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»Non-Aqueous Vanadium Redox Flow Batteries«1st International Flow Battery Forum (IFBF)

Non‐Aqueous Vanadium Redox Flow Batteries1st International Flow Battery Forum (IFBF)

June 16th, 2010

Charles Monroe, Levi Thompson, Alice Sleightholme, and Aaron ShinkleUniversity of Michigan Department of Chemical Engineering

Christian Doetsch, Sascha Berthold, Birgit BrosowskiFraunhofer Institute UMSICHT

Jens Tuebke, Jens NoackFraunhofer Institute ICT


• Framework: Cooperation between University of Michigan (United States) and Fraunhofer Gesellschaft (Germany) established

• Project Partners:University of Michigan: Department of Chemical Engineering(Prof. Levi Thompson, Prof. Charles Monroe)

Fraunhofer Institute UMSICHT and ICT(Dr. Christian Doetsch, Dr. Jens Tuebke)

• Project Aim:Examination, developing and testing of materials and stack design for a non‐aqueous redox flow battery

Project Outline 1/2


•Main advantages of non‐aqueous systems:‐ Higher Voltage level‐ No Hydrogen/oxygen production‐ Higher energy densitiy

•Work plan:‐ Redox‐Chemistry, materials, membranes: University of Michigan‐ Prototype development: Fraunhofer ICT‐ Scale up, test bench: Fraunhofer UMSICHT

• Time Frame: Start End of 2009 / Duration 24 months

Project Outline 2/2


Single‐metal Redox Flow Batteries

• Aqueous all‐vanadium redox flow battery (RFB)

Performance depends on

• Half‐cell potentials(power density)

• Active‐species concentration(energy density)

• Electrolyte reservoir volume(charge capacity)


Existing RFBs mostly use aqueous electrolytes:

• Iron/chromium

• Bromine/polysulfide

• Zinc/bromine

• All‐vanadium

Multi‐metal chemistries susceptible to crossover

Cell potential limited by water electrolysis (E° = 1.23 V)

ZBB Energy Corp, 500kWh Zn‐Br RFB

Commercial Redox Flow Battery Chemistry

Non‐aqueous electrolytes enable higher cell potentials


Non‐Aqueous Vanadium RFB

Tester et al. The MIT Press. 2005.; http://www.eia.doe.gov; http://rredc.nrel.gov

V(III)e(IV) V

Separator

CatholyteTank

AnolyteTank

Electrodes

eV(III)V(II)

Source

• Single metal RFB mitigates cross contamination

Energy density dependent on:

– Cell potential

– Electrolyte concentration

– Electrolyte reservoir volume

Energy density dependent on:

– Cell potential

– Electrolyte concentration

– Electrolyte reservoir volume

Vanadium Acetylacetonate


Equation of the solvent

• Non‐aqueous0.01 M V(acac)3 (active species)

[vanadium‐actetylacetonate]

0.1 M TEABF4/CH3CN (support)[Tetra‐ethyl‐ammonium‐tetrafluoroborate]

Glassy carbon working electrode

• Aqueous0.01 M VOSO4 (active species)

[vanadyl‐sulfat]

2 M H2SO4/ultrapure H2O (support)

Glassy carbon working electrode

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

VIII/VIV

VII/VIII

2.2V

Cur

rent

den

sity

/mA

cm

- 2

Potential/V vs.SHE-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0

-15

-10

-5

0

5

10

VIV/VV

VII/VIII

1.4V

Cur

rent

den

sity

/mA

cm -

2

Potential/V vs. SHE


Progress: Redox Chemistry

• Presence of Cl‐ ions (from membrane manufacturing) produces extra peak close to VIII/VIV redox couple

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

10 mV/s300 K

a)

Cur

rent

den

sity

/mA

cm2

Potential/V vs. SHE-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

-10

-5

0

5

10

15

500 mV/s300 K

Cur

rent

den

sity

/mA

cm-2

Potential/V vs. Ag/Ag+

• Peak circled in red corresponds to oxidation of V(acac)3 to VO(acac)2 produced from active species in presence of air


Linear Sweep Voltammetry (LSV)

Composition:0.01M V(III) (acac)30.05M TEABF4in CH3CN

• Quasi‐reversible Model

– Butler‐Volmer1

– Small reductant concentration

– Microelectrode (Steady State)

– io (Exchange current density) and φ (Standard Potential) are fit parameters

ff

cLo

eieii

ii )1(

,

11

• Current normalized by limiting current

• Diffusion Coefficient1

D = 1.8 x 10‐5 ± 3.5 x 10‐6 cm2/s

(1) Bard and Faulkner. Electrochemical Methods. 2001


Linear Sweep Voltammetry:V(III) / V(IV) Redox Couple

Carbon Gold

io= 170 A/m2io= 3 A/m2

Scan rate: 1 mV/s Scan rate: 0.5 mV/s


Linear Sweep Voltammetry:V(III) / V(IV) Redox Couple

Platinum All

io= 90 A/m2

Scan rate: 0.5 mV/s


Progress: Membrane diagnostics• Implementation of proposed one‐dimensional test cell

Critical system variables: liquid solutions

membranes(or MEA)

electrodematerials

(or endcaps)


Progress: Membrane diagnostics• Charge/discharge with anion‐exchange membrane (Neosepta AHA) underway Au electrodes, flow‐by mode, 0.1 M V(acac)3 [vanadium‐actetylacetonate] and

0.5 M TEABF4/CH3CN [Tetra‐ethyl‐ammonium‐tetrafluoroborate / Acetonitrile]

• Charge current 0.4 mA, discharge –0.05 mA; Burn‐in complete after 3 cycles

• 85% Coulombic efficiency

40 60 80 100 120 140-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.0

0.5

1.0

1.5

2.0

2.5

Cur

rent

/mA

Time/hours

Vol

tage

/V

20 40 60 800.0

0.5

1.0

1.5

2.0

2.5

Volta

ge/V

Time/hours


Progress: Prototype development task

Redox Flow Test Cell – First Results

‐10 cm² active area‐ Graphite felt (COS1006)‐ Bipolar plate (Schunk GmbH, Germany)‐Microporous membrane (Scimat)‐ 0.1 M V(Acac)3‐ 0.05 M TEABF4‐ Acetonitrile

Rct = 1590

Rs = 5

C = 1.02 mF

Impedance spectroscopy


Progress: Prototype development task

Redox Flow Test Cell – Charge / Discharge

‐ 20 mA (2 mA/cm²) galvanostatic charge up to 2 V, 2.2 V, 2.4 V, 2.6 V‐ 5 min OCV‐Measurement‐ 5 mA (0.5 mA/cm²) galvanostatic discharge down to 0.3 V

0 1 2 3 4 5

0,0

0,5

1,0

1,5

2,0

2,5

Cur

rent

[A]

Voltage Current

Vol

tage

[V]

Time [h]

-0,02

-0,01

0,00

0,01

0,02

0,03

0 1 2 3 4 5

-0,01

0,00

0,01

0,02

0,03

0,04

0,05

0,06

Cha

rge

[Ah]

Pow

er [W

]

Time [h]

0

10

20

30

40

50

22 %64 %0.292.4

23 %66 %0.232.2

24 %80 %0.192

EECEPout [mW/cm²]

Voltage [V]


Progress: Scale‐upCell / Stack Design

cell data (for a liquid, aqueous system)

number of cells 2

membrane area 1600 cm²

Voltage (charge)

3,3 V (2 x 1.65 V)

Current

0 – 200 A

Currently testing

materials, sealings,

glue for

non‐aqueous‐system


Scale‐up and test benchDesign and erecting a first test facility as a mobile test bench

• 15 kW power

• Electrolyte tank:2 x 40 l 2 kWh

• Stack size up to1 x 0.8 x 0.3 m200 kg

• Charge0 – 40 V0 – 375 A

• Discharge< 40 V0 – 440 A

• Flow rate0.35 – 5 l/min