Electrolytes anodic passivation

In a recent review, Tao etal. [34] describe the partial fluorination and the perfluorination of organics with particular emphasis on medically important compounds and pharmaceuticals. The selective electrofluorination (SEF) of olefins and active methylene groups is reviewed by Noel et al. [35] In the case of heterocycles, nuclear fluorination is known to be the predominant process. However, in aromatic compounds, nuclear substitution as well as addition proceeds simultaneously, leading to the formation of a mixture of products. The influence of solvents, supporting electrolytes, and adsorption on product yield and selectivity is summarized and evaluated. Dimethoxyethane is found to be a superior solvent for SEF processes. Redox mediators have been employed to minimize anode passivation and to achieve better current efficiencies. 

When organic solvents are used for anodic fluorination, anode passivation (the formation of a nonconducting polymer film on the anode surface that suppresses faradaic current) takes place very often, which results in low efficiency. Moreover, depending on the substrates the use of acetonitrile can yield an acetoamidation byproduct. To prevent acetoamidation and anode passivation, Meurs and his coworkers used an EtaN 3HF ionic liquid as both a solvent and supporting electrolyte (also a fluorine source) for the anodic fluorination of benzenes, naphthalene, olefins, furan, benzofuran, and phenanthroline. They obtained corresponding partially fluorinated products in less than 50% yields (Scheme 8.2) [11]. 

Figure 1.58 Anodic passivation of p-Si and n-Si in different electrolytes without illumination. In KOH corrosion takes place, 50mVs-1 (source Ref. [32]).

The VC was first explored as an electrolyte solvent,which affords a good electrolytic conductivity due to its extremely high relative permittivity (e = 127). However, it became a typical compound as an anode passivation film-forming agent, after it was found that the addition of a small amount of VC suppressed gas evolution during the initial charge with the enhanced cycle efficiency, and protected the decomposition of reduction-susceptible solvents such as trimethyl phosphate (TMP). The excellent stability of the passivating layeE was demonstrated by the fact that the addition of 1 wt% of VC in 1 M LiPE /EC + DMC + DEC (33 33 33 wt%) improved the cycle life of commercial lithium-ion polymer ceUs. ... 

Because of the presence of an oxide film, the dissolution rate of a passive metal at a given potential is much lower than that of an active metal. It depends mostly on the properties of the passive film and its solubility in the electrolyte. During passivation, which is a term used to describe the transition from the active to the passive state, the rate of dissolution therefore decreases abruptly. The polarization curve of a stainless steel in sulfuric acid, given in Figure 6.2, illustrates this phenomenon. In this electrolyte, the corrosion potential of the alloy is close to -0.3 V. Anodic polarization leads to active dissolution up to about -0.15 V, where the current density reaches a maximum. Beyond this point, the current density, and hence the dissolution rate, drops sharply. It then shows little further variation with potential up to about 1.1 V. Above that value the current density increases again because transpassive dissolution and oxidation of water to oxygen becomes possible. 

Solvent-free electrochemical fluorination is an alternative method for preventing anode passivation and acetoamidation [18, 19]. As already mentioned, handling extremely corrosive and poisonous anhydrous HF in a laboratory setting is accompanied by serious hazards and experimental difficulties. Molten salts such as 70 % HF/pyridine (Olah s reagent) and commercially available EtaN-SHF [20] are often used to replace anhydrous HF. Other molten salts with the general formula R4NF-nHF (n > 3.5, R = Me, Et, and n-Pr) are useful in selective electrochemical fluorination. These electrolytes... 

In order to achieve high production rates, high current densities are obviously desirable. However, an excessive current density causes at least two problems (I) increased impurity levels in the cathode deposit an increased roughness promotes occlusion of anode residues and electrolyte and (2) anode passivity occurs at current densities above 25-28 mAcm. ... 

At the anode, metal impurities form chlorides those of copper and rinc arc soluble and gradually build up in solution, while silver and lead chlorides are left as an insoluble slime. Platinum and rhodium also dissolve and are recovered at intervals from the electrolyte when their combined concentration exceeds, say, 75gdm. Significant silver levels may cause problems of anode passivation due to AgCl formation the coating must then be removed. 

Electrolytes which have been employed with this system are 1 molar LiBp4 in propylene carbonate (PC) and 1 Molar LiAsFg in 1 1 by volume PC-DME. The latter appears to be the electrolyte of choice at present. The addition of CO2 or substances such as dibenzyl carbonate (DBC) or benzyl succinimidyl carbonate have been reported to reduce anode passivation which causes voltage delay and increased DC resistance between 40 and 70% depth of discharge." These materials are believed to operate by lowering the impedance of the solid electrolyte interface (SEI) layer on the surface of the lithium anode. 

---