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

Abstract

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 ∼70.74 m² 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 μS 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 ∼42.9 mg g⁻¹, which was 97.5% of the initial value, proving its prominent regeneration.