Transcript Fuel cells - I.T. at The University of Toledo

Fuel cells
Fuel cell history
 First demonstrated in principle by British Scientist
Sir Willliam Robert Grove in 1839.
 Grove’s invention was based on idea of reverse
electrolysis.
What is a fuel cell
 Creates electricity through
electrochemical process
 Operates like a battery
 Emits heat and water only
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
Working of Battery
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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.
Reactions inside Zinc/carbon battery
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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."
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
Parts of fuel cells
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There are 4 main parts
Anode
Cathode
Catalyst
Proton exchange membrane
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
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.
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.
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.
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
electrons leaving positively charged protons.
to give up
 Oxygen ions at Cathode side attract the hydrogen protons.
Cont…..
 Protons pass through electrolyte membrane.
 Electrons are redirected to Cathode through external
circuit.
 Thus producing the current - power
Fuel cell working
Graphic showing working of Fuel Cell
http://americanhistory.si.edu/fuelcells/basics.htm
The Chemistry of a Fuel cell
Anode side:
2H2 => 4H+ + 4e-
Cathode side:
O2 + 4H+ + 4e- => 2H2O
Net reaction:
2H2 + O2 => 2H2O
 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
Working Diagram Of Fuel Cell
Figure 3
Types of fuel cells
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Alkaline (AFC)
Phosphoric Acid
(PAFC)
Solid Polymer
(PEMFC)
Moltan Carbonate
(MCFC)
Solid Oxide
(SOFC)
Direct Methanol
(DMFC)
Temp.°C
Application
70-90
150-210
Space
Commercially available
70-90
Automotive application
550-650
Power generation
1000-1100
Power generation
70-90
Under development
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
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
Contd …
 Disadvantages: uses expensive
platinum as catalyst
 Large size and weight
 Low power and current
 Existing PAFC’s have outputs of
200kw and 1Mw are being tested
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 polyperflourosulfonic acid.
 Catalyst: Metals (usually platinum) coated
on both sides of membrane act as catalyst
 Advantages: Use of solid electrolyte
reduces corrosion and management
problems
Contd..
 Disadvantages: Sensitive to fuel
impurities
 Cell outputs generally range from 50
to 250 kW.
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
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 kw’s -1 mw’s MCFCS have been tested
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%
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
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
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.