Study On Hydrogen Storage And Thermal Storage Properties Of Na-Mg Bimetallic Hydrogen Storage Composites Modified By Ti-based Catalysts

Confronted with the new developing demand of hydrogen economy in the new era,study on the high-capacity hydrogen storage materials has been the hotspot chased by researchers around the world.The Na/Mg-based binary metal hydride with typical perovskite-type,NaMgH₃,stands out among many hydrogen storage materials at present and becomes one of the most promising energy storage media in the field of hydrogen storage and heat storage applications,due to its high capacity,thermochemical stability,good reversibility,easy access to raw materials and low cost.Unfortunately,as a hydrogen storage material,the initial temperature and peak temperature of hydrogen ab/desorption of NaMgH₃ are much higher than expected,the kinetics of hydrogen ab/desorption reaction is slow and the cycling stability is poor.As a heat storage material,NaMgH₃ confronted with the same problem of slow kinetics and poor cycling stability.Therefore,to counter the drawbacks of this system,the kinetic,cycling and thermochemical performances are improved by doping with Ti-based transition metal catalysts.Moreover,the advanced materials microstructure analysis technologies are used to figure out the effect mechanisms of transition metal Ti-based catalysts on improving the hydrogen storage and thermal storage performances of NaMgH₃.Finally,based on the best improved NaMgH₃ composite,the model of thermal energy system is constructed preliminarily to further exhibit the thermal storage performances of NaMgH₃ in practical application.Commerical TiO₂ microparticles?MP?,commercial TiO₂ nanoparticles?NP?and TiO₂ nanotubes?NT?prepared by an alkaline hydrothermal method were introduced to NaMgH₃.Among them,TiO₂ NT shows the best catalystic on improving the hydrogen desorption kinetics of NaMgH₃.The initial temperature of dehydrogenation is decreased to 300°C.NaMgH₃-TiO₂ NT exhibits complete desorption of hydrogen with capacity of 3.4 wt.%at 350°C in 10 min,and the apparent activation energies?E_a?of both two steps reactions decrease to 91.7 kJ/mol and 142.1 kJ/mol.Also,the thermochemical property stays stable.During the ball-milling and heat treatment,TiO₂NT is partly reduced and in-situ forms Na_0.46TiO₂ and Ti,which not only change the kinetic model of dehydrogenation of NaMgH₃,but also improve the efficiency of e^-transfers and decrease the energy barrier of hydrogen desorption reaction.Hence,based on the mechanism above,the kinetics of dehydrogenation of NaMgH₃ is dramtically enhanced.The lamellar-structure Ti₃C₂ was prepared via etching the Ti₃AlC₂ with the in-situ HF.And five composites of NaMgH₃-x wt.%Ti₃C₂?x=0,3,5,7 and 9?were synthesized by ball-milling.Among them,the NaMgH₃-7 wt.%Ti₃C₂ composite exhibits the best catalystic efficiency and should represent the optimal concentration.The initial temperature of dehydrogenation is decreased to 250°C.It can also release4.8 wt.%H₂ at 365°C in 15 min and absorb 3.5 wt.%H₂ at 300°C in 6 s.The apparent activation energies?E_a?of both two steps reactions decrease to 114.08 kJ/mol and139.40 kJ/mol.Additionally,the cycling stability of NaMgH₃ is remarkably enhanced with the rate of capacity loss decreasing from 25%to 12%.The significantly improved properties of kinetics and cycling are resulted from that the lamellar-structure Ti₃C₂ are ball milled into small pieces of lamellar-structure with NaMgH₃ particles embedded in.After the microstructure analysis,it's cleared that the unique morphology not only can refine the size of NaMgH₃ particles and reduce the energy barrier of reaction,but also prevent the growth and aggregation of NaMgH₃ particles during cycling.Moreover,the thermal storage density of NaMgH₃-7 wt.%Ti₃C₂ composite still stays at a high level,confirmed to be 2562 kJ/kg.Based on catalytic effect of Ti-based catalysts?TiO₂ and Ti₃C₂?on NaMgH₃,the NaMgH₃-7 wt.%Ti₃C₂ composite was chosen to built the thermal energy storage system of NaMgH₃-7 wt.%Ti₃C₂/LaNi₅.After hydrogen ab/desorption pressure platform tests of NaMgH₃-7 wt.%Ti₃C₂ and LaNi₅,the working termperatures of this thermal energy storage system was speculated.It's proved that expended natural graphite?ENG?can optimize comprehensive thermal storage performance of NaMgH₃-7 wt.%Ti₃C₂ composite.It's indicated that NaMgH₃-7 wt.%Ti₃C₂@ENG exhibits a better thermal conductivity of 2.40 W m^-1 K^-1.The thermal energy mass density and thermal energy volumetric density of NaMgH₃-7 wt.%Ti₃C₂@ENG remain at a relatively high level and are calculated to be 2458 kJ/kg and 454.44 kWh_th m^-3.Also,NaMgH₃-7 wt.%Ti₃C₂@ENG shows an outstanding cycling endo/exothermicity performance.During the 10 cycles,the peak temperatures of NaMgH₃-7 wt.%Ti₃C₂@ENG consistently go up from 440°C to 451°C,demonstrating the excellent kinetics and heat conduction properties.The valus of heat conduction of 1.5 g NaMgH₃-7 wt.%Ti₃C₂@ENG is calculated to be 3247.37 J for the first cycle.After 10 cycles,the value of heat conduction remains to be 3146.92 J with a decay rate of 4.0%and the rate of heat conductivity is calculated to be 85.35%.