DC and AC characterization of a Vanadium Redox Flow Battery (VRFB) using a Pinflow 20 cm² test lab cell

Significance of the Topic

Redox Flow Batteries (RFBs) are emerging as a leading technology for large-scale energy storage due to their low Levelized Cost Of Storage (LCOS), decoupling of energy and power, and long cycle life. Among RFB chemistries, vanadium-based systems offer excellent durability by using the same element in both electrolytes, avoiding cross-contamination and volume changes associated with solid electrodes.

Objectives and Study Overview

This work demonstrates the electrochemical characterization of a Vanadium Redox Flow Battery (VRFB) employing a 20 cm² Pinflow test lab cell. The study covers both direct current (DC) cycling to assess charge–discharge performance and electrochemical impedance spectroscopy (EIS) to probe internal resistances under various states of charge (SoC) and flow conditions.

Methodology and Instrumentation

Electrolyte Composition and Cell Setup:

• 0.8 M V3+/VO2+ in 2 M H2SO4 with 0.3 wt% H3PO4
• Pinflow 20 cm² graphite felt electrodes, Nafion 115 membrane
• Electrolyte volume: 58.3 mL per half-cell, initial SoC at –50%
• Flow rate: 40 mL·min⁻¹, temperature 25 °C

Instrumentation:

• BCS-815 for galvanostatic pre-charge and cycling
• VSP-300 for EIS measurements
• BT-Lab and EC-Lab software for experiment control and data analysis
• Watson-Marlow 323S pumps with 313X/313DW heads

Main Results and Discussion

DC Cycling:

• Pre-charge at 3 A to reach +50% SoC under voltage and capacity limits
• Galvanostatic cycling between 0.8 V and 1.65 V at ±3 A (150 mA·cm⁻²) for 71 cycles
• Coulombic efficiency stabilized around 96.5–97.5% after initial cycles
• Capacity fade measured at –4.2 mAh per cycle, indicating robust cyclability

EIS Analysis:

• Nyquist spectra recorded by PEIS (5 mV, 200 kHz to 50 mHz) at –50% and +50% SoC
• Equivalent circuit: L1 + R1 + Q1/(Wδ2 + R2)
• Ohmic resistance (R1), charge transfer resistance (R2), and diffusion parameters (Wδ2) were extracted
• Significant variation of low-frequency diffusion constant and pseudo-capacitance with SoC
• Flow rate study (20–120 mL·min⁻¹) by GEIS (10 mV, 100 kHz to 5 mHz) showed diffusion time constant inversely proportional to flow rate

Benefits and Practical Applications

The combination of DC cycling and EIS provides comprehensive diagnostics of VRFB health, allowing quantification of efficiency losses and internal resistances. Reliable performance under varying SoC and flow conditions demonstrates applicability in grid-scale energy storage, renewable integration, and flexible renewable-grid operation.

Future Trends and Potential Applications

Advances in electrode materials, membrane selectivity, and cell architectures can further enhance VRFB efficiency and reduce costs. Real-time impedance monitoring and automated control systems will enable predictive maintenance and optimized operation. Scale-up of flow cell designs and integration with renewable energy sources will drive broader commercialization.

Conclusion

This study confirms that VRFBs can be effectively characterized using standard DC cycling and EIS techniques. High coulombic efficiency and low capacity fade underline VRFB durability. Impedance analysis reveals the influence of SoC and flow rate on internal resistances, guiding design improvements and operational strategies.

References

• Noack J. et al., Redox flow batteries for renewable energy storage, pv-tech.org (2019)
• Mazúr P. et al., J. Power Sources 414 (2019) 354
• Mazúr P. et al., MethodsX 6 (2019) 534
• BioLogic Application Note 66, Diffusion coefficient determination by EIS
• BioLogic Application Note 62, EIS measurement of battery internal resistance
• BioLogic Topic: ZSim and ZFit for inductance correction
• Barral G. et al., J. Appl. Electrochem. 21 (1991) 991