Hierarchically nanoporous pyropolymer nanofibers for surface-induced sodium-ion storage.

Surface-driven charge storage materials based on both electrochemical double layer (EDL) formation and pseudocapacitive behavior can deliver high energy and power capabilities with long-lasting cycling performance. On the other hand, the electrochemical performance is strongly dependent on the material properties, requiring sophisticated electrode design with a high active surface area and a large number of redox-active sites. In this study, hierarchically nanoporous pyropolymer nanofibers (HN-PNFs) were fabricated from electrospun polyacrylonitrile nanofibers by simple heating with KOH. The HN-PNFs have a hierarchically nanoporous structure and an exceptionally high specific surface area of 3,950.7 m 2 g −1 as well as numerous redox-active heteroatoms (C/O and C/N ratio of 10.6 and 16.8, respectively). These unique material properties of HN-PNFs resulted in high reversible Na-ion capacity of ∼292 mAh g −1 as well as rapid kinetics and stable cycling performance in the cathodic potential range (1-4.5 V vs. Na + /Na). Furthermore, energy storage devices based on HN-PNFs showed a remarkably high specific energy of ∼258 Wh kg −1 at ∼245 W kg −1 as well as a high specific power of ∼21,500 W kg −1 at ∼78 Wh kg −1 , with long and stable cycling behaviors over 2,000 cycles. [ABSTRACT FROM AUTHOR]