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

Discovering electrode materials with exceptional capacitance, an indicator of the ability of a material to hold charge, is critical for developing capacitive deionization devices for water desalination. Maganese oxides (MnO_x) have shown promise as capacitive electrode materials, but they exhibit a trade-off in which a higher loading of the active MnO_x comes at the cost of lower conductivity. To address this challenge and achieve high salt adsorption, we fabricated electrodes comprising vertically aligned core-shell nanostructures using atomic layer deposition (ALD) to coat thin films of MnO_x onto vertically aligned carbon nanotubes (VACNTs). The inherently hierarchical, anisotropic, three-dimensional macroporous structure of VACNTs and the tunable coating, a hallmark of ALD, enabled co-optimization of the hybrid material's specific capacitance with respect to mass and geometric area. The specific capacitance was optimized in 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 exhibited a remarkable sodium ion adsorption capacity of 490 ± 30 μmol of Na/g of material (2-fold higher than that of pristine VACNTs) at a functioning voltage of 1.2 V, which may ultimately enable expanded desalination applications of capacitive deionization.