Vibrational signature of hydrated protons confined in MXene interlayers

Abstract

The hydration structure of protons has been studied for decades in bulk water and protonated clusters due to its importance but has remained elusive in planar confined environments. Two-dimensional (2D) transition metal carbides known as MXenes show extreme capacitance in protic electrolytes, which has attracted attention in the energy storage field. We report here that discrete vibrational modes related to protons intercalated in the 2D slits between Ti ₃C ₂T _x MXene layers can be detected using operando infrared spectroscopy. The origin of these modes, not observed for protons in bulk water, is attributed to protons with reduced coordination number in confinement based on Density Functional Theory calculations. This study therefore demonstrates a useful tool for the characterization of chemical species under 2D confinement.

Abstract

Intercalation of protons in 2D materials plays a major role for several applications in energy storage and conversion. Here, the authors show that protons intercalated in Ti ₃C ₂T _x MXene interlayer during electrochemical cycling have a different hydration structure than protons in bulk water.