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

The local microenvironment, which is often tuned by adding alkali metal cations to the electrolyte, has recently been found to play a major role in the electrocatalytic activity of nanomaterials.(1) Modulating the microenvironment 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 in the interlayer space. Nevertheless, the nature of the aqueous microenvironment in the MXene interlayer remains poorly understood. Here, we apply Fourier transform infrared (FTIR) spectroscopy to probe the hydrogen bonding of intercalated water in Ti₃C₂T<i>_x</i> as a function different intercalated cation and relative humidity. Being highly sensitive to different H-bonding states of water confined within the MXene layers,(2,3) especially in the O-H stretching mode region, FTIR spectroscopy enables the direct characterization of the H-bonding network of water and gives information on the relative amounts of water present in the samples. Because anions do not intercalate into MXene,(4) we are able to probe the hydration shell around isolated cations in a 2D confined environment. Strong changes in the hydrogen-bonding of water molecules confined between the MXene layers is observed after cation exchange. 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 possibly be used to tune the activity or selectivity of electrochemical reactions in the interlayer space of MXenes in the future.<br/><br/>References<br/>1. Schreier, M.; Kenis, P.; Che, F.; Hall, A. S. <i>ACS Energy Lett.</i> <b>2023</b>, <i>8</i> (9), 3935-3940.<br/>2. Lounasvuori, M.; Sun, Y.; Mathis, T. S.; Puskar, L.; Schade, U.; Jiang, D.-E.; Gogotsi, Y.; Petit, T. <i>Nature Commun.</i> <b>2023</b>, <i>14</i> (1), 1322.<br/>3. Lounasvuori, M.; Mathis, T. S.; Gogotsi, Y.; Petit, T. <i>J. Phys. Chem. Lett.</i> <b>2023</b>, <i>14</i> (6), 1578–1584.<br/>4. Shpigel, N.; Chakraborty, A.; Malchik, F.; Bergman, G.; Nimkar, A.; Gavriel, B.; Turgeman, M.; Hong, C. N.; Lukatskaya, M. R.; Levi, M. D.; Gogotsi, Y.; Major, D. T.; Aurbach, D. <i>J. Am. Chem. Soc.</i> <b>2021</b>, <i>143</i> (32), 12552–12559.