The present invention discloses a new construction of ultracapacitor utilizing particles of transition metal nitride having negligible amount of halide impurities. The construction is expected to attain high specific energy density by using transition metal nitride particles and higher reliability by avoiding potential corrosion of metal components with halide impurities. The transition metal nitride particles are preferably synthesized by basic ammonothermal process, which utilizes supercritical ammonia with alkali metal mineralizers. Transition metal nitride such as vanadium nitride, molybdenum nitride, titanium nitride, nickel nitride, neodymium nitride, iron nitride, etc. can be synthesized in supercritical ammonia with reducing mineralizers such as potassium, sodium, lithium, magnesium, calcium, and aluminum. Since supercritical ammonia has characteristics of both gas and liquid, it can over complicated fine structure or fine particles. The new method is suitable for forming a protective coating on complicated structure or forming micro- to nano-sized particles.