Mass-producible polyhedral macrotube carbon arrays with multi-hole cross-section profiles: superb 3D tertiary porous electrode materials for supercapacitors and capacitive deionization cells

Supercapacitors and capacitive deionization (CDI) cells used for energy storage and water desalination, respectively, are related devices which are based on intensive adsorption of ions to highly porous electrodes. Engineering of porous carbon electrodes with high specific surface area but a robust structure is very important for the two technologies. Herein, we report on mass-producible polyhedral macrotube carbon arrays possessing multihole cross-section profiles, which can be used as superb 3D porous materials for both supercapacitors and CDI cells. They were prepared through economical and environmentally friendly processes including carbonization of lotus stems at 500 °C followed by activation at 800 °C (LS500/A800). LS500/A800 possesses a tertiary pore structure (hierarchical macro-meso-micro porosity) that provides high density of adsorption sites, and allows fast ion adsorption through channels that provide short transport distance for ions in solution. Despite the high porosity and large specific surface area (>2500 m2 g-1), they also benefit from robust frameworks that demonstrate impressive cycling stability. These unique structures and excellent wettability in aqueous solutions enable excellent electrochemical performance with specific capacitance > 370 F g-1 at a current density of 1 A g-1 and excellent cycling stability with 97% capacitance retention over 10 000 cycles. Furthermore, MCDI cells based on LS500/A800 electrodes exhibit a salt adsorption capacity (SAC) of 31.7 mg g-1 at a voltage span of 1.4 V and 97% SAC retention (SACR) after 100 desalination-regeneration cycles, which are significantly better than those of commercial activated carbon electrodes.