Citrus-peel-derived, nanoporous carbon nanosheets containing redox-active heteroatoms for sodium-ion storage

Advanced design of nanostructured functional carbon materials for use in sustainable energy storage systems suffers from complex fabrication procedures and the use of special methods and/or expensive precursors, limiting their practical applications. In this study, nanoporous carbon nanosheets (NP-CNSs) containing numerous redox-active heteroatoms (C/O and C/N ratios of 5.5 and 34.3, respectively) were fabricated from citrus peels by simply heating the peels in the presence of potassium ions. The NP-CNSs had a 2D-like morphology with a high aspect ratio of >100, high specific surface area of 1167 m² g^-1, and a large amount of nanopores between 1 and 5 nm. The NP-CNSs also had an electrical conductivity of 2.6 × 10¹ s cm^-1, which is approximately 50 times higher than that of reduced graphene oxide. These unique material properties resulted in superior electrochemical performance with a high specific capacity of 140 mAh g^-1 in the cathodic potential range. In addition, symmetric full-cell devices based on the NP-CNSs showed excellent cyclic performance over 100'000 repetitive cycles.