Porous Graphite Felt Modified with Carbon Nanoparticles for an MV/4-HO-TEMPO Flow Battery

In this paper, an engineered porous graphite felt (GF) was first prepared by a high-temperature etching method and then carbon nanoparticles were decorated on the GF surface. The physical and chemical properties of the modified carbon graphite felt (MCGF) electrode were then analyzed through various tests, including field-emission scanning electron microscopy, X-ray photoelectron spectroscopy, Brunauer–Emmett–Teller analysis, cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charging/discharging curves. The physical characterization and electrochemical performance analysis indicated that high-temperature etching influenced the hydrophilicity and reaction surface area of the electrode. The presence of carbon nanoparticles adhering to the fiber surface enhances electron transfer at the electrode/electrolyte interface during the oxidation and reduction processes of 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (4-HO-TEMPO). The peak currents of the redox reactions (147.2 and 142.3 mA for oxidation and reduction processes, respectively) on the electrode modified by MCGF are about two times those for the other electrodes. After the electrode was assembled in the battery, the MCGF electrode demonstrated a significant improvement in battery performance, notably increasing both battery capacity and efficiency. Meanwhile, the carbon nanoparticles can still firmly attach to the modified electrode after multiple cycles and exhibit powerful catalytic performance. At a current of 80 mA/cm2, the energy efficiency (EE) of the battery increased from 51.1 to 54.4% by the MCGF electrode. Furthermore, after 100 cycles, the system's cyclability testing revealed no significant fading, indicating the outstanding cycling stability of the MCGF electrode.