Fe-Mn
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@danielfp248 I ran into some of your old work on Fe-Mn batteries. I've been interested in Fe-Mn batteries for some time and was wondering if you could share some of your experience.
In particular, if we look at the pourbaix diagram of Fe and Mn overlapped, there's a region in the pH 4-6 range where Mn2+ oxidizes to MnO2 and Fe2+ reduces to Fe on charge.
Let's say we can ignore the fact that there are solid phases - then ion crossover poses a long term concern. Have you guys found any inexpensive or DIY ion selective membrane options (either specific to a particular ion or broad spectrum diy anion/cation exchange membranes?) In my experience screening for inexpensive battery chemistries, crossover of solution phase species is a problem I haven't really seen an easy DIY solution for. Not that I've looked super hard!
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@danielfp248 I ran into some of your old work on Fe-Mn batteries. I've been interested in Fe-Mn batteries for some time and was wondering if you could share some of your experience.
In particular, if we look at the pourbaix diagram of Fe and Mn overlapped, there's a region in the pH 4-6 range where Mn2+ oxidizes to MnO2 and Fe2+ reduces to Fe on charge.
Let's say we can ignore the fact that there are solid phases - then ion crossover poses a long term concern. Have you guys found any inexpensive or DIY ion selective membrane options (either specific to a particular ion or broad spectrum diy anion/cation exchange membranes?) In my experience screening for inexpensive battery chemistries, crossover of solution phase species is a problem I haven't really seen an easy DIY solution for. Not that I've looked super hard!
@muntasirms I've done several experiments on Fe/Mn and Zn/Mn chemistries. The problem with Mn2+ is the formation of the solid MnO2 phase and the presence of the metastable Mn3+. Forming solid MnO2 poses a non-trivial constraint on the battery, as it limits deposition per area to around a few mAh/cm2, very impractical for a flow battery, furthermore, Mn3+ formation causes MnO2 to form away from the electrode (as it disproportionates into Mn2+ and MnO2), causing some Mn to become lost around the battery system.
A possibility is to try to stabilize Mn3+ somehow (for example with Mn-EDTA), but the main issue is that even this stabilized Mn3+ is unstable and eventually self-degrades by oxidizing the chelate around the Mn atom. I tried creating a flow battery system with Fe-DTPA/Mn-EDTA, which has a max solubility of around 0.5M, but the system did not cycle due to the Mn-EDTA being too unstable. There are a few posts on my blog about this. The oxidized Mn-EDTA is also quite sensitive to pH, so it is hard to create conditions under which it is stable. Mn3+ can also be stabilized with HCl+H2SO4, but only at very low concentrations (there's a paper on using this in a flow battery, but only very low capacities are achieved).
Another possibility is to stabilize MnO2 as nanoparticles in solution. This can be achieved through the use of TiO2+ in sulfuric acid (using titanyl sulfate). Such systems are quite harsh from a chemical perspective, so I haven't tested them at all (I don't want to run a 3M sulfuric acid system containing reactive Ti compounds). You can read more about this system here (https://www.sciencedirect.com/science/article/pii/S0378775322000209). This is one of the most interesting and potentially viable Mn chemistries out there although only reaching around 17Wh/L.
Honestly Mn based systems are best suited for static batteries, where the formation of the MnO2 and Mn3+ phases is less problematic.