Electrolytic reduction process

Aluminum can be produced by metaHothermic, carbothermic, or electrolytic reduction processes. The earliest commercial process for producing aluminum (1855—1893) was sodiothermic reduction of aluminum haUdes. Once the HaH-HAroult process became commercial, however, sodiothermic reduction was not competitive. 

The polarographic method is applicable to the determination of inorganic anions such as bromate, iodate, dichromate, vanadate, etc. Hydrogen ions are involved in many of these reduction processes, and the supporting electrolyte must therefore be adequately buffered. 

Metallic Mg is produced industrially using both electrolytic and thermal reduction methods. The electrolytic processes differ primarily in the choice of electrolyte—anhyd MgClj, partially hydrated MgClj x HjO and MgO. The more important thermal reduction processes use FeSi, A1 alloys or C as reducing agents. 

To convert aluminum from the stuff of princes toys into recyclable kitchen foil required an inexpensive electrolytic reduction process. Two 22-year-old scientists, the American chemist Charles Hall and the French metal-lurgist Paul Heroult, discovered the same process independently in 1886. Both became famous as founders of the aluminum industry. Hall in the United States and Heroult in Europe. 

Thermal magnesium, i.e., magnesium produced by the Pidgeon process earlier and by the magnefherm process at present, constitutes only 30% of the total magnesium production. The rest is produced electrolytically in which the leading examples are (i) the Dow electrolytic reduction process, and (ii) Norsk hydro process. 

When the electrolyte solutions are not too reactive, as in the case of ethereal solutions, there is no massive formation of protective surface films at potentials above Li intercalation potential, and most of the solvent reduction processes may occur at potentials lower than 0.3 V vs. Li/Li+. Hence, the passivation of the electrodes is not sufficient to prevent cointercalation of solvent molecules. This leads to an exfoliation of the graphite particles into amorphous dust (expholiated graphene planes). This scenario is demonstrated in Figure 2a as the reduction of the 002 diffraction peak21 of the graphite electrode, polarized cathodically in an ethereal solution. 

These facts would suggest that the electrolysis of molten alkali metal salts could lead to the introduction of mobile electrons which can diffuse rapidly through a melt, and any chemical reduction process resulting from a high chemical potential of the alkali metal could occur in the body of the melt, rather than being confined to the cathode volume. This probably explains the failure of attempts to produce the refractory elements, such as titanium, by electrolysis of a molten sodium chloride-titanium chloride melt, in which a metal dust is formed in the bulk of the electrolyte. 

Electroanalytical techniques are an extension of classical oxidation-reduction chemistry, and indeed oxidation and reduction processes occur at the surface of or within the two electrodes, oxidation at one and reduction at the other. Electrons are consumed by the reduction process at one electrode and generated by the oxidation process at the other. The electrode at which oxidation occurs is termed the anode. The electrode at which reduction occurs is termed the cathode. The complete system, with the anode connected to the cathode via an external conductor, is often called a cell. The individual oxidation and reduction reactions are called half-reactions. The individual electrodes with their half-reactions are called half-cells. As we shall see in this chapter, the half-cells are often in separate containers (mostly to prevent contamination) and are themselves often referred to as electrodes because they are housed in portable glass or plastic tubes. In any case, there must be contact between the half-cells to facilitate ionic diffusion. This contact is called the salt bridge and may take the form of an inverted U-shaped tube filled with an electrolyte solution, as shown in Figure 14.2, or, in most cases, a small fibrous plug at the tip of the portable unit, as we will see later in this chapter. 

Obtained in MeCN solution containing 0.2 mol dm-3 B114NBF4 as supporting electrolyte. Solutions were 2 x 10-3 mol dm 3 in ligand, and potentials were determined with reference to SCE. Three-electron reduction process as determined by coulometric experiments. Cathodic shift in reduction potential produced by the presence of anions (4 equiv) added as their ammonium of butylammonium salts. Values obtained in DMSO solution. 

Electrolytic or chemical (by carbon and hydrogen) reduction processes have been employed in a number of cases to obtain phosphides from phosphates. 

The nanostructured Au and AuPt catalysts were found to exhibit electrocatalytic activity for ORR reaction. The cyclic voltammetric (CV) curves at Au/C catalyst reveal an oxidation-reduction wave of gold oxide at +200 mV in the alkaline (0.5 M KOH) electrolyte but little redox current in the acidic (0.5 M H2SO4) electrolyte. Under saturated with O2, the appearance of the cathodic wave is observed at -190 mV in the alkaline electrolyte and at +50 mV in the acidic electrolyte. This finding indicates that the Au catalyst is active toward O2 reduction in both electrolytes. From the Levich plots of the limiting current vs. rotating speed data, one can derive the electron transfer number (w). We obtained n = 3.1 for ORR in 0.5 M KOH electrolyte, and 2.9 for ORR in 0.5 M H2SO4 electrolyte. The intermittent n-value between 2 and 4 indicates that the electrocatalytic ORR at the Au/Ccatalyst likely involved mixed 2e and 4e reduction processes. 

The implication of such a picture of the solution structure on the microscopic level not only concerns ion transport but also further relates to the electrochemical stability of the electrolytes in lithium ion cells, because these solvent molecules in the solvation sheath, such as EC or PC, migrate with the ions to electrode surfaces and are probably more involved in the oxidative or reductive processes than the noncoordinating, low- solvent molecules, such as the linear carbonates. This could have a profound impact on the chemical nature of the electrolyte/electrode interfaces (section 6). 

Aurbach and co-workers performed a series of ex situ as well as in situ spectroscopic analyses on the surface of the working electrode upon which the cyclic voltammetry of electrolytes was carried out. On the basis of the functionalities detected in FT-IR, X-ray microanalysis, and nuclear magnetic resonance (NMR) studies, they were able to investigate the mechanisms involved in the reduction process of carbonate solvents and proposed that, upon reduction, these solvents mainly form lithium alkyl carbonates (RCOsLi), which are sensitive to various contaminants in the electrolyte system. For example, the presence of CO2 or trace moisture would cause the formation of Li2COs. This peculiar reduction product has been observed on all occasions when cyclic carbonates are present, and it seems to be independent of the nature of the working electrodes. A single electron mechanism has been shown for PC reduction in Scheme 1, while those of EC and linear carbonates are shown in Scheme 7. ... 

Because of the similar potentials between fully lithiated graphite and lithium metal, it has been suggested that the chemical nature of the SEIs in both cases should be similar. On the other hand, it has also been realized that for carbonaceous anodes this formation process is not expected to start until the potential of this anode is cathodically polarized (the discharge process in Figure 11) to a certain level, because the intrinsic potentials of such anode materials are much higher than the reduction potential for most of the solvents and salts. Indeed, this potential polarization process causes one of the most fundamental differences between the SEI on lithium metal and that on a carbonaceous anode. For lithium metal, the SEI forms instantaneously upon its contact with electrolytes, and the reduction of electrolyte components should be indiscriminate to all species possible,while, on a carbonaceous anode, the formation of the SEI should be stepwise and preferential reduction of certain electrolyte components is possible. 

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