Flow Battery Research Collective

danielfp248

Did a few cycles at low SOC and then attempted to cycle at 10Ah/L.

Catholyte (left) anolyte (right) below:

pH of catholyte was 0.24 and anolyte was 4.7. As you can see on second cycle to 10Ah/L the CE dropped from 93% to 83% and then capacity continued to decrease. On disassembly a lot of unreacted passivated iron remained on the anode, likely due to the pH increase. As Fe deposits water activity likely increases, which increases HER, which increases pH and leads to Fe passivation.

I am going to run a test adding 1M ZnCl2 to the electrolyte, to see how the additional salinity changes water activity and metal passivation.

danielfp248

I prepared a bulk amount of solution to have a more standard mix. This is 90g MgCl2, 32g FeCl2.2H2O, 2.5mL 15% HCl, 0.75g of Ascorbic acid. Volume to a total 200mL using a volumetric flask, then transferred to store here.

Initial MgCl2 addition was done in a 1L beaker over around 150mL of reverse osmosis water, added HCl after, then Fe, then ascorbic, then waited to cool, transferred to volumetric flask to finalize vol, then put into storage.

If you prepare this be very careful as MgCl2 reacts very exothermically with water. Cool the water in an ice bath and add slowly.

Final solution is perfectly clear.

The above correspond to molar concentrations of around 0.98M Fe and 4.7M Mg.

The HCl is to ensure complete dissolution of any Fe3+ hydroxyde/oxide contamination and the ascorbic acid is to reduce any present Fe3+ into Fe2+. Excess ascorbic has to react on initial charge, so it will eat into a tiny bit of the SOC.

danielfp248

@StevenHickey Awesome to have you here! Very excited to see someone here with so much experience in flow batteries. Since you have so much experience in Zn-Br I wanted to ask if you have had any chance to test the Zn+Na sulfamate chemistry that was published not long ago. I tried it with some success, but had some stability issues https://chemisting.com/2026/03/23/zn-br-sulfamate-battery-stability/. Would love to hear your opinion on that subject.

danielfp248

The cost per kWh of this is too high, the use of organic materials and ion exchange membranes puts this closer to what people do in academia and further from the cells that can be reasonably fabricated using DIY approaches. The use of an ion exchange membrane likely reduces the lifetime of this cell a lot. For a static cell, I think the sulfuric acid Cu/Mn battery we have discussed before is far more promising. That chemistry requires no exchange membranes, uses only commodity inorganic materials and cycles to 30-40Wh/L.

For pure Fe batteries I am much more inclined to the Fe flow batteries.

danielfp248

I just wrote a blog post sharing my first results testing the Zn-Br chemistry recently published by a Chinese group. Sodium sulfamate is not very easy to get, but I was able to source it from labdiscounter.nl in the EU. If any of you are in labs where you can also test this chemistry, I would be happy to hear about your thoughts and/or results.

danielfp248

Thanks for sharing your pictures! It always makes us very happy to see anyone reproducing the kit independently.

You can see my current setup below:

I am testing the double reservoir with spill-over communication (that's why the reservoirs look different and are at the center). My pumps are connected in a suction configuration and they enter and exit the cell on their sides. When you have pumps in a push-through configuration having the flow going from bottom to top is important to push air out, but when you suck through the cell the vacuum will push all air out and fill the entire cavity, almost regardless of how you pump. I haven't seen any air being trapped there (no bubbles are evident when shaking or moving the cell so that the flow is in either direction).

As you can see I have changed some connections to luer locks, except the connections for the pumps. I however like how you've made them ALL luer locks, much simpler to disassemble.

As you can see I'm also testing the new clamp-compatible end-plates and clamp. This works SO much easier than the screws, since the compression is centered I am also experiencing fewer problems with the cell and slightly better energy efficiencies since the felt compression seems to be more uniform. Opening and closing this cell is a breeze compared to the screws. I used PLA with 80% infill to print the end plates but I am sure PETG will work great too.

About the PP, it is a bit tricky to find the PP and printer settings that work best for water tight results. I am using the Ivor white PP from smart 3d, which has worked well on the prusa core one. In any case, let me know if there's anything I can help you with.

danielfp248

@rowow

We don't have the equipment or time to prepare these membranes but if you send us samples of 100-400um thick membranes we can test them out.

danielfp248

@rowow I don't know what to tell you. While sulfonated crosslinked polysterene membranes can work great for many flow cell applications (they absolutely have no problem with strongly acidic environments), they will have problems with many flow battery chemistries due to the chemical properties of these membranes and the highly concentrated oxidative environments the membranes are subjected to. You can look at the research on membrane degradation if you want to learn more about this.

These resins have existed for a long time, they have been tested, their problems are well known. You are welcome to try them in flow batteries and share your results here.

As I mentioned the concerns about cost, dendrite formation in deposition chemistries and chemical degradation are mainly why we decided to go with microporous instead of ion exchange membranes. Even if ion exchange membranes were very low cost, for an open source flow battery we believe microporous membranes actually offer better robustness.

danielfp248

This is a polystyrene crosslinked sulfonated resin. It is not going to resist the conditions of flow batteries due to the reactions the aromatic units will go through, which will decompose the resin with time. Bear in mind the problem is NOT solely about the oxidation potential itself, but about the concentration of oxidative species. Potential will control whether some reactions happen or not, but the speed of these reactions will be determined by concentration. A +1.5V potential might be survivable in the long term at mM concentrations but at M concentrations the story can be very different. Sulfonated polystyrenes are not chemically suitable for the application.

Viable low cost alternatives for flow batteries must have more structurally sound backbones, like for example sulfonated poly(ether ether ketone) (SPEEK) membranes (even these won't survive all chemistries). Since the ion exchange membrane is not an easy to remove component and it has to survive for +10 years of very harsh conditions we have therefore decided to use microporous membranes instead. Of course, for demonstration or short term applications, I am sure a polystyrene membrane would be ok with some chemistries.

danielfp248

Thanks for sharing. What is the exact chemistry of the resin you are using? Typically resins used in water softening applications will degrade significantly with the highly concentrated oxidants present in flow battery catholytes. Did you find a fluorinated resin used in water softening that would be low cost?

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