2.6 Batteries
Electrochemical cells used for power generation are called batteries. Although batteries come in many different shapes and sizes there are a few basic types. You won't be required to remember details of the batteries, but some general information and features of each type is presented here.
1. Primary batteries - (dry cell batteries)
- non-rechargeable
- electrolytes are present as a paste rather than as a liquid
- general purpose battery used for flashlights, transistor radios, toys, etc.
- The basic dry cell battery consists of: zinc case as the anode (oxidation); a graphite rod is the cathode (reduction) surrounded by a moist past of either MnO2, NH4Cl, and ZnCl2 or in alkaline dry cells a KOH electrolytic paste.
- General reactions for the battery - manganese(IV) oxide-zinc cell (different batteries have different reactions - you don't need to remember any of these reactions)
cathode |
2MnO2(s) + 2 NH4+(aq)+ 2e- → Mn2O3 (s) + H2O(l) + 2NH3 (aq) |
anode |
Zn(s) → Zn2+(aq) + 2e- |
- Maximum voltage 1.5V. By connecting several cells in series 90V can be achieved.
- Advantages of alkaline batteries - consistent voltage, increased capacity, longer shelf-life, and reliable operation at temperatures as low as -40°C
- Disadvantage - higher cost
2. Secondary Batteries (storage batteries)
- rechargeable
- an example - lead-acid battery used in cars. Anode is grid of lead-antimony or lead-calcium alloy packed with spongy lead; Cathode is lead(IV) oxide. Electrolyte is aqueous sulfuric acid. Consists of numerous small cells connected in parallel (anode to anode; cathode to cathode).
- General reaction:
cathode |
PbO2(s) + 4H+(aq)+ SO42-(aq) + 2e- → PbSO4 (s) + 2H2O(l) + 2NH3 (aq) |
anode |
Pb(s) + SO42-(aq) → PbSO4 (s) + 2e- |
- Secondary batteries are recharged by passing a current through the battery in the opposite direction. In a car battery this occurs when the engine is running.
- Other examples include the nickel-iron alkaline battery, nickel-zinc batter, nickel-cadmium alkaline battery, silver-zinc, silver-cadmium
3. Fuel Cells
- fuel cells are electrochemical cells that convert energy of a redox combustion reaction directly into electrical energy. Requires a continuous supply of reactants and a constant removal of products.
- Cathode reactant usually air or pure oxygen; anode fuel is a gas such as hydrogen, methane, or propane. Carbon electrodes typically contain a catalyst. The electrolyte is typically KOH.
- General reaction:
cathode |
O2(g) + 2H2O(l) + 4e- → 4OH-(aq) |
anode |
2H2 (g) + 4OH-(aq) → 4H2O(l) + 4e- |
| net | 2H2 (g) + O2(g) → 2H2O(l) |
- Advantages - no toxic waste products (water is the only product); very efficient energy conversion (70-80% efficient)
- Disadvantage - too expensive for large-scale use.