Mechanism of Electrochemical Hydrogenation of Epitaxial Graphene

Abstract

Mechanism of electrochemical hydrogen adsorption on epitaxial graphene (EG) was observed to be dependent on defects in EG as observed by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and Raman spectroscopy. To determine the material dependence on electrochemical hydrogenation, a set of different EG samples (Si-face EG [∼2 ML], C-face EG [∼10 ML], M-plane EG [∼25 ML], defective Si-face EG [>50 ML]) and a graphite disk [>500 ML] were characterized using a home built electrochemical cell developed in previous work, dilute perchloric acid (HClO4) solution, silver/silver chloride (Ag/AgCl) reference electrode in saturated KCl (0.198 V vs. NHE) and potentiostat. The Nyquist plots obtained from EIS of epitaxial graphene with low defect density showed only one semicircle covering the entire frequency range attributed to adsorption of hydrogen at the relatively chemically inert basal plane surface and further supported by lack of hydrogen peaks in the CV. Samples with high defect density showed an additional semicircle at the intermediate to high frequency ranges linked to adsorption and charge transfer of hydrogen to graphene. The increased presence of point defects in epitaxial graphene augments the surface area of the material resulting in increased diffusion of hydrogen ions though the graphene lattice allowing for hydrogen to adsorb to additional sites within the lattice.