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Lecture 3: Fuel Cells General
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Lecture Contents:
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Introduction
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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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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?
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A fuel cell converts the chemicals hydrogen and oxygen into water, and
in the process it produces electricity.
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The other electrochemical device that we are all familiar with is the
battery.
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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.
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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?
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Fuel cells are usually classified by the type of electrolyte they
use.
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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
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Anode side:
2H2 => 4H+ + 4e-
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Cathode side:
O2 + 4H+ + 4e- => 2H2O
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Net reaction:
2H2 + O2 => 2H2O
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Chemistry of a Fuel Cell
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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.
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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.
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The electrolyte
Ion exchange membrane. The membrane blocks electrons.
This specially treated material, which looks something like ordinary kitchen plastic wrap.
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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)
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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),
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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)
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liquid phosphoric acid as an electrolyte
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(4) Alkaline fuel cells (AFCs)
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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)
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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)
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(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)
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Operation to produce
electricity, heat & water:
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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)
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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.
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