Active Material Transport in Carbon Porous Electrode for Vanadium Redox Flow Batteries
Authors: Ishitobi Hirokazu , Ide Tomoki , Shiraishi Soshi , Tsukada Hidehiko , Nakao Takato , Nakagawa Nobuyoshi 
Published: December 2024

Introduction

The Vanadium Redox Flow Battery (VRFB) is an energy storage device that supplies vanadium electrolyte solutions and is expected to be used as a storage system for renewable energy due to its advantages such as being able to design output and capacity independently. To charge and discharge at high currents, VRFBs need to enhance not only the electrode activity but also the active material transport within the porous electrodes due to low diffusion coefficients and high viscosity of electrolyte solutions. However, there are only a few previous works focusing on material transport [1,2], and the mass transport phenomena in boundary film is still unclear. We are proposing Seamless Carbon (SC) materials as electrode materials for VRFBs, which have a uniform interconnected pore structure in all directions [3]. This pore structure should be advantage for the analysis of material transport. The objective of this work is to evaluate the limitation of material transport rate in positive and negative half-cells. By comparing the current-voltage curves of symmetric cells and conventional full cells, mass transport limiting half-cell was clarified.

Experimental

SC materials were prepared as follows; a precursor was carbonized at 800 °C under a N2 gas atmosphere, heat treated at 1500 °C under an Ar gas atmosphere, and oxidized at 420 °C under an air atmosphere. The interconnected pore diameter of the SC material was set to 13 μm. A single cell was prepared by sandwiching electrodes and a membrane (Nafion 117) between carbon blocks with interdigitated flow channels. SC material with a thickness of ca. 0.4 mm was set into both positive and negative half-cells. Current-overpotential curves were obtained under flowing electrolyte solution at rates of 5 to 20 mL min−1.

Results and Discussion

Figure 1 shows the current-overvoltage curves of the negative half-cell. While the limiting current was observed in the negative half-cell, it was not observed in the positive half-cell. Comparing with the current-voltage curves in the full cell, the insufficient active material transport at the negative half-cell dominates the limiting current of the full cell. The one-pass conversion was ca. 0.3 at the limiting current, which indicates the impact of bulk concentration change was small, while active material transport within the boundary film should impacted for the limiting current. The equilibrium of ion exchange between vanadium complex and surface oxygen groups on the negative electrode does not favor the disassociation of proton due to the acidic condition. Consequently, the vanadium concentration on the electrode surface becomes higher than zero, suggesting that the concentration gradient within the boundary layer is relatively small which resulted insufficient active material transport in the negative half-cell.

Acknowledgement

This research was supported by JSPS KAKENHI C JP22K04814.

References

[1] K. M. Tenny et al., J. Electrochem. Energy Convers. Storage, 17 (2020) 041010
[2] M. Heijden et al., J. Electrochem. Soc., 169 (2022) 04050
[3] H. Ishitobi et al., J. Electrochem. Soc., 170 (2023) 010536

Figure 1