Improving hydrogen storage thermodynamics and kinetics of Ce-Mg-Ni-based alloy by mechanical milling with TiF_(3)

Mg-based hydrides are too stable and the kinetics of hydrogen absorption and desorption is not satisfactory.An efficient way to improve these shortcomings is to employ reactive ball milling to synthesize the nanocomposite materials of Mg and additives.In this experiment,TiF_(3)was selected as an additive,and the mechanical milling method was employed to prepare the experimental alloys.The alloys used in this experiment were the as-cast Ce_(5)Mg_(85)Ni_(10),as-milled Ce_(5)Mg_(85)Ni_(10)and Ce_(5)Mg_(85)Ni_(10)+3 wt.%TiF3.The phase transformation,structural evolution,isothermal and non-isothermal hydrogenation and dehydrogenation performances of the alloys were inspected by XRD,SEM,TEM,Sievert apparatus,DSC and TGA.It revealed that nanocrystalline appeared in the as-milled samples.Compared with the as-cast alloy,ball milling made the particle dimension and grain size decrease dramatically and the defect density increase significantly.The addition of TiF_(3)made the surface of ball milling alloy particles markedly coarser and more irregular.Ball milling and adding TiF_(3)distinctly improved the activation and kinetics of the alloys.Moreover,ball milling along with TiF_(3)can decrease the onset dehydrogenation temperature of Mg-based hydrides and slightly ameliorate their thermodynamics.

Hydrogen storage thermodynamics and kinetics of as-cast Ce-Mg-Ni-based alloy

The reaction kinetics of alloys based on magnesium are known to be greatly improved by the partial substitution of Mg with rare earths and transition metals,particularly Ni.The enhanced superficial hydrogen dissociation rate,the weakened Mg-H bond and the lower activation energy following element replacement are thought to be related to the better performance.The experimental alloys Ce5Mg_(95-x)Ni_(x)(x=5,10,15)were smelted by the vacuum induction melting.The phase transformation and structural evolution of experimental alloys before and after reaction with hydrogen were char-acterized by X-ray diffraction,scanning electron microscopy and transmission electron microscopy.The cast specimens contain CeMg_(12),Mg and Mg_(2)Ni phases,and the increase in Ni content results in an obvious growth of Mg_(2)Ni phase.The isothermal and non-isothermal hydrogenation and dehydrogenation kinetics of the experimental specimens were investi-gated using the Sievert apparatus,differential scanning calorimetry and thermal gravimetric analyzer.The activation energy may be calculated using the Arrhenius and Kissinger equations.The experimental alloys have been shown to have good activation properties,with a reversible hydriding and dehydriding capacities of around 5.0 wt.%in the first cycle.The initial dehydrogenation temperature of MgH_(2) decreases from 557.5 to 537.7 K with changing Ni content from 5 to 15 at.%.The dehydrogenation activation energy also reduces from 77.09 to 62.96 kJ/mol,which explains the improved hydrogen storage performance caused by Ni substitution.It can be shown that the impact of Ni on the decomposition enthalpy of MgH_(2) is quite modest,with the absolute enthalpy(ΔHr)only decreasing from 78.48 to 76.15 kJ/mol.

Research and application of Ti-Mn-based hydrogen storage alloys

Ti–Mn-based hydrogen storage alloys are considered to be one of the most promising hydrogen storage alloys for proton exchange membrane fuel cell applications,because of their good hydrogen absorption and desorption kinetics,low price,good activation performance,possession of high electrochemical capacity,and good cycling performance.The structure,performance characteristics,crystal structure of hydrides,development and application status of Ti–Mn-based hydrogen storage alloys were reviewed,and the methods to improve Ti–Mn-based hydrogen storage alloys were discussed:optimization of the preparation process,element substitution,and surface treatment.(1)In the study of the alloy preparation process,it was found that the use of the annealing process can significantly improve the high rate discharge performance,and cycling stability performance,increasing the maximum discharge capacity of the alloy electrode.In addition,using vacuum plasma spraying to prepare the electrode has better cycling stability and kinetic performance.(2)In element substitution,the effects of using Zr elements to partially replace Ti and Mn with Cr,V,Mo,and Fe on the hydrogen storage properties of Ti–Mn-based alloys were investigated.(3)In the study of surface treatment,palladium was plated on the surface of TiMn_(1.5) alloy by chemical deposition,and the strong affinity of palladium for hydrogen accelerated the cleavage of hydrogen molecules,which significantly improved the hydrogen absorption kinetics of TiMn_(1.5) alloy.Meanwhile,a new binary alloy system was formed by adding TiMn_(2) to MgH_(2),and it was shown that the addition of TiMn_(2) significantly improved the hydrogen absorption/desorption kinetics of the MgH_(2) alloy.Finally,the prospect of the application of Ti–Mn-based hydrogen storage alloys is presented,and the insight of further development of the alloy is offered.