Electrodeposition of less noble elements

Electrodeposition of less noble elements and aluminum alloys 

Sodium and lithium Both sodium [15] and lithium [16] electrodeposition was successful in neutral chloroaluminate ionic liquids that contained protons. These elements are interesting for Na- or Li-based secondary batteries, where the metals would serve directly as the anode material. The electrodeposition is not possible in basic or acidic chloroaluminates, only proton-rich NaQ or LiQ buffered neutral chloroaluminate liquids were feasible. The protons enlarged the electrochemical window towards the cathodic regime so that the alkali metal electrodeposition became possible. For Na the proton source was dissolved HQ that was introduced via the gas phase or via 1-ethyl-3-methylimidazolium hydrogen dichloride. Triethanolamine hydrogen dichloride was employed as the proton source for Li electrodeposition. For both alkali metals, reversible deposition and stripping were reported on tungsten and stainless steel substrates, respectively. 

Callium Elemental gallium can be electrodeposited from both chloroaluminate [17] and chlorogallate [18] ionic liquids. In the latter case l-ethyl-3-methylimidazohum chloride was mixed with GaQs, thus giving a highly corrosive ionic liquid that was studied for GaAs thin film electrodeposition. In the chloroaluminates Ga can be deposited from Lewis acidic systems. It was found that the electroreduction from 

Ga(III) leads first to Ga(I), then upon further reduction the elemental Ga forms from Ga(I). On glassy carbon the electrodeposition involves instantaneous three-dimensional nucleation with diffusion-controlled growth of the nuclei. No alloying with A1 was reported if deposition of Ga was performed in the Ga(I) diffusion regime. Reproducible electrodeposition of Ga is a promising route to binary and ternary compound semiconductors. A controlled electrodeposition of GaX quantum dots (X = P, As, Sb) would be very attractive for nanotechnology. 

Aluminum alloys with niobium and tantalum Nb and Ta can be obtained in elemental form from high-temperature molten salts. Nb and Ta are widely used as coatings for corrosion protection as they form - like Al - thin oxide layers that protect the underlying material from attack. In technical processes several high-temperature molten salts are employed for electrocoating and the morphology of the deposit is strongly influenced by the composition of the baths. Some attempts have 

2 Making of Inorganic Materials by Electrochemical Methods 6.2.1.3 Electrodeposition of less noble elements 

A1 is more noble than Ti, and so at room temperature only codeposits and alloys can be obtained. Furthermore, kinetic factors also play a role in the electrodeposition of the element. 

The electrodeposition of Cr in acidic chloroaluminates was investigated in [24]. The authors report that the Cr content in the AlCr deposit can vary from 0 to 94 mol %, depending on the deposition parameters. The deposit consists both of Cr-rich and Al-rich solid solutions as well as intermetallic compounds. An interesting feature of these deposits is their high-temperature oxidation resistance, the layers seeming to withstand temperatures of up to 800 °C, so coatings with such an alloy could have interesting applications. 

It was quite recently reported that La can be electrodeposited from chloroaluminate ionic liquids [25]. Whereas only AlLa alloys can be obtained from the pure liquid, the addition of excess LiCl and small quantities of thionyl chloride (SOCI2) to a LaCl3-sat-urated melt allows the deposition of elemental La, but the electrodissolution seems to be somewhat Idnetically hindered. This result could perhaps be interesting for coating purposes, as elemental La can normally only be deposited in high-temperature molten salts, which require much more difficult experimental or technical conditions. Furthermore, La and Ce electrodeposition would be important, as their oxides have interesting catalytic activity as, for instance, oxidation catalysts. A controlled deposition of thin metal layers followed by selective oxidation could perhaps produce cat-alytically active thin layers interesting for fuel cells or waste gas treatment. 

Aluminium allays with iron, cobalt, nickel, copper, and silver 

Nb and Ta can be obtained in elemental form from high-temperature molten salts. 

Nb and Ta are widely used as coatings for corrosion protection, since they - like A1 - form thin oxide layers that protect the underlying material from being attacked. 

The authors reported that they obtained Nb contents of up to 29 wt-% in the deposits, at temperatures between 90 and 140 °C. In [22], chloroaluminate liquids were employed at room temperature and AlNb films could only be obtained if NbCl5 was prereduced in a chemical reaction. The authors reported that Nb powder is the most effective reducing agent for this purpose. Similar preliminary results have been obtained for Ta electrodeposition. Although it seems to be difficult to deposit pure Nb and Ta in low-melting ionic liquids, the alloys with A1 could have quite interesting properties. 

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Generally, the experimental results on electrodeposition of CdS in acidic solutions of thiosulfate have implied that CdS growth does not involve underpotential deposition of the less noble element (Cd), as would be required by the theoretical treatments of compound semiconductor electrodeposition. Hence, a fundamental difference exists between CdS and the other two cadmium chalcogenides, CdSe and CdTe, for which the UPD model has been fairly successful. Besides, in the present case, colloidal sulfur is generated in the bulk of solution, giving rise to homogeneous precipitation of CdS in the vessel, so that it is quite difficult to obtain a film with an ordered structure. The same is true for the common chemical bath CdS deposition methods. 

Gold electrodeposition has been reported from chloroaluminate-based liquids and from a liquid made of an organic salt and AuQs [55,56]. Although gold can be electrodeposited in high quality from aqueous solutions the latter result is interesting, especially with respect to the deposition of unusual alloys of gold with less noble elements. 

Much of the early literature of polonium describes methods for separating it from these mixtures many of these have subsequently been adapted to the separation of milligram amounts of polonium from irradiated bismuth and to its purification. The methods range from a simple chemical separation of the element with a tellurium carrier to its electrodeposition on to a more noble metal or its spontaneous electrochemical replacement on the surface of a less noble metal. 

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