Tag Archives: zinc bromide

Zinc Bromine Batteries: Making higher purity zinc bromide from readily available salts

Although Zinc Bromide is a readily available commodity chemical in some parts of the world (readily available in bulk quantities or for businesses), it is difficult to source in places like the US due to a lack of retail applications to justify its sale to the general public. Small amounts can be bought from sources on ebay/amazon, but the cost can often be above 1 USD/gram with similar costs from Alibaba when buying small quantities (<10kg). For people wanting to do small/medium scale experimentation on Zn-Br batteries, it is often impractical to buy in the necessary quantities, so a low cost synthesis of Zinc Bromide from readily available chemicals is desirable.

Crude Zinc Bromide produced through the process described in this article before being put into the desiccator.

The cheapest solution I have found comes from the use of readily available Zinc Sulfate and Sodium Bromide. Both of these salts are available in high purity at very low retail prices (<10 USD/kg even when buying sub kilogram quantities). Since Sodium Sulfate is significantly less soluble than Zinc Bromide, preparing a very concentrated solution containing both salts leads to the precipitation of Sodium Sulfate with Zinc Bromide remaining in solution. The problem with this approach is that the solution always contains a substantial amount of Sodium Sulfate and the many separation/concentration/crystallization steps involved to obtain higher purity ZnBr2 make it a rather impractical approach if higher purity Zinc Bromide is required or if you want to achieve the result quickly.

My solution to this problem is to use a readily available organic solvent – rubbing alcohol – to help speed up the process and achieve better results. Here’s a summary of the synthesis for a small amount:

  1. In a 50mL tall beaker add ~20mL of distilled water and dissolve 5.5g of Zinc Sulfate Monohydrate and 6.1g of Sodium Bromide. Note that an excess of the Zinc Sulfate is used, as it is the least soluble of the reagents.
  2. Stir the mix until everything is fully dissolved. All the formed salts should be soluble enough to dissolve in this amount of water at 25C (77F)
  3. To this solution, add ~20mL of rubbing alcohol.
  4. Stir the mix for a couple of minutes.
  5. Let the solution set until everything separates. Three distinct layers should form (one solid, two liquid). The Sodium Sulfate will precipitate and a bottom aqueous layer will form while a top alcohol layer will remain.
  6. Decant the top alcohol layer and collect it. This layer contains no salts and can be used for future batches.
  7. Transfer the bottom aqueous phase to another container. The solid precipitate is normally well formed enough as to allow for transferring the bottom aqueous layer to another container without the need for filtering. The remaining solid is sodium sulfate plus the excess zinc sulfate.
  8. The aqueous layer transferred contains all the Zinc Bromide. You can heat and evaporate most of the water, but bear in mind that the Zinc Bromide will hold very tightly to it and might start to sizzle and “erupt” aggressively as you heat more and more (it’s similar to when water splashes into hot oil). Since the aqueous layer contains some alcohol make sure you do this in an open or very well ventilated space. It is better to reduce the volume until the point where the ZnBr2 starts to crystallize and then transfer it to a desiccator so that it can finish its drying process. A container with anhydrous Calcium Chloride or anhydrous Magnesium Sulfate can do the job.

The above described process – using rubbing alcohol – has the advantage of producing the Zinc Bromide quickly, without the need to perform successive steps of cooling/decanting/filtering/crystallizing, etc. Since both Sodium Sulfate and Zinc Sulfate are almost completely insoluble in alcohol containing solutions – while Zinc Bromide is not – this leads to a significantly faster and more satisfactory synthesis from readily available chemicals. Using a slight excess of zinc sulfate is recommended to avoid the presence of Sodium Bromide in the final solution.

The only tricky part is taking the Zinc Bromide out of the final solution. I prefer not to heat it till it’s completely dry, since the sizzling and “eruptions” of the Zinc Bromide can be pretty aggressive as it tries to hold dearly to every milligram of water it can manage to. Besides, if you heat it to dryness it will quickly become wet again as it cools unless you immediately put it inside a desiccator. It is therefore preferable to put this in a desiccator as soon as Zinc Bromide crystals start to appear and let the drying agent get all the water out of the Zinc Bromide. Note that a desiccator doesn’t need to be anything fancy, some air-tight tupperware you fill with a good enough drying agent can do (the drying agent needs to be more hygroscopic than Zinc Bromide).

I haven’t scaled this process up – as I only work at very small scales – so I don’t know what problems could occur at larger scales. Since it involves alcohol I would advice working at a small scale to see if this process might fit your needs and to be careful and follow all safety precautions.

Zinc Bromine Batteries: First tests using TMPhABr

As I’ve mentioned in previous posts, tetrabutylammonium bromide (TBABr) is not a very good sequestering agent for static Zn-Br batteries due to its very low solubility in Zinc Bromide solutions. To solve this problem, I have decided to test trimethylphenylammonium bromide (TMPhABr) as a potential replacement, since this salt also forms and insoluble perbromide but – due to its significantly higher polarity and lower molecular weight – should be significantly more soluble than TBABr. I ordered it from Alibaba around one week ago and recently got it delivered.

Picture of the TMPhABr I got from China

My initial tests with it involved testing its solubility in Zinc Bromide solutions. The solubility of TMPhABr in pure water is not indicated clearly anywhere, but I assumed its solubility would be similar to that of trimethylbenzylammonium bromide (TMBABr) or tetrapropylammonium (TPABr) bromide, both which have solubilities of around 10% by mass in water at 25C. My initial tests have confirmed this suspicion with solutions at 10% by mass being easy to prepare at 20-25C. I didn’t try to prepare more concentrated pure solutions as my objective is to judge its solubility in the presence of Zinc Bromide.

The first test I performed to evaluate this was a 0.25M solution of Zinc Bromide which was able to dissolve 0.12M of TMPhABr with no problems. I then increased the amount of ZnBr2 to 0.5M – which is what the authors of the Chinese paper using ZnBr2+TPABr use – and I was able to dissolve 0.25M of TMPhABr without issues. With this result I know I will be able to at least reproduce similar experimental conditions to those achieved by the Chinese researchers, something that I could never do with TBABr due to the solubility issues mentioned before.

To test how far I could take this I then attempted to prepare a 1M solution of Zinc Bromide and see if I could get 1M of TMPhAbr to go with it. Sadly at this point the concentration of TMPhABr is already too high – would be close to 10% by weight of the solution – so it was actually not possible to get to this point. This means that the practical limit of this battery will be to have around 0.25M of TMPhABr dissolved, which is probably a realistic limit for most quaternary ammonium salts since we are unlikely to get an effective sequestering agent – not electrochemically active and with no effect on pH – with a molar mass significantly lower than that of TMPhABr at a similar price point.

First two charge/discharge curves measured (at 2mA constant current). Battery was charged to 500 uAh and then discharged to 0.5V. First curve, CE=68%, EE=57%. Second curve, CE=79%, EE = 66%.

I then used this 0.5M ZnBr2 + 0.25M TMPhABr solution to create the first battery. This battery had a diameter of 0.5 inches and was built within my Swagelok cell. I used a 0.2mm thick Zinc anode followed by 8 layers of fiberglass separator and a CC4 carbon electrode. I also made sure to sand the graphite electrodes in the Swagelok cell to make sure their exposed surface was pristine. I put 50uL of the electrolyte on the cell but I won’t know how much ended up in the separator until I open the cell after testing and weight the wet components.

The graph above shows the first – to the best of my knowledge, the first ever public – charge/discharge curves of a static Zn-Br cell prepared using TMPhABr as a sequestering agent. It is very interesting to note that the shape of the discharge curve improved immensely moving from TBABr, showing that this battery is significantly better behaved. Although the CE and EE of this first curve were particularly low, the CE of the second curve measured already showed an increase of the CE to 79% and EE 66%. I will keep cycling the battery and will show you how the CE and EE change as a function of the number of cycles. Exciting times!

Zinc Bromine Batteries: Is TBABr the best complexing agent?

Secondary Zn-Br batteries suffer from a huge problem of self-discharge due to the formation of elemental Bromine which, although largely insoluble in water, is soluble enough to migrate through the cell and react with the zinc anode, effectively self-discharging the cell.

To circumvent this issue, researchers have used chemicals that sequester the produced bromine into a product that has even less affinity for water — an insoluble or immiscible perbromide. In flow batteries this is done to generate a liquid phase that is immiscible with water, since it still needs to be a liquid to allow proper flow of the reagent. In static batteries this is undesirable, because a liquid is still able to flow through the cell and react with the Zn anode.

This is a figure taken from the Chinese paper. You can see that they do test the TBABr for its perbromide’s solubility

The 2020 Chinese paper we’ve discussed previously in this blog goes around this problem by using a sequestering agent that forms an insoluble perbromide, tetrapropylammonium bromide (TPABr). Notably the paper uses TPABr instead of tetrabutylammonium bromide (TBABr) which is almost an order of magnitude cheaper due to its significantly wider array of industrial uses compared to TPABr. Not only that, but the TBABr perbromide is even more insoluble, so the chemistry should be even better, right?

It is worth noting that they are aware of the above facts. You can see this in the image above – taken from the supporting information of the paper – where they clearly show TBABr forms an insoluble perbromide. So why did they choose to go with a significantly more expensive chemical (TPABr) and not use TBABr when its the obvious choice from a practical standpoint?

Precipitation of TBABr from a completely transparent TBABr 1M solution when in contact with a 0.5M Zinc Bromide solution

The problem – which I have lived through experimentally – is that the solubility of TBABr in the presence of ZnBr2 is quite terrible. The TBABr is extremely soluble in water – you can easily prepare a 50% solution by weight in distilled water – but it precipitates back very aggressively when put it into contact with a solution of zinc bromide. The image above shows you what happens when you mix a 1M solution of TBABr with a 0.5M solution of ZnBr2. The authors of the paper probably saw this issue and immediately recognized this as a potential problem for their batteries, my intuition is that they did run and have results for some cells using TBABr, but the results were probably so much worse than those of TPABr, due to this solubility issue, that they simply did not publish them.

The TPABr is most probably a significantly better sequestering agent because it’s likely significantly more soluble than TPABr in Zinc Bromide solutions. This agent is however unlikely to be soluble enough to support very large capacity solutions (>= 2M ZnBr2).

As I mentioned on a previous post, a better sequestering agent must allow for large solubility, be commercially available and form an insoluble perbromide. The only candidate I can think of to fulfill this role would be trimethylphenylammonium bromide (TMPhABr). I might be tempted enough to test it to order some from Alibaba if I can get a low quantity order for a reasonable price!