Synthesis, structure and electrochemical characterization of transition metal nitride supercapacitors derived by a two-step transition metal halide approach

This research has been directed to synthesize transition metal nitrides (Ti, V, Nb, Mo, Zr, Ta, and W) at relatively low temperature using a two-step approach involving the reaction of transition metal halides with ammonia. The resultant precursors are heat-treated in ammonia to yield nano-crystalline particles.; The phase evolution and the structure of transition metal nitrides generated by the two-step halide approach have been studied and compared using x-ray diffraction and HRTEM. The cubic form (Fm3m) of transition metal nitrides begins to form at temperature as low as 400°C under NH3 atmosphere for titanium and vanadium nitrides with crystallite sizes less than 10nm. These nano-crystallites are spherical in shape exhibiting specific surface areas of 128.17m2/g and 71.28m2/g for titanium and vanadium nitrides, respectively. The highest surface area of 83.46m 2/g was obtained for NbN (Fm3m) synthesized at 600°C under NH 3 atmosphere. The specific surface area decreased with increasing nitridation temperature due to sintering effect for all the transition metal nitrides.; The electrochemical responses of these transition metal nitrides were analyzed by primarily cyclic voltammetry (CV) using 1M KOH electrolyte employing various scan rates (2mV/s to 100mV/s). Among all the nitrides synthesized, the highest gravimetric capacitance of 850F/g was exhibited by vanadium nitrides synthesized at 400°C scanned at 2mV/s. Such a high capacitance is due to the pseudocapacitance behavior confirmed by the two peaks in the CV plot at -0.8 and -0.6V. The gravimetric capacitance increased with increase in surface area and decreased with increase in scanning rates for all the synthesized transition metal nitrides. Tantalum nitride (Ta3 N5) exhibited the lowest gravimetric capacitance due to its poor electronic conductivity (<10-6/Om).; The results of these studies have shown that vanadium nitride appears to be the most promising electrochemical capacitor material. Other than vanadium nitride, the titanium, niobium and molybdenum nitrides exhibit similar responses. Results of the synthesis, structure and electrochemical response of the various binary transition metal nitrides are discussed followed by a detailed study on the synthesis mechanism and electrochemical behavior of the most promising vanadium nitride.