Molecular adsorption at electrolyte/α-AlO 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 (U) of the AlO anode. U is related to the molecular adsorption at the AlO/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-AlO surface. In an acidic electrolyte (pH < 6), the AlO 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 AlO surface is negatively charged and the short chain EG molecules adopt a "tilting" orientation. The U 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 AlO surface, U 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.