N-doped carbon nanosheets assembled microspheres for more effective capacitive deionization

Abstract The effect of electrode material’s surface morphology on capacitive deionization (CDI) has been discussed here. For more effective CDI performance, the hierarchical carbon microspheres (HCMs) with vertically aligned nanosheets are designed to obtain the roughest surface. HCMs are derived from the polyimide precursor which is formed by polymerizing methyl ethylenedicarboxylic anhydride (PMDA) with ethylenediamine (EDA) directly without any template agents. Active HCMs (A-HCMs) exhibit a specific surface area of 1870.18 m2·g−1 and a capacitance of 228.18 F·g−1 in 1 M NaCl aqueous electrolyte after carbonization and KOH activation. When processed into CDI electrodes, the vertically aligned nanosheets in A-HCMs would create continuous spaces for ion diffusion, instead of the smooth surfaces reported for other electrode materials. The meso/micro pores in nanosheets still support a large electrical double layer (EDL) for ion storage. A-HCMs exhibit a NaCl capacity of 14.64 mg·g−1 at 1.0 V in the initial concentration of 400 mg·L−1. More than 50 stable deionization/regeneration cycles could be achieved. The A-HCMs were shown to have several benefits: carbon nanosheets with micro- and meso-pores that enhance SAC, N-doped carbon for lower electric resistance and better CE, and vertically aligned nanosheets that benefit ions diffusion. We have found that macromorphology is also an effective way to optimize CDI performance in addition to controlling pore's structure.