## 3 Results and Discussion
### SEM, FTIR and XRD  
To gain a better understanding of the relationship between the surface morphology and catalytic effect, the electrodes were subjected to SEM. Figure 5a-d depict the morphology of modified fibers surfaces with and without Bi particles at the same magnification. The surface of GF appeared to be clean and smooth as shown in Figure 5a, but the surface of Bi/TGFs was attached with Bi and the size of Bi increased with the increase in electrochemical deposition voltage. The bismuth of Bi/TGF-0.8V (Figure 5b) is in the grooves on the surface of the fibers, and bismuth of Bi/TGF-1.2V and Bi/TGF-1.6V attached to the surface of the fiber is crystallized into sheets in Figure 5c,d. According to the results of LSV, the HER and hydrogen ion absorption are preferentially carried out on the tiny Bi particles, as shown in Bi/TGF-0.8V. Therefore, too many flaky Bi particles, as shown in Bi/TGF-1.2V and Bi/TGF-1.6V, are not sensitive to HER and hydrogen ion absorption. Because of this, only an appropriate Bi loading can better catalyze the negative electrode reaction. This is the reason that the results of CV and EIS for Bi/TGF-1.2V are superior to that for Bi/TGF-1.6V. The XRD result of Bi/TGF-1.2V was reported in Figure 5e, which has a perfect match with the Pure Bi standard card, indicating that the metal Bi affects the catalyst on the surface of carbon fiber. And the comparison of FTIR results between GF and Bi/TGF-1.2V in Figure 5f showed that more oxygen-containing groups appeared on the GF surface after Bi deposition (-O-H stretching vibration peak: c.a. 3600-3200 cm\({}^{-1}\); alkyl C-H bending vibration peak: c.a. 2920-2850 cm\({}^{-1}\); -C=O stretching vibration peak: c.a. 1720-1750 cm\({}^{-1}\)) [50]. It can be seen that the more active metal bismuth becomes the active center, so that the hydrophobic GF can absorb oxygen in the air and become a hydrophilic electrode material. To confirm the effect of Bi on enhanced electrochemical performances, the morphology of TGF acquired after 20 charge-discharge cycles in the cell was obtained, and the results are depicted in Figure 5g,h. The size of metal bismuth particles deposited on the surface of GF has always been a focus of researchers [42]. If only the catalytic activity of Bi is considered, then the nanoscale Bi will obviously be more active. However, the flow erosion of electrolytes and the electrolytic effect during the charge and discharge process will inevitably affect the existence of metal bismuth particles on the electrode surface and in the flow cells. This work attempts to control the deposition size and amount of Bi by electrodeposition voltage so that it can adapt to the internal environment of the VRFBs. Too large or too small scale of Bi is not conducive to its catalytic effect, so Bi/TGF-1.2V can be concluded to own the highest catalytic activity. EDS results showed that the Bi particles and electrolytes still remained on the surface of Bi/TGF-1.2V after 20 charge-discharge cycles (see it in the Supplementary Material).  
Figure 5: SEM images of TGF and Bi/TGFs: (**a**) TGF, (**b**) Bi/TGF-0.8V, (**c**) Bi/TGF-1.2V, (**d**) Bi/TGF-1.6V and (**e-h**) XRD, FTIR and EDX spectrogram (after 20 charge–discharge cycles) of Bi/TGF-1.2V.