Sciencemadness Discussion Board

Fine Manganese Powder For Battery Negative Electrode

CraigX - 21-5-2012 at 10:30

...in all its gory detail...

I'm making manganese negative alkaline battery electrodes to create alkaline cells of 2+ volts, eg Ni-Mn. (Metallic Mn is -1.56 V at pH 14.) I raise the hydrogen overvoltage with 1% antimony sulfide so it'll stay charged, that is, so the metallic manganese doesn't discharge spontaneously to Mn(OH)2.

I use zinc plated metal for current collector material, and fine sintered zinc powder as a structural 'sponge' that enhances conductivity, as zinc is about the only thing that itself has high enough hydrogen overvoltage not to bubble hydrogen and discharge the Mn electrode.

In theory, since the reaction voltage of zinc (-1.28) is lower than manganese, the zinc will remain in metallic form unless the electrode is seriously overdischarged, so the cycling/zincate problems of zinc electrodes don't enter the equation.

In an unrelated theory the Mn can start as any oxide - the more oxidized it is, the more charging the cell needs to become metallic Mn. That sounded simple. I compacted together fine Zinc powder, MnO2, & Sb2S3 (50-50-1%) around a fine zinc plated mesh with a zinc plated leed wire and sintered it to fuse zinc particles, but of course in the cell the MnO2 & Zn reacted together spontaneously ("seriously overdischarged" - duh!) and oxidized the zinc, which took the zinc plating off the current collector metal, and it was ruined before it could finish charging.

Anything more oxidized than Mn(OH)2/MnO is going to oxidize some zinc, and evidently it's hard to prevent further oxidation in air before putting the electrode into the electrolyte (flooded cell) in the battery.

Ideally I'd like to start with Mn metal powder. That would sinter together best with the zinc powder and the cell would start charged. I bought some, but the #300 mesh is much too coarse - too little surface area for too much volume, giving few amp-hours. (Zinc comes as a much finer powder, but even there it's said that better zinc electrode results are obtained by starting from ZnO.)

So I have to start with a fine Mn oxide powder. Making Mn with MnO2 + heat and no air evidently leaves a blob of the metal rather than powder.

I was reading the permanganate topic, and someone mentioned a substitution method for getting the metal:

"Provided I can get my hands on Mg powder [pyro supply?], I'll now be gunning for the MnCl2 + Mg ---> Mn + MgCl2 reduction (ΔH = - 161 kJ/mol of MnCl2)."

and there was another one mentioned elsewhere using
3 Mn(OH)2(?iirc) + 2 Al ---> 3 Mn + 2 Al(OH)3.

These sound intriguing, but how might one harvest the fine Mn powder these reactions would make without having it spontaneously form Mn(OH)2?

Thanks for any ideas!

(If I have to start with MnO/Mn(OH)2, evidently I can use MnCO3 and sinter in a closed tube, since the CO2 is driven off at a temperature well below the ~490ºc I get the kiln up to.)

blogfast25 - 21-5-2012 at 12:51

Quote: Originally posted by CraigX  

"Provided I can get my hands on Mg powder [pyro supply?], I'll now be gunning for the MnCl2 + Mg ---> Mn + MgCl2 reduction (ΔH = - 161 kJ/mol of MnCl2)."

and there was another one mentioned elsewhere using
3 Mn(OH)2(?iirc) + 2 Al ---> 3 Mn + 2 Al(OH)3.




The reduction of MnCl2 with Mg works but requires anhydrous manganous chloride, not easy to obtain or make. This reaction does yield a powder of sorts but it's difficult separate it from the MgCl2 (MgCl2 is soluble in ethanol but you'd need very dry ethanol).

For get about the reduction of Mn oxides with Al, not practical at all.

Buy good purity lump Mn and size reduce it yourself, is my advice.

CraigX - 22-5-2012 at 18:58

It's hard metal. I think it might be problematic to get it finer mechanically than what I can buy, and the chemical process also sounds daunting.

I think I'll try the MnCO3 ==> MnO + CO2 route and start with a discharged - but not overdischarged - electrode.

Craig
http://www.TurquoiseEnergy.com

elementcollector1 - 22-5-2012 at 20:35

Try electroplating until you have a sizable amount, and powdering that. Pure manganese, works well, etc.

blogfast25 - 23-5-2012 at 06:13

Quote: Originally posted by CraigX  
It's hard metal. I think it might be problematic to get it finer mechanically than what I can buy, and the chemical process also sounds daunting.


http://www.TurquoiseEnergy.com


It's hard but quite brittle. Should be quite easy to size reduce.