Study Of The Hydrogen Storage Properties And Electrochemical Properties Of Mg-Ca-Ni Alloys

Mg-based hydrogen storage alloys are regard as one of the most promising alloys because of their high hydrogen storage capacity, abundant resource and low price. Nowadays, Fossil energy are depleted and increasingly serious environmental pollution, it is quite significant for developing magnesium-based alloys as storage and transport vehicle of Clean Energy Hydrogen. However, their hydriding and dehydriding kinetics are slow and the working temperatures are too high for practical applications.In this thesis, Mg-based hydrogen storage alloys Mg_67-xCa_xNi₃3(x=0, 5, 10, 15, 20,at%)were prepared by means of casting + mechanical milling and casting + melt-spinning + mechanical milling. P-C-T curve of Mg_67-xCa_xNi₃3 alloys were tested by P-C-T at 523K,573K and 623K;discharge curve of Mg_67-xCa_xNi₃3 alloys were tested by electrochemical tester and have studied phase composition of the various stages and microstructure of alloys by XRD and SEM. The purpose is to understand the Ca content,temperature and preparation technologies effect on phase structure, electrochemical properties and hydrogen storage properties. The results show that:1. Preparation technologies have little effect on the phase composition. Mg₆₇Ni₃₃ alloy mainly composes of Mg2Ni and a little quantity of Mg. The content of Mg2Ni decreases, while MgNi2 and Mg2Ca increase with the increase of Ca content in Mg_67-xCa_xNi₃3 alloys. When Ca content reached 20at%, Mg47Ca20Ni33 alloy mainly consists of MgNi2 and Mg2Ca phase. After absorption, Mg₆₇Ni₃₃ alloy mainly composes of Mg₂NiH₄, Mg-Ca-Ni ternary alloy mainly compose of Mg₂NiH₄ and CaH2 and the content of Mg₂NiH₄ decreases, while MgNi2 and CaH2 increase with the increase of Ca content in Mg_67-xCa_xNi₃3 alloys. After adsorption, Mg₆₇Ni₃₃ alloy mainly composes of Mg2Ni, Mg-Ca-Ni ternary alloy mainly compose of Mg2Ni,CaH2 and MgNi22. Ca content and preparation technologies have effect on the maximum hydrogen absorption capacity of alloys. Ca can decrease the maximum hydrogen absorption capacity of alloys owing to decreasing Mg2Ni which can hydrogen absorption. Ca also has the ability to form amorphous, the more content of Ca, the stronger ability to form amorphous, and amorphous alloy can improve the hydrogen storage capacity. But after absorption and desorption, alloys no longer maintain the amorphous structure. 3. Ca content and preparation technologies have different effect on the electrochemical properties. Ca is able to greatly improve the electrochemical cycle stability. Discharge capacity retention rate of alloys which contain Ca respectively increased from 12.11% and 12.62% of Mg₆₇Ni₃₃ to 82% and 60% of Mg_67-xCa_xNi₃3 after 30 cycles. However, Ca can reduce the discharge capacity of alloys, from 88.64 mA·h·g-1 of Mg₆₇Ni₃₃ to about 25 mA·h·g-1 of Mg_67-xCa_xNi₃3. For the alloys containing Ca, the discharge capacity will decrease with the increase of Ca content by casting + mechanical milling. However, the discharge capacity of alloys prepared by casting + mechanical milling first decrease and then increase. But Ca elements can reduce the activation properties of the alloys. Mg₆₇Ni₃₃ alloy can reach to the maximum discharge capacity at the first discharge, however the Mg_67-xCa_xNi₃3 alloys need seven cycles to reach the maximum discharge capacity. Melt-spinning can increase initial discharge capacity of Mg_67-xCa_xNi₃3 alloys, but decrease the discharge cycle stability.4. Ca content and preparation technologies both have effect on the absorption, desorption plateau pressure of alloys. Melt-spinning and higher temperature can increase the absorption, desorption plateau pressure of alloys.5. Ca can increase the hydride formation enthalpy of Mg_67-xCa_xNi₃3 alloys. The more content of Ca, the higher enthalpy,which shows Ca enhances the stability of hydride,is not benefit for charging-discharging hydrogen. Enthalpy of Mg_67-xCa_xNi₃3 alloys prepared by casting + melt spinning + mechanical milling is smaller than enthalpy of Mg_67-xCa_xNi₃3 alloys prepared by casting + mechanical milling, which shows that melt spinning helps reduce the stability of hydrides.6. Adding Ca and Melt-spinning both coarsen alloy particles and slow down the rate of hydrogen absorption alloy. In addition, the higher the temperature, the slower rate of hydrogen abso rption alloys.