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Deeper research on all-Fe flow batteries using water-in-salt electrolytes

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During the last year we have been doing some minor research in the world of Fe flow batteries to investigate the use of MgCl2 and CaCl2 water-in-salt (WiSe) electrolytes for all-Fe flow batteries. We have recently obtained a new grant from NLnet to help us better research these chemistries and get to a bigger scale using our larger scale flow cells. The objective of this blog post is to better describe the specific research we will be initially doing and how this will lead to a better understanding of how these flow batteries work.

Our research will use our small scale kits and will be mainly dedicated to the exploration of chemical variations of the all-Fe WiSe chemistries. With this research we would like to answer the following questions:

  1. Which is the highest CE and more stable WiSe electrolyte, CaCl2 or MgCl2?
  2. What is the effect of different WiSe electrolyte concentrations?
  3. What is the highest FeCl2 concentration that can be achieved in these electrolytes?
  4. Can auxiliary salts like NH4Cl improve the batteries?
  5. What is the highest stable energy density that we can achieve?
  6. What are the effects of common minor additives used in Fe flow batteries for improved stability? (citric acid, ascorbic acid, gallium chloride, indium chloride, etc)

Due to the fact that we don’t have the funding or equipment of a large university, we will focus on the more pragmatic aspects of the above questions, that is, we will focus on running the experiments and collecting the battery cycling data (which we will make available for free in our open source website), not so much on answering more complicated “why” questions that would require much more expensive characterizations.

Given the previous experiments we have done and the literature available on Fe plating using WiSe, we will start our experiments with the tests described in the flow chart above. All tests are going to be executed at a current of 10mA/cm2 and all of them will be using 5mm carbon felt that is pretreated with a heating butane torch to improve wetting. All tests will be using a Daramic 900um microporous separator, which has been graciously donated by Daramic for these experiments. All electrolytes will be freshly prepared and filtered using 0.45um PVDF filters before use.

After this we will have a better understanding of how much WiSe can improve Fe flow batteries and whether or not they can achieve stable cycling with limited hydrogen evolution under our testing conditions.

These tests will also build a framework to use for a larger scale flow battery using the all-Fe electrolyte and will inform future experiments we will run. The data for all of these experiments will also become available to anyone who wants to rerun them or analyze the results more closely. The data will be released in chunks as we advance through the different stages of the process.

As always we would like to express our gratitude to NLnet for sponsoring this project!

Reproducing Zn-Br flow batteries using Sodium Sulfamate

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A recent Chinese Nature paper showed how Sodium sulfamate can be used in Zn-Br batteries to sequester active Br2 into an N-bromosulfamate that is much less aggressive, much more water soluble and even more easily electrochemically reversible than elemental bromine. I also wrote a recent post discussing the potential use of nicotinamide to achieve this (plot twist, it doesn’t work as the nicotinamide Zn complex is not very soluble). In today’s post I want to share with you my attempts at reproducing this chemistry of the Chinese paper using our open source flow battery dev kit.

The paper is very extensive and shares multiple formulations, they share a formulation for normal asymmetric cells as well as formulations to run the batteries using microporous Daramic membranes. Thankfully I have a bunch of 900um Daramic (thanks a lot to Daramic who donated these membranes to us for research). I bought some Sodium sulfamate (NaSA), Zinc bromide (ZnBr2), Potassium bromide (KBr) and potassium acetate (KAc) and proceeded to run some tests.

Tests using 1M ZnBr2, 1M KBr, 2M KAc, 1M NaSA. Charge and discharge were both done at 30mA/cm2.

My tests using the formulations that they disclose exclusively for daramic were not very successful. Formulations using only ZnBr2, KAc and NaSA suffer from either lower capacities because of lower conductivity or issues with hydrogen evolution. This was specially the case when I tried the ZnBr2 1M, KAc 1.5M, NaSA1.5M formulation, which they suggest in the supporting information to reach >50Ah/L. However I think this is a typo and they meant 2M ZnBr2. If you read that paragraph in the supporting information closely you’ll realize why this is the case (they previously refer to a ZnBr2 1M solution and then say this is basically 2x that, but still write it as ZnBr2 1M).

I then proceeded to test using some of the electrolytes they suggest for asymmetric cells, which were much more successful. In particular the 1M ZnBr2, 2M KAc, 1M KBr and 1M NaSA was great, with high CE values and decent EE values (see graph above). I didn’t experience dendrites before reaching the Nernst limit of the cells when using the 900um thick Daramic, which suggests plating is not as aggressively dendritic as with other electrolytes. However dendrites are quite evident when using 300um Daramic, suggesting you need around 300um of Daramic for every 30mAh/cm2. This might explain why the paper restricts most plating to below 90mAh/cm2 when using the 900um Daramic. I have yet to reproduce the Chinese group capacity or cycling stability values, but I believe I have validated the electrochemical principles well.

Clear evidence of dendrites at 78-79mAh (using 8mL of total electrolyte) using a 2M ZnBr2, 1.5M KAc, 1.5M NaSA electrolyte.

It is also worth noting that the Chinese group does some fancy functionalization of their felt with both nitrogen containing groups and carbon nanotubes, which aggressively boosts the conductivity and energy efficiency of the felt for the Br reactions. This is an important different that might justify why they get energy efficiencies closer to 75-80% while mine are just shy of 60%. I also haven’t optimized the compression ratio of my felt, which means that my felt might be under or over-compressed to extract the max EE in this setup. I also lack an oven to properly do air activation of the felt, so my felt is quite suboptimal and just used as-is.

Furthermore, the paper successfully tested a true flow battery setup using Ti-Br. I cannot easily buy TiOSO4 but I decided to try to innovate and test this chemistry in a fully symmetric setup coupled with 0.5M of Fe-DTPA. While Fe-DTPA isn’t expected to be fully resistant to N-bromosulfamate, I figured it might last enough to provide me with some data. Given that the redox potential of the Fe-DTPA redox couple is quite lower than Fe2+/Fe3+, I figured it should give some appreciable voltage in an Fe-DTPA/N-Br-sulfamate battery. Fe-DTPA is also quite soluble and stable at the near neutral pH that favors the N-Br-sulfamate chemistry, so it should work nicely.

Fe-DTPA 0.5M, NaSA 1M, KBr 1M, KAc 1M. Cycled at a current of 30mA/cm2.

The results above, which have never been published before, show that this chemistry works to some extent. The low CE does suggest that a significant portion of the Fe-DTPA is somehow lost, perhaps to oxidation by atmospheric oxygen (I cannot purge my cells with N2 or Argon at the moment), but also likely from just interactions with N-Bromosulfamate across the microporous membrane. With that said, it does show that the new stabilized bromosulfamate chemistry opens up the window to some very interesting options that just didn’t exist before. Perhaps I can test nicotinamide in this setup, where there is no Zn to cause it to precipitate out of solution.

Finally, I wanted to dedicate the above post to Robert Murray-Smith, a fellow chemist in the UK who passed away recently and was a key inspiration for the start of this blog. I know his passing has been very sad for a lot of us in the DIY community, the curiosity and inspiration he instilled in a lot of us will live on. Thank you Robert!

Revisiting the idea of using chelates for the Fe/Mn flow battery

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On my last post I wrote about the potential of using Fe/Mn in acidic solution to create an Fe/Mn flow battery. I cited a paper published a few years ago which shows that you can achieve reversible Mn3+ chemistry in a solution of sulfuric acid and hydrochloric acid, I then proceeded to confirm this reversibility using cyclic voltammetry of Mn2+ solutions in hydrochloric acid.

However, it quickly became clear from analysis of the paper that this was only at very low capacities. This is because Mn3+ becomes unstable as its concentration increases in solutions, turning into MnO2 and Mn2+.

A 0.5M Fe-DTPA + 0.5M Mn-EDTA solution in an acetate buffer (prepared with 100mL of 8% acetic acid + 10g of potassium acetate)

Given the very low volumetric densities that can be achieved with the acid setup, there’s no option but to revisit the use of more stable and reversible forms of manganese. The best candidate seems to be Mn-EDTA. This complex has already been shown to work in flow batteries at the 0.5M-1.0M range (see here).

I had already thought about using this complex and wrote several posts about its potential use in combination with Fe-EDTA or Fe-EDDHA (see here). However, there is a big problem with the pH compatibility of the Mn-EDTA with the Fe-EDTA or Fe-EDDHA. The issue being that Mn3+-EDTA is only stable under acidic pH conditions, where the solubility of both Fe-EDTA and Fe-EDDHA is limited to around 0.1M. These chelates are only highly soluble under basic pH conditions, which are fully incompatible with Mn-EDTA.

CV of the solution shown in the first image. The half-wave potentials for both reactions are -0.11V and 0.61V, both Vs Ag/AgCl. The above CV was done with a scan rate of 10mV/s.

The question is whether there is any easily accessible Fe chelate that is both compatible with Mn-EDTA in solution (so that we can create a symmetric electrolyte) and that can create soluble solutions at >0.5M concentrations in a pH ~5-6 buffer. Note that I need both chelates to be dissolved at >0.5M at the same time since I want the electrolyte to be symmetric so that it can work using a microporous membrane.

The answer is Fe-DTPA. This chelate is highly soluble at acidic pH values and – best of all – it is soluble enough to actually be in >0.5M solution in the presence of Mn-EDTA at this high concentration. Above you can see a picture of the Fe-DTPA+Mn-EDTA solution. The solution also contains an acetate buffer, which should ensure pH stability on charge/discharge, which should prevent degradation of the Mn-EDTA.

The second image shows a CV of the Fe-EDTA/Mn-EDTA buffered solution, showing that both the Fe and Mn electrochemical reactions are reversible. The half wave potentials are -0.11V and 0.61V, giving us an expected potential for the flow battery of +720mV. This is close to what I had measured before for Fe-EDTA/Mn-EDTA. This proves that the DTPA does not change the electrochemical characteristics of the system very much. The above test also confirms there acetate buffer is stable to the generated Mn3+-EDTA.

The next step is to build a flow battery using the above solution and see what performance characteristics we can get. With the current solutions this system will be limited to around 8-9Wh/L. However I haven’t tested the solubility limits of the chelates in this buffer.

An Open Source DIY Flow battery

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Over the past year, I’ve collaborated with my colleagues Kirk Smith, Sanli Faez, and Joshua Hauser on developing an open-source flow battery design and kit. Our aim is to make it feasible for most individuals to construct this flow battery with readily available parts that can be either purchased online or fabricated affordably. We’re targeting a price point below 1000 EUR, inclusive of the potentiostat, to ensure accessibility.

The kit encompasses all necessary components for constructing and utilizing a flow battery for research and development purposes. This includes the battery itself, pumps, electronic components for pump operation, potentiostat, tubing, reservoirs, and a jig for orderly arrangement. Presently, similar setups cost upwards of 9000 EUR, hence our aspiration for significant cost reduction.

A polypropylene FDM printed prototype being tested with Mn/Fe chemistry. This particular test was done without reservoirs, on close circuit circulation to easily detect any leaking.
Image of one of our latest prototypes. This features polypropylene FDM printed reservoirs, a resin printed cell body and a PLA FDM printed jig.

Throughout this endeavor, we’ve explored various fabrication methods for our designs, employing FDM and resin 3D printing techniques alongside traditional CNC fabrication. While all three methods are viable, our experiments indicate that the most optimal results are achieved through traditional milling.

Charge/discharge cycle using photopaper separators and the Zn-I chemistry using the open source flow battery design.

Validation of our design involved utilizing a low-cost photopaper separator and Zn-I chemistry. We’ve achieved successful charge/discharge cycles at capacities ranging from 20-40 Wh/L. However, long-term cycling validation remains ongoing, as we’ve only been testing the final design for approximately a month.

Our design will be presented at the Flow4UBattery Event in Eindhoven, Netherlands, on April 8-9, 2024. You can register here for free, which also includes complimentary lunch (so please make sure you intend to attend if you subscribe). Day 2 of the event will feature a workshop where participants can assemble a flow battery themselves using the design from our kit. Additionally, we’ll be giving away 5 complete kits during the event, each including mystat potentiostats. We’ll also have a fully assembled kit doing cycling so that you can see the fully assembled kit in action!

After this event, we will look into selling these kits online, with all proceeds going towards the development of higher capacity kits with the objective of reaching an open source flow battery stack within the next 2 years. We will also be publishing the full designs and bill of materials online, so that anyone can create their own too!

A Zinc-Iodide flow battery using a matte photo paper membrane

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On my last post, I showed the results of charging/discharging a flow battery using a ZnCl2+NH4Cl+KI electrolyte using 4 layers of Daramic as a membrane. However, while Daramic is a low cost material, it is not easily accessible for DIY testing at this moment. For this reason, I wanted to run some tests on materials that are easier to source than Daramic.

I looked for materials around my house that had similar porosity (0.1-5um). I tested several different papers that I had around but none of them worked very well. The porosity of most traditional printer papers is high, with most having 10-20um pore sizes. This means that you need many layers to prevent fast self-discharge from migration of the triiodide across the membrane. Additionally, the papers lost structural integrity quite easily.

Test using a 2m ZnCl2 + 2m NH4Cl + 4m KI electrolyte at a current density of 20mA/cm2. Four layers of matte photo paper were used as separator.

Finally, I stumbled upon matte photo paper as a potential solution. This paper has much lower porosity with <5um pore sizes. Some of these papers might even have pore sizes that are below 1um. This is important for printing photographs, as low pore sizes implies that there is less bleeding of ink when it is applied to the paper, although ink needs to be applied much more slowly to the material (reason why printing with these papers is really slow).

For my initial test, I used 4 layers of matte photo paper. The paper does have a substantially higher ohmic resistance compared to Daramic, so I had to lower the current density to 20mA/cm2. I did 4 cycles of charge/discharge that you can see above (I only did 4 because the lower current meant cycling was quite slow). The CE of 87.54% and EE of 75.72% with a capacity of 33.8 Ah/L shows that photo paper is definitely a good choice for at least the short term cycling of these devices.

On inspection, the photo paper did not show any evident degradation although dendrite penetration happened just as much as it did with the Daramic separator. The separator was also completely black, fully permeated by the catholyte solution which contains triiodide in solution when charged.