Niobium Nitride Nb₄N₅ as a New High-Performance Electrode Material for Supercapacitors

Supercapacitors suffer either from low capacitance for carbon or derivate electrodes or from poor electrical conductivity and electrochemical stability for metal oxide or conducting polymer electrodes. Transition metal nitrides possess fair electrical conductivity but superior chemical stability, which may be desirable candidates for supercapacitors. Herein, niobium nitride, Nb₄N₅, is explored to be an excellent capacitive material for the first time. An areal capacitance of 225.8 mF cm^-2, with a reasonable rate capability (60.8% retention from 0.5 to 10 mA cm^-2) and cycling stability (70.9% retention after 2000 cycles), is achieved in Nb₄N₅ nanochannels electrode with prominent electrical conductivity and electrochemical activity. Faradaic pseudocapacitance is confirmed by the mechanistic studies, deriving from the proton incorporation/chemisorption reaction owing to the copious +5 valence Nb ions in Nb₄N₅. Moreover, this Nb₄N₅ nanochannels electrode with an ultrathin carbon coating exhibits nearly 100% capacitance retention after 2000 CV cycles, which is an excellent cycling stability for metal nitride materials. Thus, the Nb₄N₅ nanochannels are qualified for a candidate for supercapacitors and other energy storage applications.