Catalyst 201: Catalysts and Poisons from the Battery

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Catalyst 201 Catalysts and Poisons from the
Battery
Philadelphia Scientific

• Harold A. Vanasse
• Daniel Jones

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Presentation Outline

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

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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.

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Catalyst Basics
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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.

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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.

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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.

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Test Results

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

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H2S Absorbed by Positive Plate
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Test Results

• Lead oxides make up positive plate active
  material.
• Lead oxides absorb H2S.

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H2S Absorbed in a VRLA Cell
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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!

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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
  25C 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.

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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.

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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.

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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.

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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.

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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.

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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
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How long will it last?

• Theoretical Life Estimate
• Empirical Life Estimate

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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.

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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.

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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.

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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.
  

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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.