Introduction to Fuel Cells

5/27/2018 Introduction to Fuel Cells

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FUEL CELL TECHNOLOGY

BY:AHMED MOHAMED IBRAHIM

SECTION:2

UNDER SUPERVISION OF:

DR.KHAIRY FAKHRY


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CONTENTS

Introduction: What are Fuel Cells? ................................................................................................................................ 3

Overview ................................................................................................................................................................... 3

How do Fuel Cells work? ........................................................................................................................................... 3

Why arent Fuel Cells readily available commercially? ............................................................................................. 4

Common Types of Fuel Cells .......................................................................................................................................... 6

Polymer exchange membrane fuel cell (PEMFC) ................................................................................ 7

Solid oxide fuel cell (SOFC) ..................................................................................................................... 7

Alkaline fuel cell (AFC) ............................................................................................................................. 7

Molten-carbonate fuel cell (MCFC) ......................................................................................................... 7

Phosphoric-acid fuel cell (PAFC) ............................................................................................................ 8

Direct-methanol fuel cell (DMFC) ............................................................................................................ 8

Well, all that is good, any disadvantages though? ...................................................................................................... 10

Cost ........................................................................................................................................................... 10

Durability ................................................................................................................................................... 10

Hydration .................................................................................................................................................. 10

Delivery ..................................................................................................................................................... 10

Infrastructure ............................................................................................................................................ 10

Storage and Other Considerations ................................................................................................................ 11

Common Uses of Fuel Cells ......................................................................................................................................... 11


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INTRODUCTION:WHAT ARE FUEL CELLS?

OVERVIEW

A fuel cell is an electrochemical energy conversion device. A fuel cell converts the chemicals hydrogen and oxygen

into water, and in the process it produces electricity. It is very similar to the well known battery. A key difference,

however, is that a fuel cell doesnt go dead; because a fuel cell has a continuous supply of chemicals, while a

battery has all the chemicals stored inside it.

Every fuel cell has two electrodes, one positive and one negative, called, respectively, the anode and cathode. The

reactions that produce electricity take place at the electrodes. Every fuel cell also has an electrolyte, which carries

electrically charged particles from one electrode to the other, and a catalyst, which speeds the reactions at the

electrodes.

Hydrogen is the basic fuel, but fuel cells also require oxygen. One great appeal of fuel cells is that they generate

electricity with very little pollutionmuch of the hydrogen and oxygen used in generating electricity ultimately

combines to form a harmless byproduct, namely water.

One detail of terminology: a single fuel cell generates a tiny amount of direct current (DC) electricity. In practice,

many fuel cells are usually assembled into a stack. Cell or stack, the principles are the same.

HOW DO FUEL CELLS WORK?

The purpose of a fuel cell is to produce an electrical current that can be directed outside the cell to do work, such

as powering an electric motor or illuminating a light bulb or a city. Because of the way electricity behaves, this

current return to the fuel cell, completing an electrical circuit. The chemical reactions that produce this current are

the key to how a fuel cell works.

There are several kinds of fuel cells, and each operates a bit differently. But in general terms, hydrogen atoms

enter a fuel cell at the anode where a chemical reaction strips them of their electrons. The hydrogen atoms are

now "ionized," and carry a positive electrical charge. The negatively charged electrons provide the current through

wires to do work. If alternating current (AC) is needed, the DC output of the fuel cell must be routed through a

conversion device called an inverter.


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1. SIMPLE FUEL CELL

Oxygen enters the fuel cell at the cathode and, in some cell types; it there combines with electrons returning from

the electrical circuit and hydrogen ions that have traveled through the electrolyte from the anode. In other cell

types the oxygen picks up electrons and then travels through the electrolyte to the anode, where it combines with

hydrogen ions.

The electrolyte plays a key role. It must permit only the appropriate ions to pass between the anode and cathode.

If free electrons or other substances could travel through the electrolyte, they would disrupt the chemical reaction.

Whether they combine at anode or cathode, together hydrogen and oxygen form water, which drains from the

cell. As long as a fuel cell is supplied with hydrogen and oxygen, it will generate electricity.

Even better, since fuel cells create electricity chemically, rather than by combustion, they are not subject to the

thermodynamic laws that limit a conventional power plant (see "Carnot Limit). Therefore, fuel cells are more

efficient in extracting energy from a fuel. Waste heat from some cells can also be harnessed, boosting system

efficiency still further.

WHY ARENT FUEL CELLS READILY AVAILABLE COMMERCIALLY?

Fuel cell sure looks attractive as an alternative to diesel generators; but why arent they available on a commercial

scale?

The basic workings of a fuel cell may not be difficult to illustrate. But building inexpensive, efficient, reliable fuel

cells is a far more complicated business.


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Scientists and inventors have designed many different types and sizes of fuel cells in the search for greater

efficiency, and the technical details of each kind vary. Many of the choices facing fuel cell developers are

constrained by the choice of electrolyte. The design of electrodes, for example, and the materials used to make

them depend on the electrolyte. Today, the main electrolyte types are alkali, molten carbonate, phosphoric acid,

proton exchange membrane (PEM) and solid oxide. The first three are liquid electrolytes; the last two are solids.

The type of fuel also depends on the electrolyte. Some cells need pure hydrogen, and therefore demand extra

equipment such as a "reformer" to purify the fuel. Other cells can tolerate some impurities, but might need higher

temperatures to run efficiently. Liquid electrolytes circulate in some cells, which require pumps. The type of

electrolyte also dictates a cell's operating temperature"molten" carbonate cells run hot, just as the name implies.

Each type of fuel cell has advantages and drawbacks compared to the others, and none is yet cheap and efficient

enough to widely replace traditional ways of generating power, such coal-fired, hydroelectric, or even nuclear

power plants.


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COMMON TYPES OF FUEL CELLS

Fuel cells come in many varieties; however, they all work in the same general manner. They are made up of three

adjacent segments: the anode, the electrolyte, and the cathode. Two chemical reactions occur at the interfaces of

the three different segments. The net result of the two reactions is that fuel is consumed, water or carbon dioxide

is created, and an electric current is created, which can be used to power electrical devices, normally referred to as

the load.

At the anode a catalyst oxidizes the fuel, usually hydrogen, turning the fuel into a positively charged ion and a

negatively charged electron. The electrolyte is a substance specifically designed so ions can pass through it, but the

electrons cannot. The freed electrons travel through a wire creating the electric current. The ions travel through

the electrolyte to the cathode. Once reaching the cathode, the ions are reunited with the electrons and the two

react with a third chemical, usually oxygen, to create water or carbon dioxide.

2. A BLOCK DIAGRAM OF A FUEL CELL

The most important design features in a fuel cell are:

The electrolyte substance. The electrolyte substance usually defines the type of fuel cell.

The fuel that is used. The most common fuel is hydrogen.

The anode catalyst breaks down the fuel into electrons and ions. The anode catalyst is usually made up of

very fine platinum powder.

The cathode catalyst turns the ions into the waste chemicals like water or carbon dioxide. The cathode

catalyst is often made up of nickel but it can also be a nanomaterial-based catalyst.

A typical fuel cell produces a voltage from 0.6 V to 0.7 V at full rated load. Voltage decreases as current increases,

due to several factors:


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Activation loss

Ohmic loss (voltage drop due to resistance of the cell components and interconnections)

Mass transport loss (depletion of reactants at catalyst sites under high loads, causing rapid loss of

voltage).

To deliver the desired amount of energy, the fuel cells can be combined in series and parallel circuits to yield

higher voltage and parallel-channel of configurations allow a higher current to be supplied. Such a design is called a

fuel cell stack. The cell surface area can be increased, to allow stronger current from each cell. In the stack,

reactant gases must be distributed uniformly over all of the cells to maximize the power output.

There are several different types of fuel cells, each using a different chemistry. Fuel cells are usually

classified by their operating temperature and 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 main types of fuel cells include:

POLYMER EXCHANGE MEMBRANE FUEL CELL (PEMFC)

The Department of Energy (DOE) is focusing on the PEMFC as the most likely candidatefor transportation applications. The PEMFC has a high power density and a relatively low

operating temperature (ranging from 60 to 80 degrees Celsius, or 140 to 176 degrees

Fahrenheit). The low operating temperature means that it doesn't take very long for the fuel cell to

warm up and begin generating electricity. We?ll take a closer look at the PEMFC in the next

section.

SOLID OXIDE FUEL CELL (SOFC)

These fuel cells are best suited for large-scale stationary power generators that could

provide electricity for factories or towns. This type of fuel cell operates at very high temperatures

(between 700 and 1,000 degrees Celsius). This high temperature makes reliability a problem,

because parts of the fuel cell can break down after cycling on and off repeatedly. However, solid

oxide fuel cells are very stable when in continuous use. In fact, the SOFC has demonstrated the

longest operating life of any fuel cell under certain operating conditions. The high temperature

also has an advantage: the steam produced by the fuel cell can be channeled into turbines to

generate more electricity. This process is called co-generation of heat and power (CHP)and it

improves the overall efficiency of the system.

ALKALINE FUEL CELL (AFC)

This is one of the oldest designs for fuel cells; the United States space program has used

them since the 1960s. The AFC is very susceptible to contamination, so it requires pure hydrogen

and oxygen. It is also very expensive, so this type of fuel cell is unlikely to be commercialized.

MOLTEN-CARBONATE FUEL CELL (MCFC)

Like the SOFC, these fuel cells are also best suited for large stationary power generators.

They operate at 600 degrees Celsius, so they can generate steam that can be used to generate

more power. They have a lower operating temperature than solid oxide fuel cells, which means

they don't need such exotic materials. This makes the design a little less expensive.


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PHOSPHORIC-ACID FUEL CELL (PAFC)

The phosphoric-acid fuel cell has potential for use in small stationary power-generation

systems. It operates at a higher temperature than polymer exchange membrane fuel cells, so it

has a longer warm-up time. This makes it unsuitable for use in cars.

DIRECT-METHANOL FUEL CELL (DMFC)Methanol fuel cells are comparable to a PEMFC in regards to operating temperature, but

are not as efficient. Also, the DMFC requires a relatively large amount of platinum to act as a

catalyst, which makes these fuel cells expensive.


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WELL,ALL THAT IS GOOD,ANY DISADVANTAGES THOUGH?

Fuel cells might be the answer to our power problems, but first scientists will have to sort out a few major

issues:

COST

Chief among the problems associated with fuel cells is how expensive they are. Many of

the component pieces of a fuel cell are costly. For PEMFC systems, proton exchange

membranes, precious metal catalysts (usually platinum), gas diffusion layers, and bipolar plates

make up 70 percent of a system's cost [Source: Basic Research Needs for a Hydrogen

Economy]. In order to be competitively priced (compared to gasoline-powered vehicles), fuel cell

systems must cost $35 per kilowatt. Currently, the projected high-volume production price is $73

per kilowatt [Source:Garland]. In particular, researchers must either decrease the amount of

platinum needed to act as a catalyst or find an alternative.

DURABILITYResearchers must develop PEMFC membranes that are durable and can operate at

temperatures greater than 100 degrees Celsius and still function at sub-zero ambient

temperatures. A 100 degrees Celsius temperature target is required in order for a fuel cell to have

a higher tolerance to impurities in fuel. Because you start and stop a car relatively frequently, it is

important for the membrane to remain stable under cycling conditions. Currently membranes tend

to degrade while fuel cells cycle on and off, particularly as operating temperatures rise.

HYDRATION

Because PEMFC membranes must by hydrated in order to transfer hydrogen protons,

researches must find a way to develop fuel cell systems that can continue to operate in sub-zero

temperatures, low humidity environments and high operating temperatures. At around 80 degrees

Celsius, hydration is lost without a high-pressure hydration system.

The SOFC has a related problem with durability. Solid oxide systems have issues with material

corrosion. Seal integrity is also a major concern. The cost goal for SOFC?s is less restrictive than

for PEMFC systems at $400 per kilowatt, but there are no obvious means of achieving that goal

due to high material costs. SOFC durability suffers after the cell repeatedly heats up to operating

temperature and then cools down to room temperature.

DELIVERY

The Department of Energy?s Technical Plan for Fuel Cells states that the air compressortechnologies currently available are not suitable for vehicle use, which makes designing a

hydrogen fuel delivery system problematic.

INFRASTRUCTURE

In order for PEMFC vehicles to become a viable alternative for consumers, there must be

a hydrogen generation and delivery infrastructure. This infrastructure might include pipelines,

truck transport, fueling stations and hydrogen generation plants. The DOE hopes that
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development of a marketable vehicle model will drive the development of an infrastructure to

support it.

STORAGE AND OTHER CONSIDERATIONS

Three hundred miles is a conventional driving range (the distance you can drive in a car with a full tank of

gas). In order to create a comparable result with a fuel cell vehicle, researchers must overcome hydrogenstorage considerations, vehicle weight and volume, cost, and safety.

While PEMFC systems have become lighter and smaller as improvements are made, they still are too

large and heavy for use in standard vehicles.

There are also safety concerns related to fuel cell use. Legislators will have to create new processes for

first responders to follow when they must handle an incident involving a fuel cell vehicle or generator.

Engineers will have to design safe, reliable hydrogen delivery systems.

Researchers face considerable challenges. In the next section, we will explore why the United States and

other nations are investing in research to overcome these obstacles.

COMMON USES OF FUEL CELLS

Providing power for base stations or cell sites

Cogeneration

Power Generation

Distributed generation

Emergency power systems are a type of fuel cell system, which may include lighting, generators and other

apparatus, to provide backup resources in a crisis or when regular systems fail. They find uses in a wide

variety of settings from residential homes to hospitals, scientific laboratories, data centres

Telecommunication equipment and modern naval ships.

An uninterrupted power supply (UPS) provides emergency power and, depending on the topology,

provide line regulation as well to connected equipment by supplying power from a separate source when

utility power is not available. Unlike a standby generator, it can provide instant protection from a

momentary power interruption.

Base load power plants

Solar Hydrogen Fuel Cell Water Heating

Hybrid vehicles, pairing the fuel cell with either an ICE or a battery.

Notebook computers for applications where AC charging may not be readily available.

Portable charging docks for small electronics (e.g. a belt clip that charges your cell phone or PDA).

Smartphones, laptops and tablets.

Small heating appliances

Food preservation, achieved by exhausting the oxygen and automatically maintaining oxygen exhaustion

in a shipping container, containing, for example, fresh fish.

Breathalyzers, where the amount of voltage generated by a fuel cell is used to determine the

concentration of fuel (alcohol) in the sample.

Carbon monoxide detector, electrochemical sensor.

Introduction to Fuel Cells

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