HCDI performance of Na-2x3 and Na-2x4 nanowires for water desalination

Abstract

Hybrid capacitive deionization (HCDI) is an emerging water desalination technology that integrates an intercalation electrode against a capacitive carbon counter electrode. The former electrode incorporates ions into the material volume unrestricted by surface area. Tunnel manganese oxide nanowires are a promising class of intercalation materials due to their low cost, small environmental footprint, stability in aqueous solutions, and high theoretical ion removal capacity. Previous HCDI studies reported high desalination performance of disordered Na-stabilized manganese oxide (NaxMnO2) tunnel phases known as 2xn-MnO2 and hybrid-MnO2. In contrast, this study focuses, for the first time on the synthesis and HCDI water desalination performance of highly ordered NaxMnO2 phases with uniform rectangular tunnels. These tunnels are formed by 2 MnO6 octahedra on one side and 3 or 4 MnO6 octahedra on the perpendicular side called Na-2x3 and Na-2x4, respectively. The analysis includes ion removal performance in NaCl solution to understand ion intercalation and surface adsorption processes into tunnel manganese oxides and the role of the stabilizing ions compared to the two previously reported disordered phases.