Operando APXPS Reveals Ion-Specific Dynamics in Membrane Capacitive Deionization

Membrane capacitive deionization (MCDI) is a promising electrochemical approach for the selective removal of ions from dilute saline waters. However, the dynamic behavior of specific counterions within such systems, particularly under applied potentials, remains poorly understood due to the lack of in situ molecular-level probes. In this study, we use operando tender ambient-pressure X-ray photoelectron spectroscopy (APXPS) to investigate the transport and electrosorption behavior of monovalent cations (Na+ and Cs+) in a custom-designed MCDI cell. Our results reveal a markedly slower adsorption rate for Cs+ compared to Na+, which we attribute to variations in their hydration energy and their impact on overall membrane water content. Furthermore, we track local electrical potential variations during adsorption–desorption cycles, revealing potential-dependent shifts in core-level binding energies. According to our findings, local electrical potentials around PVA binder and electrolyte are similar. However, the local potential experienced by these ions upon the change in cell voltage is not as significant as it is for the bulk electrolyte and PVA matrix. This reduced response likely arises from the close proximity of the adsorbed ions to the conductive electrode, where the electric field is strongly screened, highlighting the differential impact of potential modulation on species located at varying distances from the electrode–electrolyte interface. This study provides direct insight into ion-specific dynamics in MCDI and highlights tender-APXPS as a powerful operando tool for probing electro-membrane processes in water treatment technologies.