Applications of electrochemical cells

• Two types of cells

  • Primary
  • Secondary
• Primary cells can only be used once. Secondary cells can be recharged and reused
• Structure of a primary cell

  A) Cathode

  B) Anode

  C) Porous separator

  D) Carbon electrode

  E) Cover

  
• Primary cells transfers stored chemical energy into electrical energy by a non-reversible chemical reaction

  • Usually cheap
  • More reliable
  • Useful for applications such as smoke detectors
  • Examples include carbon-zinc dry cell and alkaline dry cell
• Secondary cells

  • Are more environmentally friendly
  • Reversible reaction
  • More expensive
  • Would have to buy charging equipment
• Lithium cells are found in phones and are rechargeable
• Lithium cells

  At the positive electrode

  Li+ + CoO2 + e- --> Li[CoO2]
• Lithium cells

  At the negative electrode

  Li --> Li+ + e-
• Overall reaction when discharging the lithium cell (using it)

  Li + CoO2 --> LiCoO2
• Lithium is highly flammable and cobalt is toxic
• A battery is more than one cell connected together in a series arrangement. When a conductor is connected across the two terminals, a circuit is formed
• Lead batteries are found in cars
• Calculating e.m.f of batteries

  1. Fist calculate the Ecell
  2. Then times it by the number of cells in the battery
• Fuel cells convert chemical energy into electrical energy

  • Its an open system as reactants flow in and products flow out
  • Used in spacecraft and in hydrogen powered cars
• Acidic fuel cells

  • H2 releases 2e- and becomes 2H+
  • Electrons flow around the circuit from anode to cathode.
  • H+ ions travel through the P.E.M (proton exchange membrane) to cathode
  • H+ ions reacts with O2 to form H2O + e-
• Alkaline fuel cells

  • H2 reacts with 2 OH- to release 2e- and make 2H2O
  • Electrons flow around the circuit
  • H2O travels through the P.E.M to the cathode
  • H2O reacts with O2 and e- to reform OH-
• Hydrogen - oxygen fuel cell

  At the oxygen electrode the reaction 4e- + 4H+ + O2 --> 2H2O

  A) Hydrogen gas

  B) Voltmeter

  C) Anode

  D) Cathode

  E) P.E.M

  F) Electrolyte

  
• Two methods of manufacturing hydrogen

  • Reaction of hydrocarbons with steam
  • Electrolysis of acidified water
• Transporting and storing hydrogen

  Since H2 is an explosive gas with a low melting point its hard to transport it.

  Methods to transport and store H2

  • Liquid under pressure
  • Adsorption - adhere hydrogen to the surface of something
  • Absorption - adhere hydrogen into something
• Hydrogen rich fuels include methanol, natural gas and petrol so it would produce CO2 as part of the hydrogen production
• Direct methanal/ethanol fuel cell

  • Uses methanol or ethanol as a fuel without converting it first to hydrogen
  • Ethanol is less toxic but produce more CO2
  • Ethanol is produced by fermentation which is considered carbon neutral
  • At the ethanol electrode the following reaction occurs C2H5OH + 3H2O --> 2CO2 + 12H+ + 12e-
• Hydrogen-oxygen fuel cells

  In alkaline solutions

  • At the anode the H2 gas reacts with 2OH- to produce 2H2O and 2e- in a reversible reaction
  • At the cathode 1/2O2 reacts with H2O and 2e- to produce 2OH-

  In acidic solutions

  • At the anode the H2 dissociates into 2H+ and 2e-
  • At The cathode 1/2O2 reacts with the 2H+ and 2e- to form H2O

  Both overall equations is 1/2O2 +H2 --> H2O with  E° = 1.23v
• What are the 3 advantages of fuel cells

  • They are much more efficient that an internal combustion engine in machines as more energy is converted into kinetic energy. Combustion engines waste a lot as thermal energy.
  • Many electric vehicles are battery powered, however, unlike batteries, fuel cells don't need to be recharged. You just need a ready supply of oxygen and hydrogen.
  • The only waste product is water and no carbon dioxide is emitted directly from the cell unlike a combustion engine.
• What are the 3 disadvantages of fuel cells

  • Hydrogen is highly flammable and must be stored and transported correctly.
  • It is expensive to transport and store hydrogen. Storage of hydrogen is in pressured containers.
  • Energy is required to make the hydrogen and oxygen in the first place. Fossil fuels are generally used to pass water through an electrolysis process. The use of fossil fuels contributes to CO2 emissions.
• Some small rechargeable batteries are made from nickel/cadmium cells. With the reaction going left to right when discharging(electrons flow from Cd to Ni) and the opposite for charging
• Equations for rechargeable cells

  Cd (s) + 2OH- ⇌  Cd(OH)2 (s) + 2e-

  NiO(OH) (s) + H2O(l) + e- ⇌  Ni(OH)2 (s) + OH-

  Overall equation: Cd (s) + 2 NiO(OH) + 2H2O ⇌  2Ni(OH)2 + Cd(OH)2
• Deduce the equation for the overall reaction that occurs in the ethanol-oxygen fuel cell.

  CH3CH2OH +3O2 ---- 2CO2 + 3H2O