M. Sterlin Leo Hudson, A. Ramesh, K. Takahashi
Central University of Tamil Nadu,
Keywords: hydrogen storage, metal decorated graphene, catalysts, re-hydrogenation, kinetics
Summary:Abstract: 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, Hf¬ 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.