Voltaic cell
- aka battery, rechargeable battery, storage battery, dry cell, wet cell
We use the term "voltaic cell" to encompass any source of steady voltage potential which sources the energy from a chemical, rather than mechanical, source. Mechanical sources of voltage potential are referred to here as generators.
General notes
Terminology: Anode, Cathode, Electrode, Plate
Terminology problems (no pun intended): The terms anode and cathode are confusing in this context. Consider a voltaic cell driving a chemical reaction in an electrolysis bath. The anode from the perspective of the electrolyte in the battery is the cathode from the perspective of the electrolyte in the bath being powered. Solution: (also no pun intended) We use "positive plate" and "negative plate" to differentiate, the positive plate of the battery is connected to the positive terminal. It receives electrons from the circuit. Likewise the negative plate of the battery supplies electrons to the cicuit. Electrode is used as a neutral (third pun also not intended) term.
Internal charge carriers
- Positive charge carriers (usually H+) travel from the negative plate to the positive plate. (In the same direction as the current.)
- Negative charge carriers (usually OH-) travel from the positive plate to the negative plate. (In the opposite direction as the current.)
Plate potential and cell voltage
While you should be able to add the absolute value of the potentials of the plates to get the cell voltage, the two values are, in practice, unrelated. Too many microphysical changes take place to simply add them up and get the result.
Wet cells
Wet cells are defined herein to be "battery" type devices where the electrolyte is neither solid nor fixed within a solid.
Lead-acid cell
| Cell | |
|---|---|
| Electrolyte | sulfuric acid |
| Net reaction | Pb + PbO2 + 2 H2SO4 → 2 PbSO4 + 2 H2O |
| Voltage | 2v |
| Internal charge carrier | H+ |
| Energy density, volume(J/L) | 7 |
| Positive plate | |
| Material | lead (II) oxide |
| Reaction | PbO2(s) + H2SO4(aq) + 2 H + + 2 e- → PbSO4(s) + 2 H2O(l) |
| Negative plate | |
| Material | lead |
| Reaction | PbO(s) + H2SO4(aq) → PbSO4 + 2 H + + 2 e |
The standard by which all other voltaic cells are measured.
Advantages
- Easy to recharge at 2.17-2.34v reverse potential.
- Simple to recycle since both plates become the same substance.
Disadvantages
- The container (cell) must be able to withstand concentrated sulfuric acid
Hazards
- Can evolve hydrogen gas
Construction details
- A glass or ceramic container filled with sulfuric acid, with two plates (lead and lead (II) oxide) suspended in the solution but separated from each other.
- The lead (II) oxide plate is frequently made of a lead frame press-filled with lead (II) oxide.
LeClanch Cell
| Cell | |
|---|---|
| Electrolyte | Ammonium chloride |
| Net reaction | Zn(s) + 2 MnO2(s) + 2 NH4Cl(aq) → ZnCl2 + Mn2O3(s) + H2O + 2 NH3 |
| Voltage | 1.4v |
| Internal charge carrier | H+ |
| Positive plate | |
| Material | Manganese dioxide |
| Reaction | 2 MnO2 + 2 H + + 2 e- → Mn2O3 + H2O |
| Negative plate | |
| Material | Zinc |
| Reaction | Zn + 2 NH4Cl → 2 H + + 2 e- + ZnCl2 + 2 NH3 |
Advantages
- long shelf life
Disadvantages
- Brief duty period (internal resistance)
- Produces ammonia (low vapor pressure, irritant, noxious) during normal operation
Hazards
Construction Details
Trough Cell
| Cell | |
|---|---|
| Electrolyte | cloth or paper soaked in water containing salt |
| Net reaction | Zn(s) + Cu(s) + H2O → ZnSO4 + Cu(s) + H2 |
| Voltage | 1.4v |
| Internal charge carrier | H+ |
| Positive plate | |
| Material | Copper |
| Reaction | Cu + 2 H + + 2 e- → Cu + H2 |
| Negative plate | |
| Material | Zinc |
| Reaction | Zn + H2SO4 → ZnSO4 + 2 H + + 2 e- |
Advantages
- Requiring only copper, zinc, salt water, and a nonconductive container, a Trough cell is probably the simplest to implement.
Disadvantages
- Low available energy (internal resistance shuts down cell)
- Rapid rise in internal resistance
- This cell is not easily recycled since the zinc sulfate must be reduced to elemental zinc again
Hazards
- Produces hydrogen gas during normal operation
Construction Details
Trough containing saltwater filled with zinc-silver-cloth triplets
Dry Cells
Zinc-Carbon
| Cell | |
|---|---|
| Electrolyte | cloth or paper soaked in ammonium chloride |
| Net reaction | Zn(s) + 2 MnO2(s) + 2 NH4Cl(aq) → Mn2O3(s) + Zn(NH3)2Cl2(aq) + H2O(l) |
| Voltage | 1.5v |
| Internal charge carrier | Cl- |
| Positive plate | |
| Material | manganese dioxide |
| Reaction | 2 MnO2(s) + 2 e− + 2 NH4Cl(aq) → Mn2O3(s) + 2 NH3(aq) + H2O(l) + 2 Cl− |
| Potential(v) | +0.74 |
| Negative plate | |
| Material | Zinc |
| Reaction | Zn(s) + 2 Cl- → ZnCl2(aq) + 2 e- |
| Potential(v) | -0.7626 |
Advantages
- Simple construction dry cell
Disadvantages
- Ammonium chloride electrolye is not easy to produce
- Hard to recycle, since zinc is removed completely
Hazards
Construction Details
- Metal-capped carbon rod is conductor to manganese dioxide core
- Electrolye typically completely absorbed in paper, but could be sawdust, etc
- Zinc forms outer container, so no additional material required
Alkaline
| Cell | |
|---|---|
| Electrolyte | potassium hydroxide (not consumed) |
| Net reaction | Zn(s) + 2 MnO2(s) → ZnO(s) + Mn2O3(s) |
| Voltage | 1.5v |
| Internal charge carrier | OH- |
| Positive plate | |
| Material | mixture of carbon and manganese dioxide |
| Reaction | 2 MnO2(s) + H2O(l) + 2 e- → Mn2O3(s) + 2 OH− |
| Potential(v) | +0.15 |
| Negative plate | |
| Material | Zinc powder |
| Reaction | Zn(s) + 2 OH− → ZnO(s) + H2O(l) + 2 e− |
| Potential(v) | -1.28 |
Advantages
- High energy density
- Long shelf life
- High discharge rate
- Electrolyte is not consumed
- Mn and Zn do not intermix, but remain separated by permeable separator (easier purification)
Disadvantages
- Construction is more complicated
- may require steel
- Separate conductive electrode materials for anode and cathode
- Requires conductive neutral barrier between manganese dioxide and carbon/zinc/KOH mixture
Hazards
- Can leak potassium hydroxide
Construction Details
- Concentric cylinders (outer to inner)
- steel case
- manganese dioxide / carbon mixture
- conductive separator permeable to KOH
- Zinc powder
- steel spike electrode
Silver oxide
| Cell | |
|---|---|
| Electrolyte | potassium hydroxide |
| Net reaction | 4 Zn + Ag4O4 → 4 ZnO + 4 Ag |
| Voltage | 1.59v |
| Internal charge carrier | OH- |
| Positive plate | |
| Material | Mixture of silver peroxide and potassium hydroxide. |
| Reaction | Ag4O4(s) + 4 H2O(l) + 8 e- → 8 Ag(s) + 8 OH- |
| Potential(v) | +0.34 |
| Negative plate | |
| Material | zinc |
| Reaction | Zn + 2 OH- → ZnO + H2O + 2 e- |
| Potential(v) | -1.25 |
Advantages
- Very high energy density
- Long shelf life
- High discharge rate
- Electrolyte is not consumed
Disadvantages
Hazards
- Can leak KOH
Construction Details
See Also
- Lithium Cells[1]
References
- ↑ Types of Lithium Ion
courtesy Battery University.