100kwh-hunter
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Another Zinc Bromide Thread
- Thread starter DBT
- Start date
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Cell2 results:
The separator material is not suitable to keep bromine on one half of the cell. Vertical operation allowed the bromine to reach the cathode and discharge in minutes.
Laying the cell flat with the Anode at the bottom was much better. 100mAh in, 96mAh out. 750mA charge current, 750mA discharge current. 96% efficiency.
Repeating the test at 200mAh only resulted in 90mAh back out of the cell (500mA charge/ 500mA discharge).
Repeating the test again at 100mAh at 750/750mA recovered 95mAh back out of the cell.
I suspect with only 4-5mm between electrodes, there is very minimal area for bromine to collect. As the bromine layer climbs higher, self discharge occurs limiting available storage.
Solution? A larger area for electrolyte/bromine to collect. Or a sump, or an external tank...
I've laser cut 6mm layers of perspex and I'll use that in increase the bromine layer volume, and also add graphite felt to help capture the bromine. Some kind of sequestering agent (to convert bromine to a solid salt and drop out of solution) would make the separator much more effective. I'll see how the larger area between electrode performs.
Stats for Cell2: Charge voltage 2.00v @ 750ma. 1.90v @ 500mA. Discharge voltage 1.50v @750mA. 1.6v @ 700mA.
The separator material is not suitable to keep bromine on one half of the cell. Vertical operation allowed the bromine to reach the cathode and discharge in minutes.
Laying the cell flat with the Anode at the bottom was much better. 100mAh in, 96mAh out. 750mA charge current, 750mA discharge current. 96% efficiency.
Repeating the test at 200mAh only resulted in 90mAh back out of the cell (500mA charge/ 500mA discharge).
Repeating the test again at 100mAh at 750/750mA recovered 95mAh back out of the cell.
I suspect with only 4-5mm between electrodes, there is very minimal area for bromine to collect. As the bromine layer climbs higher, self discharge occurs limiting available storage.
Solution? A larger area for electrolyte/bromine to collect. Or a sump, or an external tank...
I've laser cut 6mm layers of perspex and I'll use that in increase the bromine layer volume, and also add graphite felt to help capture the bromine. Some kind of sequestering agent (to convert bromine to a solid salt and drop out of solution) would make the separator much more effective. I'll see how the larger area between electrode performs.
Stats for Cell2: Charge voltage 2.00v @ 750ma. 1.90v @ 500mA. Discharge voltage 1.50v @750mA. 1.6v @ 700mA.
New cell (Cell3) is charging.
6mm of graphite felt + 12mm of reservoir, separator, 3mm void. As the PEG+Tween is only need on the zinc side (i think), the lower larger bromine side is filled without the additives (to help with IR) - and also without supporting salt which might hurt IR.
Charging at CC/CV 2.00v/1amp. It is accepting 550mA due to IR.
Cell volume = 80mmx80mmx21mm = 134.4ml = 2.7Ah at this ZnBr2 concentration.
I may have pierced the separator when filling the cell :-( I don't think that'll matter too much as it didn't really block the Br anyway. It may end up mixing the additives throughout the cell, and also the assisting salts.
Before stripping down the old cell, I put 100mAh into it to check out the Zinc deposition. see below
You can see where the air bubble was in the cell. I think I've avoided that this time around. Also there is no discoloration of the perspex or the separator. They seem stable with Bromine, in the short term at least. I can also see my fingerprints on the graphite but as the zinc grows that won't be an issue.
Also, the cell really needs O-rings or to be chemically welded. Capillary action wicks the electrolyte out and forms a conductive path from negative to positive via the screws. This is a significant source of self discharge and even with the cell empty, anode to cathode was conducting current. I'll give this cell 100 cycles or so and if there is no significant damage or decrease in efficiency i'll rebuild but this time chemically welding the layers together.
6mm of graphite felt + 12mm of reservoir, separator, 3mm void. As the PEG+Tween is only need on the zinc side (i think), the lower larger bromine side is filled without the additives (to help with IR) - and also without supporting salt which might hurt IR.
Charging at CC/CV 2.00v/1amp. It is accepting 550mA due to IR.
Cell volume = 80mmx80mmx21mm = 134.4ml = 2.7Ah at this ZnBr2 concentration.
I may have pierced the separator when filling the cell :-( I don't think that'll matter too much as it didn't really block the Br anyway. It may end up mixing the additives throughout the cell, and also the assisting salts.
Before stripping down the old cell, I put 100mAh into it to check out the Zinc deposition. see below
You can see where the air bubble was in the cell. I think I've avoided that this time around. Also there is no discoloration of the perspex or the separator. They seem stable with Bromine, in the short term at least. I can also see my fingerprints on the graphite but as the zinc grows that won't be an issue.
Also, the cell really needs O-rings or to be chemically welded. Capillary action wicks the electrolyte out and forms a conductive path from negative to positive via the screws. This is a significant source of self discharge and even with the cell empty, anode to cathode was conducting current. I'll give this cell 100 cycles or so and if there is no significant damage or decrease in efficiency i'll rebuild but this time chemically welding the layers together.
Cell3 first cycle - 400mAh in, 385mAh out. 96.25% efficiency.
It's 7pm here so I've put it on charge at 150mA, 1.800v. I'll charge it for ~12-14hrs and begin a discharge tomorrow morning. That should give a good idea of how much capacity the cell will hold (limited by bromine reaching the zinc). I'll then start 1hr charge/discharge cycles and plot a few days of continuous cycling.
I'll start building Cell4 - similar to Cell3 but with an external electrolyte tank and pump to circulate. Aiming for around 10Ah.
If there's no significant decrease in performance after a few hundred cycles, I'll begin designing the large 300x200mm (A4 size) cell stack. Good for ~15-25amps charge/discharge current with a 100Ah electrolyte tank. If the reliability scales up with cell size I'll start ordering materials for a 32cell stack (48v). But first, we'll need a couple hundred cycles on Cell3.
It's 7pm here so I've put it on charge at 150mA, 1.800v. I'll charge it for ~12-14hrs and begin a discharge tomorrow morning. That should give a good idea of how much capacity the cell will hold (limited by bromine reaching the zinc). I'll then start 1hr charge/discharge cycles and plot a few days of continuous cycling.
I'll start building Cell4 - similar to Cell3 but with an external electrolyte tank and pump to circulate. Aiming for around 10Ah.
If there's no significant decrease in performance after a few hundred cycles, I'll begin designing the large 300x200mm (A4 size) cell stack. Good for ~15-25amps charge/discharge current with a 100Ah electrolyte tank. If the reliability scales up with cell size I'll start ordering materials for a 32cell stack (48v). But first, we'll need a couple hundred cycles on Cell3.
100kwh-hunter
Active member
- Joined
- Mar 2, 2019
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Well?!?!?!
I would like to see your report and progress and your ingenuities.
Please update.
With best regards Igor
I would like to see your report and progress and your ingenuities.
Please update.
With best regards Igor