Overpotential co-deposition

In the case of copper, electrodeposition at low overpotentials produces large grains with relatively well-defined crystal shapes. Further increasing the overpotential leads to the formation of cauliflower-like and carrot-like protrusions, and finally, dendritic deposits are formed in the absence of strong hydrogen co-deposition.13... 

The aim of this chapter was to give comprehensive treatment of the morphology of copper electrode posited at high overpotentials, especially in the presence of hydrogen co-deposition, obtained in the potentiostatic conditions from different electrolytes and at different temperatures. 

The deposits obtained at an overpotential of 550 mV with different quantities of electricity are shown in Figs. 3-6. At this overpotential, there is no hydrogen co-deposition at all. The deposit obtained with a quantity of electricity of 2.5 mAh cm-2 is shown... 

Nikolic ND, Brankovic G, Maksimovic VM, Pavlovic MG, Popov KI (2010) Application of pulsating overpotential regime on the formation of copper deposits in the range of hydrogen co-deposition. J Solid State Electrochem 14 331—338... 

Only elements with electrode potentials higher than lead will co-deposit with lead at the cathode. Hence, those elements that dissolve with lead from the anode are theoretically not able to deposit at the cathode and will remain and build up in solution. Ideally this automatically allows only the predominant metal to transfer from the anode to the cathode, but does require treatment of the electrolyte to remove those impurities which tend to accumulate. However, departure from equilibrium caused by high transfer currents and different electrode overpotentials for dissolution and deposition can broaden the range of elements that can be co-dissolved at the anode and co-deposited... 

The schematic indicating the potential region where OPCD occurs is shown in Fig. 2 using the example of CoFe alloy. The schematic considers electrodeposition process from the solution which is at standard conditions (P°, Cqq2+ = Cpg2+ = 1 mol). In order to obtain desired composition of CoFe (50 50) alloy, the concentrations of Co and Fe ions in the solution have to be appropriately adjusted together with the potential (overpotential) or current at which the alloy deposition occurs. Typical approach towards the solution and deposition potential (current) design involves experiments where the concentration of more noble metal, Co, is such that C(-q2+ < Cp 2+, so that Co deposition occurs under mixed control for a... 

The results of electrochemical impedance spectroscopy show that Al coating leads to an increase in the polarization resistance of a bare Mg alloy by one order of magnitude in 3.5 wt% NaCl solution. Furthermore, the potentiodynamic polarization results show that Al-coated Mg alloy can be passivated, and a wider passive region with a lower passive current density can be obtained if the Al is electrodeposited at a lower applied current or a low cathodic overpotential. The passivity of the co-deposited Al/Zn film is slightly inferior to that of the pure Al coating. 

A mercury cathode finds widespread application for separations by constant current electrolysis. The most important use is the separation of the alkali and alkaline-earth metals, Al, Be, Mg, Ta, V, Zr, W, U, and the lanthanides from such elements as Fe, Cr, Ni, Co, Zn, Mo, Cd, Cu, Sn, Bi, Ag, Ge, Pd, Pt, Au, Rh, Ir, and Tl, which can, under suitable conditions, be deposited on a mercury cathode. The method is therefore of particular value for the determination of Al, etc., in steels and alloys it is also applied in the separation of iron from such elements as titanium, vanadium, and uranium. In an uncontrolled constant-current electrolysis in an acid medium the cathode potential is limited by the potential at which hydrogen ion is reduced the overpotential of hydrogen on mercury is high (about 0.8 volt), and consequently more metals are deposited from an acid solution at a mercury cathode than with a platinum cathode.10... 

A/cm2, 90°C, 30 wt% KOH) with time during a batch experiment demonstrating in situ activation. Evidently the addition of only 10 mg/cnr of cobalt in the form of cobaltous nitrate, which is decomposed by generating soluble Co(OH)fand Co(OH)4 species, results in reduction of the anodic overpotential (+340 mV) by more than 50 mV. This is due to the anodic deposition of a thin film of electrocatalytically active cobalt spinell, C04O4, according to... 

Hinogami et al. deposited small metal particles of Cu, Ag, and Au (20-200 nm diameter) onto p-Si in order to reduce the overpotential for C02 reduction [114], such that the onset potentials were 0.5 V more positive than those of the respective bulk metal electrode. In a C02-saturated aqueous solution, mainly CO was produced with some formic acid however, at —1.05 V (versus SCE) the Ag-coated electrodes produced CO with 51% faradaic efficiency. Similarly, Au-coated electrodes yielded CO with 62% faradaic efficiency at —0.74V (versus SCE). The Cu-coated electrodes, when held at a potential of—1.2 V (versus SCE), produced formic acid and CO with faradaic efficiencies of 32% and 19%, respectively. Small amounts of methane and ethylene were also observed. 

It is claimed (165) that a composite electroplated deposit of Co plus Mo, regardless of the alloy thickness, provides far from such an active deposit as is achieved by in situ addition of very small amounts of Co and Mo anionic species during cathodic Hj evolution where at least 200 mV of overpotential... 

If the potential of an atomically smooth (non-stepped) singular face is changed, e.g., to a value more negative than the reversible potential, the enhanced deposition rate dep,free will increase the adatom concentration above its equilibrium value Co,ads until the opposite reaction of dissolution idiss,ads reestablishes the balance. The adatom concentration Cads(r/) increases and becomes a function of overpotential as given by eq. (2.29). 

Eigure 16a shows CO stripping on Pt(lll), Pt(lll)-Ru (following spontaneous deposition), and Pt(lll)-Ru (where the spontaneously deposited ruthenium has been reduced in hydrogen). Only a very small reduction in overpotential for CO electro-oxidation is observed for Pt(lll)-Ru . The overpotential for CO electro-oxidation on the Pt(l 11) surface has been reduced, however, on the Pt(lll)-Ru surface, and the latter exhibits a doublet structure. This CO stripping result on Pt(l 11)-Ru is nearly identical to that found on the Pt(l 11)-Ru surface where the ruthenium was MVD deposited (Eigure 15) It was concluded that Pt(lll)-Ru ° was decorated islands of Ru . 

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