Kittima Ngamsai1

Amornchai Arpornwichanop1, 2

PREDICTION OF THE OXIDATION STATE OF VANADIUM IN A VANADIUM

REDOX FLOW BATTERY

1 Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University

2 Computational Process Engineering, Chulalongkorn University

Outline

Materials and MethodsMaterials and Methods22

Results and DiscussionResults and Discussion33

ConclusionsConclusions44

IntroductionIntroduction11

Introduction

Renewable energy

Conventional energy

Energy storage technology

IntroductionWhat is Vanadium redox flow battery (VRB)?

Energy storage technologyElectrochemical 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

VO2+ VO2+ V2+ V3+

Negative

Positive

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 ?

V2+ V3+ V4+ V5+Charge Charge

Discharge Discharge

Balance

Imbalance

V2.5+ V4.5+

V2.5+ V 4.5+

PositiveNegative

Introduction

Electrolyte Imbalance in VRB

Side reaction

- Air oxidation of V(II) ion

- Gassing side reaction during charging

Electrolyte transfers across membrane

- Vanadium ion transfer

- Water transfer

Cause of electrolyte imbalance

Introduction

Electrolyte Imbalance in VRB

Effect of electrolyte imbalanceThe loss of energy capacityDecrease Efficiency Release heat

Side reaction Electrochemical reaction Electrolyte transfer Electrolyte mixing

Method to rebalance electrolyte

Introduction VRB Research for electrolyte imbalance

The other research groups 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 **

In this study

An electrolyte imbalance can be measured by using the modified OCV cell and Nernst’s equation

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.

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.

State of charge Versus OCV

Materials and Methods

Materials and MethodsTo investigate a correlation of OCV and the oxidation state of

vanadium in an electrolyte solution, the conventional OCV has been modified

Figure 1 (a) Conventional OCV cell and (b) modified OCV cell

(a) (b)

Materials and Methods

The initial electrolyte solutions was prepared at an oxidation state of vanadium of +3.5 (including 50% V3+ and 50% VO2+).

Experimental

Vanadium salt

Sulfuric acid

1.0 M

2.0 M

1.5 M

Materials and Methods

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.

Experimental

Materials and Methods

OCV

Cell

P P

Positive Electrolyte

Power supply/Load

Vocv

Negative Electrolyte

Data logger

Negative electrolyte

Positive electrolyte

Reference electrolyte

V

V

VVocv_neg Vocv_pos

Figure 2. Schematic diagram of the VRB system

Materials and Methods

Samples were titrated to determine the oxidation state of

vanadium using the potentiometric titration with potassium

permanganate as a titrant.

Experimental

To confirm the reliability of the time conversion method

Electrolyte solution samples were collected in different OCV

Materials and Methods

Oxidation of vanadium (from +2 to +3):

(1)

Oxidation of vanadium (from +3 to +4):

(2)

Oxidation of vanadium (from +4 to +5):

(3)

e32 VV

eH2VOOHV 22

3

eH2VOOHVO 222

Nernst equation for correlation of OCV and oxidation state of vanadium in the electrolyte

In the electrolyte system of VRB

Materials and Methods

][V

][VlnOCV

3

20

nF

RTEnn

]VO[

]V[lnOCV

2

30

nF

RTEmm

][VO

]VO[lnOCV

220

nF

RTEpp

Nernst equation for correlation of OCV and oxidation state of vanadium in the electrolyte

From (1)

From (2)

From (3)

(4)

(5)

(6)

Materials and Methods

Correlation of an OCV and oxidation state of vanadium in electrolyte

Charging-Discharging

time Conversion method

Titration method

Nernst equation

Results and DiscussionCharging-discharging time Conversion

method

Figure 3. Correlation of time and OCVs at the vanadium concentration of 1.5 M.

(

)

charge transfer

IdtQ

)(tfQ ; constant

I

Charge Discharge

00np EE

Results and Discussion

oxidation state of vanadium

1.0 M Charge

OC

V (

V)

oxidation state of vanadium

1.5 M Charge

OC

V (

V)

oxidation state of vanadium

2.0 M Charge

OC

V (

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).

Comparison of Time Conversion method & Titration

method

Results and Discussion

oxidation state of vanadium

1.0 M Discharge

OC

V (

V)

oxidation state of vanadium

1.5 M Discharge

OC

V (

V)

oxidation state of vanadium

2.0 M Discharge

OC

V (

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).

Comparison of Time Conversion method & Titration

method

Results and Discussion

Figure 6. Comparison of the OCV and oxidation state of vanadium obtained from charging and discharging

processes.

oxidation state of vanadium

1.0 M

OC

V (

V)

Charging-discharging time Conversion method

Results and DiscussionNernst

Equation

VE on 7.0

VE om 0

VE op 7.0

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

,

,

][V

][Vln7.0OCV

3

2

nF

RTn

]VO[

]V[lnOCV

2

3

nF

RTm

][VO

]VO[ln7.0OCV

22

nF

RTp

,onE

omE o

pE

V2+ to V3+

V3+ to VO2+

VO2+ to

VO2+

Results and Discussion

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).

Comparison of Titration method & Nernst equation

oxidation state of vanadium

OC

V (

V)

ConclusionsA 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.

THANK YOU