Introduction
What is Fuel Cell?
You can think of a fuel cell as a “factory” that takes fuel as input and produces electricity
as output. Like a factory, a fuel cell will continue to churn out product (electricity) as
long as raw material (fuel) is supplied.
O2
H2O
Fuel Cell
e-
H2
This is the key different between a fuel cell and a battery a fuel cell is not consumed
when it produces electricity. A fuel cell transforms the chemical energy stored in a fuel
into electrical energy.
History
1800s
In 1838, Christian Friedrich Schönbein (October 18, 1799 – August 29, 1868) was a
German-Swiss chemist who discovered the principle behind the fuel cell. He gained 40
pound from British associated for the advancement of science for a fuel cell research
that is the first official supported fuel cell research project.
Sir William Robert Grove (11 July 1811 – 1 August 1896) was a British lawyer, judge
and physical scientist who made the first demonstration of a fuel cell in 1839.
1
In Figure (a), water is being electrolysed into hydrogen and oxygen by passing an electric
current through it.
In Figure (b), the power supply has been replaced with an ammeter, and a small current
is flow. The electrolysis is reversed – the hydrogen and oxygen are recombining, and
an electric current is being produced.
In 1889, Ludwig Mond and his assistant Charles Langer were the first to coin the term
“fuel cells”. They built a fuel cell running on air and industrial coal gas also called Mond
gas.
2
Ludwig Mond
Mond and Langer understood Grove’s need for notable surface of action and made
significant changes in the experimental design of fuel cells.
1. Firstly, they used a porous non-conducting diaphragm and impregnated it with dilute
sulphuric acid, that is, the electrolyte. 2. Next on each side of the diaphragm they placed perforated leaves of platinum coated
with a thim film of platinum black. The powdered platinum black served as a catalyst
in promoting the reactivity of the fuel cell. At small intervals the platinum leaf was
brought in contact with strips of lead. In this manner Mond and Langer reduced the I2R internal losses.
3. Finally the diaphragms so prepared were “placed side by side or one above the other,
with non-conducting frames of pasteboard, wood, india rubber, etc, intervening, so
as to form chambers through which the gases to be employed (generally hydrogen
and air) are passed, so that one side of diaphragm is exposed to one gas and other to
the other gas, and the spaces between the diaphragms are connected that these gases
pass in contact with a number of diaphragms.”
It is easily understood that the modern fuel cells are very similar to the Mond and
Langer-H2/O2 cell. With this cell they obtained a current density of 6 A/ft2 at a voltage
of 0.73 V. But the cell voltage was found to decrease by 10% per hour of operation and
the electrolyte was unable to demonstrate satisfactory invariance.
3
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•
In 1902, J.H. Reid developed the concept of Alkaline Fuel Cell (AFC), and
obtained the patent by the concept.
•
In 1923, A. Schmind ADO7\XBH( •
In 1932, G. W. Heise FCZ/"T=MG
1950s
Francis Tomas (Tom) Bacon was an English engineer who developed the first
practical hydrogen-oxygen fuel cell.
• He initially experimented with Grove's use of activated platinum gauze with a
sulphuric acid electrolyte ,WR>QMKS, but quickly moved on
to use activated nickel electrodes with an aqueous potassium hydroxide
electrolyte ;LMKS.
• In January 1940, he moved to a laboratory at King's College London and there
developed a double cell, with one unit for generating the hydrogen and
oxygen gases and the other for the fuel cell proper. This could be reversed so
that it acted as both an electrolyser and a fuel cell. Problems were encountered
due to the high operating temperatures and pressures and the corrosive nature of
the chemicals.
• In 1946, under new funding arrangements, the work moved to the Department
of Colloid Science at Cambridge University. There Bacon's team were shown a
sample of porous nickel sheet whose origins were so obscure they were
protected by the Official Secrets Act. They used this sheet to develop electrodes
with large pores on the gas side and finer ones on the electrolyte side, which
created a much more stable interface than had existed previously.
• As funding levels increased the apparatus was moved again to the Department of
Chemical Engineering. There the team overcame problems of corrosion of the
oxygen electrode by soaking the new nickel electrodes in lithium hydroxide
solution followed by drying and heating.
• In 1959, with support from Marshall of Cambridge Ltd. (later Marshall
Aerospace) a 5 kW forty-cell battery, with an operating efficiency of 60%,
was demonstrated publicly.
• Later in 1959, Bacon and his colleagues demonstrated a practical five-kilowatt
unit capable of powering a welding machine.
4
In 1959, a team led by Harry Ihrig built a 15 kW fuel cell tractor for AllisChalmers which was demonstrated across the US at state fairs. This system used
potassium hydroxide as the electrolyte and compressed hydrogen and oxygen as the
reactants.
Modern History (1960s)
Willard Thomas Grubb, while working for General Electric (GE) in Schenectady, New
York as a chemist, made a major advance in fuel cells, originally designed by Sir William
Grove in 1839. Grubb developed the sulphonated polystyrene ion-exchange
membrane in 1955; it is now referred to as the Proton Exchange Membrane, or PEM
fuel cell. Three years later, another GE chemist, Leonard Niedrach, devised a way of
depositing platinum onto this membrane. The improvements made through the
combined efforts of Grubb and Niedrach ultimately produced the "Grubb-Niedrach fuel
cell".
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5
In 1965, The patents for the fuel cell were licensed by Pratt and Whitney as part of a
successful bid to provide electrical power for Project Apollo. The fuel cells were ideal in
this regard because they have rising efficiency with decreasing load (unlike heat
engines), hydrogen and oxygen gases were already on board the ship for
propulsion and life support and the by-product water could be used for drinking
and humidifying the atmosphere of the capsule.
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The Fuel Cell Structure
From Grove’s cell, the fuel cell is to say that the hydrogen fuel is being “BURNT” or
combusted in the simple reaction:
2H2 + O2 2H2O
However, instead of the heat energy being released, electrical energy is produced.
8
Even Grove developed the first cell, but the cell produced very small amount of current.
The main reasons for the small current are,
•
•
The contact area between the gas, the electrode, and the electrolyte is small.
The distance between the electrodes is large. The electrolyte resists the flow of
electric flow.
To overcome the problems, the electrodes are usually made flat, with a thin layer of
electrolyte as below figure.
Basic cathode-electrolyte-anode construction of a fuel cell
A typical fuel cell should includes several components as following:
• Membrane
• Catalytic layer
• Diffusion layer
• Flow channel
• Current corrector
9
Basic Concept
10
Proton Exchange Membrane Fuel Cell (PEMFC)
At the anode of PEMFC, the hydrogen releases electrons and H+ ions.
2H2 4H+ + 4eAt the cathode of PEMFC, oxygen reacts with electrons (from electrode), and H+ from
the electrolyte, to form water.
4H+ + 4e- + O2 2H2O
Electrons,
The electron produces at anode must pass through an electrical circuit to the cathode.
Protons,
H+ ions must pass through the electrolyte. An acid is a fluid with free H+ ions, and so
serves this purpose very well. Certain polymers can also be made to contain mobile H+
ions. These materials are called proton exchange membranes.
Alkaline Electrolyte Fuel Cell (AFC)
In an alkali, hydroxyl (OH-; ) ions are available and mobile.
At anode, the OH- ions react with hydrogen, releasing energy and electrons, and produce
water.
2H2 + 4OH- 4H2O + 4eAt cathode, oxygen reacts with electrons taken from electrode, and water in the
electrolyte, forming new OH- ions.
O2 + 4e- + 2H2O 4OH•
•
•
•
The OH- ion at cathode must able to pass through electrolyte.
There must be an electrical circuit for electrons to go from the anode to cathode.
Twice as much hydrogen is needed as oxygen.
The water is created twice as fast at the anode.
11
Electrode reactions and charge flow for an alkaline electrolyte fuel cell. Electrons flow
from anode to cathode, but conventional positive current flows from cathode to anode.
Positive Cathodes and Negative Anodes
Looking at the example shown as above, the electrons are flowing from the anode to the
cathode. The cathode is thus the electrically positive terminal, since electrons flow
from – to +.
From Concise Oxford English Dictionary defines cathode as:
• The negative electrode in an electrolyte cell or electron valve or tube.
• The positive terminal of primary cell as a battery.
Having two such opposite definitions is bound to cause confusion.
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What Limits the Current?
As one can see, the fuel cell produces electric current by hydrogen reacts at the anode,
and then releasing energy. However, it is not mean the reaction can be occurred without
limitation.
The “activation energy” must be supplied to get over the “energy hill” to make the
reaction happen.
The activation energy is the height of the potential barrier (sometimes called the energy
barrier) separating two minima of potential energy (of the reactants and of the products
12
of reaction). For chemical reaction to have noticeable rate, there should be noticeable
number of molecules with the energy equal or greater than the activation energy.
Classical energy diagram for a simple exothermic chemical reaction
If the probability of a molecule having enough energy is low, then the reaction will only
proceed slowly.
With very high temperature, the enough activation energy can be obtained! However,
this is not the case for the fuel cell reaction.
There are three ways to deal with the slow reaction rate:
• the use of catalysts.
• raising the temperature.
• increasing the electrode area.
Increasing the electrode area,
From the reaction of
2H2 + 4OH- 4H2O + 4e-
The fuel gas and OH- ions from electrolyte are needed to take a reaction at anode as well
as the necessary activation energy. The “coming together” of H2 and OH- ions must take
place on the surface of the electrode, as the electrons produced must be removed. The
reaction, involving fuel or oxygen (usually a gas), with the electrolyte (solid or liguid) and
the electrode, is sometimes called the three phases contact. The bringing together of
these three things is a very important issue in fuel cell design.
Clearly, the reaction rate will be proportional to the area of the electrode. That’s why we
define the performance of a fuel cell design is often quoted in terms of the current per
cm2.
What’s the REAL electrode area?
• It is not the straightforward area (Length width).
• The electrode is highly porous and greatly increasing the effective surface area.
• The porous electrode can be hundreds or thousands of times their
straightforward area.
13
TEM image of fuel cell catalyst. The black specks are the catalyst particles finely divided
over a carbon support. The structure clearly has a large surface area.
14
Fuel Cell Stack
The voltage of a single fuel cell is quite small, about 0.7V when drawing a useful current.
This means that to produce the useful voltage many cells have to be connected in series.
Such a collection of fuel cells in series is known as a “stack”. The most obvious way to
do this is by simply to connecting the edge of each anode to the cathode of the next cell,
all along the line.
Simple edge connection of three cells in series
15
Fuel Cell Type
Many different types of fuel cells are tried, since two fundamental technical problems:
• The slow reaction rate is leading to the low current and power.
• Hydrogen is not a readily available fuel.
Data for different types of fuel cell
* CHP systems: combined heat and power system
80 – 200 oC
160 – 220 oC 500 – 1000 oC Figures Used to Compare Systems
To compare fuel cells with each other, and with other electric power generators, certain
standard key figures are used.
Current density (current per unit area)
Unit: mA/cm2 or A/ft2 (1.0 mA/cm2 = 0.8 A/ft2)
Specific operating voltage
Unit: V
Power per unit area
Unit: mW/cm2
Power density (= Power / Volume)
Unit: kW/m3
Specific power (= Power / Mass)
Unit: W/kg
1.1 Advantage and Disadvantage
Advantage
16
•
Efficiency: More efficient than combustion engines (since produces electricity
directly from chemical energy).
•
Simplicity and Silent: No moving parts (no friction loss and silent), this yields the
potential for highly reliable and long-lasting system.
Low emissions: Virtually no undesirable products such as NOx, SOx, and
particulate emissions.
•
Disadvantage
•
•
•
•
•
High cost
Fuel storage
Operational temperature compatibility
Susceptibility to environmental poison
Durability under start-stop cycling
Applications
17
Chart to summarize the application and main advantages of fuel cells of different types,
and in different applications.
Chart to summarize the applications and main advantages of fuel cells of different types,
and in different applications
18