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?

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).

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.

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.

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.

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.

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!