Capacitance decay mechanism of vanadium nitride supercapacitor electrodes in KOH electrolytes

Abstract

Vanadium nitride (VN) is a promising pseudocapacitive material due to the high theoretical capacity, rapid redox Faradaic kinetics, and appropriate potential window. Although VN shows large pseudocapacitance in alkaline electrolytes, the electrochemical instability and capacity degradation of VN electrode materials present significant challenges for practical applications. Herein, the capacitance decay mechanism of VN is investigated and a simple strategy to improve cycling stability of VN supercapacitor electrodes is proposed by introducing VO₄³⁻ anion in KOH electrolytes. Our results show that the VN electrode is electrochemical stabilization between −1.0 and −0.4 V (vs. Hg/HgO reference electrode) in 1.0 M KOH electrolyte, but demonstrates irreversible oxidation and fast capacitance decay in the potential range of −0.4 to 0 V. In situ electrochemical measurements reveal that the capacitance decay of VN from −0.4 to 0 V is ascribed to the irreversible oxidation of vanadium (V) of N–V–O species by oxygen (O) of OH⁻. The as-generated oxidization species are subsequently dissolved into KOH electrolytes, thereby undermining the electrochemical stability of VN. However, this irreversible oxidation process could be hindered by introducing VO₄³⁻ in KOH electrolytes. A high volumetric specific capacitance of 671.9 F·cm⁻³ (1 A·cm⁻³) and excellent cycling stability (120.3% over 1000 cycles) are achieved for VN nanorod electrode in KOH electrolytes containing VO₄³⁻. This study not only elucidates the failure mechanism of VN supercapacitor electrodes in alkaline electrolytes, but also provides new insights into enhancing pseudocapacitive energy storage of VN-based electrode materials.

Graphical abstract