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Electrochemical Cells & Batteries

Key Concepts

Battery: cells in a series

Cells:

  • Primary cells: cannot be recharged

        examples: Leclanché, Alkaline cell, Lithium battery, Button cell

  • Secondary cells: can be recharged by applying voltage greater than that of the spontaneous cell reaction which reverses the half reactions

        examples: Lead-acid battery, Nickel-cadmium cell, Vanadium redox battery

  • Fuel cells: those that function continuously due to continued supply of reactants

        examples: hydrogen-oxygen fuel cell

Examples

PRIMARY CELLS

Leclanché or dry cell : Zn|(ZnCl2),NH4Cl|MnO2,C

  • Anode (-): zinc case
    Zn(s) -----> Zn2+(aq) + 2e-

  • Cathode (+): carbon rod in contact with carbon & MnO2
    NH4+(aq) + MnO2(s) + e- -----> NH3(aq) + MnO(OH)(s)

  • Electrolyte: Paste of NH4Cl, ZnCl2 (acid electrolyte)

  • Voltage: 1.5 volts

  • Uses: low drain appliances such as radios, torches, liquid crystal calculators, toys

Alkaline cell : Zn,ZnO|KOH(paste)|MnO2,C

  • Anode (-): brass rod in contact with powdered zinc
    Zn(s) + 2OH-(aq) -----> ZnO(s) + H2O(l) + 2e-

  • Cathode (+): steel case in contact with carbon & MnO2
    MnO2(s) + H2O(l) + e- -----> MnO(OH)(s) + OH-(aq)

  • Electrolyte: paste of 7M KOH (alkaline electrolyte)

  • Voltage: 1 volt

  • Uses: High current or high total capacity appliances such as electronic photographic flash units, tape recorders, radios, shavers
    Can deliver higher current than Leclanché cell without severe voltage drop.

Mercury button cell

  • Anode (-): steel cap in contact with powdered zinc
    Zn(s) + 2OH-(aq) -----> ZnO(s) + H2O(l) + 2e-

  • Cathode (+): steel case in contact with powdered HgO
    HgO(s) +H2O(l) + 2e- -----> Hg(l) + 2OH-(aq)

  • Electrolyte: paste of KOH (alkaline electrolyte)

  • Voltage: 1.3 volts

  • Uses: watches, pacemakers, hearing aids, microphones

Silver button cell: Zn,ZnO|KOH (paste)|Ag2O,Ag

  • Anode (-): steel cap in contact with powdered zinc
    Zn(s) + 2OH-(aq) -----> ZnO(s) + H2O(l) + 2e-

  • Cathode (+): steel case in contact with powdered Ag2O
    Ag2O(s) + H2O(l) + 2e- -----> 2Ag(s) + 2OH-(aq)

  • Electrolyte: paste of KOH (alkaline electrolyte)

  • Voltage: 1.3 volts

  • Uses: watches, pacemakers, hearing aids
    As cell discharges, concentrations do NOT change as reactants & products are solids.
    [OH-] in electrolyte does NOT change.
    So, the voltage of this cell remains constant throughout its lifetime.

Lithium cell : Li|Li+(nonaqueous)|KOH(paste)|MnO2,Mn(OH)3,C

  • Anode (-): lithium
    Li----->Li++e-

  • Cathode (+): carbon in contact with manganese(III),manganese(IV) electrode
    MnO2+2H2O+e------>Mn(OH)3+OH-

  • Electrolyte: paste of KOH (alkaline electrolyte)

  • Voltage:  

  • Uses: auto-wind, auto-focus cameras
    Can deliver high current.
    Have long shelf-life.

Lithium iodide solid state cell : Li|LiI(s)|I2(in polymer)

  • Anode (-): lithium
    Li(s) -----> Li+ + e-

  • Cathode (+): iodine
    ½I2(s) + e- -----> I-

  • Electrolyte: lithium iodide

  • Voltage: 2 volts

  • Uses: cardiac pacemaker, watches, calculators
    Lasts about 10 years
SECONDARY CELLS

Lead-acid battery (or accumulator) : Pb,PbSO4|H2SO4(aq)|PbO2,Pb

  • Anode (-): lead plates
    Pb(s)+HSO4-(aq) ----->PbSO4(s)+H+(aq)+2e-

  • Cathode (+): lead plates covered with PbO2
    PbO2(s)+3H+(aq)+HSO4-(aq)+2e- ----->PbSO4(s)+2H2O(l)

  • Electrolyte: 4M H2SO4

  • Voltage: 2.1 volts per cell (6 cells in series makes up a 12volt car battery)

  • Uses: motor vehicle batteries
    As cell delivers current, lead sulfate deposits on electrodes & [H2SO4] falls.
    Six 2V cells connected in series make up a 12V battery.
    To obtain high current output from the battery, the surface area of the electrodes in contact with the electrolyte must be large & the plates close together.

Nickel-cadmium cell (NiCad) : Cd,Cd(OH)2|KOH|NiO2,Ni(OH)2,Ni

  • Anode (-): grid covered with cadmium
    Cd(s)+2OH-(aq)----->Cd(OH)2(s)+2e-

  • Cathode (+): grid covered with nickel(III)hydroxide
    NiO(OH)(s)+H2O(l)+e------>Ni(OH)2(s)+OH-(aq)

  • Electrolyte: KOH

  • Voltage: 1.25volts

  • Uses: video cameras, phones, cordless drills, laptop computers
FUEL CELLS
  • promising future

  • more efficient than coal or gas-fired power stations

  • non-polluting

  • porous electrodes lined with catalysts, eg, Ni or Pt, making it expensive

  • systems required to continuously supply reactants & remove products

  • CH4, CH3OH, C3H8 can be used instead of H2(g)

Alkaline fuel cell

  • Anode (-): porous carbon anode with catalyst in contact with H2(g)
    H2(g)+2OH-(aq)----->2H2O(l)+2e-

  • Cathode (+): porous carbon cathode with catalyst in contact with O2(g)
    O2(g)+2H2O(l)+4e------>4OH-(aq)

  • Electrolyte: hot KOH(aq)

  • Voltage: ~1volt

Acid fuel cell

  • Anode (-): porous carbon anode with catalyst in contact with H2(g)
    H2(g)----->2H+(aq)+2e-

  • Cathode (+): porous carbon cathode with catalyst in contact with O2(g)
    O2(g)+4H+(aq)+4e------>2H2O(l)

  • Electrolyte: hot H3PO4(aq)

  • Voltage: ~1volt
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Oxidation and Reduction

Oxidation Numbers (States)

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Electrochemical (Voltaic/Galvanic) Cells

Corrosion

Electrolytic Cells

Calculating Electrochemical Cell EMF (voltage)

Writing cell half-equations

Faraday Laws of Electrolysis

 

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