Molecular adsorption at electrolyte/α-Al 2 O 3 interface of aluminum electrolytic capacitor revealed by sum frequency vibrational spectroscopy.

The operating voltage of an aluminum electrolytic capacitor is determined by the breakdown voltage (Ub ) of the Al2 O3 anode. Ub is related to the molecular adsorption at the Al2 O3 /electrolyte interface. Therefore, we have employed sum-frequency vibrational spectroscopy (SFVS) to study the adsorption states of a simple electrolyte, ethylene glycol (EG) solution with ammonium adipate, on an α-Al2 O3 surface. In an acidic electrolyte (pH < 6), the Al2 O3 surface is positively charged. The observed SFVS spectra show that long chain molecules poly ethylene glycol and ethylene glycol adipate adopt a "lying" orientation at the interface. In an alkaline electrolyte (pH > 8), the Al2 O3 surface is negatively charged and the short chain EG molecules adopt a "tilting" orientation. The Ub results exhibit a much higher value at pH < 6 compared with that at pH > 8. Since the "lying" long chain molecules cover and protect the Al2 O3 surface, Ub increases with a decrease of pH. These findings provide new insights to study the breakdown mechanisms and to develop new electrolytes for high operating voltage capacitors.