Controllable Hydrogen Production Of Lithium Borohydride And Its Regeneration Cycle Based On Hydrates

With the goal of"carbon peak"and"carbon neutrality",hydrogen energy has become the competitive candidate in the energy field,but its large-scale application in hydrogen production,storage,and utilization still faces a series of challenges such as technology,security,and economy,etc.Hydrogen generation by the controllable decomposition of borohydrides under liquid-phase conditions is a real-time hydrogen supply technology integrating"hydrogen production/storage/transportation",which may break the bottleneck of hydrogen storage technology for scaling-up of applications.However,chemical hydrogen storage system based on liquid phase is plagued by the problems such as high cost,irreversibility and H₂ generation rate needs to be regulated,which should be solved for the large-scale application of borohydride systems.Among the borohydrides,lithium borohydride（LiBH₄）is relatively more expensive.In this thesis,the controllable hydrogen production system of LiBH₄ was studied,especially focusing on the key scientific issues such as kinetic regulation of hydrogen production by LiBH₄hydrolysis/alcoholysis and its regeneration based on crystalline hydrate.The main research contents are as below:Firstly,the existing LiBH₄-based chemical hydrogen storage system is faced with disadvantages including low H₂ generation rate and poor reaction controllability,thereby cheap transition-metal chlorides（Co Cl₂,Ni Cl₂,etc.）are firstly adopted to catalyze the hydrogen production of LiBH₄ hydrolysis and the effects of concentration,species,and dose of the chloride solutions on the hydrogen release behaviors of LiBH₄ were systematically studied.In order to break the high dependency of LiBH₄-based hydrolysis system on catalysts and the limitation that it is unable to be applied in severe cold regions or under low-temperature conditions（<0°C）,low-molecular alcohols such as methanol,ethylene glycol,and alcohol-water mixture were used to react with LiBH₄.The influencing factors and hydrogen release mechanism of alcoholysis reaction were explored,and the feasibility of LiBH₄ regeneration from the final by-product was preliminarily verified.Furthermore,NH₃ was introduced to enhance the H₂ liberation kinetics and the effective hydrogen density of LiBH₄-based hydrolysis system by forming LiBH₄·NH₃.The hydrogen storage capacity of LiBH₄·NH₃/（Co Cl₂+H₂O）solid-liquid system is 7.1 wt.%,and the hydrogen yield is up to 97%.Secondly,the LiBH₄ hydrolysis by-product/alcoholysis ultimate product LiBO₂·2H₂O was used as raw material to react with Mg-based reductants（Mg,Mg-Al,Ca Mg₂,Mg H₂,etc.）under room temperature and Ar atmosphere via high-energy ball milling,which achieves the regeneration of LiBH₄ with the crystal hydrate as green hydrogen source.The conversion yield of LiBH₄ was up to 77%,even higher than that（75%）by the commercial method.The purified LiBH₄ shows better hydrolytic performance than the purchased product,and the formation of by-product LiBO₂·2H₂O was effectively regulated by catalytic hydrolysis.According to the thermodynamic calculations,the energy efficiency of the integrated cycle of hydrogen production/storage based on LiBH₄ hydrolysis and regeneration in this thesis is about 38%.The regeneration method of LiBH₄ based on the crystalline hydrate not only omitted the dehydration process of LiBO₂·2H₂O with high-energy consumption but broke the high dependence of the existing synthesis methods on expensive hydrides（Li H,Na BH₄,etc.）,highly toxic and flammable B₂H₆ or high temperature/high pressure H₂,which greatly reduced the synthesis cost of LiBH₄.Herein,the regeneration mechanism of LiBH₄ was also analyzed by semi-in situ XRD,FTIR,NMR,MS,and hydrogen isotope tracer method.It was clarified that that H⁺in the crystalline water was firstly reduced to H⁰（H₂）by the Mg-based reducing agents,and then the generated H₂ was adsorbed,dissociated,and further reduced to H^-（Mg H₂）on the surface of Mg.Afterward,LiBH₄ was generated after a stepwise substitution of（OH）^-in[B（OH）₄]^-by the in situ formed H^-.Finally,a new method of integrated hydrogen production/storage in a closed loop based on crystal hydrate was developed by introducing CO₂ into the hydrolysis and regeneration cycle of LiBH₄.In the thesis,the effects of CO₂ and carbonate/bicarbonate on the hydrolysis performance of LiBH₄ were analyzed in detail.Then the by-products（Li₂B₄O₇·5H₂O and Li₂CO₃）from the hydrolysis reaction of LiBH₄ promoted by CO₂ were used as raw materials to react with Mg-containing reducing agents（Mg/Mg H₂）under room temperature and Ar atmosphere by ball milling,leading to the one-step regeneration of LiBH₄.Herein,the capture of CO₂（Li₂CO₃）and the reductive conversion of the carbonate to the hydrocarbon（methanation）are also achieved while LiBH₄ is recycled.Since the hydrogen in Mg-Li₂CO₃-Li₂B₄O₇·5H₂O regeneration system cannot meet the synthesis requirements of the equivalent amount of LiBH₄and CH₄,H₂/Mg H₂ was introduced to balance the ratio of Li:B:H in the ball milling system and the regeneration yield of LiBH₄ was increased to 68%.The raw materials were further optimized,that is,Li OH·H₂O was used to replace Li₂CO₃ in the above system to balance the molar ratio of Li,B and H in the regeneration system,thereby improving the atomic utilization efficiency of Li and B in Li₂B₄O₇·5H₂O.The green hydrogen source（H⁺）from Li₂B₄O₇·5H₂O and Li OH·H₂O can be converted into H^-and stored in LiBH₄,by ball milling with Mg/Mg-Al under room temperature and Ar atmosphere,enabling the regeneration of LiBH₄ without additional hydrides and H₂ as hydrogen source.The reaction mechanism of LiBH₄ was analyzed by semi-in situ XRD,FTIR,NMR,and MS,and there are mainly two steps:firstly,Li₂B₄O₇·5H₂O was transformed into LiBO₂·2H₂O intermediate under the reduction of the Mg-based materials,then LiBO₂·2H₂O further reacted with Mg H₂ formed in situ to generate LiBH₄.