Structure and Dynamics in Functionalized Graphene Oxides through Solid-State NMR

Graphene oxide (GO), a derivative of the supermaterial graphene, has intrinsic proton conductivity, which is similar to Nafion, the most popular proton exchange membrane material currently used in fuel cells. Research into acid-functionalized GOs and determining the role of acidic groups in increasing proton conductivity will help to improve polymer electrolyte membrane performance in fuel cell systems. Multinuclear solid-state NMR (ssNMR) spectroscopy was used to analyze the structure and dynamics of GO and a number of sulfonic acid derivatives of GO, both novel and previously reported. 13C CP-MAS spectra showed the disappearance of surface-based oxygen groups upon GO functionalization and can be used to identify linker group carbon sites in previously synthesized and novel functionalized GO samples with high specificity. Dehydration of these samples allows the collection of 1H spectra with resolved acidic proton and water peaks. The effect of dehydration on the proton spectrum is partially reversible through rehydration. Deuteration of the acidic groups in high temperature and acidic conditions was virtually unsuccessful, indicating that only the surface and not the intercalated functional groups play a role in the enhanced proton conductivity of ionomer/functionalized GO composites. Increased surface area and increased delamination of functionalized GO are suggested to be important to improved proton exchange membrane fuel cell performance. This synthesis and method of analysis prove the utility of ssNMR in the study of structure and dynamics in industrially relevant amorphous carbon materials despite the obvious difficulties caused by naturally broad signals and low sensitivity.