Chemical reactions in aluminium electrolytic capacitors
Sammanfattning: Aluminium electrolytic capacitors are used in a wide variety of different applications. The aluminium electrolytic capacitor consists of two aluminium foils wound together and separated by special paper, which is impregnated with an electrolyte. Both foils are etched and one of them, the anodic foil, is anodised to a certain voltage to create an oxide layer, which acts as a dielectric. The paper impregnated with the electrolyte works as an elongation of the cathode towards the anode and assures the good contact to the highly etched oxide. In order to improve capacitor performance, extensive investigations have been performed to study the chemical reactions in the electrolyte during the lifetime of the capacitor and to describe the oxide-electrolyte interface. As a part of an ongoing research project UV, FT- IR, FT-Raman and NMR spectroscopy were applied to investigate the electrolyte ingredients and by-products in the electrolyte solution and when they adsorbed or complexed at the surface of the capacitor foil. As a first step the solvation of 1-vinyl-2-pyrrolidone (VP) and poly(1-vinyl- 2-pyrrolidone) (PVP) in solvents of various polarity, specifically water, ethylene glycol (EG), chloroform and carbon tetrachloride, was studied. The different measurements made it possible to establish the structures of the solvated molecules, the type of hydrogen bonding and the strength of the vinyl double bond in the different solvents. The 1H and 13C NMR spectra revealed that polar solvents attack the solutes at the carbonyl group of the pyrrolidone ring, whereas nonpolar solvents interact mainly with the vinyl group in VP and with the polymer chain in PVP. From the adsorption experiments carried out in EG and water, it was concluded that PVP adsorption from both aqueous and EG solutions, was negligible. It was found that the presence of a dicarboxylic acid enhances the adsorption of PVP, due to a hydrophobic interaction between the carbon chains of the polymer and the dicarboxylic acid. The simultaneous adsorption of PVP and azelaic acid was studied as a function of adsorption time, pH, solvent, temperature and concentration in order to establish a more detailed surface complexation model. Three surface complexes of the azelaic acid were identified from both solvents (water and EG), inner sphere, outer sphere and an intermediate one, and their amounts were influenced mainly by the pH. The adsorption of PVP depends only on the azelaic acid and influenced neither by the temperature nor the pH. The interaction between ƒ×-aluminium oxide and an EG based capacitor electrolyte was investigated as well. It was found that only a few ingredients of the electrolyte react with the oxide (azelaic acid, PVP, phosphoric acid), the others act as pH or conductivity buffers (boric acid, ammonia, water). The adsorption of azelaic acid and PVP from the electrolyte was studied as a function of temperature, pH and time and the result was compared to the adsorption from model solutions of simpler composition. The influence of other components such as phosphoric acid both in the electrolyte and on the aluminium oxide was also investigated, as was the presence of water. At low pH and high temperature (T > 105oC) the acid formed an ester with EG and this product adsorbed on the oxide surface. The PVP was attached to the adsorbed azelaic acid by hydrophobic interaction, which is pH independent. Ester formation was catalyzed by other electrolyte ingredients like boric acid. At high pH, surface adsorption of azelaic acid occurs through a de-protonated species, which is mainly co-ordinated through outer sphere complexation. At high temperature or after a long equilibration time, the surface of the alumina changed, resulting in less adsorption of the organic substances, independent of pH. This change is due to a selective adsorption of phosphate species from the electrolyte, which block active surface sites. At the same time, the electrolyte without alumina was also investigated as a function of temperature, experimental time and pH. The chemical reactions in the electrolyte supported the conclusions drawn from the adsorption studies.
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