The Synthesis,Structures And Electrochemical Properties Of Mg-Co-based Hydrogen Storage Electrode Materials

Mg-based hydrogen storage alloys are a kind of hydrogen storage materials with high capacity and low cost.Among these alloys,Mg-Co-based alloys attracted much attention in recent years due to its remarkable hydrogenation capacity at low temperature.Previous study indicates that their major phase structures are body-centered cubic(BCC)structure,which is favorable for the hydrogen absorption,especially when their grain sizes are nano-scaled.However,they would not desorb any stored hydrogen even in vacuum.Therefore,the present work will explore the reversible hydrogen storage(charge-discharge)performances of Mg-Co-based alloys especially in electrochemical condition.By means of mechanical alloying,a series of Mg-Co-based alloys were synthesized,such as MgxCo100-x binary alloys,Mgx-Co100-x-5at.%Pd ternary alloys,Mg45M5Co50(M= Zr,Ti,Pd)alloys,Mg50Co45Pd5 ternary alloys,Mg67-xPdxCo33(x =1,3,5,7)ternary alloys,and so on.Except for the Mg67Co33 and the short-time-milled Mg67Co33-5 at.%Pd alloys,all of the Mg-Co-based alloys possess homogeneous BCC structure.Their grain sizes are nano-scaled according to analyses by X-ray diffraction(XRD)and transmission electron microscope(TEM).The tested results of charge-dischrage show that the initial discharge capacities of binary Mg-Co alloys would increase at first and then decrease with the increment of Mg content,while the capacities of ternary Mg-Co-5 at.%Pd alloys increased.The cyclic stabilities of both binary Mg-Co alloys and ternary Mg-Co-5 at.%Pd reduced with the raise of Mg content.As for partially alloyed Mg67Co33,the capacity was only 14.7 mAh g-1.By additive 5 at.%Pd,the capacities and cyclic satbilities of alloys rose with increased ball-milled time.By substitution of Pd for Mg,the capacities of alloys first increased and then decreased with the increment of Pd content.The Mg64Pd3Co33 possesses the highest discharge capacity of 624.3 mAh g-1.The electrochemical kinetics,properties of alloys were enhanced with the increment of Mg amount.The adding or substitution by Pd could lift the electrochemical properties.The corrosion current densities of alloys tested by Tafel polarization increased with the increment of Mg content,while decreased with the doping or substitution by metallic Pd.XRD patterns of Mg67-xPdxCo33(x =1,3,5,7)ternary alloys implied the presence of Mg(OH)2 and Co(OH)2 on the surfaces of alloys.The X-ray photoelectron spectroscopy(XPS)analysis on alloys electrodes after 5 cycles of charge-discharge informs that the corrosion of Mg could be depressed by doped Pd.The dissolved magenesium ions in KOH solution decrease with the increment of Pd element,which was observed by inductively coupled plasma mass spectrometry(ICP-MS)test.Therefore,the loss of discharge capacity might be resulted from the formation of irreversible hydroxide products,such as Mg(OH)2,surface reaction deactivation and the dissolved magnesium in KOH electrolyte.