Preparation And Electrocatalytic Properties Of Ag Modified A₂B₇-type La-Mg/Y-Ni Hydrogen Storage Alloy

Direct borohydride fuel cell（DBFC）is a clean energy conversion device that converts the chemical energy of fuel into electrical energy.It has attracted much attention due to its advantages of high energy density,high conversion rate and environmental protection.The anode catalyst of DBFC is the decisive factor to determine the performance of DBFC.The search for anode catalysts with high catalytic activity and low cost is the premise of large-scale application of DBFC.In recent years,it has been found that the use of hydrogen storage alloys as DBFC anode catalysts can not only improve fuel utilisation,but also effectively reduce catalyst costs.However,the electrocatalytic performance of hydrogen storage alloys when used as DBFC anode catalysts is closely related to the electrochemical properties of the hydrogen storage alloy.In addition,hydrogen storage alloys tend to oxidise under alkaline electrolytes and form inert electrodes,leading to a reduction in the number of active sites on the alloy surface.Therefore,high-capacity A₂B₇hydrogen storage alloys with a superlattice structure are used as base alloy catalysts in this paper In order to increase the active sites on the surface of the alloy,La-Mg-Ni@Ag composite alloy was prepared by surface modification of A₂B₇hydrogen storage alloy with noble metal Ag.In this study,the structure,electrochemical properties and electrocatalytic properties of A₂B₇-type hydrogen storage alloy and La-Mg-Ni@Ag composite alloy were studied,and the intrinsic relationship between them was explored.Some main research results were obtained:（1）The A₂B₇-type La-Mg-Ni hydrogen storage alloy was prepared by vacuum induction melting method,and then treated by spark plasma sintering（SPS）and rapid quenching,respectively.The structure,electrochemical properties and catalytic oxidation of BH₄^-of the hydrogen storage alloys obtained by the three methods were studied in comparison.The results show that the as-cast alloy consists of La Ni₅,La₂Ni₇and La Ni₃phases.SPS treatment has little effect on the phase structure of the alloy.Although it is able to increase the maximum discharge capacity,it leads to a significant reduction in catalytic activity.The disappearance of La₂Ni₇and La Ni₃phases from the quenched alloy,which consists almost entirely of La Ni₅phase,causes a decrease in maximum discharge capacity and deterioration of catalytic properties.In general,the electrochemical and electrocatalytic properties of the as-cast alloy are the best.（2）In order to overcome the difficulties in the preparation of La-Mg-Ni hydrogen storage alloy,A₂B₇type La-Y-Ni hydrogen storage alloy with similar crystal structure to La-Mg-Ni hydrogen storage alloy was selected.Then,the as-cast La-Y-Ni hydrogen storage alloy was annealed at 850℃,950℃and 1050℃for 24 hours.The structure,electrochemical properties and the catalytic oxidation of BH₄^-of the La-Y-Ni hydrogen storage alloy before and after annealing were studied in comparison.The results show that the structure,electrochemical properties and catalytic oxidation of BH₄^-have been improved after annealing.Moreover,the change in catalytic performance is similar to the change in maximum discharge capacity,both increasing and then decreasing with increasing annealing temperature.When the annealing temperature is 950℃,the hydrogen storage alloy exhibits the best electrochemical and electrocatalytic properties.（3）To increase the active sites on the surface of the La-Mg-Ni hydrogen storage alloy,La-Mg-Ni@Ag composite alloy was prepared by dispersing precious metal Ag on the surface of A₂B₇type La-Mg-Ni hydrogen storage alloy using UV radiation dispersion method.The structure and electrocatalytic properties of La-Mg-Ni@Ag composite alloy were investigated.The results show that compared with La-Mg-Ni alloy,the oxidation peak current density of La-Mg-Ni@Ag composite alloy increases by about 50%,and the number of transferred electrons reaches 4.97,which is much larger than 1.91 of La-Mg-Ni alloy.Ag modified La-Mg-Ni hydrogen storage alloy effectively improved the catalytic activity and stability of the hydrogen storage alloy catalyst.