Limited Accessibility to Surface Area Generated by Thermal Pretreatment of Electrodes Reduces Its Impact on Redox Flow Battery Performance

Thermal oxidation of carbon electrodes is a common approach to improving flow battery performance. Here, we investigate how thermal pretreatment increases electrode surface area and the effect this added surface area has on electrode performance. Specifically, we rigorously analyze the surface area of Freudenberg H23 carbon paper electrodes, a binder-free model material, by systematically varying pretreatment temperature (400, 450, and 500 °C) and time (0 to 24 h) and evaluating changes in the physical, chemical, and electrochemical properties of the electrodes. We compare physical surface area, measured by a combination of gas adsorption techniques, to surface area measured via electrochemical double layer capacitance. We find good agreement between the two at shorter treatment times (0-3 h); however, at longer treatment times (6-24 h), the surface area measured electrochemically is an underestimate of the physical surface area. Further, we use gas adsorption to measure a pore size distribution and find that the majority of pores are in the micropore range (< 2 nm), and ca. 60% of the added surface area are in the sub-nanometer (< 1 nm) pore size range. We postulate that the solvated radii and imperfect wetting of electrochemical species may hinder active species transport into these recessed regions, explaining the discrepancy between electrochemical and physical surface area. These results are supported with in situ flow cell testing, where single-electrolyte polarization measurements show little improvement with increasing surface area. Further, using a simple convection-reaction model to simulate electrode overpotential as a function of surface area, we find that increasing surface area improves the performance to a point, but the mass transport to and the catalytic activity of the reaction sites offer greater comparative impact. Ultimately, this work aims to inform the design of electrodes that offer maximal accessible surface area to redox species.