(Invited) Direct Probe of Electrochemical Pseudocapacitive pH Jump at a Graphene Electrode

Abstract

Molecular-level insight into interfacial water at buried electrode interface is essential in electrochemistry, but spectroscopic probing of the buried interfaces remains challenging. Here, using surface-specific heterodyne-detected sum-frequency generation (HD-SFG) spectroscopy, we directly access the interfacial water in contact with the graphene electrode supported on calcium fluoride (CaF2). The HD-SFG signal is obtained by overlapping the infrared and visible pulses at the sample position. An SFG signal is enhanced when the infrared frequency is resonant with the molecular vibration. The HD-SFG can provide the information on the orientation of the interfacial molecules. With the molecular specificity, orientational specificity, and surface specificity of the HD-SFG, we clarify the molecular level insight into the structure of the electrified graphene-water interface. At the negative voltage, we found the negative HD-SFG peak at 3630 cm-1, which manifests that the O-H group points to the bulk water, not to the CaF2 supported graphene. This O-H group is the Ca-O-H group, indicating that the chemical reaction occurs at the CaF2 surface. By monitoring the variation of the HD-SFG spectra upon the various applied potential, We find phase transition-like variations of the HD-SFG spectra. We attribute such a variation not to charging/discharging of graphene but to charging/discharging of the CaF2 substrate through the pseudocapacitive process. The potential-dependent spectra are nearly identical to the pH-dependent spectra, evidencing that the pseudocapacitive behavior is associated with a substantial local pH change induced by water dissociation between the CaF2 and graphene. Our work evidences the local molecular-level effects of pseudocapacitive charging at an electrode/aqueous electrolyte interface.