Abstract
The present study compared the effect of different gaseous environments on physicochemical properties and subsequent hydrogen storage ability of thermally exfoliated graphene oxide (EGO). The reducing, inert or oxidizing environments were generated using hydrogen, argon or air as the carrier gas, respectively. The structure of thermally exfoliated graphene oxide depended on the type of gaseous environment. The EGO prepared in presence of Air showed the fluffiest layered structure having highest surface area. The surface area order was EGO(Air) (268 m2/g) > EGO(H2) (248 m2/g) > EGO(Ar) (155 m2/g). The average pore sizes of EGO(Air) and EGO(H2) were 2.9 and 2.8 nm, with pore volumes of 1.2 and 1.6 cm3/g, respectively. The average pore size for EGO(Ar) was highest at 4.1 nm, associated with presence of larger void space and lowest total pore volume of 1.0 cm3/g. Thus, presence of oxidative or reducing atmosphere seemed to be more conducive to exfoliation of layers by gradual removal of functional groups. The inert atmosphere of argon caused severe thermal separation of layers and functional groups, adversely affecting the layered structure as observed. The EGO(Air) also showed highest O/C ratio suggesting presence of significant amount of oxygen–containing functional groups on the surface. The hydrogen uptake order at 77 K and 30 bar was: EGO (Air) 3.34 wt.% > EGO (H2) 3.12 wt.% > EGO (Ar) 2.2 wt.%. The highest uptake of EGO(Air) might have resulted from highest surface area, highest O/C ratio and presence of considerable pore volume.
Highlights
Hydrogen is environmentally friendly and renewable source of energy [1,2,3,4]
For exfoliated graphene oxide (EGO) samples, the oxygen contents were in the range of 21.4 to 19.6 wt.% which corresponded to the O/C values between 0.18–0.20, depending on exfoliation environment
The presence of oxygen–containing functional groups in the EGO (H2) and EGO (Air) samples was confirmed by O1s peaks and the corresponding deconvolution of the peaks is shown in Fig. 3(D, F)
Summary
Hydrogen is environmentally friendly and renewable source of energy [1,2,3,4]. Hydrogen has a gravimetric energy density of 142 MJ/kg which is three times higher compared to that of gasoline (~46.8 MJ/kg) [5, 6]. Solid state storage of hydrogen is an alternative safe method Carbon based materials, such as carbon fiber, templated carbon, activated carbon, carbon nanotube, with high surface area and large pore volume have been established as potential materials for hydrogen storage [12,13,14,15,16]. Lueking et al [45] reported hydrogen uptake capacity of 1.2 wt.% at 20 bar for exfoliated graphite nanofibers prepared in presence of argon and having surface area of 555 m2/g. At room temperature the hydrogen uptake capacities of graphene oxides exfoliated in different gaseous environments were reported in the range of 0.10–0.49 wt.% at different pressures (20–90 bar) as shown in Supplementary Table S2 [12, 45, 47]. The literature survey showed that no systematic comparative studies, have done on the effect of different gaseous environments on the physicochemical properties of the thermally exfoliated graphene and their effect on the hydrogen storage properties to the best knowledge of the authors
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