Hi, I have been trying electrodeposition of boron from its aqueous solution. My electrochemical cell consists of a three electrode setup consisting copper foil as a working electrode, platinum plate as a counter electrode, and Ag/AgCl as a reference electrode. The reduction potential I am interested with regard to electrodeposition is around -1.34V vs Ag/AgCl. Can I use this experimental setup for CV in the potential window: -1.5V to +0.1V?
Is there any potential window for respective reference electrodes? How do we figure it out?
Dear Vishnu Priya H R
You can use this setup. The reference electrode can measure any potentials, from 0 to a few hundred (+/-) volts, because current does not flow through the reference electrode. Potential windows depend on your potentiostat output voltage.
Other question is that boron itself cannot be obtained from water solutions, it requires salt melts at high temperatures. You can co-deposited boron with some metals, like Ni.
Dear Prof. Branimir,
Thank you for explaining the reference electrode potential window succinctly. Regarding obtaining boron from aqueous solutions, why is it that Boron can't be electrodeposited
Any particular reference to the fused solution based boron electrodeposition?
Thanks
Vishnu
Dear Vishnu Priya H R
The standard electrode potential of boron (for example B(OH)3) is -0.889 V and involves 3H+ and 3e for reduction.
B(OH)3 + 3H+ +3e = B(s) +3H2O Er,st = -0.889 V
Er(B) = -0.889 – 2.3RT/(3F) log c^(-3)(H+)
Er(B) = -0.889 + 2.3RT/F log c(H+)
Er(B) = -0.889 – 0.059 pH
Er(H+|H2) at pH 0 = 0 V Er(B) = -0.889 V
Er(H+|H2) at pH 7 = -0.42 V Er(B) = -1.3 V
Er(H+|H2) at pH 14 = -0.827 V Er(B) = -1.716 V
So at any pH the reduction thermodynamic potential (neglecting the overpotentials) is always for ~ -0.9 V more negative than hydrogen electrode potentials. At any conditions, hydrogen will be evolved by the 100% rate. Similarly like for Al, Mg, etc.
The potential window available for the electrode (in your case copper) is understood as a region of potentials, where almost no current (only the co called capacitive current which is usually very low) flows in the absence of the studied compound (in your case boron compound). The potential window is limited by electrochemical transformation of the electrode material (usually in positive limit) or electrochemical transformation of some component in our electrolyte (e.g. water or supporting electrolyte). Because of high amount of these components, huge current flows when the potential limits are exceeded. This can prevent or interfere with your deposition.
Dear Prof. Branimir,
Based on your previous reply regarding the pH of the solution, what you mean is that no matter what is the reduction potential of the metal of interest (Boron), hydrogen evolves in the cathodic scan because the HER occurs at much greater potential than the electrodeposition of boron?
Also, if I could do a chronopotentiometry by supplying an ample amount of current depending on the area of the working electrode, Will I just see hydrogen evolution and not the electrodeposition of boron?
Thanks
Vishnu.
Yes,
But, your constatation "the HER occurs at much greater potential" should be "the HER occurs at much lower potentia"
Dear Professor Branimir,
What I meant was that assuming pH=7, since the reduction of water is happening at a negative potential (-0.42V), electrodeposition of boron from its aqueous solution happens at much lower potential (-1.3V). Kindly correct me if I am wrong.
Thanks
Vishnu,
Thanks for the clarification, Prof. Braningir.
It was a very fruitful discussion.
Regards
Vishnu,
Dear Vishnu Priya H R
I was very interested in your question.
For the copper electrode the current density at pH = 7 and E = -1.3 V (Er(H+|H2) at pH 7 = -0.42 V; Er(B) = -1.3 V), assuming Tafel slope of -0.12 V dec and exchange current density of ~0.1 mA cm^-2 for hydrogen evolution reaction will be:
j = j0 exp{-2.3(E-Er)/|bc|}= ~2.1 A
which is a very high current. Certanly accounting boron overpotential the deposition potential will be for example -1.4 to -1.5 V SHE (-1.6 to -1.7 V vs Ag|AgCl). If you try some other metal with lower exchange current density and high hydrogen overpotential (for example zinc or mercury, I am not sure does boron makes amalgam), maybe you could obtain small amounts of boron, by the electrolysis of boric acid, try galvanostatic mod with the current of 100-300 mA cm^-2?
I realize that reading the discovery of boron by Sir Humphry Davy https://en.wikipedia.org/wiki/Boron :
Boron was not recognized as an element until it was isolated by Sir Humphry Davy[8] and by Joseph Louis Gay-Lussac and Louis Jacques Thénard.[7] In 1808 Davy observed that electric current sent through a solution of borates produced a brown precipitate on one of the electrodes. In his subsequent experiments, he used potassium to reduce boric acid instead of electrolysis. He produced enough boron to confirm a new element and named the element boracium.[8] Gay-Lussac and Thénard used iron to reduce boric acid at high temperatures. By oxidizing boron with air, they showed that boric acid is an oxidation product of boron.[7][17] Jöns Jacob Berzelius identified boron as an element in 1824.[18] Pure boron was arguably first produced by the American chemist Ezekiel Weintraub in 1909.
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