and the INM -Leibniz Institute for New Materials in Saarbrücken, Germany. A trained chemist, she received her bachelor's degree and her master's degree from the Karlsruhe Institute of Technology, Germany, in 2018 and 2020, respectively. Her research explores high-performance sodium-ion battery materials, novel technologies for battery recycling, and water remediation.

has emerged as a promising alternative to traditional separation processes due to its high selectivity metrics and energy efficiency. [7] Depending on the nature of electroactive material, the ion immobilization and separation process mechanisms are different. For example, nanoporous carbons immobilize ions via electrosorption. Sub-nanometer pores may cause ion sieving or require ions to (partially) shed their solvation shell; this effect enables further tunability of the ion selectivity. [8] Even more confined sites for ion uptake are found in Faradaic materials. [9] Thereby, processes such as ion intercalation or other redox processes enable selectivity toward certain cations or anions. [9,10] For example, LiMn 2 O 4 provides facile intercalation into its crystal structure only for specific ions with matched size and valence, aligning with intrinsic ion selectivity. [11] Other materials like TiS 2 [12] show potential-dependent (tunable) ion selectivity according to the hydration energy of ions. This mechanism is linked with the specific onset potential for ion intercalation (or other redox processes), which gives rise to the unique battery-like feature in electrochemical measurements. [13] Yet, the ion selectivity of pseudocapacitive materials has remained largely unexplored. [14] MXene is a promising, quickly growing, and novel family of 2D metal carbides or nitrates. [15] The ability to reversibly Electrochemical ion separation is a promising technology to recover valuable ionic species from water. Pseudocapacitive materials, especially 2D materials, are up-and-coming electrodes for electrochemical ion separation. For implementation, it is essential to understand the interplay of the intrinsic preference of a specific ion (by charge/size), kinetic ion preference (by mobility), and crystal structure changes. Ti 3 C 2 T z MXene is chosen here to investigate its selective behavior toward alkali and alkaline earth cations. Utilizing an online inductively coupled plasma system, it is found that Ti 3 C 2 T z shows a time-dependent selectivity feature. In the early stage of charging (up to about 50 min), K + is preferred, while ultimately Ca 2+ and Mg 2+ uptake dominate; this unique phenomenon is related to dehydration energy barriers and the ion exchange effect between divalent and monovalent cations. Given the wide variety of MXenes, this work opens the door to a new avenue where selective ion-separation with MXene can be further engineered and optimized.