Endowing electrospun carbon fiber with excellent electrocatalytic properties towards VO2+/VO2+ redox reaction for vanadium redox flow battery by in situ iridium decoration

Review on the Applications of Biomass-Derived Carbon Materials in Vanadium Redox Flow Batteries

The development of vanadium redox flow batteries (VRFBs) requires the exploration of effective and affordable electrodes. In order to increase the electrochemical activity of these electrodes and decrease the polarizations, they are doped with an electrocatalyst. In this context, the use of biomass-derived materials as electrocatalysts in VRFBs has received much attention recently due to their widespread availability, renewable nature, low cost, and high energy efficiency. This paper aims to review the synthesis methods of biomass-derived carbon materials and their applications in VRFBs. In line with this aim, recent developments in carbon-based electrode modification methods and their electrochemical performance in VRFBs are summarized. The studies show that porous carbon electrocatalysts increase energy efficiency by reducing overpotentials and improving electrocatalytic activation. In addition, it is thought that biomass carbon doped electrocatalysts can improve the hydrophilicity of the electrodes, the transfer of vanadium ions, and the reaction kinetics. The highest charge voltage decrease rate of 8.61% was obtained in the Scaphium scaphigerum, whereas the highest discharge voltage increase rate of 14.29% was observed in the twin cocoon, as in all reviewed studies. Furthermore, the maximum energy efficiency (75%) was achieved in a VRFB equipped with an electrode doped with carbon derived from Scaphium scaphigerum and cuttlefish. It can be concluded from the reviewed studies that the electrochemical performances of electrodes doped with biomass-derived carbons in VRFBs are more effective than those of the bare electrodes.

Highly porous carbon nanofiber electrodes for vanadium redox flow batteries

The electrochemical performance of carbon nanofiber (CNF) electrodes in vanadium redox flow batteries (VRFBs) is enhanced by optimizing the morphological and physical properties of low-cost electrospun CNFs. The surface area, porosity and electrical conductivity of CNFs are tailored by modifying the precursor composition, especially the sacrificing agent, Fe(acac)₃, in the polymer precursor and carbonization temperature. A highly porous structure with a large surface area is generated by the catalytic growth of graphitic carbon spheres surrounding the iron nanoparticles which are removed by an acid etching process. The graphitic carbon layers formed at a high carbonization temperature improve the electrical conductivity of CNFs. The large surface area of 349 m² g^-1 together with the abundant mesopore-dominant structure leads to high wettability and high activity for redox reactions of the electrode, giving rise to enhanced electrochemical performance in VRFBs. It delivers an energy efficiency (EE) of 91.4% at a current density of 20 mA cm^-2 and 79.3% at 100 mA cm^-2, and maintains an average EE of 72.5% after 500 charge/discharge cycles at 100 mA cm^-2.

One-step activation of high-graphitization N-doped porous biomass carbon as advanced catalyst for vanadium redox flow battery

In this paper, we reported a one-step activation strategy to prepare highly graphitized N-doped porous carbon materials (KDC-FAC) derived from biomass, and adopted ferric ammonium citrate (FAC) as active agent. At high temperature, FAC was decomposed into Fe- and NH₃-based materials, further increasing graphitization degree, introducing N-containing functional groups and forming porous structure. KDC-FAC has superior electrocatalytic activity and stability towards V²⁺/V³⁺ and VO²⁺/VO₂⁺ redox reactions. High graphitization degree can enhance the conductivity of carbon material, and porous structure is conducive to increase reaction area of vanadium redox couples. Moreover, N-containing functional groups are beneficial to improve the electrode wettability and serve as active sites. The single cell tests demonstrate that KDC-FAC modified cell exhibits good adaptability under high current density and superb stability in cycling test. Compared with pristine cell, the energy efficiency of KDC-FAC modified cell is increased by 9% at 150 mA cm^-2. This biomass-derived carbon-based material proposed in our work is expected to be an excellent catalyst for vanadium redox flow battery.