Vertically-aligned growth of CuAl-layered double oxides on reduced graphene oxide for hybrid capacitive deionization with superior performance

Abstract

Designing novel nanostructured materials is very significant to enhance the desalination capacity enabled by capacitive deionization (CDI). In this work, a vertically-aligned CuAl-layered double oxide grown on reduced graphene oxide (CuAl-LDO/rGO) is proposed as a high-performance anode for hybrid CDI (HCDI). It is found that the morphology of CuAl-LDO is ultimately dependent on the loading mass of urea. As a result, the optimized CuAl-LDO/rGO exhibits a relatively high specific surface area, 3D porous structure and electrochemical behavior. When employed as an anode for HCDI, it demonstrated a high salt removal capacity of 64.0 mg g⁻¹ at a cell voltage of 1.2 V in NaCl solution with an initial concentration of 1000 mg L⁻¹. After 20 cycles, the capacity still remains at 58.0 mg g⁻¹ which is ∼90% of the initial value, suggesting the superior cycling ability. Moreover, the phase transformation of CuAl-LDO/rGO at various stages indicates that the formation of Cu(OH,Cl)₂·2H₂O may be responsible for the capacity decay upon cycling. However, the morphology characterization realizes that the aggregation of Cu(OH,Cl)₂·2H₂O nanoparticles can be effectively inhibited within CuAl-LDO/rGO compared to that of randomly oriented CuAl-LDO nanoflakes, which accounts for the superior salt removal capacity retention. Besides, a comparison of HCDI performance is performed among different anodes, highlighting the advantages of CuAl-LDO/rGO. The good performance is attributed to the unique structure of CuAl-LDO/rGO as it provides many tunnels for diffusion of salt ions and active sites for intercalation.