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JohnWW
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The hydrated Mn(II) cation, in aqueous solution, and in crystals in which it is bonded to H2O molecules or oxy-anions, is normally a very pale pink
color. Such "pale" compounds of low-valent transition metals are supposed to be due to "forbidden" energy-level transitions of d electrons, the energy
difference corresponding to that of quanta of the appropriate absorbed wavelengths of visible light. (Similarly for "pale" colors of lanthanide
metals, due to "forbidden" transitions of 4f electrons).
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Xenoid
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Hi dann2 - I guess from your posts about preparing manganese nitrate you are going to have (another) go at MnO2 anodes.
I revisited MnO2 a few months ago, in an attempt to coat some MMO mesh. This was successful, but I had the same old problems with "Purple Haze". I
wrote up some notes, but never got around to posting them at the time. They are a bit garbled, but you (and others)may find them interesting and of
some use.
I'm not sure why you are posting this MnO2 stuff in the cobalt oxide thread, but I'll post my MnO2/MMO anode material in the Multilayer Metal Oxide / Titanium Anodes for Chlorate/Perchlorate thread as it doesn't appear to have been visited recently. 
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papaya
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Reviving old thread, since I found something interesting:
http://www.scribd.com/doc/26829856/Fabrication-of-High-Quali...
and corresponding paper:
https://www.sciencedirect.com/science/article/pii/S001346861...
In short, it has been found that copper cobalt oxide type spinel coating on Ti provides successful electrodes for electrolysis of base solutions, and
incorporation of copper highly increases stability and conductivity of Co oxide, Cu0.3Co2.7O4 being most optimal composition.
Though they never mentioned the use for brine electrolysis, I decided to give it a try in chlorate cell. Ti rod was cleaned, etched in HCl, then
brushed with mixed Cu(NO3)2/Co(NO3)2 solution(the composition calculated in a way to provide 10% at. of Cu) and baked under max. 300°C. Five coats
were put on Ti, without cleaning loosely bound matter in between.
Well, many times I attempted pure Co3O4 electrodes, and what can I say is - in this case I got a coating that physically is very different from pure
Co3O4. It's really hard, and it nearly doesn't stain the wet towel when rubbed(unlike Co3O4), though it's not shiny and maybe porous.
Anyway I put it into cell and with 4V it develops 1.2-1.5A of current(for pure Co3O4 I needed about 4.2V for that), after 15hours of run the cell is
clear - only minor quantity of some fluffy thing floats at the bottom, I believe it's only some dust that developed at the start and will not be
produced anymore. Anyway I filled fresh solution and restarted the cell - it's clear now, the surface of anode after 15hours of run is still hard and
doesn't stain wet towel, I think it'll be interesting to see what will happen in the future.
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plante1999
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Quite interesting, keep us updated.
I never asked for this.
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papaya
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Anyone has access to this?
http://onlinelibrary.wiley.com/doi/10.1111/j.1744-7402.2011....
[Edited on 28-1-2014 by papaya]
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papaya
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EDIT:
The link seems not to work, it's
Rousset, A., Tenailleau, C., Dufour, P., Bordeneuve, H., Pasquet, I., Guillemet-Fritsch, S., Poulain, V. and Schuurman, S. (2013), Electrical
Properties of Mn3−xCoxO4 (0 ≤ x ≤ 3) Ceramics: An Interesting System for Negative Temperature Coefficient Thermistors. International
Journal of Applied Ceramic Technology, 10: 175–185. doi: 10.1111/j.1744-7402.2011.02723.x
It is interesting, since in the abstract it states that single phase spinel is formed :
"Single-phase spinel manganese cobalt oxides Mn3−xCoxO4 dense ceramics were prepared for the first time and their structural/electrical property
relationships characterized. The electrical properties, that is, the resistivity at 25°C, the energetic constant, and the resistance drift at 125°C,
were determined and correlated with the cation distribution. Finally, the electrical characteristics of the Mn3−xCoxO4 system were compare'd
with other important classes of manganese-based spinel oxides, Mn3−xNixO4 and Mn3−xCuxO4, already commercialized as negative temperature
coefficient (NTC) thermistors. The high values of energetic constant and low resistivities observed in Mn3−xCoxO4 ceramics present a promising
interest for such industrial applications."
As far as I know no one yet tried to use mixed Co3O4 - MnO2 oxides coating, I would like to read it if someone shares. Thanks.
[Edited on 28-1-2014 by papaya]
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papaya
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Have found it for free!
http://libgen.org/scimag/get.php?doi=10.1111%2Fj.1744-7402.2...
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papaya
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Well, I'm very pleased with preliminary results which that particular Cu0.3Co2.7O4 coated anode shows. As I told I put it into the cell five days ago,
and it's still running perfectly and it seems it is not going to die in the near future, as it looks very good and doesn't lose any noticeable
quantity from coating (the liquid is really clear, only very slight brown layer is attached to the bottom of the cell, that I think came out of
CATHODE when I turned off the cell for a minute).
BUT there's something that you must know in case if you want to try it (I encourage everyone interested to test it himself as to compare the
results).
(1) When first prepared and put into cell the anode will lose some amount of loosely bound stuff in the first 24-36hours of working - this is NOT the
real degradation rate.
After I washed it and put into a fresh cell, it showed only negligible amount of the same stuff in next 24 hours. That time the anode was running at
current density of 40-50mA/cm^2 at max., so I wanted to see what happens if I put the whole 100mA/cm^2 and adjusted the voltage to achieve that. In
the next 12 hours I found at the bottom of the cell black dense precipitation that looked like the same as coating on the anode - it seems it crackled
and detached from the surface and fell down. However anode looked still OK so I washed it, filtered the solution and put it back to 40-50mA/cm^2 mode.
After 36 hours of run cell is PERFECTLY clean, unbelievable, so the second point is
(2) never exceed 50mA/cm^2 current density, seems 30-40 will be optimal for a long run.
I cannot see the reason WHY at higher current density(2x) the wear rate becomes so unproportionally high, it may be even the thermal effect of the
hotter cell
so point (2.1) is keep the cell below 40°C (though not sure on this point)
(3, maybe important or not) I baked anode at low temps, on the hot plate, not exceeding 300°C, put 5 coats, not rubbing the surface in between. When
applying last coats it seemed like previous layers absorbed the liquid on contact, even they were HARD(in contrast to soot-like).
I think I'll run this particular cell (anode) for some more time without much intervention, only adding water/salt, but I already see it's really
tough and maybe will not run for months until it dies some day.
In addition here is the article that made my mind to try it :http://libgen.org/scimag/get.php?doi=10.1016%2Fj.electacta.2010.04.026
I also have some ideas on MnxCo3-xO4 spinel that may become next thing to try (one formulation is not enough  )as I found it exists as a single phase up to x=1.3 and is possible to make straight from mixed nitrate solutions,
however couldn't find anything on it's durability, see about it here http://libgen.org/scimag/get.php?doi=10.1016%2Fs0013-4686%28...
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