• Electrochemical cells can be used as a commercial source of electrical energy
• Cells can be?non-rechargeable?(irreversible),?rechargeable?or?fuel cells
• Type of cells used in commercial applications depend on
  • the?voltage?required
  • the?current?needed
  • the?size?of the cell
  • the?cost

   

• Although it is commonly used incorrectly, the term?battery?should be used to refer to a?collection?of cells
• A car battery is correct, because it is a collection of six cells joined together

Non-rechargeable cells

The Daniell cell

• The Daniell cell was one of the earliest electrochemical cells and consisted of a simple metal-metal ion system
• It was invented by British chemist John Daniell in 1836
• The cell consists of
  • a zinc rod immersed in a solution of zinc sulfate
  • a copper cylinder filled with copper sulfate solution
  • a porous pot that separates the copper sulfate from the zinc sulfate

   

A Daniell cell

• The zinc acts as the negative electrode and the copper is the positive electrode
• The half-cell reactions are

Zn (s)?→? ?Zn2+?(aq)? +??2e-? ? ? ? ? ?E??= -0.76 V

Cu2+?(aq)? + 2e-?→ Cu (s)? ? ? ? ? ? ??E??= +0.34 V

• The cell generates an emf of 1.1 V and the overall reaction is

Zn (s)? + CuSO4?(aq)→ Cu (s)? + ZnSO4?(aq)? ? ??E?cell?= +1.10 V

• However, the cell is impractical to use as a portable device because of the hazardous liquids in the cell

Zinc-carbon cells

• Zinc-carbon cells are the most common type of non-rechargeable cells, consisting of
  • a zinc casing which acts as the negative electrode
  • a paste of ammonium chloride which acts as an electrolyte as well as the positive electrode
  • a carbon rod which acts as an electron carrier in the cell

   

The zinc-carbon cell

• The half-cell reactions are

Zn (s)?→? ?Zn2+?(aq)? +??2e-? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?E??= -0.76 V

2NH4+?(aq) + 2e-?→ 2NH3?(g) + H2?(g)? ? ? ? ? ? ??E??= +0.74 V

• The cell generates an emf of 1.50 V and the overall reaction is

2NH4+?(aq)?+ Zn (s)? → 2NH3?(g) + H2?(g)??+ Zn2+?(aq)? ??E?cell?= +1.50 V

• As the cell discharges, the zinc casing eventually wears away and the corrosive contents of the electrolyte paste can leak out, which is an obvious disadvantage of zinc-carbon cells
• The cell provides a small current and is relative cheap compared to other cells
• Extra long life cells have a similar chemistry, but supply a higher current and use zinc chloride in the paste; they are suitable for torches, radios and clocks
• Another variation on the cell uses an alkaline paste in the electrolyte and they have a much longer operating life, but are noticeably more expensive than regular zinc-carbon cells

Rechargeable Cells

• Rechargeable cells employ chemical reactions which can be reversed by applying a voltage greater than the cell voltage, causing electrons to push in the opposite direction
• There are many types of rechargeable cells, but common ones include?lead-acid?batteries,?NiCad?cells and?lithium?cells which are covered in more detail in the next section

Lead-acid batteries

• Lead-acid batteries consist of?six cells?joined together?in series
• The cells use?lead?metal as the?negative electrode?and and?lead(IV) oxide?as the?positive electrode
• The electrolyte is sulfuric acid

A lead-acid battery

• The half-cell reactions are

Pb (s) +? SO42-?(aq)??→? ?PbSO4?(s)? +? 2e-??? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?E??= -0.36 V

PbO2?(s) +? 4H+?(aq) +? SO42-?(aq) +? 2e-?→ ?PbSO4?(s)? + 2H2O (l)? ? ? ? ?E??= +1.70 V

• The cell generates an emf of about 2 V and the overall reaction is

PbO2?(s) +? 4H+?(aq) +? 2SO42-?(aq) +? Pb (s) → ?2PbSO4?(s)? + 2H2O (l)? ? ? ?E?cell?= +2.06 V

• In a commercial car battery, the six cells in series give a combined voltage of about 12 V
• When the car is in motion, the generator provides a push of electrons that reverses the reaction and regenerates lead and lead(IV) oxide
• Lead-acid batteries are designed to produce a?high current?for a short period of time, hence their use in powering a starter motor in car engines
• The disadvantage of lead-acid batteries is that:
  • They are very?heavy
  • They contain?toxic?materials: lead and lead(IV) oxide
  • The sulfuric acid electrolyte is very?corrosive

   

• This presents challenges of disposal when lead-acid batteries come to the end of their useful life

NiCad cells

• NiCad stands for nickel-cadmium and these cells are available in many standard sizes and voltages so they can replace almost any application of traditional zinc-carbon cells
• Although they are more expensive cells, the fact they can be recharged hundreds of times means they are commercially viable
• The positive electrode consists of cadmium and the negative electrode is made of a nickel(II) hydroxide-oxide system
• The half-cell reactions are

Cd (s) +? 2OH-??(aq) → Cd(OH)2?(s)? +? 2e-E??= -0.82 V

NiO(OH) (s) + H2O (l) + e-?→ Ni(OH)2?(s) + ? OH-? (aq)? ? ? ? ?E??=? +0.38 V

• The overall reaction in the cell is

2NiO(OH) (s) + 2H2O (l) + Cd (s) → 2Ni(OH)2?(s) + Cd(OH)2?(s)? ? ? ? ?E??= +1.2 V

• Cadmium is a toxic metal so the disposal of old NiCad cells is also an environmental issue