Catalytic Effect And Mechanism Of Ni-based Solid Solutions On Hydrogen Storage Performance Of Magnesium Hydride

With the development of human society,people's awareness of environmental protection is constantly enhanced.In recent years,green travel has become a new development trend,green new energy vehicles are gradually replacing the traditional fuel vehicles.Hydrogen fuel cell vehicles have attracted much attention because of their high energy conversion rate,zero pollution of combustion products and a wide range of fuel sources.Magnesium hydride(MgH2)is considered to be one of the most promising on-board solid hydrogen storage materials in the future due to its abundant raw materials of Mg,high hydrogen storage capacity and low cost.However,the application of MgH2 is limited by the high hydrogen desorption temperature and slow hydrogen adsorption/desorption kinetics.It was found that doping catalyst is one of the most effective methods to improve the hydrogen storage properties of MgH2.Among them,the Ni-based catalysts are particularly outstanding in improving the adsorption/desorption kinetics of MgH2.It is well known that Ni can form a solid solution with some transition metals.However,there are few reports on the catalytic effect of Ni-based solid solution on the hydrogen storage performance of MgH2.Based on the excellent catalytic effect of Ni itself,this thesis systematically investigated the catalytic effects of NiFe,NiCo,NiCu solid solutions and NiCoCu ternary transition metal catalysts on the hydrogen storage performance of MgH2 by using a combination of experimental research and first-principles calculations,and the modification mechanisms of them are explored.The specific contents are as follows:Firstly,the solid solutions of Ni-25%X(X=Fe,Co,Cu)with atomic ratio of 3:1 were successfully prepared and doped into MgH2 by mechanical ball milling.The catalytic effect and mechanism of Ni-25%X(X=Fe,Co,Cu)solid solutions on improving the hydrogen storage properties of MgH2 were studied.It was found that the initial dehydrogenation temperature of the doped MgH2 systems decreased significantly under the excellent catalysis of the Ni-based solid solutions,which decreased by about 90? compared with that of as-milled pristine MgH2 system.Among them,the Ni-25%Co solid solution has the best modification effect.The MgH2/Ni-25%Co system can release 5.19wt%hydrogen within 10min at 300?,while asmilled pristine MgH2 system can only release 1.78wt%hydrogen under the same conditions.More importantly,the dehydrogenated MgH2/Ni-25%Co can absorb 5.39wt%hydrogen within 3min at 275?,while as-milled pristine MgH2 can only absorb 0.32wt%hydrogen under the same condition.The theoretical calculation results show that the hydrogen absorption/desorption energy barriers of Mg/MgH2 system doped with Ni-based solid solutions decrease significantly,which well explains the experimental results that Ni-based solid solution has excellent modification effect on MgH2.Secondly,based on the characteristics that Ni and Cu can be infinite solid solutions,three solid solutions of Ni-25%Cu,Ni-50%Cu and Ni-75%Cu were prepared and doped into MgH2 by mechanical ball milling.The influence and mechanism of catalysts on improving the hydrogen storage properties of MgH2 were studied.Compared with MgH2/Ni-25%Cu and MgH2/Ni-75%Cu systems,MgH2/Ni-50%Cu system has the lowest initial dehydrogenation temperature and peak dehydrogenation temperature,which indicates that the Ni-50%Cu solid solution has the best catalytic effect.Meanwhile,the MgH2/Ni-50%Cu system can release 5.14wt%hydrogen within 15min at 300?,while as-milled pristine MgH2 can only release 2.92wt%hydrogen under the same conditions.More importantly,the system can still absorb 4.37wt%hydrogen within 30min even at a low temperature of 250?,while as-milled pristine MgH2 can only absorb 0.4 1wt%hydrogen.The theoretical calculation results showed that the bonding strength between Mg-H is weakened by co-doping of Ni and Cu transition metal atoms,which leads to the decrease of structural stability and the enhancement of hydrogen storage properties of MgH2 system.This may be the intrinsic reason for the excellent hydrogen storage properties of the MgH2/Ni-50%Cu system.Finally,based on the excellent modification effects of NiCo and NiCu solid solutions on MgH2,NiCoCu ternary transition metal catalysts with different mass ratios(NiCo-20%Cu,NiCo-40%Cu,NiCo-60%Cu)were synthesized and doped into MgH2 to explore its influence and mechanism on hydrogen storage properties of MgH2.It was found that the hydrogen storage properties of the MgH2/NiCoCu system was significantly improved compared with that of asmilled pristine MgH2 system.The MgH2/NiCo-40%Cu system can release 5.06wt%hydrogen within 15min at 300?,while as-milled pristine MgH2 can only release 2.93wt%hydrogen under the same conditions.More importantly,the dehydrogenated MgH2/NiCo-40%Cu can absorb 4.33wt%hydrogen within 10min at 275?,which is 3.51wt%more than as-milled pristine MgH2.However,compared with MgH2/NiCo and MgH2/NiCu systems,the hydrogen storage properties of MgH2/NiCoCu system was not further improved.