Enhanced Capacitive Deionization with Hollow Carbon Spheres Derived from Melamine–Formaldehyde Templates

The architectural configuration of an electrode material significantly impacts its capacitive deionization (CDI) performance, particularly due to the disparity in ion diffusion resistance between the surface and core. To mitigate this disparity, a hollowing methodology was employed to revamp conventional porous carbon spheres. Hierarchically porous hollow carbon spheres (HCSs) were synthesized by thermal annealing phenol formaldehyde resin-coated melamine formaldehyde resin spheres (MFSs) in an inert gas at 800 °C. The advantage of employing modified MFSs as templates lies in their complete degradation during thermal annealing, a feature not observed with commercial polystyrene microspheres. Unlike mesoporous SiO₂ microspheres which require additional hydrofluoric acid treatment, these do not. HCS-100 exhibited exceptional NaCl adsorption capacity, achieving a salt adsorption capacity of 25.20 mg g^–1 and a salt adsorption rate of 2.78 mg g^–1 min^–1 under a working voltage of 1.2 V. This performance was demonstrated with an initial NaCl solution concentration of 500 mg L^–1, and it maintained impressive stability over 70 cycles. The results demonstrate that the hollowing strategy is a direct yet powerful way to enhance the CDI performance of electrode materials. The utilization of the modified MFS template simplifies the fabrication process, contributing to the overall effectiveness of this approach.