Polynitrogen (PN) species (Nn, n from 3 to 8) as highly energetic materials have attracted many theoretical calculations and predictions. N3, N4, N5 or their ions were experimentally detected under high-pressure and high-temperature conditions. Herein, a N8− PN stabilized on the positively charged sidewalls of multi-walled carbon nanotubes (MWNTs) has been synthesized using cyclic voltammetry (CV) under ambient conditions. ATR-FTIR and Raman spectroscopic data assigned on the basis of density functional theory (DFT) calculations support the successful synthesis of a C2h symmetry chain structure of the N8 anion stabilized as MWNT+N8−. Temperature programmed desorption (TPD) data show that MWNT+N8− is thermally stable up to 400 °C. Oxygen-reduction reaction (ORR) experiments carried out using MWNT+N8− as the cathodic catalyst shows that it is very active for ORR with an even higher current density than that of a commercial Pt/carbon catalyst.