Single-Entity Electrochemistry of N-Doped Graphene Oxide Nanostructures for Improved Kinetics of Vanadyl Oxidation

N-doped graphene oxides (GO) are nanomaterials of interest as building blocks for 3D electrode architectures for vanadium redox flow battery applications. N- and O-functionalities have been reported to increase charge transfer rates for vanadium redox couples. However, GO synthesis typically yields heterogeneous nanomaterials, making it challenging to understand whether the electrochemical activity of conventional GO electrodes results from a sub-population of GO entities or sub-domains. Herein, single-entity voltammetry studies of vanadyl oxidation at N-doped GO using scanning electrochemical cell microscopy (SECCM) are reported. The electrochemical response is mapped at sub-domains within isolated flakes and found to display significant heterogeneity: small active sites are interspersed between relatively large inert sub-domains. Correlative Raman-SECCM analysis suggests that defect densities are not useful predictors of activity, while the specific chemical nature of defects might be a more important factor for understanding oxidation rates. Finite element simulations of the electrochemical response suggest that active sub-domains/sites are smaller than the mean inter-defect distance estimated from Raman spectra but can display very fast heterogeneous rate constants >1 cm s^-1. These results indicate that N-doped GO electrodes can deliver on intrinsic activity requirements set out for the viable performance of vanadium redox flow battery devices.