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0581.5271 Electrochemistry for Engineers LECTURE 10 Lecturer: Dr. Brian Rosen Office: 128 Wolfson...
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Transcript of 0581.5271 Electrochemistry for Engineers LECTURE 10 Lecturer: Dr. Brian Rosen Office: 128 Wolfson...
0581.5271 Electrochemistry for Engineers
LECTURE 10
Lecturer: Dr. Brian Rosen Office: 128 Wolfson
Office Hours: Sun 16:00
HW #5 Clarifications
nF
HV
ENTROPY IN UNITS OF J / (mol*K)
Batteries
Recall Important Definitions
• Coulombs – Unit of CHARGE• Amperes – Unit of CURRENT [Coulombs per second] • Volts – Unit of POTENTIAL [Joules per Coulomb]• Watt – Unit of POWER [Joules per second]• Joule – Unit of ENERGY [Watts x seconds]
– Watt-hour (Wh) is also a unit of energy [Watts x hours]
• POWER DENSITY – Rate of Energy Transfer per unit volume or mass [kW/m3 or kW/kG]
• • ENERGY DENISTY – The amount of energy
stored in a given system [kJ/m3 or kJ/kg]
Recall Important Definitions Pt 2
Basic Operating Principle
A AO Oxidation at Anode (-)
BO BReduction at Cathode (+)
BAOBOA GGGG
The chemical energy within the bonds of the “charged” state is greater than that ofthe discharged state
Charged State Discharged State
Batteries can be classifieds as two types as primary batteries and secondary batteries.
Primary batteries
In primary batteries, the electrochemical reaction is not reversible.
During discharging the chemical compounds are permanently changed and electrical energy is released until the original compounds are completely exhausted.
Thus the cells can be used only once.
Primary Batteries
Secondary batteries
In secondary batteries, the electrochemical reaction is reversible and the original chemical compounds can be reconstituted by the application of an electrical potential between the electrodes injecting energy into the cell.
Such cells can be discharged and recharged many times.
Secondary Batteries
For Example
• Leclanché Battery (Primary)
• Nickel-Cadmium Battery (Secondary)
32
arg
22 OMnZnOMnOZnedisch
The zinc + Manganese (II) oxide system has a greater enthalpy than the zinc oxide and Mn (III) oxide
222 )()(2)(2 OHCdOHNiOHOOHNiCdd
c
Energy Density
Inside A Battery
Lead-Acid Battery
Pb-Acid Battery: The Anode
Pb-Acid Battery: The Anode
-
Pb-Acid Battery: The Cathode
Pb-Acid Battery: The Cathode
Pb-Acid Battery: Discharging
Pb-Acid Battery: Charging
Nernst Equation for Pb-Acid Battery
OHPbSOSOHHPbOPb 24422 2222
1log0592.0931.1 42SOHH aa
E
Self Discharge (Leakage Current)
eHPbSOSOHPb
OHPbSOeHSOHPbO
d
c
d
c
22
222
442
24422
OHPbSOSOHPbOPbd
c24422 222
(+)
(-)
Electrochemical reaction, permitted by thermodynamics, can occur on the electrode Surface and must be balanced by the discharge of the electrode (since the cell is at open circuit)
eAA
Since the potential of the (+) terminal is very high, side reactions can occur.
If the potential of the (+) terminal is above the reduction potential for the side reaction
the electrons produced by the side reaction will be consumed by discharging the (+) terminal
Self Discharge of (-) Terminal
eHPbSOSOHPb
HeH
22
22
442
2
Self Discharge of (+) Terminal
eHOOH
OHPbSOeHSOHPbO
222
1
2
1
222
22
24422
Store Batteries in the Fridge!
Why is Lead Advantageous for Storing Chemical Energy?
Battery Polarizability
IREE concactOCP
Why is the charging curve abovethe discharge curve?
Charge-Discharge Curve at Constant Current
activation overpotential
Depletion
• Resistive drops at electrodes (lead sulfate is a poor conductor)
• Electrolyte gradient near the electrode surface (depletion)
• Resistance of ionic movement through electrolyte (ohmic losses)
• Activation overpotentials
Mechanisms Affecting Voltage
Battery Capacity, C and Cp
Effect of Discharge Rate on C
Example Data (U. Colorado)
Theoretical Specific Capacity
M
nFq
3600
s
hr
kg
mol
mol
As
g
mAh
3600
1
M = molecular weight in kg/molF = faraday constantn = number of electronsq = specific capacity
Practical Specific Capacity
W
jAtq cutoffprac 3600
W = weight of catalyst in gA = electrochemical area area in cm2
j = current density in mA/cm2
q,prac = practical specific capacity
Why is the utilization generally below 100%?
nutilizatioq
qprac %100
Theoretical Specific Energy
W
dtitVcutofft
3600
)(0
gJ
g
ssJ
g
sW
V = voltage (function of time)i = current (held constant)T,cutoff = cutoff timeW = catalyst weight
Theoretical Specific Power
cutoff
cutoff
t
t
dtW
dtitV
0
0
)(
V = voltage (function of time)i = current (held constant)T,cutoff = cutoff timeW = catalyst weight
Battery Efficiency
Typical coulomb efficiency = 90%
Approximate voltage efficiency =(2V/2.3V) = 87%
Energy efficiency = (90%)(87%) = 78%
Charging Management
Solubility of Discharge Products
Initial Discharge Recharge
Soluble dischargeproduct
Insoluble dischargeproduct
Dendrite Formation
• Particularly susceptible when using Li or Zn electrodes
Zn dendrite formation and inhibitionby polyethylene glycol
“Short Circuit”
Cycle Testing
Need for Porous Electrode Materials
• Lead electrodes need to have high surface area for high energy density
• Without high porosity, surface would passivate quickly
Cobasys batteries
• Negative electrode: Metal Hydride such as AB2 (A=titanium and/or vanadium, B= zirconium or nickel, modified with chromium, cobalt, iron, and/or manganese) or AB5 (A=rare earth mixture of lanthanum, cerium, neodymium, praseodymium, B=nickel, cobalt, manganese, and/or aluminum)
• Positive electrode: nickel oxyhydroxide (NiO(OH))
• Electrolyte: Potassium hydroxide (KOH)
Nickel-Metal Hydride (NiMH) Battery
Nickel-Metal Hydride (NiMH) Battery
• Redox occurs in the lattice
MHOHeMOHech
edisch
arg
arg2
eOHOHNiOOHOHNiech
edisch2
arg
arg2
The negative electrode material must be an alloycapable of large amountof hydrogen adsorption
LaNi5
TiN2
ZrNiTi2Ni
Typical electrodes can adsorb up to 2wt% hydrogen when charged
• It is not advisable to charge Ni-MH batteries with a constant-voltage method. Ni-MH batteries do not accept well a high initial charging current.• Float voltage is about 1.4 V (voltage of full capacity, compensating for self discharge)• Minimum voltage is about 1 V.
Saft NHE module battery
Cobasys Nigen battery
Nickel-Metal Hydride (NiMH) Battery
http://www.panasonic.com/industrial/battery/oem/images/pdf/panasonic_nimh_overview.pdf
• Effects of temperature:
Saft NHE module battery
Nickel-Metal Hydride (NiMH) Battery
NiMH Over-charge and Over-discharge
• Advantages:• Less sensitive to high temperatures than Li-ion and Lead-acid• Handle abuse (overcharge or over-discharge better than Li-ion bat
• Disadvantages:• More cells in series are need to achieve some given voltage.• Cost
Nickel-Metal Hydride (NiMH) Battery
• 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
Li-Ion Battery
• CathodeLiCoO2 Li1-xCoO2 + xLi+ + x e-c
d
Cn + xLi+ + x e- CnLixcd
• Anode
• OverallLiCoO2 + Cn Li1-xCoO2 + CnLix
c
d
Li-Ion Battery
• 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 one kilogram
• 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.
Li-Ion Battery
• Contrary to lead-acid batteries, Li-ion batteries do not accept well a high initial charging current.• In addition, cells in a battery stack needs to be equalized to avoid falling below the minimum cell voltage of about 2.85 V/cell.• Thus, Li-ion batteries need to be charged at least initially with a constant-current profile. Hence they need a charger• Typical float voltage is above 4 V (typically 4.2 V).
Li-Ion Battery Charging
• Effects of temperature:
Li-Ion Battery Temperature Effects
Saft Intensium 3 Li-ion battery“Advanced Lithium Ion Battery Charger”
V.L. Teofilo, L.V. Merritt and R.P. Hollandsworth
“Increased Performance of Battery Packs by Active Equalization”Jonathan W. Kimball, Brian T. Kuhn and Philip T. Krein
• Controlled charging has 2 purposes:• Limiting the current• Equalizing cells
Li-Ion Battery Equalization
• Factors affecting life:• Charging voltage.• Temperature• Age (time since manufacturing)
• Degradation process: oxidation
Li-Ion Battery
• 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
Li-Ion Battery
Li-Air Batteries
Replacing the IC Engine?
GDL of a Li-Air Battery
Challenges with Li-Air Batteries
• Poor efficiency (> 70%, ORR kinetics)• Low reaction rate (0.01 – 0.1 mA/cm2)• Low cycle life (10-100 cycles)• Engineering challenges
– No moisture exposure– Instability of Li– Dendrite formation
Testing ORR Materials for Li-Air
ORR on Li-Air : A Comparison
Translate to REAL materials