Flow Battery Research Collective

Charging to 6Ah/L at 30mA/cm2 and discharging at 5mA/cm2. At most we only get 2-3 Ah/L of available capacity, same as if we charged to 4Ah/L.

The CE drops a lot when going to higher capacities (even at a current of 40mA/cm2, which means it is not due to crossover as lower currents do not imply a lower CE). This is likely because the stability of Mn3+ species in solution is very limited, so you seemingly cannot exceed a ~2.5Ah/L capacity.

I was doing a literature review of Fe/Mn the other day and happened to find this article on Fe/Mn using MSA (https://www.sciencedirect.com/science/article/pii/S001346862030637X). This article uses an asymmetric setup with FeCl3 on one side (paper says it's FeCl2 but that must be a mistake because the reaction requires Fe reduction on charge) and MnCl2 on the other, both sides using 3M methanesulfonic acid, separated by a Nafion membrane. The Mn3+ is in theory stabilized in the acid media, but given the color of the solution it might be that MnO2 nanoparticles are stabilized instead.

While the paper does not use this in a symmetric setup, I see no reason why this reaction couldn't work symmetrically so I prepared an electrolyte using the following:

• 3g MnCl2.4H2O
• 4mL FeCl3 40% w/w solution
• 4mL 75% methanesulfonic acid (MSA)
• around 1mL of water (final volume was taken to 10mL)

The above creates a solution that is around 1.5M Fe, 1.5M Mn and 3M MSA. This setup has the advantage that both reactions generate no solid products. At a 100% SOC this would give us ~20Ah/L. On charge:

Fe3+ + e- -> Fe2+
Mn2+ -> Mn3+ + e-

The potential difference between these two half reactions is not very high though, so the total expected cell voltage is ~550mV. However this is a "true flow battery" in that power and capacity are fully decoupled as the reaction products are all in solution. Note that Mn3+ is expected to have limited stability, especially at high concentrations, so I would expect capacity to degrade heavily as the Mn3+ gets converted into MnO2, unless this MnO2 is somehow stabilized in solution (which could be as nanoparticles). Interestingly Fe2+ can react with MnO2, so the battery might also self-heal if this happens, just temporarily capacity in the process.

I loaded the electrolyte in a cell with carbon felt on both anode and cathode and used Daramic as a separator (cannot use paper as it reacts with Mn3+). Below are the results of a few cycles at low capacity (0.25Ah/L at 10mA/cm2), just to test the chemistry. It seems to work quite well:

I will continue to run some tests and will let you know what I get.

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.

Running to only 7Ah/L at 10mA/cm2 (1M Fe, 4.5M CaCl2, 1M NH4Cl)

This is the result of charging the 1M Fe, 4.5M CaCl2, 1M NH4Cl cell to the Nernst limit (1.5V)

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.

I also just made a blog post on this chemistry, with some of the latest results https://chemisting.com/2025/09/15/studying-an-all-fe-chemistry-using-wise-in-our-flow-battery-kit/

@sepi To answer your questions:

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).

@sepi Thanks for writing. It is exciting in the sense that Fe systems are great because Fe is low cost, low toxicity, easy to source and very sustainable given how much Fe is present in the earth's crust. However, Fe systems suffer from big problems with hydrogen evolution, as H2 evolution occurs easily at acidic pH (which is needed for the Fe3+ species to be stable in solution). In turn, H2 evolution increases the electrolyte's pH, which then causes problems with Fe hydroxide precipitation. These problems have prevented massive adoption of Fe chemistries in flow batteries, in spite of all the above mentioned advantages.

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.

This circulated without any leaks, even lacking one of the endplates.

I will try doing a 3 cell stack, removing one of the other birch wood end plates.

First stack assembly. This stack contains two cells. My screws weren't long enough so I had to sacrifice using one of the birch wood endplates to have it seal. Hopefully it doesn't leak!

I will keep you guys posted. I will leak test this and then mail it to kirk for testing with real electrolyte.

Cycling of the 1M Fe from FeCl2.2H2O, 2.5M CaCl2 and 1M NH4Cl cell to 4Ah/L gave pretty good results, with even some improvements to CE and EE with cycling. I am now doubling the current to 20mA/cm2 and trying to take it to the Nernst limit, see how it does then.

So I realized that my mystat potentiostat lost calibration somehow, so all the experiments on this thread were done with a -0.3V bias. This means that in reality I only discharged to 0.3V and all charging and discharging potentials are actually +0.3V higher. The CV and plate/strip experiments don't have this problem as I used another mystat that was properly calibrated.

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.

@sepi Thanks for trying this. I understand what you mean, I had similar issues when I tried to do this before. The main issue is that the fit is quite tight, so depending on the resolution of both the sleeve and the hole on the endplate, you can have too low of a tolerance for it to work. The easiest fix is probably to increase the size of the holes on the endplate around 0.25mm, so that this works regardless of the printer. We will make the modification on the official endplate exported files so that this approach works more easily.

The Fe/Mg cell died when I tried the higher SOC values. The cycle took a long time (so the CE was a lot lower because of the microporous membrane) but the next cycle showed very ample deterioration of the cell. On opening of the cell there was evidently a lot of undissolved metal on the anode side and a lot of Fe hydroxide had fallen out of solution and basically accumulated inside the reservoir.

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.

Plating/stripping experiments in the Fe+Ca electrolyte show that the Fe deposition is >99% reversible. As in the CV there is no evidence of HER at all, there isn't any bubble formation of the working electrode at all, neither is there any degradation of the plating with cycling.

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.

While the above runs I also prepared a 1M Fe and 4.5M Ca electrolyte using 1.6g of FeCl2.2H2O and 6.6g of CaCl2.2H2O. I did this to perform CV experiments, as a CV was never done on the original paper that discussed the Ca/Mg WiSE electrolytes for Fe plating. The CV experiments were done with a glassy carbon working electrode, a graphite counter electrode and an Ag/AgCl reference electrode.

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.

I did a cycle to 4Ah/L at 5mA/cm2 (~30% of available SOC at 1M Fe). As you can see below the CE is above 93% (even though it took 7+ hours to do the charge/discharge) and the EE is above 50%. I am now going to charge this to the Nernst potential (cutoff at 1.4V). See what I get at 100% SOC.

Doubling the capacity, results still look really good, although the SOC is still very low (~0.4% of SOC). Never thought I would see CE >90% in an Fe plating system with a microporous membrane.

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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.

I got the MgCl2 and CaCl2 to start doing more tests (yay!). I prepared an electrolyte using 1.6g of FeCl2.2H2O and 4.3g of MgCl2 (1M Fe, 4.5M Mg). I then added water till the final volume was 10mL (took something like 5mL). It is worth noting that anhydrous MgCl2 reacts VERY aggressively with water, so make sure you add the water slowly and use an ice batch, otherwise the solution might boil aggressively and splash you. I also added 0.2mL of 15% HCl to lower the pH and solubilize hydroxide impurities in the FeCl2.2H2O.

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.