Fuel Cells
6. Fuel Cells
Learning Goals
•
the common design features of fuel cells including the use of porous electrodes for gaseous reactants to increase cell efficiency (details of specific cells not required)
•
the comparison of the use of fuel cells and combustion of fuels to supply energy with reference to their energy efficiencies (qualitative), safety, fuel supply (including
the storage of hydrogen), production of greenhouse gases and applications
•
the comparison of fuel cells and galvanic cells with reference to their definitions, functions, design features, energy transformations, energy efficiencies (qualitative)
and applications.
DP 4.1
The common design features of fuel cells including
use of porous electrodes for gaseous reactants to
increase cell efficiency (details of specific cells not
required)
3
Fuel cells
• Definition: A type of galvanic cell which
requires a continuous supply of reactants to
convert chemical energy to electrical energy.
• Fuel cells were used in the Gemini and Apollo
space missions to produce electrical energy and
water. It is clean and very efficient but requires
pure hydrogen and oxygen.
Key Features of Fuel Cells
Fuel cells are a type of galvanic cell that is continuously supplied with reactants.
Key Features:
• Reactants are kept separate
(preventing direct redox
reaction)
• Inlets to continuously supply
reactants
• Outlets to continuously
remove waste product
• Electrolyte
• Porous Electrodes
Reductant
Fuel in
(Example
H2 gas)
Oxidant
in
(O2 gas)
6
Porous Electrodes
Porous Electrodes
• Allow reactants to diffuse through
and react with ions in electrolyte
• Allows movement of ions through to
electrolyte
• Increases SA:V ratio and therefore
increases the rate of reaction
• Embedded with a catalyst to further
increase the rate of reaction
7
Porous Electrodes
e- 
• Anode (-): site of oxidation.
• Cathode (+): site of reduction.
• Electrons travel from anode to cathode.
Anode (-)
Cathode (+)
Electrolyte
Analogous to salt bridge in a standard galvanic cell
• Completes circuit by allowing
movement of ions (NOT ELECTRONS).
• Cations move to cathode.
• Anions move to anode.
• Each fuel cell will have a different
electrolyte which will influence the
balancing of half-equations.
• Pay attention to whether electrolyte is
acidic (H+) or alkaline (OH-)
Common
Electrolytes:
H+ (aq)
OH- (aq)
O2- (s)
CO32- (l)
Continuous supply of reactants
Reductant
• Inlets to continuously
supply reactants.
• Outlets to
continuously remove
waste product.
Oxidant
Energy Efficiency
• Fuels cells are generally more energy efficient than a car
engine or coal fired power station
• Car engine
• Coal powered fuel stations
• Fuel cells
+ steam used for operating turbines
25-30%
30 - 40%
40-60%
85%
• Why?
• Less energy transformations!
• Fuel cells transform chemical energy directly to electrical energy.
Hydrogen Fuel Cells
Most common type of fuel cell
Overall Reaction is always the same:
2H2 (g) + O2 (g) → 2H2O (l)
HOWEVER…
Half-equations will change
depending on the electrolyte used.
Hydrogen gas always oxidised
Oxygen gas always reduced
Hydrogen Fuel Cell:
OH
electrolyte
The Clues are in the diagram: Take note of the arrows.
At the cathode (Reduction):
O2 (g) → OH- (aq)
O2 (g) + 2H2O (l) + 4e- → 4OH- (aq)
At the anode (Oxidation):
H2 (g) → H2O (l)
H2 (g) + 2OH- (aq) → 2H2O (l) + 2eEMF: 1.23 V
The equation for the overall reaction may be written as:
O2 (g) + 2H2O (l) +2H2 (g) + 4OH- → 4OH- (aq)+ 4 H2O (g)
2H2(g) + O2(g)  2H2O(l)
13
Hydrogen Fuel Cell: Molten carbonate
Electrolyte
Interpreting the diagram:
This must be the cathode (site of reduction).
Electrons are entering into the electrode
It is the site where the oxidising agent, O2 (g), is
being continuously supplied.
Cathode:
O2 (g) + CO2 (g)
 CO32- (l)
O2 (g) + 2CO2 (g)
 2CO32- (l)
O2 (g) + 2CO2 (g) + 4e-
 2CO32- (l)
*Molten has liquid state
Hydrogen Fuel Cell: Molten carbonate
Electrolyte
Interpreting the diagram:
This must be the anode (site of oxidation).
Electrons are exiting the electrode.
Reducing agent, H2 (g), is being continuously
supplied
Anode:
H2 (g) + CO32- (l)  CO2 (g) + H2O (g)
H2 (g) + CO32- (l)  CO2 (g) + H2O (g) + 2e-
Hydrogen Fuel Cell: Molten carbonate
Electrolyte
Interpreting the diagram:
Half equations at the electrodes:
Anode:
2 x ( H2 (g) + CO32- (l)  CO2 (g) + H2O (g) + 2e- )
2H2 (g) + 2CO32- (l)  2CO2 (g) + 2H2O (g) + 4eCathode:
O2 (g) + 2CO2 (g) + 4e-
 2CO32- (l)
Overall redox equation:
2H2 (g) + 2CO32- (l) + O2 (g) + 2CO2 (g) + 4e-  2CO2 (g) + 2H2O (g) + 4e- + 2CO32- (l)
2H2 (g) + O2 (g)  2H2O (g)
Let’s practice 
Hydrogen Fuel Cell: H+ Electrolyte
1.
Acidic or alkaline conditions?
Acidic
2.
What is the electrolyte?
H+ ions
3.
What is the oxidant?
O2 gas
4.
Write the half-equations and full equation for this
fuel cell (states not required):
a. Oxidation half equation at the anode:
H2  2H+ + 2eb. Reduction half equation at the cathode:
O2 + 4H+ + 4e-  2H2O
5.
Overall equation
2H2 + O2 + 4H+  4H+ + 2H2O
2H2 + O2  2H2O
More
Let’s practice 
- Electrolyte
Hydrogen
Fuel
Cell:
OH
practice:
1.
2.
3.
4.
Acidic or alkaline conditions?
What is the electrolyte?
What are the reactants?
Write the half-equations and full equation
for this fuel cell (states not required):
a. Oxidation half equation at the anode:
• H2 + 2OH-  2H2O + 2eb. Reduction half equation at the cathode:
• O2 + 2H2O + 4e-  4OH-
5. Overall equation
• 2H2 + 4OH- + O2 + 2H2O  4H2O + 4OH• 2H2 + O2  2H2O
Other Fuel Cells
More practice:
• Other fuel cells exist besides
hydrogen fuel cells
• Often these use fuels (reductants)
such as methane or methanol
• The fuels typically still react with
oxygen gas (oxidant)
Reductant
Fuel in
(e.g.CH4 or
CH3OH)
Oxidant
in
(O2 gas)
Other Fuel Cells
More practice:
When tackling a new fuel cell try to:
1. Identify the anode and cathode
(electrons go from anode to cathode)
2. Identify half-equations at each
electrode by looking at the arrows and
substances going to or from the
electrodes
• TO an electrode - reactants
• FROM an electrode - products
3. Write the half-equations using these
reactants and products then balance
using KOHES as normal
4. Overall redox reaction is the same as
combustion reactions
Reductant
Fuel in
(e.g.CH4 or
CH3OH)
Oxidant
in
(O2 gas)
Methanol Fuel Cell Example
Other fuel cells exist besides hydrogen fuel cells
Write the half-equations occurring at the:
Cathode:
O2 (g) → H2O (g)
4H+ (aq) + 4e- + O2 (g) → 2H2O (g)
Anode:
CH3OH (l) + H2O (g) → CO2 (g)
CH3OH (l) + H2O (g) → CO2 (g) + 6H+ (aq) + 6eOverall Equation
12H+ (aq) + 3O2 (g) + 2CH3OH (aq) + 2H2O (g)
→ 6H2O (g) + 2CO2 (g) + 12H+ (aq)
3O2 (g) + 2CH3OH (l) → 4H2O (g) + 2CO2 (g)
LOOKS JUST LIKE A COMBUSTION REACTION EQUATION!
REMEMBER: COMBUSTION IS A TYPE OF REDOX REACTION
All in the electrolyte
There are six main types of fuel cells, based on the electrolyte used (DO NOT memorise – you need to be able to interpret):
1. Combine the half equations for the fuel cells that are highlighted to show the overall redox equation.
2. What do you notice for your answer to question 1.
All in the electrolyte
There are six main types of fuel cells, based on the electrolyte used (DO NOT memorise – you need to be able to interpret):
Be careful though!!! Whilst it is useful to note that most hydrogen fuel cells have the overall equation 2H2 + O2  2H2O
Other fuel cells exist!!! Interpret the Q!!! Combine the half equations for the methanol fuel cell.
DP 4.2
The comparison of the use of fuel cells and combustion of
fuels to supply energy with reference to their energy
efficiencies (qualitative), safety, fuel supply (including the
storage of hydrogen), production of greenhouse gases and
applications
Comparison
Fuel Cells
Combustion Engine/Power
Station
Energy
Transformations
Chemical → Electrical
Chemical → Thermal →
Mechanical → Electrical
Energy Efficiency
More Efficient
Less Efficient
Cost
More Expensive
• Catalysts in porous electrodes are
expensive
• Also cost of storing H2 is expensive
Less Expensive
• Infrastructure is already
established
Availability of Fuel
If using H2 depends on source
Finite reserves
Environmental Impact H2O released from hydrogen cell can be
reused for H2 production
Can also produce GHGs depending on
fuel cell
Produces GHGs:
CO2 (g) & H2O (g)
26
Fuel Cell: Advantages and disadvantages
These are the main points to take
notice of.
Concerns: Storage of H2 (g)
Considerations:
• Controlling possible ignition sources (open flames or
something that could spark) since hydrogen is highly
flammable and explosive in the presence of a spark in air.
• Ensuring storage vessels are appropriate since hydrogen is
stored under pressure
• Regular maintenance of hydrogen containers, leak detector
since the gas is colourless.
• Following appropriate hazardous materials guidelines
(MSDS – Material Safety Data Sheet)
For hydrogen fuel cells, hydrogen gas must be
sourced and stored.
Hydrogen economy… the future
The hydrogen economy is a proposed system of
PROS
delivering energy for society using hydrogen as
•Water is the only product of a hydrogen fuel cell therefore no greenhouse gases (+ other pollutants )
the source of energy opposed to hydrocarbons.
•Water produced can be used for other purposes
•Can produce H2 from electrolysis of water
•Biogas methane can be converted to H2
•Waste heat can be used for other purposes
CONS
•Unless produced renewably, H2 production releases green-house
gases
•Steam reforming (process of producing H2 from fossil fuel methane)
forms hydrogen gas with lower energy content
•Hydrogen storage would require $$ to develop infrastructure (must be
stored under high pressures and is kept away from ignition sources)
•Hydrogen gas is dangerous to store as very flammable
DP 4.3
The comparison of fuel cells and galvanic cells with
reference to their definitions, functions, design features,
energy transformations, energy efficiencies (qualitative) and
applications.
Fuel Cells vs Primary Cells
Fuel Cell
Galvanic Cell/Primary Cell
Direct energy conversion: Chemical  Electrical
Direct energy conversion: Chemical  Electrical
Porous electrodes
Electrodes are either inert or part of the
conjugate redox pair (a product or reactant)
Ongoing supply of reactants (open system)
is used to continuously generate electricity
Fixed quantity of fuel within galvanic cell
(closed system) is used to generate a finite
amount of electricity
Normally uses gaseous fuels such as Hydrogen
or Methane gas.
Normally uses two conjugate redox pairs.
Reactants can be solid, aqueous ions or gases
Half cells separated in the same vessel
Half cells in separate vessels
32
Comparing Galvanic cells and Fuel cells
eOxidising
Oxidising
agent fuel
agent
fuel
continuously
continuously
supplied
supplied
here
here
(fuel gets
(fuel
gets
reduced)
reduced)
Reducing
Reducing
agent
fuel
agent fuel
continuousl
continuously
y supplied
supplied
here(fuel
here(fuel gets
oxidised)
outlet
Comparing Galvanic cells
and Fuel cells
eOxidising
Oxidising
agent fuel
agent
fuel
continuously
continuously
supplied
supplied
here
here
(fuel gets
(fuel
gets
reduced)
reduced)
Reducing
Reducing
agent
fuel
agent fuel
continuousl
continuously
y supplied
supplied
here(fuel
here(fuel gets
oxidised)
outlet
DEMO: Time to go outside!!
More practice:
Aim: To observe a hydrogen fuel cell generate electricity and to power a
motorised turbine:
Overall redox reaction:
2H2 + O2  2H2O
1.
Why is there double of one gas produced? Which gas is this?
2.
Write the oxidation states (note: always indicate + or -) for each of the
elements involved in the overall reaction.
3.
Identify the oxidising agent.
4.
Identify the what is undergoing oxidation.
5.
State one application of a fuel cell.
6.
State one advantage of this fuel cell and disadvantage.
Continue with revision WS booklet questions for remainder of session.
Past VCAA exam: 2015 Q10
Past VCAA exam: 2015 Q10
Multiple choice questions
Multiple choice questions
Multiple choice questions
All fuel cells…
A. are rechargeable and have electrodes that are separated.
B. are galvanic cells and the required reactants are stored in the cells.
C. are rechargeable and the reactants are stored externally and continually supplied.
D. convert chemical energy into electrical energy and the reactants are continually supplied
Practice questions
A galvanic cell is constructed from the following two half cells
under standard conditions.
Half cell 1: a nickel electrode in a solution of 1.0 M nickel nitrate
Half cell 2: a cadmium electrode in a solution of 1.0 M cadmium
nitrate A sketch of the cell is given.
a) Given that the standard reduction potential of Cd2+(aq)/ Cd(s)
is –0.40V, show the direction in which electrons will flow in
the external circuit of this galvanic cell.
b) Give the equation for the half reaction that takes place at the
anode of this cell.
c) List two factors that need to be considered when selecting an
appropriate substance for use in the salt bridge.
Practice questions
2NH4+ (aq) + 2MnO2 (aq) + Zn (s) → Mn2O3 (s) + Zn2+ (aq) + 2NH3 (aq) + H2O (l)
acidic environment
Find the half-equation for the oxidation and reduction equations.
Identify the reductant and oxidant.
Practice questions
A hydrogen-oxygen fuel cell can operate with an alkaline electrolyte such as
potassium hydroxide. In this cell, the reaction at the cathode is shown below
Write the half-equation for the reaction that occurs at the anode in a
hydrogen-oxygen cell with an alkaline electrolyte
Practice questions
A fuel cell that can provide power for buses is the
phosphoric acid fuel cell, PAFC. The electrolyte is
concentrated phosphoric acid and the reactants are
hydrogen and oxygen gases.
A simplified sketch of a phosphoric acid fuel cell is
given below.
a) Give the equation for the half reaction that takes
place at the
i. anode of this cell
ii. cathode of this cell.
b) On the diagram of the fuel cell above, draw an
arrow to show the direction in which the H2PO4–
ion moves as the cell delivers an electrical
current.
c) Describe one advantage and one disadvantage of
such a fuel cell compared with a petrol-driven car
engine.
Practice questions
Submarines operate both on the surface and underwater. When
operating underwater, the submarine acts as a closed system,
where there is no interaction with the atmosphere. Most types of
submarines use both batteries and diesel engines to provide their
energy requirements. A new type of submarine uses proton
exchange membrane (PEM) fuel cells and diesel engines. Below is
a diagram of a PEM fuel cell.
a) State the function of the electrolyte in a fuel cell.
b) Write the balanced overall redox reaction that occurs in this
PEM fuel cell.
c) Give two safety considerations for the safe storage of hydrogen,
H2, gas on a submarine.
d) State two advantages of using a PEM fuel cell compared to a
diesel engine when a submarine is underwater
e) Most submarines generate more H2 gas for their fuel cells when
travelling on the surface. Explain how the H2 gas could be
generated.
Practice questions
The lithium button cell, used to power watches and calculators, is a primary
cell containing lithium metal. The lithium ion cell is a secondary cell that is
used to power laptop computers.
a) What is the difference between a primary and secondary cell?
b) b. By referring to information provided in the Data Book, give one reason
why lithium is used as a reactant in these galvanic cells.
c) Some early lithium metal batteries exploded when exposed to water.
Write a balanced equation, including states, for the reaction between
lithium metal and water to explain why an explosion might occur.