Nitro-graphene oxide in Iridium Oxide hybrids: Electrochemical modulation of N-graphene redox states and Charge capacities

Carbon plays a significant role in the development of electrode materials that include catalysis, energy storage and sensing. Graphene-based coatings and carbon nanotubes on the other hand have expanded that role beyond through nanostructuring in hybrids or the formation of composites. In particular, the formation of hybrids of nanocarbons with iridium oxide yield nanostructured materials through direct anodic deposition, with substantially improved charge capacities vs pure IrOx. Modifying the possible redox sites, new structuring of hybrids and enlarged charge capacities are expected. This work shows that N-doped graphenes, as part of a IrOx hybrid, offer a new redox chemistry on graphene oxide through electrochemical modulation of the redox states of nitrogen in graphene and yielding stable nitro groups bound to carbon, so far the largest oxidation state reported in N-doped graphene . The hybrid materials are obtained in the form of coatings thanks to the spontaneous adhesion of iridium oxo species on N-doped graphenes and further anodic electrodeposition of the mixture. While the oxidizing synthesis process already involves a modification of the oxidation state of nitrogen, further electrochemical cycling evidence the electrochemical processes for both IrOx and N groups attached to graphene oxide. All hybrids obtained present a wide range of nitrogen –based groups that include the nitro group, and a significant charge capacity that remains large upon electrochemical cycling and that involves all the faradic processes from the iridium and graphene components. Among all cases, a particular hybrid, including the highest starting oxidation state, reaches a significantly larger charge capacity, also larger than the graphene oxide hybrid, with 70% retention upon cycling. Although Nitrogen doping of graphene is considered a reducing process, this case shows that the oxidized range of nitrogen doping is also possible. IrOx and the reversible redox processes that iridium offers, are thought to be essential in stabilizing an unusual nitro-carbon-oxide system and allowing a sustained high charge storage capacity that doubles that of pristine graphene or graphene oxide hybrids.