My adventures building a Zinc-Bromine battery

Finally there was a very high increase in the potential required for charging and a big drop in the discharge potential during the 24th cycle. Reason why I stopped cycling the battery at this point.


It is interesting though that this is not the characteristic failure due to zinc dendrites, where there are sudden and sharp declines in the potential due to the dendrites touching the cathode. Opening up the cell revealed no dendrite formation, although there were a significant amount of crystals formed at the edges of the fiberglass separator. This indicates that the problem might have actually been electrolyte losses, possibly due to evaporation.

I have now put together a separator-cell less using 3 PTFE o-ring spacers - which gives me the same total cell volume - to see if we get a similar result with this 1% PEG-200 + 1% Tween 20 + 3M ZnBr2 electrolyte when no separators are used.

A cell with a 3 PTFE o-ring spacer configuration and the 1% PEG-200 + 1% Tween 20 + 3M ZnBr2 electrolyte has gone through 20 cycles now with practically the same behavior as the fiberglass separator cell. Charging to 15mAh at 15mA, discharging to 0.5V. The energy density is also around 28-30 Wh/L.



The cell is deteriorating in the same way as a function of the cycle number with the charge potential increasing and the discharge potential decreasing with each cycle. This means that the internal resistance of the device is somehow increasing with time. This increase in internal resistance has not stabilized and has already taken the average charging voltage to almost 2V. There are several hypothesis that would explain why this might happen, including zinc oxide formation at the anode or bromine intercalation at the graphite cathode. Happily no catastrophic dendrite formation seems to be happening, so that's great

I have just ordered a couple of 1" titanium electrodes to use in my device, to see if this leads to better cathode performance. If this cell fails I will also try using an untreated GFE-1 electrode as anode as well, to see if this behavior is related with the anode.

Curves started to heavily deteriorate around the 24th cycle, but not due to dendrites. Opening up the battery when charged to 15mAh revealed no dendrites present at all. It seems the PEG-200 + Tween 20 combination is effective at suppressing dendrites in this configuration while retaining acceptable CE and EE values.

With that said, the increase in charging potential is now a new challenge. To see if the anode has to do with this I am now running a battery with an untreated GFE-1 anode, to see how this changes the characteristics of the battery (I have never used a carbon felt as an anode before). I will wait for the arrival of my titanium cathodes before running experiments after this.

Changing the anode doesn't seem to prevent the above from happening (using 1% Tween20 + 1% PEG200 + 3M ZnBr2). Either with a GFE-1 anode or with a Zinc anode, the results are pretty similar. No dendrites but a consistent deterioration of the charging and discharge voltages. See the results below on a Zinc anode (8 cycles), the consistent increase in charge potential and drop in discharge potential is evident.


I will be waiting for my Ti cathode in order to test whether changing the graphite electrode in the Swagelok cell has any effect on this phenomenon.

I just wrote a post about the charge/discharge potential degradation. https://chemisting.com/2020/11/20/z...sistance-in-tw20peg-200-containing-solutions/

I am also currently running an experiment using a Titanium electrode with a GFE-1 cathode and a graphite anode. We'll see how that behaves

So I've learned that, while Titanium is "resistant" to bromine, under the oxidative potential in a Zn-Br cell it is attacked quite strongly by Br and pitted pretty aggressively. This electrode was certified as Ti-6Al-4V 0.5. See the electrode after it was put into a Zn-Br cell and run for a few cycles as a cathode, potentials were between 1.9-2.1V. The black spots are holes in the Ti electrode.



This means that, for this battery chemistry, graphite electrodes are the only realistic choice. Pure Ti might behave differently, but sadly it is quite more expensive compared to its common alloys.

I published a post about the cause of the increase in internal resistance and how I think I have solved the issues: https://chemisting.com/2020/11/22/zinc-bromine-batteries-why-internal-resistance-was-increasing/

Always bad when you have gas ?

Potentials started to deteriorate in the inverted battery, although it's made it through 29 cycles so far. I'm still curious to see if it stabilizes or just dies within the next 20 cycles. If it dies, then the hydrogen was only part of the problem.

I received some more ZnBr2 so I decided to take this battery apart and put together a battery using a correctly prepared electrolyte. I prepared a 1.5M ZnBr2 electrolyte with 1% PEG 200 and 1% Tween 20 and restarted the testing process.

I was previously using 3M but after looking at a lot of the zinc bromide literature and how the conductivity changes as a function of concentration it does not seem to make a lot of sense to use concentrations above 1.5-2M, so I'll be using these concentrations from now on.

First 29 cycles of an inverted battery (GFE-1 cathode pretreated with 10% TMPhABr on top) using the electrolyte described in #160





Charge and discharge potentials have remained within 2% of their initial values. I want to run this battery for more than 50 cycles to make sure that it will not die before doing additional experiments. Despite the lower CE and EE values, it does seem that an inverted configuration is the only viable one in a stable ZnBr static battery, due to the need to keep hydrogen from escaping the device.

A normal configuration (cathode at the bottom) makes it hard for hydrogen bubbles at the anode to leave or react, making the surface area of the anode decrease with time. If the bubbles escape, then all that hydrogen evolution makes the electrolyte irreversibly more basic, making the pH go higher and higher with time, eventually completely killing the battery (as Bromine is unstable at higher pH values and ZnO forms at the anode surface).

Here is a blog post I just published discussing why the inverted architecture is likely necessary to get these batteries to work long term. https://chemisting.com/2020/11/25/z...erted-configuration-is-likely-more-practical/

I have also been doing some tests of miscibility and solubility with propylene carbonate (PC) which is a polar aprotic solvent that I thought could be used in these batteries. Sadly the solubility of ZnBr2 in PC is not very high (I estimate it to be 0.5-1M at 20C), so making an electrolyte entirely out of PC doesn't seem possible. The solubility of TMPhABr in PC is quite high though, so I thought I could use it in a biphasic system as the cathode. However when you mix it with a ZnBr2 1.5M solution with 1% PEG-200 and 1% Tween it just forms an emulsion (not shocking given the surfactant present), so this didn't seem to work.

I then made a saturated solution of TBABr in PC, which seems to be around ~50% by weight and this solution actually does not mix at all with the ZnBr2 solution. It should be pretty good at conducting charges, so an experiment with a GFE cathode soaked in this PC solution in an inverted cell will be quite interesting. Since the TBABr/PC solution is less dense than water and the TBABr3 really hates water, it should help retain the Br significantly better.

I will be running the current cell (#161) to 50 cycles though, so I will likely do this experiment after thanksgiving ?

Hi Daniel, Great work. It reminded me of this article, apologies if you've already seen it, but just in case you haven't. https://iopscience.iop.org/article/10.1149/2.0151816jes

Adam007 said:

Hi Daniel, Great work. It reminded me of this article, apologies if you've already seen it, but just in case you haven't. https://iopscience.iop.org/article/10.1149/2.0151816jes

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Thanks for linking that article, I have read it before, but I do believe others who haven't will find the link really useful. Given my current experience with these batteries, I now believe that this article ignores some key problematic aspects that greatly limit the usability of a minimal architecture Zn-Br battery. The life cycles in the thousands under realistically high current loads are just not possible once you consider them.

The first is that Titanium current collectors will degrade significantly over time, the second is that a normal architecture with a CE > 90% and an EE > 70% will just not last that long due to deterioration caused by hydrogen evolution, this kills the batteries due to the alkalization of the electrolyte . An inverted architecture that removes this problem then cuts your CE down to 70% and your EE down to 50%, which makes all the numbers much worse.

It does become pretty clear after sometime working in these batteries why the industry decided to go with Zn-Br flow batteries rather than static configurations. The static configurations, even very simple ones, have problems that are quite formidable once you consider real-life deployments and their requirements.

I just published a new blog post showing the first results I have using a PC/TBABr electrolyte on the cathode to sequester Bromine. They are not very good, but you won't find them anywhere else ?

Here are some new results using propylene carbonate. I was now able to get a cathode saturated with an organic phase to behave quite efficiently. The fact that lower currents give better CE/EE values points to self-discharge being significantly slower, although this won't be confirmed until I actually measure it.

danielfp248 said:

Here are some new results using propylene carbonate. I was now able to get a cathode saturated with an organic phase to behave quite efficiently. The fact that lower currents give better CE/EE values points to self-discharge being significantly slower, although this won't be confirmed until I actually measure it.

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

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Daniel, the Chemisting.com is showing a Web error

Tim K said:

Daniel, the Chemisting.com is showing a Web error

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It seems to work fine for me. Let me know if you are still seeing a problem.

I've been thinking a lot recently about whether it is better to use a non-woven fiberglass separator setup or a PTFE o-ring setup. With the issue of zinc dendrites largely solved by a 1% Tween 20 addition and the use of 1.5M ZnBr2 instead of 3M, it seems that separators might be the better choice. Let me know what you guys think about this latest post.

Seems like both have specific use cases (e.g., no O-RIngs for an RV).
(website works fine for me too!)

danielfp248 said:

It seems to work fine for me. Let me know if you are still seeing a problem.

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Hi Daniel
I am still getting the error.
Other links work fine just Chemisting. This is what I get

Tim K said:

Hi Daniel
I am still getting the error.
Other links work fine just Chemisting. This is what I get

Click to expand...

Thanks for letting me know Tim! It might be your phone/browser combination that the page is having issues with. Can you access it without problems from a desktop?

Before continuing with my work with propylene carbonate - which is more time consuming - I wanted to see if the use of a ZnBr2 1.5M + 1% Tween 20 electrolyte makes the use of separators viable and, if this is the case, whether this leads to the first stable configuration that could be built by others at a larger scale. I put together a cell with this electrolyte a GFE-1 cathode pretreated with a 10% TMPhABr solution and a 16 fiberglass separator layer 2 days ago in an inverted configuration (GFE-1 cathode on top). These are the results after 27 cycles, charging/discharging at 15mA to 15mAh of charge and discharged to 0.5V. Energy density at last cycle is 20 Wh/L.




In contrast to previous experiments using fiberglass separators, this experiment has not died or showed any sort of dendrite related deterioration. Mean charge/discharge potentials evolved very positively during the first 5 cycles and have since deteriorated although the pace of deterioration seems to be slowing down and the potentials have not deteriorated past 2% of the mean potentials of the first cycle yet.

I really want to see how far this setup goes, since it has been really stable so far, so I will keep running it until it hits at least 20% deterioration from max capacity or energy density. If it runs for more than 200 cycles without that much deterioration, it could be a good candidate for someone to attempt to build a larger device with the same configuration.

I have given this some thought and I believe a glass petri dish that is 5.9" x 0.75" could be built to use the same configuration with graphite foil as anode at the bottom, 16 layers of the fiberglass separator and a GFE-1 cathode plus a graphite foil current collector at the top. This would give around 5.2Wh of energy with a total amount of charges stored of 3.6Ah.

Things don't always go as planned when scaling, it'll be interesting to see what you get!
Love reading your posts!