1. Electrochemistry for Engineers
Electrochemistry for Engineers
LECTURE 10
Lecturer: Dr. Brian Rosen
Office: 128 Wolfson
Office Hours: Sun 16:00

2. HW #5 Clarifications
ENTROPY IN UNITS OF
J / (mol*K)

3. Batteries

4. 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]

5. Recall Important Definitions Pt 2
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]

6. Basic Operating Principle
A  AO
Oxidation at Anode (-)
BO  B
Reduction at Cathode (+)
Charged State
Discharged State
The chemical energy within the bonds of the “charged” state is greater than that of
the discharged state

7. Primary Batteries
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.

8. Secondary 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.

9. For Example Leclanché Battery (Primary)
Nickel-Cadmium Battery (Secondary)
The zinc + Manganese (II) oxide system has a greater enthalpy than the
zinc oxide and Mn (III) oxide

10. Energy Density

12. Inside A Battery

13. Lead-Acid Battery

14. Pb-Acid Battery: The Anode

15. Pb-Acid Battery: The Anode

16. Pb-Acid Battery: The Cathode

17. Pb-Acid Battery: The Cathode

18. Pb-Acid Battery: Discharging

19. Pb-Acid Battery: Charging

20. Nernst Equation for Pb-Acid Battery

21. Self Discharge (Leakage Current)
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)
(+)
(-)
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

22. Self Discharge of (-) Terminal

23. Self Discharge of (+) Terminal

24. Store Batteries in the Fridge!

25. Why is Lead Advantageous for Storing Chemical Energy?

26. Battery Polarizability
activation
overpotential
Why is the charging curve above
the discharge curve?
Depletion
Charge-Discharge Curve at Constant Current

27. Mechanisms Affecting Voltage
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

28. Battery Capacity, C and Cp

29. Effect of Discharge Rate on C

30. Example Data (U. Colorado)

31. Theoretical Specific Capacity
M = molecular weight in kg/mol
F = faraday constant
n = number of electrons
q = specific capacity

32. Practical Specific Capacity
W = weight of catalyst in g
A = electrochemical area area in cm2
j = current density in mA/cm2
q,prac = practical specific capacity
Why is the utilization generally below 100%?

33. Theoretical Specific Energy
V = voltage (function of time)
i = current (held constant)
T,cutoff = cutoff time
W = catalyst weight

34. Theoretical Specific Power
V = voltage (function of time)
i = current (held constant)
T,cutoff = cutoff time
W = catalyst weight

35. Battery Efficiency Typical coulomb efficiency = 90%
Approximate voltage efficiency =(2V/2.3V) = 87%
Energy efficiency = (90%)(87%) = 78%

36. Charging Management

37. Solubility of Discharge Products
Initial
Discharge
Recharge
Soluble discharge
product
Insoluble discharge
product

38. Zn dendrite formation and inhibition by polyethylene glycol
Particularly susceptible when using Li or Zn electrodes
“Short Circuit”
Zn dendrite formation and inhibition
by polyethylene glycol

39. Cycle Testing

40. Need for Porous Electrode Materials
Lead electrodes need to have high surface area for high energy density
Without high porosity, surface would passivate quickly

41. Nickel-Metal Hydride (NiMH) Battery
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)
Cobasys batteries

42. Nickel-Metal Hydride (NiMH) Battery
Redox occurs in the lattice
The negative electrode
material must be an alloy
capable of large amount
of hydrogen adsorption
LaNi5
TiN2
ZrNi
Ti2Ni
Typical electrodes can adsorb
up to 2wt% hydrogen when charged

43. Nickel-Metal Hydride (NiMH) Battery
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.
Cobasys Nigen battery
Saft NHE module battery

44. Nickel-Metal Hydride (NiMH) Battery
Effects of temperature:
Saft NHE module battery

45. NiMH Over-charge and Over-discharge

46. Nickel-Metal Hydride (NiMH) Battery
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

47. Li-Ion 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)
discharge

48. Li-Ion Battery Cathode Anode Overall
Cathode more positive redox potential
Discharge: Li+ intercalates the positive materials -> provide outer electron flow
Charge: Li+ deintercalates from cathode and intercalates the anode.
Li-ion shuttles b/w cathode and anode during cycling -> conversion & storage of electrochemical energy within the
Cells
Energy density: storage of large amount of Li
Power density: fast ionic/electronic transfer 48

49. 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.

50. Li-Ion Battery Charging
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).

51. Li-Ion Battery Temperature Effects
Effects of temperature:

52. Li-Ion Battery Equalization
Controlled charging has 2 purposes:
Limiting the current
Equalizing cells
“Increased Performance of Battery Packs by Active Equalization”
Jonathan W. Kimball, Brian T. Kuhn and Philip T. Krein
“Advanced Lithium Ion Battery Charger”
V.L. Teofilo, L.V. Merritt and R.P. Hollandsworth
Saft Intensium 3 Li-ion battery

53. Li-Ion Battery Factors affecting life: Charging voltage. Temperature
Age (time since manufacturing)
Degradation process: oxidation

54. 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

55. Li-Air Batteries

56. Replacing the IC Engine?

57. GDL of a Li-Air Battery

58. Challenges with Li-Air Batteries
Poor efficiency (> 70%, ORR kinetics)
Low reaction rate (0.01 – 0.1 mA/cm2)
Low cycle life ( cycles)
Engineering challenges
No moisture exposure
Instability of Li
Dendrite formation

59. Testing ORR Materials for Li-Air

60. ORR on Li-Air : A Comparison

61. Translate to REAL materials