FUEL
CELLS
KNOCKHARDY PUBLISHING
2008
SPECIFICATIONS
KNOCKHARDY PUBLISHING
FUEL CELLS
INTRODUCTION
This Powerpoint show is one of several produced to help students
understand selected topics at AS and A2 level Chemistry. It is based on the
requirements of the AQA and OCR specifications but is suitable for other
examination boards.
Individual students may use the material at home for revision purposes or it
may be used for classroom teaching with an interactive white board.
Accompanying notes on this, and the full range of AS and A2 topics, are
available from the KNOCKHARDY SCIENCE WEBSITE at...
www.knockhardy.org.uk/sci.htm
Navigation is achieved by...
either
clicking on the grey arrows at the foot of each page
or
using the left and right arrow keys on the keyboard
FUEL CELLS
CONTENTS
• Introduction
• The Proton Exchange Membrane Fuel Cell (PEMFC)
• PEMFC Cell – theory and electrode reactions
• Why use fuel cells?
• Sources of hydrogen
• Fuel cell vehicles
• Fuel cells - Advantages
• Fuel cells – Disdavantages
• The future of fuel cells
• Other types of fuel cell
FUEL CELLS
INTRODUCTION
Fuel cells generate electricity from an electrochemical reaction in which
oxygen (from air) and a fuel (e.g. hydrogen) combine to form water.
The electricity produced can be used to power cars, buses, laptops and
mobile phones. The by-product, heat, can also be used.
FUEL CELLS
INTRODUCTION
Fuel cells generate electricity from an electrochemical reaction in which
oxygen (from air) and a fuel (e.g. hydrogen) combine to form water.
The electricity produced can be used to power cars, buses, laptops and
mobile phones. The by-product, heat, can also be used.
STRUCTURE
• cells consist of two electrodes, a negative anode and a positive cathode
•
•
•
•
electrodes are separated by a solid or liquid electrolyte
electrically charged particles move between the two electrodes
catalysts (e.g. Pt) are often used to speed up reactions at the electrodes
electricity is generated when oxygen and hydrogen combine
A TYPICAL FUEL CELL
The Proton Exchange Membrane Fuel Cell - PEMFC
Operation
• hydrogen (the fuel) is oxidised to H+ ions (protons) at the anode
• protons move through the electrolyte
• electrons pass through the external circuit
• oxygen is reduced at the cathode
• water is produced
• a platinum catalyst accelerates the reactions at the electrodes
PROTON EXCHANGE MEMBRANE FUEL CELL



M
E
M
B
R
A
N
E
A
N
O
D
E

C
A
T
H
T
O
D
E



CLICK ON A NUMBER FOR AN EXPLANATION OF WHAT IS TAKING PLACE
PROTON EXCHANGE MEMBRANE FUEL CELL



M
E
M
B
R
A
N
E
A
N
O
D
E


C
A
T
H
T
O
D
E



HYDROGEN GAS ENTERS THE CELL
CLICK ON A NUMBER FOR AN EXPLANATION OF WHAT IS TAKING PLACE
PROTON EXCHANGE MEMBRANE FUEL CELL



M
E
M
B
R
A
N
E
A
N
O
D
E


C
A
T
H
T
O
D
E



HYDROGEN IS OXIDISED TO H+ AT THE ANODE
2H2(g) —> 4H+(aq) + 4e¯
CLICK ON A NUMBER FOR AN EXPLANATION OF WHAT IS TAKING PLACE
PROTON EXCHANGE MEMBRANE FUEL CELL



M
E
M
B
R
A
N
E
A
N
O
D
E


C
A
T
H
T
O
D
E



ELECTRONS TRAVEL VIA THE EXTERNAL CIRCUIT
CLICK ON A NUMBER FOR AN EXPLANATION OF WHAT IS TAKING PLACE
PROTON EXCHANGE MEMBRANE FUEL CELL



M
E
M
B
R
A
N
E
A
N
O
D
E


C
A
T
H
T
O
D
E



H+ IONS TRAVEL THROUGH THE ELECTROLYTE AND MEMBRANE
CLICK ON A NUMBER FOR AN EXPLANATION OF WHAT IS TAKING PLACE
PROTON EXCHANGE MEMBRANE FUEL CELL



M
E
M
B
R
A
N
E
A
N
O
D
E


C
A
T
H
T
O
D
E



OXYGEN GAS ENTERS THE CELL
CLICK ON A NUMBER FOR AN EXPLANATION OF WHAT IS TAKING PLACE
PROTON EXCHANGE MEMBRANE FUEL CELL



M
E
M
B
R
A
N
E
A
N
O
D
E


C
A
T
H
T
O
D
E



OXYGEN IS REDUCED AT THE CATHODE
O2(g) + 4H+(aq) + 4e¯ —> 2H2O(l)
CLICK ON A NUMBER FOR AN EXPLANATION OF WHAT IS TAKING PLACE
PROTON EXCHANGE MEMBRANE FUEL CELL



M
E
M
B
R
A
N
E
A
N
O
D
E


C
A
T
H
T
O
D
E



A CATALYST SPEEDS UP THE REACTION BETWEEN OXYGEN AND H+
CLICK ON A NUMBER FOR AN EXPLANATION OF WHAT IS TAKING PLACE
PROTON EXCHANGE MEMBRANE FUEL CELL



M
E
M
B
R
A
N
E
A
N
O
D
E


WATER IS PRODUCED
C
A
T
H
T
O
D
E



2H2(g) + O2(g) —> 2H2O(l)
CLICK ON A NUMBER FOR AN EXPLANATION OF WHAT IS TAKING PLACE
THEORY
The relevant half reactions are…
O2(g) + 4H+(aq) + 4e¯
2H+(aq) + 2e¯
2H2O(l)
H2(g)
E° = +1.23V
E° = 0.00V
THEORY
The relevant half reactions are…
O2(g) + 4H+(aq) + 4e¯
2H+(aq) + 2e¯
2H2O(l)
H2(g)
E° = +1.23V
E° = 0.00V
The equation with the MORE POSITIVE E° value reverses the one with the
less positive E° value.
O2(g) + 4H+(aq) + 4e¯
H2(g)
2H2O(l)
2H+(aq) + 2e¯
THEORY
The relevant half reactions are…
O2(g) + 4H+(aq) + 4e¯
2H+(aq) + 2e¯
2H2O(l)
H2(g)
E° = +1.23V
E° = 0.00V
The equation with the MORE POSITIVE E° value reverses the one with the
less positive E° value.
O2(g) + 4H+(aq) + 4e¯
H2(g)
2H2O(l)
2H+(aq) + 2e¯
Double the second equation to balance the electrons then combine both to
give the overall reaction…
THEORY
The relevant half reactions are…
O2(g) + 4H+(aq) + 4e¯
2H+(aq) + 2e¯
2H2O(l)
H2(g)
E° = +1.23V
E° = 0.00V
The equation with the MORE POSITIVE E° value reverses the one with the
less positive E° value.
O2(g) + 4H+(aq) + 4e¯
H2(g)
2H2O(l)
2H+(aq) + 2e¯
Double the second equation to balance the electrons then combine both to
give the overall reaction…
2H2(g) + O2(g) —> 2H2O(l)
E° = +1.23V
THEORY
The relevant half reactions are…
O2(g) + 4H+(aq) + 4e¯
2H+(aq) + 2e¯
2H2O(l)
H2(g)
E° = +1.23V
E° = 0.00V
The equation with the MORE POSITIVE E° value reverses the one with the
less positive E° value.
O2(g) + 4H+(aq) + 4e¯
H2(g)
2H2O(l)
2H+(aq) + 2e¯
Double the second equation to balance the electrons then combine both to
give the overall reaction…
2H2(g) + O2(g) —> 2H2O(l)
E° = +1.23V
ELECTRODE REACTIONS
Anode (-)
2H2(g) —> 4H+(aq) + 4e¯
OXIDATION
Cathode (+)
O2(g) + 4H+(aq) + 4e¯ —> 2H2O(l)
REDUCTION
THEORY
The relevant half reactions are…
O2(g) + 4H+(aq) + 4e¯
2H+(aq) + 2e¯
2H2O(l)
H2(g)
E° = +1.23V
E° = 0.00V
The equation with the MORE POSITIVE E° value reverses the one with the
less positive E° value.
O2(g) + 4H+(aq) + 4e¯
H2(g)
2H2O(l)
2H+(aq) + 2e¯
Double the second equation to balance the electrons then combine both to
give the overall reaction…
2H2(g) + O2(g) —> 2H2O(l)
E° = +1.23V
ELECTRODE REACTIONS
Anode (-)
2H2(g) —> 4H+(aq) + 4e¯
OXIDATION
Cathode (+)
O2(g) + 4H+(aq) + 4e¯ —> 2H2O(l)
REDUCTION
CELL REACTIONS - SUMMARY
Anode (-)
OXIDATION
2H2(g) —> 4H+(aq) + 4e¯
E° = 0.00V
Cathode (+)
REDUCTION
O2(g) + 4H+(aq) + 4e¯ —> 2H2O(l)
E° = +1.23V
overall reaction
2H2(g) + O2(g) —> 2H2O(l)
E° = +1.23V
CELL REACTIONS - SUMMARY
Anode (-)
OXIDATION
2H2(g) —> 4H+(aq) + 4e¯
E° = 0.00V
Cathode (+)
REDUCTION
O2(g) + 4H+(aq) + 4e¯ —> 2H2O(l)
E° = +1.23V
overall reaction
Electrolyte
2H2(g) + O2(g) —> 2H2O(l)
E° = +1.23V
• Carries charged particles from one electrode to the other
• It must allow only the appropriate ions to pass between
the electrodes
• If other substances travel through the electrolyte,
they can disrupt the chemical reaction.
FUEL CELLS – Why use them?
•
•
•
•
•
•
our society is dependent upon fossil fuels such as coal, oil and gas
fossil fuels are a non-renewable energy resource
fuel prices are rising and resources dwindling
food, transport and electricity costs are affected by fuel prices
the atmosphere is becoming more and more polluted
carbon dioxide contributes to climate change and the greenhouse effect
FUEL CELLS – Why use them?
•
•
•
•
•
•
our society is dependent upon fossil fuels such as coal, oil and gas
fossil fuels are a non-renewable energy resource
fuel prices are rising and resources dwindling
food, transport and electricity costs are affected by fuel prices
the atmosphere is becoming more and more polluted
carbon dioxide contributes to climate change and the greenhouse effect
Limitations
• storage of hydrogen - safety considerations
• transportation of hydrogen - low density so expensive to deliver
• feasibility of liquefied hydrogen under pressure - safety considerations
• limited life of adsorber / absorber - economic considerations
• limited life cycle of cell - economic considerations
• high production costs - economic considerations
• use toxic chemicals in cell production - environmental considerations
FUEL CELLS – Manufacture of hydrogen
METHODS
• ideally from non-polluting and renewable resources; (solar, wind, hydro)
• from hydrocarbon fuels by reforming
• from natural gas (methane) or ethanol
CH4 + H2O —> CO + 3H2
• electrolysis of water
FUEL CELLS – Manufacture of hydrogen
METHODS
• ideally from non-polluting and renewable resources; (solar, wind, hydro)
• from hydrocarbon fuels by reforming
• from natural gas (methane) or ethanol
CH4 + H2O —> CO + 3H2
• electrolysis of water
REFORMING
Most of today’s hydrogen is generated by steam reforming. Unfortunately
it uses non-sustainable, natural resources.
Fuel is mixed with steam in the presence of a metal catalyst to produce
hydrogen and carbon monoxide.
This method is cost effective and efficient with conversion rates of 70-80%.
FUEL CELLS – Manufacture of hydrogen
METHODS
• ideally from non-polluting and renewable resources; (solar, wind, hydro)
• from hydrocarbon fuels by reforming
• from natural gas (methane) or ethanol
CH4 + H2O —> CO + 3H2
• electrolysis of water
REFORMING
Most of today’s hydrogen is generated by steam reforming. Unfortunately
it uses non-sustainable, natural resources.
Fuel is mixed with steam in the presence of a metal catalyst to produce
hydrogen and carbon monoxide.
This method is cost effective and efficient with conversion rates of 70-80%.
STORAGE OF HYDROGEN
• liquid stored under pressure
• adsorbed on the surface of a solid
• absorbed within a solid
or
or
FUEL CELLS – Manufacture of hydrogen
METHODS
• ideally from non-polluting and renewable resources; (solar, wind, hydro)
• from hydrocarbon fuels by reforming
• from natural gas (methane) or ethanol
CH4 + H2O —> CO + 3H2
• electrolysis of water
REFORMING
Most of today’s hydrogen is generated by steam reforming. Unfortunately
it uses non-sustainable, natural resources.
Fuel is mixed with steam in the presence of a metal catalyst to produce
hydrogen and carbon monoxide.
This method is cost effective and efficient with conversion rates of 70-80%.
STORAGE OF HYDROGEN
• liquid stored under pressure
• adsorbed on the surface of a solid
• absorbed within a solid
or
or
FUEL CELL VEHICLES (FCV’S) - ADVANTAGES
• produce less pollution from exhaust gases
(no NOx, CO, unburnt hydrocarbons)
• produce less CO2
• are more efficient
FUEL CELLS - Advantages
• eliminates pollution caused by burning fossil fuels; the only by-product
is water
• eliminates greenhouse gases if the hydrogen used comes from
electrolysis of water
• eliminates economic dependence on politically unstable countries for
fossil fuel
• have a higher efficiency than diesel or gas engines
• most operate silently compared to internal combustion engines
• some have low heat transmission - ideal for military applications
• operating times are much longer than with batteries
• maintenance is simple since there are few moving parts in the system
FUEL CELLS - Disadvantages
• production, transportation, distribution and storage of hydrogen is
difficult
• reforming is technically challenging and not environmentally friendly
• refuelling and starting times of fuel cell vehicles (FCV’s) are longer
• driving range of cars is shorter than in a traditional vehicles
• fuel cells are generally slightly bigger than comparable batteries or
engines
• currently expensive to produce, since most units are hand-made
• some use expensive materials
• the technology is not yet fully developed and few products are available
FUEL CELLS - The future
Limited supplies of fossil fuels may cause
us to move to a ‘hydrogen economy’.
However
• greater acceptance by the public and politicians is necessary
• handling and maintenance of hydrogen systems must be safe
• improvements to hydrogen manufacturing must be made
FUEL CELLS – Other types
Fuel cells are classified according to the nature of the electrolyte.
Alkaline Fuel Cells (AFC)
• uses an alkaline electrolyte such as potassium hydroxide
• used by NASA in space shuttles
Direct Methanol Fuel Cells (DMFC)
• uses a polymer membrane as an electrolyte
• a catalyst on the anode draws hydrogen from liquid methanol
• eliminates need for a fuel reformer, so pure methanol can be used as fuel
Molten Carbonate Fuel Cells (MCFC)
• uses a molten carbonate salt as the electrolyte.
• has the potential to be fuelled with coal-derived fuel gases, methane / natural gas
Phosphoric Acid Fuel Cells (PAFC)
• anode and a cathode made of a finely dispersed platinum catalyst on carbon
• has a silicon carbide structure that holds the phosphoric acid electrolyte
• used to power many commercial premises and large vehicles, such as buses
FUEL CELLS – Other types
Fuel cells are classified according to the nature of the electrolyte.
Proton Exchange Membrane Fuel Cells (PEMFC)
•
•
•
•
uses a polymeric membrane as the electrolyte, with platinum electrodes
operate at relatively low temperatures.
can vary their output to meet shifting power demands.
best for cars, for buildings and smaller applications
Solid Oxide Fuel Cells (SOFC)
•
•
•
•
•
use solid ceramic electrolytes; eg zirconium oxide stabilised with yttrium oxide
work at high temperatures
can reach efficiencies of around 60 per cent
are expected to be used for generating electricity and heat in industry
have potential for providing auxiliary power in vehicles
Regenerative Fuel Cells (RFC)
• produce electricity from hydrogen and oxygen
• can be reversed to produce hydrogen and oxygen; effectively storing energy
FUEL
CELLS
THE END
©2010 JONATHAN HOPTON & KNOCKHARDY PUBLISHING