RFB University

[sepi]

I have so many questions that are related to RGBs but don't really fit into existing threads. I'll poste here and hope that we can collect those random questions in one thread such that we don't pollute the other threads too much.

1. I read about the history of batterie and it seems that for a long time, terracotta or other ceramics were used as membranes/separators. Have you investigated their use? Once I have a functioning system I will surely try it out. The advantages would be that they are tough (at least physically) and potentially easy to produce almost anywhere in the world.

2. I noticed that some designs of for FBs don't have flow frame like in the dev kit but rather a thin meandering channel. What's the reason for these two designs an why did you select the flow frame

3. What would happen if you just removed the membrane in the Zn/I cell, would the iodine anions just progressively oxidize the metallic zinc and produce heat?

4. Do I understand correctly that the Nernst equation can be used to predict at what temperature and relative concentration between oxidized and reduced ion of two species a redox reaction takes place? Do I also understand that this does not predict or influence kinetics?

Many thanks for your precious insights!

[sepi]

5. Is voltage efficiency a function of overpotentials?

[Vorg]

If you are looking at ceramics, I stumbled across this YT video using a kind of cement. But seems to me thickness would be just one of many problems.

[danielfp248]

@sepi Let me answer your questions:

1. Several modern batteries - Like Na/S - use ceramics as separators, so do several batteries in development, like solid Li-ion batteries. Ceramics are hard but they are also very brittle, the ionic conductivity of something like terracotta is also very low, so the way they increase ohmic resistance is very high. You might be able to create a very bad battery with one, but for anything close to practical, your common pottery sources are most likely not gonna cut it without heavy modification. For this reason ceramics used as separators that can support high current densities have to be extremely thin and have to be engineered to have high ionic conductivity. For a flow battery, where membranes require flexibility, they are not generally a good fit on their own. With that said, maybe you could try sandwiching a very thin layer of terracota between two photopaper layers, or just wetting the photopaper in a terracota suspension and drying it, see what you get.

2. These flow channels are usually machined into an electrode, such as a graphite plate. This complicates the fabrication process significantly when compared with something like 3D printing. This is because the graphite has to be bipolar electrode material (cannot be regular graphite because liquids can leak through normal graphite plates) and usually gaskets are needed to properly seal these devices. This increases the cost and complexity to the point where it is only accessible to academic/industry players with significant resources or people with a lot of experience in machining. Services like Xometry will not do machining of graphite bipolar plate material.

3. Yes, that's exactly what would happen. For a 2M solution charged to max SOC delivering around 250mWh of stored energy, 10mL of solution would heat up around 20C.

4. The Nernst equation gives the electrode potential of a redox half-cell or cell under non-standard conditions, accounting for temperature and the concentrations of reactants and products via the reaction quotient. It’s a thermodynamic equation that relates to free energy, and while it doesn't describe reaction kinetics, it helps predict the direction and favorability of redox reactions. It is applicable away from equilibrium, and when the system reaches equilibrium. It doesn't predict or influence kinetics.

5. Yes. The higher the overpotentials, the lower the voltaic efficiency.

[sepi]

@danielfp248 thanks so much for your answers. I have more :).

6. Is there a potential between bulk solutions while the battery is operating or while its resting? I guess you can imagine the whole battery as an equivalent classical electrical circuit (with electrons as charge carriers) with current sources at the electrode-electrolyte junctions and resistors for the electrolytes and separator. Does that make sense? This would also mean that the electrolytes would be at equal potential when no current flows.

[danielfp248]

@sepi Thanks for your reply!

6. To measure a potential you would need to introduce a probe in each solution, these two would be alike the two current collectors in the battery. If there is a path to the membrane from the solutions (if for example all the tubing is full of liquid), then you will measure some potential between the two tanks. It will be less than the potential between the current collectors (because the probes you're using are far more separated) but the circuit is equivalent only that you have a larger series resistance in between the probes.