An Investigation On Aluminum Activation By Alloying And Its Hydrogen Generation Performance

Putting forward hydrogen energy and promoting its application are of milestonesignificance to solve current energy crisis. However, the problem of hydrogen storage andtransportation has been limited the further application of hydrogen. In order to solve theseproblems, an online on-demand hydrogen supply system was proposed, i.e. thecombination of hydrogen producing with storage and transportation. Using active metalsto split water and generate hydrogen online and to provide fuel for following devicesimplements the safe and compact storage and transportation of hydrogen. Compared withhydrogen gas, solid active metals and water are much more convenient for storage andtransportation.As a kind of common active metal, metal Al is cheap and easy to get. Al is the mostabundant metal element in the earth’s crust. In addition, Al is trivalent metal and it is lightbecause of its low density, so Al has very high energy density. Theoretically, Al can reactwith water to generate hydrogen. Nevertheless, a dense oxide layer formed on Al surfaceprevents Al from splitting water continuously. In order to solve this problem and realizeAl-water reaction to generate hydrogen, the phenomenon of self-passivation must bebroken.The thesis focused on Al activation by means of alloying, adding low melting pointmetals Ga, In, Sn and Bi to metal Al to prepare Al alloys. The hydrogen generationperformance (including hydrogen generation rate and hydrogen yield) of Al-Ga binary,Al-Ga-In-Sn quarternary and Al-Ga-In-Sn-Bi quinary alloys with different ratios wereinvestigated. By XRD, SEM with EDX and DSC analysis, we had a knowledge of phasecomposition and microstructure of alloys and then a activation mechanism was proposed.The effect of water temperature and Al content on hydrogen generation performance ofalloys was investigated.Al-Ga binary alloys are mainly composed of Al(Ga) solid solutions, whose hydrogengeneration rate is very low and hydrogen yield is poor. The melting point of Al-Ga binaryeutectic is26.6oC, above which a liquid phase would exist in the alloy. The presence of aliquid phase in the Al-Ga alloys is necessary for Al activation, because Al solvated in this liquid phase can be protected from oxidation and passivation so that Al is able to generatehydrogen by splitting water.Al-Ga-In-Sn quarternary alloys are consist of Al(Ga) solid solutions, intermetalliccompounds In3Sn and InSn4. For quarternary alloys containing both In3Sn and InSn4phases, part of In3Sn and InSn4grow together. Generally, the hydrogen generation rate ofquarternary alloys is very fast and the hydrogen yield can reach the theoretical value. Foralloys containing In3Sn and InSn4phases at the same time, its hydrogen generation rate isfar higher than that containing only one phase. The eutectic reaction of Al withintermetallic compounds is considered as the quarternary alloys activation mechanism.Al-Ga-In-Sn-Bi quinary alloys are mainly composed of Al(Ga) solid solutions,intermetallic compounds InSn4and InBi phases. In general, the hydrogen generation rateof quinary alloys is very stable, endurable and controllable. Moreover, the hydrogen yieldis very high. Thus, the quinary alloys are suitable to provide fuel for fuel cells online.With the increase of Bi content in the alloys, the hydrogen production rate increasesfirstly and then decreases, the hydrogen yield gradually increase to theoretical value. Thequinary alloys activation mechanism includes two processes: one is the melting of Al-Gabinary eutectic; another one is the eutectic reaction of Al with intermetallic compoundsInSn4and InBi.The reaction of alloy with water is essentially the reaction of Al in the alloy withwater. As the activation components, Ga, In, Sn and Bi do not participate in the reaction.The reaction is thermodynamic so that the hydrogen generation rate was accelerated withthe increase of reaction temperature. Hydrogen generation rate decreased whenincreasing Al content in the alloy.