Experimental and first principle studies on hydrogen desorption behavior of graphene nanofibre catalyzed MgH2

Abstract

With the combination of experiment and first-principles theory, we have evaluated and explored the catalytic effects of graphitic nanofibres for hydrogen desorption behaviour in magnesium hydride. Helical form of graphene nanofibres (HGNF) have larger surface area, curved configuration and high density of graphene layers resulting in large quantity of exposed carbon sheet edges. Therefore they are found to considerably improve hydrogen desorption from MgH2at lower temperatures compared to graphene (onset desorption temperature of MgH2catalyzed by HGNF is 45 °C lower as compared to MgH2catalyzed by graphene). Using density functional theory, we find that graphene sheet edges, both the zigzag and armchair type, can weaken Mg[sbnd]H bonds in magnesium hydride. When the MgH2is catalyzed with higher electronegative and reactive graphene edge of graphene, the electron transfer occurs from Mg to carbon, due to which MgH2is dissociated into hydrogen and Mg[sbnd]H component. The Mg gets bonded with the graphene edge carbon atoms in the form of C[sbnd]Mg[sbnd]H and C[sbnd]H bonds. In the as formed C[sbnd]Mg[sbnd]H, the graphene edges “grab” more electronic charge as compared to the normal charge donation of Mg to H. This leads to the weakening of the Mg[sbnd]H bond, causing hydrogen to desorbs at lower temperatures. © 2016 Hydrogen Energy Publications LLC