Oxygen-induced electrode activation and modulation essence towards enhanced anode redox chemistry for vanadium flow batteries

The vanadium flow battery (VFB) has seen a promising potential for use in large-scale energy storage. However, the sluggish anode redox chemistry still greatly limits the cycling performance of the VFBs. Herein, we realize an enhanced anode redox chemistry for the VFBs by tailoring the oxygen functional groups on carbon felts via a facile ozone-accelerated acid oxidation method, and more importantly uncover the modulation essence of p-band center in activated carbon felts. By introducing oxygen functional groups, the activated carbon felts exhibit both increased specific surface area and improved hydrophilicity over pristine carbon felts. Material and electrochemical characterizations indicate that an increased oxygen content in the carbon felts can substantially facilitate the V2+/V3+ redox kinetics. Benefiting from the enhanced V2+/V3+ kinetics, the VFB full cell delivers a superior energy efficiency of 72.8 % at 300 mA cm−2, and a long-term cycling stability is also achieved over 600 consecutive charge-discharge cycles at 150 mA cm−2 with only a 6.5% decay in energy efficiency. Most importantly, first principle calculations uncover that the oxygen function groups, especially carboxyl, can enhance the adsorption process but meanwhile to a certain extent suppress the charge transfer process for the V2+/V3+ redox reactions, which highlights the significance of delicate modulation of oxygen function groups on carbon felts to enhancing anode redox chemistry and full cell performance for the VFBs.