A Novel Aluminium Semi-Fuel Cell with Bipolar Membrane for Water Treatment

Water is a unique resource that cannot be substituted by alternatives, it requires care and conservation. Unsafe water is a substantial cause of human diseases and death. Effective methods of wastewater treatment from biologically non-degradable harmful pollutants is the application of hydroxyl radicals OH^* which are strong oxidants (oxidation potential   is E⁰ = 2,8 V). Formation of OH^* in the AOPs is based on chemical, photochemical and photo catalyzed decomposition of hydrogen peroxide in acidic media at pH 3

Fe²⁺ + H₂O₂ → Fe³⁺ + OH^* + OH^-                                                                    (1)

O₃+ H₂O₂ → 2OH^* + 3/2O₂                                                                                                       (2)

H₂O₂ 2OH^*                                                                                                                        (3)

One of the effective ways for solving the problem is development of the electrochemical method for the on-site generation of H₂O₂ by means of cathodic reduction of oxygen in the fuel cells system and for the carry out of water treatment process directly in catholyte of a fuel cell, which is the main goal of the presented work.

            In all fuel cells systems with hydrogen peroxide cogeneration, oxygen is reduced in neutral or alkaline media by the reaction (4). I such case we cannot curried out AOPS (1) in cell, because we must have pH 2.5-3.2.

O₂ + H₂O + 2ē → HO₂^‾ + OH^‾  E⁰  = -0.065 V                                                   (4)

In previous work we developed bipolar membrane aluminium-air semi fuel cell. Bipolar membrane gives us opportunity to create aluminium-air semi fuel cell, where catholyte is acidic solution with pH 2.5-3.2 and as an anolyte we can utilize strong alkaline media.

            The bipolar membrane consists of two ion exchange layers of opposite charge in intimate contact. In the applied electric field, the hydroxide ions and the protons produced by water splitting (8) in bipolar junction move towards the respective electrode in the electric field. Water is replenished in the interface by diffusion through the gel-like membrane layers. The driving forces for the transport of these ions are both, the electrical potential and the concentration gradients. The water splitting requires energy that is supplied by the applied electrical field. Do to reaction 8 we can maintain stable pH in cathodic and anodic chamber

Electrode reactions in O2( air)–Al semi- fuel cell:

Anode: 2Al + 8OH‾→ 2AlO2‾+ 4H2O + 6ē  E0 = -2.35 V                                      (5)

Cathode: 3O2 + 6H+ + 6ē → 3H2O2  E0= 0.69 V                                                   (6)

Total: 2Al + 3O2 + 2H2O + 2OH‾ → 2AlO2‾ + 3H2O2 E0= 3.04 V                       (7)

Bipolar membrane: 4H2O → 4H++ 4OH‾  E0 = 0.83 V                                           (8)

For   water  dissociation  in  the  bipolar  membrane  0.83  V  is  required,  the  electromotive  force  of  the  power source will be E0= 3.04 - 0.83 = 2.21 V. The electromotive force is absolutly enough for the cell operation

Model solutions polluted with phenol and p-nitrofenol.  Initial COD of solution with pollutant were 500 ppm, after 20 min of treatment in the fuel cell COD decreased to 10 ppm.