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.

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.

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.

There is still some decay in capacity due to increases in resistance, although much slower. I will now charge this to 6.5Ah/L, see if it can cycle in a stable manner at that capacity.

1. We are testing concentrated MgCl2 and CaCl2 electrolytes, which have never been published in all-Fe flow batteries. The experiments are therefore innovating in the space.

2. It wouldn't be an issue and in fact this is how Zn-Br commercial flow batteries work, to make them stable they are never charged beyond something like 20-30% of their SOC. The Zn-Br batteries have high enough capacities at 20-30% SOC to make them still quite dense at this much lower SOC rating. For an all-Fe flow battery this would be so low that the cost per kWh would climb a lot, but of course you can always go this route if the compromises are worth it to you.

3. The max voltage the all-Fe chemistry we're studying could give would be around 1.2V given the CV. Ohmic losses because of the solution conductivity and separator thickness are likely a meaningful component of our drop, so are likely the kinetics of plating/stripping and Fe2+/Fe3+ reactions on carbon felt.

4. As far as I know, the main patents on Fe plating flow batteries were issues in the mid 80s, so most of the original patents of this technology have expired. However there are a lot of patents in Fe batteries, particularly dealing with electrolyte modifications and electrode modifications (to reduce H2 evolution).

1. Do you do anything differently than what is already published or is the challenge mostly to get something kind of proven working in the FBRC devkit?
2. Would it in general be an issue to just never charge the battery to high SoCs? Sure your volumetric energy density will go down but you might avoid all kinds of trouble this way. I imagine habing a very simple or lowtech battery of mediocre efficiency and density could still be of a lot of value to many people.
3. Are the low voltages due to ohmic losses in the separator and maybe electrolyte?
4. How is the patent situation around this technology?

I had personally never been able to have an Fe system work with our battery system, so the excitement comes from finally having some electrolyte configurations that are sort of working well (at least cycling well at low SOC values with significant CE and EE). The big issue is that there isn't any stability in cycling at high SOC values yet, but at the current state you can run experiments with our kit and help develop Fe battery systems.

The potentials are lower than those expected from the CV experiments, but after recalibrating my potentiostat the losses are actually lower than I thought. So I am now getting potentials near the 0.95V when going to high SOC values.

take it to the Nernst limit

https://www.youtube.com/watch?v=9D-QD_HIfjA

I recalibrated my mystat to ensure it is properly zeroed and got curves that make much more sense. This is for a new electrolyte I prepared with 1M Fe from FeCl2.2H2O, 2.5M CaCl2 and 1M NH4Cl (I added this as ammonium often helps prevent metal passivation). I also added 10mg/mL of citric acid, to serve as a buffering agent.

These are curves obtained at 0.5Ah/L charge/discharge at 10mA/cm2:

As you can see the potential has now stabilized and the final CE and EE values are quite good (for all Fe at least!). The discharge potential is also now +0.87V, which is in much better agreement with my CV values.

I then cut the daramic separator in half and took a 30x image using a microscope.

Clogging of the separator by hydroxide that forms on the metal surface is likely a problem with a setup with carbon felt on both sides.

I have now started a cell with a non-conductive separator on the anode side, to see how results change.

Doing experiments at multiple plating times (5, 10, 30, 60 seconds), plating at -0.9V and stripping at -0.5V (10 plate/strip cycles per experiment), shows the behavior is basically reversible.

The stripping efficiency seems really high and the standard potentials come out at -0.68V for the Fe2+/Fe0 and +0.56V for the Fe2+/Fe3+. This means our max potential on the Fe/Ca system should be 1.2V (shouldn't be far for the Fe/Mg) system, which suggests we are suffering from kinetic problems of one of the reactions on the untreated felt. I will test some felt treatments (heat and hypochlorite) to see if these can improve the VE. Fe is known to have kinetics problems on untreated felts, the CV confirms we should be able to get a much higher potential from this chemical system.

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I am now charging to 10x the capacity (which would be ~40% of theoretical SOC at 1M) to see how it behaves.

So far I've only done very low SOC cycling but it is extremely encouraging that I see high CE values even under these conditions. This is normally a sign of a very reversible system. @muntasirms It seems this works quite well, thanks for sharing the paper and the idea of using MgCl2/CaCl2 in flow batteries with us!

I will continue cycling and then start going to higher SOC levels, see how reversible this actually is.

Will wait for the Mg, Ca chlorides before doing another test. Just urea doesn't seem to work well enough.