## 3 Results and Discussion
### Cell Performance Tests  
Figure 6a-c show CE, VE, and EE for VRFB single-cells at current densities from 20 mA/cm\({}^{2}\) to 80 mA/cm\({}^{2}\). The CE value of Bi/TGF-1.6V is significantly lower than that of other samples, as reported in Figure 6a. Excessive Bi deposition at 1.6 V increases the consumption of V\({}^{4+}\) ion infiltrated from the positive electrolyte [45], resulting in a significant decrease in CE. Under the same charge-discharge current density, the VE value increases first and then decreases with the increase in electrochemical deposition voltage. As shown in Figure 6b, batteries assembled with Bi/TGF-0.8V and Bi/TGF-1.2V have better charge and discharge efficiency than those with TGF cells. At 80 mA/cm\({}^{2}\), the VE value of the battery assembled with Bi/TGF-1.2V can reach 86.09%, which is about 8% higher than that of the battery assembled with TGF. However, the electrochemical performance of Bi/TGF-1.6V is worse than that of other felts. This result further confirms the analysis of Bi morphology described above. It can be seen from the SEM results in Figure 5 that the influence of deposition voltage on Bi morphology is significant. The higher the voltage selected, the larger the size of the Bi particles obtained. However, only the battery performance of the sample Bi/TGF-1.2V is good, which indicates that the appropriate Bi particle size can play a role in the battery performance. If the Bi particles are too small in size, then the improvement effect on the battery is not significant. Bi particles with larger sizes are more likely to fall off and may cause a sharp decline in battery performance. At the same time, SEM results of sample Bi/TGF-1.2V also confirmed that after 20 cycles, Bi still remained on the electrode surface, and the battery performance was also improved. In Figure 6c, the trend of EE changes is similar to that of VE, which is attributed to the minor influence of CE. The above results reveal that the electro-deposition of Bi particles on graphite felt can improve the electrochemical performance of vanadium batteries, especially at high current densities. At 80 mA/cm\({}^{2}\), the efficiency of Bi/TGF-1.6V was significantly reduced. It can be concluded that excessive deposition of Bi particles is not conducive to the electrochemical performance of VRFBs. Figure 6d reports the charge-discharge voltage-capacity of VRFBs using TGF and Bi/TGF electrodes at a current density of 40 mA/cm\({}^{2}\). Compared with the VRFB assembled with the TGF electrode, the VRFB assembled with the Bi/TGF electrode has a significant decrease in the over-potential during the corresponding charging and discharging process. This is mainly due to the increase in electro-catalytic activity due to the introduction of Bi particles, resulting in a decrease in the charging voltage and an increase in the discharge voltage.  
Figure 6: _Cont._To gain further understanding of the effect of Bi catalyst on cell capacity, the changes in charge and discharge capacities of VRFBs at constant current densities from 20 mA/cm\({}^{2}\) to 80 mA/cm\({}^{2}\) were investigated (Figure 7). As current density rose, the charge and discharge capacities decreased. The charge capacities of the cell made of Bi/TGF-1.2V electrodes showed the highest values. At the current density of 20 mA/cm\({}^{2}\), the charge capacities of the cell assembled with Bi/TGF-1.2V electrodes reached 23.348 Ah/L. This value was about 12.62% higher than that for TGF. At the current density of 80 mA/cm\({}^{2}\), the charge capacities of the cell made of Bi/TGF-1.2V electrodes reached 26.294 Ah/L, while the charge capacities of the cell assembled with TGF declined to yield the lowest value. The increase in capacity was certainly attributed to the effect of Bi particles as the catalyst. However, the efficiency of Bi/TGF-1.6V dropped dramatically at 80 mA/cm\({}^{2}\). Therefore, excess Bi particles did not benefit the electrochemical performances of VRFBs.