Tag Archives: Mn/Fe flow battery

Problems with the Fe-EDDHA|Mn-EDTA flow battery system

I wrote a blog post about some of my first tests of the electrochemistry of the Fe-EDDHA|Mn-EDTA system and how I planned to build a flow battery using this chemistry. I have since been able to setup a flow battery and perform some initial experiments. So far, there are several fundamental problems with the above chemistry that strongly affects its viability. I will discuss these problems below.

Fe-EDDHA solubility and pH. The Fe-EDDHA is more soluble at more basic pH values and its solubility at neutral pH is limited to perhaps only 0.1-0.2M. Furthermore, there are several less soluble impurities present – different isomers of the EDDHA – which have to be filtered before the cell is operated. Additionally, the pH changes substantially upon charge/discharge, which causes problems with the Mn-EDTA side (you’ll see later on).

Fe-EDDHA anolyte (A2) next to Mn-EDTA catholyte (C2) solutions, both at around 0.2M.

Difficulty in working with Fe-EDDHA solutions. The solutions of Fe-EDDHA are very deeply red, basically black at the concentrations we’re working with here. This means that it is very hard to tell if there are any insoluble substances in the solution. It is also hard to tell the charge state of the solution, as it will only slowly become more transparent as the Fe-EDDHA is reduced to its Fe(II) form.

Fe(II)-EDDHA sensibility to oxygen. Once the Fe-EDDHA is reduced in the anode, the liquid becomes incredibly sensitive to any oxygen dissolved in the solution, which will quickly oxidize the Fe(II)-EDDHA back to Fe(III)-EDDHA. Given that I currently have no means of purging my system of oxygen, this makes it impossible for me to run flow batteries using Fe-EDDHA right now in a reliable way.

Anolyte and catholyte of the system. You can observe the red color of Mn(III)-EDTA appear in the catholyte (C2) as the flow battery is charged.

Mn-EDTA stability at higher pH. The oxidized form of Mn-EDTA, which is Mn(III)-EDTA, is red at a pH below 6.5 and yellow at a pH above 6.5 (you can see that yellow in a previous post). However, the yellow molecule is extremely unstable, therefore it is very important for pH to remain below 6.5. However, at a pH below 6.5 the solubility of the Fe-EDDHA drops substantially. The differences in pH between charge and discharge means that both systems are virtually incompatible, because as the Fe-EDDHA charges it induces changes of pH that very negatively affect the decomposition of the Mn-EDTA catholyte.

For all the above reasons, I decided I no longer will explore the Fe-EDDHA|Mn-EDTA system, as I currently don’t have the technical setup that is necessary to properly study it. Furthermore, the low solubility, difficulty of working with the Fe-EDDHA solutions and the problems with pH, make this system more complicated than I was hoping for, especially in a system I hoped to be simple for DIY.

I will continue to study flow batteries based on Mn chemistries, including Mn-EDTA (as these are very interesting), but I will likely change the anolyte to something that is more aligned with my current setup. Most likely I will have to compromise and use a potentially higher density, low cost anolyte, that will involve plating a metal on the carbon felt anode, like Zn. Hopefully in this manner I’ll be able to find a low cost setup to bring a DIY solution with a capacity at least in the 10-20Ah/L range.

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

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).

Measuring and improving the performance of PVA/Cellulose cation exchange membranes

In a previous post I described how to create a DIY cation exchange membrane using some easy to get materials. These membranes could achieve significant permselectivity values, but still far away from those required to create membranes for a robust flow battery. Additionally, the sheet resistance of these membranes – which I measured using a 4 contact electrode method – was quite bad, with values often greater than 6000 ohm/cm2. The through plane resistance was around 3x that, although my method for through-plane resistance measurement is not reliable yet.

Some of the last membranes I produced using a PVA solution with a pH in the 6-7 range. The membrane remains an off-white yellowish color, but does not oxidize as in my previous tests.

In this post, I want to talk about the advancements I have made to improve the fabrication of these membranes. First of all, I have lowered the preparation temperature to 150C, this avoids charring the membranes and improves reproducibility. I also added 80 minutes of additional time at these temperature once all the PVA coats have been put on, this improves crosslinking and drastically reduces the solubility of the membrane in water (to the point where it becomes fully insoluble).

I have also found out that decreasing the acidity by adding some potassium hydroxide also helps retain membranes structure, increase permselectivty and decrease sheet resistance. This matches some papers on cellulose phosphorylation using potassium phosphate and ammonium phosphate salts, with solutions that have much higher pH values than phosphoric acid. The higher pH helps preserve the structure of the cellulose and PVA, as a lack of acid reduces the changes of degradation of the cellulose and PVA. The phosphorylation still happens, thanks to the urea catalyst present.

With this in mind, the membranes can probably be made using monopotassium or monoammonium phosphates, much more readily and less dangerous chemicals compared to concentrated phosphoric acid and potassium hydroxide.

One of my last experiments to measure permselectivity. The cell to the right contains a very small amount of NaFe(EDDHA), which has a very deep red color. This makes it very easy to see membrane crossover.

The best values I have achieved so far are a permselectivity of 80% and a sheet resistance of 373 ohm/cm2. These are still much worse than those of commercially available membranes, but certainly better than the values I was achieving before.

From the parameters I have tested, the cross-linking temperature and pH seem to be the most important to the qualities of the membrane, so I will try to study these too with a bit more detail to find out if I can produce membranes with better qualities. Increasing the concentration of P at higher pH values with higher urea quantities might also help achieve better cross-linking.

Thinking about a membrane for my Fe/Mn flow battery

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

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.