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 1
What is a redox flow battery? Stores energy in reduced and oxidized species in solution Solution is pumped through cell during charge/discharge Positive: A n A n+1 + e - Negative: C n+1 + e - C n charge discharge Figure taken from C. Ponce De Leon et al. / J. Power Sources 160 (2006) 716-732
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) Stability (cycle life) Corrosion resistance Energy density Solution stability Fe, Sn Accelerated Testing Electrolyte n=2 Solubility Single solution RFB, separator, membrane
Examples of flow battery chemistries All-Vanadium VO 2+ + H 2 O VO 2 + + 2H + + e - E 0 = +1.0 V vs. SHE V 3+ + e - V 2+ E 0 = -0.26 V vs. SHE Tin-Bromine 2Br - Br 2 + 2e - E 0 = +1.09 V vs. SHE Sn 4+ + 2e - Sn 2+ E 0 = +0.15 V vs. SHE Alkaline Iron-Cobalt (TEA) Fe 2+ Fe 3+ + e - E 0 = -0.76 V vs. SHE Co 3+ + e - Co 2+ E 0 = +0.25 V vs. SHE Nitrobenzene-Bromine 2Br - 3 3Br 2 + 2e - E 0 = +1.10 V vs. SHE NB + e - NB - E 0 = -0.90 V vs. SHE Speakers
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/m 2 ) 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 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. [1] Kim, S. et al., Hickner, M. A.; Electrochem. Commun., 2010, 12, 1650-1653.
Co/Fe: The Alkaline Redox Flow Battery 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)(H 2 O)]+ e - [Co(mTEA)(H 2 O)] - E = -0.04 V Discharge: C / 0.9 M [Fe(TEA)(OH)] 2-, 4 M OH - // 4 M OH -, 0.45 M [Co(TEA)(OH)] / C E cell»1.00 V mtea = TEA =
Co/Fe: The Alkaline Redox Flow Battery Cell Performance
Co/Fe: The Alkaline Redox Flow Battery
Sn/Br 2 Redox Flow Battery Cell configuration Separator C / HBr (2 M), NaBr (4 M) // HBr (2 M), NaBr (4 M), Sn 4+ / C Half-cell charge/discharge reactions Discharge Positive electrode: Br 2 + 2e - Br - Charge Negative electrode: SnBr 2-6 + 2e - Charge SnBr 2-4 Discharge Advantages 1. No cross contamination problem 2. High capacity per unit concentration (2e - electrons transfer)
Limitation of Sn(IV)/Sn(II) redox reaction Large irreversibility Sn(II) Sn(IV) + 2e - Sn(IV) + 2e - Sn(II) Potential loss during charge/discharge Voltage efficiency Operating cost Understanding the mechanism should offer guidance for solving the large irreversibility.
Scanning electrochemical microscopy (SECM) -Mechanistic study of Sn(IV)/Sn(II) reduction- Au Au +2e - Sn(IV)Br 2-6 Sn(II)Br 4 2- +e - -Br - Sn(IV)Br 2-6 Sn(III)Br 3-6 à Sn(III)Br 2-5 Sn(IV)Br 6 2- -2e - Au Sn(II)Br 4 2- Sn(IV)/Sn(II) redox reaction Sn(IV)Br 2- Sn(III)Br 3-4 6 -e - Au Sn(IV)/Sn(III) redox reaction The short-lived Sn(III) intermediate, Sn(III)Br 6 3-, was detected at small d Chang, J.; Bard, A. J., submitted.
Br 2 /Nitrobenzene Redox Flow Battery Redox active liquids for low-cost, high-energy-density flow batteries A bromine (Br 2 ) / nitrobenzene (NB) flow battery could achieve energy densities comparable to Li-ion batteries. Negative: NB + e - NB - Positive: 2Br 3-3Br 2 + 2e - charge discharge ΔE = 2.0 V
graphite flow field Advantages of Br 2 /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 cathode Li + BPh - 4 TBA + Li + BPh - Br 4 2 Br 3 - BPh - 4 Li + s e p a r a t o r anode Li + NB BPh - 4 NB - BPh - 4 Li + Li + BPh - 4 graphite flow field Solvent: nitrobenzene Anode and cathode: porous carbon Separator: low-cost polymer
Research Challenges 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 - / Br 2 reaction is not reversible in nonaqueous solvents Reaction 1: 3Br 2 + 2e - 2Br 3-2 Reaction 2: Br 3 - + 2e - 3Br - 1 Can we uncover the reaction mechanisms and make the reactions more reversible?
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 Jinho Brent