Tuning the Microenvironment of Water Confined in Ti₃C₂T_x MXene by Cation Intercalation

The local microenvironment has recently been found to play a major role in the electrocatalytic activity of nanomaterials. Modulating the microenvironment by adding alkali metal cations into the electrolyte can be used to either suppress hydrogen or oxygen evolution, thereby extending the electrochemical window of energy storage systems, or to tune the selectivity of electrocatalysts. MXenes are a large family of two-dimensional transition metal carbides, nitrides, and carbonitrides that have shown potential for use in electrochemical energy storage applications. Due to their negatively charged surfaces, MXenes can accommodate cations and water molecules between the layers. Nevertheless, the nature of the aqueous microenvironment in the MXene interlayer space is poorly understood. Here, we apply Fourier transform infrared spectroscopy (FTIR) to probe the hydrogen bonding of intercalated water in Ti₃C₂T_x as a function of intercalated cation and relative humidity. Substantial changes in the FTIR spectra after cation exchange demonstrate that the hydrogen bonding of water molecules confined between the MXene layers is strongly cation-dependent. Furthermore, the IR absorbance of the confined water correlates with resistivity estimated by 4-point probe measurements and interlayer distance calculated from XRD patterns. This work demonstrates that cation intercalation strongly modulates the confined microenvironment, which can be used to tune the activity or selectivity of electrochemical reactions in the interlayer space of MXenes in the future.