REDOX Allen J. Bard, Netzahualcóyotl Arroyo-Currás (Netz Arroyo), Jinho Chang, Brent Bennett Department of Chemistry and Center for Electrochemistry 1 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 Positive: An An+1 + e- Negative: Cn+1 + e- Cn 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 Primary Secondary Fuel RFB Battery Battery 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 Examples of flow battery chemistries • All-Vanadium 2+ + + - 0 VO + H2O VO2 + 2H + e E = +1.0 V vs. SHE V3+ + e- V2+ E0 = -0.26 V vs. SHE • Tin-Bromine - - 0 2Br Br2 + 2e E = +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 - - 0 2Br3 3Br2 + 2e E = +1.10 V vs. SHE NB + e- NB- E0 = -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/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 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)(H2O)]+ e [Co(mTEA)(H2O)] 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 Ecell »1.00 V mTEA = TEA = Co/Fe: The Alkaline Redox Flow Battery Cell Performance Co/Fe: The Alkaline Redox Flow Battery Sn/Br2 Redox Flow Battery Cell configuration Separator C / HBr (2 M), NaBr (4 M) // HBr (2 M), NaBr (4 M), Sn4+ / C Half-cell charge/discharge reactions Discharge Positive electrode: Br + 2e- Br- 2 Charge 2- - Charge 2- Negative electrode: SnBr6 + 2e SnBr4 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 Potential loss during Sn(II) Sn(IV) + 2e- charge/discharge Sn(IV) + 2e- Sn(II) 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- +e- -Br- 2- 2- 2- 3- 2- Sn(IV)Br6 Sn(II)Br4 Sn(IV)Br6 Sn(III)Br6 à Sn(III)Br5 2- 2- 2- 3- Sn(IV)Br6 Sn(II)Br4 Sn(IV)Br4 Sn(III)Br6 -2e- -e- Au Au Sn(IV)/Sn(II) redox reaction Sn(IV)/Sn(III) redox reaction 3- The short-lived Sn(III) intermediate, Sn(III)Br6 , was detected at small d Chang, J.; Bard, A. J., submitted. Br2/Nitrobenzene Redox Flow Battery 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. Negative: NB + e- NB- - - Positive: 2Br3 3Br2 + 2e charge discharge ΔE = 2.0 V 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 cathode s e p a r a t o r anode Solvent: flow field flow - nitrobenzene - NB graphite + BPh4 BPh4 + Li Li - BPh4 Anode and cathode: + + - Li - TBA Br3 NB porous carbon raphite raphite + - + g BPh4 Li Separator: flow field flow Li - Br BPh4 2 + - low-cost polymer Li BPh4 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 / Br2 reaction is not reversible in nonaqueous solvents - - Reaction 1: 3Br2 + 2e 2Br3 Reaction 2: Br - + 2e- 3Br- 2 3 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 .