Tag Archives: batteries

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: What would be realistically required?

Current commercial Zn-Br flow batteries have specific energies in the 34.4–54 W·h/kg region, with most companies being at the lower end of this range. In order for a static Zn-Br battery to be better than its current industrial counter-parts it would ideally improve on this specific energy while reducing the costs of production substantially.

My current tests using carbon cloth cathodes, Zinc anodes, fiberglass separators and Zinc Bromide electrolytes in the 0.25-0.5M range with a TBABr sequestering agent present at concentrations of around 0.1-0.2M have shown an ability to store around 0.5mAh with a weight of around 0.250g per total cell (no packaging material), which would give the cells a specific energy of around 3 W·h/kg, which is one order of magnitude lower than current commercial Zn-Br flow batteries.

An image of one of my current Zn-Br battery cells. The cell has a diameter of 0.5 inches and is placed inside a Swagelok cell with graphite electrodes for measuring

In terms of weight, I have been using a 0.2mm thick Zinc anode that is quite thicker than what would be strictly necessary for the battery, the anode thickness can be changed to 0.02mm Zinc foil (10x less mass) which would reduce the total amount of mass by more than 70%. The anode mass is currently around 180mg, so lowering this to 18mg would take the current specific energy to around 9 W·h/kg (since there is no expected loss in the current battery configuration from using a thinner Zn anode).

This improvement is still not enough, we need to increase the capacity by at least 4-6x which means increasing the amount of Zinc Bromide in the battery to at least the 1.5-2M range and increasing the amount of energy injected/extracted to at least 2.0-3.0 mAh for this battery. This means that TBABr is not going to work, reason why my tests are now going to move to using TMPhABr or TPABr. These new sequestering agents also have lower molecular weights, so they are bound to be significantly more “atom efficient” compared to TBABr. The end batteries right now contain around 50uL of electrolyte – I put 100uL but half is “pushed out” when Swagelok cells are closed (this is determined by weighting the dry and final battery cell) – so theoretically a 2-4M Zinc Bromide solution should offer a capacity of around 2.7-5.2 mAh but we are unlikely to be able to extract this amount because of the conductivity of the solution becoming lower as we plate Zn and oxidize bromide to perbromide in the cathode.

The current energy efficiency of the battery is still too low (max has been 60% in most cases) so the hope is that the higher Zinc Bromide concentration, coupled with the new sequestering agents, will help increase this efficiency to the 70-80% region while also helping improve maintain Coulombic efficiencies above 95%. The energy efficiency of current Zn-Br flow batteries is mostly below the 80% mark, so anything above this number would be highly desirable.

If the above mentioned sequestering agents can achieve these efficiencies at these concentrations then we would be able to reach specific energies of around 45 W·h/kg for the cells I’m constructing. If we can achieve energy efficiencies above 90% – already seen in published research using TPABr – this would already put them at a significantly more competitive place relative to current Zn-Br technology.

Currently Li-ion cells are in the 100-265 W·h/kg range, so this technology could only compete if significantly higher zinc bromide concentrations – in the order of 10M – can be achieved, while retaining a functional sequestering agent or if we can add a supporting electrolyte that enables the extraction of most of the zinc bromide without lowering the efficiency of the battery (although that electrolyte adds some weight). It is much more likely that a technology like this would compete in battery life and USD/kWh terms. Li-ion technology right now is at around 200 USD/kWh while a technology like Zn-Br in static cells could start at a fifth of this price. The life of a static Zn-Br battery with a viable sequestering agent is also expected to be significantly longer (>10,000 cycles) so that would also help it compete with Li-ion (with Lithium Iron Phosphate batteries surviving for around 2000 cycles when fully discharged on each cycle).

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!

Building a DIY opensource USB potentiostat/galvanostat: Part One

As I explained on my last post, I want to build a system to characterize batteries at a small scale at my home. This means being able to test things like their coulombic efficiency and measure things like charge/discharge curves. The perfect solution came as the USB potentiostat/galvanostat published in this paper so I order 3 boards using Oshapark and got the rest of the parts from microchipdirect and digikey.

PCB board I received from Oshapark

I received my PCB order last week – as shown above – and proceeded to solder the components that I received as well. After soldering all the components I then connected the board to the PicKit3 programmer using the programming leads. By the way, the square pin right below the K3 mark should match pin 1 in the programmer, something that is not mentioned within the above cited paper.

When I did this I used the MPLAB X IPE v5.4 software, downloaded from this link. Using the advanced options I made the PicKit3 provide power to the board and I then proceeded to program it at 4V because when I connected it at 5V I received some erros about VDD not matching between the microcontroller voltage and the provided voltage. In the end I was able to program and verify the chip at this voltage with the hex file provided by the authors of the paper.

After this I then connected the chip into the computer using a USB port and instantly received a USB overcurrent warning, which immediately disconnected the PCB from my computer (uh oh). After checking the board I noticed a short after the charge pumping circuit, where the +9V line was almost shorted to ground, with a resistance of around 10-100 ohm when it should be at least 10kohm given the lowest resistance connected between ground and this line (R2). You can test this by measuring resistance between the leads in R2.

After painfully taking out all the components one-by-one from one PCB and soldering them onto a second one I realized that my problem was that overheating the PCB actually created a short-to-ground in this line, more likely than not related with partial melting of the PCB in the U8 microchip leads where the +9V and ground lines are particularly close to one another. This can actually happen by heating anywhere on the board that’s connected to the ground line, even if you overheat something like the LED D3 or D4 lines. I noticed because I caused the same damage on the second board I was working on, even though the lines were not shorted right before I was working on the D4 LED but became shorted right after I spent around 20 seconds applying heat (yes, my bad).

Two boards I worked on that are now useless. Left one was the first I soldered all components on, second is the one I was testing components on when I noticed the short caused by over-heating the ground line.

Right now I sadly only have one board left (sigh) and have already desoldered and soldered a lot of the components. I now need to desolder all the remaining components from these two boards solder them onto the third board, although this case I will need to be especially careful about how I apply heat to the board as I definitely do not want to cause this shorting issue again. I will update this blog after I try again.

Building a machine to test and research batteries at home

As a chemist who loves electro-chemistry, battery technology has always seemed incredibly interesting, especially since it’s within the group of potential topics that could be researched with some degree of success at home. This is because batteries can be made within a very wide array of chemistries, some of which use very easy-to-find materials and the equipment necessary to research batteries at a small scale should not be hard to build.

Public PCB project at OshPark

However, after looking at a lot of people sharing their DIY batteries at their own houses on the internet, it seems clear that most of them don’t do any proper characterization of their batteries at all and those who do – who appear to be very few – seem to use relatively expensive pieces of equipment to do so, probably the lower end of what would be used within a regular university research environment.

The options available to minimally characterize batteries, which means at least measuring their charge/discharge curves seem to all be expensive and there is no commercial option I could find that would allow you to perform these tests for less than 1000 USD.

However, I did find a very interesting publication (here) where the researchers share the PCB, software, firmware and bill of materials for a cheap galvanostat/potentiostat that can be used for the characterization of small batteries. Given its limited current +/25mA, it cannot be used for the characterization of any larger batteries, but it should allow for some very interesting and well-done research of small batteries at home.

I added this PCB to OshaPark (you can order it here) and I have ordered the materials from Digikey using the bill of materials provided by the author within the paper (you can use this file to upload to dikigey directly) . The microcontroller used within this project also requires to be programmed using a PicKit3, so you will need to get one here. Note that due to COVID related supply constraints I had to order the MCP3550-50E/SN microchip from microchipdirect.com instead of digikey and I also changed the mini-USB port for a micro-USB port (609-4053-1-ND).

Current progress of my order at pcbway

As backup plan I have also ordered a fully assembled PCB board from pcbway.com, which charged me a total of 154 USD for the entire production of the PCB and mounting all the surface components. This is all done in China and the exported to the US, so it will take around a month for the entire process to go through. I want to compare the quality of my own assembly with the product I obtain from China.

In this process I also got quotes for several different US manufacturers for the production of these boards, but came to the conclusion that it is not economical unless I wanted to get at least 10-20 manufactured. This is because the price is often in the 1200-1500 USD range, independently of whether I get 1 or 10-12 boards done.

I still haven’t received everything I need from oshpark and digikey to assemble the board but once I do I will update you on my progress building/programming/testing this open source galvanostat/potentiostat.