Optimizing Sodium Ion Adsorption Through Robust d–d Orbital Modulation for Efficient Capacitive Deionization

Unraveling the fundamental mechanisms of sodium ion adsorption behavior is crucial for guiding the design of electrode materials and enhancing the performance of capacitive deionization systems. Herein, the optimization of sodium ion adsorption is systematically investigated through the robust d–d orbital interactions within zinc-doped iron carbide, facilitated by a novel liquid nitrogen quenching treatment. Liquid nitrogen quenching treatment can enhance the coordination number, strengthen d–d orbital interactions, promote electron transfer, and shift the d-band center of Fe closer to the Fermi level, thereby enhancing sodium ions adsorption energy. Consequently, the obtained electrode material achieves a superior gravimetric adsorption capacity of 121.1 mg g⁻¹ and attractive cyclic durability. The adsorption capacity is highly competitive compared to the vast majority of related research works in the field of capacitive deionization. Furthermore, sodium ion adsorption/desorption mechanisms are substantiated through ex situ techniques, revealing dynamic atomic and electronic structure evolutions under operational conditions. This work demonstrates that optimizing sodium ion adsorption via robust d–d orbital modulation enabled by liquid nitrogen quenching treatment is an effective approach for developing efficient capacitive deionization electrode materials.