Zinc Bromide flow batteries manufacturers usually use standard electrolytes. These electrolytes utilize the same ingredients with different ratios: Bromine, Zinc ...
ZINC BROMIDE FLOW BATTERIES: CUSTOM BROMINE COMPLEXING AGENTS Dr. Ben-Zion Magnes*, Dr. Ran Elazari*, Dr. Iris Ben-David†, Dr. Ronny Costi* *The Electrochemistry Lab and †Organic Lab, ICL-IP R&D, Beer-Sheva, Israel
Introduction Zinc Bromide flow batteries manufacturers usually use standard electrolytes. These electrolytes utilize the same ingredients with different ratios: Bromine, Zinc Bromide, Zinc Chloride, supporting electrolyte (e.g. KCl), anti-dendrite additives and Bromine Complexing Agents (BCA). According to publications, most manufacturers use a very small variety of BCAs. Main challenges of the Zinc Bromide flow batteries include (a) charge overvoltages, (b) low current densities during cycling, (c) dendrite formation, (d) high viscosity of the electrolyte polybromide phase in low states of charge, and (e) pH variations during operation. Our approach for tackling these challenges is by fitting the physico-chemical properties of the BCA to specific cell configurations. We, at ICL-IP, have developed and studied a library of BCAs. This library consists, mainly, of families of quaternary ammonium and phosphonium salts. The large variety of BCAs allows the preparation and tailoring of custom electrolytes that will overcome the met challenges at specific conditions. Here we present a comparison between two similar BCAs from the same family, and demonstrate our ability to tune the required properties of the electrolyte.
Br2 Concentration in Aqueous Phase
Viscosity and Adhesion
Bromine concentration in the aqueous phase is an extremely important parameter for cell operation. Bromine concentration depends on: • Solubility of BCA in aqueous phase of the electrolyte • Molecular structure of the BCA • Interfacial dynamics between the aqueous and polybromide phases (viscosity and relative density) BCA17 gives a higher bromine concentration at low SOCs, than BCA13, which facilitates the charge cycle by lowering overvoltages, and allows deeper discharge.
BCA17 polybromide phase has a lower viscosity at low SOC than BCA13.
0.016
State of Charge, %
Br2, aq. phase [M]
0.014 0.012 0.01
0 20 40
0.008 0.006
BCA17
0.004
13 BCA
0.002 0
0
20
40
60
80
BCA17 Viscosity, cP Density, gr/cm3 1.57 50 1.59 55 1.68 42
100
State of Charge [%]
Bromine concentration in aqueous phase of electrolyte at low states of charge
Adhesion of BCA to the Electrode 80
0.005
Retention Time [sec]
Br2, aq. phase [M]
0.006
BCA17
0.004 0.003 0.002
3
1 BCA
0.001 0
BCA13 20%SOC
10
20
State of Charge [%]
30
40
BCA17 0%SOC
40
20
0
0
BCA13 0%SOC
60
0
1000
2000
3000
Rate of rotation [rpm]
BCA13 Density, gr/cm3 Viscosity, cP 1.58 65-70 1.59 65-70 1.66 55
Adhesion of the polybromide phase to a graphite electrode can block the electrode, slow down bromine/bromide electrochemical kinetics and add resistance to the system, thus lowering efficiencies. By using RDE, we can compare adhesive strength of the polybromide phases. BCA17 shows lower adhesion even at low SOCs, while BCA13’s adhesion decreases, from a relatively high adhesion strength, as the SOC raises.
Dendrites and Overvoltages
pH changes
Dendrites can cause short circuits in cells and lower efficiencies. Typical zinc deposition pattern at the end of charge (4.5 hr, 60 mA/cm2) is presented: BCA13 (left) shows uneven pattern that can lead to
Electrolyte pH drop during a lengthy operation of the cell is a well documented issue with ZnBr2 flow cells. This pH drop can harm the construction materials of the cell and directly affects the activity of the electrolyte and the cell efficiency.
pH changes at 60C
1.5
BCA13 BCA17
1.3
Direct pH
Bromine concentration in aqueous phase of electrolyte 0.018
1.1
0.9
0.7
BCA13 BCA17
0.5 0.4 0.3 0.2 0.1 0
0
200
400
600
800
Time [s]
1000
1200
1400
Voltage profiles at the beginning of charge shows BCA17 as having a lower and more stable profile in comparison to BCA13. This profile difference leads to better charging efficiencies.
Concentration of H+
0.06
0.5
BCA17
0.04 0.03 0.02 0.01 0
1
1
2
3
4
5
2
3
4
Time [w]
5
6
Using the appropriate BCA can dampen the effect of pH drop and H+ formation. BCA13 electrolytes at 60°C (accelerated studies) exhibit deeper pH drop and higher H+ formation in comparison to BCA17 electrolytes at the same conditions.
Free Energy – Stability of Polybromide Complexes DFT theoretical calculations were used to compare the free energy (ΔG) of polybromide complexes at different coordinating structures in aqueous and organic phases. The structures studied were free BCA (with a single bromide ion), and complexed BCA with one, two or three bromine molecules in coordination. In organic phase both BCA13 and BCA17 exhibit similar stability. In aqueous phase BCA17 shows a much higher stability than BCA13 for low coordination (one and two molecules), which manifests the main population of bromine in aqueous phase. This higher stability coincides with the higher bromine concentration in aqueous phases at low SOC (as presented above), and with the lower overvoltages.
Summary
6
Time [w]
BCA13
0.05
Concentration H+ [M]
Relative Cell Potential [V]
dendrite growth, while BCA17 (right) exhibits a more conformal and uniform deposition.
Charge Volatage Profile
0.6
This work presents a comparison between two similar ICL-IP proprietary BCAs. We have demonstrated the ability to custom design a bromine complexing agent and to control many properties and behaviors. Our development and characterization capabilities enabled us to form an extensive library of BCAs and electrolyte additives which allows ICL-IP to work with our customers on improving the performance, efficiency, stability and safety of their products.
