Study On Microstructure And Electrochemical Properties Of Super-lattice La-Y-Ni Based A₂B₇ Type Hydrogen Storage Alloys

The key to develop the high power density Ni-MH batteries is researching hydrogen storage alloys with high capacity and good cycle stability.The R-Mg-Ni system hydrogen storage alloy have a high discharge capacity,but its cycle stability is poor.In this paper,based on the research status of various types of hydrogen storage alloys,especially La-Mg-Ni based hydrogen storage alloys,select the element Y to substitute for Mg,and set the La-Y-Ni system A₂B₇ type hydrogen storage alloy as the research object.In this paper,the XRD,SEM,EDS and corresponding electrochemical measurements were adopted to analysis the evolution of La-Y-Ni-based A₂B₇ hydrogen storage alloy structure and electrochemical properties.The results of Y1-xLaxNi3.25Mn0.15Al0.1?x=0,0.15,0.25,0.33,0.5,0.75,1?series of hydrogen storage alloys show that the alloys are composed by main phase of A₂B₇-type,with the increase of x,the abundance of A₂B₇-type increased first and then decreased.When x=0.33,the abundance of A₂B₇ is the highest.The electrochemical test shows that the alloy x=0.33 has the maximum discharge capacity and the best cycle life,which is related to the higher abundance of A₂B₇,the alloy also has a good high rate discharge performance,electrical analysis measured D0 consistent with its law.The ratio of La and Y has a great influence on the phase structure and electrochemical properties of the alloy.When the ratio of La and Y is1: 2,the alloy has better comprehensive electrochemical performance.The alloy La0.33Y0.67Ni3.25Mn0.15Al0.1 is annealed at different temperatures,and results show that appropriate increase in the annealing temperature is conducive to the Ce2Ni7 phase abundance increase,when the annealing temperature is 950 ?,CaCu5 and Gd2Co7 phase phase disappeared,Ce2Ni7 phase abundance achieve the maximum 93.18%.When the annealing temperature exceed 950?,the Ce5Co19 phase increases.The electrochemical PCT curve and its electrochemical properties of the alloy after annealing are closely related to the microstructure and the unit cell volume of each phase.The annealed alloy at 950? have the highest in the Ce2Ni7 phase abundance and the biggest unit cell volume.The alloy also has a lower hydrogenation platform pressure?0.0192-0.087atm?and the highest hydrogen storage capacity and the electrochemical discharge capacity of 371mAh/g,the retention rate of the alloy electrode capacity after the 100 charge / discharge cycles reached the maximum value of88.9%.The HRD performance of annealed alloy electrode is improved to a certain extent,HRD900 increases first and then decreases with the increase of annealing temperature.Among them,950 oC annealing alloy has the best performance of high current discharge,and its HRD900 reaches 83.4%.The diffusion of hydrogen in La0.25Y0.75Ni3.25Mn0.2Al0.15 alloy is the controlling factor of its high magnification discharge performance.The surface of the hydrogen storage alloy particles and electrode pieces were coatedNAFION using simple evaporation solvent phase transformation method.The effects of different concentrations of NAFION on the electrochemical properties of the alloys surface topography were investigated.The results show that with the decrease of the concentration,The NAFION formed on the surface of the alloy particles is changed from a dispersed and raised white flakes to a thin?well-spread film,but the film can not completely coated alloy particles.Electrochemical performance test results show that the cycle stability of the alloy after coated NAFION is improved.When the NAFION dilution ratio is 1: 5,alloy electrode discharge capacity is the largest and the cycle stabilityS100?89%?is best than bare alloy.After the surface coating of NAFION,the high current discharge performance of the alloy electrode was significantly decreased,and the HRD900 was reduced from 81.6% of the bare alloy to68.1% after the coating.The increase of the surface electron exchange resistance?or the exchange current density I0 decreases?at the time of the alloy electrode reaction is one of the main factors affecting the high magnification discharge performance of the coated alloy.The surface of the alloy electrode should be coated with a relatively higher concentration of NAFION solution,so that can form a complete NAFION film on the surface of the electrode,but the alloying electrode after alloying the electrochemical properties are not improved.