Effects of borate polyester mixed with water on anodizing behavior and electrical properties of ZrO2-coated Al foil

ZrO2-Al2O3 composite oxide film is a promising dielectric material for future application in aluminum electrolytic capacitors. Herein, ZrO2-Al2O3 composite oxide films are prepared on etched Al foils via ZrO2 sol-gel coating and anodization. Generally, Al foil anodization is performed in an aqueous boric acid electrolyte. However, if the Al foils are anodized above approximately 700 V, a boric acid solution is unsuitable for use as an electrolyte, because it facilitates O2 evolution. Therefore, in this study, we use a borate polyester solution as an electrolyte for anodization at a relatively high voltage, and discuss the effects of the borate polyester concentration on the anodization behavior and electrical properties of the ZrO2-coated Al foils. The results suggest that a borate polyester electrolyte with high resistivity is suitable for use in high-voltage anodization, as it effectively increases the sample withstanding voltage. Further, the specific capacitances of the samples decrease as the borate polyester to water ratio increases; the most suitable ratio is found to be 0.4. Relative to those of samples anodized in boric acid solution, the specific capacitances of the samples anodized in the borate polyester electrolyte with a 0.4 mixing ratio are increased by 12.7%, 8.9%, and 43.3% for 700, 800, and 900 V, respectively.