Tag Archives: zinc bromine

Reproducing Zn-Br flow batteries using Sodium Sulfamate

1 Reply

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!

Could we create a Zn-Br flow battery using Nicotinamide?

Leave a reply

Zinc bromide flow batteries have been researched very extensively during the past 30 years. There are many advantages to this chemistry, very high potential (~1.8V), high efficiencies, symmetric electrolyte and low reagent costs. Nonetheless, the disadvantages are also huge: zinc dendrites, hydrogen evolution, bromine corrosion, etc. Despite all the development, a lot of these disadvantages remain insurmountable.

A recent nature paper has disrupted the field by using sulfamate ions as a bromine scavenger. Unlike previously used complexing agents that sequestered Bromine as reactive Br3- species, the new scavenging method sequesters Bromine as an N-bromosulfamate, which is stable in solution in the timescales necessary for energy storage. Furthermore, the N-bromosulfamate is chemically much milder than elemental Br2 or Br3-, making the use of cheaper gasketing materials possible and preventing a lot of issues associated with the high reactivity of elemental bromine species.

A model of the nicotinamide molecule. The Br reacts primarily with the amide group (NH2) under mildly acidic conditions.

I have been very excited by these findings and have ordered some sodium sulfamate to test this chemistry myself in our FBRC development kit. However, the development of this technology is likely not open source and it is very likely that the people involved with it want to patent it and lock down the technology. This made me think about potential alternatives that could be used outside of the sulfamate family that could also exploit the mechanism of Br storage in N-Br bonds. Such a technology might be outside the scope of their original paper and therefore be exempt from intellectual property registration.

Thinking about the stability of N-Br compounds (usually not stable at all), I immediately think about NBS and analogous chemical compounds. These are very stable reagents that are routinely used in chemical synthesis, although their aqueous solubility is very low and therefore not useful in the creation of a ZnBr2 aqueous battery.

With that said, nicotinamide (vitamin B3) is a very water soluble and readily available material that also forms a stable N-Br compound in mildly acidic conditions. This 2007 paper describes how N-bromonicotinamide can be created using elemental bromine. While N-bromine compounds from amines like this would often go through a Hoffman rearrangement to yield the corresponding amine, this doesn’t happen under mildly acidic condition in the case of nicotinamide. In fact, the 2007 paper mentions that a concentrated solution of this N-Br compound was stored for months without degradation. The solubility of nicotinamide is also very high (5-6M), compared to sodium sulfamate’s solubility limit of 1.3M at 25C.

An example of a nicotinamide-Zn complex. Check this paper to learn more.

Furthermore, nicotinamide forms mono and dimeric complexes with Zn atoms through the nitrogen in their pyridine ring, which makes this nitrogen unavailable for potential deactivation with direct bromination of this N to yield the corresponding quaternary nitrogen salt (irreversible and undesirable in the context of a battery).

Given that vitamin B3 is very soluble, very low cost, already produced industrially, has a stable amide N-Br compound and is unlikely to undergo Hoffman rearrangement or similar decomposition modes, it is a great candidate to serve as a Br2 scavenger in a ZnBr2 battery. I am going to buy some vitamin B3 to test this idea out. Stay tuned for some tests of this and the sodium sulfamate chemistry.

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

12 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.

Zinc Bromine Batteries: Proposing the next scaling level, a petri dish battery

Leave a reply

My experiments with Zn-Br batteries in Swagelok cells have now matured to the point where a relatively stable battery has been achieved with no large dendrite problems and acceptable Coulombic and Energy efficiency values (~85% and ~65% respectively). I believe these results would already allow some of you – who want to explore larger scales – to construct some prototypes with my exact configuration, but at a larger scale. In this post I talk about a larger scale setup constructed using a petri dish.

The building material is a difficult choice, since Zn-Br batteries form pretty corrosive chemical species, in particular elemental Bromine and perbromide salts. In order to ensure stability, the cheapest and most easily available material that resists Bromine is glass. The cheapest casing to build a relatively small scale, yet modular and practical battery that fits the geometry of my design would be a petri dish, you can get a 100mm diameter petri dish here, which you could use to put together the battery I’ll be describing.

Suggested geometry for the petri dish cell.

Knowing the area and height of the cell (100mm x 18mm) we can calculate the energy density and charge capacity you should get. Given my previous research, this should be around 2Ah of stored charge with an energy stored of around 2.85Wh. To obtain this ratios you would charge the cell at a constant current of 1A to 2Ah, which matches a current density of around 11.8mA/cm2 in my Swagelok cells, charging to 15mAh at 15mA.

For the anode, you will want to get 0.2mm Zn metal, which you can obtain here. For the cathode you will want to get GFE-1 graphitic felt, which you can buy here. Note that this is NOT a regular carbon felt, it is a highly activated and conductive felt. For the separator a non-woven Fiber glass tissue will do the trick, like this one. The GFE-1 cathode will need to be soaked in a 10% solution of Trimethylphenylammonium bromide (TMPhABr, CAS 16056-11-4) and then air dried. I bought my TMPhABr from alibaba. Using Tetrabutylammonium bromide (TBABr) gives significantly worse results because of the much higher molar mass and lower solubility of the salt in Zinc Bromide solutions. As a current collector for the cathode and to connect it to the outside I suggest you use graphite foil, like this one. For the Zn anode make sure you leave a strip of anode when you cut the piece so that you can make the connection to the outside, cover all the metal that will go through the cell in some non-reactive insultation, electrical tape should work well enough for your first tests so that it doesn’t short with the cathode material.

Geometry of suggested components for petri dish battery

For the electrolyte you can either use Zinc Bromide at 2.7M or a solution of this concentration prepared using Sodium Bromide and Zinc Sulfate Monohydrate. I suggest you browse some of my previous posts on solution preparation if you’re interested in using these sometimes cheaper and more widely available chemicals. However, in both cases you will need to prepare the solution with 3% hydrogen peroxide instead of distilled water, to ensure all Fe is precipitated and removed from the solution before putting it into the battery (Fe impurities cause huge problems in the cells). After preparing the electrolyte, leaving it 24 hours to react and then filtering, drop some strips of Zinc foil in it so that all the peroxide reacts, then use peroxide measuring strips before loading it into the cell to ensure all hydrogen peroxide has reacted. After a couple of days all peroxide should be gone. Also, make sure you also add 1% Tween 20 to the solution, this is a key component to improve wetting and prevent dendrite formation.

To put the cell together, place the Zn foil at the bottom of the petri dish, then cut as many layer of fiberglass separator so that once you put the GFE-1 cathode on top and close the lid you’re able to lightly compress the cell. Then the easiest thing is to take out the separators and cathode, soak them in electrolyte and put them back in the cell. Now you should be able to close the lid, lightly compress it, clean any spillover and seal the petri dish using a couple of rounds of electrical tape.

I am also very aware that Bromine is denser and will want to sink into the cell, most of it won’t though as it will be sequestered as perbromide within the carbon electrode. There are very good reasons why this cell has the cathode at the top and why a normal geometry just does not work well for long term cycling at these energy densities. Please read this post to better understand why a practical static Zn-Br battery actually requires an inverted geometry to work well.