A Study On Hydrogen Generation By Hydrolysis Of Al-Li Based Mixture

In this thesis the hydrogen generation performance of theas-prepared Al Li based mixtures were systematically studied. The effects ofalloy or mixture composition, reaction temperature, preparation technologyand other processing parameters on the hydrolysis of Al Li systemperformance were clarified. Based on the experimental results of hydrolysisperformance and the analysis of electrochemistry, microscopic structure andother characterization methods used to reveal the formation mechanism ofAl Li based system, the alloy or mixture composition and preparationprocessing were optimized. The hydrolysis mechanism of the Al Li basedsystem was tentatively proposed. The main conclusions in this work are listedas follows,1. Hydrogen generation performance and hydrolysis mechanism of Al Lialloys. In the present work, the Li element is first proposed to add into Al toform Al/Li alloy in an attempt to increase the Al hydrogen evolution amountand improve the hydrolysis reactivity. The results showed that the addition ofLi could significantly improve the hydrolysis rate of AlLi-based mixtures andincrease hydrogen generation amount. When Li amount was increased up to20wt%, the efficiency was close to100%. The powder X-ray diffractionanalysis indicated that some new phases, such as AlLi, Al₂Li₃and Al4Li9,were formed during the milling process. With the increase of Li content, therewas a transformation from simple AlLi to complex alloy Al₂Li₃and Al4Li9phase, which led to the decreased particle size and increased surface areatherefore. The enhanced hydrolysis rate of Al Li alloy was mostly attributedto the presence of the new phases which acted as active reaction centers inwater. The hydrolysis byproducts LiOH and released heat from Li hydrolysiscould improve the hydrolysis kinetics of Al. LiOH did not only accelerate Alhydrolysis, but also reacted with Al（OH）₃（bayerite） to produce new compound LiAl₂（OH）₇·2H₂O. With the increase of Li content, moreLiAl₂（OH）₇·2H₂O compound was produced from the reaction of Al（OH）₃andLiOH, confirming that the addition of Li did change the hydrolysis mechanism.Different preparation processing and hydrolysis conditions affected thehydrolysis performace of Al Li alloy. Prolonging ball-milling time andincreasing solution temperature could improve the hydrolysis kinetics ofAl Li alloy.2. Effect of other metals （Sn, Bi, Ga and In） on the hydrogen generationperformance of Al Li alloy. Adding the metals into the Al Li alloy with lowLi content could improve the hydrolysis rate and increase hydrogen generationamount. The microstructure observation indicated that the addition of Li andSn was helpful to the formation of new LixSnyphase which distributed in thealuminum matrix to form LixSny Al active center. Prolonging milling timeand increasing Li/Sn weight ratio both could increase the amount of LixSnyphase and also make it distributed in Al matrix uniformly. The LixSny Alactive center was conducive to form dual micro galvanic cell of LixSnyandAl Sn, The LixSnyphase acted as the initial hydrolysis center on Al surfacebecause standard potential of the alloy was much higher than1.29V of waterdecomposition potential. The hydrolysis byproduct LiOH during thehydrolysis process did promote the corrosion of Al. There was an evolutionfrom LiSn and Li₅Sn₂phase to complex Li₁₃Sn₅phase with the increase ofLi/Sn weight ratio from1:7to3:1in the process. Hydrogen generation amountof Al Li Sn alloy was up to1147mL g^-1with99%efficiency as Li/Sn weightratio was1:1. In addition, based on the hydrolysis mechanism of microgalvanic cell, Al-Li alloys with the addition of other metal （Sn, Bi, Ga or In）also showed good hydrogen generation performance.3. Hydrogen generation and hydrolysis mechanism of Al Li/NaBH₄mixture. The results showed that the hydrolysis byproduct Al（OH）₃couldpromote the NaBH₄hydrolysis. Hydrolysis byproduct NaBO₂presentedalkaline and could accelerate Al hydrolysis. Hydrogen generation of Al15wt%Li/NaBH₄mixture （weight ratio1:1） could generate1067mL g^-1 hydrogen with64%efficiency. Adding CoCl₂salt into the Al Li/NaBH₄mixture could significantly promote the hydrogen generation performance ofthe mixture. Al10wt%Li5wt%CoCl₂/NaBH₄mixture （weight ratio of1:1）generated1747mL g^-1hydrogen with100%efficiency, and the maximumhydrogen generation rate reached262mL g^-1min^-1in the present work. Thehydrolysis byproducts LiOH and Co₂B were the major factors to improve thehydrogen generation performance of the Al Li/NaBH₄mixture. Co2B haddouble catalytic effects on Al/NaBH₄hydrolysis. Co₂B/Al（OH）₃was a goodpromoter for NaBH₄hydrolysis. It also acted as a cathode and formed microgalvanic cell with anode Al to accelerate the corrosion of Al. By optimizingthe composition design and hydrolytic conditions of the system couldeffective control the hydrolysis performance and hydrogen generation amountof Al Li/NaBH₄mixture.Aluminum lithium system is an ideal hydrogen source for portablehydrogen fuel cell system with many advantages including good hydrogengeneration performance, high hydrogen purity and high hydrogen storagevalue.