I am attempting to charge a pair of vanadium electrolytes by pumping them through a flow battery under a constant voltage.
Before charging the electrolytes, the anolyte and catholyte of equal volumes were both 1.5 M VOSO4 and 3 M H2SO4. As expected, this solution was a clear blue as the V(IIII) ions dissolved.
After charging I expected V(V) and V(III) which corresponds to yellow and green.
However, after pumping these electrolytes through the cell under ~1.7 Volts for several minutes, both solutions became much darker in appearance. In addition, the catholyte developed a strange, almost red hue (upper tube in picture). The anolyte seems to be a shade between blue and green, but is also very dark in appearance. After taking away the voltage supply, the solutions were shown to hold a voltage difference so they seem to be charging. However, my solutions don't look at all what I expected. Has anyone experienced this strange coloration before or know what might be the issue?
Hello Ethan Wise ,
Looking at this color, it might look like you are forming a V(II) complex which has more of a purple color. You may also end up with a combination of different oxidation states for the Vanadium metal; I would guess V(II) [purple], V(III) [green], V (IV) [blue] and V(V) [yellow] which gives you a brown-ish color.
The potential that you are applying to your cell might also be incorrect. Also, have you measured the resistivity of the solution?
You might want to check the surface of your electrodes. Is there any delamination?
Best regards,
Marie B.
Hello Ethan Wise ,
Looking at this color, it might look like you are forming a V(II) complex which has more of a purple color. You may also end up with a combination of different oxidation states for the Vanadium metal; I would guess V(II) [purple], V(III) [green], V (IV) [blue] and V(V) [yellow] which gives you a brown-ish color.
The potential that you are applying to your cell might also be incorrect. Also, have you measured the resistivity of the solution?
You might want to check the surface of your electrodes. Is there any delamination?
Best regards,
Marie B.
Thanks for your comments, Marie Berthuel .
I did not observe delamination on the graphite felt electrodes but I intend to rerun my experiment with new electrodes and a lower starting voltage.
As you mentioned, I think it is possible that my electrolytes had V(II), V(III), and V(IV) (purple, green, and blue) but since I started with V(IV) it is strange that I did not observe any oxidized vanadium: yellow V(V). If some air pockets were trapped in my cell, might the oxygen have acted as an oxidizer?
When I took apart the cell, I also noticed some vanadium precipitate inside the cell. I don't know if this is related to my problem.
Could you make a filtration[1], to separate solids (possible precipitates) from the liquid ?
1. FILTRATION TECHNIQUES http://faculty.sites.uci.edu/chem1l/files/2013/11/RDGfiltration.pdf
I agree with Marie Berthuel. There will be different oxidation states formed under charge/ discharge conditions with the vanadium and some of these will react with the acid and dissolve. eg. VOSO4 complex which I have personally made. Color about dark purple
On my 2nd charging attempt I confirmed that the dark coloring was due to a mixture of vanadium ions at different states of charge. In the early stages of charging, both solutions became dark again and remained this way until near the end of the charging process. Then, the solutions rapidly changed to a recognizable yellow V(V) and green V(III).
I suppose the mixture of species leads to an optical effect where almost all light is absorbed by the solution until the concentration of a single ion state approaches 100%.
Hello @Ethan Wise ,
It’s awesome. I am glad we could help you with that issue.
Best regards,
Marie Berthuel
Ethan Wise , Different oxidation states is primary reason. Also look in to electrodes physical state. Some time, because of micro etching of electrodes electrolyte appears dark in color. Filter electrolyte before concluding and also check weight of dried electrodes before and after charging discharging test.
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