Configuration of 3D hydrangea-like MoSe2@N-doped carbon nanoflowers with synergistic functional regulation for highly efficient and selective capacitive deionization

Abstract

Transition-metal diselenides (TMDs) are promising materials for capacitive deionization (CDI) technology due to their high theoretical storage capacity and reversible intercalation/deintercalation behavior. However, drawbacks of TMDs like low conductivity, interlayer self-stacking and volume expansion severely impede ion migration, undermine the structural stability, and limit the accessibility of active sites, ultimately leading to unsatisfactory CDI performance. Herein, we elaborately designed a three-dimensional hydrangea-like MoSe₂@N-doped carbon nanoflowers (MoSe₂@N-CF) composite to enhance the efficiency and selectivity of cation capture. In this configuration, N-CF can not only serve as the growth substrate to prevent the aggregation of MoSe₂ nanosheets and enhance the conductivity, but also effectively increases the interlayer spacing of MoSe₂ in the molecules level. Meanwhile, the unique pseudocapacitive intercalation behavior of MoSe₂ provides abundant active sites for cation storage and fast ion diffusion kinetics for ion transfer. Benefited from the synergistic functional regulation of MoSe₂ and N-CF, the MoSe₂@N-CF electrode demonstrates a remarkable specific capacitance (217.3 F g⁻¹), reduced ion diffusion resistance, and superior electrochemical stability. The asymmetrical CDI cell (AC//MoSe₂@N-CF) presents excellent desalination capacity (40.83 mg g⁻¹), rapid desalination rate (6.93 mg g⁻¹ min⁻¹), and demonstrates a 90 % retention rate (from 16.42 mg g⁻¹ to 14.78 mg g⁻¹) over 35 CDI cycles. Additionally, the MoSe₂@N-CF electrode exhibits unique ion selectivity in multi-ion solutions, which reflects the combined influence of intercalation effect and electric double-layer effect arising from the distinct functional structures within the composite material, where their relative contributions vary with solution concentration. Density functional theory calculations further validate the ion selectivity mechanism based on ion diffusion energy. This work offers valuable insights into leveraging unique structural characteristics of pseudo-capacitive intercalation material and carbon-based material, advancing the development of CDI.