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My adventures building a Zinc-Bromine battery

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!
 
First 80 cycles have passed. The last curve's CE is 91.55% and EE is 73.66%. This battery seems to be incredibly more stable than my previous batteries. So far, zinc dendrites do not seem to be an issue. :)

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svetz said:
Nice! Any idea what the bump at 45 is about?
Click to expand...

Thanks!

I don't know what the bump was about. However, I can speculate a bit. In other cells 45 cycles was around the point where Zinc dendrites started to become a serious problem - which was evident after opening the batteries up - in this case it's likely that dendrites also started to build to a certain point but then were somehow eliminated in subsequent charge/discharge cycles. If this was the case we are likely to see the battery oscillate 85-90% CE as time goes on. I haven't even touched the battery or measuring equipment, so whatever is happening was fully due to the battery chemistry.

I had never run one of the these Zn-Br batteries for so long, so it's really exciting to see we can cycle them 80+ times at 100% depth with no large degradation in their CE or EE :)
 
After 100 cycles the CE and EE remained stable. Final values were CE=89.79%, EE=73.68%. I am now going to try another 100 cycles at a higher current density (these were done at 2mA, I am now going to try see how it changes at 5mA).

1601556426812.png
 
i'm really interested in Zn-Br chemistry and believe that the solution is to make the battery no flow setup. I know that some manufactures are working with Zinc Bromine Gel electrolyte and they change the discharge profile of the battery with the thickness and viscosity of the gel layer.
Thanks for sharing your research Daniel, i'm also reading your blog
 
Altin961 said:
i'm really interested in Zn-Br chemistry and believe that the solution is to make the battery no flow setup. I know that some manufactures are working with Zinc Bromine Gel electrolyte and they change the discharge profile of the battery with the thickness and viscosity of the gel layer.
Thanks for sharing your research Daniel, i'm also reading your blog
Click to expand...

Thanks for following my progress and reading my blog :)

The gel batteries by gel-ion are certainly interesting. You can read more about their approach by reading the patent for their technology:


The approach certainly makes sense, being that the researchers working on this come from the ionic-liquid world.

As a humble solo chemist I am hoping to build an understanding about a much simpler approach and hopefully contribute something useful to those interested in this type of technology.
 
I had to restart the test because of a computer issue (computer restarted in the middle of it for updates :( ). After 70 cycles of the new test the CE=95.55% and the EE=66.86%. It seems the battery is perfectly stable, after going through 100 cycles at 2mA and 100+ cycles at 5 mA, the CE and EE at each current density remain close to the initial values.


1601641601620.png
 
First curve trying to charge to 2000 uAh using 2M ZnBr2 solution saturated with TMPhABr plus a solid TMPhABr layer. CE=75.8%, EE=56.4%. Let's see if it improves on subsequent curves!

1601654157014.png
 
After 5 cycles dendrites started to become a problem. You can see this in this charge cycle as the voltage drops as dendrites start to generate shorts in the battery. It seems the higher ZnBr2 concentration has reduced the solubility of TMPhABr to the point were dendrite formation is happening yet again and causing big stability issues. We are depositing 4 times more Zn per charge cycle now, so I had some suspicions this might happen! Time to reflect a bit before the next experiment ?

1601682210143.png
 
Next experiment has had a couple of modifications. First, I have added 1% PEG-200 to the electrolyte to reduce Zinc dendrite growth. Second, I have changed the battery structure to go back to 8 continuous layers of fiberglass, followed by a 50mg layer of TMPhABr, right next to the cathode.

Screenshot 2020-10-03 083018.png

These are the results after 7 cycles at 2mA, charging to 2000uAh, discharging to 0.5V:

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Last curve:
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Last cycle CE=94.43%, EE=63.08%.

There is also a small problem with the addition of the saturated electrolyte because it often carries some undissolved TMPhABr that I end up depositing on top of the CC4 cathode. Filtering the electrolyte is likely going to help with the drop in potential from these solids causing resistance between the CC4 cathode and the graphite electrode of the Swagelok cell.

There is substantial internal resistance showing up in this entire process, reason why the EE has dropped substantially from the first TMPhABr experiments at lower ZnBr2 concentrations. I am going to continue cycling the above cell and possibly run a couple of new experiments filtering the electrolyte before I start experimenting with my other cathode materials to see what difference they can make.
 
The above cell remained stable for another 5 cycles, ending at a CE=93.03% and an EE=62.43%.

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I have opened up this cell and built a new one, making sure to filter the electrolyte before applying it on top of the cathode (we'll see if that makes a difference or not)
 
I have made the new cell with filtered electrolyte and have also decided to introduce a waiting time of 5 minutes after adding the electrolyte - before closing the Swagelok cell - to ensure the electrolyte is able to properly soak through the entire device. These are the results of this cell so far, charged/discharged at 2mA to 2000uAh and discharged to 0.5V:

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Last curve had a CE=88% and an EE=71%. The device wants to continue to improve so I'll cycle it to see how it behaves and to evaluate if Zinc dendrites will become an issue. This is the last curve:

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Remember this is an electrolyte with 2M ZnBr2 that has been saturated with TMPhABr and with 1% PEG-200 added, with a 50mg layer of TMPhABr between the cathode and the first layer of fiberglass. The device uses a CC4 carbon cloth cathode.

It will likely take a week to cycle this cell. Time after which I believe it will be time to start testing differences between cathode materials.

This is a big step! A 4x improvement in specific energy ? moving way closer to something that would have specific energy greater than that of lead acid :cool:
 
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