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 π-π 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∥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.