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PROCESS SYSTEMS ENGINEERING GROUP

SCHOOL OF ENGINEERING

PROCESS SYSTEMS ENGINEERING M.Sc.

(ENERGY SYSTEMS AND THERMAL PROCESSES)

Academic Year 2010-2011

PSE 16: POWER GENERATION

OLUMIDE OLUMAYEGUN

ESSAY ON FUEL CELLS

18 November 2010

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ABSTRACT

This report seeks to give an overview of fuel cells as means of generating

electricity. The aim is to have a better understanding of fuel cells and it various

application. Fuel cell is a developing technology and this report will also enumerate

the future prospect of this mode of generating electricity. Fuel cell can be

considered as an electrochemical device which takes fuel and generate electricity

as it output. It coverts chemical energy directly to electrical energy in a more

efficient manner when compared to heat engines.

Fuel cells can be categorised based on the electrolyte used. The major types of

fuel cells based on the electrolytes are Polymer electrolyte membrane fuel cell

(PEMFC), Alkaline fuel cell (AFC), Phosphoric acid fuel cell (PAFC), Molten

carbonate fuel cell (MCFC), Solid oxide fuel cell (SOFC). The last three belong to a

class of fuel cell regarded as medium or high temperature fuel cells. Direct

methanol fuel cell is also a type of fuel but it considered separately because it is

classified not on the basis of the electrolyte used but on the basis of the fuel,

methanol.

Direct methanol fuel cell has a lot of advantages which include high net energy

density, the fuel is readily available and cheap, small weight of the fuel, and ease

of transportation. But the fuel cell has not witnessed full commercialisation because

of the high cost of producing it, the technical expertise regarding its production is

also not fully developed, and there is lack of common standard among the

organisations producing the fuel cell.

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LIST OF CONTENTS

Title page

Abstract ii

List of Contents iii

1. PRINCIPLE OF FUEL CELL 1 1.1 Introduction 2 1.2 Fuel Cell Operation 3 1.3 Gibbs Free Energy, Electrical Work, and Voltage of Fuel Cell 3 2. TYPES OF FUEL CELLS 4 2.1 Polymer Electrolyte Membrane Fuel Cell (PEMFC) 4 2.2 Alkaline Electrolyte Fuel Cell (AFC) 5 2.3 Phosphoric Acid Fuel Cell (PAFC) 6

2.4 Molten Carbonate Fuel Cell (MCFC) 6 2.5 Solid Oxide Fuel Cell (SOFC) 7 3. DIRECT METHANOL FUEL CELLS (DMFC) 7 3.1 Advantages of Direct Methanol Fuel Cells 8

3.2 Disadvantages of Direct Methanol Fuel Cells 8 3.3 Obstacles to Commercialisation of Direct Methanol Fuel Cells 9 REFERENCES 10

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1. PRINCIPLE OF FUEL CELL

1.1 Introduction

Fuel cells can be considered as an electrochemical device which takes fuel and

generate electricity as it output. In as much as there is supply of fuel it keeps

producing electricity without using up its component parts in the process. This

differentiate a fuel cell from a battery, another electrochemical devices. In a battery

the electrodes are consumed in the process of producing electricity. Fuel cells also

differ from combustion or heat engines in that the chemical energy of the fuel is

converted directly into electricity. In combustion engines the energy of the fuel is

first converted into heat energy which can then be converted to mechanical energy

to generate electricity. This is a very complex process that involves a lot of energy

losses, thus the fuel cell is a better converter chemical energy to useful work than

the heat engines.

The available energy that can be realized from a fuel cell comes from the

difference between the bonding energy of the products and the reactants in the

electrochemical reaction. Take the reaction of hydrogen with oxygen for example,

this can be regarded as a combustion process with hydrogen as the fuel:

H2 + 1

2O2 H2O (1.1)

On molecular scale hydrogen gas bonds and oxygen gas bonds are broken and

hydrogen-oxygen bonds are formed by transferring electrons between the

molecules. The bonding energy of the water molecule is lower than the bonding

energies of the hydrogen and oxygen gases. In fuel cells the excess energy is

turned to useful work by causing the electrons to be transferred across a

conducting wire thereby converting chemical energy to electrical energy.

Conversion efficiency can be as high as 60-80% compared to steam engines which

can only be as high as 50% (Winterbone, D.E., 1997).

The electrochemical half reaction for hydrogen combustion is as shown below:

H2 2H+ + 2e- (1.2)

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1

2O2 + 2H+ + 2e- H2O (1.3)

By spatially separating the reaction sites for the oxidation and reduction half

reaction the electrons released by the first half reaction is made to flow across an

external circuit and hence do useful work before completing the reaction. The

separation of the reaction sites is achieved by means of an electrolyte. Electrolyte

allows the passage of ions but will not allow the electrons to flow through. A fuel

cell will contain at least two electrodes, providing the centres for the half reaction,

separated by the electrolyte. The diagram below is a schematic representation of a

simple fuel cell. The electrodes are made of platinum immersed in aqueous acid

electrolyte.

Electric load

e-

Platinum electrode

Electrolyte

H+

H2 O2

Figure 1: Simple fuel cell

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1.2 Fuel Cell Operation

The production of electricity in a fuel cell can be simplified into four basic steps

(O'Hayre, R. P. et al, 2006):

a. Reactant delivery into the fuel cell.

b. Electrochemical reaction – the rate of the chemical reactions at the

electrodes will determine the amount of current that will be produced by the

fuel cell. Catalyst are employed at the electrodes to increase the reaction

rate.

c. Ionic conduction through the electrolyte and electron conduction through the

external circuit .

d. Product removal from the fuel cell – the product of the reaction need to be

remove from the system else they will build up and stop further reaction

from taking place.

The real voltage delivered by a fuel cell is usually less than the thermodynamically

predicted voltage due to losses associated with above steps. For a fuel cell with an

ideal voltage, Eth the real voltage, V can be expressed as:

V = Eth + ρact - ρohmic - ρconc (1.4)

where,

ρact - losses due to activation energy of the electrochemical reaction.

ρohmic- ohmic losses due to ionic conduction through the electrolyte and electron

conduction through external circuit.

ρconc- Concentration losses due to delivery and removal of material.

1.3 Gibbs Free Energy, Electrical Work, and Voltage of Fuel Cell

Gibbs free energy represent the work potential of a system. Hence the maximum

electrical work that can be performed by a fuel cell is given by the change in Gibbs

free energy of the system:

Welect = -Δg (1.5)

Electrical work can be defined as:

Welect = EQ (1.6)

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If n moles of electrons were transferred then the total charge transferred would be:

Q = nF (1.7)

where F is the Faraday constant. Combining the above three equation yields

Δg = -n FE (1.8)

Hence the reversible voltage obtainable from a fuel cell is

E = g

nF (1.9)

2. TYPES OF FUEL CELLS

Fuel cells can be classified based on their electrolyte. The main types of fuel cell

are:

1. Polymer electrolyte membrane fuel cell (PEMFC).

2. Alkaline fuel cell (AFC).

3. Phosphoric acid fuel cell (PAFC).

4. Molten carbonate fuel cell (MCFC).

5. Solid oxide fuel cell (SOFC).

Direct methanol fuel cell is another type of fuel cell but most researcher consider it

as belonging to a different class as its classification is based on the fuel and not on

the electrolyte used. Phosphoric acid, molten carbonate and solid oxide fuel cells

belong to a class of fuel cell regarded as medium or high temperature fuel cells.

2.1 Polymer Electrolyte Membrane Fuel Cell (PEMFC)

PEMFC uses a proton conducting polymer, perflourinated proton-exchange

membranes, as the electrolyte. The polymer membrane is coated on both side with

a thin layer of platinum-based catalyst and porous carbon electrode. This kind of

sandwich arrangement of electrode-catalyst-membrane-catalyst-electrode is often

referred to as membrane electrode assembly (MEA). The whole structure is less

than 1mm in thickness.

The polymer membrane must contain some amount of water in order to keep its

conductivity hence it operating temperature is limited to 90 0C and below. Also

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because of the low operating temperature the only practical catalyst are platinum-

based catalysts. Direct methanol (liquid fuel) can be used as fuel for PEMFC apart

from the conventional hydrogen fuel. Research into the suitability of liquid fuel for

this type fuel cell is still ongoing. One of the major advantage of PEMFC over other

type of fuel cell is its high power density (300-1000 mW/cm2). It also possess a

good start-stop capability. The PEMFC is suitable for portable application because

of its low-temperature operating condition.

The many advantages of PEMFC are overshadowed by its many limitations which

include stringent thermal control requirement, moisture control, expensive

materials, and susceptibility to CO poisoning (Bocarsly, A. B. and Niangar, E. V. ,

2009). Most of these limitation can be minimised if the PEMFC can be designed to

operate at elevated temperature of about 120-150 0C. Hence recent research has

been in the development high temperature PEMFC, obtaining better catalytic

action with less expensive materials and mitigation of poisoning of polymer

electrolyte membrane by impurity gases.

2.2 Alkaline Electrolyte Fuel Cell (AFC)

The electrolyte employed in AFC is potassium hydroxide and unlike the acid

electrolyte fuel cell where H+ is conducted from the anode to the cathode in the

AFC OH- is conducted from the cathode to the anode. The reaction at the anode

and cathode is shown below:

Anode: H2 + 2OH- 2H2O + 2e- (1.10)

Cathode: 1

2O2 + 2e- + H2O 2OH- (1.11)

Water is produced twice as fast as is been consumed hence the need to remove

excess water to avoid dilution of the electrolyte. The main limitation of AFC for

terrestrial application is the degradation the KOH electrolyte by CO2 poisoning as

shown below, therefore pure H2-O2 must be used.

2OH- +CO2 CO32- +H2O (1.12)

Advantages offered by AFC include improved cathode performance, non-precious

metal such as nickel can be used as catalyst and extremely inexpensive

electrolyte.

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2.3 Phosphoric Acid Fuel Cell (PAFC)

Phosphorous acid fuel cell uses liquid H3PO4 as electrolyte. The electrolyte is

embedded in SiC matrix between two porous graphite electrodes coated with a

platinum catalyst (O'Hayre, R. P. et al, 2006) . The half reaction is as follows:

Anode: H2 2H+ + 2e- (1.13)

Cathode: 1

2O2 + 2H+ + 2e- H2O (1.14)

PAFC will operate optimally in the temperature range of about 180- 210 0C with

electrical efficiency of about 40% but could be as high as 70% when used in a

combined heat and power application. The technology of PAFC is relatively

matured but current research interest is how to make it cost competitive with

conventional power technologies.

2.4 Molten Carbonate Fuel Cell (MCFC)

In MCFC the electrolyte is a molten mixture of Li2CO3 and K2CO3 sustained by

LiOAlO2 matrix. The mobile charge carrier is carbonate ion, CO32- and the reaction

at the anode and cathode is as follows:

Anode : H2 + CO32- CO2 +H2O + 2e- (1.15)

Cathode: 1

2O2 + CO2 + 2e- CO3

2- (1.16)

There is the need for recirculation of CO2 because the CO2 is produced at the

anode to be consumed at the cathode. It does not suffer from CO poisoning like

most other fuel cell, the CO is actually a fuel. The anode electrode is usually

nickel/chromium alloy while the cathode is made of lithiated nickel oxide hence the

advantages of using non-precious catalyst. It enjoys fuel flexibility as hydrogen,

methane and simple alcohol could be used as fuel. One of its advantage is the

corrosive nature of the molten electrolyte. MCFC is most suitable for stationary,

continuous power application.

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2.5 Solid Oxide Fuel Cell (SOFC)

SOFC is made of solid oxide ceramic electrolyte sandwiched between two porous

electrodes (Sammes, N.M. and SpringerLink, 2006). The common electrolyte

material is yttria-stabilised Zirconia (YSZ). The anode is usually made of Ni/8YSZ

material while typical cathode material is strontium doped LaMnO3 (LSM). In SOFC

O2- is the mobile ion conductor and the reactions at the anode and cathode are

Anode: H2 + O2- →H2O +2e- (1.17)

Cathode: O2 + 2e- → O2- (1.18)

Water is produced at the anode unlike PEMFC, AFC and PAFC in which water is

produced at the cathode. The operating temperature is between 600 and 1000 0C.

Although the high operating temperature could lead to low open circuit voltage but

at the same time it increases its performance with the possibility of inwardly

processing hydrocarbon.

3. DIRECT METHANOL FUEL CELLS (DMFC)

Direct methanol fuel cells use methanol, a liquid fuel, in place of pure hydrogen. It

require water at the anode as an additional reactant. Even though in principle

methanol could be used with any of the electrolytes its use is only practicable with

the PEMFC. If used in the high temperature electrolyte fuel cells methanol will lose

the simplicity of its being a liquid fuel. Also it produce CO2 which makes it

inapplicable in AFC. The reaction at the electrodes are

Anode: CH3OH +6OH- → CO2 +5H2O +6e- (1.19)

Cathode: 1 O2 + 6H+ 6e- → 3H2O (1.20)

The overall reaction is

CH3OH + 1 O2 → 2H2O +CO2 (1.21)

One major characteristics of DMFC is that there is a considerable voltage loss at

the anode in addition to the loss at the cathode common to all other fuel cell

(Larminie, J., Dicks, A. and Knovel, 2003).

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3.1 Advantages of Direct Methanol Fuel Cells

High net energy density – energy density of methanol is close to those of

hydrocarbon fuels. The net energy density of methanol is about 18.9 MJ/kg

compared to hydrogen which is about 0.72 MJ/kg (Larminie, J., Dicks, A.

and Knovel, 2003).

Readily available and cheap fuel – Methanol can be produced from

various sources such as wood, natural gas and coal. This makes the fuel

readily available and cheap.

Reduce weight of the fuel cell – DMFC does not need a fuel vaporiser

which is a complex, heavy and space consuming component of other fuel

cells running on pure hydrogen. This makes DMFC light weight compared to

other fuel cells.

Simpler to use and could be refill easily – Methanol being a liquid is

easier to use and could be refill easily. It does not require reformer like

hydrogen fuel and could be used at room temperature and pressure.

Ease of transportation - the technology for transporting liquid material is

well developed hence methanol could be delivered to the public using

existing infrastructure.

3.2 Disadvantages of Direct Methanol Fuel Cells

Slow rate of reaction at the anode – the reaction at the anode progresses

so much slowly when compared to hydrogen reaction. The reaction occurs

by many complex steps which can even produce CO as intermediate

product. The CO can act as a poison. The overall effect is that DMFC has

low exchange current resulting in high activation overvoltage loss at the

anode (O'Hayre, R. P., et al, 2006).

Fuel crossover – DMFC also experience significant crossing over of

methanol from the anode to the cathode through the electrolyte. The

polymer electrolyte membrane commonly use with DMFC is a very good

absorber of methanol. The methanol will mix with water and crosses rapidly

to the cathode. This will result in reduced open circuit voltage as methanol

that could have reacted at the anode to liberate electrons is being lost

through this process.

Loss of fuel through evaporation – for any reasonable increase in the

temperature of the fuel cell methanol will evaporate and escape with the

carbon dioxide gas. This amounts to loss of useful fuel and in some systems

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the methanol have to be condensed out of the carbon dioxide thereby

adding to the complexity of the system.

Poor performance compared to the hydrogen fuel – as a result of the

above limitations of DMFC its performance is very poor when compared to

proton exchange membrane fuel cells running on hydrogen.

3.3 Obstacles to Commercialisation of Direct Methanol Fuel Cells

Despite the many benefit of Direct Methanol fuel cells there are still many hurdles

to be crossed before full commercialisation can be achieved. One of the major

hindrances to commercialisation of the fuel cell is the high cost of production which

translate to high price for the end users. DMFC uses expensive platinum as the

cathode and even the more expensive platinum/ rubidium as the anode. Price has

been a major limitation to full commercialisation of DMFC; electrolyte made of thick

expensive polymer and even with that still waste a lot of fuel.

Another obstacles to commercialization of DMFC has to do with technical expertise

regarding the production of the fuel cells. Currently DMFC have low overall

efficiency and power density and there is also the need for miniaturisation. Further

technological development in electrode and electrolyte manufacturing is necessary

for rapid move toward commercialization. Zhang J. (2008) was of the opinion that

„for early DMFC commercialization the target total catalyst loading for the entire

membrane electrolyte assembly (MEA) (composed of both cathode and anode

catalyst layers as well as membrane) must be reduced from the current 2.0–8.0

mgPt cm-2 to a level < 1.0 mgPt cm-2 without performance compromise‟.

Also lack of common standard among the organisation involved in the production

of DMFC poses an obstacles to the commercialisation. According to Makuch G.

(2004) there is need for “cross-organizational coordination to ensure market

acceptance and commercial success”. There is the need for standardisation of the

packaging especially the methanol cartridge. This will allow the users flexibility in

the purchase and use of the component parts.

Furthermore government regulation/legislation regarding the use of methanol in

some countries needs to be amended in order to hasten the journey toward

commercialisation of DMFC. Methanol is classified as hazardous and inflammable

material and even restricted on some airlines. The regulations need to be amended

to delineate between the use of methanol as fuel in fuel cell in which case it has

been properly packaged in cartridges and other common use.

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REFERENCES

Bocarsly, A. B. and Niangar, E. V. (2009), "FUEL CELLS - PROTON-EXCHANGE

MEMBRANE FUEL CELLS | Membranes: Elevated Temperature", in Jürgen

Garche (ed.) Encyclopedia of Electrochemical Power Sources, Elsevier,

Amsterdam, pp. 724-733.

Larminie, J., Dicks, A. and Knovel, ( 2003), Fuel cell systems explained, 2nd ed., J.

Wiley, Chichester, West Sussex.

Makuch G. (2004), Micro Fuel Cells Strive for Commercialization

http://powerelectronics.com/power_management/battery_portable_power_manage

ment/power_micro_fuel_cells_3/ (Accessed 2010 14/11)

O'Hayre, R. P., Cha S., Colella W., and Prinz F. B., (2006), Fuel cell fundamentals,

John Wiley & Sons, Hoboken, N.J.

Sammes, N.M. and SpringerLink, ( 2006), Fuel cell technology, Springer, London

Winterbone, D.E., ( 1997), Advanced Thermodynamics for Engineers, Elsevier.

Zhang J. (2008), PEM Fuel Cell Electrocatalysis, Institute for Fuel Cell Innovation, National Research Council Canada. http://www.scitopics.com/PEM_fuel_cell_electrocatalysis.html (Accessed 2010 14/11)