Category Archives: Battery research

Zinc Bromine Batteries: Think about the electrodes!

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In order to study the chemistry of Zinc-Bromine batteries I have been using a swagelok cell that I bought from China that has a central Teflon body with stainless steel electrodes. This has been problematic due to the reactivity of these electrodes with the elemental bromine and tribromide salts produced in the battery, requiring the use of some “improvisation” in order to make the batteries work.

Charge/Discharge curve of a Zn-Br battery built using a zinc anode, carbon felt cathode, fiber glass separator and 0.5M ZnBr2+ 0.2 TBAB electrolyte in a swagelok cell with stainless steel electrodes covered with conductive HDPE.

To be able to generate the necessary chemical reactions without interference from the stainless steel I have coated the electrodes with some conductive HDPE I have, which has a relatively high volume resistance of around 10K ohm. The charge/discharge curve above shows you the type of measurements I have been able to achieve with this setup, with the above curve having a Coulombic efficiency of 88%.

This is lower than what I could achieve with the copper anode I was using previously (which gave me around 96%), but note how the charging voltage is lower and the discharge voltage higher, meaning that the overall energy efficiency is significantly better (around double). This is however not because of the zinc anode, but because with the conductive HDPE now covering both electrodes, I have now been able to tighten the cell more and achieve a lower overall internal resistance.

However the still relative low energy efficiency and voltage drop when going from charge to discharge are still pointing to significant sources of efficiency loss, possibly from the 10K ohm resistance that the conductive HDPE is giving. The weird shapes at the beginning and end of the discharge curve are also pointing to more than one chemical reaction happening, probably because the electrodes are still somehow interfering with the chemistry (maybe some micro holes in the conductive HDPE at points of stress are exposing the electrodes underneath). For this reason I have decided to get graphite electrodes for the Swagelok cell, which I will build from these 0.5 inch diameter graphite rods I found.

I am also going to change the current carbon felt cathode for carbon paper electrodes – which are on the way – but I will refrain from using the paper until I can perform some tests using actual graphite electrodes that are guaranteed to be free of any pesky side reactions, with way less resistance than this conductive HDPE.

Zinc Bromine Batteries: Current battery and experiments to follow

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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!

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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!

Building a DIY opensource USB potentiostat/galvanostat: Part Three

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Through the past month, I have been trying to build an open source potetionstat/galvanostat as described in a research paper (see here). Knowing that the probability of failure trying to manually solder such small components was high, I ordered some fully assembled PCBs from PCBway one month ago to make sure I had a plan B in case my manual attempts failed.

This is the Chinese made potentiostat-galvanostat board built by PCBway using the files I provided (which are files obtained/modified from the paper mentioned before).

If you have read my last few posts, this is exactly what happened, I completely failed at successfully assembling this board myself (not the best soldering hand in town!) but thankfully received my fully assembled PCB from China a couple of days ago. The PCB from china worked flawlessly, allowing me to perform the calibration and have a fully functioning potentiostat/galvanostat for home use.

The python software provided by the creators of this potentiostat also worked really well. Using the knowledge I obtained within the last couple of posts, I was able to easily use the drivers provided by the authors to use this software without any issues. The software implements some basic experiments, like CV, charge/discharge curves and Rate testing, but the best thing is that the entire thing is open source, allowing me to customize the experiments to do whatever I want, something I know many researchers wish they could do with the expensive software packages – all closed source – provided by normal potetionstat manufacturers.

These are some charge/discharge curves I am now measuring for a prototytpe battery I made. I modified the software in order to be able to do the charge to 350 uAh, then proceed with the discharge.

This ends my quest for the building of a – now not so much – DIY potentiostat/galvanostat, giving me the functionality of a piece of equipment that usually costs around 1000-3000 USD for just a couple of hundred dollars. Even more, this potentiostat allows me to use current in the -25 to 25 mA range, something that isn’t that common unless you go for the more expensive potentiostats above the 3K+ USD range, since the cheapest potentiostats are usually built for high sensitivity at lower currents – because these are mostly intended for analytical chemistry experiments – rather than for the charging/discharging of battery cells.

My posts will now move onto the experimental batteries I am attempting to build and their characterization. I have always noticed that DIY batteries on the internet are almost never properly characterized – no wonder given how difficult it has been up until now to get access to proper equipment to do so – but with this piece of hardware I will now be able to perform all of these experiments without issues.

Building a DIY opensource USB potentiostat/galvanostat: Part Two

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With some improved soldering skills I reattempted soldering of all the components into the brand new PCB I had left from Osha Park. After doing this I still experienced a significant amount of shorts but this time I was able to pinpoint the sources by some smoke coming off the PCB (not the greatest sign!). In the end all the shorts were coming from what seems to be the underside of the microchips, probably related with some flux residue that got carbonized and became somewhat conductive after the chips were soldered and the circuit was powered.

My somewhat successfully powered DIY potentiostat/galvanostat

With this information I now resoldered all the chips, being very careful about cleaning all the flux to ensure that there were not shorts after the chip was put into place. With all these shorts removed I was able to finally power the board without any excessive current drain.

Due to the fact that the drivers that come with this device are unsigned I had to restart windows using the “Advanced boot options” to ensure that driver signing was disabled. Also remember to install pyusb and usblib before launching the python program.

With this done I was able to successfully connect the PCB to the computer and use the software to interact with it. However after trying to do the calibration I noticed that the entire potentiostat/galvanostat functionality was actually not working and I was actually unable to set any potential without the circuit going a bit “crazy”. As you can see in the image below, everytime I tried to set the potential to some value I just got some random potential being set, with current bouncing all over the place.

Trying to set the potential of the device to 1.5V with RE connected to WE and CE connected to SE just generated a bunch of noise

Feeling the temperature of the different chips, the one that is overheating seems to be the OPA4192 chip. I tried to remove it and resolder it again, but I have the same problems and the same type of abnormal behavior. Right now it seems that the most likely scenario is that all my desoldering and soldering endeavors have fried one of the components of the board, meaning that I might not be able to get it to work at all with the current components.

Thankfully plan B is still going on – a PCB being fully assembled by pcbway – so I should be able to get a fully working board within the next couple of weeks. I am still debating whether it’s worth it to order new components and try on a new board – with my already gained experienced – but I think I’ll wait for the working PCB to ensure this board works as I expect it to before I make any further DIY attempts.