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My adventures building a Zinc-Bromine battery

Very much enjoying reading this, thanks for sharing. I'm currently working on a recycled plastic case for a minimal architecture battery from my workshop waste, using old ABS from my edge banding machines and a mold. Also some old carbon and fibreglass sheets I have lying around may come in handy. I'm hoping a surfboard/racing car part manufacture background using carbons/epoxy/fillers/foams may pay dividends when it comes to the final layups!
Keep up the good work.
 
I am now running the exact same setup described in #200 in order to confirm stability at higher cycle numbers. Battery holding really well so far after 14 cycles. Energy density is 26 Wh/L. This is confirming that the elimination of Fe impurities increases both voltaic and energy efficiencies and improves battery stability.

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Very much enjoying reading this, thanks for sharing. I'm currently working on a recycled plastic case for a minimal architecture battery from my workshop waste, using old ABS from my edge banding machines and a mold. Also some old carbon and fibreglass sheets I have lying around may come in handy. I'm hoping a surfboard/racing car part manufacture background using carbons/epoxy/fillers/foams may pay dividends when it comes to the final layups!
Keep up the good work.

Thanks for your support! How are you planning to build the battery? What geometry, cathode/anode materials, electrolyte characteristics, etc? Are you going to cover the ABS in a carbon material to prevent its reaction with Bromine? Let me know :)
 
The test that was started in #203 continues. After 25 cycles the battery is still behaving fine. So far no signs of dendrites in the curves although there is some slight deterioration of the voltaic efficiency. I will continue this test till the battery either fails or loses 20% of its max capacity. So far the battery has lost around ~3%. A drop of stored charge to 10.8 mAh would imply a loss of 20%.

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Battery from #203, still going strong at 50 cycles. Deterioration has been really small, energy efficiency has dropped but now stabilized and Coulombic efficiency has remained in the 87-90% range. Stored charge has also remained pretty consistent.

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After cycling this battery 75 times, we start to see some important deterioration in the energy efficiency and stored charge. Voltaic efficiency has dropped quite substantially as well. Charging cycles are now taking the battery past the 2.1V mark, where there is expected to be substantial hydrogen evolution. This failure is however not due to dendrites but probably due to some phenomenon related with the TMPhABr, if the TMPhABr is decomposing with time, then it will not be a viable choice for a sequestering agent and we'll need to think about something else. I am tempted to open the battery now and examine how it has evolved.

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So a Zn-Br battery without a sequestering agent will deteriorate faster, basically because it is being over-charged more if you try to charge it to 15mAh since elemental Br diffusion is much more efficient. You can see the significant deterioration of the charge potential. It becomes significantly higher with time as the battery becomes electrolyte deficient at the cathode due to the constant escape of Br from it with each charge cycle. Most probably this battery should not be charged above 8mAh. You can see that the charges stored are way lower compared to the TMPhABr cell tested in #203.

This all shows why a minimal architecture Zn-Br design is limited to a low energy density of around 7Wh/L if you want to use the battery without significantly deteriorating it with time.

However the discharge potential is significantly more stable, which does show that there are some problems when using TMPhABr as a sequestering agent. With these two devices characterized I will now try a battery with a GFE-1 cathode pretreated with a 10% TBABr solution, to see how it compares to these two batteries.

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A battery made with a GFE-1 cathode pretreated with 10% TBABr (soaked and then air-dried as I do with TMPhABr) cannot even be charged to 15mAh due to its very high resistance. The very low solubility of the TBABr in the electrolyte causes the coating to be extremely inactive and basically an insulator over the cathode's conductive surfaces. This means that charging starts at 2V and goes to 2.9V at 5mAh, effectively making TBABr a bad choice as a sequestering agent in this configuration.
 
I'm going to be moving out of the US within the next two weeks, so I will sadly be unable to perform any experiments till February :) However I will be happy to answer anyone's questions about Zn-Br batteries or any of my previous research.
 
Hi All.

Wow, interesting article. I am also working on a static ZBr2 battery which will be used in a UPS at home as we have plenty of planned power outages in SA :( Still waiting for my KBr order before starting putting my theoretical knowlege into practical experiments.

In my online "research" I came across some interesting ideas that might help improve the DIY ZnBr2 cells.

1) According to a Chinese paper, adding a magnet to the anode side helps reduce dendrites as the zinc dendrites would have to counter the magnetic field. OK, crude method to help reduce dendrites... but might be helpfull to someone with lots of magnets .

2) In flow batteries a high velocity of electrolyte circulating around the anode helps reduce dendrites. As I see the use of a separator in this post, it might be of interest on a larger battery where one could circulate the electrolyte around the anode without disturbing the bromide from settling.

3) A 3:1 surface ratio between anode:cathode helps reduce dendrites. This came from a paper investigating anode/cathode ratio for flow batteries with same material anode and cathode. The confrimed theory is that the larger surface helps spread the zinc deposition on the anode across a larger surface making each spike less effective to grow. A 4:1 ratio did not have added benefits, thus 3:1 was considered an optimal ratio.
=> My battery design will thus include a split cathode in 3 independent "sectors" and only use 1 sector during charging whereas all 3 cathode sectors are used during discharge. In addition, should a dendrite reach that cathode sector the charging can be switched to the next cathode sector. In addition, (see 4) a targeted negative charge pulse can be used to break up the dendrites that reach the cathode.

4) Coming from the electronics side, the charge logic has been a main focus of my interest. Robert Murray mentioned an important keyword for charging the ZnBr2 batteries: electroplating. Dendrites & rough surfaces are a big issue in electroplating as well. To combat the rough surface during electroplating, the quality electroplating controllers use pulses instead of uninterrupted current supply and every x pulses (10th?) are negative. Simplified, the positive pulses accelerate the ions towards the Zn anode and during the pulse break the ions can float around & spread evenly at the anode surface instead of concentrate towards the closest dendrite spike. The negative pulse targets the dendrite itself as that is the path with the lease resistance between anode/cathode.
=> I think the effect of the charge controller for ZnBr2 batteries regarding dendrites are underestimated. My presumption is that following the electroplating charge logic will help reduce dendrites more than additives in the solution. That doesn't exclude combining charge controll and additives to even enhance the dendrite reduction.

Anyway, though I'd share this.

Mike
 
Anyway, though I'd share this.

Mike
Thanks for sharing! :) In my experimental research 1% Tween 20 was enough to largely prevent dendrites at a current density of 10mA/cm2. It is a pretty cheap additive, so you might want to give it a try in your setup when you start your research. I hope you will be able to post charge/discharge curves and efficiency Vs cycle plots so that we can discuss your devices quantitatively ?

Also if you're using KBr or NaBr and Zinc sulfate or other zinc salts, make sure you use hydrogen peroxide to precipitate Fe so that you can eliminate Fe impurities when you synthesize the ZnBr2, otherwise your batteries will face issues because of these impurities.
 
Fellas,

My name is Kevin and I'm glad to join the struggle here on this thread. I've been experimenting with ZnBr2 batteries for about 6 months. I have a math background but no formal chemistry. I have a lot to share but I'll try to summarize my key findings:

1) Internal Resistance: This is the main factor to be solved in my opinion as it connects with so many of the battery's parameters. My electrodes are similar to Robert Murray Smith's design ( I agree he shares almost no usable performance data to help decision making) and involves ironing CAPLINQ's Linqstat conductive plastic onto the back of 5mm PAN based graphite felt and then sealing a metal mesh onto the back in a similar fashion. The current collector is cut to be slightly smaller than the GF with a tab portion sticking out as an electrode attachment point. Prior to sealing the electrode, I wrap the exposed tab portion in black trash bag (HDPE) and seal this with the iron and baking paper, that way there can be a sealed overlap when I lay the exposed mesh on the graphite felt. Initially I was using 2 layers of CAPLINQs 500 ohm square material with stainless steel mesh and sealing the back with 4-5 layers. My internal resistance was extremely high and I could only get maybe 5ma/cm2 safely at the 2.0v charging cutoff. I upgraded to CAPLINQs 200ohm (the best they currently offer although they say 75 ohm is under development) (https://www.caplinq.com/linqstat-xv...ive-plastic-sheeting-xvcf-3.5s200-series.html). I use ONE layer of this plastic material on the graphite side, a 200 size copper mesh (more like copper fabric) and I heat seal this onto the graphite very nicely with an iron and a sheet of baking paper. The difference in performance was astounding.

Dan,

I would love to see what performance you get using a lower resistance electrode like the type I described above. If you are not using CAPLINQs 200 ohm plastic, I doubt you are optimized. They are currently making the best conductive plastic on the market (to my knowledge).

I would also try SHORTING your batteries when they start to fail to "reset" them and see if a strip cycle restores them to life. This is the benefit of using graphite felt as anode and cathode and allows the battery to be safely cycled to 0% SOC as a way of resetting the chemistry. (This is also a way mitigate the danger of the battery when it is time to dispose of any test cells, bring it down to 0% SOC and use baking soda to neutralize the remaining electrolyte)

My Recommendations for the group:
1) I think the electrode must be optimized against a plain control electrolyte of 2M ZnBr2 FIRST before we dive into the additives. An article titled "Research Progress of Zinc Bromine Flow Battery" (Hang Lin1, Tianyao Jiang1, Qingyang Sun1, Guangzhen Zhao1 and Junyou Shi1,2,*) mentions that the conductivity of ZnBr2 is HIGHEST at 2M which I believe is why many references involve morality around this amount. I think this is where we choose a separator or a spacer design as well. The MA-ZnBr2 Optimization article talks about 1.5cm being an optimal spacing.

2) After that, we find an economic bromine sequestering agent (BSA) that improves the chemistry. I am in possession of Tetrapropylammonium bromide (TPAB) Tetraethylammonium Bromide (TEAB)and Tetramethylammonium (TMAB) and will be testing all three along with a combination. If I can soak the GF in TPAB or TEAB and get as much of the lightest molecular weight option (TMAB) dissolved into solution, this could get us the maximum amount of bromine sequestration possible.
3) After a BSA is chosen, we can start playing with PEG 200... Tween.. Ethylene Glycol and conductive salts.

Only ideas! let me know what you guys think.
 
Thanks for posting ? Glad to see someone else joining!

I would love to see what performance you get using a lower resistance electrode like the type I described above. If you are not using CAPLINQs 200 ohm plastic, I doubt you are optimized. They are currently making the best conductive plastic on the market (to my knowledge).

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.
 
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