REDOX - Feasibility, Cells and Batteries

8/10/2019 REDOX - Feasibility, Cells and Batteries

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Redox Equilibria

Feasibility, Cells and Batteries


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Recap: Using Eo values to calculate Ecell

Ecell= Eo

rightEo

left

If this value is positive then the reaction is

feasible.

.Howevera reaction may not be visible as it

may not be kinetically feasible (large activation

energy).


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When it goes wrong - Example

.... When the conditions are not standard!

e.g. Cu2++ Zn Zn2++ Cu

Example: changing concentration

if the half equations are in equilbrium...

i) increase Zn2+ conc will shift equlibirum to left. (REDUCESEASE OF ELECTRON LOSS).

Ecell becomes more negative

ii) increase Cu2+ conc will shift equilibrium to right. (INCREASESEASE OF ELECTRON GAIN).

Ecell becomes more positive.


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Recap: Using Eo values to calculate Ecell

Ecell

= Eoright

Eoleft

If this value is positive then the reaction isthermodynamically feasible.

.Howevera reaction may not be visible as itmay not be kinetically feasible (large activationenergy).

SO..Eo

values refer to standard conditionsonly..so a reaction with a neg. Ecell value maytake place if e.g. the concentration of the solutionwas > 1M.


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NOTE: You will need your electrochemical series forthis.

Its pretty simple....

The more reactive a metal is, the more likely it is tolose electrons and form a positive ion.

MORE REACTIVE metals have more NEGATIVEelectrode potentials and vice versa for nonmetals.

Reactivity and REDOXReactions


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Component of an Electrochemical Cell

Cell Potential

A cell can be made from 2 electrodes (metals orelectronic conductors) dipping into an electrolytesolution and connected by an external circuit.

The electrical potential or voltage, E, between the twoelectrodes in a cell can be measured by connecting avoltmeter.

The measured voltage is known as the cell potential, E.Older terminology refers to the electromotive force,

EMF.

The cell potential is related to the Total Entropychange for the overall reaction carried out by the cell.


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Cell Potential

S = nFEo

= Total Entropy change, Jmol-1

K-1

n = number of electrons transferred in the cellreaction

F = Faradays constant, 96,500 Coulombs mol-1

Eo = cell potential, volts, V Thus the cell potential is equivalent to a measure of

how much work can be done by the electronsflowing through the external circuit of the cell. It is

strictly a THERMODYNAMICmeasurement of the cell(ie not kinetic)


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Relationship between Sotot, K and Eo

We can find the Total Entropy Change, and the equilibriumconstant for the cell reaction.

Sotot= nFEo= RlnK

R is the gas constant (8.31 Jmol-1K-1)

Eo= potential of a cell under standard conditions (Volts)

Ln K = logarithm to the base e of the equilibrium constant. n = number of electrons transferred in the cell

reaction

F = Faradays constant, 96,500 Coulombs mol-1

So the total entropy change is proportional to the cell potentialwhich is also proportional to the equilibrium constant.If Eo is positive then Sototwill be positive and hence spontaneousand feasible (and K will be greater than 1).We can use these values to predict if the equilibrium will lie to the

left or to the right (left if negative, right if positive)


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What is a Fuel Cell?

Quite simply, a fuel cell is a device that converts chemical energy into

electrical energy, water, and heat through electrochemical reactions.

Fuel and air react when they come

into contact through a porous

membrane (electrolyte) which separates

them.

This reaction results in a transfer of

electrons and ions across the electrolyte

from the anode to the cathode.

If an external load is attached to this

arrangement, a complete circuit is formedand a voltage is generated from the flow

of electrical current.

The voltage generated by a single cell is typically rather small (< 1 volt), so many

cells are connected in series to create a useful voltage.


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Galvanic cell (battery)Hydrogen fuel cell

Open system

Anode and cathode are gases in

contact with a platinum catalyst.

Reactants are externally supplied,

no recharging required.

Closed system

Anode and cathode are metals.

Reactants are internally consumed,

need periodic recharging.

Fuel Cell Vs. Battery

Basic operating principles of both are very similar, but there are several

intrinsic differences.


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Fuel Cells in Use: Transportation Systems

Many of the major car companies are developing fuel cell car prototypes

which should come to market during the next decade. The cars use eitherpure hydrogen or methanol with an on board reformer.


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FCVFuel cell vehicle

Use hydrogen rich fuelsmethanol,

natural gas, petrol

Used reformer to convert fuel tohydrogen at 250 -300Oc

CH3OH + H2O3H2+ CO2


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Hydrogen Fuel Cell

Between the reduction and oxidation stages, the

electrons are routed through a circuit. Electrodes

Are usually porous graphite

eHH 222

Hydrogen ions (protons) permeate through the

electrolyte membrane (solid polymer)

POSITIVE electrode: Reduction reaction

NEGATIVE electrode Oxidation reaction

(facilitated by a catalyst - typically Pt or Ni)

OHeHO 22 22

1.23 V


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Polymer Electro lyte Membrane

Polymers such as polyphenylenes are

used

Water is a crucial participant in the

process

absorption of water increases the

proton conductivity

membrane is confined not free

to swell pushes electrodes


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Animation of Hydrogen Fuel

Cell


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Animation of a fuel cell workingfuel-cell-animation.swf

The pressurized hydrogen gas (H2) entering thefuel cell on the anode side.

This gas is forced through the catalyst by the

pressure. When an H2molecule comes incontact with the platinum on the catalyst, itsplits into two H+ions and two electrons (e-).

The electrons are conducted through the anode,where they make their way through the externalcircuit (doing useful work such as turning amotor) and return to the cathode side of the fuel

cell.
http://localhost/var/www/apps/conversion/tmp/scratch_10/fuel-cell-animation.swfhttp://localhost/var/www/apps/conversion/tmp/scratch_10/fuel-cell-animation.swfhttp://localhost/var/www/apps/conversion/tmp/scratch_10/fuel-cell-animation.swfhttp://localhost/var/www/apps/conversion/tmp/scratch_10/fuel-cell-animation.swfhttp://localhost/var/www/apps/conversion/tmp/scratch_10/fuel-cell-animation.swfhttp://localhost/var/www/apps/conversion/tmp/scratch_10/fuel-cell-animation.swf


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Meanwhile, on the cathode side of the fuel cell,

oxygen gas (O2) is being forced through the catalyst,

where it forms two oxygen atoms.

Each of these atoms has a strong negative charge.

This negative charge attracts the two H+ions through

the membrane, where they combine with an oxygenatom and two of the electrons from the external circuit

to form a water molecule (H2O).

This reaction in a single fuel cell produces only about0.7 volts.

To get this voltage up to a reasonable level, many

separate fuel cells must be combined to form a fuel-

cell stack.


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Problems with Fuel Cells The fuel cell uses oxygen and hydrogen to produce electricity.

The oxygen required for a fuel cell comes from the air.

In fact, in the PEM fuel cell, ordinary air is pumped into thecathode.

The hydrogen is not so readily available, however.

Hydrogen has some limitations that make it impractical for use

in most applications. For instance, you don't have a hydrogen pipeline coming to

your house, and you can't pull up to a hydrogen pump at yourlocal gas station.

Hydrogen is difficult to store and distribute, so it would be muchmore convenient if fuel cells could use fuels that are morereadily available.

This problem is addressed by a device called a reformer.

A reformer turns hydrocarbon or alcohol fuels into hydrogen,which is then fed to the fuel cell.
http://science.howstuffworks.com/fuel-processor.htmhttp://science.howstuffworks.com/fuel-processor.htm


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A Battery (cel l)

Redox (Oxidation-Reduction Reaction)

Baghdad Battery250 BC

Negative

Brass terminal

Positive


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Batteries and Fuel Cel l

Similarities

Chemical potential

energy converted in

to Electric potential

energy Cellular structure

Redox reactions

Differences

Passage of H2and O2

thru vs. storage of

chemicals in battery

Flow battery

A battery has all of its chemicals

stored inside, and it converts those

chemicals into electricity too. This means that a battery (primary)

eventually "goes dead" and you

either throw it away or recharge it.


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Standard Zinc Carbon Batteries

Chemistry

Zinc (-), manganese dioxide (+)

Zinc, ammonium chloride aqueous electrolyte

Features

+ Inexpensive, widely available

Inefficient at high current drain

Poor discharge curve (sloping)

Poor performance at low temperatures


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Heavy Duty Zinc Chloride

Batteries ChemistryZinc (-), manganese dioxide (+)

Zinc chloride aqueous electrolyte

Features (compared to zinc carbon)

+ Better resistance to leakage

+ Better at high current drain

+ Better performance at low temperature


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Standard Alkaline Batteries

Chemistry

Zinc (-), manganese dioxide (+)

Potassium hydroxide aqueous electrolyte

Features

+ 50-100% more energy than carbon zinc

+ Low self-discharge (10 year shelf life)

Good for low current (< 400mA), long-life use

Poor discharge curve


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Alkaline-Manganese Batteries

(2)


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Alkaline Battery Discharge


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How Disposable Batteries Work Both electrodes generate

electrons when they dissolve

The cathode generates more

than the anode

To remove electrons from the

anode, positive ions plate or

stick to the anode

A membrane separates the A+

and B+ions

Eventually, positive ions

accumulate near the cathode

and are depleted near the anode


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Secondary Cells

These are

rechargeable

When they are

recharged, the

current is reversed.Original chemicals

are reconstituted.


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The Lead Acid Battery (secondary cell) Two electrodes, one of

lead, the other of lead

dioxide (PbO2) immersed

in sulfuric acid Lead ions (Pb++)

dissolve, leaving two

electrons behind

Two electrons flow

through the circuit and

are used to help lead

dioxide dissolve


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Lead-Ac id Batter ies

e.g. car batteries, deep-cycle batteries

Energy-to-weight ratio very low

Energy-to-volume ratio: low

But .Power-to-Weight ratio:

LARGE

RECHARGABLE

)(2)(4)(

2

4)(2

)(4)(24)(

224:

2:

lsaqs

saqs

OHPbSOeHSOPbOcathode

ePbSOSOPbanode

(2V per cell, 6 cells per

battery)


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Lead-Acid

The formation of insoluble lead sulfate

creates a potential problem as if it builds

up it prevents the reverse process and so

the cells cannot be recharged.

Recharging = reversing the flow of current

so that electrodes are regenerated.


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PH 0101 Unit-5 Lecture-7 31

There are two types of lithium-based batteries available.

1. Lithium batteries2. Lithium-ion batteries

In lithium batteries,a pure lithium metallic element is

used as anode. These types of batteries are not

rechargeable (i.e. primary batteries).

In lithium-ion batteries, lithium compounds are used

as anode.

These batteries are known as re-chargeablebatteries. Therefore, Lithium ion batteries are

considered as best than pure Lithium based

batteries.


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32

Lithium-ion battery (Li-ion Battery)

Li-ion batteries are secondary batteries.

The battery consists of a anode of Lithium, dissolved as

ions, into a carbon.

The cathode material is made up from Lithium liberatingcompounds, typically the three electro-active oxide materials,

Lithium Cobalt-oxide (LiCoO2)

Lithium Manganese-oxide (LiMn2O

4)

Lithium Nickel-oxide (LiNiO2)


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Principle

During the charge and discharge processes, lithium ions

are inserted or extracted from interstitial space between

atomic layers within the active material of the battery.

Simply, the Li-ion is transfers between anode and cathode

through lithium Electrolyte.

Since neither the anode nor the cathode materials essentially

change, the operation is safer than that of a Lithium

metal battery.


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34

Li-Ion battery Principle

Li- ion Electrolyte


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35

Construction

The electrolytes are selected in such a way that thereshould be an effective transport of Li-ion to the cathode

during discharge.

The type of conductivity of electrolyte is ionic innature rather than electronic


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36

The lithium ion is inserted and exerted

into the lattice structure of anode andcathode during charging and

discharging

During discharge current flows through

external circuit and light glowsDuring charging, no the electrons flows

in the opposite direction


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37
http://localhost/var/www/apps/conversion/Local%20Settings/Temporary%20Internet%20Files/Content.IE5/O1KXGNFG/final/Li-Ion_big.gif


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During charging, lithium in positive electrode material is

ionized and moves from layer to layer and inserted intothe negative electrode.

During discharge Li ions are dissociated from the anode

and migrate across the electrolyte and are inserted into

the crystal structure of the host compound of cathode.

At the same time the compensating electrons travel in

the external circuit and are accepted by the host to balance

the reaction.

The process is completely reversible. Thus the lithium

ions pass back and forth between the electrodes during

charging and discharging.


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39

Because of this reason, the lithium ion batteries are called Rocking chair, Swingcells.

A typical Li-ion battery can store 150 watt-hours of

electricity in 1 kilogram of battery as compared to lead acid

batteries can sore only 25 watt-hours of electricity in onekilogram

All rechargeable batteries suffer from self-discharge

when stored or not in use.

Normally, there will be a three to five percent of self-

discharge in lithium ion batteries for 30 days of storage.


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Advantages

They have high energy density than other rechargeable

batteries

They are less weight

They produce high voltage out about 4 V as compared

with other batteries. They have improved safety, i.e. more resistance to

overcharge

No liquid electrolyte means they are immune from leaking.

Fast charge and discharge rateDisadvantage

They are expensive

They are not available in standard cell types.

5. Advantage, disadvantage and applications


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Appl icat ions

The Li-ion batteries are used in cameras, calculators

They are used in cardiac pacemakers and other

implantable device

They are used in telecommunication equipment,

instruments, portable radios and TVs, pagers

They are used to operate laptop computers and

mobile phones and aerospace application

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REDOX - Feasibility, Cells and Batteries

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