My adventures building a Zinc-Bromine battery

danielfp248

Solar Addict
According to Rob on YouTube, this design doesn't or shouldn't have much issues with dendrites problems, please if you have the time and resources do take a look this too
Thanks for your comment! The problem with this design is that the internal resistance is so high - due to the distance between electrodes - that the energy efficiency will be abysmal (probably lower than 10%). Putting 10 Wh in to get 1Wh out is not practical unless you have a lot of energy to waste. In practice if you want to get high energy efficiencies (>50%) you will need to keep your electrode distance below 5mm. This is because of limits in the conductivity of Zinc Bromide solutions. This is why these "jar batteries" are not practical Zn-Br batteries. Note that their Coulombic efficiency can still be quite high (>80%) but this does not mean the battery is any good in real terms.

With electrode distances so short, zinc dendrites are sadly going to be a problem we will have to solve.
 

danielfp248

Solar Addict

Juptron

New Member
Thanks for your comment! The problem with this design is that the internal resistance is so high - due to the distance between electrodes - that the energy efficiency will be abysmal (probably lower than 10%). Putting 10 Wh in to get 1Wh out is not practical unless you have a lot of energy to waste. In practice if you want to get high energy efficiencies (>50%) you will need to keep your electrode distance below 5mm. This is because of limits in the conductivity of Zinc Bromide solutions. This is why these "jar batteries" are not practical Zn-Br batteries. Note that their Coulombic efficiency can still be quite high (>80%) but this does not mean the battery is any good in real terms.

With electrode distances so short, zinc dendrites are sadly going to be a problem we will have to solve.
Thanks for the information, Just that this is one of the most diyest... Battery I have ever come across hence my fascination for it 😄. ... Thanks for what you are also doing! I see this https://www.sciencedirect.com/science/article/pii/S2589004220305356 is helpful for your current work too...
 
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danielfp248

Solar Addict
Thanks for the information, Just that this is one of the most diyest... Battery I have ever come across hence my fascination for it 😄. ... Thanks for what you are also doing! I see this https://www.sciencedirect.com/science/article/pii/S2589004220305356 is helpful for your current work too...

Thanks for your reply! I discuss that paper extensively on my blog. Note that their capacity, energy density and specific power numbers are incredibly misleading, since they are calculated without considering the mass of the electrolyte or total mass of the cathode (for the capacity) or the total mass of the cathode and water in the electrolyte (for the density and specific power). When you consider these factors their values drop by around 100x which matches what we know about commercial Zn-Br technology. If these numbers actually held, this paper would have been published in Nature or another top tier journal :)
 

Juptron

New Member
Thanks for your reply! I discuss that paper extensively on my blog. Note that their capacity, energy density and specific power numbers are incredibly misleading, since they are calculated without considering the mass of the electrolyte or total mass of the cathode (for the capacity) or the total mass of the cathode and water in the electrolyte (for the density and specific power). When you consider these factors their values drop by around 100x which matches what we know about commercial Zn-Br technology. If these numbers actually held, this paper would have been published in Nature or another top tier journal :)
Ok, but anything practically from 30wh/l looks good to me 😋
 

danielfp248

Solar Addict
After 20 cycles, the battery is still going strong. Energy efficiency has decreased slightly, but Coulombic efficiency remains above 92%. No sign of dendrites yet :) I will continue cycling, see how long it takes for the battery to fail!

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danielfp248

Solar Addict
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.

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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.
 

danielfp248

Solar Addict
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.

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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.
 

danielfp248

Solar Addict
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.
 

danielfp248

Solar Addict
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.

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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.
 

danielfp248

Solar Addict
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.

1605978205965.png

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.
 
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danielfp248

Solar Addict
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.

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danielfp248

Solar Addict
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.
 
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