Methods for determining state of charge and calibrating reference electrodes in a redox flow battery
View Patent ↗The invention concerns methods of determining the state of charge of a half-cell within a redox flow battery, the method comprising: (i) measuring the rate of change in equilibrium half-cell reduction potential of the electrolyte as charge is passed into the electrolyte solution within the cell; and (ii) correlating said rate of change in equilibrium half-cell reduction potential with the state of charge of said half-cell. Other aspects of the invention concern balancing the state of charge of a flow battery and methods of calibrating an oxidation/reduction probe.
1. A method of determining the state of charge of an electrolyte within an operating half-cell within a redox flow battery wherein the electrolyte exhibits an instantaneous equilibrium half-cell reduction potential at any given point in time, said method comprising:
measuring the instantaneous equilibrium half-cell reduction potential of the electrolyte as a function of charge that has passed into an electrolyte solution within the half-cell during operation of the operating redox flow battery; wherein the change in the instantaneous equilibrium half-cell reduction potential as a function of charge that has passed through the electrolyte solution is reflective of the state of charge of the electrolyte, which can be determined by solving the equation
dE
dS
=
-
RT
n
F
(
100
S
2
-
100
S
)
where,
E is the equilibrium half-cell reduction potential,
S is the percent of an electrolyte species in a reduced state,
T is the temperature of the electrolyte,
R is the gas constant,
n is the number of electrons in the reaction,
F is Faraday's constant,
dE is the differential change in equilibrium half-cell reduction potential,
dS is the differential change in the percent of a reduced electrolyte species and is determined by the equation
dS
=
A
·
h
c
·
V
·
F
·
1
3600
,
where,
A is the amount of current passed through the electrolyte solution,
h is the number of hours the current was passed into the electrolyte solution,
c is the concentration of an active material in the electrolyte solution,
V is the volume of the electrolyte solution, and
F is Faraday's constant.
2. The method of claim 1 , wherein said equilibrium half-cell reduction potential is measured using a carbon electrode and a Ag/AgCl reference electrode.
3. The method of claim 2 , wherein said carbon electrode is a glassy carbon electrode.
4. The method of claim 1 , wherein correlating said rate of change is made utilizing a set of calibration data, the set of calibration data relating at least one state of charge to at least one rate of change in equilibrium half-cell reduction potential.
5. The method of claim 1 , wherein a relationship between said rate of change in equilibrium half-cell reduction potential and the state of charge of said half-cell follows the Nernst equation.
6. The method of claim 1 , wherein a relationship between said rate of change in equilibrium half-cell reduction potential and the state of charge of said half-cell does not follow the Nernst equation.
7. The method of claim 1 , wherein said half-cell is either half-cell-in a flow battery comprising:
a first half-cell comprising a first aqueous electrolyte comprising a first redox active material, and a first electrode in contact with said first aqueous electrolyte;
a second half-cell comprising a second aqueous electrolyte comprising a second redox active material, and a second electrode in contact with said second aqueous electrolyte; and
a separator comprising a membrane disposed between said first and second aqueous electrolytes; and
wherein said first and second electrodes are not the same electrodes as those used for measuring the rate of change in equilibrium half-cell reduction potential of the electrolyte as charge is passed into the electrolyte solution within the first or second half-cell.
8. The method of claim 7 , wherein said membrane is an ionomer membrane.
9. A method of balancing the state of charge between two half-cells of a redox flow battery, said method comprising:
determining the state of charge of a first half-cell within said redox flow battery, said first half-cell comprising a first electrolyte solution, said determination being made by measuring a first rate of change in equilibrium half-cell reduction potential of said first half-cell as current is passed through said first electrolyte solution, and correlating said first rate of change in equilibrium half-cell reduction potential with the state of charge of said first half-cell;
determining the state of charge of a second half-cell within said redox flow battery, said second half-cell comprising a second electrolyte solution, said determination being made by measuring a second rate of change in equilibrium half-cell reduction potential of said second half-cell as current is passed through said second electrolyte solution, and correlating said second rate of change in equilibrium half-cell reduction potential with the state of charge of said second half-cell;
determining a difference between the states of charge of the first and second half-cells; and
if the difference in state of charge between said first and second half-cells exceeds a first predetermined value, recharging at least one half-cell such that the difference in state of charge between said first and second half-cells is less than a second predetermined value; wherein said first predetermined value is greater than said second predetermined value.
10. The method of claim 9 , wherein the rate of change in equilibrium half-cell reduction potential (dE/dS) is determined by the equation
dE
dS
=
-
RT
n
F
(
100
S
2
-
100
S
)
where
E is the equilibrium half-cell reduction potential,
S is the percent of an electrolyte species in a reduced state,
T is the temperature of the electrolyte solution,
R is the gas constant,
n is the number of electrons in the reaction,
F is Faraday's constant,
dE is the differential change in equilibrium half-cell reduction potential,
dS is the differential change in the percent of a reduced electrolyte species and is determined by the equation
dS
=
A
·
h
c
·
V
·
F
·
1
3600
,
A is the amount of current passed through the electrolyte solution,
h is the number of hours the current was passed through the half-cell,
c is the concentration of an active material in the electrolyte solution,
V is the volume of the electrolyte solution, and
F is Faraday's constant.
11. The method of claim 10 , wherein said equilibrium half-cell reduction potential is measured using an oxidation/reduction potential probe comprising a carbon electrode and a Ag/AgCl reference electrode.
12. The method of claim 11 , wherein said carbon electrode is a glassy carbon electrode.
13. The method of claim 9 , wherein correlating said rate of change is made utilizing a set of calibration data, the set of calibration data relating at least one state of charge to at least one rate of change.
14. The method of claim 9 , wherein a relationship between said rate of change in equilibrium half-cell reduction potential and the state of charge of said half-cell follows the Nemst equation.
15. The method of claim 9 , wherein a relationship between said rate of change in equilibrium half-cell reduction potential and the state of charge of said half-cell does not follow the Nernst equation.
16. The method of claim 9 , wherein a separator comprising an ionomer membrane is disposed between said first and second half-cells.
17. A method of calibrating an oxidation/reduction probe comprising:
determining the state of charge of a half-cell in which the oxidation/reduction probe resides by (i) measuring a rate of change in equilibrium half-cell reduction potential as current is passed through an electrolyte solution associated with the half-cell; and
(ii) correlating said rate of change in equilibrium half-cell reduction potential with the state of charge of said half-cell;
determining a measured potential of the electrolyte solution in said half-cell using the oxidation/reduction probe to be calibrated;
determining a theoretical potential of the electrolyte solution based on a known relationship between state of charge and potential of the electrolyte solution; and
determining a difference between said theoretical potential and said measured potential, and using said difference to calibrate said oxidation/reduction probe.
18. The method of claim 17 , wherein the rate of change in equilibrium half-cell reduction potential (dE/dS) is determined by the equation
dE
dS
=
-
RT
n
F
(
100
S
2
-
100
S
)
where
E is the equilibrium half-cell reduction potential,
S is the percent of an electrolyte species in a reduced state,
T is the temperature of the electrolyte solution,
R is the gas constant,
n is the number of electrons in the reaction,
F is Faraday's constant,
dE is the differential change in equilibrium half-cell reduction potential,
dS is the differential change in the percent of a reduced electrolyte species and is determined by the equation
dS
=
A
·
h
c
·
V
·
F
·
1
3600
,
A is the amount of current passed through the electrolyte solution,
h is the number of hours the current was passed through the half-cell,
c is the concentration of an active material in the electrolyte solution,
V is the volume of the electrolyte solution, and
F is Faraday's constant.
19. The method of claim 18 , wherein said equilibrium half-cell reduction potential is measured using a carbon electrode and a Ag/AgCl reference electrode.