1. © Philadelphia Scientific 2001 Philadelphia Scientific Hydrogen Sulfide in VRLA Cells Harold A. Vanasse Frank J. Vaccaro Volen R. Nikolov INTELEC 2001

2. © Philadelphia Scientific 2001 Philadelphia Scientific Presentation Outline H 2 S is produced in VRLA Cells. H 2 S is absorbed on the lead dioxide of the positive plate. Resultant H 2 S equilibrium concentration is less than 1 ppm.

3. © Philadelphia Scientific 2001 Philadelphia Scientific Background H 2 S is known in: –Flooded cells. –VRLA cells in thermal runaway. Measured in our lab in Negative Active Material testing (Intelec 2000). Supported in literature: –Reduction reaction. –MeS + 2H + = H 2 S + Me 2+

4. © Philadelphia Scientific 2001 Philadelphia Scientific Finding H 2 S Goals: –Prove that H 2 S could be produced at normal float voltages and temperatures. –Identify sources. Early testing eliminated many candidates as main factors. Reaction between pure lead and acid became our focus.

5. © Philadelphia Scientific 2001 Philadelphia Scientific Test Rig Test run at 40ºC. 1.300 specific gravity acid. Test run at a variety of voltages. Three identical test rigs used.

6. © Philadelphia Scientific 2001 Philadelphia Scientific Results H 2 S Concentration independent of voltage. Results repeated over multiple tests.

7. © Philadelphia Scientific 2001 Philadelphia Scientific Another Surprise H 2 S concentration declines over time or repeated rounds of testing. Results repeated over multiple tests.

8. © Philadelphia Scientific 2001 Philadelphia Scientific Interim Findings Sulfuric acid + charged negative plate = H 2 S. Liberation of H 2 S is not voltage dependent. H 2 S concentration high at first, but decreases over time. If this were the case, we would smell rotten eggs around new VRLA cells.

9. © Philadelphia Scientific 2001 Philadelphia Scientific H 2 S is removed by Positive Plate Lead dioxide reactions predict absorption: –PbO + H 2 S = PbS + H 2 O –4PbO 2 + H 2 S = PbSO 4 + 3PbO + H 2 O –4PbO 2 + 3H 2 SO 4 + H 2 S = 4PbSO 4 + 4H 2 O Two experiments lead to proof.

10. © Philadelphia Scientific 2001 Philadelphia Scientific Experiment 1: Reactor Test

11. © Philadelphia Scientific 2001 Philadelphia Scientific Experiment 1: Reactor Test Results Input: 108 ppm H 2 S in H 2 @ 50 ml/min. Output: Connected to GC. Measurements taken every 15 minutes. Test Material Amount (grams) Breakthrough Time (minutes) Empty0.00.01 PbO2.2120 PbO 2 2.0360

12. © Philadelphia Scientific 2001 Philadelphia Scientific Experiment 2: H 2 S Through a VRLA Cell

13. © Philadelphia Scientific 2001 Philadelphia Scientific Experiment 2: Results H 2 S clearly being removed in the cell. Output H 2 S significantly lower than input concentration.

14. © Philadelphia Scientific 2001 Philadelphia Scientific H 2 S Interactions H 2 S Generated at the Negative Plate. H 2 S Absorbed or Oxidized at the Positive Plate. Follow on Questions: –Which process is dominant? –What H 2 S equilibrium concentration level is established?

15. © Philadelphia Scientific 2001 Philadelphia Scientific GC Analysis of VRLA Cells Multiple cells from multiple manufacturers sampled weekly for H 2 S. All cells on float service at 2.27 VPC at either 25°C or 32° C. Age of cells: New to 6 years old.

16. © Philadelphia Scientific 2001 Philadelphia Scientific Results of GC Sampling H 2 S concentration: 0 ppm < 1 ppm, but always less than 1 ppm. Found across all cells tested. Analytical proof of the presence of H 2 S in VRLA cells. Maximum equilibrium threshold established for float conditions.

17. © Philadelphia Scientific 2001 Philadelphia Scientific Conclusions H 2 S can be produced by VRLA cells through the reduction of sulfur- containing compounds. H 2 S can be absorbed within a VRLA cell by the positive plate active material. In cells on float, H 2 S concentration levels are less than 1 ppm.

18. © Philadelphia Scientific 2001 Philadelphia Scientific Impact H 2 S is a poisonous gas that corrodes metal. H 2 S can poison precious metal catalysts. We have built a filter into our latest catalyst design to protect against H 2 S.