Tag Archives: zinc bromine

Zinc Bromine Batteries: Initial thoughts about a practical battery

During this past week I have been experimenting and thinking more about Zn-Br batteries and how a real practical battery would look like (how it would be built and what its characteristics would be like). Let’s imagine we have found a complexing agent that can be used in highly concentrated ZnBr2 solutions. What would a prototype battery look like and how much would it cost?

The first thing we need to consider is the geometry to build such a battery. Single-cell batteries for Zn-Br chemistry are impractical due to the limits that would impose on current density – and it’s not the 19th century – so the ideal battery would probably follow a configuration similar to modern lead-acid batteries, where multiple cells are put together to achieve better results. The simplest way to do this is to stack materials next to each other within a box, then flood the box with the desired electrolyte solution.

Proposed stacking of layers for a battery built in a 101x54x55mm project box. Note that the cells are laid horizontally (left to right) . Fiberglass separator thickness should be increased so all contents fit tightly inside the box.

In a 101mm x 54mm x 44mm project box you could fit a volume of around 237mL. If we decide to use a very porous carbon felt electrode – which I have experience with – with titanium current collectors, glass fiber separators and zinc anodes, we would create a cell configuration like the one showed above. This would occupy the entire cell with either separator, current collector, anode or cathode material. Given that the materials used take little real volume, as they are either very porous or very thin, I’m going to assume the solution volume we will fit will be equal to 200mL, which is realistic given the characteristics of the materials.

If we use a 2M ZnBr2 solution, this would give a maximum theoretical energy of 40Wh. If the cells are all connected in parallel, this would give us a battery with a voltage of 1.85 at a capacity of 21.6 Ah. The battery would be charged at a constant current of around 2.85A, although depending on the actual kinetics this might need to go down to even 285mA. In the above design you actually have only 6 cells that are each equal to 2 normal cells connected in parallel – as they share a current collector in the cathode – so connecting these 6 in series would give you a voltage of 11.1V with an expected charging current of 475mA.

The main caveat of the above design is that it uses a 2M ZnBr2 solution, assuming we can find a complexing agent that forms an insoluble perbromide that can be in the initial formulation at a concentration equal to at least the same as the ZnBr2 then this should be no problem. After a lot of research about the solubility of perbromides and organic ammonium salts I believe this might be possible using trimethylphenylammonium bromide, but such a complexing agent has never been tried! The 200 mL of solution used here would use 90.07g of ZnBr2 and 86.45g of TMPB.

Note that this configuration would certainly not work without a complexing agent that precipitates the tribromide formed. Without it the bromine would pool at the bottom and discharge the cell – in a horizontal configuration – or just sink and discharge the cell in a vertical configuration.

Cost (USD)ItemURL
3Project boxhttps://www.allelectronics.com/item/mb-132/abs-project-box-3.97-x-2.12-x-1.72/1.html
45Carbon felthttps://www.ceramaterials.com/product/gfe-1-pan-graphite-felt/
10.99Fiberglass tissuehttps://www.amazon.com/Multipurpose-Fiberglass-reinforcing-waterproofing-membranes/dp/B0719KWMJ7/ref=sr_1_5?dchild=1&keywords=fiberglass+paper&qid=1600038562&sr=8-5
11.49High purity Znhttps://www.amazon.com/99-9-Sheet-Plate-Metal-140x140mm/dp/B086FGDW83/ref=sr_1_5?dchild=1&keywords=Zn+sheet&qid=1600038696&sr=8-5
9.01Zinc BromidePrice with shipping confirmed from Alibaba vendor
19.99Titanium foilhttps://www.amazon.com/0-3mm-200mm-300mm-Titanium-Purity/dp/B07G8YYPFV/ref=sr_1_2?dchild=1&keywords=titanium+foil&qid=1600040029&sr=8-2
18.85TMPBPrice with shipping confirmed from Alibaba vendor
Potential materials used to construct a prototype Zn-Br cell

Using all the materials above, the cost of building such a prototype would be in the order of probably 120 USD. Probably around 200 USD after you add shipping for everything. In reality this cell is also unlikely to yield 40Wh and will most likely be in the vicinity of 20Wh if everything works as expected.

It is also important to note that an ABS project box like the one above is a risky first-choice, given that ABS can adversely react with elemental bromine, so a PTFE project box would – although much more expensive – be a safer choice for a prototype. By the time I build something like this, I hope I have already established that TMPB forms insoluble enough perbromide salts under my much more controlled Swagelok cell conditions.

Note that I am still far away from executing something like this! Currently I am even far away from testing a TMPB cell, but I wanted to write this blog post to condense all this theoretical research and serve as a referring point for me or others in the future.

Zinc Bromine Batteries: Current battery and experiments to follow

This week I published a post about my first success in the making of a Zinc Bromine battery, this first battery had a Coulombic efficiency of at least 96% and was able to show the expected charge/discharge curves, which I hadn’t been able to see before. In this post I want to talk about some of the problems I have found and the experiments that will follow to attempt to fix them.

Current structure of my battery. The cell also includes around 80-100uL of a 0.5M ZnBr2+0.2M TBAB solution.

The structure of my current battery is shown above. The first problem I have run into are side reactions due to my use of copper tape as the anode used for zinc plating in the batteries. When I discharge the battery I seem to inevitably get some Cu oxidized and into solution, which is affecting the chemistry of the battery as a function of time. This means that I am losing a lot of coulombic efficiency and my charge/discharge curves are starting to show unwanted side reactions. I will be trying to replace this copper tape anode with a conductive HDPE covering plus a zinc anode to prevent any of these side effects.

The second problem comes from the use of a conductive carbon felt cathode that is pretty heavy (500mg per electrode used in the Swagelok cell) which means that my specific capacity is currently in the 0.5-1 mAh/g of cathode material, when ideally I should be seeing specific capacities in the order of 100-500mAh/g. The battery is already very efficient at using the electrolyte though as the maximum theoretical capacity of it is in the 0.01mAh/uL, given how much zinc and TBAB there is inside of it.

I have ordered an assortment of carbon paper materials (see it here) so that I can test whether these will offer me equivalent power storage with a significantly lower mass. I also ordered the MGL 190 carbon paper (see here) which seems especially promising given that I will be able to build a cathode weighing just 11mg for this area. This should allow me to reach much higher specific capacities if I’m able to sustain the same total capacity for the cell.

When I fully open the cells after going through a full charge cycle I do not observe any accumulation of yellow TBAB tribromide within the interior of the carbon felt electrode. This is telling me that whatever storage is happening is probably only going on within the first few microns of the cathode materials, meaning most of the cathode materials is actually being wasted and not being used for charge storage.

This is the new battery structure I’ll be moving to this week after I get the zinc anode and carbon paper materials.

Another problem with the carbon felt is that it has a lot of “loose hairs” that “sneak” into the porous fiberglass separator and cause shorts between the anode and cathode unless I use 4-6 layers of fiberglass I use (which is sadly pretty porous). This substantially increases the internal resistance of the battery and the hairs, although shorting the battery to a much lesser degree, may still be causing an incredible amount of self-discharge given that they do provide significantly shorter paths between the battery anode and cathode materials.

Getting rid of all copper, changing to a zinc anode, covering both anode and cathode with conductive HDPE and changing from a carbon felt cathode to a carbon paper cathode may all be moves that should help me greatly increase the performance of this battery. Stay posted for some further updates!

Zinc Bromine Batteries: First success!

In my first article about zinc-bromine batteries I discussed why these batteries are gaining interest and how some recent articles point to their potential use as reliable and cheap batteries, especially for large scale applications. After building my own DIY potentiostat/galvanostat, I wanted to use this technology to characterize home-made zinc-bromine batteries and experiment with their chemistry.

One of my initial attempts at a Zn-Bromine battery using carbon felt electrodes as both anode and cathode. Trying to charge the battery at 1mA/cm^2 never got above 1.32V and potential declined after time.

My previous article also mentioned some of my first attempts at building these batteries, which were mostly failed attempts due to the complexity of the battery builds. Even though I was able charge the batteries a little bit – and obtained relatively high Coulombic efficiencies when injecting a small amount of charge – I was never able to sustain potential values close to the expected 1.6-1.8V of the zinc bromine system. Always topping up at around 1.3-1.35V as shown in the image above, when trying to inject charges at 1mA/cm^2.

A huge problem of my first set of designs was a complete inability to adequately reproduce my batteries. The electrode construction was very complicated and every battery I tried had slightly different geometry and different amounts of electrolyte within their construction. In order to standardize the study I decided to change to a Swagelok cell construction (which I bought from China here). I bought a cell and got it delivered to the US within one week.

Button Cell Swagelok-Type Cell for Cell Testing
These are the Swagelok cells I am using to build my batteries now. These cells have an inner diameter of half an inch.

Although the Swagelok were supposed to make things easier, I started to face issues with the electrode material of the cells being reactive towards the bromine generated within the battery charging process. In my initial attempts using a carbon felt electrodes and a fiberglass separator, the stainless steel electrodes in the cell – which are inevitably exposed to the solution – were getting corroded away by the generated bromine and tribromine salts.

I was finally able to surmount these issues by covering the Swagelok cell electrode pieces with conductive HDPE, basically by wrapping the electrode with it and then inserting it within the Swagelok cell. Using this method I was able to produce my first successful Zn-Br cell using a tetrabutylammonium bromide (TBAB)/ZnBr2 solution (0.25 and 0.5M respectively) , a copper electrode for zinc reduction a fiber-glass separator and a carbon felt electrode for the tribromide depositing.

Charge/discharge curve of my first successful cell. I charged the cell to 500uAh and then discharged it until it reached 0.5V. This process was carried out at 1mA.

The image above shows you my first successful charge/discharge curve. To the best of my knowledge, this is the only example available online for experimental data of a TBAB/ZnBr2 cell. The Coulombic efficiency of the above cell was 96%, which is great considering this is the first successful one I have built. The cell used around 80-100uL of solution and 4 layers of fiber-glass separator (see my previous post for links to these materials).

I am still facing some issues related with the cutting of the separator/electrode materials to place within the cell (I have bought a 0.5 inch cutter which should make this way easier) and I am also going to try using a zinc electrode for the zinc plating, which should make things easier. I also want to see if I can get a better non-reactive conductive coating for the cell electrodes, since the conductive HDPE I am using has a quite significant resistance. Things are looking up though!