FUEL CELL TECHNOLOGY:

P.R.SURESH, R.ASHOK KUMAR FINAL YEAR, FINAL YEAR B.TECH (AUTOMOBILE), B.TECH(AUTOMOBILE) SRM UNIVERSITY, SRM UNIVERSITY sureshautomobile@gmail.com, ashokautocellence@gmail.com MOBILE NO: 9943307299. 9994895600

FUEL CELLS_AUTOMOBILE:

Abstract:

Heat engines based on fossil fuel combustion produce harmful pollutants and greenhouse

gas emissions. Environmental concerns and sustainable development call for new

technology for energy conversion and power generation, which is more efficient,

environmentally friendly and compatible with alternative fuels and renewable energy

sources and carriers. Fuel cells meet all these requirements, and are being developed

as one of the primary energy technologies of the future. In this paper, the construction,

operational working and thermodynamic performance of fuel cells is analyzed, energy

conversion efficiency of fuel cells and heat engines is studied and compared, and

misconceptions about fuel cell efficiency clarified. It is shown that both fuel cells and

heat engines have the same maximum theoretical efficiency, which is equivalent to the

Carnot efficiency, when operating on the same fuel and oxidant. However, fuel cells are

free from the high temperature limit imposed by materials on heat engines and less

irreversibility’s associated with heat rejection. As a result, fuel cells can have higher

practical efficiencies.

Introduction:

Progress in human society, and especially modern civilization, has been marked by

ever-increasing energy consumption and power requirements. The majority of the

energy needs have been provided by combustion of fossil fuels since the industrial

revolution. Heat engines utilizing fossil fuel combustion have resulted in severe local

air pollution, threatening the health of millions of people living in many of the world’s

urban areas. They continue to contribute significantly to the increase in the

atmospheric carbon dioxide concentrations, thus intensify the prospect of global

warming. In addition to the health and environmental concerns, a steady depletion of

the world’s limited fossil fuel reserves and the very survival of humankind call for new

generation technology for energy conversion and power generation, which is more

efficient than the conventional heat engines with minimal or no pollutant emissions,

and also compatible with renewable energy sources and carriers for sustainable

development and energy security. Fuel cell has been identified as the most promising

and potential energy conversion technology, which meets all of the above

requirements. In fact, fuel cell technology has been successfully used in many specific

areas, notably in space explorations, where fuel cell operates on pure hydrogen and

oxygen with over 70% thermal to electrical energy efficiency and the only by-product

water constitutes the sole source of drinking water for the crew. There are now several

hundred fuel cell units for terrestrial applications, from stationary cogeneration,

mobile transportation to portable applications, operating in over a dozen of countries,

impressive technical progress has been achieved, and is driving the development of

competitively priced fuel cell-based power generation systems with advanced features.

Besides being efficient, clean and compatible with future energy sources and carriers,

fuel cell also offers many additional advantages for both mobile and stationary

applications. Fuel cell is an electrochemical device and has no moving components except

for peripheral compressors and motors. As a result, its operation is very quiet, and

virtually without vibration and noise, thus capable of being sited at the premises of the

consumer to eliminate power transmission lines. Its inherent modularity allows for

simple construction and operation with possible applications for dispersed, distributed

and portable power generation, because it may be made in any size from a few watts to

megawatt scale plant with equal efficiency. Its fast response to the changing load

condition while maintaining high efficiency makes it ideally suited to load following

applications. Its high efficiency represents less chemical, thermal and carbon dioxide

emissions for the same amount of energy conversion and power generation.

At present, fuel cell is being used routinely in automobile applications, and has been

under intensive development for terrestrial use, such as for utilities and zero emission

vehicles. There exist a variety of fuel cells, and they can be classified based on their

operating temperature such as low and high temperature fuel cells, the type of ion

migrating through the electrolyte, etc. However, the choice of electrolyte defines the

properties of a fuel cell. Hence, fuel cell is often named by the nature of the electrolyte

used. There are presently six major fuel cell technologies at varying stages of

development and commercialization. They are alkaline, phosphoric acid, polymer

electrolyte membrane, molten carbonate, solid oxide and direct methanol fuel cells.

History:

The developments leading to an operational fuel cell can be traced back to the early

1800’s with Sir William Grove reorganized as the discoverer in 1839. Throughout the

remainder of the century, scientists attempted to develop fuel cells using various fuels

and electrolytes. Further work in the first half of the 20th century served as the

foundation for systems eventually used in the Gemini and Apollo space flights. How-

ever, it was not until 1959 that Francis T. Bacon success-fully demonstrated the first

fully operational fuel cell.

The GM 1966 Electro van was the automotive industry's first attempt at an automobile

powered by a hydrogen fuel cell. The Electro van, which weighed more than twice as

much as a normal van, could travel up to 70mph for 30 seconds.

In 2005 the British firm Intelligent Energy produced the first ever working hydrogen run

motorcycle called the ENV (Emission Neutral Vehicle). The motorcycle holds enough fuel

to run for four hours, and to travel 100 miles in an urban area, at a top speed of 50 miles

per hour. In 2004 Honda developed a fuel-cell motorcycle which utilized the Honda FC

Stack.

The 2001 Chrysler Natrium used its own on-board hydrogen processor. It produces

hydrogen for the fuel cell by reacting sodium borohydride fuel with Borax, both of

which Chrysler claimed were naturally occurring in great quantity in the United States.

The hydrogen produces electric power in the fuel cell for near-silent operation and a

range of 300 miles without impinging on passenger space. Chrysler also developed

vehicles which separated hydrogen from gasoline in the vehicle, the purpose being to

reduce emissions without relying on a nonexistent hydrogen infrastructure and to

avoid large storage tanks.

The first public hydrogen refuelling station was opened in Reykjavík, Iceland in April 2003. This station serves three buses built by DaimlerChrysler that are in service in the public transport net of Reykjavík. The station produces the hydrogen it needs by itself, with an electrolyzing unit (produced by Norsk Hydro), and does not need refilling: all that enters is electricity and water. Royal Dutch Shell is also a partner in the project. The station has no roof, in order to allow any leaked hydrogen to escape to the atmosphere.

There are numerous prototype or production cars and buses based on fuel cell technology being researched or manufactured by motor car manufacturers. In 2008, Honda released a hydrogen vehicle, the FCX Clarity. Meanwhile there exist also other examples of bikes and bicycles with a hydrogen fuel cell engine.

PROMINENT TYPES FUEL CELLS:

The most prominent high temperature fuel cells are: Molten carbonate Solid oxide

The most prominent low-temperature fuel cells are:

Alkaline Phosphoric Acid Proton Exchange Membrane (or solid polymer)

High temperature fuel cells:

Types of fuel cells differ primarily by the type of electrolyte they employ. The type of electrolyte, in turn, determines the operating temperature, which varies widely between types.

High-temperature fuel cells operate at greater than 1100 ºF (600 °C). These high temperatures permit the spontaneous internal reforming of light hydrocarbon fuels such as methane into hydrogen and carbon in the presence of water. This reaction occurs at the anode over a nickel catalyst provided that adequate heat is always available. This is essentially a steam reforming process.

Internal reforming eliminates the need for a separate fuel processor, and can use fuels other than pure hydrogen. These significant advantages lead to an increase in overall efficiency by as much as 15%. During the electrochemical reaction that follows, the fuel cell draws on the chemical energy released during the reaction between hydrogen and oxygen to form water, and the reaction between carbon monoxide and oxygen to form carbon dioxide. High temperature fuel cells also generate high-grade waste heat, which can be used in downstream processes for co-generation purposes.

High-temperature fuel cells react easily and efficiently with-out an expensive noble metal catalyst, such as platinum. On the other hand, the amount of energy released by the electrochemical reaction degrades as the reaction temperature increases.

High-temperature fuel cells suffer from severe materials problems. Few materials can work for extended periods without degradation within a chemical environment at high temperature. Furthermore, high temperature operation does not lend itself easily to large-scale operations and is not suitable where quick start-up is required. As a result, current high temperature fuel cells applications have focused on stationary power plants where the efficiencies of internal reforming and co-generative capabilities outweigh the disadvantages of material breakdown and slow start-up.

Low-temperature fuel cells:

Low-temperature fuel cells typically operate below 480 ºF (250 ºC). These low temperatures do not permit internal reforming, and therefore require an external source of hydro-gen. On the other hand, they exhibit quick start-up, suffer fewer materials problems and are easier to handle in vehicle (automotive) applications.

In low temperature fuel cell Proton Exchange Membrane (PEM) Fuel Cells is mostly used.

The main reason to use PEM type of fuel cell is, PEM type operates at very low range of temperature and pressure. Amount of volt produced in every cell.

Construction of pem fuel cell assembly:

PEM Fuel Cell Stack Construction:

Individual fuel cells have an maximum output voltage on the order of 1 VDC. Substantial voltages and power outputs are obtained by connecting many cells electrically in series to form a fuel cell stack, much like a armature in motor’s. Different designs of fuel cell stacks use fuel cells of varying dimensions and in varying quantities.

Physically, each fuel cell consists of a membrane electrode assembly (MEA), which consists of the anode, cathode, electrolyte and catalyst, sandwiched between two flow field plates made of graphite. The plates channel the fuel and air to opposite sides of the MEA.

Coolant is used to regulate the fuel cell reaction temperature. To facilitate this, cooling plates are placed between each fuel cell. These cooling plates channel the coolant past the fuel cells to absorb or supply heat as required. Seals between the graphite plates ensure that the oxidant, fuel and coolant streams never mix within the fuel cells.

Electrical endplates are placed at either end of the series of flow field plates. These endplates are connected to the terminals from which the output power is extracted, and typically include the fluid and gas interface connections. The entire sequence of plates is held together by a series of tie rods or other mechanical means.

Membrane Electrode Assembly (MEA):

The MEA is the heart of the fuel cell. The MEA consists of a solid polymer electrolyte membrane sandwiched between two porous carbon electrodes. A platinum catalyst is integrated between the membrane and the electrodes. The electrode assemblies often include integral seals where they contact adjacent components.

The electrodes provide the interface between the reactant gases and electrolyte. As such they must allow wet gas permeation, provide a reaction surface where they contact the electrolyte, be conductive to the free electrons that flow from anode to cathode, and be constructed of compatible materials. Carbon fiber paper is typically used for this purpose since it is porous, hydrophobic (non wetable), conductive and non-corrosive. The electrode material is very thin to maximize gas and water transport.

A catalyst is added to the surface of each electrode where it contacts the electrolyte in order to increase the rate at which the chemical reaction occurs. A catalyst promotes a chemical reaction by providing ready reaction sites but is not consumed in the process. Platinum is typically used for this purpose due to its high electro-catalytic activity, stability and electrical conductivity. Platinum is very expensive, so the amount used (known as the catalyst loading) is a significant factor in the cost of a fuel cell. Fuel cell designers strive to minimize the amount of platinum used while maintaining good cell performance.

Electrolyte:

The solid polymer electrolyte is the ultimate distinguishing characteristic of a PEM fuel cell.

The electrolyte is a thin membrane of a plastic-like film that ranges in thickness from50to175 (microns). These membranes are composed of perfluorosulfonic acids, which are Teflon-like fluorocarbon polymers that have side chains ending in sulphuric

acid group(–SO3

2–). Thus, PEM fuel cells ultimately use an acidic electrolyte just like

phosphoric acid fuel cells.

All acidic solid polymer electrolytes require the presence of water molecules for hydrogen ion conductivity since hydro-gen ions move together with water molecules during the ion exchange reaction. The ratio of water to hydrogen ions for effective conductivity is typically about 3:1. For this reason, the gases in contact with the membrane must be saturated with water for effective fuel cell operation.

A Solid Electrolyte Membrane for Ballard’s PEM Fuel Cells was shown in above figure:

A number of commercial membranes are available such as Nafion, produced by DuPont, and others by the Dow Chemical Company. In addition, fuel cell manufacturers like Bal-lard Power Systems have developed their own proprietary membranes.

All electrolytes must perform the fundamental functions of being a proton conductor, an electron insulator and a gas separator. In addition, manufacturers strive to produce membranes that have reasonable mechanical strength, dimensional stability (resistance to swelling), high ionic conductivity, low equivalent weight (the weight of polymer relative to the number of acid sites), and that are easily manufacturabel. To some extent, mechanical and dimensional stability of the polymer is provided through its integration into a membrane electrode assembly which adds a supporting structure.

Flow Field Plates:

The flow field plates channel fuel and oxidant to opposite sides of the MEA. Each flow field plate contains a single gas channel of serpentine design that maximizes gas

contact with the MEA. The specific shape of the gas channels is critical for uniform power generation, stable cell performance and correct product water management. Different flow field plate designs are tailored to various fuel cell applications.

Each plate must be electrically conductive so that the current released during the electrochemical reaction can flow from one cell to the next, and ultimately to the electrical endplates from which the stack power is drawn.

The plates are typically made of graphite into which the flow channels are either machined or pressed. Graphite is the preferred material due to its excellent conductivity, low contamination and relatively low cost.

Coolant plates, placed between each fuel cell, are of similar design and construction to the gas flow field plates. Coolant flow channels are designed for effective heat management.

Flow field and coolant plates incorporate gas and water ports used to distribute evenly the fuel, oxidant and coolant that enter and exit the stack. Seals between the graphite plates ensure that these flow streams do not mix.

Humidifiers:

Humidification of the reactant gases is an important aspect of PEM fuel cell operation.

Without adequate humidification, ion conduction cannot occur and fuel cell damage can

result. The amount of water that a gas can absorb is highly dependent on the humidification

temperature particularly at low pressure. Hotter gases can hold more water than colder gases.

Since the goal of humidification is to saturate the reactant gases with as much water as possible, the gases must be humidified at or near the fuel cell operating temperature (as set by the stack coolant temperature). If humidified at a lower temperature, the gas would no longer be saturated once it reached the operating temperature. If humidified at a higher temperature, some water would condense into the gas paths once it dropped to the operating temperature.

On some fuel cell stacks, humidifiers are integrated into the stack itself. On other fuel cell stacks, humidifiers are separate, external components.

Internal humidifiers consist of an additional series of graphite plates assembled into the fuel cell stack. This separates the stack into an active section, which contains the fuel cells, and an inactive section, which contains the humidifier plates. The humidification plates are similar to flow field plates and are used to channel gas and water to either side of a hydrophilic membrane. The water migrates across the membrane and saturates the adjacent gas. A variety of membranes are commercially available for this purpose.

Internal humidifiers draw water directly from the stack cool-ant stream and results in a simple, well integrated system with excellent temperature matching characteristics. How-ever, this arrangement precludes the use of anything other than pure water as coolant. Pure water exacerbates cold weather starting problems as the cooling water would freeze. In addition, the combined stack/humidifier tends to be bulky and complicates service since both components must be repaired concurrently.

External humidifiers are most commonly of either a membrane or a contact design. Membrane humidifiers operate in the same fashion as internal humidifiers although they are packaged separately. Contact humidifiers operate by spraying humidification water onto a hot surface or into a chamber of high surface area through which one of the reactant gases flows. The water then evaporates directly into the gas causing it to saturate.

Basic Operating principle of fuel cell:

In a fuel cell, the chemical energy of a fuel and an oxidant is converted directly into

electrical energy, which is exhibited in terms of cell potential and electrical current

output. The maximum possible electrical energy output and the corresponding

electrical potential difference between the cathode and anode are achieved when the

fuel cell is operated under the thermodynamically reversible condition. This maximum

possible cell potential is called reversible cell potential, one of the significantly important

parameters for fuel cells.

A fuel cell consists of an anode, an electrolyte, and a cathode. On the anode, the fuel is

oxidized electrochemically to positively charged ions. On the cathode, oxygen

molecules are reduced to oxide or hydroxide ions. The electrolyte serves to Transport

either the positively charged or negatively charged ions from anode to cathode

reactions in a fuel cell operating on hydrogen and air with a hydrogen-ion-conducting

electrolyte.

The hydrogen flows over the anode, where the molecules are separated into ions and

electrons. The ions migrate through the ironically conducting but electronically

insulating electrolyte to the cathode, and the electrons flow through the outer circuit

energizing an electric load. The electrons combine eventually with oxygen molecules

flowing over the surface of the cathode and hydrogen ions migrating across the

electrolyte, forming water, which leaves the fuel cell in the depleted air stream. An

energy system based on fuel cell technology requires a reliable hydrogen

infrastructure. At present, hydrogen is produced mostly through extraction from fossil

fuels. However, this might change in the future when electrolysis will become

prominent as a means to produce hydrogen.

The name PEM is derived from the electrolyte used which is a proton-conducting

polymer foil. PEM stands for Proton Exchange Membrane or Polymer Electrolyte

Membrane. Both sides of the membrane are coated with a thin layer of catalyst

material. These two layers form the electrolyser’s negative and positive electrode.

Electrolysis is the decomposition of water into hydrogen and oxygen. Electrolysers

essentially consist of a negative and a positive electrode as well as an electrolyte. PEM

electrolysers are characterized by their very simple and compact construction. PEM

electrolysers decompose pure water into hydrogen and oxygen. When a DC voltage is

applied to the fuel cell water molecules at the anode are oxidized to oxygen and

protons while electrons are released. The protons (H+ ions) pass through a proton-

conducting membrane to the cathode where they are reduced to hydrogen gas by

incorporating electrons from the outer circuit. In the process, oxygen gas accumulates

at the anode.

Proton exchange membrane fuel cells, also known as polymer electrolyte membrane

(PEM) fuel cell (PEMFC), are a type of fuel cell being developed for transport applications

as well as for stationary fuel cell applications and portable fuel cell applications. Their

distinguishing features include lower temperature/pressure ranges (50-100 degrees C)

and a special polymer electrolyte membrane.

To function, the membrane must conduct hydrogen ions (protons) but not electrons as this would in effect "short circuit" the fuel cell. The membrane must also not allow either gas to pass to the other side of the cell, a problem known as gas crossover. Finally, the membrane must be resistant to the reducing environment at the cathode as well as the harsh oxidative environment at the anode. Splitting of the hydrogen molecule is relatively easy by using a platinum catalyst. Unfortunately however, splitting the oxygen molecule is more difficult, and this causes significant electric losses. An appropriate catalyst material for this process has not been discovered, and platinum is the best option. One promising catalyst that uses far less expensive materials—iron, nitrogen, and carbon—has long been known to promote the necessary reactions, but at rates that are far too slow to be practical. Recently researchers at the Institute National de la Recherche Scientifique (INRS) in Quebec have dramatically increased the performance of this type of iron-based catalyst. Their material produces 99 amps per cubic centimetre at 0.8 volts, a key measurement of catalytic activity[.That is 35 times better than the best non precious metal catalyst so far, and close to the Department of Energy's goal for fuel-cell catalysts: 130 amps per cubic centimetre also matches the performance of typical platinum catalysts.

The only problem at the moment is its durability because after only 100 hours of testing the reaction rate dropped to half. Another significant source of losses is the resistance of the membrane to proton flow, which is minimized by making it as thin as possible, on the order of 50 μm. The PEMFC is a prime candidate for vehicle and other mobile applications of all sizes down to mobile phones, because of its compactness. However, the water management is crucial to performance: too much water will flood the membrane, too little will dry it; in both cases, power output will drop. Water management is a very difficult subject in PEM systems, primarily because water in the membrane is attracted toward the cathode of the cell through polarization. A wide variety of solutions for managing the water exist including integration of electroosmotic pumps. Furthermore, the platinum catalyst on the membrane is easily poisoned by carbon monoxide (no more than one part per million is usually acceptable) and the membrane is sensitive to things like metal ions, which can be introduced by corrosion of metallic bipolar plates, metallic components in the fuel cell system or from contaminants in the fuel / oxidant.

CONSTRUCTION OF FUEL CELL IN CAR’S:

Comparison of Fuel Cells with Batteries:

Fuel cells and batteries are both galvanic cells and therefore have many

similarities. Both fuel cells and batteries consist of an anode and a cathode in

contact with an electrolyte. Both devices generate electrical energy by

converting chemi-cal energy from a high energy state to a lower energy state

using an electrochemical reaction.

These reactions occur at the anode and cathode with elec-tron transfer forced

through an external load in order to complete the reaction. Individual cells of

both batteries and fuel cells generate only small DC voltages, which are then

combined in series to achieve substantial voltage and power capacities.

Fuel cells differ from batteries in the nature of their anode and cathode. In a

battery, the anode and cathode are metals; zinc or lithium is typically used for

the anode and metallic oxides for the cathode. In a fuel cell, the anode and

cathode are composed of gases often in contact with a platinum cata-lyst to

promote the power generating reaction. Hydrogen or a hydrogen-rich gas

mixture is typically used as the anode and oxygen or air as the cathode.

Fuel cells also differ from batteries in the fundamental method in which the

chemical reactants are stored. In a battery, the anode and cathode form an

integral part of the battery structure and are consumed during use. Thus, a

battery can only operate until these materials are fully con-sumed after which

it must either be replaced or recharged, depending on the nature of the

materials.

In a fuel cell, the chemical reactants are supplied from an external source so

that its materials of construction are never consumed and do not need to be

recharged. A fuel cell continues to operate as long as reactants are supplied and

the reaction products are removed.

Comparison of Fuel Cells with Internal Combustion

Engines:

Fuel cells and internal combustion engines share similarities of form. Both fuel

cells and internal combustion engines use gaseous fuel, drawn from an external

fuel storage system. Both systems use hydrogen rich fuel. Fuel cells use pure

hydrogen or a reformate gas mixture. Internal combustion engines typically

use hydrogen-containing fossil fuels directly, although they could be configured

to operate using pure hydrogen.

Both systems use compressed air as the oxidant; in a fuel cell engine the air is

compressed by an external compressor. In an internal combustion engine, the

air is compressed internally through piston action. Both systems require

cooling, although engines operate at higher temperatures than fuel cells.

In some respects, fuel cells and internal combustion engines are fundamentally

different. Fuel cells react the fuel and oxidant electrochemically whereas

internal combustion engines react the fuel and oxidant combustive. Internal

combustion engines are mechanical devices that generate mechanical energy

while fuel cells are solid state devices that generate electrical energy (although

the systems used to support fuel cell operation are not solid state).

Pollution is related to the fuel composition and the reaction temperature. Fuel

cell engines operating on pure hydrogen produce no harmful emissions; those

that operate on hydro-gen-rich reformate produce some harmful emissions

depend-ing on the nature of the process. Internal combustion en-gines

operating on pure hydrogen can be designed to produce almost zero harmful

emissions; those that run on conventional fuels produce significantly more

pollution.

Comparison of practical energy conversion efficiency (based on LHV) of different technologies as a function of scale:

The advantages of PEM fuel cells:

Tolerant of carbon dioxide. As a result, pem fuel cells can use unscrubbed air as oxidant, and reformate as fuel.

Operate at low temperatures. This simplifies materials issues, provides for quick start-up and increases safety.

Use a solid, dry electrolyte. This eliminates liquid han-dling, electrolyte migration and electrolyte replenishment problems.

Use a non-corrosive electrolyte. Pure water operation minimizes corrosion problems and improves safety.

Have high voltage, current and power density Operate at low pressure which increases safety Have good tolerance to differential reactant gas pressures Are compact and rugged Have relatively simple mechanical design Use stable materials of construction

Disadvantages of pem fuel cell:

Can tolerate only about 50 ppm carbon monoxide,

Can tolerate only a few ppm of total sulphur compounds ,

Need reactant gas humidification: Humidification is energy intensive and increases the complexity of the system. The use of water to humidify the gases limits the operating temperature of the fuel cell to less than water’s boiling point and therefore decreases the potential for co-generation applications.

Use an expensive platinum catalyst ,

Use an expensive membrane that is difficult to work with it.