How Fuel Cells Work

How Fuel Cells Work

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Description: Fuel Cell- an electrochemical energy conversion device:- To convert the chemicals hydrogen and oxygen into water, and in the process it produces electricity. Battery: the other electrochemical device that we are all familiar. 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.

 
Author: National Tsing Hua University  | Visits: 423 | Page Views: 796
Domain:  Green Tech Category: Battery & Fuel Cell 
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Contents:
How Fuel Cells Work
Fuel Cells (燃料電池):
Making power more efficiently
and with less pollution.

Fuel Cell
- an electrochemical energy
conversion device
 To convert the chemicals hydrogen and oxygen
into water, and in the process it produces
electricity.
 Battery (電池): the other electrochemical device
that we are all familiar.
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.

For 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 in use today use hydrogen
and oxygen as the chemicals.

Fuel Cell Descriptions


Fuel Cells generate electricity through an
electrochemical process

 In

which the energy stored in a fuel is
converted directly into DC electricity.



Because electrical energy is generated
without combusting fuel,

 Fuel

cells are extremely attractive from an
environmental stand point.

Attractive characteristics
of Fuel Cell


High energy conversion efficiency



Modular design



Very low chemical and acoustical pollution



Fuel flexibility



Cogeneration capability



Rapid load response

A functioning cell
in a Solid Oxide Fuel Cell stack



It consists of three components - a cathode, an anode,
and an electrolyte sandwiched between the two.



Oxygen from the air flows through the cathode



A fuel gas containing hydrogen, such as methane,
flows past the anode.

Negatively charged oxygen ions migrate through the
electrolyte membrane react with the hydrogen to form
water,


The reacts with
the methane fuel
to form hydrogen (H2) &

carbon dioxide (CO2).

This electrochemical reaction generates electrons, which
flow from the anode to an external load and back to the
cathode,
 a final step that both completes the circuit and supplies
electric power.
 To increase voltage output, several fuel cells are stacked
together to form the heart of a clean power generator.


Cool Fuel Cells
Fuel cells promise to be the environmentallyfriendly power source of the future,
but some types run too hot to be practical.
NASA-funded research may have a solution.




All fuel cells have the
same basic operating
principle.
An input fuel is catalytically reacted
(electrons removed from the fuel elements)
in the fuel cell to create an electric current.







Fuel cells consist of an electrolyte material which is sandwiched
in between two thin electrodes (porous anode and cathode).
The input fuel passes over the anode (and oxygen over the
cathode) where it catalytically splits into ions and electrons.
The electrons go through an external circuit to serve an electric
load while the ions move through the electrolyte toward the
oppositely charged electrode.
At the electrode, ions combine to create by-products, primarily
water and CO2. Depending on the input fuel and electrolyte,
different chemical reactions will occur.

Basic Configuration

PEMFC

Animation of PEMFC



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Applications of Fuel cells

Woking Park Fuel Cell CHP schematic



U

Four primary types
of fuel cells
They are based on the electrolyte employed:


Phosphoric Acid Fuel Cell



Molten Carbonate Fuel Cell



Solid Oxide Fuel Cell



Proton Exchange Membrane Fuel Cell

Phosphoric Acid Fuel Cells
-PAFCs









The most mature fuel cell technology in terms of system
development and commercialization activities.
Has been under development for more than 20 years
Has received a total worldwide investment in the
development and demonstration of the technology in
excess of $500 million.
The PAFC was selected for substantial development a
number of years ago because of the belief that, among the
low temperature fuel cells,
It was the only technology which showed relative tolerance
for reformed hydrocarbon fuels and thus could have
widespread applicability in the near term.

PAFC Design and Operation


The PAFC uses liquid phosphoric acid as the
electrolyte.



The phosphoric acid is contained in a Teflon bonded
silicone carbide matrix.



The small pore structure of this matrix preferentially
keeps the acid in place through capillary action.



Some acid may be entrained in the fuel or oxidant
streams and addition of acid may be required after
many hours of operation.



Platinum catalyzed, porous carbon electrodes are
used on both the fuel (anode) and oxidant (cathode)
sides of the electrolyte.












Fuel and oxidant gases are supplied to the backs of the porous
electrodes by parallel grooves formed into carbon or carboncomposite plates.
These plates are electrically conductive and conduct electrons
from an anode to the cathode of the adjacent cell.
In most designs, the plates are "bi-polar" in that they have
grooves on both sides - one side supplies fuel to the anode of
one cell, while the other side supplies air or oxygen to the
cathode of the adjacent cell.
The byproduct water is removed as steam on the cathode (air
or oxygen) side of each cell by flowing excess oxidant past the
backs of the electrodes.
This water removal procedure requires that the system be
operated at temperatures around 375oF (190oC).
At lower temperatures, the product water will dissolve in the
electrolyte and not be removed as steam. At approximately
410oF (210oC), the phosphoric acid begins to decompose.











The byproduct water is removed as steam on the cathode
(air or oxygen) side of each cell by flowing excess oxidant
past the backs of the electrodes.
This water removal procedure requires that the system be
operated at temperatures around 375oF (190oC).
At lower temperatures, the product water will dissolve in
the electrolyte and not be removed as steam. At
approximately 410oF (210oC), the phosphoric acid begins
to decompose.
Excess heat is removed from the fuel cell stack by
providing carbon plates containing cooling channels every
few cells.
Either air or a liquid coolant, such as water, can be passed
through these channels to remove excess heat.

Electrochemical reactions in
PAFC









At the anode:
Hydrogen is split into two hydrogen ions (H+), which
pass through the electrolyte to the cathode, and
two electrons which pass through the external circuit
(electric load) to the cathode.
At the cathode:
the hydrogen, electrons and oxygen combine to form
water.

Electrochemical reactions in PAFC









PAFC Performance
Characteristics
PAFC power plant designs show electrical efficiencies in
the range from 36% (HHV) to 42% (HHV).
The higher efficiency designs operate with pressurized
reactants.
The higher efficiency pressurized design requires more
components and likely higher cost.
PAFC power plants supply usable thermal energy at an
efficiency of 37% (HHV) to 41% (HHV).
A portion of the thermal energy can be supplied at
temperatures of ~ 250oF to ~ 300oF.
However, the majority of the thermal energy is supplied at
~150oF.
The PAFC has a power density of 160-175 watts/ft2 of
active cell area

Molten Carbonate Fuel Cells
- MCFC
 A molten carbonate salt mixture is used as its electrolyte.
 They evolved from work in the 1960's aimed at
producing a fuel cell which would operated directly on
coal.
 While direct operation on coal seems less likely today,

 The operation on coal-derived fuel gases or natural gas
is viable.

Molten Carbonate Salt
used as Electrolyte in MCFC


A molten carbonate salt mixture is used as its electrolyte.



The composition of the electrolyte (molten carbonate salt
mixture) varies, but usually consists of lithium carbonate
and potassium carbonate.



At the operating temperature of about 650oC (1200oF), the
salt mixture is liquid and a good ionic conductor.



The electrolyte is suspended in a porous, insulating and
chemically inert ceramic (LiAlO3) matrix.

Reactions
in MCFC
 The anode process involves
a reaction between hydrogen
and carbonate ions (CO3=)
from the electrolyte.
 The reaction produces water
and carbon dioxide (CO2)
while releasing electrons to
the anode.
 The cathode process combines
oxygen and CO2 from the oxidant
stream with electrons from the
cathode to produce carbonate ions
which enter the electrolyte.
 The need for CO2 in the oxidant
stream requires a system for
collecting CO2 from the anode
exhaust and mixing it with the
cathode feed stream.

Reactions
in MCFC

Description of reactions in MCFCs
 The anode process involves a reaction between hydrogen
and carbonate ions (CO3=) from the electrolyte.
 The reaction produces water and carbon dioxide (CO2)
while releasing electrons to the anode.

 The cathode process combines oxygen and CO2 from the
oxidant stream with electrons from the cathode to produce
carbonate ions which enter the electrolyte.
 The need for CO2 in the oxidant stream requires a system
for collecting CO2 from the anode exhaust and mixing it
with the cathode feed stream.

 As the operating temperature increases,
 the theoretical operating voltage for a fuel cell decreases and
with it the maximum theoretical fuel efficiency.
 On the other hand, increasing the operating temperature
increases the rate of the electrochemical reaction and
 Thus increases the current which can be obtained at a given
voltage.
 The net effect for the MCFC is that the real operating voltage is
higher than the operating voltage for the PAFC at the same
current density.
 The higher operating voltage of the MCFC means that more
power is available at a higher fuel efficiency from a MCFC than
from a PAFC of the same electrode area.
 As size and cost scale roughly with electrode area, this
suggests that a MCFC should be smaller and less expensive
than a "comparable" PAFC.



As size and cost scale roughly with electrode area, this
suggests that a MCFC should be smaller and less expensive
than a "comparable" PAFC.



The MCFC also produces excess heat at a temperature which
is high enough to yield high pressure steam which may be fed
to a turbine to generate additional electricity.



In combined cycle operation, electrical efficiencies in excess of
60% (HHV) have been suggested for mature MCFC systems.



The MCFC operates at between 1110°F (600°C) and 1200°F
(650°C) which is necessary to achieve sufficient conductivity of
the electrolyte.



To maintain this operating temperature, a higher volume of air
is passed through the cathode for cooling purposes.



As mentioned above, the high operating temperature of
the MCFC offers the possibility that it could operate
directly on gaseous hydrocarbon fuels such as natural gas.



The natural gas would be reformed to produce hydrogen
within the fuel cell itself.



The need for CO2 in the oxidant stream requires that CO2
from the spent anode gas be collected and mixed with the
incoming air stream.



Before this can be done, any residual hydrogen in the
spent fuel stream must be burned.



Future systems may incorporate membrane separators to
remove the hydrogen for recirculation back to the fuel
stream.



At cell operating temperatures of 650oC (1200oF) noble
metal catalysts are not required.



The anode is a highly porous sintered nickel powder,
alloyed with chromium to prevent agglomeration and creep
at operating temperatures.



The cathode is a porous nickel oxide material doped with
lithium.



Significant technology has been developed to provide
electrode structures which position the electrolyte with
respect to the electrodes and maintain that position while
allowing for some electrolyte boil-off during operation.



The electrolyte boil-off has an insignificant impact on cell
stack life.



A more significant factor of life expectancy has to do with
corrosion of the cathode.



The MCFC operating temperature is about 650oC (1200oF).



At this temperature the salt mixture is liquid and is a good
conductor.



The cell performance is sensitive to operating temperature.



A change in cell temperature from 650oC (1200oF) to
600oC (1110oF) results in a drop in cell voltage of almost
15%.



The reduction in cell voltage is due to increased ionic and
electrical resistance and a reduction in electrode kinetics.

Solid Oxide Fuel Cells






The Solid Oxide Fuel Cell (SOFC) uses a ceramic,
solid-phase electrolyte which reduces corrosion
considerations and eliminates the electrolyte
management problems associated with the liquid
electrolyte fuel cells.
To achieve adequate ionic conductivity in such a
ceramic, however, the system must operate at about
1000oC (1830oF).
At that temperature, internal reforming of
carbonaceous fuels should be possible, and the waste
heat from such a device would be easily utilized by
conventional thermal electricity generating plants to
yield excellent fuel efficiency.

The fuel cell will compete with many other types of energy
conversion devices, including
 the gas turbine in city's power plant,
 the gasoline engine in your car and
 the battery in your laptop.
 Combustion engines like the turbine and the gasoline engine
burn fuels and
 use the pressure created by the expansion of the gases to
do mechanical work.
 Batteries converted chemical energy back into electrical
energy when needed.
 Fuel cells should do both tasks more efficiently.
 A fuel cell provides a DC (direct current) voltage that can be
used to power motors, lights or any number of electrical
appliances.


Classification of Fuel Cells






There are several different types of fuel cells, each using a
different chemistry.
Fuel cells are usually classified by the type of electrolyte
they use.
Some types of fuel cells work well for use in stationary power
generation plants.



Others may be useful for small portable applications or for
powering cars.



The proton exchange membrane fuel cell (PEMFC) is one
of the most promising technologies.
This is the type of fuel cell that will end up powering cars,
buses and maybe even your house. Let's take a look at how
they work...



Tiny Fuel Cell to Power Sensors








A fuel cell prototype that is the size of a pencil eraser and can deliver small
amounts of electricity was developed at Case Western Reserve University
(CWRU).
The fuel cells are 5 mm3 in volume and generate 10 mW of power with
short pulses of up to 100 mW.
The cell power is so limited
There is no practical consumer use yet.
A cell phone, e.g., needs ~ 500 mW.
The first use will be in sensors for the military.

Microfuel cell










The prototype microfuel cell uses an electrochemical process to directly
convert energy from hydrogen into electricity.
The fuel cell works like a battery, using an anode and cathode, positive and
negative electrodes (solid electrical conductors), with an electrolyte.
The electrolyte can be made of various materials or solutions. The hydrogen
flows into the anode and the molecules are split into protons and electrons.
The protons flow through the electrolyte, while the electrons take a different
path, creating an electrical current.
At the other end of the fuel cell, oxygen is pulled in from the air and flows
into the cathode.
The hydrogen protons and electrons reunite in the cathode and chemically
bond with the oxygen atoms to form water molecules.
Theoretically, the only waste product produced by a fuel cell is water.
Fuel cells that extract hydrogen from natural gas or another hydrocarbon will
emit some carbon dioxide as a byproduct, but in much smaller amounts than
those produced by traditional energy sources.

PEMFC: Proton Exchange
Membrane Fuel Cell
Animation: fuel-cell-animation.swf


The cell uses one of the simplest reactions of any fuel cell.

Four Basic Elements in a PEMFC












Anode: the negative post of the fuel cell, has several jobs.
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.
Cathode: the positive post of the fuel cell,
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.

Four Basic Elements in a PEMFC











The electrolyte is the proton exchange membrane.
This specially treated material, which looks something
like ordinary kitchen plastic wrap,
only conducts positively charged ions.
The membrane blocks electrons.
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.

Chemistry of a Fuel Cell
 Anode

side:
2H2  4H+ + 4e-

 Cathode

side:
O2 + 4H+ + 4e-  2H2O

 Net

reaction:
2H2 + O2  2H2O

Animation of a fuel cell working
fuel-cell-animation.swf
The pressurized hydrogen gas (H2) entering the
fuel cell on the anode side.
 This gas is forced through the catalyst by the
pressure. When an H2 molecule comes in
contact with the platinum on the catalyst, it
splits into two H+ ions and two electrons (e-).
 The electrons are conducted through the anode,
where they make their way through the external
circuit (doing useful work such as turning a
motor) and return to the cathode side of the fuel
cell.




Meanwhile, on the cathode side of the fuel cell,
oxygen gas (O2) is being forced through the catalyst,
where it forms two oxygen atoms.



Each of these atoms has a strong negative charge.



This negative charge attracts the two H+ ions through
the membrane, where they combine with an oxygen
atom and two of the electrons from the external circuit
to form a water molecule (H2O).



This reaction in a single fuel cell produces only about
0.7 volts.



To get this voltage up to a reasonable level, many
separate fuel cells must be combined to form a fuelcell stack (電池堆).

PEMFCs operate at a fairly low temperature
(about 176oF~80oC),
 It means they warm up quickly and don't require
expensive containment structures.
 Constant improvements in the engineering and
materials used in these cells have increased
the power density to a level where a device
about the size of a small piece of luggage can
power a car.


Problems with Fuel Cells













The fuel cell uses oxygen and hydrogen to produce electricity.
The oxygen required for a fuel cell comes from the air.
In fact, in the PEM fuel cell, ordinary air is pumped into the
cathode.
The hydrogen is not so readily available, however.
Hydrogen has some limitations that make it impractical for use
in most applications.
For instance, you don't have a hydrogen pipeline coming to
your house, and you can't pull up to a hydrogen pump at your
local gas station.
Hydrogen is difficult to store and distribute, so it would be much
more convenient if fuel cells could use fuels that are more
readily available.
This problem is addressed by a device called a reformer.
A reformer turns hydrocarbon or alcohol fuels into hydrogen,
which is then fed to the fuel cell.