I am studying an iron chloride redox battery cell (not a flow battery). It is constructed with an iron negative electrode, a carbon positive electrode with an iron (II) chloride electrolyte. The electrodes are physically isolated from each other in a beaker of iron(II) chloride. This cell can be charged and discharged and will run a motor. The output of the battery is 1.2V and can deliver 0.2A.
Unlike most redox batteries, the positive carbon electrode does not take part in the chemistry. The positive carbon electrode, as far as I understand it, simply provides a reaction site for the positive half reaction.
We are told that the half reaction at the positive electrode converts Fe2+ ions to Fe3+ ions (charge) and back again (discharge).
The half reaction at the negative electrode converts Fe2+ ions to metallic iron (charge) and back again (discharge).
The task is to fully describe the iron chloride redox chemistry at both electrodes during charge and discharge.
Where I am:
I am starting with an assumption that during charge and the application of external electrical energy, the dissociated ions in the electrolyte, Fe2+ and Cl-, must migrate to the oppositely charged electrodes. This would be independent of the electrode material and determined simply by the external EMF applied to the battery during charge. The Fe2+ ions must migrate to the negative electrode while the Cl- ions must migrate to the positive electrode.
The result is that the negative electrode will be surrounded by Fe2+ ions and the positive electrode will be surrounded by Cl- ions.
It seems clear that Fe2+ ions will react with incoming electrons, at the negative electrode, to form metallic iron.
However, I do not understand how any reaction can occur at the positive electrode with respect to any variety of "iron chloride". There should be no Fe2+ ions at the positive electrode. Therefore, no reaction involving Fe2+ ions can occur at the positive electrode.
The positive electrode is carbon surrounded by Cl- ions. It is under the influence of a positive electric potential by the external charge source. The only possible reaction I can see occurring is that the Cl- ions may liberate an electron at the positive electrode to form chlorine gas. However, this is not observed under the charging voltage applied, at 25OC and 1 atm. There are no bubbles or chlorine smell.
I have found various references such as:
https://courses.lumenlearning.com/boundless-chemistry/chapter/electrolysis/
That state:
Oxidation of ions or neutral molecules occurs at the anode, and reduction of ions or neutral molecules occurs at the cathode. For example, it is possible to oxidize ferrous ions to ferric ions at the anode:
Fe2+(aq)→Fe3+(aq)+e−
This is exactly what we are told is happening at the positive electrode but I am unable to see how this can occur.
How can a positive ion be in proximity to the positive electrode in order to react with it?? How can any reaction involving Fe2+ ions happen at the positive electrode??
