Tag Archives: TMPhABr

Zinc Bromine Batteries: Trying a configuration without a solid separator

During my quest to build and characterize a zinc-bromine battery, I have mainly focused on the use of non-woven fiberglass separators between carbon electrodes in order to hold the electrolyte in the device. This is because some of the research papers with the most promising results for these batteries use this type of separator. Having a solid separator makes the battery easier to construct and easily translates to the construction of both coin-cell and pouch devices, which are the two classic prototype devices used in the modern battery industry. Moreover, a solid separator substantially increases the mechanical stability of the cell and reduced the influence of gravity on the device, making it less susceptible to changes in orientation. The image below details the best structure I have achieved so far for this type of device.

Most successful structure so far for the batteries I have built in my Swagelok cell. Note the cell was never measured in any preferred orientation, the cell was placed horizontally for measuring.

However, one big problem of having a solid separator has been the existence of edge effects at the border of the separator. Batteries have grown Zinc dendrites around the edges of the fiberglass separator, despite my best efforts to attempt to avoid them. While using a 20% PEG-200 concentration and increasing the concentration of Zinc Bromide to 3M helped to largely eliminate dendrite formation, none of these or other modifications were able to fully prevent the problem.

Dendrites formed within a solid separator are quite problematic, because they cannot be fixed and lead to extremely limited battery lifetime. Even if the battery is fully shorted to try to eliminate these dendrites, the mechanical damage to the separator caused by the dendrite structure is permanent, unless more expensive self-healing separators are used. Furthermore, a dendrite can partially react with perbromide coming from the cathode and be cut “half-way” effectively leaving some Zinc stranded in the middle of the battery which is only going to be slowly removed by reactions with diffusing bromine or perbromides.

New battery cell structure I am trying. Note that the batteries are going to be tested in this exact orientation, as the lack of a solid separator means that bromine will accumulate in the bottom electrode, so this electrode has to be the cathode.

For the reasons highlighted above, I have decided to try separator-less batteries in order to see if these batteries can effectively avoid the dendrite problem while still retaining or improving on the coulombic and energy efficiency values I have achieved so far. To do this I have used a 2mm piece of PVC shrink tubing as a separator, cutting it so that it forms a 2mm x 35mm strip that can go around the internal diameter of my Swagelok cell (0.5 inch diameter) and prevent the upper electrode from contacting the bottom electrode. The space is then filled with electrolyte and the cell is closed, with the spacer preventing any additional compression of the cell.

The biggest advantage of this configuration is that it can be easily maintained, since the cell can be opened and the electrodes can be easily changed or cleaned. The lack of a solid spacer also means that any Zinc dendrites that form will either dissolve as they touch the lower electrode or fall and just discharge the battery, while they will never be “stranded” in the middle of the battery. Large amounts of dendrites might still lead to battery shorts, but given that I am using 20% PEG-200 and there are no longer any separator related edge effects, I hope this will stop being a huge problem.

Coulombic and energy efficiency as a function of the charge/discharge cycle for first battery under the above configuration
Charge/discharge curve for last cycle in the previous image. Charge/discharge is done at 10mA.

The first prototype battery put together in this configuration – 3M ZnBr2 + 20% PEG-200 solution – has been able to survive 6 charge/discharge cycles, charging to 10mAh and discharging to 0.5V, both at 10mA. The battery has an energy density of 21.6 Wh/L, given its current geometry (all battery components) but I am hopeful this energy density can be pushed closer to 40 Wh/L after I confirm that dendrite formation is not an issue under this electrolyte conditions. Previous batteries in a fiberglass separator were able to sustain 10-20 cycles under these conditions before showing important shorting issues due to dendrite formation, so we will see how far this prototype battery can go before disaster strikes.

Zinc Bromine Batteries: Trying to improve energy efficiency above 80%

My experiments using carbon cloth cathodes have helped me construct some decent static Zinc-Bromine batteries. In particular, the CC4 carbon cathodes have been very flexible and have been used throughout most of my experiments. The last experiment I did, with a CC4 cathode previously soaked in a 50% solution of TMPhABr solution and then air dried, have shown a CE=91% with an EE=70% at a charge/discharge current of 5mA, charging to 3000 uAh and discharging to 0.5V.

Coulombic and energy efficiencies as a function of the number of cycles for a pre-treated CC4 cathode (soaked in 50% TMPhABr and air dried) with a 3M solution ZnBr2. Charged to 3000uAh at 5mA, discharged to 0.5V.

Charge/discharge curves for all the cycles in the first figure.

Coupling these cathodes with a ZnBr2 3M solution with 10% PEG200 has allowed me to achieve specific power values in the region of 30-40 Wh/kg – total weight of cell – with more than 40 charge/discharge cycles (see above), without the formation of any Zinc dendrites (which would short batteries after only 10-20 cycles at this charge density in previous battery tests). Higher PEG200 concentrations cause significant increases in the internal resistance of the cell while lower concentrations (<5%) are just not effective at preventing Zinc dendrite formation when using metallic Zinc anodes.

Despite the good results, I have yet to achieve high energy efficiencies, mainly due to a couple of problems. The first is that significant bromine diffusion is happening due to a lot of bromine being formed at the surface of the CC4 cathode without enough presence of TMPhABr to capture it and the second, that the internal resistance of the cell was still significantly high, owing to the significant resistance of the CC4 cathode being used.

First charge/discharge curve for a GFE-1 felt electrode, pre-treated with 10% TMPhABr. Charged/discharged at 5mA, charged to 3000uAh, discharged to 0.5V.

In order to attempt to solve these problems, I have decided to change to a carbon cathode that is both significantly more conductive and possesses a significantly higher surface area compared to CC4. My choice material being the GFE-1 carbon felt. For the first test I have soaked a piece of cathode in 10% TMPhABr and air-dried it before use.

You can see the first charge/discharge curve ever produced in this configuration above. The charging potential is already significantly lower than that of the CC4 electrode and the discharge potential significantly higher, both signs of a markedly lower internal resistance. For this first cycle the Coulombic efficiency was 79% while the energy efficiency was 72%. We’ll see if the CE and EE of this battery improves as its cycled and whether or not this cathode leads to more stable cycling than the CC4 cloth electrodes!

Zinc Bromine Batteries: Solid TMPhABr layers are not the answer

My latest efforts to build higher capacity Zinc-Bromine batteries, have focused on the use of solid TMPhABr layers, because the solubility of TMPhABr is very low in the presence of high concentrations of ZnBr2 (2-4M). The idea by doing this was to provide a relatively stable source of TMPhA+ cations that could be taken to the cathode and be used to form an insoluble perbromide as bromide is reduced to elemental bromine and then sequestered by the quaternary ammonium salt.

Evident formation of perbromide oustide the cathode material due to movement of elemental Bromine to the TMPhABr solid layer.

However, the solubility of TMPhABr is too low for this and what happens is that the cathode mainly generates elemental Bromine, which then flows through the battery and is converted – outside the cathode – into TMPhABr3 as it reaches the TMPhABr solid layers. What happens is that the perbromide is fixed outside the cathode, and only the portion that is in contact with the cathode is ever able to be reduced to contribute to the battery current during the discharge phase while the part that is far away from the cathode becomes “dead capacity” and is never able to be regenerated again.

This is evident by looking at disassembled batteries – see image above – where the yellow/orange perbromide is present across the battery separator, showing that elemental bromine was produced, migrated, reacted with the organic ammonium salt to form the perbromide and was then unable to be recovered because of its distance from the cathode. This is also showed by the loss in both energy and Coulombic efficiencies for batteries that use this solid layer at higher ZnBr2 concentrations, compared with the cells that used fully dissolved ZnBr2 0.5M + TMPhABr 0.25M. The Coulombic efficiency drops from >95% to <80% while the energy efficiency drops from >80% to <70%.

New cell structure proposed, using a cathode material that has been soaked in a 50% w/v solution of TMPhABr.

The best way to implement this solid TMPhABr strategy might actually be to introduce this solid within the structure of the cathode material (see proposed structure above). For this I have prepared a 50% w/v solution of TMPhABr (it is extremely soluble in distilled water), immersed two CC4 cathodes into it and I am now waiting for these to dry. Once they are dry I will be able to place them within batteries and run an experiment – without any solid layer – to see if this actually improves the results.

Zinc Bromine Batteries: Problems at higher capacities with TMPhABr

As you saw on my previous post, I was able to generate pretty decent results with TMPhABr when using Zinc Bromide solutions at 0.5M with an addition of 0.25M of this quaternary ammonium salt. However it is pretty clear that at this concentration of Zinc Bromide the specific energy is too low, so I subsequently tried to reach higher efficiencies by trying higher concentrations of Zinc Bromide with a solid layer of TMPhABr (since at >0.5M of ZnBr2 the solubility drops too much). My experiments were done with the cell configuration showed below. The electrolyte also contained 1% of PEG-200 in order to prevent dendrite formation.

Battery structure for tests shown below.
Charge/discharge curves charging to 2000 uAh at 2mA and discharging to 0.5V at this same current. Last value was CE=86.41% and an EE=68.74%. This electrolyte contained a 2M solution of Zinc Bromide.

These experiments were quite successful, with a Coulombic efficiency of 86.41% and an energy efficiency of 68.74%. The capacity of these devices was increased by 4x over my previous experiments at 0.5M of ZnBr2 showing that the solid layer of TMPhABr does work in order to generate insoluble perbromides within the battery. However the battery performance did start to degrade at around the 10th cycle, so I stopped cycling the above battery to see if I could get better behavior at even higher Zinc Bromide concentrations since the increase in ZnBr2 concentration did show a reduction in the internal resistance of the battery.

Charge/discharge curves for a 3M Zinc Bromide electrolye, where an attempt was made to charge to 5000 uAh and discharge to 0.5V at a current density of 5mA. Highest CE=74.71%, EE=55.76%

The attempt to use higher concentrations at higher current densities were not very successful. Although the capacity was increased to around 10x of my initial battery, the problem was that both the Coulombic and energy efficiencies dropped to unacceptable levels. The charging voltage also saw substantial climbs – reaching almost 2V – which probably created a lot of unwanted reactions. The worst problem was however the zinc dendrite formation, which became apparent after I tried cycles at lower capacity and current density for the same cell. You can see below that at the fourth cycle the charge voltage drops suddenly and then the discharge is extremely inefficient. This is because dendrites have pierced the separator effectively shorting the battery.

Curves where I attempted to charge to 2000 uAh and discharge to 0.5V at 2 mA.

This dendrite issue is one of the most important problems in Zinc-Bromine batteries – both flow and static – and one of the reasons why rechargeable Zinc chemistries have not been massively adopted thus far. If the above batteries are to be practical, I need to find a setup that provides both high capacity – which means a 3M ZnBr2 electrolyte – with the elimination of Zinc dendrites. The addition of PEG-200 helps, but it is clearly not enough to eliminate this issue. Upon opening the above battery, it was evident that dendrites had completely pierced through the entire separator and shorted the electrodes.

One hypothesis I have is that local formation of Zinc dendrites should be hindered by high local TMPhABr concentrations (since they do not form when high amounts of this are dissolved) so a potential solution is to create another solid layer of the TMPhABr next to the Zinc anode (as shown below). I am currently testing the battery configuration shown below to evaluate this hypothesis.

Current testing configuration to attempt to remove Zinc dendrites by a much higher local concentration of TMPhABr close to the Zn anode.
Curve for the above cell charged to 3000 uAh and discharged to 0.5V at 2mA. CE=76.51%, EE=61.01%

Another issue that has been pointed out to be is the absence of additional support electrolyte, so I am planning to test ammonium sulfate at 2M to see how this modifies the behavior of my batteries at these higher capacities. Ammonium ions will turn my battery more acidic, so I am expecting some losses in Coulombic efficiency at higher current densities from a more favorable hydrogen evolution potential.

Zinc Bromine Batteries: Going for high capacity with TMPhABr

The initial tests using TMPhABr have been a complete success. A battery made with 0.5M ZnBr2 + 0.25M TMPhABr charged to 500 uAh and discharged to 0.5V was able to achieve stability past 100 charge/discharge cycles at 2mA and more than 100 charge/discharge cycles at 5mA. There was a significant drop in energy efficiency when going to higher current densities (from 75% at 1mA to 66% at 2mA) but overall the Coulombic efficiency remained high through the entire testing, at values greater than 90% and in some cycles greater than 95%. This was also all using a CC4 carbon cloth cathode, which means I made no effort to optimize the cathode at all. The cell showed a difference of around 50mg between the dry state and discharged wet state, meaning that overall it contained around 30-40uL of solution (I haven’t measured the density of the ZnBr2+TMPhABr so I don’t have an exact answer).

RE: My adventures building a Zinc-Bromine battery
100 charge/discharge cycles at 2mA. Charged to 500 uAh and discharged to 0.5V.
70 charge/discharge cycles at 5mA. Charged to 500 uAh and discharged to 0.5V.

These results are extremely encouraging because they show that the TMPhABr is a way better behaved sequestering agent for bromide relative to TBABr. Most notably the tests also show a lack of performance degradation from Zinc dendrite formation, which was a big problem in the TBABr experiments. The charge/discharge curves are also significantly better behaved with a much longer and more stable “discharge plateau” which implies more stable electrochemical performance. There is also a complete absence of rare shoulders or spikes in the curve, which hint that important additional electrochemical processes are absent.

The CE and EE of the cell are always significantly lower when running the first few cycles, indicating that the formation of some surfaces or species is necessary for the cell to reach peak performance. This is likely due to the need for TMPhABr3-friendly sites to form, as the Br oxidation side is expected to be the rate limited process in this type of device. Since I’m using a Zinc anode, the formation of Zn nucleation sites is not expected to be significantly difficult.

A sample charge/discharge curve measured at 5mA. Notice the long discharge voltage plateau.

The biggest issue right now is that a cell like the above has a really low specific energy (around 2.8 Wh/kg), so a very substantial increase is required to make the above cell viable. I suggested some modifications in one of my last posts but it is clear that a cell with a ZnBr2 concentration lower than 2M is simply not going to be able to provide an adequate density. Given the solubility limitations of TMPhABr, we are unlikely to be able to achieve this using just a mixed solution of this sequestering agent and Zinc bromide.

My idea to solve this problem is to include a layer of solid sequestering agent in the battery and use a saturated solution of TMPhABr in 2M ZnBr2 as an electrolyte. The TMPhABr won’t be dissolved right away, but it will be slowly transported by the Zinc Bromide solution as TMPhABr3 is deposited in the cathode of the cell. Hopefully the process reverses when the cell is discharged and we’re able to get a cell that can successfully charge/discharge at high densities without the need for all the TMPhABr to remain in solution.

Suggested cell structure using a starting solid layer of sequestering agent

I expect that a cell like this will have way longer stabilization time – as the TMPhABr migrates through the cell and forms a stable structure in the cathode, hopefully without dramatically hindering its functionality. I also hope that the much higher ZnBr2 concentration won’t increase the formation of Zn dendrites or that the formation of these dendrites will be curtailed by the presence of a TMPhABr solid layer at some point.

The above cell design is now in testing, so we should see if we can achieve charge/discharge cycles to 2000 uAh!

Zinc Bromine Batteries: First tests using TMPhABr

As I’ve mentioned in previous posts, tetrabutylammonium bromide (TBABr) is not a very good sequestering agent for static Zn-Br batteries due to its very low solubility in Zinc Bromide solutions. To solve this problem, I have decided to test trimethylphenylammonium bromide (TMPhABr) as a potential replacement, since this salt also forms and insoluble perbromide but – due to its significantly higher polarity and lower molecular weight – should be significantly more soluble than TBABr. I ordered it from Alibaba around one week ago and recently got it delivered.

Picture of the TMPhABr I got from China

My initial tests with it involved testing its solubility in Zinc Bromide solutions. The solubility of TMPhABr in pure water is not indicated clearly anywhere, but I assumed its solubility would be similar to that of trimethylbenzylammonium bromide (TMBABr) or tetrapropylammonium (TPABr) bromide, both which have solubilities of around 10% by mass in water at 25C. My initial tests have confirmed this suspicion with solutions at 10% by mass being easy to prepare at 20-25C. I didn’t try to prepare more concentrated pure solutions as my objective is to judge its solubility in the presence of Zinc Bromide.

The first test I performed to evaluate this was a 0.25M solution of Zinc Bromide which was able to dissolve 0.12M of TMPhABr with no problems. I then increased the amount of ZnBr2 to 0.5M – which is what the authors of the Chinese paper using ZnBr2+TPABr use – and I was able to dissolve 0.25M of TMPhABr without issues. With this result I know I will be able to at least reproduce similar experimental conditions to those achieved by the Chinese researchers, something that I could never do with TBABr due to the solubility issues mentioned before.

To test how far I could take this I then attempted to prepare a 1M solution of Zinc Bromide and see if I could get 1M of TMPhAbr to go with it. Sadly at this point the concentration of TMPhABr is already too high – would be close to 10% by weight of the solution – so it was actually not possible to get to this point. This means that the practical limit of this battery will be to have around 0.25M of TMPhABr dissolved, which is probably a realistic limit for most quaternary ammonium salts since we are unlikely to get an effective sequestering agent – not electrochemically active and with no effect on pH – with a molar mass significantly lower than that of TMPhABr at a similar price point.

First two charge/discharge curves measured (at 2mA constant current). Battery was charged to 500 uAh and then discharged to 0.5V. First curve, CE=68%, EE=57%. Second curve, CE=79%, EE = 66%.

I then used this 0.5M ZnBr2 + 0.25M TMPhABr solution to create the first battery. This battery had a diameter of 0.5 inches and was built within my Swagelok cell. I used a 0.2mm thick Zinc anode followed by 8 layers of fiberglass separator and a CC4 carbon electrode. I also made sure to sand the graphite electrodes in the Swagelok cell to make sure their exposed surface was pristine. I put 50uL of the electrolyte on the cell but I won’t know how much ended up in the separator until I open the cell after testing and weight the wet components.

The graph above shows the first – to the best of my knowledge, the first ever public – charge/discharge curves of a static Zn-Br cell prepared using TMPhABr as a sequestering agent. It is very interesting to note that the shape of the discharge curve improved immensely moving from TBABr, showing that this battery is significantly better behaved. Although the CE and EE of this first curve were particularly low, the CE of the second curve measured already showed an increase of the CE to 79% and EE 66%. I will keep cycling the battery and will show you how the CE and EE change as a function of the number of cycles. Exciting times!