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FUEL CELLS

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Fuel cells

Introduction

Low temperature type

• PAFC (Phosphoric Acid Fuel Cell)

• AFC (Alkaline Fuel Cell)

• PEMFC (Polymer Electrolyte Membrane Fuel Cell)

• DMFC (Direct Methanol Fuel Cell)

High temperature type

• MCFC (Molten Carbonate Fuel Cell)

• SOFC (Solid Oxide Fuel Cell)

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Fuel cells: Introduction

1. Basic structure

2. Characteristic

3. Classification

Basic structure of fuel cells

: anode (H2) ǀ electrolyte ǀ cathode (O2)

: H2 (or H2-source) and O2 are supplied from outside

: reaction of water formation in basic

(H2+1/2O2 → H2O)

: stack structure to get high voltage

High efficiency / Environmentally friendly

: direct production of electric power

(thermal power generation: fossil fuel → heat→ mechanical energy (turbine) → electric energy)

: by-product (heat, hot water, steam) can be utilized

: No exhaust fume, No noise

Classification depending on

: working temperature (low temp./ high temp.)

: electrolyte (liquid / solid)

: fuel

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Low temperature type FC

1. PAFC (Phosphoric Acid Fuel Cell)

2. AFC (Alkaline Fuel Cell)

3. PEMFC (Polymer electrolyte Membrane Fuel Cell)

4. DMFC (Direct Methanol Fuel Cell)

PAFC

: phosphoric acid electrolyte

: H2 is applied by reforming hydrocarbons

H2 → 2H++2e-

1/2O2+2H++2e- → H2O

: H2+1/2O2 → H2O

cf) generation of hydrogen

: Electrolysis of water / reforming of hydrocarbons / Coal methods / etc.

AFC

: KOH aqueous solution electrolyte

H2+2OH- → 2H2O+2e-

1/2O2+H2O+2e- → 2OH-

: H2+1/2O2 → H2O

: CO2 forms KHCO3 degrading the performance

: high purity of H2 and O2 should be supplied

: special application (spacecraft)

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Low temperature type FC

1. PAFC (Phosphoric Acid Fuel Cell)

2. AFC (Alkaline Fuel Cell)

3. PEMFC (Polymer electrolyte Membrane Fuel Cell)

4. DMFC (Direct Methanol Fuel Cell)

PEMFC

: proton-exchange membrane electrolyte (Nafion)

H2 → 2H++2e-

1/2O2+2H++2e- → H2O

: H2+1/2O2 → H2O

: poisoning of Pt catalyst by CO at low temperatures

: polyimide, polyacryl are investigated for high temperature operation over 120oC

DMFC

: similar structure with PEMFC

: methanol is used as a hydrogen source

: small size, mobile application

CH3OH+H2O → CO2+6H++6e-

3/2O2+6H++6e- → 3H2O

: CH3OH+3/2O2 → CO2+2H2O

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High temperature type FC

1. MCFC (Molten Carbonate Fuel Cell)

2. SOFC (Solid Oxide Fuel Cell)

MCFC

: molten Li2CO3/K2CO3 electrolyte (over 600oC)

: reforming of hydrocarbon inside the cell owing to high operation temperature

2H2+2CO32- → 2CO2+2H2O+4e-

O2+2CO2+4e- → 2CO32-

: 2H2+O2 → 2H2O

: CO2 should be supplied to the cathode side

(recycle from the anode)

SOFC

: ion-conductive oxide electrolyte

(Yittria Stabilized Zirconia, YSZ, for O2- conduction)

(BaCe5.2Sm0.2O3-α for proton conduction)

H2+O2- → H2O+2e-

1/2O2+2e- → O2- (O2--conduction type)

H2 → 2H++2e-

2H++2e-+1/2O2 → H2O (H+-conduction type)

: high temperature operation (800~1000oC)

: technical difficulty of sealing, heat-expansion, etc.

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* 전기화학 (오승모 저)

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* 전기화학 (오승모 저)

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