Progress & Challengesin the Development of

Flow Battery Technology

Frank WalshElectrochemical Engineering Laboratory

Energy Technology Research GroupUniversity of Southampton, UK

F.C.Walsh@soton.ac.uk

Invited Paper for 1st IFBF, 11.10 -11.40, 15 June 2010, Vienna

Contents• Principles of (and case for) RFBs and FBs• Examples of cells

– Polysulfide-bromine RFB (historical)– All vanadium RFB– Zinc-cerium RFB/FB– Soluble lead-acid FB

• Characterisation of their performance• Summary• Challenges & further work

Energy Storage TechnologiesDischarge Time vs. Power Profiles

Flow batteries could cover a wide “sweet spot” -providing a high storage capacity for <20 kW to 3 MW+applications

Source: ESA - ElectricityStorageAssociation

Principle of Redox Flow Batteries:Divided Unit Cell (<100 cm2 in lab.)

Positiveelectrolyte

tank

Ion exchangemembrane

Pump Pump

Negativeelectrolyte

tank

Positiveelectrode

Negativeelectrode

Redox Flow BatteriesBipolar Stack (<200 electrodes of <1 m2)

Positive electrolyteinlet

Endelectrode

Bipolarelectrode

Ion exchangemembrane

End plateelectrode

Bipolarelectrode

Ionexchangemembrane

+-

Negative electrolyteinlet

Electrolyte outlet

+ + + +_ _ __

A Classification (FCW) of Flow BatteriesAccording to number of solid phases & any membrane

ClassicalRedox flow battery (RFB)

Divided

½ RFB and ½ metalHybrid flow battery (HFB)

Divided

Metal−metal oxideFlow battery (FB)

Undividede.g. Vanadium species Zinc−cerium Soluble lead acid

V5+

V4+

V2+

V3+

Ce4+

Ce3+ Zn2+

Zn

Pb2+ Pb2+

PbPbO2

membrane membrane

−+ − + − +0S.1M 1S.1M 2S.0M

H+ H+

Strategies for ChoosingRedox Flow Cell Electrochemistry

• Look at the electrochemical series.• Find a pair of redox couples with a high cell voltage.• One couple can be highly oxidised.• The other couple can be highly reduced.• But both redox couples must be sustainable:

– stable themselves and, preferably, in combination– kinetically reversible at practical electrodes– reasonable in cost, easily sourced, transported, stored…

Selected standard electrode potentials (vs. SHE at 298 K)(An electrochemical series of redox couples in equlibrium)• Pb2+ + 2H2O + 2e- = PbO2 + 4H+ 1.46 V• Ce4+ + e- = Ce3+ 1.44 V

0.5O2 + 2H+ + 2e- = H2O 1.23 V• Br3

- + 2e- = 3Br- 1.09 V• VO2

+ + 2H+ + e- = VO2+ + H2O 1.00 V2H+ + 2e- = H2 0 V

• Pb2+ + 2e- = Pb -0.14 V• V3+ + e- = V2+ -0.26 V• S4

2- + 2e- = 2S22- ca. -0.50 V

• Zn2+ + 2e- = Zn -0.76 V _

+

Redox Flow Batteries: Cell Reactions

Examples of cell reactions - forward process on charge:• Bromide-polysulfide (Regenesys)

S42- + 3Br- = 2S2

2- + Br3-

• All vanadium (UNSW, VRB, Re-Fuel, Cellstrom, Cellenium, …)V3+ + VO2+ + H2O = V2+ + VO2

+ + 2H+

• Soluble lead-acid flow (Uni. Southampton, C-Tech & E-on)2Pb2+ + 2H2O = Pb + PbO2 + 4H+

• Zinc-air (Uni. Southampton, Many others) Zn2+ + H2O = Zn + 0.5O2 + 2H+

• Zinc-cerium (Plurion, Uni Soton, Uni Strathclyde)Zn2+ + 2Ce3+ = Zn + 2Ce4+

- Power capability depends on cell size, voltage and current density.- Energy storage capability depends on electrolyte tank capacityand concentrations of reactants.

Regenesys® Cell Stacks & Chemistry

10 μm

1 m1 m

+−−−−+

−−−

−−−

++=++

=−

=+

NaBrSBrSNaCell

BreBrelectrodePositive

SeSelectrodeNegative

52235:

23:

22:

32

22

4

3

22

4

C-PE composite

<20 x 0.1 m2

<60 x 0.2 m2

<200 x 0.7 m2

<100 kW<1.4 kW<8.5 kW

Regenesys technology was acquired in 2004 by VRB;VRB technology was acquired by Prudent in 2009.

Note: commodity chemicals

Regenesys XL10 Pilot Module

Flow dispersion, pressure dropand mass transport studies

ST XL10

Time, t / s0 50 100 150 200

Con

cent

ratio

n of

KC

l, / M

0.0

2.0e-6

4.0e-6

6.0e-6

8.0e-6

1.0e-5

1.2e-5

1.4e-51.0 cm s-1 2.1 cm s-1 3.1 cm s-1 4.2 cm s-1 5.2 cm s-1 6.2 cm s-1

Effect of Velocity on Flow Dispersion

Achieving High Surface Area Electrodes

• Porous, 3-dimensional materials, e.g., C– RVC, felt, paper, activated particles, microfibres

• Nanostuctured materials, e.g., TiO2 and titanates– Spheroidal, belt, fibre or tube

• Deposit or coating on the substrate– Random or ordered (templated?)

50 nm

Zn-Ce Cell in the Laboratory• Negative: Carbon-polymer composite

• Positive: Pt/Ti mesh

• Temperature: 50 oC

• Positive electrolyte: 0.8 M Ce(CH3SO3)3 + 4 M CH3SO3H

• Negative electrolyte: 1.5 M Zn(CH3SO3)2 + 1 M CH3SO3H

• Electrolyte velocity < 4 cm s-1Reference cells Flow

battery

Zn-Ce Battery. Initial Discharge Voltage vs.Current Density: Effect of electrode material

Applied current density / mA cm-2

0 10 20 30 40 50 60

Initi

al c

ell v

olta

ge /

V

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Pt-TiGraphiteCarbon polymer30 ppi RVC100 ppi RVCAlfa Aesar carbon feltSGL carbon feltPt-Ti mesh stack

• Constant current density.

• Different positive electrode materials.

• 4 hours of charge at 50 mA cm-2 .

• Temperature: 50 oC

• Positive electrolyte: 0.8 M Ce(CH3SO3)3 + 4 M CH3SO3H.

• Negative electrolyte: 1.5 M Zn(CH3SO3)2 + 1 M CH3SO3H.

• Carbon-polymer –ve composite electrodes.

• Electrolyte velocity of ca. 4 cm s-1.

Manufacture of V(II) to V(V)

V(V) V(IV) V(III) V(II)

Unit VRFB Flow Cell System Unit VRFB Flow Cell System (Lab.(Lab.))

RFBReferencecell

250 mLtank

D.c.powersupply

Variableload Clamp

meter

Electricalpump

Serialcard

Positiveelectrolyteoutlet

Graphite plate

Copperplate

PTFEgasket

Positiveelectrolyteinlet

Negativeelectrolyteoutlet

M5 stainlesssteel tie-bolts

1 cm

Nafion 115 cation membrane

Unit VRFB Flow Cell System Unit VRFB Flow Cell System (Lab.(Lab.))Unit cell (10 cm x 10 cm)

Individual fibres, 3-D electrode

Graphite felt, 3-D electrode

Vanadium RFB Lab Cell (100 cm2)Charge-Discharge Behaviour, 10 A

19

Time

Laboratory Pilot VanadiumStack and Frame (Re-Fuel)

• 40 cells, each 50 x 25 cm = 5 m2

• 25-35 oC• 37.5 L in each reservoir• 1.6M V in 4M H2SO4• Nafion 112 cation exchange membrane• Cell voltage 1.3-1.5 V (52-60 V per stack)• Max. current density 100 mA cm-2

• 3-5 kW nominal power Re-Fuel Technology Ltd, Wokingham, UKpart of Camco International Ltd

Peter RidleyGary SimmonsJohn Samuels

Charge-Discharge Curves Charge-Discharge Curves forforVRFB Laboratory Pilot Stack VRFB Laboratory Pilot Stack (Re-Fuel)(Re-Fuel)

I / A

Time / hour0 1 3 4 6 7 8

Cur

rent

/ A

-120

-80

-40

0

40

80

Sta

ck v

olat

age

/ V

0

20

40

60

Cha

rge

Dis

char

ge

Charge efficiency54% 75% 79% 76% 68%

Estack

I

DTi project with Re-Fuel and Scottish Power

Modelling All-Vanadium RFBs

• Main redox reactions

• Temperature variations

• H2/O2 evolution with bubble formation

• Reservoirs

Modelling of an All Vanadium RFB:Comparison with experimental data

2-D dynamic performance model (15 parameters)

• Charge transfer

• Mass transport

• Momentum conservation

• Kinetic model

• Porosity of electrodes

• Electrolyte transport

• Membrane characteristics

• Known vanadium reactions

• Major side reactions

60 mA cm-2

297 K4M H2SO4

A.A. Shah & F.C. Walsh, Electrochim. Acta, 53 (2008) 8087-8100.

Positive electrode Pb2+ + 2H2O - 2e- PbO2 + 4H+

Negative electrodePb2+ + 2e- Pb

Overall cell reaction2Pb2+ + 2H2O - 2e- Pb + PbO2 + 4H+

charge

dischargecharge

discharge

Soluble Lead Flow Battery: Principles

• Porous carbon or nickel electrodes• Methanesulfonic acid electrolyte• 1 x 2 cm2 to 6 x 100 cm2 to 10 x 1,200 cm2

• NO MEMBRANE• 1.5M Pb(CH3SO3)2 + 0.9M CH3SO3H• < 94% charge efficiency• < 80% voltage efficiency

charge

dischargeMSA

100 cm2 Soluble Lead Flow Battery(Uni Soton, C-Tech Innovation & E-on)

10 cm

Derek Pletcher

Duncan Stratton-CampbellJohn Collins

Soluble Lead-Acid Battery Teamat the University of SouthamptonRichard Wills John Low

Gareth Kear Ravi Tangirala

& Derek Pletcher

Charge-Discharge of a Soluble Lead Flow BatteryNi –ve;C-polymer +ve 20 mA cm-2; 1 h; 1.5 L; 23oC.

0.5 M Pb(CH3SO3)2 + 0.05 M CH3SO3H + 5 mM C16H33(CH3)3N+.

Voltage efficiency

Cycle number0 5 10 15 20

Effic

ienc

y %

50

60

70

80

90

100

10 mA cm-2

20 mA cm-2

30 mA cm-2

Depth of Discharge100 % 50 % 25 %

At low DOD:- Less build-up of Pb & PbO2- Less shedding of PbO2

-ve

+ ve

Charge-Discharge Simulations• 1.5 L electrolyte• 10 cm × 10 cm active area• Flow rate 2.3 cm s-1

• 1.2 cm inter-electrode gap• 27 °C

Dominantcomplex

oxide reactionduring 2nd

charge

Cu-PbO2 Flow BatteryCyclic voltammetry and charge-discharge data

Potential, E vs. SCE / V-1.0 -0.5 0.0 0.5 1.0 1.5 2.0

Cur

rent

, I /

mA

-0.6-0.4-0.20.00.20.40.60.81.01.2

Cur

rent

den

sity

, j /

mA

cm

-2

-0.45-0.30-0.150.000.150.300.450.600.750.90

Cu Cu2+

Pb Pb2+

PbO2 Pb2+

Cu2+ Cu Pb2+ Pb

Pb2+ PbO2

Cel

l vol

tage

, Ece

ll / V

Time, t / h0 8 16 24 32 40 48 56 64

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2Charge

Discharge

0.5 mol dm-3 Cu2+ and 0.5 mol dm-3 Pb2+

1 mol dm-3 MSA 0.5 hour charge discharge at 20 mA cm-2

0.5 V via -0.9 V to +1.9 V vs. SCE20 mmol dm-3 Cu2+; 20 mmol dm-3 Pb2+

1 mol dm-3 MSAGlassy C disc.

Variants of the Soluble Lead Acid FBCopper-Lead Flow Batteries

Cu deposit PbO2 deposit100 micron

charge2

dishchargeNegative: ( ) 2 ( ) 0.34VCu aq e Cu s E+ − ⎯⎯⎯⎯→+ =+←⎯⎯⎯⎯ o

charge22 2dishcharge

Positive : ( ) 2 2 ( ) 4 1.7VPb aq H O e Pb O s H E+ − +⎯⎯⎯⎯→+ − + = +←⎯⎯⎯⎯ o

5 kW h Pilot Cell

• Entegris carbon-polymer composite electrodes.• Ni coated for Pb (-ve) electrode• Flow distribution design from E.on / C-Tech CFD & flow visualisation.• 400 x 250 mm active electrode area.• 10 Frames – initial commissioning with 4 frames.• Electrolyte volume 50 - 100 litres. Operating between 1 & 0.3 M Pb2+

• 5.7 kW h charge capacity• 50 mA cm-2 for 450 min.

Pilot Soluble Lead-Acid Rig(C-Tech Innovations)

Soluble Lead-Acid Pilot Cell:Flow Visualisation

Velocity Contours on Plane near Cell Mid-height

Higher speedjetting flowsshown by redregions

After J. Fackrell E.ON

CFD Analysis of Pilot Cell (ANSYS)

Needs for a Flow Battery - 1• High cell voltage:

– Large difference between formal potentials of +ve and –veelectrode reactions

– Low overpotentials at both electrodes– High solution conductivity

• High cell current:– High current density and electrode area– All reactants highly soluble to avoid mass transport limitations

• High energy efficiency:– Low overpotentials and low ohmic drops– High charge efficiency

Needs for a Flow Battery - 2• High cycle life:

– No change in battery (electrodes or electrolyte) during cycling.• A complete charge/discharge cycle:

– 100% efficiency for both electrode chemistries.– All oxidation states must be completely stable.– No losses of species through the membrane/separator.– No electrode corrosion or membrane damage.– No accumulation of impurities in the electrolyte.

• Practicality:– Low cost and wide availability; safe and non-toxic materials.

• High energy storage capability per litre of electrolyte:– All reactants must be highly soluble.

Summary• Redox flow cells are progressively developing

– Their scientific history is 35+ years long!• Many types have been the subject of lab. R & D

– The scientific literature and the web can be confusing.• Few types have survived commercial scale-up

– Performance, expense, longevity and user-friendliness are issues.• The most developed types include:

– All-vanadium, polysulfide-bromine, zinc-bromine.• Academic progress includes:

– V-V has been modelled; Zn-Ce and Zn-air are underdevelopment

– V-Br and V-air demonstrated, soluble Pb-acid has scaled-up.– Cu-Pb, Zn-Pb... considered.

Challenges• Large scale RFB installations to provide increased confidence

– In flow battery technology for competitive energy storage.• Improved stack and cell design

– Simpler, undivided, more production oriented, modular, etc.• Better mathematical models and simulations

– Simpler, multi-physics, multi-scale, effect of gases, dynamic...• Higher performance, yet practical, electrodes

– Nanostructured, layered, 3-dimensional, non-coated....• Specialised miniature RFB systems

– Ionic liquids, organic redox couples, biochemical, biological...

Cell Design and Complexity• 2-D or 3-D electrodes?• Uncoated electrodes or coated electrodes?• Commodity electrolytes or specialised chemical ones?• Simple electrolytes or complex (e.g. 2-phase) ones?• Aqueous electrolyte or non-aqueous (ionic liquid)?• Single phase or two-phase electrolyte operation?• Undivided or divided cell?• Microporous polymer or ion-exchange membrane?• Bipolar or monopolar electrode connections?• Internal or external manifolds?

Acknowledgements:Funding and Industrial Partners

• Soluble Lead-Acid FB– UK TSB– John Bateman & John Fackrell of E-on– John Collins & Duncan Stratton-Campbell of C-Tech Innovations

• All Vanadium RFB– UK Dti & UKTi– Peter Ridley, Gary Simmons & John Samuels of Re-Fuel Ltd– Scottish Power

• Zn-Ce– Research Institute for Industry & University of Southampton

Acknowledgements:Academic Colleagues and Research

Workers at Southampton• Prof Derek Pletcher & Carlos Ponce de León• Dr Akeel Shah, Dr Gareth Kear & Osman Mohamed• Dr Richard Wills & Dr Matt Watt-Smith• Dr Hantou Zhou & Dr Xiaohong Li• Dr John Low & Jacky Leung• Ravi Tangirala, Hasan Al-Fetlawi & Caiping Zhang• Dr Suleiman Sharkh & Rusllim Mohammed