The feasibility of hollow echinus-like NiCo2O4 nanocrystals for hybrid capacitive deionization

Hybrid capacitive deionization (HCDI) has emerged to address the shortcomings of CDI in high-salinity solutions. Cathodes for HCDI with active Faradaic reaction ability are desirable. In this work, we propose a facile method to prepare hollow echinus-like NiCo2O4 nanocrystals as high-performance cathodes for HCDI. It was determined that the specific surface area of NiCo2O4 is similar to 70.74 m(2) g(-1) with the mean pore diameter of 20.72 nm. The electrochemical analysis demonstrated that NiCo2O4 possesses high specific capacity, remarkable capacitance retention and low impedance. As a cathode for HCDI, NiCo2O4 exhibited high salt removal capacity of 44.3 mg g(-1) with charge efficiency of 98.7% at 1.2 V in an NaCl solution with initial conductivity of 1000 mu S cm(-1). This is attributed to the definite redox reaction between NiCo2O4 and sodium ions during the charge/discharge process. Moreover, the HCDI behavior of NiCo2O4 in various solutions containing K+, Na+ and Mg2+ was explored, illustrating that the ionic radius has priority in HCDI. Furthermore, the salt removal capacity of the NiCo2O4 electrode after 20 cycles was still similar to 42.9 mg g(-1), which was 97.5% of the initial value, proving its prominent regeneration.