Enhancement of the Carbon Nanowall Film Capacitance. Electron Transfer Kinetics on Functionalized Surfaces

The effects of electrochemical oxidation and surfactant adsorption on behavior of vertically oriented carbon-nanowall (CNW)-based electrodes are studied. Electrochemical oxidation is carried out by the electrode polarization in aqueous solutions at high anodic potentials corresponding to water electrolysis, whereas the modification of surface by surfactants is accomplished by the adsorption of molecules characterized by the cage-like structure. Using the methods of cyclic voltammetry and impedancemetry, it is shown that a substantial increase in the capacitance of CNW-based electrodes is observed in both cases (30–50-fold and 3–5-fold, respectively). The as-grown and modified electrodes are characterized by scanning electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. A substantial increase in a number of oxygen-containing functional groups is observed on the CNW surface after the electrode polarization at high anodic potentials. The kinetics of redox reactions on the CNW film surface is studied by comparing the behavior of systems [Ru­(NH₃)₆]^2+/3+, [Fe­(CN)₆]^4–/3–, Fe^2+/3+, and VO₃^–/VO²⁺. It is demonstrated that oxidation of nanowalls makes the electron transfer in the redox reaction VO₃^–/VO²⁺ and the redox system Fe^2+/3+ considerably easier due to coordination of discharging ions of these systems with the functional groups; however, no such effect is observed for the redox-systems [Fe­(CN)₆]^3–/4– and [Ru­(NH₃)₆]^2+/3+

Oxygen Reduction by Lithiated Graphene and Graphene-Based Materials

Oxygen reduction reaction (ORR) plays a key role in lithium-air batteries (LABs) that attract great attention thanks to their high theoretical specific energy several times exceeding that of lithium-ion batteries. Because of their high surface area, high electric conductivity, and low specific weight, various carbons are often materials of choice for applications as the LAB cathode. Unfortunately, the possibility of practical application of such batteries is still under question as the sustainable operation of LABs with carbon cathodes is not demonstrated yet and the cyclability is quite poor, which is usually associated with oxygen reduced species side reactions. However, the mechanisms of carbon reactivity toward these species are still unclear. Here, we report a direct in situ X-ray photoelectron spectroscopy study of oxygen reduction by lithiated graphene and graphene-based materials. Although lithium peroxide (Li2O2) and lithium oxide (Li2O) reactions with carbon are thermodynamically favorable, neither of them was found to react even at elevated temperatures. As lithium superoxide is not stable at room temperature, potassium superoxide (KO2) prepared in situ was used instead to test the reactivity of graphene with superoxide species. In contrast to Li2O2 and Li2O, KO2 was demonstrated to be strongly reactive