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) --> Zn 2+ (aq) + 2 e - 2 NH 4 + (aq) + 2 MnO 2(s) + 2 e - --> Mn 2 O 3(s) + 2 NH 3(aq) + H 2 O (l) Overall reaction: Zn (s) + 2 NH 4 + (aq) + 2 MnO 2(s) --> Zn 2+ (aq) + Mn 2 O 3(s) + 2 NH 3(aq) + H 2 O (l) Zinc Case (Anode) Carbon Rod (Cathode) ZnCl 2, MnO 2, & NH 4 Cl 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 MnO 2 Cathode mixture KOH electrolyte Brass current collector Zn (s) + 2 OH - (aq) --> ZnO (s) + H 2 O (l) + 2 e - Anode Rxn (oxidation) MnO 2(s) + 2 H 2 O (l) + 2 e - --> Mn(OH) 2(s) + 2 OH - (aq) + H 2 O (l) Cathode Rxn (Reduction) Overall Rxn: Zn (s) + MnO 2(s) + H 2 O (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) + H 2 O (l) + 2 e - Anode Rxn (Oxidaion) HgO (s) + H 2 O (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) + H 2 O (l) + 2 e - --> Ni(OH) 2(s) + OH - cathode rxn (reduction) Cd (s) + OH - (aq) + NiO(OH) (s) + H 2 O (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) + SO 4 2- (aq) --> Pb(SO 4 ) (s) + 2 e - anode rxn (oxidation) Pb (s) + PbO 2(s) + 4 H + (aq) + 2 SO 4 2- (aq) --> 2 Pb(SO 4 ) (s) + 2 H 2 O (l) PbO 2(s) + 4 H + (aq) + SO 4 2- (aq) + 2 e - --> Pb(SO 4 ) (s) + H 2 O (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 NH 4 + (aq) + 2 MnO 2(s) --> Zn 2+ (aq) + Mn 2 O 3(s) + 2 NH 3(aq) + H 2 O (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 H 2(g) + 4 OH - (aq) --> 4 H 2 O (l) + 4e - Anode Rxn (oxidation) 2 O 2(g) + 2 H 2 O (l) + 4 e - --> 4 OH - (aq) Cathode Rxn (reduction) Overall Rxn:2 H 2(g) + O 2(g) --> 2 H 2 O (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 H 2 gas (they use fossil fuels for this now). Storage capacity for H 2 gas is still a problem (Capacity and dangers from the gaseous H 2 ). The fuel cell still has problems with reliability.