Boosting enhanced capacitive deionization of H₂TiO₃/carbon electrodes by yolk-shell construction

Capacitive deionization (CDI) is a highly efficient approach for sustainable water desalination and ion-selective separation from complex brines. Layered metatitanic acid (H₂TiO₃, HTO) is a promising platform for electrochemical separation of monovalent ions due to its unique structural properties, but it faces lower conductivity when applied solely to CDI. Coupling HTO with carbon offers a valid strategy to address its charge transfer limitation and thereby enhance the CDI performance. Herein, we report a yolk-shell structured HTO/carbon electrode (denoted as ys-HTO@C) based on a stepwise nanoscale architectonic strategy that demonstrates superior charge transfer and storage properties and, therefore, better CDI performance relative to its core-shell structured and bulk counterparts (denoted as cs-HTO@C and HTO, respectively). Electrochemical desalination reveals that ys-HTO@C electrode has a Na⁺ adsorption capacity of 53.1 mg g^-1 in 500 mg L^-1 of NaCl solution at 1.2 V, a charge efficiency of 0.68, and moderate cycling stability (70.5 % of capacity retention over 40 cycles) due to its unique void space inside. Structural analyses indicate that capacity fading during cycling is primarily attributed to carbon oxidation and HTO degradation over time. Additionally, lithium extractions in synthetic and actual brines show that the HTO/carbon electrodes exhibit preferential capture of Li⁺ over Na⁺, Ca²⁺, Mg²⁺, and Pb²⁺ ions, with a separation factor of Li⁺/Na⁺ up to 60.7 for the ys-HTO@C electrodes in natural brine from East Taijinaier Salt Lake. It was observed that Li⁺ selectivity is concentration-dependent and likely driven by both the HTO's intrinsic Li⁺ preference and the ion's properties. The findings underscore the potential of the yolk-shell construction strategy for preparing novel and highly efficient electrodes for electrochemical desalination and selective Li⁺ extraction from complex brines.