1

REDOX

Allen J. Bard, Netzahualcóyotl Arroyo-Currás (Netz Arroyo), Jinho Chang, Brent Bennett

Department of Chemistry and Center for Electrochemistry

The University of Texas at Austin

What is a redox flow battery?

• Stores energy in reduced and oxidized species in solution

• Solution is pumped through cell during charge/discharge

Figure taken from C. Ponce De Leon et al. / J. Power Sources 160 (2006) 716-732

Positive: An An+1 + e- Negative: Cn+1 + e- Cn

charge

discharge

Redox Flow Battery Schematic

Experimental Flow-Through Cell

Advantages of flow batteries

• High current and voltage efficiency

• High usage of active materials

• Stable voltage throughout charge and discharge

• Long useful life (> 20 years with maintenance)

• No phase change in electrodes

• Simple replacement of materials

• Modularity and scalability

• Energy capacity is a function of the size of tanks

• Power capacity is a function of the number of cells

Speakers

Redox Flow Batteries vs. Alternatives

RFB Primary Battery

Secondary Battery

Fuel Cell

Rechargeable ✔ ✖ ✔ ✖

Separate sizing of power and energy (capacity) ✔ ✖ ✖ ?

No phase change on cycling ✔ ✖ ✖ ✔

Simple outer sphere redox reactions ✔ ✖ ✖ ✖

High energy density ✖ ✔ ✔ ✔

Cost (inexpensive materials) Fe, Sn…

Stability (cycle life) Accelerated Testing

Corrosion resistance Electrolyte

Energy density n=2… Solubility

Solution stability Single solution RFB, separator, membrane

Speakers

Examples of flow battery chemistries • All-Vanadium

VO2+ + H2O VO2+ + 2H+ + e- E0 = +1.0 V vs. SHE

V3+ + e- V2+ E0 = -0.26 V vs. SHE

• Tin-Bromine

2Br- Br2 + 2e- E0 = +1.09 V vs. SHE

Sn4+ + 2e- Sn2+ E0 = +0.15 V vs. SHE

• Alkaline Iron-Cobalt (TEA)

Fe2+ Fe3+ + e- E0 = -0.76 V vs. SHE

Co3+ + e- Co2+ E0 = +0.25 V vs. SHE

• Nitrobenzene-Bromine

2Br3- 3Br2 + 2e- E0 = +1.10 V vs. SHE

NB + e- NB- E0 = -0.90 V vs. SHE

General Challenges and Motivation

• Cost is the primary driving factor

• High capital costs (> $300/kWh or > $1000/kW)

• Long payout (10+ years)

• Polymer membranes for separators ($1000/m2)

• Pumps and other system equipment

• Focus of research efforts

• Cheaper materials

• Higher energy density (most flow batteries < 100 Wh/L)

• Better energy efficiency (most flow batteries ~ 80%)

Co/Fe: The Alkaline Redox Flow Battery Motivation and Goals

[1] Kim, S. et al., Hickner, M. A.; Electrochem. Commun., 2010, 12, 1650-1653.

State-of-the-art RFBs suffer from capacity fading due to species crossover. Our

goal was to develop a low-cost, gas-free, crossover-free technology in strongly

alkaline electrolyte.

Complexes of Fe and Co as Redox Species

(L) is an organic ligand. The maximum solubility of the Co/Fe system is ≈ 0.45 M.

Co/Fe: The Alkaline Redox Flow Battery

Example: Half-Cells and Net Cell Reaction

[Fe(TEA)(OH)]- + e- [Fe(TEA)(OH)]2- E = -1.05V

[Co(mTEA)(H2O)]+ e- [Co(mTEA)(H2O)]- E = -0.04V

Ecell »1.00V

Discharge:

C / 0.9 M [Fe(TEA)(OH)]2-, 4 M OH- // 4 M OH-, 0.45 M [Co(TEA)(OH)] / C

mTEA = TEA =

Co/Fe: The Alkaline Redox Flow Battery

Cell Performance

Co/Fe: The Alkaline Redox Flow Battery

Co/Fe: The Alkaline Redox Flow Battery

Sn/Br2 Redox Flow Battery

Advantages 1. No cross contamination problem 2. High capacity per unit concentration (2e- electrons transfer)

Cell configuration C / HBr (2 M), NaBr (4 M) // HBr (2 M), NaBr (4 M), Sn4+ / C

Separator

Half-cell charge/discharge reactions

Positive electrode: Br2 + 2e- Br-

Negative electrode: SnBr6

2- + 2e- SnBr42-

Discharge

Charge

Discharge

Charge

Sn(IV) + 2e- Sn(II)

Sn(II) Sn(IV) + 2e-

Understanding the mechanism should offer guidance for solving the large irreversibility.

Large irreversibility

Potential loss during charge/discharge

Voltage efficiency

Operating cost

Limitation of Sn(IV)/Sn(II) redox reaction

Scanning electrochemical microscopy (SECM) -Mechanistic study of Sn(IV)/Sn(II) reduction-

Sn(IV)/Sn(II) redox reaction Sn(IV)/Sn(III) redox reaction

The short-lived Sn(III) intermediate, Sn(III)Br63-, was detected at small d

Chang, J.; Bard, A. J., submitted.

Au

Au

Sn(IV)Br62-

+2e-

Sn(II)Br42- Sn(IV)Br6

2-

-2e-

Sn(II)Br42-

Au

Au

Sn(IV)Br62- Sn(III)Br6

3- +e-

Sn(III)Br63- Sn(IV)Br4

2-

-e-

à Sn(III)Br52-

-Br-

Redox active liquids for low-cost, high-energy-density flow batteries

• A bromine (Br2) / nitrobenzene (NB) flow battery could achieve energy densities comparable to Li-ion batteries.

ΔE = 2.0 V

Br2/Nitrobenzene Redox Flow Battery

Negative: NB + e- NB- Positive: 2Br3

- 3Br2 + 2e-

charge

discharge

Advantages of Br2/NB RFB

• High energy density lower cost, smaller footprint

• Vanadium RFB: 25-35 Wh/L

• RFB with redox liquids (theoretical): ~ 200 Wh/L

• Simple chemistry new cell designs

• Low-cost membrane or membrane-free

se

pa

ra

to

r

grap

hit

e fl

ow

fie

ld grap

hite

flow

field

cathode anode

Li+

Li+

Li+

Li+

Li+

TBA+

NB

NB-

BPh4-

BPh4-

BPh4-

BPh4-

Br3-

Li+

BPh4-

BPh4-

Br2

Solvent: nitrobenzene Anode and cathode: porous carbon Separator: low-cost polymer

Research Challenges

1

2

Reaction 1: 3Br2 + 2e- 2Br3-

Reaction 2: Br3- + 2e- 3Br-

Can we uncover the reaction mechanisms and make the reactions more reversible?

• Low solution conductivity low power density

• We can have lower $/kWh, but can we achieve lower $/kW?

• Energy density is limited by solubility of electrolyte salt

• Br- / Br2 reaction is not reversible in nonaqueous solvents

Acknowledgment

We are grateful for support of this research from the Global Climate and Energy Project, administered by Stanford University, under subaward 27777240-51978A.

Netz Brent Jinho