1. Kittima Ngamsai1 Amornchai Arpornwichanop1, 2
PREDICTION OF THE OXIDATION STATE OF VANADIUM IN A VANADIUM REDOX FLOW BATTERY
Kittima Ngamsai1
Amornchai Arpornwichanop1, 2
1 Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University
2 Computational Process Engineering, Chulalongkorn University

2. Results and Discussion
Outline 1
Introduction
Materials and Methods 2
Results and Discussion 3
Conclusions 4

3. Introduction Renewable energy Conventional energy
Energy storage technology

4. Introduction Energy storage technology Electrochemical cell
What is Vanadium redox flow battery (VRB)?
Energy storage technology
Electrochemical cell
(Reduction & Oxidation= Redox reaction)
Energy is stored in electrolyte solution
(Vanadium salt dissolved in sulfuric acid)
Power depends on the cell
Energy depends on the electrolyte

5. Introduction
Principle of VRB
V2+
V3+
VO2+
VO2+
Negative
Positive

6. the oxidation state of vanadium
Introduction
Problem of electrolyte system
Electrolyte Imbalance
Prediction of electrolyte imbalance
Prediction of
the oxidation state of vanadium

7. Introduction What is Electrolyte Imbalance in VRB ? Balance Imbalance
Negative
Positive
V2+
V3+
V4+
V5+
Charge
Charge
Discharge
Discharge
Balance
Imbalance
V2.5+
V4.5+
V 4.5+

8. Introduction Electrolyte Imbalance in VRB
Cause of electrolyte imbalance
Side reaction
- Air oxidation of V(II) ion
- Gassing side reaction during charging
Electrolyte transfers across membrane
- Vanadium ion transfer
- Water transfer

9. Introduction Electrolyte Imbalance in VRB
Effect of electrolyte imbalance
The loss of energy capacity
Decrease Efficiency
Release heat
Method to rebalance electrolyte
Side reaction Electrochemical reaction
Electrolyte transfer Electrolyte mixing

10. The conventional open circuit voltage (OCV) cell has been modified.
Introduction
VRB Research for electrolyte imbalance
The other research groups
In this study
Sukkar and Skyllas-Kazacos developed membrane to improve the transfer behavior of vanadium ion and water.*
Skyllas-Kazacos and co-worker added some chemical reactants to restore the electrolyte balance **
The conventional open circuit voltage (OCV) cell has been modified.
A correlation of the OCV and the oxidation state of vanadium in an electrolyte solution is investigated.
An electrolyte imbalance can be measured by using the modified OCV cell and Nernst’s equation
Note:*T. Sukkar and M. Skyllas-Kazacos, J. Membr. Sci. J. 222 (2003)
** M. Skyllas-Kazacos and L. Goh, J. Membr. Sci. J (2012)

11. Materials and Methods
State of charge Versus OCV

12. Materials and Methods
To investigate a correlation of OCV and the oxidation state of vanadium in an electrolyte solution, the conventional OCV has been modified
(a)
(b)
Figure 1 (a) Conventional OCV cell and (b) modified OCV cell

13. Materials and Methods Experimental
The initial electrolyte solutions was prepared at an oxidation state of vanadium of +3.5 (including 50% V3+ and 50% VO2+).
Vanadium salt
Sulfuric acid
1.5 M
2.0 M

14. Materials and Methods Experimental
The VRB single cell with effective area of 1 dm2 and the modified OCV cell were employed.
The electrolyte solutions were fed into the cell by two peristaltic pumps.
A constant current was applied to charge and discharge for one cycle.
Data logger was used to record OCVs for every 10 seconds.
The charging time (or discharging time) can be then converted to the vanadium oxidation state.

15. Reference electrolyte
Materials and Methods
OCV
Cell P
Positive Electrolyte
Power supply/Load
Vocv
Negative Electrolyte
Data logger
Negative electrolyte
Positive electrolyte
Reference electrolyte V
Vocv_neg
Vocv_pos
Figure 2. Schematic diagram of the VRB system

16. Materials and Methods Experimental
To confirm the reliability of the time conversion method
Electrolyte solution samples were collected in different OCV
Samples were titrated to determine the oxidation state of vanadium using the potentiometric titration with potassium permanganate as a titrant.

17. Materials and Methods In the electrolyte system of VRB (1) (2) (3)
Nernst equation for correlation of OCV and oxidation state of vanadium in the electrolyte
In the electrolyte system of VRB
Oxidation of vanadium (from +2 to +3):
(1)
Oxidation of vanadium (from +3 to +4):
(2)
Oxidation of vanadium (from +4 to +5):
(3)

18. Materials and Methods
Nernst equation for correlation of OCV and oxidation state of vanadium in the electrolyte
From (1)
(4)
(5)
From (2)
From (3)
(6)

19. Materials and Methods
Charging-Discharging time Conversion method
Titration method
Nernst equation
Correlation of an OCV and oxidation state of vanadium in electrolyte

20. Results and Discussion( )
Results and Discussion
Charging-discharging time Conversion method
Charge
Discharge
charge transfer
; constant
Figure 3. Correlation of time and OCVs at the vanadium concentration of 1.5 M.

21. Results and Discussion
oxidation state of vanadium
1.0 M Charge
OCV (V)
Comparison of
Time Conversion method & Titration method
oxidation state of vanadium
1.5 M Charge
OCV (V)
oxidation state of vanadium
2.0 M Charge
OCV (V)
Figure 4. Correlation of OCV and oxidation state of vanadium at the vanadium concentration of 1.0 M, 1.5 M and 2.0 M (charging time conversion method and titration method).

22. Results and Discussion
oxidation state of vanadium
1.0 M Discharge
OCV (V)
Comparison of
Time Conversion method & Titration method
oxidation state of vanadium
1.5 M Discharge
OCV (V)
oxidation state of vanadium
2.0 M Discharge
OCV (V)
Figure 5. Correlation of OCV and oxidation state of vanadium at the vanadium concentration of 1.0 M, 1.5 M and 2.0 M (discharging time conversion method and titration method).

23. Results and Discussion
Charging-discharging time Conversion method
1.0 M
OCV (V)
oxidation state of vanadium
Figure 6. Comparison of the OCV and oxidation state of vanadium obtained from charging and discharging processes.

24. Results and Discussion,
Results and Discussion ,
Nernst Equation
The experimental data is used to determine the values of and from (4) to (6) Based on the oxidation state of
vanadium of +3.5, as the reference electrolyte
V2+ to V3+
V3+ to VO2+
VO2+ to VO2+

25. Results and Discussion
Comparison of Titration method & Nernst equation
OCV (V)
oxidation state of vanadium
Figure 7. Comparison of OCV and oxidation state of vanadium at the vanadium concentration of 1.0 M, 1.5 M and 2.0 M (Nernst equation and titration method).

26. Conclusions
A correlation of the OCV and the oxidation state of vanadium is investigated.
Nernst equation is used to describe this relationship
The standard potential of each half cell is obtained from experimental data.
The prediction of OCV by Nernst equation agrees reasonably with the experimental data at different oxidation states of vanadium.

27. Conclusions
Nernst equation with standard potential of each half cell from these experiments can be utilized to evaluate the oxidation state of vanadium in each side by measurement of the OCV at each half cell compared with the reference electrolyte.
Electrolyte imbalance can thus be measured by modified OCV and Nernst equation.

28. Acknowledgements
Financial support from
Cellennium (Thailand) Co., ltd., is gratefully acknowledged.
The authors would like to thank Dr. Suradit Holasut for his support and suggestions.

29. Thank you