Palmerston North Boys’ High School
Chemistry Level 3
Achievement Standard 91393 v1 (C3.7)
Demonstrate understanding of oxidation-reduction processes
By Wassa Tunsiri
Non-rechargeable battery: Aluminium air
In an aluminium air battery, aluminium metal (or foil, electrode) is immersed in electrolyte near the porous electrode which has air on one side and electrolyte on the other side (aluminium’s side). The choice of electrolyte is various. It can be common salt (NaCl) or alkaline solution (KOH), sea water can also be used but only specific type of the alloys of aluminium can be used. The porous electrode can be made of granules of activated charcoal (mostly made of carbon) which prevents the leakage of electrolyte.
The silvery aluminium (Al) reacts with OH- (KOH, which is electrolyte) ions to form a white precipitate aluminium hydroxide (Al(OH)3(s) ) and release 3 electrons as the oxidation number has increased from 0 to +3, Al(s) to Al3+(aq).The OH- ions are present because the electrolyte is alkaline solution which contains OH- ions. This oxidation reaction occurs at the anode. During the reaction, silvery aluminium is being coated by white precipitation Al(OH)3(s). Oxidation half equation: Al(s) + 3OH−(aq) → Al(OH)3(s) + 3e−
At the porous electrode (activated charcoal) the water in the electrolyte reacts with oxygen on another side from the air. The colorless water (O2) absorbs one electron and gets reduced into colorless OH- ion. This reduction reaction occurs at the cathode.
Reduction half equation: O2(g) + 2H2O(l) + 4e− → 4OH−(aq)
The overall equation is: 4Al(s) + 3O2(g) + 6H2O(l) → 4Al(OH)3(s)
The electrons flow from anode through the battery and back to the cathode.
Conventional cell diagram (IUPAC): Al(s)/ Al(OH)3(s)//O2(g),OH-(aq)/C(s)
Calculating cell potential:
Eocell = Eored – Eoox = 0.40 – (-2.31) = 2.71v
This positive value of standard reduction potentials suggests that the oxidation-reduction reaction occurs spontaneously as the redox couples of O2(g)/OH-(aq) and Al(OH)3(s)/Al(s) produce a positive resultant cell potential of +2.71v.
Aluminium metal will get oxidized in the electrolyte (KOH) forming Aluminium 3+ ions and OH 1- ions (Al(OH)3). The electrons produced from oxidation reaction will travel from the aluminium electrode (anode) through the electricity device and then to cathode or activated charcoal, water in the electrolyte will react with oxygen in the air which travel through the activated charcoal and this is where electrons are used up in the reduction reaction to form OH- ions.
Rechargeable battery: Zinc-bromine battery
There are 2 tanks in the zinc bromine battery. The solution of zinc bromide (electrolyte) is stored in each of 2 tanks. When the battery is charged or discharged the solutions are pumped through the electrode and back to the tanks. A tank that is used to store solution for positive electrode is called catholyte reservoir and tank that is used to store solution for negative electrode is called anolyte reservoir. These reservoirs are separated by a micro porous separator (salt bridge) which is made of polyethylene coated with sulfonated polysulfone (a cationic polyelectrolyte). All battery components are made from a bromine inert plastic.
Anolyte reservoir contains zinc metal which, during the charge, is plated from the electrolyte solution and deposits onto the surface of cell stacks. Catholyte reservoir contains bromine solutions which is converted into bromide ions at the positive electrode which is then immediately stored. The zinc-bromine battery uses carbon plastic electrodes that cannot and do not take part in the reactions but act as substrates for the reactions.
In zinc bromine flow battery, reaction at the anode (reversible), the plated zinc at the anode dissolves in the electrolyte (KOH) to form Zn2+ ions. Silvery Zinc metal (Zn) undergoes oxidation reaction forming