Hydrogen Fuel Cells Basic Principles

Transcription

1 Hydrogen Fuel Cells Basic Principles first demonstration by William Grove in 1839 Electrolysis Fuel Cell currents small: low contact area (gas/electrode/electrolyte) large distance between electrodes (electrolyte resistance) From: J. Larminie, A: Dicks, Fuel Cell Systems Explained,Wiley (2003)

2 Basic cathode-electrolyte construction Note: Cathode positive terminal Cathode is always the eletrode into which e flow Porous electrode structures Anode negative terminal Anode the electrode from which e flow From: J. Larminie, A: Dicks, Fuel Cell Systems Explained,Wiley (2003)

3 What limits the current? Fuel cell reactions proceed only slowly use of catalyst raising the temperature increasing the electrode area From: J. Larminie, A: Dicks, Fuel Cell Systems Explained,Wiley (2003)

4 Connecting Cells in Series: A fuel cell stack Single cell voltage: ~ 0.7 V in operation Most obvious way: Useful voltage: connecting cells in series Problem: Voltage drops method not used From: J. Larminie, A: Dicks, Fuel Cell Systems Explained,Wiley (2003)

5 Bipolar plates Simple design: From: J. Larminie, A: Dicks, Fuel Cell Systems Explained,Wiley (2003)

6 Fuel cell stacks with bipolar plates From: J. Larminie, A: Dicks, Fuel Cell Systems Explained,Wiley (2003)

7 Anode/Electrolyte/Cathode assemblies From: J. Larminie, A: Dicks, Fuel Cell Systems Explained,Wiley (2003)

8 Bipolarplatten mit interner Gasverteilung From: J. Larminie, A: Dicks, Fuel Cell Systems Explained,Wiley (2003)

9 Principles of Basic Fuel Cell Types

10 Fuel Cell Types generally distinguished by type of electrolyte and conducting ion: fuel cell type temp. anode reaction conducting ion cathode reaction SOFC (Solid Oxide FC) MCFC ( Molten Carbonate FC) PAFC ( Phosphoric Acid FC) PEMFC ( H + Exchange Membrane FC) 1000 C H 2 + O -2 H 2 O + 2e - O -2 (Y-stabilized ZrO 2 ) 650 C H 2 + CO 3-2 H 2 O + CO 2 + 2e - CO 3-2 (alkali carbonates) 200 C H 2 2H + + 2e - H + (H 3 PO 4 ) 80 C H 2 2H + + 2e - H+ (solid polymer) ½ O 2 + 2e - O -2 ½ O 2 + CO 2 + 2e - CO 3-2 ½ O 2 + 2H + + 2e - H 2 O ½ O 2 + 2H + + 2e - H 2 O DMFC ( Direct Methanol FC) AFC ( Alkaline FC) 80 C CH 3 OH + H 2 O CO 2 + 6H + + 6e - H+ (solid polymer) 80 C H 2 + 2OH - 2H 2 O + 2e - OH- (KOH) 1.5 O 2 + 6H + + 6e - 3H 2 O ½ O 2 + H 2 O + 2e - 2OH -

11 high-temperature fuel cells (SOFC, MCFC): temperature required to obtain sufficient electrolyte conductivity ability to oxidize CO and to use CH 4 reactant via internal reforming low/medium-temperature fuel cells: temperature maximum dictated by H 2 O-loss (no conductivity without H 2 O) require clean H 2 : CO-tolerance of 1% for PAFC and <<100ppm for others; CO 2 -free O 2 /air for AFCs (carbonate formation: ph change, precipitation)

12 Schematic PEMFC Energiewissenschaften, Vorlesung SS 2011, Prof. K. Krischer anode: H 2 2H + + 2e - cathode: ½O 2 + 2e - + 2H + H 2 O porous composite of catalyst and H + -conducting polymer solid H + -conducting polymer (polymer membrane) porous composite of catalyst and H + -conducting polymer from: Dannick Bouchard, laboratoire Brisard, Univ. de Sherbrooke

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14 DMFC porous composite of catalyst and H + -conducting polymer solid H + -conducting polymer (polymer membrane) porous composite of catalyst and H + -conducting polymer from: P. Piela and P. Zelenay, Fuel Cell Review 1 (2004) 17

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16 SOFC anode: H 2 +O -2 H 2 O + 2e - cathode: ½O 2 + 2e - O -2 porous metal/oxide composite (cermet) solid oxide ion conductor (ceramic membrane) porous oxide (e - conductor) from: Dannick Bouchard, Laboratoire Brisard, Univ. de Sherbrooke

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18 Applications and advantages of different fuel cell typs From: J. Larminie, A: Dicks, Fuel Cell Systems Explained,Wiley (2003)