Origin of superior pseudocapacitive mechanism of transition metal nitrides

Large-scale deployment of Internet of Things (IoT), a revolutionary innovation for a better world, is hampered by the limitation of energy self-sufficiency. Constructing transition metal nitride (TMN)-based micro-supercapacitors is a possible solution by taking advantage of the high conductivity, large specific capacitance, and large tap density of the materials. However, the pseudocapacitive storage mechanism of TMNs is still unclear consequently impeding the design of microdevices. Herein, the functions and mechanism of TMNs with different metal oxynitride (TMNO_x) concentrations in pseudocapacitive electrodes are investigated systematically by in situ Raman scattering, ex situ X-ray photoelectron spectroscopy, as well as ion isolation and substitution cyclic voltammetry. It is found that the specific capacitances of TMNs depend on the TMNO_x concentrations and the N–M–O site is responsible for the large pseudocapacitance via the Faradic reaction between TMNO_x and OH^–. Our study elucidates the mechanism pertaining to pseudocapacitive charge storage of TMNs and provides insights into the design and optimization of TMNO_x as well as other electrode materials for pseudocapacitors.