Transcript Philadelphia Scientific Catalyst 201: Catalysts and Poisons from the Battery

Philadelphia Scientific
Catalyst 201:
Catalysts and Poisons from
the Battery
Harold A. Vanasse
Daniel Jones
© Philadelphia Scientific 2003
Philadelphia Scientific
Presentation Outline
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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
H2S concentration (ppm)
600
• High concentration
of H2S produced.
• H2S concentration
independent of
voltage.
• H2S produced at
normal cell voltage!
500
400
300
200
100
0
2.25
2.35
2.45
2.55
2.65
2.75
Cell voltage (V)
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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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Test
Material
Amount
(grams)
Breakthrough
Time
(minutes)
Empty
0.0
0.01
PbO
2.2
120
PbO2
2.0
360
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H2S Absorbed in a VRLA Cell
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Test Results
160
90
H2S Concentration (ppm)
140
Gas Flowrate (ml/min)
120
80
70
100
60
50
80
40
60
30
40
20
20
10
0
Inlet gas flowrate (ml/min)
H2S concentration in the outlet gas
(ppm)
100
• 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!
0
0
5
10
15
20
25
30
Time (hours)
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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
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.
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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
Gas / Vapor Path
Porous
Disk
Filter
Material
Catalyst
Material
• Chamber created by
non-porous walls.
• Gas enters through
one opening.
• Microporous disk
further restricts flow.
• Gas passes through
filter before reaching
catalyst.
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.
© Philadelphia Scientific 2003
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