Molecular adsorption at electrolyte/α-Al₂O₃ interface of aluminum electrolytic capacitor revealed by sum frequency...

The operating voltage of an aluminum electrolytic capacitor is determined by the breakdown voltage (U_b) of the Al₂O₃ anode. U_b is related to the molecular adsorption at the Al₂O₃/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-Al₂O₃ surface. In an acidic electrolyte (pH < 6), the Al₂O₃ 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 Al₂O₃ surface is negatively charged and the short chain EG molecules adopt a "tilting" orientation. The U_b 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 Al₂O₃ surface, U_b 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.