Sec 5 (Shahed) AFCs Presentation March 03

8/7/2019 Sec 5 (Shahed) AFCs Presentation March 03

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Alkaline Fuel CellAlkaline Fuel Cell

5Factors Affecting Long-Term Performance

&

Technical Barriers for Commercialization

Shahed Islam

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MAJOR FACTORSMAJOR FACTORSAFFECTING LONG TERM PERFORMANCEAFFECTING LONG TERM PERFORMANCE

Major Concern Effect

1) Electrolyte:Contamination/poisoning

Low powerdensity

Stack Life2) Electrode/catalyst:

Degradation

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1.1. ELECTROLYTE POISONING: WITH COELECTROLYTE POISONING: WITH CO22

Source: CO2 Poisoningy Carbon dioxide source

Presence in the Fuel or Air stream

[or as a result of carbon catalyst corrosion]

Chemical Reaction:Carbon dioxide Poisoning in AirBreathing Alkaline Fuel Cells

CO2+ 2OH CO3-2 + H2O

or,

CO2

+ 2KOH K2CO3 + H2O

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Effects:

1) Reduced OH concentration, interfering with

kinetics Lower pH of electrolyte

Reduction of ionic conductivity

2) Electrolyte viscosity increase, resulting in lowerdiffusion rate and lower limiting currents;

1. ELECTROLYTE POISONING: WITH CO2

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1.1. ELECTROLYTE POISONING WITH COELECTROLYTE POISONING WITH CO22

Degradation in AFC Electrode Potential with CO2 Containing and CO2 Free Air Source

[FC Handbook: p124]

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1.1. ELECTROLYTE POISONING: WITH COELECTROLYTE POISONING: WITH CO22

Effects: contd.

3) Precipitation of carbonate salts out ofthe solution: Block pores in electrodex Change electrolyte Volume

x Limit reactant accessx reduced oxygen solubility (cathode block)

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1. ELECTROLYTE POISONING WITH

CO2

Polarization curves of the fuel cells (HKU-002C) with different electrolytes at 298K

[Accelerated Studies - A. Tewari: Quantification of CO2, Fig-1]

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2.2. ELECTRODE/CATALYST DEGRADATIONELECTRODE/CATALYST DEGRADATION

y Common non-noble electrode materialsinvestigated

Silver :Most often considered as cathodic catalyst

Nickel :Most often considered as anodic catalyst

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2.2. ELECTRODE/CATALYST DEGRADATIONELECTRODE/CATALYST DEGRADATION

Postulation

y Crystallized carbonate might destroy the layersof the electrode mechanically

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-- EFFECTS OF CARBONATE PRECIPITATIONEFFECTS OF CARBONATE PRECIPITATION

y

1% CO2 in oxidant stream at 25C Significant half cell performance decrease seen in just 2 hours

x Reactive site blockage due to presence of carbonates on electrode(verified by XRD)

Al-Salehet. al., 1994

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TECHNICAL ISSUESTO BEOVERCOME FORTECHNICAL ISSUESTO BEOVERCOME FOR

COMMER

CIALIZAT

IO

NCOMMER

CIALIZAT

IO

N

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OVERCOMINGTHE BARRIERSOVERCOMINGTHE BARRIERS

FOR COMMERCIALIZATIONFOR COMMERCIALIZATION

y CO2 Poisoning

y Stack life/Cost

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STRATEGIES FOR CONTROLLING CO2STRATEGIES FOR CONTROLLING CO2POISONINGPOISONING

a) Filtration of oxidant stream to remove traceCO2 amounts Soda lime absorption towersx 1kg soda lime / 8 kwh operation time

y Pros: Significantly lower amount of CO2 introduced into

fuel cell system

y Cons: Space/design issues

Higher capital and operational costs

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b) Molecular sieve regenerative adsorbers

y

Pros: Significantly lower amount of CO2 introduced into

fuel cell system

y

Cons: Space/design issues

Not feasible

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C) Circulating electrolyte / Electrolyte replacement

y Pros:

Possible to filter the circulating electrolyte

Can also perform a batch replacement of electrolyte

Removes by product: water; easy mgmt of excess heat

Mitigates problems caused by CO2 poisoning

y Cons:

Design is similar to water circulation loops present in PEMFCs,

however circulating liquid is corrosive

Space/design issues

Increased capital and operational costs

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STRATEGIES FOR CONTROLLING CO2 POISONINGSTRATEGIES FOR CONTROLLING CO2 POISONING

d) Solid anion exchange membrane as electolyte hydrocarbon polymeric membrane is being developed

y Pros:

No free OH- ions in solution so carbide formation is limited Solid membrane so handling of potentially corrosive liquids is

eliminated

y Cons: Lower ionic conductivity

x Decreased cell performance

Low thermal stability

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OTHER TECHNICAL BARRIERS:OTHER TECHNICAL BARRIERS:

Power Density: to increase COST: to decrease

- Electrolyte Conductivity increase - Mass production

- Research into novel/cheaper

materials

- Higher catalyst activity - Cost of component materials

- Cost of manufacturing

- Improve design - Decreased costs will accelerate

commercialization ofAFCs

THESETECHNICAL BARRIERS CAN BEOVERCOME,WITH

MORERESEARCH AND INTEREST.

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SUMMARY/CONCLUSIONSUMMARY/CONCLUSION

CO2 is not a problem, slightly lower performance is due

to reduced conductivity of the electrolyte and the

electrolyte can be changed periodically as changing oil inIC engine

For wider application, research on anion exchange

membrane is required

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REFERENCESREFERENCES1) Meng Ni. et al, WHEC 16, June 2006: Technological development and prospect of AFSs

2) Gulzow, E. et al, 2004 [243 251]: Long-term operation of AFC electrode .. ..

3) Scott,K. et al, Journal of power Sources, 175, 2008

4) Al-Saleh,M. et al, 1994 [575 580]: Effect of CO2 on the performance of Ni and Agin an AFC

5) Schulze,M. et al, Journal of power Sources, 127, 2004

6) Tewary, A. et al, Journal of Power Source, 153, 2006: Quantification of CO2 in airbreathing AFCs

7) K.Kordesch, J. Gsellmann and B.Kraetschmer, in Power Sources, 9, Edited by J.Thompson, Academic Press, New York, NY, 379, 1983

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Sec 5 (Shahed) AFCs Presentation March 03