Lecture 3: Fuel Cells General

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Lecture 3 Fuel Cells General

• Lecture Contents
• Introduction
• Examples from Journals

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The concepts of Battery and Fuel Cell
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Materials Theoretical Capacity (Ah/g)
H2 26.59
Li 3.86
Na 1.16
Mg 2.20
Al 2.98
Ca 1.34
Fe 0.96
Zn 0.82
Cd 0.48
Pb 0.26
(Source D. Linden Handbook of Batteries2nd Ed. P1.8)
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Charge mode
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Discharge mode
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Batteries
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Where is your Liquid Electrolyte??
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Where is your Gel Polymer Electrolyte??
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Where is your Gel Polymer Electrolyte??
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Where is your Solid Polymer Electrolyte??
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Fuel Cells
O2 4e- 4H ? 2H2O
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What is a Fuel Cell?

• A fuel cell converts the chemicals hydrogen and
  oxygen into water, and in the process it produces
  electricity.
• The other electrochemical device that we are all
  familiar with is the battery.
• A battery has all of its chemicals stored inside,
  and it converts those chemicals into electricity
  too. This means that a battery eventually "goes
  dead" and you either throw it away or recharge
  it.
• With a fuel cell, chemicals constantly flow into
  the cell so it never goes dead -- as long as
  there is a flow of chemicals into the cell, the
  electricity flows out of the cell. Most fuel
  cells in use today use hydrogen and oxygen as the
  chemicals

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What is a Fuel Cell?

• Fuel cells are usually classified by the type of
  electrolyte they use.
• Most fuel cells are powered by hydrogen, which
  can be fed to the fuel cell system directly or
  can be generated within the fuel cell system by
  reforming hydrogen-rich fuels such as methanol,
  ethanol, and hydrocarbon fuels. ions or for
  powering cars.

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Chemistry of a Fuel Cell

• Anode side2H2 gt 4H 4e-
• Cathode sideO2 4H 4e- gt 2H2O
• Net reaction2H2 O2 gt 2H2O

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Chemistry of a Fuel Cell

• The anode
• It conducts the electrons that are freed from
  the hydrogen molecules so that they can be used
  in an external circuit.
• It has channels etched into it that disperse the
  hydrogen gas equally over the surface of the
  catalyst.
• The cathode
• the positive post of the fuel cell, has channels
  etched into it that distribute the oxygen to the
  surface of the catalyst. It also conducts the
  electrons back from the external circuit to the
  catalyst, where they can recombine with the
  hydrogen ions and oxygen to form water.
• The electrolyte
• Ion exchange membrane. The membrane blocks
  electrons.
• This specially treated material, which looks
  something like ordinary kitchen plastic wrap.
• The catalyst
• is a special material that facilitates the
  reaction of oxygen and hydrogen.
• It is usually made of platinum powder very
  thinly coated onto carbon paper or cloth.

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Chemistry of a Fuel Cell
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(1) Polymer Electrolyte Membrane fuel cells
(PEMFCs) 

• Solid polymer as an electrolyte and porous carbon
  electrodes containing a platinum catalyst. They
  need only hydrogen, oxygen from the air, and
  water to operate

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(2) Direct methanol fuel cells (DMFCs),

• Powered by pure methanol, which is mixed with
  steam and fed directly to the fuel cell anode.

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(3) The phosphoric acid fuel cells (PAFCs)

• liquid phosphoric acid as an electrolyte

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(4) Alkaline fuel cells (AFCs)

• These fuel cells use a solution of potassium
  hydroxide in water as the electrolyte and can use
  a variety of non-precious metals as a catalyst at
  the anode and cathode.

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(5) Molten carbonate fuel cells (MCFCs)

• an electrolyte composed of a molten carbonate
  salt mixture suspended in a porous, chemically
  inert ceramic lithium aluminum oxide (LiAlO2)
  matrix.

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(6) Solid oxide fuel cells (SOFCs)

• (SOFCs) use a hard, non-porous ceramic compound
  as the electrolyte

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Example from Journals
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Case study 1 Proton Exchange Membrane Fuel Cells
(PEMFCs)

• Operation to produce electricity, heat water
• H fuel is oxidized at the anode
• Oxygen is reduced at the cathode

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Case study 1 Proton Exchange Membrane Fuel Cells
(PEMFCs)
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Case study 1 Proton Exchange Membrane Fuel Cells
(PEMFCs)
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Case study 2 Alkaline Fuel Cells (AFCs)

• Sodium Borohydride
• There are two possible routes for oxidation of
  NaBH4.
• In the first route NaBH4 is directly oxidized
• NaBH4 8OH-? NaBO2 6H2O 8e-
• In the second route, hydrogen is liberated at
  high temperature or pH less than 7.
• NaBH4 2H2O ? NaBO2 4H2
• 4H2 8OH-? 8H2O 8e-
• The hydrogen produced by this route may escape
  from the system thus the fuel may not be fully
  utilized.