I know that it is 2 years later since your last post, but over the last year I have been fascinated by, and experimenting with MgSO4 and other sulphate electrolytes in Lead-acid batteries for solar off-grid energy storage.
I wanted to ask if you have done any more experimenting since your last post?, or if you got further info on the actual chemistry?,
I've chatted to some good friends of mine who are chemists, and attempted to discover some possible equations for the process, and my current thoughts are this: (Note, I am not a chemist by any stretch of the imagination
, but I do learn fast, and have been doing a lot of research, hopefully other people with greater understanding in this area can enlighten me)
So, the standard equation for Lead-acid chemistry (Discharge) (both half equations together), according to wikipedia, is:
Pb(s) + 2H2SO4 (aq) + PbO2(s) <=> 2PbSO4(s) + 2H2O @ 2.0v
Half equations for Discharge are:
@ Neg plate (anode)
Pb(s) + HSO4- => PbSO4(s) + H+ + 2e- @ 0.356v
@ Pos plate (cathode)
PbO2 (s) + 3H+ + HSO4- + 2e- => PbSO4 (s) + 2H2O @ 1.685v
Since Pb compounds are mostly insoluble, there is never any significant amount of lead in the solution, so the electrolyte is mostly made up of H2O + H2SO4.
Now, if we look at adding MgSO4 to the battery, instead of H2SO4 (fully replacing the electrolyte, not just adding to it), I wonder if we get the following balanced equations:
Pb(s) + PbO2(s) + 2MgSO4(aq) + 2H2O => 2PbSO4(s) + 2Mg(OH)2
The Half equations being:
@ Neg Plate
Pb(s) + SO42-
=> PbSO4(s) + Mg(OH)2
@ Pos plate
PbO2(s) + SO42-
=> PbSO4(s) + 2e-
An interesting thing here is that if these equations are correct, then the presence of MgSO4 instead of H2SO4, causes a Mg(OH)2 solution, which is an Alkaline, along with removing any H2SO4 or HSO4-
ion from the equations, would turn this battery into a lead-alkaline battery instead yeah?.
A fully charged battery of this type would have PbSO4(s) in precipitate, with Mg(OH)2 in solution, and upon discharge, would have MgSO4 in solution, and Pb back on the plates.
Would it, therefore, theoretically be more tolerant of deeper-discharges (say, instead of a 30% DOD, a 70% DOD), without causing epic damage to the plates / electrolyte?, since the plates aren't being subjected to the H2SO4. I assume though, that Alkaline solutions would still have an effect on the Pb (MgSO4 seems to have a pH of between 9 to 11, depending on concentration, which makes it a pretty effective alkaline).
I understand how tetravalent Pb4+ (at the Pos plate, from PbO2) becomes reduced to form divalent Pb2+
liberating + charge to the + plate, and that (at the Neg plate, from Pb(s) ), Pb is oxidised to form divalent Pb2+
liberating - charge to the - plate,
but I have wondered what processes the Mg is undergoing during the charge/discharge process, and if it would return so cleanly into MgSO4 under discharge, or if, instead, it would remain as Mg(OH)2, and the SO4 ion from the PbSO4 would react with the water, to form HSO4-
Looking at the Metal Activity Series Table:
is way at the bottom (as in, it needs a strong oxidising acid to react with), and it is less reactive than Mg2+
(which reacts with a far greater range of acids)
So, my thoughts suggest that having the Pb + Mg together in the same electrolyte means that the Mg2+
(as an ion in solution) is going to have a greater displacing effect on the Pb2+
, and that the Pb2+
will be more submissive in this arrangement.
Maybe this accounts for how the Mg2+
ion will remove the sulphation on the Pb plates, due to it's greater reactivity (its greater ability to lose e-
more readily to form + ions, corrode more easily and become a stronger reducing agent (electron donor)).
Perhaps it is, literally, displacing the Pb2+
of the PbSO4 at the surface area of where the Mg2+
in electrolyte meets the PbSO4 on plates, like the Pb moves out of the way, and lets the Mg take it's partner, which, being so easily water soluble, has no trouble dissolving into solution, thus removing the SO4 ion from the Pb plate.
I have wondered, if that is the case, why the Mg2+
as an ion in solution, would form Mg(OH)2
, rather than remain as MgSO4(aq)?
Also, I wonder if this means that the Mg would NOT displace onto the Pb plates themselves, unlike a less reactive metal such as Cu, which seems to be displaced onto the Pb plates, when the Pb comes into solution when using CuSO4.
I have read online about other people having big success of lead-acid battery rejuvenation using Alum (K2
Both K and Al are way higher on the reactivity table of metals than Pb, and so I wonder if it is having a similar effect to the Mg, but with an even greater effect due to K and Al being even greater in reactivity than that of Mg as well?
MgSO4 is highly water soluble compared to PbSO4, which tends to remain more as a precipitate.
Lastly, in my experiments, I have different batteries of differing molarity going on, to test the results.
Just for reference, in case it's helpful to anyone else, molar mass of MgSO4.7H20 (epsom salts) = 246.47g/mol
This means that:
0.3mol solution of MgSO4 = 75g per 1 Litre of H2O
1mol solution of MgSO4 = 246.47g per 1 Litre of H2O
5mol solution of MgSO4 = 1.2Kg per 1 Litre of H2O
In my experiment with the 0.3mol solution, I found that the battery (it was a 100A 12v deep cycle) did only present a small spark when shorted out across the terminals, and when shorted out through a shunt, it initially sparked very little, and had a small Amp reading across the shunt, approx. 3amp, however this began to rise, and over a course of 5 minutes, the shunt was reading approx. 50amps, and the 4Gauge wire connecting the terminals was feeling quite warm.
The battery itself felt warm, and the hottest point was in the cell that was the most sulphated of them all when I did the electrolyte conversion, from its previous lead-acid life.
I have wondered if the solubility of the PbSO4(s) back into solution, and then for the Pb to return to the plate is playing a role in throttling the "speed" of the reaction, and therefore the maximum short-circuit amp bandwidth, so to speak.
I have not yet run the same experiment on the 1mol and 5mol solution setups to see if the same thing occurs.
Can anyone add to / enlighten / correct my equations and calculations for all of this?,
I too am interested in alternative battery electrolytes, and with all the armchair scientists on internet forums out there naysaying to the possibility of using MgSO4 as an additive, let alone a stand-alone electrolyte, it would be great to refine and come up with an understanding of what processes and equations are actually going on.
Anyone got any funky ideas?
- Sylph H