Category Archives: Battery research

First tests of a Fe-EDDHA|Mn-EDTA system, towards a Fe/Mn flow battery at neutral pH

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I have recently been working on a project to create a DIY flow battery using Fe/Mn salts. The idea is to be able to achieve a close to or neutral pH system, with low cost salts, high concentrations of active species and good cycling ability. In today’s post I will describe some of my very preliminary results using a split cell system.

The image below shows you the experimental setup I am using. Both the right and left side contain graphite rod electrodes. The two chambers are separated by the DIY high permselectivity membrane I prepared using PVA/citric acid/phosphoric acid. The chamber on the left contains a solution of NaFeEDDHA from a commercial fertilizer source at a concentration of 0.05m + 3.5m of NaCl, while the cell on the right contains a solution with 0.05m of Na2MnEDTA + 3.5m NaCl. The pH was set to 7 using potassium carbonate (only a few milligrams were needed). Both chambers are stirred using magnetic stirring bars (tiny ones at 2mm).

A picture of the Fe-EDDHA|Mn-EDTA system. The left side has the Fe and the right side has the Mn. Both solutions are prepared at 0.05m concentration with 3m NaCl. The pH of the system is 7. System is showed after 2mAh of charge.

The idea of these first experiments at low concentration was to put some charge into the system to observe if there was any precipitation of Mn oxides on the cathode, or any other noticeable side reactions. We can also determine if there is any self-discharge due to crossing of Fe-EDDHA over the membrane by seeing the color change on the Mn-EDTA side and tracking the potential. I also wanted to observe what the potential was after charging (predicted standard potential is around 1.2V).

It is worth noting that the separation between the electrodes is quite large and the electrode area is low, so there are expected to be very substantial ohmic losses in this type of configuration. This means it is not useful for charge/discharge cycle data. However we should be able to get some important information about the reversibility of the chemistry and the presence of any bad side reactions, as mentioned above.

The capacity of the system at this (15mL per side) configuration would be 20.1mAh. I charged it to 2mAh at 2.3V, which was able to introduce current at a rate between 700-800mA. After stopping the charging process, the potential dropped to around 1.1V fast and then very slowly from that point. It will take more charge for the potential to hold steady there, but this already shows the chemistry is working. Changing the electrodes for new graphite rods had the potential still holding at similar values, which means the potential is not due to any deposits on the graphite electrodes.

Despite the big charging over-potential – due to ohmic losses – there was no depositing of metallic Fe on the anode or the evolution of any hydrogen gas (no bubbling was observed). I also could not observe the formation of any MnO2 precipitate on the cathode. This therefore means that the Mn3+ is stable, at least in the short term, in the catholyte (as expected from literature experienced with Mn-EDTA).

Thinking about a membrane for my Fe/Mn flow battery

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To build an Fe/Mn flow battery we need a cation exchange membrane to separate the catholyte and anolyte chambers of the device. In this post I want to talk about my initial thoughts about how to create a DIY membrane for this purpose.

Chemical representation of PVA (Polyvinyl alcohol) not to be confused with polyvinyl acetate (what PVA glue is made of).

Commercial cation exchange membranes do exist. Nafion membranes are the most commonly used, but their cost is too high. Just a small 10cm x 10cm square of Nafion can cost upwards of 50 USD, depending on the type of Nafion used. Lower cost membranes (like SPEEK based membranes) have been tested in the literature, but I cannot find any place that actually sells these “lower cost” membranes at a truly lower cost than Nafion.

To be able to make a viable DIY flow battery we need a membrane that we can make, that is lower cost. The requirements of a cation exchange membrane for the Fe/Mn system would be as follows:

  1. Not dissolve in water at neutral pH.
  2. Made from readily available, low cost materials.
  3. Mechanically stable.
  4. No reaction with any of the redox species in solution.
  5. Contain anionic groups (which makes it selective to cations)
  6. Have high conductivity

I looked at potential materials to build this membrane and PVA has become the most prominent base material. It is a polymer with OH functional groups, which I can use to react with readily available chemicals to create a functionalized polymer. My first experiments will involve using phosphoric acid, urea and potassium silicate to create functionalized membranes.

I will prepare 10% w/w solutions of PVA in distilled water, then add different amounts of the above mentioned additives to determine which compositions cast best and have the best properties. I will be casting the films in petri dishes, as this seems to be the most common method in the PVA membrane literature. I will also possibly anneal the membranes by heating them at different temperatures after they have settled.

Double chamber electrochemical cell I bought (haven’t received it yet)

I have also bought a double chamber electrochemical cell to perform experiments using these membranes. The idea is to measure if there is any crossover across the membranes and possibly also measure the ionic conductivity of the membrane.

To measure crossover of ions I can setup one side with the Fe salt and another with the Mn salt, then carry out cyclic voltammetry measurements on the Mn side as a function of time, to measure the appearance of the Fe peak (if there is any crossover). I can compare times between membranes as well. I can also test microporous membranes and non-functionalized PVA membranes, to obtain some baseline measurements. If I setup one side with just NaCl and the other with Fe, I can likely obtain more sensitive measurements (as I will have no current from reactions with Mn species).

Additionally if I use Fe-EDDHA I could sample the solution and measure the appearance of the Fe-EDDHA visible absorption peak near 500nm, which is highly sensitive given the chelate’s very high molar extinction coefficient. Although for this I would near to purchase a Uv-Vis spectrometer, which would cost me 500-1000 USD.

I can also measure ion diffusion by setting up distilled water on one side and a 3M NaCl on the other side and measuring conductivity as a function of time on the distilled water side. This will allow me to compare different membranes and see which ones transport ions faster. If I add Fe chelate to the NaCl I could perhaps measure both ion transport and selectivity simultaneously.

It will be a very interesting journey!

The best low cost Fe/Mn flow battery: Some perspectives about solubility and chelates

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I have previously discussed my project to create a DIY flow battery using Fe/Mn chemistry. On this post I want to expand on the potential limits of this chemistry and some modifications that should enhance our ability to increase its energy density and performance.

My first idea is to attempt to create a flow battery using an NaFe(EDTA) solution as anolyte and an Na2Mn(EDTA) solution as catholyte. This battery would have a potential of around 0.74V, as I measured by cyclic voltammetry (CV) of the species involved. I commented on how the limit of solubility of these chemicals – without any additives – is limited to at best around 0.5M, which limits the battery power density to around 10 Wh/L.

This image shows some NaFe(EDDHA)

However, it is interesting to note that the solubility of these EDTA salts increases aggressively with pH, such that both can be dissolved above 1M at a pH of 7. I confirmed that the solubility increases aggressively as a function of pH, being able to create a solution that was around 1M for both compounds with 3M NaCl supporting electrolyte. To do this I used potassium carbonate to increase the pH gradually to the 7-7.5 range. I also confirmed that the reversibility of the electrochemistry was unaffected through CV, although both standard half-cell potentials are shifted negatively by around 50mV.

This increase in solubility is interesting, as it increases the power density of the battery substantially. If the compounds can be dissolved at 2M, then it would give the battery a density closer to lead acid, at 40Wh/L.

Sadly there are no published studies that show the solubility of EDTA salts as a function of pH, however one of the few published studies of Mn-EDTA in flow batteries (here) shows that you can dissolve Na2MnEDTA at concentrations past 1M. I have bought some additional Mn-EDTA to perform my own solubility experiments, I will let you know what I find out.

Image from this study, using a Zn/Mn flow battery at slightly acidic pH.
Image from this study using Fe-EDDHA at a slightly basic pH.

Another interesting note is to look at other Fe chelate candidates. While EDTA is the lowest cost chelate, the Fe-EDDHA chelate is interesting, as it has a significantly more negative potential Vs Ag/AgCl (-0.6V instead of -0.1V for Fe-EDTA). Recent literature of Fe-EDDHA chelate characterization and its use in flow batteries already shows its practical application (here and here). This increases the potential of an Fe/Mn battery from 0.74V to around 1.2V, which is a decent potential to achieve within the stable window of water at pH 7.

This means that, if using Fe-EDDHA, we could potentially achieve a power density of up to 80Wh/L at a solubility of 2M. If the solubility limit is around 1M, then it should still allow us to get to 40 Wh/L. With this in mind, the Fe/Mn chemistry should match lead acid power density and be a strong competitor against Vanadium based chemistries. This is especially given the fact that Fe/Mn are super abundant and this battery is based on already commercially available chemicals in water, at a neutral pH.

As you can see above, the anolyte and catholyte I propose have been tested, so this is definitely a system that can be built in a rather straightforward manner.

My learning curve with Zinc-Iodine batteries

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In July 2021, I wrote about my goal to reproduce the results of a paper using a high surface area carbon, loaded with elemental iodine, to create a practical Zinc-Iodine battery.

This was not successful because my material of choice – a GFE-1 cathode – did not absorb any iodine when placed in an iodine chamber at 55C for 10 minutes. This is because the cathode material is likely already passivated, as it has been in air for more than a year now, so activation would likely be necessary to get it to load with Iodine. Performing this process is outside my current technical possibilities (no ovens or vacuum chambers available).

Best results I obtained for a ZnI2 2.5M cell. The electrolyte was also saturated with NaCl. This used a Zn anode, GFE-1 cathode and 10 layers of fiberglass separator.

Instead of continuing down this path, I decided to create a battery using Zinc Iodide. This chemical is hard to get though, so I synthesized it from metallic Zinc and elemental Iodine. With the 2.5M solution ZnI2 I created – concentration determined by its density – I then proceeded to create batteries in my Swagelok cell configuration. The best result I got, where energy density was around 29 Wh/L, are shown above.

Despite the above, I was never able to achieve high CE or EE values, with the values for my batteries at >20Wh/L capacities being around 55% for CE and 35% for EE. Low coulombic efficiency happened because of diffusion of elemental iodine away from the cathode – through the formation and diffusion of I3– – as evidenced by the strong coloring of the fiber glass separators when taking the batteries apart. In a similar fashion to Zn-Br batteries, the elemental halogen diffuses and this inevitably lowers the CE, EE and increases the self-discharge of the battery. Even the high surface GFE-1 carbon felt, is just uncapable of holding to the halogen on its own.

Besides making most properties of the batteries worse, the diffusion eventually kills the battery, as so much reagent is lost per cycle that at some point the battery is simply unable to properly function, as any generated zinc is consumed by Iodine that reaches the anode.

I have tried several things so far to eliminate the problem. Thicker carbon material, more dilute ZnI2, TBABr as a complexing agent, none of these have worked, since I3– forms very efficiently under any excess of I– in the solution, which is inevitable as when the reaction starts there is ample iodide in solution. Adding TBAB does precipitate the TBAI3 salt, but this greatly increases the series resistance of the battery.

Is there anyway to stop the I2 from becoming I3– and migrating? Can we somehow plate elemental I2 at the cathode and avoid self-discharge and losses in CE and EE? Stay tuned for more research results.

A new plan, moving to Zinc-Iodine batteries

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In late 2020, I stopped my work on Zn-Br batteries because I decided to move out to the US and back to my home country. Because of the entire move and the pandemic, I didn’t have the time to continue my journey in the research of home batteries. However, I am now fully settled back home and have all my equipment ready to start testing batteries again.

However, I decided against continuing the development of Zn-Br batteries, due to issues I posted on a forum (read here). In summary, the issues with Zn-Br batteries are not something I believe I can surmount or better understand in a DIY environment. The solutions I explored, all involved big compromises between different aspects of the battery that all seemed very unattractive to me. Best of luck to all of you who wish to continue doing experiments using Zn-Br chemistry.

Image taken from the Zn-I battery paper mentioned below

However, there is no shortage of exciting chemistries in the world of Zn aqueous batteries. After doing a thorough review of the current literature, I have decided to start researching Zinc-Iodine batteries, which have achieved some milestone development during the past 3 years, and that should be easy to test in a DIY environment.

My first goal will be to reproduce the results shown in this paper. These researchers use nothing fancy, a highly conductive, high surface area activated carbon that is loaded with iodine as a cathode, a zinc sulfate electrolyte, and a zinc anode.

Given that I have a GFE-1 cathode material (see here), I will try loading it with elemental iodine by sublimation. I will then use a glass fiber separator, 1M ZnSO4 solution as the electrolyte and a 0.2mm Zn plate as the anode. Wish me luck!