Aqueous all-iron flow batteries using surface-modified carbon electrodes improved via electrochemical exfoliation

Abstract

The exploration of cost-effective and high-performance electrodes is essential for advancing aqueous flow battery (AFB) technology, particularly for the AFBs employing iron–organic complexes as the active material for electrolytes. In this study, an electrochemically exfoliated carbon paper (Exf-CP) electrode with a porous carbon nanosheet structure fabricated via a simple high-voltage treatment in a phosphate buffer is proposed as a new electrode for AFBs. Exf-CP exhibits significantly enhanced electrochemical activity and wettability owing to the introduction of abundant carbon defects and oxygen-containing functional groups. To evaluate whether its surface modification and defect generation are well accomplished, various analyses, such as cyclic voltammetry, electrochemical impedance spectroscopy, and X-ray photoelectron spectroscopy, are considered. When the performance of this Exf-CP is compared with that of pristine carbon paper, AFBs employing this Exf-CP that are operated under 150 mA cm⁻² conditions demonstrate a 25.1%p improvement in energy efficiency (75.2%) and a lower charge–discharge overpotential, while the AFBs operated at 100 mA cm⁻² are stably operated for 400 cycles without significant performance loss. Based on that, this work substantiates that electrochemical exfoliation is a facile, low-cost, and scalable surface modification strategy that eliminates the conventional requirements for carbon nanotube incorporation or high-temperature treatment, offering substantial performance enhancement for iron-based AFB systems.