Paper Title
One-Step Chemical Precipitation of Vanadium Nitrides for Electrochemical Capacitors

Abstract
With an increasing in needs for the development of renewable and sustainable energy storage systems, there is an incessant demand for emerging technologies including secondary batteries, electrochemical capacitors (ECs), and fuel cells. Among these energy storage systems, ECs hold a unique position due to their high power density, short charge time, and long and stable cycle life. ECs can store charges either by the non-Faradaic charge separation at the electrode/electrolyte interface or by the reversible Faradaic reaction at the electrode surface. The former is called electric double-layer capacitors (EDLCs) and the latter is called redox capacitors (or pseudocapactitors). Various transition metal oxides and conducting polymers have been investigated as electrode materials for redox capacitors. Recently, transition metal nitrides such as titanium nitride (TiN), molybdenum nitrides (MoN and Mo2N), and vanadium nitride (VN) have received much attention as alternatives to metal oxides for electrochemical capacitor applications because of their excellent electrical conductivity and specific capacitance. Especially, VN is considered to be a good candidate for electrode materials due to its high theoretical capacity, fast redox Faradic response, and high hydrogen evolution overpotential. One of the typical techniques for preparing vanadium nitrides is the reduction of V2O5 powders using ammonia at high temperatures. Calcination of V2O5 xerogels under an ammonia or nitrogen atmosphere was also reported to yield VN powders. These methods require high temperatures and take long time. It is necessary to develop simple and cost-effective approach to synthesize vanadium nitrides. In this study, vanadium nitrides are directly prepared at low temperature (70°C) via one-step chemical precipitation using vanadium tetrachloride and urea. The structural and morphological properties of the vanadium nitrides were examined. The electrochemical properties of the vanadium nitrides were also investigated to evaluate them as electrode materials for electrochemical capacitors. Structural and morphological analyses showed that mesoporous vanadium nitride nanowhiskers with diameters in the range of 5-20 nm and lengths of 50-300 nm had a specific surface area of 144 m2/g and a pore diameter of 4.7 nm. The cyclic voltammetry and charge-discharge tests indicated that the vanadium nitrides obtained by one-step chemical precipitation stored charges via both electric double-layer capacitance and pseudocapacitance. The vanadium nitride electrode exhibited a specific capacitance of 598 F/g. After 5000 charge-discharge cycles, the vanadium nitride electrode had an equivalent series resistance of 1.42  and retain 83% of its initial specific capacitance. Keywords - Electrochemical capacitor, one-step chemical precipitation, specific capacitance, vanadium nitrides