Study On Structure And Electrochemical Performance Of Ti-V-Cr-Ni Solid Solution Alloy

In this work, Ce, Dy,Zr,Nb,Al, Pd and Cu elements have been used as theadditional element to Ti_0.25V_0.35Cr_0.1Ni_0.3 alloy to improve the electrocatalytic activity andthe kinetic performance of the alloy electrode. The microstructure and electrochemicalproperties of the alloy have been investigated by XRD, FESEM-EDS, TEM, EIS,charge/discharge test and ICP-MS measurements.The substitution of rare earth Ce for V has been conducted to investigate thestructure and electrochemical characteristics of Ti_0.25V_0.35-xCe_xCr_0.1Ni_0.3(x=0.06⁰.12)hydrogen storage alloy. The result shows that Ce segregates to the surface of the alloyseriously during arc-melting, while little Ce content affects the microstructure andelectrochemical properties.To avoid Ce segregating to the alloy surface, the master alloy of CeNi3 has beenused to prepare the alloy during arc-melting. Ti_0.25V_0.35-xCe_xCr_0.1Ni_0.3(x=0, 0.005) alloyare mainly composed of V-based solid solution with body-centered-cube (BCC) structureand TiNi-based secondary phase. Ce does not exist in the two phases, instead it exists asthe Ce-rich small particles with irregular edge distributed near the grain boundary ofV-based solid solution phase. The discharge capacity, the cycle stability and the high-ratedischarge ability of the alloy electrode have been effectively improved with thesubstitution of Ce for V at 293K. EIS indicates that the substitution of Ce for V improvesthe dynamic performance, which makes the charge transfer resistance (R_T) decrease andthe exchange current density (I₀) increase markedly, and the apparent activation energy isfar higher than that for AB5 type alloy.The self-discharge characteristics of Ti0.25V0.35-xCexCr0.1Ni0.3 (x=0.005) alloyelectrode are abnormal. A larger discharge capacity has been gotten after standing at opencircuit for 24h. EIS has been used to investigate the abnormal self-dischargecharacteristic of Ti_0.25-xZr_xV_0.35Ce_0.01Cr_0.1Ni_0.3 (x=0.005⁰.04) alloy electrode, and theresults show that the charge transfer resistances (RT) have been lowed after standing atopen circuit for 24h.Effects of a serial of additional elements with different atomic radius on themicrostructure and electrochemical properties of Ti0.25V0.34M0.01Cr0.1Ni0.3 (M=Dy, Zr, Nb,Al, Pd and Cu) hydrogen storage alloy have been investigated. The addition of Pd isbeneficial for Ti0.25V0.34M0.01Cr0.1Ni0.3 alloy to improve the kinetic performance of thealloy electrode.Ti0.17Zr0.08V0.34Cu0.01Cr0.1Ni0.3 and Ti0.17Zr0.08V0.34Nb0.01Cr0.1Ni0.3 alloys aremainly composed of V-based solid solution phase with body-centered-cubic (BCC)structure and C14 Laves phase with hexagonal structure. The alloy electrode has a higherdischarge capacity within a wide temperature region from 303K to 343K. The apparentactivation energy(?rH) for electrochemical reaction of Ti0.17Zr0.08V0.34Cu0.01Cr0.1Ni0.3 andTi0.17Zr0.08V0.34Nb0.01Cr0.1Ni0.3 alloy electrode are lower than that onTi0.25V0.35-xCexCr0.1Ni0.3 (x=0, 0.005) alloy electrode.The microstructure and electrochemical properties of Ti0.25V0.34Al0.01Cr0.1Ni0.3and Ti0.17Zr0.08V0.34Al0.01Cr0.1Ni0.3 alloys have been investigated. The results show thatthe alloy electrode with C14 Laves phase has a higher discharge capacity than that thealloy with TiNi-based secondary phase. The electrochemical reaction on the alloy inwhich C14 Laves phase exists as the secondary phase has a lower apparent activationenergy and lower charge transfer resistance than that on alloy with TiNi-based phase.Ti0.17Zr0.08V0.34RE0.01Cr0.1Ni0.3 (RE=Ce, Dy) hydrogen storage alloy have beeninvestigated. Rare earth Ce as Ce-rich small white lard embeds mainly in C14 Lavesphase, while rare earth Dy exists in both phases of the alloy. At higher discharge current,hydrogen diffusion in the bulk of the alloy may be a rate-limiting step.The performances degradation of Ti0.17Zr0.08V0.34Pd0.01Cr0.1Ni0.3 alloy electrodehave been investigated. The decrease of hydrogen diffusion coefficient (D), the increaseof charge transfer resistances (RT), and the dissolutions of V and Zr element to KOHsolution with charge/discharge cycling would be responsible for the performancesdegradation of the alloy electrode.