FC Advantages and Applications-1

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Fuel Cells: Advantages and Applications

Energy is the pervasive element of a modern industrial economy. A

substantial proportion of our present day energy is met through fossil fuels

derived from ultimately finite reserves and thus cannot be sustained

indefinitely in the longer term. Besides, the deleterious effects of excessive

consumption of carbonaceous fuels on the economy and ecology of a large

part of the world is already apparent and too well known to be recounted

here. While, much debate surrounds the utilization of fully renewable energy

sources, like solar, hydro, biomass, etc. these will be unable to provideadequate energy in the foreseeable future. Instead, it is being increasingly

realized that there is an urgent need to extract most usable energy from the

available carbonaceous fuels.

Of all the sources of carbonaceous fuels, petroleum is by far the most

convenient and therefore valuable. However, it is the least abundant and not

widely distributed, rendering countries with the largest reserves

disproportionate economic and political sway. Most estimates have

suggested that, at present and projected discovery, production and

consumption rates, world oil-supply will fail to meet the demand by about

ten years from now. Concerns of this kind have brought into sharp focus the

need to develop new, more energy efficient and environmentally benign

energy systems.

Electrochemical conversion of the chemical energy directly to

electrical energy is achieved through fuel cells. The words, fuel cells and

batteries, lead to the misconception that there must be a close connection

between the two devices. However the purpose of both devices is entirely

different. A fuel cell is an electrical energy producer. It takes a fuel (example

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H2, CH3OH), oxidizes it at anode and oxygen (or an oxidizing agent) is

reduced at the cathode. The free energy of the oxidation comes out directly

as electrical energy.

Batteries, on the other hand must have their electricity produced

elsewhere. It receives electrical energy, which drives a reaction on each of

the two electrodes respectively, up a free energy gradient for the overall

reaction made from the electrochemical reaction at each of the two

electrodes. This charged battery can then be put aside and the electricity is

stored till it is needed.

Figure 1(a) and 1(b)

The discovery of the first fuel cell principle making electricity

directly from chemicals is assigned to a British judge, Sir William Grove.

He knew that if one passes electricity through water it decomposes into

hydrogen and oxygen gases. Water was first electrolyzed by Nicholson and

Carlisle in 1800. Sir William knew that hydrogen and oxygen combines with

explosive zeal to make water. It struck him: If putting electricity in water

made hydrogen and oxygen, could putting hydrogen and oxygen into a cell

make water? He decided to try it. He took an electrolysis cell, disconnected

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the power source, bubbled hydrogen around one electrode and oxygen

around another as shown in this view graph (Fig. 1 (a) and (b)). He

measured the current between the two. To his great surprise a current flowed

through the wires. He found that the voltage between the two electrodes 1

was 0.6 V. He put several of these devices in series. With 50 of such

devices, each supplied with hydrogen and oxygen at the two electrodes he

produced some 25 30 V.

The principle that Grove discovered remains unchanged even today.

By definition, fuel cell is an electrochemical device that continuously

converts chemical energy into electrical energy for as long as fuel and

oxidant are supplied to it. Fuel cells bear similarities to batteries, with

which they share the electrochemical nature of the power generation process

and to the engines that, unlike batteries, will work continuously consuming a

fuel of some sort. A fuel cell operates quietly and efficiently and when

hydrogen is used as fuel, it generates only power and drinking water. Thus, a

fuel cell is a so called zero-emission engine. Thermodynamically, the most

striking difference is that thermal engines are limited by the Carnot

efficiency while fuel cells are not. Thermodynamics also tells us that an

energy conversion process that occurs at constant temperature is more

efficient than a process that relies on large temperature differences.

In the past, several fuel cell concepts have been tested in the

laboratory but the systems that are being potentially considered for

commercial development are: (i) Alkaline Fuel Cells (AFCs), (ii) Phosphoric

acid fuel cells (PAFCs), (iii) Polymer electrolyte fuel cells (PEFCs), (iv)

Solid polymer-electrolyte direct methanol fuel cells (SPE-DMFCs), (v)

Molten Carbonate Fuel Cells (MCFCs) and (vi) Solid Oxide Fuel Cells

(SOFCs).

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An alkaline fuel

cell, also known as

Bacon fuel cell after its

British inventor is

shown schematically in

this view graph (Fig.

2). The cell works

optimally around 80 oC Figure 2

and uses relatively inexpensive materials. The two electrodes are separated

by a porous matrix saturated with an aqueous alkali solution, such as

potassium hydroxide. In the electrolyte, hydroxyl (OH-) ions are available

and mobile. At the anode, these react with hydrogen, releasing energy and

electrons, and producing water, following the reaction.

2H2 + 4OH- 4H2O + 4e

-

At the cathode, oxygen reacts with electrons taken from the electrode, and

water in the electrolyte, forming new OH- ions following the reaction.

O2 + 2H2O + 4e- 4OH-

Note that although water is consumed at the cathode, it is created twice as

fast at the anode. Accordingly, the net cell reaction is,

O2 + 2H2 2H2O (1.23 V)

Alkaline fuel cells are most promising systems for space applications and

have been used successfully in Apollo and other space missions.

Alkaline Fuel Cells, however, suffer from carbonate fouling. When

carbon dioxide, CO2 is present in the fuel or oxidant, the alkaline electrolyte

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(KOH or NaOH) becomes poisoned through the conversion to carbonates

(K2CO3 / Na2CO3). Because of this, alkaline fuel cells typically operate on

pure oxygen, or at least purified air. The fuel when used from reformer is

also purified to remove carbon monoxide and carbon dioxide. The main

mechanism of poisoning is by blocking the pores of the catalyst layers with

K2CO3 / Na2CO3 which is not reversible. It reduces the ionic conductivity of

the electrolyte.

A Phosphoric Acid Fuel Cell (PAFC) is shown schematically in this

view graph (Fig. 3). The electrolyte consists of concentrated phosphoric acid

(98% H3PO4 and 2% water) in a silicon carbide matrix, which is used to

retain the acid while both the electrodes which also function as catalysts are

made from Pt or its alloys. A combination of 98% H3PO4 and 2% water

provides a liquid that can be heated to about 200oC at atmospheric pressure.

At these high temperatures phosphoric acid polymerizes to pyrophosporic

acid H4P2O7 which has considerably higher ionic conductivity than H3PO4.

At lower temperatures, phosphoric acid tends to be a poor ionic conductor

and CO poisoning of the Pt electrocatalyst in the anode becomes severe.

At the anode of a

Phosphoric Acid Fuel Cell,

hydrogen gas ionizes

releasing electrons and

creating protons followingthe reaction,

2H2 4H+ + 4e- Fig 3

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At the cathode, oxygen reacts with electrons taken from the electrode and

protons from the electrolyte to form water following the reaction.

O2 + 4H+ + 4e- 2H2O

Accordingly, the net cell reaction is given as,

O2 + 2H2 2H2O (1.23V)

Temperatures of about 390 deg F or 200 deg C is commonly used now

while the operating pressure has exceeded 8 atm in a 11 MW electric utility

demonstration plant.

The porous electrodes used in PAFCs contain a mixture of the

electrocatalyst supported on carbon black and a polymeric binder to bind the

carbon black particles together forming an integral structure A porous

carbon paper substrate serves as a structural support for the electrocatalyst

layer and as the current collector. The composite structure consisting of a

carbon black/binder layer on carbon paper substrate forms a three phase

interface, with the electrolyte on one side and the reactant gases on the otherside of the carbon paper.

Several designs for the bipolar plate and stack components are being

used. For example, in the multicomponent bipolar plates, a thin impervious

plate serves to separate the reactant gases in adjacent cells within the stack,

and separate porous plates with ribbed channels direct the gas flow. The

porous structure allows rapid gas permeability and also provides storage for

additional acid to replace the acid lost by evaporation during the operating

life of the cell.

Heat generated during cell operation is removed by either liquid or

gas coolants which are routed through cooling channels in the cell stack.

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Complex manifolds and connections are required for liquid cooling but

better heat transfer is achieved than with air cooling. Gas cooling on the

other hand has simplicity, reliability, and relatively low cost.

The conversion efficiency of fuel bound energy to electricity of a

PAFC is typically 40 to 47% on a fuel (natural gas) LHV basis.

Phosphoric Acid Fuel Cell systems have been under development for

many years, and some of the earliest demonstrations were in the 1970s under

the Target the acronym for Team to Advance Research for Gas Energy

Transformation program funded by the American Gas Association. Many

organizations have been involved in the development of Phosphoric Acid

Fuel Cells but the thrust of commercialization is now borne mainly by two

companies, UTC Fuel Cells which is based in Connecticut, USA, and Fuji

Electric in Japan. Alongside this development, Sanyo, Toshiba and

Mitsubishi Electric have built Phosphoric Acid Fuel Cell stacks in the 1980s

and early 1990s. However, as technical progress mushroomed in Polymer

Electrolyte Fuel Cells technology, many organizations have shelved their

activities in Phosphoric Acid Fuel Cells.

A Polymer Electrolyte Fuel Cell is shown schematically in this view

graph (Fig. 4). This type of

fuel cell consists of a

proton conducting

membrane, such as a

perfluorosulphonic acid

Polymer (commercially

called Nafion as the Figure 4

electrolyte which has good proton conducting properties, contained between

two Pt impregnated porous electrodes. The back of the electrodes are coated

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with a hydrophobic compound such as Teflon forming a wet proof coating

which provides a gas diffusion path to the catalyst layer.

The half-cell reactions occurring at the anode and cathode in a

Polymer Electrolyte Fuel Cell are given as follows.

At anode: 2H2 4H+ + 4e-

At the cathode: O2 + 4H+ + 4e- 2H2O

Accordingly, the net cell reaction is given as,

O2 + 2H2 2H2O (1.23 V)

Some of the advantages of the cell are that it may be operated at high

current densities resulting in a cell that has a fast start capability, compact

and light weight design, and that there is no corrosive fluid spillage hazard

because the only liquid present in the cell is water. Thus, a PEMFC is well

suited for use in vehicles. A disadvantage associated with this type of fuel

cell, however, is that Pt catalysts are required as promoters for the

electrochemical reaction.

The ionic conductivity of the electrolyte increases with the water

content. It is necessary to maintain high enough water content in the

electrolyte to avoid membrane dehydration and maintain proper ion

conductivity without flooding the electrodes. Thus, the balance between

production of the water by the oxidation of the H2 and its evaporation has to

be controlled.PEFCs are capable of operation at temperature of 175 deg F or 80 oC

and at pressures from 0.10 to 1.0 MPa (10 to 100 psig) and with suitable

current collectors and supporting structure, these fuel cells may be capable

of operating at pressures as high as 3000 psi.

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The conversion efficiency of fuel bound energy to electricity of a

PEMFC is 40 to 47% on a fuel (natural gas) LHV basis.

Over the years, Polymer Electrolyte Fuel Cells have advanced

substantially. The largest

Polymer Electrolyte Fuel

Cell system

demonstrated in the fuel

cell engine for buses

produced by Ballard

Power Systems is shown in this view graph (Fig. 5). The fuel cell in this

case has a power of about 260 kW, and the final motor drive is built into the

system hence, the manufacturers call this a fuel cell engine. The power

of the electric motor is 205 kW. Its dimensions are about 2.5 m wide, 1.6 m

deep, and 1.33 m high. The maximum output power of the motor is 205 kW

with the difference driving the compressors and various pumps needed by

the system. The main disadvantage is to humidify the electrolyte during its

operation.

Most fuel cells are powered by hydrogen, which can be fed to the fuel

cell system directly or can be generated within the fuel cell system by

reforming hydrogen-rich fuels such as methanol, ethanol, and hydrocarbon

fuels. Direct methanol fuel cells (DMFCs), however, are powered by pure

methanol, which is mixed with steam and fed directly to the fuel cell anode.

DMFCs do not have many of the fuel storage problems typical of some fuel

cells since methanol has a higher energy density than hydrogen though

less than gasoline or diesel fuel. Methanol is also easier to transport and

supply to the public using our current infrastructure since it is a liquid, like

gasoline.

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A Solid

Polymer

Electrolyte Direct

Methanol Fuel

Cell is shown

schematically in

this view graph

(Fig. 6).

The half-cell

reactions, occurring in a Solid Polymer Electrolyte Direct Methanol Fuel

Cell are as follows,

At the anode: CH3OH + H2O CO2 + 6H+ + 6e-

At the cathode: 3/2 O2 + 6H+ + 6e- 3H2O

Accordingly, the net cell reaction is,

3/2 O2 + CH3OH CO2 + 2H2O (1.21 V)

Since the methanol is mixed with ample amount of water, it avoids complex

humidification and thermal management problems associated with the

Polymer Electrolyte Fuel Cells. But, Solid Polymer Electrolyte Direct

Methanol Fuel Cells suffer from methanol crossover across the Nafion

membrane, which affects the cathode and hence the cell during its operation.

Molten carbonate fuel cells (MCFCs) are currently being developedfor natural gas and coal-based power plants for electrical utility, industrial,

and military applications. MCFCs are high-temperature fuel cells that use an

electrolyte composed of a molten carbonate salt mixture suspended in a

porous, chemically inert ceramic lithium aluminum oxide (LiAlO2) matrix.

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Since they operate at extremely high temperatures of 650C (roughly

1,200F) and above, non-precious metals can be used as catalysts at the

anode and cathode, reducing costs.

A Molten Carbonate Fuel

Cell and its operating principle

are depicted in this view graph

(Fig.7). The electrolyte

employed is generally a

mixture of alkali metal

carbonates and the ion conduction is through the carbonate ions. These salts

function as electrolyte only in liquid phase with the cell operating at 600-

700oC.

The half-cell reactions occurring in a Molten Carbonate Fuel Cell are

as follows.

At the anode: 2H2 + 2CO32- 2H2O + 2CO2 + 2e

-

At the cathode: O2 + 2CO2 + 2e- 2CO3

2-


2H2 + O2 2H2O ( 1.15V)

Improved efficiency is one of the reason MCFCs offer significant cost

reductions over phosphoric acid fuel cells (PAFCs). Molten carbonate fuel

cells can reach efficiencies approaching 60 percent, considerably higher than

the 37-42 percent efficiencies of a phosphoric acid fuel cell plant. When the

waste heat is captured and used, overall fuel efficiencies can be as high as 85

percent.

Unlike alkaline, phosphoric acid, and polymer electrolyte membrane

fuel cells, MCFCs don't require an external reformer to convert more

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energy-dense fuels to hydrogen. Due to the high temperatures at which

MCFCs operate, these fuels are converted to hydrogen within the fuel cell

itself by a process called internal reforming, which also reduces cost.

Molten carbonate fuel cells are not prone to carbon monoxide or

carbon dioxide "poisoning" they can even use carbon oxides as fuel

making them more attractive for fueling with gases made from coal. Because

they are more resistant to impurities than other fuel cell types, scientists

believe that they could even be capable of internal reforming of coal,

assuming they can be made resistant to impurities such as sulfur and

particulates that result from converting coal, a dirtier fossil fuel source than

many others, into hydrogen.

The primary disadvantage of current MCFC technology is durability.

The high temperatures at which these cells operate and the corrosive

electrolyte used accelerate component breakdown and corrosion, decreasing

cell life. Scientists are currently exploring corrosion-resistant materials for

components as well as fuel cell designs that increase cell life without

decreasing performance.

Currently, Fuel Cell Energy in the US is actively pursuing the

commercialization of Molten Carbonate Fuel Cells. Europe and Japan also

have several developers of this

technology. This view graph (Fig.

8) shows a 250 kW Molten

Carbonate Fuel Cell installed in a

hospital in Bielefield, Germany.

Figure 8

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A Solid Oxide Fuel Cell is shown schematically in this view graph

(Fig.9). The Solid Oxide Fuel Cellis a complete solid-state device that uses

an oxide ion-conducting ceramic material as the electrolyte. Stabilized

zirconia is commonly used as the electrolyte and the operating temperatures

of the Solid Oxide Fuel Cellsare between 900-1100oC.

Figure 9

The half-cell reactions occurring in a Solid Oxide Fuel Cellare given

as follows.

At the anode: 2H2 + 2O2- 2H2O + 4e

-

At the cathode: O2 + 4e- 2O2-


2H2 + O2 2H2O ( 1.15V)

Various types of Solid Oxide Fuel Cell systems are being developed in the

US, Europe and Australia. Siemens Westinghouse in the US has now

expanded its production facilities to cope with the demand for increased

number of Solid Oxide Fuel Cellsdemonstration systems.

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The efficiencies and envisaged applications of the aforesaid fuel cell systems

are summarized in this view graph (Table 1).

Table 1: The efficiencies and envisaged applications of various fuel cell

systems.

PAFC PEFC MCFC SOFC AFC DMFC

Electrical System

Efficiency (%) 36-45 32-40 43-55 43-55 26-31 40

Some Applications3

Cogeneration 2* 1* 3* 4*

Utility

Power

3* 2* 1*

Distributed Power 3* 4* 2* 1*

Utility Repowering 3* 2* 1*

Passenger Vehicles 3* 1* 2*

Heavy Duty Vehicles 2* 4* 1* 3*

Portable Power 2* 1*

Specialty Power 3* 1* 2*3 The degree of priority increases with number of asterisks.

Certain other hydrogen carrying fuels, such as sodium borohydride, which

has a hydrogen content of

about 11 wt. % and a

capacity value of 5.67 Ah/g,

are also being explored. A

Direct Borohydride Fuel

Cell is shown schematically

in this view graph (Fig.10).

The half-cell reactions occurring in the cell are as follows:

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At the anode: 8NaOH 8Na+ + 8OH-

NaBH4 + 8OH- NaBO2 + 6H2O + 8e

-

At the cathode: 2O2 + 4H2O + 8e- 8OH-


NaBH4 + 2O2 NaBO2 + 2H2O (1.64V)

The use of a liquid oxidant such as hydrogen peroxide, in Direct

Borohydride Fuel Cells extends their application domain to locations wherefree convection of air is limited, e.g. under-water applications. The

operating principle of such a Direct Borohydride Fuel Cell is shown

schematically in this view graph (Fig.11).

The half-cell reactions in such a Direct Borohydride Fuel Cell are as follows.

At the anode: 8NaOH 8Na+ + 8OH-

NaBH4 + 8OH- NaBO2 + 6H2O + 8e

-

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delivery to individuals who need regular doses, e.g. AIDS patients, is

depicted in this view graph (Fig.13).

There are varied advantages of fuel cells, which include efficiency,

simplicity, low emissions and silence. The advantages of fuel cells impact

particularly strongly on combined heat and power systems, and on mobile

power systems, especially for vehicles and electronic equipment, such as

portable computers, mobile telephones, and military communication

equipment. A key point is the very wide range of applications of fuel cell

power, from systems of a few watts up to several megawatts. In this respect,

fuel cells are quite unique as energy converters. As shown in this view

graph (Fig.14), their range of applications indeed far exceeds all others.

The most important limitation of fuel cells at the present time is their

cost, and therefore R&D efforts are being expended to make them cost-

effective.

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In the past, fuel cell technology has largely been the preserve of a

limited groups consisting primarily of electro and catalyst chemists, and

chemical engineers. There is a need to develop more people with knowledge

of fuel cell technology. The lack of a comprehensive exposure of fuel cells

and their applications has been a limiting factor in the inclusion of this

subject in academic undergraduate and graduate courses in science and

engineering. To me, this forum undoubtedly forms the ideal launching pad

to induct fuel cell science and technology into appropriate courses and post-

graduate activities.

References:

1. W. Grove,Phil. Mag. 14: 127 (1839).

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FC Advantages and Applications-1