Sciencemadness Discussion Board

Magnesium-Copper cell questions

Sulaiman - 29-2-2016 at 05:18

I am experimenting with primary electochemical/galvanic/voltaic cells,
presently I am constructing a Magnesium cathode, which is going to be made of a solid 19mm dia x 100mm rod, which has oxidised a little on the surface.
I have read of using iodine to 'activate' the surface / reduce MgO and I wonder if
iodine vapour, or a dip in Lugol's Iodine solution (KI + I2 in H2O) would work ?

As a sanity check, please comment on this simple cell;
Mg cathode (activated with iodine), MgSO4, Porous Pot, CuSO4, Cu.
I am expecting the CuSO4 to deposit Cu on the Cu electrode/anode
and the sulphate ions to migrate through the porous pot to oxidise Mg to MgSO4 at the cathode. Destroying CuSO4 and creating MgSO4.
The odd part is that this cell is intended to run 'dry'
... not liquid electrolytes/solutions but the hydrated sulfates with atmospheric moisture, like a 'crystal cell'
This is for very low current long-term discharge, e.g. 100 uA.century ,starting with 2 Moles of Mg. (50g)
If performance is poor I will add H2SO4.
How wrong am I ?

Marvin - 29-2-2016 at 05:32

It's a weird idea, usually the point of the pot is to prevent ion migration.

I think if you add acid you'll just dissolve the magnesium.

Sulaiman - 29-2-2016 at 09:48

If there is no ion flow then there is no current

In this case the porous pot is mainly to prevent the two electrolytes from mixing
to prevent the magnesium from being copper plated


Using iodine to 'activate' the magnesium I discovered from Grignard reactions,
I do not know if it will help at all in this case
.... anyone ?

deltaH - 29-2-2016 at 13:45

Neither copper sulphate nor magnesium sulphate are deliquescent salts, so they probably won't conduct sufficiently when dry. I'd use a concentrated calcium chloride brine as the electrolyte because that will remain wet indefinitely when exposed to the atmosphere. You can moisten sand with it if you want it semi solid or even just soak into earthenware pots.

My two cents worth: at such a low current, you can use air as the oxidant and iron as the anode (exposed to air). Your setup would then mimic that of cathodic protection using a magnesium bar. Just submerse the magnesium bar under the brine or brine sand fully, connected to a metal wire and have the other end connected to a vertical iron bar/object that is only partially immersed in the sand and a portion exposed to air.

I wouldn't bother with special activation of the magnesium but I would give it a light sanding before using.

[Edited on 29-2-2016 by deltaH]

100-year cell

deltaH - 29-2-2016 at 14:02

Sorry for the terrible drawing, but this was done on a device not meant for drawing, many brave ewoks died creating this :D

The cell uses fine iron wool in the top to hold the sand in place (acts like a filter) and at the same time provide a large surface area for reaction with air.

100 year cell.jpg - 32kB

You could probably even draw tens of milliamps from a cell like this if you wanted to.

If you want it light, simply use cotton wool to soak up the electrolyte solution instead of sand.

I would make the casing a glass jar if you want it to last 100 years. Plastic may become brittle after so long a time, but if you want to use plastic, use polyethylene or PP container.

[Edited on 29-2-2016 by deltaH]

Marvin - 29-2-2016 at 14:26

Sulaiman,

The diaphragm would need to conduct sulphate ions but I don't see any cell with water and magnesium lasting a long time. While dry I don't see what would make your pot conduct at all. Would traditional zinc/copper/paper cells sealed with sulphur not work?

deltaH,

Your air cell appears to be backwards, the magnesium would anodically protect the iron. You may have hit on a workable solution with iron air though.

blogfast25 - 29-2-2016 at 15:07

Quote: Originally posted by deltaH  
Sorry for the terrible drawing, but this was done on a device not meant for drawing, many brave ewoks died creating this :D

The cell uses fine iron wool in the top to hold the sand in place (acts like a filter) and at the same time provide a large surface area for reaction with air.



You could probably even draw tens of milliamps from a cell like this if you wanted to.

If you want it light, simply use cotton wool to soak up the electrolyte solution instead of sand.

I would make the casing a glass jar if you want it to last 100 years. Plastic may become brittle after so long a time, but if you want to use plastic, use polyethylene or PP container.



Unless you very effectively cut off air during non-use, that iron wool will just keep on rusting like mad.

I prefer the Mg/air cell:

2 Mg + O2 + 2 H2O ===> 2 Mg(OH)2

http://chemistry.stackexchange.com/questions/33066/magnesium...

deltaH - 29-2-2016 at 21:40

Quote: Originally posted by blogfast25  
Quote: Originally posted by deltaH  
Sorry for the terrible drawing, but this was done on a device not meant for drawing, many brave ewoks died creating this :D

The cell uses fine iron wool in the top to hold the sand in place (acts like a filter) and at the same time provide a large surface area for reaction with air.



You could probably even draw tens of milliamps from a cell like this if you wanted to.

If you want it light, simply use cotton wool to soak up the electrolyte solution instead of sand.

I would make the casing a glass jar if you want it to last 100 years. Plastic may become brittle after so long a time, but if you want to use plastic, use polyethylene or PP container.



Unless you very effectively cut off air during non-use, that iron wool will just keep on rusting like mad.

I prefer the Mg/air cell:

2 Mg + O2 + 2 H2O ===> 2 Mg(OH)2

http://chemistry.stackexchange.com/questions/33066/magnesium...


The cell simply has to be constantly shorted if not connected to the load, it will then enjoy cathodic protection and not rust. It slowly and continuously will discharge, faster when not connected to a load because the shorted currents will be higher than under load.

I think op intends to leave it permanently hooked up to the load anyhow (to a clock?). The current that will flow will be less than the cathodic protection current since there is now a resistance in series, but 100uA should be easily attainable under load. One of the reasons for using iron wool instead of just a piece of iron is to increase the surface area and work with thin films of electrolyte over the iron so that the cathodic protection currents shoots up because you've essentially optimised the conditions for iron to rust as fast as possible, so the magnesium has to pump lots of electrons into the iron to counter this.
Then when you connect the load in series, the current should drop.

BTW

2 Mg + O2 + 2 H2O ===> 2 Mg(OH)2

is similar to what will happen in the cell, well at least the half reactions, i.e. Mg => Mg2+ + 2e- and 1/2O2 + H2O + 2e- => 2OH-

The cell I describe above will initially form slaked lime around the immersed parts of the iron wool near the top (due to the high pH that will occur near the surface of the anode) and in time the electrolyte will gradually enrich with MgCl2 as more and more Mg2+ enters into solution and Ca2+ is sunk out of solution, so eventually Mg(OH)2 will start to deposit near the iron as well.

So it is basically a form of a magnesium air cell, a very low current variation anyhow.

One could also substitute MgCl2 as the electrolyte if one has it instead of the CaCl2.

One possible problem with my cell that occurs to me is that the lime deposit that forms near the anode would over very long periods of time react with CO2 to form scale. This scale may insulate the iron surface from electrolyte and so impede operation, but possibly not, it very much would depends on the porosity of the scale that forms, if it's only slightly porous 100uA flow is not much of an ask.

I am very curious about this type of cell, since it's so easy to make, I might give it a try myself soon. I think I will substitute the Mg with Zn and the electrolyte with ZnCl2 because I have those and not Mg.

The emf of the zinc cell will be much lower, but it should still work.

The nice thing about using the zinc variation is that I don't worry about the formation of carbonate impeding operation at the anode.

Alternatively I might also try a piece of aluminium and ZnCl2 as deliquescent electrolyte. Like the CaCl2/Mg cell, the electrolyte will initially precipitate ZnO at the anode and eventually aluminium hydroxide as it gradually transforms from a solution of zinc chloride to aluminium chloride.

In summary, this type of cell is just about making the small cathodic protection currents work for you by making the conditions for corrosion more favourable around the anode.

Incidentally, even a copper anode would work, but honestly, why bother wasting copper when iron wool would do the job just fine and probably even better, delivering higher currents.

[Edited on 1-3-2016 by deltaH]

Sulaiman - 29-2-2016 at 22:33

I foresee a similar problem in my proposed cell, as SO4 ions combine with the Mg cathode, MgSO4 will be formed around the Mg rod
adding to the MgSO4 already present, possibly cracking the porous pot due to the pressure of the expanding MgSO4.
My plan is to have initially empty space beneath the Mg rod by supporting it with a short glass cylinder / shot-glass or similar.

Although the MgSO4.7H2O and CuSO4.5H2O are not hygroscopic
I expect there to be enough water adsorbed on the surfaces of the crystals
to give more than enough conductivity for my intended maximum loading of 100 uA.
The H2SO4 option is only if the cell performance is not up to my expectations, in which case it may be 'boosted' with H2SO4 for short-term high discharge rate use ... for what I do not know !
(theoretically 107 Amp.hours available)

My cell is just a Daniel Cell https://en.wikipedia.org/wiki/Daniell_cell
with magnesium instead of zinc, for higher voltage, I had initially planned to use silver as the anode, but too costly.


[Edited on 1-3-2016 by Sulaiman]

deltaH - 1-3-2016 at 01:04

I built the zinc version in a can of baked beans this morning. The scrap zinc metal charge was 112g enclosed in a cotton rag pouch sitting near the bottom.

It uses sand moistened with 33 wt.% ZnCl2 solution as electrolyte with fine electrolyte moistened iron wool at the top as anode (exposed to air.)

It delivers an open circuit emf of 400mV and 33mA current on shorting, which I will leave connected to prevent the iron wool from rusting to kingdom come.

I must say, the low value of the emf was surprising, but then I looked up the overpotential for oxygen over an iron anode (wiki on overpotential) which is a very high +750mV... in crude terms, that almost algebraically negates the potential that zinc can deliver and what you're left with is oxygen's half potential in alkaline conditions which is about 400mV.

By my estimate, if I'd used Mg and either MgCl2 or CaCl2, I could have had a open cell emf of about 1V.

I will try to upload photos tonight, have to get to work now :(

[Edited on 1-3-2016 by deltaH]