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 evolution V3+/V2+ VO2+/VO2+

VO2+/V3+

Fe3+/Fe2+

Mn3+/Mn2+

MnO4-/MnO2

Ce4+/Ce3+

Co3+/Co2+

Cu2+/Cu+

TiOH3+/Ti3+

Ti3+/Ti2+

Cr3+/Cr2+

Zn2+/Zn

S/S2-

Br2/Br-

BrCl2-/Br-

Cr5+/Cr4+

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 RFBV specie Vn+, M H+, M SO4

2-, M Temp, oC Time for p.p.

V2+

2 6 5 -5 419 hr2 6 5 25 Stable (>30 d)2 6 5 40 Stable (>30 d)

V3+

2 4 5 -5 634 hr2 4 5 25 Stable (>30 d)2 4 5 40 Stable (>30 d)

V4+ (VO2+)2 6 5 -5 18 hr2 6 5 25 95 hr2 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 hr1.8 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.

Stability of Vn+ Cations in HCl Solution

Vn+ specie Vn+, M H+, M Cl-, M T, oC Time for precipitationV2+ 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 hr2.3 3.1 10 -5 96 hr2.3 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+

specieVn+, M T, oC Time for

precipitationV2+ 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+

specieVn+, M T, oC Time for

precipitationV4+

(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 using SO42-/Cl- mixed electrolytes.

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

V

O

O

O

O

O

O

O

O

O

Cl

In sulfate solution, V5+ exists as [VO2(H2O)3]+, which tends to convert to V2O5-3H2O precipitation via:

[VO2(H2O)3]+ → VO(OH)3 + [H3O]+

2VO(OH)3 → V2O5-3H2O ↓

Solution Chemistry of the Mixed Electrolytes-DFT Study

V

51V in mixed acid

51V in sulfuric acid

51V in VOCl3standard

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

51V

Che

mic

al S

hift

(ppm

)

0

10

20

30

40

50

60

70

80

51V

Line

Wid

th (k

Hz)

Chemical Shift-Sulfuric AcidChemical Shift Mixed AcidLine Width Sulfuric AcidLine 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 mixedsystems, the stability of V4+ is controlled by the solubility ofVOSO4, the stability of V3+ is controlled by the solubility ofV2(SO4)3 and VOCl, the stability of V2+ is controlled by thesolubility of VSO4.

The improvement of stability of V2+, V3+, and V4+ in the mixedsystem over the current sulfate systems is due to thedecrease of SO4

2- 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

Curr

ent D

ensi

ty (A

/cm

2 )

Potential (V vs. SHE)

V5+V4+

V4+ V5+

V2+ V3+

V3+V2+

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

Volta

ge (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

Ef

ficie

ncy

(%)

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

Effic

ienc

y, %

Coulombic Efficiency

Voltage Efficiency

Energy Efficiency

40oC 0oC5oC50oC

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 sulfate2.5MV 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 0.87

25 39.2 43.1 22.6 0.96 0.97 0.94 0.90 0.89 0.88

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

sulfate system, while still being highly efficient.

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 the newly developed mixed electrolyte.

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

Prepare for larger systems demonstration within 2-3 years.

AcknowledgementsFinancial 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 MolecularScience Laboratory, a national scientific user facility sponsored bythe DOE’s Office of Biological and Environmental Research (BER).