Spectroelectrochemical study of carbon structural and functionality characteristics on vanadium redox reactions for flow batteries

Vanadium redox flow batteries have applications for large-scale electricity storage. This paper reports the influence of carbon structural characteristics of sustainable walnut shell-derived carbons in carbon/polyvinylidene fluoride composite electrodes on vanadium redox reactions. Pyrolysis, gasification, and chemical treatment procedures were used to modify the structural characteristics of carbons. Carbon functional groups were modified by chemical treatment with HNO3, heat treatment with K2CO3, and high-temperature NH3 treatment. Carbon porous structures were characterized using gas adsorption studies. Raman spectroscopy and X-ray diffraction were used to characterize the carbon molecular structure. Functional groups were characterized using X-ray photoelectron spectroscopy, acid/base titrations, temperature-programmed desorption, and Fourier transform infrared spectroscopy. The influence of carbon structure, porosity, and surface functional groups on the redox reactions of vanadium was investigated using cyclic voltammetry and electrical impedance spectroscopy. The VO2+/VO2+ and V2+/V3+ couples had well-defined peaks in cyclic voltammetry, with the former being the most intense, but the V3+/VO2+ couple was not observed for samples carbonized under nitrogen. The results show that V2+/V3+ and VO2+/VO2+ couples observed in cyclic voltammograms were enhanced for carbonization temperatures up to 800 degrees C. Electrical impedance spectroscopy also showed impedance trends. The electrochemistry results are primarily related to changes in carbon structure and the catalysis of V3+ oxidation by surface functional groups in the carbon structure. The V3+/VO2+ couple was limited by slow kinetics, but it occurs on specific oxygen and nitrogen sites in the carbon structure. The oxidation of V(iii) to V(iv) only occurs on a limited number of surface sites, and the outer-sphere electron transfer to oxidize V(iii) takes place at much more positive potentials. The coulombic, voltage, and energy efficiency of the carbon electrodes were suitable for batteries. The oxidation of V(iii) to V(iv) only occurs on a limited number of specific functional group sites and the outer-sphere electron transfer to oxidize V(iii) only takes place at much more positive potentials.