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Vanadium Redox Flow Battery:
Optical, State-of-Charge Sensor
Prof. Noel Buckley, Dr Xin Gao, Dr Robert Lynch
Department of Physics and Energy
Materials and Surface Science Institute
University of Limerick
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Introduction
•Interest in flow batteries
• Large-scale storage of energy from intermittent
sources (e.g., wind, ocean, solar)
• Separate sizing of power and energy
•Vanadium redox system
• Positive (VIV/VV): VO2+ + 2H+ + e– = VO2+ + H2O
• Negative (VII/VIII): V2+ = V3+ + e–
• Typically 1.5 mol dm-3 Vanadium or greater
• Separated by proton-conducting membrane
• Cross-contamination problems minimized
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CatholyteReservoir
Reference Electrode
Flow Cell
Hydrogen Collection Tube
Anolyte Reservoir
Catholyte: VIV/VV Anolyte: VII/VIII
Experimental
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State of Charge (SoC) of VRFBs
Traditional Method 1: Open-circuit Voltage
Positive: Eo(VV/VIV) = 1.00 V vs SHE
Negative: Eo(VIII/VII) = -0.26 V vs SHE
− Small offsets and drifts in electrodes potentials
can be equivalent to the effect of a significant
change in mixture ratio (especially for mixture
ratios close to 50%)
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State of Charge (SoC) of VRFBs
Traditional Method 2: Coulometry
Charge passed is tracked and remaining
charge is estimated
− Current inefficiencies lead to
overestimates of the remaining charge
− Unequal half cell efficiencies are not
accounted for so SoC imbalance is not
detected
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State of Charge (SoC) of VRFBs
Traditional Method 3: Conductivity
The conductivity of the anolyte and catolyte
is different for different states of charge
− Other effects such as electrolyte dilution
and impurity levels also alter conductivity
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State of Charge (SoC) of VRFBs
Traditional Method 4: UV-visible spectroscopy
Each of the vanadium species has a
characteristic absorption spectrum
Accurate determination of concentration of each
vanadium species at low concentration
− In-situ (high concentration) measurements of
vanadium IV/V mixtures display spectra that do
not correspond to the predicted values
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UV-Visible Spectroscopy of VRFB Electrolytes:
Vanadium Species Dissolved in H2SO4
VIII
200 300 400 500 600 700 800 9000.0
0.5
1.0
Abs.
Wavelength (nm)
0
100%
50%
Anolyte 1.5 M in 3 M H2SO4
VII
0% SoC 100% SoC
Each of the vanadium species has a characteristic absorption spectrum
Accurate determination of concentration of each vanadium species at low concentration
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UV-Visible Spectroscopy of VRFB Electrolytes:
Vanadium Species Dissolved in H2SO4
VIV VV
Catholyte 1.24 M in 3 M H2SO4
200 400 600 8000
1
2
3
min
Ab
s.
Wavelength (nm)
521 nm
0
100%
50%
Predicated
0% SoC 100% SoC
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UV-Visible Spectroscopy of VRFB Electrolytes:
Vanadium Species Dissolved in H2SO4
VIV VV
Catholyte 1.24 M in 3 M H2SO4
200 400 600 8000
1
2
3
Ab
s.
Wavelength (nm)
50%
Experimental
min
0
100%
50%
Predicated
521 nm0% SoC 100% SoC
−In-situ (high concentration) measurements of vanadium IV/V mixtures display spectra that
do not correspond to the predicted values
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Advancement of UV-Vis Technique:
In-Situ SoC & Vanadium Concentration
(Patent Pending ) Advantages:
Instantaneous measurement of SoC and vanadium concentration without knowing past operation;
Possible in-situ measurement (i.e. at high vanadium concentration) without the need for electrolyte dilution;
Easy to connect to existing systems without need for electrodes;
Greater accuracy than other techniques (e.g. conductivity and potential measurement);
Separate readings for the SoC and concentration of anolyte and catholyte;
Measurement is independent of system’s electrochemistry;
The technique does not require knowledge of concentration of vanadium.
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Catholyte Anolyte
In-Situ VRFB State of Charge (SoC)
& Vanadium Concentration
Optical Probe Technique Anolyte SoC
VII+VIII Conc.
Catholyte SoC
VIV+VV Conc.
65%
1.67 mol dm-3
69%
1.68 mol dm-3
Could be supplied with one or
two probes.
Probes could be used to
analyse electrolyte at any
access point in the system.
Typically for use during
vanadium redox flow battery
maintanence, testing and
troubleshooting.
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‘Fuel Gauge’: In-Line
Determination of SoC &
Concentration To be used in vehicles that store their energy
in a vanadium redox flow battery
An optical fibre probe could be placed in each
of the two ‘fuel’ tanks or in the ‘fuel’ lines
Two needles on a gauge (or some other type
of display) could show the remaining charge
so that the operator would know when to refill
Also, if the operator refilled before ‘zero
charge’ they would know how much charge
was in the electrolyte they exchange at the
filling station
A similar system could be operated at the
filling station’s pumps to determine the charge
of the fuel being exchanged
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