State-of-the-art aluminum solid polymer electrolytic capacitors are superior to conventional liquid aluminum electrolytic capacitors in ways of lower equivalent series resistance (ESR), higher operational temperature, and higher reliability. In many low-voltage applications, solid polymer electrolytic capacitors are thus preferably used. However, this technology is not available for rated voltages higher than 200 V. One reason for that is the limited breakdown voltage of aluminum oxide in contact with the conducting polymer electrolyte and the limited knowledge about that interface. This study investigated the anodic oxide growth on aluminum at forming voltages up to 1200 V. It was shown that the oxide quality and breakdown voltage drastically decrease at voltages above 900 V. By the introduction of a step-voltage anodization with subsequent intermediate hydrations, the oxide quality was significantly enhanced, improving electrical properties and reaching average breakdown voltages of over 1000 V. With cross-section investigations, it was shown that by the intermediate hydration steps, the void structure within the oxide is changed, so that the voids are smaller and more homogeneously distributed. Additionally, the structure of the hydrated layer, formed by the hydrothermal process on top of the oxide, is changed, so that it is thicker and less porous with the additional hydrations. The polymer likely penetrates less into this changed hydrated layer, which lowers the amount of polymer in direct contact with the oxide. This introduces a more resistive layer between the oxide and the polymer, effectively keeping the polymer from possible breakdown points and defects in the oxide, which could potentially increase the breakdown voltage. This process of performing additional hydrations along the formation of the oxide could be a promising step in achieving the highest-voltage polymer solid electrolytic capacitors.
Kruse et al. (Fri,) studied this question.