Vanadium nitride has displayed many interesting characteristics for its use as a pseudocapacitive electrode in an electrochemical capacitor, such as good electronic conductivity, good thermal stability, high density and high specific capacitance. Thin films of VN were prepared by D.C. reactive magnetron sputtering. The electrochemical stability of the films as well as the influence of dissolved oxygen in 1 M KOH electrolyte were investigated. In order to avoid material as well as electrolyte degradation, it was concluded that vanadium nitride should only be cycled between −0.4 and −1.0 V vs. Hg/HgO. After a 24 hours stabilization period, the prepared VN thin film showed an initial capacitance of 19 mF.cm −2 and a capacity retention of 96% after 10000 cycles. Furthermore, dissolved oxygen in the electrolyte was demonstrated to cause self-discharge up to a potential above −0.4 V vs. Hg/HgO, where VN was shown to be unstable. Additionally, the presence of oxygen was shown to shift the open circuit potential of a VN electrode to about 0 V through self-discharge processes. Electrochemical capacitors are currently being developed to complement other energy storage or conversion systems such as batteries and fuel cells. In the past two decades, several electrode materials have been investigated. The mostly studied materials include carbons, 1-8 conducting polymers 9-12 as well as pseudocapacitive 13 transition metal oxides such as ruthenium dioxide 14-18 and manganese dioxide. [19][20][21][22][23][24] In the effort to improve the performance of electrochemical capacitors, a widely used approach has been to develop synthetic methods to develop nanomaterials that are believed to lead to increased energy density and higher rate capability. 14,23,[25][26][27][28] Another approach has been to investigate the charge storage properties of novel materials. Accordingly, several research groups considered materials such as MXenes 29,30 and transition metal nitrides. [31][32][33][34][35][36][37] In the latter class of compounds, molybdenum nitride was firstly investigated 32,33,35,38,39 and in the past decade a great deal of attention has focused on other nitrides with an intensive focus on vanadium nitride 37,40-58 as a consequence of the impressive capacitance of 1340 F.g −1 reported for nanosized VN particles in 1 M KOH. 58 Indeed, VN is an interesting electrode material due to this high reported specific capacitance coupled with its close to metallic electronic conductivity (1.18 S.m −1 ), 59 high density (6.13 g.cm −3 ) and high melting point (2619 K).
59The hypothesis proposed by Choi et al.58 to explain such a high specific capacitance of VN, is that, in addition to electrochemical double layer capacitance, successive fast reversible redox reactions are taking place, involving surface oxide groups and OH − ions from the electrolyte. This hypothesis was supported by the observation of surface oxide via ex-situ XPS 58 and FTIR 41,58 post cycling measurements.While most studies concentrated on new synthesis of VN with the goa...
