## 3 Results and Discussion  
### Electrochemical Performance  
In Figure 2, the CV curves of TGF and Bi/TGFs are plotted in 1 M V\({}^{3+}\) + 3 M H\({}_{2}\)SO\({}_{4}\) solution. It can be seen that the curves of Bi/TGFs exhibit apparent current peaks, especially corresponding to the oxidation peak of V\({}^{2+}\)\(\rightarrow\)V\({}^{3+}\). The oxidation peak currents of Bi/TGFs are 186 mA/cm\({}^{2}\) (Bi/TGF-0.8V), 218 mA/cm\({}^{2}\) (Bi/TGF-1.2V) and 207 mA/cm\({}^{2}\) (Bi/TGF-1.6V), respectively. The minimum oxidation peak is observed on the TGF (56 mA/cm\({}^{2}\)). Meanwhile, the TGF has the lowest electric double layer capacity (EDLC) of 4.085 F/cm\({}^{2}\). However, the EDLC values of Bi/TGFs are increased to 9.262 F/cm\({}^{2}\) (Bi/TGF-0.8V), 12.502 F/cm\({}^{2}\) (Bi/TGF-1.2V) and 11.480 F/cm\({}^{2}\) (Bi/TGF-1.6V), respectively. These results indicate that the Bi deposition on TGFs promotes the negative reaction of vanadium ions. In a comparison of the peak current and capacitance values, Bi/TGF-1.2V shows better electro-catalytic activity than the other samples. Figure 3 shows the EIS spectrograms of samples in 1 M V\({}^{3+}\) + 3 M H\({}_{2}\)SO\({}_{4}\) solution at a polarization potential of \(-\)0.4 V (vs. SCE). The EIS spectrograms provide further analysis concerning the influence of Bi on the electrochemical properties of TGFs on the negative side. As shown in Figure 3, each EIS spectrogram includes a semicircle and a linear part. The semicircle at high frequencies (100 Hz to 100 kHz) reflects the charge transfer resistance, while the linear part at low frequencies (0.01 Hz to 100 Hz) is related to the diffusion of vanadium ions through the solution. Thus, the EIS spectrograms indicate that negative V\({}^{3+}\)/V\({}^{2+}\) redox couples on samples were mix-controlled by charge transfer and diffusion steps. The EIS spectrograms are fitted by an equivalent circuit in Figure 3, where \(R_{s}\) represents the bulk solution resistance, \(R_{p}\) stands for the charge transfer resistance of the electrochemical reactions, \(\mathit{CPE}\) is the constant phase element that accounts for double-layer capacitance, and \(\mathit{W}\) is Warburg impedance. The fitting results for all samples are given in Table 1. Although the pristine graphite felt had been pre-treated by the thermal method, the Rp of TGF for the V\({}^{3+}\)/V\({}^{2+}\) redox reaction was the largest, resulting in poor electrochemical activity. However, the \(\mathit{Rp}\) values of Bi/TGFs were smaller, suggesting that Bi is more effective in catalyzing negative electrode reactions. In Table 1, the \(\mathit{Rp}\) for Bi/TGF-0.8V, Bi/TGF-1.2V, and Bi/TGF-1.6V are 1.24 \(\Omega\cdot\)cm\({}^{2}\), 0.89 \(\Omega\cdot\)cm\({}^{2}\), and 1.39 \(\Omega\cdot\)cm\({}^{2}\), respectively. The results indicate that the electrodeposition potential of 1.2 V can enhance the electron transfer rate of the V\({}^{3+}\)/V\({}^{2+}\) redox reaction. To deeply analyze the electrocatalytic mechanism of Bi toward the V\({}^{3+}\)/V\({}^{2+}\) redox couples, LSV is conducted for samples in 3 M H\({}_{2}\)SO\({}_{4}\) and 1 M V\({}^{3+}\)+ 3 M H\({}_{2}\)SO\({}_{4}\), as shown in Figure 4. The potential reactions have been proposed in the literature [48]. The chemical equations are shown as follows: \begin{table}
\begin{tabular}{c c c c c c c c} \hline \multirow{2}{*}{**Samples**} & **R\({}_{\mathbf{s}}\)** & **CPE (T)** & **CPE** & **R\({}_{\mathbf{p}}\)** & \multirow{2}{*}{**W (R)**} & \multirow{2}{*}{**W (T)**} & \multirow{2}{*}{**W (P)**} \\  & **(\(\Omega\cdot\)cm\({}^{2}\))** & **(\(\times\)10\({}^{-5}\))** & **(P)** & **(\(\Omega\cdot\)cm\({}^{2}\))** & & \\ \hline TGF & 0.68 & 8.49 & 1.11 & 1.69 & 2.48 & 0.002 & 0.365 \\ Bi/TGF-0.8V & 0.66 & 1.41 & 1.08 & 1.24 & 17.72 & 0.040 & 0.652 \\ Bi/TGF-1.2V & 0.64 & 1.13 & 1.11 & 0.89 & 1.50 & 0.001 & 0.384 \\ Bi/TGF-1.6V & 0.69 & 7.38 & 1.12 & 1.39 & 17.94 & 0.240 & 0.624 \\ \hline \end{tabular}
\end{table} 
Table 1: The parameters obtained from fitting EIS plots with the equivalent circuit.