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
Outline
1
Introduction
2
Materials and Methods
3
Results and Discussion
4
Conclusions
Introduction
Renewable energy
Conventional energy
Energy storage technology
Introduction
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+
Negative
VO2+ VO2+
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 ?
Negative
V2+ Charge
Positive
V3+
V4+ Charge
Discharge
V5+
Discharge
V2.5+
Balance
V2.5+
Imbalance
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
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 **
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) 4348.
In this study
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
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
1.0 M
Vanadium salt
1.5 M
Sulfuric acid
2.0 M
The initial electrolyte solutions was
prepared at an oxidation state of
vanadium of +3.5 (including 50%
V3+ and 50%VO2+).
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.
Materials and Methods
Power supply/Load
Data logger
Cell
Vocv
V
P
P
Negative Reference Positive
electrolyte electrolyte electrolyte
OCV
Negative
Electrolyte
Positive
Electrolyte
V
Vocv_neg
V
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
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):
V2  V3  e
(1)
 Oxidation of vanadium (from +3 to +4):
V3  H2O  VO 2  2H  e
(2)
 Oxidation of vanadium (from +4 to +5):
VO 2  H 2O  VO 2  2H   e 
(3)
Materials and Methods
Nernst equation for correlation of OCV and oxidation state of
vanadium in the electrolyte
From (1)
2
RT
[V
]
OCVn  En0 
ln 3
nF [V ]
(4)
From (2)
OCVm 
RT
[V3 ]

ln
nF [VO 2 ]
(5)
From (3)

RT
[
VO
2]
OCV p  E 0p 
ln
nF [VO 2 ]
(6)
Em0
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
Charging-discharging time Conversion method
Charge
Discharge
E E
0
p
0
n
charge transfer

Q  Idt
Figure 3. Correlation of time and OCVs at the vanadium
concentration of 1.5 M.
Q  f (t )
; constant
I
Results and Discussion
Comparison of
Time Conversion method &
Titration method
OCV (V)
1.0 M Charge
oxidation state of vanadium
2.0 M Charge
OCV (V)
OCV (V)
1.5 M Charge
oxidation state of vanadium
oxidation state of vanadium
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
Comparison of
Time Conversion method &
Titration method
OCV (V)
1.0 M Discharge
oxidation state of vanadium
2.0 M Discharge
OCV (V)
OCV (V)
1.5 M Discharge
oxidation state of vanadium
oxidation state of vanadium
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
OCV (V)
1.0 M
oxidation state of vanadium
Figure 6. Comparison of the OCV and oxidation state of vanadium obtained from
charging and discharging processes.
,
,
Results and Discussion
Nernst Equation
The experimental data is used to determine the values of
o
m
o
and
p from (4) to (6) Based on the oxidation state of
vanadium of +3.5, as the reference electrolyte
E
E
V2+ to V3+
Eno  0.7 V
V3+ to VO2+
Emo  0 V
VO2+
to VO2
+
E  0.7 V
o
p
o
n
E ,
RT [V 2 ]
OCVn  0.7 
ln 3
nF [V ]
RT
[ V 3 ]
OCVm 
ln
nF [VO 2 ]
RT [VO 2 ]
OCVp  0.7 
ln
nF [VO 2 ]
Results and Discussion
OCV (V)
Comparison of Titration method & Nernst equation
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.