Research On The Hydrogen Generation Properties And Mechanisms Of Al, Mg Based Materials

Metallic Mg and Al have been considered as promising hydrogen generation materials because they show the merits of light weight, easy accesss, low cost and high theoretical hydrogen generation capacity. Mg or Al based hydrogen generation materials can be applied as hydrogen source for protable devices or polymer electrolyte fuel cell (PEFC). However, due to the dense oxide film on the surface of Mg or Al and the hydroxide formed during the hydrolysis process, the hydrolysis cannot be fully carried out. In this work, Mg or Al powder is used as the starting material and different hydrides or salts are added into the powder to form composites by high energy ball milling. XRD, SEM-EDS, BET and XPS are adopted to analyze the microstructures. The following contents are studied systematically in this thesis: (1) the hydrogen generation properties and mechanism of Mg-AlCl3 composite, (2) the influences of hydrides addition on the hydrolysis properties of Mg, and the emphasis is played on Mg-LiBH4 composite, (3) the improvement and mechanism of the hydrolysis properties of Mg-LiBH4 composite by adding salts, (4) the influences of hydrides addition on the hydrolysis properties of Al, especially the Al-LiH and Al-CaH2 composites are systematically studied, (5) the influences and mechanism of salts addition on the hydrolysis properties of Al-LiH and Al-CaH2 composite.Firstly, the influences of different salts addition on the hydrolysis properties of Mg are studied. The experimental results show that AICl3 is the best addition to improve the hydrolysis properties of Mg. It is found that the hydrogen generation yield of the mixture first increases with increasing the milling time or the addition amount of AICl3 and then decreases. At room temperature (RT) the 6 h milled Mg-3 mol%AlCl3 composite has the highest hydrogen generation yield of 93.86% and maximum hydrogen generation rate mHGR of 455.9 ml min-1g-1. What’s more, the hydrogen generation yield can also be greatly improved by raising the reaction temperature. The sample can react with water quickly and thoroughly when the temperature is increased to 80℃.Then the effects of different hydrides addition on the hydrolysis properties of Mg are studied. The results show that there is a better synergistic effect between Mg and LiBH4 than others. The hydrogen generation yield increases with increasing milling time and decreasing the addition amount of LiBH4, which can be attributed to the fact that LiBH4 cannot be effectively activated by Mg itself. As a result, the excessive LiBH4 cannot react with water timely and cover the suface of Mg grains, which leads to the decrease of hydrogen generation yield. At RT the 10 h milled Mg-3 wt.% LiBH4 composite has the highest hydrogen generation yield of 45.4% while 3 h milled Mg-36 wt.% LiBH4 mixture shows the highest mHGR of 475 ml min-1g-1.As Mg powder cannot fully activate LiBH4, AICl3 is then added to the Mg-LiBH4 composite. The results show that the hydrogen generation yield of LiBH4 can be greatly improved after milling with AICl3. What’s more, the hydrogen yields are all improved when the original Mg-LiBH4; composite are milled with AICl3. The hydrogen generation yield of the Mg- LiBH4-AICl3 mixture increases with increasing milling time or decreasing the addition amount of AICl3. At RT, the 6 h milled Mg-9 wt.% LiBH4-1 wt.%AlCl3 composite has the highest hydrogen generation yield of 87% and mHGR of 1083.5 ml min-1g-1. Besides, the effects of adding NiCl2 into the Mg-LiBH4 composite are even better than adding AICl3. With the addition of NiCl2, LiBH4 which formerly does not react with water can carry out a fast hydrolysis. More importantly, the in-situ generation of metallic Ni greatly promoted the hydrolysis of Mg powder. Firstly, Mg and Ni can form galvanic cells which intensify the corrosion of Mg. Secondly, the metallic Ni can prevent activated Mg from repassivation. At RT, the 6 h milled Mg-18 wt.% LiBH4-15 wt.%NiCl2 mixture has a yield of 96.1 and mHGR of 1113.3 ml min-1g-1.The influeces of different hydrides addition on the hydrolysis of Al powder are also studied in addition to Mg-based materials. The resutls show that both LiH and CaH2 are ideal additives. The hydrogen generation yield of Al-LiH composite increases with raising the reaction temperature and witness a big improvement at 75 ℃. Apart from activating Al power, LiH can also reacts with water quickly to generate hydrogen and create numerous passages for water to penetrate in to react with freshly exposed Al. More importantly, LiAl2(OH)7 can be generated during the hydrolysis of Al-LiH mixture, which is beneficial for the removal of Al(OH)3. At 75℃, the 3 h milled Al-30 mol% LiH composite has a yield of 96% and mHGR of 4556.3 ml min-1g-1. Similarly, the addition of CaH2 into Al powder also achieves a good activation effect. The hydrogen generation yield increases with increasing the addition amount of CaH2. Prolonging the milling time can also increase the hydrogen yield. At 75℃, the 15 h milled Al-10 mol% CaH2 composite reaches a yield of 97.8% and mHGR of 2074.3 ml min-1g-1.Adding salts into the Al-LiH and Al-CaH2 composite also proves to be effective. The results show that both Al-LiH-KCl and Al-CaH2-NiCl2 are promising composites. With respect to the Al-LiH-KCl composite, the hydrogen generation yield first increases with increasing the addition amount of LiH or KCl and then decreases. Prolonging the milling time will decrease the grain size and the hydrogen yield increases. At 60℃, the 10 h milled Al-10 mol% LiH-10 mol%KCl composite reached the hydrogen generation yield of 97.1% and mHGR of 1500 ml min-1g-1. With respect to the Al-CaH2-NiCl2 composite, the hydrogen yield first increases with increasing the addition amount of NiCl2, reaches the highest yield at 10 mol% and then decreases. Similarly,3 h proves to be the optimum milling time. At 75℃, the 3 h milled Al-10 mol% CaH2-10 mol% NiCl2 composite shows a hydrogen yield of 92.1% and mHGR of 1566.3 ml min-1g-1。...