Applications: Batteries

1
Applications Batteries Corrosion

• Battery a group of galvanic cells connected in
  series ( to ).
• Total potential ?E of the battery sum of the
  individual cell potentials.
• In common usage, a single galvanic cell is often
  referred to as a battery.
• Sources of direct current, important because of
  portability.

2

• Primary Battery can not be recharged
• Secondary Battery rechargeable (storage
  batteries)
• Fuel Cell reactants supplied from an external
  source

3

• Lead-Acid Storage Battery (a secondary battery)
• Provides current for starter motor in cars
• Anode Pb(s) HSO4(aq) ? PbSO4(s) H(aq) 2
  e
• E 0.356 V
• Cathode PbO2(s) HSO4(aq) 3 H(aq) 2 e
  ?
• PbSO4(s) 2 H2O(l)
• E 1.685 V
• Overall Pb(s) PbO2(s) 2 H(aq) 2
  HSO4(aq) ?
• 2 PbSO4(s) 2 H2O(l)
• ?E 2.041 V

4
Figure 17.13One of the Six Cells in Storage
Battery a 12-V Lead Storage Battery
5
Some Primary Batteries

• The Zinc-Carbon (LeClanche) Dry Cell 1866
• Zn outer shell, C rod in center, paste of NH4Cl,
  ZnCl2, and MnO2 ?E 1.5 V
• Anode Zn(s) ? Zn2(aq) 2 e
• Cathode 2 NH4(aq) 2 e ? 2 NH3(g)
  H2(g)
• Also 2 MnO2(s) H2(g) ? Mn2O3(s) H2O(l)
• Zn2(aq) 2 NH3(aq) 2 Cl(aq) ?
  Zn(NH3)2Cl2(s)
• Net 2 MnO2 2 NH4Cl Zn? Mn2O3 H2O
  Zn(NH3)2Cl2

6
Figure 17.14A Common Dry Cell Battery
7
Figure 17.15A Mercury Battery
8

• The Mercury Button Battery 1.35 V
• Anode Zn(s) 2 OH(aq) ? ZnO(s) H2O(l) 2 e
• CathodeHgO(s) H2O(l) 2e? Hg(l) 2 OH(aq)
• Net Zn(s) HgO(s) ? ZnO(s) Hg(l)

9
Fuel Cells Reactants Continuously Supplied

• Ex. The H2/O2 Fuel Cell
• Anode 2 H2(g) 4 OH(aq) ? 4 H2O(l) 4 e
• Cathode 4 e O2(g) 2 H2O(l) ? 4 OH(aq)
• Overall 2 H2(g) O2(g) ? 2 H2O(l)

10
Figure 17.16Schematic of the Hydrogen-Oxygen
Fuel Cell
11
Corrosion Unwanted Oxidation

• Ex rusting of iron
• Fe2(aq) 2 e ? Fe(s) E 0.44 V
• Figure 17.17 The Electrochemical Corrosion of
  Iron

12

• Protection of steel (iron) by galvanizing
  (plating with a layer of zinc)
• Zn2(aq) 2 e ? Zn(s) E 0.76 V
• Since this is a lower potential, iron in contact
  with zinc is forced to be cathodic.
• Net Reaction Zn(s) Fe2(aq) ? Zn2(aq)
  Fe(s)
• ?E 0.44 (0.76) 0.32 V
• Thus, the zinc oxidizes instead of the iron.

13

• Sacrificial Anode Allowing a more active metal
  to oxidize in order to preserve a less active
  metal.
• Ex Magnesium blocks may be attached to an
  underground iron pipe in order to keep it from
  rusting away.
• Mg Fe2 ? Mg2 Fe
• ?E 0.44 (2.37) 1.93 V
• Mg sacrificial anodes are also found inside home
  water heaters.

14
Figure 17.18Cathodic Protection