Electrochemical Characterization of Vanadium Nitride as Pseudocapacitive Electrode for Aqueous Electrochemical Capacitor

Whereas most of the supercapacitors on the market are designed with activated carbon electrodes, current research is focused toward the development of alternative materials exhibiting high energy and power densities and long term cycling efficiency. Some transition metal oxides such as RuO₂¹ or MnO₂² exhibit high specific capacity while there density is up to 8 times higher than the density of activated carbon.

These interesting features are explained by fast and reversible redox reactions taking place in the near surface of the active material particles. This phenomenon is referred to as pseudocapacitance. The main drawbacks are the  high cost of RuO₂, and  the poor electronic conductivity of MnO₂ thus considerably limiting the power capability of an electrode using such a material.

In 1998, Lui et al. reported similar pseudocapacitive behavior of a molybdenum nitride electrode in acidic electrolyte³ and only a single article was published with reference to this topic. ⁴. In 2006, nitride compounds regained some interest concerning energy storage after Choi et al. reported an impressive capacity of 1340 F/g for a vanadium nitride electrode in alkaline electrolyte⁵. Considering its attractive capacity but also its high electronic conductivity, VN has been intensively studied since that time concerning new syntheses and processes involving xerogel⁶, nanotubes coating⁷ and TiN/VN core/shell nanostructure⁸. Choi et al. suggested that the pseudocapacitive behavior of the vanadium nitride electrode can be explained by an equilibrium reaction implicating OH^- ions adsorption and reversible redox reactions of an oxidized surface resulting from VN oxidation. However, only few studies were aimed at understanding the charge storage mechanism. Indeed, while Pande et al. confirmed the role of OH^- anions in the reversible redox process⁹, large differences in specific capacity and stability are reported in the literature. These differences can be attributed to the large diversity within the synthesis method, which often used oxides as a precursor to oxy-nitrides rather than nitrides. However, as demonstrated by the observation of an unexplained irreversible anodic process in different studies^6,10, a lack of understanding in the different charge storage mechanisms involved exists.   

In this communication, the synthesis of a model material VN deposited as thin film will be described, followed by electrochemical investigations accompanied with in-situ and ex-situ characterizations. The main goal of this work is to get some insight in the different reactions involved in the charge storage mechanism and to determine the suitable conditions of utilization of VN so as to obtain a highly stable active electrode material.

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