Characterization of the Co-Prussian Blue Analogue Interface: Insights from In Situ Vibrational Sum Frequency Generation Spectroscopy

In secondary ion battery systems, understanding the electrode–electrolyte interfaces is crucial for enhancing the cycle life and performance. However, obtaining molecular insights into the interfacial structure is challenging. In this study, we employed surface-sensitive vibrational sum frequency generation (VSFG) spectroscopy to characterize a cobalt Prussian blue analogue (PBA) material with varying K+-ion content. The cathode material was characterized both at the solid TiO2/Au interface and the liquid electrolyte interface by monitoring the C≡N-stretching vibrations of the Coa–CN–Feb unit (where a and b are the oxidation states). Their variations of intensities and frequencies as functions of K+-ion loading and applied potential provide structural information about PBA before and during electrochemical oxidation. The analysis reveals an inhomogeneous distribution of PBA at open circuit potential, consisting primarily of FeII–CN–CoII, FeII–CN–CoIII, and FeIII–CN–CoII units. Additionally, the combination of attenuated total reflectance infrared spectroscopy (ATR-IR) and Raman spectroscopy yields further information about both the surface of the materials and its bulk structure, which is significantly impacted by the presence of K+ ions. The effect of increased K+ concentrations is highlighted in the VSFG spectroscopy, which indicates structural distortions upon increasing the K+ concentration. In-situ analysis under a bias characterized the electrochemical oxidation process, showing systematic formation of FeIII–CN–CoIII and FeII–CN–CoIII species alongside the depletion of FeII–CN–CoII. This investigation underscores the potential of advanced spectroscopic techniques to improve the understanding of molecular processes at electrode interfaces, thereby advancing battery research.