Category Archives: Battery research

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

Leave a reply

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

4 Replies

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

8 Replies

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!

Zinc Bromine Batteries: About my Swagelok cell for small scale battery testing

11 Replies

In order to properly research batteries at a small scale, it is necessary to have setups that have a reproducible geometry and that can be put together and taken apart many times, in the exact same way. Through all the research published in this blog, I have been using Swagelok cells to achieve this goal. In today’s post I will be talking about my Swagelok cell and how I use it to put together my batteries.

The Swagelok cell model I have used for my tests.

My Swagelok cell – showed above – is just a piece of PTFE pipe with an internal diameter of 0.5 inches, that has been threaded externally in order to enable the screwing of two stainless steel caps, which are the contact points that allow the connection between the internal electrodes and the external testing equipment.

Besides this external stainless steel caps, the Swagelok cell comes with some internal stainless steel electrodes, that you can use to contact the cathode and anode materials of the battery, if the chemistry allows for this. However, given the corrosive nature of elemental bromine, I use 0.5 inch graphite rods that serve as carbon electrodes. The rods have been rounded on one side – to make a nice contact with the stainless steel caps – and they are flat on the other side to fully contact the cathode and anode electrodes.

Vertical cross-section of the Swagelok cell once assembled with a separator-less inverted configuration.

The image above shows you a diagram of a vertical cross section of the Swagelok cell when it is fully assembled with a separator-less inverted Zn-Br battery configuration. As you can see, the bottom graphite electrode is wrapped heavily in PTFE tape, in order to provide a water-tight seal, while the top electrode is only wrapped once, in order to insulate everything but the tip of the electrode from making contact with solution, allowing for an air gap to allow any excess solution to pool after compression.

As you can see, the battery comprises a small space in the middle of the Swagelok cell. The space for the electrolyte is provided by the use of PTFE o-ring spacers and when the cell is closed the space has a perfectly defined volume, since the rings do not compress. When a separator containing setup is used, these o-ring spacers are replaced by layers of non-woven fiberglass separator, which is the other material I have tried. In an inverted configuration the anode is placed at the bottom, while in a normal configuration the anode is placed on top.

Given how the battery is configured, it is easy to take it apart and put together a new battery with brand new materials to either repeat and experiment or perform an entirely new experiment with the exact same geometry. Geometry and mass are very important aspects of battery research – as they determine specific power and energy – so being able to do experiments where these two variables can be guaranteed to be as reproducible as possible is conducive to better results.

Zinc Bromine Batteries: What happens if you don’t use a sequestering agent?

Leave a reply

Almost all of my efforts in the construction of Zn-Br batteries have focused on the used of sequestering agents in order to enhance the performance of the batteries and obtain higher energy densities and lower self-discharge rates. However, a couple of people have asked me what the results of a “minimal architecture” Zinc-Bromine battery would be with my current battery design. I therefore put together a battery using a 0.2mm Zn anode, 15 layers of fiberglass separator, a GFE-1 cathode without any pretreatment and a 2.7M ZnBr2 electrolyte with 1% Tween20 (which is needed to prevent dendrite formation).

The results, shown below, illustrate the problem of trying to create a Zn-Br battery without a sequestering agent. The lack of a sequestering agent means that the formed bromine is easily able to exit the cathode material and go into the separator, migrating towards the anode. The cell’s behavior is similar to my batteries with sequestering agents, as during the first few cycles the cathode losses a lot of bromine to the media due to the lack of any oxidized bromine species in the separator and therefore starts at a lower Coulombic and Energy efficiency.

Resulting first 15 charge/discharge curves for a Zn-Br battery containing no sequestering agents, charged to 15mAh and discharged to 0.5V, both at a rate of 15mA.

However, as the battery progresses, the Zn-Br battery assembled without a sequestering agent stabilizes at a CE of around 60% and an EE of around 45%, while with a sequestering agent, these values go up all the way to 90% and 65%. It is therefore evident that the sequestering agent does a good job of keeping the formed bromine in the cathode material, while the lack of a sequestering agent makes the battery significantly less efficient.

The energy density also changes quite dramatically, with the sequestering agent battery reaching around 25Wh/L and this design without one reaching only close to 17Wh/L. The use of a sequestering agent improves almost all aspects of the battery, except perhaps the stability of the battery which is lower if the sequestering agent reacts in any way with the electrodes or the bromine as a function of time.

Evolution of charge and discharge potentials for a battery with no sequestering agent.

I am going to continue cycling this battery in order to see if it reaches the same stability limits as my other devices or if it is able to run significantly longer. Batteries containing sequestering agents and charged to 15mAh have shown to deteriorate substantially at around 60-70 cycles, particularly batteries using TMPhABr as the sequestering agent. If these batteries without the sequestering agent are more stable then the stability issues of my designs using a sequestering agent could be assigned to chemical instabilities of this agent within the electrochemical environment.