The Electrochemical Series

VELECTRONS

IONS

The metal HIGHER in the electrochemical series LOSES electrons to the metal lower down.

ELECTRONS flow through the wires.

IONS flow through the solution to complete the circuit.

A cell

The electrochemical series (ECS)

AluminiumZincIronTinLead

Electrons flow from one metal to another to create a voltage

Imagine you connect different metals to TIN...

Mg Cu Sn

1.2 V -0.5 V 0 V

Mg Cu Sn

1.2 V -0.5 V 0 V

+ve – the metal is above the other metal in the ECS

-ve – the metal is below the other metal in the ECS

0 – the metal is the same as the

other metal

large number – large gap

between the metals in the ECS

small number – small gap

between the metals in the ECS

MagnesiumAluminiumZincIronTinLeadCopper

Above tin

Large gap

Below tin

Small gap

Introduction to Electrochemistry An electric cell converts chemical energy into electrical

energy Alessandro Volta invented the first electric cell but got his inspiration

from Luigi Galvani. Galvani’s crucial observation was that two different metals could make the muscles of a frog’s legs twitch. Unfortunately, Galvani thought this was due to some mysterious “animal electricity”. It was Volta who recognized this experiment’s potential.

An electric cell produces very little electricity, so Volta came up with a better design:

A battery is defined as two or more electric cells connected in series to produce a steady flow of current Volta’s first battery consisted of several bowls of brine (NaCl(aq))

connected by metals that dipped from one bowl to another His revised design, consisted of a sandwich of two metals

separated by paper soaked in salt water.

8 © Alexis Kwasinski, 2012

Li-ion batteries• Positive electrode: Lithiated form of a transition metal oxide (lithium cobalt oxide-LiCoO2 or lithium manganese oxide LiMn2O4)

• Negative electrode: Carbon (C), usually graphite (C6)

• Electrolyte: solid lithium-salt electrolytes (LiPF6, LiBF4, or LiClO4) and organic solvents (ether)

http://www.fer.hr/_download/repository/Li-ION.pdf

discharge

9 © Alexis Kwasinski, 2012

Li-ion batteries• Chemical reaction (discharge)

• Positive electrode

• Negative electrode

•Overall

• In the above reaction x can be 1 or 0

• With discharge the Co is oxidized from Co3+ to Co4+. The reverse process (reduction) occurs when the battery is being charged.

LiCoO2 Li1-xCoO2 + xLi+ + xe-

xLi+ + xe- + 6C LixC6

Through electrolyte

Through load

LiCoO2 + C6 Li1-xCoO2 + C6Lx

10 © Alexis Kwasinski, 2012

Li-ion batteries

• Advantages with respect to lead-acid batteries:• Less sensitive to high temperatures (specially with solid electrolytes)• Lighter (compare Li and C with Pb)• They do not have deposits every charge/discharge cycle (that’s why the efficiency is 99%)• Less cells in series are need to achieve some given voltage.

• Disadvantages:• Cost

What is a hydrogen fuel cell?•Hydrogen fuel cells (HFCs) are a type of electrochemical cell.

•HFCs generate electricity by reduction and oxidation reactions within the cell.

•They use three main components, a fuel, an oxidant and an electrolyte.

•HFCs operate like batteries, although they require external fuel.

•HFCs are a thermodynamically open system.

•HFCs use hydrogen as a fuel, oxygen as an oxidant, a proton exchange membrane as an electrolyte, and emit only water as waste.

How do they work?•Fuel (H2) is first transported to the anode of the cell

•Fuel undergoes the anode reaction

•Anode reaction splits the fuel into H+ (a proton) and e-

•Protons pass through the electrolyte to the cathode

•Electrons can not pass through the electrolyte, and must travel through an external circuit which creates a usable electric current

•Protons and electrons reach the cathode, and undergo the cathode reaction

Chemistry behind the technologyOxidation

At the anode of the cell, a catalyst (platinum powder)

is used to separate the proton from the electron in

the hydrogen fuel. Anode half-reaction:

2H2 4H+ + 4e-

Eo = 0.00V

Reduction

At the cathode of the cell, a second catalyst (nickel) is used to

recombine the protons, electrons, and oxygen atoms to

form water. Cathode half- reaction:4H+ + O2 + 4e- 2H2O

Eo = 0.68V

In electrochemistry, the Eocell value (energy) of a fuel cell is equal to the Eo

of the cathode half-reaction minus the Eo

of the anode half-reaction. For a hydrogen fuel cell, the two half reactions are shown above. So to calculate the energy of one fuel cell, we need to subtract the anode energy from the cathode energy. For a HFC, the Eo

cell = 0.68V – 0.00V which equals 0.68V

Uses of hydrogen fuel cellsThere are many different uses of fuel cells being utilized right now. Some of these uses are…

•Power sources for vehicles such as cars, trucks, buses and even boats and submarines

•Power sources for spacecraft, remote weather stations and military technology

•Batteries for electronics such as laptops and smart phones

•Sources for uninterruptable power supplies.

Voltaic Cells (Galvanic Cell) A device that spontaneously produces electricity by redox Uses chemical substances that will participate in a spontaneous redox

reaction. The reduction half-reaction (SOA) will be above the oxidation half-reaction (SRA) in the

activity series to ensure a spontaneous reaction.

Composed of two half-cells; which each consist of a metal rod or strip immersed in a solution of its own ions or an inert electrolyte. Electrodes: solid conductors connecting the cell to an external circuit Anode: electrode where oxidation occurs (-) Cathode: electrode where reduction occurs (+) The electrons flow from the anode to the cathode (“a before c”) through an

electrical circuit rather than passing directly from one substance to another A porous boundary separates the two electrolytes while still allowing ions to flow

to maintain cell neutrality Often the porous boundary is a salt bridge, containing

an inert aqueous electrolyte (such as Na2SO4(aq) or KNO3(aq)),

Or you can use a porous cup containing one electrolyte which sits in a container of a second electrolyte.

Voltaic cells can be represented using cell notation:

The SOA present in the cell always undergoes reduction at the cathode

The SRA present in the cell always undergoes oxidation at the anode

Voltaic Cells (Galvanic Cells)

The single line represents a phase boundary (electrode to electrolyte) and the double line represents a physical

boundary (porous boundary)

The single line represents a phase boundary (electrode to electrolyte) and the double line represents a physical

boundary (porous boundary)