Download - Commercial Voltaic Cells
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