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diy solar

My adventures building a Zinc-Bromine battery

Tim K said:
Hi Daniel
Works fine on laptop. Still not on phone. Will have to bing read once a week rather than daily on phone.
Click to expand...

Seems like the issue was a php memory limit problem. I have fixed it so it should work on your phone now!
 
There's been some deterioration but the battery is still working fine. Capacity and energy density are still within 20% of the highest value. Threshold for a 20% drop from highest values are 46.4% for energy efficiency and 18 Wh/L for energy density. Current values are 50.58% for EE and 19.55 Wh/L for energy density after 53 cycles. Potentials have been deteriorating, but not more than 5% from their initial values. No critical failure has happened up until now, no evidence of dendrites or irreversible changes from hydrogen bubbles.

1607009091740.png
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The battery lasted for 60 cycles and then died within a couple of cycles. Opening the battery revealed large amounts of elemental Bromine and perbromides which points to the fact that the charge/discharge cycling always generated more bromine that could be recovered. The 15mA current or the current capacity might be too much given the current battery characteristics. This might be the reason why the Princeton minimal architecture batteries were only charged to an energy density of 7-8 Wh/L.

This might have to do with the fact that the TMPhABr forms a solid as well, this might be good for sequestering the bromine, but it might be too inaccessible for the battery chemistry to be as reversible as necessary for the battery to properly work. I am reflecting upon this findings to think about what I will be experimenting with next. Long term stability is definitely a high priority before other aspects of the battery can be improved.
 
wondering if are you considering a periodic shorting of the battery in the design.
that could be easily automated, and 've been under the impression it would solve much of the dendrite issues.

want to learn about all this soon (far from being a chemist.. but would love to diy my offgrid battery), so thank you for the awesome research. :)
 
tiago said:
wondering if are you considering a periodic shorting of the battery in the design.
that could be easily automated, and 've been under the impression it would solve much of the dendrite issues.

want to learn about all this soon (far from being a chemist.. but would love to diy my offgrid battery), so thank you for the awesome research. :)
Click to expand...

Thanks for your support :)

I specifically want to avoid any processes that make the batteries more complicated, I want to get a battery that works and lasts for a long time without the need for things like periodic shorting. The use of Tween 20 greatly reduced dendrite formation, so those don't seem to be a problem anymore, even when using fiberglass separators.
 
It also seems that I have been underestimating compression in my cells by a very large amount when using fiberglass separators. I measured the inner cell height as a function of the distances between the caps of the swagelok cell on the outside and found out that I am actually working with substantially smaller cell gaps than I anticipated, as I am able to compress the device a lot when the cell is closed (fiberglass and carbon felt both compress a lot). This means that I have been working with cells that are actually only around 2.5-3mm tall, which means that I have been measuring and aiming for energy densities in the 40-50Wh/L range, while I previously thought the cells were around 5.8mm tall, as this is what I measured with a caliper when I took apart the devices, but compression makes the devices way shorter.

I put together a cell with double the amount of separator, in an inverted configuration using a GFE-1 cathode pretreated with a 10% TMPhABr solution, using a 4.2M ZnBr2 solution with 1% TW20 which gave me a compressed height of 4.8mm (energy density of 29.81Wh/L). Note that I used a much more concentrated electrolyte because I wanted to try the straight electrolyte from a ZnBr2 synthesis from sodium sulfate and zinc sulfate, the approximate concentration was obtained by measuring the density using a pycnometer. The results are below.

1607282834172.png
1607282844283.png
I stopped cycling the cell here because I wanted to open it to see if any deterioration had happened up until this point - given the higher concentration of ZnBr2 - I saw no evidence of dendrites though. The CE is acceptable at 86.78%, and the EE is the highest within the cells I've measured in inverted configurations at 64.15%.

During the next few days I will put together another cell in this configuration and run it for longer, to see if I can reproduce the above results and see whether these taller cells can produce better results. So far no cell has been stable enough to last more than 100 cycles, we'll see if this one is the charm ?
 
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danielfp248 said:
It also seems that I have been underestimating compression in my cells by a very large amount when using fiberglass separators. I measured the inner cell height as a function of the distances between the caps of the swagelok cell on the outside and found out that I am actually working with substantially smaller cell gaps than I anticipated, as I am able to compress the device a lot when the cell is closed (fiberglass and carbon felt both compress a lot). This means that I have been working with cells that are actually only around 2.5-3mm tall, which means that I have been measuring and aiming for energy densities in the 40-50Wh/L range, while I previously thought the cells were around 5.8mm tall, as this is what I measured with a caliper when I took apart the devices, but compression makes the devices way shorter.

I put together a cell with double the amount of separator, in an inverted configuration using a GFE-1 cathode pretreated with a 10% TMPhABr solution, using a 4.2M ZnBr2 solution with 1% TW20 which gave me a compressed height of 4.8mm (energy density of 29.81Wh/L). Note that I used a much more concentrated electrolyte because I wanted to try the straight electrolyte from a ZnBr2 synthesis from sodium sulfate and zinc sulfate, the approximate concentration was obtained by measuring the density using a pycnometer. The results are below.

View attachment 29488
View attachment 29489
I stopped cycling the cell here because I wanted to open it to see if any deterioration had happened up until this point - given the higher concentration of ZnBr2 - I saw no evidence of dendrites though. The CE is acceptable at 86.78%, and the EE is the highest within the cells I've measured in inverted configurations at 64.15%.

During the next few days I will put together another cell in this configuration and run it for longer, to see if I can reproduce the above results and see whether these taller cells can produce better results. So far no cell has been stable enough to last more than 100 cycles, we'll see if this one is the charm ?
Click to expand...
Your CFE-1, I assume as a beginner is, carbon felt electrode but what does the -1 signify?
 
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Trying to get reproducible compression heights from this non-woven fiberglass separator is quite difficult, the separator in reality breaks down as it is compressed, turning into a sort of pile of mushed up fiberglass, since not all the layers break down in the same way every time, the separator sometimes compresses too much and sometimes too little, the variations are actually quite significant, which makes reproducibility difficult in this sort of setup. I'm trying to find a way to make this compression more reproducible. Maybe if I get a woven fiberglass separator - which would be a sort of compromise between a separator and spacer setup - it would work better? Your opinions are welcome!
 
danielfp248 said:
Trying to get reproducible compression heights from this non-woven fiberglass separator is quite difficult, the separator in reality breaks down as it is compressed, turning into a sort of pile of mushed up fiberglass, since not all the layers break down in the same way every time, the separator sometimes compresses too much and sometimes too little, the variations are actually quite significant, which makes reproducibility difficult in this sort of setup. I'm trying to find a way to make this compression more reproducible. Maybe if I get a woven fiberglass separator - which would be a sort of compromise between a separator and spacer setup - it would work better? Your opinions are welcome!
Click to expand...
I was thinking maybe Geotextile but would be too thin.
 
After running some experiments with compression and looking at cell results at different compression ratios, I believe that it is best to work with cells at almost no compression. These are far more reproducible. I can achieve this by closing the swagelok cell while measuring resistance between the electrodes and stopping the closing whenever the resistance drops. This means that the cell geometry is basically that of the uncompressed materials, which is easy to determine.
 
I've been talking to a few people about their desire to build DIY setups using these cells and the fact that doing any large scale processing of solutions to create ZnBr2 solutions is probably not going to be an option. With this in mind it does seem that testing an electrolyte that requires as little processing (heating/cooling/filtering) using zinc sulfate and sodium bromide might be interesting. I will try to do an experiment with an electrolyte that goes through a single filtering step - with no heating, cooling or IPA additions required - to see if a device like this would work despite the large amount of sodium sulfate within the electrolyte.
 
It's amazing how much precipitate you get from ZnBr2 solutions prepared from Zinc Sulfate and Sodium Bromide when you add hydrogen peroxide. This is a 3.5M solution - estimated by density - after precipitating the Fe with a 3% hydrogen peroxide addition of around 30% by volume. The solution goes from very yellow, to almost completely transparent after all the Fe is gone. Note that although peroxide can also oxidize Bromide to elemental Bromine, it does so very slowly and the reaction with Fe proceeds much faster at this pH. The fact that this is a red solid that can be filtered shows that this is not elemental Bromine that is being formed.

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I treated my ZnBr2 solution manufactured from ZnSO2.H2O and NaBr2 with 3% hydrogen peroxide to remove all iron, waited 24 hours, filtered all the generated Fe solids, then added some metallic Zn foil to ensure all peroxide reacted and any elemental bromine was eliminated and waited another 24 hours. I then measured the resulting density of the electrolyte which puts its concentration at around 2.7M ZnBr2. I then added 0.5% Tween 20 to a sample of this electrolyte to build a battery.

The battery was built using a GFE-1 cathode pretreated with a 10% TMPhABr solution, a 0.2mm Zn anode and 15 layers of fiberglass separator. The cell was sealed with minimal compression of the layers and was set in an inverted configuration (cathode on top). Here are the results charging to 15mAh at 15mA, discharging to 0.5V:

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The elimination of all the Fe within the electrolyte did give us better stability, improving potentials as a function of time and better voltaic efficiency. Overall energy density was around 25 Wh/L for this device. I opened up the device at this point to observe how the electrodes looked, absolutely no Iron oxide/hydroxide formation on the Zn anode, no dendrite formation and a slighter yellow coloring of the separator layers. If you're going to be preparing your own electrolyte from NaBr2 and ZnSO4.H2O it is absolutely fundamental to remove all the Fe using hydrogen peroxide.

I am going to build another battery with this makeup tomorrow and take it to larger cycle numbers to verify stability.
 
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