High Temperature Vacuum Annealing and Hydrogenation Modification of Exfoliated Graphite Nanoplatelets

Highly active defect sites on the edges of graphene automatically capture oxygen from air to form various oxygen groups. A two-step procedure to remove various oxygen functional groups from the defect sites of exfoliated graphene nanoplatelets (GNPs) has been developed to reduce the atomic oxygen concentration from 9.5% to 4.8%. This two-step approach involves high temperature vacuum annealing followed by hydrogenation to protect the reduced edge carbon atoms from recombining with the atmospheric oxygen. The reduced GNPs exhibit decreased surface resistance and graphitic potential-dependent capacitance characteristics compared to the complex potential-dependent capacitance characteristics exhibited by the unreduced GNPs as a result of the removal of the oxygen functional groups present primarily at the edges. These reduced GNPs also exhibit high electrochemical cyclic stability for electrochemical energy storage applications. 1. Introduction Graphene can be considered a polycyclic aromatic hydrocarbon [1–4]. The basal plane consists of sp2-hybridized carbon atoms in a honeycomb lattice structure [1–4]. However, the size of the basal plane of graphene is finite, and some of the carbon atoms in the highly aromatic basal plane terminate at the edges in a nonaromatic state. The carbon atoms present at the edges are highly chemically active as compared to the chemically inert carbon atoms present in the basal plane [5, 6]. During the preparation of graphene nanosheets, the edge carbon atoms can react spontaneously with atmospheric oxygen to create carboxyl, keto, or hydroxyl group [5–7]. GNPs developed by the Drzal research group in Michigan State University are prepared by acid intercalation and rapid exfoliation process in a microwave environment. The acid-intercalated graphite particles undergo substantial expansion to form worm-like products, which are then mechanically grounded to generate small stacks of graphenes that are 1 to 15 nanometers thick, with diameters ranging from submicrometer to 100 micrometers. The size of the resulting GNPs can be further reduced to a few hundred nanometers by vibratory ball milling over extended period of a few days. The exfoliation process and morphologies of GNPs are detailed elsewhere [7]. The exfoliation process minimizes the presence of basal plane defects so that the number of edge sites is fixed for a given platelet size. Surface analysis indicates a proportional increase in the atomic oxygen concentration with decreasing graphene nanosheets dimension [8, 9]. For large GNPs with an average lateral dimension