diy solar

diy solar

My adventures building a Zinc-Bromine battery

I just got the sample kit of carbon cloth electrodes I will be testing within my cells. It contains the following carbon cloth choices, all from AvCarb (https://www.fuelcellearth.com/fuel-cell-products/carbon-cloth-variety-kit/):

CCP – Carbon Cloth Plain 15 mil thick
CC4P – Carbon Cloth Plain 25 mil thick
CC6P – Carbon Cloth Plain 35 mil thick
HCB-1071 – Carbon Cloth Plain 15 mil thick

Carbon felt they are being compared to (https://www.ceramaterials.com/product/gfe-1-pan-graphite-felt/)

These are all untreated and I will use them as-is.

Cell structure will be as follows:

1600187970796.png

Electrolyte will be 100 uL of ZnBr2 0.5M + TBAB 0.2M, all drawn from the exact same solution.

The basic test will be to do 20 cycles at 1mA, charging to 500uAh.
 
A picture of the first test cell I made for these tests, now evaluating the CC6 carbon cathode. Note this cell goes into the Swagelok cell for testing.

WhatsApp Image 2020-09-15 at 3.44.45 PM.jpeg
 
Best results for CC6

Coulombic efficiency = 73.54%
Energy efficiency = 45.26%
Current = 1mA
Charged to 500 uAh

Zinc anode (0.2 mm)
Fiber glass separator (8 layers)
Electrolyte (0.5M ZnBr2 + 0.2M TBABr)
0.5 inch diameter
Measured in a Swagelok cell with graphite electrodes

1600221068010.png
 
The voltage drop above reminds me more of the behavior of a capacitor than a cell. For example with LiFePO4 there's a nominal voltage, similar to how the temperature of water holds steady at 0° C as it freezes.

I take it that at 1 mA what you're really measuring is the ion flow through the electrolyte?
discharge_voltage_of_lithium_iron_phosphate.jpg
 
svetz said:
The voltage drop above reminds me more of the behavior of a capacitor than a cell. For example with LiFePO4 there's a nominal voltage, similar to how the temperature of water holds steady at 0° C as it freezes.

I take it that at 1 mA what you're really measuring is the ion flow through the electrolyte?
discharge_voltage_of_lithium_iron_phosphate.jpg
Click to expand...

I measured the capacitance of the cells using cells without ZnBr2. Their capacity due to capacitance is just around 1uAh, which is reasonable given the electrode separation and dielectric constant of the solution.The charge extracted here is more than 30x that, so it is due to a chemical process. When I take charged cells apart I can see the plated Zn and deposited TBABr3 so the chemistry is definitely happening.

Some batteries have strong potential plateaus while others have more linear downslopping curves. This depends on how well the battery can sustain the chemical reactions through the discharge phase at the current density demanded by the discharge process.

The CC6 carbon electrode is bad at this, possibly because of a much lower surface area and conductivity, but the process is no doubt based on the chemistry of the battery.

The discharge at 1mA is not limited by ion flow through the battery but most likely by the kinetics of TBABr3 reduction in the CC6 electrode. Self-discharge is likely to also be an important factor limiting efficiency at these charge/discharge currents.
 
Last edited:
Best results for CC4

Coulombic efficiency = 69.96%
Energy efficiency = 43.43%
Current = 1mA
Charged to 500 uAh

Zinc anode (0.2 mm)
Fiber glass separator (8 layers)
Electrolyte (0.5M ZnBr2 + 0.2M TBABr)
0.5 inch diameter
Measured in a Swagelok cell with graphite electrodes

1600263018909.png
 
These are the CC4 electrode results at 10mA. The fact that the Coulombic efficiency and Energy efficiency are better at higher current is a direct indication that there are some serious self-discharge problems with this cell.

This is because both processes charge/discharge the same amount of charge but the fact that the low current process takes longer means there is more time for ion diffusion related discharge processes. Most likely we have very poor formation of the TBABr3 and what we're forming is mostly elemental bromine within the surface of the electrode, which can migrate and self-discharge the battery by reacting with the Zn deposited on the anode.

Coulombic efficiency = 80%
Energy efficiency = 56%

1600267843510.png
 
Last edited:
NMNeil said:
The original 1885 patent for those interested.
And a later one
Click to expand...

Thanks for posting these! It is worth noting that the reason these Zn-Br batteries never saw commercial use was because of the self-discharge experienced when Br2 migrates across the cells. This is a show-stopper for these batteries as they lose 20%+ of their capacity per day under ideal conditions.

The Princeton publications on "minimal architecture" Zn-Br batteries, points to this problem not being a huge issue if you're interested in commercial scale deployments and have "energy to waste" - only caring about relatively short term storage - but even then, it's hardly easy to sell a battery with strong self-discharge problems, even if the cost per watt is low.
 
Last edited:
So I opened the CC4 cell when it was fully charged and did see a lot of TBABr3 formation, so it is certainly working, however, since the concentration of TBABr is substantially lower than that of ZnBr2, it might not be able to sequester all the Br2 that is being formed (see my last post - https://chemisting.com/2020/09/15/zinc-bromine-batteries-is-tbab-the-best-complexing-agent/ - as to why I don't prepare more concentrated TBABr solutions).

I decided to try a small experiment and just put 20uL of 1M TBABr solution right on top of the CC4 cathode and reseal the cell and remeasure it. The cell was discharged and then put through some charge/discharge cycles at 1mA to 500 uAh.

Coulombic efficiency > 99%
Energy efficiency = 62%


1600274961303.png

The internal resistance increased a lot though - probably due to the precipitation of TBABr within the cell - but at least the self-discharge problem seems to have been largely solved. The more experiments, the more I realize TBABr might not be the best sequestering agent to use within these cells because of its solubility limit in the presence of ZnBr2.
 
I think I will work more on the chemistry before using more of my carbon materials, which I have a small supply of, because without the chemistry properly working, it is not smart to optimize the cathode choice. There are two main roads to try:
  1. Increase the solubility of the TBABr in the ZnBr2 solution. This can be done by reducing the polarity of the solvent with additives or reducing the polarity of the Zn ion with the use of Zn complexes. However this can change the solubility of the TBABr3, which we would want to avoid at all costs.
  2. Change the complexing agent to something more soluble. This involves changing TBABr to something more soluble in ZnBr2 solution that still forms an insoluble perbromide. The main candidates would be TPABr (the one used in the chinese paper) or TMPhABr (which I think is going to be even more soluble).
I am going to think about the experimental design to try 1, since trying 2 is going to be significantly more expensive.
 
If it were setup as a flow battery, wouldn't that eliminate the self discharge issue?
 
svetz said:
If it were setup as a flow battery, wouldn't that eliminate the self discharge issue?
Click to expand...

To some extent, but not entirely. It eliminates the self-discharge issue in the sense that you can move the bromine away to store it somewhere - so the self-discharge across long periods of time is eliminated - but when the battery is charged/discharged you can still have bromine diffusion across the flow battery membrane because the zinc-bromine flow battery still has a solid zinc anode where zinc metal is plated that is separated from a bromine solution by a membrane. Bromine can go across that and reduce the efficiency of the battery. This is why flow battery systems also use sequestering agents but in this case they want the sequestering agent to form an immiscible liquid (like tetramethylammoniumbromine does) rather than a solid (like TBABr does).

So your self-discharge on storage is eliminated but your energy efficiency loses due to self-discharge during the actual charging/discharging process are not. The complexity, weight and cost of the system are also increased substantially. A big disadvantage of Zn-Br flow batteries vs Vanadium ones is that your catholyte capacity is determined by a tank (where the bromine is stored) but your anolyte is still going to be limited by how much zinc you can plate onto the actual anode while in Vanadium flow batteries both the catholyte and anolyte are stored in tanks and your capacity is determined by the size of these tanks, not the dimensions of the flow cell electrodes (which determine your power output instead).

If we can find a solution for Zn-Br that requires no flow battery setup, then that will be much cheaper and much more energy dense.
 
Zinc bromine flow batteries require a fair bit of maintenance too. It can all be automated with a BMS but with the cycling that is needed you really need two batteries both rated to do the job alone so that when one is doing a maintenance cycle the other is still there to carry your loads.
 
gnubie said:
Zinc bromine flow batteries require a fair bit of maintenance too. It can all be automated with a BMS but with the cycling that is needed you really need two batteries both rated to do the job alone so that when one is doing a maintenance cycle the other is still there to carry your loads.
Click to expand...

This is part of the reason why, in practice, zinc-bromine flow batteries ended up being really complicated, hard to maintain and offer low specific energy and power values. A static system would get rid of most of these problems, provided the self-discharge issues can be solved effectively.
 
danielfp248 said:
This thread is not about flow-batteries though! It's about static zinc-bromine systems.
Click to expand...
Understood. But the link was for those who inquired about the self discharge rate in zinc bromine flow batteries, and if it was the same for non flow batteries.
 
You must log in or register to reply here.

Similar threads

D
My adventures building a DIY Zn/I flow battery
2
Replies
36
Views
2K
danielfp248
D
D
My adventures building a Zinc-Iodine battery
6 7 8
Replies
140
Views
11K
danielfp248
D
D
My adventures building a DIY Mn/Fe flow battery
7 8 9
Replies
161
Views
10K
Vorg
Vorg
D
My adventures building a Zn-MnO2 battery
2 3 4
Replies
65
Views
6K
danielfp248
D
Ionic Battery Maker
190 Watt Hours Per Kg On My Homemade Zinc Ion Battery
Replies
11
Views
809
SamDeleted
S
Back
Top