A coupled capacitive desalination (C-CDI) for enhanced desalination performance at ultralow voltage

A coupled capacitive desalination (CCDI) technique has been developed based on an innovative liquid–solid hybrid electrode design, wherein a thin layer of redox-active solution is confined near the surface of a porous carbon electrode. In this configuration, the desalination behavior is significantly enhanced through synergistically coupling of electrical double layer of the porous carbon and the redox reactions of the solution-phase redox species at the liquid-solid interface. To demonstrate the feasibility and superiority of this novel approach, the desalination behaviors of this liquid-solid hybrid have been investigated in a model system, where hierarchically porous hollow carbon spheres (HCS) are covered with a thin layer of ferrocene derivative (FcN₂Br₂) solution. The results reveal that the thickness of the redox electrolyte plays a critical role in determining the overall desalination performance. When confined within 500 µm, the thin layer of FcN₂Br₂ solution can effectively couple with the HCS electrode to achieve “overlay effects” in terms of both ion storage kinetics and capacity. Furthermore, the CCDI can achieve most of its desalination capacity with ultralow energy consumption, owing to the intense redox reaction of FcN₂Br₂ in a narrow potential range. Consequently, this setup attains a high desalination capacity of 52.2 mg g^-1 and a rapid desalination rate of 6.6 mg g^-1 min^-1 at an ultralow voltage of 0.6 V, surpassing most reported benchmark devices. Overall, this pioneering work underscores the significant benefits of integrating liquid and solid electrodes, paving a groundbreaking and promising path for the future CDI evolution.