Fuel cells

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Fuel cells
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Fuel cell history

• First demonstrated in principle by British
  Scientist Sir Willliam Robert Grove in 1839.
• Groves invention was based on idea of reverse
  electrolysis.

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What is a fuel cell

• Creates electricity through electrochemical
  process
• Operates like a battery
• Emits heat and water only

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Battery

• A battery is essentially a can full of chemicals
  that produce electrons. Chemical reactions that
  produce electrons are called electrochemical
  reactions.
• Battery has two terminals. One terminal is marked
  (), or positive, while the other is marked (-),
  or negative.

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Working of a battery
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Working of Battery

Electrons collect on the negative terminal of the
battery. Normally some type of load like a motor
or bulb is connected using wire from positive
terminal of the battery to its negative terminal
Inside the battery itself, a chemical reaction
produces the electrons. The speed of electron
production by this chemical reaction (the
battery's internal resistance) controls how many
electrons can flow between the terminals.
Electrons flow from the battery into a wire, and
must travel from the negative to the positive
terminal for the chemical reaction to take place.
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Reactions inside Zinc/carbon battery

• Take a jar filled with sulfuric acid (H2SO4).
  Stick a zinc rod in it.
• The acid molecules break up into three ions two
  H ions and one SO4-- ion.
• The zinc atoms on the surface of the zinc rod
  lose two electrons (2e-) to become Zn ions.
• The Zn ions combine with the SO4-- ion to
  create ZnSO4, which dissolves in the acid.
• The electrons from the zinc atoms combine with
  the hydrogen ions in the acid to create H2
  molecules (hydrogen gas). We see the hydrogen gas
  as bubbles forming on the zinc rod.
• Now stick a carbon rod and connect a wire between
  zinc and carbon rods
• The electrons flow through the wire and combine
  with hydrogen on the carbon rod, so hydrogen gas
  begins bubbling off the carbon rod.
• There is less heat. You can power a light bulb or
  similar load using the electrons flowing through
  the wire.
• The electrons go to the trouble to move to the
  carbon rod because they find it easier to combine
  with hydrogen there. There is a characteristic
  voltage in the cell of 0.76 volts. Eventually,
  the zinc rod dissolves completely or the hydrogen
  ions in the acid get used up and the battery
  "dies."

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Fuel Cell And battery

• A fuel cell is an electrochemical energy
  conversion device. A fuel cell converts the
  chemicals hydrogen and oxygen into water, and in
  the process it produces electricity.
• 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

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Parts of fuel cells

• There are 4 main parts
• Anode
• Cathode
• Catalyst
• Proton exchange membrane

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The Anode

• The anode is the negative post of the fuel cell.
  
• 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 cathode is the positive post of the fuel
  cell.
• It 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 Catalyst

• 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. The catalyst
  is rough and porous so that the maximum surface
  area of the platinum can be exposed to the
  hydrogen or oxygen.
• The platinum-coated side of the catalyst faces
  the PEM.

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The Proton Exchange Membrane

• The electrolyte is the proton exchange membrane.
• This is a specially treated material that only
  conducts positively charged ions.
• The membrane blocks electrons.

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Fuel Cell Theory

• A fuel cell consists of two electrodes - Anode
  and Cathode.
• Hydrogen and Oxygen are fed into the cell.
• Catalyst at Anode causes hydrogen atoms to give
  up electrons leaving positively charged protons.
• Oxygen ions at Cathode side attract the hydrogen
  protons.

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Cont..

• Protons pass through electrolyte membrane.
• Electrons are redirected to Cathode through
  external circuit.
• Thus producing the current - power

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Fuel cell working
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Graphic showing working of Fuel Cell

http//americanhistory.si.edu/fuelcells/basics.htm
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The Chemistry of a Fuel cell

• Pressurized hydrogen gas (H2), enters the fuel
  cell on the anode side
• Oxygen gas (O2) is forced through the catalyst
  on the Cathode side
• This reaction in a single fuel cell produces
  about 0.7 volts

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Working Diagram Of Fuel Cell
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Types of fuel cells

• Temp.C Application
• Alkaline (AFC) 70-90 Space
• Phosphoric Acid 150-210 Commercially
  available
• (PAFC)
• Solid Polymer 70-90 Automotive application
• (PEMFC)
• Moltan Carbonate 550-650 Power generation
• (MCFC)
• Solid Oxide 1000-1100 Power generation
• (SOFC)
• Direct Methanol 70-90 Under development
• (DMFC)

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Alkaline Fuel Cell

• Used in spacecraft to provide drinking water and
  electricity
• Electrolyte Aqueous solution of alkaline
  potassium Hydroxide
• Output of 300w -5KW
• Power generation efficiency of about 70
• Too expensive for commercial applications

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Phosphoric Acid Fuel cell

• Used in hospitals, nursing homes and for all
  commercial purposes
• Electrolyte Liquid Phosphoric acid
• Catalyst platinum
• Electrical efficiency of 40
• Advantages using impure hydrogen as fuel and 85
  of the steam can be used for cogeneration

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Contd

• Disadvantages uses expensive platinum as
  catalyst
• Large size and weight
• Low power and current
• Existing PAFCs have outputs of 200kw and 1Mw are
  being tested

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Proton Exchange Membrane Cells

• Also called as Solid Polymers and used for quick
  startup in automobiles, light duty vehicles and
  potentially to replace rechargeable batteries
• Electrolyte Solid organic polymer
  poly-perflourosulfonic acid.
• Catalyst Metals (usually platinum) coated on
  both sides of membrane act as catalyst
• Advantages Use of solid electrolyte reduces
  corrosion and management problems

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Contd..

• Disadvantages Sensitive to fuel impurities
• Cell outputs generally range from 50 to 250 kW.

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Molten Carbonate Fuel cell

• Majorly used for electric utility applications
• Electrolyte Liquid solution of lithium, sodium
  and/or potassium carbonates.
• Catalyst Inexpensive metals can be used as
  catalyst other than Platinum
• Advantages High operating temperature allow for
  inexpensive catalysts

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Contd..

• Higher efficiency and flexibility to use more
  type of fuels like carbon monoxide, propane,
  marine gas due to high temperatures
• Disadvantage Higher temperature enhances
  corrosion and breakage of cell components
• High fuel to electricity generation of about 60
  or 85 with cogeneration.
• 10 kws -1 mws MCFCS have been tested

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Solid Oxide Fuel Cell

• Highly promising fuel cell
• Used in big, high-power applications including
  industrial and large-scale central electricity
  generating stations
• Some developers also see SOFC use in motor
  vehicles
• Power generating efficiencies could reach 60 and
  85

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Cont..

• Two Variations
• One type of SOFC uses an array of meter-long
  tubes, and other variations include a compressed
  disc that resembles the top of a soup can
• Closer to commercialization
• Demonstrations of tubular SOFC technology have
  produced as much as 220 kW

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Direct Methanol Fuel Cells

• Similar to the PEM cells in that they both use a
  polymer membrane as the electrolyte
• The anode catalyst itself draws the hydrogen from
  the liquid methanol, eliminating the need for a
  fuel reformer.
• Efficiency of about 40
• typically operate at a temperature between
  120-190 degrees F

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Cont..

• Relatively low range
• Attractive for tiny to mid-sized applications, to
  power cellular phones and laptops
• Higher efficiencies are achieved at higher
  temperatures
• Major problem Fuel crossing over from the anode
  to the cathode without producing electricity.