Synthesis And Regeneration Of Metal Borohydride Combining Hydrogen Storage And Production Based On Hydrates

Metal borohydride(NaBH₄ or Mg(BH₄)₂)is one of the most studied hydrogen storage materials;however,it is still challenging to produce metal borohydride with low cost and high efficiency,which are largely determined by the sources of boron and hydrogen and reducing agents used.In this study,hydrated borax(Na₂B₄O₇·10H₂O or Na₂B₄O₇·5H₂O),CO₂ treated hydrolytic product of NaBH₄(Na₂B₄O₇·10H₂O)and boric acid(B₂O₃·3H₂O)are used for NaBH₄ or Mg(BH₄)₂(re)generation by mechanochemical method under ambient conditions.In the first section,this thesis reports an economical method to generate NaBH₄ by ball milling hydrated borax(Na₂B₄O₇·10H₂O and/or Na₂B₄O₇·5H₂O)with different reducing agents such as MgH₂,Mg,and NaH under ambient conditions.The direct use of natural hydrated borax avoids the dehydration process(at 600°C)and consequently reduces cost and improves overall energy efficiency.A high yield of 93.1%can be achieved for a short ball mill duration(3.5 h)for Na₂B₄O₇·5H₂O-NaH-MgH₂ system.The reasons for such high yield lie in several aspects.Firstly,in contrast to anhydrous borax used in the literature,hydrated borax in this work tends to react more effectively with MgH₂ which facilitates the conversion from B-O to B-H.Secondly,H₂ is simultaneously generated in this process which acts as an extra hydrogen source and minimizes the agglomeration of ball milled powders and leads to high surface areas.In Na₂B₄O₇·5H₂O-NaH-MgH₂ system,H₂ is generated in situ which subsequently reacts with Mgforming MgH₂.Low cost Mgis therefore employed to replace the majority of MgH₂,leading to an attractive yield of 78%.To further reduce the cost of raw materials and improve the utilization of hydrogen source in the hydrated borax,Na₂B₄O₇·10H₂O is used to partially substitute for Na₂B₄O₇·5H₂O,leading to a complete replacement of MgH₂.The optimized recipe features low cost and high efficiency since it utilizes hydrogen from the hydrated water in natural borax and avoids high temperatures.Mechanistic studies of NaBH₄ synthesis on the different systems were carried out by ex situ XRD,FTIR,NMR,and MS analysis.The results show that the reaction mechanisms of Na₂B₄O₇·10H₂O and Na₂B₄O₇·5H₂O systems is different.In the Na₂B₄O₇·10H₂O systems,Na₂B₄O₇·10H₂O transforms into NaB(OH)₄ by reaction with NaH,whereas Na₂B₄O₇·5H₂O converts into amorphous Na₂B₄O₅(OH)₄ first.Then,NaB(OH)₄and Na₂B₄O₅(OH)₄ react with MgH₂ to form NaBH₄.In the second section,the hydrolytic product NaBO₂ in aqueous solution reacts with CO₂(in the air),forming Na₂B₄O₇·10H₂O and Na₂CO₃,both of which are ball-milled with low-cost Mgunder ambient conditions to regenerate NaBH₄ and simultaneously convert CO₂ into CH₄.A new closed-loop integrated technology for hydrogen storage/generation is developed.Compared with previous studies,this approach avoids expensive reducing agents such as MgH₂,bypasses the energy-intensive dehydration procedure to remove water from Na₂B₄O₇·10H₂O,and does not require high pressure H₂ gas,therefore leading to much reduced costs.Ex situ XRD,FTIR,NMR,and MS analysis are carried out to explore the mechanism of NaBH₄regeneration.The results show that Mgreacts with Na₂B₄O₇·10H₂O to form Na₂B₄O₇·5H₂O in the beginning,followed by reaction with in situ formed MgH₂ to generate NaBH₄.Particularly,the intermediate compound,H₂BOH,is successfully observed for the first time during the regeneration process.Furthermore,the reasons for high yield of NaBH₄ are analyzed.In the third section,based on the fact that Al is more abundant and can be oxidized into a higher valance state,this thesis extends the method of(re)generation NaBH₄ by ball milling Na₂B₄O₇·10H₂O with Al and its alloys(Al₈₈Si₁₂,Mg₁₇Al₁₂ or Mg₂Al₃),instead of Na,Mgor Ca.An effective strategy is developed to facilitate mass transfer during the reaction by introducing NaH to enable the formation of NaAl O₂ instead of dense Al₂O₃ on Al surface,and by using Si as a milling additive to prevent agglomeration and also break up passivation layers.The highest yield of NaBH₄(?62%)is achieved by ball milling a mixture of Na₂B₄O₇·10H₂O,NaH,Si and Al under Ar atmosphere.And a higher yield of 85.2%can be achieved for using Mg17Al12.Finally,to confirm the universality for the(re)generation of borohydride from hydrate,B₂O₃·3H₂O is ball-milled with MgH₂ under room temperature to form Mg(BH₄)₂ with considerable yield(close to 70%).Besides,Mg(BH₄)₂ is successfully generated by replacing expensive MgH₂ with Mg,without the need of any external H₂ source.Moreover,the yield of Mg(BH₄)₂ increases to 80%when B₂O₃·3H₂O is substituted with B₂O₃ to react with MgH₂.Mechanistic studies of Mg(BH₄)₂ synthesis on the B₂O₃-MgH₂ system were carried out by ex situ XRD,FTIR,NMR,and MS analysis.The results show that H^-in MgH₂ plays a crucial role in Mg(BH₄)₂ formation.