Towards an advanced electrochemical horizon: ion selectivity and energy harnessing through hybrid capacitive deionization with carbon-coated NaTi2(PO4)3 and N-rich carbon nests

Abstract

For both water softening and energy storage, to date, a variety of capacitive devices have been developed; however, their dual functionality has been rarely investigated. An enhanced selective sodium-ion removal along with charge-storage was achieved by combining sodium-ion capture and release through sorption and regeneration steps of a capacitive deionization (CDI) process, respectively. Leveraging their unique and reversible Na⁺-removal capability, sodium superionic conductors (NASICONs) hold immense promise for hybrid capacitive deionization (HCDI). Despite the great desalination ability of HCDI systems, the unbalanced ion-capture and the possibility of co-ion expulsion have led to a real bottleneck that can effectively be tackled by placing an ion exchange membrane (IEM) between the electrolyte and the electrode. Herein, the state-of-the-art Na⁺ selective technology has been engineered using well-matched carbon-coated NaTi₂(PO₄)₃ (NTP-C) and N-rich carbon nests (NCNs) as negative and positive electrodes, respectively. The fabricated HCDI cells benefit from a commendable salt adsorption capacity (SAC) of 96.8 mg g⁻¹, a salt adsorption rate (SAR) of 2.42 mg g⁻¹ min⁻¹, and a specific energy consumption (E_s) of 18.5 j mg_NaCl⁻¹ in the sorption step. These devices also achieve a remarkable energy storage capacity (Q) of 46.52 C g⁻¹ at a low concentration of NaCl (500 ppm) in the regeneration step. The NTP-C//NCN HCDI systems achieved remarkable cycle stability with almost 92.3 and 91.3% retention of their salt adsorption and charge storage capacities, respectively, after 30 continuous cycles. The Na⁺ selective removal capability of the fabricated HCDI systems was evaluated by comparing their Na⁺ removal capacity in the absence and presence of Mg²⁺, Ca²⁺, and K⁺ ions (S_Na⁺/X > 2.5) which resulted in a superior sodium removal efficiency (SRE%) of almost over 50% from both pure and contaminated mixtures. As a direct consequence of high charge storage capacity, the fabricated HCDI is well-suited for energy applications, so it marks the beginning of a pioneer horizon towards the commercialization of HCDI technologies.