Chemistry for Fuel CellsChemistry for Fuel Cells

Alternative Energy and Fuel Alternative Energy and Fuel CellsCells

Hocking Technical CollegeHocking Technical College

July 23, 2007July 23, 2007

What is Chemistry?What is Chemistry?

Chemistry is the study of matter and Chemistry is the study of matter and its transformations.its transformations.

Matter can simply be defined as Matter can simply be defined as “stuff” – anything that has mass and “stuff” – anything that has mass and occupies space. occupies space.

– Some matter can be seen and felt. Some matter can be seen and felt.

– Some matter is difficult to detect, but it Some matter is difficult to detect, but it still exists.still exists.

In order to define fuel In order to define fuel cell technology, some cell technology, some basic chemistry and basic chemistry and

physics must be physics must be understood.understood.

  A fuel cell produces electricity A fuel cell produces electricity

by means of a chemical by means of a chemical reaction.reaction.

2H2 + O2 2 H2O

All chemical compounds are All chemical compounds are made up of 2 or more elements, made up of 2 or more elements,

from a pool of about 100 from a pool of about 100 different elements, combined different elements, combined together in what is called a together in what is called a

chemical bond.chemical bond.   HH22– element– element

OO22– element– element

HH22O– chemical compoundO– chemical compound

Periodic Table of the Periodic Table of the ElementsElements

A chart that A chart that

organizesorganizes

elements intoelements into

columns columns

and rowsand rows

The nature of the The nature of the chemical bond must chemical bond must be understood to be understood to describe how a fuel describe how a fuel cell works.cell works.

Each element is made Each element is made up of atoms which are up of atoms which are

unique to that unique to that element. element.

All atoms of a given element are All atoms of a given element are essentially the sameessentially the same

All atoms of other elements are All atoms of other elements are very different from the atoms of very different from the atoms of the given element. the given element.

Atoms are made up of a Atoms are made up of a nucleus and electronsnucleus and electrons

surrounding the nucleus.surrounding the nucleus.

Electrons

Nucleus

The nucleus The nucleus In the very center of the atom In the very center of the atom Very small compared to the size of the Very small compared to the size of the

atom atom Does not take part in chemical reactions Does not take part in chemical reactions Contains protons (positively charged Contains protons (positively charged

particles)particles) Number of protons determines which Number of protons determines which

elementelement Contains neutrons (particles not charged)Contains neutrons (particles not charged)

Proton Neutron

Subatomic ParticlesSubatomic Particles

SubatomSubatomicic

ParticleParticle

ElectricalElectrical

ChargeChargeRelativeRelative

Mass Mass (amu)(amu)

Location Location in Atomin Atom

ProtonProton 11 1 1 In nucleusIn nucleus

NeutronNeutron 00 11 In nucleusIn nucleus

ElectronElectron 11 1/18371/1837 OutsideOutside

nucleusnucleus

Atomic NumberAtomic Number

Each element has its own atomic Each element has its own atomic number (no. of protons). That’s what number (no. of protons). That’s what makes the element unique.makes the element unique.

Atoms are uncharged (neutral).Atoms are uncharged (neutral).

Therefore, Therefore, # protons = # electrons# protons = # electrons

Law of Electrical NeutralityLaw of Electrical Neutrality

The ElectronsThe Electrons

Some of the electrons are on the Some of the electrons are on the edge of the atom.edge of the atom.

Very small mass compared to the Very small mass compared to the proton and neutron.proton and neutron.

Take part in a chemical reaction.Take part in a chemical reaction. The carrier of current in The carrier of current in

electricity.electricity.

Organization of ElectronsOrganization of Electrons

Electrons are found in energy levels (shells).Electrons are found in energy levels (shells).

Valence electronsValence electrons

– Electrons in outer shell Electrons in outer shell

– Involved in formation of compoundsInvolved in formation of compounds

Core electrons Core electrons

– Not found in outer energy levelsNot found in outer energy levels

Valence electrons (VE)Valence electrons (VE)

Valence Valence electronselectronsdeterminedeterminechemical chemical properties.properties.

No. of VE No. of VE equals equals Group NoGroup No..

Group Group ##

IAIA IIAIIA VAVA VIIIAVIIIA

# VE# VE 11 22 55 88

ExamplExamplee

HH MgMg NN NeNe

ExamplExamplee

NaNa CaCa PP ArAr

Oxygen atom has 8 Oxygen atom has 8 electronselectrons

Nucleus

Oxygen Atom

6 valence electrons (in outer 6 valence electrons (in outer shell)shell)

2 core electrons inside2 core electrons inside

Valence Electrons Valence Electrons

Core Electrons

Oxygen Atom

Hydrogen has 1 Hydrogen has 1 electronelectron

Nucleus

Hydrogen Atom

1 valence electron and 1 valence electron and no core electronsno core electrons

Valence Electron

No Core Electrons

Hydrogen Atom

Chemical BondingChemical Bonding

Occurs when the electrons of two or more Occurs when the electrons of two or more different atoms interact and form a different atoms interact and form a compound containing the atoms.compound containing the atoms.

The Valence Electrons form the bond and The Valence Electrons form the bond and can be completely removed from one can be completely removed from one atom and added to another to form the atom and added to another to form the bond (ionic).bond (ionic).

The Valence Electrons form the bond and The Valence Electrons form the bond and can be shared between the atoms to form can be shared between the atoms to form the bond (covalent bond). the bond (covalent bond).

Noble Gas TheoryNoble Gas Theory

G. N. Lewis discovered that atoms lose, G. N. Lewis discovered that atoms lose, gain, or share electrons to attain a gain, or share electrons to attain a noblenoble

gas arrangement.gas arrangement.

What does this mean?What does this mean?

– Noble gases have a full outer shell of Noble gases have a full outer shell of

8 electrons (except He which has 2 in a full 8 electrons (except He which has 2 in a full outer shell) – great stability!outer shell) – great stability!

An Ion Is Formed When An Ion Is Formed When An Atom Loses One Or An Atom Loses One Or

More ElectronsMore Electrons This will form a positively charged This will form a positively charged

ion.ion.

Cu Cu+2 + 2e-

Zn Zn+2 + 2e-

An Ion Is Formed When An Ion Is Formed When An Atom Gains One Or An Atom Gains One Or

More ElectronsMore Electrons This will form a negatively charged This will form a negatively charged

ion.ion.Cl + e- Cl-

O + 2e- O-2

Ionic CompoundIonic Compound

Formed when one atom pulls Formed when one atom pulls electrons from another atom to electrons from another atom to

obtain the octet of electronsobtain the octet of electrons

Cu Cu+2 + 2e-

O + 2e- O-2

Cu + O Cu+2O-2

Ionic Compound

Ions may be replaced by Ions may be replaced by another atom changing another atom changing

to an ion.to an ion.Zinc metal put into a solution of copper Zinc metal put into a solution of copper

(+2) ions (CuCl(+2) ions (CuCl22) will lose 2 electrons to ) will lose 2 electrons to the copper (+2) and become zinc (+2) the copper (+2) and become zinc (+2) ions.ions.

The copper (+2) ions will gain the The copper (+2) ions will gain the electrons from the zinc metal to electrons from the zinc metal to become copper metal.become copper metal.Cu+2Cl2

-1 + Zn Cu + Zn+2Cl2-1

Ion Replacement can be Ion Replacement can be Predicted by the Predicted by the

Electromotive SeriesElectromotive Series

Na+1 + 1 e-1 Na(s) –2.71 vZn+2 + 2 e-1 Zn(s) –.76 vFe+2 + 2 e-1 Fe(s) –.44 vCd+2 + 2 e-1 Cd(s) –.40 vPb+2 + 2 e-1 Pb(s) –.126 v2 H+1 + 2 e-1 H 2 (g) 0.000 vCu+2 + 2 e-1 Cu(s) +.337 vFe+3 + 1 e-1 Fe+2 +.771 vAg+1 + 1 e-1 Ag(s) +.799 vO2 (g) + 4 H +1 + 4 e-1 2 H 2 O(l) +1.229 v

Cu Zn battery uses this Cu Zn battery uses this reaction by separating the two reaction by separating the two

solutions by an electrolytic solutions by an electrolytic bridge.bridge.

Electrons do not cross the Electrons do not cross the electrolyte on the bridge.electrolyte on the bridge.

Ions cross the electrolyte bridge.Ions cross the electrolyte bridge.

Electrons move through the wire Electrons move through the wire since they cannot cross the bridge.since they cannot cross the bridge.

The Cu Zn cell is The Cu Zn cell is shown belowshown below

Cu+2

Cu metal

2e-

V

Zn+2

Zn metal1.10 volts

Na2SO4 salt bridge

Cu+2 + Zn Cu + Zn+2

In the Laboratory, we In the Laboratory, we use a will use a Porous use a will use a Porous

CupCup

Cu+2

SO4-2

Porous Cup

Beaker

Zn+2

SO4-2

Zinc Strip(anode)

Copper Strip(cathode)

Voltmeter+ -

Wire

Covalent CompoundCovalent Compound

Formed when neither atom Formed when neither atom reacting can pull the electron reacting can pull the electron from the other atomfrom the other atom

The atoms involved form a The atoms involved form a molecule and share the electrons molecule and share the electrons to obtain the “octet of electrons”to obtain the “octet of electrons”

2H2 + O2 2H2O

The reaction of The reaction of hydrogen and oxygenhydrogen and oxygen

In this reaction the oxygen and In this reaction the oxygen and hydrogen share their electrons.hydrogen share their electrons.

Hydrogen and oxygen react to Hydrogen and oxygen react to form water.form water.

This reaction can occur by burning This reaction can occur by burning the hydrogen in oxygen.the hydrogen in oxygen.

Energy is given off during the Energy is given off during the reaction.reaction.

When oxygen bonds with another When oxygen bonds with another atom it needs 2 extra electrons added atom it needs 2 extra electrons added to the outer shell to have 8 electrons to the outer shell to have 8 electrons in the outer shell. (inert gas Neon)in the outer shell. (inert gas Neon)

When hydrogen bonds with another When hydrogen bonds with another atom it needs 1 extra electron added atom it needs 1 extra electron added to the outer shell to have 2 electrons to the outer shell to have 2 electrons in the outer shell. (inert gas Helium)in the outer shell. (inert gas Helium)

Formation of waterFormation of water

Valence Electrons are Valence Electrons are shared in the formation shared in the formation

of water.of water.

Shared Electrons

Hydrogen Atom

Hydrogen Atom

Oxygen Atom

A fuel Cell works in the A fuel Cell works in the same way as the same way as the

batterybattery

Hydrogen and oxygen are Hydrogen and oxygen are separated by an electrolyte that separated by an electrolyte that will not conduct electrons across it.will not conduct electrons across it.

The electrons pass from one side The electrons pass from one side to the other by means of a wire.to the other by means of a wire.

Hydrogen and oxygen react to Hydrogen and oxygen react to form water.form water.

Fuel Cell Operates as a Fuel Cell Operates as a BatteryBattery

ElectrolyteH2

H2

O2

Catalyst Catalyst

Anode Cathode

Hydrogen and Oxygen Hydrogen and Oxygen are separated by an are separated by an electrolyteelectrolyte

H2

Electrolyte

H2

O2

CatalystCatalyst

Anode Cathode

A catalyst helps the A catalyst helps the hydrogen and electrons hydrogen and electrons

separate.separate.

H+

H+

e-

e-

Electrolyte

H+

H+

e-

e-OO

CatalystCatalyst

Anode Cathode

Hydrogen ions travel Hydrogen ions travel across the electrolyte. across the electrolyte.

Electrons travel through Electrons travel through the wire.the wire.

Electrolyte

H+

H+

H+

H+

e-e-

e-

e-

O

O

Anode Cathode

The hydrogen ions, The hydrogen ions, atomic oxygen, and atomic oxygen, and

electrons combine at the electrons combine at the cathode.cathode.

Electrolyte

H+

H+

e-

e-

H+

H+e-

e-

O

O

Anode Cathode

The hydrogen ions, The hydrogen ions, atomic oxygen, and atomic oxygen, and

electrons combine at the electrons combine at the cathode.cathode.

Electrolyte

H+

H+

e-

e-

H+

H+e-

e-

O

O

Anode Cathode

The reaction is The reaction is completed as water is completed as water is formed at the cathode.formed at the cathode.

Electrolyte

OHH

OHH

Water

Water

Anode Cathode

The Fuel Cell Produces The Fuel Cell Produces a Voltage and Heata Voltage and Heat

The maximum voltage of the cell The maximum voltage of the cell is 1.229 volts.is 1.229 volts.

Since some of the energy is given Since some of the energy is given off as heat, this maximum voltage off as heat, this maximum voltage is not realized.is not realized.