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My adventures building a Zinc-Bromine battery
#1
Hi Guys,

My name is Daniel, I am a chemist with a passion for battery technology and currently trying to build a highly efficient Zinc-Bromine battery at home using readily available materials. I have a blog where you can follow my progress (https://chemisting.com/). I am using a DIY USB potentiostat/galvanostat (read about how to build it here https://www.sciencedirect.com/science/ar...7217300317) in order to properly characterize the batteries I build and in this way systematically modify/improve my builds. 

I am using Swagelok cells for the construction of the batteries (0.5 inch diameter). This is the current configuration I have tested:



So far I have achieved a 96% Coulombic efficiency, although my specific capacity sucks, at around 1.0 mAh/g of cathode material. I am going to change to a Zn anode and to carbon paper cathodes (see my latest blog post), which should help increase the specific capacity by a factor of 10-100x.

Any suggestions/comments/questions are welcome!
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#2
https://secondlifestorage.com/showthread.php?tid=8772
https://secondlifestorage.com/showthread.php?tid=8776

Sorry to be the bringer of bad news and disappoint you.
It is very good to do and be build very easy with great success.
The biggest obstical would be to obtain large quantities of znbr, in small amounts it is to expensive.
The other biggest problem with znbr cells is you have to discharge them completely (0.0V) before you can charge them again.
You will be able to charge without discharging but it will take longer and longer, the zinc must dissolve back into the bromine.
To counter this problem you would have to build two power walls, and yes some of you are going to have a laugh right now....
You can deplete one power wall to charge the other power wall.
You also must put in 3 times more energy than you will get out. 3 to 1 ratio.
The charging is very slow but they can hold big amount of power, discharging is also.
You would need a lot of gallons of that stuff to make a decent power wall.
But those where mine experiments with mine results.

Where are you from, i can send you some stuff like carbon paper and such, if needed of course.
If you are willing to pay for shipping.
Needles to say znbr2 is not included for postal reasons.


I am going to follow this with interest, thanks for bringing that "itch" back!
Probably i quit to soon.

With best regards and best success, Igor

Ps,
I really strongly suggest you get a: at least a 4 times bigger diameter.
0.5 inch will do nothing start with 3 or even 4 inch.
Make your electrodes as big as possible but dont let them get into contact with the bromine.

Hdpe water pipe is really cheap, and does the job exelent...

You know what i also remembered...i still have them, incl the solution in them.....
C Ya, if they are looking for me, i am in the shed, bye.
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#3
Thanks for your reply! I did read your posts with great interest.

I'm not so much interested in the building of large batteries right now, basically because I want to understand and optimize the chemistry/materials at a small scale I can characterize well before moving onto anything larger. I want to have a well characterized Coulombic efficiency, energy efficiency, specific power and specific capacity before I try to build anything that is larger. The Swagelok cell I use is 0.5 inch, which is pretty convenient to build and test a lot of small cells without a lot of mess. 

About your cell construction, I think that many of the problems you describe are actually related with a Zn-Br battery without any complexing agents. If you do not use any complexing agents the batteries will strongly self-discharge plus you'll have to deal with elemental Br, which is a nuisance to say the least. I am using tetrabutylammonium bromide (TBAB) - which completely eliminates this issue - since the TBAB forms an insoluble tribromide salt in the battery cathode. I can charge/discharge my cells from any point within the charge/discharge cycles, so I definitely do not seem to be experiencing the issues you refer to related with having to fully discharge the cell to 0V before charging. 

Another issues is the separator, the flower foam is - in my opinion - not a good choice, because of how much it impairs ion mobility and increases internal resistance. I am using a thin fiberglass separator with good results (so far).

Do you have any charge/discharge curves for your cells? I would love to see how they look under the conditions you were experimenting with.

Also, I'm in the US west coast atm. So getting materials shipped from the Netherlands will probably be more expensive than buying them locally, but thanks a lot for offering!
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#4
Thanks for the replay, feedback is always appreciated.
Only in the first experiments i used flower foam, after that i did not use it anymore.
Indeed it hold back the ion movement, when i stop using that the conductivity was getting a lot better.
Nice thought to use a separator, aldo i think that if your cell(s) would be high enough there is no need for it.

With the next set of experiments i altered the complete cell, the results where promising.
My mistake btw....every couple of cycles you must reset them by depleting them, sorry for the goofing up.
In my setup they wanted to do almost nothing anymore after a couple of cycles, nothing in nothing out, sort of speaking.
The tbab was something i heard about after i quit/give up with them, and it is very hard to get here.
Not kidding:it is easier on this side of the pond to get the papers to buy stuff like nitrocellulose, then to get papers to obtain znbr/tbab.

My charge discharge curve was very liniair, almost 45 degrees both on m volt and m amps.
They where both equal with charge and discharge, the more volts that where in the higher in amps it would have, how ever now that i am thinking on it, the last 10-15% when the v went up the a went down.
Not sure on this one, i owe you a proper answer if i can find those papers.

I used nothing fancy, i just wrote down every hour what the charging and or discharging was, both on mV and mA, never made it into a graphic.
I think i did not throw away those results.
The charger was telling what he give atm and the discharge measurement was followed by 2 multi-meters.
Still regret the selling of that ir tester.
Your usb galvanometer is really wonderful, but as a (average, yah right) construction worker with a good set of brains this is way out of my ballpark.
After building and programming robotics with gw basic and older i lost interest.

If i remember correctly i wanted to start up a experiment (one of the???) with the inside of a "dead" li ion cell, they have excellent rods in them.
Excellent site btw, going to read it again and don't be surprised if i start with some experiments again, found my cells btw
Sorry in advance for my English, it is not my native language and i refuse to use a translation program.

It is Friday, cheers
Igor
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#5
Thanks for the reply Igor!

If you want to restart your experiments, I would suggest finding a complexing agent, because otherwise it will be very hard for this to be viable. The self-discharge is just a complete nightmare and it will likely never get to a viable system with that going on. Even though bromine is not very soluble in water, it will migrate through the electrolyte and discharge the cells very effectively. The formation of zinc dendrites is also a very important limiting factor when no complexing agents are used. Tetrabutylammonium is not the only possible choice, I am not aware of what chemicals you have access to in the Netherlands but certainly some other choices are possible. TBAB has the added benefit that it also inhibits zinc dendrites quite effectively.

About charge/discharge, these batteries should be charged at constant current, usually researchers do this at around 10 mA/cm^2 although lower current densities usually lead to better cycling. If you use a constant current supply and monitor the potential you can create some charge/discharge curves. Usually they are charged at the constant current until the potential increases past some threshold (mostly 1.9-2.1V, but that depends on the internal resistance of your battery), they are also usually discharged to 0.5V although you can also discharge them to 0 if you wish although this leads to poorer cycling as the initial nucleation of the Zn on the electrode needs to be repeated.
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#6
I'm hooked once more! Looking forward to all your experiments, and corresponding data. BTW, that 2020 paper was an awesome find!

Static Zn-Br2 (very optimistic figures)

Max. Energy Density: 142 Wh kg−1
Max. Power Density: 13,000 W kg−1
Coulombic Efficiency: 99.9%
Energy Efficiency: 94%
Cycling Life: 11,000

Remarkable. Watching https://chemisting.com
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#7
An interesting fact is that the theoretical maximum energy density can be calculated using the ZnBr2 concentration. Assuming absolutely all the zinc bromide gets transformed into metallic Zinc and elemental bromine you can calculate how much power this could deliver per liter of solution, assuming you would perfectly discharge at the potential given by the reaction (around 1.85V). This gives you the following theoretical concentration Vs Wh/L plot. 



This shows that - given the results of the paper which uses a 0.5M concentration of ZnBr2 - the specific energy they discuss is impossible, given the amount of water that is present, of course, they get these values because they count only the mass of the ZnBr2+TPAB (which is misleading as the water is a vital component of the system). This does show that to get to Li-ion territory in practice, we would need concentrations greater than 1.5M, which are achievable with the ZnBr2 but pretty much not possible with the TBAB or TPAB complexing agents. A battery that seeks to reach Li-ion densities therefore requires a complexing agent with a solubility at least an order of magnitude greater than TPAB that still forms insoluble and solid tribromides. 

The solubility of ZnBr2 in water is 4470g/L at 20C (wikipedia) which gives us 19.86M (it is THAT soluble), so the first step to get a very high density ZnBr2 battery is to get a complexing agent that can get to at least 1-2M while retaining the necessary chemical properties of the tribromides formed.

I will continue with all my TBAB experiments in the meantime as characterizing/understanding this system well will be key to advance in this direction.
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#8
Just published a post about some thoughts in building an actual practical battery (https://chemisting.com/2020/09/13/zinc-b...l-battery/). Still very far away - I need to understand TBAB cells, other carbon cathode choices, different ZnBr2 concentrations, etc - but worth thinking about.
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#9
I would go with a higher molar solution, 2.0m, but you will need longer cells.
Keep up the work please, i am following with a lot of interested.
Cheers and thanks
Btw next week i am going after tbab
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#10
(09-15-2020, 05:06 PM)100kwh-hunter Wrote: I would go with a higher molar solution, 2.0m, but you will need longer cells.
Keep up the work please, i am following with a lot of interested.
Cheers and thanks
Btw next week i am going after tbab

I would like to prepare 2M solutions but TBAB is not soluble at the concentration that would be required to support a 2M solution of ZnBr2. Note TBAB is REALLY soluble on its own, in distilled water, but will precipitate easily in the presence of ZnBr2. To use ZnBr2 at concentrations higher than 0.5M we need to use another complexing agent that's more soluble with the zinc present.

Thanks for all the support!

This is a picture of a cell I just made using a CC6 carbon electrode. This week I will be testing an array of different carbon electrodes to determine which one works best.

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