01 - Célula a Combustível - Introdução - 10-07-14

Chemical Engineering Department | University of Jordan | Amman 11942, Jordan

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Special Topics ( Fuel Cell Fundamentals and Technology)

Dr.-Eng. Zayed Al-Hamamre

Fuel Cell Principles: Introduction


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Anode and cathode Degradation corrosion and erosion

Fuel cell irreversibility

Hydrogen Production and storage techniques

Photovoltaic fuel cell systems

Fuel cell Mathematical Modeling (SOFC)

Course Projects


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Contents

What is a fuel cell

A Simple fuel cell

Fuel cell advantage and disadvantage

Basic Operating Features

Fuel Cell Stack

Fuel Cell Systems Introduction

Energy/Environmental Context

Fuel Cell Types


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What is a fuel cell?

Fuel cells are electrochemical devices that convert chemical energy stored in fuels

into electrical energy directly.

A fuel cell will continue to churn out product (electricity) as long as raw material

(fuel) is supplied (Continuous Process)

Unlike batteries, a fuel cell is not consumed when it produces electricity.

Input (Fuel) Output

H2-O2 Fuel cell


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Compared with conventional thermal power system:

Each step introduces losses that adversely affect the overall conversion efficiency.

Motion of Piston or Turbine

that derive a generator

Fuel Conversion


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Fuel is burned releasing heat.

In H2 Combustion: H2 + .5O2 H2O, hydrogen molecules are oxidized,

producing water and releasing heat.

Conversion in conventional thermal power system (combustion engine):

1. O2 and H2 gases

2. Bonds are Brocken

3. H-O bonds are formed

associated with energy

release as heat (3 and 4)

4. These bonds are broken and

formed by the transfer of

electrons between the

molecules.

E H2O bonding < E H2 and O2 bonding

Conventional Thermal Power System


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Overall reaction:

2H2 + O2 2H2O

is split into two electrochemical half reactions

Anodic reaction:

2H2 4H+ + 4e-

flow through an external

circuit as current

transfer through

the electrolyte

Cathodic reaction:

O2 + 4H+ + 4e- 2H2O

Both reaction are catalized

Conversion in Fuel Cell


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H2 – O2 Fuel cell:

Two platinum electrodes dipped into Sulfuric acid (an

aqueous acid electrolyte).

Hydrogen gas, bubbled across the left electrode, is split

into protons (H+) and electrons.

The protons can flow through the electrolyte, but the

electrons cannot.

The electrons flow from left to right through a piece of

wire that connects the two platinum electrodes.

When the electrons reach the right electrode, they

recombine with protons and bubbling oxygen gas to

produce

The flowing electrons will

provide power to the load

Simple Fuel Cell


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At the anode oxidation reaction (electrons are

removed from a species) is taking place

At the cathode reduction reaction (electrons are

added to a species) is taking place.

Consists of an electrolyte layer in contact with

an anode and a cathode on either side

Fuel Cell Components

Electrolyte layer

1. liquid electrolyte fuel cells,

• The reactant gases diffuse through a thin electrolyte film that wets portions of

the porous electrode and react electrochemically on their respective electrode

surface.


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• The electrochemical performance could be reduced if the porous electrode

contains an excessive amount of electrolyte, due to the restriction of gaseous

species transportation.

2. Solid electrolyte fuel cells,

• A large number of catalyst sites into the interface that are electrically and

ionically connected to the electrode and the electrolyte, is required

• A high-performance interface requires the use of an electrode which, in the zone

near the catalyst, has mixed conductivity (conducts both electrons and ions)

Reducing the thickness of the electrolyte, and developing improved electrode and

electrolyte materials which broaden the temperature range over which the cells can

be operated.



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The electrolyte not only transports dissolved reactants to the electrode, but also

conducts ionic charge between the electrodes, and thereby completes the cell electric

circuit

Fuel Cell Electrodes

The electrodes are highly porous to

1. Further increase the reaction surface area and ensure good gas access.

2. Ensure that reactant gases are equally distributed over the cell

3. Ensure that reaction products are efficiently led away to the bulk gas phase

Electrode must be made of materials that have good electrical conductance in order

to conduct electrons away from or into the electrode electrolyte interface (three-

phase interface) once they are formed and provide current collection and connection

with either other cells or the load.



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FC Basic Operation Separate liquid- or gas-phase fuel and oxidizer streams enter through flow channels,

separated by the electrolyte/electrode assembly.

Reactants are transported by diffusion and/or convection to the catalyst layer (electrode),

where electrochemical reactions take place to generate current

Electrons are produced at the anode

e- flow through the bipolar plate

(also called cell interconnect) to the

external circuit driving the load ,

the ions generated migrate through

the electrolyte to complete the

circuit.

At the cathode, e- recombine with

the oxidizer in the cathodic oxidizer

reduction reaction (ORR)


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FC Basic Operation

Fuel cells produce electricity by converting a primary energy source (a fuel) into a

flow of electrons.

This conversion necessarily involves an energy transfer step, where the energy from

the fuel source is passed along to the electrons constituting the electric current

This transfer has a finite rate and must occur at an interface or reaction surface.

The amount of electricity produced proportional with the amount of reaction surface

area or interfacial area available for the energy transfer. Larger surface areas translate

into larger currents.

To provides large reaction surfaces that maximize surface-to-volume ratios:

• Fuel cells are usually made into thin, planar structures.

• The electrodes are highly porous to further increase the

reaction surface area and ensure good gas access.


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e- e- e- e- e- e- e- e-

e-

e-

e-

e-

3. Ionic conduction

through the electrolyte

and electron

conduction through the

external circuit

4. Product removal from

the fuel cell

1. Reactant delivery (transport) into the fuel cell

2. Electrochemical reaction

+ve ions

or

-ve ions Depleted

Oxidant and

Product Gases

Out

Depleted Fuel and

Product Gases Out

Cross-sectional view of a planar fuel cell

Inside the Stack


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Reactant Transport

Efficient delivery of reactants is accomplished by using flow field plates

Flow field plates contain many fine channels or grooves to carry the gas flow and

distribute it over the surface of the fuel cell.

The shape, size, and pattern of flow channels can significantly affect the

performance of the fuel cell

Electrochemical Reaction

The current generated by the fuel cell is directly related to how fast the

electrochemical reactions proceed. High current output is desirable.

Catalysts are used to increase the speed and efficiency of the electrochemical

reactions.

The reactions kinetics represent greatest limitation to fuel cell performance.

Inside the Stack


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Ionic (and Electronic) Conduction.

The electrochemical reactions either produce or consume ions and electrons.

To maintain charge balance, these ions and electrons must therefore be transported

from the locations where they are generated to the locations where they are

consumed.

Ions transport tends to be more difficult than that of electrons. This is because ions

are much larger and more massive than electrons.

ionic transport can represent a significant resistance loss, reducing fuel cell

performance

Thus, the electrolytes in technological fuel cells are made as thin as possible to

minimize the distance over which ionic conduction must occur.

Inside the Stack


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Product Removal

In addition to electricity, all fuel cell reactions will generate at product species.

The H2–O2 fuel cell generates water.

Hydrocarbon fuel cells will typically generate water and carbon dioxide (CO2).

These products must be removed to prevent fuel cell blocking

Inside the Stack


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Fuel Cell Stacking

For most practical fuel cell applications, unit cells must be combined in a modular

fashion into a cell stack to achieve the voltage and power output level required for

the application (Increase voltage (and power) to useful levels).

Generally, the stacking involves connecting multiple unit cells in series via

electrically conductive interconnects.

Stacking Arrangements

Planar-Bipolar Stacking

The most common fuel cell stack design

Individual unit cells are electrically connected with interconnects (separator plate).


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Fuel Cell stack

arrangements

Single Cell

GDL

Membran

Flow Fields

The separator plate:

1. To provide an electrical series connection between adjacent cells, specifically for

flat plate cells, and

2. To provide a gas barrier that separates the fuel and oxidant of adjacent cells.

3. The interconnect also includes channels that distribute the gas flow over the cells

Fuel Cell Stacking


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20Larminie and Dicks,2003

Fuel Cell Stacking


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• Serpentine flow. Air

or fuel follow a zig-

zag path

• Spiral flow. Applies

to circular cells

• Cross-flow. Air and fuel flow perpendicular to

each other

• Co-flow. Air and fuel

flow parallel and in the

same direction. In the

case of circular cells,

this means the gases

flow radially outward

• Counter-flow. Air and

fuel flow parallel but

in opposite directions.

Again, in the case of

circular cells this

means radial flow

Larminie and Dicks,2003

Fuel Cell Stacking


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Fuel Cell Stacking


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Fuel Cell Stacking

Fuel cell stack in series. The total current is the same in each fuel cell; the voltage is

additive for each fuel cell plate in series.


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Fuel Cell Stacking


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Fuel Cell Stacking


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Fuel Cell Stack



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Stacks with Tubular Cells

Developed for high-temperature fuel cells

Have significant advantages in sealing and in the structural integrity of the cells.

They represent a special

geometric challenge to the

stack designer when it comes

to achieving high power

density and short current

paths.

Single tubular Cell

Fuel Cell Stack


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Tubular Stack

Bundle (24 Cells)

Fuel Cell Stacking


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Fuel Cell TypesThere are five major types of fuel cells, differentiated from one another by their electrolyte:

1. Phosphoric acid fuel cell (PAFC)

2. Polymer electrolyte membrane fuel cell (PEMFC)

3. Alkaline fuel cell (AFC)

4. Molten carbonate fuel cell (MCFC)

5. Solid-oxide fuel cell (SOFC)


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Fuel Cell Types


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Fuel Cell Systems

Practical fuel cell systems require several other sub-systems and components; the

so-called balance of plant (BoP), i.e. heat exchangers, controls, valves, fans,… etc

The precise arrangement of the BoP depends heavily on :

• Fuel cell type

• Fuel choice

• The application

• Specific operating conditions and

• Requirements of individual cell and stack designs

Fuel cell systems (power plant) contain:

Fuel cell stack

Fuel preparation, i.e. impurities removal and fuel processing


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Thermal management, controlling the stack temperature (POB), heat exchangers,

controls, valves, fans.

Electric power conditioning equipment (converting the variable DC voltage output

to AC current.

Air supply.

Water management

A simple rendition of a

fuel cell power plant.

Fuel Cell Systems


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Fuel Cell Systems


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FC and Environmental

Fuel cells are environmentally friendly (if H2 is used as fuel).

The environmental impact of fuel cells depends strongly on the context of their use

H2 fuel cells are coupled with

electrolyzers and renewable energy

technologies (such as wind and solar

power) to provide a completely closed-

loop

pollution-free energy

economy


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Fuel Cell: Advantages

Fuel cells they are often far more efficient than combustion engines produce

electricity directly from chemical energy,

Fuel cells can be all solid state and mechanically ideal, meaning no moving parts

highly reliable, silent operation and long-lasting systems

Undesirable products such as NOx, SOx , and particulate emissions are virtually

zero.

Fuel cells offer potentially higher energy densities compared to batteries and can

be quickly recharged by refueling

Fuel cells allow easy independent scaling between power (determined by the fuel

cell size) and capacity (determined by the fuel reservoir size).


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Fuel Cell: Disadvantages

Fuel cell technology is currently only economically competitive in a few highly

specialized applications (e.g., onboard the Space Shuttle orbiter) due to the cost.

Operational temperature compatibility concerns, susceptibility to environmental

poisons, and durability under start–stop cycling.

Fuel availability and storage:

• Fuel cells work best on hydrogen gas, a fuel which is not widely available, has

a low volumetric energy density, and is difficult to store

• Alternative fuels (e.g., gasoline, methanol, formic acid) are difficult to use

directly and usually require reforming. These problems can reduce fuel cell

performance and increase the requirements for ancillary equipment.


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Fuel Cell Challenges


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Energy, Power, Energy density, and Power Density

Energy is defined as the ability to do work (J).

Power is the rate at which energy is expended or produced intensity of energy use or

production (J.s-1 = W)

Energy = power × time, Wh = 3600 J

Power density refers to the amount of power that can be produced by a device per

unit mass (gravimetric) or volume (volumetric).

Energy density refers to the total energy capacity available to the system per unit

mass or volume.

Power density and energy density are more important than power and energy because

they provide information about how big a system needs to be to deliver a certain

amount of energy or power


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Energy density comparison of selected fuels (lower heating value).

Energy density


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Improvements are required if fuel cells are to compete in portable and automotive

applications

Power density


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Automobile

DaimlerChrysler

GM Opel Ford

Fiat

Toyota

Volkswagen

DaimlerChrysler Ballard

MAN Siemens

Fuel Cell Applications


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Sulzer Hexis

HGC (Energiepartners)

Houshold Applications

Vaillant



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H Power

Corporation

Fraunhofer Institut

für Solare

Energiesysteme

Portable Devices



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Howaldswerke Deutsche

Werft

Space and Marine Applications

International Space Station ISS

Space Shuttle



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Major engineering disciplines involved in fundamental fuel cell science.

Fundamentals of Fuel Cell Science,

Documents

01 - Célula a Combustível - Introdução - 10-07-14