In aqueous media nobody has really achieved it at energy densities that would matter and in non-aqueous media it is incredibly expensive and still, not without reversibility problems.

What's expensive about the non-aqueous media? Do you speak about deep eutectic solvents?

I see a number of groups working on aluminum flow batteries. Seems they don't have the dendrite problem.

I will keep you posted on how the cycling goes and if it degrades in capacity as other similar tests have done in the past.

I am now testing it with a non-conductive felt on the Zn side, to increase the time it takes for dendrites to form, as they now have to cross the entire cell to reach the membrane. I expect some loss in conductivity but this can be worth the tradeoff. This also increases the time it takes for Fe3+ to react with Zn as it now has to entirely cross the separator and cannot easily meet Zn half way through.

As you can see the solution is not entirely translucid, because there is some slight Fe oxide impurity in the FeCl2 I use.

I am trying to charge to 5Ah/L which would be 75% SOC of a 0.5M symmetric electrolyte.

If you just googled you would believe that we have plenty of technologies that were much better than Vanadium, but reality shows only Vanadium flow batteries at large scale. This is part of why I believe our dev kit is important, to lower the entry bar for flow battery research enables people with other priorities to also research the technology and find and study solutions with the idea of scale up in mind, without the pressure to publish over them.

he Fe3+Gly complex is extremely red while the Fe2+Gly complex is transparent

Got it - I actually didn't know that about Fe-glycine complexes. Thank you for teaching me something new today. Could lend to some cool/inexpensive colorimetric characterization.

The plating makes sense given that Zn plating tends to be much faster than Fe plating. The main other thing I could think of is some invisible amount of oxide depositing over time and fouling the surface (may especially be difficult to see in the felt) since at high Fe2+ concentrations oxides will form (at least thermodynamically) according to its Pourbaix diagram (below). Of course kinetics of that deposition is all affected by complexing with glycine and the like so I'm not certain. Intuitively I would say it could be alleviated by decreasing the pH. I wish I could help more but thank you for being so responsive and detailed. Good luck!

High-energy and low-cost membrane-free chlorine flow battery:
https://www.nature.com/articles/s41467-022-28880-x#:~:text=To meet the needs of,La France in 188428.

A high-energy and low-cost polysulfide/iodide redox flow battery:
https://www.sciencedirect.com/science/article/abs/pii/S2211285516304153#:~:text=Highlights * • The polysulfide/iodide redox flow,reversibility of polysulfide and iodide redox chemistries.

Air-Breathing Aqueous Sulfur Flow Battery for Ultralow-Cost Long-Duration Electrical Storage:
https://www.sciencedirect.com/science/article/pii/S2542435117300326

And 2 Google search options with a lot of results over my head:
Sulfur-Air Hybrid Redox Flow Batteries: https://www.google.com/search?num=10&cs=1&sca_esv=db6c98fd308e1745&q=Sulfur-Air+Hybrid+Redox+Flow+Batteries&sa=X&ved=2ahUKEwjS8bKDhoGPAxXpL0QIHXvhGwgQxccNegQIEhAB&mstk=AUtExfDy0ZazObNsltp9BYwtpQylX9u5AJT5yaQVQRbdARMJvJuEzcAf-Z2d4utwzTSnhCKcn4qOInT9b9zVkToTFJswogvnK4pun2X3x25KeDNW8dbrTS5jwMmIp6riqcFc6wG0Rm8FPMwC2B67TUxmok7FRMfFyfzTWq_Ub_BP2zY5rP4&csui=3

All-Iron Redox Flow Batteries: https://www.google.com/search?num=10&cs=1&sca_esv=db6c98fd308e1745&q=All-Iron+Redox+Flow+Batteries&sa=X&ved=2ahUKEwjS8bKDhoGPAxXpL0QIHXvhGwgQxccNegQIIhAD&mstk=AUtExfDy0ZazObNsltp9BYwtpQylX9u5AJT5yaQVQRbdARMJvJuEzcAf-Z2d4utwzTSnhCKcn4qOInT9b9zVkToTFJswogvnK4pun2X3x25KeDNW8dbrTS5jwMmIp6riqcFc6wG0Rm8FPMwC2B67TUxmok7FRMfFyfzTWq_Ub_BP2zY5rP4&csui=3

Google sure like long URL's

• Does your anolyte yellow over time? I know we're ruling out crossover but that could be another easy way to check - FeCl3 is a deep yellow vs. FeCl2. I'm assuming you've thought of this but I thought I'd suggest it

Chlorides here don't play a strong role as the Gly complexes are much stronger than the chloride ones. The Fe3+Gly complex is extremely red while the Fe2+Gly complex is transparent. There is always some additional Fe3+ - because my starting FeCl2 is slightly oxidized - so the starting electrolyte is already quite red. On charging the anolyte then turns transparent and the catholyte turns completely blood red. They never turn orange. On discharge the anolyte also never turns red, it remains transparent.

• You mentioned seeing a lot of undissolved metal on the anode side. Any possibility of dead metal flaking off the felt? In acidic media I wouldn't expect it but who knows. What does the metal on the graphite felt look like usually? any pics?

No, all the metal remains on the felt. I didn't get any peaks, but the metal looks just like regular plated Zn. It doesn't rust on exposure to oxygen, so it probably contains very little Fe. At Zn/Fe ratios higher than 1.5, Zn seems to plate exclusively, which I confirmed with CV measurements (I see no oxidation peaks for metallic Fe when doing CV, which you do see at lower ratios).

To be honest, I am only interested in systems that use microporous membranes (no ion selective membranes) as selective membranes not only drive the initial cost of systems up a lot but can make them very unreliable, as even slight problems with selectivity can lead to solutions going to waste for non-symmetric electrolytes.

Me too, and that's the route I chose as well (trying to avoid chemistries that required them). Just wanted to ask in case you found an easy/inexpensive solution!

@danielfp248 said in Alternative Electrolytes:

Also note I've seen Fe oxides form at pH values as low as 2.5

Right, that's not too surprising either. Kinetics/rate of formation is difficult to deconvolute from thermodynamic "possibility to form" as it were. Just to clarify, seeing the oxides = orange particulates in the graphite felt?

Some other ideas:

• Does your anolyte yellow over time? I know we're ruling out crossover but that could be another easy way to check - FeCl3 is a deep yellow vs. FeCl2. I'm assuming you've thought of this but I thought I'd suggest it
• You mentioned seeing a lot of undissolved metal on the anode side. Any possibility of dead metal flaking off the felt? In acidic media I wouldn't expect it but who knows. What does the metal on the graphite felt look like usually? any pics?

Thanks for being so responsive. This is interesting

To be honest, I am only interested in systems that use microporous membranes (no ion selective membranes) as selective membranes not only drive the initial cost of systems up a lot but can make them very unreliable, as even slight problems with selectivity can lead to solutions going to waste for non-symmetric electrolytes. So, I only study systems where the composition of the discharged anolyte and catholyte is identical and a microporous separator can be used, even if this means sacrificing some CE.

Also note I've seen Fe oxides form at pH values as low as 2.5, there is only a lack of Fe oxide formation because of the Glycine here. Without glycine you do see Fe hydroxide precipitation on the catholyte side on charging as time goes by, even though the measured pH stays below 4.

There is clearly some crossover of Fe3+ and this of course causes some corrosion of the Zn2+, but this should only cause some loss of the CE, it shouldn't lead to increases of ohmic resistance and the cell losing capacity as a function of time. The electrolyte is perfectly symmetrical (both anolyte and catholyte have the exact same composition when discharged), so migration of solution shouldn't be a big problem for short term cycling.

The problem is not that my CE isn't perfect - I obviously expect this from a microporous separator - the problem is that something is happening that is making the capacity go down as a function of time. I have no idea what this should be, but corrosion shouldn't do this, it should just lower the CE (let me know if I'm wrong here).

Also I have equipment to do CVs, but our flow battery does not have the ability to do three electrode experiments atm.

I didn't see any Fe oxides forming

This aligns with what I'd expect - most iron oxides form at pHs above ~ 5 ish. The most stable ones (magnetite is usually the major problem) form in strongly basic conditions, ph >12 ish.

@danielfp248 said in Alternative Electrolytes:

This is 2M FeCl2, 3M ZnCl2, 2M Glycine. At 20mA/cm2. Felt on both sides, daramic membrane. The pH of this is 3.2.

Hmm...your daramic membrane isn't ion-selective, right? Is it just a size exclusion separator? My first thought is ion crossover (or if you already have both electrolytes mixed, just corrosion). Similar to my question in the Fe-Mn post actually! If your separator isn't ion selective (or you already have all active species mixed together), then any Fe2+ and Fe3+ in electrical contact with Zn metal can drive galvanic corrosion because their reaction voltages differ:

2Fe3+  + Zn <-> 2Fe2+ + Zn2+ 

A couple ways you could ad-hoc test this:

• After charging, let the cell rest but track the open circuit potential over time - if the cell voltage drops fairly quickly (over the course of hours), that could suggest corrosion
• If you want to be more quantitative, you could take a look at mixed potential theory - it predicts what the open circuit potential of a system would be if you have two redox couples that are electrically shorted. Here's a nice preprint from a buddy at @quinnale 's lab that explains it nicely (though fair warning, it's still complicated).
• Try having two separate solutions of the FeCl2+Glycine and ZnCl2+Glycine in each half cell. Unless you have an ion selective membrane, you'll still have ion crossover over time, but it will be slower and your capacity loss shouldn't be as significant.

I think this also explains your lower, but very stable coulombic efficiency.

Granted, if that hypothesis is right, you would've seen this problem from the first cycle. So someone else might have a more accurate view of the problem! Do you guys have the equipment for 3 electrode experiments? That could help with diagnostics. I might make a new post with a list of affordable electrochemical equipment like reference electrodes that I've run into.