Green synthesis of vertical graphene nanosheets and their application in high-performance supercapacitors

Vertical standing graphene sheets are highly desirable in energy storage applications because without p-p stacking their surface can be fully utilized. In this work, vertical graphene nanosheets (VGS) are successfully synthesized on nickel foam via a simple plasma enhanced chemical vapor deposition (PECVD) technique. Instead of hazardous and costly hydrocarbon gases, we adopt a green approach by using a low-cost, non-toxic, sustainable and environmentally-friendly natural organic material, M. alternifolia essential oil (containing a hydrocarbon monomer), as the precursor. The 4 minute deposition duration results in multilayered horizontal graphene (h-GS) with sparsely distributed vertical graphene while 16 minute deposition leads to fully covered vertical graphene nanosheets (f-VGS). To demonstrate their application as a conductive and high-surface-area substrate in energy storage, MnO2 thin films are hydrothermal grown to form MnO2@f-VGS core-shell structure and MnO2@h-GS. The core@shell electrode of MnO2@f-VGS demonstrates a significantly higher specific capacitance of 203 F g(-1) at a current density of 10 A g(-1) compared to that of 82 F g(-1) at 10 A g(-1) shown by MnO2@h-GS. Moreover, the assembled full supercapacitors containing MnO2@f-VGS parallel to active carbon as electrodes can deliver a reasonably high specific capacitance of 250 F g(-1) at 2 A g(-1). Such f-VGS may have application also in battery and fuel cell electrodes.