Preparation Of Mg-Ni And La-Ni Hydrogen Storage Alloys By Molten Salt Electrolysis And Study On Hydrogen Storage Properties

Nowadays,fossil energy is over-consumed and causes prodigious damage to the environment.As a resource-rich,green and environmentally friendly clean energy,hydrogen energy has become one of the preferred new energy sources to replace fossil energy.Solid hydrogen storage materials are widely used because of the advantages of safety,high efficiency,and high density,and have become one of the hotspots in the research of hydrogen storage technology.In this paper,Mg₂Ni and La Ni₅ hydrogen storage alloys were synthesized selectively by molten salt electrolysis.Different electrochemical methods such as cyclic voltammetry,square wave voltammetry,and open-circuit chronopotentiometry were employed to study the electrochemical behavior of Mg(?)and La(?)on Mo electrodes,and co-electrodeposition potential of Mg-Ni and La-Ni alloys.The composition and micro-morphology of the alloys were characterized,and the hydrogen storage performance was also studied.(1)The electrochemical behavior of Mg(?)on the Mo working electrode was studied in Li Cl-KCl molten salt at 650?,and the diffusion coefficient of Mg(?)was calculated.The potentials of co-deposition of Mg(?)and Ni(?)ions on the Mo wires in the molten salt were also investigated.The reduction peak potentials of the two intermetallic compounds of Mg-Ni alloys were-1.5 and-1.54 V(vs.Ag/Ag Cl),respectively.The Mg-Ni alloys were synthesized by potentiostatic and galvanostatic electrolysis.XRD proved that the electrolytic product was Mg₂Ni.Compared with the potentiostat,the constant current electrolysis reduces the time to 2hours,which indicates that the Mg₂Ni alloy can be more effectively prepared by the constant current method.(2)The electrochemical behavior of La(?)on the Mo working electrode was investigated in Li Cl-KCl melts at 650?,and the diffusion coefficient of La(?)was calculated.The electrochemical behavior of La(?)on Ni electrode in Li Cl-KCl system was studied,and the reduction peak potentials of the two intermetallic compounds of La-Ni alloy are-1.58 and-1.83V(vs.Ag/Ag Cl),respectively.The results of XRD showed that single La Ni₅ alloy was obtained through potentiostatic electrolysis with Ni sheet as working electrode at-1.6 V.However,the products of galvanostatic electrolysis were La₂Ni₃ and La Ni₅,which indicated that it is not feasible to synthesize a single La Ni₅ alloy by galvanostatic electrolysis.(3)For the Mg₂Ni and La Ni₅ alloys prepared by the molten salt electrolysis,the capacity storage method was used for the hydrogen storage performance test.The TPD curve was employed to test the initial hydrogen release temperature and the end hydrogen release temperature of Mg₂Ni and La Ni₅.It can be seen from the activation curve that Mg₂Ni needs 4cycles to be fully activated,while La Ni₅ needs 3 cycles to be fully activated.It can be known from the hydrogen absorption/desorption rate curve that the hydrogen storage capacity of Mg₂Ni alloy is 2.702 wt.%at a temperature of 350?;the hydrogen storage capacity of La Ni₅ alloy is1.24 wt.%at a temperature of 0?.From the PCT curve,the Mg₂Ni alloy has one plateau pressure at 1.61 MPa at 350?and the La Ni₅ alloy has a plateau pressure at 0.072 MPa at 0?.The activation energy of the reaction between Mg₂Ni and H₂ to form Mg₂Ni H₄ is calculated by JMAK theory to be 42.3 k J/mol,and derived from the Gibbs Free Energy Equation,the reaction enthalpy of La Ni₅ and H₂ to form La Ni₅H₆ is-23.61 k J/mol and the reaction entropy is-79.56J/(mol·K)..