Only Fe system

[danielfp248]

This test showed some deterioration on cycling:

I took out the catholyte and anolyte when charged (you can see the anolyte (left) and catholyte (right) in the pictures below). There isn't any hydroxide precipitation in either one. However there are some pieces of detached Fe metal on the anolyte, which I think are what causes the slight loss in capacity and increases in ohmic resistance as a function of time.

[danielfp248]

I prepared a bulk amount of solution to have a more standard mix. This is 90g MgCl2, 32g FeCl2.2H2O, 2.5mL 15% HCl, 0.75g of Ascorbic acid. Volume to a total 200mL using a volumetric flask, then transferred to store here.

Initial MgCl2 addition was done in a 1L beaker over around 150mL of reverse osmosis water, added HCl after, then Fe, then ascorbic, then waited to cool, transferred to volumetric flask to finalize vol, then put into storage.

If you prepare this be very careful as MgCl2 reacts very exothermically with water. Cool the water in an ice bath and add slowly.

Final solution is perfectly clear.

The above correspond to molar concentrations of around 0.98M Fe and 4.7M Mg.

The HCl is to ensure complete dissolution of any Fe3+ hydroxyde/oxide contamination and the ascorbic acid is to reduce any present Fe3+ into Fe2+. Excess ascorbic has to react on initial charge, so it will eat into a tiny bit of the SOC.

[danielfp248]

Did a few cycles at low SOC and then attempted to cycle at 10Ah/L.

Catholyte (left) anolyte (right) below:

pH of catholyte was 0.24 and anolyte was 4.7. As you can see on second cycle to 10Ah/L the CE dropped from 93% to 83% and then capacity continued to decrease. On disassembly a lot of unreacted passivated iron remained on the anode, likely due to the pH increase. As Fe deposits water activity likely increases, which increases HER, which increases pH and leads to Fe passivation.

I am going to run a test adding 1M ZnCl2 to the electrolyte, to see how the additional salinity changes water activity and metal passivation.