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 N2 sorption measurements, are correlated to changes in wettability, conductivity, and electrochemical kinetics, investigated by H2O sorption measurements, cyclic voltammetry, and electrochemical impedance spectroscopy in the VO2+ electrolyte, respectively. We found that the kinetics of the VO2+/VO2+ reaction increases with an increase in sp2-C content and therefore an increase in crystallite size and conductivity of the mesoporous N-doped carbon. Nevertheless, the largest current for the VO2+/VO2+ 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.