Chapter 15 Electrochemical Engineering
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Transcript of Chapter 15 Electrochemical Engineering
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Chapter 15
Electrochemical Engineering
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What we will study
Electrochemical principlesAnodes and cathodes
Half cells and simple electrochemical cells
Fuels CellsRagone plot and battery capacities
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Why electrochemical engineering?
Batteries and fuel cells are deeply embedded in “GreenEnergy” because solar and wind energy systems need tostore electrical energy
It’s also a hot topic because some vehicles use batteriesfor propulsion such as in hybrid cars and trucks
Electrochemical engineering is the study of whathappens inside batteries, fuel cells and ‘ultracapacitors’
So be prepared for a little more chemistry!
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Green Energy
“Green energy” refers to renewable energy suppliesthat do not spew forth greenhouse gases nor toxicimpurities
Wind and solar energy are two favored sources
Big disadvantage #1: Only works when the wind blowsor the sun shines
Big disadvantage #2: May make too much electricityexactly when you don’t need it.
Solution: Store the electrical energy until you do needit.
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Load leveling
Electricity not used whengenerated, nor available
when needed if the sun orwind go down
Solution: Battery storage
These may be very
large batteries!
Mismatch of green electric power and use
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Solar and Wind Need Equipment!
Batteries
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Why Do Batteries Work?
Matter is inherently electrically charged. Simplestcase is ionic bonding (i.e., attraction) in compoundssuch as common table salt: Na+Cl- in which Coulombicforces hold together positively charged sodium ionsand negatively charged chlorine ions. The forcebetween these ions is:
where e is the charge on an electron and r is theinterionic distance. k is the dielectric constant, whichis about 80 for water.
2
2
kr
eF =
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Electrochemistry
When Na+Cl- dissolves in water with k ≈ 80, the forcesbetween ions lessen allowing free ions as Na+ and asCl-
Ion-containing solutions are called “Electrolytes”
Overall ion-containing solutions are electricallyneutral
Locally ion-containing solutions have both charged
species at short distances to each otherBatteries use these ions when they can beseparated
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Electrolytes, Anodes and Cathodes
Electrodes are classifiedas anodes and cathodes
Anodes are “sources” ofelectrons, and cathodesare “sinks” for electrons
e- = electrons
E = Electrolyte
C = Cathode
A = Anode
e- flow
E
A C
- +
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Electrolytes, Anodes and Cathodes
Anodes are “sources”of electrons, andcathodes are “sinks”
for electronse- = electrons
E = Electrolyte
C = Cathode
A = Anode
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e- flow
E
A C
+-
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Electrolytes, Anodes and Cathodes
Beware Franklin’serror!
e- = electrons
E = ElectrolyteC = Cathode
A = Anode
Conventional Current flow
E
A C
- +
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Lead-Acid Batteries
These are the batteries you find in a car
Both electrodes are based on lead, Pb one with aPbO2 coating
The electrolyte is sulfuric acid written H2SO4,
which dissolved in water is 2H+
/SO=
4 (the sulfateion has two electrons/molecule)
The principle anodic reaction is: Pb → Pb++ + 2e-
The two electrons flow through the external
circuit to the cathode on which:PbO2 + 4H
+ + SO4= + 2e- = PbSO4 + 2H2O
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Lead-Acid Batteries
Product of reaction is PbSO4
which precipitates duringdischarge and dissolves during charging.
The anodic voltage at the anode is 0.36V above areference cell and the cathodic is 1.69 V below.
Overall cell voltage = ~2.0 VA C
E
Anodic
0.36 V Reference
cell
Cathodic
1.69V
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Can You Power a Car Using Batteries ?
Property Lead-acidbattery
Gasoline
Mass 25. kg 25. kg
Energy 3,000 kJ 1.2 105 kJ
Mass energy storage density 120 kJ/kg 46,500 kJ/kg
Volumetric energy storagedensity
250 kJ/liter 34,400 kJ/liter
Power 5 kW Typically >100 kW
This battery is too heavy, contains too little energy, anddelivers too little power – that’s why hybrids are a
popular substitute.
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The Electrochemical SeriesCell voltage set by the tendency to transfer
electrons
Half Cell Chemistry Potential in volts
Li+ + e
-Li(s) –3.05 V
Na+ + e
- Na(s) –2.71 V
Mg++ + 2e- Mg(s) –2.37 VZn
++ + 2 e
- Zn(s) –0.76 V
Fe++
+ 2 e- Fe(s) –0.44 V
Ni++
+ 2e- Ni(s) –0.25 V
2H+ + 2 e
- H2(g) 0.00V (Hydrogen ½ cell is defined as zero)
Cu
++
+ 2 e
-
Cu(s) 0.34 VCu
+ + e
- Cu(s) 0.52 V
Ag+ + e
- Ag(s) 0.80 V
Pd++
+ 2e- Pd(s) 0.95 V
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The Electrochemical Series
These half-cell reactions in principle arereversible. The more negative the more theywant to lose electrons; the more positive themore to gain them
This determines how cells will behave
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Daniell cell
The electrolytes areZnSO4(aq) with a Zn anodeand CuSO
4
(aq) with a Cucathode. Write down thereactions in each half celland explain what happensin the salt bridge. What is
the voltage?
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Daniell cell
In bulk aqueous solution , Zn
++
and SO4=
must be inbalance with each other.
( )4 4ZnSO Aq Zn SO++ =↔ +
But the anode is also dissolving and thus yields
some locally extra Zn++ ions according to:
Zn(s) Zn 2e (V 0.76V)
++ −↔ + = +
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Daniell cell
Electrons from the anode flow through the externalcircuit precipitating copper at the cathode:
( )Cu 2e Cu s 0.34V++ −+ ↔ +We have removed copper ions from solution; therefore
there must be a corresponding reduction in SO4= ions
from the electrolyte. They must move into the salt
bridge to exactly counteract the Zn++ ions from the
anodic side.
The cell potential is equal to:
(+0.76 V) + (+0.34 V) = +1.10 V.
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Electroplating
An electroplater wants to coat a 10.0 cm by10.0 cm copper plate with 12.5 micrometersof silver. How many electrons must pass in
the external circuit? How many coulombsare passed? If the plating takes 1,200. swhat’s the electrical current in amperes in
the external circuit?
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Electroplating
Know: Atomic mass of Ag is108 kg/kmol. Its density is10,500 kg/m3. Avogadro’snumber (NAv) is 6.02 × 10
26
atoms/kmol. What we callcurrent is nothing but therate of flow of electrons, so1.00 A = 1.00 coulomb/s and
one electron carries –1.60 ×10-19 coulombs.
Ag+, NO3- , H2O
Ag Cu
+ -
Electron
flow
Conventional
current flow
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Electroplating
The reaction at the anode is Ag(s) → Ag+ + e-
and the reaction at the cathode is Ag+ + e- →Ag(s); hence one atom of silver dissolves at
the anode and one atom of silver isdeposited at the cathode. For each atom ofsilver dissolving at the anode and depositingat the cathode, one electron must circulate in
the external circuit.
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Electroplating
[ ]-6
22 324
-3
12.5×10 ×100.×10,500 cmmMass =μm cm kg/m /μm m1.00×10
=1.31×10 kg
• Next convert to kmols:
[ ] kmols1022.1kg
kmolskg
108
1031.1kmols 5
3−
−
×=
×=
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Electroplating
atoms1032.7
]/][[1032.7
1002.61022.1
deposited atomsAg
21
21
265
×=
×=×××
=
−
kgatomskg
Next convert to atoms of Ag(s):
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Electroplating
A975.0]C/s[
.1200
1017.1currentHence
coulombs1017.1
]coulombs/e][[6010.11032.7
3
3
-1921
=×
=
×=
×× −− e
Next convert to mass of Ag(s):
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Fuel CellsFuel cells are just continuously refueled batteries.
They will not discharge while electrochemicalfuel is being fed to them.
Most fuel cells depend on “Proton Exchange
Membrane” or “PEM” to catalyze electrodereactions
RxnCathodic 022244
RxnAnodic442
22
2
H OH H Oe H
e H H
⇒+⇒++
+⇒−+−+
−+
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Fuel Cells
http://aq48.dnraq.state.ia.us/prairie/images /fuelcell.jpg
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Fuels cells
Note: Only H2 and O2(i.e.,air) in and only H2Oout.
Cell voltage is 1.23 V for overall
rxn H2 + O2 = H2O
Apparently no green house gas pollution!
Unfortunately to make H2 needs copious CO2
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The Ragone ChartBatteries must supply both energy and power
Typically batteries supply current measured at mA/cm2 ofelectrode area at a few volts
The more electrode area, the greater the current; this maybe internal area packing or simply more cells placed in
parallelThe more cells in series the higher the voltage
But it’s the application that demands whether the cellcan deliver both enough energy (say mileage between
charges on an electric car) and power (say to passanother car)
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The Ragone Chart
The best measures of energy and power efficiency aretheir mass densities: e = E /Wt (Whr/kg) and p = P/Wt(W/kg)
The energy density delivered by a power source for a
time t is simply e = p × t.Take log base 10 of this equation:
log10 e = log10 p + log10 t
Plot the log10 energy density of a battery vs. log10power density for the same battery and you getthe Ragone Plot
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The Ragone Chart
Modified from a graphic of Maxwell Technologies: http://www.maxwell.com.
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The Ragone Chart
Very convenient way to compare differentelectrochemical sources
Ideally you want to have your cake and eat it by
being in the upper right cornerReality shows what can be achieved by competingelectrochemical sources
“Ultracapacitors” are storage devices that can
store thousands of times the energy capabilityof an electrical capacitor.
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Ragone Chart
That that the form oflog10 e = log10 p + log10t is y = mx + c and has a
slope of m =1 giventhe log10 scaling inchart.
The battery’s discharge
time is given by e/ p
1,000
100
10
1
0.1
0.01
E n e r g y , W
h r / k g
10 100 1,000 10,000
Power, W/kg
1 h o u
r
3 6 0 s
3 6 s
3. 6 s
0. 3 6 s
3 6 m s
1 0 h o
u r s
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Summary
Green energy and load storage and leveling
Electrochemical series
Simple electrical cells
Simple electrochemistry
Principles of fuel cells
Ragone chart to characterize
E l i E i i