My adventures building a Zinc-Bromine battery

Timtam264 said:

Danial, Thank you for all the awesome insights so far.

I am super curious about the recent video of Robert Murry using a ceramic separator would be interesting to see if a dendrite could ever plate its way through that structure.

I have also been thinking about applications for even a ZnBr2 cell with high self discharge and i think it would be perfect time shifting power.
In New Zealand and we have 15 minute power pricing so in theory you could build a battery, buy power when its cheaper and use it when its quite expensive. during the day it could charge from solar for the evening peek usage. and again charge over night from very cheap renewable (We have lots of geothermal and hydro) for the morning peeks.

Great work, i cant wait to get my hands on some supplies and have a go my self.

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Thanks for your message and kind words! I really appreciate your support.

I would certainly want to try a ceramic separator sometime, if just to see how it does in terms of dendrites and internal resistance, but sadly ceramics require more equipment than I have or can procure in the small space I have available. Working with them - cutting, forming, heating, etc - is something my small apartment cannot handle.

About self-discharge, this is hardly an issue in a battery like the one showed in Rob's latest video on the subject. The main issue in this case is the energy efficiency of the battery, which for a configuration like the one he showed is pretty dismal (<15%). This is due to both the distance between the electrodes and the ceramic separator. A battery where you need to put in 10Wh to get out 1-2Wh is hardly practical. To get to decent energy efficiency values (>60%) you really need to do a lot of work to reduce internal resistance, maximize cathode surface area, etc. In that case - with a more engineered battery - what you propose might be a very interesting use case.

I have received my PTFE spacers (0.5 outside diameter o-rings, 1.17mm thick). This is the first cell constructed using them, this cell uses 3 PTFE separators, GFE-1 cathode treated with 10% TMPhABr and a 3M ZnBr2, 1% PEG-200 solution (I ran out of ZnBr2 3M 20% PEG-200 solution, I am waiting for more ZnBr2 to arrive to make more).

So far, so good. Charging to 15mAh, at 15mA, discharging to 0.5V. Energy density at this point is ~30 Wh/L.

I was just reading this,
They claim addition of zinc chloride improves efficiency by about 5%

A cell according to any one of the preceding claims wherein said bromine complexing agent is selected from N-methyl, N-ethyl morphilinium bromide, N-methyl, N-ethyl pyrrolidinium bromide, N-methyl, N-ethyl pyrrolidium bromide, N-methyl, N-ethyl piperidinium bromide and mixtures thereof.

Importantly, in addition to the zinc bromide, chloride ions are added to the electrolyte in amounts sufficient to reduce the amount of free bromine present in the electrolyte during charging of the cell.

also they talk about only charging the cell to 75% of the rated capacity.

Timtam264 said:

I was just reading this,
They claim addition of zinc chloride improves efficiency by about 5%

EP0159442A1 - Improved electrolyte for zinc-bromine batteries - Google Patents

The coulombic efficiency of aqueous zinc bromine batteries is increased by the addition of chloride ions to the electrolyte in amounts sufficient to reduce the amount of free bromine present in the electrolyte during operation of the cell.

patents.google.com

A cell according to any one of the preceding claims wherein said bromine complexing agent is selected from N-methyl, N-ethyl morphilinium bromide, N-methyl, N-ethyl pyrrolidinium bromide, N-methyl, N-ethyl pyrrolidium bromide, N-methyl, N-ethyl piperidinium bromide and mixtures thereof.

Importantly, in addition to the zinc bromide, chloride ions are added to the electrolyte in amounts sufficient to reduce the amount of free bromine present in the electrolyte during charging of the cell.

also they talk about only charging the cell to 75% of the rated capacity.

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Thanks for your suggestions!

I have confirmed the effect of chloride ions to an extent. In some experiments I made, saturating 3M ZnBr2 with NaCl, I was able to increase the efficiency values of the devices (to CE = 94.37%. EE=74.60%), those curves are posted on the thread. The main problem was that dendrite formation was made significantly worse by this electrolyte addition when I used a fiberglass separator, although it might not be the case in the separator-less devices. I don't have ZnCl2 - to test the exact hypothesis you mention here - but I think I can prepare it from NaCl and ZnSO4.

About the sequestering agent, all those agents mentioned create perbromide liquids - which are very useful for flow batteries - but in this particular battery I require the perbromide to be a solid, reason why I'm using TMPhABr.

About the "rated capacity" I have really not carried out experiments to charge the cell till I reach some breakout voltage, to try to determine what the max amount of possible charge is. I have mainly calculated the expected max capacity given the volume and concentration of electrolyte I add, then aim for around 50% of that value (this is probably around 90%+ of the cell's real capacity). Once I finish cycling the current cell I might try a charging to breakout experiment, just to see how much charge I can really put in there.

Current test cell (described in #122), still alive and well after 18 cycles (CE=87.69%, EE=68.17%):

Still measuring battery described in #122. Trying to improve my graphs a little bit. Now at cycle 20, CE and EE still improving

I cycled 25 times to 15mAh, no problems at all:


I have decided to stop cycling at this point and run a "max capacity" experiment. I am going to charge the battery to 2.1V, see how much charge I am able to store and how much I can recover, test the limits of the battery's energy density.

So I ran a cycle until the battery reached 2.1V at 15mA, discharging at this same current to 0.5V. The battery had a CE=78.57% with an EE=58.59%. Total charge I was was able to extract was 26.56mAh, which would give the battery a capacity of 62Wh/L. However the battery failed during the second cycle - classic dendrite related failure - so charging to this capacity is not going to be viable. The injected charge was 33.78mAh, so if this is the max capacity, 75% of this value would be around 25mAh, which is 10mAh higher than what I've been charging the battery to. For the following experiments I will be charging to this capacity instead.



I have now put together a new battery with the same configuration, will charge/discharge to 25mAh at 15mA. We'll see what we get

First curve is done, the battery behaved very well on first charge/discharge. CE =85.86% EE = 68.22% ?. Total charge recovered was 21.13 mAh, energy density is at ~45Wh/L. For battery configuration see #122

Just published an update to my blog with some recap about current experimental results and what I am testing right now. https://chemisting.com/2020/10/31/z...eflon-o-ring-separators-capacity-and-peg-200/

This is another post I wrote, concerning a test with PEG-200 at 6% and some air bubbles that cause problems with electrode surface area. https://chemisting.com/2020/10/31/zinc-bromine-batteries-peg-200-bubbles-and-over-potential/

I've been thinking about starting an Instagram account to share pictures and stories about the fabrication of these devices and my experiments. Do you guys think this would be interesting for you? Let me know

danielfp248 said:

I've been thinking about starting an Instagram account to share pictures and stories about the fabrication of these devices and my experiments. Do you guys think this would be interesting for you? Let me know

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Would be cool too, I love the blog post and your post here it's more detailed, having to see more pictures on Instagram would be very fine as well...

danielfp248 said:

I've been thinking about starting an Instagram account to share pictures and stories about the fabrication of these devices and my experiments. Do you guys think this would be interesting for you? Let me know

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Hi Daniel.

The more the better! I still owe you some pictures and discharge curves of my setup.

I think your carbon felt is really making the difference. I have graphite felt which doesn't seem to have a very large surface area. The results with TPABr are very poor unless I coat the felt with activated carbon (nitrocellulose binder). I still haven't managed to get anywhere near my 34 Wh/L I can get without a sequestering agent, so your results are truly impressive!

I also think TMPhA Br is a better sequestering agent than TPA Br. I say that simply based on TMPhA being more polar and clearly sequestering (as you have shown).

I have done some reading and I can activate my felt with simple heating or chemical treatment in permanganate, but I don't see anything with a surface area like 1000 m^2 like your felt.

I would eventually like to make the electrode myself. Let's see how that goes.

I started having some weird stability issues with devices in the separator-less setup Trying to figure out what's going on. Here is a pretty plot showing what I'm experiencing:

Faulty/loose connection?

svetz said:

Faulty/loose connection?

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Doesn't seem like it. It seems to be an issue with electrolyte leaking or cathode/graphite contact. It'll take me some time to figure out and fix.

I just published a blog post about the above issues and how they were majorly fixed by going back to a metallic Zn anode. However, the question of dendrites is still there and higher PEG-200 concentrations or NaCl/NaBr additives are ineffective at preventing them at higher ZnBr2 concentrations.

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

@Juptron danielfp248 is trying to make a more energy dense version, and is aware of Robert see the third post on this thread. https://diysolarforum.com/threads/my-adventures-building-a-zinc-bromine-battery.11910/#post-132445