Tuning The Thermodynamic And Kinetic Properties Of Metal Hydrides And Complex Hydrides

Hydrogen is an ideal energy source for social development,while its large scale application requires high efficient hydrogen production and storage approaches.Solid-state hydrogen storage is one of the most promising ways to realize hydrogen economy,but their performances are still far from the targets for on-board applications.Metal hydride systems like Mg based alloys,Ti Cr2 based and Zr Fe2 based AB2 type alloys and unstable metal hydrides are potential candidates for solid-state hydrogen storage.To promote the applications of these metal hydrides,in this thesis,the thermodynamic and kinetic properties of them are tuned in term of forming solid solution,catalyzing,alloying and nanoconfinement.The dual-tuning of thermodynamic and kinetic properties of Mg2 Ni was achieved by the dissolution of In to form a Mg2In0.1Ni solid solution.The dehydrogenation enthalpy change for Mg2 Ni reduced from 64.5 to 38.4 k J/mol H2,and the dehydrogenation activation energy decreased from 80 to 28.9 k J/mol.However,the synthesis of Mg2In0.1Ni solid solution required the combination of prolonged sintering and ball milling.To improve the preparation efficiency,dielectric barrier discharge plasma assisted milling(P-milling)was employed to synthesize Mg-In-F and Mg-In-Al-Ti systems.P-milling was shown to simultaneously accomplish the efficient synthesis of Mg(In)and Mg(In,Al)solid solutions and the in-situ generation of Mg F2 catalyst as a dispersed dopant due to the continual impact force of ball-milling and the rapid heating effect of discharge plasma.Therefore,P-milling realized the dual-tuning of thermodynamic and kinetic properties of Mg based alloys: the dehydrogenation enthalpy change of Mg-In-F decreased from 79 to 69.2 k J/mol,and that of Mg-In-Al-Ti further reduced to 65.2 k J/mol;the dehydrogenation activation energy of Mg-In-F declined to 127.7 k J/mol from the initial value of 160 k J/mol,and that of Mg-In-Al-Ti was 125.2 k J/mol.The thermodynamic and kinetic properties of Ti Cr2 and Zr Fe2 alloys were improved by the way of alloying to develop Ti-Cr-Mn based and Zr-Fe based high-pressure alloys for use in hybrid metal hydride tank system.Among the Ti-Cr-Mn based alloys,(Ti0.85Zr0.15)1.1Cr0.9Mo0.1Mn was the optimized composition with a hydrogen capacity of 1.78 wt% and a hydrogen desorption pressure of 0.95 MPa at 0 oC.According to our calculations,if a hybrid hydrogen metal hydride tank were to be filled with 28% of(Ti0.85Zr0.15)1.1Cr0.9Mo0.1Mn alloy,the volumetric H2 density would reach the DOE 2017 target(40 kg/m3),and the gravimetric density was 2.72 wt%.In Zr-Fe based alloys,(Zr0.7Ti0.3)1.04Fe1.8V0.2 showed the best overall properties with a desorption plateau pressure and a reversible capacity of 1.12 MPa and 1.51 wt%,respectively,at 0 °C.Besides,this alloy owned a high cycling stability without obvious capacity loss even up to 200 cycles.By combining the(Zr0.7Ti0.3)1.04Fe1.8V0.2 alloy with a 35 MPa high pressure tank,the gravimetric density of this hybrid system reached 1.95 wt% at a volumetric H2 density of 40 kg/m3.Unstable metal hydrides M(Al H4)3(M = Sc,Y)were prepared by ball milling under high hydrogen pressure.Sc(Al H4)3 was extremely unstable,which decomposed into Sc H2,Al and H2 during the milling process.Y(Al H4)3 stayed in an amorphous state after milling,and its decomposition proceeded via a four-stage dehydrogenation process upon heating over the temperature range of 80-400 °C.At 80-170 °C,Y(Al H4)3 firstly decomposed into YAl H6,2Al and 3H2.With increasing temperature up to 250 °C,YAl H6 continued to release hydrogen to form YH3 and additional Al metal.Further increasing to 300 °C,YH3 released hydrogen to form YH2.As the temperature reached 350 °C,the newly formed YH2 reacted with Al to generate YAl3,and this reaction proceeded completely heating up to 400 °C.An amount of 3.4 wt% H2 was released within 30 min at 140 °C during the first dehydrogenation step.The activation energy of the first dehydrogenation step of Y(Al H4)3 was 92.1 k J/mol.The first dehydrogenation step showed a reversibility of 75% even at a low temperature of 145 °C.