A New Vanadium Redox Flow Battery Using Mixed Acid Electrolytes Liyu Li, Soowhan Kim, Wei Wang, M. Vijayakumar, Zimin Nie, Baowei Chen, Jianlu Zhang, Jianzhi Hu, Gordon Graff, Jun Liu, Gary Yang* Funded by the Energy Storage Systems Program of the U.S. Department Of Energy through Pacific Northwest National Laboratories November 2, 2010 US DOE Energy Storage Systems (ESS) Program Review Washington DC Redox Flow Battery (RFB) A redox flow battery is a promising technology for large scale energy storage . Potential RFB Systems 3+ 2+ + 2+ V /V VO2 /VO H2 evolution All V Redox Flow Battery O2 evolution - Br2/Br 2+ 3+ 3+ 2+ Zn2+/Zn VO /V Mn /Mn 2+ + - 3+ 2+ Cu /Cu MnO4 /MnO2 Cr /Cr Cl2/Cl - - - 4+ 3+ BrCl2 /Br Ce /Ce 3+ 2+ Ti /Ti 2- S/S Cr5+/Cr4+ TiOH3+/Ti3+ Fe3+/Fe2+ Co3+/Co2+ -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 Standard potential (V) of redox couples The use of vanadium in both the anolyte and catholyte effectively eliminate the cross-contamination between the electrolytes through the ion-exchange membrane. Challenges for All Vanadium Sulfate RFB n+ + 2- o V specie V , M H , M SO4 , M Temp, C Time for p.p. 2 6 5 -5 419 hr V2+ 2 6 5 25 Stable (>30 d) 2 6 5 40 Stable (>30 d) 2 4 5 -5 634 hr V3+ 2 4 5 25 Stable (>30 d) 2 4 5 40 Stable (>30 d) 2 6 5 -5 18 hr V4+ (VO2+) 2 6 5 25 95 hr 2 6 5 40 Stable (>30 d) 2 8 5 -5 Stable (>30 d) 5+ + 2 8 5 25 Stable (>30 d) V (VO2 ) 2.2 7.8 5 40 95 hr 1.8 8.4 5 40 358 hr Low energy density: Vn+ concentration <1.7M, decided by the low solubility of V4+ at low temperatures and the poor stability of V5+ at high temperatures. Limited operation temperature window: 10 to 40oC, requiring active electrolytes temperature management during hot/cold weathers. Stability of Vn+ Cations in HCl Solution Vn+ specie Vn+, M H+, M Cl-, M T, oC Time for precipitation V2+ 2.3 5.4 10 -5 Stable (>10 d) 2.3 5.4 10 25 Stable (>10 d) 2.3 5.4 10 40 Stable (>10 d) V3+ 1.5 3.0 7.5 -5 Stable (>10 d) 1.8 3.0 8.4 -5 124 hr 2.3 3.1 10 -5 96 hr 2.3 3.1 10 25 Stable (>10 d) 2.3 3.1 10 40 Stable (>10 d) V4+ (VO2+) 2.3 5.4 10 -5 Stable (>10 d) 2.3 5.4 10 25 Stable (>10 d) 2.3 5.4 10 40 Stable (>10 d) 5+ + V (VO2 ) 2.3 7.7 10 -5 Stable (>10 d) 2.3 7.7 10 25 Stable (>10 d) 2.3 7.7 10 40 Stable (>10 d) Cl- anions can effectively stabilize V5+, V4+, and V2+. It is likely that high concentration of V2+, V3+, V4+, and V5+ cations can be stabilized in a mixed sulfate and chloride electrolyte solution. n+ 2— - V Stability in SO4 Cl Mixed Solutions Vn+ Vn+, M T, oC Time for Vn+ Vn+, M T, oC Time for specie precipitation specie precipitation V2+ 3 -5 Stable (>10 d) V4+ 3 -5 Stable (>10 d) 2+ 2.5 -5 Stable (>10 d) (VO ) 2.5 -5 Stable (>10 d) 2.5 25 Stable (>10 d) 2.5 25 Stable (>10 d) 2.5 40 Stable (>10 d) 2.5 40 Stable (>10 d) 3 40 Stable (>10 d) 3 40 Stable (>10 d) V3+ 3 -5 192 hr (8 d) V5+ 3 -5 Stable (>10 d) + 2.5 -5 Stable (>10 d) (VO2 ) 2.5 -5 Stable (>10 d) 2.5 25 Stable (>10 d) 2.5 25 Stable (>10 d) 2.5 40 Stable (>10 d) 2.5 40 Stable (>10 d) 3 40 Stable (>10 d) 3 40 Stable (>10 d) 2.7 V5+ The overall stability of vanadium is 50 Stable (>10 d) 0.3 V4+ decided by V3+ at low temperatures. 2.7 V5+ 60 Stable (>10 d) 0.3 V4+ 2- - 5+ 4+ 3+ 2+ SO4 /Cl mixtures can effectively stabilize >2.5M V , V , V and V . Much broad operation temperature window (-5 to 60 oC) can be achieved using SO42-/Cl- mixed electrolytes. Solution Chemistry of the Mixed Electrolytes- DFT Study O Cl O O V V O O O O O O 5+ + In sulfate solution, V exists as [VO2(H2O)3] , which tends to convert to V2O5-3H2O precipitation via: + + [VO2(H2O)3] → VO(OH)3 + [H3O] 2VO(OH)3 → V2O5-3H2O ↓ Solution Chemistry of the Mixed Electrolytes- NMR Study 51V in mixed acid 51V in sulfuric acid 51 V in VOCl3 standard 35Cl in mixed acid 35 Cl in VOCl3 35Cl in HCl Temperature Dependence of VO2Cl(H2O)2 Formation -540 80 Chemical Shift-Sulfuric Acid Chemical Shift Mixed Acid 70 -545 Line Width Sulfuric Acid Line Width Mixed Acid 60 -550 50 -555 40 30 V Line Width (k Hz) 51 V Chemical Shift (ppm) -560 51 20 -565 10 -570 0 -30 -20 -10 0 10 20 30 40 50 60 Temperature, oC VO2Cl(H2O)2 complex starts to form in the mixed solutions when temperatures reaching ~20oC. Solution Chemistry of the Mixed Electrolytes- DFT and Stability Studies Same V2+, V3+ and V4+ -containing structures was predicted 2+ 3+ in both electrolyte solutions: [V(H2O)6] , [V(H2O)6] and 2+ [VO(H2O)5] . According to SEM-EDS, and XRD analysis, in the mixed systems, the stability of V4+ is controlled by the solubility of 3+ VOSO4, the stability of V is controlled by the solubility of 2+ V2(SO4)3 and VOCl, the stability of V is controlled by the solubility of VSO4. The improvement of stability of V2+, V3+, and V4+ in the mixed system over the current sulfate systems is due to the 2- decrease of SO4 concentration in the solutions. Electrochemical Properties of Mixed Solutions 0.3 V4+ V5+ 0.2 2+ 3+ ) V V 2 0.1 A/cm ( 0.0 -0.1 V4+ V5+ -0.2 4+ 2- - Current Density 2.5 M V + 2.5 M SO + 6 M Cl -0.3 4 4+ 2- 2+ 3+ 1.5 M V + 5 M SO V V 4 -0.4 -0.8 -0.4 0.0 0.4 0.8 1.2 1.6 Potential (V vs. SHE) No chlorine gas evolution. Electrode and Cell Reactions (T<10oC) 2+ Charge + + Cathode: VO + H2O – e Discharge VO2 + 2H E° = 1.00V 3+ Charge 2+ Anode: V + e Discharge V E°=-0.25V 2+ 3+ Charge + + 2+ Cell: VO + H2O + V Discharge VO2 + 2H + V E°=1.25V Electrode and Cell Reactions (T>10oC) 2+ - Charge + Cathode: VO + Cl + H2O – e Discharge VO2Cl + 2H 3+ Charge 2+ Anode: V + e Discharge V 2+ - 3+ Charge + 2+ Cell: VO + Cl + H2O + V Discharge VO2Cl + 2H + V Cell Testing and RFB Evaluation - Cell and reservoirs are inside the environmental chamber. - Circulating pump is out side of the chamber. - Pressure monitoring. Cell Performance at Ambient Temperature 100 95 1.8 1.6 Charging 90 1.4 Discharging 1.2 85 Voltage (V) 1.0 0.8 0.6 80 Coulomb 0 10 20 30 40 50 60 70 80 90 100 Efficiency (%) Energy Capacity / Charging Capacity (Ah%) Voltage 75 70 10 15 20 25 30 35 40 45 50 55 Cycle Number Stable performance with 88% energy efficiency at 50 mA.cm-2 Cell Performance at Varied Temperatures 100 95 Coulombic Efficiency Voltage Efficiency Energy Efficiency 90 85 Efficiency, % 80 40oC 50oC 5oC 0oC 75 70 0 10 20 30 40 50 60 70 80 90 Cycle number A VFB with 2.5 M V mixed acid electrolyte can be operated under a broad temperature range of 0 to 50 oC. Redox reactions are temperature dependent. No noticeable gas evolution over 25 days. Cell Performance at Varied Current Densities Energy Density Columbic Energy Discharge (Wh.L-1) Efficiency Efficiency Current, (mA.cm-2) Mixed sulfate Mixed sulfate Mixed sulfate 2.5MV 3MV 1.6MV 2.5MV 3MV 1.6MV 2.5MV 3MV 1.6MV 100 36.2 39.5 22.3 0.95 0.95 0.94 0.81 0.76 0.83 75 37.5 40.8 22.4 0.96 0.96 0.94 0.84 0.81 0.85 50 38.5 41.8 22.6 0.96 0.97 0.94 0.87 0.85 0.87 25 39.2 43.1 22.6 0.96 0.97 0.94 0.90 0.89 0.88 Cell operation conditions: 10 cm2 flow cell, Charged to 1.7V by 50 mA/cm2 current. The VRBs using mixed sulafte-chloride electrolytes were able to deliver 70 to 80% more energy than the sulfate system, while still being highly efficient. Summary A new vanadium redox flow battery with a significant improvement over the current technology was developed. This battery utilizes sulfate-chloride mixed electrolytes, which are capable of dissolving 2.5 M vanadium, representing about 70% increase in energy density over the current sulfate system. More importantly, the new electrolyte remains stable over a wide temperature range of -5 to 60oC, potentially eliminating the need of energy-consuming solution temperature management. Battery tests indicated no concern of chlorine gas evolution during the battery operation. Further Work Optimize the mixed electrolyte for further improvement in energy density and stability.