to modulate the microstructure,while ball milling with AB₂ alloy and electroless coating were used to modify the surface of the alloy.The microstructure and electrochemical performance of V_2.1TiNi_0.4Zr_0.06Mn_0.05 alloys before and after modified were investigated by means of XRD,SEM,EDS analyses and electrochemical measurements.All of the work was made to improve the overall performance of V_2.1TiNi_0.4Zr_0.06Mn_0.05 alloy,with cycling stability and high-rate discharge ability increasing and the maximum discharge capacity being invariable.The study on the microstructure and electrochemical performance of the cast and quenched V_2.1TiNi_0.4Zr_0.06Mn_0.05 alloys shows that all of these alloys consist of a V-based solid solution main phase with a bcc structure and a C14-type secondary phase in the form of three-dimensional network structure,and the secondary phase precipitates along the grain boundaries of the main phase.After quenching treatment,the unit cell of both the main phase and secondary phase increase gradually.Quenching treatment at 1173 K and 1223 K does not change the activation behavior,while the alloy quenched at 1273 K reaches the maximum discharge capacity at the third cycle.As the quenching temperature increasing,the maximum discharge capacity decays,and the alloy quenched at 1173 K decreases slightly to 452 mA·h·g^-1,compared with 462 mA·h·g^-1 for the cast alloy. However,quenching treatment improves the cycling stability of all the alloys,and quenching treatment at 1173 K increases the high-rate dischargeability and exchange current density.Therefore,the alloy quenched at 1173 K shows the best overall electrochemical properties.The influence of ball milling with x wt.%Zr_0.7Ti_0.3Ni_1.1V_0.2Mn_0.7Cr_0.1(x =10,30,50) on the microstructure and electrochemical properties of V_2.1TiNi_0.4Zr_0.06Mn_0.05 alloy have been studied.The microstructure analysis shows that the alloys milled with AB₂ are consist of a V-based solid solution main phase,a C14-type Laves secondary phase and a C15-type Laves phase.Moreover,the smaller particles of Zr_0.7Ti_0.3Ni_1.1V_0.2Mn_0.7Cr_0.1 alloy are distributing homogeneously on the surface of the V_2.1TiNi_0.4Zr_0.06Mn_0.05 alloy.The unit cells of both the main phase and secondary phase expand with the increase of Cu content. The electrochemical measurements reveal that ball milling do not change the activation behavior of the alloys ball milled and impairs the discharge capacity and high-rate dischargeability.After ball milling,the capacity retention after 20 charging/discharging cycles(S₂₀) increases,and among the alloys studied,the alloy with x=30 have the highest S₂₀,increasing from 14.18%to 40.83%.Electroless coating with Ni and Ni-Cu has been carried out on the V_2.1TiNi_0.4Zr_0.06Mn_0.05 alloy,and their influence on the microstructure and electrochemical properties have been investigated.It is found all of these alloys consist of a V-based solid solution main phase with a bcc structure and a C14-type secondary phase.In the XRD patterns of the coated alloys,Cu diffraction peak is found,while Ni diffraction is not found. Further more,lots of fine grain particles are coating on the surface of the V_2.1TiNi_0.4Zr_0.06Mn_0.05 alloy.Electrochemical test shows that the maximum discharge capacities of the coated alloys decreases from 462 mAh·g^-1 to 420.8 mAh·g^-1 and 407.3 mAh·g^-1 at the second cycle,indicating that the activation of the coated alloy behavior is almost invariable,and the cycling stability and high-rate dischargeability are improved significantly,with the cycling capacity retention S₃₀ increasing from 8.61%for the uncoated alloy to 41.09%and 45.64%and the high-rate dischargeabilities(HRD₄₀₀) at 400 mA·g^-1 discharge current increasing from 38.96%to 72.73%and 76.62%.