Establishment Of LiBH₄Destabilization Systems And Their Hydrogen Storage Properties And Mechanism

Taking an overview of the state-of-the-art of hydrogen energy, the largest challenge is torealize a reversible high hydrogen capacity system with favorable thermodynamic and kineticcharacteristic via solid-state hydrogen storage. In this doctoral dissertation, the hydrogenstorage property of LiBH4, which possesses very high density (18.4wt.%and121kg/m3),was improved through destabilization and catalysis, aiming at achieving a reversible highhydrogen system with superior de/rehydrogenation performance.A destabilizing reversible system of LiBH4-NdH2+xis established to reduce thedehydrogenation enthalpy change of LiBH4from74kJ mol-1H2to64kJ mol-1H2, andfacilitate6.0wt.%hydrogen emission within1.5h at370oC. The reversibility of thisdestabilization reaction is nanosize-controlling by NdH2+xnessecitating particle size of10nm.During the subsequent cycles, NdH2+xobviously coarsens and the destabilization effectdiminishes, but it still has positive effect on the dehydrogenation of LiBH4. LiBH4-NdH2+xsystem shows superior reversibility under rehydrogenation conditions of400oC and10MPa,with cyclic dehydrogenation capacity as high as5.3wt.%.We demonstrate a significant destabilization effect of alkali metal hydroxides(LiOH/NaOH/KOH) on the solid-state dehydrogenation of LiBH4. This destabilization effectis originated from the interaction between H+in [OH]ˉand Hˉin [BH4]ˉ. The destabilizationeffect is in the order of LiOH> NaOH>KOH, because of larger Pauling’s electronegativity ofalkali metal (Li> Na>K) leading to more acidic of the proton donor [OH]ˉsite. Besides, thestoichiometry of [BH4]ˉ:[OH]ˉhas a great effect on the destabilization effect. With theincremental amount of LiOH, the dehydrogenation kinetics is enhanced, but the temperatureis increased. LiBH4-LiOH and LiBH4-4LiOH composites liberate4.1wt%and6.5wt%hydrogen in10minutes at207°C and250°C, respectively. This is ascribed to the different[BH4]ˉ[OH]ˉconfigurations showing different interaction. LiBH4-alkali metal hydroxidessystem is irreversible under rehydrogenation conditions of400oC and10MPa.Nanostructured CoSxis synthesized and doped into LiBH4by ball milling. Thedehydrogenation of LiBH4-CoSxsystem goes through three stages. The first stage takes placeat a peak temperature of175oC, and6.7wt.%hydrogen is released from LiBH4within1h with less B2H6emission. This is the reaction between LiBH4and CoSxresulting in theformation of elemental Co. The second stage shows a peak temperature of300oC, which isthe reaction between LiBH4and Co forming CoB compound. The third stage is above350oC,and it is speculated that the catalysis effect of CoB facilitates the dehydrogenation of LiBH4.LiBH4-CoSxsystem shows partial reversibility under the rehydrogentaion conditions of400oC and10MPa, and the cyclic dehydrogenation is3.0-4.0wt.%.Based on the speculation of catalysis of CoB, nanostructured CoB with5differentmorphologies is synthesized by chemical reduction process, and then doped into LiBH4byball milling. CoB plays as catalyst and its activity is in the order of mulberry-like>waxberry-like> chain-like> flake-like> rod-like, which is in accordance with their specificsurface area from high to low. Waxberry-like and mulberry-like CoB dramatically enhancedehydrogenation of LiBH4at200°C, leading4.8and4.6wt.%hydrogen liberation in3hrespectively, and more than2wt.%hydrogen is reversible for the former under260°C and10MPa. Mulberry-like CoB shows the best catalytic effect at350°C, with which10.4wt.%hydrogen liberated from LiBH4in1h. Significantly, almost full reversibility was realizedunder400°C and10MPa, remaining9.6wt.%at the fourth cycle.A novel system of LiBH4-h-BN possessing high capacity and excellent reversibility isestablished through adding nanoscale h-BN into LiBH4by ball milling. The stoichiometricratio of LiBH4:h-BN shows great effect on the improving effectiveness. As the ratio ofLiBH4:h-BN increases from1:0to1:3, the on-set dehydrogenation temperature is reducedfrom290°C to175°C, and the peak temperature decreases from470°C to390°C, and thedehydrogenation kinetics is gradually increased at400°C. LiBH4-3h-BN system showsexcellent reversibility under moderate conditions of400oC and10MPa, which is as high as11.5wt.%exceeding its theoretical value of4.2wt.%. This work suggests that LiBH4-h-BNsystem is a novel candidate for hydrogen storage with high capacity and excellentreversibility.