© Philadelphia Scientific 2003 Philadelphia Scientific Catalyst 201: Catalysts and Poisons from the Battery Harold A. Vanasse Daniel Jones Philadelphia Scientific

© Philadelphia Scientific 2003 Philadelphia Scientific Presentation Outline A Review of Catalyst Basics Hydrogen Sulfide in VRLA Cells Catalyst Poisoning Filter Science A Design to Survive Poisons Catalyst Life Estimates

© Philadelphia Scientific 2003 Philadelphia Scientific Catalyst Basics By placing a catalyst into a VRLA cell: –A small amount of O 2 is prevented from reaching the negative plate. –The negative stays polarized. –The positive polarization is reduced. –The float current of the cell is lowered.

© Philadelphia Scientific 2003 Philadelphia Scientific Catalyst Basics

© Philadelphia Scientific 2003 Philadelphia Scientific Catalysts in the Field 5 years of commercial VRLA Catalyst success. A large number of cells returned to good health. After 2-3 years, we found a small number of dead catalysts. –Original unprotected design. –Indicated by a rise in float current to pre-catalyst level.

© Philadelphia Scientific 2003 Philadelphia Scientific Dead Catalysts No physical signs of damage to explain death. Unprotected catalysts have been killed in most manufacturers’ cells in our lab. –Catalyst deaths are not certain. –Length of life can be as short as 12 months. Theoretically catalysts never stop working …. unless poisoned. Investigation revealed hydrogen sulfide (H 2 S) poisoning.

© Philadelphia Scientific 2003 Philadelphia Scientific H 2 S Produced on Negative Plate Test rig collects gas produced over negative plate. Very pure lead and specific gravity acid used. Test run at a variety of voltages. Gas analyzed with GC.

© Philadelphia Scientific 2003 Philadelphia Scientific Test Results High concentration of H 2 S produced. H 2 S concentration independent of voltage. H 2 S produced at normal cell voltage!

© Philadelphia Scientific 2003 Philadelphia Scientific H 2 S Absorbed by Positive Plate

© Philadelphia Scientific 2003 Philadelphia Scientific Test Results Lead oxides make up positive plate active material. Lead oxides absorb H 2 S. Test Material Amount (grams) Breakthrough Time (minutes) Empty PbO PbO

© Philadelphia Scientific 2003 Philadelphia Scientific H 2 S Absorbed in a VRLA Cell

© Philadelphia Scientific 2003 Philadelphia Scientific Test Results H 2 S clearly being removed in the cell. 10 ppm of H 2 S detected when gassing rate was 1,000 times normal rate of cell on float!

© Philadelphia Scientific 2003 Philadelphia Scientific GC Analysis of VRLA Cells Cells from multiple manufacturers sampled weekly for H 2 S since November All cells on float service at 2.27 VPC at either 25°C or 32° C. Results: –H 2 S routinely found in all cells. –H 2 S levels were inconsistent and varied from 0 ppm to 1 ppm, but were always much less than 1 ppm.

© Philadelphia Scientific 2003 Philadelphia Scientific H 2 S in VRLA Cells H 2 S can be produced on the negative plate in a reaction between the plate and the acid. H 2 S is absorbed by the PbO 2 of the positive plate in large quantities. An equilibrium condition exists where H 2 S concentration does not exceed 1 ppm.

© Philadelphia Scientific 2003 Philadelphia Scientific How do we protect the Catalyst? Two possible methods: –Add a filter to remove poisons before they reach the catalyst material. –Slow down the gas flow reaching the catalyst to slow down the poisoning.

© Philadelphia Scientific 2003 Philadelphia Scientific Basic Filter Science Precious metal catalysts can be poisoned by two categories of poison: –Electron Donors: Hydrogen Sulfide (H 2 S) –Electron Receivers: Arsine & Stibine A different filter is needed for each category.

© Philadelphia Scientific 2003 Philadelphia Scientific Our Filter Selection We chose a dual-acting filter to address both types of poison. –Proprietary material filters electron donor poisons such as H 2 S. –Activated Carbon filters electron receiver poisons.

© Philadelphia Scientific 2003 Philadelphia Scientific Slowing Down the Reaction There is a fixed amount of material inside the catalyst unit. Catalyst and filter materials both absorb poisons until “used up”. Limiting the gas access to the catalyst slows down the rate of poisoning and the rate of catalyst reaction.

© Philadelphia Scientific 2003 Philadelphia Scientific Microcat ® Catalyst Design Chamber created by non-porous walls. Gas enters through one opening. Microporous disk further restricts flow. Gas passes through filter before reaching catalyst. Gas / Vapor PathPorous Disk Filter Material Catalyst Material Housing

© Philadelphia Scientific 2003 Philadelphia Scientific How long will it last? Theoretical Life Estimate Empirical Life Estimate

© Philadelphia Scientific 2003 Philadelphia Scientific Theoretical Life Estimate Microcat ® catalyst theoretical life is 45 times longer than original design. –Filter improves life by factor of 9. –Rate reduction improves life by factor of 5.

© Philadelphia Scientific 2003 Philadelphia Scientific Empirical Life Estimate: Stubby Microcat ® catalysts developed for accelerated testing. –1/100 th the H 2 S absorption capacity of normal. –All other materials the same. –Placed in VRLA cells on float at 2.25 VPC & 90ºF (32ºC). –Two tests running. Float current and gas emitted are monitored for signs of death.

© Philadelphia Scientific 2003 Philadelphia Scientific Stubby Microcat ® Catalyst Test Results Stubby Microcats lasted for: –Unit 1: 407 days. –Unit 2: 273 days. Translation: –Unit 1: 407 x 100 = 40,700 days = 111 yrs –Unit 2: 273 x 100 = 27,300 days = 75 yrs.

© Philadelphia Scientific 2003 Philadelphia Scientific Catalyst Life Estimate Life estimates range from 75 years to 111 years. We only need 20 years to match design life of VRLA battery. A Catalyst is only one component in battery system and VRLA cells must be designed to minimize H 2 S production. –Fortunately this is part of good battery design.

© Philadelphia Scientific 2003 Philadelphia Scientific Conclusions Catalysts reduce float current and maintain cell capacity. VRLA Cells can produce small amounts of H 2 S, which poisons catalysts. H 2 S can be successfully filtered. A catalyst design has been developed to survive in batteries.