A combined morphological and electrochemical characterization of carbon electrodes in vanadium redox flow batteries: Insights into positive and negative electrode performance

Reduced power density is one of the main critical issues of vanadium redox flow batteries and it is mainly due to critical electrolyte distribution over the porous electrodes and sluggish kinetics. Therefore, it is crucial to elucidate the interplay between morphological and electrochemical features of carbon electrodes and their influence on battery performance. In this work, three considerably different carbon electrodes were characterized in order to highlight the most relevant electrode properties governing system operation, distinguishing between positive and negative electrode performance, that are differently affected by kinetic and mass transport losses, especially at local level. Scanning electron microscopy and Raman spectroscopy provided insights on the electrodes structure, while mercury intrusion porosimetry and thermogravimetric analyses permitted to evaluate porous volume, surface area, skeletal density and pores diameter. Local electrochemical characterization was performed by means of throughplate hydrogen reference electrodes: innovative in-situ cyclic voltammetry allowed to decouple electrode operation from fluid-dynamic phenomena, whose influence on battery performance was evaluated with polarization curves at different flow rates. The best electrode design is the result of a proper combination between high surface area and pores diameter, that on one hand has to be sufficiently large to promote electrolyte permeation from distribution channel, but on the other hand has to be small to reduce mass transport losses at pore scale. The negative electrode performance is strongly affected by kinetic losses, while the operation of positive electrode is mainly influenced by mass transport phenomena.