1. Increase in Positive Active Material Utilization in Lead-Acid Batteries Simon McAllister, Rubha Ponraj, I. Francis Cheng, Dean B. Edwards Department of Chemistry, University of Idaho, Moscow, ID 83844-2343 Email: ifcheng@uidaho.edu, Tel: 208-885-6387ifcheng@uidaho.edu Background Experimental Preparation of the Positive plate Lead Strips - Pb and 4-6% Sb with Teflon ring attached. Paste contains PbO, 0.5% Dynel fibers, additives for total mass of 10 g, 1 ml of 1.4 specific gravity H 2 SO 4 1.2 ml of DI water Teflon ring is filled with paste Hydroset for 24 hours to convert Pb 0 to Pb II Dried overnight Results The lead-acid battery is a highly successful rechargeable electrochemical storage system. Despite its design dating from Plante in 1859 the system can benefit from new approaches. The basic electrode reactions during charge and discharge of a lead-acid battery with 1.3 specific gravity H 2 SO 4 are shown in reactions 1-3 [1,2] Purpose The performance of these batteries is limited by the positive plate reaction due to low PbO 2 positive active material (PAM) utilization which is around 30% or less at the 1 hour rate. Utilization is the amount of charge we obtain at a certain discharge rate relative to the theoretical charge. Reasons for low utilization include slow diffusion of electrolyte into the interior, and electrical isolation from the buildup of PbSO 4 [3,4]. To improve PAM utilization [4-8]: 1) Porosity and conductivity of the PAM have to be increased. 2) The unreacted material has to be replaced with a filler. Porosity of PAM is increased by the addition of highly porous additives thereby enhancing the diffusion of acid to the interior of active material and providing fine local reservoirs of acid within porous particles. Characteristics of Ideal Additives Lead-acid batteries have a harsh acid and oxidative environment, therefore the following characteristics have to be considered while selecting additives [4]: Increase utilization Chemically and oxidatively stable Good adhesion to active material Increase PAM utilization without affecting cycle life Low cost Light weight Diatoms are our choice as an additive because of its porosity, abundance and stability towards battery conditions. In this work, we added size sorted diatoms (Melosira) at different loadings and studied their effect on the positive plate performance. Figure 3 - Teflon ring filled with paste and hydroset. Internal volume of ring is 0.24 ml Formation and Conditioning Positive plate polyethylene separator b/w negative and positive plates Glass mat with 90% porosity Figure 5 - Formational cell (cross sectional view) Formation step with 1.1 specific gravity H 2 SO 4 to convert Pb 0, PbO, and PbSO 4 to PbO 2 Theoretical capacity - 0.2241 Ah/g Fast charge with constant current to obtain 100% theoretical capacity in 24 hrs Slow charge with constant current (half of fast charge) applied for 12 hrs to reach 125% theoretical. Conditioning Acid is changed to 1.3 specific gravity H 2 SO 4 and plates are cycled individually 4 to 5 times in a cylindrical cell. Discharged at 10 mA g -1 Charged at fast rate to 125% previous discharge capacity working electrode (positive plate) Counter electrode (Pt coil) reference electrode (Ag/AgCl) Figure 6 – Picture of cylindrical cell Performance measurements Four size fractions, 20-30 µm, 30-53 µm, 53-74 µm and 74-90 µm, at 3 wt% and 5 wt% were tested. 4 plates in each group were pasted. Capacity measurements are taken at a 50 mA cm -2 discharge and a 10 mA cm -2 discharge. Plates are cycled at each rate until the capacity reaches a maximum. Figure 7 - Performance changes due to the diatoms. Specific capacity is the amount of charge that can be produced for a given mass, in mAh g -1. The addition of 3% diatoms of 53-74 µm exhibited the best performance and they are believed to favor acid diffusion inside the active mass. The scanning electron micrographs (Figure 8a, b) show the porous structure of diatoms. The SEM of diatoms recovered from the active material after the performance tests (Figure 8d) proves that diatoms survive in the battery environment. Conclusion Diatoms are an inexpensive filler material that increases positive active material utilization by replacing unreacted active material, while maintaining pores for diffusion. Utilization increases by over 12.7% at a fast rate discharge of 50 mA cm -2 with a corresponding 9.3% increase in specific capacity. Future work Duplicate diatoms effect in full positive plate. Add conductive additives to increase electron flow References 1. H. Bode, Lead-Acid Batteries, translated by R.J. Brodd and K.V. Kordesch, 1997, page 4. 2. S.V. Baker, P.T. Moseley and A.D. Turner, J. Power Sources, 27 (1989) 127. 3. P.T.Moseley, J. Power Sources, 64 (1997) 47. 4. K.McGregor, J. Power Sources, 59 (1996) 31. 5. H.Dietz, J.Garche, K.Weisner, J. Power Sources, 14 (1985) 305. 6. D.Berndt, Maintenance-Free Batteries, second edition 1997, page 103. 7. P.W. Appel, D.B. Edwards, J. Power Sources, 55 (1995) 81. 8. D.B. Edwards, V.S. Srikanth, J. Power Sources, 34 (1991) 217. Acknowledgement Office of Naval Research Award Number: N00014-04-1-0612, Department of Chemistry, Microelectronics Research and Communications Institute (MRCI), Dr. and Mrs. Renfrew. negative plate in between separators Figure 4 - Formation cell (side view) Figure 2 – Diagram of porous lead dioxide with diffusion of HSO 4 ¯ Figure 8 - Scanning electron micrograph of diatoms of different sizes: a) 20-30 µm b) 53-74 µm c) >90 µm d) Diatoms recovered from active material after the performance tests. Figure 1 – Schematic of lead-acid battery Gri d PbO 2 PbSO 4 layer HSO 4 ¯ e-e- HSO 4 ¯ diffusion enhanced by additives Electrically isolated PbO 2