Fuel cell technology.

PEMFC TECH.

INTRODUCTION

A fuel cell by definition is an electric cell, which unlike storage cells can be continuously fed with the fuel so that the electrical power output is sustained indefinitely. It converts the hydrogen or hydrogen containing fuels directly into electrical energy plus heat through the electrochemical reaction of a hydrogen and oxygen into water. Fuel cells are one of the cleanest and most efficient technologies for generating electricity. A fuel cell produces electricity by means of an electrochemical reaction much like a battery. But there is an important difference. Rather than extracting the chemical reactants from the plates inside the cells, a fuel cell uses hydrogen fuel and oxygen extracted from the air to produce electricity. Unlike a battery, however, fuel cells never lose their charge. As long as there is a constant source of fuel – usually natural gas for the hydrogen and air for the oxygen – fuel cells will generate electricity. Since there is no combustion, there are none of the pollutants commonly produced by boilers and furnaces. For systems designed to consume hydrogen directly, the only products are electricity, water and heat.

Fuel Cell Origins:

In 1839, Sir William Grove, a British physicist, first discovered the principle of the fuel cell. Grove utilized four large cells, each containing hydrogen and oxygen, to produce electric power which was then used to split the water in the smaller upper cell into hydrogen and oxygen. However, it was Ludwig

Mond and Charles Langer first used the term "fuel cell" in 1889. It was not until 1932 that Francis Bacon developed the first successful fuel cell. NECAR3, the first commercially available fuel cell vehicle was introduced by Diamler-Benz of Germany. General Motors, Ford and Ballard Power Systems(Canada) too are putting up stiff resistance.

Types of Fuel Cells:

Broadly Fuel Cells are classified as:

Polymer Exchange Membrane Fuel Cell (PEMFC)
Molten Carbonate Fuel Cell (MCFC)
Solid Oxide Fuel Cell (SOFC)
Phosphoric Acid Fuel Cell (PAFC)
Direct Methanol Fuel Cell (DMFC)
Alkaline Fuel Cell (AFC)
Protonic Ceramic Fuel Cell (PCFC)

Comparison of General Types of Fuel Cells

PRINCIPAL AND OPERATION :

At the anode the hydrogen molecules give up electrons and form hydrogen A PEM (Proton Exchange Membrane, also called Polymer Electrolyte Membrane) fuel cell uses a simple chemical process to combine hydrogen and oxygen into water, producing electric current in the process. For those interested in the chemistry, it works something like electrolysis in reverse:

Ions, a process which is made possible by the platinum catalyst.
The electrons travel to the cathode through an external circuit, producing electrical current. This current can perform useful work by powering any electrical device (such as an electric motor).
The proton exchange membrane allows protons to flow through, but stops electrons from passing through it. As a result, while the electrons flow through an external circuit, the hydrogen ions flow directly through the proton exchange membrane to the cathode, where they combine with oxygen molecules and the electrons to form water.
In this way, hydrogen fuel's natural tendency to oxidize and form water is utilized to produce electricity and useful work.
No pollution is produced and the only resulting products are water and heat.
The equations look like this:
Anode: 2H2 --> 4H+ + 4e-
Cathode: 4e- + 4H+ + O2 --> 2H2O

Overall: 2H2 + O2 --> 2H2O

Construction Of PEMFC:

PEMFC basically consists of three parts namely Fuel Processor, Fuel Stack (Membrane) and Power Conditioner.

The fuel processor portion of a fuel cell system has two operating components: the fuel reformer and the carbon monoxide (CO) cleanup unit. The fuel reformer processes a hydrocarbon fuel, such as natural gas, into a hydrogen-rich gas known as reformate. Reformate contains heavy concentrations of CO so a CO cleanup system is pplied to reduce the CO concentrations to acceptable levels (under 50 ppm). Modern PEMFC employ high ...

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Overall: 2H2 + O2 --> 2H2O

Construction Of PEMFC:

PEMFC basically consists of three parts namely Fuel Processor, Fuel Stack (Membrane) and Power Conditioner.

The fuel processor portion of a fuel cell system has two operating components: the fuel reformer and the carbon monoxide (CO) cleanup unit. The fuel reformer processes a hydrocarbon fuel, such as natural gas, into a hydrogen-rich gas known as reformate. Reformate contains heavy concentrations of CO so a CO cleanup system is pplied to reduce the CO concentrations to acceptable levels (under 50 ppm). Modern PEMFC employ high performance fuel processors that consistently produce the highest concentrations of H2 at the lowest levels of CO.

2) The PEMFC stack is the heart of the fuel cell system. It is made up of a membrane electrode assembly (MEA) sandwiched between two gas diffusion layers (GDLs) with bipolar plates on each side.

The reformate arriving from the CO cleanup system feeds the fuel side of the fuel cell. The hydrogen in the fuel passes through the GDL, which typically serves three functions within a PEM fuel cell:

a) Diffuse the reactant gases across the surface of the membrane.

b) Manage the water around the membrane.

c) Provide a highly conductive path between the membrane and bipolar plates.

3) The last step is the power conditioner. The power conditioner first converts the low voltage direct current (DC) produced by the PEM fuel cell to a high voltage alternating current (AC). Batteries are used to ensure that power surges from such things as air conditioner start-ups can be handled. Batteries also meet any extended peak period of demand, which are higher than stack peak output.

Along with these three major subsystems, there are several supporting auxiliary subsystems that are required to make the entire PEM fuel cell power system function properly.

PEMFC Membranes:

The PEFC uses as its electrolyte a polymer membrane. This membrane is an electronic insulator, but an excellent conductor of hydrogen ions. The materials used to date consist of a fluorocarbon polymer backbone, similar to Teflon, to which are attached sulfonic acid groups. The acid molecules are fixed to the polymer and cannot "leak" out, but the protons on these acid groups are free to migrate through the membrane. With the solid polymer electrolyte, electrolyte loss is not an issue with regard to stack life.

The electrolyte membrane looks rather like a thick sheet of food wrap and can be handled easily and safely. The anode and cathode are prepared by applying a small amount of platinum black to one surface of a thin sheet of porous, graphitized paper, which has previously been wet-proofed with Teflon/Nafion. The electrolyte is then sandwiched between the anode and cathode, and the three components are sealed together under heat and pressure to produce a single "membrane/electrode assembly" (MEA). This assembly, which is the heart of the fuel cell, is less than a millimeter thick.

Membrane Characteristics:

The basic characteristics required include:

Low cost for the MEA.
High conductivity (resistivity of 0.1 ohm-cm2 or less under operating conditions)
Good barrier properties.
High mechanical strength.
Thermal and chemical stability (>150°C, 5000 hr durability)
Electronic insulation.

In addition, Polymer Electrolyte Membranes must have:

Properties making it amenable to MEA formation, including good dimensional stability, appropriate mechanical properties (membranes that are too stiff typically present difficulties for preparation of MEAs), and reasonable thermal stability.
Favorable interaction with water, i.e. the membrane should not lose desirable properties in the presence of water; the membrane should (ideally) provide good conductivity under a range of hydration conditions; and the membrane should allow adequate transport of water to prevent polarization.

Perfluorosulfonic acid (PFSA)-based membranes (e.g. Nafion) have long been the standard. Membranes based on aromatic sulfonic acid moieties have been the primary class of materials synthesized as people search for alternatives to PFSA’s. The latter include a wide range of materials, including sulfonated poly (phosphazenes), sulfonated poly (sulfones), sulfonated poly (ether ketones), sulfonated poly (trifluorostyrenes) and so on. Available PFSA’s have almost all of the necessary properties for a good fuel cell membrane.

There are four elements of the polymer electrolyte that can be varied:

Primary structure of the polymer
Morphology of the polymer
Nature of the acid group used
Nature of the medium, such as water, used to foster dissociation and transport of protons within the polymer phase.

MEA and Pt-Catalyst :

MEA has the generic structure : electrode/electrolyte/electrode, packaged in the orm of a sandwich of three thin films.

For catalyst layers containing about 0.15 mg Pt/cm2, the thickness of catalyst layer is close to 10 microns, less than half the thickness of a sheet of paper.

MEA, with a total thickness of about 200 microns or 0.2mm, can generate more than half ampere of current for every square cm of MEA at 0.7 volts.

Membrane Electrode Assembly(MEA)

The function of the electrode is to bring about reaction between the reactant (fuel or oxygen) and the electrolyte, without itself being consumed or corroded. It must also, be an electronic conductor and bring the gaseous fuel, electrolyte and electrode itself to contact.

The MEA consists of a solid polymer, proton exchange membrane. A layer of platinum-based catalyst and a gas-porous electrode support material are on both sides of the membrane, forming the anode and cathode of the cell.

Applications of PEM Fuel Cells:

PEM cells may have a mixed record in space, but several companies have been testing the cells in more down-to-earth vehicles. In 1995, Ballard Systems tested PEM cells in buses in Vancouver and Chicago and later in experimental vehicles made by DaimlerChrysler. PEM cells have also supplied power to unmanned blimps called aerostats and to sonobuoys, which are nautical buoys that generate and receive sonar signals.

Early in 2000, AeroVironment selected PEM technology to provide nighttime power for its solar-powered Helios long-duration aircraft. The goal is to make the unpiloted aircraft fly continuously for up to six months. Photovoltaic panels during the day will run electric motors and electrolyze water. At night, the fuel cell will run the motors by converting the hydrogen and oxygen back into water. Test flights are planned for 2003. Automotive research has taken on new urgency as air quality regulations grow steadily stricter, particularly in California. Energy Partners and the U. S. Department of Energy's Office of Advanced Automotive Technologies provided two 20 kw fuel cell stacks to Virginia Tech and Texas Tech universities to evaluate performance in hybrid electric cars. Major automakers like Ford and Volkswagen are also testing PEM vehicles.

Recently, NASA used PEMFC’s to power on-board electronics for the Gemini and Apollo spacecraft. In fact, NASA still uses PEMFC’s to provide electricity and water for its space shuttle missions.

Residential: fuel cells operate silently, they reduce noise pollution as well as air pollution and the waste heat from a fuel cell can be used to provide hot water or space heating for a home.

Portable Power: several manufactures are also developing fuel cell power supplies for portable applications, providing a few watts up to several kw of electricity, fuelled by stored natural gas, propane, methanol or hydrogen gas. Portable power packs using fuel cell can be lighter and smaller in volume.Miniature fuel cells, once available to the commercial market, will help consumers talk for up to a month on a cellular phone without recharging. Fuel cells will change the telecommuting world, powering laptops and palm pilots hours longer than batteries. These miniature fuel cells generally run on methanol.

Fuel Cell Powered Mobile Robots: Sandia National Laboratories' Intelligent Systems and Robotics Center (ISRC) has developed the RATLER™ ("Robotic All Terrain Lunar Exploration Rover") which is powered by a PEMFC system. Fuel cells offer a way to improve Sandia's electric robotic vehicle performance, to extend operating range and life, and to reduce observable signatures so that robotic vehicles are used more effectively in a variety of demanding applications. Because power and energy density are limiting factors when deploying robots in field applications, fuel cell technology helps in addressing these issues.

Transportation: All the major automotive manufacturers have a fuel cell vehicle either in development or in testing right now — Honda, Toyota, Daimler Chrysler, General Motors, Ford, Hyundai, BMW, Volkswagen, etc. The NECAR 5is the latest prototype fuel cell automobile by Daimler Chrysler. This automobile is fuelled with liquid methanol, which is converted into hydrogen and carbon dioxide through the use of an onboard fuel processor. The efficiency of a fuel cell engine is about a factor of two higher than that of an internal combustion engine. The NECAR 5 drives and feels like a normal car. It has top speed of over 150 km/h(90mph) with a power output of 75 KW (100 hp) . it requires less maintenance low emission level, quietness and smoothness.

Benefits:

Fuel cells are efficient: they convert hydrogen and oxygen directly into electricity water and heat, with no combustion in the process. The resulting efficiency is between 50 to 80 %, about double that of an internal combustion engine.

Fuel cells are clean: If hydrogen is the fuel , there are no pollutant emissions from a fuel cell itself, only the production of pure water . In contrast to an internal combustion engine, a fuel cell produces no emissions of sulphur dioxide, which can lead to acid rain, nor nitrogen oxides, which produce smog nor dust particulates.

Fuel cells are quiet: A fuel cell itself has no moving parts. As a result, electrical power produced relatively silently. Many hotels and resorts in quiet locations , for example, could replace diesel engine generators with fuel cells for both main power supply or for back up power in the event of power outage.

Fuel cells are modular: That is fuel cells of varying sizes can be stacked together to meet require power demand as mention earlier fuel cells systems can provide power over a large range from a few watt to mega watts.

Fuel cells are environmentally safe: They produce no hazardous waste products and their only by product is water and heat.

PEMFC’s do not use corrosive liquid electrolytes, and their components are easily

recycled at the end of their operating life, avoiding environmental problems

associated with battery disposal.

Known Limitations and Experimented Solutions:

The PEM fuel cell requires an aqueous membrane to operate efficiently. The fuel cells cannot be operated at high temperatures because the water inside the cell boils at 100oC. As a result, if the cell operates on a hot or sunny day, this will cause the water inside to boil and evaporate. This will result in a dry membrane, which will eventually lose its conductivity. Therefore, a humidifier is used to help keep the membrane in an aqueous environment. However, humidifiers add to additional mass and consume power from the fuel cell.

A major limitation of the fuel cell is the onboard hydrogen storage. Hydrogen can be stored in a rechargeable metal hydride or in a hydride compound that releases hydrogen when reacted with water. Physically, hydrogen can be stored as compressed gas, cryogenically cooled liquid, or through the absorption of a surface.

The Future of Fuel Cells:

Fuel cells operate at high efficiency, regardless of size and load. The fuel cell has the potential to help decentralize the power industry, protect the environment, provide low cost electricity for consumers, and offer a highly efficient power technology. Transportation, low emissions, mobile connectivity, etc are just a few of the promises of this exciting technology. Since there is no combustion, there are none of the pollutants commonly produced by boilers and furnaces. When natural gas or other hydrocarbons are used, fuel cells produce some carbon dioxide, though much less than would be created if the fuel were burned. Advanced fuel cells using natural gas, for example, could potentially reduce carbon dioxide emissions by 60% compared to a conventional coal plant and by 25% compared to today's natural gas plants. Moreover, the carbon dioxide is emitted in concentrated form, which makes its capture and sequestration much easier. Also, there is good scope for development of fuel cell in India and other developing countries – especially considering their use in fuel cell buses.

CONCLUSION

"Of all the technologies, the fuel cell car seems to be the most promising; it has the chance of becoming the mass market car", says Bryon McCormick, co-director of General Motors, summing up one of the most important advancements in the automobile industry, spearheaded by fuel cell technology.

As our demand for electrical power grows, it becomes increasingly urgent to find new ways of meeting it both responsibly and safely. In the past, the limiting factors of renewable energy have been the storage and transport of that energy. With the use of fuel cells and hydrogen technology, electrical power from renewable energy sources can be delivered where and when required, cleanly, efficiently and sustainable.

Since conventional energy sources are exhaustible, renewable energy sources form to be the technology of tomorrow’s world. Fuel Cells promote energy diversity and a transition to renewable energy sources. Fuel cells are an environmentally clean, quiet, and highly efficient method for generating electricity and heat from natural gas and other fuels. Fuel cells can also utilize fuels containing hydrogen, including methanol, ethanol, and natural gas, and even gasoline or diesel fuel. Since fuel cells operate silently, they reduce noise pollution as well as air pollution. Fuel cells are creating new markets for steel, electronics, electrical and chemical industries. Thus fuel cells are creating thousands of high quality jobs around the globe in the process.