Graphene decorated metal nanoclusters as an effective novel catalyst for improving the hydrogen storage behaviour of MgH2

摘要

High volumetric and gravimetric hydrogen density in MgH2 motivates significant research attention over the recent years to make it as a viable hydrogen storage material for fuel cell applications. However, the high thermodynamic stability of MgH2 (△Hf = 74.06 ± 0.42 kJmol~(-1)H2) is a major bottleneck for its practical applications. Several metal and non-metal catalysts are being investigated for improving the hydrogen sorption kinetics and recyclability of MgH2 for hydrogen storage applications. Here, we propose graphene decorated with Fe clusters (G-Fe) to be an effective alternative catalyst for dehydrogenation and re-hydrogenation reaction of MgH2. Dehydrogenation kinetics of MgH2 with 5 wt.% G-Fe is increased significantly and the formation enthalpy, △H_f is lowered to 50.4 ± 2.9 kJmol~(-1)H2. Additionally, re- hydrogenation of MgH2 with 5 wt.% G-Fe takes only 4 minutes to absorb 5 wt% of hydrogen at 300 °C and 25 atm. Hence, graphene decorated with Fe clusters is effective for both the hydrogenation and dehydrogenation processes. TEM micrograph shows that graphene decorated Fe cluster behaves like a shell while MgH2 is its core, resulting in the reduction of crystallite growth during cycling. After six re- hydrogenation cycles, the crystallite size of MgH2 increased only by 15 nm, showing a reduction in crystallite growth when compared to other metal and non-metal catalysts. Density functional theory shows that defects of graphene act as the active sites for dehydrogenation of MgH2 while Fe clusters reduce the adsorption of dissociated H atoms, resulting in low temperature dehydrogenation.Thus, graphene decorated with metal clusters could open up a new way of designing a new type of catalysts which could replace transition metal catalysts.