Enhancement in hydrogen sorption behaviour of MgH2 catalyzed by graphene quantum dots

The present investigation reports the synthesis of graphene quantum dots (GQDs) by the microwave-assisted green synthesis method, and its catalytic effect in improving the hydrogen storage behaviour of magnesium hydride (MgH2). Transmission electron microscopy, UV–Vis, Raman, FTIR and XRD analysis was done to characterize the samples. It has been observed that the as-synthesized GQDs are in the size range of 3–12 nm. The catalytic activity of GQDs on improving the de/re-hydrogenation kinetics of MgH2 has been investigated with different additive concentrations. It has been observed that there is an improvement in the hydrogen sorption characteristics of ball-milled MgH2 when 7 wt.% GQDs was employed. The onset dehydrogenation temperature of 7 wt.% GQDs admixed ball milled MgH2 (7 % GQDs-MgH2) was observed at 300 ᵒC, which is 60 ᵒC lower than that of additive free ball-milled MgH2 under identical condition. Furthermore, the 7 % GQDs-MgH2 sample reabsorbs nearly 5 wt.% hydrogen in 2.5 min at 300 ᵒC and 15 atm hydrogen pressure. The GQDs catalyzed MgH2 sample exhibit good de/re-hydrogenation cyclic stability. No significant loss in hydrogen capacity was noticed even after 25 de-/re-hydrogenation cycles. From the Van't Hoff plot, the formation enthalpy of 7 % GQDs-MgH2 was estimated to be 58 kJmol-1.