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

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

Introduction Renewable energy Conventional energy Energy storage technology

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

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

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

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+

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

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

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) 235-247. ** M. Skyllas-Kazacos and L. Goh, J. Membr. Sci. J. 399-400 (2012) 43-48.

Materials and Methods State of charge Versus OCV

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

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

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.

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

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.

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)

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)

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

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.

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).

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).

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.

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+

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).

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.

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.

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.

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