Commercial Voltaic CellsA voltaic cell can be a convenient,

portable source of electricity. We know them as batteries. Batteries have been in use for over 100

years in various forms.The technology of batteries remained

fairly stagnant until about 1990. Why???

Lead-Acid BatteryThis type of cell has been around for over 80

years. It uses lead as the anode and lead(IV) oxide

as the cathode. Highly caustic H2SO4 is also involved in the

overall reaction. The reaction produces a reliable 2.0 V.

Lead-Acid Battery

Lead-Acid BatteryThe half-reactions are:Pb(s) + HSO4

-(aq) PbSO4(s) + H+

(aq) + 2e (anode)PbO2(s) + 3H+

(aq) + HSO4-(aq) + 2e

PbSO4(s) + 2H2O(l) (cathode)Overall reaction is:PbO2(s) + Pb(s) + 2H+

(aq) + 2HSO4-(aq) 2PbSO4(s) + 2H2O(l)

During recharging, water is consumed. This used to require that water occasionally was added to the battery.

The new batteries use Pb/Ca alloy as the anode which resists the consumption of water. This has led to the “maintenance-free” batteries.

Lead-Acid BatteryAdvantages: produces steady voltage, very

high current, many recharges, relatively low cost.

Disadvantages: environmental concerns, massive, reverse reaction can produce H2.

Zinc-Carbon Dry Cell

Known also as the LeLanche cell (for its inventor), uses a zinc can as the anode and a graphite rod as the cathode.

A paste containing NH4Cl and MnO2 separates the two electrodes.

Zinc-Carbon Dry CellThe anode and cathode reactions are:Zn(s) Zn+2

(aq) + 2e- (anode)2 NH4

+(aq) + 2 MnO2(s) + 2e-

Mn2O3(s) + H2O(l) + 2 NH3(aq

(cathode)Advantages: inexpensive, produces a reliable

1.5 V.Disadvantages: performs poorly under high

demand, poor in cold weather, prone to leak when it gets old, environmental (disposal).

The Zinc-Carbon Dry Cell

Alkaline Dry Cell Similar, but uses KOH as the paste

between the electrodes. The reactions are:

Zn(s) + 2OH-(aq) Zn(OH)2(s) + 2e (anode)

2MnO2(s) + H2O(l) + 2e Mn2O3(s) + 2 OH-(aq) (cathode)

Advantages: better under high demand, better in cold weather.

Disadvantages: higher cost, environmental (disposal).

Alkaline Dry Cell

NiCad CellNickel-Cadmium (Nicad) batteries were some of the

first widely used rechargeable batteries.The reactions are:

Cd(s) + 2OH-(aq) Cd(OH)2(s) + 2e (anode)

NiOOH(s) + H2O(l) + e Ni(OH)2(s) + OH-(aq) (cathode)

Advantages: easy to recharge, many recharge cycles, good current supply.

Disadvantages: longer recharge times, cost, weight, toxicity of Cd, and “memory loss.”

NiMH CellNewer version is the Nickel-Metal

hydride (NiMH) battery that has longer life and eliminates the Cadmium which is replaced with a ZrNi2 metal alloy. This alloy absorbs Hydrogen anions that are oxidized.

Most hybrid automobiles use these type of batteries.

Advantages: Have a very long-life and can last for up to eight years.

Disadvantage: Replacement costs in an auto can be upwards of $8,000.

Lithium-Iodine CellA “true” dry cell.The anode is lithium metal and the cathode is

an I2 crystal. Current is carried by diffusion of Li+ ions.This battery is used in pacemakers as well as

the BIOS in computers.

Lithium-Iodine CellThe anode and cathode reactions are:

Li(s) Li+(aq) + 1e (anode)

I2(s) + 2e 2 I-(aq) (cathode)

Advantages: environmentally friendly, produces a large voltage (3.0 V), long life, rechargeable, large power to mass ratio.

Disadvantages: produces low current, cost.

Lithium-Ion CellA newer version of the previous type.Graphite serves as one electrode with LiCoO2 as

the other electrode.During charging, the Li+ ions migrate to the

anode (graphite) and the Cobalt is oxidized.During discharge, the Li+ migrate spontaneously

to the cathode. These are the batteries of choice for most

portable computers and PDA’s. Can be recharged many times for up to two

years.

Lithium-Ion Cell

Lithium-Ion CellAdvantages: Store more energy per gram of

weight, hold their charge of long periods, and each cell has a large voltage (3.6V).

Disadvantages: Degrade even without use, last two to three years, cannot be completely discharged, and may catch fire if they fail.

Cell Voltages / Currents

Most devices require voltages of 3.0, 6.0, or even 12.0V as well as high currents.

To produce these values, cells are placed in both series as well as in parallel.

Fuel CellsEnergy choice of the future.Not a true battery as it requires a

constant supply of reactants.Used by NASA on space vehicles to

generate electricity.May soon be mass produced for

automobile propulsion.Smaller versions could power laptops and

cell phones.

Fuel CellsThe overall reaction converts H2 and O2 into

H2O. 2 H2(g) + 4 OH-

(aq) 4 H2O(l) + 4e (anode) O2(g) + 2 H2O(l) + 4e 4 OH-

(aq) (cathode)

Overall Reaction is: 2 H2(g) + O2(g) 2 H2O(l)

Fuel Cells

Fuel Cells

Fuel Cell Organizationwww.fuelcells.org

Fuel Cell Producer / Researcherwww.ballard.com

Fuel CellsAdvantages: best for the environment -

produces water!, relatively low mass, much more efficient than the internal combustion engine, greatly simplify car design.

Disadvantages: cost, storage / use of hydrogen, mass production, acceptance.

CorrosionElectrochemical process of corrosion is

essentially a mini voltaic cell. When a drop of water comes into contact

with iron, the corrosion process begins.At the center of the drop, iron metal is

oxidized: Fe Fe+2 + 2e.At the edges, oxygen is reduced: O2 + 4H+ + 4e 2H2O

Corrosion

CorrosionCorrosion of iron is more favored when:

Moisture is presentConcentrations of electrolytes (salt) is presentLower pH’s

Prevention of corrosion can be achieved by:Paint – prevents oxygen and water from interacting with

the ironUse of a sacrificial metal – any more active metal in

contact with the iron will be oxidized in preference to the iron. This is sometimes called cathodic protection.

Cathodic Protection

Cathodic Protection