From hydroxyl group to carbonyl group: Tuning the supercapacitive performance of holey graphene

Both hydroxyl group and carbonyl group are the inborn active sites on graphene for energy storage. However, it is difficult to distinguish the definite influence of these functional groups on the supercapacitive performance of graphene due to the considerable changes of total oxygen content (TOC) when adjusting the content of hydroxyl group and carbonyl group on graphene. In this paper, holey graphene was prepared by reoxidation method with ammonium persulfate as oxidant. During the reducing process, four kinds of reductant (formaldehyde, sodium borohydride, vitamin C and glucose) were used to optimize the TOC of graphene. Then, the directional tuning of the content of hydroxyl group and carbonyl group on graphene under a stable TOC was achieved by controlling the addition amount of specific reductant. Results from morphological and electrochemical analysis suggest that high carbonyl content leads to a fluffier graphene structure, bigger specific surface area, higher specific capacitance and lower ion diffusion resistance while high hydroxyl group content results in wider operating potential window and better electrical conductivity. After balancing the content of hydroxyl and carbonyl group in the holey graphene, the assembled supercapacitor owns the high energy density of 22 Wh·Kg−1 and excellent cycling performance (no capacity loss after 10 000 charge-discharge cycles).