Investigation On The Preparation And Electrochemical Preperties Of V-based Solid Solution Hydrogen Storage Alloy Electrodes

V-based hydrogen storage alloys were used in Ni/MH due to their high dischargecapacity. Whereas, the high costs and poor cycling stability of this type of alloys preventthem from developing in industry. In this thesis, in order to reduce costs and improvethe cycling stability, based on an overall review of the research and development ofV-based hydrogen storage alloy electrodes, The elements of Mo, Mn, Sn and V2O5metal oxides have been used as the additional element to VTiCrNi alloy, which iscomposed of lower O content. The microstructure and electrochemical properties of thealloy electrodes have been investigated by SEM, XRD, EIS, liner polarization andpotential step and ICP measurements. Moreover, the addition of ZnO to the alkalineelectrolyte has been investigated.The structural and electrochemical properties of the V2Ti1-xNiCrx（x=0.1-0.7）alloy electrodes have been systematically studied. The results show that all of the alloysmainly consist of a V-based solid solution phase with a BCC structure and a TiNisecondary phase. With increasing the content of Cr substitution for Ti, theelectrochemical activity and maximum discharge capacity is decreased, while thecharge/discharge cycling stability first increased and then decreased, and geting its bestvalue （91.3%） at x=0.5. Electrochemical kinetic measurements show that withincreasing the content of Cr substitution for Ti, the high rate dischargeability （HRD）,the exchange current density I0and the hydrogen diffusion coefficient D in the alloyelectrodes are all first increased and then decreased. It is found that a moderate contentof Cr substitution for Ti is favorable for improving the electrochemical property of thealloy electrodes, and reaching a relatively good overall electrochemical properties atx=0.5.The effects of Mo, Sn and Mn element subsitution on microstructure andelectrochemical properties of V2Ti0.5Cr0.5Ni alloy electrodes are studied in this paper. Itis found that all of these alloys composed of V-based solid solution with BCC structureas main phase and TiNi secondary phase, and the TiNi phase precipitates along the grainboundary of the BCC main phase and forms a three-dimension network. Moreover, it isfound that an substitution of Mn for Ni can increase the maximum discharge capacity ofthe alloy electrodes obviously, and reach the maximum value429.3mAh/g at x=0.2, butdecrease the cycling stability at the same time. The electrochemical kinetic measurements show that a moderate content of Mo substitution for Ni can improve theHRD, exchange current density and hydrogen diffusion coefficient, and reach the bestperformance at x=0.04. With increasing the content of Mn and Sn subsitution for Ni, theHRD, exchange current density and hydrogen diffusion coefficient is decreased, so itnot benefit for improving the electrochemical properties of the alloy electrodes.The different content of V2O5substitution for V on the microstructure andelectrochemical properties of V2Ti0.5Cr0.5Ni alloy electrodes have been investigatedsystematically. It is found that the alloy V2Ti0.5Cr0.5Ni without V2O5consists of aV-based solid solution phase with BCC structure and TiNi secondary phase, withincreasing the content of V2O5substitution for V, a new phase Ti4Ni2O appears in thealloys. The electrochemical measurements show that with increasing the content ofV2O5the maximum discharge capacity of the alloy electrode decreases from366.8mAh/g （x=0） to225.3mAh/g （x=0.35）; The cycling stabiling improves remarkably,when x=0.2, the ratio of remaining capacity after100cycles is83.7%; The high ratedischargeability （HRD）, exchange current density and hydrogen diffusion coefficientfirst increased and then decreased. Among the alloys studied, the V1.95Ti0.5Cr0.5NiO0.05alloy electrode shows a relatively good overall electrochemical properties, of which theCmax, the C100/Cmax, the HRD400, the I0and D are352mAh/g，73.7%,70.3%,33.31mA/gand6.51×10-11cm2/s, respectively.In order to explain the cycling degradation mechanism of the V-based hydrogenstorage alloy electrode, V2Ti0.5Cr0.5Ni0.9Mn0.1and V2Ti0.5Cr0.5Ni0.8Mn0.2alloy electrodeshave been studied by means of XRD, SEM and EIS measurements. It is found that thedegradation of the discharge capacity is mainly caused by the pulverization alloyparticles and the oxidation/corrosion of the alloys during cycling in the KOH alkalineelectrolyte. The results of ICP analysis on the electrolyte indicate that the dissolutioncontent of main hydrogen absorbing elements V and Ti increasing with charge/dischargecycling, which is responsible for the cycling degradation mechanism of the alloyelectrode.The effects of introducing ZnO into electrolyte on the surface and electrochemicalproperties of alloy electrodes have been investigated. It is found that introducing ZnOinto electrolyte can improve the cycling stability and electrochemical kinetic propertiesof the alloy electrode. When1.2g ZnO is added into50mL KOH electrolyte, the ratio ofremaining capacity of the alloy electrode is97.8%after30cycles charge/discharge.With increasing the content of ZnO, the EIS decreases first and then increases, while the exchange current density first increased and then decreased. When0.8g ZnO is addedinto50mL KOH electrolyte, the alloy electrode shows a relatively good overallelectrochemical properties.