Graphite felts (GFs) are a widely used electrode material in vanadium redox flow batteries (VRFB) by virtue of their high porosity, chemical stability, low cost and good electrical conductivity. However, the hydrophobicity and the low electrochemical activity are the main obstacles for the use of pristine GF in VFRB applications. To tackle such problems, bismuth has been reported to be a promising catalyst for V²⁺/ V³⁺ redox reactions [1]. Post-processing parameters, such as air/vacuum drying, exposure time to water during rinsing step, etc. can impact the performance of the Bi modified GFs, which will correspondingly affect morphology, purity and distribution of the catalyst leading to different kinetics. These parameters are rarely studied and reported in the literature.

In this work, an effective synthesis method, the so-called dynamic hydrogen bubble template (DHBT) electrodeposition, has been employed for the deposition of high surface area Bi on GFs [2]. This method involves galvanostatic deposition of the metal at high current densities which is accompanied by hydrogen evolution, and the hydrogen bubbles serve as dynamic template for metal deposition. Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) were carried out in a half-cell setup. The kinetics of Bi modified GFs could be obtained qualitatively by using Friedl’s method [3], in which the charge transfer resistance and double layer capacitance from impedance measurements are correlated, and quantitatively from the CV fitting routine in Polarographica developed by Tichter et al. [4]. The kinetic trends derived from both methods are consistent, with Polarographica exhibiting specific quantitative values as compared to Friedl’s method.

It has been found that the post processing and drying conditions after Bi electrodeposition affect the performance of the GFs significantly. Therefore, the impacting parameters including electrochemically active surface area, Bi distribution, impurity, etc. are further studied by excluding certain parameters under different conditions. By the two previously mentioned methods of obtaining kinetics and the characterization of SEM and XRD, it is demonstrated that long time exposure to water during the rinsing step combined with vacuum drying after synthesis will promote a relatively higher kinetics than the other post-processing treatments on the Bi modified GFs.

Full-cell tests with a VRFB test bench have been carried out. According to the polarization curves and charge-discharge curves at 25 mA/cm² and 50 mA/cm², the electrodes with Bi show pronounced advantages over pristine GF, namely, lower overpotential, lower charge voltage, higher discharge voltage and higher electrolyte utilization. Long term cycling will be compared and discussed in detail as well.

[1] B. Li et al., “Bismuth Nanoparticle Decorating Graphite Felt as a High-Performance Electrode for an All-Vanadium Redox Flow Battery,” Nano Lett., vol. 13, no. 3, pp. 1330–1335, Mar. 2013, doi: 10.1021/nl400223v.

[2] M. Yang, “Fern-shaped bismuth dendrites electrodeposited at hydrogen evolution potentials,” J. Mater. Chem., vol. 21, no. 9, p. 3119, 2011, doi: 10.1039/c0jm03213a.

[3] J. Friedl and U. Stimming, “Determining Electron Transfer Kinetics at Porous Electrodes,” Electrochimica Acta, vol. 227, pp. 235–245, Feb. 2017, doi: 10.1016/j.electacta.2017.01.010.

[4] T. Tichter et al., “Real-space simulation of cyclic voltammetry in carbon felt electrodes by combining micro X-ray CT data, digital simulation and convolutive modeling,” Electrochimica Acta, vol. 353, p. 136487, Sep. 2020, doi: 10.1016/j.electacta.2020.136487.