###### Abstract  
In this paper, bismuth (Bi) was successfully deposited on graphite felts to improve the electrochemical performances of vanadium redox flow batteries. Modified graphite felts with different Bi particle loadings were obtained through electrochemical deposition at voltages of 0.8 V, 1.2 V and 1.6 V in 0.1 M BiCl\({}_{3}\) solution for 10 min. The optimal Bi particle loading was confirmed by scanning electron microscopy (SEM), single cells and electrochemical tests. The SEM images revealed the deposition of granular Bi particles on the fiber surface. The Bi-modified felts which were electrochemically deposited at 1.2 V (Bi/TGF-1.2V) showed excellent electrochemical performances in cyclic voltammetry curves and impedance spectroscopy. Meanwhile, the single cells assembled with Bi/TGF-1.2V as negative electrodes exhibited higher voltage efficiencies than the others. The optimized Bi particle loading induced better catalysis of the V\({}^{3+}\)/V\({}^{2+}\) reaction and hence significantly improved the cell performances. In addition, the prepared Bi-modified felts showed stable cell performances and slower charge-discharge capacity declines than the other electrodes at current densities between 20 mA/cm\({}^{2}\) and 80 mA/cm\({}^{2}\). Compared with the pristine felt, the voltage efficiency of the vanadium redox flow battery assembled with Bi/TGF-1.2V graphite felt was 9.47% higher at the current density of 80 mA/cm\({}^{2}\). The proposed method has considerable potential and guiding significance for the future modification of graphite felt for redox flow batteries. vanadium redox flow battery; graphite felt; electro-chemical deposition; bismuth; electrochemical performance; cyclic voltammetry curve; impedance spectroscopy; capacity decay; voltage efficiency +
Footnote †: journal: Applied Physics Letters  
the electrolyte is easy to recycle to restore the battery capacity and prolong the lifetime of the VRFB [4; 5; 6]. In particular, VRFBs have been widely studied as a new type of efficient, economical and environmentally friendly secondary batteries due to the wide standard reduction potentials of vanadium redox couples [7; 8; 9]. Key materials in VRFBs include electrodes, electrolytes, ion exchange membranes and current collector plates. The electrode does not participate in the redox reactions, only provides the reaction place, and electrons pass through the electrode/electrolyte interface to form a potential difference. After such a process, the VRFB can be charged and discharged based on the redox reactions of the vanadium redox couples. The electrolyte is composed of active substances and supporting electrolytes, in which the content of active substances determines the battery capacity, and supporting electrolytes can improve the conductivity of the electrolyte. The ion exchange membrane is used to separate positive and negative active substances, while selectively allowing the protons as the charge carriers to migrate through the membrane under the electric driving force to maintain electric neutrality and electrolyte balance. The electrode materials play important roles in the performances of VRFBs because the redox reactions of vanadium ions occur on the electrode surface [10; 11; 12]. Graphite felt (GF) is used as the most common electrode material in VRFBs [13; 14]. GF is characterized by a high specific surface area, strong oxidation resistance, corrosion resistance and high electrical conductivity. However, large-scale application of GF is limited by its poor hydrophilicity and electrochemical activity [15; 16]. Tremendous efforts have been devoted to improving the electrochemical properties of GF. The modification of GF is mainly achieved by increasing the amount of active functional groups, enhancing the effective area, and introducing surface catalysts [17; 18; 19]. [PERSON] et al. proposed that -OH and C=O functional groups are the main reasons for improving the electrochemical activity of carbon felt [20; 21].