Cells & Batteries

Cells & Batteries

Primary Cells

these cells cannot be easily re-charged; once they die… they stay dead.

I. Regular Carbon – Zinc Dry Cell

Primary Cells

Zn(s) --> Zn2+(aq) + 2 e-

2 NH4+

(aq) + 2 MnO2(s) + 2 e- --> Mn2O3(s) + 2 NH3(aq) + H2O(l)

Overall reaction:

Zn(s) + 2 NH4+

(aq) + 2 MnO2(s) --> Zn2+(aq) + Mn2O3(s) + 2 NH3(aq) + H2O(l)

Zinc Case (Anode)

Carbon Rod (Cathode)

ZnCl2, MnO2, & NH4Cl in paste

(Anode Rxn)

(Cathode Rxn)

Primary Cells

More efficient; better in high load situations

II. Alkaline Cells

Primary Cells Zn – KOH anode paste

Steel (Fe) Case

MnO2 Cathode mixture

KOH electrolyte

Brass current collector

Zn(s) + 2 OH-(aq) --> ZnO(s) + H2O(l) + 2 e- Anode Rxn (oxidation)

MnO2(s) + 2 H2O(l) + 2 e- --> Mn(OH)2(s) + 2 OH-(aq) + H2O(l)

Cathode Rxn (Reduction)

Overall Rxn: Zn(s) + MnO2(s) + H2O(l) --> ZnO(s) + Mn(OH)2(s)

Primary Cells III. Mercury Battery

– smaller; used to power hearing aids & calculators

Zn(s) + 2 OH-(aq) --> ZnO(s) + H2O(l) + 2 e-

Anode Rxn (Oxidaion)

HgO(s) + H2O(l) + 2 e- --> Hg(l) + 2 OH-(aq)

Zn(s) + 2 HgO(s) --> ZnO(s) + Hg(l)

Cathode Rxn (Reduction)

Overall Rxn:

Secondary Cells

These cells can be re-charged by running a current backwards through them.

IV. Ni-Cad The first practical rechargeable “drycell”.

Ni-Cad Demo

Secondary Cells

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

(oxidation)

NiO(OH)(s) + H2O(l) + 2 e- --> Ni(OH)2(s) + OH-

cathode rxn (reduction)

Cd(s) + OH-(aq) + NiO(OH)(s) + H2O(l) ↔ Cd(OH)2(s) + Ni(OH)2(s)

Overall Rxn:

These reactions can be reversed. The cell can be re-charged many times over (about 1000 x). The cells are more expensive to manufacture but last much longer. The reactions are not 100% reversible and eventually the cells can not be made to “hold” a charge and must be replaced. These are being replaced by the new Li-Metal hydride cells.)

Secondary Cells

V. Lead Acid Storage Battery

the first practical re-chargeable battery; still used in cars, motorcycles, boats etc.

This is the first true “battery” made up of 6 separate cells. It is a good choice for motor vehicles because it provides a large initial supply of energy to start the engine, has a long shelf life, and is reliable at low temperatures. The downside is the weight of the batteries (they contain lead after all) and the environmental problems of lead.

Secondary Cells

Pb(s) + SO4

2-(aq) --> Pb(SO4)(s) + 2 e- anode rxn (oxidation)

Pb(s) + PbO2(s)+ 4 H+(aq) + 2 SO

42-

(aq) --> 2 Pb(SO4)(s) + 2 H2O(l)

PbO2(s) + 4 H+(aq) + SO4

2-(aq) + 2 e- --> Pb(SO4)(s) + H2O(l)

(reduction @ cathode)

Overall Rxn:

Secondary Cells

A cross section of the newer Li-Metal hydride rechargeable cell.

VI. Lithium Metal

an alternative to the Lead-Acid battery; a much higher power to mass ratio than the lead-acid; better for use in electric cars, small electric devises such as cameras

Secondary Cells

Li(s) --> Li+(aq) + 2 e- (oxidation @ anode)

These cells output almost 3.0 V compared to the 1.5 V of regular carbon – zinc cells.

Zn(s) + 2 NH4+(aq) + 2 MnO2(s) --> Zn2+(aq) + Mn2O3(s) + 2 NH3(aq) + H2O(l)

(reduction @ cathode)

VII. Fuel Cells

Newer Technologies

combining hydrogen gas with oxygen gas without “burning” but with the production of electricity.

The electricity is then used to power an electric motor.

A clean re-newable type of power source.

Fuel Cell demo

Fuel Cells

The reaction occurs in two half cells as follows:

2 H2(g) + 4 OH-(aq) --> 4 H2O(l) + 4e- Anode Rxn (oxidation)

2 O2(g) + 2 H2O(l) + 4 e- --> 4 OH-(aq) Cathode Rxn (reduction)

Overall Rxn: 2 H2(g) + O2(g) --> 2 H2O(l)

(same as “burning hydrogen gas with oxygen!!)

The great thing about fuel cells is that they are very low polluters. The only product is water. The fuel sources are atmospheric oxygen and breaking down water can generate hydrogen. Large solar cells using the sun energy could be used to do this.

Fuel Cells

Advantages:

Disadvantages:

Still need a source of H2 gas (they use fossil fuels for this now).

Storage capacity for H2 gas is still a problem (Capacity and

dangers from the gaseous H2). The fuel cell still has problems with

reliability.

The End!