1. MOST SIGNIFICANT RESEARCH ACCOMPLISHMENTS IN POWER SOURCES

Transcription

1 1. MOST SIGNIFICANT RESEARCH ACCOMPLISHMENTS IN POWER SOURCES Currently, Dr. Popov is professor at USC and Director of the Center for Electrochemical Engineering at USC. His research group is comprised of two postdoctoral fellows, eight PhD students, and four BS students. His research interest in the area of power sources focuses on new materials for cathodes and anodes for primary and secondary batteries capacitors and fuel cells and development of performance models for capacitors and molten carbonate fuel cells and capacity fade mathematical models for lithium ion batteries. His research group is supported by numerous government agencies such as: Department of Energy, Office of Naval Research, US Army Corps of Engineers, Reconnaissance Office (NRO), NASA, Sandia National Laboratory, and the South Carolina Department of Transportation an private industries. In 2003, he was awarded with a major award from DOE to develop novel non precious metal catalyst for fuel cells. The goal of the proposed work is to construct and test membrane electrode assemblies (MEAs) with different non-precious metallic nanoclusters and to demonstrate the potential to perform at least as good as the conventional Pt catalysts currently in use in MEAs. 1. Battery Research Novel high performance anode materials were developed for Nickel-Metal Hydride (Ni-MH) batteries with high capacity, longer cycle life, low self discharge, uniform operation at high temperatures and corrosion resistance. From this research one step electroless process was discovered for deposition of Ni-P, Co-P and Ni-Co-P composites on MH particles. 1-4 This process controls the particle size of the alloy and forms protective coating on the surface. Using this process it is possible to decrease the particle size to a level at which the rate of pulverization dramatically decreases. Since the particles are encapsulated at their optimized particle size, the corrosion and consequently the capacity fade was drastically reduced. These studies revealed that Ni-Co-P composite alloys deposited by a one-step process offer the best possible means of optimizing the alloy particle size and in obtaining MH composites with high cycle life Theoretical models were developed for measuring diffusion coefficients and to explain the processes occurring at the hydride electrode/electrolyte interface. 2,8,11 Constant potential and constant current discharge were used to determine the hydrogen diffusion coefficients in an MH electrode. 8 Porous electrode theory was applied to estimate the exchange current density, the polarization resistance, and symmetry factor for MH electrode in alkaline solutions. The exchange current density, polarization resistance, and symmetry factor were determined from polarization curves, which were obtained at low overpotentials. 2 A theoretical model for the metal hydride electrode was developed assuming that hydrogen diffusion in the alloy and charge transfer at the surface control the discharge 1

2 process. Theoretical equations for the dependence of equilibrium potential and exchange current density on the surface hydrogen concentration have been derived. 11 For DOE Office of Basic Energy Science and Sandia National Laboratories, SNL a novel materials for Li-ion battery were developed that utilize a composite anode based on carbon encapsulated with Ni composite and cathodes based on Li x CrOy, or Li x Co 1- xcro y A wide range of Co doped LiMn 2 O 4 spinels were synthesized and electrochemically characterized. These Co-doped spinels showed improved specific capacity, superior charge transfer and capacity retention over pure spinels. 27 Chromium oxides and lithiated chromium oxides were synthesized by thermal decomposition of chromium trioxide at high temperatures and oxygen pressures. 21 A novel approach for suppressing the solvated lithium intercalation in graphite was developed by microencapsulation graphite with nanosized Ni-composite particles. 71 A Pd encapsulated graphite electrode was developed as the negative electrode in Li-ion cells. Through dispersion of ultra fine nanoparticles of palladium on the surface of graphite, the interfacial properties of the carbon surface were modified. The presence of Pd dramatically reduces the initial irreversible capacity. 24 A complex mathematical model for spherical particles was used to determine the lithium ion diffusion coefficient in graphite as a function of the state of charge. 23 A mathematical model was developed for the lithium intercalation in a single spinel particle as a microelectrode. 26 A simple theoretical model was developed to simulate the galvanostatic discharge behavior of the Ni-composite graphite electrode. 29 For the past tree years, Dr. Popov research group was involved in elucidating the mechanisms behind the capacity loss of these batteries during cycling To achieve this goal, we have determined changes in the structure/reactivity relationships among the various battery components after cycling. For the first time we have shown that the failure of these batteries arises due to lost of the active material and increase in resistance at both electrodes. Extensive electrochemical material characterization studies were carried out on both the anode and cathode. These studies indicate that capacity fade Li-ion cells can be attributed to: (i) structural degradation at the cathode and the anode (ii) loss of active materials at both electrodes due to electrolyte oxidation. 30,31 At present our research efforts are focused on optimizing the charging protocol of Li-ion cells under different operating conditions. The capacity fade of the lithium ion cells was found to increase with increase in temperature. Capacity fade in Sony Li-ion cells is attributed to oxidation of cathode (LiCoO 2 ) during overcharge. Both primary (Li + ) and secondary active material (LiCoO 2 ) are lost during charging. The higher capacity fade for the cell cycled at 50 o C and 55 o C is due to repeated film formation over the surface of anode, which results in increased rate of lithium loss A semi-empirical and first principles-based model has been developed to simulate the capacity fade of Li-ion batteries. Incorporation of a continuous occurrence of the solvent reduction during constant current and constant voltage (CC-CV) charging explains the capacity fade of the battery. The effects of parameters such as end of charge voltage and depth of discharge, the film resistance, the exchange current density and the over 2

3 voltage of the parasitic reaction on the capacity fade and battery performance were estimated quantitatively. 32,34,36,37 2. Capacitors Current commercial capacitors do not satisfy the twin requirements of both high power and low cost. The later factor is critical in successful commercialization of an electric vehicle. Research done in my laboratories has led to a unique and economical synthesis route that leads to the development of hybrid capacitor materials with superior performance compared to state-of-art devices. This would translate directly into capacitors with superior energy and power densities capable of satisfying the high power requirements of electronic devices and electric vehicles. Previous methods for synthesizing hybrid capacitors were either expensive or the process parameters were difficult to control leading to difficulties in replicating the final material composition and thickness. For DOE supercapacitors based on microencapsulated carbons with Ni, Co or Ru were developed. The super-capacitors constructed so far have specific capacitance larger than 200 F/g and a power density larger than 500 W/kg. In order to achieve these goals a one-step technique for electroless deposition of Ru, Ni and Co on carbons was developed by our group at the University of South Carolina. Since thin films of Ni, Co or Ru will be deposited on carbon particles, the supercapacitors have better utilization of the active materials than those constructed from bare Co(OH) 2, Ni(OH) 2 or hydrous RuO The double layer capacitance was correlated to the pore structure of sel-gel derived carbon xerogels. Mesopore and micropore size distributions and cumulative surface areas were extracted from a density functional theory analysis Very fine oxide xerogels were prepared using a unique solution chemistry associated with the sol-gel process. This process was optimized by studying the effect of thermal treatment on the surface area, pore volume, crystallinity, particle structure, and corresponding electrochemical properties of the resulting xerogels High power density sol gel derived high surface area carbon-ruthenium xerogels were prepared from carbonized resorcinol-formaldehyde resins containing an electrochemically active from of ruthenium oxide Using one step electroless process developed in our laboratories, high energy density ruthenium oxide-carbon composites with different loadings of RuO 2 were synthesized. Both, the electrochemical oxidation and temperature treatment increase the specific capacitance of the composites significantly. Ru valence changes from 2 to 4 on complete oxidation with proton diffusion within the bulk being rate determining step for the faradic reaction A mathematical model of an electrochemical capacitor with hydrous RuO 2 electrodes including both double-layer and surface faradic processes was developed to predict the behavior of the capacitor under conditions of galvanostatic charge discharge. 41 A mathematical model was also developed for charge discharge of electrochemical capacitor that explicitly accounts for particle-packing effects in a composite electrochemical capacitor consisting of hydrous RuO 2, nanoparticles dispersed within the porous activated carbon. 47 3

4 A mathematical model was also developed to investigate the effect of nonuniform distribution of salt electrolyte phase of the electrode in the context of dilute solution theory. 44 Recently amorphous nanostructured composite materials with different RuO 2 laodings on carbon were prepared by colloidal method. The specific capacitance was estimated to be 863 F/g Cathode Optimization in Polymer Electrolyte Membrane Fuel Cells (PEMFC) and in Molten Carbonate Fuel Cells Mathematical models were used to identify optimum electrode formulations, thereby accelerating fuel cell development by identifying important variables that can be used to optimize the performance. I was collaborating with Fuel Cell Energy (FCE) in studying and improving the corrosion characteristics of FCE s current collectors. Novel coatings that reduce/eliminate the corrosion of various materials used in the fuel cell were developed in the past three years Plating Cobalt/Co-Ni alloys over the NiO electrode used in MCFC leads to a longer lifetime at a similar performance. Through this research, the lifetime of the cathode used in MCFC was increased. Apart from this, LiNiCO 2 and LiCoO 2 were evaluated as cathode materials in MCFC. The choice of these two materials was based on my prior experience in working in Li-ion Batteries in a DOE funded project. We have shown that LiNiCoO 2 has much lower dissolution rate than NiO and is a prospective candidate to replace NiO in MCFC. A three phase homogenous model was developed to simulate the performance of the molten carbonate fuel cell cathode. The homogenous model is based on volume averaging of different variables in the three phases over a small volume element. This approach can be used to model porous electrodes as it represents the real system much better than the conventional agglomerate model. Using the homogenous model, the polarization characteristics of the MCFC cathode and MCF cell performance were optimized under different operating conditions. 53 For UTC corporation and DOE in the last two years a novel process was developed for preparation of thin film membrane assemblies with novel nanostructured Pt-alloy catalysts with Co, Ni and Cr. This effort resulted in development novel method based on pulse electro -deposition technique for preparation of membrane electrode assemblies. In this approach, platinum was deposited directly on the surface of the carbon electrode. The method ensures most of the platinum to be in close contact with the membrane. Using this method it is possible to increase the Pt/C ratio up to 75% near the surface of the electrode resulting in a 5 times thinner electrode than commercial state-of the art electrodes. 4

5 References 1. G. Zheng, B. N. Popov, and R. E. White, "Electrochemical Determination of the Diffusion Coefficient of Hydrogen through a LaNi 4.25 Al 0.75 Electrode in Alkaline Aqueous Solution," J. Electrochem. Society, 142, 2695 (1995). 2. G. Zheng, B. N. Popov, and R. E. White, "Application of Porous Electrode Theory on Metal Hydride Electrodes in Alkaline Solution," J. Electrochem. Soc., 143, 435 (1996). 3. G. Zheng, B. N. Popov, and R. E. White, "Determination of Transport and Electrochemical Kinetic Parameters of Bare and Copper Coated LaNi 4.27 Sn 0.24 Electrodes in Alkaline Solution," Journal of the Electrochemical Society, 143, (1996). 4. B. N. Popov, G. Zheng, and R. E. White, "Determination of Transport and Electrochemical Kinetic Parameters of M-H Electrodes," J. Appl. Electrochem. 26, 603 (1996). 5. G. Zheng, B. N. Popov and R. E. White, "Effect of Temperature on Performance of LaNi 4.76 Sn 0.24 Metal Hydride Electrode," J. of Appl. Electrochem. 27, 1328 (1997). 6. B. N. Popov, G. Zheng, and R. E. White, Electroplating of Thin Films of Bismuth onto AISI 4340 Steel and Inconel 718, Corrosion Science, 51, 6, 429 (1995). 7. G. Zheng, B. N. Popov and R. E. White, "Electrochemical Investigation of Bare and Palladium Coated LaNi 4.25 Al 0.75 Electrodes in Alkaline Solutions," J. of Appl. Electrochem. 28, 381 (1998). 8. B. S. Haran, B. N. Popov and R. E. White, "Theoretical Analysis of Metal Hydride Electrodes: Studies on Equilibrium Potential and Exchange Current Density," in J. Electrochem. Soc., 145 (12) 4082 (1998). 9. B. S. Haran, B. N. Popov and R. E. White, "Studies on Electroless Cobalt Coatings for Microencapsulation of Hydrogen Storage Alloys," J. of Electrochem. Soc., 145, 3000 (1998). 10. G. Zheng, B. S. Haran, B. N. Popov and R. E. White, "Studies on Metal Hydride Electrodes with Different Weights and Binder Contents," J. Appl. Electrochem., 29, 361 (1999). 11. B. S. Haran, B.N. Popov and R. E. White, "Determination of Hydrogen Diffusion Coefficient in Metal Hydrides by Impedance Spectroscopy," J. of Power Sources, 75, 56 (1998). 12. A. Durairajan, B. Haran, B. N. Popov and R. E. White, Cycle Life and Utilization Studies on Cobalt Microencapsulated AB 5 Type Metal Hydride, J. Power Sources, 83, 114 (1999). 13. G. Zheng, B. S. Haran, B. N. Popov, and R. E. White, Studies on Metal Hydride Electrodes with Different Weights and Binder Contents, J.of Applied Electroch., 29(3), 361 (1999). 5

6 14. B. Veeraraghavan, J Paul, B. Haran, Branko Popov, Study of Polypyrolle Graphite Composite as Anode Material for Secondary Lithium-ion Batteries, J. Power Sources, 104 (2), 377 (2002). 15. B. Veeraraghavan, J Paul, B. Haran, Branko Popov, Study of Polypyrolle Graphite Composite as Anode Material for Secondary Lithium-ion Batteries, J. Power Sources, 104 (2), 377 (2002). 16. B. N. Popov, W. Zhang, E. C. Darcy, and R. E. White, "AC-Impedance Spectroscopy as a Nondestructive Health Interrogation Tool for Lithium-BCX Cells," J. Electrochem. Soc., 140, 11, (1993). 17. B. N. Popov and R. E. White, "Battery Work at University of South Carolina," Automotive Technology Development, 1, 371 (1997). DOE, Energy Efficiency and Renewable Energy, Office of Transportation Technologies, Washington, DC. 18. B. N. Popov and R. E. White, "Development of Novel Cathode Materials for Li-ion Batteries," Ann. Battery Conf. on Appl. and Adv. IEEE 98TH8299, 387 (1997). 19. P. Arora, B. N. Popov and R. White, "Electrochemical Investigation of Cobalt- Doped LiMn 2 O 4 as Cathode Material for Li-ion Batteries," J. Electrochem. Soc., 145, 807 (1998). 20. D. Zhang, B. N. Popov and R. E. White "Electrochemical Investigation of Chromium Doped LiMn 2 O 4 as a Cathode Material for Lithium-Ion Batteries," J. of Power Sources, 76, 81 (1988). 21. P. Arora, B. N. Popov and R.E. White, "Chromium Oxides, Promising Cathode Materials for Secondary Lithium Batteries, Electrochemical and Solid State Latters, 1(6), 249 (1998). 22. D. Zhang, B.N. Popov, Pankaj Arora, Yuri M. Podrazhansky, and Ralph E. White, "Cobalt Doped Chromium Oxides as Cathode Materials for Secondary Batteries," J. Power Sources, 83, 121 (1999). 23. P. Yu, B. N. Popov, J. A. Ritter and R. E. White, "Determination of the Lithium Ion Diffusion Coefficient in Graphite, J. Electrochem. Soc., 146(1), 8, (1999). 24. Ping Yu, J. A. Ritter, R. E. White and B. N. Popov, Ni-Composite Microencapsulated Graphite as the Negative Electrode in Lithium-Ion Batteries: Electrochemical Impedance and Self Discharge Studies, J. Electrochem. Soc., 147, 2081 (2000). 25. Ping Yu, J.A. Ritter, R. E. White and B. N. Popov, Ni-Composite Microencapsulated Graphite as the Negative Electrode in Lithium-Ion Batteries: Initial Irreversible Capacity Study, J. Electrochem. Soc., 147(4), 1280, (2000). 26. Dong Zhang, Branko N. Popov and Ralph E. White, Modeling Lithium Intercalation of a Single Spinel Particle under Potentiodynamic Control, J. Electrochem. Soc., 147, 831 (2000). 27. D. Zhang, B. S. Haran, A. Durairajan, R. E. White, Y. Podrazhansky and B. N. Popov, Studies on Capacity Fade of Lithium-ion Batteries, J. Power Sources, 91, (2000),

7 28. Ping Yu, Bala S. Haran, James Ritter, Ralph E. White and Branko N. Popov, Palladium-Microencapsulated Graphite as the Negative Electrode in Li-ion Cells, J. Power Sources, 91, 107 (2000). 29. V. R. Subramanian, P. Yu, B. N. Popov and R. E. White, Modeling Lithium Diffusion in Nickel Composite Graphite, J. Power Sources, 96(2), 396 (2001). 30. P. Ramadass, A. Durairajan, B. Haran, R. White and B. Popov, Studies on Capacity Fade of Spinel-Based Li-ion Batteries, J. Electrochem. Soc., 149 (1), A54 (2002), 31. P. Ramadass, Bala Haran, Ralph White, Branko N. Popov, Performance Study of Commercial LiCoO 2 and Spinel-Based Li-ion Cells, J. Power Sources, 104, 2, G. Sikha, P. Ramadass, B. S. Haran, B. N. Popov, Comparison of the Capacity Fade of Sony Cells Charged with Different Protocols, J. Power Sources, 122, 67 (2003). 33. R. P. Ramasamy, B. Veeraraghavan, B. Haran, B. Popov, Electrochemical Characterization of Polypyrrole-Co 0.2 CrO x Composites as Cathode Material for Lithium Ion Batteries, J. Power Sources, 124, 197 (2003). 34. N. Gang, B. Haran, B. Popov, Capacity Fade Study of Lithium Ion Batteries Cycled at High Discharge Rates, J. Power Sources, 117, 160 (2003). 35. R. P. Ramasamy, P. Ramadass, B. S. Haran, B. N. Popov, Synthesis, Characterization and Cycling Performance of Novel Chromium Oxide Cathode Materials for Lithium Batteries, J. Power Sources, 124, 155 (2003). 36. P. Ramadass, B. Haran, R. White and B. N. Popov, Mathematical Modeling of the Capacity Fade of Li-ion Cells, J. Power Sources, 123, (2003). 37. P. Ramadass, B. Haran, P. M. Gomadam, R. White, B. Popov, Development of First Principles Capacity Fade Model for Li-Ion Cells, J. Electrochem. Soc., 151, (2) A196- A203 (2004). 38. E. Hristova, Lj. Arsov, B. N. Popov and R. E. White,. "Ellipsometric and Raman Spectroscopic Study of Thermally Formed Films on Titanium," J. of Electrochem. Soc.144, 2318, (1997). 39. Ch. Lin, J. A. Ritter, Characterization of Sol-Gel-Derived Cobalt Oxide Xerogels as Electrochemical Capacitors, J. of Electrochem. Soc.145, (12) 4097 (1997). 40. Ch. Lin, J. A. Ritter and B. N. Popov, "Development of Carbon-Metal Oxide from Sol-gel Derived Carbon-Ruthenium Oxide Xerogels, J. of Electrochem. Soc.,146 (9) 3155 (1999). 41. Ch. Lin, J. A. Ritter and B. N. Popov and R. E. White, "A Mathematical Model of an Electrochemical capacitor with Double Layer and Faradic Processes," J. Electrochem. Soc., 146, (9), 3168 (1999). 7

8 42. Ch. Lin. J. A. Ritter and B. N. Popov, "Correlation of Double-Layer Capacitance with the Pore Structure of Sol-Gel Derived Carbon Xerogels," J. Electrochem. Soc., 146 (10), 3639 (1999). 43. M. Ramani, B. S. Haran, R. E. White and B. N. Popov, Synthesis and Characterization of Hydrous Ruthenium Oxide-Carbon Supercapacitors, J. Electrochem. Soc., 148, A374 (2001). 44. Ch. Lin, Branko N. Popov, Harry J. Ploehn Modeling the Effects of Electrode Composition and Pore Structure on the Performance of Electrochemical Capacitors, J. Electrochem. Soc.,, 149, A 167 (2002). 45. H. Kim and B. N. Popov, Characterization of Hydrous Ruthenium Oxide/Carbon Nanocomposite Supercapacitors Prepared by a Colloidal Method, J. Power Sources, 104 (1), 52 (2002). 46. H. Kim and B. N. Popov, Synthesis and Characterization of MnO 2 Based Mixed Oxides as Supercapacitors, J. Electrochem. Soc., 150(3), D56-D62 (2003). 47. H. Kim, B. N. Popov, Mathematical model of RuO 2 /Carbon Composite Electrode for Supercapacitors, J. Electrochem. Soc., 150, A1153 (2003). 48. N. Subbramanian, B. S. Haran, P. Ganesan, R. E. White, Analysys of Molten Carbonate Fuel Cell Performance Using a three Phase Homogeneous Model, J. Electrochem. Soc., 150(1), A46-A56 (2003). 49. A. Durairajan, H. Colon-Mercado, B. Haran, R. White and B. Popov, Electrochemical Characterization of Cobalt-Encapsulated Nickel as Cathodes for MCFC, J. Power Sources, 104 (2), 157 (2002). 50. P. Ganesan, Hector Colon, Bala Haran, Ralph White, Branko N. Popov, Study of Cobalt-Doped Lithium-Nickel Oxides as Cathodes for MCFC, J. Power Sources 104, 296 (2002). 51. Prabhu Ganesan, Hector Colon, Bala Haran, Branko N. Popov, Performance of La 0.8 Sr 0.2 CoO 3 Coated NiO as Cathodes for Molten Carbonate Fuel Cells, J. Power Sources, 115, (2003). 52. N. Subramanian, B. S. Haran, P. Ganesan, R. E.White, B. N. Popov, A Full Cell Mathematical Model of MCFC, J. Electrochem. Soc., 150, A1360 (2003). 53. H. Kim, B. N. Popov, Development of Novel Method for Preparation of PEM Fuel Cell Electrodes, accepted for publication in Electrochemical and Solid State Letters, 7, (4) (1) (2004). 8