Enhanced performance of membrane-free capacitive deionization with citrate-chelated high-entropy Prussian blue electrodes

Capacitive deionization (CDI) technology has gained wide attention for producing clean and portable water to relieve the pressure on water resource shortages. Faradaic materials are considered to be a promising alternative to traditional carbon materials storing charges in the electrical double layers (EDL) that have limitations due to co-ion repulsion. Herein, we have synthesized High-entropy Prussian blue analogues (HEPBA) with the chelation of citrate using a simple one-step hydrothermal method to control their size, crystallinity and composition. The optimized HEPBA-based electrodes used in half-cells show a high specific capacity of ~283 F g −1 at 1 mV s −1 . A high salt adsorption capacity of 36.8 mg g −1 stable for the 50 cycles tested here was reached in the membrane-free desalination cell based on HEPBA and Ag@C electrodes. This architecture overcomes the typical limitations of low adsorption capacity and co-ion repulsion, enabling efficient operation without the need for an ion-exchange membrane. The improved salt-adsorption capacity is due to the high optimized crystallinity of the HEPBA material and maintenance of the configurational entropy. A series of ex-situ experiments revealed that the desalination and regeneration processes were governed by the redox of the Fe 3+ /Fe 2+ couple, Mn 3+ /Mn 2+ couple, Co 3+ /Co 2+ couple and Cu 2+ /Cu + couple while the Ni 3+ /Ni 2+ couple was electrochemically inactive for the HEPBA. Electrodes fabricated using this simple one-step process provide a possibility to obtain optimized HEPBAs for real-life applications.