Electrochemical and Fuel cells, Electrolysis

• Electrochemical cells are used to convert chemical energy into electrical energy
• The electrical energy which comes from electrochemical cells comes from the movement of electrons from one half cell to another
• Electrochemical cells use chemical reactions which transfer electrons from one species to another by redox reactions.
• Electrochemical cells are made from two separate half cells, each of which are one of the two half equations in the redox reaction.
• Each half cell contains an element in two different oxidation states.
• There are three types of half cells: Metal/metal ion half cells, Ion/ion half cells, and gas half cells.
• In metal / metal ion half cells, a metal rod is dipped into a solution of its aqueous metal ions. The metal rod enables electrons to move to or from the half cell, creating a voltage.
• In ion / ion half cells, there is a solution of ions of the same element in two different oxidation states. For example Fe 2+ and 3+
• In ion / ion half cells, there must be a platinum electrode, which is largely unreactive, to allow the movement of electrons between half cells.  
• The ( standard ) electrode potential tells us how readily a chemical substance gains electrons.
• The more positive the electrode potential, the greater the tendency for a chemical substance to gain electrons.
• Standard electrode potential is the EMF of a half cell measured against the standard hydrogen electrode under standard conditions of 298K, 100kPa and 1 moldm-3
• The cell potential, Ecell, = most positive electrode potential - least positive electrode potential.
• Electrons flow away from the half cell with the least positive half cell to the most positive one.  
• The most positive half cell in terms of electrode potential is the Cathode, the positive electrode.
• The least positive half cell is the negative electrode, the anode.
• Red cat: reduction occurs at the Cathode.
• Oxidation takes place at the anode.
• If Fe3+ has an electrode potential of 0.77 and Iodine has one of 0.54, iodine cannot oxidised a solution of Fe2+ to Fe3+ as iodine is not a powerful enough oxidising agent, as iodines standard electrode potential would need to be more positive to ensure it is the forward reaction.
• Standard electrode potential is under standard conditions, if the cell doesnt operate under these conditions, the electrode potentials will be different to the standard values.
• As the cell operates the concentrations of solutions change from the 1 moldm-3, and so equillibrium position would shift causing the electrode potential to change.
• If the rate of reaction is very slow, it may appear as if a feasible reaction in an electrochemical cells is not occuring, eventhough it is.
• If the activation energy of a reaction is high, it may prevent a feasible reaction from taking place.