G.G. Jang, J.K. Keum, B. Song, L. Li, D.K. Hensley, Y. Jiang, A. Hunt, K-S Moon, C.P. Wong, M.Z. Hu
Oak Ridge National Laboratory,
Keywords: graphene, vertical alignment, supercapacitor
Summary:This work develops a molecular engineering approach to control the interspacing of graphene sheets and then an external field approach to induce the re-orientation of the graphene sheets during the deposition process to form a coating with aligned graphene sheets. Four different ligand molecules (e.g. L1, L2, L3 and L4) were effectively grafted on the surfaces of well-dispersed graphene oxides (GOs) in liquid suspension and chemically reduced to GOs (rGOs). The obtained products (i.e., the graphene sheets that are grafted with molecular ligands as spacer and then reduced to modified rGOs (m-rGOs)) have shown well controlled nanometer sized (up to 2nm) interlayer spacings between the sheets. The external field renders the graphene sheets aligned and enhanced diffusion and accessibility of electrolyte ions into the graphene surfaces. It results in improved the electrode surface usability and power density. In this study, we investigated the effects of external field on the nano-micro-structures and the electrochemical properties of graphene nanosheets synthesized. The alignment of nanostructured graphene sheets in the deposited coatings enhanced supercapacitor performance. The field aligned mrGOs exhibited a vertically oriented microstructure in three dimensional networks, which showed enhancement in electrochemical performance (i.e., higher specific capacitance and faster electron transport kinetics), compared to the baseline coatings without the field alignment. For example of m-rGO (L1), up to ~300% higher specific capacitance at 100 mV/s, a specific capacitance of 240±7 F/g at 10 mV/s and 20% enhanced capacitance retention were achieved, compared to ones of non-external field. Aligning deposited graphene nanosheets via external field could be a novel versatile fabrication technique to tailor microscopic architecture of graphene coatings for high performance supercapacitor. We will present the experimental results on electrochemical characterizations and nano-micro-structures of molecular engineered graphene sheets in conjunction with the synthesis routes and external fields and discuss on perspectives on the developed technology.