Electrodeposition of copper -- Why are the anodic and cathodic Tafel curves different?

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Screenshot 2020-01-15 at 11.22.42 AM.png


The shapes of the anodic and cathodic Tafel curves are different. What does it mean? Does it mean that the electrodeposition of the copper onto a surface of an electrode is uneven? If yes, I am also thinking that this has something to do with the macrothrowing power? Since it was done in an acidic plating bath it has a poor macrothrowing power. The slope on the right hand side is anodic and it has a greater slope, which shows that the small difference in potential will lead to an uneven deposit. Is my thinking correct? but why the cathodic curve is not like anodic? I am a bit confused here.

Anodic reactions:
1) Cu0 -> Cu2+ + 2e-
2)2H2O -> O2 + 4H+ + 4e-

Cathodic reactions:
1) 4H+ + 2e- -> 2H2
2) O2+ + 2H2O + 4e- -> 4OH-
 
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  • #2
chemisttree
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You should be able to find out the implications for alpha=<0.5 on the tafel plot in a textbook.
 
  • #3
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I was going to start a new thread but I think this may be relevant. I have used ##CaCO_{3}## solution to precipitate ##CaCO_{3}## on the cathode. It is a white layer and removed easily. Another group have a black layer which I want to try replicate:

d0aoSey.png

I believe that the reason for their black colour is because they use a copper electrode which corrodes to give Cu2+ then forms Cu at the cathode. But I can't get the black colour, even with a copper electrode. I think they may add something to the solution to perhaps corrode the electrode even more, or maybe the corrode their matal surface somehow so it reacts with the copper in solution?

Would anyone have any ideas about this? Thanks.
 
  • #4
Borek
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Black often suggests use of graphite electrodes. Presence of copper (especially in the carbonates) would mean some hue of bluish green.
 
  • #5
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Black often suggests use of graphite electrodes. Presence of copper (especially in the carbonates) would mean some hue of bluish green.
Thanks Borek, and indeed, that's why I started with graphite electrode, even then I still get a white layer. They must have added something to their solution that caused the copper (Cu2+ from their copper electrode) to react at the cathode and turn the calcium carbonate black. I've no idea what that could be.

I've tried increasing the voltage, length of time, even tried burning the deposited white layer with a hand held blowtorch, still doesn't turn black.

What chemical / process could turn a white calcium carbonate layer black? I guess that is the question.
 
  • #6
Borek
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Black impurities.

Perhaps copper sulfide could produce brownish/black tint, but sulfides are not between common contaminants in carbonates.
 
  • #8
Tom.G
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Specifically it's CuO, Cupric Oxide. Try Google.
 
  • #9
Borek
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Somehow I doubt in CuO in the solution full of carbonates and designed to produce scale (which from what I remember is what @rwooduk is working with).
 
  • #10
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Black impurities.


Perhaps copper sulfide could produce brownish/black tint, but sulfides are not between common contaminants in carbonates.
It looks like some form of copper oxide is the impurity, maybe copper sulfide could be introduced somehow here.

I should have shown the full image of the sample, it's not the best picture but it actually has a reddish hue (left side), which supports the copper aspect:


k0pAXXk.png



As you probably know the degree / type of deposition at the cathode will depend on the stable species in solution, as given by the Pourbaix diagram:


Sdawee1.png



They are electrodepositing copper oxide. See here: https://www.sciencedirect.com/science/article/pii/S1877705815008450
Specifically it's CuO, Cupric Oxide. Try Google.
Somehow I doubt in CuO in the solution full of carbonates and designed to produce scale (which from what I remember is what @rwooduk is working with).
That paper is actually quite useful, thank you!

However, Borek is correct, yes, copper oxide was my first guess, but the paper I'm working from states that the black deposit is calcite (calcium carbonate). They use a calcium carbonate solution to get a black calcite deposit. A standard calcium carbonate solution is produced over several hours with ##CaCO_{3}## in solution with ##CO_{2}## bubbling, it reaches a pH value of around 6. You could also use ##Ca(OH)_{2}## in a similar way. Alternatively you can mix solutions, such as ##CaCl_{2}## and ##NaHCO_{3}##.

When I use a copper (or graphite) electrode (anode) to precipitate calcium carbonate at the metal surface (cathode), I get a white layer, even though the copper or graphite (anode) shows signs of corrosion. I must be getting some reduction of copper at the metal surface (see Pourbaix), but all I get is a white layer of calcium carbonate.

To get a black calcite layer the precipitated carbonate must have some reaction with a corrosion layer at the surface of the metal. However the metal surface will not readily corrode because it is kept at a negative potential. So I think the blackened layer is achieved by either pretreatment of the metal surface or caused by something added to the calcium carbonate solution.

What if I first apply a copper oxide layer, then use calcium carbonate solution to deposit a calcite layer on the copper oxide? Perhaps they would interact? Any other suggestions would be welcome.
 
  • #11
chemisttree
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Somehow I doubt in CuO in the solution full of carbonates and designed to produce scale (which from what I remember is what @rwooduk is working with).
Right. CuO in solution would be unusual but other copper carbonate species would not. The oxide could be produced from the decomposition of those species.
 
  • #12
chemisttree
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It looks like some form of copper oxide is the impurity...

...but the paper I'm working from states that the black deposit is calcite (calcium carbonate). They use a calcium carbonate solution to get a black calcite deposit. A standard calcium carbonate solution is produced over several hours with ##CaCO_{3}## in solution with ##CO_{2}## bubbling, it reaches a pH value of around 6. You could also use ##Ca(OH)_{2}## in a similar way. Alternatively you can mix solutions, such as ##CaCl_{2}## and ##NaHCO_{3}##.

When I use a copper (or graphite) electrode (anode) to precipitate calcium carbonate at the metal surface (cathode), I get a white layer, even though the copper or graphite (anode) shows signs of corrosion. I must be getting some reduction of copper at the metal surface (see Pourbaix), but all I get is a white layer of calcium carbonate.
If all you are seeing at the electrode is white, you aren’t redoxing copper at all. Does the carbonate ppt on both electrodes and the walls of the container? What exactly is in your solution and what potential and current are you observing? Supporting electrolyte?
 
  • #13
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Right. CuO in solution would be unusual but other copper carbonate species would not. The oxide could be produced from the decomposition of those species.
I agree that copper species is likely causing the black layer not seen in my experiment, but the black layer is not (purely) copper oxide because it is stated to be calcite (##CaCO_{3}##).

If all you are seeing at the electrode is white, you aren’t redoxing copper at all. Does the carbonate ppt on both electrodes and the walls of the container? What exactly is in your solution and what potential and current are you observing? Supporting electrolyte?
Apologies, I should be more clear because this is not electroplating (as in the OP). The copper electrode (anode in this case) is positive to apply a negative bias on the metal (cathode) surface (stainless steel). The solution is calcium carbonate solution (pH 6) or I have also used ##CaCl_{2}.NaHCO_{3}## solution. Both solutions have ions in excess (carbonate or chloride ions) so there is no need to add an electrolyte. When a (negative) potential is applied this generates hydrogen and leads to an increase in pH and ##CaCO_{3}## precipitation at the cathode (stainless steel surface). I get a thin layer of white calcium carbonate on my metal sample. Carbonate does not precipitate anywhere else other than at the cathode because that is the region of increased pH. Although the solution does begin to turn cloudy after some time.

The copper electrode (anode) becomes noticably corroded, but does not blacken. As would be expected at the +ve, especially for ##CaCl_{2}.NaHCO_{3}## solution where there are more aggressive chloride ions. The solution turns turquoise blue and confirms corrosion of the anode. However, you are correct the absence of black layer at the cathode indicates a lack of interaction with any copper species present.

Unfortunately I am using a basic power supply, and not a potentiostat, so I have limited ability to control (or measure) the voltage / current.

To obtain a blackened layer of calcite I think ##CaCO_{3}## solution must interact with the copper species / oxide layer, but I am unsure how. I thought someone here may have some ideas on how to get a blackened calcite deposit and it seemed relevant to the OP.
 

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