My adventures building a Zinc-Bromine battery

[metaros]

Thanks for posting ? Glad to see someone else joining!



I don't use any conductive plastics, the Swagelok cell I use - described in my last blog post in chemisting.com - uses graphite electrodes and a GFE-1 carbon felt electrode, the resistance of the graphitic felt electrode is below 10 ohm, so it is much more conductive than these plastics (almost an order of magnitude more than the ones you mention). Conductive plastics are not going to work well, even at 75ohm, that is still way too high. If all electrodes are graphitic felts or clothes, the conductivity is going to be way better. You'll notice no papers use conductive plastics, precisely for this reason. Any configuration that expects to work with good energy efficiencies at current densities > 10mA/cm will use graphitic felts and/or cloths.

About shorting, I know this can help eliminate dendrites and "reset" the chemistry, but it is my goal to try to come up with a chemistry that does not require this short circuiting step as this is a significant limiting factor of this chemistry's implementations up until now. There are also some chemical processes - such as hydrogen evolution when the top electrode is the zinc electrode - that are irreversible, the battery will never recover from these processes. The Princeton papers on minimal architecture ZnBr2 batteries acknowledge that large scale setups will require inverted geometries - Zn electrode at the bottom - due to this reason. Iron impurities also generate a lot of issues in long term cycling when the ZnBr2 source is not high purity (>99.98%), this has been a big issue for commercial ZnBr2 flow batteries.

About your recommendations:

1. I think it is well established that anything between 1.5-3M ZnBr2 will work fine. I have seen significant difference in dendrite formation between concentrations though, so it is probably worth it to run experiments to determine cycle life at different ZnBr2 concentrations. The spacer or separator design will also depend a lot on your ZnBr2 concentration. It is likely better to commit to a ZnBr2 concentration, say 2M, and optimize the spacer or separator design based on that. Separator materials, cell design, etc, will all play a role in this.

2. The problem with these sequestering agents is their solubility in the electrolyte. At a 2M ZnBr2 concentration these are basically insoluble in the electrolyte. In a flow cell this does not matter because you have an anolyte that contains only the BSA but in a static cell, the insolubility of the BSA creates a lot of problems with the kinetics of the sequestering and irreversible processes in the device.

In batteries with a normal configuration - Zn electrode on top - hydrogen evolution damages the battery, as H2 escapes and makes the electrolyte more basic -, in an inverted configuration - Zn electrode at the bottom - the BSA migration from top to bottom creates a significant issue as the BSA gets "lost" in the middle of the battery. The Trimethylphenylammonium bromide (TMPhABr) I used, was the best I could find to reduce these issues, but they are still present and seemingly quite insurmountable for any BSA of this type. The best option might be to actually functionalize a graphitic felt with a sequestering compound so that you can have effective sequestering without needing anything to dissolve or migrate in the electrolyte.

3. These additives can be quite critical, I wouldn't give them lower priority than the BSA. Through my research I believe I've established quite conclusively that 0.5-1% Tween 20 is effective at reducing dendrites. I've also established that PEG-200 reduces conductivity too much to be useful and conductive salt additions such as NaBr or NaCl greatly increase dendrite formation due to their effects on Zn ion migration.

I believe that perhaps the most important part is to define an experimental setup that we can all use and reproduce because while everyone is using different electrodes, geometries, etc, it is quite difficult to share and reproduce results. I believe for initial small scale experiments, a setup like mine has big advantages as everyone can pretty much build it and guarantee we all share the same geometry and measuring instruments. Swagelok cells are a standard in battery research for small scale experiments and the open source galvanostat I used provides everyone with the ability to measure charge/discharge curves and perform other standard experiments. The total cost of the testing setup is probably around 400 USD (swagelok cell, electrodes, etc). If anyone is interested in how to get everything, just let me know and I'll guide you as best as I can.

These are great points dan. Do we know at what voltage hydrogen is generated? If the cell is properly sealed is there an internal pressure at which the hydrogen will recombine?

As to the conductive plastic, the thruplane resistance is very low especially since I only use one layer between the graphite felt and copper mesh. Otherwise I dont know how we can seal the cell and get positive and negative leads exposed without some nonpourous layer, is that the function of the graphite foil? (I forget if you use this or not.)

The swagelok is ideal I think for getting reps on certain trials but at some point we need to validate the design in a prototype that can scale economically. We are a ways away from that yet but just something to think about. As far as conductive salts I have seen conflicting papers that feature Cl. Ill track down the one with NaBr but the only effect was lower internal resistance at the expense of lower power density. Cathode side kinetics are sluggish and having extra Br in the mix could help idk about self discharge though.

 

[metaros]

Also I agree... I do not want to have to short the cell periodically to make it usable. I thought maybe it could be a way to help troubleshoot why your cells are breaking down. If you reset and see a return to some previous level, you might be able to rule out the BSA decomposition as an explaination

 

[danielfp248]

These are great points dan. Do we know at what voltage hydrogen is generated? If the cell is properly sealed is there an internal pressure at which the hydrogen will recombine?

As to the conductive plastic, the thruplane resistance is very low especially since I only use one layer between the graphite felt and copper mesh. Otherwise I dont know how we can seal the cell and get positive and negative leads exposed without some nonpourous layer, is that the function of the graphite foil? (I forget if you use this or not.)

The swagelok is ideal I think for getting reps on certain trials but at some point we need to validate the design in a prototype that can scale economically. We are a ways away from that yet but just something to think about. As far as conductive salts I have seen conflicting papers that feature Cl. Ill track down the one with NaBr but the only effect was lower internal resistance at the expense of lower power density. Cathode side kinetics are sluggish and having extra Br in the mix could help idk about self discharge though.

Thanks for your reply ?

About the hydrogen, the generation only becomes substantial above 1.95V, but the hydrogen overpotential is determined by a lot of factors, including additives, so this can change substantially depending on the final configuration of the cell. Keeping hydrogen inside the cell is also not viable, it will go through almost any material, including graphite and many metals. The only viable solution to this problem I can think of in aqueous cells is to have an inverted geometry and have the generated hydrogen bubble up to meet the anode and recombine with the generated bromine/perbromides to generate HBr and regenerate the chemistry. The other solution is non-aqueous chemistry, but this has proven very hard to achieve for these cells in a static configuration (I have done experiments with Propylene carbonate and Me-THF) and this would also increase cell construction costs by an order of magnitude.

I am not using graphite foil, just GFE-1 felt electrodes and graphite electrodes. For a larger scale prototype, I suggested cells built using petri dishes - which have a defined volume - you can also build the electrodes without having to use any conductive polymers, carbon cloths or graphite foil should be enough to expose leads. You can read more about that proposal here https://chemisting.com/2020/12/17/z...-the-next-scaling-level-a-petri-dish-battery/.

A big problem with copper mesh current collectors is that any exposure of them - even microscopic ones - will lead to corrosion of the collector and many irreversible problems within the cell chemistry. Copper current collectors are avoided in most published research I have seen due to this reason. They can be used if you only discharge cells to 1V, to avoid copper oxidation. Whenever metallic current collectors are used, titanium foil is a more common choice.

As I see it, jumping to a larger scale is really not justified while a small scale hasn't been properly optimized. Experiments using larger geometries are significantly more wasteful and expensive. If we optimized things on Swagelok cells then this step would become way easier as we would face only problems of scaling, not problems inherent to the basic chemistry. This is the path battery research usually takes, optimize the small scale, then increase scale and deal only with the problems of scaling, repeat until you reach the desired final scale.

What do you say? Would you like to get a Swagelok cell setup?

 

[Tim K]

Thanks for your reply ?

About the hydrogen, the generation only becomes substantial above 1.95V, but the hydrogen overpotential is determined by a lot of factors, including additives, so this can change substantially depending on the final configuration of the cell. Keeping hydrogen inside the cell is also not viable, it will go through almost any material, including graphite and many metals. The only viable solution to this problem I can think of in aqueous cells is to have an inverted geometry and have the generated hydrogen bubble up to meet the anode and recombine with the generated bromine/perbromides to generate HBr and regenerate the chemistry. The other solution is non-aqueous chemistry, but this has proven very hard to achieve for these cells in a static configuration (I have done experiments with Propylene carbonate and Me-THF) and this would also increase cell construction costs by an order of magnitude.

I am not using graphite foil, just GFE-1 felt electrodes and graphite electrodes. For a larger scale prototype, I suggested cells built using petri dishes - which have a defined volume - you can also build the electrodes without having to use any conductive polymers, carbon cloths or graphite foil should be enough to expose leads. You can read more about that proposal here https://chemisting.com/2020/12/17/z...-the-next-scaling-level-a-petri-dish-battery/.

A big problem with copper mesh current collectors is that any exposure of them - even microscopic ones - will lead to corrosion of the collector and many irreversible problems within the cell chemistry. Copper current collectors are avoided in most published research I have seen due to this reason. They can be used if you only discharge cells to 1V, to avoid copper oxidation. Whenever metallic current collectors are used, titanium foil is a more common choice.

As I see it, jumping to a larger scale is really not justified while a small scale hasn't been properly optimized. Experiments using larger geometries are significantly more wasteful and expensive. If we optimized things on Swagelok cells then this step would become way easier as we would face only problems of scaling, not problems inherent to the basic chemistry. This is the path battery research usually takes, optimize the small scale, then increase scale and deal only with the problems of scaling, repeat until you reach the desired final scale.

What do you say? Would you like to get a Swagelok cell setup?

Yes I would but lots of items here in France are very difficult to obtain due to Covid and Brexit

 

[metaros]

Thanks for your reply ?

About the hydrogen, the generation only becomes substantial above 1.95V, but the hydrogen overpotential is determined by a lot of factors, including additives, so this can change substantially depending on the final configuration of the cell. Keeping hydrogen inside the cell is also not viable, it will go through almost any material, including graphite and many metals. The only viable solution to this problem I can think of in aqueous cells is to have an inverted geometry and have the generated hydrogen bubble up to meet the anode and recombine with the generated bromine/perbromides to generate HBr and regenerate the chemistry. The other solution is non-aqueous chemistry, but this has proven very hard to achieve for these cells in a static configuration (I have done experiments with Propylene carbonate and Me-THF) and this would also increase cell construction costs by an order of magnitude.

I am not using graphite foil, just GFE-1 felt electrodes and graphite electrodes. For a larger scale prototype, I suggested cells built using petri dishes - which have a defined volume - you can also build the electrodes without having to use any conductive polymers, carbon cloths or graphite foil should be enough to expose leads. You can read more about that proposal here https://chemisting.com/2020/12/17/z...-the-next-scaling-level-a-petri-dish-battery/.

A big problem with copper mesh current collectors is that any exposure of them - even microscopic ones - will lead to corrosion of the collector and many irreversible problems within the cell chemistry. Copper current collectors are avoided in most published research I have seen due to this reason. They can be used if you only discharge cells to 1V, to avoid copper oxidation. Whenever metallic current collectors are used, titanium foil is a more common choice.

As I see it, jumping to a larger scale is really not justified while a small scale hasn't been properly optimized. Experiments using larger geometries are significantly more wasteful and expensive. If we optimized things on Swagelok cells then this step would become way easier as we would face only problems of scaling, not problems inherent to the basic chemistry. This is the path battery research usually takes, optimize the small scale, then increase scale and deal only with the problems of scaling, repeat until you reach the desired final scale.

What do you say? Would you like to get a Swagelok cell setup?

For sure, Ill look over your blog, I think I remember seeing instructions for the build. Ill take any documents / tips you have. For testing, Ive already purchased a ZKETECH battery tester using their free software (EB Tester v1.85)

 

[danielfp248]

For sure, Ill look over your blog, I think I remember seeing instructions for the build. Ill take any documents / tips you have. For testing, Ive already purchased a ZKETECH battery tester using their free software (EB Tester v1.85)

Can you measure charge/discharge curves with it? These are the most fundamental measurements in battery science.

 

[danielfp248]

Yes I would but lots of items here in France are very difficult to obtain due to Covid and Brexit

I ordered everything from China and it was sent through DHL, I believe you should be able to get everything to France without much trouble if this mail couriers are working fine.

 

[metaros]

Can you measure charge/discharge curves with it? These are the most fundamental measurements in battery science.

Yes! It does a lot. I work full time as active duty military and I do this stuff on the side on weekends. Ill share my next trials in a few days with actual data. For reference the electrodes ive made with the condutive plastic laminated copper mesh and graphite felt are 8x8 cm in a small 9x9x2cm PP container. From the exposed tab to farthest edge I register 1.5 ohms.

 

[danielfp248]

Yes! It does a lot. I work full time as active duty military and I do this stuff on the side on weekends. Ill share my next trials in a few days with actual data. For reference the electrodes ive made with the condutive plastic laminated copper mesh and graphite felt are 8x8 cm in a small 9x9x2cm PP container. From the exposed tab to farthest edge I register 1.5 ohms.

Awesome! Looking forward to seeing your charge/discharge curves!

 

[tiago]

400USD seems a bit much for testing here...
Maybe we can consider group buys?

 

[danielfp248]

400USD seems a bit much for testing here...
Maybe we can consider group buys?

Most expensive part is the shipping, if you are able to buy say, 10, it would be way cheaper.

 

[Eiskuh]

Hi All

I was looking at some prices for potassium bromide today and was thinking of maybe experimenting with chloride instead of bromide to keep experimental costs a bit down before switching to bromine. Has anyone tried other halogens yet?

I know chloride is a bit more aggressive so more precautions need to be taken. I then remembered seeing some experiments where Zinc Iodine and Zinc bromine where mixed which helps the Iodine/Bromine to settle better as they interact... Maybe a mixed approach would also be good to investigate.

I also checked the potential difference between the halogens salts and how many Watt hours theoretically could be stored per mole of solution (correct me if I am wrong: 2 electrons... coulombs...= +- 53A/h per mole * voltage):
Zinc Florine: 3.63v => 192 Watt/h per mole
Zinc Iodine: 1.29v => 68 Watt/h per mole
Zinc Bromine: 1.82v => 96 Watt/h per mole
Zinc Chlorine: 2.15v => 113 Watt/h per mole

When mixing these salts, the cell voltage can be calculated in the molar ratio e.g. 1x ZnBr : 2 ZnI2 => (1x1.82v+2x1.29v)/3 = 1.46v

ZnFl2 would store nearly double the W/h, but I think Florine is riskier/more expensive compared to chlorine/iodine/bromine? (speaking as non chemist here). The chlorine is readily available and stores more watts... so it sound worth investigating.

Has anyone tried alternative halogens? Any thoughts?

Anyway, I will be sharing my first 3D printed experimental battery design soon

 

[danielfp248]

Hi All

I was looking at some prices for potassium bromide today and was thinking of maybe experimenting with chloride instead of bromide to keep experimental costs a bit down before switching to bromine. Has anyone tried other halogens yet?

I know chloride is a bit more aggressive so more precautions need to be taken. I then remembered seeing some experiments where Zinc Iodine and Zinc bromine where mixed which helps the Iodine/Bromine to settle better as they interact... Maybe a mixed approach would also be good to investigate.

I also checked the potential difference between the halogens salts and how many Watt hours theoretically could be stored per mole of solution (correct me if I am wrong: 2 electrons... coulombs...= +- 53A/h per mole * voltage):
Zinc Florine: 3.63v => 192 Watt/h per mole
Zinc Iodine: 1.29v => 68 Watt/h per mole
Zinc Bromine: 1.82v => 96 Watt/h per mole
Zinc Chlorine: 2.15v => 113 Watt/h per mole

When mixing these salts, the cell voltage can be calculated in the molar ratio e.g. 1x ZnBr : 2 ZnI2 => (1x1.82v+2x1.29v)/3 = 1.46v

ZnFl2 would store nearly double the W/h, but I think Florine is riskier/more expensive compared to chlorine/iodine/bromine? (speaking as non chemist here). The chlorine is readily available and stores more watts... so it sound worth investigating.

Has anyone tried alternative halogens? Any thoughts?

Anyway, I will be sharing my first 3D printed experimental battery design soon

Bear in mind elemental chlorine and fluorine are gases, very poisonous and corrosive gases, so it is really hard to keep them properly contained in a battery system. For chlorine or fluorine you will have to go with flow battery designs - to properly store these gases - this is not easy, safe, convenient or cheap to do. If you want to go this route, make sure you know what you're doing.

Also consider that Bromine and Iodine are both quite reactive and will eat through most 3D printer materials, so you will need to either apply PTFE or similar coatings to your materials or print in materials like PTFE or CPVC (which is sadly not very easy). Iodine is the safest of the halogens, so go with that one if you're new to working with these materials.

 

[Eiskuh]

Bear in mind elemental chlorine and fluorine are gases, very poisonous and corrosive gases, so it is really hard to keep them properly contained in a battery system. For chlorine or fluorine you will have to go with flow battery designs - to properly store these gases - this is not easy, safe, convenient or cheap to do. If you want to go this route, make sure you know what you're doing.

Also consider that Bromine and Iodine are both quite reactive and will eat through most 3D printer materials, so you will need to either apply PTFE or similar coatings to your materials or print in materials like PTFE or CPVC (which is sadly not very easy). Iodine is the safest of the halogens, so go with that one if you're new to working with these materials.

Gas.... wasn't thinking of that and looking at the solubility of chlorine at room temperature is not that great at either.

Regarding plastics, I am testing it with ABS. I did some research but all plastic manufacturers have no data available for halogens in solution. Only 1 manufacturer does list chlorine in solution as stable with ABS but chlorine out of solution as corrosive... hence I will be testing with ABS.

My final battery design intent is to remove the cathode and/or anode from the electrolyte after charging to reduce self-discharge. This too would reduce the contact of the halogen with the plastic. This will however depend on my tests where I will also see how the plastics cope.

 

[danielfp248]

Gas.... wasn't thinking of that and looking at the solubility of chlorine at room temperature is not that great at either.

Regarding plastics, I am testing it with ABS. I did some research but all plastic manufacturers have no data available for halogens in solution. Only 1 manufacturer does list chlorine in solution as stable with ABS but chlorine out of solution as corrosive... hence I will be testing with ABS.

Both bromine and iodine will react with ABS, so you sadly cannot use that without any treatment. They will also react quite fast with this plastic. If you want a cheap material for battery construction I would go with glass, which is stable against both bromine and iodine.

My final battery design intent is to remove the cathode and/or anode from the electrolyte after charging to reduce self-discharge. This too would reduce the contact of the halogen with the plastic. This will however depend on my tests where I will also see how the plastics cope.

This is why Zn-Br batteries ended up with flow battery designs, to prevent discharge reactions from happening. However moving around elemental Bromine or perbromides is not easy to do, because of how reactive they are (special pumps are required) manipulating any meaningful quantity of these chemicals by hand is also not advisable because of how dangerous they can be.

My advice would be, do this at a very small scale - like using the swagelok cells I describe in my blog - as this will involve only milligram amounts of material and will allow you to experiment without putting yourself in any significant risk. Working with larger scales, even if it only in the 1-10g scale, can already lead to health hazards if you're not careful. If you learn about the batteries at the smallest practical scales you will be way better prepared for when you scale up.

 

[Nitrous]

I have watched all of Robert's videos, I even decided to pay to become a member of his community to get access to several extra videos about this topic not available to the general public.

The videos have not been very useful though, mainly because of the following reasons:

1. While Robert shares a lot of info about fabrication, he shares practically zero information about real results (no charge/discharge curves, coulombic efficiency info, self discharge rates, specific power/capacity, etc) so I have no idea how good or bad the batteries he made are.
2. His choices are sometimes very odd, in particular his battery architecture deviates very substantially from what a modern battery looks like. For example the choice of a very thick phenolic foam separator causes very big internal resistance and heavily reduces ion mobility. These choices are likely to very heavily reduce both energy efficiency and specific power of the battery. The charge/discharge rates are also likely to suffer.

Because of these issues I have instead decided to reproduce the results of well documented research with high specific capacity values. In particular this recent chinese paper on highly efficient static Zinc bromine batteries (https://www.sciencedirect.com/science/article/pii/S2589004220305356). Since TPAB is hard to obtain, my first objective is to see what a battery made using TBAB can achieve within this same architecture.

Note that I am not saying Robert's batteries are "bad" just that because of the lack of information I can't tell and given some odd choices he has made I would rather go with reproducing published research instead.

Funny, I’ve had the same feelings. I smile each time he describes the “3 types of people”. clearly, he’s saying that the least trustworthy of the 3 are those involved in battery development - and yet we accept all things on the channel at face value. Reminiscent of the ”frog and the scorpion”. . That being said, I have no particular reason to doubt him.... I think his primary goal (and he‘s quite successful at it) is to inspire others to experiment. To take the published papers and dare to ask “why?”. After all, Flash Graphene synthesis wasn’t optimized on the basis of some quantum revelation. It was a grad student who said something like this... “if we can produce graphene with the power of high intensity lasers, why not try a simple spark discharge?“. Anyone wanting to look at Graphene seriously is in need of a cheap Raman Spectroscope. My old physchem graduate advisor wrote two books on various spectroscopic measurement systems.... he’d be impressed with what you can do with a 3D printer these days!

 

[Nitrous]

Gas.... wasn't thinking of that and looking at the solubility of chlorine at room temperature is not that great at either.

Regarding plastics, I am testing it with ABS. I did some research but all plastic manufacturers have no data available for halogens in solution. Only 1 manufacturer does list chlorine in solution as stable with ABS but chlorine out of solution as corrosive... hence I will be testing with ABS.

My final battery design intent is to remove the cathode and/or anode from the electrolyte after charging to reduce self-discharge. This too would reduce the contact of the halogen with the plastic. This will however depend on my tests where I will also see how the plastics cope.

bromine is an element (like most other gases) that exists as a liquid Under specific temps/pressures. The TPAB in the paper being referred to seems to be a pretty effective barrier to bromine migration. Is TMAP or TEAB equally effective? TBAP was a bit pricey, here in Canada.

 

[Nitrous]

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:

View attachment 30221

View attachment 30219
View attachment 30220

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.

How are you affixing the current collector to the Carbon?

 

[danielfp248]

bromine is an element (like most other gases) that exists as a liquid Under specific temps/pressures. The TPAB in the paper being referred to seems to be a pretty effective barrier to bromine migration. Is TMAP or TEAB equally effective? TBAP was a bit pricey, here in Canada.

The problem with TPAB is that its solubility in ZnBr2 solutions is really low, so in practice you cannot get more than a 0.5M solution of TBAP+ZnBr2 which seriously limits your capacity. TEABr's perbromide is a liquid, so it will not be an effective way to sequester the Br unless you're able to move it away since otherwise its diffusion is going to be substantial, then TBABr is way more insoluble with ZnBr2 compared with TPABr. I was never able to get it to work in any effective manner because of this, even trying to add it as a solid.

 

[danielfp248]

How are you affixing the current collector to the Carbon?

I don't add any current collector to the carbon, see my Swagelok cell configuration here https://chemisting.com/2020/12/25/z...wagelok-cell-for-small-scale-battery-testing/