Cells and batteries

Cells and batteries

1. Cells

   • A cell is a system that contains chemicals which react and produce electricity
   • Contains two different electrodes in contact with an electrolyte
     • The two electrodes must be able to conduct electricity- usually metals
     • Electrolyte - liquid that conducts electricity and contains ions which react with the electrodes
   • The chemical reactions between the electrodes and the electrolyte set up a charge difference between the electrodes
     • If the electrodes are connected by a wire the charge is able to flow and electricity is produced
       • A voltmeter can be connected to the circuit to measure the voltage of the cell

2. Electrodes and voltage

   • Different metals will react differently with the same electrolyte - causes the charge difference (voltage) of the cell
     • Type of electrode will affect voltage - the bigger the difference in reactivity the bigger the voltage
       • If there is no difference in reactivity (electrodes the same metal or reactivity of metals the same) the voltage of the cell will be 0

3. Electrolytes and voltage

   • The voltage of the cell also depends on the concentration of ions in the electrolyte - the lower the concentration the lower the voltage will be
     • As ions in the electrolyte react, their concentration will slowly decrease, and so will the voltage of the cell
       • Eventually the voltage will drop to zero and the cell will stop working

4. Batteries

   • A battery is formed by connecting two or more cells together in series
     • The voltage of the battery is greater than any of the individual cells
       • Equal to all the voltages added togetehr
   • Non-rechargeable and rechargeable batteries
     • Non-rechargeable batteries
       • Once any one of the reactants is used up, the reaction can't append and so no electricity is produced
         • Alkaline batteries are non-rechargeable
     • Rechargeable batteries
       • The reaction can be reversed by connecting it to an external electronic current

5. Fuel cells

   • What is a fuel cell?
     • An electrical cell that's supplied with a fuel and oxygen and efficiently transfers the energy released by the reaction between them as electricity
     • The fuel enters the cell at one side, where it becomes oxidised and sets up a potential difference within the cell
       • The potential difference is what powers the device that the cell is connected to
     • The oxygen enters the cell at the other side where it reacts to form water
   • Hydrogen-oxygen fuel cell
     • The fuel is hydrogen which is oxidised when it combines with oxygen to produce water - not polluting and reduces energy
     • Electrolyte often a solution of potassium hydroxide
     • Electrodes often porous carbon with a catalyst
     • Hydrogen goes to the anode compartment and oxygen goes to the cathode compartment
     • At the negative electrode (anode) hydrogen is oxidised. H+ ions in the electrolyte move to the cathode
       • At the positive electrode (cathode) oxygen is reduced and reacts with the H+ ions to make water
         • The electrons flow through an external circuit from the anode to the cathode - this is the electric current
         • Cathode: O2 + 4H+ + 4e >> 2H2O
       • Anode : H2 >> 2H+ + 2e
     • Hydrogen fuel cells and batteries
       • Disadvantages of batteries
         • Fuel cells don't produce as many pollutants (only water and heat). Although electric vehicle don't produce many pollutants either their batteries are much mor polluting to dispose of
           • Made from highly toxic metal compounds
         • Rechargeable batteries have a limit to how many times they can be recharged before the need replacing
         • Batteries store less energy so would need to be recharged more often (can take a long time)
       • Disadvantage of hydrogen fuel cells
         • Takes up lots more space than to store a rechargeable battery
         • Hard to store safely - explosive when mixed with air
         • Use fossil fuels - made from either hydrocarbons or by electrolysis which uses electricity