© Philadelphia Scientific 2003 Philadelphia Scientific Catalyst 201: Catalysts and Poisons from the...

© Philadelphia Scientific 2003 Philadelphia Scientific

Catalyst 201: Catalysts and Poisons from

the Battery

Harold A. Vanasse

Daniel Jones

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


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!

0

100

200

300

400

500

600

2.25 2.35 2.45 2.55 2.65 2.75

Cell voltage (V)

H2S

con

cent

ratio

n (p

pm)


H2S Absorbed by Positive Plate

Material to be tested

Reactor

H2 +

H2S

10

0 p

pm

GC

H2 Gas with 100 ppm of

H2S


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

H

2 +

H2S

10

0 p

pm

H2 Gas with 100 ppm of

H2S

VRLA cell

GC

2.27V


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!

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25 30

Time (hours)

H2S

co

nce

ntr

ati

on

in

th

e o

utl

et

ga

s (p

pm

)

0

20

40

60

80

100

120

140

160

Inle

t ga

s flow

rate

(ml/m

in)

H2S Concentration (ppm)

Gas Flowrate (ml/min)


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

FilterMaterial

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.

© Philadelphia Scientific 2003 Philadelphia Scientific Catalyst 201: Catalysts and Poisons from the...

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