Investigating the Effect of Microstructure and Surface Functionalization of Mesoporous N-Doped Carbons on V4+/V5+ Kinetics

State-of-the-art electrode materials for all-vanadium redox flow batteries are based on carbon. Unfortunately, the impact of the carbon structure, i.e., microstructure/crystallinity, surface functional groups, and porosity/morphology/surface area, on the electrochemical performance is still unclear. This is due to the fact that usually several structural characteristics are varied due to synthesis or post-treatment procedures at the same time. Therefore, this paper shows systematically how microstructure, porosity, and surface functional groups vary with carbonization and graphitization temperature (ranging from 700 to 1500 °C) for a mesoporous N-doped carbon (MPNC). Changes in the material’s structure (e.g., morphology, porosity, crystal structure, surface functionalization), determined by scanning and transmission electron microscopy, X-ray diffraction (pair distribution function analysis), X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, and N 2 sorption measurements, are correlated to changes in wettability, conductivity, and electrochemical kinetics, investigated by H 2 O sorption measurements, cyclic voltammetry, and electrochemical impedance spectroscopy in the VO 2+ electrolyte, respectively. We found that the kinetics of the VO 2+ /VO 2 + reaction increases with an increase in sp 2 -C content and therefore an increase in crystallite size and conductivity of the mesoporous N-doped carbon. Nevertheless, the largest current for the VO 2+ /VO 2 + reaction for the same amount of carbon during cyclic voltammetry is observed for the MPNC carbonized at an intermediate temperature, 1000 °C, as a result of its larger wettability and thus available surface area compared to the MPNC carbonized at 1500 °C.