Effects Of Graphene Additives On Hydrogen Storage And Electrochemical Properties Of La-Mg-Ni Based Alloy

As some of the most promising negative electrode materials in Ni/MH batteries,La-Mg-Ni metal hydride alloys were of great interest for investigations recently compared with the traditional AB 5-type alloys owing to their high capacities and good hydrogenation/dehydrogenation properties.Graphene is a kind of new carbon material with excellent electrical conductivity,high specific surface area,which can be used to form nanocomposites and improve the high-rate dischargeability?HRD?of Ni/MH batteries.This paper mainly studied the effects of graphene and graphene nanohybrid additives on hydrogen storage and electrochemical properties of La_0.7Mg_0.3(Ni_0.85Co_0.15)₃.5 alloys from three different parts.In order to improve the overall hydrogen storage properties of La_0.7Mg_0.3(Ni_0.85Co_0.15)₃.5 metal hydride alloys,the effects of graphene addition and surface treatment on their hydrogen storage and electrochemical properties have been investigated.After the addition of graphene with the surface treatment,the discharge capacity of La_0.7Mg_0.3(Ni_0.85Co_0.15)₃.5 alloy electrodes at a high discharge current density was significantly improved as well as the high-rate dischargeability?HRD?,the limiting current density IL,the exchange current density I0,the hydrogen diffusion coefficient D and the anodic peak current Ip of the alloy electrodes were also increased.Among of them,the electrode with 2%graphene addition exhibits the most significantly advantages for electrochemical properties and kinetics.Due to good catalysis of metal nickel for the electrochemical reduction/oxidation reactions between H2O and H in alkaline electrolyte,graphene/nickel?Gp/Ni?nanocomposite have been successfully synthesized by depositing nickel nanoparticles onto graphene sheets using ethylene glycol as solvent and hydrogen as reducing agent.The effects of graphene and graphene/nickel nanohybrid additives on the hydrogen storage and electrochemical properties of La-Mg-Ni based alloy are systematically investigated by XRD,SEM,EDS and a list of conventional electrochemical tests.The results show that the discharge capacity,cycle life,discharge characteristics and electrochemical kinetics of the composite electrodes are all improved with the addition of graphene and graphene/nickel nanohybrid.Among of them,the improvement of electrochemical kinetics is most obvious after the 2%graphene/nickel nanohybrid added.Cobalt is one of the most effective elements to improve the cycle life of hydrogen storage alloys,and graphene can significantly improve the high-rate dischargeability?HRD?of Ni/MH batteries.In order to improve the overall electrochemical performance of La_0.7Mg_0.3(Ni_0.85Co_0.15)₃.5 alloy electrode,graphene/cobalt?Gp/Co?nanocomposite have been successfully synthesized by using ethylene glycol as solvent and hydrogen as reducing agent.Subsequently,we prepared as-cast and the La_0.7Mg_0.3(Ni_0.85Co_0.15)₃.5 composite electrodes with different content of Gp/Co nanocomposite by 10 min ball milling.By P-C-T and a series of electrochemical tests,we found that as the content of Gp/Co nanocomposite increased,the hydrogen storage capacity of La_0.7Mg_0.3(Ni_0.85Co_0.15)₃.5 alloys first increase and then decrease;the largest charge-discharge capacity Cmax,the high-rate dischargeability?HRD?,the limiting current density IL,the exchange current density I0 and the anodic peak current Ip of the alloy electrodes were significantly improved except the hydrogen diffusion coefficient D.