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My adventures building a Zinc-Bromine battery

Just throwing this out as a 4th way (sorry if it's crazy-talk), wouldn't the Zn+2 cation be attracted to something with a strong negative charge? Would it be possible to create that electrically, like a capacitor does with a barrier to where they're attracted such they move through the electrolyte but can't actually electrically interact (no actual power consumed)? Seems they'd push the TBABr out of the area thereby concentrating them where you wanted them to be and of course, nothing new added into the chemistry for side-reactions.
 
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Just throwing this out as a 4th way (sorry if it's crazy-talk), wouldn't the Zn+2 cation be attracted to something with a strong negative charge? Would it be possible to create that electrically, like a capacitor does with a barrier to where they're attracted such they move through the electrolyte but can't actually electrically interact (no actual power consumed)? Seems they'd push the TBABr out of the area thereby concentrating them where you wanted them to be and of course, nothing new added into the chemistry for side-reactions.

Zinc ions are indeed attracted to negative charges, however in solution things are "neutralized" at a very small scale. A Zinc ion will not just be like an isolated point charge in solution, it's charge will be neutralized by a solvation sphere made by water molecules with their oxygen atoms oriented towards the Zinc, this sphere will grow until the effect of the zinc ion charge is close to zero. Furthermore, there are also solvated negative ions in solution that will further contribute to the net charge of the solution always being zero.

That said, when close to a charged plate, the ions closest will indeed be attracted to it - as a function of how strong the electric field is - and this is why they will get close enough to be reduced. If you put a field that is really high but provides no current, you can indeed create a small amount of ions that get "stuck to it" because of the potential and no power is consumed, but this effects falls very rapidly as you move further into the solution and the ions are "neutralized' enough by the water molecules not to "feel" anything from the plate. You indeed create a super small capacitor - nanometers wide - but the effect is not useful to capture any large number of ions. Furthermore, this capturing is somewhat selective due to the diffusion coefficient of the ions, but you are bound to capture a distribution of everything in solution with the same charge sign.
 
These are some efficiency measurements as a function of the number of cycles for a cell with CC4 that was initially prepared with 100uL of ZnBr 0.5M + TBABr 0.2M electrolyte with 50uL of TBABr 1M added on top of the cathode after assembly. The cell was charged to 500 uAh at 1mA and discharged to 0.5V at 1mA. Second plot shows all the charge/discharge cycles.

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Also, you can get a 1.0M TBABr + 1.0M ZnBr2 solution if you use 50% Isopropyl alcohol / 50% water mixture. Sadly the TBABr3 is soluble in this mixture as well, so the battery strongly self-discharges :( This is evident in the terrible drop in Coulombic and energy efficiencies at lower charging currents. The next test will be using PEG200, which I will do next week (shipping takes a while!).

I am also ordering trimethylphenylammonium bromide (TMPhABr) and tetrapropylammonium bromide (TPABr) so we should have some very interesting experiments coming up within the next couple of weeks :)
 
In the meantime - while I get the chemicals for the next round of experiments - I created a battery using carbon felt as the cathode, where the electrolyte was prepared by dissolving 0.720g of ZnBr2 in 1mL of water, then taking that to 10 mL in a volumetric flask using a 1M TBABr solution. As expected a lot of the TBABr precipitated out but it became suspended whenever the solution was agitated. The 100uL of the electrolyte were put into the cell right after agitating the volumetric flask, just putting the suspension on top of the cathode and letting it wick through. I want to test the long cycling properties of this device. I am running these cycles charging to 500 uAh, discharging to 0.5V.

So far these are the results:

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As you can see both the CE and the EE of the cell have been increasing as a function of the cycle number. My hypothesis is that the precipitated TBABr is initially randomly distributed within the battery but is then reorganized within the cell as the cell precipitates and dissolves TBABr3 as it goes through charge/discharge cycles. When this happens some TBABr can go into solution and then when the cell discharges this TBABr precipitates selectively at the cathode, where the concentration of TBABr increases locally when the TBABr3 is dissolved.

This process seems to slowly increase the cell's internal resistance - probably because it's forming a layer of solid TBABr at the cathode after discharge - but the overall CE and EE of the cell increase as the formation of TBABr3 from TBABr becomes more efficient. I want to see when this process reaches a steady-state and how stable the CE and EE of the final cell are. It will be at least 4-5 days till I get new materials so I will cycle this cell continuously till then :)
 
So the discharge curves reaching towards 500 are higher cycle numbers? How temperature sensitive is the precipate?

Why not add some sodium like you were talking about in your blog to see what happens?
 
So the discharge curves reaching towards 500 are higher cycle numbers? How temperature sensitive is the precipate?

Yes, these ones are the higher cycle numbers. I have no idea how sensitive to temperature the precipitate is (TPABr3). From a review of the literature it seems stable - melts at 71C without decomposing - but I don't know how its solubility in water changes as a function of temperature (could be an important factor). The battery tests have been done inside my house, which is kept at 69-75F but I have done no efforts to actively measure or control temperatures.

Why not add some sodium like you were talking about in your blog to see what happens?

Since I need to buy all the reagents and perform all the experiments (and my time is sadly limited!) I limit myself to the experiments I consider have the highest chances of success, for this reason I have decided to pursue the PEG200 addition and the TMPhABr experiments next and leave aside any experiments dealing with ZnEDTA2Na2 or NaBr. This is because these experiments require me to purchase new reagents and complicate the chemistry more than I would like, given the limited characterization tools at my disposal. I might get to them one day though!
 
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These are all the charge/discharge plots for the last described battery up until now with the CE and EE information, in case you guys want a better look into the evolution of the charge/discharge curves as a function of time.

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In my research of Zinc-Bromine battery literature I also discovered that Robert Murray Smith filed an application for a patent for an improved electrolyte for a Zinc-Bromine battery where the sequestering agent is made of - the most British thing ever - black tea (https://worldwide.espacenet.com/pub...T=D&ND=3&date=20191211&DB=EPODOC&locale=en_EP). The theory is that the polyphenols in black tea will serve as sequestering agents. The patent does contain some summarized experimental data, although no charge/discharge data or stability data is given. Without detailed data, it is hard not to be skeptical of these results or whether these batteries have any meaningful cycle life.

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I strongly doubt this works to build a stable battery (as described) because polyphenols would not be present in either high enough concentrations or offer reversible enough chemistry to be useful for this purpose. Plus, there are a lot of compounds in black tea that will react irreversibly with elemental bromine.

This is a patent though, so the application might be misleading on purpose and they might be using a specific pure polyphenol - which might be present in black tea and grape juice - as a sequestering agent (meaning he would just be using this odd black tea description to protect the IP). Definitely makes me curious about this chemistry.
 
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So, the formed solid finally became too much at the cathode and caused the CE and EE to start dropping heavily:

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You can see the charge cycle became extremely noisy (last one measured);

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It seems this method of battery building is definitely not viable. Next tests should be the PEG200 and then the TMPhABr after that :)
 
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After opening the cell I believe the above behavior wasn't caused by TBABr solid formation in the cathode but by zinc dendrites making their way all across the separator and finally shorting the cell during the charging cycle. I could see the zinc dendrites present across all the layers of separator, all the way back to the Zn anode. It seems the TBABr is not enough to avoid this problem, another reason to test the PEG200 addition.
 
I've received my PEG-200 and have prepared a solution as follows:
  1. Added 0.720g of ZnBr2 to a 10mL volumetric flask
  2. Added 1mL of water and dissolved the ZnBr2
  3. Added 2mL of PEG-200
  4. Completed to volume with 1M TBABr solution
The solution was sadly still very cloudy - TBABr precipitated upon addition and did not solubilize back - but the difference was that this suspension seems to be stable for significantly longer because of the higher viscosity of the PEG-200. I am going to cycle this battery for a while, since the TMPhABr I ordered from China might still take a while to get here. The PEG-200 should also reduce the formation of Zinc dendrites dramatically so we can see how this affects the cycle life of the cell (previous one failed at 50 cycles).

This cell was built with C4 carbon cloth as cathode, 8 layers of fiberglass and a 0.2mm Zinc anode. I added 100uL of electrolyte on top of the cathode after placing all the components in my Swagelok cell.
 
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Using this so much PEG-200 and solid in the cell does lead to a large amount of internal resistance. However the chemistry seems to be better behaved with better shaped charge discharge curves and more consistent efficiencies. Cells were charged to 500uAh at 1mA and discharged to 0.5V at the same current.

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I stopped the test at this point because the charging potentials were getting to high (close to 2.4V) due to the internal resistance increasing within the cell, which meant that a lot of energy was probably being wasted in irreversible reactions at the anode (such as H2 evolution). I am resuming these tests at half the current (0.5mA) to see if I can get the charging potentials to be lower.

This internal resistance problem - due to a lack of enough supporting electrolyte - means that TBABr will probably not work, even in the presence of PEG-200. Using more PEG-200 would also hugely increase the viscosity of the electrolyte, which decreases efficiency due to mobility issues.

I will post partial results of the 0.5mA tests in a 2-3 days or when the cell fails.
 
The resistance did not decrease substantially at lower currents so I stopped this experiment. It seems PEG-200 cannot be used at a concentration higher than a few percent without large increases in internal resistance. Next experiments will involve TMPhABr when I get it in a few days :)
 
After six cycles the battery cell is already at a Coulombic efficiency of 85% with an energy efficiency of 70%. Let's see how it evolves as more cycles are done! I'm super excited, my hypothesis seems to have worked :eek:

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After 28 cycles, the battery is now at 90% Coulombic efficiency with an energy efficiency of 74%.

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This is the last curve measured:
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This seems to be a complete success so far :) I will keep on cycling to see how stable it is!
 
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