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

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

Catalyst Basics By placing a catalyst into a VRLA cell: A small amount of O2 is prevented from reaching the negative plate. The negative stays polarized. The positive polarization is reduced. The float current of the cell is lowered.

Catalyst Basics

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.

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 (H2S) poisoning.

H2S Produced on Negative Plate Test rig collects gas produced over negative plate. Very pure lead and 1.300 specific gravity acid used. Test run at a variety of voltages. Gas analyzed with GC.

Test Results High concentration of H2S produced. H2S concentration independent of voltage. H2S produced at normal cell voltage!

H2S Absorbed by Positive Plate

Breakthrough Time (minutes) Test Results Lead oxides make up positive plate active material. Lead oxides absorb H2S. Test Material Amount (grams) Breakthrough Time (minutes) Empty 0.0 0.01 PbO 2.2 120 PbO2 2.0 360

H2S Absorbed in a VRLA Cell

Test Results H2S clearly being removed in the cell. 10 ppm of H2S detected when gassing rate was 1,000 times normal rate of cell on float!

GC Analysis of VRLA Cells Cells from multiple manufacturers sampled weekly for H2S since November 2000. All cells on float service at 2.27 VPC at either 25°C or 32° C. Results: H2S routinely found in all cells. H2S levels were inconsistent and varied from 0 ppm to 1 ppm, but were always much less than 1 ppm.

H2S in VRLA Cells H2S can be produced on the negative plate in a reaction between the plate and the acid. H2S is absorbed by the PbO2 of the positive plate in large quantities. An equilibrium condition exists where H2S concentration does not exceed 1 ppm.

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.

Basic Filter Science Precious metal catalysts can be poisoned by two categories of poison: Electron Donors: Hydrogen Sulfide (H2S) Electron Receivers: Arsine & Stibine A different filter is needed for each category.

Our Filter Selection We chose a dual-acting filter to address both types of poison. Proprietary material filters electron donor poisons such as H2S. Activated Carbon filters electron receiver poisons.

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.

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 Path Porous Disk Filter Material Catalyst Material Housing

How long will it last? Theoretical Life Estimate Empirical Life Estimate

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.

Empirical Life Estimate: Stubby Microcat® catalysts developed for accelerated testing. 1/100th the H2S 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.

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.

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 H2S production. Fortunately this is part of good battery design.

Conclusions Catalysts reduce float current and maintain cell capacity. VRLA Cells can produce small amounts of H2S, which poisons catalysts. H2S can be successfully filtered. A catalyst design has been developed to survive in batteries.