Zinc Bromine Batteries: About my Swagelok cell for small scale battery testing

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In order to properly research batteries at a small scale, it is necessary to have setups that have a reproducible geometry and that can be put together and taken apart many times, in the exact same way. Through all the research published in this blog, I have been using Swagelok cells to achieve this goal. In today’s post I will be talking about my Swagelok cell and how I use it to put together my batteries.

The Swagelok cell model I have used for my tests.

My Swagelok cell – showed above – is just a piece of PTFE pipe with an internal diameter of 0.5 inches, that has been threaded externally in order to enable the screwing of two stainless steel caps, which are the contact points that allow the connection between the internal electrodes and the external testing equipment.

Besides this external stainless steel caps, the Swagelok cell comes with some internal stainless steel electrodes, that you can use to contact the cathode and anode materials of the battery, if the chemistry allows for this. However, given the corrosive nature of elemental bromine, I use 0.5 inch graphite rods that serve as carbon electrodes. The rods have been rounded on one side – to make a nice contact with the stainless steel caps – and they are flat on the other side to fully contact the cathode and anode electrodes.

Vertical cross-section of the Swagelok cell once assembled with a separator-less inverted configuration.

The image above shows you a diagram of a vertical cross section of the Swagelok cell when it is fully assembled with a separator-less inverted Zn-Br battery configuration. As you can see, the bottom graphite electrode is wrapped heavily in PTFE tape, in order to provide a water-tight seal, while the top electrode is only wrapped once, in order to insulate everything but the tip of the electrode from making contact with solution, allowing for an air gap to allow any excess solution to pool after compression.

As you can see, the battery comprises a small space in the middle of the Swagelok cell. The space for the electrolyte is provided by the use of PTFE o-ring spacers and when the cell is closed the space has a perfectly defined volume, since the rings do not compress. When a separator containing setup is used, these o-ring spacers are replaced by layers of non-woven fiberglass separator, which is the other material I have tried. In an inverted configuration the anode is placed at the bottom, while in a normal configuration the anode is placed on top.

Given how the battery is configured, it is easy to take it apart and put together a new battery with brand new materials to either repeat and experiment or perform an entirely new experiment with the exact same geometry. Geometry and mass are very important aspects of battery research – as they determine specific power and energy – so being able to do experiments where these two variables can be guaranteed to be as reproducible as possible is conducive to better results.

Zinc Bromine Batteries: What happens if you don’t use a sequestering agent?

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Almost all of my efforts in the construction of Zn-Br batteries have focused on the used of sequestering agents in order to enhance the performance of the batteries and obtain higher energy densities and lower self-discharge rates. However, a couple of people have asked me what the results of a “minimal architecture” Zinc-Bromine battery would be with my current battery design. I therefore put together a battery using a 0.2mm Zn anode, 15 layers of fiberglass separator, a GFE-1 cathode without any pretreatment and a 2.7M ZnBr2 electrolyte with 1% Tween20 (which is needed to prevent dendrite formation).

The results, shown below, illustrate the problem of trying to create a Zn-Br battery without a sequestering agent. The lack of a sequestering agent means that the formed bromine is easily able to exit the cathode material and go into the separator, migrating towards the anode. The cell’s behavior is similar to my batteries with sequestering agents, as during the first few cycles the cathode losses a lot of bromine to the media due to the lack of any oxidized bromine species in the separator and therefore starts at a lower Coulombic and Energy efficiency.

Resulting first 15 charge/discharge curves for a Zn-Br battery containing no sequestering agents, charged to 15mAh and discharged to 0.5V, both at a rate of 15mA.

However, as the battery progresses, the Zn-Br battery assembled without a sequestering agent stabilizes at a CE of around 60% and an EE of around 45%, while with a sequestering agent, these values go up all the way to 90% and 65%. It is therefore evident that the sequestering agent does a good job of keeping the formed bromine in the cathode material, while the lack of a sequestering agent makes the battery significantly less efficient.

The energy density also changes quite dramatically, with the sequestering agent battery reaching around 25Wh/L and this design without one reaching only close to 17Wh/L. The use of a sequestering agent improves almost all aspects of the battery, except perhaps the stability of the battery which is lower if the sequestering agent reacts in any way with the electrodes or the bromine as a function of time.

Evolution of charge and discharge potentials for a battery with no sequestering agent.

I am going to continue cycling this battery in order to see if it reaches the same stability limits as my other devices or if it is able to run significantly longer. Batteries containing sequestering agents and charged to 15mAh have shown to deteriorate substantially at around 60-70 cycles, particularly batteries using TMPhABr as the sequestering agent. If these batteries without the sequestering agent are more stable then the stability issues of my designs using a sequestering agent could be assigned to chemical instabilities of this agent within the electrochemical environment.

Zinc Bromine Batteries: Proposing the next scaling level, a petri dish battery

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My experiments with Zn-Br batteries in Swagelok cells have now matured to the point where a relatively stable battery has been achieved with no large dendrite problems and acceptable Coulombic and Energy efficiency values (~85% and ~65% respectively). I believe these results would already allow some of you – who want to explore larger scales – to construct some prototypes with my exact configuration, but at a larger scale. In this post I talk about a larger scale setup constructed using a petri dish.

The building material is a difficult choice, since Zn-Br batteries form pretty corrosive chemical species, in particular elemental Bromine and perbromide salts. In order to ensure stability, the cheapest and most easily available material that resists Bromine is glass. The cheapest casing to build a relatively small scale, yet modular and practical battery that fits the geometry of my design would be a petri dish, you can get a 100mm diameter petri dish here, which you could use to put together the battery I’ll be describing.

Suggested geometry for the petri dish cell.

Knowing the area and height of the cell (100mm x 18mm) we can calculate the energy density and charge capacity you should get. Given my previous research, this should be around 2Ah of stored charge with an energy stored of around 2.85Wh. To obtain this ratios you would charge the cell at a constant current of 1A to 2Ah, which matches a current density of around 11.8mA/cm2 in my Swagelok cells, charging to 15mAh at 15mA.

For the anode, you will want to get 0.2mm Zn metal, which you can obtain here. For the cathode you will want to get GFE-1 graphitic felt, which you can buy here. Note that this is NOT a regular carbon felt, it is a highly activated and conductive felt. For the separator a non-woven Fiber glass tissue will do the trick, like this one. The GFE-1 cathode will need to be soaked in a 10% solution of Trimethylphenylammonium bromide (TMPhABr, CAS 16056-11-4) and then air dried. I bought my TMPhABr from alibaba. Using Tetrabutylammonium bromide (TBABr) gives significantly worse results because of the much higher molar mass and lower solubility of the salt in Zinc Bromide solutions. As a current collector for the cathode and to connect it to the outside I suggest you use graphite foil, like this one. For the Zn anode make sure you leave a strip of anode when you cut the piece so that you can make the connection to the outside, cover all the metal that will go through the cell in some non-reactive insultation, electrical tape should work well enough for your first tests so that it doesn’t short with the cathode material.

Geometry of suggested components for petri dish battery

For the electrolyte you can either use Zinc Bromide at 2.7M or a solution of this concentration prepared using Sodium Bromide and Zinc Sulfate Monohydrate. I suggest you browse some of my previous posts on solution preparation if you’re interested in using these sometimes cheaper and more widely available chemicals. However, in both cases you will need to prepare the solution with 3% hydrogen peroxide instead of distilled water, to ensure all Fe is precipitated and removed from the solution before putting it into the battery (Fe impurities cause huge problems in the cells). After preparing the electrolyte, leaving it 24 hours to react and then filtering, drop some strips of Zinc foil in it so that all the peroxide reacts, then use peroxide measuring strips before loading it into the cell to ensure all hydrogen peroxide has reacted. After a couple of days all peroxide should be gone. Also, make sure you also add 1% Tween 20 to the solution, this is a key component to improve wetting and prevent dendrite formation.

To put the cell together, place the Zn foil at the bottom of the petri dish, then cut as many layer of fiberglass separator so that once you put the GFE-1 cathode on top and close the lid you’re able to lightly compress the cell. Then the easiest thing is to take out the separators and cathode, soak them in electrolyte and put them back in the cell. Now you should be able to close the lid, lightly compress it, clean any spillover and seal the petri dish using a couple of rounds of electrical tape.

I am also very aware that Bromine is denser and will want to sink into the cell, most of it won’t though as it will be sequestered as perbromide within the carbon electrode. There are very good reasons why this cell has the cathode at the top and why a normal geometry just does not work well for long term cycling at these energy densities. Please read this post to better understand why a practical static Zn-Br battery actually requires an inverted geometry to work well.

Zinc Bromine Batteries: Iron impurities in Zinc Bromide solutions derived from Zinc Sulfate

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An important issue with Zinc-Bromine batteries is the need for a high purity Zinc Bromide solution in order for the battery to work properly. The use of low-cost ZnBr2 sources, in particular the use of Zinc Sulfate and Sodium Bromide to generate Zinc Bromide solutions, can be problematic due to the presence of large amounts of Iron impurities. In this post I will discuss what happens when you have these impurities in a battery and how you can purify a Zinc Bromide solution to remove iron and achieve better results.

Layers of a Zn-Br battery using a 4.2M ZnBr2 electrolyte derived from the reaction of commercial grade Zinc Sulfate monohydrate and Sodium Bromide. The battery was taken apart after 15 charge/discharge cycles at an energy density of around 20 Wh/L.

The image above shows you the layers of a Zn-Br battery that was taken apart after 15 cycles, charging the cell to 15mAh at 15mA and discharging to 0.5V, the cell has a diameter of half an inch. As you can see, the fiberglass separator layers look very yellow and, although it would be tempting to assign this color to the presence of elemental Bromine, which is true in part, cells constructed with high purity ZnBr2 solutions do not show such a strong yellow coloring. The yellow color is in large part caused by the oxidation of Fe+2 to Fe+3.

Moreover, the Zinc anode is usually a grey or black color in batteries created with high purity ZnBr2 solutions, while in this case, it looks a lot like a rusted piece of Iron. This is no coincidence, as Fe is actually reduced in the anode and then oxidized to insoluble Fe oxide/hydroxide species. The first two layers of the battery show the strong presence of these red Fe+3 oxides, which are non-conductive and significantly hamper the performance of the battery.

Battery performance characteristics for the battery that was dismantled and showed above.

The characteristics of this inverted battery were actually not bad to begin with, achieving max CE and EE values of 81% and 61% respectively. However the discharge curves start showing significant deterioration around the 10th cycle, with strong drops in the discharge voltage around 3-4 mAh into the discharge cycles. This was also shown as a strong decay in the average discharge potential, which during the 15th cycle was around 6% lower than the starting value. This speed of decay is dramatically faster than for Zn-Br batteries created using high purity ZnBr2 solutions.

The above implies that you need to get rid of these Fe impurities if you expect to be able to run a Zn-Br battery that lasts for a long amount of time. Thankfully there is a pretty easy way to do this, which we know from the literature surrounding the purification of Zinc brines (see here). The trick is to use a 3% hydrogen peroxide solution for the preparation of your ZnBr2 solution, instead of distilled water, when you dissolve the Zinc Sulfate and Sodium Bromide salts. This causes all the Fe to be oxidized to Fe+3 and to precipitate out of solution.

A reaction containing 62g of NaBr, 52g of ZnSO4.H2O and around 50mL of a 3% hydrogen peroxide solution. You might want to add around 50-100mL of distilled water before filtering to make your processing easier.

The image above shows you how this reaction looks. The Fe compound generated is very red, while the precipitated sodium sulfate is white, giving you this sort of look. The reaction takes around 6 hours to fully complete – for all the Fe to be precipitated out – time after which you can filter the solution and measure the density of your resulting brine to figure out what approximate concentration you have (per my previous post). It’s crazy how much Iron the “high purity” agricultural grade Zinc Sulfate Monohydrate contains!

It is also worth noting that this ZnBr2 solution will contain a significant amount of peroxide so you either need to heat the solution up to fully decompose the peroxide or wait till all the peroxide is decomposed at room temperature before actually using the solution in a battery. To be absolutely sure, you can also use H2O2 testing scripts (like these), to figure out whether you have any peroxide left before considering a solution to be battery-ready.

With that said, this process is absolutely necessary to build Zn-Br batteries if you’re deriving your Zinc Bromide from an impure source, as a Zn-Br battery containing large amounts of Fe will degrade and stop working pretty fast as a function of time. If you want to test your ZnBr2 solution you can add a couple of drops of H2O2 to a sample of it and see if any Fe precipitate forms.

Zinc Bromine Batteries: Is there anything that can work just like propionitrile?

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Trying to improve on Zinc-Bromine batteries, it seems that the use of non-aqueous solvents in order to better sequester the bromine would be ideal since successfully sequestering the bromine in a static battery configuration is hard, even when ammonium salts are used as sequestering agents. This is because the salts are largely insoluble in ZnBr2 solutions, so the sequestering agent needs to be included as a solid, which can only be done in very limited amounts without strongly degrading battery performance. This is what I have tried to do with TMPhABr saturated GFE-1 cathodes, but the amount of Bromine I can sequester before it leaks out into the rest of the cell is quite limited. This is evident when I take apart fully charged cells and the entire cell is tinted with a strong orange color.

Ideally we would want to have a phase that is separate from the aqueous ZnBr2 phase but that has a higher affinity for bromine, is able to dissolve a significant amount of ZnBr2 (>1.5M) and is non-toxic. Propionitrile has been shown to be a decent candidate to achieve this, see here, but it is not an ideal candidate for a DIY solution because it is hard to get in most cases and can be a significant hazard for human health. It is catalogued as a dangerous chemical for transport in the US and EU and its use therefore poses a lot of additional restrictions and problems.

Most polar solvents that can dissolve ZnBr2 to some extent and are electrochemically stable – meaning they won’t get oxidized at the cathode – are sadly also miscible with water. This means that they don’t form a separate phase, which is an absolute requirement. I tried using propylene carbonate mixed with tetrabutylammonium bromide (TBABr) – which does not mix with ZnBr2 solutions – but this was a complete failure due to the fact that the bromine and perbromides formed actually prefer the ZnBr2 phase to this organic phase, even at lower concentrations of TBABr.

2-Methyltetrahydrofuran BioRenewable, anhydrous, >= 99 %, Inhibitor-free | 96-47-9 | Sigma-Aldrich
Me-THF, a new organic solvent candidate I will be trying in Zn-Br batteries to serve as a cathode side electrolyte and sequestering media.

I then continued my search for the “perfect solvent” for this, reviewing a lot of the literature on non-aqueous solvents in batteries and on “green chemistry” to find the solvent with the perfect polarity, ability to dissolve Zn-Br, affinity for Bromine, immiscibility with water and low toxicity. I think I might have found a solution in the form of 2-methyltetrahydrofuran or Me-THF. This solvent is a highly polar ether, it can dissolve high amounts of ZnBr2, it has a high affinity for elemental Br, it’s significantly less toxic than propionitrile and, alike propionitrile, it does not mix with water or ZnBr2 solutions. It is also electrochemically stable and is also safe enough that it’s available for purchase on ebay/amazon, so I decided to buy some of it to do some basic experiments and see if this solvent finally provides the answer we have been looking for,

Interestingly the density of Me-THF is quite low – around 80-90% that of water – so it will naturally rest on top of it. This means that it is naturally suited for an inverted cell configuration (cathode on top, anode on the bottom), which is what we want (see here). I couldn’t find what the partition coefficient with ZnBr2 solutions in water is, but the hope is that we should be able to get enough ZnBr2 in the ether to have a successful battery starting with a 3M solution of ZnBr2 in water. Batteries with Me-THF likely won’t require the use of any ammonium salts – as in the propionitrile paper mentioned above – since the affinity of the Br for Me-THF is high enough to keep it away from the aqueous phase.

Alike with PC, it is hard to know what to expect, so I would advice waiting for some of my results before going out to buy this solvent. Who knows how it will work in practice!