Tag Archives: TBABr

Zinc Bromine Batteries: Can we just put solid TBABr in there?

I have mentioned how the usability of TBABr in Zn-Br batteries is limited due to the poor solubility of TBABr in the presence of large concentrations of zinc bromide. In my experiments the most concentrated solution I was able to get was around 0.1M ZnBr2 + 0.1M TBABr. This is problematic since we aren’t going to achieve high specific energy or power values with an electrolyte that is this dilute in terms of Zn concentration. However, what if we put a suspension into the cell as the electrolyte?

Image of the TBABr + ZnBr2 suspension prepared

When the cell is charging, the concentration of TBABr in the electrolyte will go down as TBABr3 precipitates out of solution. However, if there is extra TBABr within the cell, that solid will dissolve to replace the TBABr that precipitated. When the cell is discharged, the process will reverse, TBABr3 will redissolve and some TBABr will precipitate again as it is pushed out of solution by the perbromide that needs to go back into solution. The conductvity of the solution should be less affected, because it will only be reduced as a function of the loss of ZnBr2, without an actual loss of TBABr. The problem of course, is that there will be some solid TBABr in the cell, which is likely to increase the series resistance of the cell (because the solid salt is not a conductor).

How do we achieve this? To do this I first put 0.720g of ZnBr2 into a 10mL volumetric flask, then dissolved that into 1mL of distilled water. I then added as much 1M TBABr solution as needed to fill the volumetric flask to 10mL. The total concentration of ZnBr2 is around 0.33M but a lot of “solid” precipitates out of solution, forming a high viscosity phase with the consistency of honey that is made almost entirely out of TBABr. If we agitate the flask, this phase gets suspended into solution quite easily, forming a cloudy suspension (see image above).

Evolution of CE and EE as a function of the cycle number. The cell was charged to 500 uAh and discharged to 0.5V, both at 1mA.

I then built a battery within my graphite electrode Swagelok cell using a zinc anode, 8 layers of fiber glass separator and a carbon felt cathode. I then added 100uL of the above prepared suspension right after agitating the flask vigorously, allowing the material to wick through the cell for a minute before closing the Swagelok cell.

I have since started doing charge/discharge cycles of this cell with very interesting results (see above). The cell initially had relatively low coulombic efficiency (CE) and energy efficiency (EE) values, but these started improving as the cell was cycled. My hypothesis is that – per my previous explanation – the solid is first randomly distributed within the cell but gets organized and deposited within the cathode as the number of charge/discharge cycles increases. I believe this greatly improves the formation of the TBABr3 within the cathode and prevents the solubilization of the perbromide, which reduces self-discharge and therefore increases the cell’s efficiency.

All charge/discharge curves for the cell up until now.

I believe we can see some experimental evidence for this hypothesis as we see a “shoulder” emerge at the start of the charge phase as the number of cycles increases. I think this is consistent with a significant amount of TBABr deposited close to the cathode interface after discharge, which creates a higher resistance to current flow that subsides as the TBABr3 starts forming and this TBABr dissolves back into solution. This is of course an interpretation based on very limited information and I would be thrilled to know what any of you think about the evolution of the charge/discharge curves and what you believe they are telling us. I will continue cycling this cell during the next 2-3 days, to see how the cell stabilizes and whether the CE and EE start going down after.

With that said, it seems pretty clear that TBABr by itself is not going to be an adequate sequestering agent. I will be trying to use PEG200 to increase its solubility – as discussed in some of my previous posts – but I also already ordered TMPhABr (trimethylphenylammonium bromide) as I believe this will be a way better sequestering agent for these devices.

Zinc Bromine Batteries: How can we increase the solubility of TBABr?

As I mentioned in a previous post, the most important issue with the use of tetrabutylammoniumbromide (TBABr) in static Zn-Br batteries, is that the solubility of TBABr drops very sharply when zinc bromide is also in solution. While you can prepare 50% w/w solutions of TBABr in distilled water, the max concentration drops to around 0.15M when preparing solutions in the presence of 0.5M of zinc bromide. This is very bad because – in order to function as an effective sequestering agent – we would want the concentration of TBABr to be able to be significantly higher in solution.

Tetra-n-butylammonium bromide - Wikipedia
Graphic representation of the TBABr salt. You can see that the TAB+ cation has a strong aliphatic component

The solubility of TBABr drops because of a sharp increase in the polarity of the solution due to the introduction of the Zn+2 ions, which are small and – due to their double charge – substantially increase the dielectric constant of the medium. The TBA+ cation is actually not that polar, being spherical and with a strong aliphatic component, meaning it cannot very successfully interact with this new, much more polar medium. As a consequence the TBABr drops out of solution.

In order to prevent this from happening, we need to find solutions that either make the Zn cation less polar or make the media less polar by introducing a less polar additive that can compensate for the increase in polarity brought by the Zn cation. These two potential solutions however, need to avoid the TBABr3 becoming soluble as the perbromide needs to remain insoluble for the battery to work as designed (create an insoluble perbromide to prevent self-discharge).

To make the solvent less polar by adding something else, we need to consider our potential choices and their polarity. We could add another solvent that doesn’t react with perbromide, like an alcohol, but we would need to be very careful with the amount to ensure that it does not make the perbromide soluble (since we know TBABr3 is slightly soluble in alcohols (see here)). We could also decrease the polarity by adding a polymer – like PEG 200 – which also has the benefit of decreasing the formation of dendrites in the Zinc anode. Both of these solutions are potential avenues for experimentation.

Zn(II)-EDTA | Dojindo
The EDTA complex formed between EDTA and Zinc ions

To decrease the polarity by masking the Zinc ion we can use a chelating agent that can react with the Zinc in order to reduce its effect on the dielectric constant of the medium. We could do this by replacing ZnBr2 with ZnEDTANa2 which replace bromides by the Zn(EDTA)-2 complex and requires the addition of two sodium ions, which are bound to be significantly less polar than the Zn+2 cation. However this would imply we would have less bromide available, so it might require the addition of NaBr to recover the equivalent moles of bromide we have lost. Alternatively we can also just add NaH2EDTA2 but we would require to make pH adjustments to the electrolyte, which is not something we would like to do. Additionally, the ZnEDTANa2 reagent is cheap and easily available – as it’s used as a fertilizer in agriculture – and the NaBr is also really low cost. This solution decreases the specific energy/power of the battery though, as the weight is increased by the use of additional reagents.

So there you have it, three potential experiments to try to make TBABr a viable sequestering agent for high energy/power density Zn-Br static batteries. Will they work? I plan to test them out one by one!

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!