Tag Archives: TBABr

Zinc Bromine Batteries: Why propylene carbonate will not work as a cathode electrolyte

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I have written several blog posts in the past about the potential use of propylene carbonate (PC) as a potential non-aqueous solvent in Zn-Br batteries. However, through my research I have now discovered that this solvent will not work in these devices as a cathode electrolyte, due to the way it interacts with the chemicals that are generated within the cell. In this post I will explain to you the experiments I did and why I reached this conclusion.

The idea with using PC was initially to completely replace the electrolyte within the cell. This was discarded right away due to ZnBr2 solubility issues and low conductivity issues of the constructed cells. The idea then evolved to using a tetrabutylammonium bromide (TBABr) saturated PC solution (PC-TBABr) as an organic layer in an inverted device, since this layer can rest on top of a Zn-Br solution and can even remain on top after mixing if the Zn-Br solution is concentrated enough (>4M).

Small cell I used to visually study what happened with a PC-TBABr containing device. The cell was not characterized as the geometry is not reproducible and the surface area is too big for my testing equipment.

The organic layer is completely immiscible with the highly concentrated ZnBr2 layer and it was my hope that the TBABr3 produced in the cathode would be substantially more soluble in the PC-TBABr compared to the aqueous phase. Given that the PC phase is more than 50% TBABr, it seemed very likely that the produced perbromide would have a significantly higher affinity for the PC-TBABr.

To confirm whether this was happening, I constructed an inverted cell using a glass vial in order to be able to see what was going on (which you can see above). I placed a zinc anode at the bottom, used fiberglass as a separator and placed a GFE-1 cathode saturated with PC-TBABr on top using some C4 carbon cloth as a current collector. Before placing the GFE-1 cathode I filled the cell with a 4.2M ZnBr2 electrolyte, which makes the PC-TBABr remain less dense even after prolonged mixing. Since my objective was not to measure efficiencies with this device, but just to observe the chemical processes, I did not strive for a reproducible geometry.

After charging for a while with 3 charged AA batteries at more than 3V – just to make the process go fast – I noticed a lot of TBABr3 forming and precipitating within the cell. Sadly, the perbromide seems to form on the cathode and then immediately migrate out and into the aqueous phase. To my absolute surprise, the TBABr3 – which is more of a liquid rather than a solid – has higher affinity for the aqueous phase, although it doesn’t solubilize within it but rather forms a suspension with it.

I then proceeded to take the electrolyte out of the cell and perform an extraction using some additional PC-TBABr and surprisingly, all the perbromide stays in the aqueous phase after mixing and just refuses to get into the PC-TBABr. For this reason, PC is not going to work as a cathode electrolyte within the device, as the perbromide just exits it and never returns. This is probably why my devices trying to do this never seemed to work for too long.

Zinc Bromine Batteries: First successful static cell using a non-aqueous solvent for Br sequestration

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During my latest experiments, I have moved to an inverted geometry setup, given that hydrogen evolution is a problem that needs to be eliminated for cells that are expected to last for long periods of time. However, an inverted geometry carries with it the problem of more favorable diffusion of elemental bromine – due to the fact that the cathode is now on top – reason why aggressively sequestering bromine is now a top priority.

In order to do this I have been trying to use a TBABr saturated propylene carbonate (PC) solution (which I will be calling TBABr-PC). My expectation was that by soaking the cathode in this solution I would be able to prevent any elemental bromine formed from escaping. The TBABr-PC behaves a lot like an ionic liquid (it’s > 50% TBABr) so its high conductivity and much higher affinity for elemental Br should allow the battery to work properly while keeping all the Br2 from reaching the anode or the aqueous electrolyte.

After mixing an aqueous Zinc Bromide solution with a TBABr saturated PC solution, two new phases form, with the organic phase now at the bottom.

The problem with these initial tests was that the battery seemed to suffer from initially low conductivity and charge retention with substantial changes through time that appeared to improve on these parameters. My guess was that there was a lot of ion migration between the initial TBABr-PC and the ZnBr2 aqueous electrolyte and that the battery was just not stable while these were happening.

To solve this issue I prepared 10mL of 1.5M ZnBr2, 1% Tween 20 solution and mixed them with 2mL of TBABr-PC. The TBABr-PC was initially above the aqueous electrolyte, as expected from its lower density. After adding them together I then proceeded to mix them vigorously, which lead to the separation of two new phases. The PC phase now became denser, with the aqueous phase resting on top. This shows that there was some transfer of ZnBr2 into the PC solution, although thankfully the phases do remain immiscible.

First cycle of a cell using a GFE-1 cathode saturated with the bottom phase resulting from mixing a saturated TBABr-PC solution with a 1.5M ZnBr2+1% Tween 20 solution.

I then proceeded to fill an inverted cell with the top solution, saturated a GFE-1 cathode with the bottom solution and placed the saturated GFE-1 cathode on top before compressing the Swagelok cell. The cell had no separator but 4 PTFE o-rings as spacers. Since the PC had proved to have low conductivity before, I decided to cycle this device at 5mA to 15mAh. You can see the result of the first cycle above.

Although the CE and EE are now significantly better than before, there are still big questions about how a cell like this will evolve over time and whether the TBABr-PC is as effective at sequestering elemental bromine as I believe it might be. The fact that the organic phase is now denser also begs the question of whether the organic phase will just pool at the lower half of the cell with time. Hopefully affinity for the GFE-1 cathode is high enough. A potential solution to this problem is to try this experiment again with a 3M ZnBr2 solution, which is going to have significantly higher density.

I will first cycle this cell for some time to gauge its stability before running a self-discharge experiment to test whether the TBABr-PC does significantly impair self-discharge of the device.

Zinc Bromine Batteries: First results ever using Propylene Carbonate

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Earlier this month, I wrote an article about the use of non-aqueous solvents in Zn-Br batteries. The only published result I could find was an article dealing with Zn-Br flow batteries using propionitrile as the catholyte solvent but I wanted to avoid the use of propionitrile due to its toxicity and scarcity (hard to find/buy for an individual in the US). However I thought propylene carbonate (PC) could be a suitable replacement, so I bought some to test whether this was true or not.

The first experiments I carried out were to figure out whether PC could be used as the sole solvent within the battery. Sadly the solubility of ZnBr2 is not high enough – at most in the 0.5-1M range at 20C – and the conductivity of these ZnBr2 solutions was also not high enough, with very noisy charge/discharge curves with very high charge voltages that retained almost no charge at all.

The solubility of both TMPhABr and TBABr in PC is better, although TBABr is by far the most soluble. With TBABr I was able to achieve saturated solutions with almost 50% of TBABr, giving them a very decent amount of conductivity. Sadly this wasn’t enough to make PC usable as a single electrolyte though, as the bad behavior of the charge/discharge was also apparent when using this as the sole solvent.

Charge/discharge curves for a cell built with a 1% Tween20 + 1% PEG 200 + 1.5M ZnBr2 electrolyte with a GFE-1 cathode fully saturated with a 50% TBABr in PC solution.
Evolution of CE and EE values for the curves shown before.

The idea then came to use this concentrated PC TBABr solution to saturate the GFE-1 cathode and use this in an inverted cell. It is interesting that although PC is infinitely miscible with water, a 50% solution of PC TBABr is actually not miscible with a 1.5M ZnBr2 solution in water. This is because the affinity of TBABr for PC is much higher than that of ZnBr2 and the affinity of ZnBr2 for water is also significantly higher as well.

This experiment was better behaved with actually measurable charge/discharge curves. I did 4 curves charging/discharging to 15mAh at 15mA – the results are shown above – with the best CE and EE values being 61% and 25% respectively. The charging voltages do show that the internal resistance is significantly higher than when using water so there is likely a lot more of hydrogen evolution at the anode. The generation of elemental bromine at the cathode is also probably significantly slower, given the much higher viscosity and lower conductivity of the PC electrolyte.

Given the higher charge density used, I thought It might be the case that the PC electrolyte is just not able to support as high of a current density as the normal aqueous electrolyte and therefore a much lower charge density needs to be used to use this successfully. I am going to be evaluating this hypothesis within my next few tests.

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

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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?

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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!