Research Of Rapid Hydrogen Generation Through Al-based Materials Hydrolysis

With increasingly serious environmental deterioration and fossil fuel depletion, Hydrogen, one of regenerative and ultimate clean secondary energy, has been attracting much attention by scientists. But at present, fast generation, safe storage and delivery of hydrogen have become the bottleneck problems, which restrict hydrogen energy application. The "on-board" production and usage of hydrogen can be an approach to resolve these problems.Aluminum, abundant in earth, has the characteristics such as low density, high capacity and good activity. Thermodynamically, aluminum can react with water to produce hydrogen. However, a layer of inert thin oxide film is easily formed on the aluminum surface and prevents water from contacting with Al metal surface. If the inert thin oxide film could be eliminated, the reactivity of Al with water would be improved greatly, and then Al will be one of the most promising excellent hydrogen generation materials.The key of hydrogen generation by splitting water with Al is to destroy or eliminate the inert thin oxide film, which can make water penetrate into the core so as to contact with fresh Al surface. The existing forms and hydrolysis environment of Al have great effects on formation of inert thin oxide film. Based on this understanding, we design two ways to destroy or eliminate the inert thin oxide film and achieve fast hydrogen generation by Al hydrolysis. One is to change the existing forms of Al by adding activated elements (such as Li or Ca) to form alloy, and the other is to change the solution environment of reaction micro-area by adding oxide (such as CaO) which can produce OH-by hydrolysis to form composite materials.From above two points, three kinds of Al-based materials, Al-Li alloy, Al-Ca alloy and Al-CaO composite material, which can fast split water to produce hydrogen, are successfully prepared by adding activated metal Li, Ca or low price oxide CaO in this dissertation. The hydrogen generation rate and yield of Al-based materials are further promoted by activation modification of ball milling. The hydrogen generation performances of Al-based materials splitting water have been researched and its reaction model has been built. The main research contents and results of this dissertation are as follows:(1) Al-Li alloy prepared by vacuum induction furnace has segregated Al. Only as Li content reaches 20.6%, Al-Li alloy can rapidly split water to generate hydrogen and hydrogen yield can exceed 90%. The reactivity of Al-Li alloy prepared by mechanical ball milling is improved greatly, and its hydrogen generation rate and yield are promoted obviously. As Li contents are more than or equal to 10%, ball milling for 1.0h, hydrogen yield can reach 100%. But as Li contents are lower than 10%, adding NaCl is needed during the process of ball milling. After adding NaCl, hydrogen generation rate is also improved and hydrogen yield can reach 100% too. With the increase of Li contents, hydrogen generation rate increases, accordingly, NaCl addition can be decreased and ball milling time can be shorter. Even in the water at 0℃, hydrogen can also be produced rapidly. Ca2+, Mg2+, NO3-and ethanol are harmful to hydrogen generation. Placed in the air for a short time, hydrogen yield decreases slightly, and Al-Li alloy has a good air oxidation resistance.The maximum hydrogen mass density and mass yield of Al-Li alloy can respectively reach 7.7% and 100%, which is more than at least 1.1% and 20% compared with other Al-based materials. The hydrogen mass density completely exceeds U.S. Department of Energy's 2010 goal (6%).(2) During the process of preparing Al-Ca alloy by mechanical ball milling, longer ball milling time will cause agglomeration of alloy powder, which will affect its hydrogen generation performance. In the process of preparing Al-Ca alloy, NaCl addition can reduce particle sizes of powder and improve hydrogen generation, and hydrogen yield can reach 100%. Hydrogen generation rate and yield will be increased with the increase of Ca content or NaCl addition. As Ca content is higher, NaCl addition and ball milling time can be reduced accordingly. With the increase of initial water temperature, hydrogen generation rate increases, and hydrogen yield first increases and then decreases. C1-and SO42-can promote the hydrogen generation of alloy, but Ca2+and Mg2+will inhibit the hydrogen generation. Al-Ca alloy has poorer air oxidation resistance and need to be stored in the dried and closed environment.The maxium hydrogen mass density and mass yield of Al-Ca alloy can respectively reach 6.9% and 93%, which is more than at least 0.2% and 16% compared with other Al-based materials. The hydrogen mass density completely also exceeds U.S. Department of Energy's 2010 goal. In addition, Al-Ca alloy can be easily prepared and has low cost.(3) Al and cheap CaO are directly used as starting materials to prepare Al-CaO hydrogen production materials by mechanical ball milling. The prepared materials have too long reaction induction period and low hydrogen yield, no more than 80%. Adding NaCl into raw materials, the hydrogen generation rate and yield of prepared Al-CaO composite materials are improved obviously, when ball milling for 1.0h, hydrogen yield can reach 100%. With the increase of CaO content and NaCl addition, hydrogen generation rate and yield is increased gradually. The higher the CaO content is, the less the NaCl addition may be accordingly, and high hydrogen yield can also be obtained. The hydrogen mass density of Al-CaO composite materials is 4.3-5.8% and has excellent air oxidation resistance. C1- and SO42- are helpful to hydrogen generation, but Mg2+ is harmful. With the increase of initial water temperature, reaction induction period and hydrogen yield will first increase and then decrease.Compared with other Al-based materials, although hydrogen mass density and mass yield of Al-CaO composite materials has no obvious advantages, its hydrogen production cost is very little.(4) There are two existing forms of Al, activated and un-activated, in these Al-based hydrogen production materials. During the hydrogen generation reaction, Li or Ca (CaO in Al-CaO system) and activated Al react rapidly with water to generate hydrogen, at the same time the un-activated Al reacts completely with formed LiOH or Ca(OH)2 to produce hydrogen and LiAl2(OH)7 or Ca2Al(OH)7 byproducts. And the remaining LiOH or Ca(OH)2 react with Al(OH)3 to produce LiAl2(OH)7 or Ca2Al(OH)7 byproducts. The reaction of Al with LiOH belongs to first order chemical reaction and is controlled by surface chemical reaction. And the apparent activation energy is about 50kJ/mol. Increasing LiOH concentration and temperature is helpful to the reaction between Al and LiOH.The research results can provide some theoretical evidences for preparing Al-based materials with optimal hydrogen generation performance.