Sciencemadness Discussion Board

Advice for Aluminum Air Battery

Ty314159 - 12-9-2023 at 09:30

I'm working on making an aluminum-air battery using aluminum foil and porous carbon foam. Right now, it's very crude and uses brown paper towel soaked in ~1M NaOH stock solution sandwiched between the electrodes.

I'm working on optimizing the electrolyte and the membrane right now with my club group.
Based on an old research paper, (https://doi.org/10.1016/0378-7753(92)80147-4) "Alkali citrate cum stannate" based on 4M NaOH should be the best electrolyte. I'm not sure exactly how much sodium citrate and sodium stannate is in that electrolyte though, but I think it would be very valuable to know to prevent my aluminum from just dissolving in the hydroxide solution, ruining my battery.

Also, should the carbon foam be soaked in electrolyte too? I know it must adsorb air to work, but I'm not sure what a good middle ground between air exposure and the carbon cathode touching the electrolyte is, since it seems like a tradeoff.

In the final iterations of this battery, we will of course get 99.9% pure aluminum sheets to use as our anode to make the process just that bit more efficient and permanent.

I'm thinking of using agar to thicken the electrolyte so that the jelly can be used by itself as the membrane, but I worry about dendrites and the system shorting out. I'm also thinking about using polyester felt as a membrane. Please let me know if you've got any ideas or knowledge about this. Many of the sources I encounter use professional ion exchange membranes, which is not something we want to do, since that would go against the spirit of this battery being totally home-made.

If anybody has any experience or advice in aluminum-air batteries or the design of makeshift metal-air batteries in general, please do let me know! It'll be really appreciated. Apologies in advance if I'm ever inane!

[Edited on 9-12-2023 by Ty314159]

Rainwater - 12-9-2023 at 13:47

Using a think layer of pva glue(alcohol not acetate) on your membrane material, then soaking it consentrated NaOH solution, should stiffin the material while making it permemibule, permeable, per me a bull, crap my brain just reset.

If you want to go all out and make a ion exchange membrane, i found this some time ago and have had success ising it to generate h2so4, and naoh via electrolysis

has some good instructions in this paper and shows their results from a larger list of reagents, some easy to get( i used consentrated lemon juice and boric acid(ant killer))
https://www.mdpi.com/2073-8994/12/6/960

Attachment: Pva membrane study.pdf (7.4MB)
This file has been downloaded 308 times

sceptic - 12-9-2023 at 14:53

If the carbon foam electrode is at the top of the cell, and is porous enough, you might be able to "wetness" gradient, where the top of the foam is dry, the bottom is soaked in electrolyte, and the intermediate volume becomes progressively drier.

As far as the membrane or agar goes, I don't see why its necessary. They're normally used to keep the catholyte and anolyte separate, which it doesn't sound like your battery needs. If you do need one, I don't think dendrites will be a problem. They only form during charging, which would be impossible using an aqueous electrolyte anyway.

This sounds like an interesting project. Please report how it turns out!

Ty314159 - 14-9-2023 at 06:22

Thank you very much, rainwater and sceptic!

I agree that the membrane isn't really necessary, so I think we will instead use thin polyester felt instead.

I'm having trouble acquiring the sodium stannate to prevent my aluminum from dissolving. Hopefully I can ask around for it specifically or for tin metal. Let me know if you guys have any ideas regarding that.

I'll be sure to post here a guide and results report for my makeshift aluminum air batteries if the project goes well!

sceptic - 14-9-2023 at 11:07

I don't know whether it's the same where you are, but in America, most outdoors shops sell tin sinkers in their fishing section. Even Walmart has them. They're probably overpriced for the metal, but they are a convenient source.

Rainwater - 14-9-2023 at 15:23

If you want enough to last a lifetime for cheap.
https://www.rotometals.com/tin-popcorn-flakes-99-9-pure/

I use these to make stannous chloride solution.
Easy to weight, large surface area to so they react quick.

Looks like your in luck
Page 1694( pdf page 1741) of Handbook of Preparative Inorganic Chemistry gives detailed instructions.
Looks like an air sensitive reaction.
I have successfully performed about 60 preparations from this book. Some things are easy, some not.
But I have yet to succeed when it warns about air/co2/o2 and I ignore it.
Quote:

Sodium Hexahydroxostannate (IV)
Na2[Sn(OH)6]
Sn(OH)4 + 2 NaOH = Na2[Sn(OH)6]

A solution of SnCl4 in very dilute hydrochloric acid is neutralized
to methyl orange with carbonate-free NaOH. The SnO2 •
aq. precipitate is filtered off, washed until chloride-free with
H2O, and added in portions to an excess of concentrated, 100°C
NaOH, in which it dissolves rapidly, affording a clear solution.
The crystalline hexahydroxostannate precipitates after a short
time. The crystal slurry is filtered in the absence of CO2 and
washed with 30% NaOH and then several times with ethanol and ether.


Looks like a first step is to make stannous chloride unless you have some on hand.
I use a reflux setup, toss in an excess of tin metal, cover it with HCl(hardware store) and boil for about 20 minutes, until the solution starts to turn cloudy white.
The cloudyness is caused by tin hydroxide percipitating and can be cleared by adding HCl drop wise with stirring.

Decant the solution and lable it.

Ty314159 - 31-1-2024 at 19:23

Hello again! The aluminum air battery does work, although there seems to be a mass transfer issue in regards to oxygen getting dissolved in the electrolyte and then actually being able to participate in the chemical reaction, leading to very poor current (<20 mA with a 6 square inches). This could be fixed by pumping air through the cell, but honestly that is a pain in the butt!

So I have decided to personally switch gears away from aluminum air batteries towards aluminum permanganate batteries that have the oxidizer readily available in the solution.

Thanks again for the instructions to make the tin compound, but unfortunately it would be too much of a pain to make given the restrictions of the school in regards to safety for this particular club at this time. The conditions required to get tin to the proper oxidation state would be just a little bit too harsh.

Luckily, Agar is an effective inhibitor to the reaction between Al(s) and NaOH(aq), and agar is easier and was a lot more readily available than tin IV (https://iopscience.iop.org/article/10.1149/1945-7111/ab9cc7/...)

I think I will start a new thread for the aluminum permanganate battery eventually.