Tag Archives: Fe chemistry

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.

Towards a DIY Manganese/Iron flow battery. First experiments using cyclic voltammetry.

Flow batteries are a great approach for large scale energy storage. While in a battery the amount of energy is constrained by the mass of the anode and the cathode, in a flow battery the cathode and anode are stable electrodes (most commonly graphitic foams) and the energy is stored in solutions that are pumped through these electrodes.

General diagram of a flow battery.

Many of the lowest cost approaches to the chemistry of a flow battery are unable to fully take advantage of this, because they reduce a metal to its solid form on the anode. Approaches using Fe and Zn where this happens are common. The deposition of a solid metal then creates additional issues with both passivation and with dendrites, which can end up shorting the flow battery down the line.

To solve these issues, we need a chemistry where both the oxidation and the reduction happen in solution (with no solid formation on the electrodes). Additionally both of the half-reactions need to be reversible. From a DIY perspective, they should ideally happen under mild conditions and, to make things even more difficult, we need materials that are low cost and that can be easily purchased.

Relative abundance of elements in the earth’s crust

Manganese (Mn) and Iron (Fe) are some of the most common elements on the Earth’s crust, so they fulfill the cost issue. However, when building a flow battery with Mn, we find that the oxidation reactions that Mn is involved in generally involve the formation of insoluble Mn oxides. This happens because Mn3+ is generally unstable in solution and reacts with water to create Mn2+ plus an Mn4+ oxide or hydroxide.

However, a few papers have been published on the use of Na2Mn(EDTA) in flow batteries. This chelate – a commonly available fertilizer – protects the Mn3+ from reacting with water and enhances the reversibility of the reaction. Given the potential where the oxidation of Mn(EDTA)-2 happens, I thought it could certainly be coupled with the reduction of Fe3+ to create a flow battery. Additionally NaFe(EDTA) is also a low cost highly available fertilizer we can use.

On a previous post, I spoke about a setup for electrochemistry that I created, which allows me to carry out several measurements in solution. Using cyclic voltammetry of a solution containing Na2Mn(EDTA) and NaFe(EDTA) I was able to characterize the system and obtain half reaction potential values for the Fe and Mn reactions mentioned above.

Half reactions and half reaction potentials measured Vs an Ag/AgCl reference electrode. For Mn the reduction is shown.
Cyclic voltammetry used to obtain the E1/2 measurements. The Fe reaction happens to the left while the Mn reaction happens to the right.

If we add the two potentials above, we can obtain the expected potential for our battery, which would be 0.74V. This is not very high, which means that the energy density of our flow battery system is going to be low. If we consider the solubility of both compounds, then we expect the power density of this battery to be around 10Wh/L. This means that you would need 100L of 0.5M NaFe(EDTA) and 100L of 0.5M Na2MnEDTA to get a 1kWh battery. This means 18.3kg of the Fe salt and 19.5kg of the Mn salt. You will also need around 35kg of NaCl as a supporting electrolyte.

At retail you can find both Fe and Mn salts for a price of around 6-15 USD/kg (if you buy 25-55lb bags). On the low end this means the cost would be 226.8 USD/kWh and on the high end 567 USD/kWh at a retail price point. In pallet amounts, the cost for both is around 2 USD/kg, so the cost goes down to 75.6 USD kWh. Note that this is only for the Fe and Mn salts.

The challenge is now to create a small electrochemistry setup with two electrochemical chambers separated by an ion exchange membrane where we can carry out some initial charge/discharge measurements and measure the cross-over of ions without the need to do any sort of pumping. This is also going to involve the design of some DIY low cost membranes, since Nafion membranes would be extremely expensive. Additionally, since the conditions are so mild (pH 5-6), we can use some modified PVA or cellulose cation transport membranes that can be produced for very low cost.