Flow Battery Research Collective

Hello everyone! It’s been a while
I’ve returned to testing the flow cell and even built a second one to check repeatability. I’m wondering what could cause such a large difference in performance between them. The second cell showed only 0.85 V during discharge (compared to 1.15 V for the first one).

There was also a warning during the discharge of the first (better) cell:

“The command b'RANGE 2' resulted in an unexpected response. The expected response was ‘OK’; the actual response was ‘WAIT’.”

After closing this warning and restarting the charge/discharge cycles, the system no longer stopped charging the cell at 10 mAh as before. I manually stopped the cycle at 22 mAh.

The cell was left in the workshop, and I was remotely connected from home to the Raspberry Pi controlling the pumps and Mystat. The next day, when I returned to the workshop, both tubes from the pumps had detached from the cell, and the electrolyte had spilled on the table.
Do you have any idea what might have happened?

I also wanted to ask whether you’ve tried different gasket materials—such as FKM (Viton) rubber or expanded PTFE (ePTFE)?

@sepi Sorry for the late reply. I'm glad the first attempt was successful and that I’ve learned how it works! I believe flow batteries are the future - at least for stationary systems. So, what’s next? Maybe I’ll try designing my own small battery…

@sepi , thanks for the kind words. @danielfp248, @kirk It is really cool that you have done and made this project available! Building and testing it personally is an experience much more valuable than just reading an article.
I'll be honest — before the first test, I was a bit worried that a leak might suddenly appear.
I'm also glad, @danielfp248 , that you confirmed the accuracy of my results.
I'm looking forward to seeing your project develop!

I’ve finally completed the first tests of the battery.

@danielfp248, I used the parameters you suggested, but I had to enter one of the currents (either charge or discharge) with a negative sign. With each subsequent cycle, the efficiency improved (as shown below). Is this difference in capacity from cycle to cycle expected?

I expected the program to automatically adjust the pump RPMs, but it turned out I had to set them manually. I'm not sure if the values I chose were appropriate.

Now I understand why you mentioned that the connection combination didn’t make a difference
However, I still have some doubts about the resulting voltage value—doesn’t it seem too low?

Before turning on the pumps, I zeroed Mystat. I’ve just realized that it should also be calibrated with a 1.000 kOhm resistor.

I used the tubing that came with the pumps. After completing five cycles, I emptied the electrolyte and rinsed the system several times with deionized water.

Now that I know the system works, I’ll look for proper electrolyte-resistant tubing and continue with further testing.

Here’s how the electrolyte’s color changed after just a few cycles:

Ok, thank you @danielfp248 ! I know I'm probably being a pain, but I just wanted to be absolutely sure:) So this part is already solved.

However, I have found out that mystat.py was not able to save results to a file.

On the Raspberry Pi that I am using right now, I was unable to install PyQt5 in a virtual environment following your readme.txt and requirements.txt file. The problem is known and I have not found a working solution for the venv setup. As a workaround, I was using the default version of Python and libraries. This was probably causing the file saving issue.

Happily, the script seems to be working on the Raspberry Pi's default system Python and libraries after changing the "choose_file" function to the following:

def choose_file(file_entry_field, questionstring):
"""Open a file dialog and write the path of the selected file to a given entry field."""
filedialog = QtWidgets.QFileDialog()
# Get the tuple (filename, filter) and take only the filename
filename, _ = filedialog.getSaveFileName(mainwidget, questionstring, "", "ASCII data (*.txt)",options=QtWidgets.QFileDialog.DontConfirmOverwrite)
file_entry_field.setText(filename)

I hope there will be no more incompatibility issues during battery testing

@danielfp248 Ok, thank you. So there is no need to assign one peristaltic pump to WE/SE and the other to CE/RE? Because if the electrodes are connected arbitrarily, this information will not be passed to mystat.py, right?

Edit: I suppose that I should connect the WE/RE to the electrode associated with the positive (P) pump circuit, and the CE to the electrode associated with the negative (N) pump circuit.

Another question before I start the test: I understand that I need to connect WE/SE to one electrode and CE/RE to the other. Does it matter which electrode is connected to which pair? Does the Mystat.py script differentiate between them?

@danielfp248 said in Following your documentation – feedback & questions:

After that you can then start going to high SOC values at 40000uA. I would recommend first cycling to 100mAh (set Upper bound to 100000uAh). Enclose the battery when cycling to higher SOC - you can put it inside a plastic tub - because leaks due to any problem will spray highly charged electrolyte, which, even if the volume is low, can be dangerous.

Thank you for this advice. To be honest, I’m a little bit afraid of leaks that could damage the Arduino, above all. Do you propose covering just the cell, or the whole system? I’ve placed the system in an IKEA Samla box, and I can cover it, just making some holes for the cables. Do you recommend extra covering for the cell as well?

@sepi, @danielfp248 thanks :), @danielfp248, I have an exact electrolyte from the documentation https://fbrc.codeberg.page/rfb-dev-kit/electrolyte.html .

It's been a while, but I’ve just configured a Raspberry Pi and got the mystat.py script working alongside Mystat and the Arduino. It looks like the chemistry is ready as well. Could I ask you for some instructions on how to proceed with the test using the mystat.py script? There are multiple options available, and I want to make sure everything is set up correctly

@kirk said in Following your documentation – feedback & questions:

Could you share the file you modified? It looks like you added a big cylinder.

I have send you an email,

@kirk said in Following your documentation – feedback & questions:

The program here * should * work on linux when run as root (in order to have proper USB access). You can control the pump speeds via the Arduino with the software, even without the MYSTAT connected (under the "charge/discharge" tab) (this will all hopefully be in a video soon!)

Thank you. I am planning to use a Raspberry Pi for long-term tests. Are you using a Raspberry Pi too, or a PC?

@kirk said in Following your documentation – feedback & questions:

In the CAD files we have specified a 2D endplate (https://codeberg.org/FBRC/RFB-dev-kit/src/branch/main/CAD/exports/Metal Endplate.step), that could be laser cut or milled from aluminum. @danielfp@chemisting.com has received endplate versions of this in aluminum, the only catch is then you need insulating washers so as to not short them. Another rigid polymer could work too.

Thank you. For now, if there are no leaks during the long-term tests, I will not change these endplates.

And also, I wanted to ask you about the current collector's material – is brass a better option than pure copper? What was the criterion for the material type and thickness selection?

Thank you @kirk . Yes, I had already understood it from your previous message.

I’ve finished the assembly and performed a leak test using demineralised water. The first attempt failed due to leaks in both reservoirs (see photo). After that, I made a small design change and increased the 3D printer’s flow rate for the infill. The new reservoirs seem fine now.
Did you encounter this problem too? How did you solve it?

The cell itself doesn’t leak. I used a torque of 3 Nm for now, but it already looks slightly distorted. Have you considered using more bolts?

I also wanted to ask: is silicone resistant to this type of chemistry? Since it’s used for gaskets, maybe silicone tubing could be used as well?

Regarding the Arduino program: it seems there are four defined pins — In1 (pin 9), In2 (pin 8), In3 (pin 7), and In4 (pin 6) — but they don't appear to serve any purpose.

As for the main Python program — should it work with no issues once the Arduino and MYSTAT are connected to the computer?

While assembling the flow cell, I noticed that the manual doesn't mention anything about the Membrane Frame. I also have doubts whether the Membrane Frame model provided on the website isn't too thin (0.8 mm) to ensure a proper seal when using 4 membranes made from 230g/m² matte photo paper. Each membrane, according to my measurements, is 0.3 mm thick — which means the total thickness is 0.4 mm greater than the Membrane Frame's thickness. I also used 0.1 mm gaskets (as specified in the Bill of Materials), so such a large difference in thickness is unlikely to be compensated.
@kirk, do I need a thicker Membrane Frame or thicker inner gaskets?

Thank you for the comprehensive answer — I think you've convinced me to go with PP

Hi! I'm happy to have the opportunity to learn this based on your experience

I'm attaching a photo of my 3.2 x 6.4 mm tubing—hopefully it's the correct one.

It seems that the default tubes in the Kamoer KPK200 pumps are already Pharmed BPT, so I assume there's no need to replace them - is that correct?

Oh, you mentioned a barbed fitting adapter now, but it wasn’t listed in the bill of materials - do I actually need it, or can I manage without it?

And yes, I’m using the MYSTAT potentiostat.

A guide showing how to connect all the cables between the pumps, Arduino, MYSTAT, and the power supply would be really helpful for me.

I'm also interested in this topic. Polypropylene is not a trouble-free material. There are some other filaments classified as chemically resistant (https://help.prusa3d.com/filament-material-guide). Is PP the only suitable option?

Hi! I already have all the components. I had the brass plates CNC-cut, and I cut the 0.1 mm silicon gasket and grafoil with a knife (I 3D printed a template for that). I noticed that the documentation hasn't yet been updated to include the new Kamoer KPK200 pumps.

@kirk, could you send a photo showing how the tubing is connected (what length should the hoses be cut to?) and what the electrical connections are?

The documentation says to cut 4 separator sheets, but then only 3 are to be used. So is the 4th one not needed?

Also, I assume I don't need to worry about the graphite felt appearing white in your photos? It's not, for example, actually polypropylene felt mistakenly labeled as graphite felt?

Hi! Thanks @kirk for that info. I still don't have all the components to start assembling the cell. I'll figure out how it will go and eventually reprint the jig.

Good to hear the packing tape hasn’t caused any leakage issues in your case. I ended up getting a PP adhesive foil that’s originally meant for covering book covers.