High-Performance Capacitive Deionization via Manganese\nOxide-Coated, Vertically Aligned Carbon Nanotubes

Abstract

Discovering electrode\nmaterials with exceptional capacitance, an\nindicator of the ability of a material to hold charge, is critical\nfor developing capacitive deionization devices for water desalination.\nMaganese oxides (MnO_<i>x</i>) have shown promise\nas capacitive electrode materials, but they exhibit a trade-off in\nwhich a higher loading of the active MnO_<i>x</i> comes at the cost of lower conductivity. To address this challenge\nand achieve high salt adsorption, we fabricated electrodes comprising\nvertically aligned core–shell nanostructures using atomic layer\ndeposition (ALD) to coat thin films of MnO_<i>x</i> onto vertically aligned carbon nanotubes (VACNTs). The inherently\nhierarchical, anisotropic, three-dimensional macroporous structure\nof VACNTs and the tunable coating, a hallmark of ALD, enabled co-optimization\nof the hybrid material’s specific capacitance with respect\nto mass and geometric area. The specific capacitance was optimized\nin this study to 215 ± 7 F/g and 1.1 ± 0.1 F/cm² in a 1 M NaCl electrolyte at a scan rate of 5 mV/s. This material\nexhibited a remarkable sodium ion adsorption capacity of 490 ±\n30 μmol of Na/g of material (2-fold higher than that of pristine\nVACNTs) at a functioning voltage of 1.2 V, which may ultimately enable\nexpanded desalination applications of capacitive deionization.