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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

1

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

2

CatholyteReservoir

Reference Electrode

Flow Cell

Hydrogen Collection Tube

Anolyte Reservoir

Catholyte: VIV/VV Anolyte: VII/VIII

Experimental

3

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%)

4

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

5

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

6

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

7

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

8

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

9

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

10

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.

11

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.

12

‘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

E


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