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A New Vanadium Redox Flow BatteryUsing Mixed Acid Electrolytes

November 2, 2010

US DOE Energy Storage Systems (ESS) Program Review

Washington DC

Liyu Li, Soowhan Kim, Wei Wang, M. Vijayakumar, Zimin Nie, Baowei

Chen, Jianlu Zhang, Jianzhi Hu, Gordon Graff, Jun Liu, Gary Yang*

Funded by the Energy Storage Systems Program of the U.S. Department Of Energy through Pacific

Northwest National Laboratories



Redox Flow Battery (RFB)

A redox flow battery is a promising technology for large scale energy

storage .



Potential RFB Systems

-1.0 -0.5 0.0 0.5 1.0 1.5 2.0

Standard potential (V) of redox couples

H2 evolution O2 evolutionV3+/V

2+VO2

+/VO

2+

VO2+/V

3+

Fe3+/Fe

2+

Mn3+/Mn

2+

MnO4-/MnO2

Ce+/Ce

+

Co3+/Co

2+

Cu2+/Cu

+

TiOH3+/Ti

3+

Ti+/Ti

+

Cr+/Cr

+Zn

2+

/Zn

S/S2-

Br2/Br-

BrCl2-/Br

-

Cr+/Cr

+

Cl2/Cl-

All V Redox Flow Battery

The use of vanadium in both the anolyte and catholyte effectively

eliminate the cross-contamination between the electrolytes through

the ion-exchange membrane.



Challenges for All Vanadium Sulfate RFB

V specie Vn+, M H+, M SO42-, M Temp, oC Time for p.p.

V2+

2 6 5 -5 419 hr

2 6 5 25 Stable (>30 d)

2 6 5 40 Stable (>30 d)

V3+

2 4 5 -5 634 hr

2 4 5 25 Stable (>30 d)

2 4 5 40 Stable (>30 d)

V4+ (VO2+)

2 6 5 -5 18 hr

2 6 5 25 95 hr

2 6 5 40 Stable (>30 d)

V5+ (VO2+)

2 8 5 -5 Stable (>30 d)

2 8 5 25 Stable (>30 d)

2.2 7.8 5 40 95 hr

1.8 8.4 5 40 358 hr

Low energy density: Vn+ concentration <1.7M, decided by the low

solubility of V4+ at low temperatures and the poor stability of V5+ at high

temperatures.

Limited operation temperature window: 10 to 40oC, requiring active

electrolytes temperature management during hot/cold weathers.



Stabili ty of Vn+ Cations in HCl Solution

Vn+ specie Vn+, M H+, M Cl-, M T, oC Time for precipitation

V2+ 2.3 5.4 10 -5 Stable (>10 d)

2.3 5.4 10 25 Stable (>10 d)2.3 5.4 10 40 Stable (>10 d)

V3+ 1.5 3.0 7.5 -5 Stable (>10 d)

1.8 3.0 8.4 -5 124 hr

2.3 3.1 10 -5 96 hr

2.3 3.1 10 25 Stable (>10 d)

2.3 3.1 10 40 Stable (>10 d)

V4+ (VO2+) 2.3 5.4 10 -5 Stable (>10 d)

2.3 5.4 10 25 Stable (>10 d)

2.3 5.4 10 40 Stable (>10 d)

V5+ (VO2+) 2.3 7.7 10 -5 Stable (>10 d)

2.3 7.7 10 25 Stable (>10 d)2.3 7.7 10 40 Stable (>10 d)

Cl- anions can effectively stabilize V5+, V4+, and V2+.

It is likely that high concentration of V2+, V3+, V4+, and V5+ cations can be

stabilized in a mixed sulfate and chloride electrolyte solution.



Vn+ Stability in SO42—Cl- Mixed Solutions

Vn+

specie

Vn+, M T, oC Time for

precipitation

V2+ 3 -5 Stable (>10 d)

2.5 -5 Stable (>10 d)

2.5 25 Stable (>10 d)

2.5 40 Stable (>10 d)

3 40 Stable (>10 d)

V3+ 3 -5 192 hr (8 d)

2.5 -5 Stable (>10 d)

2.5 25 Stable (>10 d)

2.5 40 Stable (>10 d)

3 40 Stable (>10 d)

Vn+

specie

Vn+, M T, oC Time for

precipitation

V4+

(VO2+)

3 -5 Stable (>10 d)

2.5 -5 Stable (>10 d)

2.5 25 Stable (>10 d)

2.5 40 Stable (>10 d)

3 40 Stable (>10 d)

V5+

(VO2+)

3 -5 Stable (>10 d)

2.5 -5 Stable (>10 d)

2.5 25 Stable (>10 d)

2.5 40 Stable (>10 d)

3 40 Stable (>10 d)

2.7 V5+

0.3 V4+ 50 Stable (>10 d)

2.7 V5+

0.3 V4+

60 Stable (>10 d)

SO42-/Cl- mixtures can effectively stabilize >2.5M V5+, V4+, V3+ and V2+.

Much broad operation temperature window (-5 to 60 oC) can be achieved usingSO42-/Cl- mixed electrolytes.

The overall stability of vanadium isdecided by V3+ at low temperatures.





51V in mixed acid

51V in sulfuric

acid51V in VOCl3

standard

35Cl in mixed acid

35Cl in VOCl3

35Cl in HCl

Solution Chemistry of the Mixed Electrolytes-

NMR Study



Temperature Dependence of VO2Cl(H2O)2 Formation

-570

-565

-560

-555

-550

-545

-540

-30 -20 -10 0 10 20 30 40 50 60Temperature,

oC

5 1 V C h e m i c a l S h i f t ( p p m )

0

10

20

30

40

50

60

70

80

5 1 V

L i n e W i d t h ( k H z )

Chemical Shift-Sulfuric Acid

Chemical Shift Mixed Acid

Line Width Sulfuric Acid

Line Width Mixed Acid

VO2Cl(H2O)2 complex starts to form in the mixed solutions when

temperatures reaching ~20oC.



Solution Chemistry of the Mixed Electrolytes-

DFT and Stability Studies

Same V2+, V3+ and V4+ -containing structures was predictedin both electrolyte solutions: [V(H2O)6]

2+, [V(H2O)6]3+ and

[VO(H2O)5]2+.

According to SEM-EDS, and XRD analysis, in the mixed

systems, the stability of V4+

is controlled by the solubility of VOSO4, the stability of V3+ is controlled by the solubility of

V2(SO4)3 and VOCl, the stability of V2+ is controlled by the

solubil ity of VSO4.

The improvement of stabi li ty of V

2+

, V

3+

, and V

4+

in the mixedsystem over the current sulfate systems is due to the

decrease of SO42- concentration in the solutions.



Electrochemical Properties of Mixed Solutions

-0.8 -0.4 0.0 0.4 0.8 1.2 1.6-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

2.5 M V4+

+ 2.5 M SO2-

4 + 6 M Cl

-

1.5 M V4+ + 5 M SO2-4

C u r r e n t D e n s

i t y ( A / c m

2 )

Potential (V vs. SHE)

V5+V

4+

V4+

V5+

V2+

V3+

V3+V

2+

No chlorine gas evolution.



Electrode and Cell Reactions (T<10oC)

Cathode: VO2+ + H2O – e VO2+ + 2H+ E° = 1.00V

Anode: V3+ + e V2+ E°=-0.25V

Cell: VO2+ + H2O + V3+ VO2+ + 2H+ + V2+ E°=1.25V

Charge

Discharge

Charge

Discharge

Charge

Discharge



Cathode: VO2+ + Cl- + H2O – e VO2Cl + 2H+

Anode: V3+ + e V2+

Cell: VO2+ + Cl- + H2O + V3+ VO2Cl + 2H+ + V2+

Charge

Discharge

Charge

Discharge

Charge

Discharge

Electrode and Cell Reactions (T>10oC)



- Cell and reservoirs are inside the environmental chamber.

- Circulating pump is out side of the chamber.

- Pressure monitoring.

Cell Testing and RFB Evaluation



0 10 20 30 40 50 60 70 80 90 1000.6

0.8

1.0

1.2

1.4

1.6

1.8

Charging

V o l t a g e ( V )

Capacity / Charging Capacity (Ah%)

Discharging

10 15 20 25 30 35 40 45 50 5570

75

80

85

90

95

100

Coulomb Energy Voltage

E f f i c i e n

c y ( % )

Cycle Number

Stable performance with 88% energy efficiency at 50 mA.cm-2

Cell Performance at Ambient Temperature



70

75

80

85

90

95

100

0 10 20 30 40 50 60 70 80 90

Cycle number

E f f i c i e n c y , %

Coulombic Efficiency

Voltage Efficiency

Energy Efficiency

40oC 0

oC5

oC50

oC

A VFB with 2.5 M V mixed acid electrolyte can be operated under a

broad temperature range of 0 to 50 oC.

Redox reactions are temperature dependent.

No noticeable gas evolution over 25 days.

Cell Performance at Varied Temperatures



Cell Performance at Varied Current Densities

Cell operation conditions: 10 cm2 flow cell, Charged to 1.7V by 50 mA/cm2 current.

Discharge

Current,

(mA.cm -2)

Energy Density

(Wh.L-1)

Columbic

Efficiency

Energy

Efficiency

Mixed sulfate Mixed sulfate Mixed sulfate

2.5MV 3MV 1.6MV 2.5MV 3MV 1.6MV 2.5MV 3MV 1.6MV

100 36.2 39.5 22.3 0.95 0.95 0.94 0.81 0.76 0.83

75 37.5 40.8 22.4 0.96 0.96 0.94 0.84 0.81 0.85

50 38.5 41.8 22.6 0.96 0.97 0.94 0.87 0.85 0.8725 39.2 43.1 22.6 0.96 0.97 0.94 0.90 0.89 0.88

The VRBs using mixed sulafte-chloride electrolyteswere able to deliver 70 to 80% more energy than the

sulfate system, while sti ll being highly effic ient.



A new vanadium redox flow battery with a significant improvement

over the current technology was developed.

This battery utilizes sulfate-chloride mixed electrolytes, which are

capable of dissolving 2.5 M vanadium, representing about 70%

increase in energy density over the current sulfate system.

More importantly, the new electrolyte remains stable over a wide

temperature range of -5 to 60oC, potentially eliminating the need of

energy-consuming solution temperature management.

Battery tests indicated no concern of chlorine gas evolution during the

battery operation.

Summary



Further Work

Optimize the mixed electrolyte for further improvement in energy

density and stability.

Demonstrate a 2.0 kWh (0.3 kW) bench-top prototype FRB with thenewly developed mixed electrolyte.

Build up strong collaborations with industry, university, and othernational laboratory partners.

Prepare for larger systems demonstration within 2-3 years.



Acknowledgements

Financial support :

1. DOE Office of Electricity Delivery and Energy Reliability

Energy Storage Program (Manager: Dr. Imre Gyuk)

2. PNNL LDRD program for NMR and DFT-related work.

The NMR work was carried out at the Environmental and Molecular Science Laboratory, a national scientific user facility sponsored by

the DOE’s Office of Biological and Environmental Research (BER).

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