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